16 Stainless Steelmaking Topics to discuss... Introduction Thermodynamics of decarburization of chromium melt Technology of stainless steel making
Introduction Stainless steels contain typically 10-30 % chromium besides other elements like C, Mn, Si, S etc. Chromium imparts corrosion resistance to steel. Varying amounts of other alloying elements like Ni, Mo, V, Ti, Ni, etc may be added to obtain certain specific property. It may be noted that all stainless steels contain chromium and carbon besides other elements. Production of stainless steels requires controlling chromium and carbon. Types of stainless steels: Austenitic stainless steels: Cr = 18%, Ni = 8%, C = 0.03-0.15% Ferritic stainless steels: Cr = 12-30%, C = 0.08-0.12% Martensitic stainless steels: Cr = 13%, C = 0.15-0.25% Duplex stainless steels: Cr ~ 25% Precipitation hardenable stainless steel: Cr = 18-20%,Ni = 8-10 % Ni and Cu, Ti, Al 3/17 Thermodynamics of Decarburization of Chromium Melt In stainless steel making, both chromium and carbon oxidizes when decarburization of melt is done. so the central problem is to achieve enough oxidation to bring the carbon content down to 0.03% without excessive loss of chromium The Ellingham diagram for oxide formation indicates that carbon oxidation in preference to chromium oxidation can occur at temperatures greater than 1220 C, when both elements are in pure state. Under all practical conditions, however, carbon oxidation can occur at temperatures above 1800 C in presence of chromium. 4/17
2C + O 2 = 2CO 2Cr + 1.5 O 2 = Cr 2 O 3 or 3Cr + 2O 2 = Cr 3 O 4 Equilibrium distribution between Cr and C is represented by considering Cr 3 O 4 (Cr 3 O 4 ) + 4[C] = 3[C]r + 4{CO} Assuming pure Cr 3 O 4 and using Henry s law for carbon and chromium, it follows K = [Cr] 3 f Cr 3 [Cr] 3 p CO 4 f C 4 [C] 4 [C] 4 = f 3 Cr p 4 CO K x f C f C and f Cr are activity coefficient of C and Cr in liquid iron at 1wt % standard state. 4 Hilly and Kaveney proposed the following equation for distribution of Cr and C: log [Cr] [C] The effect of Ni log [Cr] [C] = 13800 + 8.76 0.925 log p T CO = 13800 T + 4.21[Ni] + 8.76 0.925 log p CO The above two equations describe distribution ratio of Cr and C in Fe-Ni-Cr-C melt. 5/17 lowering of p co allows the same to be achieved at a much lower temperature very high temp. needed if C is decreased <0.04% with high Cr content Chromium-carbon-temperature relationship in oxygen saturated steel melts (p CO = 1 atm) Influence of pressure and temperature on the retention of chromium by oxygen saturated steel melts at 0.05% C 6/17
Table showing the influence of temperature and p CO on the distribution ratio R = [Cr]/[C] without Ni and R 1 = [Cr]/[C] in the presence of 10% Ni. Table: chromium / carbon distribution ratio p CO = 1 atm p CO = 0.25 atm p CO = 0.10 atm T (K) R R 1 R R 1 R R 1 1873 25 36 89 129 207 301 1973 55 82 209 295 488 690 2073 128 173 460 619 1077 1477 2173 240 339 863 1220 2019 2852 We can infer the following form the table: a) Increasing temperature increases R and R 1 at all p CO. High temperature is required to suppress Cr oxidation in favor of C oxidation b) Decreasing p CO increases R and R 1 at all temperatures c) Nickel increases R and R 1 7/17 The above observations suggest that high temperature is needed to remove carbon in presence of chromium, if stainless steel is produced at atmospheric pressure. If reduced pressures are used, lower temperature can cause oxidation of carbon. 8/17
For the reduction of chromium oxide from slag by ferrosilicon during the last stages of VOD/AOD operation, Hilty et al. assumed the following reaction (Cr 2 O 3 ) + 2[Si] = 3[Cr] + 2(SiO 2 ) ; K = h Cr 3 a SiO2 2 h Si 2 a Cr2 O 3 Noting that activities of SiO 2 and Cr 2 O 3 in slag are functions of slag basicity, equilibrium relation can be arrived at from the equation for K. However, for practical purposes, statistically fitted empirical coefficients are recommended, such as: log [Cr] slag = 1.283 log [Cr] 0.748 log [Si] 1.709 log V 0.923 where V = slag basicity = (CaO + MgO)/SiO 2 9/17 Technology of Stainless Steelmaking Stainless steel can be produced by two routes: 1) electric arc furnace (EAF) route 2) electric arc furnace plus argon oxygen decarburiser (EAF + AOD) route, commonly known as the duplex process. There is another process, called triplex refining, where electric arc furnace melting and converter refining are followed by refining in a vacuum system, is often desirable when the final product requires very low carbon and nitrogen levels. This third step makes the cycle time even longer and adds to cost. 10/17
EAF Route Electric arc furnace was used to produce stainless steel by melting scrap of the desired composition. EAF was used only as a melting unit. Typically, charge consists of stainless steel scrap carbon steel scrap reduction slag mix (FeSi - FeCrSi - CaO - Al) containing low carbon low carbon FeMn and FeCr Ferrochrome, containing ~55-70% Cr is the principal source of chromium. Classification of FeCr based primarily on their carbon content, such as: Low carbon ferrochrome (about 0.1% C) most expensive Intermediate carbon ferrochrome (about 2% C) High carbon ferrochrome (around 7% C) least expensive 11/17 The scrap is melted in EAF and after melt-down period, the melt contains around 10% Cr, all Nickel and carbon. Melt consists of Fe-Cr-Ni-C alloy. At low temperature, Cr is protected by ensuring sufficient Si in the melt. Oxygen is blown onto the melt and the temperature is raised quickly to about 2000 C. Initially chromium oxidizes until bath temperature rises to 1800 C. Carbon oxidation occurs at the expense of chromium once the bath temperature rises to 1800 C. The blow is discontinued when carbon level is below specification. In spite of high temperature, the slag contains about 50% Cr 2 O 3. To recover Cr from slag, the reduction slag mix is added [ 2Cr 2 O 3 + 3Si = 3SiO 2 + 2Cr ]. After 5-10 minutes reaction time, low carbon coolants (SS scrap) are added to bring the temperature down to about 1600 C. In the finishing stage, low carbon ferrochrome is added to make the chromium content of steel to a desired value. 12/17
The disadvantages with this technology are Slow process (tap-to-tap time = 5-6 hours) High temperature is required which cause damage to the refectory lining. Low carbon ferrochrome is required which in expensive Low Cr yeild (~87%) Thus a technology which can use high carbon ferro-chrome and to decarburize the bath at selectively lower temperatures would be required. If carbon is to be oxidized in preference to Cr at low temperatures, a reduction in low CO pressure would be required. This can be achieved either by vacuum or by diluting the oxygen gas by adding argon gas. The former one is vacuum oxygen decarburisation (VOD) and the later is argon oxygen decarburisation (AOD). 13/17 EAF + AOD Route The process in carried out in a converter type of vessel, lined with magnesite brick. A mixture of argon + oxygen gas is injected through the tuyeres located on the side of the converter shell. Fe-Cr- Ni-C alloy melt is prepared in EAF. The unrefined melt is charged in AOD vessel. High carbon-ferrochrome is charged. In the initial stage a mixture of O 2 : Ar in 3:1 ratio is blown through the side tuyeres. When carbon reduces to 30% of the original value the ratio of O 2 : Ar is changed to 2:1 (as Cr needs more protection). Blow continued until C reduced to 0.09-0.12%. 14/17
First stage of blow generates sufficient amount of heat due to oxidation of Si, C, and small quantities of Cr, Mn and Fe. Coolants (SS steels scrap, pure Ni, other alloys if needed) are added (5% of the change) at several points to avoid excessive damage of the refractory lining. In the final stage, the ratio of O 2 :Ar is changed 1:3 to bring C to the desired value. Fe- Si is added to recover Cr from slag and slag basicity is maintained at 1.5-2 by adding a reduction slag mix (CaO+FeSi). Slag formation and slag metal reactions are facilitated by argon stirring of the bath. The bath is desulphurized to levels well below 0.015% in a few minutes by preparing a new slag (CaO+FeSi+CaF 2 ). 15/17 AOD vessel Illustration of duplex and triplex methods of stainless steelmaking 16/17
Advantage of EAF+AOD Process Process yield for Cr is ~93% (EAF+AOD), ~97-99% in AOD vessel. High carbon ferroalloys can be used. Cycle time of AOD is roughly matches EAF (2-2.5 hrs). Thus, the use of AOD can roughly double the stainless steel production from one EAF. Mixing is violent during AOD process, producing cleaner steel with low dissolved gas. Very low C and S level can be obtained. 17/17