Steel sheet and plated steel sheet
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° 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 precipitate density of Ti(C,N) and Nb(C,N) each having a circle-equivalent diameter of 10 nm or less is 10 10 precipitates/mm 3 or more. A ratio (Hvs/Hvc) of a hardness at 20 μm in depth from a surface (Hvs) to a hardness of the center of a sheet thickness (Hvc) is 0.85 or more.
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: 0.015 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: 5 to 60%, and
bainite: 40 to 85%, wherein
a region that is surrounded by a grain boundary having a misorientation of 150 or more and having a circle-equivalent diameter of 0.3 μm or more is defined as a crystal grain, and 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 precipitate density of Ti(C,N) and Nb(C,N) each having a circle-equivalent diameter of 10 nm or less is 10 10 precipitates/mm 3 or more, and
a ratio (Hvs/Hvc) of a hardness at 20 μm in depth from a surface (Hvs) to a hardness of the center of a sheet thickness (Hvc) is 0.85 or more.
2. The steel sheet according to claim 1 , wherein
an average dislocation density is 1×10 14 m −2 or less.
3. The steel sheet according to claim 1 , wherein
a tensile strength is 480 MPa or more,
a ratio of the tensile strength and a yield strength is 0.80 or more,
a product of the tensile strength and a limit form height in a saddle-type stretch-flange test is 19500 mm·MPa or more, and
a fatigue strength ratio is 0.45 or more.
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
Cr: 0.05 to 1.0%, and
B: 0.0005 to 0.10%.
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
Mo: 0.01 to 1.0%,
Cu: 0.01 to 2.0%, and
Ni: 0.01% to 2.0%.
6. 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%.
7. The steel sheet according to claim 1 , wherein
a plating layer is formed on a surface of the steel sheet.
8. The steel sheet according to claim 7 , wherein
the plating layer is a hot-dip galvanizing layer.
9. The steel sheet according to claim 8 , wherein
the plating layer is an alloyed hot-dip galvanizing layer.