Multiscale investigation of grain boundary pinning behavior and strengthening-toughening effects by V–Nb–Ti complex microalloying in 25CrNiMo steel

This study investigates the multi-scale regulation mechanisms of V, Nb, Ti, and their combined additions on the austenite grain growth behavior and the heat-treated microstructural characteristics of 25CrNiMo low-alloy steel. Using OM, SEM, EBSD, TEM, and thermodynamic calculations, the multi-scale correlation mechanisms,ranging from nano-precipitates and grain-boundary pinning to microstructural evolution and macroscopic properties,were revealed. The results show that both the microalloying composition and the austenitizing temperature jointly determine the growth kinetics of prior austenite grains. In the steel with only V addition, the rich-V carbonitride M (C, N)/MN exhibits a dissolution rate as high as 97% at 1100 °C, resulting in low thermal stability. Consequently, the austenite grains undergo explosive coarsening at 1100 °C, with an average grain size of 106.6 μm. In contrast, the V-Nb-Ti micro-alloyed steel forms thermally stable multi-component precipitates such as TiN and NbC, which remain abundant even at 1100 °C (0.104 wt.%). Thus, fine and uniform prior austenite grains (7.98 μm-15.22 μm) are obtained at 900 °C-1100 °C. TEM analysis confirms that nano-scale composite precipitates preferentially distribute along grain boundaries, providing outstanding grain-boundary pinning force. Further analysis of the high-temperature tempered microstructure indicates that the grain-refinement effect of austenite is directly inherited by the final microstructure. After quenching at 950 °C and tempering at 550 °C, the V-Nb-Ti micro-alloyed steel acquires a fine and uniform tempered martensite structure, with prior austenite grain sizes of only 8 μm-10 μm and an optimized proportion of high-angle grain boundaries. EBSD analysis reveals the refinement at the microstructural scale and the substructural characteristics. While achieving refinement, the high density of nano-precipitates in the V-Nb-Ti micro-alloyed steel also suppresses dislocation recovery to some extent, as reflected by higher KAM values (1.67). The study demonstrates that multi-component microalloying elements form thermally stable composite precipitates at the atomic/nano-scale, exert strong pinning at the grain-boundary scale, thereby achieving grain refinement and stabilization at the microstructural scale, and ultimately lay a microstructural foundation for obtaining excellent mechanical properties. This research provides theoretical and experimental support for the development of high strength and toughness, low-process-sensitivity low-alloy steels through multi-scale collaborative design.