COLD-FORMED SQUARE STEEL TUBE HAVING EXCELLENT EARTHQUAKE RESISTANCE

摘要

PROBLEM TO BE SOLVED: To provide a square steel tube which can contribute to the improvement of earthquake resistance by achieving the coexistence of high strength and a low yield ratio as cold-worked and further securing the high impact absorbing properties and plastic deformability in the square part.;SOLUTION: The square steel tube is obtained from the one obtained by subjecting a steel sheet to cold bending. The steel tube has a regulated chemical composition. The bent part lies in a state as-worked at a right angle, and also, the following requirements (A) to (C) are satisfied: (A), in the flat part of the steel tube, yield strength is ≥355 MPa and tensile strength is ≥520 MPa; (B), in the microstructure of the flat part, the area fraction of a bainitic structure is ≥40%; and (C), in the surface layer part at the square part of the steel tube, Vickers hardness is ≤350 Hv, elongation in a tensile test is ≥10%, and charpy absorption energy vE₀ at 0°C is ≥70 J.;COPYRIGHT: (C)2011,JPO&INPIT