STEEL FOR HIGH STRENGTH BOLTS HAVING EXCELLENT DELAYED FRACTURE RESISTANCE AND HIGH STRENGTH BOLT

摘要

Provided are: steel for high strength bolts, which exhibits excellent delayed fracture resistance without addition of an expensive alloy element such as Mo or V even if the steel has a high strength, namely a tensile strength of 1,100 MPa or more; and a high strength bolt which is formed of this steel and has excellent delayed fracture resistance.Steel for high strength bolts, which has excellent delayed fracture resistance and contains, in mass%, 0.20-0.35% of C, 0.3-1.0% of Si, more than 0% but 0.6% or less of Mn, more than 0% but 0.02% or less of P, more than 0% but 0.02% or less of S, 0.3-1.5% of Cr, 0.01-0.1% of Al, 0.05-0.1% of Ti, 0.0003-0.005% of B and more than 0% but 0.01% or less of N with the balance made up of iron and unavoidable impurities, while satisfying formula (1) and formula (2). This steel for high strength bolts has a mixed structure of ferrite and pearlite. In formula (1) and formula (2), [ ] denotes the content (mass%) of each element.1/([C] × [Mn]) ≥ 5.5 (1)[C] + [Si]/2 + [Mn]/2 + [Cr]/3 ≥ 0.82 (2)