To provide a high strength wire rod having excellent delayed fracture resistance as well as ≥1200 N/mm2 tensile strength, a useful method for obtaining a high strength wire rod like that, and a high strength bolt having the above characteristics.
This wire rod is composed of a steel containing 0.5-1.0% C, and the area ratio of a pearlitic structure having ≤200 nm pearlite lamellar spacing is regulated to ≥80% by inhibiting the formation of structures of one or ≥2 kinds among pro-eutectoid ferrite, pro-eutectoid cementite, bainite, and martensite and also strength is regulated to ≥1200 N/mm2 by means of heavy wire drawing. Moreover, the steel is heated to ≥800°C, cooled rapidly down to 520-650°C, and isothermally held at the temperature, by which the formation of structures of one or ≥2 kinds among pro-eutectoid ferrite, pro-eutectoid cementite, bainite, and martensite is inhibited to regulate the area ratio of a pearlitic structure having ≤200 nm pearlite lamellar spacing to ≥80%. Then, strength is regulated to ≥1200 N/mm2 by means of heavy wire drawing.
IBARAKI NOBUHIKO
MAKII KOICHI
KAKO HIROSHI
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