| With the development of continuous casting theory and technology,the quality_of continuous casting billet has been improved continuously.In the industrial production,surface and subcutaneous defects of continuous casting billet are still one of the important factors restricting the quality of finished products,the rate of finished products and the economic benefits of enterprises.These defects are closely related to the shape of meniscus and its initial solidification behavior.The shape of meniscus depends on the interfacial tension between mold powder and steel,which is governed by the compositions and temperature of mold powder and steel.The crystallizer has a strong cooling capacity,and there must be a continuous temperature gradient across the solidus temperature and the liquidus temperature on the meniscus.The existence of this temperature gradient will affect the shape of the meniscus and the overflow behavior of molten steel.This study focused on the meniscus region in the continuous casting mold,and investigated the evolution behavior of surface tension and interfacial tension between slag and steel during temperature-changing process.The evolution mechanism of surface tension of slag and steel was proposed.On the basis of this,the relationship of the overflow behavior and the formation of oscillation marks was discussed.The research results are of great significance for analyzing the initial solidification behavior of molten steel in the mold and guiding the practice of continuous casting production.In order to improve the accuracy and efficiency of the surface tension measurement of molten steel,according to the principle of surface tension measurement by sessile drop method,a calculation program for solving the surface tension of droplets by curve fitting method was developed.The program includes extracting droplet image data,constructing objective function,selecting variable parameters,establishing and solving numerical equations.Finally,an ideal droplet edge contour curve can be fitted to minimize the objective function value,and then the surface tension can be calculated.The experimental verification results show that the developed program runs stably and reliably with high computational efficiency,which lays good foundation for the subsequent determination of surface tension of molten steel.Based on the developed surface tension calculation program and thermodynamic analysis,the evolution behavior and influence mechanism of surface tension of Fe-C-S steels during continuous cooling process were investigated.The results show that in the liquid phase,the surface tension of molten steel decreases with the decrease of temperature.In the mushy zone,there are significant differences in the evolution behavior of surface tension of molten steel:when the mass fraction of carbon is high(0.39%and 0.95),the temperature range of the mushy zone is wide,and C and S have sufficient time to segregate during solidification.The mass fraction of solute in the liquid phase increases with the decrease of temperature,which leads to the significant decrease of surface tension of molten steel.When the mass fraction of carbon is low(0.0021%,0.063%and 0.148%),the temperature range of the mushy zone is relatively narrow,and the solidification time is relatively short.Thus,C and S don't have sufficient time to segregate or the segregation is less during the solidification process,which has no significant impact on the surface tension of molten steel.With the increase of mass fraction of sulfur in steel,the effect of segregation of S on the surface tension of molten steel is gradually weakened;the peritectic reaction can slow down the reduction of surface tension by affecting the cooling process of molten steel.In addition,the influence of non-metallic inclusions in steel on the surface tension of molten steel was investigated.The results show that the floating of non-metallic inclusions on the surface of molten steel can reduce the surface tension of molten steel.With the prolongation of standing time after deoxidation of aluminum,the cleanliness of molten steel gradually increases,and the surface tension of molten steel gradually increases and finally becomes stable.According to the composition of the mold powder in industrial production,the surface tension of CaO-SiO2-Na2O-CaF2 slag system was measured by the barrel method,and the influence of temeprature was investigated.The results show that the surface tension of slag decreases with the increase of temperature.In the lower temperature range(1350?1410?),the surface tension of slag with higher basicity(1.03 and 1.16)is obviously affected by temperature,while the surface tension of slag with lower basicity(0.67 and 0.85)is less affected by temperature.In the higher temperature range(1410?1580?),the decreasing trend of surface tension of slag becomes gentle.In order to reveal the microscopic nature of the variation of surface tension of slag with temperature,the structure of slag was measured and analyzed.The results show that the evolution of surface tension had a significant consistency with the transformation of the structure of melt.When the basicity of slag is lower(0.67 and 0.85),the polymerization degree of melt is higher,and the influence of temperature on the structure of the slag is less.When the basicity of slag is higher(1.03 and 1.16),the temperature has a great effect on the structure of the slag.As the temperature increases,the polymerization degree of melt decreases rapidly,the average radius of the anion groups in the melt decreases,the molecular thermal motion increases,and the intermolecular interaction reduces,which ultimately leads to a decrease in the surface tension of the slag.According to the results of surface tension measurement of molten steel and slag,the evolution behavior of interfacial tension between slag and steel during continuous cooling process was determined,and the shape equation of meniscus based on temperature-changing process was constructed.The results show that the sudden change of surface tension of molten steel during the continuous cooling process determines the changing trend of interfacial tension between slag and steel,which determines the discontinuity of the shape of meniscus.When the temperature range of mushy zone is sufficiently wide,the meniscus in the zone tends to be close to the mold wall,and the molten steel has a tendency to overflow.Combined with the actual solidification process of the meniscus in the mold,it is shown that the interfacial tension determined by the compositions of the molten steel and the width of the solidification interval control the' overflow mode of the molten steel and the types of the oscillation marks.For the ultra-low carbon steel represented by IF steel,the interfacial tension between slag and steel is large,the temperature range of mushy zone is small,and the solidification distance of meniscus is long.Under a certain degree of supercooling,the overflow of molten steel occurs late,and it is easy to form periodic covering oscillation marks.For the medium-carbon sulfur-containing steel,the temperature range of mushy zone is wide,and the interfacial tension between slag and steel is obviously reduced.Acertain length of solidification distance can be formed on the meniscus,and the overflow of molten steel occurs earlier,which leads to the formation of periodic concave oscillation marks without obvious coverage.For the high carbon steel similar to bearing steel,the temperature range of mushy zone is wide,the interfacial tension between slag and steel is much smaller,and the meniscus is closer to the crystallizer wall,showing that there is no significant overflow behavior of molten steel,which leads to the formation of the hook oscillation marks without coverage and periodic form. |
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