Cold-rolled steel sheet

1. A cold-rolled steel sheet of carbon steel or low-alloy steel comprising C: 0.01 to 0.25% by mass, the main phase of which is ferrite, and is characterized in that the average ferrite crystal grain diameter D (μm) at the depth of ¼ of the sheet thickness from the steel sheet surface satisfies the relations respectively defined by the formulas (5) and (6) given below and the increase rate X (μm/min) in average ferrite crystal grain diameter at 700° C. at the depth of ¼ of the sheet thickness from the steel sheet surface and said average ferrite crystal grain diameter D (μm) satisfy the relation defined by the formula (3) given below:

1.2≦D≦9.3  formula (5)

D≦ 5.0−2.0·Cr+5000/(5+350·C+40·Mn) 2   formula (6)

D·X≦ 0.1  formula (3)

and, further, that, at the depth of ¼ of the sheet thickness from the steel sheet surface, the area percentage of ferrite crystal grains the crystal grain diameter d (μm) of which satisfies the relation defined by the formula (4) given below is at least 80%:

D/ 3 ≦d ≦3 D   formula (4)

wherein C, Cr and Mn represent the contents (in % by mass) of the respective elements in the steel.

2. A cold-rolled steel sheet according to claim 1 , characterized in that it contains, as a second phase other than ferrite, a total of less than 50% of one or more species selected from the group consisting of less than 50% of bainite, less than 30% of pearlite, less than 5% of granular cementite, less than 5% of martensite and less than 3% of retained austenite and has a yield ratio lower than 0.75, % in each occurrence being % by volume.

3. A cold-rolled steel sheet according to claim 1 , characterized in that it contains, as a second phase other than ferrite, 5 to 40% by volume of martensite and has a yield ratio lower than 015.

4. A cold-rolled steel sheet according to claim 1 , characterized in that it contains, as a second phase other than ferrite, 3 to 30% by volume of retained austenite and has a product, TS×El, of tensile strength TS (MPa) and total elongation El (%) of not less than 18000 (MPa·%).

5. A cold-rolled steel sheet according to claim 1 , characterized in that the average crystal grain diameter Ds (μm) at the depth of 1/16 of the sheet thickness from the steel sheet surface and the average crystal grain diameter Dc (μm) at the center of the sheet thickness satisfy the relation Ds≦0.9Dc.

6. A cold-rolled steel sheet according to claim 1 , characterized in that the carbon equivalent Ceq(I) defined by the formula (7) given below is 0.06 to 0.6%:

Ceq(I)=C+Mn/6+Si/24+Cr/5+Mo/4+Ni/40+V/14  formula (7)

wherein the symbols of elements in the above formula represent the contents (in % by mass) of the respective elements in the steel.

7. A cold-rolled steel sheet according to claim 1 , characterized in that the C content is not higher than 0.17% by mass, the carbon equivalent Ceq(II) defined by the formula (8) given below is 0.03 to 0.20% and the base metal resistance indicator Rsp defined by the formula (9) given below is not higher than 45:

Ceq(II)=C+Mn/100+Si/90+Cr/100  formula (8)

Rsp= 13.5×(Si+Al+0.4Mn+0.4Cr)+12.2  formula (9)

wherein the symbols of elements in the above formulas represent the contents (in % by mass) of the respective elements in the steel.

8. A cold-rolled steel sheet characterized in that it comprises a Zn, Al, Zn—Al alloy or Fe—Zn alloy coat layer formed on the surface of a cold-rolled steel sheet according to claim 1 .