HOT ROLLED STEEL SHEET FOR SQUARE COLUMN FOR BUILDING STRUCTURAL MEMBERS AND METHOD FOR MANUFACTURING THE SAME

摘要

Provided is a hot-rolled steel plate suitable as a raw material for a square steel tube for use as a building structural member. This hot-rolled steel plate contains, in mass%, 0.07 to 0.18% of C, 0.3 to 1.5% of Mn, 0.01 to 0.06% of Al and at most 0.006% of N with the balance being Fe and unavoidable impurities, and comprises a primary phase consisting of ferrite and a secondary phase consisting of pearlite or both pearlite and bainite. The frequency of the second phase is 0.20 to 0.42, and the mean grain diameter of the primary and secondary phases is 7 to 15µm. A square steel tube which combines a low yield ratio and high toughness can be produced by subjecting the hot-rolled steel plate to cold forming.