Method for manufacturing high strength bolt excellent in resistance to delayed fracture and to relaxation

A method for producing a high-strength bolt having excellent delayed fracture resistance and stress relaxation resistance, comprising:

• preparing a steel material comprising
• 0.50 to 1.0% by mass of C,
• 0.5% by mass or less of Si, not including 0% by mass,
• 0.2 to 1% by mass of Mn,
• 0.03% by mass or less of P, including 0% by mass;
• and
• 0.03% by mass or less of S, including 0% by mass;
• said steel material comprising
• a total of less than 20% by mass of a pro-eutectoid ferrite structure, a pro-eutectoid cementite structure, a bainite structure and a martensite structure;
• and
• a remainder of a pearlite structure;
• drawing said steel material severely to obtain a steel wire;
• forming said steel wire into a bolt shape through a cold heading, to obtain a shaped steel wire;
• and
• subjecting said shaped steel wire to a blueing treatment at a temperature within a range of 100 to 400° C.;
• thereby producing a high-strength bolt having excellent delayed fracture resistance and stress relaxation resistance in addition to a tensile strength of 1200 N/mm2 or higher.