DESCRIPTION

The present invention relates to a reinforcement cord. The invention also relates to a tyre for vehicle wheels comprising such a reinforcement cord.

Preferably, the reinforcement cord of the invention is used as a structural reinforcement element in the manufacturing of semi-finished products intended to be used for building tyres for two or four-wheeled vehicle wheels.

The reinforcement cord of the invention can, however, also be used as a reinforcement element of other types of elements made from elastomeric material, like for example pipes (in particular underwater pipes and high-pressure hydraulic pipes) , conveyor belts, transmission belts and cables .

Throughout the present description the following definitions apply.

The term "elastomeric material" is used to indicate a composition comprising at least one elastomeric polymer and at least one reinforcing filler. Preferably, such a composition also comprises additives like, for example, a cross-linking agent and/or a plasticizer. Thanks to the presence of the cross-linking agent, the elastomeric material can be cross-linked by heating.

The expression "initial diameter" of a cord is used to indicate the transversal size of a cord made up of steel wires. The diameter of the cord is measured with the BISFA E10 method (The International Bureau For The Standardization Of Man-Made Fibres, Internationally Agreed Methods For Testing Steel Tyre Cords, 1995 edition) .

The expression "breaking load" of a reinforcement cord is used to indicate the load at which the cord breaks, measured with the BISFA E6 method (The International Bureau For The Standardization Of Man-Made Fibres, Internationally Agreed Methods For Testing Steel Tyre Cords, 1995 edition) . The expression "elongation at break" of a reinforcement cord is used to indicate the percentage elongation at which the cord breaks, again measured with the BISFA E6 method.

The expression "part load elongation" of a reinforcement cord is used to indicate the difference between the percentage elongation obtained by subjecting the cord to a traction of 50 N and the percentage elongation obtained by subjecting the cord to a traction of 2.5 N. The part load elongation is evaluated with the BISFA E7 method (The International Bureau For The Standardization Of Man-Made Fibres, Internationally Agreed Methods For Testing Steel Tyre Cords, 1995 edition) .

The expression "rigidity" of a reinforcement cord is used to indicate the resistant moment to a bending with predetermined angle (normally 15°) evaluated with the BISFA E8 method (The International Bureau For The Standardization Of Man-Made Fibres, Internationally Agreed Methods For Testing Steel Tyre Cords, 1995 edition) .

The expression "fraying" of a reinforcement cord is used to indicate the tendency of the single wires that make up the cord to not stay stably interwoven when the cord is subjected to cutting by clippers. Fraying is evaluated with the BISFA E3 method (The International Bureau For The Standardization Of Man-Made Fibres, Internationally Agreed Methods For Testing Steel Tyre Cords, 1995 edition) . The expression "low carbon content" is used to indicate a carbon content lower than 0.40 % by weight.

As already stated, the reinforcement cord has a preferred, although not exclusive, application in tyres for vehicle wheels . Such tyres typically comprise a carcass structure and, in a radially outer position with respect to the carcass structure, a belt structure, In a radially outer position with respect to the belt structure a tread band is provided, through which the tyre makes contact with the road surface.

The behaviour of tyres for vehicle wheels is greatly influenced by the particular type and configuration of the carcass structure and of the belt structure. Such structures are in particular intended to give the tyre the desired characteristics of structural strength and comfort.

Both the carcass structure and the belt structure typically comprise a plurality of reinforcement cords, made from textile or metallic material, embedded in a matrix of elastomeric material. The reinforcement cords of the carcass structure are typically oriented substantially parallel to the rotation axis of the tyre, whereas the reinforcement cords of the belt structure are typically oriented obliquely with respect to the equatorial plane of the tyre, in the so-called "crossed" belt structures, or substantially parallel to the equatorial plane of the tyre (i.e. inclined with respect to the aforementioned equatorial plane by a very small angle, generally lower than about 10°), in the so-called "zero-degree" belt structures .

Reinforcement cords can be used in other components of the tyre, like for example, in the specific case of use on heavy vehicles, gravel guard strips and side reinforcement strips of the belt structure.

The particular type and configuration of the reinforcement cords used in the tyres for vehicle wheels has a noticeable impact on the production cost of the tyre.

Such reinforcement cords are typically made by twisting a plurality of wires of textile or metallic material. With particular reference to reinforcement cords made from metallic material, they typically comprise steel wires.

Each of such steel wires generally comprises a steel core typically coated by at least one metallic layer that takes care of both protecting the underlying steel against corrosion and providing adequate adhesion to the elastomeric material that incorporates such a wire, as well as facilitating and improving the drawing process which the wire itself is subjected to.

The Applicant has observed that an important mechanical characteristic in reinforcement cords made from metallic material used in tyres for vehicle wheels, and in particular in the belt structures of such tyres and, in the specific case of tyres for heavy vehicles, in the gravel guard strips and in the side reinforcement strips of the belt structure, is the rigidity. The Applicant has indeed verified that the belt structure, the gravel guard strips and the side reinforcement strips of the belt structure of the tyre are subjected, during travel, to peak loads generated by lateral stresses to which the aforementioned components must provide adequate structural resistance, and this happens as much effectively as the rigidity of such components, and therefore of the reinforcement cords comprised therein, increases.

The Applicant has for a long time used in its tyres reinforcement cords comprising steel wires having medium- high carbon content twisted together with a winding pitch equal to about 14 mm in its tyres. In such cords, in particular, steel C80 or C70 (i.e. with a carbon content equal to 0.80% by weight or to 0.70 % by weight, respectively) is used. Such reinforcement cords (hereafter also called "HT and NT cords") have excellent mechanical characteristics, also in terms of rigidity. Tyres in which such reinforcement cords are used are indeed greatly appreciated by customers. The Applicant has observed that the use of steel having high carbon content involves high production costs of the reinforcement cords and, therefore, of the tyres incorporating such reinforcement cords.

It has also been proposed to use cords with wires made from stainless steel, or inox, that is steel having low carbon content and high chrome content, typically greater than about 11 % by weight, as well as other components.

The Applicant has however observed that the use of stainless steel involves high production costs, as well as a manufacturing process of the wires and/or of the cords that is more complex and problematic when manufacturing the reinforced semi-finished product and using it in the tyre due to the difficulty in getting the stainless steel to stick to the elastomeric materials and in maintaining the coupling over the time.

Non-stainless steel wires having low carbon content are described in WO 2009/109495, WO 84/02354 and EP 0222429.

The Applicant has observed that non-stainless steel wires having low carbon content (hereafter reference will be made to steel wires having low carbon content meaning non- stainless steel) typically have mechanical characteristics not suitable for being used in reinforcement cords of tyres .

The Applicant has however found that by using particularly long and unconventional winding pitches, it is possible to manufacture reinforcement cords comprising steel wires having low carbon content (hereafter also indicated with the expression "LC cords") that have excellent behaviour in terms of rigidity, comparable to behaviour obtainable using HT or NT steel wires .

The Applicant has also found that the mechanical behaviour of reinforcement cords comprising steel wires having low carbon content twisted with more conventional winding pitches, and typically used in cords comprising HT and/or NT steel wires, is insufficient in terms of strength of the material and/or of maximum peak load allowed, due to the torsional stress which they are subjected to. Moreover, during production, the cords twisted with conventional winding pitches are of poorer quality, which causes a more frequent breaking that, together with the lower advancing speed of the steel wires in the machine carrying out the winding, cancels out one of the advantages of using low carbon content steel, i.e. the increased efficiency (in terms of cost and/or productivity) .

The Applicant has however observed that as the winding pitch increases, the risk of there being the undesired phenomenon of fraying in the reinforcement cords increases. For example, the Applicant has found that cords comprising HT and NT steel wires can have significant problems of fraying when the HT or NT steel wires are twisted with winding pitches of more than 14 mm.

Surprisingly, the Applicant has however found that such a problem of fraying does not occur when the cords are made with steel having low carbon content.

The Applicant therefore believes that reinforcement cords comprising steel wires with a carbon content lower than about 0.40 % (non-stainless, for example with a chrome content lower than about 2%) twisted with winding pitches of at least about 14 mm, having characteristics of structural strength substantially identical to those of the greatly appreciated HT and NT (with particular reference to rigidity) and not being subject to problems of fraying, can be effectively used in tyres for vehicle wheels, and in particular in the belt structure of such tyres and, in the specific case of tyres for heavy vehicles, in the gravel guard strips and in the side reinforcement strips of the belt structure. The present invention therefore relates, in a first aspect thereof, to a reinforcement cord comprising at least two steel wires having a carbon content lower than or equal to about 0.40 % by weight and a chrome content lower than about 2% by weight. Preferably, said at least two steel wires are twisted together with a winding pitch greater than or equal to about 14 mm.

The Applicant observes that the use of winding pitches greater than those typically used in HT and NT cords advantageously allows a significant decrease of the production costs of the cords, and, consequently, of the tyres. The Applicant has indeed observed that the winding pitch is directly correlated to the advancing speed of the machine carrying out the twisting of the steel wires so that, being equal the twisting time, the greater the winding pitch the greater the advancing speed of the aforementioned machine, i.e. the greater the length of twisted portion that can be obtained or, vice-versa, being equal the length of the twisted portion, the greater the winding pitch and the less the time required for twisting.

According to the Applicant, therefore, an increase in the length of the winding pitch causes an advantageous increase in productivity and, therefore, a reduction of the production costs. More specifically, the Applicant has observed that, when compared to HT and NT cords, the reinforcement cords of the invention effectively allow the desired reduction of the production costs to be obtained both due to the lower cost of the steel wires and to the possibility of using greater winding pitches than those used in HT and NT cords without having risks of fraying.

In a second aspect thereof, the present invention relates to a tyre for vehicle wheels, comprising:

- a carcass structure comprising at least one carcass layer having axially opposite side edges associated with respective annular anchoring structures comprising at least one annular anchoring element and at least one elastomeric filler;

- a tread band applied in a radially outer position with respect to said carcass structure.

Optionally, the tyre comprises at least one of the following structural components:

- a belt structure applied in a radially outer position with respect to said carcass structure, said belt structure comprising at least one belt layer;

- at least one structural element wound around said annular anchoring element and the elastomeric filler so as to at least partially surround them;

- at least one protective layer of the annular anchoring structure;

- at least one gravel guard strip configured to protect the belt structure from the entry of foreign bodies;

at least one side reinforcement strip of the belt structure at an axial end thereof.

At least one from said at least one carcass layer, said at least one belt layer, said at least one structural element, said at least one protective layer, said at least one gravel guard strip and said at least one side reinforcement strip of the belt structure comprises an elastomeric material reinforced with at least one reinforcement cord, wherein said at least one reinforcement cord comprises at least two steel wires having a carbon content lower than or equal to about 0.40 % and chrome content lower than or equal to about 2 % by weight, wherein said at least two steel wires are twisted together with a winding pitch greater than or equal to about 14 mm.

The present invention, in at least one of the aforementioned aspects, can have at least one of the following preferred characteristics. Preferably, said winding pitch is greater than or equal to about 16 mm.

Preferably, said winding pitch is lower than or equal to about 25 mm. In preferred embodiments, said winding pitch is comprised between about 16 mm and about 25 mm.

In particularly preferred embodiments, said winding pitch is equal to about 18 mm. In alternative embodiments, said winding pitch is equal to about 20 mm. Preferably, the carbon content is greater than about 0.18% by weight .

Preferably, the carbon content is lower than about 0.25 % by weight.

In preferred embodiments, the carbon content is comprised between about 0.18 % and about 0.25 % by weight.

Preferably, said at least two steel wires have a diameter greater than or equal to 0.295 mm.

Preferably, said at least two steel wires have a diameter lower than or equal to 0.40 mm. In preferred embodiments, said at least two steel wires have a diameter comprised between about 0.295 mm and about 0.40 mm.

For example, said at least two steel wires have a diameter whose value is preferably selected from: 0.295 mm, 0.30 mm, 0.32 mm, 0.35 mm and 0.38 mm.

Preferably, the cord comprises a number of steel wires lower than or equal to twenty, more preferably lower than or equal to twelve, even more preferably lower than or equal to seven. In some embodiments, the number of steel wires is equal to two .

In other embodiments, the number of steel wires is equal to at least three.

In this last case the cord preferably comprises at least two first steel wires twisted together with said predetermined winding pitch to form a core of wires and at least one second steel wire twisted around said core of wires with said predetermined winding pitch.

For example, said core of wires can comprise two first steel wires .

In this case the cord comprises, in a preferred embodiment thereof, a second steel wire twisted around said core of wires with said predetermined winding pitch or, in another preferred embodiment thereof, two second steel wires twisted together around said core of wires with said predetermined winding pitch.

Alternatively, the cord can comprise two first steel wires twisted together with said predetermined winding pitch to form said core of wires and three second steel wires twisted together around said core of wires with said predetermined winding pitch.

In some embodiments the core of wires comprises three first steel wires .

Preferably, the cord in this case comprises at least one second steel wire twisted around said core of wires with said predetermined winding pitch. For example, the cord can comprise a second steel wire twisted around said core of wires with said predetermined winding pitch or, in a preferred embodiment thereof, two second steel wires twisted together around said core of wires with said predetermined winding pitch.

Alternatively, the cord can comprise three steel wires twisted together with said predetermined winding pitch.

In further embodiments, the cord can comprise a first steel wire and at least two second steel wires twisted around said first steel wire with said predetermined winding pitch. For example, the cord can comprise two or three second steel wires twisted around said first steel wire with said predetermined winding pitch or, in a preferred embodiment thereof, four steel wires twisted together around said first steel wire with said predetermined winding pitch.

Alternatively, the cord can comprise four first steel wires twisted together with said predetermined winding pitch to form said core of wires and at least one second steel wire twisted around said core of wires with said predetermined winding pitch.

In further embodiments, the cord comprises four or five steel wires twisted together with said predetermined winding pitch.

Preferably, the cord has a diameter comprised between about 0.5 mm and about 1.5 mm.

Further characteristics and advantages of the reinforcement cord of the present invention will become clearer from the following detailed description of preferred embodiments thereof, made with reference to the attached drawings . In such drawings :

- figure 1 is a side view of an illustrative embodiment of a reinforcement cord in accordance with the present invention, with an enlarged section thereof taken at the section plane indicated with the lines A-A;

- figure 2 is a block diagram of a process for producing a steel wire which can be used in the cord of figure 1;

- figure 3 illustrates a radial half-section of a tyre for vehicle wheels comprising a plurality of reinforcement cords in accordance with the present invention. In figure 1, reference numeral 10 indicates an illustrative embodiment of the reinforcement cord of the invention.

The cord 10 has a preferred application in a tyre for vehicle wheels, like for example the tyre 100 illustrated in figure 3.

The tyre 100 of figure 3 is, in particular, an illustrative embodiment of a tyre for four-wheeled vehicles. The cord 10 described below can in any case also apply to embodiments of tyres different from those of figure 3, as well as tyres for two-wheeled vehicles.

In figure 3 "a" indicates an axial direction and "r" indicates a radial direction. For the sake of simplicity, figure 3 shows just a portion of the tyre, the remaining portion not shown being identical and arranged symmetrically with respect to the radial direction "r".

Throughout the present description, the terms "radial" and "axial" and the expressions "radially inner/outer" and "axially inner/outer" are used with reference to the radial direction and to the axial direction of the tyre 100. The terms "circumferential" and "circumferentially", on the other hand, are used with reference to the direction of the annular development of the tyre 100, which corresponds to a direction lying on a plane parallel to the equatorial plane of the tyre and perpendicular to the radial direction "r". Such a direction also corresponds to the rolling direction of the tyre 100.

The tyre 100 comprises at least one carcass structure 101, in turn comprising at least one carcass layer 111.

Hereafter, for the sake of simplicity of explanation, reference will be made to an embodiment of the tyre 100 comprising a single carcass layer 111, being nevertheless understood that what is described has analogous application in tyres comprising more than one carcass layer. The carcass layer 111 has axially opposite end edges engaged with respective annular anchoring structures 102, called bead cores, possibly associated with an elastomeric filler 104. The area of the tyre comprising the bead core 102 and the elastomeric filler 104 forms an annular reinforcing structure 103 called "bead" and intended to allow the tyre 100 to be anchored on a corresponding assembly rim, not shown.

The carcass layer 111 comprises a plurality of reinforcement elements (not shown) coated with an elastomeric material or incorporated in a matrix of a cross-linked elastomeric material.

The carcass structure 101 is usually of the radial type, i.e. the reinforcement elements of the carcass layer 111 are arranged on planes comprising the rotation axis R of the tyre and substantially perpendicular to the equatorial plane M of the tyre. Said reinforcement elements generally consist of textile cords, for example rayon, nylon, polyester (for example polyethylene naphthalate (PEN) ) , but it is not to be excluded that they can consist of metallic cords .

Each annular reinforcing structure 103 is associated with the carcass structure 101 by bending back (or turning up) opposite side edges of the at least one carcass layer 111 around the annular anchoring structure 102, so as to form the so-called turns-up 101a of the carcass structure 101, as illustrated in Figure 3.

In an embodiment, the coupling between carcass structure 101 and annular reinforcing structure 103 can be made through a second carcass layer (not shown in figure 3) applied in an axially outer position with respect to the carcass layer 111.

An anti-abrasion strip 105 is arranged so as to wrap around the annular reinforcing structure 103 along the axially inner and outer and radially inner and outer areas of the annular reinforcing structure 103, thus being arranged between the latter and the rim of the wheel when the tyre 100 is mounted on the rim. In a radially outer position with respect to the carcass structure 101 a belt structure 106 is associated, the belt structure 106 comprising one or more belt layers 106a, 106b arranged radially one on top of the other.

The layers 106a, 106b comprise a plurality of reinforcement cords (not shown) . Such reinforcement cords can have a crossed orientation with respect to the circumferential direction of the tyre 100.

In a radially outer with respect to the belt layers 106a, 106b at least one zero degrees reinforcement layer 106c, commonly known as "0° belt" can be applied. Such a layer generally incorporates a plurality of reinforcement cords oriented in a substantially circumferential direction. Such cords thus form an angle of few degrees (typically lower than about 10°, for example comprised between about 0° and 6°) with respect to the equatorial plane M of the tyre 100.

The aforementioned reinforcement cords are coated with an elastomeric material or embedded in a matrix of cross- linked elastomeric material .

In a radially outer position with respect to the belt structure 106 a tread band 109 in elastomeric mixture is applied, as well as other constituent semi-finished elements of the tyre 100.

Respective sidewalls 108 in elastomeric mixture are also applied onto the side surfaces of the carcass structure, in an axially outer position with respect to the carcass structure 101. Each sidewall 108 extends from one of the side edges of the tread band 109 up to the respective annular reinforcing structure 103. The anti-abrasion strip 105 extends at least up to the respective sidewall 108.

The rigidity of the sidewall 108 can be improved by providing in the bead of the tyre a reinforcement layer 120 generally known as "flipper" or additional strip-shaped insert .

The flipper 120 is a reinforcement layer that is wound around the respective bead core 102 and the elastomeric filler 104 so as to at least partially surround them, said reinforcement layer being arranged between the carcass layer 111 and the annular reinforcing structure 103. Usually, the flipper is in contact with the carcass layer 111 and said annular reinforcing structure 103.

The flipper 120 typically comprises a plurality of metallic or textile cords embedded in a cross-linked elastomeric material .

The bead of the tyre can also comprise a further protective layer 121 that is generally known by the term "chafer", or protective strip, and which has the function of increasing the rigidity and integrity of the annular reinforcing structure 103.

The chafer 121 usually comprises a plurality of cords embedded in a cross-linked elastomeric material; such cords are generally made from textile material (for example aramid or rayon) , or from metallic material (for example steel cords ) .

The tread band 109 has, in a radially outer position thereof, a rolling surface 109a intended to come into contact with the ground. On the rolling surface 109a circumferential grooves are formed, which are connected by transversal grooves (not shown in figure 3) so as to define on the rolling surface 109a a plurality of blocks of various shapes and sizes. For the sake of simplicity, in figure 3 the rolling surface 109a is shown slick. A sub-layer 107 is arranged between the belt structure 106 and the tread band 109.

A strip consisting of elastomeric material 110, commonly known as "mini-sidewall", can possibly be provided in the connection area between the sidewalls 108 and the tread band 109. This mini-sidewall is generally obtained by co- extrusion with the tread band 109 and allows an improvement of the mechanical interaction between the tread band 109 and the sidewalls 108 to be obtained. Preferably, an end portion of the sidewall 108 directly covers the side edge of the tread band 109.

In the case of tubeless tyres, a layer of rubber 112, generally known as "liner", can also be provided in a radially inner position with respect to the carcass layer 111 to provide the necessary impermeability to the inflation air of the tyre 100.

In the case in which the tyre is intended to be mounted on a heavy vehicle, preferably, the belt structure comprises, in a radially outer position thereof, at least one gravel guard strip (not shown) adapted to protect the belt layers from the entry of foreign bodies. The gravel guard strip incorporates a plurality of metallic reinforcement cords incorporated in an elastomeric material.

Preferably, the belt structure also comprises a side reinforcement strip (not shown) that can be radially overlapped to the radially outermost belt layer, at an axial end thereof. The side reinforcement strip incorporates a plurality of reinforcement elements, preferably high elongation metallic cords , incorporated in an elastomeric material. Preferably, moreover, an insert is arranged substantially at the shoulder portion, i.e. the portion where the side end of the tread band connects to the sidewall. In particular, the insert has a portion that is arranged substantially in a radial direction between the belt structure and the tread band and a portion that is arranged substantially in an axial direction between the carcass structure and the sidewall of the tyre.

The expression "heavy vehicle" is used to generally indicate a vehicle belonging to categories 2-M3, N1~N3 and 02~04 defined in "Consolidated Resolution of the Construction of Vehicles (R.E.3) (1997)", Annex 7, pages 52-59, "Classification and definition of power-driven vehicles and trailers", like for example, lorries, trucks, tractors, buses, vans and other vehicles of this type.

The reinforcement cords of the belt layers 106a, 106b, and/or of the zero-degree reinforcement layer 106c, and/or of the flippers 120, and/or of the chaffers 121, and/or of the carcass layer 111, and/or in the specific case of tyres for heavy vehicles, of the gravel guard strip and of the side reinforcement strips of the belt structure, can be made in accordance with the present invention. They can thus consist of cords 10 of the type illustrated in figure

For the sake of simplicity, in figure 3 the cords 10 of the invention are illustrated as belonging to the zero-degree reinforcement layer 106c and are identified with the reference numeral 10. For further simplicity, the reference numeral 10 is only associated with some of such cords.

With reference to figure 1, an embodiment of a cord 10 in accordance with the present invention is shown. The cords 10 comprises two identical steel wires 1. The wires 1 are made from steel with chrome content lower than or equal to about 2 % by weight, preferably lower than or equal to 1.5 % by weight, for example lower than or equal to about 0.1 % by weight, and carbon content lower than or equal to about 0.40 % by weight, preferably a variation ranging from C18 to C25 (i.e. from 0.18 % by weight to 0.25 % by weight) .

The steel wires 1 are twisted together with a winding pitch P greater than or equal to about 14 mm, preferably comprised between about 16 mm and about 25 mm, for example equal to about 18 mm or 20 mm.

The wires 1 preferably have a diameter comprised between about 0.295 mm and about 0.40 mm, for example equal to 0.295 mm, 0.30 mm, 0.32 mm, 0.35 mm or 0.38 mm.

In the specific example of figure 1 the wires 1 have a diameter equal to about 0.30 mm.

The cord 10 of figure 1 therefore has a construction that can be identified with the symbology 2X0.30. In accordance with such symbology, indeed, the number before the symbol "X" indicates the number of wires twisted together and the number after the symbol "X" indicates the diameter of such wires . Analogous constructions to that of figure 1 can be made using wires 1 of different diameter.

There are also alternative embodiments (not shown) of the cord 10 of the invention, which differ from the one described above and shown in figure 1 only in that they use a different number of steel wires.

The number of steel wires used in the cord 10 of the invention can indeed also be greater than two. Preferably, such a number is not greater than twenty, more preferably lower than or equal to twelve, even more preferably lower than or equal to seven.

The mutual arrangement of the wires can in that case be of various kinds .

For example, in some embodiments that are not shown, all of the steel wires are twisted together with the same winding pitch P, in such a way making constructions for example of the type 3X0.30, 4X0.30, 5X0.30, 7X0.30. Among these, the last three constructions are particularly preferred. Analogous constructions can be made with steel wires of different diameter.

In other embodiments that are not shown, at least two of the steel wires are twisted together with the aforementioned winding pitch so as to define a core of wires and at least one other steel wire is twisted around the aforementioned core of wires with a winding pitch that can be different or, preferably, equal to the winding pitch of the wires of the core. Also in this case, it is possible to make various constructions of cords 10 in accordance with the invention.

Preferably, the following constructions are made: 2+1X0.30, 2+2x0.30, 1+4X0.30, 3+2X0.30, 4+3X0,30, 1+6X0,30, where the number before the symbol "X" and the symbol "+" indicates the number of steel wires twisted together that define the aforementioned core of wires, the number before the symbol "X" and after the symbol "+" indicates the number of steel wires twisted around the aforementioned core of wires and the number after the symbol "X" indicates the diameter of the steel wires. Also in this case it is possible to make analogous constructions with steel wires of different diameter .

With reference to figure 1, each of the wires 1 of the cord 10, in addition to being twisted around the other wire 1, can be individually twisted on itself with a predetermined winding pitch that may or may not be equal to the winding pitch P. Wires twisted on themselves can also be used in the other embodiments of the cord 10 described above and not shown. The diameter of the cord 10 of the invention obviously varies as a function of the diameter of the steel wires, the number of steel wires and the reciprocal arrangement of such wires. Such a diameter is preferably comprised between about 0.50 mm and about 1.5 mm. With reference to figure 2, an example of a process for producing a wire 1 which can be used in the cord 10 of the invention will be described.

The process initially comprises providing a steel wire 50 (rod) having carbon content lower than or equal to 0.40 % by weight and having for example a diameter equal to about 5.50 mm. Such a steel wire 50 is subjected to a scaling operation to remove possible oxides from the surface thereof. The scaling is carried out in a suitable scaling station 150 comprising a plurality of properly positioned rollers 150a. The steel wire 50 is passed through the rollers 150a with a speed for example equal to about 180 metres per minute.

The wire 50, after having been subjected to scaling, is fed into a pickling and boriding station 152 comprising an area 152a where the wire 50 is subjected to sulphuric pickling and an area 152b where the wire is subjected to boriding. The outer surface of the wire is in this way cleaned and prepared for the subsequent drawing operations .

The wire coming out from the pickling and boriding station 152 is then collected in a reel 154, from which it is taken to be fed into a dry drawing machine 156. When coming out from the dry drawing machine 156 the wire 50 has a diameter comprised between about 1 mm and about 2.60 mm, for example equal to 1.93 mm. Such a wire 50 is collected in a reel 158.

The wire is taken from the reel 158 to be subjected to brassing in a suitable brassing station 160, without having previously been subjected to any patenting operation.

The brassing initially comprises repeatedly passing the wire 50 in a bath 160a of H ₂ S0 ₄ and in a single bath 160b of brass consisting of many deposition tanks (or alternatively a bath of just zinc or of just copper) to form on the wire a suitable surface coating layer. The speed of the wire in the brassing station 160 is about 40 metres per minute.

The wire coming out from the brassing station 160 is then collected in a reel 162, from which it is taken to be passed into a thin drawing machine 164, with a speed egual for example to about 16 metres per second, with 21 steps.

When coming out from the thin drawing machine 164 a wire 1 is obtained having a reduced diameter, preferably comprised between 0.295 mm and 0.40 mm. Such a wire 1 is collected in a reel 166.

The wire 1 is subsequently taken from the reel 166 and twisted together with at least one other identical wire taken from a respective reel to make the cord 10 of the invention shown in figure 1. In the case in which the cord comprises more than two wires, each of them is taken from a respective reel and twisted to the others to make any of the alternative constructions described above and not shown .

In particular, the wires 1 are twisted together with a winding pitch equal to at least 14 mm, so as to obtain the desired rigidity without having the risk of fraying.

The twisting process is carried out according to well- established technology in a suitable machine (typically called "twisting machine") , for example unwinding the wires (or the strands) from the respective supply reels on which they are wound through one or more pulling capstans and sending them to twisting groups capable with their rotation of determining the winding pitch of the cord, to then wind onto a collection reel the cord so made.

Irrespective of its specific construction, the cord 10 of the invention is then coated with elastomeric material (or embedded in a matrix of elastomeric material) to make a semi-finished product intended for example to the building of the tyre 100 of figure 3. EXAMPLES

The Applicant has compared the mechanical behaviour of some samples of cords 10 in accordance with the present invention (hereafter "LC cords") with that of the HT and NT cords used in tyres produced and commercialised for a long time by the same Applicant and appreciated by customers for their excellent characteristics of structural strength.

The Applicant firstly compared LC cords with construction 2X0.30 and winding pitch equal to 14 mm (leftwards) and HT and NT cords of identical construction and winding pitch. On each of such cords three tests were carried out, according to tables 1, 2 and 3 given below

TABLE 1 (cord 2X0.30 HT - pitch 14/s)

Initial diameter (mm) 0.59 0.59 0.59

Breaking load under traction

443 444 444

(N)

Simultaneous breaking YES YES YES

Elongation at break (%) 2.1 1 2.12 2.10

Part load elongation (load 2-

0.047 0.047 0.046

16 N) (%)

Rigidity (tsu) 20 22 21

Fraying (%) 0 0 0

TABLE 2 (cord 2X0.30 NT - pitch 14/s)

Initial diameter (mm) 0.58 0.58 0.58

Breaking load under traction

402 399 404

(N)

Simultaneous breaking YES YES YES Elongation at break (%) 2.09 2.11 2.09

Part load elongation (load 2-

0.045 0.047 0.047

16 N) (%)

Rigidity (tsu) 19 21 21

Fraying (%) 0 0 0

TABLE 3 (cord 2X0.30 LC - pitch 14/s)

Such tests confirmed the excellent mechanical behaviour of the HT and NT cords and equally excellent mechanical behaviour of the LC cords of the invention. The only parameter that in the cords LC proved to be penalised is the breaking load. However, according to the Applicant such a parameter is not at all critical in applications in which the cord is subject to peak load (like for example in belt structures of tyres for vehicle wheels) . In such applications, indeed, the absolutely critical parameter is rigidity. In this respect, the tests demonstrated that the cords LC have substantially identical behaviour in terms of rigidity to that of HT and NT cords.

Together with the breaking load, the simultaneous breaking of the wires making up the cord was evaluated. In all of the tests carried out, the LC cords behaved positively, like the NT, HT cords, breaking simultaneously. Such behaviour of the LC cords is advantageous since if some wires were to remain intact when others break, there would be a lack of uniformity of distribution of the loads over the single components of the cord that would have a negative impact on the resistance to fatigue of the structure, for example in the case of a carcass structure that is typically stresses by cyclical traction and compression forces.

It has also been found, as expected, that both in the HT and NT cords and in the LC cords a winding pitch equal to 14 mm does not cause problems of fraying.

The Applicant carried out analogous tests on cords having a greater winding pitch, to evaluate the effects of the increase in winding pitch on the mechanical behaviour of the cords .

In this respect, the Applicant firstly carried out tests on HT and NT cords with construction 2X0.30 and winding pitch equal to 16 mm (leftwards), according to tables A and B given below.

TABLE A (cord 2X0.30 HT - pitch 16/s)

Initial diameter (mm) 0.58 0.58 0.58

Breaking load under traction (N) 445 443 443

Simultaneous breaking YES YES YES

Elongation at break (%) 20.1 2.02 2.05

Part load elongation (load 2-16

0.047 0.049 0.052

N) (%)

Rigidity (tsu) 20.5 22 21

Fraying (%) 60 70 70 TABLE B (cord 2X0.30 NT - pitch 16/s)

As expected, it was found that in HT and NT cords it is not possible to use winding pitches greater than 14 mm as the undesired phenomenon of fraying would occur.

The Applicant then wanted to verify the effects of an increase in winding pitch on the mechanical behaviour of LC cords .

In this respect, the Applicant compared LC cords with construction 2X0.30 and winding pitch equal to 18 mm (leftwards) and HT and NT cords of identical construction and winding pitch, according to tables Al, Bl and CI given below .

TABLE A1 (cord 2X0.30 HT - pitch 18/s)

Initial diameter (mm) 0.58 0.58 0.58

Breaking load under traction (N) 447 445 445

Simultaneous breaking YES YES YES

Elongation at break (%) 1.87 1.78 1.82

Part load elongation (load 2-16

0.049 0.054 0.055

N) (%)

Rigidity (tsu) 21 22 22 Fraying (%) 100 100 100

TABLE B1 (cord 2X0.30 NT - pitch 18/s)

Initial diameter (mm) 0.58 0.58 0.58

Breaking load under traction (N) 410 415 390

Simultaneous breaking YES YES YES

Elongation at break (%) 1.89 1.85 1.92

Part load elongation (load 2-16

0.050 0.054 0.051

N) (%)

Rigidity (tsu) 21 20 21

Fraying (%) 100 100 100

TABLE C1 (cord 2X0.30 LC - pitch 18/s)

Initial diameter (mm) 0.58 0.58 0.58

Breaking load under traction (N) 332 331 331

Simultaneous breaking YES YES YES

Elongation at break (%) 1.87 1.78 1.82

Part load elongation (load 2-16

0.049 0.054 0.055

N) (%)

Rigidity (tsu) 22 21 21

Fraying (%) 0 0 0

Such tests firstly highlighted the excellent behaviour of LC cords with winding pitch 18 mm in terms of rigidity; the values obtained are indeed absolutely comparable with those of the greatly appreciated HT and NT cords with winding pitch 14 mm.

The tests also highlighted that it is not possible to use a winding pitch equal to 18 mm in HT and NT cords due to the problem of fraying (as expected)

The tests also highlighted that in LC cords with winding pitch equal to 18 mm there is no fraying phenomenon.

Analogous results were obtained with winding pitches equal to 20 mm.

With winding pitches lower than 14 mm, on the other hand, it was noted a reduction in strength of the material (in particular, a reduction in the allowable maximum loads) and a higher frequency of breaking that, together with the lower advancing speed of the steel wires in the machine that carries out the twisting, would cancel out one of the advantages of using steel having low carbon content, i.e. the increased productivity.

The Applicant has moreover had confirmation, through the aforementioned tests, of its intuition, i.e. that reinforcement cords comprising steel wires with a carbon content lower than about 0.40% and chrome content lower than or equal to about 2 % by weight, twisted with winding pitches equal to, or greater than, about 14 mm have characteristics of structural strength substantially identical to those of the greatly appreciated HT and NT cords (with particular reference to rigidity) and are not subject to fraying problems. Such cords can therefore be used in tyres for vehicle wheels, and in particular in the belt structure of such tyres, and in the specific case of tyres for heavy vehicles, in the gravel guard strips and in the side reinforcement strips of the belt structure, obtaining a substantial reduction of production costs.

The present invention has been described with reference to some preferred embodiments. Various changes can be made to the embodiments described above, whilst still within the scope of protection of the invention as defined by the following claims .