Effect of increased N content on microstructure and tensile properties of low-C V-microalloyed steels

The processing of four low-carbon vanadium-microalloyed steels with varying nitrogen (N) content of 30-165 ppm, including preheating at 1200 °C, rapid cooling to 550 °C for isothermal transformation, and air cooling, was simulated using a Gleeble 3500 system. The microstructure of each sample was characterized and the tensile properties measured; a microstructure consisting of granular bainitic ferrite (GBF), acicular ferrite (AF), and martensite/austenite (M/A) constituent formed in all of the samples. Moreover, the increasing N content led to an increase in the effective grain size of the GBF and AF and a decrease in the density of dislocations, as well as promoted the precipitation of nanoscale particles; however, the overall yield strength decreased slightly. The increased N content also resulted in an increased amount of M/A constituent, which improved the overall strain-hardening capacity of the microstructure of GBF+AF/M/A, and resulted in an elevated tensile strength and a simultaneously lowered yield-to-tensile strength ratio. In addition, the mechanisms governing the effect of increased N in controlling the final microstructure and tensile properties were discussed.