Sub-entry nozzle (SEN) clogging is one of the most persistent operational challenges in the continuous casting process. It directly affects steel flow stability, casting speed, slab quality, and tundish life. In severe cases, it forces early strand stoppage, leading to significant production losses. For steel grades requiring ultra-low inclusion content—such as automotive sheets, pipeline steel, electrical steel, and bearing steel—SEN clogging becomes even more critical due to strict cleanliness requirements.

1. Understanding the Mechanisms of SEN Clogging

SEN clogging is caused by the accumulation of solid inclusions, reaction products, or solidified steel along the internal surface of the nozzle. Over time, these deposits grow and reduce the flow area, resulting in unstable casting conditions.

1.1 Alumina-Based Clogging

Alumina clogging is the most common issue in aluminum-killed steels. During deoxidation, dissolved oxygen reacts with aluminum to form fine Al₂O₃ particles. These inclusions have poor wettability to the nozzle’s refractory materials and tend to accumulate in areas with turbulent or low-velocity flow.

Mechanisms include:

• Inclusion agglomeration and collision near the SEN inlet

• Deposition on the nozzle wall due to poor Al₂O₃ wetting

• Growth of deposit layers by continuous trapping of inclusions

This type of clogging is dangerous because it develops quietly and becomes noticeable only after the flow becomes restricted.

1.2 Calcium Aluminate Clogging

Calcium treatment modifies solid alumina inclusions into liquid or semi-liquid CaO–Al₂O₃–SiO₂ phases. However, improper calcium control results in:

• solid CaS

• high-melting CaO-rich compounds

• unstable liquid phases during temperature drops

These phases solidify inside the SEN and form sticky layers that grow rapidly.

1.3 Steel Solidification Clogging

Insufficient superheat or high heat loss through the nozzle wall leads to partial freezing of steel:

• low casting speed

• long ladle-change time

• inadequate tundish heating

• high heat extraction by the refractory

Solidified steel narrows the nozzle bore and disturbs the flow pattern.

1.4 Refractory Reaction and Erosion

SEN materials such as ZrO₂-C or Al₂O₃-C may react with molten steel, forming:

• FeO-based reaction layers

• ZrO₂ agglomerates

• carbon oxidation regions

These reaction products create rough surfaces that trap more inclusions.

2. Key Factors Influencing SEN Clogging

Avoiding clogging requires an understanding of the process variables that trigger or accelerate it.

2.1 Steel Cleanliness

High inclusion content increases clogging frequency. Factors include inadequate ladle refining, slag carryover, and poor desulfurization.

2.2 Calcium Treatment Control

Both insufficient and excessive calcium cause nozzle buildup.

2.3 Argon Injection Rate

Argon gas helps reduce clogging but must be carefully controlled:

• too low → inadequate inclusion flotation

• too high → nozzle erosion and mold level oscillation

2.4 Casting Speed Stability

Inconsistent casting speed causes fluctuating flow regimes inside the SEN, affecting attachment rates.

2.5 Superheat Control

Low superheat increases the risk of solid-steel deposition.

2.6 Refractory Properties

The chemical and physical characteristics of SEN material influence inclusion adhesion capability.

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3. Industrial Strategies to Prevent SEN Clogging

A successful strategy integrates metallurgy, refractory technology, tundish operation, and caster control.

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3.1 Improving Steel Cleanliness: Foundation of Anti-Clogging Practice

3.1.1 Ladle Refining

Processes such as RH, VD, and LF refining improve inclusion removal by:

• deep vacuum deoxidation

• argon stirring for inclusion flotation

• desulfurization and slag-metal reactions

3.1.2 Slag Engineering

Slag composition must ensure:

• low FeO (< 0.5%)

• strong basicity

• good absorption capacity for Al₂O₃ and CaO inclusions

3.1.3 Tundish Metallurgy

Tundish equipment that enhances cleanliness:

• turbulence suppressors

• weirs/dams

• gas curtains

• slag retaining furniture

Cleaner steel means significantly fewer particles available to form clogging layers.

3.2 Optimizing Calcium Treatment

Calcium treatment aims to modify Al₂O₃ inclusions into liquid phases at casting temperature.

3.2.1 Principles of Effective Calcium Treatment

• Proper timing after desulfurization

• Control of Ca/Al ratio

• Maintain steel temperature to keep inclusions molten

• Prevent over-calcium leading to CaS solidification

3.2.2 Key Operational Tips

• Use high-purity CaSi wire

• Apply consistent wire feeding speed

• Perform real-time inclusion morphology analysis (if available)

Proper calcium treatment dramatically reduces SEN clogging in Al-killed steel grades.

3.3 Selection of High-Performance SEN Refractory Materials

Modern SENs are engineered with materials offering improved resistance to deposition and reaction.

3.3.1 ZrO₂-Based Refractory Advantages

• high thermal stability

• excellent corrosion resistance

• improved non-wetting behavior for alumina

3.3.2 Anti-Wetting Coatings

Surface coatings significantly reduce inclusion adhesion, such as:

• BN (boron nitride) coatings

• graphite-based layers

• newly developed anti-Al₂O₃ coatings

3.3.3 Composite Layer Designs

Some SENs incorporate:

• a smooth inner bore

• an anti-clogging coating

• reinforced zirconia working layer

Such layered designs extend nozzle life and delay clogging.

3.4 Optimize Argon Injection Through the SEN

Argon injection is a critical parameter for minimizing clogging and stabilizing steel flow.

3.4.1 Benefits of Argon Injection

• reduces inclusion adhesion by generating a protective gas barrier

• helps float inclusions into the tundish or mold

• prevents steel solidification near the nozzle wall

3.4.2 Best Practices

• Maintain flow rates within 5–25 L/min depending on steel grade and nozzle size

• Monitor for gas leakage in slide gates and joints

• Avoid sudden argon flow fluctuations

• Use dual-line argon supply for reliability

A stable and optimized argon system is essential for preventing nozzle deposition.

3.5 Maintain Proper Superheat and Temperature Stability

Temperature control is essential in avoiding steel solidification clogging.

3.5.1 Target Superheat Levels

• Typical superheat during casting: +20°C to +40°C

• Higher superheat required for ultra-clean or high-alloy steels

3.5.2 Heat Loss Management

• Use ladle and tundish preheating

• Ensure insulation of SEN and tundish walls

• Minimize time between tapping and casting

3.5.3 Prevent Temperature Fluctuations

Temperature changes increase inclusion formation and solid deposition.

3.6 Ensure Stable Casting Operation and Flow Control

Stable operation minimizes turbulence, which reduces inclusion re-entrainment and adhesion.

3.6.1 Avoid Frequent Speed Adjustments

Casting speed should remain constant to maintain steady flow in the SEN.

3.6.2 Maintain Precise Slide Gate or Stopper Control

Misalignment or erratic adjustments increases turbulence and accelerates clogging.

3.6.3 Smooth Ladle Change Operation

Avoid extended tundish waiting time which cools the steel and increases clogging potential.

Stable casting leads to a substantially lower clogging rate.

3.7 Adopt Advanced SEN Designs

Refractory manufacturers continue to innovate SEN designs for anti-clogging performance.

3.7.1 Improved Geometry

• optimized inlet transition

• smooth internal bore

• controlled outlet port angles

• hydrodynamic designs for laminar flow

3.7.2 Multi-Stage Gas Injection SENs

These designs introduce argon at various heights to maintain stable bubbles and reduce clogging.

3.7.3 Anti-Clogging Coatings

Advanced coatings can prolong SEN life by preventing alumina buildup.

4. Integrated Anti-Clogging Strategy

The most effective method to avoid SEN clogging is to treat it as a metallurgical and operational system problem instead of a single-parameter issue. A successful strategy includes:

• producing clean steel in the ladle

• optimizing calcium treatment to modify inclusions

• using high-quality SEN refractories

• stabilizing argon flow for inclusion removal

• maintaining constant superheat

• enforcing stable casting operation

• applying advanced SEN geometry designs

When these factors are controlled together, SEN clogging can be reduced dramatically—by up to 50–80% in many industrial plants.

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5. Conclusion

SEN clogging remains a major challenge in continuous casting, especially for high-grade steel production. However, with proper metallurgical treatment, refractory selection, tundish operation, argon optimization, and casting stability, clogging can be significantly minimized.

Avoiding SEN clogging is not a single action but a coordinated approach involving steel refining, nozzle technology, thermodynamic control, and precise process operation. By adopting the strategies described in this article, steel plants can achieve:

• improved casting stability

• longer SEN service life

• fewer strand interruptions

• better surface and internal steel quality

• higher productivity and reduced cost