Co-precipitation of nanosized Cu and carbides improving mechanical properties of 1 GPa grade HSLA steel

Two Cu-containing (1.5 wt% and 3.0 wt%) high strength low alloy steels were studied to explore the influence of co-precipitation of nanosized Cu particles and carbides on microstructures and mechanical properties, compared with a Cu-free counterpart, by using scanning electron microscopy, electron back-scattered diffraction, transmission electron microscopy, scanning transmission electron microscopy, tensile tests and low-temperature impact tests. Cu addition increased the yield strength and that increased with the increase of Cu content. Despite of the highest yield stress of 1044 MPa for 3.0Cu steel after aging at 550 °C for 120 min, 1.5Cu steel showed a good combination of yield stress and low temperature impact toughness with a yield strength reaching 1003 MPa and low-temperature toughness with 185 J at −40 °C and 65 J at −84 °C after aging at 550 °C for 240 min. The co-precipitation of nanosized Cu particles and MC/M2C carbides induced the synergic precipitation strengthening, leading to higher strength while maintaining good impact toughness in 1.5Cu steel. However, with further increasing Cu addition, the impact toughness of 3.0Cu decreased due to the occurrence of excessively nanosized Cu particles at the interfaces of martensitic laths, which caused stress concentration that deteriorated impact toughness. With increasing aging time, the precipitation of nanosized MC/M2C-type carbides was promoted. Thus, the softening of the matrix was counteracted by precipitation strengthening from nanosized Cu particles and carbides. In addition, the martensitic variants selection was inhibited because of Cu addition, thus, the fraction of high angle grain boundaries increased, which was helpful for enhancing impact toughness.