Study On The Microstructure Evolution And Work Hardening Behavior Of TWIP/TRIP Steels

Recently,automobile lightweight is one of the main directions of automobile industry development,and thinning of high strength steel plate is the main method of automobile lightweight.TWIP/TRIP steels have received striking attention in the field of automotive application due to their superior ductility,decent work hardening rate and unique plastic deformation mechanism.However,the traditional TWIP/TRIP steels are usually used in the state of complete recrystallization.It suffers from quite low yield strength(about 200 ~ 500MPa),limiting their further industry development.Therefore,development of the TWIP/TRIP steels with high yield strength and excellent working hardening rate has become the goal of researchers.In this paper,Fe-30Mn-4Si-2Al TWIP/TRIP steels are used as experimental steels.The main contents and results of this study are as follows:(1)We observed and analyzed the microstructure evolution of experimental steels during the tensile deformation and its relationship with work hardening rate.The high density dislocations formed from 0 to 0.08 true strain(stage I),and the rate of dislocation proliferation is the highest.The amount of deformation induced ?-martensite is small and the work hardening rate decreases sharply.The deformation induced ?-martensite begins to proliferate promptly from 0.08 to 0.38 true strain(stage II and III).The occurrence of ?-martensite reduces the mean free path of dislocations.Therefore,the work hardening is due to dynamic microstructure refinement(dynamic Hall-Petch effect).Finally,over 0.38 true strain,?-martensite nearly saturates,and the proliferation of dislocation is difficult.Thus the strain hardening rate decreases again.(2)The main governing factor responsible for the deformation mechanisms of TWIP/TRIP steel is the stacking fault energy(SFE)of the austenitic matrix.The SFE sensitively increases with increasing deformation temperature.Therefore,experimental steels were rolled at various temperatures over a wide temperature range of 298 ~ 1073 K,and then deformed by tensile test at room temperature.The combined effects of two deformation mechanisms(?-martensitic transformation and twinning),will be developed in the material.It was found that the elevating of rolling temperature changed the deformation microstructure from ?-martensite to deformation twins,as well as a decrease in dislocation density.It is interesting to note that the asynchronous synergy of deformation twins induced by warm rolling at 673 K and ?-martensite which occurs at room temperature tensile deformation renders this material distinctly yield strength with an improved work hardening rate.(3)We studied the influence of pre-strain rolling reduction and annealing temperature on the microstructure and mechanical properties of experimental steels.At 673 K,the experimental steels were rolled with different reductions(20 ~ 60%).With the increase of rolling reduction,the strength and hardness of the experimental steels is obviously increased.Then,the samples were annealed with a pre-strain of 60% at 623 ~ 1023 K.The working hardening rate is improved and a good combination of strength and plasticity is achieved.