A STEEL CORD FOR RUBBER REINFORCEMENT

Abstract

The invention provides a steel cord with a construction of n×1, n is the number of the steel filaments of the steel cord, the steel cord has an elongation at 2.5N-50N of less than 1.2% and a twist pitch of greater than 16 mm, each of the steel filaments has a form of helical wave with a wave length L expressed in mm and a wave height H expressed in mm when being unravelled from said steel cord, L is greater than 16 mm, each of the steel filaments has a space volume Vs satisfying that, Vs=L×H.sup.2×π/4, and Vs>20 mm.sup.3. The invention steel cord is beneficial for the stress distribution.

Claims

1.-15. (canceled)

16. A steel cord with a construction of n×1, n being the number of the steel filaments of said steel cord, said steel cord having an elongation at 2.5N-50N of less than 1.2% and a twist pitch of greater than 16 mm, each of said steel filaments having a form of helical wave with a wave length L expressed in mm and a wave height H expressed in mm when being unravelled from said steel cord, characterized in that, said L is greater than 16 mm, each of said steel filaments has a space volume Vs satisfying that,
Vs=L×H.sup.2×π/4, and Vs>20 mm.sup.3.

17. The steel cord as claimed in claim 16, wherein said Vs is greater than 23 mm.sup.3.

18. The steel cord as claimed in claim 17, wherein said Vs is greater than 30 mm.sup.3.

19. The steel cord as claimed in claim 18, wherein said Vs is greater than 35 mm.sup.3.

20. The steel cord as claimed in claim 16, wherein said Vs is smaller than 200 mm.sup.3.

21. The steel cord as claimed in claim 16, wherein said wave length L is greater than 20 mm.

22. The steel cord as claimed in claim 21, wherein said wave length L is greater than 24 mm and smaller than 40 mm.

23. The steel cord as claimed in claim 16, wherein said twist pitch of steel cord is greater than 20 mm.

24. The steel cord as claimed in claim 23, wherein said twist pitch of steel cord is greater than 24 mm and smaller than 40 mm.

25. The steel cord as claimed in claim 16, wherein said steel cord has an elongation at break of less than 5.0%.

26. The steel cord as claimed in claim 16, wherein n ranges from 2-7.

27. The steel cord as claimed in claim 16, wherein said steel filament has a tensile strength of more than 4000-2000×D MPa when being unravelled from said steel cord, D being the diameter of the steel filament expressed in mm.

28. The steel cord as claimed in claim 27, wherein said steel filament has a tensile strength of more than 4200-2000×D MPa.

29. Use of the steel cord as claimed in claim 16 is for rubber reinforcement.

30. A tire comprising at least one belt layer, at least one carcass layer, at least one tread layer and a pair of bead portions, wherein said belt layer and/or said carcass layer comprises at least one steel cord as claimed in claim 16.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0022] FIG. 1 describes a steel cord of the invention.

[0023] FIG. 2a-2b describes the steel filament unravelled from the steel cord and the corresponding wave length and wave height.

MODE(S) FOR CARRYING OUT THE INVENTION

[0024] The steel filaments for steel cord are made from a wire rod.

[0025] The wire rod is firstly cleaned by mechanical descaling and/or by chemical pickling in a H.sub.2SO.sub.4 or HCl 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.

[0026] At this first intermediate diameter D1, 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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 ternary alloy coating, including copper, zinc and a third alloy of cobalt, titanium, nickel, iron or other known metal.

[0031] 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. The final product is a steel filament with 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, with a tensile strength (TS) typically above 3000 MPa and adapted for the reinforcement of rubber products.

[0032] Steel filaments adapted for the reinforcement of tires typically have a final diameter D 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. Better that the diameter the steel filament D is in the range of 0.10 mm-0.50 mm.

[0033] A number of steel filaments are twisted by the existing steel cord making process, i.e. cabling or bunching process, to form a steel cord, and the steel filaments are pre-formed in the existing preforming method prior to being twisted to form a cord. The pre-determined wave height of the steel filament is realized by adjusting the preforming on the steel filament, the pre-determined wave length of the steel filament is realized by adjusting the preforming on the steel filament and the twisting process, thereby the unravelled steel filament with the pre-determined space volume is reached. Although the wave length and wave height of the steel filament are measured when the steel filament is unravelled from the steel cord, the unravelling operation won't bring any substantial change to the wave length and the wave height of the steel filament.

[0034] FIG. 1 illustrates one embodiment of the invention. The steel cord 100 comprises four steel filaments 105 with a diameter of 0.38 mm. FIG. 2a illustrates the steel filament 105 unravelled from the steel cord 100, FIG. 2b illustrates the wave length L and wave height H of the steel filament 105 according to the measurement method. The detail of the measurement method of the wave length and wave height is: [0035] first, cut the steel cord 100 with a certain length as a sample, such as 100-150 mm, unravel the steel filaments 105 from the steel cord sample, [0036] second, project one steel filament 105 and make the silhouette of the middle part of the steel filament 105 on the screen, [0037] third, measure the distance of two subsequent wave lengths (i.e. from one peak to the next second peak) and divide by two to determine the wave length; measure the distance between the valley and the imaginary base line between the two peaks to determine the wave height, [0038] fourth, repeat 5 pieces of the same steel filament 105 and calculate the average values, the average values are the wave length L and the wave height H.

[0039] Following Table 1 summarizes the performance of the invention steel cord comparing with the reference.

TABLE-US-00001 TABLE 1 Invention 1 Invention 2 Reference Construction 4 × 1 6 × 1 4 × 1 Filament diameter (mm) 0.38 0.38 0.38 L (mm) 27.97 29.04 20.43 H (mm) 1.40 1.90 0.98 Vs (mm.sup.3) 43 82 15 Fatigue test (k circles) 7306 16682 1986 Elongation at break of cord (%) 2.51 3.41 3.49 Elongation at 2.5N-50N (%) 0.35 0.63 0.30 Twist pitch of cord (mm) 27 28 20 Tensile strength of steel filament 3592 3593 3588 (MPa)

[0040] Fatigue test is a test to know to what extent the steel cord will separate from the rubber. The fatigue test has the following steps: [0041] fuse the steel cord into 15 small pieces with a length of 330 mm, cut small pieces at the central portion to get 30 small steel cord samples, make sure that the cut end of the small steel cord sample is not flare; [0042] prepare a rubber strip with a size of 203 mm×35 mm×6.4 mm (length×width×height) and a rubber cover with the same size as the rubber strip; [0043] put the end portions of the small steel cord samples (with a length of 12.5 mm and including the cut end) on the rubber strip, arrange the 15 small steel cord samples separately along one side of the rubber block and put another 15 small steel cord samples separately along the opposite side of the rubber strip, make sure that one small steel cord sample on one side of the rubber strip has the same horizontal axis as the corresponding small steel cord sample on the opposite side, put the rubber cover on the rubber strip and the small steel cord samples to make the rubber block, and then cure the rubber block; [0044] identify the No. 2, 4, 6, 9, 11, 13 small steel cord samples along the length of the rubber block, cut the rubber block to take out the small blocks with a length about 22 mm, make sure that each small block is inserted with two small steel cord samples in its central position, then fix the steel cords of each small block by two clamps along the vertical axis, and then give a vibration force to the small block with a pre-determined frequency and amplitude under room temperature, and record the repeated circles of the vibration force till the steel cord separate with the small block.

[0045] Compared with the reference steel cord, the invention steel cords have a better capability to suffer more times of vibrations until they separate with the rubber blocks, and this proves that the invention steel cord contributes to better stress distribution.