Casting billet surface defects vary in shape and are mainly affected by the solidification process of the mould liquid, and are related to the formation of the steel shell, liquid level fluctuations of the mould, immersion nozzle design, performance of protective slag and other factors.
The wave of the liquid level will cause uneven growth of the billet shell, slag will also be involved into the billet shell. If steel liquid level fluctuates in less scope, slag entrapment at the liquid will be less. Therefore, it is very important to choose a sensitive and reliable control system of the level to ensure that the fluctuations are within the allowable range.
The vibration of the mould will come into being some traces on the surface of the cast billet. The bubble, slag particles will gather area at lower position of the trace and influencing quality of the billet. To equip the vibration mechanism of high frequency and small amplitude can less depth of traces.
Incipient billet shells surface of the mould is uneven to lead to longitudinal cracks and sunken in the cast billet, resulting in liquid breakout. Uniformity of shell growth is determined by the steel composition, mould cooling, steel surface stability and lubrication performance of protective slag.
The residues on the liquid surface should not be involved into the interior when the liquid is poured. The insertion depth of the Immersion nozzle is need to mind carefully, if the depth is too shallow, slag powder will be involved in the solidification shell forming slag inclusion; another key parameter is the exit angle of the nozzle.
The protection slag must be a good ability to absorb inclusions and lubricate slag film.
