After refining and argon blowing outside the furnace, the chemical composition of the steel becomes uniform throughout the ladle. However, after solidification, the chemical composition of a continuous casting billet varies from the center to the surface, and in some cases, the differences can be significant. This non-uniformity in composition is referred to as segregation.
Segregation can be classified into two types:
- Microsegregation – This refers to the composition differences between the trunk and branches of dendrites. The scale of segregation is very small, typically within a few microns.
- Macrosegregation – This involves composition differences over a larger scale, usually measured in centimeters or even meters. To examine segregation, a longitudinal or cross-sectional specimen can be taken from the billet. Sulfur print or acid etching methods are then used for inspection, and the segregation pattern can be observed with the naked eye.
Main Causes of Elemental Segregation:
- Differences in solubility between liquid and solid states: Elements like sulfur (S), phosphorus (P), oxygen (O), and carbon (C) are highly prone to segregation.
- Cooling rate: A faster cooling rate leads to reduced segregation.
- Diffusion in high-temperature solids: Elements that diffuse quickly at high temperatures can reduce segregation. For example, carbon has a partition coefficient (K value) of 0.13, indicating strong segregation. However, during high-temperature annealing, carbon atoms have a high diffusion rate, which promotes homogenization.
- Flow in the liquid phase before solidification: Intense flow during this phase can worsen macrosegregation. For instance, severe centerline segregation in a casting billet may result in a bulging defect known as “belly bulge.”
