High strength steel having a superior delayed fracture resistance and bolt made of the steel and method for manufacturing working product by using it

摘要

PURPOSE: A process for preparing a fine composite structure steel having excellent delayed fracture resistance and capable of being used in a delayed fracture strength of 150kg/mm¬2 or more by basically solving hydrogen embrittlement and process for production of steel articles using the same are provided. Therefore, the obtained steel can be used as a high strength bolt. CONSTITUTION: The subject high strength steel comprises by weight, C:0.65 to 1.50%, Si:2.0 to 4.0%, Mn:0.1 to 0.8%, P and S:0.01% or less, N:0.005 to 0.01%, O:0.05% or less, one or two or more selected from Ni:0.3 to 2.0%, B:0.001 to 0.003%, V:0.01 to 0.5%, Nb:0.01 to 0.5%, Mo:0.01 to 0.5%, Ti:0.01 to 0.2%, W:0.01 to 0.5% and Cu:0.01 to 0.2% and the balance of Fe with other inevitable impurities. The fine structure of the steel is a composite structure of ferrite and retained austenite having a phase fraction of 20 to 35%.