Steel cord having four steel filaments for rubber reinforcement

Abstract

The invention is relating to a steel cord comprising four steel filaments with a filament diameter d expressed in mm, and the steel cord has a cord lay length LL in the range of 10-40mm, the four steel filaments of the steel cord have an average tensile strength TS expressed in MPa, 4100-2000xd≤TS≤4900-2000xd, the steel cord has a pre-determined total strain. The invention provides a solution to reduce the problem of steel filament fracture, cracks on the surface of steel filament or the loose or flare of steel cord during the production of steel cord.

Description

[0001] Title Steel cord having four steel filaments for rubber reinforcement

[0002] Description

[0003] Technical Field

[0004]

[0001] The invention relates to a steel cord for rubber reinforcement, particular the steel cord having four steel filaments. The invention also relates to a tire reinforced by that steel cord.

[0005] Background Art

[0006] [2] Steel cords are widely used as a reinforcement for rubber products, such as rubber belt, rubber tire, hose etc. As a reinforcement for rubber product, the steel cord is required to have a certain strength, corrosion resistance, fatigue resistance, rubber penetration property, rubber adhesion property and etc.

[0007] [3] A trend in rubber tire field is lighter tire, a lighter tire has advantages such as lower rolling resistance and energy saving for carbon emissions reduction. Steel cords, as the reinforcement of the tire, are developed to be finer and lighter accordingly. One solution is to increase the tensile strength of the steel filaments of the steel cord, and due to the use of steel filaments with higher tensile strength, the finer steel filaments are able to provide sufficient cord breaking load, as thus the diameter and the weight of the steel cord can be reduced. The use of the finer and lighter steel cords makes the tire being thinner and lighter.

[0008] [4] LIS6102095 discloses a steel cord having a single twisted construction obtained by twisting 3 to 6 steel filaments, each steel filament has a tensile strength of 4000-4800 N / mm2for tire weight reduction, the steel filament is processed to have a residual compressive stress inside the twisting helix such that the ratio of curvature Ro of a helix in each helically formed steel filament obtained by untwisting of the cord to a radius of curvature Ri of the helix after the removal of surface layer from the inside of the helix in the steel filament satisfy Ri / Ro<1 , in this case the space between mutually adjacent steel filaments is bigger for penetrating rubber into the inside of the cord.

[0009] [5] US4966216 discloses a steel cord with single layer construction obtained by twisting 3-5 metallic filaments for heavy duty radial tire which attempts a large reduction of tire weight, improves the fracture resistance, corrosion fatigue resistance and fretting resistance at the end of the carcass ply. Such steel cord is obtained by making the filament tensile strength and filament diameter satisfy specific relations. The steel filament has an ultra- high tensile strength of 380 kg / mm2when the filament diameter is 0.25mm.

[0010] [6] The steel filaments with higher tensile strength are beneficial for tire weight reduction. However, the steel filaments with higher tensile strength are difficult to be processed like being twisted to form a steel cord, since the higher tensile strength leads to more filament fracture problem during processing for making a steel cord.

[0011] Disclosure of Invention

[0012] [7] The primary object of the invention is to solve the problem of fracture of steel filaments with higher tensile strength during the making of steel cord.

[0013] [8] The first object of the invention is to provide a steel cord comprising ultra- high tensile strength steel filaments, while the filament fracture problem during the steel cord production is reduced.

[0014] [9] The second object of the invention is to provide a tire reinforced by steel cords comprising steel filaments with ultra-high tensile strength.

[0015]

[0010] According a first aspect of the invention a steel cord is provided, the steel cord comprises four steel filaments with a filament diameter d expressed in mm, the steel cord has a cord lay length LL in the range of 10-40mm, the four steel filaments of the steel cord have an average tensile strength TS expressed in MPa, 4100-2000xd<TS<4900-2000xd, wherein when 0.35mm<d<0.55mm, the steel cord has a total strain ToSr in the range of 2.90%-3.90%, or when 0.18mm<d<0.35mm, the steel cord has a total strain ToSr in the range of 3.10%-4.30%, wherein the total strain ToSr is calculated by the formula, ToSr=(TSr2+ SSr2)0 5, wherein,

[0016] TSr=2x(1-cos(arctan(7ixD / LL))), wherein D=2.414xd;

[0017] SSr=TSx7i / 360000.

[0011] According a first aspect of the invention a steel cord is also provided, the steel cord comprises four steel filaments with a filament diameter d expressed in mm, the steel cord has a cord lay length LL in the range of 10-40mm, the four steel filaments of the steel cord have an average tensile strength TS expressed in MPa, 4100-2000xd<TS<4800-2000xd, wherein when 0.35mm<d<0.55mm, the steel cord has a total strain ToSr in the range of 2.90%-3.90%, or when 0.18mm<d<0.35mm, the steel cord has a total strain ToSr in the range of 3.10%-4.30%, wherein the total strain ToSr is calculated by the formula, ToSr=(TSr2+ SSr2)05, wherein,

[0018] - TSr=2x(1-cos(arctan(7ixD / LL))), wherein D=2.414xd;

[0019] - SSr=TSx7i / 360000.

[0020]

[0012] A steel cord is made by twisting multiple steel filaments via applying external force on the steel filaments. For one steel filament, on the one hand, it is self-rotating; on the other hand, it is twining with other steel filaments to form a steel cord. As a result, the steel filament is deformed, and the torsion strain is generated in the steel filament due to steel filament’s self-rotating, and the bending strain is generated in the steel filament due to the twining with other steel filaments. The deformation of the steel filament firstly enters the phase of elastic deformation, and then the deformation of the steel filament enters the phase of plastic deformation in case of sufficient deformation. For “elastic deformation”, once the external force on the steel filament decreases or even disappears, the steel filament will partially or completely return to its original state, and the elastic deformation of the steel filament caused by the external force will also completely or partially disappear. For “plastic deformation”, the deformation of the steel filament is stable, i.e., the plastic deformation of the steel filament caused by the external force will not change even if the external force decreases or disappears. As thus for obtaining a stable steel cord, the strain of steel filament including the torsion strain and the bending strain shall be sufficient to make the deformation of the steel filament entering the phase of plastic deformation. With the increase of tensile strength of steel filament, such as super high tensile, ultra-high tensile or ever mega-high tensile, the steel filament becomes harder due to the use of higher carbon steel wire rod or higher strain hardening of drawing for reaching higher tensile strength, and the ductility of steel filament thereby becomes worse, as a result, in the process of twisting multiple steel filaments into steel cord, the conventional deformation of steel filament, i.e. steel filament’s own rotation and twining of the steel filament with other steel filaments, is not enough to make the steel filament generating sufficient plastic strain for entering the phase of plastic deformation, and then the steel cord will be unstable, for example the steel cord becomes very loose or even flare, and that results in the properties of the steel cord such as breaking load and cord diameter are unstable and even do not meet the requirements. One solution is to reduce the lay length of the steel cord to make the steel filament generating sufficient plastic strain, however, this may lead to other defects such as fracture of steel filament or cracks on the surface of steel filament, due to that the steel filament is harder because of the increase of tensile strength.

[0021]

[0013] The invention provides a solution to more comprehensively balance the various parameter factors such as cord lay length, filament diameter and filament tensile strength which mostly influence the plastic strain generated in steel filament particular the steel filament with higher tensile strength, and the steel filament is able to generate sufficient plastic strain in case of such a construction and parameters to make sure that the problem of steel filament fracture, cracks on the surface of steel filament or the loose or flare of steel cord is reduced during the production of steel cord. The invention introduces a total strain ToSr of steel cord with a predetermined value range according to the cord lay length, steel filament diameter and the average tensile strength of the steel filaments of the steel cord. Specifically, for the steel cord having steel filaments with a filament diameter of more than 0.35mm and less than or equal to 0.55mm, the steel cord has a total strain ToSr in the range of 2.90%-3.90%; for the steel cord having steel filaments with a filament diameter in the range of 0.18-0.35mm, the steel cord has a total strain ToSr in the range of 3.10%- 4.30%. By doing this, the problem of fracture of steel filament, cracks on the surface of steel filament or the loose or flare of steel cord in the production of the steel cord by twisting multiple steel filaments with super- high tensile, ultra-high tensile or ever higher is reduced. Too high ToSr will lead to the problem of fracture of steel filament or cracks on the surface of steel filament, while too low ToSr will lead to the problem of the loose or flare of steel cord.

[0022]

[0014] According to the present invention, the four filaments of the steel cord have the same diameter. The total stain ToSr of steel cord varies depending on the filament diameter.

[0023]

[0015] When 0.35mm<d<0.55mm, preferably the cord lay length LL is in the range of 21 -36mm. In such a case, the steel cord preferably has a total strain ToSr in the range of 2.90%-3.32%.

[0024]

[0016] In one preferable embodiment, when 0.40mm<d<0.55mm, the steel cord has a cord lay length in the range of 22-36mm. In such a case, the steel cord has a total strain ToSr preferably in the range of 2.90%-3.70%, more preferably in the range of 2.90%-3.30%.

[0025]

[0017] In another preferable embodiment, when 0.35mm<d<0.40mm, the steel cord has a cord lay length in the range of 21 -29mm. In such a case, the steel cord has a total strain ToSr preferably in the range of 3.00%-3.90%, preferably in the range of 3.02%-3.32%.

[0026]

[0018] When 0.18mm<d<0.35mm, preferably the cord lay length LL is in the range of 10-28mm. In such a case, the steel cord preferably has a total strain ToSr in the range of 3.25%-3.85%.

[0027]

[0019] In one preferable embodiment, when 0.28mm<d<0.35mm, the steel cord has a cord lay length in the range of 10-28mm. In such a case, the steel cord has a total strain ToSr preferably in the range of 3.10%-4.00%, more preferably in the range of 3.25%-3.60%.

[0028]

[0020] In another preferable embodiment, when 0.18mm<d<0.28mm, the steel cord has a cord lay length in the range of 10-24mm. In such a case, the steel cord has a total strain ToSr preferably in the range of 3.35%-4.30%, preferably in the range of 3.55%-3.85%.

[0021] The tensile strength of the steel filaments of the steel cord can not be too low, otherwise, the effects of reducing tire weight and lowering rolling resistance of tire are not as expected; the tensile strength of the steel filaments of the steel cord can not be too high, otherwise, the steel filaments will be too hard to generate sufficient plastic strain for reducing the problem of fracture of steel filament, cracks on the surface of steel filament or the loose or flare of steel cord. Although the steel filaments are produced by the same process, the tensile strength of the steel filaments may vary. As thus the present invention would use the average tensile strength of the four steel filaments for the determination of the total strain of steel cord. The steel filaments of the steel cord may have the same or substantial same tensile strength, i.e. with a difference no more than 5%; alternatively, the steel filaments of the steel cord may have different tensile strength, i.e. with a difference more than 5%, or even more than 10% but better no more than 20%.

[0029]

[0022] According to the present invention, TS, the average tensile strength of the four steel filaments of the steel cord, 4100-2000xd<TS<4900-2000xd MPa, more preferably 4100-2000xd<TS<4800-2000xd MPa, most preferably, 4150-2000xd<TS<4750-2000xd MPa. More preferably, 4200- 2000xd<TS<4500-2000xd MPa. Alternatively, 4500-2000xd<TS<4800- 2000xd MPa.

[0030]

[0023] The invention is for a steel cord comprising four steel filaments, more accurately, the steel cord comprises a number of steel filaments, the number is four; or in other words, the number of steel filaments of the steel cord is four. The steel cord may have a construction of 1x4 or 2+2. No matter what the construction of the steel cord is, the steel cord has a cord lay length in the range of 10-40mm. The cord lay length can not be too small, otherwise, the problem of steel filament fracture or cracks on the surface of steel filament would be too high.

[0031]

[0024] According a second aspect of the invention, a tire is provided, the tire comprises a tread portion, a carcass comprising at least one carcass ply, a pair of beads, and a belt package located between the carcass and the tread portion, the belt package comprises at least one belt ply, wherein the carcass ply and / or the belt ply comprises at least one steel cord, the steel cord comprises four steel filaments with a filament diameter d expressed in mm, the steel cord has a cord lay length LL in the range of 10-40mm, the four steel filaments of the steel cord have an average tensile strength TS expressed in MPa, 4100-2000xd<TS<4900-2000xd, wherein when 0.35mm<d<0.55mm, the steel cord has a total strain ToSr in the range of 2.90%-3.90%, or when 0.18mm<d<0.35mm, the steel cord has a total strain ToSr in the range of 3.10%-4.30%, wherein the total strain ToSr is calculated by the formula, ToSr=(TSr2+ SSr2)05, wherein,

[0032] - TSr=2x(1-cos(arctan(7ixD / LL))), wherein D=2.414xd;

[0033] - SSr=TSx7i / 360000.

[0034]

[0025] According a second aspect of the invention, a tire is also provided, the tire comprises a tread portion, a carcass comprising at least one carcass ply, a pair of beads, and a belt package located between the carcass and the tread portion, the belt package comprises at least one belt ply, wherein the carcass ply and / or the belt ply comprises at least one steel cord, the steel cord comprises four steel filaments with a filament diameter d expressed in mm, the steel cord has a cord lay length LL in the range of 10-40mm, the four steel filaments of the steel cord have an average tensile strength TS expressed in MPa, 4100-2000xd<TS<4800-2000xd, wherein when 0.35mm<d<0.55mm, the steel cord has a total strain ToSr in the range of 2.90%-3.90%, or when 0.18mm<d<0.35mm, the steel cord has a total strain ToSr in the range of 3.10%-4.30%, wherein the total strain ToSr is calculated by the formula, ToSr=(TSr2+ SSr2)05, wherein,

[0035] - TSr=2x(1-cos(arctan(7ixD / LL))), wherein D=2.414xd;

[0036] - SSr=TSx7i / 360000.

[0037]

[0026] Since the tensile strength of the steel filament is higher, the equivalent cord strength can be achieved with the use of finer steel filaments, and the weight of steel cord can be reduced, and accordingly the weight of the tire can be reduced.

[0038]

[0027] Preferably the belt ply of the tire is reinforced by the invention steel cords.

[0039] Brief Description of Figures in the Drawings

[0040]

[0028] Figure 1 describes an invention cord with a construction of 1x4.

[0041]

[0029] Figure 2 describes an invention cord with a construction of 2+2.

[0042] Mode(s) for Carrying Out the Invention

[0043]

[0030] The steel cord is made by twisting four steel filaments into a cord.

[0044]

[0031] The steel filaments for steel cord are made from a wire rod. The wire rod can be made from pure fresh steel which doesn’t contain any recycled steel such as direct reduced iron, hot briquetted iron or pig iron, or the wire rod can be made from steel containing recycled steel. Preferably, the wire rod is made from steel with a recycled steel content of at least 50%, or at least 75% or even 100%. As thus at least one or all of the steel filaments for steel cord contains a recycled steel content of at least 50%, or at least 75% or even 100%.

[0045]

[0032] The wire rod is firstly cleaned by mechanical descaling and I 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.

[0046]

[0033] At this first intermediate diameter, e.g. at about 3.0 to 3.5 mm, the dry drawn steel filament 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 filament is then ready for further mechanical deformation.

[0047]

[0034] Thereafter the steel filament is further dry drawn from the first intermediate diameter until a second intermediate diameter in a second number of diameter reduction steps. The second diameter typically ranges from 1 .0 mm to 2.5 mm.

[0035] At this second intermediate diameter, the steel filament 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.

[0048]

[0036] If the total reduction in the first and second dry drawing step is not too big a direct drawing operation can be done from wire rod till second intermediate diameter.

[0049]

[0037] After this second patenting treatment, the steel filament is usually provided with a brass coating: copper is plated on the steel filament and zinc is plated on the copper. A thermos-diffusion treatment is applied to form the brass coating. Alternatively, the steel filament can be provided with a coating comprising two, three or more metals selected from the group consisting of copper, zinc, cobalt, titanium, nickel, iron, tin, silver or other known metal.

[0050]

[0038] The brass-coated or the ternary alloy coated steel filament is then subjected to a final series of cross-section reductions by means of wet drawing machines. Wet drawing process includes a series of drawing passes by various drawing dies.

[0051]

[0039] The final steel filament has a carbon content higher than 0.70 percent by weight, or no less than 0.80 percent by weight, or even higher than 0.90 percent by weight.

[0052]

[0040] Steel filaments adapted for the reinforcement of tires typically have a final diameter d preferably ranging from 0.18 mm to 0.55 mm. Examples of filament diameters are 0.16 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, 0.45 mm.

[0053]

[0041] Four steel filaments are twisted by the existing steel cord producing process, i.e. cabling or bunching process, to form a steel cord with a construction of 1x4 or 2+2.

[0054]

[0042] One embodiment is as illustrated in figure 1 , that the steel cord 100 has a construction of 1x4, each of the four steel filaments 105 of the steel cord 100 has a diameter of 0.45mm, the cord lay length is 30mm, the average tensile strength of the four steel filaments 105 TS is 3300 MPa. The steel cord 100 has a total strain ToSr of 3.15%.

[0043] Another embodiment is as illustrated in figure 2, that the steel cord 200 has a construction of 2+2 consisting of two groups of steel filaments, the first group has two steel filaments 205, and the second group has two steel filaments 210, each of the four steel filaments of the steel cord 200

[0055] 5 has a diameter of 0.225mm, the cord lay length is 12mm, the average tensile strength of the four steel filaments TS is 3800 MPa. The steel cord 200 has a total strain ToSr of 3.87%.

[0056]

[0044] Table 1 summarizes the properties and the performance the invention steel cords and the reference steel cords. io

[0045] Table 1

[0057]

[0046] From Table 1, invention steel cords have improved performance in the problem of steel filament fracture or cord flare.

[0058]

[0047] TS is the average tensile strength of the four steel filaments of the steel

[0059] 15 cord. The method of measuring and calculating the average tensile strength TS includes:

[0060] 1 . Unravel the four steel filaments with a certain length out of the steel cord as the samples for testing;

[0061] 2. Measure the breaking load of one sample of one steel filament, repeat

[0062] 20 5 times breaking load measurement for each steel filament and calculate the average as the breaking load of one individual steel filament, and then calculate the tensile strength of individual steel filament by dividing the steel filament breaking load by the steel filament cross-sectional area;

[0063] 25 3. Calculate the average of tensile strength of the four steel filaments as the average tensile strength of the four steel filaments TS.

Claims

Claims1 . A steel cord for rubber reinforcement, said steel cord comprising four steel filaments with a filament diameter d expressed in mm, said steel cord having a cord lay length LL in the range of 10-40mm, said four steel filaments of said steel cord having an average tensile strength TS expressed in MPa, 4100- 2000xd<TS<4900-2000xd, characterized in that, when 0.35mm<d<0.55mm, said steel cord has a total strain ToSr in the range of 2.90%-3.90%, or when 0.18mm<d<0.35mm, said steel cord has a total strain ToSr in the range of 3.10%-4.30%, wherein said total strain ToSr is calculated by the formula, ToSr=(TSr2+ SSr2)05, wherein,- TSr=2x(1 -cos(arctan(7ixD / LL))), wherein D=2.414xd;- SSr=TSx7i / 360000.

2. A steel cord as claimed in claim 1 , characterized in that said average tensile strength TS of said four steel filaments of said steel cord satisfies: 4100-2000xd<TS<4800-2000xd MPa.

3. A steel cord as claimed in claim 2, characterized in that said average tensile strength TS of said four steel filaments of said steel cord satisfies: 4200-2000xd<TS<4500-2000xd MPa, or 4500-2000xd<TS<4800-2000xd MPa.

4. A steel cord as claimed in claim 1 , 2 or 3, characterized in that when 0.35mm<d<0.55mm, said cord lay length LL is in the range of 21 -36mm.

5. A steel cord as claimed in claim 4, characterized in that said steel cord has a total strain ToSr in the range of 2.90%-3.32%.

6. A steel cord as claimed in claim 4, characterized in that when 0.40mm<d<0.55mm, said cord lay length LL is in the range of 22-36mm.

7. A steel cord as claimed in claim 6, characterized in that said steel cord has a total strain ToSr in the range of 2.90%-3.70%, preferably in the range of 2.90%-3.30%.

8. A steel cord as claimed in claim 4, characterized in that when 0.35mm<d<0.40mm, said cord lay length LL is in the range of 21 -29mm.

9. A steel cord as claimed in claim 8, characterized in that said steel cord has a total strain ToSr in the range of 3.00%-3.90%, preferable in the range of 3.02%-3.32%.

10. A steel cord as claimed in claim 1 , 2 or 3, characterized in that when 0.18mm<d<0.35mm, said cord lay length LL is in the range of 10-28mm.11 .A steel cord as claimed in claim 10, characterized in that said steel cord has a total strain ToSr in the range of 3.25%-3.85%.

12. A steel cord as claimed in claim 10, characterized in that when 0.28mm<d<0.35mm, said cord lay length LL is in the range of 10-28mm.

13. A steel cord as claimed in claim 12, characterized in that said steel cord has a total strain ToSr in the range of 3.10%-4.00%, preferably in the range of 3.25%-3.60%.

14. A steel cord as claimed in claim 10, characterized in that when 0.18mm<d<0.28mm, said cord lay length LL is in the range of 10-24mm.

15. A steel cord as claimed in claim 14, characterized in that said steel cord has a total strain ToSr in the range of 3.35%-4.30%, preferable in the range of 3.55%-3.85%.

16. A steel cord as claimed in any one of claims 1 to 15, characterized in that said steel cord has a construction of 1x4 or 2+2, the four steel filaments of the steel cord have the same diameter.

17. A steel cord as claimed in any one of claims 1 to 16, characterized in that at least one of the steel filaments contains a recycled steel content of at least 75% or even 100%.

18. A tire comprising a tread portion, a carcass comprising at least one carcass ply, a pair of beads, and a belt package located between said carcass and said tread portion, said belt package comprising at least one belt ply, characterized in that said carcass ply and / or said belt ply comprises at least one steel cord as claimed in any one of claims 1 to 17.

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