Effect of initial microstructures on grain refinement in a stainless steel by large strain deformation

Initial grain size effect on submicrocrystalline structure evolution was studied in multiple compressions of a 304 stainless steel at 873 K (0.5 T_m). Four sets of specimens with different initial microstructures were used, i.e. annealed samples with grain sizes of D₀ = 15 and 2.2 μm, and dynamically recrystallised ones with D₀ = 3.5 and 1.5 μm. The new ultra-fine-grains (D = 0.25 μm) develop as a result of a continuous increase in the misorientations between the subgrains that evolved during deformation. In the samples with D₀≤3.5 μm, the fraction of the strain-induced high-angle boundaries increases rapidly to more than 60% with a straining to about 1.5. On the other hand, their fraction does not exceed 20% at ε = 1.5 in the sample with D₀ = 15 μm. The latter needs much more straining to around 6 to obtain 60% of high-angle (sub)grain boundaries.