Effect of B2O3 and CaO/Al2O3 on the Melting Properties, Viscosity, and Electrical Conductivity of ESR Slag for Reduced Activation Ferritic/Martensitic Steel with Boron

This study investigates the effect of B₂O₃ content and CaO/Al₂O₃ ratio on the melting temperature, viscosity, and electrical conductivity of CaF₂–CaO–Al₂O₃–MgO slag systems for electroslag remelting of boron-containing reduced activation ferritic/martensitic (RAFM) steel. The physicochemical properties were characterized using a slag melting point analyzer, a high-temperature melt property tester, and an electrical conductivity meter. The underlying mechanisms were elucidated through X-ray diffraction (XRD), confocal laser Raman microscope, and thermodynamic calculations using FactSage 8.2. The results demonstrate that increasing B₂O₃ content leads to a progressive decrease in the high-melting-point phase Ca₁₂Al₁₄O₃₂F₂ while increasing the low-melting-point phase CaF₂. The low-melting-point region in the isothermal section diagram expands significantly and shifts upward, resulting in a reduction in the melting temperature of the slag. When the CaO/Al₂O₃ ratio increases from 0.8 to 1.1, the content of low-melting-point phase Ca₁₂Al₁₄O₃₃ increases, decreasing the melting temperature from 1346 °C to 1310 °C. With the increase of B₂O₃ content and CaO/Al₂O₃ ratio, the viscous flow activation energy of the slag gradually decreases. The aluminate structure in the slag becomes depolymerized, leading to an increase in the number of non-bridging oxygen atoms. The highly polymerized \(Q_{{{\text{Al}}}}^{3}\) and \(Q_{{{\text{Al}}}}^{4}\) structural units are transformed into less polymerized structural units such as \(Q_{{{\text{Al}}}}^{0}\), \(Q_{{{\text{Al}}}}^{1}\), and \(Q_{{{\text{Al}}}}^{2}\). The overall degree of polymerization of the slag is reduced, and the viscosity decreases. The electrical conductivity increases with temperature from 1430 °C to 1520 °C. With increasing B₂O₃ content, the activation energy for electrical conduction decreases, reaching a minimum of 36.34 kJ mol⁻¹ at 8 wt pct B₂O₃, where maximum conductivity is observed. The electrical conductivity increases first and then decreases with the increase of CaO/Al₂O₃ ratio. When CaO/Al₂O₃ is 0.9, the lowest activation energy of slag conductivity is 52.13 kJ mol⁻¹, and the conductivity is the highest. The variation in activation energy fundamentally governs the changes in electrical conductivity.