Evaluating Effect of Niobium on Microstructure and Mechanical Properties of Ultrafine/Nanostructured AISI 301 Austenitic Stainless Steel through Martensite Treatment In recent years, a great attention has been paid on the development of high strength nanostructured steels. However, the lack of high ductility in the ultrafine/nano grained steels restricts their wide spread use in constructional applications. In this study, an ultrafine/nano grained 301 austenitic stainless steel containing 0.15wt% niobium was fabricated by the thermomechanical martensite treatment. This treatment includes strain-induced martensitic transformation by cold rolling followed by reversion annealing. Effect of niobium on the reversion behavior and mechanical properties of the ultrafine grained AISI 301 was investigated. For this purpose, ingots of the AISI 301 steel containing 0.15%wt niobium were prepared in vacuum induction furnace, homogenized at 1200?C for 10h, forged at 1000-1200?C, and finally solution treated at 1200?C for 6h. After every stage, specimens were quenched at water to prevent precipitation of second phases. The annealed specimens were then cold rolled at 0 and 25?C with different reduction in thickness up to 90%. The cold rolled sheets were then annealed at 700-900?C for 15-1800s. Start and finish temperatures of the reversion transformation was determined by a dilatometric test. The microstructures and mechanical properties were evaluated by means of optical and scanning electron microscopy with EDS analysis, X-ray diffraction, ferritscope, microhardness and tensile test. The results showed that the amount of delta ferrite was decreased from 6.8% in the cast ingots to 3.5%, 0.51% and 0.12% after homogenization, hot forging and solution annealing, respectively. The grain size after 6 hrs solution annealing at 1200?C was 200?m. The EDS analysis confirmed that the chromium carbides, decomposed after solution annealing, were precipitated at grain boundaries of the hot forged specimen. Increasing thickness reduction, decreasing rolling temperature and cross rolling resulted in an increase in the volume fraction of martensite, but a decrease in ? c . In addition, increasing annealing time and temperature led to an increase in the volume fraction of reversed austenite. A fully austenitic structure was observed in the specimen annealed at 850?C for 300s with the austenite grain size of 130 ± 19 nm. By increasing the annealing time to 1800s, the grain size was reached to 1900 ± 170 nm. The ultrafine grained steel annealed at 850?C for 300s possessed an exceptional elongation of 46%, high tensile strength of 1012 MPa, due to both nano-sized precipitates and strain-induced martensite that formed during tensile test. The cold rolled specimen at 0?C and annealed at .