Study On The Microstructure And Properties Of Cold Rolled TWIP Steeland Annealing Treatments

The designed composition of the tested Fe-19Mn-0.6C TWIP steel was based on the calculated stacking fault energy(SFE), and the recrystallization temperature of various cold-rolling reduction is determined by microstructure and micro-hardness, thus determined the annealing process at various cold-rolling reduction. Meanwhile, the microstructure evolution behaviour and mechanical properties of cold-rolled and as-annealed samples were invistigated via optical microscopy(OM), X-ray diffraction(XRD) and scanning electron microscope(SEM).The results showed that the stacking fault energy of the TWIP steel was 23.732 mJ/m2,which meet the requirement that the stacking fault energy between 20 mJ/m2 and 35mJ/m2, thus resulted in the twinning as the main deformation mechanism of the TWIP steel. The recrystallization temperature of the TWIP steel at various reduction of 25%,50% 75% and 88% were 500-525 ℃, 400 ℃-425 ℃, 400-415 ℃and 400-410 ℃,respectively; and the temperatures of 580-600 ℃, 530 ℃-550 ℃, 520-540 ℃ and500-520 ℃ were the recrystallization end temperature of the tested steel, respectively.With the increase in strain, the twinning evolution was from the first-twinning to the second-twinning, and numerous of nano-twinning formed at last. More twinning can be obtained as the greater speed was operated at cold-rolling. Comprision the properties in the normal direction and parallel direction at the reduction of 50% of the TWIP steel, the yield strength and tensile strength were larger in normal direction whenever the rolling speed was 0.2 m/s or 0.4 m/s; whereas the elongation rate was lower. As the temperature increased to the recrystallization start temperature during annealing of the cold-rolled TWIP steel, a little martensite generated from the segregation of carbon atoms; the martensite gradually disappeared during the annealing temperature further increased to the recrystallization end temperature, thus the fully recrystallization austenite grains were obtained. The formation of twinning promoted by the cold-rolling process with intermediate annealing treatment for the TWIP steel with the same reduction, which can significantly enhanced the yield strength and tensile strength, conversely, the ductilityslightly declined.