| Argon gas injection on fluid flow in continuous slab casting mold plays an important role in the casting process and the slab quality. While it can prevent clogging of the SEN (Submerged Entry Nozzle) and secondary oxidation of molten steel, the conversion of flow pattern of molten steel which is changed from double recirculation to single recirculation is still a technical problem. The bubbles in the double recirculation can promote the inclusion to be removed, meanwhile, some of them are entrapped on the solidified shell to form internal defects in steel products, and how to strength the beneficial function of bubbles and circumvent their weaknesses has been a serious problem. Therefore, it is highly emphasized to reveal the behavior character of steel-gas-inclusion three-phases and bubble attachment on the solidified shell, as well as interaction of inclusion removal by bubble. They are three core questions conducted to understand and optimize the metallurgical factors on slab quality and productivity in this paper. The gas injection on fluid flow in a continuous slab casting mold was analyzed by means of water model simulation and numerical simulation. The mathematical model under the condition of produce process was built after two simulation methods matched.After the establishment of physical model for the mold according to similarity principle, the flow pattern with different air flowrates have been observed by using red color matter to trace and the critical air flowrate have been achieved under the different casting conditions in the mold model. Meanwhile, the mathematical model to simulate the gas-liquid two-phase flow in the water model has been developed by conservation equations for mass continuity and momentum, turbulence model and DPM (Discrate Phase Model). Since the results of physical model are greatly essential to validate and perfect the numerical model, the mathematical model can describe the behavior chariactor of molten steel-argon two-phase flow, so the formed conditions of a single or double recirculation flow pattern in the slab continuous casting mold with argon gas injection are quantitive described. As a result, the functional relationship of dimensionless criterion number [(Jgρg+Jlρl)(Jg+Jl)cos2θ]/ρlgD and gas-liquid volume ratio Qg/Ql, which the number is integrated with the relationships including flowrate of gas and liquid, density of gas and liquid, slab size, and tilt angle of the nozzle port, is proposed to discriminate the boundary of single and double recirculation flow pattern. The criterion condition, the area of single recirculation flow pattern is above the critical curve and that of double recirculation flow pattern is below the curve, is adapted to the conventional mold for slab caster.The bubble distribution and motion including velocity, pathlength and gas fraction parameters in the water model were invested by using the high-speed video camera measurement and the image processing. The results show that the bubble distribution is dominated by water flowrate as the nozzle outlet area and its angle is fixed, which is hardly influenced by other factors such as gas flowrate, SEN shape, and surbumerged depth, so the bubble distribution can be regulated by controlling the casting speed in the mold. The gas fraction in water model can be calculated by using HFM(Homogeneous Flow Model) whose initial condition is the relationship between the bubble terminal velocity and diameter measured in the physical experiment, so can the gas fraction in the mold. The bubbles weaken turbulence fluctuation of water flow, which can be observed from the variation of the flow velocity direction and the velocity value at the wide face, the free surface and the narrow face.When the spherical bubble attaches to solid wall in liquid, the free energy change formula in system is deduced to prove that the bubble attachment process at the balance between the buoyancy and the surface tension on the bubble can occur spontaneously with surface area reduced. It is because of this thatΔG<0 in system at the condition of the contact angle ranging from 0°to 180°. Meanwhile, the phenomenon can not happen becauseΔG=0 in system when the contact angle is equal to 0°or 180°. The attached bubble can not break off from the wall unless the extra power is applied. Moreover, the bubble attachment to metal wires in the water model was recorded by the high-speed video camera, which the process could be described as four steps including the bubble approaching to and attachment to the metal wires, forming liquid film between the bubble and the wires, forming new gas-liquid interface, the bubble sticking to the wires stably.Finally, the removal efficiency of inclusion by bauycancy and attachment to bubbles was studied quantitatively at five different water flowrates, by using two different purging plug types in SEN, and at four different gas flowrates. At first, the removal efficiency decreases rapidly as the water flowrate increases to a critical value, while it intends to slow as the water flowrate continues to enlarge. Secondly, the removal efficiency by using porous brick is increased than that by using multihole plug. Thirdly, as the gas flowrate enlarges, there is a modest increase of the removal efficiency due to three reasons including the probability of an inclusion attachment to a bubble, the bubbles distribution and their quantities. Moreover, the inclusion attachment to the bubble, which was the main method of inclusion removal, was recorded and vertified by the high-speed vedio camera. The process of an inclusion attachment to a bubble can be described as five steps including the bubble approaching and attachment to the inclusion, forming liquid film between the bubble and the inclusion, forming new gas-liquid interface after liquid film becoming thin, the inclusion slipping from the bubble top surface to bottom, the inclusion oscillation at the bottom of the bubble and float with the bubble. The inclusion can leave off from liquid film if there is any disturbution when the inclusion slips on the bubble surface. The removal efficiency of inclusion in the molten steel can be simulated by using DPM and HFM, and present the similar rule with the physical exiperiment. |
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