| 摘要 |
PROBLEM TO BE SOLVED: To obtain a bolt steel having high tensile strength and excellent delayed fracture resistance and reduced in costs by specifying the amount of S-free Ti type fine precipitates of specific size or the number of Mn type inclusions having a major axis of a specific value or above in a specific inspection area in a longitudinal section, in a hot or cold rolled wire rod. SOLUTION: This steel satisfies either of the following required conditions (1), (2): (1) Among Ti type fine precipitates of a size of 200-500nm, >=10% of precipitates are free from S; (2) The number of sulfides composed essentially of Mn or multiple inclusions composed essentially of the sulfides, having >=10μm major axis, in an inspection area of 20mm<2> between a position at a depth of 0.3mm from the surface layer and the central axis in the longitudinal section passing through the central axis of the wire rod is regulated to <=5 pieces. This steel has a composition consisting, e.g. of, by weight, 0.08-0.45% C, >0-1<1% Si, 0.2-1.5% Mn, <=0.015% P, <=0.015% S, 0.01-0.1% Ti, 0.0005-0.01% B, and the balance Fe. In spite of its high strength of (100 to 130)kgf/mm<2> , this steel has excellent delayed fracture resistance.
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