The Sub-Entry Nozzle (SEN) is one of the most critical refractory components used in continuous casting of steel. Installed between the tundish and the mold, the SEN performs essential metallurgical and operational functions: stabilizing molten steel flow, minimizing reoxidation, reducing heat loss, suppressing surface turbulence, and preventing mold powder entrapment. To meet these functional requirements under severe conditions—including temperatures above 1,550°C, corrosive slag atmosphere, high levels of argon injection, and erosive molten steel flow—the SEN must be manufactured from highly engineered refractory compositions.
The composition of a Sub-Entry Nozzle is not a single, uniform formula. Instead, it is a carefully designed multi-material system that includes the outer body, the bore (hot-face) material, the anti-clogging or anti-carbon deposition layer, and sometimes a slag line reinforcement structure. Variations in steel grades, casting speeds, mold dimensions, tundish setup, and casting powder chemistry further influence the material design of every SEN.
This article provides an in-depth examination of the principal compositions used in SEN manufacturing, their functional roles, and why different material systems are selected for different operational conditions.
1. Functional Requirements of SEN Materials
To understand why specific compositions are used, it is necessary to first consider the performance requirements. SEN materials must:
- Resist corrosion from molten steel and mold flux.
- Minimize erosion caused by high-speed steel flow, turbulence, and argon bubbles.
- Prevent clogging, especially from alumina buildup in Al-killed steels.
- Maintain thermal shock resistance, particularly during start-up.
- Provide mechanical strength to support system loads.
- Prevent carbon pickup, which can modify steel quality.
- Ensure non-wettability, reducing inclusion adhesion.
- Withstand long casting sequences, such as 8–20 hours in modern casters.
These requirements result in the use of complex refractory compositions combining alumina, zirconia, carbon, SiC, SiO2, spinel-forming oxides, and advanced coating technologies.
2. Main Material System of SEN: Zirconia–Graphite (ZrO₂–C)
The dominant composition for SENs is Zirconia–Graphite, which provides high thermal shock resistance and excellent corrosion protection.
2.1 Zirconia (ZrO₂)
Zirconia is the most important mineral phase in the hot-face of SENs.
Key characteristics:
- Melting point > 2,700°C
- Exceptional chemical stability
- Extremely low wettability by molten steel
- High resistance to alumina deposition
- Low thermal conductivity
- High refractoriness under load
Since SENs are directly exposed to high-temperature molten steel inside the mold, zirconia ensures minimal chemical attack and structural degradation.
Zirconia types used:
- Fused zirconia for maximum purity
- Sintered zirconia for cost-effective performance
- Partially stabilized zirconia (PSZ) for improved toughness
ZrO₂ content in SEN composition typically ranges between 60–90% depending on the steel type and casting duration.
2.2 Graphite (C)
Graphite is added to enhance thermal shock resistance and reduce steel adhesion.
Roles of graphite:
- Prevents thermal cracking in the bore
- Provides lubrication during erosion
- Reduces slag wetting
- Supports anti-clogging behavior
Graphite content typically ranges from 10–20%, but it must be controlled carefully to avoid oxidation. High graphite improves shock resistance but decreases oxidation resistance; therefore antioxidants are required (see below).
3. Anti-Oxidation Additives in SEN Composition
Because SENs contain graphite, anti-oxidation additives are necessary.
3.1 Common antioxidants include:
- SiC (Silicon Carbide)
- Al (Metallic Aluminum)
- Mg (Metallic Magnesium)
- Ca-Ba compounds
- ZrB₂ (Zirconium Diboride) – high-performance option
Function:
These additives create protective layers such as SiO₂ or Al₂O₃ on the surface of graphite during oxidation, shielding carbon from direct contact with oxygen. They also modify inclusion behavior and support anti-clogging.
4. Slag-Line Reinforcement Materials
The slag line is the region of the SEN body in direct contact with tundish slag. It requires additional corrosion and erosion resistance.
4.1 Alumina–Carbon (Al₂O₃–C)
Alumina-carbon is often used in less demanding slag-line applications.
Advantages:
- Good thermal shock performance
- Excellent mechanical strength
- Cost-effective
However, alumina-carbon can be prone to corrosion by CaO-based tundish slags.
4.2 Zirconia–Carbon (ZrO₂–C) for slag line
For aggressive casting scenarios—high-speed slab casters, peritectic steel grades—ZrO₂–C is preferred at the slag line as well.
Benefits:
- Superior slag resistance
- Low wettability
- High erosion resistance
4.3 Spinel-Forming Oxides (MgO·Al₂O₃)
Magnesia-alumina spinel systems are increasingly used.
Benefits:
- Self-forming spinel improves corrosion resistance
- Reduced carbon content for better oxidation resistance
5. Body Material of SEN: Al₂O₃–SiO₂–ZrO₂ Systems
While the hot-face uses high ZrO₂–C, the outer structural body typically employs a different material.
5.1 Alumina–Silica Refractory Body
This provides:
- Mechanical strength
- Structural stability
- Low thermal conductivity
- Lower cost
Typical composition:
- 60–80% Al₂O₃
- 10–20% SiO₂
- Minor SiC or carbon
Body materials must resist cracking during preheating and casting. Increasing demand for longer sequence casting has led to the adoption of high-alumina and ZrO₂-enriched bodies.
6. Anti-Clogging Layer Composition (Most Critical Part)
Modern steelmaking requires precise control of SEN clogging. Therefore, SENs include an inner anti-clog coating.
6.1 Types of anti-clogging coatings
- MgO-based coatings
- CaO or CaZrO₃ coatings
- Zirconium diboride (ZrB₂) coatings
- Al₂O₃–MgO spinel coatings
- In-situ formed CaO–Al₂O₃–SiO₂ glass films
Functions:
- Reduce inclusion adhesion (especially alumina)
- Promote fluid flow and self-cleaning
- Provide non-wetting properties
6.2 Carbon Modification
Some coatings include modified carbon or nano-carbon structures to reduce clogging and improve smooth flow.
7. Ceramic Filters or Inserts (Optional SEN Component)
Some advanced SEN designs incorporate ceramic inserts or turbulence-control filters.
Materials include:
- Porous zirconia filters
- Alumina ceramic plates
- SiC flow modifiers
These features reduce turbulence at the steel-mold interface and enhance steel cleanliness.
8. Binder and Resin Systems
SENs are shaped using resin-bonded systems.
8.1 Typical binders:
- Phenolic resin
- Pitch
- Carbon-rich polymer binders
After firing:
These resins leave controlled carbon residue that enhances mechanical strength and thermal performance.
9. Summary of Typical SEN Composition (Example Range)
Below is a representative formulation for a modern high-performance SEN:
| Component | Typical Percentage | Function |
|---|---|---|
| Zirconia (ZrO₂) | 60–90% | Hot-face corrosion & erosion resistance |
| Graphite (C) | 10–20% | Thermal shock resistance, anti-clogging |
| SiC | 3–10% | Antioxidation, wear resistance |
| Carbon Residue (from binders) | 3–8% | Structural stability |
| Spinel (MgO·Al₂O₃) | 5–15% | Slag-line corrosion resistance |
| Al₂O₃–SiO₂ body | — | Structural body of SEN |
| Anti-clogging coating | Thin layer | Reduces alumina buildup |
| ZrB₂ (optional) | 1–5% | Premium anti-clog additive |
The exact composition is customized for:
- Steel grade (Al-killed, Si-killed, IF steel)
- Casting speed
- Mold dimensions
- Slag chemistry
- Sequence length
- Argon injection strategy
10. Why SEN Composition Continues to Evolve
Continuous casting demands are intensifying:
- Higher casting speeds
- Lower inclusion tolerances
- More complex alloy systems
- Minimization of clogging shutdowns
- Longer tundish sequences
These industry pressures drive ongoing improvements in SEN materials, including:
- Nano-structured coatings
- Graphite-reduced oxidation-resistant bodies
- Spinel-enriched hot-face zones
- Advanced ZrO₂ forms (TZP, PSZ)
- Multi-layer SEN designs
Conclusion
The composition of a Sub-Entry Nozzle is a sophisticated multi-layer system engineered to withstand extreme thermal, chemical, and mechanical stresses during continuous casting. While the core material remains ZrO₂–C, advanced systems integrate spinel, SiC, antioxidants, high-alumina bodies, anti-clogging coatings, and optional ceramic inserts to provide optimal performance.
Understanding SEN composition is essential for:
- Reducing nozzle clogging
- Enhancing casting stability
- Increasing sequence length
- Improving steel cleanliness
- Lowering operational costs
The refractory design of SENs continues to evolve with steelmaking demands, ensuring enhanced reliability and metallurgical quality for modern continuous casting operations.
More information please visit Henan Yangyu Refractories Co.,Ltd
