Crystallisation Behaviour and Heat Transfer Phenomenon of Fluoride-free Mould Fluxes for Steel Continuous Casting
Mould fluxes provide indispensable functions for steel continuous casting, such as heat transfer control, lubrication, thermal insulation, inclusion entrapment, and oxidation prevention. Most of the commercial mould fluxes contain fluorides to ensure high steel quality, but they also bring about environmental and health hazards due to the high volatility of fluorides. This project aims at understanding the relationship amongst chemical composition, crystallisation behaviour and heat transfer of fluoride-free mould fluxes by using hot thermocouple technique and infrared emitter technique together with x-ray diffraction and scanning electron microscopy. A commercial mould flux for low carbon steel casting was studied as a benchmark. The critical cooling was 7 degree Celsius/s and the shortest incubation time was 15 s at 1000 degree Celsius. Dominant precipitate was Ca2SiO4 at 950 and 1100 degree Celsius with minor Na2CaSi3O8 and Ca2MgSi2O7. Cuspidine was only found at 950 degree Celsius. For fluoride-free mould fluxes, the effects of CaO/SiO2 ratio, Na2O and B2O3 on the crystallisation kinetics of mould fluxes were investigated by establishing continuous cooling transformation and time-temperature transformation diagrams. The increase of CaO/SiO2 ratio or Na2O content promoted crystallisation tendency of mould fluxes by increasing the critical cooling rate and shortening the incubation time. Whilst the increase of B2O3 content had a reverse effect on crystallisation tendency. Phase identification showed that the dominant phase changed from CaSiO3 to Ca2MgSi2O7 and Ca11Si4B2O22 with increasing CaO/SiO2 ratio or Na2O content, also with decreasing B2O3 content. The heat transfer measurement showed that the heat flux decreased first with the increase of CaO/SiO2 ratio or Na2O content, but it started to increase when CaO/SiO2 ratio or Na2O content exceeded 1.1 or 9 mass%. A similar behaviour of heat transfer was also found when varying B2O3 content. The heat transfer rate reached the lowest value when CaO/SiO2 ratio was at 1.1, Na2O content at 9 mass% and B2O3 at 6.8 mass% in current system. The heat transfer across mould flux film depends on not only crystallisation tendency of mould fluxes but also crystal morphology.
Crystallisation, Mould fluxes, Fluoride-free, Heat transfer
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