Hot-rolled steel sheet with excellent press formability and production method thereof
Abstract:
The issue of the present invention is to provide a hot-rolled steel sheet with excellent press formability and method for producing the steel sheet, wherein the steel sheet has not only hole expandability but also stretch flanging workability by not assessing hole expandability for stretch flanging as conventional but an actual phenomena of side-bend elongation.To solve the issue, it is confirmed that the steel sheet are excellent in hole expandability and stretch flanging workability, wherein the steel sheet with a certain content of C, Si and Mn is characterized in that, in a metallic structure of said steel sheet, the area fraction of ferrite is 70% or more, the area fraction of bainite is 30% or less, the area fraction of either one or both of martensite and retained austenite is 2% or less, and with regard to respective average intervals (Lθ, Li and LMA), average diameters (Dθ, Di and DMA) and number densities of a cementite, an inclusion and either one or both of martensite and retained austenite (nθ, ni and nMA), a void formation/connection index L defined by formula 1 is 11.5 or more: L = n θ ⁢ L θ / D θ 2 + 2.1 ⁢ ⁢ n i ⁢ L i / D i 2 + n MA ⁢ L MA / D MA 2 n θ + n i + n MA ( formula ⁢ ⁢ 1 )
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