| In order to achieve low cost, high safety and low energy consumption, the steel with high strength and good toughness can be obtained by adjusting alloy-element and working procedure, meeting the demands of automobile industry. In this study, a medium Mn steel, as a candidate of new generation of motor vehicle steel, has been designed to improve the strength and toughness based on TRIP (Transformation Induced Plasticity) effect related to austenite phase.The Fe-7Mn steel after hot-rolling or warm rolling has been annealed at various temperatures. Micro structures and the content of austenite phase of the steel were investigated by means of optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Moreover, the distribution of phases has been characterized by electron backscattered diffraction (EBSD) technique. Mechanical properties of the steel under different heat-treatment processes were investigated by using tensile machine.The obtained results are shown as follows:Ac1 and Ac3 temperatures of the steel are determined by the dilatometer as 585℃ and 720℃, respectively. A continuous cooling transformation (CCT) curves have been drawn by various cooling rates tests using the dilatometer. It has shown that the martensitic transformation occurs at 300℃ under a cooling rate of 0.1℃/s～60℃/s.The investigated steel with an initial thickness of 6mm has been hot-rolled at 800℃ to a final thickness of 1.5mm, subsequently annealed at various temperatures for 30min. The obtained specimen consists of ferrite, austenite and martensite, which converted from the metastable austenite with large size. The highest volume fraction of austenite is 36%, resulting from the annealing at 630℃ for 30min. This specimen exhibits the highest yield stress of 560 MPa and the highest ultimate tensile stress of 1470 MPa, with a total strain of 30% at a strain rate of 1×10-3/s.Warm-rolling the investigated steel with an initial thickness of 6mm at 630℃, the obtained specimen(1.5mm) subsequently annealed at various temperatures for 30min. For the steel annealed at 600℃ for 30 min, the highest volume fraction of austenite is 37%, accompanied with ultrafine laths of ferrite and martensite (λ≤1μm). This specimen exhibits the highest yield stress of 910MPa and the highest ultimate tensile stress of 1600MPa, with a total strain of 28% at a strain rate of 1×10-3/s.In order to analyze the impact of austenitizing temperature on the mechanical properties of experimental steel, annealing the sample at 800℃ and 900℃ for 30 min, respectively, the sample have subsequently been cooled to room temperature by air. Afterwards, the sample has been annealed at 630℃ for 30 min to promote the reverse transformation of martensite into austenite. It shows that the volume fraction of austenite decreased from 28% to 15% with increasing temperature from 800℃ to 900℃, due to the decreasing nuclei site related to the increasing austenite size at the higher temperature. The sample annealed at 800℃ for 30 min add cooling to room temperature shows the highest yield stress of 910MPa and the highest UTS of 1330 MPa, with a total elongation of 30%.By analyzing the effect of different heat treatments on strengthening mechanisms, one can see that the initial higher work hardening is due to the flaw in the fine ferrite and austenite grains. The enhanced work-hardening ability of the investigated steel is closely related to the interaction of the TRIP effect, martensitic transformation and dislocations in the ferrite, resulting in the excellent combination of strength and ductility. |
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