Optimization of working roll cooling in hot rolling

Abstract

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.

The cooling of working rolls is an important process in the hot rolling technology. The optimal cooling of rolls should be designed with respect to two aspects. The first is the wearing of a roll where high temperature decreases the durability of the surface layer. The second aspect is a thermal deformation of a roll. This is critical for the shape and tolerance of flat products. Cooling at rolling mill should be designed with consideration to both aspects. Finding optimum pressure and flow rate is a difficult task. In regards to water quantity, the experience shows that the phrase “more is better” is not valid here. In other word – an increase in the amount of water can even cause a decrease in cooling intensity. Water nozzles are typically used in this case. There are many of factors which can influence the efficiency of the nozzle cooling system: Type of a nozzle, geometrical configuration (nozzle pitch, distance from the roll, orientation, number of manifolds), coolant pressure and temperature. Cooling intensity is mostly specified through Heat transfer coefficient (HTC) or heat flux (HF) distribution. Coolant flow on the rotating roll surface makes the problem complex. Surface temperature of the cylinder plays an important role in the heat transfer mechanism, especially for higher temperatures where boiling must be considered. No analytical or numerical solution of heat transfer and fluid flow for this case is known. The task can be successfully solved experimentally. An experimental bench and methodology of realistic boundary conditions determination was developed in the Heat Transfer and Fluid Flow Laboratory (HEATLAB). The strategy of optimization is based on two steps. First is investigation of present situation of work roll cooling system and second is design of a new system. Criterion of optimization is saving of cooling water with remaining or increasing of cooling intensity. Comparison of the original design and new design was done numerically, using special software and experimentally by temperature measurement of working roll after specified rolling campaign. Optimized cooling system was applied on hot flat rolling mill in voestalpine Stahl GmbH.