Simulation of Austenite Flow Curves under Industrial Rolling Conditions Using a Physical Dynamic Recrystallization Model

Abstract

Hot compression tests were carried out on three steels: i) a 0.038% Nb-0.11%C microalloyed grade; ii) a Nb-modified TRIP steel; and iii) a Ti-stabilized low carbon steel. The tests were performed at strain rates up to 1 s -1 and over the temperature range 880-1 200°C. The initiation of dynamic recrystallization (DRX) was observed under all testing conditions. Two sets of equations were derived from the experimental curves: i) a work hardening relation pertaining to the grains in which DRX has not yet nucleated; and ii) a separate work hardening expression describing the mean flow stress applicable to the grains in which DRX is taking place. With the aid of the temperature and strain rate dependences determined from the data, and using the law of mixtures, extrapolated flow curves were calculated applicable to strain rates up to 100 s -1 , i.e. to those involved in strip mill rolling. The simulations show that, once DRX has been initiated, the flow stress is controlled by the kinetics of the softening mechanisms.

The Critical Strain for Dynamic Transformation in Hot Deformed Austenite

“…6 of the earlier publication. 22) Although the DRX critical strains illustrated here are essentially identical to those reported previously, the DT critical strains are lower and entirely new.…”

“…When the order is increased into the 7-10 range, the DRX minima are detectable, but the DT ones are not. This was the approach taken inadvertently in the previous publication, 22) which is why only one set of minima were reported at that time. From Fig.…”

“…The first is that the critical strain behaviors of the three steels reported previously 22) require correction in light of the current results. For this purpose, the present critical strains are plotted as functions of the Zener-Hollomon parameter Z in Fig.…”

“…6 was re-analyzed using the approach of Ref. 22). The first important difference between the present calculations of those of the earlier treatment is that, when the previous fitting was carried out, the polynomial was required to reproduce the entire flow curve.…”

“…When the present results were first analyzed using the double derivative method and presented in the preliminary report published earlier, 22) only a single set of minima were reported and these were identified as being associated with DRX. Here we consider the question of how it is possible to arrive at two differing sets of critical strains from a single set of flow curves and using the same basic method of analysis.…”