Study On Ductile Damage Of Heat-treated 22MnB5 Boron Steel With Tailored Mechanical Properties

At present,hot stamping technology is an important way to realize the lightweight of automobile and meet the demand of energy saving,emission reduction and safety performance in automobile industry.With the wide application of hot stamping technology,it is found that hot stamping parts with high strength and low ductility cannot meet the safety requirements.Therefore,in order to further improve the safety performance of hot stamping parts,people designed and manufactured parts with“customized” mechanical properties.The part protects passengers by increasing the plasticity of local regions to absorb the energy generated during collision.The mechanical properties of tailor-tempered parts are different in different regions.The microstructure in the transition region is generally composed of multiple phases,and the volume fraction,distribution and microstructure morphology of each phase determines the mechanical properties of the parts.Therefore,it is necessary to quantitatively study the relationship between microstructure and mechanical properties.Based on the above background,heat-treated 22 MnB5 boron steels with tailored mechanical properties are taken as the research object,and its ductile damage behavior is studied from the micro-to the macro-scale.(1)A series of 22 MnB5 boron steel sheets with tailored properties were prepared by different heat-treatment processes,including pure bainite,pure martensite and five mixed bainite/martensite heterogeneous products with different volume fractions.Metallographic and scanning electron microscopy(SEM)characterization experiments were carried out on the dual-phase microstructure samples after heat-treatment processes.It was found that with the increase of holding time,the proportion of bainite increased significantly,and the granular characteristics became more obvious.And with the increase of holding time,the proportion of martensite decreases and the microstructure morphology becomes smaller and dispersed.In addition,it was found by hardness test that vickers hardness value decreased with the increase of holding time.(2)Quasi-static tensile tests were carried out on boron steel sheets with tailored properties prepared by heat-treatment process and combined with digital image correlation(DIC)technology.The stress-strain datas of the corresponding materials were obtained by dogbone tensile test.In addition,in order to characterize the ductile damage fracture behavior of heat-treated 22 MnB5 boron steel,the specimens with different stress states from shear to plane strain were prepared and tensile tests were carried out.The constitutive models of pure bainite and pure martensite were established from the strain perspective by using DIC technology,and the constitutive models of mixed bainite/martensite microstructure were established by linear interpolation based on volume fraction and Vickers hardness.In order to verify the accuracy of the two types of constitutive models,the specimens with mixed microstructure under different stress states were modeled and simulated respectively,and the constitutive model based on Vickers hardness was closer to the real properties of materials than the constitutive model based on volume fraction.The fracture strain,stress triaxiality and Lode parameters of tailored boron steel were obtained by the hybrid numerical-experimental method.It was found that with the increase of holding time,the proportion of bainite in tailored boron steel increased continuously,and the fracture strain increased significantly only when the proportion of bainite was higher(80%,90%,96%).The fracture parameters of pure bainite and pure martensite were obtained by the least square method,and the fracture surfaces of pure bainite and pure martensite were constructed in the three-dimensional space of Lode parameters,stress triaxiality and fracture strain respectively.(3)The representative volume element(RVE)model was established based on the real microstructure morphology of mixed bainite/martensite microstructure,and the effect of mesh density on the simulation results was studied.In order to accurately predict the elastic-plastic behavior of mixed bainite/martensite microstructure in the RVE model,the constitutive models of pure bainite and pure martensite were reconstructed based on dislocation strengthening theory,and material parameters were optimized according to the mode FRONTIER optimization platform.On this basis,the material parameters of each phase in the mixed bainite/martensite microstructure were determined.In order to further study the damage and fracture behavior of mixed bainite/martensite microstructure,a method combining the micro-and the macro-scale to obtain fracture strain was proposed.(4)The void nucleation and microcrack propagation of mixed bainite/martensite microstructure specimens with different volume fractions before fracture were studied and analyzed by using finite element simulation software and SEM.It is found that there are three kinds of void nucleation modes in mixed bainite/martensite microstructure: at the phase boundary between bainite and martensite,in bainite phase and in martensite phase.For the mixed microstructure consisting of 80% bainite and20% martensite(80%B+20%M),the voids tend to initiate at the bainite/martensite boundary at low stress triaxiality,and are more likely to initiate in bainite at medium stress triaxiality and high stress triaxiality.In addition,with the increase of martensite volume fraction,the martensite distribution changes from discrete to continuous,and the nucleation location of voids gradually moving from bainite to martensite,and microcracks also pass through the martensite phase during the propagation process.