WO2015173143A1

WO2015173143A1 - Multi-strand steel cord

Info

Publication number: WO2015173143A1
Application number: PCT/EP2015/060206
Authority: WO
Inventors: Roel DESPIEGELAERE; Dirk Meersschaut; Jef Wilmots
Original assignee: Nv Bekaert Sa
Priority date: 2014-05-14
Filing date: 2015-05-08
Publication date: 2015-11-19
Also published as: CN106460320A

Abstract

The invention relates to a multi-strand steel cord comprising J central strands and K outer strands positioned around the J central strands, with J ≥1 and K ≥3. The J central strands comprise a core of L central filaments and at least a first layer of M filaments positioned around said core of L central filaments, with 1 ≤ L ≤ and 4≤ M ≤ 20. The K outer strands comprise a core of P central filaments and at least one layer of Q filaments positioned around said core of P central filaments, with 1 ≤ P≤ 5and 4≤ Q≤ 20. The circumferential surface of the core of L central filaments of the J central strands is coated with polymer material whereas the circumferential surface of the J central strands is free or substantially free of polymer material. Furthermore the K outer strands are free or substantially free of polymer material. The invention further relates to a method of manufacturing such multi-strand steel cord.

Description

Multi-strand steel cord

Description

Technical Field

[0001] The invention relates to multi-strand steel cords, and more particularly to multi-strand steel cords adapted to be used to reinforce rubber articles, in particular rubber tires, such as truck, passenger or off-the road tires, rubber tracks or belts. The cords according to the present invention are in particular suitable for the reinforcement of off-the-road tires.

The invention further relates to a method of manufacturing multi-strand steel cords.

Background Art

[0002] To obtain a good performance of a steel cord adequate penetration of the rubber into the cord during the vulcanization of the reinforced rubber article is essential.

[0003] Rubber penetration can be influenced by proper constructional features of the cord, for example by providing free spaces between individual filaments of the cord. However, for multi-strand cords rubber penetration is difficult.

A multi-strand steel cord comprises a plurality of strands of twisted filaments, whereby the strands are further twisted in a so-called multiple twisting structure.

[0004] As a multi-strand cord comprises a high number of filaments and a

complicated twisting structure due to the multiple-twisting operation, rubber penetration is much more complex compared to a cord having a single-strand structure.

[0005] To solve the problem of allowing rubber penetration, in-situ rubberized strands have been proposed.

With in-situ rubberized strands is meant strands that are coated on the inside during the manufacturing by applying rubber or a rubber compound in the uncrosslinked (green) state, before the cords are incorporated into rubber articles, such as tires, which they are intended to reinforce. [0006] US2002/160213 describes a method of manufacturing a cord whereby the core filaments are individually coated with uncured rubber before being stranded in a core. Subsequently the core is stranded with outer filaments around the core. Such method leads however to the use of an excessively large amount of rubber. The overspill of rubber at the periphery of the final strand is unacceptable. This results not only in a waste of rubber material but causes also problems in the manufacturing process, for example during cabling or winding because of the undesirable adhesion of the uncured rubber for example with manufacturing tools or between strands.

[0007] WO201 1/000963 and US2010/0170215 describe multi-strand cords

whereby the core of filaments (the inner layer of filaments) of all strands, either central strands as outer strands is rubberized in situ. Such multi- strand cords have the drawback that a high amount of rubber is required. Furthermore the method of manufacturing of the multi-strand cord is complex and the high amount of rubber in the cord makes the

vulcanization of the reinforced article complex.

Disclosure of Invention

[0008] It is an object of the present invention to provide a multi-strand steel cord allowing full rubber penetration.

It is another object of the invention to provide a multi-strand steel cord having a minimal amount of polymer material inside the cord.

It is a further object of the invention to provide a multi-strand steel cord allowing to obtain good adhesion with rubber.

Furthermore it is an object of the present invention to provide a method of manufacturing a multi-strand steel cord.

[0009] According to a first aspect of the present invention, a multi-strand steel cord is provided.

The multi-strand steel cord comprises J central strands and K outer strands. The K outer strands are positioned around the J central strands. Preferably, the K outer strands are helically twisted around the J central strands. The number of central strands J is higher than or equal to 1. The number of outer strands K is higher than or equal to 3.

The number of central strands J ranges preferably from 1 to 3. The number of outer strands K ranges preferably from 5 to 9 and is for example 5, 6, 7, 8 or 9.

[0010] For the purpose of this application all capitalised letters (J, K, L, M, N, P, Q, R, S, T,..) represent natural numbers (1 , 2, 3, 4, 5, 6,...).

[001 1] Each strand of the multi-stand cord, either a central strand or an outer strand, comprises a core and a number of filaments positioned around this core of filaments. Preferably, the filaments positioned around the core are twisted around the core of filaments.

The core comprises one filament or a number of filaments that are twisted together. The number of filaments of a core ranges preferably from 1 to 5, and is for example equal to 1 , 2 or 3.

The filaments positioned around the core of filaments are for example arranged in a layer positioned around the core. Possibly, the filaments positioned around the core of filaments are arranged in a number of layers, for example in a first and a second layer whereby the first layer is positioned around the core of filaments and the second layer is positioned around the first layer.

[0012] The core of a strand has a circumferential surface. The circumferential surface of the core of a strand is defined as the outer surface of the core formed by the central filaments.

Also the strands have a circumferential surface. The circumferential surface of a strand is defined as the outer surface of the strand, i.e. the core surrounded by the filaments, for example the filaments of the first layer or the filaments of the first and the second layer. [0013] In a preferred embodiment of a multi-strand cord according to the present invention the J central strand or central strands comprise(s) a core of L central filaments and a first layer of M filaments positioned around the core of L central filaments, with 1 < L < 5 and 4 < M < 20. More preferably, M ranges between 6 and 15. Examples comprise strands having a core of 1 central filament L and a first layer of 6 filaments M or strands having a core of 3 central filaments L and a first layer of 9 filaments M.

In another preferred embodiment, the J central strand or central strands comprise(s) a core of L central filaments, a first layer of M filaments positioned around the core of L central filaments and a second layer of N filaments positioned around the first layer of M filaments, with 1 < L < 5 and 4 < M < 20 and 4 < N < 20. More preferably L, M and N are specified as follow : 1 < L < 5, 4 < M < 10 and 4 < N < 20, for example 1 < L < 3 and 4 < M < 9 and 6≤ N < 15. Examples comprise strands having a core of 1 central filament L, a first layer of 6 filaments M and a second layer of 12 filaments N, strands having a core of 3 central filaments M, a first layer of 9 filaments M and a second layer of 15 filaments N or strands having a core of 3 central filaments M, a first layer of 8 filaments M and a second layer of 14 filaments N.

[0014] In a preferred embodiment, the K outer strands comprise a core of P

central filaments and a first layer of Q filaments positioned around the core of P central filaments, with 1 < P < 5 and 4 < Q < 20. More preferably, Q ranges between 6 and 15. Examples comprise strands having a core of 1 central filament P and a first layer or 6 filaments Q or strands having a core of 3 central filaments P and a first layer of 9 filaments Q.

In another preferred embodiment, the K outer strands comprise a core of P central filaments, a first layer of Q filaments positioned around the core of P central filaments and a second layer of R filaments positioned around the first layer of M filaments, with 1 < P < 5 and 4 < Q < 20 and 4 < R < 20. More preferably P, Q and R are specified as follow : 1 < L < 5, 4 < M < 10 and 4 < N < 20, for example 1 < P < 3 and 4 < Q < 9 and 6 < R < 15.

Examples comprise strands having a core of 1 central filament P, a first layer of 6 filaments Q, a second layer of 12 filaments R, strands having a core of 3 central filaments P, a first layer of 9 filaments Q and a second layer of 15 filaments R or strands having a core of 3 central filaments P, a first layer of 8 filaments Q and a second layer of 14 filaments R.

[0015] A multi-strand cord according to the present invention is characterized in that

- the circumferential surface of the core of L central filaments of the J central strands is provided with polymer material;

- the circumferential surface of the J central strands is free or substantially free of polymer material; and

- the K outer strands are free or substantially free of polymer material.

[0016] As mentioned above the circumferential surface of the core of L central filaments of the J central strands is provided with polymer material.

Preferably, polymer material is also present between the L central filaments of the core of the J central strands. More preferably, the voids created between the L central filaments of the core of the J central strands are completely filled with polymer material.

[0017] In a further preferred embodiment the first layer of M filaments as well as the second layer of N filaments in the central strand remain free or substantially free of polymer material. This limits the amount of extrusion steps further as well as reduces the amount of polymer material that is introduced. In addition, the fact that the surface of the central strand is free or substantially free of polymer material greatly eases the use of the central strand in the further fabrication of the steel cord.

[0018] The polymer material is preferably applied by extrusion, for example on the L central filaments of the J central strands, on the core of the J central strands or both on the L central filaments of the J central strands and on the core of the J central strands. [0019] The K outer strands are free or substantially free of polymer material. This means that the P central filaments of the core of said K outer strands and the filaments of said K outer strands positioned around the core of P central filaments (Q and R filaments) are free or substantially free of polymer material.

Consequently, the circumferential surface of the core or P central filaments of the K outer strands is free or substantially free of polymer material. Furthermore it is clear that also the circumferential surface of the K outer strands is free or substantially free of polymer material.

[0020] The filaments are preferably made of steel. In a preferred embodiment, the filaments are made from plain carbon steel. Such a steel preferably comprises a minimum carbon content of 0.40wt% C (for example at least 0.70 wt% C or at least 0.80wt% C) with a maximum of 1.1 wt% C, a manganese content ranging from 0.10 to 0.90 wt% Mn, the sulphur and phosphorus contents are each preferably kept below 0.030 wt%.

Additional alloying elements such a chromium boron, cobalt, nickel, vanadium -a non-exhaustive enumeration- may also be added.

[0021] In an alternative embodiment, the filaments are made from stainless steel.

Stainless steels contain a minimum of 12wt%Cr and a substantial amount of nickel. More preferred stainless steel composition comprise are austenitic stainless steels. The most preferred compositions are known in the art as AISI (American Iron and Steel Institute) 302, AISI 301 , AISI 304 and AISI 316.

[0022] The filaments have a diameter preferably ranging between 0.04 mm and

1.20 mm depending on the application. More preferably, the filaments have a diameter ranging between 0.15 and 0.6 mm, for example 0.15 mm, 0.175 mm and 0.35 mm. The different filaments of a strand or a cord may have the same diameter although this is not necessary. [0023] The filaments have preferably a tensile strength larger than 2000 N/mm², more preferably larger than 2500 N/mm².

[0024] The filaments are obtained in a conventional drawing process.

[0025] The filaments and/or the strands can be uncoated or they can be coated with a suitable coating. Preferred coatings are for example zinc or zinc alloy coatings such as zinc coatings, brass coatings, brass based coatings, zinc aluminium coatings or zinc aluminium magnesium coatings. Preferred brass based coatings are ternary or quaternary brass based coating, i.e. coatings comprising copper and zinc and one or two additional metals.

The additional metal is for example cobalt (Co) or nickel (Ni). Also iron (Fe), tin (Sn), manganese (Mn), indium (In), bismuth (Bi), and

molybdenum (Mo) can be considered as additional element. A preferred quaternary brass based coating comprises a Cu-Zn-Ni-Co alloy.

[0026] The zinc or zinc alloy coating is for example applied in a conventional galvanizing process.

[0027] The polymer material can be any type of polymer material, for example any elastomer or rubber. For the purpose of this invention the terms "elastomer" and "rubber" are interchangeable. Examples of polymer material comprise natural rubbers, polyisoprene, polybutadiene, butadiene copolymers, isoprene copolymers and blends of one or more of these elastomers.

Examples of butadiene copolymers or isoprene copolymers are

butadiene-styrene copolymers, isoprene-styrene copolymers, isoprene- butadiene-styrene copolymers.

[0028] The polymer material is preferably crosslinkable, that is to say it

comprises by definition a suitable crosslinking system enabling the composition to be cross-linked while it is being cured. [0029] The polymer material may further comprise one or more additives for example additives that are known additives used in rubbers. Examples of additives comprise organic reinforcing fillers such as carbon black or inorganic fillers such as silica, coupling agents, anti-ageing agents, antioxidants, plasticizers, tackifying resins, antireversion agents, adhesion promoting agents.

[0030] The formulation of the polymer material present in the central strand may be chosen to be similar or even identical to the formulation of the polymer material of the article the multi-strand cord according to the present invention is intended to reinforce. In such case, there is no compatibility problem between the polymer material in the cord and the polymer material of the article to be reinforced.

[0031] A multi-strand steel cord according to the present invention has the

advantage that the cord comprises only a minimum amount of polymer material and nevertheless allows to obtain good rubber penetration after vulcanisation.

As there is no polymer material on the circumferential surface of the individual strands, either the central strands or the outer strands, there is no increase in the diameter of the multi-strand cord. So there is no increase in the amount of rubber material to embed the multi-strand steel cord and consequently there is no increase in costs.

As there is no polymer material on the circumferential surface of the individual strands or on the outer circumferential surface of the multi- strand cord good adhesion with fresh rubber during ply production (for example calendering or extrusion) is obtained.

Compared to multi-strands cords known in the art whereby all the strands either central strands or outer strands are in-situ rubberized the method of manufacturing the multi-strand cord is easier and cheaper.

Furthermore the manufacturing of rubber articles reinforced with the multi- strand cords according to the present invention is simplified because of the limited amount of polymer material present in the multi-strand cord.

[0032] In preferred embodiments the P central filaments of the core of the K outer strands are preformed. By such preforming the rubber penetration in the K outer strands, more particularly into the core of the K outer strands is increased. Within the context of this application with 'preformed' or 'preforming operation' on an element (an element being a filament or a strand) is meant':

- either forming the element into a wave form possibly followed or not followed by a rotation of the element. In the former case a plane wave is obtained, while in the latter a spatial wave is obtained. The wave forming can be applied by guiding the element through a pair of intermeshing crimper wheels, or;

- a polygonal preforming as for example described in WO 95/16816. Such preforming is obtained by combining the bending operation obtained by guiding the element under tension over a polygonal performer wheel followed with a rotation of the element.

[0033] In other preferred embodiments at least one and preferably all of the K outer strands are preformed.

[0034] In still further preferred embodiments first the filaments of the outer

strands are preformed and subsequently once the strand is formed, the outer strands are preformed.

[0035] According to a second aspect of the present invention, a method of

manufacturing a multi-strand cord is provided. The method comprises the following steps

- providing J strands, with J > 1 , said J strands comprising a core of L central filaments and at least one layer of M filaments positioned around said core of L central filaments; with 1 < L < 5 and 4 < M < 20, the circumferential surface of said core of L filaments being coated with a polymer material;

- providing K strands, with K > 3, said K outer strands comprising a core of P central filaments and at least one layer of Q filaments positioned around said core of P central filaments, with 1 < P < 5 and 4 < Q < 20, said P central filaments and said Q outer filaments being free of substantially free of polymer material;

- assembling said J strands and said K strands in a cabling or twisting operation to manufacture a multi-strand cord, whereby said J strands are central strands and said K strands are outer strands, said K outer strands being positioned around said J central strands.

[0036] The J central strand or strands may comprise one layer of filaments

positioned around the central filaments, for example a first layer

comprising M filaments positioned around the core of L central filaments. Alternatively the J central strand or strands comprise(s) a first layer comprising M filaments positioned around the core of L central filaments and a second layer comprising N filaments positioned around the first layer of filaments.

[0037] Similarly, the K outer strands may comprise one layer of filaments

positioned around the L central filaments, for example a first layer comprising M filaments positioned around the core of L central filaments. Alternatively the K outer strands comprise a first layer comprising M filaments positioned around the core of L central filaments and a second layer comprising N filaments positioned around the first layer of filaments.

[0038] Preferably, the circumferential surface of said J central strands is

substantially free of polymer material.

[0039] A central strand J is for example obtained by

- assembling a core of L central filaments, for example by means of a

cabling or twisting operation; - applying polymer material on the core of L central filaments, for example by extrusion;

- assembling the central strand by twisting M filaments around the core of L central filaments provided with polymer material.

[0040] An alternative method to manufacture a central strand J comprises the steps of

- applying polymer material on L filaments, for example by extrusion,

- assembling said L filaments, for example by means of a cabling or

twisting operation to form the core of said strand;

[0041] In case the J central strand or strands comprise(s) two layers of filaments around the core of central filaments, the strand or strands is/are

assembled by twisting M filaments around the core of L central filaments and subsequently twisting N (4<N<20) filaments around the layer of M filaments.

[0042] The K outer strands can be manufactured by a known method. The

filaments of the K strands, either the P central filaments of the core or the Q filaments do not comprise polymer material.

[0043] The J central strands and K outer strands are assembled to form the multi- strand cord for example by means of a conventional twisting process, e.g. by twisting first the individual strands, the J central strands and the K outer strands, followed by twisting the strands into the cord. This twisting can be done by means of a conventional tubular twisting machine or by means of a double twisting machine.

[0044] According to a third aspect of the present invention the use of a multi- strand steel cord as described above for reinforcing rubber articles such as tires or conveyor belts. The multi-strand steel cord according to the present invention is in particular suitable for reinforcing off-the road tires.

Brief Description of Figures in the Drawings

[0045] The invention will now be described into more detail with reference to the accompanying drawings whereby

- Figure 1 shows a first embodiment of a multi-strand cord according to the present invention;

- Figure 2 shows a second embodiment of a multi-strand cord according to the present invention;

- Figure 3 is a schematic illustration of a method to manufacture a

central strand for a multi-strand cord according to the present invention.

Mode(s) for Carrying Out the Invention

[0046] The present invention will be described with respect to particular

embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[0047] Figure 1 shows the cross-section perpendicular to the axis of a multi- strand cord 100 according to the present invention. The multi-strand cord 100 has a 1x(3+9) + 6x(3+9) construction, ZZSSZ, lay lengths (mm) 6.3/12.5/6.3/12.5/28. The multi-strand cord 100 has thus one central strand 102 surrounded by 6 outer strands 104. The filaments have for example a diameter equal to 0.245 mm. Possibly, the multi-strand cord 100 comprises a wrapping filament (not shown). [0048] The central strand 102 has a 3+9 configuration, i.e. a core 106 of three central metal filaments 108 and nine outer metal filaments 1 10 positioned in a layer around the core 106 of the three central metal filaments 108. The circumferential surface 1 12 of the core 106 of the three central metal filaments 108 of the central strand 102 is provided with polymer material 103, for example rubber. Preferably, the polymer material 103 is not only present on the circumferential surface 1 12 of the core 106 of the three central filaments 108 of the central strand 102 but also between the central filaments 108 of the core 106 of the central strand 102. In a preferred embodiment the voids created between the three central filaments 108 of the core 106 of the central strand 102 are completely filled with polymer material 103.

No polymer material is applied on the nine outer metal filaments 1 10 of the central strand 102 and no polymer material is applied on the

circumferential surface 1 13 of the central strand 102.

[0049] In the example shown in Figure 1 , the outer strands 104 of the multi- strand cord 100 have the same configuration as the central strand 102. The outer strands 104 comprise three central filaments 1 14 forming the core 1 16 of the outer strands 104 and nine outer filaments 1 18 positioned in a layer around the core 1 16 of the three central metal filaments 1 14. The outer strands 104 are distinguished from the central strands 102 in that there is no polymer material on or between the filaments of the outer strands 104, i.e. not on or between the three central metal filaments 1 14 of the core and not on or between the nine outer metal filaments 1 18. Consequently, there is no polymer material on the circumferential surface 120 of the core 1 16 of the outer strands 104 and not on the circumferential surface of the outer strands 104.

[0050] Figure 2 shows the cross-section perpendicular to the axis of a second example of a multi-strand cord 200 according to the present invention. The multi-strand cord 200 has a 1x19+6x19 construction, ZZSSZ, lay lengths (mm) 6.3/10/22. The multi-strand cord 200 has thus one central strand 202 surrounded by 6 outer strands 204. The filaments have a diameter of 0.20 mm. Possibly, the multi-strand cord 200 comprises a wrapping filament (not shown).

[0051] The central strand 202 has a 1 +6+12 configuration, i.e. a core 206 having one central filament 207, a first layer comprising six filaments 208 positioned around the core 206 and a second layer comprising twelve filaments 210 around the first layer.

The circumferential surface 212 of the core 206 comprising the central metal filament 207 of the central strand 202 is provided with polymer material 203, for example provided with rubber.

No polymer material is applied on the six filaments 208 of the first layer or on the twelve filaments 210 of the second layer and no polymer material is applied on the circumferential surface 213 of the central strand 202.

[0052] In the example shown in Figure 2, the outer strands 204 of the multi- strand cord 200 have the same configuration as the central strand 202, i.e. a 1 +6+12 configuration. The outer strands 204 comprise a core 214 of one filament 215, a first layer comprising six filaments 216 positioned around the core 214 and a second layer comprising twelve filaments 218 around the first layer.

[0053] The outer strands 204 are distinguished from the central strands 202 in that there is no polymer material on or between the filaments of the outer strands 204, i.e. not on the central filament 216 of the core 214, not one filaments of the first layer 216 or on the filaments of the second layer 218 and not between the filaments of the outer strand.

[0054] Consequently, there is no polymer material on the circumferential surface

220 of the core 214 of the outer strands 204 and not on the circumferential surface 221 of the outer strands 214. [0055] Figure 3 is a schematic illustration of the equipment to manufacture a central strand for a multi-strand cord according to the present invention. In the example shown in Figure 3 the strand to be manufactured has a 3+9 construction.

In the device 300, filaments 302 to be filaments of the core of the strand are fed from feed means (not shown). These filaments 302 are gathered and twisted to form the core 304 of the strand. The core 304

subsequently passes through a coating zone 306, for example through an extrusion head where polymer material is applied around the core 304. Subsequently, the filaments 310 to be the outer filaments of the strand are delivered and the filaments 310 are twisted around the core 306 by means of twisting machine 312 to form strand 314. Care is taken that the outer surface of the strand 314 is not contaminated with the polymer material.

[0056] Possibly a second layer of filaments is twisted around the first layer. As with the first layer, precautions are taken to avoid that the second layer is contaminated with polymer material.

[0057] It is clear for a person skilled in the art that the manufacturing of the

strands, either the central strands or the outer strands or both the central and the outer strands and the manufacturing of the multi-strand cord can be done in one process.

Claims

1. A multi-strand steel cord comprising J central strands and K outer strands, said K outer strands being positioned around said J central strands, with J > 1 and K > 3,

said J central strand or stands comprising a core of L central filaments and at least a first layer of M filaments positioned around said core of L central filaments, with 1 < L < 5 and 4 < M < 20,

said K outer strands comprising a core of P central filaments and at least one layer of Q filaments positioned around said core of P central filaments, with 1 < P < 5 and 4 < Q < 20,

characterized in that the circumferential surface of the core of L central filaments of said J central strands is coated with polymer material, the circumferential surface of the J central strands is free or substantially free of polymer material and that said K outer strands are free or substantially free of polymer material.

2. A multi-strand steel cord according to claim 1 , wherein said J central strand or strands comprise(s) a second layer of N filaments positioned around said first layer of M filaments, with 4 < N < 20 and wherein the circumferential surface of said first layer of M filaments and the circumferential surface of said second layer of N filaments are free or are substantially free of polymer material.

3. A multi-strand steel cord according to claim 1 or claim 2, wherein said K outer strands comprise a second layer of R filaments positioned around said first layer of Q filaments, with 4 < R < 20.

4. A multi-strand steel cord according to any one of the preceding claims, wherein said polymer material comprises an elastomer.

5. A multi-strand steel cord according to claim 4, wherein said elastomer is selected from the group consisting of natural rubbers, polyisoprene, polybutadiene, butadiene copolymers, isoprene copolymers and blends of one or more of these elastomers.

6. A multi-strand steel cord according to any one of the preceding claims, wherein said P central filaments of the core of said K outer strands are preformed.

7. A multi-strand steel cord according to any one of the preceding claims, wherein said K outer strands are preformed.

8. A multi-strand steel cord according to any one of the preceding claims, wherein said P central filaments of the core of said K outer strands and said K outer strands are preformed.

9. A method of manufacturing a multi-strand steel cord as defined in any one of claims 1 to 9, said method comprising the steps of

- providing J central strands, with J > 1 , said J strands comprising a core of L central filaments and at least one layer of M filaments positioned around said core of L central filaments; with 1 < L < 5 and 4 < M < 20, the circumferential surface of said core of L filaments being coated with a polymer material; whereby the circumferential surface of said J strands is free or is substantially free of polymer material.

- providing K strands, with K > 3, said K outer strands comprising a core of P central filaments and at least one layer of Q filaments positioned around said core of P central filaments, with 1 < P < 5 and 4 < Q < 20, said P central filaments and said Q filaments being free or substantially free of polymer material;

10. The method according to claim 9 whereby the J central strands are further provided with a second layer of N filaments positioned around said first layer of M filaments, and wherein the circumferential surface of said first layer of M filaments as well as the circumferential surface of said second layer of N filaments is kept free or substantially free of polymer material. 1. Use of a multi-strand steel cord as in any one of claims 1 to 8 for reinforcing a rubber article.