Nano-sized precipitation and properties of a low carbon niobium micro-alloyed bainitic steel

The present work focuses on microstructure evolution and precipitation strengthening during tempering at region of 550–680°C to elucidate the structure–property relationship in the steel. The effect of tempering on the development of a 700MPa grade high strength hot rolled cost-effective bainitic steel was studied for infrastructure applications. Granular bainite with dispersed martenisit–austenite (M–A) constituents in the bainitic ferrite matrix was obtained after hot rolling and air cooling to room temperature. The decomposition of M–A constituents to cementite carbides and the precipitation of nano-sized NbC carbides in bainitic matrix on tempering were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Nano-sized precipitates of NbC precipitated during tempering were in average diameter of ~4.1–6.1nm. There were ~86–173MPa increases in yield strength after tempering at region of 550–680°C. It is noticeable that those nano-sized NbC precipitates provide an effective way to significantly increase the strength of the low carbon bainitic steel. High yield strength of 716MPa with high ductility (uniform elongation of 9.3% and total elongation of 22.4%), low yield to tensile ratio of 0.9 and good low temperature toughness of 47J (half thickness) at –40°C was obtained after tempering at 680°C for 30min.