In-situ Study of Crystallisation Behaviour of CaO-SiO_2-Na_2O-B_2O_3-TiO_2-Al_2O_3-MgO-Li_2O Fluorine-free Mould Fluxes with Different CaO/SiO_2 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_2O-B_2O_3-TiO_2-Al_2O_3-MgO-Li_2O 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_2MgSi_2O_7 and CanSi_4B_2O_(22). It revealed that CaSiO_3 was the major phase at low CaO/SiO_2 ratio. The amount of Ca_2MgSi_2O_7 and Ca_(11)Si_4B_2O_(22) increased with increasing CaO/SiO_2 ratio.