Motions of alloying additions in the CAS steelmaking operations
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INTRODUCTION
IN an earlier study ill it was shown that buoyant alloy additions (e.g., A1, Fe-Si) projected into a liquid steel bath during furnace tapping operations experienced short immersion times (1 to 3 seconds) and are therefore unlikely to melt subsurface. Consequently, possible air oxidation of such additions in the vicinity of upper free surface and their chemical reactions with any oxidizing slag must adversely affect any efforts by steelmakers producing quality steel. By and large, these two factors are responsible for the low and erratic recoveries of light metal additions, particularly aluminum and ferrosilicon, that are observed in practice. Moreover, conventional argon stirring into a filled ladle of steel, while an effective means for producing clean, homogeneous steel, is not a viable alternative for introducing buoyant alloy additions, owing to the following inherent drawbacks. (1) By mixing in an upper (often oxidizing) slag phase with an upwelling plume of deoxidized steel, reoxidation of solutes can take place. This is particularly true at high bubbling rates, when slag droplets can become entrained in steel. Ezi (2) Oxidation of exposed liquid steel and associated solutes at the "eye" of the plume is possible, leading to alloy fade effects. (3) Entrapment of alloying elements into the slag, particularly of light metal (pp < 3000 k g / m 3 approx.), is D. M A Z U M D A R , A s s o c i a t e P r o f e s s o r on leave from the Department of Metallurgical Engineering, Indian Institute of T e c h n o l o g y , K a n p u r , 208016, India, is V i s i t i n g P r o f e s s o r , Department of Mining and Metallurgical Engineering, McGill University, Montreal. R.I.L. GUTHRIE, Macdonald Professor of Metallurgy, is with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2A7. Manuscript submitted July 29, 1986. METALLURGICAL TRANSACTIONS B
possible if any attempts are made to adjust steel chemistry within a full ladle of steel. To overcome such difficulties, a technically superior method of alloy addition making 13j has been introduced by Nippon Steel Corporation, Tokyo, Japan. This addition making technique, known as the CAS (composition adjustment by sealed argon bubbling systems), bubbles argon gas into molten steel through a porous plug or submerged lance. The rising gas/liquid plume creates an opening in the top slag cover, over which a refractory lined cylinder is lowered into the steel. The essential idea is to make bulk alloy additions inside this slag-free region under an inert atmosphere. This procedure of alloy addition is currently in practice at the Gary Works of U.S. Steel Corporation, 14Jthe Newcastle Works of BHP, Australia, the Wakayama Works of NSC, Japan, and others. Although industry has reported superior and more reproducible aluminum recoveries, the nature of particleliquid interactions in such systems remains to be quantified. The subsurface motion of such additions and their subsequent dissolution and dispersion are clearly fe
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