Effects of V–Nb microalloying on the microstructure and properties of spring steel under different quenching-tempering times

The effects of V–Nb microalloying on the microstructure and mechanical properties of ultra-high-strength spring steel at various austenitizing and tempering times were investigated. To achieve this, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron backscatter diffraction, and tensile tests were performed. The equations for austenite grain coarsening at 900 °C and (V,Nb)C carbide growth at 400 °C were obtained. The results indicated that the tensile properties of the investigated steel significantly improved after V–Nb microalloying with the same quenching-tempering treatment. With increasing austenitization or tempering time, the tensile properties of the experimental steels dramatically declined owing to the coarsening of the grain size or of the martensite lath width and carbide size. Because V–Nb microalloying increased the densities of the grain boundaries and carbides, the full width at half maximum 211α, microstrain, and dislocation density decreased slowly with prolonged tempering time. The strength increment among the investigated steels mainly resulted from the grain refinement strengthening, dislocation strengthening, and most importantly, precipitation strengthening of (V,Nb)C carbide.