CAS-OB Process of Secondary Steelmaking
CAS-OB Process of Secondary Steelmaking
The CAS-OB (Composition Adjustment by Sealed Argon Bubbling with Oxygen Blowing) process was developed in the 1980s by Nippon Steel Corporation to improve on the existing argon rinsing stations used for deoxidizing and alloying of liquid steel. 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 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. Typical schematic diagram of a CAS-OB installation are shown in Fig 1.
Fig 1 Schematic diagram of a CAS-OB installation
CAS-OB is a ladle treatment process which is designed for heating and alloying liquid steel. The CAS-OB process has many advantages which include (i) high and predictable yield of alloying materials, (ii) low aluminum consumption, (iii) more consistent attainment of the target temperature for casting and low total oxygen content after treatment. 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 (e.g. IR-UT and REHeating), although the heating rates are higher in CAS-OB process. Furthermore, slag often sticks to bell structure which causes increase in weight and volume of the bell. This can have undesirable effects on the CAS-OB operation.
The process allows alloy additions to be made under an inert argon environment. It allows simultaneous addition of Al and O2 gas blown through a top lance. These react to form Al2O3 and generate a considerable amount of heat due to exothermic nature of the reaction. The CAS-OB process, therefore results into chemical heating of the liquid steel. Principle of the CAS-OB process is at Fig 2.
Fig 2 Principle of 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 or bell fixed to the movable bracket. To the top of the bell, a port is provided, which serves the purpose of feeding of ferro alloys into the bell and for removal of gases to the gas cleaning system. The design of the bell has provision for lowering of oxygen lance and process and instrument (PI) 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 CaSi wire.
Bell consists of two parts. The upper part is lined only from the inside, while bottom is lined both inside and outside. Lining of the bell is usually done with of high-alumina castables reinforced with 2 % stainless steel needles. These castables are also used for the lining of the oxygen lance and 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 bell.
The service life of lining of the top of the bell is usually 400 to 600 heats while the lining life of the bottom of the bell is 50 to 150 heats. The lining life of oxygen lance is usually 100 heats minimum and that of lance for blowing argon is 150 heats minimum.
The process is commonly 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 main feature of the process is the refractory bell underneath which alloy additions to the bath is made. The steel ladle is positioned such that the bell 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 bell. Bottom bubbling gas creates an ‘open eye’ in the slag layer. The bell is lowered onto the liquid steel over this open eye in the slag. In particular, it allows the simultaneous addition of Al and blowing of O2 gas through a top lance. Due to the heat generated by the reaction of Al and O2, the liquid steel temperature can be raised by up to 10 deg C per minute. The Al2O3 produced need to float out to produce clean steel. Additional argon stirring, if necessary, is achieved through a specially shaped submerged lance. Further addition of ferro alloys into this slag free region achieves higher yield.
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 bell 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 bell is lowered. After having lowered the bell 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 bell 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 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.
The following are the special features of the CAS-OB process.
- The diameter of the bell 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, bell position changing system is adopted. With this bell position can be switched to good bubbling area.
- Total oxygen content of CAS-OB process is normally similar to bubbling process and ladle furnace but it is slightly inferior to that of RH process.
- During the process the skull is attached to upper part of the bell, while some oxide material is attached to lower part of the bell. Oxide growth in lower part of bell causes clash with the rim of teeming ladle and break down of the bell.