Study On Transport Phenomena In Continuous Casting Tundish Using Argon Bubbling From Ladle Shroud

With increasing requirements for the cleanness of liquid steels,a continuous casting tundish,originally acting as a liquid steel distributor,was highly valued due to its metallurgical functions.In tundish operations,big inclusions(larger than 50?m)can be removed by promoting tundish flow using conventional flow controllers,such as weirs,dams,turbulence inhibitors et al.However,inclusions smaller than 50?m do not have enough buoyancy to float up by themselves.Hence,they cannot be effectively removed through tundish flow optimization.Inert gas bubbling technique is a practical method for the removal of small inclusions in tundishes.Because,the nonmetallic small inclusions which are non-wetted with the liquid steel can be brought to the top surface by attaching to bubble surfaces,or by being captured within a rising bubble's wake,and finally be absorbed by tudnish slag.This article tried to generate micro-bubbles within liquid steel passing through a ladle shroud into the tundish,in order to produce clean steel.Transport phenomena in continuous casting tundishes under micro-bubble conditions was investigated through water modeling and numerical simulations.The main research contents were drawn as follows:(1)Physical experiments were performed to generate micro-bubbles using a newly designed ladle shroud in a full-scaled,four-strand water model tundish.Gas was injected through small orifices located at the upper section of a ladle shroud.Bubbles were refined under the effect of shearing action caused by the entry flow,combined with the turbulent flow on bubble break-up.Average sizes of forming bubbles were investigated,considering the influence of gas flows,gas injection positions and ports numbers.A novel approach was applied to precisely measure bubble sizes,eliminating the measuring error caused by the distance between bubbles and camera lens.Under an appropriate gas injection scheme,the average size of bubbles was as small as 0.675 mm,in the range of micron scale.Compared with bubbles in regular size(around 3mm),micro-bubbles have a longer residence time in the tundish bath,a bigger surface area per unit volume,which are conducive to inclusion removal.(2)Three-dimensional numerical model,corresponding to the water experiment,was established to study the movement behaviors of the bubbles of different sizes in tundishes.The results showed that,micro-bubbles tended to flow with the flow path lines.As such,they are spread more widely in the tundish than bubbles in regular size.According to the comparison between flow field with and without micro-bubbles,flow patterns in tundish would not be impacted by micro-bubbles.Because,the driven force provided by micro-bubbles on water flow was very wake.Most of the micro-bubbles generated from the ladle shroud distributed in the pouring region of the tundish.Here,the fluid flow had a high velocity and a strong turbulence,which was difficult to be impacted by bubble flotation.Furthermore,the large amount of rising bubbles cannot form an upward fluid flow,due to their wide distribution.(3)A full-scaled water modeling was carried out,in order to study the removal of inclusions by micro-bubble swarms.Hollow glass borosilicate microspheres,smaller than 100?m,was used to simulated nonmetallic inclusions in liquid steel.Inclusion detection was implemented using a novel Aqueous Particle sensor(APS ?),with some modifications and calibrations.APS III,developed based on Coulter principle,can in-situ detect the number and size distribution of the inclusion at the tundish nozzle,under different micro-bubble conditions.The results revealed that the removal of small inclusion was enhanced by micro-bubbles.The effect of micro-bubbles on inclusion removal depended greatly on the gas injection protocols used.When gas was injected through four ports located at the first injection layer,with a gas flow rate of 0.2L/min(optimum gas injection scheme),the number density of inclusion at outlet was as low as 7.85/ml,corresponding to an inclusion removal ratio of 79.56%.(4)The behaviors of slag-metal interfaces with micro-bubble swarms were investigated in a water model tundish.Various micro-bubble conditions were achieved using varying gas injection schemes,in order to quantitatively analyze the effect of bubble sizes on the metal-slag interface.Linear low density polyethylene beads were used to simulate slag phase in tundish operations.The results indicated that micro-bubbles perform better on maintaining a stable slag layer,compared with bubbles in regular size.The formation of a slag eye was effectively limited by reducing gas flow rates,as well as increasing the number of gas injection ports.The critical condition for forming a slag eye was obtained,according to the slag behaviors under different micro-bubble conditions.The interaction between slag,metal and bubbles was considered in a multi-phase numerical simulation by coupling DPM model with VOF model.Slag eyes obtained by water modeling and numerical displayed similarity in shape.The relative error between slag eyes of two systems was only 17.6%,which indicates the validity of numerical simulation.A novel ladle shroud was designed based on a deep understanding of the dynamics when bubbles generated from a submerged nozzle.Bubble sizes were reduced dramatically by the shearing action from high speed cross flow and the break-up action from turbulent flow,producing micro-bubbles in tundishes.Owing to the effect of micro-bubbles,the ratio of inclusion removal was raised and the slag layer became more stable,which lay a great foundation for the production of clean steel.