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Key Points for Dephosphorization and Desulfurization Operations in Medium-Frequency Induction Furnace Smelting
Sep 01,2020
(1) Melting period Dephosphorization
According to the metallurgical reaction mechanism of dephosphorization, the dephosphorization operation should be scheduled during the melting stage, with particular attention paid to four key steps: oxidation of the molten steel, preparation of the initial slag, removal of slag, and secondary slag formation.
(1) Oxidation of molten steel
To achieve rapid dephosphorization and ensure that the molten steel actively participates in the dephosphorization reaction throughout the entire melting process, iron oxide scale or iron ore can be added to the furnace along with scrap steel before melting begins. For each ton of molten steel, 8 to 10 kg of iron oxide scale or iron ore should be prepared—half of the total amount required for the scrap steel initially charged into the furnace. If neither iron oxide scale nor iron ore is available, oxygen can be blown into the melt at the early stage of melting to oxidize the molten steel.
(2) Preparation of the initial slag
Once the scrap steel begins to melt, the initial slag can be formed. Weigh out approximately 0.6% to 0.8% of the total charge in lime and fluorite (with a lime-to-fluorite ratio of 10:1), and add them gradually into the furnace along with the scrap steel as it melts. When the scrap steel has melted to about 70% of the required total amount, prepare to skim off the slag, so that the secondary slag can effectively remove phosphorus. Note that the lime used for slag formation must be dry and have a proper particle size. Powdered lime is strictly prohibited because it readily absorbs moisture; therefore, lime must be dried and used immediately after drying. Iron oxide scale and fluorite must also be thoroughly dried before use.
(3) Skimming slag
Steel liquefaction Stop adding material and reduce the power when the level reaches the required 60–70%, maintaining the molten state, stirring the slag liquid, and then tilting the furnace to discharge the slag.
(4) Secondary slag formation
After most of the slag has been removed, re-add the scrap steel, increase the power to melt it, and then add a slag batch equal to 0.5% of the total charge—along with any remaining iron oxide scale or iron ore—to the furnace. Once all the scrap steel has been completely melted, tilt the furnace to remove the slag, take a sample for analysis to determine whether the phosphorus content is within the allowable range. Generally speaking, after performing the above operations on carbon-containing scrap steel, the dephosphorization efficiency can reach up to 50%.
(2) Desulfurization during the reduction period
Practice has shown that the quality of deoxidation in molten steel directly affects the desulfurization efficiency. Due to the melting characteristics of medium-frequency induction furnaces, the slag is directly exposed to the air. In contrast to arc furnaces, where the furnace door can be closed during the formation of reducing slag to minimize the exchange of the furnace atmosphere with the outside environment, the external conditions for forming reducing slag in medium-frequency induction furnaces are relatively poorer. Therefore, desulfurization operations in medium-frequency induction furnaces have their own specific requirements, and the following procedures should be carefully carried out:
(1) After the late stage of melting when all the oxide slag has been removed, add an appropriate amount of lime and fluorite—just enough to cover the entire liquid surface—to prevent the molten steel from absorbing air and undergoing oxidation. Once a thin slag layer has formed, perform pre-deoxidation using silicon-calcium powder, then add various alloys to adjust the chemical composition so that, except for sulfur, the molten steel’s chemical composition meets the company’s internal control standards. Subsequently, sprinkle Si-Ca powder for pre-deoxidation to prepare for the formation of a reducing slag that will effectively remove sulfur.
(2) Gradually add the prepared reducing slag—composed of quicklime, aluminum oxide, sodium carbonate, and fluorite in a ratio of 20:5:2:2—at 1.5% of the total molten steel volume. Raise the temperature of the molten steel to a relatively high level and continuously stir the slag mixture for 20 to 30 minutes. During this period, to ensure the stability of the slag, sprinkle Si-Ca powder onto the slag surface at intervals of about 5 minutes.
(3) During steel tapping, final deoxidation is carried out in the ladle. Typically, aluminum is added to the ladle at a rate of 0.3 kg per ton of steel. It is essential to ensure that slag and steel are tapped simultaneously. If the amount of sulfur to be removed is relatively large, the slag in the ladle can be removed, the molten steel returned to the furnace, and the slag-making process described in Step 2 can be repeated.
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Kangda Electric Furnace specializes in the manufacture of the following equipment: KGCL series medium-frequency power supplies, KGPS series medium-frequency power supplies, GTR series medium-frequency induction heating furnaces...
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Address: Yuhe Industrial Park, Weicheng District, Weifang City, Shandong Province
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