Stainless Steel Production Processes
Stainless Steel Production Processes
Stainless steel represents quite an interesting material family because of its excellent qualities. Among the outstanding qualities which makes stainless steel ideally suited for several applications are resistance to corrosion, resistance to abrasion or erosion, good strength in a broad range of temperatures, non-contaminating, sanitary acceptability, cleanability, fabricability availability, cost effectiveness, and visual appeal. Because of these properties, stainless steels have been indispensable for technological progress during the last century and their annual consumption has increased faster than other materials. They find application in all fields needing materials with good corrosion resistance, together with the ability to be worked into complex geometries.
Stainless steels are used in a wide variety of applications. Most of the structural applications occur in the chemical and power engineering industries, which account for more than a third of the market for stainless steel products. These applications include an extremely diversified range of uses, including nuclear reactor vessels, heat exchangers, oil industry tubulars, components for chemical processing, and pulp and paper industries, furnace parts, and boilers used in fossil fuel electric power plants.
Stainless steel is iron-base alloy which contains a minimum of around 12 % chromium (Cr), the quantity needed to prevent the formation of rust in unpolluted atmosphere (hence the designation stainless). Few stainless steels contain more than 30 % chromium or less than 50 % iron (Fe). Stainless steel is made to contain no more than 0.12 % carbon (C). A large fraction of production of stainless steels is produced to a maximum carbon content of 0.07 % or 0.08 %.
The stainless steel gets its stainless characteristics through the formation of an invisible and adherent chromium-rich oxide film. This oxide film forms and heals itself in the presence of oxygen. Other elements which are added to improve particular characteristics include nickel (Ni), manganese (Mn), molybdenum (Mo), copper (Cu), titanium (Ti), silicon (Si), niobium (Nb), aluminum (Al), sulphur (S), and selenium (Se). Carbon is normally present in quantities ranging from less than 0.03 % to over 1 % in certain grades. Stainless steels are mainly classified as austenitic, ferritic, martensitic, duplex, or precipitation hardening grades.
With specific restrictions in certain types, the stainless steels can be shaped and fabricated in the conventional ways. They are produced in cast, powder metallurgy (P/M), and wrought forms. Available wrought product forms include plate, sheet, strip, foil, bar, wire and wire rod, semi-finished products (blooms, billets, and slabs), and pipe and tube. Cold rolled flat products (sheet, strip, and plate) account for more than 60 % of stainless-steel product forms.
Stainless steels were developed in the beginning of the 20th century. Their unique properties have led to their use in several different applications. Fundamental studies in the thermo-dynamics and kinetics led to understanding of the phenomenon associated with chromium oxidation and how to minimize it. The development of the argon oxygen decarburization (AOD) process revolutionized the production of stainless steel. This process, based on the dilution principle, allowed steelmakers to use lower cost raw materials and to shorten the steelmaking time compared to the other processes in use at that time. This led to lowering of the cost of producing stainless steels. This, in turn, led to dramatic increases in the applications for stainless steels. Over the years, other processes have been developed for making stainless steels. Majority of these processes use vessels or reactors in which gases are introduced to carry out the necessary decarburization reactions. Some of these processes for making stainless steels utilize decarburization under vacuum.
In the early stages of its production, stainless steel was melted using an electric arc furnace (EAF). In those early days, carbon steel scrap, iron ore, and calcined lime were charged into the EAF. After the scrap was molten, carbon was removed by adding ore until the carbon content reached 0.02 %. The electrodes were then raised and the slag removed as completely as possible. Desired quantities of ferro-silicon (Fe-Si), calcined lime, and fluorspar (CaF2) were added and the temperature of the bath was raised so that a large quantity of low carbon ferro-chromium (Fe-Cr) could be added for achieving the aim chromium level. The desired quantity of the low carbon ferro-chromium was added in two or three separate batches. The bath had to be mixed thoroughly by rabbling or reladling and slag had to be kept fluid by continuous additions of ferro-silicon, calcined lime, and fluorspar. After all the desired chromium was dissolved in the steel, a sample was taken for preliminary analysis. Final additions were made and the heat was brought to the desired tapping temperature and tapped.
In fact, majority of all stainless steels produced is processed by EAF melting followed by refining in an argon oxygen decarburization (AOD) converter. Presently, production of stainless steels is normally a two-stage process involving the melting of scrap and ferro-alloys in an EAF or induction furnace (IF) followed by refining in AOD converter for adjusting the carbon content and removing impurities. Alternatively melting and refining steps include vacuum induction melting, vacuum arc remelting, electroslag remelting, and electron beam melting. However, melting and refining of stainless steels is most frequently accomplished by the EAF / AOD processing route.
During the final stages of producing basic mill forms (sheet, strip, plate and bar) and bringing these forms to specific size and tolerances, the materials are subjected to hot reduction with or without subsequent cold rolling operations, annealing, and cleaning. Further steps are needed to produce other mill forms, such as wire and tube. Fig 1 shows the normally used production processes for making different wrought stainless-steel products.
Fig 1 Production processes for making different wrought stainless-steel products
Until around 1970, the majority of stainless steel was produced in the EAF. With the arrival of tonnage oxygen (O2) production, the electric furnace stainless steel melting practice changed from the above. Gaseous oxygen could be used to improve the rate of decarburization. This could be achieved by injecting oxygen gas into the liquid steel using a water-cooled lance. The faster oxidation of carbon with high oxygen potential was accompanied by the adverse reaction of extensive oxidation of chromium to the slag. This necessitated a well-defined reduction period in which ferro-silicon was used to reduce the oxidized chromium from the furnace slags.
In the late 1960s, a number of laboratory studies have been performed to understand the thermo-chemistry of the stainless steels. One of these studies has studied the carbon-chromium-temperature relationships. The experiments of the study have involved blowing oxygen onto the surface of the baths of liquid chromium alloys. The study has tried to perform the experiments under isothermal conditions but has found it difficult because of the exothermic nature of the oxidation reactions. The study has added argon (Ar) to oxygen in order to control the temperature. The study has found that with argon dilution it is possible to decarburize the liquid steel to even lower levels of carbon without excessive oxidation of chromium.
The observations of the above study have led to the initial experiments where argon-oxygen mixtures have been injected through a lance into the bath in the EAF. It has been found that argon injection in the wide and shallow bath of an EAF has not influenced the decarburization reaction completely as predicted. Hence, after several experiments, the developers have decided that a separate refining reactor is necessary to develop a commercial process. At Joslyn Steel (later Slater Steels), a 15-ton converter with three tuyeres was built. The first successful heat was made in October 1967. These successful trials led to patents for the AOD process for the refining of stainless steels and other specialty alloys by the industrial gases division of the Union Carbide Corporation ((now Praxair Inc.).
In the refining process of stainless steel, chromium is one of the main alloying elements, but it hinders the activity of carbon making it difficult to decarburize steel, especially in the low-carbon range chromium is oxidized in priority hampering decarburizing reactions.
The history of stainless-steel production technology is essentially that of the development of methods to effectively decarburize steel while preventing oxidation of chromium. The AOD and VOD (vacuum-oxygen decarburization) processes, their combination (the AOD-VOD process), and the combination of a converter and the VOD process (the converter-VOD process), are the results of these development efforts. The EAF-AOD and converter-VOD processes now account for the major part of stainless-steel production. Of these, the converter-VOD process was instrumental in the development of high purity ferritic stainless steels.
The AOD process is superior to the VOD in efficiency and in being capable of decarburizing from a high-carbon range, but its decarburization rate decreases and fluctuates considerably in the low carbon range, and the carbon content achieved is higher than that achieved by the VOD process. In addition, while oxygen is blown in during VOD, it is necessary to inject nitrogen (N2) or argon as diluting gas together with oxygen to decrease the partial pressure of carbon mono-oxide (CO) gas, but this is costly especially when the gas volume is high.
Process route for stainless steel production – There are several different process routes available for stainless steel melting and refining. The choice of the process route depends on several factors which determine capital costs as well as operating costs. The choice of process route is influenced by raw material availability, desired product, down-stream processing, existing shop logistics, and capital economics. Normally, some degree of flexibility in process route is desirable, since these factors can change rapidly. In general, stainless steelmaking process flow can be classified as duplex or triplex routes. The comparison of these two process routes is shown in Fig 2.
Fig 2 Comparison of duplex and triplex process routes for stainless steelmaking
In duplex process route, melting in EAF/IF is followed by refining in a converter. The duplex process tends to be flexible with respect to raw material selection. In triplex process, melting in EAF/IF and refining in converter is followed by refining with a vacuum degassing system. The triplex process is frequently desirable when the final product has very low carbon and nitrogen specifications. Triplex process tends to have overall cycle times longer than duplex process since there is an extra transfer from refining converter to vacuum degassing unit. It also tends to have slightly higher refractory costs since there are two reactors performing decarburization.
Raw material availability affects the choice of process route. For example, high argon costs shift the economics of stainless production away from dilution type processes in favour of vacuum processes. A lack of scrap forces the use of more high carbon ferro-chrome, increasing the carbon load in the charge. In such a case, process routes which have high decarburization rates are to be favoured. Lack of scrap, which can be used for cooling the bath, also can favour processes which are not thermally efficient, as there is no economical addition for cooling the bath. Where scrap and ferro-alloys are expensive or not available, process routes have been developed which use chromium ore and blast furnace (BF) hot metal. These processes typically incorporate a smelting unit, frequently a converter, to reduce the ore, followed by decarburization in another converter, possibly followed by vacuum processing.
The desired product mix affects the choice of process. High production levels of ultra-low carbon and nitrogen stainless steels tend to favour vacuum processes for avoiding high consumption of argon. For typical levels of carbon and nitrogen in stainless steels, the processing cost are normally lower for the converter processes than triplex processes.
Down-stream processing considerations also affect the choice of process. Sequence continuous casting needs a stainless-steel process route which provides liquid steel at the proper temperature, composition, and time for sequence casting. Availability of ladle furnaces and ladle stations can shift some of the non-decarburization time from the converter to the ladle furnace or station. Some high productivity shops tap the heat out of the converter just after the reduction stage. The final additions are made to the ladle. This frees up the converter for the next heat.
If existing melting capacity is not sufficient for desired production levels, the refining process needs to melt large quantities of cold material. This favours processes which are thermally efficient or ones which can generate heat for melting. If melting is mainly by IF, raw material choices can be limited. Availability of BF hot metal also influences the type of refining process. Available vacuum or bag-house capacity can influence the choice between vacuum or dilution methods. Crane capacity can limit vessel size. Crane traffic and shop layout affect process flow. Existing ladle cars can help or hinder material movement. An existing continuous casting machine (CCM) also affects process choice.
Cost of capital always affects process choices. Systems for making ferro-alloy additions can be very expensive. Bag-house and vacuum capacity is also expensive. There is frequently a balance between capital and operational cost. The process with the lower operating cost sometimes has the highest capital cost.
In some installations, capital spending considerations have eclipsed those of all other factors, including flexibility and operating costs, in the selection of process route. For major turn-key installations, which can include steel melting, refining, casting, hot and cold rolling, annealing and pickling lines, as well as the associated technology, the choice of equipment suppliers is limited to large supplier organizations. These supplier organizations aggressively promote their own secondary steelmaking process as superior relative to operating costs, quality, and productivity etc. However, some of these suppliers manufacture and install any other type of secondary steelmaking, if the customer insists.
Depending on geographical location, order backlogs, previous equipment sales, and manpower availability etc., any one of these suppliers can be in a better position to manufacture and install equipment at a substantially lower price than the others. For maximum flexibility, a shop is required to have a process flow which can incorporate a duplex or triplex route. This offers the most flexibility in raw materials, production capability, and process flow. In this case, only products which need vacuum degassing for economic and / or quality reasons use a triplex route.
The AOD process revolutionized stainless steelmaking. It lowered the cost of production of stainless steels considerably. It allowed operators to use EAF for melting down of stainless and carbon steel scraps with desired quantities of low-cost high carbon ferro-chromium. The decarburization operation has moved out of the EAF and into the newly designed converter. The oxidation-reduction operation has been conducted at very high productivity rates. Additionally, the quality of the alloy steels produced has improved. The process has adopted by major stainless producers at a very rapid rate. Fig 3a shows AOD converter which is the predominant process for producing stainless steel.
Fig 3 Converters for producing stainless steel
Duplex processes are used for making stainless steels. There is an EAF or IF which melts down scrap, ferro-alloys, and other raw materials to produce the liquid steel. The liquid steel, which contains majority of the chromium and nickel as well as some other alloying elements, is the charge to the converters. The converters are used to achieve low carbon stainless steels which is tapped into a teeming ladle. The EAF-AOD process is one such duplex route. The versatility of the AOD process has led the steelmakers to re-examine the use of different converters for melting of stainless steels. This has led to the development of several other converters for duplex processes. These include: KCB-S process developed by Krupp Stahl (Fig 3b), K-BOP process used by Kawasaki Steel Corporation (now JFE Steel Corporation), K-OBM-S promoted by Voest Alpine (now Primetals Technologies), metal refining process (MRP) developed by Mannesmann Demag (now SMS group), Creusot-Loire-Uddeholm (CLU) process (Fig 3c), Sumitomo top and bottom (STB) blowing process by Sumitomo Metals (now Nippon Steel & Sumitomo Metal Corporation), top mixed bottom inert (TMBI) process used by Allegheny Ludlum Corporation (now Allegheny Technologies), VODC process tried by Thyssen (now ThyssenKrupp) where vacuum is applied to the converter, and AOD-VCR process developed by Daido Steel (Fig 3d).
The development work to make stainless steels using conventional BOF (basic oxygen furnace) converters had begun in the late 1950s and early 1960s. By the mid-1960s, some steelmakers have been using existing BOF converters for a partial-decarburization followed by decarburization in a ladle under vacuum to make the low carbon stainless steels. These processes are known as triplex processes since three process units, such as EAF, a converter for pre-blowing, and a vacuum decarburization unit for final refining, are involved. The steels undergo treatment for final decarburization, final trimming, homogenization, and flotation of inclusions before the ladle is taken for the continuous casting operation.
In almost all of the triplex processes, vacuum processing of steels in the teeming ladle is the final step before casting. The vacuum oxygen decarburization (VOD) process has been developed by Thyssen (now ThyssenKrupp). The other processes using vacuum include the use of Ruhrstahl-Heraeus-oxygen blowing (RH-OB) process for making stainless steels at Nippon Steel Corporation, and the use of a strong stirring-vacuum oxygen decarburization (SS -VOD process by Kawasaki Steel Corporation (now JFE Steel Corporation). Fig 4 shows these processes.
Fig 4 Vacuum processes for refining stainless steel
With all these developments over several decades, now there are several different processes to make stainless steels. The available processes can be divided into three groups namely (i) the converter processes, (ii) converter with vacuum processes, and (iii) vacuum processes.
Special considerations in refining stainless steels – Stainless steel decarburization is to account for or minimize the oxidation of chromium. It is normally accepted, that when oxygen is injected into stainless steel, a mixture of chromium and iron is oxidized. Decarburization occurs when dissolved carbon reduces the chromium and iron oxides which form. The decarburization sequence is hence ‘3/2O2(g) + 2Cr = Cr2O3 (equation 1), and ‘Cr2O3 + 3C = 2Cr + 3CO (g) (equation 2). Decarburization occurs on the surface of rising bubbles which form from either the inert gas which is injected or on the surface of chromium oxide particles which are being reduced and generating CO gas.
The decarburization of stainless steel involves techniques to minimize chromium oxidation. Thereare three basic techniques namely (i) temperature, (ii) dilution, and (iii) vacuum. The temperature technique has been used by EAF stainless steelmaking before the development of duplex methods.
As the temperature increases, the equilibrium carbon content at a particular chromium content decrease. However, this leads to operational difficulties and high costs. The dilution technique is used by the AOD and all converter processes. The injection of inert gas (argon or nitrogen) lowers the partial pressure of CO gas in the bath, hence allowing higher chromium contents to be in equilibrium with lower carbon contents. Applying a vacuum to the metal bath also removes CO gas, allowing high chromium contents to be in equilibrium with low carbon contents. It is especially effective when the carbon content is low.
Careful manipulation of the slag, as it participates in the reaction, is important. Any chromium oxide not reduced by carbon ends up in the slag, which can form a complex spinel. Subsequent processing (called reduction) is needed to recover oxidized elements such as chromium, iron, and manganese etc. The effectiveness of the reduction step is dependent on several factors including slag basicity and composition, temperature, mixing conditions in the converter, and solid addition dissolution kinetics.
Raw materials – Raw materials used in all stainless-steel making operations can be divided into two main groups namely non-metallic raw materials and metallic raw materials. The non-metallic raw materials typically needed are calcined lime, dolomitic lime, and fluorspar. The metallic raw materials needed are stainless steel scrap, carbon steel scrap, and a variety of alloying materials and deoxidizers. The major alloying elements used are chromium, nickel, manganese, and silicon additives. Ferro-chromium is the prime source of chromium and it comes in a range of addition agents. The chromium is normally charged to the EAF and AOD as low carbon ferro-chrome, medium carbon ferro-chrome, or high carbon ferro-chrome.
Nickel is present in considerable quantities in the AISI (American Iron and Steel Institute) 200 and AISI 300 series stainless steels. In addition to austenitic stainless-steel scrap, nickel is available as electrolytic nickel, nickel powder, nickel oxide, briquetted nickel, and ferro-nickel. Manganese is present in considerable quantities in the AISI 200 series of stainless steels. Silicon is used in all the stainless steelmaking processes to chemically reduce the chromium which is present in the slag at the end of the de-carburization period. For this purpose, silicon is added in the reduction mix as ferro-silicon or ferro-chromium-silicon. Additional silicon is needed for meeting any silicon specification on several grades of the stainless steels. The most common raw materials used to supply the silicon are its ferro-alloys which contain nominally either 50 % silicon or 75 % silicon.
Molybdenum is added to some of the stainless-steel grades. The materials which can be used to add molybdenum includes molybdenum oxide in powder or briquette form, and ferro-molybdenum. Aluminum is a common deoxidant and high purity aluminum pig or bar is added for this purpose. Titanium is a very frequent addition to several stainless-steel grades for the purpose of stabilization. The most commonly used alloys of titanium are 70 % titanium-30 % iron, and 90 % titanium-6 % aluminum-4 % vanadium alloy. The other elements which are normally added in the ladle include carbon, sulphur, nitrogen, niobium, tungsten, and copper. Selenium and tantalum are added to a very limited number of grades.
In stainless steelmaking, majority of the cost of the final product is because of the raw materials. In recent past, computer programmes, majority of which use linear programming techniques, have been developed and used to utilize the raw materials in such a way that the total raw material cost of production is minimized.
Melting – For the EAF, typical charges are scrap and ferro-alloys. As there is further refining, least cost charges incorporate high carbon ferro-alloys for chromium and manganese units. Normally, the least cost source of material is used, for scrap this can consist of oily turnings and furnace skulls. Sometimes, hot metal from the blast furnace or from other iron source is available. In this case, the EAF can be used to melt only ferro-alloys with the resulting melt then mixed with the liquid iron in a converter process for refining. The EAF can also be by-passed altogether.
When hot metal is the source and stainless scrap or ferro-alloys are not available or very expensive, then chromium, manganese, and nickel ores can be used as sources of material.
EAF melting – The delivery of liquid steel for further processing needs melting facilities of some kind. The EAF is by far the most popular choice because of its flexibility of raw material sources and widespread use. IF melting is popular in smaller shops and foundries, however, it is less capable of handling some scrap sources, such as turnings and oily scrap.
The job of the furnace operator, whether it is EAF or other method, is to melt down the basic raw materials as quickly and as economically as possible. The potential of chromium oxidation places some limits on the use of oxygen injection into the EAF. Carbon injection and foamy slag practices, while normal during making of carbon steel, are rarely used during stainless steel production. The slags which form in the melting of stainless steel are high in chromium oxide and are not as fluid as the slag formed during carbon steel production. The stainless-steel melt slags do not foam easily. CO gas generation for formation of the foamy slags also does not occur if the chromium content is high.
The charges for stainless steel melting in several countries are mainly scrap based. Stainless steel scrap is frequently a cheaper source of chromium and nickel units than virgin material such as ferro-chromium and nickel. Melting of stainless-steel scrap is a choice of the least cost sources of metal units. There is normally some consideration of impurities such as copper and phosphorus, which are not removed during subsequent processing.
For a AISI 300 series stainless steel, a typical melt-in mixture can consist of 50 % AISI 300 series stainless steel scrap (18 % Cr, 8 % Ni, and 1 % Mn), 30 % carbon steel scrap, 14 % high carbon ferro-chrome (7 % C, and 65 % Cr), 4 % nickel (commercial purity), and 1 % high carbon ferro-manganese (7 % C, and 65 % Mn). After melting, this mixture yields around 18 % Cr, 8 % Ni, 1 % Mn, and 1 % C. In the early stages of melting, some steel melting shops use burners and oxygen lancing. The added energy input increases the melt-in rate in the early stages of melting. After the second bucket however, burner and oxygen use are limited to avoid chromium oxidation. At tapping, ferro-silicon is frequently added for recovering oxidized chromium from the slag.
Converter melting – Converter melting needs oxygen and carbon injection coupled with post-combustion for generating heat. Converter melting is rarely economical but high electrical power costs or lack of electric melting capacity, combined with hot metal availability, can make this option viable. In converter melting, liquid hot metal is charged into a smelting converter and the ores are added. Carbon is also injected or charged as the reductant and as fuel. Oxygen is injected by a lance for burning carbon and post-combustion. This generates the heat necessary for the smelting reaction. Ores typically contain some quantity of gangue, so appropriate slag conditioners are also added. The slag is tapped and the metal, now close to the desired composition, is transferred to another converter for refining.
Dilution refining processes – The melting processes supply the liquid stainless steel, which now contains desired quantities of chromium, nickel, and other alloying elements, to the next process step. The latter is directed at decarburizing the stainless steel to the desired carbon content. It is accompanied by oxidation of chromium, iron, silicon, aluminum, titanium, and manganese which are present in the charge.
In a converter, decarburization is carried out using the dilution principle. There are different converters based on the gases used. Another difference is whether the converter is side or bottom-blown. Supporters of side blowing argue that side blowing results in higher carbon removal efficiencies (quantity of oxygen reacting with carbon divided by the total quantity of gases blown) in the range of 0.1 % to 0.005 % carbon, because of longer inert gas bubble residence time, and improved desulphurization in the range of 0.005 % to less than 0.001 % sulphur because of improved mixing. The most popular converters for making stainless steels are AOD, KCB-S, K-BOP / K-OBM-S, MRP and CLU converters.
AOD converter process – The AOD converter shown in Fig 3a is used for the production of stainless steel in the second step of a duplex process. AOD converter has a top lance for oxygen injection and several bottom tuyeres for blowing oxygen or inert gas during the operation. Argon and nitrogen are normally used as inert gas. In the early days of the AOD converter process, the converter was tilted for making raw material additions as well as for taking samples and temperature measurements using immersion thermo-couples. The desire to increase the productivity has led to continuous charging of raw materials during the blow periods as well as reduction periods. These days instrumentation has been developed which can take melt samples as well as steel temperatures using a specially designed sub-lance with the converter in the upright position.
AOD converter is a pear-shaped vessel normally lined with basic refractories. It has a removable, conical cover in place. The important feature of an AOD converter is that it is normally side blown. In case of those steel grades which can tolerate nitrogen, a mixture of oxygen and nitrogen can also be blown. Since liquid stainless steels do not generate foam, and since the majority of the stainless-steel refining processes are side or bottom-blown, the dimensions of a stainless refining converter are smaller than a comparable BOF converter. Typical internal volumes of AOD converters are in the range 0.4 cubic meter per ton (cum / ton) to 0.8 cum / ton bath weight.
The liquid steel, which contains majority of the chromium and nickel needed for meeting the final heat composition, is tapped from the EAF into a transfer ladle. The transfer ladle is lifted with a crane and the liquid steel is poured into the AOD converter. The converter can be rotated downwards so that the side mounted tuyeres are above the bath level during charging of the liquid steel. After the charging, liquid steel is subjected to three steps of refining namely decarburization, reduction, and desulphurization. Normally, the gas blowing stage can be divided into oxidizing stage and reduction stage. For oxidizing stage, pure oxygen or nitrogen diluted oxygen is blown from lance and bottom tuyeres. At the reduction stage, only argon is blown from bottom tuyeres for removing extra nitrogen and reducing oxidized chromium by ferro-silicon (Fe-Si). A hood connected to the exhaust gas system is placed above the mouth of the converter to collect the exhaust gas from AOD. Normally, there is a gap between AOD converter mouth and the exhaust hood.
A major modification of the original AOD process involves the use of a top-blowing lance in addition to the side blowing tuyeres. The lance can be used to inject oxygen at desired flow rates for increasing the decarburization and / or for post-combustion. The top lance can also be designed for blowing mixed gases such as inert-oxygen gas mixtures. The installation of a lance and the introduction of oxygen in the early stages of decarburization can reduce the time for a heat. The technology can be used for increasing the productivity (tons per hour).
Another modification of the AOD involves applying vacuum on the converter for reducing the argon and silicon consumption as well as process time when making low carbon grades. This modification is identified as AOD-VCR (vacuum converter refiner). The AOD-VCR converter is shown in Fig 3d.
K-BOP and K-OBM-S process – K-BOP process began as a conventional top oxygen blown BOF. It has been modified to have seven bottom tuyeres of the OBM (Q-BOP) type. These tuyeres can blow oxygen with LPG (liquefied petroleum gas) for tuyere cooling. Powdered calcined lime can also be injected through these tuyeres. In its initial development stage, an 85 ton ultra-high-powered EAF (UHP-EAF) has been used to supply the liquid steel to the process reactor. The top and bottom oxygen blowing has been used for achieving the high decarburization rates. The developmental work has been for reducing the chromium ore and coke which are added to the reactor. The reactor has been hence used for the reduction of chromium ore and after that switching over to decarburization. Additional developmental work has led to a smelting reduction process for stainless steelmaking using separate reactors. Final decarburization (below 0.1 % C) has been carried out in a top-blown RH degassing unit (the KTB process).
Eventually a process using separate converters and a VOD unit has been developed. The K-OBM-S has been evolved from the K-BOP process. The K-OBM-S process initiated with tuyeres installed in the converter bottom. However, two installations, are side-blown reactors. Hence, a K-OBM-S converter is top-blown with a lance and bottom or side blown with tuyeres. It is very similar to a modern AOD. However, in the K-OBM-S process, hydro-carbons, such as natural gas or LPG, are used for tuyere protection and this can be helpful in increasing the refractory life.
Metal refining process (MRP) converter – The metal refining process is also a duplex process where scrap and raw materials are melted in an EAF or similar unit. Liquid steel, which contains chromium and nickel, is charged to the MRP converter. Decarburization is carried out using oxygen and inert gases. In early stages of development, the gases have been alternately blown through the tuyeres in the bottom of the reactor. The oxygen is blown into the melt without dilution with any inert gas. The desired oxygen blow is followed by blowing with inert gas only. The cycle of oxygen blow followed by inert blow is called cyclic refining or pulsing and the developers have claimed that the flushing with pure inert gases can lead to achieving low CO gas partial pressure and faster decarburization and hence lower chromium oxidation and consumption of silicon for reduction. The original version of the converter has now evolved into the MRP-L process in which all oxygen is top-blown and inert gas is injected through the porous elements in the bottom.
The process can be used for higher blowing rates than those used in the AOD process, which has side-wall tuyeres. The bottom tuyeres can be replaced easily through the use of an exchangeable bottom. With bottom tuyeres, there is less likely to be erosion on the sidewalls of the vessel. In recent years, the MRP-L converters have been coupled with a vacuum unit as part of the triplex process for making stainless steels, especially those needing lower carbon and nitrogen levels. In these plants, the heats are tapped at an intermediate carbon level appropriate for subsequent vacuum decarburization.
Creusot-Loire-Uddeholm (CLU) converter – The CLU converter process is similar to the AOD converter process for making stainless steels. It also uses liquid steel from an EAF or similar melting furnace. The converter is bottom-blown hence differentiating it from the side-blown AOD converter. The major motivation for its development has been the idea to substitute steam as the diluting gas rather than argon. Like the AOD process, CLU process also uses the dilution technique. The process is based on the principle that the decarburization is to take place at a reduced partial pressure of CO gas since the carbon-chromium equilibrium in a steel bath is very much dependent on the partial pressure of the CO gas in the gas bubbles created during the decarburization.
The decarburization period consists of injecting an oxygen-steam mixture. The process is energy inefficient as the reaction of steam with the liquid steel bath is endothermic. Chromium oxidation is higher than in the AOD process when decarburization is continued below around 0.18 % carbon. Although the original objective of reducing argon consumption can be met, the increased silicon requirement for the reduction step does not necessarily lead to overall cost savings. Further, the use of steam throughout the entire period has been found to lead to undesirable hydrogen content in the refined steel. Hence, practices have evolved which use different quantities of steam, argon, and nitrogen in the process.
Krupp combined blowing-stainless (KCB-S) process – The production of stainless steels in the BOF converter using the top lance has been practiced prior to the arrival of the AOD process. After introduction of the AOD process, Krupp Stahl AG modified the converter at its Bochum works so that combined blowing through the lance and tuyeres can be practiced for refining stainless steels. The process has been named Krupp combined blowing-stainless or KCB-S process (Fig 3b).
The simultaneous introduction of process gases helped in the increase of the decarburization rate. The blowing through a top lance and through the tuyeres below the bath surface helped in achieving very high decarburization rates. The increased decarburization rate has led to a reduction of up to 30 % in the refining time compared to a conventional AOD alone.
Liquid steel from an EAF is charged to the converter. At the start of the blow, pure oxygen is injected simultaneously through the lance and side-wall tuyeres. After a desired process temperature is reached, different additions are made during the blow. The additions consist of calcined lime, ferro-alloys, and scrap. After a critical carbon level is reached, the oxygen content of the process gas is reduced by using inert gases such as nitrogen or argon. Oxygen to inert gas ratios of 4:1, 2:1, 1:1, 1:2 and 1:4 is used as decarburization to lower levels is pursued. When the carbon content of 0.15 % is reached, the use of the lance is discontinued and the process gases are introduced only through the tuyeres. When the desired aim carbon level is reached, the oxygen blow is discontinued and silicon is added as ferro-silicon to reduce the chromium oxide in the slag and to achieve the needed silicon specification level. The addition of calcined lime and other fluxing agents with the ferro-silicon leads to the lowering of the dissolved oxygen content and improves the desulphurization.
Argon secondary melting (ASM) converter – This process is similar to the AOD process, except that the tuyeres are in the bottom of the converter. When using top-blown oxygen, it is identified as the ASM-L process.
Sumitomo top and bottom blowing process (STB) converter – Sumitomo has developed to overcome the disadvantages of a pure top or pure bottom blowing process by combining the two concepts into one process. It also tried to overcome two disadvantages of the AOD process at that time namely (i) tuyere erosion, and (ii) limited oxygen flow rate. The additional supply of oxygen rich gases from the top lance led to shorter decarburization time compared to the AOD process as practiced in late 1970s. The process has been developed in a 250-ton converter at Kashima works. The process has been renamed Sumitomo metal refining (SMR) and has been used for a short time in the 160-ton converter at Wakayama works.
Top mixed bottom inert (TMBI) converter – Allegheny Ludlum Corporation used a BOF converter to make ferritic stainless steels whenever incremental stainless-steel capacity was necessary. In the early stage of development, oxygen, and oxygen-argon mixtures were introduced through a supersonic top lance. However, the process was less efficient than the AOD process. Hence, one BOF converter was equipped with bottom tuyeres to inject only inert gases such as argon or nitrogen. The majority of the process gas was introduced through the top lance. The top lance could be used to introduce the desired mixture of gases. The process was called top mixed bottom inert (TMBI) process. The process is similar to the other processes which use combined gas blowing in a converter. The particular plant operated by Allegheny Ludlum has coreless induction furnaces which melt carbon steel scrap and supply chromium-free hot metal to the BOF converters.
Combined converter and vacuum units – The converter processes have one disadvantage in that stainless steels with very low carbon and nitrogen residuals are difficult to produce. The decarburization period are longer while chromium oxidation and argon consumption are increased as the desired carbon and nitrogen levels are decreased. Some steelmakers have tried to overcome this disadvantage by applying vacuum to the converter at the very late stages of the decarburization process. The concept was promoted by Leybould-Heraus as an alternative to AOD or VOD. The concept of applying vacuum to a converter is being pursued by the installation of AOD / VCR process.
The AOD-VCR process has been developed in 1991 as a process combining the advantages of AOD and VOD. This process is a low-oxygen decarburizing process wherein vacuum facilities are added to an AOD furnace, and carbon content is decreased at low pressure utilizing oxygen solute in steel and oxides in slag without injecting oxygen, taking advantage of the strong stirring effect of the AOD process. In 2001, a low-pressure accelerated decarburizing process has been developed whereby the furnace pressure is decreased from a mid-carbon range ([C] – 0.6 %) and steel is decarburized with oxygen blown in without injecting diluting gas. Using this process, a considerable improvement has been achieved in oxygen consumption, leading to a remarkable decrease in both processing time and consumption of silicon, which has been used as a decarburizing agent.
Fig 5 schematically shows the processing stages of the low-pressure, accelerated decarburizing using the AOD-VCR process. Whereas when using the conventional VOD process, it is necessary to mix a diluting gas with oxygen blown in from the bottom in the carbon range of 0.6 % to 0.1 %, with the AOD-VCR process, no diluting gas is needed because of the low furnace pressure, and hence high oxygen efficiency for decarburizing is achieved.
Fig 5 Comparison of conventional AOD and AOD-VCR processes
The AOD-VCR operates as a conventional AOD down to 0.08 % C to 0.1 % C. The process is stopped for sampling and a vacuum lid are put into place. The lid is sealed to a flange located about half way up the conical section of the converter. A vacuum is pulled and used for the remaining of the decarburization and reduction. Desulphurization is carried out in the transfer ladle prior to AOD charge. The major advantages of this process relative to converter processes are decreased argon and silicon consumption. Disadvantages include higher refractory consumption, decreased ability to melt scrap and added maintenance and costs associated with steam production. When compared to separate converter and VOD units, the AOD-VCR has higher operating costs (silicon, refractory, and argon), lower productivity and higher nitrogen contents. Capital costs can be somewhat lower than having two separate units.
Vacuum refining processes – The use of vacuum for decarburization of steels was developed by different steelmakers in Germany. The early processes included RH degassing, DH degassing, and the Allegheny vacuum refining (AVR) as a second step in the duplex process. These processes involve lowering the pressure above the steel bath to promote evolution of the CO gas. The liquid stainless steels going into the vacuum process normally contain carbon at a level of around 0.5 % or lower. Majority of vacuum processes are performed in a chamber with a ladle full of metal as opposed to a separate refining reactor used in the dilution / converter processes.
In mid 1960s, the vacuum decarburization concept was used to develop the AVR process. It was used to make the regular grades of stainless steels and it lowered the consumption of low carbon ferro-chrome associated with the oxidation-reduction practice used in the EAF. This process became non-competitive with the introduction of the AOD process. In late 1970s, Allegheny Ludlum built a plant with an AOD reactor and discontinued the use of the AVR process.
Early duplex processes where vacuum processing has been used as the second step is too slow and had very limited flexibility with respect to raw materials which can be used. The vacuum processes cannot keep up with the improving productivity of EAFs and the operating costs are very high. Hence, later developments focused on the use of converters to decarburize the liquid steel from EAFs as a second stage followed by vacuum degassing for the finishing stage. Such processes are known as triplex processes for making stainless steels as they use three processes to achieve the desired finished chemistry.
Nippon Steel Corporation introduced the RH-OB process for making of stainless steels. Hot metal from a blast furnace has been fed to a BOF converter where the metal is alloyed with chromium and blown down to a carbon level of 0.5 % to 0.6 %. The final decarburization has been conducted using the RH-OB process. Nippon Steel converted an existing RH degasser, which was used for carbon steels, so that oxygen could be injected under vacuum.
The VOD process has been developed in the mid-1960s. In the early stages of its development, VOD has been used to decarburize liquid steel from EAF. Later, preliminary decarburization has done in a BOF and the EAF-BOF-VOD triplex process became more productive. In early 1970s, Kawasaki Steel Corporation modified the VOD process using multiple porous plug bubblers in the ladle. They called this the SS-VOD (strong stirring VOD) process. In 1988, ALZ (Allegheny-Longdoz, now a subsidiary of Cockerill) in Belgium modified the facilities to make stainless steels by a triplex process consisting of EAF melting, MRP-L converter, and VOD process.
The major advantages of the vacuum processes include low consumption of argon and low oxidation of chromium during the final decarburization to low carbon levels. The latter leads to less consumption of reduction elements for recovering chromium from slag. The teeming of steel from the ladle used in the vacuum processes eliminates the pick-up of nitrogen and oxygen from air which is associated with tapping of the converters. The SS-VOD process, because of the strong stirring achieved using multiple bubblers in the ladle, further improved the ability to produce even lower levels of carbon, nitrogen, and hydrogen at higher chromium levels.
A major disadvantage of VOD processing is that it is less flexible than an AOD or other converter processes with respect to raw materials usage. Typical additions to the VOD are around 4 % to 8 % of tap weight. Typical vacuum treatment times are 50 minutes to 70 minutes with start carbon contents of 0.3 %, compared with 40 minutes to 60 minutes with a converter starting at 1.5 % C to 2.5 % C. This added time makes it difficult to sequence continuous casting of heats.
Several steelmakers have also realized that vacuum processes frequently have high operating costs and cannot compete with the ease of operating a converter process at atmospheric pressure. However, the vacuum processes, especially the SS-VOD process, have the unique ability of achieving lower carbon and nitrogen levels in stainless steels which cannot be easily achieved by the AOD process or other converter processes.
Direct stainless steelmaking – In recent years, there have been efforts to use chromium and nickel ores for the production of stainless steel in place of the ferro-alloys. In Japan, a number of organizations have developed and are using such processes commercially. In particular, Kawasaki Steel (now JFE) has developed a process (KCS process) which smelts chromite ore (FeCr2O4 plus other oxides). Stainless steel is produced with 160-ton converters, without any EAF operation, but using dephosphorized hot metal, chromite ore, and ferro-alloys as the major charge materials.
The hot metal is desulphurized and then fed into a K-BOP / K-OBM converter (SR-KCB). Chromite pellets, which are partially pre-reduced up to 60 % chromium content in a rotary kiln, are charged to the first reactor. Coke is also charged and serves as a heat source in the reactor. The charging rate of ore and coke and the oxygen blow rate are controlled to achieve suitable temperature for the melt. The liquid steel, which now contains around 11 % chromium to 16 % chromium and 5 % carbon to 6 % carbon is charged to the second KBOP / K-OBM converter. Primary decarburization (DC-KCB) is carried out using oxygen, argon, and nitrogen gases in this converter. Top and bottom oxygen blowing is used for lowering the carbon content of the liquid steel to around 1 % level. This is followed by mixed gas blowing only from the bottom. This is called the DC-KCB process and is similar to a modern AOD operation. The process reduces the carbon to around 0.15 %. Final decarburization and reduction are carried out in a VOD unit. Fig 6 shows an outline of the KCS process.
Fig 6 KCS process for direct stainless steelmaking
The KCS process uses lesser quantity of scraps than the EAF-AOD process route. However, it needs liquid hot metal from a blast furnace and more process steps and more capital investment than the EAF-AOD route.
NKK Corporation has developed and commercialized a similar process which also smelts both chromite ore and nickel ore. After hot metal dephosphorization, the metal is transferred to a smelting furnace and nickel ore is smelted. As the nickel ore typically contains 1 % to 3 % nickel, a considerable quantity of ore is added and the slag is to be removed several times to achieve the desired nickel level. After reaching the needed nickel content, chromite or chromium ore is smelted. The smelting takes around six hours (four hours for nickel and two hours for chromium). This is followed by the decarburization.
Similar to the KSC process, the NKK process uses lesser scrap and only a small quantity of nickel metal and ferro-chrome. However, the process needs hot metal and a very long process time. These direct stainless steelmaking processes need lesser scrap and ferro-alloys and can be considered when the stainless steel scrap and ferro-alloys are not readily available in sufficient quantities. However, chromium and nickel ores are needed and can be used in an integrated steel plant where the hot metal is available. The processes are more capital intensive than the other processes. Hence, their implementation in other plants and locations is limited.
Equipment for EAF-AOD process – The EAF-AOD, EAF-CLU and similar duplex process routes are the most popular for making stainless steels. The equipments used in the EAF-AOD process route, which is by far the most popular process in the world for making stainless steels are described below.
For converter refining processes, the converter consists of a refractory-lined steel vessel. Several types and qualities of basic refractory have been used in the converter. Process gases are injected through submerged tuyeres which are installed in the side-wall or bottom of the converter. Side-wall injection imparts maximum stirring energy to the bath for highest efficiency of mixing. Bottom injection normally improves wear characteristics of the refractories in the barrel section of the converter. The number and relative positioning of tuyeres is determined by the converter size, range of heat sizes, process gas flow rates, and the types of alloy steels to be refined. Process gases are oxygen, nitrogen, carbon di-oxide (CO2), air, hydro-carbons and argon. Majority of recent converter installations include the use of a top lance for blowing oxygen.
The gas control system supplies the process gases at nominal rates of 1 N cum/min/ton to 3 N cum/min/ton. The system accurately controls the flow-rates and monitors the quantity of gas injected into the bath to enable the operator to control the process and measure the total oxygen injected. In contrast, vacuum processes typically use much lower total input rates, e.g., 0.02 N cum/min/ton to 0.6 N cum/min/ton (typical top oxygen blow rate of 0.3 N cum/min/ton to 0.4 N cum/min/ton) and seldom use gases other than oxygen, nitrogen, and argon.
A high production shop typically has three inter-changeable converters for availability of the process. At any given time, one of the converters is in the tiltable trunnion ring refining steel, a second newly lined converter is at a preheating station, and the third converter is at a reline station. The converter in the trunnion ring typically can be replaced with a pre-heated converter in less than an hour. Process gas control, converter activities and ancillary equipment can range from manual to fully automated. Majority of the installations are equipped with a computer for calculating oxygen requirements as well as ferro-alloy additions. Some installations have computer controls capable of setting flow rates to the gas control system.
Converter size and shape – The converter consists of a refractory-lined steel shell. With a removable, conical cover in place, the converter outline is sometimes described as pear-shaped. Modified BOF converters have also been used. Since liquid stainless steels do not generate foam, and majority of the stainless-steel refining processes are side or bottom-blown, the dimensions of a stainless-steel refining converter are smaller than a comparable tonnage BOF or OBM (Q-BOP). Typical internal volumes of the stainless-steel refining converters are in the range 0.4 cum/ton to 0.8 cum/ton bath weight.
For converters which tap into a ladle held by a crane, a sliced cone top section is frequently used. The slice portion allows the crane to come close to the converter. Fig 3a shows an AOD vessel with a sliced cone top section. Converters which tap into a ladle car normally have a BOF-type concentric cone top section.
Refractories – High temperatures at the tuyere tip and high bath agitation place large demands on the converter refractory. While typical BOF refractory campaigns are months or years long, stainless-steel converter campaigns are several days or weeks long. Refractory costs are a considerable fraction of the total operating costs. There are two basic choices of refractory type namely magnesite-chromite, and sintered dolomite. The choice of refractory is dependent on the converter operation pattern, final product specifications, and economics. Magnesite-chromite refractories have high wear resistance but have a higher unit cost than the sintered dolomite refractories. Chromium pick-up from the brick is possible. Magnesite-chromite bricks are simultaneously acidic and basic and hence strict slag compositions are to be maintained for preventing rapid wear.
Sintered dolomite refractories are normally less costly than magnesite-chromite refractories and chromium pick-up is not a factor. Desulphurization to very low levels is normally easier in sintered dolomite refractories since very basic slags can be used without detrimental effects to the brick. Several major stainless-steel producers use sintered dolomite refractories in the converter. Converters are typically zoned lined by thickness and brick quality for maximizing lining life and minimizing of the costs. High wear areas of the converter, normally the tuyere wall, slag line, and transfer pad (where the metal stream strikes the converter during transfer) are zoned lined thicker and with higher quality refractory than other parts of the converter.
Tuyeres and plugs – The AOD process and the majority of other converter processes have tuyeres mounted in the side-wall or in the bottom. These tuyeres typically consist of a copper tube with a stainless-steel outer tube. An annulus is formed between the copper and stainless tubes. Cooling gases blown through the outer annulus (shroud) form a metal or oxide accretion (called a mushroom) at the tuyere tip. This accretion protects the tuyere and surrounding refractory. Process gases of oxygen / inert mixtures blow through the inner annulus. Special designs exist for normalizing the flow in the annular gap. Tuyere size and number depend on specific process parameters. There are normally between two to nine tuyeres in an AOD converter.
Side-wall mounted tuyeres are submerged while processing. When the converter is rotated, the tuyeres are above the bath. At this point, the process gases can be shut off and a small cooling flow protects the tuyeres. Bottom-blown converters have a variety of tuyere configurations depending on flow rates needed. There are normally two to four tuyeres in the bottom. Majority have some variation of OBM (Q-BOP) tuyeres. Processes with low flow rates which use only inert gases, such as VOD, for the bottom-blown gases can use a plug instead of a tuyere.
Top lances A lance, while not needed, highly improves converter processes. In the high carbon region, a lance can increase the oxygen input rate, increasing the decarburization rate. A lance can be used for post-combustion of CO gas, increasing the scrap melting rate or temperature of the bath. For smaller converters, post-combustion lowers fueling need and increases process efficiency. The quantity of oxygen which reacts with the bath can range from 30 % to 100 % of the oxygen blown. This quantity depends on lance height, nozzle design, and gas pressure used.
There are two basic lance types, sonic / sub-sonic and supersonic. Sonic / sub-sonic lances consist of straight pipes, while supersonic lances use converging / diverging (Laval) nozzles. Supersonic lances are always water cooled. Sonic / sub-sonic lances can be water cooled depending on the lance operating position. Smaller sonic / sub-sonic lances frequently are not water cooled. Several sonic / sub-sonic lances are simple pipes with nickel or calorized tips to increase heat resistance.
A water-cooled lance is typically positioned 1 m (metre) to 4 m from the bath surface. Non-water cooled lances are positioned close to the converter mouth. Supersonic lances are used for high oxygen input rates where post-combustion is not needed. A large majority of the oxygen from a supersonic lance reacts with the bath. As in BOF operation, the lance can have post-combustion ports. With sonic lances, some of the oxygen does not react with the bath, some post-combustion is inevitable. Heat balances indicate that 75 % to 90 % of available energy from post-combustion is transferred to the liquid bath. The simplicity of a non-water-cooled sonic lance makes it attractive to smaller production shops.
Inert gases are sometimes injected with the oxygen from the top lance for extending the period of the top blown gases being used. Argon top blowing can reduce air infiltration into the converter and improve carbon removal efficiency in the late stages of the decarburization blow. This is especially true for converters where high-rate bottom argon blowing causes excessive splashing, vibration, or refractory wear. A recent development in top lance technology is the development of top heating. An oxy-fuel burner in the top of the converter provides heat to the converter during operation. This results in benefits similar to post-combustion.
Gases – Pure argon, oxygen, and nitrogen are used by virtually all the production shops. In rare cases, nitrogen is not used since the product mix or operation pattern does not warrant the extra cost of supplying nitrogen. Some other gases used are crude argon, high-pressure air, non-cryogenic nitrogen and oxygen, hydro-carbons, and CO2 gas. These other gas choices depend on operational pattern, relative cost of supply, and product mix.
Crude argon contains 95 % to 97 % argon, 1 % to 3 % oxygen, and 0.5 % to 2 % nitrogen. It can be used in the middle to late stages of decarburization and reduction, depending on final product specifications. The choice of using crude argon is dependent on availability, relative cost of pure argon, and product specifications.
High-pressure air can be used to partially replace nitrogen, but the capital and maintenance cost of compressors and associated equipment limit this option to larger steel melting shops. Non-cryogenic nitrogen and oxygen sources also need some compression for use in the refining processes, and the choice of this supply method again depends on pure against non-cryogenic prices, duty cycle, power costs, and production rates compared to pure gases.
CO2 gas and hydro-carbons can be used as process gases down to certain carbon levels. In stainless-steel production, increased chromium oxidation below 0.3 % C by CO2 gas or hydrogen and carbon pick-up from hydro-carbons limits the use of these gases. The replacement of nitrogen by these gases depends on the relative cost of the gases. Low nitrogen specification grades of stainless-steel, which typically are made using only argon as an inert gas, can use CO2 gas as a replacement for argon in the early stages of decarburization.
Converter drive system – The drive system is similar to a BOF converter except that in majority of the cases the converter is removable from the trunnion. Torque requirements and safety interlocks are an important design criterion. Some converters are designed to rotate 360-degree. Side-blown converters normally have replaceable shells. With removable converters, crane access and capacity are considerations in the placement and size of the operating unit. The trunnion bearings and drive are to be designed to handle the vibration and rocking motion of the converter during processing.
Emissions collection – Emissions collection strategies fall into two categories. Close capture hoods have a small gap, typically 0.2 m to 0.4 m, between the hood and converter mouth. They are normally water cooled since the hot gases cannot entrain enough dilution air to cool the gas. Ferro-alloy addition chutes and lance positioning for alloy additions are to be integrated with the hood. Other types of capture hoods are further from the converter mouth. These are normally refractory lined and not water cooled. Canopy systems have a substantial distance from the converter mouth to the collection duct which is located in the roof of the shop. An accelerator stack is sometimes used as a chimney to direct the smoke plume to the collection duct.
Bag-house fan capacity, crane location, emissions regulations, and cost dictate the choice of emission collection system. Several systems use wet or dry precipitators in place of bags for dust collection. Top lance placement can hamper optimal placement of the collection hood. The lance has to penetrate the collection hood at some point. Close capture systems need less bag-house capacity and are likely to emit less smoke, but water-cooling duct work can be expensive.
Canopy systems do not operate effectively if the crane is to pass through the plume periodically. Bag-house requirements are normally higher since the bag-house is also to take in dilution air which is entrained into the rising smoke plume. On the plus side, water cooling of the canopy is normally not necessary. However, placement of a lance with an accelerator stack system can be problematical. The gas evolved from the converter is mainly CO gas along with argon or nitrogen, unless substantial post-combustion occurs. A major portion of the combustion and cooling takes place at the mouth exit and / or at the hood intake. Further cooling takes place in the duct. By the time the gas reaches the bag-house, the gas temperature is typically 120 deg C. The gas is composed mainly of CO2 gas and inert gas. Solids emissions from the converter are mainly particulates of metallic (e.g., iron, chromium, and manganese) and non-metallic oxides (e.g., lime and silica). Total solids emissions average between 6 kg/ton to 10 kg/ton of metal. Dust loading is about 50 grams/cum.
For a new installation, the total cost of the fume system is typically around 40 % of the total converter installation cost. In vacuum processes, the fume collection system is an integral part of the equipment and typically is used in conjunction with a water treatment plant.
Converter operation – No matter what converter-based process is used for refining stainless steel, there are several common steps during the refining process. Decarburization is the carbon removal step. As decarburization is not completely efficient, some metallic oxides are formed. Reduction recovers these oxides from the slag phase. Refining of the metal occurs throughout processing.
Decarburization – After the liquid steel charge is in the converter, an oxygen-inert gas (argon, nitrogen, or carbon di-oxide) mixture is injected into the converter. The initial oxygen to inert ratio normally varies in the range of 5:1 to 3:1 and is lowered with the progress of decarburization. The main benefit associated with the dilution process comes into play when the oxygen to inert gas ratio is 1:1 and then further reduced to 1:3 or lower. Oxidation of carbon continues, but oxidation of chromium is limited. The latter is because of the very low oxygen potential of the gas mixture, which minimizes chromium oxidation.
For a AISI 300 series stainless steel, typical steps arre 1.5 N cum/min/ton oxygen mixed with 0.5 N cum/min/ton inert gas from the start carbon content to 0.35 % C, then 0.5 N cum/min/ton oxygen mixed with 0.5 N cum/min/ton inert gas to 0.15 % C, followed by 0.25 N cum/min/ton oxygen mixed with 0.75 N cum/min/ton inert gas to 0.05 % C, and then 100 % argon to final specification.
More recent blow programmes consist of using a starting oxygen to inert gas ratio of higher than 3:1 and continuous adjustment of the oxygen to inert gas ratio during the blowing period. Depending on the aim nitrogen content, nitrogen can be used as the inert gas for a period of the oxygen blow. Beginning carbon levels in converter processing can range from 0.7 % to 4.5 % C, with a typical range of 1 % to 2.5 %. Typical silicon levels range 0.2 % to 0.4 % before decarburization. If a top lance is available, it is used during the high oxygen to inert gas periods. Some converters use a top lance to add heat. The top heating lance is a modified oxy-fuel burner.
During decarburization, additions are made for getting the proper final chemical composition. These additions normally consist of desired quantities of high carbon ferro-chromium, stainless steel scrap, carbon steel scrap, nickel, iron, high carbon ferro-manganese, and molybdenum oxide. These additions serve to reduce the bath temperature since carbon and chromium oxidation are exothermic. In general, the bath temperature is controlled to less than 1,720 deg C. Total ferro-alloy addition weights are in the range of 5 % to 30 % of tap weight.
Calcined lime or dolomitic lime are normally added just before the oxygen blow to flux the transfer slag and silicon in the metal. During the oxygen blow, silicon is oxidized before carbon. Calcined lime and dolomitic lime is sometimes added before the end of the blow to cool the bath and to reduce the volume of reduction additions. Slag fluxing additions, such as calcined lime, dolomitic lime and spar, are typically in the range of 3 % to 7 % total bath weight.
It follows that the length of the blow period is determined by the starting carbon and silicon levels of the hot metal charged to the AOD. Decarburization times range from 20 minutes to 35 minutes in modern converters (start carbon 1.5 % to 2.5 % and aim carbon 0.04 %). Normally, the converter is turned down to a horizontal position and a sample of the liquid steel is taken for analyses at a carbon level of around 0.1 %. Some high productivity shops can sample without rotating the converter by using a sub-lance. The steel sample is analyzed for carbon. The temperature of the steel bath is also measured.
The next step is the reduction step, in which the reduction additions are charged and stirred with an inert gas for a desired time. The reduction mix consists of silicon alloys, such as ferro-silicon or chromium-silicon, and / or aluminum, which are added for the reduction of metallic oxides from the slag and fluxing agents such as limestone, calcined lime, dolomitic lime, and fluorspar. The bath is then stirred with inert gas, typically for only five minutes to eight minutes.
The formation of a high basicity slag and the reduction of oxygen potential in the metal bath are good conditions for the removal of sulphur. For example, with a start sulphur of 0.03 %, a reduction treatment of 2 kg to 3 kg aluminum/ton, 2 kg to 3 kg fluorspar/ton, final slag basicity of around 1.7, and temperature of 1,700 deg C, finish sulphur contents of 0.003 % to 0.005 % can be achieved. If the grade to be produced needs an extra low sulphur level, the bath is deslagged after the reduction step and another basic slag is added. The liquid steel and the fluxes are then mixed to complete the de-sulphurization reaction. In modern practices a sulphur level of 0.001 % or less is easily achieved with this double slag practice.
Ideally, at this stage of the process, the chemistry of the liquid steel meets final specifications so that the heat can be tapped. If necessary, additional raw materials are required to be charged for small chemistry adjustments. Depending on the grade, additional deoxidation or alloying additions can be needed. Such final additions are stirred in the AOD converter just before tap or are added in the tap ladle. Following tap, the ladle is frequently stirred for composition homogenization and temperature uniformity along with flotation of inclusions. This is done in a ladle equipped with stirring facilities with or without the use of a ladle furnace. After the ladle treatment, the steel is ready to be cast.
Refining – The quantity of stirring energy from the gas blown through the sub-surface tuyeres and the formation of CO gas deep within the metal bath results in the converter processes being among the most intensely stirred metallurgical reactors. The intimate gas-metal contact and very good slag-metal mixing facilitate refining reactions.
Sulphur removal is a slag-metal reaction which occurs during the reduction phase of the process. Phosphorus, which needs oxidizing conditions, cannot be removed in the converter processing. Nitrogen control is a gas-metal reaction. Depending on final nitrogen specifications for the stainless-steel grade, the inert gas during the initial stages of de-carburization can be nitrogen. After a certain carbon level is achieved, the nitrogen gas is replaced by argon. Such an approach is practiced by all operators for reducing the argon usage and costs and still achieving a desired nitrogen specification. After the change from nitrogen to argon, nitrogen is removed from the bath by both evolved CO gas and argon. Volatile elements with high vapour pressures, such as lead, zinc, and bismuth are removed during the decarburization period.
Process control – The nature and number of reactions makes mathematical modeling and process control of stainless-steel refining more complicated than standard pneumatic steelmaking. Static calculation (i.e., no feedback) methods were used in the early days of stainless processing. These calculations were standardized on worksheets which allowed operators to estimate temperature changes, oxygen to inert gas ratio changes and nitrogen to argon changes. Reduction materials calculations were also performed.
With the introduction of powerful and inexpensive computers, dynamic calculations with automated or manual correction are used. These allow the incorporation of sophisticated models for temperature, blow programme, nitrogen, and alloy addition control. Coupled with automated additions systems, process variability has been much reduced and process efficiency has steadily improved.
Process control computer models (static, dynamic or hybrid) can utilize artificial intelligence to control end-point carbon and temperature levels and can eventually eliminate the need for sub-lances and / or off-gas analyses. At a minimum, these programmes advise the operator of blowing procedures, ferro-alloy and flux addition needs and nitrogen switch-points, as well as provide data logging. Several operators integrate the process control computer model with the gas control system (valve rack) and some with other converter functions (converter rotation, additions, and fume system interfaces, and sampling, etc.). These programmes can be integrated with automated sampling equipment and / or equipment for off-gas analyses. The degree of control and process algorithm sophistication varies.
Post-converter treatments – Post converter treatments after converter processing are similar to treatments for other steel grades. Post-treatment facilities have several benefits such as reduced converter tap-to-tap time, improved quality, and an inventory buffer of liquid steel between the converter and the continuous casting machine. The use of secondary steelmaking furnaces for post converter treatments is increasing, especially in production shops with continuous casting facilities.
Ladle treatment stations normally include an automated additions system, an injection lance, or porous plug for stirring with inert gas, and wire feeding equipment. Some producers use lances to inject lime, fluorspar, and calcium alloys for quality improvements such as reduced inclusion. Ladle additions of deoxidizing compounds are common. Wire feeding is used for adding alloys of titanium, aluminum, calcium, and sulphur for improving ferro-alloy recoveries and quality. Some operators use ladle furnaces for providing additional heat to perform these operations. Carbon pick-up from the electrode arcs can be minimized by close control of slag thickness, arc length, and stirring rates, even when treating ultra-low carbon (ULC) grades. Production shops having vacuum refining units normally use these for the last stage of decarburization on ultra-low carbon and ultra-low nitrogen (ULN) grades or as part of the triplex (EAF/IF-converter-VOD) process.
Latest developments in stainless steel production technology – As regards steelmaking, expensive alloying elements such as nickel, chromium, manganese, and molybdenum etc. are consumed in large quantities in the production of austenitic stainless steel, and for this reason, this type of stainless steel is mainly produced in EAFs from recycled stainless-steel scrap. The thermal capacity of EAFs for stainless steel production, which has been mostly 30 tons to 60 tons in the past, has increased to 90 tons to 160 tons, and several old furnaces have been expanded to increase production capacity. At the same time, supplementary burners and oxygen-blowing facilities have become standard equipment for several furnaces for accelerating scrap melting, as well as aluminum conductor frames for decreasing power consumption. These are specific examples of the efforts for improving productivity and reducing costs in steelmaking processes.
In addition to the EAF-AOD and converter-VOD processes mentioned earlier, a steelmaking process using a smelting-reduction (SR) furnace, a decarburizing (DC) furnace and a VOD furnace, called the SR-DC-VOD process, has been developed and used for commercial production. This process is characterized by the use of chromium ore instead of metallic chromium.
For future development of steelmaking processes, it is necessary to adopt a broad view covering recycling of raw materials and waste. While a carefully elaborated recycling system covering the collection routes of steel scrap has been established for general-purpose austenitic stainless steel, no such system is in place for new stainless steels now under development and those for which applications are being developed, these new grades of stainless-steels include resource-saving type steels such as ferritic, dual-phase stainless steels as well as low-nickel, high-manganese steels commonly known as the AISI 200 series steels.
Since the AISI 200-series steels are not magnetic, like other austenitic stainless steels, no magnetic classification is applicable, and their scrap can easily be mixed with those of common austenitic steels, causing degradation of scrap quality and possibly jeopardizing the stainless-steel waste recycling system. Another important issue needing more emphasis from the view-point of zero emissions is the recycling of slag, scale, dust, and sludge as side products of stainless-steel production processes.
Hot rolling of sheet products – Hot-rolled stainless-steel sheets are produced using two types of rolling mills namely (i) tandem hot strip mills designed for ordinary carbon steel, and Steckel mills exclusively for stainless steel. Although inferior in productivity to a tandem hot strip mill, a Steckel mill needs smaller capital investment and offers wider freedom in rolling operation since the number of passes can be selected as desired because of reversing rolling, and in view of these advantages, several stainless-steel sheet producers opt for this type of mill.
In order to significantly improve the accuracy of strip thickness and crown, some of the latest Steckel mills have the dynamic pair-cross function, whereby the centre-lines of the upper and lower sets of work and back-up rolls are crossed with each other at angles changeable during rolling. The recently developed on-line roll grinder system is effective in preventing local wear of work rolls, which problem is serious especially with Steckel mills.
Cold rolling of sheet products – In 1958, Nisshin Steel Co. Ltd. (formerly called Nippon Teppan) introduced 20-high Sendzimir mills for cold rolling of large-width stainless steel strips for the first time in Japan, resulting in a remarkable improvement in productivity. In consideration of the significant work hardening austenitic stainless steel, mono-block type Sendzimir mills, which use small-diameter work rolls and are capable of applying heavy reduction, were widely used at that time. Then, several Japanese stainless-steel producers introduced 12-high cluster mills of a split-housing type around 1990, for better shape control capacity, easy automatic operation and higher rolling speeds. NSSC (Nippon Steel & Sumikin Stainless Steel Corporation) also constructed this type of cluster mill at Kashima Works in 1992 (rolling speed 1,000 m/min) and another at Hikari Works in 1993 (rolling speed 1,200 m/min).
On the other hand, different improvements have been introduced to Sendzimir mills, the conventional single ‘As-U’ shape control mechanism has been renovated into the double ‘As-U’ mechanism and the flexible shaft backing assembly (FSBA), a split housing design has been introduced. The maximum rolling speed was increased to 1,000 m/min. Fig 7 shows schematic illustration of flatness control system.
Fig 7 Schematic illustration of flatness control system
Lubricant is an important factor in the cold rolling of stainless steel. Presently, neat oils are widely used to get lustrous surfaces characteristic of the product. For getting good luster, it is necessary to maintain thin oil films on the strip surfaces during rolling, but the oil films break locally under heavy rolling loads at high speeds, leading to heat scratching and poor and uneven luster. In view of this, rolling lubricants suitable for high-speed rolling are being developed employing measures such as the addition of long-chain, dibasic dimethyl ester or such like capable of maintaining good lubricating properties at high temperatures. In order to increase rolling speed further, however, better resistance to heat scratching is indispensable, and it is necessary to develop emulsion lubricants with high cooling capacities.
NSSC’s Hikari Works developed a soluble rolling lubricant, and by adequately controlling the size of colloid in the developed emulsion lubricant, successfully produced high-luster stainless steel sheets at high speed under a heavy rolling load for the first time with the rolling speed of 120 m/min.
Since the late 1980s, more emphasis has been placed on the environmental friendliness of cars and higher performance of their engines, and in this context, use of stainless steel for exhaust systems expanded rapidly. Higher workability is now needed of these materials in consideration of the complex product shape, especially of exhaust manifolds etc. In response, parallel with development of new steels, the tandem process, cold rolling of stainless steel using a tandem cold mill for common carbon steel with large-diameter work rolls, has been developed to improve workability and other material properties while reducing costs. This new process spurred rapid expansion of the use of ferritic stainless steels for automotive components.
With respect to the annealing and pickling processes, the annealing temperatures for stainless steel are higher than those for common carbon steel, and hence, a longer descaling section is necessary to remove the scale which forms during annealing. For this reason, in terms of a capacity increase in the annealing and pickling line for hot-rolled strips (hot annealing pickling line, HAP line), in addition to an increase in furnace capacity and improved heating efficiency, high-performance abrasive brushes and longer pickling tanks have been introduced to increase the mechanical and chemical descaling capacities. With respect to the chemical descaling capacity of a HAP line, while the use of sulphuric acid and a mixture of nitric and hydro-fluoric acids for the pickling baths remains unchanged, a highly efficient acid recovery system has been effective in stabilizing the pickling capacity, and new technologies such as turbulence pickling using forced cross flows in pickling tanks and spray pickling are being employed to increase capacity. Some new HAP lines, not restricted by space limitations, have a processing capacity of as much as one million tons per year or so.
Higher efficiency is pursued with respect also to finishing annealing and pickling (FAP) lines. The increase in the processing speed of FAP lines has been made possible by advances in heating control and descaling technologies. In the field of heating control, using newly developed quick-response impinging burners, NSSC’s Kashima Works commercially applied the direct flame heating method to commercial production.
It is necessary for a FAP line to de-scale steel strips without damaging the surfaces, and for this reason, it is necessary to change the oxide scale which has formed on the annealing furnace to a water-soluble component. Conventionally, this has been mainly done through a salt bath, but as the line speed increased, the Ruthner process, which afforded easier operation, became more popular. It has to be noted, however, that the Ruthner process cannot reform silicon oxide, and consequently, descaling performance is severely diminished when processing strips with special chemical compositions such as high-silicon stainless steels. For resolving this problem, NSSC built a Ruthner section at the exit from the salt bath of its FAP line at Kashima works, and as a result, a line speed of 70 m/min has been maintained without being restricted by steel chemistry. However, to respond to the need for higher descaling speeds and processing of a wider variety of steel grades, more efficient descaling processes are sought.
Another technical trend worth mentioning is integration of process steps for cold-rolled sheets, which is seen with major stainless-steel producers. There are different types of process integration. In the first type, to improve the efficiency and yield of cold rolling, several cold rolling mill-stands with small-diameter work rolls are combined into a tandem mill train. In the second type, a skin pass mill and a tension leveler are incorporated in an FAP line.
In the third type, annealing and cold rolling processes are integrated to form a continuous processing line for high-efficiency production of stainless steels for general applications. This kind of integration is sub-divided into two types namely (i) integration of up-stream processes from HAP to cold rolling, and (ii) that of down-stream processes from cold rolling through annealing, pickling, skin pass rolling to tension leveling. Fig 8 schematically shows the integrated rolling, annealing, and pickling (RAP) line, where cold rolling on a 3-stand tandem mill, annealing, pickling, skin pass rolling, and leveling are integrated into one continuous line.
Fig 8 Schematic configuration of integrated RAP line
Several of the latest skin pass mills (SPMs) are integrated in the delivery sections of high-speed FAP lines. With either an in-line or off-line skin pass mill, dry rolling on a 2-high mill with large-diameter work rolls has been the main-stream practice to get lustrous surfaces characteristic of stainless steel. For off-line skin pass rolling, the latest mill design trend is reversing rolling on a convertible 2 high / 4-high mill and wet / dry rolling. Because of its higher flatness control ability, 6-high universal crown control mill (UC-mill) is the preferred skin-pass mill solution for meeting the continually increasing demands placed on strip flatness, elongation, and roughness, especially for hard-material grades. This mill has a very good flatness control function.
The development of production technologies for cold-rolled stainless-steel sheets has mainly focused on mass production of steels for general applications at the lowest cost possible. However, the market also typically began to demand highly functional stainless steels such as high-purity ferritic and dual-phase steels, each in comparatively small quantities. Hence, technical development, which has mainly focused on improving the productivity of commercial quality steels, has to place more emphasis on efficient production of technically demanding steels in small lots.
Plate rolling – Recent developments in the field of stainless-steel plate production have included the development of new products and improvement of plant operation technologies mainly related to the production of the developed products, with such new products including high nitrogen content AISI grade 316L steel for chemical tanker applications and super stainless steel for use in highly corrosive environments. Production technologies which have demonstrated considerable advances in recent years include, specifically, improvement of hot workability through steel chemistry design and content control of individual alloying elements, a reduction in tramp elements, and slab soaking etc. and the stable production of high-strength materials through the thermo-mechanical control process (TMCP).
Alloy elements indispensable for stainless steel production, such as chromium, nickel, and molybdenum are expected to be in short supply in the future. In consideration of this, one of the most important technical trends in the field of stainless-steel plate production is switching to new low-cost and high-relative-strength materials which can sustain the present performance of the product. Expanding the applications of dual-phase steels is a typical example of such a technical trend, and actually, there is a specific move towards improving the descaling capacities with a view to increasing the production of dual-phase steels.
In the consumer market for stainless steel plates, the size of plant facilities and transport vessels made of stainless steel is increasing, and hence, the need for larger-sized plates to decrease the fabrication costs for welding and other work is becoming ever stronger. In stainless steel plate production, further technical development is expected in the size increase of production equipment, capacity increase for high-strength products, development of production technology for technically demanding new materials, namely those more difficult to work, pickle etc. or needing stricter dimensional tolerance or higher surface quality.
Wire rod rolling – After shipment from a steel works, wire rods undergo different secondary working steps such as drawing, forging, cutting, and heat treatment etc. at the downstream processing plants to become final products. The hot rolling processes of stainless-steel wire rods at a steel plant used to be as breakdown rolling of blooms into billets, roughing, intermediate, and finishing rolling of billets into wire rods, and off-line solution heat treatment. Over the last few years, advanced technologies such as direct rolling from blooms into wire rods and in-line heat treatment have been included in commercial production practice.
Some of the stainless-steel producers have introduce helical rolling mills (HRMs) capable of applying a heavy reduction in one pass to the roughing mill train of its wire rod mill line. On the other hand, an in-line heat treatment process has been developed by combining it with the HRMs. This has brought the direct rolling and in-line heat treatment into commercial operation. Fig 9a schematically shows an HRM. Three conical rolls are arranged helically, and each of the rolls turns to reduce the rolled material while revolving around it, keeping its position relative to the others. This configuration enables application of a heavy reduction per pass and a heavy strain to the surface layer of the rolled material. The heavy working of the surface layer accelerates recrystallization of the texture and hence increases the ductility of the material at subsequent rolling passes, which is effective in preventing cracking and wrinkles because of the coarse crystal grains.
Fig 9 Schematics of HRM and 3-roll stand used in RSB
The in-line heat treatment, also known as direct solid solution treatment (DST), is a technology to apply controlled cooling to hot-rolled wire rods which takes advantage of its sensible heat. It replaces solution heat treatment, and hence, is effective for raising production efficiency and saving energy. In the later stages of wire rod rolling, the latest technical trend is to provide a high-rigidity, high-functionality block mill before or after a finishing mill train. The reducing sizing block (RSB) mill is shown in Fig 9b. This mill is effective for better product size accuracy, size-free rolling, and controlled rolling and cooling for material quality control. High-rigidity, high-functionality block mills have been introduced to the several stainless-steel wire rod mills.
The 3-roll, 4-stand RSB is a precision rolling mill characterized by less flattening of material during rolling than by mills with a two-roll configuration, and is capable of realizing an ovality (difference between diameters measured in different directions) of 0.15 mm or less. The four stands are driven individually, and the roll screw-down can also be controlled individually, enabling size-free rolling, whereby wire rods of different sizes can be rolled without having to change rolls. When it is necessary to change the rolls because of roll wear or a large change in product size, the down time is minimal because of the quick changing with pre-assembled spare stands, which has proved effective in considerably improving the productivity.
Production technology of stainless steel has developed for enabling production of stainless-steel products of better properties at lower costs in response to growing demand. While the demand for stainless-steel products and their applications are expected to expand yet more in the long run, with new steel materials having specific functionality, such as high-purity ferritic and dual-phase steels, accounting for a good part of that expansion. This means that a wider variety of products have to be produced in smaller lots, and technically demanding products are going to account for a larger proportion of all production.
Hence, besides aiming for efficient mass production of products for general applications as in the past, the technical development of the stainless-steel production is to focus on efficient production of technically demanding highly functional products in small lots. Another important aspect is the development of recycling technologies for stainless steel scrap and waste arising from the stainless steel industry itself. This is going to be fundamental for maintaining and strengthening the material’s advantage of excellent recyclability.
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