Research And Application Of Cooling Control Method Of Hot Rolling Strip Steel Based On UFC

Ultra-Fast cooling(UFC)is the core aspect of New Generation Thermal Mechanical Controlled Processing(NG-TMCP)technology,which plays an important role in the development and production of resource-saving&environment-friendly 'hot rolled steel materials.Base on the significant technology "The equipment and process technology of NG-TMCP for hot rolled strip",which was the subject of "Green production technology and application demonstration of steel industry" for "12th Five-Year Plan" National Key Technology R&D Program,and the cooperative development project of UFC with Shougang Qian'an and Jingtang Iron&Steel Company,the following research is conducted.Focused on the high-precision and intelligent control methods of cooling process after rolling for hot rolled strip,the mathematical model of cooling process based on UFC,cooling control strategies,coil temperature control methods,adjustment strategies and methods of cooling water and the uniformity of through-thickness microstructure and mechanical properties influenced by cooling path were systematically studied.The results of this study have been successfully applied in industrial-scale production with excellent effects.(1)The hot rolled strip steel was characterized by high rolling speed and large range of thickness,and the jet impact cooling was the main heat transfer mechanism during ultra-fast cooling process.At present,there was little research on the mathematical model of UFC for hot rolled strip steel.Based on heat transfer in cooling process of hot rolled strip,the finite difference equation characterized by differential discrete was established,which effectively improved the efficiency and accuracy of online calculation.Meanwhile,combined with the heat exchange mechanism of hot rolled strip,the theoretical statistic model of convective heat transfer coefficient in finite difference equation was established,and correction coefficient of the new model was optimized.(2)UFC equipment delivery temperature(UFC-T)was an important process factor affecting the product microstructure and mechanical properties under NG-TMCP process.However,there was little reference about the high-precision control strategy and method of UFC-T for hot rolled strip steel.In consideration of the high heat transfer intensity and through-thickness temperature gradient of strip steel during UFC process,the influence laws of single header on UFC-T under different setting strategies of UFC headers were systematically studied by the finite difference method,and the optimization valve matrix setting strategy was developed.Furthermore,combined with the features of UFC equipment,such as compact and fast response rate,the intelligent self-adaption system was developed,which accomplished high-precision and self-adaption control of UFC-T.(3)In terms of hot rolled strip steels,the high-precision temperature control of strip head special cooled section was an important way to improve the production stability and yield rate.According to the process requirements of strip head temperature on different productions,and combining with the layout characteristics of cooling equipment,un-cooled strategy on laminar cooling zone,poor cooled strategy on UFC zone and un-cooled strategy based on pressure pre-compensation on UFC zone were developed,respectively.The control strategies achieved high-precision control of head special cooled section(temperature up to 720 ?,length of 2.0 m),and the yield rate was approximately increased by 5%.(4)The conventional coil temperature(CT)PID feedback controller could not achieve excellent effect if the gain parameters were tuned improperly.Aiming at the characteristics of highly nonlinearity and large scale time delay in CT feedback control,an expert self-tuning anti-delay PID controller was developed.The results showed that a high-efficiency and stable control effect was achieved,and the control precision of CT(±20 ?)was approximately increased by 5%.(5)The application of UFC technology has become the most effective means to make up the deficiency of cooling capacity for hot rolled strip line with short length arrangement of cooling equipment.On the basis of analysis of the production line,in consider of the problems,such as short regulating time and insufficient temperature correction method,an intelligent CT feed forward control method based on UFC technology was developed,and process temperature control precision within 20 ? was approximately increased by 8%.(6)In the production of hot rolled strip steel,equipment working condition and calculation deviation of control system,etc.usually affect the CT control precision of the first strip when changing specification or batch of product.In order to solve the above problem,the big data technology combined with characteristics of hot rolling line data information was employed to guide the current production.Based on big data,an intelligent CT self-adaption control method including collecting data records,intelligent searching and guiding the production were developed through the establishment of database and the development of relevant algorithms.The application results showed that CT within ± 20 ? control precision of some steel grade was approximately increased by 2%.(7)The reasonable and efficient regulation of the cooling water pressure and header flux was the important premise to improve the utilization rate of cooling water and ensure the accuracy of the temperature control during strip cooling process.According to the process characteristics of UFC,such as dynamic water supply with high pressure and nonlinear regulation of header flux,an evaluation method of adjusting efficiency for cooling medium was proposed.Based on the pressure adjusting efficiency(PAE),the energy-saving and high-precision water pressure control strategy was developed.Under the premise to ensure the control precision for water pressure,the total amount of energy-saving reached 20%.Furthermore,based on the flux adjusting efficiency(FAE),a PI controller for header flux was developed.The optimal parameters of the controller were systematically studied-by using the newly established simulation model,which realized optimization of the parameters in PI controller under different initial opening rates of flux adjusting valve and interference value.(8)The development and utilization of reasonable cooling path control method based on UFC technology to achieve comprehensive control of microstructure and mechanical properties for hot rolled strip steel was the ultimate goal of the NG-TMCP.The influence of various cooling path on the uniformity of though-thickness microstructure and mechanical properties of X80 pipeline steel with thickness of 22.0 mm was studied by combining hot rolled experiment and temperature field simulation.The results showed that the uniform through-thickness microstructure and mechanical properties were obtained under the cooling path of UFC with medium cooling capacity(cooling rate of?23 0C/s)followed by ultimate cooling capacity(cooling rate of-50 ?/s).Compared with conventional laminar cooling,the yield strength,tensile strength and-20 ? Charpy impact energy of the experimental steel obtained by above mentioned cooling path were increased by 80 MPa,40 MPa and 45 J,respectively,and the deviation of through-thickness hardness was controlled within 20 HV.This result laid a solid guidance for high-quality pipeline steel production under ultra-fast cooling technology.The results of this study had been successfully applied to many domestic conventional hot strip rolling lines.According to different products,specifications and processes,the parameters of cooling after rolling including UFC-T,CT,cooling water pressure and UFC header flux were high-precision and stably controlled.Utilizing the platform provided by UFC technology with mature equipment and high-precision cooling control,a series of low-cost and high-quality products were developed,and a stable large-scale industrial production was achieved,which made outstanding contributions to enterprises for reducing cost and increasing quality of products.