Induction furnaces are widely used in foundries and for special steel production due to their advantages, such as rapid heating, strong electromagnetic stirring, and ease of temperature and composition control. Their core metallurgical tasks can be summarized as ละลาย, การกลั่นกรอง, and Conditioning. Among these, deoxidation, การผสม, and element control are the critical steps for achieving the final product quality.
Ⅰ. การทำออกซิเดชั่น
Deoxidation is a crucial step in the melting process. Excessive oxygen content in molten steel or alloy will react with elements like iron, ซิลิคอน, and carbon during cooling and solidification, forming oxide inclusions (เช่น, เฟ2O, SiO2, Al2O3). These inclusions severely degrade the material’s toughness, ความแข็งแกร่ง, and fatigue performance.
Sources of Oxygen:
- Charge Materials: Rust (Fe2O3) on the surface of scrap steel and return materials.
- Atmosphere: Contact between the molten bath surface and the air during melting.
- Refractories: Certain unstable oxides within the furnace lining.
Deoxidation Methods and Sequence: Deoxidation is typically carried out using precipitation deoxidation. This involves adding elements with a stronger affinity for oxygen than iron, causing them to form stable oxides that float up as inclusions into the slag to be removed.
The selection and addition sequence of deoxidizers follows the principle of “sequential deoxidation,” generally proceeding from the weakest to the strongest deoxidizing agent:
- Manganese (Mn) การทำออกซิเดชั่น: Ferromanganese (FeMn) is added during the mid-stage of melting. Manganese has a moderate deoxidizing power, capable of removing a large portion of the oxygen. Its reaction product, manganese silicate (MnO⋅SiO2), has a low melting point, making it easy to float to the surface.
- Chemical Reactions: [เฟ]+[O]→(เฟ2O)
- [Mn]+(เฟ2O)→(MnO)+[เฟ]
- ซิลิคอน (Si) การทำออกซิเดชั่น: Ferrosilicon (FeSi) is added after manganese deoxidation. Silicon is a strong deoxidizer that can reduce the oxygen content to a lower level.
- ปฏิกิริยาเคมี: [Si]+2(เฟ2O)→(SiO2)+2[เฟ]
- อลูมิเนียม (Al) การทำออกซิเดชั่น (Final Deoxidation): Metallic aluminum is added just before tapping. Aluminum is an extremely strong deoxidizer capable of lowering the oxygen content in the steel to very low levels (typically < 20 ppm). อย่างไรก็ตาม, it should be noted that the product, alumina (Al2O3), has a high melting point and can form fine, dispersed inclusions that may impair the fluidity of the molten steel if not properly controlled. ดังนั้น, aluminum is typically added at the final stage, either just before tapping or in the ladle.
- ปฏิกิริยาเคมี: 2[Al]+3(เฟ2O)→(Al2O3)+3[เฟ]
Key Operational Points:
- Timing: Preliminary deoxidation must be completed before adding any major alloying elements, especially those that are easily oxidized.
- Stirring: The electromagnetic stirring in an induction furnace aids the collision, agglomeration, and flotation of deoxidation products.
- Slag Removal (Deslagging): The slag formed from these reactions should be removed promptly to prevent oxygen from re-entering the melt (reversion).
Ⅱ. การผสม
Alloying is the process of adding specific elements to the molten metal to adjust its chemical composition, thereby achieving the desired mechanical and physical properties (such as corrosion or heat resistance).
Keys to Precise Alloy Addition:
- Yield Rate (Recovery): This is the most important concept. Yield rate refers to the percentage of an added alloying element that actually dissolves into the molten metal. It is influenced by several factors:
- Chemical Affinity: Elements that are easily oxidized, เช่นอลูมิเนียม (Al), ไทเทเนียม (Ti), and boron (B), have a lower and less stable yield rate. ในทางตรงกันข้าม, elements that are not easily oxidized, like nickel (Ni), โมลิบดีนัม (Mo), และทองแดง (Cu), have very high yield rates (typically >95%).
- อุณหภูมิหลอมละลาย: The higher the temperature, the more easily elements are oxidized, resulting in a lower yield rate.
- Melt Condition: The better the deoxidation of the melt, the higher the yield rate for subsequently added, easily-oxidized elements.
- Addition Method and Sequence: Adding alloys into a well-deoxidized bath with a protective slag cover helps to improve the yield rate.
- Order of Addition:
- Non-oxidizable Elements: Nickel (Ni), โมลิบดีนัม (Mo), ทองแดง (Cu), ฯลฯ, can be added with the initial charge materials.
- Moderately Oxidizable Elements: Chromium (Cr), manganese (Mn), และซิลิคอน (Si) are typically added after the charge is molten and preliminary deoxidation is complete.
- Strongly Oxidizable Elements: อลูมิเนียม (Al), ไทเทเนียม (Ti), boron (B), เซอร์โคเนียม (Zr), ฯลฯ, must be added at the last moment, after final deoxidation and just before tapping, to minimize oxidation losses.
- Calculating the Addition Amount: Accurate alloying calculations are fundamental to meeting grade specifications.
- Addition Amount=Element Content in Alloy%×Yield Rate%(Target%−Actual%)×Weight of Molten Metal
- Example: For a 1-ton (1000 กิโลกรัม) heat of steel, the target manganese content is 1.5%, and the current analysis shows 0.3%. Ferromanganese with 75% Mn content is used, with an estimated yield rate of 90%.
- Weight of pure Mn needed: (1.5%−0.3%)×1000 kg=12 kg
- Weight of FeMn to add: 75%×90%12 kg≈17.8 kg
- การควบคุมกระบวนการ:
- Preliminary Analysis: Use a spectrometer to take samples during the melting process to monitor the chemical composition in real-time and make fine adjustments as needed. This is essential for precise control.
- การควบคุมอุณหภูมิ: Strictly control the tapping temperature. An excessively high temperature will accelerate element loss and can damage the furnace lining.
Ⅲ. Element Control
In addition to the main alloying elements, controlling elements like carbon, sulfur, and phosphorus is equally important.
Carbon (C):
- การทำคาร์บูร์ (Increasing Carbon): When carbon content is low, carburizing agents like graphite, petroleum coke, or anthracite coal can be added. To improve absorption, they should be added after the melt has heated up but before deoxidation. Electromagnetic stirring significantly promotes the dissolution and diffusion of carbon.
- Decarburizing (Reducing Carbon): An induction furnace does not have the oxidative decarburizing capability of a converter. Carbon loss mainly occurs through reaction with oxygen: [C]+[O]→{CO}. If carbon content needs to be lowered, it is typically managed by charge selection (using low-carbon scrap) หรือ, in special cases, by oxygen lancing (ที่, อย่างไรก็ตาม, accelerates lining erosion).
Sulfur (ส): Desulfurization is a challenge in induction furnaces, especially with an acidic lining (silica sand, SiO2).
- Principle of Desulfurization: The desulfurization reaction requires a strongly reducing atmosphere และ high-basicity slag.
- ปฏิกิริยาเคมี: [ส]+(O2−)→(S2−)+[O] or more specifically: [เฟส]+(แคลเซียมโอ)→(ซีเอเอส)+[เฟ2O]
- Implementation Methods:
- Basic Lining: To achieve effective desulfurization, a basic (magnesia, MGO) or neutral (alumina, Al2O3) lining must be used. The SiO2 in an acidic lining will react with the basic slag, reducing its efficiency.
- Synthetic Slag: A pre-melted, high-basicity synthetic slag (เช่น, CaO−Al2O3−CaF2 system) is added to the surface of the melt.
- Deep Deoxidation: The desulfurization reaction is favorable in a low-oxygen environment. ดังนั้น, it must be performed after deep deoxidation. The lower the oxygen content in the steel, the more effective sulfur removal into the slag. Sulfur is typically removed effectively only after final deoxidation with aluminum.
Phosphorus (P): Dephosphorization is generally not feasible in an induction furnace. The removal of phosphorus requires an oxidizing atmosphere with low temperature, high oxygen potential, and high-basicity slag. These conditions are the exact opposite of the normal reducing melting environment in an induction furnace. ดังนั้น, phosphorus control relies entirely on strictly selecting low-phosphorus raw materials.
สรุป: Operational Flow for Meeting Specific Grade Requirements
Combining the points above, a typical melting process in an induction furnace aimed at producing high-quality alloy steel is as follows:
- Charge Calculation: Accurately calculate and select clean, low-sulfur, and low-phosphorus charge materials based on the target grade and element yield rates.
- ละลาย: Rapidly melt the charge materials, adding non-oxidizable alloys (Ni, Mo, ฯลฯ) with the charge.
- Heating and Carburizing: Raise the temperature to the process target and add carburizing agents as needed.
- Preliminary Deoxidation and Composition Adjustment: Take a sample for analysis. แล้ว, add ferromanganese, ferrosilicon, ฯลฯ, for initial deoxidation and adjust major alloying elements like chromium.
- Deslagging and Refining: Remove the initial slag. ถ้าจำเป็น (เช่น, for desulfurization), add a new refining slag.
- Final Composition and Temperature Adjustment: Take another sample for analysis and make final micro-adjustments to the composition. Adjust the temperature to the target tapping temperature.
- Final Deoxidation and Special Element Addition: Just before tapping or in the furnace, add aluminum for final deoxidation and make last-minute additions of micro-alloying elements like titanium and boron.
- Tapping: Pour the molten metal, which now meets all composition, temperature, and purity specifications, into a ladle or mold.
Through systematic control of deoxidation, การผสม, slag chemistry, temperature, and analytical methods, the induction furnace is fully capable of producing high-quality metallic materials that meet a wide range of demanding grade requirements.
