Unraveling the significance of cobalt on transformation kinetics, crystallography and impact toughness in high strength steels

This work reveals the significant effects of cobalt (Co) on the microstructure and impact toughness in as-quenched high strength steels by both experimental characterizations and thermo-kinetic analyses. The results show that the Co-bearing steel exhibits finer blocks and a lower ductile-brittle transition temperature than the steel without Co. In addition, the Co-bearing steel shows higher transformation rates at the intermediate stage with bainite volume fraction ranging from around 0.1 to 0.6. The improved impact toughness of the Co-bearing steel results from the higher dense block boundaries dominated by V1/V2 variant pair. The addition of Co induces a larger transformation driving force and a lower bainite start (BS) temperature, which contributes to the refinement of blocks and the increase of V1/V2 variant pair. These findings would be instructive for the composition, microstructure design and properties optimization of high strength steels.