A further study of the kinetics of recrystallization and grain growth of cold rolled TWIP steel

Abstract

Hot rolled, laboratory-cast, TWIP steel specimens with composition 22% Mn-0.6% C (in mass %) was cold rolled to reductions of 40%, 50%, 60% and 70% and afterwards isothermally annealed for various times in the interval of temperatures 450°C ≤ T ≤ 1100°C. The purpose was to study the precipitation behavior and its plausible effect in the static recrystallization and grain growth kinetics. Two types of precipitates were found in 600 °C ≤ T ≤ 700 °C for long times: (Fe, Mn)3C - Cementite and Vanadium Carbonitrides. Recrystallized grain size was very fine, D0 ≤ 2 μm. Also, a weaken retained rolling texture in the recrystallisation process was found. Calculated value of activation energy for recrystallization, Qsoft = 281 ± 70 kJmol -1 was obtained which corresponds practically with the activation energy for bulk self-diffusion in austenite (270 kJmol -1 ) and for Mn diffusion in the austenite lattice (265 kJmol -1 ). Nevertheless, higher calculated activation energy for grain growth, QGG = 384 ± 60 kJmol -1 was found with a grain growth exponent of nGG ∼4. Consequently, the most plausible explanation is that the quantity of precipitates is enough to have relevant pinning effect of migrating grain boundaries during grain growth due to the mean length between precipitates, Lprec, is smaller than some threshold value of grain size, Lprec < Dthreshold, being, D0 << Dthreshold.