Towards a better width control in cold rolling of flat steel strips

摘要

Strip width decrease, also called 'width necking', may reach 15 to 20 millimetres all along the steel production cold route. First reduction rolling on tandem cold mills (fig. 1) is considered one of the most critical process operations for that phenomenon (until 8 mm for the first reduction rolling on DWI, packaging steel grade used for food cans). Continuous annealing and galvanising furnaces, temper mills and tension levellers are also known to have an effect on strip width variations. These width contractions are scattered and sometimes badly estimated; consequently they lead to under-width coils that do not satisfy customer's width requirements and coils must be diverted or rejected. To compensate for these width contractions, cold plants use to order from hot strip mills coils with significant over-widths; these over-widths are often overestimated due to width variation scatter. The hot finishing strip mill, with its own width scatter, tends to increase also the width ordered by cold mills. All these over-widths and the associated side trimming operation produce an important and non'optimised yield all along the route. Thus, better mastering this yield requires clarifying the width variation phenomena, especially during cold rolling operation since it is a major contributor to width change. Spray cooling of moving surfaces is a difficult heat transfer task which can only be precisely solved using experimental techniques. A large number of spray parameters influence heat transfer intensity. Experimental technique providing boundary conditions for numerical models, mentioned by Horsky (8) is described. Heat transfer in the sprayed area is seriously dependent on the surface temperature. The Leidenfrost temperature for spray cooling is typically higher than 500 deg C and for intensive sprays exceeds 1100 deg C. Even in the temperature interval below the Leidenfrost temperature there was found to be a serious dependence of the heat transfer coefficient on surface temperature. The experiments proved that decreasing values of heat transfer coefficient occurred with increasing temperature of the sprayed moving surface. The study of the velocity influence of the sprayed surface, proved a decrease in heat transfer intensity on a rotating cylinder proportional to the speed of rotation. Inclination of coolant jets against and along the direction of movement proved the surprising fact that higher HTC values are obtained when the jets follow the sprayed cylindrical surface. Any superposition cannot be used and only the experiment with experimental investigation into the mutual interference of several sprays can be used.