JPH08226085A - Ultra-high-strength steel cord and radial tire reinforced therewith - Google Patents

Abstract

PURPOSE: To produce a steel cord for rubber reinforcement, having a simple structure at low cost, capable of efficiently manifesting ultra-high mechanical strength at least comparable to those of conventional cords composed of four wires, also excellent in both corrosion resistance and rigidity, and high in flexural fatigue resistance despite being relatively larger in diameter of each of the wires. CONSTITUTION: By using a wire made of a carbon steel wire 0.80-0.89wt.% in carbon content, having tensile strength satisfying the relationship: Y>=-200d+400, and <7% in the torque drop-off when subjected to such a twisting-torque test as to apply one-way torsion followed by reverse torsion to a specimen, two of such wire are bundled in nearly parallel with each other and a single wire is spirally wrapped around the resultant bundle at a pitch of 45-65 times the diameter of this wire, thus obtaining the objective steel cord.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel cord used for reinforcing a rubber product and a radial tire using the same.

[0002]

2. Description of the Related Art Conventionally, a steel cord has been used as a rubber reinforcing material in a radial tire for a vehicle. Of these, a steel cord for reinforcing a belt of a tire for a passenger vehicle is 4 to 4 in consideration of steering stability. A steel cord having a 1 × 4 structure or a 1 × 5 structure in which five strands (steel wires or filaments) are twisted together is often used. However, the steel cord having the 1 × 4 or 1 × 5 structure generally has a cross-sectional shape as shown in FIG. 1, and has almost no gap between the cord strands. Therefore, in the vulcanization step after tire molding, the rubber hardly penetrates into the inside of the cord, and a void not filled with the rubber exists in the longitudinal direction at the center of the cord. For this reason, if stones or metal fragments cause damage during running, moisture will infiltrate from the scratches, and the moisture will reach the cord, and the moisture will further propagate through the void in the center of the cord, which will cause rust to occur in the cord length. Will be propagated to. As a result, the bond between the vulcanized cord and the rubber around it is broken, causing so-called separation, which significantly shortens the life of the tire and the function of the tire as a composite. It will also cause the problem of a significant reduction.

Moreover, recently, in order to reduce the fuel consumption of automobiles, there is a strong demand for weight reduction of tires, and at the same time, there is a strong demand for cost reduction. As a measure against the former, it is effective to increase the strength of the steel cord to reduce the amount of cord used per unit and to reduce the diameter of the cord to reduce the amount of rubber covering it. However, as a measure for improving the strength, conventionally, in the relationship between the wire tensile strength Y and the wire diameter d, Y ≧ −200d + 3
High strength strands of about 65 (kgf / mm ² ) have been put into practical use, and this strength level was still insufficient. Also, in order to reduce the cost of the latter, it is effective to set the number of strands that compose the steel cord to three.
In the case of the x3 structure, the cross-sectional shape is as shown in FIG. 2, and therefore the problem of rubber permeability still arises. In addition, in order to maintain the strength required for the steel cord, it is necessary to increase the wire diameter, but when the wire diameter increases, the wire diameter effect greatly reduces the fatigue resistance against bending. Therefore, although it is necessary that the wire constituting the steel cord has extremely high strength and good toughness, there has been no wire having such characteristics in the past, and the above requirements could not be met.

The present invention was devised in order to solve the above-mentioned problems, and its purpose is to have an ultrahigh strength equal to or higher than that of a conventional cord having four strands with a simple structure. It is an object of the present invention to provide a steel cord for rubber reinforcement, which is capable of efficiently exhibiting the above-mentioned properties, has excellent corrosion resistance and rigidity, and has excellent bending fatigue resistance in spite of a large wire diameter, and is inexpensive. Another object of the present invention is to provide a radial tire which has a long life, is excellent in steering stability, and can be reduced in weight. The steel cord of the present invention is suitable for radial tires, but is also suitable as a reinforcing material for rubber products such as conveyor belts.

[0005]

In order to achieve the above object, the present invention provides a steel cord consisting of three strands of carbon steel wire rods containing carbon in an amount of 0.80 to 0.89% by weight. Made, the tensile strength satisfies the following formula,
In addition, after twisting in one direction, use a twisting-torque test that gives a twist in the opposite direction with a torque reduction rate within the range of 7%. Bundle these two strands in parallel and The present invention is characterized in that a single wire is wound in a spiral shape at a pitch of 45 to 65 times the wire diameter. Y ≧ −200d + 400 [Y: tensile strength (kgf / mm ² , d: strand diameter (m
m)] Another feature of the present invention is that the steel cord is used for reinforcing the belt layer.

[0006]

Function: By reducing the number of strands and increasing the diameter of the strands, the workability can be lowered, so that the manufacturing cost is reduced. Moreover, since the 2 + 1 structure is used, the rubber penetrates well into the inside of the cord, and therefore has excellent separation resistance. Also, since the two wire bundles are almost parallel to each other, there is little decrease in strength that occurs when twisting the wires of the wire, and one wire is wound at a long pitch of 45 to 65 times the wire diameter. In addition, since the strength utilization rate is high, a cord having higher strength can be obtained. Moreover, the strength of the strands forming the cord is Y ≧ −200d + 400 (kg
f / mm ² ) and higher strength than conventional high strength materials,
In addition, a material having excellent toughness is used, in which a torque reduction rate in a twist-torque test in which a backward twist is applied after a unidirectional twist is within a range of 7%. For this reason, there is little decrease in the strength of the strand when the cord is used, and a higher cord strength is obtained in combination with the effect of the long winding pitch described above, and there is little reduction in fatigue resistance even when the strand diameter increases. It can be a practical steel cord.

The present invention will be described in detail below with reference to the accompanying drawings. FIG. 3 (a) shows an example of a steel cord according to the present invention, which basically has three wires 1a, 1a, 1 having the same diameter.
Although it is made of b, it has a 2 + 1 structure in which two strands 1a, 1a are bundled substantially in parallel, and one strand 1b is spirally wound around this. The winding pitch P of the wire 1b is in the range of 45 to 65 times the wire diameter d. The winding pitch was limited to 4 of the wire diameter d.
This is because if it is less than 5 times, the twisting efficiency is low, or the rubber permeability is slightly low, and if the pitch is larger than 65 times the wire diameter d, the cords are likely to come apart.
Generally, the wire diameter d is appropriately selected from the range of 0.22 to 0.35 mm. FIG. 3B schematically shows the cross-sectional shape at each position where one pitch of the steel cord is divided into five. If three steel wires are simply twisted at a time to form a steel cord with a 1x3 structure, the cross-sectional shape will be as shown in Fig. 2, and rubber will hardly penetrate into the center of the cord. Although a void remains, in the cord of the present invention, the two strands 1a, 1 bundled in substantially parallel to each other are used.
Since one strand 1b is wound around a, there is no closed contour portion in which three strands are adjacent to each other. Therefore, the adhesion area with the rubber is wide, the rubber can be adhered to every corner, and the separation resistance is excellent. Also, since the bundles of two strands are almost parallel,
The strength reduction that occurs when twisted into a cord is small, and the winding pitch can be made large enough to prevent the cord from becoming loose, and the strength utilization factor is high, so that a cord having a higher strength can be obtained. The two strands of wire 1a, 1a that are bundled substantially in parallel may be in close contact with each other at all times, or may have a portion that is appropriately separated, such as the cross section of.

Strands 1a constituting the steel cord,
1a and 1b have ultrahigh strength and higher toughness, which is higher than the conventional strength. More specifically, first, the strands 1a, 1a, 1b of the present invention have a carbon content of 0.80-0.
It was obtained by drawing a carbon steel wire rod of 89% by weight to a predetermined intermediate diameter, and then subjecting it to heat treatment, plating and drawing.
The lower limit of the carbon content of the carbon steel wire rod is set to 0.80%, so that when the carbon content is below this range, the tensile strength is Y ≧ −200d + 4 even if suitable final wire drawing conditions described later are adopted.
This is because 00 (kgf / mm ² ) cannot be obtained. The upper limit is set to 0.89% for the following reason. That is, as a method for increasing the tensile strength, a wire having a high carbon content such as 0.90% by weight or 1.0% or more by weight is used, and the wire drawing workability is also increased to increase the strength by work hardening. It is possible to raise it. However, a wire having a high C content is expensive and its heat treatment is difficult. Also in the wire drawing process, since high strength (high hardness) material is drawn out using a die, the drawing force becomes high and the die wear becomes severe, or the wire cannot be pulled out and disconnection occurs frequently. This is because the target wire is likely to be practically unobtainable. As specific chemical component composition, C: 0.80 to 0.89% (preferably 0.80 to 0.85%), Si: 0.15 to 0.3
5%, Mn: 0.3 to 0.9%, balance iron and unavoidable impurities.
A predetermined amount of Ni, Ni, or the like may be added as an alloy element.

The wires 1a, 1a, 1b in the present invention are
As shown in FIG. 9, the tensile strength is Y ≧ −200d + 400.
It has a characteristic of (kgf / mm ² ). This is because it is necessary to obtain a good reinforcing effect with a small number.
The upper limit of this strength level is -200d + 4 at the above carbon amount.
It is about 30. It is a feature of the present invention to use those having this strength and having a torque reduction rate within a range of 7% in a twist-torque test in which a backward twist is applied after a unidirectional twist. . A twist-torque test in which a backward twist is given after a unidirectional twist is to grasp the wire 8 at a predetermined gripping interval as shown in FIG. 8, and in this state, apply a slight tension in the wire axis direction at a constant speed. After twisting in one direction, for example, clockwise for a predetermined number of times, twist it in the opposite direction, for example, counterclockwise, until the wire breaks-
This is a test for measuring torque.

The reason why the test method and the result thereof are adopted as parameters is as follows. That is, when a twist-torque curve is measured by twisting in one direction as shown in FIG. 4A, a normal curve in which the torque continuously rises to the right leads to breakage, and a breakage results. It appears that a torque drop occurs between them. It is considered that such a decrease in torque is caused by cracking due to fine defects generated inside the wire due to the wire drawing strong working. However, when we actually used a wire that did not show a decrease in torque in this test and twisted or wound it to make a steel cord, there were many wires that were broken or had insufficient fatigue properties. Appeared. Therefore, the judgment of the toughness was insufficient and inaccurate in the torque reduction judgment by this test.

Therefore, as shown in FIG. 4 (b), the present inventor continuously detects not only the twisting-torque in one direction but also the twisting-torque in the opposite direction, and the reverse direction is detected. The torque decrease in the twisting-torque process was measured. As a result, in such a unidirectional-reverse twisting torque test, the wire that had little or no decrease in torque during the reverse twisting process had high strength and good fatigue resistance, and was used in the steel cord manufacturing process. It was found that there is little decrease in breaking strength due to twisting, twisting, or winding, and fatigue resistance tends to be good. On the other hand, a wire that does not show torque failure in the one-way twisting process but has a large torque decrease in the reverse twisting process has a wire breakage in the cord manufacturing process, and the twisting efficiency is poor. Did not sufficiently exhibit the strength of the wire, and the improvement in fatigue resistance was still insufficient.

Based on this knowledge, the present inventor further conducted a twist-torque test by a unidirectional-reverse-direction twist method on a large number of wires having different diameters and materials, and measured the reduction rate of the twist-torque. saw. As a result, it was found that, in any case, if the torque reduction rate is 8% or more, the above-mentioned good characteristics cannot be obtained. Here, the torque decrease rate ΔT
In the torsion-torque curve of FIG. 4 (b), T is the torsional elastic limit in the first unidirectional twist, that is, the torque value at the upper limit of the straight line rising to the right in the figure is T. When the minimum value is t, the torque decrease rate ΔT is expressed by the following equation. However, when there is no torque decrease, t = T. ΔT = [(T− | t |) / T] × 100 (%) If the torque decrease rate ΔT is 8% or more, the above-described problem occurs, and therefore, in the present invention, the torque decrease rate ΔT is 7%. %
The parameters that normalize the toughness of only the steel wire showing the characteristics below were adopted.

A strand having the above-described tensile strength characteristics and toughness characteristics in which a torque reduction rate in a twist-torque test in which a backward twist is applied after a unidirectional twist is within 7% is a carbon steel wire rod. Of 4.0 to 5.5 mm, pickled and coated as usual, and continuously dry drawn to obtain an intermediate wire having a diameter of 1.2 to 2.3 mm. The wire is patented to form a uniform fine pearlite structure that does not contain different structures such as bainite, and then plated with good adhesion to rubber (usually brass plating) to obtain the final raw material wire. It is obtained by wet drawing a wire. And it is suitable to employ the following conditions in the wet drawing process. The approach angle (2α) is 8 ~ as a drawing die.
Use a bearing of 10 ° and a bearing length of 0.3d ₁ (d ₁ = drawing hole diameter). In the final drawing, a double die in which two dies are arranged in series is used, and a light skin pass is performed with the drawing area reduction rate at the exit side die being in the range of 1.2 to 3.9%. As the drawing dies to be used, at least two double dies and one to five upstream dies are sintered diamond nibs. Alternatively, a conventional alloy nib may be used. By keeping the temperature of the lubricating liquid low, the temperature of the wire immediately after passing through the final drawing die is controlled to be 150 ° C or lower.

Explaining these conditions in detail, FIG. 5 shows a drawing die (including a double die for final drawing which will be described later) used in the wet drawing process, and 1 is a die containing a nib 2. , Nib 2 is approach part 2
The angle 2α of 0 is 8 to 10 °, and the length l of the bearing portion 21 is 0.3d ₁ . Conventionally, an approach angle of 12 ° is generally adopted because the drawing force is the lowest, but rather, it is important that the surface and inside of the wire undergo uniform processing and the surface residual stress is also low. Therefore, the present invention sets the approach angle to 8 to 10.
It was something that The bearing length is shortened in order to suppress the increase in pull-out resistance due to this, and the normal pull-out resistance of 0.5d ₁ is too large, which is not suitable.

FIG. 6 shows a double die (finishing die) 3 for final drawing. A normal die 5a and a skin pass die 5b are arranged in series in the casings 4 and 4 in close proximity to each other to achieve a predetermined surface reduction. The rate is divided into two parts. The nibs 2a and 2b of the normal die 5a and the skin pass die 5b are made of sintered diamond, and have the approach angle and the bearing length described above. As described above, 4 including the two nibs 2a and 2b of the double die 3 and the drawing die upstream of this.
The reason why the sintered diamond nibs are used for about one piece is that, firstly, the surface roughness of sintered diamond is much smoother than that of alloy dies, so that the pulling force can be lowered, This is because the surface becomes smooth and is effective in improving fatigue resistance. Secondly, since the sintered diamond is hard, there is almost no wear due to continuous drawing,
This is because it is possible to prevent an increase in die diameter due to wear and a change in surface reduction rate due to this.

The reason why the double die is used as the final drawing die to perform the skin pass is to reduce the heat generation of the wire due to the drawing and to keep the residual stress on the wire surface low. The reason why the drawing area reduction rate (skin pass area reduction rate) by the skin pass die 5b is set in the range of 1.2 to 3.9% is that when the amount is 1.1% or less, the amount of processing is too small to reduce the residual stress relaxation effect. This is because the residual stress relaxation effect is small even if it is too large at 4.0% or more. The most preferable skin pass area reduction rate is 1.5 to 3.0%. The temperature of the wire immediately after passing the final die is set to 15
The temperature of the lubricating liquid is kept low so as to be 0 ° C or less by controlling the temperature of the lubricating liquid to be low in combination with the use of a skin pass, thereby suppressing the wire temperature immediately after passing through the final die to a certain value or less. This is because there is an advantage that the brittleness of the wire due to aging can be prevented. In this way, the method of keeping the temperature of the lubricating liquid low is to install a circulation pump and a cooler outside the tank of the wet wire drawing machine, forcibly draw the circulating liquid from the tank, cool it and return it to the tank. The temperature of the lubricating liquid may be controlled to, for example, 35 ° C. or less during operation. Under the above conditions, it is possible to efficiently manufacture a practical ultrahigh-strength reinforcing material with little deterioration in fatigue resistance even when the wire diameter is increased. FIG. 7 shows a steel cord according to the present invention on a tread 12
The radial tire used for at least one of the belts 13a and 13b below is shown, and it is possible to obtain a long-life lightweight tire having excellent steering stability and corrosion resistance.

[0017]

EXAMPLES Examples of the present invention will be described below. << Specific Example 1 >> C: 0.84, Si:
A carbon steel wire rod consisting of 0.21, Mn: 0.53, balance iron and unavoidable impurities was used. This wire rod was continuously dry-drawn to a predetermined intermediate diameter. Furthermore, after heat-treating this so as to form a fine pearlite structure, it is brass-plated with a predetermined composition for vulcanizing and adhering to rubber to make the final raw material
This raw material was wet-drawn into an ultra-high-strength wire, and then a 2 + 1 structure steel cord was manufactured with a buncher type twisting machine. In the structure of this wire, the die for rising in the final wet drawing was a double die as shown in FIG. 6, and the skin pass area reduction rate was 2.5%. As the dice, two double dies and one upstream die were dies using a diamond nib, and the other dies were conventional alloy dies. The approach angles (2a) of the dies used were all 10 °, and the bearing length was 0.3d ₁ . The rising surface temperature of the wire was 139 ° C. The thus obtained three strands having a diameter of 0.28 mm were twisted into a 2 + 1 steel cord. At this time, the winding pitch of one winding wire was set to 5 levels, and it was set as Examples 1-3 and Comparative Examples 1-2. In addition, Comparative Example 4 was prepared using a conventional high-strength wire having the same diameter. In Comparative Example 3, two pieces of diamond nib dice were simply used from the ascent without using an ascending skin pass in wet drawing, and a steel cord using an element wire produced under the same conditions as in Examples 1 and 2 was used. Furthermore,
A conventional 1 × 3 structure was made using the same strands as in Examples 1 to 3 to make Comparative Example 5, and a conventional high strength material having a 1 × 4 structure was made to be Conventional Example 1. The characteristics of these are shown in Table 1.

<Specific Example 2> As a raw material, C by weight%:
Example 1 using a carbon steel wire rod composed of 0.82, Si: 0.20, Mn: 0.51, balance iron and unavoidable impurities.
A wire having a diameter of 0.23 mm (rising surface temperature: 135 ° C.) was made under the same conditions as in (1), and a cord was obtained using this wire. About them, it was set as Example 4, 5 and the comparative example 6.
In addition, a cord having a 1 × 3 structure was manufactured using the same strands and used as Comparative Example 9. Further, as in the specific example 1, a sample not using the skin pass wire drawing was set as a comparative example 7, and a steel cord using a conventional high strength material was set as a comparative example 8. The characteristics of these are shown in Table 2.

<Specific Example 3> Using the same material as in Specific Example 1, an ultrahigh-strength wire and a cord having a diameter of 0.32 mm were produced and set as Example 6. In addition, a cord having a 1 × 3 structure was manufactured using the same strands and used as Comparative Example 10. Diameter 0.30
mm high-strength wire and a steel cord having a 1 × 4 structure using the same were prepared and used as Conventional Example 2. These characteristics are shown in Table 3.
Shown in

<Specific Example 4> As a raw material, C: 0.
87, Si: 0.23, Mn: 0.50, carbon steel wire consisting of balance iron and unavoidable impurities, and specific examples 1 to 3
A wire and a cord having a diameter of 0.35 mm were produced under the same conditions as in Example 7 to obtain Example 7. In addition, a steel cord having a 1 × 3 structure was manufactured using the same strands and used as Comparative Example 11. These characteristics are shown in Table 4.

In Tables 1 to 4, "Twist-
In the "torque test", as shown in FIG. 8, a straight wire is gripped with a gripping interval L between the fixed-side grip 6 and the movable-side grip 7 of 300 d (d is a wire diameter, mm), and the fixed side is fixed. While suspending the weight and applying a slight tension, the movable grip 7 is rotated by the motor 9 at a speed of 30 rpm to twist the wire 10 times in one direction, then the rotation is stopped once, and the wire is moved in the opposite direction. It is twisted at the above-mentioned twisting speed until the line breaks, and it is determined by taking a twist-torque curve.
○ in “One-way-reverse direction torsion test result” is the torque decrease rate △
T indicates 0 to 7% (good), and x indicates that the torque decrease rate ΔT is 8% or more (defective). "Twisting efficiency" is a value obtained by dividing the actual strength of the cord by the collective strength of the strands before twisting and multiplying by 100. The "bending rigidity index" is an index in which a code sample having a length of 70 mm is bent at a certain angle and the magnitude of the bending moment required for this is determined, and one of the conventional example or the comparative example is set to 100. did. The "fatigue resistance index" is the cord breaking load of 1 on three rolls of a constant diameter in which zigzag samples obtained by vulcanizing steel cords in rubber are arranged in a zigzag pattern.
It is the result of measuring the number of repetitions until the cord is broken by stretching the roll under a load of 0% and repeatedly moving the roll to the left and right. One was set as 100 and expressed by an index. “Rubber permeability” means vulcanizing a linear cord in rubber to make a sample, then disassembling this cord in the longitudinal direction, visually observing the permeability of the rubber, and observing the entire strand of the cord. 100 when the circumference is completely covered with rubber
It was judged as%. In the “strandability”, ⊚ indicates no problem, Δ indicates disconnection, and × indicates many disconnections.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

[Table 3]

[0025]

[Table 4]

As is clear from Table 1, in Examples 1 to 3, the strands have the desired strength and good toughness, and the winding pitch is appropriate, so that the strength and resistance of the steel cord are high. Good balance of fatigue resistance, bending rigidity, twisting efficiency and rubber penetration. On the other hand, in Comparative Examples 1 and 2, although the strand itself is appropriate, the winding twist pitch is out of the specified range of the present invention, so that the twisting efficiency and the rubber permeability are low, and the cord is easy to be separated. Is insufficient. In Comparative Example 3, the conditions of the twisting efficiency and the fatigue resistance are not sufficient because the die conditions in the wet wire drawing process are inappropriate. In Comparative Example 4, although the toughness is good, the tensile strength of the wire is insufficient, so that the properties of the cord strength and the fatigue resistance are poor. In Comparative Example 5, the penetrability is particularly poor because the cord structure is inappropriate.

Further, as is clear from Table 2, the present invention is effective for strength, fatigue resistance, bending rigidity, even when the wire diameters are different.
Good balance of twisting efficiency and rubber permeability. On the other hand, the winding pitch is outside the range of the present invention (too large)
Comparative Example 6 of No. 3 is not suitable because it easily disperses. Further, in Comparative Example 7 in which the skin pass was not performed in the wire drawing step, the properties of the wire were poor, and the twisting efficiency and fatigue resistance properties were insufficient even when used as a steel cord. Comparative Example 8, which uses a wire having a tensile strength outside the scope of the present invention as the wire, is inferior in cord strength and fatigue resistance even though the toughness and the twist pitch are appropriate. In Comparative Example 9, since the cord structure is 1 × 3, the characteristics of the wire are good, but the rubber permeability is particularly poor.

As is clear from Table 3, in Example 6 in which the wire has a large diameter, the steel cord has a good balance of strength, fatigue resistance, bending rigidity, twisting efficiency, and rubber permeability. On the other hand, in Comparative Example 10 in which a suitable wire is used, the cord structure is inconvenient and the rubber permeability is poor. Conventional Example 2 is inferior in cord strength and rubber permeability. As is clear from Table 4, Example 7 in which the wire has a larger diameter is well balanced in all of the strength, fatigue resistance, bending rigidity, twisting efficiency, and rubber permeability of the steel cord. On the other hand, Comparative Example 11 has an inadequate cord structure and thus has poor rubber permeability.

[0029]

As described above, according to claim 1 of the present invention, in the steel cord using three strands, one having ultrahigh strength and truly good toughness is used as the strand, and Since it has a 2 + 1 structure with a predetermined winding pitch instead of 3 twists, it has the same or higher cord strength and rigidity than the high strength steel cord with a 4 twist structure, and there is little decrease in strength during cord production. It has a high utilization rate, good rubber permeability, little deterioration in fatigue resistance, a small number of strands, and a large diameter to reduce the workability. In combination with the fact that the amount of carbon in C is the same as in the past and that it has a simple cord structure, it is possible to obtain an excellent effect such that the manufacturing cost can be reduced.
According to the second aspect, an excellent effect that a tire having good corrosion resistance, good steering stability and lighter weight can be obtained can be obtained.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a conventional steel cord.

FIG. 2 is a cross-sectional view showing a conventional steel cord.

FIG. 3 (a) is a side view showing an example of a steel cord according to the present invention, and FIG. 3 (b) is a sectional view thereof.

FIG. 4A is a diagram showing a twist-torque curve in a one-way torsion torque test, and FIG. 4B is a diagram showing a twist-torque curve in a one-way / reverse-direction torsion torque test according to the present invention. is there.

FIG. 5 is a cross-sectional view of a drawing die used in the present invention.

FIG. 6 is a cross-sectional view of the final drawing die used in the present invention.

FIG. 7 is a cross-sectional view of a radial tire to which the present invention has been applied.

FIG. 8 is an explanatory diagram showing an outline of a torsion-torque tester.

FIG. 9 is a diagram showing the relationship between the wire diameter and the tensile strength.

[Explanation of symbols]

 1a, 1a, 1b Strand T Torque value at torsional elastic limit t Minimum torque value at lowering part

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] April 15, 1996

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0025

[Correction method] Change

[Correction content]

[0025]

[Table 4]

Claims (2)

[Claims] 1. A steel cord consisting of three strands, made from a carbon steel wire rod containing carbon in an amount of 0.80 to 0.89% by weight, having a tensile strength satisfying the following formula, and after unidirectional twisting: , Twist that gives reverse twist-Use a wire whose torque reduction rate in the torque test is within 7%,
An ultrahigh-strength steel cord characterized by bundling such two strands substantially in parallel and winding one strand around this in a spiral shape at a pitch of 45 to 65 times the diameter of the strand. Y ≧ −200d + 400 [Y: tensile strength (kgf / mm ² , d: strand diameter (m
m)] 2. A radial tire comprising the steel cord according to claim 1 for reinforcing a belt layer.