Non-isothermal Crystallization Kinetics of Mold Fluxes for Casting High-Aluminum Steels
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UMINUM, as an effective additive, plays a key role in improving the tensile strength, yielding strength, and crash resistance of steel structures, and it tends to reduce the weight of the steel.[1–3] However, aluminum in molten steel may enter the mold flux as Al2O3 by reducing the components of the mold flux, such as SiO2 and MnO. In addition, the already existing Al2O3 inclusions formed in the primary and secondary steelmaking processes can be absorbed into the mold flux during casting. Such a pick-up of Al2O3 in the mold flux drastically deteriorates the thermal properties of the mold flux and leads to unstable heat transfer and poor mold lubrication, surface defects being introduced on the slab, or even the breakout of the molten steel.[4–6] To
LEJUN ZHOU, HUAN LI, WANLIN WANG, and JIE YU are with the School of Metallurgy and Environment, Central South University, Changsha 410083, China and also with the National Center for International Research of Clean Metallurgy, Central South University. Contact e-mail: [email protected] ZHAOYANG WU is with the School of Metallurgy and Environment, Central South University and also with the Key Laboratory of Metallurgical Emission Reduction & Resources Recycling, Anhui University of Technology, Ma’anshan 243002, China. SENLIN XIE is with the School of Metallurgy and Environment, Central South University. Manuscript submitted April 24, 2017.
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resolve these problems, two strategies have been proposed: (1) traditional mold flux with low CaO/SiO2, which contains 40 mass pct SiO2 to minimize the effects that the reaction between SiO2 (in the mold flux) and Al (from the molten steel) has on the mold flux properties,[7,8] and (2) a non-reactive mold flux, which is CaO-Al2O3-based to inhibit the slag metal reaction.[9] A series of studies have been conducted to investigate the crystallization behavior of these two kinds of mold fluxes. Omoto et al.[7] designed a low CaO/SiO2-low viscosity mold flux for casting high-aluminum-containing electrical steel, and their results suggested that the crystallization ability of the mold flux became stronger with the addition of Li2O, whereas the slag rim became thicker. Seo et al.[10] indicated that a higher CaO/SiO2 ratio could enhance the crystallization capability of traditional mold fluxes but that B2O3 or Li2O would inhibit it. Ryu et al.[11] reported that the crystallization temperature increased, whereas the incubation time for crystallization decreased with the increase of the CaO/ SiO2 mass ratio and alumina content. However, the change in the chemical composition of the reactive CaO-SiO2-based mold fluxes could not prevent the aluminum from the molten steel from migrating to the reactive mold flux. According to a report from ArcelorMittal,[12] the pick-up of Al2O3 in the spent mold flux could surpass 30 mass pct. during the continuous casting of high-aluminum transformation-induced plasticity (TRIP) steel. Therefore, to
prevent a chemical reaction from occurring between the molten steel and the mold flux
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