Stirring-roller For A Continuous Slab-casting Machine

Abstract

A split stirring roll including two half-rolls aligned at a distance from one another, each incorporating a polyphase linear inductor mounted fixedly a short distance from outer tubular bodies of each half-roll to arrange between them an annular space for circulation of a cooling fluid; each inductor can generate a traveling magnetic field sliding along the width of a cast slab. The two half-rolls are mounted externally in end rolling bearings and internally in a common intermediate rolling bearing attached between them, the inductors including narrowed-diameter hollow terminal portions to rest in the three rolling bearings and communicate with a cooling circuit including the annular spaces. The split stirring roll is configured for continuous casting of extra-wide slabs and can form a preferred tool for controlled electromagnetic stirring of molten metal for optimized exchange of material between a top and bottom of a casting machine.

Description

[0001] The invention applies to the area of continuous casting of flat metal products, such as slabs, particularly made of steel. More precisely, it relates to the controlled electromagnetic stirring of the molten metal still liquid in the solidification pool by inductors housed coaxially in supporting and guiding rolls of the secondary cooling zone of the continuous casting machine, made hollow for this purpose.

[0002] The first embodiments of such rolls, called "stirring rolls" already date from the state of the 1970s (FR No. 2185467). Schematically, a stirring roll consists mainly of:

[0003] an element in axial rotation designed to come into rolling contact with the large surface of the product being cast, consisting of a tubular body (with a diameter which is in principle equal to, in any case close to the diameter of the usual supporting roll that it replaces), this tubular body being held at its ends by two spindles, of generally frustoconical shape, the small bases of which are housed in two end rolling bearings attached to the rigid structures of the frame of the casting machine;

[0004] an electromagnetic stirring equipment consisting of a polyphase linear inductor able to generate a magnetic field sliding along its axis and lodged in the free internal space of the tubular body, coaxially with the latter, means for immobilizing the inductor in axial rotation being advantageously provided, as proposed for example by FR No. 80/14487;

[0005] and a cooling circuit by circulation of a cooling liquid, usually water, in order to secure the heat level of the tubular body in contact with the hot slab and of the electromagnetic stirring equipment.

[0006] It should be noted that the inductor is of the three-phase or two-phase type with one or more pairs of magnetic poles per phase of the external electricity supply to which its connection means connect it, and that it therefore generates a movable magnetic field travelling along the axis of the roll, either in one direction or in the other, depending on the order of connection of these windings to the phases of the electrical supply. The travelling speed of the field depends, for its part, on the pole pitch of the inductor (a parameter fixed by construction) and on the frequency of the electric current that supplies it and which, for its part, is the variable for the adjustment of this speed.

[0007] The application of the stirring rolls to a continuous casting machine of slabs consists schematically in subjecting the slab, in one or more zones of the machine situated downstream of the ingot mould, to one or more travelling magnetic fields, sliding horizontally along the width of the slab, each in a determined direction. The still liquid portion of the molten metal situated in the heart of the slab being cooled is therefore driven in a direction that is identical to that of the sliding of the magnetic field acting on it.

[0008] Therefore, depending on the circulation movements that it is desired to impress on the liquid metal, the stirring rolls can be placed in groups, face-to-face on either side of the large faces of the slab at one and the same height level (see FIG. 1, coming from the aforementioned No. 2185467), or superposed on one and the same side of the slab, or distributed on either side of the slab at different stages of the machine, etc.

[0009] A preferred use, still normal these days, of such stirring rolls on a machine for the continuous casting of steel slabs is clearly set out for example in EP-A-0 097 561 dating from 1982. By conveniently distributing the stirring rolls over the metallurgical length of the slab, the user thus manages to create within it circulatory movements of the molten metal, the overall configuration of which, called "in triple zero" shown in FIG. 2, is developed in parallel with the large faces of the slab and tends to homogenize the liquid metal pool, both thermally and chemically, between the top and the bottom of the casting machine via loops of metal streams which are established in parallel with the large faces of the slab, with, along the two short lateral faces, an ascending branch of one side and a descending branch of the other, on either side of the stirring roll.

[0010] It is now well accepted that such a double homogenization of the molten metal ensures that a cast product is obtained that has a good internal health allied with a widened equiaxe solidification structure favourable to a reduction or even a removal of the central porosities and of the central segregation.

[0011] However, it is known today that this double homogenization of the cast metal between the top and the bottom of the machine can be substantially improved by installing within the liquid metal a central main current, rising or descending, but located at mid-width of the slab or in this vicinity, rather than only along the short lateral faces where it is opposed by the roughness of a solidification front of the cast product that is present virtually in all parts in this location.

[0012] However, stirring rolls do not allow it. Through their design, they are capable of making only circulation loops that occupy the whole width of the slab, therefore with vertical branches, which are only laterally localised, along and against the short faces.

[0013] The question that arises therefore is that of producing a new item of equipment for the controlled electromagnetic stirring of the liquid metal which can produce a central vertical circulation of the molten metal in the core of the cast slab, without harming or overly disrupting existing slab casting machines.

[0014] The present invention proposes to achieve this by converting a conventional stirring roll into a double stirring roll or, in other words, into a "split stirring roll".

[0015] As the mechanical rolls on a slab caster that are divided are commonly called "split rolls", that's why, in this technical specification, they are called "split stirring rolls".

[0016] Accordingly, the subject of the invention is a split stirring roll designed to be mounted on a machine for the continuous casting of flat products with a wide cross section, such as a steel slab, as a replacement of a usual supporting and guiding roll, the said split stirring roll comprising:

[0017] an element in axial rotation designed to come into rolling contact with the large surface of the cast slab, the said element consisting of at least one tubular body held at its ends by two spindles, of generally frustoconical shape, the small bases of which rest in two end rolling bearings;

[0018] an electromagnetic stirring equipment occupying the internal free space of the tubular body, coaxially with the latter;

[0019] and a cooling circuit by circulation of a cooling liquid, usually water, in order to secure the heat level of the tubular body in contact with the hot slab and of the electromagnetic stirring equipment,

characterized in that said electromagnetic stirring equipment consists of two polyphase linear inductors (40, 50) placed end-to-end and aligned, electrically independent and each capable of generating a travelling magnetic field sliding independently one of the other.

[0020] As will be undoubtedly understood, nothing visible will change on the continuous casting machine relative to an installation fitted with usual stirring rolls. However, simply by conveniently connecting the windings of the two internal inductors to the external electricity power supply so that they produce magnetic fields sliding in opposite directions, for example convergent directions, a main rising circulation of the molten metal will necessarily be generated within the cast slab in the middle region of the slab beginning where the two inductors meet. If the two inductors are of identical size, the sought rise of the molten metal towards the ingot mould will naturally locate itself just in the portion located at the middle of the large faces of the slab.

[0021] The multi-purpose nature of the electromagnetic stirring equipment in this regard should be emphasized here: the desired homogenization of the molten metal within the liquid pool in the slab can be obtained in the same way if the two inductors are adjusted to produce travelling magnetic fields sliding in opposite divergent directions, that is to say moving away from one another from the middle joining position of the inductors. The main central circulation of the liquid metal will then flow down towards the point of junction of the two inductors on the upper side and upwards on the lower side (see FIG. 6b)

[0022] According to a variant embodiment, the electromagnetic stirring equipment has a magnetic core (or yoke) common to the two inductors.

[0023] According to another embodiment, each inductor has its own magnetic core, distinct from and distant from that of the other inductor.

[0024] In the latter case, and according to a particularly advantageous variant embodiment, the split stirring roll with two inductors at a distance according to the invention comprises an element of rolling contact with the surface of the slab on which it rests, consisting of two distinct aligned tubular bodies, and a third rolling bearing that separates them, preferably situated in the middle of the width of the slab, and delimiting, with the end bearings, two half stirring rolls, this intermediate bearing receiving the small abutted bases of two other spindles, of generally frustoconical shape similar to those of the spindles engaged in the end bearings and of which the large bases are attached to the ends facing the two tubular bodies.

[0025] As will also be understood here, this variant with an intermediate bearing is totally appropriate for satisfying a new nascent requirement in the field of the continuous casting of slabs, namely that of the casting of wide slabs, greater than 2 m wide, that can now exceed 50% and more of the conventional widths, which are of the order of 1.6 m.

[0026] Moreover, the known stirring rolls conventionally have diameters of 300 mm and more. But nowadays rolls of smaller diameter (of the order of 240 mm) are increasingly used that can be easily used in conjunction with small-diameter supporting rolls which have the advantage of being able to be more numerous on one and the same machine height portion and, therefore, perform their function better.

[0027] Usually, machines for the continuous casting of steel slabs are designed for casting products the width of which is conventionally between approximately 0.9 and 1.6 m, and even sometimes up to 2.0 m. But, in a constant search to increase the productivity of steel-works, the current trend is to hope to have machines that can cast products of greater width, for example of 2.4 m. This would involve having stirring rolls that are correlatively longer (let us say 2.5 m and more in order to give an idea).

[0028] However, the structure of current stirring rolls of relatively small diameter would not make it possible to prevent them from sustaining more or less quickly a prohibitive bend because of the distance that is too great between the bearings, due to the forces to which the rolls would be subjected in contact with the cast product.

[0029] It is therefore a supplementary object of the invention to be able thus to propose a new design of stirring rolls, with three rolling bearings that flank two half-rolls, and that could be used on machines for the continuous casting of very wide slabs, because they would not sustain deformations in service which would make them ineffective and would compromise their mechanical strength.

[0030] It should be understood that "half-roll" includes both the case in which two aligned tubular bodies forming the stirring roll in its entirety have equal lengths and the case in which they would have unequal lengths for reasons indicated below.

[0031] Each half-roll has its own electricity supply terminals for supplying the inductor that it encloses, preferably all grouped at the end of the half-roll concerned facing the outside of the machine.

[0032] On the other hand, the circulation of the cooling fluid is common to the two half-rolls, and the means that provide the circulation of the fluid in each half-roll are connected to one another inside the central bearing.

[0033] These features make it possible to produce a compact device that can easily find its place on the casting machine, because it is not necessary to place electric connections and fluid-supply ducts in the central zone of the segment supporting the stirring rolls and the usual rolls providing the support and the guidance of the slab when it descends in the casting machine.

[0034] A clarification must be made here on the subject of the terminology "split stirring roll" chosen to qualify the stirring rolls according to the invention. This terminology applies both to the embodiment with two half-rolls and to that with a single tubular body. The adjective "split" specifically relates to the qualifier "stirring" which precedes it in order to express the fact that the two internal inductors are electrically independent, without, for all that, necessarily being physically distant from one another. This technological option with separated inductors at a distance however remains, as has been said, designed and hence suited to the casting of extra-wide slabs, because of the presence of an additional intermediate rolling bearing that it underlies.

[0035] The invention will be better understood on reading the following description, making reference to the attached drawings of figures in which:

[0036] FIG. 1 is a schematic view illustrating the prior art represented by document FR No. 2185467 already cited to create a stirring of the liquid metal in the secondary cooling zone of the casting machine according to a configuration with a double loop, called "butterfly wings";

[0037] FIG. 2 is a schematic view similar to the preceding figure, but illustrating the prior art represented by document EP-A-0 097 561 already cited also in order to install a stirring of liquid metal according to a configuration with three loops, called "triple zero";

[0038] FIG. 3 is an overview, in a plane perpendicular to the large faces of the cast slab, of an embodiment of a split stirring roll according to the invention with three rolling bearings, one of which is intermediate, and two tubular bodies;

[0039] FIG. 4 is a view similar to FIG. 3, but illustrating an embodiment of a split stirring roll according to the invention with a single tubular body and with no intermediate rolling bearing;

[0040] FIG. 5 illustrates a first stirring mode with central circulation of cast liquid metal that can be achieved with the split stirring roll according to the invention, the portion 5a being a view in a plane perpendicular to the large faces of the cast slab, the corresponding portion 5b being a view in the plane of the large faces;

[0041] FIG. 6, in its two portions 6a and 6b, is a figure similar to FIG. 5 illustrating a second stirring mode with central circulation of cast liquid metal that can be achieved with the split stirring roll according to the invention;

[0042] FIG. 7, in its two portions 7a and 7b, is a figure similar to FIG. 6 illustrating another mode of stirring liquid metal, of the type rotating about the casting axis, that can also be achieved with the split stirring roll according to the invention.

[0043] FIGS. 1 and 2 have already been mentioned in the introduction of the present specification as representative of the prior art.

[0044] One will therefore self-limit in this instance to indicate that: [0045] a) FIG. 1 shows the circulation loops (in dashed lines) of the stirred molten metal, with main rises and falls along the short faces 1 and 2 which form over the whole width of the cast slab 3 under the action of two conventional stirring rolls 4 and 5 mounted facing one another, on either side of the large faces 6 and 7 of the slab being cast in the secondary cooling zone of the casting machine, the two stirring rolls producing magnetic fields which slide longitudinally in the same direction shown by the solid horizontal arrows 8 and 9; [0046] b) and FIG. 2 shows the circulation loops (in dashed lines) of the stirred molten metal, with main rises and falls along the short faces 1 and 2 which form over the whole width of the cast slab 3 under the action of two conventional stirring rolls, separated from one another, on one and the same large face 6 of the slab (in this instance on the intrados) and represented here only by the solid horizontal arrows 10 and 11 showing that the magnetic fields that they produce slide longitudinally in opposite directions.

[0047] In the FIGS. 3 and 4, to which reference is now made, the identical or equivalent elements are indicated with identical reference numbers, so that the following description can be read considering these two figures jointly.

[0048] In these figures, the tubular splited stirring roll 1 is attached to a rigid support 2 incorporated into the frame of the machine for the continuous casting of continuous-cast slabs and enclosing the usual supporting and guide rolls.

[0049] The cast slab 3 progresses perpendicularly to the plane of the figure from the ingot mould (not shown) to the bottom of the casting machine, and the split stirring roll 1 covers the whole width while remaining in rolling contact with its surface via a generatrix of the element in axial rotation that forms it.

[0050] According to the embodiment of the invention shown in FIG. 3, this element in rotation is formed by two cylindrical tubular bodies 6 and 12 at a distance from one another, therefore forming two stirring half-rolls 4 and 5 that are tubular, contiguous and aligned and of equal length in the example shown.

[0051] Three rolling bearings attached to the support 2 ensure the alignment and retention in axial rotation of these two half-rolls: two end bearings, the left bearing 9 and right bearing 15, and an intermediate bearing 10, in the central position here, facing the middle of the slab 3.

[0052] Each half-roll 4 or 5 incorporate a polyphase linear inductor, mounted coaxially with it, capable of generating a magnetic field sliding longitudinally. The left half-roll 4 contains the inductor 40; the right half-roll 5 contains the inductor 50.

[0053] In the rest of the description, the terms "left" and "right" will be used with reference to the device as it is seen in FIG. 3 or 4. Moreover, it is specified that everything that will be said concerning the left half-roll 4 and concerning its appendages will be exactly transposable to the right stirring half-roll 5, the design and the production of one being identical to that of the other, although, in certain cases, the user could opt for different lengths between them.

[0054] The cylindrical body 6, forming the left stirring half-roll 4, is held at its ends by two spindles, of the same general frustoconical shape, a left spindle 7 and a right spindle 8. Each spindle is mounted at one end of said body via its large base 7a, 8a, the small base 7b, 8b being, for its part, engaged in an end rolling bearing, respectively left 9 and intermediate 10, secured to the support 2 of the rigid frame of the casting machine.

[0055] As is usual, these rolling bearings 9 and 10 allow the stirring roll 4 to rotate freely about its longitudinal axis B under the effect of the rolling contact of its tubular body 6 with the slab 3 progressing towards the lower portion of the continuous casting machine.

[0056] However, the inductor 40 with a travelling magnetic field, centred in the tubular body 6 and taking up virtually the whole of the volume available inside the roll (except for an annular space 60 which separates it by a few mm, let us say 4 or 5 mm, from said body in order to allow the circulation of cooling water) is, for its part, prevented from rotating.

[0057] Accordingly, its left terminal extension 40a and right terminal extension 40b, of a smaller diameter than that of the inductor, form supporting shafts which rest, one in a flange with keys 11, placed on the outside of the left bearing 9, therefore after having traversed this bearing via the inside of the small base 7b of the spindle 7, and, the other, after having likewise traversed the small base 8b of the right spindle 8, in the intermediate bearing 10 in order to fit onto the facing extension, coming from the right inductor 50, via a male-female coupling (not shown) with splines and therefore removable simply by translation.

[0058] Similarly, the right stirring half-roll 5 comprises a cylindrical body 12 engaged between two spindles, a left spindle 13 and right spindle 14 (similar to the spindles 7 and 8) via their respective large bases 13a and 14a. The small base 14b of the right spindle 14 is inserted into a right end rolling bearing 15 secured to the support 2, and the small base 13b of the left spindle 13 is inserted into the central intermediate rolling bearing 10.

[0059] As shown in FIG. 3, the internal spindles 8 and 13 engaged in the intermediate bearing 10 but against one another. But this is not an absolute obligation, provided that their connection is sealed, because these parts form the cooling circuit of the assembly as will be seen below.

[0060] Each stirring half-roll 4, 5 has its own electric circuit providing the electric power supply to its inductor. The left stirring half-roll 4 therefore comprises, mounted on the end of the end bearing 9, a panel 16 with two pairs of connecting terminals 16' and 16'', having two pairs of terminals in order to provide the connection to the electricity network for the two-phase windings of the inductor 40.

[0061] It should be noted that the polyphase linear inductor used by the invention, which may be of two- or three-phases type, may use different numbers of electrical terminals, namely four (as two pairs) or three with a common neutral for a two-phases type, and six (as three pairs), or four with a common neutral for a three-phases type, it being nevertheless emphasized that, in the case of an electrical star connection of the inductor, its number of terminals will be reduced

[0062] To be noted also that an inductor of this type can schematically be made up of an alternating series of windings 41, 51 and of apparent circular magnetic poles 42, 52 coming from the internal magnetic core of the inductor, which also serves as a winding support for the windings in the reception spaces between two contiguous magnetic poles. All that protrudes from this ordered assembly are the electric connecting wires, or cables, 41a, 41b; 51a, 51b which exit and enter each series of windings that are interconnected in series-opposition (therefore connected to the same phase), and meet as close as possible the terminals of the connection panel.

[0063] It will be noted that, usually in the case of a single two-phase inductor covering the width of the slab, that is an inductor of approximately 1.6 to 2.0 m, the number of windings interconnected in series-opposition is four, that is having two pole pitches over the active length of the inductor.

[0064] Note also that, in the known case of a single stirring roll covering the entire width of the slab, the latter comprises a panel with electrical connection terminals at each of its ends, one receiving the wires of phase one, the other receiving the wires of phase two, or one receiving the input wires of the interconnected phase windings, the other receiving the output wires. On the other hand, in the case of a split stirring roll according to the invention, it is preferable to combine within one and the same panel all of the connection terminals relating to one and only one of the two inductors and to dedicate a given panel to the inductor that is closest to it.

[0065] This gives only one connection panel per inductor. In the case shown of a split stirring roll with an intermediate bearing 10, therefore with two distinct inductors at a distance from one another, it is understood that in this manner, it is not necessary to provide connection through the essential bearing 10, which would complicate the design and the mounting of the installation.

[0066] The right stirring half-roll 5 comprises, at the right end of the assembly, a connection panel 17 furnished with two pairs of terminals 17' and 17'', similar to the panel 16 which, dedicated to the left stirring half-roll 4, is on the left side of the installation.

[0067] As for the cooling of the stirring half-rolls 4, 5, this is provided, according to the invention, by a common circuit, comprising the following elements taken in an order corresponding to the direction of circulation of the cooling water: [0068] a first inlet waterbox 18, mounted (in the example shown) externally on the left rolling bearing 9 and in which the water for cooling the stirring rolls enters through a supply duct 19 connected to a pressurized source of treated water not shown; [0069] an axial passageway 20 of the bearing 9 connecting the waterbox 18 to the blind hole 21 drilled in the left terminal extension 40a of the left inductor 40, this drilled blind hole comprising a radial daughter drilled duct 22 which emerges in the annular space 60 already mentioned, arranged between the tubular body 6 and its concentric inductor 40 in order to be capable of providing a water circulation in the vicinity of the periphery of the stirring roll 4 and cool it during and after its contact with the hot slab 3; [0070] a second radial drilled duct 23 which collects the water that has circulated in the annular space 60 in order to bring it into a second blind axial hole 24, symmetrical with the blind hole 21, but provided in the right terminal extension 40b of the engaged left inductor 40, as already mentioned, via a sealed male-female fitting, with the left terminal extension 50a of the right inductor 50 within the intermediate bearing 10; [0071] a third drilled blind hole 25 arranged axially in this left terminal extension 50a and from which is tapped a third radial daughter drilled duct 26, similar to the daughter ducts above, and which emerges in the annular space 61 arranged between the tubular body 12 of the right stirring roll 5 and its concentric internal inductor 50 in order to be capable of providing a circulation of water in the vicinity of the stirring roll periphery 5 and to cool it during and after its contact with the hot slab 3; [0072] a fourth radial drilled daughter duct 27 opening into a fourth blind drilled axial hole 28 in the right terminal extension 50b of the stirring roll 5, a second axial passageway 29, equivalent to the left axial passageway 20, arranged in the axis of the right end bearing 15 in order to connect an outlet waterbox 31 furnished with a water discharge duct 30.

[0073] Note that, in the exemplary embodiment in question, it is a face of the wall of the waterbox, the inlet box 18 and outlet box 31, that forms the connection terminal panel 16 and 17 respectively, the connecting wires 42a, 42b; 51a, 51b for the inductors naturally being insulated in order to be able to join them while passing through the waterboxes, as is the case here.

[0074] For the slabs 3 of very great width (for example 2 400 mm), the split stirring roll 1 according to the exemplary embodiment that has just been described makes it possible to prevent deformations, particularly a bend, which would inevitably occur in the absence of the central bearing 10. The latter makes it possible to dispense with these deformations by providing each stirring roll forming the assembly with a relatively short length, comparable to or even less than that of the conventional stirring rolls used on machines for continuous casting of slabs of normal widths (up to 2 m and more frequently 1 600 mm).

[0075] The design of the split stirring roll 1 in its embodiment in two stirring half-rolls 4 and 5 that has just been described is not substantially modified relative to the usual design, except for: [0076] the fact that the electrical connections of a given inductor are all on the same side of the half-roll that incorporates this inductor; [0077] and the fact that it is necessary to provide means of communication between the cooling circuits of each stirring half-roll, for example by placing end-to-end and sealingly the two end spindles facing one another inside the intermediate central bearing in order to form a single sealed duct.

[0078] This solution consisting in providing a cooling circuit common to the two stirring half-rolls 4, 5 prevents having to place inlet and/or outlet water ducts in the central bearing 10, in the same manner as it is not necessary to install therein electrical connection terminals, as already mentioned above.

[0079] But the invention also recommends another embodiment of the split stirring roll, that is simpler than the previous one. Shown in FIG. 4, it is characterized by a single tubular body 6, as in the case of a conventional stirring roll, therefore without an intermediate rolling bearing.

[0080] This single tubular body encloses, mounted axially with it and a short distance away in order to arrange the annular separating space 60, two polyphase linear inductors, a left inductor 45 and right inductor 55, electrically separate from one another, but produced in a single piece, that is to say not at a distance because they have a single and common magnetic core (not shown) therefore without requiring a removable mechanical connection between them, nor any particular cooling circuit in this magnetic core.

[0081] Such an assembly of "left-right" inductors coupled mechanically does not differ visually from a single inductor. Simply, the connecting wires of the windings of one to the electrical supply phases will all preferably be directed to an electrical connection panel present at an end of the roll and the connecting wires of the second inductor all directed to the other connection panel mounted at the other end. In this instance, the connecting wires 45a, 45b of the left inductor 45 are directed to the closest panel 16 mounted on the left rolling bearing 9, and the connecting wires 55a, 55b of the right inductor 55 are directed to the closest panel 17 mounted on the right rolling bearing 15.

[0082] The split stirring rolls according to the invention, both in their embodiment with intermediate bearing and without, can be incorporated without particular difficulties in a segment of the structure of the caster conventionally also supporting other usual rolls for supporting and guiding the slab. The reader will have understood that the split stirring rolls with single tubular body, therefore without an intermediate bearing according to one embodiment of the invention, must be preferably reserved for the casting of slabs of conventional width, that is to say up to 1 600 mm just for giving idea. Beyond that, it would be preferable to opt for split stirring rolls with two tubular bodies and an intermediate bearing. As a example, for the casting of wide slabs, namely 2 400 mm wide, it is possible to provide split stirring rolls consisting of two half-rolls, such as 4 and 5, which are of 1150 mm length each, with an intermediate rolling bearing that is approximately 200 mm wide. The diameter of the stirring rolls 4, 5 can, for its part, be of the order of 230 mm.

[0083] As a variant, it is possible to provide that, for at least some of the split stirring rolls with two half-rolls and intermediate bearing fitted to a continuous caster of slab, the lengths of the two tubular bodies are unequal. Specifically, the absence of contact between the central bearing and the slab means that the portion of the slab passing opposite this bearing is not cooled and not supported. Therefore, providing split stirring rolls having contiguous stirring rolls of unequal length and having their intermediate bearing offset between two successive split stirring rolls will make it possible to ensure that it is not always the same portion of the width of the slab that is affected by this absence of cooling and support. This will, as required, improve the evenness of cooling of the slab and prevent local bulges.

[0084] Now, with reference to FIGS. 5, 6 and 7, some examples will be given illustrating the new possibilities offered to controlled electromagnetic stirring in the secondary cooling zone of a machine for the continuous casting of slabs by the split stirring rolls according to the invention when several of them are used in a coordinated manner.

[0085] In this instance, they will be, in the three examples currently being considered, used as pairs 70a and 70b, placed at the same level on the metallurgical height, facing one another, on either side of the large faces of the cast slab.

[0086] In order to simplify, suppose that they are split stirring rolls of the type with a single tubular body, therefore without intermediate rolling bearing, each incorporating two monolithic inductors 71 and 72 in the roll 70b, and 73 and 74 in the paired roll 70a. The two inductors facing one another on distinct rolls will be connected to one and the same polyphase electric power supply. In this instance, via their respective connection boxes not shown, the inductors 71 and 73 are connected to the power supply 78 and the inductors 72 and 74 are connected to the power supply 79. The latter will preferably be identical to the power supply 78, but nevertheless separate in order to be able to easily ensure the various desired adjustments concerning the direction of sliding of the magnetic fields, as will be seen.

[0087] The slab 3 is first formed in a continuous casting ingot mould 75 with the aid of an immersion nozzle 76 centred on the casting axis A and bringing the molten metal to the ingot mould via lateral outlet openings. The slab leaves the ingot mould to join, below, the secondary cooling zone while progressing towards the bottom of the casting machine at a controlled speed so as to keep at a controlled level the height position of the liquid meniscus 77 within the ingot mould. It is in this secondary cooling zone of the casting machine that the retention and guide rolls are found, including the two split stirring rolls 70a and 70b of the invention.

[0088] If the operator wishes to produce a stirring configuration ensuring the establishment of a vertical circulation of the liquid metal in the middle of the liquid pool, all he has to do is to connect the two inductors in each split stirring roll so that they produce traveling magnetic fields sliding in opposite directions in one and the same roll and sliding in the same direction in the two facing rolls on either side of the large faces of the slab (or which are facing one above the other on the same side of the slab, according to a configuration not shown here).

[0089] More precisely, as shown in FIG. 5a, if he wishes to obtain above the stirring rolls a rising circulation of the molten metal at the centre of the slab, the operator will give the travelling fields converging opposite directions, namely sliding on a given inductor 71 (and 73) while going towards its companion inductor 72 (and 74 respectively) of the same roll 70b (and 70a).

[0090] Conversely, as shown in FIG. 6a, if he wishes to produce above the stirring rolls a go dawn circulation of the molten metal at the centre of the slab, the operator will give the travelling magnetic fields divergent opposed directions, namely sliding on a given inductor 71 (and 73) while moving away from its companion 72 (and respectively 74) of the same roll 70b (and 70a).

[0091] Moreover, he can switch from one configuration to the other, even during the same casting, simply by reversing one phase via a flip-flop provided in each of the electrical power supplies 78 and 79.

[0092] In the case of thick slabs in particular, the operator might also wish, permanently or temporarily, to impose a rotary stirring in a plane perpendicular to the casting axis A. In order to achieve his objective, he will then simply have to modify the electrical connections so that, as shown in FIG. 7a, the two magnetic fields slide in the same direction on the two inductors of one and the same roll and in the opposite direction between the two rolls.

[0093] It goes without saying that the invention is not limited to the exemplary embodiments described, but that it extends to many variants or equivalents to the extent that its definition given by the following claims is respected. For example, it will have been understood that the term "spindle" used to qualify the supporting elements mounted at the ends of the or both tubular bodies forming a split stirring roll will have to be understood to cover any transmission member capable of providing a sealed rigid connection between the tubular bodies and the rolling bearings that support them in free axial rotation.

Claims

1-7. (canceled)

8. A split stirring roll for a machine for continuous casting of large cross section flat products or a slab, comprising: an element in axial rotation configured to come into rolling contact with a large surface of the cast slab, the element including at least one cylindrical tubular body held at its ends by two spindles, of generally frustoconical shape, small bases of which rest on two end rolling bearings attached to rigid structures of the frame of the casting machine; an electromagnetic stirring equipment filling up an internal free space of the tubular body, coaxially with the tubular body and at a short distance to arrange an annular space between them; a cooling circuit, including the annular space, to circulate a cooling liquid to secure a heat level of the tubular body in contact with the slab and of the electromagnetic stirring equipment; and electric connection terminals to ensure electric connection of an item of the electromagnetic stirring equipment with an external electricity supply, wherein the electromagnetic stirring equipment includes two polyphase linear inductors placed end-to-end and aligned, electrically independent and each capable of generating a travelling magnetic field sliding independently of the other.

9. A split stirring roll according to claim 8, wherein the item of electromagnetic stirring equipment includes a magnetic core that is common to the two inductors that form it.

10. A split stirring roll according to claim 8, wherein each of the two inductors forming the item of electromagnetic stirring equipment includes its own magnetic core distinct from that of the other inductor.

11. A split stirring roll according to claim 8, wherein the element in axial rotation, configured to come into rolling contact with the surface of the cast slab on which it is resting, includes two distinct aligned cylindrical tubular bodies and includes a third intermediate rolling bearing between the two other end rolling bearings and delimiting with the two other end roller bearings, two half stirring rolls situated on either side of the intermediate bearing and each formed by one of the two tubular bodies, each tubular body being held at its ends by two spindles of generally frustoconical shape, one of small bases of which rests in an end rolling bearing and the other rests in the intermediate rolling bearing.

12. A split stirring roll according to claim 10, wherein the element in axial rotation, configured to come into rolling contact with the surface of the cast slab on which it is resting, includes two distinct aligned cylindrical tubular bodies and includes a third intermediate rolling bearing between the two other end rolling bearings and delimiting with the two other end roller bearings, two half stirring rolls situated on either side of the intermediate bearing and each formed by one of the two tubular bodies, each tubular body being held at its ends by two spindles of generally frustoconical shape, one of small bases of which rests in an end rolling bearing and the other rests in the intermediate rolling bearing.

13. A split stirring roll according to claim 12, wherein the intermediate rolling bearing is situated in a middle of a width of the slab.

14. A caster for continuous casting of metal slabs fitted with stirring rolls, wherein at least two of the stirring rolls are split stirring rolls according to claim 8.

15. A caster for continuous casting of metal slabs according to claim 14, wherein the two split stirring rolls are placed facing one another, either on either side of large faces of the cast slab, or one above the other on one and a same large face, and wherein the two polyphase linear inductors which each incorporates are connected to electricity supplies to generate travelling magnetic fields which slide in opposite directions on the two inductors of one and a same split stirring roll and in a same direction on the inductors facing one another in distinct split stirring rolls.