| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a circular steel pipe capable of improving earthquake resistance by reducing the hardness on the external face side of a circular steel pipe with a tensile strength in a class of 90 MPa equivalent to a high strength class in application for a building steel frame as the problem characteristic of the circular steel pipe as being high in strength. Ž<P>SOLUTION: In the circular steel pipe, a chemical componential composition is controlled while satisfying prescribed relational equation, and further, the following requirements (A) to (C) are satisfied: (A) the microstructure of a steel sheet is composed of bainite with an aspect ratio of at least ≥3 by ≥50% in an area fraction, and dislocation density ρ is 1.0×10<SP>5</SP>(m<SP>-2</SP>) to 6.0×10<SP>5</SP>(m<SP>-2</SP>); (B) the average Vickers hardness Hv in the central part other than the surface layer part from the surface or back surface of the steel sheet to a depth of 2 mm is 180 to 280; and (C) the average vickers hardness Hv in the surface layer part from the surface or back surface of the steel sheet to a depth of 2 mm is ≤1.4 times the average Vickers hardness of the central part. Ž<P>COPYRIGHT: (C)2010,JPO&INPIT Ž
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