Study On The Multiphase Flow, Heat Transfer And Solidification, Motion And Entrapment Of Inclusions During Continuous Casting

Continuous casting consists of many related transport phenomena, such as fluid flow, heat transfer, solidification, and so on. In the current study, according to the features of continuous caster machine, the segment-based method was adopted to simulate the phenomena from the meniscus to the solidification end for slab continuous casting with Flow Control Mold (FC-Mold) and round billet centrifugal continuous casting, respectively. Besides that, some industrial measurements were also carried out.In the first part, a three-dimensional model coupling fluid flow, heat transfer, solidification for slab continuous casting process with FC-Mold was constructed.The current work investigated the influences of FC-Mold on fluid flow, level fluctuation at meniscus, heat transfer, and subsurface cleanliness of slab were discussed with casting parameters, such as magnetic field intensity, casting speed and slab cross section. Main results show that after applying FC-Mold, the formation of vortex on meniscus was suppressed, and the average temperature of molten steel at the top surface increased by 2 K. The distribution of temperature on the slab surface became symmetrical, and the calculated metallurgical length changed little. While the application of FC-Mold could lead an increase of reheat temperature near the slab corner about 450 mm below meniscus.When the slab cross section was 1300 mm, after applying FC-Mold, the solidification end changed little. While the metallurgical length increased by 2.5 m as the casting speed increased from 1.8 m/min to 2.0 m/min.For the real-time industrialdata, the methods of smoothing and variance were proposed to deal with the time-dependent level position and thermocouple temperature, respectively. It found that when the upper magnetic field intensity was half of the lower one, FC-Mold could decrease the power of the low-frequency fluctuations and the temperature in the upper mold region where the magnetic field worked.Combined with the simulation results, the reason for the decrease of>10 ?m inclusions at the subsurface 2 mm below loose side after applying FC-Mold was explained. With the application of FC-Mold, as the temperature of molten steel on meniscus increased, the growth of hook was damped, for which both the average length and depth of hook reduced. Thus, the amount of inclusions entrapped by the hook decreased, resulting in an improvement of cleanliness for the slab subsurface.The effect of FC-Mold on improving the symmetry of total oxygen in slab was remarkable. Moreover, after applying FC-Mold, the average total oxygen in slab decreased from 12.2 ppm to 10.6 ppm.The second part involved the systematic scientific study about the centrifugal continuous casting process. Firstly, a three-dimensional multi-phase turbulent model was established to study the influences of parameters (nozzle position, casting speed and rotational speed) on the transport phenomena. The simulation results show that the casting speed and rotational speed had the most effects. After increasing the casting speed twice, the penetration depth of jet flow from nozzle increased sharply from 126 mm to 220 mm, and the volume of entrained gas increased from 0.0003 m3 to 0.0012 m3. The reduction of rotational speed was beneficial to decrease the volume of entrained gas, and the penetration depth was the minimum with 120 mm when the rotational speed was 60 rpm. The larger the angular moving angle of the nozzle, the more dispersed the distribution of inclusion was. While the casting speed showed a reversed trend.Then the characteristics of heat transfer and solidification under the rotating system was studied. The jet flow from the nozzle hardly impinged on the same location on the shell due to the rotation of the shell during solidification. Thus, the shell thickness on the same height was uniform around and the thinning shell and a hot spot on the surface of shell was avoided. The calculated metallurgical length was approximately 8.5 m. Subsequently, the distribution of inclusions within the shell was predicted based on the critical liquid fraction and flow velocity at the solid-liquid front, as well as the inclusion diameter. It indicated that with a larger diameter, inclusions tended to be entrapped towards the center area of the billet.Finally, the automated particle analysis was employed to detect non-metallic inclusions the cross section of unsteady and steady round billets, in which the distributions of sulfide, oxide, and dual phase oxide-sulfides inclusions were obtained. The calculated solidification velocity was higher than the upper limit at which inclusions were entrapped by the solidifying front, revealing that for oxide inclusions smaller than 8 ?m in this study, the centrifugal force had little influence on its final distribution in billets. Etching experiment was performed to reveal the dendrite structure of the billet, which explained the band-shape pattern of MnS distribution at the outside 25 mm layer.