WO2001073141A1

WO2001073141A1 - Method and device for thermal treatment of steel wire

Info

Publication number: WO2001073141A1
Application number: PCT/BE2001/000055
Authority: WO
Inventors: René BRANDERS; Jacques Bauden
Original assignee: Le Four Industriel Belge
Priority date: 2000-03-27
Filing date: 2001-03-27
Publication date: 2001-10-04
Also published as: EP1268865A1; CA2404500A1; AU2001246255A1; KR20020093865A; CA2404500C; JP5060007B2; JP2003531960A; KR100735434B1; BE1013361A3; EP1268865B1

Abstract

The invention concerns a method for thermal treatment of steel wire, which consists in raising the temperature of at least one steel wire to be treated up to a temperature for austenitizing the steel, and maintaining the wire at said temperature for a temperature equalizing period within the metal mass of said at least one wire. The invention is characterised in that it comprises forming a fluidised bed, heating the fluidised bed, and moving through said heated fluidised bed at least one metal wire to be treated so as to ensure at least said temperature rise, said heating being at least partly obtained by at least heating the fluidised bed which is produced substantially tangential to an upper average area thereof.

Description

HEAT TREATMENT METHOD AND DEVICE

OF STEEL WIRE

The present invention relates to a process for the heat treatment of steel wire, comprising a rise in temperature of at least one wire to be treated up to a temperature of austenitization of the steel, and maintaining the wire at this temperature. temperature during a period of temperature equalization within the metallic mass of said at least one wire, during which there occurs an at least partial dissolution of carbides present in the steel. It also relates to a device for implementing this method.

Conventionally, the heating of the wire or of the layer of wires in the methods of patenting wires is carried out either in ovens with open fire, or in tube ovens, without the intervention of a fluidized bed. In the case of unalloyed steel, the heating is generally carried out in ovens with open fire whose atmospheres, obtained by combustion of a gas and an oxidizer, are regulated so as to be: oxidants in the first heating phase of the products so as to burn residues of various kinds which may be on their surface, b) reducing with respect to the product, when the latter has passed the allotropic transformation phase in order to avoid oxidation and decarburization on the surface of the products.

When it comes to alloy steel, a particular surface finish is generally sought. The products are, therefore, heated by radiant tubes traversed by a protective gas, thus allowing the products to maintain an impeccable surface state throughout its heating phase. In both cases, the heating time is relatively long, given the low heat transfer coefficients, either by convection in the case of open fire ovens, or by pure radiation in the case of tube ovens. The length of the oven necessary for heating the wire to an austenitization temperature, for example of the order of 950 ° C., then assumes an importance which plays to the detriment of the following process, the period of temperature equalization within the metallic mass of the wire ("soaking") which can only be short (normally a few seconds) if the dimensions of the oven are to be kept within limits acceptable to oven users. This results in poor dissolution of the cementite present at the joints of the austenite grains during the heating of the wires.

Furthermore, wire heating furnaces using a fluidized bed have already been known for a few years and the present applicant has already placed such ovens on the market, in particular for the diffusion of Cu-Zn coatings in wires. steel.

The Applicant has in fact found that good heat exchange can be obtained between heated particles of the fluidized bed, which are in a way bombarded on the surface of the threads running in the bed, and the threads themselves (see DE -A-3623890).

A method, as indicated above, has also been described in Japanese patent 2623004. The heating of the bed is carried out according to one embodiment using the fluidizing agent which consists of flue gases resulting from the combustion of a combustion gas in air, this fluidizing agent being injected into the bottom of the fluidized bed by perforated lances, very sophisticated to prevent penetration of particles from the bed into the lance. According to another embodiment, the fluidizing agent is air and a heat source external to the fluidized bed is formed of burners located above the fluidized bed and projecting a flame or hot gases vertically on it.

The process according to this prior document makes it possible to reach in the fluidized bed a temperature only moderate from 470 to 550 ° C. In its first embodiment, it is limited by the very complexity of the lances supplying the bed with a fluidizing agent, such lances being unable to withstand the aggressiveness of flue gas at too high a temperature, at the risk of having to be replaced very regularly. As for vertical heating from above, it has the drawback of being ineffective and above all of causing a lack of uniformity in the temperature of the bed. Indeed, the burners used each have a punctual impact on the bed with the consequence of small very hot surfaces of the bed and the neighboring parts much less, which is obviously to be avoided to obtain an adequate treatment of the wires.

Also known are fluidized beds heated by combustion, inside the oven or on the surface thereof, of a non-combustible fluidizing agent when it is introduced into the bed, but subsequently rendered combustible during its ascent (v for example Reynoldson RW, Anwendung von gasbeheizten Wirbelbetten zur Wârmebehandlung von etallen, in Hartenei-Technischen Mitteilungen, Vol. 37 (1982), Munich, p. 109-119; Patent abstracts of Japan vol. 015, n ° 231 (= JP03072036 )). These heating processes have the disadvantage of being dangerous, because they present a real danger of explosion and they produce huge flames directed upwards. They do not allow a control of the uniformity of heating in the parts of the bed where the objects to be heated are located. Also known are fluidized beds contained in small diameter tanks and produced using a non-combustible fluidizing agent. These beds are heated by surface injection by burners of a combustion mixture in order to heat treat large metal parts and mass which remain static throughout the duration of a treatment, which can last up to 30 minutes. (v. Reynoldson RW, op. cit.).

The object of the present invention is to offer a solution to the often large dimensions of ovens, in particular ovens used in the austenitization of steel wires, while achieving equivalent or even superior qualities of treated wires and allowing processing during the continuous scrolling of the threads.

The present invention also aims to allow rapid and uniform heating of the fluidized bed and of the wires to be treated, and this at a good energy yield, so as to allow a period of temperature equalization within the metallic mass largely greater than what is applied now, and without this prejudicing the bulk of the overall apparatus.

The problems mentioned above are solved by a process for the heat treatment of steel wires, as indicated at the beginning, this process further comprising

- formation of a fluidized bed by passing a fluidizing agent through a fluidizable particulate medium,

- a heating of the fluidized bed, and - a passage through the heated fluidized bed of said at least one metal wire to be treated, in one direction, so as to ensure at least said temperature rise, said heating being at least partially obtained by at least one heating of the fluidized bed which is produced substantially tangentially to an average upper surface thereof.

As is known, a fluidized bed is formed of particles of solid material which are brought into suspension by the entrainment of a fluidizing agent, normally a gas, which crosses the bed from bottom to top. During this crossing, the fluidizing agent carries upwards the particles, some of which are projected in the form of waves or protuberances higher than others. There is therefore obtained, in cross section of the bed, a very animated upper band, the upper surface of which is continuously variable in height, and which represents a part of the bed less dense in particles.

By the expression “average upper surface of the fluidized bed”, it is therefore to be understood according to the invention the surface which is situated at the average height between the hollows and the tops of the waves existing at the upper surface of the bed.

By tangential heating to this upper medium surface, the particles of the fluidized bed, which are there in a sparse, highly dispersed state, can be heated to extremely high temperatures, close to incandescence. The particles thus strongly heated, transfer the heat to the other particles which will be in contact with the wires by obtaining an optimal heat exchange at a speed impossible to envisage in accordance with the teaching of the prior art. Furthermore, the tangential heating is independent of the fluidizing agent and the perforated lances projecting this agent are not attacked by flue gases brought to excessive temperatures.

Given the excellent heat transfer coefficient via the heated fluidized bed according to the invention, the temperature for heating the wires, during which the austenitization of the wires takes place, can be drastically shortened, and therefore we can now envisage a period of maintaining the austenitization temperature sufficient to obtain a deep dissolution of the carbides of Fe from the steel wires , without increasing the cost of the installation to a commercially indefensible level and without making it of an unbearable size. In addition, this embodiment makes it possible to decompose the installation into two units, one relatively short, in the form of a fluidized bed oven and the other consisting of a simple tube or insulated box where the wires pass in being kept at temperature by simple means such as electric heating elements, gas burners of the radiating type, the fluidizing agent recirculated from the elongated oven, and / or a second fluidized bed.

According to one embodiment of the invention, said at least one tangential heater is oriented transversely relative to the direction of travel of said at least one metal wire to be heated. By its transverse and tangential orientation, the heating of the fluidized bed allows uniform heating and spread over the entire width and length of the bed of particles suspended in the bed, which allows uniform heating of the threads running in the oven.

According to an advantageous embodiment of the invention, said at least one tangential heating comprises a projection of at least one flame and / or jet of smoke tangentially to said average upper surface of the fluidized bed. Using such a process, the burners directly heat the particles of the fluidized bed, without gases burned at high temperature having to pass through a perforated tube, fragile under these conditions. The layout is simple and highly efficient. According to a particular embodiment of the invention, said at least one tangential heating comprises a heat exchange between a heating medium and the fluidized bed by means of at least one tube which is disposed tangentially to said average upper surface of the fluidized bed and through which said heating medium passes. It is thus possible to efficiently and quickly heat alloy steel wires which cannot come into contact with flue gases. The heat exchange takes place remarkably between a simple, non-perforated tube, through which the heating medium and the sparse particles of the bed pass which lick the walls of the heated tube.

According to an improved embodiment of the invention, the method comprises at least one pumping of fluidized particulate medium above the fluidized bed and a discharge of a jet of this particulate medium towards the fluidized bed in a direction opposite to said direction of scrolling. Said at least one tangential heating is carried out at least partially through the jet of repressed particulate medium. It should be known that, during the passage of the wires in the fluidized bed, by mechanical transport, the particles of the bed tend to be entrained, which causes them to migrate from the inlet of the installation to the outlet. The present embodiment not only makes it possible to counteract this disadvantageous effect, but it makes it possible to take advantage of this pumping to increase the efficiency of the tangential heating. Indeed, by pumping, the particulate matter is discharged in the form of rain at a level higher than the ridges of the fluidized bed and in the middle of the bed, which has the effect of raising in this case the aforementioned average upper surface of the bed above. above the ridges of the bed. It follows that the heads of the burners can according to the invention be arranged at a level also raised. It then becomes possible to arrange the burner heads outside particles, and therefore the embodiment has significantly less risk of fouling of these burner heads by particles from the bed.

Other features of the process according to the invention are indicated in claims 1 to 9 which follow.

According to the invention, there is also provided a device for heat treatment of steel wire, comprising

- an elongated heating oven,

- a temperature maintenance zone, and - means for driving at least one steel wire to be treated which make them pass through the oven and the temperature maintenance zone, so that said at least one wire to be treated undergoes a temperature rise in the furnace up to a steel austenitization temperature and is maintained at this temperature in the temperature maintenance zone, this device further comprising

a fluidized bed formed in the heating furnace by passing a fluidizing agent through a fluidizable particulate medium, and

- Heating means of the fluidized bed comprising at least heating means which produce heating substantially tangentially to an upper average surface thereof.

. Other details concerning the devices according to the invention are indicated in claims 10 to 17 which follow.

Other details and particularities of the invention will emerge from the description given below, without implied limitation, of some exemplary embodiments of devices according to the invention, with reference to the appended drawings.

Figures 1 to 3 show a cross-sectional view of three alternative embodiments of a fluidized bed oven according to the invention. FIG. 4 represents a top view of the furnace shown in FIG. 1.

FIG. 5 represents a device comprising two successive units, one for heating the wires and the other for maintaining the temperature.

FIG. 6 represents a graph illustrating the variation in temperature of the wire passing through the oven as a function of time.

Figures 7 and 8 show a cross-sectional view and a longitudinal sectional view of a fourth alternative embodiment of a fluidized bed furnace according to the invention.

In the various drawings, identical or analogous elements are designated by the same references.

In Figure 1, there is shown in cross section an elongated oven 1 containing a fluidized bed 2. The fluidized bed 2 is formed of particles relatively finely divided into a material resistant to high temperatures and preferably inert with respect to the son to heat. One can consider for example particles of silica, zirconia, alumina or other refractory materials having a good ability to transmit heat to the bodies which are in contact with them. Mention may very particularly be made of alumina sands, for example of grain size F90 according to standard FEPA 42F 1984, or zircon sands.

A fluidizing agent, such as for example air, nitrogen, ammonia, is introduced at high pressure and in this case at room temperature at the base of the fluidized bed, via a perforated lance 3. Any protective gas judiciously chosen according to the nature of the steel can be used for this purpose. By its pressure and its flow, the fluidizing agent carries the particles from the bed upwards and brings them into suspension. This swirl type suspension is manifested at the top of bed 2 by the formation of waves, the mean upper surface of the bed being represented by a dashed line 4.

It should be noted that the fluidizing agent is not combustible under the process conditions. The latter, cold or at room temperature, does not in any way stress the lance by which it is projected into the fluidized bed and the lifespan of the lances is consequently very greatly increased compared to the lances according to the prior art which are used for the both fluidization and heating of the bed.

The very hot fluidizing agent after passing through the bed is then recovered at the top of the furnace by an exhaust duct 5 (see FIG. 5).

In the examples illustrated in the figures, a layer of wires 6 passes through the oven in the longitudinal direction of the oven which is perpendicular to the plane of the drawing, at a speed which is calculated as a function of a constant fixed by production requirements and therefore the capacity of the processing line. This constant is equal to the product of the diameter of the wire expressed in mm and the speed of the wire expressed in m / minute. Therefore, the thinner the wire, the faster its running speed and vice versa. It should be noted that one could consider the scrolling of a single wire at a time in the oven, or the scrolling of several wires of different diameters, or the scrolling of several overlapping layers at the same time.

As illustrated in FIG. 1, the means for heating the fluidized bed consist of several burners 7 (see also FIG. 4) which are, in this example, arranged on either side of the ply of wires to be heat and which project their flame or smoke transversely to the direction of travel of the wires. The flames or fumes are in particular projected tangentially to said upper average surface 4 of the fluidized bed, thereby directly heating the particles. suspended in a strip of the bed where they are less dense and therefore more easily carried almost incandescent. It is obvious that, if in the examples illustrated the so-called tangential direction of the flame or of the fumes coincides perfectly with the surface 4 shown, it is possible to envisage having the burners slightly oblique with respect to this flame or these fumes, provided that one always obtains a well spread heating on the surface of the bed and that the threads are not reached by the flame or the fumes.

The flame of the burners and the fumes produced at the outlet of the burner commonly result from the combustion of a fuel and an oxidizer, for example a combustible gas or liquid, such as CH ₄ , fuel, etc. ., and air, enriched or not with oxygen, or even oxygen considered industrially pure. The flue gases formed are then recovered by the exhaust pipe 5, simultaneously with the heated fluidizing agent.

As can be seen in FIG. 4, the burners are arranged alternately to the left and to the right of the sheet of wires and the flame and the very hot jet of burnt gas which accompanies it extend transversely over the entire width from the oven. This arrangement offers the advantage of allowing very uniform heating of the fluidized bed and therefore of the wires to be treated.

In the embodiment illustrated in FIG. 2, the wires to be heated cannot come into contact with burnt gases to maintain their surface state. The heating of the bed takes place here also tangentially to the mean upper surface 4 of the bed, but by means of an unperforated tube 8 which the waves of particles of the bed come to lick, which are thus brought to high temperature. The burnt gases exit through an outlet conduit 9 through which they can be recovered. The embodiment illustrated in Figure 3 is of the same type as that shown in Figure 2. However, in this case, there is provided a box 10 located below the fluidized bed and in communication with the spray lance 3. A or burners 11 are arranged in this box and make it possible to obtain hot fumes up to a moderate temperature, higher than ambient temperature, for example of the order of 400 to 500 ° C., where the fumes are not yet exaggeratedly aggressive for the lances 3. This embodiment allows an even greater efficiency in heating the fluidized bed. It is understood that this arrangement could also be provided with that of open flame burners as illustrated in FIG. 1.

The wires to be treated in the ovens according to the invention are for example steel wires which are going to be subjected to a patenting operation.

The patenting operation is a well-known process in the world of wire drawing, it consists in bringing, at first, the temperature of a wire or a sheet of metal wires to a level such that one obtains a transformation of ferrite and perlite or perlite and cementite into austenite.

In a second step, what is called isothermal quenching is carried out so as to transform the austenite (previously obtained by heating) into fine perlite in order to obtain good mechanical properties and, especially, excellent drawing ability. .

During the wire heating step, it is necessary to reach a temperature of the order of 950 ° C. to obtain an austenitization of the steel. However, before going to quenching, a period of temperature equalization within the metallic mass, called "soaking" is favorable for dissolving the carbides present at the joints of austenite grains.

As illustrated in FIG. 5, in an example of a device according to the invention, it will be possible to provide two separate units, a heating unit 1 and a temperature equalization unit 12.

The heating unit consists of a fluidized bed oven 1 2, as previously described, which can be relatively short, given the excellent heat transfer coefficient of this type of oven. The wire 6 runs continuously through this oven which may have a length of for example 5 to 6 m. In the bottom of the oven, a box 10 of the type shown schematically in Figure 3 is arranged to allow prior heating of the fluidizing medium. The temperature of the heated fluidized sand according to the invention can advantageously reach approximately 1000 ° C.

The wire 6 thus leaves the oven 1 at a temperature of approximately 950 ° C. and then passes into the temperature equalization unit 12, by means of a sealed hopper 13. The unit 12 consists of a perfectly insulated box or tube from the thermal point of view. In this one it suffices to maintain the acquired wire temperature of 950 ° C. and a heat exchange allowing heating is no longer necessary. In the illustrated case, this temperature is maintained by recycling the burnt gases from unit 1 through the exhaust duct 5. The burnt gases released are brought to a filtering means, for example a cyclone 14, and the filtered gases that are still hot or partially heated in a complementary manner are brought using a pump 15 to the unit 12 for maintaining the temperature of the wire. The dissolution of carbides (cementite) is carried out in unit 12, which can have a length of approximately 4 m and in which, by scrolling, the wire remains for a sufficient time to obtain the dissolution / on carbides. On leaving the unit 12, the wire passes to a quenching device 16 only partially shown.

The total device of this example therefore has a completely acceptable length in terms of overall dimensions of approximately 9 to 10 m.

In FIGS. 7 and 8 another embodiment has been shown in which pumping devices 17 are provided to counteract the migration of the fluidized bed particles from the inlet of the furnace to the outlet. The particles are in fact mechanically entrained by the scrolling movement of the wires.

These pumping devices 17 comprise in the example illustrated a tube 18 disposed substantially vertically, which is connected in its lower part to a collection cone 19. The latter is advantageously arranged above a perforated lance 3. In its upper part, the tube 18 is connected to a sprinkler nozzle 20 which is oriented in a direction opposite to the direction of travel 21 of the wires 6 to be treated. Advantageously, the sprinkler nozzle 20 is also oriented obliquely towards the central part of the oven, as can be seen from FIG. 7. In the present embodiment, the perforations in the lances 3 are arranged downwards.

The particles of the fluidized bed and the fluidizing agent rush into the collection cone. Since the mass of the particles then has an apparent density in the tube which is significantly lower than that of the fluidized bed, this mass is projected higher than the crests of the fluidized bed: the sprinkler nozzles then return jets 22 of particles towards the back and towards the center of the bed, which helps to counter the migration of particles in the direction of travel of the son. As can be seen, this pumping does not imply, in the illustrated case, any additional energy expenditure.

These jets have the effect of raising the average upper surface of the bed which in the example illustrated is at the level of the dashed line 4 '. As can be seen in Figure 7, this level is higher than the wave crests of the fluidized bed.

As is apparent from FIGS. 7 and 8, the heating by the burners 7 is therefore now carried out, in accordance with the invention, tangentially to this raised upper average surface 4 ′, so that the gas jet coming from the burner crosses the projection of particles from the pumping device.

The tangential heating thus obtained is particularly effective. In addition, it offers the advantage that the heads of the burners 7 are out of reach of the particles which are projected towards the center of the furnace. Soiling of the burner heads can thus be favorably avoided.

Non-limiting exemplary embodiments will now be given to explain the invention in more detail.

Example 1

A steel wire of eutectoid type and with a diameter of 3 mm is introduced into a fluidized bed furnace equipped according to the invention, the constant DV (wire diameter (mm) x running speed (m / min)) is 36. The fluidized bed consists of Al ₂ 0 ₃ - F90, with a particle size of 106 to 250 μm. It is kept in suspension by a fluidizing gas, here air, having a minimum pressure of 600 mm H ₂ 0 and a flow rate of 64 NrrïVh.m ² . The fluidizing gas is introduced from the bottom of the fluidized bed at room temperature. Burners project tangentially to the upper average surface of the fluidized bed of the fumes resulting from the combustion of gas and combustion air, so as to heat the particles of the fluidized bed almost to incandescence and to obtain a bed of a temperature average of about 1000 ° C. The fumes from the burners are in direct contact with the fluidized bed.

As can be seen from curve A of the graph given in Figure 6, the wire reaches a temperature of 950 ° C after approximately 20 seconds of travel, maximum 30 seconds, that is to say that the length of the oven with a fluidized bed according to the invention can be limited to a value of approximately 5 m, for the constant DV chosen above.

Over a length of 4 m, which corresponds to a passage period of 20 seconds of the wire, it is maintained in unit 12 at a temperature of 950 ° C, for the dissolution of tertiary cementite. By way of comparison, an identical wire was run through a conventional open fire oven, without a fluidized bed, of the high convection type. It can be noted on curve B that the wire must be heated for approximately 50 seconds to reach the austenitization temperature and that the period of temperature equalization within the metallic mass cannot be carried out in a furnace. the same length and with the same constant DV, only for 3 to 4 seconds, which is insufficient to obtain good dissolution of the carbides (tertiary cementite).

Example 2

A steel wire of type XC70 (0.76% of C) is subjected to the same processing conditions as in Example 1. The wire has a diameter of 1 mm and its constant DV is 36. The wire reaches a temperature of 950 ° C. after approximately 5 seconds of travel and the length of the fluidized bed oven can be limited to a value equal to or even less than 5 m.

Over a length of 4 m, which corresponds to a passage period of approximately 7 seconds, the wire is kept at the temperature of 950 ° C. for the dissolution of the tertiary cementite.

Example 3

A steel wire of type XC70 is subjected to the same processing conditions as in Example 1. The wire has a diameter of 5 mm and its constant DV is 36.

The wire reaches a temperature of 950 ° C. after approximately 40 to 50 seconds of travel and the length of the fluidized bed oven can be limited to a value of approximately 5.5 m. Over a length of 4 m, which corresponds to a passage period of approximately 35 seconds, the wire is kept at the temperature of 950 ° C.

It should be understood that the present invention is in no way limited to the embodiments described above and that many modifications can be made thereto without departing from the scope of the appended claims.

Claims

1. A method of heat treatment of steel wire, comprising - a rise in temperature of at least one wire to be treated up to a temperature of austenitization of the steel, and

- Maintaining the wire at this temperature during a temperature equalization period within the metallic mass of said at least one wire, during which there occurs an at least partial dissolution of carbides present in the steel, characterized in that 'He understands

- A heating of the fluidized bed, and - a passage through the heated fluidized bed of said at least one metal wire to be treated, in one direction, so as to ensure at least said temperature rise, said heating being at least partially obtained by at minus a heating of the fluidized bed which is produced substantially tangentially to an average upper surface thereof.

2. Method according to claim 1, characterized in that said at least one tangential heating is oriented transversely relative to the direction of travel of said at least one metal wire to be heated.

3. Method according to one of claims 1 and 2, characterized in that said at least one tangential heating comprises a projection of at least one flame and / or jet of smoke tangentially to said average upper surface of the fluidized bed.

4. Method according to one of claims 1 and 2, characterized in that said at least one tangential heating comprises a heat exchange between a heating medium and the fluidized bed by means of at least one tube which is arranged tangentially to said upper middle surface of the fluidized bed and through which said heating medium passes.

5. Method according to any one of claims 1 to 4, characterized in that it comprises at least one pumping of particulate medium fluidized above the fluidized bed and a discharge of a jet of this particulate medium towards the fluidized bed in a direction opposite to said direction of travel, and in that said at least one tangential heating is carried out at least partially through the jet of repressed particulate medium.

6. Method according to any one of claims 1 to 5, characterized in that the fluidizing agent is non-combustible during the process.

7. Method according to any one of claims 1 to 6, characterized in that the heating of the fluidized bed further comprises heating of the fluidizing agent prior to its passage through the particulate fluidizable medium.

8. Method according to any one of claims 1 to 7, characterized in that the heating makes it possible to reach a temperature of the fluidized bed greater than 950 ° C, preferably at 1000 ° C, in a time of at most 30 seconds.

9. Method according to any one of claims 1 to 8, characterized in that it comprises maintaining said at least one wire at the austenitization temperature in said fluidized bed and / or outside this fluidized bed.

10. Steel wire heat treatment device, comprising

- an elongated heating oven (1),

- a temperature maintenance zone (12), and - means for driving at least one steel wire to be treated which make them pass in one direction through the oven and the temperature maintenance zone, so that said at least one wire to be treated undergoes a temperature rise in the furnace up to a temperature of austenitization of the steel and is maintained at this temperature in the temperature maintenance zone, characterized in that it comprises outraged

a fluidized bed (2) formed in the heating furnace (1) by passage of a fluidizing agent through a particulate fluidizable medium, and

11. Device according to claim 10, characterized in that the tangential heating means comprise at least one burner (7) which projects a flame and / or fumes tangentially to said average upper surface (4) of the fluidized bed (2).

12. Device according to claim 10, characterized in that it comprises several burners (7) arranged on either side of the wire or wires (6) in movement, preferably alternately, and projecting their flame and / or smoke transversely to the direction of travel of the wire or wires.

13. Device according to claim 10, characterized in that the tangential heating means comprise tubes (8) which are arranged tangentially to said upper surface medium (4) of the fluidized bed (2), transversely to the direction of travel of the wire or wires, and through which a heating medium passes.

14. Device according to any one of claims 10 to 13, characterized in that it comprises at least one device for pumping particulate matter which pumps it over the fluidized bed and discharges it in a jet towards the bed fluidized in a direction opposite to said direction of travel and in that a heating means produces said tangential heating through the jet of particulate medium discharged by at least one aforementioned pumping device.

15. Device according to any one of claims

10 to 14, characterized in that it further comprises means for preheating (11) the fluidizing agent.

16. Device according to any one of claims 10 to 15, characterized in that it comprises an elongated oven (1) of short length in which said at least one wire to be heated quickly reaches the austenitization temperature and, at the following the oven, a thermally insulated box (12) in which the wire is maintained at said predetermined maximum temperature by means of temperature maintenance (5, 14, 15).

17. Device according to claim 16, characterized in that the means for maintaining the temperature are electric heating elements, gas burners of the radiating type, recirculation means (5, 14, 15) of the fluidizing agent leaving the elongated oven (1), and / or a second fluidized bed.