JP2593207B2 - High-strength steel wire and steel cord for reinforcing rubber products - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a steel wire used as a rubber product reinforcing material and a steel cord formed by twisting the steel wire, and more particularly to a steel cord having a high tensile strength. The present invention also relates to a rubber product reinforcing steel wire having improved fatigue resistance and a steel cord formed by twisting the steel wire.

(Prior art) In recent years, the conditions of use of steel wires for reinforcing rubber products or steel cords obtained by twisting the steel wires have become increasingly severe. For example, steel wire reinforced hoses, tires reinforced with steel cords, As the performance of conveyor belts and the like increases, higher tensile strength is required.

For this reason, studies on increasing the strength of steel wires have been under way. For example, methods for increasing the strength of steel wires include increasing the carbon content of steel wires, changing the composition ratio of steel wires,
Alternatively, the face area ratio is increased in the wire drawing process. Further, in steel cords, the strength utilization rate of the steel wire by twisting differs depending on the twisted structure of the cord, so that the twisted structure has been improved.

In Test No. 32 in the examples of JP-A-62-77442, C: 0.65%, Si: 0.66%, Mn: 0.80%, P: 0.012%, S: 0.008%
It is disclosed that a steel wire having a composition of%, N: 0.0035%, and Mg: 0.006%, having a wire diameter of 0.06 mm, had a tensile strength of 408 kg / mm ² .

Japanese Patent Application Laid-Open No. 58-120735 discloses a technique for improving the fatigue resistance by reducing the decarburization depth of a steel wire to a specific value or less.

Further, in JP-A-61-52348, in Examples, C
It is disclosed that a carbon steel wire containing 0.0005 to 0.015% by weight of B in carbon steel of 0.6 to 1.2% by weight has a tensile strength of 402 kg / mm ² and a bending fatigue limit per tensile strength of 0.35.

(Problems to be Solved by the Invention) However, in the method of increasing the carbon content of the steel or increasing the area reduction rate in the wire drawing process, the improvement in the fatigue resistance although the tensile strength is increased is not achieved. There was a problem that the fatigue limit per tensile strength was lowered.

Further, the improvement of the tensile strength by the improvement of the twist structure of the cord has a small effect, and it cannot be said that it is a sufficient means for increasing the strength of the steel wire cord for reinforcing rubber products.

Furthermore, the technique disclosed in Japanese Patent Application Laid-Open No. 62-77442 has a disadvantage that the composition of the steel wire is special and the production cost of the steel wire is increased. According to the example, the tensile strength of the invention steel wire is 195 kg / mm ² ,
Fatigue strength per tensile strength is 0.21, 0.24, which is becoming unsatisfactory due to the recent severe use conditions of rubber products.

As described above, various studies have been made on increasing the strength of steel wires.However, since the fatigue limit per strength decreases as the strength increases, or because the steel wire is a specific composition, There is a problem that the production cost becomes high and the device cannot be used industrially, and the solution has been insufficient.

Accordingly, an object of the present invention is to provide a high-strength steel wire for reinforcing rubber products having high tensile strength and improved fatigue resistance while maintaining adhesiveness with rubber, and a cord made of the steel wire at a low manufacturing cost. It is to provide without rising.

(Means for Solving the Problems) The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, under a predetermined carbon content, a steel wire before final drawing has a predetermined metal structure. It has been found that a high-strength steel wire for reinforcing rubber products having improved not only tensile strength but also fatigue resistance without increasing the production cost and a cord made of the steel wire can be obtained, thereby completing the present invention. Reached.

That is, the present invention relates to a steel wire obtained by drawing a steel wire, wherein the steel wire is a high-carbon steel having a carbon content of 0.75 to 1.00% by weight, and after a patenting treatment before final drawing. The metal structure is a uniform structure from the center of the cross section of the wire rod to the surface layer made of fine pearlite with a small crystal grain size, and a metal coating on the peripheral surface of the steel wire to maintain adhesion with rubber. And the tensile strength (T _S ) of the steel wire is 35
0 kg / mm ² or more and a tensile strength (T _S) per fatigue limit (F _L) of the ratio (F _L / T _S) is rubber products reinforced for high strength steel wire is 0.27 or more, and the steel wire The present invention relates to a high-strength steel cord for reinforcing rubber products in which at least two strands are used and T _S and F _L / T _S have the same values as described above.

(Operation) The steel wire used in the present invention is required to be a high carbon steel having a carbon content of 0.75 to 1.00% by weight. This is because if the carbon content is less than 0.75% by weight, it is difficult to increase the tensile strength to 350 kg / mm ² or more under heat treatment conditions and wire drawing conditions that are industrially possible. If it exceeds 10% by weight, the fatigue resistance decreases, and the fatigue limit per tensile strength is 0.27.
This is because it cannot be done more.

In the present invention, the reason why the tensile strength of the steel wire and the steel cord is set to 350 kg / mm ² or more is to make a remarkable difference in performance as compared with a rubber product reinforced by a conventional steel wire or a steel cord. For this reason, the diameter of the steel wire or steel cord must be 95% or less, or the amount used for rubber products when the strength of the reinforcing material is set to the design standard should be 90% or less. Is required. The reason why the fatigue limit per tensile strength of steel wire and steel cord is 0.27 or more is that at least a fatigue limit equivalent to that of conventional steel wire is required, and the durability of rubber products is greatly improved. 0.30
That is all.

Further, the steel wire used as the starting material of the steel wire of the present invention is a hard steel wire (SWR) having the above carbon content and described in JIS G3506.
It is preferable to use a piano wire (SWRS) shown in H) and JIS G3502. The reason for this is that these are generally used industrially and do not cause an increase in manufacturing cost.

In the present invention, it is preferable that the carbon content in the surface layer portion of the steel wire and the surface portion of the wire rod subjected to the patenting process before the final drawing be 0.70% by weight or more. This is because the surface of the wire tends to decarburize during the heat treatment, and in particular, the carbon content in the surface layer tends to be insufficient. Here, the surface portion is a portion from the surface of the wire from which plating is removed to a predetermined depth, and specifically, the predetermined depth is a depth at which the surface portion has a volume of about 10% of the entire volume of the wire. Means

Then, the carbon content of the surface layer of the steel wire or wire rod is reduced to 0.70.
In order to make the weight% or more, it is desirable to reduce the patenting treatment involving decarburization as much as possible, and it is preferable to use a wire that has been subjected to a direct heat treatment that does not require a patenting treatment in intermediate drawing. As a method of suppressing the decarburization of the steel wire, a heating furnace atmosphere in the patenting process may be a reducing gas or an inert gas, or a method of removing the decarburized layer of the patented wire with a peeling die. However, both are not preferred because the production cost increases.

As described above, the reason why the carbon content in the surface layer portion of the steel wire and the steel wire constituting the steel cord is preferably 0.70% by weight or more is because the fatigue limit of the steel wire and the steel cord can be significantly improved. is there.

Further, in the present invention, it is necessary that the steel wire rod subjected to the patenting treatment before the final drawing is made of fine pearlite having a small crystal grain size and has a uniform structure from the center to the surface layer of the cross section of the wire rod. tissue and the first improvements and intensity of fatigue resistance improvement are accomplished simultaneously by a tensile strength of 350 kg / mm ² or more and a tensile strength per a steel wire and steel cord ratio of 0.27 or more fatigue limit It was found that it was possible to obtain In order to simply increase the strength, it is known to strengthen the ferrite ground of the steel wire or to reduce the cementite lamellar spacing in the pearlite structure. This leads to a decrease and is not preferred.

The fine pearlite structure satisfying the conditions of the present invention as described above can be obtained by selecting appropriate heat treatment conditions. Hereinafter, the heat treatment conditions will be described in detail.

In the austenitizing treatment in which the processed structure obtained in the drawing before the heat treatment is homogenized by the heat treatment, it is necessary that the solution is at least not sufficiently insufficient to meet this purpose, and at most it is excessively solution-rich. It must be within a range that does not result in a proper solution. In other words, if the solution is insufficient, there is a limit in the efficient strengthening by wire drawing, and excessive solution formation has a limit in the wire workability due to coarsening of crystal grains.

By the way, in general heat treatment, it is customary to perform a large amount of solution treatment in consideration of a variation in process capability, a variation in material, and the like.

However, forming a fine pearlite structure according to the present invention,
And, a uniform structure from the center to the surface layer in the cross section of the wire is achieved by reducing the solution so small that it does not become undissolved, instead of the conventional multiple solution treatment, and specifically, By adjusting the conditions of the solution treatment and the patenting treatment so that the pearlite crystal grain size of the wire before final drawing, that is, the grain size number obtained by a measurement method in accordance with JIS G 0551-1965 becomes 8 or more. Achieved.

Here, the pearlite crystal grains mean a region where cementite and ferrite are present in a regular arrangement, that is, a single crystal block.

Next, in the present invention, it is necessary to apply a metal coating on the steel wire in order to enhance the adhesiveness with rubber or the adhesiveness with rubber and the lubricity at the time of drawing to the patented wire. Examples thereof include brass, a ternary alloy obtained by adding a third element to brass, zinc, and the like, and such a metal coating treatment can be performed by a conventionally known method.

The twist structure of the steel cord of the present invention is not particularly limited, and may be any twist structure such as single twist, multiple twist, layer twist, bundle twist, etc. Is also good.

Further, the steel wire and the steel cord of the present invention are passed through a straightening roll to improve the straightness of the steel wire and the steel cord after the final drawing of the steel wire or after being twisted. By adjusting the diameter of the roll, the arrangement of the rolls, or the tension of the steel wire or steel cord, the steel wire can be given compressive residual stress, and the steel wire with the compressive residual stress has improved corrosion fatigue resistance. As a result, the durability of the rubber product can be improved.

(Examples) Next, the present invention will be specifically described with reference to examples.

The steel wire used is a hard steel wire specified in JIS G3506, and the steel wire with intermediate heat treatment needs to be patented to the extent of drawing from 5.5 mm in diameter to the steel wire diameter before final heat treatment. What does not require an intermediate heat treatment is a steel wire that does not require patenting treatment by direct heat treatment.

The steel wire drawn to the steel wire diameter before the final heat treatment is subjected to a heat treatment under the heat treatment conditions shown in Table 1 for final drawing, and oxides generated on the steel wire by the heat treatment are removed. Brass plating was performed and the wire was drawn to a final wire diameter by wet drawing. Although final drawing rate were carried out at usually about 80% of less, because is for the purpose of suppressing age hardening phenomenon during wire drawing, the age hardening phenomenon especially tensile strength exceeding 350 kg / mm ² This is because it becomes remarkable, disconnection frequently occurs at the time of drawing, and the workability also becomes remarkably impaired.

In Table 1, the pearlite block size of the patented steel wire before final drawing and the carbon content in the steel wire surface layer after final drawing, and the fatigue limits of steel wires and steel cords are as follows. Was measured as described above.

Measuring method of pearlite block size of steel wires Measured according to the austenitic grain size test method of JIS G0551-1956 steel.

Photographs of the pearlite block size measurement were obtained from the following equation using 20 photographs taken at 400 times magnification with an optical microscope.

First, a steel wire or a steel cord was loosened, and then the plating of the steel wire was dissolved and removed. Then 12N H ₂ SO ₄
10% of the total volume of the steel wire in an aqueous solution under ultrasonic agitation
% Was dissolved. The steel wire in which the surface layer was dissolved in this way was washed with water and ethanol, dried, and the carbon content was determined using a carbon analyzer manufactured by LECO.

 The carbon content of the steel wire surface layer was determined by the following equation.

Carbon content of surface layer (% by weight) = [(carbon content of steel wire from which plating is removed) (% by weight)-(carbon content of steel wire from which surface layer is removed) (% by weight) × 0.9] × 10 Fatigue limit measurement method Using a rotating bending fatigue tester, temperature 25 ° C, relative humidity 60%
The maximum stress at which none of the samples broke at a total rotation speed of 200,000 rotations at a rotation speed of 5,000 rotations / minute under the atmosphere of was used as the fatigue limit.

 The fatigue limit was determined by the following equation.

d: diameter of steel wire or steel wire constituting steel cord (m
m) R: The bending radius of curvature (mm) at which no ten samples break in the rotating bending fatigue test, and the distance from the bending neutral axis of the steel wire or steel cord to the bending center.

Next, evaluation of the steel cord formed by twisting the steel wires will be described.

First, a steel wire (diameter 0.18 mm) of Experiment No. 11 in Table 1 was twisted to form a cord, and the strength and fatigue limit were evaluated. Table 2 shows the evaluation results.

Similarly, a steel wire (diameter 0.20 mm) of Experiment No. 8 in Table 1
The steel wire of Experiment No. 7 (diameter 0.25 mm) and the steel wire of Experiment No. 19 (diameter 0.22 mm) were each twisted, and the strength and fatigue limit of the cord were measured. The results are shown in Tables 3, 4 and 4 below. 5
It is shown in the table.

The steel wire of Experiment No. 5 shown in Table 1 was subjected to a bending treatment by the method disclosed in Japanese Patent Application Laid-Open No. 57-149578, and the residual stress value of the steel wire was changed to evaluate the corrosion fatigue resistance.

The residual stress value was changed by appropriately selecting the tension applied to the steel wire and the degree of bending in the bending process of the steel wire.

Corrosion fatigue resistance is 31 kg on steel wire using a rotary bending fatigue tester
/ a mm ² bending stress to, Cl ^-, NO ₃ ^- and SO ₄ ^2-ions steel wire was immersed in pH7 aqueous solution containing, rotated at a rotational speed of 1000 rpm, a total rotation up to the steel wire is broken The number was defined as the corrosion fatigue life.

Residual stress value of steel wire is as follows: enamel is applied to the half circumference over the length of 150 mm steel wire, immersed in 30% nitric acid within 30 seconds,
It was indicated by the amount of change in bending of the steel wire. When it turned to the enamel coating side, it turned to the plus side, and when it turned to the opposite side, it turned to the minus side.

The results are shown in FIG. As can be seen from this figure, the corrosion fatigue resistance becomes better as the residual stress value is set to the minus side, and the improvement effect is particularly greater than -10 mm.

FIG. 1 shows the results of Table 1 and a representative example of the same table. As is clear from FIG.
Both the tensile strength and the fatigue resistance were remarkably improved as compared with the steel wires Nos. 1 to 3.

Further, when the breakability of the steel wire of the comparative example was broken at 1,000 to 2,000 m, the breakability of the steel wire of the present invention was 10,000 m. Was not found.

Next, in the steel cords obtained by twisting the steel wires of the present invention shown in Tables 2 to 5, the tensile strength was 350 kg / mn ^2.
As described above, it can be seen that the fatigue limit is 0.30 or more, which is an excellent property.

Furthermore, since the steel wire and the steel cord of the present invention are brass-plated and have excellent adhesion to rubber, the durability of the rubber product can be further improved.

Fig. 2 shows the effect of the carbon content of the steel wire surface layer on the fatigue limit. From this figure, it can be seen that the carbon content of 0.70% or more significantly improves the fatigue resistance. It is clear that.

(Effect of the Invention) As described above, in the high-strength steel wire for reinforcing rubber products of the present invention and the steel cord formed by twisting the steel wire, the steel wire rod before the final drawing under a predetermined carbon content. Specified metallographic structure, and by specifying the carbon content of the surface of the steel wire after the final wire drawing, the steel wire and the steel cord have the same tensile strength without impairing the wire drawing workability. Since the fatigue resistance is improved and the steel wire is coated with a metal such as brass plating or zinc plating, the adhesion between the steel wire and the steel cord and the rubber is extremely excellent. Furthermore, corrosion fatigue resistance can be improved by applying a compressive residual stress to a steel wire or a steel cord.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the tensile strength of the steel wire and the fatigue limit, FIG. 2 is a graph showing the relationship between the carbon content of the surface portion of the steel wire and the fatigue limit, and FIG. It is a graph which shows the relationship between a stress value and corrosion fatigue life.

Claims (5)

(57) [Claims] 1. A high-strength steel wire for reinforcing rubber products obtained by drawing a steel wire, wherein the steel wire is a high-carbon steel having a carbon content of 0.75 to 1.00% by weight and a final drawing. The metallographic structure after the patenting process before the wire is a uniform structure from the center of the wire rod cross section to the surface layer made of fine pearlite with a small crystal grain size. It has a metal coating to maintain adhesion, and the tensile strength (T _S ) of the steel wire is 350 kg /
A high-strength steel wire for reinforcing rubber products, characterized in that the ratio (F _L / T _S ) of the fatigue limit (F _L ) per tensile strength (T _S ) is 0.27 or more, which is not less than mm ² . 2. The high-strength steel wire for reinforcing rubber products according to claim 1, wherein the carbon content of the surface layer of said steel wire is 0.70% by weight or more. 3. The high-strength steel wire for reinforcing rubber products according to claim 1, wherein a compressive residual stress is applied. 4. A steel wire according to any one of claims 1 to 3, wherein at least two of the steel wires are twisted, and the stranded wire cord has a tensile strength (T _S ) of 350 kg / mm ² or more and a tensile strength of at least 350 kg / mm ^2. A high-strength steel cord for reinforcing rubber products, wherein a ratio (F _L / T _S ) of a fatigue limit (F _L ) per strength (T _S ) is 0.27 or more. 5. The high-strength steel cord for reinforcing rubber products according to claim 4, which is provided with a compressive residual stress.