EP0506768B1 - Steel wire having a structure of the cold-drawn lower bainite type; method for producing such wire - Google Patents

Abstract

Wire presenting the following caracteristics: a) it is comprised at least partly of steel having a carbon content at least equal to 0.1 % and at the most equal to 0.6 % and a boron content lower than 8 ppm; b) the steel of the wire has a structure (7) of the cold-drawn lower bainite; c) the diameter of the wire varies between 0.10 and 0.40 mm; d) the breaking resistance of the wire is at least equal to 2800 MPa; e) the elongation at rupture of the wire is at least equal to 0.4 %. The method according to the invention for producing such wire comprises the cold-drawing of a machine wire including from 28 % to 90 % of proeutectoïd ferrite and from 72 % to 10 % of perlite, and carrying out a heat treatment in order to obtain a structure of the lower bainite type, and a cold-drawing on the wire, the temperature of the wire during such cold-drawing being lower than 0.3 TF, TF being the melting temperature of steel expressed in Kelvin.

Description

The invention relates to metal wires and methods for obtaining these wires. These threads are used, for example, to reinforce plastic or rubber articles, in particular pipes, belts, plies, tire casings.

The son of this type commonly used today are made of steel containing at least 0.6% carbon, this steel having a hardened pearlitic structure. The breaking strength of these wires is around 2800 MPa (megapascals), their diameter generally varies from 0.15 to 0.35 mm, and their elongation at break is between 0.4 and 2%. These wires are produced by drawing a starting wire, called "machine wire", the diameter of which is of the order of 5 to 6 mm, the structure of this machine wire being a hard structure, consisting of perlite and ferrite. with a high rate of perlite which is generally greater than 72%. During the production of this wire, the drawing operation is interrupted at least once to carry out one or more heat treatments which make it possible to regenerate the initial structure.

• la matière première est coûteuse, car le taux de carbone est relativement élevé ;
• les paramètres ne peuvent pas être modifiés facilement, en particulier le diamètre du fil machine et le diamètre final sont maintenus dans des limites rigides, le procédé manquant donc de souplesse ;
• la grande dureté du fil machine due à sa structure fortement perlitique rend le tréfilage difficile, avant le traitement thermique, de telle sorte que le taux de déformation ε de ce tréfilage est nécessairement inférieur à 3 ; d'autre part les vitesses de ce tréfilage sont faibles et il peut y avoir des casses du fil lors de cette opération.

This process has the following drawbacks:

• the raw material is expensive because the carbon content is relatively high;
• the parameters cannot be easily modified, in particular the diameter of the wire rod and the final diameter are kept within rigid limits, the process therefore lacking flexibility;
• the high hardness of the wire rod due to its highly pearlitic structure makes wire drawing difficult, before the heat treatment, so that the rate of deformation ε of this drawing is necessarily less than 3; on the other hand, the speeds of this drawing are low and there may be breaks in the wire during this operation.

On the other hand, the wires themselves have a resistance to breaking which is sometimes insufficient, and their resistance to fatigue is limited, probably as a result of damage to these wires during the drawing before heat treatment, because of the high hardness of the wire rod.

Japanese patent application published under No. 54-79119 describes a process for preparing a boron steel wire of bainitic structure by heating in a fluidized bed. The wires obtained are characterized by weak mechanical properties.

The Patent Abstracts of Japan, Vol. 13, No. 42 (C-564) (3390) of January 30, 1989 and JP-A-63241136 describe a wire with a high carbon content (0.7 to 0.9%), this wire having a higher bainite structure .

The document Stahl u. Eisen, Vol. 70, n ° 2, of January 19, 1950, pages 52-57 describes the production of steel wires by wire drawing and heat treatment. The deformation rates described in this document are low and the low mechanical values given correspond to very large wire diameters, of around 1 mm.

The object of the invention is to provide a hardened wire having a non-pearlitic structure and having a breaking strength and an elongation at break at least as high as known hardened pearlitic steel wires, and less damage than known sons.

Another object of the invention is to propose a method which does not have the aforementioned drawbacks for producing this yarn.

• a) il est constitué au moins en partie par un acier ayant une teneur en carbone au moins égale à 0,1 % et au plus égale à 0,6 %, et une teneur en bore inférieure à 8 ppm (parties par million) ;
• b) l'acier du fil présente une structure de type bainite inférieure écrouie.

The metal wire according to the invention having a diameter of 0.10 to 0.40 mm, a breaking strength at least equal to 2800 MPa, and an elongation at break at least equal to 0.4%, has the following features:

• a) it is constituted at least in part by a steel having a carbon content at least equal to 0.1% and at most equal 0.6%, and a boron content of less than 8 ppm (parts per million);
• b) the steel of the wire has a structure of the strained lower bainite type.

• a) on écrouit un fil machine en acier, cet acier ayant une teneur en carbone au moins égale à 0,1 % et au plus égale à 0,6 % et une teneur en bore inférieure à 8 ppm (parties par million), cet acier comportant de 28 % à 90 % de ferrite proeutectoïde et de 72 % à 10 % de perlite ; le taux de déformation ε de cet écrouissage étant au moins égal à 3 ;
• b) on arrête l'écrouissage et on effectue un traitement thermique structural unique sur le fil écroui ; ce traitement consiste à chauffer le fil au dessus du point de transformations AC3 pour lui donner une structure d'austénite homogène, puis à le refroidir rapidement à une température comprise entre 350°C et 450°C, la vitesse de ce refroidissement étant au moins égale à 250°C/seconde, et à le maintenir dans cette plage de température pendant un temps au moins égal à 30 secondes, de façon à obtenir une structure de type bainite inférieure comportant des précipités de carbures répartis de façon pratiquement homogène dans une matrice ferritique ;
• c) on refroidit le fil à une température inférieure à 0,3 T_F, T_F étant la température de fusion de l'acier, exprimée en Kelvin ;
• d) on effectue un écrouissage sur le fil ayant subi ce traitement thermique, la température du fil lors de cet écrouissage étant inférieure à 0,3 T_F, le taux de déformation ε de cet écrouissage étant au moins égal à 3.

The process according to the invention for producing this yarn is characterized by the following points:

• a) a steel wire rod is hardened, this steel having a carbon content at least equal to 0.1% and at most equal to 0.6% and a boron content less than 8 ppm (parts per million), this steel with 28% to 90% proeutectoid ferrite and 72% to 10% perlite; the rate of deformation ε of this work hardening being at least equal to 3;
• b) the work hardening is stopped and a unique structural heat treatment is carried out on the work hardened wire; this treatment consists in heating the wire above the transformation point AC3 to give it a homogeneous austenite structure, then in rapidly cooling it to a temperature between 350 ° C and 450 ° C, the speed of this cooling being at least equal to 250 ° C / second, and to maintain it in this temperature range for a time at least equal to 30 seconds, so as to obtain a structure of the lower bainite type comprising precipitates of carbides distributed almost homogeneously in a matrix ferritic;
• c) the wire is cooled to a temperature below 0.3 T _F , T _F being the melting temperature of the steel, expressed in Kelvin;
• d) a work hardening is carried out on the wire having undergone this heat treatment, the temperature of the wire during this work hardening being less than 0.3 T _F , the rate of deformation ε of this work hardening being at least equal to 3.

The invention also relates to assemblies comprising at least one wire according to the invention.

The invention also relates to articles reinforced at least in part by wires or assemblies in accordance with the preceding definitions, such articles being, for example, hoses, belts, plies, tire casings.

The invention will be easily understood with the aid of the following embodiment examples, and all schematic figures relating to these examples.

• la figure 1 représente la structure de l'acier d'un fil avant traitement thermique, lors de la mise en oeuvre du procédé conforme à l'invention ;
• la figure 2 représente la structure de l'acier d'un fil après traitement thermique, lors de la mise en oeuvre du procédé conforme à l'invention ;
• la figure 3 représente la structure de l'acier d'un fil conforme à l'invention.

On the drawing :

• FIG. 1 represents the structure of the steel of a wire before heat treatment, during the implementation of the method according to the invention;
• FIG. 2 represents the structure of the steel of a wire after heat treatment, during the implementation of the method according to the invention;
• FIG. 3 represents the structure of the steel of a wire according to the invention.

In the following, all the percentages and ppm of composition indicated are by weight and the measurements of breaking strength and elongation at break are carried out according to the AFNOR NFA 03-151 method.

Ln étant le logarithme népérien,
So étant la section initiale du fil avant cet écrouissage et
Sf étant la section du fil après cet écrouissage.By definition, the rate of strain ε of a hardening is given by the formula ε = Ln $$\frac{\text{So}}{\text{Sf}}\text{,}$$

Ln being the natural logarithm,
So being the initial section of the wire before this work hardening and
Sf being the section of the wire after this work hardening.

The purpose of the examples which follow is to describe the preparation and the properties of three wires in accordance with the invention.

• teneur en carbone : 0,4 %
• teneur en bore : inférieure à 8 ppm ;
• teneur en manganèse : 0,4 % ;
• teneur en silicium : 0,2 % ;
• teneur en phosphore : 0,015 % ;
• teneur en soufre : 0,02 % ;
• teneur en aluminium : 0,015 % ;
• teneur en azote : 0,005 % ;
• teneur en chrome : 0,05 % ;
• teneur en nickel : 0,10 % ;
• teneur en cuivre : 0,10 % ;
• teneur en molybdène : 0,01 % ;
• teneur en ferrite proeutectoïde : 53 %
• teneur en perlite : 47 %
• température de fusion de l'acier, T_F : 1795 K
• résistance à la rupture R_m : 700 MPa ;
• allongement à la rupture A_r : 17 %

In these examples, an unworked wire rod 5.5 mm in diameter is used. This wire rod is made of steel, the characteristics of which are as follows:

• carbon content: 0.4%
• boron content: less than 8 ppm;
• manganese content: 0.4%;
• silicon content: 0.2%;
• phosphorus content: 0.015%;
• sulfur content: 0.02%;
• aluminum content: 0.015%;
• nitrogen content: 0.005%;
• chromium content: 0.05%;
• nickel content: 0.10%;
• copper content: 0.10%;
• molybdenum content: 0.01%;
• proeutectoid ferrite content: 53%
• perlite content: 47%
• melting temperature of steel, T _F : 1795 K
• breaking strength R _m : 700 MPa;
• elongation at break A _r : 17%

Three wire in accordance with the invention are produced with this wire rod as follows:

Example 1

The machine wire is descaled, it is coated with a drawing soap, for example borax, and it is drawn dry to obtain a wire with a diameter of 1.1 mm, which corresponds to a slightly higher rate of deformation ε at 3.2.

The drawing is easily carried out thanks to the relatively ductile structure of the wire rod. By way of example, a steel with 0.7% carbon which is not hardened has a breaking strength R _m of approximately 900 MPa and an elongation at break of approximately 8%, that is to say that it is much less ductile.

The drawing described above is carried out at a temperature below 0.3 T _F , for the purpose of simplification, although this is not essential, the drawing temperature possibly being able to equal or exceed 0.3 T _F.

FIG. 1 represents the longitudinal section of a portion 1 of the structure of the wire thus obtained. This structure consists of elongated blocks 2 of cementite and elongated blocks 3 of ferrite, the largest dimension of these blocks being oriented in the drawing direction.

• on chauffe le fil pour le porter à 900°C, c'est-à-dire au dessus du point de transformation AC3, et on le maintient pendant 1 minute à cette température de façon à obtenir une structure d'austénite homogène ;
• on refroidit ensuite le fil à 400°C dans un bain de sel en moins de 2 secondes, et on maintient le fil à cette température pendant 1 minute, puis on le refroidit à environ 20°C, c'est-à-dire à la température ambiante.

The following heat treatment is then carried out on the wire thus obtained:

• the wire is heated to bring it to 900 ° C., that is to say above the transformation point AC3, and it is maintained for 1 minute at this temperature so as to obtain a homogeneous austenite structure;
• the wire is then cooled to 400 ° C in a salt bath in less than 2 seconds, and the wire is kept at this temperature for 1 minute, then it is cooled to about 20 ° C, i.e. Room temperature.

FIG. 2 represents a section of a portion 4 of the structure of the wire thus obtained. This structure, of the lower bainite type, consists of precipitates of carbide 5, distributed in a practically homogeneous manner in a matrix 6 of ferrite. This structure is obtained thanks to the preceding heat treatment, and it is preserved during cooling to room temperature. The precipitates 5 generally have dimensions at least equal to 0.005 μm (micrometer) and at most equal to 0.5 μm.

The wire thus obtained by this heat treatment and this cooling to room temperature is coated with a layer of brass. The thickness of this layer of brass is small (of the order of μm) and it is negligible compared to the diameter of the wire before brass plating. A wet wire drawing of this wire is then carried out so as to obtain a final diameter of 0.2 mm, which practically corresponds to ε = 3.4. The wire drawing is facilitated by the brass layer. The temperature of the wire, during this drawing, is necessarily less than 0.3 T _F.

FIG. 3 represents a longitudinal section of the portion 7 of this wire according to the invention thus obtained. This portion 7 has a structure of the strained lower bainite type consisting of carbides 8 of elongated shape which are practically parallel to each other and whose largest dimension is oriented along the axis of the wire, that is to say along the direction of drawing shown schematically by the arrow F in FIG. 3. These carbides 8 are arranged in a hardened ferritic matrix 9.

This wire according to the invention has a breaking strength of 3200 MPa and an elongation at break of 0.7%.

Example 2

• on chauffe le fil de la même façon que dans l'exemple 1 pour obtenir une structure d'austénite homogène ;
• on refroidit ensuite le fil à 370°C en moins de 2 secondes et on le maintient à cette température pendant 90 secondes, puis on le refroidit à la température ambiante.

The machine wire is descaled, it is coated with a layer of wire-drawing soap, for example borax, and it is drawn dry to obtain a wire with a diameter of 0.9 mm, which corresponds to a rate of deformation ε slightly higher than 3.6. The structure obtained is similar to that shown in FIG. 1. The following heat treatment is then carried out on the wire thus obtained:

• the wire is heated in the same way as in Example 1 to obtain a homogeneous austenite structure;
• the wire is then cooled to 370 ° C in less than 2 seconds and kept at this temperature for 90 seconds, then cooled to room temperature.

The structure obtained is similar to that shown in FIG. 2. The wire is then brassed and drawn in a manner similar to Example 1 to obtain a final diameter of 0.17 mm, which corresponds practically to ε = 3.3. The temperature of the wire during this drawing is less than 0.3 T _F. The wire according to the invention thus obtained has a structure similar to that shown in FIG. 3.

This wire has a breaking strength equal to 3000 MPa and an elongation at break equal to 0.9%.

Example 3

A wire according to the invention is produced in the same way as in Example 1 but with the difference that the wire drawing carried out after the heat treatment is continued until the final diameter of 0.17 mm, which corresponds practically to ε = 3.7. This wire according to the invention has a breaking strength equal to 3500 MPa and an elongation at break equal to 0.7%. The intermediate structures and the final structure are analogous to the structures previously described.

• on part d'un fil machine à faible taux de carbone, et donc d'un coût peu élevé ;
• on bénéficie d'une grande souplesse dans le choix des diamètres des fils, c'est ainsi par exemple qu'on peut utiliser des fils machines dont le diamètre est notablement supérieur à 6 mm, ce qui réduit encore les coûts, et on peut réaliser des fils très variés en diamètre ;
• le tréfilage avant le traitement thermique structural est relativement aisé, de telle sorte que le taux de déformation ε de ce tréfilage peut être supérieur à 3 ; d'autre part, ce tréfilage peut être réalisé avec des vitesses élevées ; enfin on réduit la fréquence des casses de fils et des changements de filières, ce qui réduit encore les coûts ;
• le fil obtenu présente une résistance à la rupture et un allongement à la rupture de valeurs au moins égales à celles des fils classiques, ce qui se traduit donc par une énergie de rupture au moins égale à celle des fils classiques ;
• le fil est moins endommagé lors du tréfilage avant traitement thermique ;
• le fil obtenu présente une meilleure résistance à la corrosion que les fils classiques par suite de sa faible teneur en carbone.

The invention has the following advantages:

• we start from a low carbon wire rod, and therefore of a low cost;
• there is great flexibility in the choice of wire diameters, this is for example that we can use machine wires whose diameter is significantly greater than 6 mm, which further reduces costs, and we can achieve wires very varied in diameter;
• wire drawing before structural heat treatment is relatively easy, so that the rate of deformation ε of this drawing may be greater than 3; on the other hand, this wire drawing can be carried out with high speeds; finally, the frequency of wire breaks and changes of dies is reduced, which further reduces costs;
• the wire obtained has a breaking strength and an elongation at break of values at least equal to those of conventional wires, which therefore results in a breaking energy at least equal to that of conventional wires;
• the wire is less damaged during the drawing before heat treatment;
• the wire obtained has better resistance to corrosion than conventional wires due to its low carbon content.

Preferably, the steel of the wire according to the invention has a carbon content at least equal to 0.2% and at most equal to 0.5%.

Preferably, in the steel of the wire according to the invention and therefore in the starting machine wire, there are the following compositions: 0.3% ≦ Mn ≦ 0.6%; 0.1% ≦ Si ≦ 0.3%; P ≦ 0.02%; S ≦ 0.02%; Al ≦ 0.02%; N ≦ 0.006%.

Advantageously in the steel of the wire according to the invention and therefore in the starting machine wire, there are the following relationships: Cr ≦ 0.06%; Ni ≦ 0.15%; Cu ≦ 0.15%; Mo ≦ 0.015%.

• le fil machine de départ a une teneur en carbone au moins égale à 0,2 % et au plus égale à 0,5 % ;
• le fil machine de départ a une teneur en ferrite proeutectoïde au moins égal à 41 %, et au plus égale à 78 % et une teneur en perlite au moins égale à 22 % et au plus égale à 59 % ;
• le taux de déformation ε lors de l'écrouissage avant le traitement thermique structural est au plus égal à 6 ;
• le taux de déformation ε lors de l'écrouissage après le traitement thermique structural est au plus égal à 4,5.

Preferably, in the process according to the invention, there is at least one of the following characteristics:

• the starting wire rod has a carbon content at least equal to 0.2% and at most equal to 0.5%;
• the starting wire rod has a proeutectoid ferrite content at least equal to 41%, and at most equal to 78% and a perlite content at least equal to 22% and at most equal to 59%;
• the rate of deformation ε during work hardening before the structural heat treatment is at most equal to 6;
• the rate of deformation ε during work hardening after the structural heat treatment is at most equal to 4.5.

In the examples described above, the wire after heat treatment was brass-plated to facilitate its drawing, however the invention covers the cases where other wire drawing flows are used than brass, for example copper, zinc, ternary copper alloys -zinc-nickel, copper-zinc-cobalt, copper-zinc-tin, these flows being other than steel.

The wire work hardening in the previous examples is carried out by drawing, but other techniques are possible, for example rolling, possibly associated with drawing, for at least one of the work hardening operations.

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

Claims (21)

1. A metal wire having a diameter of 0.10 to 0.40 mm, a resistance to rupture of at least 2,800 MPa, and an elongation upon rupture of at least 0.4%, this wire being characterised by the following features:

   (a) it is formed at least in part by a steel having a carbon content of at least 0.1% and of at most 0.6%, and a boron content of less than 8 ppm (parts per million);

   (b) the steel of the wire has a strain-hardened lower bainite-type structure.
2. A metal wire according to Claim 1, characterised in that the steel has a carbon content of at least 0.2% and at most 0.5%.
3. A metal wire according to any one of Claims 1 or 2, characterised in that the steel satisfies the following relationships: 0.3% ≦ Mn ≦ 0.6%; 0.1% ≦ Si ≦ 0.3%; P ≦ 0.02%; S ≦ 0.02%; Al ≦ 0.02%; N ≦ 0.006%.
4. A metal wire according to Claim 3, characterised in that the steel satisfies the following relationships: Cr ≦ 0.06%; Ni ≦ 0.15%; Cu ≦ 0.15%; Mo ≦ 0.015%.
5. A metal wire according to any one of Claims 1 to 4, characterised in that it is coated with a metal layer other than steel.
6. A metal wire according to Claim 5, characterised in that it is coated with a layer of brass.
7. A method of producing a metal wire according to any one of Claims 1 to 6, characterised by the following features:

   (a) a steel machine wire having a carbon content of at least 0.1% and at most 0.6% and a boron content of less than 8 ppm (parts per million) is strain-hardened, said steel comprising 28% to 90% proeutectoid ferrite and 72% to 10% perlite; the deformation ratio ε of this strain-hardening being at least equal to 3;

   (b) the strain-hardening is stopped and a single structural heat treatment is carried out on the strain-hardened wire; this treatment consists in heating the wire to above the AC3 transformation point in order to impart to it a homogenous austenite structure, then cooling it rapidly to a temperature of between 350°C and 450°C, the rate of this cooling being at least equal to 250°C/second, and maintaining it within this temperature range for a period of time of at least 30 seconds so as to obtain a structure of lower bainite type having carbide precipitates distributed practically homogenously in a ferrite matrix;

   (c) the wire is cooled to a temperature below 0.3 T_F, T_F being the melting point of the steel expressed in Kelvin;

   (d) strain-hardening is carried out on the wire which has undergone this heat treatment, the temperature of the wire upon this strain-hardening being less than 0.3 T_F, the deformation ratio ε of this strain-hardening being at least equal to 3.
8. A method according to Claim 7, characterised in that the machine wire has a carbon content of at least 0.2% and at most 0.5%.
9. A method according to any one of Claims 7 or 8, characterised in that the machine wire satisfies the following relationships: 0.3% ≦ Mn ≦ 0.6%; 0.1% ≦ Si ≦ 0.3%; P ≦ 0.02%; S ≦ 0.02%; Al ≦ 0.02%; N ≦ 0.006%.
10. A method according to Claim 9, characterised in that the machine wire satisfies the following relationships: Cr ≦ 0.06%; Ni ≦ 0.15%; Cu ≦ 0.15%; Mo ≦ 0.015%.
11. A method according to any one of Claims 7 to 10, characterised in that a metal coating other than steel is effected on the wire after the structural heat treatment before strain-hardening.
12. A method according to Claim 11, characterised in that said coating is a coating of brass.
13. A method according to any one of Claims 7 to 12, characterised in that the machine wire has a proeutectoid ferrite content of at least 41% and at most 78% and a perlite content of at least 22% and at most 59%.
14. A method according to any one of Claims 7 to 13, characterised in that the deformation ratio ε upon the strain-hardening before the structural heat treatment is at least 3 and at most 6.
15. A method according to any one of Claims 7 to 14, characterised in that the deformation ratio ε upon the strain-hardening after the structural heat treatment is at least 3 and at most 4.5.
16. A method according to any one of Claims 7 to 15, characterised in that at least one strain-hardening operation is effected at least in part by drawing.
17. A method according to any one of Claims 7 to 16, characterised in that the structure of lower bainite type obtained after the rapid cooling is such that the carbide precipitates have, in general, dimensions of at least 0.005 µm (micrometre) and at most 0.5 µm.
18. An assembly comprising at least one wire according to any one of Claims 1 to 6.
19. An article reinforced with at least one wire according to any one of Claims 1 to 6.
20. An article reinforced with at least one assembly according to Claim 18.
21. An article according to any one of Claims 19 or 20, characterised in that it is an automobile tyre.

Images