Research On Flatness Evolution Of Q&P High Strength Steel Plate During Heat Treatment

Quenching and partitioning steel,also known as Q&P steel,has the majority of martensite with a certain amount of residual austenite through the quenching and partitioning heat treatment(also known as the Q&P heat treatment)of cold-rolled carbon silicon manganese steel.As one of the third generation advanced high strength steel,Q&P steel is a high strength and high toughness steel with the TRIP effect and has broad application prospect.In Q&P steel production,flatness problem is one of the main technical problems.In this paper,the thermal elasto-plastic constitutive model of QP980 high strength steel is studied.A coupled finite element model of the temperature,microstructure and stress-strain field has been established to simulate the evolution of two common cold-rolled plate flatness defects during Q&P heat treatment.The mechanism of flatness change and the influence of heat treatment process are revealed,based on which the corresponding flatness control strategies are proposed.As a result,theoretical support for the optimization of Q&P steel heat treatment process is provided.The main work and conclusions are as follows.(1)Taking QP980 steel as the typical Q&P high strength steel in China,the whole process of heat treatment is experimentally studied.A creep strain equation coupled with temperature and time is proposed to describe the volume change of the material in partitioning process.The transformation kinetics of the quenching process considering initial quenching temperature,the equations of transformation strain and transformation plasticity,the thermal expansion coefficient of each phase are gained through experimental research.(2)The thermal deformation behavior of QP980 steel austenite,martensite and tempered martensite is studied by isothermal tensile test.The Johnson-Cook constitutive equation is modified by introducing temperature-strain coupling and improvement of strain strengthening.As a result,the calculation precision of material flow stress under high temperature is improved.The Arrhenius hyperbolic sine constitutive equation considering the influence of strain rate is established.The calculated values of the constitutive equations are both in good agreement with the experimental values.(3)According to the coupling principles of temperature,microstructure and stress-strain field,a numerical simulation model for the whole process of Q&P heat treatment was developed.In this model,the physical simulation of QP980 steel thermal-elastoplastic incremental constitutive equations are implemented to commercial finite element software ABAQUS as the user subroutines.(4)Based on this numerical model,a simulation study on the heat treatment process of QP980 steel plate with initial edge wave flatness defect is carried out.from which the evolution of the edge wave defect during the whole process of heat treatment is obtained.In the analysis of the mechanism of edge wave evolution,it is found that the variation of the total longitudinal strain distribution in the width direction which leads to plate flatness change is attributed to the variation of one or several longitudinal strain component distributions,including the thermal strain distribution caused by temperature distribution,the distribution of phase transformation strain and the transformation plasticity caused by the phase transformation sequence and the distribution of creep strain caused by partitioning sequence.The quantitative influence of quenching temperature,partitioning temperature and partitioning time on the edge wave defect is obtained.Furthermore,the control strategies of the edge wave flatness defect during Q&P heat treatment process are proposed.(5)Based on the numerical model of Q&P heat treatment,a simulation study is conducted on the heat treatment process of QP980 steel plate with C warping defect,from which the evolution of the warping defect during Q&P heat treatment is obtained.In the analysis of the mechanism of C warping evolution,it is revealed that the evolution of the total longitudinal strain distribution along thickness direction which leads to the flatness change is attributed to the evolution of one or several longitudinal strain component distributions,including thermal strain,plastic strain,transformation strain,transformation plastic strain and creep strain.The quantitative influence of quenching temperature,partitioning temperature and time on the C warping defect is obtained.Accordingly,the C warping defect control strategies of Q&P heat treatment are put forward.(6)Q&P heat treatment process experiment platform has been set up in the laboratory.Firstly,the salt bath heat treatment experiment with different process parameters is carried out for QP980 steel plate with edge wave defect.The effect of heat treatment parameters on the edge wave is consistent with the numerical simulation results.Also the heat treatment experiment of different parameters was accomplished for QP980 steel plate with C warping defect.The effect of parameters on C warping defect is in good agreement with the numerical simulation.The experimental results validate the numerical simulation results well.