| 摘要 |
PROBLEM TO BE SOLVED: To provide a welded steel-pipe superior in crushing resistance, which can be manufactured with high productivity and at a low cost, without degrading the circularity and deformability of the pipe. SOLUTION: The welded steel-pipe has a component composition comprising, by mass%, 0.03 to 0.08% C, 0.01 to 0.50% Si, 0.50 to 1.75% Mn, 0.08% or less Al, 0.005 to 0.060% Nb, 0.005 to 0.040% Ti and the balance Fe with unavoidable impurities; and is controlled so that a volume fraction of a second phase structure is 2% or less, a difference between Vickers hardnesses in the surface layer and in a central part in a pipe thickness direction is 40 or less, and a compression yield stress in a pipe perimeter direction is 0.85 times or more than a tensile yield stress in the perimeter direction. COPYRIGHT: (C)2010,JPO&INPIT
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