Steel sheet and plated steel sheet
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° or more and has a circle-equivalent diameter of 0.3 μm or more is defined as a crystal grain, the proportion of crystal grains each having an intragranular misorientation of 5 to 14° 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/nm 2 or more and 4.5 pieces/nm 2 or less. An average grain size of cementite precipitated at grain boundaries is 2 μm or less.
1. A steel sheet, comprising:
a chemical composition represented by, in mass %,
C: 0.008 to 0.150%,
Si: 0.01 to 1.70%,
Mn: 0.60 to 2.50%,
Al: 0.010 to 0.60%,
Ti: 0 to 0.200%,
Nb: 0 to 0.200%,
Ti+Nb: 0.015 to 0.200%,
Cr: 0 to 1.0%,
B: 0 to 0.10%,
Mo: 0 to 1.0%,
Cu: 0 to 2.0%,
Ni: 0 to 2.0%,
Mg: 0 to 0.05%,
REM: 0 to 0.05%,
Ca: 0 to 0.05%,
Zr: 0 to 0.05%,
P: 0.05% or less,
S: 0.0200% or less,
N: 0.0060% or less, and
balance: Fe and impurities; and
a structure represented by, by area ratio,
ferrite: 0 to 30%, and
bainite: 70 to 100%, wherein
a region that is surrounded by a grain boundary having a misorientation of 15° or more and has a circle-equivalent diameter of 0.3 μm or more is defined as a crystal grain, and wherein the proportion of crystal grains each having an intragranular misorientation of 5 to 14° 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/nm 2 or more and 4.5 pieces/nm 2 or less, and
an average grain size of cementite precipitated at grain boundaries is 2 μm or less.
2. The steel sheet according to claim 1 , wherein
a tensile strength is 480 MPa or more, and
the product of the tensile strength and a limit form height in a saddle-type stretch-flange test is 19500 mm·MPa or more.
3. The steel sheet according to claim 1 , wherein
the chemical composition contains, in mass %, one type or more selected from the group consisting of
Cr: 0.05 to 1.0%, and
B: 0.0005 to 0.10%.
4. The steel sheet according to claim 1 , wherein
the chemical composition contains, in mass %, one type or more selected from the group consisting of
Mo: 0.01 to 1.0%,
Cu: 0.01 to 2.0%, and
Ni: 0.01% to 2.0%.
5. The steel sheet according to claim 1 , wherein
the chemical composition contains, in mass %, one type or more selected from the group consisting of
Ca: 0.0001 to 0.05%,
Mg: 0.0001 to 0.05%,
Zr: 0.0001 to 0.05%, and
REM: 0.0001 to 0.05%.
6. The steel sheet according to claim 1 , wherein
a plating layer is formed on a surface of the steel sheet.
7. The plated steel sheet according to claim 6 , wherein the plating layer is a hot-dip galvanizing layer.
8. The plated steel sheet according to claim 6 , wherein the plating layer is an alloyed hot-dip galvanizing layer.