To obtain excellent delayed fracture resistance by working a steel of specific composition at a temperature in the range lower than the Ac1 transformation point and not lower than a temperature at which inverse transformation is brought about simultaneously with working, in the course of temperature raise from a temperature lower than the Ac1 transformation point of the steel, and then cooling the steel without delay.
A steel, having a composition containing, by weight, 0.2-0.5% C, 0.2-2.0% Si, 0.2-2.0% Mn, ≤0.01% P, ≤0.01% S, 0.01-0.1% Al, 0.002-0.01% N, and 0.5-12% Ni, is worked. When warm working at a temperature not higher than the transformation point is applied to the Ni-containing steel stock under proper conditions, dynamic inverse transformation occurs, that is, inverse transformation occurs simultaneously with working and extremely fine equiaxed austenite grains are formed, and, after cooling, the high strength steel having ≥1200 MPa strength and excellent in delayed fracture resistance can be manufactured by an easy method by taking advantage of both the grain-boundary strengthening by grain refining and the hydrogen infiltration/diffusion inhibiting effect by Ni.
SHIRAGAMI TETSUO
NIIKURA MASAKAZU
FUJIOKA MASAAKI
ADACHI YOSHITAKA
MATSUKURA NARIKAZU
NIPPON STEEL CORP
SUMITOMO METAL IND
KOBE STEEL LTD
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