The Study Of Several Dynamic Issues During Initial Solidification In Continuous Casting

It is important to control the initial solidification in continuous casting, during this period, not only the fluid flow, heat transfer and mass transfer were involved, but also the complex system, such as high temperature and multi-phase, was concerned. Most of billet defects were formed in the initial solidification stage, and would get worse during later process. Based on the mold oscillation and argon blowing techniques, the dynamic process and its influence on the billet qualities were investigated, then the dynamic formation process of billet defects were analyzed. It was helpful to improve the billet quality.Up to the present, metallurgists payed attention to the fluid flow, heat transfer, and the force in the phase transfer, however, most of them were affected by many factors, it was difficult to study directly through experiments or numerical simulation, thus some important information was lost. In this paper, mold oscillation and billet withdrawing were considered, and the transient heat transfer mathematical model was established, with which the heat transfer character was quantitatively revealed, then a new formation mechanism of oscillation marks was proposed.Besides, improper application of argon blowing would lead to the billet defeats. In order to reduce the influence of gas bubble on the billet quality, it was very important to investigate the dynamic behavior of gas bubble in the mold. The motion of bubble in mold was a three phase flow problem, it was extremely difficult to calculate this process by numerical simulation. Meanwhile, the water modeling experiment was hard to reflect the real motion characteristics of gas bubble in the liquid metal. In this paper, combing with physical modeling and numerical simulation, bubbles distribution in the liquid metal (mercury) was measured, and bubble deformation during floating was calculated, its influence on the steel-slag interface was analyzed. On the other hand, bubble behavior in solidifying front was observed in the experiments. With which the dynamic behavior of gas bubble in the mold was comprehensively described.This paper is composed of four parts. In the first part, a mathematical model was established to calculated the heat transfer under mold oscillation, the mold oscillation and billet withdrawing were considered through convection term in the governing equations. The heat transfer in continuous casting of Tin was calculated with this model, it was found that temperature of initial solidification area was varied periodically along with mold oscillation. The frequency spectrum analysis of temperature data showed that the frequency was the same as mold oscillation. In order to verify the mathematical model, experiments were carried out on a small-scale caster. Thermocouples column were adopted to measure the temperature of initial solidification area. Repeat experiments results showed that mold oscillation leads to temperature fluctuation, the amplitude was about 4 to 10℃, and the frequency of temperature variety was almost the same as mold oscillation. Compared with calculation and experiment results, it was found that calculation temperature was agreed with measured data, the mathematical model was suitable to calculate the heat transfer under mold oscillation during continuous casting.The heat transfer of steel continuous casting was simulated with above model, the temperature fluctuation phenomenon was confirmed theoretically. Different continuous casting operation parameters were considered, and its influence on temperature fluctuation was analyzed. The results showed that the maximum temperature fluctuation appeared in vicinity of the initial solidification point, both increasing mold oscillation frequency and decreasing the oscillation amplitude would helpful to reduce the temperature fluctuation.In the second part of the paper, combined with heat transfer simulation and analytical calculation of peritectic phase transformation, a special mathematical model was proposed to investigate the effect of temperature fluctuation on the initial solidification during continuous casting of peritectic steel. Pretectic phase transfer with different temperature fluctuation was calculated with above mathematical model, the interface of L/δandδ/γwere changed along with temperature fluctuation. While the temperature fluctuation amplitude was larger, part of theγ-phase would be re-melted . Different chemical compositions with different carbon content were calculated, and it was found that the peritectic steel with carbon content of 0.1-0.17% was most sensitive to the temperature fluctuation, it maybe the reason why some of peritectic steels and stainless steels were difficult to continuous cast in industry production. In the third part of the paper, the samples were cut from the industrial billet, and the surface morphology was observed, solidified microstructures in the oscillation marks area were analyzed, the possible formation mechanism of oscillation marks and cracks were proposed.Besides the depression type and hook type oscillation marks, a new type of oscillation marks was found on the billet surface, we called it as "melt-overflow" type oscillation marks. The temperature in meniscus area may changed near the ZST ( Zero Strength Temperature) or ZDT ( Zero Ductility Temperature) because of temperature fluctuation in initial solidification area, and lead to the formation of "melt-overflow" type oscillation marks. And in this situation, any force exert on the shell might brake the dendrite, then the new type oscillation marks formed.Based on the analysis of calculation and experiment results, the formation mechanism of all kinds of oscillation marks could be explained, and the temperature fluctuation was considered to be a key factor to the formation of oscillation marks. The ITF( Index of Temperature Fluctuation) was proposed to evaluate the effect of temperature fluctuation on billet surface defects, the variation of ITF lead to the non-uniform shell thickness, and then the billet mechanical properties was heterogeneous, furthermore lead to the formation of surface defects. So, it was useful to improve the billet quality by depressing the ITF as low as possible.In the fourth part of this paper, the impact of argon blowing on the slab quality was studied in experiment and numerical simulation respectively, the deformation of bubbles during floating and its influence on the velocity field in the area of steel-slag interface were calculated, and the mechanism of bubble entrapment in the solidification front was proposed.The experiment device of argon blowing was designed, mercy was used in the experiment, and a resistance probe was developed to measure the quantity of bubbles. The results showed that most of bubbles were escaped form liquid surface, some of the bubbles which moved to the solid-liquid interface were captured and then entrapped by the solidified shell. The level set method was used to simulate the three phase flow, and the shape deformation of bubbles with different diameter was calculated, vertical velocity in the steel-slag interface was analyzed, also the possible influence on slag entrapment was discussed. The dip test was carried to observe bubble behavior in front of solid-liquid interface, it was found that some bubbles were captured, while other bubbles could escape form the interface. Observation on the slab specimens, it was found that bubbles were existed at the bottom of oscillation marks, and the bubble was surrounded by dendrites, maybe the floating bubbles were blocked by the dendrites in the meniscus ,it might to be an important reason why bubbles were entrapped in the solidified shell.Based on the investigation of above key dynamic issues, the temperature distribution and variation in initial solidification area under mold oscillation was revealed, and then the formation mechanism of oscillation marks was proposed. Also, the dynamic behavior of gas bubble in the liquid metal and in solidification front was described comprehensively, and it would be used to determine the reasonable argon blowing parameters.