Steel sheet, hot-dip galvanized steel sheet and galvannealed steel sheet
A steel sheet having a chemical composition of the base metal including, in mass %, C: 0.17 to 0.40%, Si: 0.10 to 2.50%, Mn: 1.00 to 10.00%, P: 0.001 to 0.03%, S: 0.0001 to 0.02%, Al: 0.001 to 2.50%, N: 0.0001 to 0.010%, O: 0.0001 to 0.010%, Ti: 0 to 0.10%, Nb: 0 to 0.10%, V: 0 to 0.10%, B: 0 to 0.010%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 2.00%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, REM: 0 to 0.50%, the balance: Fe and impurities, wherein the steel sheet has an internal oxidized layer in which at least one part of a crystal grain boundary is covered by oxides, and in which a grain boundary coverage ratio of oxides is 60% or more in a region from the surface of the base metal to a depth of 5.0 μm.
1. A steel sheet in which a chemical composition of a base metal comprises, in mass %,
C: 0.17 to 0.40%,
Si: 0.10 to 2.50%,
Mn: 1.00 to 10.00%,
P: 0.001 to 0.03%,
S: 0.0001 to 0.02%,
Al: 0.001 to 2.50%,
N: 0.0001 to 0.010%,
O: 0.0001 to 0.010%,
Ti: 0 to 0.10%,
Nb: 0 to 0.10%,
V: 0 to 0.10%,
B: 0 to 0.010%,
Cr: 0 to 2.00%,
Ni: 0 to 2.00%,
Cu: 0 to 2.00%,
Mo: 0 to 2.00%,
Ca: 0 to 0.50%,
Mg: 0 to 0.50%,
REM: 0 to 0.50%, and
the balance: Fe and impurities,
wherein the steel sheet has, from a surface of the base metal to a depth of 5.0 μm or more, an internal oxidized layer in which at least one part of a crystal grain boundary is covered with oxides, and wherein:
in a region from the surface of the base metal to a depth of 5.0 μm, a grain boundary coverage ratio of the oxides is 60% or more.
2. The steel sheet according to claim 1 , comprising:
a decarburization layer from the surface of the base metal to a depth of 50 μm or more.
3. A hot-dip galvanized steel sheet, comprising:
a hot-dip galvanized layer on a surface of the steel sheet according to claim 2 .
4. The hot-dip galvanized steel sheet according to claim 3 , wherein:
a coating mass of the hot-dip galvanized layer is not more than 70 g/m 2 .
5. A galvannealed steel sheet, comprising:
a galvannealed layer on a surface of the steel sheet according to claim 2 .
6. The galvannealed steel sheet according to claim 5 , wherein:
a coating mass of the galvannealed layer is not more than 70 g/m 2 .
7. The galvannealed steel sheet according to claim 5 , wherein:
the galvannealed layer contains, in mass %, Fe: 7.0 to 15.0%.
8. The steel sheet according to claim 1 , comprising:
a nickel electroplating layer on the surface of the base metal.
9. A hot-dip galvanized steel sheet, comprising:
a hot-dip galvanized layer on a surface of the steel sheet according to claim 8 .
10. The hot-dip galvanized steel sheet according to claim 9 , wherein:
a coating mass of the hot-dip galvanized layer is not more than 70 g/m 2 .
11. A galvannealed steel sheet, comprising:
a galvannealed layer on a surface of the steel sheet according to claim 8 .
12. The galvannealed steel sheet according to claim 11 , wherein:
a coating mass of the galvannealed layer is not more than 70 g/m 2 .
13. The galvannealed steel sheet according to claim 11 , wherein:
the galvannealed layer contains, in mass %, Fe: 7.0 to 15.0%.
14. The steel sheet according to claim 1 , wherein the steel sheet has a tensile strength of 980 MPa or more.
15. A hot-dip galvanized steel sheet, comprising:
a hot-dip galvanized layer on a surface of the steel sheet according to claim 1 .
16. The hot-dip galvanized steel sheet according to claim 15 , wherein:
a coating mass of the hot-dip galvanized layer is not more than 70 g/m 2 .
17. A galvannealed steel sheet, comprising:
a galvannealed layer on a surface of the steel sheet according to claim 1 .
18. The galvannealed steel sheet according to claim 17 , wherein:
a coating mass of the galvannealed layer is not more than 70 g/m 2 .
19. The galvannealed steel sheet according to claim 17 , wherein:
the galvannealed layer contains, in mass %, Fe: 7.0 to 15.0%.