590MPa CLASS HIGH YIELD RATIO CIRCULAR STEEL FOR CONSTRUCTION STRUCTURE EXCELLENT IN EARTHQUAKE-PROOF PERFORMANCE, AND PROCESS FOR PRODUCING THE SAME

摘要

The circular steel pipe of the present invention satisfies the following requirements (A) to (C) while adjusting the chemical composition while satisfying a predetermined relational expression.;(A) The microstructure of the steel sheet has at least an area fraction of bainite having an aspect ratio of 3 or more of 50% or more, and a dislocation density p of 1.0 × 10 ⁵ (m ⁻² ) or more and 6.0 × 10 ⁵ (m ⁻² ) or less. ego,;(B) The average Vickers hardness Hv of the center part except the surface layer part from each of the front and back surfaces of a steel plate to a depth of 2 mm is 180-280,;(C) The average Vickers hardness Hv of the surface layer part from each of the front and back surfaces of the steel plate to a depth of 2 mm is 1.4 times or less of the average Vickers hardness Hv of the said center part.;With such a configuration, it is possible to reduce the hardness of the steel pipe outer surface side, which is a tensile stress field, which is a problem inherent to circular steel pipes as a high strength, with respect to a steel pipe having a tensile strength of 590 MPa class, which is located in a high strength class in building steel applications. Can contribute to improvement.