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
