Crystallization Behaviors of CaF2-CaO-Al2O3-MgO-B2O3 Slag for Electroslag Remelting of B-Containing Rack Steel

The crystallization behaviors of CaF2-CaO-Al2O3-MgO-B2O3 slag for electroslag remelting of B-containing rack steel was investigated through a series of non-isothermal and isothermal crystallization experiments. The techniques employed for this determination included differential scanning calorimetry, X-ray diffraction, scanning electron microscopy-energy dispersive spectroscopy, and FactSage 8.2. The results indicated that an increase in B2O3 content suppressed the crystallization of CaF2-CaO-Al2O3-MgO slag. The crystallization temperature decreased as the B2O3 content in the slag increased from 2 to 7 mass pct. In the slag containing 2 mass pct B2O3, spherical CaF2 precipitates first, followed by reticulate Ca12Al14F2O32 phase. Increasing B2O3 addition promoted the formation of Ca-5(BO3)(3)F and calcium aluminate (Ca12Al14O33 or CaAl4O7), and decreased the crystallization of Ca12Al14F2O32 phase. The crystallization sequence transformed into CaF2 -> CaAl4O7 -> MgAl2O4 + Ca-5(BO3)(3)F in the case of 7 mass pct B2O3. B2O3 addition inhibits the crystallization of the dominated phase CaF2, which would improving the lubrication and heat transfer performance of ESR-type CaF2-CaO-Al2O3-MgO slags.