| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide: a high-strength welded steel pipe for an API X80 grade (tensile strength 700 MPa) or higher high-strength line pipe, achieving both excellent deformability which satisfies a ratio of a 1.5% yield strength relative to 0.5% yield strength of a base material of 1.10 or more and a product of a tensile strength and uniform elongation of 7,500 MPa% or more, and excellent weld zone toughness in a low temperature region down to -20°C; and a method for manufacturing the high-strength welded steel pipe. <P>SOLUTION: The welded steel pipe includes a base material with a specific component composition and a weld metal with a specific component composition. The section of the base material has a micro structure in which a first phase is bainite and a second phase is island martensite dispersed in the first phase at an area ratio of 5-20% and having an average aspect ratio of 2.0 or less, while 90% or more of the island martensite is present at a prior austenite grain boundary. The section of the weld metal has a micro structure in which acicular ferrite and bainite have an area ratio of 80% or more in total and island martensite has an area ratio of 5% or less. <P>COPYRIGHT: (C)2013,JPO&INPIT |
