Steel cord with four steel filaments for rubber reinforcement

Abstract

The present invention relates to a steel cord comprising four steel filaments having a filament diameter d expressed in mm, and having a cord lay length LL ranging from 10 mm to 40 mm, the four steel filaments of the steel cord having an average tensile strength TS expressed in MPa, 4100-2000 * d < = TS < = 4900-2000 * d, the steel cord having a predetermined total strain. The invention provides a solution to reduce the problems of steel monofilament breakage, steel monofilament surface cracks or steel cord loosening or strand scattering in the steel cord production process.

Description

TECHNICAL FIELD

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

[0002] Steel cords are widely used as reinforcements of rubber products such as rubber belts, rubber tires, hoses, etc. As a reinforcement of rubber products, a steel cord is required to have certain strength, corrosion resistance, fatigue resistance, rubber permeability, rubber adhesion, etc.

[0003] A trend in the field of rubber tires is lighter tires, which have advantages such as lower rolling resistance and energy saving to reduce carbon emissions. Therefore, steel cords as reinforcements of tires are developed to be thinner and lighter. One solution is to increase the tensile strength of steel filaments of a steel cord, and since a steel filament having a higher tensile strength is used, a thinner steel filament can also provide sufficient cord breaking load, so that the diameter and weight of the steel cord can be reduced. The use of thinner and lighter steel cords makes the tire thinner and lighter.

[0004] US6102095 discloses a steel cord having a single-twisted structure obtained by twisting 3 to 6 steel filaments, each having a tensile strength of 4000 N / mm 2 to 4800 N / mm 2 to reduce tire weight, the steel filaments being processed to have residual compressive stress within the twisted spiral portion, so that the ratio of the curvature R0 of the spiral portion in each steel filament formed by untwisting the cord to the radius of curvature R1 of the spiral portion after removing the surface layer from within the spiral portion in the steel filament satisfies R1 / R0≤1, in which case the space between mutually adjacent steel filaments is larger to allow rubber to penetrate into the interior of the cord.

[0005] US4966216 discloses a steel cord having a single-layer structure obtained by twisting 3 to 5 metal filaments for heavy load radial tires, which attempts to greatly reduce tire weight, improve the breaking resistance, corrosion resistance, and anti-fretting wear resistance at the end of the body ply. This steel cord is obtained by satisfying a certain relationship between the tensile strength of the filaments and the diameter of the filaments. When the filament diameter is 0.25 mm, the steel filaments have an extremely high tensile strength of 380 kg / mm 2 .

[0006] A steel filament having a higher tensile strength is beneficial for the reduction of tire weight. However, a steel filament having a higher tensile strength is difficult to process like twisting to form a steel cord, because the higher tensile strength causes more filament breakage problems in the processing of manufacturing a steel cord. SUMMARY

[0007] The main object of the present invention is to solve the problem of breaking of steel filaments having higher tensile strength in the process of manufacturing steel cord.

[0008] The first object of the present invention is to provide a steel cord comprising steel filaments having very high tensile strength, while reducing the problem of filament breakage in the process of manufacturing steel cord.

[0009] The second object of the present invention is to provide a tire reinforced by a steel cord comprising steel filaments having very high tensile strength.

[0010] According to the first aspect of the present invention, there is provided a steel cord, the steel cord comprising four steel filaments having a filament diameter d in mm, the cord lay length LL of the steel cord being in the range of 10 mm to 40 mm, the four steel filaments of the steel cord having an average tensile strength TS in MPa, 4100-2000 x d ≤ TS ≤ 4900-2000 x d, wherein

[0011] when 0.35 mm < d ≤ 0.55 mm, the total strain ToSr of the steel cord is in the range of 2.90% to 3.90%, or

[0012] when 0.18 mm ≤ d ≤ 0.35 mm, the total strain ToSr of the steel cord is in the range of 3.10% to 4.30%,

[0013] wherein the total strain ToSr is calculated by the following formula:

[0014] ToSr = (TSr 2 + SSr 2 ) 0.5 , wherein,

[0015] - TSr = 2 x (1 - cos(arctan(π x D / LL)), where D = 2.414 x d;

[0016] - SSr = TS x π / 360000.

[0017] According to the first aspect of the present invention, there is also provided a steel cord, the steel cord comprising four steel filaments having a filament diameter d in mm, the cord lay length LL of the steel cord being in the range of 10 mm to 40 mm, the four steel filaments of the steel cord having an average tensile strength TS in MPa, 4100-2000 x d ≤ TS ≤ 4800-2000 x d, wherein

[0018] when 0.35 mm < d ≤ 0.55 mm, the total strain ToSr of the steel cord is in the range of 2.90% to 3.90%, or

[0019] the total strain ToSr of the steel cord ranges from 3.10% to 4.30% when 0.18 mm < d < 0.35 mm,

[0020] wherein the total strain ToSr is calculated by the following formula:

[0021] ToSr = (TSr 2 + SSr 2 ) 0.5 wherein,

[0022] - TSr = 2 x (1 - cos(arctan(n x D / LL))), wherein D = 2.414 x d;

[0023] - SSr = TS x n / 360000.

[0024] The steel cord is manufactured by twisting a plurality of steel filaments by applying an external force on the steel filaments. For one steel filament, it is self-rotating in one aspect; in another aspect, it is entangled with other steel filaments to form the steel cord. Thus, the steel filament is deformed, and a torsional strain is generated in the steel filament due to the self-rotation of the steel filament, and a bending strain is generated in the steel filament due to the entanglement with other steel filaments. The deformation of the steel filament first enters an elastic deformation stage, and then the deformation of the steel filament enters a plastic deformation stage in the case of sufficient deformation. For “elastic deformation”, once the external force on the steel filament is reduced 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. even if the external force is reduced or disappears, the plastic deformation of the steel filament caused by the external force will not change. Therefore, in order to obtain a stable steel cord, the strain (including the torsional strain and the bending strain) of the steel filament should be sufficient to make the deformation of the steel filament enter the plastic deformation stage. With the increase of the tensile strength of the steel filament, such as ultra-high tensile strength, super-high tensile strength or extremely high tensile strength, the steel filament becomes harder due to the use of a steel wire rod with a higher carbon content or a higher degree of drawing strain hardening to achieve a higher tensile strength, so that the ductility of the steel filament becomes poorer, therefore, in the process of entangling a plurality of steel filaments into a steel cord, the conventional deformation of the steel filament (i.e. the rotation of the steel filament itself and the entanglement of the steel filament with other steel filaments) is not sufficient to make the steel filament generate sufficient plastic strain to enter the plastic deformation stage, therefore, the steel cord will be unstable, for example, the steel cord becomes very loose or even strands, which can lead to unstable performance (such as breaking load and cord diameter) of the steel cord, or even does not meet the requirements. One solution is to reduce the twist pitch of the steel cord to make the steel filament generate sufficient plastic strain, however, this can lead to other defects, such as the breakage of the steel filament or the cracks on the surface of the steel filament, because the steel filament is harder due to the increase of the tensile strength.

[0025] The present invention provides a solution to more fully balance various parameter factors (e.g., cord twist, filament diameter, and filament tensile strength) that mainly affect the plastic strain generated in steel filaments (particularly steel filaments having a higher tensile strength) that are capable of generating sufficient plastic strain to ensure a reduction in the problem of filament breakage, cracks on the surface of the filaments, or relaxation or strand separation of the steel cord during the production of the steel cord under such structure and parameters. The present invention introduces a total strain ToSr of the steel cord having a predetermined value range according to the cord twist, the filament diameter, and the average tensile strength of the filaments of the steel cord. Particularly, for a steel cord having filaments with a filament diameter greater than 0.35 mm and less than or equal to 0.55 mm, the total strain ToSr of the steel cord ranges from 2.90% to 3.90%; for a steel cord having filaments with a filament diameter ranging from 0.18 mm to 0.35 mm, the total strain ToSr of the steel cord ranges from 3.10% to 4.30%. By doing so, the problem of filament breakage, cracks on the surface of the filaments, or relaxation or strand separation of the steel cord during the production of the steel cord by twisting a plurality of steel filaments having an ultra-high tensile strength, a super-high tensile strength, or a higher tensile strength is reduced. Too high a ToSr will result in the problem of filament breakage or cracks on the surface of the filaments, and too low a ToSr will result in the problem of relaxation or strand separation of the steel cord.

[0026] According to the present invention, the four filaments of the steel cord have the same diameter. The total strain ToSr of the steel cord varies according to the filament diameter.

[0027] When 0.35 mm < d ≤ 0.55 mm, preferably, the cord twist LL ranges from 21 mm to 36 mm. In this case, preferably, the total strain ToSr of the steel cord ranges from 2.90% to 3.32%.

[0028] In a preferred embodiment, when 0.40 mm < d ≤ 0.55 mm, the cord twist LL of the steel cord ranges from 22 mm to 36 mm. In this case, preferably, the total strain ToSr of the steel cord ranges from 2.90% to 3.70%, more preferably, ranges from 2.90% to 3.30%.

[0029] In another preferred embodiment, when 0.35 mm < d ≤ 0.40 mm, the cord twist of the steel cord ranges from 21 mm to 29 mm. In this case, preferably, the total strain ToSr of the steel cord ranges from 3.00% to 3.90%, preferably, ranges from 3.02% to 3.32%.

[0030] When 0.18 mm < d < 0.35 mm, preferably the cord lay length LL ranges from 10 mm to 28 mm. In this case, preferably the total strain ToSr of the steel cord ranges from 3.25% to 3.85%.

[0031] In a preferred embodiment, when 0.28 mm < d < 0.35 mm, the cord lay length of the steel cord ranges from 10 mm to 28 mm. In this case, preferably the total strain ToSr of the steel cord ranges from 3.10% to 4.00%, more preferably from 3.25% to 3.60%.

[0032] In another preferred embodiment, when 0.18 mm < d < 0.28 mm, the cord lay length of the steel cord ranges from 10 mm to 24 mm. In this case, preferably the total strain ToSr of the steel cord ranges from 3.35% to 4.30%, preferably from 3.55% to 3.85%.

[0033] The tensile strength of the steel filaments of the steel cord cannot be too low, otherwise the effects of reducing the weight of the tire and lowering the rolling resistance of the tire would not be as expected; the tensile strength of the steel filaments of the steel cord cannot be too high, otherwise the steel filaments would be too stiff to generate sufficient plastic strain to reduce the problems of breakage of the steel filaments, cracks on the surface of the steel filaments, or relaxation or uncoiling of the steel cord. The tensile strength of the steel filaments can vary, although they are produced by the same process. Therefore, the present application uses the average tensile strength of the four steel filaments to determine the total strain of the steel cord. The steel filaments of the steel cord can have the same or substantially the same tensile strength, i.e. a difference of no more than 5%; alternatively, the steel filaments of the steel cord can have different tensile strengths, i.e. a difference of more than 5%, or even more than 10%, but preferably no more than 20%.

[0034] According to the present application, the average tensile strength TS of the four steel filaments of the steel cord is 4100 - 2000 x d < TS < 4900 - 2000 x d MPa, more preferably 4100 - 2000 x d < TS < 4800 - 2000 x d MPa, most preferably 4150 - 2000 x d < TS < 4750 - 2000 x d MPa. More preferably 4200 - 2000 x d < TS < 4500 - 2000 x d MPa. Alternatively, 4500 - 2000 x d < TS < 4800 - 2000 x d MPa.

[0035] The invention relates to a steel cord comprising four steel filaments, more precisely, the steel cord comprises a plurality of steel filaments, the number being four; or in other words, the number of steel filaments of the steel cord is four. The steel cord can have a 1 x 4 or 2 + 2 structure. Regardless of the structure of the steel cord, the cord lay length of the steel cord ranges from 10 mm to 40 mm. The cord lay length cannot be too small, otherwise the problem of breaking of the steel filaments or cracks on the surface of the steel filaments would be too much.

[0036] According to a second aspect of the invention, there is provided a tire comprising a tread portion, a carcass comprising at least one carcass ply, a pair of beads and a belt assembly located between the carcass and the tread portion, the belt assembly comprising at least one belt ply, wherein the carcass ply and / or the belt ply comprises at least one steel cord, the steel cord comprising four steel filaments having a filament diameter d expressed in mm, the cord lay length LL of the steel cord ranging from 10 mm to 40 mm, the four steel filaments of the steel cord having an average tensile strength TS expressed in MPa, 4100 - 2000 x d < TS < 4900 - 2000 x d, wherein

[0037] when 0.35 mm < d < 0.55 mm, the total strain ToSr of the steel cord ranges from 2.90% to 3.90%, or

[0038] when 0.18 mm < d < 0.35 mm, the total strain ToSr of the steel cord ranges from 3.10% to 4.30%,

[0039] wherein said total strain ToSr is calculated by the following formula:

[0040] ToSr = (TSr 2 + SSr 2 ) 0.5 wherein,

[0041] - TSr = 2 x (1 - cos(arctan(π x D / LL)), with D = 2.414 x d;

[0042] - SSr = TS x π / 360000.

[0043] According to a second aspect of the application, there is also provided a tire comprising a tread portion, a carcass comprising at least one carcass ply, a pair of beads and a belt assembly located between the carcass and the tread portion, the belt assembly comprising at least one belt ply, wherein the carcass ply and / or the belt ply comprises at least one steel cord, the steel cord comprising four steel filaments having a filament diameter d expressed in mm, the cord lay length LL of the steel cord ranging from 10 mm to 40 mm, the four steel filaments of the steel cord having an average tensile strength TS expressed in MPa, 4100-2000 x d ≤ TS ≤ 4800-2000 x d, wherein

[0044] when 0.35 mm < d ≤ 0.55 mm, the total strain ToSr of the steel cord ranges from 2.90% to 3.90%, or

[0045] when 0.18 mm ≤ d ≤ 0.35 mm, the total strain ToSr of the steel cord ranges from 3.10% to 4.30%,

[0046] wherein said total strain ToSr is calculated by the following formula:

[0047] ToSr = (TSr 2 + SSr 2 ) 0.5 wherein,

[0048] - Tsr = 2 x (1 - cos(arctan(π x D / LL)), with D = 2.414 x d;

[0049] - SSr = TS x π / 360000.

[0050] Thanks to the high tensile strength of the steel filaments, the same cord strength can be obtained by using thinner steel filaments and the weight of the steel cord, and thus the weight of the tire, can be reduced.

[0051] Preferably, the belt ply of the tire is reinforced by the steel cord of the application. BRIEF DESCRIPTION OF DRAWINGS

[0052] Figure 1 A cord of the application having a 1 x 4 construction is described.

[0053] Figure 2 A cord of the application having a 2 + 2 construction is described. DETAILED DESCRIPTION

[0054] The steel cord is made by twisting the four steel filaments into a cord.

[0055] The steel filaments for steel cords are made from wire rods. The wire rods can be made from virgin new steel, e.g. direct reduced iron, hot briquetted iron or pig iron, without any recycled steel, or the wire rods can be made from steel containing recycled steel. Preferably, the wire rods are made from steel having a recycled steel content of at least 50%, or at least 75%, or even 100%. Thus, at least one or all of the steel filaments for steel cords have a recycled steel content of at least 50%, or at least 75%, or even 100%.

[0056] The wire rods are first cleaned by mechanical descaling and / or by chemical pickling in a solution of H2SO4 or HC1 to remove oxides present on the surface. The wire rods are then rinsed in water and dried. The dried wire rods are then subjected to a first series of dry drawing operations to reduce the diameter to a first intermediate diameter.

[0057] At this first intermediate diameter, e.g. in the range of about 3.0 mm to 3.5 mm, the dry drawn steel filaments are subjected to a first intermediate heat treatment, called lead quenching. Lead quenching refers to austenitizing first up to a temperature of about 1000°C, followed by a phase transformation from austenite to pearlite at a temperature of about 600°C to 650°C. The steel filaments can then be used for further mechanical deformation.

[0058] Thereafter, the steel filaments are further dry drawn from the first intermediate diameter to a second intermediate diameter in a second number of diameter reduction steps. The second diameter is typically in the range of from 1.0 mm to 2.5 mm.

[0059] At this second intermediate diameter, the steel filaments are subjected to a second lead quenching treatment, i.e. austenitizing again at a temperature of about 1000°C, followed by quenching at a temperature of 600°C to 650°C to allow transformation into pearlite.

[0060] If the total reduction in the first dry drawing step and the second dry drawing step is not large enough, a direct drawing operation can be performed from the wire rod to the second intermediate diameter.

[0061] After this second lead quenching treatment, the steel filaments are typically provided with a brass coating: copper plating on the steel filaments, followed by zinc plating on the copper. A thermal diffusion treatment is applied to form the brass coating. Alternatively, the steel filaments can be provided with a coating containing two, three or more metals selected from the group comprising copper, zinc, cobalt, titanium, nickel, iron, tin, silver or other known metals.

[0062] The brass-coated or ternary alloy-coated steel filaments are then subjected to a final series of cross-section reductions by a wet drawing machine. The wet drawing process comprises a series of drawing passes through various drawing dies.

[0063] The final carbon content (wt.%) of the steel filaments is higher than 0.70, or not less than 0.80, or even higher than 0.90.

[0064] The final diameter d of the steel filaments suitable for tire reinforcement is generally preferably in the range of 0.18 mm to 0.55 mm. Examples of the diameter of the filaments 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.

[0065] The four steel filaments are twisted by the existing steel cord production process (i.e., cabling or beaming process) to form a steel cord having a 1x4 or 2+2 structure.

[0066] One embodiment is shown in FIG. 1. Figure 1 As shown, the steel cord 100 has a 1x4 structure, each of the four steel filaments 105 of the steel cord 100 has a diameter of 0.45 mm, the cord lay is 30 mm, and the average tensile strength TS of the four steel filaments 105 is 3300 MPa. The total strain ToSr of the steel cord 100 is 3.15%.

[0067] Another embodiment is shown in FIG. 2. Figure 2 As shown, the steel cord 200 has a 2+2 structure composed of two groups of steel filaments, the first group having two steel filaments 205 and the second group having two steel filaments 210, each of the four steel filaments of the steel cord 200 has a diameter of 0.225 mm, the cord lay is 12 mm, and the average tensile strength TS of the four steel filaments is 3800 MPa. The total strain ToSr of the steel cord 200 is 3.87%.

[0068] Table 1 summarizes the properties and performances of the steel cords of the present application and the reference steel cords.

[0069] Table 1

[0070]

[0071]

[0072] As can be seen from Table 1, the steel cords of the present application have improved performances in terms of the problem of steel filament breakage or cord strand separation.

[0073] TS is the average tensile strength of the four steel filaments of the steel cord. The method of measuring and calculating the average tensile strength TS includes:

[0074] 1. Untwist four steel filaments having a certain length from the steel cord as a test sample;

[0075] 2. Measure the breaking load of one sample of one steel filament, repeat the measurement of the breaking load 5 times for each steel filament and calculate the average value as the breaking load of one individual steel filament, then calculate the tensile strength of the individual steel filament by dividing the breaking load of the steel filament by the cross-sectional area of the steel filament;

[0076] 3. Calculate the average value of the tensile strengths of the four steel filaments as the average tensile strength TS of the four steel filaments.

Claims

1. A steel cord for rubber reinforcement, said steel cord comprising four steel filaments having a filament diameter d expressed in mm, the cord lay length LL of said steel cord ranging from 10 mm to 40 mm, said four steel filaments of said steel cord having an average tensile strength TS expressed in MPa, 4100-2000 x d < TS < 4900-2000 x d, wherein, when 0.35 mm < d < 0.55 mm, the total strain ToSr of said steel cord ranges from 2.90% to 3.90%, or when 0.18 mm < d < 0.35 mm, the total strain ToSr of said steel cord ranges from 3.10% to 4.30%, wherein said total strain ToSr is calculated by the following formula: ToSr = (TSr 2 + SSr 2 ) 0.5 wherein, - ToSr = 2 x (1 - cos(arctan(π x D / LL)), with D = 2.414 x d; - SSr = TS x π / 360000.

2. The steel cord according to claim 1, wherein, said average tensile strength TS of said four steel filaments of said steel cord satisfies: 4100-2000 x d < TS < 4800-2000 x d MPa.

3. The steel cord according to claim 2, wherein, said average tensile strength TS of said four steel filaments of said steel cord satisfies: 4200-2000 x d < TS < 4500-2000 x d MPa, or 4500-2000 x d < TS < 4800-2000 x d MPa.

4. The steel cord according to claim 1, 2 or 3, wherein, when 0.35 mm < d < 0.55 mm, the cord lay length LL ranges from 21 mm to 36 mm.

5. The steel cord according to claim 4, wherein, the total strain ToSr of said steel cord ranges from 2.90% to 3.32%.

6. The steel cord according to claim 4, wherein, when 0.40 mm < d < 0.55 mm, the cord lay length LL ranges from 22 mm to 36 mm.

7. The steel cord according to claim 6, wherein the total strain ToSr of said steel cord ranges from 2.90% to 3.70%, preferably from 2.90% to 3.30%.

8. The steel cord according to claim 4, wherein, when 0.35 mm < d < 0.40 mm, the cord lay length LL ranges from 21 mm to 29 mm.

9. The steel cord according to claim 8, wherein, the total strain ToSr of said steel cord ranges from 3.00% to 3.90%, preferably from 3.02% to 3.32%.

10. The steel cord according to claim 1, 2 or 3, wherein, when 0.18 mm < d < 0.35 mm, the cord lay length LL ranges from 10 mm to 28 mm.

11. The steel cord according to claim 10, wherein, the total strain ToSr of said steel cord ranges from 3.25% to 3.85%.

12. The steel cord according to claim 10, wherein, when 0.28 mm < d < 0.35 mm, the cord lay length LL ranges from 10 mm to 28 mm.

13. The steel cord according to claim 12, wherein, the total strain ToSr of said steel cord ranges from 3.10% to 4.00%, preferably from 3.25% to 3.60%.

14. The steel cord according to claim 10, wherein, when 0.18 mm < d < 0.28 mm, the cord lay length LL ranges from 10 mm to 24 mm.

15. The steel cord according to claim 14, wherein, the total strain ToSr of said steel cord ranges from 3.35% to 4.30%, preferably from 3.55% to 3.85%.

16. The steel cord according to any one of claims 1 to 15, wherein, said steel cord has a 1 x 4 or 2 + 2 structure, said four steel filaments of said steel cord having the same diameter.

17. The steel cord according to any one of claims 1 to 16, wherein, 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 assembly located between the carcass and the tread portion, the belt assembly comprising at least one belt ply, wherein, The carcass plies and / or the belt plies comprise at least one steel cord according to any one of claims 1 to 17.

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