Effect of SiO2 and B2O3 on crystallization and structure of CaF2–CaO–Al2O3-based slag for electroslag remelting of ultra-supercritical rotor steel

Regarding development of the dedicated slag for electroslag remelting, crystallization characteristics of the slag exert a strong influence on the initial solidification of liquid metal in mold and surface quality of as-cast ingot. The crystallization behavior of CaF2–CaO–Al2O3-based slag with varying SiO2 and B2O3 contents and their correlation with the slag structure were investigated. Increasing SiO2 (0.24–8.95 mass%) and B2O3 (0–3.20 mass%) contents lowers the crystallization temperature and suppresses the crystallization of the slag melts, as well as decreases the sizes of the crystalline phases. The crystalline phases precipitated during continuous cooling of the slag melts are faceted 11CaO·7Al2O3·CaF2, faceted or spherical CaF2, and non-faceted MgO·Al2O3 (or MgO) in sequence irrespective of the SiO2 and B2O3 contents of the slag. The polymerization degree of slag melts increases with increasing either SiO2 or B2O3 contents. The crystallization of the slag melts is increasingly retarded with increasing the SiO2 and B2O3 contents of the slag caused by increased component diffusion resistance originating from increased polymerization degree of the slag melts.