METALLURGY
EAF TECHNOLOGY
Electrical Steelmaking in place of Basic Oxygen Process – Danieli Vision from Quality Point of View Integrated iron and steelmaking production represents the globally predominant technology. Global requirements for CO2 reduction will influence needed changes in iron and steelmaking philosophy using new technologies that generate less CO2. Based on forecasts, hydrogen application in primary steelmaking and scrap melting process will have higher shares in parts of the world where the integrated iron and steelmaking process is considered as traditional. A transition from traditional and well-established processes to the process based on DRI / HBI production, together with scrap melting in electric arc furnaces, must be followed by the means for producing high quality grades with low residual element contents. Danieli presents a way to keep capability for producing high-quality grades with low residual element contents using different charge materials. AUTHORS: Bojan Vucinic, Franco Gan-
din, Danieli & C. Officine Meccaniche S.p.A CONTACT: Bojan Vucinic, Senior Tech-
Team Manager, phone (0039) 04321955519, b.vucinic@danieli.it
A
pproximately 30% of the world’s crude steel production is carried out via electrical arc furnace (EAF) as the primary unit for steelmaking. The ratio between the two main primary steelmaking units, basic oxygen furnaces (BOF) versus electric arc furnaces, has changed in recent years and this trend seems to be confirmed for upcoming years due to changed conditions in raw material price and production costs, and due to strict requirements for pollution control. Making high-quality steel always has been the domain of integrated mills organized by the blast furnace-converter-secondary metallurgy-casting route. Now, numerous electric steelmaking operations around the world have successfully entered this product market. The main reason for this progress is the high flexibility in operation and raw materials options available to EAF steelmakers. Table 1 shows allowable content of residual elements for some steel grades(1) which are mainly produced via BOF steelmaking process and must be pro-
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Element, %
IF steel
DDQ
Drawing Commercial Structural Fine wire
Rebar
Cu
0,03 0,04 0,06
Ni
0,03 0,04 0,06 0,08 0,1 0,07 0,35
Mo
0,008
0,1 0,12
0,07
0,4
0,01 0,015 0,015 0,04 0,015 0,08
Cr
0,03 0,04 0,06 0,08 0,1 0,08 0,15
Sn
0,008 0,015 0,015 0,02 0,03 0,015 0,08
Table 1. Typical allowed residual elements content for most flat steel grades
duced via electric arc furnaces as well. Nitrogen absorption during melting can be reduced by the application of controlled slag foaming, controlled carbon removal rate during process, carbon boiling control, proper EAF design, and controlled raw material application. The installation of vacuum degassers (RH or VTD) in the EAF steelmaking route improves the quality of steel products with ultra-low carbon and low-gas contents. Sulphur contents of lower than 10 ppm for certain steel grades can be produced in vacuum tank degassers (VTD.)
Integrated plant – Converter melt shops The first step in the oxygen blow of the BOF process is silicon oxidation. The results show that no nitrogen is removed during the initial phase of the blow, when silicon oxidation is the primary reaction. The rate of nitrogen removal jumps as soon as carbon oxidation is increased. As the dissolved carbon content decreases,
the decarburization rate, and therefore the CO evolution and nitrogen removal rates, also decrease. At the end of the oxygen blow process nitrogen present in the oxygen jet dissolves in the liquid bath, when the nitrogen removal efficiency of the bath is reduced due to lower CO generation rate(2). Usually, nitrogen contents before tapping from the BOF are in the range of 15 to 40 ppm, including reblown heats. Figure 1 shows the typical nitrogen content before tapping from BOF (1.a.), and sulphur content after sulphur removal from hot metal and before tapping from BOF (1.b.) Thanks to low nitrogen and sulphur content before tapping from the BOF, sulphur removal during secondary metallurgy and nitrogen removal during the vacuum process usually are not required for BOF based melt shops. The charge mix for the electric arc furnace process is different than for the BOF process so nitrogen content before tapping from
November 2020 / MPT International
