Microstructure and Mechanical properties of Medium Manganese Steels

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Medium manganese steel is a 3rd generation advanced high strength steel, which contains 3–12 wt% manganese. This steel is highly suitable material for the automobile industry due to good balance between the cost of material and the tensile properties. It has got the optimum amount of strength and elongation. Initially the microstructure of this steel after cold rolling is deformed martensite (ά). Intercritical annealing is done to get moderate amount of retained austenite (γr) having good stability, which leads to higher strength and better ductility due to transformation-induced plasticity (TRIP). The formation of stable retained austenite (γr) by intercritical annealing process is our prime interest. Medium Manganese steel usually exhibits austenitic-ferrite microstructure after the intercritical annealing process, which makes the steel suitable for 3rd generation advanced high strength steels as this microstructure gives good amount of elongation. From this study we get best combination of intercritical annealing and quenching to get the required microstructures and properties. Thermo-calc software is used to find the optimum temperature for intercritical annealing at which we get optimum amount of retained austenite (γr) containing maximum manganese. Tensile tests were carried out at different pre-strain level from the samples taken in both rolling and transverse directions. Samples were prepared for SEM and optical microscope characterization to see the changes from initial microstructure of the steel. X-ray diffraction was also done to find out the amount of retained austenite that we are getting from tensile sample after pre-strain it to different level. It was concluded that after intercritical annealing at 650 °C for 2 h the steel sample have approximately 22% retained austenite and ultimate tensile strength (UTS) of 843 MPa and total elongation of around 36%.