Effect of TiO2 on the crystallisation behaviour of CaF2–CaO–Al2O3–MgO slag for electroslag remelting of Ti-containing tool steel

Abstract

The crystallisation characteristics of CaF2–CaO–Al2O3–MgO slags with various TiO2 contents from 0 to 9.73 mass% were studied using a single hot thermocouple technique, SEM-EDS and X-ray diffraction. The crystallisation mechanism of TiO2-bearing slag was identified based on kinetic analysis. It was found that increasing TiO2 from 0 to 6.43 mass% inhibited the crystallisation ability of electroslag remelting-type CaF2–CaO–Al2O3–MgO slag, whereas further increasing TiO2 content up to 9.73 mass% enhanced the slag crystallisation signally. When increasing TiO2 content to 6.43 mass%, the crystalline phase shift from CaO to Ca12Al14O32F2 and CaTiO3 at high temperatures. At lower temperatures, the crystalline phase change from polygonal Ca12Al14O32F2 to need-like CaTiO3. Further increasing TiO2 content to 9.73 mass%, the crystalline phase are Ca12Al14O32F2 and CaTiO3 in the range of 1473–1613 K. The crystallisation of crystalline phase in the isothermal crystallisation is surface nucleation and controlled by interface reaction when TiO2 content is lower than 6.43 mass% in the slag. It is bulk nucleation and diffusion-controlled one-dimensional growth in slag with 9.73 mass% TiO2.