Delayed fracture excellent in boron-added high strength bolts for steel and high- strength bolts

摘要

PROBLEM TO BE SOLVED: To provide boron-containing steel for a high-strength bolt excellent in delayed fracture resistance even having a high strength of 1,100 MPa or higher without adding a large amount of an expensive alloying element such as Cr or Mo.;SOLUTION: The boron-containing steel includes 0.20-0.40% (exclusive of 0.40%) of C, 0.20-1.50% of Si, 0.30-2.0% of Mn, 0.03% or less (exclusive of 0%) of P, 0.03% or less (exclusive of 0%) of S, 0.05-1.0% of Ni, 0.01-1.50% of Cr, 1.0% or less (inclusive of 0%) of Cu, 0.01-0.10% of Al, 0.01-0.1% of Ti, 0.0003-0.0050% of B and 0.002-0.010% of N, respectively, and includes one ore more elements selected from a group composed of Cu, Ni and Cr in a total amount of 0.10-3.0% with the balance composed of iron and unavoidable impurities. The boron-containing steel has a ratio ([Si]/[C]) of the Si content [Si] to the C content [C] of 1.0 or more, and has a ferrite-pearlite structure.;COPYRIGHT: (C)2012,JPO&INPIT