Steel cord for rubber reinforcement

Abstract

The invention provides a steel cord, the steel cord comprises two or more steel filaments, at least one of the steel filaments has a twist pitch of 6 mm to 40 mm, each of the steel filaments has a tip rise of less than 5 mm with a gauge length of 200 mm after being unravelled out of the steel cord. The invention steel cord has an improved straightness with reduced steel filament fracture risk.

Claims

1. A steel cord comprising two or more steel filaments, at least one of said steel filaments has a twist pitch of 6 mm to 40 mm, wherein each of said two or more steel filaments has a tip rise of less than 5 mm, the tip rise of steel filament being measured by unravelling the steel filament from the steel cord, cutting the steel filament to have 6 samples with a gauge length of 200 mm, measuring a tip rise for each sample, and taking the highest value of the tip rise of 6 samples as the tip rise of the steel filament.

2. The steel cord according to claim 1, wherein each of said steel filaments has a tip rise of less than 4 mm.

3. The steel cord according to claim 2, wherein each of said steel filaments has a tip rise of less than 3 mm.

4. The steel cord according to claim 1, wherein the average of the tip rise of said two or more steel filaments is less than 2.5 mm and more than 0.02 mm.

5. The steel cord according to claim 4, wherein the average of the tip rise of said two or more steel filaments is less than 2.0 mm.

6. The steel cord according to claim 1, wherein each of said two or more steel filaments has a tensile strength, measured on the steel filament unravelled out of the steel cord, of more than 4000-2000D MPa, D being the diameter of the steel filament expressed in mm.

7. The steel cord according to claim 6, wherein each of said two or more steel filaments has a tensile strength of more than 4200-2000D MPa.

8. The steel cord according to claim 7, wherein each of said two or more steel filaments has a tensile strength of more than 4300-2000D MPa.

9. The steel cord according to claim 1, wherein each of said steel filament has a diameter D ranging from 0.17-0.45 mm.

10. The steel cord according to claim 1, wherein each of said steel filaments has a twist pitch of 6 mm to 40 mm.

11. The steel cord according to claim 1, wherein said steel cord has a construction of n1, each of the steel filaments has a twist pitch of 10 mm to 36 mm.

12. The steel cord according to claim 11, wherein said n is 3-6.

13. The steel cord according to claim 12, wherein said n is 4.

14. The steel cord according to claim 1, wherein said steel cord has an arc height of less than 15 mm.

15. 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 according to claim 1.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

(1) FIGS. 1a-1b describe the invention steel cord and one steel filament unravelled out of the steel cord.

(2) FIG. 2 describes the measurement of tip rise of steel filament.

MODE(S) FOR CARRYING OUT THE INVENTION

(3) The steel filaments for steel cord are made from a wire rod.

(4) 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.

(5) 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.

(6) 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.

(7) 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.

(8) 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.

(9) 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.

(10) 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 cast of wet drawn steel filament is controlled to be more than 250 mm. The multi-dimensions straighters, i.e. two, three or more straighters, are used to adjust the cast of the wet drawn steel filament. Alternatively, the high cast can be realized by adjusting the position of the last drawing die. Wet drawing process includes a series of drawing passes by various drawing dies. Drawing dies are kept in the drawing die holders for well positioning, i.e. the drawing die holders are used for fixing the drawing dies. The drawing die holder of the last drawing die (for final drawing pass) is adjustable on its position, while the position of the drawing die holders of the other drawing dies is not adjustable. Normally the postion of the last drawing die holder is adjusted manually according to the operator's personal experience, however, this manual operation makes uncertainty of the postion of the last drawing die holder and the postion of the last drawing die correspondingly, for example, the central axis of the last drawing die is not substantially in the same line with the central axis of the previous drawing die, and this makes the filaments unstraight after final drawing pass. The invention solves the problem, the relative position between the last drawing die and the previous drawing die is more accurately adjusted by a laser line. A laser transmitter is set at the outlet of the last drawing die, while a laser receiver is set at the inlet of the previous drawing die. A laser line starts from the laser transmitter, and transfers through the last drawing die and then the previous drawing die, and finally arrives at the laser receiver. By doing this, it is sure that the central axis of the last drawing die and the central axis of the previous drawing die are in the same line. After this, a very straight wet drawn steel filament is obtained. By making the cast of wet drawn steel filament bigger, the wet drawn steel filament is more straight, this is benefit to have the steel filament unravelled from the steel cord with a lower tip rise.

(11) The final steel filament is a very straight 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) preferably of more than 4000-2000D MPa, and adapted for the reinforcement of rubber products.

(12) 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, 0.45 mm, 0.50 mm. Better that the diameter the steel filament D is in the range of 0.17 mm-0.45 mm.

(13) Two or more steel filaments are twisted by the existing steel cord producing process, i.e. cabling or bunching process, to form a steel cord. Very important to control the length of each steel filament starting from pay-off spools to the bunching point wherein the steel filaments gather and start to winding, and to make sure that the difference of such length among the steel filaments being smaller than 100 mm. This is benefit to have the steel filament unravelled from the steel cord with a lower tip rise.

(14) The invention avoids the usage of the straightener prior to steel cord taking up which is for improving the straightness of the steel cord as mentioned in JP2009249799, thereby avoids the damage from the straightening operation to the steel filament, as thus the filament fracture risk during the use of the tire is reduced. Particularly, the steel filaments with higher tensile strength is relatively more easier to be damaged comparing with the steel filaments with relatively lower tensile strength. Furthermore, as mentioned previously, for the steel cord with the steel filaments having a higher tensile strength, i.e., higher than 4200-2000D MPa, the straightening operation can't bring the desired effect, that the steel cord is still not straight after straightening operation. The invention is beneficial for the steel cord with higher tensile strength steel filaments. The invention avoids the use of rolling and rotating process as mentioned in JP2005169484 before the cabling or bunching process, thereby avoids the damage to the straightness of the steel cord.

(15) Table 1 shows the comparison between the invention steel cord and the references.

(16) TABLE-US-00001 TABLE 1 Reference 1 Reference 2 Invention Construction 4 1 4 1 4 1 Steel filament diameter (mm) 0.225 0.225 0.225 Use of rotary rollers No No No before cabling or bunching process Steel filament 12 12 12 twist pitch (mm) Use of straightener No Yes No after cabling or bunching process TS of each 3790 3796 3792 steel filament (MPa) Highest tip rise among 7 6 2 the steel filaments unravelled out of steel cord (mm) Average of the tip-rise 3.5 3.4 1.3 of steel filaments unravelled out of steel cord (mm) Steel cord straightness (mm) 43 12 10 Steel cord arc height (mm) 23 8 9 Rubber ply tip rise (mm) 10-15 <5 <5

(17) The above table shows that the invention steel cord is very straight by reducing the tip rise of the individual steel filament. Reference 2 steel cord is straight, however, the straightness of steel cord is realized by using the straightener, and the straightening operation may damage the steel filaments, and such damage brings the risk of steel filament fracture or breaking during the running of the tire, and both the tip rise of the individual steel filament and the average of tip rise of steel filaments are higher.

(18) FIG. 1a shows the invention steel cord 100 with a construction of 41 comprising four steel filaments 105, FIG. 1b shows the steel filament 105 which is unravelled out of the steel cord 100.

(19) A method of measuring the tip rise of steel filaments unravelled of the steel cord is provided, and FIG. 2 illustrates the measurement, a) Unravel all steel filaments out of the steel cord, b) Cut one unravelled steel filament consecutively to have 6 samples 205 with a length 200 mm+/5 mm, this length is so-called gauge length of 200 mm; put one sample 205 on the horizontal table top of the worktable 215, and measure the distance from each of the two ends of the sample 205 to the horizontal table top by the ruler and record the value of distance as T1 and T2, and the measurement is according to the highest point of each end of the sample 205, thereby the steel filament diameter is included into the distance, the bigger value between T1 and T2 of is deemed as the tip rise of the sample 205; and then measure the rest 5 samples, the highest value of the tip rise of the 6 samples is deemed as the tip rise of the unravelled steel filament. c) Measure the rest unravelled steel filaments in the same way and get the tip rise of the rest unravelled steel filaments, calculate the average of the tip rise of all the unravelled steel filaments, and this is the average of the tip rise of steel filaments.

(20) The method of testing and calculating the tensile strength of each steel filament includes: Unravel the steel filaments from the steel cord, Measure the filament breaking load according to the principle mentioned in the standard ISO6892-1:2019 with some particular setting like the clamp length being 250 mm and the test speed being 100 mm/min, test 5 times for each filament and calculate the average as the breaking load of the individual steel filament, Calculate the tensile strength of individual filament by dividing the filament breaking load by the filament cross-sectional area.

(21) The steel cord straightness and steel cord arc height are measured according to the method mentioned in China standard GB/T 33159-2016.