Effect of nitrogen contents on the microstructure and mechanical properties of V-Ti microalloyed steels for the forging of crankshafts

The microstructure and mechanical properties of vanadium-titanium microalloyed crankshaft steels with two different nitrogen concentrations were studied. The increasing nitrogen not content only refined the austenite grain size but also facilitated the formation of intragranular ferrite (IGF) idiomorphs. The V-Ti-N steel has a better combination of strength, plasticity and toughness than that of the V-Ti steel. Increasing the nitrogen content also facilitated the precipitation of V(C, N) particles in ferrite, which significantly strengthened it. This outcome remedied the strength reduction caused by the decrease in the pearlite volume fraction. The IGF idiomorphs can deform along with the outside pearlite compatibly and restrain the crack growth in pearlite while in tensile deformation. As a result, the plasticity is significantly improved from the increase of the IGF idiomorphs. Moreover, the effective grain size can be refined by the increase of the IGF idiomorphs, which improves the toughness.