JP2002069570A - Free cutting steel for machine structure having excellent mechanical property and partibility of chip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide Pb free cutting steel for machine structure capable of stably and securely exhibiting mechanical properties and partibility of chips equal to those of the conventional Pb-added steel. SOLUTION: In this free cutting steel for machine structure in which sulfide inclusions are present, operation is performed so that the following inequalities are satisfied: -7×Af-0.03×Aav+5.6/aspect ratio+9.2μF1-3.2>=1.5 (1), and 5.8×Af+0.18×Aav-32/aspect ratio-87×F1+61>=10 (2); wherein, as to the explanation of the variables (Af, Aav, aspect ratio and F1) in the relationships, refer to the text. The operation is preferably performed so as to be incorporated with, by mass, 0.01 to 0.7% C, 0.01 to 2.5% Si, 0.1 to 3% Mn, 0.01 to 0.16% S, 0.05% P (including 0%) and 0.1% Al (including 0%) respectively as the chemical components in the steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a free-cutting steel for a machine structure to be subjected to a cutting process, such as a part of an industrial machine, an automobile, an electric product, etc. It relates to a free-cutting steel for machine structures that has excellent properties (especially lateral impact properties) and excellent machinability during cutting (especially chip breaking properties). The aim is to provide satisfactory steel materials.

[0002]

2. Description of the Related Art With the recent increase in the speed and automation of cutting, the machinability of steel materials used for machine structural parts has become more important, and free cutting has been improved in machinability. Demand for steel is growing. On the other hand, the required strength of steel materials is becoming stricter, and in this case, machinability tends to deteriorate in inverse proportion to the increase in strength of steel materials. For this reason, there is a demand for providing a steel material that satisfies both conflicting properties of high strength and machinability.

Many free cutting steels for machine structures satisfying both mechanical properties and machinability have been proposed and put to practical use. For example, proposals focusing on sulfides and oxides in free-cutting steel include:
JP-A-59-205453, JP-A-62-23970, JP-A-2000-205453
No. 87179, which is not a free-cutting steel, but has been proposed focusing on sulfides and oxides in steel as disclosed in JP-A-7-18
Nos. 8853 and 7-238342.

Japanese Patent Application Laid-Open No. 59-205453 discloses a low-carbon sulfur free-cutting steel in which Te, Pb and Bi are all added to S in combination, and the content of Al is reduced to reduce oxide-based intercalation. MnS-based inclusions whose major axis is 5μ or more, minor axis is 2μ or more, and the ratio of major axis / minor axis is 5 or less account for 50% or more of all MnS-based inclusions. Free cutting steel has been proposed. Although the machinability is expected to be improved in this free-cutting steel, the minor axis size of the large MnS-based inclusions (minor axis is 2 μm
Above) is difficult to suppress,
There is a concern that sufficient lateral impact properties may not always be obtained.

[0005] JP-A-62-23970 discloses a low-carbon sulfur-
In order to improve lead free-cutting steel, C, Mn, P, S, Pb,
By specifying the respective contents of O, Si, and Al, the average size of MnS-based inclusions, and the ratio of sulfide-based inclusions that are not bonded to oxides, the machinability can be improved. Proposed. In this free-cutting steel, although the effect of significantly improving the finished surface roughness during cutting can be expected,
No mention is made of the size of the sulfide containing MnS as a main component, and the variation in the minor axis size increases,
There is a concern that sufficient lateral impact properties may not always be obtained.

[0006] All of these technologies are free-cutting steels to which Pb and S are added in a combined manner. However, as the problem of environmental pollution due to Pb has been highlighted, the use of Pb in steel materials tends to be restricted. Yes, research on so-called Pb-free technology for improving machinability is being actively pursued.

[0007] Japanese Patent Application Laid-Open No. 2000-87179 is directed to carbon steel for machine structure and alloy steel for machine structure, which is combined with Ca, Mg, and REM (rare earth element) to provide abrasion resistance as a cemented carbide tool. There has been proposed a steel for machine structural use which is excellent in abrasion property and chip processing property. However, only the composition of sulfide-based inclusions is described, and the size and form of sulfide-based inclusions that have an important effect on mechanical properties and machinability are not considered in detail.

[0008] JP-A-7-188853 discloses that C, Si, Mn, C
r, P, S, TO, as basic components, and 0.0015 as T.Mg
Gear carburizing steels containing up to 0.0350% have been proposed.
In the present invention, the inclusion of Mg in the steel material reduces the size of the oxide-based inclusions (mainly alumina), suppresses the extensibility of MnS, dramatically improves surface fatigue strength, and reduces tooth bending. It is said that improvement in the degree of fatigue can be expected, but there is no mention of improving the lateral impact resistance or machinability.

JP-A-7-238342 discloses the above-mentioned JP-A-7-238342.
In order to further improve the steel for carburizing of gears described in Japanese Patent No. 188853, oxides and sulfides contained in the steel material are expressed by the following formula as the number ratio (MgO + MgO.Al ₂ O ₃ ) number / total oxide number ≧ 0.80... 0.20 ≦ (Mn · Mg) S number / total sulfide number ≦ 0.70... In this steel, it is said that by defining the number ratio of oxide and sulfide by the above formula, a dramatic improvement in surface fatigue strength and an improvement in tooth bending fatigue strength can be expected. No mention is made of improving the machinability.

[0010] In the past research on S free-cutting steels, as described above, the technology of improving the machinability by controlling the size and shape of sulfide-based inclusions such as MnS has been mainstream. Free cutting steel that exhibits machinability comparable to Pb free cutting steel has not been realized. In the technology to improve machinability by controlling the form of sulfide-based inclusions, when rolling or forging steel, the sulfide-based inclusions are deformed long with the plastic deformation of the base material. It has also been pointed out that anisotropic mechanical properties of components cause impact values in certain directions to decrease.

The machinability is (1) cutting resistance, (2)
The tool life and finished surface roughness are evaluated based on items such as tool life and (3) finished surface roughness. Conventionally, tool life and finished surface roughness have been regarded as important among these items. In the midst of the progress of swarfing, (4) chip breaking is an important issue that cannot be neglected from the viewpoint of work efficiency and safety. That is, the chip breaking property is a property that evaluates that the chip is cut into short lengths during cutting, but when this property is deteriorated, obstacles such as the chip being elongated helically long and becoming entangled with the cutting tool occur. Hinders safe operation of cutting.
In conventional Pb-added steels, comparatively good machinability was also exhibited in terms of chip breaking properties.
No free steel material with good properties has been realized.

[0012]

DISCLOSURE OF THE INVENTION The present invention has been made under such a circumstance, and the object of the present invention is to provide a Pb-free mechanical property and a chip breaking property comparable to those of conventional Pb-added steel. An object of the present invention is to provide a free-cutting steel for machine structures that can be stably and surely exerted.

[0013]

The free-cutting steel for machine structures according to the present invention, which has achieved the above object, is a free-cutting steel for machine structures in which sulfide-based inclusions are present and satisfies the following expression. It has a gist. −0.7 × Af−0.03 × Aav + 5.6 / aspect ratio + 9.2 × F1−3.2 ≧ 1.5 (1) 5.8 × Af + 0.18 × Aav−32 / aspect ratio−87 × F1 + 61 ≧ 10 (2) where Af = Average area ratio of sulfide inclusions (%) Aav = average area per sulfide inclusion (μm ² ) Aspect ratio = ratio of major axis to minor axis of sulfide inclusions (major axis /
Average value of minor axis) F1 = X ₁ / (A / n) ^1/2 X ₁ : For each particle in the observation field, the distance to another particle that is closest to the particle is set in the observation field. A value measured by averaging all particles present (μm) A: Observed area (mm ² ) n: Number of sulfide particles observed in the above-mentioned observed area (pieces)

In the free-cutting steel for machine structures of the present invention, C: 0.01 to 0.7% as a chemical component of the steel material from the viewpoint of securing physical properties required for the free-cutting steel for machine structures.
(The meaning of mass%, the same applies hereinafter.), Si: 0.01 to 2.5%, Mn:
It is preferable that each contains 0.1 to 3%, S: 0.01 to 0.16%, P: 0.05% or less (including 0%), and Al: 0.1% or less (including 0%). Also, if necessary, further Mg:
0.0005 to 0.02%, Ca: at least 1 of 0.0005 to 0.02%
If a seed is contained, more excellent effects can be exhibited.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In order to solve the above-mentioned problems, the present inventors have made various studies on the relationship between "mechanical properties and chip breaking properties" and "sulfide-based inclusions in free-cutting steel". Considered from an angle. As a result, when the size and shape (morphology) and distribution of sulfide-based inclusions such as MnS can be controlled in a well-balanced manner, a free-cutting steel for machine structures with excellent mechanical properties and chip breaking properties can be obtained. It revealed that. Hereinafter, the operation and effect of the present invention will be described in detail.

As described above, in the mechanized cutting work, it is required that one of the evaluation items of the machinability is to cut chips at the time of cutting finely. The present inventors have confirmed that the cutting of the chips is caused by the occurrence of cracks due to the concentration of stress near the inclusions present in the steel. In addition, if the inclusions are elongated in the steel, good chip breaking properties can be obtained for cutting in a certain direction, but the chip cutting properties suddenly decrease when the cutting direction changes. It was also confirmed that there was a problem of doing so. On the other hand, in the case of spherical inclusions, it was also found that although there is no anisotropy such that the machinability changes depending on the cutting direction, the chip breaking property is not always good.

Therefore, the present inventors have considered variously that it is necessary to newly examine not only the size and shape of the sulfide-based inclusions in the free-cutting steel but also their distribution. As a result, it was revealed that fine inclusions form clusters, which work together to promote chip separation. By controlling the state of these clusters, the area ratio, the size, and the extensibility of inclusions, it has been found that the impact value in the lateral direction can also be improved. The above-mentioned cluster means a state in which the distribution state of the inclusions is not spatially uniform and the distribution state is non-uniform to form the aggregation region.

The present inventors have studied from various angles based on the above idea. As a result, to obtain a free-cutting steel excellent in both mechanical properties and chip breaking property, a form satisfying the following equation is required. It has been found that the above object can be achieved satisfactorily if the clusters are distributed and the area ratio, size, and aspect ratio of the inclusions are controlled. Hereinafter, for convenience, the left side of equation (1) is F ₍₁₎ and the left side of equation (2) is F ₍₂₎ . F ₍₁₎ = − 0.7 × Af−0.03 × Aav + 5.6 / aspect ratio + 9.2 × F1−3.2 ≧ 1.5 (1) F ₍₂₎ = 5.8 × Af + 0.18 × Aav−32 / aspect ratio−87 × F1 + 61 ≧ 10 (2)

Here, Af is the average area ratio (%) of sulfide-based inclusions, and Aav is the average area (μm ² ) per sulfide-based inclusion. Further, in the free-cutting steel for machine structures of the present invention, when the ratio of the major axis to the minor axis (major axis / minor axis: aspect ratio) of the sulfide-based inclusions is controlled, excellent lateral impact characteristics and chip breaking properties are obtained. Be demonstrated. Further, F1 means the distribution index of the sulfide-based inclusion particles, and for each inclusion particle in the observation visual field, the distance from the particle closest to the particle is measured for all the particles present in the observation visual field. And the average value X ₁ (μm) and the distance between particles (A / n) ^1/2 when all the observed particles are uniformly aligned at lattice points [where A: observation area (mm ² ), n: it is the observation area in the sulfide particles number observed in (number) ratio of the values of _{[X 1 / (a / n} ) 1/2].

As an example, a case where there are 12 sulfide-based inclusion particles in the observation visual field will be described with reference to FIG. The actual observation field and sulfide type inclusion particles are distributed as shown in (a) of FIG. 1, when the closest distance for each sulfide inclusions and x ₁ ~x _12, the mean value X ₁ becomes _{X 1 = (x 1 + x} 2 + ··· + x 12) / 12. Assuming that the sulfide-based inclusion particles are uniformly distributed as shown in FIG. 1 (b), the closest distance of each sulfide-based inclusion particle is x ₁ = x ₂ = ··· · · = x ₁₂ becomes, when the observation area and a, closest distance X ₂ is the _{X 2 = (x 1 + x} 2 + ··· + x 12) / 12 = (a / 12) 1/2 Can be represented. The ratio of X ₁ and X ₂ and distribution index F1 for the sulfide type inclusion particles.

The distribution index F1 of the sulfide-based inclusion particles defined as described above is 1 when the sulfide is completely uniform.
And when it is not uniform, it deviates from 1 and becomes a value smaller than 1.

In order to obtain a free-cutting steel having excellent mechanical properties (lateral impact properties), it is necessary to satisfy F ₍₁₎ ≧ 1.5. In other words, sulfide-based inclusions satisfying F ₍₁₎ ≧ 1.5 are those in which the average area of each sulfide-based inclusion is small, the average area ratio of sulfide-based inclusions in the whole is low, and sulfides that are nearly spherical are included. Free-cutting steel uniformly distributed.

On the other hand, in order to obtain a free-cutting steel excellent in chip controllability, it suffices to satisfy F ₍₂₎ ≧ 10. That is, it is preferable that the average area per one piece is large, the average area ratio of the sulfide-based inclusions in the whole is high, and the sulfide-based inclusions extending in a band shape are unevenly distributed.

Therefore, in order to obtain a free-cutting steel excellent in both mechanical properties and chip breaking properties, F ₍₁₎ ≧ 1.5 and F ₍₂₎
What is necessary is just to control to the form and distribution state which satisfy ≧ 10.

The free-cutting steel for machine structures of the present invention is characterized in that the form and distribution of the sulfide inclusions are specified as described above, and the type of steel is not particularly limited. From the viewpoint of satisfying the required characteristics as a free-cutting steel for machine structures, C: 0.01 to 0.7%, Si: 0.01 to 2.5%, M
n: 0.1 to 3%, S: 0.01 to 0.16%, P: 0.05% or less (including 0%) and Al: 0.1% or less (including 0%). By adjusting the chemical composition of the steel, better properties can be obtained with the tensile strength required for free-cutting steel for machine structures, the distribution and shape of sulfide-based inclusions can be improved, and mechanical properties and machinability can be improved. Both properties are excellent. The action of each of these components is as follows.

C: 0.01 to 0.7% C is the most important element for securing the strength of the final product, and from such a viewpoint, the C content is preferably 0.01% or more. However, when the C content becomes excessive,
Since toughness is reduced and machinability such as tool life is adversely affected, the content is preferably 0.7% or less. Note that a more preferable lower limit of the C content is 0.05%, and a more preferable upper limit is 0.5%.

Si: 0.01-2.5% In addition to being effective as a deacidifying element, Si is an element that contributes to increasing the strength of mechanical parts by solid solution strengthening. It is preferable that the content is not less than 0.1%, more preferably 0.1% or more.
However, if it is contained excessively, the machinability will be adversely affected. Therefore, the content is preferably 2.5% or less, more preferably 2% or less.

Mn: 0.1-3% Mn is an element that not only enhances the hardenability of the steel material and contributes to the increase in strength, but also forms sulfide-based inclusions and contributes to the improvement in chip breaking performance. In order to exhibit these effects effectively, it is preferable to contain 0.1% or more. However, if contained excessively, the machinability is rather reduced, so it is preferably 3% or less, more preferably 2%
It is better to keep it below.

S: 0.01 to 0.16% S is an element effective for forming sulfide-based inclusions and improving machinability.
It is preferable to contain 0.01% or more. However, S
Is excessive, the cracks tend to occur starting from sulfides such as MnS, so that the content is preferably 0.16% or less.

P: 0.05% or less (including 0%) Since P tends to cause grain boundary segregation and deteriorate impact resistance, it should be suppressed to 0.05% or less, more preferably 0.02% or less.

Al: 0.1% or less (including 0%) Al is important as a deacidifying element when smelting steel, and is also effective for forming nitrides and refining austenite crystal grains. However, when it becomes excessive, the crystal grains become coarser and adversely affect the toughness. Therefore, the content is preferably suppressed to 0.1% or less, more preferably 0.05% or less.

The preferred chemical composition of the free-cutting steel for machine structural use according to the present invention is as described above, and the balance is basically composed of iron and unavoidable impurities. The chemical composition is not limited to the present invention because it has characteristics as a technical idea in defining the mutual balance between the form and the distribution state of the system inclusions. Depending on the required characteristics, the composition may slightly deviate from the above preferable composition of the chemical component. In addition to the above, it is also effective to further include the following elements as necessary.

Mg: 0.0005 to 0.02% Mg-containing oxide is an element serving as a nucleus for the formation of sulfide-based inclusions, and has an action of controlling the form by dissolving Mg in the inclusions. If the Mg content is less than 0.0005%, solid solution Mg
Insufficient amount makes it impossible to control the morphology of sulfide inclusions. Therefore, in order to obtain a free-cutting steel for machine structures that is excellent in both transverse impact properties and machinability (chip breaking properties), Mg must be 0.00
It is preferable to contain not less than 05%, more preferably 0.001
%. However, if it is contained excessively, the sulfide-based inclusions are hardened and the chip breaking performance is reduced. Therefore, the content is preferably set to 0.02% or less, more preferably 0.01% or less.

Ca: 0.0005 to 0.02% Ca is also an element that exerts the same effect as Mg, and the upper limit of the content is preferably 0.02%, more preferably 0.02%.
1% is good. The lower limit is preferably 0.0005%, more preferably 0.001%.

The method for producing the free-cutting steel for machine structures of the present invention may be any of a powder method and a melting method, and is not particularly limited, but includes rolling / forging temperature, rolling reduction, cooling rate during casting. It is important to control the addition order of each element in a well-balanced manner, and the form and distribution of the sulfide inclusions may satisfy the above requirements. The type of the sulfide targeted in the present invention is not specified, and the sulfide of Mn, Ca, Mg, Zr, REM, or other elements (Ni, Cr, Cu, Mo, V, Nb, Ti, Zr, Pb, B
i, B, etc.) and their sulfides, as well as their complex sulfides, carbosulfides, oxysulfides and the like.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention, and any change in the design based on the above and following points is not limited to the present invention. It is included in the technical range of.

[0037]

EXAMPLE Free-cutting steel for machine structures having the chemical composition shown in Tables 1 and 2 was rolled and forged at the following temperatures, reduction ratios,
Production was performed while controlling the cooling rate during casting and the order of addition of each element in a well-balanced manner. The morphology and distribution of sulfide-based inclusions were measured, and the transverse impact value and the number of chips were measured.

As the smelting furnace, one of a converter, a 20 kg VIF and a 150 kg VIF is used (VIF is a vacuum high frequency induction melting furnace), and hot rolling is performed at a reduction ratio of 73 to 96%. 1100
℃ heating) or hot forging (1100 ℃ or 1200 ℃)
Then, a round bar having a diameter of 80 to 50 mm was prepared. The round bar was cut to an appropriate size, quenched and tempered, and the Vickers hardness was adjusted to 270 ± 10. Then, materials for morphological measurement, impact test, and cutting test of sulfide-based inclusions were prepared.

[0039]

[Table 1]

[0040]

[Table 2]

The morphological measurement test piece of the sulfide-based inclusions has a cross section parallel to the direction in which it is rolled or forged and expanded.
Using an optical microscope, magnification: 100 times, 0.5 mm per visual field
An area of × 0.5 mm was observed in 100 visual fields at a time, and the shape and distribution of the sulfide-based inclusions were image-analyzed as follows.

(Shape of sulfide-based inclusion) The shape of the inclusion was 1.0 area for all 100 visual fields observed.
The major axis, minor axis, area, and number of sulfides of μm ² or more were measured. When the inclusion particles are present over two observation fields, the inclusion particles that overlap the two sides that are in contact with the adjacent image are not measured out of the four sides of the field of view so that the number is not counted repeatedly. . That is, the inclusion particles (shown in white) in contact with the right side and the bottom side as shown in FIG. 2A are not counted, but are counted as inclusions in the next observation field. Specifically, the number of sulfide-based inclusion particles in the visual field was counted as shown in FIG. 2 (b).

(Distribution state of sulfide-based inclusions) The distribution state of the inclusions was evaluated in the following manner.
Was evaluated. [F1] Area: For each visual field of 0.5 mm × 0.5 mm, area: 1.0 μm ²
The center of gravity of the above sulfide-based inclusions was determined, the distance between the centers of gravity of each sulfide and other sulfides was measured, and the distance of each particle to the closest particle was determined. Then, the average value X ₁ measured value for the distance between nearest grains for each field, nearest distance between particles in the case where the same number of sulfide particles are uniformly dispersed in a grid in the same area [(A / n) ^{1 / 2]} and the ratio of [X ₁ / (a /
n) ^1/2 ] to obtain the sulfide particle distribution index F1. This was measured for five visual fields and the average value was determined. The area of the target sulfide was set to 1.0 μm ² or more because controlling a sulfide smaller than this has little effect.

In the impact test, a Charpy impact test was carried out using a test piece cut at a right angle to the direction of expansion by rolling or forging, and the impact value in the lateral direction was determined. The cutting test was performed using a straight drill made of HSS (diameter: 10 mm), and the number of chips for two holes was counted.
The cutting conditions were as follows: speed: 20 m / s, feed rate: 0.2 mm / r
ev and hole depth: 10 mm, and dry cutting was performed.

Table 3 shows the results of measurement of the form and distribution of the sulfide inclusions, and the values of F ₍₁₎ and F ₍₂₎ obtained from the results. The transverse impact value and the number of chips measured as described above are also shown.

[0046]

[Table 3]

FIG. 3 shows the impact value in the lateral direction with respect to the value of F ₍₁₎ .
FIG. 4 is a graph showing the number of chips with respect to the value of ₍₂₎ . Here, those satisfying the expression (1) or (2) are indicated by ●, and those not satisfying the expression are indicated by ○.

As can be seen from FIG. 3, the free-cutting steel satisfying the formula (1) (ie, F ₍₁₎ is 1.5 or more) has a high lateral impact value and a high mechanical property (particularly, a lateral impact property). ) Is excellent.
In addition, as can be seen from FIG. 4, the free-cutting steel satisfying the formula (2) (that is, F ₍₂₎ is 10 or more) has a large number of chips, and has a high machinability (particularly, chip breaking). Are better.

Further, the free-cutting steel for machine structure of the present invention satisfying both the formulas (1) and (2) is represented by ●, which does not satisfy one of the formulas and is out of the range of the present invention (Comparative Example). ) Is marked with ○, and the transverse impact value and the number of chips are graphed as shown in FIG. 5. As can be seen from FIG. 5, the free-cutting steel satisfying the requirements of the present invention has mechanical properties (lateral impact ) And machinability (chip breaking property), but those that do not meet the requirements of the present invention are free-cutting steels in which both properties are not balanced and at least one of the properties is inferior. there were.

[0050]

The present invention is configured as described above. If the size and form and distribution of sulfide-based inclusions in free-cutting steel are controlled in a well-balanced manner, both mechanical properties and chip breaking properties are improved. Excellent stable free-cutting steel for machine structures could be provided.

[Brief description of the drawings]

FIG. 1 is a diagram for specifically explaining a method of calculating a distribution index F1 of sulfide-based inclusion particles.

FIG. 2 is a diagram for explaining a method of counting the number of sulfide-based inclusions present in an observation visual field.

FIG. 3 is a graph of a lateral impact value with respect to a value of F ₍₁₎ .

FIG. 4 is a graph showing the number of chips with respect to the value of F ₍₂₎ .

FIG. 5 is a graph showing the relationship between mechanical properties (lateral impact properties) and machinability (chip breaking properties).

Continued on the front page (72) Inventor Takehiro Tsuchida 1-5-5 Takatsukadai, Nishi-ku, Kobe City Inside Kobe Steel Research Institute Kobe Steel Co., Ltd. (72) Inventor Takahiro Kudo 1-5-5 Takatsukadai, Nishi-ku, Kobe City No. Kobe Steel, Ltd.Kobe Research Institute (72) Inventor Masato Kaiso 2nd Nadahama-cho, Nada-ku, Kobe-shi Kobe Steel Works, Ltd.Kobe Works (72) Inventor Masami Somegawa 2, Nadahama-cho, Nada-ku, Kobe Address Kobe Steel Works Kobe Works

Claims (3)

[Claims] Claims: 1. A free-cutting steel for a machine structure having sulfide-based inclusions, wherein the free-cutting steel for a machine structure is excellent in mechanical properties and chip breaking characteristics, characterized by satisfying the following expression. −0.7 × Af−0.03 × Aav + 5.6 / aspect ratio + 9.2 × F1−3.2 ≧ 1.5 (1) 5.8 × Af + 0.18 × Aav−32 / aspect ratio−87 × F1 + 61 ≧ 10 (2) where Af = Average area ratio of sulfide-based inclusions (%) Aav = Average area per sulfide-based inclusion (μm ² ) Aspect ratio = Ratio of major axis to minor axis of sulfide-based inclusions (major axis / minor axis) F1 = X ₁ / (A / n) ^1/2 X ₁ : For each particle in the observation field, the distance to another particle closest to the particle is determined by the total A value measured by averaging the particles (μm) A: Observed area (mm ² ) n: Number of sulfide particles observed in the above observed area (pieces) 2. In mass%, C: 0.01 to 0.7% Si: 0.01 to 2.5% Mn: 0.1 to 3% S: 0.01 to 0.16% P: 0.05% or less (including 0%) Al: 0.1% or less ( 2. The free-cutting steel for machine structures according to claim 1, wherein the free-cutting steel contains 0%. 3. A mass%, Mg: 0.0005 to 0.02%, Ca: 0.00
3. The free-cutting steel for machine structures according to claim 1, wherein the steel contains at least one of 05 to 0.02%.