Hot-rolled steel sheet and cold-rolled steel sheet and manufacturing method thereof

1. A hot-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 (1) and (2) 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 crystal grain diameter D (μm) satisfy the relation defined by the formula (3) given below:

1.2≦D≦7  formula (1)

D≦ 2.7+5000/(5+350·C+40·Mn) 2   formula (2)

D·X≦ 0.1  formula (3)

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

2. A hot-rolled steel sheet according to claim 1 , characterized in 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 D represents the average ferrite crystal grain diameter (μm) at the depth of ¼ of the sheet thickness from the steel sheet surface.

3. A hot-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 50% 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.

4. A hot-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 0.75.

5. A hot-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·%).

6. A hot-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, the average crystal grain diameter D (μm) at the depth of ¼ 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 relations Ds≦0.75D and D≦0.9Dc.

7. A hot-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 0.60%:

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.

8. A hot-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.

9. A hot-dip plated hot-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 hot-rolled steel sheet according to claim 1 .