| Continuously cast round bloom has taken the place of steel ingot in the field of some high value-added products, such as heavy caliber seamless tubes, large ring forgings, high-speed locomotive wheels, etc. However, round bloom continuous casting, with the characteristic of large section size and low casting speed, has large mushy zone and big volume shrinkage in the solidification process, which could cause quality problems easily in production, e.g. central porosity, central crack, center segregation, etc. And electromagnetic stirring(EMS) is commonly used in continuous casting to improve the quality of cast products. Furthermore, there is a strong need to control the non-round degree of round bloom in continuous casting. Therefore, in order to improve the quality of round bloom, it is very important to study the macro-transport phenomena in continuous casting with EMS and analyze mechanical parameters in the withdrawing and straightening process for specific continuous casting process. With a Φ600 mm round bloom caster of some steel company as the research object, the dissertation concentrates on the macro-transport phenomena in continuous casting. The characteristics of the electromagnetic field, the turbulent flow, the heat transfer and macroscopic solidification in a Φ600 mm round bloom caster with M-EMS(mold electromagnetic stirring) and F-EMS(final electromagnetic stirring) were numerically studied respectively. The one dimensional solidification characteristics of a Φ600 mm round bloom were numerically discussed, and the method for determining the target surface temperature of a round bloom in the secondary cooling zone was obtained. Furthermore, the dissertation proposes the quantitative method for determining pressure distribution of the straightening and withdrawal machine by analyzing mechanical parameters in the withdrawing and straightening process of Φ600 mm round bloom continuous casting. The main research content and the results obtained are as follows: ① On the basis of deeply analyzing the characteristic of magnetohydrodynamics in continuous casting with EMS, the decoupling conditions of electromagnetic field and fluid flow were obtained, that is the magnetic Reynolds number is less than 1 and the dimensionless frequency is much less than 1. Through the comparative study of various turbulence models for the actual conditions, the low Reynolds number k-ε model is best for investigating fluid flow and solidification problems in continuous casting with EMS. Based on the above research, a coupled three dimensional mathematic model was developed to describe electromagnetic field, fluid flow, heat transfer and solidification in a Φ600 mm round bloom continuous casting with EMS. ② The numerical simulation of three dimensional electromagnetic field, fluid flow, heat transfer and solidification in a Φ600 mm round bloom continuous casting mold with M-EMS was conducted. The calculated magnetic flux density matches well with the measured value, and the accuracy of both mathematic model and solving method of electromagnetic field were verified. Moreover, the predicted solidified shell thickness is in good agreement with the analytic formula, and also the accuracy of both mathematic model and solving method of fluid flow and solidification were proved. The results show that, with the increase of the current frequency, electromagnetic force density increases firstly and then decreases and reaches the maximum at 2.5 Hz. The Joule heating generated by M-EMS is very small, and it is feasible to ignore the Joule heating in the computations of heat transfer. When considering solidification in continuous casting mold with M-EMS, the rotation velocity of molten steel reduces significantly, which proves that the solidified shell influences stirring intensity obviously. With the increase of stirring intensity, the steady flow of molten steel becomes unstable, and the bias flow appears. The invasion depth of the stream from submerged entry nozzle(SEN) becomes shallow due to the rotating M-EMS, and the temperature of molten steel in the mold is raised, which is favorable for heat transfer. The rotating M-EMS leads to depression in the center of meniscus, and the bigger the stirring intensity the deeper the depression, which may cause mold slag entrapment and secondary oxidation of molten steel. When the geometric certer of M-EMS does not coincide with the center of round bloom, electromagnetic force density still appears circumferential distribution in the round bloom cross-section, but both magnetic flux density and electromagnetic force density present asymmetrical distribution. ③ Numerical calculation of three dimensional electromagnetic field, fluid flow, heat transfer and solidification in a Φ600 mm round bloom continuous casting with F-EMS was carried out, and the accuracy of coupled mathematic model and solution method were verified by the measured magnetic flux density and surface temperature of round bloom. The researches show that, electromagnetic force density at 1/2 and 1/3 radius of the Φ600 mm round bloom reaches the maximum at 10 Hz and then decreases gradually with the increasing current frequency. The latent heat release model affects the mushy zone configuration significantly, and the equilibrium solidification model is more reasonable. The install location of F-EMS is closer to the meniscus, the larger the mushy zone is, and the stirring intensity is greater. At the actual continuously stirring model, the rotation velocity of molten steel varies between the peaks and troughs periodically. ④ A one dimensional transient mathematic model was established to describe heat transfer and solidification in the secondary cooling zone of a Φ600 mm round bloom. The growth regularity of a Φ600 mm round bloom solidified shell was numerically studied, and the predicted shell thickness was compared with multiple sets of measured data in literatures. The method for determining aim surface temperature of round bloom in the secondary cooling zone was given, that is the aim surface temperature could be obtained by the following heat flux density expression of the round bloom 22 2212( 0.5) 0.5ar= -é ù- +ê ú? ?R R LKq R K t Rr where 2 2 22.15 1.04 R Rr =R -RK t +K t Using the proposed model and method, the partition of secondary cooling zone in a Φ600 mm round bloom continuous casting was optimized, and the water table of secondary cooling zone was established. Finally, the influences of technological parameters on heat transfer and solidification in continuous casting were discussed. ⑤ Based on the distribution of temperature and stress in the round bloom cross-section at the straightening zone, the slip resistance formula in a Φ600 mm round bloom continuous casting was deduced, and the computational model of the straightening force and torque of a round bloom were established. Finally, the quantitative method for determining pressure distribution of the straightening and withdrawal machine was proposed. To sum up, the dissertation focuses on improving the quality of continuously cast round bloom and adopts numerical simulation. The macro-transport phenomena in continuous casting with EMS were studied, and the mechanical parameters in the withdrawing and straightening process were analyzed. Furthermore, the key process parameters in a Φ600 mm round bloom continuous casting were determined, and good application effect was obtained in practical production. |
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