HIGH-STRENGTH STEEL TUBE FOR LOW-TEMPERATURE USE WITH SUPERIOR BUCKLING RESISTANCE AND TOUGHNESS IN WELD HEAT-AFFECTED AREAS, AND MANUFACTURING METHOD FOR SAME

摘要

PROBLEM TO BE SOLVED: To provide an APIX100 grade high-strength steel tube with a plate thickness of about 20 to 40 mm having superior buckling resistance and toughness in weld heat-affected areas, and suitable for a steel tube for conveying natural gas and crude oil used in an earthquake zone and a frozen ground zone where ground settlement is severe, and to provide a manufacturing method therefor.;SOLUTION: The steel tube is provided with: a base metal part having a specified componential composition, having tensile properties satisfying 760 to 930 MPa tensile strength and ≥5% uniform elongation and having a yield ratio of ≤85%, and in which Charpy absorbed energy at a test temperature of -40°C is ≥210 J in the case plate thickness is 25 mm, and is ≥150 J in the case plate thickness is ≥25 mm; and a weld heat-affected area composed of a weld metal by seam welding having a specified composition, and whose microstructure in which old austenite grain sizes reach ≥50 μm in the vicinity of a welding line in the seam weld area of the steel tube is provided with a mixed structure provided with lower bainite or a mixed structure of, in area percentage, 50% or higher bainite, upper bainite and/or martensite.;COPYRIGHT: (C)2010,JPO&INPIT