Effect of Temperature and Strain Rate on Dynamic Re-Crystallization of 0.05C- 1.52Cu-1.51Mn Steel

Mechanics, Materials Science & Engineering, September 2016

ISSN 2412-5954

Effect of Temperature and Strain Rate on Dynamic Re-Crystallization of 0.05C1.52Cu-1.51Mn Steel Pawan Kumar1, a, Peter Hodgson1, b 1

Institute For Frontier Materials, Deakin University, Australia

a

pkumar@deakin.edu.au

b

peter.hodgson1@deakin.edu.au DOI 10.13140/RG.2.1.4905.2403

Keywords: dynamic re-crystallization, strain rate, temperature

ABSTRACT. Dynamic re-crystallization (DRX) is one of the most efficient methods to achieve ultra-fine ferrite grain in the steel. The DRX associated with the formation of new grains in hot working condition. The factors influencing the grain size achievable through thermo-mechanical controlled processing are known to be work hardening and softening by dynamic process of recovery. The point at which the combine effect of strain hardening and recovery are unable to accommodate more immobile dislocation is the starting point of DRX process. In present investigation, critical stress for initiation of DRX is calculated for 0.05C-1.52Cu-1.51Mn steel and the influence of strain rate and temperature is studied.

It was observed that at lower strain rate, critical stress for initiation of Dynamic re-crystallization (DRX) is increases initially and then it become saturated at higher strain rate. It is also absorbed that higher temperature and lower strain rates are the favourable condition for typical DRX process. It is also hinted that Cu precipitation take place process adopted in the experiments.

Introduction. Dynamic re-crystallization (DRX) is one of the most efficient method to achieve ultrafine ferrite grain in the steel [1-2]. The DRX associated with the formation of new grains (in hot working condition); the size of grain is expressed as:

where A is a constant; G is the shear modulus; n is the grain size exponent, which is about 0.7 for hot working conditions [3-5].

The factors influencing the grain size achievable through thermo-mechanical controlled processing are known to be work hardening and softening by dynamic process of recovery [6]. The three mechanisms with strain hardening, dynamic recovery and dynamic re-crystallization are different in their softening mechanisms.

When the combine effect of strain hardening and recovery are unable to accommodate more immobile dislocation is the starting point of DRX process. Low stacking-fault energy materials generally exhibit discontinuous DRX. The mechanism corresponding to DDRX is bulging (local migration).

Bulging of grain boundaries generate nuclei which further grows and consumes at deformed matrix; leading to increase in the dislocation density. The morphology governing by DDRX shows nearly constant average grains size, which is due to the further deforming of large grains due to further straining and taken up by new DRX nuclei. This process considered as a Discontinuous process [7, 8-9].

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High stacking-fault energy materials generally exhibit Continuous DRX [10-13]. In this phenomenon the formation of three-dimensional arrays of deformation low-angle boundaries (LABs) takes place, which is further transformed into high-angle grain boundaries (HABs). The high orientation gradient and the strain incompatibility between joint grains evolves the strain induced LABs. Upon further straining their mis-orientation increases leading to transformation into HABs; this leads to the development of recrystallized grains. The CDRX phenomena generally exhibits an equi-axed morphology throughout the structure.

The Cu is use to provide precipitation hardening in steel. Setuo Takaki et. al has studied the effect of pre-strain with Cu addition on 0.007C-0.01Mn-1.5Cu steel aged at 300C at 20 mins [14]. There is no Cu clusters/precipitates observed in non-prestrained steel; although existence of Cu clusters of size around 0.7nm are reported in prestrained steel, it has shown any change in distribution upon ageing as 500 oC for 20 mins. It is observed that Cu clusters tend to distribute coarsely in non-pre-strained steel. It is also observed that at peak age condition; clusters of copper tend to grow homogeneously in pre-strained samples. However it found that in non-pre-strained samples; a coarsening behavior is observed. The mechanism of grain refinement in steel by Cu precipitation is not known till now. It is proposed by some workers that precipitate dislocation interaction tends to create deformation bands during straining and this leads to fine re-crystallized grains [14]. Setuo Takaki et. al. also reported strengthening of heavily deformed and re-crystallized ferrite due to precipitates of copper [14].

In the present investigation the effect of temperature and strain rate is studied for the flow behaviour of material under investigation. The critical stress and strain is also calculated for the initiation of dynamic re-crystallization process. Also the influence of temperature and strain rate on the critical stress and strain for DRX is investigated.

Materials and Methods.

The material under investigation is 0.05C-1.52Cu-1.51Mn steel. Thermo-mechanical simulator (Gleeble) was used for hot compression test in plain strain condition. The specimens were austenitized at 1100oC for 5 min and cooled at the rate of 5oC/Sec; it is then subjected to hot compression as shown in Fig. 1. Single hot compression tests were conducted at temperature 800-1000oC with strain rates of 0.01, 0.1, 1 s-1.

Fig 1. Thermo-Mechanical process used in experiments.

Result and Discussion. From Fig. 2, It is observed that DRX taken place at strain rate of 0.01/Sec at different temperature up to 800 0C. Effect of temperature and strain rate on DRX of experimental steel can also be observed from fig. 3, fig. 4 and fig. 5. When the deformation temperature is comparatively low , DRX seemingly take place only at a slower strain rate of 0.01/Sec; for higher MMSE Journal. Open Access www.mmse.xyz

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strain rates however offend indication of dynamic recovery is noticed is noticed for higher strain rate of 0.1/sec. Increasing the strain rate at low deformation temperature as restricted DRX is observed from Fig. 3. Upon increasing deformation temperature to 950 0C the dynamic re-crystallization is recorded at low strain rates till 0.1/Sec; whereas at higher strain rates of 1/Sec occurrence of dynamic recovery is indicated as shown in Fig. 4.

As expected higher deformation temperature like 1000 0C envisages the occurrence of DRX at all strain rates which 0.01/Sec, 0.1/Sec and 1/Sec. It therefore follows from the above diagram that dynamic re-crystallization of the experimental steel is favored at higher temperature and lower strain rate. The combination of deformation temperature and strain rate is essentially an important aspect in deciding dynamic re-crystallization is set in or not. It is known that DRX is thermally activated from therefore it is accentuated by higher deformation temperature and higher availability of time at deformation temperature. It is obvious that the slower strain rates provides longer time for DRX phenomena to take place and hence above observations are made in present investigation.

-

or Initiation of DRX: -

as: The inflection point is detected by fitting 3rd degree polynomial to - curve (1) At critical stress for initiation of DRX the second derivative becomes zero; so

(2)

Becomes zero, therefore,

(critical) = B/3A

(3)

Following the same argument the critical stress for DRX as well as critical strain for the same has been calculated for all cases where DRX could be observed. It appears from Fig. 7 that the critical stress decreases with increase in deformation temperature. Fig. 8 exhibits that critical strain for occurrence of DRX at a constant strain rate of 0.01/Sec decreases with deformation temperature tending to assume some constant value at higher deformation temperature. Fig. 9 shows that effect of strain rate on critical stress for occurrence of DRX at fixed highest deformation temperature 10000C; rise in the magnitude of critical stress for DRX with increasing strain rate is logically consistent with the fact that higher strain rate provides less time for DRX to take place at any specific deformation temperature.

In fig. 10 transmission electron micrographs of steel deformed at strain rates of 0.01/Sec at 900 0C shows that precipitation of Cu has taken place concurrently with DRX or just after DRX and during austenite to ferrite transformation. In the first case the precipitates would have sited at the grain boundaries while in the second case the precipitate impend transformation growth of DRX grains although conclusive evidence has not been derived in the present investigation. The either of the above two events could lead to achievement of fine grained ferrite from austenite this is why SEM image by Fig. 10 shows that ferrite grain size of 2deformed at 900 0C. MMSE Journal. Open Access www.mmse.xyz

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180 160 140

stress(Mpa)

120 100 80 (950C-0.01/sec)

60

(1000C-0.01/sec)

40

(850C-0.01/sec) 20

(800C-0.01/sec)

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

strain

Fig. 2. Flow curve at temperature 800 oC-1000 oC and strain rate of 0.01/sec.

850C-strain rate 0.01/sec 850C-strain rate 0.1/sec

300

850C-strain rate 1/sec

stress(Mpa)

250 200 150 100 50 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

strain

Fig. 3. Flow currve at temperature 850 oC and strain rate of 0.01, 0.1 and 1/sec.

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250

stress(Mpa)

200

150

100 950C-strain rate 0.01/sec 50

950C-strain rate 0.1/sec 950C-strain rate 1/sec

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

strain

Fig. 4. Flow curve at temperature 950 oC and strain rate of 0.01, 0.1 and 1/sec.

180 160 140

stress(Mpa)

120 100 80 60

(1000C- strain rate 0.01/sec)

40

1000c-strain rate 0.1/sec

20 1000C-strain rate 1/sec 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

strain

Fig. 5. Flow curve at temperature 1000 oC and strain rate of 0.01, 0.1 and 1/sec.

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300

work hardening rate

250 200 150 100 50 0 100

110

120

130

140

150

stress (Mpa)

Fig. 6. Work hardening rate Vs True stress.

critical strain for DRX

0.35 0.3 0.25 0.2 0.15 0.1 800

850

900

950

1000

1050

1100

Temperature in C

Fig. 7. Critical strain for DRX Vs Temperature.

140

critical stress for DRX

130 120 110 100 90 80 70 60 50 40 800

900 Temperature in C

Fig. 8. critical stress for DRX Vs temperature.

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1000

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160

critical stress for DRX

140 120

100 80 60 40 20 0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

strain rate

Fig. 9. critical stress for DRX Vs strain rate at 1000 0C.

Fig. 10. TEM micrograph at temperature 900 oC and strain rate of 0.01/sec.

Summary.

1. High deformation temperature and low strain rate is the favorable condition for dynamic recrystallization for the material under investigation which is 0.05C-1.52Cu-1.51Mn steel.

2. The critical stress for dynamic re-crystallization decreases with increase in deformation temperature. The critical strain for occurrence of DRX at a constant strain rate of 0.01/Sec decreases with deformation temperature tending to assume some constant value at higher deformation temperature.

3. Precipitation of Cu has taken place concurrently with DRX or just after DRX and during austenite to ferrite transformation. The ferrite grain size of 2in the experimentation.

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[13] Setuo Takaki, Masaaki Fujioka, Shuji Aihara, Yasunobu Nagataki, Takako Yamashita,Naoyuki Sano, Yoshitaka Adachi, Masahiro Nomura and Hiroshi Yaguchi, Effect of Copper on Tensile Properties and Grain-Refinement of Steel and its Relation to Precipitation Behavior, Materials Transactions, Vol. 45, No. 7 (2004) pp. 2239- 2244, doi: 10.2320/matertrans.45.223 Cite the paper Kumar, P., & Hodgson, P. (2016). Effect of Temperature and Strain Rate on Dynamic ReCrystallization of 0.05C-1.52Cu-1.51Mn Steel. Mechanics, Materials Science & Engineering, Vol 6. doi:10.13140/RG.2.1.4905.2403

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