Ladle Metallurgy

Ladle Metallurgy

Ladle metallurgy plays a crucial role in ensuring the quality of steel products. After tapping of steel from a primary steelmaking furnace such as basic oxygen furnace (BOF), electric arc furnace (EAF), induction furnace (IF), or energy optimization furnace (EOF), liquid steel for high quality or specialty applications is subjected to further refining in a number of alternative processes collectively known as ladle metallurgy. Ladle metallurgy is sometimes also called ladle refining or secondary steelmaking. Ladle metallurgy processes are normally performed in ladles. Tight control of ladle metallurgy is associated with the production of high grades of steel in which the tolerances in chemistry and consistency are narrow.

The precise role of the ladle metallurgy in a modem steel plant varies depending on the configuration and the product range of the steel plant but there are three parameters which are required to be controlled in the processes of ladle metallurgy for ensuring high quality of steel products. These three parameters are (i) homogeneity and final value of temperature, (ii) homogenization of chemistry controlling the final levels of carbon, sulphur, phosphorus, oxygen, nitrogen, calcium, silicon, manganese, aluminum, and other alloying elements, and (iii) control of inclusions characteristics consisting of number, size, morphology, and chemistry.

During the ladle metallurgy, control of these key parameters is achieved through the manipulation of three main variables namely (i) addition of chemical reagents and fluxes, (ii) heating through electrodes mounted in the roof of the ladle metallurgy station or through chemical methods, and (iii) stirring of the liquid steel in the ladle by the injection of an inert gas mainly argon gas. For achieving these controls, the operator at the ladle metallurgy station is normally provided with (i) the incoming steel chemistry, (ii) the incoming steel temperature, and (iii) the quantity of carry-over slag and its chemistry.

The objectives of ladle metallurgy are (i) homogenization of chemical composition and temperature of liquid steel in the ladle, (ii) deoxidization or killing of steel for the removal of dissolved oxygen, (iii) adjustment of superheat, i.e., heating of the liquid steel to a temperature suitable for its continuous casting, (iv) additions of ferro-alloys and carburizer for making adjustments in the chemistry of liquid steel, (v) vacuum degassing for the removal of dissolved hydrogen and nitrogen gases, (vi) decarburization for the removal of carbon for meeting the requirement of certain grades of steel, (vii) desulphurization for the reduction of sulphur concentrations to levels as low as 0.002 %, (viii) micro-cleanliness i.e., removal of undesirable non-metallic inclusions, (ix) inclusion morphology i.e., changing the composition of remaining non-metallic inclusions for improving the micro-structure of the steel, and (x) improvement in the mechanical properties such as toughness, ductility, and transverse properties etc.

Since the ladle metallurgy is a step after primary steelmaking furnace and before casting, it is the final opportunity to modify the steel chemistry. Hence, it is necessary that the chemistry of the liquid steel in terms of (i) dissolved elements, and (ii) inclusion chemistry and size distribution, is corrected in this step of steelmaking. During ladle processing, the liquid steel is normally stirred by an inert gas such as argon through a porous plug(s) at the bottom and / or a lance inserted deep into the liquid steel from the top of the ladle. The purging serves several purposes such as homogenization of the liquid steel, flotation of inclusions, improvement in the rates both of refining reactions and dissolution of alloying elements and promoting inter-phase interactions in the ladle.

Since the steel contains several elements beside iron and carbon, the reactions taking place during the refining operations in the ladle are complex. The situation is further complicated by the refractory, as it reacts with the liquid steel and the slag. It even affects the compositions of inclusions and their stabilities.

History of ladle metallurgy

The treatment of steel in the ladle started when the first ladle-to-ladle and ladle-to-ingot mould vacuum degassing processes for the removal of hydrogen appeared on the scene. A very efficient method for the treatment of liquid metal is the injection of a powdered reagent or an ferro-alloy. Already in his time, Henry Bessemer suggested that powdered material is to be added to the steel. In the 1930s and 1940s, injection of powdered material was used in many different ways. One example is injection of lime powder into hot metal for its desulphurization.

In 1950s, the injection technique was introduced in foundries. The main purpose was desulphurization and alloying with magnesium. During that decade, injection metallurgy was not yet a great success, mainly owing to technical problems. A new era for injection metallurgy started in the late 1960s. The technique was developed and improved in many ways and in several countries at the same time. The advantages of adding powdered material deeply into liquid metal are that metallurgical operations can thereby be carried out faster, with higher yields, with better reproducibility, and to meet special requirements for the products.

In the late 1950s more efficient vacuum degassers such as the Dortmund Hoerder (DH) and Ruhrstahl-Heraeus (RH) processes became popular. In the middle of 1960s, degassing processes such as vacuum arc degassing (VAD), the ASEA-SKF process, and the vacuum oxygen decarburization (VOD) process for treating high-chromium steels were successfully implemented. Converter processes such as the argon oxygen decarburization (AOD) process were introduced in the early 1970s. The AOD process is now the preferred route in the production of several specialty steels and stainless-steels.

Granulated flux injection into the liquid steel, combined with argon stirring, started in the early 1970s. This was soon followed by the application of cored-wire feeding of alloying elements for better control of composition and inclusion morphology. All the above innovations have had a pronounced effect on the steelmaking process, particularly with respect to the ladle or the furnace in which the steel is produced. For example, the implementation of ladle metallurgy and its related aspects enabled electric furnace steelmakers to use their furnaces for fast melting without the need to perform any refining in the furnace. In addition, ladle refining and degassing make it possible for the steelmaker to exert much tighter control over the properties of the final product through improved accuracy in the composition of the final product as well as its cleanliness and also by being able to control inclusion morphology.

Treating steel in the ladle is as old as the use of ladles in steelmaking. The main purposes for ladle treatment of hot metal and liquid steel include desulphurization, deoxidation, alloying, and inclusion shape control. One of the first ordinary ladle metallurgical processes was the Perrin process, in which steel was tapped into a ladle containing pre-melted slag. The kinetic energy of the steel was used to produce a large reaction surface and intensive stirring in the ladle.

In ladle metallurgy, the ladles used for the refining of steels are normally lined with magnesia (MgO) based refractories. Then, as the slag is close to or saturated with MgO, exchange reactions with the liquid steel cause magnesium and other elements in the slag to transfer to the liquid steel to some extent. As the content of dissolved magnesium is in ppm (parts per million) level in the majority of the steels, it is difficult to distinguish between magnesium from non-metallic inclusions and the part dissolved in the steel. As making cleaner steel and improving the steel properties have become one of the foremost important tasks of the steel industries, the formation of Al2O3.MgO spinel inclusions and their stability have been the focus of several studies in recent times.

Operations of ladle metallurgy are directed towards two main goals namely (i) setting of needed concentration of basic components, including micro-additions, and (ii) maximum removal of components which deteriorate the quality of steel, such as sulphur, phosphorus and mainly oxygen.

The target level of oxygen always is to be as low as possible. However, the level of nitrogen has to be set in some concentration range, corresponding to the expected functions of nitrides in grain size and structure control.

Removal of oxygen through oxide formations offers the variety of possibilities, even within one target composition of steel. Simultaneous action of manganese, silicon, and titanium, or aluminum results in shift of thermodynamic equilibrium towards deeper deoxidization as well as in formation of liquid precipitates, which are more easily removed from steel.

The deoxidization process is to be properly designed for particular kind of steel. This is particularly with regards  to the steel with titanium, which is also strong deoxidizer. The thermodynamic background of deoxidization processes including basic parameters of phases description, as well as the computer software for equilibrium calculations are normally available.

Depending on the types of steel needed, one or more of the following ladle metallurgy processes are used. These are (i) rinsing or stirring, (ii) ladle furnace, (iii) ladle injection (iv) ladle refining, (v) degassing processes, (vi) AOD process, and (vii) CAS-OB (Composition adjustment by sealed argon bubbling with oxygen blowing) process.

Rinsing or stirring – The liquid steel after tapping is stratified in the teeming ladle because of the additions of the ferro-alloys and the carburizer in the teeming ladle at the time of tapping of the liquid steel. This stratified steel is agitated by purging of argon gas. Argon purging through the liquid steel bath help generate enough bath turbulence for getting homogenous temperature, composition, and promotion of slag metal refining reaction.  Stirring with argon also improves mixing rate for chemical additions. Different variables for argon rinsing include gas purging rate, quantity of liquid steel (heat size), quantity of superheat available in the liquid steel, quantity of carry-over slag, quantity of synthetic slag or ladle covering compound added, and the quantity of mixing needed for chemical additions.

The rinsing of liquid steel in the teeming ladle is carried out through injection of the gas into the steel bath. Argon gas is preferred for rinsing since it is not only inert in nature but its solubility in steel is also very low. Rinsing results from the expansion of gas because of heating and decrease in the pressure as the gas rises.

The rinsing of liquid steel by argon gas is reported to be an excellent process for floatation and separation of non-metallic inclusions. Experienced operators and metallurgists recognize the importance of accurate and consistent argon rinsing in the teeming ladle. Clean steel and good castability in the continuous casting machine (CCM) depend on a consistent and gentle rinse stir. A good argon rinsing control system facilitates reproducible and accurate argon rinsing rates and durations.

Argon can be introduced in the teeming ladle either through a deeply inserted refractory lance from the top into the liquid steel bath or through a bottom purge-plug. The top refractory lance can be of ‘T’, ‘Y’ or straight bore type. For moderate gas bubbling rates (e.g., less than 0.6 N cubic metre per minute), porous refractory plugs are used, normally mounted in the bottom of the ladle. The function of the porous plug is to provide gas stirring of the liquid steel for promoting homogenization. Normal stirring operations are performed by percolating argon gas through the porous plug. These days majority of the ladles are equipped with bottom plugs for argon bubbling.

Argon introduced through a bottom purge plug is a more effective method of gas rinsing than an argon drip on top of the bath through the top lance. Normally rinsing operation is performed by percolating argon gas through the porous purge plug arrangement in the bottom of the teeming ladle and the top lance mechanism serves as a back-up means for liquid steel bath rinsing in the event the plug circuit in the ladle is temporarily inoperable. The gas supply connection to the teeming ladle can be either manually done with quick coupling system or is automatically made when the transfer car with the teeming ladle placed on it, arrives at the rinsing station.

The data with the top stirring with argon results in a slightly decreased free open performance of the ladle. A schematic illustration of a porous plug assembly in the ladle bottom and different types of porous purge plugs are shown in Fig 1. The figure shows standard shapes of 6 types of porous plugs. They are isotropic plugs (nos. 1 and 2), component plugs consisting of sliced (no. 3) and concentric (no. 4) and capillary plugs consisting of conical (no. 5) and rectangular (no.6).

  Fig 1 Porous plug assembly and different types of porous plugs

Ladle furnace – A ladle furnace is used to relieve the primary process of steelmaking of several of the secondary refining operations. It is one of the key components of ladle metallurgy processes. Sound ladle furnace operation delivers benefits in terms of steel quality, productivity, and cost-efficiency. Process requirements and heat-cycle times are considered to be the main factors when installing a ladle furnace.

The main functions of a ladle furnace are (i) reheating of liquid steel by electric power which is conducted by graphite electrodes, (ii) homogenization of steel temperature and chemistry through argon gas rinsing, (iii) formation of slag layer which protects refractory from arc damage, concentrates and transfer heat to the liquid steel, trap inclusions and metal oxides, and provides means for desulphurization, (iv) additions of ferro-alloys to provide for bulk or trim chemical control, (v) cored wire addition for trimming and morphology control, (vi) provide a means for deep desulphurization, (vii) provide a mean for dephosphorization, and (viii) act as a buffer for downstream equipment and process.

Specialized ladle furnace process tools and design features include minimized draft roof design, sampling and temperature manipulators, stirring equipment, multi-line wire feeders and modern automated systems. All these features maximize performance and operational safety, while ensuring lowest possible energy consumption and emissions.

The ladle roof is typically of water-cooled design with a refractory centre or delta section and is configured to coordinate with existing ladles such that the roof covers completely cover the top portion of the ladle when in the operating (i.e., fully lowered) position.

An important issue in arc reheating of a steel bath is whether the thermal energy which is supplied at, or near, the surface of the melt can be dispersed rapidly enough such that no significant temperature gradients are created within the steel in the ladle. Fig 2 shows schematics of ladle furnace equipment with top stirring.

Fig 1 Schematics of ladle furnace equipment with top rinsing 

Ladle injection – Liquid steel can be reheated by oxidizing aluminum and / or silicon by means of oxygen injection through a lance. Reheating of steel in the ladle with submerged oxygen injection is being practiced in some steel plants.  In RH-OB process, an average thermal efficiency of 20 % to 30 % is being achieved. Also, a reheating efficiency of around 80 % for the RH-OB operation has been reported. In RH-KTB process oxygen is supplied through a top lance instead of through submerged tuyeres as in the RH-OB. The thermal efficiency for the RH-KTB process appears to be similar to that for submerged oxygen injection into the ladle.

Injection techniques have the advantages of dispersing the reactants in the steel bath and at the same time provide a large reaction surface area. The type of powders used is governed by the purpose of injection. For dephosphorization, the powder used is either a mixture containing CaO+CaF2+Fe2O3+ mill scale or soda. For desulphurization, the powders used are (i) CaO + Al, (ii) CaO + CaF2 + Al, (iii) CaC2, (iv) Mg + (MgO, Al2O3, chloride slag), (iv) CaC2+ CaCO3, and (v) CaO. For alloying, the powders used are Fe-Si, CaCN2, NiO, MoO2, Fe-B, and Fe-Ti etc. For deoxidation, the powders used are Al, Ca Si, Ca, Si, Mn, and Al, while for shape control, the powders used are Ca-Si and Ba.

It is to be noted that methods for injection of powder are also to be developed. The slag forming materials are lighter than steel and deep injection is needed for the efficiency of the reaction. Powder can be injected either through cored wire or pneumatic transport. Fig 3 shows arrangement of ladle injection carried out either by injecting cored wire or by pneumatic injection through a top lance. In both, argon is bubbled through a porous plug fitted at the bottom of the ladle. The process efficiency is normally estimated by the mass balance.

Fig 3 Ladle injection technique

A comparison of total oxygen contents measured in the cast steel from oxygen reheated heats and heats which are not reheated has shown no significant differences between the two sets of values.

Ladle refining – The refining of steel in the ladle is broadly defined here as comprising of the operations such as deoxidation, desulphurization, dephosphorization, controlled additions of alloying elements and inclusion modification. There are a number of benefits which are available with the use of ladle refining furnace (LRF) are (i) homogenizing the composition and temperature of the liquid steel, (ii) making the steel cleaner, by the removal of oxygen inclusions, (iii) improving grain refinement in the micro-structure, (iv) degassing of the steel, through inert gas purging, (v) saving on ferro-alloys consumption, and (vi) increasing productivity, as the melting furnace gets emptied earlier. A ladle refining furnace looks similar to a ladle furnace as shown in Fig 2 but without the electric heating facility.

The refining steel in the ladle is normally done by deoxidation of steel with ferro-manganese, ferro-silicon, silico-manganese, and aluminum. The steel is first deoxidized partially with silico-manganese, ferro-manganese, and / or ferro-silicon followed by a final deoxidation with aluminum. Such a practice has several advantages including minimization of nitrogen pick up, minimization of phosphorus reversion and minimization of aluminum losses during primary steel making.

Today use of synthetic slags in the ladle has become an integral part of the ladle metallurgy. The use of synthetic slag consisting of calcium-alumino silicate helps in the dissolution of the deoxidation products which helps in the deoxidation activity. Partially deoxidized steel can also be further deoxidized with calcium silicide (Ca-Si) which is injected in the ladle in the form of cored wire. Killed steels deoxidized with aluminum normally have less than 5 ppm of dissolved oxygen.

In certain steel grades, a very low sulphur content is specified e.g., 20 ppm and less. This low sulphur contents can only be achieved by steel desulphurization in the ladle in the presence of a calcium aluminate slag when the steel is fully killed. For the required degree of desulphurization to take place within a practical time span, good mixing of steel and slag is essential. The rate, at which the sulphur can be removed, is strongly recommended by the gas flow rate during rinsing of steel. Another method for achieving very low sulphur content is by the injection of fluxes into the ladle. A typical flux used for desulphurization contains 70 % CaO and 30 % CaF2. Desulphurization achieved through powder injection is around 15 % faster than the desulphurization with a top slag only, combined with the gas rinsing. Desulphurization of steel in the ladle is accompanied by a decrease in the temperature of the steel bath and hence needed reheating.

Dephosphorization in ladle is needed when the phosphorus content of input hot metal during primary steel making is high. Removal of phosphorus from the steel in the ladle is achieved by treating the steel with lime based oxidizing slags containing iron oxide.

Calcium treatment of liquid steel is normally adopted to modify the morphology of the inclusions. As a result of the treatment with calcium, the alumina and silica inclusions are converted to liquid calcium aluminates or calcium silicates. These liquid inclusions are globular in shape because of sulphur tension effects. This change in inclusion composition and shape is normally known as inclusion morphology control or modification. Since the boiling point of calcium is 1491 deg C, calcium is a vapour at the steel making temperature. Hence when adding calcium to the liquid steel, special measures are required to be taken for ensuring its proper recovery in the steel bath. Calcium or calcium alloys are added to the liquid steel bath at the greatest possible depth so as to make use of the increased pressure from the ferrostatic head to prevent the calcium from evaporating. Further calcium retention frequency decreases with increasing quantity of calcium injected. The quantity of calcium to be injected has to be adjusted in accordance with the degree of cleanliness of the steel and its total oxygen content.

Vacuum degassing of liquid steel – During the primary steelmaking process, gases like oxygen, hydrogen, and nitrogen dissolve in the liquid steel. These gases have a harmful effect on the mechanical and physical properties of steel. Dissolved oxygen from liquid steel cannot be removed as molecular oxygen and its removal is termed as deoxidation. The term degassing is used for the removal of hydrogen and nitrogen gases from the liquid steel. Since the degassing process of the liquid steel is carried out under vacuum, it is also known as vacuum degassing of liquid steel. Vacuum degassing processes are carried out in steel teeming ladles.

Degassing is employed to remove nitrogen and hydrogen from steel. Initially, vacuum degassing was used primarily for hydrogen removal. However, during the last two to three decades, there has been an increased use of vacuum degassing for the production of ultra-low-carbon (ULC) steels with carbon contents of 30 ppm or less.

Removal of hydrogen and nitrogen gases from liquid steel is necessary since both of these gases harm the properties of steel. Solubility of hydrogen in steel is low at ambient temperature. Excess hydrogen is rejected during solidification and results in pinhole formation and results into the porosity in the solidified steel. Few ppm of hydrogen gas causes blistering and loss of tensile ductility. In case of nitrogen gas, maximum solubility of nitrogen in liquid iron is 450 ppm and less than 10 ppm at room temperature. During solidification excess nitrogen is rejected which can cause formation of either blow holes or nitrides. Excess nitrogen also causes embrittlement of heat affected zone during welding of steels and also impairs cold formability of steel.

Vacuum degassing is an important secondary steelmaking process. This process was originally used for hydrogen removal from the liquid steel but presently it is also being used for secondary refining and has become increasingly important process of secondary steelmaking. Pressure dependent reactions are the reason for the treatment of liquid steel in this process. Presently, a vacuum degassing treatment has become an essential facility for a steel melting shop producing quality steel.

Desorption of gases is a gas / metal interfacial reaction. The atomic hydrogen and nitrogen from the liquid steel has to diffuse at the gas / metal interface, where it is converted to molecular hydrogen or nitrogen which can then be desorbed. The effectiveness of vacuum treatment increases with increase in surface area of liquid exposed to vacuum. The increased surface area of liquid steel exposed to vacuum e.g., in the form of a thin stream or gas induced stirring accelerates the degassing process.

Temperature of liquid steel drops during vacuum degassing process. More is the surface area of stream exposed to vacuum higher is the temperature drop. The degassing time need to be kept at minimum. The degree of degassing increases with the degree of vacuum. Vacuum of the order of 1 mm mercury or even less than 1 mm mercury (1 mm mercury = 1 torr) is employed in the practice. Vacuum pumping capacity is required to be adequate.

Vacuum degassing processes which are presently being used can be classified into the three types namely (i) stream degassing practice, (ii) circulation degassing practice, and (iii) ladle or tank degassing practice.

Degassing can be carried out either by placing ladle containing molten steel under vacuum (non recirculating system) or by recirculation of molten steel in vacuum (recirculating system). Examples of recirculating systems are RH, RH-OB, RH-KTB, and DH etc. processes and examples of non-recirculating systems are ladle or tank degassers, including VAD (vacuum arc degassing) and VOD (vacuum oxygen decarburization), and stream degassers. Fig 4 shows different vacuum degassing processes.

Fig 4 Vacuum degassing processes

In ladle degassing, the effectiveness of degassing decreases from top to bottom of the liquid steel bath. Bottom layers of steel are very much less affected by vacuum since these layers are under the influence of ferrostatic pressure because of the column of liquid steel. Hence bath agitation helps exposing the entire content of liquid steel to the vacuum.

In both recirculating and non-recirculating systems argon, is used as the lifting or stirring gas. In recirculating systems, the argon is used as the so-called lifting gas to lower the apparent density of the liquid steel to be lifted up from the ladle into the vacuum vessel. In non-recirculating systems argon is used as the stirring gas to promote the removal of hydrogen and / or nitrogen and to homogenize the bath.

There is not much difference between recirculating and non-recirculating systems in terms of the effectiveness with which hydrogen or nitrogen can be removed. If the primary function of the degasser is to remove hydrogen and sometimes nitrogen, the choice of system is determined mainly by the desired match between the primary steelmaking furnace and the continuous casting machine as well as by considerations in regard to capital and operating costs.

One of the purposes to treat steel in an RH or RH-OB (KTB) degasser is to lower the dissolved oxygen content of the steel by means of carbon deoxidation before adding aluminum to kill the steel completely. With such a carbon deoxidation practice there are considerable cost savings as a result of the decreased usage of aluminum.

Some nitrogen removal from liquid steel during vacuum degassing is possible, provided the steel is fully killed and has low content of sulphur.

Argon oxygen decarburization process – Argon oxygen decarburization (AOD) is a process mainly used the production of stainless steel and other high grade alloy steels with oxidizable elements such as chromium and aluminum. After initial melting in the primary steelmaking furnace, the liquid steel is transferred to an AOD vessel where it is subjected to three steps of refining namely (i) decarburization, (ii) reduction, and (iii) desulphurization. AOD was invented in 1954 by the Linde division of The Union Carbide Corporation, which became known as Praxair in 1992.

The liquid steel is decarburized and refined in the AOD vessel to less than 0.05 % carbon. The key feature in the AOD vessel (Fig 5) is that the oxygen for decarburization is mixed with argon or nitrogen inert gases and injected through submerged tuyeres. This argon dilution minimizes unwanted oxidation of precious elements contained in specialty steels, such as chromium.

Fig 5 Argon oxygen decarburization vessel

AOD process uses dilution technique for the decarburization of the 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 processes being among the most intensely stirred metallurgical reactors. The intimate gas – metal contact and excellent slag – metal mixing facilitate refining reactions.

AOD process is widely used for the production of stainless steels and specialty alloys such as silicon steels, tool steels, nickel-base alloys and cobalt-base alloys. The process is popular since it combines higher metallic yields with lower material costs. Other benefits include accuracy in chemistry control down to 0.01 % carbon and lower, rapid desulphurization to less than 0.001 %, and lead removal to less than 0.001 %. The end result is cleaner steel coupled with increased productivity. 

CAS-OB process – The CAS-OB process is a ladle treatment process in secondary metallurgy which is used for the heating of steel through chemical means. The abbreviation CAS-OB stands for ‘Composition Adjustment by Sealed Argon Bubbling – Oxygen Blowing’. The process was developed and patented by Nippon Steel Corporation in the 1980s. During the CAS-OB process, the most important functions are the adjustment of the temperature to an optimum level and the accurate addition of alloying elements. The purpose of the heating is to ensure sufficient temperature of the liquid steel when it is sent to the continuous casting machine. The CAS-OB process belongs among the processes which operate at the atmospheric pressure.

The CAS-OB process is designed for homogenization and control of the composition and temperature of steel. It is a ladle treatment process which is designed for heating and alloying of liquid steel. The process is widely used for steel grades which do not need vacuum degassing treatment. Recently, because of wider application of vacuum degassing treatment, the use of the CAS-OB process has decreased.

The CAS-OB process enables consistently high alloy recoveries and the reheating of steel using the exothermic reaction between oxygen and aluminum. With this capability of good chemical composition control, steel homogeneity, and reheating, the CAS-OB process becomes an ideal buffer station in the secondary metallurgy of steelmaking. The objective of the CAS-OB process is to homogenize and control the steel composition and temperature. It has been reported that the CAS-OB process enables a better scheduling, improved temperature control, and higher inclusion purity.

CAS-OB is a ladle treatment process which is designed for heating and alloying liquid steel. The process allows alloy additions to be made under an inert argon environment. It allows simultaneous addition of aluminum and oxygen gas blown through a top lance. These react to form alumina and generate a considerable quantity of heat because of the exothermic nature of the reaction. The CAS-OB process, hence results into chemical heating of the liquid steel.

In chemical heating processes the steel is heated by way of an exothermic reaction of a dissolved element by oxygen blowing. The use of aluminum is preferred as an element for chemical heating. It has been reported that a concentration of 0.1 % of dissolved aluminum within the liquid steel is able to produce a temperature rise of +34 deg C by reacting with oxygen gas. Obviously, there is also heat losses caused by radiation and through the ladle walls. Fig 6 shows principle and schematic diagram of CAS-OB process.

Fig 1 Principle and schematic diagram of CAS-OB process

In the CAS-OB process liquid steel processing is carried out in ladles, equipped with slide gates and a porous plug for blowing argon. Equipment for the process consists of a snorkel (also called a bell) fixed to the movable bracket. At the top of the snorkel, a port is provided, which serves the purpose of feeding of aluminum and ferro-alloys (if necessary) into the snorkel and for the removal of gases to the gas cleaning system. The design of the snorkel has provision for lowering of oxygen lance and process and instrument lance for sampling, measuring of the temperature and for measuring of the dissolved oxygen as well as a lance for injecting a metal powder, desulphurizing compound, and calcium silicide (Ca-Si) wire.

Snorkel consists of two parts. The upper part is lined only from the inside, while bottom is lined both inside and outside. Lining of the snorkel is normally done with high-alumina castables reinforced with 2 % stainless steel needles. These castables are also used for the lining of the oxygen lance and sub-merged lance for blowing argon into the liquid steel, which is used when argon cannot be supplied to the liquid steel through the bottom porous plug. Chrome magnesite bricks have also been used for the lining of bottom of the snorkel. There is a specially shaped sub-merged lance for additional argon stirring.

Comments on Post (1)

  • yogesh sajjanwar

    Nice one Sir.

    • Posted: 23 April, 2014 at 15:29 pm
    • Reply

Leave a Comment