Effect of the Ni content on the microstructure and mechanical properties of Cu-bearing high-strength steels

Cu-bearing high-strength low-alloy (HSLA) steels are widely used in demanding applications requiring a balance of strength and low-temperature toughness, where Ni is a key alloying element influencing both microstructural evolution and mechanical properties. The effect of Ni content (1.5–3.5 wt%) on the strengthening mechanisms of Cu-bearing high-strength steels was systematically investigated. As Ni increased, the hardenability of the tested steels was significantly enhanced, and the microstructure gradually transformed into fully lath martensite from bainite/martensite mixed structure. Prior austenite grains as well as martensitic substructures were refined. Grain refinement strengthening was the dominant contributor to the increased yield strength. Higher Ni content also increased dislocation density and slightly raised the number density of Cu-rich nanoprecipitates, while having little effect on their average size (∼12 nm). Hall-Petch analysis combined with boundary misorientation evaluation identified the block as the effective strengthening unit, due to its high-angle boundaries that effectively impede dislocation motion.