Study On Rolling Process And The Microstructure Evolution Of Trip/Twip Steels

Recently,TRIP/TWIP steels which exhibit an exceptional combination of high strength,outstanding ductility and work hardening,have been suggested as one of the most promising candidates for structural components in modern automobile industry.However,TRIP/TWIP steel suffers from low yield strength(about 200-500 MPa),limiting the industry development.Therefore,improving the yield strength of TRIP/TWIP steel and ensuring good plasticity have become the main task of researchers.In this paper,the effects of rolling reductions and rolling temperature on the microstructure evolution and mechanical properties of three different compositions of Fe-30Mn-Si-Al based TRIP/TWIP steel are studied.The main results of this study are as follows:(1)The effect of warm rolling reduction ratio ranging from 20% to 55% on microstructure evolution,the tensile deformation mechanism,and the associated mechanical properties of an Fe-30Mn-4Si-2Al TRIP/TWIP steel were studied.The densities of dislocation and twins were raised with the increasing of warm rolling reductions.The investigated steel,with a fully recrystallized state,exhibited a single ?-TRIP effect during the room temperature tensile deformation.However,it should be noted that deformation twins are induced during warm rolling at 673 K with rolling reduction of 35%.Therefore,asynchronous synergy of two deformation mechanisms,namely twinning and ?-martensite transformation,is achieved,leading to novel bimodal-microstructured TWIP/TRIP steels with high performance.Furthermore,a tensile deformation mechanism that started from a single ?-martensitic transformation moved to a bi-mode of ?-martensitic transformation accompanied with deformation twinning,and finally to dislocation glide with the increasing warm rolling reduction was proposed.(2)The effect of caliber rolling temperature(773K?1373K)on microstructure evolution and mechanical properties of three test steels are analyzed.With the increasing rolling temperature,the dislocation density decreases and the microstructure changes from the fiber crystal to the equiaxed crystal.The deformation mechanism of the three test steels are affected by the rolling temperature.The ?-martensite,deformation twins and dislocations are induced by tensile test during room temperature.Therefore,a good combination of strength and plasticity is achieved.(3)We studied the influence of annealing temperature on the microstructure and mechanical properties of Fe-30Mn-0.3Si-2Al-0.1C with a pre-strain of 75%.The experimental results show that the aspect ratio of the microstructure grains increases and the dislocation density decreases with the increasing annealing temperature.The recrystallization temperature is 973 K.It is interesting to note that the plasticity is improved significantly owing to the occurrence of deformation twin during tensile deformation,achieving a perfect combination of the strength,when the annealing temperature is 1273 K.