Crystallization Characteristics of CaO-Al₂O₃-Based Mold Flux and Their Effects on In-Mold Performance during High-Aluminum TRIP Steels Continuous Casting

Crystallization behaviors of the newly developed lime-alumina-based mold fluxes for high-aluminum transformation induced plasticity (TRIP) steels casting were experimentally studied, and compared with those of lime-silica-based mold fluxes. The effects of mold flux crystallization characteristics on heat transfer and lubrication performance in casting high-Al TRIP steels were also evaluated. The results show that the crystallization temperatures of lime-alumina-based mold fluxes are much lower than those of lime-silica-based mold fluxes. Increasing B₂O₃ addition suppresses the crystallization of lime-alumina-based mold fluxes, while Na₂O exhibits an opposite effect. In continuous cooling of lime-alumina-based mold fluxes with high B₂O₃ contents and a CaO/Al₂O₃ ratio of 3.3, faceted cuspidine precipitates first, followed by needle-like CaO·B₂O₃ or 9CaO·3B₂O₃·CaF₂. In lime-alumina-based mold flux with low B₂O₃ content (5.4 mass pct) and a CaO/Al₂O₃ ratio of 1.2, the formation of fine CaF₂ takes place first, followed by blocky interconnected CaO·2Al₂O₃ as the dominant crystalline phase, and rod-like 2CaO·B₂O₃ precipitates at lower temperature during continuous cooling of the mold flux. In B₂O₃-free mold flux, blocky interconnected 3CaO·Al₂O₃ precipitates after CaF₂ and 3CaO·2SiO₂ formation, and takes up almost the whole crystalline fraction. The casting trials show that the mold heat transfer rate significantly decreases near the meniscus during the continuous casting using lime-alumina-mold fluxes with higher crystallinity, which brings a great reduction of surface depressions on cast slabs. However, excessive crystallinity of mold flux causes poor lubrication between mold and solidifying steel shell, which induces various defects such as drag marks on cast slab. Among the studied mold fluxes, lime-alumina-based mold fluxes with higher B₂O₃ contents and a CaO/Al₂O₃ ratio of 3.3 show comparatively improved performance.