A billet induction melting furnace is a steel-making equipment that uses an induction electric heating effect to heat and melt metals. It is especially suitable for smelting a small amount of high-quality steel and alloy with high-quality raw materials (high-quality scrap steel, ferroalloy, etc.). A vacuum induction furnace equipped with a vacuum system is an important piece of equipment for smelting high-quality alloy.
Structure of Billet Induction Melting Furnace
There are two types of billet induction melting furnaces, namely, melting ditch type and crucible type, whose working principle is similar to that of the transformer.
The primary winding (induction coil) of the trench-type induction furnace is equipped with an iron core to reduce magnetic leakage and improve the power factor. The secondary winding is a trench filled with molten metal. When alternating current is passed in the induction loop, induction electromotive force is generated in the liquid metal in the melting ditch to generate Joule heat and heat the furnace charge. Therefore, the melting ditch shall always be filled with molten metal, and the furnace shall not be shut down at will, nor shall the variety of molten metal be changed frequently. At the same time, the energy is transformed in the melting ditch and then transferred to the molten pool. The heat load of the melting ditch is very high. Therefore, although the power factor and electrical efficiency of this induction furnace are high, it is not suitable to smelt alloy steel and alloy.
A Crucible induction furnace is used to place the crucible in the induction loop. When the induction loop is powered by alternating current, the induction electromotive force is generated in the furnace charge in the crucible, and then the joule heat is generated to melt the furnace charge. Crucible induction furnace has a low power factor and thermal efficiency, but they can smelt high melting point metals, easily change the varieties, and is suitable for discontinuous operation. It plays an important role in smelting a small amount of high-quality steel and alloy.
Metallurgical Characteristics of Billet Induction Melting Furnace
(1) The heat first reaches the metal molten pool and then transfers to the slag, so the slag temperature is low;
(2) It is a cylindrical melting pool, which determines that there is a small specific steel slag interface area in the crucible smelting;
(3) The molten pool is subject to strong electromagnetic stirring. The lower the power frequency is, the higher the power is, and the stronger the stirring is, which is the main factor limiting the maximum specific power;
(4) Compared with arc heating, induction heating has no hot spot, no arc, less environmental pollution, and uniform temperature;
(5) No carbonization, no local overheating, simple operation, and less alloy burning loss;
(6) The refining reaction cannot be carried out, the furnace lining is strictly required, the capacity is small, and the production is discontinuous, so the cost is high.
Induction Furnace Body
The main components of the billet induction melting furnace include the furnace frame, water-cooled induction coil, magnetic yoke, crucible, furnace cover, and shaft tilting mechanism under the tapping nozzle surrounding the furnace body. The furnace frame and coil fixing device shall have sufficient strength to bear the weight of molten steel. During the production of the crucible, a series of factors must be considered when selecting refractory materials: the chemical characteristics of the material should match the alloy being melted, and it should have a certain strength, stability, and insulation, which is easy to make, install and repair, and the cost is low.
Crucibles are generally divided into acid (quartz sand, containing SiO2>98%), neutral (high alumina material, Al2O3>95%), and alkaline (magnesia, MgO>85%). In addition, there is zirconia (ZrO265%, SiO2 about 30%), magnesium aluminum, and chromium aluminum crucibles. The acid crucible is not suitable for steelmaking because it is easy to be eroded by slag and has low fire resistance. Neutral high aluminum and zirconia crucibles are suitable for smelting various steel grades and high nickel alloys. Alkaline magnesium oxide or aluminum-magnesium crucible with high magnesium oxide content can withstand the corrosion of alkaline slag and is suitable for smelting various steel grades and alloys. The price is also moderate, but the thermal stability is poor, the thermal conductivity is strong, and cracks are prone to occur. When making the crucible, the particle size ratio of the refractory must be appropriate to achieve the highest density.
The laboratory induction furnace can use preformed and pre-sintered crucibles, and the large induction furnace can use integrally rammed crucibles. There are two methods of tamping: the wet method and the dry method. First, the bottom of the crucible is formed, and then the steel mold for tamping the crucible wall is placed at the bottom, and the side wall is tamped layer by layer. According to the different materials and adhesives, different sintering systems are used to dry and sinter the crucible. The first furnace shall be heated slowly to avoid cracks. This method has a low cost.
Billet Induction Melting Furnace Power Supply
Generally, there are three types: high frequency (>104Hz), intermediate frequency (>50-104Hz), and power frequency (50-60Hz). A thyristor frequency converter has been used as a medium-frequency power supply to replace the original motor generator set. As the production of high-grade alloy steel is small in scale, with many varieties and generally discontinuous production, the application of an intermediate frequency power supply during smelting can obtain a higher power factor and larger output power, so the intermediate frequency power supply is the main power supply for induction furnace. The frequency shall be selected according to furnace capacity, furnace charge size, and acceptable mixing strength. The higher the frequency is, the higher the specific power can be achieved (the specific power is generally 200~1000kW/t).
Thyristor medium frequency power supply converts three-phase power frequency alternating current into single-phase medium frequency alternating current and adopts AC-D-AC frequency conversion circuit to send medium frequency electric energy output by inverter circuit to load. Since an induction furnace is an inductive load with a low power factor (about 0.05~0.1), capacitors are used to improve the power factor. The advantages of thyristor medium frequency power supply are low cost, high electrical efficiency, automatic frequency adjustment to maintain a high power factor, easy automatic control, easy installation, and use, etc.
Key Points of Process
Desulfurization and deoxidation are carried out at the same time during billet induction melting furnace operation. Generally, decarburization and dephosphorization are not carried out. Therefore, good raw materials must be selected, melted as soon as possible, and deoxidation and tapping shall be carried out at the proper temperature.
Feeding
In addition to meeting the chemical composition requirements, the scrap and recycled materials used should also have appropriate dimensions to enable the furnace materials to be heated effectively. For this reason, some crushed materials can also be used to fill the gap to improve the density of furnace materials in the crucible. When a molten pool is formed at the bottom of the crucible, the charge size has no effect on the heating effect. Since there is no oxidation period during smelting, the elements have little burning loss and can be accurately batched. Alloy materials that are not easy to be oxidized, such as nickel and ferromolybdenum, can be added together with scrap steel.
In order to improve productivity, reduce hydrogen content in steel, and avoid explosion of scrap containing water or grease when heating in the furnace, scrap can be preheated to below 550 ℃. If the preheating temperature is too high, the furnace charge will be seriously oxidized, reducing the thermal efficiency. During discontinuous steelmaking, wet materials and grease-containing materials must be added in the first batch to facilitate drying, and only dry materials can be added after forming a molten pool. Small furnaces are usually loaded manually, and the furnace materials are loaded into the furnace with boxes or chutes. The large furnace can be loaded with a conveyor belt or basket.
Melting
Pay close attention to the melting of the furnace charge, continuously feed and timely, and continuously loosen the furnace charge to make it fall steadily into the molten bath, keep the melting running smoothly and avoid bridging. The so-called “bridging” refers to the furnace charge in the semi-molten state being bonded above the molten pool so that the furnace charge cannot contact the molten pool. This will cause a sharp rise in the temperature of the molten pool and damage the crucible. Once the bridge is erected, the crucible can be tilted to melt the gap on the erected furnace charge so as to continue feeding, raise the steel water surface to the bridge, and melt the unbonded furnace charge.
Slagging
Slagging shall be carried out before all furnace materials are melted to avoid serious oxidation of molten steel. Slag can be added to the bottom of the crucible in advance. The slag system used is mainly CaO-Al2O3-CaF2. The composition of the slag is different according to different crucible materials and deoxidizers. The slag shall be dried in advance to avoid hydrogen increase in molten steel.
Slagging, Deoxidation and Tapping
After the furnace charge is fully melted, temperature measurement and sampling shall be carried out. Skilled operators can coordinate the operation in front of the billet induction melting furnace by adjusting the input power and correctly predicting the time of temperature rise to tapping temperature. According to the analysis results of steel samples before the furnace, alloy materials are added, and ferrosilicon powder and various composite deoxidizers are used for deoxidation. Due to strong stirring in the molten pool, the alloy can be rapidly melted and evenly distributed, and the deoxidized products can also float up and be absorbed by the slag. After the ingredients are qualified and the temperature is appropriate, the slag can be removed. If the slag is thin, pour it out by tilting the crucible or adding lime, and remove it after the slag is thickened. During tapping, 1~2kg/t aluminum or other strong deoxidizers shall be added to the steel flow for final deoxidization.