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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著者

    • RADOVIC Nenad
    • Department of Physical Metallurgy, Faculty of Technology and Metallurgy, University of Belgrade
    • DROBNJAK Djordje
    • Department of Physical Metallurgy, Faculty of Technology and Metallurgy, University of Belgrade

抄録

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

収録刊行物

  • ISIJ international  

    ISIJ international 39(6), 575-582, 1999-06-15 

    The Iron and Steel Institute of Japan

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