Research And Application Of Coating Weight Control System For Hot-Dip Galvanizing Line

Cold-rolled galvanized strip has good mechanical and anticorrosion properties, is widely used in automotive, appliance, building and other industries. Zinc coating weight is a key performance indicator of cold rolled galvanized strip. Coating weight too much will affect the spot welding,stickiness properties and also cause the zinc raw material waste, coating weight deficiency will affect anticorrosion property and can not meet the requirements of customers. Zinc coating weight accuracy is directly related to cold-rolled galvanized strip quality and production cost. Therefore, how to improve the zinc coating weight accuracy is an important study to hot-dip galvanizing production enterprises and metallurgical equipment suppliers.AnSteel NO.5hot dip galvanizing line put into operation in2008, using air knife of German FOEN company for zinc coating weight control, coating weight control is open-loop system, the operator manually adjust the air knife pressure and air knife spacing parameters for coating weight control according to the coating weight actual value. Manual mode completely relies on the operator’s experiences and the coating weight fluctuation can’t be duly controlled. Air knife is the core equipment in hot dip galvanizing line coating weight control system, by means of adjusting the air knife pressure and knife spacing coating weight can be controlled. However, it difficult to establish the precise mathematical model because of the complexity of galvanizing process and the non-linear between air knife factors and coating weight. Besides hot dip galvanizing is a big lag process, the coating weight measurement is nearly140meters far away from the air knife and the system lag time over one minute, using conventional means of control is difficult to achieve the desired performance.Base on An gang NO.5hot dip galvanizing line, FOEN air knife principle and system structure is detailed analyzed and using prediction model and adaptive control theory the advanced coating weight automatic control system is developed to meet the industrial applications of AnSteel NO.5CGL.Firstusing statistic regression analytical method, the coating weight mathematical model of different product specifications are established based on the real measured data. Then air knife pressure closed-loop control system structure and working elements are analyzed and coating weight control system is established. The main circuit is coating weight control and the secondary circuit is the gas pressure control. The main circuit includes model preset control; strip speed feed forward control and coating weight adaptive feedback control to eliminate the large hysteresis and non-linear effects of the process objects. The secondary loop is a passive system designed using the conventional PID control and eliminates the disturbance rapidly using intensive sampling advantage of conventional PID. Combined with operational experience in the production process, the control strategy of strip changing process is developed. Finally, automation system of the cold-rolled hot-dip galvanizing line has been designed and integrated into the production line automation systems. The result of field operation shows that the control system has high control accuracy especially when speed change or top coating weight differed with the bottom, eliminates the defect with man control system of coating weight, saves lots of zinc material and increases the product regular proportion.