High-strength cold-rolled steel sheet excellent in workability and method for manufacturing the same
In a cold-rolled steel sheet in relation with the present invention, metallurgical structure of the steel sheet is made a mixture structure including bainite, residual austenite and tempered martensite, particularly, when the metallurgical structure is observed with a scanning electron microscope, bainite is constituted of composite structure of high temperature range forming bainite with 1 μm or above average distance between neighboring residual austenite and/or carbide and low temperature range forming bainite with below 1 μm average distance between neighboring residual austenite and/or carbide, and when the area ratio of the high temperature range forming bainite with respect to total metallurgical structure is made a and the total area ratio of the low temperature range forming bainite and the tempered martensite with respect to the total metallurgical structure is made b, a: 20-80%, b: 20-80%, and a+b: 70% or above are satisfied. The cold-rolled steel sheet in relation with the present invention is improved in all of the elongation (EL), stretch flange formability (A), and bending properties (R) with a good balance, is excellent in the overall workability, and has 980 MPa or above tensile strength.
1. A cold-rolled steel sheet containing in mass %:
C: 0.10-0.3%,
Si: 1.0-3.0%,
Mn: 1.5-3%,
Al: 0.005-0.2%,
and satisfying:
P: 0.1% or below,
S: 0.05% or below
the balance comprising iron with inevitable impurities, wherein;
the metallurgical structure of the steel sheet comprises bainite, residual austenite and tempered martensite,
(1) when the metallurgical structure is observed with a scanning electron microscope;
bainite is constituted of complex structure of high temperature range forming bainite with 1 μm or above average distance between neighboring residual austenite and/or carbide and low temperature range forming bainite with below 1 μm average distance between neighboring residual austenite and/or carbide, and
when the area ratio of the high temperature range (400° C. ≦T1(° C.) ≦540° C.) forming bainite with respect to total metallurgical structure is made a and the total area ratio of the low temperature range (200° C.≦T2(° C.)≦400° C.) forming bainite and the tempered martensite with respect to the total metallurgical structure is made b,
a: 20-80%, b: 20-80%, and a+b: 70% or above are satisfied, and
(2) the volumetric ratio of residual austenite measured by a saturation magnetization method is 3% or above with respect to the total metallurgical structure.
2. The cold-rolled steel sheet according to claim 1 , wherein, when a MA mixture phase in which quenched martensite and residual austenite are compounded is present in the metallurgical structure, a number ratio of the MA mixture phase satisfying that an equivalent circle diameter d at an observed cross-section exceeds 3 μm with respect to the number of the total MA mixture phase is below 15%.
3. The cold-rolled steel sheet according to claim 1 , wherein an average equivalent circle diameter D of former austenite grains is 20 μm or below.
4. The cold-rolled steel sheet according to claim 1 further containing as other elements at least either one of:
Cr: 1% or below, and
Mo: 1% or below.
5. The cold-rolled steel sheet according to claim 1 further containing as other elements at least one kind selected from a group consisting of:
Ti: 0.15% or below,
Nb: 0.15% or below, and
V: 0.15% or below.
6. The cold-rolled steel sheet according to claim 1 further containing as other elements at least either one of:
Cu: 1% or below, and
Ni: 1% or below.
7. The cold-rolled steel sheet according to claim 1 further containing as other element:
B: 0.005% or below.
8. The cold-rolled steel sheet according to claim 1 further containing as other elements at least one kind selected from a group consisting of:
Ca: 0.01% or below,
Mg: 0.01% or below, and
rare earth metal: 0.01% or below.
9. The cold-rolled steel sheet according to claim 1 comprising a hot-dip galvanizing layer or a hot-dip galvannealing layer on a surface of the cold-rolled steel sheet.
10. A cold-rolled steel sheet containing in mass %:
C: 0.10-0.3%,
Si: 1.0-3.0%,
Mn: 2.0-3%,
Al: 0.005-0.2%,
and satisfying:
P: 0.1% or below,
S: 0.05% or below
the balance comprising iron with inevitable impurities, wherein;
the steel sheet has an Ac 3 point from 852° C. to 918° C.;
the metallurgical structure of the steel sheet comprises bainite, residual austenite and tempered martensite,
(1) when the metallurgical structure is observed with a scanning electron microscope;
bainite is constituted of complex structure of high temperature range forming bainite with 1 μm or above average distance between neighboring residual austenite and/or carbide and low temperature range forming bainite with below 1 μm average distance between neighboring residual austenite and/or carbide, and
when the area ratio of the high temperature range (400° C. ≦T1(° C.) ≦540° C.) forming bainite with respect to total metallurgical structure is made a and the total area ratio of the low temperature range (200° C.≦T2(° C.)≦400° C.) forming bainite and the tempered martensite with respect to the total metallurgical structure is made b,
a: 20-80%, b: 20-80%, and a+b: 70% or above are satisfied, and
(2) the volumetric ratio of residual austenite measured by a saturation magnetization method is 3% or above with respect to the total metallurgical structure.
11. A cold-rolled steel sheet containing in mass %:
C: 0.10-0.3%,
Si: 2.0-3.0%,
Mn: 1.5-3%,
Al: 0.005-0.2%,
and satisfying:
P: 0.1% or below,
S: 0.05% or below
the balance comprising iron with inevitable impurities, wherein;
the steel sheet has an Ac 3 point from 852° C. to 918° C.;
the metallurgical structure of the steel sheet comprises bainite, residual austenite and tempered martensite,
(1) when the metallurgical structure is observed with a scanning electron microscope;
bainite is constituted of complex structure of high temperature range forming bainite with 1 μm or above average distance between neighboring residual austenite and/or carbide and low temperature range forming bainite with below 1 μm average distance between neighboring residual austenite and/or carbide, and
when the area ratio of the high temperature range (400° C. ≦T1(° C.) ≦540° C.) forming bainite with respect to total metallurgical structure is made a and the total area ratio of the low temperature range (200° C.≦T2(° C.)≦400° C.) forming bainite and the tempered martensite with respect to the total metallurgical structure is made b,
a: 20-80%, b: 20-80%, and a+b: 70% or above are satisfied, and
(2) the volumetric ratio of residual austenite measured by a saturation magnetization method is 3% or above with respect to the total metallurgical structure.
12. The cold-rolled steel sheet according to claim 10 , wherein the cold rolled steel sheet contains an Si content in the range of 2.0-3 mass %.
13. The cold-rolled steel sheet according to claim 1 , wherein the steel sheet has an Ac 3 point from 860° C. to 918° C.
14. The cold-rolled steel sheet according to claim 1 , wherein the cold-rolled steel sheet contains an Si content in the range of 2.0-3 mass %.
15. The cold-rolled steel sheet according to claim 13 , wherein the cold rolled steel sheet contains an Mn content in the range of 2.0-3 mass %.
16. The cold-rolled steel sheet according to claim 13 , wherein the cold-rolled steel sheet contains an Si content in the range of 2.0-3 mass %.
17. The cold-rolled steel sheet according to claim 1 , wherein the cold-rolled steel sheet. contains an Mn content in the range of 2.0-3% and a Si content in the range of 2.0-3 mass %.
18. The cold-rolled steel sheet according to claim 13 , wherein the cold-rolled steel sheet. contains an Mn content in the range of 2.0-3 mass % and an Si content in the range of 2.0-3 mass %.