High-strength steel sheet and high-strength galvanized steel sheet
A high-strength steel sheet includes: a specific chemical composition; and a microstructure represented by, in a ⅛ thickness to ⅜ thickness range with ¼ thickness of a sheet thickness from a surface being a center, in volume fraction, ferrite: 85% or less, bainite: 3% or more and 95% or less, tempered martensite: 1% or more and 80% or less, retained austenite: 1% or more and 25% or less, pearlite and coarse cementite: 5% or less in total, and fresh martensite: 5% or less, in which the solid-solution carbon content in the retained austenite is 0.70 to 1.30 mass %, and to all grain boundaries of retained austenite grains having an aspect ratio of 2.50 or less and a circle-equivalent diameter of 0.80 μm or more, the proportion of interfaces with the tempered martensite or the fresh martensite is 75% or less.
1. A high-strength steel sheet, comprising:
a chemical composition represented by, in mass %,
C: 0.075 to 0.400%,
Si: 0.01 to 2.50%,
Mn: 0.50 to 3.50%,
P: 0.1000% or less,
S: 0.0100% or less,
Al: 2.000% or less,
N: 0.0100% or less,
O: 0.0100% or less,
Ti: 0.000 to 0.200%,
Nb: 0.000 to 0.100%,
V: 0.000 to 0.500%,
Cr: 0.00 to 2.00%,
Ni: 0.00 to 2.00%,
Cu: 0.00 to 2.00%,
Mo: 0.00 to 1.00%,
B: 0.0000 to 0.0100%,
W: 0.00 to 2.00%,
one type or two types or more selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM: 0.0000 to 0.0100% in total,
a balance: Fe and impurities, and
a parameter Q0 expressed by (Expression 1): 0.35 or more; and
a microstructure represented by, in a ⅛ thickness to ⅜ thickness range with ¼ thickness of a sheet thickness from a surface being a middle, in volume fraction,
ferrite: 85% or less,
bainite: 3% or more and 95% or less,
tempered martensite: 1% or more and 80% or less,
retained austenite: 1% or more and 25% or less,
pearlite and coarse cementite: 5% or less in total, and
fresh martensite: 5% or less, wherein
a solid-solution carbon content in the retained austenite is 0.70 to 1.30 mass %, and
in all grain boundaries of retained austenite grains having an aspect ratio of 2.50 or less and a circle-equivalent diameter of 0.80 μm or more, a proportion of interfaces with the tempered martensite or the fresh martensite is 75% or less,
Q0=Si+0.1Mn+0.6Al (Expression 1)
wherein Si, Mn, and Al are set to the contents of the respective elements in mass %.
2. The high-strength steel sheet according to claim 1 , further containing:
in mass %,
one type or two types or more selected from the group consisting of
Ti: 0.001 to 0.200%,
Nb: 0.001 to 0.100%, and
V: 0.001 to 0.500%.
3. The high-strength steel sheet according to claim 1 , further containing:
in mass %,
one type or two types or more selected from the group consisting of
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Cu: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
B: 0.0001 to 0.0100%, and
W: 0.01 to 2.00%.
4. The high-strength steel sheet according to claim 1 , further containing:
in mass %,
0.0001 to 0.0100% in total of one type or two types or more selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM.
5. The high-strength steel sheet according to claim 1 , wherein
a density of the retained austenite grains having an aspect ratio of 2.50 or less and a circle-equivalent diameter of 0.80 μm or more is 5.0×10 10 piece/m 2 or less.
6. A high-strength galvanized steel sheet, comprising:
a galvanized layer formed on a surface of the high-strength steel sheet according to claim 1 .
7. The high-strength galvanized steel sheet according to claim 6 , wherein
an Fe content in the galvanized layer is 3.0 mass % or less.
8. The high-strength galvanized steel sheet according to claim 6 , wherein
an Fe content in the galvanized layer is 7.0 mass % or more and 13.0 mass % or less.