Co-precipitation mechanism of nanoscale particles and mechanical properties in multicomponent ultra-high strength low carbon steel

Abstract

The superior mechanical performance of ultra-high strength low carbon steel was attained through the co-precipitation strengthening and lath-martensitic microstructure refinement. The research results showed that the precipitates enriched with Cu, Ni, Mn and Al were observed at 525 °C aging by atom probe tomography (APT), the face-centered cubic (FCC) Ni3Al phase precipitated from matrix firstly, and then the body-centered cubic (BCC) Cu-rich precipitates nucleated on the surface of Ni3Al particles to form composite precipitation phase of nanoscale Ni3Al and Cu particles eventually. The co-precipitation strengthening increment of nanoscale Cu and Ni3Al particles was calculated as 413 MPa. With the addition of multiple micro-alloyed elements, such as Nb, Ti and Mo, a relatively high amount of (Nb, Ti, Mo)C nanoscale co-precipitates below 10 nm were obtained in the matrix aged at 525 °C. The substitution of Mo for Nb, Ti would decelerate the coarsening kinetics of (Nb,Ti)C and cause high amount of finer precipitates. While those fine (Nb, Ti) C particles could evidently impede the grain growth, the finer martensitic blocks were subsequently obtained at 820 °C quenching, and the grain size and effective grain size were 5.6 µm and 3.08 µm in this condition, respectively. In this research, the Ni-Al-Cu steel with best matching of strength and toughness has been achieved when it was aged at 600 °C after 820 °C quenching, the yield strength and −80 °C impact toughness could reach 1138 MPa and 74 J, respectively.