Study On Strain Hardening Behaviors Of Twinning Induced Plasticity Steels

Twinning Induced Plasticity is that deformation induced twinning in austenite of TWIP steels helps to enhance material's strength and ductility,increase its energy absorption capacity and hinder its crack growth during forming.These excellence behaviors promote TWIP steels to have chance applying in the light-weight of auto-body.However,the process of twinning induced plasticity is a complex interaction not only between twinning and slip,twin and twin,and twin and matrix,but also between microstructure and macroscopic strain hardening behavior.Therefore,the key factors for applying TWIP steel into the auto-body manufacturing are how to comprehend the deformation induced twinning and twinning induced plasticity in the deformation process of TWIP steels,and how to represent the strain hardening behaviors of material.This thesis aims to explore the mechanism of strain induced twinning,find the influence of deformation twinning on strain hardening behavior of material,and provide the optimization method in the deep drawing process of TWIP steels.Firstly,an in-situ observation platform is set up to catch the deformation twinning of TWIP steel.Secondly,a new deformation twinning model is developed to explore the effects of mechanical parameters(such as strain,stress,deformation temperature,strain rate and so on)on twinning nucleation and growth based on the shear-band theory.Thirdly,a dislocation-based mixed strain hardening model is established considering twinning and twinning-free microstructure features,and then an elasto-plasticity constitutive model is presented with twinning induced plasticity.Finally,a failure factor for TWIP steel forming is presented as well as the twinning model and strain hardening model,which could be used to calculate the forming limit diagram of TWIP steel and study the effects of material parameters,mould geometry parameters and forming parameters on the formability of material.And then the optimization method of parameters for sheet forming of TWIP steel is given.The main contents of this thesis include:(1)In-situ test for deformation twinning volume fraction of TWIP steelsIn order to get the volume fraction of deformation twinning in froming,an in-situ platform for the tension test of TWIP steels with optical microscope has been built based on the function of the in-situ method and the defects of SEM/TEM facility in deformation twinning test.The two fixtures of the equipment move in the same speed and create an almost constant filed in the middle of specment,whose ends connected with the fixtures.And the test location is found with an on-line tracing auxiliary device controlled by a high precision 3D position technique.The twinning area of grains can be recognized directly by the processed Metallograph and is calculated by the percentage of twinning area within the picture,on which is superimposed by the rectangular grids with half grain size spacing,and the volume fraction of twinning is calculated by the twinning volume fraction of grains considering the thickness and space of twinning.The test results also show that the twinning volume fraction decreases with manganese content and tension speed.In conclusion,the in-situ equament can support the experimental platform in observing and testing the twinning volume fraction of TWIP steels in deformation process.(2)Deformation twinning model of TWIP steelsIn order to get deformation induced twinning mechanicsm of TWIP steels in forming,a stacking fault energy(SFE)model of TWIP steels is presented with the influencing factors including alloying elements and temperature,and the SFE increment caused by the temperature rise in deformation and by plastic strain considered.Based on the shear-band theory proposed by Olson and Cohen,the deformation induced twinning model is built,in which twinning drive force are controlled by the stacking fault energy and stress state and the shear-band nucleation and growth are controlled by temperature,strain rate and stress state.Finnaly,the effect of Mn content,stress state,and tension speed(strain rate)on deformation induced twinning is given based on the twinning mocel,which present a theoretical method in studying the influence of microstructure change on the macro mechanical behaviors.(3)Constitutive equation of TWIP steelIn order to catch the effect of microstructure changes on the work-hardening behaviors of TWIP steels,a strain hardening model of TWIP steel is built,considering the influence of deformation twins on the formation of dislocation based on the typical dislocation model and mutiphase mixture law.And the elastic-plastic constitutive relationship of TWIP steels is developed with the combination of the influence of deformation twins on the strain component of TWIP steels,and the classics incremental theory of plasticity.Then the influences of the Mn content,tensile speed,and stress state on the mechanical behaviors of TWIP steels are investigated.Moreover,the influences of deformation twinning on the strain hardening properties are also analyzed.And the effect of deformation twinning on the failure factor is studied in uniaxial tension and cup-drawing.These proposed models are helpful to study the formability of TWIP steels.(4)Study on stamping formability of TWIP steelsThe material deformation behavior in sheet forming process is related to the stress/strain state.It is very difficulty to formulate the proper process parameters to make sure the best formability of TWIP steel at present.To address this issue,the forming failure factor,considered as the basis of the analysis of stamping performance,is constructed,the forming limit of the materials is calculated,and the influence of deformation twinning on the failure factor of forming limit diagram is analyzed.On this basis,the influences of material parameters,forming parameters and mold parameters on the failure factors are studied.Finally,the optimization of the process parameters in cup-drawing is realized.New methods are given here to control the better formability in TWIP steels sheet forming process.