| 摘要 |
PROBLEM TO BE SOLVED: To improve formability, impact resistance, amounts of work hardening and baking hardening, and a balance between strength and elongation by providing a specific composition containing C, Si, Mn, and Cr, also providing a structure where primary phase is composed of pro-eutectoid ferrite and secondary phase is composed of martensite, etc., and regulating crystalline grain size of pro-eutectoid ferrite and plate crown to specific values, respectively. SOLUTION: This steel sheet has a composition consisting of, by mass, 0.05-0.40% C, 1.0-3.0% Si, 0.6-3.0% Mn, 0.2-2.0% Cr, and the balance essentially Fe and also has a steel structure where primary phase is composed of pro- eutectoid ferrite and secondary phase is composed of martensite, acicular ferrite, and retained austenite. Further, the crystalline grain size of pro-eutectoid ferrite is regulated to <=5μm, and also plate crown is regulated to <=140μm. In order to obtain such grain size and plate crown, heavy-draft rolling is performed, in the latter stage of the hot rolling process, at e.g. >=15% draft at the final pass.
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