The tensile strength is within the delayed fracture properties as the preferred 600 MPa or more, such as a machine for building industry provides excellent high-strength steel material and a manufacturing method thereof. More specifically, C, Si, Mn, Al, N, P, containing S and, if necessary Mo, Nb, V, Ti, Cu, Ni, Cr, W, B, Ca, REM, 1 species of Mg or it made of containing two or more elements, and the balance of Fe and inevitable impurities, old in the austenite grain 3+ over the average value the plate thickness direction of the aspect ratio of, or, added to the cementite in the Ras interface and a coating ratio of 50% or less, or, in a more delayed fracture in steel safety index is not less than 75%. Further, after the steel casting, Ar 3 without cooling below the transformation point, or Ac 3 and then re-heated above transformation temperature, rolling reduction is carried out for 30% or more hot rolling in the non-recrystallized station, and continues to Ar 3 or more transformation point after cooling to a temperature of from cooling rate of 1 ℃ / s or more to less than 350 ℃, Ac 1 is within the delayed fracture properties are excellent method for producing a high-strength steel material in which the tempering transformation point or less.
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