EP0953651B1 - Stainless steel for manufacturing drawn wire, especially tyre reinforcement wire, and wire manufactured by said process - Google Patents

Abstract

A drawn steel wire is produced by drawing an austenitic stainless steel of specified composition to produce a high tensile fracture stress without excessive martensite formation. A drawn steel wire is produced from a greater than 5 mm diameter wire-rod or a pre-drawn wire of a steel of composition (in wt.%) 0.005-0.050 C, 0.005-0.050 N, 0.1-2 Si, 0.1-5 Mn, 5-12 Ni, 10-20 Cr, 0.01-4 Cu, 0.01-3 Mo, 0.0001-0.030 S, 0.005-0.10 P, less than 0.5 each (less than 1 total) impurities and balance Fe, the sum of C% + N% being ≤ 60 x 10<-3> and the JM value being -55 to -30 exclusive, where JM = 551 - 462 x (C% + N%) - 9.2 x Si% - 20 x Mn% - 13.7 x Cr% - 29 x (Ni% + Cu%) - 18.5 x Mo%. The process comprises (a) initial drawing with a cumulative reduction epsilon of 2-4 exclusive at below 600 degrees C without intermediate anneals to obtain a 0.7-2 mm diameter wire; (b) intermediate annealing at above 700 degrees C; (c) optional conditioning; and (d) final drawing with a cumulative reduction epsilon of 3-4.5 exclusive at below 600 degrees C without intermediate anneals to obtain a 0.1-0.4 mm diameter wire. An Independent claim is also included for a steel wire obtained by the above process, the wire optionally being coated with copper, brass or zinc and diffusion treated prior to the final drawing stage.

Description

The present invention relates to a method for producing a wire drawn wire, in particular tire reinforcement wire of diameter less than 0.3 mm, by drawing a basic wire-rod of a diameter greater than 5 mm or a previously drawn wire.

Metal reinforcing threads of elastomers for tires, which can be used in the field of production of parts subjected to fatigue, must have a small diameter, generally between 0.1 mm and 0.4 mm, and characteristics mechanical high. The tensile load at break can be greater than 2200 MPa, the residual ductility, measured by the narrowing in tension, torsion or by loopback test must be non-zero, the fatigue endurance limit by rotary or alternating bending shall be greater than 1000 MPa.

These characteristics are necessary to support the efforts static or alternating to which the wire is subjected in assemblies incorporated in the tires.

In addition, wire drawing of stainless steel wire up to diameter between 0,1 and 0,4 mm must be possible under conditions industrial, ie with such low case frequencies as possible.

The patent application FR 93 12 528 (EP-A-648891) deals with the use of a wire stainless steel with a diameter of between 0.05 mm and 0.5 mm the breaking strength Rm is greater than 2000 MPa. Steel of which is composed the yarn contains in its composition at least 50% of martensite obtained, by drawing, at a reduction rate higher than 2.11 with intermediate annealing, the sum of the nickel and chromium being between 20% and 35%.

Patent No. 97 01 858 (European Application EP-A-859064) deals with the production of a steel wire austenitic stainless steel in the form of drawn hardened wire containing a certain proportion of martensite formed during drawing, wire drawing being performed without annealing, with a higher cumulative reduction rate to 6.

By cumulative deformation by wire drawing ε, the value of the natural logarithm of the report of the initial and final sections. (s = Log [So / Sf])

The described process specifies compositions particularly stable vis-à-vis martensite hardening that allow obtaining tensile loads greater than 2200 MPa when the cumulative deformation is very high and greater than 6.

In general, it is known a drawing process in two phases with cumulative deformation of 0.7-3, EP-A-474530.

Tire reinforcements are generally made by stranding of wires of diameter between 0.1 mm and 0.30 mm. In In the case of stainless steels, a breaking load of 2200 MPa is sufficient in view of the fact that the service behavior of the steel is not, or only slightly, degraded by the environment wet.

Higher breaking loads can be interesting industrially but there are difficulties in the development by wire drawing with very high mechanical properties because these become brittle, especially by an excess of martensite.

It may be useful to provide manufacturers with reinforcements pneumatic steel wires which may be processed on their equipment, taking into account treatment operations physical or chemical properties of their own.

The object of the invention is to produce a drawn wire, in particular tire reinforcement thread of diameter less than 0.4 mm per wire drawing of a basic machine wire of diameter greater than or equal to 5 mm or a pre-drawn steel wire of composition given, and having a mechanical characteristic in charge at the rupture greater than 2200 MPa and preferably greater than 2400 MPa without fragility, that is to say with a narrowing non-zero in traction.

0,005% ≤ carbone ≤ 0,050%

0,005% ≤ azote ≤ 0,050%,
   le carbone et azote satisfaisant de préférence la relation C% + N% ≤ 60 10^-3%

0,1% ≤ silicium ≤ 2,0%,

0,1% ≤ manganèse ≤ 5%,

5% ≤ nickel ≤ 12%,

10% ≤ chrome ≤ 20%

0,01 % ≤ cuivre ≤ 4%

0,01 % ≤ molybdène ≤ 3%,

0,0001 % ≤ soufre ≤ 0,030%,

0,005% ≤ phosphore ≤ 0,10%,

des impuretés inhérentes à la fabrication en teneur inférieure à 0,5% pour chaque élément et inférieure à 1% au total, la composition satisfaisant la relation suivante: JM = 551 - 462*( C% + N% ) - 9,2 * Si% - 20 * Mn% - 13,7 * Cr% - 29*( Ni% + Cu% )- 18,5 * Mo%, avec
   -55 < JM < -30,
   le fil de base étant soumis à:

• un tréfilage préalable sous un taux de déformation cumulé ε supérieur à 2 et inférieur à 4, pour obtenir un fil de diamètre compris entre 2 mm et 0,7 mm,
• un traitement de recuit intermédiaire à plus de 700°C permettant la reconstitution d'une structure adoucie principalement austénitique,
• éventuellement, un conditionnement avant réduction finale,
• un tréfilage final, sous un taux de déformation cumulé ε inférieur à 4,5 et supérieur à 3, pour obtenir un fil de diamètre compris entre 0,1 mm et 0,4 mm,
• le fil étant, pendant les deux opérations de tréfilage, maintenu à une température inférieure à 600°C, sans recuit entre les passes de tréfilage.

The subject of the invention is a process for producing a drawn wire, in particular a tire reinforcing wire with a diameter of less than 0.4 mm by drawing a base wire-machine with a diameter greater than 5 mm. or a pre-drawn base wire of a steel of the following weight composition:

0.005% ≤ carbon ≤ 0.050%

0.005% ≤ nitrogen ≤ 0.050%,
the carbon and nitrogen preferably satisfying the relationship C% + N% ≤ 60 10 ^-3 %

0.1% ≤ silicon ≤ 2.0%,

0.1% ≤ manganese ≤ 5%,

5% ≤ nickel ≤ 12%,

10% ≤ chromium ≤ 20%

0.01% ≤ copper ≤ 4%

0.01% ≤ molybdenum ≤ 3%,

0.0001% ≤ sulfur ≤ 0.030%,

0.005% ≤ phosphorus ≤ 0.10%,

impurities inherent in the production at a content of less than 0.5% for each element and less than 1% in total, the composition satisfying the following relationship: Y = 551 - 462 * (C% + N%) - 9.2 * If% - 20 * Mn% - 13.7 * Cr% - 29 * (Ni% + Cu%) - 18.5 * Mo%, with
-55 <JM <-30,
the basic thread being subject to:

• prior drawing under a cumulative deformation rate ε greater than 2 and less than 4, to obtain a wire having a diameter of between 2 mm and 0.7 mm,
• an intermediate annealing treatment at more than 700 ° C allowing the reconstitution of a mainly austenitic softened structure,
• optionally, conditioning before final reduction,
• a final drawing, under a cumulative deformation rate ε less than 4.5 and greater than 3, to obtain a wire with a diameter of between 0.1 mm and 0.4 mm,
• the wire being, during the two drawing operations, maintained at a temperature below 600 ° C, without annealing between the drawing passes.

• en outre, la composition satisfait la relation suivante: JM = 551 - 462*( C% + N% ) - 9,2 * Si% - 20 * Mn% - 13,7 * Cr% - 29*( Ni% + Cu% )- 18,5 * Mo% , avec
     -55 < JM < -30, le taux de déformation au tréfilage étant lié à JM par la relation : - 9,23 ε - 17 < JM < - 9,23 ε + 3
• la composition comprend de 3% à 4% de cuivre.
• le conditionnement avant tréfilage final, est en outre, une opération de revêtement du fil recuit par un métal ou d'un alliage métallique choisi parmi : le cuivre, le laiton, le zinc.
• le traitement de recuit intermédiaire est réalisé à une température comprise entre 700°C et 1350°C en un temps adapté à la température et à la méthode de chauffage,
• le conditionnement avant tréfilage final, comprend, en outre, un traitement de diffusion à moins de 700°C, des dépôts de Cu, Zn, ou laiton sur fil recuit.

The preferred features of the invention are:

• in addition, the composition satisfies the following relationship: Y = 551 - 462 * (C% + N%) - 9.2 * If% - 20 * Mn% - 13.7 * Cr% - 29 * (Ni% + Cu%) - 18.5 * Mo%, with
  -55 <JM <-30, the rate of wire drawing deformation being related to JM by the relation: - 9.23 ε - 17 <JM <- 9.23 ε + 3
• the composition comprises from 3% to 4% copper.
• the conditioning before final drawing, is in addition, a coating operation of the wire annealed by a metal or a metal alloy selected from: copper, brass, zinc.
• the intermediate annealing treatment is carried out at a temperature of between 700 ° C. and 1350 ° C. in a time adapted to the temperature and to the heating method,
• the conditioning before final drawing, comprises, in addition, a diffusion treatment at less than 700 ° C, deposits of Cu, Zn, or brass on annealed wire.

0,005% ≤ carbone ≤ 0,050%

0,005% ≤ azote ≤ 0,050%,
   le carbone et azote satisfaisant la relation C% + N% ≤ 60 10^-3% 0,1 % ≤ silicium ≤ 2,0%,

0,1% ≤ manganèse ≤ 5%,

5% ≤ nickel ≤ 12%,

10% ≤ chrome ≤ 20%

0,01 % ≤ cuivre ≤ 4%

0,01% ≤ molybdène ≤ 3%,

0,0001 % ≤ soufre ≤ 0,030%,

0,005% ≤ phosphore ≤ 0,10%,

des impuretés inhérentes à la fabrication en teneur inférieure à 0,5% pour chaque élément et inférieure à 1% au total;
   -le fil ayant subi, lors de son élaboration un recuit puis éventuellement, avant tréfilage final, un conditionnement comprenant une opération de revêtement d'un métal ou d'un alliage métallique choisi parmi : le cuivre, le laiton, le zinc, pouvant être suivi d'un traitement de diffusion.The invention also relates to a steel wire obtained by this method, namely a tire reinforcing wire with a diameter of less than 0.4 mm obtained by drawing a basic machine wire of diameter greater than 5 mm or a pre-drawn base wire characterized by the following weight composition:

0.005% ≤ carbon ≤ 0.050%

0.005% ≤ nitrogen ≤ 0.050%,
the carbon and nitrogen satisfying the relationship C% + N% ≤ 60 10 ^-3 % 0.1% ≤ silicon ≤ 2.0%,

0.1% ≤ manganese ≤ 5%,

5% ≤ nickel ≤ 12%,

10% ≤ chromium ≤ 20%

0.01% ≤ copper ≤ 4%

0.01% ≤ molybdenum ≤ 3%,

0.0001% ≤ sulfur ≤ 0.030%,

0.005% ≤ phosphorus ≤ 0.10%,

impurities inherent in the production of less than 0.5% for each element and less than 1% in total;
the wire which has undergone, during its production, annealing and possibly, before final drawing, a packaging comprising an operation for coating a metal or a metal alloy chosen from: copper, brass, zinc, which may be followed by a diffusion treatment.

The description which follows and the attached figures, all given as non-limiting example will make clear the invention.

FIG. 1 shows the maximum cumulative deformation rate ε that can be achieved by industrial drawing between the two drawing operations, as a function of the index JM defined by the relationship satisfying the composition.

FIG. 2 shows, as a function of the cumulative deformation rate ε the evolution of the load at break, in the process according to the invention (steel A and B), compared to that of reference steels, excluding the invention.

Drawing of a reinforcing stainless steel whose diameter varies between 0.1 and 0.4 mm, must meet a service life of the endurance view in fatigue in flexion or in traction or torsion as well as holding in a humid environment or in solicitation combined: wet environment and fatigue and wire-to-wire friction.

The fine wire is made by wire drawing from a wire rod or a pre-drawn steel wire. Because of the composition of steel, after wire drawing, the final drawn wire has improved properties of tensile strength and residual ductility sufficient for its in assembly, for example in the form of plies, cables.

According to the invention the drawing is carried out with a stainless steel of general weight composition A and B presented in Table 1, the steel C, E, F, G being taken as a reference. Steel VS NOT Yes mn Cr Or Cu MB S P JM AT 0,023 0,032 0.38 0.54 18.2 10.0 0.36 0.23 0.0090 0,023 -36 B 0,024 0,024 0.47 1.24 18.3 9.7 0.31 0.39 0.0011 0,025 -50 VS 0,011 0,027 0.40 1.83 17.2 8.1 3.24 0.36 0.0040 0,025 -78 E 0,011 0.016 0.35 0.54 17.1 9.5 3.16 0.19 0.0020 0,027 -81 F 0,085 0,038 0.85 1.05 17.5 8.2 0.38 0.21 0.0020 0,023 -27 BOY WUT 0.082 0,045 0.67 0.78 18.5 8.8 0.34 0.20 0.0030 0,025 -52

The invention makes it possible to define an austenitic stainless steel capable of being drawn without annealing from the diameter wire rod greater than 5 mm to a diameter of between 0,7 mm and 2 mm, then, to be annealed at this intermediate diameter, and, possibly coated, for example brass, and finally to be drawn to new without annealing, between drawing passes, up to a diameter between 0.4 mm and 0.1 mm. Thus, we obtain a mechanical characteristic load at break Rm greater than 2200 MPa, and preferably greater than 2400 MPa, without fragile character. The composition according to the invention satisfies a relationship JM included within a certain limited range such as for reduction rates cumulated specific during final drawing between ε greater than 3, (from 1.6 mm to less than 0.357 mm, from 1.2 mm to less than 0.268 mm; from 0.8 mm to less than 0.179 mm), and ε less than 4.5, 1.6 mm to more than 0.169 mm; from 1.2 mm to more than 0.126 mm; 0.8 mm to more than 0.0084 mm), live wire drawing of final diameter wire between 0.1 mm and 0.4 mm is possible without excessive brittleness, with a breaking load greater than 2200 MPa.

Direct wire drawing refers to a drawing operation comprising a succession of drawing passes for each of which the wire engagement temperature is between ambient temperature and 200 ° C, without the wire being brought to a temperature above 600 ° C.

Table 1 presents, for comparison, steel compositions not satisfying the characteristics of the invention (steels C, E, F, G).

Table 2 shows some examples of drawing on steels according to the invention and outside the invention. Steel Initial diameter (mm) Final diameter (mm) cumulative ε Rm MPa Martensite% drawing AT 4.36 1.19 2.60 2214 53 correct 4.36 0.68 3.72 2500 69 breakages B 1.0 0.18 3.43 2064 - correct 5.67 1.0 3.47 1828 16 correct VS 5.56 0.59 4.49 2165 69 correct 5.56 0.55 4.63 2211 72 - 5.56 0.25 6.24 2666 87 breakages E 5.6 0.672 4.24 2069 62 correct 5.6 0,355 5.52 2424 86 correct 5.6 0.178 6.90 2644 90 breakages F 5.5 1.8 2.14 1950 22 cques breaks BOY WUT 1.95 0.7 2.10 2064 35 cques breaks

With the steels A and B according to the invention, it is possible to draw, without excessive breakage during drawing, with cumulative deformation greater than 3 and obtain wires with a breaking load greater than 2200 MPa with cumulative deformations less than 4.5.

With steel E outside the invention, whose coefficient JM is less than -55, no breaking load greater than 2200 can be obtained MPa with cumulative strain greater than 4.6. With steel C, outside the invention, whose coefficient JM is less than -55, it is not possible to obtain higher breaking loads at 2200 MPa, with cumulative deformation less than 4.5.

With steel F, with a high carbon content, except for the invention, the fragility is reached in wire drawing for cumulative deformations of 3, and it is not possible to reach ε greater than 3.

With G steel, with high carbon content, excluding the invention, it is similarly, although the JM index is between -30 and -55.

The drawing of the wire is preferably carried out on a machine multipasses the wire being, on the one hand, lubricated with soap or lubricant liquid, and on the other hand, temperature controlled between 20 ° C and 180 ° C.

The wire may also be brassed between the two operations of drawing. The brass layer improves the drawing ability and the adhesion of the yarn with the elastomers of the tires.

From a metallurgical point of view, it is known that certain elements of alloys used in the composition of steels favor the appearance of the ferrite phase of metallographic structure of cubic type center. These elements are called alpha-genes. Among these are the chromium, molybdenum, silicon.

Other so-called gamma-gene elements favor the appearance of the austenite phase of metallographic structure of cubic type with faces centered. These elements include carbon, nitrogen, manganese, copper, nickel.

Carbon, nitrogen, chromium, nickel, manganese, silicon are the usual elements for obtaining a steel austenitic stainless.

It has been noticed that the compositions forming a quantity excessive martensite wire drawing become brittle and brittle at drawing. This amount of martensite is a function of the total content carbon and nitrogen in steel and is in the order of 70% for a total carbon and nitrogen content less than or equal to 0,060%, and by example, by 30% for a total carbon and nitrogen content of approximately 0.100%.

According to the invention, the steel has a total carbon and nitrogen content of less than or equal to 0.060%, the drawing conditions satisfying the following relation: -55 <JM <-30

It has also been noted that compositions having a subscript JM greater than the value determined above and a total content of carbon and nitrogen in the order of 0.040% become brittle before to achieve wire drawing at the final diameter.

In the same way, the presence in excessive quantities of silicon, ie in quantities greater than 2%, has the effect of weaken the wire in the hardened state by wire drawing in the presence of a quantity important of martensite.

The contents of manganese, chromium and sulfur are chosen in proportion to generate deformable sulfides of composition well determined.

Copper is added to the composition of the steel according to the invention because it stabilizes the austenite and as a result, improves the properties of cold deformation. However the copper content is limited to 4% to avoid difficulties of hot transformation because copper, in amount greater than 4% substantially lower the upper limit of reheating temperature of the steel before rolling, beyond which is a local merger.

According to a form of the invention, the sulfur content must be less than 0.030% to obtain thick sulphide inclusions not exceeding 5 μm on rolled product.

Coarse inclusions such as oxides and sulphides are generally considered to be harmful to property in the field of fine wire drawing and in the field of fatigue resistance, especially in flexion and / or torsion.

The composition of the stainless steel according to the invention, containing more than 5% nickel, more than 0.01% copper, more than 10% of chromium, a total carbon and nitrogen content of less than 0,060%, a JM index less than -30, can be drawn by the method of the invention up to the final diameter with a reduced breakage rate, the wire retaining mechanical characteristics that allow its use in the field of tire reinforcement.

The JM should be in the range -55 to -30. Indeed, if JM is less than -55, the amount of martensite formed remains low and the load at break can not reach high values greater than 2200 MPa, even after final drawing with a cumulative deformation ε close to 4.5.

This remark justifies the limit of the chromium content to less than 20% and total copper and nickel less than 16%.

The process applied to the drawing of the stainless steel according to the invention makes it possible to obtain a yarn having excellent fatigue strength measured by rotary bending with an endurance stress at 2.10 ⁶ cycles greater than 1000 MPa.

The yarn obtained contains less than 50% of austenite or more than 50% of martensite. The steel used is austenite slightly unstable with a total carbon and nitrogen content of less than 0.060%.

• une faible tendance à la formation de martensite, formation en quantité suffisante pour durcir l'acier, et en quantité insuffisante pour provoquer une fragilisation du fil après tréfilage,
• une consolidation très progressive de telle sorte que la résistance à la rupture peut être comprise entre 2200 MPa et 3000 MPa pour un fil tréfilé de 0,18 mm tréfilé depuis 5,5 mm avec un recuit intermédiaire, ou pour d'autres tréfilés obtenus avec un taux de réduction cumulé de 3 à 4,5 après le dernier recuit.

The method according to the invention from a composition of steel optimized for cold deformation and fine wire drawing ensures:

• a weak tendency to the formation of martensite, formation in sufficient quantity to harden the steel, and in insufficient quantity to cause weakening of the wire after wire drawing,
• a very progressive consolidation so that the breaking strength can be between 2200 MPa and 3000 MPa for a drawn wire of 0.18 mm drawn from 5.5 mm with an intermediate annealing, or for other drawn products obtained with a cumulative reduction rate of 3 to 4.5 after the last annealing.

Claims (7)

1. Process for producing a drawn wire, especially a tyre reinforcement cord, less than 0.4 mm in diameter by the wire-drawing of a base rod stock greater than 5 mm in diameter or of a pre-wire-drawn base wire of a steel having the following composition by weight:

   0.005% ≤ carbon ≤ 0.050%

   0.005% ≤ nitrogen ≤ 0.050%,
   the carbon and nitrogen preferably satisfying the relationship C% + N% ≤ 60 x 10^-3%;

   0.1% ≤ silicon ≤ 2.0%,

   0.1% ≤ manganese ≤ 5%,

   5% ≤ nickel ≤ 12%,

   10% ≤ chromium ≤ 20%,

   0.01% ≤ copper ≤ 4%,

   0.01% ≤ molybdenum ≤ 3%,

   0.0001% ≤ sulphur ≤ 0.030%,

   0.005% ≤ phosphorus ≤ 0.10%,

   the balance being iron with impurities inherent in the manufacture, having a content of less than 0.5% for each element and less than 1% in total, the composition satisfying the following equation: JM = 551 - 462 × (C% + N%) - 9.2 × Si% - 20 × Mn% - 13.7 × Cr% - 29 × (Ni% + Cu%) - 18.5 × Mo%,    with -55 < JM < -30,
   the base wire being subjected to:

   a prior wire-drawing operation, with a cumulative reduction ratio ε of greater than 2 and less than 4 in order to obtain a wire having a diameter between 2 mm and 0.7 mm;

   an intermediate annealing treatment at above 700°C, allowing a softened, mainly austenitic structure to be reformed;

   optionally, a conditioning operation before the final reduction;

   a final wire-drawing operation with a cumulative reduction ratio ε of less than 4.5 and greater than 3, in order to obtain a wire having a diameter of between 0.1 mm and 0.4 mm;

   the wire being, during the two wire-drawing operations, maintained at a temperature below 600°C, without annealing between the wire-drawing passes.
2. Process according to Claim 1, characterized in that the composition furthermore satisfies the following equation: JM = 551 - 462 × (C% + N%) - 9.2 × Si% - 20 × Mn% - 13.7 × Cr% - 29 × (Ni% + Cu%) - 18.5 × Mo%,    with -55 < JM < -30, the wire-drawing reduction ratio being linked to JM through the relationship: - 9.23 ε - 17 < JM < - 9.23 ε + 3.
3. Process according to Claims 1 and 2, characterized in that the composition comprises from 3% to 4% copper.
4. Process according to either of Claims 1 and 2, characterized in that the conditioning before the final wire-drawing operation is furthermore an operation of coating with a metal or with a metal alloy chosen from: copper, brass, zinc.
5. Process according to one of Claims 1 to 4, characterized in that the intermediate annealing treatment is carried out at a temperature between 700°C and 1350°C for a time suited to the temperature and to the heating method.
6. Process according to one of Claims 1 to 5, characterized in that the conditioning before the final wire-drawing operation furthermore includes a treatment at below 700°C for diffusion of the copper, zinc or brass platings on the annealed wire.
7. Steel wire obtained by the process according to one of Claims 1 to 6, especially a tyre reinforcement cord, less than 0.4 mm in diameter obtained by the wire-drawing of a base rod stock greater than 5 mm in diameter or of a pre-wire-drawn base wire, characterized by the following composition by weight:

   0.005% ≤ carbon ≤ 0.050%

   0.005% ≤ nitrogen ≤ 0.050%, the carbon and nitrogen preferably satisfying the relationship C% + N% ≤ 60 x 10^-3%;

   0.1% ≤ silicon ≤ 2.0%,

   0.1% ≤ manganese ≤ 5%,

   5% ≤ nickel ≤ 12%,

   10% ≤ chromium ≤ 20%,

   0.01% ≤ copper ≤ 4%,

   0.01% ≤ molybdenum ≤ 3%,

   0.0001% ≤ sulphur ≤ 0.030%,

   0.005% ≤ phosphorus ≤ 0.10%,

   the balance being iron with impurities inherent in the manufacture, with a content of less than 0.5% for each element and less than 1% in total, the wire having optionally undergone, during its production, a conditioning treatment before the final wire-drawing, comprising an operation of coating with a metal or with a metal alloy chosen from: copper, brass, zinc, possibly combined with a diffusion treatment.

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