High-strength steel and high-strength bolt with excellent resistance to delayed fracture, and manufacturing method therefor

Abstract

The disclosed steel contains, by mass, 0.10-0.55% carbon, 0.01-3% silicon, and 0.1-2% manganese. Said steel also contains one or more of the following: 0.05-1.5% chromium; 0.05-0.2% vanadium; 0.05-0.40% molybdenum; 0.001-0.05% niobium; 0.01-4% copper; 0.01-4% nickel; and 0.0001-0.005% boron. The remainder comprises iron and unavoidable impurities. The structure of the disclosed steel is primarily tempered martensite. Said steel, which is a high-strength steel that exhibits excellent resistance to delayed fracture, is characterized by the following being formed in a surface layer thereof: (a) a nitrided layer with a thickness of at least 200 [mu]m from the surface of the steel and a nitrogen concentration by mass that is at least 0.02% higher than that of the steel and no greater than 12.0%; and (b) a low-carbon region, at a depth between 100 [mu]m and 1000 [mu]m from the surface of the steel, with a carbon concentration by mass that is at most 0.9 times that of the steel and no less than 0.05%.

Description

technical field [0001] The present invention relates to high-strength steel materials used for wire rods and PC steel rods (steel rods for prestressed concrete), especially high-strength steel materials having a tensile strength of 1,300 MPa or more and high-strength bolts that are excellent in delayed fracture resistance and their manufacturing methods . Background technique [0002] High-strength steels that are often used in machinery, automobiles, bridges, and building structures are medium-carbon steels with a C content of 0.20 to 0.35%, such as SCr and SCM specified in JIS G 4104 and JIS G 4105, etc. Handled steel. However, regardless of the type of steel, when the tensile strength exceeds 1300 MPa, the risk of delayed fracture (delayed failure) increases. [0003] As a method of improving the delayed fracture resistance of high-strength steel, a method of making the steel structure a bainite structure, or a method of making prior austenite grains finer are effective...

AI Technical Summary

Problems solved by technology

However, the surface hardness of the coil spring after nitriding is Hv900 or more, and the delayed fracture characteristics are low under high load stress such as bolts and PC steel rods, and there is a problem that delayed fracture occurs when the corrosive environment becomes severer.

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