High strength welded steel pipe and a method of manufacturing the same weld heat-affected zone toughness to excellent tensile strength for the following low temperature more than 570MPa 760MPa

摘要

PPROBLEM TO BE SOLVED: To provide a high strength welded steel pipe for low temperature use having excellent weld heat affected zone toughness which has a pipe thickness of ≤50 mm and a tensile strength of 570 to 760 MPa, and is suitable for transport use to natural gas and crude oil, and to provide its production method. PSOLUTION: The welded steel pipe has a base metal part whose fundamental componential system is composed of Nb-Ti-B and further having a composition comprising one or more kinds selected from Cu, Ni, Cr, Mo, V, Ca, REM, Zr and Mg, and satisfying 0.12≤PSBCM/SB≤0.20, and the balance Fe with inevitable impurities, and, regarding the microstructure of a longitudinal seam weld zone in which an old austenite grain size in the vicinity of a welding line is ≥50 μm, the area ratio of bainite in which carbides essentially consisting of cementite are precipitated within the laths and/or between the laths of bainitic ferrite with the average grain size of ≤5 μm is ≥50%, and the balance insular martensite and/or upper bainite including cementite between the laths or martensite or pearlite or their mixed structure. PCOPYRIGHT: (C)2009,JPO&INPIT