| 摘要 |
PROBLEM TO BE SOLVED: To stably improve local ductility such as stretch-flange formability, and to enable hardening treatment by short-time heating, such as induction hardening, by constituting the steel sheet of a hypo-eutectoid steel having a specific composition containing C and S, dispersing carbides in ferrite so that carbide spheroidizing rate becomes a specific value or above, and regulating the average grain size of carbides to a value in a specific range. SOLUTION: This steel sheet is composed of a hypo-eutectoid steel having a composition containing, by mass, 0.1-0.8% C and <=0.01% S, and carbides are dispersed in ferrite so that carbide spheroidizing rate becomes >=90%. The carbide spheroidizing rate is the proportion (%) of the number of carbides, where the ratio between the maximum length (p) of carbide and the maximum length (q) in a direction perpendicular to it satisfies p/q<3, which is comprised in the total number of carbides within the observational field of metallic structure in the cross-sectional area of the steel sheet. Further, the average grain size of carbides is regulated to 0.4 to 1.0μm. The average grain size of carbides means the average value of circle-equivalent diameters, with respect to all the measured cabides, measured on respective carbides within the observational field.
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