Basis of development of new technology

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 at low temperatures in preference to Cr , a reduction in pressure of CO from 1 atmosphere to lower value would be required. Reduction in pressure of CO can be achieved either by vacuum or by using a mixture of . The former one is vacuum oxygen decarburization (VOD) and the later is (AOD)

AOD Process

The process in carried out in a converter type of vessel. Vessel is lined with magnesite brick.  A mixture of argon +oxygen is injected through the tuyeres located on the side of the converter shell.

Fe-Cr- Ni-C alloy melt is prepared in EAF. Melt is charged in AOD vessel. High carbon-ferrochrome is charged. In the initial stage a mixture of : Ar in 3:1 ratio is blown through the side tuyeres. When carbon reduces to 30% of the original value the ratio of : Ar is changed to 2: 1 and blow is continued to attain 0.09 to 0.12% C. First stage of blow generates sufficient amount of heat due to oxidation of Cr and hence coolants are added (5% of the change). Stainless steel scrap is used.

In the final stage, the ratio of : 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 to 2 by adding lime.

,            

Slag formation and slag metal reactions are facilitated by argon stirring of the bath. The bath is desulphurized to levels well below 0.015%.   

Reference:  

• Chakrabarti: steel making
• A.Ghosh and A. Chatterjee: Ironmaling and steelmaking
• S.K.Dutta and A.B.Lele: Trans. IIM, vol. 56(1), Feb. 2003, pp 19-22