EP4689273A1

EP4689273A1 - A high elongation steel cord for rubber reinforcement

Info

Publication number: EP4689273A1
Application number: EP24715216.8A
Authority: EP
Inventors: Aijun Zhang; Yaodong LI
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 2023-04-04
Filing date: 2024-04-02
Publication date: 2026-02-11
Also published as: WO2024208815A1; WO2024207219A1; CN120981627A

Abstract

The invention provides a steel cord with a construction of 1xn is provided, n is the number of the steel filaments of the steel cord, the steel cord has an elongation at break At of more than 5.0%, the steel cord has a structural elongation As ranging from 0.86% to 3.50%, At and As satisfy the following formulas: As/At≥17%; and (At-As)/At≥50%. The invention provides a HE cord with sufficient elongation performance meeting the demands of a tire in the different phase of the tire.

Description

16952-LZ

Title A high elongation steel cord for rubber reinforcement

Description

Technical Field

[1 ] The invention relates to a high elongation steel cord for rubber reinforcement. The invention also relates to a tire reinforced by high elongation steel cords.

Background Art

[2] Steel cords are widely used as a reinforcement for rubber products, such as rubber belt, rubber tire, hose and etc.

[3] A tire for truck or passenger car generally comprises a tread, crown, belt and carcass, a pair of beads and two sidewalls. The belt comprises one or more rubber plies reinforced by steel cords, and the belt comprises at least one working belt ply and possibly one or more other belt plies, for example one protection belt ply, or one or more zero degree belt plies or one transition belt ply.

[4] The high elongation (HE) steel cord, with an elongation at break of higher than 5%, is generally used for reinforcing zero degree belt ply or protection belt ply of a tire.

[5] One type of high elongation steel cord has a construction of nxm, such as 3x7, 3x4 or 4x2. Such type of HE cord has multiple strands , as thus such steel cord has a big cord diameter, and accordingly the zero degree belt ply or the protection belt ply of a tire is thick. This doesn’t satisfy with the trend of lighter tire and lower rolling resistance tire.

[6] Another type of HE cord has a construction of 1 xn, such as 1 x4 or 1 x5. Such type of HE cord has one strand.

[7] US2002150786 discloses an open 1 xn steel cord, the steel cord has full rubber penetration performance because of the open construction, and the steel filaments of the steel cord have a pearlitic structure for reaching a total elongation at break of at least 5%. However, such HE cord is less used due to the high fracture ratio during the tire running.

[8] It is still desired to develop a higher performance 1 xn HE cord with better performance. Disclosure of Invention

[9] The primary object of the invention is to provide a steel cord with sufficient elongation performance meeting the demands of a tire in the different phase of the tire.

[10] Another object of the invention is to provide a tire comprising the steel cord with sufficient elongation performance.

[1 1 ] According to a first aspect of the invention, a steel cord with a construction of 1 xn is provided, n is the number of the steel filaments of the steel cord, the steel cord has an elongation at break At of more than 5.0%, the steel cord has a structural elongation As ranging from 0.86% to 3.50%, At and As satisfy the following formulas: As/At>17%; and (At-As)/At>50%.

[12] It was found that As/At of at least 17% is beneficial for reducing the risk of breakage of steel cord in the tire. As/At reflects the low load elongation performance of a steel cord. During the manufacture of a tire, the rubber is turned to be solid from viscous fluid via vulcanization. In the vulcanization process, the tire will have certain expansion in the radial direction, and the steel cords in the tire are subjected to a low load due to the viscous fluidity of the rubber during the expansion of the tire. The HE steel cords in the zero degree belt ply are arranged along the circumferential direction of the tire with about zero degree or a very minor angle, so that the HE steel cords shall have sufficient low load elongation to accommodate the elongation in the circumferential direction of the tire. If the low load elongation of the HE steel cord is too small, on the one hand, the HE steel cords are not able to reach the predetermined position which is very important for realizing the function of a zero degree belt ply; on the other hand, the other steel cords in the belt ply adjacent to the radial inward side of zero degree belt ply expand outward, and thereby those steel cords may be in contact with the HE steel cords in zero degree belt ply, and this will lead to the abrasion between the HE steel cords in zero degree belt ply and the other steel cord in the adjacent belt ply during the running of a tire, and this increases the risk of breakage of steel cord in the tire. Therefore, the HE steel cords for zero degree belt ply shall have sufficient low load elongation to accommodate the tire expansion and also reduce the possibility of the contact between the steel cords in the zero degree belt ply and the other steel cords in the adjacent belt ply.

[13] To further reduce the risk of breakage of steel cord in the tire, it was found that (At-As)/At shall be greater than 50%. (At-As)/At reflects the high load elongation of the steel cord. Tires are subjected to a lot of impact from various obstacles on the road during service life. If the HE steel cords in the zero degree belt ply or protection belt ply don’t have sufficient high load elongation, the zero degree belt ply or protection belt ply can’t provide sufficient extension deformation to absorb the impact energy, and this leads to a risk of breakage of HE steel cords in the tire and thereby results in a reduced tire life. (At-As)/At being greater than 50% is beneficial for the reduction of such risk.

[14] Therefore, the invention provides a HE steel cord with sufficient elongation performance meeting the demands of a tire in the different phase of the tire.

[15] The invention is for HE steel cord, ‘HE’ means the elongation at break At of the steel cord is more than 5%. Preferably, At is more than 5.5%. More preferably, At is more than 6.0% and less than 9.0%.

[16] Preferably, As/At>18%. More preferably, As/At>20%. More preferably, As/At>22%.

[17] Preferably, (At-As)/At>55%. More preferably, (At-As)/At>60%.

[18] Preferably, As ranges from 1 .0% to 3.0%. More preferably, As ranges from 1 .6% to 2.8%.

[19] The steel cord has a lay length of more than 4mm and less than 10mm, as thus the steel cord has a relatively close construction or a little open construction rather than a loose construction. More preferably, the steel cord has a lay length of more than 7mm and less than 10mm.

[20] Preferably, each of the steel filaments has a tensile strength of more than 3400-2000xD MPa, D being the diameter of the steel filament expressed in mm. More preferably each of the steel filaments has a tensile strength of more than 3600-2000xD MPa. The tensile strength of the steel filament can’t be too high, otherwise the breakage of the steel filament in the manufacture of the HE steel cord will be very high. [21 ] Each of the steel filaments has a diameter D ranging from 0.15-0.45mm.

[22] Preferably, each of the steel filaments is a preformed steel filament.

[23] According to the invention, n is preferably from 2 to 7, and n is more preferably 4 or 5.

[24] The invention HE cord is for reinforcing the zero degree belt ply, and it also available for reinforcing the other belt ply of the tire such as the protection belt ply.

[25] According to a second aspect of the invention, a tire comprises a belt, a carcass, a tread and a pair of bead, while the belt comprises at least one working belt ply and at least one zero degree belt ply and/or a protection belt ply, wherein the zero degree belt ply or the protection belt ply comprises at least one steel cord with a construction of 1 xn, n is the number of the steel filaments of the steel cord, the steel cord has an elongation at break At of more than 5.0%, the steel cord has a structural elongation As ranging from 0.86% to 3.50%, At and As satisfy the following formulas: As/At>17%; and (At-As)/At>50%.

[26] The steel cords in the zero degree belt ply are wound along the tire circumferential direction in 0 degree or with a very minor angle, i.e. ±15 degree.

Brief Description of Figures in the Drawings

[27] Figure 1 describes a load-elongation diagram of a steel cord according to the invention.

Mode(s) for Carrying Out the Invention

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

[29] 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.

[30] 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.

[31 ] 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.

[32] 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.

[33] 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.

[34] 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.

[35] 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.

[36] 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.

[37] Steel filaments adapted for the reinforcement of tires typically have a final diameter D preferably ranging from 0.15 mm to 0.45 mm. Examples of wire diameters are 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.

[38] 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. The desired elongation at break and the structural elongation of the steel cord can be reached by the known mehtods. After the cabling or bunching process of forming a steel cord, the false twister is used for reaching the desired the structural elongation, in particular, the steel cord is firstly overtwisted and secondly untwisted at the false twister which is commonly used in the steel cord manufacturing, i.e., the steel cord is overtwisted to a lay length which is smaller than the final lay length, and then the steel cord is untwisted to the final lay length. The overtwisting and untwising of the steel cord are offset, so that the use of false twister won’t bring any substantial change to the lay length of the steel cord. Furthermore, the desired structural elongation of the steel cord can be obtained by adjusting the preforming on the steel filaments. The desired elongation at break of the steel cord can be obtained by adjusting the lay length of the steel cord.

[39] The first embodiment is a 5x0.35 HE cord, the steel cord has five steel filaments with a diameter of 0.35mm, the lay length of the steel cord is 8mm, the elongation at break of steel cord is 6.62%, the structural elongation of steel cord is 2.32%.

[40] The second embodiment is 5x0.38 HE cord, the steel cord has five steel filaments with a diameter of 0.38mm, the lay length of the steel cord is 8mm, the elongation at break of steel cord is 6.31%, the structural elongation of steel cord is 1 .78%.

[41] The third embodiment is 4x0.40 HE cord, the steel cord has four steel filaments with a diameter of 0.40mm, the lay length of the steel cord is 8mm, the elongation at break of steel cord is 5.46%, the structural elongation of steel cord is 1 .49%.

[42] The fourth embodiment is 5x0.365 HE cord, the steel cord has five steel filaments with a diameter of 0.365mm, the lay length of the steel cord is 6.5mm, the elongation at break of steel cord is 6.66%, the structural elongation of steel cord is 1 .44%.

[43] The load-elongation diagram of the first embodiment is illustrated in FIGURE 1 . The load-elongation measurement starts at a pre-load of 0.5- 1 N. The load-elongation curve of a steel cord of the first embodiment is depicted, while the horizontal ordinate is the elongation s in percentage and the vertical ordinate is the load F in newton. When a tangent line 105 is drawn to the straight part of the curve, this can be extended towards the elongation s axis, and the crossing e₀ corresponds to the structural elongation As. The extreme elongation e_x of the steel cord prior to break is the elongation at break At. [44] Table 1 sumarrizes the invention steel cords and the reference steel cords. The steel cords are in the zero degree belt ply of the tire for the tire endurance test.

[45] Table 1

Claims

1 . A steel cord with a construction of 1 xn, n being the number of the steel filaments of said steel cord, said steel cord having an elongation at break At of more than 5.0%, characterized in that said steel cord has a structural elongation As ranging from 0.86% to 3.50%, said At and said As satisfy the following formulas:

As/At>17%; and, (At-As)/At>50%.

2. A steel cord as claimed in claim 1 , characterized in that At is more than 5.5%.

3. A steel cord as claimed in claim 2, characterized in that At is more than 6.0% and less than 9.0%.

4. A steel cord as claimed in any one of claims 1 to 3, characterized in that As/At>18%.

5. A steel cord as claimed in claim 4, characterized in that As/At>20%.

6. A steel cord as claimed in claim 5, characterized in that As/ At>22%.

7. A steel cord as claimed in any one of claims 1 to 6, characterized in that (At-

As)/At>55%.

8. A steel cord as claimed in claim 7, characterized in that (At-As)/At>60%.

9. A steel cord as claimed in any one of claims 1 to 8, characterized in that said

As ranges from 1 .0% to 3.0%.

10. A steel cord as claimed in claim 9, characterized in that said As ranges from 1 .6% to 2.8%.

11 . A steel cord as claimed in any one of claims 1 to 10, characterized in that the steel cord has a lay length of more than 4mm and less than 10mm.

12. A steel cord as claimed in claim 11 , characterized in that the steel cord has a lay length of more than 7mm and less than 10mm.

13. A steel cord as claimed in any one of claims 1 to 12, characterized in that each of said steel filaments has a diameter D ranging from 0.15-0.45mm.

14. A steel cord as claimed in any one of claims 1 to 13, characterized in that said n is 4 or 5.

15. A tire comprising a belt, a carcass, a tread and a pair of bead, said belt comprising at least one working belt ply and at least one zero degree belt ply and/or a protection belt ply, characterized in that said zero degree belt ply or said protection belt ply comprises at least one steel cord as claimed in any one of claims 1 to 14.