Continuous casting is one of the most critical processes in modern steel production. The stability of this process directly determines billet quality, production efficiency, and overall operating costs. Among the many quality issues encountered in billet casting, warping and bending of billets remain common challenges that can negatively affect downstream rolling operations.
Warped billets not only reduce product quality but may also lead to equipment instability, rolling difficulties, and additional processing costs. Understanding the hazards, causes, and corrective measures of billet warping is therefore essential for improving the performance of continuous casting machines.
A few years ago, I visited a steel plant operating a 170 mm billet continuous casting machine equipped with a dual tundish and 10 strands. During the visit, several operational practices highlighted how small details in process control can significantly influence billet quality, including warping behavior.
Although the equipment itself was not outdated, many operational habits resembled practices common in the 1990s. This situation is not unique to one facility. In many steel plants around the world, long-standing operating habits can quietly affect efficiency, yield, and product quality.
The following observations illustrate how operational details in mold operation, tundish practice, nozzle management, and secondary cooling influence billet shape stability and overall casting performance.
1. Start Casting Method and Process Stability
During the visit, the caster was still using a swing trough system to start casting.
Historically, this was a widely adopted method. However, in modern billet casting operations, especially for rebar production, many plants have switched to metered nozzles combined with stopper rod control.
The difference in tundish performance between the two systems can be substantial.
Typical tundish life:
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Stopper rod control: 15–18 hours
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Metered nozzle casting: over 60 hours
Longer tundish life improves process stability and reduces interruptions during casting sequences. Stable flow conditions also contribute to more uniform solidification in the mold, which helps minimize billet distortion and warping.
Frequent casting interruptions or unstable flow conditions can increase thermal fluctuations in the strand, making billet shape control more difficult.
2. Quartz Submerged Entry Nozzles as Emergency Backup
Another observation was the plant’s continued use of quartz submerged entry nozzles (SENs) as backup equipment.
Most experienced casting engineers are aware that quartz SENs have several limitations:
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Rapid erosion during operation
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Short service life
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High risk of flow instability
Because of these limitations, quartz SENs are typically used only in emergency situations. Their presence as backup equipment suggests that occasional process disturbances still occur in the operation.
Flow instability during SEN replacement can disrupt molten steel distribution in the mold. These disturbances can influence shell growth symmetry and may contribute to billet bending or internal stress development.
Maintaining stable nozzle performance is therefore an important factor in preventing warping defects.
3. Mold Spray Nozzle Alignment
The spray system near the mold foot rollers included 24 spray nozzles covering the billet faces and corners.
However, several nozzles appeared slightly misaligned, indicating that spray distribution might not be perfectly uniform.
In continuous casting, uneven cooling distribution can create temperature gradients across the billet section. These gradients lead to uneven thermal contraction, which is one of the primary causes of billet warping.
Poor spray alignment may lead to:
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Surface cracks
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Internal stress accumulation
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Temperature instability along the strand
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Billet bending during cooling
Even small mechanical adjustments to spray nozzle positioning can significantly improve cooling uniformity and metallurgical quality.
4. Billet Loss During Submerged Entry Nozzle Replacement
Another operational issue was billet loss during submerged entry nozzle replacement.
Approximately 4–6 meters of billet per strand were discarded during each replacement operation.
Over long casting sequences, this loss can represent a considerable reduction in production yield.
In many modern plants, billets produced during nozzle replacement are inspected and conditioned rather than automatically scrapped. If surface quality is acceptable after grinding or conditioning, these billets can still be used, reducing unnecessary losses.
Improving operational procedures during SEN replacement can therefore increase yield while maintaining product quality.
5. Head and Tail Billet Cutting Optimization
Another hidden cost identified during discussions with the site team was the length of head and tail billet cutting.
Head and tail sections are typically removed because they may contain defects such as:
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Inclusion concentration
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Segregation
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Surface irregularities
However, excessive cutting can significantly increase material losses.
Process analysis suggested that optimizing the cutting length could reduce costs by 5–8 million RMB annually, without requiring major equipment investment.
This improvement would rely mainly on improved process control and better monitoring of billet quality.
6. Billet Bending During Cooling Bed Handling
The casting machine used a cooling bed approximately 7 meters long with a bidirectional pushing system to collect and stack billets.
This arrangement resulted in noticeable billet bending.
Interestingly, the downstream rolling mill appeared capable of handling these billets without major issues. However, in many rolling mills, excessive billet curvature can cause problems such as:
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Difficulties in mill entry guides
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Reduced rolling stability
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Increased equipment wear
Improving billet straightness at the casting stage can therefore help ensure smoother rolling operations.
7. Surface Slag Inclusions
Some billets also showed surface slag inclusions, most likely formed during nozzle replacement operations.
Surface inclusions are relatively common during transitional operations in continuous casting. Fortunately, minor surface defects can often be removed through grinding or conditioning processes rather than scrapping the entire billet.
Effective slag control and stable casting conditions can significantly reduce the occurrence of these defects.
8. Secondary Cooling Optimization
The secondary cooling zone showed visible temperature variations along the billet surface.
Temperature inconsistency during secondary cooling is a major contributor to billet warping. When one side of the billet cools faster than the other, uneven contraction occurs, leading to bending or distortion.
After technical discussions, the plant adjusted the secondary cooling water distribution, improving temperature uniformity along the strand.
As a result, the billet macrostructure quality improved noticeably.
This case highlights the critical importance of secondary cooling control in continuous casting operations.
9. Measures to Reduce Billet Warping
Based on practical plant experience, several measures can help reduce billet warping in continuous casting:
1. Improve mold level and flow stability
Stable molten steel flow promotes uniform shell formation.
2. Optimize secondary cooling water distribution
Uniform cooling minimizes thermal stress and distortion.
3. Maintain accurate spray nozzle alignment
Proper alignment ensures balanced cooling.
4. Reduce casting interruptions
Continuous and stable casting sequences improve strand quality.
5. Improve nozzle management and replacement procedures
Stable SEN operation reduces process disturbances.
6. Optimize billet cutting length
Reducing unnecessary head and tail cuts improves yield.
Final Thoughts
Continuous casting performance rarely depends on a single parameter. Instead, it is influenced by a combination of many small operational details.
Key factors include:
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Start casting methods
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Nozzle operation practices
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Spray alignment
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Secondary cooling control
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Billet cutting strategy
When these factors are properly managed, steel plants can significantly improve billet quality, reduce warping defects, and increase production efficiency.
In many cases, the most effective improvements come not from major equipment upgrades but from refining operational practices and strengthening process control.
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