Composition Design and Property Investigation of Mold Fluxes for the Continuous Casting of Rare-Earth Weathering Steel

Conventional CaO–SiO2-based mold fluxes used in the continuous casting of rare-earth weathering steel are prone to severe slag–steel interfacial reactions, resulting in marked compositional changes and progressive property deterioration after rare-earth oxide pickup, which compromises lubrication stability and casting operability. In this study, a novel low-reactivity CaO–Al2O3-based mold flux was designed through phase-diagram-guided composition design, IMCT-based thermodynamic screening, and experimental investigation of melting, viscosity, and crystallization behavior. The originality of this work lies in establishing a design–validation framework for fluxes that remain stable after Ce2O3 absorption, rather than simply replacing the conventional CaO–SiO2 system. Validation against literature SiO2 activity data showed good trend consistency, supporting the use of the IMCT model as a semi-quantitative tool for composition screening. The results showed that CaO, Li2O, and Ce2O3 exhibited relatively high activities, whereas B2O3 showed extremely low activity. When the Ce2O3 content exceeded 5 wt.%, the viscosity remained about 0.30 Pa·s, and when the w(CaO)/w(Al2O3) ratio was higher than 1.6, it stabilized at about 0.25 Pa·s. The minimum melting temperature, 1122.6 °C, was obtained at 5 wt.% Ce2O3. Compared with a conventional CaO–SiO2-based flux, the developed flux showed similar initial processability but much better stability after absorbing 15 wt.% Ce2O3, with less severe deterioration in melting, viscosity, and crystallization behavior. These results provide scientific insight into mold-flux design under rare-earth oxide pickup conditions and practical guidance for improving the continuous casting of rare-earth weathering steels.