EP0326005B1 - Methods and device for heat treating carbon steel wires in order to obtain a fine perlitic structure - Google Patents

Abstract

The wire (1), prior to this treatment, has been held at a temperature greater then the transformation temperature AC3. The wire (1) is cooled and then the pearlitization treatment is carried out, this cooling and pearlitization treatment being carried out by causing the wire (1) to pass into at least one tube (3) containing a gas (12) virtually free of forced ventilation, the tube (3) being surrounded by a heat transfer fluid (9). <??>Device (100) permitting the implementation of this method. <??>Methods and complete installations for thermal treatment of carbon steel wires (1) using this method or this device. Steel wires (1) obtained according to these methods and/or with these devices and these installations. <IMAGE>

Description

The invention relates to methods and devices for heat treating carbon steel wires so as to obtain a fine pearlitic structure. These threads are used in particular to reinforce rubber and / or plastic articles, 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.

• une première phase qui consiste à chauffer le fil et à le maintenir à une température supérieure à la température de transformation AC3 de façon à obtenir une austénite homogène ;
• une deuxième phase qui consiste à refroidir le fil pour obtenir une structure perlitique fine.

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 the most used processes is a so-called "patenting" heat treatment which consists of austenitization of the wire at a temperature of 900 to 1000 ° C. followed by immersion in a bath of lead or of molten salts maintained at a temperature of 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 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.

European patent application no. EP-A-0 270 860 describes a process for thermally treating a carbon steel wire so as to obtain a fine pearlitic structure by regulating the temperature of the wire during the transformation of austenite into pearlite so that it does not differ not more than 10 ° C, by excess or by default, of a given temperature lower than the transformation temperature AC1 and higher than the temperature of the pearlitic nose, 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. This process makes it possible to avoid the use of metals or of molten salts, and it therefore eliminates the problems of hygiene and cleaning of the abovementioned wires, while leading to simpler installations and more flexible operation. However, this process requires the use of compressors or turbines to obtain modulated ventilation, which can lead to relatively high investment and operating costs. Furthermore, this process can only be used on an industrial scale for wires of relatively small diameter, for example at most equal to 3 mm.

Application DE-A-2 111 631 describes a process for thermally treating a steel wire so as to obtain a pearlitic structure. In this process, during the cooling preceding the pearlitization, the wire is passed through a tube containing a reducing or inert gas which may be practically immobile. This document gives no general relationship to be respected between the characteristics of the tube, the wire and the gas for this cooling.

The object of the invention is to make it possible to carry out a heat treatment for the transformation of austenite into perlite which avoids the use of metals or molten salts, as well as the use of forced ventilation, while making it possible to treat wires whose diameter can vary within wide limits.

• a) on refroidit le fil depuis une température supérieure à la température de transformation AC3 jusqu'à une température inférieure à la température de transformation AC1, pendant un temps inférieur à 5 secondes ;
• b) on effectue ensuite le traitement de perlitisation à une température inférieure à la température de transformation AC1 ;
• c) ce traitement de refroidissement et de perlitisation est effectué en faisant passer le fil dans au moins un tube contenant un gaz pratiquement dépourvu de ventilation forcée, le tube étant entouré par un fluide caloporteur de telle sorte qu'un transfert de chaleur s'effectue depuis le fil, à travers le gaz et le tube, vers le fluide caloporteur ;
  ce procédé étant caractérisé en ce que :
• d) les caractéristiques du tube, du fil et du gaz sont choisies de telle sorte que les relations suivantes soient vérifiées, au moins lors du refroidissement précédant la perlitisation :
  
  $\text{1,05 ≦ R < 15 (1)}$
  
  
  $\text{5 ≦ K ≦ 10 (2)}$
  
  
  
  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 étant le diamètre intérieur du tube exprimé en millimètres, D_f étant le diamètre du fil exprimé en millimètres, ce diamètre étant au plus égal à 6 mm, λ étant la conductibilité du gaz déterminée à 600°C, cette conductibilité étant exprimée en watts.m⁻¹. ⁰K⁻¹ , Log étant le logarithme népérien.

Consequently, the invention relates to a method for thermally treating at least one carbon steel wire so as to obtaining a fine pearlitic structure, the wire, prior to this treatment, having been maintained at a temperature higher than the transformation temperature AC3 to obtain a homogeneous austenite, this process comprising the following points:

• a) the wire is cooled from a temperature above the transformation temperature AC3 to a temperature below the transformation temperature AC1, for a time of less than 5 seconds;
• b) the pearlitization treatment is then carried out at a temperature below the transformation temperature AC1;
• c) this cooling and pearlitization treatment is carried out 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 so that heat transfer takes place from the wire, through the gas and the tube, to the heat transfer fluid;
  this process being characterized in that:
• d) the characteristics of the tube, wire and gas are chosen so that the following relationships are verified, at least during the cooling preceding the pearlitization:
  
  $\text{1.05 ≦ R <15 (1)}$
  
  
  $\text{5 ≦ K ≦ 10 (2)}$
  
  
  
  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 millimeters, D _f being the diameter of the wire expressed in millimeters, this diameter being at most equal to 6 mm, λ being the conductivity of the gas determined at 600 ° C., this conductivity being expressed in watts.m⁻¹. ⁰ K⁻¹, Log being the natural logarithm.

• a) des moyens permettant de refroidir le fil depuis une température supérieure à la température de transformation AC3 jusqu'à une température inférieure à la température de transformation AC1, pendant un temps inférieur à 5 secondes ;
• b) des moyens permettant d'effectuer le traitement de perlitisation à une température inférieure à la température de transformation AC1 ;
• c) ces moyens de refroidissement et de perlitisation comportent au moins un tube et des moyens pour faire passer le fil dans le tube, ce tube contenant un gaz pratiquement dépourvu de ventilation forcée, ce tube étant entouré par un fluide caloporteur de telle sorte qu'un transfert de chaleur s'effectue depuis le fil à travers le gaz et le tube, vers le fluide caloporteur ;
  ce dispositif étant caractérisé par les points suivants :
• d) les caractéristiques d'au moins un tel tube, du fil et du gaz sont choisies de telle sorte que les relations suivantes soient vérifiées, au moins lors du refroidissement précédant la perlitisation :
  
  $\text{1,05 ≦ R < 15 (1)}$
  
  
  $\text{5 ≦ K ≦ 10 (2)}$
  
  
  
  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 étant le diamètre intérieur du tube exprimé en millimètres, D_f étant le diamètre du fil exprimé en millimètres, ce diamètre étant au plus égal à 6 mm, λ étant la conductibilité du gaz déterminée à 600°C, cette conductibilité étant exprimée en watts.m⁻¹. ⁰K⁻¹ , Log étant le logarithme népérien ;
• e) un ou plusieurs tels tubes sont agencés de telle sorte qu'après refroidissement du fil depuis une température supérieure à la température de transformation AC3 jusqu'à une température donnée inférieure à la température de transformation AC1, ils permettent de maintenir le fil à une température qui ne diffère pas de plus de 10°C par excès ou par défaut de cette température donnée, pendant un temps supérieur au temps de perlitisation, en modulant les échanges thermiques, ce ou ces tubes constituant plusieurs zones tubulaires permettant ladite modulation, les relations suivantes étant vérifiées dans la ou les zones tubulaires où la vitesse de perlitisation est la plus rapide :
  
  $\text{1,05 ≦ R ≦ 8 (3)}$
  
  
  $\text{3 ≦ K ≦ 8 (4).}$
  

The invention also relates to a device making it possible to heat treat at least one carbon steel wire so as to obtain a fine pearlitic structure, the wire, prior to this treatment, having been maintained at a temperature higher than the transformation temperature AC3 to obtain a homogeneous austenite, this device comprising:

• a) means making it possible to cool the wire from a temperature above the transformation temperature AC3 to a temperature below the transformation temperature AC1, for a time of less than 5 seconds;
• b) means for performing the pearlitization treatment at a temperature below the transformation temperature AC1;
• c) these cooling and pearlitization means comprise at least one tube and means for passing the wire through the tube, this tube containing a gas practically without forced ventilation, this tube being surrounded by a heat transfer fluid so that heat is transferred from the wire through the gas and the tube to the heat transfer fluid;
  this device being characterized by the following points:
• d) the characteristics of at least one such tube, of the wire and of the gas are chosen so that the following relationships are verified, at least during the cooling preceding the pearlitization:
  
  $\text{1.05 ≦ R <15 (1)}$
  
  
  $\text{5 ≦ K ≦ 10 (2)}$
  
  
  
  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 millimeters, D _f being the diameter of the wire expressed in millimeters, this diameter being at most equal to 6 mm, λ being the conductivity of the gas determined at 600 ° C., this conductivity being expressed in watts.m⁻¹. ⁰ K⁻¹, Log being the natural logarithm;
• e) one or more such tubes are arranged such that after the wire has cooled from a temperature above the transformation temperature AC3 to a given temperature below the transformation temperature AC1, they make it possible to maintain the wire at a temperature which 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, by modulating the heat exchanges, this or these tubes constituting several tubular zones allowing said modulation, the relationships The following are verified in the tubular zone (s) where the pearlitization speed is the fastest:
  
  $\text{1.05 ≦ R ≦ 8 (3)}$
  
  
  $\text{3 ≦ K ≦ 8 (4).}$
  

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 or 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 des courbes de transformation d'austénite en perlite, ainsi qu'une courbe montrant l'évolution de la température en fonction du temps pour un fil d'acier traité de façon à obtenir une structure perlitique fine ;
• La figure 2 représente un dispositif conforme à l'invention, cette figure étant une coupe effectuée selon l'axe du dispositif ;
• La figure 3 représente le dispositif de la figure 2, selon une coupe perpendiculaire à l'axe du dispositif, cette coupe étant schématisée par les segments de ligne droite III-III à la figure 2 ;
• La figure 4 représente un autre dispositif conforme à l'invention, cette figure étant une coupe effectuée selon l'axe du dispositif ;
• La figure 5 représente le dispositif de la figure 4 selon une coupe perpendiculaire à l'axe du dispositif, cette coupe étant schématisée par les segments de ligne droite V-V à la figure 4.
• Les figures 6 et 7 représentent chacune un autre dispositif conforme à l'invention.
• La figure 8 représente une installation complète pour traiter thermiquement un fil d'acier, cette installation utilisant au moins un dispositif conforme à l'invention ;
• La figure 9 représente en coupe une portion de la structure perlitique fine d'un fil traité conformément à l'invention ;

On the drawing :

• FIG. 1 represents curves of transformation of austenite into perlite, as well as a curve showing the evolution of the temperature as a function of time for a steel wire treated so as to obtain a fine perlitic structure;
• 2 shows a device according to the invention, this figure being a section taken along the axis of the device;
• 3 shows the device of Figure 2, in a section perpendicular to the axis of the device, this section being shown schematically by the straight line segments III-III in Figure 2;
• FIG. 4 shows another device according to the invention, this figure being a section taken along the axis of the device;
• FIG. 5 represents the device of FIG. 4 according to a section perpendicular to the axis of the device, this section being shown diagrammatically by the straight line segments VV in FIG. 4.
• Figures 6 and 7 each show another device according to the invention.
• FIG. 8 represents a complete installation for heat treating a steel wire, this installation using at least one device according to the invention;
• FIG. 9 represents in section a portion of the fine pearlitic structure of a wire treated in accordance with the invention;

FIG. 1 represents the curve φ showing the evolution of the temperature of a steel wire as a function of time, when this wire is subjected to a pearlitization treatment. 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 1, the abscissa axis corresponds to time T and the ordinate axis corresponds to temperature ϑ.

Prior to the pearlitization treatment, the wire was 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 the point A in FIG. 1. The point called "pearlitic nose" corresponds to the minimum time T _m of the curve X₁, the temperature of this pearlitic nose being referenced ϑ _p . The origin O of times T corresponds to point A.

The wire is 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. 1 as being greater 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 the 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 is likely to 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 schematically for example by the curved line segment CD, the temperature at D being referenced ϑ _D.

Figures 2 and 3 show a device 100 according to the invention. This device 100 is a heat exchanger comprising an enclosure 3 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 runs along arrow F, the diameter of the wire 1 being referenced D _f , this wire 1 being a carbon steel wire.

Figure 2 is a section taken along the axis xx 'of the wire 1 which is also the axis of the device 100, and Figure 3 is a section made perpendicular to this axis xx', the section of Figure 3 being shown schematically by the straight line segments III-III, in Figure 2, the axis xx 'being shown schematically by the letter "x" in Figure 3. The drive means of the wire 1 are known means not shown in these Figures 2 and 3 for the purpose of simplification, these means comprising for example a winder actuated by a motor, for winding the wire after treatment. The space 6 between the wire 1 and the tube 3 is filled with a gas 12 which is directly in contact with the wire 1 and the inner wall 30 of the tube 3. The gas 12 remains in the space 6 during the treatment of the wire 1, the device 100 being devoid of means capable of allowing forced ventilation of the gas 12, that is to say that the gas 12 without forced ventilation is possibly set in motion in space 6 only by the displacement of the wire 1 according to arrow F. During the heat treatment of the wire 1, a heat transfer takes place from the wire 1 to the gas 12. λ is the conductivity of the gas 12 determined at 600 ° C. This conductivity is expressed in watts.m⁻¹. ⁰ K⁻¹. The wire 1 is guided by two wire guides 2 made for example of ceramic or tungsten carbide, these guides 2 being located one at the inlet, the other at the outlet of the wire 1 in the tube 3. The tube 3 is cooled externally by a heat transfer fluid 9, for example water circulating in an annular sleeve 4 which surrounds the tube 3. This sleeve 4 has a length L _m , an internal diameter D _mi , an external diameter D _me . The sleeve 4 is supplied with water 9 through the tubing 8, the water 9 leaves the sleeve 4 through the tubing 10, the flow of water 9 along the tube 3 thus taking place in the opposite direction to the direction F The seal between the zone 7 containing water 9 (internal volume of the sleeve 4) and the space 6 containing the gas 12 is obtained using seals 5 made for example of elastomers. The length of the tube 3 in contact with the fluid 9 is referenced L _t in FIG. 2.

The exchanger 100 can in itself constitute a device according to the invention. It is also possible to assemble several exchangers 100, along the axis xx ′, by means of the flanges 11 constituting the ends of the sleeve 4, the wire 1 then passing through several exchangers 100 arranged in series along the axis xx ′.

These devices allow the thermal treatment of the wire 1 represented by the part of the curve φ located between the points A and C, that is to say the treatment comprising a cooling followed by a pearlitization. These devices can also be used to cool the wire 1 after pearlitization, if desired, this cooling corresponding to the part CD of the curve φ.

$\text{5 ≦ K ≦ 10 (2)}$

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, λ étant la conductibilité du gaz déterminée à 600°C et exprimée en watts.m⁻¹.⁰K⁻¹, Log étant le logarithme népérien. D_f est au plus égal à 6 mm.The characteristics of the tube 3, the wire 1 and the gas 12 are chosen so that the following relationships are verified, at least during the cooling preceding the pearlitization and shown diagrammatically by the part AB of the curve φ:

$\text{1.05 ≦ R ≦ 15 (1)}$

$\text{5 ≦ K ≦ 10 (2)}$

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 and D _f being expressed in millimeters, λ being the gas conductivity determined at 600 ° C and expressed in watts.m⁻¹.⁻K⁻¹, Log being the natural logarithm. D _f is at most equal to 6 mm.

The gas 12 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 is close to 1, and the use of a very conductive gas 12, for example hydrogen, becomes necessary .

Figures 4 and 5 show another device 200 according to the invention with an axis yy ′, Figure 4 being a section along this axis and Figure 5 being a section perpendicular to this axis, the section of Figure 5 being shown schematically by the straight line segments VV in FIG. 4, the axis xx ′, being shown diagrammatically by the letter "x" and the axis yy ′ being shown diagrammatically by the letter "y", in FIG. 5.

This exchanger 200 is similar to the exchanger 100 previously described with the difference that it comprises six tubes 3 surrounded by the cylindrical sleeve 4, a wire 1 being disposed along the axis xx ′ of each of these tubes, this axis xx ′ therefore also being the axis of the wire 1 placed in this tube 3. Each of these tubes 3 is filled with gas 12, as for the exchanger 100, and the volume 7 inside the sleeve 4, outside the tubes 3 is the seat of a circulation of heat transfer fluid, for example water.

Like the exchanger 100, the exchanger 200 can alone constitute a device according to the invention, or be assembled coaxially with other exchangers 200 by means of flanges 11 constituting the ends of the sleeves 4, the wires 1 thus passing through several 200 exchangers arranged in series.

To obtain a transformation of austenite into perlite under the best conditions, it is preferable that the steps of transformation of the wire shown diagrammatically by the line BC in FIG. 1 are carried out at a temperature which varies as little as possible, the temperature of 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 BxCx segment. 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. Figure 1 shows for example the ideal case where the temperature is constant and equal to ϑ _B during the steps schematized 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 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 the gas 12 being maximum when the pearlitization speed is maximum, this in order to avoid the phenomenon of recalescence due to an excessive rise in temperature of the wire 1 during pearlitization .

This modulation can preferably be carried out by varying either the internal diameter of the tubes 3 through which the wire passes, or the length of the various tubes 3 through which the wire passes.

• le diamètre D_ti diminue de l'échangeur 100-2 à l'échangeur 100-4, de telle sorte que la puissance de refroidissement par unité de longueur croisse de l'échangeur 100-2 à l'échangeur 100-4 ;
• le diamètre D_ti augmente de l'échangeur 100-4 à l'échangeur 100-6, ce qui permet de réaliser des puissance de refroidissement par unité de longueur décroissantes.

FIG. 6 represents a device in which this modulation is carried out by varying the internal diameter of the tubes. This device 300 according to the invention comprises seven heat exchangers similar to the exchanger 100 previously described and shown in Figures 2 and 3. These exchangers referenced 100-1 to 100-7 are connected in series by their flanges 11, the wire 1 passing from the heat exchanger 100-1 to the heat exchanger 100-7 in the direction of arrow F, the pipe 10 for the water outlet of an exchanger being connected to the inlet pipe 8 of the heat exchanger previous, in the opposite direction to that of arrow F, the water 9 therefore flowing in series in these exchangers 100. For each of the exchangers 100, the internal diameter D _ti of the tube 3 is constant, but this diameter D _ti varies from heat exchanger 100-1 to heat exchanger 100-7 as follows:

• the diameter D _ti decreases from the heat exchanger 100-2 to the heat exchanger 100-4, so that the cooling power per unit of length increases from the heat exchanger 100-2 to the heat exchanger 100-4;
• the diameter D _ti increases from the exchanger 100-4 to the exchanger 100-6, which makes it possible to achieve decreasing cooling power per unit of length.

The lengths of elements, referenced Lm₁ to Lm₇, are constant for elements 100-1 to 100-7, as well as the lengths of tube 3 in contact with water, referenced L _t1 to L _t7 .

The 100-4 heat exchanger with the highest cooling power therefore corresponds to the zone where the pearlitization speed is greatest.

$\text{3 ≦ K ≦ 8 (4)}$

R et K ayant les mêmes définitions que précédemment.In this area, we have the following relationships:

$\text{1.05 ≦ R ≦ 8 (3)}$

$\text{3 ≦ K ≦ 8 (4)}$

R and K having the same definitions as above.

The device 400 shown in FIG. 7 has the same structure as the device 300 previously described, with seven exchangers referenced 100-1 to 100-7 connected in series by their flange 11. The difference with the device 300 comes from the fact that the exchangers 100 of this device 400 all have the same internal diameter D _ti for the tubes 3, and that the length L _t , measured parallel to the wire 1, of the tubes 3 in contact with the fluid 9 is varied, without making vary the diameter D _ti and this for an element length 100 which can be constant for all these elements, the element lengths, referenced Lm₁ to Lm₇ in FIG. 7 therefore having for example the same value, for the device 400.

In FIG. 7, the lengths of tubes 3 are referenced L _t1 to L _t7 for the exchangers 100-1 to 100-7 of the device 400. The exchangers 100-2 to 100-4 have lengths of tubes L _t2 to L _t4 increasing in the direction of arrow F, so that there is an increase in the average cooling power, reported per meter of wire, from exchanger 100-2 to exchanger 100-4. On the contrary, the lengths L _t4 to L _t6 decrease in the direction of arrow F, so that there is a decrease in the average cooling power, compared to the meter of wire, from the exchanger 100-4 to '' at exchanger 100-6. The heat exchanger 100-4, whose cooling power is the highest, here again corresponds to the zone where the pearlitization speed is the greatest and the relations (3) and (4) previously indicated for the device 300 are still here respected.

In devices 300 and 400 with modulation, relations (3) and (4) need only be checked for exchangers 100-4 where the pearlitization speed is the fastest.

In devices 300 and 400, exchangers 100-1 and 100-7 lead to small heat exchanges per unit of length, either because the corresponding diameter D _ti is high, in the case of device 300, or because the length L _ti corresponding is small, in the case of the device 400 and it is possible that these exchangers 100-1 and 100-7 do not verify any of the relations (1) to (4). These exchangers 100-1 and 100-7 correspond to the practically isothermal maintenance of the wire 1 before and after pearlitization, that is to say for the parts BBx and CxC of the segment BC situated outside the zone ω (FIG. 1 ) the temperature is therefore practically constant on the BC segment. The CxC segment corresponds to a practically isothermal maintenance after pearlitization, to avoid applying to the wire 1 premature cooling for the case where the pearlitization is not finished, since the pearlitization time is likely to vary from steel to steel. other as said before.

To obtain a constant temperature of wire 1 in exchangers 100-1 and 100-7, it may be advantageous to pass an electric current through wire 1, when it crosses these exchangers, it is also for this purpose replacing these exchangers 100-1 and 100-7 by muffle furnaces maintained at the temperature ϑ _B , the devices making it possible to pass the electric current, or these muffle furnaces not being represented in FIGS. 6 and 7 for the purpose of simplification.

The invention covers cases where both the diameter D _ti and the length L _t are varied in the same device. On the other hand, in devices 300 and 400, exchangers 200 connected in series could be used, so as to treat several wires simultaneously.

On the other hand, instead of using several tubes 3 of different diameters, it is possible to use a single tube, the diameter of which varies along its axis, to effect the modulation of the heat exchanges previously described while respecting relations (3) and (4) in the area where the pearlitization speed is maximum.

FIG. 8 represents the diagram of a complete installation for treating a wire 1, this installation according to the invention using at least one of the devices described above.

This installation 500 comprises five zones referenced Z₁ to Z₅. The wire 1 coming from the coil 13 is heated in the zone Z₁, in a known manner, for example by means of a gas or muffle furnace up to a temperature of 900 to 1000 ° C to obtain a homogeneous austenite corresponding to point A in FIG. 1, this temperature being higher than the transformation temperature AC3.

The wire 1 is then cooled in the zone Z₂ to a temperature of 500 to 600 ° C., so as to obtain a metastable austenite corresponding to point B in FIG. 1.

The wire 1 then passes through the zone Z₃ where it undergoes the treatments corresponding to the segment BC of FIG. 1. The wire then passes through the zone Z₄ where it is cooled to a temperature for example of around 300 ° C. The wire then enters the zone Z₅ where it is brought to a temperature close to room temperature, for example from 20 to 50 ° C, by immersion in water. The cooling effected in zones Z₄ and Z₅ corresponds to the segment CD in FIG. 1.

The wire 1 leaving the bath Z₅ is then wound on the coil 14.

The zones Z₂ to Z₄ can for example use exchangers of the same type as the exchangers 100, 200 previously described with optionally for the zone Z₃ a device with modulation 300 or 400.

• 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, D_f étant au plus égal à 6 mm, et de préférence au moins égal à 0,4 mm ;
• 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 invention 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, D _f being at most equal to 6 mm, and preferably at least equal to 0.4 mm;
• 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 (1) and (2) are verified during the cooling diagrammed by the portion AB of the curve φ (Figure 1). When using tubes containing a gas without forced ventilation, the tubes being surrounded by a heat transfer fluid, but the relationships (1) and (2) not being 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.

The invention is illustrated by the following nine exemplary embodiments which are all in accordance with the invention.

The wires treated in these examples are made of steel, the composition of this steel being given in Table 1, according to the examples, as well as the transformation temperatures AC1 and AC3.

• a) la vitesse du fil est de 1 mètre par seconde.
• b) la longueur des différentes zones Z₁ à Z₅, mesurée en suivant le fil est la suivante :
  pour la zone Z₁ : 3 m ; pour la zone Z₂ : 2,6 m ; pour la zone Z₃ : 3 m ; pour la zone Z₄ : 3 m ; pour la zone Z₅ : 1 m ; ces longueurs sont référencées L₁ à L₅ à la figure 8.
• c) les températures des fils sont les suivantes :
  • à la sortie de la zone Z₁ = 975°C
  • à la sortie de la zone Z₂ et dans toute la zone Z₃ = 550°C
  • à la sortie de la zone Z₄ = 300°C.

All the examples are produced with an installation 500 according to the invention having the five zones Z₁ to Z₅ previously described. This installation uses heat exchangers 100 or 200 for zones Z₂ and Z₄ and devices 300 or 400 for zone Z₃, in the case of examples 1 to 8 which are carried out while avoiding the phenomenon of recalescence, that is to say say with a practically constant temperature in the zone Z₃. Example 9, on the contrary, is carried out without combating recalescence, the temperature varying in the zone Z₃. The conditions of Example 9 will be defined later. As for examples 1 to 8, the conditions are as follows:

• a) the wire speed is 1 meter per second.
• b) the length of the different zones Z₁ to Z₅, measured by following the wire is as follows:
  for zone Z₁: 3 m; for zone Z₂: 2.6 m; for zone Z₃: 3 m; for zone Z₄: 3 m; for zone Z₅: 1 m; these lengths are referenced L₁ to L₅ in Figure 8.
• c) the temperatures of the wires are as follows:
  • at the exit of zone Z₁ = 975 ° C
  • at the exit from zone Z₂ and throughout zone Z₃ = 550 ° C
  • at the exit from zone Z₄ = 300 ° C.

For all examples 1 to 9 the duration of the cooling time in the zone Z₂ is less than 5 seconds, this cooling corresponding to the portion AB of the curve φ (Figure 1).

The examples are carried out as follows:

EXAMPLE 1

• . Tube 3 réalisé en verre type pyrex, les diamètres étant les suivants : D_ti = 5 mm, D_te = 10 mm.
• . Diamètres du manchon 4 :D_mi = 35,2 mm; D_me = 42,4 mm.
• . Pour une température du fil de 975°C, les températures du tube 3 sont les suivantes : face interne 190°C, face externe 65°C.

- Les caractéristiques de la zone Z₃ sont les suivantes :
utilisation du dispositif 300, à modulation par variation de D_ti, les valeurs de D_ti et D_te étant les suivantes pour les échangeurs 100-1 à 100-7 :
pour les échangeurs 100-1 et 100-7 : D_ti = 25 mm, D_te = 35 mm,
pour les échangeurs 100-2 et 100-6 : D_ti = 5 mm, D_te = 10 mm,
pour les échangeurs 100-3 et 100-5 : D_ti = 4 mm, D_te = 8 mm
pour l'échangeur 100-4 : D_ti = 3 mm, D_te = 8 mm.
L'échangeur 100-4 est celui où la vitesse de perlitisation est maximum.
Les diamètres des manchons 4 ont, dans tous les cas, les valeurs suivantes : D_mi = 35,2 mm, D_me = 42,4 mm.
Les diverses longueurs L_m des manchons 4 sont les suivantes : pour les échangeurs 100-1 et 100-7, L_m = 0,75 m. Pour les échangeurs 100-2 à 100-6, L_m = 0,30 m, ce qui correspond donc à une longueur totale de 3 m.
- Les caractéristiques de l'échangeur 100 formant la zone Z₄ sont les suivantes :
Tube 3 en verre de type pyrex avec D_ti = 5 mm, D_te = 10 mm. Les diamètres du manchon 4 sont les suivants :
D_mi = 35,2 mm, D_me = 42,4 mm.
La valeur de λ à 600°C est égale à 0,28 watt.m⁻¹.⁰K⁻¹. Le tableau 2 suivant donne les valeurs de R et K pour les zones Z₂ à Z₄ avec l'indication des relations (1) à (4) éventuellement vérifiées dans ces zones TABLEAU 2 Zone R K Relations (1) à (4) éventuellement vérifiées Z₂ 3,85 8,13 (1), (2), (3) Z₃ échangeurs 100-1 et 100-7 19,23 17,84 aucune relation vérifiée échangeurs 100-2 et 100-6 3,85 8,13 (1), (2), (3) échangeurs 100-3 et 100-5 3,08 6,78 (1) à (4) échangeur 100-4 2,31 5,05 (1) à (4) Z₄ 3,85 8,13 (1), (2), (3)
Après traitement dans l'installation 500, le fil 1 a une résistance de rupture à la traction de 1350 MPa (mégapascals). Ce fil est ensuite laitonné puis tréfilé de façon connue pour obtenir un diamètre final de 0,20 mm. La résistance de rupture à la traction pour ce fil tréfilé est de 3500 MPa. Le rapport des sections correspond par définition au rapport :

Pour l'exemple 1 le rapport des sections est égal à 42,25.
- Diameter of wire 1 treated: 1.3 mm
- Gas 12 conductor of heat: NH₃ cracked (volume percentages: H₂ = 75%, N₂ = 25%).
- Water flow 9 at 20 ° C: 8 liters per minute, all the sleeves 4 being in series.
- The characteristics of exchanger 100 in zone Z₂ are as follows:

• . Tube 3 made of pyrex type glass, the diameters being as follows: D _ti = 5 mm, D _te = 10 mm.
• . Sleeve 4 diameters: D _mi = 35.2 mm; D _me = 42.4 mm.
• . For a wire temperature of 975 ° C, the temperatures of tube 3 are as follows: internal face 190 ° C, external face 65 ° C.

- The characteristics of the Z₃ zone are as follows:
use of the device 300, modulated by variation of D _ti , the values of D _ti and D _te being the following for the exchangers 100-1 to 100-7:
for exchangers 100-1 and 100-7: D _ti = 25 mm, D _te = 35 mm,
for exchangers 100-2 and 100-6: D _ti = 5 mm, D _te = 10 mm,
for exchangers 100-3 and 100-5: D _ti = 4 mm, D _te = 8 mm
for the 100-4 exchanger: D _ti = 3 mm, D _te = 8 mm.
The 100-4 exchanger is the one where the pearlitization speed is maximum.
The diameters of the sleeves 4 have, in all cases, the following values: D _mi = 35.2 mm, D _me = 42.4 mm.
The various lengths L _m of the sleeves 4 are as follows: for the exchangers 100-1 and 100-7, L _m = 0.75 m. For exchangers 100-2 to 100-6, L _m = 0.30 m, which therefore corresponds to a total length of 3 m.
- The characteristics of the exchanger 100 forming the zone Z₄ are as follows:
Tube 3 in pyrex type glass with D _ti = 5 mm, D _te = 10 mm. The diameters of the sleeve 4 are as follows:
D _mi = 35.2 mm, D _me = 42.4 mm.
The value of λ at 600 ° C is equal to 0.28 watt.m⁻¹.⁰K⁻¹. The following table 2 gives the values of R and K for the zones Z₂ to Z₄ with the indication of the relations (1) to (4) possibly verified in these zones TABLE 2 Zoned R K Relations (1) to (4) possibly verified Z₂ 3.85 8.13 (1), (2), (3) Z₃ exchangers 100-1 and 100-7 19.23 17.84 no verified relationship exchangers 100-2 and 100-6 3.85 8.13 (1), (2), (3) heat exchangers 100-3 and 100-5 3.08 6.78 (1) to (4) exchanger 100-4 2.31 5.05 (1) to (4) Z₄ 3.85 8.13 (1), (2), (3)
After treatment in installation 500, wire 1 has a tensile breaking strength of 1350 MPa (megapascals). This wire is then brass plated and then drawn in a known manner to obtain a final diameter of 0.20 mm. The tensile breaking strength for this drawn wire is 3500 MPa. The report of the sections corresponds by definition to the report:

For example 1 the ratio of the sections is equal to 42.25.

EXAMPLE 2

This example is carried out under the same conditions as Example 1, further varying the diameter D _f of the wire and the composition of the hydrogen / nitrogen mixture. In all cases the exchangers of zones Z₂ and Z₄ check the relations (1), (2) and the exchanger 100-4 where the pearlitization speed is maximum, in the device 300 of zone Z₃, checks the relations (3 ) and (4). Table 3 gives the values of D _f , R and K for the exchangers of zones Z₂, Z₄ and for the exchanger 100-4 of device 300, the volumetric% of hydrogen in the gas mixtures, as well as the values of λ at 600 ° C. The values of R and K for the zones Z₂ and Z₄ are referenced respectively R _M , K _M , and the values of R and K for the exchanger 100-4 are referenced respectively R _m and K _m .

• la résistance à la rupture (résistance de rupture à la traction) du fil après traitement thermique, exprimée en MPa ;
• le diamètre de tréfilage du fil, exprimé en mm, c'est-à-dire le diamètre du fil après tréfilage ;
• le rapport des sections dû au tréfilage ;
• la résistance à la rupture (résistance de rupture à la traction) du fil au diamètre final, c'est-à-dire après tréfilage, exprimée en MPa.

Table 3 also gives the following values:

• the breaking strength (tensile breaking strength) of the wire after heat treatment, expressed in MPa;
• the wire drawing diameter, expressed in mm, that is to say the wire diameter after drawing;
• the ratio of the sections due to the drawing;
• the breaking strength (tensile breaking strength) of the wire at the final diameter, that is to say after drawing, expressed in MPa.

Example 3

• . Tous les tubes 3 sont en alumine, les diamètres D_ti et D_te identiques pour les sept échangeurs 100 ayant les valeurs suivantes : D_ti = 3 mm, D_te = 8 mm. Les longueurs L_t de tube varient de la façon suivante :
  pour les échangeurs 100-1 et 100-7, L_t = 0,15 m ;
  pour les échangeurs 100-2 et 100-6, L_t = 0,20 m ;
  pour les échangeurs 100-3 et 100-5, L_t = 0,25 m ;
  pour l'échangeur 100-4, L_t = 0,28 m.
  Tous les échangeurs 100-1 à 100-7 vérifient les relations (1) à (4), avec : λ = 0,28 ; R = 2,31 ; K = 5,05.
  Après traitement dans l'installation 500 le fil 1 a une résistance de rupture à la traction de 1340 MPa.
  Le fil 1 ainsi obtenu puis laitonné et tréfilé de façon connue pour avoir un diamètre de 0,2 mm a une résistance de rupture à la traction égale à 3480 MPa, le rapport des sections étant égal à 42,25.

This example is carried out under the same conditions as Example 1, except for the zone Z₃ which is carried out with the device 400. The characteristics of the exchangers 100 of this device 400 are the following:

• . All the tubes 3 are made of alumina, the diameters D _ti and D _te identical for the seven exchangers 100 having the following values: D _ti = 3 mm, D _te = 8 mm. The lengths L _t of tube vary as follows:
  for exchangers 100-1 and 100-7, L _t = 0.15 m;
  for exchangers 100-2 and 100-6, L _t = 0.20 m;
  for exchangers 100-3 and 100-5, L _t = 0.25 m;
  for the 100-4 exchanger, L _t = 0.28 m.
  All the exchangers 100-1 to 100-7 verify the relations (1) to (4), with: λ = 0.28; R = 2.31; K = 5.05.
  After treatment in installation 500 the wire 1 has a tensile breaking strength of 1340 MPa.
  The wire 1 thus obtained and then brass-plated and drawn in a known manner so as to have a diameter of 0.2 mm has a tensile breaking strength equal to 3480 MPa, the ratio of the sections being equal to 42.25.

Example 4

• Zone Z₂ : Utilisation de trois échangeurs 100 en série, chacun ayant les caractéristiques suivantes : tube 3 en acier vitrifié à l'intérieur. D_ti = 4,5 mm ; D_te = 10 mm.
  Diamètres du manchon 4 : D_mi = 35,2 mm ; D_me = 42,4 mm.
• Zone Z₃ : Utilisation d'un dispositif 300, avec des tubes 3 en acier vitrifiés à l'intérieur, les diamètres de ces tubes 3 étant les suivants :
  pour les échangeurs 100-1 et 100-7 : D_ti = 25 mm, D_te = 35 mm
  pour les échangeurs 100-2 et 100-6 : D_ti = 3,5 mm, D_te = 10 mm
  pour les échangeurs 100-3 et 100-5 : D_ti = 3 mm, D_te = 10 mm
  pour l'échangeur 100-4 : D_ti = 2,8 mm, D_te = 10 mm
  Diamètres des manchons 4 : D_mi = 35,2 mm, D_me = 42,4 mm.
• Zone Z₄ : Utilisation de trois échangeurs 100 en série, chacun ayant les caractéristiques suivantes : tubes 3 en acier vitrifié à l'intérieur. D_ti = 4,5 mm ; D_te = 10 mm. On a λ = 0,42 watt.m⁻¹. ⁰K⁻¹.

A wire of diameter D _f = 2 mm is used. The cooling gas 12 is pure hydrogen. The water flow at 20 ° C is 19 liters per minute. The characteristics of the example are as follows:

• Zone Z₂: Use of three heat exchangers 100 in series, each having the following characteristics: tube 3 in vitrified steel inside. D _ti = 4.5 mm; D _te = 10 mm.
  Sleeve 4 diameters: D _mi = 35.2 mm; D _me = 42.4 mm.
• Zone Z₃: Use of a device 300, with vitrified steel tubes 3 inside, the diameters of these tubes 3 being as follows:
  for exchangers 100-1 and 100-7: D _ti = 25 mm, D _te = 35 mm
  for exchangers 100-2 and 100-6: D _ti = 3.5 mm, D _te = 10 mm
  for exchangers 100-3 and 100-5: D _ti = 3 mm, D _te = 10 mm
  for the 100-4 exchanger: D _ti = 2.8 mm, D _te = 10 mm
  Sleeve diameters 4: D _mi = 35.2 mm, D _me = 42.4 mm.
• Zone Z₄: Use of three heat exchangers 100 in series, each having the following characteristics: tubes 3 in vitrified steel inside. D _ti = 4.5 mm; D _te = 10 mm. We have λ = 0.42 watt.m⁻¹. ⁰ K⁻¹.

The exchangers of zones Z₂ and Z₄ verify relations (1) and (2). the following table 4 gives, for the exchangers 100-1 to 100-7, of the device 300 the values of R and K as well as the relationships (1) to (4) possibly verified. TABLE 4 n ° of exchangers R K Relations (1) to (4) possibly verified 100-1 and 100-7 12.5 24.05 (1) 100-2 and 100-6 1.75 5.33 (1) to (4) 100-3 and 100-5 1.50 3.86 (1), (3), (4) 100-4 1.40 3.20 (1), (3), (4)

After heat treatment, the wire 1 has a tensile breaking strength equal to 1340 MPa. After brass plating and drawing in a known manner to obtain a diameter of 0.3 mm, the tensile breaking strength is 3450 MPa, the section ratio being 44.44.

Example 5

This example is produced with an installation using exchangers 200 for zones Z les, Z₃, Z₄, so as to treat six wires 1 simultaneously.

The water flow rate at 20 ° C is 110 liters per minute, and the diameters of the sleeves 4 are as follows:
D _mi = 82.5 mm, D _me = 88.9 mm
Apart from these points, the conditions of the example are the same as for example 4.

After heat treatment, the wire 1 has a tensile breaking strength of 1350 MPa. After brass plating and wire drawing carried out in a known manner to have a diameter of 0.3 mm the tensile breaking strength is 3500 MPa for a section ratio of 44.44.

Example 6

The conditions are identical to those of Example 4 by varying the diameter D _f of the wires as well as the composition of the gas (mixture of hydrogen and nitrogen).

In all cases, the exchangers of zones Z₂ and Z₄ check the relations (1) and (2), and the exchanger 100-4 where the pearlitization speed is maximum, in the device 300 of zone Z vérif, checks the relations (3) and (4).

Table 5 below gives the values of D _f , of R and K for the exchangers of zones Z₂, Z₄ and for the exchanger 100-4 of device 300, the volumetric% of hydrogen in the gas mixtures, as well as the values from λ to 600 ° C.

The values of R and K for the zones Z₂ and Z₄ are referenced respectively R _M , K _M and the values of R and K for the exchanger 100-4 are referenced respectively R _m and K _m .

• la résistance à la rupture (résistance de rupture à la traction) du fil après traitement thermique, exprimée en MPa ;
• le diamètre de tréfilage du fil, exprimé en mm, c'est-à-dire le diamètre du fil après tréfilage ;
• le rapport des sections dû au tréfilage ;
• la résistance à la rupture (résistance de rupture à la traction) du fil au diamètre final, c'est-à-dire après tréfilage, exprimée en MPa.

Table 5 also gives the following values:

• the breaking strength (tensile breaking strength) of the wire after heat treatment, expressed in MPa;
• the wire drawing diameter, expressed in mm, that is to say the wire diameter after drawing;
• the ratio of the sections due to the drawing;
• the breaking strength (tensile breaking strength) of the wire at the final diameter, that is to say after drawing, expressed in MPa.

Example 7

This example is carried out under the same conditions as Example 1, but the cracked ammonia which is a decarburizing gas has been replaced by a gas maintaining the thermodynamic equilibrium with respect to the carbon of the steel at 800 ° C. . The volumetric composition of this gas being H₂ = 74%, N₂ = 24%, CH₄ = 2%. The values of R and K as well as the relationships that are verified are identical to what is shown in Table 2. The figures concerning the drawing and the resistance of the wire are identical to within 2% of those obtained for Example 1.

Example 8

This example is carried out under the same conditions as Example 1, but the cracked ammonia has been replaced by a fuel gas making it possible to correct a decarburization which has occurred in the treatments prior to the heat treatment according to the invention. Volumetric composition of the gas: H₂ = 63.75%, N₂ = 21.25%, CH₄ = 15%. No graphite deposition is observed on the surface of the wire, the thickness of recarburization is of the order of 3 μm.

The values of R, K as well as the verified relationships are identical to what is given in Table 2. After heat treatment, the wire has a breaking tensile strength of 1320 MPa. After brass plating and wire drawing carried out in a known manner to have a diameter of 0.2 mm, the section ratio being 42.25, the tensile breaking strength is 3450 MPa.

Example 9

This example is performed without erasing the recalescence. Diameter D _f of wire 1 = 5.5 mm; wire speed 1 = 1.5 m / s.

The zones Z₂, Z₃, Z₄ each use an exchanger 100, these exchangers all being identical, with tubes 3 made of vitrified steel internally with D _ti = 6 mm, D _te = 12 mm. Water flow at 20 ° C = 120 liters / minute, cooling gas: pure hydrogen. Total heat treatment time = 9.9 seconds. Length of the heat treatment installation (zones Z₂ to Z₄) = 14.85 m.

• à la sortie de la zone Z₁ : 975°C,
• au début de la transformation de l'austénite métastable en perlite (point B_x de la figure 1) : 550°C,
• à la sortie de la zone Z₄ : 350°C.

The wire temperatures are as follows:

• at the exit of zone Z₁: 975 ° C,
• at the start of the transformation of metastable austenite into perlite (point B _x of FIG. 1): 550 ° C,
• at the exit from zone Z₄: 350 ° C.

The difference between the minimum temperature and the maximum temperature during the transformation of austenite into perlite (recalescence) is 60 ° C. λ = 0.42; R = 1.091; K = 6.27

After heat treatment, the wire has a tensile breaking strength equal to 1310 MPa. After brass plating and drawing in a known manner to have a diameter of 0.84 mm, the section ratio being 42.87, the wire has a tensile breaking strength equal to 3350 MPa.

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. 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 100 nanometers (1000 Å), with a standard deviation of 25 nm (250 Å).

All the examples 1 to 9 previously described make it possible to obtain a structure corresponding to that previously described for the portion 50, but the structure reached is the finest in the case where combating recalescence is combated.

• 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.

Preferably, the invention 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.

By way of comparison, the two examples 10 and 11 which follow are not in accordance with the invention. These two comparative examples are produced with an installation similar to the installation 500 previously described comprising the zones Z₁ to Z₅. The zones Z₂, Z₃, Z₄ each use a heat exchanger 100, these heat exchangers all being identical with glass tubes 3 of pyrex type, with D _ti = 25 mm and D _te = 35 mm. The diameters of the sleeves have in all cases the following values: D _mi = 50 mm, D _me = 60 mm. The length of the installation is 18 m (zones Z₂ to Z₄).

In the two comparative examples, the heat conducting gas 12 is cracked ammonia comprising 75% hydrogen and 25% nitrogen (% by volume). The conductivity λ at 600 ° C is equal to 0.28 watt.m⁻¹. ° K⁻¹. The steel contains 0.7% carbon, it is identical to that used for the previous examples 4, 5, 6 (Table 1).

The conditions specific to Comparative Examples 10 and 11 are as follows:

Example 10

Diameter of the treated wire: 1.3 mm; wire progression speed: 1 m / sec. We therefore have R = 19.23 and K = 17.8, none of the relations (1) to (4) being verified. Temperature of the wire leaving the zone Z₁: 975 ° C. The cooling time corresponding to zone Z₂ is 6.7 sec, the wire leaving this zone Z₂ having a temperature of approximately 600 ° C.

The passage time in zone Z₃ is 4, 6 sec, the perlitization being completed at the exit of this zone Z₃.

The recalescence is significant, the temperature difference between the minimum temperature and the maximum temperature of the wire, during the transformation of the austenite into perlite (zone Z₃) being 80 ° C.

(valeur moyenne), l'écart type étant de 25,5 nm (255 Å), cette structure n'étant donc pas conforme à la structure précédemment décrite.After the heat treatment described, the wire has a tensile breaking strength equal to 1100 MPa. The wire is then brass plated and then drawn in a known manner up to a diameter of 0.23 mm and it then has a tensile breaking strength equal to 2765 MPa for a section ratio of 31.95. This example not in accordance with the invention therefore results in excessive recalescence, and low breaking strength values, before and after drawing. On the other hand the structure of the wire, after the heat treatment described in this example checks the relationship $\text{i + e = 135 nm (1350 Å)}$

(average value), the standard deviation being 25.5 nm (255 Å), this structure therefore not conforming to the structure previously described.

Example 11

Diameter of the treated wire: 2.8 mm, speed of progression of the wire: 0.5 m / sec.

We therefore have R = 8.93 and K = 61.3. The relation (1) is therefore the only one of the relations (1) to (4) which is verified.

The temperature of the wire leaving the zone Z₁ is 975 ° C as in the previous example.

The passage time in zone Z₂ is 11.5 sec, the wire, at the exit of this zone Z₂, having a temperature of approximately 630 ° C.

The passage time in zone Z₃ is 8.5 sec, the perlitization being completed at the exit from this zone Z₃. In this zone Z₃, during pearlitization, the temperature difference between the minimum temperature and the maximum temperature of the wire is 60 ° C., that is to say that the recalescence is less significant than in Example 10 above. , by following a low pearlitization speed in the zone Z₃, which is due to a higher transformation temperature.

After heat treatment, the wire has a tensile breaking strength of 1010 MPa. The wire is then brass plated then drawn in a known manner up to a diameter of 0.42 mm and it then has a tensile breaking strength equal to 2500 MPa for a section ratio of 44.44.

This example not in accordance with the invention results in a very long treatment time and a low tensile breaking strength.

(valeur moyenne),
l'écart type étant de 30 nm (300 Å),
c'est-à-dire que la structure du fil n'est pas conforme à la structure précédemment décrite.On the other hand, the structure of the wire, after the heat treatment described in this example, checks the relationship:
$\text{i + e = 145 nm (1450 Å)}$

(average value),
the standard deviation being 30 nm (300 Å),
that is to say that the structure of the wire does not conform to the structure previously described.

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

Claims (11)

1. A process for heat treating at least one carbon steel wire so as to obtain a fine pearlitic structure, the wire having been maintained prior to this treatment at a temperature above the transformation temperature AC3 so as to obtain a homogeneous austenite, the process comprising the following features:

   a) the wire is cooled from a temperature above the transformation temperature AC3 to a temperature below the transformation temperature AC1, for a period of less than 5 seconds;

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

   c) this cooling and pearlitisation treatment is carried out by passing the wire through at least one tube which contains a gas which is practically without forced ventilation, the tube being surrounded by a heat-exchange fluid in such a manner that a transfer of heat takes place from the wire, through the gas and through the tube, towards the heat-exchange fluid;
   this process being characterised in that:

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

   

   
   $\text{5 ≦ K ≦ 10 (2)}$

   

   
   
   in which, 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, this diameter being at most 6 mm, λ being the conductivity of the gas determined at 600° C, this conductivity being expressed in watts.m⁻¹.°K⁻¹, and Log being the natural logarithm.
2. A process according to Claim 1, characterised in that after having cooled the wire from a temperature above the transformation temperature AC3 to a given temperature below the transformation temperature AC1, the wire is held 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 in which the rate of pearlitisation is fastest:
   
   $\text{1.05 ≦ R ≦ 8 (3)}$

   

   
   $\text{3 ≦ K ≦ 8 (4).}$

   
3. A process according to Claim 2, characterised in that the wire is held at a temperature which does not vary by more than 5° C plus or minus from said given temperature.
4. A process according to any one of Claims 2 or 3, characterised in that the modulation is effected by varying the inside diameter of the tube or of at least one tube.
5. A process according to any one of Claims 2 to 4, characterised in that the modulation is effected by using several tubes, the lengths of which are varied.
6. A process of heat-treating at least one carbon steel wire, characterised by the following features:

   - the wire is heated to bring it to a temperature above the transformation temperature AC3 so as to obtain a homogeneous austenite;

   - a treatment according to any one of Claims 1 to 5 is then effected;

   - the wire is then cooled.
7. A device for heat treating at least one carbon steel wire so as to obtain a fine pearlitic structure, the wire having been held, prior to this treatment, at a temperature above the transformation temperature AC3 in order to obtain a homogeneous austenite, this device comprising:

   a) means for cooling the wire from a temperature above the transformation temperature AC3 to a temperature below the transformation temperature AC1, for a period of less than 5 seconds;

   b) means which make it possible to effect the pearlitisation treatment at a temperature below the transformation temperature AC1;

   c) these cooling and pearlitisation means comprise at least one tube and means for passing the wire through the tube, this tube containing a gas which is practically without forced ventilation, the tube being surrounded by a heat-exchange fluid in such a manner that a transfer of heat takes place from the wire, through the gas and through the tube, to the heat-exchange fluid;
   this device being characterised by the following features:

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

   

   
   $\text{5 ≦ K ≦ 10 (2)}$

   

   
   
   in which, 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, this diameter being at most 6 mm, λ being the conductivity of the gas determined at 600° C, this conductivity being expressed in watts.m⁻¹.°K⁻¹, and Log being the natural logarithm;

   e) one or more such tubes are arranged such that, after cooling the wire from a temperature above the transformation temperature AC3 to a given temperature below the transformation temperature AC1, they make it possible to maintain the wire at a temperature which does not differ by more than 10° C plus or minus from said given temperature, for a time greater than the pearlitisation time, by modulating the heat exchanges, said tube or tubes forming a plurality of tubular zones permitting said modulation, the following relationships being satisfied in the tubular zone or zones in which the pearlitisation rate is fastest:
   
   $\text{1.05 ≦ R ≦ 8 (3)}$

   

   
   $\text{3 ≦ K ≦ 8 (4)}$

   
8. A device according to Claim 7, characterised in that the tube or tubes are arranged such that the temperature of the wire does not differ by more than 5° C plus or minus from said given temperature.
9. A device according to any one of Claims 7 or 8, characterised in that the inside diameter of the tube or of at least one tube varies.
10. A device according to any one of Claims 7 to 9, characterised in that it comprises several tubes, the lengths of which vary.
11. An installation for heat-treating at least one carbon steel wire comprising at least one device according to any one of Claims 7 to 10, the installation furthermore comprising means for bringing the wire to a temperature above the transformation temperature AC3 before pearlitisation, and means for cooling the wire after pearlitisation.

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