Research On Toughness Damage Of Quenched Martensitic Boron Steel 22MnB5 Based On Micromechanics

With the increasing demand for low energy consumption and high safety in automobiles,advanced high-strength steel(AHSS)is widely used in the automotive industry.The application of advanced high-strength steel guarantees the strength of the body with fewer materials,and thus significantly reduces the weight of the body.The advanced high-strength steel processed by hot stamping process can produce rewarding ultra-high strength parts,so it is very popular in the automotive industry.However,unlike traditional metals,its characteristics of high strength and low ductility making the forming process laborious.These characteristics also bring great challenges to the simulation of fracture behavior.This paper takes the quenched martensite 22MnB5 obtained by the hot stamping process as the research object,and conducts research on its ductile damage behavior from the aspects of experiment,numerical simulation and micromechanical damage mechanism.(1)Ductile fracture models of martensite 22MnB5 were studied.Uniaxial tensile specimens was texted to determine the hardening parameters.The inverse method was used to extrapolate its true stress and strain data.Tensile specimens,such as the central hole specimen,in-plane shear specimen,notched specimen,equal biaxial tensile specimen and hole expansion specimen were implemented to determine the fracture behavior of the material under various stress conditions.Digital image correlation(DIC)method was used to monitor the change of surface speckle.At the same time,the history of strain evolution of the sample surface was recorded during the experiment.A group of specimens were cut every 15 ° along the rolling direction of the sheet.By conducting tensile experiments on uniaxial tensile specimens in different directions,it was proved that the martensite 22MnB5 can be simplified as an isotropic material,so the isotropic Mises yield function was used in this study.With the material parameters of the fracture model were calibrated by the least square method,the hybrid numerical-experimental method and the DIC method were used to obtain the fracture strain under different stress states and respectively constructed two different ductile fracture surfaces.The fracture surfaces obtained by the two methods were compared in the three-dimensional space of Lode parameters,stress triaxiality and fracture strain.The comparison results showed that the fracture surface obtained by the hybrid numerical-experimental method was in good agreement with the experimental value.The user subroutine VUMAT was written in the Fortran language based on the Lou-Huh(DF2015),Modified Mohr-Coulomb(MMC),and Hosford-Coulomb(HC)ductile fracture criterion.The ABAQUS/Explicit module was used for numerical calculations and the simulation data and experimental data were extracted for comparative study.The results showed that the DF2015 ductile fracture criterion can accurately describe the ductile fracture behavior of martensite 22MnB5 from shear to equi-biaxial tensile stress.(2)Based on the macro and micro observations of fracture surfaces,the study of the ductile fracture mechanism was conducted.First,through macro analysis,the relation between the direction of the maximum shear stress and the fracture surface was revealed.Based on the macro analysis results,in order to further explore the micro-mechanism of martensite 22MnB5 ductile fracture,a scanning electron microscope(SEM)was used to observe and analyze the fracture at different angles.The comparison of the orientation of the void and the direction of the maximum shear stress revealed the effect of the maximum shear stress on the accumulation of voids during the ductile fracture process.Finally,based on the DF2015 ductile fracture criterion,the ABAQUS/Explicit module was used to conduct finite element simulation research.The results of numerical simulation fully explained the formation of the fracture morphology of martensite 22MnB5 in macro and micro scale.It further proved the relation between the process of void accumulation and the maximum shear stress during ductile fracture.The results of macroscopic experiment,SEM observation and finite element simulation overall revealed the effect of maximum shear stress on ductile fracture.Therefore,for the ductile fracture criterion based on micro-mechanism,the expression of stress triaxiality,Lode parameter and maximum shear stress should be considered in the description.(3)The applicability of the DF2015 ductile fracture criterion to complex stress states was verified.The U-shaped beam of martensite 22MnB5 was manufactured by hot stamping process,and the U-shaped beam and DP590 back plate were connected at the molecular level by means of spot welding.Performed three-point bending experiment on the welded U-shaped beam assembly,so there produced a complex local load history,and finally formed two symmetrical macro cracks under the punch.The finite element model of the three-point bending experiment was established,and the DF2015 ductile fracture criterion was used to perform finite element simulation analysis on the three-point bending experiment.The comparison of experimental and simulated crack position,shape and load-stroke history data showed that the DF2015 ductile fracture criterion can achieve an accurate prediction of fracture behavior under complex stress conditions.