Wire rod having superior hydrogen delayed fracture resistance, method for manufacturing same, high strength bolt using same and method for manufacturing bolt

A method for manufacturing a wire rod having superior hydrogen delayed fracture resistance, the method comprising:

• heating steel including C:
• 0.3 to 0.7 wt %, Si:
• 0.05 to 2.0 wt %, Mn:
• 0.7 to 1.5 wt %, La:
• 30 to 70 ppm, Ni:
• 0.01 to 0.1%, and a remainder configured of Fe and inevitable impurities to a temperature of Ae3+150° C. to Ae3+250° C.;
• cooling the heated steel at a rate of 5 to 15° C./s and rolling, the steel at a temperature of Ae3+50° C. to Ae3+150° C. to manufacture a wire rod;
• and
• cooling the rolled wire rod to 600° C. or less at a rate of 0.5 to 3° C./s.

A bolt comprising C:

• 0.3 to 0.7 wt %, Si:
• 0.05 to 2.0 wt %, Mn:
• 0.7 to 1.5 wt %, La:
• 30 to 70 ppm, Ni:
• 0.01 to 0.1%, and a remainder configured of Fe and inevitable impurities, and having a tensile strength of 1200 MPa or greater and superior hydrogen delayed fracture resistance.

A method for manufacturing a bolt having superior hydrogen delayed fracture resistance, the method comprising:

• heating steel including C:
• 0.3 to 0.7 wt %, Si:
• 0.05 to 2.0 wt %, Mn:
• 0.7 to 1.5 wt %, La:
• 30 to 70 ppm, Ni:
• 0.01 to 0.1%, and a remainder configured of Fe and inevitable impurities to a temperature of Ae3+150° C. to Ae3+250° C.;
• cooling the heated steel at a rate of 5 to 15° C./s and rolling the steel at a temperature of Ae3+50° C. to Ae3+150° C. to manufacture a wire rod;
• cooling the rolled wire rod to 600 ° C. or less at a rate of 0.5 to 3° C./s;
• and
• bolt-forming using the cooled wire rod;
• performing a heat treatment on the formed bolt at a temperature of 850 to 950° C.;
• and
• performing quenching after the heat treatment, and then performing tempering at a temperature of 300 to 500° C.