Effect of Interpass Time and Cooling Rate on Apparent Activation Energy for Hot Working and Critical Recrystallization Temperature of Nb-microalloyed Steel

Abstract

Two-stage linear ln[sinh(ασ)] vs. 1/T relations indicative of double Q_HW behavior, are obtained from anisothermal multipass flow curves generated by testing Nb/Ti microalloyed steel in torsion (in the range 1250-800ºC), except for the case of the high cooling rate-short interpass time tests, which gave rise to single-stage plots, indicative of single Q_HW behavior. Above the T_nr, the Q_HW^U (apparent activation energy for hot working corresponding to the upper temperature range) is little affected by test variables (interpass time and cooling rate) and type of deformation (isothermal continuous and anisothermal multipass deformation give virtually equivalent Q_HW^U, values above the T_nr). However, below the T_nr, the Q_HW^L, corresponding to the lower temperature range, becomes sensitive to test variables, and may show considerable deviation from Q_HW^L obtained in continuous tests. Therefore, the temperature dependence of the flow stress, below the T_nr, relevant to a multipass hot working operation, can be well described only by the Q_HW^L obtained from the multipass flow curves, because it is interpass time dependent. The interpass time dependence of both the Q_HW^L and the T_nr can be divided into three regions. Within the short interpass time region (1.8 to 10 sec), the two parameters show opposite trends, while beyond 10 sec they correlate well. Within the 1.8 to 10 sec region the T_nr is assumed to be controlled by Nb in solid solution, while the Q_HW^L is controlled by both solute and precipitation effect. Beyond 10 sec precipitation is the only controlling mechanism. In addition to the T_nr derived from Mean Flow Stress (MFS) and ln[sinh(ασ)] vs. 1/T plots, the recrystallization limit, T_rl, which correlates well with both T_nr's, and the recrystallization stop, T_rs, temperatures are derived from % Fractional Softening (FS) vs. 1/T plots.