Comparison of Strengthening Mechanism of the Nb, V, and Nb-V Micro-Alloyed High-Strength Bolt Steels Investigated by Microstructural Evolution and Strength Modeling
Hui Wen, Qian Wang, Qian Wang, Yueyuan Dou, Qiang Wang, Qiang Wang, Xiaochun Xu, Qingfeng Wang, Qingfeng Wang
Abstract
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.