Study On The High Speed Deformation Behavior And α’-M Reversion In High Manganese TRIP Steel

Due to the strain induced martensitic transformation,superior mechanical properties such as high strength,large elongation and better work hardening rate are maintained in high manganese TRIP(transformation-induced plasticity)steel.The excellent plasticity of high manganese TRIP steel is not reduced during high speed deformation,which expands its application areas.This study investigates the orientation dependence,variant selection,and kinetics of martensitic transformation at high strain rate(ε>103 s-1),as well as the propagation of cracks.Further more,the corresponding crystallographic characteristics are calculated based on the crystallography of martensitic transformation.Additionaly,the deformation and phase transformation during cold rolling(10 s-1<ε<102s-1)are analyzed.Then,the reversion of deformed α’-M(α’-M→γ)and the recrystallization of γ are studied.These can provide theoretica basis for the industrial application of α’-M reversion.The results are shown as follows:The strain rate has different influences on the transfomation of γ→ε-M andε-M→α’-M.γ→ε-M transformation is inhibited while the ε-M→α’-M transformation is promoted at high strain rate.Orientation dependence of martensitic transformation exists even at high strain rate.α’-M variant selection becomes weaker and a pair of α’-M variants with<111>60° misorientation is preferentially selected during high speed deformation.The kinetics of martensitic transformation is significantly affected by strain rate during compression.The TRIP effect especially ε-M→α’-M transformation is accelerated at the early stage of high speed compression,then the martensitic transformation is restrained.The behaviors of crack propagation in cylindrical and hat shaped specimens during high speed compression are different.For the cylindrical specimens,the cracks always propagate along the fine y austenite grains in the adiabatic shear band.For the hat shaped specimens,the cracks are observed near α’-M in matrix and the propagation can be transgranular or intergranular.Calculations based on the crystallography of martensitic transformation indicate that the criteria for variant selection during γ→ε-M and ε-M→α’-M transformation are different.The variant selection of γ→ε-M transformation is determined by the mechanical work induced by external force and transformation strain during γ→ε-M.α’-M variant selection is affected by the mechanical work and the strain energy during ε-M→α’-M transformation because higher strain energy must be overcome.The occurrence of ’α-M variant pairs with misorientations of<111>60° and<110>50° reduces the strain energy effectively,thus these α’-M variant pairs are selected preferentially.In α’-M variant pair with zigzag morphology,the α’-M variant with the highest mechanical work shows nucleation superiority;while higher mechanical work and strain energy reduction are both essential for the nucleation of the other variant in the pairs.The occurrence of ε-M reversion(ε-M→γ)in<110>γ grains during tension is affected by two factors.First,the γ→ε-M transformation is promoted and the ε-M→α’-M transformation is inhibited,thus the twinning of ε-M is promoted and its orientation favors ε-M reversion.Second,the distribution of ε-M plates also affects the phase transformation,less collisions between ε-M plates inhibit the nucleation of α’-M and lead to the twinning of ε-M and its reversion.Investigations of microstructure and texture evolution during cold rolling shows that y phase is almost completely transformed into α’-M phase at medium reduction;and a higher rolling reduction results in the dominant deformation of γ,ε-M and α’-M.The main texture components in α’-M are {113}<110>,{554}<225>and rotated cube({001}<110>)at medium rolling reduction,which are the typical phase transformation textures.The {113}<110>texture rotates toward a more stable orientation {223}<110>and leads to a strong cold rolling texture(<110>//RD)with the increase of reduction.The reversion of α’-M occurs by diffusional mechanism,accompanying with the diffusion of Mn and Al elements.And α’-M grains with {111}<112>orientation,which have high stored energy,’show the advantage of reversion.Deformed α’-M is merged by the adjacent γ,and elongated γ grains with a large amount of subgrains are obtained.Subsequently,recrystallization of γ grains occurrs by sub-grain coalescence and equiaxed γ grains are obtained.The textures of equaxied γ grains are approximately same as that of the deformed γ grains,which is generated by the growth of residual γ grains.