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What are the measures to improve the surface quality of continuous casting slab

The surface defects of casting slab mainly refer to slag inclusion and crack. Such as serious surface defects. Finishing must be carried out before hot working, otherwise the metal yield and cost will be affected. It is the precondition of hot delivery and hot charging to produce casting slab with no defect on the surface. The shape of the surface defects of the casting slab is different, and the causes are complex. Generally speaking, the surface defects of the billet are mainly controlled by the solidification process of the molten steel in the mold. To ensure the surface quality, the following points must be paid attention to in operation:
(1) Stability of crystallizer liquid level: fluctuation of steel liquid level will cause uneven growth of billet shell, and slag will also be drawn into billet shell. The test indicates that the relationship between the liquid level fluctuation and the slag inclusion depth under the skin of the casting slab is as follows: the liquid level fluctuation range is mm, the slag inclusion depth under the skin is mm ± 20 < 2 ± 40 < 4 > 40 < 7. When the slag inclusion depth under the skin is less than 2mm, the casting slab can be eliminated during heating, and the surface of the casting slab must be cleaned when the slag inclusion depth is 2-5mm. The steel level fluctuates within ± 10mm, which can eliminate the slag inclusion under the skin. Therefore, it is very important to select a sensitive and reliable liquid level control system to ensure that the liquid level fluctuation is within the allowable range.  
(2) Mold vibration: the weak point on the surface of the casting slab is the "vibration trace" formed by the meniscus shell. The harm of vibration marks to surface quality is as follows: ① the valley of vibration marks is the origin of transverse cracks; ② the valley of vibration marks is the accumulation area of bubbles and slag particles. Therefore, the mold vibration mechanism with high frequency and small amplitude can reduce the vibration mark depth.  
(3) Uniformity of the primary shell: the uneven primary shell on the meniscus of the mold will cause longitudinal cracks and depressions of the casting slab, resulting in leakage. The uniformity of the shell growth depends on the steel composition, the cooling of the mold, the stability of the steel surface and the lubrication performance of the mold flux.
(4) Molten steel flow in the mold: the forced flow caused by the injection flow in the mold shall not involve the slag on the liquid surface. When the insertion depth of the submerged nozzle is less than 50mm, the slag powder on the liquid surface will be drawn into the solidified shell, forming a slag inclusion under the skin; When the insertion depth of submerged nozzle is more than 170mm, the slag inclusion under the skin will also increase. Therefore, the immersion depth and outlet inclination angle are very important parameters.
(5) Performance of protective slag: it shall have good ability of absorbing inclusions and lubricating ability of slag film.

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