| 摘要 |
PURPOSE:To improve the workability of a coll rolled steel sheet having a specified composition in case where a thin slab is used as starting material to manufacture the steel sheet, by specifying the relation between the cooling rate of the slab and the hot rolling reduction ratio when the slab is directly hot rolled. CONSTITUTION:A thin steel slab consisting of, by weight, 0.0010-0.070% C, 0.05-0.50% Mn, <=0.015% S, 0.005-0.090% Al and the balance Fe or further contg. one or more among <=50ppm Ca, REM satisfying REM/S<=2.0, <=50ppm Mg, Zr satisfying Zr/S<=10 and B satisfying B/N=0.3-1.5 is directly hot rolled. At this time, hot rolling is carried out so that the average cooling rate V( deg.C/min) of the slab in the range of 1,350-1,000 deg.C and the hot rolling reduction ratio R (R>=1.0) satisfy relation represented by the formula. The hot rolled slab is then cold rolled and annealed.
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