After refining and blowing outside the furnace, the composition of the steel in any position in the ladle is uniform. And after solidification, chemical compositions of continuous casting billet from center to surface isn’t same, even some of the differences are very large. This non-uniformity of the composition is called segregation.
The segregation can be divided into two kinds: one is called micro segregation, is the difference between the trunk of the dendrite and the dendrite composition, the general distance is very small is a few microns range of segregation. The other is called macroscopic segregation, which is a long distance range (in centimeters or meters to calculate) within the composition of the difference. Take a longitudinal section or cross-sectional specimen from the cast billet, do a sulfur seal or acid dip inspection, and use the naked eye to observe the condition of segregation.
Main reasons for the element segregation:
Difference in solubility of elements in liquid and solid state, generally S, P, O and C are strongly segregation elements.
The cooling speed is faster, the degree of segregation is smaller.
The diffusion speed of elements in high-temperature solids is quickly able to mitigate the segregation degree. For example, carbon K value is 0.13, as a strong segregation element, but in high temperature annealing, carbon atoms have strong diffusion ability, which is conducive to homogenization.
The flow in liquid phase before solidifying is stronger and the macroscopic segregation is worse. For example, the serious central segregation of casting billet can cause the drum belly of the billet.
