| The development and application of tundish metallurgy technology should focus on improving the billet quality and increasing the billet purity.The basis and premise of improving the billet quality is to realize the casting with low superheat and constant speed,in which the uniformity of molten steel composition and temperature in each strand of a tundish should be increased to reduce the unsteady casting.The effective methods to achieve the purification of molten steel in a tundish include the extension of the average residence time of molten steel in a tundish by changing the flow state of molten steel to promote the floating of inclusions,the capture and absorption of inclusions by micro bubbles,and the prevention of secondary oxidation.In this study,starting with the optimization of tundish flow field,a modified combination model was built by taking the consideration of the fully-mixed zone.The evaluation method for the consistency of each strand in a tundish was determined and adopted to confirm that the large-size inclusions could be effectively removed by the optimized flow field.Furthermore,an industrial trial of large-flow argon injection into ladle shroud was conducted in Aosen Steel.The micro bubbles generated in the molten steel were successfully captured in order to investigate their morphological characteristics,size distribution,and inclusion removal effect in the molten steel.The main research work includes the following aspects:(1)Model for analyzing the flow characteristics of molten steel in a multi-strand asymmetric tundish.For the physical simulation experiment of the multi-strand asymmetric tundish,the proportion of dead zone was calculated to be oversize when the classical combination model and Sahai modified combination model were used to describe the flow characteristics.The primary reason for this problem was that the dead zones in the two models included not only the defined domains but also the contribution of fluids with residence time more than twice the theoretical residence time in the fully-mixed flow.Therefore,in order to eliminate this partial contribution,a new modified combination model was developed on the basis of the classical combination model to modify the calculation method of dead zone ratio.The new developed model was applied to the experiments on a single-strand tundish and a multi-strand asymmetric tundish,and compared with the classical combination model and Sahai modified combination model.The proportion of dead zone calculated by the new modified combination model was slightly smaller than those calculated by the other two models for the single-strand tundish,but significantly smaller for the multi-strand tundish with the fully-mixed flow.This was more consistent with the experimental results and more accurate in reflecting the flow characteristics in the tundish.Thus,the new modified combination model could overcome the limitation that the classical combination model and Sahai modified combination model were inaccurate in analyzing the flow characteristics of multi-strand tundish.(2)Structure optimization of multi-strand asymmetric tundish.The new modified combination model was applied to solve the problems of short-circuit strand and high proportion of dead zone in Aosen Steel.The optimal structure scheme of the tundish with the diversion hole of 60 mm and the 4#-dam height of 300 mm was determined after a series of research on the asymmetry analysis,the diversion plate optimization,and the orthogonal design optimization of retaining wall with diversion hole and dam.As a result of using the optimal scheme,the proportion of dead zone decreased from 28.4%to 12.9%,the standard deviation of the time to reach peak decreased from 30.7 s to 24.8 s,the standard deviation of peak value decreased from 0.231 to 0.139,and the average residence time increased from 709.7 s to 879.9 s.Furthermore,the large-size inclusions in the casting billets produced at 3#-strand and 1#-strand decreased from 21.21 mg·(10kg)-1 and 15.96 mg·(10kg)-1 to 11.42 mg·(10kg)-1 and 12.21 mg·(10kg)-1,respectively,and the average difference in temperature decreased from 2.9℃ to 2.1℃.It was shown that the structure optimization of the tundish could increase the uniformity of molten steel composition and temperature,improve the consistency of each strand in the tundish,and strengthen the ability to remove large-size inclusions.(3)Industrial trial on inclusion removal by large-flow argon injection into ladle shroud.Based on the large-size inclusion removal by optimizing flow field in the tundish,the micro-size inclusion removal technology was further investigated by injecting large-flow argon into ladle shroud.A large number of dispersed tiny argon bubbles were successfully generated in the tundish,and the cold steel sheet dipping method was introduced to characterize the bubbles in the molten steel.The microscope analysis showed that the size of bubbles inside the dipping cold steel sheet was in the range of 100-1000 μm,with an average of 500μm.The turbulence fragmentation is the main mechanism of forming micro bubbles by the large-flow argon injection into ladle shroud.Inclusions were found at the inner wall of some bubbles.The probability of bubbles adhering to Al2O3 inclusions was significantly higher than that of bubbles adhering to aluminosilicate composite inclusions.Applying both the tundish structure optimization and the argon injection into ladle shroud,the inclusions above 20 μm in the steel were completely removed,the removal rate of inclusions at 5-10 μm increased by 48.58%,and the nitrogen increase in the continuous casting process significantly decreased by 45.83%.(4)Simulation of micro bubbles motion and distribution behaviors.The dual-phase coupling flow field between molten steel and bubbles produced by the large-flow argon injection into ladle shroud was simulated using the Eulerian-Lagrangian method and the discrete phase model.When the flow rate of argon injection into ladle shroud was 3.0m3/h,the small-size bubbles with a diameter of 0.134 mm had the strongest tendency to spread in the molten steel and extend to the pouring zone,with an average residence time of 24 s and a maximum residence time of 200 s.The bubbles larger than 0.938 mm were gathered at the impact zone.The residence time and trajectory length of bubbles with the diameter of 0.134-0.670 mm in the tundish decreased significantly with the increasing bubble diameter,while that of bubbles with the diameter of 0.938-1.742 mm varied gently.The bubbles above 0.670 mm would impact to the bottom of the tundish along with the high-speed molten steel and then quickly float upwards.The bubbles with a diameter of around 0.670 mm had the largest filtration volume in unit time and more dispersed distribution in the tundish,resulting in the best filtration and purification effects on the molten steel.The volume of argon bubbles generated in the tundish per second to filter the molten steel was about 0.44 times of the volume of the impact zone.The fine argon bubbles generated by the large-flow argon injection into ladle shroud could play a good role in filtering and removing inclusions. |
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