| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for cooling a superhigh strength cold rolled steel sheet solving the defect of the strip shape generated in continuous annealing for a superhigh strength steel sheet in which the total volume ratio of both phases of martensite and bainite is controlled to >=70% and having >=100 N/mm2 tensile strength. SOLUTION: In continuous annealing, after heating at Ac1 to 900 deg.C, water cooling or gas-water cooling is executed to the temp. range of (Ms+10 deg.C) to (Ms+100 deg.C) at the average cooling rate of 30 to 500 deg.C/s, next, gas cooling is executed to a temp. range of (Ms-30 deg.C) to (Ms-100 deg.C), and, successively, water cooling or gas-water cooling is executed to <=400 deg.C at the average cooling rate of 30 to 1,000 deg.C/s. Preferably, the steel sheet in the process of the gas cooling is brought into contact with a pair or more rolls held to the temp. of (Ms+10 deg.C) to (Ms+100 deg.C), where Ms denotes the transformation starting temp. of martensite.
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