Effect of annealing temperature on nano/ultrafine grain of Ni-free austenitic stainless steel

In the present work, martensite thermomechanical treatment was employed to refine the microstructure of a metastable Ni-free austenitic stainless steel down to nano/ultrafine size. For this purpose, the studied alloy was cold rolled to about 80% reduction followed by reversion annealing at 973 K (700 °C)-1273 K (1000 °C) for 1-104 s. It was observed that the finest austenitic microstructure is achieved in the sample reversion annealed at 1173 K (900 °C) for 100 s. It had a bimodal microstructure composed of nano and ultrafine grains with an average grain size of about 240±117 nm. Also, formation of nano-size (Cr,Fe,Mn)-rich carbonitrides was observed in the microstructure of this sample. The reversion mechanism was determined to be shear-type. The nano/ultrafine grained sample exhibited an ultrahigh strength (yield strength of 1150 MPa and tensile strength of 1293 MPa) and a good elongation of 21%. It was shown that grain refining down to nano/ultrafine range promotes deformation-induced martensitic transformation, which can improve the ductility of the alloy.