Effects of (CaO + BaO)/Al₂O₃ Ratio on the Melting, Crystallization, and Melt Structure of CaO–Al₂O₃-10 wt% SiO₂-Based Mold Fluxes for Advanced High-Strength Steels

Advanced high-strength steels (AHSSs) have been applied to the automobile industry due to the advantages of low energy consumption and high safety. The non-reactive CaO–Al₂O₃-based mold fluxes with low/no SiO₂ content have been proposed for developing the efficient casting technology of sustainable AHSSs, due to the occurrence of the reaction between AHSSs and conventional CaO–SiO₂-based mold flux. Then, the effects of (CaO + BaO)/Al₂O₃ ratio on the melting, crystallization, and melt structure of a designed non-reactive CaO–Al₂O₃-10 wt% SiO₂-based mold flux system have been studied. Results suggested that a higher (CaO + BaO)/Al₂O₃ ratio would lower the melting temperatures of the mold fluxes, and the slag system showed a good thermal stability during the melting process with the evaporation less than 5.10%. Besides, the crystallization behavior was promoted with the increase of (CaO + BaO)/Al₂O₃ ratio, which is associated with the reduction of the polymerization degree of the melt structure, because of the high polymerization degree of Q²(Si) structural units, and the symmetric and asymmetric [AlO₄]⁵⁻-tetrahedral structures were destroyed by the released O²⁻ ions. Thus, the migration resistance of ions and atom groups was reduced, and then the growth kinetics of LiAlO₂ and Ca₂SiO₄ crystals were improved. When 7.42 wt% BaO was added to Sample D with (CaO + BaO)/Al₂O₃ ratio at 1.75, the crystallization was promoted with the decomposition of melt structure, and the Ba-bearing phase of Ba₃Al₂O₆ crystal was precipitated to replace the main crystalline phase of LiAlO₂ due to the tight junction of Ba²⁺ and [AlO₄]⁵⁻ for compensating the Al³⁺ charge. Graphical Abstract: [Figure not available: see fulltext.].