JPS5843519B2 - Steel cord for reinforcing rubber articles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to steel cord for reinforcing rubber articles such as conveyor belts, tires, and hoses, and more particularly to a 7x7 stranded configuration consisting of a central strand and side strands. Regarding steel cord.

There are various types of steel cords for reinforcing rubber articles such as conveyor belts and tires, such as those with a 3 x 7 twist structure, those with a 7 x 7 twist structure, and those with a 7 x 19 twist structure.

As shown in FIGS. 1 to 3, the steel cord with a 7×7 twisted structure, which is the subject of the present invention, has six side strands 3 arranged outside one central strand 2, and The center strand 2 and each side strand 3 are each composed of one center wire (center strand) 4 and six side wires (side strands) 5.

A steel cord 1 having such a 7×7 strand configuration is generally indicated by , and the diameter of each strand is indicated by .

Conventional steel cord with such a 7x7 strand configuration is
In terms of strength characteristics, fatigue resistance, etc., the following structures were common.

(a) The center strand 2 and the side strands 3 all have the same twist coefficient (here, the twist coefficient of the strands is
In FIG. 3, when the twist length of the central strand 2 is 1 and the diameter is m, it is expressed in l/m.

The same applies to the side strands 3.

). (b) The twist coefficient of each strand 2 and 3 is 17, and the twist coefficient of cord 1 is 7. Therefore, the ratio of the twist coefficient of center strand 2 to the cord twist coefficient is 17/7/2.
4 was thought to be optimal (here, the twist coefficient of the cord is expressed as L/M, where the twist length of cord 1 is the length of cord 1 in FIG. 3, and the diameter is M.

). (/→ The strand diameter of the center strand 2 is greater than or equal to the strand diameter of the side strands 3.

(→ In each strand 2, 3, the wire diameter of the center wire 4 is greater than or equal to the wire diameter of one side wire 5 (the center wire is thicker than the side wire (
This is called center diameter increase, and the wire diameter difference is about 10%.

). With conventional steel cords as described above, when the steel cord is vulcanized (heated) bonded to rubber, the cutting load after vulcanization is lower than the cutting load before heating (hereinafter referred to as initial cutting load) due to heating. In particular, when the heating temperature during vulcanization is 140° C. or higher in a steel cord having a 7×7 twisted structure, the cutting load due to vulcanization is significantly reduced.

However, it is common to heat rubber articles at heating temperatures of 150'C.

In this way, the cutting load of the steel cord is vulcanized (heated)
If the initial cutting load decreases by a wide range than the initial cutting load, the initial cutting load data cannot be used for zero-strength design of rubber articles such as conveyor belts and tires, and if the degree of decrease in cutting load changes, the strength calculation of the rubber article will be incorrect. It turns out.

Furthermore, if a large reduction in cutting load is expected, the amount of steel cord used will be greater than when designing strength based on initial cutting load, and control will be increased accordingly.

Furthermore, in order to prevent a large drop in the cutting load of the steel cord, the vulcanization temperature must be lowered (which necessitates a longer vulcanization time, which reduces productivity and increases manufacturing costs. .

As described above, the conventional steel cord having a 7×7 strand structure causes a large reduction in cutting load due to vulcanization (heating), which poses a major problem in terms of the quality and cost of the rubber article.

In order to understand the cause of the large decrease in cutting load due to heating in the conventional steel cord with the above-mentioned 7 x 7 twisted configuration, the present inventors determined that the relationship between the wire diameter and the twist coefficient of the cord is as follows. The twist coefficient ratio of the central strand is approximately 2
．． Tensile tests were conducted on various conventional steel cords with a 7x7 strand structure (4) before and after heating (heated at 160°C for 30 minutes and then left to cool).
The cutting situation of steel cord was considered.

This consideration was made by photographing the steel cord with a high-speed camera at 120 frames per second and observing the image, and by observing the tensile load at the point when the wire starts cutting (the point at which the cutting sound of the wire first occurs). This was done by removing the steel cord and observing the steel cord at that time.

As a result, when we compared data such as elongation and load obtained from tensile tests and observed the cutting state of the strands, we found that the elongation of the steel cord before heating was greater than that after heating. Moreover, there was an indentation at the cut end, which was thought to be caused by the interlocking action between the strands and between the strands.

This revealed that when conventional steel cords are heated, the elongation rate of the steel cords is significantly reduced.

In addition, in the film taken with a high-speed camera, the state of each strand of the various steel cords before heating is photographed in the same frame of the film at the time of cutting, whereas the various steel cords after heating are photographed in the center strand. A few frames after the frame in which the condition of the side strand when it was cut was photographed, the condition of the side strand when it was cut was photographed.

As a result, it was found that in the steel cord before heating, each strand was cut almost simultaneously, whereas in the steel cord after heating, the center strand was cut first, and then the side strands were cut.

Furthermore, at the time when the strands started cutting, uncut strands and cut strands were mixed, and the number of uncut strands was different between the center strand and the side strands.

As a result, in the steel cord after heating, the entire cord does not break instantly, but several strands begin to break first, and during the cutting process, the number of strands cut in the center strand is equal to the number of strands in the side strands. As a result, it was found that the side strands were cut after the center strand was cut as described above.

As mentioned above, the reason why the elongation rate during cutting drastically decreases when the steel cord is heated is because it is made of high carbon steel (each strand of the steel cord is made of high carbon steel).

) strain aging occurs even below 150°C (that is, the general vulcanization temperature for rubber) (see, for example, Journal of the Japan Institute of Metals, Vol. 24, No. 8, p. 515, 1960).
This is thought to be due to a decrease in ductility due to strain aging in high carbon steel, that is, embrittlement.

In addition, when heating a conventional steel cord, the center strand breaks and then the side strands break.The reason why the side strands break after the center strand breaks is because when a tensile load is applied to a steel cord, the center strand generally receives the largest load, and after heating, the steel cord becomes ductile due to strain aging. It is thought that the center strand cannot withstand the load that it could withstand before heating and breaks off before the side strands.

From this, the reason why the cutting load after heating is much lower than the initial cutting load in the conventional steel cord with a 7 x 7 twisted configuration is that the ductility of each strand is reduced and the This is thought to be due to the fact that the maximum load is applied to the center strand and the center strand breaks first, and then the side strands break, even though the tensile stress is reduced due to the indentation caused by the interlocking of the strands. .

Based on the above considerations, the present inventors conducted various experiments and research, and found that when the ratio of the twist coefficient of the cord to the twist coefficient of the center strand is set to a predetermined value, the cutting load due to heating is reduced. The present invention has been completed based on the discovery that this can be significantly improved.

An object of the present invention is to provide a steel cord having a 7x7 strand structure that does not significantly reduce the cutting load when a tensile load is applied even when vulcanized at a typical temperature.

The above-mentioned object, according to the invention, is to improve the twist coefficient (a
) to the twist coefficient (b) of the central strand (b/
This is achieved by setting a) to between 1 and 1.8:.

If the 1t::(b/a) exceeds 1.8, the cutting load will be significantly reduced due to heating, as in conventional steel sheets, and if the ratio (b/a) is less than 1, the code The twisting loss of its own mechanical strength is large, the initial cutting load is small,
Therefore, it is not sufficient as a reinforcing cord for rubber articles.

The reason why the ratio of the twist coefficient of the center strand to the twist coefficient of the cord from 1 to 1.8 can prevent a large decrease in the cutting load after heating with respect to the tensile load is that the twist coefficient of the center strand is Since it approximates the twist coefficient of the cord, it prevents the center strand from breaking earlier than the center strand when a tensile load is applied (premature breakage of the center strand), i.e. each strand breaks almost simultaneously. This is thought to be for the purpose of

In fact, on the film in which the cutting state was photographed with the aforementioned high-speed camera, the state of the steel cord according to the present invention at the time of cutting of each strand was photographed in the same frame.

Experiments have shown that not only should the ratio of the twist coefficient of the center strand to the twist coefficient of the cord be 1 to 1.8, but also that all the strands of the side strands have the same wire diameter or that all the strands of the center strand have the same diameter. Equivalent (wire diameter is JISG-3521
It has been found that the deterioration of cutting load due to heating is further improved when the wire diameter is set to 1, which is considered to be equivalent within the tolerance in G-3522.

The reason for this is thought to be that, in addition to preventing premature breakage of the center strand, by making the diameters of each strand in the strand the same, the effect of the interlocking action caused by interlocking between the strands is reduced. .

Next, a specific explanation will be given based on an example.

Example 1 In a steel cord with the configuration shown below, the twist length of the center strand was changed, and (a) 160°C x 30 minutes and (
b) Rate of change in cutting load after heating at 180°C for 30 minutes ((cutting load after heating Xl0Q/initial cutting load) -10
0) was investigated.

The results are shown in FIG.

From this result, it was found that when the twist coefficient of the center strand exceeds 12.8, the cutting load significantly deteriorates.

○Steel cord 7×7 (Note 0.28φ) O wire diameter
dl, = 0.28mm dB (, = d0 1 = 0.31mm dO, = 0.34myn ○ Cord twist coefficient - 7,00 0 Initial cutting load (for steel cord with strand twist coefficient of 17) = 732 kg Note: This is the wire diameter of the wire with the most common diameter, and is generally referred to as the representative wire diameter.

Example 2 In order to examine whether the combination of steel cord strand diameters affects the cutting load after heating, we investigated whether or not the center diameter of the side strands was increased and the twist coefficient (twist length) of the center strands. The combination of A, A', B below
, B' was created, and the rate of change in cutting load due to heating was investigated.

Codes A and A' have a ratio of the center strand twist coefficient to the cord twist coefficient of 2.43, codes B and B' have this ratio of 1.36, and A and B have center strands centered on the side strands. The diameter was increased (0.29φ), and A' and B' were used without the diameter increase (0.26φ).

The test results are shown in Table 1.

As is clear from the table, codes A' and B', which do not have a center diameter increase in the side strands, are better than codes A and B, which have a center diameter increase.
The reduction in cutting load is smaller than that of B (and the ratio of the twist coefficient of the center strand to the twist coefficient of the cord is 1.8 or less).

The code B' has this ratio greater than 1.8.

Cutting load decreases significantly less than cord A'**
I understand.

○ Steel cord 7X7 (0.26φ) ○ Wire chemical composition (%) CO, 73, Mn O, 50, So,
oio,si O,23, P O,013,CuO,02 0 Cord twist coefficient -7,0 0 Side strand twist coefficient -17 0 Diameter of each strand (mm) d, , =0.26do ,
= 0.29 do , = 0.31 Example 3 In Example 1.2 above, the heating conditions were only 160°C x 30 minutes and 180°C x 30 minutes, but the heating conditions reduced the cutting load. The effects of the following two codes C and D were tested.

Code C has a ratio of the twist coefficient of the center strand to the cord twist coefficient of 1.50 within the range of 1 to 1.8,
In addition, all of the side strands had the same strand diameter, and code D had the above-mentioned ratio of 2.43, and the side strands had a center diameter increase.

The test results are shown in FIG. As is clear from the drawings, for code C, there is almost no decrease in the cutting load due to variations in heating conditions, but for code D, it was found that the cutting load deteriorates rapidly depending on the heating conditions.

Example 4 For codes C and D above, before heating and after heating (160°C x 30 minutes), then a tensile test was conducted.

The results are shown in FIG.

As is clear from the diagram * *, there is no change in the cutting load for code C before and after heating, but for code D, the cutting load significantly decreases after heating.

Example 5 A bending fatigue test was conducted on the above cords C and D.

As a result, as shown in Table 2, Code C has an improved cutting load after heating, and the number of bending breaks is also greater than Code D.

Example 6 A bending fatigue test was conducted on the following cords E and F, in which the ratio of the twist coefficient of the center strand to the cord twist coefficient was set to 1.13 and 2.06, and the wire diameter was different from that of the above-mentioned example.

The results are shown in Table 3.

As is clear from the table, cord E, in which the ratio of the twist coefficient of the center strand to the cord twist coefficient is in the range of 1 to 1.8, has better cutting load change rate after heating and bending fatigue resistance than cord F. It's improving.

As explained above, in a steel cord with a 7x7 configuration, by setting the ratio of the twist coefficient of the center strand to the twist coefficient of the cord to 1 to 1.8, the heating when vulcanizing the rubber article can The steel cord can obtain a cutting load comparable to the initial cutting load without significantly lowering the cutting load than the initial cutting load, and also has improved bending fatigue resistance, which improves the quality of rubber articles. This has the advantage of preventing deterioration and easing restrictions on vulcanization conditions.

[Brief explanation of the drawing]

Figures 1 to 3 show a steel cord with a 7x7 strand structure, with Figure 1 being a perspective view, Figure 2 being a sectional view, and Figure 3 being a cross-sectional view.
The figure is an explanatory diagram of the twisting structure. Figure 4 is a diagram showing the relationship between the ratio of the center strand twist coefficient to the cord twist coefficient and the rate of change in cutting load, Figure 5 is a diagram showing the relationship between heating temperature and rate of change in cutting load, and Figure 6 is a graph showing the relationship between the heating temperature and rate of change in cutting load. It is a figure which shows the tensile curve of the cord before and after heating. 1: cord, 2: center strand, 3: side strand, 4: center wire (strand), 5: side wire (strand).

Claims (1)

[Claims] In a steel cord having a 17×7 twist configuration, the ratio of the twisting coefficient of the center strand to the twisting coefficient of the cord is 1 to 1.
18. A steel cord for reinforcing rubber articles, characterized in that the steel cord is 18. 2. A steel cord for reinforcing a rubber article according to claim 1, wherein all the wires of the side strands have the same wire diameter.