Description Technical Field

[0001] The present invention relates to a steel cord adapted for the reinforcement of elastomer products, especially for reinforcing the carcass plies of medium to heavy truck and bus radial tires. The steel cord comprises a core filament with a core filament diameter and two layers of filaments twisted around the core filament in the same direction and with the same step.

Background Art

[0002] Steel cords where all the steel filaments are twisted in the same direction and with the same step are known in the art and are often referred to as "compact steel cords" since they may lead to a compact cross-sectional configuration with a high amount of steel within a limited cross-sectional surface area.

[0003] Within the group of compact steel cords, following configurations are

widely used in the art of elastomer reinforcement:

- 1 xdc I 18*di CC

- 1 xdc I 15xdi CC

A 1 xdc I 18xdi CC is a compact cord comprising one core filament with a core filament diameter d _c and eighteen layer filaments with a layer filament diameter di, arranged in an intermediate layer of six filaments and an outer layer of twelve filaments. Such a compact cord is disclosed in

EP-B1 -0 627 520. The compact cord constitutes a stable construction because the cross-section of such a steel cord has the typical form of regular hexagon which is centrosymmetric and quite stable.

The 1 *dc I 15*di CC disclosed in WO-A1-03 069 055 comprises a core filament with a core filament diameter and fifteen layer filaments having a layer filament diameter twisted around the core filament. The fifteen layer filaments are arranged in an intermediate layer of five filaments and in an outer layer of ten filaments.

The core filament diameter is smaller than the layer filament diameter. Although such a configuration is technically possible, it is, however, not always technically practicable. Such a compact cord doesn't always constitute a stable construction for the cross-section of the cord has the typical form of regular pentagon which is not centrosymmetric, it would easily lose balance by being disturbed from the twisting machine during the manufacture process. Inversion or kinks would suddenly appear.

Further, it would sometimes change into a ( 1 +4+1 1 ) CC construction or (1 +6+9) CC construction instead of the desirable (1 +5+10) CC

construction as described above. This can be explained as follows: take the desirable construction 0.2+(5+10)x0.245 mm CC for example, there is a small gap "g" between the adjacent filaments as illustrated in Figure 1a. However, during the process of constitution, at a time each small gap comes together to form a big gap "G" as shown in Figure 1 b, and "G" is big enough to allow one filament of intermediate layer in, which leads to a construction of 0.2+(4+1 1)x0.245 mm CC construction shown as Figure 1 c. Because of the unstable construction, the rubber penetration is affected a lot. The eleven outer layer filaments become so close that it is not allowed the desirable rubber to penetrate into the inside, which results in moisture traveling along hollow spaces inside and along the steel cord. Thus, the resistance against corrosion is not quite good. Such quality defects of the steel cord which are caused by the construction itself can not be avoided by adjusting the technological parameters during the manufacture process.

Disclosure of Invention

[0004] The invention aims at avoiding the disadvantages of the prior art.

[0005] It is an object of the present invention to provide for a stable steel cord

construction with an acceptable level of fatigue resistance. [0006] It is a further object of the present invention to provide for a steel cord with good corrosion resistance.

[0007] According to the present invention there is provided a steel cord adapted for the reinforcement of elastomer products. The steel cord comprises a core filament with a core filament diameter Do and 3N layer filaments twisted around the core filament in the same direction and with the same step, N ranges from four to five. The 3N layer filaments are arranged in an intermediate layer of N filaments having an intermediate layer diameter Di and in an outer layer of 2N filaments having an outer layer diameter D2, and the steel cord has the following characteristics (Do, Di , D2 in mm ) :

Do≤Di<D ₂

[0008] The invention cord is a compact cord. The cord has a good corrosion

resistance, a high breaking load and a small diameter.

[0009] N is preferably equal to five, which means that the steel cord comprises sixteen filaments. Five is preferred to four since five filaments in the intermediate layer fill more the intermediate layer in a stable way.

[0010] In a first preferable embodiment of the invention, the core filament and the intermediate layer filaments have a same diameter (Do=Di). This first preferable embodiment may further satisfy following formula:

0.76 < ( Do/ D ₂ )< 0.93.

Due to optimization of the ratio of the diameters Do/ D2, optimum

penetration of the rubber through the intermediate layer and the outer layer and as far as the centre, which ensures very high protection against corrosion and against the possible propagation thereof.

Besides, by selecting such a range of the ratio of the diameters Do/ D2, it is technically practicable to constitute a stable construction (1+5+10) CC thereby avoiding undesirable or unstable configurations.

The first embodiment further has the advantage that only two different filament diameters are required. [0011] In a second preferable embodiment of the invention, the core filament diameter Do is smaller than the intermediate layer diameter Di. This second preferable embodiment may further satisfy following formula:

0.69 < ( Do/ Di )< 0.81 ;

0.59 < ( Do/ D ₂ )< 0.73.

Due to optimization of not only the ratio of the diameters Do/ Di but also the ratio of the diameters Do/ D2, it is always technically desirable since it results in constituting a stable configuration with good corrosion resistance.

Brief Description of Figures in the Drawings

[0012] The invention will now be described into more detail with reference to the accompanying drawings:

[0013] Figure 1 a andl b andl c are different cross-sections of the steel cord during the constitution in the prior art;

[0014] Figure 2 is a cross-section of a first embodiment of a steel cord according to the invention;

[0015] Figure 3 is a cross-section of a second embodiment of a steel cord

according to the invention.

Mode(s) for Carrying Out the Invention

[0016] Figure 2 shows a cross-section of a first embodiment of a steel cord 10.

The steel cord comprises a core filament 12 with a core filament diameter Do. Five intermediate layer filaments 14 having an intermediate layer diameter Di form an intermediate layer around core filament 12. The diameter Di is equal to the diameter Do. Ten outer layer filaments 16 form an outer layer having an outer layer diameter D2 around the intermediate layer. All intermediate layer filaments 14 and all outer layer filaments 16 are twisted in the same twisting direction and with the same twisting step so that a compact configuration is formed. [0017] The core filament diameter Do and the outer layer diameter D2 further satisfy the following relationship:

0.76 < ( Do/ D ₂ )< 0.93.

[0018] The steel cord 10 according to the invention was built as follows. Starting product is a steel wire rod. This steel wire rod has following steel composition: a minimum carbon content of 0.65% (e.g. at least 0.80%, or at least 0.92% with a maximum of 1.2%); a manganese content ranging from 0.20% to 0.90%; a silicon content ranging from 0.15% to 0.90%; a maximum sulphur content of 0.03%; a maximum phosphorus content of 0.30%; additional elements such as chromium (up to 0.2 to 0.4%) , boron, copper, cobalt, nickel, vanadium ... may be added to the composition in order to minimize the amount of deformation needed to obtain a predetermined tensile strength; all percentages being percentages by weight.

[0019] The steel rod is drawn in a number of consecutive steps until the required final diameter. The final diameters of the steel filaments range from 0.05 mm to 0.60 mm. In case of reinforcement of carcass plies of tires, the filament diameters preferably range from 0.05 to 0.35 mm.

[0020] The drawing steps may be interrupted by one or more heat treatment steps such as patenting.

[0021] The steel filaments are preferably provided with a coating which promotes the adhesion to rubber or with a coating which gives corrosion resistance to the wire. A rubber adherable coating is e.g. brass; a corrosion resistant coating is e.g. zinc.

[0022] Suitable examples of a cord according to the first embodiment of the

invention are:

0.18+5x0.18+10x0.215 CC 0.19+5x0.19+10x0.23 CC

0.21 +5x0.21 +10x0.25 CC

[0023] Figure 3 shows a cross-section of a second embodiment of the steel cord 20 according to the invention. The steel cord 20 comprises a core filament 22 with a core filament diameter Do. Five intermediate layer filaments 24 with an intermediate layer diameter Di have been twisted around the core filament 22. An outer layer of outer layer filaments 26 having an outer layer diameter D2 is formed around the intermediate layer.

[0024] The core filament Do is smaller than the intermediate layer diameter Di and the diameters Dos DK D2 satisfy the following relationships:

0.69 < ( Do/ Di )< 0.81 ;

0.59 < ( Do/ D ₂ )< 0.73.

In contrast with Figure 2, the ratio of the diameters Do/ D2 is smaller.

Due to optimization of the ratios of the diameters Do/ Di and Do/ D2, a stable construction (1+5+10) CC is obtained thereby avoiding

undesirable constructions.

[0025] Depending upon the steel composition used and upon the degree of final drawing, the steel cord can be carried out in various tensile versions, ranging from a normal tensile strength to an ultra high tensile strength.

[0026] The steel cords according to the invention can be manufactured by means of a tubular twisting machine or preferably by means of a double-twisting device.

[0027] Both embodiments may be provided or not with a wrapping filament

wrapped around the other filaments. However, for use in carcass plies, the non wrapped version is preferred. [0028] A suitable example of a cord according to the second embodiment of the invention is

0.175+5x0.23+10x0.25 CC twisting step = 16 mm twisting direction = Z

[0029] Other examples are:

0.17+5x0.22+10x0.24 CC

0.175+5x0.225+10x0.245 CC

0.18+5x0.23+10x0.255 CC

[0030] Table 1 hereunder summarizes some properties measured on the steel cord according to the second embodiment of the present invention.

0.175+5 ^* 0.23+10 ^* 0.25 CC ST

Dimensions Max. Min.

Diameter cord mm 1.143 1.136

Linear density g/m 5.7728 5.7721

Lay length mm 16.18 16.08

PLE % 0.05 0.046

Tensile test expanded

Weight per length g/m 5.7723 5.7723

Breaking load N 2491 2484

Tensile strength MPa 3392 3383

E-modulus MPa 204550 185247

Total elongation at fracture % 3.1 1 2.74

Three point bending test

not embedded

E _k MPa 634 61 1

Ev MPa 1879 1659

Knee value M _v Nmm 1.22 0.78

A % 20 14 Three point bending test

embedded

E _k MPa 1092 1040

Ev MPa 5168 4485

Knee value M _v Nmm 7.1 6.8

A % 40 38

Compression test

Rk MPa 277 259

Wk % 1.12 0.84

E compression modulus MPa 83066 76568

Rubber penetration test

Pressure drop % 20 0

Sections with porosity 5 0

Impact

Pm N 980 888

E _m J/mm ² 6.25 5.52

Fatigue Test ( Rest Verformung )

Curvature 1/m 0.05 0.03846

Number of cycles before 16728 80453

fracture A high resistance against corrosion is obtained by this compact steel cord structures which allow elastomer or rubber to penetrate inside the steel cord between the individual steel filaments at least in the outer layer, so that moisture is prevented from traveling along hollow spaces inside and along the steel cord. This compact character of the steel cord, however, by seriously selecting the range of the ratio of the diameters, since it increases not only the compactness and the amount of steel cross-section per total cross-section, but also avoids constituting undesirable or unstable configurations because of the suitable gap between the adjacent filaments, thus making it possible to achieve a structure with improved resistance against corrosion. Most of the above-cited tests are described more in detail in the paper by Bourgois Luc, "Survey of Mechanical Properties of Steel Cord and Related Test Methods", Tire Reinforcement and Tire Performance, ASTM STP 694, R.A. Fleming and D.I. Livingston, Eds., American Society for Testing and Materials, 1979, pp.19-46.