Note: Descriptions are shown in the official language in which they were submitted.
CA 02375661 2001-11-29
METHOD AND INSTALLATION FOR MEASURING AND REGiTLATING THE FLOW
RATE OF A LIQUID METAL IN A CONTINUOUS CASTING INGOT MOULD
The present invention relates to metallurgic installa-
tions and, more specifically, to installatwons of continuous
casting of a liquid metal in an ingot mould.
Fig. 1 very schematically and partially shows in a
perspective view, the inlet section of a continuous metallurgic
casting ingot mould 1. The ingot mould essentially includes a
mould 2, open at its two ends in the case of a continuous
casting. The liquid metal is brought into the ingot mould by an
immersed nozzle 3, plunged into the mould 2. Nozzle 3 has
lateral ports 4 which aim at giving a horizontal component to
the speed of the liquid metal at the outlet of nozzle 3.
Fig. 2 is a simplified cross-sectiori view of a conven-
tional ingot mould 1 illustrating, with arrows, the motions of
the liquid metal in the inlet section of mould 2. As illustrated
in Fig. 2, the horizontal component of the liquid metal speed,
given by ports 4 of nozzle 3, limits the vertical penetration
depth of the metal supply stream into mould 2. Liquid metal 1
comes, for example, from a crucible 5 (for example, of blast
furnace type). In the example shown in Fig. 2, crucible 5
2C includes, in its lower portion, an opening 6 associated with a
controllable closing means 7 for controllinci the liquid metal
poured into nozzle 3. In conventional installations, the speed
CA 02375661 2001-11-29
2
of the liquid metal at the outlet of nozzle 3 can reach several
meters per second. It is thus important to be able to control
the liquid metal penetration in the cast. Indeed, too large a
penetration of this liquid metal raises several problems. Among
these, the dragging of non-metallic particles coming from the
powder or skin (not shown) which covers ingot: 8 cast in mould 2
should be noted. These particles are trapped in the obtained
metal. Too large a penetration of the liquid. metal also causes
an inverted thermal gradient since the hot liquid metal has an
effect upon the deep regions of the cast and causes, in particu-
lar, a local remelting deep into the at least partially
solidified ingot, which also adversely affects the quality of
the product.
To limit the liquid metal speed, braking systems and,
in particular, electromagnetic brake systems, are used.
A first type of electromagnetic brake uses a D.C.
magnetic field in a direction perpendicular to the metal flow,
which generates induced currents. The induceci currents interact
with the applied magnetic field and generate an electromagnetic
force which is a braking force aiming at nullifying the speed
having caused the induced currents. Such D.C. magnetic field
systems are generally formed of an electro--magnet totally or
partially surrounding the ingot mould, and which generates a
magnetic field transversal to the liquid metal. Such systems
have the disadvantage of being passive, that is, the magnetic
field has a geometry and a position which are set once and for
all, whereby any divergence at a given operating point reduces
the braking efficiency. Accordingly, this braking appears to be
inefficient when the supply conditions (spE:ed, nozzle shape,
nozzle port immersion depth, etc.) change.
A second category of so-called sliding field electro-
magnetic brakes uses an A.C. magnetic field generated by a
polyphase power supply applied to inductors exhibiting an
adapted space distribution. The magnetic field is thus given a
rotation or translation motion according to whether the inductor
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shape is cylindrical or planar. Such magnetic fields enable accelerating or
slowing down the
liquid metal flow in continuous metallurgic castings. Thus, the system is
active since the
mechanical effect induced in the liquid metal is independent from the liquid
speed and is
controlled by the operator.
The present invention more specifically relates to continuous casting
installations
equipped with a sliding magnetic field electromagnetic brake system.
In practice, in industrial continuous metallurgic casting installations, a
sliding magnetic
field brake is formed of four sliding field inductors associated by pairs on
each side of mould 2
of the ingot mould. In Fig. 1, two of these inductors have been schematically
illustrated and
designated with reference 9. In Fig. 2, the two inductors have been
illustrated in dotted lines On a
same side of the ingot mould, the two inductors are, as illustrated in Fig. 1,
symmetrically
arranged with respect to the axis of nozzle 3 on either side thereof to
balance the metal
distribution.
An example of an electromagnetic brake in a metallurgic casting installation
is described,
for example, in European patent application N 0,550,785 .
A problem posed is that the geometry of ports 4 of nozzle 3 varies along time,
in
particular, de to an erosion of these ports due to the fast flow of the liquid
steel in the nozzle.
This erosion does not necessarily evolve symmetrically, which then results in
a hydrodynamic
dissymmetry in the ingot mould due to a stronger flow on one side of nozzle 3
than on the other.
Such an imbalance adversely affects the quality of the finished product, since
it results, not only
in the introduction of non-metallic particles coming from the liquid metal
skin, but also in
different solidification durations from one side of the formed ingot to the
other.
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4
It is thus desirable to be able tc) differentiate the
actions of sliding magnetic field inductors 9 to restore a
balanced injection in the ingot mould.
For this purpose, it could be thought to separately
supply the four inductors to provide many combinations in the
organization of the liquid metal motions. In particular, the
braking of the liquid metal flow on one side or the other of
nozzle 3 could then be individualized.
However, the theoretical individualizing of the
1C) effects of the different inductors on the metal cast poses
implementation problems due, in particular, to the need to then
know the actual speed of the metal cast at a given time.
Further, the current metal injection speed must be known on
either side of nozzle 3.
A conventional method to adjust the sliding
electromagnetic field in an ingot mould of the type illustrated
in Figs. 1 and 2 consists of modeling the flow in a test
structure using, for example, water to determine the excitation
frequency of the inductors. Such a method is described, in
particular, in above-mentioned European patent application
N 0,550,785.
Clearly, such a method cannot enable knowing in real
time the speed of the flow through both ports 4 of nozzle 3 and,
more specifically, detecting an imbalance in this flow.
A first solution to know this speed would be to use
stress gauges attached to rods dipped into the liquid steel of
the ingot mould. By measuring a signal linked to the hydrody-
namic effort exerted by the liquid steel on the rods, any flow
dissymmetry can be detected and, accordingly, corrected by modi-
3C fying the power injected in inductors 9. However, the use of
rods, for example, alumina rods, poses several. problems.
A first problem is that such rods form an intrusive
element in the ingot mould which is likely to introduce
pollution in the obtained product, in particular, by an erosion
of the rods due to the liquid metal cast.
CA 02375661 2001-11-29
Another disadvantage is that the wearing by erosion of
these measurement rods makes this solution, in practice, hardly
viable from an economical point of view, due to the high
consumption of alumina rods in an industrial process.
5 The present invention aims at overcoming the disad-
vantages of conventional continuous metallurgic casting
installations. The present invention more specifically aims at
enabling individualized control of the inductors of a sliding
field electromagnetic brake of such an installation.
The present invention also aims at providing a
solution which causes no pollution of the liquid metal during
casting.
The present invention also aims at providing a solu-
tion which is particularly economical and requires no consumable
material replacement.
The present invention further ainls at providing a
solution which is particularly adapted to an individualized
control of the powers injected into the inductors generating the
sliding magnetic field.
To achieve these objects, the present invention
provides a method for measuring the flow speed of a liquid
molten metal in an ingot mould equipped wit:h a sliding field
electromagnetic brake, consisting of measuring the voltage or
the current of at least one power source of the electromagnetic
brake and extracting the flow speed from this measurement.
According to an embodiment of the present invention,
the method is applied to an electromagnetic brake, at least one
inductor of which includes two batches of several conductors in
a vertical direction, and consists of applying, for each
conductor, the following relation:
gradV = -i(co-vk)A-pj,
where co represents the A.C. excitation pulse of the sliding
field, v represents the metal speed, k represents the wave
number of the inductive sliding magnetic field, A represents the
vector potential, p represents the resistivity of the metal, j
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represents the density of the excitation current of the conductor, and
represents the
voltage across the inductor.
According to an embodiment of the present invention, the speed measurement is
used to
servocontrol the excitation of the inductors onto a predetermined value.
The present invention also provides a method for regulating the continuous
casting speed
of a molten metal in an ingot mould, consisting of controlling the voltage or
the current of at
least one supply source of a sliding field electromagnetic brake including
several inductors, with
a measurement of the current or of the voltage in each inductor.
The present invention also provides a continuous casting installation of the
type using a
sliding filed electromagnetic brake to control the flow of a liquid metal
provided by two ports of
a nuzzle, characterized in that each inductor of the electromagnetic brake is
powered by an
individual circuit; and in that the installation includes means for regulating
the supply voltage or
current of each inductor to maintain the liquid metal flow speeds balanced
between the two ports.
According to an embodiment of the present invention, each supply circuit of
each
inductor includes its own means for regulating the electromagnetic excitation
power of this
inductor.
According to an embodiment of the present invention, the installation includes
a central
station for controlling the supply circuits of the different inductors to
regulate the liquid metal
flow speed.
As such, there is provided a method for measuring the flow speed of a liquid
molten
metal (1) in an ingot mould (1) equipped with a sliding field electromagnetic
brake,
characterized in that it consists of: supplying the electromagnetic brake with
current, respectively
voltage, from at least one constant power source; measuring the voltage,
respectively the current,
of the power source (31, 32); and extracting the flow speed from the
variations of this
measurement.
Alternatively, there is also provided a method for regulating the continuous
casting speed
of a molten metal in an ingot mould (1) equipped with a sliding field
electromagnetic brake
including several inductors (9), characterized in that it consists of:
supplying the electromagnetic
brake with current, respectively voltage, from at least one constant power
source; and controlling
the voltage or the current of the power source (31, 32) with a measurement of
the current or of
the voltage in each inductor.
DOCSMTL: 2447728\1
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There is also provided a continuous casting installation of the type using a
sliding field
electromagnetic brake to control the flow of a liquid metal (1) provided by
two ports (4) of a
nozzle (3), characterized in that each inductor (9) of the electromagnetic
brake is powered by an
individual circuit (21); and in that the installation comprises means (26, 35,
36) for regulating the
supply current, respectively voltage, of each inductor, from a measure of the
current, respectively
voltage, inductor supply variations so as to maintain the liquid metal flow
speeds balanced
between the two ports.
The foregoing objects, features and advantages of the present invention, will
be discussed
in detail in the following non-limiting description of specific embodiments in
connection with
the accompanying drawings, among which:
Figs. 1 and 2, previously described, show an example of a continuous
metallurgic cast
installation of the type to which the present invention applies; and i
designates the imaginary part
of a complex number.
CA 02375661 2001-11-29
7
Fig. 3 very schematically shows the respective
positions of the inductors in a continuous casting system to
which the present invention applies;
Fig. 4 is a top view of an ingot mould equipped with a
casting speed control system according to the present invention;
and
Fig. 5 schematically shows an embodiment of a circuit
for controlling an inductor according to the present invention.
The same elements have been designated with the same
1C1 references in the different drawings. For clarity, only those
elements which are necessary to the understanding of the present
invention have been shown in the drawings and will be described
hereafter. Reference can be made to literature, in particular,
to European patent application N 0,550,785, for the forming of a
continuous casting iristallation with an active sliding field
electromagnetic brake, since the present invention does not
modify the structure of conventional installations.
A feature of the present invention is to take
advantage of an individual power supply of the different
inductors of a sliding field electromagnetic brake to extract,
from the electric characteristics of this inductor supply,
information concerning the flow speed of the liquid metal in the
ingot mould.
According to the present invention, the fact that the
currents induced by the conductive liquid metal in the magnetic
field created by the inductors depend, among others, on the
liquid metal flow speed, is used. In particular, assuming that
the system is stabilized for a metal speed corresponding to a
permanent liquid metal flow state, any disturbance which causes
a variation in this speed translates as a variation in the
impedance of the inductor(s) responsive to the corresponding
induced current. Thus, according to the present invention, a
constant power source, either in current, or in voltage, is used
to supply the inductors, and the possible variation of the other
3 5 variable (voltage or current) is examined to deduce a variation
CA 02375661 2001-11-29
8
in the liquid metal flow speed. Further, due to the fact that
the inductors are powered separately from one another this speed
can be localized. This information may, in a preferred
embodiment, be used as the feedback of a system of control of
the power supply of the different inductors to control the metal
flow speed with a point of equilibrium corresponding to a given
speed reference, for example, calculated based on a modeling
such as described in European patent application N 0,550,785.
Fig. 3 very schematically illustrates the position of
four inductors in a continuous casting installation. For simpli-
fication, only inductors 9 and a parallelogram symbolizing
liquid metal 1 between these inductors have been shown.
Conventionally, each inductor 9 is formed of several
imbricate turns adapted to being respectively supplied by
different phases. In the example of Fig. 3, a two-phase electro-
magnetic brake system has been assumed. Eac:h inductor 9 thus
includes two circuits, respectively 10 and 11, of conductive
imbricate turns in a magnetic yoke 12 opposite to metal 1 with
respect to plane x-z in which are inscribed conductive circuits
10 and 11. A first conductive circuit 11 corresponding to a
first phase is formed of three packs of conductors 13, 14, 15.
The number of conductors of the central pack 15 corresponds to
twice the number of conductors of packs 13 and 14 which surround
two packs 16, 17 of conductors of second circuit 10 intended to
be supplied by the second phase of the two-phase power supply.
To form the adapted ampere-turns, the conductor batches are
directly connected by phase to one of their ends and, via the
supply source (not shown in Fig. 3), to their other respective
ends. Thus, in the example of Fig. 3 where the vertical axis z
is in the cast direction and the horizontal axes x, y, are
respectively in the largest direction of liquid metal 1 corre-
sponding to the alignment of the ports (4, Fig. 1) of the
injection nozzle and in the smallest direction of liquid metal
1, the conductor packs of the different indluctors are in the
vertical direction z. They are, for example, directly connected
CA 02375661 2001-11-29
9
by their respective lower ends. By the connection of the
conductor packs, the turns are run through by a current which,
in the vertical sections, is inverted according to whether
conductors 13, 14, or 15 for first circuit 11, and 16 or 17 for
second circuit 10 are involved. To illustrate this flow in
opposite directions, an example of current flow, symbolized by a
"." or a"x" according to the flow direction in the vertical
sections, has been indicated in Fig. 3.
The current flow as shown in Fig. 3 is conventional
and will not be detailed any further. It should only be noted
that the present invention may be implemE:nted in a system
including a greater number of phases, for example, in a three-
phase or polyphase system while respectinq the usual phase
imbrication to obtain a polyphase sliding field. It should also
be noted that, as illustrated by the representations of the
current flow directions in Fig. 3, axis x corresponds to a
longitudinal axis of symmetry which in fact is an axis of
antisymmetry for inductors 9 which are opposite two by two.
In a sliding field electromagnetic field such as
illustrated by the preceding drawings, it can be considered that
the potential vector A, the current density and the electric
field E have a single component along vertical axis z, that the
speed of induced metal v has a single component along
longitudinal axis x, and that the magnetic induction B has two
components along horizontal axes x and Y.
The synchronism speed vs of the sliding
electromagnetic field is equal to the product of the operating
frequency f of the A.C. excitation of the two phases by the
wavelength X of the sliding field wave. It should be noted that
the actual speed v of the metal is opposite to this synchronism
speed which also has one component only alonq longitudinal axis
X.
The equations which govern the operation of the elec-
tromagnetic field,,respectively in the inductor, in the air, in
the magnetic yoke, and in the induced metal, rnay be expressed as
CA 02375661 2001-11-29
follows in projection on vertical axis z where the single
unknown value is component A along Oz of poteritial vector A.
In the inductor, one may write:
-div o (gradA) =J;,
5 where Ji represents the current density imposed in the inductor
by the power supply, and where 0 represents the permeability of
vacuum.
In the air, one may write:
- div (gradA) = 0 .
10 In the magnetic yoke, one may write:
- div 1 (graclA) = 0 ,
L o r
where r is the relative permeability of the magnetic medium.
In the induced metal, on may write:
- div 1(gradA) = i(o A v 8A ~
o P P ax
where w represents the electric pulse of the A.C. power supply
(co = 2nf) and where p represents the resistivity of the liquid
metal.
As a first approximation, to neglect edge effects, it
can be considered that the potential vector A. is a sliding wave
due to an infinitely long inductive sheet following longitudinal
direction x. It can then be considered that the only component A
of the vector potential according to vertical axis z can be
written as:
A = Aoe'((o'-k )
where k represents the wave number of the inductive sliding
magnetic field (k = 2n/k).
With this approximation, the precedi_ng relation in the
induced metal can be expressed in projection on the vertical
axis as being equal to:
- div 1(gradA )+ 1((o - vk)A = 0.
o P
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11
Introducing the synchronism speed of the inductor in
this equation provides:
- div 1(gradA) + i 2 (vs - v)A = 0.
o kP
All the above expressions show that the only variable
quantities for a given current are potential A and speed v of
the liquid metal.
It should be noted that, rather than the current, the
current density must be set. However, the ntunber of conductors
per pack (that is, the number of turns) has no incidence since
1C) the voltage variation of each phase will be compared in a
relative manner for a metal speed variation.
Voltage gradient gradV can thus be calculated based on
the respective values of potential vector A, of imposed current
density j, and of the previously established relations.
By projecting on vertical axis z the following Maxwell
equation:
j=6( icoAxB-gradV)=6(- iwA+vxrotA-gradV)
which links values gradV and A, and by replacing vxrotA by
ivkA, the following relation giving the voltage gradient on each
20 conductor is obtained:
gradV = -i (c.)-vk) A - pj.
It is then sufficient to sum up the values obtained
for all the conductors in each pack to obtain the total voltage
of the respective phases. If need be, the impedance of each
25 phase, rather than the voltage, may be deduced by dividing this
voltage by the currents imposed by current sources 31 and 32.
As a specific example of implementation, taking for
each conductor pack a rectangle of 160x100 mm2 (except for end
packs 13 and 14 which each correspond to a rectangle of 80x100
30 mm2), the current density is 6.75x106 amperes rms per m2.
Assuming a relative permeability r of 1,000, the wavelength k
of the sliding field is approximately 1.3 m. For an operating
frequency of, for example, 0.65 Hz, the synchronism speed vs is
84.5 cm/s.
CA 02375661 2001-11-29
12
Assuming, in a simplified manner, that the induced
metal is a solid of constant resistivity p = 100.10-8 S2m (which
corresponds to a conductivity of 1.106 (0m)-1, that is, substan-
tially that of liquid steel), the respective values of the total
voltage for the conductor packs may be calculated for two
modules of liquid metal speed of 10 and 9 cm/s. For example, for
batches 16 and 17 of 40 conductors having a square cross-section
of 20x20 mm2 in series, which amounts to considering batches of
40 spirals in each of which flows a current of 2700 A rms, volt-
ages of 38.66 volts and 36.74 volts in modulus are obtained for,
respectively, 10 cm/s and 9 cm/s. According:Ly, the modulus of
the voltage of the corresponding phase decreases by
approximately 2/38, that is, approximately 5 ,. On the impedance
of the corresponding phase, the variation also is on the order
of 5% for a same metal speed variation.
Accordingly, it can be considered that with industrial
values, a variation on the order of 10% of the metal speed
causes a variation on the order of from 5 to 6% in the voltage
and in the impedance. This variation is substantial enough to be
2C used to control the regulation circuits to bring the speed back
to its mean reference value, or to bring down to zero an
interval between two values.
Fig. 4 illustrates, in a top view of an ingot mould,
the respective electric connections according to the present
invention of the two-phase inductors illustrated in Fig. 3. The
direct connections between the difference conductor packs, for
example, in the lower portion of the system, have been
symbolized by dotted lines.
In the top view of Fig. 4, nozzle 3 has been
schematically at the center of mould 2. Each .Lnductor 9 has been
symbolized by its magnetic yoke 12 and its two conductive
circuits 10, 11 formed, in the vertical direction, respectively
of two packs 16, 17 of a same number of conductors and of three
packs 13, 14, 15, the central pack 15 having a number of
conductors which is twice that of end packs 13 and 14.
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13
As indicated previously, sections 16 and 17 of each
circuit 10 are directly connected, for example, by a cable 18 in
their lower portion. Similarly, packs 13 and 14 are each
connected to pack 15, for example, by cables, respectively 19
and 20. In the upper portion of the vertical_ conductive packs,
said packs are conriected by their ends to supply means.
According to the present invention, conductive circuits 10 and
11 of each inductor 9 are individually connected to a supply
circuit 21 specific to the concerned inductor. Thus, packs 13
and 14, pack 15, pack 16, and pack 17 are connected to a circuit
21 by respective cables 22, 23, 24, and 25.
According to the present inventiori, all circuits 21
have an identical structure which will be described hereafter in
relation with Fig. 5. Each circuit is individually connected to
a central control station 26, for example, by cables 27. Cables
27 have been illustrated as including several independent
conductors to bring, to each supply circuit 21, the different
necessary A.C. supply phases as well as, if necessary,
appropriate control signals provided by central station 26. It
should however be noted that only the control signals could be
individualized and that the polyphase supply conductors could be
common to the different circuits 21, said circuits then being in
charge of adapting the respective powers to be provided to each
of the inductors.
For clarity, the different references of inductors 9
have been indicated only once in Fig. 4, each inductor having a
similar structure and differing from the others by the current
flow direction only, as illustrated in Fig. 3.
Fig. 5 very schematically shows the structure of a
supply circuit 21 of an inductor according to the present inven-
tion.
In the example of Fig. 5, it is assumed that each
inductor phase is supplied by a low-frequency A.C. signal, of
which the rms current value is set to a predetermined value
according to the nominal braking characteristics desired for the
CA 02375661 2001-11-29
14
ingot mould. Thus, circuit 21 of Fig. 5 includes two current
sources 31 and 32 supplying, for example, cables 23 and 25
respectively associated with conductor pac:ks 15 and 16 as
illustrated in relation with Fig. 4. Current sources 31 and 32
'i are, according to the present invention, controllable, respec-
tively, by signals 33 and 34 provided by regulation circuits,
respectively 35 and 36. Each circuit 35, 36, measures the
voltage between, respectively, conductors 22 and 23 and
conductors 24 and 25. These voltage measurements are intended to
evaluate the liquid metal speed opposite to the corresponding
inductor.
In the embodiment illustrated in Fig. 5, each
regulator 35, 36 receives a reference 37, 38 from the control
station 26 (Fig. 4) and is in charge of controlling the current
1S provided by sources 31 and 32 to enable a regular and balanced
speed in the ingot mould. However, the regulation may also be
provided to be performed directly by central station 26, or a
voltage regulation may be provided to be used to calculate the
speed, so that it is exploited by the central station 26.
Of course, the inductors may also be supplied by a
controllable voltage of predetermined value, and a current
measurement, the variations of which will then depend on the
speed, may be used, thus enabling feedback on the supply voltage
source.
The practical implementation of the method according
to the invention, by the forming of electronic circuits or the
programming of computing tools necessary for the calculation, is
within the abilities of those skilled in the art based on the
functional indications given hereabove. It should be noted that
the complexity of this electronic circuit or of the programming
computations will depend on the desired accuracy for the
control, as for any conventional control.
An advantage of the present invention is that it
enables measuring the liquid metal speed in the ingot mould
without any physical contact with the liquid ntetal.
CA 02375661 2001-11-29
Another advantage of the present irivention is that it
is particularly well adapted to a control of continuous casting
systems since it is very easy to have a feedback on the current
or the voltage in the inductors.
5 Another advantage of the present irivention is that it
requires no modification of conventional continuous casting
installations with a sliding field electromagnetic brake, except
for the control circuits of the different inductors.
Of course, the present invention is likely to have
1C) various alterations, improvements and modifications which will
readily occur to those skilled in the art. In particular, the
adaptation of the method according to the number of phases of
sliding field electromagnetic brake systems is within the abili-
ties of those skilled in the art according to the application
15 and to the functional indications given hereabove. Further, the
numerical values indicated in the foregoing description have
been indicated only to show the industrial feasibility of the
present invention and for illustration only. Further, it should
be noted that the present invention may be implemented in any
continuous casting system, whatever the shape of the ingot
mould, provided that it uses an active sliding field
electromagnetic brake system.
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