Effect of Rice Husk Ash Insulation Powder on the Reoxidation Behavior of Molten Steel in Continuous Casting Tundish
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IT is important to improve the cleanliness of molten steel in tundish for manufacturing high-quality steel products. The tundish has an important role in cleaning the molten steel by coagulation and floating up of nonmetallic inclusions in conjunction with distributing the molten steel to single or multiple casting strands.[1] However, the contamination of molten steel occurs in tundish by various factors, including the entrapment of carryover ladle slag, reaction with tundish flux, degradation of refractory, reaction with ladle well packing material, oxidation of molten steel by residual slag and metal, and oxidation of molten steel by air.[1–4] The changes in inclusion characteristics in the tundish can even influence the submerged nozzle clogging. The reoxidation behavior of molten steel in the tundish can be divided into an initial teeming stage at the beginning of the casting or transition period and a stable teeming stage. The oxidation by air occurs at the initial teeming stage, because the molten steel becomes exposed to ambient as it is devoid of tundish flux by the
TAE SUNG KIM and JOO HYUN PARK are with the Department of Materials Engineering, Hanyang University, Ansan 15588, Korea. Contact email: [email protected] YONGSUG CHUNG is with the Department of Advanced Materials Engineering, Korea Polytechnic University, Siheung 15073, Korea. LAURI HOLAPPA is with the School of Chemical Technology, Aalto University, Espoo 00076, Finland. Manuscript submitted September 1, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
turbulent flow.[5–8] The effect of the oxidation by air, however, is negligible to the reoxidation of molten steel under dilute oxygen condition, i.e., p(O2) = 7.4 9 104 atm, which was reported by Higuchi et al.[9] Thus, it can be assumed that the reaction between molten steel and tundish flux is a predominant factor affecting the reoxidation phenomena in tundish when the flow of the air into the tundish was prevented by Ar protection. At the stable teeming stage, the molten steel in tundish would react with tundish flux; hence, tundish flux needs less amounts of reducible oxide, such as iron oxide, manganese oxide, or silica, for preventing reoxidation. Kuchar and Holappa mentioned that double layer tundish slag, which consists of a solid powder layer at the upper part and a liquid slag layer at the reaction zone, in the vicinity of molten steel could prevent the steel in the tundish from both heat loss and reoxidation, as shown in Figure 1.[6] However, rice husk ash (RHA), which contains up to 90 pct silica, is commonly used for tundish flux to provide excellent protection from heat loss.[10] Thus, the reoxidation of molten steel should be reconsidered for a long casting sequence because of continuous dissolution of SiO2 from the RHA powder layer to the molten slag pool. Because the dissolution of SiO2 from RHA to the molten slag increases silica activity in the slag, understanding the interfacial reaction between Al-containing steel and silicate slags is necessary to reveal the reoxidati
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