Continuously cast billets are the primary product of steel production. After the molten steel in the steelmaking furnace is smelted, it is cast through a continuous casting machine to obtain continuously cast billets. Continuously cast billets come in various shapes as needed, such as round billets, square billets, and slabs. The solidification of continuously cast billets generally goes through three stages: the molten steel is rapidly cooled in the crystallizer to form the initial billet shell; the billet shell with a liquid core continues to grow and thicken stably in the secondary cooling zone; and the billet shell grows rapidly at the end of solidification.
Figure 1. Schematic diagram of continuous casting process.
Correspondingly, the grain distribution characteristics of continuously cast billets are: from the surface to the center, there are three typical regions with different grain structures: a fine equiaxed grain region on the surface, a columnar grain region in the middle, and an equiaxed grain region in the center. This structure is also known as the “three-layer solidification structure.”
The grain distribution of continuously cast billets has very significant and regular characteristics, which directly determine the performance of the subsequently rolled steel.
Three Regions of Continuously Cast Billets
Fine Equiaxed Grain Region on the Surface
The rapid cooling effect of the crystallizer causes the surface layer of the billet to solidify rapidly, forming a rapid cooling grain zone. This part of the grain is located in the outermost layer of the billet, and its thickness is usually very thin, generally a few millimeters to tens of millimeters. It consists of fine, non-directional equiaxed grains. The grain size is small, and the structure is dense.
The reason for the formation of fine equiaxed grains is that the molten steel comes into contact with the copper wall of the forced-cooling crystallizer, producing extremely high supercooling; at the same time, the extremely high supercooling and the cooling wall of the crystallizer cause the molten steel to instantly form a large number of crystal nuclei in the meniscus region. These crystal nuclei can grow freely in all directions in the initial stage, but due to extremely rapid cooling and a high nucleation rate, the grains do not have enough time and space to grow, thus forming fine equiaxed crystal zones.
Intermediate Columnar Crystal Zone
The cooling and contraction of the chilled crystals creates an air gap between the billet and the mold, reducing the heat transfer rate of the billet shell. Simultaneously, the internal molten steel raises the solid-liquid interface temperature, reducing the nucleation rate, and the grains begin to form columnar crystal bands along the direction of fastest heat dissipation. Columnar crystals form the main part of the continuously cast billet, with the greatest thickness and strong directionality. The grains grow like pillars, extending inward from the fine-grained zone.
The growth direction of the columnar crystals is basically perpendicular to the billet surface and along the direction opposite to the heat flow, i.e., towards the center of the billet.
The reasons for the formation of columnar crystals are: 1. Directional heat dissipation. After the formation of the surface fine-grained zone, the billet shell is formed, and the heat flow direction becomes stable, mainly conducting inward perpendicular to the billet surface. 2. Preferred growth. Crystals exhibit anisotropy. Grains whose crystal axes align with the direction of heat flow (i.e., the temperature gradient direction) grow the fastest, “displacing” grains with other orientations, thus forming coarse, uniformly oriented columnar crystals. 3. Compositional supercooling. At the solidification front, solute elements (such as C, S, P, etc.) are displaced into the molten steel ahead, lowering the solidification point of the liquid phase and forming a compositionally supercooled zone, providing conditions for the stable forward growth of columnar crystals.
Central equiaxed crystal region.
During the later stages of solidification, a large number of equiaxed crystals form in the central region of the billet cross-section. These consist of relatively coarse, non-directional equiaxed grains, typically much larger than the finer equiaxed crystal region on the surface.
Why do equiaxed crystals form eventually?
The reason is that when solidification reaches the center, the temperature of the remaining molten steel is already low, and the solute is enriched, increasing the overall supercooling. The source of crystal nuclei increases, mainly from two sources: 1. Dendritic melting/remelting. During columnar crystal growth, dendrite arms break off and detach due to solidification shrinkage or molten steel convection, being washed into the central liquid phase region to become new crystal nuclei; 2. Spontaneous nucleation: When the molten steel in the central region reaches sufficient undercooling and contains more impurities, crystal nuclei can spontaneously form.
In the central region of the molten steel, the directionality of heat dissipation is no longer obvious, and crystal nuclei can grow freely in all directions within the liquid, eventually forming equiaxed crystals. A low-magnification image of one end of the cast billet is shown in Figure 2.
Key Factors Affecting Grain Distribution
This three-layer structure is not fixed; its relative width and grain size are affected by the following process parameters:
Superheat (the temperature at which the molten steel is poured above its liquidus).
High superheat: Favors columnar crystal growth, widens the columnar crystal region, and reduces or even eliminates the central equiaxed crystal region, which is undesirable as it deteriorates the central quality.
Low superheat: Favors the formation and expansion of central equiaxed crystals, improving the density of the billet center.
Cooling Intensity
Strong cooling in the secondary cooling zone: Increases the temperature gradient, promoting columnar crystal growth.
Weak cooling in the secondary cooling zone: Reduces the temperature gradient, favoring the development of central equiaxed crystals.
Electromagnetic Stirring
This is the core technology for controlling grain structure in modern continuous casting. By applying a rotating or traveling electromagnetic field at the solidification front, the molten steel is forced to flow.
Function: Breaks up growing columnar dendrites, carrying them as nuclei into the central region; simultaneously, stirring helps to homogenize the central temperature. The effect is a significant expansion of the central equiaxed grain region, refinement of grains, and a substantial improvement in central porosity and segregation.
Casting Speed
Increased casting speed elongates the liquid core of the billet, shifting the solidification endpoint downwards. This effectively prolongs the growth time of columnar crystals, resulting in a longer and wider columnar grain region.
Steel Composition: Alloying elements affect the thermal conductivity, solidification range, and undercooling tendency of steel, thus influencing grain morphology.
Influence of Grain Distribution on Steel Properties
Fine equiaxed grain region on the surface: Dense structure, good performance, but the region is too thin, contributing limited to overall performance.
Columnar grain region: Due to the coarse and directional grains, the steel exhibits anisotropic properties. Impurities and segregated elements easily accumulate at the columnar grain boundaries, forming weak surfaces. This is a high-risk area for internal cracks, central porosity, and segregation, severely impairing the steel’s transverse properties and toughness.
Central equiaxed grain region: Grains are non-directional, resulting in good isotropic properties. This effectively interrupts the continuous growth of columnar crystals, eliminates weak areas, and thus improves the central density and overall mechanical properties of the cast billet.
Summary
The grain distribution of continuously cast billets exhibits a typical three-layer structure: from the surface inwards, from fine to coarse, and from undirected to directed and back to undirected. One of the core objectives for metallurgists is to suppress the growth of columnar crystals and promote and expand the central equiaxed crystal zone by optimizing processes (such as low-superheat casting, application of electromagnetic stirring, and gentle reduction techniques), thereby obtaining high-quality cast billets with uniform structure, density, and excellent properties.
