The equiaxed zone is the part of an as-cast steel cross-section where grains have roughly similar dimensions in all directions. This distinguishes them from elongated columnar grains, which grow mainly along one axis. The balance between these structures is an important aspect of solidification and internal quality.
From columnar to equiaxed growth
In continuous casting, heat is extracted from the steel through the cooled mold walls. A solid shell forms at the surface and thickens toward the center as cooling continues.
Columnar grains grow inward from the solidifying outer region toward the hotter liquid core. Farther inside, an equiaxed structure can develop when crystals within the remaining liquid survive and grow, rather than remelting.
The resulting section commonly contains an outer columnar region and an inner equiaxed zone. Their relative extent depends on the steel grade and the conditions during casting.
Why superheat matters
Steel is cast at a temperature above its liquidus temperature: the temperature above which that steel grade is fully liquid. The difference between the temperature of the liquid steel being cast and its liquidus temperature is called superheat temperature.
Superheat temperature = liquid-steel casting temperature − liquidus temperature
Small crystals entering liquid steel that is still too hot tend to remelt. For them to survive and grow, the surrounding liquid must reach conditions below its liquidus temperature.
As the superheat increases, the proportion of equiaxed grain typically decreases. Removing superheat is therefore essential to equiaxed-zone development. The presence of crystals alone is not enough: the thermal conditions must also allow them to grow.
How electromagnetic stirring contributes
Electromagnetic stirring, or EMS, puts liquid steel in motion without mechanical contact. By mixing hotter and colder regions, it redistributes heat and improves heat transfer during solidification.
In billet and bloom casting, mold electromagnetic stirring (M-EMS) acts where heat extraction is particularly effective. It promotes early superheat removal, helping create the conditions for equiaxed grains to develop farther along the caster.
In slab casting, strand electromagnetic stirring (S-EMS) acts below the mold, mixing hotter and colder regions of the remaining liquid core to promote equiaxed-zone development. The appropriate configuration depends on the steel grade and casting conditions.
Why the equiaxed zone matters for quality
A broad, fine equiaxed zone is a desirable feature in many steel grades. In billets and blooms, promoting this structure is one of the objectives of EMS, alongside reducing center segregation (the uneven distribution of chemical elements) and porosity, or internal voids. These are related quality objectives, not interchangeable measurements.
For ferritic stainless steel slabs, developing an equiaxed structure can also help reduce ridging and roping: surface irregularities that become apparent during subsequent processing.
A larger equiaxed zone does not, by itself, guarantee a defect-free product. Its significance must be assessed for the particular steel grade, casting conditions and intended application.
Examining the result
The structure is examined on transverse samples taken from the cooled cast product. The sample surface is prepared and chemically etched to reveal the columnar and equiaxed regions.
This examination shows the extent and distribution of the equiaxed zone. Comparing these observations with the steel grade, casting superheat, section size and EMS settings helps establish whether the process is producing the required internal structure.
