CAS-OB Process of Secondary Steelmaking
CAS-OB Process of Secondary Steelmaking
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 not requiring 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 amount of heat due to 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. Principle of the CAS-OB process is shown in Fig 1.
Fig 1 Principle of the CAS-OB process
The process equipment
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. To 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 (CaSi) 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.
The service life of lining of the top of the snorkel is normally 400 heats to 600 heats while the lining life of the bottom of the snorkel is 50 heats to 150 heats. The lining life of oxygen lance is normally 100 heats minimum and that of lance for blowing argon is 150 heats minimum.
The CAS-OB process
Liquid steel from a primary steelmaking process (basic oxygen furnace or electric arc furnace) is initially poured into a ladle. The ladle consists of a steel casing with a refractory brick or castables layer on the inside. The refractory layer provides insulation and is resistant to the corrosive environment of the steel bath, having a high temperature of around 1600 deg C. During treatment, steel components react with added oxygen to form a slag phase. Additions in form of oxides also dissolve into the slag phase. Typical slag components are FeO, SiO2, MgO, Al2O3, and CaO. In the CAS-OB-process, the slag phase takes part in the reactions and protects the liquid steel from the atmosphere and works as an insulating layer.
The CAS-OB process is designed to create an inert atmosphere above the steel to allow the addition of alloys without contact with atmospheric oxygen or an oxide slag. This is accomplished by first creating a slag free area (known as an eye) at the surface of the liquid steel by the introduction of argon into the steel through a porous plug at the bottom of the ladle. Argon bubbles reaching the surface of the steel push aside the slag layer on top of the ladle, creating a slag free area. The amount of argon flow needed to produce the required size of eye on top of the ladle varies with the condition of the porous plug, the depth of slag on top of the ladle, and the fluidity of the slag. Once the eye is created, the snorkel can be lowered into the slag free area.
The operation of the process is accomplished through the use of three items namely (i) a refractory snorkel on top of the ladle, which can be used to contain an inert atmosphere of argon or the chemical reaction between oxygen and aluminum, (ii) supply of argon gas, and (iii) a water-cooled oxygen lance. These functional items are the heart of the process.
The main characteristic of the process is the refractory snorkel underneath which alloy addition to the bath is made. The snorkel provides a protected environment for adding alloying materials, where the steel surface is open, but still protected from contact with the surrounding atmosphere. This also ensures that the amount of absorbed nitrogen can be kept at a low level. The argon bubbling also provides stirring for homogenizing temperature and composition of the steel.
The steel ladle is positioned in such a way that the snorkel is situated right above the porous stirring plug. This ensures that the agitated surface of the steel bath is confined to the area underneath the snorkel. Additional argon stirring, if necessary, can be done through the specially shaped sub-merged lance. Reheating of the steel is accomplished by injecting oxygen in conjunction with aluminum additions.
Bottom bubbling argon gas creates an ‘open eye’ in the slag layer. The snorkel is lowered onto the liquid steel over this open eye in the slag. In particular, it allows the simultaneous addition of aluminum and blowing of oxygen gas through a top lance. The alumina produced need to float out to produce clean steel. Further addition of ferro alloys into this slag free region achieves higher yield.
The main stages of the process are heating, reduction of slag, and (possible) alloying. The purpose of the heating stage is to increase the temperature of the liquid steel to its target value before its continuous casting. Before the actual heating begins, the liquid steel is stirred by bottom blowing of argon to form a slag-free open-eye area on the surface of the steel bath. Consequently, the refractory snorkel is partly submerged within the liquid steel.
During the heating stage, solid aluminum particles are fed onto the free steel surface inside the snorkel. The aluminum is oxidized under the snorkel by blowing oxygen with a supersonic lance and the exothermic reaction causes an increase in the steel temperature. Aluminum oxide (alumina) formed during the heating goes into the slag phase lying on top of the steel surface and some amount of aluminium is dissolved into the steel. Due to the heat generated by the reaction of aluminum and oxygen, the liquid steel temperature can be raised by upto 10 deg C per minute without excessive equipment wear.
Due to the intensive lance blowing, in addition to aluminum, some portion of other metals from the steel phase, manganese, silicon, and iron in particular, are oxidized into the slag. This is undesirable from the economical point of view and, hence, it is normally necessary to perform slag reduction after the heat-up stage.
In addition to increasing the alumina content in the slag phase, the oxygen blowing leads to an increase in the amount of FeO, SiO2 and MnO in the slag. In order to avoid excessive losses of the metal components, the reduction of slag is performed after heating. During the reduction stage, the snorkel structure is lifted and the steel is stirred using argon blowing from the porous plugs at the bottom of the ladle. Vigorous argon stirring results in a circulating motion of the steel in the ladle.
In the slag reduction stage, the steel phase is strongly stirred by blowing some inert gas, normally argon, from the bottom of the ladle. The gas stirring forces the steel phase into a circular motion. At the interface of steel and slag, the flowing steel causes the disengagement of small droplets from the top slag layer. The slag droplets and steel form an emulsion where a large interfacial area between phases occurs. The increased interfacial area accelerates the mass transfer between the steel and slag and, hence, provides preferable conditions for the reduction reactions.
As a result of shear stresses which the turning flow of steel imposes on the top slag, small droplets disengage from the slag layer, leading to an immense increase in the interfacial area between slag and steel. This large interfacial area provides favourable conditions for a high reduction rate.
The process is normally divided into heat-up, alloying, and the reduction of slag. The objective of the heat up stage is to increase the temperature of the steel bath by chemical heating, which is conducted by feeding aluminum particles into the melt and employing simultaneous oxygen-blowing through a top lance. In practice, the rate of chemical heating is limited in order to avoid introducing excessive thermal stresses to the wall structures by means of heat transfer processes, particularly radiation and convection.
The procedure of the CAS-OB treatment is started by defining the steel bath level for immersing the snorkel to sufficient depth. Before the snorkel is lowered, the argon flow rate is increased in such a way that a slag-free area, i.e. an open-eye, is formed into which the snorkel is immersed. After the snorkel is lowered, the bottom blowing is decreased and deoxidation of the steel is carried out by aluminum or aluminum-silicon addition. Depending on the temperature measurement, the process of deoxidation is followed by the heating of the steel. In the heat-up stage solid aluminum particles are fed onto the steel surface and oxygen is simultaneously blown through the top lance. A possible alloying stage follows after the heat-up stage has been completed. Steel samples are taken before heating and after alloying just before the snorkel is lifted.
After the delivery of the ladle to the position of the liquid steel processing position, blowing of argon through the porous plug in the bottom of the ladle is started and the presence of purging is visually monitored. Simultaneously the height of free board is measured and the value of the movement of the bell is calculated. It is to be ensured that the lower end of the bell shall be immersed into the liquid steel by at least 200 mm. Also the temperature of the liquid steel and the activity of dissolved oxygen in it are measured.
Before the snorkel is lowered, argon flow is increased so that the surface of the liquid steel in the purge has the ‘open eye’. In the area of ‘open eye’, the snorkel is lowered. After having lowered the snorkel in the liquid steel to the desired depth, the flow of argon is reduced. The free surface of the liquid steel from the slag inside the snorkel serves the place for the addition of granular aluminum and other additives for deoxidation. After this, homogenization is carried out of the liquid steel for 4 minutes to 5 minutes. Then the argon flow is reduced further for taking a sample and measuring the temperature of the liquid steel. The result of temperature measurements is calculated for chemical heating, the required amount of aluminum metal and oxygen. Fig 2 shows schematic diagram of a CAS-OB process.
Fig 2 Schematic diagram of a CAS-OB installation
Special features of CAS-OB process
There are several special features of the CAS-OB process. In this process, the snorkel goes down to the steel to produce an inert area over the ‘slag eye’ (area without slag since the bubbling plume pushed it aside). The alloy addition and chemical reheating by oxygen and aluminum injection are produced underneath the snorkel. It produces more alumina than a ladle metallurgy furnace, and this alumina is required to float out to get clean steel.
The diameter of the snorkel is critical to secure the slag free region in the slag layer. It is to be determined by knowing the diameter of the open eye during bottom bubbling. The diameter of the open eye can be estimated with the help of geometry of the bubbling plume (plume cone) which depends on the gas flow rate.
In CAS-OB process, opening of bottom bubbling plug is important to secure open eye. To avoid the risk of failure of bubbling plug opening, snorkel position changing system is adopted. With this arrangement, the position of the snorkel can be switched to good bubbling area.
Total oxygen content of CAS-OB process is normally similar to argon bubbling process and ladle furnace but it is slightly inferior to that of Ruhrstahl-Heraeus (RH) vacuum degassing process.
During the process the skull is attached to upper part of the snorkel, while some oxide material is attached to lower part of the snorkel. Oxide growth in lower part of the snorkel causes clash with the rim of teeming ladle and break down of the bell.
Advantages and disadvantages
The CAS-OB process has many advantages which include (i) decrease in tapping temperature of around 15 deg C, (ii) lesser re-blowing of the heats in the basic oxygen furnace, (iii) fast and reliable homogenization of alloys, (iv) high and predictable yield of alloying materials, (v) low consumption of aluminum, (vi) more consistent attainment of the target temperature for continous casting, (vii) low total oxygen content after treatment, (viii) enables alloying with narrow tolerances, (ix) reduced alloy consumption and costs, (x) lesser aborted heats, and (xi) buffering between the basic oxygen furnace and continuous casting machine results in improved teeming conditions.
Disadvantages of the CAS-OB process include (i) slag formers need to be added before the ladle is transported to the station, and (ii) sulphur removal cannot be carried out with the process. Investment costs for setting up a CAS-OB station are higher compared to some other heating processes such as IR-UT (injection refining-up temperature) process, although the heating rates are higher in CAS-OB process. Moreover, slag frequently sticks to bell structure which causes increase in weight and volume of the bell. This can have undesirable effects on the CAS-OB operation.