THE FORMATION MECHANISM OF AUSTENITE STRUCTURE WITH MICRO/SUB MICROMETER BIMODAL GRAIN SIZE DISTRIBUTION

Nano-crystalline (<100 nm) and ultrafine grained (100 similar to 500 nm) materials have high strength and toughness, but its work hardening ability and uniform elongation decreased relative to the coarse grained material. Through the deformation, phase transformation and recrystallisation combination mode of development of bimodal grain size distribution of ferrite, bainite steel, the elongation rate is greatly improved. These studies are generally in order to improve the mechanical properties of material through change microstructure, but lack of study for the bimodal grain size distribution formation mechanism. This research work by cold rolling with annealing at 820 similar to 870 degrees C, in 316L austenitic stainless steel to achieve micro (3 similar to 5 mu m) and sub-micro (300-500 nm) bimodal grain size distribution. In the austenite deformation process, deformation twinning and strain induced martensite transformation occurred in large deformation stage. Accordingly inferred austenite deformation twinning is the micro mechanism of strain induced martensite. Annealing at 820 similar to 870 degrees C, the hardness of the samples and the grain size distribution remains nearly constant. Through the comparative analysis of induced martensite austenite evolution driving force and strain deformation during annealing, determined the source of bimodal grain size distribution. The micro scale grains came from the recrystallization of deformed austenite in the cold deformation does not change, and sub-micron grain size is mainly composed of strain induced martensite reverse transformation.