Study On The Evolution Of Residual Stress During Continuous Cooling And Annealing Process Of High Strength Steel

High-strength steel,an important basic material,is widely used in the energy,transportation and electric power industries,among others.To reduce the cost and improve the performance of high-strength steel,the thermos-mechanical control process has become the primary manufacturing method for this material.During rapid cooling,temperature stress interacts with microstructural transformations,introducing high residual stresses that reduce material performance and restrict the application and development of high-strength steel.Therefore,studying the residual stress evolution characteristics during high-strength steel manufacturing and establishing the corresponding relationships between microstructural transformations and residual stress evolution have extremely important academic and engineering significance for controlling the residual stress levels of high-strength steel during manufacturing.With a focus on the problem of residual stress,which restricts the quality and performance of high-strength steel products,this study considered the development of a residual stress testing system for high-strength steel,the effects of temperature stress on the behavior of continuous cooling transformations and residual stress,the evolution of the microstructure during annealing,the control mechanism of residual stress and the regulation technology of residual stress at low temperature.Two scientific problems were solved:a description of the kinetic transformation model of ferrite considering the hysteresis effect during continuous cooling process and revealing the regulation mechanism of residual stress during annealing based on precipitation plasticity and the precipitation induced creep effect,which provide a theoretical basis for the regulation of residual stress in the manufacturing process of high-strength steel.A low-temperature residual stress control technology based on the mechanism of deformation-induced carbide cryogenic dissolution was developed,which enabled the residual stress to be accurately controlled and reduced the cost of residual stress control in actual production.The main research results of this study are as follows:(1)A residual stress measurement system based on the crack compliance method(CCM)was developed.The test principle,test procedure and test error of the CCM based on the elastic fracture mechanics principle were systematically analyzed.The Abaqus user subroutine was compiled,and the CCM residual stress measurement system was developed by combining Abaqus and MATLAB methods to solve the difficult problem of characterizing the residual stress in the thickness direction of a high-strength steel plate.This approach lays a foundation for the systematic analysis of the phase transition behavior of high-strength steel and its influence on the residual stress.(2)The effects of temperature stress on the transformation plasticity and phase transformation kinetics of microalloyed low-carbon steel during continuous cooling were studied.It was found that under the temperature stress below the yield strength of the material,transformation plasticity existed in 700L low carbon microalloyed steel,and the transformation plasticity strain was linearly positively correlated with temperature stress.The transformation plasticity coefficient was determined to be k=1.337×10-4.The ferrite structure has a hysteretic effect under externally applied stress,and the initial transformation temperature of ferrite shifts to a low temperature.A kinetic model was constructed to model the ferrite structure phase transformation during continuous cooling considering the hysteretic effect,?=1-exp[-b(FS-T)n].This method solves the problem of accurately describing the transformation during continuous cooling process.The Deform finite element software was used to predict the value of residual stress during continuous cooling.The results show that the predicted value of residual stress considering transformation plasticity is close to the test data.The calculated residual stress value without considering transformation plasticity is approximately 40%lower than the experimental data.(3)The annealing structural transformation behavior of microalloyed low-carbon steel was studied.Two important phenomena were revealed:precipitation induced dislocation multiplication during annealing,and the carbon concentration in the microzone determined the precipitation behavior of carbide.The steps of annealing process of 700L can be described as follows:carbon separation and carbon segregation,cementite ? precipitation,residual austenite decomposition,cementite ? precipitation,alloy carbide precipitation and Mn partitioning.The reaction activation energies of the first five stages were 102.50 KJ/mol,117.24 KJ/mol,147.29 KJ/mol,197.55 KJ/mol,and 343.41 KJ/mol,respectively.During the precipitation of alloy carbides,the dislocation density in the microstructure rapidly increased from 7.44×1013 m-2 to 2.99×1014 m-2,and there was precipitation-induced dislocation multiplication.During cementite precipitation,the carbon concentration in the micro-zone determines the precipitation behavior of cementite.For the same carbon content,increasing the dislocation density can change the micro-zone carbon concentration and promote the precipitation of cementite.(4)The evolution law of residual stress during annealing was studied,and a new mechanism of residual stress regulation based on precipitation plasticity and the precipitation-induced creep effect was revealed.The experimental data show that there are two residual stress adjustment stages during the annealing of microalloyed low-carbon steel.The first stage is located in the cementite precipitation zone,the temperature range is 200?-350?,and the residual stress regulation mechanism is the precipitation plasticity mechanism.After annealing at 300? for 30 min,the residual stress in the thickness direction decreases from 487 MPa to 200 MPa,but the elastic strain energy remains unchanged.The second stage overlaps with the alloy carbide precipitation stage,and the tenperature range is 500?-650?.The residual stress control mechanism is the precipitation-induced creep mechanism.After annealing at 600? for 30 min,the residual stress in the thickness direction decreases from 487 MPa to 174 MPa,and the elastic strain energy decreases from 1609 MPa·mm to 439 MPa·mm,a 72.72%reduction.(5)Prestrain induced carbide dissolution,which promoted low-temperature carbide precipitation during subsequent annealing;this phenomenon was used to develop a new technology for low-temperature residual stress regulation.A 10%prestrain increased the dislocation density in a 700L structure from 1.37×1014 m-2 before deformation to 2.39×1014 m-2,while increasing the distortion energy,partially breaking and dissolving the carbide and changing the carbon concentration of the microzone.During annealing,the activation energies of cementite ? precipitation,cementite ? precipitation and alloy carbide precipitation decreased from 117.24 kJ/mol,197.55 kJ/mol,and 343.41 kJ/mol to 93.06 kJ/mol,155.59 kJ/mol and 300.80 kJ/mol,respectively.The cementite precipitate,alloy carbide precipitate and Mn partitioning temperatures decreased from 300?,500 ?,and 625? to 100 ?,475?,and 575?,respectively.Driven by low-temperature carbide precipitation,the residual stress control temperature decreased from 300? to 100?;the elastic strain energy regulation temperature decreased from 600? to 450?.