Abstract
SUS316L is generally believed as a stable austenitic stainless steel, but strain-induced martensitic transformation can occur when large deformation is given. The predominant mechanisms, especially, in terms of the effect of grain size have been reported, however, it is not still clear. In this study, strain-induced transformation behavior of SUS316L steel was investigated from the points of grain size and dislocation density. Microstructures with various grain sizes and dislocation densities were fabricated by warm multi-pass multi-directional caliber rolling and annealing. Fully recrystallize microstructures with austenite grain sizes of 12, 18 and 27 µm were fabricated. Bimodal structures with ultrafine grains and coarse grains including high density dislocations were also fabricated. These specimens were rolled at a reduction strain from 0.12 to 2.7 at liquid nitrogen temperature to occur enough amount of strain-induced martensitic transformation. To evaluate martensite volume fraction in these cryogenic rolled materials, transmission type X-ray diffraction in synchrotron radiation of Spring-8 was used. Through thick XRD data can be obtained by this method. Regarding fully recrystallized materials, volume fraction of strain-induced martensitic transformation decreased with decreasing in grain size. It is very clear that gran refinement improves the mechanical stability of austenite. On the other hand, regarding bimodal structure, accumulated dislocations promote transformation, and transformation rates were higher with dislocation density at lower reduction strain area. However, saturated volume fractions decreased with decreasing in grain size. Therefore, strain-induced transformation is affected by both grain size and dislocation density having opposite effects, retarding and promoting the transformation.
