Simulation Of Microsegregation And Microstructure Of Continuous Casting Strand And Theoretical Investigation On Soft Reduction

Soft reduction technology, as an effective way to eliminate the centerline segregation and porosity, has been widely carried out in steel continuous casting process. However, the solute segregation behavior of the mushy zone at the final stage of solidification, closely affected by dendrite growth, solute redistribution, fluid flow and strand shell deformation caused by soft reduction or bulge, is very complicated. The theoretical research works of the soft reduction technology are relatively rare, and the application of the soft reduction technology mainly relys on plant trials and experience, so it is difficult to develop a reasonable soft reduction process according to the continuous casting process and steel grade. In this paper, on the basis of the detailed description of the microsegregation and dendrite growth of the mushy zone at the final stage of solidification and strand shell deformation caused by soft reduction, a mathemathical model accounting for the discharge of solute-enriched liquid from the mushy zone with the soft reduction by one pair of pinch rolls was established, and the criterion for the theoretical determination of the soft reduction zone was firstly proposed according to the improvement degree of the solute segregation ratio of the mushy zone after the application of soft reduction. Finally, the achievements of the theoretical work have been successfully used in bloom continous casting. The main contents and achievements are as follows:(1) Based on the regular hexagon dendrite morphology proposed by Uehsima, a solute microsegregation model of the mushy zone was developed with the consideration of delta-ferrite/gamma-austenite phase transformation and MnS inclusion precipitation during the solidification of continuous casting steel. The simulation results show that the solute segregation of mushy zone at the final stage of solidification is closely related to the solidification mode of steel. With the increase of nominal carbon content, the solidification mode of steel changes from lâ†'δ-ferrite, to l+8-ferriteâ†'γ-austenite, and finally to lâ†'y-austenite. The interdendritic solute segregation ratios of Si, P and S at the final stage of solidification increase slowly, but the interdendritic solute segregation ratio of C decreases slowly and the interdendritic solute segregation ratio of Mn almost keeps the same level. Finally, the comparison between the calculated results and experimental datas of thermomechanical temperature of steel shows that it is reasonable to predicate the ZST and ZDT by calculated temperature at solid fraction of0.75and1.0, respectively.(2) A combined cellular automaton-finite difference (CA-FD) model has been developed to simulate microstructure of continuous casting steel, with a consideration of the solutal and curvature undercooling of the dendrite tip. The simulated microstructure of prestressing wire steel SWRH82B shows that the increase of the superheat or the secondary cooling ratio will accelerate the dendrite growth, but restrain the equiaxed dendrite nucleation and growth in the center of strand, and finally result in columnar coarse, columnar zone larger and equiaxed zone smaller. With the increase of superheat from10℃to40℃, the equiaxed ratio decreases from44.3%to21.7%, and the average grain diameter also decreases from1.64mm to1.94mm. With the increase of secondary cooling ratio from0.45L/kg to0.73L/kg, the equiaxed ratio decreases from35.5%to16.5%, and the average grain diameter increases from1.77mm to2.07mm. Therefore, the reasonable superheat and soft cooling are effective methods to achieve the high equiaxed ratio of strand.(3) The three-dimensional deformation behavior of continuous casting bloom during the soft reduction process was theoretically characterized, and the theoretical equations for lateral elongation efficiency, longitudinal elongation efficiency, reduction efficiency and longitudinal elongation ratio of bloom were proposed. The FEM analysis of the prestressing wire steel SWRH82B strand deformation behavior caused by the soft reduction shows that when the non-solidification ratio (liquid zone area/total cross section area) is same, the lateral elongation efficiency and reduction efficiency of bloom increase firstly and later keep the same level, but the longitudinal elongation efficiency of bloom decreases firstly and later keeps the same level and longitudinal elongation ratio of bloom increases linearly with the increase of reduction amount.(4) From the centerline segregation elimination point that the solute-enriched liquid in mushy zone shoud be squeenzed into the upstream liquid region with soft reduction, a mathemathical model accounting for the discharge of solute-enriched liquid from the mushy zone with soft reduction by one pair of pinch rolls has been established as following: Where Ki is solute segregation ratio at the center of bloom after the soft reduction, ps and pl are solid and liquid density, respectively, ε is solute discharge ratio, fs is solid fraction, and Cs,i and Cl,i are the average solute concentration in solid and liquid phase immediately before the soft reduction, respectively. From the improvement degree of the solute segregation ratio of the mushy zone after the application of soft reduction, the theoretical criterion for the reduction zone was firstly proposed that the start point of the reduction zone should not be greater than the minimum optimum reduction position and the end point of the reduction zone should not be less than the maximum optimum reduction position. And the study on the effect of chemical composition on the optimum reduction zone shows that the optimum reduction positions for each element are affected by nominal carbon content, because the steel solidification mode changes with the variation of nominal carbon content. But the other elements contents have no effect on the optimum reduction positions of each element.(5) The industrial trials and application of the theoretical model of the soft reduction zone show that the centerline segregation and porosity are eliminated and internal quality is improved significantly, when the soft reduction are performed at optimum soft reduction zone, which includes the optimum reduction positions of each element of steel. For the bearing steel GCr15, the cord steel SWRH72A and the prestressing wire steel SWRH82B, the carbon segregation ratio of the cross section decreases from0.908～1.269,0.987～1.280and0.867～1.459to0.924～1.060,0.947～1.045and0.918～1.069, respectively. The carbon segregation ratio of the longitudinal section decreases from0.875～1.390,0.901～1.378and0.875～1.309to0.872～1.086,0.869～1.084and0.872～1.086, respectively. The ratio of the center carbon segregation ratio of the longitudinal section between0.95～1.05increases from18.7%,23.3%and21.2%to94.6%,92.7%and92.9%, respectively. The ratio of the center porosity between0～1.5increases from49.45%,48.78%and52.78%to85.00%,86.95%and90.91%, respectively. And the ratio of the center cavity between0-1.5increases from63.74%,68.29%and66.67%to87.5%,96.65%and96.36%, respectively.