| abstract |
[PROBLEMS] To provide a high-strength bolt steel and a method of manufacturing a bolt having sufficient delayed fracture resistance even with a high-strength steel having a tensile strength of 1350 MPa or more. SOLUTION: In mass%, C: 0.35 to 0.5%, S i: 0.05 to 0.3%, Mn: 0.4% or less, P: 0.03% or less, S: 0.03% or less, Cr: 0.15 0.55%, Mo: 0.2-0.8%, V: 0.2- 0.5%, sol. Al: 0.01 to 0.05%, and if necessary, one or more of Nb, Ti, Zr, one or two of Cu, Ni and B A high-strength steel for bolts, the balance being Fe and impurities and satisfying the following formulas (1) and (2) (where formula 2 includes B, Ti and Zr). 0.2 ≦ 2.5Cr 2 + Mo 4 ≦ 1 (1) Ti + Zr ≧ 3.4N (nitrogen) (2) |