Tundish Technology for Casting Clean Steel: A Review
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US casting of steel is a widely used process and an important step in steel production. Over 90 pct of the steel produced in the entire world is continuously cast. At the same time, the quality requirements of steel are also increasing. Thus, steel cleanliness and strict composition control are now becoming the primary concerns of steelmakers. Tundish is the last metallurgical vessel through which molten metal flows before solidifying in the continuous casting mold. During the transfer of metal through the tundish, molten steel interacts with slag, refractories, and the atmosphere. Thus, the proper design and operation of a tundish are important for delivering steel of strict composition and quality. Two to three decades of the last century have seen major advances in tundish technology for clean steel production. Growth in tundish technology has slowed down as it is becoming a mature technology now and as a result, there has been slow advance in this century or so. Some of the technological advancements are discussed in this paper. The term quality of steel refers to steel which has strict composition control and small number of evenly distributed small-sized non-metallic inclusions. High-quality clean steel is necessary to meet the stringent requirements of better mechanical properties, particularly ductility and durability of steel. Non-metallic inclusions include oxides, sulfides, nitrides, carbides, and their compounds or composites. Sulfides, carbides, and nitrides precipitate under normal conditions during cooling of steel below solidus temperatures. Small particles of particular oxide inclusions, sulfides, carbides, and nitrides, have been utilized to control
YOGESHWAR SAHAI, Professor Emeritus, is with the Materials Science & Engineering Department, The Ohio State University, Columbus, OH 43210. Contact e-mail: [email protected] Manuscript submitted October 30, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
microstructure for improving steel properties. However, most of the large oxide inclusions and some sulfide inclusions form while the steel is in liquid state. If these large inclusions are not removed before solidification, they give rise to processing difficulties and degrade product quality. Sulfide inclusion can be minimized by desulfurizing melt in ladle prior to its transfer to the tundish. Oxide inclusions are of two types; exogenous and indigenous. Exogenous inclusions form either by reoxidation of the deoxidized and refined melt by air and/or oxidizing slag, or by entrainment of slag or refractory particles. Indigenous inclusions form in ladle from reactions between dissolved oxygen and deoxidizers such as aluminum or silicon. Indigenous inclusions are much smaller in size and are less harmful unless they agglomerate and form larger inclusions. Indigenous inclusions are generally smaller than ~50 lm in diameter, while exogenous inclusions are larger than this size. Functions of a tundish include delivery of clean melt to different strands at required flow rate, temperature, and composition. So in terms of
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