• Cobalt addition refines bainitic/martensitic blocks and lowers the ductile-brittle transition temperature, enhancing impact toughness.
• Co accelerates transformation kinetics at intermediate stages (bainite fraction 0.1-0.6) by increasing driving force and reducing bainite start temperature.
• Improved toughness is attributed to a higher density of block boundaries dominated by the V1/V2 variant pair, as revealed by crystallographic analysis.
• The findings provide a theoretical basis for designing high-strength steels with superior toughness via cobalt alloying.