Effect of SiO2 and BaO/CaO Mass Ratio on Structure and Viscosity of B2O3-Containing CaF2–CaO–Al2O3-Based Slag for Electroslag Remelting of Rotor Steel

The structure of 20 mass pct CaF2–CaO–Al2O3-based glasses with different SiO2 contents and BaO/CaO mass ratios were studied by Raman spectroscopy and MAS-NMR spectroscopy. Aluminum exists exclusively as AlIV species (mainly AlO4 and secondly AlO3F). The relative fractions of $${\text{Q}}_{\text{Si}}^{1}$$ , $${\text{Q}}_{\text{Si}}^{2}\text{(1Al)}$$ , $${\text{Q}}_{\text{Si}}^{2}$$ and $${\text{Q}}_{\text{Si}}^{3}$$ structural units significantly increase at the expense of $${\text{Q}}_{\text{Si}}^{0}$$ species with increasing SiO2 content or BaO/CaO mass ratio, leading to enhanced polymerization of aluminosilicate networks. Boron is present in three-coordinated boron species (BIII), except for 1 to 2 pct of four-coordinated boron (BIV) in the glasses with 9.82 and 14.56 mass pct SiO2 or 8.62 mass pct BaO. BIII is mainly orthoborate containing zero bridging oxygen, and the others are pyroborate with one bridging oxygen. The relative fraction of orthoborate decreases and the relative fraction of pyroborate increases with increasing the SiO2 content or BaO/CaO mass ratio, bringing an increase in the polymerization of borates. The degree of polymerization of both aluminosilicate networks and borate networks increases with increasing the SiO2 content or BaO/CaO mass ratio. The viscosity of the slag increases with increasing the SiO2 content due to increased polymerization degree of the slag melts and bond strength of individual structural unit. The increase in the viscosity with increasing the BaO/CaO mass ratio is dominated by enhanced polymerization degree of the slag.