| 摘要 |
PROBLEM TO BE SOLVED: To provide high strength steel in which high strength and delayed fracture resistance are compatible. SOLUTION: In steel having tensile strength of >=1,200 N/mm2, the center line average roughness Ra in the surface roughness of the steel is controlled to <=1.0μm, and PPI50 to <=175. In the steel, when immersed into an aqueous solution of 5 weight % NaCl having pH 3 by 24 hours, the amount of diffusible hydrogen after the immersion may be <=0.5 ppm (mass%). The components in the steel, e.g. contains 0.05 to 0.8% C, <=1.5% Si, <=2% Mn, <=0.03% P and <=0.04% S. Further, the steel may contain Mo, V, Ti, Nb, Cu or the like.
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