Manufacturing method of press-bend cold-formed circular steel pipe with excellent earthquake resistance

摘要

A method of manufacturing a press-bended cold-formed circular steel pipe with favorable earthquake-proof performance is provided to realize a low yield ratio over 490 MPa of tensile strength. In a method of manufacturing a press-bended cold-formed circular steel pipe with favorable earthquake-proof performance, the steel pipe includes 0.02~0.20 mass% of carbon, 0.05~0.5 mass% of silicon, 0.50~2.0 mass% of manganese, 0.01~0.1 mass% of aluminium, 0.002~0.007 mass% of nitrogen, the small quantity of iron, and a foreign material. A ratio of included phosphorous is limited under 0.02 mass%, and the ratio of included sulfate is limited under 0.008 mass%. Yield strength is between (YSo-980t/D) and (YSo-980t/D+120) MPs, and tensile strength thereof ranges from (Ts0-560t/D) to (TSo-560t/D+100) MPs, as an outer diameter of the steel pipe is D(mm), the yield strength of a product standard for the steel pipe is over YSo(MPa), the tensile strength of the steel pipe is over TSo(MPa), and the thickness of the steel pipe is t(mm). A yield ratio of the steel pipe is under 75~82t/D%. Hardness of the steel pipe at 1 millimeter of depth from a surface is in the range of 140~200 HV. The steel pipe is formed in a circular pipe by a press bending cold-forming operation. A heat treating operation is not performed after the press bending cold-forming operation.