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Yuichi Namimura

Nobuhiko Ibaraki

Seiichi Koike

Mitsuo Takashima

Katsuhiro Tsukiyama

A high-strength bolt having excellent delayed fracture resistance and stress relaxation resistance in addition to a tensile strength of 1200 N/mm or higher is disclosed. A steel material for the high-strength bolt includes C: 0.50 to 1.0% by mass (hereinafter, referred to simply as &ldquo;%&rdquo;), Si: 0.5% or less (not including 0%), Mn: 0.2 to 1%, P: 0.03% or less (including 0%) and S: 0.03% or less (including 0%). The steel material has pro-eutectoid ferrite, pro-eutectoid cementite, bainite and martensite structures at less than 20% in total and a pearlite structure as the remainder. The high-strength bolt is produced by drawing the steel material severely to obtain a steel wire, forming the steel wire into a bolt shape through a cold heading, and subjecting the shaped steel wire to a blueing treatment at a temperature within a range of 100 to 400&deg; C.

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Kobe, Japan

Yuichi Namimura