Enhanced Mechanical Properties of Microalloyed Austenitic Stainless Steel Produced by Martensite Treatment

A thermomechanical treatment based on the reversion of strain-induced martensite (SIM) was employed to produce ultrafine-grained structures in a microalloyed austenitic stainless steel (ASS). The smallest average grain size of approximate to 480nm was achieved by annealing the heavily-rolled samples at 650 degrees C for 240min while an outstanding combination of strength and ductility was acquired by annealing at 850 degrees C for 1min due to the synergic effects of the following phenomena: refinement of austenite matrix grains down to an average grain size of 620nm, presence of small amount of thermally-induced martensite, formation of a bimodal grain size distribution by primary and secondary recrystallization processes, and dislocation accumulation. The kinetics of strain-induced martensitic transformation was studied and the classical rule of mixtures was used to elucidate the hardening contribution of martensite. Subsequently, it was shown that the Hall-Petch relation is valid for a wide range of grain size for AISI 304 type stainless steel.