Tundish powders
A tundish cover powder is placed on top of molten steel in the tundish.
In general, the tundish covering powder should have the following functions:
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To act as a thermal insulator.
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To act as a barrier between air and liquid steel to prevent reoxidation.
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To assimilate the inclusions that separate from the steel during casting.
The tundish powders are mainly divided into two broad categories: acid and basic. The main component of acid powders is the silica (SiO2) while calcium oxide (CaO) based powders are the basic powders.
When using an acid covering powder the steel cleanness can be weakened by oxygen pick-up – reoxidation – from the covering powder because the oxygen potential of an acid slag is by far higher than that of the steel.
For high grade steels basic tundish covering powder should be preferred. Basic covering powders in the tundish are also beneficial for the lining because they do not readily react with basic refractories.
The basic powder,by their natural,are not a good thermal then be covered with acid powders (so called double layer).
In order to satisfy the three functions listed above you generally take these steps:
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Powder basic (low melting temperature) over steel, to form a liquid layer with a high capacity to absorb impurities and an effective barrier against the phenomenon of reoxidation. This layer not acts as a thermal insulator.
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Adding a layer of powder acid (direct contact with the external environment) that offers a high thermal insulation.
