In this work, microstructure and mechanical properties of metastable austenitic Fe-Cr-Mn-N steels containing different amount of N (less than ~0.5 wt%) was studied. In addition, possibility of application of martensite thermomechanical treatment for grain refinement of these steels down to nano/ultrafine range was explored. It was observed that stacking faults and ?-martensite needles decreases with N, conversely, short range ordering and deformation twinning increases. Further, it was found that yield strength and hardness of the alloys linearly increases with N. In contrast, elongation and tensile strength of the alloys changed nonlinearly with increasing N. Influence of N on mechanical properties was mainly related to its effect on solid solution strength and deformation-induced ?´-martensitic transformation. Martensite thermomechanical treatment was employed to refine the microstructure of the steels down to nano/ultrafine size. It was observed that the finest austenitic microstructure is achieved in the 0.36 wt% N steel reversion annealed at 1173 K (900?C) for 100 s with an average grain size of about 217 nm. However, the superior mechanical properties was achieved in the 0.44 wt% N steel, exhibiting an ultrahigh strength (yield strength of 1324 MPa and tensile strength of 1467 MPa) with a good elongation of 17%. Keywords: Austenitic steels; Microstructure; deformation; Martensite; Twinning; Grain refinement.
