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

摘要

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–SiO_(2)–Na_(2)O–B_(2)O_(3)–TiO_(2)–Al_(2)O_(3)–MgO–Li_(2)O fluorine-free mould fluxes with CaO/SiO_(2) 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/SiO_(2) 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/SiO_(2) 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 CaSiO_(3), Ca_(2)MgSi_(2)O_(7) and Ca_(11)Si_(4)B_(2)O_(22). It revealed that CaSiO_(3) was the major phase at low CaO/SiO_(2) ratio. The amount of Ca_(2)MgSi_(2)O_(7) and Ca_(11)Si_(4)B_(2)O_(22) increased with increasing CaO/SiO_(2) ratio.