JP3001572B1 - High-strength, high-ductility ultrafine steel wire, stranded wire, and method for producing the same - Google Patents

Abstract

An object of the present invention is to provide an ultrafine steel wire and a stranded wire having extremely high tensile strength as a reinforcing material for a tire, a hose, and the like, and having excellent twistability. SOLUTION: In weight%, C: 0.8-1.50%,
Si: 0.10 to 0.50%, Mn: 0.20 to 0.7
0%, Al: 0.005% or less, P: 0.02% or less,
S: 0.02% or less, O: 0.005% or less, N: 0.
005% or less, 5 ≦ (Ca + Mg + REM) ≦ 50pp
m, the balance being iron and unavoidable impurities.
40mm or less diameter, 4000MPa or more tensile strength,
It has a kink strength retention of 20% or more, a hooking strength of 80% or more, and a Vickers hardness (HVs) at a position of 0.4 × (diameter of extra fine steel wire) from the center of the C section and a Vickers hardness at the center ( A high-strength and high-ductility ultrafine steel wire, wherein the difference (HVs-HVc) in HVc) is 30 or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength ultrafine steel wire used for a steel cord of an automobile tire. Specifically, the wire diameter is 0.40 mm or less reinforced by cold drawing using a die. Tensile strength 4000 MPa
The present invention relates to an ultrafine steel wire of grade or higher and a stranded wire obtained by twisting the wire.

[0002]

2. Description of the Related Art In ultrafine steel wires for automobile steel cords, the need for higher tensile strength of ultrafine steel wires has recently increased more and more due to the demand for weight reduction of tires. For this reason, compared to the conventional strength level, the occurrence of disconnection particularly in the stranded wire process tends to be extremely high, which is a major obstacle to attaining high strength of the steel cord.

[0003] Conventionally, in order to suppress breakage in the twisting process of a high-strength steel cord, in the torsion-torque curve of the torsion test, cracks are generated in the longitudinal direction of the steel wire on the way to breakage so that torque is reduced. It is considered to be most effective to suppress the delamination detected by the phenomenon of abrupt decrease, and various techniques have been developed.

[0004] For example, as described in JP-A-2-10220 and JP-A-4-371549,
It is described that by adding a special alloying element such as o to control the area ratio of proeutectoid cementite, it is possible to suppress longitudinal cracking and wire breakage during twisting test of high strength ultrafine steel wire. ing.

Japanese Patent Application Laid-Open No. Hei 6-336649 discloses that in a wet drawing process, a finishing die is divided into two dies to form a double die, and the inlet and outlet of the first finishing die and the inlet of the second finishing die are wet-lubricated. It is described that by doing so, it is possible to suppress heat generation during processing and prevent the occurrence of delamination.

JP-A-8-120407 discloses that by setting the average particle size of carbide in pearlite to 10 to 50 nm, it is possible to suppress delamination in a high-strength steel wire and improve the torsion value. It is stated that there is.

[0007]

However, in the actual stranded wire process, disconnection occurs in the stranded wire process even if no delamination occurs in the twist test as described in Japanese Patent Application Laid-Open No. Hei 8-26009. Has been experienced on a daily basis, especially at a strength of 4000 MPa or more. In other words, in reality, suppression of delamination is not a necessary and sufficient condition for ensuring the twistability of a high-strength steel cord of 4000 MPa or more. The present invention focuses on the current situation as described above, and aims to provide an ultrafine steel wire and a stranded wire having extremely high tensile strength as a reinforcing material for tires, hoses, and the like, and having excellent twistability. .

[0008]

Means for Solving the Problems The constitution of the high-strength, high-ductility ultrafine steel wire and the stranded wire according to the present invention, which can solve the above-mentioned problems, is C: 0.8 to 1.50 by weight%. %, S
i: 0.10 to 0.50%, Mn: 0.20 to 0.70
%, Al: 0.005% or less, P: 0.02% or less,
S: 0.02% or less, O: 0.005% or less, N: 0.
005% or less, 5 ≦ (Ca + Mg + REM) ≦ 50 ppm, with the balance being iron and unavoidable impurities.
0 mm or less diameter, 4000 MPa or more tensile strength, 2
It has a kink strength retention of 0% or more, a hooking strength of 80% or more, and a Vickers hardness (HVs) at a position of 0.4 × (diameter of extra fine steel wire) from the center of the C section and a Vickers hardness at the center ( A high-strength and high-ductility ultrafine steel wire, wherein the difference (HVs-HVc) in HVc) is 30 or less.

In the above-mentioned ultrafine steel wire, the bending value is 8
A high-strength, high-ductility ultrafine steel wire characterized by having zero or more turns. The bending value means that one end of the ultrafine steel wire is fixed to a pair of grips having an arc having a radius five times the diameter of the ultrafine steel wire, and the other end of the ultrafine steel wire is 1% of the tensile breaking load. This is the number of repetitions up to the fracture when bending is repeated alternately in the forward and reverse directions by 90 degrees along the arc while applying the load of.

[0010] There is a gist of a stranded wire, which is obtained by twisting these ultrafine steel wires.

[0011] Further, the friction coefficient between the steel wire and the die in the finishing wire drawing step is set to 0.07 or less, and furthermore, the ultra-fine steel wire is subjected to a straightening process during the wire drawing process. A method for producing the high-strength, high-ductility ultrafine steel wire.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reasons for limiting the components will be described. In the present invention, the components are specified for the purpose of performing a patenting heat treatment or the like to obtain a pearlite structure having good drawing workability.

C: 0.8 to 1.5% When the C content is less than 0.8%, the strength of the patenting material
4000MP due to small work hardening rate in wire drawing process
In order to secure a tensile strength of a or more, an extremely large drawing strain is required. For this reason, ductility is deteriorated, and disconnection occurs frequently in the wire drawing process and the twisting process, and it is extremely difficult to produce on an industrial scale. When C is contained in excess of 1.5%, proeutectoid cementite precipitates in the rolled wire or the patented material, and the wire drawing and twisting properties are significantly deteriorated. 0.
It was specified as 8 to 1.5%.

Si: 0.10 to 0.50% Si is added in order to utilize the action as a deoxidizing element and a solid solution strengthening element. Particularly, in the present invention, since Al is not added, it is necessary to add at least 0.10% or more of Si. However, since the ductility of the ferrite in the pearlite structure to be added in excess of 0.50% is deteriorated, the wire drawing workability, the twist workability, and the like are deteriorated.
It was defined as 0.50%.

Mn: 0.20 to 0.70% Mn is added not only to utilize the deoxidizing element and the effect of fixing S in the steel as MnS, but also to improve the hardenability of the steel and improve the strength. Is an effective element to increase the
If it is less than 0.20%, the effect will be insufficient. On the other hand, 0.
If it exceeds 70%, drawability and twistability are significantly deteriorated due to generation of micro-marten due to segregation.
The amount added was defined as 0.10 to 0.70%.

Al: 0.005% or less Al is originally added as a deoxidizing element, but Al ₂ O ₃ , which is a deoxidizing product, significantly deteriorates the drawability and twistability of ultrafine steel wire. In the present invention, since it is necessary to deoxidize with Si and Mn to minimize the amount of Al added, Al is specified to be 0.005% or less. Al does not have to be contained.

P: 0.02% or less P is an element that lowers the toughness and ductility of the ultrafine steel wire.
It is an element that easily segregates. Therefore, in the present invention, the P content needs to be 0.02% or less. Preferably, it is 0.01% or less.

S: 0.02% or less S, like P, is an element that lowers the toughness and ductility of the ultrafine steel wire, and is an element that tends to segregate. Therefore, in the present invention, the S content needs to be 0.02% or less. Preferably, it is 0.01% or less.

O: 0.005% or less When O exceeds 0.005%, hard oxide-based nonmetallic inclusions such as SiO ₂ increase, and the drawability of ultrafine steel wire increases.
It is necessary to set the content to 0.005% or less, since the twistability is significantly deteriorated.

N: 0.005% or less If N exceeds 0.005%, the strain aging adversely affects the toughness and ductility of the ultrafine steel wire, so it must be specified to 0.005% or less.

5 ≦ (Ca + Mg + REM) ≦ 50 ppm The content of Al and O is specified, and in addition to minimizing the amount of hard nonmetallic inclusions, the addition of Ca, Mg, and REM oxidizes these oxides. It is effective to combine the material with Al ₂ O ₃ , SiO _2, etc. and soften it to improve the drawability and twistability of the ultrafine steel wire. In order to sufficiently achieve composite of nonmetallic inclusions, (Ca +
Mg + REM (Mg + REM). However, if the content exceeds 50 ppm, the effect is saturated and it is not economical.
m ≦ (Ca + Mg + REM) ≦ 50 ppm.

Next, the reason for defining the kink strength and the hook strength will be described.

Since the shear strain caused by twisting is maximum on the surface, decreases as it goes inward, and becomes zero on the axis, the twisting test corresponds to a test for evaluating the deformability of the surface layer of a fine steel wire. If the deformability is extremely low, the limit will be reached early and delamination will occur. To the extent that delamination does not occur in the area of conventional tensile strength,
In other words, if the surface deformability could cope with relatively small shear strain, the occurrence of disconnection during twisting could be suppressed. But 4000
When manufacturing a cord from a super-fine steel wire with a buncher type twisting machine or higher, the rigidity of the ultra-fine steel wire is so high that a larger amount of overtwisting is required to prevent the cord from coming apart. A larger shear strain is locally applied to the wire surface layer. In the torsion test, the shear strain is uniformly distributed on the surface layer of the ultrafine steel wire between the chucks, and as described above, the judgment based on the presence or absence of delamination can evaluate only the surface deformability for a small shear strain. Therefore, delamination does not occur.
Since it was not possible to satisfy the necessary and sufficient conditions for ensuring good twisting workability of ultrafine steel wire having a tensile strength of 000 MPa class or more, it was necessary to establish another evaluation index and clarify the required level .

The inventors of the present invention have researched and investigated the required characteristics for ensuring the twistability of a high-strength ultrafine steel wire.
The kink test and the hook test were found to be effective as test methods for simulating the state of shear strain locally applied to the surface layer of ultrafine steel wire. The kink test is a test in which a loop is formed with a single ultrafine steel wire and both ends are pulled, and the breaking strength is evaluated. Shear strain is locally applied to the surface layer of the loop portion. In addition, the hooking test is a test in which two ultrafine steel wires are crossed with each other and both ends are pulled to evaluate the breaking strength. Since the shear strain is locally applied to the surface layer at the crossed portion, the situation in the twisting process is changed. Can be reproduced more accurately, and the twisting workability of the ultrafine steel wire can be quantitatively evaluated by the breaking load in the kink test and the hook test.

The kink strength is a value obtained by dividing a breaking load in a kink test by a tensile breaking load in%.
The hooking strength is a value obtained by dividing the breaking load in the hooking test by the tensile breaking load and expressed as a percentage.

In particular, by setting the kink strength to 20% or more and the hooking strength to 80% or more, good twist workability can be ensured. FIG. 1 shows the relationship among the number of disconnections, kink strength, hook strength, and delamination when a 0.20 mm 4300 MPa class ultrafine steel wire was twisted. If delamination occurs, the number of disconnections is high. However, even when delamination does not occur, if the kink strength and hooking strength are low, the number of disconnections is still high. That is, the number of disconnections can be reduced for the first time by controlling the kink strength and the hook strength within the range of the present invention.

Recently, an open structure having a high rubber permeability to the cord has been adopted for the purpose of improving the corrosion fatigue life of the steel cord, but in order to maintain the open structure of the cord, a bending process is performed in a twisting process. There is an example in which it is manufactured by applying strain. In this case, as described above, a large shear strain is locally applied to the surface of the ultrafine steel wire.However, as a method for evaluating the deformability of the surface of the ultrafine steel wire with respect to bending strain more accurately than the kink test and the hook test, Evaluation of the bending value by a bending test is effective.

Here, the bending value means that one end of the ultrafine steel wire is fixed to a pair of gripping tools having an arc having a radius of five times the diameter of the ultrafine steel wire, and further, the other end of the ultrafine steel wire is tensilely broken. This is the number of repetitions up to fracture when a load of 1% of the load is applied and alternately repeated in the forward and reverse directions by 90 degrees along an arc.

In particular, in order to reduce the number of disconnections, it is effective to define the kink strength and the hooking strength within the range of the present invention and to define the bending value at 80 times or more. FIG. 2 shows 0.30 m without delamination
The relationship between the number of disconnections and the bending value when a 4200 MPa class ultrafine steel wire was twisted with an open structure of 1 × 4 was shown. It can be seen that, in addition to controlling the kink strength and the hooking strength within the range of the present invention, by setting the bending value to 80 or more, the number of twisted wire breaks of the 0.30 mm 4200 MPa class open structure cord is reduced. In other words, even without delamination, kink strength, hooking strength,
If the bending value is not within the range of the present invention, the number of disconnections is extremely high.

In order to secure the above kink strength, hooking strength, and bending value, in addition to the above-described component ranges, uniform deformation in the finish drawing step is promoted, and It is effective that the difference (HVs-HVc) between the Vickers hardness (HVs) at the position of 0.4 × (diameter of the ultrafine steel wire) and the Vickers hardness (HVc) at the center is 30 or less. FIGS. 3, 4, and 5 show the relationship between the hardness difference of the C section of 0.20 mm 4200 MPa class and the kink strength, hook hardness, and bending value. If the hardness difference between the surface layer and the center of the C section is controlled within the range of the present invention, it is possible to secure the kink strength, the hooking strength, and the bending value within the range of the present invention due to effects such as formation of a uniform texture. Become.

In order to secure a difference in hardness of the cross section of the ultrafine steel wire C within the scope of the present invention, for example, there is a method of promoting uniform deformation in the finish drawing step. In particular, by reducing the emulsion particle size of the wet lubricant, etc.
It is effective to reduce the shear strain applied to the surface of the steel wire when the die is drawn by setting the friction coefficient between the dies to <0.07.

As a method of securing the hardness difference of the cross section of the ultrafine steel wire C within the scope of the present invention, in addition to the above-described finish drawing method, straightening of the ultrafine steel wire is performed during the drawing process. Is also effective.

For example, it is possible to soften the surface layer of the ultrafine steel wire by the Bausinger effect or the like by applying a torsion strain by a twist straightening process as shown in FIG. 6 and then performing a wire drawing process, thereby reducing HVs-HVc. As a result, an effect equivalent to promoting uniform deformation in the finish drawing step can be obtained. In particular, the surface layer is softened by applying the straightening process, and HVs-HVc can be set to a negative value, and the ductility of the surface layer can be further improved.

The ultrafine steel wire having the above characteristics is 4000M
It has good twistability even with extremely high strength of Pa or more, and can be used as a reinforcing material for tires, belts, hoses, etc. by twisting several to tens of them.

[0035]

【Example】

[0036]

[Table 1]

(Embodiment 1) The effects of the present invention will be described in more detail with reference to the following embodiments. A steel wire having a wire diameter of 1.8 to 1.4 mm of the chemical components shown in Table 1 was subjected to lead patenting treatment and brass plating treatment, and then 0.20 mm by wet drawing.
Stretched. The ultrafine steel wire produced in the above process was twisted to 1 × 12 in the twisting process, and the number of disconnections was evaluated. Table 1 shows the results. Inventive material No. 1-8 and comparative material N
o. Comparing Nos. 9 to 13, it can be seen that disconnection in the twisting step is extremely low by controlling the components, kink strength and hook strength retention within the range of the present invention. Further, it can be seen that it is important to control the hardness difference of the C section of the ultrafine steel wire to HV30 or less in order to secure the kink strength and the hooking strength within the range of the present invention. No. 10 has a C content of 1.5
%, Proeutectoid cementite is generated in the patenting material, so that wire drawing cannot be performed to 0.20 mm.
No. In No. 11, the Al content is too large, and the disconnection in the twisting process frequently occurs due to hard inclusions. No. In No. 12, the content of (Ca + Mg + REM) is small, the softening of the nonmetallic inclusions is insufficient, and the disconnection frequently occurs in the twisting step. No. 13 has a (Ca + Mg + REM) content exceeding 50 ppm. The number of disconnections in the stranding process is N
o. It can be seen that there is no difference from 1 and 2, and it is meaningless to contain more than 50 ppm. No. No. 14 has a large hardness difference in the C section and does not cause delamination, but has a low kink strength. No. In No. 15, the difference in hardness in the C section is even greater and delamination does not occur, but the kink strength and hooking strength are both low, so that the number of disconnections in the stranded wire process is high.

[0038]

[Table 2]

(Embodiment 2) Further, twist straightening is applied,
By further reducing the hardness difference in the C section, the kink strength and the hook strength can be increased, and the bending value can be controlled within the range of the present invention. Lead patenting treatment of a steel wire having a wire diameter of 2.5 mm having the chemical composition shown in Table 2,
After the brass plating, it was drawn to 0.35 mm by wet drawing. A straightening process was applied at 0.60 mm during drawing. The conditions of the straightening process were as follows. The back tension applied to the ultrafine steel wire was 2% of TS, and five straightening dies having a curvature of 10 mm were arranged at intervals of 30 mm. The pressing amount of the dice was set to 5 mm, and after a single twist of 400 mm was applied, the wire was drawn to 0.35 mm. Then, in the twisting step, the wire was twisted into a 1 × 4 open structure, and the number of disconnections was evaluated. Table 2 shows the results.
In addition to increasing the kink strength and the hooking strength, by controlling the bending value within the range of the present invention, it is possible to reduce the number of disconnections in the twisting process even in an open structure. N
o. In Nos. 19 and 20, no delamination occurs because no twist correction processing is performed, but the hardness difference in the C section is large, and the kink strength, hooking strength and bending value are low, so that the number of disconnections in the twisting step is high.

[0040]

As described in detail above, in addition to controlling the components of the ultrafine steel wire, the hardness difference in the C section is reduced, and the kink strength, hook strength, and bending value are controlled to predetermined values. By doing so, it is possible to manufacture a high-strength steel cold that has never been seen before.

[Brief description of the drawings]

FIG. 1 shows a 0.20 mm 4300 MPa class ultrafine steel wire
4 is a graph showing the relationship among the number of disconnections, kink strength, hook strength, and delamination when twisting is performed to × 12.

FIG. 2 0.30 mm without delamination
4 is a graph showing the relationship between the number of disconnections and the bending value when a 4300 MPa class ultrafine steel wire is twisted into 1 × 4.

FIG. 3 is a graph showing a relationship between a difference in hardness of a C section and a kink strength.

FIG. 4 is a graph showing a relationship between a hardness difference and a hooking strength of a C section.

FIG. 5 is a graph showing a relationship between a hardness difference and a bending value of a C section.

FIG. 6 is a diagram showing an outline of a straightening process.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ultrafine steel wire 2 Straightening die 3 Straightening die set holder

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/06 C22C 38/06 D07B 1/06 D07B 1/06 A (56) References JP-A-2-10220 (JP, A JP-A-4-371549 (JP, A) JP-A-6-336649 (JP, A) JP-A-8-120407 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 301 B21C 1/00 B21C 9/00 B21C 19/00 B60C 9/00 C22C 38/06 D07B 1/06

Claims (5)

(57) [Claims] 1. Weight%: C: 0.8-1.50%, Si: 0.10-0.50%, Mn: 0.20-0.70%, Al: 0.005% or less, P: 0.02% or less, S: 0.02% or less, O: 0.005% or less, N: 0.005% or less, 5 ≦ (Ca + Mg + REM) ≦ 50ppm, with the balance being iron and unavoidable impurities It has a diameter of 0.40 mm or less, a tensile strength of 4000 MPa or more, a kink strength of 20% or more, a hooking strength of 80% or more, and 0.4 × (diameter of extra fine steel wire) from the center of the C section. Vickers hardness (HVs) at center and Vickers hardness (HVc) at center
(HVs-HVc) is 30 or less. 2. The high-strength and high-ductility ultra-fine steel wire according to claim 1, wherein the bending value has at least 80 turns. The bending value means that one end of the ultrafine steel wire is fixed to a pair of grips having an arc having a radius five times the diameter of the ultrafine steel wire, and the other end of the ultrafine steel wire is 1% of the tensile breaking load. This is the number of repetitions up to the fracture when bending is repeated alternately in the forward and reverse directions by 90 degrees along the arc while applying the load of. 3. A stranded wire obtained by twisting the ultrafine steel wire according to claim 1 or 2. 4. The method for producing a high-strength, high-ductility ultrafine steel wire according to claim 1, wherein the coefficient of friction between the steel wire and the die in the finishing wire drawing step is 0.07 or less. . 5. The method according to claim 1, wherein a coefficient of friction between the steel wire and the die in the finishing wire drawing step is set to 0.07 or less, and the ultra-fine steel wire is subjected to a straightening process during the wire drawing process. Or 2
3. The method for producing a high-strength and high-ductility ultrafine steel wire according to item 1.