Effect of N content on microstructure and properties of bainite steel for V microalloyed springs

This study investigates the effect of nitrogen (N) content on the phase transformation, microstructure, and mechanical properties of vanadium-containing bainitic spring steel. Three experimental steels with different N contents (N1: 0.0084%, N2: 0.0158%, N3: 0.0245%) were designed and produced. A comprehensive analysis was performed using a fully automated phase transition instrument, optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and tensile and impact testing. The results showed that all experimental steels exhibited a bainite + martensite/austenite (M/A) mixed microstructure. Increasing the N content led to the precipitation of VN up to 0.1402%, a reduction in grain size, and enhanced bainitic phase transformation. This resulted in an accelerated transformation rate, a higher final transformation percentage of 82.4%, and a more refined, dispersed M/A structure. These microstructural changes improved toughness and increased the yield-to-ultimate strength ratio to 0.64. As a secondary phase, VN contributed to precipitation strengthening, grain refinement, and dislocation strengthening, thus enhancing strength, plasticity, and overall performance. The N3 experimental steel exhibited the most favorable combination of strength and toughness.