Submerged Entry Shroud in Steelmaking: Mechanisms, Degradation, and Clean Steel Control

1. Introduction

In modern continuous casting and secondary steelmaking operations, maintaining steel cleanliness between the ladle and tundish is of critical importance. One of the most essential refractory components serving this function is the Submerged Entry Shroud (SES), also referred to as the sub entry shroud or ladle-to-tundish shroud. The SES provides a protected flow channel for molten steel, preventing reoxidation, minimizing inclusion formation, and stabilizing steel flow.

Although the submerged entry shroud is often considered a “passive” refractory component compared with the submerged entry nozzle (SEN), recent experimental and industrial studies—particularly those focusing on decarburization, oxidation, and clogging mechanisms—have demonstrated that the internal condition of the shroud plays a decisive role in downstream steel quality and nozzle performance. Many of the degradation phenomena identified for SENs are directly transferable to SES behavior, especially regarding carbon oxidation, coating interactions, and steel–refractory reactions.

This article presents a comprehensive technical discussion of the sub entry shroud, grounded in the scientific insights provided by the referenced experimental work, with particular attention to material degradation, internal surface reactions, and their influence on clean steel production.

2. Function and Position of the Submerged Entry Shroud

The sub entry shroud is installed between the ladle slide gate or stopper system and the tundish inlet nozzle. Its main functions are:

Preventing molten steel contact with atmospheric oxygen and nitrogen

Suppressing reoxidation reactions

Reducing temperature loss

Minimizing turbulence and slag entrainment

Protecting downstream refractory components (tundish nozzle, SEN)

The SES typically operates under the following conditions:

Steel temperature: 1550–1650 °C

Strong thermal gradients during preheating and start-up

Exposure to oxidizing gases during preheating

Long contact times with liquid steel

These conditions make the SES highly vulnerable to chemical and structural degradation, particularly at the internal bore surface.

3. Materials Used in Sub Entry Shrouds

3.1 Typical Material Systems

Most sub entry shrouds are manufactured from Al‚O–C, MgO–C, or ZrO‚-containing Al‚O–C refractories. Carbon is intentionally added to:

Improve thermal shock resistance

Reduce steel wettability

Enhance spalling resistance

However, as demonstrated in the learned article, carbon is also the weakest link under oxidizing conditions.

3.2 Role of Carbon in Shroud Degradation

Carbon oxidation begins at temperatures as low as 873–973 K, particularly during preheating in oxygen- or CO‚-containing atmospheres. Once carbon is oxidized:

Open porosity increases

Oxygen diffusion accelerates

Steel penetration becomes possible

Chemical reactions with steel intensify

This decarburization phenomenon, extensively studied for SENs, is equally relevant for SESs.

4. Decarburization of Sub Entry Shrouds

4.1 Mechanism of Decarburization

The decarburization of SES refractories occurs primarily during preheating and standby periods, when the shroud is exposed to hot oxidizing gases. The reaction can be simplified as:

C (solid) + O‚ / CO‚ ’ CO / CO‚ (gas)

As shown in the referenced study:

Initial decarburization is reaction-controlled

Later stages are diffusion-controlled

Porosity and pore connectivity dominate oxidation kinetics

Once the internal surface loses carbon, it becomes chemically active, increasing its affinity for molten steel and inclusions.

5. Influence of Internal Coatings on Sub Entry Shroud Performance

5.1 Conventional Glass and Silicon-Based Coatings

Glass or silicon powder coatings are often applied to SESs to protect carbon during preheating. While these coatings can temporarily reduce oxidation, experimental evidence indicates several negative effects:

Alkali-rich glass penetrates refractory pores

Reaction with graphite generates CO gas

Local pressure buildup causes microcracking

Inhomogeneous coating thickness leads to uneven protection

These effects mirror the coating-related issues observed in SENs and explain why coated SESs may still contribute to cleanliness problems.

5.2 Formation of Reactive Internal Surfaces

Once coatings degrade or infiltrate the refractory matrix, the SES internal surface can transform into:

Alkali-rich reaction layers

Silicate phases

Decarburized alumina-rich zones

Such surfaces act as nucleation sites for inclusion attachment, even before steel reaches the SEN.

6. Interaction Between Sub Entry Shroud and Molten Steel

6.1 Reoxidation and Inclusion Formation

If the SES fails to provide complete sealing, atmospheric oxygen may enter the steel stream, causing:

Formation of Al‚O inclusions

Growth of complex oxides

Increased inclusion loading entering the tundish

These inclusions later contribute to nozzle clogging, a phenomenon often incorrectly attributed only to the SEN.

6.2 REM-Alloyed Steels and Shroud Reactivity

The learned article demonstrates that steels containing rare earth metals (REM) are particularly sensitive to refractory interactions. In SESs with decarburized or coated internal surfaces:

REM elements reoxidize rapidly

REM oxides adhere to refractory walls

Initial accretion layers form upstream of the SEN

Thus, the SES can act as the first stage of the clogging process, not merely a transport channel.

7. Accretion and Deposit Formation Inside Sub Entry Shrouds

Although less severe than in SENs, deposit formation inside SESs has been observed, especially during long casting sequences. These deposits consist of:

Oxide-rich layers near the refractory wall

Steel-enriched solidified phases

Reaction products derived from coatings

Such deposits increase flow resistance and promote turbulent flow into the tundish, indirectly affecting mold-level stability.

8. Thermal Shock and Mechanical Damage

Sub entry shrouds experience:

Rapid heating during steel opening

Localized cooling during flow interruptions

Mechanical stresses from assembly and alignment

If decarburization has already weakened the matrix, thermal shock can cause:

Internal cracking

Spalling of the bore surface

Accelerated erosion during casting

This mechanical degradation further exposes fresh reactive surfaces.

9. Engineering Strategies for Improving Sub Entry Shroud Performance

9.1 Atmosphere Control During Preheating

Based on experimental findings:

Oxygen content in preheating gas must be minimized

Short, high-temperature preheating is preferable

Long low-temperature holding should be avoided

These measures significantly reduce carbon oxidation.

9.2 Advanced Coating Technologies

The article demonstrates that YSZ (yttria-stabilized zirconia) coatings, applied via plasma-based processes, offer substantial advantages:

Chemically inert surface

High resistance to steel and REM reactions

Effective barrier against oxygen diffusion

Smooth internal bore surface

Although more expensive, such coatings represent a promising future direction for SES design.

9.3 Material Optimization

ZrO‚-containing Al‚O–C systems show improved oxidation resistance

Antioxidants such as ZrSi‚ provide volumetric expansion upon oxidation, sealing pores

Optimized carbon content balances thermal shock resistance and oxidation risk

10. Role of the Sub Entry Shroud in Clean Steel Production

From a systems engineering perspective, the sub entry shroud must be viewed as an active metallurgical component, not a simple refractory pipe. Its internal condition directly influences:

Steel reoxidation behavior

Inclusion population entering the tundish

Downstream SEN clogging

Overall casting stability

Failures or degradation at the SES stage often propagate through the entire casting process.

11. Educational Significance for Engineering Students

For engineering students, the SES provides a valuable case study in:

High-temperature materials degradation

Multiphase chemical reactions

Interaction between process design and materials selection

Importance of upstream control in complex metallurgical systems

Understanding SES behavior reinforces the concept that clean steel production begins before the tundish and mold.

12. Conclusion

The submerged entry shroud plays a far more critical role in steelmaking than traditionally assumed. Experimental and industrial evidence shows that decarburization, coating degradation, and refractory–steel interactions inside the SES can significantly affect steel cleanliness and casting performance. Many phenomena previously attributed solely to SEN clogging originate, at least in part, within the SES.

By applying advanced materials, optimized preheating practices, and improved coating technologies, the SES can be transformed from a vulnerability into a robust barrier protecting steel quality. For modern steelmaking, a scientifically informed approach to sub entry shroud design and operation is essential. More information please visit Henan Yangyu Refractories Co.,Ltd