The internal quality of billets refers to their macrostructure, component segregation, central porosity, centerline segregation, and internal cracks. The formation of these defects is influenced by heat transfer, mass transfer, and stress during solidification. After thermal processing, some defects may disappear, some may become distorted, and others may remain intact—ultimately affecting product performance.
The internal defects of billets are mainly determined by the cooling process in the secondary cooling zone and the billet support system. Optimizing the secondary cooling water distribution, aligning support rolls, and preventing billet bulging are essential prerequisites for improving internal billet quality.
Improving the Internal Quality of Cast Billets
Controlling Billet Structure
A key measure is to expand the equiaxed crystal zone to suppress the growth of columnar crystals. This helps reduce centerline segregation and central porosity.
Using low-superheat casting, electromagnetic stirring, and similar technologies has proven effective in expanding the equiaxed zone.
Optimizing the Secondary Cooling System
In the secondary cooling zone, it is critical to maintain a uniform surface temperature across the billet. The temperature at the straightening point should ideally exceed 900°C, and straightening should be avoided while the billet still has a liquid core. The distribution of secondary cooling water is typically computer-controlled to ensure precision.
Managing Billet Stress and Deformation
During solidification in the secondary cooling zone, billet stress and deformation are major causes of cracking. Many plants adopt technologies such as multi-point bending straightening, precise arc alignment, roll gap alignment, and soft reduction casting to improve product quality.
Controlling Liquid Flow
By controlling liquid flow, non-metallic inclusions can be encouraged to float and be more evenly distributed. Technologies such as mold electromagnetic stirring and optimized immersion nozzles are practical and effective solutions.
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