| Because of its superior mechanical properties including good formability, high absorption ability for the unit impact-energy, the (transformation-induced-plasticity, TRIP/twin-induced-plasticity, TWIP)TRIP/TWIP steels containing high content of Mn becomes the mainly developed steel for the potential application in automobile field. Up to date, extensive researches on the TRIP/TWIP steel have been focused on either the TRIP effect or the TWIP effect. The reported results are still scarce to simultaneously discuss the both effects. Consequently, the present study mainly investigates the mechanical properties and deformation mechanism for a new kind of TWIP/TRIP steel which has a chemical composition of Fe-20Mn-0.6C. The obtained results may promote the application of this new kind of steel in the automobile industry.The microstructures of the steel before and after deformation have been investigated by using the optical micrograph, X-ray diffraction (XRD), electron-backscattering diffraction (EBSD) and transformation electron micrograph (TEM). The investigated steel has been rolled at room temperature with different reductions. And the mechanical properties have been characterized by controlling the strain during uni-axial tensile tests.The obtained result show that the microstructure of the investigated steel is mainly composed of austenitic matrix and annealing twins after hot rolling. However, a significant amount of deformation twins are observed after cold rolling. Moreover, the twin density increases with increasing rolling reduction. Under a rolling reduction of10%and20%, numerous martensite laths have occurred. Annealing the cold-rolled sheet at750℃, the grain size becomes smaller and a lot of grain boundaries present with the special relationships such as Σ3and Σ9types, in comparison with the hot-rolled steel.The as-prepared steel exhibits excellent mechanical properties including a yield stress of365MPa and a tensile stress of924MPa, with the elongation-to-failure over80%. Very impressively, the product of strength and strain is as high as73.9GPa-%. After annealing at750℃, the product of strength and strain increases up to84.4GPa-%which can be attributed to the strain higher than90%, despite of the decrease for the tensile stress from924MPa to907MPa.Both the yield stress and tensile stress decrease with increasing rolling reduction whereas the elongation-to-failure goes along the contrary way. For instance, the yield stress and tensile stress is1423MPa and1454MPa, respectively, for the sample with a rolling reduction of40%. Nevertheless, it is disappointed to note that the strain decreases to13%, resulting in the production of stress and ductility as low as18.9GPa-%.The postmortem observations indicate that both the TRIP and TWIP effects have played a role during the deformation of the hot-rolled sheet. The stress concentration has been induced by the pile-up of dislocations at twin boundaries and grain boundaries, which results in the twinning process. Therefore, the twin density reasonably increases with increasing strain and multi-twinning systems have been observed. The martensite phase increases with increasing strain and occur at the vicinity of twin boundaries and/or grain boundaries. The deformation process can be divided into4stages according to the strain:(a) εtrue≤0.025, the deformation process is dominated by the slip and multiplications of dislocations,(b)0.025<εtrue≤0.39, the formation of stacking faults and the slip of dislocations, as well as the twinning process,(c)0.39<εture≤0.49, the twinning process accompanying with the martensite transformation,(d)0.49<εtrue, the initialization of the secondary twinning system and the martensite transformation. |
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