Recrystallization, Precipitation Behaviors, and Refinement of Austenite Grains in High Mn, High Nb Steel
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IN recent years, the high Mn, high Nb approach was applied well in the development of high-grade line pipe steel[1,2] because of its advantages as a low-cost material allowing substantial grain refinement. A large volume of literature[3,4] demonstrated that microalloying retards static recrystallization (SRX), in which Nb is the most efficient among the microalloying elements. Based on this characteristic, a new rolling technique for high Mn, high Nb steel, called high-temperature processing (HTP), was applied in the production of X80 pipeline steel. Differing from traditional thermomechanical control processing for the steel with low/medium Nb content, in this process, the start temperature of finish rolling under nonrecrystallization conditions is typically 50 K to 100 K (50 C to 100 C) higher than that for traditional processing.[5,6] High-temperature rolling requires a smaller mill force. However, the drawbacks of the process are lack of strain accumulation because of increased recovery and no full pancaking of austenite grains at relative higher rolling temperature. These points are important for the production of higher grade pipeline steel (X100/X120). Moreover, besides the effect of Nb, SRX behavior[7,8] depends on precipitation, initial grain size, reduction rate, time interval, etc. The addition of high Mn[9] can retard precipitation, and subsequently it can influence the recrystallization behavior of high Nb steel. A finer initial grain or larger reduction also can accelerate C.L. MIAO, Doctor, C.J. SHANG, Professor, and G.D. ZHANG, Postgraduate, are with the School of Materials Science and Engineering, University of Science and Technology Beijing, 100083 Beijing, P.R. China. Contact e-mail: [email protected] H.S. ZUROB, Professor, and S.V. SUBRAMANIAN, Professor, are with the Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L814L7, Canada. Manuscript submitted December 7, 2010. Article published online September 30, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
recrystallization.[10] Recent research[11] on static recrystallization for industrial high Mn, high Nb pipeline steel indicates fast and complete recrystallization still occurs at 1273 K (1000 C), provided that the initial starting grains are fine and no precipitation occurs. These results are in contrast to the traditionally held view that no recrystallization or partial recrystallization takes place at 1273 K (1000 C) for high-content, Nb-bearing approach. Therefore, the effect of Nb on the recrystallization behavior in a high Mn system should be reconsidered and reinterpreted, especially with respect to the effect of initial grain size and the amount of Nb in solid solution and as precipitates. In this article, the recrystallization and precipitation behaviors of Nb-bearing steel were clarified for the high Mn system, particularly the mechanism about the change of recrystallization behaviors caused by the initial austenite grain size and the influence of high Mn on precipitation behavior. This research wi
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