Correlation between apparent activation energy for hot working and temperature of no recrystallization in microalloyed steels

Abstract

Two-stage linear ln[sinh(as)] vs 1/T plots, based on the stress data derived from either continuous or multipass flow curves generated by testing low carbon Nb/Ti microalloyed steel (Nb steel) and medium carbon V-microalloyed steel (V-steel) in torsion, indicative of double Q(HW) behavior, are revealed in all but the high cooling rate-short interpass time multipass tests, which gave rise to single-stage plots, indicative of single Q(HW) behavior. Above the Tor, the Q(HW)(U) is little affected by steel composition, type of test, or test variables. Below the Tnr, the Q(HW)(L) becomes sensitive to all variables, and the temperature dependence of the flow stress relevant to a multipass hot working operation, can be well described only by the Q(HW)(L) based on the multipass flow curves, because it is interpass time dependent. The interpass time dependence of both the Q(HW)(L) and the Tor can be divided into three regions. Within the short interpass time region (1.8 to 10 seconds), the two parameters show opposite trends, while beyond 16 seconds they correlate well. Within the 1.8 to 10 second region the Tnr is assumed to be controlled by solutes, while the Q(HW)(L) is controlled by both solute and precipitation effect. Beyond 10 seconds precipitation is the only controlling mechanism. Also, high Tnr does not necessarily mean a high Q(HW)(L) or vice-versa. It seems reasonable to assume that the Tnr temperature indicates the beginning of suppression of recrystallization, while Q(HW)(L) indicates the extent of suppression.