Effect Of Cooling Mode On The Plastic Behavior Of Phase Transformation And Residual Stress Of DQ400

Wear-resistant steel is widely used in metallurgical machinery,coal mining and transportation due to its excellent performance.Generally,low alloy composition design,controlled rolling,rapid cooling,fine grain strengthening and precipitation strengthening are used to reduce material cost and improve welding performance.However,in the continuous cooling process of hot rolling,due to the rapid cooling rate will introduce residual stress inside the material,resulting in serious flatness defects and distortion problems,which seriously affect the subsequent processing and use of the material.Therefore,how to reduce residual stress is an urgent problem for steel enterprises.At present,the research on residual stress of strip during continuous cooling process takes the phase transformation process as an“ideal phase transformation”process,ignoring the influence of cooling process parameters on phase transformation behavior and residual stress.In this paper,DQ400 wear-resistant steel is taken as the research object.Aiming at the problem of shape difference caused by two cooling modes in the continuous cooling process of hot rolled strip,the phase transformation kinetics and phase transformation plasticity equations under different final rolling temperatures and cooling rates are established.The ABAQUS subroutine was developed to study the influence of key process parameters on the residual stress of DQ400,and the calculation results were verified by the crack compliance method based on the principle of fracture mechanics.The above work enriches the phase transformation theory of wear-resistant steel and provides theoretical data support for strip shape control and residual stress control in laminar cooling process.The main conclusions are as follows:（1）For martensite,the higher the starting temperature of phase transformation,the faster the phase transformation rate,and the higher the residual stress level.When the starting point of DQ400 phase transformation increases by 12°C and the phase transformation time shortens by 0.2 s,the surface tensile stress increases by 75 MPa,the core compressive stress increases by 67 MPa,and the overall stress along the thickness direction of the strip increases by 48.86%.（2）The phase transformation plasticity coefficient of DQ400 is negatively correlated with the cooling rate,and the larger the cooling rate is,the smaller the phase transformation plasticity coefficient is.When the cooling rate is 30°C/s,40°C/s and45°C/s,the phase transformation plasticity coefficient is 9.7734×10^-5,9.14142×10^-5 and8.32594×10^-5,respectively.The influence equation of phase transformation plasticity coefficient is k=1.2597×10^-4-9.174×10^-7ν.The influence of phase transformation plastic strain on residual stress depends on the direction and magnitude of deviatoric stress tensor at phase transformation time.（3）The evaluation index of residual stress is formulated,and three characteristic parameters are defined:the upper and lower surface stress imbalance BT_S,the thickness direction elastic strain energy imbalance BT_E and the width direction elastic strain energy change rate CEn.These three characteristic parameters can be used as indicators to control the warping and bending distortion of strip steel,and provide theoretical data support for actual production.（4）The final rolling temperature affects the beginning temperature and rate of phase transformation of DQ400,which results in the residual stress sensitive to the final rolling temperature.When the finish rolling temperature is between 840°C and 860°C,the start temperature of phase transformation increases and the phase transformation rate increases,resulting in the maximum surface tensile stress reaching+228 MPa and the core compressive stress reaching-205 MPa,which are higher than those at the finish rolling temperatures of 880 and 820°C.（5）Increasing the cooling rate can shorten the phase transformation time of DQ400,but it will increase the temperature stress,resulting in the superposition of thermal strain,phase transformation strain and phase transformation plastic strain,thus deteriorating the residual stress.Compared with sparse cooling,the surface tensile stress increased by 242MPa,the core compressive stress increased by 319 MPa,and the overall residual stress in the thickness direction increased by 118.39%.