Bake-hardenable high-strength cold-rolled steel sheet and method of manufacturing the same

1. A bake-hardenable high-strength cold-rolled steel sheet having excellent bake hardenability, cold aging resistance, and deep-drawability, which is represented by an average r value of not less than 1.4, and reduced planar anisotropy represented by |Δr|≦0.5, with chemical components consisting of, in % by mass:

C: 0.0016% to 0.0040%,

Si: 0.005% to 0.05%,

Mn: 0.1% to 0.45%,

P: 0.01% to 0.07%,

S: 0.001% to 0.008%,

Al: 0.01% to 0.08%,

N: 0.0010% to 0.0050%,

Nb: 0.002% to 0.020%,

Mo: 0.005% to 0.050%,

Cu: 0% to 1.00%,

Ni: 0% to 1.00%,

Cr: 0% to 1.00%,

Sn: 0% to 0.100%,

V: 0% to 0.50%,

W: 0% to 1.00%,

Ca: 0% to 0.0100%,

Mg: 0% to 0.0100%,

Zr: 0% to 0.0500%, and

REM: 0% to 0.0500%,

a value of [Mn %]/[P %] being in a range of 1.6 to 45, where [Mn %] is an amount of Mn and [P %] is an amount of P,

an amount of C in solid solution obtained from [C %]−(12/93)×[Nb %] being in a range of 0.0005% to 0.0025%, where [C %] is an amount of C and [Nb %] is an amount of Nb,

with a balance consisting of Fe and inevitable impurities, wherein

the bake-hardenable high-strength cold-rolled steel sheet satisfies following Equation (1), where X(222), X(110), and X(200) represent ratios of integrated intensity of X-ray diffraction of {222} plane, {110} plane, and {200} plane, respectively, being parallel to a plane located at a depth of ¼ plate thickness measured from a surface of the steel sheet, and

the bake-hardenable high-strength cold-rolled steel sheet has tensile strength in a range of 300 MPa to 450 MPa,

14.9≧ X (222)/{ X (110)+ X (200)}≧3.0Equation  (1).

2. The bake-hardenable high-strength cold-rolled steel sheet as claimed in claim 1 , wherein the chemical components include, by mass, at least one chemical component selected from:

Cu: 0.01% to 1.00%,

Ni: 0.01% to 1.00%,

Cr: 0.01% to 1.00%,

Sn: 0.001% to 0.100%,

V: 0.02% to 0.50%,

W: 0.05% to 1.00%,

Ca: 0.0005% to 0.0100%,

Mg: 0.0005% to 0.0100%,

Zr: 0.0010% to 0.0500%, and

REM: 0.0010% to 0.0500%.

3. The bake-hardenable high-strength cold-rolled steel sheet as claimed in claim 1 or 2 , wherein a coated layer is provided on at least one surface.

4. A method of manufacturing a bake-hardenable high-strength cold-rolled steel sheet, including:

hot rolling a slab containing chemical components as claimed in claim 1 or claim 2 at a heating temperature of not less than 1200° C. and at a finishing temperature of not less than 900° C. to obtain a hot rolled steel sheet;

coiling the hot rolled steel sheet at a temperature in the range of 700° C. to 800° C.;

cooling the hot rolled steel sheet that has been coiled at a cooling rate of not more than 0.01° C./sec so as to decrease the temperature at least from 400° C. to 250° C.;

performing cold rolling under a condition that a cold rolling reduction ratio CR % at the time of cold rolling after acid pickling satisfies the following Equations (2) and (3), where [Mn %] is an amount of Mn, [P %] is an amount of P, and [Mo %] is an amount of Mo;

performing continuous annealing in a temperature range of 770° C. to 820° C.; and

performing temper rolling in a rolling reduction ratio of 1.0% to 1.5%,

CR %≧75−5×([Mn %]+8[P %]+12[Mo %])  Equation (2)

CR %≦95−10×([Mn %]+8[P %]+12[Mo %])  Equation(3).

5. The method of manufacturing a bake-hardenable high-strength cold-rolled steel sheet as claimed in claim 4 , further including providing a coated layer on at least one surface before performing the temper rolling.

6. The bake-hardenable high-strength cold-rolled steel sheet as claimed in claim 1 , wherein the amount of Mn is not more than 0.40% by mass.