| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile strength steel plate having a tensile strength of≥590 MPa, a low YR (yield ratio) of≤80% and having excellent high heat input joint toughness without performing reheating/tempering after hot rolling, and to provide its production method. SOLUTION: The low YR high tensile strength steel plate contains, by mass, 0.04 to 0.20% C, 0.0002 to 0.003% B, 0.0003 to 0.0025% Ca or the like, and the balance Fe with inevitable impurities, and in which the value of C+100×B is 0.1 to 0.32, and the fraction of bainite in a region other than range to 5 mm below the plate surface is≥50%, and has a plate thickness of≥70 mm. The steel plate comprising the above components is hot-rolled and is thereafter rapidly cooled from a temperature of an Ar<SB>3</SB>point or higher, thus the fraction of bainite in the steel can be controlled to≥50% even if the plate thickness of the steel plate is≥70 mm to increase its strength without causing a strength difference in the plate thickness direction even as-quenched, and, by the movable dislocation of the bainite structure, the YR can be reduced. COPYRIGHT: (C)2006,JPO&NCIPI
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