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Strand electromagnetic stirring (S-EMS) for thick slab casters: Box-type or In-roll stirrers?Use of In-roll stirrers provides operators with the ability to produce a higher proportion of equiaxed microstructure in the cast slab. For superheats below 25°C, a single stage stirrer should suffice. For superheats in excess of 25°C, the use of a double-stage system is recommended.

S-EMS can be designed as Box-type or In-roll EMS and as single- or double-stage EMS. The choice depends on

mechanical and metallurgical considerations. In-roll EMS is a patented technology of Danieli Rotolec; Box-type EMS is also available from other suppliers. Both Box-type and In-roll EMS use linear inductors with horizontally travelling magnetic fields that produce a stirring force that pushes the liquid steel horizontally along the slab width and generates a butterfly-type stirring pattern in the liquid steel (see Figure 1).

MECHANICAL AND CASTER DESIGN CONSIDERATIONSThe main differences between Box-type and In-roll EMS are as follows:Box-type stirrer This type is installed behind the rolls (see Figure 2). The rolls and the entire steel structure between the slab and the stirrer must be made of non-magnetic stainless steel. The distance between the stirrer and the slab surface must be as short as possible to maintain reasonable efficiency (ie, reasonable electrical power and cost). This is why Box-type stirrers are recommended for installation high in the machine, close to the mould (3-4m below meniscus, in segment zero) where roll diameters are small and the stirrer-to-slab distance is small (typically ≤220mm). If the distance is larger (270mm, for example) the stirrer has to be positioned behind these bigger rolls in segments 1 or 2, and the electrical power requirement and cost of the stirrers is approximately double.

Modifications to the machine segment, installation of the stirrer support structure, and the need for the adjacent rolls to be non-magnetic may be expensive and increase maintenance cost. Box-type stirrers are designed for high current intensity (typically, 800-1,000A per phase, and so need big power cables) and require de-ionised water (ie, dedicated pure water system) for coil cooling; box cooling uses industrial water and box purging uses dry air or nitrogen.In-roll stirrers These are always used in pairs, either

Author: Siebo Kunstreich Danieli-Rotelec

r Fig 1 Single-stage S-EMS r Fig 2 Box-type strand EMS

r Fig 3 Single-stage in-roll stirrers

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a

a wavy surface. Internal quality of plate and tube grades has generally been addressed by use of soft reduction since the 1990s.

The amount of equiaxed zone that can be obtained with S-EMS depends, in addition to steel grade, mainly on stirrer position, stirring force and superheat.

A stirrer in too low a position in the caster or with insufficient stirring force obviously cannot generate a large equiaxed zone. However, a stirrer installed too close to the mould and with too strong a stirring force must also be avoided as it would generate steel flows that would perturb mould steel flow and induce meniscus fluctuations.

Provided that position and stirring force are optimised, results, for a given steel grade, then depend on steel superheat: larger superheats mean smaller equiaxed zone widths.

Figure 6 shows a strongly asymmetrical equiaxed structure with respect to slab thickness. Because of the curvature of the strand, a larger equiaxed zone is always observed on the outer (lower) radius of the machine where equiaxed crystals sink down and accumulate by sedimentation. But

placed side-by-side on the inside radius or face-to-face on both sides of the slabs (see Figures 3a and 3b). Since the inductors are located inside the mechanically rotating guide rolls, a minimum roll diameter (typically ≥240mm) is required. If placed in segments with comparable roll diameter, In-roll stirrers do not need any modifications to the existing machine structure.

In-roll EMS has much better efficiency than Box-type EMS because the inductor is very close to the slab surface and, therefore, needs three to six times less electrical power. Current intensity is typically 400A per phase (hence smaller power cables), and coil and roll cooling use the same water circuit/quality (typical mould water quality is good enough); dry air or nitrogen for purging is not required.

Some earlier concerns that it should not be used in caster segments with split rolls and in rolls whose diameters were less than 240mm are now resolved and the product is ready for market.

The main advantage of In-roll EMS compared to Box-type EMS is that it can easily be used in the lower part of the caster with larger roll diameters and without detrimental increase of power requirement and cost. It is, therefore, very suitable for use as a second stirrer in a lower position (see Figures 4 and 5) and permits double-stage EMS at reasonable cost. Single-stage vs double-stage EMS is a metallurgical consideration that will now be addressed.

METALLURGICAL CONSIDERATIONSStrand stirrers address the effect of superheat on internal solidification structure, ie, equiaxed zone, segregation and centerline porosity. Strand stirrers are now mainly used for ferritic stainless and silicon steels where the increase in equiaxed zone width is known to reduce ridging or roping, a defect caused by solidification dendrites that ‘print-out’ on the metal surface after rolling or drawing and generate

r Fig 6 Macrograph of stainless steel cast at 35°C superheat with single-stage S-EMS

r Fig 5 Double-stage in-roll stirrers

r Fig 4 Double-stage S-EMS

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there is another asymmetry. The equiaxed structure that starts developing roughly symmetrically on both sides inside of the columnar structure C1 and C3 remains equiaxed until the slab centre (E3) on the outer side, whereas it is interrupted by a new secondary columnar structure (C2) on the inner side. Consequently, the total equiaxed zone width (E1+E2+E3) is smaller than the expected width W. The missing part, the secondary columnar solidification C2, is larger when steel superheat is greater. Therefore, the total equiaxed zone width decreases with increasing superheat. The detailed mechanism of this phenomenon is complex, but basically it means that the first stirrer could not negate the effect of the large superheat and dendrites grow again below the stirrer position because the liquid steel is still superheated.

Increasing the stirring force of the first stirrer, as some people believe, does not stop the phenomenon; it only disturbs the flow conditions in the mould without stopping the secondary dendrites (at best it would only marginally decrease the secondary columnar zone C2). The only way to improve the situation is to use a second stirrer below the first one as in Figure 4, ie to use double-stage EMS. This solution extends the stirring pattern over a longer range, recycles more cold steel and equiaxed crystals from the bottom of the machine to the top and, hence, reduces the remaining superheat faster. The secondary dendrites will disappear, or at least become much shorter, and the equiaxed zone width remains large even at high superheat as shown in Figures 7 and 8. Casting conditions for these steels were: ferritic stainless steel, superheat 25-49°C, casting speed 0.8-1m/min.

CONCLUSIONS` If one can afford always to cast at low superheat

below 25°C one should choose single-stage EMS and use either Box-type or In-roll stirrers, whichever is more convenient with respect to mechanical caster design and cost.

r Fig 7 Single-stage vs double-stage stirring with In-roll EMS

a) Single-stage EMS (one pair of rolls at 3.4m below meniscus)

b) Double-stage EMS (two pairs of rolls at 3.4m and 5.2m below meniscus)

` If one casts at higher superheats, either one stays with single-stage EMS and accepts variable equiaxed zone width according to actual superheat, or one chooses double-stage EMS and achieves reproducible results with little variation. Since the lower stirrer in Box-type design would be very expensive, one should choose In-roll stirrers.

These simple guidelines cover most applications. Other EMS solutions do exist for special cases but details are outside the scope of this article. MS

Siebo Kunstreich is Managing Director at Danieli-Rotelec, Paris, France.

CONTACT: siebo.kunstreich@danieli-rotelec.fr

r Fig 8 Single-stage vs double-stage stirring with In-roll EMS

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