Quantitative Analysis of Microstructures and Strength of Nb-Ti Microalloyed Steel with Different Ti Additions

A quantitative analysis was carried out in the present study to determine the effects of titanium (Ti) addition on microstructures and strength of Nb-Ti microalloyed steel. The obtained results revealed that strength was significantly improved with an increase in Ti content from 0.041 to 0.079 wt pct. The difference in the yield strength between the two steel samples occurred due to the different strengthening effects of grain refinement, precipitation, and dislocation strengthening, among which the grain refinement and precipitation strengthening contributions were dominating. With a further increase in the Ti content, ferrite grains became refined. Consequently, a homogeneous ferrite microstructure was attained for high Ti contents. Moreover, large-sized (Ti, Nb)C particles manifested the Kurdjumov–Sachs (KS) relationship, whereas fine (Ti, Nb)C particles held the Baker–Nutting (BN) relationship; thus, abundant fine nanoscale (Ti, Nb)C particles formed after coiling. Furthermore, the high dislocation density facilitated the precipitation of (Ti, Nb)C particles along dislocation lines.