In the competitive landscape of modern steelmaking, controlling the quality of molten steel is paramount for meeting stringent industry standards. Among the various control parameters, the dosage of desulfurizers plays a pivotal role in determining the final chemical composition, purity, and mechanical properties of the steel. This article provides a technical analysis of how optimizing desulfurizer usage—specifically Lime, Ca-Al alloys, and Magnesium—can drive operational excellence and product reliability for B2B steel producers.

1. Types and Characteristics of Desulfurizers

Selecting the right type of desulfurizer is the first step in optimizing the refining process. Different agents offer distinct advantages depending on the steel grade and desired outcome.

1.1 Common Desulfurizers in Steelmaking

The effectiveness of desulfurization relies heavily on the agent’s ability to react with sulfur and modify inclusions.

• ●Lime (CaO): Ideal for high-carbon and ordinary carbon steels. The recommended dosage is typically 1.5% to 2.5% of the total molten steel mass. To maximize efficiency, slag basicity should be maintained between 1.8 and 2.2.
• ●Ca-Al Alloy: Best suited for low-carbon structural steels. When the molten steel temperature is between 1550°C and 1580°C, a dosage of 0.5% to 1.2% yields rapid reaction rates. This agent promotes the formation of spherical CaS inclusions, which is crucial for controlling steel purity.
• ●Magnesium (Mg): Used primarily for special steels and alloy refining. A lower dosage of 0.2% to 0.5% can significantly reduce residual sulfur and minimize hard inclusions.
• ●Rare Earth Composites: Containing elements like CeO₂ and La₂O₃, these agents (dosage 0.05%–0.2%) refine inclusion size and improve cleanliness. They are often used in combination with Ca-Al or Lime to modify inclusion morphology.

1.2 Performance Comparison

The choice of desulfurizer directly influences inclusion morphology, which subsequently affects the steel’s structural integrity.

• ●Ca-Al Alloy: Generates spherical CaS inclusions (10–20μm) that are uniformly distributed, facilitating easier slag removal.
• ●Lime: Tends to form plate-like or irregular CaS inclusions (length 50–80μm). Without proper control, these can act as stress concentrators.
• ●Mg and Rare Earth: Produce micro-spherical or composite spherical inclusions. By adjusting stirring intensity, these agents minimize the risk of stress concentration within the steel matrix.

2. Impact of Desulfurizer Dosage on Molten Steel Quality

Achieving the optimal balance in desulfurizer dosage is critical. Both insufficient and excessive amounts can lead to quality defects, affecting everything from chemical homogeneity to mechanical strength.

2.1 Influence on Chemical Composition

The dosage regulates sulfur conversion reactions and the dissolution of intermetallic elements. In the LF refining process of low-carbon steel, adding 0.8% Ca-Al alloy at 1550–1580°C with a stirring speed of 100–120 rpm can reduce sulfur content from 0.020% to 0.006% within 10 minutes.

• ●Insufficient Dosage (e.g., 0.4%): Leads to incomplete desulfurization, leaving sulfur content around 0.012%. This results in uneven inclusion distribution and the formation of irregular CaS, which can become sources of micro-cracks during rolling.
• ●Excessive Dosage (e.g., >1.2%): While sulfur may drop below 0.004%, excess Calcium and Aluminum generate large amounts of CaO-CaS inclusions. This increases slag thickness and can cause local temperature drops or inclusion aggregation.

2.2 Influence on Steel Purity

Steel purity is evaluated by the quantity, size, shape, and distribution of inclusions.

• ●Low Dosage Risks: When Ca-Al alloy is under-dosed (0.4%–0.5%), residual inclusions can grow to 50–80μm. These irregular shapes are difficult to remove, leading to high-density inclusion zones that risk cracking during cooling.
• ●Optimal Dosage Benefits: A dosage of 0.8%–1.0% Ca-Al reduces inclusion size to 10–20μm. The spherical morphology allows these particles to float to the slag layer easily. Maintaining oxygen content between 0.002% and 0.004% ensures high spheroidization rates, significantly enhancing the steel’s cleanliness.

2.3 Influence on Mechanical Properties

The mechanical performance of the final product is perhaps the most critical metric for end-users.

• ●Optimized Performance: Controlling sulfur content between 0.005% and 0.010% with spherical inclusions maintains tensile strength at 450–480 MPa and impact toughness at 27 J/cm². This indicates a ductile fracture mode, ideal for structural applications.
• ●Consequences of Imbalance:

• ○Under-dosing: Increases residual sulfur (>0.012%), creating plate-like inclusions that act as crack origins. Impact toughness drops to 18–22 J/cm².
• ○Over-dosing: Excessive CaO-CaS inclusions (20–30μm) can lower impact toughness to 20–25 J/cm² and increase strength variability.

3. Conclusion

The study confirms that precise control over desulfurizer type and dosage is essential for high-quality steel production. For low-carbon steel LF refining, a Ca-Al alloy dosage of 0.8%–1.0% is the optimal range, ensuring residual sulfur stays below 0.006% while maintaining favorable spherical inclusion morphology. By adhering to these technical parameters, steel manufacturers can achieve superior chemical stability, high purity, and robust mechanical properties, ultimately delivering greater value to downstream clients.