| The metallurgical process in the mold is key process to the improvement of slab quality.The flow behavior of molten steel plays a very important role in continuous casting process.Therefore,it is of great significance to achieve controlling of molten steel flow in the mold.In the continuous casting process,the flow behavior of molten steel in the mold will influence the movement and distribution of inclusions and argon bubbles,and then influence the quality of continuous casting slab.The high-temperature molten steel which exits from the side port of submerged entry nozzle(SEN)impacts the narrow face of mold and forms upward backflow and downward backflow.The upward backflow directly impacts the meniscus and causes the instability and fluctuation of the meniscus and easily causes the defects of continuous casting slab,such as the oscillation marks on the slab surface and the subsurface inclusion.The downward backflow can carry the non-metallic inclusions and argon bubbles deep into the mold cavity and forms the subsurface defects if they are captured by the initially solidified shell.To ensure a high quality of continuous casting slab,the electromagnetic brake(EMBr)technology which was born in the 1980s has been widely used to control molten steel flow in the mold as an effective technique,has become one of the important means to control the molten steel flow in the mold of continuous casting process with high casting speed.EMBr technology has developed from the first generation of Local Magnetic Field to the applied second generation of Level Magnetic Field and the third generation of Flow Control Mold.However,owing to the limitation of effective region of magnetic pole,the braking effect of the conventional EMBr was obviously affected by process parameters and electromagnetic parameters,such as SEN depth,SEN port angle and casting speed,reduce the braking effect of conventional EMBr.Based on the basic principle of electromagnetic brake technology,the thesis propose a new pattern of electromagnetic brake(EMBr)device and focus to study the behavior of molten steel flow,fluctuation of steel/slag interface and braking effect of EMBr in high casting speed slab mold.The numerical simulations was carried out to investigate the behavior of molten steel flow,fluctuation of steel/slag interface and braking effect in continuous casting slab mold with vertical electromagnetic brake(V-EMBr)and EMBr-Ruler.The physical model for describing the fluctuation of mold level with V-EMBr was set up.The thermal physical simulation experiment was carried out to study the behavior of level fluctuation and meniscus shape with V-EMBr.The main research results are as follows:(1)The three-dimensional mathematical model was established to describe magnetic field in the continuous casting mold with electromagnetic brake.The distribution of magnetic field with V-EMBr and EMBr-Ruler in the mold were investigated.The research results are as follows:1)The magnetic field that generated by V-EMBr is mainly distributed in the coverage area of vertical magnetic poles.The value of magnetic flux density was gradually reduced from the pole center to the wide face center of the mold.The magnetic field acting region along the mold width direction is expanded as the current indensity is increased.2)The magnetic field that generated by EMBr-Ruler mainly distributed in the coverage area of horizontal magnetic poles.The value of magnetic flux density was gradually reduced from the pole center to the edges of the horizontal magnetic pole.(2)The three-dimensional mathematical model was established to describe the behavior of molten steel flow and the fluctuation of steel/slag interface in the mold with V-EMBr.The influence of process parameters,such as casting speed,SEN depth,the SEN port angle and magnetic flux density,on the molten steel flow,the fluctuation of steel/slag interface and the braking effect of V-EMBr were investigated.The research results are as follows:1)The Lorentz force that generated by V-EMBr is mainly distributed in the jet flow impact region,upward and downward backflow zone at the same time,the Lorentz force has a reverse direction to the velocity of molten steel.For a given magnetic flux density,the magnitude of Lorentz force in upward and downward backflow region is obviously larger than that generated by EMBr-Ruler.2)The application of V-EMBr is helpful to reduce the velocity of upward backflow,to weaken the impact of upward backflow on the liquid slag layer and to increase the thickness of liquid slag layer.The braking effect of V-EMBr was slightly influenced by the variation of the SEN depth and the SEN port angle.The impact direction of jet flow will be deflected downwards when a strong magnetic flux density was applied,which leads to an excessive depression on the upward backflow.3)The effective magnetic field region along the height direction of mold was expanded with V-EMBr,which reduces the diffusion extent of jet flow along the casting direction and the thickness direction of the mold.Meanwhile,the impact effect of jet flow on the narrow face and wide face of mold was also decreased.(3)The thermal physical simulation experimental system of V-EMBr with SnPbBi low melting temperature alloy was set up.The influence of the SEN depth,the SEN port angle and magnetic flux density on the instantaneous fluctuation of liquid metal and meniscus shape were studied.The research results are as follows:1)The peak values of instantaneous fluctuation of liquid metal during the measurement process were greatly reduced as the application of V-EMBr.The instantaneous fluctuation intensity of mold level was gradually decreased as magnetic flux density increased.When the Bmax reached to 0.31 T,the abnormal fluctuation disappeared.2)The decrease of the SEN depth and the SEN port angle will increase the fluctuation intensity of liquid metal level in mold.The application of V-EMBr can reduce the average wave height of liquid metal level caused by the variation of the SEN depth and the SEN port angle,so that the fluctuation of mold level was stabilized.3)When the V-EMBr is not applied,the meniscus showed an irregular shape owing to the high surface velocity of liquid metal and the height of meniscus reached to 22 mm.With V-EMBr,the surface velocity of liquid metal is gradually decreased as the magnetic flux density increased,and the fluctuation intensity of liquid metal level is also decreased.The meniscus shape was more smooth and the height of meniscus was reduced to 8 mm.(4)The three-dimensional mathematical model was established to describe the behavior of molten steel flow and the fluctuation of steel/slag interface in the mold with EMBr-Ruler.The influence of process parameters(casting speed,SEN depth and SEN port angle)and magnetic flux density on the molten steel flow,the fluctuation of steel/slag interface and the braking effect of EMBr-Ruler were investigated.The research results are as follows:1)With EMBr-Ruler,the Lorentz force is mainly distributed in the jet flow impact region and the Lorentz force in upward and downward backflow region is much smaller compared with V-EMBr.2)As EMBr-Ruler was applied,the molten steel flow and the fluctuation of steel/slag interface were obviously influenced by the variation of the SEN port angle.The variation of the SEN port angle cause the change of jet flow direction and the flow intensity of upward backflow.A high magnetic flux density can easily lead to a low velocity at meniscus region which will influence the melting of mold powder and lubrication between the mold wall and the initially solidified shell.3)When EMBr-Ruler was applied,the diffusion extent of jet flow along casting direction was reduced,however the diffusion extent of jet flow along the thickness direction was increased.Therefore,the impact effect of jet flow on the wide face of mold is increased.(5)The comparation of braking effect between the V-EMBr and EMBr-Ruler was analyzed to clarify the advantage of V-EMBr.The research results are as follows:1)The braking effect of V-EMBr was slightly influenced by the SEN depth and the SEN port angle.The application of V-EMBr can effectively reduce the diffusion extent of jet flow along the casting direction and the thickness direction of the mold,and decrease the impact effect of jet flow on the narrow face and wide face of mold.2)The braking effect of EMBr-Ruler was obviously influenced by the SEN depth and the SEN port angle,increase the surface velocity of molten steel and exacerbate the fluctuation of steel/slag interface.The application of EMBr-Ruler can increase the diffusion extent of jet flow along the thickness direction of mold and the impact effect of jet flow on the wide face owing to the limitation of effective magnetic field region.3)Compared with EMBr-Ruler,the application of V-EMBr is more conducive to reduce the height of meniscus and stabilize the fluctuation of steel/slag interface with the same magnetic flux density.The meniscus height is easily tends to be a stable value which means that the application of V-EMBr is more conducive to stabilize the fluctuation of steel/slag interface. |
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