Mechanical Properties And Microstructure Of Fe-20Mn-2.6Al-2.6Si TRIP/TWIP Steel

High manganese Fe-Mn-Al-Si TRIP/TWIP(transformation induced plasticity/twinning induced plasticity)steel with an excellent combination of strength andductility and extraordinary strain hardening rate can effectively reduce the weight ofcar body and fuel consumption, as well as improve the safety performance of thevehicle. So the Fe-Mn-Al-Si steels have very good application prospect. TheMicrostructure evolution and strain hardening rate of the forged Fe-20Mn-2.6Al-2.6Sisteel with different deformation levels were studied by optical microscopy, X-raydiffraction (XRD), transmission electron microscopy (TEM) and high-resolutiontransmission electron microscopy (HRTEM). The grain size, mechanical propertiesand corresponding fracture morphology of the cold rolled Fe-20Mn-2.6Al-2.6Si steelunder different annealing time at different tension strain rate were studied by electronback-scattered diffraction (EBSD), energy dispersive spectrometry (EDS) andscanning electron microscopy (SEM).The results showed that there were a quantity of stacking faults and a smallamount of dislocations in low deformation level of high manganeseFe-20Mn-2.6Al-2.6Si steel. When the deformation levels was below14percent, thephase-transformation of γâ†'ε occurred and the mount of ε-martensite was graduallyincreased. The density of dislocation increased and dislocation entangled with eachother in high deformation level. After the deformation levels was above14percent，the mount of ε-martensite was gradually decreased, while the mount of α-martensite isgradually increased. So the γâ†'α and εâ†'α transformations occurred in the highdeformed samples. The phase transformation (formation of ε-martensite andα-martensite) resulted in the increase of strain hardening rate. The ε-martensitenucleated at the stacking fault with stress concentration. With the increase ofannealing time at the same annealing temperature, the size of grains of cold rolledsamples was growing. Smaller the size of grains, higher the yield strength of samples,and lower the tensile strength and the elongation. The yield strength increasing andtensile strength was decreasing with the strain rate increasing. The strain-hardeningrate of the deformed samples with high strain rate hardly increased after smalldeformation level, and began to decrease after medium deformation level. Theequiaxed dimples and parabolic dimple of fracture morphology were obsearved in the samples at low strain rate. The size of dimple become smaller and shallower with thestrain rate increase.