Thermal Simulation Investigation On Solidification Structure And Defects Control Of Continuous Casting Bearing Steel Billet

With the rapid development of equipment manufacturing industry in China,the demand for high-quality bearing steel is increasing,which drives bearing steel industry to develop in the direction of high output and high quality.How to improve product quality and performance has increasingly become the focus of research and development of many steel enterprises.Taking the industrial grade GCr15 bearing steel billet as the research object,aiming at the key parameters of superheat and cooling intensity,electromagnetic stirring and solidification end reduction in industrial production,a new thermal simulation method is developed.Based on the limitation of the current research on the solidification process of continuous casting billet,a new industrial thermal simulation equipment with various process control method for continuous casting process is established.The influence mechanism of different process conditions on solidification structure and defects of continuous casting bearing steel billet was studied.The main conclusions are as follows:Without electromagnetic stirring,the equiaxed grain ratio has a linear relationship with superheat and cooling intensity respectively,the effect of superheat on equiaxed grain ratio is greater than that of cooling intensity.With the increase of superheat,the secondary dendrite arm spacing at the surface decreases gradually,and at 1/2 radius and in the center increase gradually.The local cooling rate at 1/2 radius of billets decreases linearly with superheat;With the increase of cooling intensity,the secondary dendrite arm spacing at the surface,at 1/2 radius and in the center both decreases,the local cooling rate at 1/2 radius of billets increases linearly with the cooling intensity;The central segregation increases with the increase of superheat and cooling intensity,and the best linear fitting model between the maximum carbon segregation ratio and equiaxed grain ratio is: r_Cma=-0.0018P_e+1.097.Based on Hunt model,the simplified criterion of CET transition without electromagnetic stirring is: G^3.4/V≤3.62×10¹⁵.The three-dimensional coupling model of magnetic field-flow-heat transfersolidification is established.The magnetic field,flow field and temperature field under electromagnetic stirring in the mold are numerically simulated,and the influence of molten steel flow on electromagnetic force is considered.The results show that with the increase of current intensity,the magnetic induction intensity in molten steel increases linearly and the electromagnetic force increases in the form of second power,The central temperature changes slightly in the range of 50 A ～ 150 A and decreases rapidly in the range of 150 A ～ 250 A.when it exceeds 250 A,the temperature change slows down and tends to be stable;With the increase of stirring frequency,the magnetic induction intensity inside the molten steel decreases exponentially,the electromagnetic force firstly increases and then decreases,reaches the maximum at 3 Hz,and the central temperature has little change in the range of 1Hz ～ 5 Hz.If it exceeded 5 Hz,the central temperature begins to rise sharply;the superheat dissipation of mold electromagnetic stirring makes the solidified shell thinner and inhibits the growth of shell,resulting in the "spindle" distribution of liquid steel flow field when stirring is completed,and with the increase of stirring intensity,the secondary flow gradually increases,the depth of the central high temperature zone gradually becomes shallow,and shrinks to the central area of the stirrer in a double concave shape;The velocity of liquid steel and the heat transfer between liquid steel and mold are both reduced with considering the solidification effect,which is more consistent with the actual situation.Under the mold electromagnetic stirring,with the increase of stirring intensity,the equiaxed grain ratio increases linearly and the mixed grain ratio decreases linearly;the secondary dendrite arm spacing at the surface,at half radius and in the center decreases,the local cooling rate at 1/2 radius of billets increases exponentially with stirring intensity;the fractal dimension of dendrites gradually increases;the central segregation firstly decreases and then increases,reaching the minimum near 0.28m/s.With the increase of superheat,the equiaxed grain ratio decreases linearly,the mixed grain ratio increases exponentially;the secondary dendrite arm spacing at the surface gradually decreases,at 1/2 radius and in the center gradually increase,the local cooling rate at 1/2 radius of billets increases exponentially with stirring intensity;the fractal dimension of dendrites firstly increases and then decreases;the central segregation firstly decreases and then increases and reaching the minimum value near 30 ℃.GPD method can be used to predict the variation trend of central segregation of billets under different conditions,and it is more accurate to use the maximum value of carbon segregation ratio;In numerical simulation,in the range of experimental liquid steel flow velocity,the dendrites are difficult to be broken without necking,and the stress at the root of dendrites will increase greatly when necking occurs,which can reach 2 ～ 3 times of its elastic limit.Therefore,dendritic fracture likely to be a necking fracture mechanism.Under the final electromagnetic stirring,the magnetic induction intensity in the liquid pool increases linearly and the electromagnetic force increases in the form of second power with the increase of current intensity;With the increase of stirring frequency,the magnetic induction intensity in the liquid pool decreases exponentially,and the electromagnetic force first increases and then decreases;With the increase of the liquid pool width,the magnetic induction intensity in the liquid pool does not change,and the electromagnetic force and flow velocity increase linearly;The central segregation firstly decreases and then increases with the increase of stirring intensity and reaching the minimum near 0.11 m/s.The central segregation gradually decreases with the increase of liquid pool width.Compared with changing the liquid pool width,increasing the stirring intensity has a more significant refined effect on the solidification structure in the center of the billets,and both help to reduce the equivalent diameter of the central porosity and improve its sphericity.In the process of solidification end reduction,the reduction efficiency increases with the increase of liquid pool width,and firstly increases and then decreases with the increase of reduction amount,reaching the maximum value at 6 mm.The reduction efficiency,reduction amount and liquid pool width meet the first-order polynomial relationship.At the same time,the tendency of intermediate cracks increases with the reduction efficiency;the central segregation decreases gradually with the increase of reduction amount,reaches the minimum value when the reduction amount is 8 mm,continues to increase the reduction amount,the central segregation does not change;with the increase of liquid pool width,the central segregation firstly decreases and then increases,and reaches the minimum value near20 mm;compared with changing liquid pool width,increasing reduction amount is more helpful to the refinement of solidification structure in the center,the decrease of equivalent diameter of central porosity and the improvement of sphericity.Comparing the improvement effects of final electromagnetic stirring and solidification end reduction on central segregation under different solidification ratio（ratio of shell thickness to equivalent radius）,it is found that when the value is greater than 0.82,solidification end reduction is better than final electromagnetic stirring,and when it is less than 0.82,final electromagnetic stirring is better than solidification end reduction.