The co-precipitation evolution of NiAl and Cu nanoparticles and its influence on strengthening and toughening mechanisms in low-carbon ultra-high strength martensite seamless tube steel

The designed low-carbon ultra-high strength martensite seamless tube steel was manufactured by hot rolling and quenching-tempering processes. The multiple strengthening mechanisms are evaluated depending on the microstructure and co-precipitation evolution mechanism of Cu and NiAl, and the toughening mechanisms associated with multiscale microstructures are systematically discussed. The results show that the microstructure of the experimental steel in the quenched state consists of 87.8% lath martensite (LM) and 12.2% granular bainite (GB), while the microstructure in the QT state includes tempered martensite (TM), GB and a small amount of reversed austenite. The TEM morphology of QT steel shows three types of nanoparticles co-precipitated by Cu-rich, NiAl and Cu-NiAl, and the nanoparticles coarsen significantly and the number density decreases dramatically as the aging temperature increases from 500°C to 650°C. The co-precipitation evolution mechanism of nanoparticles elucidates that high density of small-sized BCC-Cu and B2-NiAl particles is optimal for strengthening increment. The experimental steel has an ultimate yield strength of 1332.5 MPa aged at 500°C, which is attributed to high precipitation strengthening of 651.2 MPa (general superposition of shear strengthening and Orowan strengthening) and dislocation strengthening of 454.8 MPa. The experimental steel has obvious low-temperature toughening, and the impact energy at -40°C increases from 5 J to 237 J as the aging temperature increases from 500°C to 650°C. The excellent low-temperature toughness is attributed to the reduction of dislocation density, the weakening of the shear mechanism and the transformation of a small amount of reversed austenite to increase the crack nucleation energy, and the increase of the number fraction of HAGB and the significant plastic deformation increase the crack propagation energy.