Calcium Silicon Alloy CaSi | Deoxidizer & Desulfurizer
Calcium Silicon Alloy CaSi
A premium composite deoxidizer and desulfurizer for modern steelmaking. Dual-action metallurgy with silicon-led deoxidation and calcium-driven inclusion control.
Quick Answer: What Is Calcium Silicon Alloy?
Calcium silicon alloy (also called CaSi alloy or silicon calcium alloy) is a ferroalloy composed primarily of calcium, silicon, and iron. It is widely used in steelmaking as a composite deoxidizer and desulfurizer. Standard grades such as Ca31Si60, Ca28Si60, and Ca24Si60 deliver both silicon and calcium simultaneously to molten steel, enabling primary deoxidation, deep desulfurization, and transformation of solid alumina inclusions into liquid calcium aluminates that improve castability.
Product Overview & Chemical Composition
Calcium silicon alloy is produced by reducing silica and lime in submerged-arc furnaces. The resulting alloy is a grey, brittle material with a metallic luster, typically supplied in lump form for ladle addition or in powder form for cored wire filling and injection processes.
Our technical team supplies calcium silicon alloy in three primary grades designed to match different steelmaking requirements. Each grade maintains tight compositional control for predictable recovery and consistent inclusion modification.
| Grade | Ca (%) | Si (%) | C (max) | P (max) | S (max) | Al (max) |
|---|---|---|---|---|---|---|
| Ca31Si60 | ≥31 | 55-66 | 0.8% | 0.04% | 0.06% | 2.5% |
| Ca28Si60 | ≥28 | 55-65 | 0.8% | 0.04% | 0.06% | 2.5% |
| Ca24Si60 | ≥24 | 55-65 | 0.8% | 0.04% | 0.06% | 2.4% |
Standard reference: YB/T 5051-2016 (China ferroalloy industry standard for calcium silicon alloy). Other specifications available on request.
Deoxidation Mechanism: How CaSi Works in Molten Steel
The effectiveness of calcium silicon alloy stems from the complementary metallurgical behaviors of its two active elements. Silicon and calcium act in sequence to remove oxygen and modify non-metallic inclusions.
Stage 1: Silicon-Led Primary Deoxidation
When calcium silicon alloy is added to liquid steel, the silicon component reacts first with dissolved oxygen. Silicon has a strong affinity for oxygen and readily forms silica (SiO₂) according to the reaction [Si] + 2[O] → (SiO₂). This primary deoxidation step rapidly reduces the dissolved oxygen content in the melt, lowering the oxygen potential and creating favorable thermodynamic conditions for the subsequent calcium reactions.
The silicon deoxidation product, SiO₂, is generally liquid or partially dissolved in the slag at steelmaking temperatures. Because silicon is present in substantial quantities within CaSi alloy, this initial deoxidation proceeds efficiently without separate ferrosilicon additions.
Stage 2: Calcium-Driven Deep Deoxidation & Desulfurization
Calcium possesses an even stronger affinity for oxygen than silicon. Once the silicon has reduced the bulk oxygen level, calcium engages in deep deoxidation, further lowering total oxygen to very low levels. The reaction [Ca] + [O] → (CaO) proceeds with a highly negative Gibbs free energy, making calcium one of the most powerful deoxidizers available in ferroalloy form.
Beyond deoxidation, calcium also reacts with dissolved sulfur to form calcium sulfide (CaS), which partitions into the slag phase. Combined with an appropriate basic slag system, this mechanism can achieve sulfur contents below 0.001% in the final steel product. The dual deoxidation-desulfurization capability distinguishes CaSi alloy from single-element deoxidizers.
Inclusion Modification: The Critical Advantage
Perhaps the most important metallurgical function of calcium in steelmaking is inclusion modification. In aluminum-killed steels, dissolved aluminum reacts with oxygen to form solid alumina (Al₂O₃) inclusions with melting points exceeding 2050°C. These hard clusters accumulate in the tundish nozzle during continuous casting, causing blockages and surface defects.
When calcium from CaSi alloy reacts with these alumina clusters, it transforms them into liquid calcium aluminate compounds such as 12CaO·7Al₂O₃ (mayenite), with melting points below 1400°C. These liquid inclusions are spherical, deform readily during hot rolling, and do not clog nozzles. The result is cleaner steel and more reliable continuous casting.
CaSi vs. Other Deoxidizers: A Technical Comparison
Selecting the right deoxidizer depends on steel grade, process route, and quality targets. The table below compares calcium silicon alloy against ferrosilicon (FeSi) and pure aluminum.
| Property | CaSi Alloy | Ferrosilicon (FeSi75) | Pure Aluminum |
|---|---|---|---|
| Primary Deoxidation Strength | Strong | Strong | Very Strong |
| Deep Desulfurization | Yes | No | No |
| Inclusion Modification (Al₂O₃) | Yes | No | Creates Al₂O₃ |
| Nozzle Clogging Prevention | Prevents | Neutral | Can Cause |
| Cost per Ton of Steel | Moderate | Low | Moderate |
| Application Complexity | Moderate | Simple | Moderate |
| Best For | Clean steel, IF/ULC | General grades | Al-killed grades |
For steel mills producing high-quality flat products, pipeline steels, or automotive grades, the inclusion-modification benefit of calcium silicon alloy typically outweighs the incremental cost compared to ferrosilicon alone.
Applications by Steel Grade
Different steel grades place different demands on deoxidation and inclusion control. Our technical team recommends CaSi grades and addition practices based on the target application.
- IF / ULC Steel
Interstitial-Free & Ultra-Low Carbon Steels
These grades require extremely low total oxygen and controlled inclusion morphology. Ca31Si60 is typically added after aluminum deoxidation to modify alumina inclusions into liquid calcium aluminates, ensuring defect-free cold-rolled sheet for automotive exposed panels.
- Pipeline Steel
API Line Pipe & Sour-Service Grades
Pipeline steels demand very low sulfur for hydrogen-induced cracking resistance. CaSi alloy provides combined desulfurization and inclusion spheroidization. Typical practice adds 0.5–2.0 kg per tonne depending on initial sulfur and slag conditions.
- Bearing Steel
Bearing & Gear Steels
Non-metallic inclusions are the primary origin of fatigue failure in bearings. Calcium treatment transforms harmful alumina stringers into benign spherical calcium aluminates, significantly improving fatigue life and contact durability.
- Stainless Steel
Austenitic & Duplex Stainless Grades
In stainless steelmaking, calcium silicon serves as a deoxidizer and inclusion modifier during AOD or ladle refining. The low-carbon specification of standard CaSi grades (C≤0.8%) minimizes carbon pickup during treatment.
- Pressure Vessel
Boiler & Pressure Vessel Steels
These grades require controlled sulfur and inclusion content for weldability and toughness. Calcium silicon treatment in the ladle furnace achieves the required cleanliness levels while maintaining process stability.
- Rail Steel
Heavy-Haul & High-Speed Rail
Rail steels benefit from reduced oxide and sulfide inclusions to resist fatigue cracking under cyclic wheel loading. Calcium treatment improves inclusion deformability and reduces internal crack initiation sites.
Standard Sizes, Packaging & Logistics
Calcium silicon alloy is supplied in forms suited to electric furnace and ladle furnace addition. Consistent sizing ensures predictable melting and recovery.
Lump Sizes
Standard lump sizes include 10–50 mm and 50–100 mm. Fine fractions (0–3 mm, 1–3 mm, 3–10 mm) are available for powder applications and cored wire filling.
Bulk Packaging
Primary packaging uses 1000 kg bulk bags with inner moisture barrier. Smaller steel drums of 20–25 kg are available for trial quantities and specialized foundry use.
Moisture Protection
Calcium silicon is hygroscopic. All export shipments use sealed inner liners, stretch-wrapped pallets, and desiccant packs to prevent oxidation during transit.
Quality Documentation
Each batch is accompanied by a mill test certificate showing chemical composition by spectrometry, size distribution, and moisture content on request.
Frequently Asked Questions
What is calcium silicon alloy used for in steelmaking?
Calcium silicon alloy serves as a composite deoxidizer and desulfurizer in steelmaking. The silicon component performs primary deoxidation by reacting with dissolved oxygen, while calcium provides deep deoxidation and transforms solid alumina inclusions into liquid calcium aluminates, improving castability and reducing nozzle clogging during continuous casting.
How does calcium silicon compare to ferrosilicon as a deoxidizer?
Ferrosilicon provides economical deoxidation but cannot modify inclusions or desulfurize effectively. Calcium silicon alloy delivers both strong deoxidation and inclusion modification in a single addition, making it superior for clean-steel grades where alumina control is critical.
What are the standard grades of calcium silicon alloy?
The most common grades are Ca31Si60 (Ca≥31%, Si 55-66%), Ca28Si60 (Ca≥28%, Si 55-65%), and Ca24Si60 (Ca≥24%, Si 55-65%). All standard grades control carbon ≤0.8%, phosphorus ≤0.04%, sulfur ≤0.06%, and aluminum ≤2.5%.
Why is calcium effective at removing sulfur from molten steel?
Calcium has a stronger affinity for sulfur than most other common elements in steel. When added to the melt, calcium reacts with dissolved sulfur to form calcium sulfide, which partitions into the slag. Combined with a proper slag system, this mechanism can achieve sulfur levels below 0.001%.
In which steel grades is calcium silicon alloy most commonly applied?
Calcium silicon is widely used in aluminum-killed steels, interstitial-free (IF) steels, ultra-low carbon (ULC) steels, pipeline steels, pressure vessel steels, bearing steels, and stainless steels where inclusion morphology control and low sulfur are required.
What is the typical recovery rate of calcium when adding CaSi alloy to the ladle?
When calcium silicon alloy is added directly to the ladle as lump material, calcium recovery typically ranges from 15% to 25%. The relatively low recovery is due to calcium's low density and high vapor pressure at steelmaking temperatures. Using cored wire injection can improve recovery to 10-25% or higher with optimized parameters.
What sizes and packaging options are available for calcium silicon alloy?
Calcium silicon alloy is typically supplied in lump form with sizes including 10-50 mm and 50-100 mm. Standard packaging uses 1000 kg bulk bags or 20-25 kg steel drums on pallets. Moisture-resistant wrapping and seaworthy packing are available for export shipments.
Request a Quotation for Calcium Silicon Alloy
Our technical team can recommend the optimal CaSi grade, sizing, and addition practice for your steelmaking process. Send us your specifications and annual volume requirements for a competitive offer.
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