Description

Technical Field

[1 ] The invention relates to a steel cord with a flat cross section used for

reinforcing rubber product.

Background Art

[2] A steel cord with a flat cross section comprising a core and an outer layer has an advantage on stiffness than the steel cord with the round cross section. There will be different stiffness in the different direction of the flat steel cord, and this feature will lead to a good handling, riding comfort or driving stability when such flat steel cord is used for reinforcing rubber tyre. In a direction perpendicular to the major axis of the flat steel cord, i.e.

perpendicular to the road surface, there is a decreased stiffness and an increased flexibility so that any obstacles such as cobble stone on the road can be taken easily. In the direction along the major axis of the flat steel cord, i.e. parallel to the road surface, there is an increased stiffness and decreased flexibility leading to an improved stability when taking a bend. Additionally, the rubber ply reinforced by the steel cord with a flat cross-section will be thinner than that reinforced by the steel cord with a round cross section for the same volume or weight of steel cord. Therefore, rubber can be saved, weight of the tyre can be reduced, and gas

consumption can be improved.

[3] Prior art EP0264145 discloses a steel cord with an elongated cross

section comprising a flat wire as the core. It is a drawback that the flat core wire will have a mechanical property loss, e.g. loss of fatigue resistance and less tensile strength, during the manufacturing. To limit the

mechanical property loss due to the stress-concentration at the corners of the flat core wire, another prior art US2012/0177940A1 discloses a steel cord with an elongated cross section comprising core wires with oval cross section. But the oval core wire needs profiled drawing dies, which is costly.

[4] Prior art JP09-268485A provides another way for producing a steel cord with oval cross section, wherein round steel wires are arranged along an ellipse. The drawbacks are that the ply reinforced by such steel cord is still very thick, the manufacturing process of this steel cord is not very easy to operate, and the producing cost is very high.

Disclosure of Invention

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

[6] It is also an object of present invention to provide a steel cord with

different stiffness in different direction.

[7] It is a further object of present invention to provide a simple process to

manufacture a steel cord with different stiffness in different direction.

[8] It is a further object of present invention to provide a steel cord with

improved ability on corrosion prevention.

[9] According to one aspect of present invention, a steel cord for reinforcing rubber products has a flat cross section with a structure of m+n, wherein n outer layer wires are twisted around m core wires, and at least one of the m core wires is a zinc wire. The flat cross section of the steel cord has a major axis and a minor axis, and the ratio R between the width of the steel cord along the major axis and the height of the steel cord along the minor axis may range from 1.05 to 2.50, and preferably from 1.10 to 1.80. The width of the steel cord along the major axis may range from 0.2mm to 2.5mm, and preferably from 0.3 to 1.5mm. The m may range from 1 to 6, while the n may range from 3 to 15. The at least one zinc core wire may be deformed in elongated or flat cross section.

[10] According to another aspect of present invention, the method of

manufacturing a steel cord with flat cross section, comprises the step (a) twisting m core wires, including at least one zinc core wire, and n outer layer wires together into a steel cord with approximate round cross section, and (b) flattening the steel cord with approximate round cross section into flat cross section by pressing or rolling.

Brief Description of Figures in the Drawings

[1 1 ] Figure 1 schematically illustrates a magnified cross sectional view of a

steel cord with flat cross section and at least one zinc core wire.

[12] Figure 2 schematically illustrates a device to make a steel cord with

approximate round cross section comprising at least one zinc core wire. Mode(s) for Carrying Out the Invention

[13] Figure 1 schematically illustrates a magnified cross sectional view of a steel cord with flat cross section and at least one zinc core wire. The steel cord 10 has a flat cross section with a structure of 1 +6, where in six outer layer wires 14 are twisted around one core wire 12, and the one core wire 12 is a zinc wire. The zinc wire 12 may be made of pure zinc or zinc alloy. The zinc alloy may be binary alloy compositions, for example, Zn-Ni, Zn- Fe, Zn-Mn, Zn-AI, and Zn-Mg. The zinc alloy may also be ternary alloy compositions, for example, Zn-Fe-AI, and Zn-AI-Mg.

[14] The flat cross section of steel cord 10 has a major axis and minor axis, and the ratio R between the width W of the steel cord along the major axis and the height H of the steel cord along the minor axis many range from 1 .05 to 2.50, preferably from 1 .10 to 1 .80. Here the major axis means the longest line between the surfaces of outer layer wires, while the minor axis is perpendicular to the major axis.

[15] The manufacturing process for a steel cord 10 with flat cross section, comprises following steps. Firstly twisting one zinc core wire 12, and six outer layer wires 14 together into a steel cord with approximate round cross section, and then flattening the steel cord with approximate round cross section into flat cross section by pressing or rolling. Since the steel cord is flattened by pressing or rolling and zinc is comparatively soft material to deform under pressure, the zinc core wire 12 is deformed in elongated or flat cross section. Comparatively, rolling is a more favorable process than pressing to flattening the steel cord. In the rolling process, the rollers rotate in the same direction and at the same speed of movement of the steel cord. There is no slippage between the rollers and the steel cord and the rolling process is continuous. On the contrary, in the pressing process, either there is a slippage between the tools and the steel cord to cause damages on the surface of the steel cord or the steel cord can be fed in step movement with a complex process.

[16] Figure 2 schematically illustrates a conventional way of manufacturing a steel cord with approximate round cross section comprising at least one zinc core wire. The 1 +6 steel cord 10 has one zinc core wire 12 and six outer layer wires 14. The manufacturing is carried out by means of a well- known tubular stranding or twisting machine 18, sometimes referred to as a "cabling machine". The core wire 12 is unwound from a supply spool 16. The core wire 12 is passed to the tubular stranding machine 18 where the core wire 12 is guided on the cylindrical surface (guiding elements not shown). The cylindrical main body of the tubular stranding machine 18 comprises six stationary supply spools 20 where the outer layer wires 14 are unwound. These outer layer wires 14 are also guided via the cylindrical surface of the stranding machine 14. Thereafter the outer layer wires 14 are laid around the core wire 12 at the cord forming die 24 in order to form the 1 +6 steel cord 10. Optionally, before the forming die 24, the outer layer wires 14 can be plastically deformed by a pre-forming device 22, and the pre-forming device 22 can plastically deform the outer layer wires 14 by bending. The plastically deformed outer layer wires 14 can make spaces between the outer layers wires 14 of the cord 10 to allow rubber penetration. As is known in the art, a tubular stranding machine 18 does not subject the individual wires 12, 14 to a rotation around their own axes, which is suitable for core wire 14 made of zinc or zinc alloy.

[17] A steel cord according to present invention may have following

advantages. First, a zinc core wire is soft to deform under pressure, and it is easier to flatten the steel cord comprising a zinc core wire by pressing or rolling. Therefore, the manufacturing process is very easier and requires less force. Secondly, a zinc core wire is soft to deform under pressure, outer layer wires may impress on the surface of the zinc core wire, and the impress may help the steel cord to maintain the flat geometry after pressing or rolling. Therefore, the profile of the flat steel cord is more stable. Thirdly, zinc is a metal higher in the electro-potential series than steel, and zinc core wire can provide cathodic protection to other steel wires, e.g. zinc core wire scarifies itself and improves the corrosion resistance of other steel wires in the steel cord.

[18] A comparison test between present invention and prior art confirms above advantages. Cord diameter (mm) / Cord breaking load Lifetime

flat cord width x height (Newton)

(mm)

Prior art 0.30+6x0.26 0.82 1240 1

HT

Present invention 1.25x0.58 985 19.6

0.30(Zinc)+6x0.26 HT

Present invention 1.25x0.58 1050 21.5

0.30(Zinc)+6x0.26 ST

Present invention 1.25x0.58 1 130 20.2

0.30(Zinc)+6x0.26 UT

[19] In the table, HT means high tensile steel wire, wherein the tensile strength (MPa) of the steel wire is at least TS=3800-2000xd and d is the wire diameter in mm. ST means super tensile steel wire, wherein the tensile strength (MPa) of the steel wire is at least TS=4100-2000xd and d is the wire diameter in mm. UT means ultra-tensile steel wire, wherein the tensile strength (MPa) of the steel wire is at least TS=4400-2000xd and d is the wire diameter in mm. Prior art 0.30+6x0.26 HT is a steel cord comprising one steel core wire with diameter of 0.30mm and six steel outer layer wire with diameter of 0.26mm, wherein the wires are high tensile steel wire. Present invention 0.30(Zinc)+6x0.26 is a flattened steel cord comprising one zinc core wire with diameter of 0.30mm and six steel outer layer wires with diameter of 0.26mm, wherein the outer layer steel wires can be high tensile steel wire, super tensile steel wire, or ultra-tensile steel wire.

[20] Prior art 0.30+6x0.26 HT cord has a round cross-section with a diameter of 0.82mm, while the flattened present invention cord 0.30(zinc)+6x0.26 has a flat cross section with width 1.25mm and height 0.58mm.

Comparatively the height of the flattened present invention cord is 30% lower than the diameter of the prior art cord, and accordingly the height of belt ply reinforced by the flat present invention cord is lower than that of prior art cord. Therefore, rubber can be saved and weight of tire can be reduced because of the thinner belt ply.

[21] Because present invention replaces the steel core wire with zinc core wire.

The breaking load of present invention is lower than that of comparable prior art cord. In the comparison test, the breaking load of present invention 0.30(zinc)+6x0.26 HT cord is 985 newton, while the breaking load of prior art 0.30+6x0.26 HT cord is 1240 newton. To compensate the loss on breaking load of the cord, present invention cord can use ST or UT steel wire. Accordingly, the breaking load of present invention

0.30(zinc)+6x0.26 ST cord is 1050 newton and the breaking load of present invention 0.30(zinc)+6x0.26 UT cord is 1 130 newton.

[22] Besides, present invention substantially improves the corrosion resistance ability. The lifetime of corrosion test on present invention cord is about twenty times that of prior art cord. The zinc core wire scarifies itself and drastically improves the corrosion resistance of other steel wires in the steel cord.

[23] A typical steel composition of the steel wire for rubber reinforcement 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 composition for high-tensile steel cord has a minimum carbon content of around 0.80 weight %, e.g. 0.78 - 0.82 weight %.

[24] The steel wires are manufactured according to following steps from a steel rod with above composition. The wire rod is firstly cleaned by mechanical descaling and / or by chemical pickling in a H ₂ SO ₄ 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.

[25] At this first intermediate diameter di, 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 austenizing 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.

[26] Thereafter the steel wire is further dry drawn from the first intermediate diameter di until a second intermediate diameter d ₂ in a second number of diameter reduction steps. The second diameter 02 typically ranges from 1 .0 mm to 2.5 mm.

At this second intermediate diameter 02, the steel wire is subjected to a second patenting treatment, i.e. austenizing 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.

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 02.

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.

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 wire with 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.

Steel wires adapted for rubber reinforcement typically have a final diameter ranging from 0.05 mm to 0.60 mm, e.g. from 0.10 mm to 0.40 mm. Examples of wire 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.

Besides the above example, the possible m+n construction can be.

n=3 n=4 n=5 n=6 n=7 n=8 n=9 m=1 1 +3 1 +4 1 +5 1 +6 m=2 2+5 2+6 2+7 2+8 2+9 m=3 3+6 3+7 3+8 3+9 m=4 4+8 4+9 When m=1 , the steel cord has one zinc core wire. When m=2, the steel cord has one or two zinc core wire. When m=3, the steel cord has one, two or three zinc core wire. ...