The concave surface on a continuous casting billet, often referred to as “surface depression,” is primarily caused by uneven solidification and shrinkage during the casting process. Here are the key reasons:
1. Non-Uniform Cooling
-
Excessive or uneven cooling in certain areas can cause localized shrinkage, leading to depressions.
-
Improper secondary cooling water distribution (e.g., too much water in some zones) accelerates surface solidification while the inner metal remains liquid, creating inward pulling forces.
2. Irregular Solidification Shell Formation
-
If the initial solidification shell is too thin or uneven, it may deform under ferrostatic pressure from the molten core, causing depressions.
-
Fluctuations in casting speed can disrupt stable shell growth, exacerbating this issue.
3. Mold-Related Issues
-
Poor mold taper design or excessive wear can lead to inadequate support for the solidifying shell.
-
Inconsistent mold oscillation (e.g., incorrect frequency or stroke) may cause sticking or uneven shell formation.
-
Improper mold flux lubrication can increase friction, leading to uneven shell development.
4. Thermal Contraction & Stress
-
Rapid cooling causes significant thermal contraction, and if stress distribution is uneven, depressions form.
-
Alloy composition (e.g., high carbon or microalloyed steels) affects solidification shrinkage behavior.
5. Process Parameter Fluctuations
-
Variations in superheat (molten steel temperature) or casting speed destabilize solidification.
-
Sudden changes in withdrawal force or misalignment of rollers can mechanically induce depressions.
Solutions to Mitigate Concavity
-
Optimize secondary cooling water distribution to ensure uniform solidification.
-
Adjust mold taper and oscillation parameters for better shell support.
-
Use high-quality mold flux to improve lubrication and heat transfer.
-
Stabilize casting speed and superheat to minimize fluctuations.
-
Ensure proper alignment and maintenance of rollers and mold.
By addressing these factors, surface concavity in continuous casting can be significantly reduced, improving billet quality.
