Calcium silicon alloys are composite alloys composed of silicon (Si) and calcium (Ca), typically containing 28%-35% Ca, 55%-65% Si, with the remainder being iron and small amounts of impurities.
This combination of two elements is by no means accidental, but rather a carefully designed "golden pairing" by metallurgists:
| Elements | Disadvantages of using it alone | Advantages of the combination |
| Calcium (Ca) | Low boiling point (1482℃), violent vaporization at molten steel temperatures, extremely low yield, difficult to control. | Silicon, acting as a "carrier element," lowers the vapor pressure of calcium, allowing it to dissolve stably in molten steel. |
| Silicon (Si) | Moderate deoxidation capacity; cannot achieve deep deoxidation when used alone. | Working synergistically with calcium, it first creates favorable conditions for calcium during initial deoxidation, increasing deoxidation efficiency by 30%-40%. |
Key takeaway: The presence of silicon allows calcium to dissolve "quietly" in molten steel, rather than instantly vaporizing and escaping. This is the technological basis for CaSi alloys to play a dual role.
Why consider the order of deoxidation and desulfurization?
In ladle refining processes, silicon calcium alloy (SiCa) is hailed as a "universal refining agent." It can simultaneously perform deoxidation, desulfurization, and inclusion modification, making it an indispensable auxiliary material for producing high-purity steel. Adding only 0.2%-0.5% per ton of steel is sufficient for deep refining, making it a core auxiliary material in the production of mid-to-high-end steel.
However, a fundamental question has consistently troubled on-site engineers and process designers: when calcium silicon alloy is added to molten steel, do deoxidation and desulfurization occur simultaneously, or sequentially? If the latter, which occurs first?
The answer to this question directly determines:
Timing of addition: Should it be added in the early or late stages of refining?
Method of addition: Should it be added all at once or in batches?
Cost-effectiveness: How to maximize calcium utilization?
Whose reaction is more "urgent"?
1. In molten steel, calcium participates in the following key reactions simultaneously:
Deoxidation reaction
| Reaction Types | Chemical reaction equation | Explanation |
| Basic deoxidation of silicon |
Si + 2FeO → SiO₂ + 2Fe |
This process occurs spontaneously in molten steel at 1500-1600℃. SiO₂ has a low density and easily floats to form slag. |
| Enhanced deoxidation of calcium |
2Ca + O₂ → 2CaO |
Calcium has a stronger affinity for oxygen than silicon and aluminum, and can remove residual oxygen from the molten steel. |
| Inclusion denaturation |
Ca + Al₂O₃ → CaO·Al₂O₃ |
It transforms brittle Al₂O₃ into low-melting-point liquid calcium aluminate. |
Desulfurization reaction
| Reaction Types | Chemical reaction equation | Explanation |
| Calcium-dominated desulfurization |
Ca + FeS → CaS + Fe |
CaS has a melting point of 2450℃ and is almost insoluble in molten steel, floating as solid particles. |
| Silicon-assisted desulfurization |
Si + 2FeO → SiO₂ + 2Fe |
It reduces the oxygen content of the molten steel, creating a reducing environment for desulfurization and preventing the formation of CaSO₄. |
2. In metallurgical thermodynamics, the more negative the Gibbs free energy change (ΔG) of a reaction, the stronger the spontaneous tendency of the reaction, and the more "urgent" it is.
Calcium's reaction affinity order:
Calcium's reaction with oxygen: ΔG is very negative; at steelmaking temperatures (1600℃), calcium has an extremely strong affinity for oxygen.
Calcium's reaction with sulfur: ΔG is also negative, but less negative than that of the calcium-oxygen reaction.
Conclusion: From a purely thermodynamic perspective, calcium preferentially reacts with oxygen, and then with sulfur.
3. Critical threshold: Oxygen's "priority passage"
Studies show that desulfurization only occurs on a large scale when the oxygen content in molten steel decreases to a certain level:
When the initial oxygen content is ≤50ppm, the desulfurization rate is 25% higher than when the oxygen content is 80-100ppm. Silicon's deoxidation role is crucial in this process, creating the necessary reducing environment for the calcium-sulfur reaction.
Comparison of Deoxidation and Desulfurization Effects
1 Quantitative Data on Deoxidation Effect
According to industrial practice statistics, the deoxidation effect of siliconcalcium alloys is closely related to the steel grade and the amount added:
| Steel Grades | CaSi addition amount | Initial oxygen content (ppm) | Oxygen content after refining (ppm) | Deoxidation efficiency |
| Plain Carbon Steel (Q235) |
0.2%-0.3% |
80-100 |
40-50 |
45%-60% |
| Low Alloy High-Strength Steel (Q355) |
0.3%-0.4% |
90-110 |
35-45 |
55%-68% |
| Stainless Steel (304) |
0.4%-0.5% |
100-120 |
25-35 |
65%-79% |
| Alloy Structural Steel (40Cr) |
0.3%-0.4% |
85-105 |
30-40 |
58%-71% |
2 Quantitative Data on Desulfurization Effect
The effects of the desulfurization reactions carried out concurrently are as follows:
| Steel Grades | CaSi addition amount | Initial sulfur content (%) | Sulfur content after refining (%) | Desulfurization efficiency | Core value |
| Plain Carbon Steel (Q235) |
0.2%-0.3% |
0.03-0.05 |
0.015-0.025 |
30%-50% |
Avoid hot brittleness |
| Low Alloy High Strength Steel (Q355) |
0.3%-0.4% |
0.02-0.04 |
0.008-0.015 |
55%-70% |
Improve weldability |
| Stainless Steel (304) |
0.4%-0.5% |
0.015-0.03 |
0.003-0.008 |
70%-85% |
Enhance corrosion resistance |
| Wear-Resistant Steel (NM450) |
0.3%-0.4% |
0.02-0.04 |
0.006-0.012 |
65%-80% |
Improve wear resistance |
3 Deep Desulfurization Capacity
For high-end steel grades, siliconcalcium alloys can achieve deeper desulfurization:
| Process Scenarios | CaSi addition amount | Refining conditions | Sulfur content after desulfurization | Desulfurization efficiency |
| Routine Addition |
0.1%-0.3% |
- |
<0.01% |
80%-90% |
| High-End Steel Refining |
0.3%-0.5% |
LF furnace refining |
<0.005% |
≥93% |
| Continuous Casting Protective Casting |
0.05%-0.1% |
Feeding speed3-5m/s |
<0.003% |
Ultra-low sulfur steel standard |
Key Insight: Comparing the two tables reveals that, at the same dosage, the deoxidation reaction occurs earlier and faster, and the deoxidation efficiency generally reaches a considerable level before the desulfurization reaction starts. This confirms the thermodynamic order of deoxidation taking precedence over desulfurization.
The answer is revealed: Which occurs first, deoxygenation or desulfurization?
From the reaction order, deoxygenation occurs before desulfurization.
| Comparison Dimensions | Deoxygenation reaction | Desulfurization reaction |
| Thermodynamic Tendency | Calcium has a stronger affinity for oxygen, resulting in a more negative ΔG | Secondary affinity |
| Time Sequence | It occurs throughout the entire process, but is dominant in the early stages | Active in the middle stage, requires oxygen level to decrease |
| Oxygen Content Dependence | It can still occur under hyperoxia conditions | Requires oxygen content ≤50ppm for efficient operation |
| The Role of Silicon | Core deoxygenation element | Auxiliary (creating a reducing environment) |
The behavior of calcium in molten steel can be imagined as a "priority treatment" process:
First priority: Deoxidation-After entering the molten steel, calcium first "seeks" oxygen atoms to combine with, while silicon initially deoxidizes, creating conditions for calcium.
Second priority: Desulfurization-When oxygen is consumed to a low level (≤50ppm), calcium begins to combine with sulfur in large quantities.
Third priority: Modification-Finally, the remaining calcium is used to modify residual Al₂O₃ inclusions, forming low-melting-point calcium aluminate, optimizing the inclusion morphology.
Process Implications
This scientific principle suggests to on-site engineers:
Do not expect to complete deoxidation and desulfurization simultaneously with a single addition-the priority of calcium dictates that it must be done in stages.
Controlling oxygen is a prerequisite for efficient desulfurization-if deoxidation is incomplete in the early stages, the efficiency of desulfurization in the later stages will inevitably be affected.
Calcium treatment in the later stages of refining is equally important-even after deoxidation and desulfurization are completed, an appropriate amount of calcium is crucial for improving casting performance.
FAQ
Q1: Why is calcium treatment performed in the later stages of refining?
A: Because calcium preferentially reacts with oxygen. Only after the oxygen content has decreased to a low level can calcium efficiently perform desulfurization and inclusion modification.
Q2: How to improve calcium yield?
A: Use the cored wire feeding method (15%-20% more efficient than the direct feeding method), control the steel temperature at 1500-1600℃, and start adding calcium when 1/3 of the steel has been tapped.
Q3: What are the consequences of adding excessive silicon-calcium alloy?
A: Excessive addition (>0.6%) will lead to excessively high calcium content in the steel, forming CaO inclusions, and reducing impact toughness by 10%-15%.
Q4: What role does silicon play in silicon-calcium alloy?
A: Silicon acts as a carrier element, reducing the high vapor pressure of calcium, allowing it to dissolve stably in molten steel; simultaneously, silicon performs preliminary deoxidation, creating conditions for calcium desulfurization.
