Research Of Simulating Inclusion Floating, Collision And Aggregation Law In Molten Steel Based On Fractal Theory

Nonmetallic inclusion in steel is the most important factor that influences the quality of steel, as a result, the floating, collision, aggregation and other removal dynamic process of inclusions in molten steel have been studied by many domestic and foreign metallurgists. Due to different desoxidants used in the steel making process, deoxidization products which stagnated in the molten steel are solid inclusions with high melting point. The interfacial tension between solid inclusions and liquid steel is generally big, so large-dimension and complex inclusions are easily formed after all kinds of collision and aggregation. At present, inclusion clusters are often simplified to spherical inclusions by most researchers. However, there is a big difference between the morphology and structure of inclusion clusters and that of spherical inclusions, this simplification can’t reflect the real floating, collision and aggregation behavior of inclusion clusters.In this paper, the common alumina inclusion clusters in aluminum killed steel are chosen as the research project. On the basis of building the structure model and describing the alumina inclusion cluster morphology using fractal theory. the floating, collision and aggregation behaviors are investigated using the simulation method. At last, taken the influence of morphology on the floating, collision and aggregation into consideration, the space distribution and moving trajectory of inclusion clusters in the continuous casting slab mold are discussed. The concrete contents are as follows:Firstly, in view of the difficulty of describing the morphology and structure of inclusion clusters using the traditional Euclidean geometry, the fractal theory is introduced. Taken the alumina inclusion clusters for example, the feasibility of using the fractal theory to study the morphology and structure of inclusions is analyzed. The results show that:the structure of inclusion clusters has obvious fractal characteristics, and the fractal dimension can be calculated using the box counting method based on image processing; The fractal dimension of inclusion clusters is a important parameter to the quantitatively describe its morphology, the larger the fractal dimension, the denser the structure of clusters alumina. Based on the fractal characteristics of inclusion clusters, a three dimensional fractal structure model is built, which can quantitatively describe the structure of inclusion clusters.Based on the fractal structure model of inclusion clusters, different morphologies of fractal aggregates were built to represent real inclusion clusters in molten steel. In order to investigate the floating behavior of inclusion clusters in molten steel, the lattice Boltzmann method was introduced and whether this method can be used to simulate the floating behavior directly in molten steel is investigated. The results show that:the floating behavior of different fractal aggregates with fractal dimension smaller than three and equal to three can be simulated directly using the lattice Boltzmann method.Based on the fractal structure model of inclusion clusters, different morphologies of fractal aggregates can be built, and the floating behavior of those fractal aggregates in molten steel were simulated using the lattice Boltzmann method to investigate the floating behavior of inclusion clusters. The results show that:the fractal dimension is the important factor which has a great influence on the floating behavior of inclusion clusters. The structure of inclusion clusters is loose, so the floating velocity of inclusion clusters is smaller than the spherical inclusion particles with the same volume. In the traditional research, inclusion clusters with small fractal dimension are often simplified into spherical inclusions, which will overestimate the floating velocity of inclusion clusters. The floating velocity model can be expressed as V=N0.9468-0.9197/1)r 2g(ρm-ρs)r2/9μm Re<2, which can reflect the relationship between the floating velocity of inclusion clusters and its morphology.The simulation research of collision and aggregation during the floating process of inclusion particles shows that:because of the floating velocity difference between different size inclusion particles, inclusion particles can collide and aggregate with each other into complex morphologies of inclusions. The collision process can significantly increase the floating velocity of small size inclusions. The aggregate structure formed from the collision of different size inclusions is loose, and the fractal dimension decreases. There is a great difference between the collision of solid inclusions and liquid inclusions. Liquid inclusions can rapidly integrate into big size spherical liquid inclusions because of the surface tension.The space distribution and motion trajectory of inclusion clusters in the slab continuous casting mold were simulated. The results show that:the complex morphology inclusions can accumulate easily in the conjunction area between the nozzle hole and outlet, the nozzle bottom and the top area of the nozzle outlet. So in those areas, the nozzle clogging will occur easily. The fluid flow in the mold exist upper and lower circulation zones, the number density of big size and complex morphology inclusions formed from collision is large in the vortex center of the circulation zones.When alumina inclusion clusters with diameter of 10,40,70, 100μm were compared with spherical alumina inclusions of the same size, the floating removal rate absorbed by the protection slag decreases 0.3%,0.5%,1.6%,3.3% in the continuous casting slab mold. With the increasing of inclusion size, the floating removal rate of alumina inclusion clusters compared with that of the same size spherical inclusions decreases more.