JPH09241982A - High-strength steel cord for rubber reinforcement and radial tire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an inexpensive steel cord for rubber reinforcement, capable of efficiently realizing its mechanical strength comparable to or higher than that of conventional high-strength steel cords even with simple structure, excellent in corrosion resistance, and high in fatigue resistance even in the case of larger wire diameter, by making use of a carbon steel wire rod <0.80wt.% in carbon content as constituent wires. SOLUTION: This high-strength steel cored is obtained by using wires 1a, 1b made of a carbon steel wire rod containing 0.70-0.76wt.% carbon having such characteristics that the tensile strength Z prior to be laid into cord satisfies the relationship: Z>=-200d+365(kgf/mm<2> ) and the ratio of the core strength B after laid to the collective strength A prior to being laid into cord: B/A is >=0.935, and by bundling two of the wires nearly in parallel with each other and wrappingly laying another wire on the above two wires.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength, high-performance and inexpensive steel cord suitable for use in reinforcing rubber products such as automobile tires and conveyor belts for transportation, and a radial tire for automobiles using the steel cord. Regarding

[0002]

2. Description of the Related Art In radial tires for automobiles and passenger cars, as shown in FIG.
a and 13b are indispensable, and as a reinforcing material for these, generally 1 × 4 in which four or five strands are twisted at the same time
Steel cords of structure or 1x5 structure were used. However, in such a structure, as shown in the cross section in FIGS. 2 (a) and 2 (b), since there is almost no gap between the strands, the reinforcing layer of the manufactured tire has rubber up to the inside of the steel cord. It has not penetrated. The function of the composite of the rubber and the reinforcing material in such a state cannot be sufficiently exerted for a long period of time. That is, when the tire is damaged and moisture intrudes into the tire from here, rust propagates through the voids in the steel cord where the rubber has not penetrated. This not only lowers the strength of the steel cord and sometimes breaks it, but also the progress of rust destroys the adhesive layer between the surface of the wire and the rubber, resulting in so-called peeling.
(Separation) phenomenon occurs. As a result, the integrity of the rubber and the steel cord as the reinforcing material is impaired, and the function as a tire is greatly reduced.

Therefore, recently, a steel cord having a structure in which rubber easily penetrates into the cord has been adopted. FIG. 4 shows an example thereof, (a) is a steel of 2 + 2 structure in which two strands b, b are wound around two strands a, a, which are bundled almost in parallel. Is the code. (b) shows five strands a, b, c,
It is a steel cord with a 1 × 5 loose open structure in which d and e are each oversized and twisted loosely.

On the other hand, recently, in order to reduce 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 width in the reinforcing layer. That is, a conventional steel cord strand generally uses carbon steel containing 0.70 to 0.76% by weight of carbon as a raw material, and as shown in FIG. mm) in relation to Z = -200d + (33
5 to 355) (kgf / mm ² ). On the other hand, using carbon steel containing 0.80 to 0.86% by weight of carbon, Z = -200d + (365 to 395) (kgf / m
It is currently becoming widespread to make steel cords using high-strength wires of about m ² ). Further, as a measure for the latter cost reduction, a simple structure, such as a 1 × 3 structure, is used in which the number of strands forming the steel cord is reduced.

However, as described above, the use of a carbon steel wire having a high carbon content as a raw material for the high strength steel cord inevitably increases the raw material cost. Moreover, as the carbon content of the raw material wire becomes higher, segregation is more likely to occur in the steelmaking process, and also in the step of processing the raw material wire rod to produce a wire, it becomes difficult to perform heat treatment, and subsequent drawing and twisting The adverse effect of non-metallic inclusions may appear significantly in the process. Further, if the number of strands of the steel cord structure is reduced to a conventional 1 × 3 structure, the cross-sectional shape as shown in FIG. 3 does not occur and no gaps are formed between the strands. The rubber permeability problem arises.
Moreover, when the number of strands forming the steel cord is reduced, it is necessary to increase the strength of the strand and increase the strand diameter in order to maintain a predetermined strength as the steel cord. If it becomes large, the fatigue resistance against bending will be greatly reduced due to the wire diameter effect. In order to solve this problem, it is necessary to have good toughness and particularly good fatigue resistance, but the fact is that no one having such characteristics has been obtained in the past.

[0006]

SUMMARY OF THE INVENTION The present invention was made by research to solve the above problems, and its purpose is to have a carbon content of 0.8 as a wire.
By effectively utilizing less than 0% of carbon steel wire rod, it is possible to efficiently realize strength equal to or higher than the conventional high strength steel cord with a simple structure, excellent corrosion resistance, and even if the wire diameter is large. An object of the present invention is to provide an inexpensive steel cord for rubber reinforcement having excellent fatigue resistance. The steel cord according to the present invention is suitable not only for radial tires but also as a reinforcing material for rubber products such as conveyor belts. A second object of the present invention is to provide a radial tire which has a long life and is light in weight.

[0007]

In order to achieve the above-mentioned object, the present invention is a steel cord consisting of three strands, made from a carbon steel wire rod containing 0.70 to 0.76% by weight of carbon, The tensile strength before twisting the cord satisfies the following formula, and the ratio B / A of the aggregate strength A of the strand before twisting the cord to the strength B of the cord after twisting is 0.935 or more. A wire is used, two of the wires are bundled substantially in parallel, and one wire is wound around the wire so that the wires are twisted together. Z ≧ −200d + 365 [Z: Tensile strength (kgf / mm ² ), d: Diameter (mm)]

In the present invention, preferably, the strands constituting the cord have a torque reduction rate of 7% or less in a twist-torque curve in a twist test in which twisting in one direction is followed by twisting in the opposite direction. The cord twist pitch is in the range of 40d to 65d with respect to the wire diameter d. Further, in order to achieve the second object, the present invention uses the above steel cord for reinforcing the belt portion.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 (a) shows an example of a rubber-reinforcing steel cord according to the present invention, which is composed of three strands having the same wire diameter, but in particular two strands 1a, 1a.
Are bundled almost in parallel, and a single wire 1b is twisted around this bundle in a spiral shape 2 + 1
It has a structure. In this example, the two strands 1a and 1a are always in close contact with each other, but the invention is not limited to this, and there may be portions separated from each other. FIG. 1B schematically shows a cross-sectional shape at each position where one pitch of the steel cord is divided into five. As described above, if three strands are simply twisted together at the same time to form a steel cord with a 1 × 3 structure, the cross-sectional shape will be as shown in FIG. 3, and most of the rubber will be in the center of the cord. Although voids that do not penetrate are generated in the longitudinal direction, in the present invention, since one strand 1b is wound around the two strands 1a, 1a which are bundled substantially in parallel, the three strands 1a are formed. , 1a, 1b are not adjacent to each other, a so-called closed contour portion does not occur, and a void s reaching the center of the cord always exists at any cross-section position. Therefore, the adhesion area with the rubber is wide, and the rubber can be adhered to every corner, and the separation resistance is excellent. Further, since the two strands 1a, 1a are bundled and are substantially parallel to each other, the pitch P is long enough to prevent the cord from being structurally distorted because the strength is not reduced by twisting the cord. In addition, since the strength utilization factor of the wire is good, the strength of the steel cord can be made higher.

Strands 1a constituting the steel cord,
1a and 1b have strength and toughness comparable to or higher than the wire of a high-strength steel cord using a raw material wire containing carbon in an amount of 0.80 to 0.86% by weight. More specifically, first, the wire strands 1a, 1a, 1b of the present invention are drawn to a predetermined intermediate diameter using a carbon steel wire rod containing carbon in an amount of 0.70 to 0.76% by weight, and then heat treated and plated. It is also made by wire drawing and has a high tensile strength Z before twisting of Z ≧ −200d + 365 (kgf / mm ² ). The lower limit of the carbon content of the carbon steel wire rod is 0.7
0% by weight means that when the carbon content is below this range, the tensile strength is Z ≧ −20 even if suitable final wire drawing conditions are adopted.
This is because 0d + 365 (kgf / mm ² ) cannot be obtained. The upper limit is set to 0.76% because there is a problem that the cost is increased if the amount of carbon exceeds the upper limit. The specific chemical composition is C: 0.70 to 0.76 by weight.
%, Si: 0.12 to 0.35%, Mn: 0.3 to 0.9
%, The balance being iron and unavoidable impurities, but a predetermined amount of Cr, Ni or the like as an alloying element may be added to the basic composition. When the tensile strength Z before twisting is Z <-200d + 365 (kgf / mm ² ), there is little difference from the conventional one, and it is not possible to exert an effect on weight saving of the tire due to high strength.

The present invention further includes three strands 1a, 1a,
1b has a ratio (B / A) of the aggregate strength A before cord twisting and the cord strength B after cord twisting is 0.935.
That is all. In other words, this means that the twisting efficiency is such that the twisting is reduced, and the B / A
Is less than 0.935, the strength is insufficient as a twisted cord even if the tensile strength of the wire satisfies the above conditions, which is not preferable. Further, the twist pitch P of the cord, that is, the winding pitch of the wire 1b is the wire diameter d.
The range of 40 to 65 times is preferable. If the twisting pitch P of the cord is less than 40d with respect to the wire diameter d, the pitch is too short and the twisting efficiency (B / A × 100%) becomes low, or the rubber permeability tends to decrease, which is not possible. . On the other hand, 6
If the pitch P is too long to exceed 5d, the cords tend to come apart, which is not possible. Generally, the wire diameter d is appropriately selected from the range of 0.20 to 0.35 mm.

Further, in the present invention, it is preferable that each strand 1a,
1a and 1b have good toughness before and after twisting. Specifically, this means that in a twisting test in which twisting in one direction and then twisting in the opposite direction is performed, when a continuous curve of twisting-torque is taken, the decrease in torque value before breakage is 7% or less. Is. Explaining in detail, conventionally there is no appropriate evaluation method and scale for toughness,
The toughness was evaluated by the number of times the strands were cut while the strands were twisted in a certain direction while applying a slight tension to the strand axis with a predetermined gripping interval. On the other hand, in the present invention, as a means for judging the toughness, a twisting test that gives a twist in one direction and the opposite direction is adopted, and the torque reduction rate in the twist-torque curve in this test is 7% or less. Was toughness. By adopting this parameter, the strand has both high strength and toughness, the strength decreases little when twisted with the cord (high twisting efficiency), the strength is fully exhibited, and the conventional cord also has fatigue resistance. It is possible to obtain something much better than.

In the toughness judging method according to the present invention, as shown in FIG. 7, the wire W is gripped by the fixed-side grip 6 and the movable-side grip 7 facing each other at a predetermined interval L, and the fixed-side wire is grasped. By hanging a weight on the wire, lightly apply tension in the axial direction of the wire as shown by the arrow.
In this state, the movable-side grip 7 is rotated at a constant speed in a fixed direction (for example, clockwise direction) a predetermined number of times by a motor 9 and twisted, whereupon the rotation of the movable-side grip 7 is temporarily stopped, and then the reverse direction ( For example, the twist-torque curve is taken until the strand is broken by twisting in the counterclockwise direction). When a twist-torque curve is measured by twisting in one direction as shown in Fig. 6 (a), a normal curve in which the torque continuously rises to the right is drawn, and the torque decreases during breakage. Appears. It is considered that such a decrease in torque is caused by cracking from a fine defect generated inside the strand by strong drawing. However, when actually twisting and twisting strands that do not show a decrease in torque in the one-way torsion test, breakage occurs, the strength of the cord decreases greatly, and fatigue resistance also decreases. Many were inadequate. Therefore, the determination of toughness by this test is insufficient and inaccurate.

The inventor tried to obtain a twist-torque curve by twisting in one direction and the opposite direction as shown in FIG. 6B for a number of wires having different diameters and materials. As a result, even in the one-way-reverse twist test, the torque reduction rate was 7%.
It has been found that the following strands have sufficiently high strength and good toughness, and even when twisted into a cord, there is little decrease in strength and the fatigue resistance is also good. On the other hand, although the torque drop does not appear in the twisting test with only one direction,
The toughness of the strands with a torque reduction of 8% or more in the reverse twisting process of the reverse twisting test is obviously insufficient, and even in the twisting process, breakage occurs, and the twisting efficiency is poor, The obtained steel cord did not exhibit the strength of the strand sufficiently, and the fatigue resistance was not sufficient. The torque decrease rate ΔT is the elastic limit of the twist in the first one-way twist in the twist-torque curve of FIG. 6 (b), that is, the torque value at the upper limit of the straight-up straight line portion in the figure is T,
Letting t be the minimum torque value of the lowered portion in the opposite twist,
The torque decrease rate ΔT is expressed by the following equation. However, when the torque drop is 0, t = T. ΔT = [(T− | t |) / T] × 100 (%) The above-described problem occurs in a wire in which the torque reduction rate is 8% or more, and the other wires have sufficient toughness and have a steel cord. Even so, it is most suitable as a reinforcing material.

Next, a method of manufacturing a wire for a steel cord as described above will be described. First, a carbon steel wire rod having a diameter of 4.0 to 5.5 mm of the above-described component composition is pickled and coated as usual, and is subjected to continuous dry drawing to obtain an intermediate wire having a diameter of, for example, 1.2 to 2.3 mm. Get the line. Then, the intermediate line is subjected to a patenting treatment to obtain a uniform fine pearlite structure that does not include a different structure such as bainite.
An alloy (usually brass) plating with good adhesion to rubber is applied to obtain the final raw material wire. Next, the final raw material wire is wet drawn to obtain a target diameter, for example, a diameter of 0.20 to 0.35 m.
m of plated steel wire is obtained. The following conditions are employed in the wet drawing process. Approach angle (2α) is 8 to 1 as a drawing die
0 °, the bearing length is 0.3d ₁ (d ₁ = withdrawal pore size)
Use those. Finish drawing is performed using a double die in which two dies are stacked, and the area reduction rate is 1.2 to 3.9 at the exit side die.
Perform skin pass with%. As a drawing die to be used, a sintered diamond nib is used for at least two double dies and several upstream dies. Otherwise, a conventional alloy nib may be used. The temperature of the wire immediately after passing through the final drawing die is 150 °
C is controlled to be equal to or less than C.

The manufacturing conditions according to the present invention will be described in detail. FIG. 8 shows a drawing die (including a double die for finish drawing which will be described later) used in the wet drawing process. 1 is a die containing a nib 2, and the nib 2 has an approach portion 20 with an angle 2α of 8 to 10 °,
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, and a bearing length of 0.5d ₁ is generally used. The workability of the surface and the inside of the wire drawing wire can be made uniform, and the surface residual stress is also reduced, so that the toughness can be maintained even if the total workability is increased. Further, by shortening the bearing length, the contact length with the wire can be shortened, and the pull-out resistance can be reduced.

FIG. 9 shows a double die 3 for finish drawing, in which normal dies 5a and skin pass dies 5b are arranged in close proximity to casings 4 and 4, respectively, and the predetermined area reduction rate is divided into two. I try to get it. The nibs 2a and 2b of the normal die 5a and the skin pass die 5b are made of sintered diamond, and have the above-described approach angle and bearing length. As described above, by using the sintered diamond nibs for about four or more pieces including the two nibs 2a and 2b of the double die 3 and the drawing dies upstream of the double dies 3, first, the sintered diamond is alloyed. The surface roughness is much smoother than that of, so that the drawing force can be reduced. In addition, the surface of the drawn wire becomes smooth, which is effective in improving fatigue resistance. Secondly, since the sintered diamond is particularly hard, there is almost no wear due to continuous drawing, and it is possible to prevent an increase in the die diameter and a change in the area reduction rate due to the wear. In addition, it is possible to reduce the time and labor required for exchanging the dies and the production stop time. Diamond is expensive in itself, but in view of the above, it is inexpensive overall.

Further, a double pass is used as a finish pulling die, and a skin pass with a reduction in area of 1.2 to 3.9% is performed. Thereby, the heat generation of the wire due to the drawing can be reduced, and the wire temperature immediately after the drawing can be reduced by about 25 to 40 ° C. as compared with the case of the single die. In addition, the residual stress on the wire surface can be suppressed to the negative side. The reason why the drawing area reduction ratio by the skin pass die 5b is set in the range of 1.2 to 3.9% is that if the amount is 1.1% or less, the working amount is too small and the residual stress relaxation effect is small.
This is because the residual stress relaxation effect is small even if it is 4.0% or more, which is too large. Then, the lubricating liquid temperature is kept low so that the temperature of the wire immediately after passing through the final die becomes 150 ° C. or lower. Thereby, the embrittlement of the wire due to aging can be prevented in combination with the use of the skin pass. The method of keeping the lubricating fluid temperature low is to install a circulation pump and a cooler outside the tank of the wet wire drawing machine, forcibly extract the circulating liquid from the tank, cool it, and return it to the tank. May be controlled to, for example, 35 ° C. or less during operation. By adopting the above-mentioned final wire drawing step conditions, while using a carbon steel wire rod having a carbon content of 0.70 to 0.76 wt% which does not increase the manufacturing cost as a raw material, the strength is high and the toughness is high. It is possible to obtain excellent strands, and therefore, even with a cord having a small number of strands, the number of strands is less reduced by twisting and the fatigue resistance is also very excellent.

[0019]

Next, examples of the present invention will be described. [Specific Example 1] 1) As a raw material, the chemical composition is C: 0.75, S in weight%
i: 0.21, Mn: 0. A wire rod having a diameter of 5.5 mm and including 52, the balance Fe, and unavoidable impurities was used. After subjecting the raw wire material to a pretreatment such as pickling and coating,
Continuous dry drawing was performed to obtain an intermediate wire. After heating this intermediate wire in a gas furnace, quenching it in a fluidized bed furnace (patenting treatment), followed by electrolytic pickling, followed by two-layer electroplating of a predetermined amount of copper and zinc, and then The plating was thermally diffused in a fluidized bed furnace to brass plating, which was the final raw material. 2) Further, the final raw material is continuously wet-drawn to have a diameter of 0.2.
An 8 mm wire was manufactured. Change the wire drawing conditions at this time
(However, the temperature of the wire immediately after passing through the final die was controlled to 150 ° C. or less by the measurement value of the heat flux thermometer). Then, a steel cord having a 2 + 1 structure was manufactured by using a buncher type twisting machine by using these three wires, which were referred to as Examples 1 to 5 and Comparative Examples 1 to 3, respectively. Further, using the same raw material wire rod, a conventional strength element wire was manufactured by a conventional method to manufacture a steel cord, which was referred to as Conventional Example 1. Furthermore, the chemical composition is C: 0.83, Si: 0.21, M in weight%.
A high-strength wire and a cord were prepared by a conventional method using a wire rod having a diameter of 5.5 mm, which was composed of n: 0.50, the balance being iron and unavoidable impurities. Table 1 shows these manufacturing conditions, strand characteristics, and cord characteristics.

[0020]

[Table 1]

[Specific Example 2] As a raw material, a chemical component is contained by weight%
In addition to using a wire rod having a diameter of 5.5 mm, which is composed of C: 0.72, Si: 0.23, Mn: 0.53, the balance Fe, and unavoidable impurities, a diameter of 0.25 m is obtained in the same process as in Example 1.
m was produced. At this time, in the final continuous wet drawing, the conditions were changed to manufacture the wire, and then 2+
A steel cord with one structure was manufactured. These were designated as Examples 6 and 7, respectively. Further, using the same raw material wire rod, a conventional strength element wire was manufactured by a conventional method to make a cord, which was referred to as Conventional Example 2. Furthermore, a high-strength wire and a cord were prepared by the conventional method using the same wire rod as in Comparative Example 4 of Concrete Example 1 and made into Comparative Example 5. Table 2 shows these conditions, strands and cord characteristics.

[0022]

[Table 2]

[Specific Example 3] As a raw material, the same wire rod as that used in the example of specific example 2 was used to manufacture a wire having a diameter of 0.32 mm and a steel cord of 2 + 1 made of the same in the same manner as in examples 8 and 9 and the related art. Example 3 is set. Table 3 shows these manufacturing conditions, strand characteristics, and cord characteristics.

[0024]

[Table 3]

In Tables 1 to 3, the "twisting test" is the distance L between the fixed-side grip 6 and the movable-side grip 7 in FIG.
Was set to 300d (d is the wire diameter mm), and the straight wire W (the untwisted wire after being twisted and not subjected to any processing such as hand stretching) was grasped. In this state, a weight of 400 gr is hung on the fixed side and light tension is applied, and in this state, the grip 7 on the movable side is rotated by the motor 9 at a speed of 30 rpm and a twist-torque curve is taken until it breaks in one direction. Also, after twisting 10 times in one direction, the rotation was stopped once, and the wire was twisted in the opposite direction at the above-mentioned twisting speed until the strand was broken, and the twist-torque curve was used to make the determination. is there. In the “Twist test results” in the table, ◯ means that the torque reduction rate is 7% or less.
(Good), and × means that the torque reduction rate is 8% or more (bad). "Rubber permeability" means 1 straight cord
After putting it in unvulcanized rubber under a tension of 00 gr to vulcanize it to make a sample, take out the cord in the rubber, disassemble this cord in the longitudinal direction, and visually check the penetration of the rubber into the cord. It was determined by observing that the completely permeated product was 100%. The "fatigue resistance index" is that one straight cord is put into unvulcanized rubber to make a vulcanized sample, which is stretched over three rotatable rolls arranged in a zigzag pattern. Under the condition that a tensile load of 10% of the breaking load is applied to the cord, the roll is repeatedly moved to the left and right to repeatedly bend the sample, and the number of repetitions until the cord breaks is measured. In the above, each conventional example is set to 100 and is represented by an index.

As can be seen from Tables 1 to 3, Examples 1 to 9 have a breaking load, twisting efficiency, rubber permeability, and fatigue resistance, all of which contain 0.80% by weight or more of carbon as a raw material. Compared with Comparative Examples 4 and 5 using the contained raw material wire, it has the same or more characteristics (fatigue resistance is remarkably good). Is also excellent. On the other hand, Comparative Examples 1 to 3 have poor toughness by the unidirectional-reverse twist test, and have a low ratio of strength before and after twisting, so that even if rubber permeability is satisfied. The fatigue resistance is poor. In the twisting-torque curve in the twisting test, the wire before twisting the cord and the wire after twisting have almost the same curve. It was poor even after aging, and the toughness was not restored by twisting.

[0027]

According to claims 1 to 3 described above, the rubber permeation is good because of the 2 + 1 structure, and the strand is made of a normal raw material with a low carbon content which is low in cost. Despite this, it has high strength and good toughness, twisting efficiency is not inferior to conventional strength materials, has characteristics equal to or higher than high strength materials using raw material wire with higher carbon content, and fatigue resistance Since it is particularly excellent, an excellent effect that the reinforcing effect on rubber is high can be obtained. According to claim 4, since the cord strength is increased by 10% or more, the reinforcing effect can be maintained even if the usage amount of the steel cord is reduced,
In addition, the excellent effect that a lightweight radial tire with a long life can be obtained is obtained.

[Brief description of drawings]

FIG. 1 (a) is a side view showing a part of a steel cord according to the present invention in an enlarged manner, and FIG. 1 (b) is a schematic enlarged sectional view of the same one pitch.

2A and 2B are cross-sectional views showing an example of a conventional rubber-reinforced steel cord.

FIG. 3 is a cross-sectional view showing another example of a conventional rubber-reinforced steel cord.

FIG. 4 is a cross-sectional view showing another example of a conventional rubber-reinforcing steel cord.

FIG. 5 is a diagram showing a relationship between a wire diameter and a tensile strength.

6A is a one-way torsional torque curve diagram, and FIG. 6B is a one-way-reverse torsional torque curve diagram according to the present invention.

FIG. 7 is an explanatory diagram showing an outline of a torsion-torque tester for toughness evaluation according to the present invention.

FIG. 8 is a sectional view of a drawing die used in the present invention.

FIG. 9 is a sectional view of a finish drawing die used in the present invention.

FIG. 10 is a cross-sectional view of a radial tire to which the present invention is applied.

[Explanation of symbols]

 1a, 1a, 1b Strands Z Tensile strength d Strand diameter T Torque value at torsional elastic limit t Minimum torque at lowered part A Collective strength of strands before stranding B Cord strength after stranding

Claims (4)

[Claims] 1. A steel cord consisting of three strands made of a carbon steel wire rod containing carbon in an amount of 0.70 to 0.76% by weight, and the tensile strength before twisting the cord satisfies the following formula. In addition, using a strand having a ratio B / A of the aggregate strength A of the strand before the cord is twisted to the strength B of the cord after the strand is 0.935 or more, the two strands are almost parallel to each other. A steel cord for rubber reinforcement, which is bundled and twisted so that one strand is wound around it. Z ≧ −200d + 365 [Z: Tensile strength (kgf / mm ² ), d: Diameter (mm)] 2. The wire constituting the cord is twisted in one direction,
The steel cord for rubber reinforcement according to claim 1, wherein a reduction rate of torque is 7% or less in a twist-torque curve in a twist test in which a reverse twist is applied. 3. The twist pitch of the cord is 4 with respect to the wire diameter d.
The steel cord for rubber reinforcement according to claim 1 or 2, which is in a range of 0d to 65d. 4. A radial tire for an automobile, wherein the steel cord according to claim 1 is used for reinforcing a belt portion.