| 摘要 |
PROBLEM TO BE SOLVED: To obtain excellent roll formability and bulking resistance capable of effectively preventing both the generation of edge waves at the time of roll forming and the generation of bulking at the time of bending by allowing the steel to have a mixed structure of ferrite and bainite contg. a specified amt. of ferrite and controlling the yield ratio, tensile strength and the ratio between the elastic limit and tensile strength respectively to specified values. SOLUTION: This steel is the one having a steel structure mainly formed of a mixed one of ferrite and bainite contg. >=10% ferrite and having physical properties of <=75% yield ratio(YR), >=580 MPa tensile strength(TS) and >=45% (elastic limit/TS) ratio. As to the componential compsn. of the steel, the one contg., by weight, 0.03 to 0.18% C, <=2.0% Si, 1.0 to 3.5% Mn, <=0.05% P, <=0.02% S and <=0.10% Al and moreover contg. at least one kind among 0.005 to 0.1% Nb, 0.005 to 0.1% Ti and 0.0001 to 0.005% Ca is suitable.
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