Nippon Steel Corporation

Ryoichi Nishiyama

Masahiro Nakata

Yuji Yamaguchi

Kohichi Sano

Natsuko Sugiura

Makoto Uno

A steel sheet has a specific chemical composition and has a structure represented by, by area ratio, ferrite: 5 to 60%, and bainite: 40 to 95%. When a region that is surrounded by a grain boundary having a misorientation of 15&#xb0; or more and has a circle-equivalent diameter of 0.3 &#x3bc;m or more is defined as a crystal grain, the proportion of crystal grains each having an intragranular misorientation of 5 to 14&#xb0; to all crystal grains is 20 to 100% by area ratio. A precipitate density of Ti(C,N) and Nb(C,N) each having a circle-equivalent diameter of 10 nm or less is 10precipitates/mmor more. A ratio (Hvs/Hvc) of a hardness at 20 &#x3bc;m in depth from a surface (Hvs) to a hardness of the center of a sheet thickness (Hvc) is 0.85 or more.

Steel sheet and plated steel sheet

A steel sheet has a specific chemical composition and has a structure represented by, by area ratio, ferrite: 30 to 95%, and bainite: 5 to 70%. When a region that is surrounded by a grain boundary having a misorientation of 15&#xb0; or more and has a circle-equivalent diameter of 0.3 &#x3bc;m or more is defined as a crystal grain, the proportion of crystal grains each having an intragranular misorientation of 5 to 14&#xb0; to all crystal grains is 20 to 100% by area ratio. An average aspect ratio of ellipses equivalent to the crystal grains is 5 or less. An average distribution density of the total of Ti-based carbides and Nb-based carbides each having a grain size of 20 nm or more on ferrite grain boundaries is 10 carbides/&#x3bc;m or less.

A steel sheet has a specific chemical composition and has a structure represented by, by area ratio, ferrite: 0 to 30%, and bainite: 70 to 100%. When a region that is surrounded by a grain boundary having a misorientation of 15&#xb0; or more and has a circle-equivalent diameter of 0.3 &#x3bc;m or more is defined as a crystal grain, the proportion of crystal grains each having an intragranular misorientation of 5 to 14&#xb0; to all crystal grains is 20 to 100% by area ratio. A grain boundary number density of solid-solution C or a grain boundary number density of the total of solid-solution C and solid-solution B is 1 piece/nmor more and 4.5 pieces/nmor less. An average grain size of cementite precipitated at grain boundaries is 2 &#x3bc;m or less.

A steel sheet having a specific chemical composition and structure represented by, by area ratio, ferrite: 5 to 95%, and bainite: 5 to 95%. The proportion of crystal grains having an intragranular misorientation of 5 to 14&#xb0; to all crystal grains is 20 to 100% by area ratio. Hard crystal grains A in which precipitates or clusters with a maximum diameter of 8 nm or less are dispersed in the crystal grains with a number density of 1&#xd7;10to 1&#xd7;10pieces/cm. Soft crystal grains B in which precipitates or clusters with a maximum diameter of 8 nm or less are dispersed in the crystal grains with a number density of 1&#xd7;10pieces/cmor less are contained. The volume % of the hard crystal grains A/(the volume % of the hard crystal grains A+the volume % of the soft crystal grains B) is 0.1 to 0.9.

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Tokyo, Japan

Ryoichi Nishiyama