In-situ Study of Crystallisation Behaviour of CaO–SiO2–Na2O–B2O3–TiO2–Al2O3–MgO–Li2O Fluorine-free Mould Fluxes with Different CaO/SiO2 Ratios

The increasing environmental concern for the fluorine emission in steel continuous casting makes the development of fluorine-free mould fluxes imperative. The main challenge in the development of fluorine-free mould fluxes is controlling heat transfer rate which is closely related to the crystallisation behaviour of mould fluxes. In this study, the crystallisation behaviour of CaO–SiO2–Na2O–B2O3–TiO2–Al2O3–MgO–Li2O fluorine-free mould fluxes with CaO/SiO2 mass ratios from 0.9 to 1.2 was examined using single hot thermocouple technique (SHTT) and double hot thermocouple technique (DHTT). Continuous cooling transformation (CCT) and time-temperature transformation (TTT) diagrams developed using SHTT showed that the crystallisation temperature increased and the incubation time decreased with the increase of CaO/SiO2 ratio. DHTT was used to simulate the temperature gradient between copper mould and strand in steel continuous casting. Analysis of the crystallinity evolution in the simulated temperature field showed an increased crystallinity of fluxes with the increase of the CaO/SiO2 ratio at certain times. The crystal phases and crystal morphologies formed in different conditions were analysed by X-ray diffraction (XRD), scanning electron microscope (SEM), and X-ray energy dispersive spectroscopy (EDS). Phases formed in the process of the flux crystallisation included CaSiO3, Ca2MgSi2O7 and Ca11Si4B2O22. It revealed that CaSiO3 was the major phase at low CaO/SiO2 ratio. The amount of Ca2MgSi2O7 and Ca11Si4B2O22 increased with increasing CaO/SiO2 ratio