EP0486658B1 - Process for obtaining a high strength hard-drawn steel wire - Google Patents

Abstract

PCT No. PCT/FR91/00446 Sec. 371 Date Feb. 5, 1992 Sec. 102(e) Date Feb. 5, 1992 PCT Filed Jun. 6, 1991 PCT Pub. No. WO91/19014 PCT Pub. Date Dec. 12, 1991.A process is disclosed for obtaining a high-strength, strain-hardened steel wire by drawing a rod wire, optionally with intermediate thermal treatment. The rod wire is produced from a steel that is devoid of proeutectoid cementoid, and which comprises 0.45-0.75% C, less than 0.010% each of S and P, and conventional proportions of Mn, Si, Ca, Mo, and Al. The wire-drawing operation is carried out in two distinct phases, separated from each other by a single brassing-patenting treatment. In the final wire-drawing step the level of cross-sectional reduction is greater than 97 per cent. The level of cross-sectional reduction is also exceptionally high during the first wire-drawing phase as a result of elimination of conventional intermediate patenting.

Description

The present invention relates to a process for obtaining a hardened steel wire, with high resistance, usable in particular for the manufacture of strands and cables for reinforcing elastomeric articles such as tires; it also relates to the reinforcing elements (strands and cables) made from wires obtained by such a process.

• C : 0,68 à 0,84 %
• Mn : 0,40 à 0,70 %
• Si : 0,20 a 0,30 %
• Al : moins de 0,005 %
• S : moins de 0,020 %
• P : moins de 0,020 %
• N : moins de 0,010 %
• Ni, Cu, Cr, Mo, Co : moins de 0,050 % chacun
• le reste étant du fer.

The strands and cables for the reinforcement of tires usually consist of steel wires comprising, in addition to iron:

• C: 0.68 to 0.84%
• Mn: 0.40 to 0.70%
• If: 0.20 to 0.30%
• Al: less than 0.005%
• S: less than 0.020%
• P: less than 0.020%
• N: less than 0.010%
• Ni, Cu, Cr, Mo, Co: less than 0.050% each
• the rest being iron.

• application d'une première série de passes de tréfilage pour amener le fil à un diamètre voisin de 3 mm (taux de réduction de section de 70 % environ) ;
• patentage, par exemple patentage au plomb pour redonner au fil une capacité d'étirabilité ;
• application d'une deuxième série de passes (taux de réduction de section de 85 à 94 % environ) ;
• patentage laitonnage ;
• application d'une troisième série de passes de tréfilage pour atteindre le diamètre final désiré (taux de réduction de section final 90 à 96 % voire 97 %).

These wires, the diameter of which is between approximately 0.10 and 0.50 mm, more generally between 0.15 and 0.40 mm, are produced from a wire called "machine wire" of common diameter 5.5 mm. , by drawing in several stages with patenting or intermediate heat treatment. The conventional process includes the following steps:

• application of a first series of drawing passes to bring the wire to a diameter close to 3 mm (section reduction rate of approximately 70%);
• patenting, for example lead patenting to restore the yarn's ability to stretch;
• application of a second series of passes (section reduction rate of 85 to 94% approximately);
• patenting brass plating;
• application of a third series of wire drawing passes to reach the desired final diameter (reduction rate of final section 90 to 96% or even 97%).

In some cases, for ordinary steel grades and depending on the final diameter desired, the first three stages can be replaced by a single wire drawing stage up to a diameter of approximately 1.35 to 1.80 mm.

It is noted that, in general, the more the diameter decreases, the greater the reduction rate of section applied can be.

A distinction is made between traditional threads of ordinary or current quality and second generation threads called high resistance. These two types of son are both obtained from steel chosen in the above composition, by varying the carbon content and possibly the final reduction rate of section.

Ordinary quality wires which have, for example, a typical breaking strength of 2850 MPa are obtained from a steel with 0.72% (+ 0.02) carbon with application of a reduction rate of final section from 95 to 96.5%.

The high resistance wires are obtained from a steel with 0.82% (± 0.02) of carbon with application of a final reduction rate of between 96 and 97%, the latter value being in practice only very rarely affected. They have a breaking strength of up to 3200 to 3450 MPa. However, this last grade of steel dangerously approaches the metal of a physical limit called "eutectoid point" beyond which fragile phases of proeutectoid cementite appear, making the wire drawing and especially the stranding almost impossible because of the high number of breaks . This limit which depends on the diameter, is higher the smaller the diameter, it is difficult to determine since a certain amount of wire can pass the axial deformation test during the drawing but not the deformation constraints imposed during assembly operations.

The manufacture of high-resistance wires therefore requires surrounding oneself with precautions, which increases the cost, both at the level of the steelmaker, for the supply of a wire rod free of segregation (fragile phases) and at the level of the cable manufacturer during patenting, wire drawing, stranding, wiring.

In the range of hard and semi-hard steels (carbon content> 0.4%) and more particularly in the composition cited, usually used for tire reinforcement wires: C content from 0.68 to 0.84% (see 0.6 to 0.9%), the high resistances are always obtained with high carbon contents, although research has been carried out in parallel to improve the work hardening capacity, which also goes in the direction of high resistances.

• C : 0,6 à 0,9 %
• Si : 0,1 à 0,5 %
• Mn : 0,3 à 1 %
• P : < 0,010 %
• S : < 0,005 %
• Cu + Ni + Cr : < 0,10 %
• 0₂ : < 30 ppm
• N₂ : < 30 ppm.

Thus, Japanese patent no. 60.152659 relates to a steel wire usable as reinforcement for tires, said steel having the following composition:

• C: 0.6 to 0.9%
• If: 0.1 to 0.5%
• Mn: 0.3 to 1%
• P: <0.010%
• S: <0.005%
• Cu + Ni + Cr: <0.10%
• 0₂: <30 ppm
• N₂: <30 ppm.

The low P and S contents are presented as notably improving the resistance to fatigue. This patent leads to a high resistance wire, of around 3800 MPa, for a carbon content of 0.84%. But with this content the metal is located in the area of the eutectoid point and the problems mentioned above are not resolved.

• C : 0,4 à 1,4 %
• Mn : 0,1 à 1 %
• Si : 0,1 à 0,4 %
• Al : < 0,005 %
• S : < 0,015 % (de préférence < 0,010 %),

l'originalité revendiquée résidant dans la faible teneur en soufre, laquelle permet d'augmenter les capacités de déformation à froid, en particulier les déformations non axiales (torsion, pliage). Ce brevet conduit à un fil dont la résistance R en MPa est supérieure à 2325-1130 log d où d est le diamètre du fil exprimé en mm ; ce qui pour des petits diamètres : (d inférieur à 0,15 mm) permet d'atteindre une résistance élevée (pour d = 0,15 R ≧ 3250 MPa). Cependant, la consultation des exemples permet de constater qu'en pratique les hautes résistances sont obtenues avec un acier dont la teneur en carbone se situe dans la gamme 0,80 à 0,85 %, ce qui est somme toute un résultat banal et ne résoud pas le problème évoqué ci-dessus, lorsqu'on se rapproche de la limite constituée par le point eutectoïde.European patent no. 0144 811 relates to a steel wire usable as reinforcement for tires, said steel having the following composition:

• C: 0.4 to 1.4%
• Mn: 0.1 to 1%
• If: 0.1 to 0.4%
• Al: <0.005%
• S: <0.015% (preferably <0.010%),

the originality claimed resides in the low sulfur content, which makes it possible to increase the cold deformation capacities, in particular the non-axial deformations (torsion, bending). This patent leads to a wire whose resistance R in MPa is greater than 2325-1130 log where d is the diameter of the wire expressed in mm; which for small diameters: (d less than 0.15 mm) allows reach a high resistance (for d = 0.15 R ≧ 3250 MPa). However, consulting the examples shows that in practice the high strengths are obtained with a steel whose carbon content is in the range 0.80 to 0.85%, which is after all a banal result and does not does not solve the problem mentioned above, when we approach the limit constituted by the eutectoid point.

US patent no. 3617230 relates to a high-strength steel wire obtained both by a high carbon content: 0.9 to 1.1% and by the application of high work hardening rates in the final phase: more than 97%, these rates being made possible by the use of a steel with a low content of S (<0.010%) and P (<0.005%). However, it is a steel wire which goes beyond the scope of the tire reinforcement application both by its use: piano wire and by its composition, in particular its carbon content which comes out of the usual range of steel for tire wires given at the beginning of the text. In addition, the patent says nothing about the behavior of such a wire under bending or twisting constraints.

Numerous other documents such as for example EP-A-0 169 587, EP-A-0 232 558, EP-A-0 292 039 and DE-A-2 739 484, also describe different possibilities making it possible to obtain steel wires with high tensile strengths.

However, all the solutions proposed to date imply using steels which are difficult to produce, the carbon content of which is higher the higher the tensile strength desired, which entails the risk of dangerous segregation. by following the presence of proeutectoid cementite which may appear in compositions with a high carbon content (for example 0.82% carbon), causing the risk of breakage during the stranding operation. Furthermore, the solutions proposed for obtaining such high-resistance yarns involve the implementation of drawing and / or intermediate treatment processes which are increasingly complex and delicate to implement.

However, it has been found, and this is the subject of the present invention, that it was possible to obtain such high-strength steel wires, usable in particular as reinforcements for tires (and what these wires are both of the so-called current quality and of the so-called "high resistance" quality) by using steel grades having a reduced carbon content (compared to the usual levels recommended for obtaining a given resistance) and allowing therefore to overcome the drawbacks linked to the risks of the presence of proeutectoid cementitis.

• C : 0,45 à 0,67% ou 0,68 à 0,75%
• Mn, Si, Ca, Mo, Al ... dans des teneurs conventionnelles ; et dans lequel :
• la nuance d'acier du fil machine est sélectionnée parmi ceux dont la teneur en S et P est inférieure à 0,010 % ;
• l'opération de tréfilage est réalisée en deux phases distinctes séparées l'une de l'autre par un seul traitement de patentage-laitonnage et;
• le taux de réduction de section dans l'étape finale de tréfilage est supérieur à 97 % (donc plus élevé que celui normalement utilisé pour un type d'acier similaire pour réaliser un fil de même diamètre final, le taux de réduction de section lors de la première phase de tréfilage étant donc également plus élevé du fait de la suppression d'un patentage intermédiaire).

In general, the invention therefore relates to a process for obtaining a high strength cold-worked steel wire obtained by drawing a wire rod, possibly with heat treatment and intermediate surface treatment, said wire rod being of a steel with a structure free from proeutectoid cementite, and the composition of which comprises, in addition to iron:

• C: 0.45 to 0.67% or 0.68 to 0.75%
• Mn, Si, Ca, Mo, Al ... in conventional contents; and in which:
• the steel grade of the wire rod is selected from those with an S and P content of less than 0.010%;
• the drawing operation is carried out in two distinct phases separated from each other by a single patenting-brass plating treatment;
• the section reduction rate in the final wire drawing stage is greater than 97% (therefore higher than that normally used for a similar type of steel to produce a wire of the same final diameter, the section reduction rate during the first drawing phase is therefore also higher due to the elimination of an intermediate patenting).

Advantageously, the final section reduction rate is between 97 and 98%.

• pour réaliser des fils dits "variété courante" présentant une résistance à la rupture comprise entre 2400 et 3000 MPa on utilise un acier ayant une teneur en carbone comprise entre 0,45 et 0,67 % selon la revendication 1, alors qu'antérieurement, pour obtenir une même résistance à la rupture, on utilisait un acier ayant une teneur en carbone comprise entre 0,50 et 0,75 % ; à titre indicatif, pour une résistance à la rupture de l'ordre de 2850 MPa, on utilise conformément à l'invention un acier avec une teneur en carbone de l'ordre de 0,6 % au lieu d'un acier de 0,72 % ;
• pour la réalisation de fils dits "à haute résistance", dont la résistance à la rupture est comprise entre 3000 et 3500 MPa, conformément à l'invention, on utilise un acier ayant une teneur en carbone comprise entre 0,68 et 0,75 % selon la revendication 2, alors qu'antérieurement, il était nécessaire d'utiliser un acier ayant une teneur en carbone de l'ordre de 0,80 ; 0,82 %.

The method according to the invention is characterized in that:

• to produce so-called "common variety" wires having a breaking strength of between 2400 and 3000 MPa, steel having a carbon content of between 0.45 and 0.67% according to claim 1 is used, whereas previously, to obtain the same breaking strength, a steel having a carbon content of between 0.50 and 0.75% was used; by way of indication, for a breaking strength of the order of 2850 MPa, a steel with a carbon content of the order of 0.6% is used in accordance with the invention instead of a steel of 0, 72%;
• for the production of so-called "high strength" wires, the breaking strength of which is between 3000 and 3500 MPa, in accordance with the invention, a steel having a carbon content of between 0.68 and 0.75 is used % according to claim 2, whereas previously it was necessary to use a steel having a carbon content of the order of 0.80; 0.82%.

In accordance with the invention, in the wire drawing stage preceding the final stage, the reduction rates of section are higher than those currently practiced, and this as said previously.

All the treatments entering into the implementation of the process according to the invention, such as surface treatments, heat treatments, lubricants, are carried out under conditions similar to the conventional process. Thus, the patenting is advantageously carried out under conditions making it possible to obtain the finest pearlitic structure possible. Likewise, advantageously, the pickling and brass plating operations are carried out under conditions leading to a good quality coating, favoring passage through the drawing dies.

The wire obtained by implementing the process according to the invention is used to make strands, cables, and this under standard production conditions.

The high rate of section reduction in the final drawing step is made possible by the use of a steel grade conforming to the new composition selected, with an S and P content of less than 0.010% for each of these two. constituents.

The application of high rates of cross-section reduction in the final wire drawing stage leads to high strength wires, this for carbon contents lower than what is usually necessary. Thus, a steel having a carbon content of 0.45 to 0.67% is used instead of a steel of 0.72% for obtaining wires of ordinary quality: typical resistance of the order of 2850 to 2900 MPa and a steel having a carbon content of 0.68 to 0.75% instead of 0.82% for obtaining high resistance wires, resistance of at least 3000 MPa and more often than not at least 3200 MPa.

In the latter case, in addition to saving on the cost of steel (steel at 0.82% of C being expensive), the invention has an important technical advantage because a steel grade conforming to the new composition selected is easier to produce and allows a considerable reduction, even elimination of the risks of segregation caused by the presence of proeutectoid cementite which was encountered in steels of usual composition with a carbon content higher than 0.80% about. Such a shade, far from the eutectoid point, better withstands the stresses of traction, folding and torsion. This results in a reduction in wire drawing and stranding breaks and an improvement in productivity.

Other advantages are linked to the invention.

The greater deformation capacity resulting from the reduction (or even the elimination) of segregation allows the elimination of intermediate patenting (or equivalent heat treatment) and that of a wire drawing step.

Thus, by the claimed process and for relatively small final diameters, it is possible to obtain the wire according to the invention from a machine wire of current diameter 5.5 mm in only two drawing steps (instead of three ), and a single patenting treatment (instead of two), that associated with brass plating. This applies to ordinary quality wire as well as to high resistance wire.

In the case where, in order to obtain the fine wire in only two stages of drawing, previously used non-standard 5 mm diameter machine wire was used, it can be replaced by a standard wire of 5.5 mm diameter and obtain the same end product by applying higher section reduction rates.

In addition, while retaining all the usual steps, since the cross-section reduction rate in the final stage is greater than the current rate, the cross-section reduction rate in the intermediate stages will be lower. This combined with the lower carbon content results in easier drafting conditions for blanks for intermediate diameters before patenting-brass plating.

It should also be noted that the use of a steel with S and P content, less than 0.010% for each of these components, not only allows the application of high rates of reduction in section leading to an increase in the resistance to traction, but also increases the resistance of the wire to bending and twisting stresses, which improves behavior in stranding and wiring by reducing the number of breaks.

However, the invention and its advantages will be better understood with the aid of the examples below given by way of illustration and not limitation.

In these examples, the properties of the wires: resistance, elongation, necking are measured by means of the usual dynamometry tests. The folding and twisting properties are measured according to ISO 7801 and ISO 7800 standards respectively.

The steel grades are defined by the contents of C, Mn, Si, P, S on the understanding that the contents for the other components: Al, Ni, Cu, Cr, Mo, Co, N comply with the values given in the general definition of the wire.

Example 1 :

This example illustrates the manufacture of a standard (ordinary) quality wire by the method according to the invention compared to a wire manufactured by the usual method.

The wire according to the invention is produced from a steel of composition A: in accordance with the composition selected for the invention: 0.64% C, 0.55% Mn, 0.20% Si, 0.008% P, 0.008% S.

The current wire is made from a steel of current composition B: 0.72% C, 0.60% Mn, 0.22% Si, 0.015% P, 0.012% S.

The starting wire rod for the two compositions A and B has a diameter of 5.5 mm.

The machine wire of composition A is drawn directly from the diameter 5.5 mm to the diameter 1.30 mm with the application of a cross-section reduction rate of 94.4%, made possible by the composition of the steel and in particular the low S and P content

The machine wire of composition B is drawn by 5.5 mm with an intermediate diameter of 3.0 mm, then it undergoes an intermediate patenting before being drawn with the final diameter of 1.15 mm. Finally, the two sons are patented for brass plating under standard conditions.

The properties of patented brass plated wire are summarized in the following table: STEEL ⌀ (mm) RESISTANCE (MPa) ELONGATION (%) STRICTION BENDS TORSIONS AT 1.30 1160 9.6 60.1 17 67 B 1.15 1250 9.1 57.9 19 70

Then, the fine wire drawing is carried out in a humid environment to obtain the 0.22 mm diameter with standard resistance (ordinary quality).

On wire A, a section reduction rate of 97.14% is applied, in accordance with the invention. On wire B, a standard rate of 96.34% is applied.

For the two wires, the rates of wire drawing breakage are low and of the same order. The mechanical properties are as follows: STEEL SECTION REDUCTION% RESISTANCE (MPa) ELONGATION% BENDS TORSIONS AT 97.14 2930 2.6 75 137 B 96.34 2920 2.5 80 123

It is noted that the mechanical properties of the two wires are very close, or even identical with regard to strength and elongation.

The results of the bending and twisting tests suggest good behavior of wire A in stranding and wiring. This is confirmed during the production of a 1 x 27 strand with each of the wires A and B. There is a significant improvement in the number of breaks with the wire A.

These two wires A and B are completely interchangeable in terms of their use. On the other hand, the wire A according to the invention also has the advantage of eliminating the intermediate patenting operation and a drawing step.

• tréfilage du diamètre 5 mm au diamètre 1,22 mm (taux de réduction de section : 94 %) ;
• patentage laitonnage ;
• tréfilage du diamètre 1,22 mm au diamètre 0,25 (taux de réduction de section : 95,8 %).

Up to now, however, it has been possible to save a wire drawing step and a patenting treatment by passing directly from the wire of diameter 5.5 mm machine to the intermediate wire of diameter 1.35 to 1.37 mm approximately. However, given the applicable final reduction rate (approximately 96.5% maximum), it was not possible to obtain a final product with a diameter less than or equal to 0.25 mm. Also, the implementation of this "shortened" process for obtaining fine wires (0.25 mm diameter) necessarily required the use of a machine wire with a diameter of 5 mm, which is non-standard and therefore expensive. The steps were as follows:

• wire drawing from diameter 5 mm to diameter 1.22 mm (section reduction rate: 94%);
• patenting brass plating;
• drawing of the diameter 1.22 mm to the diameter 0.25 (reduction rate of section: 95.8%).

With the present invention, it is therefore possible to obtain the finished wire (diameter 0.22 to 0.25 mm for example) in two drawing stages only, with a single heat treatment, this from a machine wire of standard diameter of 5.5 mm, i.e. at no additional cost.

Example 2 :

This example illustrates the implementation of the invention in its most advantageous form. It relates to the manufacture of a high resistance wire, by the method according to the invention, compared to a wire produced by the usual method.

The wire according to the invention is produced from a steel of composition Al, in accordance with the composition selected for the invention: 0.72% C, 0.54% Mn, 0.23% Si, 0.007% P, 0.006% S.

The wire manufactured by the usual process is made of a steel of composition Bl: 0.82% C, 0.55% Mn, 0.25% Si, 0.005% P, 0.009% S. This wire although having a content of S and P of less than 0.01% for each of these components is not made of a steel of composition in accordance with that selected for the invention. It can not withstand high rates of section reduction because of its C and Mn content, it dangerously approaches the eutectoid point with the associated risks.

The starting wire rod for the two compositions Al and Bl has a diameter of 5.5 mm.

The machine wire of composition Al is drawn directly from 5.5 mm to the final diameter of 1.48 mm (section reduction rate: 92.75%).

The machine wire of composition Bl undergoes an intermediate patenting with a diameter of 3.0 mm before being drawn to a diameter of 1.30 mm. Then the two sons undergo patenting for brass plating under standard conditions; the mechanical properties of these are: STEEL ∅ (mm) RESISTANCE (MPa) ELONGATION% STRICTION% BENDS TORSIONS Al 1.48 1260 9.2 54.0 18 60 Bl 1.30 1350 8.8 48.4 14 53

The fine wire drawing is carried out in a humid environment in order to obtain the 0.25 mm high resistance product. Breakage rates are low and identical for the two sons.

The threads have the following characteristics: STEEL FINAL SECTION REDUCTION (%) RESISTANCE (MPa) ELONGATION% BENDS TORSIONS Al 97.15 3250 2.5 80 110 Bl 96.30 3270 2.6 84 107

It can be seen that the two wires have practically identical characteristics and are therefore interchangeable as regards their uses. The stranding behavior of the wire A1 according to the invention is good, the breakage rate even being reduced compared to the wire Bl. In addition, this wire offers the advantage of eliminating the intermediate patenting as well as a wire drawing step. Above all, it avoids the use of wire rod with 0.82% carbon which not only presents an additional cost on purchase, but also risks of fragile phases (proeutectoid cementite) causing breaks in wire drawing and stranding despite all the precautions that can be taken, these breakages causing a reduction in productivity.

Example 3 :

This example illustrates the production of wires of small diameters such as 0.175 mm with high resistance and 0.150 mm of current quality by implementing very high reduction rates of section (97.5% to 98%) while having an aptitude for correct stranding and with an acceptable breakage rate, these wires being used in particular in the production of products such as the compact cables 1 x 27 and 1 x 12 (all the wires being twisted together in the same direction and with the same pitch) .

• A2 : 0,73 %C, 0,55 %Mn, 0,20 %Si, 0,009 %P, 0,007S
• A3 : 0,56 %C, 0,57 %Mn; 0,24 %Si, 0,007 %P, 0,008S

   Les fils de départ sont des fils machine de diamètre 5,5 mm pour les deux types d'acier A2 et A3 ; ces fils après un premier tréfilage conduisent à des fils de diamètre 3,0 mm.The steels used both have a composition in accordance with that selected for the invention:

• A2: 0.73% C, 0.55% Mn, 0.20% Si, 0.009% P, 0.007S
• A3: 0.56% C, 0.57% Mn; 0.24% Si, 0.007% P, 0.008S

The starting wires are 5.5 mm diameter machine wires for the two types of steel A2 and A3; these wires after a first drawing lead to wires with a diameter of 3.0 mm.

The intermediate wires come from the patented 3.0 mm diameter. After dry drawing, a classic brassage patenting is carried out, resulting in the following mechanical properties: STEEL ⌀ (min) RESISTANCE after patenting (MPa) ELONGATION (%) STRICTION (%) BENDS TORSIONS A2 1.19 1265 9.0 53.0 26 75 A3 1.04 1080 9.8 60.4 30 80

When wet drawing, the wire A2 gives the diameter 0.175 mm high resistance used in particular for stranding for construction 1 x 27. The wire A3 is transformed up to the diameter 0.15 mm used as covering wire. The mechanical properties of these fine wires are: STEEL SECTION REDUCTION (%) ⌀ (mm) RESISTANCE (MPa) ELONGATION (%) BENDS TORSIONS A2 97.84 0.175 3440 2.6 129 145 A3 97.92 0.150 2915 2.3 131 174

It can be seen that, on the one hand, a high resistance wire is obtained from a steel with only 0.73% carbon, which places it very far from the problem zone located near the eutectoid point and on the other hand, ordinary quality wire from only 0.56% carbon steel.

The cable produced without any particular problem from the A2 wire wrapped with the A3 wire has a resistance comparable or even greater than that of the cables produced from the usual wires with 0.82% carbon. In all cases the other properties of the cable, such as fatigue, adhesion (at the origin and after aging), remain unchanged.

• utilisation d'acier plus facile à produire ;
• moins de risques de ségrégation dangereuse à cause de la présence de cémentite proeutectoïde, qui risque d'apparaître dans des compositions d'acier à teneur élevée en carbone (0,82 %), donc diminution des casses au toronnage ;
• élimination plus facile du patentage intermédiaire en raison de l'augmentation des diamètres intermédiaires et du tréfilage facilité par un taux de carbone plus bas.

It is clear from the above description and examples that the invention therefore has many advantages, among which there may be mentioned:

• easier use of steel;
• less risk of dangerous segregation due to the presence of proeutectoid cementite, which is likely to appear in steel compositions with a high carbon content (0.82%), therefore a reduction in stranding breaks;
• easier elimination of intermediate patenting due to the increase in intermediate diameters and wire drawing facilitated by a lower carbon content.

Claims (3)

1. Process for obtaining a high strength hard-drawn steel wire, produced by drawing a wire rod in conjuction with optional heat treatment and intermediate surface treatment, said wire rod being made of steel having a structure devoid of proeutectoid cementite, whose chemical composition is such that, in addition to iron, it comprises :

   - C : 0.45 to 0,67 %

   - Mn, Si, Ca, Mo, Al .... of conventional contents ; and in which :

   - the steel grade of the wire rod is selected from those whose S and P content is less than 0.010 % ;

   - the wire-drawing operation is carried out in two distinct phases separated from each other by a single brassing-patenting treatment and ;

   - the level of cross-section reduction in the final wire-drawing step is greater than 97 %, the final fracture strength of the wire produced being between 2400 and 3000 MPa.
2. Process for obtaining a high strength hard-drawn steel wire, produced by drawing a wire rod in conjuction with optional heat treatment and intermediate surface treatment, said wire rod being made of steel having a structure devoid of proeutectoid cementite, whose chemical composition is such that, in addition to iron, it comprises :

   - C : 0,68 to 0,75 %

   - Mn, Si, Ca, Mo, Al .... of conventional contents ; and in which :

   - the steel grade of the wire rod is selected from those whose S and P content is less than 0.010 % ;

   - the wire-drawing operation is carried out in two distinct phases separated from each other by a single brassing-patenting treatment and ;

   - the level of cross-section reduction in the final wire-drawing step is greater than 97 %, the final fracture strength of the wire produced being between 3000 a,d 3500 MPa.
3. Cables, strands or similar articles produced from steel wires obtainted by the process according to one of claims 1 and 2.