Thermal Modeling And Numerical Simulation Of Strip Steels In The Hot Rolling

In the production and control of continuous hot slab rolling, temperature is one of the most important parameters controlling the kinetics of metallurgical transformations and the flow stress of the rolled metal. Temperature setting models and cooling control of steels with high accuracy not only improve the rolling stability but change the metallurgical structure of steel products and refine the crystal grain and finally increase the mechanical characteristics and physical property as well as workability, in terms of ductility, drawability and formability.Combined with rolling technology of the new built hot slap rolling mill at Baosteel, the heat transfer phenomena and metallurgical behaviors occurring during the rolling and laminar cooling process were experimentally studied and modeled. Hybrid temperature models in the finish stands were built to calculate the thermal profile along the strip thickness direction and precisely predict the finishing and coiling temperatures. Then the on-line temperature models had been evaluated and tested in hot strip production. In addition, considering the thermal influence on variation of steel material properties and metallurgical behaviors, numerical models to describe nonlinearly coupled aspects of temperature and phase transformation had been proposed for research and development of new steel products. The main work and innovation are briefly stated as following.1) A differential model was proposed to replace the former thermal model of rolling conduction and an off-line adaption technique based on steel grade and thickness class was adopted to correct the radiation factor of air cooling model. Furthermore, the original water cooling model was replaced by a new exponential model with on-line adjustment. Through the conversion model of surface and average temperature, the enhanced hybrid temperature model presented higher prediction accuracy as well as acceptable response speed. In addition, the short-term and long-term adaption models in exponential form, as well as a neural network adaption system characterized by 'Relay' initialization learning, were put forward to adjusting the heat transfer coefficients of models.2) The thermal and metallurgical behaviors of the strip occurring during the cooling process in hot strip mill were analysed and coupled mathematical models of temperature and phase transformation had been built. The variational method is utilized for the discretization of the governing transient conduction-convection equation, with heat transfer coefficients adaptively determined by the actual mill data. To consider the thermal effect of phase transformation during cooling, a constitutive equation for describing austenite decomposition kinetics of steel in air and water cooling zones is coupled with the heat transfer model by iterative procedure. In addition, numerical simulation was performed in the special cooling condition of dual phase steel, with results confirming that the setup accuracy of temperature prediction system was satisfactory.3) It should be noted that temperature models relied heavily on thermo-physical parameters such as specific heat, conductivity and density of the metal being rolled. These properties were very limited in the literature and when available they were often quoted at a single temperature such as room temperature. This immediately compromised the accuracy and relevance of theoretical predictions. Thus, three example carbon steels were chosen for experiments and analyses. Experiments are performed on the electro-thermo-mechanical test system and the method of least squares has been used to statistically model the properties as functions of temperature.4) The temperature proposed had been applied in the new built hot strip mill at Baoshan Iron & Steel Co. Ltd. (Baosteel), with results showing that the finishing temperature prediction accuracy of hybrid temperature models in finish stands and the coiling temperature prediction accuracy of finite differential models in laminar cooling zones were 7.5℃and 9.0℃, separately. The comparison among the new temperature models and that of 2050mm and 1580mm hot strip mill at Baosteel as well as other mills in domestic and abroad indicated that the new temperature models had achieved the international advanced level.