Patent ID: 7510614

Claim:
A high strength bolt excellent in delayed fracture resistance containing, by mass %, C: 0.2 to 0.6%, Si: 0.05 to 0.5%, Mn: 0.1 to 2%, Mo: 0.5 to 6%, Al: 0.005 to 0.5%, and a balance of Fe and unavoidable impurities, having a tensile strength of 1400 MPa or more, and having a compressive residual stress of the surface layer of the thread root of 10 to 90% of the tensile strength. wherein the surface layer part of the thread root from the surface down to at least 50 pm having pre-austenite grains with an aspect ratio of the axial direction and radial direction of 2 or more and having a Vickers hardness of 460 or more.