Effect of interpass time and cooling rate on apparent activation energy for hot working and critical recrystallization temperature of Nb-microalloyed steel
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Two-stage linear In[sinh(alpha sigma)] 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 tin the range 1250-800 degrees C), 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 T-nr the QV (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(H)(W)(U), values above the T-nr). However, below the T-nr the O-HW(L) corresponding to the lower temperature range, becomes sensitive to test variables, a nd may show considerable deviation from O-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 O-HW(L) is controlled by both solute and precipitation effect. Beyond 10sec precipitation is the only controlling mechanism. In addition to the T-nr derived from Mean Flow Stress (MFS) and in [sinh(alpha sigma)] 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.
Keywords:microalloyed steel / hot working / recrystallization
Source:ISIJ International, 1999, 39, 6, 575-582
- Iron Steel Inst Japan Keidanren Kaikan, Tokyo