Steel sheet
A steel sheet according to an aspect of the present invention has a chemical composition within a predetermined range; in which a metallographic structure at a thickness ¼ portion includes, by unit area %, ferrite: 10% or more and less than 50%, granular bainite: 5% or more and less than 50%, and martensite: 20% or more and less than 60%; in the metallographic structure at the thickness ¼ portion, a total of upper bainite, lower bainite, residual austenite, and pearlite is 0% or more and less than 15% by unit area %; at the thickness ¼ portion, a product of an area ratio Vm of the martensite and an average hardness Hv of the martensite is 12,000 to 34,000; and a tensile strength is 980 MPa or higher.
1. A steel sheet comprising, as a chemical composition, by unit mass %,
C: 0.06% to 0.15%,
P: 0.040% or less,
S: 0.0100% or less,
N: 0.0100% or less,
O: 0.0060% or less,
Si and Al in total: 0.20% to 2.50%,
Mn and Cr in total: 1.50% to 3.00%,
Mo: 0% to 1.00%,
Ni: 0% to 1.00%,
Cu: 0% to 1.00%,
Nb: 0% to 0.30%,
Ti: 0% to 0.30%,
V: 0% to 0.50%,
B: 0% to 0.0100%,
Ca: 0% to 0.0400%,
Mg: 0% to 0.0400%,
REM: 0% to 0.0400%, and
a remainder including Fe and impurities,
wherein a metallographic structure at a thickness ¼ portion includes, by unit area %,
ferrite: 10% or more and less than 50%,
granular bainite: 5% or more and less than 50%, and
martensite: 20% or more and less than 60%,
in the metallographic structure at the thickness ¼ portion, a total of upper bainite, lower bainite, residual austenite, and pearlite is 0% or more and less than 15% by unit area %,
at the thickness ¼ portion, a product of an area ratio Vm of the martensite and an average hardness Hv of the martensite is 12,000 to 34,000, and
a tensile strength is 980 MPa or higher,
wherein the thickness ¼ portion is a portion of the steel sheet between a plane at a depth of ⅛ in a sheet thickness t from a first rolled surface and a plane at a depth of ⅜ in the sheet thickness t from the first rolled surface, wherein the sheet thickness t is a distance between first and second rolled surfaces of the steel sheet, and
wherein the area ratio is the area ratio in the metallographic structure at the thickness ¼ portion of the steel sheet, the unit of Vm is area %, and the unit of the average hardness Hv is Vickers hardness.
2. The steel sheet according to claim 1 , comprising, as the chemical composition of the steel sheet, by unit mass %, one or more of
Mo: 0.01% to 1.00%,
Ni: 0.05% to 1.00%, and
Cu: 0.05% to 1.00%.
3. The steel sheet according to claim 1 , comprising, as the chemical composition of the steel sheet, by unit mass %, one or more of
Nb: 0.005% to 0.30%,
Ti: 0.005% to 0.30%, and
V: 0.005% to 0.50%.
4. The steel sheet according to claim 1 , comprising, as the chemical composition of the steel sheet, by unit mass %,
B: 0.0001% to 0.01%.
5. The steel sheet according to claim 1 , comprising, as the chemical composition of the steel sheet, by unit mass %, one or more of
Ca: 0.0005% to 0.04%,
Mg: 0.0005% to 0.04%, and
REM: 0.0005% to 0.04%.
6. The steel sheet according to claim 1 ,
wherein a hot-dip galvanized layer is provided on a surface of the steel sheet.
7. The steel sheet according to claim 1 ,
wherein a galvannealed layer is provided on a surface of the steel sheet.