780MPa CLASS LOW YIELD RATIO CIRCULAR STEEL FOR CONSTRUCTION STRUCTURE EXCELLENT IN EARTHQUAKE-PROOF PERFORMANCE, AND PROCESS FOR PRODUCING THE SAME

摘要

PURPOSE: A resistance yield ratio circular steel pipe and a manufacturing method thereof are provided to secure ductility by reducing hardness of the outer side of the pipe. CONSTITUTION: A resistance yield ratio circular steel pipe comprises C: 0.01~0.06%, Si: 0.10~0.40%, Mn: 1.60~2.50%, Al: 0.025~0.090%, Cu:0.15~0.70%, Ni: 0.90~1.60%, Cr: 0.50~1.35%, Mo: 10~0.30%, Ti: 0.008~0.025%, B: 0.0005~0.0025%, N:0.0030~0.0060%, Ca: 0.0005~0.0040% and iron or impurities. A superficial layer is located in 2mm point from a surface of the pipe and average Vickers hardness Hv of a central part is 230~310.