Method of producing cold-rolled steel sheet

1. A method of producing a cold-rolled steel sheet comprising:

(a) producing a hot-rolled steel sheet comprising:

subjecting a carbon steel or low-alloy steel slab, comprising C: 0.01 to 0.25% by mass, to multi-pass hot rolling characterized in that the final rolling pass is finished at a temperature not lower than the Ar 3 point and not lower than 780° C.;

cooling the rolled sheet to 720° C. or below within 0.4 second at a cooling rate of not lower than 400° C./second; and

maintaining the rolled sheet in a temperature range of 600 to 720° C. for at least 2 seconds, to yield:

a hot-rolled steel sheet, the main phase of which is ferrite, 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;

(b) pickling the hot-rolled steel sheet;

(c) cold-rolling at a reduction of 40 to 90%; and

(d) heat-treating at a temperature of not higher than 900° C., to yield:

a cold-rolled steel sheet, the main phase of which is ferrite, 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 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 method of producing a hot-dip-plated cold-rolled sheet comprising:

(a) producing a hot-rolled steel sheet comprising:

subjecting a carbon steel or low-alloy steel slab, comprising C: 0.01 to 0.25% by mass, to multi-pass hot rolling characterized in that the final rolling pass is finished at a temperature not lower than the Ar 3 point and not lower than 780° C.;

cooling the rolled sheet to 720° C. or below within 0.4 second at a cooling rate of not lower than 400° C./second; and

maintaining the rolled sheet in a temperature range of 600 to 720° C. for at least 2 seconds, to yield:

a hot-rolled steel sheet, the main phase of which is ferrite, 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;

(b) pickling the hot-rolled steel sheet;

(c) cold-rolling at a reduction of 40 to 90%; and

(d) hot-dip plating in a continuous hot-dip plating line, to yield:

a hot-dip-plated cold-rolled sheet, the main phase of which is ferrite, 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 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.