Casting billet surface defects vary in shape and are primarily influenced by the solidification process within the mould. These defects are closely related to the formation of the initial steel shell, fluctuations in the mould’s liquid level, immersion nozzle design, the performance of protective slag, and other related factors.
Proper control of these key parameters can significantly reduce surface defects on the billets.
Mould Level Control
Fluctuations in the mould’s liquid level can cause uneven shell growth and lead to slag entrapment at the solidification front. If the liquid steel level remains stable within a narrow range, the risk of slag inclusion is reduced. Therefore, it is crucial to adopt a sensitive and reliable level control system to ensure that level fluctuations remain within the permissible limits.
Mould Vibration Mechanism
Mould vibration may leave marks on the billet surface. Bubbles and slag particles tend to accumulate at the lower parts of these marks, affecting the surface quality. Using a high-frequency, low-amplitude vibration system can help reduce the depth and severity of vibration marks.
Uniformity of Initial Shell Formation
Uneven formation of the initial solidified shell can lead to longitudinal cracks and surface depressions, potentially causing steel breakout. The uniform growth of the shell is influenced by factors such as steel composition, mould cooling conditions, steel surface stability, and the lubrication properties of the protective slag.
Liquid Flow Behavior in the Mould
Surface residues should not be entrained into the molten steel during casting. The insertion depth of the immersion nozzle must be carefully controlled—if too shallow, slag powder may be drawn into the solidifying shell, causing slag inclusions. Another critical parameter is the nozzle’s exit angle, which affects flow direction and turbulence.
Functions of Protective Slag
Protective slag must possess good absorption capability for inclusions and provide effective lubrication to form a stable slag film between the mould and the billet surface.
