Microstructure evolution during reheating of a Nb-bearing steel Ⅰ. Metallographic examination

Shanwu Yang, Enlong Dong, Huibin Wu, Chengjia Shang, Xuemin Wang, and Xinlai He Materials Science and Engineering School, University of Science and Technology Beijing, Beijing 100083, China Anshan Iron & Steel Group Corporation, Anshan 114003. China

摘 要：<正>Cooled in water after the isothermal relaxation of deformed austenite for different timea, a Nb-bearing microalloyed steel always exhibits synthetic microstructures of bainitic ferrite, granular bainite and acicular ferrite. When these samples were reheated to and held at 650 or 700℃, the non-equilibrious microstructures tend to evolve into equilibrious ones. The sample relaxed for 60 s displays the highest thermostability, while the microstructure evolution is the quickest in the sample relaxed for 1000 s even though it is the softest before reheating. Softening is not a single process occurring during reheating, in which the hardness fluctuates with time. There are two peaks in the hardness-time curve of each sample having undergone relaxation, while a single peak occurs in the curve of the sample having not been relaxed. Pre-strain accelerates the evolution process. These results indicate that the thermostability of microstructures is determined by their history of formation to a considerable degree.

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