Effect of Nb and V on the continuous cooling transformation of undercooled austenite in Cr–Mo–V steel for brake discs |
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作者姓名: | Dan Wu Fu-ming Wang Jin Cheng Chang-rong Li |
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作者单位: | School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China |
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基金项目: | This work was financially supported by the National Na-ture Science Foundation of China;the National Key Research and Develop-ment Program of China |
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摘 要: | The increasing speed of trains necessitates the development of brake-disc materials that meet more stringent requirements. Therefore, Nb and V have been added to Cr–Mo–V steel to improve its thermal fatigue performance when used in brake discs. In this paper, the influences of Nb and V on the static continuous cooling transformation (CCT) behaviors of undercooled austenite were studied. The microstructures, hardness, and dislocation densities at different cooling rates and with the addition of different alloying elements were also investigated. The results show that the transformation products of ferrite, granular bainite, lower bainite, and martensite form under different cooling conditions. With increasing Nb and V contents, the CCT curves are shifted to the left, ferrite and bainite transformations are promoted, and the critical cooling rate of total martensite formation is increased. The added V mainly forms V-rich M8C7 precipitates and reduces the dissolved C content; therefore, the Ac1, Ac3, and Ms-point temperatures increase. Moreover, the stability of retained austenite is also reduced; its content therefore decreases. Compared with V, the effect of added Nb is weaker because of its smaller content. However, the addition of Nb improves the hardness at lower cooling rates because of the precipitation of fine NbC particles and refining of the microstructure.
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关 键 词: | brake-disc steel niobium vanadium microstructure continuous cooling transformation |
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