| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a high-strength steel pipe having a seam portion and excellent in workability. <P>SOLUTION: The high-strength steel pipe has a composition containing, by mass, 0.05-0.25% C, 0.5-2.0% Si, 1.0-2.5% Mn, ≤0.020% P, ≤0.002% S, 0.005-0.08% Al and ≤0.004% N. The structure of the region except a range of ±15° in the circumferential direction based on the center position of the seam is a structure containing 20-60% by area of a martensitic phase, the region except the range has a tensile residual stress of ≤450 MPa in the circumferential direction, and the hardness difference ΔHV between the region and the remaining region is ≤250 points by Vickers hardness. The steel pipe is preferably obtained by using a steel sheet having a structure containing a predetermined range of martensite as blank, and adopting a manufacturing process including a roll forming step in which little work strain is applied to the blank and a seam annealing step. A roll forming method by a cage roll system or a roll forming method in which a formed flower is made oval may be used in the roll forming step in which little work strain is applied to the blank. Accordingly, the high-strength steel pipe has such high strength as a tensile strength of ≥590 MPa, has a high elongation value suitable for the high strength, and is excellent in workability. <P>COPYRIGHT: (C)2010,JPO&INPIT |
