Description

Technical Field

[1 ] The invention relates to a steel cord for reinforcing pneumatic tires, this invention also relates to use of the steel cord for carcass reinforcement of pneumatic tires.

Background Art

[2] Steel cord for pneumatic tire reinforcement is known, and multi-layer steel cord for carcass reinforcement of pneumatic tires is also known.

Prior art US5595057A discloses a 3+9+15 cord for carcass reinforcement. In this cord, because the filaments around the core are pre-formed according to a forming ratio, the structure of the cord is maintained without a wrap wire. Therefore, the fretting wear between the filaments in the outer layer and the wrap wire is avoided. But because the twisting direction of intermediate layer is opposite to the twisting direction of the outer layer, fretting wear from the point contact between the filaments in the

intermediate layer and the filaments in the outer layer remains.

Prior art US5318643A discloses a 27 CC compact cord, wherein the 27 filaments are twisted in the same direction with the same twisting pitch. Because all the filaments are twisted in the same direction with the same twisting pitch, filaments maintain line contact with adjacent filaments, and the fretting between filaments is limited. But this compact structure has inherent drawback on rubber penetration, because the line contact between filaments seals the routes for rubber to permeate through the cord.

Prior art JP59223503A discloses a 4+9+14 cord, wherein the intermediate layer and the outer layer of the cord are unsaturated. There are gaps between the filaments of the intermediate layer and the filaments of the outer layer for rubber penetration. But the breaking load of the steel cord needs further improvement. Disclosure of Invention

[3] It is an object of the present invention to overcome the drawbacks of prior arts.

[4] It is also an object of the present invention to provide a multi-layer steel cord suitable for carcass reinforcement in a pneumatic tire.

[5] It is yet another object of the present invention to provide a multi-layer steel cord, which not only achieves a good balance amongst tensile strength, rubber penetration, and fatigue resistance but also is cost- effective to manufacture and use.

According to a first aspect of the invention, a steel cord for reinforcing pneumatic tires is a multi-layer steel cord comprising a core, an

intermediate lay and an outer lay. The core has four filaments and preferably consists of four steel filaments, the intermediate lay has nine filaments and preferably consists of nine steel filaments, and the outer lay has fourteen filaments and preferably consists of fourteen steel filaments. There is preferably no wrapping wire. At least one filament in the intermediate lay and the outer lay of the steel cord has a tensile strength no less than (3800-2000xd) MPa where d is the diameter of the filament expressed in mm. The other filaments in the intermediate lay and the outer lay may have a tensile strength that is lower.

Preferably, all the filaments in the intermediate lay and outer lay of the steel cord have a tensile strength no less than (3800-2000xd) MPa where d is the diameter of the respective filaments expressed in mm. Here the tensile strength of a filament in the intermediate lay and the outer lay means the tensile strength of the filaments after twisting into a steel cord: to measure, firstly release the filament from the steel cord and secondly make a tensile test according to ISO6892-1 :2009. To achieve the tensile strength no less than (3800-2000d) MPa, the original tensile strength of the filament to be stranded into the steel cord should be no less than (4100-2000d) MPa. The super tensile filaments give the steel cord the basis for a high breaking load, meanwhile the 4+9+14 structure provides gaps between filaments for rubber penetration. The filaments of the core may have the same or a different tensile strength level.

[7] The twist direction of the intermediate lay may be the same as the twist direction of the outer lay, and preferably the twist direction of the core may be the same as the twist direction of the intermediate lay. The same twist direction provides more line contact between filaments in different lays, in contrast to point contacts. Line contacts limit the fretting wear.

[8] All the filaments may have preferably the same filament diameter, and the filament diameter may range between 0.10mm and 0.40mm, and preferably between 0.15mm and 0.35mm. The same filament diameter simplifies the preparation of filaments for this steel cord. Since the 4+9+14 structure already provides gaps for rubber penetration, it is not necessary to enlarge the filament diameter for core.

[9] The ratio between the lay length of intermediate lay and the lay length of the outer lay may preferably range between 0.70 and 0.90, and most preferably between 0.75 and 0.85. The higher ratio between the lay length further extends the line contact between filaments in different lays, and further limits the fretting wear. Besides, the higher ratio also limits the tensile strength loss due to the twisting of the cord. This advantage is even higher with the high tensile strength levels applied in this invention, since the twisting loss tends to be higher with higher tensile strengths.

[10] As disclosed in US5595057, a forming ratio is defined as a ratio of

amplitude H1 of wave formed in the filament to ideal diameter D1 of the lay. The forming ratio of the filaments in the intermediate lay and that of filaments in the outer lay may range between 0.75 and 0.95, and preferably between 0.85 and 0.95. This applied forming ratio of the filaments maintains the structure of the steel cord without a wrap wire.

The steel cord may have no flare. Flare means the spreading of the filament ends or the strand ends at the cut end of the cord, expressed as the unravelled length in millimetres. No flare means the filament ends at the cut end of cord do not spread.

[1 1 ] At least one of the filaments may show a twist around its own axis in

addition to the twist around the central axis of the steel cord, and preferably all the filaments may show a twist round its own axis in addition to the twist around the central axis of the steel cord. The cord can be made on a double twisting machine, wherein the filament has a twist around its own axis in addition to the twist around the central axis of the steel cord. The double twisting machine provides the most cost-effective way to manufacture a multi-layer cord.

[12] With the combination of above, the multi-layer steel cord, not only

achieves a good balance between breaking load and rubber penetration, but also is cost-effective to manufacture and use.

[13] According to another aspect of present invention, use of a steel cord for carcass reinforcement of pneumatic tires, and the steel cord is a multilayer steel cord comprising a core, an intermediate lay and an outer lay. The core has four filaments, the intermediate lay has nine filaments, and the outer lay has fourteen filaments. At least one filament of the

intermediate lay and outer lay of the steel cord has a tensile strength no less than (3800-2000xd) MPa where d is the diameter of the filament expressed in mm.

Brief Description of Figures in the Drawings

[14] Figure 1 schematically illustrates a cross-sectional view of a steel cord incorporating present invention.

[15] Figures 2A, 2B and 2C schematically illustrate a device and process to manufacture a steel cord incorporating present invention.

[16] Figures 3A, 3B and 3C schematically illustrate another device and process to manufacture a steel cord incorporating present invention. Mode(s) for Carrying Out the Invention

[17] A typical steel tire cord composition has a minimum carbon content of 0.65%, a manganese content ranging from 0.40% to 0.70%, a silicon content ranging from 0.15% to 0.30%, a maximum sulphur content of 0.03%, a maximum phosphorus content of 0.30%, all percentages being percentages by weight. There are only traces of copper, nickel and / or chromium. A typical steel tire cord composition for high-tensile steel cord has a minimum carbon content of between 0.80 and 0.85 weight %. To further improve the tensile strength of the steel filaments, the minimum carbon content may range between 0.85 and 0.90 weight %, or even between 0.90 and 0.95 weight %. Besides, other alloy ingradients may be added, for example Cr.

[18] The process to manufacture steel filaments for a steel cord always starts with a wire rod with above steel composition. The wire rod is firstly cleaned by mechanical descaling and / or by chemical pickling in a H2SO4 or HCI solution in order to remove the oxides present on the surface. The wire rod is then rinsed in water and is dried. The dried wire rod is then subjected to a first series of dry drawing operations in order to reduce the diameter until a first intermediate diameter.

[19] At this first intermediate diameter d1 , e.g. at about 3.0 to 3.5 mm, the dry drawn steel wire is subjected to a first intermediate heat treatment, called patenting. Patenting means first austenitizing until a temperature of about 1000 °C followed by a transformation phase from austenite to pearlite at a temperature of about 600 - 650 °C. The steel wire is then ready for further mechanical deformation.

[20] Thereafter the steel wire is further dry drawn from the first intermediate diameter d1 until a second intermediate diameter d2 in a second number of diameter reduction steps. The second diameter d2 typically ranges from 1 .0 mm to 2.5 mm.

[21 ] At this second intermediate diameter d2, the steel wire is subjected to a second patenting treatment, i.e. austenitizing again at a temperature of about 1000 °C and thereafter quenching at a temperature of 600 to 650 °C to allow for transformation to pearlite. [22] If the total reduction in the first and 2nd dry drawing step is not too big a direct drawing operation can be done from wire rod till diameter d2.

[23] After this second patenting treatment the steel wire is usually provided with a brass coating: copper is plated on the steel wire and zinc is plated on the copper. A thermo-diffusion treatment is applied to form the brass coating.

[24] The brass-coated steel wire is then subjected to a final series of cross- section reductions by means of wet drawing machines. The final product is a steel filament with a carbon content above 0.60 per cent by weight, with a tensile strength typically above 2000 MPa and adapted for the reinforcement of elastomer products.

[25] Besides, the brass-coating may contain other ingrediants, for example Co, to form a ternary alloy coating comprising Cu, Zn and Co to further improve the adhesion between the steel filaments and the polymer matrix when the steel cord is embeded into a polymer matrix.

[26] Steel filaments adapted for the reinforcement of tyres typically have

filaments with a final diameter ranging from 0.05 mm to 0.60 mm, e.g. from 0.10 mm to 0.40 mm. Examples of filament diameters are 0.10 mm, 0.12 mm, 0.15 mm, 0.175 mm, 0.18 mm, 0.20 mm, 0.22 mm, 0.245 mm, 0.28 mm, 0.30 mm, 0.32 mm, 0.35 mm, 0.38 mm, 0.40 mm.

[27] Figure 1 schematically illustrates a cross-sectional view of a steel cord incorporating present invention. The steel cord 10 is a multi-layer steel cord comprising a core, an intermediate layer and an outer layer. The core has four core filaments 12, the intermediate layer has nine intermediate filaments 14, and the outer layer has fourteen outer filaments 16. There are gaps between the intermediate filaments 14 and outer filaments 16 for rubber penetration.

[28] One of the embodiments of present invention is a 4+9+14x0.20 ST cord with following specifications. The filament diameter is 0.20 mm with an original tensile strength no less than 3700 Mpa. All the filaments are twisted in the same direction. The four core filaments are twisted with a lay length 6mm. The nine intermediate lay filaments are twisted with a lay length 12mm, and the fourteen outer lay filaments are twisted with a lay length 15mm.

[29] Figures 2A, 2B and 2C schematically illustrate a device and process to manufacture a steel cord incorporating present invention.

Figure 2A shows a cabling machine 18 to make the 4x1 core strand 20, wherein four spools of core filaments 12 are installed inside the drum 22. The four core filaments 12 are led through the surface of the drum 22, to the cord forming point 24. When the drum 22 rotates, the four core filaments 12 are twisted together at the cord forming point 24.

[30] Figure 2B shows a cabling machine 18 to make the 4+9 strand 26,

wherein nine spools of intermediate filaments 14 are installed inside the drum 22 and the 4x1 core strand 20 is installed outside the drum 22. The nine intermediate filaments 14 are led through the surface of drum 22, to the cord forming point 24. The 4x1 core strand 20 is led through the surface of the drum 22, to the cord forming point 24. At the cord forming point 24, the 4x1 core strand 20 is located in the centre and the nine intermediate filaments 14 are surrounding the 4x1 core strand 20. When the drum 22 rotates, the nine intermediate filaments 14 are twisted around the 4x1 cord strand 20 at the cord forming point 24 to form the 4+9 strand 26.

[31 ] Figure 2C shows a cabling machine 18 to make the 4+9+14 steel cord 10, wherein fourteen spools of outer lay filaments 16 are installed inside the drum 22 and the 4+9 strand 26 is installed outside the drum 22. The fourteen outer lay filaments 16 are led through the surface of drum 22 to the cord forming point 24. The 4+9 strand 26 is led through the surface of the drum 22 to the cord forming point 24. At the cord forming point 24, the 4+9 strand 26 is located in the centre and the fourteen outer lay filaments 16 are surrounding the 4+9 strand 26. When the drum 22 rotates, the fourteen outer lay filaments 16 are twisted around the 4+9 strand 26 at the cord forming point 24 to form the 4+9+14 steel cord 10.

[32] Figures 3A, 3B and 3C schematically illustrate another device and process to manufacture a steel cord incorporating present invention. Figure 3A shows a double twist machine 28 to make the 4x1 core strand 20, wherein the four core filaments 12 are located outside the machine and are led through the cord forming point 24, the first twist point 30, the flyer 32, the second twist point 34, and to the spool 36 winding the 4x1 core strand 20. When the flyer 32 rotates, the four core filaments 12 firstly receive the first twist at the first twist point 30, and secondly receive the second twist at the second twist point 34. Therefore, the 4x1 core strand 20 receives two twists when the flyer 32 rotates once.

[33] Figure 3B shows a double twist machine 28 to make the 4+9 strand 26, wherein the 4x1 core strand 20 and the nine intermediate filaments 14 are located outside the machine and are led through the cord forming point 24, the first twist point 30, the flyer 32, the second twist point 34, and to the spool 38 winding the 4+9 strand 26. At the cord forming point 24, the 4x1 core strand 20 is located in the centre and the nine intermediate filaments 14 are surrounding the 4x1 core strand 20. When the flyer 32 rotates, the 4x1 core strand 20 and the nine intermediate filaments 14 are firstly twisted together at the first twist point 30, and secondly receive the second twist at the second twist point 34. Therefore, the 4+9 strand 26 receives two twists when the flyer 32 rotates once. Besides, since the 4x1 core strand 20 receives the same twist as the intermediate filament 14 in Figure 3B, the lay length of 4x1 core strand 20 for Figure 3B can be longer than that for Figure 2B.

[34] Figure 3C shows a double twist machine 40 to make the 4+9+14 cord, wherein the 4+9 strand 26 is located outside the machine while the fourteen spools of outer filaments 16 are located inside the machine. The 4+9 strand 26 is led through the first twist point 42, the first flyer 44, the second twist point 46, to get together with the fourteen outer filaments 16 at the cord forming point 24. The 4+9 strand 26 and the fourteen outer filaments 16 are stranded together at the third twist point 47, and are further led through the second flyer 48 and the second twist point 49 to the spool 50 winding the finished 4+9+14 steel cord 10. When the first flyer 44 rotates, the 4+9 strand 26 receives the first twist at the first twist point 42 and a second twist at the second twist point 46. When the second flyer 48 rotates, the 4+9 strand 26 and the fourteen outer filaments 16 receive the first twist at the third twist point 47 and the second twist at the fourth twist point 49. Since the first flyer 44 and the second flyer 48 form a loop, both the first flyer 44 and the second flyer 48 rotate at the same direction. Since the 4+9 strand 26 on the first flyer 44 runs in the opposite direction against the 4+9 strand 26 on the second flyer 48. The second flyer 48 gives reverse twist on the 4+9 strand 26. i.e. the first flyer 44 gives two twists to the 4+9 strand 26 while the second flyer 48 gives two reverse twists to the 4+9 strand 26. Therefore, the 4+9 strand 26 maintains the lay length from Figure 3B in the final product 4+9+14, and the fourteen outer filaments are twisted around the 4+9 strand 26.

Below table shows the output difference in relative data between figures 2 process and figures 3 process, wherein the output of figures 3 process is twice that of figures 2 process. The increase of the ratio between lay length of intermediate lay and lay length of outer lay may further increase the outputs, because the increase on 4x1 strand and 4+9 strand may out- weight the decrease on 4+9+14 cord.

[36] Besides, since the drum of figure 2C process is heavier than the flyer of figure 3C process, the energy consumption of figures 3 process is also less than that of figures 2 process.

[37] Below table confirms that present invention achieves a good balance

between tensile strength and rubber penetration. The same twist direction between the intermediate lay and outer lay can improve the breaking load from 2500N to 2700N, while the increase of the ratio between the lay length of intermediate lay and the lay length of outer lay can further improve the breaking load from 2700N to 3000N.

Prior art Present invention Present invention preferable embodiment

Cord structure 4+9+14x0.20 ST 4+9+14x0.20 ST 4+9+14x0.20 ST

Twist direction S/S/Z or Z/Z/S S/S/S or Z/Z/Z S/S/S or Z/Z/Z

Lay length 5/10/16 5/10/16 6/12/15

Breaking load >=2500N >=2700N >=3000N

Rubber good good Good

penetration