Abstract: CrNiMo low-alloy marine steels without Cu(961-Base) and with 1wt% Cu(961-Cu) quenched twice and then tempered at high temperature were prepared, and then the effect of Cu on microstructure, mechanical properties, precipitated phase and texture characteristics of the steel was investigated by thermal dilatometer, optical microscope(OM), scanning electron microscope(SEM), electron backscatter diffractometer(EBSD) and transmission electron microscope(TEM). The results show that the Ac₁ and Ac₃ temperatures of the 961-Base steel are 767 ℃ and 905 ℃, and that of 961-Cu steel are 762 ℃ and 895 ℃, respectively. The ferrite/bainite transformation temperatures of the 961-Cu steel are lower than that of the 961-Base steel. The microstructure of the tested steels after heat treatment is dominated by lath bainite(LB) and a small amount of quasi-polygonal ferrite(QPF). The lath of the 961-Cu steel is more refined, which increases the proportion of large-angle grain boundaries by 6.8% and is more beneficial to hinder crack propagation. Compared with that of the 961-Base steel, the yield strength and tensile strength of the 961-Cu steel are increased by 84 MPa and 68 MPa, respectively, and the low-temperature impact absorbed energy are increased from 220 J to 267 J. The impact fracture of the tested steels is dominated by the shear lip and fiber zone, with the 961-Cu steel having a greater shear section ratio. The Cu-rich phase (fcc structure) is non-uniformly precipitated in the matrix and on dislocation line in the form of globular or rods, producing a precipitation strengthening effect. Both the tested steels are characterized by α and γ textures, with the 961-Cu steel containing a higher proportion of γ textures that is conducive to deformation.

Key words: high strength low-alloy steel, microstructure, mechanical properties, Cu-rich phase, texture