Study On Deformation Behavior Of Heat-treated Boron Steel Based On Representative Volume Element Method

Due to the high strength-to-density ratio of advanced high-strength steels,the demand to manufacture automobile structural components has been increasing over the last few decades.The lightweight design can reduce vehicle weight,improve safety,and crashworthiness qualities to achieve low fuel consumption and high environmental protection.Hot stamping is the key forming technology to manufacture automobile components with high strength using advanced high strength steels.With the help of the traditional hot stamping process,the car components could achieve a higher strength caused by the transformation of martensite through rapid cooling after austenitization.However,the energy absorption in car crash has been an intractable issue due to the poor ductility of the advanced high strength steels.In order to acquire the comprehensive safety performance of the car components,the tailored tempering process(TTP)in hot stamping with gradient mechanical properties has been developed.In the production process of hot stamping TTP components,the sheet metal was first austenitized,and then other heating or cooling tools were used to control the cooling rate to achieve different mechanical properties in different areas of the same part.Since heterogeneous TTP components includes multiple phases,the influences of volume content,morphology,size,and distribution of each phase on mechanical properties are complicated and confusing issues.Therefore,the elastoplastic constitutive relationship with customized material properties,as well as the failure modes under different loading conditions should be investigated in depth.The research issues are as follows:(1)Seven sets of different heat-treated 22 MnB5 boron steels with various hardness were prepared.Through metallographic characterization and uniaxial tensile tests equipped with digital image correlation technology,the influences of holding time on the transformation of bainite/martensite mixed structure,toghether with the mechanical properties of components was analyzed.It was noted that as the holding time increased,the volume content of martensite decreased,while bainite increased.In addition,the yield strength and ultimate tensile strength decreased with the rise of holding time,with a better ductility.(2)In order to predict the elastoplastic relations of heterogeneous material,a feasible approach was proposed to describe the constitutive relationships.The mixed structures of bainite and martensite with different volume fractions were characterized.According to the volume fraction of each phase,different homogenization methods were carried out to predict the mechanical properties of multi-phase steels.In addition,the RVE(Representative Volume Element)model considering the morphology,size and distribution of the constituent phases was employed to predict the elastoplastic constitutive relationships of the mixed structures.The volume expansion of the martensite would cause local plastic deformation of the surrounding bainite during the phase transformation.It was noted that the hardness of the bainite around the martensite was higher than that inside the bainite.In order to predict the mechanical properties,the phase boundary was introduced into the modified secant model.Based on the measured mechanical properties of pure phase and the volume fraction of each phase in the mixed structure,the modified secant method was used to predict the complicated responses of the bainite/martensitic steel.(3)Based on the characterized microstructural morphology of mutli-phases,the RVE model was established to analyze the relationship between microstructure morphology and overall mechanical properties with different phases.Metallographic diagrams were randomly selected from different positions of the sample to establish the RVE model,and the validity of the RVE model was verified.Furthermore,the sensitivity of different sizes and types of FE elements were studied.The simulation results showed that the deformation mainly occurred in the bainite phase.The failure modes of the tested cases were local shear failure.(4)The RVE model was used to study the failure modes under different stress conditions.The tensile tests found that the failure modes at different positions of the specimen were different.The boundary conditions of three typical positions in the sample were selected.It was found that the failure mode under the plane stress state without side restraint was shear.The failure mode under the plane stress state with side restraint was vertical fracture,and the strain value corresponding to the ultimate tensile strength was the smallest.The failure modes in the plane strain state without side restraint were necking and shear modes,and the strain value corresponding to the ultimate tensile strength was the largest.(5)The effects of bainite ductility on the overall ductility and the final failure mode of the material were studied by changing the maximum strain value of bainite.The ductility of bainite significantly affected the ductility of the material,however there was almost no effect on the failure mode.The overall ductility of the material would increase as the bainite strain value increased.When the input strain of bainite exceeded 60%,the overall ductility of the material would no longer change.(6)The RVE models with different martensite content were simulated under different stress states.It was found that when the volume fraction of martensite was higher than70%,the shear in thickness direction was extensively obvious.However,when the volume fraction of martensite was lower than 20%,the vertical fracture in the middle of the sample was particularly significant.Moreover,when the martensite volume fraction was reduced to 4.3%,the main fracture mode was the synergistic effects of necking and shear.In addition,as the volume fraction of martensite increased,the strain value corresponding to the ultimate tensile strength decreased.