High-strength cold-rolled steel sheet, high-strength hot-dip galvanized steel sheet and high-strength galvannealed steel sheet
A high-strength cold-rolled steel sheet is provided that has a chemical composition that consists of, by mass %, C: 0.050 to 0.40%, Si: 0.01 to 3.0%, Mn: 1.0 to 5.0%, sol. Al: 0.001 to 1.0%, Ti: 0.005 to 0.20%, B: 0.0005 to 0.010%, P: 0.1% or less, S: 0.01% or less, O: 0.1% or less, N: 0.01% or less, Cr: 0 to 1.0%, Mo: 0 to 1.0%, Ni: 0 to 1.0%, Cu: 0 to 1.0%, Sn: 0 to 0.50%, Nb: 0 to 0.20%, V: 0 to 0.50%, W: 0 to 0.50%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, Bi: 0 to 0.01%, Sb: 0 to 0.10%, Zr: 0 to 0.01%, and REM: 0 to 0.01%, with the balance being Fe and impurities, and that satisfies the formulas [sol. Bs/B≤0.50] and [sol. Bq/B>0.50] (where, B: B amount in the steel; sol. Bs: soluble B amount in a surface layer portion of the steel; and sol. Bq: soluble B amount in the steel interior).
1. A cold-rolled steel sheet, having a chemical composition consisting of, by mass %,
C: 0.050 to 0.40%,
Si: 0.01 to 3.0%,
Mn: 1.0 to 5.0%,
sol. Al: 0.001 to 1.0%,
Ti: 0.005 to 0.20%,
B: 0.0005 to 0.010%,
P: 0.1% or less,
S: 0.01% or less,
O: 0.1% or less,
N: 0.01% or less,
Cr: 0 to 1.0%,
Mo: 0 to 1.0%,
Ni: 0 to 1.0%,
Cu: 0 to 1.0%,
Sn: 0 to 0.50%,
Nb: 0 to 0.20%,
V: 0 to 0.50%,
W: 0 to 0.50%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
Bi: 0 to 0.01%,
Sb: 0 to 0.10%,
Zr: 0 to 0.01%,
REM: 0 to 0.01%, and
the balance: Fe and impurities,
and satisfying formula (i) and formula (ii) below:
sol. Bs/B≤0.50 (i)
sol. Bq/B>0.50 (ii)
where, the meaning of each symbol in the above formulas is as follows:
B: B content (mass %) contained in steel sheet;
sol. Bs: B content (mass %) present as a solid solution from a surface down to a depth of 30 μm of the steel sheet wherein the value of sol. Bs s determined calculating the B amount consumed as a precipitate by measuring the mass of boride by an electrolytic extraction residue method to a depth of 30 μm without grinding the steel sheet surface, and thereafter deducting the calculated B amount from the B content contained in the steel; and
sol. Bq: B content (mass %) present as a solid solution at a position of ¼ thickness of the steel sheet, wherein the value of sol. Bq is determined by calculating the B amount consumed as a precipitate by measuring the mass of boride by an electrolytic extraction residue method by mechanically grinding the steel sheet to the position of ¼ thickness, and thereafter performing electrolytic extraction to a depth of 30 μm, and thereafter deducting the calculated B amount from the B content contained in the steel.
2. The cold-rolled steel sheet according to claim 1 , wherein the chemical composition contains, by mass %, one or more elements selected from:
Cr: 0.001 to 1.0%,
Mo: 0.001 to 1.0%,
Ni: 0.001 to 1.0%,
Cu: 0.001 to 1.0%,
Sn: 0.001 to 0.50%,
Nb: 0.001 to 0.20%,
V: 0.001 to 0.50%,
W: 0.001 to 0.50%,
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%,
BI: 0.0001 to 0.01%,
Sb: 0.0001 to 0.10%,
Zr: 0.0001 to 0.01%, and
REM: 0.0001 to 0.01%.
3. The cold-rolled steel sheet according to claim 1 , wherein:
a steel micro-structure at a position from a surface down to a depth of 30 μm of the steel sheet is, in area %:
polygonal ferrite: 10 to 95%, and
the balance: one or more kinds selected from martensite, bainite and retained austenite,
with a proportion of tempered martensite to the martensite overall being 50% or more; and
a steel micro-structure at a position of ¼ thickness of the steel sheet is, in area %:
polygonal ferrite: 60% or less, and
the balance: one or more kinds selected from martensite, bainite and retained austenite,
with a proportion of tempered martensite to the martensite overall being 50% or more.
4. The cold-rolled steel sheet according to claim 3 , wherein:
a steel micro-structure at a position from the surface down to the depth of 30 μm of the steel sheet is, in area %:
polygonal ferrite: 30 to 95%; and
a steel micro-structure at the position of ¼ thickness of the steel sheet is, in area %,
polygonal ferrite: 10 to 60%.
5. The cold-rolled steel sheet according to claim 4 , wherein:
a tensile strength is 980 MPa or more, and a ratio R/t between a sheet thickness t and a minimum bending radius R is 2.5 or less.
6. The cold-rolled steel sheet according to claim 3 , wherein:
a steel micro-structure at a position from the surface down to the depth of 30 μm of the steel sheet is, in area %:
polygonal ferrite: 10 to 80%, and
a steel micro-structure at the position of ¼ thickness of the steel sheet is, in area %:
polygonal ferrite: 20% or less,
martensite: 50% or more,
bainite: 40% or less, and
retained austenite: 20% or less.
7. The high strength cold-rolled steel sheet according to claim 6 , wherein:
a tensile strength is 1180 MPa or more, and a ratio R/t between a sheet thickness t and a minimum bending radius R is 3.5 or less.
8. A high-strength hot-dip galvanized steel sheet that has a hot-dip galvanized layer on a surface of the cold-rolled steel sheet according to claim 1 .
9. A high-strength galvannealed steel sheet that has a galvannealed layer on a surface of the cold-rolled steel sheet according to claim 1 .