Effect of SiO₂ Content and Mass Ratio of CaO to Al₂O₃ on the Viscosity and Structure of CaO–Al₂O₃–B₂O₃–SiO₂ Slags

Abstract

The development of mold fluxes for continuous casting is one of major challenges to produce high aluminum steel. The CaO–Al₂O₃–B₂O₃ based mold flux is one of the potential candidates for casting high aluminum steel but its composition and properties still need to be optimized. In this work, the effect of silica and mass ratio of CaO to Al₂O₃ on the viscosity and structure of slag are studied. The viscosity of CaO–Al₂O₃–B₂O₃–SiO₂ slag with varying SiO₂ content (3%, 5%, and 7%) and mass ratio of CaO to Al₂O₃ (0.8, 1, and 1.2) were measured by rotating cylinder method at temperatures between 1723 K and 1873 K. It was found that the addition of SiO₂ leads to the increase of the slag viscosity and the activation energy increases from 178.6 to 203.4 kJ/mol. In contrast, the increase of mass ratio of CaO to Al₂O₃ will reduce the viscosity of slag and the activation energy decreases from 227.1 to 191.0 kJ/mol. The structures of glassy CaO–Al₂O₃–B₂O₃–SiO₂ slags were investigated by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Deconvolutions on Raman spectral reveal that silicon mainly exists as Q⁰(Si) and Q¹(Si) in glasses. According to deconvolution results of XPS, as SiO₂ content in glassy slag increases, the number of bridging oxygens increases, indicating a more polymerized structure. In contrast, the increase of the ratio of CaO to Al₂O₃ contributes to the depolymerization of glassy slag. The structural variations with different SiO₂ contents and mass ratio of CaO to Al₂O₃ can be correlated to the viscosity variation of slag.