Influences of Austenization Temperature and Annealing Time on Duplex Ultrafine Microstructure and Mechanical Properties of Medium Mn Steel

A duplex ultrafine microstructure in a medium manganese steel （0.2C-5Mn） was produced by austenite re- verted transformation annealing （ART-annealing）. The microstructural evolution during annealing was examined by scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and X-ray diffraction （XRD）. Based on the microstructure examination, it was found that some M3C type carbides appeared in the martensitic matrix at the beginning of the ART-annealing. But with further increasing annealing time, these carbides would be dissolved and finally disappeared. Meanwhile, the austenite lath was developed in the ART-annealing process and the volume fraction of austenite increosed with the increase of the annealing time, which resulted in a duplex microstructure con- sisting of ultrafine-grained ferrite and large fraction of reverted austenite after long time annealing. The mechanical property examinations by uniaxial tensile tests showed that ART-annealing （6 h, 650 ℃） resulted in a superhigh product of strength to elongation up to 42 GPa ·%.