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ON THE EFFECT OF LIQUID STEEL FLOW PATTERN ON SLAB QUALITY AND
THE NEED FOR DYNAMIC ELECTROMAGNETIC CONTROL IN THE MOLD.
Siebo Kunstreich and Pierre H. Dauby (x)
Danieli Rotelec, Paris Bagnolet, France -(x)
Formerly with LTV Steel, Cleveland, USA.
1. INTRODUCTIONThis paper addresses surface/subsurface quality (slivers, pipe, pinholes, blowholes, inclusion
content) and diversion rate (slab casting abnormality codes) of products cast on high or low-
throughput thick slab machines. Examples from LTV, NKK and TKS pertaining to curved-mold
and vertical/bending casters will be reviewed which make apparent the paramount effect of steel
flow stability and steel flow pattern in the mold on defect occurrence. The first conclusion,
which pertains mostly to slow-speed or wide-slab machines, is that creating or maintaining a
stable double-roll flow is key to eliminating slab defects. The second conclusion, relating mostlyto high-speed casters, is that the intensity of the double-roll flow pattern must not be excessive.
With this in mind, the efficiency of an existing versatile electromagnetic-based technology that
allows accelerating, slowing down and stirring the liquid steel was examined. The analysis
shows good response of the liquid steel to various imposed magnetic fields, presents
corresponding slab/coil quality improvements, and demonstrates the relevance of the proposed
hydrodynamic and metallurgical concepts.
2. THE MOLD FLOW PATTERN CONCEPTSingle Roll and Flow Instabilities as Causes of Steel Defects
It is now well established that steel flow pattern obtained from conventional bifurcated SENs in
slab caster molds is not of any sort (1 - 6). It can be single or double-roll, with high speeds, narrow
widths, low argon flows, deep SENs and steep angles promoting double-roll flow. Little known
is the effect of steel flow pattern on steel quality.
In a paper presented in Madrid in 1998 (1), TKS-Dortmund/Amepa indicated that slabs cast
during a double-roll flow pattern in the mould were identified to be less frequently defective in
comparison to the average production of the caster. The study was referring to fine scale
sliver-type defects on non-stabilized LCAK steel grades. The slabs were very wide (2700 x 220
mm) and typically cast at 1.00 m/min with a 15 downward SEN and 8-10 l/min argon. Thestudy sadly concluded that under the existing casting conditions, the single-roll flow was the
dominant pattern for all SEN designs.
In 2000, the results of a 4-year research project conducted at LTV Steel with PIV and MFC
measurements were published in great detail (Particle Image Velocimetry in the Technology
Center water models and electromagnetic-based steel velocity measurements in the Cleveland-
East curved-mold caster 2, 3). Two conclusions were:
Steel quality is directly related to steel flow stability: unstable flows typically generate fourtimes more defects than stable flows(3).
Pencil pipe rejections on ULC titanium-stabilized grades are directly related to steel flow
pattern(2)
: single-roll flow leads to significantly more defects than double-roll flow.
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New, unpublished data from LTV Steel Indiana Harbor Works offer an opportunity to confirm
and sharpen these conclusions in an extended range of casting speeds and slab widths (Table I).
The numbers were generated using nail board measurements to determine the steel flow pattern
in the mold of the Indiana Harbor No. 2 machine that was revamped to vertical/bending in April
2000 (7, 8). The analysis was prompted by unexpected instances of pencil pipe defects although
all defective slabs had been cast under the same steady casting conditions.EventNo.
SENDepth(mm)
CastingSpeed
(m/min)
ArgonFlow
(l/min)
Slab Width(mm)
Flow PatternSingle ; Double
1. 144 1.16 20.4 1168 S
2. 127 1.65 9.6 1397 D
3. 127 1.65 9.6 1397 D
4. 162 1.65 8.5 1422 D/S
5. 142 1.50 9.6 1422 D
6. 144 1.47 25.5 1422 D
7. 155 1.57 26.9 1422 D
8. 162 1.52 30.3 1422 D9. 144 1.16 31.7 1422 S
10. 165 1.14 19.8 1448 D
11. 155 1.16 31.7 1448 D/S
12. 150 1.27 15.8 1525 D/S
13. 152 1.35 19.8 1525 D
14. 173 1.16 20.4 1525 S/D
15. 157 1.52 19.8 1575 S
16. 173 1.65 28.3 1575 S/D
17. 137 1.19 18.7 1600 S
18. 162 1.04 14.7 1727 D
19. 111 1.14 18.4 1778 S
20. 111 1.14 18.4 1778 S/D21. 122 1.12 10.5 1879 S
22. 122 1.12 10.5 1879 S
23. 109 1.12 10.5 1879 S
24. 122 1.12 10.5 1879 S
25. 137 1.14 17.0 2133 S
26. 137 1.12 17.0 2133 S
27. 137 1.12 17.0 2133 S
28. 137 1.12 17.0 2133 S
29. 122 1.12 17.0 2133 S
Table I Industrial (nail board) data show that steel flow pattern/stability depends
on slab width, argon flow rate, casting speed and SEN immersion depth.
The data clearly confirm the effect of the various casting parameters described above: eachevent, except one, is explainable line by line considering slab width, casting speed, argon
flow and SEN depth.
Interestingly, the comparison of events 2-3 and 21-24 shows that slabs cast at samethroughput, same argon and same SEN depth, had different flow patterns, with the narrower
slabs being double-roll and the wider slabs being single-roll. In more general terms, Fig. 1
shows that slow speeds and wide widths promote single-roll, independently from machine
throughput.
Again, flow pattern was found to affect steel quality: the six pencil-pipe defective slabs that
occurred at Indiana Harbor during the first year of operation (although of much less
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Fig. 1 Slow casting speeds and wide slab widths promote single-roll flow pattern in the mold (nail boardmeasurements; 23 dippings; 1170/2135-mm wide, 260-mm thick slabs; 1.10/1.65 m/min; 110/165-mm
SEN depth; 8/30 l/min argon). LTV data.
Fig. 2 Left-hand side; curved mold machine: single-roll flow pattern increases pencil pipe defects in
frequency and severity (36 defective coils; 1850-mm wide, 230-mm thick slabs; 1.0 m/min; 6.5 l/minargon). Right-hand side; vertical/bending machine: of the six defective slabs, all were single-roll(1880/2135-mm wide, 260-mm thick slabs; 1.17 m/min; 10/17 l/min argon). LTV data.
Fig. 3 Identification of steel flow pattern with nail board measurements. In the double-roll pattern, steel hits
first the narrow faces and creates a standing wave that locally reduces molten slag layer thickness. In thesingle-roll pattern, steel is first lifted to the meniscus and molten slag layer thickness is reduced in thevicinity of the SEN. LTV data.
0
2
4
6
8
10
12
14
Numberofcas
ts
1.1-1.2 1.3-1.5 1.5-1.65
Casting speed [m/min]
Double roll
Single roll
0
1
2
3
4
5
6
7
Numberofcas
ts
1170-1450 1525-1730 1780-1880 2135
Slab width [mm]
Double roll
Single roll
0
20
40
60
80
100
Frequencyofdefectivecoils[%]
1 2 3 4
Pencil pipe severity rating
Double roll Single roll, unstable, unidentified
0
20
40
60
80
100
Frequencyofdefectivecoils[%]
1 2 3 4
Pencil pipe severity rating
Double roll Single roll
0 100 200 300 400 500 600 700
Distance From Narrow Face (mm)
Mold Powder
Steel
Liquid Mold Flux
0
20
40
60
80
100
Thickness
(mm)
0
20
40
60
80
100 DOUBLE ROLL FLOW
0
20
40
60
80
100
0 100 200 300 400 500 600 700
Distance From Narrow Face (mm)
Thickness(mm)
Liquid Mold Flux
0
20
40
60
80
100
0 100 200 300 400 500 600 700
Distance From Narrow Face (mm)
MoldPowder
Liquid Mold Flux
Steel
SINGLE ROLL FLOW
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severity than those observed in the Cleveland curved-mold machine study) were all cast in
single-roll flow conditions (Fig. 2).
Fig. 3 which describes the impact of steel flow pattern on steel meniscus profile, may explain
why double-roll leads to fewer defects than single-roll flows.
Single-roll flow directs steel upwards from the SEN to the meniscus and narrow faces.Typically it will push mold powder, argon bubbles and inclusions to the narrow faces, which
will result in a lack of lubrication around the SEN and a preferential accumulation of
inclusions and bubbles in the edges of the slabs (9, 10). Maintaining a "stable" single-roll flow
is difficult: for given casting speed and slab width, it requires high argon flows and shallow
SEN immersion depths - two conditions that promote slag emulsification (11), pulsating
meniscus velocities and do not reduce meniscus velocities, nor mold level fluctuations.
In contrast, a double-roll flow that maintains the steel flow deep until it hits on the narrow
faces and splits into two loops will more typically weaken the steels momentum and result
in a relatively slower downward steel stream. A potential negative effect of the double flow
pattern is thinning of the molten slag layer near the narrow faces and powder shearing alongthe meniscus if high narrow-face standing waves and excessive steel meniscus velocities are
generated. However, hyperactive double-roll flow situations can easily be mastered by
electromagnetic forces, as will be described in section 4.
3. FLOW INSTABILITIESTransitioning Flows and Transient Casting Operations as Causes of Instabilities
Several instances of transient casting conditions, such as ladle, tundish, SEN tube, SEN depth,
slab width, or casting speed changes are well known from the operating and quality control
personnel. Accordingly, in each steel company, comprehensive lists of slab abnormalities have
been developed and used to redirect/downgrade coded slabs. Generally, the number of slabs that
are diverted after each transient event is determined by cause-and-effect analysis; strictly, it
should be based on the time necessary for the steel flow to stabilize again after a transient
operation (4 minutes after a casting speed change (1), 15 minutes after a tundish change (1), 35
seconds after a slide gate plate movement (12)). Always the number of slabs affected by transient
casting conditions represents several percents of the annual production of a caster and should not
be overlooked.
Little known is the fact that, depending on casting conditions, the steel flow can establish itself
somewhere between single and double-roll. At that point, the steel flow becomes steadilyunstable unsuspected by the operating and quality control personnel! Events 11 thru 16 in Table
I are such examples of unfortunate combinations of speeds, widths, depths and argon flows. The
observation is critical as, very often, the cause/origin of a coil defect cannot be found. It is as if
the defect did occur during "stable" casting conditions. While this should indicate that the defect
is the result of an intrinsic process flaw (like an inadequate steel flow pattern), it is interesting to
observe that mold flow pattern that combines all casting conditions into one single parameter can
correlate well with defects (2, 3).
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Fig. 4 The EMLA mode promotes heattransfer to the meniscus. NKK data.
Fig. 5 The EMLS mode reduces powder-basedinclusions and related pinholes in D&Imaterial (0.18-mm thick beverage canproduct). NKK data.
Fig. 6 The EMRS mode reduces subsurface
blowholes. Test result on low carbon steel withonly traces of soluble aluminium. NKK data.
100
80
60
40
20
0
WITHOUT
EMLSWITH
EMLS
SLIVER-BASEDPINHOLES
ONBEVERAGECANPRODUCTS(Index)
WITHOUT
EMRS
WITH
EMRS
6
4
2
0900 1800 2700
EMLA Current setting [A]
MeniscusTem
perature
Increase
[C]
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4. EMLA, EMLSMaintaining a Stable Double-Roll Flow
It becomes clear from the precedent section that steelmakers need a technology that can maintain
a stable double-roll flow in the mold, independently from slab width, casting speed, SEN depthor argon flow rate. A technology exists that is an effective method to meet that objective (13, 14).
The technology is based on the use of four linear stirrers that are positioned approximately at
mid-height of the mold; on each side of the SEN, behind each broad face. The stirrers induce
horizontally-traveling (AC) magnetic fields. They create forces in the steel that can accelerate or
slow down the steel according to the traveling direction of the fields. In contrast with "passive"
DC electromagnetic systems that can only brake what goes too fast with a braking effect that is
proportional to the actual speed of the liquid steel, these AC fields are "active." They brake with
considerably higher efficiency than DC systems and can also accelerate the liquid steel (actually,
they can put in motion dead zones that do not move). Switching between accelerating and
slowing down functions is possible on-line by appropriate electronic control in function of all
operating parameters as they change during casting.
Benefits of this technology proposed by NKK in 1991 have been described in great detail in the
literature (13-16). Examples include:
Fig. 4: The electromagnetic level accelerating function (EMLA) uses magnetic fieldstraveling from the SEN to the narrow slab faces. It promotes heat transfer to the steel
meniscus, i.e., improves mold powder melting, shortens the length of the solidification hooks
and reduces slivers and frozen meniscus situations.
Fig. 5: The electromagnetic level stabilizer/slowing-down function (EMLS) uses magnetic
fields traveling from the narrow slab faces to the SEN. It decreases mold level fluctuations,
i.e., reduces powder-based inclusions and sticker alarms on high-speed casters.
5. EMRSThe 3
rdFunction of a Flexible Electromagnetic Stirring Technology
The technology of the four linear electromagnetic stirrers mentioned in the precedent section
offers also a third function of stirring (EMRS, an electromagnetic rotative stirring function)
making it a very flexible electromagnetic stirring technology. Indeed, by appropriate selection of
the direction of the four traveling fields (going in the same direction all over one broad face, but
in opposite direction on the other broad face) forces can be generated at the meniscus that will
rotate the steel in a horizontal plane.
Benefits of meniscus rotative stirring, that was initially proposed by Nippon Steel in 1982 (with,
however, a stirrer located high in the mold), concernthe very early stages of steel solidification
at the meniscus (17). Fig. 6 is an example of test results obtained with EMRS at NKK(18) with
stirrers installed at mid-height of the mold. Observe how EMRS, that makes steel velocity and
heat transfer more uniform along the perimeter of the mold and "washes" the solidification front,
significantly reduces subsurface pinholes, blowholes and inclusion content (alumina and
calcium-aluminates, typically). The data refute the opinion that the "washing" effect of the
rotative stirring can only be obtained with a stirrer located high in the mold and confirm that a
same electromagnetic system can be used efficiently for all three stirring functions on a same
caster.
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6. SELECTING THE OPTIMUM EMLA, EMLS, EMRS MODESBreaking Free from the Constraints of a Demanding Product Mix
The selection and the drive of the EMLA, EMLS and EMRS functions on a same caster may
look complex. Actually it is very simple if one remembers the goal of maintaining a stable
double-roll steel flow pattern to minimize defects.
The control concept can easily be explained on a slab-width/casting-speed diagram and includes
three operating modes.
Fig. 7 represents the single- and double-roll flow events of table I in such a slab-width/casting-speed diagram; for simplification, the unstable S/D or D/S events were not
represented. A hyperbola line for a constant casting throughput of 4.6 t/min was added for
reference. The schematic was chosen to show the limits of a "naturally-existing single-roll
domain at 1550 mm slab widths, and of a "naturally-
existing double-roll domain at >1.3 m/min casting speeds and
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case of high-throughput/low-argon casting or the use of EMLA mode to accelerate in case of
low-throughput/high-argon casting. In industrial mode, an F-value that combines slab width,
casting speed, argon flow and SEN design/depth is calculated every 5 seconds, and stirring
intensity/direction is actuated accordingly. Indeed, water modeling and mathematical
simulations conducted at NKK-Fukuyama related F-value to SEN spouting
stream velocity,meniscus flow velocity and the height of the standing wave at the narrow
faces of the mold. Fig. 9 schematizes the break-even line between EMLS and EMLA
operations. For given SEN depth/design and argon flow, this line can be understood as the
iso-throughput line of the ideal operating "window" where no electromagnetic flow control is
required. Moving away from this line, higher throughputs require stronger
Fig. 9 Schematic representation of the three
operating modes. In the double-roll domain,EMLS-EMLA is operated by automatic F-value controller to maintain meniscus flowvelocity within the optimized operationalwindow. In the single-roll domain,permanent EMLA mode is used to convertsingle-roll and unstable flows into optimizeddouble-roll flows. In both domains, EMRS
mode is used for specific metallurgicalpurposes.
500
1000
1500
2000
2500
0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,8casting speed [m/min]
slabwidth[mm]
EMLS
EMLA
EMLA
permanent
DOUBLE ROLL
DOMAIN
SINGLE ROLL
DOMAIN
F-value control
EMRS
Fig. 10 The EMLS mode (at 280 A in thisexample) reduces steel meniscus velocitiessignificantly. Excessive braking (at 360 A in thisexample) reverses the double-roll pattern into a
detrimental "returning" single-roll proving theexistence of an optimum window of operation.
The left-hand side diagrams show liquid steeltrajectories in a 2-D mathematical simulation (theSEN is at the far left). Right-hand side diagrams
represent steel meniscus velocities (positivevelocities go from the SEN to the narrow face).
Meniscus velocities without EMLS at 0 Amp coilcurrent are represented in all three diagrams forreference.
Meniscusflowvelocity
(m/min)
Half mold width (m) Half mold width (m)
Coil current = 0 AMoldheihtm
Meniscusflow
velocity
(m/min
)
Half mold width (m) Half mold width (m)
Coil current = 280 AMoldhei
htm
Meniscusflowvelocity
(m/min)
Half mold width (m) Half mold width (m)
Coil current = 360 AMoldheihtm
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EMLS settings; lower throughputs require stronger EMLA settings. Wrong settings must be
avoided, as they generate defects. Interestingly, mathematical simulations show that too
strong EMLS settings (i.e., too much braking) induce a third loop in the mold (Fig. 10) which
disturbs the principal double-roll flow, thereby confirming mathematically the existence of
the optimum window represented on Fig. 8.
The second operating mode is permanent EMLA operation and covers the "naturally-existing single-roll domain, specifically pertaining to wide-slab, slow-speed casters. In this
case, the steel flow exiting the SEN needs to be accelerated to transform the single-roll flow
into a steady and optimized double-roll flow. This operating mode also addresses the various
transient casting conditions described in section 3 such as slow-speed head/tail slabs in
sequence casting or the steady unstable situations. In contrast to the general opinion that
EMLS/EMLA technology is only of interest to high-speed high-throughput machines, this
second EMLA mode addresses the majority of the currently existing casters. Industrial
results are not yet available for publication, however mathematical simulations show that the
EMLA function can easily transform a single-roll flow into a double-roll flow (Fig. 11).
The third operating mode is the rotative stirring EMRS mode. Its role is not to maintain thedouble-roll flow pattern in its optimized operating window, but rather to generate a flow
pattern that homogenizes steel temperature gradient along the mold perimeter and washes the
solidification front. For this reason, currently, the EMRS mode is limited to specific steel
grades such as pseudo-rimming, enameling, peritectic, tin plate, and beverage cans.
Coil current = 250 A
Half mold width (m)
Meniscusf
lowvelocity
Half mold width m
Moldh
eight(m)
Coil current = 450 A
Half mold width m
Meniscusflowvelocity
Half mold width m
Moldheight(m)
Coil current = 0 A
Half mold width (m)
Meniscusflowvelocity
Half mold width m
Moldheight(m)
Fig. 11 The EMLA mode (at 450 A in this example)can transform a single-roll flow pattern into a double-roll flow.
Left-hand side diagrams show liquid steel trajectoriesin a 2-D mathematical simulation. Right-hand sidediagrams represent steel meniscus velocities (positive
velocities go from the SEN to the narrow face).Meniscus velocities without EMLA at 0 Amp coilcurrent are represented in all three diagrams forreference.
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7. CONCLUSIONS
The contribution of this paper was meant to be twofold: (1) improve our understanding of the
effect of mold flow pattern on steel quality and, (2) present an electromagnetic-based technology
that allows to stabilize the preferred flow pattern and minimize slab/coil rejections. Specific
findings are:
Steel flow in the mold is affected by slab width, casting speed, argon flow rate, and SENangle/immersion depth.
Steel flow can be single-roll or double-roll, or it can establish itself somewhere betweensingle and double flow making it steadily unstable. Casting conditions changes such as ladle,
slab width, casting speed, argon flow rate, or SEN immersion depth changes are other
conditions that make the flow unstable.
Steel quality is directly correlated to steel flow stability and steel flow pattern, thereforemaintaining a stable double-roll flow is key to optimizing steel product quality.
The need for a versatile electromagnetic technology that can accelerate, slow-down, or rotatethe steel stream in the mold was demonstrated, and the benefits of the three functions were
analyzed.
Acceleration (EMLA) and braking (EMLS) modes can optimize the double-roll flow patternsthat are prevalent on high-speed high-throughput machines. The effect is to minimize slivers,
prevent excessive mold level fluctuations and reduce powder shearing and breakouts alarms.
A new application of EMLA was described that eliminates the negative impact of slow-speedmachines, wide-slabs, and transient-casting conditions. It consists of accelerating single-roll
and unstable/transitioning flow patterns to widen the domain of the preferred double-roll
flow conditions.
Rotative stirring EMRS which is a third mode of stirring available with the same equipment,
was confirmed to be efficient although the stirrers are installed at mid-mold height. Iteliminates the negative constraints of difficult-to-cast grades or demanding applications such
as pseudo-rimming, enameling, peritectic, tin plate, and beverage cans by reducing
subsurface pinholes, blowholes and inclusion content
A qualitative fluid-flow approach (single/double roll) was used in conjunction with aquantitative mathematical approach (F-Factor and 2-D simulation) to propose optimum use
of the three functions.
In its optimum use, the new, versatile technology is a unique tool to free caster operatorsfrom incompatibilities and inherent conflicting constraints between demanding product
mixes (slab width/casting speed/steel quality incoherence), productivity goals, and steel flow
patterns that are naturally unsuitable for defect minimization.
ACKNOWLEDGEMENTS
The authors wish to thank Dr. J. Kubota from the Fukuyama Works Steelmaking Department
and Messrs. T. Kondo and S. Mizuoka from the Steelmaking Group Consulting Department of
NKK Corporation in Japan for their valuable suggestions and discussions and support of this
paper.
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