| At the end of teeming from refining ladle, slag with steel will pour through the nozzle according to vortex in varying degrees when steel free surface gets certain level. Slag entrapment that is caused by vortex in ladle is related to function of subsequent tundish and mold technology directly, and has lots of disadvantages to casting process and final steel quality. In this paper, study of essence and law of vortex and slag entrapment, revealing of liquid flowing mechanism and features, investigating of relevant influencing factors and effective controlling method is proceeded by theoretical analysis, physical experiment and numerical simulation. This work has some theoretical and guiding significance to further illustrate mechanism of slag entrapment in ladle, take measures to control slag down, improve productivity and steel purity.Theoretical analysis show that during pouring process in ladle, liquid vortex generated at free surface, and developed gradually according to the relationship of ω1’=(r/r’)2ω, and became funnel-shaped finally. The rotary direction of vortex was anticlockwise with the Coriolis force caused by earth rotation. The subsequent physical experiment concluded that the rotary direction of vortex was uncertain with resting time was short if other condition was certain. With longer resting time, little nozzle eccentricity ε and other condition was not obvious, and the vortex rotated anticlockwise. With increasing nozzle eccentricity, the vortex was found right above the nozzle and its rotary direction became indefinite due to existence of ladle’s side wall.The development of vortex and its influencing factors in ladle were investigated by physical experiment with capacitive liquid level sensor device which was used to measure critical height of vortex. The results showed that vortex development was divided into four stages, which included generation of surface rotation, production of vortex, scallops of vortex core and vortex through the nozzle. The critical height of vortex rised with increasing of nozzle diameter, but descended with increasing of eccentricity ratio of nozzle position; initial liquid level had little effect on it; slag inhibit the formation and development of vortex to some extent, but the effect of the thickness of slag layer on critical height of vortex was not obvious. The relationship between critical height of vortex (/?) during casting and nozzle diameter of ladle (d), position of nozzle (ε) was expressed as h=65.276+1.427d-76.778ε.Unsteady heat conduction of steel in ladle during standstill and fluid steel flow dynamics during ladle drainage operations were studied by using numerical simulation method. Numerical results of unsteady heat conduction showed that temperature gradient in steel during standstill of ladle lead to steel’s circular flow and heat loss at the bottom of ladle was the most serious, which existed some small circular flow. Numerical results of steel pouring in ladle showed that mass flow rate at nozzle by a very small value, increased to maximum rapidly, and then decreased slowly due to conversion between potential energy and kinetic energy of steel. When vortex was appeared at the end of teeming, tangential velocity of steel was generated near the nozzle, which led to inflection point at flow-time curve.Numerical results of pouring in ladle with inclined plane bottom showed that steel at the bottom of ladle flowed into nozzle along the inclined plane and some vortex existed near the ladle’s side wall far away from nozzle, which dispersed free surface flow velocity of steel above the nozzle and inhibited vortex formation.Structure of ladle’s optimized nozzle was designed by physical experiment. The results showed that trapezoidal nozzle with upper height of10-30mm could prevent generation of vortex positively using trapezoidal nozzle with lager upper cross section instead of ordinary nozzle, and when upper height was20mm, critical height of vortex was lowest in all of other cases. According to different upper shape, which was circle, square and hexagon respectively, the effect of square shape on vortex formation was most obvious. When suitable structural trapezoidal nozzle was adopted, effect of preventing vortex was very evident and critical height of vortex and residual steel in ladle was decreased greatly compared with common nozzle.The influence of optimized bottom and model brick involved in ladle on vortex was studied by physical experiment. The results showed that the effect of preventing vortex was much clear with appropriate bottom structure of ladle and proper arrangement of model. Changing height, length of model and distance from the nozzle could influence flow field, and effect of height of model on vortex was obvious. Critical height of vortex was decreased with increasing height of model. When the height of model brick was about critical vortex height, the effect was best, which reduced1/3of the initial critical height. After ladle’s bottom was optimized and the model brick was added, the effect was satisfying too, and critical height of vortex was reduced about50%.The influence of floating anti-vortex apparatus and stopper rod on vortex in ladle was researched by physical test. The results showed that floating anti-vortex apparatus could prevent development of vortex core effectively and restrain generation of vortex. Polygon floating device compared with sphere floating device, could astrict development of fluid rotating velocity to a higher degree, and make free surface more stationary and reduce slag emulsification to enter into the nozzle. The existence of stopper rod could control vortex actively. Sink-vortex was eliminated when stopper rod was above the nozzle and at this time, teeming process of ladle was optimized immensely, and slag entrapment is least, and steel yield was maximum, and steel was more purer. |
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