Study On Crystallization Properties Of Mold Fluxes Using Single Hot Thermocouple Technique

The heat transfer and lubrication properties of the mold/strand gap are directly influenced by crystallization properties of mold fluxes. The increase in crystallization of mould fluxes is a usual way to minimize surface defects especially for crack-sensitive steel grades. But it will increase the frequency of sticking type breakout by lack of lubrication.between strand and mold. Therefore, the crystallization properties of mold fluxes should be controlled in a reasonable range. The influence of the chemical composition on crystallization properties of mold fluxes was focused on in the paper, in order to establish the theoretical basis of lubrication and heat transfer properties of mold fluxes.The ways of studying on crystallization properties of mold fluxes are prior research, including Differential Thermal Analysis (DTA), temperature-viscosity and Single Hot Thermocouple Technique (SHTT). The results show that the crystallization temperature of a mold flux measured by DTA was lowest, and that of SHTT was in the middle. The maximum cooling rate of SHTT can achieve 150℃/s. Moreover, crystallization occurring in the sample can be observed in situ under defined thermal conditions, and the growth rate of crystals and the changes in the crystallized fraction can be measured. Therefore, the solidification simulation of mold fluxes can be provided by using SHTT.The influence of the chemical composition on the critical cooling rate of mold fluxes was obtained by constructing of Continuous Cooling Transformation (CCT) using SHTT. The critical cooling rate of mold fluxes was increased with the increase of basicity, F-, Li2O and MgO. However, Na2O, Al2O3, B2O3 and MnO have a negative effect on the crystallization properties of mold fluxes.In addition, the influence of the chemical composition on the incubation time was also investigated by constructing Temperature Time Transformation (TTT) diagram using SHTT. The results show that TTT diagrams moved to higher temperature and shorter incubation time with the increase of basicity, F-, Na₂O, Li₂O and MgO. While the crystallization speed of mold fluxes was decreased with the addition of Al₂O₃, B₂O₃ and MnO.It was confirmed by X-ray diffraction and SEM that the crystal phases precipitated were calcium silicate (CaO-SiO₂) and cuspidine (Ca₄Si₂O₇F₂), respectively. When the basicity reached 1.5 or the content of F- was 10 mass%, cuspidine was the only crystal phase precipitated. Ca₂Mg(Si₂O₇) was promoted to prevent the precipitation of cuspidine when the content of MgO in the sample was very high.The analysis of crystallization kinetics shows that the rate coefficient K and the exponent n increased with the increase of basicity. The ratio of Tnose to TLiquidus is close to 0.80, which indicates that there is under cooled in the crystallization process of mold fluxes. The critical cooling rate of mold fluxes increased and the average incubation time was decreased with the increase of basicity, resulting in the increase of the thickness of the crystalline layer and the interface thermal resistance to reduce the heat flux. The crystallization properties and the thickness of mold fluxes film can be represented by the critical cooling rate and incubation time.