Transformation Plastic Behaviour And Its Effect On Residual Stress Of 700L Under Temperature Stress

High-strength steel has been widely used in the automotive,high-speed rail,marine,aerospace and other industrial fields due to its excellent characteristics.The mechanical properties,fatigue strength,stress corrosion resistance,dimensional stability and service life of mechanical parts and large components produced from high-strength steel are greatly affected by residual stress.During rolling,cooling,forming,welding,etc,the existence of residual stress raises the possibility of hydrogen embrittlement of bearing components(such as bolts,suspension springs,aircraft landing gear,etc.)or bearing steel structures(such as pressure vessels,building structures,etc.),which deteriorates the material performance and changes the fracture mechanism,so that fractures generally occur suddenly and without warning,and often causes serious consequences.Therefore,the harm caused by residual stress cannot be ignored,and residual stress in high-strength steel has aroused extensive attention.In the detection of strip residual stress,the existing detection methods,such as hole-drilling method,X-ray diffraction method,magnetic measurement method,ultrasonic method and other mechanical or physical detection methods,can only characterize the surface stress of the strip,and there is a lack of understanding of the thickness of the strip.During the hot rolling process of high-strength strip,the factors causing residual stress are complex and involve a lot.At present,there are many studies on residual stress during rolling,but relatively few studies in the continuous cooling.The distribution law of residual stress in the strip during continuous cooling is not yet clear.In addition,the existing research on the residual stress of the steel strip during continuous cooling regards the phase transformation process as a "free phase transformation" process,and only considers the spherical expansion during the transformation of different structures.However,the phase transformation takes place under the temperature stress during continuous cooling of strip steel,which makes the phase transformation strain have a certain direction.The temperature stress changes the plastic deformation behavior of microstructure,which in turn changes the residual stress distribution of the strip during the phase transformation.At present,in the continuous cooling process of hot-rolled strip,there still exist the following two problems: 1)The distribution of residual stress in high-strength steel after rapid cooling is not clear;2)The influence law of temperature stress on the phase transformation plasticity behavior and residual stress of the material is unclear.This article takes 700 L low-carbon microalloy high-strength steel as the research object,aiming at the product quality problems for hot-rolled strip caused by residual stress.Centering on the residual stress distribution law and the influence of temperature stress on the 700 L transformation plastic behavior and residual distribution during continuous cooling,the thermal simulation test,the development of the crack compliance method test system based on the principle of fracture mechanics,the measurement of the thermal properties of the material and the establishment of the finite element simulation model are adopted to explore transformation plastic behaviour of 700 L and its effect on residual stress distribution under temperature stress.The main research conclusions are as follows:(1)A characterization system for residual stress in the thickness direction of highstrength steel was established.Abaqus user subroutine was compiled.Based on the principle of elastic fracture mechanics,Abaqus and Matlab are combined to develop a crack compliance method for residual stress test and calculation system,which solves the problem of characterizing residual stress in the thickness direction of high-strength steel plates.At the same time,the residual stress distribution law after continuous cooling of 700 L was obtained by this method,and the uncertainty of the crack compliance calculation was analyzed and evaluated.According to the analysis of the test results,it is proposed that phase transformation,coiling tension and plate thickness are the three key factors that affect the distribution of residual stress in high-strength steel coils,which lays the foundation for systematic analysis of the phase transformation behavior of high-strength steel and its impact on residual stress.Simultaneously,it also provides ideas for shape and residual stress controlling during laminar cooling.(2)The effect of temperature stress on the transformation plastic behavior for 700 L was studied.Through thermal expansion test to simulate the effect of temperature stress,it was found that the temperature stress changes the plastic behavior of the austenite decomposition process,making it directional,and the transformation plasticity is linearly related to the uniaxial load,the transformation plasticity coefficient k=1.357×10-4.At the same time,the "hysteresis effect" was observed under the temperature stress,that is,the temperature stress lowered the temperature while the phase transformation started,and delayed the transformation start time.Based on this,the empirical dynamic function of the JMAK form of ferrite transformation under continuous cooling is summarized.(3)A finite element residual stress calculation model based on the influence of temperature stress on the phase transformation behavior is established,it overcomes the calculation deviation caused by adopting "free phase transformation" that only considers spherical expansion,so that it improves the accuracy of residual stress prediction in the continuous cooling process.The simulation results show that the average residual stresses on the surface and core are 73.39 MPa and-24.68 MPa when the transformation plasticity is not considered,and the average values on the surface and core are 164.23 MPa and-39.19 MPa when the transformation plasticity is not considered.The measured values of the residual stresses on the surface and core are 214.01 MPa and-49.95 MPa.It can be seen that the calculated residual stress after considering the phase change plasticity is closer to the measured value..Therefore,the temperature stress changes the plastic behavior of high-strength steel during phase transformation,determines the residual stress distribution after phase transformation,and provides a new direction for the residual stress control during the preparation of high-strength steel.