Comparison of Strengthening Mechanism of the Nb, V, and Nb-V Micro-Alloyed High-Strength Bolt Steels Investigated by Microstructural Evolution and Strength Modeling

The strengthening mechanism of Nb, V, and Nb-V micro-alloyed high-strength bolt steels was investigated and compared using microstructural evolution and strength modeling. Optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to characterize the microstructure and precipitations. The results show that Nb-V composite micro-alloyed steel possessed a higher yield strength compared with Nb or V micro-alloyed steel when quenched at 870 °C and tempered at 450–650 °C. Furthermore, the strength increment of Nb-V micro-alloyed steel with respect to Nb or V micro-alloyed steel reached the maximum at a tempering temperature of 600 °C, and precipitation strengthening and dislocation strengthening presented higher strength contributions in Nb-V micro-alloyed steel than in Nb micro-alloyed steel and V micro-alloyed steel owing to the higher volume fraction and finer precipitate size. When V was added in combination with Nb in steel, the number of Nb-rich carbonitrides increased, which resulted in a higher volume fraction of the effective pinning particles-Nb-rich (Ti,Nb,V)(C,N) with diameters smaller than 50 nm and led to an enhanced refinement of the prior austenite grain. In addition, Nb could reduce the consumption of V during quenching, allowing more V to be solid-solved in the matrix after quenching, thereby further enhancing the precipitation strengthening effect of V during tempering.