FUNDAMENTAL PHENOMENA GOVERNING HEAT-TRANSFER DURING ROLLING

To quantify the effect of roll chilling on the thermal history of a sl ab during hot rolling, tests were conducted at the Canada Center for M ineral and Energy Technology (CANMET) and at the University of British Columbia (UBC). In these tests. the surface and the interior temperat ures of specimens were recorded during rolling using a data acquisitio n system. The corresponding heat-transfer coefficients in the roll bit e were back-calculated by a trial-and-error method using a heat-transf er model. The heat-transfer coefficient was found to increase along th e arc of contact and reach a maximum, followed by a decrease, until th e exit of the roll bite. Its value was influenced by rolling parameter s, such as percent reduction, rolling speed, rolling temperature, mate rial type, etc. It was shown that the heat-transfer coefficient in the roll gap was strongly dependent on the roll pressure, and the effect of different variables on the interfacial heat-transfer coefficient ca n be related to their influence on pressure. At low mean roll pressure , such as in the case of rolling plain carbon steels at elevated tempe rature, the maximum heat-transfer coefficient in the roll bite was in the 25 to 35 kW/M2-degrees-C range. As the roll pressure increased wit h lower rolling temperature and higher deformation resistance of stain less steel and microalloyed grades, the maximum heat-transfer coeffici ent reached a value of 620 kW/m2-degrees-C. Obviously, the high pressu re improved the contact between the roll and the slab surface, thereby reducing the resistance to heat flow. The mean roll-gap heat-transfer coefficient at the interface was shown to be linearly related to mean roll pressure. This finding is important because it permitted a deter mination of heat-transfer coefficients applicable to industrial rollin g from pilot mill data. Thus, the thermal history of a slab during rou gh rolling was computed using a model in which the mean heat-transfer coefficient between the roll and the slab was determined from an estim ate of the rolling load. It was found that the heat loss of a slab to the roll was 33 pct of the total, which emphasizes the importance of a ccurately characterizing the heat-transfer coefficient in the roll bit e during hot rolling.