Patent ID: 8728257

Claim:
A high tensile strength steel material having a tensile strength of at least 900 Mpa and having an excellent delayed fracture resistance property, comprising: elements of, in mass percent, C of 0.02 to 0.25%, Si of 0.01 to 0.8%, Mn of 0.5 to 2.0%, Al of 0.005 to 0.1%, Mo of 0.2 to 1%, Cr of 0.3 to 2%, N of 0.0005 to 0.008%, P of 0.03% or less, 0.0004%≦S≦0.0025%, 0.0010%≦Ca≦0.0030%, and 0.0008%≦O≦0.0030%, and optionally at least one element selected from Nb of 0.001 to 0.1%, V of 0.001 to 0.5%, and Ti of 0.001 to 0.1%, in mass percent, and a value of ACR which satisfies the following expression 0.2≦ACR≦1.0, and the remainder including Fe and inevitable impurities, and the microstructure of said steel material comprises martensite or bainite, [includes residual austenite in a volume fraction of 0.5 to 5%,] and precipitates contained in said steel include those of Mo and optionally of at least one element selected from Nb, V and Ti, an average grain size of said precipitates is 20 nm or less, and the number of said precipitates is at least 5 per 250000 nm 2 , and ACR =(Ca−(0.18+130*Ca)*O)/1.25/S, wherein Ca, O or S is the content (mass percent) of each component; and said high tensile strength steel material having an excellent delayed fracture resistance property is manufactured by a method comprising: a step of quenching said steel material from a temperature of Ar3 transformation temperature or higher to a temperature of 500° C. or less, and a step of tempering the steel material while a central portion of the steel material is heated from a tempering start temperature to a predetermined tempering temperature at an average heating rate of 1° C./s or more, after the quenching.