Study On The Inverse Calculation Of Heat Transfer Coefficients Of Secondary Cooling During Continuous Casting Process Based On Thermal Imaging Thermometry

As an important technological parameter during the continuous casting process, the heat transfer coefficients of secondary cooling have a great effect on both the calculation of the on-line solidification and heat transfer model and the accuracy of the secondary cooling spray water system and dynamic control model. Therefore, it is of great necessity to accurately calculate the heat transfer coefficients of different secondary cooling zones. Based on the practical situations of a280mm×325mm bloom continuous casting machine in a steel plant, an inverse calculation model of the heat transfer coefficients of secondary cooling during continuous casting process was developed by the thermal imaging thermometry. The effects of casting speed, super heat and spray water density on the heat transfer coefficients were also investigated. The major contents and conclusions are summarized as follows:(1) A calibration method for thermal imager and a screening stragety for bloom surface temperature were established to optimize the thermal imaging thermometry. The surface temperature of the bloom was measured in field with a FLIR thermal imager under different technological situations of the continuous casting process.(2) A two-dimensional solidification and heat transfer model and an inverse calculation model of heat transfer coefficients of the secondary cooling were built. A solidification and heat transfer model, which calculates the temperature distribution all over the streamline of the casting machine, was constructed by the methods of tracking element on the basis of the finite element software named ANSYS. Based on the solidification and heat transfer model and the measured surface temperature, an inverse calculation model which gives the heat transfer coefficients of different secondary cooling zones was established as well.(3) The convergence behavior during solving of inverse calculation model of heat transfer coefficients of the secondary cooling was discussed. It is concluded from the comparison of the effect of three different calculating methods that give the iterative increment for the heat transfer coefficients on the convergence rate that the solving rate can be significantly improved with the proper choose of calculating relaxation factor.(4) The effect of casting speed, super heat and spray water density on the heat transfer coefficients of the secondary cooling process was analyzed. It is found that the heat transfer coefficients decrease slightly with the increasing surface temperature of the bloom. Compared with the effect of casting speed and super heat, the effect of spray water density on the heat transfer coefficients is of much more significance. The heat transfer coefficients increase dramatically as the spray water density increases. With the neglection of the effect of surface temperature, the following expressions are obtained through the fitting between the heat transfer coefficients h (W/m2·℃) and the spray water density w (L/m2-min). zone1:h=29.953w0.699(R=0.9986) zone2:h=56.739w0.526(R=0.9990) zone3:h=87.053w0.371(R=0.9969) zone4:h=109.753w0.260(R=0.9955) zone5:h=118.218w0.209(R=0.9966)(5) The inverse calculation model was verified through the thermomety and pin-shooting experiments. The deviation between the calculated temperature and the measured temperature is restricted in±6℃of which the percentage of deviation is less than1%while the deviation beteween the claculated shell thickness and the measured shell thickness is less than±2mm of which the deviation rate is less than2%, indicating the model is of great accuracy.