Steel Crystallization Two-phase Zone

Crystalline two-phase zone
During the solidification of molten steel, there is a transition zone - a two-phase zone (Figure 1) between the solid phase (solidification layer) near the mold wall and the internal liquid phase. In the solidified steel ingot, there are three zones: solid phase zone, two-phase zone, and liquid phase zone. The crystallization process of molten steel, namely nucleation and nucleation growth, only occurs in the two-phase region. The solidification of ingot is the process of two-phase zone moving from the surface of ingot to the center of ingot: when the liquid isotherm reaches a certain position in the ingot, crystallization begins; When the solid isotherm reaches a certain position, the crystallization at that position will end and all will be transformed into solid. The time interval between the liquidus temperature line and the solidus temperature line reaching a specified point in the ingot, that is, the time that the point experienced from the liquidus temperature to the solidus temperature line, is called the local solidification time of the point, which is often expressed in q. The local solidification time is inversely proportional to the average cooling rate at that location. Due to the significant differences in heat transfer conditions at different parts of the ingot, the local solidification time at different parts can vary greatly, resulting in different crystal structures. The distance between the liquid isotherm line and the solid isotherm line in the ingot is called the width of the two-phase zone, which is represented by △ x. And there is. Narrow two-phase zone is conducive to the development of columnar crystals, while wide two-phase zone is conducive to the development of equiaxed crystals.
Cause of formation
During alloy solidification, selective crystallization (also known as desolvation or liquid precipitation) occurs due to the different solubility of solutes in the solid and liquid phases. With the progress of crystallization, solute precipitation (K) continuously occurs at the forefront of solidification