Microstructural Changes and Strengthening of Austenitic Stainless Steels during Rolling at 473 K

The microstructural changes in 304L and 316L austenitic stainless steels during plate rolling with 95% rolling reduction at a temperature of 473 K and their effect on strengthening were studied. The microstructure evolution was associated with deformation twinning and microshear banding. The latter ones involved ultrafine crystallites, which rapidly evolved in strain-induced ultrafine austenite grains as a result of fast increase in misorientations between them. Besides the ultrafine austenite crystallite evolution, the microshear bands assisted local appearance of deformation martensite, which attained about 25 vol.% and 3 vol.% at total strain of 3 in 304L and 316L steels, respectively. Both the microshear banding and the martensitic transformation promoted the formation of ultrafine grains with a size of less than 1 mu m. The strain dependence of the ultrafine grain fraction obeyed a modified Johnson-Mehl-Avrami-Kolmogorov function. The deformation grain size and dislocation density that develop during rolling could also be expressed by exponential functions of true strain. Incorporating the revealed relationships between the strain and the microstructural parameters into modified Hall-Petch-type equation, unique expression for the yield strength of processed steels was obtained. The dislocation strengthening was the largest contributor to the strength, especially at small to medium strains, although grain size strengthening increased during rolling approaching that from dislocations at large strains.