Effect of SiO₂ substitution with Al₂O₃ during high-Al TRIP steel casting on crystallization and structure of low-basicity CaO–SiO₂-based mold flux

The crystallization and structure of non-conventional lime–silica-based mold fluxes after undergoing slag–steel interaction in casting high-Al transformation induced plasticity (TRIP) steel were studied. The results showed that the crystallization temperatures of the mold fluxes decreased with decreasing the SiO₂/Al₂O₃ ratio, and CaO/MnO₂ ratio had an opposite effect on the crystallization temperatures. The crystalline phases precipitated in the mold flux were Ca₄Si₂O₇F₂ and NaAlSiO₄. Decreasing SiO₂/Al₂O₃ ratio and increasing CaO/MnO₂ ratio in the mold fluxes have no influence on the types of crystalline phases. The dominant crystalline phase precipitated in each mold flux was Ca₄Si₂O₇F₂ with dendritic morphology, except for part of that with globular morphology in the mold flux without MnO₂ addition. NaAlSiO₄ crystals are distributed in the space among Ca₄Si₂O₇F₂ crystals. The size of Ca₄Si₂O₇F₂ crystals in the slag with higher SiO₂/Al₂O₃ ratio is smaller, which is attributed to the polymerization degree of the mold flux with increasing SiO₂/Al₂O₃ ratio. [SiO₄]-tetrahedral, [AlO₄]-tetrahedral and T–O–T bending (T denotes Si or Al) depolymerized gradually with decreasing SiO₂/Al₂O₃ ratio, and an opposite trend was observed for the case with increasing CaO/MnO₂ ratio. The polymerization degree of the mold fluxes decreased, which would result in the decrease in the viscosity of the mold fluxes.