| 摘要 |
Disclosed is an ultra high strength cold rolled steel sheet having high flatness and a martensite single phase structure, which is obtained by suppressing deterioration in steel sheet shape during the hardening process in continuous annealing. Specifically disclosed is a method for producing an ultra high strength cold rolled steel sheet that has a tensile strength of 980 MPa or more by continuously annealing a steel sheet after cold rolling, said steel sheet having a component composition that contains, in mass%, 0.05-0.40% of C, 2.0% or less of Si, 0.05% or less of P, 0.02% or less of S, 0.01-0.05% of Al, less than 0.005% of N and 1.0-3.0% of Mn, with the balance made up of Fe and unavoidable impurities. In the continuous annealing, the steel sheet is primarily cooled from a soaking temperature that is not less than the Ac3 transformation point to the temperature range from the Ms point to the Ms point + 200°C at an average cooling rate of 20°C/second or more, and after maintaining the resulting steel sheet within the above-described temperature range for 0.1-60 seconds, the steel sheet is secondarily cooled to 100°C or less at an average cooling rate of 100°C/second or more, thereby obtaining an ultra high strength cold rolled steel sheet having a flatness of 10 mm or less. |
