Investigation Of Multi-step Forming Process Of The Third-generation Automotive Medium-Mn Steel

At present,the demand for light weight and safety in the automotive industry is improved,so that all kinds of new materials are continuously applied to vehicle body.The third generation of high strength steel has attracted much attention due to its large number of metastable austenite,which can enhance strength and plasticity.However,most of the researchers'research on medium manganese steel is limited by composition and organization control,and little is known about its forming process.Combined with microstructure characterization,mechanical property test and simulation analysis,the multi pass forming ability of medium manganese steel is explored.The medium manganese steel with a thickness of 1.6mm was used as the research object,and the duplex microstructure of ultrafine grained ferrite matrix and metastable austenite was obtained by reverse phase transformation annealing process.The influence of different pre deformation amount on the austenite transformation and strain hardening during the multi-pass deformation process of middle manganese steel was investigated by tensile test,pre deformation design experiment,microstructure characterization and phase content determination.And the forming process of medium manganese steel was simulated by finite element method.The following conclusions are obtained:(1)Determination of anisotropy or isotropy of medium manganese steel:The experimental results show that the mechanical properties of medium manganese steel are similar in three directions,and they all have good strong plastic products.The maximum difference is only about 0.45%.The composite plastic strain ratio is 0.900,which is close to the index of isotropic material,and?R is only-0.061,which can be regarded as isotropic material.(2)Effect of strain rate on mechanical properties:In the current range of stamping speed,three strain rates of 0.01/s,0.005/s and 0.001/s were selected.The uniform elongation of tensile specimens is almost the same,and the relative error is only 1.02%.Therefore,the effect of strain rate on the formation of medium manganese steel is negligible.(3)In order to study the influence of pre deformation on subsequent forming ability,the design pre-distortion is 0,5%,10%and 15%respectively.It is found that with the increase of the amount of pre-deformation of the medium manganese steel,the yield strength of the steel increases,the tensile strength almost does not change,which makes the ratio of the flexion strength increase.And there is a phenomenon of yield plateau growth,and there are obvious fluctuations in the work hardening curve.The length of the yield platform increases with the increase of pre-deformation,which will lead to deformation instability and reduce the formability.It has a negative effect on multi-step forming of medium manganese steel.(4)The microstructure of the medium manganese steel after different pre-deformation was characterized by XRD,OM,SEM,EBSD and TEM.It is found that austenite is transformed into martensite continuously with the increase of pre deformation.And the change of austenite transformation is nonlinear with the increase of pre-strain.The fitting formula for the relationship between them is f_?(28)17.36 exp(-13.64?).(5)Combined with numerical simulation method,simulation analysis is carried out by ABAQUS and DYNAFORM.The results of the actual stamping parts are compared with the actual test results.It is verified that the variation of hardness and the stress distribution of the simulation results are basically corresponding.The stress at the bottom two corners is concentrated and the amount of thinning is large,which verifies the accuracy of the measured data.It is proved that the medium manganese steel has good one-step forming ability.