The flow of molten steel in the mould tubes has a great influence on the quality of the mold. The unreasonable flow of molten steel in the mold will lead to many defects in the slab, such as bubbles and inclusions in the slab, entrainment of mold slag, surface defects, and leakage. Therefore, studying the control technology of molten steel flow in the mold plays an important role in improving the quality of the cast slab.
A reasonable flow field of the mold tube can make the molten mold slag form a uniform slag film between the mold wall and the solidified shell, which plays a lubricating role, reduces the drawing resistance, and prevents the occurrence of breakout accidents. Improve the heat transfer conditions of the billet and improve the surface quality of the billet. The unreasonable flow field of the mold will entrain the mold flux into the molten pool, which may be captured by the front of the solidified billet shell, forming subcutaneous slag inclusions and affecting the quality of the casting billet. In addition, the high-temperature molten steel from the nozzle outlet collides with the mold shell, and the backflow washes the shell to cause uneven growth, and cracks and surface defects are prone to appear in weak places. Therefore, it is necessary to control the injection flow to a reasonable impact depth and impact pressure. At the same time, by changing the flow state of molten steel, the dendrites are interrupted, the formation of equiaxed grains is promoted, and the internal structure of the slab is improved. Reasonable molten steel flow can also fully float the inclusions.
At present, the use of electromagnetic stirring to optimize the flow field of the mold has become an important means to improve the quality of the slab, and has been widely used in continuous casting production. The electromagnetic stirring of the mold is mainly at the initial stage of molten steel solidification, through the action of electromagnetic stirring, the flow and heat transfer conditions of the steel/slag interface are improved, so that the initial solidification shell tends to be uniform. Electromagnetic stirring technology can also improve the equiaxed crystallinity of the billet, which is one of the most effective means to improve the internal structure of the billet. The electromagnetic stirring of the mold is mostly used in billet continuous casting. The flow generated by the electromagnetic stirring scours the solidification front, breaks the dendrites, and promotes the formation of equiaxed grains. In addition, a rotating magnetic field is set at a certain height in the middle section of the submerged nozzle, which can control the flow state of molten steel entering the mold to form a swirling motion. The flow of the nozzle and the crystallizer is controlled by rotating the magnetic field, and the impact depth is about 50% lower than that of the straight nozzle under the same outflow flow conditions. At the same time, with the increase of the pulling speed, the horizontal flow velocity of the liquid surface generated by the swirl nozzle is faster, which is beneficial to the melting of slag on the surface of the molten steel in production.
With the increase of the casting speed, the flow rate of the molten steel outflow from the submerged nozzle increases continuously, and the impact of the high temperature liquid flowing out of the nozzle on the solidified shell is intensified. It is easy to lead to the remelting of the shell and even the leakage phenomenon, which aggravates the capture of inclusions by the solidified shell. An electromagnetic brake is set on the mold, and the upward electromagnetic force is used to stop the molten steel flowing out of the submerged nozzle and change its direction. In this way, the penetration depth of molten steel is reduced, the inclusions are promoted to float and separate, and at the same time, the fluctuation of the meniscus is suppressed to prevent slag entrainment. Electromagnetic braking is mainly used for thin slab continuous casting with high drawing speed. The new type of mold electromagnetic braking device is composed of two pairs of upper and lower bar-shaped magnetic poles. The upper magnetic pole is installed at the liquid level of the mold to control the fluctuation of the liquid level, and the lower magnetic pole is installed in the nozzle area to control the impact depth of the lower backflow. Full control of the internal flow field.
At present, the soft contact electromagnetic continuous casting technology has received extensive attention. This technology uses the electromagnetic pressure perpendicular to the surface of the slab to push away the molten steel near the inner wall of the mold, so that the width of the mold slag channel increases, the slag liquid seeps smoothly, improves the lubricating conditions of the slab, and reduces the drawing resistance. This technology can significantly improve the surface quality of the slab, and at the same time, the internal structure of the metal is dense and the quality is significantly improved.
