Nippon Steel Corporation

Takahiko Kohtake

Koutarou Hayashi

Makoto Okonogi

Yoshihiro Suwa

Kazuto Kawakami

A steel sheet has a predetermined chemical composition containing, in mass %, C: more than 0.18% and less than 0.30%, Mn: more than 2.50% and 4.00% or less, and other elements. The steel micro-structure at a position at a depth of ¼ of the sheet thickness from the surface in an L cross section of the steel sheet is, in area %, tempered martensite: 25 to 90%, ferrite: 5% or less, retained austenite: 10 to 50%, and bainite: 5% or less. At a position at a depth of ¼ of the sheet thickness from the surface of the L cross section, the proportion of a total area of retained austenite grains having an area of 1 μmor more and having a grain shape circularity of 0.1 or more is less than 50% with respect to the entire area of retained austenite. The steel sheet satisfies the formula C/C≥1.2.

Steel sheet and method for producing same

A steel contains, in a chemical composition, C, Si, Mn, and Al, and contains pearlite as a metallographic structure, and a value obtained by dividing an Mn content in a cementite in the pearlite in terms of at % by an Mn content in a ferrite in the pearlite in terms of at % is higher than 0 and equal to or lower than 5.0.

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Amagasaki, Japan

Takahiko Kohtake