Steel cord for rubber reinforcement

Abstract

A steel cord containing a core layer and an sheath layer, the core layer containing a plurality of core wires with a number of n and the sheath layer comprises a plurality of sheath wires with a number of m, and the steel cord has a flat cross-section with a major axis and a minor axis, the flat cross-section has a flat ratio being the ratio of the length of the major axis and the length of the minor axis, the flat ratio is more than 1.2, the steel cord has a breaking load being BL.sub.cord, the core wires and the sheath wires have a sum breaking load being Sum BL.sub.wires when the core wires and the sheath wires are un-twisted from the steel cord, BL.sub.cord and Sum BL.sub.wires satisfies the following formula: BL.sub.cord/Sum BL.sub.wires>96%. The steel cord has higher breaking load.

Claims

1. A steel cord, comprising a core layer and a sheath layer, said core layer comprising a plurality of core wires with a number of n and said sheath layer comprising a plurality of sheath wires with a number of m, said n ranges from 2 to 3, said m ranges from 6 to 12 said steel cord having a flattened cross-section with a major axis and a minor axis, said flattened cross-section having a flat ratio being the ratio of the length of said major axis and the length of said minor axis, said flat ratio being more than 1.2, said steel cord having a breaking load being BL.sub.cord, the sum of the breaking load of said core wires and said sheath wires which are unraveled from said steel cord being Sum BL.sub.wires wherein, said BL.sub.cord and said Sum BL.sub.wires satisfies the following formula:
BL.sub.cord/SUM BL.sub.wires>96%.

2. The steel cord as claimed in claim 1, wherein said BL.sub.cord and said Sum BL.sub.wires satisfies the following formula:
BL.sub.cord/SumBL.sub.wires>97%.

3. The steel cord as claimed in claim 1, wherein said flat ratio of said steel cord is less than 1.8.

4. The steel cord as claimed in claim 3, wherein said flat ratio of said steel cord is ranging from 1.25 to 1.50.

5. The steel cord as claimed in claim 1, wherein said a plurality of core wires have a twist pitch being more than 300 mm, said a plurality of sheath wires have a twist pitch being less than 30 mm.

6. The steel cord as claimed in claim 1, wherein said a plurality of core wires and said a plurality of sheath wires have the same twist pitch and the same twist direction.

7. A method for making a steel cord as claimed in claim 1, comprising the following steps: a) providing core wires and sheath wires; b) twisting said sheath wires around said core wires thereby to form a steel strand with substantially round cross-section; c) using two reverse pulleys one after another for improving the length matching of said core layer and said sheath layer; d) flattening said steel strand by roller straightener to form said steel cord with a flattened cross-section.

8. A reinforcing rubber article comprising the steel cord as claimed in claim 1.

9. A tire comprising a belt layer, a carcass layer, a tread layer and a pair of bead portions, wherein said belt layer is embedded with the steel cord as claimed in claim 1.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

(1) FIG. 1 shows an invention steel cord with a structure of 3/7 compact cord.

(2) FIG. 2 shows an invention steel cord with a structure of 3/6 compact cord.

(3) FIG. 3 shows an invention steel cord with a structure of 3+7 cord.

(4) FIG. 4 shows a flow chart of a disclosed method for making a steel cord.

MODE(S) FOR CARRYING OUT THE INVENTION

(5) The core wires and the sheath wires for steel cord are made from a wire rod.

(6) 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.

(7) At this first intermediate diameter d1, 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 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 wire is then ready for further mechanical deformation.

(8) Thereafter the steel wire is further dry drawn from the first intermediate diameter d1 until a second intermediate diameter d2 in a second number of diameter reduction steps. The second diameter d2 typically ranges from 1.0 mm to 2.5 mm.

(9) At this second intermediate diameter d2, the steel wire 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.

(10) 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 diameter d2.

(11) 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. Alternatively, the steel wire can be provided with a ternary alloy coating, including copper, zinc and a third alloy of cobalt, titanium, nickel, iron or other known metal.

(12) 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 a carbon content above 0.60 percent by weight, e.g. higher than 0.70 percent by weight, or higher than 0.80 percent by weight, or even higher than 0.90 percent by weight, with a tensile strength typically above 2000 MPa, e.g. above 3800-2000 d Mpa, or above 4100-2000 d MPa or above 4400-2000 d MPa (d is the diameter of final steel wire) and adapted for the reinforcement of elastomer products.

(13) Steel wires adapted for the reinforcement of tyres 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.

(14) After the preparation of the core wires and the sheath wires, the core wires and the sheath wires are subjected to the twisting process, the sheath wires are twisted around the core wires to form a steel strand with substantially round cross-section. For n+m construction, the core wires are firstly twisted and then un-twisted to have the twist pitch more than 300 mm. For n/m construction, the core wire are twisted in the same twist direction and same twist pitch in one step as the sheath wires to have a twist pitch less than 30 mm.

(15) After that, two reverse pulleys are used one after another for making the length of the core layer and the length of the sheath layer being better matched.

(16) Finally, the steel strand is flattened by roller straightener to form the steel cord with a flat cross-section.

(17) FIG. 1 illustrates the first embodiment of the invention. Steel cord 100 has a structure of 3/7. The steel cord 100 has three core wires 105 and seven sheath wires 110. The core wires 105 have a diameter of 0.20 mm while the sheath wires 110 have a diameter of 0.32 mm. The steel cord 100 has an elongation under the load of 50N with a preload of 2.5N is 0.361%. The flat ratio of the steel cord 100 is 1.52, and BL.sub.cord/Sum BL.sub.wires is 100.6%, Sum BL.sub.wires is measured on the core wires and sheath wires which are untwisted from the cord. The core wires 105 and the sheath wires 110 have the same twisting pitch of 16 mm.

(18) A second embodiment is also a 3/7 cord. The core wires have a diameter of 0.20 mm while the sheath wires have a diameter of 0.32 mm. The steel cord has an elongation under the load of 50N with a preload of 2.5N is 0.236%. The flat ratio of the steel cord 100 is 1.53, and BL.sub.cord/Sum BL.sub.wires is 101.7%, Sum BL.sub.wires is measured on the core wires and sheath wires which are untwisted from the cord. The core wires 105 and the sheath wires 110 have the same twisting pitch of 18 mm.

(19) FIG. 3 illustrates the third embodiment of the invention. Third embodiment is 3+7. The steel cord 300 has three core wires 305 and seven sheath wires 310. The core wires 305 have a diameter of 0.20 mm while the sheath wires 310 have a diameter of 0.32 mm. The steel cord 300 has an elongation under the load of 50N with a preload of 2.5N is 0.06%. The flat ratio of the steel cord 300 is 1.37, and BL.sub.cord/Sum BL.sub.wires is 98.3%, BL.sub.cord is 2256N. The core wires 305 have a twist pitch more than 300 mm, the sheath wires 310 have a twist pitch of 16 mm.

(20) A comparison test is done. Table 1 shows the test result.

(21) TABLE-US-00001 TABLE 1 First Reference Second Reference Third Reference embodiment 1 embodiment 2 embodiment 3 Structure 3/7 3/7 3/7 3/7 3 + 7 3 + 7 Core wire 0.20 0.20 0.20 0.20 0.20 0.20 diameter (mm) Sheath wire 0.32 0.32 0.32 0.32 0.32 0.32 diameter (mm) Core wire twist 16 16 18 18 >300 >300 pitch (mm) Sheath wire twist 16 16 18 18 16 16 pitch (mm) Steel cord flat 1.52 1.53 1.50 1.44 1.37 1.47 ratio Length matching Yes No Yes No Yes No is improved BL cord (N) 2194 2078 2141 2053 2256 2172 Sum BL of core 321.9 321.9 323.1 317.4 364.0 365.0 wires (N) Sum BL of sheath 1859.2 1858.5 1838.9 1842.4 1931.3 1932.0 wires (N) Sum BL of core 2181.1 2180.4 2162.0 2159.8 2295.3 2297.0 wires and sheath wires (N) BL.sub.cord/ 100.6% 95.3% 99.0% 95.1% 98.3% 94.6% Sum BL.sub.wires

(22) From the table, Sum BL of core and sheath wires has similar value between the invention steel cord and the reference cord, but the breaking load of the invention steel cord is much higher than the breaking load of the reference cord. This proves that improved length matching between the core layer and sheath layer plays a key role in improving the flat cord breaking load.

(23) FIG. 2 illustrates the forth embodiment of the invention. Forth embodiment is 3/6. The steel cord 200 has three core wires 205 and six sheath wires 210. The core wires 205 have a diameter of 0.20 mm while the sheath wires 210 have a diameter of 0.30 mm. The steel cord 200 has an elongation under the load of 50N is 0.220%. The flat ratio of the steel cord 200 is 1.44, and BL.sub.cord/Sum BL.sub.wires is 99.9%, BL.sub.cord is 1921N. The core wires 205 and the sheath wires 210 have the same twisting pitch of 16 mm.