Effect Of Annealing Temperatures And Strain Rates On Tensile Properties Of Fe-20Mn-0.6C TWIP Steel

With the rapid development of economy and the improvement of people’s living standard, car has being become a necessity of life in the country. Consequently, a higher standard must be facing in order to meet the demand for automotive safety and environmental protection. TWIP steels have received extensive attentions in recent years due to their outstanding mechanical properties, such as high strength, high plasticity, good formability, and super specific energy-absorption-capability.In this study, the microstructures of the steel after different cold rolling reductions and different heating treatment have been investigated by using the optical micrograph, X-ray diffraction (XRD) and electron-backscattering diffraction (EBSD). Subsequently, the mechanical properties have been measured by tensile tests with various strain rates. And the corresponding microstructure variations have been investigated using XRD, optical micrograph, transmission electron micrograph (TEM). The major results can be summarized as follows:The steel plate with a hot-rolling reduction of50%and a cold-rolling reduction of40%has been annealed at700℃,750℃,800℃,850℃and900℃for30min, respectively. It is noted that both the grain size and twin lamellar spacing (λ) increase with increasing annealing temperatures. For instances, the grain size is about10μm accompanied with λ=3μm for the sample annealed at700℃for30min. However, a grain size of50μm and X=30μm can be observed for the sample annealed at900℃for30min. After annealing, the steel exhibits excellent mechanical properties. At a strain rate of5×10-3S-1, the yield stress is338MPa,303MPa,238MPa,194MPa and212MPa while the tensile stress is995MPa,976MPa,940MPa,884MPa and844MPa for the sample annealed at700℃,750℃,800℃,850℃and900℃for30min, respectively. Meanwhile, it is noted that the elongation-to-failure (εf) increases with increasing annealing temperature. For example, a εf value of83%is obtained for the sample annealed at700℃for30min. However, the εf significantly increases to97%with increasing annealing temperature to900℃. Obviously, the yield stress and tensile stress increase with the increase of annealing temperature, while the strain goes along a contrary way.The steel plate with a hot-rolling reduction of50%has been rolled at room temperature with a reduction of20%,40%,60%and80%. It is noted that, with increasing rolling-reduction from20%to80%, both the yield stress and tensile stress increase from862MPa to1440MPa and from1267MPa to2040MPa, whereas the strain terrifically decreases from35%to3%. To get a trade-off between the stress and strain, the cold-rolled plate has been annealed at800℃for5min. It is interesting to note that the obtained samples exhibit excellent balance between stress and strain. For example, the sample with a cold-rolling reduction of60%shows a yield stress of355MPa, a tensile stress of976MPa and a strain of93%.After a process of50%hot rolling and40%cold rolling and annealing at850℃for30min, the mechanical properties of the obtained steel have been measured by tensile tests at various strain rates of5×10-3S-1,5×10-2S-1and5×10-1S-1. It is very interesting that both tensile stress and the εf decrease with increasing strain rate while the yield stress goes along a contrary way. As the sample deformed at a strain rate of5×10-3S-1,5×10-2S-1’and S×10-1S-1, the yield stress increases from194MPa to205MPa and then to255MPa with increasing strain rate. Simultaneously, the tensile stress decreases from884MPa to842MPa, and then to827MPa, accompanied with decreasing εf value of95%,87%and81%, respectively.For the sample annealed at various temperatures (700℃,750℃,800℃,850℃and900℃) for30min, TEM observations show that twin density in the deformed sample increases with increasing annealing temperature. It is believed that the critical force for twinning decreases with increasing grain size resulted from increasing annealing temperature. Consequently, the twinning process has been enhanced during the tensile deformation, leading to the excellent mechanical properties combined with high strength and good ductility.