Effects of direct-quenching and tempering on the microstructure and mechanical properties of an ultra-low carbon Ti containing bainite steel

Ultra-low carbon Ti containing bainite steel was prepared by multipoint dispersion supplying Ti wire in the melt, followed by controlled rolling, relaxation, direct-quenching (DQ), and tempering at temperatures in the range of 450 °C–700 °C. The evolution of microstructures under various heat treatment were characterized using scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Tensile property and impact toughness test were conducted. Results showed the following: (i) nanoparticles form during solidifying, controlled rolling, and tempering; (ii) microstructure consisted of acicular ferrite (AF), retained austenite (RA), and lath bainite (LB) that was associated with a high density of dislocations was obtained by controlled rolling and DQ; (iii) an optimum balance between strength and toughness can be achieved in an ultra-low carbon Ti containing bainite steel via controlled rolling, relaxation, DQ, and tempering. Effects of tempering temperature on mechanical properties are discussed in terms of the recovery of bainite lath (BL), precipitation of nanoparticles, evolution of martensite-austenite (M-A) constituent, and variations in dislocation density.