| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for producing a high-tensile steel sheet having a tensile strength of≥550 MPa class and having suitable SSC (Sulfide Stress Cracking) resistance. SOLUTION: A steel having a composition containing, by mass, <0.030% C, 0.01 to 0.55% Si, 0.5 to 2.0% Mn, 0.005 to 0.05% Ti, 0.005 to 0.06% Nb, 0.0005 to 0.003% B, 0.005 to 0.1% Al and 0.0005 to 0.006% N, and the balance Fe with inevitable impurities is heated at 1,000 to 1,350°C, is thereafter hot-rolled, is subjected to accelerated cooling from an Ar<SB>3</SB>transformation point to 650°C, is subsequently heated by an induction heating apparatus in such a manner that the maximum arrival temperature of the surface in the steel sheet reaches (an Ac<SB>1</SB>transformation point -20°C) or above, the temperature at the inside of the steel sheet reaches less than (an Ac<SB>1</SB>transformation point -20°C), and also, the difference between the maximum arrival temperature between the surface of the steel sheet and the central position of the sheet thickness reaches≥50°C, and is thereafter air-cooled, thus the maximum hardness in the sheet thickness direction of the steel sheet and the maximum hardness in the weld heat affected zone are controlled to≤248 Hv. COPYRIGHT: (C)2007,JPO&INPIT
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