In continuous casting of steel, controlling the transfer of molten steel from the tundish to the mold is one of the most critical tasks for ensuring product quality, process stability, and operational safety. Among the refractory flow-control and protection devices used in this zone, the Sub Entry Shroud (SES) plays a decisive role. The sub entry shroud is not merely a mechanical connection between the tundish nozzle and the mold; it is a key metallurgical component that protects molten steel from reoxidation, stabilizes flow, and directly influences inclusion behavior and surface quality of the cast strand.

As steel cleanliness requirements become increasingly stringent—especially for automotive, electrical, and high-strength low-alloy (HSLA) steels—the performance of the sub entry shroud has become a focal point in continuous casting technology. This article provides a comprehensive and technical explanation of how the sub entry shroud works, including its structure, operating principles, materials, interaction with fluid flow, and its impact on steel quality.

2. Definition of Sub Entry Shroud

A Sub Entry Shroud (SES) is a tubular refractory component installed between the tundish bottom nozzle and the mold’s submerged entry nozzle (SEN), or directly integrated with the SEN depending on casting design. Its primary function is to provide a fully enclosed pathway for molten steel as it flows from the tundish into the mold, preventing contact with atmospheric oxygen.

In slab casting, the SES is usually connected to the SEN and extends below the steel meniscus in the mold. In billet or bloom casting, the SES may serve as the submerged nozzle itself or as a protective shroud above it.

3. Why a Sub Entry Shroud Is Necessary

3.1 Prevention of Reoxidation

Molten steel is highly reactive with oxygen. Even brief exposure to air during transfer from tundish to mold can cause rapid oxidation, leading to the formation of non-metallic inclusions such as Al₂O₃, SiO₂, or complex oxides. The SES isolates the steel stream from the atmosphere, significantly reducing reoxidation.

3.2 Suppression of Nitrogen and Hydrogen Pickup

In addition to oxygen, exposure to air can result in nitrogen absorption and hydrogen pickup, which negatively affect mechanical properties and increase the risk of defects such as pinholes and embrittlement. The SES creates a controlled environment that minimizes gas absorption.

3.3 Flow Stability and Mold Level Control

An unprotected steel stream is prone to turbulence, splashing, and jet instability. The SES provides a smooth, guided flow path, which stabilizes the steel jet entering the mold and contributes to consistent mold level control.

4. Structural Design of a Sub Entry Shroud

4.1 Shroud Body

The main body of the SES is a refractory tube with a carefully controlled internal diameter. The internal bore size is designed to balance flow velocity, pressure loss, and residence time.

4.2 Upper Connection (Tundish Interface)

The upper end of the SES connects to the tundish nozzle, usually via a seating block or gasket system. This joint must be gas-tight to prevent air aspiration due to the Venturi effect created by high-velocity steel flow.

4.3 Lower Connection (Mold or SEN Interface)

At the lower end, the SES either connects to the SEN or directly discharges steel into the mold below the meniscus. Proper alignment is critical to avoid asymmetric flow, erosion, or jet impingement on mold walls.

4.4 Argon Injection Ports (Optional)

Many SES designs incorporate argon injection ports near the upper or middle section. Controlled argon flow serves to:

• Prevent air aspiration
• Reduce nozzle clogging
• Modify steel flow characteristics

5. Operating Principle of the Sub Entry Shroud

The working mechanism of the SES is governed by fluid dynamics, thermodynamics, and metallurgical interactions.

5.1 Enclosed Steel Transfer

Once molten steel leaves the tundish nozzle, it immediately enters the SES. Because the shroud is sealed, the steel is fully enclosed throughout its descent into the mold. This eliminates direct contact with ambient air.

5.2 Ferrostatic Pressure-Driven Flow

The steel flow through the SES is driven by the ferrostatic pressure head created by the liquid steel column in the tundish. The SES does not actively control flow rate; instead, it ensures that flow remains stable and protected.

5.3 Suppression of Turbulence

The smooth internal surface of the SES reduces boundary-layer disturbances. Compared with an open stream, flow inside the shroud is more uniform and less prone to breakup, which minimizes splashing and air entrainment at the mold entry.

5.4 Submerged Discharge Below Meniscus

A critical aspect of SES operation is that steel exits below the mold meniscus. This prevents surface turbulence and reduces slag entrainment, contributing to cleaner steel and improved surface quality.

6. Materials Used in Sub Entry Shrouds

6.1 Alumina-Carbon (Al₂O₃–C)

Alumina-carbon materials are widely used due to their excellent thermal shock resistance and low wettability with molten steel. Carbon reduces adherence of inclusions and minimizes clogging.

6.2 Zirconia-Containing Materials

Zirconia additions enhance resistance to erosion and chemical attack, particularly in high-speed casting or aggressive steel grades.

6.3 Oxidation Protection Systems

Because carbon-containing materials are susceptible to oxidation, SES products often incorporate:

• Antioxidants (Al, Si, B₄C)
• Protective coatings
• External slag or glaze layers

7. Argon Protection and Anti-Clogging Function

7.1 Air Aspiration Prevention

High-velocity steel flow can create negative pressure inside the SES, drawing air into poorly sealed joints. Argon injection creates a positive pressure barrier that prevents air ingress.

7.2 Nozzle Clogging Mitigation

Argon bubbles reduce the adhesion of alumina inclusions to the inner wall of the shroud and SEN, slowing clog formation and extending casting time.

7.3 Flow Pattern Modification

Small amounts of argon can modify the flow regime, reducing jet velocity fluctuations and improving flow symmetry.

8. Influence of SES on Steel Quality

8.1 Inclusion Control

By minimizing reoxidation and turbulence, the SES significantly reduces the formation and entrainment of non-metallic inclusions.

8.2 Surface Quality Improvement

Stable mold flow and reduced slag entrainment lead to fewer surface defects such as slivers, oscillation mark cracks, and pinholes.

8.3 Internal Quality Enhancement

Uniform flow distribution promotes even solidification and reduces centerline segregation and porosity.

9. Common Operational Issues

Despite its advantages, SES performance can be compromised by:

• Poor sealing and air aspiration
• Excessive erosion due to high casting speed
• Oxidation of carbon-containing materials
• Improper argon flow causing bubble entrapment

These issues highlight the importance of proper design, installation, and operational control.

10. Comparison with Open Pouring Systems

Compared to open pouring:

• SES systems offer dramatically lower reoxidation rates
• Improved steel cleanliness
• Better mold level stability
• Higher operational safety

As a result, open pouring is now largely obsolete in high-quality steel production.

11. Future Trends in Sub Entry Shroud Technology

Ongoing developments include:

• CFD-optimized internal geometry
• Advanced anti-clogging coatings
• Smart SES systems integrated with flow sensors
• Longer-life materials for ultra-long casting sequences

12. Conclusion

The sub entry shroud is a critical metallurgical and flow-control component in continuous casting. By providing a sealed, controlled pathway for molten steel from the tundish to the mold, it prevents reoxidation, stabilizes flow, reduces inclusion formation, and enhances both surface and internal quality of cast products.

Understanding how the sub entry shroud works—from its structural design to its interaction with fluid flow and metallurgical phenomena—is essential for achieving stable casting operations and producing high-cleanliness steel.