Fundamental Research On High-temperature Reduction Pretreatment Of Continuous Casting Billet

For thick plates,internal quality problems,such as segregation and porosity,which occur inevitably during the continuous casting process,would deteriorate mechanical properties and shorten the product service life circle.Subsequent rolling process affected hardly on the improvement of internal quality.To solve the internal quality problems of thick plates,the high-temperature reduction pretreatment process was conducted on 42CrMo billet in this work.The improvement mechanism of different process parameters on the internal quality of billet and the influence on the microstructure evolution was systematically studied through finite element simulation and experiment.The healing mechanism of internal porosity and the improvement mechanism of center segregation were studied.To obtain the high-temperature finite element parameters of 42CrMo steel,the Gleeble-3500 thermal simulator was employed in operating the hot compression experiment.The high-temperature flow behavior of 42CrMo steel at the temperature of 1200-1350? and the deformation rate of 0.001-10s-1 was studied.Moreover,two typical constitutive models,strain compensated Arrhenius constitutive model and back propagation artificial neural network(BPANN)model,were used to predict the curve,and the prediction performance was evaluated.Based on the hot processing map of Prasad instability criterion,the optimal process range at high-temperature reduction pretreatment was obtained,namely,at 12501350?,0.01-1s-1.Based on the temperature field obtained by solidification cooling simulation,the influence of different process parameters on the equivalent plastic strain in the thickness direction was studied by simulation and the actual high-temperature reduction pretreatment process experiment.With the increase of surface temperature,the reduction amount and the decrease of the reduction pass,the internal equivalent plastic strain and the density increased.The defect improvement mechanism is that the internal equivalent strain is increased by temperature gradient deformation so that the small plastic deformation can change the size and shape of the hole and press the hole completely.To study the improvement effect of high-temperature reduction pretreatment process on the center segregation of slab,the element distribution was analyzed by scanning electron microscopy(SEM),original position statistic distribution analysis(OPA),and electron probe microanalysis(EPMA).Four kinds of microstructures are observed in this study.Lath bainite matrix and granular structure on segregation bands mainly dominate the microstructure of the original billet.With the increase of surface temperature,the reduction of reduction passes and the reduction amount,the segregation coefficients of Mo,Mn,Cr,and Si elements show a decreasing trend.The improvement mechanism of center segregation is that plastic deformation promotes austenite recrystallization.The increase of austenite grain boundaries increases the diffusion coefficient of solid solution elements,thus improving the micro-region segregation at the core.