Research On Dynamic Mechanical Properties And Phase Transformation Behavior Of High Strength TRIP Steel

Thermodynamics of phase diagram and Landau theory are combined together with experimental study to explore the dynamic mechanical properties and transformation mechanism of high strength steel for automobiles during deformation.By using universal material test machine,high-speed material testing machine Zwick HTM 5020 and rotation disk bar–bar tensile impact apparatus with infrared radiation thermometer,OM,XRD,SEM,TEM and XRD techniques,the mechanical response of experimental steel was investigated.The martensitic transformation was analysed by using calculation of phase diagram and Ginzburg-Landau phase field simulation.The main conclusions can be obtained as follows:1 Under quasi static tensile load,the experimental TRIP steel A exhibits a ultimate tensile strength of 975 MPa and a total elongation of 28%.And the experimental TRIP steel B exhibits a ultimate tensile strength of 968 MPa and a total elongation of 24%.They all have a higher product of strength and elongation because of TRIP effect resulting from above 15 volume fraction retained austenite with about 1.1 weight percent carbon content.Under dynamic tensile load,the strengthening of ferrite are obtained because of the instantaneous proliferation of dislocation formed in matrix of the steel.2 The total elongation of experimental TRIP steel firstly decreased with the strain rate increasing because the "gradual transformation" of retained austenite is inhibited by the deformation localization.Then with the increase of the strain rate,the temperature at the fracture of tensile example can reach from 150 ? to 260 ?.So the total elongation of experimental TRIP steel is improved by the great adiabatic heating.3 The transformation model from retained austenite to martensite is established based on Ginzburg-Landau simulation,which,combined with the measured adiabatic temperature rising of the test steels,is successfully to be applied to obtain the effect of strain rate on the mechanical behavior of TRIP steels of different grades: in the quasi-static state the transformation occurs gradually under tensile load,while high strain rate is applied transformation of martensite accomplishes instantaneously;the strength of TRIP steel is increased with the increase of strain rate;under dynamic loading,the instantaneous proliferation of dislocation leads to localized deformation and inhibits the TRIP effect,which damages the ductility largely and even overcomes the benefit effect of adiabatic temperature rising.4.In the light of thermodynamics of martensite transformation,formula for Gibbs free energy in TRIP steels is established.This formula is engaged in Thermo-Calc software through an interface connection and its thermodynamic calculation result shows: the difference of work,say,(35)E done by external stress on TRIP and DP steels under quasi-static state is very similar to the transformation energy from retained austenite to martensite at room temperature.But at the high strain rate and tensile loading condition,the(35)E on TRIP and DP steels is almost equal to zero.Calculation result by Landau theory proves that which calculates the transformation energy by the multiplication of the critical elastic energy density and the mole fraction of martensite.Based on the calculation results combined the traditional thermodynamics with Landau theory,the relationship among the external stress,compositions of TRIP steels and Ms temperature of retained austenite is established offering the basics of optimization of composition as well as the processing parameters for steels.