Patent Publication Number: US-11021770-B2

Title: Method for the heat treatment of a steel reinforcement element for tires

Description:
RELATED APPLICATIONS 
     This is a U.S. National Phase Application under 35 USC 371 of International Application PCT/EP2015/053457 filed on Feb. 19, 2015. 
     This application claims the priority of French application no. 1451380 filed Feb. 21, 2014, the entire content of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a method for the heat treatment of a steel reinforcing element for a tire. 
     BACKGROUND OF THE INVENTION 
     A method is known from the prior art for manufacturing a steel reinforcing element for a tire, for example a steel wire. 
     The manufacturing method makes it possible to manufacture, from a wire, referred to as wire stock, having an initial diameter between 4.5 and 7.5 mm, a wire that can be used for reinforcing plies of the tire having a diameter of between 0.08 mm and 0.50 mm. 
     Firstly, the wire stock of predominantly pearlitic microstructure is drawn, for example in a dry environment, so as to reduce its initial diameter to an intermediate diameter, for example equal to 1.3 mm. At the end of this drawing step, the steel of the wire has a microstructure comprising several mixed phases. 
     Next, the wire of intermediate diameter is heat treated so as to modify the microstructure of the steel. In this instance, the predominantly pearlitic microstructure of the steel is regenerated. 
     After having coated the wire of intermediate diameter with a metal layer, the coated wire of intermediate diameter is drawn, for example in a wet environment, so as to reduce its diameter to a final diameter, for example equal to 0.20 mm. 
     A method for the heat treatment of the wire of intermediate diameter is known from U.S. Pat. No. 4,767,472 that comprises three steps and is carried out by means of a heat treatment facility. 
     The heat treatment facility comprises, in the run direction of the wire, upstream means for storing the untreated wire, for example upstream reels, a heating device, a cooling device, and downstream means for storing the treated wire, for example downstream reels. 
     During a first step, the temperature of the wire is increased above the austenitizing temperature of the steel in order to obtain a predominantly austenitic microstructure. For this purpose, the heat treatment facility comprises a device for heating the wire comprising a gas-fired furnace. 
     Then, in a second step, the temperature of the wire is reduced in order to obtain a metastable austenitic microstructure by means of a cooling device comprising a water bath. The bath comprises pure liquid water at a temperature above 80° C. through which the wire is made to run. 
     In a third step, carried out downstream of the bath, the temperature of the wires is left to drop in ambient air or else in a thermally insulated device. During this exposure to the ambient air, the predominantly austenitic microstructure is transformed to a predominantly pearlitic microstructure or else this transformation, pre-initiated in the bath, is continued by passing through the pearlite transformation range. 
     However, during the transformation, it is not possible to precisely control the rate of temperature reduction, in particular in order to control the recalescence. In order to resolve this problem, document U.S. Pat. No. 6,228,188 proposes successive passes of the wire through water baths alternating with passes through air during the transformation. 
     However, this process is relatively tedious to control. Indeed, depending on the desired rate of temperature reduction, it is necessary to control numerous parameters such as the length of the baths, the length of the passes through air, the water temperature of the baths, the temperature of the ambient air, the run speed of the wire. These parameters must be changed as soon as the nature of the wire varies, in particular its diameter and the composition of the steel. 
     SUMMARY OF THE INVENTION 
     The objective of the present invention is to provide a more flexible heat treatment method. 
     This and other objects are attained in accordance with one aspect of the present invention directed to a method for the heat treatment of a steel reinforcing element for a tire comprising a transformation of the steel microstructure and in which the temperature of the reinforcing element is reduced during the transformation of the steel microstructure by simultaneously extracting heat from the reinforcing element and supplying heat to the reinforcing element. 
     The method according to the invention is easy to control. It makes it possible to change the nature of the reinforcing element without having to modify too many parameters. Indeed, the temperature reduction can be easily controlled by determining the amount of heat that has to be extracted from the reinforcing element. This amount can then be easily adjusted by simultaneously controlling the extraction and the supply of the heat, unlike the method from the prior art in which heat is only extracted by means of the baths and the ambient air. 
     The invention applies to any type of method in which the temperature of the reinforcing element is reduced during the transformation of the steel microstructure and more advantageously to a continuous cooling heat treatment (abbreviated to “CCHT”) method. Unlike an isothermal heat treatment (abbreviated to “IHT”) method that uses a TTT (time-temperature-transformation) diagram and comprises one or more changes of rate during the temperature reduction step, a CCHT-type method uses a CCT (continuous cooling transformation) diagram and has a continuous rate during the temperature reduction step. 
     In the present description, any range of values denoted by the expression “between a and b” represents the field of values ranging from more than a to less than b (that is to say limits a and b excluded) whereas any range of values denoted by the expression “from a to b” means the field of values ranging from a up to b (that is to say including the strict limits a and b). 
     In one embodiment, the heat is extracted from the reinforcing element by thermal convection in contact with at least one cold source. 
     In one embodiment, heat is supplied to the reinforcing element by the Joule effect through the reinforcing element. 
     Supplying heat by the Joule effect allows a direct supply of heat to the reinforcing element through the latter. Thus, it is possible to increase the temperature very rapidly which makes it possible to use a high run speed and therefore to obtain a high unit mass throughput. Indeed, supplying heat by the Joule effect is extremely effective since it is carried out through the wire without convection. 
     In addition, supplying heat by the Joule effect leads to a relatively low and controlled energy expenditure relative to other means for supplying heat, in particular of the convection type such as gas-fired furnaces. 
     Preferably, the heat is extracted from the reinforcing element by virtue of means for extracting heat from the reinforcing element comprising: 
     a run chamber of the reinforcing element containing an intermediate cold source arranged between the reinforcing element and an external cold source, 
     a chamber for circulation of the external cold source arranged around the run chamber of the reinforcing element. 
     Such extraction means are compatible with a high run speed. Indeed, the heat extraction means have a heat-extraction capacity that is substantially greater than the water bath used in the prior art which makes it possible to increase the run speed of the reinforcing element. 
     Furthermore, a high speed in the bath used in the prior art leads to a turbulent flow of the water in contact with the wire and therefore to an insufficient and poorly controlled reduction in the temperature of the wire, then leading to the appearance of surface defects of the wire. By eliminating the water bath, the problem of the appearance of turbulent flow at high speed, and therefore the problem of the appearance of surface defects of the reinforcing element, are eliminated. 
     In addition, the extraction means are much safer than the baths customarily used, whether they are water baths, lead baths or molten salt baths. Indeed, the circulation chamber makes it possible to physically isolate the reinforcing element from the operators. 
     Unlike the lead or molten salt baths that may pose environmental and safety problems, the extraction means are safe and environmentally friendly. Furthermore, the heat extraction means used make it possible to avoid any cleaning step that aims to eliminate the lead or the molten salts covering the reinforcing element. 
     In one embodiment, the intermediate cold source comprises a heat exchange gas. In one embodiment, the heat exchange gas may comprise one or more gaseous constituents. 
     Advantageously, heat exchange gas comprises a gas selected from reducing gases, inert gases and mixtures of these gases, preferably from reducing gases, and more preferentially is dihydrogen. 
     In one embodiment, the external cold source comprises a heat exchange liquid. 
     Preferably, heat is supplied to the reinforcing element by virtue of means for supplying heat by the Joule effect through the reinforcing element comprising two electrically conductive terminals. 
     Advantageously, each electrically conductive terminal comprises an electrically conductive rotatable pulley. 
     The rotatable pulleys allow both the electrical conduction and also the passage and guiding of the reinforcing element, irrespective of the run speed of the latter. 
     Advantageously, the reinforcing element is made to run at a mean run speed strictly greater than 40 m·min −1 , preferably strictly greater than 90 m·min −1 , more preferentially greater than or equal to 200 m·min −1  and more preferentially still greater than or equal to 300 m·min −1 . 
     The mean speed should be understood to mean the ratio of the distance travelled by one point of the reinforcing element to the time taken by this point to travel this distance. 
     Unlike the method described in U.S. Pat. No. 4,767,472 and the IHT methods that inevitably require a relatively long transformation time, for example of the order of several tens of seconds, the transformation in a CCHT-type method may be relatively short, for example of the order of several seconds, which makes it possible to use high run speeds in a facility having a reduced size. 
     In one embodiment, the mean rate of temperature reduction during the transformation of the microstructure of the steel is greater than or equal to 30° C.s −1 , preferably greater than or equal to 50° C.s −1  and more preferentially greater than or equal to 70° C.s −1 . 
     The use of too low a mean rate of reduction does not make it possible to rapidly carry out the transformation of the steel microstructure. Thus, the risk of obtaining a steel having undesired mechanical properties is minimized. 
     In one embodiment, the mean rate of temperature reduction during the transformation of the microstructure of the steel is less than or equal to 110° C.s −1 , preferably less than or equal to 100° C.s −1  and more preferentially less than or equal to 90° C.s −1 . 
     The use of too high a rate of reduction has the risk of resulting in a quenching of the steel which according to the desired properties of the steel is not desirable. 
     The mean rate of reduction is understood to mean the ratio of the difference in degrees Celsius between the temperature before transformation and after transformation to the time taken to carry out the transformation. 
     Thus, several embodiments could be envisaged in which the mean rate of reduction is within ranges extending from 30° C.s −1  to 90° C.s −1 , from 30° C.s −1  to 100° C.s −1 , from 30° C.s −1  to 110° C.s −1 , from 50° C.s −1  to 90° C.s −1 , from 50° C.s −1  to 100° C.s − , from 50° C.s −1  to 110° C.s −1 , from 70° C.s −1  to 90° C.s −1 , from 70° C.s −1  to 100° C.s −1  and from 70° C.s −1  to 110° C.s −1 . 
     In one embodiment, the temperature is reduced by more than 30° C., preferably by more than 50° C., more preferentially by more than 75° C. and more preferentially still by more than 100° C. during the transformation of the steel microstructure. 
     In one embodiment, the transformation of the steel microstructure takes place in a temperature range extending from 800° C. to 400° C., preferably from 750° C. to 500° C. and more preferentially from 650° C. to 550° C. 
     The expression “by more than X° C.” means that the temperature is reduced by a temperature range strictly greater than X° C., the value of X° C. therefore being excluded. 
     Preferably, the method comprises a step of reducing the temperature of the reinforcing element by continuous cooling: 
     from an initial temperature of an initial stability range of the steel, 
     to a final temperature of a final stability range of the steel, 
     the temperature reduction step comprising a transformation of the steel microstructure from a microstructure of the initial range to a microstructure of the final range. 
     The method is thus relatively robust and simple to control. Indeed, unlike the method described in U.S. Pat. No. 4,767,472 and certain heat treatment methods from the prior art, referred to as isothermal transformation methods (abbreviated to “IHT” for isothermal heat treatment), in which the transformation of the steel takes place at a substantially constant temperature, the method here involves continuous cooling (abbreviated to “CCHT” for continuous cooling heat treatment). The two types of methods are easily distinguished, in particular by means of the time-temperature diagrams used to represent them. An IHT-type method uses a TTT (time-temperature-transformation) diagram and comprises one or more changes of rate during the temperature reduction step. A CCHT-type method uses a CCT (continuous cooling transformation) diagram and has a continuous rate during the temperature reduction step. Thus, among other features of a CCHT-type method, the temperature of the reinforcing element is reduced during the transformation of the steel microstructure. 
     In the method described above, once the rate of temperature reduction is defined, it is then relatively easy to control it considering its continuity during the temperature reduction step. 
     In addition, unlike the IHT-type methods in which a very large amount of heat is supplied to the reinforcing element in order to keep it at substantially constant temperature during the transformation, the method described being of CCHT type makes it possible to reduce the energy consumption of the method. 
     In one embodiment, the initial temperature is greater than or equal to 750° C., preferably greater than or equal to 800° C. and more preferentially greater than or equal to 850° C. 
     In one embodiment, the final temperature is less than or equal to 650° C., preferably less than or equal to 550° C. and more preferentially less than or equal to 450° C. 
     Thus, several embodiments could be envisaged in which the initial temperature/final temperature pairs are 750° C./450° C., 750° C./550° C., 750° C./650° C., 800° C./450° C., 800° C./550° C., 800° C./650° C., 850° C./450° C., 850° C./550° C., 850° C./650° C. 
     In one embodiment, the initial range is the austenite stability range of the steel. Thus, the steel preferentially has a predominantly austenitic initial microstructure. 
     In one embodiment, the final range is the ferrite-pearlite stability range of the steel. Thus, in this ferrite-pearlite stability range, the steel has a predominantly ferritic-pearlitic final microstructure. The steel may also comprise, depending on its composition and on the various parameters of the method, bainite and/or ferrite and/or martensite. In the case where one or more of these microstructures is present, the ferritic-pearlitic microstructure is preferably predominant. 
     A predominantly austenitic/ferritic-pearlitic microstructure is understood to mean, in a manner known to a person skilled in the art, that the steel comprises a predominant austenitic/pearlitic phase relative to the other phases that make up the steel. A predominant phase is understood to mean that the steel comprises, by weight, a higher percentage of this phase relative to the sum of the percentages of the other phases, i.e. more than 50%, preferably more than 75%, or even more than 95% and more preferentially more than 98% by weight of this phase relative to the sum of the percentages of the other phases. 
     Advantageously, the temperature reduction step comprises a reduction in the temperature of the reinforcing element in the initial stability range of the steel. 
     Advantageously, the temperature reduction step comprises a reduction in the temperature of the reinforcing element in the final stability range of the steel. 
     In one embodiment, the steel microstructure is transformed by passing through at least one transformation range. 
     Advantageously, the temperature for entering the transformation range, i.e. the temperature delimiting the passage between the initial stability range and the transformation range, is greater than or equal to 550° C., preferably greater than or equal to 600° C., more preferentially greater than or equal to 650° C. and more preferentially still greater than or equal to 700° C. 
     Advantageously, the temperature for leaving the transformation range, i.e. the temperature delimiting the passage between the transformation range and the final stability range, is greater than or equal to 400° C., preferably greater than or equal to 500° C., more preferentially greater than or equal to 600° C. and more preferentially still greater than or equal to 650° C. 
     In one preferred embodiment, the transformation range comprises the ferrite transformation range. 
     In one preferred embodiment, the transformation range comprises the pearlite transformation range. 
     In one embodiment, prior to the step of reducing the temperature of the reinforcing element, the method comprises a step of increasing the temperature of the reinforcing element to a temperature greater than or equal to the austenitizing temperature of the steel. 
     By heating the steel above the austenitizing temperature of the steel, a predominantly austenitic, or even completely austenitic microstructure is obtained. 
     Optionally, heat is supplied to the reinforcing element during at least one portion of the step of reducing the temperature of the reinforcing element. 
     The supply of heat to the reinforcing element makes it possible to control the reduction in temperature of the reinforcing element and therefore to obtain the desired microstructure of the steel. In particular, the supply of heat makes it possible not to reduce the temperature too rapidly which would result in the creation of undesired phases in the microstructure of the steel. 
     Unlike the supply of heat of an IHT-type method, the supply of heat of the method is relatively small since it does not have the objective of keeping the temperature of the reinforcing element constant. 
     According to other optional features of the method:
         The steel reinforcing element is a steel wire.   The steel wire has a diameter ranging from 0.5 to 5.5 mm, preferably from 0.5 to 3 mm and more preferentially from 1 to 2.5 mm.   The steel comprises from 0.4% to 1.2%, preferably from 0.4% to 1% and more preferentially from 0.4% to 0.8% of carbon by weight.       

     Another subject of the invention is a steel reinforcing element capable of being obtained by a method as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood from reading the following description, given solely by way of non-limiting example and with reference to the drawings in which: 
         FIG. 1  is a diagram of a heat treatment facility according to a first embodiment for the implementation of the method according to an embodiment of the invention; 
         FIG. 2  is a diagram of a device for heating the facility in  FIG. 1 ; 
         FIG. 3  is a diagram of a temperature-maintaining device of the facility in  FIG. 1 ; 
         FIG. 4  is a diagram of a device for cooling the facility in  FIG. 1 ; 
         FIG. 5  is a cross-sectional view along V-V of the cooling device in  FIG. 4 ; 
         FIG. 6  is a diagram illustrating various steps of a method for manufacturing a reinforcing element comprising steps of a heat treatment method according to an embodiment of the invention; 
         FIG. 7  is a CCT time-temperature diagram illustrating the heat treatment method of  FIG. 6 ; and 
         FIG. 8  is a diagram of a heat treatment facility according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Example of a Heat Treatment Facility for the Implementation of the Method According to the Invention 
     Represented in  FIG. 1  is a first embodiment of a facility for the heat treatment of a reinforcing element for a tire, denoted by the general reference  10 . 
     The treatment facility  10  is capable of treating steel reinforcing elements F, here steel wires. The steel wires F have a diameter ranging from 0.5 to 5.5 mm, preferably from 0.7 to 3 mm and more preferentially from 1 to 2.5 mm. 
     The facility  10  comprises, in the run direction of the element F in the facility  10 , from upstream to downstream, upstream means  12  for storing the element F, a device  14  for heating the element F, a device  15  for maintaining the element F at temperature, a device  16  for cooling the element F and downstream means  18  for storing the heat-treated element F. 
     The upstream  12  and downstream  18  storage means each comprise a reel for storing the element F respectively making it possible to unwind and wind up the element F. In the upstream storage means  12 , the element F is then connected to the potential P 0 . 
     The heating device  14  is represented in  FIG. 2 . The device  14  makes it possible to heat the element F at a temperature greater than or equal to the austenitizing temperature of the steel. 
     The heating device  14  comprises means  20  for supplying heat to the element F. The heat supply means  20  comprise means  21  for supplying heat by the Joule effect through the element F. These heat supply means  21  comprise two electrically conductive terminals  22 ,  24  powered by a current source, here a transformer  26 . Each terminal  22 ,  24  is respectively connected to the phase conductor P 1  and the neutral conductor N 1 . In this instance, each terminal  22 ,  24  respectively comprises an electrically conductive rotatable pulley  23 ,  25 . Each terminal  22 ,  24  is arranged so that each pulley  23 ,  25  is in contact with the element F during the operation of the facility  10 . 
     The temperature-maintaining device  15  is represented in  FIG. 3 . The device  15  is arranged between the heating device  14  and the cooling device  16  and makes it possible to maintain the temperature of the element F at a temperature greater than or equal to the austenitizing temperature of the steel. 
     The maintaining device  15  comprises means  31  for supplying heat to the element F. The heat supply means  31  comprise means  33  for supplying heat by the Joule effect through the element F. These heat supply means  31  comprise two electrically conductive terminals  35 ,  37  powered by a current source, here a transformer  39 . Each terminal  35 ,  37  is respectively connected to the phase conductor P 2  and the neutral conductor N 2 . In this instance, each terminal  35 ,  37  respectively comprises an electrically conductive rotatable pulley  41 ,  43 . Each terminal  35 ,  37  is arranged so that each pulley  41 ,  43  is in contact with the element F during the operation of the facility  10 . 
     The facility  10  also comprises means  49  for applying the element F against the terminals  24 ,  35  and more specifically in contact with the pulleys  25 ,  41 . The application means  49  here comprise a rotatable pulley  51 . 
     The cooling device  16  is represented in  FIGS. 4 and 5 . 
     The cooling device  16  comprises means  30  for supplying heat to the element F and means  32  for extracting heat from the element F to at least one cold source, here two cold sources. The cold source(s) are different from ambient air. 
     The heat supply means  30  comprise means  34  for supplying heat by the Joule effect through the element F. The heat supply means  34  comprise two electrically conductive terminals  36 ,  38  respectively positioned upstream and downstream of an inlet  40  and an outlet  42  of the element F into/from the cooling device  16 . Each terminal  36 ,  38  is respectively connected to the phase conductor P 3  and the neutral conductor N 3 , the neutral conductor N 3  being at the same potential as the earth T. Each terminal  36 ,  38  respectively comprises an electrically conductive rotatable pulley  53 ,  55 . The heat supply means  34  also comprise a current source, here a transformer  44 , powering the two terminals  36 ,  38 . Each terminal  36 ,  38  is arranged so that each pulley  53 ,  55  is in contact with the element F during the operation of the facility  10 . 
     The facility  10  also comprises means  61  for applying the element F against the terminals  37 ,  36  and more specifically in contact with the pulleys  43 ,  53 . The application means  61  here comprise a rotatable pulley  63 . 
     The heat extraction means  32  are of the type having convective exchange between the element F and the cold source(s). 
     The heat extraction means  32  comprise a run chamber  46  of the element F. The run chamber  46  forms a jacket and has a general shape with axial symmetry with respect to the run axis X of the element F, in this instance having a circular general cross section. The run chamber  46  contains an intermediate cold source  48 . The heat exchange between the element F and the intermediate cold source  48  occurs by convection, here by forced convection due to the circulation of the intermediate cold source  48  in the run chamber  46  from upstream to downstream in the run direction of the element F. As a variant, the cold source is devoid of forced convection. For example, the intermediate cold source  48  comprises a heat exchange gas  50 . Preferably, the gas  50  is selected from reducing gases, inert gases and mixtures of these gases. More preferentially, the gas  50  is a reducing gas, here dihydrogen H 2 . As a variant, the gas  50  comprises several gaseous constituents, for example an H 2 +N 2  mixture. In another variant, the gas  50  is nitrogen N 2 . 
     The heat extraction means  32  also comprise a chamber  52  for circulation of an external cold source  54 . The circulation chamber  52  forms a jacket and has a general shape with axial symmetry with respect to the run axis X of the element F, in this instance having a circular general cross section. The circulation chamber  52  is radially external with respect to the run chamber  46  and arranged around the latter. The intermediate cold source  48  is arranged between the element F and the external cold source  54 . The heat exchange between the intermediate cold source  48  and the external cold source  54  occurs by convection, here by forced convection due to the circulation of the external cold source  54  in the circulation chamber  52 , from downstream to upstream in the run direction of the element F. 
     For example, the external cold source  54  comprises a heat exchange liquid  56 , here water. 
     The heat supply means  30  and heat extraction means  32  are arranged so that the temperature of the element F at the outlet  42  is strictly less than the temperature of the element F at the inlet  40 . 
     Example of a Heat Treatment Method According to the Invention 
     An example of a method for the heat treatment of the steel reinforcing element F for a tire will now be described with reference to  FIGS. 6 and 7  within the context of a method for manufacturing the element F. 
     The steel comprises for example from 0.4% to 1.2%, preferably from 0.4% to 1% and more preferentially from 0.4% to 0.8% of carbon by weight. The steel may also comprise specific alloying elements such as Cr, Ni, Co, V, or various other known elements (see, for example,  Research Disclosure  34984—“ Micro - alloyed steel cord constructions for tires ”—May 1993 ; Research Disclosure  34054—“ High tensile strength steel cord constructions for tires ”—August 1992). In this instance, a conventional steel containing 0.7% of carbon is used. 
     Prior to the heat treatment method, the element F is drawn, for example in a dry environment, so as to reduce its initial diameter, equal to 5.5 mm, to an intermediate diameter, here equal to 1.3 mm, in a step  100 . At the end of this drawing step  100 , the steel of the element F has a microstructure comprising several mixed phases. 
     The heat treatment method is then carried out in which the element F of intermediate diameter is heat treated so as to modify the microstructure of the steel. In this instance, the predominantly pearlitic microstructure is regenerated. 
     The heat treatment method comprises a step  200  of increasing the temperature of the element F from a temperature T 0  to a temperature T 1  greater than or equal to the austenitizing temperature of the steel. As illustrated in  FIG. 7 , this step lasts from t 0  to t 1  and is carried out by means of the heating device  14 . 
     During at least one part of this step  200 , heat is provided to the element F by the Joule effect through the element F. Each terminal  22 ,  24  is in contact with the element F during this step  200 . 
     The temperature of the element F is increased from T 0  to T 1  at a mean rate of increase ranging from 100 to 1000° C.s −1 , preferably 500 to 950° C.s −1  and more preferentially from 700 to 900° C.s −1 . Here, the rate of increase is equal to 836° C.s −1 . 
     Preferably, T 0  is less than or equal to 100° C. and more preferentially less than or equal to 50° C. Here, T 0 =20° C. Preferably, T 1  is greater than or equal to 850° C. and more preferentially greater than or equal to 900° C. Here, T 1 =975° C. 
     The method comprises a step  202  of maintaining the temperature of the element F at a temperature greater than or equal to the temperature T 1 . As illustrated in  FIG. 7 , this step  202  lasts from t 1  to t 2  and is carried out by means of the temperature-maintaining device  15 . 
     As a variant, the method may not comprise a step  202  of maintaining the temperature of the steel at the austenitizing temperature of the steel. Thus, a method could be envisaged without a temperature-maintaining step, that is to say in which the step of increasing the temperature of the element F from a temperature T 0  to a temperature T 1  greater than or equal to the austenitizing temperature of the steel and the step of reducing the temperature described below are carried out consecutively. In this variant, the facility  10  does not comprise a device  15  arranged between the heating device  14  and the cooling device  16 . 
     Then, the element F arrives at the inlet  40  of the heat extraction means  32  of the device  16  at the time t 2 . The method then comprises a step  204  of reducing the temperature of the element F by continuous cooling from an initial temperature T 2  to a final temperature T 3 . As illustrated in  FIG. 7 , this step  204  lasts from t 2  to t 3  and is carried out by means of the cooling device  16 . 
     In order to reduce the temperature from T 2  to T 3 , heat is extracted from the element F by thermal convection in contact with the intermediate cold source  48 , in the example by virtue of the heat extraction means  32 . 
     The temperature is reduced from T 2  to T 3  at a mean rate of reduction greater than or equal to 30° C.s −1 , preferably greater than or equal to 50° C.s −1  and more preferentially greater than or equal to 70° C.s −1 . In this preferred embodiment, the rate of reduction is less than or equal to 110° C.s −1 , preferably less than or equal to 100° C.s −1  and more preferentially less than or equal to 90° C.s −1 . 
     In the example described, T 2  is greater than or equal to 750° C., preferably greater than or equal to 800° C. and more preferentially greater than or equal to 850° C. Here, T 2 =T 1 =975° C. 
     In the example described, T 3  is less than or equal to 650° C., preferably less than or equal to 550° C. and more preferentially less than or equal to 450° C. Here, T 3 =400° C. 
     Preferably, heat is supplied to the element F during at least one part of the reduction step  204 , in the example by virtue of the means  34  for supplying heat by the Joule effect through the element F which is then in contact with the terminals  36 ,  38 . 
     The temperature of the reinforcing element F is strictly decreasing during the reduction step  204 . 
     The temperature reduction step  204  is after the temperature-maintaining step  202  which is itself after the temperature increase step  200 . 
     The element F is made to run at a mean run speed preferably strictly greater than 40 m·min −1 , preferably strictly greater than 90 m·min −1 , more preferentially greater than or equal to 200 m·min −1  and more preferentially still greater than or equal to 300 m·min −1 . Here, the run speed is equal to 315 m·min −1 . 
     The inlet temperature of the water is between 20° C. and 40° C., and here substantially equal to 15° C. The inlet temperature of the gas is substantially equal to 20° C. 
     The element F then arrives at the outlet  42  of the heat extraction means  32  of the device  16  at the time t 3 . The element F obtained by the method has, in this example, a tensile strength Rm equal to 1150 MPa. 
     After the heat treatment method, in a step  300 , the heat-treated element F of intermediate diameter is coated with a metal layer, for example a layer of brass. 
     Then, in a step  400 , the coated, heat-treated element F of intermediate diameter is drawn, for example, in a wet environment so as to reduce its diameter to a final diameter, for example equal to 0.23 mm. 
     The element F thus obtained could be used as an individual wire for reinforcing tire plies or else for the manufacture of a layered cord or else a stranded cord for reinforcing tire plies. 
     The heat treatment method has been illustrated in  FIG. 7  by a curve C representing the change in the temperature of the element F as a function of the time. The reduction step  204  will be described with reference to  FIG. 7 . 
     The initial temperature T 2  belongs to an initial stability range of the steel, here the austenite stability range I in which the steel has a predominantly austenitic microstructure. 
     The final temperature T 3  belongs to a final stability range of the steel, here the pearlite stability range IV in which the steel has a predominantly pearlitic microstructure. 
     The reduction step  204  comprises a reduction of the temperature of the element F in the initial stability range, here in the austenite stability range. This reduction is illustrated by the portion C 1  of the curve C between the points (t 2 , T 2 ) and (t 2 ′, T 2 ′). 
     Next, the reduction step  204  comprises a transformation of the steel microstructure from the microstructure of the initial range, here austenitic, to a microstructure of the final range, here ferritic-pearlitic. The steel microstructure is transformed by passing through at least one transformation range. In this instance, the ferrite transformation range II (portion C 2  of the curve C) and pearlite transformation range III (portion C 3  of the curve C) are passed through successively between the points (t 2 ′, T 2 ′) and (t 3 ′, T 3 ′). The transformation ranges of the steel are distinct from the bainite range and preferably from the martensite range. 
     This transformation of the steel microstructure takes place in a temperature range [T 2 ′, T 3 ′] extending from 800° C. to 400° C., preferably from 750° C. to 500° C. and more preferentially from 650° C. to 550° C. The temperature T 2 ′ for entering the ferrite transformation range II, i.e. the temperature delimiting the passage between the initial stability range I and ferrite transformation range II, is greater than or equal to 550° C., preferably greater than or equal to 600° C., more preferentially greater than or equal to 650° C. and more preferentially still greater than or equal to 700° C. The temperature T 3 ′ leaving the pearlite transformation range III, i.e. the temperature delimiting the passage between the pearlite transformation range III and the final stability range IV, is greater than or equal to 400° C., preferably greater than or equal to 500° C., more preferentially greater than or equal to 600° C. and more preferentially still greater than or equal to 650° C. In this instance, T 2 ′=710° C. and T 3 ′=600° C. 
     During this transformation of the steel microstructure, the temperature of the element F is reduced, by simultaneously extracting heat from the element F by thermal convection in contact with the intermediate cold source  48  and supplying heat to the element F by the Joule effect through the element F. In order to reduce the temperature of the element F, more heat is extracted than is supplied thereto. 
     During this transformation of the steel microstructure, the temperature of the element F is reduced for example by more than 30° C., preferably by more than 50° C., more preferentially by more than 75° C. and more preferentially still by more than 100° C. In this instance, the temperature is reduced by 123° C. 
     During this transformation, the mean rate of temperature reduction is greater than or equal to 30° C.s −1 , preferably greater than or equal to 50° C.s −1  and more preferentially greater than or equal to 70° C.s −1 . 
     During this transformation, the mean rate of temperature reduction is less than or equal to 110° C.s −1 , preferably less than or equal to 100° C.s −1  and more preferentially less than or equal to 90° C.s −1 . 
     In this instance, the mean rate of temperature reduction is equal to 86° C.s −1 . 
     Next, the reduction step  204  comprises a reduction of the temperature of the element F in the final stability range, here in the pearlite stability range. This reduction is illustrated by the portion C 4  of the curve C between the points (t 3 ′, T 3 ′) and (t 3 , T 3 ). 
     It will be noted that the method described here is a continuous cooling method. Thus, as illustrated in the CCT diagram of  FIG. 7 , there is no sudden change in the rate of cooling of the element F between the temperatures T 2  and T 3 . In addition, the transformation takes place completely between the inlet  40  and the outlet  42  of the cooling device  16 . In other words, the element F leaves the initial stability range downstream of the inlet  40  and upstream of the outlet  42  and reaches the final stability range downstream of the inlet  40  and upstream of the outlet  42 . 
     By transforming the steel microstructure between the inlet  40  and the outlet  42  of the cooling device  16 , the formation of oxides at the surface of the steel is limited, or even avoided, unlike a heat treatment that takes place entirely or partly in an oxidizing medium, for example in ambient air. 
     A person skilled in the art is capable of distinguishing the austenitic, pearlitic, bainitic, ferritic and martensitic microstructures described above, in particular by microscopic observation by known means, for example a scanning electron microscope (SEM) or electron backscatter diffraction (EBSD). This observation could, in a known manner, be preceded by a chemical etching. 
     The features relating to the operation of the facility  14 , such as the intensity of the current in the devices  14  and  16 , the temperature of the intermediate cold source  48  and external cold source  54 , the flow rate of the external cold source  54 , are in particular a function of the size of the element F, of its diameter in the case of a wire, and of the run speed of the element F. The values of these features are within the capabilities of a person skilled in the art who will be able to determine them by successive tests or else by calculation. 
     Represented in  FIG. 8  is a second embodiment of a facility for the heat treatment of a reinforcing element for a tire. Elements similar to those represented in the first embodiment are denoted by identical references. 
     Unlike the first embodiment, the facility according to the second embodiment comprises several cooling devices  16  arranged in series downstream of the heating device  14 . 
     The cooling devices  16  are connected together by ducts  58  for circulation of the element F. These ducts also enable the circulation of the intermediate cold source  48 . The cooling devices  16  are also connected together by ducts  60  for circulation of the external cold source  54 . 
     The invention is not limited to the embodiments described above. 
     In particular it could be envisaged to heat-treat several reinforcing elements simultaneously in one and the same cooling device. Thus, several reinforcing elements may run through the heat extraction means which makes it possible to further increase the total mass throughput without increasing the size of the heat treatment facility. 
     It could also be envisaged to superimpose several heat treatment facilities on top of one another in order to reduce the floor space requirement. 
     The reinforcing element could be different from a wire, in particular different from a wire of circular cross section. 
     The heat treatment method according to the invention could be carried out within the context of a manufacturing method different from that described above. In particular, a manufacturing method comprising two dry-drawing steps or two wet-drawing steps could be used. 
     It may also be possible to combine the features of the various embodiments described or envisaged above, as long as these are compatible with one another. 
     The great ease of operation of the method and also the reduced size of the facility described above will be noted, unlike the method and facility from U.S. Pat. No. 4,767,472 in which the relatively low value of the run speed of the facility is compensated for by the simultaneous treatment of several tens of wires that makes it possible to obtain a high total mass throughput despite a mass throughput per wire, i.e. a unit mass throughput, that is low. Furthermore, in U.S. Pat. No. 4,767,472, when the upstream storage reels are empty, it is necessary to join the end of each wire to another wire originating from a full reel. It is therefore necessary to carry out as many joining operations as there are wires, which makes the operation of the heat treatment facility from U.S. Pat. No. 4,767,472 relatively complex and tedious. In addition, it is necessary to have as many reels as there are wires, which makes the heat treatment facility from U.S. Pat. No. 4,767,472 relatively large. 
     The scope of protection of the invention is not limited to the examples given hereinabove. The invention is embodied in each novel characteristic and each combination of characteristics, which includes every combination of any features which are stated in the claims, even if this feature or combination of features is not explicitly stated in the examples.