Study On The Effects Of Material Compositions On The Strain Hardening Behaviors Of Twinning/Transformation Induced Plasticity Advanced High Strength Steels

Safety,energy-saving and environmental protection have been the main themes of modern automotive technology,especially in the development of environmental friendly lightweight cars.The high strength steels with the advantages of high strength,superior energy absorption and excellent initial strain hardening rate,have been the dominant material for the manufacturing of lightweight cars.As the investigations on high strength steels proceed,deformation twinning and phase transformation are found during the plastic deformation of TWIP(TWinning Induced Plasticity)steels and TRIP(Transformation Induced Plasticity)steels,which can bring significant influence on the strain hardening properties of materials.Because of the complex coupling of deformation twinning,martensitic transformation and dislocation movement during the plastic deformation of TWIP/TRIP steels,and the complicated interactions between different deformation mechanisms,it’s difficult to build a mechanical model which can describe the deformation behavior and the mechanical property of TWIP/TRIP steels comprehensively with the interactions considered.In addition,though tremendous works have been done to investigate the influences of different material compositions on the mechanical properties of TWIP/TRIP steels,most of them only focus on the regularities of experimental results instead of giving reasonable explanations for the exhibited mechanical behaviors result from different chemical compositions.Further more,studies show that the deformation mechanisms of TWIP/TRIP steels are closely related to the material compositions.Therefore,developing a model with material compositions and factors influencing SFE(Stacking Fault Energy)considered become the key to evaluate the deformation behaviors of TWIP/TRIP steels.Base on the shear band theory and the similarities between deformation twinning and martensitic transformation,a TWIP/ TRIP model controlled by SFE is developed in this work.During the calculation of SFE in this work,the effects of chemical composition,deformation temperature,micro strain and the temperature increment result from deformation is taken into account.According to the multi phases mixed law,a strain hardening model with both TWIP effects and TRIP effects considered is established in this work.Validated by the experimental results of different TWIP/TRIP steels,the model established in this work can describe and predict the stress-strain relationship and the strain hardening behaviors of TWIP/TRIP steels accurately.Considering the effects of material composition on the SFE,the effects of Mn contents,Ni contents and C contents on the deformationbehaviors and strain hardening properties of TWIP,TRIP and TWIP/TRIP steels are investigated in this work base on the results of the developed model and the experiments.The mechanisms of material composition influencing the strain hardening behaviors of material are investigated.Results show the generations of deformation twinning and martensitic transformation are controlled by SFE.Due to the complex coupling between dislocation slip,deformation twinning and martensitic transformation,the material compositions determine the macro mechanical behaviors and strain hardening abilities of TWIP/TRIP steels by influencing the nucleation of deformation twinning and martensitic transformation.