The thin layer of small crysrtals forms when a melt comes to a contact with a wall of a cold metallic mold. The crystals (grains) have no favorable direction (equiaxed) and their chemical composition is close to that of the liquid steel. Heat liberated as a result of Crystallization depresses the nucleation and crystal growth.
Columnar grains start to grow when a stable and directed heat flow is formed as a result of heat transfer through the zone of small equiaxed grains. Direction of the columnar grains growth is oppsite to the direction of heat flow. Columnar grains continue to grow untill the heat flow decreases due to the following causes:
When the temperature of the melt, adjacent to the solidification front, increases due to the liberation of the latent heat, constitutional undercooling will end and the columnar grains growth will stop.
Low temperature gradient (low heat flow) and low cooling rate of the solute-enriched liquid in the cenral zone of the ingot result in formation of equiaxed grains. This process is slow due to slow heat extraction therefore the number of nuclei (seed crystals) is low and the grains size is large.
Zone of large equiaxed grains is enriched by the impurities (sulfur, phosphorous, carbon).
This cone-shaped zone is a mixture of small equiaxed garins grown as a result of the contact with a bottom of a cold metallic mold and crystals and crystals fragments, which sedimintate from other ingot zones.
Bottom cone is characterized by negative segregation of the impurities.
Shrinkage cavity is located in the top part of the ingot (which is later discarded) where last portion of liquid solidifies. The mold design should provide upwards direction of solidification at its last stage. Below the shrinkage cavity the zone of shrinkage porosity is located. This zone forms when the feeding of solidifying metal by the residual liquid is insufficient. Isolated pockets of liquid metal separated from the liquid pool by “bridges” form their own shrinkage cavities (shrinkage pores).
The mold shape, which is wider in upper levels and thermal isolation of the “hot top” favor to diminish the shrinkage porosity.