The metal lumps or slag lumps embedded under the slab skin are called “slag inclusion”. The reason is:
(1) During open pouring, the splash of steel flow sticks to the surface of the mould and the cold steel is embedded in the solidified shell;
(2) The liquid level of the copper mould tube fluctuates too much, and the insoluble matter in the slag is involved in the solidification shell.
In terms of the composition of the slag inclusions, the appearance of manganese-silicate inclusions is large and shallow; Al2O3 inclusions are small and deep. If not removed, it will cause surface defects in the finished product and increase the scrap rate of the product. The thermal conductivity of slag inclusions is lower than that of steel, which causes the shell of the slag inclusion to grow slowly and the solidified shell to be weak, which is often the cause of leakage. Generally, the high melting point of casting powder makes it easy to form surface slag inclusions.
When casting the protective powder, the fundamental reason for slag inclusions is the instability of the copper mould tube liquid level. Therefore, the shape and size changes of the water outlet hole, the insertion depth, the amount of Ar gas blowing, the loss of control of the stopper rod, and the sudden change of the pulling speed will cause fluctuations in the crystallizer liquid level, which will cause slag inclusions in severe cases; in terms of the content of the slag inclusions, there are unmelted powdered protective powder, Al2O3 inclusions that float up and are not absorbed by the liquid slag in time, and excessive high-melting-point Al2O3 that is absorbed and dissolved.
When the depth of subcutaneous slag inclusion is less than 2mm, the ingot can be eliminated during the heating process; when the depth of subcutaneous inclusion is 2-5mm, the ingot must be surface finished before hot working. To eliminate the slag inclusion on the surface of the ingot, the following measures should be taken:
a. Minimize the fluctuation of the liquid level in the copper mould tube, preferably control it to less than, and keep the liquid level stable;
b. The insertion depth of the submerged nozzle should be controlled at the optimal position of (125±25)mm;
c. The inclination angle of the outlet of the submerged nozzle should be properly selected, with the principle that the outlet stream does not stir the meniscus slag layer;
d. The amount of Ar gas blown by the intertank plug should be controlled appropriately to prevent the strong stirring and turning of the steel slag interface when the bubbles float up;
f. Select protective powder with good performance, and the original content of ω (Al2O3) should be less than 10%, and at the same time control a certain thickness of the liquid slag layer.
