EP0044786B1 - Process and means for casting thin, chilled ribbons by continuous casting - Google Patents

Description

The invention relates to the manufacture of thin ribbons by brutal casting and freezing, on a cold substrate running continuously, at high speed. It concerns in particular the obtaining of metallic materials, in the vitreous state, according to a process generally designated by the term of hyperhardening.

We know, in fact, that by cooling at very high speed, of the order of 106 ° C / second, of certain metals or molten alloys, it is possible to give them a glassy structure, it is that is, having no crystalline character on X-rays (“Metallic glasses”, Praveen Chandhari, Bill Giessen and David Turnbuli, “Pour la science juin 1980, n ° 32, p. 68).

In general, such an amorphous structure is obtained by projecting a jet of molten metal, which spreads out in the form of a very thin layer, on a cooled surface, good conductor of heat and moving at high speed.

Various methods of quenching on mobile cold surfaces have been proposed in the technique (quenching inside a wheel, on a drum, on a disc, between two rollers, etc.), the simplest and most commonly used consisting of projecting a jet of molten metal onto the external surface of a cold metal wheel rotating at high speed. The molten metal, ejected under pressure from a crucible, forms in contact with the wheel a stationary bulb, which gives rise to a hyper-soaked metallic ribbon. This, under the effect of centrifugal force, tending to detach from the cold wheel, to the detriment of the performance of the treatment, various solutions have been proposed to prolong this contact, for example the use of a tunnel with scanning. co-current gas described by document US-A-4,077,462.

The studies carried out on these different types of process, which have shown the influence of the nature of the gaseous boundary layer in contact with the cold surface on the properties of the product obtained and in particular the quality of the edges and the surface condition of the ribbon. , have led to proposing to operate under inert and controlled atmospheres, and in particular under low pressure, as provided for in document FR-A-2410368, of which FIG. 2 represents the whole apparatus in a closed enclosure. A major drawback of this latter technique however lies in the volume of the enclosure to be produced, in particular in the case of exploitation of the process on an industrial scale. In addition, in the case of its implementation under vacuum, the method, as described, can only be applied discontinuously, the vacuum having to be broken each time it is desired to recover the ribbon produced. . This document served as the basis for the preparation of the preamble of claim 1.

Finally, it has been found, during the tests which led to the present invention, that in the hyper-tempering process on a wheel, when operating under vacuum, the separation of the ribbon occurs more quickly than when the process is carried out the open air, that the quenching is less energetic, and that in general the detachment of the metal strip from the wheel is an unstable phenomenon.

This last drawback, in particular, has led to research, in order to implement hyper quenching under a controlled atmosphere, the means of controlling the effects of centrifugal force and, for this purpose, to resort to hyper quenching on a moving moving strip. at high speed under the jet of molten metal. This method, in principle known per se, in particular from document EP-A-0 002 785, however has notable drawbacks, among which we can notably mention the vibrations of the support strip, and more generally the insufficient precision of its positioning. , resulting in particular from its drive by pulleys rotating at high speed, the difficulty of effectively cooling the strip, and a greater complexity of implementation than hyperhardening on a wheel.

According to a particular aspect of the present invention, the latter aims to overcome these difficulties in using the hyper-tempering process on a moving strip, with a view in particular to implementing it under a controlled atmosphere, possibly under reduced pressure. It provides precise positioning of the moving strip, making it possible to make its vibrations negligible, while at least partly ensuring its cooling. It provides for this purpose to have opposite at least one of the faces of this strip at least one box comprising one or more orifices (holes, slots, etc.), through which a pressurized fluid, preferably a gas at low temperature, is ejected in the direction of the strip, in order to produce, between the latter and the box, a fluid cushion using the Coanda effect which maintains it at a precise distance and without friction on said box . Said effect is described for example in an article in "Science and Life" of August 1974, pp. 68-73 (publ. Excelsior Publications 5, rue de la Baume Paris 8 °).

It should be noted in passing that proceeds from a substantially different principle the device used according to document FR-A-2382297 which comprises a mold for continuous casting of metal plates consisting of two parallel metal strips hydrostatically guided, which pass vertically from top to bottom, and two block chains, driven at the same speed as the bands, said blocks serving to laterally delimit the space available for the meta! liquid. In this device, in fact, the liquid cushions supporting said bands have essentially the function of opposing the hydrostatic pressure of the molten metal contained in said mold, in order to avoid deformation and mechanical friction, and of applying said strips against said blocks, in order to ensure the lateral sealing of the mold.

The invention also relates to a method of manufacturing thin metallic ribbons by spraying a jet of molten metal or metal alloy onto a cold substrate moving at high speed, the impact of the jet and the forming of the ribbon. in contact with the substrate being carried out in an atmosphere under reduced pressure compared to atmospheric pressure, according to which before the temperature of the ribbon reaches the vitrification temperature of said metal or metal alloy, the ribbon is brought into an atmosphere greater than said reduced pressure.

The invention also relates to a device for hyper-dipping a metal or a metal alloy, during its shaping of thin ribbon by projection of a jet of said metal or metallic alloy in fusion on a mobile substrate moving at high speed at below a metal jet ejection orifice, said orifice and the area of the mobile substrate receiving the impact of the jet being arranged inside an enclosure comprising means for ensuring control of the nature and the pressure of the atmosphere in said enclosure according to which said substrate consists of a strip facing at least one of the faces of which and in the vicinity of the impact zone, is disposed at least one box comprising at least one orifice for ejecting a pressurized fluid, advantageously gaseous and at low temperature, creating between said box and said strip a fluid cushion ensuring, by Coanda effect, its maintenance without mechanical friction against said box, in the prec position ise with respect to it, and with respect to an entry slit and to an exit door having an internal threshold allowing the passage of said strip for, respectively, its entry and exit from said enclosure.

Preferably, such a box is arranged upstream of the impact zone of the molten metal and, moreover, advantageously situated opposite the impact face, so as to be able, by supplying it with a chosen gas, to modify the nature of the gas of the boundary layer in said zone.

The eject holes for the pressurized fluid may consist of rectilinear slots or small holes, possibly aligned in one or more rows.

Preferably, said box comprises a plurality of orifices for ejecting pressurized fluid aligned along at least one straight line parallel to the direction of advancement of said strip, at least one orifice having the form of a rectilinear slot disposed under the line median of said band.

Usually, said strip will be constituted by a continuous metallic strip driven by a motor member such as a drum or a pulley and passing over one or more return members. Advantageously, the return members will be constituted by fixed curved boxes comprising one or more orifices for ejecting a gas under pressure, preferably at low temperature, creating under said strip a cushion of Coanda effect gas, which keeps it at fixed distance from said box, without direct contact with its surface.

In an advantageous embodiment of the device according to the invention, it will comprise, downstream of the impact zone of the molten metal and opposite the face opposite to the impact face, a Coanda effect box, of preferably concave, arranged in such a way that the movable strip follows, following the impact of the liquid metal, a portion of path having a curvature corresponding to a concavity of the impact face of said strip, and thus tending, by inertia effect, to keep the ribbon in intimate contact with the band.

Of course, it is generally not desirable for the gas ejection orifices of the Coanda effect cushion to open inside the enclosure, and this possibility is even practically excluded when the installation is used under reduced pressure. In this case, it is desirable that a Coanda cushion is placed under the strip downstream of the enclosure and as close as possible to the exit door to avoid friction of the upper face of the strip against the door. outlet, and it is advantageous that a second cushion is disposed upstream of the enclosure and as close as possible to the inlet slot.

This entry slot, which aims only to allow the free passage of the support strip, of well defined section and position, can be produced in the form of various devices of the known art, such as seals, airlocks or chambers intermediates, which keep the air intake inside the enclosure low.

The exit door is of more delicate embodiment, because it must allow not only the passage of the support strip, but also that of the ribbon manufactured inside the enclosure. In particular, when the enclosure is placed under reduced pressure, due to the play that must necessarily be provided above the strip for the exit of the ribbon, a gas flow occurs, coming from outside the enclosure, which tends to peel off the tape from the tape and oppose its release, and therefore its recovery. However, tests have shown that the difficulty is overcome when the distance between the impact zone and the outlet is less than a critical value. This is generally quite low, on the order of a centimeter. and seems to correspond to the zone where the ribbon is still hot enough to adhere to the strip, its temperature being higher than the vitrification temperature of said metal or metal alloy.

In addition, to prevent the gaseous flow coming from outside the enclosure through the exit door from disturbing the jet of molten metal, the bulb which forms during its impact on the strip, and the spreading and cooling the ribbon, it is desirable that at least one of the vacuum outlets of the enclosure is arranged in the immediate vicinity of the exit door. Preferably, these vacuum intakes are arranged, in identical pairs, symmetrically with respect to ! has support strip, and near its edges.

In practice, keeping below a maximum critical value of the distance between the impact zone and the outlet is made difficult because of the bulk of certain organs, in particular the crucible containing the molten metal and its heating means, to be placed in this region of the enclosure.

To overcome this difficulty, it is advantageous to use a structure with an exit door offset towards the inside of the enclosure, preferably removable and interchangeable, so as to allow the device to be easily adapted to the chosen working conditions: dimensions and strip speed, nature of the alloy, and processing temperature, strip width, etc.

Tests have shown the advantage of such an interchangeable and at least partially retractable structure, capable of accommodating the size of the various organs, some of which are at high temperature, located inside the enclosure in the exit area. It appeared, in particular, that the production of the ribbon and its extraction from the enclosure under reduced pressure can both be obtained under good conditions, even if the distance between the impact zone and the downstream wall of the enclosure is quite large, when there is, above and at a very short distance from the strip supported by the strip, a part which is preferably partially retractable in the form of an awning having a surface substantially parallel to it ci, and covering it, from the immediate vicinity of the impact zone and at most from the critical distance defined above, to the outside of the enclosure.

The use of such awnings is particularly advantageous, because it makes it possible to place the lateral vacuum sockets, located in the vicinity of the outlet and on either side of the strip, in very direct communication with the slot through which the ribbon exits. of the enclosure.

The tests carried out under reduced pressure with such a device have proved to be perfectly satisfactory, since it is found that the metallic glass ribbon which forms in contact with the strip remains stuck to it over a sufficient distance to allow it to be extracted from the vacuum box, so that it can then be continuously recovered, for example by centrifugal ejection.

• la figure 1 est une vue schématique illustrant un dispositif d'hypertrempe conforme à l'invention équipé d'un caisson à effet Coanda disposé au-dessous de la bande mobile ;
• les figures 2 à 5 représentent diverses variantes d'un tel caisson ;
• la figure 6 est une vue schématique d'un dispositif conforme à l'invention pour l'hypertrempe d'un métal ou d'un alliage sous atmosphère contrôlée ;
• la figure 7 est une vue schématique partielle, depuis l'intérieur de l'enceinte, montrant une porte de sortie décalée, pour travail sous pression réduite et les prises de vide prévues à proximité ;
• la figure 8 est une vue similaire montrant une structure de sortie sous auvent ;
• la figure 9 est une vue schématique avec arrachement, montrant un organe de renvoi courbe fixe à effet Coanda pour la bande mobile ;
• la figure 10 est une vue de détail éclatée montrant des formes simples de pièces d'entrée et de sortie de l'enceinte.

The accompanying drawings show embodiments of the invention, which will now be described in more detail. In these drawings:

• FIG. 1 is a diagrammatic view illustrating a hyper-tempering device according to the invention equipped with a Coanda effect box placed below the movable strip;
• Figures 2 to 5 show various variants of such a box;
• FIG. 6 is a schematic view of a device in accordance with the invention for the hyperhardening of a metal or an alloy under a controlled atmosphere;
• FIG. 7 is a partial schematic view, from inside the enclosure, showing an offset exit door, for working under reduced pressure and the vacuum connections provided nearby;
• Figure 8 is a similar view showing an awning outlet structure;
• FIG. 9 is a diagrammatic view with cutaway, showing a fixed curved deflection member with Coanda effect for the movable band;
• FIG. 10 is an exploded detail view showing simple shapes of inlet and outlet parts of the enclosure.

We will first refer to FIG. 1, on which we see a crucible 1, surrounded externally by a solenoid 2, making it possible to heat the metal 3 contained in the crucible to a temperature higher than the melting temperature. molten metal can be ejected under pressure by a nozzle 4 in the direction of a metal strip 5, driven at high speed by means not shown below the nozzle 4. In contact with the strip 5, the molten metal undergoes hyperquenching and freezes to form a metallic ribbon 6 in the vitreous state, which adheres to the strip 5 and which is entrained by the latter.

In accordance with one aspect of the invention, a box 7, pierced with holes 8 arranged along the center line of the strip 5 (FIG. 2), is placed below this, and a gas under pressure ( air, helium, nitrogen or other), preferably at low temperature, is projected by the holes 8 in the direction of the strip 5, so as to form under this strip a gas cushion, which applies it, but without mechanical contact with it , against box 7, by Coanda effect. The gas cushion guides this strip as it travels at high speed under the nozzle 4 and thus eliminates its vibrations, in particular those which come from the drive device. It also contributes to the cooling of the strip 5, in order to evacuate the calories provided by the metal. in fusion.

It is, of course, possible to use a plurality of boxes 9 pierced with holes 10 aligned parallel to the direction of advancement of the strip 5 (FIG. 3) or boxes 11, provided with orifices 12, arranged perpendicular to the strip 5 ( figure 4).

One can also use studs 13, provided with holes 14 (Figure 5) possibly arranged in staggered rows.

As indicated above, this device, in the context of the invention, is particularly suitable for over-tempering carried out under reduced pressure compared to atmospheric pressure, or under any other controlled atmosphere.

FIG. 6 illustrates such an implementation. The movable strip 16, driven by a pulley 17, passes over two return pulleys, one fixed 18, the other mounted on a tensioner 19a. It passes through an enclosure 20, the lower part of which consists of the plate of a cooled box 21, comprising orifices, supplied with pressurized fluid, forming the gas cushion with Coanda effect. These orifices, arranged under the strip 16 only upstream and downstream of the grip of the enclosure 20, are not visible in the figure.

In the case of the drawing, the enclosure 20 comprises a framework 22, laterally provided with transparent walls 23, making it possible to observe the operations. In the enclosure 20, as previously, is arranged a crucible 24, surrounded by a solenoid 25, which melts the metal or alloy contained in the crucible.

The enclosure 20 comprises, for the passage of the strip 16, an inlet orifice 26a (FIG. 6), closed by a removable part 26b (FIG. 10), the lower face of which has a groove of suitable width and depth , with a slight clearance, with the dimensions of the strip 16, applies to the support box 21, and an outlet orifice 27a (FIG. 6), closed by a door 27b (FIG. 10), also mounted on the box so to let the passage of the tape and the ribbon.

Improved exit door variants are described below.

FIG. 7 shows an embodiment of an exit door according to the invention, having a tunnel with an opening offset towards the interior of the enclosure. This tunnel belongs to a removable part 28, preferably partially retractable, in the form of an angle iron having, on the one hand, a wing 28a substantially parallel to the support box 7 and resting on it by its two sides 29, and whose lower face has a profile groove adapted to the section of the strip 16 and to that of the ribbon 6 and, on the other hand, a wing 30a, arranged in the same way as the door 27b of FIG. 10, the side of which faces towards the inside the angle iron is rectified so as to be applied in a sealed manner under the effect of the vacuum prevailing in the enclosure against the external wall 22 of the enclosure, itself rectified on its surface in contact with the wing 30a. Because of its removable nature, this exit door has the advantage of easily adapting to changes in working conditions, without requiring any other modification of the essential of the device, and of avoiding a blockage of the strip thanks to freedom of movement in the event of an operational incident.

In the variant comprising an awning represented by FIG. 8, the general shape of the removable part 30 is close to that of FIG. 7, with a wing 30b applied to the wall 22. Its wing 31 does not, however, have any flanks in contact with the box 7, but takes the form of a plate, the lower face of which is flat, substantially parallel to the ribbon and located a short distance from the latter. The angle of the angle iron can advantageously be slightly less than 90 °, for example of the order of 85 to 88 °.

In FIGS. 7 and 8, the impact zones of the molten metal jet on the strip 16 have been identified by the letter I, and by the letter S the points where the ribbon 6 engages under the wings 28a and 31 of the exit doors, that is to say in fact the internal thresholds of said doors. According to the invention, the distances IS must be less than a critical distance which depends on the working conditions.

The enclosure 20 is naturally equipped with vacuum sockets 32, two in number, arranged laterally with respect to the strip 16, in the case of FIGS. 7, 8, and 10. As indicated above, the orifices 32 must be arranged as close as possible to the door of the enclosure.

It has also been found that the best results are obtained when the jet of molten metal is inclined relative to the strip 16, by an angle of 60 ° for example. Under these conditions, the metallic strip is formed on the strip 16 with less risk of projection, on the sides and towards the rear, of drops of molten metal.

Advantageously, as already indicated, it is possible to replace the return pulleys 18 and 19 with fixed curved return members 33, convex (FIG. 9) or concave, pierced with orifices 34 for ejecting a gas under pressure. , preferably at low temperature, which applies by Coanda effect the strip 16 against the member 33. This avoids any friction of the strip against the return members, which contributes to limiting the vibrations and to cooling the strip 16.

An example of operation will now be described. It uses a device comprising an endless steel strip, about 4 meters long, and of section 16 mm x 1 mm, capable of being driven at a speed of between 0 and 3000 m / minute, sliding on a box -planar support 10 cm wide and 50 cm long, which includes pressure gas ejection orifices, 1.5 mm in diameter and 2 cm apart. These orifices are arranged along the axis of the strip, over the entire length of the box, except at the level of the enclosure and the inlet and outlet parts, that is to say over approximately 15 cm. Crucibles 24 were used, pierced with an orifice with a diameter varying between 0.3 and 0.8 mm and spaced about 5 mm from the strip, and arranged in such a way that the jet of molten metal made an angle of 60 ° with it. A 1.5 kW vacuum pump allows you to easily obtain an absolute pressure in the enclosure of 0.05 bar. The overpressure of ejection of a molten metal through the orifice makes it possible to adjust the flow rate and was chosen for these tests in the range of 0.5 to 1 bar.

The devices of the invention making it possible to obtain metallic glasses, in particular with alloys of the type A _x 8 ₁ - _x where A consists of one or more transition metals (Fe, Cr, Ni, Mn, Co, etc. .), and B of one or more metalloids (P, C, Si, B, etc.), and where x, which is the atomic fraction of A, is of the order of 0.8. These alloys are known to give, by brutal quenching, products with very high content in the vitreous phase.

The best results have been obtained under reduced pressure, for example of the order of 0.05 bar, in particular using the devices illustrated in FIGS. 7 and 8. For belt speeds of 1,000 to 3,000 m / minute, and with a distance IS less than a critical value varying between 10 and 20 mm and a tunnel or awning length of the order of 5 cm, it was possible to obtain with these alloys ribbons of 1 to 7 mm width, and from 30 to 100 micrometers thick; these ribbons had regular edges and flat faces, qualities that can be attributed to vacuum work. In addition, the products obtained had a higher ductility than those of ribbons of the same kind, which are produced under vacuum in fully enclosed enclosures. This advantage seems to be attributable to the very rapid exit of the ribbon from the enclosure under reduced pressure, which allows a more effective quenching, close to that obtained by quenching in an uncommon atmosphere, thanks to an increase in the cooling rate. of the metallic alloy in the temperature zone situated above the so-called vitrification temperature.

Claims (15)

1. A method of making thin metal ribbons by projection of a jet of fused metal or metallic alloy on to a cold substrate moving at high speed, the impact of the jet and formation of the ribbon on contact with the substrate being carried out in an atmosphere at lower pressure than atmospheric pressure, characterised in that before the temperature of the ribbon reaches the vitrification temperature of said metal or alloy the ribbon is brought into an atmosphere having a pressure greater than said lower pressure.

2. A device for hypertempering a metal or a metallic alloy during formation thereof into a thin ribbon (6) by projection of a jet of said metal or metallic alloy in the fused state on to a moving substrate (16) moving at high speed below an orifice (4) for ejection of the metallic jet, said orifice (4) and the zone (I) of the moving substrate receiving the impact of the jet being arranged inside an enclosure (20) comprising means (32) for ensuring control of the nature and pressure of the atmosphere in said enclosure, characterised in that said substrate comprises a strip (16) facing at least one of the surfaces of which, in the neighbourhood of the impact zone (I), there is arranged at least one box (7) comprising at least one orifice (8) for ejecting a fluid under pressure, advantageously gaseous and at a low temperature, creating between said box (7) and said strip (16) a fluid cushion ensuring, by the Coanda effect, its support without mechanical rubbing against said box (7) and in a precisely defined position relative to the box and relative to an input slot (26a) and to an output port (27a) having an internal threshold (S) allowing passage of said strip (5, 16) for, respectively, its entry into and its exit from said enclosure (20).

3. A device according to claim 2, characterised in that said box (7) is arranged upstream of the impact zone (I) of the molten metal on to said strip (5, 16) and facing the impact surface, and in that said fluid is a gas.

4. A device according to claim 2 or 3, characterised in that said box (7) comprises a plurality of orifices (8, 10, 12, 14) for ejection of fluid under pressure aligned along at least one straight line parallel to the direction of advance of said strip, at least one orifice being in the form of a rectilinear slot arranged below the median line of said strip.

5. A device according to one of claims 2 to 4, in which said strip (5, 16) comprises a continuous metal strip driven by a motor member (17) passing over return members (18, 19) characterised in that at least one of said return members comprises a fixed curved box (33) comprising one or more orifices (34) for ejecting a fluid, advantageously gaseous and under pressure, preferably at a low temperature, in order to create between said box (33) and said strip (5, 16) a fluid cushion which holds the strip in position without rubbing against said box.

6. A device according to one of claims 2 to 5, characterised in that it comprises, downstream of the zone of impact of the molten metal and facing the surface opposite the impact surface, a concave, Coanda effect box (7, 33) arranged so that the moving strip follows, after impact (I) of the liquide metal, a portion of its trajectory having a curvature corresponding to a concavity in the impact surface of said strip (5, 16).

7. A device according to one of claims 2 to 6, characterised in that the means for ensuring control of the atmosphere comprise at least one vacuum port (32) arranged immediately adjacent the outlet port (27a) of said enclosure (20).

8. A device according to claim 7, characterised in that it comprises two vacuum ports (32) arranged laterally relative to said strip (16) along each edge of the latter, substantially in its plane.

9. A device according to one of claims 2 to 8, characterised in that it comprises at least two Coanda effect boxes of which one is upstream and the other downstream of the enclosure (20), in the immediate proximity of the input slot (26a) and outlet port (27a).

10. A device according to one of claims 2 to 9, characterised in that said enclosure (20) comprises, as a lower wall, the plate of a cooled support (21) of which the ends outside the enclosure comprise Coanda effect boxes such as those provided for said strip (16) adjacent the impact zone (I).

11. A device according to one of claims 2 to 10, characterised in that it comprises, in the enclosures (20), above the ribbon (6) supported by the strip (16), a member (28, 30), preferably partially retractable, having facing the ribbon a surface substantially parallel thereto and covering it from the immediate neighbourhood of the impact zone (I) to the exterior of the enclosure, said surface of said member (28, 30) providing relative to the ribbon (6) and the strip (16) a play sufficient for their passage.

12. A device according to claim 11, characterised in that the lower surface of the member (28, 30) covering the ribbon (6) forms an angle from 0 to 50 with the ribbon, which angle opens towards the metallic jet.

13. A device according to at least one of claims 11 and 12, characterised in that the output member (28, 30) covering the ribbon is movable, with the possibility of play in the vertical direction.

14. A device according to one of claims 1 to 13, characterised in that the axis of ejection of the molten metal or alloy is inclined relative to said strip, forming an acute angle, opening upstream, with the strip (5, 16).

15. A device for sudden cooling of a molten material during forming thereof into a thin ribbon (6), the device comprising a strip (5, 16) moving at high speed below an orifice (4) for ejection under pressure of a metal or alloy in the molten state and being characterised in that, facing at least one of the surfaces of said strip (5, 16) and in the neighbourhood of the impact zone (I) of the molten metal or alloy on this strip (5, 16) there is arranged at least one box (7) comprising at least one orifice (8) for ejection of a fluid, advantageously gaseous under pressure, preferably at a low temperature, creating between said box (7) and said strip (5, 16) a fluid cushion ensuring, by the Coanda effect, its support without mechanical rubbing against said box (7) in a precisely defined position relative thereto.

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