Phase Transformation Behavior Of 700T Under Stress And Its Effect On Residual Stress

Residual stress is a common problem in the preparation process of low carbon microalloyed 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 and the continuous cooling process after rolling is the phase transformation process under the combined action of temperature stress,unit tension and temperature field.Ignoring the influence of stress will inevitably lead to the theoretical model unable to accurately predict the actual stress.Based on this,this paper takes 700T as the research object,studies the influence of stress on phase transformation kinetics and phase transformation plasticity of 700T by combining thermal simulation experiment and microstructure analysis,analyzes the evolution law of phase transformation stress under stress,and discusses the regulation method of phase transformation stress.The main conclusions are as follows:(1)Stress reduces the start temperature of 700T phase transition but accelerates the phase transition process.It is found that the compressive stress will cause the phase transformation starting point of ferrite to move to the low temperature region,and the loading of-20 MPa load can reduce the phase transformation starting temperature by 20℃.The phase transformation time can be shortened by 1.4 s by loading-70 MPa load.The phase transformation kinetic model under the influence of stress is:ξ=1exp[-b(FS-T)]1.21099,where b=0.000606×s2+0.06314×s+3.141.In the experimental stress range,the transformation plastic strain is positively correlated with the loading stress,and the fitting coefficient k=1.69×10-4 MPa-1.(2)The phase transformation dynamic model considering the influence of stress can improve the calculation accuracy of residual stress.The phase change residual stress was calculated by ABAQUS finite element software and compared with the measured data of residual stress by crack flexibility method.The measured values of residual stress considering stress influence,without considering stress influence and crack flexibility method are-63 MPa,-20 MPa and-56 MPa,respectively.Therefore,considering the influence of stress can improve the prediction accuracy of residual stress.(3)It is found that the phase transformation strain on the strip surface at the beginning of phase transformation at the core determines the final residual stress level.The evolution of residual stress during phase transformation includes three stages:temperature stress dominant stage,surface phase transformation dominant stage,and core phase transformation dominant stage.The phase transformation strain on the surface of the strip at the beginning of the phase transformation at the center determines the level of the final residual stress.The phase transformation stress can be controlled by interfering with the phase transformation strain at this stage.(4)Based on the above evolution law of residual stress,a residual stress control method using external compressive stress to intervene the strain difference between the surface and the core of the strip at the beginning of phase transformation is proposed.After applying-30 MPa compressive stress during the phase transformation process,the residual stress on the lower surface of the strip at the specified measuring point is reduced from 99 MPa to 15 MPa,and the residual stress in the center is reduced from-56 MPa to-12 MPa.