Design And Realization Of Laminar Cooling Process Control System For Large-scale Hot Strip Rolling

Laminar cooling is an important section of hot rolling process control system, whose core functions is cooling the strip from finish-milling delivery temperature to intended target coiling temperature by opening or closing the water valves or changing their spray mode or volume. For its control accuracy has significant effects on the physical performances of the strip, the actual coiling temperature must be controlled in a certain tolerance range in order to get qualified products. However, the laminar cooling is a time-varying, nonlinear, large time-delay complex process, which is quite difficult to control at a high accuracy and some problems still to be further studied.At first, this paper introduces the principles and claim of laminar cooling and the popular differential temperature heat exchanger model; then combining with the design and development practices of Baosteel No.3hot rolling line, this paper gives a more detailed exposition of the design and implementation methods. The main contents are as follows:1) In the cooling control strategy step, in order to achieve precise cooling, this paper uses the combination mode of steel grade, thickness class and temperature class to determine the different cooling pattern, spray pattern, start valve number, based on the incoming PDI data and calculation results from finishing-mill;2) In pre-set and dynamic control step, this paper uses differential temperature heat exchanger model to solve the weak points of traditional temperature models;3) In cooling water calculation step, a fast iterative calculation method of water has been designed to solve problem of the strip temperature distribution in rapid movement process and the control period with the limit of no more than100ms;4)In the feedback control step, for the laminar cooling characteristics of large time-delay, this paper uses a new method combined Smith Predictor with fuzzy PI method to avoid accurate modeling of system for the conventional Smith Predictor and overshoot problems of the conventional PI control. Production practice has proved that the laminar cooling process control system designed by Baosteel has good stability and high control accuracy, whose coiling temperature controlled accuracy of±20℃has been stable at more above98.5%. In a word, the designed system has meet the request of Baosteel large-scaled hot rolling production, and its successful experience is a positive reference for the promotion of other hot strip rolling lines.