Influence of Cooling Uniformity at Runout Table on Strip Flatness

In a hot strip mill, edge wave and center buckle are sometimes observed in rolling and in water cooling. The mechanism of flatness deterioration due to water cooling has not been made clear. The influence of cooling uniformity at a runout table on strip flatness was experimentally and numerically analyzed. The facial temperature distributions of hot strips at coiling were measured using an infrared camera. The results showed that the temperature around the widthwise center of the strip is lower than that at other parts except for the edges due to non-uniform water cooling. The coiling temperature deviation increases in proportion to the drop in temperature during water cooling. The shape of strip becomes worse, where the temperature varies periodically in the longitudinal direction. A theoretical model was developed to simulate the strip temperature, phase transformation, thermal stress, and flatness. The distribution of heat transfer coefficients in the strip width direction is given so that the calculated coiling temperature distribution agrees with the measured distribution. This work clarified the following points : Non-uniform cooling in the strip width direction causes temperature deviation, and consequently flatness becomes poor. The temperature deviation must be minimized to produce hot strips with good flatness. In the case of a low-carbon steel 3.2 mm thick and 1000 mm in width, when the coiling temperature deviation in the strip width direction is under 20°C, the strip flatness is good.