Microstructure analysis of (CaO-SiO2)-xTiO2-based mold flux microstructures using in situ high temperature Raman spectroscopy

Abstract

During continuous casting of high-Ti steels, conventional CaO-SiO₂-based mold flux reacts strongly with molten steel, forming CaO-SiO₂-TiO₂ slags. This transformation increases crystallinity and raises the melting and break temperatures, while reducing slag viscosity. Using rotational viscometry, XPS and in situ high-temperature Raman spectroscopy, this study clarified the structure-property relationships underlying these changes. Increasing the TiO₂/SiO₂ mass ratio decreased viscosity from 0.42 Pa·s to 0.12 Pa·s, while increasing the melting temperature from 945 °C to 1187 °C and the break temperature from 1151 °C to 1212 °C. Slag morphology transitioned from a glass structure (N1, N2) to a crystals (N3) with high-melting-point perovskite (CaTiO₃) precipitated. Structurally, increased TiO₂/SiO₂ ratios depolymerized the silicate network, increasing non-bridging oxygen (NBO) and free oxygen (FO) content. At lower TiO₂ levels, TiO₂ acted as a network modifier, forming depolymerized [TiO₆]⁸⁻ octahedra associated with NBO. At higher contents, TiO₂ functioned as a network former, where $Q_{Si}^2$ and $Q_{Si}^3$ silicate structural units decreased and replaced by [TiO₄]⁴⁻ and [Ti₂O₆]⁴⁻ clusters, forming complex Ti-O networks. These Ti-O units, with larger volumes and lower bond energies than Si-O tetrahedra, created wider structural gaps and edge-sharing configurations, facilitating oxygen ion occupancy rate, further increasing NBO and FO concentrations. This network depolymerization improved slag flowability, with the lowest viscosity observed in N3. However, rising [TiO₆]⁸⁻ octahedra content promoted perovskite nucleation, increasing slag crystallinity. This work provided critical theoretical insight for designing mold fluxes for high-Ti steel, targeting non-/low-reactivity and suppressed crystallization.