METHOD FOR RAPIDLY COOLING STEEL STRIP IN CONTINUOUS ANNEALING

PROBLEM TO BE SOLVED: To execute rapid cooling without impairing the material performance of a steel strip by controlling the average cooling speed of the steel strip with gas jet cooling and the average cooling speed of the steel strip with roll cooling so as to satisfy a specific relation between the above both average cooling speeds. SOLUTION: The relation between the average cooling speed CRg ( deg.C/sec) of the steel strip with the gas jet cooling and the average cooling speed CRr ( deg.C/sec) of the steel strip with the roll cooling is to satisfy the formula I and the formula II. In the formulas, V is the passing speed (m/sec) of the steel strip, Lg is the steel strip length (m) from the starting point of the gas jet cooling to the starting point of the roll cooling, Lr is the steel strip length (m) from the starting point of the roll cooling to the stopping point of the roll cooling, Te is the steel strip temp. ( deg.C) at the starting point of the gas jet cooling, Td is the steel strip temp. ( deg.C) at the stopping point of the roll cooling, Tm is the steel strip temp. ( deg.C) at the starting point of the roll cooling, Tc is the allowable temp. ( deg.C), at which the roll cooling of the steel strip can be started, decided with the steel strip dimension and the tension of the cooling roll.