Microstructural characteristics and impact fracture behaviors of low-carbon vanadium-microalloyed steel with different nitrogen contents

Abstract The influence of nitrogen content between 0.0032 and 0.0081 wt % on the microstructures and the impact toughness of low-C V-microalloyed steel was investigated. The isothermal transformation experiments in the medium temperature were performed at Gleeble-3800. The microstructures and impact fracture behaviours were obtained through a series of characterization means and the mechanism were investigated. Results indicated that increasing the nitrogen content created a larger number of finer precipitated (Ti,V) (C,N) particles as well as micro-sized particles. The former decreased the average diameter of prior austenite grains (PAGs) and the latter promoted the nucleation of intragranular acicular ferrite (AF), which led to an increased amount of AF and a reduced mean equivalent diameter (MED) of ferrite grains. Moreover, the increased nitrogen content increased the amount of fine martensite-austenite (M/A) constituents and tended to change the internal substructure of M/A constituent from twin-type to lath-type. Thereby these variations in microstructure resulted in the decrease in 50% fracture appearance transition temperature (50% FATT), and alterations in the fracture behavior in the upper shelf region and lower shelf region. Moreover, the increasing N content led to changes in the mechanism of crack initiation from micro-cleavage to microvoid, and the mechanism of crack propagation from predominantly transgranular cleavage to predominantly intragranular microvoid coalescence at the test temperature of -20 °C. As a result, the initiation and propagation energy increased significantly with the increased nitrogen content.