1. Introduction

The Sub-Entry Nozzle (SEN) is a critical functional refractory component in the continuous casting process of steel. Positioned between the tundish and the mold, the SEN controls the flow of molten steel into the mold cavity while protecting the steel stream from secondary oxidation and regulating flow patterns to ensure stable solidification. Despite its relatively small size compared to other casting equipment, the SEN has a disproportionate influence on casting quality, productivity, and safety.

Problems associated with the SEN—such as clogging, erosion, cracking, air aspiration, and abnormal flow behavior—can lead to severe operational consequences, including mold level fluctuation, inclusion entrapment, breakout accidents, surface and internal defects, and unplanned casting interruptions. Therefore, understanding how to avoid SEN-related problems is of paramount importance for steelmakers.

This article provides a systematic and technical discussion of the major SEN problems, their root causes, and practical measures to prevent or mitigate these issues through material selection, design optimization, steel cleanliness control, operational practices, and maintenance management.

2. Typical Problems of the Sub-Entry Nozzle

Before discussing preventive strategies, it is necessary to understand the main categories of SEN problems encountered in industrial practice:

1. Clogging and partial blockage

2. Chemical and mechanical erosion

3. Thermal cracking and spalling

4. Air aspiration and reoxidation

5. Unstable or asymmetric flow pattern

6. Premature SEN breakage or leakage

Each of these problems has distinct mechanisms but is often interconnected with others.

3. Avoiding SEN Clogging

3.1 Mechanism of SEN Clogging

SEN clogging is the most common and troublesome problem in continuous casting, particularly for Al-killed steels. Clogging mainly results from:

• Deposition of alumina (Al₂O₃) inclusions on the inner bore

• Reaction between molten steel and SEN refractory

• Steel reoxidation due to air aspiration

• Precipitation of complex oxides (e.g., Al₂O₃–CaO–MgO spinels)

As deposits accumulate, the effective flow area is reduced, leading to flow instability, mold level fluctuation, and eventually casting interruption.

3.2 Material Optimization

To reduce clogging, SEN materials must exhibit excellent non-wettability and chemical stability:

• Al₂O₃–C with low wettability is widely used due to its resistance to steel penetration.

• ZrO₂ inserts in the bore region improve resistance to chemical attack and reduce inclusion adhesion.

• Anti-clogging additives, such as BN or special oxide modifiers, can further reduce alumina adhesion.

3.3 Steel Cleanliness Control

Steel composition and cleanliness have a direct impact on clogging tendency:

• Optimize calcium treatment to modify solid Al₂O₃ inclusions into liquid calcium aluminates.

• Control total oxygen (T.O.) levels in the tundish.

• Avoid excessive aluminum pickup during secondary metallurgy.

3.4 Operational Measures

• Maintain stable casting speed to prevent flow stagnation.

• Use argon gas injection through the SEN wall or stopper rod to suppress inclusion deposition.

• Avoid sudden temperature drops that promote oxide precipitation.

4. Preventing SEN Erosion

4.1 Erosion Mechanisms

SEN erosion occurs due to:

• High-velocity molten steel flow

• Chemical dissolution of refractory phases

• Mechanical wear from turbulent flow and inclusion impact

Severe erosion changes the internal geometry of the SEN, leading to asymmetric flow and increased inclusion entrapment.

4.2 Design Optimization

• Optimize port angle and port shape (e.g., well-rounded edges) to reduce local turbulence.

• Increase wall thickness in high-wear zones.

• Apply ZrO₂-reinforced inserts in the port and slag line regions.

4.3 Material Selection

• Use high-purity fused alumina or partially stabilized zirconia in critical zones.

• Reduce low-melting-point impurities such as SiO₂ and alkali oxides.

5. Avoiding Thermal Cracking and Spalling

5.1 Causes of Thermal Damage

Thermal cracking and spalling result from:

• Rapid temperature changes during preheating or casting start

• High thermal gradients between the SEN surface and core

• Inadequate thermal shock resistance of refractory materials

Cracks not only shorten SEN life but also allow steel penetration, accelerating failure.

5.2 Preheating Control

• Implement controlled and uniform preheating curves.

• Avoid localized flame impingement.

• Ensure sufficient soaking time to equalize temperature throughout the SEN body.

5.3 Material Improvements

• Use carbon-containing refractories with high thermal shock resistance.

• Optimize grain size distribution to improve fracture toughness.

• Introduce flexible bonding systems to absorb thermal stress.

6. Preventing Air Aspiration and Reoxidation

6.1 Mechanism of Air Aspiration

Air aspiration occurs when negative pressure develops inside the SEN due to high casting speed or improper sealing. This leads to:

• Reoxidation of molten steel

• Formation of new inclusions

• Accelerated SEN clogging

6.2 Structural and Assembly Measures

• Ensure tight connection between tundish well block, gasket, and SEN.

• Use high-quality refractory gaskets with good compressibility and sealing performance.

• Avoid misalignment during SEN installation.

6.3 Process Control

• Maintain adequate steel head in the tundish.

• Avoid excessive argon flow that may induce pressure fluctuations.

• Monitor oxygen pickup during casting as an indirect indicator of air aspiration.

7. Controlling Flow Pattern and Mold Hydrodynamics

7.1 Importance of Flow Control

Improper flow pattern caused by SEN design or wear can result in:

• Meniscus instability

• Inclusion entrapment

• Slag entrainment

• Surface defects such as slivers and oscillation marks

7.2 SEN Design Considerations

• Select appropriate port angle (typically 10°–25° downward) based on slab thickness and casting speed.

• Use two-port or multi-port designs to balance flow symmetry.

• Consider special designs such as swirl SENs to improve flow uniformity.

7.3 Monitoring and Adjustment

• Use mold level sensors and flow modeling results to optimize SEN parameters.

• Replace SENs showing severe internal deformation or erosion.

8. Extending SEN Service Life

8.1 Quality Control and Inspection

• Conduct dimensional and structural inspection before use.

• Reject SENs with visible cracks, density variation, or machining defects.

8.2 Proper Storage and Handling

• Store SENs in dry, temperature-stable environments.

• Avoid mechanical impact during transportation and installation.

8.3 Operational Discipline

• Avoid emergency casting conditions whenever possible.

• Train operators on correct SEN handling, installation, and replacement procedures.

9. Role of Simulation and Digital Tools

Advanced numerical simulation has become an important tool for avoiding SEN problems:

• CFD modeling helps predict flow patterns, erosion zones, and pressure distribution.

• Thermal stress analysis assists in optimizing preheating and material design.

• Data-driven monitoring enables early detection of abnormal SEN behavior.

Integrating simulation results with plant experience significantly enhances SEN reliability.

10. Conclusion

Avoiding problems of the Sub-Entry Nozzle requires a holistic and systematic approach that integrates refractory material engineering, SEN structural design, steel cleanliness control, and disciplined operational practices. No single measure can completely eliminate SEN-related issues; instead, success depends on coordinated optimization across the entire continuous casting process.

By selecting appropriate SEN materials, minimizing clogging mechanisms, controlling erosion and thermal damage, preventing air aspiration, and ensuring stable mold flow, steelmakers can significantly improve casting stability, product quality, and overall production efficiency. As continuous casting technology evolves, the role of the SEN will remain central, making ongoing innovation and process control essential for modern steel plants