In steelmaking, the deoxidation process plays a decisive role in determining molten steel cleanliness and the final steel quality. Excess oxygen in molten steel forms inclusions, reduces castability, and weakens mechanical properties.
To achieve efficient and complete deoxidation, specific environmental and process conditions must be met, along with a proper selection of deoxidizing agents.
I. Basic Environmental Requirements for Molten Steel Deoxidation
To ensure optimal deoxidation performance, the steelmaking process must provide the following conditions:
- Low Inclusion Content
Deoxidation should occur in a clean molten steel environment. Existing inclusions can act as nuclei for oxidation reactions, hindering the removal of deoxidation products. - High Affinity Between Deoxidizer and Oxygen
The deoxidizer must have a strong affinity for oxygen. Even a small addition should effectively reduce the residual oxygen level in steel, ensuring high deoxidation efficiency. - Easily Floatable Deoxidation Products
The oxides formed during deoxidation should quickly agglomerate, grow, and float up. Otherwise, they remain as nonmetallic inclusions, lowering steel cleanliness and castability. - Prevention of Secondary Oxidation
After deoxidation, molten steel should be protected from reoxidation by using refining slag or inert gas shielding to maintain a low oxygen atmosphere.
II. Advantages of Using Ba Alloy in Deoxidation
Among various deoxidation methods, Ba-containing alloy deoxidation stands out due to its composite deoxidation mechanism and high efficiency in inclusion removal.
When a Ba alloy is added to molten steel, the solubility of Ba is very low, and only a small amount of BaO forms at the beginning. Other elements in the alloy first react with oxygen, producing their respective oxides. As the reaction continues, Ba combines with these oxides to form complex deoxidation products, which provide several benefits:
- Faster Inclusion Floatation:
Ba has a high atomic mass, producing larger composite oxide particles that float up more rapidly. - Cleaner Molten Steel:
The composite deoxidation process significantly reduces inclusions, improving steel cleanliness and castability. - Enhanced Calcium Deoxidation Effect:
Ba lowers the vapor pressure of Ca, increasing its solubility in molten steel. This enhances calcium’s deoxidizing ability and promotes the modification of inclusions.
Thus, Ba–Ca composite alloys are regarded as highly efficient deoxidizers in modern steelmaking, combining powerful deoxidation capability with excellent inclusion control.
III. Characteristics and Limitations of Aluminum Deoxidation
Metallic aluminum is widely used for secondary refining due to its strong deoxidizing ability and fast reaction rate. However, in clean steel production, Al deoxidation presents several drawbacks:
- Al₂O₃ Inclusions Affect Workability
Alumina inclusions severely reduce the drawability of spring steel and hard wire, often causing wire breakage. To avoid brittle Al₂O₃ inclusions, LF furnace refining with non-aluminum deoxidation or slag-washing processes is applied, keeping total oxygen (T[O]) ≤ 0.003%. - Reduced Fatigue Resistance
Al₂O₃ particles often act as nuclei for fatigue cracks, particularly harmful to bearing steel, rail steel, and wheel steel. - Creep Brittleness and Lower High-Temperature Strength
Aluminum deoxidation products and residual Al can cause creep brittleness in heat-resistant steels, reducing high-temperature strength and service life.
Risk of Nozzle Clogging During Continuous Casting
At steelmaking temperatures, Al₂O₃ particles are sharp-edged solids that tend to accumulate in the tundish nozzle, causing blockage and casting interruptions.
