Effect of Nb addition on microstructure and mechanical properties of 25CrNiMoV (DZ2) steel for high-speed railway axles

The microstructure, precipitates and properties of 25CrNiMoV (DZ2) steel for high-speed railway axles with different Nb contents were investigated by means of optical microscopy, scanning electron microscopy, electron back-scattering diffraction, transmission electron microscopy and physicochemical phase analysis. The results show that the grain size of the original austenite of the test steels decreases from 20.5 to 14.2 and 10.8 μm after adding 0.026 and 0.039 wt.% Nb to a 25CrNiMoV steel, respectively. Moreover, the block width of the tempered martensite in the test steels is refined from 1.91 to 1.72 and 1.60 µm, respectively. MC-type precipitates in 25CrNiMoV steel are mainly VC, while (Nb,V)C gradually precipitates when Nb is microalloyed, and the amount of precipitates increases with increasing Nb content. Through strengthening mechanism analysis, it is found that grain refinement strengthening is the primary way to increase the strength. The improvement in the yield strength with increasing Nb content is attributed to a significant increase in precipitation strengthening, grain refinement strengthening and dislocation strengthening.