WO2021008853A1

WO2021008853A1 - A steel cord for rubber reinforcement

Info

Publication number: WO2021008853A1
Application number: PCT/EP2020/067887
Authority: WO
Inventors: Haidong XI; Ming Xu; Jing Yang; Zhigao YU
Original assignee: Nv Bekaert Sa
Priority date: 2019-07-17
Filing date: 2020-06-25
Publication date: 2021-01-21
Also published as: CN212713929U; CN112239907A

Abstract

The invention provides a steel cord comprising one core steel filament and sheath steel filaments with a number of n, the core steel filament is untwisted and the sheath steel filaments are twisted around the core steel filament, the core steel filament is substantially linear along its longitudinal direction, the core steel filament is a flattened filament with an aspect ratio of width to thickness at its cross section being in the range of 1.08 to 1.90. The invention steel cord has reduced tip rise problem when it is embedded into the rubber ply for tire making.

Description

Title A steel cord for rubber reinforcement

Description

Technical Field

[1 ] The invention relates to a steel cord for rubber reinforcement. The

invention also relates to a tire reinforced by the steel cord.

Background Art

[2] Steel cord comprising at least two steel filaments is used as a

reinforcement for rubber products, such as rubber belt, rubber tire, rubber hose, etc. The at least two steel filaments are twisted together to form a steel cord.

[3] It is already known that the steel cord for reinforcing rubber tire is required to have a certain tensile strength, bending stiffness, corrosion resistance, fatigue resistance, rubber penetration degree, rubber adhesion property and etc.. Rubber penetration degree is one property of the steel cord that characterizes to what extent the rubber can penetrate into the steel cord. The rubber penetrates into the steel cord and fills the gaps among the steel filaments and thereby reduces the cavity inside the steel cord.

Thereby the moisture is prevented to enter into the steel cord, which avoids that the steel filaments of the steel cord become corroded, and this ensures a long lifetime of the steel cord and the rubber product. Therefore high rubber penetration degree is always desired for a steel cord.

[4] A type of steel cord with a construction of 1 +n is widely used. For reaching high rubber penetration property, the core steel filament is made to have a diameter bigger than the sheath steel filament, such as the steel cord in CN205000184. However, this steel cord suffers from the tip rise problem during the tire making process. A rubber ply which is embedded with steel cords in parallel is one component for tire making. The rubberized steel cord ply is further processed by being cut into small pieces with a certain length, width and thickness. The rubberized steel cord ply is being cut with an angle oblique with the longitudinal axis of the rubberized steel cord ply or being cut perpendicularly to the longitudinal axis of the rubberized steel cord ply. Subsequently, the small pieces of rubber ply, all with the same shape are spliced to a desired length for one tire by machine.

[5] It occurs that after cutting, one or some of the four corners of the small piece of rubberized steel cord ply sometimes rise out of the plane. If the corner rises a certain height, such as ten or more millimetres, this will make the automatic machine splicing impossible, then the splicing can only be done manually, and this leads to a reduced working efficiency.

This is so-called“tip rise” problem.

Disclosure of Invention

[6] The primary object of the invention is to provide a steel cord to solve the problems mentioned above and in particular the tip rise problem.

[7] The second object of the invention is to provide a tire reinforced by the invention steel cord.

[8] According to the first aspect of the invention, a steel cord is provided. The steel cord comprises one core steel filament and sheath steel filaments with a number of n, the core steel filament is untwisted and the sheath steel filaments are twisted around said core steel filament, the core steel filament is substantially linear along its longitudinal direction, the core steel filament is a flattened filament with the cross section having an aspect ratio of width to thickness being in the range of 1.08 to 1.90.

[9] Normally a steel filament for a steel cord has a round or so-called

substantially round cross-section,“substantially round cross-section” means the cross-section is not a perfect round shape because of the nature of the production, wherein the ratio of width to thickness of the filament is about 1 with minor and acceptable difference which is due to production tolerances.

[10] For the invention steel cord, the core steel filament is a flattened filament, i.e. the cross section in a plane perpendicular to the length dimension of the core steel filament shows two flat faces that are substantially parallel to one another, the surfaces of the core steel filament for connecting the two flat faces could be curved surfaces. The perpendicular distance between two parallel lines tangent to the cross section of the wire is the ‘calliper distance’ as it corresponds to the distance between the anvils of a calliper when touching the core steel filament. When now considering the calliper distances in all directions perpendicular to the length of the core steel filament one can find a minimum that will be called‘thickness’ and a maximum that will be called‘width’. Width and thickness of the core steel filament are conveniently measured by micro calliper, the measurement method is as follows: first the steel cord is unravelled and the core steel filament identified; then the maximum calliper distance and the minimum calliper distance at a first place of the core steel filament is determined with micro calliper by turning the core steel filament over all angles and repeatedly measuring the distance; next, at a second and third place the same measurement is performed. The first, second and third place are sufficiently far from one another for example at least one lay length of the sheath steel filaments apart. The average value of the three maximum calliper distances is the width of the core steel filament, and the average value of the three minimum calliper distances is the thickness of the core steel filament. The ratio of the width to the thickness of the core steel filament is in the range of 1.08 to 1.90 and will be called‘aspect ratio’.

[11 ] Surprisingly the inventors found that by introducing a flat or flattened core steel filament that is untwisted, the tip rise problem of the steel cord when it is embedded into the rubber ply is solved.

[12] The sheath steel filaments have a round or substantially round cross- section.

[13] For the invention, the core steel filament is untwisted, this means that the core steel filament is almost straight without twisting, or the twist pitch of the core steel filament is more than 250mm or infinite. According to the present invention,“untwisted” does not mean that the steel filament is not subjected to any twisting process; an untwisted steel filament could be a steel filament firstly being subjected to twisting and then being subjected to un-twisting. Besides flattening, there is no other preforming on the core steel filament, so that along the longitudinal direction the core steel filament is substantially linear or straight,“’substantially linear along the longitudinal direction of the core steel filament” means the core steel filament is not in a wavy or spiral form but in a linear or straight form with minor deviation caused by the production. [14] Preferably, the aspect ratio of width to thickness of the core steel filament is in the range of 1.12-1.55. More preferably the aspect ratio of the core steel filament is in the range of 1.15-1.50. This reduces the tip rise problem of the steel cord in rubber ply very much.

[15] Although the core steel filament is flat, the steel cord is not as flat as the core steel filament. The steel cord has an aspect ratio of width to thickness being in the range of 1.0-1.1. The measurement method of the width and thickness of the steel cord is almost the same as the measurement method of the width and the thickness of the core steel filament, just replacing the core steel filament by the steel cord.

[16] The core steel filament has an equivalent diameter bigger than the

diameter of the sheath steel filament. The equivalent diameter of the core steel filament is the diameter of an imaginary circle that has the same area as the perpendicular cross section of the core steel filament, wherein the area of the core steel filament is determined by dividing the core steel filament linear density by the steel density. Preferably equivalent diameter of the core steel filament is bigger than the sheath steel filament diameter with a difference of at least 0.03mm, preferably the equivalent diameter difference is at most 0.08mm, for example 0.05 or 0.06mm. This can have better improvement on tip raise problem. Preferably, the core steel filament has an equivalent diameter in the range of 0.25mm-0.50mm.

[17] The width of the core steel filament is bigger than the equivalent diameter of the core steel filament. As a preferably solution, the width of the core steel filament is smaller than or equal to 0.55mm. If the width of the core steel filament is too big, it will be difficult to control the cord formation quality.

[18] The thickness of the core steel filament is smaller than the equivalent diameter of the core steel filament. As another preferably solution, the thickness of the core steel filament is at least 0.25mm. If the thickness of the core steel filament is too small, the tire reinforced by the steel cord may have a smaller stiffness in the radial direction that leads to a worse tread wear and bigger rolling resistance. [19] The core steel filament is a flattened filament, the stiffness in width is bigger than the stiffness in thickness. The different stiffness in different direction leads to better handling comfort and turning handling of the tire.

[20] The steel cord can be any one of the existing 1 +n construction steel cord.

Preferably n is 5, 6 or 7.

[21 ] The steel cord has a twist pitch in the range of 10 to 20mm, preferably in the range of 12 to 18mm.

[22] According to the second object of the invention, a tire is provided. A tire comprising the belt layer, the carcass layer, the tread layer and a pair of bead portions, the belt layer and/or the carcass layer is embedded with at least one steel cord comprising one core steel filament and sheath steel filaments with a number of n, the core steel filament is untwisted and the sheath steel filaments are twisted around said core steel filament, the core steel filament is substantially linear along the longitudinal direction of the core steel filament, the core steel filament is a flattened filament with an aspect ratio of width to thickness being in the range of 1.08 to 1.90.

Brief Description of Figures in the Drawings

[23] Figure 1 describes a first preferred embodiment of the invention.

[24] Figure 2 describes the flattened core steel filament of the first embodiment

[25] Figure 3 describes a prior art with a construction of 1 +6.

[26] Figure 4 shows tip rise.

[27] Figure 5 describes a second preferred embodiment of 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 / or by

chemical pickling in a FI2SO4 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 Di, 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 Di 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.

[32] At this second intermediate diameter D2, 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 diameter D2.

[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 thermo-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. The final product is a steel filament with a carbon content above 0.60 per cent by weight, e.g. higher than 0.70 per cent by weight, or higher than 0.80 per cent by weight, or even higher than 0.90 per cent by weight, with a tensile strength typically above 2000 MPa, e.g. above 3800-2000xD (HT) MPa, or above 4100-2000xD MPa (ST) or above 4400-2000xD (UT) MPa (D is the diameter of the final steel filament) and adapted for the reinforcement of elastomer products.

[36] Steel filaments 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. Better that the diameter the steel filament is in the range of 0.25mm-0.50mm.

[37] Then one steel filaments as a core steel filament and n steel filaments as sheath steel filaments are prepared for further so-called cabling or bunching process to form a steel cord. The steel filament for core steel filament is flattened (with a cross-section in flat shape) by passing through a group of rollers to have an aspect ratio of width to thickness being in the range of 1.08 to 1.90 before the sheath steel filaments are twisted around it, while the steel filaments for sheath steel filaments are not flattened (with a round cross-section). Then the sheath steel filaments are twisted around the core steel filament by so-called cabling or bunching process, then a steel cord with a construction of 1 +n is obtained, wherein the core steel filament being untwisted and substantially linear and said sheath steel filaments being twisted around said core steel filament, the core steel filament has a cross-section in flat shape with an aspect ratio of width to thickness being in the range of 1.08 to 1.90.

[38] Preferably, the sheath steel filaments could be pre-formed or without

preforming before being twisted around the core steel filaments.

“Preforming” means the steel filament is preformed in a certain manner, so that the steel filament has a two-dimensional or three-dimensional form, for example wavy or spiral, regularly or repeatedly presenting along the length of the steel filament. Preforming could be any one of existing preforming, such as crimping by a pair of gears, polygonal preforming described in W095/16816, double crimping described in W099/2854 or preforming described in US4258543 or KR100635328.

[39] The core steel filament is only flattened without any preforming, and this means there is no other above-mentioned preforming on the core steel filament, so that the core steel filament is substantially linear but not in a wavy or spiral form along the longitudinal direction of the core steel filament.

[40] Figure 1 shows the first embodiment with a construction of 1 +6. The steel cord 100 has one core steel filament 105 with flat cross-section and 6 sheath steel filaments 100 with round cross-section. The core steel filament 105 is un-twisted and is linear along its longitudinal direction. Figure 2 shows the cross-section of the core steel filament 105.

[41 ] Figure 3 shows the prior art steel cord with a construction of 1 +6. The

steel cord 300 has one core steel filament 305 with round cross-section and 6 sheath steel filaments 300 with round cross-section.

[42] The comparison test is done for the prior steel cord 300 and invention

steel cord 100. Table 1 summarises the test result.

Table 1

[43] Figure 4 shows the measurement of tip rise value. The small piece 400 cut from a rubber ply which is embedded with steel cords 100 has a

determined length, width and thickness. Two of the four corners 405 of the small piece 400 rise. The value T measured from the thickness direction of the small piece 400 is the tip rise value.

[44] Air Drop test method according to the invention has the similar working principle with the Air permeability method described in US2012227885, the differences are: first, the specimen with steel cord in rubber has a length being 90% of lay length of steel cord and with a minimum of 8.0mm; second, for the invention air drop method the air pressure DR showing in the display is the result when the testing time is 60 seconds. The DR result of“100” in the display is recorded as“0%” which means full rubber penetration of the sample steel cord, the DR result of“0” in the display is recorded as“100%” which means no rubber penetration, the DR result of “60” in the display is recorded as“40%” which means 60% rubber penetration.

[45] From Table 1 , it is clear that the invention steel cord has less tip rise

problem compared with the prior art steel cord.

[46] Figure 5 shows the second embodiment of 1 +5. The steel cord 500 has one core steel filament 505 and five sheath steel filaments 510. The core steel filament 505 is un-twisted and is linear along its longitudinal direction The core steel filament 505 has a flat cross-section with an aspect ratio of

1 .21 .

Claims

1. A steel cord, comprising one core steel filament and sheath steel filaments with a number of n, said core steel filament being untwisted and said sheath steel filaments being twisted around said core steel filament, said core steel filament being substantially linear along its longitudinal direction, characterized in that said core steel filament is a flattened filament with a cross-section having an aspect ratio of width to thickness in the range of 1.08 to 1.90.

2. A steel cord according to claim 1 , characterized in that said aspect ratio is in the range of 1.12-1.55.

3. A steel cord according to claim 2, characterized in that said aspect ratio is in the range of 1.15-1.50.

4. A steel cord according to any one of claims 1 -3, characterized in that said width of said core steel filament is smaller than or equal to 0.55mm.

5. A steel cord according to any one of claims 1 -4, characterized in that said

thickness of said core steel filament is at least 0.25mm.

6. A steel cord according to any one of claims 1 -5, characterized in that n is 5, 6 or 7.

7. A steel cord according to any one of claims 1 -6, characterized in that said core steel filament has an equivalent diameter bigger than the diameter of said sheath steel filament.

8. A tire comprising the belt layer, the carcass layer, the tread layer and a pair of bead portions, characterized in that said belt layer and/or said carcass layer is embedded with at least one steel cord as claimed in any one of claims 1 to 7.