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Stainless steel production by IF-AOD / MRK - LRF route

Dr S K Dutta and A B Lele, Metalluical Engg. Deptt, Faculty of Tech. & Engg, M S University of Baroda

28 Dec' 2004

Stainless steel displays unique engineering properties in terms of strength, toughness and overall corrosion resistance over a wide range of temperatures along with good forming and welding characteristics. Against growing competition from alternative materials like aluminums and plastics, stainless steel remains the only choice where corrosion resistance properties are required. Among all steel categories, stainless is the most growth intensive. Although tonnage wise, stainless steels are a very small member of the steel family, accounting for barely 2 to 3 per cent of total steel output worldwide. The total quantity of world crude stainless steel output jumped from about 1.0 million tonne (mt) in 1950 to 21.6 mt in the year 2000, registering and impressive annual growth rate of over 6 per cent The stainless steel sector in India has experienced a quantum jump in the last two decades. Production during 1978 was only 15,000 tonnes, which reached level of around 1.2 mt during 2002-2003 representing a share of 5.6 per cent out of the total world stainless steel production of 21.5 mt [2]. There is a tremendous potential to increase the consumption of stainless steel in the sectors such as construction, transport, processing industries etc. The Present per capita consumption in India is lowest in the world, around 0.66 Kg as compared to 10 to 18 Kg in developed countries [4]. The expanding processing sectors like chemical, petro-chemical, oil and gas, food processing, transport, etc. require a large amount of stainless steel. The extent of stainless steel demand in India is projected 1.69 mt for 2006 - 2007. Stainless steel making technologies have been exploited to enhance overall efficiency, tonnage capability, flexibility of input materials, reduction in processing cost, and improvement in product quality. New technologies are available and also under development for meeting over increasing demands of stainless steel making, so as to achieve the following goals: Saving on the material cost, Reducing the energy consumption, Increasing the productivity, Improving the product quality, Reducing environmental emissions, Introducing automation, computer aided quality control and production planning, Reducing the investment cost. Two types of processes, developed for stainless steel making are as follows: Duplex process (EAF / IF - AOD / VOD) Triplex process (EAF / IF - AOD / MRK - LRF) In duplex process, after melting in primary unit, refining is carried out in different vessel. Duplex process have certain disadvantages with respect to the ratio of ferro-alloys to scrap in the case of VOD and higher argon, ferro-silicon consumption and shorter convertor life in case of AOD, lengthy process time in secondary vessel convertor. However, in order to overcome these demerits and produce different grades with grater economy and better quality, triplex process is used. In triplex process, after melting in primary unit, refining are carried out in two different vessels. In the first vessel, decarburisation and major refining take place and final stages of desulphurisation and degassing take place in second vessel. Compare to duplex, triplex process has following merits: Increase productivity Increase number of heats per day Increase scrap to liquid metal yield Improved quality of metal High operational flexibility Comparatively lower cost of production. Thermodynamics and reaction kinetics of stainless steel making process Conditions in the production of stainless steels are largely governed by thermodynamics equilibrium of oxygen reactions with carbon and chromium at different temperatures and partial pressure of CO. With rising temperatures and decreasing CO partial pressures, equilibrium conditions can therefore shifted to lower carbon levels at the same amount of chromium. The decarburisation of stainless steel involves the techniques to minimize chromium oxidation. There are three basic techniques: increasing the temperature, decreasing of CO-partial pressure by dilution with inert gas and using the vacuum. In the past the temperature technique was used for production of stainless steel in arc furnace, but it was abandoned due to uneconomical nature of production . The dilution technique is used by AOD / MRK process and all converter processes. The injection of inert gas (argon or nitrogen) lowers the partial pressure of CO in the bath, thus allowing higher chromium contents to be in equilibrium with low contents of carbon. Applying vacuum to metal bath also remove CO, allowing high chromium contents to be in equilibrium with low carbon contents. It is especially effective when carbon content is low. Classification of stainless steels Stainless steels can be divided into five families. Four are based on the characteristic crystallographic structure/microstructure of the alloys in the family: ferritic, martensitic, austenitic or duplex (austenitic plus ferritic). The fifth family, the precipitation - hardenable alloys, is based on the type of heat treatment used, rather than microstructure. IF - AOD I MRK - LRF Route Induction Furnace (IF): Technology of melting of metals and alloys by electric induction system is fairly old in the world. In India also many steel plants and ordnance factories to manufacture high alloy and tool steel installed high frequency induction melting furnaces. The requirement of stainless steel in country was fairly high. But stainless steel production was not good. Induction melting furnaces started making stainless steels since early eighties by using imported stainless steel scrap. At present stainless steel is produced by melting stainless steel scrap, mild steel scrap and ferro alloys in induction furnace; and transferring the liquid metal to AOD / MRK vessel for refining and purging by inert gas to the liquid metal. After AOD / MRK treatment, further refining is done in LRF and then produce billet by con cast process. In India stainless steel produced, in induction furnace route, is 277,000 tonnes in 2002-2003. Now a day's contribution by the secondary producer for production of crude steel is drastically increased (37.6 per cent in 2002-2003). Out of those induction furnace units contributes (15.6 per cent). The cost of melting the charge in induction furnace is lower than that in arc furnace due to the following reasons: Reasonable power consumption due to faster melting, lower tapping temperature and higher power density. Fe-Cr melting is difficult in an arc furnace due to the absence of stirring. Also, lime and fluxes added to protect the lining take away substantial amount of heat. Lower tapping temperature of 15300C. In arc n furnace, chromium comes in contact with atmospheric oxygen and gets oxidized. To recover this Cr, at the end of melting, metal is super heated above 15300C and reductant like carbon or Fe-Si is added. No use of graphite electrodes in induction furnace. No flux / lime requirement for melting in induction furnace. Lower refractory cost as acidic ramming mass is much cheaper when compared to basic ramming mass and bricks. 4 per cent higher Cr recovery compared to that of arc furnace as negligible amount of Cr is oxidized in induction furnace. Reduced pollution due to fumes, dust, slag. Gases formed and sucked in the arc furnace cause greater pollution. Argon Oxygen Decarburisation (AOD) / Metal Refining Konvertor (MRK): After melting the scrap in induction furnace the liquid metal of temperature 1600-1650oC is tapped and then subsequently charged in to the AOD / MRK. To carry out decarburisation and other reactions either oxygen or mixture of oxygen-nitrogen / argon is blown through the bottom tuyer of the converter. Excellent gas mixing promotes fast decarburisation, deoxidation and desulphurisation and improves recoveries of alloying elements. The steel refined by AOD / MRK process is low in oxygen, nitrogen and argon. AOD / MRK process ensures low content of suspended oxides, high level of purity in stainless and other steels. Ladle Refining Furnace (LRF): Processed metal, as above, is then transferred to ladle refining furnace for desulphurisation of melt. Gentle stirring with nitrogen / argon in the LRF removes inclusions and thus improves the quality. LRF can be operated with single graphite electrode working on dc power supply. Single electrode reduces the refractory erosion and carbon pick up. Treatment of AOD / MRK heats in LRF reduces cost and improves quality of the melt. Table 6 shows the progress of chemistry changes during 200 series stainless steel production in LF-MRK-LRF route [11]. Remarks With the high projected demand of stainless steel, numbers of expansion plans from the existing producers have come to force. At present, in spite of some of the demerits, duplex processes are used for production of stainless steel. However, triplex processes can be considered to be the most advanced process route for high quality stainless steelmaking.