EP0347699B1 - Process and device for making a homogeneous austenitic structure - Google Patents

Abstract

Process and device (100) for heat-treating at least one carbon steel wire (1) so as to obtain a homogeneous austenite structure, characterised in that the wire (1) is heated in a tube (2) containing a gas (4) virtually free from forced ventilation, the gas (4) being directly in contact with the wire (1), the time for heating the wire (1) being less than 4 seconds per millimetre of diameter of the wire (1). Pearlitising installation (300) using such a process and such a device. <??>Steel wires obtained according to this process, this device or this installation. <IMAGE>

Description

The invention relates to methods and devices for thermally treating carbon steel wires so as to obtain a homogeneous austenite structure, these wires being for example capable of undergoing another heat treatment to obtain a fine pearlitic structure. .

• chauffage par induction dans lequel le fil est soumis à un champ magnétique ayant une fréquence de 5000 à 200 000 Hz ; ce procédé ne s'applique dans de bonnes conditions qu'à des fils d'un diamètre supérieur à 3 mm et pour des températures inférieures au point de Curie.
• chauffage dans un four à moufle à l'aide de résistances électriques ; ce procédé évite les inconvénients du chauffage par induction, mais il conduit à des temps de chauffage élevés de l'ordre de 10 à 15 secondes par millimètre de diamètre des fils.
• chauffage dans un four à gaz ; ce procédé conduit ici encore à des temps de chauffage élevés, du même ordre que ceux des fours à moufle, car la température des gaz à la sortie du four doit être faible si l'on veut obtenir un rendement thermique convenable, d'autre part la conductibilité thermique des gaz de combustion est moins bonne que celle des gaz utilisables dans un four à moufle (hydrogène, mélange d'hydrogène et d'azote, hélium) ; il est possible, dans les fours à gaz, de contrôler le pouvoir désoxydant des gaz de combustion, mais cela demande une surveillance très attentive du réglage des brûleurs à gaz.

The known processes for austenitizing steel wires in the process are notably the following:

• induction heating in which the wire is subjected to a magnetic field having a frequency of 5000 to 200,000 Hz; this process only applies under good conditions to wires with a diameter greater than 3 mm and for temperatures below the Curie point.
• heating in a muffle furnace using electric resistances; this process avoids the drawbacks of induction heating, but it leads to high heating times of the order of 10 to 15 seconds per millimeter of diameter of the wires.
• heating in a gas oven; this process again leads to high heating times, of the same order as those of muffle furnaces, because the temperature of the gases at the outlet of the oven must be low if one wants to obtain a suitable thermal efficiency, on the other hand the thermal conductivity of the combustion gases is less good than that of the gases usable in a muffle furnace (hydrogen, mixture of hydrogen and nitrogen, helium); it is possible, in gas ovens, to control the deoxidizing power of the combustion gases, but this requires very careful monitoring of the adjustment of the gas burners.

Document DE-A-2 111 631 describes a device for thermally treating metal wires so as to obtain a pearlitic structure. At the start of this treatment, the wires pass through a combustion oven or an electric oven to undergo an austenitization. The wires thus reach a temperature of 900 ° C to 1000 ° C.

Patent EP-B-0 326 005 describes a process and a device making it possible to heat treat at least one carbon steel wire so as to obtain a fine pearlitic structure by passing the wire through at least one tube containing a gas practically without forced ventilation, the tube being surrounded by a heat transfer fluid.

The object of the invention is to obtain heating times of less than 4 seconds per millimeter of wire diameter, during an austenitization treatment, which makes it possible to have higher production rates than with known installations, and which also makes it possible to reduce the lengths of the installations.

• a) on chauffe le fil en le faisant passer dans au moins un tube contenant un gaz pratiquement dépourvu de ventilation forcée, le gaz étant directement au contact du fil, le temps de chauffage du fil étant inférieur à 4 secondes par millimètre de diamètre du fil ;
• b) les caractéristiques du tube, du fil et du gaz sont choisies de telle sorte que les relations suivantes soient vérifiées :
  
  $\text{1,05 ≦ R ≦ 7 (1)}$
  
  
  
  $\text{0,6 ≦ K ≦ 8 (2)}$
  
  
  
  avec par définition
  
  ${\text{R = D}}_{\text{ti}}{\text{/D}}_{\text{f}}$
  
  
  ${\text{K = [Log (D}}_{\text{ti}}{\text{/D}}_{\text{f}}{\text{)]<figure-callout id="2" label="xD f" filenames="imgf0001.png,imgf0002.png" state="{{state}}">xD</figure-callout>}}_{\text{f}}\text{²/λ}$
  
  
  
  D_ti étant le diamètre intérieur du tube exprimé en millimètres, D_f étant le diamètre du fil exprimé en millimètres, λ étant la conductibilité du gaz déterminée à 800°C, cette conductibilité étant exprimée en watts.m⁻¹.°k⁻¹, Log étant le logarithme népérien.
  L'invention concerne également un dispositif permettant de traiter thermiquement au moins un fil d'acier au carbone, de façon à obtenir une structure d'austénite homogène, le dispositif comportant les points suivants :
  • a) il comporte au moins un tube et des moyens permettant de faire passer le fil dans le tube ; le tube contient un gaz pratiquement dépourvu de ventilation forcée, directement au contact du fil, le dispositif comportant des moyens pour chauffer le gaz ; les moyens permettant de faire passer le fil dans le tube sont tels que le temps de contact du fil avec le gaz soit inférieur à 4 secondes par millimètre de diamètre du fil ;
  • b) les caractéristiques du tube, du fil et du gaz sont choisies de telle sorte que les relations (1) et (2) précédentes soient vérifiées D_ti, D_f, λ et Log ayant les mêmes définitions que précédemment indiqué.

Consequently, the process according to the invention for heat treating at least one carbon steel wire, so as to obtain a homogeneous austenite structure comprises the following points:

• a) the wire is heated by passing it through at least one tube containing a gas practically without forced ventilation, the gas being directly in contact with the wire, the wire heating time being less than 4 seconds per millimeter of wire diameter ;
• b) the characteristics of the tube, wire and gas are chosen so that the following relationships are verified:
  
  $\text{1.05 ≦ R ≦ 7 (1)}$
  
  
  
  $\text{0.6 ≦ K ≦ 8 (2)}$
  
  
  
  with by definition
  
  ${\text{R = D}}_{\text{ti}}{\text{/ D}}_{\text{f}}$
  
  
  ${\text{K = [Log (D}}_{\text{ti}}{\text{/ D}}_{\text{f}}{\text{)] <figure-callout id="2" label="xD f" filenames="imgf0001.png,imgf0002.png" state="{{state}}">xD </figure-callout>}}_{\text{f}}\text{² / λ}$
  
  
  
  D _ti being the inside diameter of the tube expressed in millimeters, D _f being the diameter of the wire expressed in millimeters, λ being the conductivity of the gas determined at 800 ° C, this conductivity being expressed in watts.m⁻¹. ° k⁻¹ , Log being the natural logarithm.
  The invention also relates to a device making it possible to heat treat at least one carbon steel wire, so as to obtain a homogeneous austenite structure, the device comprising the following points:
  • a) it comprises at least one tube and means making it possible to pass the wire through the tube; the tube contains a gas practically without forced ventilation, directly in contact with the wire, the device comprising means for heating the gas; the means for passing the wire through the tube are such that the contact time of the wire with the gas is less than 4 seconds per millimeter of wire diameter;
  • b) the characteristics of the tube, the wire and the gas are chosen so that the preceding relations (1) and (2) are verified D _ti , D _f , λ and Log having the same definitions as previously indicated.

The term "practically without forced ventilation" means that the gas in the tube is either immobile or subjected to weak ventilation which practically does not modify the heat exchanges between the wire and the gas, this weak ventilation being for example due only to the movement of the wire itself.

The invention also relates to the methods and complete installations for heat treatment of carbon steel wires using the methods and / or the devices described above.

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 dispositif conforme à l'invention, cette figure étant une coupe effectuée selon l'axe du dispositif ;
• la figure 2 représente en coupe le dispositif représenté à la figure 1, cette coupe qui est effectuée perpendiculairement à l'axe du dispositif, étant représentée par les segments de ligne droite II-II à la figure 1 ;
• la figure 3 représente en coupe un autre dispositif conforme à l'invention, cette coupe étant effectuée selon l'axe du dispositif ;
• la figure 4 représente en coupe le dispositif représenté à la figure 3, cette coupe, qui est effectuée perpendiculairement à l'axe du dispositif, étant représentée par les segments de ligne droite IV-IV à la figure 3 ;
• la figure 5 représente une installation complète de traitement thermique d'un fil métallique, cette installation comportant un dispositif conforme à l'invention ;
• la figure 6 représente une courbe montrant l'évolution de la température en fonction du temps pour le fil traité dans l'installation de la figure 5 ;
• la figure 7 représente un dispositif utilisé dans l'installation de la figure 5, cette figure étant une coupe effectuée selon l'axe du dispositif ;
• la figure 8 représente le dispositif de la figure 7 selon une coupe perpendiculaire à l'axe du dispositif, cette coupe étant indiquée par les segments de ligne droite VIII-VIII à la figure 7 ;
• la figure 9 représente en coupe une portion de la structure perlitique fine du fil traité dans l'installation représentée à la figure 5.

On the drawing :

• Figure 1 shows a device according to the invention, this figure being a section taken along the axis of the device;
• 2 shows in section the device shown in Figure 1, this section which is made perpendicular to the axis of the device, being represented by the straight line segments II-II in Figure 1;
• Figure 3 shows in section another device according to the invention, this section being taken along the axis of the device;
• 4 shows in section the device shown in Figure 3, this section, which is made perpendicular to the axis of the device, being represented by the straight line segments IV-IV in Figure 3;
• FIG. 5 represents a complete installation for heat treatment of a metal wire, this installation comprising a device according to the invention;
• FIG. 6 represents a curve showing the evolution of the temperature as a function of time for the wire treated in the installation of FIG. 5;
• 7 shows a device used in the installation of Figure 5, this figure being a section taken along the axis of the device;
• 8 shows the device of Figure 7 in a section perpendicular to the axis of the device, this section being indicated by the straight line segments VIII-VIII in Figure 7;
• FIG. 9 represents in section a portion of the fine pearlitic structure of the wire treated in the installation shown in FIG. 5.

Figures 1 and 2 show a device 100 according to the invention for implementing the method according to the invention. FIG. 1 is a section of the device 100 along the axis xx ′ of this device, FIG. 2 is a section perpendicular to this axis xx ′, the section of FIG. 2 being shown diagrammatically by the straight line segments II-II to Figure 1. The device 100 comprises a tube 2, for example ceramic, refractory steel or tungsten carbide, in which the wire 1 runs in carbon steel along arrow F, along the axis xx ′.

The wire drive means 1 are known means not shown in these Figures 1 and 2 for the purpose of simplification, these means comprising for example a winder actuated by a motor, for winding the wire after treatment.

The space 3 between the wire 1 and the internal wall 20 of the tube 2 is filled with a gas 4. This gas 4 is directly in contact with the wire 1 and the internal wall 20. The gas 4 remains in the space 3 during the treatment of the wire 1, the device 100 being devoid of means capable of allowing forced ventilation of the gas 4, that is to say that the gas 4 without forced ventilation is possibly set in motion in space 3 as by the displacement of the wire 1 according to arrow F. This gas is for example hydrogen, a mixture of hydrogen and nitrogen, a mixture of hydrogen and methane, a mixture of hydrogen, nitrogen, and methane, helium, a mixture of helium and methane.

The wire 1 is guided by two wire guides 5, for example made of ceramic or tungsten carbide located at the entry and the exit of the wire 1 in the tube 2. The tube 2 is heated externally by an electric resistance 6 wound around the tube 2 and outside this tube 2 against the external wall 21 of the tube 2. The tube 2 is thermally insulated from the outside by the sleeve 7 surrounding the tube 2 and by the two plates 8 located at the ends of the tube 2. Tube 2 is also electrically isolated in case where it is metallic. The plates 8 and the sleeve 7 are for example made with sintered refractory fibers. The tube 2, the heating resistor 6, the sleeve 7 and the plates 8 are placed inside a metal tube 9 which is cooled by a hollow tube 10 wound around the tube 9, this hollow tube 10 being traversed by a cooling fluid 11, for example water.

The device 100 is closed at both ends by circular plates 12 which are applied to the flanges 90 of the tube 9, by means of gas-tight seals 13. The sealed passage 14 allows the electrical supply of the resistor 6. This passage 14 is crossed by two electric wires 15 each connected to one end of the resistor 6 (this connection is not shown in the drawing for the purpose of simplification) . This sealed passage 14 is fixed to one of the two circular plates 12 with gas-tight seals 16.

The device 100 comprises an expansion clearance 17, the springs 18 act on the plate 19 serving for the distribution of the forces, which makes it possible to maintain the tube 2 in the middle of the sleeve 7 whatever its temperature.

In FIG. 2, D _f represents the diameter of the wire 1, D _ti represents the internal diameter of the tube 2 (diameter of the internal wall 20), D _te represents the external diameter of the tube 2 (diameter of the external wall 21). λ is the conductivity of gas 4 determined at 800 ° C, this conductivity being expressed in watts.m⁻¹. ° K⁻¹.

$\text{0,6 ≦ K ≦ 8 (2)}$

avec par définition

${\text{R = D}}_{\text{ti}}{\text{/D}}_{\text{f}}$

${\text{K = [Log(D}}_{\text{ti}}{\text{/D}}_{\text{f}}{\text{)]xD}}_{\text{f}}\text{²/λ}$

D_ti et D_f étant exprimés en millimètres, Log étant le logarithme népérien.In accordance with the invention, D _ti , D _f , and λ are chosen so as to verify the following relationships:

$\text{1.05 ≦ R ≦ 7 (1)}$

$\text{0.6 ≦ K ≦ 8 (2)}$

with by definition

${\text{R = D}}_{\text{ti}}{\text{/ D}}_{\text{f}}$

${\text{K = [Log (D}}_{\text{ti}}{\text{/ D}}_{\text{f}}{\text{)] <figure-callout id="2" label="xD f" filenames="imgf0001.png,imgf0002.png" state="{{state}}">xD </figure-callout>}}_{\text{f}}\text{² / λ}$

D _ti and D _f being expressed in millimeters, Log being the natural logarithm.

The invention thus makes it possible, unexpectedly, to heat the wire 1 from a temperature below the transformation temperature AC3, for example from ambient temperature, to a temperature above the transformation temperature AC3, so as to obtain a homogeneous austenite structure, and this for a very short time less than 4 seconds per millimeter in diameter of the wire D _f . On the other hand, one can choose, if desired, the nature of the gas 4 so that it exerts a chemical action on the surface of the wire, for example a deoxidizing, fuel or decarburizing action.

• simplicité, coûts d'investissement et de fonctionnement peu élevés, car on se dispense d'employer des compresseurs ou des turbines qui seraient nécessaires avec une circulation de gaz forcée ;
• on peut obtenir une loi de réchauffement précise ;
• le réchauffement est rapide, ce qui permet d'augmenter les cadences de fabrication et de diminuer la longueur des installations ;
• le réchauffement rapide peut s'appliquer à des fils dont le diamètre D_f varie dans de larges limites, le même dispositif permettant notamment de traiter des fils dont les diamètres D_f varient dans un rapport de 1 à 5.

The invention therefore has the following advantages:

• simplicity, low investment and operating costs, because there is no need to use compressors or turbines which would be necessary with forced gas circulation;
• you can get a precise warming law;
• the heating is rapid, which makes it possible to increase the production rates and reduce the length of the installations;
• rapid warming can be applied to wires whose diameter D _f varies within wide limits, the same device making it possible in particular to treat wires whose diameters D _f vary in a ratio of 1 to 5.

For wires with a large diameter D _f , greater than 4 mm, the ratio R is close to 1 and the use of a very good heat conducting gas, for example hydrogen, then becomes necessary.

Preferably the diameter D _f of the wire is at least equal to 0.4 mm and at most equal to 6 mm.

FIGS. 3 and 4 represent another device 200 according to the invention, this device making it possible to simultaneously treat several wires 1, for example six, FIG. 3 being a section of this device along the axis yy ′ of this device and the FIG. 4 being a section perpendicular to the axis of this device, the axis yy ′ being represented by the reference "y" in FIG. 4.

The structure of this device 200 is similar to that of the device 100 with the difference that six tubes 2 are arranged in the enclosure 9 constituted by a steel tube, around the axis yy ′ which is the axis of this tube 9. A wire 1 passes through each tube 2, the gas 4 being placed inside the tubes 2 which are each heated by a resistor 6 as previously described for the device 100, the insulating sleeve 7 being arranged around the six tubes 2 .

The examples which follow make it possible to better understand the invention.

Examples 1 to 4

Four examples of treatment of a carbon steel wire 1 are carried out with the device 100 previously described. The characteristics of the wire 1 and of the device 100 are given in the following table 1.

• . exemples 1, 2, 3 : ammoniac craqué (75 % d'hydrogène, 25 % d'azote, ces % étant exprimés en volumes)
• . exemple 4 : 78 % d'hydrogène, 2 % de méthane (% en volumes)

Le temps de chauffage T_c correspond au temps nécessaire pour que le fil passe de la température ambiante (environ 20°C) qu'il a, à l'entrée du tube, à la température qu'il a à la sortie du tube (980°C), cette température étant suffisante pour mettre les carbures en solution.The nature of gas 4 was as follows for the examples.

• . Examples 1, 2, 3: cracked ammonia (75% hydrogen, 25% nitrogen, these% being expressed by volume)
• . Example 4: 78% hydrogen, 2% methane (% by volume)

The heating time T _c corresponds to the time necessary for the wire to go from the ambient temperature (about 20 ° C.) which it has, at the inlet of the tube, to the temperature which it has at the outlet of the tube ( 980 ° C), this temperature being sufficient to put the carbides in solution.

Example 5

In this example, the diameter D _f of the wire 1 and the nature of the gas 4 which is a mixture of hydrogen and nitrogen are varied and therefore the values of λ, R and K. The characteristics of the wire 1 and of the device 100 are as follows: Carbon content of the steel of wire 1 = 0.85%; alumina tube 2, D _ti = 2.5 mm, D _te = 6 mm; the outer face 21 of the tube 2 is heated to 1100 ° C with an electrical resistance 6 having a power of 33 kW; wire running speed 1: 2.35 m / sec; length of tube 2: 6 m; heating time: 2.55 sec; temperature of wire 1: at the inlet of tube 2: 20 ° C, at the outlet of tube 2: 980 ° C.

The following table 2 gives the values of D _f , the volumetric% of gas 4 in hydrogen, the values of λ, R, K, as well as the production of wire 1.

For all the tests corresponding to this example, the heating time per millimeter of wire diameter (T _c / D _f ) varies from 1.46 to 3.1 sec / mm; Table 2 Wire diameter 1 (mm) (D _f ) R % H₂ λ at 800 ° C (Wm⁻¹. ° K⁻¹) K Yarn production 1 in kg / hour 1.75 1.43 100 0.487 2.24 158.0 1.55 1.61 98 0.472 2.43 124.0 1.30 1.92 90 0.418 2.64 87.0 0.94 2.66 69 0.297 2.91 45.8 0.82 3.05 62 0.263 2.85 35.0

Example 6

A multitubular device similar to the device 200 described above is used, but with ten tubes 2. The characteristics of the example are as follows:
Carbon content of the steel of wire 1: 0.70%; wire diameter D _f : 1.75 mm; identical tubes 2 in alumina, D _ti = 2.5 mm, D _te = 6 mm; the external faces 21 of the tubes are heated to 1100 ° C. using 10 resistors 6 (one resistance per tube 2), each resistance having a unit power of 27 kW (total power 270 kW); gas 4: cracked ammonia: wire running speed: 2.02 m / sec; length of each tube 2: 6 m; heating time 2.97 sec; production of wire 1: 1360 kg / hour; wire temperature at the inlet of each tube 2: 20 ° C, at the outlet of each tube 2: 980 ° C; λ = 0.328; R = 1.43; K = 3.33. The heating time per millimeter of wire diameter (T _c / D _f ) is equal to 1.70 sec / mm.

Example 7

This example is carried out under the same conditions and with the same results as example n ° 2 but by replacing the cracked ammonia with a gas 4 maintaining the thermodynamic equilibrium with the carbon of the steel at 800 ° C., this gas 4 having the following composition (% by volume): 74% hydrogen; 24% nitrogen; 2% methane.

Example 8

This example is carried out under the same conditions as example n ° 2 but the cracked ammonia is replaced by a fuel gas making it possible to correct a decarburization which occurred in the previous operations. The composition of gas 4 is as follows in this example (% volumetric): 85% hydrogen, 15% methane. The other conditions and results are the same as for Example 2 with the following differences: the heating time goes from 2.97 to 2.75 seconds, the ratio T _c / D _f then being equal to 1.57 sec / mm, the wire running speed is 2.18 m / sec. a thickness of superficial recarburization is obtained on the order of 2 μm. No graphite deposition is observed on wire 1.

The invention makes it possible to obtain a very precise wire temperature at the outlet of the treatment, this temperature not varying by more than 1.5 ° C. by excess or by default of the temperature indicated at the outlet of the tubes 2, for the examples 1 to 8, which guarantees good consistency in the quality of the wire.

Examples 9 to 12 which follow are produced in a device similar to the device 100 previously described, but these examples are not in accordance with the invention. The characteristics of the wire 1 and of this device are given in the following table 3. These examples are characterized by a ratio T _c / D _f notably greater than 4 seconds per mm of diameter of the wire, the values of the ratios R and K not corresponding to the set of relations (1) and (2) previously indicated and the austenitization does not can not then be performed with the advantages described above.

• . exemple 9 N₂ pur
• . exemple 10 N₂ = 50 % H₂ = 50 %
• . exemple 11 N₂ = 65 % H₂ = 35 %
• . exemple 12 N₂ = 50 % H₂ = 50 %
  (% volumétriques)

Dans tous les exemples conformes à l'invention, on obtient une structure d'austénite homogène.The nature of gas 4 was as follows for these examples 9 to 12

• . example 9 pure N₂
• . example 10 N₂ = 50% H₂ = 50%
• . example 11 N₂ = 65% H₂ = 35%
• . example 12 N₂ = 50% H₂ = 50%
  (% volumetric)

In all the examples according to the invention, a homogeneous austenite structure is obtained.

• La zone Z₁ correspond à l'échauffement du fil 1 pour obtenir une structure d'austénite homogène ;
• la zone Z₂ correspond au refroidissement du fil 1 jusqu'à une température de 500 à 600°C de façon à obtenir une austénite métastable ;
• la zone Z₃ correspond à la transformation d'austénite métastable en perlite ;
• la zone Z₄ correspond à un refroidissement du fil 1, après perlitisation, jusqu'à une température par exemple d'environ 300°C ;
• la zone Z₅ correspond à un refroidissement final du fil 1 pour l'amener à une température proche de la température ambiante, par exemple de 20 à 50°C.

FIG. 5 represents a complete installation making it possible to heat treat a wire 1 of carbon steel in order to obtain a fine pearlitic structure. This installation 300 comprises the zones Z₁, Z₂, Z₃, Z₄, Z₅, the wire 1 passing through these zones, in the direction of the arrow F from the starting coil 30, to the coil 31 where the wire 1 is wound. treated, this coil 31 being actuated in rotation by the motor 310 which therefore allows the wire 1 to travel along the arrow F. The wire 1 passes successively and in this order through the zones Z₁ to Z₅.

• Zone Z₁ corresponds to the heating of wire 1 to obtain a homogeneous austenite structure;
• zone Z₂ corresponds to the cooling of the wire 1 to a temperature of 500 to 600 ° C so as to obtain a metastable austenite;
• zone Z₃ corresponds to the transformation of metastable austenite into perlite;
• zone Z₄ corresponds to cooling of the wire 1, after pearlitization, to a temperature for example of around 300 ° C;
• zone Z₅ corresponds to a final cooling of the wire 1 to bring it to a temperature close to ambient temperature, for example from 20 to 50 ° C.

FIG. 6 represents the curve φ showing the evolution of the temperature of the steel wire 1 as a function of time, when this wire crosses the zones Z₂ to Z₅. This figure also represents 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, for the steel of this wire. In this figure 6, the abscissa axis corresponds to time T and the ordinate axis corresponds to temperature ϑ, the origin of times corresponding to point A.

Prior to the pearlitization treatment, the wire 1 is heated and maintained at a temperature higher than the transformation temperature AC3 so as to obtain a homogeneous austenite, this temperature ϑ _A , for example between 900 ° C and 1000 ° C, corresponding to point A in FIG. 6. The point known as "pearlitic nose" corresponds to the minimum time T _m of the curve x₁, the temperature of this pearlitic nose being referenced ϑ _P.

The wire 1 is then cooled until it reaches a temperature below the transformation temperature AC1, the state of the wire after this cooling corresponding to point B, the temperature obtained at this point B at the end of time T _B being referenced ϑ _B. This temperature ϑ _B has been shown in FIG. 6 as being higher than the temperature ϑ _P of the pearlitic nose, which is the most frequent in practice, without being absolutely necessary. During this cooling of the wire between points A and B there is transformation of stable austenite into metastable austenite, as soon as the temperature of the wire drops below the transformation point AC3, and "seeds" appear at the grain boundaries of the metastable austenite. The area between the curves x₁, x₂ is referenced ω. Perlitization consists in passing the thread from the state represented by point B, to the left of zone ω, to a state represented by point C, to the right of zone ω. This transformation of the wire is for example shown diagrammatically by the straight line segment BC which intersects the curve x₁ at B _x and the curve x₂ at C _x , but the invention also applies to cases where the variation in temperature of the wire between the points B and C is not linear.

The formation of germs continues in the part of the segment BC situated to the left of the zone ω, that is to say in the segment BBx. In the part of the segment BC crossing the zone ω, that is to say in the segment B _x C _x , there is transformation of a metastable austenite into perlite, that is to say perlitization. The pearlitization time may vary from one steel to another, so the treatment represented by the segment C _x C aims to avoid applying premature cooling to the wire in case the pearlitization is not completed . Indeed, residual metastable austenite which would undergo rapid cooling would transform into bainite which is not a structure favorable to wire drawing after heat treatment, nor to the use value and the mechanical properties of the final product.

Rapid cooling between points A and B followed by isothermal maintenance in the field of metastable austenite, that is to say between points B and B _x allows an increase in the number of germs and a decrease in their cut. These 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 numerous and smaller.

After the pearlitization treatment, the wire is cooled, for example to room temperature, this cooling, preferably rapid, being shown diagrammatically for example by the curved line segment CD, the temperature at D being referenced ϑ _D.

In the installation 300, the zone Z₁ corresponds to the heating of the wire 1 to bring it to the state corresponding to the point A, the zone Z₂ corresponds to the cooling represented by the portion AB of the curve φ, the zone Z₃ corresponds to the portion BC of the curve φ, the zones Z₄ and Z₅ together correspond to the cooling represented by the portion CD of the curve φ.

The zone Z₁ is produced for example with the device 100 according to the invention described above.

The zone Z₂ is produced for example in accordance with the French patent application n ° 88/00904. The device 32 corresponding to this zone Z₂ is shown in FIGS. 7 and 8.

This device 32 is a heat exchanger comprising an enclosure 33 in the form of a tube with an internal diameter D ′ _ti and an external diameter D ′ _te in which the wire 1 to be treated travels along the arrow F, of diameter D _f .

FIG. 7 is a section taken along the axis xx ′ of the wire 1 which is also the axis of the device 32, and FIG. 8 is a section made perpendicular to this axis xx ′, the section of FIG. 8 being shown diagrammatically by the straight line segments VIII-VIII, in FIG. 7, the axis xx ′ being shown diagrammatically by the letter "x" in FIG. 8. The space 34 between the wire 1 and the tube 33 is filled with a gas 35 which is directly in contact with the wire 1 and the inner wall 330 of the tube 33. The gas 35 remains in the space 34 during the treatment of the wire 1, the device 32 being devoid of means capable of allowing forced ventilation of the gas 35, that is to say that the gas 35 practically without forced ventilation is possibly set in motion in the space 34 only by the displacement of the wire 1 according to arrow F. During the heat treatment of the wire 1, heat is transferred from wire 1 to gas 35. λ ′ is the conductivity of gas 35 d complete at 600 ° C. This conductivity is expressed in watts.m⁻¹. ^o K⁻¹. The wire 1 is guided by two wire guides 36 made for example of ceramic or tungsten carbide, these guides 36 being located one at the inlet, the other at the outlet of the wire 1 in the tube 33. The tube 33 is cooled externally by a heat transfer fluid 37, for example water circulating in an annular sleeve 38 which surrounds the tube 33. This sleeve 38 has a length L ′ _m , an internal diameter D ′ _mi , an external diameter D ′ _Me . The sleeve 38 is supplied with water 37 through the tubing 39, the water 37 leaves the sleeve 38 through the tubing 40, the flow of water 37 along the tube 33 thus taking place in the opposite direction to the direction F The seal between the zone 41 containing water 37 (interior volume of the sleeve 38) and the space 34 containing the gas 35 is obtained using seals 42 produced for example in elastomers. The length of the tube 33 in contact with the fluid 37 is referenced L ′ _t in FIG. 7.

The exchanger 32 can in itself constitute a device for the zone Z₂. It is also possible to assemble several exchangers 32, along the axis xx ', by means of the flanges 43 constituting the ends of the sleeve 38, the wire 1 then passing through several exchangers 32 arranged in series along the axis xx'.

$\text{5 ≦ K' ≦ 10 (4)}$

avec, par définition :

${\text{R' =D'}}_{\text{ti}}{\text{/D}}_{\text{f}}$

${\text{K' = [Log (D'}}_{\text{ti}}{\text{/D}}_{\text{f}}{\text{)]xD}}_{\text{f}}\text{²/λ'}$

D'_ti et D_f étant exprimés en millimètres, λ' étant la conductibilité du gaz déterminée à 600°C et exprimée en watts.m⁻¹.°K⁻¹, Log étant le logarithme népérien.The characteristics of the tube 33, of the wire 1 and of the gas 35 are chosen so that the following relationships are verified, during the cooling preceding the pearlitization and shown diagrammatically by the part AB of the curve φ:

$\text{1.05 ≦ R '<15 (3)}$

$\text{5 ≦ K '≦ 10 (4)}$

with, by definition:

${\text{R '= D'}}_{\text{ti}}{\text{/ D}}_{\text{f}}$

${\text{K '= [Log (D'}}_{\text{ti}}{\text{/ D}}_{\text{f}}{\text{)] <figure-callout id="2" label="xD f" filenames="imgf0001.png,imgf0002.png" state="{{state}}">xD </figure-callout>}}_{\text{f}}\text{² / λ '}$

D ' _ti and D _f being expressed in millimeters, λ' being the conductivity of the gas determined at 600 ° C and expressed in watts.m⁻¹. ° K⁻¹, Log being the natural logarithm.

The gas 35 is for example hydrogen, nitrogen, helium, a mixture of hydrogen and nitrogen, hydrogen and methane, nitrogen and methane, helium and methane, d 'hydrogen, nitrogen and methane.

For wires 1 of large diameter, the ratio R 'between the internal diameter D' _ti and the diameter D _f of the wire must be close to 1, and the use of a very conductive gas 35, for example hydrogen , becomes necessary.

The zone Z₃ of the installation 300 is produced for example by using several exchangers 32 arranged in series, under the conditions described below.

To obtain a transformation of austenite into perlite under the best conditions, it is preferable that the steps of transformation of the wire 1 shown diagrammatically by the line BC in FIG. 1 are carried out at a temperature which varies as little as possible, the temperature of the wire 1, for example, not differing by more than 10 ° C by excess or by default of the temperature ϑ _B obtained after the cooling shown diagrammatically by the line AB. This limitation of the variation in temperature is therefore carried out for a time greater than the pearlitization time, this pearlitization time corresponding to the segment B _x C _x . Advantageously, the temperature of the wire 1 does not differ by more than 5 ° C by excess or by default of the temperature ϑ _B on this line BC. FIG. 6 represents for example the ideal case where the temperature is constant and equal to ϑ _B during the stages shown diagrammatically by the line BC which is therefore a line segment parallel to the abscissa axis.

The transformation of austenite into perlite which takes place in the domain ω gives off an amount of heat of approximately 100,000 J.Kg⁻¹, with a transformation speed which varies in this domain as a function of time, this speed being low. near the points B _x and C _x and maximum towards the middle of the segment B _x C _x . Under these conditions, if one wants a practically constant temperature during this transformation, it is necessary to carry out modulated heat exchanges, that is to say heat exchanges whose power per unit of length of the wire 1 varies along the device where this transformation takes place, the cooling due to gas 35 being maximum when the pearlitization speed is maximum, this in order to avoid the phenomenon of recalescence due to an excessive temperature rise of the wire 1 during pearlitization .

This modulation can preferably be carried out by varying either the internal diameter D ′ _ti of the tubes 33 through which the wire passes, or the length L ′ _t of the various tubes 33 through which the wire passes, as described in the patent application French aforementioned ^No. 88/00904.

• si la modulation est réalisée en faisant varier le diamètre intérieur D′_ti des tubes 33, ce diamètre diminue depuis l'entrée de la zone Z₃ jusqu'à l'échangeur 32 où la vitesse de perlitisation est la plus grande, puis ce diamètre augmente ensuite en direction de la sortie de la zone Z₃, dans le sens de la flèche F ;
• si la modulation est réalisée en faisant varier la longueur L′_t des tubes 33, cette longueur augmente depuis l'entrée de la zone Z₃ jusqu'à l'échangeur 32 où la vitesse de perlitisation est la plus rapide, puis cette longueur diminue ensuite en direction de la sortie de la zone Z₃ dans le sens de la flèche F.

In the zone Z₃, the exchanger 32 whose cooling power is the highest corresponds to the region where the pearlitization speed is the greatest. In these conditions :

• if the modulation is carried out by varying the internal diameter D ′ _ti of the tubes 33, this diameter decreases from the entry of the zone Z₃ to the exchanger 32 where the pearlitization speed is the greatest, then this diameter increases then towards the exit from zone Z₃, in the direction of arrow F;
• if the modulation is carried out by varying the length L ′ _t of the tubes 33, this length increases from the entry of the zone Z₃ to the exchanger 32 where the pearlitization speed is the fastest, then this length then decreases towards the exit from zone Z de in the direction of arrow F.

In both cases, in the direction of arrow F, an increase in the cooling power is caused from the entry of zone Z₃ to the exchanger 32 where the pearlitization speed is the fastest, then this power then decreases towards the exit from zone Z₃.

$\text{3 ≦ K′ ≦ 8 (6)}$

R′ et K′ ayant les mêmes définitions que précédemment.In this exchanger 32 where the pearlitization speed is the fastest, there are preferably the following relationships:

$\text{1.05 ≦ R ′ ≦ 8 (5)}$

$\text{3 ≦ K ′ ≦ 8 (6)}$

R ′ and K ′ having the same definitions as above.

The zone Z₄ is constituted for example by an exchanger 32 verifying the relationships (3) and (4) previously defined.

The wire 1 then enters the zone Z₅ where it is brought to a temperature close to ambient temperature, by example from 20 to 50 ° C, by immersion in water.

The wire 1 treated in the installation 300 has the same structure as that obtained by the known lead patenting process, that is to say a fine pearlitic structure. This structure consists of cementite lamellae separated by ferrite lamellae. By way of example, FIG. 9 represents in section a portion 50 of such a fine pearlitic structure. This portion 50 comprises two substantially parallel cementite lamellae 51 separated by a ferrite lamella 52. The thickness of the cementite lamellae 51 is represented by "i" and the thickness of the ferrite lamellae 52 is represented by "e". The pearlitic structure is fine, that is to say that the average value i + e is at most equal to 1000 Å, with a standard deviation of 250 Å.

Such a wire can be used, for example, to reinforce articles made of plastics or rubbers, in particular tire casings.

• Après traitement thermique et avant tréfilage, le fil présente une résistance de rupture à la traction au moins égale à 1300 MPa ;
• Le fil peut être tréfilé de façon à avoir un rapport des sections au moins égal à 40 ;
• Le fil, après tréfilage, présente une résistance de rupture à la traction au moins égale à 3000 MPa.

The installation 300 also makes it possible to obtain at least one of the following results:

• After heat treatment and before drawing, the wire has a tensile breaking strength at least equal to 1300 MPa;
• The wire can be drawn so as to have a section ratio at least equal to 40;
• The wire, after drawing, has a tensile breaking strength at least equal to 3000 MPa.

L'installation 300 présente les avantages suivants :

• simplicité, coûts d'investissement et de fonctionnement peu élevés, car :
  • . on évite l'emploi de métaux ou de sels fondus ;
  • . on se dispense d'employer des compresseurs ou des turbines qui seraient nécessaires avec une circulation de gaz forcée ;
• on peut obtenir une loi de refroidissement précise et éviter le phénomène de recalescence ;
• possibilité d'effectuer avec la même installation un traitement de perlitisation sur des diamètres D_f de fils qui peuvent varier dans de larges limites ;
• on évite tout problème d'hygiène et un nettoyage du fil n'est pas nécessaire puisqu'on évite l'emploi de métaux ou de sels fondus.

The report of the sections corresponds by definition to the report:

Installation 300 has the following advantages:

• simplicity, low investment and operating costs, because:
  • . the use of molten metals or salts is avoided;
  • . there is no need to use compressors or turbines which would be necessary with forced gas circulation;
• one can obtain a precise cooling law and avoid the phenomenon of recalescence;
• possibility of carrying out with the same installation a pearlitization treatment on diameters D _f of wires which can vary within wide limits;
• any hygiene problem is avoided and cleaning of the wire is not necessary since the use of metals or molten salts is avoided.

These advantages are only obtained when relations (3) and (4) are verified during the cooling shown diagrammatically by the portion AB of the curve φ (FIG. 6). When using tubes containing a gas without forced ventilation, the tube being surrounded by a heat transfer fluid, but the relationships (3) and (4) are not verified during the cooling preceding the pearlitization and corresponding to the portion AB of the curve φ, it is not possible to perform a correct perlitization.

Claims (27)

1. A method for the heat treatment of at least one carbon steel wire (1) so as to obtain a homogenous austenite structure, comprising the following features:

   a) the wire (1) is heated by passing it through at least one tube (2) containing a gas (4) which is practically without forced ventilation, the gas (4) being directly in contact with the wire (1), the wire heating time being less than 4 seconds per millimetre of diameter of the wire;

   b) the characteristics of the tube (2), the wire (1) and the gas (4) are so selected that the following relationships are satisfied:
   
   $\text{1.05 ≦ R ≦ 7 (1)}$

   

   
   
   $\text{0.6 ≦ K ≦ 8 (2)}$

   

   
   
   with, by definition,
   
   ${\text{R = D}}_{\text{ti}}{\text{/D}}_{\text{f}}$

   

   
   ${\text{K = [Log (D}}_{\text{ti}}{\text{/D}}_{\text{f}}{\text{)]xD}}_{\text{f}}\text{²/λ}$

   

   
   
   D_ti being the inside diameter of the tube expressed in millimetres, D_f being the diameter of the wire expressed in millimetres, λ being the conductivity of the gas determined at 800°C, this conductivity being expressed in watts.m⁻¹.°k⁻¹, Log being the natural logarithm.
2. A method according to Claim 1, characterised in that the tube (2) is heated on the outside by an electric resistor (6).
3. A method according to any one of Claims 1 or 2, characterised in that the gas (4) is in thermodynamic equilibrium with the carbon of the steel of the wire (1).
4. A method according to any one of Claims 1 or 2, characterised in that the gas (4) permits a surface recarburisation of the steel of the wire.
5. A method according to any one of Claims 1 to 4, characterised in that the gas exerts a deoxidising action on the surface of the wire (1).
6. A method according to any one of Claims 1 to 5, characterised in that a pearlitisation treatment is then carried out on the wire (1).
7. A method according to Claim 6, comprising the following features:

   c) the wire (1) is cooled from a temperature above the AC3 transformation temperature to a temperature below the AC1 transformation temperature;

   d) the pearlitisation treatment is then carried out at a temperature below the AC1 transformation temperature;

   e) this cooling and pearlitisation treatment is carried out by passing the wire (1) through at least one tube (33) containing a gas (35) which is practically without forced ventilation, the tube being surrounded by a heat transport fluid in such a manner that a transfer of heat takes place from the wire, through the gas and the tube, towards the heat transport fluid;

   f) the characteristics of the tube (33), the wire (1) and the gas (35) are so selected that the following relationships are satisfied at least upon the cooling preceding the pearlitisation:
   
   $\text{1.05 ≦ R' < 15 (3)}$

   

   
   
   $\text{5 ≦ K' ≦ 10 (4)}$

   

   
   
   with, by definition:
   
   ${\text{R' = D'}}_{\text{ti}}{\text{/D}}_{\text{f}}$

   

   
   ${\text{K' = [Log(D'}}_{\text{ti}}{\text{/D}}_{\text{f}}{\text{)]xD}}_{\text{f}}\text{²/λ'}$

   

   
   
   D'_ti being the inside diameter of the tube expressed in millimetres, D_f being the diameter of the wire expressed in millimetres, λ' being the conductivity of the gas determined at 600°C, this conductivity being expressed in watts.m⁻¹.°k⁻¹, Log being the natural logarithm.
8. A method according to Claim 7, characterised in that after having cooled the wire (1) from a temperature above the AC3 transformation temperature to a given temperature below the AC1 transformation temperature, the wire is maintained at a temperature which does not differ by more than 10°C plus or minus from said given temperature for a period of time greater than the pearlitisation time by modulating the heat exchanges, the following relationships being satisfied in the zone or zones of the tube or tubes (33) where the rate of pearlitisation is the fastest:
   
   $\text{1.05 ≦ R' ≦ 8 (5)}$

   

   
   
   $\text{3 ≦ K' ≦ 8 (6),}$

   
9. A method according to Claim 8, characterised in that the wire (1) is maintained at a temperature which does not differ by more than 5°C plus or minus from said given temperature.
10. A method according to any one of Claims 8 or 9, characterised in that the modulation is effected by varying the inside diameter (D'_ti) of the tube, or of at least one tube (33).
11. A method according to any one of Claims 8 to 10, characterised in that the modulation is effected using several tubes (33), the length (L'_t) of which is varied.
12. A method according to any one of Claims 6 to 11, characterised in that the wire (1) is then cooled.
13. A device (100) for the heat treatment of at least one carbon steel wire (1) so as to obtain a homogenous austenite structure, the device comprising the following features:

    a) it comprises at least one tube (2) and means for passing the wire through the tube; the tube (2) contains a gas (4) which is practically without forced ventilation, directly in contact with the wire, the device comprising means for heating the gas; the means for passing the wire through the tube are such that the time of contact of the wire with the gas is less than 4 seconds per millimetre of diameter of the wire;

    b) the characteristics of the tube (2), the wire (1) and the gas (4) are so selected that the following relationships are satisfied:
    
    $\text{1.05 ≦ R ≦ 7 (1)}$

    

    
    
    $\text{0.6 ≦ K ≦ 8 (2)}$

    

    
    
    with, by definition,
    
    ${\text{R = D}}_{\text{ti}}{\text{/D}}_{\text{f}}$

    

    
    ${\text{K = [Log(D}}_{\text{ti}}{\text{/D}}_{\text{f}}{\text{)]xD}}_{\text{f}}\text{²/λ}$

    

    
    
    D_ti being the inside diameter of the tube expressed in millimetres, D_f being the diameter of the wire expressed in millimetres, λ being the conductivity of the gas determined at 800°C, this conductivity being expressed in watts.m⁻¹.°k⁻¹, Log being the natural logarithm.
14. A device (100) according to Claim 13, characterised in that it comprises an electric resistor (6) arranged on the outside of the tube in order to heat it.
15. A device (100) according to any one of Claims 13 or 14, characterised in that the gas (4) is in thermodynamic equilibrium with the carbon of the steel of the wire.
16. A device (100) according to any one of Claims 13 or 14, characterised in that the gas (4) permits a surface recarburisation of the steel of the wire (1).
17. A device (100) according to any one of Claims 13 to 16, characterised in that the gas (4) is capable of exerting a deoxidising action on the surface of the wire (1).
18. A device (200) according to any one of Claims 13 to 17, characterised in that it comprises an enclosure (9) within which several tubes (2) are arranged.
19. A device (100, 200) according to any one of Claims 13 to 18, characterised in that the diameter D_f of the wire (1) varies from 0.4 to 6 mm.
20. A device (100, 200) according to any one of Claims 13 to 19, characterised in that it makes it possible to treat wires (1) within a diameter ratio D_f of 1 to 5.
21. An installation (300) for the heat treatment of at least one carbon steel wire (1) comprising at least one device (100, 200) according to any one of Claims 13 to 20.
22. A heat treatment installation (300) according to Claim 21, characterised in that behind the austenitisation device (100, 200) it comprises means for cooling the wire (1) and for obtaining a fine pearlitic structure, these means comprising the following features:

    c) these cooling and pearlitisation means comprise at least one tube (33) containing a gas (35) which is practically without forced ventilation, the tube being surrounded by a heat transport fluid (37) in such a manner that a transfer of heat takes place from the wire, through the gas and the tube, towards the heat transport fluid;

    d) the characteristics of the tube (33), the wire (1) and the gas (35) are so selected that the following relationships are satisfied, at least upon the cooling preceding the pearlitisation:
    
    $\text{1.05 ≦ R' ≦ 15 (3)}$

    

    
    
    $\text{5 ≦ K' ≦ 10 (4)}$

    

    
    
    with, by definition:
    
    ${\text{R' = D'}}_{\text{ti}}{\text{/D}}_{\text{f}}$

    

    
    ${\text{K' = [Log(D'}}_{\text{ti}}{\text{/D}}_{\text{f}}{\text{)]xD}}_{\text{f}}\text{²/λ'}$

    

    
    
    D'_ti being the inside diameter of the tube expressed in millimetres, D_f being the diameter of the wire expressed in millimetres, λ' being the conductivity of the gas determined at 600°C, this conductivity being expressed in watts.m⁻¹.°k⁻¹, Log being the natural logarithm.
23. An installation (300) according to Claim 22, characterised in that one or more tubes (33) are arranged in such a manner that after having cooled the wire (1) from a temperature above the AC3 transformation temperature to a given temperature below the AC1 transformation temperature, they make it possible to maintain the wire (1) at a temperature which does not differ by more than 10°C plus or minus from said given temperature for a period of time greater than the pearlitisation time by modulating the heat exchanges, the following relationships being satisfied in the zone or zones of the tube or tubes where the rate of pearlitisation is the fastest:
    
    $\text{1.05 ≦ R' ≦ 8 (5)}$

    

    
    
    $\text{3 ≦ K' ≦ 8 (6).}$

    
24. An installation (300) according to Claim 23, characterised in that said tube or tubes (33) are so arranged that the temperature of the wire (1) does not differ by more than 5°C plus or minus from said given temperature.
25. An installation (300) according to any one of Claims 23 or 24, characterised in that the inside diameter (D'_ti) of the tube or of at least one tube (33) varies in the pearlitisation means.
26. An installation (300) according to any one of Claims 23 to 25, characterised in that it comprises several tubes (33), the lengths (L'_t) of which vary, in the pearlitisation means.
27. An installation (300) according to any one of Claims 21 to 26, characterised in that it comprises means for cooling the wire (1) after pearlitisation.

Images