| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for producing a high strength cold-rolled steel excellent in ductility and hole expansibility. SOLUTION: A steel blank which contains C, Mn, P, S, N adjusted to the respective suitable contents and contains 0.005-0.20% Nb and 0.2-1.5% Si, 0.2-2.0% Al so as to satisfy Si+1/2(Al)≥0.50% and the balance substantially iron, is subjected to hot-rolling and cold-rolling, wherein the finish rolling completing temperature is higher than the A<SB>3</SB>transformation point and a cooling temperature is 400-650°C. Further, the cold-rolled steel sheet is heated to an annealing-soaking temperature of 800-900°C temperature range at 20°C/s heating speed in≥Ac<SB>1</SB>transformation point, and after keeping for 60-300s at 800-900°C, this steel sheet is slowly cooled at 1-10°C/s cooling speed to the slowly cooling-stop temperature in the temperature zone of 600-700°C. Successively, the steel sheet is rapidly cooled at 1-10°C/s cooling speed to a slowly cooling-stop temperature in the temperature zone of 350-500°C, and after keeping for 30-300s in the temperature zone of 350-500°C, this steel sheet is cooled to apply the annealing treatment. Thus, the high strength cold-rolled steel sheet excellent in the ductility and the hole expansibility, is obtained. The steel blank may contai one or more elements selected among Cr, Ni, Mo, B and one or two elements selected from Ti and V. COPYRIGHT: (C)2006,JPO&NCIPI
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