1. Introduction

In modern continuous casting of steel, the Submerged Entry Nozzle (SEN) plays a critical role in controlling the transfer of molten steel from the tundish into the mold. Beyond its material composition and port geometry, the positional accuracy of the SEN relative to the mold centerline has a profound influence on the mold flow field, heat transfer, inclusion behavior, and surface quality of the cast product.

One of the most frequently overlooked yet highly influential parameters is the SEN offset, defined as the lateral or angular deviation of the nozzle from the ideal central alignment within the mold. Even small offsets—on the order of a few millimeters—can significantly alter the flow pattern inside the mold cavity. This article provides an in-depth analysis of the influence of SEN offset on the flow field, discussing fluid dynamics mechanisms, metallurgical consequences, operational causes, and mitigation strategies.

2. Definition and Types of SEN Offset

2.1 What Is SEN Offset?

SEN offset refers to the deviation of the nozzle’s bore or ports from the mold’s geometric centerline. In an ideal casting setup, the SEN is perfectly aligned vertically and horizontally, ensuring symmetrical flow into the mold.

However, in real industrial conditions, offsets may occur due to:

• Mechanical tolerances in mounting systems

• Thermal expansion of refractory components

• Wear or deformation of the SEN

• Improper installation or alignment

• Mold oscillation and casting vibrations

2.2 Types of SEN Offset

SEN offset can be classified into several categories:

1. Lateral Offset
   Horizontal displacement of the SEN from the mold centerline.

2. Angular (Tilt) Offset
   Inclination of the SEN axis relative to the vertical direction.

3. Port-Level Asymmetry
   Uneven erosion or blockage causing effective offset of jet direction.

4. Dynamic Offset
   Time-dependent displacement due to vibration, wear, or thermal distortion.

Each type of offset affects the flow field in a distinct manner.

3. Flow Field Characteristics in Continuous Casting Molds

3.1 Ideal Symmetrical Flow Pattern

In a perfectly aligned SEN system, the flow field exhibits:

• Symmetrical double-roll or single-roll circulation

• Balanced jet impingement on mold walls

• Uniform velocity distribution near the meniscus

• Stable slag layer at the mold top

This balanced flow minimizes inclusion entrapment and promotes uniform solidification.

3.2 Key Flow Field Parameters Affected by SEN Offset

• Jet angle and penetration depth

• Turbulence intensity

• Velocity distribution at the meniscus

• Recirculation zone symmetry

• Shear stress at the solidification front

Even minor misalignment can disturb these parameters.

4. Influence of Lateral SEN Offset on the Flow Field

4.1 Jet Deflection and Asymmetric Flow

A lateral offset causes unequal distances between the SEN ports and the mold walls, resulting in:

• One jet impinging closer to a narrow face

• Increased jet momentum on one side

• Reduced flow strength on the opposite side

This asymmetry leads to an imbalanced double-roll flow structure, with one dominant circulation loop.

4.2 Meniscus Velocity Imbalance

Lateral offset increases meniscus velocity on the closer side, causing:

• Local slag layer thinning

• Slag entrainment risk

• Enhanced surface turbulence

On the opposite side, stagnant flow zones may form, increasing the risk of surface freezing and hook formation.

4.3 Inclusion Transport and Entrapment

Asymmetric flow affects inclusion motion by:

• Driving inclusions toward one side of the mold

• Increasing inclusion capture near the high-velocity jet

• Reducing flotation efficiency on the low-flow side

This results in non-uniform cleanliness across the slab or billet width.

5. Influence of Angular SEN Offset (Tilt)

5.1 Downward and Upward Tilt Effects

An angular offset changes the effective jet angle:

• Downward tilt increases jet penetration depth, strengthening lower recirculation loops

• Upward tilt increases meniscus turbulence and slag-metal interaction

Both conditions can destabilize the mold flow field.

5.2 Asymmetric Port Discharge

When the SEN is tilted, even nominally symmetric ports discharge jets with different effective angles, causing:

• Unequal impingement points

• Distorted recirculation zones

• Increased shear stress on one side of the shell

This can promote longitudinal cracks and internal defects.

6. Turbulence and Energy Dissipation Effects

6.1 Local Turbulence Intensification

Offset SEN conditions typically increase turbulence intensity:

• Higher Reynolds numbers near one jet

• Increased velocity gradients

• Enhanced energy dissipation

Excessive turbulence near the meniscus promotes slag entrainment and mold powder emulsification.

6.2 Impact on Flow Stability

Unstable flow fields may exhibit:

• Oscillating meniscus behavior

• Flow pattern switching (single-roll ↔ double-roll)

• Periodic asymmetry in shell growth

Such instability complicates process control and quality consistency.

7. Thermal and Solidification Consequences

7.1 Non-Uniform Heat Transfer

SEN offset leads to uneven heat flux distribution:

• Higher convective heat transfer near the dominant jet

• Reduced cooling on the opposite side

This results in asymmetric shell thickness, increasing breakout risk.

7.2 Shell Growth and Crack Formation

Non-uniform flow and cooling can cause:

• Uneven solidification fronts

• Increased tensile stress in the shell

• Higher susceptibility to longitudinal and transverse cracks

8. Operational Causes of SEN Offset

8.1 Installation and Alignment Errors

Common causes include:

• Inaccurate mounting of tundish or mold

• Misaligned stopper rod or slide gate systems

• Worn centering devices

8.2 Refractory Wear and Deformation

During casting:

• SEN erosion changes port geometry

• Asymmetric clogging alters effective flow area

• Thermal expansion causes gradual displacement

These factors lead to progressive offset over the casting sequence.

9. Detection and Diagnosis of SEN Offset

9.1 Online Monitoring Techniques

• Mold level fluctuation analysis

• Thermocouple heat flux mapping

• Electromagnetic flow sensors

9.2 CFD and Physical Modeling

Computational Fluid Dynamics (CFD) and water model studies are widely used to:

• Quantify flow asymmetry

• Predict offset sensitivity

• Optimize SEN positioning

10. Mitigation Strategies and Best Practices

10.1 Mechanical Alignment Control

• Precision alignment tools during SEN installation

• Regular inspection of centering devices

• Tight dimensional tolerances on refractory components

10.2 SEN Design Optimization

• Flow-balanced port geometry

• Anti-clogging bore designs

• Wear-resistant materials to maintain symmetry

10.3 Process Control Measures

• Optimized casting speed

• Controlled argon injection rates

• Adaptive mold level control

11. Industrial Case Studies and Practical Implications

Industrial studies have shown that reducing SEN offset from 5 mm to less than 1 mm can:

• Reduce surface defects by over 30%

• Improve inclusion distribution uniformity

• Increase casting stability and sequence length

These results highlight the economic and quality impact of precise SEN alignment.

12. Conclusion

The offset of a Submerged Entry Nozzle is a critical yet often underestimated parameter in continuous casting. Even small deviations from ideal alignment can significantly alter the mold flow field, leading to asymmetric circulation, increased turbulence, uneven heat transfer, and higher defect rates.

Through proper mechanical alignment, robust SEN design, advanced monitoring, and CFD-based optimization, steelmakers can effectively control SEN offset and achieve stable flow conditions, improved product quality, and safer casting operations.

A thorough understanding of the influence of SEN offset on the flow field is therefore essential for modern, high-performance continuous casting operations.More information please visit Henan Yangyu Refractories Co.,Ltd