Patent ID: 7754029

Claim:
A steel with an excellent delayed fracture resistance and a tensile strength of the 1801 MPa class or more consisting essentially of, by mass %, C: 0.20 to 0.60%, Si: 0.50% or less, Mn: over 0.10% to 3%, Al: 0.005 to 0.045%, in an amount effective to deoxidize and form nitrides to make pre-austenite grains finer, Cu: 0.05 to 0.30%, in an amount effective to improve corrosion resistance, Mo: over 3.0% to 10%, and further consisting essentially of a quenched and tempered microstructure, treated by tempering at 600% to 750° C., containing fine Mo carbide precipitates, and a balance of Fe and unavoidable impurities.