Effects of B 2 O 3 and BaO on the Crystallization Behavior of CaO-Al 2 O 3 -Based Mold Flux for Casting High-Al Steels
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OLD flux has been widely used in continuous casting,[1] to play important roles such as lubricating the strand, absorbing inclusions from liquid steel, preventing the steel from oxidation, and moderating the heat transfer between the steel and mold.[2] In order to meet the production needs of advanced auto body steel, the research regarding to Advanced High Strength Steels (AHSS) has been conducted intensively, especially for the transformation-induced plasticity (TRIP) steels.[3] However, a large quantity of aluminum has been added in TRIP steel, and the high Al in the molten steel has the tendency to react with SiO2 in the mold slag during the process of continuous casting; therefore, the content of SiO2 in the mold slag decreases markedly and Al2O3 increases obviously. Consequently, the properties of the resulting mold flux vary dramatically, and would tend to cause the lubrication and heat transfer problems, which in turn introduce poor surface quality of the cast slabs. Previous studies regarding to the optimization of the ratio of Al2O3/SiO2[4,5] or CaO/SiO2[6,7] and the amount of fluidizers[8,9] for the traditional lime–silica-based mold flux to solve these problems have been conducted. However, the variance of crystallization and heat flux behaviors of mold flux that introduced by slag/steel reaction make it difficult to optimize above mold fluxes system. Therefore, the non-reactive lime–alumina-based mold flux has been proposed, where SiO2 has been partially replaced by Al2O3 to inhibit the reaction between Al and SiO2 (slag/steel interaction) in the casting of high-Al DAN XIAO and BOXUN LU, Graduate Students, and WANLIN WANG, Professor, are with the School of Metallurgy and Environment, Central South University, Changsha 10083, P.R. China. Contact e-mail: [email protected] Manuscript submitted December 7, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
TRIP steels. Cho et al.[10] investigated a series of lime– alumina-based mold fluxes with CaO/Al2O3 (C/A) ratio of 1 and 3.3 in casting experiments; and the trial results of the mold flux with C/A of 1 indicate that the fluctuation of final spent mold flux has been inhibited comparing with lime–silicate-based system, but the final spent mold fluxes show strong crystallization property that leads to the deterioration of the mold lubrication. Meanwhile, the trail results of C/A 3.3 suggest that even though the problem of lubrication became better, however, the fluctuation of heat flux and slag/steel reaction was getting stronger than those of C/A 1, also the spent mold fluxes show strong crystallization tendency. Therefore, the study of the lime–alumina-based mold flux with C/A of 2 is worthy to be studied, as it has the potential to improve the lubrication property in the casting of high-Al TRIP steel compared with mold fluxes with C/A of 1. Meanwhile, it can reduce slag/steel reaction comparing with mold fluxes with C/A of 3.3. As the problem of strong crystallization of previous developed mold flux system, fluxing agents like B2O3 and BaO have been widely used to lower the mel
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