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Argon Oxygen Decarburization Process

• satyendra
• April 28, 2014
• 36 Comments
• AOD converter, CO dilution, decarburization, degassing, Side blown, stainless steel, tuyeres,

Argon Oxygen Decarburization Process

Argon oxygen decarburization (AOD) is a process mainly used in production of stainless steel and other high-grade alloys such as silicon (Si) steels, tool steels, nickel (Ni) base alloys and cobalt (Co) base alloys with oxidizable elements such as chromium (Cr) and aluminum (Al). Today, over 75 % of the world’s stainless steel is made using the AOD process. Metallurgical converters such as the AOD converter normally utilize gas blowing for the mixing and refinement of the liquid steel.

AOD provides an economical way to produce stainless steel with a minimal loss of precious elements. It is part of a duplex process in which scrap or virgin raw materials are first melted in the electric arc furnace (EAF) or induction furnace (IF). The electric arc furnace / induction furnace – AOD duplex process consists of melting scrap and raw materials in an electric arc furnace or induction furnace and thereafter decarburization and secondary refining of the melt in the AOD converter. This allows using a broad range of materials such as inexpensive high-carbon (C) ferro-chromium (Fe-Cr) and green scrap used to increase the chromium content, both in the furnace and the converter.

The key feature in the AOD converter is that oxygen (O2) for decarburization is mixed with argon (Ar) or nitrogen (N2) inert gases and injected through the submerged tuyeres. This argon dilution minimizes unwanted oxidation of precious elements contained in specialty steels, such as chromium.

The AOD converter is with side-wall tuyeres and top blowing lance. A mixture of argon-oxygen is injected into steel through several side-wall tuyeres located near the bottom of the converter. Simultaneously, pure oxygen (or mixed with inert gas) is blown into the bath through a top lance to maximize oxygen delivery into the steel. The molten metal in the AOD converter is stirred strongly and encounters circulatory motions because of bottom gas injection. Hence, melt temperature and composition is homogeneous and the rate of reactions is improved.

The AOD process is a crucial refining method in modern stainless-steel production. It has been widely used to remove carbon in the past few decades. The AOD converter can provide excellent mixing conditions through turbulent stirring using submerged tuyeres. In the AOD process, the flow characteristics in the bath have a considerable influence on the mass transport, momentum exchange, and heat transfer, which are closely linked with the gas-metal reaction kinetics and the refining efficiency. A deep understanding of jet behaviour, bubble flow characteristics, and mixing efficiency facilitates further optimization of the decarburization and desulphurization operations. This increases the AOD productivity and lower its energy and material consumption as well as the manufacturing cost.

In order to reduce the cost, argon can be replaced by nitrogen for the early stages of the process. However, using argon is necessary during the last stage of decarburization and reduction stage to eliminate the pickup of nitrogen. However, for some grades of stainless steels, argon is replaced with nitrogen to improve the mechanical properties. The molten metal is decarburized and refined in the AOD converter to less than 0.05 % carbon. Fig 1 shows the electric arc furnace / induction furnace – AOD duplex process line for stainless steelmaking.

Fig 1 Electric arc furnace/induction furnace-AOD duplex process line for stainless steelmaking

The AOD process was invented in 1954 by the Linde Division of The Union Carbide Corporation (which became known as Praxair in 1992). The process is commercialized in the early 1970s and since then, it has been the most efficient and reliable method for the refining of stainless and specialty steels. From then on, this kind of refining equipment rapidly substituted the position of the EAF in the field of stainless steelmaking because of its higher efficiency on the decarburization and its simple parts. The fundamental idea of the AOD to make efficient decarburization is diluting the carbon mono oxide (CO) with the inert gas (argon or nitrogen) which is the outcome of the decarburization chemical reaction in the steel to decrease the partial pressure of the carbon mono oxide. By this method, the carbon in the stainless steel can be removed efficiently without excessive chromium oxidation.

Today, the AOD process remains a prominent method in the stainless-steel industry. It offers steelmakers higher flexibility in raw material selection, enabling the use of cost-effective inputs and ensuring accurate and consistent results. The process has also contributed to increased production capacity with relatively small capital investments compared to conventional electric furnace methods.

AOD process is so popular since it combines higher metallic yields with lower material costs. Other benefits include pinpoint accuracy in chemistry control down to 0.01 % carbon and lower, rapid desulphurization coupled to less than 0.001 %, and lead (Pb) removal to less than 0.001 %. When the end result is a cleaner metal with increased productivity, it is no wonder why AOD is at the heart of the stainless-steel production industry.

AOD converter (Fig 2) 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. After initial melting the metal is then transferred to an AOD converter where it 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 blow from lance and bottom tuyeres. At the reduction stage, only argon is blown from bottom tuyeres to remove extra nitrogen and reduce 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 exhaust hood.

Fig 2 Argon oxygen decarburization converter

The AOD process is the first production process for stainless steel grades in which the treatment time in a converter under atmospheric conditions corresponds to the casting time of a slab caster for several steel grades. In this way, a necessary prerequisite for the economically efficient production of stainless, acid, and heat-resistant steel grades are fulfilled.

Fundamentals of AOD process – The computational fluid dynamics (CFD) technique has been proven effective in solving the complex flow field in AOD converters, and a series of CFD-based models have been developed. In one of the studies, a 3D single tuyere AOD model has been developed, in which a separate AOD tuyere model verified by ‘laser doppler anemometer’ (LDA) measurements has been used for describing the pressure inlet boundary conditions. In the later studies, this model has been extended to simulate a six tuyeres AOD converter, and the fluid slag phase has also been included. The model has successfully predicted the fluid flow, turbulence, bubble behaviour, and liquid steel-slag dispersion.

Another study has carried out a sequence of investigations on the combined side and top blowing AOD process through numerical simulations. The influence of the tuyere angle, side-tuyere number, and gas flow rate on the fluid characteristics and mixing efficiency have been reported. The tuyere number and the tuyere angle have little effect on the overall mixing characteristics but certainly causes local variations in the mixing efficiency. This study has also shown that the top lance does not change the necessary features of mixing created by the side blowing but considerably increases the turbulent kinetic energy and changes the local flow pattern.

AOD refining is based upon (i) carbon mono oxide dilution, (ii) mixing, and (iii) degassing. Each of these AOD phenomena and a reaction mechanism are discussed below.

Carbon mono oxide dilution – AOD process uses dilution technique for the decarburization of steel bath. The injection of inert gas (argon or nitrogen) lowers the partial pressure of carbon mono oxide in the bath, hence allowing higher chromium content to be in equilibrium with lower carbon contents. The quantity of stirring energy from the gas blown through the sub-surface tuyeres and the formation of the carbon mono oxide deep within the metal bath results in the converter being among the most intensely stirred metallurgical reactor. The intimate gas – metal contact and excellent slag – metal mixing facilitate refining reactions.

During historical stainless-steel decarburization by conventional oxygen blowing, the atmosphere in equilibrium with the melt is necessarily pure carbon mono oxide at one atmosphere (0.1 MPa) pressure. Under these conditions, the quantity of chromium existing in the melt is limited by equilibrium with a given carbon content at a given temperature. The curves shown in Fig 3 show these equilibrium effects. Any excess of chromium is rapidly oxidized. As the carbon content of the melt decreases, the quantity of chromium also decreases. Raising the melt temperature raises the equilibrium chromium concentration, but temperatures are soon reached which are uneconomically high in terms of refractory life.

Fig 3 Carbon-chromium equilibrium curves

Although equilibrium relationships provide an adequate picture of the direction in which a process moves, they do not indicate the actual path or rate of the reactions. It is to be remembered that gaseous di-atomic oxygen first dissolves as mono-atomic oxygen in the molten metal. Equations 2 and 3 break down equation 1 into the two reactions believed to be controlling the over-all equilibrium. Equation 1 is ‘1/4 Cr3O4 (s) + C = 3/4 Cr + CO (g)’. Equation 2 is ‘1/4 Cr3O4 (s) = 3/4 Cr + O’ and equation 3 is ‘C + O = CO (g)’. As written, these reactions show that oxygen is absorbed by the melt before it reacts with carbon and subsequently leaves the system as carbon mono oxide. Top blown oxygen-bottom blown argon tests have shown that this reaction sequence does, in fact, occur.

The results of laboratory induction furnace tests using several dilution ratios, it has been found that for a given dilution ratio, there are distinct break points in the decarburization reaction where the oxygen efficiency for carbon removal drops rapidly and more chromium and other metallic oxidation occur. These break points are related to equilibrium carbon mono oxide pressures at the test composition and temperature, and it has been found that this calculated pressure cannot be predicted by assuming 100 % conversion of the input oxygen to carbon. With a mixture of oxygen and argon in the proportions of 1:2, for example, the break point does not occur at a calculated pressure of 0.05 MPa.

However, it has been found that for any given gas mixture the carbon level at which this decrease in decarburization rate occurs is reproducible and predictable. Consideration of these results has led to the development of the programmed blow. This involves increasing the oxygen dilution during the decarburization so that continually lower carbon levels can be achieved without excessive chromium oxidation. The consequences of this improvement are that the optimum proportion of argon can be determined and used for the instantaneous carbon and chromium content. This needs continuously changing oxygen / argon proportions for minimum argon usage. It has been found that this can be approximated by two or more stepwise changes in gas ratios.

In summary, decarburization of stainless-steel melts containing specification level is of chromium can be accomplished. Through variation of the diluent gas volume, excessive oxidation of chromium and attendant bath temperature increase can be controlled. The need for low-carbon ferro-chromium in stainless steel production is hence eliminated.

Mixing – The submerged injection of gases into molten metals is an intrinsic aspect of the majority of the metal refining operations. With the increase in the number of ladle steelmaking operations, the theoretical and industrial aspects of submerged gas injection have been considered at some length at a number of studies. Gases are injected into the AOD converter through tuyeres located in the lower side wall. There have been several studies on the hydro-dynamic phenomena associated with submerged gas jets in liquid metals, normally these studies have used aqueous systems as a means of predicting jet and bubble behaviour in molten metals.

Investigations have shown that there is a large difference in the physical behaviour of bubbles and jets between gases in water and liquid metals. The bubbles which form on the tip of the tuyere in liquid metal systems are larger than those formed in aqueous systems. This is mainly because of the non-wettability of the tuyeres in liquid metal systems. Jet penetration in liquid metals is much less than the jet penetration of gases in water.

Gases injected into molten metals are believed to form very large spherical-capped bubbles because of the high surface tension of liquid metals. One of the studies has calculated the minimum theoretical volume of a bubble forming on a 3 mm orifice in water, iron, steel, aluminum, and copper. These calculated bubble volumes are shown in Tab 1. It is clearly obvious that the minimum bubble size forming at an orifice is much larger in molten metal systems than aqueous systems.

At low flows, the sum of bubble volumes remains constant. At sufficiently higher flows, surface tension is less important and a constant bubble frequency regime is achieved, where the rate of bubble formation in iron and steel remains steady at ten per second.

Stirring is known to improve the kinetics of alloy deoxidation, permitting equilibrium conditions to be more closely approached. Extremely low oxide contents are achievable if the oxides formed can escape prior to solidification and if reoxidation is prevented during casting.

Degassing – The mechanism of argon degassing is quite similar to that of vacuum degassing. There are several inherent differences with vacuum degassing, particularly with respect to carbon deoxidation. The removal of dissolved gases from steel by argon results from (i) the effect of argon bubbles in providing nucleation sites for reactions which transfer the dissolved gases from the liquid steel to the gas phase, (ii) the improved reaction kinetics resulting from stirring, and (iii) the driving force for these reactions provided by the low partial pressure of carbon mono oxide, nitrogen, and hydrogen (H2) in the argon bubbles.

During the oxygen blow of AOD refining, evolving carbon mono oxide supplements the argon to assist in degassing. The carbon mono oxide and argon bubbles absorb hydrogen and nitrogen, (as well as carbon mono oxide up to saturation) as they rise through the bath. Argon injection causes considerable stirring of the bath, hence improving the kinetics of the degassing reaction by (i) decreasing the effect of diffusion boundary layers around the reaction sites, (ii) transporting the reaction sites to the reactants, and (iii) transporting the saturated bubbles away from the newly injected reaction sites.

Degassing in AOD is important not only for the production of low gas content (especially low hydrogen content) low alloy steels but also to control the nitrogen content in stainless steels. Nitrogen dissolves in steel with harmful effects in several cases. Hence, it is frequently necessary to purge a portion of the dissolved nitrogen by blowing argon through the bath during the latter portion of decarburization and subsequent refining. By switching from nitrogen to argon at some point during the decarburization blow and continuing to use argon during reduction and any subsequent steps, purging of nitrogen by argon and carbon mono oxide is accomplished. In order to finish at a specific nitrogen level, the switch-over point from nitrogen to argon is required to be determined with reasonable accuracy.

In certain grades, on the other hand, nitrogen is desired as a strengthening or austenitizing agent. Production of these steels previously needed the addition of expensive nitrogen-bearing ferro-alloys. However, these grades can now be made by alloying with gaseous nitrogen at the end of the heat.

The first commercial use of nitrogen as a replacement for argon in AOD refining was reported in 1972. While it is reasonable to use the solubility of nitrogen in pure liquid iron for the majority of the plain carbon steels, it is clearly unacceptable in considering stainless grades. The effect of deoxidizing and alloying elements on the activity of carbon, oxygen, nitrogen, and hydrogen has been reported. For example, the presence of alloying elements such as chromium and vanadium (V) hinder carbon deoxidation and increased nitrogen solubility while the presence of nickel aids the carbon mono oxide reaction and decreases nitrogen solubility.

Elements such as titanium (Ti) and vanadium increase the solubility of nitrogen in steel, hence making its removal more difficult. The formation of stable oxides and / or nitrides hinder the removal of carbon mono oxide or hydrogen. For grade 304 stainless steel, the solubility of nitrogen after the AOD reduction step is 0.23 %.

The solubility of nitrogen in all steels is known to follow Sieverts law given by equation 4 which is ‘%N = constant x under root (P N2)’. This relationship is evident from the reaction given by equation 5 and equation 6 and its equilibrium constant. Equation 5 is ‘1/2 N2 (g) = N’, and equation 6 is K1 = aN / (PN2)to the power 0.5 = (fN x %N) / (PN2)to the power 0.5’. Where ‘N’ represents nitrogen dissolved in the steel, a is the activity of dissolved nitrogen ‘N’, ‘f’ is the activity coefficient of dissolved nitrogen ‘N’, ‘%N’ is the weight percent of dissolved nitrogen, ‘K1’ is the equilibrium constant of reaction at equation 5’, and ‘PN2’ is the partial pressure of nitrogen in atmosphere (0.1 MPa).

For the AOD, ‘PN2’ of the exit gas is of interest and is given by equation 6 which is ‘PN2 = [QN2 / (QN2 + QAr+ QCO)] x Psystem’, where ‘QN2’ and ‘QAr’ is the volumetric flow rate of the gas injected and ‘QCO’ is the volumetric flow rate of carbon mono oxide generated. Equation 5 clearly reduces to Sieverts law since ‘fN’ is a constant for a specific bath chemistry and temperature. For the specific case of nitrogen dissolving in pure liquid iron (the binary Fe-N system), the ‘constant’ of equation 4 is ‘K1’ (implicit in this statement is that ‘fN’ is 1 for Fe-N alloys).

In pure liquid iron, the Gibb’s free energy for equation 5 is ‘delta G1 degree = 860 + 5.71T (cal.g-atom) +/- 100 cal.’ (equation 7). Knowing that ‘delta G degree = -RT in K (kelvin)’ at equilibrium, an expression can be derived for the logarithm of ‘K1’ in the pure liquid iron which is log K1 = -188/T – 1.25’. (equation 8). At 1,600 deg C (1,873 deg K), the solubility of nitrogen in pure liquid iron is 0.045 %. In stainless steels, the activity coefficient of nitrogen is not unity, hence thus in order to use equation 5, it is necessary to calculate ‘fN’.

The effect of alloying elements on the solubility of nitrogen in liquid iron alloys is shown in Fig 4. It can be noted that the common intercept is the solubility of nitrogen in pure iron as discussed above. A negative interaction coefficient indicates that the alloy element ‘j’ increases the solubility (or decreases the activity) of nitrogen. Conversely, a positive interaction coefficient indicates that the alloy element ‘j’ decreases the solubility (or increases the activity) of nitrogen.

Fig 4 Solubility of nitrogen in alloy melts

Each curve for a specific element, ‘j’, on the diagram defines an activity coefficient of nitrogen in the ternary alloy ‘Fe-N-j’. From this activity coefficient, one can determine the interaction coefficient, ‘ejN’, which describes the effect of the alloying element ‘j’ on the behaviour of ‘N’ in the steel.

These effects can be combined for a complex alloy such as stainless steel. The combined effects of the various alloy elements in most stainless steels can be summarized by the equation 9 which is ‘log %Neq = – 188/T – 1.25 – [(3280/T-0.75) x (0.13 x %C + 0.047 x %Si + 0.0 x %Ni – 0.011 x %Mo – 0.023 x %Mn – 0.047 x %Cr)] – [(+0.00017) x %Cr square]’.

Using equation 9, one can calculate the solubility of nitrogen at 1 atmosphere (0.1 MPa). Based on this equation, three generalizations can be made. These are (i) chromium increases the nitrogen solubility, (ii) increased temperature decreases the nitrogen solubility (it is to be noted that the opposite is true for pure iron or some plain carbon steels), and (iii) carbon decreases the nitrogen solubility.

AOD converter – AOD converter is a pear-shaped vessel normally lined with basic refractory lining. 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. As molten stainless steels do not generate foam, and 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 (basic oxygen furnace) 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.

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 mouth. Converters which tap into a ladle car normally have a BOF type concentric cone top section.

A high production shop typically has three interchangeable converters for 100 % 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 preheated converter in less than an hour.

The AOD converter has 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 and 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.

A major modification of the AOD process involves the use of top blowing lance in addition to the side blowing tuyeres. The lance can be used to inject oxygen at desired blow rates to increase the decarburization and / or post combustion. The top lance can also be designed for blowing mixed gases such as inert gas – oxygen mixtures. The installation of a lance and introduction of oxygen in the early stages of decarburization can reduce the time for a heat. The technology can be used to increase the productivity (tons / hour) of the steel melting shop. Majority of the recent converter installations include the use of a top lance for blowing oxygen.

Another modification of the AOD process involves applying vacuum on the converter to reduce the consumption of argon and silicon as well as the process time when making low carbon grades. The modification is known as AOD-VCR. Fig 5 shows a AOD-VCR converter.

Fig 5 AOD-VCR converter

AOD converter refractories – High temperatures at the tuyere tip and high bath agitation place high demands on the refractories of the AOD converter.  While typical BOF refractory campaigns are months or years long, stainless 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, magnesite-chromite, and dolomite. The choice of refractory is dependent on the vessel operation pattern, final product specifications, and economics. Magnesite chromite refractories have high wear resistance but have a higher unit cost than dolomitic refractories. Chromium pick-up from the brick is possible. Magnesite chromite bricks are simultaneously acidic and basic and strict slag compositions are to be maintained to prevent rapid wear.

Dolomitic 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 dolomitic refractories since very basic slags can be used without detrimental effects on the bricks.

Converters are typically zoned by thickness and brick quality to maximize lining life and minimize costs. High wear areas of the converter, normally the tuyere wall, slag line, and transfer pad are zoned thicker and with higher quality refractory than other parts of the converter.

AOD process – Stainless production in the AOD converter can be broken into three phases namely (i) decarburization which consists of reduction of carbon level to specification level , control of bath temperature, and additions to adjust heat weight, bath and slag composition, and control temperature, (ii) reduction / desulphurization which consists of recovering virtually all oxidized metallics, degassing, and sulphur control to any level from 0.001 % to 0.02 % with a single or double slag practice as appropriate, and (iii) trimming which consists of minor adjustments to chemistry and temperature.

The input of the AOD process is the output of the electric arc furnace or induction furnace process. The liquid steel, which contains majority of the chromium and nickel needed to meet the final heat composition, is tapped at a temperature of 1,500 deg C to 1,600 deg C from the electric arc furnace or induction furnace into a transfer ladle. The liquid metal is transferred from transfer ladle to AOD converter. The AOD converter can be rotated downwards so that the side mounted tuyeres are above the bath level during charging of the liquid steel.

After the transfer of liquid steel containing iron, chromium, carbon, and nickel from electric arc furnace or induction furnace to the AOD converter, high carbon ferro-chrome is added and the blow is started with the blowing of inert gas (argon, nitrogen) and oxygen mixture. In the initial stage, oxygen to argon in the ratio ranging from 5:1 to 3:1 is blown through the side tuyeres. The ratio is lowered with the progress of the decarburization. Since the blowing is done along with argon, it is possible to carry out the decarburization at a lower temperature. When carbon reduces to 30 % of the original value, the ratio of oxygen to argon is changed to 2:1. The major benefit associated with the dilution process comes into play when the oxygen to inert gas ratio is 1:1. Oxidation of carbon continues, but oxidation of chromium is limited. This is because of the very low oxygen potential of the gas mixture, which minimizes chromium oxidation. The blow is continued to achieve 0.09 % C to 0.012 % C.

Process gases are injected through submerged tuyeres which are installed in the side wall or bottom of the converter. Side wall injection normally imparts maximum stirring energy to the bath for highest efficiency of mixing. Bottom injection normally improves wear characteristics in the barrel section of the converter. The number and relative positioning of tuyeres is determined in part by converter size, range of heat sizes, process gas flow rates, and types of alloys refined.

The gas control system supplies the process gases at nominal rates of 1 normal cubic meter per minute per ton (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.

Decarburization is a common step for carbon reduction in the steelmaking practice. Decarburization involves oxygen injection to reduce the carbon content in the steel melt and is crucial for secondary steelmaking processes such as the AOD process. In general, decarburization is similar to other secondary steelmaking processes such as the basic oxygen furnace (BOF). However, in the AOD process, it is also a requirement to maintain the stainless properties acquired by elements such as chromium. Hence, the difficulty is to oxidize the excess carbon without oxidation of chromium described by the main reactions in the AOD. The reactions are ‘2[C] + {O2} = 2{CO}’ (equation 10), and ‘4[Cr] + 3O2 = 2(Cr2O3)’ (equation 11). These reactions combine to ‘3[C] + (Cr2O3) = 2[Cr] + 3{CO}’ (equation 11), where {} is the gaseous form, [] is dissolved in liquid, and () is in the slag.

Fruehan studied the reaction sequence and mechanisms for oxidation of chromium and carbon and found that chromium oxidation is considerably faster than carbon oxidation. The study states the possibility that the main reaction at the nozzle region is the oxidation of chromium at the bubble interface. Further, as the bubble rises in the bath, chromium oxide (Cr2O3) is reduced by carbon. In this way, there are local reactions taking place at one part of the process which further up in the bath reacts with other products because of the species transport and implies that total equilibrium exists in the AOD converter. The combined reaction above states that a lower partial pressure of carbon monoxide benefits decarburization. The partial pressure of carbon mono oxide is partly governed by the ferro-static pressure, which implies that decarburization is more effective at the surface because of the lower ferro-static pressure. On the contrary, another hypothesis is that local equilibrium exists in the AOD instead of total equilibrium. For this hypothesis, majority of the decarburization occurs at the nozzle region which is affected by the ferro-static pressure imposed by the bath height above the nozzle. Hence, decreasing the height between the nozzle and the bath surface can introduce more efficient decarburization.

Decarburization occurs when dissolved carbon reduces the chromium and iron oxides which form. Decarburization occurs on the surface of rising bubbles that form from either the inert gas that is injected or on the surface of chromium oxide particles that are being reduced and generating carbon mono oxide.

During decarburization, additions are made for achieving the proper final chemical composition. These additions normally consist of desired quantities of high carbon ferrochromium, stainless steel scrap, carbon steel scrap, nickel, iron, high carbon ferro-manganese (Fe-Mn), and molybdenum oxide. These additions also serve to reduce the bath temperature as carbon and chromium oxidations are exothermic. In general, the bath temperature is controlled to less than 1,720 deg C. Total weight of alloy addition is in the range of 5 % to 30 % of the tap weight. During the final stage of blowing, the ratio of oxygen to argon is changed to 1:3 to 1:2 for bringing carbon to the desired value which can be less than 0.03 %.

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 lime, dolomitic lime, and fluorspar. The bath is then stirred with inert gas, typically for around five to eight minutes. The reduction reaction is Cr3O4 + 2Si = 3Cr + 2 SiO2. Additional silicon addition is needed if requirement of silicon is there to meet the silicon specification of some of the stainless steels.

Careful manipulation of slag, as it precipitates in the reaction, is important. Any chromium oxide not reduced by carbon ends up in the slag, which can form a complex spinel. The effectiveness of reduction step is dependent on several factors including slag basicity and composition, temperature, mixing conditions in the converter and solid addition dissolution kinetics.

Lime and / 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. Lime and dolomitic lime are 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 lime, dolomitic lime and spar, are typically in the range of 3 % to 7 % of total bath weight.

Additions of lime are made to dilute the sulphur in the liquid steel bath. The formation of a high basicity slag and the reduction of oxygen potential in the metal bath are good conditions for sulphur removal. For example, with starting sulphur of 0.03 %, and with a reduction treatment of 2 kg to 3 kg aluminum / ton, 2 kg to 3 kg fluor spar / 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. Also, aluminum or silicon can be added to remove oxygen. If the grade to be produced requires 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 desulphurization reaction. In modern practices, a sulphur level of 0.001 % or less is easily achieved with this double slag practice. Other trimming alloy additions can be added at the end of the step. After sulphur levels have been achieved the slag is removed from the AOD vessel and the metal bath is ready for tapping.

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 AOD converter processing.

Nitrogen control is a gas – metal reaction. Depending on final nitrogen specification for the stainless-steel grade, the inert gas during the initial stages of decarburization can be nitrogen. After a certain carbon level is achieved, the nitrogen gas is replaced by argon. Such an approach is normally practiced by steelmakers to reduce argon usage and costs and still achieve a desired nitrogen specification. After the change from nitrogen to argon, nitrogen is removed from the bath both by evolved carbon mono oxide and argon. Volatile elements with high vapour pressures, such as lead, zinc, and bismuth, are removed during the decarburization period.

The formation of high basic slag and the reduction of oxygen potential in the liquid steel bath are good conditions for sulphur removal. These are achieved by having a high lime concentration in the slag and a low oxygen activity in the metal bath. The transfer of sulphur to slag takes place as per the reaction ‘S(bath) + CaO(slag) = CaS (slag) + O(bath)’. (equation 12)

The length of the blow period is determined by the starting carbon and silicon levels of the hot metal charged to the AOD converter. Decarburization time ranges from 20 minutes to 35 minutes in modern converters (starting carbon from 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 %.

Ideally at this stage of the process, the chemistry of the liquid steel meets the final specifications so that the heat can be tapped. If necessary, additional raw materials may be charged for small chemistry adjustments before tapping. After tapping, the ladle is frequently stirred for composition homogenization and temperature uniformity along with flotation of the 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. In the early days of the AOD process, the converter was tilted for raw material additions as well as for taking samples and for measurement of temperature using immersion thermocouples. The desire to increase the productivity has led to continuous charging of raw materials during the blow period as well as reduction period. Modern 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.

Blowing in the AOD converter – The AOD process uses the dependence of the chromium – carbon equilibrium on the partial pressure by breaking the process down into different phases and progressively adding more inert gas to the process gas as the carbon content decreases. This process reduces the partial pressure of carbon mono oxide so that the decarburization toward low carbon contents is promoted, while at the same time the oxidation of chromium is limited. However, for kinetic reasons, the oxidation of chromium cannot be fully avoided. For reducing the loss of chromium, the input of oxygen at low-carbon contents of less than, for example, 0.4 % is, depending on the chromium content of the melt, increasingly reduced since the achievable decarburization rate here is a function of the melt bath carbon-content proper and, moreover, chromium oxidation increases because of the thermo-dynamic equilibrium situation.

Fig 6 shows the typical blowing stages of a 120-ton AOD converter with seven side-wall nozzles. Down to a carbon content of around 0.4 %, a total volume of 240 cubic meters per minute (cum/min) of oxygen at standard temperature and pressure (STP) is blown for refining, with the larger proportion of oxygen blown through the single hole top lance. The decarburization rate is proportional to the oxygen supplied. The maximum quantity of oxygen is limited by the steel grade, the starting conditions, the oxygen supply, the waste gas exhausted, and the slag slopping. At this stage, the low quantity of inert gas through the side-wall nozzles serves for cooling. In conformity with the laws of kinetics and thermo-dynamics at decreasing carbon contents, the quantity of oxygen is reduced in stages, and at the same time, the process gas is enriched with inert gas (dynamic blow). The aspect ratio of oxygen and inert gas drops from the original 8:1 through 1:1 and 1:2 down to 1:3.3, with the total quantity of process gas remaining almost constant during the dynamic stages. At the end of decarburization period, the carbon content is roughly around 0.02 % for a lot of stainless-steel grades.

Fig 6 Blowing pattern of the 120-ton AOD converter with characteristic vibration intensity levels

Decarburization is followed by a pure inert gas stirring for reduction. During the reduction stage, the oxidized metals, chromium, above all, are reduced mainly by the addition of silicon carriers. In this process, the initial Cr2O3-containing refining slag changes from a solid to a liquid state. Reduction and subsequent deslagging are followed by a separate desulphurization using a two-slag method. The basicity of the slag and, hence, the sulphur capacity is increased by adding lime and fluxing elements, with which simultaneous inert gas stirring causes an almost complete transition of sulphur from the melt into the slag. The last inert gas stirring stage (alloying) provides an opportunity for analysis trimming of the melt before tapping.

The AOD converter is a metallurgical reactor which offers excellent mixing conditions because of the high melt turbulence caused by the injection of large quantities of process gas through the side-wall nozzles. However, the AOD process is accompanied by intense converter vibrations, which from the outside can be recognized by more or less staggering movements of the vessel around the axis of rotation. Vessel vibrations act as torques on the structural components and have to be taken into account during the design phase, especially with regard to the dimensioning of the torque support arm.

Fig 6 also shows the vibration intensity throughout the process schematically as a red curve. The carbon content of the starting melt is around 3 %. Although the highest quantity of process gas is injected during the main blow, the vibration intensity during this stage is relatively low. It is typically already during the main blowing period that a clear reduction of the vibration amplitude is to be noticed, which can be attributed to damping effects caused by the increasing weight of the slag with high-slag viscosity. Another reduction of the amplitudes normally is observed toward the end of the first blowing stage, which can be explained by a reduced decarburization rate and the decreasing formation of carbon mono oxide bubbles. Similar phenomena were observed in vibration measurements on basic oxygen furnace.