JP2007031890A - Steel cord and pneumatic radial tire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel cord excellent in durability in which metal fatigue of a core is prevented by improving fatigue resistance of a straight and large-diameter core filament, and a pneumatic radial tire using the steel cord.
SOLUTION: A core 1 made of one steel filament 2 having a diameter larger than that of sheath filaments 4 and 6, and two-layer sheaths 3 and 5 made of a plurality of filaments arranged around the core 1 are provided. The sheath filaments 3 and 5 are steel cords 10 in which all of the constituent filaments 4 and 6 have substantially the same diameter and are twisted in the same direction and the same pitch, and the core filament 2 has a carbon content of 0.70. It consists of a carbon steel wire of 0.75% by weight, and the sheath filaments 4 and 6 are made of a carbon steel wire having a carbon content of 0.80 to 0.85% by weight, and the strength of the core and the sheath filaments 2, 4, 6 Are 3000 to 3400 N / mm ² .
[Selection] Figure 2

Description

  The present invention relates to a steel cord and a pneumatic radial tire, and more particularly to a steel cord having excellent fatigue resistance and cord productivity, and a pneumatic radial tire having excellent durability using the steel cord.

  As a reinforcing material for carcasses of large pneumatic radial tires for trucks and buses, steel cords with a multi-layer structure such as two-layer twist and three-layer twist have been widely used in the past, especially for tires with severe use conditions. A three-layer steel cord 50 having a small filament diameter and a large number of filaments, such as the 3 + 9 + 15 × 0.175 + 1 structure shown in FIG.

  In the above three-layer steel cord 50, since the twist direction is generally twisted at different pitches in the S / S / Z / S direction, the filament cross-sectional area is increased by fretting between the core and the inner and outer sheaths and the wrapping wire. The cord strength decreases gradually and the strength of the cord gradually decreases, and the plating is cut to reduce the corrosion fatigue resistance, and when fretting wear progresses, the cord is damaged due to impact and compression strain during tire use, leading to tire failure. There's a problem. Moreover, the twisted wire process at the time of cord manufacture increases from the twisted structure, and the cost of the tire is also affected as a result of increasing the cord cost.

  In order to improve the deterioration of fatigue resistance due to fretting and the problem of cost, a so-called bundle-twisted cord having a 1 × 12 or 1 × 19 structure in which a large number of filaments are twisted in the same direction and the same pitch will be used. Therefore, various attempts have been made to improve rubber penetration and comparative examples. For example, two or more coaxial layers (sheaths) are arranged around a core made of one steel filament, and the outermost layer has one or two fewer filaments than can be disposed on the circumscribed circle of the coaxial layer. A steel cord having a 1 + 6 + (10-11) structure in which this coaxial layer is twisted in the same direction and at the same pitch is disclosed (Patent Document 1), and a single core (dx) is arranged around the core. A 1 + 18 structure steel cord consisting of a first sheath made of 6 filaments (da) and 12 filaments (db, dc) arranged on the outer periphery of the first sheath, and the diameter of the filament is dx ≧ da ≧ db> It has been proposed to improve the durability by making the cord cross-sectional shape closer to a circular shape from a hexagonal shape using at least three types of filament diameters as dc (Patent Document 2).

In recent years, there has been a demand for weight reduction of tires due to social demands for improving the fuel efficiency of automobiles. Steel cords with higher strength than conventional strength (2500-2800 N / mm ² ) have been developed, and steel in tires has been developed. The amount of code used is reduced to cope with it.
JP-A-8-232179 Japanese Patent Laid-Open No. 2004-9879

  In a steel cord having a 1 + 18 structure (also referred to as a 1 × 19 structure) in which the core at the center of the cord as described above is formed of a single filament, the core filament at the center of the cord is not twisted and the cord is formed in a straight state. Further, in order to provide a gap between the sheath filaments for infiltrating rubber inside the cord to maintain the filament restraint property and reduce the contact pressure, a core filament diameter larger than that of the sheath filament is employed.

  When such a steel cord is used for a tire carcass, when the tire is used on a vehicle that is used for long-distance continuous running or under heavy load conditions, stress strain is particularly likely to occur due to repeated stress applied to the tire. From the side portion to the shoulder portion, the stress strain is concentrated on the core filament that is straight and has a large diameter. As a result, the core filament in the cord breaks ahead of the sheath filament due to metal fatigue, and the core protrudes from the tire side part, causing air leakage, or promoting the fatigue phenomenon of the sheath filament in the cord to cause cord damage There is a problem.

  The present invention has been made in view of the above problems, and by improving the fatigue resistance of a straight and large-diameter core filament, it avoids preceding fracture due to metal fatigue of the core and improves the fatigue resistance of the steel cord. Another object of the present invention is to provide a pneumatic radial tire excellent in durability using this steel cord as a tire reinforcing material.

  The inventors of the present invention have made extensive studies on improving the fatigue resistance of steel cords in a steel cord having a straight core filament such as a 1 + 18 structure. As a result, the materials have high toughness and high strength. The present invention has been completed by finding that the balance of the fatigue resistance of the entire steel cord is improved by arranging the steel filament having excellent fatigue resistance in the core.

That is, the steel cord of the present invention has a core made of one steel filament having a diameter larger than that of a sheath filament, and at least one layer of a sheath made of a plurality of filaments arranged around the core, All the constituent filaments of the sheath are steel cords having substantially the same diameter and twisted in the same direction and the same pitch, and the core filament is made of a carbon steel wire having a carbon content of 0.70 to 0.75% by weight. The sheath filament is made of a carbon steel wire having a carbon content of 0.80 to 0.85% by weight or more, and the strength of the core and the sheath filament are both 3000 to 3400 N / mm ² .

  The steel cord of the present invention has a bundle-twisted structure in which the filaments constituting the sheath are twisted in the same direction and the same pitch, and the fretting wear is made uniform by making line contact between the filaments. The strength of the cord can be maintained by significantly reducing the cord than the layered cord, and the production can be performed at a low manufacturing cost.

The carbon content of the core filament is 0.70 to 0.75% by weight, the carbon content of the sheath filament is 0.80% by weight or more, and the strengths of the core and the sheath filament are both 3000 to 3400 N / mm ² . As a result, the high strength of the cord is ensured, and the fatigue resistance of the core filament made of a high carbon steel wire having a carbon content of 0.70 to 0.75% by weight, which is excellent in toughness, is improved and becomes a neck of fatigue. It is possible to prevent preceding destruction due to metal fatigue of the straight and large-diameter core.

  In the steel cord of the present invention, the diameter (D1) of the core filament is 0.175 to 0.34 mm, and the ratio D2 / D1 between the diameter of the core filament and the sheath filament (D2) is 0.75 to 0. .95 is preferable, and in particular, by making the core filament slightly thicker than the sheath filament, the contact pressure between the filaments can be lowered to further improve the fatigue resistance. However, if the difference in filament diameter between the core and the sheath is too large, the sheath filament will move easily and the fretting resistance, fatigue resistance and process passability will be reduced. Therefore, the lower limit of D2 / D1 should be about 0.75. Is preferred.

  The pneumatic radial tire of the present invention is a tire using the steel cord as a reinforcing material, and in particular, by using it as a carcass ply, the durability of the carcass is improved, and the tire life is prolonged and the renewability is improved. Moreover, since the said steel cord improves corrosion resistance, it can be used suitably also as reinforcement members, such as a belt and a chacher. When used for a carcass, it is useful for a tire having a flatness ratio of 80% or less where stress strain tends to concentrate from a sidewall to a so-called buttress portion of a shoulder portion.

  According to the steel cord of the present invention, the fretting wear between filaments is reduced, the strength of the cord is maintained, the fatigue resistance of the core filament is improved, and the preceding failure due to the metal fatigue of the core filament is prevented to prevent the steel cord. As a result, the service life of the pneumatic radial tire using the steel cord can be greatly extended.

  Hereinafter, a steel cord and a pneumatic radial tire according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a half sectional view of a tire T showing an example of a pneumatic radial tire for truck / bus according to the present invention.

  The tire T has a single carcass ply 24 in which steel cords 10 extending in the tire width direction are arranged at predetermined intervals in the tire circumferential direction between a pair of left and right bead portions 25, and both end portions thereof are bead portions 25. A bead filler 27 is sandwiched around a bead core 26 embedded in the tire so as to be folded and locked from the inside to the outside of the tire. Four belt plies 21 made of a steel cord for a belt such as 3 × 0.20 + 6 × 0.35 are disposed on the outer peripheral side of the carcass ply 24 of the tread portion 23.

  The steel cord used for the carcass ply 24 is a steel cord 10 having a 1 × 0.20 + 18 × 0.18 structure shown in the cross-sectional view of FIG.

  The steel cord 10 includes one core filament 2 that constitutes the core 1 that is the central structure of the cord, six filaments 4 that constitute the inner sheath 3 disposed around the core filament 2, and the inner sheath 3. The sheath filaments 4 and 6 have the same diameter, and the core filament 2 has a larger diameter than the sheath filaments 4 and 6.

  In the steel cord 10, the filaments 4 and 6 of the inner and outer sheaths 3 and 5 arranged around the core filament 2 are all twisted in the same direction and at the same pitch, and the cord cross-sectional outline is substantially hexagonal as shown in the figure. The core filament 2 is located at the center of the cord 10 and extends straight in the cord axis direction to form a cord.

  This steel cord 10 uses a normal tubular-type strand wire machine or buncher-type strand wire machine, and simultaneously supplies 19 filaments to the strand wire machine, and each filament is arranged concentrically in a 1 + 6 + 12 configuration from the end plate. The cords are then converged and manufactured by a single stranded wire process, and the manufacturing cost can be reduced by reducing the number of stranded wires in the cord.

  The diameter (D1) of the core filament and the diameter (D2) of the sheath filament constituting the steel cord 10 are each in the range of 0.175 to 0.34 mm, and a filament thinner than 0.175 mm is produced by wire drawing. It is not suitable for the cost as a steel cord for tires, and if it exceeds 0.34 mm, it becomes rigid and deteriorates the workability of the stranded wire, and also adversely affects the tire characteristics and fatigue resistance.

  When used as a carcass cord or a chacher cord, a cord of about 0.175 to 0.25 mm, more preferably 0.23 mm or less is preferable from the viewpoint of ensuring fatigue resistance and flexibility. The belt cord is preferably 0.22 to 0.34 mm in order to ensure belt rigidity.

Each of the steel filaments 4 and 6 constituting the sheaths 3 and 5 is made of a carbon steel wire having a carbon content of 0.80 to 0.85% by weight or more, for example, a high carbon steel made of a piano wire defined in JIS G3502. Can be used. Steel wire with a strength of 3000 to 3400 N / mm ² is obtained by repeatedly applying patenting (heat treatment) and dry wire drawing to an intermediate wire of the desired diameter, coating the brass plating, and then final processing by wet wire drawing. Can be produced by conventional methods.

If the filament strength is less than 3000 N / mm ² , the amount of cord used for securing the tire strength is not sufficiently reduced, and the weight of the tire cannot be reduced. On the other hand, a strength exceeding 3400 N / mm ² will be obtained from the wire. If this is the case, strong processing is required at the time of wire drawing, resulting in deterioration of wire drawing workability and brittleness, which may reduce fatigue resistance and tire durability.

In the steel cord 10 of the present invention, the filament 2 constituting the core 1 is made of a carbon steel wire having a carbon content of 0.70 to 0.75% by weight and has a strength of 3000 to 3400 N / mm ² . It consists of a filament.

  In the present invention, fatigue resistance is achieved by performing appropriate patenting and wire drawing without impairing the toughness of the carbon steel wire having a carbon content of 0.70 to 0.75% by weight, which is excellent in material toughness. A high-strength filament excellent in properties can be obtained.

  By adopting the steel cord 10 as described above, the fatigue limit of the core filament 2 is improved, and the fatigue resistance of the straight and large-diameter core filament 2 is improved. In particular, the repeated stress strain applied to the tire when used in a carcass Fatigue failure of the core filament 2 that is likely to occur prior to the other sheath filaments 4 and 6 that are twisted in the portion where the strain tends to concentrate from the tire side portion to the shoulder portion. Can be prevented.

The strength of the core filament 2 obtained from the carbon steel wire having a carbon content of 0.70 to 0.75% by weight is 3000 to 3400 N / mm ² by optimizing the patenting and drawing conditions of the carbon steel wire. By this optimum processing, the pearlite structure in the steel becomes a uniform fine and strong structure, and defects in the structure are difficult to be connected to each other, and the strength and fatigue limit can be improved. However, if the workability is increased too much, even if the strength increases, the ductility of the steel deteriorates and the fatigue limit cannot be increased, adversely affecting the fatigue resistance, and disconnection in the cord manufacturing process. Therefore, the upper limit of the strength is set to 3400 N / mm ² .

  In order to obtain a filament having the above-mentioned strength level from a carbon steel wire having a carbon content of 0.70 to 0.75% by weight, for example, a method described in JP-A-9-241982 can be used. Moreover, it is possible to obtain a steel filament having the above-mentioned strength level by applying a carbon steel wire having a carbon content of 0.70 to 0.75% by weight to the method described in Japanese Patent No. 3437247.

  In the steel cord 10 of the present invention, the ratio D2 / D1 between the diameter (D1) of the core filament 2 and the diameter (D2) of the sheath filaments 4 and 6 is preferably 0.75 to 0.95.

  Thus, by making the core filament diameter D1 slightly larger than the sheath filament diameter D2, a slight gap S is provided between the sheath filaments 4, 4 and 6, 6 to reduce the contact pressure between the filaments. In addition to improving fatigue, it is possible to improve the shape stability of the cord by intruding rubber into the gap S and improve corrosion fatigue resistance. The filament restraint is improved, the movement of the filament in the cord is suppressed, and the cord can be used as a reinforcing material such as a belt or a chacher in which the cord end is free.

  However, if the difference between the filament diameters of the core 1 and the sheaths 3 and 4 is too large, the gap S becomes too large, and the sheath filaments 4 and 6 are easily moved during cord manufacturing and processing. Reduces fatigue and deteriorates processability in calendering. Further, if D1 of the core 1 is made thicker, the fatigue resistance of the core filament 2 itself and the balance of the entire cord are lowered and twisting is liable to occur. Therefore, D2 / D1 is preferably limited to about 0.75. When D2 / D1 exceeds 0.95, the rubber penetration is insufficient and the effect cannot be obtained.

  The pneumatic radial tire according to the present invention uses the steel cord for the carcass 24 of the tire T shown in FIG. 1 to improve the fretting wear of the steel cord 10 and the fatigue resistance of the core, and has excellent carcass durability. This is a pneumatic radial tire with a long service life. In particular, as shown in FIG. 1, it is suitable for heavy-duty large tires for heavy loads used for trucks and buses having a flatness ratio ((tire height / tire cross-sectional width) × 100) of 80% or less.

  Furthermore, the effect of rubber penetration into the inside of the cord improves the restraint of the core and inner sheath filament and suppresses the movement of the filament, so that the cord 21 can be used as a reinforcing material for the belt 21 and the chacher 22 used in a free state. It can be used and contributes to cost reduction of tires.

  In the above embodiment, the present invention has been described with a steel cord having a 1 + 18 structure. However, the present invention is a two-layer structure such as 1 + 6, a three-layer structure such as 1 + 6 + (10-11), a 1 + 6 + 12 + (16-18), or the like. The present invention can also be implemented in a steel cord having one straight core filament, such as a four-layer structure.

  Next, the present invention will be specifically described with reference to examples. The present embodiment is an example in which the steel cord of the present invention is applied to the carcass of a large pneumatic radial tire, but the present invention is not limited thereby.

  Each steel cord having the 1 + 18 structure shown in Table 1 was manufactured using a normal tubular type stranded wire machine. Each filament used in these steel cords is a piano wire rod SWRS72A material and SWRS82A material diameter 5.5mm rod specified in JIS G3502, and repeats patenting and dry wire drawing to obtain an intermediate wire with a predetermined diameter. After brass plating (copper ratio 64%, plating adhesion amount 4.5 g / Kg), the number of dies, the degree of processing and the wire drawing speed were changed using an ordinary wet wire drawing machine, and wire drawing was performed. Each filament described in the table was produced. In the table, piano wire materials are abbreviated as 72A material and 82A material.

The strength of each filament was determined by measuring the filament strength (cutting load) according to the method of JIS G3510 and using the following equation (1). Strength (N / mm ² ) = Strength of filament (N) / Cross sectional area of filament (mm ² ) (1)

  The following evaluation was performed using each steel cord. The results are shown in Table 1.

[Belt fatigue]
A belt strip-shaped vulcanized sample with a width of 2.5 cm and a length of 40 cm, in which steel cords are embedded in rubber at a rate of 17 cords / 2.5 cm, is prepared, and a 1 inch pulley is mounted on a firestone belt fatigue tester. It was used for bending fatigue, and the number of life cycles (times) until the cord was broken and the sample was broken was measured. Table 1 shows an index with the conventional example set to 100. The larger the index, the better the fatigue resistance.

[Process passability]
Next, in the calendering process using each steel cord, a topping operation with an opposite width of 1 m arranged at 17 / 2.5 cm was performed, and the processability of each steel cord was evaluated as follows.
○: No problem in process passability.
(Triangle | delta): Some disorder | damage | failure generate | occur | produced in the code density at the time of calendar roll passing.
X: Disturbance of code density occurred when passing through the calendar roll, and it was necessary to temporarily stop the operation of the calendar and correct the operation conditions.

[Tire durability]
A radial tire of size 225 / 80R17.5, in which each topping coated with rubber in the calendar process was applied to a carcass ply, was prototyped, and durability was evaluated by a drum test under the following conditions. A common member was used for each part other than the carcass, and the belt was a four-sheet belt made of steel cord 3 × 0.20 + 6 × 0.35HT.
Drum test conditions: A drum tester having a rotating drum with a diameter of 1700 mm made of steel with a smooth surface was set at a constant ambient temperature of 38 ± 3 ° C., a tire internal pressure of 900 KPa, and a speed of 56 Km / h, and 85% of the maximum load specified by JATMA. 4 hours, then 90% of the maximum load for 6 hours, and then 100% of the maximum load for 24 hours. At this time, if there was no abnormality in the appearance and the inner surface, the vehicle was further run at 140% of the maximum load until a failure occurred. The travel distance until the failure occurred is shown in Table 1 as an index with the conventional example 1 as 100. The larger the index, the better the durability.

  As shown in the table, compared to conventional products, in the examples, the fatigue resistance of the core is improved, so that both belt fatigue and tire durability are excellent, the process passability is good, and the tire performance and processability are improved. It can be compatible. On the other hand, in Comparative Example 1 using the conventional strength level filament made of 72A material for the core, since the core strength is low, both belt fatigue and tire durability tend to decrease. Further, in Comparative Example 2 in which D2 / D1 is small, the sheath filament tends to be biased and the process passability is difficult. In Comparative Example 3 in which D2 / D1 exceeds 1, the cord cross-sectional shape is collapsed and becomes distorted. When used in a carcass, the individual filaments are not evenly strained and the fatigue resistance deteriorates. In addition, the filament easily moves in the tire manufacturing process, and the process passability is inferior.

  As described above, the steel cord according to the present invention is excellent in fatigue resistance, and is therefore suitable as a carcass material for pneumatic radial tires. Particularly, heavy load pneumatic for trucks and buses having a flatness ratio of 80% or less. The durability life of the radial tire can be improved. In addition to the carcass, it can be used as a belt or chacher, or as a reinforcing material for rubber products such as industrial belts and rubber crawlers.

1 is a half sectional view of a pneumatic radial tire according to an embodiment. It is sectional drawing of the 1 + 18 structure steel cord of embodiment. It is sectional drawing of the conventional 3 + 9 + 15 + W structure steel cord.

Explanation of symbols

10 …… Steel cord 1 …… Core 2 …… Core filament 3 …… Inner sheath 4 …… Inner sheath filament 5 …… Outer sheath 6 …… Outer sheath filament S …… Gap

Claims (5)

A core composed of one steel filament having a diameter larger than that of the sheath filament, and at least one layer of sheath composed of a plurality of filaments arranged around the core;
All the constituent filaments of the sheath are substantially the same diameter, and are steel cords twisted together in the same direction and the same pitch,
The core filament is made of a carbon steel wire having a carbon content of 0.70 to 0.75% by weight, the sheath filament is made of a carbon steel wire having a carbon content of 0.80 to 0.85% by weight, and the core and sheath A steel cord characterized in that the strength of each filament is 3000 to 3400 N / mm ² . The diameter (D1) of the core filament is 0.175 to 0.34 mm, and the ratio D2 / D1 between the diameter of the core filament and the sheath filament (D2) is 0.75 to 0.95. The steel cord according to claim 1. A pneumatic radial tire using the steel cord according to claim 1 or 2 as a reinforcing material. The pneumatic radial tire according to claim 3, wherein the steel cord is used for a carcass ply. The flat radial tire according to claim 4, wherein the flatness is 80% or less.

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