Effect of Li 2 O on the Behavior of Melting, Crystallization, and Structure for CaO-Al 2 O 3 -Based Mold Fluxes
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HIGH aluminum containing steels, such as TRIP/ TWIP steels, are considered as new generation automotive steels, as they can lower the weight of automotive bodies for better fuel efficiency without sacrificing safety.[1,2] The content of aluminum in steels is beneficial for improving their mechanical properties; however, it also introduces other problems during the continuous casting process, in which the migration of aluminum from molten steel into liquid mold flux is the crucial one. According to the report from ArcelorMittal,[3] the pickup of Al2O3 content in the spent mold flux could surpass 30 mass pct during the continuous casting of TRIP steel. The increase of Al2O3 content in the mold flux leads to the significant variation of its properties, such as melting and crystallization behavior, which degenerates the quality of the shell and, consequently, affects the smoothness of the casting process.[4–6] In order to modify the negative effects of the mold flux properties
LEJUN ZHOU, HUAN LI, WANLIN WANG, DAN XIAO, LEI ZHANG, 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, Changsha, 410083 China. Contact e-mail: [email protected] Manuscript submitted November 2, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
caused by the pickup of Al2O3, intensive studies have been carried out. Shi et al.[7] investigated the effect of Na2O and B2O3 on the crystallization of CaO-Al2O3based mold flux for the casting of TRIP steel and showed that the crystallization of the mold flux was enhanced with the addition of Na2O, whereas it was inhibited by B2O3. Yan et al.[8] evaluated the influence of B2O3 and CaF2 on the crystallization behavior of CaO-Al2O3-based mold flux, and the results suggested that the crystallization of mold flux was weakened by both of them. Li et al.[9] studied the viscosity variation of CaO-Al2O3-based mold flux with different TiO2 contents, and the results indicated that the viscosity decreased with the addition of TiO2. Lu et al.[10] investigated the impact of Li2O on the crystallization behavior of CaO-Al2O3-based mold flux for casting high aluminum steels and showed that Li2O could increase the incubation time of mold flux crystallization in the high-temperature zone and decrease it in the low-temperature zone. The evolution of molten slag structure was studied recently. Kim and Sohn[11] reported that the addition of CaF2 and Li2O could decrease the viscosity of CaOSiO2-based mold flux, since they were effective in modifying complex silicate structures into simpler silicate structures. Also, in their following research work about CaO-Al2O3-based mold flux, the results suggested that an appropriate addition of Li2O could decrease the viscosity of mold flux by breaking the [AlO4]-tetrahedral network structure.[12] Similar results were also obtained
Table I.
Designed Chemical Compositions of Mold Fluxes with Different Li2O Contents (Mass Percent)
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