WO2017169667A1

WO2017169667A1 - Steel wire material, and methods respectively for producing steel wire material and steel wire

Info

Publication number: WO2017169667A1
Application number: PCT/JP2017/009780
Authority: WO
Inventors: 智一増田; 悠一福住
Original assignee: 株式会社神戸製鋼所
Priority date: 2016-03-31
Filing date: 2017-03-10
Publication date: 2017-10-05
Also published as: JP2017179524A

Abstract

A steel wire material comprising 0.5 to 0.9% by mass of C, 1.5 to 2.5% by mass of Si, 0.1 to 0.6% by mass of Mn, 0.05% by mass or less (excluding 0% by mass) of P, 0.05% by mass or less (excluding 0% by mass) of S, 1.2 to 2.5% by mass of Cr, 0.2 to 0.6% by mass of V and a remainder made up by Fe and unavoidable impurities, wherein the largest diameter of particles of a spheroidized carbide is 100 nm or less in a portion of the steel wire material which is located at a depth of 0.5 mm from the surface of the steel wire material, the areal fraction of particles of the spherical carbide, each of which has a longer diameter of 100 nm or less, relative to the whole area of the carbide is 90% or more, the largest diameter of particles of a V-containing carbonitride is 20 nm or less, and the number density of particles of the V-containing carbonitride, each of which has a longer diameter of 20 nm or less, is 30 particles/μm² or more.

Description

Steel wire, steel wire and method for manufacturing steel wire

The present disclosure relates to a steel wire material, and a method of manufacturing a steel wire material and a steel wire, in particular, a spring steel wire material used for a spring that requires excellent toughness and high fatigue strength, such as a valve spring and a clutch spring. The present invention relates to a steel wire that can be used, and a method of manufacturing a steel wire and a steel wire.

Higher stress is required for springs such as valve springs and clutch springs used in engines, clutches (including torque converters), fuel injection devices, etc., as the weight of automobiles and the output of automobile engines increase. ing. In particular, there is a need for a spring that is less susceptible to fatigue failure due to higher fatigue strength, i.e., internal defects, etc. as the load stress on the spring increases.

For these springs, oil tempered wires (hereinafter sometimes referred to as “steel wires”) are used.
The structure of the oil tempered wire is mainly a tempered martensite structure, and it is easy to ensure high strength, and has the advantages of excellent fatigue strength and sag resistance. However, as the strength increases, the toughness decreases, the sensitivity to internal defects such as inclusions or surface defects such as wrinkles increases, and there is a tendency that fatigue breakage during use of the spring and breakage during processing are likely to occur.
Recently, as a concrete example of increasing the stress on spring steel wires used in engine valves and fuel injection devices, even with a repetition rate of several hundreds of millions to several billions, which is 100 to 1000 times that of the prior art, There is a demand for high fatigue strength that provides fatigue. At the same time, it is also required to have excellent toughness so that breakage does not occur during spring processing.

In response to such demands, the following techniques have been proposed so far.
For example, Patent Document 1 discloses a spring steel having high strength and high toughness by controlling V carbide after quenching and tempering to an appropriate amount.
Patent Document 2 discloses a steel material having high fatigue characteristics by limiting Cr-containing carbonitrides.

Japanese Patent No. 5114665 Japanese Patent Laid-Open No. 2015-196840

However, in the control of V carbide by quenching and tempering described in Patent Document 1, the precipitation site of the carbide that precipitates during tempering is limited, so that sufficiently high toughness may not be obtained.
Further, in Patent Document 2, since the Cr content is limited to 0.2% by mass or less, if nitriding is performed during the manufacture of a valve spring, softening (decrease in strength) is large, and fatigue resistance is not sufficient. There is.

The present disclosure has been made paying attention to the above-mentioned problems, and an object thereof is to provide a steel wire material that is a material of a steel wire having excellent fatigue resistance and excellent toughness. It is another object of the present invention to provide a method for manufacturing a steel wire having excellent fatigue resistance and excellent toughness, and a method for manufacturing a steel wire used as a material for the steel wire.

The steel wire material according to one embodiment of the present invention includes C: 0.5 to 0.9 mass%, Si: 1.5 to 2.5 mass%, Mn: 0.1 to 0.6 mass%, P : 0.05% by mass or less (excluding 0% by mass), S: 0.05% by mass or less (not including 0% by mass), Cr: 1.2 to 2.5% by mass, and V: 0.0. 2 to 0.6% by mass, the balance is Fe and inevitable impurities, and the maximum diameter of the spheroidized carbide is 100 nm or less in the part from the surface to a depth of 0.5 mm, and the major axis is 100 nm or less with respect to the entire carbide. The spherical carbide has an area fraction of 90% or more, the maximum diameter of the V-based carbonitride is 20 nm or less, and the number density of the V-based carbonitride having a major axis of 20 nm or less is 30 pieces / μm ² or more.

The steel wire rods are Cu: 0.5% by mass or less (not including 0% by mass), Ni: 1.0% by mass or less (not including 0% by mass), and Mo: 1.0% by mass or less (0 It may further contain one or more selected from the group consisting of:

The steel wire material further includes at least one selected from the group consisting of Ti: 0.1% by mass or less (not including 0% by mass) and Nb: 0.5% by mass or less (not including 0% by mass). May be included.

The method for producing a steel wire according to one embodiment of the present invention includes: C: 0.5 to 0.9 mass%, Si: 1.5 to 2.5 mass%, Mn: 0.1 to 0.6 mass% %, P: 0.05% by mass or less (not including 0% by mass), S: 0.05% by mass or less (not including 0% by mass), Cr: 1.2 to 2.5% by mass, and V A step of preparing a steel material containing 0.2 to 0.6% by mass, the balance being Fe and inevitable impurities, and after heating the steel material to 1150 to 1300 ° C., at a hot working temperature of 1000 to 1300 ° C. A step of hot working, a step of cooling from a controlled cooling start temperature of 1000 ° C. or higher to a slow cooling start temperature of 450 to 600 ° C. at an average cooling rate of 1 to 5 ° C./second after the hot working; Cooling from the slow cooling start temperature to 300 ° C. at an average cooling rate of 10 to 100 ° C./hour, No.

The steel materials are Cu: 0.5% by mass or less (not including 0% by mass), Ni: 1.0% by mass or less (not including 0% by mass), and Mo: 1.0% by mass or less (0% by mass) 1% or more selected from the group consisting of:

The steel material further contains at least one selected from the group consisting of Ti: 0.1% by mass or less (not including 0% by mass) and Nb: 0.5% by mass or less (not including 0% by mass). You can do it.

A method of manufacturing a steel wire according to one embodiment of the present invention includes a step of skinning a surface of a steel wire obtained by the steel wire or the method of manufacturing a steel wire, and a steel wire subjected to the skinning. The method may include a step of annealing or patenting, a step of drawing the annealed or patented steel wire, and a step of quenching and tempering the steel wire subjected to the drawing.

According to the embodiment of the present invention, it is possible to provide a steel wire that is a material of a steel wire having excellent fatigue resistance and excellent toughness. It is also possible to provide a method for producing a steel wire having excellent fatigue resistance and excellent toughness and a method for producing a steel wire used as a material for the steel wire.

Embodiments of the present invention will be described below. However, it should be noted that the embodiments described below are intended to embody the technical idea of the present invention and are not intended to limit the technical scope of the present invention.
In steel wires, by reducing internal defects, especially inclusions, which are the starting point of fatigue fracture, steel wires using the steel wires can obtain high fatigue strength, and breakage during processing caused by inclusions. It is known that the occurrence of can be reduced.
Although it is possible to reduce the size and amount of inclusions, it is still unavoidable that a small amount of inclusions are present, and a small amount of inclusions when the stress applied during use as a spring is high. Fatigue breakage may occur during long-term operation. For this reason, in order to obtain high fatigue strength, it is required to suppress fatigue fracture due to internal defects by the matrix structure. In addition to obtaining high fatigue properties, it is also required to have sufficient toughness so that breakage during spring processing or the like can be sufficiently suppressed.

In fatigue fracture, it is known that a crack generated from an internal defect propagates at a very low speed immediately after the occurrence (extremely low-speed crack growth region), and then the growth rate is increased to cause a fracture. Since most of the life until fracture is spent in the extremely low-speed crack growth region, if the crack growth rate in the extremely low-speed crack growth region can be suppressed, the fatigue life can be extended, and for long-term use. Reliability can be improved.

From such a viewpoint, the inventors conducted various studies to obtain a steel wire having excellent fatigue characteristics. As a result of investigating the crack growth rate in the extremely slow crack growth region during long-term use, it was found that the crack growth rate per cycle (cycle) was shorter than the interatomic distance. This is considered to indicate that the crack growth is intermittent and has progressed at the atomic level. In such a growth mode, if the resistance is sufficiently large with respect to the interatomic distance (precipitates, inclusions, etc. on the order of μm), the crack can easily pass around it, thus suppressing the crack growth. Difficult to do.

In order to suppress such extremely low-speed crack growth, it is effective to uniformly disperse many small resistances (fine precipitates, solid solution elements).
As will be described in detail later, the inventors set the components in a predetermined range and appropriately control the hot working conditions such as rolling, thereby setting the maximum diameter of the spheroidized carbide (the maximum value of the major axis) to 100 nm or less. The ratio of the spherical carbide having a major axis of 100 nm or less in the entire carbide can be made a certain value or more, the maximum diameter of the V-based carbonitride is made 20 nm or less, and the number density of the V-based carbonitride having a major axis of 20 nm or less is more than a certain value. It has been found that a steel wire can be obtained. And it discovered that the steel wire obtained using such a steel wire has the outstanding fatigue-resistant characteristic and the outstanding toughness.
Below, the detail of the manufacturing method of the steel wire which concerns on embodiment of this invention, and a steel wire and a steel wire is demonstrated.
In this specification, “steel wire” means a steel material that has been hot-formed, such as rolled, and processed into a predetermined size, and “steel wire” means drawing (drawing). And so-called oil tempered wire obtained by quenching and tempering. In addition, when processing a steel wire material and obtaining a steel wire, you may perform a shaving (shaving) and heat processing (for example, annealing or patenting) as needed before a drawing process.
In the present specification, the “major axis” of the carbide means the maximum dimension of each target carbide, and the “maximum diameter” means the maximum value of the major axis of the entire target carbide. The long diameter and the maximum diameter can be measured from, for example, a TEM image.

1. Steel wire [the maximum diameter of the spheroidized carbide is 100 nm or less, and the area fraction of the spherical carbide having a major axis of 100 nm or less with respect to the entire carbide is 90% or more]
In the steel wire according to the embodiment of the present invention, the maximum diameter of the spherical carbide is 100 nm or less in the region between the surface and the depth of 0.5 mm. Moreover, the area fraction of the spherical carbide | carbonized_material whose major axis is 100 nm or less with respect to the whole carbide | carbonized_material is 90% or more.
By forming spherical carbides with a major axis of 100 nm or less in advance in steel wires, many of these spherical carbides remain in the matrix after quenching and tempering treatment to obtain steel wires, improving the toughness of the steel wires. In addition, the fatigue resistance can be improved.

If the maximum diameter of the spherical carbides exceeds 100 nm, the distance between the spherical carbides becomes too large, and the uniformity of the distribution of the spherical carbides after quenching and tempering is impaired, so that fatigue cracks easily progress.
In order to maximize the effect of improving fatigue resistance and toughness, the ratio of spherical carbide having a major axis of 100 nm or less in the entire carbide needs to be considerably high. Specifically, the spherical carbide having a major axis of 100 nm or less The area ratio with respect to the entire carbide is 90% or more. This is because if the area ratio of the spherical carbide having a major axis of 100 nm or less is lower than 90%, the uniformity of the distribution of the spherical carbide is impaired, so that fatigue cracks easily develop.

The region defining the area fraction of the spherical carbide having the maximum diameter of the spheroidized carbide of 100 nm and the major axis of 100 nm or less with respect to the entire carbide is used as a spring from the surface of the steel wire to a depth of 0.5 mm. In this case, high stress is applied to the surface of the steel wire and a portion close to the surface, and bending stress is applied to the steel wire during spring forming (coiling), and this bending stress is also applied to the surface and surface of the steel wire. This is because the portion close to is high stress. That is, this is because the portion to which high stress is applied during use and processing is defined.
In addition, the maximum diameter of the above-mentioned spherical carbide is preferably 80 nm or less, more preferably 60 nm or less. Further, the area fraction is preferably 92% or more, and more preferably 95% or more.

In the composition described in detail later, the formed spherical carbide is (Fe, Cr) ₃ C in which a part of Fe of cementite is mainly substituted by Cr.
Measurement of the maximum diameter of spheroidized carbide and the area fraction of spherical carbide with a major axis of 100 nm or less with respect to the entire carbide is a cross section (transverse section) perpendicular to the hot working direction (for example, the rolling direction when hot working is rolling). ), A sample for microscopic observation is prepared by an extraction replica method from an area within 0.5 mm from the surface. The obtained sample for microscope observation was observed with a transmission electron microscope equipped with a composition analyzer such as EDX, and for elements other than Fe, C and Cr were each 10% or more by mass ratio, and the major axis (one For the carbide, the maximum diameter and the area fraction are determined with the ratio of the vertical dimension (the dimension of the portion having the maximum dimension on the TEM image) and the vertical diameter ratio (aspect ratio) of 2.0 or less as the spherical carbide. . Observation with a transmission electron microscope is performed by observing 3 or more fields of 5 μm × 5 μm, that is, an area of 25 μm ² .
Note that image analysis software may be used as necessary.

[In the portion from the surface to a depth of 0.5 mm, the maximum diameter of the V-based carbonitride is 20 nm or less, and the number density of the V-based carbonitride having a major axis of 20 nm or less is 30 / μm ² or more]
In the steel wire according to the embodiment of the present invention, the maximum diameter of the V-based carbonitride is 20 nm or less in a region between the surface and a depth of 0.5 mm. The number density of V-based carbonitrides having a major axis of 20 nm or less is 30 / μm ² or more.
In the steel wire, by previously forming a V-type carbonitride having a major axis of 20 nm or less, these V-type carbonitrides remain in the matrix even after quenching and tempering treatment to obtain a steel wire. It is possible to improve the toughness and fatigue resistance.

If the major axis of the V-based carbonitride is larger than 20 nm or the number density of the V-based carbonitride is smaller than 30 / μm ² or more, a sufficient fatigue crack growth suppressing effect cannot be obtained.
The maximum diameter of the V-based carbonitride is preferably 18 nm or less, more preferably 16 nm or less. The number density of the V-based carbonitride is preferably 35 pieces / μm ² or more, more preferably 40 pieces / μm ² or more.

The region where the maximum diameter of the V carbonitride is 20 nm and the number density is 35 pieces / μm ² or more is defined as the portion from the surface of the steel wire to a depth of 0.5 mm. This is the same reason as the case of the area fraction of the spherical carbide having a major axis of 100 nm or less with respect to the whole.

The maximum diameter and number density of V-based carbonitrides are measured by preparing a sample for microscopic observation by the extraction replica method from a region within 0.5 mm from the surface in a cross section (transverse section) perpendicular to the hot working direction. Do it.
The obtained sample for microscope observation was observed with a transmission electron microscope equipped with a composition analyzer such as EDX, and for elements other than Fe, V was 10% or more by mass ratio, and N and C peaks were observed. The deposited precipitate is defined as V-based carbonitride. The maximum diameter of the V-based carbonitride is obtained, the number of V carbonitrides having a major axis of 20 nm or less is obtained, and the number density per 1 μm ² is obtained using the observed visual field area. Image analysis software may be used as necessary.
Observation with a transmission electron microscope is performed by observing three or more fields of 1.2 μm × 1.2 μm, that is, an area of 1.44 μm ² .

[composition]
Steel wire and steel wire according to an embodiment of the present invention, C: 0.5-0.9 mass%, Si: 1.5-2.5 mass%, Mn: 0.1-0.6 mass%, P: 0.05% by mass or less (not including 0% by mass), S: 0.05% by mass or less (not including 0% by mass), Cr: 1.2 to 2.5% by mass, and V: 0 .2 to 0.6% by mass, with the balance being Fe and inevitable impurities.

Cu: 0.5% by mass or less (not including 0% by mass), Ni: 1.0% by mass or less (not including 0% by mass), and Mo: 1.0% by mass or less (not including 0% by mass) 1) or more selected from the group consisting of:

One or more selected from the group consisting of Ti: 0.1% by mass or less (not including 0% by mass) and Nb: 0.5% by mass or less (not including 0% by mass) may be further included.
Each element is described in detail below.

(1) C: 0.5 to 0.9% by mass
C (carbon) is an element effective for improving the strength of a steel wire, and is an element necessary for obtaining a spheroidized carbide and a V-based carbonitride. In order to effectively exhibit such an effect, the C content is 0.5% by mass or more, preferably 0.55% by mass, more preferably 0.6% by mass or more. If the C content is excessive, the strength of the steel wire becomes too high, and the required toughness cannot be ensured. Therefore, the C content is 0.9% by mass or less, preferably 0.85% by mass or less, and more preferably 0.80% by mass or less.

(2) Si: 1.5 to 2.5% by mass
Si (silicon) is an element effective for improving the strength of a steel wire. In order to effectively exhibit such an effect, the Si content is 1.5% by mass or more, preferably 1.6% by mass or more, and more preferably 1.7% by mass or more. On the other hand, when the Si content is excessive, the strength becomes too high and the toughness necessary for the steel wire cannot be ensured. Therefore, Si content is 2.5 mass% or less, Preferably it is 2.4 mass% or less, More preferably, it is 2.3 mass% or less.

(3) Mn: 0.1 to 0.6% by mass
Mn (manganese) has the effect of improving the hardenability and improving the strength of the steel wire. In order to effectively exhibit such an effect, the Mn content is 0.1% by mass or more, preferably 0.15% by mass or more, and more preferably 0.20% by mass or more. On the other hand, if the Mn content is excessive, the hardenability becomes too high, so that a supercooled structure such as martensite is easily generated during the production of the steel wire, and it is difficult to cut and draw to obtain a steel wire. Become. Therefore, the Mn content is 0.6% by mass or less, preferably 0.55% by mass or less, more preferably 0.5% by mass or less.

(4) P: 0.05% by mass or less (excluding 0% by mass)
P (phosphorus) is an inevitable impurity, and its content should be as low as possible. P is an element that easily segregates at the grain boundaries, and may reduce toughness and workability of the steel wire. Therefore, the P content is 0.05% by mass or less. The content of P is preferably 0.04% by mass or less, more preferably 0.03% by mass or less. The smaller the content of P, the better. However, it is difficult to industrially make it less than 0.001% by mass, and considering the mass productivity, the content is generally about 0.001% by mass or more.

(5) S: 0.05% by mass or less (excluding 0% by mass)
S (sulfur) is an inevitable impurity and is preferably as small as possible. In particular, since S forms sulfide inclusion MnS and segregates during hot working, the steel wire may be embrittled, so the S content is 0.05% by mass or less. The S content is preferably 0.04% by mass or less, more preferably 0.03% by mass or less. The smaller the S content, the better. However, since it is difficult to industrially make it less than 0.001% by mass, it is generally contained in an amount of about 0.001% by mass or more in consideration of mass productivity.

(6) Cr: 1.2 to 2.5% by mass
Cr (chromium) is an element useful for improving the strength of a steel wire. In addition, Cr replaces part of Fe of cementite to form spheroidized carbide (Fe, Cr) ₃ C having a major axis of 100 nm or less, and this is uniformly dispersed in the matrix of the steel wire. Contributes to improvement. In order to effectively exhibit such an effect, the Cr content is set to 1.2% by mass or more. The content of Cr is preferably 1.3% by mass or more, and more preferably 1.4% by mass or more. However, if the Cr content is excessive, the hardenability is increased too much, so that a supercooled structure such as martensite is likely to be produced during the production of the steel wire, and it is difficult to cut and draw to obtain a steel wire. Become. Therefore, the Cr content is set to 2.5% by mass or less. The content of Cr is preferably 2.2% by mass or less, and more preferably 2.0% by mass or less.

(7) V: 0.2 to 0.6% by mass
V (vanadium) contributes to improving the toughness of the steel wire by being finely precipitated as carbonitride. In order to exert this effect, the V content is 0.2% by mass or more, preferably 0.25% by mass or more, more preferably 0.3% by mass or more. On the other hand, when the V content is excessive, coarse V-based carbonitrides increase, fine V-based carbonitrides decrease, and toughness and fatigue characteristics decrease. Therefore, the V content is 0.6% by mass or less, preferably 0.55% by mass or less, more preferably 0.5% by mass or less.

The steel wire material and steel wire concerning one embodiment of the present invention may contain the following ingredients selectively further.
(8) Cu: 0.5% by mass or less (excluding 0% by mass), Ni: 1.0% by mass or less (not including 0% by mass), and Mo: 1.0% by mass or less (0% by mass) 1 or more members selected from the group consisting of (not including) These selected components are useful elements for increasing the strength of the steel wire. These may be contained alone or in combination of two or more. Details of the content of each element when added are shown below.

Cu: 0.5% by mass or less (excluding 0% by mass)
Cu (copper) is an element useful for increasing the strength of a steel wire. In order to exhibit such an effect, the content of Cu is more than 0% by mass. The Cu content is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and further preferably 0.2% by mass or more. On the other hand, when the Cu content is excessive, it becomes a liquid phase at a constant temperature (1356 K) and segregates at the austenite grain boundaries during deformation during hot rolling to generate surface cracks. It is 5 mass% or less, Preferably it is 0.4 mass% or less, More preferably, it is 0.3 mass% or less.

Ni: 1.0% by mass or less (excluding 0% by mass)
Ni (nickel) is an element useful for increasing the strength and toughness of a steel wire. In order to exert such an effect, the Ni content is more than 0% by mass. The Ni content is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and still more preferably 0.2% by mass or more. On the other hand, if the Ni content is excessive, the steel wire surface is unevenly concentrated, the irregularities on the steel wire surface become large, the surface properties are deteriorated, and the fatigue properties are adversely affected. The amount is 1.0% by mass or less. The Ni content is preferably 0.9% by mass or less, and more preferably 0.8% by mass or less.

Mo: 1.0% by mass or less (excluding 0% by mass)
Mo (molybdenum) is an element useful for increasing the strength and toughness of a steel wire. In order to exert such an effect, the Mo content is more than 0% by mass. The Mo content is preferably 0.05% by mass or more, more preferably 0.08% by mass or more, and still more preferably 0.10% by mass or more. On the other hand, if the Mo content is excessive, the toughness and fatigue properties are adversely affected, so the Mo content is 1.0% by mass or less. The Mo content is preferably 0.8% by mass or less, more preferably 0.5% by mass or less.

(9) One or more selected from the group consisting of Ti: 0.1% by mass or less (not including 0% by mass) and Nb: 0.5% by mass or less (not including 0% by mass) These selected components Is an element useful for improving the toughness of steel wires. These may be contained alone or in combination of two or more. Details of the content of each element when added are shown below.

Ti: 0.1% by mass or less (excluding 0% by mass)
Ti (titanium) is an element that contributes to improving the toughness of a steel wire by forming carbonitrides and refining crystal grains. In order to exhibit such an effect effectively, the Ti content is more than 0% by mass. The content of Ti is preferably 0.02% by mass or more, more preferably 0.03% by mass or more. On the other hand, when the Ti content is excessive, the toughness of the steel wire is lowered, so the Ti content is 0.1% by mass or less. The Ti content is preferably 0.08% by mass or less, and more preferably 0.05% by mass or less.

・ Nb: 0.5% by mass or less (excluding 0% by mass)
Nb (Nb) is an element that contributes to improving the toughness of the steel wire by forming carbonitrides to refine crystal grains. In order to effectively exhibit such an effect, the content of Nb is more than 0% by mass. The Nb content is preferably 0.02% by mass or more, more preferably 0.04% by mass or more, and still more preferably 0.06% by mass or more. On the other hand, when the content of Nb is excessive, not only the cost increases, but also the yield point (yield ratio) is raised to deteriorate the workability for skin cutting and drawing, so the content of Nb is 0.5. It is below mass%. The Nb content is preferably 0.4% by mass or less, and more preferably 0.3% by mass or less.

The basic components of the steel wire and the steel wire according to the embodiment of the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that steel contains inevitable impurities such as Ca and Na which are inevitably mixed depending on the situation of materials such as iron raw materials (including scrap), auxiliary materials, and manufacturing equipment.
In addition, about P and S, it is so preferable that there is little content, Therefore It is an inevitable impurity, However, The composition range is prescribed | regulated separately as mentioned above. For this reason, in the present specification, the “unavoidable impurities” constituting the balance is a concept excluding elements whose composition range is separately defined.

2. Next, a method for manufacturing the above-described steel wire will be described. In the method of manufacturing a steel wire according to an embodiment of the present invention, when hot working a steel material satisfying the above composition, the steel material is heated to 1150 to 1300 ° C. and then heated at a hot working temperature of 1000 to 1300 ° C. Then, cool from the controlled cooling start temperature set to 1000 ° C or higher to the slow cooling start temperature set between 450 to 600 ° C at an average cooling rate of 1 to 5 ° C / second, and then start slow cooling It is characterized by cooling from temperature to 300 ° C. at an average cooling rate of 10 to 100 ° C./hour.
Thereby, in the part from the surface to a depth of 0.5 mm, the maximum diameter of the spheroidized carbide is 100 nm or less, the area fraction of the spherical carbide having a major axis of 100 nm or less with respect to the entire carbide is 90% or more, and the V-based carbonitride The maximum diameter is 20 nm or less, and the number density of V-based carbonitrides having a major axis of 20 nm or less can be 30 / μm ² or more.
Below, the detail of the manufacturing method of a steel wire is demonstrated. In the following description, hot working will be described by taking rolling as an example. However, the hot working is not limited to rolling, and other hot working methods such as forging may be used.

A steel material satisfying the predetermined chemical composition for hot working is obtained by a steelmaking process, a casting process, and the like. For example, a billet of a predetermined size may be produced by performing ingot rolling (bloom) obtained by a steelmaking process and a continuous casting process.
In the hot working process such as rolling, the V-based carbonitride is dissolved, and then the V-based carbonitride is reprecipitated and a spherical carbide structure is obtained. As shown below, heating, hot working and cooling are performed. Condition control is required.

(1) Heating at a heating temperature of 1150 to 1300 ° C. In heating before rolling, it is necessary to dissolve coarse V-based carbonitrides and Cr-based carbides present in billets (steel materials for hot working). The heating temperature is 1150-1300 ° C. When the heating temperature is lower than 1150 ° C., V-based carbonitrides remain and cannot be finely precipitated by subsequent cooling. Also, Cr-based carbides remain, and uniform spherical carbides cannot be obtained by subsequent cooling.
The heating temperature is preferably 1175 ° C or higher, more preferably 1200 ° C or higher. The higher the billet is heated, the easier it is to dissolve the V-based carbonitride and Cr-based carbide, so the upper limit of the heating temperature is not particularly limited, but considering the heat-resistant temperature of the heating furnace, the upper limit of the heating temperature is It is 1300 degrees C or less, Preferably it is 1275 degrees C or less, More preferably, it is 1250 degrees C or less. In addition, what is necessary is just to select the holding time in heating temperature suitably according to operation conditions, for example, is 30 minutes or more. The heating temperature may be confirmed, for example, by measuring the temperature of the steel material at the outlet from the heating furnace with a radiation thermometer or the like. If the holding time is sufficiently long, the heating furnace temperature may be used as the heating temperature. Good.

(2) Hot working at a hot working temperature of 1000 to 1300 ° C. Hot working conditions have a great influence on the size and number density of V-based carbonitrides. V-based carbonitrides are likely to precipitate at austenite grain boundaries when hot working such as rolling is not applied, and are easily coarsened when precipitated at austenite grain boundaries. In order to precipitate fine V-based carbonitrides, it is necessary to apply processing strain in the austenite grains. For this reason, the hot working temperature is 1000 ° C or higher, preferably 1025 ° C or higher, more preferably 1050 ° C or higher, and 1300 ° C or lower, preferably 1275 ° C or lower, more preferably 1250 ° C or lower. When the hot working temperature is lower than 1000 ° C., V-based carbonitride precipitates during hot working and the strength of the hot working material is increased, so that processing to a predetermined wire diameter becomes difficult. On the other hand, when the hot working temperature exceeds 1300 ° C., the strain introduced by the working disappears again by heat, so that the V-based carbonitride easily precipitates at the austenite grain boundaries, and the target size and number density are obtained. It becomes impossible.
In addition, you may confirm hot processing temperature by measuring the temperature of the steel materials just before hot processing with a radiation thermometer etc., for example.

(3) Cooling at an average cooling rate of 1-5 ° C / sec from a controlled cooling start temperature of 1000 ° C or higher to a slow cooling start temperature of 450-600 ° C Cooling is performed at an average cooling rate of 1 to 5 ° C./second from the cooling start temperature to the slow cooling start temperature set between 450 and 600 ° C. When the cooling start temperature is lower than 1000 ° C., precipitation of V-based carbonitrides becomes difficult, and a predetermined number density cannot be obtained. The controlled cooling start temperature is preferably 1025 ° C. or higher, more preferably 1050 ° C. or higher.
By setting the slow cooling start temperature to a temperature between 450 and 600 ° C., bainite formed in a region between the surface and a depth of 0.5 mm can be changed to a spherical carbide. When the slow cooling start temperature exceeds 600 ° C., the spherical carbide aggregates and is likely to be coarsened, so that a spheroidized carbide having a predetermined size cannot be obtained. The slow cooling start temperature is preferably 575 ° C. or lower, more preferably 550 ° C. or lower.
On the other hand, when the slow cooling start temperature is lower than 450 ° C., bainite cannot be changed to spherical carbide by subsequent cooling, and it becomes difficult to perform the shaving and drawing to obtain a steel wire. The slow cooling start temperature is preferably 475 ° C. or higher, more preferably 500 ° C. or higher.

As long as an appropriate controlled cooling temperature is selected, there is no influence on the structure control from the completion of hot working to the start of controlled cooling. Therefore, it is possible to select a cooling condition suitable for the operation such as natural cooling or controlled cooling.
The strain at the time of hot working is important for increasing the precipitation sites of V-based carbonitrides. However, if a cross-sectional reduction rate of approximately 50% or more is ensured, a predetermined V-based carbonitride size and The number density can be obtained more reliably.

By performing cooling from the controlled cooling temperature to the cooling stop temperature set between 450 ° C. and 600 ° C. at an average cooling rate of 1 to 5 ° C./second, a predetermined V-based carbonitride size and number density are obtained. In addition, bainite can be formed in a region having a depth of 0.5 mm from the surface. This bainite can be converted to a spherical carbide in subsequent cooling.
When the average cooling rate is slower than 1 ° C./second, V carbonitride grows coarsely, so that a predetermined size cannot be obtained, the toughness of the steel wire is reduced, and the depth from the surface to 0.5 mm is reduced. The structure of the region becomes pearlite, and the distribution of Cr in the steel wire becomes non-uniform, resulting in a decrease in toughness. The average cooling rate is preferably 1.2 ° C./second or more, more preferably 1.5 ° C./second or more.
On the other hand, at an average cooling rate faster than 5 ° C./second, a cooling time sufficient for precipitation of the V-based carbonitride cannot be obtained, the number density is insufficient, and the toughness of the steel wire is lowered. Moreover, since many bainite generate | occur | produces in a part deeper than 0.5 mm in depth, and it becomes easy to break at the time of a drawing process, it becomes difficult to obtain a steel wire. The average cooling rate is preferably 4.5 ° C./second or less, more preferably 4 ° C./second or less.

The average cooling rate may be adjusted, for example, by controlling the conditions of the Stealmore cooling (air volume, wind speed, and presence / absence of cover).
The average cooling rate is measured, for example, by measuring the temperature T1 of the coil (rolled material) immediately after placing the stealmore cooling conveyor and the temperature T2 of the coil immediately before the converging machine, and the coil conveys the difference between the temperature T1 and the temperature T2. It may be obtained by dividing by the time t1 required to move from immediately after being mounted on the lens to immediately before the condenser (average cooling rate = (T1-T2) / t1).
The temperature T1 may be obtained by measuring the temperature of 10 rings with a radiation thermometer for the sparse and dense portions of the coil and using the average value. Similarly, the temperature T2 may be obtained by measuring the temperature of 10 rings with a radiation thermometer for the sparse and dense portions of the coil and using the average value.
As an example of the radiation thermometer, there is a high precision digital portable radiation thermometer IGA15 + manufactured by Hazama Sokki Co., Ltd. However, it is not limited to this.

(4) Cooling from the slow cooling start temperature to 300 ° C. at an average cooling rate of 10 to 100 ° C./hour In the process of cooling from the slow cooling start temperature to room temperature, it is formed in a region between the surface and a depth of 0.5 mm. It is necessary to change the bainite to spherical carbide. For this purpose, cooling is performed at an average cooling rate of 10 to 100 ° C./hour between the slow cooling start temperature and 300 ° C. When the average cooling rate is slower than 100 ° C./hour, bainite cannot be changed to spherical carbide, and it becomes difficult to perform the shaving and drawing to obtain a steel wire. The average cooling rate is preferably 90 ° C./hour or less, more preferably 80 ° C./hour. On the other hand, the slower the cooling rate, the easier it is to obtain a spherical carbide structure. However, when the cooling rate is slower than 10 ° C./hour, not only the productivity is inhibited, but also the spherical carbide begins to aggregate, and a predetermined size is obtained. Cannot be obtained. The average cooling rate is preferably 30 ° C./hour or more, more preferably 50 ° C./hour or more.

Such an average cooling rate may be adjusted, for example, by controlling conditions for holding the rolled material taken up.
The average cooling rate is determined by measuring the temperature T2 of the coil immediately before the converging unit and the temperature T3 of the coil immediately after the transfer of the hook conveyor, and the difference between the temperature T2 and the temperature T3 is determined as follows. It may be obtained by dividing by the time t2 required to move to just after being transferred to (average cooling rate = (T2-T3) / t2).
The temperature T3 may be measured by using a contact-type thermometer and bringing a contact-type temperature sensor into contact with the coil.
As an example of a contact-type thermometer, a surface thermometer made by TESTTO INC can be mentioned. However, it is not limited to this.

3. Manufacturing method of steel wire By using a steel wire obtained in this manner, a steel wire having excellent fatigue resistance and toughness can be obtained by manufacturing a steel wire.
A known general manufacturing method may be used for the process for obtaining the steel wire from the steel wire. In spring steel, the steel wire (rolled wire) is subjected to skin removal for the purpose of removing the decarburized layer, flaws, etc. in the vicinity of the surface, and then softened or annealed to soften the processed layer generated on the surface by the skin removal. Heat treatment such as patenting is performed. Then, the steel wire is obtained by further drawing (drawing) to a desired wire diameter, followed by quenching and tempering.
Note that, depending on the application and use conditions of the spring, the skin treatment and the heat treatment such as softening annealing or patenting may be omitted.

Below, the manufacturing conditions of a steel wire are illustrated.
Softening annealing conditions can be arbitrarily set, but the heating temperature is 450 to 750 ° C., the holding time at the heating temperature is 10 seconds or more and 60 seconds or less, and the average cooling rate is 1 ° C./second or more and 10 ° C./second or less. It is preferable to do.
In the quenching and tempering treatment, the heating temperature during quenching is 850 ° C. or higher, preferably 870 ° C. or higher, more preferably 890 ° C. or higher in order to suppress the regeneration and growth of Cr-based carbides. On the other hand, when the heating temperature becomes excessively high, the V-based carbide dissolves and the effect of improving toughness decreases, so the heating temperature is 1000 ° C. or lower, preferably 980 ° C. or lower, more preferably 960 ° C. or lower. Quenching is performed in oil that has been heated to a predetermined temperature and then heated, for example, approximately 50 to 80 ° C., and then tempered. Tempering may be performed by appropriately adjusting the temperature and time so that a desired tensile strength such as 2100 MPa or more is obtained. For example, tempering is generally preferably performed at a heating temperature of 350 ° C. or higher and 450 ° C. or lower. By carrying out such treatment, the desired tensile strength is obtained.

The steel wire obtained in the embodiment of the present invention exhibits excellent fatigue properties and excellent toughness as shown in the examples. The steel wire according to the embodiment of the present invention can be manufactured into various springs such as a valve spring, a clutch spring, an engine spring, and a transmission spring by processing the steel wire into a desired coil diameter, free height, and number of turns. You may perform well-known various surface hardening processes, such as shot peening and nitriding, after a spring process as needed.

Hereinafter, the embodiments of the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and may be appropriately changed within a range that can meet the gist of the present invention. In addition, it is of course possible to carry out them, all of which are included in the technical scope of the present invention.

(1) Sample preparation Melting and casting was performed in a small vacuum melting furnace, and 150 kg of steel ingots having the chemical composition shown in Table 1 were prepared. In addition, the underline attached | subjected under the numerical value of each table | surface has shown that the said numerical value has remove | deviated from embodiment of this invention mentioned above. This ingot was forged to produce a 155 mm square steel piece. A dummy billet was welded to the steel slab, and a steel wire having a diameter of 7 mm was manufactured by actual rolling under the hot working conditions (rolling conditions) and cooling conditions shown in Tables 2 and 3. “Control cooling rate” in Tables 2 and 3 means an average cooling rate from the control cooling start temperature to the slow cooling start temperature, and “Cooling rate to 300 ° C.” means 300 ° C. from the slow cooling start temperature. Mean average cooling rate up to.
Next, the steel wire was subjected to skin cutting, softening annealing, and drawing processing, followed by quenching and tempering treatment to obtain a steel wire having a wire diameter of φ3.0 mm. The tempering conditions of the quenching and tempering treatment were adjusted so that the tensile strength of the steel wire was 2100 to 2150 MPa.

Note: The balance is Fe and inevitable impurities

The maximum diameter and number density of the obtained V-based carbonitride of the steel wire rod and the maximum diameter and area fraction of the spherical carbide were evaluated by the following methods. Moreover, the tensile test, 3-point bending evaluation, and fatigue characteristic evaluation were performed with the following method about the obtained steel wire.

(2) Maximum diameter and number density of V-based carbonitride Samples were collected from a region from the surface of the steel wire to 0.5 mm in a cross section (cross section) perpendicular to the rolling direction of the steel wire. Using this sample, a sample for microscopic observation was produced by the extraction replica method.
Specifically, cutting, mechanical polishing, electrolytic polishing, etching, carbon deposition, peeling, and washing were performed in this order to prepare a microscope sample. For electrolytic polishing, 10% perchloric acid-90% ethanol is used as the electrolytic solution, 10% acetylacetone-90% methanol-1 mass% tetramethylammonium chloride is used as the etching solution, and 1% nitric acid-99% methanol is used as the stripping solution. did. V-carbonitrides were observed using a sample prepared by the above-described extraction replica method, using a field emission transmission electron microscope HF-2000 with an acceleration voltage of 200 kV, an imaging magnification of 20000 times, and an overall magnification of 30000 times. Performed under conditions.

Identification of the V-based carbonitride was performed using an EDX analyzer (Sigma) manufactured by Kevex, which is attached to the field emission transmission electron microscope. Regarding elements other than Fe, carbonitrides containing 10% or more of V by mass ratio were defined as V-based carbonitrides. The evaluation was performed by observing three visual fields of 1.2 μm × 1.2 μm, that is, an area of 1.44 μm ² . As a result, the number density of the V-based carbonitride having the maximum diameter of the V-based carbonitride and the major axis of 20 nm or less was determined. Regarding the number density, the number of V-based carbonitrides having a major axis of 20 nm or less was determined using image analysis software (Image Pro Plus) manufactured by Media Cybernet, and the number density per 1 μm ² was obtained.
The measurement results are shown in Tables 4 and 5.

(3) Maximum Diameter and Area Fraction of Spherical Carbide Evaluation of the spherical carbide was carried out using the same sample for microscope observation used in the evaluation of the V-based carbonitride described above. That is, the measurement was performed with a field emission transmission electron microscope HF-2000 under the conditions of an acceleration voltage of 200 kV, an imaging magnification of 20000 times, and an overall magnification of 30000 times.
This was carried out using an EDX analyzer (Sigma) manufactured by Kevex Inc. attached to a field emission transmission electron microscope. Regarding the elements other than Fe, those having a mass ratio of C and Cr of 10% or more and a ratio of the major axis to the perpendicular diameter (aspect ratio) of 2.0 or less were defined as spherical carbides. The evaluation was performed by observing 3 visual fields of 5 μm × 5 μm, that is, an area of 25 μm ² . As a result, the maximum diameter of the spheroidized carbide and the area fraction of the spheroidized carbide having a major axis of 100 nm or less in all the carbides were obtained. Regarding the area fraction, the area of the spherical carbide having a major axis of 100 nm or less and the area of all carbides were calculated using image analysis software (Image Pro Plus) manufactured by Media Cybernet, and the area fraction was calculated. The measurement results are shown in Tables 4 and 5.

(4) Tensile test Using an autograph (manufactured by Shimadzu Corp.), a test piece having a distance between scores of 200 mm was used to perform a tensile test in accordance with JIS Z 2241 (2011) to measure the tensile strength.
The measurement results are shown in Tables 4 and 5.

(5) Three-point bending evaluation A three-point bending jig, which is a universal testing machine, combined with a U-notch-type pushing jig with a distance between grades of 50 mm and φ3 mm, is installed. A point bending test was performed. The specimen that did not break during the three-point bending was designated as “No”, passed (good toughness), and the specimen was broken as “Yes” and failed.
The evaluation results are shown in Tables 4 and 5.

(6) Fatigue property evaluation Fatigue property was evaluated using the obtained steel wire and an ultrasonic axial force fatigue tester (UFT-2000) manufactured by Shimadzu Corporation. The fatigue test was performed at a frequency of 20 kHz, a stress ratio of −1 (complete swinging), a room temperature atmosphere, a stress amplitude of 800 to 1000 MPa, and a censored cycle of 10 billion cycles (1 × 10 ¹⁰ cycles). A fatigue test (n = 10) was performed at an arbitrary stress amplitude within the above range. When less than 10 test pieces were broken from the internal inclusions in 5 × 10 ⁷ to 1 × 10 ¹⁰ cycles, the test pieces were added as appropriate. Then, for the 10 samples broken from the inclusions in the steel material, the inclusion size was obtained as described later, the stress intensity factor range ΔK was calculated, and an approximate curve (ΔK = A × NB: stress intensity factor range ΔK and fatigue life N). A stress intensity factor range ΔK10 of 1 billion cycles was obtained from A and B as approximation coefficients) and used as fatigue characteristics.

The stress intensity factor range was calculated as follows.
The fracture surface of the test piece fractured in the fatigue test was observed with SEM, and the inclusion size was measured using image analysis software (Image Pro Plus) manufactured by Media Cybernet. The inclusion size was the square root √ (a × b) of the product of the short side a and the long side b on the fracture surface. From the obtained inclusion size and stress amplitude, the stress intensity factor range ΔK was calculated from the following equation. Specimens that were fatigue fractured starting from the structure of the specimen surface or inclusions facing the surface were excluded. In the determination of fatigue characteristics, the fatigue resistance is good when ΔK10 is 2.0 MPa√m or more as compared with the result of the conventional material, and it is determined to be acceptable (◯), and the others are not acceptable (× ).

ΔK10 = 0.5σ _a × √ {π × √ (a × b)}
Here, σ _a is the test stress. Tables 4 and 5 show the evaluation results.

Test Nos. 1 to 28 in Table 4 are samples in which the heat treatment conditions were variously changed as shown in Table 2 using the steel type K in Table 1. Of these, test no. 2 to 3, 6 to 8, 11 to 12, 14 to 16, 19 to 21, and 24 to 27 are all samples manufactured under the conditions according to the above-described embodiment of the present invention, and V-based carbonitrides. Since the maximum diameter and number density of each and the maximum diameter and area fraction of the spherical carbide were all controlled within the range of the embodiment of the present invention, excellent toughness and fatigue characteristics were exhibited.

On the other hand, Test No. whose heating temperature is lower than the lower limit. No. 1 is insufficiently heated and cannot sufficiently dissolve the V-based carbonitride, so the maximum diameter of the V-based carbonitride is excessive, the number density is excessively low, and the toughness and fatigue characteristics of the steel wire are insufficient. Met.
On the other hand, test No. whose heating temperature is higher than the upper limit. In No. 4, heating is excessive and the austenite grain size becomes coarse, and subsequent hot working cannot form sufficient V-based carbonitride precipitation sites, resulting in an excessively low number density of V-based carbonitrides. It was. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.

Test No. whose hot working temperature is lower than the lower limit. No. 5 cannot form sufficient V-based carbonitride precipitation sites by hot working, and the number density of V-based carbonitrides is too low. As a result, the toughness and fatigue characteristics of the steel wire are insufficient. Met.
Test No. whose hot working temperature is higher than the upper limit. No. 9, because the heating was excessive and the austenite grain size became coarse even after hot working, and the precipitation site of V-based carbonitride was insufficient, so the maximum diameter of V-based carbonitride was excessive and the number density was too small, As a result, the toughness and fatigue characteristics of the steel wire were insufficient.

Test No. with controlled cooling start temperature lower than lower limit. No. 10, because the precipitation sites of V-based carbonitrides became excessive and the V-based carbonitrides were agglomerated and coarsened, the maximum diameter of the V-based carbonitrides was too large and the number density was too small. Insufficient toughness and fatigue properties.
Test No. in which the average cooling rate from the controlled cooling start temperature to the slow cooling start temperature is lower than the lower limit. No. 13, because the V-based carbonitride was agglomerated and coarsened, the maximum diameter of the V-based carbonitride was excessive, and the pearlite transformation was completed during cooling, so the spherical carbide in the region from the surface to a depth of 0.5 mm The maximum diameter was too large, and the spheroidized carbide area fraction was too small. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.

Test No. in which the average cooling rate from the controlled cooling start temperature to the slow cooling start temperature is faster than the upper limit. In No. 17, the precipitation time of the V-based carbonitride was insufficient and the number density was too low. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.
Test No. whose slow cooling start temperature is lower than the lower limit. No. 18 could not sufficiently change the bainite on the surface to spherical carbide, and the area fraction of the spherical carbide was too small. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.

Test No. whose slow cooling start temperature is higher than the upper limit. In No. 22, since the spherical carbide agglomerates and becomes coarse, the maximum diameter of the spherical carbide becomes excessive and the size becomes large. As a result, the toughness and fatigue characteristics of the steel wire are insufficient.
Test No. in which the average cooling rate from the slow cooling start temperature to 300 ° C. is lower than the lower limit. No. 23 caused aggregation and coarsening of the spherical carbide, and the maximum diameter of the spherical carbide was excessive. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.

Test No. whose average cooling rate from the slow cooling start temperature to 300 ° C is higher than the upper limit. In No. 28, the bainite formed on the surface portion could not be sufficiently changed to the spherical carbide, and the area fraction of the spherical carbide was too small. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.

Test No. in Table 5 Nos. 29 to 45 are samples in which the hot working conditions and the cooling conditions are constant as shown in Table 3, and the steel types are changed from A to R (excluding K). Of these, test no. Each of 29 to 39 satisfies the above-described component range according to the embodiment of the present invention, and all of the maximum diameter and number density of the V-based carbonitride and the maximum diameter and area fraction of the spherical carbide are present. Because it is controlled within the scope of embodiments of the invention, it exhibits excellent toughness and fatigue properties.

On the other hand, test No. using a steel type M having a C amount lower than the lower limit. In No. 40, the hardenability at the time of cooling was insufficient, and pearlite was the main phase, so the area fraction of the spherical carbide was too small. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.
Test No. using steel type N having a Cr content lower than the lower limit. No. 41 had insufficient hardenability during cooling and pearlite was the main phase, so the area fraction of the spherical carbide was too small. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.

Test No. using a steel type O having a V amount lower than the lower limit. In No. 42, the amount of V for forming the V-based carbonitride was insufficient, so the number density of the V-based carbonitride was too low. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.
Test No. using steel type P with more C than the upper limit. In No. 43, C was present excessively, and spherical carbides were easily agglomerated and coarsened, and the maximum diameter of the spherical carbide was excessive. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.

Test No. using steel type Q with more Cr than the upper limit. In No. 44, Cr was excessively present, and spherical carbides easily aggregated and coarsened, and the maximum diameter of the spherical carbides was excessive. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.
Test No. using steel type R with more V than upper limit. In No. 45, V was excessively present, and the V-type carbonitride easily aggregated and coarsened, and the maximum diameter of the V-type carbonitride was excessive. As a result, the toughness and fatigue characteristics of the steel wire were insufficient.

The present invention includes the following aspects.
Aspect 1:
C: 0.5 to 0.9% by mass
Si: 1.5 to 2.5% by mass
Mn: 0.1 to 0.6% by mass
P: 0.05% by mass or less (excluding 0% by mass)
S: 0.05% by mass or less (excluding 0% by mass)
Cr: 1.2-2.5% by mass, and V: 0.2-0.6% by mass
And the balance consists of Fe and inevitable impurities,
In the portion from the surface to a depth of 0.5 mm, the maximum diameter of the spheroidized carbide is 100 nm or less, and the area fraction of the spherical carbide having a major axis of 100 nm or less with respect to the entire carbide is 90% or more. A steel wire having a maximum diameter of 20 nm or less and a number density of V-based carbonitrides having a major axis of 20 nm or less of 30 / μm ² or more.
Aspect 2:
Cu: 0.5% by mass or less (excluding 0% by mass)
Ni: 1.0% by mass or less (not including 0% by mass), and Mo: 1.0% by mass or less (not including 0% by mass)
The steel wire according to aspect 1, further containing one or more selected from the group consisting of:
Aspect 3:
Ti: 0.1% by mass or less (not including 0% by mass), and Nb: 0.5% by mass or less (not including 0% by mass)
The steel wire according to aspect 1 or 2, further containing one or more selected from the group consisting of:
Aspect 4:
C: 0.5 to 0.9 mass%, Si: 1.5 to 2.5 mass%, Mn: 0.1 to 0.6 mass%, P: 0.05 mass% or less (including 0 mass%) S), 0.05% by mass or less (excluding 0% by mass), Cr: 1.2 to 2.5% by mass, and V: 0.2 to 0.6% by mass with the balance being Fe And a step of preparing a steel material made of inevitable impurities,
Heating the steel material to 1150-1300 ° C. and then hot working at a hot working temperature of 1000-1300 ° C .;
After the hot working, cooling from a controlled cooling start temperature of 1000 ° C. or higher to a slow cooling start temperature of 450 to 600 ° C. at an average cooling rate of 1 to 5 ° C./second;
Cooling from the slow cooling start temperature to 300 ° C. at an average cooling rate of 10 to 100 ° C./hour;
A method for manufacturing a steel wire material.
Aspect 5:
The steel material is
Cu: 0.5% by mass or less (excluding 0% by mass)
Ni: 1.0% by mass or less (not including 0% by mass), and Mo: 1.0% by mass or less (not including 0% by mass)
The manufacturing method of the steel wire of aspect 4 which further contains 1 or more types chosen from the group which consists of.
Aspect 6:
The steel material is
Ti: 0.1% by mass or less (not including 0% by mass), and Nb: 0.5% by mass or less (not including 0% by mass)
The manufacturing method of the steel wire of aspect 4 or 5 which further contains 1 or more types chosen from the group which consists of.
Aspect 7:
A step of skinning the surface of the steel wire obtained by the steel wire according to any one of aspects 1 to 3 or the steel wire according to any of aspects 4 to 6, and
Annealing or patenting the shaved steel wire; and
A step of drawing the annealed or patented steel wire;
A step of quenching and tempering the drawn steel wire;
The manufacturing method of the steel wire containing.

This application is accompanied by a priority claim based on the Japanese patent application No. 2016-070590 whose application date is March 31, 2016. Japanese Patent Application No. 2016-070590 is incorporated herein by reference.

Claims

C: 0.5 to 0.9% by mass
Si: 1.5 to 2.5% by mass
Mn: 0.1 to 0.6% by mass
P: 0.05% by mass or less (excluding 0% by mass)
S: 0.05% by mass or less (excluding 0% by mass)
Cr: 1.2-2.5% by mass, and V: 0.2-0.6% by mass
And the balance consists of Fe and inevitable impurities,
In the portion from the surface to a depth of 0.5 mm, the maximum diameter of the spheroidized carbide is 100 nm or less, and the area fraction of the spherical carbide having a major axis of 100 nm or less with respect to the entire carbide is 90% or more. A steel wire having a maximum diameter of 20 nm or less and a number density of V-based carbonitrides having a major axis of 20 nm or less of 30 / μm 2 or more.
The steel wire rod according to claim 1, further comprising at least one of the following (a) and (b).
(A) Cu: 0.5% by mass or less (excluding 0% by mass), Ni: 1.0% by mass or less (not including 0% by mass), and Mo: 1.0% by mass or less (0% by mass) (B) Ti: 0.1% by mass or less (not including 0% by mass) and Nb: 0.5% by mass or less (not including 0% by mass) One or more selected from the group
C: 0.5 to 0.9 mass%, Si: 1.5 to 2.5 mass%, Mn: 0.1 to 0.6 mass%, P: 0.05 mass% or less (including 0 mass%) S), 0.05% by mass or less (excluding 0% by mass), Cr: 1.2 to 2.5% by mass, and V: 0.2 to 0.6% by mass with the balance being Fe And a step of preparing a steel material made of inevitable impurities,
Heating the steel material to 1150-1300 ° C. and then hot working at a hot working temperature of 1000-1300 ° C .;
After the hot working, cooling from a controlled cooling start temperature of 1000 ° C. or higher to a slow cooling start temperature of 450 to 600 ° C. at an average cooling rate of 1 to 5 ° C./second;
Cooling from the slow cooling start temperature to 300 ° C. at an average cooling rate of 10 to 100 ° C./hour;
A method for manufacturing a steel wire material.
The method for producing a steel wire according to claim 3, wherein the steel further contains at least one of the following (a) and (b).
(A) Cu: 0.5% by mass or less (excluding 0% by mass), Ni: 1.0% by mass or less (not including 0% by mass), and Mo: 1.0% by mass or less (0% by mass) (B) Ti: 0.1% by mass or less (not including 0% by mass) and Nb: 0.5% by mass or less (not including 0% by mass) One or more selected from the group consisting of
Cutting the surface of the steel wire obtained by the steel wire according to claim 1 or 2 or the method of producing a steel wire according to claim 3 or 4, and
Annealing or patenting the shaved steel wire; and
A step of drawing the annealed or patented steel wire;
A step of quenching and tempering the drawn steel wire;
The manufacturing method of the steel wire containing.