Research On Spray Control Of Secondary Cooling For Billet Casting Speed Disturbance And Molten Steel Temperature Unsteady State In Tundish

Spray control in continuous casting secondary cooling is one of the key technology affect the quality and production of the billet. As the improvement of technologies and the requirement for the product quality becomes higher, the static spray control method based on the steady heat transfer model could not meet the requirement. Contrast with slab casting, the billet casting is more susceptible to the factors such as equipment, procedure and the rhythm variety, etc. The temperature in tundish fluctuates more greatly as the casting speed change more frequently which requires better spray control method to guarantee the quality of the billets. Therefore, it has great practical significance to study the dynamic spray control system in secondary cooling of billets.This thesis has studied the solidification process of billet based on the heating transfer model of continue casting. The research is focused on the feed forward control scheme and the reliable measurement tundish temperature as applying the secondary cooling control system. The main content and innovation are listed as below:(1) The3D heat transfer model for solidification and dynamic analyze for temperature field has been established.The3D heat transfer model for solidification is the foundation of the spray control system. The large computation complexity and long calculation time obstruct the application of the dynamic3D heat transfer model. To overcome the difficulty, the thesis proposed "Prior Small-Angle" method for choosing nodes while meshing automatically. It guarantees the meshed billet grid approaching to equilateral triangle which makes the meshing more reasonable and reducing the memories. Therefore, the model is faster and more accurate. Since the3D model has considered the heat transfer along the casting direction, the mesh imension of longitudinal section may has suitable increment.With dynamic analyzing the heat transfer model for solidification, the dynamic character of temperature field is found as the results of casting speed and amount of secondary cooling water. Furthermore, the change of water amount has larger effect than casting speed. When the casting speed and water amount change stepped, the surface temperature of the billet appears’Peak’.(2) The feed forward compensation model for casting speed disturbed has been established.A feed forward controller is designed to handle the’Peak’surface temperature of billet when casting speed change suddenly. It affects the billet surface temperature by controlling the water amount to improve the static cooling water control method which is suddenly changed with casting speed. Applied the "Least Square Method" to recognize the controller channel and the transfer function of disturbance. The simulated results show that the application of this feed forward compensation controller can reduce the bias between the billet surface temperature and the target temperature. Therefore, the defect of the billet decreased.(3) The coupling model for temperature field and fluid field has been establishedThe similarity of the detected and casting outlet temperature is the premise for both continuous production and introduction the superheat into secondary cooling control system. The coupling model for temperature field and fluid field has been established based on the typical T type five strands tundish. The effect of fluid field to temperature field has been analyzed and the uniformities of tundish temperature and casting outlet temperature are discussed.The analyzed results show that the tundish temperature is not simply stratified since the effect of fluid field. There are two symmetry high temperature areas corresponding to the two swirls in tundish. At the beginning time of casting, the molten steel temperature at the two outer channels is lower than the other three inner channels since the heat absorb of lining. At the normal casting stage, as the lining getting heat balance, the temperature difference between five outlets is within5℃.(4) The error compensation model has been established.The reliability of the continuous molten steel temperature measurement in tundish is the fundament to establish a reliable control system. Two compensation models have been developed to handle the measurement error caused by inserting the sensor and the inserting depth of the sensor while changing ladles. The temperature function’s coefficients are determined based on the analysis of sensors dynamic response of temperature field. The model applies the beginning measured data and genetic algorithm to find the real molten steel temperature.The results show that the tundish temperature compensation model can reduce the average respond time from210s to90s and the error is within±5℃when sensors inserting molten steel. While changing ladles, the respond time is reduced from3-5min to40s and the error is within±2℃.(5) A feed forward-backward control method for spray water based on casting speed and the disturbance of tundish temperature has been proposed.A feed forward mode was used to control secondary cooling water for the disturbance of casting speed. When casting speed is stable, the control is changed to steady mode based on casting speed and the tundish temperature for surmount the bias of billet surface temperature caused by the disturbance of coefficients such as tundish temperature. It found that the coefficients of secondary cooling water are optimized according to plastic temperature and its dynamic properties.(6) Dynamic control system of secondary cooling water was taken in to practicial application.The dynamic secondary cooling water control system based on the disturbance of casting speed and the superheat of tundish is successfully applied on the2#caster of one steel plant. Two years usage showed that the stability of the control system secondary cooling water has been enhanced. The caster can work smoothly for different steel type, superheat and casting speed. The quality of the steel is improved obviously.