Calcium silicon alloys use calcium (Ca) and silicon (Si) as their core components, with some barium and aluminum elements supporting their highly efficient desulfurization function:
Composition range: Ca 28%-35%, Si 55%-65%, impurities Al≤2.0%, S≤0.04%, P≤0.04%;
Physical properties: Melting point 1250-1350℃, density 2.5-2.8 g/cm³, in lump (5-30 mm) or granular (1-10 mm) form, with strong chemical activity at high temperatures;
Core advantages: Calcium's desulfurization capacity far exceeds that of manganese and iron, while silicon's deoxidizing effect optimizes the desulfurization environment, achieving a synergistic effect of "desulfurization + deoxidation".
Core Mechanism of Desulfurization in Silicon Calcium Alloy Steelmaking
(1) Calcium-Sulfur Chemical Reaction: The Core Basis of Desulfurization
Main Reaction: Ca + S → CaS, the generated CaS has a melting point of 2450℃ and a solubility of only 0.0002% (in molten steel), making it almost insoluble in molten steel;
Synergistic Reaction: Silicon reacts with FeO in molten steel (Si + 2FeO → SiO₂ + 2Fe), reducing the oxygen content in the molten steel and preventing the formation of difficult-to-remove sulfates (such as CaSO₄) by oxygen and sulfur, thus creating a reducing environment for the calcium-sulfur reaction;
Thermodynamic Advantage: The electrode potential of calcium is much lower than that of sulfur, ensuring that calcium preferentially combines with sulfur, making the desulfurization reaction irreversible.
(2) CaS Flotation and Separation: A Key Step in Desulfurization
Density Difference Driving Factors: CaS has a much lower density than molten steel, naturally exhibiting a tendency to float;
Enhancing Separation Factors: Stirring molten steel accelerates CaS particle aggregation, increasing the flotation speed by 2-3 times;
Slag Adsorption: After floating to the surface of molten steel, CaS is adsorbed by CaO-SiO₂-Al₂O₃ slag and discharged with the slag, completing desulfurization.
(3) Quantifying Desulfurization Effect and Influencing Factors
Core Effect Data:
Conventional Addition Amount (0.1%-0.3% of molten steel mass): Can reduce the sulfur content of molten steel from 0.05%-0.08% to below 0.01%, achieving a desulfurization rate of 80%-90%;
High-End Steel Refining (Addition Amount 0.3%-0.5% + LF Furnace Refining): Sulfur content can be reduced to below 0.005% (ultra-low sulfur steel standard), with a desulfurization rate ≥93%.
Key influencing factors:
Alloy composition: A SiCa alloy with 30%-32% Ca and 60%-62% Si content exhibits the best desulfurization efficiency. Too low a calcium content (<28%) will lead to a 15%-20% decrease in desulfurization rate.
Steel temperature: The desulfurization reaction rate is fastest at 1550-1600℃. Below 1500℃, the reaction efficiency decreases by 30%.
Oxygen content: When the initial oxygen content of the molten steel is ≤50ppm, the desulfurization rate is 25% higher than when the oxygen content is 80-100ppm. The deoxidizing effect of silicon is crucial.
Application and Adaptation of CaSi Alloy in Different Steelmaking Stages
(1) Converter Steelmaking (Final Desulfurization)
Application Logic:
Added in the later stage of converter tapping, utilizing the turbulent mixing of molten steel to initially reduce sulfur content, laying the foundation for subsequent refining;
Process Parameters:
Addition amount 0.1%-0.2%, molten steel temperature 1600-1650℃, argon stirring for 5-8 minutes during tapping, desulfurization rate can reach 70%-75%;
Suitable Scenarios:
Initial desulfurization of ordinary carbon steel and low alloy steel, reducing sulfur content to 0.02%-0.03%.
(2) LF Furnace Refining (Deep Desulfurization)
Application Logic:
Under the reducing atmosphere of the LF furnace, siliconcalcium alloy is combined with slag-forming agents such as lime and fluorite to achieve deep desulfurization;
Process Parameters:
Addition amount 0.2%-0.5%, slag basicity controlled at 1.8-2.2, argon stirring intensity 0.4-0.6 m/s, refining time 30-40 minutes, sulfur content can be reduced to below 0.005%;
Suitable Scenarios:
Production of ultra-low sulfur steels such as high-end alloy steel, stainless steel, and bearing steel.
(3) Continuous Casting Protective Casting (Terminal Desulfurization)
Application Logic:
CalciumSilicon alloy wire (diameter 10-13 mm) is fed into the crystallizer through a wire feeder to remove trace amounts of residual sulfur in the molten steel;
Process Parameters:
Wire feeding speed 3-5 m/s, wire feeding amount 0.05%-0.1%, which can further reduce the sulfur content to below 0.003%, avoiding hot brittle defects in the continuously cast billet.
Selection and Usage Control Points
(1) Selection Logic: Match alloy grade according to steel requirements
| Steel Grade | Recommended Silicon-Calcium Alloy Grades | Core Component Requirements (Ca/Si) | Desulfurization Targets |
| Ordinary carbon steel | CaSi3060 | 30%/60% | S≤0.02%, desulfurization rate≥75% |
| Low alloy steel | CaSi3262 | 32%/62% | S≤0.01%, desulfurization rate≥85% |
| High-end ultra-low sulfur steel | CaSi3560 | 35%/60% | S≤0.005%, desulfurization rate≥93% |
(2) Usage Precautions
Addition Amount Control:
Excessive addition (>0.6%) can easily lead to excessive calcium content in the molten steel, generating CaO inclusions and affecting the impact toughness of the steel (decreasing by 10%-15%).
Addition Timing:
Start adding when the converter has tapped 1/3 of the steel to avoid premature addition that could cause calcium oxidation (calcium burn-off rate increases from 10%-15% to over 30%).
Storage Protection:
Store in a dry, sealed environment to avoid moisture oxidation (generating Ca(OH)₂, reducing desulfurization activity). Storage period should not exceed 6 months.
Safety Protection:
Calcium is flammable. Keep away from open flames during addition and use a dry powder fire extinguisher to prevent alloy particles from splashing and causing a fire.
Industry Trends: Upgrading Directions for Desulfurization of Calcium Silicon Alloys
Composite Processing: Developing composite desulfurizing agents of "silicon-calcium-barium" and "silicon-calcium-aluminum." Barium and aluminum can further improve the utilization rate of calcium (from 60%-70% to over 80%), increasing the desulfurization rate by 5%-10%.
Refined Processing: Customizing compositions for different steel grades (such as ultra-low aluminum silicon-calcium alloys and low-sulfur silicon-calcium alloys) to meet the stringent impurity requirements of high-end steels.
Green Processing: Employing green electricity to smelt silicon calcium alloys, reducing carbon emissions, and optimizing production processes to reduce the content of harmful impurities (such as P and S) in the alloy.
