Recrystallization and Grain Growth Behavior in Severe Cold-rolling Deformed SUS316L Steel under Anisothermal Annealing Condition

Abstract

Ultrafine grained microstructure seems difficult to be obtained in austenite stainless steel by hot rolling process because the steel has a high recrystallization temperature and high grain growth rate. Instead of severe plastic deformation at medium temperatures, anisothermal annealing of cold-rolled steel could be an effective way to obtain the ultrafine microstructure in the steel. In order to clarify the recrystallization and grain growth in SUS316L stainless steel in anisothermal annealing condition, variations of hardness and average grain size of the steel at annealing temperatures ranging from 1073 to 1223 K and heating rates in the range of 0.031–9.3 K/s were systematically investigated, and the results were summarized in contour maps. The recrystallization finish temperature was recognized by a hardness criterion, which shows a linear relationship with the logarithm of the heating rate. Moreover, the average grain size at the critical temperature is approximately 2.2 μm at low heating rates, but decreases quickly with the increase of heating rates above 1 K/s. A proposed grain growth kinetics equation for anisothermal annealing is applied in the present investigated SUS316L steel. The proportional term, i.e., Π=(Dⁿ−D₀ⁿ)/(Γ_f−Γ_i), in the equation is calculated and is found to follow the equation Π=K·exp(0.5/θ). Meanwhile, the grain growth exponent, n, for the anisothermally annealed SUS316L steel is also determined and is found to lie between 2.5 and 3.0. On the other hand, EBSD analysis of the evolved microstructure at different heating rates indicates that low heating rate caused partial recrystallization with preferred orientations at the recrystallization finish temperature, while high heating rates above 1 K/s induced the homogeneously nucleated recrystallization microstructure with random orientations and a lognormal type grain size distribution.