STAINLESS STEEL MAKING | DUPLEX PROCESS

Stainless Steel Making Technology


An introduction of rustless process of steelmaking, stainless steel scrap could not be reused for recovery of chromium values. This process founded the basis for reuse of stainless steel scrap without much difficulties.

Principle:

● The proportion of HCFeCr in the charge could be the extent of getting nearly 0.4%C, which is carburized by pure oxygen lancing without loss of chromium by way of oxidation. Therefore charge consists of stainless scrap, high carbon FeCr or charge chrome along with ordinary scrap may be melted in an arc furnace. The molten metal (free from slag) is transfered to the decarburising unit for further refining.

● The VOD technique can successfully decarburised the bath with transfer carbon around 0.7 to 0.8%. The AOD, CLU and MRP techniques can deal with transfer carbon as high as 2.0%. These are possible by adopting the scheme of duplex refining melting in electric arc furnace followed by decarburization and finishing by any one of the above processes.

● In the absence of stainless steel scrap even BOF processes have been used as primary processes along with this technique to produce stainless steel using charge chrome as a source of Cr. The modern technique of decrabudecarbur have decreased the FeSi consumption and additional cost of vacuum or argon is added in overall process.

Duplex Refining Processes 


Argon Oxygen Decarburisation (AOD)

This is carried out in a special AOD converter which is soild bottom vessel with tuyeres provided on the sides. The tuyeres are few in number depending upon the capacity.

Working Principle :

● The vessel is lined with basic magnetic refractory molten charge from arc furnace is transferred to this vessel and blown with argon oxygen mixture, the proportion of which varies from 1:3 in the beginning through one or two discharge stages to 3:1 for the final part of the blow.
AOD furnace - stainless steel making
AOD Vessel

● The temperature of the bath rises at the end of the first stage to releasing 1710℃ and coolants like nickel, stainless scrap, HCFeCr and other addition are added in predetermined proportion to finally arrive at the correct turn down temperature. The heat is finished by deoxidation through FeSi addition.

● It also reduces back whatever chromium is in slag at the end and get recovery of 97%. Total duration of AOD heat is around 2 hours and the lining life around 80 to 100 heats. It is also effective in obtaining good sulphur removal by using argon stirring at the end. The average consumption of argon is 18 to 20Nm3/t. A major change in blowing practice has been of argon and nitrogen in the first stage because many stainless steel have high nitrogen contain of 0.025%, so argon and nitrogen mixed later stage or in the end of blow.

Vacuum Oxygen Decarburisation (VOD)

Principle :

VOD Furnace - duplex refining process
VOD vessel

In this method metal treatment is a combination of vacuum treatment of metal and oxygen blowing. It allows obtaining a high degree of decarbursation because in vacuum oxygen treatment the equilibrium of the reaction
C + 1/2O2 = {O} shifts to right. This process is used for making stainless heat resistance and alloys steels by oxidation of C without the losses of Cr. Because in vacuum treatment the equilibrium of the reaction
Cr2O3 + 3C = 2Cr + 3(CO) . Shift towards right.

Since Pw decrease, so it is suitable for making low carbon steel without loss of Cr.

Construction :

The process has an ladle with special refractory lining. The bottom and walls are lined with chrome magnesite bricks. Life of wall lining is 40 to 60 heats and that of bottom roughly 15 heats. Bottom carries a porous plug for argon blowing which is replaced after cool heat.

Process :

● The metal with 0.3 to 0.5%C is poured in to the ladle and this is transported in to vacuum plant. Where C is oxidized by O2 fed through a water cooled Lance which is introduced through the chamber cover. In the time of O2 blowing, the metal is deoxidized in vacuum and it's composition is correct by addition of ferro alloys and argon stirring. Final steel contains not more than 0.01%C and very low N2 loss of Cr during C oxidation. Total yield of Cr is roughly 98%. After deoxidation the metal is desulphurise in vacuum by a highly basic slag with argon stirring. Total VOD process varies 2 to 2.5 hours. There is no external heat facility but the regeneration heat is available from excellent reaction which proceed very fast due to O2 Lance to arrive at correct tapping temperature.
Schematic diagram VOD - stainless steel making
Details of VOD

Advantage :

1. Better utilisation of furnace.
2. Practically whole Cr requirements is with respect to scrap and ferro alloys
3. Cheaper and easy of making extra low C steel.
4. High Cr yield 98% and superior quality of steel.

CLU Process :

● The charge as usual is melted in the electric arc furnace and then transferred to a Bessemer type vessel with bottom blowing facility. Refining is entirely carried out in this vessel which has chrome manganese lining. A mixture of oxygen, steam, argon, nitrogen and air is blown from bottom. The composition of the mixture are present for the entire blow depending upon the chemistry of the bath. The presence of steam controls the temperature around 1650 to 1680℃. What ever chromium is oxidised earlier is reduces back by the addition of FeSi and FeCr as in the usual practice.

● The applicability of this process using the idle Bessemer converter could be assessed to increase stainless steel production.





References :
1. Modern Steel Making :  Dr R.H. Tupkary and V.R. Tupkary.
2. Ironmaking and Steelmaking Theory an d Practice : A. Ghosh and A. Chatterjee
3. Steel Making : A.K.Chakravorty

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