Simulation Study On Heat Transfer And Formation Of Slag Film During Solidification Of Mold Flux Used For Continuous Casting

Mold flux is a kind of important material used in continuous casting,and it plays a significant role in sequence casting and the improvement of slab surface quality.Two functions can be realized when the liquid slag infiltrates into the gap between the shell and the mold.One is lubricating the solidified shell,and the other is controlling the horizontal heat transfer.Both the two functions are related to the organization and the structure of the solid slag film between the shell and the mold wall.Therefore,in order to identify the relationship among the physical properties,organization and heat transfer behavior,it is necessary to analyze the formation process of slag film and the heat transfer mechanism of molten slag during solidification.In this work,numerical simulation and experimental methods are combined to investigate the formation of slag film and the heat transfer mechanism of mold flux during solidification.Firstly,the parameters related to crystallization kinetics of CaO-SiO₂-Al₂O₃ based mold fluxes were determined by using the thermal analysis and the hot thermocouple technique.Secondly,the formation of solid slag film was simulated in laboratory.For this process,a conduction-radiation heat transfer model was established to analyze the variation of conductive heat flux and the variation of the radiative heat flux with time.Then,the change of the crystallization fraction during solidification and the variation of the slag film thickness were discussed according to the kinetic parameters of mold fluxes and the temperature field obtained from the mathematical model.In addition,a method used to determine the value of crystallization fraction of the solid slag film was proposed.Finally,the mass transfer equation was introduced into the heat transfer model,and the growth and shrinkage behavior of bubble in mold flux during solidification were calculated.Meanwhile,the effects of size and distribution of pores on the optical properties of the solid slag film were analyzed according to the geometrical optics.The main results of this study are concluded as follows:?1?The Avrami equation,the Ozawa equation and the isoconversional method used to study non-isothermal crystallization kinetics could be written as an unified form:The crystallization rate during non-isothermal process could be expressed as a product of three one-variable functions,and the three variables are relative crystallinity,temperature,and time respectively.Compared with the rate equation based on the mass action law,the improved rate equation is better for the description of the crystallization rate due to the high relation coefficient between the measure data and the fitting results.The nonisothermal crystallization kinetics of CaO-SiO₂-Al₂O₃ based mold flux was investigated by the improved rate equation.Results indicate that when mold flux contains no Na₂O,the addition of Li₂O could enhance the activation energy of mold flux,which inhibits the crystallization of liquid slag under high cooling rate.?2?A conduction-radiation heat transfer model based on fixing mesh and moving zone was established for the solidification process of mold flux.The values of total heat flux obtained from the model agree well with experimental data,and the correlation coefficient reaches 0.960.The calculated values of slag film thickness and the crystallization of the solid slag film are close to the measured results,and the relative error is no more than 10%.In the initial stage of solidification?0-3s?,the main way of heat transfer is radiation,and the proportion of the radiative heat flux in the total heat flux is higher than 50%.After the solid slag film is formed,the proportion of the radiative heat flux is lower than 40%.In the later stage of solidification?>30 s?,the conduction of the solid slag film dominates the heat transfer from the liquid slag.In this stage,the conductive heat flux in the total heat flux is higher than 75%,and the proportion of the radiative heat flux is lower than 30%.For solid slag film with high crystalline fraction,the radiative heat flux through the slag film is lower than that for slag film with high crystallization fraction.?3?The temperature field obtained from the heat transfer model was combined with the TTT curve to analyze the formation process of the solid slag film.During the formation process of the solid slag film,on the detector side,the cooling rate first increases,and then decreases with time.The maximum value of cooling rate exceeds100?/s,and the crystalline fraction of liquid slag is lower than 0.05.After 5 s,the solid slag film began to form.After 30 s,the cooling rate on the detector side is lower than5?/s.At 45 s,the temperature of the solid slag film on the detector side is between600? and 750?.On the interface between the solid and the liquid,the cooling rate of mold flux is lower than 10?/s,and the temperature is lower than the upper limit temperature of TTT curve.The temperature difference is about 50?.For mold flux with high incubation time,the crystallization fraction of slag film is high because liquid slag could easily crystallize on rapid cooling process.The crystallization fraction could be measured by XRD method and the DSC method.For the XRD method,the diffraction peak corresponding to crystalline phase could be judged by two parameters:One is the intensity of a single diffraction peak y_h,the other is the ration of the area of the single diffraction peak and the cube of half-width y_c.When the y_h is higher than 100 and the y_c is higher than 1000,the crystalline fraction obtained from the XRD method agrees well with that from the DSC method,and the difference between the two methods is in the range of-0.07 and 0.07.?4?When the main composition of bubble is SO₂,the decomposition of the sulphate under high temperature leads to the growth of bubble during isothermal process.When the main composition of bubble is CO₂,bubbles are likely to shrinkage during isothermal and nonisothermal process.If pores with constant radius are evenly distributed in the solid slag film,the reflectivity increases with the porosity,and the transmissivity decreases with porosity for a given size.When the diameter of pore is in the range of 0.51 and 0.95 and the diameter is in the range of 50mm and 220mm,the reflectivity is between 0.07 and 0.48,and the transmissivity is between 0.52 and 0.93.When the porosity is in the range of 0.28 and 0.32,and the sauter mean diameter is in the range of 70mm and 76mm,the reflectivity is between 0.15 and 0.38,and the transmissivity is between 0.62 and 0.85.