High strength bolt excellent in delayed fracture resistance and method of production of same

摘要

The present invention provides a high strength bolt excellent in delayed fracture resistance able to advantageously prevent hydrogen embrittlement as represented by the delayed fracture phenomenon occurring along with an increase in strength and causing a particular problem, and a method of production of the same, 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%, and Al: 0.005 to 0.5%, 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. Further, a surface layer part of the thread root from the surface down to at least 50 µm has pre-austenite grains with an aspect ratio of the axial direction and radial direction of 2 or more and that part has a hardness of Hv 460 or more. Further, the method of production comprises using the steel material having the above ingredients to shape the bolt head and shaft, then heat the bolt to 900 to 1100°C, quench it, temper it by a 580°C or higher temperature, then thread roll it.