Fluoride evaporation and crystallization behavior of CaF₂–CaO–Al₂O₃–(TiO₂) slag for electroslag remelting of Ti-containing steels

To elucidate the behavior of slag films in an electroslag remelting process, the fluoride evaporation and crystallization of CaF₂–CaO–Al₂O₃–(TiO₂) slags were studied using the single hot thermocouple technique. The crystallization mechanism of TiO₂-bearing slag was identified based on kinetic analysis. The fluoride evaporation and incubation time of crystallization in TiO₂-free slag are found to considerably decrease with decreasing isothermal temperature down to 1503 K. Fish-bone and flower-like CaO crystals precipitate in TiO₂-free slag melt, which is accompanied by CaF₂ evaporation from slag melt above 1503 K. Below 1503 K, only near-spherical CaF₂ crystals form with an incubation time of less than 1 s, and the crystallization is completed within 1 s. The addition of 8.1wt% TiO₂ largely prevents the fluoride evaporation from slag melt and promotes the slag crystallization. TiO₂ addition leads to the precipitation of needle-like perovskite (CaTiO₃) crystals instead of CaO crystals in the slag. The crystallization of perovskite (CaTiO₃) occurs by bulk nucleation and diffusion-controlled one-dimensional growth.