A Mathematical Model for the Reduction Stage of the CAS-OB Process
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DUCTION
THE CAS-OB process (composition adjustment by sealed argon bubbling–oxygen blowing) was developed by Nippon Steel.[1] It is a ladle treatment process that is designed for heating and alloying molten steel. The CAS-OB process has many advantages, including high and predictable yield of alloying materials, low aluminum consumption, more consistent attainment of the target temperature for casting, and low total oxygen content after treatment.[1] There are also some disadvantages related to CAS-OB 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.[2,3] Furthermore, slag often sticks to bell structure which causes increase in weight and volume of the bell. This may have undesirable effects on the CAS-OB operation.[4–6]
PETRI SULASALMI, VILLE-VALTTERI VISURI, and AKI KA¨RNA¨, Doctoral Students, and TIMO FABRITIUS, Professor, are with the Research Unit of Process Metallurgy, University of Oulu, Oulu, Finland. Contact e-mail: petri.sulasalmi@oulu.fi MIKA JA¨RVINEN, Associate Professor, is with the Department of Mechanical Engineering, Aalto University, Espoo, Finland. SEPPO OLLILLA, Technology Expert, is with SSAB Europe Oy, Rautaruukintie 115, 92101 Raahe, Finland. Manuscript submitted February 3, 2016. Article published online August 12, 2016. 3544—VOLUME 47B, DECEMBER 2016
The main stages of the process are heating, reduction of slag, and (possible) alloying. The purpose of the heating stage is to increase the steel temperature to its target value before continuous casting. Before the actual heating begins, the steel is stirred by bottom blowing to form a slag-free open-eye area on the surface of the steel bath. Consequently, a refractory bell is partly submerged within the steel (see Figure 1). During the heating stage, solid aluminum particles are fed onto the free steel surface inside the bell. The aluminum is oxidized under the refractory bell by blowing oxygen with a supersonic lance and the exothermic reaction causes an increase in the steel temperature.[7] Heating rates of up to 10 K/min (10 C/min) can be obtained without excessive equipment wear.[2] In addition to increasing the Al2O3 content in the slag phase, the oxygen blowing leads to an increase in the amount of FeO, SiO2 and MnO in the slag.[8] In order to avoid excessive losses of the metal components, the reduction of slag is performed after heating. During the reduction stage, the bell 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. As a consequence of shear stresses that 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 favorable conditions for a high reduction rate.
METALLURGICAL AND MATERIALS
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