Patent ID: 8257517

Claim:
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.