Nitrogen-alloyed austenitic stainless steels are new materials that have favorite mechanical properties such as high strength and ductility, desirable toughness and work hardening, good corrosion resistance. All of these features are of great importance for sea water systems, automobile and nuclear industries. It is well confirmed that such high efficiency results from the role of N in solid solution. Nitrogen is known as an austenite stabilizer, solid solution strengthener and it improves pitting corrosion resistance, fracture toughness, creep and fatigue strength. The advanced thermo-mechanical process that is based on the SIM transformation, is nowadays one of the main methods for producing nano/ultra?ne-grained austenitic stainless steels. This method involves cold rolling followed by reversion annealing of the SIM to austenite. The 200 series austenitic stainless steel due to having thermodynamically metastable austenite at room temperature can be transformed to martensite below the M d temperature. The purpose of the present study is to investigate the effects of nitrogen addition on fabrication of ultrafine/nano grained AISI 201L stainless steel by advanced thermo-mechanical process. To this end, cast samples were first homogenized at 1200 °C for 4 h, then hot-rolled at 1100 °C and finally solution-annealed at 1150 °C for 2.5 h to acquire a suitable microstructure for the subsequent thermomechanical treatment. Unidirectional and cross multi-pass cold rolling at 25, 0 and -15 ?C with a strain rates of 0.1- 0.7 s-1 was carried out to 90% reduction in thickness, followed by annealing at temperature range of 700–900 ?C for different times of 15–1800 s. Microstructures were characterized by optical and scanning electron microscopy, field emission scanning electron microscopy, X-ray diffraction and feritscope measurements. Mechanical properties of the final products were examined by hardness and shear punch tests. The results showed that increasing nitrogen reduced delta ferrite and austenite grain size after solution annealing. It was found that Increasing nitrogen, strain, strain rate, initial austenite grain size, decreasing rolling temperature, and the use of cross rolling resulted in an increased volume fraction of the SIM and reduced ? s during cold rolling. The increase in hardness, yield and ultimate shear strength and decrease in elongation with increasing nitrogen concentration was observed to continue up to 0.35%wt N. The results showed that with increasing nitrogen concentration a bimodal grain size distribution appeared after annealing the 90% cold rolled specimens at 800 ?C for 60 s. Keywords: Advanced Thermomechanical Treatment, Stainless Steel, Nitrogen, Cold Working, Reversion Annealing
