Thermomechanical Processing, Microstructure, and Mechanical Properties of Medium-Mn Steel

Medium manganese (M-Mn) steels have been developed to achieve an excellent combination of strength and ductility for automotive components. This study investigates the microstructural evolution and mechanical properties of hot-rolled M-Mn steels subjected to a thermomechanical control process followed by isothermal holding at different subcritical temperatures. M-Mn steel alloy is fabricated at a laboratory scale and subjected to heat treatments using a deformation dilatometer. After the hot rolling, a fine multiphase structure is obtained after an isothermal holding at 500 degrees C, 300 degrees C, and 200 degrees C for 30 min. Microstructural characterization was examined using light microscope, scanning electron microscope, electron backscatter diffraction, and X-ray diffraction. The final microstructures have a multiphase structure from different phases: bainite, fresh martensite, tempered martensite, and film-like retained austenite with no significant M/A islands. Mechanical properties are determined by hardness and tensile tests. The sample held at 300 degrees C exhibited the best combination of strength and ductility, attributed to its higher bainite and retained austenite content.