Study On Phase Transformation Behavior And Residual Stress Evolution Of X510L During Continuous Cooling Process

High strength steel has been widely used in vehicles,bridges and other industrial fields because of its excellent performance.Residual stress is a common problem in the preparation process of high strength steel.The continuous cooling phase transformation process is the key to the formation of residual stress.However,the existing research mainly focuses on the ideal phase transformation process.In order to ensure the final rolling temperature and coiling temperature in actual production,the cooling rate is often changed during the phase transformation process.Therefore,it is the key technical basis for breaking through the regulation of residual stress of hot rolled strip to master the phase transformation behavior under non-equal cooling rate and its influence on residual stress.In view of the above problems,this paper takes X510 L high strength steel as the research object,and studies the phase transformation behavior under variable cooling rate through thermal simulation test,establishes the residual stress prediction model of continuous cooling process by ABAQUS finite element simulation,and uses the crack flexibility method to characterize the residual stress for model verification,which provides data basis and theoretical basis for solving the problem of high residual stress level caused by unreasonable setting of cooling process after rolling.The main conclusions are as follows:(1)In the ideal phase transformation process of X510 L,the phase transformation rate increases with the increase of cooling rate,but the transformation plasticity coefficient k decreases,and the overall residual stress level increases,indicating that for a single structure,the transformation kinetics has a more significant effect on the residual stress than the transformation plasticity.In actual production,it is more effective to control and improve the residual stress by adjusting the cooling process through phase transformation kinetics control.(2)The evolution of residual stress during continuous cooling process can be divided into three stages : temperature field driving stage,surface phase transformation driving stage and surface and core phase transformation difference driving stage.The plastic strain generated on the surface at the beginning of core phase transformation is the key link to determine the residual stress level.The regulation of phase transformation residual stress can be realized by intervening the plastic strain with cooling rate.When the cooling rate of this stage changes from 20 °C/s to 10 °C/s,the surface stress after laminar cooling decreases from 19 MPa to 7 MPa,and the core stress decreases from 10 MPa to 4 MPa.(3)During the phase transition of X510 L,the cooling rate is changed,and the phase transition termination temperature is basically unchanged,but it has a significant effect on the phase transition rate.At a cooling rate of 10 °C/s,the phase transition is changed from 50% to 2 °C/s,and the phase transition completion time is extended from 22.4 s to102.8 s.Therefore,the transition from fast cooling to air cooling during laminar cooling will prolong the phase transition time,resulting in the completion of phase transition in the asymmetric cooling section of the strip,resulting in asymmetric residual stress distribution.(4)Under the premise that the length of the cooling section is fixed,a reasonable critical cooling rate should be set to avoid the high residual stress level caused by the unfinished phase change before coiling.Based on the F7 outlet speed of 8 m/s in the X510 L production site,the critical cooling rate is 15 ℃/s.Therefore,in order to minimize the residual stress level of X510 L after continuous cooling,the most suitable cooling rate should be set to 15 ℃/s.