When strand electromagnetic stirring is required on a slab caster, the choice is not simply whether to install S-EMS. A second question follows immediately: should the stirring system be integrated into the rolls, or mounted externally behind them?
S-EMS In Roll and S-EMS Box Type share the same broad objective. Both act below the mold, where the focus has moved away from meniscus flow and toward internal solidification structure. Both use linear electromagnetic action to mix hotter and colder liquid-steel regions and support equiaxed-zone development. But they reach that objective through different mechanical and electrical arrangements.
The Electromagnetic Stirring and Braking for Slab Casters family separates mold-flow-control systems from strand-stirring systems for exactly this reason. The process position may be similar, but installation architecture changes what is possible in the plant.
What the two solutions have in common
In both configurations, electromagnetic forces act through the slab thickness and generate circulating loops in the remaining liquid steel. In a single-stage arrangement, the resulting movement is commonly described as a butterfly pattern: liquid circulates upward and downward in loops that mix regions at different temperatures.
The metallurgical purpose is internal rather than superficial. Strand stirring supports additional superheat removal, thermal homogenization and a larger equiaxed zone. In sensitive slab grades, this can reduce the directional solidification structures associated with ridging and roping and can improve the consistency of internal quality.
Neither system is intended to replace mold-flow control. Technologies such as MULTI-MODE® EMS act in the mold to influence meniscus behavior and flow pattern. In Roll and Box Type S-EMS act later, while the internal structure of the strand is still developing.
S-EMS In Roll: the stirrer becomes part of the roll segment
S-EMS In Roll integrates the electromagnetic device directly inside slab-supporting rolls. The stirring rolls are installed within the segment and are used in pairs to generate the required horizontal AC magnetic field.
This arrangement gives the technology a compact footprint. Because the active elements are inside the rolls, the system can be positioned at different locations along the strand with relatively limited interference with the surrounding machine structure, provided the segment and roll diameters have been engineered for the installation.
In Roll units can be installed side by side or face to face. They are also well suited to single-stage and double-stage stirring. This positioning flexibility is particularly important when the process requires action lower in the caster or when two stirring stages are needed to obtain stronger thermal homogenization and a more symmetrical equiaxed-zone distribution.
S-EMS Box Type: a strong external field behind the rolls
S-EMS Box Type uses a linear inductor mounted externally behind the rollers of a caster segment. The magnetic field crosses the distance between the inductor and the slab and creates the same broad internal mixing objective through a different installation concept.
The Box Type solution is especially suitable in the upper part of the caster, where roll diameters are smaller and the distance between the stirrer and the slab can remain short.
This distance matters because electromagnetic effectiveness decreases as the active device moves farther from the steel. Beyond a certain stand-off, substantially more electrical power is required.
The external arrangement can generate a strong through-thickness field, but it requires more space around the segment and more substantial mechanical adaptation. The segment must accommodate the inductor, non-magnetic materials may be required in the field region, and a manipulator can be needed to remove the stirrer during segment handling.
Integration is the first major difference
The most visible contrast is therefore not the metallurgical purpose but the way each solution becomes part of the caster.
In Roll stirring is integrated into the normal roll environment. On a retrofit, implementation can be relatively close to the logic of replacing selected normal rolls with stirring rolls, especially when the existing roll diameter is compatible. The position can also be changed more easily if the engineering phase has provided suitable options.
Box Type stirring sits outside the roll line. That can be effective where a strong field is required close to the upper strand, but it demands a dedicated segment concept, space for the inductor and, in many cases, provisions for withdrawal and maintenance.
This difference means that the better solution cannot be selected from metallurgical objectives alone. Available space, segment geometry, roll diameter, access, retrofit constraints and the desired strand position are equally important.
The selected strand position changes the available process window
The higher the stirrer is placed in the caster, the earlier it can act on the remaining superheat and the more time the modified thermal conditions have to influence solidification. An upper position can therefore favor equiaxed-zone development, provided the induced upper loop does not interfere with mold flow.
Lower positions may be useful when roll geometry or segment arrangement makes installation easier, but the liquid pool is smaller and the solidification state is more advanced. The choice is therefore not based only on where equipment fits. It must also consider the metallurgical length and the process zone that still remains responsive to stirring.
In Roll technology offers more freedom to distribute stirring positions along the strand. Box Type is more naturally concentrated where an external unit can remain close to the slab. That difference strongly influences the usable operating window of each concept.
Power demand follows the distance to the slab
Electromagnetic stirring force depends not only on the inductor design but also on its distance from the liquid steel. An integrated In Roll unit works close to the slab, which supports lower power demand for a given installation objective. Rotelec describes optimized In Roll configurations at about 120 kVA per pair, although the actual design always depends on the caster and process target.
A Box Type inductor is farther from the steel because rolls and the segment structure lie between the device and the slab. As stand-off distance increases, the required electrical power rises significantly. This is why Box Type is most rational where the device can be installed close enough to the strand.
The comparison is not simply low power versus high power. It is a balance between field strength, position, available space and the mechanical changes the plant can accept.
Cooling and maintenance follow different architectures
In Roll stirrers are water-cooled within the roll-based arrangement. Their integration reduces external bulk, but the specialized rolls and their segment interfaces must be included in the plant maintenance strategy.
Box Type stirrers use dedicated demineralized-water cooling and may require an external withdrawal arrangement for segment access. Their maintenance strategy therefore follows the external inductor, cooling circuit and dedicated machine interface.
Neither architecture is maintenance-free. The important point is that maintenance follows a different path: roll integration for In Roll, external equipment and segment interface for Box Type.
Single-stage and double-stage stirring change the process effect
Both the stirring position and the number of stages influence the internal flow. A single-stage system can generate the butterfly pattern required for additional mixing at one metallurgical location. A double-stage arrangement acts at two strand positions and can create a more complex triple-zero flow pattern.
Double-stage stirring is especially relevant when superheat is higher or when the steelmaker needs a more consistent equiaxed structure across the slab thickness. In Roll units are particularly suited to this strategy because their position can be distributed across different segments.
The underlying metallurgical concept is explained in The Equiaxed Zone: What It Is and Why It Matters for Internal Quality. The key point is that more stages are not automatically necessary; they are selected when the thermal and solidification conditions justify them.
There is no universal winner
A product comparison can easily become a list of advantages, but that would miss the real decision logic. In Roll is attractive when compact integration, lower power demand, flexible positioning or double-stage stirring are priorities. Box Type is attractive when a strong external field can be applied close to the upper strand and the machine layout can accept the required segment modifications.
Steel grade also matters. Strand stirring on slabs is now used mainly where a larger equiaxed zone has a clear downstream value, especially for ferritic stainless and high-silicon steels and for selected carbon grades. The solution must therefore be linked to product mix, superheat range and internal-quality target.
Two technologies, one process question S-EMS In Roll and S-EMS Box
Type are not competing answers to a generic equipment question. They are two engineering routes to the same process objective: controlling internal solidification along the slab strand.
The correct choice emerges only when electromagnetic performance is considered together with strand position, mechanical integration, cooling, power and steel-grade requirements. That is what turns strand stirring from an isolated device into a process-specific solution.
