Study On Surface Cracking And Microstructure Control For Peritectic Steels During Continuous Casting

The control of surface transverse cracks in continuous casting of steel grades with hypo-peritectic or peritectic point composition has been a long-term problem in the production of low-alloy high-strength steels.Due to the limitations of the traditional method of strand surface temperature control,improving the hot ductility of strand surface from the microstructure has become an important approach to reduce the crack sensitivity of such steels in the continuous casting,charging,and reheating processes.The microstructure factors related to as-cast ductility mainly include the prior austenite grain size and the formation of pro-eutectoid ferrite and carbonitride at the austenite grain boundary.Considering the difficulty in modifying the ferrite transformation and carbonitride precipitation behaviors during the cooling of conventional continuous casting,this study proposes a research route of suppressing the primary austenite grain coarsening,the carbonitride precipitation in continuous casting,and the formation of film-like ferrite in reheating process for controlling the surface cracks.The cooling rate significantly affects the coarsening of primary austenite grains,which mostly occurs in the depressions or oscillation marks on the strand surface.By analyzing the contributions of shrinkage at different positions in the solidified shell on the surface depression during solidification and cooling,a model for predicting the surface depression degree was constructed.This model was verified using the measured data such as the heat flux density in the mold.The results show that the degree of shell depression is closely related to the carbon content.The two largest depressions appear in the ultra-low carbon and hypo-peritectic regions,respectively,which is in agreement with the measurements.Then,a polynomial model for predicting the carbon content corresponding to the maximum depression was obtained through the linear regression method.The above models provide a reliable basis for the composition optimization in hypo-peritectic steels which owns the great surface depression sensitivity.The formation mechanism of coarse austenite grains on the strand surface is still controversial.Based on the kinetics analysis and solidification experiments,the coarsening mechanism of primary austenite grains of the peritectic steels was studied.According to the experimental results of isothermal grain growth at high temperatures of 1300?1450?,the kinetics of primary austenite grain growth was analyzed by the solute dragging theory,which was revealed as an ideal grain growth at the high-temperature stage.This behavior can well describe the austenite grain growth during slow-cooling solidification.However,as for the rapid-cooling solidification condition,which corresponds to the initial solidified shell in continuous casting,the actual grain coarsening should be faster than the ideal grain growth.Through the solidification experiments with the different cooling rates of 0.15?10?/s,the existence of two mechanisms of primary austenite grain growth was demonstrated.The grain coarsening rate during rapid-cooling solidification is much greater than the one during slow-cooling solidification.The critical cooling rate between the grain growth mechanisms was determined as 0.309?0.483?/s.According to the comparison between this value and the critical condition for massive-like transformation,the two mechanisms for grain growth at different cooling rates should correspond to the formation of austenite through diffusion-controlled peritectic transformation or massive-like transformation,respectively.The large amount of strain formed during the latter should be the reason for the rapid coarsening of primary austenite grains during rapid-cooling solidification.Based on the grain growth mechanism of primary austenite,by adding the strain energy term in the driving force model of grain boundary migration,the grain growth model which is applicable for the primary austenite grain from massive-like transformation was established.The model parameters were determined based on the multiphase-field simulation and solidification experiment data.In terms of grain size and morphology,this model can well describe the characteristics of austenite grains on the strand surface and reproduce the experimental observations such as the rapid grain coarsening at the completion temperature of austenite formation(Ty and the columnar grain morphology.Based on this model,the coarsening timing and influencing factors of the austenite grains on the strand surface were analyzed.The results show that the intensified cooling for preventing the austenite grain coarsening should be carried out before the temperature of the strand surface region drops to T?,i.e.,in the upper region of mold in continuous casting.And the critical measure is to reduce the surface oscillation marks and depressions.Besides,the potential approach of forming austenite through the diffusion-controlled peritectic transformation was further discussed,which can significantly reduce the tendency of grain coarsening.Aiming at reducing the corner cracking susceptibility in continuous casting of aluminum-containing gear steels with peritectic point composition,the critical content of aluminum and nitride elements was investigated by considering the requirement of gear carburizing treatment.Taking 20Cr as the investigated steel,the pseudo-carburizing experiment was carried out to study the influence of aluminum and nitrogen content on the austenite grain size during simulated carburizing.According to the grain structure feature and the kinetics theory,the abnormal grain growth was revealed as the grain coarsening mode during carburizing.Then,based on the multiphase-field simulation,the critical condition for the occurrence of abnormal grain growth was obtained.Combined with the experimental data,the prediction models for grain-size control in carburizing were proposed,concerning the critical mass fraction and pinning strength of AIN precipitates,respectively.According to the verified experiments,the accuracies of these models are 92%and 75%,respectively.The minimum requirement for aluminum and nitrogen content of this type of gear steels was revealed through the models.Through the simulated hot charging experiment,the relation between the strand charging temperature and the surface crack sensitivity was analyzed.It is found that the reverse growth of untransformed austenite dominates the austenite formation during hot charging from the two-phase temperature region,while the nucleation-growth of austenite from the pearlite region and ferrite grain boundaries is suppressed.In this reverse transformation behavior,the tendency of forming film-like ferrite maintains high when charging at the two-phase temperatures,and decreases with the dropping of charging temperature.Based on the strain partition simulation of the microstructure scale,the influence of the microstructure characteristics on the crack sensitivity during reheating was evaluated.Then the microstructure relation between the charging temperature and the surface crack sensitivity was revealed.Accordingly,hot charging at the two-phase temperature region should be avoided in the production,and it is suggested that the pearlite in the surface microstructure should be more than the untransformed austenite at the charging state.