CPFEM Simulations On Martensitic Transformation In TRIP Steel

Transformation induced plasticity steel is a popular automotive steel.The phase transformation induced plasticity(TRIP)effect causes the retained austenite in the steel plate to induce martensite transformation under the plastic deformation and local hardening.Uniform diffusion of phase transformation improves the toughness,ductility and shock resistance of the steel sheet.The crystal plasticity finite element model is established by combining crystal plasticity theory with finite element method.The nature of plastic deformation of materials can be studied by dislocation slip and deformation twinning.In this study,a crystal plastic finite element method based on phase transformation model is used to establish a numerical model describing the mechanical behavior and lattice characteristics of retained austenite in TRIP steel,considering the mechanism of slip and twin deformation.With the aid of the secondary development platform of ABAQUS/UMAT,it was simulated that of the phase transition characteristics,microstructure evolution and the microscopic influence mechanism of austenite initial orientation on martensite transformation during uniaxial tension of TRIP single crystal.At the same time,the textures of the polycrystalline TRIP steel under different loading modes were simulated.The evolution of the structure was simulated to reveal the relationship between the microstructure and the mechanical properties of TRIP steel,and to understand the microscopic mechanisms of martensitic transformation.The main research contents are as follows:Based on the crystal plasticity theory with considering the mechanism of slip and twin deformation,a crystal plastic finite element model of martensitic transformation constitutive relation based on stress criterion is established.The uniaxial tensile results of TRIP steel at room temperature show that the stress-strain curve is basically consistent with the experimental results,indicating that the model built in this study can accurately describe the martensitic transformation process of TRIP steel.In the initial stage of tensile deformation of TRIP steel,the stress concentration occurs at the four corners of the fixed end face,which causing the local stress to reach the austenite yield stress,and the plastic strain is accumulated in the austenite grains.Thus,martensitic transformation begins at about 5% strain of the austenite grains.As the strain increasing,the stress concentration region extends from the middle to the tensile direction with the volume fraction of martensite increases gradually.Then martensitic transformation occurs in the upper half of the grain,and the martensite volume reaches the maximum of about 40%.When the initial orientation of austenite grains is different,the initial strain of martensite transformation and the maximum martensite volume fraction are different.The grain orientation with higher yield stress has the maximum value of martensite transformation.The larger of the initial strain,the phase transformation begins at the smaller strain.Compared to the austenite grains with hard orientations of[112](Brass) and [201](Copper),the austenite grains with soft orientations of[110](Cube),[101](Goss) and [110](Rotated goss) have faster twinning rate for the lower shear strain was required in the twinning initiation.Thus,the cumulative shear strain of twinning is higher,which promotes easier formation of martensite.During the tensile process of TRIP polycrystalline model,the residual austenite has less phase deformation at the initial stage,and then gradually increases.While in the single crystal model,partial martensitic transformation can occur under small strain conditions.Deformation texture of the polycrystalline TRIP steel is analyzed during the tensile deformation with different strain conditions,and the obtained{111} pole figure is agrees well with the experimental results.