Improving strength, ductility and toughness of ultrafine bainitic steel via tailoring retained austenite and nano-carbide precipitation

Ultrafine bainitic steel exhibits promising applications in bearings, springs, and railway crossings due to its excellent strength-toughness balance and fatigue resistance. However, the presence of coarse blocky retained austenite (RA) in the microstructure limits further improvements in strength and ductility. This research investigated the effects of small deformation and rapid tempering on the microstructure and mechanical properties of medium-carbon ultrafine bainitic steel using transmission electron microscopy, X-ray diffraction, and electron backscatter diffraction microscopy. The results indicated that small deformation transformed coarse blocky RA into twin martensite and fine RA, reducing the number of blocky RA with an apparent area exceeding 1 μm 2 by 79%. Subsequent high-temperature rapid tempering converted twin martensite into ductile tempered martensite while inducing the precipitation of dispersed nano-carbides in bainitic ferrite. The refined microstructure promoted uniform distribution of geometrically necessary dislocations during deformation. The precipitation of nano-carbides alleviated lattice distortion, enabling self-toughening of bainitic ferrite. Consequently, the modified ultrafine bainitic steel successfully retained a high tensile strength of a 1.5 GPa grade, while its yield strength, total elongation and impact toughness were remarkably improved from 1022 MPa to 1189 MPa, 11.9% to 18.6%, and 60.7 J/cm 2 to 108.3 J/cm 2 , respectively. This approach provides a novel pathway for advancing the performance of steel materials.