Effect Of Various Heat Treatments On The Microstructure And Tensile Properties Of Fe-10Mn Steel

Facing to the demand of environmentally friendly materials and safety regular in vehicles filed, the urgent challenge lies on how simultaneously to achieve the energy conservation, emission reduction and weight reduction. It is well accepted that the future trend of automotive steel should pursue the combination of high strength, good ductility and low cost. The medium Mn steel, as a candidate of new generation of motor vehicle steel, is hopeful to make the above target true. Consequently, it is conceivable that there will be many advantageous including the saving resource, reducing cost, lighten vehicle weight and increasing safety.In this study, the microstructures and tensile properties of a Fe-lOMn steel after various heat treatments. The Ac1, AC3and CCT curve have been determined by a dilatometer. The steels with a reduction of50%and75%have been annealed at various temperatures. The effect of annealing duration has also been evaluated for the steel with a reduction of75%.Microstructures and the content of different phases of the steel were investigated by means of optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Meanwhile, mechanical properties of the steel under different heat-treatment processes were investigated using tensile machine.The experimental results show that Ac1and AC3temperature of the steel are590℃and710℃respectively. A continuous cooling transformation (CCT) curves have been obtained. After annealing at various temperatures, the steel mainly consists of austenite and ferrite laths with an average thickness of200nm, companied with a few of martensite. XRD analysis indicated that the volume fraction of austenite reaches the highest value of48%for the sample with a reduction of50%annealed at600℃, which surprisingly reaches the highest value of57%for the sample with a reduction of75%annealed at580℃. At the same annealing temperature, it has noted that the volume fraction of austenite in the sample with a reduction of75%keeps higher than that in the sample with a reduction of50%.The studied steel exhibits extremely high strength and ductility due to the ultrafine ferrite (λ=200nm) and the numerous of metastable austenite. For example, after annealing at630℃for30min, the sample with50%rolling reduction shows the UTS of1310MPa and a total elongation of39%, resulting in a production of strength and strain of51090MPa·%. The sample with a reduction of75%shows UTS of1380MPa and a total strain of33%after annealing at the same conditions. With decreasing the temperature to600℃, the UTS decreases to1290MPa but the strain increasing to37%.Annealing the sample with a reduction of75%at600℃for various duration, it shows that the volume fraction of austenite increases with increasing time and reaches the highest value of53%at holding time of60min. The austenite content decreases with the annealing time increasing over60min.XRD analysis and TEM observation have indicated that the volume fraction of austenite significantly decreases after deformation, accompanied with a few of deformation twins in the fractured sample. Consequently, it is easy conceivable that the excellent combination of strength and ductility mainly originates from the transformation and twinning processes. A lots of dimples can been seen from the fracture surface of the deformed specimen, indicative of ductile rupture closely related to micro-porous gathered type.