Research On Heat Transfer And Inverse Problem For Secondary Cooling Process Of Continuous Casting

Iron and steel industry is an important industry related to the national economy and people's livelihood.It is the main way to produce iron and steel by continuous casting technology.The molten steel is poured into the mould at a certain casting speed from the casting disc.High temperature molten steel will be cooled rapidly in the mould by the cooling water outside.The slab surface temperature will drop rapidly,and a thin shell will be formed on the surface.Then,the slab comes out of the mould.In the secondary cooling zone,because of external cooling water and radiation cooling of itself,the unsolidified molten steel inside the slab gradually solidifies.As soon as the slab leaves the secondary cooling zone,the internal temperature of the slab is significantly higher than the surface temperature.Heat flux continuously transmits from the inside of the slab to the surface which increases the surface temperature of the slab.As the continuous casting process proceeds,the internal and external temperature of the slab tends to be uniform and the overall temperature decreases.The solidification of slab is mainly completed in the secondary cooling zone.The secondary cooling zone is also the area where the slab is most prone to defects.Therefore,whether the secondary cooling process is reasonable or not has significant impact on the quality of slab.The optimization of cooling water quantity in secondary cooling zone is to ensure that the surface temperature in secondary cooling zone conforms to the target temperature and avoid all kinds of defects caused by the improper or uneven cooling intensity by design the cooling intensity of each zone reasonably.Whether the heat transfer coefficient between slab and cooling water is correct or not directly affects the determination of secondary cooling water flow rates.In order to improve the quality of slab and avoid defects,this paper studies the optimization of cooling water flow rates in secondary cooling zone based on the inverse problem method of heat transfer.This paper constructs an inversion system of spray heat transfer coefficient in secondary cooling zone based on Decentralized Fuzzy Inference(DFI)Method.Based on DFI,the optimization of cooling water flow rates in secondary cooling zone is studied.The main research work is as follows:(1)A two-dimensional heat transfer model of continuous casting process in the mould and secondary cooling zone was established.The spatial coordinate system was established on the moving slab section,and the temperature field of the whole continuous casting process was obtained by tracking the temperature of the slab section.(2)The region of slab section is discretized by triangular mesh element.The temperature field distribution of slab is solved by finite element method.The calculating time step and the spatial mesh are verified independently.The effects of casting speed,superheat of molten steel and cooling water flow rates on the temperature distribution of slab are studied by numerical tests.(3)The basic principle and reasoning process of the decentralized fuzzy inference method are introduced.Inversion system for the heat transfer coefficient of spray water in the secondary cooling zone based on DFI is constructed.The influence of initial guess value of heat transfer coefficient,measurement error and distribution of heat transfer coefficient on the inversion results is analyzed through simulation experiments.The simulation results show that the initial guess value of DFI method based on sensitivity analysis weighting is insensitive.This method also has better anti-interence ability,and is insensitive to the distribution of inversion parameters.Good inversion results can be obtained under different distribution forms,which proves that DFI method is universal.(4)According to the measured temperature,the heat transfer coefficients of each cooling section in the secondary cooling zone are identified and the casting machine coefficients are estimated.On this basis,the optimization of cooling wate flow rates in the secondary cooling zone is studied.The optimization of cooling water flow rates in the secondary cooling zone under different working conditions are also studied.The optimized results are replaced by the heat transfer simulation model of continuous casting.The calculated surface temperatures of the slabs all reach the target surface temperatures.It is proved that the DFI method is effective in optimizing the cooling water flow rates in the secondary cooling zone,and the ideal optimization results can be obtained under various operating conditions.