Low-cost secondary hardening steel under double ageing: Dual-precipitation control, austenite modification and strength-toughness improvement

Reducing cobalt content in secondary hardening ultra-high strength steels (UHSSs) is eager for cost reduction, but meanwhile the M₂C precipitation decreases resulting in a decrease in strength. Adding aluminum to induce NiAl precipitation can increase the strength but the toughness is reduced. Therefore, it remains a challenge to keep decent toughness through NiAl and M₂C dual precipitation when reducing Co content. Here, double ageing is proposed in a low-cost UHSSs to achieve good combination of strength and toughness through dual-precipitation control and austenite modification. Ageing at 482 °C for 32 h followed by ageing at 600 °C for 20 min (DA482 + 600) exhibits a smaller size of M₂C carbide and a smaller number density of NiAl particle than that ageing at 600 °C for 20 min followed by ageing at 482 °C for 32 h (DA600 + 482). Additionally, the DA482 + 600 sample obtains a larger volume fraction of film reversed austenite and a smaller size of blocky reversed austenite. These make it have higher impact toughness. Although the precipitation strengthening for NiAl particles and M₂C carbides is different in two samples, when considering the interaction during dislocation movement through these precipitates, Pythagorean addition rule indicates that the total precipitation strengthening is similar. Additionally, the course of heat treatment is the same and, in turn, the solid solution, dislocation density and width of martensitic laths are comparable, leading to similar strengthening contributions. As a result, the yield strength remains comparable high. When comparing strength-toughness and raw materials cost with representative secondary hardening UHSSs, the DA482 + 600 sample exhibits superior combination.