Research And Application On Multistage Cooling Control Strategy Of Hot Rolled Plate

Controlled cooling was an important part of TMCP technology by which the effect factors of phase transformation and the precipitation behavior of carbide were controlled to improve the microstructure and mechanical property of steel. Multistage cooling control was to control the phase transformation process accurately and to improve cooling uniformity so as to satisfy the need of the production of new products such as complex steel and high-strength steel etc. Based on the significant technology "Saving Type Steel Products and Its Reduced Steel Rolling Technology" of "11th Five-Year Plan" Key Technology R&D Program, the new generation cooling system was developed and the multistage cooling control stragety of hot plate was researched in this article. The mechanisms of heat transfer during accelerated cooling and ultra-fast cooling process and the analytical model for the variation of temperature inside plate were analyzed. The key technology basis for controlled cooling was researched, where the emphasis was the variation of temperature filed under different cooling strategy. Based on theoretical and technological researched, multistage cooling control system was developed. To satisfy the need of industrial application, the high-precision and uniform cooling technology was researched. Optimum control of multistage temperature profile was accomplished based on optimal control theory. The main work and results are as follows:(1) The theoretical models of the controlled cooling system were researched. First, the heat transfer between plate and coolant during the process of accelerated cooling, ultra-fast cooling and air cooling etc. were analyzed profoundly. Based on energy conservation principle, the on-line regression equation for the coefficient of heat transfer was established. (2) The finite element method to analyze temperature field was established. The differential equation and its boundary condition, initial condition to analyze thermal conduction inside hot plate were introduced. The differential equation was analyzed by the finite element method. The models for calculating latent heat during various phase transformation inside hot plate deduced from an important model of which was the transmutation product and its quota of phase transformation. The finite element mesh and time step for iterative computation were decided appropriately so that the finite element method could be used on-line with high-precision. Based on internal energy, the average temperature inside hot plate was calculated. The calculated results could reflect the variation of real average temperature.(3) The variation regular of temperature was researched for building of the multistage cooling control system. The variation regular of temperature inside hot plate under different cooling condition was researched such as accelerated cooling, ultra-fast cooling, and air cooling. The variation regulars of effective cooling thickness, balance temperature difference and red-back critical line were disclosured.(4) The technology feature of hot plate under the condition of multistage cooling was researched. Firstly, the variation regular of temperature under the combined type cooling system developed by RAL was researched. Secondly, the cooling rate was researched under different cooling strategy. Base on cooling rate, the effects of cooling method on phase transformation were analyzed. The ultra-fast cooling and the accelerated cooling were recommended to control the incubation period of phase transformation and the process of phase transformation respectively.(5) The multistage cooling control system was established to satisfy the need of industrial production of plate. Base on cooling equipment layouts of some plate plant, a multistage cooling control system was developed. The function of electric automatic control system and technology automatic control system, and trigger mechanism according to time sequence were described. The key technology to improve control accuracy and cooling uniformity of the control system was introduced. The working principle of the technology automatic control system was researched. How to select the cooling pattern and to realize the control process of ultra-fast cooling and accelerated cooling were discussed in details. The working principle of multistage cooling control system was introduced as an example to high strength C-Mn steel.(6) The optimal control based on linear quadratic was introduced to control the multistage temperature profile of hot plate. To control cooling profile with high degree of accuracy using optimal cooling efficiency, the controlled targets of performance fuction were cooling efficiency of cooling unit, finish cooling temperature and cooling profile. The finish cooling temperature and the cooling rate of 20mm DH36 of each stage were controlled with high-precision. The ideal microstructure and mechanical property of DH36 were obtained during the industrial experiment at some factory.