Cold-rolled steel sheet and hot-dip galvanized cold-rolled steel sheet
A cold-rolled steel sheet is provided that has a tensile strength of 980 MPa or more, and has a prescribed chemical composition. The microstructure is composed of, in area %, ferrite: 1 to 29%, retained austenite: 5 to 20%, martensite: less than 10%, pearlite: less than 5%, and the balance: bainite and/or tempered martensite. The total sum of the lengths of phase boundaries where ferrite comes in contact with martensite or retained austenite having a circle-equivalent radius of 1 μm or more is 100 μm or less per 1000 μm 2 . The cold-rolled steel sheet is excellent in workability and low-temperature toughness, and in particular is excellent in low-temperature toughness after introduction of plastic strain.
1. A cold-rolled steel sheet having a tensile strength of 980 MPa or more, and comprising a chemical composition consisting of, in mass %:
C: 0.10 to 0.30%,
Si: 0.50 to 2.50%,
Mn: 1.50 to 3.50%,
Al: 0.001 to 1.00%,
P: 0.05% or less,
S: 0.01% or less,
N: 0.01% or less,
O: 0.01% or less,
Cr: 0 to 1.00%,
Mo: 0 to 1.00%,
Sn: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
B: 0 to 0.005%,
Ti: 0 to 0.30%,
V: 0 to 0.50%,
Nb: 0 to 0.10%,
W: 0 to 0.50%,
Ca: 0 to 0.010%,
Mg: 0 to 0.010%,
Sb: 0 to 0.200%,
Zr: 0 to 0.010%,
Bi: 0 to 0.010%,
REM: 0 to 0.100%, and
the balance: Fe and impurities,
wherein:
a microstructure consists of, in area %:
ferrite: 1 to 29%,
retained austenite: 5 to 20%,
martensite: less than 10%,
pearlite: less than 5%, and
the balance: bainite and/or tempered martensite; and
a total sum of lengths of phase boundaries where ferrite comes in contact with martensite or retained austenite, said martensite or retained austenite having a circle-equivalent radius of 1 μm or more, is 100 μm or less per 1000 μm 2 .
2. The cold-rolled steel sheet according to claim 1 , wherein:
a thickness of the steel sheet is in a range of 0.5 to 3.2 mm.
3. A hot-dip galvanized cold-rolled steel sheet, comprising:
a hot-dip galvanized layer on a surface of the cold-rolled steel sheet according to claim 2 .
4. A hot-dip galvanized cold-rolled steel sheet, comprising:
a galvannealed layer on a surface of the cold-rolled steel sheet according to claim 2 .
5. A hot-dip galvanized cold-rolled steel sheet, comprising:
a hot-dip galvanized layer on a surface of the cold-rolled steel sheet according to claim 1 .
6. A hot-dip galvanized cold-rolled steel sheet, comprising:
a galvannealed layer on a surface of the cold-rolled steel sheet according to claim 1 .
7. A cold-rolled steel sheet having a tensile strength of 980 MPa or more, and comprising a chemical composition comprising, in mass %:
C: 0.10 to 0.30%,
Si: 0.50 to 2.50%,
Mn: 1.50 to 3.50%,
Al: 0.001 to 1.00%,
P: 0.05% or less,
S: 0.01% or less,
N: 0.01% or less,
O: 0.01% or less,
Cr: 0 to 1.00%,
Mo: 0 to 1.00%,
Sn: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
B: 0 to 0.005%,
Ti: 0 to 0.30%,
V: 0 to 0.50%,
Nb: 0 to 0.10%,
W: 0 to 0.50%,
Ca: 0 to 0.010%,
Mg: 0 to 0.010%,
Sb: 0 to 0.200%,
Zr: 0 to 0.010%,
Bi: 0 to 0.010%,
REM: 0 to 0.100%, and
the balance comprising: Fe and impurities,
wherein:
a microstructure comprising, in area %:
ferrite: 1 to 29%,
retained austenite: 5 to 20%,
martensite: less than 10%,
pearlite: less than 5%, and
the balance: bainite and/or tempered martensite; and
a total sum of lengths of phase boundaries where ferrite comes in contact with martensite or retained austenite, said martensite or retained austenite having a circle-equivalent radius of 1 μm or more, is 100 μm or less per 1000 μm 2 .