TW201732053A - Steel wire for mechanical structural components - Google Patents

Abstract

Provided is a steel wire for mechanical structural components, substantially comprising C: 0.3-0.6 mass%, Si: 0.05-0.5 mass%, Mn: 0.2-1.7 mass%, P: greater than 0 mass% to no more than 0.03 mass%, S: 0.001-0.05 mass%, Al: 0.005-0.1 mass%, and N: 0-0.015 mass%, the balance being iron and unavoidable impurities. The metal structure is composed of ferrite and cementite; the standard deviation σc of the cementite count contained in a 5 * 5 [mu]m area satisfies expression (1) below; and the average particle size of the cementite is 0.5 [mu]m or greater. 1.5 ≤ [sigma]c ≤ 4.5 (1), where [C%] indicates the carbon content expressed in mass%.

Description

Steel wire for mechanical structural parts

The content disclosed in the application of the present invention relates to a steel wire which is used as a material for mechanical structural parts. More specifically, it relates to the cold workability in the cold working after the wire material produced by the rolling is applied to the spheroidizing annealing, in particular, the low cold deformation resistance and the excellent crack resistance. Steel wire for mechanical construction parts.

Most of the mechanical structural parts such as automotive parts and construction machinery parts are subjected to spheroidizing annealing in order to impart cold-processability to hot-rolled wire materials such as carbon steel and alloy steel in the manufacturing process. . Further, the rolled wire after the spheroidizing annealing is a steel wire, and cold-working such as cold forging, cold-pressing, and cold-rolling is performed, and then machining is performed by performing machining such as cutting. After the shape, the quench hardening and tempering treatment is carried out to make the final strength adjustment, and it becomes a mechanical structural component.

It has the following effects when it has cold workability, especially low deformation resistance and excellent crack resistance. If the deformation resistance of the steel wire is low, the processing is easy, and the life of the metal mold can be improved. Life. In addition, by improving the crack resistance of the steel wire, it is possible to improve the yield of various parts.

Therefore, various methods have been proposed as techniques for improving the cold workability of steel wires. For example, the technique of the steel wire material excellent in cold-intermediate workability disclosed in Patent Document 1 is a ferrite-grained iron structure having an average particle diameter of 15 μm or less, an average aspect ratio of 3 or less, and an average particle diameter of 0.6 μm or less. The spherical stellite is composed of, and the number of the spherical stellites is 1.0 × 10 ⁶ × C content (%) or more per 1 mm ² .

The technique disclosed in Patent Document 1 is a method for producing the above-described metal structure by hot rolling and coiling a preliminary rolled steel or steel ingot, and then preparing the rolled wire at 400 ° C or higher and 600 ° C. The molten salt bath is immersed for 10 seconds or longer, and further cooled in a molten salt bath of 450 ° C or more and 600 ° C or less in a constant temperature state for 20 seconds or more and 150 seconds or less, and then cooled at 600 ° C or higher. Annealing is carried out at a temperature below 700 °C.

The steel wire disclosed in Patent Document 2 has a metal structure which is a value obtained by dividing the standard deviation of the distance between the snow and iron by the average value of the distance between the snow and the iron, and is 0.50 or less.

According to the technique disclosed in Patent Document 2, the method for producing the above metal structure is in a cooling process after hot rolling, in a temperature range of 750 to 1000 ° C up to 400 to 550 ° C, at 20 ° C / Cooling at a cooling rate of s or more, and maintaining it in a temperature range of 400 to 550 ° C for 20 seconds or more to complete the thermostatic metamorphosis, and then cooling to room temperature, and then performing the rough drawing at a profile reduction ratio of 40% or less. Line Thereafter, spheroidizing annealing is performed, and then the final purification line is performed at a profile reduction ratio of 20% or less.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2009-275252

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2006-316291

According to the steel wire obtained by the method described in Patent Document 1, the ferritic iron is almost uniformly distributed, and the soft ferrite iron structure is less, and there is a concern that the deformation resistance is increased during cold processing. In addition, the steel wire produced by this method, because the stellite iron particles are very fine, the deformation resistance increases when processed in the cold.

In addition to the steel wire described in Patent Documents 1 and 2, the steel wire of the conventionally proposed embodiment has an effect of improving cold workability such as cold forging. However, the steel wire currently required is a steel wire which is required to further improve the cold workability, and in particular, it is not only required to reduce the deformation resistance during cold working, but also excellent in crack resistance.

The embodiment of the present invention has been developed under such circumstances, and an object of the present invention is to provide a mechanical structural part having excellent cold-workability which is excellent in deformation resistance during cold working and excellent in crack resistance. Steel wire.

The steel wire for machine structure according to the embodiment of the present invention contains C: 0.3% by mass to 0.6% by mass, Si: 0.05% by mass to 0.5% by mass, Mn: 0.2% by mass to 1.7% by mass, and P: high. 0% by mass and 0.03% by mass or less, S: 0.001% by mass to 0.05% by mass, Al: 0.005% by mass to 0.1% by mass, N: 0% by mass to 0.015% by mass, and the balance being substantially iron and inevitable The impurity, the metal structure is composed of ferrite iron and ferritic iron, and the standard deviation σ _{c of the} ferritic iron number contained in the area of 5 μm × 5 μm conforms to the following formula (1), and the average of the ferritic iron The particle diameter is 0.5 μm or more.

1.5≦σ _c ≦4.5...the formula (1)

The steel wire for a mechanical structural part according to the embodiment of the present invention may contain, if necessary, from Cr: more than 0% by mass and 0.5% by mass or less, Cu: more than 0% by mass and 0.25 mass% or less, and Ni: high. One or more selected from the group consisting of 0% by mass and 0.25% by mass or less, Mo: more than 0% by mass and 0.25 mass% or less, and B: more than 0% by mass and 0.01% by mass or less, and satisfying the following numbers The relationship of equation (2).

[Cr%]+[Cu%]+[Ni%]+[Mo%]+[B%]×50≦0.75...the formula (2)

Here, [Cr%], [Cu%], [Ni%], [Mo%], and [B%] represent the contents of Cr, Cu, Ni, Mo, and B in mass%, respectively.

The steel wire for a mechanical structural component according to the embodiment of the present invention has excellent deformation resistance during cold working and excellent fracture resistance, and therefore has excellent cold workability.

Fig. 1 is a graph showing the relationship between the C concentration and the standard deviation of the amount of ferritic iron in a sample having good crack resistance and a poor sample.

Fig. 2A is a view showing the observation of the metal structure observed by FE-SEM of Test No. 15.

Fig. 2B is a view showing the observation results of the metal structure observed by FE-SEM of Test No. 16.

The inventors of the present invention have reviewed the steel wire which can reduce the deformation resistance during cold working and the effect of improving the crack resistance, and has been reviewed from various angles.

For the tissue after cold working, FE-SEM (Field-Emission Scanning Electron Microscope) and EBSD method (Electron Back Scatter Diffraction Patterns) are used. After the analysis, we found a kind of originality, that is, found: in the mother phase, the steel wire system with the larger azimuth difference around the snow-mining iron has its cold room. The crack resistance at the time of processing tends to be deteriorated, and cracks due to the connection of the pores are likely to occur. The reason for this is considered to be that the snow alloy, which has a larger local azimuth difference, is more likely to become the starting point of the hole, and the easier it is to form a hole. Further, it has been found that the local azimuth difference around the densely-packed ferritic iron is large as compared with the periphery of the sparsely dispersed ferritic iron, and the accumulation portion of the ferritic iron will cause deterioration of crack resistance. That is, it has been found that the more the Schwann iron accumulation portion in the metal structure, the larger the local azimuth difference around the snow-melamine iron, and the crack resistance at the time of cold-working processing will be deteriorated.

Therefore, it is aimed at reducing the accumulation of the ferritic iron in the structure of the steel wire before the cold processing, and reviewing the metal structure in which the distribution of the ferritic iron is uniform. As a result, a transcendence was found, which was discovered: if the distribution of the ferritic iron is excessively uniform, the ferritic iron will be distributed comprehensively in the metal structure, and there will be fermented grains of ferritic iron in the crystal grains. Iron crystal grains will increase. Moreover, we found a kind of originality, that is, we found that the ferrite-grained iron crystal grains precipitated from the feldspar in the crystal grains are harder than the ferrite-grained iron crystal grains which are not precipitated from the ferritic iron. Therefore, the distribution of the ferritic iron Excessive state uniformity will result in increased deformation resistance during cold processing.

As an index indicating the distribution state of the ferritic iron, the standard deviation of the amount of ferritic iron contained per unit area (area of 5 μm × 5 μm) was examined. That is, the standard deviation of the amount of ferritic iron obtained by measuring the amount of ferritic iron per unit area in a plurality of unit fields is used as an index indicating the distribution state of ferritic iron. (Detailed later). As a result, a transcendence was found, which was discovered: the more the organization of many layers of ferritic iron like Borne, the more the standard deviation of the amount of ferritic iron contained in the unit area The tendency to get bigger. The Bora iron (layered ferritic iron) structure is harder than the spherical stellite structure and is a structure that increases the deformation resistance during cold processing. Therefore, as mentioned above, in order not to increase the deformation resistance, it is necessary to control the standard deviation of the amount of ferritic iron to be: not too small, and not too large.

In addition to improving the crack resistance by properly controlling the distribution of Schönmöhle, the softening was also reviewed. As a result, a transcendence was found, which was discovered: based on the viewpoint of the particle dispersion strengthening mechanism, increasing the average particle size of the ferritic iron can be used as an effective means for reducing the deformation resistance.

According to the above-mentioned novelty, a technical idea is obtained, that is, in order to achieve the effect of reducing the deformation resistance and improving the crack resistance, the distribution state of the ferritic iron in the metal structure is properly controlled, that is, per unit It is important that the standard deviation of the amount of ferritic iron contained in the area (area of 5 μm × 5 μm) and the average particle size of the ferritic iron are as large as possible.

Hereinafter, each of the requirements defined in the embodiments of the present invention will be described in detail.

In addition, the term "wire material" as used in the present specification means a linear steel material which is cooled to room temperature after hot rolling and is intended to be rolled. The term "steel wire" refers to the implementation of spheroidizing annealing for rolled wire. Linear steel after quenching and tempering treatment.

1. Metal structure and distribution of ferritic iron

The metal structure of the steel wire for a mechanical structural component (hereinafter, simply referred to as "steel wire") according to the embodiment of the present invention is a so-called spheroidized structure and is composed of ferrite iron and ferritic iron. The spheroidized structure is a metal structure which can reduce the deformation resistance of steel and contribute to the improvement of cold workability. In addition, in the present specification, the term "made of ferrite iron and ferritic iron" means that a localized boehmite structure (including pseudo-like iron) may be contained in the metal structure. As long as the adverse effect on the cold workability is small, it is also possible to contain a precipitate such as AlN having an area ratio of less than 3%.

However, if it is simply a metal structure composed of ferrite iron and ferritic iron, it is not possible to improve the cold workability. For this reason, it is necessary to properly control the standard deviation of the amount of ferritic iron per unit area (area of 5 μm × 5 μm) in this metal structure and the average of ferritic iron as described in detail below. Particle size.

When the distribution state of the stellite iron is not uniform, the accumulation portion of the stellite iron which tends to accumulate strain during cold processing increases. As a result, a large number of holes having the starting point of the ferritic iron present in the accumulating portion are generated, and cracks are easily generated, and the crack resistance is deteriorated. On the other hand, if the distribution state of the ferritic iron is too uniform, the fracture resistance can be improved, but the soft ferrite iron structure which is easily deformed is reduced, and the deformation resistance during cold working is increased.

Based on this point of view, the standard deviation σ _{c of the} amount of ferritic iron contained per unit area (area of 5 μm × 5 μm) must conform to the relationship of the following formula (1).

1.5≦σ _c ≦4.5...the formula (1)

In the cross-sectional observation, the standard deviation σ _{c of the} amount of ferritic iron contained per unit area (area of 5 μm × 5 μm) is in accordance with the relationship of the formula (1), thereby improving the crack resistance during cold working. It can suppress the increase of deformation resistance.

In the formula (1), the upper limit of the standard deviation σ _c of the amount of the ferritic iron per unit area (area of 5 μm × 5 μm) is 4.5, but the upper limit of the standard deviation σ _c is preferably 4.3 or less. Below 4.0 is better.

Further, in the formula (1), the lower limit of the standard deviation σ _c of the amount of the ferritic iron contained per unit area (the area of 5 μm × 5 μm) is 1.5, but the lower limit of the standard deviation σ _c is 1.7 or more. Ok, 1.9 or better is better.

Even if it is in accordance with the relationship of the formula (1), if the snow-capped iron particles are still very fine, the deformation resistance during cold-working processing will increase due to the particle dispersion strengthening mechanism. Therefore, in the state in which the formula (1) is satisfied, the snow-capped iron particles are controlled to be coarsened, and it can be achieved that simply controlling the dispersion state of the snow-light iron cannot be achieved (only in accordance with the relationship of the formula (1). It is impossible to achieve) to reduce the deformation resistance during cold processing.

Based on this point of view, the average particle size of the fermented iron must be controlled to be 0.5 μm or more. By controlling the average particle size of the fermented iron 0.5 μm or more can reduce the deformation resistance during cold working.

A preferred lower limit of the average particle diameter of the fermented iron is 0.6 μm, and a more preferred lower limit is 0.7 μm. The upper limit of the average particle diameter of the fermented iron is not particularly limited, and for example, it may be 2.0 μm. A better upper limit is 1.8 μm, and a more preferable upper limit is 1.6 μm.

Further, the standard deviation σ _{c of the} amount of the ferritic iron may be as described in detail in the embodiment described later, and in the cross section, the scanning electron is used at the D/4 position of the radius D of the steel wire. Microscope (SEM) photographed tissue observations of five fields of 60 μm × 45 μm (5 observation fields) at a magnification of 2000 times, and placed them in every longitudinal direction and horizontal direction at intervals of 5 μm in photographs of various fields. A mesh line is divided into 108 unit areas of 5 μm × 5 μm, and then the number of ferritic irons contained in each unit area is measured, and the measured values of all fields of 5 fields × 108 unit fields are used to calculate the standard deviation.

The average particle diameter of the stellite iron is an electron microscope (SEM) photograph of five observation fields taken to obtain the standard deviation σ _{c of the} amount of ferritic iron, as described in detail in the examples described later. For example, an image analysis software such as Image-Pro Plus manufactured by Media Cybernetics, Inc. can be obtained. The total area of the snow-light iron in the photograph can be measured first, and then the average value of the area of the five observation fields relative to the total number of the ferritic iron is calculated, and the area is used to calculate the equivalent circle diameter of the ferritic iron. Think of it as the average particle size of ferritic iron.

The two aspects of the standard deviation of the amount of the ferritic iron contained in the unit area (the area of 5 μm × 5 μm) and the average particle diameter of the ferritic iron are not particularly the form of the total ferritic iron to be measured. In addition to the spherical stellite, it also includes a rod-shaped ferritic iron having a relatively large length and a wide width, a layered ferritic iron forming a ferritic structure, and the like, and the shape of the stellite iron is not limited. In addition, the standard of the size of the ferritic iron to be measured is not limited, and the standard deviation σ _{c of the} number of ferritic irons per unit area (area of 5 μm × 5 μm) to be described later and the average of the ferritic iron can be used. The method for measuring the particle size is regarded as the minimum size of the size of the ferritic iron. Specifically, it is a snow-melting iron having a size of 0.1 μm or more.

2. Chemical composition

The embodiment of the present invention is a steel wire used as a material for a mechanical structural component, and may be an ordinary chemical component of a steel wire for a mechanical structural component, but for C, Si, Mn, P, S, Al, and N should be adjusted to a proper range. Based on this point of view, the reason for the proper range of these chemical components and the reason for limiting the content thereof will be described below. In addition, in this specification, "%" which shows the chemical composition point means mass %.

An embodiment which can satisfy such a suitable chemical composition includes C: 0.3% by mass to 0.6% by mass, Si: 0.05% by mass to 0.5% by mass, and Mn: 0.2% by mass to 1.7% by mass. P: more than 0% by mass and 0.03% by mass or less, S: 0.001% by mass to 0.05% by mass, Al: 0.005% by mass to 0.1% by mass, N: 0% by mass to 0.015% by mass, and the rest is substantially Chemical composition of iron and unavoidable impurities (or C: 0.3% by mass to 0.6% by mass, Si: 0.05% by mass to 0.5% by mass, Mn: 0.2% by mass to 1.7% by mass, P: more than 0% by mass and 0.03% by mass or less, S: 0.001% by mass to 0.05% by mass, and Al: 0.005% by mass to 0.1% by mass % by mass, and N: 0% by mass to 0.015% by mass, and the rest is composed of iron and inevitable impurities.

Further, another embodiment which can conform to such a suitable chemical composition is exemplified by the fact that the above chemical component further contains from Cr: more than 0% by mass and less than 0.5% by mass, and Cu: more than 0%. % and 0.25 mass% or less, Ni: more than 0% by mass and 0.25 mass% or less, Mo: more than 0% by mass and 0.25 mass% or less, and B: more than 0% by mass and 0.01% by mass or less, the group One or more selected chemical constituents in accordance with the relationship of the following formula (2).

[Cr%]+[Cu%]+[Ni%]+[Mo%]+[B%]×50≦0.75...the formula (2)

Here, [Cr%], [Cu%], [Ni%], [Mo%], and [B%] represent the contents of Cr, Cu, Ni, Mo, and B in mass%, respectively.

C: 0.3~0.6%

C is a useful element to ensure the strength of the steel, that is, the strength of the final product. In order to effectively exert this effect, the C content must be set to 0.3% or more. The C content is preferably 0.32% or more, more preferably 0.34% or more. However, if the C content is excessive, the strength becomes too high, and the cold workability deteriorates. Therefore, it must be set to 0.6% or less. The C content is preferably 0.55% or less, more preferably 0.50% or less.

Si: 0.05~0.5%

Si is useful as a deoxidizing element and is based on its use of solid solution strengthening to increase the strength of the final product. In order to effectively exert this effect, the Si content is set to 0.05% or more. The Si content is preferably 0.07% or more, more preferably 0.10% or more. On the other hand, when the Si content is excessive, the hardness is excessively increased to deteriorate the cold workability. 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: 0.2~1.7%

Mn is an effective element which can increase the strength of the final product by improving the quench hardenability. In order to effectively exert this effect, the Mn content is set to 0.2% or more. The Mn content is preferably 0.3% or more, more preferably 0.4% or more. On the other hand, when the Mn content is excessive, the hardness increases and the cold workability deteriorates. Therefore, the Mn content is set to 1.7% or less. The Mn content is preferably 1.5% or less, more preferably 1.3% or less.

P: higher than 0% and less than 0.03%

P is an inevitable element contained in steel, which causes grain boundary segregation in steel, causing deterioration of ductility. Therefore, the P content is set to be 0.03% or less. The P content is preferably 0.02% or less, more preferably 0.017% or less, and particularly preferably 0.01% or less. Although the P content is as small as possible, it is sometimes 0.001% due to factors such as constraints in the manufacturing process.

S: 0.001~0.05%

S is an unavoidable element contained in steel, and is present in steel in the form of MnS, which deteriorates ductility and 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, more preferably 0.03% or less. However, S has an effect of improving machinability, and therefore it is preferable to contain 0.001% or more. The S content is preferably 0.002% or more, more preferably 0.003% or more.

Al: 0.005~0.1%

Al can be used as a deoxidizing element, and the solid solution N present in the steel can be fixed into an AlN form. In order to effectively exert this effect, the Al content is set to 0.005% or more. The Al content is preferably 0.008% or more, more preferably 0.010% or more. However, if the Al content is excessive, Al ₂ O _{3 is} excessively formed to deteriorate the cold workability. Therefore, the Al content is set to 0.1% or less. The Al content is preferably 0.090% or less, more preferably 0.080% or less.

N: 0~0.015%

N is an element which is unavoidably contained in the steel. When the solid solution N is excessively contained in the steel, the hardness is increased due to the strain aging effect, the ductility is deteriorated, and the cold workability is deteriorated. Therefore, the N content is set to 0.015% or less. The N content is preferably 0.013% or less, more preferably 0.010% or less. Although the N content is as small as possible, that is to say 0% is the best, but Due to factors such as restrictions in the manufacturing process, it is sometimes 0.001%.

The basic components of the steel wire of the embodiment of the present invention are as described above, and the remainder is substantially iron. In addition, the term "substantially iron" means that a trace component (for example, Sb, Zn, or the like) which does not impair the characteristics of the present invention may be contained in addition to iron, and may contain P, The inevitable impurities (such as O, H, etc.) other than S and N. Further, in the embodiment of the present invention, any of the following elements may be contained as necessary, and the characteristics of the steel wire may be further improved depending on the components contained.

Further, as described above, although P, S, and N are elements (unavoidable impurities) which are unavoidably contained, the content range thereof is defined as described above. Therefore, in the present specification, the "inevitable impurities" contained in the rest of the specification mean the elements which are inevitably contained in addition to the elements whose content ranges are specified.

From Cr: above 0% and below 0.5%, Cu: above 0% and below 0.25%, Ni: above 0% and below 0.25%, Mo: above 0% and below 0.25% and B: above 0 One or more selected from the group of % and less than 0.01%.

Cr, Cu, Ni, Mo, and B. These elements are effective elements for increasing the strength of the final product by improving the quench hardenability of the steel, and may also contain Cr, Cu, Ni, Mo as necessary. And one or more selected from B. Improve the effect of quench hardenability The content of these elements increases and becomes larger. The content of Cr is preferably 0.015% or more, and more preferably 0.020% or more, in order to provide a suitable content for effective effect. The suitable content of the Cu content, the Ni content, and the Mo content is 0.02% or more, more preferably 0.05% or more. A suitable content of the B content is 0.0003% or more, more preferably 0.0005% or more.

However, when the content of Cr, Cu, Ni, Mo, and B is excessive, the strength becomes too high, and the cold workability is deteriorated. Therefore, the Cr content is preferably 0.5% or less, and the Cu, Ni, and Mo contents are preferably 0.25% or less, and the B content is preferably 0.01% or less. A more preferable content of Cr is 0.45% or less, more preferably 0.40% or less. The content of Cu, Ni and Mo is preferably 0.22% or less, more preferably 0.20% or less. A more desirable content of B is 0.007% or less, more preferably 0.005% or less.

In addition, it is preferable to satisfy the relationship of the following formula (2). Because a more positive strength can be obtained.

[Cr%]+[Cu%]+[Ni%]+[Mo%]+[B%]×50≦0.75...the formula (2)

Here, [Cr%], [Cu%], [Ni%], [Mo%], and [B%] represent the contents of Cr, Cu, Ni, Mo, and B in mass%, respectively.

Further, as described above, Cr, Cu, Ni, Mo, and B may be selectively added elements, and among these elements, the unadded element has a content of zero in the formula (2).

The upper limit value (the value on the right side of the formula (2)) defined by the formula (2) is more preferably 0.65 mass% or less, and most preferably 0.50 mass% or less.

3. Manufacturing method

In the steel wire according to the embodiment of the present invention, it is required to have a structure after spheroidizing annealing. Therefore, in order to change the structure, it is necessary to appropriately control the conditions of the spheroidizing annealing described later.

Further, in order to secure the above-described structure, it is necessary to appropriately control the conditions at the stage of manufacturing the rolled wire, so that the microstructure of the rolled wire is changed to a spheroidal iron during spheroidizing annealing. It can be uniformly distributed, and a variable coarsened tissue is suitable.

3-1. Rolling

In the manufacturing stage of the rolled wire, the steel which meets the above composition is controlled at a proper temperature during the hot rolling, and the subsequent cooling rate is changed into three stages for cooling. It is appropriate. By manufacturing the rolled wire material under such conditions, the structure before the spheroidizing annealing (or the structure after rolling) can be made into a main phase with the ferrite and the ferrite iron (from the ferrite iron and the snow The iron composition can be controlled, and the particle size of the bcc-Fe crystal can be controlled within a proper range, and the percentage of the precipitated ferrite iron can be controlled within a proper range, and the interval of the layering of the Borne iron can be expanded. With respect to such a structure, spheroidizing annealing is performed under the conditions described later, and a steel wire in which the ferritic iron is uniformly distributed and coarsened can be obtained more surely. The manufacturing conditions of the specific rolled wire are as follows.

(1) The refining rolling is performed at a refining rolling temperature _Tf which can satisfy the following formula (2A), 800 ° C ≦ T _f ≦ 1200-500 × [C%] ... (2A)

(The [C%] here is the C content in mass%.)

(2) The first cooling with an average cooling rate of 11 ° C /sec or more and the second cooling with an average cooling rate of 4 ° C / sec or more and 10 ° C / sec or less and an average cooling rate of 3 ° C are sequentially performed. The third cooling is equal to or less than sec, and the end of the first cooling and the start of the second cooling are performed in the range of 700 to 750 ° C. The end of the second cooling and the start of the third cooling are 600. It is preferable to carry out the above-mentioned third cooling to 500 ° C in the range of ~650 °C. The details of the refining rolling temperature and the first to third cooling will be described below.

(a) Refined rolling temperature:

The refining rolling temperature _Tf is in accordance with the following formula (2A).

800°C≦T _f ≦1200-500×[C%]......The formula (2A)

(The [C%] here is the C content in mass%.)

In order to reduce the average equivalent circle diameter of the body-centered cubic (bcc)-Fe crystal grain of the metal structure of the rolled wire (hereinafter, simply referred to as "bcc-Fe average particle diameter"), That is, in order to prevent the regenerated wave from being easily precipitated in the spheroidizing annealing, it is necessary to appropriately control the refining rolling temperature. The regenerative wave iron is the cause of the uneven distribution of the ferritic iron, which causes the deterioration of the crack resistance. Therefore, it is important to prevent it from being precipitated. If the refining rolling temperature exceeds (1200-500 × [C%]) ° C, it is difficult to make the bcc-Fe crystal grain size small. In other words, the upper limit of the refining rolling temperature is increased with the carbon content. Add and get lower.

On the other hand, when the refining rolling temperature is lower than 800 ° C, the bcc-Fe crystal grain size is too small and it is difficult to soften. Therefore, it is preferable to set it at 800 ° C or higher. A better lower limit of the refining rolling temperature is 820 ° C, more preferably 840 ° C. A better upper limit of the refining rolling temperature is (1180-500 × [C%]) ° C, and a more preferable upper limit is (1160 - 500 × [C%]) ° C or less.

(b) first cooling

The first cooling is performed from a refining rolling temperature, that is, a temperature of 800 ° C or more and (1200 - 500 × [C%]) ° C or less, and is carried out to an end temperature falling within a temperature range of 700 to 750 ° C. until. In the first cooling, if the cooling rate is too slow, the bcc-Fe crystal grains will be coarsened, and the bcc-Fe crystal grain size will become large. In the spheroidizing annealing, the regenerated wave iron will be precipitated per unit area. (The area of 5 μm × 5 μm) The standard deviation of the amount of ferritic iron contained will have a concern beyond the proper range. Therefore, it is preferable to set the average cooling rate at the time of the first cooling to 11 ° C /sec or more. The average cooling rate of the first cooling is more preferably 15 ° C / sec or more, and more preferably 20 ° C / sec or more. The upper limit of the average cooling rate of the first cooling is not particularly limited, but it is preferably 200 ° C / sec or less in terms of the actual range. Further, the cooling method performed at the time of the first cooling may be performed so that the cooling rate can be changed as long as the average cooling rate can be 11 ° C /sec or more. This cooling rate at the time of the first cooling can be achieved by applying appropriate blown cooling to the rolled wire on the conveyor belt.

(c) second cooling

The second cooling is performed from the end temperature of the first cooling in the temperature range of 700 to 750 ° C until the end temperature in the temperature range of 600 to 650 ° C. Further, the area ratio of the initial precipitated iron in the metal structure of the rolled wire is lowered, that is, in order to increase the percentage of the ferrite, the second cooling is preferably performed at an average cooling rate of 4 ° C /sec or more. . The better average cooling rate for the second cooling is 5 ° C / sec or more, and the more preferable cooling rate is 6 ° C / sec or more. On the other hand, if the average cooling rate at the time of the second cooling is too fast, the percentage of the ferrite is too high, and in the spheroidizing annealing, the regenerated wave will be precipitated per unit area (5 μm × 5 μm). The standard deviation of the amount of ferritic iron contained in the area) may be more than a reasonable range. Therefore, the average cooling rate at the time of the second cooling is preferably 10 ° C / sec or less. The average cooling rate of the second cooling is more preferably 9 ° C / sec or less, more preferably 8 ° C / sec or less. In addition, the cooling method at the time of the second cooling may be performed so that the cooling rate can be changed as long as the average cooling rate is 4° C./sec or more and 10° C./sec or less. This cooling rate of the second cooling can be achieved by applying appropriate blown cooling to the rolled wire on the conveyor belt.

(d) third cooling

The third cooling is performed from the end temperature of the second cooling in the temperature range of 600 to 650 ° C to 500 ° C.

By performing this third cooling, the average stratification of the Borne iron can be made. When the gap is increased, more stellite is left, and many nucleus of spheroidal ferritic iron remains in the crystal grains. Therefore, by performing a proper spheroidizing annealing treatment, the presence of ferritic iron can also be present in the ferrite grains, and the distribution state of the ferritic iron can be appropriately controlled. In order to increase the interval between the average stratification of the ferritic iron, the temperature is in the range of 600 to 650 ° C, and the third cooling up to 500 ° C is performed at an average cooling rate of 3 ° C / sec or less. Cooling is appropriate. If the cooling rate is faster than 3 ° C / sec, it is difficult to increase the interval of the average delamination of the ferrite. The average cooling rate of the third cooling is more preferably 2.5 ° C / sec or less, more preferably 2 ° C / sec or less. This cooling rate of the third cooling can be achieved by providing a cover on the conveyor belt for controlling the heat dissipation of the rolled wire.

After the third cooling is performed, it is cooled to room temperature by performing general cooling such as cooling. Further, it is also possible to continue cooling by a cooling rate equivalent to that of the third cooling until a temperature lower than 500 ° C (for example, 400 ° C).

After the cooling reaches the room temperature, the wire drawing process may be performed at room temperature as necessary, and the cross-sectional reduction rate at this time may be, for example, 30% or less. When the wire drawing process is performed, the carbide in the steel is broken (crushed), and the subsequent spheroidizing annealing promotes the aggregation of the carbide, so that the soaking time of the spheroidizing annealing can be effectively shortened. However, if the section reduction ratio of the wire drawing process exceeds 30%, the strength after annealing becomes high, and the cold workability may be deteriorated. Therefore, the profile reduction rate of the wire drawing process is set to 30% or less. should. Further, the lower limit of the profile reduction ratio is not particularly limited, and is preferably set at 2%. As a result, the effect of the wire drawing process can be obtained more reliably.

3-2. Spheroidizing annealing

The rolled wire material produced by the above-mentioned preferable conditions is subjected to spheroidizing annealing treatment, and a part of the wave iron remains in the metal structure, and is transformed into a Worthite iron. In the process of metamorphosis into ferrite iron + ferritic iron, the ferritic iron will be uniformly deposited in the remaining core and crystal grain boundaries, and the formation of the ferritic iron can be easily controlled. Uniform state.

However, even if the rolled wire rod obtained under the conditions other than the above-described preferable conditions is subjected to spheroidizing annealing under appropriate conditions, the steel wire according to the embodiment of the present invention can be obtained.

This spheroidizing annealing condition is performed by using an atmospheric furnace for the rolled wire material, for example, SA1 described later, and is, for example, 740 ° C, which is higher than the upper temperature 730 of the A1 point. When the temperature is maintained at a higher holding temperature of °C, at least 750 ° C from 500 ° C, the average heating rate is 50 ° C / hour or more, and then the average heating rate is 6 ~ 10 ° C / In the hour, heat is applied until the temperature is maintained (for example, 740 ° C), and after maintaining for 1 to 2 hours at this holding temperature, the average cooling rate is 20 ° C / hour or more, and cooling is continued until 720 ° C, and then the average cooling is performed. The speed is 8~12 ° C / hour, and it is cooled until it reaches 640 ° C. Then, it is preferable to carry out cooling.

In the above spheroidizing annealing conditions, heating from room temperature to In the process of 730 ° C, the average heating rate from at least 500 ° C to 730 ° C is set to 50 ° C / hour or more, thereby suppressing 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 fast, the temperature followability of the rolled wire tends to be difficult, so it is preferably 200 ° C / hr or less, more preferably 150 ° C / hr or less.

Further, the average heating rate when heated from room temperature to 500 ° C is usually 100 ° C / hour or more, and the average heating rate in this temperature range has little effect on the grain growth of the metal structure. However, considering the productivity, the heating rate at this time is faster, for example, 120 ° C / hour or more, more preferably 140 ° C / hour or more. The average heating rate at this time may be set to 200 ° C /hr or less, more preferably 150 ° C / hr or less, similarly to the average heating rate from 500 ° C to 730 ° C. The average heating rate when heating from room temperature to 500 ° C may be the same as or different from the average heating rate from at least 500 ° C to 730 ° C.

In addition, by controlling the average heating rate from 730 ° C above the A1 point to the temperature of 6 to 10 ° C / hr, it is possible to suppress the grain growth of the metal structure as much as possible, and to control the wave appropriately. Decomposition and solid solution of ferritic iron in the iron structure. When the average heating rate is faster than 10 ° C / hour, it is difficult to ensure the decomposition and solid solution time of the ferritic iron in the Borne iron structure. If the average heating rate is slower than 6 ° C / hour, the temperature is from 730 ° C. The heating time until the temperature is maintained becomes long, and the decomposition and solid solution of the ferritic iron will be excessive. The average heating rate at this time, more Preferably, it is 7 ° C / hour or more and 9 ° C / hour or less.

It is advisable to keep it for 1~2 hours while maintaining the temperature. When the holding time at this holding temperature is less than 1, the decomposition and solid solution of the ferritic iron in the Borne iron structure are insufficient, and if it is higher than 2 hours, the decomposition and solid solution of the ferritic iron are too excessive. Therefore, the holding time at this time is preferably 1.2 times or more and 1.8 times or less.

After the above-described holding method is carried out, by setting the preferred average cooling rate up to 720 ° C to 20 ° C /hr or more, the grain growth of the metal structure can be suppressed, and the cooling process can be suppressed. Regeneration of the wave of iron. The average cooling rate at this time is more preferably 30 ° C /hr or more. However, if the average cooling rate is too fast, the temperature followability of the rolled wire tends to be difficult, so it is preferably set at 100 ° C / hour or less.

Then, the average cooling rate from 720 ° C to 640 ° C is controlled to 8 to 12 ° C / hour, thereby allowing the ferrihydrite to preferentially precipitate the core and grain boundaries remaining in the heating process, but The precipitation of iron from the regenerative wave is suppressed. When the average cooling rate is less than 8 ° C / hour, the metal structure may have unnecessary grain growth, and when the reverse spheroidizing annealing described later is performed, there is a concern that the regenerated wave is precipitated. If the average cooling rate is higher than 12 ° C / hour, the long-width and long-term snow-like iron similar to the Borne iron structure will precipitate a lot. The average cooling rate at this time is more preferably 9 ° C / hour or more and 11 ° C / hour or less.

The spheroidizing annealing described above can also be repeated a plurality of times. By repeating the plurality of annealings in this way, the respective particle diameters of the fermented iron When it is enlarged, the distribution state can be made to a certain degree of uniformity.

In the case of Test Nos. 36 to 38 (steel type Q, R, S) in the examples described later, even if the rolling conditions fall outside the range of the above-mentioned preferable conditions, it is possible to repeat the plural times. The spheroidizing annealing is performed under the above conditions, so that the metal structure is composed of ferrite iron and ferritic iron, and the standard deviation of the amount of ferritic iron per unit area (area of 5 μm × 5 μm) and the average of ferritic iron When the particle diameter falls within a proper range, as a result, a steel wire for a mechanical structural part capable of reducing both the deformation resistance and the fracture occurrence rate can be obtained.

The number of times of the spheroidizing annealing is preferably at least three times, but the standard deviation of the amount of ferritic iron per unit area (area of 5 μm × 5 μm) and the average of ferritic iron are preferably performed at least three times. The particle size does not change much, so it is preferably 10 or less. Further, when the spheroidizing annealing is performed in plural times, it may be repeated under the same conditions within the above-described preferable conditions, or may be repeated under different conditions.

As long as it is in contact with the above-described steel wire for a mechanical structural component according to the embodiment of the present invention and a method for manufacturing the same, it is also possible to adopt a manufacturing method different from the above-described manufacturing method by trial and error. The steel wire for the mechanical structural part of the present invention is obtained.

[Examples]

Hereinafter, embodiments of the present invention will be described more specifically by way of examples. The present invention is not limited by the following embodiments, before the It is needless to say that the scope of the invention described below can be appropriately changed and implemented, and these are also included in the technical scope of the present invention.

The steel having the chemical composition shown in the following Table 1 was rolled according to the conditions described in Table 2 to prepare a diameter. 17.0mm wire. The steel grades N and O are comparative examples in which the chemical composition falls outside the range of the embodiment of the present invention.

The steel grades P, Q, R, S, T, U, V, and W are rolled wire rods produced under conditions other than the above-described preferable rolling conditions. The steel grade P is a condition in which the average cooling rate at the time of the second cooling is slower than the preferred range. The steel type Q is a refined rolling temperature higher than the preferred range. The steel type R is a condition in which the average cooling rate at the time of the first cooling is slower than the preferred range. The steel grade S is a condition that the average cooling rate at the third cooling is faster than the preferred range. Further, the steel type T is a condition that the average cooling rate at the time of the second cooling is faster than the preferred range.

The steel type U is a holding process in which the first cooling reaches 435 ° C, that is, a temperature lower than the preferred range of the end temperature, and then held at 435 ° C for the same temperature for 120 seconds, and the cooling reaches the chamber. At the same temperature, a thickening line treatment with a section reduction rate of 20% was performed. The steel type V is maintained at 500 ° C after the first cooling, that is, after reaching a temperature lower than the preferred range of the end temperature, and then maintained at 500 ° C for the same temperature for 120 seconds, and allowed to cool to room temperature. Then, the thickening line treatment with a section reduction rate of 20% is performed. Further, the steel type W is maintained at 480 ° C for the first cooling, that is, after reaching a temperature lower than the preferred range of the end temperature, and then held at 480 ° C for the same temperature for 120 seconds. The process was maintained, and it was allowed to cool to room temperature, and then subjected to a thick drawing process in which the section reduction rate was 20%.

Next, each of the rolled wires other than the steel types U, V, and W is subjected to spheroidizing annealing in one of the annealing conditions SA1 to SA3 shown below in an atmospheric furnace.

(a) Condition SA1

In the process of heating from room temperature to 730 ° C, the range of 500 ° C from room temperature is heated at an average heating rate of 110 ° C / hour, and the range from 500 ° C to 730 ° C is 80 ° C / The average heating rate of the hour is heated. Then, the mixture was heated to 740 ° C at an average heating temperature of 8 ° C / hour, and maintained at 740 ° C for 2 hours, and then cooled to 720 ° C at an average cooling rate of 30 ° C / hour, and then cooled at an average temperature of 10 ° C / hour. The temperature was cooled to 640 ° C and then allowed to cool.

(b) Condition SA2

Condition SA1 was repeated three times.

(c) Condition SA3

In the process of heating from room temperature to 730 ° C, the range of 500 ° C from room temperature is heated at an average heating rate of 110 ° C / hour, and the range from 500 ° C to 730 ° C is 80 ° C / hour. The average heating rate is heated. Then, the mixture was heated to 740 ° C at an average heating temperature of 8 ° C / hour, held at 740 ° C for 2 hours, and then cooled to 640 ° C at an average cooling rate of 30 ° C / hour, and then allowed to cool.

The annealing conditions SA1 and SA2 are annealing conditions used for the spheroidizing annealing according to the embodiment of the present invention, and the annealing condition SA3, which is an average cooling rate of 640 ° C from 720 ° C, is annealed compared to the embodiment of the present invention. The range of conditions is faster.

Further, for the steel grades U, V, and W, spheroidizing annealing is performed in an atmospheric furnace by the annealing condition SA4 shown below.

(d) Condition SA4

The mixture was heated from room temperature to 720 ° C at an average heating rate of 150 ° C / hour, held at 720 ° C for 1 hour, and then allowed to cool. Then, a final purification line treatment with a section reduction rate of 10% was performed.

The annealing condition SA4 falls outside the range of the annealing conditions of the embodiment of the present invention.

The steel wire after the spheroidizing annealing described above was measured according to the following method: (1) the standard deviation of the amount of ferritic iron contained in an area of 5 μm × 5 μm, and (2) the average of ferritic iron. grain Diameter, (3) deformation resistance during cold working, and (4) incidence of cracking during cold working.

In addition, when the standard deviation of the amount of ferritic iron contained in the area of 5 μm × 5 μm of the steel wire after the spheroidizing annealing is measured, and the average particle diameter of the ferritic iron, the steel is observed in order to observe the cross section. The wire is buried in the resin, and the cut surface is mirror-polished by using a sandpaper or a diamond polishing pad. Further, the position of the D/4 of the radius D of the steel wire was measured.

(1) Determination of the standard deviation of the amount of ferritic iron contained in an area of 5 μm × 5 μm

The cross section of the ferritic iron was etched using bitter acid, and the structure was observed by FE-SEM, and five fields of 60 μm × 45 μm (five observation fields) were taken at a magnification of 2000 times. In the photograph, a mesh line is drawn every 5 μm along the longitudinal direction and the lateral direction, and each observation field is divided into 108 unit fields of 5 μm × 5 μm. The amount of ferritic iron contained in each unit area was measured, and the standard deviation was calculated using all the measured values of 5 fields of view × 108 unit areas. Will exist in the realm of the unit area, that is, only a part of the snow-light iron in a unit area, the snow-light iron on the upper and left realm, is considered to exist in the snow in the unit area Ming Tie came to make the measurement, and the snow-light iron on the lower and right sides was regarded as the snow-mining iron that did not exist in the unit area and was not measured. That is, the undetermined ferritic iron is considered to exist in another unit area. The minimum equivalent circle diameter of the fermented iron was measured to be 0.1 μm.

(2) Determination of the average particle size of ferritic iron

In the measurement of the average particle size of the ferritic iron, all the semolina in the photograph were measured based on the photograph taken in the above (1), using Image-Pro Plus, an image analysis software manufactured by Media Cybernetics, Inc. The area of the iron was used to find the average of the area of the five observation fields relative to the total amount of the ferritic iron. Using this area, the equivalent circle diameter of the ferritic iron was calculated as the average particle diameter of the ferritic iron. The ferritic iron that is measured is the snow feldsock that appears in the photo as a whole, and is located at the edge of the photograph. If only a part of ferritic iron appears in the photograph, it is not taken as a measurement. Object. The minimum equivalent circle diameter of the fermented iron to be measured was set at 0.1 μm.

(3) Determination of deformation resistance

Making diameter from steel wire A cold forging test sample of 10.0 mm × 15.0 mm in length was subjected to a cold forging test at a processing rate of 60% at a room temperature of 5/sec to 10/sec using a forging press. The deformation resistance was measured based on the processing rate-deformation resistance data obtained from the cold forging test of a processing ratio of 60%, and the deformation resistance at the time of 40% processing was measured five times, and the average value of five times was obtained. In addition, depending on the content of C, Si, and Mn, the obtained deformation resistance is also different. Therefore, the upper limit of the target deformation resistance (described as "the upper limit value of the deformation resistance" in Table 3) is based on the following. The equation (3) is obtained.

Upper limit of deformation resistance target value (MPa) = 400 × Ceq + 420... Equation (3)

Ceq=[C%]+0.2×[Si%]+0.2×[Mn%], where [C%], [Si%], and [Mn%] represent the contents of C, Si, and Mn, respectively. %).

(4) Determination of the incidence of rupture

The rupture rate was measured by performing a cold forging test at a processing ratio of 60% under the same conditions as in the above (3), and then performing surface observation five times using a stereoscopic microscope to determine whether or not there was a surface at a magnification of 20 times. For the rupture, divide the "number of samples with surface rupture" by 5 and find the average value. The target rupture rate in all steel grades is set below 20%.

These results are shown together with the spheroidizing annealing conditions in Table 3 below. In Table 3, the upper limit value and the lower limit value of the standard deviation σ _c of the amount of the ferritic iron obtained by the formula (1) are also described. In addition, in the comprehensive evaluation column of Table 3, both the deformation resistance and the cracking resistance rate reach the target value, and the good samples are marked as "OK", and at least one of the deformation resistance and the cracking resistance rate are at least one of them. Samples that did not reach the target value are marked as "NG (poor)".

Fig. 1 is a statistical graph showing the relationship between the C concentration and the standard deviation of the amount of ferritic iron in the sample having good crack resistance obtained from the results shown in Table 3. Among the two broken lines in the graph, the broken line on the lower side corresponds to the lower limit value 1.5 shown on the left side of the equation (1), and the dotted line on the upper side corresponds to the upper limit shown on the right side of the equation (1). The value is 4.5.

It can be seen from Fig. 1 that the samples satisfying the formula (1) and having the specified chemical composition, and the average particle size of the ferritic iron being 0.5 μm or more are all evaluated as OK, but not in the numerical formula. For the sample of (1), the comprehensive evaluation is NG (poor). Even if it meets the formula (1), the comprehensive evaluation is NG (poor) in one of the chemical composition and the average particle size of the ferritic iron.

Further, when the structure observation was carried out by the FE-SEM described above, it was confirmed that each sample was composed of ferrite iron and ferritic iron.

Fig. 2A is an observation result of observation of metal structure by FE-SEM of Test No. 15, and Fig. 2B, observation result of observation of metal structure by FE-SEM of Test No. 16. In Test No. 15, too much layered ferritic iron was not observed, but in Test No. 16, more stratified ferritic iron was observed.

From the results of Table 3, the following investigations can be made. Test Nos. 1 to 3, 5 to 7, 9, 10, 12 to 15, 17 to 20, 22 to 24, and 36 to 38 are examples of all the requirements specified in the embodiments of the present invention, and it can be seen that It can simultaneously achieve the effect of reducing the deformation resistance and improving the crack resistance.

Among these, No. 36 to 38 are examples in which steel types Q, R, and S are used, and steel types Q, R, and S are not rolled under the optimum conditions, but the balls are repeatedly subjected to annealing conditions of SA2. Annealing, therefore, the regenerative wave of hard tissue to decompose and reduce the result of iron, The distribution of Xueming Iron is uniform, and the deformation resistance and the incidence of rupture all reach the target value.

Here, for the spheroidizing annealing, the conditions of SA1 or SA2 are used, and in other respects, the same conditions (that is, the same steel type) are used in Test No. 2, 3 (steel type B), and test No. 6 , 7 (steel type C), test No. 9, 10 (steel type E), test No. 14, 15 (steel type H), test No. 19, 20 (steel type K), and test No. 23, 24 (steel type M) To observe, it can be seen that each example is: steel that is repeatedly subjected to SA1 three times, that is, after annealing of SA2, which has lower deformation resistance and cracks than steel which is annealed only once. The incidence is also low.

Test Nos. 4, 8, 11, 16, 21, and 25 to 35 are comparative examples in which one of the requirements specified in the embodiment of the present invention is lacking, and it can be seen that the deformation resistance and the incidence of cracking are One or both of them did not reach the target value.

That is, Test Nos. 4, 8, 11, 16, 21, and 25 were subjected to spheroidizing annealing under the condition of unsatisfactory annealing condition SA3, and the standard deviation of the amount of ferritic iron contained in an area of 5 μm × 5 μm. , which is greater than the upper limit value specified by the formula (1), neither the occurrence rate of cracking, or the deformation resistance and the incidence of cracking have reached the target value. In the metal structure of the test in which neither of them reached the target value, it was observed that there were many regenerated waves of iron, and thus it was not possible to satisfy the formula (1), and it was considered that the deformation resistance was increased.

Test Nos. 26 and 27 are steel types N with excessive Mn content and excessive steel content of Cr, and are still maintained at high temperatures during cold working. Deformation resistance.

Test Nos. 28 to 32 are examples in which steel types P, Q, R, S, and T which were rolled under unfavorable conditions were used. Later, although spheroidizing annealing of SA1 was performed, every 5 μm × 5 μm was used. The standard deviation of the amount of ferritic iron contained in the area is still greater than the upper limit value specified by the formula (1), and neither the rupture rate nor the deformation resistance and the rupture rate reach the target value.

Test No. 33-35 is an example in which spheroidal annealing is performed using an unsuitable annealing condition SA4 using steel grades U, V, and W which are not suitable for rolling conditions, and fine ferritic iron is uniformly dispersed. The standard deviation of the amount of ferritic iron contained in an area of 5 μm × 5 μm is less than the lower limit value defined by the formula (1), and the average particle diameter of the ferritic iron is also smaller than a predetermined value. No. 34 and 35 are standard deviations of the amount of ferritic iron contained in an area of 5 μm × 5 μm, which is less than the lower limit value, and the deformation resistance is high. No. 33 and 34 are the average particle diameters of the ferritic iron, which are smaller than the lower limit value, and the deformation resistance is high.

Priority is claimed on the basis of Japanese Patent Application No. 2015-238445, which is filed on Dec. 7, 2015. Therefore, the content of Japanese Patent Application No. 2015-238445 is incorporated herein by reference.

[Industrial availability]

The steel wire for mechanical structural parts according to the embodiment of the present invention is suitable for use as cold forging, cold press, cold rolling, and the like. Materials for various mechanical structural parts such as automotive parts and construction machinery parts manufactured by cold processing. Such mechanical structural parts, specifically, bolts, screws, nuts, sleeves, ball joints, inner tubes, torsion bars, clutch boxes, cages, housings, hubs, housings, outer boxes, metal Seat, tappet, saddle, valve, inner box, clutch, bushing, outer ring, sprocket, core, stator, anvil, star wheel, reciprocating rod, main body, flange, drum, Joints, connectors, pulleys, hardware items, yokes, metal covers, valve ejector pins, spark plugs, pinion gears for rails, steering shafts, mechanical parts for common rails, and electrical components. The steel wire according to the embodiment of the present invention is suitably used as a material for the mechanical structural component, and is industrially usable as a steel wire for a high-strength mechanical structural component, and is manufactured when the above-described various mechanical structural components are manufactured. The deformation resistance at room temperature is low, and cracking of the material can be suppressed, and excellent cold workability can be exhibited.

Claims (2)

A steel wire for mechanical structural parts, the composition thereof comprising: C: 0.3% by mass to 0.6% by mass, Si: 0.05% by mass to 0.5% by mass, Mn: 0.2% by mass to 1.7% by mass, P: higher than 0 mass % and 0.03 mass% or less, S: 0.001 mass% to 0.05 mass%, Al: 0.005 mass% to 0.1 mass%, N: 0 mass% to 0.015 mass%, and the rest: substantially consisting of iron and inevitable impurities The metal structure is composed of ferrite iron and ferritic iron, and the standard deviation σ _c of the amount of ferritic iron contained in an area of 5 μm × 5 μm is in accordance with the following formula (1), Xue Mingtie The average particle diameter is 0.5 μm or more, 1.5 ≦ σ _c ≦ 4.5... Formula (1). The steel wire for mechanical structural parts according to claim 1 further contains from Cr: more than 0% by mass and less than 0.5% by mass, Cu: more than 0% by mass and less than 0.25% by mass, and Ni: more than 0% by mass. And 0.25 mass% or less, Mo: more than 0 mass% and 0.25 mass% or less, and B: more than one selected from the group of more than 0 mass% and 0.01 mass% or less, and conforms to the following formula (2) Relationship: [Cr%]+[Cu%]+[Ni%]+[Mo%]+[B%]×50≦0.75... Equation (2) Here, [Cr%], [Cu%], [Ni%], [Mo%], and [B%] represent the contents of Cr, Cu, Ni, Mo, and B in mass%, respectively.