WO2017038436A1 - Steel wire for mechanical structure parts - Google Patents

Abstract

Provided is a steel wire for mechanical structure parts, which is reduced in deformation resistance during cold working, has improved cracking resistance, and can exhibit excellent cold workability. The steel wire for mechanical structure parts according to the present invention comprises, in % by mass, 0.3 to 0.6% of C, 0.05 to 0.5% of Si, 0.2 to 1.7% of Mn, more than 0% and 0.03% or less of P, 0.001 to 0.05% of S, 0.01 to 0.1% of Al, 0 to 0.015% of N, and a remainder made up by iron and unavoidable impurities. In the steel wire, the metallic structure of the steel contains ferrite and cementite, with the remainder made up by impurities. In the steel wire, the average equivalent circle diameter of bcc-Fe crystal grains is 15 μm or less and the number of cementite grains per 25 μm² is 2.0 × 10 × [C%] or less.

Description

Steel wire for machine structural parts

The present invention relates to a steel wire used as a material for machine structural parts. More specifically, the wire rod manufactured by temper rolling is subjected to spheroidizing annealing and then cold-worked, so the deformation resistance during cold working is low, crack resistance is good, and cold workability is good. The present invention relates to a steel wire useful as a material for machine structural parts. In addition, in this specification, a "wire" is used for the meaning of a rolled wire, and points out the linear steel material cooled to room temperature after hot rolling. The “steel wire” refers to a linear steel material obtained by subjecting the rolled wire material to a tempering treatment such as spheroidizing annealing.

When manufacturing various machine structural parts such as automobile parts and construction machine parts, spheroidizing annealing is usually applied to hot rolled wire rods such as carbon steel and alloy steel for the purpose of imparting cold workability. . And it cold-processes with respect to the rolling wire after spheroidizing annealing, ie, a steel wire, and after that, it forms into a predetermined shape by giving machining, such as cutting, and also performs quenching and tempering processing, and is final. The strength is adjusted to obtain a machine structural part.

In cold working, the life of the mold can be improved by lowering the deformation resistance of the steel wire. Moreover, the yield improvement of various components can be expected by improving the crack resistance of the steel wire.

So far, various methods have been proposed as techniques for improving the cold workability of steel wires. As such a technique, for example, in Patent Document 1, the metal structure is substantially composed of ferrite grains and spherical carbides, and the ferrite grains have an average particle size of 15 μm or more, and the spherical carbides have an average particle size of 0.1. 8 μm or less, the maximum particle size is 4.0 μm or less, and the number per 1 mm ² is 0.5 × 10 ⁶ × C% to 5.0 × 10 ⁶ × C%. A steel wire technique in which the maximum distance between spherical carbides having a particle size of 0.1 μm or more is 10 μm or less is disclosed.

In Patent Document 2, in the region from the surface to 20% of the cross-sectional radius, the average particle diameter of ferrite is 3 to 15 μm, the average particle diameter is 0.3 to 0.6 μm, and the average aspect ratio is 2. 5 or less spherical cementite is contained at a number density of 7 × 10 ⁵ pieces / mm ² or less, and the value obtained by dividing the standard deviation of the area ratio of cementite by the average area ratio of cementite is 0.25 or less. In the inner region from 75% of the radius to the center, the average particle diameter of ferrite is 20 μm or more, and the steel wire rod / steel bar has excellent cold workability and contains spherical cementite having an average particle diameter of 0.3 μm or more. Technology is disclosed.

Further, Patent Document 3 includes a ferrite structure having an average particle diameter of 15 μm or less, and spherical cementite having an average aspect ratio of 3 or less and an average particle diameter of 0.6 μm or less, and the number of the spherical cementite is 1 mm. A technique of steel wire rods having a cold workability of 1.0 × 10 ⁶ × C content (%) or more per ² is disclosed.

Japanese Patent No. 5026626 JP 2013-234349 A Japanese Patent No. 5407178

An object of the present invention is to provide a steel wire for machine structural parts capable of reducing deformation resistance during cold working, improving crack resistance, and exhibiting excellent cold workability.

The steel wire for machine structural parts according to the embodiment of the present invention that has solved the above problems is in mass%, C: 0.3 to 0.6%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.7%, P: more than 0%, 0.03% or less, S: 0.001 to 0.05%, Al: 0.01 to 0.1%, and N: 0 to 0.015 %, The balance is composed of iron and inevitable impurities, the steel microstructure is composed of ferrite and cementite, the balance is composed of impurities, and the average equivalent circle diameter of the bcc-Fe crystal grains is 15 μm or less, The main point is that the number of cementite per 25 μm ² satisfies 2.0 × 10 × [C%] or less.
However, [C%] indicates the C content in mass%.

In a preferred embodiment of the present invention, the steel wire further comprises, in mass%, Cr: more than 0%, 0.5% or less, Cu: more than 0%, 0.25% or less, Ni: more than 0%, 0 0.5% or less, Mo: more than 0%, 0.25% or less, and B: more than 0%, containing 0.01% or less, and satisfying the following formula (X) To do.
[Cr%] + [Cu%] + [Ni%] + [Mo%] ≦ 0.75 (X)
However, [Cr%], [Cu%], [Ni%], and [Mo%] respectively indicate the contents of Cr, Cu, Ni, and Mo expressed in mass%.

In a preferred embodiment of the present invention, the steel wire further contains, by mass%, Ti: more than 0% and 0.1% or less.

In a preferred embodiment of the present invention, in the steel wire, the number ratio of cementite having an aspect ratio of 3.0 or less in the metal structure is 70% or more with respect to the total cementite number.

In the steel wire for machine structural parts according to the embodiment of the present invention, the chemical composition is appropriately adjusted, the steel microstructure contains ferrite and cementite, and the balance is made of impurities, and bcc (body-centered cubic) : Body-centered cubic lattice)-by controlling the average equivalent circle diameter of Fe crystal grains (hereinafter sometimes simply referred to as "bcc-Fe average grain diameter") to be small and reducing the number of cementite per 25 μm ² Further, it is possible to provide a steel wire that realizes an improvement in crack resistance as well as a reduction in deformation resistance. Since the deformation resistance of the steel wire for machine structural parts according to the embodiment of the present invention is reduced, wear and breakage of a plastic working tool such as a die can be suppressed. Moreover, since the steel wire for machine structural components according to the embodiment of the present invention is excellent in crack resistance, it is possible to suppress the occurrence of cracking during forging and to exhibit characteristics excellent in cold workability.

The present inventors have studied from various angles in order to realize a steel wire that has both improved deformation resistance and reduced deformation resistance during cold working. As a result, it was found that the ductility is improved and the crack resistance is improved by controlling the average particle size of bcc-Fe to be small during cold working. Further, the present inventors have found that as the number density of cementite is higher, the deformation resistance is increased by the dispersion strengthening mechanism, and further, macro cracks are formed by connecting the voids starting from the cementite. In other words, if the number density of cementite is controlled to be low, the average distance between the cementite particles is expanded and the voids of the cementite origin are difficult to connect, so the deformation resistance during cold working can be reduced, and macro cracks can occur. The idea that it can be suppressed was obtained. Therefore, in order to achieve both reduction of deformation resistance and improvement of crack resistance, it is important to control the number density of cementite within an appropriate range while controlling the bcc-Fe average particle size small. As a result of the examination, the present invention was completed.

Hereinafter, each requirement prescribed | regulated by embodiment of this invention is demonstrated.

The metal structure of a steel wire for machine structural parts according to an embodiment of the present invention (hereinafter sometimes simply referred to as “steel wire”) contains ferrite and cementite, and the balance is impurities. As described above, the steel wire according to the embodiment of the present invention is obtained by spheroidizing and annealing a rolled wire, and changes to a spheroidized structure mainly composed of ferrite and cementite by spheroidizing annealing. The steel metal structure contains ferrite and cementite, and the balance is made of impurities. As a result, the deformation resistance of the steel is reduced and the cold workability is improved. A preferable total area ratio of ferrite and cementite with respect to the entire metal structure is 97% or more, more preferably 98% or more, and further preferably 99% or more. Examples of the impurities include compounds such as AlN. The compound can be tolerated as long as it does not adversely affect cold workability. For example, the preferred area ratio of the compound with respect to the total metal structure is less than about 3%. In addition to the above compounds, a pearlite structure before spheroidizing annealing may be included as long as the cold workability is not adversely affected.

Average equivalent circle diameter of bcc-Fe crystal grains: 15 μm or less The average equivalent circle diameter (bcc-Fe average grain diameter) of bcc-Fe crystal grains of steel wire is 15 μm or less to improve ductility and during cold working Generation of cracks can be suppressed. The bcc-Fe average particle diameter is preferably 13 μm or less, more preferably 11 μm or less. The bcc-Fe average particle diameter is preferably 5 μm or more in consideration of production costs. The standard for the size of the bcc-Fe crystal grains to be measured is not particularly limited, but the size that can be discriminated by the measurement method described later is the minimum size. Specifically, a size of 1 μm or more is a measurement target. In addition, the circle equivalent diameter of a crystal grain means the diameter of the circle of the same area as each crystal grain.

The metal structure to be controlled by the above-described average bcc-Fe grain size is bcc-Fe crystal grains surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystal grains is larger than 15 °. This is because the effect on cold workability is small at the small-angle grain boundary where the orientation difference is 15 ° or less. The above-mentioned “azimuth difference” is also called “deviation angle” or “bevel angle”, and the EBSP method (Electron BackScatter diffraction Pattern method) may be employed for measuring the orientation difference.

Number of cementite per 25 μm ² : 2.0 × 10 × [C%] or less In the steel wire according to the embodiment of the present invention, the number of cementite per 25 μm ² in the metal structure is 2.0 × 10 × [C %] Or less. By controlling the number of cementites per 25 μm ² within the above range, the deformation resistance can be reduced. Furthermore, by controlling to the above range, the occurrence of cracks at the cementite starting point may be suppressed. The number of cementite per 25 μm ² is preferably 1.8 × 10 × [C%] or less, more preferably 1.6 × 10 × [C%] or less. The number of cementite per 25 μm ² is preferably 0.6 × 10 × [C%] or more in consideration of productivity. In addition, although the reference | standard of the magnitude | size of the cementite used as a measuring object is not specifically limited, The size of the cementite which can be discriminate | determined with the measuring method of the number of cementite per 25 micrometers ^{2 mentioned} later becomes the minimum size. Specifically, cementite having an equivalent circle diameter of 0.1 μm or more is a measurement target.

Number ratio of cementite having an aspect ratio of 3.0 or less: 70% or more The steel wire according to the embodiment of the present invention has a number ratio of cementite having an aspect ratio of 3.0 or less to the total number of cementites (hereinafter simply referred to as “aspect ratio”). The ratio of the cementite with a ratio of 3.0 or less ”(sometimes referred to as“ the ratio of cementite ”) is preferably 70% or more. As the aspect ratio of cementite decreases, voids originating from the cementite become more difficult to generate. Therefore, macro cracks can be further suppressed by increasing the number ratio of cementite having a small aspect ratio. The number ratio of cementite having an aspect ratio of 3.0 or less is more preferably 75% or more, still more preferably 80% or more, still more preferably 90% or more, and most preferably 100%. In addition, the reference | standard of the magnitude | size of the cementite used as a measuring object is not specifically limited. However, similarly to the above-described measurement of the number of cementites per 25 μm ² , the size of the cementite that can be discriminated by the measurement method of the cementite ratio with an aspect ratio of 3.0 or less described later is the minimum size. Specifically, cementite having an equivalent circle diameter of 0.1 μm or more is a measurement target.
The aspect ratio of cementite is a value obtained by dividing the length of the major axis of the equivalent ellipse by the length of the minor axis of the equivalent ellipse when the cementite shape is an equivalent ellipse. The equivalent ellipse is an ellipse having the same area as the target cementite and having the same moment of inertia as the target cementite. For example, Media Cybernetics, Inc., which will be described later. You may obtain | require by the image analysis using "Image-Pro Plus" (brand name) made from a product.

Next, the chemical component composition of the steel wire according to the embodiment of the present invention will be described. The embodiment of the present invention is intended for a steel wire used as a material for a machine structural component, and may have a normal chemical composition as a steel wire for a machine structural component. The appropriate range of chemical components and the reasons for limiting the range are as follows. In the present specification, “%” for the chemical component composition means mass%.

C: 0.3 to 0.6%
C is an element useful for ensuring the strength of the steel, that is, the strength of the final product. In order to exhibit such an effect effectively, the C content needs to be 0.3% or more. The C content is preferably 0.32% or more, and more preferably 0.34% or more. However, if C is excessively contained, the strength is increased and the cold workability is lowered, so that it is necessary to be 0.6% or less. The C content is preferably 0.55% or less, more preferably 0.50% or less.

Si: 0.05 to 0.5%
Si is useful as a deoxidizing element and an element for improving the strength of the final product by solid solution hardening. In order to effectively exhibit such an effect, the Si content was set to 0.05% or more. The Si content is preferably 0.07% or more, and more preferably 0.10% or more. On the other hand, when Si is contained excessively, the hardness is excessively increased and the cold workability is deteriorated. Therefore, the Si content is set to 0.5% or less. The Si content is preferably 0.45% or less, more preferably 0.40% or less.

Mn: 0.2 to 1.7%
Mn is an effective element for increasing the strength of the final product through improvement of hardenability. In order to effectively exhibit such an effect, the Mn content is set to 0.2% or more. The Mn content is preferably 0.3% or more, and more preferably 0.4% or more. On the other hand, when Mn is contained excessively, the hardness is increased and the cold workability is deteriorated. Therefore, the Mn content is set to 1.7% or less. The Mn content is preferably 1.5% or less, and more preferably 1.3% or less.

P: more than 0% and 0.03% or less P is an element inevitably contained in steel, causes segregation of grain boundaries in steel, and causes ductility deterioration. Therefore, the P content is set to 0.03% or less. The P content is preferably 0.02% or less, more preferably 0.017% or less, and still more preferably 0.01% or less. The smaller the P content, the better. However, there may be a case where approximately 0.001% remains due to restrictions on the manufacturing process.

S: 0.001 to 0.05%
S is an element inevitably contained in the steel, and is present as MnS in the steel and deteriorates ductility. Therefore, S is an element harmful to cold workability. Therefore, the S content is set to 0.05% or less. The S content is preferably 0.04% or less, and more preferably 0.03% or less. However, since S has the effect | action which improves a machinability, it is made to contain 0.001% or more. The S content is preferably 0.002% or more, and more preferably 0.003% or more.

Al: 0.01 to 0.1%
Al is useful as a deoxidizing element and is useful for fixing solute N present in steel as AlN. In order to exhibit such an effect effectively, the Al content is determined to be 0.01% or more. The Al content is preferably 0.013% or more, and more preferably 0.015% or more. However, when the Al content is excessive, Al ₂ O ₃ is excessively generated and the cold workability is deteriorated. Therefore, the Al content is determined to be 0.1% or less. Al content becomes like this. Preferably it is 0.090% or less, More preferably, it is 0.080% or less.

N: 0 to 0.015%
N is an element inevitably contained in the steel. When solid solution N is contained in the steel, the hardness increases due to strain aging and the ductility decreases, and the cold workability deteriorates. Therefore, the N content is set to 0.015% or less. The N content is preferably 0.013% or less, and more preferably 0.010% or less. The N content is preferably as low as possible, and is most preferably 0%, but it may remain about 0.001% due to restrictions on the manufacturing process.

The basic components of the steel wire according to the embodiment of the present invention are as described above, and the balance is substantially iron. In addition, “substantially iron” means that the presence of trace components such as Sb and Zn, for example, other than iron, which does not inhibit the characteristics of the present invention, and that other than P, S, N, etc. It means that inevitable impurities such as O and H may be included. Furthermore, in the embodiment of the present invention, the following elements may be selectively contained as necessary. Depending on the type of the selected arbitrary element (selected component), the characteristics of the steel wire are further improved.
As described above, P, S, and N are elements inevitably contained (unavoidable impurities), but their composition ranges are separately defined as described above. For this reason, in the present specification, “inevitable impurities” included as the balance mean elements inevitably included except for elements whose composition range is separately defined.

Cr: more than 0%, 0.5% or less, Cu: more than 0%, 0.25% or less, Ni: more than 0%, 0.25% or less, Mo: more than 0%, 0.25% or less and B: One or more selected from the group consisting of more than 0% and less than 0.01% Cr, Cu, Ni, Mo and B all increase the strength of the final product by improving the hardenability of the steel. It is an effective element. If necessary, these elements may be contained alone or in combination of two or more. Such effects increase as the content of these elements increases. A preferable content for effectively exhibiting the above-described effect is such that the Cr content is 0.015% or more, more preferably 0.020% or more. The preferable contents of Cu, Ni and Mo are all 0.02% or more, more preferably 0.05% or more. The preferable content of B is 0.0003% or more, more preferably 0.0005% or more.

However, if the contents of Cr, Cu, Ni and Mo are excessive, the strength becomes too high and the cold workability may be deteriorated. Therefore, the Cr content is preferably 0.5% or less, and the Cu, Ni and Mo contents are preferably 0.25% or less. A more preferable content of Cr is 0.45% or less, and further preferably 0.40% or less. The more preferable contents of Cu, Ni and Mo are all 0.22% or less, more preferably 0.20% or less.

Also, if the B content is excessive, the toughness may be deteriorated. Therefore, the B content is preferably 0.01% or less. A more preferable content of B is 0.007% or less, more preferably 0.005% or less.

[Cr%] + [Cu%] + [Ni%] + [Mo%] ≦ 0.75
When the steel wire which concerns on embodiment of this invention contains 1 or more types of Cr, Cu, Ni, and Mo in the range mentioned above, it is preferable that the following formula (X) is satisfied.
[Cr%] + [Cu%] + [Ni%] + [Mo%] ≦ 0.75 (X)
Here, [Cr%], [Cu%], [Ni%], and [Mo%] respectively indicate the contents of Cr, Cu, Ni, and Mo expressed in mass%.
When the total content of Cr, Cu, Ni and Mo satisfies the above formula (X), it is possible to suppress the strength of the steel wire from becoming too high and improve the cold workability.

Ti: More than 0%, 0.1% or less The steel wire according to the embodiment of the present invention may contain Ti in a range of more than 0% and 0.1% or less, if necessary. Since Ti forms a compound with N, solid solution N can be reduced by containing Ti. Therefore, the steel wire can be made softer. The content of Ti is preferably 0.01% or more, more preferably 0.02% or more. On the other hand, if the Ti content is excessive, the hardness of the steel wire may increase due to the compound formed. Therefore, the preferable Ti content is 0.1% or less, more preferably 0.08% or less, and still more preferably 0.05% or less.

Next, a method for manufacturing the steel wire according to the embodiment of the present invention described above will be described. As described repeatedly, the steel wire according to the embodiment of the present invention is obtained by spheroidizing and annealing a rolled wire. Therefore, in order to appropriately control the metal structure after spheroidizing annealing as described above, it is preferable to appropriately control the spheroidizing annealing conditions described later. However, in order to ensure the above-described structure form by spheroidizing annealing, it is preferable to further appropriately control the production conditions (that is, rolling conditions) of the rolled wire rod. By controlling the rolling conditions, the microstructure of the rolled wire can be in a state where the bcc-Fe average particle diameter can be easily reduced and the number density of cementite can be easily reduced after spheroidizing annealing.

Specifically, it is preferable to adjust the finish rolling temperature at the time of hot rolling the steel that satisfies the above-described component composition, and to appropriately adjust the cooling rate and the temperature range with three subsequent cooling rates. . By producing a rolled wire rod under these conditions, the metal structure before spheroidizing annealing has pearlite and ferrite as the main phase, the crystal grains are refined, and the proeutectoid ferrite crystal grains are equiaxed. The average lamellar interval can be set to a predetermined value or less. By subjecting a rolled wire having such a metal structure to spheroidizing annealing under the conditions described later, the bcc-Fe average particle diameter is controlled to an appropriate range, and the number density of cementite is controlled to an appropriate range. Steel wire is obtained. The rolling wire manufacturing conditions for this are as follows: (a) finish rolling at 800 ° C. or more and 1000 ° C. or less, (b) first cooling with an average cooling rate of 7 ° C./sec or more, and (c) average cooling rate of 1 Second cooling at a rate of not less than 5 ° C / second and not more than 5 ° C / second, and (d) a third cooling having an average cooling rate faster than that of the second cooling and not less than 5 ° C / second in this order, The end of the first cooling and the start of the second cooling are performed within a range of 700 to 750 ° C., the end of the second cooling and the start of the third cooling are performed within a range of 600 to 650 ° C., and the third The end of cooling is preferably set to 500 ° C. or lower.

Hereinafter, detailed description will be given in the order of steps.

First, the heating temperature during hot rolling is preferably 800 ° C. or higher and 1100 ° C. or lower in consideration of productivity and rolling quality.

(A) Finishing rolling temperature: 800 ° C. or higher and 1000 ° C. or lower In order to refine the crystal grains of the metal structure of the rolled wire rod and reduce the bcc-Fe average grain size of the steel wire after spheroidizing annealing, it is especially finished. It is preferable to appropriately control the upper limit of the rolling temperature. When the finish rolling temperature exceeds 1000 ° C., it becomes difficult to reduce the bcc-Fe average particle diameter of the steel wire. On the other hand, when the finish rolling temperature exceeds 1000 ° C., cementite having a large aspect ratio may be easily precipitated after spheroidizing annealing. Therefore, the finish rolling temperature is preferably 1000 ° C. or less. The finish rolling temperature is more preferably 970 ° C. or less, and further preferably 940 ° C. or less. The number ratio of cementite having an aspect ratio of 3.0 or less after spheroidizing annealing is further increased by performing finish rolling at 970 ° C. or less, which is a more preferable finish rolling temperature, and performing spheroidizing annealing under appropriate conditions described later. It tends to be higher. However, when the finish rolling temperature is less than 800 ° C., the load on the rolling mill becomes too high and the effect of improving crack resistance after spheroidizing annealing is almost saturated. The finish rolling temperature is more preferably 830 ° C. or higher, and further preferably 860 ° C. or higher.

(B) First cooling The first cooling starts from 800 ° C. or more and 1000 ° C. or less, which is the finish rolling temperature, and ends in the temperature range of 700 to 750 ° C. In the first cooling, when the cooling rate is slow, the crystal grains of the metal structure of the rolled wire are coarsened, and the bcc-Fe average particle diameter of the steel wire after spheroidizing annealing may be increased. Therefore, the average cooling rate in the first cooling is preferably 7 ° C./second or more. The average cooling rate of the first cooling is more preferably 10 ° C./second or more, and further preferably 20 ° C./second or more. The upper limit of the average cooling rate of the first cooling is not particularly limited, but is preferably 200 ° C./second or less as a realistic range. In the cooling in the first cooling, the cooling rate may be changed as long as the average cooling rate is 7 ° C./second or more.

(C) Second cooling The second cooling starts from a temperature range of 700 to 750 ° C. and ends in a temperature range of 600 to 650 ° C. In order to reduce the number density of cementite in the steel wire after spheroidizing annealing, it is preferable to perform slow cooling at an average cooling rate of 5 ° C./second or less in the second cooling. Thereby, the average lamellar interval of the pearlite of a rolled wire rod can be made as narrow as possible, and the cementite can be easily dissolved during spheroidizing annealing, and the number density of cementite in the steel wire after spheroidizing annealing can be lowered. The average cooling rate of the second cooling is more preferably 4 ° C./second or less, and further preferably 3.5 ° C./second or less. On the other hand, if the average cooling rate in the second cooling is too slow, the bcc-Fe crystal grains are coarsened, and the average bcc-Fe grain size surrounded by the large-angle grain boundaries may be too large. Therefore, the average cooling rate in the second cooling is preferably 1 ° C./second or more. The average cooling rate of the second cooling is more preferably 2 ° C./second or more, and further preferably 2.5 ° C./second or more. In the cooling in the second cooling, the cooling rate may be changed as long as the average cooling rate is 1 ° C./second or more and 5 ° C./second or less.

(D) Third Cooling In this third cooling, the average lamellar spacing of the pearlite of the rolled wire rod is made as narrow as possible, the cementite is easily dissolved during spheroidizing annealing, and no spherical cementite nuclei are left in the grains ( That is, the number density of spherical cementite nuclei remaining in the grains is reduced). The third cooling starts from a temperature range of 600 to 650 ° C. and ends at 500 ° C. or less. In order to reduce the average lamellar spacing of the pearlite of the rolled wire rod, it is preferable that the third cooling is faster than the second cooling and at an average cooling rate of 5 ° C./second or more. Cooling slower than 5 ° C./second makes it difficult to narrow the average lamellar spacing of the pearlite of the rolled wire rod. The average cooling rate of the third cooling is more preferably 10 ° C./second or more, and further preferably 20 ° C./second or more. The upper limit of the average cooling rate of the third cooling is not particularly limited, but is preferably 200 ° C./second or less as a practical range. In the third cooling, the cooling rate may be changed as long as the average cooling rate is 5 ° C./second or more. Although the minimum of the completion | finish temperature of 3rd cooling is not specifically limited, For example, it is 200 degreeC or more.

After the third cooling, normal cooling such as cooling is performed to cool to room temperature. In general, the average cooling rate of cooling is often slower than the average cooling rate of the third cooling.

After cooling to room temperature, wire drawing may be performed at room temperature as necessary, and the area reduction rate at that time may be, for example, 30% or less. When the wire is drawn, the carbides in the steel are destroyed, and the agglomeration of the carbides can be promoted by the subsequent spheroidizing annealing. However, if the area reduction ratio of the wire drawing process exceeds 30%, the strength after annealing increases and the cold workability may be deteriorated. Therefore, the area reduction ratio of the wire drawing process is preferably 30% or less. The lower limit of the area reduction rate is not particularly limited, but the effect is preferably obtained by setting it to 2% or more.

As a spheroidizing annealing condition to be applied to a rolled wire manufactured under the preferable conditions as described above, for example, when heating from room temperature to 740 ° C. in an atmospheric furnace as SA1 described later, at least from 500 ° C. to 740 ° C. Is heated at an average heating rate of 50 ° C / hour or more, then heated to 750 ° C at an average heating rate of 2 to 5 ° C / hour, held at 750 ° C for 10 to 60 minutes, and then at an average cooling rate of 20 ° C / hour or more. It is preferably cooled to 720 ° C., cooled to 700 ° C. at an average cooling rate of 3 to 7 ° C./hour, cooled to 640 ° C. at an average cooling rate of 8 to 12 ° C./hour, and then allowed to cool. However, the spheroidizing annealing condition used in the embodiment of the present invention is not limited to this.

In the above spheroidizing annealing condition, when heating from room temperature to 740 ° C., the average heating rate from at least 500 ° C. to 740 ° C. is set to 50 ° C./hour or more, thereby suppressing the grain growth of the metal structure. The average heating rate at this time is more preferably 60 ° C./hour or more. However, if the average heating rate is too high, it becomes difficult to follow the temperature of the rolled wire, and therefore it is preferably 200 ° C./hour or less, more preferably 150 ° C./hour or less.

In addition, by controlling the average heating rate from 740 ° C to 750 ° C immediately above the A1 point at 2-5 ° C / hour, the decomposition and solid solution of cementite in the pearlite structure is sufficiently suppressed while suppressing the grain growth of the metal structure as much as possible. It can be carried out. When the average heating rate is higher than 5 ° C / hour, it is difficult to secure a sufficient time for decomposition and solid solution of cementite in the pearlite structure, and from 740 ° C when the average heating rate is lower than 2 ° C / hour. The heating time up to 750 ° C. becomes longer, and it becomes difficult to suppress the grain growth of the metal structure. The average heating rate at this time is more preferably 3 ° C./hour or more and 4 ° C./hour or less.

It is preferable to hold at 750 ° C. for 10 to 60 minutes. When this holding temperature is shorter than 10 minutes, decomposition and solid solution of cementite in the pearlite structure is insufficient, and when it is longer than 60 minutes, it becomes difficult to suppress grain growth of the metal structure. The holding time at this time is more preferably 20 minutes or more and 50 minutes or less.

After holding as described above, by setting the preferable average cooling rate up to 720 ° C. to 20 ° C./hour or more, grain growth of the metal structure can be suppressed. The average cooling rate at this time is more preferably 30 ° C./hour or more. However, if the average cooling rate is too high, it becomes difficult to follow the temperature of the rolled wire, and therefore it is preferably 100 ° C./hour or less.

Thereafter, by controlling the average cooling rate from 720 ° C. to 700 ° C. at 3 to 7 ° C./hour, the number of re-deposited cementite can be controlled and the cementite can be coarsened. When the average cooling rate is slower than 3 ° C./hour, it is difficult to suppress the grain growth of the metal structure, and when the average cooling rate is higher than 7 ° C./hour, the number of cementite to be reprecipitated increases. The average cooling rate at this time is more preferably 4 ° C./hour or more and 6 ° C./hour or less.

Thereafter, by controlling the average cooling rate from 700 ° C. to 640 ° C. at 8 to 12 ° C./hour, precipitation of cementite having a large aspect ratio such as a pearlite structure can be suppressed. When the average cooling rate is lower than 8 ° C./hour, it is difficult to suppress the grain growth of the metal structure, and when the average cooling rate is higher than 12 ° C./hour, cementite with a large aspect ratio such as a pearlite structure is formed. Reprecipitates a lot. The average cooling rate at this time is more preferably 9 ° C./hour or more and 11 ° C./hour or less.

After cooling to 640 ° C., normal cooling such as cooling may be performed to cool to room temperature. In general, the average cooling rate for cooling is often slower than the average cooling rate from 700 ° C to 640 ° C.

The spheroidizing annealing as described above may be repeated a plurality of times. By performing such repetition, the individual aspect ratio of cementite becomes small, and the number density of cementite becomes low. The number of repetitions at this time is preferably at least 3 times. Since the number density of cementite does not change so much even if it is excessively repeated, it is preferably 10 times or less. When the spheroidizing annealing is repeated a plurality of times, it may be repeated under the same conditions or within different conditions within the range of the above preferable conditions.

Hereinafter, embodiments of the present invention will be described more specifically with reference to examples. Embodiments of the present invention are not limited by the following examples, and can be implemented with appropriate modifications within a range that can be adapted to the gist described above. It is included in the technical scope of the embodiment.

Using steel having the chemical composition shown in Table 1 below, rolling was performed under various processing conditions shown in Table 2 below to produce a wire having a diameter of 17.0 mm. For steel types A to T and V to X, the end temperature of “third cooling” was set to 450 ° C., and then allowed to cool to room temperature.

Steel types O and P are comparative examples in which the chemical composition deviates from the specified value. Steel types Q, R, S, T, and U are examples in which a rolled wire was not manufactured under appropriate manufacturing conditions in the embodiment of the present invention. That is, steel types Q, T, and U are examples in which rolled wire rods are manufactured under conditions with a high finish rolling temperature, and steel types S and U are examples in which rolled wire rods are manufactured under conditions where the second cooling rate is high. R is an example in which a rolled wire was manufactured under conditions where the first cooling rate and the third cooling rate were slow.
Steel types B, E, H, I, K, L, P, Q, V, and W are examples containing at least one of Cr, Cu, Ni, Mo, and B. As shown in Table 1 below, among these steel types, all the steel types except P have [Cr%] + [Cu%] + [Ni%] + [Mo%] of 0.75 mass% or less. The above-described formula (X) is satisfied. In the steel type P, [Cr%] + [Cu%] + [Ni%] + [Mo%] exceeds 0.75 mass% and does not satisfy the formula (X).

In steel type U, after the second cooling to 550 ° C., a holding step of holding at 580 ° C. for 120 seconds was performed, and then the mixture was allowed to cool to room temperature, and a wire drawing step with a surface reduction rate of 40% was performed.

Next, spheroidizing annealing of any of the following (a) to (c) was performed on each rolled wire rod excluding the steel type U in an atmospheric furnace.
(A) When heating from room temperature to 740 ° C., after heating from room temperature to 500 ° C. at an average heating rate of 100 ° C./hour and from 500 ° C. to 740 ° C. at an average heating rate of 80 ° C./hour, an average heating temperature of 4 After heating to 750 ° C./hour and holding at 750 ° C. for 30 minutes, cooling to 720 ° C. at an average cooling rate of 30 ° C./hour, then cooling to 700 ° C. at an average cooling rate of 5 ° C./hour, average cooling After cooling to 640 ° C. at a rate of 10 ° C./hour, it is allowed to cool. Hereinafter, this annealing condition is abbreviated as “SA1”.
(B) Repeat SA1 five times. Note that the second and subsequent heating of SA2 (that is, the heating during the annealing of the second to fifth SA1 conditions in the annealing of SA2 that repeats SA1 five times) started at 640 ° C. Hereinafter, this annealing condition is abbreviated as “SA2”.
(C) When heating from room temperature to 710 ° C., heating from room temperature to 500 ° C. at an average heating rate of 100 ° C./hour, heating from 500 ° C. to 710 ° C. at an average heating rate of 80 ° C./hour, and 710 ° C. for 5 hours After being held, it is cooled to 640 ° C. at an average cooling rate of 10 ° C./hour, and then allowed to cool. Hereinafter, this annealing condition is abbreviated as “SA3”.
Here, the annealing conditions SA1 and SA2 are preferable annealing conditions of the embodiment of the present invention, and the annealing condition SA3 is an example in which the heating temperature (holding temperature) is low and not properly controlled.

The steel type U was subjected to spheroidizing annealing of either (d) or (e) below in an atmospheric furnace.
(D) Heat from room temperature to 680 ° C. at an average heating rate of 80 ° C./hour, hold at 680 ° C. for 5 hours, cool to 640 ° C. at an average cooling rate of 10 ° C./hour, and then cool. Hereinafter, this annealing condition is abbreviated as “SA4”.
(E) Heat from room temperature to 700 ° C. at an average heating rate of 80 ° C./hour, hold at 700 ° C. for 5 hours, cool to 640 ° C. at an average cooling rate of 10 ° C./hour, and then cool. Hereinafter, this annealing condition is abbreviated as “SA5”.
Here, the annealing conditions SA4 and SA5 are examples in which the heating temperature (holding temperature) is low and not properly controlled.

For the steel wire after the spheroidizing annealing, (1) the total area ratio of ferrite and cementite, (2) the bcc-Fe average particle diameter of the metal structure, (3) the number of cementite per 25 μm ² , ( 4) The ratio of cementite having an aspect ratio of 3.0 or less, (5) deformation resistance during cold working, and (6) crack occurrence rate during cold working were measured by the following methods.

In measuring the total area ratio of ferrite and cementite, bcc-Fe crystal grain size, number density of cementite, and number ratio of cementite in the steel wire after spheroidizing annealing, the cross section (that is, in the radial direction of the wire) The resin was embedded so that the cut cross-section could be observed, and the cut surface was mirror-polished with emery paper and diamond buff. With respect to the diameter D of the steel wire, the position of D / 4 was measured in the radial direction from the outer peripheral surface of the steel wire.

(1) Measurement of the total area ratio of ferrite and cementite The measurement of the total area ratio of ferrite and cementite was performed by making cementite appear by picral etching, and using FE-SEM (Field-Emission Scanning Electron Microscope, field emission scanning electron microscope). The tissue was observed, and a 60 μm × 45 μm region was photographed at 5 magnifications at a magnification of 2000 times. On these photographs, mesh lines every 3 μm were put in the vertical direction and the horizontal direction, and the number of intersections of the mesh lines existing on the ferrite or cementite was measured. A value obtained by dividing the number of intersections of mesh lines existing on ferrite or cementite by the number of intersections of all mesh lines was calculated as the total area ratio of ferrite and cementite.

(2) Measurement of bcc-Fe average particle diameter The bcc-Fe crystal grain diameter was measured using an EBSP analyzer and an FE-SEM. As an analysis tool, OIM software of TSL Solutions Inc. was used. When the crystal grain difference is defined as a boundary where the crystal orientation difference (also referred to as “bevel”) exceeds 15 °, that is, a large-angle grain boundary, and the area of the bcc-Fe crystal grain is converted into a circle The average value of the diameters, that is, the average equivalent circle diameter was calculated. The measurement area at this time was 200 μm × 400 μm, and the measurement step was 1.0 μm. Measurement points having a confidence index (Confidence Index) indicating the reliability of the measurement direction of 0.1 or less were deleted (ie, excluded) from the analysis target.

(3) Number of cementite per 25 μm ^{2 In} the measurement of the number of cementite per 25 μm ² , cementite appeared by picral etching, and the structure was observed with FE-SEM, and 29 μm × 22 μm at a magnification of 4000 times. The field was photographed with 5 fields of view. Based on these photographs, the number of all cementite in the photograph was measured by image analysis (“Image-Pro Plus” (trade name) manufactured by Media Cybernetics, Inc.), and the average value of the number of cementite in the five fields of view was determined. Asked. The number of cementite was measured with one piece showing the entire cementite in the photo and 0.5 piece containing no cementite in the photo. From the average value, the unit area was 25 μm ² and the number density of cementite was calculated. The minimum equivalent circle diameter of the cementite to be measured was 0.1 μm.

(4) Cementite ratio with an aspect ratio of 3.0 or less In measuring the cementite ratio with an aspect ratio of 3.0 or less, the measurement was performed based on the photograph taken in (3) above. Specifically, the aspect ratios of all cementites in the five photographs were measured by image analysis, and the average value of the ratios of the number of cementites having an aspect ratio of 3.0 or less to the total number of cementites in the five visual fields was determined. The total cementite and the cementite with an aspect ratio of 3.0 or less were targeted for those in which the entire cementite was shown in the photograph, and those that were not partially visible in the photograph. From the average value, a cementite ratio with an aspect ratio of 3.0 or less was calculated. The minimum equivalent circle diameter of the cementite to be measured was 0.1 μm.

(5) Measurement of deformation resistance A sample for cold forging test of φ10.0 mm x 15.0 mm was prepared from a steel wire and processed at a strain rate of 5 / sec to 10 / sec at room temperature using a forging press. A cold forging test at a rate of 60% was performed five times. The deformation resistance was measured by measuring the deformation resistance at 40% processing 5 times from the data of the processing rate-deformation resistance obtained from the cold forging test at the 60% processing rate, and obtaining the average value of 5 times. Since the required deformation resistance differs depending on the contents of C, Si and Mn, the target deformation resistance (described as “target deformation resistance” in Table 3) was defined by the following formula (1).
Target deformation resistance = 400 × Ceq + 430 (1)
However, Ceq = [C%] + 0.2 × [Si%] + 0.2 × [Mn%], and [C%], [Si%] and [Mn%] are C, Si and Mn, respectively. Content (mass%) is shown.

(6) Measurement of crack occurrence rate The crack occurrence rate was measured by performing a cold forging test with a processing rate of 60% under the same conditions as in (5) above, and observing the surface of each sample with a stereomicroscope at a magnification of 20 times. It was performed 5 times and the presence or absence of surface cracks was measured. Then, the “number of samples having surface cracks” was divided by 5 to obtain the average. The target crack generation rate for all steel types was 20% or less.

These results are shown in Table 3 below together with the spheroidizing annealing conditions. In the overall evaluation column of Table 3, an example in which both the reduction in deformation resistance and the improvement in crack resistance are good is indicated as “OK”, and at least one of the reduction in deformation resistance and the improvement in crack resistance is indicated. An example of deterioration is indicated as “NG”.

From the results in Table 3, it can be considered as follows.

First, test no. 1, 2, 4 to 8, 10, 11, 13 to 16, 18 to 20, 22 to 25, 37, 38, 40, and 41 are examples that satisfy all of the requirements defined in the embodiments of the present invention. It can be seen that both a reduction in deformation resistance and an improvement in crack resistance are achieved. Although not shown in Table 3, test no. FE-SEM observation of the metal structures 1 to 36 confirms that all contain ferrite and cementite in a total ratio of 99 area% or more.

Here, the test No. which performed both SA1 and SA2 annealing conditions. 1 and 2 (steel type A), test no. 7 and 8 (steel type E), test no. 10 and 11 (steel type F), test no. 15 and 16 (steel type I), test no. 19 and 20 (steel grade K), test no. 24 and 25 (steel grade N), and test no. Focusing on 37 and 38 (steel type V), in any case, when annealing SA2 was repeated 5 times compared to SA1, at least one of deformation resistance and crack generation rate was further reduced.

Of these, test no. 11 (steel type F) is an example in which the number ratio of cementite having an aspect ratio of 3.0 or less satisfies the preferable requirements of the embodiment of the present invention. Therefore, the test No. in which the number ratio of cementite is not properly controlled. Compared to 10 (steel type F), the crack generation rate was further reduced.

In contrast, test no. 3, 9, 12, 17, 21, 26 to 36 and 39 are comparative examples lacking any of the requirements defined in the embodiments of the present invention, and target is either deformation resistance, crack generation rate, or both. It can be seen that the value has not been reached.

Test No. 3 is an example in which spheroidizing annealing was performed using SA3 whose conditions were not appropriate using steel type A in Tables 1 and 2. Therefore, the number of cementite per 25 μm ² is larger than the specified value, and the deformation resistance and crack occurrence rate do not reach the target values.

Test No. No. 9 is an example in which spheroidizing annealing was performed using SA3 whose conditions are not appropriate using steel type E in Tables 1 and 2. Therefore, the number of cementite per 25 μm ² is larger than the specified value, and the deformation resistance does not reach the target value.

Test No. No. 12 is an example in which spheroidizing annealing was performed with SA3 whose conditions were not appropriate using the steel type F in Tables 1 and 2. Therefore, the number of cementite per 25 μm ² is larger than the specified value, and the deformation resistance and crack occurrence rate do not reach the target values.

Test No. No. 17 is an example in which spheroidizing annealing was performed using SA3 whose conditions were not appropriate using steel type I in Tables 1 and 2. Therefore, the number of cementite per 25 μm ² is larger than the specified value, and the deformation resistance and crack occurrence rate do not reach the target values.

Test No. No. 21 is an example in which spheroidizing annealing was performed using SA3 whose conditions are not appropriate using the steel type K shown in Tables 1 and 2. Therefore, the number of cementite per 25 μm ² is larger than the specified value, and the deformation resistance does not reach the target value.

Test No. No. 26 is an example in which spheroidizing annealing was performed with SA3 whose conditions were not appropriate using the steel type N in Tables 1 and 2. Therefore, the number of cementite per 25 μm ² is larger than the specified value, and the deformation resistance and crack occurrence rate do not reach the target values.

Test No. 27 is an example using the steel type O of Tables 1 and 2 with an excessive Mn content. For this reason, appropriate hot rolling and spheroidizing annealing of SA1 were performed, but the deformation resistance did not reach the target value.

Test No. No. 28 is an example using the steel type O in Tables 1 and 2 with an excessive Mn content. Therefore, although appropriate hot rolling and spheroidizing annealing of SA2 were performed, the deformation resistance did not reach the target value.

Test No. 29 is an example using the steel type P in Tables 1 and 2 with an excessive Cr content. For this reason, appropriate hot rolling and spheroidizing annealing of SA1 were performed, but the deformation resistance did not reach the target value.

Test No. 30 is an example using the steel type P of Tables 1 and 2 with an excessive Cr content. Therefore, although appropriate hot rolling and spheroidizing annealing of SA2 were performed, the deformation resistance did not reach the target value.

Test No. 31 is an example using the steel types Q of Tables 1 and 2 having a high finish rolling temperature at the time of rolling wire production. For this reason, appropriate spheroidizing annealing of SA1 was performed, but the average bcc-Fe particle diameter was coarser than the specified value, and the crack generation rate did not reach the target value.

Test No. 32 is an example using the steel types R of Tables 1 and 2 in which the cooling rate of the first cooling during the production of the rolled wire rod is slow and the cooling rate of the third cooling is slow. Therefore, although appropriate spheroidizing annealing of SA1 was performed, the average particle diameter of bcc-Fe was coarser than the specified value, and the number of cementites per 25 μm ² was larger than the specified value. The target value has not been reached.

Test No. 33 is an example using the steel types S of Tables 1 and 2 in which the cooling rate of the second cooling during the production of the rolled wire rod is fast and the cooling rate of the third cooling is the same as the cooling rate of the second cooling. Therefore, although appropriate SA1 spheroidizing annealing was performed, the number of cementite per 25 μm ² was larger than the specified value, and the deformation resistance did not reach the target value.

Test No. 34 is an example using the steel types T of Tables 1 and 2 having a high finish rolling temperature during the production of the rolled wire rod. Therefore, appropriate spheroidizing annealing of SA1 was performed, but the bcc-Fe average particle diameter was coarser than the specified value, and the crack generation rate did not reach the target value.

Test No. No. 35 uses the steel type U of Tables 1 and 2 that has a high finish rolling temperature at the time of rolling wire production, a high cooling rate of the second cooling, and does not perform the third cooling. This is an example. Therefore, the average particle diameter of bcc-Fe is coarser than the specified value, fine cementite is uniformly dispersed, the number of cementites per 25 μm ² is larger than the specified value, and the deformation resistance and crack occurrence rate are the target values. Not reached.

Test No. No. 36 uses the steel type U of Tables 1 and 2 that has a high finish rolling temperature during the production of the rolled wire, the second cooling rate is high, and the third cooling is not performed, and the spheroidizing annealing is performed with SA5 where the conditions are not appropriate. This is an example. Therefore, the average particle diameter of bcc-Fe is coarser than the specified value, fine cementite is uniformly dispersed, the number of cementites per 25 μm ² is larger than the specified value, and the deformation resistance and crack occurrence rate are the target values. Not reached.

Test No. No. 39 is an example in which spheroidizing annealing was performed using SA3 whose conditions are not appropriate using steel type V in Table 1. The number of cementites around 25 μm ² is larger than the predetermined value, and the deformation resistance does not reach the target value.

Steel wires for machine structural parts according to embodiments of the present invention are various machine structural parts such as automobile parts and construction machine parts manufactured by cold working such as cold forging, cold forging and cold rolling. It is suitably used for the material. Specifically, these mechanical structural parts include bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, covers, cases, washers, tappets, saddles, bulgs, Inner case, clutch, sleeve, outer race, sprocket, core, stator, anvil, spider, rocker arm, body, flange, drum, fitting, connector, pulley, metal fitting, yoke, base, valve lifter, spark plug, pinion gear, steering Examples include mechanical parts such as shafts and common rails, and electrical parts. A steel wire according to an embodiment of the present invention is industrially useful as a steel wire for a high-strength mechanical structural component that is preferably used as a material for the mechanical structural component. When manufacturing the various mechanical structural components described above, Excellent cold workability can be exhibited by low deformation resistance at room temperature and by suppressing cracking of the material.

The disclosure of this specification includes the following aspects.
Aspect 1:
% By mass
C: 0.3 to 0.6%,
Si: 0.05 to 0.5%,
Mn: 0.2 to 1.7%,
P: more than 0%, 0.03% or less,
S: 0.001 to 0.05%,
Al: 0.01 to 0.1% and N: 0 to 0.015%, respectively, the balance consisting of iron and inevitable impurities,
The metallographic structure of the steel contains ferrite and cementite, the balance is made of impurities, the average equivalent circle diameter of the bcc-Fe crystal grains is 15 μm or less, and the number of cementite per 25 μm ² is 2.0 × 10 × [C %] Steel wires for machine structural parts that are less than the number.
However, [C%] indicates the C content in mass%.
-Aspect 2:
Furthermore, in mass%,
Cr: more than 0%, 0.5% or less,
Cu: more than 0%, 0.25% or less,
Ni: more than 0%, 0.25% or less,
It is described in the aspect 1 which contains 1 or more types selected from the group which consists of Mo: more than 0%, 0.25% or less and B: more than 0%, 0.01% or less, and satisfy | fills following formula (X). Steel wire for machine structural parts.
[Cr%] + [Cu%] + [Ni%] + [Mo%] ≦ 0.75 (X)
However, [Cr%], [Cu%], [Ni%], and [Mo%] respectively indicate the contents of Cr, Cu, Ni, and Mo expressed in mass%.
Aspect 3:
Furthermore, in mass%,
The steel wire for machine structural parts according to aspect 1 or 2, containing Ti: more than 0% and 0.1% or less.
Aspect 4:
The steel wire for machine structural parts according to any one of aspects 1 to 3, wherein the number ratio of cementite having an aspect ratio of 3.0 or less in the metal structure is 70% or more with respect to the total cementite number.

The present application includes a Japanese patent application with a filing date of September 3, 2015, Japanese Patent Application No. 2015-173962, and a Japanese patent application with a filing date of June 23, 2016, Japanese Patent Application No. 2016- Japanese Patent Application No. 2015-173962 and Japanese Patent Application No. 2016-124960 are incorporated herein by reference with a priority claim based on No. 124960.

Claims (5)

% By mass
C: 0.3 to 0.6%,
Si: 0.05 to 0.5%,
Mn: 0.2 to 1.7%,
P: more than 0%, 0.03% or less,
S: 0.001 to 0.05%,
Al: 0.01 to 0.1% and N: 0 to 0.015%, respectively, the balance consisting of iron and inevitable impurities,
The metallographic structure of the steel contains ferrite and cementite, the balance is made of impurities, the average equivalent circle diameter of the bcc-Fe crystal grains is 15 μm or less, and the number of cementite per 25 μm ² is 2.0 × 10 × [C %] Steel wires for machine structural parts that are less than the number.
However, [C%] indicates the C content in mass%. Furthermore, in mass%,
Cr: more than 0%, 0.5% or less,
Cu: more than 0%, 0.25% or less,
Ni: more than 0%, 0.25% or less,
The element according to claim 1, which contains at least one selected from the group consisting of Mo: more than 0% and 0.25% or less and B: more than 0% and 0.01% or less, and satisfies the following formula (X). Steel wire for machine structural parts as described.
[Cr%] + [Cu%] + [Ni%] + [Mo%] ≦ 0.75 (X)
However, [Cr%], [Cu%], [Ni%], and [Mo%] respectively indicate the contents of Cr, Cu, Ni, and Mo expressed in mass%. Furthermore, in mass%,
The steel wire for machine structural parts according to claim 1 or 2, containing Ti: more than 0% and 0.1% or less. The steel wire for machine structural parts according to claim 1 or 2, wherein the number ratio of cementite having an aspect ratio of 3.0 or less in the metal structure is 70% or more with respect to the total cementite number. The steel wire for machine structural parts according to claim 3, wherein the number ratio of cementite having an aspect ratio of 3.0 or less in the metal structure is 70% or more with respect to the total cementite number.