中文摘要：

Nb-V微合金钢在1200℃固溶0.5 h后淬火,在450—650℃回火不同时间,用显微硬度和TEM测试并观察析出强化和组织软化现象,用三维原子探针（3DAP）对产生二次硬化的合金碳化物的成分进行定量分析,研究其析出长大规律.结果显示,二次硬化主要是合金碳化物析出强化的作用.随着回火温度的升高或回火时间的延长,合金碳化物的成分动态变化,即强碳化物形成元素取代或部分取代较弱的碳化物形成元素.首先,V和Nb取代Mo.然后Nb部分取代V,最后形成具有一定原子比的合金碳化物.相对回火温度,回火时间对碳化物内合金元素的相对含量影响不大.在合金碳化物长大过程中,薄片状碳化物优先沿径向方向生长,然后沿厚度方向长大并开始粗化.

英文摘要：

The mechanical properties of quench-tempered high strength low alloy steels are commonly optimized by fine and dispersively distributed alloy carbides. The role of the alloying elements in determining the alloy carbide precipitation sequence is of great significance. The co- addition of carbide-forming elements such as Mo, V and Nb complicates the precipitation behavior. The mutual inter-solutions and growth rates of various MC- and/or M2C-type carbides are qualitatively affected by the intrinsic solubility and diffusion at certain tempering condition. However, comprehensive study of the precipitation sequence must be followed with atomic scale resolution techniques. The 3D atom probe （3DAP） is a unique tool capable of obtaining chemical information at the atomic level, offering a powerful method to investigate microstructural and compositional changes occurring at nano scale. And the sizes, morphology and composition of individual alloy carbide may be visualized and quantified by 3DAP. In this paper, a quenched Nb-V microalloyed steel was chosen to investigate the precipitation behavior of alloy carbide after tempering at 450-- 650 ℃ for different times. 3DAP, micro hardness test and TEM were applied to characterize the phenomena of hardening and softening during tempering, and the composition evolution and growth behaviors of the alloyed carbides were also studied. The results indicated the second hardening of the 500-600 ℃ tempering martensite is mainly resulting from precipitation strengthening of alloy carbides.The alloy carbides composition dynamically changed with elevated temperature or prolonged time, that is, the stronger carbide-forming elements replaced or partly replaced the weaker ones. At first V and Nb replaced Mo, and then Nb partly replaced V, and at last the carbides with certain composition were formed. Tempering time has relatively less effect on the carbides composition compared with temperature. When the tempering temperature elevated or tempering time prolonged, the alloying dement