DELAYED-FRACTURE-RESISTANT BORON-CONTAINING STEEL FOR HIGH-STRENGTH BOLTS, AND HIGH-STRENGTH BOLTS

摘要

The present invention provides, without adding an expensive alloying element such as Cr or Mo in a large amount, a boron-containing steel for high-strength bolts which combines excellent delayed fracture resistance and high strength of 1100MPa or more. This boron-containing steel contains 0.20 to less than 0.40% of C, 0.20 to 1.50% of Si, 0.30 to 2.0% of Mn, up to 0.03% (exclusive of 0%) of P, up to 0.03% (exclusive of 0%)of S, 0.05 to 1.0% of Ni, 0.01 to 1.50% of Cr, up to 1.0% (inclusive of 0%) of Cu, 0.01 to 0.10% of Al, 0.01 to 0.1% of Ti, 0.0003 to 0.0050% of B and 0.002 to 0.010% of N, and contains one ore more elements selected from the group consisting of Cu, Ni and Cr in a total amount of 0.10 to 3.0% with the balance consisting of iron and unavoidable impurities and the [Si]/[C] ratio being 1.0 or more. Further, the boron-containing steel has a ferrite/pearlite dual-phase structure.