Induction Furnace Steel Making

Induction furnace steel making is a steelmaking method that uses induction electrothermal effect to heat and melt metals. The structure of an induction furnace can be divided into two methods: cored and coreless. The cored induction furnace is suitable for smelting non-ferrous metals and their alloys, while the coreless induction furnace is suitable for smelting ferrous metals and alloys, especially for high-grade and high-quality alloy steels and special alloys. The induction furnaces mentioned in this article refer to coreless induction furnaces.

According to the frequency of input current, induction furnace can be divided into three types: high frequency (above 10000Hz, vacuum tube type high-frequency generator is used), medium frequency (500~10000Hz, generator type medium frequency generator is used), and normal frequency (i.e. power frequency).

Advantages and Disadvantages of Induction Furnace

The Advantages of Induction Furnace Steel Making

1) No carbon electrode is used, and no carbon will be added during smelting, so steel grades and alloys with low carbon content can be smelted.

2) Without an electric arc, the decomposition of gas in the high-temperature area of the electric arc can be avoided, so that steel with low gas content can be smelted.

3) The adjustment of power and temperature is simple and convenient, and its range is wide, which can accurately control the temperature required by metal.

4) The electric stirring of molten steel in the crucible can accelerate the metallurgical reaction between molten steel and slag, promote the removal of non-metallic inclusions and gases, and also facilitate the homogenization of molten steel temperature and chemical composition.

5) The recovery rate of alloy elements is high. The easily oxidized alloy elements can be added under the condition of good deoxidation of molten steel. The unit surface area of the molten pool of the induction furnace is small, so the burning loss of alloy elements is small and the recovery rate is high.

6) It can be smelted under a vacuum or in an atmosphere favorable to smelting.

The Disadvantages of Induction Furnace Steel Making

① The slag cannot be heated by induction, but only by molten steel. Therefore, the slag temperature is low, which is not conducive to the reaction between the slag and steel interface.

② The crucible wall is thin, coupled with the continuous stirring and scouring of metal flow, the scouring, and erosion of slag, and the large temperature difference between the inside and outside of the crucible wall, so the life of the crucible is short, usually only dozens of times.

Smelting Process

1) The Furnace Lining Materials Required Shall Be Selected According to the Type of Smelting Steel.

Some steel grades are only suitable for smelting in alkaline furnaces. When smelting high manganese steel grades and alloys in an acid furnace, (MnO) will combine with SiO2 in the furnace lining to form a low melting point silicate, thus rapidly damaging the furnace lining. When smelting steel grades and alloys with higher aluminum and titanium content, the aluminum and titanium content will reduce a large amount of silicon dioxide in the furnace lining, which will not only damage the furnace lining, but also make it difficult to control the content in the steel. In addition, the high chromium-nickel steel produced in an acid furnace contains more silica inclusions than that in an alkaline furnace; Therefore, it is better to smelt the steel in the alkaline furnace.

2) Charge

Induction furnace smelting is basically a melting process. The oxidation method is rarely used for smelting. The slag temperature is low, the ability to remove sulfur and phosphorus is poor, and the smelting time is very short. The chemical composition cannot be controlled by furnace front analysis. For these reasons, the charge requirements are very strict.

The sulfur and phosphorus content in the furnace charge shall be as low as possible, at least 0.005%~0.01% lower than the lower specification limit. The weight and chemical composition of all furnace materials, including various ferroalloys, must be accurately mastered so as to control the composition of steel through calculation during smelting. The furnace charge shall be clean, dry, and rust-free, especially for gas-sensitive steels and alloys such as nickel base alloys. The furnace charge and slag must be fully preheated and baked before use.

The lump size of the furnace charge shall be appropriate to ensure the charging density. The contact area between materials increases when the furnace charge is too fine, and the bulk specific gravity of the entire column decreases when the block is too large, both of which increase the resistance value of the column.

3) Charging and Melting

Before charging, the residual steel residues in the furnace shall be removed, and the furnace lining damage shall be checked. The locally damaged parts blackened due to fast cooling shall be repaired. The particle size of furnace repairing materials shall be smaller than that of knotting materials, and a little more binder shall be used. Large furnaces with serious damage can be lifted into the iron mold for filling and knotting.

As the temperature of the induction furnace drops rapidly after tapping, it must be charged quickly. As far as possible, the material shall be loaded in drums. In order to accelerate the melting, the material distribution should be reasonable according to the temperature distribution in the furnace. The temperature distribution in the induction furnace is shown in Figure 2. Due to the skin effect of the current, the surface (Zone I) around the column close to the crucible wall is a high-temperature area, the bottom and middle (Zone D and ID) are relatively high-temperature areas with poor heat dissipation, and the upper (Zone IV) is a low-temperature area with less magnetic flux and large heat loss.

In order to form slag in advance, slag material accounting for 1% of the material weight can be added to the furnace bottom before charging, lime and fluorite can be added to the alkaline furnace, and broken glass can be added to the acid furnace.

At the beginning of induction furnace steel making, because the inductance and capacitance on the line are not matched properly and the current is unstable, the power can only be supplied with low power in a short time. After the current is stable, it shall be switched to full-load power transmission. During the melting process, the electrical capacity should be constantly adjusted to keep the electrical equipment with a high power factor. The molten steel will be heated to a certain extent after the charge is fully melted, and then the input power will be reduced according to the smelting requirements.

Proper melting time should be controlled. If the melting time is too short, it will cause difficulties in the selection of voltage and capacitance, and if it is too long, it will increase the unhelpful heat loss.

It shall be timely handled that “bridging” will occur if the distribution is improper or the furnace charge contains too much rust. Otherwise, overheating of the molten steel at the bottom will not only damage the furnace lining, but also make the molten steel absorb a large amount of gas.

As the middle part of the molten steel rises under the action of electromagnetic stirring, the slag often flows to the edge of the crucible and sticks to the furnace wall. Therefore, during the induction furnace steel making process, the slag should be continuously added according to the furnace conditions.

4) Refining and Deoxidation

Deoxidation is one of the important tasks of induction furnace steel making. In order to achieve a good deoxidation effect, slag with proper composition should be selected first. The temperature of the induction furnace slag is low, so slag with a low melting point and good flow should be selected. Generally, 70% lime and 30% fluorite are selected as alkaline slag materials. Due to the continuous volatilization of fluorite in the smelting process, it should be supplemented at any time. However, in consideration of the corrosion and penetration of fluorite on the crucible, the additional amount should not be too much.

The alloying of an induction furnace is the same as that of an electric arc furnace. Some alloy elements can be added during charging, and some can be added when deoxidation is good. The alloying operation can be carried out after the steel slag is completely reduced. Before the easily oxidized elements are added, the reducing slag can be removed completely or partially to improve its recovery. Due to electromagnetic stirring, the added ferroalloys generally melt faster and are distributed more evenly.

The temperature before tapping can be measured by an optical pyrometer or plug-in thermocouple. Aluminum can be finally inserted for deoxidation before tapping. Small furnaces directly pour molten steel into the upper ingot mold, and large furnaces can also be poured with steel buckets.