La Metallurgia Italiana - Numero 3, Marzo 2020 by aimnet3

Metallurgia Italiana

International Journal of the Italian Association for Metallurgy

n. 3 marzo 2020 Organo ufficiale dellâ&#x20AC;&#x2122;Associazione Italiana di Metallurgia. Rivista fondata nel 1909

La Metallurgia Italiana International Journal of the Italian Association for Metallurgy Organo ufficiale dell’Associazione Italiana di Metallurgia. House organ of AIM Italian Association for Metallurgy. Rivista fondata nel 1909

Direttore responsabile/Chief editor: Mario Cusolito Direttore vicario/Deputy director: Gianangelo Camona Comitato scientifico/Editorial panel: Livio Battezzati, Christian Bernhard, Massimiliano Bestetti, Wolfgang Bleck, Franco Bonollo, Bruno Buchmayr, Enrique Mariano Castrodeza, Emanuela Cerri, Lorella Ceschini, Mario Conserva, Vladislav Deev, Augusto Di Gianfrancesco, Bernd Kleimt, Carlo Mapelli, Jean Denis Mithieux, Marco Ormellese, Massimo Pellizzari, Giorgio Poli, Pedro Dolabella Portella, Barbara Previtali, Evgeny S. Prusov, Emilio Ramous, Roberto Roberti, Dieter Senk, Karl-Hermann Tacke, Stefano Trasatti Segreteria di redazione/Editorial secretary: Valeria Scarano Comitato di redazione/Editorial committee: Federica Bassani, Gianangelo Camona, Mario Cusolito, Carlo Mapelli, Federico Mazzolari, Valeria Scarano Direzione e redazione/Editorial and executive office: AIM - Via F. Turati 8 - 20121 Milano tel. 02 76 02 11 32 - fax 02 76 02 05 51 met@aimnet.it - www.aimnet.it

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Editoriale / Editorial

Intelligenza, resistenza, leggerezza. Tre compagni di viaggio per la nuova mobilità

Prof. Carlo Mapelli ................................................................................................................................ pag.4

Memorie scientifiche / Scientific papers Mobilità / Automotive

Heat treatments of EN AW 6082 aluminum forging alloy: effect on microstructure and mechanical properties

S. Cecchel, D. Ferrario, G. Cornacchia....................................................................................... pag.6

n. 3 marzo 2020

Anno 112 - ISSN 0026-0843

A study on PVD coatings for reduction of friction and wear of swashplate axial piston pumps and motors

R. Sola, P. Veronesi, B. Zardin, M. Borghi................................................................................ pag.14

Acciai Duplex / Duplex Stainless Steel Il test FIMEC per valutare la qualità di giunti saldati in acciaio duplex UNS S31803

R. Montanari, A. Varone, F. Bonollo, P. Ferro........................................................................... pag.24

indice

Attualità industriale / Industry news New approach for online tensile-structure properties evaluation on HSLA/ AHSS steel grades

edited by: Alessandro Ferraiuolo.......................................................................................... pag.32

Metallurgical design and production of AHSS grades DP800 and CP800 ISP and ESP thin slab technology at Acciaieria Arvedi in Cremona, Italy

R. Venturini, A. Bianchi, M. Andraghetti, C. Guarnaschelli, M.C. Cesile, P.E. Di Nunzio................... pag.43

Safety and Lightweight innovations for future mobility by using stainless steels

edited by: Marco Frigo, Stefan Lindner.................................................................................. pag.52

Scenari / Experts' Corner Effetti del Niobio negli acciai di terza generazione

a cura di: Fabio D'Aiuto................................................................................................. pag.61

Atti e notizie / AIM news Normativa / Standards ........................................................................................... pag.68 Comitati tecnici / Study groups........................................................................... pag.70

editoriale - editorial

Intelligenza, resistenza, leggerezza. Tre compagni di viaggio per la nuova mobilità La sfida della sostenibilità viaggia anche su strada e non coinvolge solo i costruttori di veicoli, ma l’intera filiera dei fornitori e dei soggetti che devono smaltire il veicolo al termine della sua vita. Questo è il dato che è emerso da MEMO, il workshop internazionale dedicato da AIM

Prof. Carlo Mapelli Politecnico di Milano

alla mobilità su strada del futuro. L’industria del veicolo continuerà a giocare un ruolo determinante nell’innovazione tecnologica ed integrerà al proprio interno diverse tecnologie che si struttureranno lungo due linee guida: fornire assistenza al conducente per migliorare la sicurezza durante la guida e diminuire le emissioni inquinanti in termini di particolato, anidride carbonica ed emissioni di metalli inquinanti. La prima sfida è lasciata allo sviluppo della sensoristica e dell’automazione, mentre è sulla secon-

“

L’industria del veicolo continuerà a giocare un ruolo determinante nell’innovazione tecnologica ed integrerà al proprio interno diverse tecnologie. Prof. Carlo Mapelli

da che il ruolo dei materiali metallici giocherà un ruolo determinante. Il 60% della diminuzione che si realizzerà nell’emissione dei veicoli sarà legato alla decremento della massa dei veicoli stessi, quindi questo richiederà l’introduzione di materiali resistenti e tenaci adeguati per la realizzazione di componenti strutturali di nuova concezione, concepiti secondo disegni che garantiscano un’adeguata rigidezza al veicolo. In un contesto in cui le leghe di alluminio e gli acciai ad alta resistenza copriranno un ruolo centrale, anche in forza della facilità di riciclare questi materiali, gli investimenti si concentreranno sulla progettazione e realizzazione di nuove leghe e trattamenti. Al contrario, appare sopravvalutato il ruolo che potrà essere coperto da componenti metallici realizzati con addittive manufacturing e nello specifico mediante la stampa 3D, a causa dei costi rilevanti della materia prima, dell’energia consumata per la produzione dei componenti, dei costi di investimento degli impianti e dell’intrinseca fragilità di componenti che a causa della struttura di solidificazione sono afflitti da una fragili-

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editoriale - editorial

tà che ne confina l’applicazione a componenti caratterizzati da limitate sollecitazioni dinamiche. Quindi, la tradizionale filiera di fornitura non verrà scardinata ma sarà comunque chiamata ad un notevole sforzo volto a contenere le emissioni di gas serra in un’ottica di diminuzione complessiva all’interno dell’intero ciclo produttivo. La diffusione dei veicoli a trazione elettrica appare meno scontata e pervasiva di quanto potrebbe sembrare, non solo per la limitata autonomia, ma anche perché il sistema di accumulo a batterie espone la filiera di fornitura a significativi rischi geopolitici, in quanto il litio, il nickel ed il cobalto provengono da pochi paesi e sono controllati da oligopoli strutturati su scala globale a differenza del mercato degli idrocarburi. Appare assai più conveniente e sicuro puntare sulle piattaforme per i cicli di circolazione urbana e su metano e GPL per i veicoli commerciali o comunque per tutti i trasporti su lunga percorrenza al di fuori delle aree urbane. Tali soluzioni sono assai più sostenibili a livello economico e, soprattutto, il sistema italiano dovrebbe puntare sulla loro difesa anche sulla base dell’elevato livello di qualità della componentistica in getti per motori endotermici che il sistema delle fonderie europee (ed in particolare italiane) ha saputo sviluppare negli ultimi decenni. E’ doveroso che vi sia un impegno a tutti i livelli per aiutare la filiera metalmeccanica nazionale legata al settore del veicolo, non in una logica protezionistica ma di promozione delle piattaforme tecnologiche più efficienti e sostenibili da un punto di vista economico ed ambientale in cui l’industria italiana ha molto da offrire e in cui possediamo un patrimonio di conoscenze che non deve essere svalutato e mortificato da portatori di interesse che attraverso l’imposizione di ambigui quadri regolatori promuovono altri sistemi solo per colmare uno scarto tecnologico e di efficienza produttiva che non hanno saputo colmare sul campo con l’industria della mobilità italiana ed europea.

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Memorie scientifiche - MobilitĂ

Heat treatments of EN AW 6082 aluminum forging alloy: effect on microstructure and mechanical properties S. Cecchel, D. Ferrario, G. Cornacchia Aluminum components for structural applications, such as some automotive suspension parts, are usually made by forging. In this context, the use of aluminum EN AW 6082 alloy application for highly demanding structural componen-

ts is increasing due to lightweighting needs. Anyway, its response to age hardening is still not well investigated and the information about the time-temperature parameters and the relative mechanical properties are generally scarcer. In order to extend the knowledge in this field, during the present research a Design Of Experiment methodology was applied and T6 and T5 heat treatment were analysed. Microstructural and mechanical behaviour were studied. The output

of these experiments constitutes a useful database of the properties of EN AW 6082 alloy in the considered range of

data. This information can be used for the designing of different structural components where either an optimization of the elongation or a maximum yield strength could be required depending on the specific application. The results also suggest that for many applications a T5 heat treatment, with high mechanical properties, a lower thickness of recrystalized area and a reduced cost, could be a good option.

PAROLE CHIAVE: ALUMINUM ALLOYS, MECHANICAL CHARACTERIZATION, AUTOMOTIVE, FORGING, MICROSTRUCTURE, 6XXX SERIES.

INTRODUCTION Usually, aluminum components for structural applications, such as some automotive suspension parts, where reliability and human safety are critical are made by forging. This process offers higher tensile, yield, fatigue strength and ductility respect to castings, thanks to an improved soundness and chemical uniformity [1]. Indeed, deformation processes refine the microstructure of alloys and reduce defects typical of castings, such as voids and porosity (gas and shrinkage), oxide films and inclusions, non-uniform microstructure, and coarse intermetallic phases [2]. Aluminium forging guarantees near net shape parts, high

Silvia Cecchel, Davide Ferrario Streparava SpA, Adro (BS), Italy s.cecchel@streparava.com

Giovanna Cornacchia

Department of Mechanical and Industrial Engineering, University of Brescia, Brescia, Italy

mechanical properties, a good surface finishing and are thus suitable for highly stressed parts [3]. The aluminium forging use is an increasingly need of the last years also for highly demanding structural products [3] mainly to further extend the automotive lightweighting at these challenging applications. Indeed, one of the major current trends of the

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Scientific papers - Automotive automotive industry is the need of lighter components to

extruded billet used for the forging process as a recrystalli-

reduce the vehicle emissions [4-9].

zed surface, due to the high temperature and high strains of

In this context one of the most widely employed alloy

the process. Consequently, the elevate solubilization tem-

forging stock is aluminium EN AW 6082, extruded from a

peratures generate inevitably a grain growth [11-13]. Thus,

Direct Chill (DC) billet [10], due to its good corrosion re-

the potential benefit of a T5 heat treatment on the micro-

sistance and mechanical properties. This is an age-harde-

structural behaviour and mechanical properties could be

nable alloy usually subjected to T6 heat treatment to im-

an interesting topic for the present application. Again, very

prove the as-forged properties.

few investigations can be found about the properties on EN

Although the application of this material is increasing in the

AW 6082 -T5 forging [11-13].

field of structural automotive components, its response

To better understand all these aspects and to analyse the

to age hardening is still not well investigated. Indeed, the

opportunity to extend the use of this alloy at even further

information about the time-temperature parameters and

stressed applications, the general knowledge about the ef-

the relative mechanical properties is generally scarcer. In

fect of the heat treatments on EN AW 6082 aluminum for-

particular, some exception can be found in Y. Birol et al. [1,

ging need to be improved. At this purpose, during the pre-

11-13] that studied the relation between some heat treat-

sent research a Design Of Experiment methodology was

ment parameters and the mechanical properties of EN AW

applied in order to analyse a wide range of heat treatment

6082 alloy. As regards T6 treatment, temperatures between

parameters applied at an actual EN AW 6082 aluminum for-

510°C and 530°C and 170°C and 190°C were investigated

ged components. T6 and T5 heat treatment were studied.

for solubilization and age hardening respectively. In gene-

T5 heat treatments are expected to reduce the typical sur-

ral, a detailed microstructural investigation was performed,

face recrystallization induced by the high solubilization

but few data about the mechanical properties achieved

temperature, while the use of real automotive parts is a fun-

after any heat treatment condition are available.

damental aspect of the research aimed to be as reliable as

G. Mrówka-Nowotnik et al. [14] deepened the tensile pro-

possible in the perspective of a final industrial application.

perties of EN AW 6082 -T6 forging samples, after a solu-

Microstructural and mechanical behaviour of these diffe-

bilization at 575°C followed by agings between 130°C and

rent heat treatments were examined and compared.

170°C. Anyway, the solubilization temperature is very closed to the eutectic transition (577°C [15]) this treatment ta-

MATERIALS AND METHODS

kes a chance to induce a partial melting of the eutectic into

Samples description

the aluminum matrix. Indeed, further investigation about

About 50 hot forged suspension automotive componen-

the mechanical properties at lower solubilization tempe-

ts made of aluminum EN AW 6082 alloy were used for the

ratures are required.

current investigation. The chemical composition of this al-

Another relevant aspect to be taken into account is that the

loy is reported in Tab. 1.

Tab.1 - Chemical composition of the EN AW 6082 components.

EN AW 6082

1.108

0.207

0.067

0.926

0.891

0.009

0.019

0.003

0.014

0.185

0.032

After the forging process, the parts were heat treated in a

Tab. 2. The solubilization time was fixed at 4 hours. As it can

preheated oven with different T6 and T5 time and tempera-

be noted, a Design Of Experiment (DOE) methodology was

ture parameters reported in

applied to assess the influence of the time and temperature

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Memorie scientifiche - MobilitĂ of heat treatment conditions on the final alloy's mechanical

used hereafter. At least three components for each condi-

properties.

tion were tested.

Tab. 2 also reports the samples nomenclature that will be

Tab.2 - Time-temperature factors for each heat treatment conditions.

Tsolubilization

Taging

taging

Samples

550

160

550

160

550

190

550

190

520

160

520

160

520

190

520

190

180

200

For the tensile test, according to UNI EN ISO 6892-1:2009

load applied for 15 s on the etched traverse surface, by me-

standard, cylindrical samples with a total length of 120 mm

ans of a Mitutoyo HM-200 instrument according to ASTM

and a gauge diameter of 10 mm were machined from the

E92-16. At least six measurements were taken and avera-

same section of the automotive components studied.

ged for each heat treatment condition.

For the metallographic observations, samples before and

The tensile tests were carried out on three samples for

after heat treatment were sectioned orthogonally to the

each heat treatment condition, following UNI EN ISO 6892-

forging direction and, also for this analysis, in the same area

1:2009. An electromechanical testing machine Instron 3369

of the parts. The sample sections have average dimensions

at a strain rate of 2 mm/min was used.

of about 25x60 mm. RESULTS AND DISCUSSION Microstructural observation

Microstructural observation

The samples surfaces were prepared with standard metal-

The grain structure of transversal sections of the forged

lographic techniques (ground with SiC papers and poli-

suspension components analysed are reported in Fig. 1

shed with 1 Âľm diamond paste) and were etched with SAPA

for the as forged, T5 and two T6 conditions. The as-forged

solution. The microstructure was examined using a Leica

sample has a very fine structure in the middle and a very

DMI 5000M Optical Microscope (OM) and a Leica G.26 Ste-

thin (about 0.5 mm) area at the surface boundary characte-

reo Microscope (SM).

rized by recrystallized grain, due to the high temperatures reached during the process. After the heat treatments, the

Hardness and tensile test

extension of the coarse-grains area increases with the tem-

Vickers microhardness tests were carried out under 0.3 Kgf

perature and reach elevate thickness after T6 heat treat-

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Scientific papers - Automotive ment (up to about 3.5 mm). The effect of T5 heat treatment

growth in comparison with T6 heat treatment (3 mm).

is very similar, with only a slight less extension of the grain

Fig.1 - Grain structure across section of samples in the as forged and heat-treated conditions. The microstructures of as forged condition, “D” T6 and “I”

grain boundaries) can be attributed to the bounding of si-

T5 heat treatment are reported Fig. 2 a, b, c respectively.

licon in the α-AlFeMnSi compounds. After the heat treat-

In general, it can be noted a fine dispersion of Mg2Si [1] and

ment a more uniform distribution of finer particles than the

plate-like particles, probably cubic α-AlFeMnSi [16], at the

as-forged condition can be observed. Similar considera-

grain boundaries. These particles are mainly aligned in the

tion can be advanced for both the heat treatment, with only

forging direction especially for the as-forged condition.

a slightly more pronounced effect for T6 heat treatment.

The presence of precipitate-free zones (white areas at the

Fig.2 - Microstructure of as forged condition (a), “D” T6 heat treatment (b) and “I” heat treatment (c).

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Memorie scientifiche - Mobilità Hardness

increases up to 115±3 HV and 114±5 HV respectively, as it

Fig. 3 reports the microhardness profile along a transversal

can be gathered from “D” and “I” heat treatment reported

section for As Forged (AF), “D” T6 heat treatment and “I” T5

as example in Fig. 3. In these cases, the hardness is more

heat treatment. The average hardness in the As Forged (AF)

uniform across the section without any peaks at the sample

condition is 87±6 HV. Peaks of hardness are observed at the

edges in comparison with the as-forged condition (“AF”

sample edges and could be related to the high strain ratio

sample).

of the surface. After T6 and T5 heat treatment the hardness

Fig.3 - Microhardness profile of As Forged (AF) condition, “D” T6 heat treatment and “I” heat treatment. Tensile test

with the solubilization temperature (from 520°C to 550°C).

The tensile properties (yield strength, stress at break,

Only in the case of 190°C of aging temperature, a slight

Young modulus and elongation%) of EN AW 6082 alloy

over-aging is observed after 6 hours of heat treatment (re-

after solubilization and artificial aging (samples from A to

duction of both yield strength and elongation%). It is wor-

H) and artificial aging only (sample I to N) at various time

thwhile to note that these considerations can be useful for

and temperatures are summarized in Tab. 3. For a better

the setup of the heat treatment parameters also for all the

understanding of the data, the effect of temperature on

condition comprised within the range of time-tempera-

yield strength and elongation of EN AW6082 T6 Al alloy is

tures considered for the present activity. Good mechani-

schematized in Fig. 4. In particular, the grey and black poin-

cal properties are observed also after T5 heat treatments.

ts represent 520°C and 550°C solubilization temperatures

In this case, higher aging temperature and time lead to a

respectively. From the analysis of T6 results it can be clear-

decrease of the mechanical properties, probably due to an

ly noted the effect of the aging: the yield strength increases

overaging. The results suggest that for many applications a

while the elongation percentage decreases with the time;

T5 heat treatment, with high mechanical properties, a lower

an opposite trend is observed with the aging temperature.

thickness of recrystalized area, could be a good option. It

Keeping constant the aging parameters, the yield strength

should be noted that thermal treatment T5 would also lead

increases and the elongation percentage slightly decreases

to cost savings.

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Scientific papers - Automotive Tab.3 - Mechanical properties of EN AW 6082 alloy after T5 and T6 heat treatment

σp02 [MPa]

σm [MPa]

E [GPa]

A(%)

Avg.

550°C 4h

160°C 2h

249

352

550°C 4h

160°C 6h

324

383

550°C 4h

190°C 2h

372

386

550°C 4h

190°C 6h

354

367

520°C 4h

160°C 2h

217

311

520°C 4h

160°C 6h

297

344

520°C 4h

190°C 2h

305

332

520°C 4h

190°C 6h

302

323

180°C 4h

326

349

180°C 8h

321

340

200°C 4h

295

317

200°C 8h

272

303

Fig.4 - Effect of temperature on yield strength and elongation of EN AW6082 T6 Al alloy.

CONCLUSIONS

• The microstructural investigation revealed that:

The use of aluminum EN AW 6082 alloy application for hi-

o From a macrostructural point of view, the as-forged

ghly demanding structural components is increasing due

sample has a very fine structure in the middle and a

to lightweighting needs. Thus, further investigation about

very thin (about 0.5 mm) area at the surface boundary

the most proper heat treatment parameters for different

characterized by recrystallized grain. After the heat

application are required. At this purpose, during the pre-

treatments, the extension of the coarse-grains area

sent research a Design Of Experiment methodology was

increases with the temperature having elevate thick-

applied and T6 and T5 heat treatment were analysed. Mi-

ness both after T5 (3 mm) and T6 (up to about 3.5 mm)

crostructural and mechanical behaviour were studied and

heat treatments. Indeed, T5 heat treatment led only to

the following conclusions can be retrieved.

a slight reduction of the grain growth in comparison

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Memorie scientifiche - Mobilità with T6 heat treatment, probably due to the high reduction ratio adopted in the area considered for the microstructural investigation.

after 6 hours of aging at 190°C. o After T5 heat treatments good mechanical properties are reached, similar to many T6 conditions. In this

o The microstructure is composed of a fine dispersion

case, higher aging temperature and time lead to a de-

of Mg2Si and plate-like particles, probably cubic

crease of the mechanical properties, probably due to

α-AlFeMnSi, at the grain boundaries. After the heat

an overaging.

treatment a more uniform distribution of finer parti-

o The average values ranged from 271 MPa, 303 MPa and

cles than the as-forged condition can be observed for

8% to 372 MPa, 386 MPa and 16% for yield strength,

both T5 and T6 heat treatments.

stress at break and elongation percentage respecti-

• The hardness analysis revealed that:

vely.

o Values of 87±6 HV, 115±3 HV and 114±5 HV were me-

Finally, it can be concluded that the output of these experi-

asured in the AF, D-T6 and I-T5 conditions respecti-

ments constitutes a useful database of the properties of EN

vely.

AW 6082 alloy in the considered range of data. This infor-

• The tensile properties revealed that:

mation can be used for the designing of different structural

o For T6 the yield strength increases while the elonga-

components where either an optimization of the elongation

tion percentage decreases with the aging time, while

or a maximum yield strength could be required depending

an opposite trend is observed with the aging tempe-

on the specific application. The results also suggest that for

rature. Keeping constant the aging parameters, the

many applications a T5 heat treatment, with high mechani-

yield strength increases and the elongation percenta-

cal properties, a lower thickness of recrystalized area and a

ge slightly decreases with the solubilization tempera-

reduced cost, could be a good option.

ture (from 520°C to 550°C). An over aging is observed

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Scientific papers - Automotive References [1]

Y. Birol, O. Ilgaz, Effect of cast and extruded stock on grain structure of EN AW 6082 alloy forgings, Materials Science and Technology, (2014), 30: 860-866, DOI:10.1179/1743284713Y.0000000407

[2]

P. N. Anyalebechi, Effect of process route on the structure, tensile, and fatigue properties of aluminum alloy automotive steering knuckles, International Foundry Research (2011), 63: 32-43

[3]

Y. Birol, E. Gokcil, S. Akdi, Potential of twin-belt-cast EN AW 6082 blanks for the manufacture of wishbone suspension forgings, Int J Adv Manuf Technol (2017) 92:3693–3701, DOI 10.1007/s00170-017-0446-3

[4]

S. Cecchel, D. Ferrario; Numerical and experimental analysis of a high pressure die casting Aluminum suspension cross beam for light commercial vehicles, La Metallurgia Italiana, 2016, vol. 108, issue 6, pp. 41-44

[5]

M. Faccoli, D. Dioni, S. Cecchel, G. Cornacchia, A. Panvini; An experimental study to optimize the heat treatment of gravity cast Sr-modified B356 aluminum alloy, Transactions of Nonferrous Metals Society of China, 2017, 27 (8): 1698-1706, https://doi. org/10.1016/S1003-6326(17)60192-4.

[6]

G. Cornacchia, S. Cecchel, A. Panvini; A comparative study of mechanical properties of metal inert gas (MIG)-cold metal transfer (CMT) and fiber laser-MIG hybrid welds for 6005A T6 extruded sheet, The International Journal of Advanced Manufacturing Technology, 2017, vol. 94, Issue 5–8, pp. 2017–2030, https://doi.org/10.1007/s00170-017-0914-9

[7]

D. Dioni, S.Cecchel, G.Cornacchia, M. Faccoli, A. Panvini; Effects of artificial aging conditions on mechanical properties of gravity cast B356 aluminum alloy, Transactions of Nonferrous Metals Society of China, 2015, vol. 25, issue 4, pp. 1035-1042

[8]

S. Cecchel, G. Cornacchia, M. Gelfi; Corrosion behavior of primary and secondary AlSi High Pressure Die Casting alloys, Materials and corrosion, 2017, vol. 68, issue 9, pp. 961-969, https://doi:10.1002/maco.201709526

[9]

S. Cecchel, G. Cornacchia, M. Gelfi, A study of a non-conventional evaluation of results from salt spray test of aluminum High Pressure Die Casting alloys for automotive components, Material and Corrosion, vol. 70, p.70-78, 2019 https://doi.org/10.1002/ maco.201810307

[10]

UK Aluminium Industry Fact Sheet 12 (2016) Aluminium forging. http://www.alfed.org.uk/files/Fact%20sheets/12-aluminiumforging.pdf

[11]

E. Gokcil, S. Akdi & Y. Birol, A novel processing route for the manufacture of EN AW 6082 forged components; 19, 311-314, (2015)

[12]

Y. Birol, E. Gökçil, M. A. Guvenc, S. Akdi, Processing of high strength EN AW 6082 forgings without a solution heat treatment, Materials Science & Engineering A 674 (2016) 25–32

[13]

Y. Birol, E. Gokcil, S. Akdi, Potential of twin-belt-cast EN AW 6082 blanks for the manufacture of wishbone suspension forgings, Int J Adv Manuf Technol (2017) 92:3693–3701 DOI 10.1007/s00170-017-0446-3

[14]

G. Mrówka-Nowotnik, J. Sieniawski, A. Nowotnik, Effect of heat treatment on tensile and fracture toughness properties of 6082 alloy, Journal of achievements in materials and manufacturing engineering, 32, 162-170, (2009)

[15]

G. Mrówka-Nowotnik, J. Sieniawski, M. Wierzbiñska, Intermetallic phase particles in 6082 aluminium alloy, Archives of Materials Science and Engineering, 28, 69-76, (2007)

[16]

Y. Birol, S. Akdi, Cooling slope casting to produce EN AW 6082 forging stock for manufacture of suspension components, Trans. Nonferrous Met. Soc. China 24, 1674−1682, (2014) DOI: 10.1016/S1003-6326(14)63240-4

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Memorie scientifiche - Mobilità

A study on PVD coatings for reduction of friction and wear of swashplate axial piston pumps and motors R. Sola, P. Veronesi, B. Zardin, M. Borghi

Swashplate axial piston pumps and motors are widely used under severe conditions because of their capability to operate at high pressure values and various ranges of speeds, maintaining a good efficiency. In some operating conditions however, the machine efficiency may be relatively low because of insufficient lubrication and this causes rapid wear and high friction losses. PVD coatings may help in reducing the friction thus having a positive impact on the machine efficiency. For this reason, pin on disk testing were conducted to investigate the friction and wear behavior of different PVD coatings on various metallic substrates. A copper and tin alloy coated with PA-CVD DLC, a carburized 17NiCrMo7 with a PVD DLC coating and a nitrided 41CrAlMo7 PVD coated with TiCN were studied. Results of tribological testing, carried out at 0.2 and 0.5 m/s as sliding speed and 5000, 10000 and 20000 rounds as ending conditions, showed that wear and friction had been reduced; moreover, microstructure features were analyzed for understanding underlying mechanism and to allow selecting the most suitable coating for the application on the swashplate axial piston machine.

KEYWORDS: PVD COATINGS, WEAR, FRICTION, STEEL, BRONZE, SWASHPLATE AXIAL PITON PUMP, LUBRICATION INTRODUCTION Axial piston pumps are positive displacement pumps, which typically transfer a definite volume of fluid from a low pressure port to a high pressure port at each shaft rotation by means of variable volume chambers (1). They convert the mechanical power coming from a prime mover into hydraulic power. The shaft rotation is transmitted to the cylinder block of the pump which drives into rotation the piston-shoes assemblies. As the shoes slides on the swashplate, the pistons are forced to translate within the cylinder block and to de-liver fluid. The valve plate then allows distribution of the fluid in the circuitry. Hydraulic fluid act

R. Sola

Industrial Research Centre for Advanced Mechanics and Materials (CIRI-MAM), University of Bologna, Bologna (Italy) e-mail address: ramona.sola@unibo.it

P. Veronesi, B. Zardin, M. Borghi Engineering Department “E. Ferrari”, University of Modena and Reggio Emilia (Italy)

also as a lubricant. However, these interfaces can be critical and influence the efficiency of the pump. Moreover, the complex lubrication mechanism involved in this kind of pumps play a relevant role also in determining the wear of their components and the leakage flows. Deterioration

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Scientific papers - Automotive of the lubrification mechanism leads to poor performance

stance and damping capacity, because the combination of

and reduced life of the component (2). Also, the presence

the properties of carbons allotropes of sp2 and sp3 hybridi-

of wear debris in the hydraulic fluid worsen the wear on the

zations (3). Titanium ni-tride coating (TiN) are still regarded

piston oil pumps components

as coatings suitable con anti-wear protection (10) in order

This study consists in an experimental comparison of ther-

to extend the service life of cutting tools, stamping tools,

mochemical treatments and coatings to improve wear resi-

bearings. However, com-parative tests of multiple different

stance and to reduce friction of axial pistons pumps. Solu-

solutions that solve a problematic or improve a specific pro-

tions are chosen based on the most common metallic alloy

perty, such as durability of the coatings, lack in literature.

used in fabrication of axial pistons pumps. Ankint Tyagi et

Finally, this literature review displays a lack of combination

al. in (3) reveal that even 15-20% reduction in wear friction

between laboratory comparative studies of surface treat-

can significantly reduce economic costs in relation to envi-

ments and test for industrial applications. This study com-

ronmental benefits and the application of coatings is one

bines two approach by proposing wear tests on solutions

of the most widely used route. Among the employed tri-

(DLC and TiCN coatings) to improve wear resistance on an

bology techniques for coating deposition the most known

industrial axial piston pump.

are physical vapor deposition/chemical vapor deposition (PVD/CVD), ion beam deposition, radio frequency magne-

EXPERIMENTAL TECHNIQUES

tron sputtering and electro deposition. Nesbit in (4) propo-

In the current study several types of metallic materials with

ses a list of suitable materials for pump design, Kalin et al in

different heat treatment and coatings are investigated: a

(5) studied the influence of wear resistance of DLC-coated

bronze CuSn12 (nominal composition wt%: C 0.25%, Pb

shoes by following wear evolution with optical microscopy

0.35%, P 0.30%, Cu balanced), a carburized 16NiCrMo7

and leakage monitoring while Sun in (6) focuses on the ef-

(nominal composition wt%: C 0.20%, Si 0.75%, Cr 1.10%,

fect of a N based coatings on the barrel surface in the con-

Ni 1%, Fe balanced) and a 41CrAlMo7 quenched and tem-

tact barrel/valve plate, Sola et al. in (7,8) studied the effect

pered (nominal composition wt%: C 0.40%, Mn 0.75%, Cr

of â&#x20AC;&#x153;duplex treatmentâ&#x20AC;? of wear and corrosion resistance of

1.70%, Mo 0.40%, Al 1%, Fe balanced). In Fig. 1 the micro-

steels and Tonelli et al. in (9) tested new solutions coating/

structures of as received materials, by optical microscope,

substrate for Improvement of wear resistance of compo-

are reported: bronze CuSn12 presents a rough as casted

nents for hydraulic actuators. DLC coatings are famous for

microstructure, while two steel exhibit tempered martensi-

their excellent wear resistance, low friction, corrosion resi-

te with different morphology due to carbon content.

CuSn12 16

NiCrMo7

41CrAlMo7

Fig.1 - Optical micrographs of as received materials. For all the metal alloys disc-shaped samples were cut from

cleaned with ultrasonic alkaline bath and de-ionized water.

a 40 mm in diameter bar and then hardened with different

Commercial anti-wear coatings are selected for the analysis

treatment and coatings. Before the thin coating deposition,

purpose. This has been done to obtain credible analysis on

the substrate was carefully polished (Ra maximum 0.2) and

the impact on the performance of the analysed film. Bron-

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Memorie scientifiche - Mobilità ze CuSn12 was coated with functional coating deposited by

Tribological test was performed on a ball-on-disk tribome-

plasma-assisted chemical vapor deposition (PACVD) using

ter (CSM Instruments, Switzerland) in the rotating mode. An

acetylene of 99.6% purity with substrate pulsed bias in the

alumina ball with 6 mm diameter were used as friction pairs.

frequency range (20-100) kHz. The deposition temperatu-

All of the disks were cleaned with acetone and dried. The

re was 350°C and the acetylene flow rate was 8.3x10-6 m3/s.

tests were conducted at room temperature and ambient

PLC PA-CVD coating on 16NiCRMo7 steel was deposited in

humidity under dry sliding conditions of 10N load, with two

same condition as bronze but the deposition temperature

sliding velocity 0.2 m/s e 0.5 m/s. After the total distances

was lower than 200°C to not soften carburized substrate.

of 5000, 10 000, 20 000 laps (for durability analysis) friction,

41CrAlMo7 was treated with a duplex treatment combining

morphology of wear tracks using SEM (Oxford instrument)

plasma nitriding and TiCN deposition. Nitriding was carri-

and cross-section profile of wear tracks by confocal micro-

ed out in an industrial reactor using DC pulse discharge at

scope (Leika) are evaluated to understand the tribological

390°C for 10h in a 25% N2 + 75% H2 gas mixture. Then thin

behavior and the durability of the solutions proposed. Tri-

coating was deposited by cathodic arc evaporation (PVD)

bological parameter are chosen to simulate the real com-

from Ti target in Ar + N2 mixture. The process was con-

mon operating conditions of industrial oil pumps. The wear

ducted at temperature of 350°C, under working pressure

rate was calculated from formula (1):

range from 0.1 - 0.2 Pa. The substrate bias was -70 V. 1)

where V is the wear volume [mm3], P is normal force [N] and

of analyzed samples. 41CrAlMo7 “duplex” treated steel

L is total sliding distance [m]. Much more information on

present a homogeneous diffusion layer, 100 µm thick, un-

the application of pin on disk test as method for tribological

der a little porous white layer, 4 µm thick (Figs. 2 - 3). Thin

characterizations of heat treated and hardened metallic ma-

and homogeneous TiCN layer is visible above white layer,

terials are visible in (11-12).

well adherent to the substrate with 1-2 µm thickness. Car-

The adhesion of the coatings was determinded by scratch

burized 16NiCrMo7 steel, Figs. 4 - 5, presents a hardened

test. In the scratch testing, as described in (13-16), a dia-

layer 400 µm thicks underlying PLC coating, homogeneous,

mond Rockwell C stylus (120° cone, 100 µm radius spheri-

well adherent to substrate and 2-3 µm thick. DLC coating

cal tip) was pulled over the coated surface with continuou-

on CuSn12 bronze, visible in Fig. 6, is homogeneous, not

sly increasing normal load from 1 to 30 N. Scratch test were

porous, well adherent to substrate with 2-3 µm as thickness.

performed on a Micro-combi Tester (Anton Paar Gmbh,

In Fig. 7 and 8 microhardness profile of 41CrAlMo7 “duplex”

Germany) according to the ASTM C1624 – 05 (2015) stan-

treated steel and 16NiCrMo7 carburized steel respectively

dard for ceramic coatings scratch testing. The loading rate

are reported. Total hardening depth of 41CrAlMo7 is about

was 11,6 N/min and stylus progressive speed was 2 mm/

250 µm and effective hardening depth is 120 µm. In carburi-

min. The scratch length was 5 mm. Optical (OM, MCT, An-

zed 16NiCrMo7 total hardening depth is near 1 mm.

ton Paar GMBH, Germany) microscopy examination was

Figs. 9 – 11 exhibit friction trends during 20 000 laps as sli-

carried out for the coatings after each scratch testing in or-

ding for all the samples examined in untreated and coated

der to determine the crack behavior and the critical loads

conditions, and in Tab. I mean friction values are reported.

Lc1, corresponding to the cohesive failure (occurrence of

From Table I and friction behavior of Fig. 9 it can be seen

the first cracks on the surface), Lc2 – the first symptoms of

that “duplex” treatment does not improve friction compa-

adhesive failure (spalling or chipping), Lc3 – severe exposi-

re to untreated 41CrAlMo7 steel, but in some case friction

tion of the removal.

value of TiCN sample increase in 10-20%. TiCN coatings are well-known to increase wear resistance and not for re-

RESULT AND DISCUSSION

duction of friction but improving surface finishing could be

Figs. 2 – 6 show optical and scanning electron micrographs

possible decrease friction.

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Scientific papers - Automotive

Fig.2 - Optical Micrograph of 41CrAlMo7 with “duplex” treatment

Fig.3 - SEM-BSE Micrograph of 41CrAlMo7 with “duplex” treatment

Fig.4- Optical Micrograph of carburized 16NiCrMo7 with

Fig.5 - SEM-BSE Micrograph of carburized 16NiCrMo7 with

PLC coating

Fig.6 - SEM-BSE Micrograph of bronze with DLC coating

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Memorie scientifiche - Mobilità

Fig.7 - Microhardness profile of 41CrAlMo7 duplex treated steel

Fig.8 - Microhardness profile of carburized 16NiCrMo7

Carbon based coatings (PLC and DLC) exhibited a different

low wear resistance. In Figs. 12 – 14 W values during sliding

behavior from TiCN because they greatly decrease friction

for all the sample analyzed in the treatment condition con-

values in more than 50%. This implies that the frictional

sidered are reported. W value of CuSn12 bronze with DLC

forces during the dry sliding test against the Al2O3 ball are

coating is greatly lower than untreated sample (Fig. 14),

dramatically reduced after deposition on the PLC and DLC

especially at 0.5 m/s as sliding speed where the reduction

coatings. The best anti-frictional properties are exhibited by

of W value (and so wear resistance increase) is about 91%,

the DLC coating – the coefficient of friction (COF) is reduced

89% and 99% after 5000 laps, 10 000 laps and 20 000 laps

by more than 5 times. Regarding PLC on carburized 16Ni-

respectively, compared to untreated bronze. At 0.2 sliding

CrMo7, it can be seen that friction during sliding is always

speed adhesion mechanism are favored and the improve-

lower that uncoated sample, also after 10 000 laps when fri-

ment in W value is near to 90% at 10 000 and goes to 40%

ction value little increase due to wear of coatings. Similar

at the end of the test. PLC coating (Fig. 14) improves wear

friction trend of CuSn12 bronze in which DLC coating con-

resistance strongly decreasing W value about 90% to 10 000

tributes to decrease friction and after 10 000 laps friction va-

laps and then, due to little wear of coating, near 60%, com-

lue increase because of wear of coating, especially at lower

pare to carburized steel. “Duplex” treatment on 41CrAlMo7

sliding speed because adhesion mechanism is help. At 0.5

(Fig. 14) does not alter friction coefficient but it greatly im-

m/s at sliding speed DLC coating is almost untouched from

proves wear resistance because it promotes a reduction of

tribological test.

W value greater than 90%, compare to untreated sample,

At the end of tribologial test, wear track of the sample are

in any sliding condition tested. “Duplex” treatment, com-

analyzed with a confocal microscope to estimate the wear

bining plasma nitriding and TiCN coating, improves wear

volume and then calculate the Wear rate value (W). High W

resistance of 41CrAlMo7 quenched and tempered steel and

value means high volume removed during sliding and so

this solution guarantees a high duration.

Fig.9 - Friction behavior during sliding for 41CrAlMo7 untreated (NT) and “duplex” treated (TiCN) La Metallurgia Italiana - marzo 2020

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Scientific papers - Automotive

Fig.10 - Friction behavior during sliding for 16NiCrMo7 carburized (NT) and with PLC coating (PLC)

Fig.11 - Friction behavior during sliding for CuSn12 bronze as received (NT) and with DLC coating (DLC)

Tab.1 - coefficient of friction means (COF) values

alloy

treatment NT

16NiCrMo7 carburized PLC

NT 41CrAlMo7 TICN ("DUPLEX")

NT CuSn12 bronze DLC

La Metallurgia Italiana - March 2020

sliding speed [m/s]

5000 laps

10 000 laps

20 000 laps

0.2

0.62

0.59

0.71

0.5

0.69

0.74

0.69

0.2

0.22

0.21

0.33

0.5

0.21

0.23

0.29

0.2

0.56

0.67

0.57

0.5

0.69

0.79

0.2

0.71

0.67

0.70

0.5

0.60

0.69

0.80

0.2

0.58

0.64

0.71

0.5

0.56

0.50

0.49

0.2

0.27

0.25

0.34

0.5

0.29

0.25

0.22

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Memorie scientifiche - MobilitĂ

Fig.12 - Wear rate graph of CuSn12 bronze

Fig.13 - Wear rate graph of 16NiCrMo7 steel

Fig.14 - Wear rate graph of 41CrAlMo7 steel Reduction of the frictional forces resulted in a significant re-

soft bronze; during sliding substrate and coating are deformed

duction of the wear rate in tribological tests as confirmed obser-

in different way, and this causes high stresses in DLC coating and

ving the surface topography of wear track reported in the Figs.15

so its cracking. The morphology of the wear track of the DLC co-

â&#x20AC;&#x201C; 19, in which it is clearly visible that the wear tracks differ signifi-

ating shows that the film was practically deformed along with the

cantly for all the samples. Severe wear on the uncoated CuSn12

substrate. The black areas are attributed to the presence of DLC

can be observed in Fig. 15. Some of the wear products are plasti-

coating and small patches of the coating were detached from

cally deformed and form chip-like areas. Figs. 18-19 show wear

the substrate and acted as an abradant in the bell-disk contact

tracks of coated with DLC. A great abrasive wear is visible in un-

site. Tensile cracks emerged as a result of ploughing.

treated sample while DLC coating is little worn and its main we-

Wear tracks of carburized 16NiCrMo7 and PLC coated steel are

aring mechanism is delamination (Fig. 16) because of cracking

reported in Figs. 17 and 18. Abrasive and triboxidative wear is

of the coating. A high hardness coating, indeed, is layered on a

visible in uncoated steel with deep grooves inside track and ac-

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Scientific papers - Automotive cumulation of oxidized wear debris at boarder track. Steel with

A comparison of the wear rate of the all solutions considered for

PLC coating is little worn and its wear track presents only a little

application in industrial pistons pumps in reported in Fig. 20, in

triboxidation without abrasion grooves.

which it is clearly visible that CuSn12 exhibits high wear resistan-

Wear tracks of 41CrAlMo7 untreated and â&#x20AC;&#x153;duplexâ&#x20AC;? treated are

ce at the beginning of the sliding but, due to the cracking and de-

shown in Fig. 19. Tracks analysis confirms the great improve-

tachment of the coating, it suffer a low durability. Duplex treated

ment in wear resistance promoted by nitriding combined with

41CrAlMo7 is a good solution for high number of laps. The best

TiCN coating. Untreated steel is abraded while coated is very

wear performance, among the solutions considered, is presen-

wore after 20 000 laps. This solution guarantees a high durability

ted by 16NiCrMo7 carburized and coated with PLC.

of the coating.

Fig.15 - SEM-SE micrographs of untreated CuSn12 bronze wear track (A). (B) wear track at high magnification

Fig.16 - SEM-SE and SEM-BSE micrographs of CuSn12 bronze with DLC coating. A. SEM-SE micrograph. B. SEM-BSE micrograph. C. SEM-BSE micrograph at high magnification

Fig.17 - SEM-BSE (A) and SEM-SE (B) micrographs of carburized 16NiCrMo7 steel

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Memorie scientifiche - Mobilità

Fig.18 - SEM-SE (A) and SEM-BSE (B) micrographs of 16NiCrMo7 steel with PLC coating

Fig.19 - SEM-SE micrograph of wear track of 41CrAlMo7 untreated steel (A) and “duplex” treated (B)

Fig.20 - wear rate comparison of the solutions analyzed

CONCLUSIONS

samples a thin, homogeneous and well adherent to substrate

Present research work studied different combination of metal

coating is present. DLC and PLC coatings promote a great re-

substrates and coatings for reduction of friction and wear and

duction of friction coefficient. All the solutions investigated gua-

efficiency optimization of axial motors and oil pumps. The pro-

rantee a high and long-lasting improvement of wear resistance.

posed solutions are CuSn12 bronze with DLC coating, carburi-

A reduction of friction and wear rate could increase global effi-

zed 16NiCrMo7 with PLC coating and quenched and tempered

ciency of oil pups and motors. The solution combining 16NiCr-

41CrAlMo7 with “duplex” treatment combined plasma nitriding

Mo7 carburized and coated with PLC presents the best tribolo-

and TiCN coating. Microstructural analyses show that in all the

gical performance in friction and wear.

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Scientific papers - Automotive References [1]

Gherardini Francesco, Zardin Barbara, Leali Francesco (2016). A parametric CAD-based method for modelling and simulation of pos tive displacement machines. JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY, vol. 30, p. 3253-3263, ISSN: 1738494X, doi:10.1007/s12206-016-0634-3.

[2]

G. Schuler, A. Jourani, S. Bouvier, J. - M. Perrochat, Efficacy of coating and thermochamical treatments to improve wear resistance of axial poston pumps, Tribology Internationl, 126 (2018) 376-385.

[3]

Ankit Tyagi, R. S. Walia, Qasim Murtaza, Shailesh M. Pandey, Pawan K. Tyagi, Bharat Bajaj, A critical review of diamond like carbon coating for wear resistance applixation, International Journal of Refractory Metals and Hard Materials, 78 (2019) 107 – 122.

[4]

Nesbitt B, editor. 7-Materials for pumps. Hand. Pumps pump. Oxford: Elsevier Science Ltd; 2006. p. 203-25.

[5]

E.Strmčnik, F.Majdič, M.Kalin, Water-lubricated behaviour of AISI 440C stainless steel and a DLC coating

for an orbital hydraulic motor application, Tribology International, Volume 131, March 2019, Pages 128-136.

[6]

S.Q.Sun,Y.W.Ye, Y.X.Wang, M.Q.Liu, X.Liu, J.L.Li, L.P.Wang, Structure and tribological performances of CrAlSiN coatings with different Si percentages in seawater, Tribology International, Volume 115, November 2017, Pages 591-599.

[7]

R. Sola, P. Veronesi, R. Giovanardi, G. Parigi, A Novel Duplex Treatment of C20 Steel Combining Low-Pressure Carburizing and Laser Quenching, Journal of Materials Engineering and Performance, Volume 26, Issue 11, 1 November 2017, Pages 5396-5403.

[8]

R. Giovanardi, P. Veronesi, R. Sola, G. Parigi, Nitriding of stainless steels: Combined plasma-gaseous process, Metallurgia Italiana, Volume 110, Issue 9, 2018, Pages 30-36.

[9]

L. Tonelli, C. Martini, L. Ceschini, Improvement of wear resistance of components for hydraulic actuators: Dry sliding tests for coating selection and bench tests for final assessment, Tribology International 115 (2017) 154–164.

[10]

M. Łępicka, M. Grądzka-Dahlke, D. Pieniak, K. Pasierbiewicz, A. Niewczas, Tribological performance of titanium nitride coatings: A comparative study on TiN-coated stainless steel and titanium alloy, Wear, Volumes 422–423, 2019, Pages 68-80.

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Sola R., Poli G., Giovanardi R., Veronesi P.,Calzolari, C. ,Zanotti A., Wear and corrosion resistance modification of nitrided and nitrocarburized steels, Metallurgia Italiana, Volume 102, Issue 4, April 2010, Pages 29-39.

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R. Sola, P. Veronesi, R. Giovanardi, S. Defanti, G. Parigi, Effect of deep cryogenic treatment on the properties of AISI M2 steel, European Conference on Heat Treatment 2015 and 22nd Heat Treatment and Surface Engineering from Tradition to Innovation Congress, IFHTSE 2015; Venice; Italy; 20 May 2015 through 22 May 2015; Code 124811.

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R. Sola, R Giovanardi, P. Veronesi, A. Morri, M. Melin, C. Garagnani, C. Soffritti, G. Parigi, Influence of a post-tempering cryogenic treatment on the toughness of the AISI M2 steel, 7th International Congress on Science and Technology of Steelmaking, ICS 2018; Venice; Italy; 13 June 2018 through 15 June 2018; Code 144934.

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Memorie scientifiche - Acciai Duplex

Il test FIMEC per valutare la qualità di giunti saldati in acciaio duplex UNS S31803 R. Montanari, A. Varone, F. Bonollo, P. Ferro

Gli acciai inossidabili duplex (DSS) sono materiali dalla struttura bifasica (austenite e ferrite) ampiamente utilizzati in campo chimico e petrolchimico grazie alla buona combinazione tra proprietà meccaniche e di resistenza alla corrosione. Durante il processo di saldatura può verificarsi la precipitazione di carburi, nitruri e altre fasi secondarie che alterano e pregiudicano le caratteristiche di questi materiali. In questo lavoro le proprietà meccaniche di un giunto saldato in acciaio UNS S31803 (1.4462), sottoposto a successivo trattamento di solubilizzazione a 1050 °C per 600 s, sono state valutate mediante test di indentazione strumentata FIMEC. I risultati ottenuti hanno confermato la possibilità di utilizzare il FIMEC come tecnica idonea a valutare la qualità dei giunti saldati e l’efficacia dei trattamenti termici successivi alla saldatura.

PAROLE CHIAVE: ACCIAI SUPER-DUPLEX, SALDATURA, TEST FIMEC

INTRODUZIONE

Gli acciai inossidabili duplex (DSS) e superduplex (SDSS)

possiedono una microstruttura, costituita in parti circa uguali di ferrite α e austenite γ, che consente di ottenere

ottime proprietà meccaniche e di resistenza alla corrosione [1], particolarmente vantaggiose per applicazioni indu-

striali nei settori chimici e petrolchimici. Il loro impiego è strettamente legato alla possibilità di realizzare giunti sal-

dati, ovvero riuscire a preservare il corretto bilanciamento tra le due fasi anche a seguito dei severi cicli termici indotti dal processo di saldatura.

Durante i processi di giunzione, nella zona fusa (ZF) e nel-

R. Montanari, A. Varone

Dipartimento di Ingegneria Industriale, Università di Roma “Tor Vergata”, Roma.

F. Bonollo, P. Ferro

Dipartimento di Tecnica e Gestione dei Sistemi Industriali, Università degli Studi di Padova, Vicenza

la zona termicamente alterata (ZTA), avvengono impor-

tanti trasformazioni di fase che influenzano le proprietà

finali del materiale [2]. In particolare, la precipitazione di fasi secondarie come la fase σ e x, risulta essere deleteria per le proprietà meccaniche e di resistenza alla corrosio-

La Metallurgia Italiana - marzo 2020

pagina 24

Scientific papers - Duplex Stainless Steel ne dell’acciaio [3-13].

secondarie (~1%) sia dalla diversa morfologia della fase σ

arricchiti di elementi austenitizzanti [14] e si fa seguire al

ne di modelli semi empirici di precipitazione e dissoluzio-

Per tale ragione si usano solitamente dei fili d’apporto processo di saldatura un trattamento termico in modo da

ripristinare il corretto bilanciamento tra ferrite e austenite [15-17].

I giunti saldati vengono generalmente esaminati mediante

test di piegatura al fine di valutarne la qualità e prevenire la rottura dei componenti in esercizio. Tuttavia, questi test

[20]. Inoltre risulta un ottimo strumento per la calibrazione di fasi secondarie [21-23].

In questo lavoro, si è analizzato un giunto saldato in ac-

ciaio duplex S31803 (1.4462) solubilizzato dopo saldatura

ad arco sommerso (SAW - Submerged Arc Welding), a 1050°C per 600 s, per valutare l’efficacia del test.

non sempre consentono di rilevare strutture non adegua-

IL TEST FIMEC

nella zona fusa del cordone. Una tecnica come il FIMEC

impiega una punta cilindrica con diametro di 1 mm [18-

te in quanto le sollecitazioni massime sono localizzate

(Flat-top cylinder Indenter for MEchanical Characterisation) [18-19], che consente di determinare le proprietà

meccaniche su scala locale, risulta essere una valida alternativa ai metodi classici in quanto permette di carat-

terizzare singolarmente le diverse zone del giunto, ZF,

ZTA e metallo base (MB) [20]. La tecnica è così sensibile da evidenziare le variazioni della tensione di snervamento

indotte sia dalla precipitazione di piccole quantità di fasi

Il test FIMEC è una prova di indentazione strumentata che 19]. La sostanziale differenza dalle altre tecniche di in-

dentazione, che impiegano punte con geometrie diverse

(piramide, sfera, cono ecc.), è che in questo caso l’area di contatto tra la punta e il materiale non varia in funzione

della profondità di penetrazione ma resta costante. Durante il test FIMEC, il carico (L) e la profondità di penetra-

zione (h) vengono registrati ed è possibile ottenere curve pressione (p) vs. h come quella mostrata in Fig. 1.

Fig.1 - Tipica curva FIMEC pressione vs. penetrazione, presa da ref. [18] / Typical pressure-penetration FIMEC curve, taken from ref.[18]. La curva sperimentale può essere divisa in quattro stadi:

valori di pressione, l’andamento è pressoché costante ed

mo tratto plastico termina quando la pressione è pari a pY

teriale.

un primo tratto elastico (fino a pL) e tre tratti plastici. Il prie corrisponde alla comparsa di una deformazione permanente del materiale. Quando p > pY, la curva mostra una

marcata diminuzione della pendenza e sulla superficie del campione, intorno all’impronta, sarà visibile la protrusio-

ne del materiale. Nell’ultimo stadio, corrispondente ad alti

La Metallurgia Italiana - March 2020

è caratterizzato da un importante flusso plastico del ma-

Considerando una velocità di penetrazione della punta

pari a 0.1 mm/min, è possibile determinare la tensione di snervamento (Rs) di un materiale metallico mediante la seguente equazione:

pagina 25

Memorie scientifiche - Acciai Duplex Se si opera con una punta standard (diametro 1 mm) e si

quanto le impronte sono sufficientemente grandi da

penetra a una profondità massima di 1 mm mantenendo

prendere in considerazione migliaia di grani. Particolar-

una distanza di sicurezza ( 5 mm) tale da non determina-

mente utile è il suo impiego nello studio di giunti saldati,

re la sovrapposizione dei campi di sforzo tra due prove

consentendo di caratterizzare le diverse zone del giunto,

FIMEC adiacenti, i risultati delle prove sono altamente

MB, ZTA e ZF (vedi per esempio refs. 24-25).

riproducibili [19]. Inoltre se si effettua il test con una velocità pari a 0,1 mm

MATERIALI E METODI

/min, i dati della tensione di snervamento ottenuti dalla

Il campione analizzato è un giunto saldato testa a testa in

curva FIMEC possono essere confrontati con quelli rica-

acciaio duplex S31803, precedentemente solubilizzato a

vati dalle prove di trazione e la deviazione standard tra i

1050°C e poi temprato in acqua. La geometria del cam-

dati ottenuti dalle prove di trazione e quelli ottenuti dal

pione è riportata nel disegno di Fig. 2 e in Tab.1 vengono

test FIMEC sono sempre inferiori al 7% [19], che è pari

mostrate le composizioni chimiche nominali dell’acciaio

allo scarto osservabile in prove di trazione eseguite su

e del materiale d’apporto. Con le lettere A e B vengono

diversi provini dello stesso materiale.

identificate le zone associate a quattro e a tre passate di

Inoltre, nonostante il carattere locale della prova, il FI-

saldatura, rispettivamente.

MEC fornisce misure rappresentative del materiale in

Fig.2 - Schema del giunto saldato testa a testa con metallo d’apporto nelle zone A (quattro passate) e B (tre passate) / Schematic representation of butt welded joint with filler metal in the area A (four runs) and B (three runs) Tab.1 - Composizione chimica (% in peso) dell’acciaio duplex S31803 (1.4462) e del materiale d’apporto / Chemical composition (wt%) of the duplex steel S31803 (1.4462) and the filler material. C

UNS 31803

<0.03

21-23

<0.2

2.5-3.5

0.15-020

4.5-6.5

<0.1

<0.03

<0.02

Materiale d’apporto

0.014

22.95

0.10

1.52

3.08

0.163

8.61

0.42

0.015

0.0008

Dopo il processo di saldatura, il materiale è stato sotto-

Per l’osservazione metallografica il giunto saldato è sta-

posto a un trattamento termico (1050 °C per 600 s), come

to preparato mediante carte abrasive, lucidato con pasta

richiesto dalla normativa per questa tipologia di giunti

diamantata e successivamente attaccato chimicamente

(ASTM A928/A928M [26] e NORSOK MDS D42 [27]). Suc-

con una soluzione di 80 ml H 2O 2, 30ml HCl e 1 g K 2S 2O5.

cessivamente è stato caratterizzato mediante test FIMEC,

Dopo l’attacco chimico sono state effettuate 15 prove FI-

analisi al microscopio ottico e prove di microdurezza Vi-

MEC in diverse zone del giunto saldato.

ckers per valutare possibili variazioni nella microstruttu-

L’evoluzione microstrutturale, a seguito del processo di

ra e delle proprietà meccaniche nelle diverse zone del

saldatura e del trattamento post-saldatura, è stata effet-

giunto saldato.

tuata confrontando le prove FIMEC con le micrografie e

La Metallurgia Italiana - marzo 2020

pagina 26

Scientific papers - Duplex Stainless Steel con tre profili di microdurezza Vickers, realizzati a tre di-

da 1 a 14 (numeri gialli). L’impronta effettuata sul metallo

verse altezze (A1, A2, A3) attraverso il cordone, mostrate

base è identificata con la lettera A.

in Fig. 3b, con un carico di 300 g e un passo di 0.5 mm.

Utilizzando l’Eq. (1) è stata determinata la tensione di snervamento associata a ciascuna curva FIMEC. I valo-

RISULTATI FIMEC

ri ottenuti, espressi in MPa, sono riportati nella foto del

In Fig. 3a vengono riportate le curve FIMEC ottenute in

campione (numeri bianchi in Fig. 3b) in prossimità delle

diversi punti del cordone (Fig. 3b), identificati con numeri

rispettive impronte.

Fig.3 - (a) Curve FIMEC ottenute nei punti identificati numericamente in (b). (b) Foto del giunto saldato dopo attacco chimico e test FIMEC. I numeri in bianco sono i valori della tensione di snervamento (MPa), ricavati dalle curve FIMEC / FIMEC curves obtained in the points numerically identified in (b). Photo of the welded joint after chemical etching and FIMEC test. The white numbers are the values of yield stress (MPa). I valori massimi della tensione di snervamento R s si rag-

base (594 MPa). La figura 4 mostra il tratto utile ai fini del

giungono nella zona fusa (~ 660-670 MPa) e diminuisco-

calcolo di R s, di tre curve FIMEC ottenute in queste diver-

no gradualmente spostandosi dal centro della saldatura

se zone del campione (MB, ZTA e ZF).

alla zona termicamente alterata (~620 MPa) fino al metallo

Fig.4 - Tratto utile per il calcolo di R s delle curve FIMEC ottenute in: MB (impronta A), ZTA (impronta 5) e ZF (importa 6). / Initial stage of FIMEC curves used for measuring Rs in: MB (A imprint), ZTA (5 imprint) e ZF (6 imprint).

La Metallurgia Italiana - March 2020

pagina 27

Memorie scientifiche - Acciai Duplex Al fine di associare i risultati sperimentali della tensione

diverse micrografie in prossimità delle impronte FIMEC

di snervamento alle microstrutture presenti nelle diver-

(Fig. 5). La Fig. 6 mostra il metallo base e la ZF con mag-

se zone del cordone di saldatura, sono state effettuate

gior dettaglio.

Fig.5 - Microstruttura dell’acciaio duplex S31803 (1.4462) nelle zone in cui sono state effettuate le prove di indentazione FIMEC / Microstructure of the duplex stainless steel S31803 (1.4462) in the areas where FIMEC indentation tests were performed. a)

Fig.6 - Microstruttura dell’acciaio duplex S31803 (1.4462): metallo base (a) caratterizzato da grani di austenite e di ferrite; ZF (b) caratterizzata da grani di ferrite, austenite, austenite secondaria e austenite di tipo Widmanstätten / Microstructure of the duplex stainless steel S31803 (1.4462): parent metal (a) constituted by austenite and ferrite grains; FZ (b) constituted by grains of ferrite, austenite, secondary austenite and Widmanstätten type austenite. La microstruttura del metallo base (impronta A) è carat-

dosi della regione soggetta a quattro passate di saldatura

terizzata da grani di austenite, di colore chiaro, e grani di

a quella soggetta a tre passate.

ferrite, di colore scuro, (Fig. 6a) mentre la microstruttu-

Le microdurezze Vickers sono state effettuate lungo le

ra nella ZF risulta caratterizzata da grani di ferrite, grani

tre linee (A1, A2 e A3) attraverso il cordone di saldatura.

di austenite, austenite secondaria e grani di austenite di

Ciascuna linea (A1, A2 e A3) è posta a una distanza ( d ) pari

tipo Widmanstätten (Fig. 6b). Non si osservano spiccate

a 5 mm, che risulta essere pari alla distanza minima di si-

differenze microstrutturali lungo la zona fusa, muoven-

curezza ,tra i centri di due impronte FIMEC adiacenti, al

La Metallurgia Italiana - marzo 2020

pagina 28

Scientific papers - Duplex Stainless Steel fine di evitare la sovrapposizione dei campi di sforzo [28].

con la corrispondente deviazione standard. La linea oriz-

Gli andamenti dei profili di microdurezza sono mostrati

zontale indica il valore di microdurezza Vickers ottenuto

in Fig. 7. In ciascun grafico sono identificate ZF e ZTA e

nel MB.

riportato il valor medio di microdurezza in ciascuna zona

Fig.7 - Profili di microdurezza Vickers attraverso il cordone lungo le linee A1, A2, A3 mostrate in Fig. 3 b / Vickers microhardness profiles across the bead along the A1, A2, A3 lines shown in Fig.3 b. La Tab. 2 mette a confronto i valori di tensione di snerva-

Entrambe le grandezze misurate, R s e HV, risentono di

mento con i valori medi di microdurezza ottenuti in ZF,

queste variazioni microstrutturali, e mostrano tra di loro

ZTA e MB, che ricadono in prossimità di alcune impronte

una certa correlazione (vedi il rapporto HV / R s) anche se

FIMEC.

le zone su cui sono state rilevate non sono esattamente

Le variazioni della tensione di snervamento sono cor-

coincidenti. Inoltre, l’impronta di ciascuna microdurezza

relate alle variazioni microstrutturali che si osservano

riguarda o la ferrite o l’austenite, come dimostrato dal-

muovendoci dal metallo base alla ZF attraverso la ZTA.

la significativa dispersione dei dati, mentre l’impronta

Gli apporti termici a cui è sottoposto il cordone di sal-

FIMEC comprende numerosi grani di ambo le fasi ed è

datura determinano infatti variazioni microstrutturali nel

meno influenzata dalla casualità. Questo aspetto sarà ap-

rapporto tra ferrite e austenite rispetto a quello ottima-

profondito in futuro.

le oltre che l’eventuale precipitazione di fasi secondarie.

Tab.2 - Tensione di snervamento Rs determinata con il test FIMEC e valore medio di microdurezza Vickers in prossimità delle impronte FIMEC (d=5 mm) / Yield stress Rs obtained through FIMEC test and the average value of Vickers microhardness near the FIMEC imprint (d=5 mm)

Posizione indentazione

Rs (MPa)

HV300

HV300 / Rs

A (metallo base)

594

256

0,43

1 (ZTA)

607

278

0,46

3 (ZF)

628

284

0,45

5 (ZTA)

607

289

0,48

6(ZF)

662

290

0,44

10 (ZTA)

599

256

0,43

12 (ZF)

658

280

0,43

La Metallurgia Italiana - March 2020

pagina 29

Memorie scientifiche - Acciai Duplex CONCLUSIONI

e data la sua elevata affidabilità è possibile il suo impiego

Le proprietà meccaniche di un giunto saldato in acciaio

in alternativa alla prova di trazione.

UNS S31803 (1.4462), sottoposto a successivo trattamen-

Inoltre, nonostante il carattere locale della prova, il FI-

to di solubilizzazione a 1050 °C per 600 s, sono state va-

MEC fornisce misure rappresentative del materiale in

lutate mediante test di indentazione strumentata FIMEC.

quanto le impronte sono sufficientemente grandi (diame-

I risultati ottenuti indicano che il test FIMEC è uno stru-

tro di 1 mm ) da coinvolgere un numero elevato di grani e

mento efficace per studiare giunti saldati in acciai duplex

non risentire della presenza di una struttura bifasica come

e superduplex a struttura bifasica, così come si è dimo-

invece avviene per le prove di microdurezza.

strato in passato per altre tipologie di acciai a struttura monofasica. Infatti a differenza di altre prove meccaniche

Ringraziamenti

(trazione, fatica, resilienza) che forniscono valori medi

Alessandra Varone ringrazia l’Associazione Italiana di Me-

non rappresentativi delle diverse zone interessate dal

tallurgia per aver finanziato una borsa di studio annuale

processo di saldatura, il FIMEC test consente di ottenere

grazie alla quale è stato svolto questo lavoro.

dati sperimentali relativi alle singole zone (ZF, ZTA e MB) Bibliografia [1]

Solomon HD, Devine TM. Duplex Stainless Steels-A tale of two phases. In Proceedings of the Conference on Duplex Stainless Steels, St. Louis, Missouri, 1982 23-28 October; 693–756.

[2]

Capello E, Chiarello P, Previtali B, Vedani M. Laser welding and surface treatment of a 22Cr-5Ni-3Mo duplex stainless steel. Mater. Sci. Eng. A 2003; 351:334–343.

[3]

Karlsson L, Bengtsson L, Rolander U, Pak S. The kinetics of intermetallic phase formation in duplex stainless weld metals and their influence on mechanical properties. In Proceeding of the Applications of Stainless Steels, Stockholm, Sweden, 1992 9–11 June; 335–344.

[4]

Fargas G, Anglada M, Mateo A. Effect of the annealing temperature on the mechanical properties, formability and corrosion resistance of hot-rolled duplex stainless steel. J. Mater. Process. Technol. 2009;209:1770–1782.

[5]

Ahn YS, Kang JP. Effect of aging treatments on microstructure and impact properties of tungsten substituted 2205 duplex stainless steel. Mater. Sci. Technol. 2000;16:382–388.

[6]

Chen TH, Yang JR. Effects of solution treatment and continuous cooling on σ-phase precipitation in a 2205 duplex stainless steel. Mater. Sci. Eng. A 2001;311:28–41.

[7]

Chen TH, Weng KL, Yang JR. The effect of high-temperature exposure on the microstructural stability and toughness property in a 2205 duplex stainless steel. Mater. Sci. Eng. A 2002;338:259–270.

[8]

El Koussy MR, El Mahallawi IS, Khalifa W, Al Dawood MM; Bueckins M. Effect of thermal aging on microstructure and mechanical properties of duplex stainless steel weldments. Mater. Sci. Technol. 2004;20:375–381.

[9]

Nowacki J, Rybicki P. The influence of welding heat input on submerged arc welded duplex steel joints imperfections. J. Mater. Proc. Technol. 2005;164:1082–1088.

[10]

Badji R, Bouabdallah M, Bacroix B, Kahloun C, Belkessa B, Maza H. Phase transformation and mechanical behavior in annealed 2205 duplex stainless steel welds. Mater. Charact. 2008; 59:447–453.

[11]

Chan KW, Tjong SC. Effect of Secondary Phase Precipitation on the Corrosion Behavior of Duplex Stainless Steels. Materials 2014;7:5268–5304.

[12]

Calliari I, Pellizzari M, Zanellato M, Ramous E. The phase stability in Cr-Ni and Cr-Mn duplex stainless steels. J. Mater. Sci. 2011;46:6916–6924.

[13]

Muthupandi V, Bala Srinivasan P, Seshandri SK, Sundaresan S. Materials Science and Engineering A 2003;358:9.

[14]

Bonollo F, Ferro P, Cervo R, Vianello B, Durante M. Caratterizzazione metallurgica e meccanica di giunti saldati in acciaio inox UNS S32750 ottenuti mediante materiale d’apporto innovativo. La metallurgia italiana 2009;55-62.

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pagina 30

Scientific papers - Duplex Stainless Steel [15]

Cervo R, Ferro P, Tiziani A, Zucchi F. Annealing temperature effects on superduplex stainless steel UNS S32750 welded joints. II: pitting corrosion resistance evaluation. Journal of Materials Science 2010;45:4378–4389.

[16]

Cervo R, Ferro P, Tiziani A, F. Zucchi. Annealing temperature effects on superduplex stainless steel UNS S32750 welded joints. I: microstructure and partitioning of elemnts. Journal of Materials Science 2010;45: 4369–4377.

[17]

Ferro P, Tiziani A, Bonollo F. Influence of Induction and Furnace Postweld Heat Treatment on Corrosion Properties of SAF 2205 (UNS 31803). Welding Jounal 2008;87:298s-306s

[18]

Gondi P, Donato A, Montanari R, Sili A. A miniaturized test method for the mechanical characterization of structural materials for fusion reactors. J. Nucl. Mater. 1996;233:1557–1560.

[19]

Riccardi B, Montanari R. Indentation of metals by flat-ended cylindrical punch. Materials Science & Engineering A 2004;381:281291.

[20]

Angella G, Fava A, Montanari R, Richetta M, Varone A. Flat-Top Cylinder Indenter Examination of Duplex Stainless Steel 2205 after Different Heat Treatments. Metals 2017;7:178-191.

[21]

Ferro P. A dissolution kinetics model and its application to duplex stainless steels. Acta Materialia 2013; 61:3141-3147.

[22]

Ferro P, Bonollo F. A semi-empirical model for sigma phase precipitation in duplex and superduplex stainless steels. Metallurgical and Materials Transactions A 2012; 43:1109-1116.

[23]

Ferro P, Fabrizi A, Bonollo F. Non-isothermal dissolution modelling of sigma phase in duplex stainless steels. Acta Metall. Sin. (Engl. Lett.) 2016; 29(9):859-868.

[24]

Filacchioni G, Montanari R, Tata ME, Pilloni L. Structural and Mechanical Properties of Welded Joints of Reduced Activation Martensitic Steels. J. of Nuclear Materials 2002;307-311:1563.

[25]

Montanari R, Filacchioni G, Riccardi B, Tata ME, Costanza G. Characterization of EUROFER 97 TIG-welded joints by FIMEC indentation tests. J. of Nuclear Materials 2004;329-333:1529.

[26]

ASTM A928 / A928M - 08a, Standard Specification for Ferritic/Austenitic (Duplex) Stainless Steel Pipe Electric Fusion Welded with Addition of Filler Metal (www.astm.org/Standards/A928.htm).

[27]

Normativa NORSOK MDS D42, https://www.standard.no/en/webshop/search/?search=NORSOK+MDS+D42

[28]

Gondi P, Montanari R, Sili A, Foglietta S, Donato A, Filacchioni G. Applicability of the FIMEC indentation test to characterize materials irradiated in the future IFMIF high intensity neutron irradiation source. Fusion technology 1996; 1607-1610.

The FIMEC test to reveal the quality of duplex stainless steel UNS S31803 welded joints Duplex stainless steels (DSS) are alloys with biphasic structure (austenite and ferrite) widely used in chemical and petrochemical fields thanks to their good mechanical properties and corrosion resistance. During welding, the precipitation of carbides, nitrides and other secondary phases can occur with detrimental effects on their characteristics. In this work the mechanical properties of a UNS S31803 (1.4462) steel welded joint, after solution treatment at 1050°C for 600 s, were analyzed through instrumented indentation test FIMEC. The results show that FIMEC test is able to assess the quality of welded joints and the effects of post-welding heat treatments.

KEYWORDS: DUPLEX STAINLESS STEELS, WELDING, FIMEC TEST

La Metallurgia Italiana - March 2020

pagina 31

AttualitĂ industriale - MobilitĂ

New approach for online tensile-structure properties evaluation on HSLA/AHSS steel grades edited by: Alessandro Ferraiuolo

This paper presents the results of the development of an innovative methodology using a mathematical model based on skin pass mill process data in order to measure online the tensile-structure properties of flat steel products. The proposed approach could represent a new solution to define the annealing process conditions (CAL, HDG, BA) to achieve the target microstructure, improved mechanical properties as well as enhanced forming-ability properties of high strength steels. The online measurement of flow stress allowed to evaluate important microstructural features such as the ferrite grain size in the final product and the austenite fraction formed at the soaking temperature. Laboratory tensile tests and metallographic investigations confirmed a good agreement with the online structure-properties characteristics calculated with the mathematical model. The proposed method is promising to produce products with exceptional uniform tensile properties and at the same way guarantee the best annealing process efficiency and low global energy consumption.

PAROLE CHIAVE: ONLINE TENSILE PROPERTIES, EPITAXIAL FERRITE, AUSTENITE FRACTION MEASURE, GRAIN SIZE MEASURE, COLD ROLLING, SKIN PASS.

INTRODUCTION

The increasing automobile market demands for redu-

ced fuel consumption as well as the need to comply with the international environmental regulations regarding

greenhouse gas emissions, have motivated and/or even

forced the automotive industry to produce more fuel-ef-

ficient vehicles by reducing their weight. To achieve this goal a new vehicle architecture based on novel design concepts has been developed. The wide introduction of

high strength steels allowed the automotive designers

Alessandro Ferraiuolo

Marcegaglia Ravenna S.p.A, via Baiona 141,Ravenna - ITALY.

to compensate the more and more increasing weight of comfort components. Parallel to this market trend, in the

last years is also increased the demand from OEM on the uniformity of tensile properties within the coils and also the consistency coil to coil. To achieve these ambitious

targets the metallurgists needs to know, in real time, both

the strip tensile properties and the evolution of micro-

La Metallurgia Italiana - marzo 2020

pagina 32

Industry news - Automotive structure during the annealing process. It is nowadays

the mathematical model.

properties can be achieved only through a tight control

EXPERIMENTAL

quite recognized that uniform tensile-microstructure of the annealing furnace temperature and also the strip

The experimentation in the present study was carried out

emissivity uniformity in order to achieve reproducible

The product mix is mainly constituted of automotive ste-

surface conditions (such as roughness, cleanliness) for annealing process conditions. The importance of tempe-

rature control is strictly related to the fact that industrial continuous annealing conditions of HSLA steels are inter-

critical (Ae1<T<Ae3) and therefore a significant austenite fraction is produced during strip processing.

The metallurgical effect of the austenite fraction represented the object of several studies [1-2] and nowadays it is quite recognized that it plays a strong effect on the final

microstructure even if not significant secondary phases hardening is produced (bainite, martensite). During the

intercritical annealing of cold-rolled HSLA steels in a hotdip galvanizing line a sequence of metallurgical phenomena including recovery, recrystallization, ferrite-to-austenite transformation and austenite decomposition takes place.

The aim of this paper is to investigate on the quite high sensitivity of HSLA grades to intercritical annealing process conditions. This study was carried out by means of a

mathematical model in the skin pass process allowing to measure online the flow stress and tensile strength corre-

at the HDG line N.4 of Marcegaglia Ravenna plant (fig.1).

el grades both for deep drawing (DX52D…DX57D) and for automotive structural applications (HSLA, AHSS). In figu-

re 1 is shown schematically the HDG line constituted of a total furnace length of 126 m (free flame + radiant tubes)

with the last vertical section (electric resistance heating) followed by a vertical cooling section constituted by 3

high power jet coolers (50÷100°C/s). The product mix of the HDG line includes both cold rolled strip and hot rol-

led strip with a wide strip dimensional range (thickness 0.7÷4.0mm and width 800÷1550mm). The activities consisted in the characterization (microstructure and tensile

properties) of hot rolled, cold rolled and galvanized strip. During the galvanization process the tensile properties

were evaluated online by means of the newly developed

mathematical model (TENSIL-PRO: TENSILe-structure PROperties model) in order to correlate the tensile properties and some microstructural features to HDG process conditions. MATERIALS

lating them to the processing conditions.

The steel chemical composition of the steel investigated

luate also further important microstructural features such

was pickled, cold rolled (reduction 55%) and finally gal-

The online measurement of flow stress allowed to evaas the ferrite grain size in the final product, the austenite fraction formed at the soaking temperature and recrystallized fraction. Laboratory tensile tests and metallographic

investigations confirmed a good agreement with the onli-

ne structure-properties characteristics calculated with

is reported in the following figure 2. The hot rolled coil vanised. The hot rolled strip was characterized in terms of

tensile properties and microstructure both at head and tail coil. Similarly the cold rolled and galvanized strip samples

were characterized by means of tensile test and microstructure analysis in the head and tail coil.

Fig.1 - HDG line N.4 at Marcegaglia Ravenna plant.

La Metallurgia Italiana - March 2020

pagina 33

Attualità industriale - Mobilità In the following figure 2 is shown the microstructure of the hot rolled strip sample (steel A) constituted of a fine grained ferrite and perlite mixture. In the table 1 are repor-

ted the results of tensile tests carried out on the hot rolled strip cut out at the tail of hot rolled coil.

Element

Steel A (wt%)

0.05

0.4

0.015

Fig.2 - Microstructure and chemical composition of hot rolled steel sample. Tab.1 - Tensile properties of hot rolled strip. Steel

thickness (mm)

ReL (MPa)

Rm (MPa)

A80%

3,0

385

445

ONLINE EVALUATION OF TENSILE-STRUCTURE PRO-

shown the results of application of a mathematical model

The industrial experience clearly suggests that a tight

led tensile properties evaluaiton) and to skin pass process

PERTIES

control of strip temperature during the annealing process

must be considered mandatory for high strength steel

grades because even variation of 20°C could yield to quite significant product inhomogeneity. Strip excursion out

of the optimal temperature range could yield to low recrystallization and high flow stress in case of temperature

below lower bound. Conversely, if the annealing temperature overcome the upper bound, quite low tensile properties are achieved due to a larger grain size.

This approach aiming to a steady annealing temperature

within the range of acceptable quality is also pursued to guarantee a high-quality process, energy efficiency and product throughput capability. In the present study it is

La Metallurgia Italiana - marzo 2020

(Tensil-Pro) applied to the cold rolling mill (for hot rol(fig.3) aiming to evaluate the tensile properties of the strip

(yield strength and tensile strength), the strain hardening

properties (dσ/dε) [3-4], grain size, austenite fraction and

the recrystallized fraction. Basically the developed model [6-7-8] evaluates the tensile properties using an approximated solution of Orowan equation and additional origi-

nal constitutive equations characterized by parameters related to physical properties of the steel under processing. The model parameters are determined for each steel

grade by means of correlation analysis with the laboratory microstructure and tensile test results.

The interesting characteristic of the developed methodology is that it can be applied, in line of principle, to all steel

pagina 34

Industry news - Automotive grades (ferritic and austenitic) and metallic alloys.

Fig.3 - Schematic view of the Tensil-Pro mathematical model architecture.

HOT AND COLD ROLLED STRIP TENSILE PROPERTIES

ved hot rolled conditions and after HDG process. The im-

quisition system, can evaluate in real time the main ten-

effective tool to understand the real metallurgical effect of

The developed mathematical model, through a data acsile properties of hot rolled strip and final properties after HDG/CAL process. In figure 4 is shown a comparison of

the tensile properties of the same coil (DP600) in as recei-

plementation of the Tensil-Pro represent a powerful and hot rolling process conditions and the CAL/HDG process conditions on the final product properties.

Fig.4 - Tensil-Pro results on DP600: a) hot rolled strip and b) final HDG strip.

ONLINE EVALUATION OF STRUCTURE PROPERTIES

The online evaluation of flow stress is used to evaluate the

ferrite grain size dÎą reversing the generalized Hall-Petch equation.

La Metallurgia Italiana - March 2020

pagina 35

Attualità industriale - Mobilità The parameter σo includes the friction stress, solid solu-

graphic grain size measurements. The results of the least

A, evaluated on the basis of steel chemical composition,

are reported in the table 2.

tion hardening and precipitation hardening for the steel hot rolling process, laboratory tensile tests and metallo-

square fit for the calculation of the Hall-Petch coefficient

Tab.2 - Hall-Petch parameters of the steel A.

Steel A

σo (MPa)

K (MPa*mm1/2)

HOT DIP GALVANIZING LINE RESULTS

ous grain structure with zone with fine grains and other

a total reduction of about 55% and then processed at the

annealed at higher soaking temperature (795°C), presen-

Steel A was cold rolled in a continuous tandem mill with galvanizing line N.4. The soaking preset temperature and

the process speed were respectively 780°C and 80m/min. In the figure 4a is shown the plot of strip soaking temperature (at end of radiant tube zone) as a function of time.

As it can be noted during processing the strip temperature

changed from about 745°C up to 795°C. In the figures 4b and 4c are shown respectively the evolution of the flow

zone with larger grains. The sample cut out at the end coil,

ts a significantly larger grain size (7 µm). The agreement between Tensil-Pro results and the optical microscope

measurements seems to suggest that the variation of flow stress could be ascribed entirely to ferrite grain size variation.

DISCUSSION OF RESULTS

stress, evaluated with the mathematical model Tensil-Pro

The data presented in the figure 4 and 5 indicates that mi-

It is noteworthy that the flow stress is quite sensitive to

king temperature during continuous annealing.

and the grain size.

process conditions variation: for a soaking temperature change of about ∆T= 50°C the flow stress change is 50MPa.

The strip microstructural characterization was carried out on samples cut out from the head and tail of the HDG coil

(Fig.5). The grain size was measured by means of linear

croalloyed steels are quite sensitive to variations of soaThe explanation of this phenomenon could be ascribed to two different mechanisms:

• Ferrite grain growth during the soaking time or

• Epitaxial grain growth occurring during reverse austenite transformation.

mean intercept method and the results, in the head and

In the following sections it will be shown that the second

crostructure of the sample annealed at head coil (soaking

experimental results currently found during HSLA and

tail coil, are respectively dα = 5.5µm and 7.0µm. The mitemperature 745°C) is characterized by a not-homogene-

mechanism explains, with a quite good agreement, the AHSS strip processing.

Fig.4 - Evolution of strip temperature, flow stress and grain size as a function of the time. La Metallurgia Italiana - marzo 2020

pagina 36

Industry news - Automotive

Fig.5 - Microstructure of steel A after HDG process: a) head coil; b) tail coil; c) tensile test results. MECHANISM OF FERRITE GRAIN GROWTH

As it is well recognised, the grain growth occurring during

isothermal annealing conditions follows a power relation such as:

2) In which d is the diameter of ferrite grain, K is the rate con-

stant for grain growth related to material properties and

process parameters through the equation

3) Where Qgg is the activation energy for ferrite grain growth.

occur through ferrite epitaxial growth. This assumption

negligible ferrite grain growth for soaking time less than

fractions at the exit of annealing furnace using the basic

At temperature below Ac1 it is found in literature a quite 1000s. In the intercritical temperature range the pinning effect of Nb carbide-nitrides is still quite strong and the activation energy Qgg ranges between 380-450 KJ/mol.

At temperature of 800Â°C a rate constant K= 4.1 10 m/s it 17

is reported. For a soaking time of about 100s (typical for continuous industrial process) the expected grain growth

is only 0,011Âľm. This result is quite far from the experi-

mental values reported above and demonstrates that the mechanism of grain growth could

not explains the high sensitivity of HSLA steel tensile properties to intercritical annealing conditions.

New theory for online evaluation of the austenite volume

allows to calculate the evolution of ferrite and austenite

tools of stereology. The parameters used as a metric for the quantitative characterization of the 3D mixed structure are the ferrite/austenite grain boundary area per unit volume SV and austenite volume fraction XV.

Letâ&#x20AC;&#x2122;s assume now the following additional hypothesis:

1) At the entry of annealing furnace the strip microstructure is constituted of a ferrite-perlite mixture.

2) During annealing the austenite phase forms randomly on the pearlite particles and on the ferrite grain boundaries.

3) The microstructure of HSLA at the end of HDG process is constituted of a mixture of ferrite + carbides.

fraction

When the strip temperature cross the intercritical range

stenite fraction formed during an intercritical annealing

nite specific grain boundary

In this section is presented the theory to evaluate the au-

process. The basic assumption of the theory is that the

reverse austenite-to-ferrite transformation is assumed to

(fig.2), the austenite start to form and the ferrite and austeand

are assumed

to change according with the following equation:

La Metallurgia Italiana - March 2020

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Attualità industriale - Mobilità This equation is valid if no grain growth occur and we as-

pletely converted into

every new formed austenite grain substitute a ferrite grain.

boundary plays a role and this justify the assumption that

sume dα=dγ i.e. during ferrite to austenite transformation This assumption could appear very restrictive but, if it is considered that during reverse transformation, nuclea-

tion and growth of new ferrite grains does not occur, the component

will disappear while

is com-

. This would mean that, for

each austenite region, only the external surface grain

the actual austenite grain size is not important and then, without any loss of generality, we can use the equation (1).

On the basis of these arguments introducing the following variable change

2) the equation (2) becomes: 3) This equation can be written as a differential form as follows 4) Introducing the austenite volume fraction

, the variation

of austenite specific grain boundary can be expressed as

follows

5) Substituting the (5) within the (4) becomes 6) Passing to derivatives the equation (6) becomes finally 7) This equation relates the rate of the ferrite grain boundary

well recognized that the relationship between the specific

during the reversed phase transformation in the cooling

as follows [1, 2]:

area per unit volume SV with the rate of austenite fraction

section. On the basis of stereological considerations it is

grain boundary area and average grain size dα can be written

8) Where K is a constant related to 3D grain shape. For polyhedron shaped grains the value K=3.35 is generally adopted.

Substituting the equation (8) into (7) we found

La Metallurgia Italiana - marzo 2020

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Industry news - Automotive Furthermore differentiating the equation (8) 10) and substituting in the equation (9) and adopting the Newton derivative notation it is finally found the final first

order differential equation (Ferrite epitaxial - Feral equation):

11) The integration of this equation is quite direct. The equa-

cess. Therefore for the initial state the grain size is equal to

The boundary conditions can be fixed assuming that the

volume fraction is

tion (11) can be rewritten as

initial state is coincident with the end of soaking and the

final state is coincident with the end of strip cooling pro-

the recrystallized grain size (d(t=0) = drex), and the austenite

(t). For the finale state the ferrite

grain size is dÎą (t), while gration limits we have

(t)=0. Introducing these inte-

12) The problem of the evaluation of the austenite fraction,

equilibrium is reached, for example the austenite fraction

a problem of ferrite grain size evaluation as a function of

(12) can be rewritten in order to calculate the effect of au-

at the end of soaking, as a function of time becomes now

time (recrystallized grain size drex, and ferrite grain size dÎą

(t)). Alternatively, if it is assumed that the thermodynamic

is calculated by means of ThermoCalc, the Feral equation

stenite reverse transformation on final ferrite grain size and it becomes:

13)

Fig.6 - Mechanisms of apparent grain growth in HSLA due to IC austenite formation and following epitaxial ferrite grain growth.

La Metallurgia Italiana - March 2020

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Attualità industriale - Mobilità The integration of this equation needs the knowledge

of recrystallized grains occurs on grain boundaries. The

adopted for the calculation of recrystallized grain size is

lated through

of recrystallized ferrite grain size

. The approach

reported below. First of all it is considered that nucleation

specific grain boundary surface after a strain

is calcu-

14) Where Do is the ferritic grain size of hot rolled strip. Recry-

alloyed steels for higher Zener pinning force, due to the

med microstructure. These can be associated with second

nucleus diameter increases. The critical diameter can be

stallization will nucleate at inhomogeneities in the deforphases particles or grain boundaries. In the case of micro-

action of the small particles volume fraction Fv, the critical calculated by the following equation [11]: 15)

Fv and r are respectively the volume fraction and average radius of carbonitride particles. The dislocation density is

evaluated by means of

16) Where ∆σ is the cold rolling work hardening (in this case

350MPa). The recrystallization nuclei for volume unit is the-

refore

17) Where τ assumed to be 0.05, represent the fraction of nuclei that grow up to final recrystallized grain size taking into

account also the deformation inhomogeneity (effective nu-

cleation site).

Finally the recrystallized grain size is calculated by means:

18) The time evolution of the austenite fraction formed at the

matical model. Following this approach in the figure 7 are

grain size measurement carried out with skin pass mathe-

lated by equation 3) and soaking temperature. It can be

end of soaking can be calculated on the basis of the online

reported the time evolution of the austenite fraction calcu-

Fig.7 - Evolution of austenite fraction and soaking temperature. La Metallurgia Italiana - marzo 2020

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Industry news - Automotive noted that below 750Â°C the austenite fraction is below 3%

te good at temperature higher than 765Â°C. At lower tem-

te fraction increases significantly. In figure 8 the austeni-

the ferrite-to-austenite transformation kinetics is sluggish

but, as the soaking temperature increases, the austeni-

te fraction at equilibrium (ThermoCalc) and the austenite fraction calculated with equation (3) are compared. The

agreement of the results in terms of trend and values is qui-

perature (below 760Â°C) the results seems to suggest that and during soaking the strip does not achieve the expected thermodynamic value.

Fig.8 - Comparison between austenite fraction calculated by equation (3) and thermodynamic austenite fraction. CONCLUSIONS

the same crystallographic orientation. This phenomenon

nisms underlying the high sensitivity of HSLA steel grades

therefore a decrease of the tensile properties. The mathe-

This paper is focused on the investigation of the mechato intercritical annealing process conditions in terms of microstructure and tensile properties. The results confirm

that this behaviour is related to the apparent grain growth due to epitaxial ferrite grain growth during the reverse au-

stenite transformation. During cooling from intercritical annealing, the new ferrite forms from the pre-existing ferrite without nucleation through epitaxial growth, i.e. with

La Metallurgia Italiana - March 2020

determines a larger ferrite grain size (equation 13) and

matical relationship between austenite fraction, formed during soaking, and final ferrite grain size was determined.

Using a skin pass mathematical model it was possible to

evaluate online tensile properties, final ferrite grain size and austenite fraction. The experimental results shown

a quite good agreement with the laboratory tensile test, thermodynamic calculations and metallographic analysis.

pagina 41

AttualitĂ industriale - MobilitĂ References [1]

T. Ogawa, et al., ISIJ International, vol. 50 (2010),n.3, pp. 469-475.

[2]

P.A. Manohar, ISIJ International, vol. 36 (1996), n.2, pp. 194-200.

[3]

A. Ferraiuolo: Device and method for online measurement of tensile and microstructure properties of steels and metallic alloys. IT Patent office N. 102017000035735, 2017.

[4]

A. Ferraiuolo, Optimization of AHSS annealing cycle by on line control of tensile properties, November 2017, GALVATECH 2017.

[5]

M. F. Ashby Philos. Mag. 1966, 14, 1157.

[6]

Orowan, E., "The Calculation of Roll Pressure in Hot and Cold Flat Rolling," Proc. Inst Mech. Eng., Vol. 150, 1943, p. 140.

[7]

W. Roberts: Cold rolling of Steel, M. Dekker, New York, (1978).

[8]

Alexander, J. M., "On the Theory of Rolling," Proc. R. Soc. Lond., A326, 1972.

[9]

M. F. Ashby: Strengthening methods in crystals, 137; 1971, Wiley.

[10]

T. Gladman: The physical metallurgy of microalloyed steels, Book, Institute of Mat. (London, England), 615.

[11]

Humphreys, Hatherley, recrystallization and related annealing phenomena, Elsevier, 2004.

La Metallurgia Italiana - marzo 2020

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Industry news - Automotive

Metallurgical design and production of AHSS grades DP800 and CP800 ISP and ESP thin slab technology at Acciaieria Arvedi in Cremona, Italy R. Venturini, A. Bianchi, M. Andraghetti, C. Guarnaschelli, M.C. Cesile, P.E. Di Nunzio The production of advanced high strength steels (AHSS) with the compact layout of ISP and ESP thin slab casting plants

is nowadays consolidated. The development of ever stronger and metallurgically challenging steels needs a careful alloy design in order to satisfy all the requirements of the automotive industry.

In this context, Acciaieria Arvedi has developed the DP800 grade as cold rolled hot dip galvanized strip in the thickness range from 0.8 to 1.2 mm and the CP800 grade as hot rolled strip in the thickness range from 2 to 4 mm, both for automo-

tive applications. Generally speaking, for the alloy design of such grades produced by thin slab casting, it is essential to optimize the use of alloying elements and to keep the overall chemical composition of the steel at an appropriate level.

The DP800 grade is currently produced starting from hot strip from both the ISP and ESP lines and, after cold reduction, is zinc-coated on a hot dip galvanizing line. To achieve the desired level of strength, the main characteristic feature of the

plant is exploited, namely the ease in obtaining an extremely fine microstructure in the hot strip. Consequently, a fine

microstructure in the cold rolled and recrystallized strip can be obtained as well, thus improving both yield and ultimate

tensile stresses. On the other hand, the properties of the as-hot rolled CP800 are mainly related to the achievement of a mixed microstructure composed of bainite as the major constituent, in association with ferrite and martensite. The metallurgical design of this grade has been completed and the first industrial trials are ongoing.

In this work, some aspects of the development of both grades are illustrated and the results of the microstructural and mechanical characterization of the DP800 grade are presented and discussed..

KEYWORDS: AHSS, ALLOY DESIGN, THIN SLAB, DP800, CP800.

INTRODUCTION

In recent years, vehicle weight reduction has become a

primary issue for improving fuel efficiency together with

the need to improve safety [1]. The current tendencies in the automotive industry are towards the use of High Strength steels (HSS) for 100% of vehicle structures. About 80% of these materials are Advanced High Strength

steels (AHSS), i.e. Dual Phase (DP), Transformation Induced Plasticity (TRIP), Complex Phase (CP), Ferrite-Bainite (FB) and martensitic steels.

The microstructure of DP steels consists of soft ferrite as

R. Venturini, A. Bianchi, M. Andraghetti Acciaieria Arvedi S.p.A., Cremona, Italy

C. Guarnaschelli, M.C. Cesile, P.E. Di Nunzio Rina Consulting - Centro Sviluppo Materiali S.p.A., Rome, Italy

a primary phase with interspersed grains of a mixture of martensite and retained austenite (MA constituent) the volume fraction of which ranges from about 5 to 30 vol.%

depending on the grade. This combination allows to achieve high strength levels comparable with conventional High Strength Low Alloyed (HSLA) steels of equivalent

La Metallurgia Italiana - March 2020

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AttualitĂ industriale - MobilitĂ grades but with higher elongation. The very low YS/TS ra-

grade. The typical UTS range for CP800 is 800-900 MPa.

Flat rolled sheets of DP steel grades are very attractive for

by adding Nb, Ti or V. The precise control of the thermo-

tio of about 0.6-0.7 is the key to their good formability. the automotive industry also due to their excellent fatigue resistance which makes them suitable for other components such as wheel discs. Besides the alloy design, the

selection of an appropriate thermal treatment in the infracritical temperature range, for recrystallization and con-

current partial transformation to austenite before hot dip galvanizing, is also very important.

Complex Phase steels have a fine microstructure consisting mostly of a ferrite-bainite matrix with possible

small amounts of martensite, retained austenite and pearlite. This microstructure allows very high ultimate ten-

sile strengths to be obtained, depending on the specific

Additional precipitation strengthening can be exploited mechanical processing and a careful balance of alloying elements are of fundamental importance to obtain hot

rolled strips of multiphase steels. These grades are cha-

racterized by good formability, high-energy absorption and high residual deformation capacity. Potential applica-

tions in automotive manufacturing include parts requiring high energy absorption in the elastic and low-plastic range, such as bumper and B-pillar reinforcements.

An overview of the tensile properties of hot-rolled high strength steels is reported in ref. [1] and a graphical sketch is shown in Fig. 1.

Fig.1 - Tensile properties of hot-rolled high strength steels (Source: WorldAutoSteel). In this context Acciaieria Arvedi already routinely pro-

The strength of Arvediâ&#x20AC;&#x2122;s technological choice relies on

rolled and zinc coated sheets, as for example the DP600

(Inline Strip Production), started in 1992, and the more re-

duces AHSS grades both as hot rolled strips and as cold

grade. The Ferrite-Bainite hot rolled strip is also available in the current product mix.

two innovative routes for producing hot strip: the ISP cent ESP (Endless Strip Production).

Fig.2 - ISP and ESP Layout.

La Metallurgia Italiana - marzo 2020

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Industry news - Automotive Both routes aim at achieving an extremely compact and

for this grade: hyper-peritectic and hypo-peritectic. In the

(energy, water, etc.) and higher compatibility with the is-

in the second case it is below 0.1% [3].

flexible plant layout leading to a significant cost reduction

sues of environmental protection (low level of gaseous

emissions). As a matter of fact, the ESP line, which fulfils the requirements of the Kyotoâ&#x20AC;&#x2122;s protocol, achieves an overall energy reduction ranging from 40% to 70% (depending on the strip thickness) and a reduction of 50% in

water consumption in comparison with conventional hot

rolling plants. All these advantages are combined with a

high strip quality and the possibility of producing hot rolled strips in low gauges down to 0.8 mm, with a high level of length-wise uniformity of microstructure and mechanical

properties [2]. In addition, the most striking feature of the

compact processes is that extremely fine microstructures are commonly obtained in the hot strip. Grain refinement

not only allows an increase in tensile properties but also an improvement in ductility and fatigue resistance.

In the continuous effort to improve its product mix, Ac-

ciaieria Arvedi has recently developed the DP800 grade as cold rolled strip hot dip galvanized in the thickness range from 0.8 to 1.2 mm and the CP800 grade as hot rolled strip in the thickness range from 2 to 4 mm.

As previously mentioned, the use of alloying elements and an accurate process control are crucial for achieving the desired target microstructure.

The alloy design of such grades produced by thin slab casting is even more important since it requires a further

optimization of the alloy additions in order to keep the

overall chemical composition of the steel at an appropria-

te level and to comply with the inherent features of the ISP and ESP lines, especially regarding the casting process. As a matter of fact, in compact processes the casting speed

is much higher (above 5 m/min) than that of conventional continuous casting machines.

The first coils of DP800 grade have been produced star-

ting from hot strips produced by both the ISP and the ESP lines and, after cold reduction, have been zinc-coated on a hot dip galvanizing line. The fine-tuning of the proces-

sing conditions is under way. The metallurgical design of the as-hot rolled CP800 grade has been completed and the first industrial trials are ongoing.

In this work the alloy design strategy and some significant results are described and analyzed. DP 800

Generally speaking, there are two alloy design strategies

La Metallurgia Italiana - March 2020

first case, the carbon content is about 0.15-0.16% whereas In order to avoid drawbacks during thin slab casting of near-peritectic alloys, such as hot cracking, Acciaieria Arvedi

has decided to develop a hypo-peritectic composition for this grade with a typical carbon content of about 0.07%.

It has to be noted that hypo-peritectic formulations have additional advantages regarding formability and perfor-

mance, for example with respect to the hole-expansion

test [4]. As a consequence, in order to achieve the tensile properties, it has been necessary to increase the fraction of

martensite in the final product since its carbon content, as

well as its hardness, are accordingly lower. In addition, the martensite start temperature (Ms) increases as carbon decreases and this has to be taken into account in designing

the overall composition if the strips have to be processed

on a hot dip galvanizing line. Therefore, in order to avoid a too early formation of martensite that would undergo tem-

pering before the final cooling of the strip, thus impairing

the mechanical performance, the Ms temperature must be

kept below 460Â°C, i.e. the temperature of the zinc pot. This is generally accomplished by increasing the Mn content to around about 2%, with an additional solid solution stren-

gthening effect. The fraction of MA in the hypo-peritectic DP800 grade typically ranges between 15 and 30 vol%.

From the microstructural viewpoint, a fine grain size is be-

neficial since it reduces the tendency to fracture and the crack propagation during cold forming operations (hole

expansion and folding tests). Moreover, a homogeneous dispersion of the martensite islands has a positive effect. This is usually obtained by exploiting the refining effect

of Nb during hot rolling and phase transformation on the run-out table of the hot strip mill. Another important pa-

rameter for modulating the yield strength in Nb-alloyed DP steels is the coiling temperature of the hot strip. If the

coiling temperature is low (i.e. below about 630Â°C) nio-

bium remains in solid solution and the fine precipitation of NbC which occurs during recrystallization annealing gives an additional strengthening contribution of about 10 to 15 MPa.

The chemical composition of the DP800 grade developed

at Acciaieria Arvedi, in compliance with the EN 10346 stan-

dard (Tab. 1 and Tab. 2), has been designed with the help of thermodynamic and kinetic models in association with empirical relationships for relating the predicted microstructure to the mechanical properties. The composition

pagina 45

Attualità industriale - Mobilità range of the main alloying elements is reported in Tab. 3.

permits to easily distinguish between the white MA and

processing parameters, the industrial trials have been star-

racterized by very fine ferrite grains. The MA particles ap-

After the simulations and laboratory tests to set up the ted to optimize the processing conditions.

A 1 mm thin strip cold rolled and zinc coated is here re-

ported as a typical example. The steel was cast on the ISP line and hot rolled to 2.5 mm thickness with a finishing rol-

ling temperature of 850±20°C and a coiling temperature of

600°C. The hot strip was then cold rolled with a reduction of 52% to the final thickness and processed by the HDG

line no. 2 at the Cremona works. The tensile properties obtained after skinpass are 790 MPa for Rm, with a Rp02/

the darker ferrite. The structure, fully recrystallized, is cha-

pear evenly dispersed and homogeneous. In a few cases, the largest MA islands exhibit a central region with a bainitic structure due to a locally lower carbon enrichment.

The metallographic examination has ascertained a fraction of MA of 22%, an average size of the MA islands of 1.2±0.4 µm and an average ferrite grain size of 2.7±0.8 µm.

The size distributions of ferrite grains and MA islands are shown in Fig. 4.

Rm ratio of about 0.70, and 15% elongation. An example of the microstructure is shown in Fig. 3. The LePera etching

Tab.1 - Chemical composition requirements (mass %) for the DP800 grade according to the EN 10346 standard. Grade

Cr+Mo

Nb+Ti

HCT780X

≤ 0.18

≤ 2.5

≤ 0.80

≤ 1.0

≤ 0.15

≤ 0.20

≤ 0.005

≤ 2.00

≤ 0.08

≤ 0.015

Tab.2 - Mechanical properties of the DP800 grade according to the EN 10346 standard. Rp0.2 (MPa)

Rm (MPa)

A80 (%)

450-560

≥ 780

≥ 14

Tab.3 - Composition ranges (mass%) of the most relevant alloying elements in DP800 grade developed at Acciaieria Arvedi. C

Cr+Mo

Nb+Ti

0.05-0.07

1.7-2.0

0.8-0.9

0.03-0.04

Fig.3 - Microstructure of the 1 mm thin HDG DP800 steel (LePera etching): a) sub-surface; b) mid thickness.

La Metallurgia Italiana - marzo 2020

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Industry news - Automotive

Fig.4 - Size distribution of ferrite grains and MA islands in the -mm thin Zn-coated DP800 steel. CP 800

with some ferrite and less than 10% martensite as minor

ried out through an extensive feasibility study based on

mined at the end of the process, after the microstructural

The development of hot strips of CP800 grade was carthermo-mechanical, microstructural and phase transformation models, all developed by RINA Consulting CSM.

This is a preliminary activity necessary in order to comply with all the layout conditions. The analysis started by

hypothesizing two chemical compositions suitable for obtaining the desired mix of microstructural constituents for the mechanical properties to be obtained. The target

microstructure is composed of bainite as the main phase

constituents. The actual mix of constituents will be detersimulations have indicated quantitatively their amount.

Generally speaking, the procedure adopted for develo-

ping a new grade is based on a set of metallurgical and

thermo-mechanical models, customized for the ISP or ESP line layouts, which consider the production process from casting down to the hot strip. A sketch of the process is shown in Fig. 5 as a flow diagram.

Fig.5 - 5 Flow diagram illustrating the procedure adopted for developing a new grade based on a set of metallurgical and thermo-mechanical models customized for the ISP or ESP line layouts.

La Metallurgia Italiana - March 2020

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Attualità industriale - Mobilità The castability of the steels is evaluated through the cal-

complex since a small amount of tensile strain is sufficient

composition has to be balanced in order to avoid the pe-

lationships based on the chemical composition can be

culation of the equivalent carbon content. The chemical ritectic region of the phase diagram. Steels that undergo

peritectic transformation are particularly susceptible to cracking due to additional strains resulting from volume changes during the solid state transformations. Conse-

quently, the continuous casting of peritectic steels is

to cause hot cracking of the material. Many empirical re-

found in literature. Some of the most recent have been obtained by exploiting thermodynamic databases of com-

mercial software packages such as ThermoCalc. Some of them are listed below.

The most used is that of Wolf [5, 6] 1)

Among the relationships obtained by fitting data generated by commercial software are worth of mention that by

Xu et al. [7]

2) that by Blazek et al. [8] 3) and that by Sarkar et al. [9] 4)

According to the diagram in Fig. 5, the rolling schedules

sed chemical compositions can produce the desired mi-

stenite grain size at the last finishing stand where the re-

must be adopted for the two lines and also depending on

are defined together with a prediction of the average aufining effect of Nb additions is considered. The optimum cooling pattern on the run-out table is identified by using

a coupled thermal-metallurgical model to predict the fraction of microstructural constituents during a generic cooling pattern. This model has shown that the propo-

crostructure but, as expected, different cooling patterns

the strip thickness. The composition range of the CP800 grade developed at Acciaieria Arvedi, in compliance with

the EN 10346 standard (Tab. 4 and Tab. 5), is shown in Tab. 6.

Tab.4 - Chemical composition requirements (mass %) for the CP800 grade according to the EN 10346 standard. Grade

Cr+Mo

Nb+Ti

HDT760C

≤ 0.18

≤ 2.5

≤ 1.0

≤ 0.25

≤ 0.20

≤ 0.005

0.015 2.00

≤ 0.08

≤ 0.015

Tab.5 - Mechanical properties of the CP800 grade according to the EN 10346 standard. Rp0.2 (MPa)

Rm (MPa)

A80 (%)

660-830

≥ 760

≥ 10

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Industry news - Automotive Tab.6 - Composition range (mass%) of the most relevant alloying elements in the CP800 grade developed at Acciaieria Arvedi. C

Cr+Mo

0.05-0.07

≤ 2.0

0.5-1.0

≤ 0.03

To find the optimum range of microstructures compatible

strip thickness of 3 mm and to an average austenite grain

cification EN 10346 (Tab. 5), a sensitivity analysis on the

tensite volume fractions is calculated as a function of the

with the target mechanical properties according to spe-

metallurgical parameters was carried out. It showed that the hot rolling finishing temperature has a second order

effect, whereas the coiling temperature is the most sensitive quantity and permits the tuning of the final microstructure.

On the other hand, the austenite grain size after the last

rolling pass has an indirect effect on the hardenability of the steel since larger austenite grains transform more easily into bainite or martensite on fast cooling. The microstructural model of austenite evolution during hot deformation has indicated that, if the steel is microalloyed with

Nb, at the exit of the hot strip mill the average austenite grain size is reduced by a factor 2, i.e. from about 10 µm

to about 5 µm. Therefore, to evaluate the effect of Nb ad-

dition on the phase transformation during cooling on the run-out table, two simulations were carried out.

An example of microstructural calculation for one of the

Nb-free compositions is reported in Fig. 6. It refers to a

size of 10 µm. The evolution of Ferrite, Bainite and Martarget coiling temperature in the range from 350 to 600°C. It can be observed that a fraction of martensite below the

established limit of 10% can be achieved by coiling the strip at temperatures slightly above 500°C.

A comparison with the martensite fraction calculated as a function of the coiling temperature for the two levels of average austenite grain size, 5 and 10 µm, is shown in Fig.

7 together with the calculated UTS. It clearly reveals that the smaller the austenite grain size, the smaller the martensite fraction and therefore the lower the minimum coi-

ling temperature required to have 10% martensite. On the other hand, for both grain sizes, the predicted UTS falls

within the prescribed range 760 960 MPa, indicating that the process is quite robust and allows for some limited variations in the hot rolling conditions and composition.

The first industrial trials have been carried out recently obtaining a minimum thickness of 2.5 mm.

Fig.6 - Calculated effect of the coiling temperature on the phase transformation of the 3 mm thin strip of the CP800 steel grade (austenite grain size at the last finishing stand of 10 µm).

La Metallurgia Italiana - March 2020

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AttualitĂ industriale - MobilitĂ

Fig.7 - Calculated effect of the austenite grain size at the last finishing stand (5 and 10 Âľm) on the martensite fraction (a) and on the predicted UTS (b) as a function of the coiling temperature for the 3 mm thin strip of the CP800 steel grade. CONCLUSIONS

On the other hand, the metallurgical design of the as-

for the metallurgical design of both the cold rolled and

mum chemical composition and the industrial hot rolling

The present work has illustrated the approach adopted zinc-coated DP800 and of the hot rolled CP800 advanced

high strength steels for their production at the Acciaieria Arvedi plants.

The DP800 grade is already in standard production with

hot strips coming from either the ISP or ESP line. Its properties are compliant with the EN 10346 standard.

La Metallurgia Italiana - marzo 2020

hot rolled CP800 grade has been completed. The opti-

practice have been defined for strip thicknesses of 3.0 and 2.5 mm with respect to both the integrated casting and rolling lines.

In the first industrial trials the minimum thickness of 2.5 mm has been obtained.

pagina 50

Industry news - Automotive References [1]

C. Mesplont, “Phase transformations and microstructure - mechanical properties relations in Complex Phase high strength steels”, PhD thesis, Ghent University, 2002.

[2]

A. Jungbauer, B. Linzer, T. Pfatschbacher, “Thinnest High-Quality Hot-Rolled Coils at Lowest Production Costs with Arvedi ESP Technology”, Mitsubishi Heavy Industries Technical Review Vol. 55 No. 1 (March 2018).

[3]

R.N. Yellakara, N. Kulkarni, J. Tripathy, A. Bader, J. Roach, J. Principe, H. Doude, H. Rhee, “Development of Dual-Phase Steel Using Non-Peritectic Carbon Chemistry for Thin-Slab Continuous Casting Compact Strip Production (CSP) Steel Mill”, Proc. AISTech 2017 2965-2973.

[4]

H. Mohrbacher, “Advanced metallurgical concepts for DP steels with improved formability and damage resistance”, Proc. Intl. Symp. on New Developments in Advanced High-Strength Sheet Steels, AIST, 2013, p. 319-329.

[5]

M.M. Wolf, “Estimation of Crack Susceptibility for New Steel Grades”, 1st European Conf. on Continuous Casting, Florence, 1991, 2489-2499.

[6]

M.M. Wolf, “Initial Solidification and Strand Surface Quality of Peritectic Steels” Continuous Casting vol. 9, Iron and Steel Society, Warrendale, USA, 1997

[7]

J. Xu et al., “Effect of Elements on Peritectic Reaction in Molten Steel Based on Thermodynamic Analysis”, ISIJ Int. 52 (2012) 18561861.

[8]

K.E. Blazek et al., “Calculation of the Peritectic Range for Steel Alloys”, Iron and Steel Technology, July 2008, 80-85.

[9]

R. Sarkar, A. Sengupta, V. Kumar, S.K. Choudhary, “Effects of Alloying Elements on the Ferrite Potential of Peritectic and Ultra-Low Carbon Steels”, ISIJ International, 55 (2015), pp. 781–790.

La Metallurgia Italiana - March 2020

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Attualità industriale - Mobilità

Safety and Lightweight innovations for future mobility by using stainless steels edited by: Marco Frigo, Stefan Lindner

Werner von Siemens with his electrically powered carriage (1882) or the electric cars developed by Ludwig Lohner and Ferdinand Porsche for the world exhibition 1900 in Paris can be considered as two famous examples of successful development on electric vehicles during the 19th century. Independent of such developments, passenger cars with combu-

stion engines dominate the 20th century because of their significant expanded range, the quick refuel process as well as

availability and price of the fossil fuels. With a view to the social desirability as well as the political and legal framework, but also the increased price and limitedness of fossil fuels, electric vehicles experience a renaissance as one enabler for

future mobility (1). This trend is reinforced by simultaneously staged megatrends like autonomous driving, car sharing,

or the topic of “last-kilometre” transportation for goods. As a direct result of this development, the demands for applica-

tion-specific material concepts also changed and are getting more challenging. Lightweight, passenger safety, but also recyclability and the material-specific CO2-footprint (life cycle engineering) can be highlighted as some major require-

ments for the mentioned application field of transportation (2, 3). A new generation of ultra-high strength stainless steels can fulfil the high expectation on materials for future mobility by offering a fully austenitic microstructure connected with a specific hardening mechanism and 100 % recyclability.

PAROLE CHIAVE: ULTRA-HIGH STRENGTH STAINLESS STEEL – LIGHTWEIGHT – SAFETY – HEAT RESISTANCE – BATTERY ELECTRIC VEHICLES – TWIP HARDENING MECHANISM – RECYCLABILITY

STAINLESS STEELS

Steels with a chromium content of at least 10.5 % are clas-

sified as stainless steels according to DIN EN 10088-1 (4). Moreover, the presence and level of other alloying elements which can be sub-divided into ferrite and austenite

formers determine the resulting microstructure, which

in turn defines the respective physical and mechanical properties of the stainless steel grade. Thereby, stainless

steels can be classified depending on their microstructure

into ferritic, martensitic, meta-stable or fully austenitic and “Duplex” (ferrite + austenite) stainless steels, view Fig. 1. In contact with atmospheric oxygen, stainless steels offer

Marco Frigo Outokumpu, Italy

Stefan Lindner

Outokumpu Nirosta GmbH, Germany

a natural and stable chromium-oxide surface layer resulting in a naturally high level of corrosion protection. Since their invention in 1912, stainless steels represent with

their 100 % recyclability the most recycled material in the world whereby the quality is preserved in the process.

With a proportion > 85 % of recycled content (> 2,000,000

La Metallurgia Italiana - marzo 2020

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Industry news - Automotive tons per year), OutokumpuÂ´s stainless steel significantly decreases the carbon footprint of the complete transpor-

tation supply chain.

Fig.1 - Microstructure and lattice structure of stainless steels. STAINLESS STEELS IN AUTOMOTIVE

ts. In addition to their enormous formability, they are also

ce material group in transportation engineering for de-

paintability. Thereby, the properties of stainless steels

Stainless steels represent an established high-performancades. Their extensive range of properties enables their use in exhaust systems, airbag tubes and trim systems as

well as crash-relevant structural and chassis componen-

notable for good weldability, adhesion properties and

perform a continuous reinvention to support the ongoing improvement and changing demands in automotive manufacturing, view Fig. 2.

Fig.2 - Timeline of stainless steel properties in the automotive world. FORTA H-SERIES â&#x20AC;&#x201C; ULTRA-HIGH STRENGTH STAIN-

TWIP-effect (Twinning-Induced Plasticity) into the mate-

A new generation of stainless steels with a stable

so-called Forta H-series enables an outstanding relation

LESS STEEL

one-phase, fully austenitic microstructure can be deve-

loped because of a special hardening mechanism called

La Metallurgia Italiana - March 2020

rial category of ultra-high strength steels, view Fig. 3. The

of mechanical-technological values for automotive and transportation construction.

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AttualitĂ industriale - MobilitĂ

Fig.3 - Mechanical-technological values of Forta H-series Responsible for this characteristic profile are the coordi-

gy absorption can be realized. Furthermore, the com-

reaching in a specific stacking fault energy. As a result, the

properties of the material in dependence of the cold-for-

nated alloying elements like chromium and manganese cold-formable material enables an intensive work-harde-

ning during cold-rolling of the material but also during cold-forming of the component or during impact situation of the vehicle. At the same time an enormous ener-

ponent-manufacturer is able to adjust the local desired

ming degree, view figure 4. Design engineers can create higher strength or higher ductility areas inside one component where the usage conditions of the components require it.

Fig.4 - Resulting microstructure and mechanical values of Forta H-series depending on the cold-forming. The Forta H-series is applicable over the complete au-

surface. Based on this passivation layer the material works

dissimilar weldability. Further, a high formability for com-

ting or galvanizing. In combination with the established

tomotive process chain because of its good similar and plex formed parts which contributes directly to component stiffness is given. Because of the well-balanced al-

loying system, the conditions defined in (5) are fulfilled to avoid any kind of delayed cracking. While manganese

works as an austenite former, the other main alloying ele-

ment chromium influences the mechanical-technological

values of the TWIP material in a positive way (6), support the solubility and homogeneity of the other elements and build up a chromium-oxid passivation layer on the steel

La Metallurgia Italiana - marzo 2020

without any kind of coating like an electrolytic zinc coacathodic dip coating in the automotive manufacturing process, the passivation layer presents a very good cor-

rosion protection system even after a lattice cut or stone chipping. The natural chromium-oxid passivation layer repassivates again after a mechanical injury and avoids an undercut of the injured dip coating process.

The Forta H-series features a very high crash performance connected with directly high energy absorption poten-

tial. Fig. 5 illustrates a dynamic high-speed axial crash of a

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Industry news - Automotive square profile manufactured of Forta H800 in a thickness

lation to the impact force and subtend a high resistance

behavior, high energy absorption combined without any

gy. Therefore it can be shown for the example of a b-pil-

of 1.5 mm. As a result the profile shows a great forming cracks in the formed as well as welded areas. During the impact situation, the Forta H-series with their fully austenitic microstructure and their TWIP hardening effect develop their full benefit: The material will hardened in re-

to the impact simultaneously absorbing the impact ener-

lar that a direct lightweight potential of 35 % in relation

to press-hardened steels like 22MnB5 is possible without having a brittle failure like other metal materials.

Fig.5 - Crash performance and energy absorption during a high-speed axial crash. CONCEPTS FOR BATTERY ELECTRIC VEHICLES

The battery compartment, sometimes also called battery

of customer acceptance for battery-powered electric

the battery cells and should therefore have an installation

Range and availability represent two key demands in point

vehicles (BEV). To fulfil those expectations, a high degree of space utilization in the vehicle floor to host the

battery modules is required and results in complex design and engineering demands. Further, the battery modules have to be protected from environmental influences such as corrosion, extreme temperatures and stone chips

as well as deformation to prevent thermal collapse. Mo-

reover, to sustain crash requirements constitute a further

challenge, especially for side and underfloor intrusion where the intrusion space is limited. Additionally, thermal management needs to ensure that the batteries are maintained at their ideal operating temperature offering maximum range. All of this leads to the battery housing as a

key component in the design of battery-powered electric

vehicles. The material stainless steel and its associated construction concepts can make a significant contribution to enable the application requirements of BEV. Stainless

steels further offer advantages in mass production as well as recycling, carbon footprint and overall operating costs. BATTERY COMPARTMENT

La Metallurgia Italiana - March 2020

pack or battery housing, covers the battery modules with space as large as possible to increase the driving range. A battery housing simple in point of its geometry can be

designed as a deep-drawn shell solution which is then scalable up to mass-produced volumes offering short manufacturing times. With the before mentioned fully austenitic stainless steels and their high tendency to

work-harden during deep-drawing, such a shell can be manufactured from highly ductile materials resulting in an ultra-high strength and therefore intrusion resistance

component. Thereby the interior radii as a safety space

between the shell wall and the integrated battery modu-

les must be dimensioned to ensure the necessary safety.

With austenitic stainless steels, an interior radius range of between seven and 12 mm represent not a limitation during deep-drawing. The advantage of a deep-drawn bat-

tery compartment beside the fast cycle times is the saved

packaging space, but also further production steps like welding in the battery â&#x20AC;&#x153;clean roomâ&#x20AC;? can be avoided. Thus,

thermal distortion during welding and subsequent clea-

ning operations which increase usually production costs are excluded with a shell construction. Fig. 6 illustrates a

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Attualità industriale - Mobilità complete stainless steel battery compartment design including the shell solution but also offering the integration

of necessary surrounding parts like the thermal manage-

ment system, the underride protection or the crash frame at the sides.

Fig.6 - Lightweight stainless steel battery compartment. THERMAL MANAGEMENT

tion is passive cooling, which presents additional advan-

lost to waste heat of 5% of the efficiency of lithium-ion

stainless steel in contact with the battery modules and a

Having more than 7,000 battery cells inside a BEV and a traction batteries, a critical amount of heat inside the battery compartment must be lead away. At temperatures

above 45 °C, battery damage occurs due to chemical reactions inside the cells. At the other extreme with a tem-

perature below −5 °C, performance and charging capacity decrease and thus lowering vehicle range. In a damage case, the mostly liquid coolant must not come into direct

contact with the battery cells. Therefore, the ideal solu-

tages in production and assembly. By using a thin ferritic thicker austenitic stainless steel as a protection to the en-

vironment, the advantages of stainless steels in point of

their physical properties can be used. The different metal thicknesses and physical properties like heat conductivity, deliver highly efficient cooling of the battery modules,

while the insulating effect of the outer-located austenitic

stainless steel helps to reduce the heat transfer with the environment, view Fig. 7.

Fig.7 - Heat conduction influenced by material thickness and physical properties. La Metallurgia Italiana - marzo 2020

pagina 56

Industry news - Automotive HEAT-RESISTANCE AS SAFETY PROTECTION

a critical deformation of the batteries, heat-resistance is

choice for exhaust systems as they are offering heat-resi-

structure must be functional since a fire department can

Stainless steels are used since decades as the material of stance as well as high-temperature corrosion resistance properties. Heat resistance means stable mechanical technological values even at higher temperatures. For the

field of battery-powered electric vehicles and a potential self-ignition of the battery cells after a crash situation with

directly connected to passenger safety. After a crash, the

rescue the occupants out of the vehicle which means in time between 9 and 12 minutes. Fig. 8 points out the resul-

ts of an austenitic stainless steel in relation to aluminum. During a self-ignition of the batteries, the expectable temperature stays over 1,200 Â°C.

Fig.8 - Result of SAG structural test after 10 minutes for different metal materials and temperatures.

INNOVATIVE VEHICLE CONCEPTS

Aachen University by using a lowest possible number of

nefits of heat, corrosion and crash resistance, but also

view Fig. 9. This structure, winning the ISSF (International

By using high-performing stainless steels with their besuperior mechanical technological values with the possibility to realize lightweight and stiff constructions, such

materials can give a significant distribution to future mobility concepts. As one example, a demonstrator of a BE-

V-passenger compartment as a structural frame design was realized in cooperation with PEM Institute of RWTH

production methods with laser technology and bending,

Stainless Steel Forum) Gold-Award of 2017, combines a high stiffness and a high crash safety with best possible

lightweight. Because of avoiding expensive investment costs for plants, tooling and painting lines, it represents

also a cost-effective solution to build-up small series with a fast cycle time.

Fig.9 - Bionic-inspired demonstrator structure of a BEV made with Forta H1000.

La Metallurgia Italiana - March 2020

pagina 57

AttualitĂ industriale - MobilitĂ Another concept worked out in cooperation with fka Aa-

stainless steels in combination with the folding principle

ly refines the advantages of the demonstrator mentioned

relation to state-of-the art vehicles. This design approach

chen GmbH is pointed out with Fig. 10 which consequentbefore. The idea behind is to create a folding box structu-

re inspired by paper packing cases which offer because of a nested design a high stiffness combined with low weight

and a high load-bearing capacity. By using pre-assembled stainless steel sheets, a simple but effective possibility for

local manufacturing without cost-intensive forming tools and presses as well as painting lines is feasible. With such a pre-constructed approach, local start-ups without established manufacturing lines can manufacture just with bending and one welding method, in the best case resistance

spot welding or laser beam welding, a car body structure for battery-powered electric vehicles. The design can be

adopted also for transport of goods or small people mover. The material properties of temper-rolled austenitic

halve the number of individual components and welds in

results in a much lighter structure, lower than 180 kg, and

can be therefore categorized into the L7e vehicle class of

the European Union representing small urban electrified vehicles with a total vehicle weight â&#x2030;¤ 450 kg without bat-

teries. At the same time, the stiff bending structure fulfills the crash-requirements of a M1 category vehicle. Because of the local manufacturing approach, just pre-assembled sheets must be send around the world instead of comple-

te vehicles, what lower the CO2-emissions for logistic. In

general, the created structure increases the sustainability

over the whole vehicle lifetime. The stainless steel vehicle structure can be recycled by 100 % or can be used in multi-generation usage.

Fig.10 - Bend-formed vehicle structure (left side) with integrated common part strategy (right) .

SUMMARY

ty represent not a contradiction in structural design. Fur-

ties make an important contribution towards satisfying the

applicable for high-temperature resistance which will be

Innovative material concepts with a wide range of propercomplex issues of future mobility concepts. Stainless steels represent a key enabler for future mobility and can increase the acceptance and efficiency of those transporta-

tion concepts. With their properties, the design demands of safety in combination with lightweight and sustainabili-

La Metallurgia Italiana - marzo 2020

ther, stainless steels as high performance materials are for battery-powered electric vehicles directly connected with passenger safety. It also offers significant improvements in areas of overall cost of mass production, carbon footprint and recycling.

pagina 58

Industry news - Automotive References [1]

Karle A. Elektromobilität – Grundlagen und Praxis, Carl-Hanser-Verlag, 2015

[2]

Müller B, Zachäus C, Meyer G. European strategic processes towards competitive, sustainable and user-friendly electrified road transport, 30th International electric vehicle Symposium, Stuttgart, 10.09.2017

[3]

Zhou Q. Electric Mobility in China – Developments, Opportunities & Challenges, 30th International electric vehicle Symposium, Stuttgart, 10.09.2017

[4]

DIN EN ISO 10088-1. Stainless steels - Part 1: List of stainless steels; German version EN 10088-1:2014

[5]

Ratte, E. Wasserstoffinduzierte verzögerte Rissbildung austenitischer Stähle auf CrNi(Mn)- und Mn-Basis, dissertation IEHK RWTH Aachen, Shaker Verlag, Band 8/07, 2007

[6]

Bracke L, de Cooman B, Liebeherr M, Akdur N. Phase transformations in High Strength Austenitic FeMnCr Steels, Solid-solid phase transformations in in-organic materials, Volume 1, S.905-910, 2005

La Metallurgia Italiana - March 2020

pagina 59

Scenari - Mobilità

Effetti del Niobio negli acciai di terza generazione a cura di: Fabio D’Aiuto - Senior Market Development Manager, CBMM Europe

Il Niobio é un metallo, noto anche con il nome di Colom-

vola periodica con il numero atomico 41; in natura si pre-

Charles Hatchett (fig. 1) ed é presente all’interno della ta-

do di Niobio (Nb2O5).

bio. Il Niobio é stato scoperto nel 1801 dallo scienziato

senta sotto forma di ossido e il piú comune é il pentaossi-

Fig.1 - Charles Hatchett Esistono al mondo 85 siti ricchi di questo elemento, ad

bio da qualche punto percentuale del minerale estratto, a

striale: due miniere si trovano in Brasile e la terza in Ca-

alluminotermica, che porta ad una concentrazione di circa

oggi tre di questi sono utilizzati per la produzione indunada. L’estrazione e il processo di raffinazione sono re-

lativamente semplici, consistono in una prima fase di concentrazione, che porta il contenuto di ossido di Nio-

circa 50% ed una seconda fase che prevede una reazione

65% Niobio e 35% Ferro, punto eutettico del diagramma binario Ferro – Niobio (fig. 2).

Fig.2 - Diagramma Ferro - Niobio

La Metallurgia Italiana - marzo 2020

pagina 60

Experts’ Corner - Automotive E’ stata scelta la composizione eutettica per la commer-

mento che va aggiunto in acciaieria, nel metallo fuso, sotto

fusione ed é quindi pienamente compatibile con il proces-

FeNb si dissolve nel liquido, pronto quindi per passare alla

cializzazione, in quanto presenta la minore temperatura di so produttivo dell’acciaio. Il Niobio é una elemento detto

microlegante, in quanto é sufficiente aggiungerne quantitá molto ridotte, per apprezzarne i benefici, tipicamente 0,2

Kg ogni 1.000 Kg di acciaio prodotto. Il processo produtti-

vo nasce in acciaieria, dove sapienti “cuochi metallurgisti” cucinano giornalmente acciai con diverse proprietá, seguendo le ricette e scegliendo gli ingredienti corretti, proprio come in cucina quando si prepara una torta. La produzione degli acciai prevede infatti l’aggiunta di elementi chimici e processi specifici, a seconda delle caratteristiche

che si vogliono ottenere. Il Niobio ad esempio é un ele-

forma di ferroniobio (FeNb). All temperatura di 1.510°C, il fase di colata. Durante il processo di colata e conseguentemente solidificazione, il Niobio si combina con il Carbo-

nio e l’Azoto, formando carburi e nitruri di Niobio. Queste fasi rimangono tali finché la bramma prodotta non viene

riscaldata alla temperatura di 1.150°C, a questa tempera-

tura il Niobio torna in soluzione solida e nella successiva

fase di laminazione a caldo, si apprezzano i primi benefici. Il Niobio in soluzione solida ritarda la ricristallizzazione durante la laminazione a caldo, portando ad un affinamento del grano (fig. 3).

Fig.3 - Affinamento del grano L’affinamento del grano porta all’incremento del carico

maggiore resistenza e le ritroviamo negli acciai HSLA (High

ti per i componenti automotive. L’affinamento del grano

dalle acciaierie, hanno permesso con l’utilizzo di chimiche

di rottura e della tenacitá, caratteristiche molto importanostacola inoltra la propagazione delle cricche, che si trova-

no a dover percorrere un percorso piú lungo e articolato, favorendo quindi ulteriormente l’assorbimento di energia. Le caratteristiche appena menzionate, si traducono in una

La Metallurgia Italiana - March 2020

Strength Low Alloy). Gli sviluppi successivi, portati avanti piú complesse, lo sviluppo di acciai a resistenza maggiore.

Grazie alla presenza di piú fasi in contemporanea (fig. 4), gli AHSS (Advanced High Strength Steel) sono stati introdotti nell’automotive.

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Scenari - Mobilità

Fig.4 - Strutture degli Advanced High Strength Steel L’evoluzione ulteriore di questi acciai, ha portato alla rea-

la rottura differita.

luzione di queste ultime due categorie, agli acciai detti di

mondo Automotive di ridurre il peso dei veicoli, con mol-

lizzazione degli UHSS (Ultra High Strength Steel) e l’evo-

terza generazione (3rd Generation Steel). La coesitenza di

piú fasi con caratteristiche differenti, porta da un lato ad un miglioramento delle proprietá meccaniche, che sono la media di tutte proprietá delle fasi presenti, da un altro ad una piú complessa gestione e fenomeni di rottura nuove.

Raggiungendo resistenze mecaniche superiori a 1.000 MPa, si incontra il fenomeno della rottura differita: atomi di idrogeno monoatomico possono entrare all’interno della struttura degli acciai e, combinandosi tra loro, formare la mole-

cola H2, che porta alla rottura del materiale causando stress interno. Il Niobio, precipitando sotto forma di carburo di Niobio, agisce come trappola irreversibile per l’idrogeno,

limitandone la mobilitá e quindi riducendo il fenomeno del-

Alla base degli sviluppi sopra riportati, c’é la richiesta del teplici benefici. Utilizzando gli AHSS e UHSS é possibile

raggiungere le stesse performance tecniche o maggio-

ri, rispetto gli acciai utilizzati in passato, utilizzando meno

materiale e quindi risparmiando peso. Minor peso vuol dire minori consumi, proprietá molto importante per l’utilizzatore finale, ma vuol dire anche minori emissioni di CO2,

soluzione utile a ridurre gli effetti nocivi sull’ambiente.

Parlando di veicoli commerciali, minor peso del veicolo,

vuol dire maggiore carico utile e quindi business maggiori. Acciai con migliori caratteristiche tecniche permettono di

raggiungere standard piú elevati di sicurezza con una durata dei componenti maggiori, a causa della maggiore resistenza a fatica. Tutti questi aspetti sono riportati nella fig. 5.

Fig.5 - Automotive driver La Metallurgia Italiana - marzo 2020

pagina 62

Experts’ Corner - Automotive In aggiunta a quanto detto fin’ora, bisonga tenere in consi-

oggi raggiunti dagli acciai si aggirano intorno ai 1200 MPa

posto il raggiungimento del livello di emisioni medie pari a

caldo, 2000 Mpa per gli acciai stampati a caldo. Nuove sfide

derazione le richieste della Comunitá Europea, che ha im95 gr CO2/Km, pena il pagamento di sanzioni. Le emissioni medie delle auto sono correlate a molti fattori:

- Motorizzazioni, l’euro 6 é oggi lo standard, si sta

evolvendo verso motori piú piccoli sovralimentati

- Ibridizzazioni dei motori: Mild Hybrid (MHEV), Plug In Hybrid (PHEV)

- Veicoli completamente elettrici a emissioni zero (BEV) - Sistemi di recupero dell’energia in frenata

- Riduzione del peso: 50 Kg in meno si traducono in una riduzione di circa 5 gr CO2/Km

Le future richieste dalla comunitá europea, porteranno ad

un ulteriore 15% di riduzione delle emissioni di CO2 entro

il 2025 e 30% entro il 2030. Per raggiungere questi targets sará fondamentale lavorare su piú fronti, lo sviluppo di nuove ibridizzazioni e soluzioni elettriche saranno la base e la riduzione del peso raggiunta grazie all’utilizzo

di nuovi acciai di terza generazione dará un ulteriore

per lo stampaggio a freddo, 1500 MPa per lo stampaggio a

tecnologiche sono nate, perché questi livelli di resistenza presentano problematiche nell’utilio quali:

- Fragilitá conseguente il processo di saldatura (LME) - Infragilimento da idrogeno (HE)

- Cricche dovute alla presenza di scarsa coesione delle

diverse fasi, durante le operzioni di espansione del foro o stampaggio (hole expansion & stretch flanging)

- Ritorni elastici

La presenza del niobio ridure il fenomeno dell’LME in

quanto affina il grano e lo omogenizza, favorendo una distribuzione uniforme del calore ed inoltre rende il materiale piú tenace. Il Niobio riduce l’infragilimento da idroge-

no agendo come trappola irreversibile. Il Niobio migliora il comportamento del materiale riducendo la presenza di

cricche durante le operazioni di hole expansion e stretch flanging (fig.6a).

contributo. La previsione é quindi avere acciai di terza

generazione sempre piú performanti e i livelli di resistenza

Fig.6a - Fenomeni connessi all’utilizzo degli acciai di terza generazione Negli acciai AHSS risultano evidenti i vantaggi in termini di migliore piegabilitá e tenacitá (fig. 6b). Queste proprietá sono migliorate grazie all’affinamento del grano e all’aumento del-

La Metallurgia Italiana - March 2020

la resistenza della fase ferritica, dove ferrite e fasi piú resistenti coesistono.

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Scenari - Mobilità

Fig.6b - fenomeni connessi all’utilizzo degli acciai di terza generazione. Lo sviluppo di soluzioni completamente elettriche dovrá

con anodo al Niobio Titanio (NTO) sono state sviluppate da

rica capillare oggi non presente e la ricerche di nuove bat-

molto promettenti, maggiore capacitá e ricariche nell’ordine

essere supportato dalla creazione di un’infrastruttura di rica-

terie a ricarica rapida potrá fare la differenza. Nuove batterie

CBMM e Toshiba e i test di laboratorio mostrano risultati dei minuti possono essere raggiunte.

Per maggiori informazioni visitate

www.cbmm.com e www.niobium.tech

La Metallurgia Italiana - marzo 2020

pagina 64

Atti e notizie - AIM news

Normativa / Standards AIM – UNSIDER – Norme pubblicate e

prEN 10357 rev

Austenitic, austenitic-ferritic and ferritic

technical delivery conditions for hot-rolled

28 febbraio 2020)

longitudinally welded stainless steel tubes

products.

progetti in inchiesta (aggiornamento

Norme pubblicate e progetti allo studio (elenco)

Norme UNSIDER pubblicate da UNI

Structural

steels

—

Part

General

for the food and chemical industry. ISO/DIS 630-2

Progetti UNSIDER in inchiesta prEN e ISO/DIS – marzo 2020

Structural steels — Part 2: Technical delivery conditions for structural steels for general purposes.

nel mese di febbraio 2020

prEN – progetti di norma europei

UNI EN 10216-2:2020

prEN ISO 19901-2

ISO/DIS 630-3 and

Structural steels — Part 3: Technical

Petroleum

a pressione - Condizioni tecniche di

fornitura - Parte 2: Tubi di acciaio non

structures

legato e legato per impieghi a temperatura

procedures and criteria (ISO/DIS 19901-

ISO/DIS 630-4

elevata

2:2020).

Structural steels — Part 4: Technical

Specific

natural

gas

requirements -

Part

industries

delivery conditions for fine-grain structural

Tubi di acciaio senza saldatura per impieghi

for

offshore

Seismic

steels.

design

delivery conditions for high-yield-strength EC 1-2020 UNI EN ISO 24817:2017 Industrie del petrolio, della petrolchimica

ISO/DIS

–

progetti

internazionali

norma

quenched and tempered structural steel plates.

e del gas naturale - Riparazioni di tubazioni

Progetti UNSIDER al voto FprEN e

per mezzo di materiale composito -

ISO/DIS 23071

Conformità ai requisiti di qualifica e

Refractory

progettazione, installazione, collaudo e

of reduced species in carbon containing

ispezione.

refractories by XRD.

FprEN – progetti di norma europei

Norme UNSIDER ritirate da UNI nel

ISO/DIS 22407

FprEN ISO 10113

mese di febbraio 2020

Norme UNSIDER pubblicate da CEN

products

—

Determination

ISO/FDIS – febbraio 2020

Metallic materials — Fatigue testing — Axial

Metallic materials - Sheet and strip -

plane bending method.

Determination of plastic strain ratio (ISO/ FDIS 10113:2019).

e ISO nel mese di febbraio 2020

ISO/DIS 19901-2

EN ISO 10113:2020

— Specific requirements for offshore

Metallic materials - Sheet and strip -

structures

Determination of plastic strain ratio (ISO

procedures and criteria.

Petroleum

and —

natural

Part

gas

industries

Seismic

ISO/FDIS

internazionali

progetti

norma

design ISO/FDIS 29001 Petroleum,

10113:2020) ISO/DIS 15177

–

petrochemical

and

natural

gas industries — Sector-specific quality

EN ISO 439:2020

Steel sheet, hot-rolled twin-roll cast, of

management systems — Requirements for

Steel and cast irons - Determination of

commercial quality.

product and service supply organizations.

ISO/DIS 12135

ISO/FDIS 16573-1

silicon content - Gravimetric method (ISO 439:2020)

Progetti UNSIDER messi allo studio dal CEN (Stage 10.99) – marzo 2020

La Metallurgia Italiana - March 2020

Metallic materials — Unified method of test

Steel — Measurement method for the

for the determination of quasistatic fracture

evaluation

toughness.

resistance of high strength steels — Part 1:

hydrogen

embrittlement

Constant load test. ISO/DIS 630-1

pagina 65

Atti e notizie - AIM news ISO/FDIS 16134 Earthquake-resistant

and

subsidence-

resistant design of ductile iron pipelines.

products for use as casing, tubing, coupling

ISO/FDIS 4947

stock and accessory material — Technical

Steel and cast iron — Determination of

delivery conditions.

vanadium

content

—

Potentiometric

titration method. ISO/FDIS 13680

ISO/FDIS 7369

Petroleum and natural gas industries —

Pipework

Corrosion-resistant alloy seamless tubular

assemblies — Vocabulary.

—

Metal

hoses

and

hose

AIM FORMA E INFORMA ANCHE A DISTANZA Programma FaReTra Fair Remote Training

L’attività di formazione ed aggiornamento di AIM non si fer-

Si può scegliere di partecipare ai singoli moduli o di seguire

I nostri Comitati Tecnici e la Segreteria lavorano a pieno rit-

I moduli saranno attivati con un minimo di 10 partecipanti

ma.

mo per dare una pronta e concreta risposta alla richiesta di

formazione a distanza delle aziende associate, che desiderano sfruttare questo periodo per incrementare le conoscenze e le competenze dei propri tecnici.

L’attività di AIM non si ferma, ma si trasforma e rimodula, of-

frendo nuovi strumenti alle proprie associate e all’industria

di riferimento, per cogliere nella situazione di forte difficoltà

che stiamo vivendo un’opportunità di crescita e innovazione. L’offerta formativa a distanza di AIM è suddivisa per macro argomenti e programmata in moduli di massimo 120 minuti

l’uno (comprensivi di Q&A e test finale di apprendimento). Ai partecipanti, che abbiano superato il test finale, verrà rila-

sciato un attestato di partecipazione in formato pdf a mezzo email.

La Metallurgia Italiana - marzo 2020

più moduli a un prezzo più vantaggioso.

ed un massimo di 50, per consentire la migliore interazione possibile tra docente e discenti.

Istruzioni dettagliate su come collegarsi e partecipare verranno inviate a mezzo email direttamente all’iscritto non ap-

pena completata la registrazione. Il nostro staff è a completa disposizione dei partecipanti per informazioni ed assistenza.

Il catalogo, che sarà presto disponile sul nostro sito www.

aimnet.it, è in costante aggiornamento, perché andrà arricchendosi via via di nuovi temi e moduli.

Vi invitiamo a monitorare la nostra proposta sul sito e sui nostri canali social.

Siamo poi a disposizione per accogliere vostri suggerimenti e richieste!

pagina 66

Atti e notizie - AIM news

Comitati tecnici / Study groups CT METALLI LEGGERI (ML) (riunione del 18 febbraio 2020)

Iniziative future • La GdS “progettazione e materiali lightweight” potrebbe essere organizzata a ottobre/novembre 2020 a Milano, incentrata sul confronto tra vari materiali leggeri, compresi i compositi, e non solo sull’alluminio. Il coordinatore Vedani ha preparato una possibile scaletta con 8 argomenti, e di conseguenza propone anche i possibili relatori per ogni tematica. Si deve definire se è opportuno coinvolgere altri enti/associazioni che potrebbero essere interessati. • GdS “Anime da fonderia”: Amalberto (coordinatore insieme a Spaccasassi) precisa che l’obiettivo è realizzare una giornata di studio su problematiche e novità nella produzione di leganti organici e inorganici per leghe di alluminio e magnesio. Al momento non è ancora disponibile una scaletta degli interventi, mentre ci sono contatti con una azienda privata che potrebbe ospitare la manifestazione e permettere una visita allo stabilimento o comunque mostrare le metodologie di produzione delle anime. La GdS potrebbe svolgersi a novembre/dicembre 2020. Stato dell’arte e notizie • Il segretario si è dimesso dal Comitato. Al suo posto, come segretario, viene nominata Elisa Fracchia, che accetta l’incarico.

CT METALLI E TECNOLOGIE APPLICATIVE (MTA) (riunione del 18 febbraio 2020)

Iniziative future • GdS “Trattamenti di rimozione del piombo dagli ottoni da riciclo”: il coordinatore Loconsolo ha preparato una scaletta provvisoria degli interventi, che spaziano dall’inquadramento normativo ai materiali alternativi, dall’importanza del riciclo ai processi di eliminazione del piombo. La giornata potrebbe tenersi a fine ottobre o inizio novembre 2020. • GdS sulla saldatura: la scaletta dovrebbe comprendere degli interventi sulla metallurgia generale delle leghe in esame, sulla corrosione e poi sulla saldatura, sia dal punto di vista delle tecniche che della metallurgia. Si pensa di arrivare a sette presentazioni. La data e il programma saranno fissati nella prossima riunione, così come la locandina di presentazione. Stato dell’arte e notizie • Nella prossima seduta si discuterà del ricambio delle cariche nel CT MTA.

La Metallurgia Italiana - March 2020

pagina 67

convegno nazionale

trattamenti termici

27th AIM National Conference & Exhibition on Heat Treatment

www.aimnet.it/tt.htm AIM è lieta di annunciare la 27° edizione del Convegno Nazionale Trattamenti Termici, il più autorevole ed affermato evento sui trattamenti termici a livello nazionale. Il Convegno-Mostra si svolgerà nei giorni 18 e 19 novembre 2020 a Genova, dove si tenne la prima edizione nel 1960. Sede dell’evento saranno i Magazzini del Cotone, nel bellissimo contesto del porto antico di Genova. Sul sito dell’evento sono disponibili tutte le proposte per le aziende che desiderano far parte dello spazio espositivo: www.aimnet.it/tt.htm

Segreteria organizzativa

Via F. Turati 8 - Milano (Italy) Tel. +39 02 76021132 info@aimnet.it www.aimnet.it

Sponsor dell’evento