U.S. patent application number 10/577461 was filed with the patent office on 2007-04-05 for electromagnetic agitation method for continuous casting of metal products having an elongate section.
This patent application is currently assigned to ROTELEC. Invention is credited to Siebo Kunstreich.
Application Number | 20070074845 10/577461 |
Document ID | / |
Family ID | 34400826 |
Filed Date | 2007-04-05 |
United States Patent
Application |
20070074845 |
Kind Code |
A1 |
Kunstreich; Siebo |
April 5, 2007 |
ELECTROMAGNETIC AGITATION METHOD FOR CONTINUOUS CASTING OF METAL
PRODUCTS HAVING AN ELONGATE SECTION
Abstract
A stirring method provides overall stirring of metal over a
metallurgical length, thereby ensuring both thermal and chemical
uniformity between a top and bottom of a liquid pool without
correspondingly being deprived of beneficial effects specific to
stirring in a mold and in a secondary cooling zone respectively,
and without disturbing, but rather stabilizing, local flow mode in
the mold. During a continuous slab casting operation, in which
molten metal is introduced into a mold via a submerged nozzle
having lateral discharge outlets opening towards narrow faces, a
stirring uses moving magnetic fields that act, in pairs, at least
in a secondary cooling zone of a casting plant, by travelling
collinearly between them in opposite directions so as to forcibly
establish a middle longitudinal circulation in the liquid pool as
two opposing collinear streams, which produce a global movement in
the form of a "four-leaf clover", the upper lobes of which extend
into the mold to near discharge jets coming from outlets of the
nozzle, brake the jets or accelerate them, as required.
Inventors: |
Kunstreich; Siebo; (SAINT
OUEN, FR) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
ROTELEC
TOURS MERCURIALES, 40 RUE JEAN JAURES
BAGNOLET
FR
F-93170
|
Family ID: |
34400826 |
Appl. No.: |
10/577461 |
Filed: |
October 22, 2004 |
PCT Filed: |
October 22, 2004 |
PCT NO: |
PCT/FR04/02728 |
371 Date: |
April 27, 2006 |
Current U.S.
Class: |
164/468 ;
164/504 |
Current CPC
Class: |
B22D 11/115
20130101 |
Class at
Publication: |
164/468 ;
164/504 |
International
Class: |
B22D 27/02 20060101
B22D027/02; B22D 11/00 20060101 B22D011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 27, 2003 |
FR |
0312555 |
Claims
1-10. (canceled)
11. A method of electromagnetic stirring in a secondary cooling
zone of a plant for continuous casting of metal products of
elongate cross section, a mold of which is provided with a
submerged casting nozzle having lateral discharge outlets directed
towards narrow faces of the mold, and utilizing travelling magnetic
fields generated by multiphase inductors placed near the cast
metal, the method comprising: for promoting liquid metal exchange
within the liquid pool between a secondary cooling zone and the
mold, forcibly establishing a longitudinal metal flow in the
secondary cooling zone, the metal flow being localized in a middle
region of the cast product as two opposing collinear streams, and
providing circulation of the liquid metal as a four-leaf clover
configuration design having two upper lobes and two lower lobes,
the upper lobes extending into the mold right up to a level of jets
coming out from the discharge outlets of the submerged casting
nozzle.
12. A stirring method according to claim 11, wherein the
longitudinal opposing collinear streams in the middle region of the
cast product, which move away from each other, are created such
that the two upper lobes that extend into the mold right up to the
level of the jets coming out from the discharge outlets of the
casting nozzle merge concurrently with the jets to reinforce the
jets.
13. A stirring method according to claim 11, wherein the
longitudinal opposing collinear streams in the middle region of the
cast product, which converge on each other, are created such that
the two upper lobes that extend into the mold up to the level of
the jets emanating from the discharge outlets of the casting nozzle
are superposed counter-currently on the jets to slow the jets
down.
14. A stirring method according to claim 11, wherein the location
of the longitudinal flow in the secondary is shifted laterally
towards one or other of the small sides of cast product.
15. A stirring method according to claim 11, wherein the
longitudinal metal flow is created as the two opposing collinear
streams by collinear moving magnetic fields that travel
longitudinally in the central region, either coming closer
together, or further apart.
16. A stirring method according to claim 12, wherein the
longitudinal metal flow is created as the two opposing collinear
streams by collinear moving magnetic fields that travel
transversely over the width of the cast product, either coming
closer together from an edge towards the center of the cast
product, or moving further apart from the center towards the edge
of the cast product.
17. A stirring method according to claim 11, wherein the travelling
magnetic fields are generated by multiphase linear inductors placed
facing large faces of the cast product.
18. A stirring method according to claim 17, wherein the inductors
are supplied with electric currents of different intensities.
19. A stirring method according to claim 11, wherein other
travelling magnetic fields are also used that act directly in the
mold on the jets of metal discharging from the outlets of the
nozzle.
20. A flat metal product obtained from a continuous casting plant,
the secondary cooling zone of which being the location of an
electromagnetic stirring operation according to that defined in
claim 11.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from PCT Application
n.sup.o PCT/FR2004/002728 based on French Patent Application
n.sup.o 03 12555
BACKGROUND OF THE INVENTION
[0002] 1.--Field of the Invention
[0003] The present invention relates to the continuous casting of
metals, especially steel. It relates more particularly to the
electromagnetic stirring of flat products (i.e. of elongate cross
section) while they are being cast, and relates even more precisely
to the establishment in the metallic liquid pool of a particular
distribution of the flows by means of applied magnetic fields.
[0004] It is reminded the general expression "product of elongate
cross section" has to be understood to designate metallurgical
products whose width is at least twice the thickness, especially
slabs, narrow slabs, thin slabs, etc. . . .
[0005] 2.--Description of Related Art
[0006] Coming out in the field of continuous steel casting at the
start of the seventies, electromagnetic stirring has rapidly
confirmed its position as an almost indispensable tool for
controlling the flows in the liquid pool undergoing solidification.
It will be recalled that the principle most commonly employed is
the well-known principle of MHD (magnetohydrodynamic) which, by
means of a moving (rotating or travelling) magnetic field generated
by a polyphase inductor, or more generally by several polyphase
inductors, placed in the immediate vicinity of the cast product,
drives the liquid metal with its displacement. Suitably located on
the metallurgical height of the casting machine, these inductors,
supplied with electrical current at an adjustable frequency,
therefore allow various types of stirring modes that can be matched
to the requirements of the metallurgist.
[0007] Moreover, constant progress in understanding the mechanisms
of metal solidification during continuous casting has specifically
demonstrated the important role played by the circulatory movements
of the liquid metal for the general quality (i.e. internal
soundness, surface cleanness or lack of inclusions, solidification
structure, etc.) of the final solidified product.
[0008] In this regard, the movements applied to the molten metal
during continuous casting may be schematically classed into two
categories, depending on whether we consider the mould or, beneath
it, the secondary cooling stages of the casting machine.
[0009] The movements settled on the liquid metal by means of
electromagnetic stirring within the mould, at a level where the
liquid portion of the cast metal is greatly predominant, are
essentially designed to control the flows in this critical area.
Indeed, here, where the free surface of the cast metal is found,
its internal cleanliness depends on the geometrical shape of this
surface. It is also here where the first solidification skin
occurs, the major importance of which being well known as regards
both the surface quality of the final cast product and the control
of the casting process itself.
[0010] On the other hand, by stirring the metal in the liquid pool
beneath the mould, therefore in the secondary cooling zone (usually
called "in the secondary"), the aim is first to improve the
internal metallurgical structure of the cast product via the
development of a largest equiaxed solidification, this being known
to be favourable both to the micro-segregation of the alloying
elements and to the absence of central porosity in the cast
product, for example. Thus, electromagnetic stirring is used for
the continuous casting of slabs more and more frequently whenever
products that require an internal structure free of porosity have
to be produced, such as for example thick plates for making
boilers, or large welded pipes.
[0011] It should be only reminder here for a better understanding
of the invention, as will be explained below, that it is well
known, as shown by the diagram of the appended FIG. 3 taken from
the document FR 72/20546, to use, in the secondary cooling zone of
a continuous slab casting machine, linear inductors 41, 41' placed
facing each other, on either side of the large faces of the cast
product, and producing transversal magnetic fields that travel over
the width of the product. The aim is thus to set up, within the
liquid metal, flows which essentially develop as two adjacent loops
rotating in opposite directions. These loops 42, 43 are established
parallel to the large faces and extend in stages along the length
of the cast product on either side of a common transverse zone of
driving action of the magnetic field, the flows of each loop rising
along one small face and descending along the opposite small face
Such a movement configuration is conventionally termed a "butterfly
wings" configuration.
[0012] It is possible, as shown in the appended FIG. 4 extracted
from document FR 82/10844, to multiply, depending on the length of
the casting machine, the transverse zones 51, 52 of the driving
action of the magnetic fields. In this case, said zones are,
pairwise, in opposite directions of rotation, between the closest
neighbouring loops so as to generate the largest possible stirred
volume for a given available stirring power. Thus, a pattern flow
referred to as a "triple-zeroes configuration" is produced, this
being formed from three adjacent loops rotating pairwise in
opposite directions, namely a central loop 60 located between the
two transverse driving zones 51 and 52, and two outer loops 61 and
62 on either side of the central loop and rotating in the same
direction.
[0013] Whatever the implementation form adopted, this can be
achieved just as well with inductors placed behind the support
rollers of the secondary cooling zone of the casting machine as
between these rollers (FR 72/20547) or inductors housed within the
actual rollers (FR 72/20546). The same also applies as regards the
means of implementing the invention, which will be explained
below.
[0014] Historically, it seems that the discovery of this type of
movement, based on recirculation of the metal in loops set up in a
plane parallel to the large faces of the slab, stems from the fact
that, unlike in long products, in the continuous casting of flat
products the elongate shape of the cross section of the product
does not easily lend itself to the establishment of a stable
rotational movement about the casting axis. The main reason
probably lies in the large velocity gradients that this requires in
the thickness of a product, which barely exceeds some twenty
centimeters for the thickest products.
[0015] However, a staged-loop configuration of the type shown in
FIGS. 3 and 4, which develops over the metallurgical length
parallel to the large faces of the product, does not suffer from
such a handicap. It also has the advantage of ensuring better heat
exchange between the top and bottom areas of the casting machine.
The hottest molten metal from the top is droved by forced
convection downwards by the descending running 42a and 43b, while
the rising running 42b and 43b seed the top with crystallites of
solidified metal that have collected in the bottom, thus favouring
the early development of extensive uniform equiaxed solidification
from the periphery right to the centre of the cast product.
However, these loops 42, 43 cannot be developed too vigorously near
the top as one would wish, owing to the risk of disturbing the free
surface of the metal in the mould. At the present time, it is known
how much the preservation of the fragile hydrodynamic equilibrium
of the in-mould flows prevailing at this level of the mould is
necessary for obtaining a good quality of the surface, of the
sub-skin and of the core of the cast product.
[0016] Precisely, the introduction of the metal to be cast via the
top of a mould using a submerged nozzle having lateral discharge
outlets opening onto the narrow faces of the mould has become
virtually general practice at the present time, replacing the
straight nozzle with a single axial discharge, consequently
reserved practically only for long products. A major advantage
obtained over in-mould flows lies in the fact that, as shown by the
diagram in FIG. 1 appended hereto, by means of a rebound effect
onto the narrow faces of the mould, the jet of hot liquid metal
coming out from each lateral hole 27, 27' in the nozzle 26 is
therefore spread out naturally into two fractions. A main fraction
21 is directed downwards, in the direction of extraction of the
cast product. The other fraction 22 is reflected upwards so as to
provide, near the free surface 23 of the in-mould metal, the
enthalpy needed to prevent the phenomenon of cast metal solidifying
at the meniscus, which are very often the cause of drastic
stoppages of the casting process. The aim is thus to produce, in
the mould, a circulation mode called "double roll" as opposed to
the "single roll" mode.
[0017] The latter mode, shown in FIG. 6, is firstly manifested by
the phenomenon of metal rising up towards the meniscus upon being
discharged from the outlets in the nozzle, very often resulting
from an injection of argon to prevent clogging of the nozzle from
the casting tundish located above it. This first upward rise is
then continued by a surface current towards each narrow face, and
after by a going down flow along the latter. In this way, a
velocity map is quickly established in the mould, in which the
velocities are generally directed downwards in the direction of
extraction of the product, with the absence of the upper roll 22
for supplying "hot" metal to the meniscus.
[0018] However, the "double roll" mode lasts during casting only if
the casting conditions (casting speed, width of the slab, depth of
immersion of the casting nozzle, flow rate of anti-clogging argon,
etc.) lend themselves thereto. Random transitions in "single roll"
mode may appear during the actual course of casting if these
conditions fluctuate, which in fact corresponds to a general
case.
[0019] In addition, an essential aspect for controlling the
in-mould "double roll" flows, lies in the preservation within the
mould of a "left-right" symmetry of the re-circulating movements at
the meniscus on either side of the nozzle. This is because it is
known that the occurrence of "left-right" asymmetries is the
grounds of oscillations in the metal bath that may result in
unacceptable rolling of the surface, well known to the operator
standing on the casting platform. This means that care must be
taken to ensure that the partial recirculation flows 22, 22' near
the top are, above all, steady over time in order to avoid the
occurrence of "left-right" asymmetries. These ascending
circulation, while still being thermally effective enough to
deliver the desired heat to the meniscus, must however not be too
intense from the hydrodynamics standpoint in order to avoid
excessive agitation of the line of first solidification 25 that
forms around the border of the meniscus against the cooled copper
wall of the mould. The regularity of this line of first
solidification is in fact the warrant of uniformity of formation of
the first skin in the top of the mould, without which there is
inevitably a risk of break-outs beneath the mould by encrustations
of slag or by local thinning of the thickness of the solidified
skin.
[0020] Stated more simply, by casting with a submerged nozzle
having lateral discharge outlets, it is possible to achieve, over
the course of any one casting run, randomly or, in any case, not
necessarily desirable, in-mould flows that are either of the
"double roll" type, or of the "single roll" type, or unstable flows
owing to "left-right" asymmetries.
[0021] It is in particular because of these difficulties in
controlling flows in the top area of continuous casting machines
that electromagnetic stirring systems have more recently appeared
that act in the mould, already on the lateral discharge jets coming
from the nozzle. As the diagrams of the appended FIGS. 2a and 2b
show, which are extracted from document JP 1534702, magnetic fields
moving horizontally are produced by multiphase linear inductors
30a, 30b and 30a', 30b' placed along the large faces of the mould
32 facing the discharge path of the metal jets on either side of
the nozzle 31. By adjusting the direction of travel of the fields,
it is then possible to slow down the current of said jets of metal
(counter-current travel of the fields, going from the small face to
the nozzle (FIG. 2b.sub.1) or, on the contrary, to speed it up
(co-current travel in the direction going from the nozzle towards
the small face (FIG. 2b.sub.2). In principle, this allows the
amount of enthalpy supplied to the surface of the cast metal to be
adjusted, for example according to the casting conditions, without
excessively disturbing the in-mould flow mode that has to be
preserved as a matter of priority.
[0022] The above rapid review of the prior art therefore clearly
shows the separation, if not the conflict, that exists when casting
products having an elongate cross section (like flat products)
between the stirring of the metal in the mould on the one hand and
the stirring in the secondary cooling zone, on the other.
BRIEF SUMMARY OF THE INVENTION
[0023] The object of the present invention is specifically to
overcome such a handicap. Stated another way, applicable to the
continuous casting of flat products, particularly slabs, the object
of the invention is, via a studied overall stirring movement of the
molten metal over the metallurgical length, to provide good
exchange of still-liquid metal in both directions between the
secondary cooling zone and the mould. This will consequently
achieve thermal and chemical uniformity between the top and bottom
of the pool of cast liquid metal without disturbing the in-mould
flow mode and, where possible, without correspondingly being
deprived of the cumulative beneficial effects specific to stirring
in the mould and to stirring in the secondary cooling zone
respectively.
[0024] One complementary object of the invention is to help to
improve the metallurgical quality of steel grades that it is
desired to produce with good internal quality, such as grades for
thick plate or for large welded pipes, ferritic stainless steel, or
silicon electric steel.
[0025] Another complementary object is to be able to vary the flows
in the secondary cooling zone in order to use them level with the
casting jets emanating from the nozzle, either as an accelerating
agent or on the contrary as a braking agent for the metal entering
the mould, or else as a means for counteracting the "left-right"
asymmetry tendencies of the metal movements within the mould.
[0026] With these objectives in mind, the subject of the invention
is a method of electromagnetic stirring in the secondary cooling
zone of a plant for the continuous casting of slabs, or other
similar flats products, the mould of which is provided with a
submerged casting nozzle having lateral discharge outlets directed
towards the narrow faces of the mould, which stirring method is
implemented by means of travelling magnetic fields generated by
multiphase inductors placed near the cast metal, characterized in
that a longitudinal liquid metal flow is forcibly established in
the said secondary cooling zone, said forced flow being localized
in the middle region of the cast product as two opposing collinear
streams.
[0027] This one naturally establishes a circulation of the entire
liquid metal in the secondary, having the configuration of a
"four-leaf clover" with two upper lobes and two lower lobes, the
upper lobes of which extend into the mould right up to the level of
the jets emanating from the discharge outlets of the casting
nozzle.
[0028] According to a implementation form of the invention, these
two longitudinal opposing collinear streams in the central part of
the product, which move away from each other, are created in such a
way that the two upper lobes which extend into the mould right up
to the level of the jets emanating from the discharge outlets of
the casting nozzle merge concurrently with the said jets in order
to reinforce them.
[0029] According to another implementation form, these two
longitudinal opposing collinear streams in the middle part of the
product, which converge on each other, are created in such a way
that the two upper lobes that extend into the mould up to the level
of the jets emanating from the discharge outlets of the casting
nozzle are superposed counter-currently on the said jets in order
to slow them down.
[0030] According to one particular embodiment of the method, the
location of the longitudinal flow in the secondary is shifted
laterally towards one or other of the narrow faces of the cast
product so as to counteract the "left-right" asymmetry tendencies
of the metal movements within the mould.
[0031] According to one method of implementation, the longitudinal
metal flow in the middle region of the cast product is created as
two opposing collinear streams by means of collinear moving
magnetic fields that travel longitudinally in the said central
region, either coming closer together, or further apart.
[0032] According to the preferred implementation, the longitudinal
metal flow in the middle region of the cast product is created as
two opposing collinear streams by means of collinear moving
magnetic fields that travel transversely over the width of the cast
product, either coming closer together from the edge towards the
centre of the cast product, or moving further apart from the centre
towards the edge of the cast product.
[0033] According to another preferred implementation form, the
travelling magnetic fields are generated by means of multiphase
linear inductors that are placed facing the large facess of the
cast product.
[0034] As another implementation form, the inductors are supplied
with electric currents of different intensities, so as to vary, in
a different manner, the action on the two opposing collinear metal
streams created by the travelling magnetic fields that they
generate.
[0035] The term "collinear" applied to the travel of the fields or
to the metal flows should be understood to mean that the magnetic
fields, or alternatively the streams of metal, do not travel
parallel to one another but instead travel along the same line, in
the manner of two collinear vectors as opposed to two parallel
vectors.
[0036] As will have been understood, the invention consists, in its
principal basics, in creating, in the secondary cooling zone, a
"stirring cross" having two transverse branches and two
longitudinal branches. The transverse branches (or horizontal
branches if it is assumed that the casting axis is vertical)
develop across the width of the cast product and the two
longitudinal (or vertical) branches develop within the central
region (usually the axial region) of the cast product.
[0037] Indeed, this "stirring cross" in the secondary zone of the
cast machine leads to the development of a recirculation flows in
the liquid pool in the form of a quadrilobate configuration, and
then creates an global configuration of the movements that also get
to the mould region, such that the aforementioned objectives
intended by the invention are reached.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0038] The invention will be more clearly understood and other
aspects will become more clearly apparent in the light of the
description that is given with reference to the appended plates of
drawings in which:
[0039] FIGS. 1 to 4 are representative of the prior art, already
considered above.
[0040] More precisely:
[0041] FIG. 1 is a standard diagram showing, in summary form and as
a vertical central section parallel to the large faces of the
mould, the known map of the circulatory movements of the liquid
metal entering a mould for the continuous casting of slabs via a
submerged nozzle provided with lateral discharge outlets that open
towards the narrow faces;
[0042] FIGS. 2a, 2b.sub.1 and 2b.sub.2 are diagrams, in two views
(on the left in perspective and on the right in cross section), of
known in-mould electromagnetic stirring modes for the continuous
casting of slabs with a submerged nozzle having lateral outlets
(cf. FIG. 1) by means of linear multiphase inductors located on
either side of the nozzle on each large face of the mould and
producing traveling magnetic fields that travel horizontally in
opposed directions, pairwise, over the same large face, either in
the same direction as the discharging jet of metal to which the
field is applied (FIG. 2b.sub.2), or in the opposite direction
(FIGS. 2b.sub.1 and 2a);
[0043] FIG. 3 is a simplified diagram showing, in perspective, a
slab during continuous casting as it can be seen in the secondary
cooling zone of the casting machine. This zone is provided with a
pair of linear inductors facing each other on each side of the
product over the width of the latter and generating a magnetic
field gliding horizontally so as to produce a "butterfly wings"
shaped electromagnetic stirring mode known for example from the
aforementioned document FR 7220546;
[0044] FIG. 4 is a diagram similar to the previous one in FIG. 3,
but showing a "triple roll" electromagnetic stirring mode, such as
that produced for example by implementing the teaching of the
aforementioned document FR 8210844;
[0045] The other figures, numbered 5 to 9, are specific to the
invention.
[0046] More precisely:
[0047] FIG. 5 is a general diagram, seen in axial vertical section
parallel to the large faces of a mould for the continuous casting
of slabs, the said mould being provided with a submerged nozzle
having lateral discharge outlets that open towards the narrow
faces, and showing the principle of a global stirring in the form
of a four-leaf clover in the secondary cooling zone according to
one of the implementation modes of the invention in which the
opposing longitudinal streams move away from each other, and the
map of the circulatory movements of the liquid metal that results
therefrom within this zone just below the mould;
[0048] FIG. 6 is a diagram similar to that of FIG. 5, but in the
case in which the in-mould flow mode is no longer of the "double
roll" type but is of the "single roll" type;
[0049] FIG. 7a is a diagram which, on the basis of a repeat of the
FIG. 5, shows by means of implementing the stirring in the form of
a four-leaf clover by means of linear inductors having a
horizontally travelling magnetic field;
[0050] FIG. 7b is a diagram similar to FIG. 7a, but illustrating
another embodiment of implementing the invention, this time using
linear inductors having a vertically travelling magnetic field;
[0051] FIG. 8 is also a diagram which, on the basis of a repeat of
the FIG. 5, illustrates a preferred embodiment of the invention,
setting up a complementary in-mould flow in "double roll" mode by
means of linear inductors generating a horizontally travelling
field, which act directly on the jets of metal discharging from the
outlets in the casting nozzle; and
[0052] FIG. 9 illustrates another implementation of the invention
which consists in creating opposing longitudinal streams of metal
in the middle part of the cast product, these no longer being
divergent but convergent.
DETAILED DESCRIPTION OF THE DRAWINGS
[0053] It should be reminded that FIGS. 1 to 4 were used to support
the explanation of the prior art already made at the beginning of
this document. They will therefore not be referred to again in the
following text.
[0054] In FIGS. 5 to 9 representative of the mode of stirring in
the secondary cooling zone specific to the invention in these two
implementation modes (divergent or convergent metallic streams at
the middle), the travelling magnetic fields, just like the linear
inductors that produce them, are represented by thick vertical or
horizontal arrows. The convective movements produced are themselves
shown by their main paths in the form of lines carrying arrowheads
indicating the direction of circulation of the movement over the
carrying path. The solid lines represent active convection zones,
and therefore circulation zones subjected to the action of the
travelling magnetic fields. The broken lines represent the passive
convection zones, in other words recirculation zones which are
necessarily complementary to the active zones in order to close the
loop of the movements.
[0055] In these figures, the same elements are denoted by identical
references. Where necessary, in order not to unnecessarily overload
certain figures, recurrent references have not been indicated so as
to make the essential elements of the invention shown in these
figures clearer.
[0056] Each of the figures shows a continuous slab casting mould I
followed beneath it by the secondary cooling zone 2 of the casting
machine, here intentionally shown without the support rolls in
order not to unnecessarily reduce the clarity of the drawing. Since
the views are in a plane parallel to the large faces of the mould,
only the narrow faces are visible at 3 and 3', these faces
determining the narrow sides 18, 18' of the cast product 6. Since
the large faces are in the plane of the figures, they are not
referenced in the figures. Moreover, for greater clarity, the
reference 6 will denote either the cast slab itself or its
still-liquid core, more generally called "liquid pool".
[0057] A submerged nozzle 4 centred on the casting axis A (which is
coincident here, as is conventionally the case, with the
longitudinal axis of the cast product) supplies the mould with
molten metal from a tundish (not shown) located above it. This
nozzle is provided with lateral discharge outlets 5 and 5' each
facing one or other of the narrow faces 3 and 3' respectively. The
size of the cast product is determined by the inside dimensions of
the mould that defines the casting space into which the molten
metal enters in the form of jets 7, 7' discharging from the outlets
of the nozzle 4, conventionally along a more or less horizontal
mean direction, or slightly inclined downwards. The cast product
thus advances from the top, level with the meniscus 8, downwards,
in the extraction direction of the casting machine, along the
vertical or along a curved path, in a plane orthogonal to that in
the figures, at an extraction rate (casting rate) usually of the
order to one metre per minute. As it advances, the product
progressively solidifies from its periphery up to the centre, by
extraction of its internal heat, firstly into the mould 1 in
contact with the cooled copper walls and then in the secondary
cooling zone 2 under the effect of the water spray rails.
[0058] It will be reminded that the metallurgical length (or depth
of the liquid pool) is conventionally defined as the difference in
the dimensions along the vertical between the level of the free
surface of the cast metal in the mould (or meniscus) and that of
the bottom of the liquid pool below the secondary cooling zone, at
the point where the finishing solidification fronts, which develop
over each of the large faces of the cast product as the
solidification progresses, meet.
[0059] Located arbitrarily along the longitudinal axis of the
product (which is coincident with the casting axis A), about 3 or 4
m below the meniscus 8, and therefore within the secondary cooling
zone 2, is a point P that will be termed the centre of the
"stirring cross" 9 which represents the specific creation of the
invention. This stirring cross 9 is a cross with four branches,
these being collinear in pairs, namely two longitudinal (and here
vertical) branches 10a, 10b, forming a pair aligned with the
casting axis A, and two transverse (and here horizontal) branches
11a, 11b forming a pair that develops over the width of the cast
product. In each of the two branches of any one pair, the liquid
metal stream circulates therein, pairwise, in opposite directions.
Moreover, the circulation of the stream in one pair is in the
opposite direction to that of the other pair.
[0060] Owing to the necessarily "finite" dimensional character of
the cast product, these branches, as may be seen, are as it were
connected together by recirculation loops in order to form an
overall flow that develops in the plane of the large faces of the
cast product in a four-leaf clover configuration, the leaves
constituting the lobes L1, L2, L3, L4, the upper two of which, L1
and L4, extend up to the mould level with the discharge jets 7 and
7'.
[0061] Thus, in the stirring mode shown in FIGS. 5 to 8, the pair
of vertical branches is on a "divergent" convection type--the
streams of metal move away from each other from the centre P. One,
10a, flows away towards the mould 1 lying above it while the other
10b flows away downwards, in the direction of extraction of the
cast product, towards the closure point of the liquid pool. In the
horizontal pair 11a, 11b, the convection of the metal is therefore
of a "convergent" type--the metal streams flow towards each other
in the direction of the centre of confluence P, flowing from the
small faces of the product towards the longitudinal axis A.
[0062] As already mentioned, the metal streams that form these
branches are created by travelling magnetic fields, which are
themselves generated by linear inductors placed in the immediate
vicinity of the cast product facing these large faces (preferably
both sides). Of course, it is unnecessary for the two pairs of
branches to be simultaneously activated by the magnetic fields.
Only one may be activated, for example the vertical branches 10a,
10b, the other branches 11a, 11b then becoming, of course, the site
of recirculation by reaction, since the centre P acts as a current
passage node that maintains the mass flow rates and the movement
quantities, and vice versa.
[0063] However, in this first stirring mode of the invention, it is
important for the vertical branches 10a and 10b to flow away from
each other, as shown in FIGS. 5 to 8. In the upper lobes L1 and L4
that are close to the mould, the metal rises along the centre and
descends along the narrow faces, the opposite being the case in the
lower lobes L2 and L3.
[0064] Under these conditions, it turns out that the implementation
of the invention maximizes the exchange of metal material between
the bottom and top of the liquid pool. Firstly, the circulation of
metal in any one lobe takes place in the direction of rotation
opposite to that established in the two closest neighbouring lobes.
Secondly, since the force of the casting jets 7 and 7' is then
systematically reinforced by the co-current rising central flux
10a, the recirculation loops L5 and L6 in the mould near the
meniscus 8 are in turn reinforced. Consequently, the "double roll"
mode L5, L1, L4 and L6 present within the mould are thus
additionally stabilized.
[0065] It will therefore be readily understood that any liquid
metal element (conceptually isolated at an arbitrary point along
the metallurgical length) will have a high probability of being
present, by randomly following the successive ascending or
descending running, at least once in the mould before re-descending
if it is initially in the secondary cooling zone, and vice versa if
it is initially chosen to be in the mould, it being understood that
overall the element will necessarily undergo a mean downward
displacement in the direction of extraction with a mean speed equal
to the casting speed. In other words, this implementation of the
invention maximizes the exchange of molten metal material between
the hot zones of the mould and those cooler zones of the secondary
cooling zone and does so by reinforcing, in the mould, the known
means suitable for stabilizing the "double roll" mode.
[0066] Such an exchange contributes in particular to better removal
of the excess heat and to the initiation of early and ample
equiaxed solidification of the metal, without any risk of
disturbing the in-mould flow mode, by instead reinforcing the
stability of the "left-right" symmetry of the movements on either
side of the nozzle, and to do so whatever the local mode present,
namely "double roll" (cf. FIG. 5) or "single roll" (cf. FIG. 6),
and therefore counteracting the natural random tendency for
transition from one mode to the other.
[0067] As already mentioned, the branches 10 and 11 of the stirring
cross 9 are generated by the action applied at these points by
travelling magnetic fields. The lines of force of these fields are
orthogonal to the surface of the cast product, or at the least have
a predominantly orthogonal component, in order to maximize the
electro-magnetic coupling with the liquid metal.
[0068] It is well known that such fields can be easily produced by
conventional multiphase linear inductors.
[0069] FIG. 7a illustrates a first implementation of the invention
in which two identical linear inductors 12 and 13 are placed
horizontally at the same vertical level on the casting machine
(collinear inductors) on either side of the casting axis and
mounted in opposition so as to create collinear magnetic fields
travelling transversely over the width of the cast product, from
the small sides 18, 18' towards the centre.
[0070] These inductors are advantageously designed so as to each
generate a travelling magnetic field, in an active convection
branch (11a or 11b), having a length equal to slightly less than
one half of the half-width of the cast slab 6.
[0071] In this case, the driving force for the stirring is given by
the convergent transverse branches 11a, 11b of the stirring cross,
and the longitudinal diverging flows 10a, 10b are then obtained
after passing the point of confluence P.
[0072] FIG. 7b illustrates a second type of implementation of the
invention, equivalent to the previous one as regards the effects
obtained. According to this second implementation, the linear
inductors 14 and 15, mounted collinearly but in opposition, are
placed vertically along the casting axis. In this way, the vertical
branches 10a and 10b (the presence of which within the secondary is
at the very basis of the invention) are this time activated
directly, the upper inductor 14 then generating a magnetic field
travelling towards the top of the casting machine in the direction
of the mould, the lower inductor 15 producing a field that travels
downwards towards the bottom of the pool.
[0073] FIG. 8 illustrates a preferred embodiment of the invention.
This consists in converting the upper edge of the upper
re-circulating lobes L1 and L4 (which reinforce the casting jets 7
and 7') into active convection zones. To do this, added to the pair
of inductors already present in the secondary cooling zone, for
creating the stirring cross 9, are two additional linear inductors
16, 17 generating horizontally travelling fields, these two
inductors being placed collinearly on either side of the nozzle 4
level with the jets of metal 7 and 7' discharging from the outlets
5 and 5' and travelling co-currently with the said jets, from the
nozzle towards the narrow faces 3, 3' of the mould 1. The effect of
convergence between the jets and the central flow rising up from
the bottom is thus further enhanced, and consequently the local
in-mould "double roll" mode likewise.
[0074] FIG. 9 is similar to FIG. 5 but is distinguished therefrom
however in an essential manner by the fact that the directions of
circulation of the metal in each of the four branches of the cross
9 are reversed. The FIG. 9 thus illustrates the second main
implementation form of the invention, which consists in creating
opposing longitudinal collinear streams 20a, 20b in the central
part of the cast product 6, which this time converge on each other
towards the point P so as to provide an overall circulation of the
liquid metal that is extended, in the mould 1, by flows rising
along the small sides 18, 18' up to level with the jets of metal 7,
7' emanating from the discharge outlets 5, 5' in the nozzle, which
they oppose as a counter-current in order to brake them.
[0075] Overall there is again a stirring configuration in the
secondary cooling zone consisting of four lobes L1 to L4, the loops
of which therefore rotate in opposite directions to those of the
first implementation. However, because of the opposing effect of
the upper lobes L1 and L4 on the jets 7 and 7', the downward return
flows of the metal in the central part of the liquid pool are less
channelled and confined, instead much more diffuse and dispersed in
that section of the product than in the said first variant.
[0076] It will be understood that these two main implementation
modes are in fact only two different and complementary facets of
the same invention and may be jointly present when implementing the
stirring method. It will in fact be easy to modify, in terms of
dynamics, the directions of travel of the acting magnetic fields,
for example by reversing the polarities of the inductors that
produce them, so as to be able, on demand, to brake or accelerate
the running of the casting jets 7, 7' by acting on the stirring
localized in the secondary cooling zone, far away from these
jets.
[0077] It will therefore be seen that a key advantage of the
invention is that it ensures good top/bottom exchange in the liquid
pool while still being able to act remotely on the casting jets in
the mould, and to do so by a simple and unsophisticated arrangement
of the electromagnetic stirring equipment, the components of which
are widely available commercially.
[0078] As will have been understood, the invention consists, in
summary, in judiciously using the electromagnetic stirring means
currently available in order to make, in the secondary cooling
zone, a cut in the long direction of the product into two
juxtaposed strands and, in each strand, to install a "butterfly
wings" type stirring configuration. By doing this, an overall flow
system is created in the secondary cooling zone consisting of four
lobes, the core of which is the "stirring cross" 9 with its centre
P.
[0079] Preferably, for obvious reasons of symmetry, this division
into two strands will take place at mid-width of the cast product,
that is to say along the longitudinal axis of the latter, as this
axis generally coincides with the casting axis.
[0080] This said, it will be sufficient to unbalance the stirring
forces between the two transverse branches 11a, 11b, for example by
a differential adjustment of the intensities of the electrical
currents supplying the inductors 12, 13 in order for the central
position of the centre P to be shifted laterally towards one narrow
face, 5, or towards the other, 5', and thus to obtain a more
selective effect on the in-mould movements on one side of the
nozzle than the other.
[0081] Likewise, a similar imbalance in the vertical branches 10a,
10b will make it possible, using given stirring equipment, to cause
an upward or downward displacement from the centre P of the
stirring cross without having to modify the position of this
equipment on the casting machine.
[0082] If it is desired to be able to use both these options of
adjusting the position of the centre P of the stirring cross, it
will admittedly be necessary to provide the secondary cooling zone
with equipment consisting of four inductors so as to be able to
electromagnetically activate each of the four branches 10a, 11b,
11a and 11b.
[0083] Whatever its mode of implementation, the invention provides
overall stirring of the metal over the metallurgical length capable
of ensuring both thermal and chemical uniformity between the top
and bottom of the liquid pool without correspondingly being
deprived of the beneficial effects specific to stirring in the
mould and stirring in the secondary cooling zone respectively, and
without disturbing, indeed by stabilizing, the local flow mode in
the mould.
[0084] It goes without saying that the invention is not limited to
the examples described above, rather it extends to many
implementation forms or equivalents provided that its definition,
given in the claims that follow, is respected.
[0085] Thus, for example, although the linear inductors to be used
conventionally have a plane structure, this arrangement is only a
preferred one. Also suitable may be inductors of curved shape in
order to better match the shape of the surface of the slab at the
point where they are placed along the metallurgical length.
* * * * *