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

• Published in: ISIJ International 1999/06/15, Vol.39(6), pp.575-582
• Main Authors: Radovié, Nenad, Drobnjak, Djordje
• Format: Article
• Language: English
• Subjects: hot working; microalloyed steel; recrystallization
• ISSN: 0915-1559; 1347-5460
• DOI: 10.2355/isijinternational.39.575

Two-stage linear ln[sinh(ασ)] vs. 1/T relations indicative of double QHW 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 QHW behavior. Above the Tnr, the QHWU (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 QHWU, values above the Tnr). However, below the Tnr, the QHWL, corresponding to the lower temperature range, becomes sensitive to test variables, and may show considerable deviation from QHWL obtained in continuous tests. Therefore, the temperature dependence of the flow stress, below the Tnr, relevant to a multipass hot working operation, can be well described only by the QHWL obtained from the multipass flow curves, because it is interpass time dependent. The interpass time dependence of both the QHWL and the Tnr 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 Tnr is assumed to be controlled by Nb in solid solution, while the QHWL is controlled by both solute and precipitation effect. Beyond 10 sec precipitation is the only controlling mechanism. In addition to the Tnr derived from Mean Flow Stress (MFS) and ln[sinh(ασ)] vs. 1/T plots, the recrystallization limit, Trl, which correlates well with both Tnr's, and the recrystallization stop, Trs, temperatures are derived from % Fractional Softening (FS) vs. 1/T plots.