Steel Induction Furnace

Large-capacity steel induction furnace are used respectively for melting, holding, and combined melting and holding.
a) medium frequency induction melting furnace in Melting

b) Holding

c) Combined melting/holding for coreless induction furnaces.

The steel induction furnace functions are combined in this type of coreless furnace, either to reduce the carbon content of cast iron molten iron by adding steel scrap, or to include both melting and holding functions in one melting system. For this purpose, coreless furnaces are usually equipped with a two-section coil. One example of application is a 40 t capacity crucible coreless furnace that uses low-cost nighttime electricity to melt a full batch of molten iron, which can then be tapped at the start of the first shift the next day.

Here, the frequency of the variable-frequency power supply of steel induction furnace is kept as close as possible to the mains frequency in order to promote stirring of the molten iron. To illustrate another method, the calculated flow pattern in a 50 t electric furnace, including different applications of a two-section coil at 3500 kW as a half-furnace coreless furnace. It shows that the half of the crucible with a single-section coil operates at a frequency of 100 Hz. A meniscus is hardly visible, and the flow velocity of the molten iron is low. This kind of furnace is suitable for holding. However, it can only combine melting and holding storage under conditions where no property adjustment is required after melting. Stirring in the furnace is sufficient for uniformity of molten iron temperature and chemical composition.

A two-section coil, in which the lower section coil is supplied by 50 Hz. The lower section coil produces relatively strong stirring of the molten iron at the bottom of the coreless furnace; however, as can be imagined, no meniscus is visible at the molten iron surface, and the flow of the molten iron above is relatively slow.

The upper coil of medium frequency induction melting furnace is supplied by 3500 kW at 50 Hz. A relatively obvious meniscus of the molten iron and a relatively strong flow velocity can be seen above. Therefore, an electric furnace with these characteristics is very suitable for duplex melting.
The same coil was used, except that the frequency was 100 Hz instead of 50 Hz. The surface bulge and the molten iron flow velocity were somewhat reduced.

Recycled material for steel induction furnace should be sorted by alloy and stored dry to prevent moisture and rust. Entrained or caked sand can lead to increased slag formation, resulting in higher energy consumption and deposit buildup. However, when charging clean material, the slag composition after shot-blast cleaning should be kept within the acidic range, i.e., with a sufficiently high proportion of SiO₂ (adding a small amount of sand can help with this). Residues from exothermic additives or ceramic filters can lead to aggressive slag, which will shorten the service life of the refractory lining. If the size of the returned material is too large, it should be broken up during charging to prevent tangling and to improve electrical efficiency due to the higher bulk density.

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