EP0083898B1 - Process and apparatus for continuously casting hollow products employing a magnetic field - Google Patents

Description

The subject of the present invention is a method for manufacturing hollow bodies by continuous casting with the use of a magnetic field which acts on the liquid metal in an annular zone adjacent to an internal mandrel, as well as the device for implementing said method. .

The method according to the invention can be applied to all metals capable of being continuously cast by the conventional methods of casting solid bodies and among which mention may be made of aluminum, copper and steels.

Although it can be applied, in a very general manner, to the manufacture of hollow bodies having sections of very different shapes, the method according to the invention will be applied with particularly great interest to the manufacture of hollow bodies of circular section and, in particular, by operating by rotary continuous casting, the hollow bodies obtained being able, for example, to serve as blanks having good qualities of inner and outer skins for the manufacture of seamless tubes.

The manufacture of hollow bodies of circular section, that is to say, having an internal hollow generally concentric with the external section, has been the subject of numerous and various technical descriptions.

Generally, in these known methods, a cylindrical or cylindrical conical metal mandrel, for example made of copper, is used, internally cooled with water and arranged coaxially inside the ingot mold or external casting mold. Arrangements are also made to cool the inner wall of the hollow product obtained, generally with water, after the formation of a solidified surface layer. As it is poured, the initially liquid metal solidifies on contact with the mandrel, the solidification front then progressing radially with respect to said mandrel.

This solidification starting from the free surface of the metal bath, this results in a trapping in the solidified surface layer, which constitutes the inner skin of the hollow body obtained, of all the dross made up of slag, inclusions or other non-metallic particles, present at the surface of the bath and, in general, an inner skin having defects, such as incrustations. dairy, folds, which must be removed by means of difficult and expensive surface treatments before subsequent use of the hollow body obtained.

The inner skin of these products therefore has the same types of defects that are observed on the outer skin of solid bodies in conventional castings. These defects are further aggravated by the small size of the available space which prevents the introduction of any mechanical device making it possible to eliminate them at least partially.

Certain methods have been developed to try to solve these difficulties. This is the case of that described in Swiss Patent No. 618,363 of 6/01/1977, which uses the electromagnetic effect of an external single-coil inductor and an internal single-coil inductor to achieve continuous casting of hollow bodies without use of external ingot mold or mandrel.

The inductors used in this process are supplied by a single-phase alternating current and therefore create a stationary sinusoidal magnetic field, generally qualified as a pulsating field.

This pulsating field mainly promotes the creation of pressure forces within the liquid metal, which move it away from the fixed walls in which the inductors are contained, without generating significant circulatory movements within the mass of liquid metal.

Thus, according to this technique, a ring of liquid metal is kept in equilibrium by a magnetic field, the free surface of this metal having a convex shape, as shown in FIG. 1 of the cited patent.

Given the small radius of action of the magnetic field, this requires that the liquid metal column be of reduced height.

Such a technique is probably usable for aluminum, which has a shallow solidification well and a relatively flat solidification front.

On the other hand, in the case of steel, a metal of high density (at least compared to aluminum), and much less conductive of heat than aluminum, the solidification well, distance measured in the current bar solidification from the free surface of the metal bath to the end of solidification zone, is very deep and much higher than that of aluminum. This would result in the need for extremely slow casting speeds to obtain a solidified skin sufficiently resistant to contain the liquid still metal, taking into account the pressure forces developed by the pulsating magnetic field, process, assuming that it is achievable, in the case of steel, totally unusable economically.

Another solution for improving the quality of the inner skin of cast hollow bodies, described in French patent n ° 2180 494, consists in using a rotary continuous casting process, in which a central mandrel is used by continuously introducing a slag between the annular surface of the metal being solidified and the outer wall of the mandrel.

This process has the disadvantage of disturbing heat exchange and delaying the progression of the solidification front from the mandrel. In addition, it is necessary to carry out a treatment of the internal surface of the product obtained before use to remove, among other things, the layer of slag deposited on the internal skin.

Also known from FR-A-2 358 222 and EP-A-0 022 711 are methods of manufacturing solid bodies by continuous casting comprising electromagnetic stirring in which forces generated by the magnetic field act on the liquid metal at vicinity of the wall of the mold for example by creating a movement of the metal in said mold. Such forces having as a consequence a flow of liquid metal in an ingot mold are mainly intended to distribute the inclusions in the mass of the product.

Note, moreover, the general difficulty of the problem to be solved, given the hostile environment: small space available both in height and in diameter, at the mold, danger of explosion by using water in case of contact with liquid metal, particularly in the case of steel.

We therefore sought a method of manufacturing hollow bodies by continuous casting, which does not have the drawbacks described above and allows, in particular, to obtain hollow bodies whose inner skin is of satisfactory quality.

We have sought, in particular, the possibility of obtaining an inner skin quality such that it allows the use of hollow bodies without any particular surface preparation or by reducing this surface preparation to a minimum.

A device has also been sought for implementing such a simple and economical process, and applicable to the casting of numerous metals or alloys.

The object of the invention is a method of manufacturing metallic hollow bodies by vertical continuous casting, in which liquid metal is continuously introduced into an annular space comprised between an exterior metallic mold cooled by fluid circulation and an interior mandrel. also cooled by circulation of fluid, this metal gradually solidifying in contact with the walls of the mold and the mandrel with the formation of a hollow body which is extracted below the mold and in which, in an annular zone close to the external surface of the mandrel, the liquid metal is subjected to the direct action of a mobile magnetic field or sliding field which creates inside this metal in said annular zone of forces, having a vertical component directed from bottom to top, which cause this metal to the free surface of the metal bath.

Thus, according to the method according to the invention, the liquid metal located in the vicinity of the internal mandrel is entrained from bottom to top, in a direction opposite to the direction of extraction of the hollow product formed. This upward movement of the liquid metal in this annular zone accelerates the ascent towards the free surface of the metal bath, of inclusions or dross present in the liquid metal in the vicinity of the external surface of the mandrel.

The circulatory movement of the liquid metal, which occurs in the vicinity of the mandrel from bottom to top, is then deflected in a radial direction as it approaches the free surface of the metal bath. On the surface of the metal bath, in the area close to the mandrel, the radial movement of the liquid metal eliminates the inclusions or particles of floating slag. Thus, these various inclusions or particles no longer risk being trapped in the inner skin area of the hollow body obtained.

In addition, the displacement of the liquid metal from bottom to top, in the immediate vicinity of the external surface of the internal mandrel, causes the formation on the surface of the metal bath, of an annular zone in relief. Thus, to the effect of the radial displacement of the liquid metal towards the periphery is added the barrier effect of this relief which prevents inclusions or particles of floating slag from coming close to the wall of the mandrel in the formation zone. of the inner skin of the hollow body.

This results in a skin quality markedly superior to that which is obtained without the use of a magnetic field having the cited effects.

The liquid metal is, in general, introduced continuously and controlled by a jet coming, for example, from a pouring nozzle which makes it possible to adjust the flow rate and the impact of the jet, both in angle and in position.

The free surface of the metal bath can either be in contact with the atmosphere or be protected by any known means such as, for example, a protective neutral gas introduced in the liquid or gaseous state, or else a slag.

The mobile magnetic field, which plays an essential role, can be created by any suitable means consisting of inductor systems, fixed or mobile relative to the liquid metal, supplied with polyphase alternating current, or in mobile inductor systems constituted by powered windings by direct current or by a magnetic magnetic material.

A particularly simple and effective embodiment of the mobile magnetic field consists in using a magnetic rotor constituted by a rotor of revolution on which a magnetized magnetic material is fixed, this magnetic rotor being contained in the internal mandrel, and animated by a movement of rotation around its axis thanks to a drive means.

In a preferred solution, said magnetic rotor is driven in rotation by the cooling fluid of the internal mandrel via a turbine or any other suitable direct or indirect drive means.

Generally, in the case of the use of a magnetic rotor, it is arranged to favor the vertical component of the mobile magnetic field with respect to the horizontal component which tends to drive the liquid metal in rotation around the mandrel .

Such a rotational movement of the liquid metal in the mold not being useful for the operation of the process, this movement can be reduced or blocked by any suitable means. To this end, it is possible in particular to orient the jet of liquid metal which penetrates into the mold so that the direction of movement of this metal has a tangential component of direction opposite to the direction of rotation due to the magnetic field.

The speed of rotation adopted for the rotor is such that the mobile magnetic field also called sliding field when one considers essentially its vertical component, has a sufficient frequency to have an effect of ascent of the metal along the notable mandrel, without however that this frequency is too high, the field then being absorbed, for the most part, by the metal screen that constitutes the mandrel and also the layer of metal solidified along the outer wall of the mandrel.

Rotational speeds of 1,000 to 3,000 rpm. corresponding to frequencies from 17 to 50 Hz, are generally adopted; higher or lower speeds may however be advantageous in some cases.

It may be advantageous to continuously carry out, during the casting, a lubrication of the external wall of the internal mandrel, in contact with the metal, by a vegetable oil, for example, rapeseed oil, to be known for this. application.

The internal mandrel will be given the conicity necessary to allow good release of the products.

The process according to the invention, which has just been described, applies most generally to any type of continuous casting and, in particular, to rotary continuous casting.

Rotary continuous casting which is commonly practiced for the production of solid bodies of circular section, generally comprises a vertical ingot mold animated by a uniform rotational movement around its axis, the cast metal being extracted vertically under the ingot mold by a continuous downward rotation-translation helical movement.

Such a technique is described in numerous publications such as FR 1,440,618, 2,119,874, and also in the “Revue de Métallurgie CtT February 1981 (pages 119 to 136).

In the case of the application of the method according to the invention to rotary continuous casting, the liquid metal is introduced into the annular space comprised between an external mold with vertical axis, of cooled circular section, rotating at a uniform angular speed around of this axis and an internal vertical mandrel, the axis of which is, most often, coincident with the axis of the external mold, said mandrel being cooled by internal circulation of fluid and rotating on itself around its axis, in the same direction as the external mold, the hollow shaped blank being extracted vertically by a downward helical movement, by extraction means.

As mentioned above, the liquid metal is subjected to a mobile magnetic field having its source inside the mandrel, so as to create forces such that they impart to the liquid metal a movement having a vertical, parallel component to the axis of the mandrel, directed from the bottom up. In the case of rotary casting, the angular speed of the internal mandrel is generally substantially equal to that of the external mold, this movement being either controlled by a mechanical device or the result of the friction product being driven in solidification course on the mandrel.

Advantageously, the hollow product in the course of solidification is subjected, along and close to the internal mandrel, not only in the vicinity of the surface, but over a height corresponding substantially to the entire height of the external mold, to the mobile magnetic field.

In a preferred solution of rotary continuous casting, directions of rotation are adopted such that the rotation of the liquid metal due to the horizontal component of the mobile magnetic field and the rotational movement of the external mold and of the mandrel are in opposite directions. The effect of the metal rising along the mandrel is then most marked despite the generally concave shape of the meniscus due to the rotation of the outer mold and the mandrel.

The rotational speed of the outer mold is generally between 30 and 120 rpm.

The advantageous solutions for operating the process according to the invention in the general case are applicable, of course, in the case of rotary continuous casting and constitute preferential solutions for making it. It will be noted in this process that, due to the presence of the sealed inner mandrel, any direct contact of the inner surface of the hollow product being formed with the cooling fluid, such as water, is avoided, this cooling takes place. making by interposed mandrel. To perfect the cooling, an anti-radiation screen can be provided, as an extension of the internal mandrel, with or without the addition of a gaseous cooling aid allowing the calories to be drained more easily.

The invention also relates to a device for implementing the method described above. This device comprises a vertical exterior mold with a metallic interior wall cooled by internal circulation of fluid, an interior mandrel with metallic wall cooled by circulation of fluid, means for introducing a liquid metal into the upper part of the annular space included between the mandrel and the mold, means for extracting down the hollow body during solidification and means for creating a mobile magnetic field housed inside the mandrel.

In this device, the mobile magnetic field can be created by inductive windings, supplied with polyphase current, fixed or mobile relative to the outer wall of the mandrel.

Preferably, the mobile magnetic field is created by means of an inductor system rotating relative to the outer wall of the mandrel and comprising either windings supplied with direct current, or a magnetic material permanently magnetized.

In the case of rotary casting, the device which is the subject of the invention further comprises means for driving the external mold in rotation as well as extraction means making it possible to extract vertically downwards, with a movement helical, the hollow body in the process of solidification. The inner mandrel is preferably arranged coaxially with the mold.

According to a preferred solution, the rotation of the rotor is ensured by the fluid of the cooling circuit via a turbine located inside the internal mandrel.

The inner mandrel is imperatively made of a non-magnetic material advantageously having good heat conductivity and as low an electrical conductivity as possible. The internal part of the mandrel, that is to say the part corresponding to the magnetic rotor, advantageously extends over a height substantially equal to that of the external mold, the rotor projecting above the free level of the metal bath.

A preferred solution for creating the mobile magnetic field consists in using permanent magnets as magnetized magnetic material, in the form of parallelepipeds with rectangular faces, on the periphery of a rotor made up of a part of revolution made of magnetic material, according to a propeller having a homogeneous north-south magnetization, preferably radial.

To increase the intensity of the magnetic field, the magnetized magnetic material is placed along two offset helices wound around the rotor like a screw with two threads, each propeller having, in this case, a homogeneous radial magnetization, one helices being magnetized so that at each point a north pole is closer to the axis of the rotor, and the other helix so that a south pole is closest to the axis of the rotor at each point. It is also possible to provide more than two offset propellers. In this case, there is an even number of propellers each having a homogeneous radial magnetization, the direction of the magnetization alternating from one propeller to the next. In this way, a mobile polyphase magnetic field is obtained by means of permanent magnets, which is simpler to produce than by using a plurality of polyspire inductors offset in space and which would have to be supplied by polyphase currents.

In order to avoid the risks of the magnetic material being magnetized by centrifugal force, it is important to secure it with the rotor by means of a hoop made of a material based on natural or synthetic fibers covering the material. magnetic magnetized and surrounding the magnetic rotor. The connection between the hoop and the substrate is preferably ensured by a polymerized synthetic resin which impregnates the hoop. The magnetic material which constitutes the rotor is preferably a mild steel or a carbon steel such as a structural steel. The intervals between the successive turns of the propeller or propellers made of magnetized magnetic material are preferably filled with a filling material such as a polymerizable mastic reinforced with fiberglass.

Between the hoop and the magnetized magnetic material, there is preferably a felt made of non-woven fibrous material. Preferably, fibers with high mechanical characteristics, such as glass fibers or polyamides, are used to form the hoop. The connection between the felt and the hoop and the substrate is preferably ensured by a polymerized synthetic resin which impregnates both the hoop and the felt.

According to a particularly advantageous solution, it is possible to use as magnetic magnetized material a magnetic rubber for example in the form of ribbons or else a cobalt-based alloy containing at least one rare earth metal, such as for example samarium.

From this general design of the device according to the invention, it results in great simplicity, both from the construction and implementation point of view and great compactness.

This ensures high reliability and operational security, while obtaining a very favorable cost of use.

• La figure 1 est une vue d'ensemble, en coupe axiale verticale, du dispositif suivant l'invention.
• La figure 2 est une vue de la turbine d'entraînement du rotor magnétique en coupe, suivant C-C', comme montré figure 1.
• La figure 3 est une vue du système d'entraînement en rotation du mandrin de la figure 1, qui se place entre les plans de coupe D-D' et E-E' de la figure 1.
• La figure 4 est une vue de face en coupe axiale partielle, du rotor magnétique de la figure 1.
• La figure 5 est un rotor magnétique suivant un premier mode de réalisation de la présente invention, en coupe axiale partielle dans le haut de la figure, comportant deux hélices en caoutchouc magnétique.
• La figure 6 est un rotor magnétique suivant un deuxième mode de réalisation de la présente invention, en coupe axiale partielle dans le haut de la figure, comportant deux hélices en alliage magnétique cobalt-terres rares.

The figures and examples below describe, without limitation, embodiments of the device according to the invention, applied to the production of hollow bodies of circular section by rotary continuous casting.

• Figure 1 is an overall view, in vertical axial section, of the device according to the invention.
• FIG. 2 is a view of the drive turbine of the magnetic rotor in section, along C-C ', as shown in FIG. 1.
• FIG. 3 is a view of the system for driving in rotation the mandrel of FIG. 1, which is placed between the cutting planes DD 'and EE' of FIG. 1.
• FIG. 4 is a front view in partial axial section of the magnetic rotor of FIG. 1.
• Figure 5 is a magnetic rotor according to a first embodiment of the present invention, in partial axial section at the top of the figure, comprising two magnetic rubber propellers.
• Figure 6 is a magnetic rotor according to a second embodiment of the present invention, in partial axial section at the top of the figure, comprising two propellers of cobalt-rare earth magnetic alloy.

The device according to the invention, described here in the case of rotary continuous casting for obtaining hollow steel bars, is shown as a whole in FIG. 1, which has been cut in its lower part to facilitate representation.

The device for the continuous rotary casting of solid steel bodies, of circular section, is known per se, in particular from the publications whose references have been given above.

The description below will therefore relate essentially to the new means used for carrying out the method and the device according to the invention.

FIG. 1 represents a device for rotary continuous casting of hollow bodies according to the invention, which comprises an external mold 1, or ingot mold, rotating around a vertical axis of generally tubular shape and of circular section, cooled, an internal mandrel 2 , a liquid metal supply system shown diagrammatically by arrow 3 and a vertical helical extraction system for the cast products. These last two systems being the same as those used for the continuous rotary casting of solid round bars, are known to those skilled in the art and, therefore, not shown. The ingot mold 1 or external mold is simply represented by its wall 4 limited to 5 and 6. This wall generally has a slight taper, with a reduction in section in the lower part, which ensures contact with the metal being solidified. Its cooling system and its rotary drive means, known to those skilled in the art, have not been shown. The free surface of the metal is at 7 and the hollow body of circular section, partially solidified is at 8.

The hollow inner mandrel 2 consists of two parts: the lower part, located at the level of the mold 1 immersed in the metal being solidified, which constitutes the active part of the mandrel, and the upper part, located above the mold 1 , carrying the control and support mechanisms of the lower part.

In its lower part, the mandrel comprises a sleeve (9), of generally tubular shape, of circular section and of height slightly greater than the height of the mold 1.

The sleeve 9 advantageously has a taper with narrowing of the section downward to allow the removal of the metal during solidification. The sleeve 9 is generally made of a non-magnetic material having good heat conductivity, for example, copper or copper alloy.

The mandrel 2 is held in position in the mold by support means shown in FIG. 2, so that the sleeve 9 is perfectly coaxial with the mold 1.

The sleeve 9 is assembled, for example, by sleeving at 10 with a static seal 11 with a support tube of revolution 12 which constitutes the upper part of the mandrel and the upper end of which penetrates into the head of the mandrel 13. A double lip seal 14 allows the free rotation of the mandrel relative to the head 13 while ensuring the tightness with respect to the pressurized fluid which circulates inside.

The rotation of the sleeve 9 is controlled by a motor system shown in Figure 3, which ensures both the mechanization in rotation of the mandrel 2 and its general maintenance in vertical position and centered relative to the mold 1, the axis of the mandrel being coincident with that of the mold 1. This mechanical drive device is described below.

The head 13, fixed to the motor device of FIG. 3 by a fixing lug P, carries the supply lines 15 and start lines 16 of the cooling fluid.

Inside the hollow mandrel 2, a central tube 17, of circular section and co-axial with the sleeve 9, supports, in its lower part, a magnetic rotor 18 which surrounds it, and which is mounted free in rotation relative to to tube 17.

The tube 17 is sealed in its lower part 19; it is secured to the support tube 12 by means of radial plates 20-21, which do not prevent the axial flow between 12 and 17 of the cooling fluid.

The sleeve 9 and the tube 17 are tightly joined to the lower part by the annular bottom piece 22 with O-ring seals 23 and 24. At its upper end, the tube 17 is centered by an annular piece 25 with respect to which it is free to rotate thanks to a static O-ring 27 inside the head of the mandrel 13.

A nut 28 screwed at 29 to the tube 17 blocks the bottom piece 22.

Thus, the sleeve 9, the support 12, the tube 17 and the bottom piece 22 are perfectly integral and can rotate at the same speed of rotation.

The magnetic rotor 18 is constituted by a hollow cylinder free to rotate on the tube 17 and carries on its outer surface a magnetic material. Its particular structure will be described later. The length of the rotor is chosen so that its upper part clearly exceeds the level corresponding to the free surface of the liquid metal in the vicinity of the sleeve 9. In construction, arrangements are made so that the interval between rotor 18 and sleeve 9 is as small as possible, taking into account the need to maintain a sufficient cross-section for the coolant.

The speed of the rotor 18 is not linked to the speed of the tube 17 and said rotor turns on rings of suitable material, for example of resin-based material plus fiber celeron type, 31 and 32 positioned on the tube 17. The rotor 18, the speed of rotation of which must be high, of the order of 1000 to 3000 rpm, is driven in rotation by the cooling fluid by means of a turbine 33 machined in the lower part of the rotor, and therefore integral with it.

Figure 2 gives, in section, the profile of the turbine. The cooling fluid, which is under a suitable pressure inside the tube 17, leaves it by radial holes such as 34 distributed in suitable number at the periphery of the tube 17. A set of orifices, such than 35, in profile suitable, are distributed around the periphery of the rotor 18 and oriented so as to cause the rotor drive to react.

The profile of the orifices 35, as well as the adjustment of the pressure of the cooling fluid used, make it possible to control the speed of rotation of the magnetic rotor 18 in the desired speed range.

Thus, according to this device, the cooling fluid, generally water, entering at 15, descending inside the tube 17 and rising in the interval 30 to exit at 16, ensures both the cooling of the sleeve. 9, to allow the elimination of calories from the metal bath, and the cooling of the rotor and of the magnetized magnetic material.

A suitable drawing of the parts makes it possible, with a water pressure of 2 to 3 bars, to reach a speed of approximately 3000 rpm, keeping the temperature of the magnetic rotor as a whole below 100 ° C., the circulation speeds adopted making it possible to avoid the presence of air in the cooling circuit.

Preferably, as the rotational speed of the rotor, that which allows a sufficiently high upward displacement speed of the liquid metal to be obtained. The ratio between the upward movement speed of the liquid metal and the speed of rotation of the rotor is a function of this speed of rotation. Beyond a critical speed of rotation, the speed of upward movement of the liquid metal no longer increases and, on the contrary, begins to decrease rapidly. This critical speed of rotation depends in particular on the nature of the material which constitutes the wall of the sleeve 9 and on the thickness of the latter.

« e étant l'épaisseur de la paroi du manchon 9 exprimée en millimètres.In the case of a copper sleeve, this critical speed of rotation of the rotor "Ne" expressed in rpm is determined approximately by the formula:

"E being the thickness of the wall of the sleeve 9 expressed in millimeters.

The rotation of the mandrel 2 is ensured by the mechanism of FIG. 3. This assembly is placed between the planes DD 'and EE' of FIG. 1. This mechanism essentially consists of a toothed crown 36 hooped on the molten part 12 by a drive shaft 37, at the end of which there is a bevel gear 38.

The crown is supported in its rotation by two tapered roller boxes 39 and 40, which keep the mandrel 2 in a fixed vertical position. The shaft 37 also rotates in a box with two tapered rollers 41 and 42, a casing waterproof and cooled 43-44 closing everything. Seals 45-46 provide sealing during rotation of the mandrel.

The head of the mandrel 13 is fixed to the motor shaft housing by the lugs P and 47 and the bolts 48.

The mandrel 2 is positioned on the mold 1 by a system, not shown, of legs moored on the one hand, on the work floor which may be at the height of the mold 1, and on the other hand, on the casing 43-44 or on the head 13 of the mandrel. Thus, it maintains a well defined vertical position of the mandrel.

Many embodiments of the magnetic rotor used in the method according to the invention are possible.

Several advantageous embodiments of this magnetic rotor are described below.

In a first embodiment, the structure of the magnetic rotor 18, creating the mobile field, is shown in elevation, Figure 4, the upper part of the figure being in section.

This rotor consists of a hollow cylinder 49 made of structural steel, the ends of which are profiled to allow the accommodation of the friction rings 31-32 making it possible to center in rotation said rotor with a minimum of friction.

The magnetized magnetic material consists of permanent magnets such as 50 positioned in housings such as 51, made side by side in a helix, on the surface of the cylinder. These magnets are fixed in their housing, for example by gluing. Advantageously used magnets of rectangular shape with rectangle faces, the long sides of which are oriented parallel to the generatrices, the north-south axis, perpendicular to the large faces, corresponding to the shortest distance between faces of the parallelepiped, and being radial, c that is to say perpendicular to the axis of the rotor.

In this embodiment, the propellers are two in number, coaxial 52 and 53, arranged around the rotor in the manner of a double-threaded thread having a pitch to the right, each propeller being magnetically homogeneously oriented, it that is to say that the poles closest to the axis of the rotor of all the magnets of the same propeller are of the same name. On the other hand, the magnetic orientation of the two propellers is opposite. Thus, in the case of FIG. 4, the poles of the propeller 52, the closest to the axis of the rotor, are south, while those of the propeller 53 the closest to the axis of the rotor, are North.

Any permanent magnet that is sufficiently stable can be used.

The direction of winding of the propeller or propellers on the magnetic rotor must be the same as the direction of rotation of the rotor around its axis seen from above. Thus, if the rotor seen from above turns clockwise, the propeller or propellers must have a right pitch.

This rotor structure creates by rotation, a mobile magnetic field also called sliding field whose direction of movement is at each point perpendicular to the threads of the propeller and contained in the plane tangent to the surface of the cylinder. The direction of movement of this sliding field therefore has, on the one hand, a vertical component which drives the liquid metal from bottom to top, on the other hand a horizontal component which tends to drive the liquid metal in rotation.

The pitch of the propeller or propellers, that is to say the distance between two turns of the same propeller along a generator, is chosen so that the horizontal component of the magnetic field remains weak, while not bringing the poles of opposite names too close on the same generator of the rotor, so as to have field lines penetrating deep into the liquid metal. The distance on the same generator, between the ends closest to a north magnet and a south magnet, is preferably not taken less than the great length of the basic parallelepiped.

The quality of the results obtained in the process according to the invention depends in particular, as will be seen below, on obtaining a sufficiently high speed of upward movement of the liquid metal along the sleeve. It is in fact this upward movement which causes dross and inclusions up to the free surface of the metal and which creates an annular relief around the sleeve which prevents dross floating on the surface of the metal bath from depositing on the interior surface. of the hollow body being solidified.

It has been seen that in order to obtain a sufficiently large upward displacement speed, it is generally necessary to rotate the magnetic rotor at an optimum speed which is often very close to the critical speed calculated by the formula given above. In many cases, this optimal speed is such that the magnetic layer which covers the rotor is liable to be torn off by centrifugal force. This risk is all the greater since, given the low permeability of the air gap formed by the gap between the rotor and the inner wall of the mandrel, the wall of the mandrel and the layer of metal already solidified in contact with the wall. exterior of the mandrel, it is necessary to use a sufficient volume of magnetic material of relatively high density to obtain the desired induction, while the actual structure of the rotor must remain as light as possible and of reduced volume. In fact, the rotor is housed inside a mandrel of relatively large length, which is secured, by only one of its ends, to a fixing means. It is therefore necessary, in many cases, to limit the speed of rotation of the rotor to a value less than the optimal speed which would give the greatest speed of upward movement of the liquid metal to avoid tearing.

In order to achieve the optimum speed of rotation, a relatively light magnetic rotor has been developed in the context of the present invention capable of rotating at high speed without the risk of the magnetized magnetic material being torn off.

This magnetic rotor comprises a part of revolution made of magnetic material, capable of rotating around its axis, on the surface of which is arranged, along at least one helix, a magnetized magnetic material; this magnetized magnetic material is secured to the rotor by at least one hoop consisting of a material based on natural or synthetic fibers as well as by a synthetic resin, this hoop covering the magnetized magnetic material and surrounding the rotor.

Two embodiments of this improved magnetic rotor according to the invention are described below without limitation.

FIG. 5 represents a first embodiment of this magnetic rotor.

This rotor comprises a part of revolution made of magnetic metal, consisting of a cylinder 64 made of carbon steel such as steel type XC 35 (AFNOR standard). This cylinder has at each of its two ends a housing 65-66 intended to receive a friction ring or a ball bearing allowing it to rotate at high speed around its axis with the minimum of friction. A turbine, machined in the lower part of the rotor, has orifices represented schematically at 67-68, oriented and dimensioned so that the fluid which passes through them, as described above, causes the rotational drive of the rotor at the desired speed. On the surface of this cylinder, two parallel grooves are machined in a helix 69-70. These grooves have a relatively small depth e and a large width 11, the distance 1 ₂ between two successive grooves is preferably close to 11, the magnetic material is bound in part in these grooves. For example, a magnetic rubber ribbon is used, the active material of which is most often a ferrite which is glued by a suitable means into the groove. In order to obtain a sufficient volume of magnetic material, several thicknesses of magnetic rubber are preferably glued. In the case of FIG. 5, two magnetic helices 71-72 are produced, each consisting of three layers of magnetic rubber 71 ₁ -71 ₂ -71 ₃ and 72 ₁ -72 ₂ -72 ₃ , Within each helix, l he North-South magnetization axis is radial and in the same direction along the helix. On the other hand, the direction of magnetization changes from one propeller to another. Thus, in the case of FIG. 5, the propeller 71 has on the outside a North pole N and the propeller 72, on the contrary, a South pole S.

In order to effectively secure the magnetic helices to each other and to the steel cylinder, the gap 73 between the helices is filled with a filling and binding material such as a mixture of fibrous material and of a polymerizable resin having good wetting power vis-à-vis the surface of the steel cylinder and also vis-à-vis the magnetic material. To improve adhesion, knurling can be carried out on the surface of the cylinder. After hardening of the resin, this bonding material makes it possible, in particular, to avoid any displacement of the magnetic helices relative to one another.

The hooping of the magnetic material and the filling material on the carbon steel cylinder is carried out by means of a hoop 74 comprising a fabric based on high modulus fibers of elasticity which completely covers the cylindrical surface formed by the two magnetic helices and the filling material. This hoop 74 is shown in partial section in FIG. 5.

To improve the connection between the fabric of the hoop 74 and the underlying materials, it is possible to lodge between the two a thin layer, not shown in FIG. 5, of a nonwoven felt based on fiberglass, the assembly then being impregnated with a liquid synthetic resin, which, after polymerization, ensures an excellent bond between the hoop, the felt and the substrate, that is to say the steel cylinder surrounded by the magnetic helices and the material filling.

The thickness of the hoop is calculated so as to keep the magnetic propellers pressed against the cylinder despite the centrifugal force which is exerted on the magnetic material when the rotor turns at its speed of speed. Among the fibers with high mechanical characteristics, which make it possible to produce the hoop, it is possible in particular to use glass fibers, polyamide fibers, or even carbon or boron fibers.

Preferably, fibers with a high modulus of elasticity are used. Certain natural fibers may also be suitable.

The relative dimensions of the various elements constituting the magnetic rotor are chosen by a person skilled in the art as a function of the various parameters of the installation for continuous casting of hollow bodies which it is a question of producing and may vary within wide limits. It is thus possible to use for the continuous casting of hollow steel bodies an internal copper mandrel, in which is housed a magnetic rotor of 144 mm of external diameter and 600 mm in height. This rotor is rotated about its axis at a speed of the order of 3000 rpm. by a turbine, as described above. This rotor has a cylindrical core of structural steel, 87 mm in diameter and 600 mm high.

On this core are machined two parallel helical grooves, with a cylindrical bottom 1.5 mm deep and 50 mm wide. Each of these helical grooves is machined around the cylinder in steps of 200 mm so that the distance between the edges closest to two grooves is 50 mm.

In each of these grooves is housed three superimposed layers of a magnetic rubber band of about 9 mm thick and whose width corresponds to that of the groove.

These ribbons are glued to the back of the throat and also glued together. The gap between the ribbons is filled with a polymerizable putty reinforced with fiberglass. The whole is then wrapped by a thin layer of about 1 mm thick in a glass felt itself covered with a fabric made of polyamide fibers with high mechanical strength and high elastic modulus, about 2 mm thick which constitutes the hoop.

The hoop and the felt are impregnated with a polymerizable liquid resin which, after hardening, ensures the connection between the hoop, the felt, and the substrate.

The thickness of the hoop and that of the felt are adjusted so that the outside diameter of the magnetic rotor reaches approximately 144 mm. Thanks to this hoop, the magnetic tape forms a block with the rotor core and supports without displacement the centrifugal forces resulting from the rotation at 3000 rpm. of the magnetic rotor.

The clearance between the outer surface of the magnetic rotor and the inner surface of the mandrel in which it is housed must be as small as possible, taking into account the need to leave sufficient passage for the circulation of the cooling fluid, most often of the water. In the present case, the flow rate of this fluid must be determined taking into account not only the calories to be removed but also the need to drive the turbine at the desired speed.

As mentioned above, the distance between the polar surfaces of the magnetic helices and the surface of the molten metal opposite must be kept to a minimum. This distance, also called air gap, corresponds to the sum of 3 terms: the thickness of metal solidified in contact with the external surface of the wall of the mandrel, the thickness of this wall of the mandrel and the distance between the internal surface of this wall of the mandrel and the outer surface of the magnetic propellers. Each of these terms must therefore be optimized by applying the usual knowledge of those skilled in the art in terms of resistance of materials, thermal, and hydrodynamics.

In a second embodiment of the improved magnetic rotor according to the invention, it is proposed to use a much stronger magnetic field than that which can be obtained by means of magnetic rubber. For this, use is made in particular of magnets based on cobalt-rare earths such as CORAMAG magnets (registered trademark of UGIMAG S.A. MAGNETS). These magnets, thanks to their very large coercive induction field, of approximately 8,000 Oe and to their very large residual education of the order of 8,300 G, make it possible to multiply by a factor of 4, for equal volume, the field magnetic product.

This means that the use of these magnets allows, thanks to a very high specific energy of around 17 MG.Oe, to achieve very significant weight and inertia gains.

FIG. 6 represents in partial section a magnetic rotor comprising such magnets.

The general arrangement is similar to that described in FIG. 5. In this case, a rotor consisting of a carbon steel cylinder 75, of the same design as the cylinder 64 of FIG. 5, is used. The lower part of the cylinder, which comprises the drive turbine similar to that schematically described in Figure 5, is not shown.

This rotor comprises, like that of FIG. 5, two parallel helical grooves 76 and 77, of shallow depth and relatively large width in which are housed parallelepiped plates. ques in cobalt-rare earth magnetic alloy such as those marketed under the brand CORAMAG. These alloys are based on cobalt and contain rare earths such as samarium combined with cobalt at least partly in the form of intermetallic compounds such as TRCo _s or TR ₂ C _O17 , TR being a rare earth metal.

In the case, for example, of a diameter at the bottom of the groove of approximately 80 mm, parallelepipedic plates of 18 × 19 × 10 mm are magnetized in the direction of the smallest thickness (10 mm in the present case). In order to obtain a maximum effect, three layers of plates such as 78, 79 and 80 are superimposed, the largest dimension of the plates being parallel to the generatrices of the cylinder, and the shortest, which corresponds to the magnetization axis, being oriented radially. As in the case of the previous example, the direction of magnetization is the same within the same propeller and changes from one propeller to another.

In the case of FIG. 6, the propeller 81 comprises plates whose North pole N is on the side furthest from the axis of the rotor, while, for the propeller 82, it is on the contrary the pole South S which is furthest from the rotor axis. We can see better, in the lower half of Figure 6, the helical arrangement side by side of the magnetic plates such as 83, 84, 85, 86 on the periphery of the rotor. These plates are preferably bonded to the rotor, and to each other by means of a synthetic adhesive. However, given the high density of these magnetic alloys (of the order of 8.4), the risks of tearing are very important and it is necessary to tighten with force these magnetic plates on the rotor by means of a hoop containing fibers with high mechanical resistance. As in the previous example, use is made of a filling and bonding material 87 such as a polymerizable putty reinforced with glass fiber which fills the gap between the turns, and then there is arranged around the assembly a hoop 88 constituted by a layer of fabric based on fibers with high mechanical characteristics and, in particular, with high modulus of elasticity, which completely covers the cylinder. This hoop can, for example, be constituted by a ribbon wound helically around the cylinder or else have the shape of a sleeve which is threaded around the cylinder. For example, a fabric based on glass fibers can be used for this.

In FIG. 6, the hoop 88 is shown only partially in the zone in axial section. It obviously covers the entire cylindrical surface of the rotor so as to strongly tighten the magnetic plates and to keep them firmly in contact with the bottom of the grooves 76 and 77, even when the rotor is rotated at 3000 rpm. min. or more. The hoop 88 is preferably secured to the substrate by impregnating this hoop with a polymerizable liquid resin of known type.

To improve the connection between the hoop and the underlying materials, a nonwoven felt based on glass fibers can be placed between the two, for example, which makes it possible to achieve elastic tightening at all points. The connection between the hoop, the felt and the underlying materials is preferably carried out by impregnation using liquid polymerizable resin.

Many embodiments of the magnetic rotor can be envisaged.

To make the rotor, different metals or magnetic alloys can be used; it is generally preferred to use steels of common types.

Numerous types of magnets can be used as magnetized magnetic material, the magnetic or dimensional characteristics of which can be extremely varied.

One can consider having, not two magnetic helices of opposite polarities, but only one of single polarity. The variation of the field in the liquid metal is then at least half as small and the efficiency reduced. It is also possible to have more than two coaxial helices by alternating the polarity between adjacent turns. Such a solution may be advantageous for rotors of large diameters.

Likewise, the rotational drive of the magnetic rotor can be achieved by many different means. We can, in particular, perform this drive, not by means of a turbine driven by the coolant, but by means of an electric motor, which can be connected directly to the rotor, or, on the contrary, be connected to the latter by a mechanical drive means of suitable length. Finally, the hoop can also be produced in a large number of different ways using a very wide variety of synthetic or even natural fibers. All these variant embodiments do not allow to depart from the field of the invention as defined in the claims.

The continuous casting device according to the invention can be further improved by providing, as shown in FIG. 1, under the rotary mandrel, a screen 54, the function of which is to reduce the radiation from the internal surface of the hollow bar. , once out of the mandrel. Such a screen, consisting of a hollow metal cylinder with a solid bottom, can be fixed by screwing at 55 to an extension of the central tube 17.

It is also possible, whether or not there is a screen 54 advantageously to provide a secondary cooling device by neutral protective gas. The distribution of such a protective gas, as shown in FIG. 1, is ensured by a tube 56 threaded at 57 and screwed into an axial hole 58 drilled in the bottom 19 of the tube 17. Radial channels such as 59 put in communication hole 58 with the outside. The gas, which leaves through these holes, strikes the inner wall during solidification of the hollow body and therefore accelerates this solidification.

This protective gas is brought to the head 13 at 60. In this way, the cooling water cannot escape from the mandrel 2 and there is no risk of untimely penetration of water into the interior cavity of the bars being solidified. At the upper end of the tube 56, a seal 61 prevents the penetration of the cooling water from the tube 17.

Advantageously, a lubrication device using vegetable oil, rapeseed oil type, can be provided in the sleeve 9-skin metal interface during solidification, for example, by a drip dispenser.

The device which has just been described, with regard to the magnetic rotor, has the advantage of being particularly simple and compact.

In the case of the embodiments of this rotor which have been described, it is not necessary to use an electrical energy source to create the magnetic field, and no more to drive the magnetic rotor in rotation. This design is particularly interesting because of the environment prevailing in the mold: high temperature, very limited space available, danger of water infiltration on the liquid metal.

Furthermore, another advantage of the device described is its simplicity of implementation. Indeed, to the same support tube 12, it is possible to adapt the dimensions of the sleeves 9 which have a different working diameter, that is to say the diameter of the part immersed in the metal being solidified, corresponding to the different diameters. interior of the hollow bodies to be manufactured. For this, the sleeve 9, instead of having the shape of a cylinder of revolution of constant section, as in FIG. 1, has over its entire part which is in contact with the cast metal, a corresponding shape of revolution to the inner section of the hollow bar to be manufactured and, in its upper zone, a section corresponding to the sleeving 10 of the support tube 12, the two parts of said sleeve 9 being connected, in this case, by a shoulder.

Of course, the diameter of the rotor 18 is adapted to the inside diameter of the sleeve 9. The same rotor can be used for several dimensions of the sleeves 9, therefore of hollow bars.

The disassembly of the assembly takes place very easily by unscrewing the nut 28, disengaging the bottom piece 22 and disengaging the sleeve 9, the rotor 18 then coming by itself and the tube 17 remaining integral with the support tube 12.

We now describe the operation of the method implemented by means of the above device.

The liquid metal is fed continuously by 3 into the mold 1, which is rotated at a constant speed. The inner mandrel 2 is also driven by a rotation movement at a constant speed substantially equal to that of the mold 1 and in the same direction. This rotation of the mandrel is ensured either by the mechanism described in FIG. 3, or simply by the friction of the metal being solidified on the internal mandrel, the mechanism described in FIG. 3 only serving in this case to keep it in the vertical position and centered the rotating mandrel. Due to the continuous rotation of the mold 1 and the mandrel 2, any localized overheating of the mold and the mandrel is avoided, in particular, by radiation at the place where the metal-liquid is introduced by 3 into the mold. As a result, the process has great symmetry, both thermal and geometric.

On contact with the cooled wall 4 of the mold 1 and of the sleeve 9 also cooled, a solid crust 8 is formed and solidification progresses as the hollow bar is extracted from the mold from below.

The free surface of the metal 7, which can optionally be protected by a stream of protective gas brought to the gaseous or liquid state, then takes, due to the rotation of the mold, the generally concave shape, as seen in FIG. 1, the outer edges rising at 62. As a result, inclusions, dross or any non-metallic particles floating on the surface of the metal, tend to move away from the periphery. The result is a particularly neat exterior surface that does not require surface preparation before further processing. This is well known and exposed, inter alia, in the article of the “Revue de Métallurgie-CIT”, already quoted.

On the side of the mandrel, the vertical component of the mobile magnetic field created by the rotating rotor 18 has the effect of totally modifying the normal solidification conditions in the vicinity of the outer surface of the sleeve 9. In fact, the ascending current of liquid metal, which occurs along this sleeve, causes all the dross and inclusions that may be present, rapidly to the free surface of the metal, and, moreover, this current, which is then deflected radially towards the periphery, causes the rise of the level of the liquid metal in the vicinity of the mandrel 2, the annular relief 63 thus formed preventing dross floating on the free surface of the metal bath 7 from depositing on the interior surface of the hollow body in the course of solidification. This mechanical barrier effect is added to the effect of entrainment by the surface current which keeps dross away from the mandrel, being on the bath.

In order to obtain a relief of maximum amplitude at 63, the rotation is made to work. of metal, due to the horizontal component of the mobile magnetic field is counteracted by the general movement of opposite direction of the hollow bar during solidification. It is therefore necessary that the direction of rotation of the hollow bar 8, and, consequently, that of the wall of the mold 1 which drives it, and also that of the mandrel 2, are opposite to the direction of rotation of the magnetic rotor 18.

The liquid metal distribution jet is oriented in such a way that it keeps the updrafts and convection currents, in the vicinity of the mandrel, their maximum efficiency. For this, the jet 3 is preferably oriented so that the movement of the metal poured into the mold has a radial centrifugal component, the tangential component, which tends to rotate the bath, being directed in the direction of rotation of the mold 1. Furthermore, the stirring carried out on the liquid metal being solidified, in the vicinity of the mandrel, has the effect to refine the structure of the inner skin of the hollow body obtained.

This results in a very beautiful inner skin of the hollow body, which does not require prolonged surface treatment to continue the manufacturing cycle.

The process of rotary continuous casting of hollow bodies applies particularly well to the case of steel. One can, for example, manufacture steel bars having an outside diameter of 350 to 400 mm and an inside diameter of 115 to 200 mm.

For an outside diameter of 400 mm and an inside diameter of 200 mm, the operating parameters are as follows:

Although the example which has just been given relates to the application of the method, to rotary continuous casting, that is to say in the case where the cast hollow body is driven in rotation as well as the mold, the following method the invention also applies more generally to processes in which the mold is fixed.

As described above, it is also possible to carry out the method according to the invention by using a mobile magnetic field obtained not by means of a magnetic rotor, but by means of an inductor comprising windings supplied with polyphase alternating current.

The use of such inductors comprising, for example, windings supplied with three-phase current is known for pumping liquid metals such as sodium and even aluminum.

Their structure corresponds substantially to that of a portion of a polyphase alternating current motor stator whose curvature is canceled so as to obtain a sliding magnetic field whose translational movement is linear.

In the case of the method according to the invention, an inductor constituted by a cylinder with a vertical axis made of magnetic material may be housed inside the mandrel, in place of a rotor, having notches projecting in its cylindrical outer wall. which are arranged in series of coils supplied with polyphase alternating current.

One can in particular, in the case of a three-phase current supply, use 3 series of coils which are arranged in the notches of the magnetic cylinder so that, when they are supplied with alternating current, one obtains a sliding electromagnetic field which moves parallel to the generatrices of the cylinder. The speed “V of translation of the field is equal to the product of the pitch of the winding“ 1 ”by the frequency“ f of the alternating current. The coils are connected to the three-phase current source so that the vertical sliding of the field occurs from bottom to top. The translation speed is adjusted by acting on the one hand on the winding pitch and on the other hand, possibly, on the frequency of the polyphase current used.

Preferably, the cylinder is fixed so as to vertically entrain the liquid metal in the region adjacent to the mandrel. In this case, the mobile magnetic field does not have a horizontal component tending to drive the liquid metal in rotation.

When using a rotary continuous casting process, the inductor preferably accompanies the mandrel in its rotational movement.

Claims (31)

1. A process for the production of hollow metal bodies by vertical continuous casting wherein a liquid metal is continuously introduced into an annular space between an outer metal mould (1) which is cooled by fluid circulation and an inner mandrel (2) which is also cooled by fluid circulation, said metal progressively solidifying in contact with the walls of the mould and the mandrel with the formation of a hollow body (8) which is removed beneath the mould, characterised in that in an annular zone which is close to the outside surface of the mandrel, the liquid metal is subjected to the direct action of a movable magnetic field which within said metal in said annular zone generates forces having a vertical upwardly directed component, which forces entrain said metal towards the free surface of the metal bath.

2. A process according to claim 1 characterised in that a raised annular zone (63) is formed at the surface of the liquid metal bath in the vicinity of the inner mandrel (2).

3. A process according to claim 1 or claim 2 characterised in that the outer mould (1) rotates.

4. A process according to claim 3 characterised in that the inner mandrel (2) rotates in the same direction as the outer mould (1) at substantially equal speed.

5. A process according to claim 3 or claim 4 characterised in that the speed of rotation of the mould and the mandrel is from 30 to 120 rpm.

6. A process according to one of claims 1 to 5 characterised in that the movable magnetic field has its source within the mandrel.

7. A process according to one of claims 1 to 6 characterised in that the movable magnetic field is generated by an inductor comprising windings which are supplied with polyphase alternating current.

8. A process according to one of claims 1 to 6 characterised in that the movable magnetic field is generated by a rotary inductor comprising windings which are supplied with direct current or a magnetized magnetic material.

9. A process according to claim 8 characterised in that when the outer mould and the mandrel are driven in rotation their common direction of rotary moment is opposite to that of the rotary inductor system.

10. A process according to one of claims 1 to 9 wherein the cast metal is either ordinary or alloy steel or stainless or refractory steel or Ni and/or Co-base alloy.

11. Vertical continuous casting apparatus for carrying out the process according to claims 1 to 10, which comprises a vertical outer mould (1) having a metal inner wall which is cooled by internal circulation of fluid, an inner mandrel (2) which is also cooled by internal circulation of fluid, means (3) for introducing a liquid metal in the upper part of the annular space provided between the mandrel (2) and the mould (1), means for removal in a downward direction of the hollow body (8) in the course of solidification characterised in that means for generating a movable magnetic field are disposed within the mandrel (2).

12. Apparatus according to claim 11 characterised in that rotary drive means act directly or indirectly on the mould (1) and/or on the hollow body (8) which is removed from the mould and/or on the mandrel (2).

13. Apparatus according to claim 11 or claim 12 characterised in that the inner mandrel (2) is disposed coaxially with respect to the mould (1).

14. Apparatus according to one of claims 11 to 13 characterised in that the movable magnetic field is generated by an inductor comprising windings which are supplied with polyphase alternating current.

15. Apparatus according to claim 14 characterised in that the windings are positioned in recesses provided in the outside wall of a cylinder which is housed within the mandrel, said windings being disposed and connected to the polyphase alternating current source in such a way as to generate a sliding electromagnetic field which moves in an upward direction.

16. Apparatus according to claim 14 or claim 15 characterised in that when the mandrel is rotatable it is accompanied in its rotary movement by the cylinder which carries the inductor windings.

17. Apparatus according to one of claims 11 to 13 characterised in that the movable magnetic field is generated by an inductor which is driven in rotation by a drive means, said inductor comprising windings which are supplied with direct current or a magnetized magnetic material.

18. Apparatus according to claim 17 characterised in that the drive means rotates the inductor at a speed of about 1 000 to 3 000 rpm.

19. Apparatus according to claim 17 or claim 18 characterised in that the inductor is fixed with respect to a turbine (33) through which passes the cooling fluid for the mandrel (2), which drives same in rotation.

20. Apparatus according to one of claims 17 to 19 characterised in that the rotary inductor is formed by a magnetic rotor (18) which rotates within the mandrel (2) and on which the magnetized magnetic material (50) is disposed around the axis of the rotor along at least one helical path (52, 53).

21. Apparatus according to claim 20 characterised in that the direction of rotation of the rotary inductor as viewed from above its axis of rotation is the same as the direction of the pitch of the helix.

22. Apparatus according to claim 20 or claim 21 characterised in that the magnetized magnetic material has a radially oriented north-south axis and that the closest poles to the axis, relative to the whole of the magnetized magnetic material of the same helix, are like poles.

23. Apparatus according to one of claims 20 to 22 characterised in that the magnetized magnetic material is disposed along an even number of coaxial helical paths which are wound in the manner of a screw thread having a plurality of threads around the rotor, the closest poles to the axis changing in name in going from one helix to the helix with adjacent threads.

24. Apparatus according to one of claims 17 to 23 characterised in that the magnetic rotor which is driven in rotation about its axis by a drive means comprises a portion of revolution which is made of a magnetic material and around which a magnetized magnetic material is disposed along at least one helical path, said magnetized magnetic material being fixed with respect to the rotor by at least one band formed by a material based on natural or synthetic fibres having high mechanical characteristics, said band covering the magnetized magnetic material and surrounding the rotor.

25. Apparatus according to claim 24 characterised in that the magnetic material is a metal or metal alloy such as mild steel or a carbon steel such as a structural steel.

26. Apparatus according to one of claims 22 to 24 characterised in that the,spaces between the successive turns of the helical configuration or configurations of magnetized magnetic material are filled by a filling material such as a mixture of fibrous material and polymerized synthetic resin.

27. Apparatus according to claim 26 characterised in that the filling material is a mastic, comprising a polymerized synthetic resin and reinforced with glass fibre.

28. Apparatus according to one of claims 24 to 27 characterised in that a felt of non-woven fibrous material is disposed between the band and the magnetized magnetic material.

29. Apparatus according to one of claims 24 to 28 characterised in that the fibrous material which constitutes the band comprises fibres with high mechanical characteristics such as glass fibres or fibres of polyamides.

30. Apparatus according to claim 28 or claim 29 characterised in that the connection between the band, the felt and the substrate is formed by a polymerized synthetic resin.

31. Apparatus according to one of claims 17 to 30 characterised in that the magnetized magnetic material is a magnetic rubber or a cobalt-based alloy containing at least one rare earth metal such as samarium.

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