A convex surface (outward bulging) on a continuous casting billet is less common than concavity but can occur due to several process-related factors. The main causes include:
1. Excessive Internal Pressure (Ferrostatic Pressure)
-
The liquid core of the billet exerts ferrostatic pressure on the solidifying shell.
-
If the shell is too thin or weak (due to high casting speed or insufficient cooling), it may bulge outward under this pressure.
2. Insufficient or Uneven Secondary Cooling
-
Inadequate cooling in certain zones slows solidification, keeping the shell soft and allowing internal pressure to push it outward.
-
Uneven spray cooling can cause localized bulging where cooling is weaker.
3. Mold-Related Issues
-
Improper mold taper: If the mold is too tight or has incorrect taper, it may not properly contain the solidifying shell, leading to bulging.
-
Poor mold flux performance: If lubrication is insufficient, friction increases, causing uneven shell growth and potential bulging.
4. High Casting Speed
-
Faster casting reduces shell thickness, making it more susceptible to deformation under ferrostatic pressure.
-
The liquid core remains larger for longer, increasing internal pressure on the weaker shell.
5. Roller Misalignment or Weak Support
-
If the support rollers are misaligned, too widely spaced, or improperly adjusted, the billet may bulge between them.
-
Excessive roll gap allows the semi-solid strand to deform outward.
6. Alloy Composition & Solidification Behavior
-
Some steel grades (e.g., low-carbon or peritectic steels) undergo uneven shrinkage or phase transformations that can contribute to surface irregularities, including convexity.
Solutions to Prevent Convexity
-
Optimize cooling (ensure uniform secondary spray cooling).
-
Adjust mold taper to better support the solidifying shell.
-
Reduce casting speed if shell thickness is insufficient.
-
Check roller alignment and spacing to provide proper strand support.
-
Use appropriate mold flux to ensure smooth shell formation.
Conclusion
Convex bulging on a continuous casting billet typically results from weak shell strength, excessive internal pressure, or inadequate support. By optimizing cooling, mold design, and roller alignment, this defect can be minimized.
