Study On Thermoplastic And Deformation Damage Mechanism Of DP980 Microalloyed Steel Continuous Casting Billet

DP980 microalloyed steel is mainly used in various medium and high-end automobiles,however,during the production of DP980 microalloyed steel by a domestic steel plant,severe edge cracks appeared on the steel coil edge of the hot-rolled strips,which greatly reduced the utilization ratio of the material and the product quality.Based on the Gleeble-3800 high temperature tensile test,the thermoplasticity and damage mechanism of DP980 microalloyed steel are studied.The combination of numerical simulation and industrial experiments is used to analyze the straightening damage of the continuous casting slab to investigate the cause of edge cracks in hot rolled coils.The hot rolling process was modified based on the thermoplasticity of the experimental steel,which significantly improved the edge cracking problem of the DP980 hot rolled coils.The main research contents and results of this article are as follows:(1)Gleeble-3800 thermal simulation test machine was used for high temperature singlepass tensile test to study the thermoplastic law of DP980 steel.It was confirmed that the third brittle temperature range of the DP980 was 650-950? under the conditions of a cooling rate of 3?/s and a strain rate of 0.005s'1 As the strain rate increased,the thermoplastic groove in the third brittle zone of the experimental steel gradually increased and narrowed.The effect of cooling rate on thermoplasticity is not obvious;(2)Based on the high temperature tensile test,according to the straightening characteristics of the continuous casting slab,the stress-strain curves with strain rates of 0.001s-l,0.005s-1,and 0.05s-1 and deformation temperatures of 750-1050? were selected to establish the experimental steel as-cast Constitutive model provides material parameters for numerical simulation;(3)Based on the high-temperature tensile test,the analysis was made from four angles of ferrite,second phase particles,sulfur segregation,and inclusions to determine the damage mechanism of the experimental steel:During the deformation process,point-shaped ferrites are precipitated on the grain boundaries which are torn before it grow up,then a crack along the crystal is formed;On the one hand,the second phase particles in the steel are uniformly precipitated in the grains,strengthening the grains,making deformation easier to concentrate on the grain boundaries,and on the other hand,the second phase particles pin the austenite grain boundaries,so that the grain boundaries will expulsion during Deformation process.Sulfur segregation weakens the atomic bonding force at the grain boundaries or the crack tip,and the brittle inclusions are separated from the matrix,which promotes crack initiation;(4)Based on the damage analysis of eight common fracture criteria,the Oyane damage criterion was used to derive the damage model according to the characteristics of the damage distribution.The deformation temperature and strain rate were introduced into the damage model,and the model coefficient was determined through multi-pass tensile experiments Finally,the final damage model for straightening analysis is obtained.The temperature field simulation and deformation damage analysis of the continuous casting process of the experimental steel revealed that no damage occurred during the continuous casting process of the experimental steel;(5)The hot-rolled control experiment of continuous casting slab after single-side grinding at the industrial site shows that the edge crack of the hot-rolled coil is caused by the unreasonable hot-rolling process.According to the theoretical research on the thermoplastic law of DP980 steel,the final rolling temperature of the strip is increased from 890? to 940?,and the edge temperature is increased from 830? to 880? the rolling speed is increased from 10m/s to 11.5m/s.The deformation of the steel edge is no longer in the trough of plasticity,and finally the edge crack defect of the strip is significantly improved.