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

Abstract

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.