What is solidification segregation?

After refining outside the furnace and stirring by blowing air, the composition of molten steel at any position in the ladle is uniform. After solidification, the chemical composition of the ingot or continuous casting slab is different from the surface to the center, and some are very different. The inhomogeneity of this composition is called segregation. Segregation can be divided into two types: one is called microsegregation, which is the difference between the trunk of the dendrite and the composition of the dendrite. The other, called macrosegregation, is the difference in composition over long distances (measured in centimeters or meters). Take longitudinal or cross-sectional samples from the casting billet, do sulfur printing or acid leaching inspection, and observe the segregation condition with the naked eye, also called low-magnification segregation. The causes of segregation are:

(1) The solubility difference of elements in liquid and solid state. The partition coefficient K is defined to characterize the degree of segregation: K=Cι (element concentration in liquid phase)/CS (element concentration in solid phase) If K=1, then Cι=CS means that there is no segregation in the solidified product, and K<1 means solidification The product has segregation. The K values ​​of different elements were determined: C, 0.13; S, 0.02, O, 0.02, P, 0.13, Si, 0.66, N, 0.28, Mn, 0.84, Cr, 0.95. It can be seen that S, P, O, and C are strong segregation elements.​​

(2) Cooling speed. The faster the cooling rate, the smaller the degree of segregation.​​

(3) The diffusion rate of elements in the solid phase. Elements diffuse quickly in high-temperature solids, which reduces segregation. For example, the K value of carbon is 0.13, which is also a strong segregation element, but when annealed at high temperature, the diffusion ability of carbon atoms is strong, which is conducive to homogenization.​​

(4) The stronger the flow in the liquid phase at the solidification front, the more serious the macrosegregation. For example, the bulging belly of the continuous casting billet is an important reason for the serious center segregation of the billet.

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