HIGH STRENGTH BOLT HAVING EXCELLENT DELAYED FRACTURE RESISTANCE AND METHOD FOR IMPROVING ITS DELAYED FRACTURE RESISTANCE

摘要

PPROBLEM TO BE SOLVED: To provide a high strength bolt having satisfactory delayed fracture resistance, and having a strength of 1,500 MPa, and to provide a method for improving its delayed fracture resistance. PSOLUTION: The high strength bolt having excellent delayed fracture resistance is composed of a steel having a composition comprising, by mass, 0.3 to 0.6% C, 0.05 to 2% Si, 0.1 to 2% Mn, 0.002 to 0.1% Al and 1 to 3% Mo, and further comprising one or more kinds of metals selected from 0.05 to 1% V, 0.01 to 1% Ti and 0.01 to 1% Nb, and the balance Fe with inevitable impurities, and in which the mass% ratio of the C also satisfies the inequality (A) 0.2(0.06Mo+0.18V+0.25Ti+0.13Nb)/C, and having a tensile strength of 1,500 MPa. The compressive residual stress of the surface layer in the under head part of the bolt is 20 to 90% the tensile strength of the steel. Further, the invention relates to a method for improving its delayed fracture resistance. PCOPYRIGHT: (C)2006,JPO&NCIPI