| The quality of tinplate steel has an important effect on the properties of tinplate,and the inclusions in tinplate steel often lead to defects in products during the stamping process.In recent years,with the increasing demand for the high performance tinplate,both tinplate steel and tin coating are becoming thinner,resulting in the fact that tinplate become increasingly sensitive to the inclusions in tinplate steel.Therefore,the strict control of inclusions in tinplate steel during production process is critical to improve the product quality.In the present work,by taking the evolution characteristics and modification kinetics of Al2O3 inclusion during RH refining of tinplate steel as the research focus,the composition,size distribution,quantity,and morphology evolution of inclusion were studied based on sampling analysis in a domestic steel plant.Based on the kinetics model of calcium transfer from top slag to liquid steel,the rate-limiting steps of calcium transfer process were investigated,and the time of calcium transfer from top slag to liquid steel to meet the modification requirement of Al2O3 inclusion was determined.The distribution of calcium element on the sides of molten slag and liquid steel was studied by high temperature experiment,and thus the results of kinetics model analysis were verified.In addition,a kinetic model of Al2O3 inclusion modification in liquid steel was established based on un-reacted core model,and the time required for Al2O3 inclusion modification to liquid calcium aluminate inclusion completely was calculated.The main research results are as followings.(1)Al2O3 inclusions are the main inclusion types after Al addition during the first refining stage.MgO·Al2O3 spinel inclusions and CaO-Al2O3 inclusions are the main inclusions types in the mid-refining stage.At the end of refining process,the main inclusions in the steel are Al2O3,and meanwhile,small amounts of CaO-MgO-Al2O3 system inclusions with low melting point and the of CaO-MgO-Al2O3 composite inclusions with high melting point are found to exist in liquid steel.(2)The calculation results of calcium transfer from top slag by the established kinetic model show that the transport of calcium in liquid steel side is the rate-determining step of calcium transfer process.Calcium content in liquid steel is only 17.39 ppm after 25 min of RH refining process,which is much smaller than the minimum calcium content required to modify Al2O3 inclusions.(3)According to the result of calcium transfer from high basicity top slag through a high temperature experiment,no calcium element is detected in liquid steel with the refining time of 60 min,whereas the increment of calcium element content in the liquid steel side is observed with the refining time of 120 min.The experiment results indicate that the ineffective calcium transfer from high basicity top slag to liquid steel,which is consistent with the analysis result based on the kinetic model.(4)The calculation result of kinetic model of Al2O3 inclusion modification shows that the time required for modification of Al2O3 inclusion is much shorter the time required for calcium transfer from top slag to liquid steel.When the calcium content in liquid steel is 47 ppm,the modification of Al2O3 inclusions with particle size less than 15 μm is complete within 70 s.(5)The modification of Al2O3 inclusion to liquid calcium aluminate inclusion is invalid merely by the calcium transfer from high basicity top slag to liquid steel.In order to realize the modification of Al2O3 inclusion to liquid calcium aluminate inclusion completely,the calcium treatment technology is proposed by combining the calcium transfer from high-basicity top slag and calcium-feeding operation. |
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