Tailoring Variant Pairing to Enhance Impact Toughness in High-Strength Low-Alloy Steels via Trace Carbon Addition

Alloying can make conventional metals reach ultra-high strength, but this usually comes at dramatic loss of toughness. In this work, a desirable strength–toughness combination in high-strength low-alloy steel achieved via trace carbon addition. The significance of carbon in tailoring variant pairing and tuning impact toughness was elucidated from the perspective of crystallography and thermodynamics. As the carbon content increases, the packets and blocks are refined, and the − 40 ℃ impact toughness improves. The enhancement of impact toughness results from the higher density of block boundaries, and the fracture mode shifts from brittle fracture to ductile–brittle combined fractures, then to ductile fracture due to the increased carbon. Increasing the carbon content would lower the martensite start temperature ( M S ) temperature and driving force for martensitic transformation, and increase the strength of austenite matrix, which in turn contributes to producing more V1/V2 variant pairs to accommodate the transformation strain.