The surface defects of the slab mainly refer to slag inclusions, cracks, etc. Such as serious surface defects. Finishing must be done before hot working, otherwise metal yields and costs are affected. The production of surface-defect-free billets is a prerequisite for hot charging. The surface defects of the slab have different shapes, and the reasons for their formation are complicated. Generally speaking, the surface defects of the slab are mainly controlled by the solidification process of the molten steel in the mold. In order to ensure the surface quality, the following points must be paid attention to in operation:
(1) The stability of the liquid level of the mold: the fluctuation of the molten steel level will cause uneven growth of the shell, and the slag will also be involved in the shell. The test points out that the relationship between the liquid level fluctuation and the subcutaneous slag inclusion depth of the slab is as follows: Liquid level fluctuation range, mm subcutaneous slag inclusion depth, mm ±20<2±40<4>40<7 When the subcutaneous slag inclusion depth is less than 2mm, the casting The billet can be eliminated during heating, and the surface of the billet must be cleaned when the slag inclusion depth is 2 to 5 mm. The molten steel level fluctuates within ±10mm, which can eliminate subcutaneous slag inclusions. Therefore, it is very important to choose 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 slab is the “vibration trace” formed by the meniscus shell. The damage of vibration marks to the surface quality is:
1) The trough of vibration marks is the origin of transverse cracks。
2) The trough is the accumulation area of bubbles and slag particles. To this end, the use of high-frequency and small-amplitude mold vibration mechanism can reduce the depth of vibration marks.
(3) Uniformity of the primary shell: The unevenness of the primary shell on the meniscus of the mold will cause longitudinal cracks and depressions in the casting billet, resulting in leakage. The uniformity of shell growth is determined by the steel composition, mold cooling, steel level stability and flux lubrication properties.
(4) The flow of molten steel in the mold: the forced flow of the mold caused by the injection flow should not involve the slag on the liquid surface into the interior. The insertion depth of the immersion nozzle is less than 50mm, and the slag powder on the liquid surface will be involved in the solidification shell, forming subcutaneous slag inclusion; the insertion depth of the immersion nozzle is greater than 170mm, and the subcutaneous slag inclusion will also increase. Therefore, the insertion depth of the immersion nozzle and the inclination angle of the outlet are very important parameters.
(5) Molding slag performance: It should have good ability to absorb inclusions and lubricity of slag film.