Effect of Coarse γ Grain Size on the Dynamic and Static Recrystallisation during Hot Working in Microalloyed Nb and Nb-Ti Steels

The effect of coarse austenite grain size on the dynamic and static recrystallization kinetics of two microalloyed Nb and Nb-Ti steels is investigated. To characterize the dynamic recrystallization behavior of the austenite, continuous-torsion tests were carried out after the reheating of the specimen at different temperatures in the range 1000-1420 C. It was observed that the occurrence of dynamic recrystallization is dependent on the initial grain size and the deformation conditions (temperature and strain rate). Decreasing values of the Zener-Hollomon parameter (Z) and grain size promote dynamic recrystallization. However, for the coarser grain sizes no peaks appear on the flow curves above a determined value of Z. This value seems to decrease with increasing grain size. An equation to predict the epsilon-p peak strain for a wide range of grain sizes was obtained for both steels. The effect of strain on the static recrystallization of austenite having a large grain size is also studied. Interrupted-torsion tests were performed to determine the fractional softening. (Author)