Comparative study of the role of niobium in low-carbon ferritic and bainitic steels

Abstract

The investigation attempts to compare the role of niobium (Nb) in ferritic and bainitic steels. The influences of Nb on the transformation, microstructure and mechanical properties of ferritic and bainitic steels were studied by dilatometry, optical microscopy, field emission scanning electron microscopy, electron backscattering diffraction, high-resolution transmission electron microscopy and tensile tests. The results show that as the Nb content increases, the transformation kinetics of ferrite and bainite exhibited a similar trend, i.e. inhibited by Nb first and then accelerated. However, the influences of Nb addition on ferritic and bainitic transformations were different. Compared to the Nb-free steels, the addition of 0.046 wt.% Nb accelerated the ferritic transformation kinetics significantly, but it inhibited the bainitic transformation. In addition, the strength of steels increased obviously at first and then changed little as the Nb content increased in both ferritic and bainitic steels. However, the strengthening mechanisms of Nb in the two types of steels were different. The precipitation strengthening is the major mechanism for the increase of strength in ferritic steels, whereas the dislocation strengthening and phase transformation strengthening are considered to be the major sources for the increase of strength in bainitic steels. Moreover, the addition of Nb reduced the work hardening capacity of ferritic and bainitic steels.