| 摘要 |
PROBLEM TO BE SOLVED: To improve the delayed fracture resistance of steel by allowing steel to contain a specified weight ratio of Ni as a steel component and moreover concentrating Ni on the grain boundaries or sub-grain boundaries into a network structure. SOLUTION: This steel contains., by weight, >=0.5% Ni. Preferably, this is the steel contg., as steel components, 0.2 to 0.5% C, 0.2 to 2% Si, 0.2% Mn, <=0.01% P, <=0.01% S, 0.01 to 0.1% Al, 0.002 to 0.01% N and 0.5 to 12% Ni, and in which Ni is concentrated on the grain boundaries or sub-grain boundaries into a network structure. By concentrating Ni on the grain boundaries or sub- grain boundaries into a network structure, the infitration and diffusion of diffusible hydrogen are suppressed to improve its delayed fracture characteristics. From the viewpoints of the securance of the strength of the steel and the improvement of the delayed fracture characteristics thereof, e.g. heat machining treatment using inverse transformation of fining the crystal grains is preferably executed. By specifying the compsn. of the steel and the structure of the grain boundaries, the steel having >=1200 MPa strength can be obtd.
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