The Critical Strain for Dynamic Transformation in Hot Deformed Austenite

概要

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The 63 flow curves published previously in ISIJ Int. (2012) are re-analyzed for the presence of departures from simple work hardening (dynamic recovery) behavior. These curves were determined on a 0.019%C plain C, a 0.11%C Nb microalloyed, and a 1.56%Mn-1.56%Si Nb-modified TRIP steel by means of hot compression tests carried out from 900 to 1150°C and at strain rates up to 1 s–1. Two sets of second derivative minima are shown to be associated with all 63 curves, in contrast to the single minima reported earlier. It is shown that double minima can only be obtained when the polynomial order of the fitting procedure on the whole flow curve is 11 or higher. By contrast, when only the ascending portion of the flow curve is fitted, double minima are found as long as the polynomial order is at least 8. The first set of minima corresponds to the initiation of dynamic transformation (DT), as shown in an earlier analysis of torsion flow curves. The second set of minima is associated with the nucleation of dynamic recrystallization (DRX); these were the only minima reported earlier. The temperature dependences of the two sets of minima intersect at higher temperatures, indicating that the only softening mechanism operating at high austenite deformation temperatures (in addition to dynamic recovery) is DRX.