• During the austenitization process, (Ti, Nb, V)C carbides with TiC particles as the core preferentially precipitate. • During the tempering stage, M 3 C, M 7 C 3 , and VC carbides sequentially precipitate, reducing the carbon content in the matrix and helping to suppress crack initiation. • The cleavage fracture dominated by coarse (Ti, Nb, V) C particles accelerates the brittle fracture process, thereby reducing the impact toughness. In this study, three types of microalloyed martensitic steels with different Nb and Ti contents were prepared using powder metallurgy techniques, and comprehensive comparative analyses of their microstructures and mechanical properties were conducted. The results show that Nb and Ti can effectively refine the sizes of prior austenite grains (PAGs) and martensitic laths, promote the formation of (Ti, Nb, V)C precipitates, and increase the strength of steel via precipitation strengthening. However, excessive Ti content can lead to the coarsening of (Ti, Nb, V)C particles, thereby deteriorating the plasticity and toughness of the 8Nb12Ti sample. During the austenitization process, (Ti, Nb, V)C carbides with TiC particles as the core preferentially precipitate. During the tempering stage, M 3 C, M 7 C 3 , and VC carbides sequentially precipitate, reducing the carbon content in the matrix and helping to suppress crack initiation. The 8Nb6Ti sample exhibits excellent tensile properties and impact toughness due to the high proportion of high-angle grain boundaries and the refined microstructure. In contrast, the 8Nb12Ti sample exhibits a relatively low level of impact work at room temperature, which stems from its internal coarse (Ti, Nb, V)C particles. Moreover, the quasi-cleavage fracture dominated by particles accelerates the fracture process, which decreases the impact toughness.
Geng et al. (Sun,) studied this question.
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