Study On Slab Quality Control Based On Dynamic Secondary Cooling And Dynamic Soft Reduction

The defects such as surface cracks,central porosity and segregation of continuous casting billet/slab have important influence on the mechanical properties and yield of steel.The generation of surface crack is closely related to the surface temperature fluctuation during continuous casting process,mainly affected by the secondary cooling.The central porosity and segregation are the inherent defects in solidification process of billet/slab,which mainly affected by the steel composition and technological parameters of continuous casting,the soft reduction is one of the most effective method for the inhibition of central porosity and segregation.In the actual production,the unsteady casting state such as the changes of casting speed occurs from time to time,the secondary cooling water flow rate and the soft reduction parameters need to be adjusted dynamically according to the casting state,that is,the dynamic secondary cooling and the dynamic soft reduction control are required,which is of great significance for the stable control of billet/slab surface temperature and reduction effect.In this research,in order to give full play to the performance of the slab continuous caster,to further improve the quality of continuous casting slab,taking the opportunity of the slab continuous caster upgrade,the solidification and heat transfer mathematical model for continuous casting slab was built,based on this,the dynamic secondary cooling control system and dynamic soft reduction control system for the slab continuous caster were developed independently,and the on line testing and application were conducted.The development and application of the dynamic secondary cooling control system and the dynamic soft reduction control system,greatly promote the enhancement of continuous casting technical level and the quality of continuous casting slab,and the master of the core technology of dynamic secondary cooling and dynamic soft reduction lay a good foundation for the subsequent technology transplants.The foundation of dynamic secondary cooling and dynamic soft reduction is the online tracking of the solidification state of the continuous casting slab,therefore,a three-dimensional solidification and heat transfer mathematical model of the continuous casting slab was established primarily based on the analysis of the slab solidification and heat transfer process and some reasonable hypothesis,which realized the prediction of temperature and solid fraction on the arbitrary position node of the casting slab,and has good adaptability to the casting parameters.The test results show that,the prediction results of the surface temperature and the thickness of the solidified shell are basically consistent with the experimental results.In order to meet the requirements of on-line application of the solidification and heat transfer model on the solution and reaction speed,an on-line thermal tracking model was developed by the simplification of the 3D mathematical model,and its prediction precision is in accord with the 3D model.Based on the on-line thermal tracking model,the slab surface temperature control objective was drawn up,and an increment PID algorithm for the secondary cooling water flow rate calculation is developed,and the fluctuation control strategy of the secondary cooling water flow rate was established,eventually,the dynamic secondary cooling control system was established including management layer,control layer and execution layer based on the modular design idea.On line test results show that the system can quickly adjust the flow rate of the secondary cooling water,and the slab surface temperature can be controlled stably.The application results shows that the fluctuation range of the slab surface temperature is significantly decreased,and the occurrence probability of the surface cracks of the continuous casting slab are significantly decreased.Based on the on-line thermal tracking model,the calculation method of dynamic soft reduction parameters,the control strategies of the reduction adjustment about the changes of casting speed and changes of the main control steel grades,the security strategies of the system operation about the message blocking,abnormal of casting length and casting time,and the abnormal state treatment strategies were established respectively.Integrating the calculation method of soft reduction parameters and control strategies,the dynamic soft reduction control system was established including management layer,control layer and execution layer based on the modular design idea.On line test results showed that,the fast and accurate adjustment of the soft reduction parameters can be realized.The application results showed that,the central segregation and porosity are significantly inhibited,the macrostructure yield and the stability of the slab are significantly improved.The quality tracing of final steel indicated that,after the application of the dynamic soft reduction,the inspection yield of hot rolled plate increased significantly,the mechanical properties and its stability such as yield strength,tensile strength,elongation rate and impact toughness increased significantly also.