In this study, influence of Nb-microalloying on formation of nano/ultrafined grain microstructure and mechanical properties during martensite reversion process in a 201-type austenitic stainless steel microalloyed with Nb was investigated. For this purpose, the 90 pct cold-rolled samples with almost fully martensitic microstructure were reversion annealed at 1023 K to 1173 K (750 °C to 900 °C) for 5 to 1800 seconds. The microstructural evolution was characterized using X-ray diffractometer, Ferritescope, optical microscope, scanning, and transmission electron microscopes. Mechanical properties were evaluated using hardness and tensile tests. The reversion mechanism was found to be diffusion controlled. In comparison with other types of 201 steel, the kinetics of grain growth at 1173 K (900 °C) was much slower in the Nb-bearing steel, being related to the rapid precipitation of nano-sized Nb-rich carbonitrides during reversion process. At this temperature, the finest austenitic microstructure was achieved in the specimen reversion annealed for 60 seconds, possessing a microstructure composed of nano and ultrafined grains with an average grain size of 93 nm. This specimen exhibited an excellent combination of ultrahigh strength (yield strength of 1 GPa and tensile strength of 1.5 GPa) and good ductility (tensile elongation of 35 pct).
