Effect of direct quenching on microstructure and mechanical properties of copper-bearing high-strength alloy steels

This study aims to investigate the effects of direct quenching on microstructural modification and mechanical properties of copper-bearing high-strength alloy steels. Two direct quenched and tempered (DQ&T) steel plates were rolled at different finish rolling temperatures, and their microstructures and mechanical properties were compared with those of a reheat quenched and tempered (RQ&T) steel plate. The as-quenched microstructure of the DQ plates consisted of refined lath martensite with high density of dislocations, which acted as preferred precipitation sites for NbC or ε-Cu particles during tempering. These fine precipitates were not coarsened much up to the tempering temperature of about 650°C, and thus played a role in improving the tempering resistance. Especially in the DQ&T plate quenched at 760°C and tempered at 660°C, yield strength reached 1050 MPa, and Charpy impact energy at −18°C showed 140 J, indicating the potent effect of the DQ&T process. These findings indicated that the copper addition and the application of the DQ&T process to low-carbon alloy steels contributed to the production of steel plates with excellent strength and toughness.