EP0270860A1 - Method and apparatus for the thermal treatment of steel wire - Google Patents

Abstract

A process for heat treating a carbon steel wire to obtain a fine pearlite structure is characterized by the following steps: (a) cooling the wire until the wire reaches a given temperature which is below the AC1 transformation temperature; (b) regulating the temperature of the wire to not more than 10 DEG C. above or below said given temperature by passing an electric current through the wire and effecting a modulated ventilation thereof; (c) cooling the wire.

Description

The invention relates to methods and installations for heat treatment of metallic wires and more particularly of carbon steel wires, these wires being used to reinforce articles made of rubber (s) and / or of plastic material (s). ), for example tire casings.

The purpose of these heat treatments is on the one hand to increase the wire drawing ability and on the other hand to improve their mechanical characteristics and their endurance.

Known treatments of this type have two phases:
a first phase which consists in heating the wire and in maintaining it at a temperature higher than the transformation temperature AC3 so as to obtain a homogeneous austenite;
- a second phase which consists in cooling the wire to obtain a fine pearlitic structure.

One of these most used processes is a so-called "patenting" heat treatment which consists of austenitization of the wire at a temperature of 800 to 950 ° C., followed by immersion in a bath of lead or molten salts maintained at a temperature from 450 to 600 ° C.

The good results obtained, particularly in the case of lead heat treatment, are generally attributed to the fact that the very high convection coefficients which are produced between the wire and the coolant allow on the one hand rapid cooling of the wire between the temperature AC3 transformation and a temperature slightly higher than that of lead, on the other hand a limitation of the "recalescence" during the transformation of the metastable austenite into perlite, the recalescence being an increase in the temperature of the wire due to the fact that the he energy provided by metallurgical transformation is greater than the energy lost by radiation and convection.

Patenting unfortunately results in high cost prices because the handling of liquid metals or molten salts leads to heavy technologies and the need to clean the wire after patenting.

On the other hand, lead is very toxic and the hygiene problems it poses lead to significant expenses.

The object of the invention is to carry out a heat treatment without using metals or molten salts, during the transformation of austenite into perlite, while obtaining results at least as good as with the patenting processes.

Consequently, the invention relates to a process for thermally treating a carbon steel wire so as to obtain a fine pearlitic structure, this process being characterized by the following three steps:
a) the wire, which has been previously maintained at a temperature above the transformation temperature AC3 to obtain a homogeneous austenite, is cooled until it reaches a given temperature below the transformation temperature AC1 and above temperature of the nose of the curve at the start of the transformation of metastable austenite into perlite, the wire then having a metastable austenite structure without perlite;
b) the temperature of the wire is then adjusted so that it does not differ by more than 10 ° C, by excess or by default, from this given temperature, this adjustment being obtained by passing an electric current through the wire, during a time greater than the pearlitization time and by performing a modulated ventilation during part of this time;
c) the wire is then cooled.

The invention also relates to a device for implementing the method defined above.

This device for thermally treating a carbon steel wire so as to obtain a fine pearlitic structure is characterized in that it comprises:
a) means for cooling the wire, which has been previously maintained at a temperature above the transformation temperature AC3, these cooling means allowing the wire to reach a given temperature below the transformation temperature AC1 and above the temperature of the nose of the curve at the start of the transformation of metastable austenite into perlite, the wire then having a metastable austenite structure without perlite;
b) means then making it possible to adjust the temperature of the wire so that it does not differ by more than 10 ° C by excess or by default of this given temperature, for a time greater than the pearlitization time, these means comprising electric means for passing an electric current through the wire and modulated ventilation means;
c) means for subsequently cooling the wire.

The invention also relates to the wires obtained with the method and / or the device according to the invention.

The invention will be easily understood with the aid of the following nonlimiting examples and all schematic figures relating to these examples.

• - la figure 1 représente un diagramme schématisant la mise en oeuvre du procédé conforme à l'invention ;
• - la figure 2 représente en fonction du temps les variations de la température du fil, de l'intensité électrique circulant dans le fil, et de la vitesse de ventilation lors de la mise en oeuvre du procédé conforme à l'invention ;
• - la figure 3 représente en coupe une partie d'un dispositif conforme à l'invention avec cinq enceintes de refroidissement et un axe, cette coupe étant effectuée selon cet axe ;
• - la figure 4 représente en coupe la première enceinte du dispositif conforme à l'invention représenté en partie à la figure 3, cette coupe étant effectuée selon l'axe de ce dispositif ;
• - la figure 5 représente en coupe la première enceinte du dispositif conforme à l'invention représenté en partie à la figure 3, cette coupe qui est effectuée perpendiculairement à l'axe de ce dispositif étant schématisée par les segments de lignes droites V-V à la figure 4 ;
• - la figure 6 représente en coupe la deuxième enceinte du dispositif conforme à l'invention représenté en partie à la figure 3, cette coupe étant effectuée selon l'axe de ce dis­positif ;
• - la figure 7 représente en coupe la deuxième enceinte du dispositif conforme à l'invention représenté en partie à la figure 3 ; cette coupe est effectuée perpendiculairement à l'axe de ce dispositif et elle est schématisée par les segments de lignes droites VII-VII à la figure 6 ;
• - la figure 8 représente en coupe un appareillage permettant d'obtenir un anneau gazeux rotatif, cet appareillage pouvant être utilisé dans le dispositif conforme à l'invention représenté en partie à la figure 3, cette coupe étant effectuée perpendiculairement à l'axe de ce dispositif :
• - la figure 9 représente un autre dispositif conforme à l'invention, ce dispositif comportant un appareillage de répartition avec un cylindre ;
• - la figure 10 représente plus en détail en coupe l'appareillage de répartition du dispositif représenté à la figure 9, cette coupe étant effectuée selon l'axe du cylindre de cet appareillage de répartition;
• - la figure 11 représente plus en détail en coupe l'appareillage de répartition du dispositif représenté à la figure 9, cette coupe, qui est effectuée perpendiculairement à l'axe du cylindre de l'appareillage de répartition, étant schématisée par les segments de lignes droites XI-XI à la figure 10 ;
• - la figure 12 représente en coupe une portion de la structure perlitique fine d'un fil traité conformément à l'invention.

On the drawing :

• - Figure 1 shows a diagram schematically the implementation of the method according to the invention;
• - Figure 2 shows as a function of time the variations in the temperature of the wire, the electric current flowing in the wire, and the ventilation speed during the implementation of the method according to the invention;
• - Figure 3 shows in section part of a device according to the invention with five cooling chambers and an axis, this section being made along this axis;
• - Figure 4 shows in section the first enclosure of the device according to the invention shown in part in Figure 3, this section being taken along the axis of this device;
• - Figure 5 shows in section the first enclosure of the device according to the invention shown in part in Figure 3, this section which is made perpendicular to the axis of this device being shown schematically by the segments of straight lines VV in Figure 4;
• - Figure 6 shows in section the second enclosure of the device according to the invention shown in part in Figure 3, this section being taken along the axis of this device;
• - Figure 7 shows in section the second enclosure of the device according to the invention shown in part in Figure 3; this cut is made perpendicular to the axis of this device and is shown diagrammatically by the segments of straight lines VII-VII in FIG. 6;
• - Figure 8 shows in section an apparatus for obtaining a rotary gas ring, this apparatus can be used in the device according to the invention shown in part in Figure 3, this section being made perpendicular to the axis of this device:
• - Figure 9 shows another device according to the invention, this device comprising a distribution apparatus with a cylinder;
• - Figure 10 shows in more detail in section the distribution apparatus of the device shown in Figure 9, this section being taken along the axis of the cylinder of this distribution apparatus;
• - Figure 11 shows in more detail in section the distribution apparatus of the device shown in Figure 9, this section, which is made perpendicular to the axis of the cylinder of the distribution apparatus, being shown schematically by the line segments lines XI-XI in Figure 10;
• - Figure 12 shows in section a portion of the fine pearlitic structure of a wire treated according to the invention.

FIG. 1 represents a diagram schematizing the operations carried out during the implementation of the method according to the invention.

A wire 1 is used which is a carbon steel wire. This wire 1 runs in the direction of arrow F on a path which includes the points A, B, C, D.

The process according to the invention comprises three stages:
a) the wire 1 which has been previously maintained at a temperature higher than the transformation temperature AC3 to obtain a homogeneous austenite, is cooled between points A and B until it reaches a given temperature below the temperature of transformation AC1 and higher than the temperature of the nose of the curve of the beginning of the transformation of metastable austenite into perlite. This cooling is shown diagrammatically by the arrow R _a . This given temperature allows the subsequent transformation of metastable austenite into perlite. The cooling R _a is carried out in a sufficiently short time so that there is no transformation of the austenite into perlite, the wire at point B then having a metastable austenite structure without perlite.
b) Between points B and C, the temperature of wire 1 is adjusted so that it does not differ by more than 10 ° C, by excess or by default of this given temperature, this adjustment being obtained by passing an electric current through the wire 1 for a time greater than the pearlitization time and by performing a cooling diagrammed by the arrow R _b . This cooling is carried out by a modulated ventilation, that is to say a ventilation whose speed is varied over time when the wire 1 passes between points B and C. This ventilation is only carried out during a game of the time when we pass the electric current through wire 1.

The passage of the electric current in the wire 1 between the points B, C is shown diagrammatically by the electric circuit 1 _{e of} which the wire 1 is a part and by the arrows I, I representing the intensity of the electric current circulating in the circuit 1 _e and so in thread 1.
c) Between points C and D, this wire 1 is cooled to a temperature which is for example close to ambient temperature, this cooling being shown diagrammatically by the arrow R _c .

• - La figure 2 représente en fonction du temps trois diagrammes 2A, 2B, 2C correspondant aux trois variations suivantes lors de la mise en oeuvre du procédé conforme à l'invention ;
• - La figure 2A représente la variation de la tempé­rature du fil 1 ;
• - La figure 2B représente la variation de l'intensite électrique circulant dans le fil 1 ;
• - La figure 2C représente la variation de la vitesse de ventilation lors des refroidissements R_a, R_b, R_c, c'est-à-dire la vitesse du gaz de refroidissement.

For example, the cooling R _a and R _c are also carried out by ventilation.

• - Figure 2 shows a function of time three diagrams 2A, 2B, 2C corresponding to the following three variations during the implementation of the method according to the invention;
• - Figure 2A shows the variation of the temperature of the wire 1;
• - Figure 2B shows the variation of the electrical current flowing in the wire 1;
• - Figure 2C shows the variation of the ventilation speed during cooling R _a , R _b , R _c , that is to say the speed of the cooling gas.

In these diagrams, time is represented by T, temperature by ϑ, electrical intensity by I, ventilation speed by V. In all these diagrams, time T is represented by the abscissa axis, and the variations of ϑ, I, V are represented by the ordinate axis. For the sake of simplicity, it is assumed that the temperature ϑ of the wire between points B and C is constant.

The three stages of the process are then reflected on the temperature diagram ϑ (fig. 2A) by a temperature level ϑ _b corresponding to step (b) preceded and followed by a drop in temperature corresponding to steps (a) and ( vs). These three steps are also expressed on the intensity diagram I by a non-zero intensity level I _b corresponding to step (b) preceded and followed by a level of zero intensity corresponding to steps (a) and (c). During step (b), the modulated ventilation is applied neither at the beginning, nor at the end of this step, it is only applied in the time interval T _B1 , T _B2 , step ( b) therefore comprising three phases. The method thus comprises five phases delimited on the diagrams of FIG. 2 by times 0 (corresponding to time T _A taken as origin), T _B1 , T _B1 , T _B2 , T _C , T _D , times T _B1 and T _B2 occurring during step (b). The implementation of the method during these five phases leads to modifications of the steel structure of the wire 1 shown diagrammatically in FIG. 2A.

Phase 1

Before the wire 1 arrives at point A, it has previously been brought to a temperature higher than the transformation temperature AC3, the wire 1 having been brought for example to a temperature between 800 and 950 ° C., and it has been maintained at this temperature so as to obtain a homogeneous austenite. When the wire 1 arrives at point A, its temperature is therefore higher than the transformation temperature AC3 and it has a structure comprising homogeneous austenite.

FIG. 2A shows the curve X₁ corresponding to the start of the transformation of metastable austenite into perlite, and the curve X₂ corresponding to the end of the transformation of metastable austenite into perlite, the nose of the curve X₁, c ' that is to say the temperature ϑ _p corresponding to the minimum time T _m of this curve X₁.

Between points A and B, that is to say between times O and T _B , the wire 1 is cooled, the average speed of this cooling, preferably rapid, being for example from 100 to 400 ° Cs C¹ so that the wire 1 reaches a given temperature ϑ _b lower than the transformation temperature AC1, and higher than the temperature of the perlitic nose ϑ _p , this temperature ϑ _b allowing the transformation of metastable austenite into perlite.

Phase 1, the duration of which is referenced P₁ on the time axis T of FIG. 2C is reflected in the diagrams of FIG. 2 by a drop in temperature ϑ, by an intensity I zero, and by a high plateau V _a of ventilation speed, this phase 1 corresponding to step (a).

During this cooling, preferably rapid, it develops at the grain boundaries of the metastable austenite "seeds" which are smaller and more numerous the higher the cooling rate. The germs are the starting points for the subsequent transformation of metastable austenite into perlite and it is well known that the fineness of perlite, therefore the use value of the wire, will be all the greater as these germs are more many and smaller. Obtaining high cooling speeds, in particular in the case of wire diameters greater than 1 mm, is due to the joint use of a cooling gas having good performance in forced convection, and to the use of speeds fast ventilation between for example between 2 and 50 ms⁻¹ for radial ventilation and between 10 and 100 ms⁻¹ for axial ventilation. The following phases 2, 3, 4 correspond to step (b).

Phase 2

The wire 1 is maintained at the chosen treatment temperature ϑ _{b by} virtue of the circulation of the electric intensity I _b without any ventilation being carried out.

On the diagram of FIG. 2C, the duration of this phase 2 is represented by the time interval P₂ from time T _B to time T _B1 , the temperature of the wire 1 has the fixed value ϑ _b , the electric current the value fixed I _b , and the ventilation speed is zero.

This phase of the heat treatment is advantageously carried out in a cooling enclosure in natural convection. During this phase, the germination speed is very high and their size is minimum.

Phase 3

During this phase, metastable austenite is transformed into perlite. To avoid an increase in the temperature of the wire, that is to say a recalescence, as a result of the energy provided by the metallurgical transformation of austenite into perlite, a modulated ventilation is carried out while maintaining the electrical intensity I _b in wire 1. In the diagram in FIG. 2C, the duration of this phase 3 is represented by the time interval P₃ between times T _B3 and T _B2 , the temperature of wire 1 is maintained at the fixed value ϑ _b , the electric current is maintained at the fixed value I _b . The ventilation is modulated as follows. The ventilation speed has a low or zero value at time T _B1 , at the start of this phase. It then increases to reach a maximum V _M , and then decreases to reach a low or zero value at time T _B2 , at the end of this phase.

This ventilation is modulated, that is to say that it has a value at all times such that the energy lost by the wire by convection and radiation is equal to the energy brought to the wire by Joule effect plus the energy brought to the wire by the metallurgical transformation austenite -> perlite.

The maximum speed V _M is for example between 2 and 50 ms⁻¹ in the case of radial ventilation, or between 10 and 100 ms⁻¹ in the case of axial ventilation. The ventilation speed V is obtained by preferably using a rotary gas turbine or injection ring in the case of radial ventilation, or a gas flow parallel to the axis of the wire, in the case of axial ventilation. , as described later.

Phase 4

This phase corresponds to the time interval T _B2 , T _C. The wire 1 is always traversed by the intensity of electric current I _b , and the temperature of the wire 1 is always equal to ϑ _b , but no ventilation is carried out, the ventilation speed therefore being zero. Since the perlitization time is likely to vary from one steel to another, the purpose of this phase 4 is to avoid applying premature cooling to the wire 1, corresponding to the phase 5 described later, in the event that the perlitization does not would not be finished at time T _B2 .

The duration of this phase 4 is represented by the time interval P₄ on the diagram in FIG. 2C. In FIG. 2A, the line segment BC crosses the region ω arranged between the curves X₁, X₂, the time T _B1 corresponding to the intersection of the segment BC with the curve X₁, the time T _B2 corresponding to the intersection of the segment BC with the curve X₂. In the direction of increasing times T, the point B is located before the region ω, therefore in an area where there is no perlite, the austenite being in the metastable state, and the point C is located after the region ω, that is to say in an area where all of the austenite is transformed into stable perlite. The ventilation modulated in FIG. 2C corresponds to the time interval where the segment BC crosses the region ω, but this ventilation modulation could be carried out during a time interval which does not correspond exactly to the crossing of this region ω, by example during a shorter time interval located entirely in the region ω, to take account of the inertia of exothermicity, or during a time interval longer than this crossing to take into account the possible variations of qualities of steel.

Phase 5

This phase corresponds to step (c). No electric current flows through the wire 1, and the wire is preferably ventilated at a high speed V _c , greater than the speed V _a of phase 1 so as to have rapid cooling. Rapid cooling is not absolutely necessary during this last phase, but it makes it possible to reduce the total time of the heat treatment and consequently the length of the installation. For example, V _c has a value between V _a and V _M on diagram 2C, but we can envisage different cases.

The duration of this phase 5 is represented by the time interval P₅ on the diagram in FIG. 2C, and it corresponds to the time interval T _C , T _D. The temperature of the wire 1 at the end of this phase 5 can for example be close to room temperature, or equal to room temperature.

Since the values of ϑ, T, I, V as well as the values of AC3, AC1, as well as the shape of the curves X₁, X₂ can vary depending on the steels, the actual values have not been plotted on the axes diagrams 2A, 2B, 2C.

For the simplicity of the presentation and of the production, the temperature of the wire 1 was assumed to be constant and equal to ϑ _b , during phases 2, 3, 4, that is to say during step (b) , but the invention applies to the case where during this step (b), the temperature of the wire 1 varies in an interval of 10 ° C by excess or by default around the temperature ϑ _b obtained at the end of phase 1 It is however preferable that the temperature of the wire 1 is as close as possible to this temperature ϑ _b . Preferably the temperature of the wire 1 does not differ by more than 5 ° C, by excess or by default, from this temperature ϑ _b , during step (b).

In the embodiment described above, no electric current flows through the wire 1 during steps (a) and (c), that is to say during phases 1 and 5, but the invention covers the cases where pass an electric current through the wire 1 during at least part of one of these phases, or of these two phases, which can have the advantage of regulating the conditions of the process, in a flexible manner, in the same device, for adapt it to several steel grades. The means making it possible to obtain the cooling Ra, R _c are then determined by taking account of this passage of electric current.

A device according to the invention for implementing the method according to the invention described above is shown in FIGS. 3 to 7.

This device 2, which is capable of treating eight wires 1 simultaneously, has a cylindrical shape with a rectilinear axis xxʹ, FIG. 3 being a section of the device 2 made along this axis, two wires 1 being represented in this FIG. 3.

The device 2 comprises five speakers referenced E₁, E₂, E₃, E₄, E₅, the wires 1 progressing from the enclosure E₁ towards the enclosure E₅, in the direction of the arrow F, the references P₁, P₂, P₃, P₄, P₅ corresponding to the durations of phases 1 to 5 in these enclosures E₁ to E₅ (Figure 3).

The enclosure E₁ is shown in detail in Figures 4 and 5, Figure 4 being a section along the axis xxʹ, and Figure 5 being a section perpendicular to this axis, this section of Figure 5 being shown schematically by the segments of straight lines VV in FIG. 4, the axis xxʹ being shown diagrammatically by the letter O in FIG. 5.

The enclosure E₁ is externally limited by a cylindrical sleeve 3 comprising an external wall 4 and an internal wall 5. The sleeve 3 is cooled by a fluid 6 for example water, which circulates between the walls 4 and 5. The wall internal 5 includes a multitude of fins 7 in the form of rings of axis xxʹ.

The enclosure E₁ comprises a motor-fan group 8. This motor-fan group 8 is constituted by a motor 9, for example an electric motor, making it possible to drive two turbines 10 rotating around the axis xxʹ, each of these turbines 10 being provided with fins 11, the wires 1 being arranged between the fins 11 and the internal wall 5.

The motor-fan unit 8 makes it possible to stir the cooling gas 12 in the form of a gaseous ring rotating in the direction of the arrows F₁ (FIG. 5), this ring 120 corresponding to the volume which separates the fins 11 and the internal wall 5. There is thus thus a radial ventilation of the wires 1.

The fins 7 allow good heat exchange between the gas 12 and the water 6.

The enclosure E₁ is aerodynamically isolated from the outside and from the next enclosure E₂ by two hollow circular plates 13 filled with a cooling fluid 14, for example water. These circular plates 13 are provided with eight openings 15 allowing the passage of the wires 1.

The enclosure E₁ corresponds to phase 1. The wires 1 have, when they enter the enclosure E₁, a temperature higher than the transformation temperature AC3, so that they then have a homogeneous austenitic structure, and they are rapidly cooled in the enclosure E₁ until they reach the temperature ϑ _b below the transformation temperature AC1 and above the temperature of the pearlitic nose ϑ _p . The temperature ϑ _b allows the transformation of metastable austenite into perlite, but this transformation does not yet take place in the enclosure E₁, because the incubation time T _B1 at the temperature of the wire ϑ _b has not yet been reached and the wires 1 keep an austenitic structure.

The wires 1 then pass into the enclosure E₂. This enclosure E₂ is shown in detail in FIG. 6, which is a section along the axis xxʹ, and in FIG. 7 which is a section perpendicular to the axis xxʹ, of this enclosure E₂, the axis xxʹ being shown diagrammatically by the letter O in this FIG. 7, the section of FIG. 7 being shown diagrammatically by the segments of straight lines VII-VII in FIG. 6. This enclosure E₂ is devoid of motor-fan group. Each wire 1 passes between two rollers 16 of material conducting electricity, for example copper, at the entrance to the enclosure E₂, these rollers 16 making it possible to circulate in each wire 1 the electric current of intensity I _b , from this enclosure E₂ to the enclosure E₄ which will be described in more detail later. The electric currents flowing in the wires 1 are supplied by transformers 17 each delivering the electric voltage U, each of these transformers 17 being controlled by a Thyristor device 18.

It is thus possible to establish at any time the equality between the heat received by the wires 1, as a result of the Joule effect, and the heat emitted by the wires 1, this emission being due to radiation and convection. The temperature of the wires 1 is thus adjusted to the same value as that reached at the output of the enclosure E₁, that is to say ϑ _b . For the simplicity of the drawing, a single transformer 17 and a single Thyristor device 18 are shown in FIG. 3. The enclosure E₂ is limited by a hollow cylindrical sleeve 19 in which circulates a cooling fluid 20, for example water . This cylindrical sleeve 19 is devoid of fins because in the enclosure E₂ the heat exchanges between the wires 1 and the cooling gas 12 are low since they are carried out in natural convection, that is to say without using mechanical means to set the gas 12 in motion.

The enclosure E₂ corresponds to phase 2, that is to say that there is in this enclosure E₃ accelerated formation of germs at the grain boundaries of the metastable austenite, but without there still being transformation d austenite in perlite.

The wires then pass into enclosure E₃. This E₃ enclosure is analogous to the E₁ enclosure but with the following differences:
- There are several motor-fan groups 8 arranged one after the other, along the axis xxʹ;
- The wires 1 are each traversed by an electric current of intensity I _b .

the ventilation due to the groups 8 is modulated, that is to say that the speed of rotation of the turbines 10 is low at the entrance to the enclosure E₃, it increases to go through a maximum, following the axis xxʹ , so that the ventilation speed passes through a maximum V _M , and then decreases towards the exit of the enclosure E₃, according to arrow F. This maximum V _M is for example different from the value of the ventilation speed in enclosure E₁. The speed of the motor-driven fan groups 8 can be adjusted for example using speed variators 21 sharpening on the electric motors 9 (FIG. 3), which allows modulation of the ventilation as a function of the thermal power to be extracted. The enclosure E₃ corresponds to phase 3, that is to say that in this enclosure E₃ there is transformation of metastable austenite into perlite which takes place at the temperature ϑ _b of the wires. This transformation gives off an amount of heat of around 100,000 J.kg⁻¹ and this at a variable speed between the entry and exit of the wires 1 of this enclosure E₃. The production of heat inside the wires 1 in this case is the sum of the heat due to the Joule effect, as a result of the electric currents flowing in these wires 1, and of the heat given off by the austenite-perlite transformation which can reach 2-4 times the Joule effect. It is therefore necessary to accelerate the heat exchanges, which is obtained thanks to the modulated radial ventilation previously described, obtained with the motor-fan groups 8.

The wires 1 then pass into the enclosure E₄ which is identical to the enclosure E₂ described above with the difference that the rollers 16 are arranged towards the outlet of the enclosure E₄, the electric currents therefore flowing in the wires for practically all of the time P₄ during which they are in this enclosure E₄. The wires 1 are here again kept at the temperature ϑ _b .

The enclosure E₄ corresponds to phase 4, its purpose is to maintain the wires 1 at the temperature ϑ _b to be sure that the pearlitization is complete before starting the cooling corresponding to phase 5.

The wires 1 then pass into the enclosure E₅ which is analogous to the enclosure E₁. This enclosure E₅ corresponds to phase 5, it allows the cooling of the wires 1 to a temperature, for example close to ambient temperature. It is not necessary for this cooling to be rapid, but it is however preferable for the cooling to be carried out rapidly to reduce the length of the device 2.

To simplify the disassembly and assembly of the device 2, each sleeve 3 is constituted by a plurality of elementary sleeves 3 _a , which can be assembled with flanges 22.

Circular plates 13, similar to the plates 13 limiting the chamber E₁ are arranged between the chambers E₂, E₃, between the chambers E₃, E₄, between the chambers E₄, E₅ and at the exit from the chamber E₅. Variable speed drives 21 allow the speeds of the motors 9 in the chambers E₁, E₅ to be varied if desired (FIG. 3).

The fixing of each motor 9 in the enclosures E₁, E₃, E₅ can be carried out with a plate 23 symmetrical around the axis xxʹ, this plate 23 comprising a bottom 24 where the motor 9 is fixed and an outer ring 25 fixed to the sleeve cylindrical 3 by the flanges 22 (Figure 4). This outer ring 25 is pierced with holes 26 for the passage of the wires 1.

The term "gas" for the cooling gas 12 must be taken in a very general sense, it covers either a single gas or a mixture of gases, for example a mixture of hydrogen and nitrogen.

Examples

The following three examples will make it possible to better understand the invention, the processing being carried out in the device 2 previously described.

The composition of the steels used is given in Table 1 below (% by weight).

The various characteristics of the wires used and the data concerning the austenitization are indicated in the following table 2:

In all cases of treatment in accordance with the method of the invention, for each example, the following characteristics were respected.

Number of wires: 8; scrolling speed of each thread: 1 ms⁻¹; the characteristics of the cooling gas 12 for the entire device 2 are given in table 3 below, this gas being a mixture of hydrogen and nitrogen of variable composition depending on the diameter of the wires 1.

The number of fan-motor groups 8 was one for the enclosures E₁, E₅ and five for the enclosure E₃, the numbering of these groups 8 then being from 8-1 to 8-5 in the direction of the arrow F, for the enclosure E₃ as shown in Figure 3 (for the simplicity of the drawing, group 8-3 is not shown in this Figure 3).

The processing characteristics of the wires 1 during phases 1 to 5 are indicated in the following table 4:

The mechanical characteristics of the wires obtained are given in table 5 below:

The invention is therefore characterized by a process which avoids the use of molten metals, for example lead, or molten salts, during the transformation of austenite into perlite, thanks to the combination of the heating of the wire by the Joule effect and of modulated ventilation, so that the invention leads to the following advantages:
- simple and flexible installations;
- It is not necessary to clean the treated wire which can therefore be for example brass plated and then drawn as it is;
- there is no hygiene problem because no toxicity is to be feared.

Preferably, we have the following relationships:
- The diameter of the wires 1 is at least equal to 0.3 mm and at most equal to 3 mm; advantageously, the diameter of the wires 1 is at least equal to 0.5 mm and at most equal to 2 mm;
- During phase 1: the wire is cooled at an average speed of 100 to 400 ° Cs⁻¹;
- in phases 2 to 4, the temperature of the wire ϑ _b is between 450 and 600 ° C;
- the effective speed of the gas ring at its maximum, in phase 3, varies from 2 to 50 ms⁻¹;
- the effective speed of the gas ring for phase 1 varies from 2 to 50 ms⁻¹.

Rotating gas rings can be obtained by other methods than turbines. Thus, FIG. 8 shows by way of example an apparatus 30 making it possible to obtain a rotary gas ring without using a turbine, this apparatus 30 being able to be used for example to replace at least one of the enclosures E₁, E₃ , E₅ previously described, FIG. 8 being a section made perpendicular to the axis xxʹ of the device 2, this axis being represented by the letter O in FIG. 8. The apparatus 30 is externally limited by a cylindrical sleeve 31 comprising an external wall 32 and an inner wall 33. A cooling fluid 34, for example water, circulates between these walls 32, 33. The apparatus 30 is internally limited by a cylinder 35. A series of injectors 36 allows the arrival of the cooling gas 12 in the annular space 37 delimited by the cylinders 33, 35, the wires 1 being arranged in this space 37 parallel to the axis xxʹ. The speed of the gas 12, at the outlet of the injectors 36 is represented by the arrow F₃₆. This speed has an orientation practically perpendicular to the axis xxʹ, and therefore to the wires 1 and it is practically tangent to the fictitious cylinder of axis xxʹ where the wires 1 are located which are equidistant from this axis xxʹ, that is to say say that the injection is tangential. A gas ring 38 of axis xxʹ is thus obtained, the speed of which is practically perpendicular to the axis xxʹ. The speed of the gas jet at the outlet of the injectors 36 has a value between twice and ten times the value of the speed of the gas ring 38. The outlet of the gas 12 towards the outside of the apparatus 30 is carried out thanks to the pipes 39, the gas outlet speed 12 being represented by the arrow F₃₉. The openings 360 of the injectors 36 are arranged on a line parallel to the axis xxʹ, two successive openings 360 being separated for example by a distance of 20 to 30 cm. It is the same for the openings 390 of the outlet pipes 39. For the simplicity of the drawing, a single injector 36 and a single return pipe 39 have been shown in FIG. 8.

A compressor 40 supplies the injectors 36 with gas 12 and receives the gas 12 which leaves the apparatus 30 through the pipes 39.

The distribution of the gas 12 to the injectors 36 is done through the manifold 41, and the modulation of the ventilation speed in the apparatus 30 can be obtained using the valves 42 disposed at the inlet of each injector 36, these valves allowing the gas flow 12 to be adjusted in these injectors 36.

The manifold 43 receives the gas 12 from the pipes 39, before this gas enters the compressor 40.

When the compressor 40 is of the volumetric type, there is a pressure regulator 44 which maintains a constant pressure difference between the injection manifold 41 and the return manifold 43.

Fins 45, in the form of rings of axis xxʹ are fixed to the inner wall 33 to promote heat exchange.

To have a good adaptation of the compressor 40 to the requirements of the apparatus 30, it may be advantageous to drive this compressor 40 by a variable speed motor, or else to use a gearbox between this motor and the compressor 40.

In the device 2 and the apparatus 30 described above, the circulation of the cooling gas was carried out radially, in the form of gaseous rings rotating around an axis parallel to the metal wires.

The invention also applies to cases where the circulation of the cooling gas takes place at least partially axially, as shown in FIG. 9. The device 50 of this FIG. 9 comprises a blower 51 which makes it possible to introduce the gas cooling 12 in a distribution apparatus 52. This apparatus 52 is shown in more detail in FIGS. 10 and 11. The apparatus 52 comprises a cylinder 53 of axis yyʹ, disposed in an annular chamber 54. The axis yyʹ is parallel to the wire 1 which passes through the annular chamber 54 10 is a section of the apparatus 52 along a plane passing through the axis yyʹ and the wire 1, FIG. 11 is a section perpendicular to the axis yyʹ, the section of FIG. 11 being shown diagrammatically by the segments of straight lines XI-XI in Figure 10.

The gas 12 leaving the pipe 55 is introduced tangentially into the chamber 54, the arrow F₅₅, which represents the direction of the gas leaving the pipe 55, being practically tangent to the cylinder 53 and having a direction perpendicular to the axis yyʹ, shown by the letter Y in FIG. 11. The gas 12 introduced into the chamber 54 then forms a gas ring 520 which rotates around the axis yyʹ, this rotation being shown diagrammatically by the arrow F₅₂ in FIGS. 10, 11. The wire 1, outside the chamber 54, passes through two tubes 56 disposed before and after the chamber 54, in the direction of the arrow F, and communicating with this chamber 54. The circulation of the gas 12 around the wire 1 in the chamber 54 is therefore partly radial. The gas 12 then flows along the tubes 56, moving away from the chamber 54, the flow then being parallel to the wire 1, according to the opposite arrows F₅₆, that is to say that the circulation of the gas 12 is then axial.

Withdrawal pipes 57 leaving the tubes 56 allow the gas 12 to flow out of these tubes 56, these pipes 57 opening onto the collecting pipe 58 connected to the outlet pipe 59. The gas leaving through the pipe 59 is reinjected into the fan 51 to be recycled, this path not being shown in the drawing for the purpose of simplification. The modulation of the ventilation along the tubes 56 and therefore along the wire 1 is obtained by regulating with the aid of the valves 60 the gas flow 12 in each of the withdrawal pipes 57. It is thus possible to obtain, in the tube sections 56 referenced 56-1 to 56-4 of the gas flow rates 12 which decrease as one moves away from the apparatus 52, in the direction of the arrows F₅₆, that is to say say that ventilation, and therefore cooling, decrease in this direction. The cooling effect is maximum in the apparatus 52 which makes it possible to subject the wire 1 to ventilation in a radial part, the ventilation in the tubes 56 being axial, that is to say that the gas 12 flows in parallel in wire 1, in the direction of the arrows F₅₆. The heat supplied by the hot wire 1 to the cooling gas 12 is removed using a water / gas heat exchanger 61. For the simplicity of the description, only four sections 56-1 to 56-4 have been shown. on either side of the apparatus 52, these sections moving away from the apparatus 52 in the direction of progression 56-1 to 56-4, but one could use a number of sections different from four on each tube 56.

The device 50 can be used for phase 3 of the method according to the invention, replacing the motor-fan units 8, which allows a simpler technical implementation.

A ventilation similar to that of the device 50 could also be used in phases 1 and / or 5 of the process according to the invention, but in this case, a modulation of the ventilation is not necessary and it suffices to have only one pipe. withdrawal 57 at each end of the tubes 56 furthest from the apparatus 52.

The axial gas flow technique is easier to implement than that of radial flow, but it is not sufficient to cool the metal wires, the diameter is greater than 2 mm and in this case, a radial flow technique must be used for the cooling gas.

As previously described, it may be advantageous to pass an electric current through the wire 1 during steps (a) and / or (c), in this case the device for implementing the method according to the invention comprises means for passing an electric current through the wire 1 during these steps, these means possibly comprising, for example, the rollers 16 previously described.

In the embodiments described above, the passage of current through the wires 1 was obtained from a voltage source U, by Joule effect, but the passage of current could also be obtained by induction, the Joule effect devices being however preferred because they are easier to carry out.

The wire 1 treated according to the invention has the same structure as that obtained by the known lead patenting process, that is to say a fine pearlitic structure. This structure includes cementite lamellae separated by ferrite lamellae. By way of example, FIG. 12 represents in section a portion 70 of such a fine pearlitic structure. This portion 70 comprises two substantially parallel cementite strips 71 separated by a ferrite strip 72. The thickness of the cementite strips 71 is represented by "i" and the thickness of the ferrite strips 72 is represented by "e" . The pearlitic structure is fine, that is to say that the average value of the sum i + e is at most equal to 1000 Å, with a standard deviation of 250 Å.

Of course, the invention is not limited to the embodiments previously described.

Claims (25)

1. Process for thermally treating a carbon steel wire in order to obtain a fine pearlitic structure, this process being characterized by the following three stages: a) the wire, which has been previously maintained at a temperature above the transformation temperature AC3 to obtain a homogeneous austenite, is cooled until it reaches a given temperature below the transformation temperature AC1 and above temperature of the nose of the curve at the start of the transformation of metastable austenite into perlite, the wire then having a metastable austenite structure without perlite; b) the temperature of the wire is then adjusted so that it does not differ by more than 10 ° C, by excess or by default, from this given temperature, this adjustment being obtained by passing an electric current through the wire, during a time greater than the pearlitization time and by performing a modulated ventilation during part of this time; c) the wire is then cooled. 2. Method according to claim 1 characterized in that it comprises the following five successive phases:
- during phase 1, the wire, which has been previously maintained at a temperature above the transformation temperature AC3, is cooled until it reaches the given temperature, the temperature of the wire is then adjusted so that it does not differ by more than 10 ° C, by excess or by default, from this given temperature, this adjustment being obtained by passing an electric current through the wire, during the following three phases 2, 3, 4:
- during phase 2, no ventilation is carried out;
- during phase 3, a modulated ventilation is carried out;
- during phase 4, no ventilation is carried out;
the wire is then cooled during phase 5. 3. Method according to any one of claims 1 or 2 characterized in that the cooling of the wire, after pearlitization, is carried out to a temperature close to room temperature. 4. Method according to any one of claims 1 to 3 characterized in that the modulated ventilation is at least partly a radial ventilation. 5. Method according to claim 4 characterized in that the radial ventilation results in the formation of a rotary gas ring whose maximum speed is at least equal to 2 m.s⁻¹ and at most equal to 50 m.s⁻¹. 6. Method according to any one of claims 1 to 5 characterized in that the modulated ventilation is at least partly an axial ventilation. 7. Method according to claim 6 characterized in that the maximum speed of the axial ventilation is at least equal to 10 m.s⁻¹ and at most equal to 100 m.s⁻¹. 8. Method according to any one of claims 1 to 7, characterized in that the cooling before pearlitization and / or the cooling after pearlitization are carried out at least in part by radial and / or axial ventilation. 9. Method according to claim 8 characterized in that during the cooling before pearlitization, the ventilation is at least partially radial with the formation of a rotary gas ring whose speed is at least equal to 2 ms⁻¹ and at most equal at 50 ms⁻¹, or axial with a speed between 10 and 100 ms⁻¹. 10. Method according to any one of claims 1 to 9 characterized in that the diameter of the wire is at least equal to 0.3 mm and at most equal to 3 mm. 11. Method according to claim 10 characterized in that the diameter of the wire is at least equal to 0.5 mm and at most equal to 2 mm. 12. Method according to any one of claims 1 to 11 characterized in that the cooling before pearlitization is carried out at an average speed of 100 to 400 ° C.s⁻¹. 13. Method according to any one of claims 1 to 12, characterized in that during step (b) the temperature of the wire does not differ by more than 5 ° C, by excess or by default, from this given temperature . 14. Device for thermally treating a carbon steel wire so as to obtain a fine pearlitic structure, characterized in that it comprises: a) means for cooling the wire, which has been previously maintained at a temperature above the transformation temperature AC3, these cooling means allowing the wire to reach a given temperature below the transformation temperature AC1 and above the temperature of the nose of the curve of the beginning of the transformation mation of the metastable austenite into perlite, the wire then having a metastable austenite structure without perlite; b) means then making it possible to adjust the temperature of the wire so that it does not differ by more than 10 ° C, by excess or by default, from this given temperature, for a time greater than the pearlitization time, these means comprising electric means for passing an electric current through the wire and means of modulated ventilation; c) means for subsequently cooling the wire. 15. Device according to claim 14 characterized in that the means for cooling the wire before and / or after pearlitization are ventilation means. 16. Device according to any one of claims 14 or 15, characterized in that the ventilation means make it possible to obtain at least partially a radial ventilation. 17. Device according to claim 16 characterized in that the ventilation means comprise at least one turbine. 18. Device according to claim 17 characterized in that the modulated ventilation means comprise several turbines and means making it possible to vary the speed of the turbines. 19. Device according to claim 16 characterized in that the ventilation means comprise at least one injector making it possible to obtain a tangential gas injection, setting in motion a rotary gas ring, the injection speed being perpendicular to the wire. 20. Device according to claim 19 characterized in that the modulated ventilation means comprise several tangential injection injectors, and means for adjusting the gas flow in these injectors. 21. Device according to any one of claims 14 to 20 characterized in that the ventilation means make it possible at least partially to obtain axial ventilation. 22. Device according to claim 21 characterized in that the modulated ventilation means comprise withdrawal pipes making it possible to modify the gas flow rate along the wire. 23. Steel wire obtained according to the process according to any one of claims 1 to 13. 24. Steel wire obtained with the device according to any one of claims 14 to 22. 25. Steel wire according to any one of claims 23 or 24 characterized in that it has a fine pearlitic structure, with cementite lamellae of thickness "i" and ferrite lamellae of thickness "e" such that the average value of the sum i + e is at most equal to 1000 Å with a standard deviation of 250 Å.

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