JPH0649592A - High carbon steel wire rod for steel wire having high strength and high ductility - Google Patents

Abstract

PURPOSE:To obtain a high carbon steel wire rod for steel wire having high strength and high ductility and excellent in fatigue characteristics by specifying a composition consisting of C, Si, Mn, P, S, Cr, N, B, and Fe. CONSTITUTION:This wire rod is a high carbon steel wire rod for steel wire having high strength and high ductility and has a composition consisting of, by weight, 0.80-1.10% C, <=0.25% Si, <=0.45% Mn, <=0.010% P, <=0.010% S, 0.15-0.35% Cr, <=0.0040% N, (0.0005) to (0.01XCr+0.001)% B, and the balance Fe with inevitable impurities. In this steel, the growth of cementite in pearlite is accelerated by adding prescribed amounts of B in combination with Cr, and lamellar spacing is refined and wire drawability is strengthened. This steel wire rod can be used as a material for the steel wire having characteristics of >= about 370kgf/mm<2> strength and >= about 30 times twisting value and excellent in fatigue characteristics as to have about 1/3 endurance ratio (fatigue strength/tensile strength) by performing conventional wire drawing of about 3.6 draft Inepsilon.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high carbon steel wire rod that can be used as a raw material for a steel wire used as a reinforcing cord wire for a steel tire for automobiles,
In particular, it relates to a high carbon steel wire rod that can provide a steel wire having higher strength and higher ductility than ever before.

[0002]

2. Description of the Related Art Steel wires such as cord wires and bead wires which have been conventionally used as reinforcing materials for rubber products such as steel tires for automobiles, conveyor belts and high pressure hoses are generally about 0.2 mm in diameter. This is a high carbon steel filament, that is, a strand obtained by twisting a cord wire, and at present, the strength of the filament is about 310 kgf / mm ² .

For example, a cord wire is generally

[0004]

[Equation 1]

It is manufactured in a process called. The numbers in the above table indicate the dimensions (mm).

Eutectoid carbon steel typified by AISI standard-C1080 has been used as a steel wire material for the cord wire. Table 1 shows a typical example of the composition.

[0007]

[Table 1]

That is, in the steel wire rod having such a composition,
After applying a patenting treatment to a strength of 115-125 kgf / mm ² , brass plating is performed to secure the adhesive force with the rubber, and then the wet method is used to finalize the adhesion of the lubricant to the surface as much as possible. For example, a steel wire having a strength of 310 kgf / mm ² and a wire diameter of 0.2 mm was manufactured by wire drawing.

As part of recent automobile weight reduction, automobile tires are also required to be reduced in weight. In order to meet the demand for weight reduction, steel wires with higher strength of 370 kgf / mm ² or more are required. Has become. However, when the conventional steel wire having the steel composition shown in Table 1 was used, even if the manufacturing conditions were optimally set, only a strength of about 350 to 360 kgf / mm ² was practically obtained.

In order to obtain this strength, the workability during wire drawing: In (base material cross-sectional area / cross-sectional area after wire drawing) is at least 4.
Although 0 is required, currently, a workability of 3.6 (working of a drawn wire with a diameter of 1.2 mm to a steel wire with a diameter of 0.2 mm) is commonly used, and in order to obtain a workability of 4.0 or higher, wet drawing It was necessary to modify the machine to increase the number of passing dice.

On the other hand, FIG. 2 shows the strength TS and the reduction RA of the steel wire obtained by drawing a steel wire having the composition shown in Table 1.
Is a graph showing the relationship between the workability and the limit workability ln (A ₀ / A _n ) during wire drawing. As is clear from FIG.
The strength obtained by wire drawing with a workability of about 300k
It was about gf / mm ² . Therefore, various techniques aiming at increasing the strength of the steel wire have been conventionally proposed.

In the technique proposed in Japanese Patent Laid-Open No. 2-194147, Cr is 0.10 to 0.30% (hereinafter, "%" means "% by weight" unless otherwise specified. ) Use the added steel wire.

JP-A-3-271329 discloses that the C content is
0.90 to 1.25% and Cr content 0.1 to 1.0
%, And if necessary, wire drawing is performed on a steel wire rod having a B content of 0.1% or less, and heat treatment is performed by limiting the cooling rate according to the C content in order to prevent pro-eutectoid cementite from occurring. A technique for manufacturing a high-strength steel wire by enabling wire drawing with a reduction in area has been proposed.

Further, the inventor of the present invention has previously disclosed in Japanese Laid-Open Patent Publication No.
According to the publication No. 19, in brief, C content: 0.7 to 0.9%
We have proposed a technology for manufacturing high strength steel wire by drawing wire with a workability of about 4.8 on the steel wire rod.

[0015]

In the technique proposed by Japanese Patent Laid-Open No. 2-194147, although wire drawing with a workability of about 3.6 is performed, only strength of about 360 kgf / mm ² is obtained,
Further, the twist value, which is an index showing ductility, is a normal level of around 25 times, and it is impossible to manufacture a desired high strength / high ductility steel wire. Furthermore, since there is no mention of the fatigue property, which is one of the most important properties as a steel wire,
Its practicality is not clear.

According to the technique proposed in Japanese Patent Laid-Open No. 3-271329, even if the formation of pro-eutectoid cementite can be suppressed, the amount of cementite in pearlite increases, and micro defects that deteriorate fatigue characteristics during processing are generated. It occurs at the interface between ferrite and cementite. Since nothing is said about this point, its practicality is not clear.

Further, according to the technique proposed in Japanese Patent Laid-Open No. 3-240919, it is true that wire drawing with a workability of 4.8 will produce
A steel wire having a strength of more than 0 kgf / mm ² is provided. However, even with this technology, a wire drawability of the same level as the current situation: 3.6, only a strength of about 300 kgf / mm ² can be obtained, and the high strength required unless a special method is required for heat treatment. I can't get it.

As described above, in order to increase the strength of the steel wire obtained by wire drawing, it is basically important to increase the strength before drawing and increase the workability. . FIG. 3 is a graph showing the relationship between the strength or lamella spacing of the drawn wire before drawing and the wire drawing workability or the ultimate strength of the steel wire. Steel wire made of eutectoid carbon steel containing no alloying elements. If the strength before wire drawing is too high, the workability cannot be increased and the strength of the steel wire will not improve. On the other hand, if the strength before wire drawing is lowered, the workability can be increased, but the ultimate strength of the steel wire does not increase.

In order to solve these problems, alloy elements such as Cr are usually added to eutectoid carbon steel, but it takes a long time for transformation, and cementite in the pearlite structure does not grow sufficiently, resulting in ductility. However, there was a problem that the steel wire cannot be used as a steel wire due to deterioration of fatigue characteristics and the like.

Here, the object of the present invention is to provide conventional manufacturing conditions.
With (working degree) as it is, especially under the condition of working degree Inε = 3.6,
Wire drawing is performed while suppressing the generation of microdefects at the ferrite-cementite interface, and the strength is 370 kgf / mm ² or more, the twisting value is 30 times or more, and the durability ratio (fatigue strength / tensile strength) is It is to provide a steel wire rod that is a raw material of a steel wire having a good fatigue property of about 1/3.

[0021]

The present inventor has found that C: 0.88.
%, Si: 0.30%, Mn: 0.50%, and a Cr content of 0.
Eight kinds of steel wire rods with different composition levels of 40, 0.55, 0.75, 1.00, 1.10, 1.30 and 1.50% were drawn (workability: 3.6) and heat treated repeatedly to produce steel wires. The ultimate strength and workability of this steel wire were measured.
The results are shown graphically in FIG. 4 in relation to the Cr content.

As is clear from the figure, the Cr content is 0.5
The ultimate strength is extremely high in the range of ~ 1.0%, but this is because the lamella spacing is 0.12 μm (LP material TS ≈ 135 kgf / mm
² ) It was a relatively rough case. If the lamella spacing is made fine, the strength before drawing (LP material TS) naturally increases. But,
When the Cr content is less than 0.5%, cementite in pearlite does not grow sufficiently and ductility is low, and the critical workability ln (A ₀
Since / A _n ) is rather decreased, it is not possible to make the tissue thinner than this and the ultimate strength is decreased.

Therefore, the inventors of the present invention have earnestly studied to improve the strength before wire drawing, the limit workability, and the strength after wire drawing by refining the lamella spacing in the region where the Cr content is less than 0.5%. Continued.

As a result, as shown in the graph of FIG.
By adding a certain amount of Cr in a specified amount, the Cr content in the range of 0.15% to 0.35%,
Since it can promote the growth of cementite in pearlite,
It was found that the ductility and wire drawability of the steel wire can be improved by making the lamella spacing finer, and further strength increase due to the wire drawing of the steel wire becomes possible, and the present invention was completed by further studies.

The gist of the present invention is as follows.
C: 0.80 to 1.10%, Si ≦ 0.25%, Mn ≦ 0.45%, P ≦ 0.01
0%, S ≦ 0.010%, Cr: 0.15 to 0.35%, N ≦ 0.0040
%, B: 0.0005 to 0.01 x Cr [%] + 0.001% A high carbon steel wire rod for a high strength and high ductility steel wire characterized by comprising the balance Fe and unavoidable impurities.

As described above, according to the present invention, in particular, the lower limit of the Cr content is determined in order to increase the work hardening to increase the strength, and the heat treatment (patenting) conditions are almost the same as those of conventional alloy elements. To determine the upper limit of Cr content so that it can be treated in the same way as in the case of eutectoid steel, and to supplement the insufficient growth of cementite in the pearlite structure, which is a drawback of Cr-containing steel, to ensure ductility and drawability. The feature is that B is added in an appropriate amount according to the Cr content.

The high carbon steel wire material for high strength and high ductility steel wire according to the present invention can be used as a steel wire for reinforcing rubber products such as steel tires for automobiles, conveyor belts and high pressure hoses.

[0028]

The function of the present invention will be described in detail below.
First, in the present invention, the reason for limiting the composition of the steel wire rod as described above will be described.

C: C is an element necessary for ensuring the strength of the steel wire. The lower limit is set to 0.80% because if the C content is less than this, a steel wire having a strength of more than 370 kgf / mm ² cannot be obtained even if the wire is drawn to a target limit workability of 3.6. On the other hand, the upper limit of 1.1% is to suppress the precipitation of proeutectoid cement. Generally, at a cooling rate of about air cooling, if the C content exceeds 0.95%, ductility deterioration in the wire drawing process due to proeutectoid cementite becomes remarkable, but by making the cooling rate equal to or higher than that of lead patenting, 1.
Up to 1% can be suppressed. Therefore, the upper limit is 1.
It is set to 1%, preferably less than 0.95%. Therefore, the C content is 0.80% or more and 1.1% or less, preferably 0.80% or more,
It is less than 95%.

FIG. 5 shows the relationship between the C content and the RA (%) of the aperture: Si: 0.25%, Mn: 0.43%, P: 0.010%, S: 0.010
%, Cr: 0.25%, N: 0.0040%, B: 0.0025% diameter 1.
It is a graph shown about a 2 mm steel wire rod. As is clear from the figure, in the case of air cooling, when the C content exceeds 0.95%, RA
(%) Is significantly deteriorated, but it is improved when the C content is 0.90 to 0.95%. Also, in the case of lead patenting, it can be seen that the deterioration of RA (%) is remarkable when it exceeds 1.1%.

Si: Si has the effect of forming a solid solution with ferrite to increase the strength of the steel, but on the other hand, when added to eutectoid steel, it also has the property of particularly degrading ductility. However, since Si is indispensable as a deoxidizing agent, the upper limit is set to 0.25% in the present invention to prevent reduction in ductility while maintaining the deoxidizing effect. Therefore, the Si content is limited to 0.25% or less.

Mn: Mn has the property of lowering the A ₁ transformation point, and in the case of eutectoid steel, it roughens the structure (pearlite) and lowers the ductility. Further, Mn causes segregation together with P and reduces ductility. Therefore, it is desirable that the Mn content is low, but it is not easy to completely reduce Mn because Mn is unavoidably mixed in at the hot metal stage. Therefore, the Mn content is limited to 0.45% or less, which has no practically harmful effect.

P, S: P and S are both preferable because they deteriorate the drawability of the steel wire made of eutectoid steel and the ductility of the steel wire after drawing. Especially each content is 0.
If it exceeds 010%, it deteriorates remarkably, so the contents of P and S are limited to 0.010% or less.

Cr: Cr is added to enhance the strength of the steel wire after drawing. By compositely adding a certain amount of B, which will be described later, with Cr, it becomes possible to increase the strength by refining the lamella spacing in the Cr content range of 0.15% to 0.35%. That is, if the Cr content is less than 0.15%, the target strength of 370 kgf / mm ² at the workability of 3.6 cannot be achieved, while the Cr content is 0.
If it exceeds 35%, the growth of cementite in pearlite with a lamella spacing of 0.1 μm will be insufficient, the ductility will decrease, and the limit workability will rather decrease. Therefore, the Cr content is 0.
Limited to 15% or more and 0.35% or less.

N: N forms a solid solution with ferrite in pearlite and causes strain aging during and after wire drawing. Therefore, if a certain amount or more is contained, ductility such as drawing and twist value, which are important for the steel wire, deteriorate. Therefore, in order to secure ductility, the N content is limited to 0.0040% or less.

B: Generally, with Cr-added steel, the growth of cementite in pearlite is slow, and only pearlite consisting of short-length cementite can be obtained. The strength of pearlite is determined by the layer spacing of cementite, but the length of cementite affects ductility and wire drawability, and the longer the length, the better ductility and wire drawability. Therefore, in order to increase the strength of the product obtained by drawing a Cr-added steel,
Increasing the length of cementite is also effective.

The strength of Cr-added steel can be increased by adding an appropriate amount of B. That is, as shown in the graph of FIG. 1, even if the Cr content is the same and the lamella spacing is the same (0.10 μm in the example shown in FIG. 1), when B is not added, the critical workability is low, and The ultimate strength is also less than 350 kgf / mm ² . This is because the cementite length is short and the ductility and wire drawability are insufficient when B is not added even if the lamella spacing is the same.

On the other hand, when B is added, the cementite grows sufficiently long, so that the wire drawability is improved, and the target strength: 370 kgf / mm ² or more can be achieved in the Cr content range of 0.15 to 0.35%. Thus, in the present invention, the addition of B promotes the growth of cementite in pearlite,
It improves the ductility and drawability of a steel wire rod, and further improves the ductility and fatigue properties of the steel wire obtained by drawing.

In the present invention, the most remarkable effect of the addition of B is that the cementite in pearlite grows for a sufficiently long time, so that the micro defects generated during the wire drawing process are reduced and the fatigue property is improved.
(Durability ratio, fatigue strength) is greatly improved. That is, as described above, in order to overcome the shortcomings of cementite, which is a disadvantage of Cr-added steel, by adding B according to the amount of Cr added, the growth of cementite in pearlite is promoted, and ferrite and cementite are drawn during wire drawing. The generation of minute defects at the interface with and is suppressed, and fatigue characteristics (durability ratio,
Fatigue strength) is improved.

The amount of B added depends on the Cr content. B [%]: 0.00
The reason for limiting the range represented by 05% or more and {0.01 × Cr [%] +0.001}% or less is that if the B addition amount is below the lower limit, cementite cannot grow to the required level,
Further, if B is added in excess of the upper limit, not only a certain level of effect cannot be expected, but also the amount of B present at the grain boundaries cannot be ignored and wire drawability is reduced. Therefore, the amount of B added is limited to the range of B [%]: 0.0005% or more and {0.01 × Cr [%] + 0.001}% or less. Compositions other than the above are Fe and inevitable impurities. Examples of inevitable impurities include Al ₂ O ₃ and SiO ₂ .

The high carbon steel wire for high strength and high ductility steel wire according to the present invention having the above composition is made into a steel wire by ordinary heat treatment and wire drawing, and it is not necessary to undergo any special process. Absent. For example, a steel wire rod is repeatedly heat treated (patented) and drawn to obtain a strength of 140-
After setting to 150kgf / mm ² , brass plating is applied and then final wire drawing is performed to obtain a normal workability of about 3.6.
A steel wire of 0 kgf / mm ² or more and a wire diameter of 0.2 mm may be used. Further, the present invention will be described in detail with reference to examples, but this is an example of the present invention and the present invention is not limited thereto.

[0042]

EXAMPLE Steels each having the chemical composition shown in Table 2 were melted in a 150 kg vacuum melting furnace and hot-rolled into a bar having a diameter of 5.5 mm.

[0043]

[Table 2]

[0044]

[Table 3]

This rod was repeatedly subjected to cold wire drawing and heat treatment to be rolled to a diameter of 1.2 mm, followed by lead patenting treatment as the final patenting treatment, and then pickling and lubricating treatment with 20% sulfuric acid. After that, wire drawing was performed using a wet continuous wire drawing machine to manufacture sample wires No. 1 to No. 27 which are cord wires having a diameter of 0.20 to 0.26 mm. The lead patenting was carried out by adjusting the temperature of the lead bath so that the lamella spacing was 0.1 μm by the usual method. The sample manufacturing process is shown below.

[0046]

[Equation 2]

The strength TS after drawing, the drawing RA and the lamella spacing of these samples were measured, and the strength TS after drawing, the drawing RA and the twist value TN (distance between chucks: 100 × diameter). dmm) was measured. Further, the rotational bending fatigue strength (strength that does not break after repeated 10 ⁷ times) was measured for each sample, and the durability ratio was obtained. The results are summarized in Tables 2 and 3.

Of the samples shown in Tables 2 and 3, the examples of the present invention all have a limit workability In (A ₀ /An)=3.6,
It had a strength exceeding the target strength of 370 kgf / mm ² and a high twist value of more than 33 times. Furthermore, the fatigue property, which is one of the most important properties in the steel wire, has a durability ratio (fatigue strength / tensile strength) of around 0.25 in the comparative example, but in the examples of the present invention, a high value of around 0.33 is obtained. Shows. The fatigue strength was about 70 to 100 kgf / mm ^{2 in the} comparative example, while it was 125 to 135 kgf / mm ^{2 in the} present invention.

On the other hand, in sample No. 1, since the C content was below the lower limit of the range of the present invention, the strength of the steel wire was insufficient. In sample No. 6, the C content exceeds the upper limit of the range of the present invention, so the ductility of the steel wire deteriorated.

In sample No. 9, since the Si content exceeds the upper limit of the range of the present invention, the ductility of the steel wire deteriorated. Sample N
In o.12, since the Mn content exceeds the upper limit of the present invention, the ductility of the steel wire deteriorated.

In sample No. 13, the P content exceeded the upper limit of the range of the present invention, so the ductility of the steel wire deteriorated. sample
In No. 14, since the S content exceeds the upper limit of the range of the present invention, the ductility of the steel wire deteriorated.

In sample No. 15, the Cr content was below the lower limit of the range of the present invention, so the strength of the steel wire was insufficient. sample
In No. 18, the Cr content exceeded the upper limit of the range of the present invention, so the ductility of the steel wire deteriorated and the strength became insufficient.

In sample No. 19, the N content exceeded the upper limit of the range of the present invention, so the ductility of the steel wire deteriorated. sample
In No. 20, since the B content was below the lower limit of the range of the present invention, the fatigue strength of the steel wire deteriorated.

In sample No. 23, the B content was above the upper limit of the range of the present invention, so the fatigue strength of the steel wire deteriorated.
In sample No. 24, the B content was less than the lower limit of the range of the present invention, so the strength of the fatigue wire of the steel wire deteriorated.

In sample No. 27, the B content exceeded the upper limit of the range of the present invention, so the fatigue strength of the steel wire deteriorated.
As described above, by using the steel wire material having the steel composition satisfying the range of the present invention, even if the patenting treatment under the normal condition and the conventional workability Inε = 3.6 are performed,
It has become possible to manufacture a steel wire that exhibits a strength of 370 kgf / mm ² or more and a twist value of 35 or more times, and a fatigue strength of 124 to 133 kgf / mm ² that is higher than conventional values.

[0056]

As described above in detail, according to the present invention, the strength: 370
kgf / mm ² or more, twist value: 30 times or more, fatigue strength 120 kgf / mm
It has become possible to manufacture high strength and high ductility steel wires of ² or more. By using a high-strength, high-ductility steel wire manufactured by utilizing the present invention as a reinforcing cord wire for a steel tire for automobiles, it is possible to remarkably contribute to weight reduction of automobiles.

[Brief description of drawings]

FIG. 1 is a graph showing the influence of the amount of Cr when Cr and B are added in combination on the workability and the ultimate strength of a steel wire according to the present invention.

FIG. 2 is a graph showing the relationship between the strength and drawing of a steel wire obtained by drawing a steel wire having the composition shown in Table 1 as a conventional method, and the workability during wire drawing.

FIG. 3 is a graph showing the relationship between the strength or lamella spacing of the drawn wire before drawing and the workability or the ultimate strength of the steel wire in the conventional method.

[Fig. 4] As a preliminary test of the present invention, the Cr content is 0 to 1.50%.
The results of measuring the ultimate strength and the workability of the steel wire in this case are shown in which the steel wire is manufactured by repeatedly drawing and heat treating 8 kinds of steel wire rods having compositions changed to 8 levels within the range It is a graph.

FIG. 5 is a graph showing the relationship between the C content and the aperture RA (%) in the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Terutaka Tsumura 4-53-3 Kitahama, Chuo-ku, Osaka Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] 1. By weight%, C: 0.80 to 1.10%, Si ≦ 0.25%, Mn ≦ 0.45%, P ≦ 0.01
0%, S ≦ 0.010%, Cr: 0.15 to 0.35%, N ≦ 0.0040%, B: 0.0005 to 0.01 ×
Cr [%] + 0.001% High carbon steel wire rod for high strength and high ductility steel wire, which consists of balance Fe and inevitable impurities.