Study Of Crystallization Kinetics And Structure Of Slag Film In The Continuous Casting Mold

Regarded as functional material of the continuous casting process, continuous casting mold fluxes plays an important role in controlling the heat transfer and lubrication of the slab in the mold. The increase in crystallization behavior of mold fluxes, which can reduce the heat transfer between mold and slab, is a common method to minimize surface defects, especially for crack-sensitive steel grades at home and broad. The crystallization behavior of mold fluxes is not related on the composition, but also on the cooling rate, that is, The crystallization behavior of mold fluxes is related on the crystallization kinetics. However, the investigation on this dissertation is not further enough. Therefore, the influence of the chemical composition on crystallization kinetics of mold fluxes has been focused on by the application of Single Hot Thermocouple Technique (SHTT), and the effect of composition of the mold fluxes on the slag film structure and heat transfer properties has been studied by the use of homemade Heat Flux Simulator of Mold Slag Film Equipment. The conclusions can lay theoretical foundation for the control of the heat transfer and lubrication between the mold and the slab.The influence of the chemical composition and cooling rate on the crystallization temperature and critical cooling rate of mold fluxes has been approached through constructing the Continuous Cooling Transformation (CCT) of mold fluxes. With the increase of basicity, [F-], the critical cooling rate has been be increased, and the crystallization temperature of mold fluxes has been increased at the same cooling rate, hence the improvement of crystallization behavior. The effect of basicity is stronger than that of [F-]. Owing to the increase of Na2O, Al2O3, B2O3 and MnO, the critical cooling rate has been reduced, and the crystallization temperature of mold fluxes has been reduced at the same cooling rate, which has contributed to the inhibition of its crystallization capacity. The Li2O(<1.5 mass%),MgO(<3mass%), in its low level, could inhibit the crystallization of mold fluxes and improve its lubricating property with the increase of content and the decrease of the critical cooling rate. Whereafter, the critical cooling rate has had a tendency to increase. When Li2O is increased from 1.5mass% to 2mass%, the critical cooling rate has had an obvious increase and then its changes has been a little bit slow. It is put forward that the crystallization capability of mold fluxes could be improved by adding Li2O, and the content of Li2O in the mold fluxes should be equal to or more than 2 mass%. As for the effect of MgO, the critical cooling rate of mold fluxes has had an obviously increasing tendency with the increase of MgO level. When the MgO is equal to 6mass%, the critical cooling rate of the sample has reached the upper limit of mold fluxes's cooling rate(₂0℃/s)beyond which the mold fluxes could not well control the heat transfer of mold. To the same mold fluxes, different cooling rates have resulted to different crystallization temperature. And the increase of cooling rate has led to the decrease of crystallization temperature.The influence rule of chemical composition of mold fluxes on the incubation time, namely the crystallization rate, has been investigated by constructing its Temperature Time Transformation (TTT) curve. TTT curve of mold fluxes shows that the temperature is becoming higher and incubation time is getting shorter with the increase of basicity, F-, Na₂O, Li₂O and MgO. When the basicity is 1.4, Na₂O 9mass%, Li₂O ₂mass%, MgO 6mass%, at a certain low temperature, the incubation time is zero, and the slag film which is at the meniscus near the mold wall has been crystallized quickly, and the interfacial thermal resistance has been improved to control the heat transfer. It is suggested that in order to cast the crack-sensitive steel grades, both controlling heat transfer of meniscus and preventing the surface longitudinal crack should be considered, and the basicity of the mold fluxes should be above 1.4. The Al₂O3 and B₂O3 could expand the incubation time of the mold fluxes, and TTT curve shows that the temperature is reducing and the crystallization rate of mold fluxes is slowed down. With the increase of Li₂O, the incubation time has been shortened and the crystallization rate has been accelerated, and then the temperature become lower when the incubation time is zero. It shows that with the increase of Li₂O the liquid slag film will increase so as to ensure the lubrication in the continuous casting process, and resolve the contradiction that appears when the heat transfer of mold fluxes has been controlled conventionally by improving the basicity, which leads to the reduction of liquid slag film and the impairment of lubricating property of mold fluxes. The research results have been proved by the practice experiment.. The TTT curve can discribe the crystallization kenetics of the powder through analyzing the kinetics functions of the isothermal crystallization process.It has been confirmed by X-ray diffraction and SEM that the crystal phases precipitated from the sample mainly consists of cuspidate(Ca₄Si₂O7F₂)and calcium silicate (CaO·SiO₂) or cuspidine (Ca₄Si₂O7F₂), and when the basicity reaches 1.5, or the content of F- is equal to 10mass%, there is only one crystal phase precipitated, namely cuspidine. Ca₂Mg(Si₂O7) has precipitated from the sample with high MgO level, which prevents the precipitation of cuspidine.The value of heat flux, slag film structure, crystal types, tested by the Heat Flux Simulator of Mold Slag Film Equipment, are in good agreement with the influence rules of the slag film in practice. However, as the slag film got from the laboratory stayed in high temperature longer than that got from the practice did, the size of the crystal grain has been smaller. As this test has only took the heat transfer property of slag film into consideration, ignoring the effect of some relevant technological parameters, such as the casting speed, the value of the heat flux, measured in the laboratory, has not been equal to that got from the actual production, but the rule has been in a good agreement. The influence rules of basicity, content of [F^-], Na₂O, trace component of Li₂O, MnO, B₂O3, MgO on the slag film structure (solid slag film thickness, crystal ratio, surface roughness ) and heat transfer have been analyzed and investigated systematically. The relation between powder compositions and the powder lubrication, represented by the thickness of solid slag film, and the capacity of controlling heat has been established. The research results shows that the main factors to affect the heat transfer and flux of slag film are the thickness of slag film of the thermal resistance and the surface roughness of slag film of the interface resistance. However, when the MnO is added into the mold fluxes, the absorption coefficient of the slag film has been improved, and the cut-off wavelength of the infrared ray has been decreased, and the thermal radiation capability has been reduced. Therefore, the effect of MnO component on the radiation heat transfer should be considered. With the raise of the melting point, the thickness of solid slag film and the crystal ratio has been increased, while the heat flux has been reduced. With the raise of the viscosity , the thickness of solid slag film has been increased, and the crystal ratio has been reduced. The coaction of solid slag film thickness and the crystal ratio could cause the heat transfer flux and the surface roughness of slag film not to be the not monotone function of the viscosity.It has been put forward that taking heat control into consideration, the method of high crystal ratio and low basicity of the mold fluxes or low crystal ratio and high basicity of the mold fluxes could be devised. Considering the improvement of lubricating property (thick liquid slag film), the method of high crystal ratio and low basicity of the mold fluxes has been more suitable. The adding of ₄mass%Al₂O3 has prevented the precipitation of the tabular spar and Ca₂(SiO₄). And the compact cuspidine is the only precipitated crystal phase. The surface the slag film has been rougher and the comprehensive heat transfer flux had been decreased. If the liquid slag film is thick enough to ensure the lubrication, the heat transfer could be controlled by adding Li₂O, which is in agreement with the results of the TTT curve. If there is 8mass% Na₂O added, and the content of Li₂O is limited between ₂mass% to 3mass%, both the good lubricating property and relatively stable heat transfer control will be ensured.Through the test and analysis on industrial mold power, the relation between the average heat flux density of slag film of the slab, tested in laboratory at different casting speeds, and comprehensive heat flux density of industrial mold. Thus, the measured heat flux density of mold fluxes in laboratory can correspond with heat flux of on-line detection in practice.Based on the study of the distribution rules of heat flux density on mold fluxes, the quantitative equation of transient average heat flux density, which was q=a-b t , has been approached. The research results have a good agreement to the production practice, which will lay a solid foundation for the design and development of mold fluxes, from experience to science.