| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-tensile steel plate having high strength and high plasticity with TS of 720 MPa or more, excellent in low temperature toughness at HAZ and arrestability of a mother material surface layer at a low cost without using Ni which is harmful to sulfide stress corrosion cracking.SOLUTION: A high-tensile steel plate has a chemical composition containing, by mass%, C:0.10 to 0.20%, Si:0.02 to 0.25%, Mn:0.7 to 1.6%, P:0.02% or less, S:0.008% or less, Cr:0.5 to 1.2%, Mo:0.1% to 0.3%, Ti:0.004 to 0.025%, B:0.0005 to 0.003%, Al:0.01 to 0.05%, N:0.01 or less and the balance Fe with impurities, consists of a mixture structure where a bainite phase has an area ratio of 85 to 99% and a ferrite phase has the area ratio of 1% to 15% in a region within 1 mm from a surface layer parallel to a rolling direction of the steel plate, and consists of the mixture structure of bainite and martensite in the region from the (1/4)t position to the (1/2)t position of a plate thickness t of the steel plate, and an average ferrite crystal particle diameter in the plate thickness direction in the region within 1 mm from the surface layer parallel to a rolling direction of the steel plate is 10 μm or less. A ratio of a particle diameter Pr in the rolling direction to a particle diameter Pt in the plate thickness direction of crystal particles at the (1/4)t position of the plate thickness t of the steel plate is 4 or more. |
