JP2000282171A - Steel for machine structure excellent in parting property of chip and mechanical property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow a steel to stably and securely exhibit excellent parting properties of chips and mechanical properties in a state of being freed of Pb by controlling the sulfide grain distributing index having a specified relation to a specified value. SOLUTION: The sulfide grain distributing index F1 expressed by the formula I [in the formula X1 denotes the value (μm) obtd. in such a manner that the distance per grain in the observing field with an another grain present most closely to the grain is measured as to all grains present in the observing field, and this is averaged, A denotes the observing area (mm2), and (n) denotes the number (pieces) of sulfide grains observed in the observing area] is controlled to <=0.5. Preferably, the sulfide grain distributing index F2 expressed by the formula II [in the formula, σ denotes the standard deviation of the number of the sulfide grains per unit area, and X2 denotes the average value of the number of the sulfide grains per unit area] is >=1.5 and it contains respectively, by mass, 0.01 to 0.7% C, 0.01 to 2.5% Si, 0.1 to 3% Mn, 0.01 to 0.2% S, <=0.05% (including zero) P, <=0.1% (including zero) Al and 0.002 to 0.02% N.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel for machine structural use which is useful as a material for parts manufactured by cutting, such as parts for industrial machines, automobiles, electric appliances and the like. The present invention relates to a so-called Pb-free steel that does not substantially contain Pb as a property improving component and is excellent in chip disposability at the time of cutting and excellent in mechanical properties.

[0002]

2. Description of the Related Art Parts such as industrial machines, automobiles, and electric products are manufactured by cutting, so that good machinability is required. As a method of improving the machinability of steel for machine structural use as a material of such components, a method of incorporating Pb, S, or the like as a machinability improving component in steel has been conventionally adopted, and particularly, a small amount of Pb is used. It is known that excellent machinability is exhibited by the addition of.

[0003] For example, Japanese Patent Application Laid-Open No.
No. 205453, MnS-based inclusions in which Te, Pb, and Bi are added to S in combination and the ratio of the major axis to the minor axis is greater than a certain value and the (major axis / minor axis) ratio is 5 or less are all MnS inclusions. Account for 50% or more of Al ₂ in the oxide inclusions.
A free-cutting steel having an O ₃ content of 15% or less has been proposed.

Japanese Patent Application Laid-Open No. Sho 62-23970 discloses a low carbon sulfur-lead free-cutting steel produced by a continuous casting method.
By defining the component ranges of P, S, Pb, O, Si, Al, etc., and defining the average size of MnS-based inclusions and the proportion of sulfide-based inclusions not bonded to oxides, Techniques for improving the performance have been proposed.

[0005] All of these technologies are free cutting steels to which Pb and S are added in combination, but the problem of environmental pollution due to Pb has been highlighted, and Pb has also been used in steel materials.
There is a tendency that the use of steel is restricted, and research on a technology for improving the machinability in a so-called Pb-free manner is being actively promoted.

Under these circumstances, a technique for improving machinability by controlling the size and shape of sulfide-based inclusions such as MnS in sulfur free-cutting steel has become mainstream.
Free cutting steels exhibiting machinability comparable to Pb free cutting steels have not been realized. In the technology of improving machinability by controlling the morphology of sulfide-based inclusions, MnS is deformed long with plastic deformation of the base material when rolling or forging steel materials, which causes mechanical properties It has been pointed out that the anisotropy is caused in the steel and the impact value in a certain direction is reduced.

Incidentally, machinability includes (1) cutting resistance,
It is evaluated by items such as (2) tool life, (3) finished surface roughness, and (4) chip breaking property. Conventionally, the tool life and finished surface roughness of these items are regarded as important. However, in recent years, automation and unmanned machining have been advanced, and from the viewpoint of work efficiency and safety, chip breaking has become an important issue that cannot be neglected. That is, the chip breaking property is a property that occurs when the chip is cut into short lengths during cutting, but if this property is deteriorated, the chip may be elongated in a spiral shape and cause troubles such as entanglement with the cutting tool. become. Even in view of such chip breaking property, the conventional P
Although the comparatively good machinability was exhibited by the b-added steel, the fact is that the Pb-free steel material having good characteristics has not been realized.

[0008]

DISCLOSURE OF THE INVENTION The present invention has been made under such a circumstance, and it is an object of the present invention to provide a Pb-free, excellent chip breaking property and mechanical strength comparable to conventional Pb-added steel. An object of the present invention is to provide a steel for machine structural use capable of stably and reliably exhibiting characteristics.

[0009]

The steel for machine structural use according to the present invention, which has achieved the above object, is a steel for machine structural use in which sulfide-based inclusions are present. The gist is that the matter particle distribution index F1 is 0.5 or less.

F1 = X ₁ / (A / n) ^1/2 (1) where X ₁ is the distance between each particle in the observation visual field and another particle that is closest to the particle. A value obtained by actually measuring all particles present in the observation visual field and averaging the values (μm) A: Observed area (mm ² ) n: Number of sulfide particles observed in the observed area (number) The object of the present invention can also be achieved in a machine structural steel having a sulfide particle distribution index F2 defined by the following formula (2) of 1.5 or more.

F2 = σ / X ₂ (2) where σ: standard deviation of the number of sulfide particles per unit area X ₂ : average value of the number of sulfide particles per unit area Also, the ratio (L1 / L2) of the major axis L1 and the minor axis L2 of the sulfide-based inclusions is 1.5 to 8.
It preferably satisfies the requirement of 0, which can further improve the chip breaking property and mechanical properties. .

In the steel for machine structural use of the present invention, from the viewpoint of securing physical properties required for the steel for machine structural use, C: 0.01 to 0.7%,
Si: 0.01 to 2.5%, Mn: 0.1 to 3%, S:
0.01 to 0.2%, P: 0.05% or less (including 0%), Al: 0.1% or less (including 0%) and N:
It is preferable to contain 0.002 to 0.02% each. The chemical component may further include, if necessary, one or more selected from the group consisting of (a) 0.002 to 0.2% Ti and a rare earth element: 0.0002 to 0.2% in total; ) Bi: 0.3% or less (excluding 0%), etc. is also useful.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In order to solve the above-mentioned problems, the present inventors have studied from various angles the relationship between the chip breaking property and the form of sulfide inclusions. As a result, it was clarified that there is a correlation between the distribution state of inclusions and the chip breaking property, not the size or shape of MnS or the like as in the conventional case. Hereinafter, the operation and effect of the present invention will be described.

As described above, in the mechanized cutting process, it is required that one of the evaluation items of the machinability is to divide the chips at the time of cutting into fine pieces. 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 when the cutting direction changes, the chip breaking properties suddenly increase. There is a problem of lowering. On the other hand, in the case of the spherical inclusion, although there is no anisotropy such that the machinability changes depending on the cutting direction, the chip breaking property was not always good.

Therefore, the present inventors focused on the distribution of sulfide-based inclusions, and by forming fine inclusions into clusters, it was possible to promote the cutting of chips in cooperation with each other, At the same time, it has been found that unlike the case of elongated elongated inclusions, since the clusters are formed in three dimensions, the anisotropy is not large and the impact value in the lateral direction can be good. Here, the cluster means a state in which the distribution state of the inclusions is not spatially dispersed uniformly, the distribution state is non-uniform, and an aggregation region is formed.

The present inventors have examined the means for evaluating the cluster based on the above idea from various angles, and found that the sulfide particle distribution index F1 defined by the above formula (1) or (2). Alternatively, it has been found that the above object can be achieved satisfactorily if F2 falls within a predetermined range, and the present invention has been completed. Next, these sulfide particle distribution indexes F1,
F2 will be described.

First, the meaning of the sulfide particle distribution index F1 is that, for each inclusion particle in the observation field, the distance between the particle and the particle existing closest to the particle is determined for all the particles present in the observation field. The average value X ₁ measured and the inter-particle distance (A) when all observed particles were uniformly aligned at lattice points
/ N) ^1/2 [where A: observation area (mm ² ), n: number of sulfide particles (number) observed in the observation area] [X ₁ / (A / n) ^1/2 ].

The sulfide particle distribution index F1 defined as described above takes a value close to 1 when the sulfide is completely uniform, and deviates from 1 when the sulfide is not uniform, and takes a value smaller than 1. According to the study of the present inventors, when the value of F1 is 0.5 or less, both the chip breaking property and the lateral impact value become favorable due to the formation of clusters. It was clarified that the chip breaking property was reduced when exceeding.

On the other hand, the meaning of the sulfide particle distribution index F2 is that a visual field of a certain area is divided into a grid, and the standard deviation σ of the number of the sulfide-based inclusions present in each grid is expressed by a unit area. is a normalized value by an average value X ₂ number of sulfide particles per. In this case, if the sulfide-based inclusions have a completely uniform distribution, the value of F2 will approach zero. When the value of F2 is 1.5 or more, it is apparent that both the chip breaking property and the lateral impact value become favorable due to the formation of clusters, and that the chip breaking property decreases when the value is less than 1.5. I made it.

In the steel for machine structural use of the present invention,
The ratio (L1 / L) of the major axis L1 and the minor axis L2 of the sulfide-based inclusions
2: aspect ratio) is preferably controlled to 1.5 to 8.0, whereby more excellent chip breaking property and lateral impact value are exhibited. That is, the sulfide-based inclusions are deformed to some extent by rolling or forging, but when the sample is cut in an appropriate direction and observed, the aspect ratio of the sulfide-based inclusions is less than 1.5 on average. When the value is too large, the chip breaking property is reduced. On the other hand, when this value is too large and exceeds 8.0, the impact value in the lateral direction is reduced.

The mechanical structure of the present invention is characterized in that the distribution of sulfide-based inclusions is defined as described above, and the type of steel material is not particularly limited. From the viewpoint of satisfying the required characteristics of C: 0.01 to 0.7%, Si: 0.01 to 2.
5%, Mn: 0.1-3%, S: 0.01-0.2%,
It is preferable that P: 0.05% or less (including 0%), Al: 0.1% or less (including 0%), and N: 0.002 to 0.02%. By adjusting the chemical composition as described above, better properties can be obtained with the tensile strength required for steel for machine structural use, and the distribution and shape of sulfide-based inclusions can be improved, and machinability and mechanical properties can be improved. All of the properties are better. 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 is excessive, the toughness is reduced and the machinability such as tool life is adversely affected. Note that a more preferred lower limit of the C content is 0.05%, and a more preferred 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. Is preferably contained at 0.01% or more, more preferably 0.1% or more. However, if it is contained excessively, the machinability will be adversely affected. Therefore, the content is preferably set to 2.5% or less, more preferably 2.0% or less.

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

S: 0.01% to 0.2% S is an element effective for forming sulfide-based inclusions and improving machinability. % Or more, more preferably 0.03% or more. However, S
Is excessive, the cracks tend to occur starting from sulfides such as MnS. Therefore, the content is preferably 0.2% or less, more preferably 0.12% or less.

P: 0.05% or less (including 0%) Since P tends to cause grain boundary segregation and deteriorate impact resistance, P is 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 in forming nitrides and refining austenite crystal grains. However, if it becomes excessive, the crystal grains become coarser and adversely affect toughness. Therefore, the content is preferably suppressed to 0.1% or less, more preferably 0.05% or less.

N: 0.002 to 0.02% N forms fine nitrides with Al, Ti, and the like, and contributes to refinement of the structure and improvement of the strength. In order to exhibit such an effect, it is preferable that the content be 0.002% or more. However, if the content is excessive, coarse nitrides may be formed. Therefore, the content should be suppressed to 0.02% or less.

The preferred chemical composition of the 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 when the distribution state of the substance is defined. It may be slightly different from the composition. Also,
In addition to the above, it is effective to further include the following elements as necessary.

Ti: 0.002 to 0.2% and rare earth
Element: selected from the group consisting of 0.0002 to 0.2% in total
When one or more types of steel materials are produced by melting, the addition of Ti or a rare earth element changes the form such as the distribution state of sulfides, and provides superior characteristics as compared with the case where no addition is made. However, if the Ti content is less than 0.002%, the effect of the addition is insufficient, and if the Ti content exceeds 0.2%, the impact value is significantly reduced. Also, Ce, L
In the case of rare earth elements such as a, Pr and Nd, if the total content is less than 0.0002%, the effect of the addition is insufficient. Will decrease. These elements may be added to either Ti or the rare earth element, or both may be added simultaneously.

Bi: not more than 0.3% (not including 0%) Bi is an element effective for improving machinability, but if contained excessively, not only does the effect become saturated, but also heat Since the forgeability is deteriorated and the mechanical properties are reduced, the content should be set to 0.3% or less.

The method for producing the steel for machine structural use of the present invention may be any of a powder method and a melting method, and is not particularly limited. In short, the sulfide forms satisfying the above requirements. Is fine. The types of sulfides targeted in the present invention are not specified, and sulfides of Mn, Ca, Zr, Ti, and other elements, composite sulfides, carbosulfides, and oxysulfides thereof are not specified. An object may be used as long as the distribution state of the inclusions satisfies the requirements defined by the above equation (1) or (2).

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples do not limit the present invention. It is included in the technical range of.

[0034]

EXAMPLES Example 1 In order to make various comparisons by changing the distribution of sulfide-based inclusions, powder was used as a raw material to prepare a material as follows. First, as raw materials, pure iron powder containing a certain amount of impurities, Mn
S powder, graphite powder, Fe-Si alloy powder, Fe
-Mn alloy powder, Fe-P alloy powder, etc. were prepared.
And, in order to intentionally change the distribution of sulfide,
The ratio of pure iron powder to MnS powder is 2: 1, 4: 1, 10: 1
The mixture was preliminarily mixed and passed through a mesh having an opening of about 400 μm to prepare a granulated powder of about 300 μm.

The premixed powder and other raw material powders were mixed at a predetermined ratio to obtain mixed powders having different sulfide distributions. At this time, the components were C: 0.3%, Si: 0.85
%, Mn: 0.65%, S: 0.06%, P: 0.01
%, The balance being iron and inevitable impurities.

Each of the mixed powders prepared as described above is subjected to CIP molding (cold isostatic pressing) in a rubber mold, and then vacuum-encapsulated in an iron capsule and HIP molded at 1100 ° C. (hot isostatic pressure). Pressure molding). Furthermore, in order to change the aspect ratio (major axis / minor axis) of the sulfide-based inclusions, forging is performed by changing the forging ratio to reduce the diameter of the round bar, and quenching / tempering is performed to perform Vickers hardness. Was adjusted to 270 ± 10. Then, materials for the cutting test piece and the Charpy impact test piece were produced. At this time, the impact test piece was cut out in a direction perpendicular to the longitudinal direction of the round bar.

Using the test piece obtained as described above,
The cutting test and the impact test were performed, and the sulfide morphology was measured. At this time, the cutting test was performed using a straight drill made of HSS (diameter: 10 mm).
The number of chips for the holes was counted. The cutting conditions were as follows: speed: 20 m / min, feed speed: 0.2 mm / r
ev and hole depth: 10 mm, and dry cutting was performed. In the impact test, a lateral impact value was obtained.

On the other hand, the morphology of sulfide was measured using an optical microscope at a magnification of 100 × 0.5 mm × 1 field of view.
The area of 0.5 mm was observed in 100 visual fields at a time, and the shape and distribution of the sulfide were image-analyzed in the following manner.

(Shape of Sulfide) Regarding the shape of the sulfide, the area was 1.0 for all of the 100 visual fields observed.
The major axis, minor axis, area, and number of sulfides having a size of ² μm ² or more were measured.

(Sulfide Distribution State) The sulfide distribution state was evaluated by the sulfide particle distribution index F1 or F2 as described below.

[F1] For each visual field having an area of 0.5 mm × 0.5 mm, the center of gravity of a sulfide having an area of 1.0 μm ² or more is determined, and the distance between the centers of gravity of each sulfide and another sulfide is measured. Then, the distance between each particle and the closest particle was determined. Then, the average value X ₁ of the actually measured values of the distances between the closest particles in each field of view and the distances between the closest particles when the same number of sulfide particles are uniformly dispersed in the same area in a grid pattern [(A
/ N) ^1/2 ] and the ratio [X ₁ / (A / n) ^1/2 ]
The sulfide particle distribution index F1 was used. This was measured for five visual fields and the average value was determined. The area of the target sulfide is set to 1.0 μm ² or more because controlling a sulfide smaller than this has little effect.

[F2] Area: Each field of view of 0.5 mm × 0.5 mm is divided into 25 grids of 0.1 mm × 0.1 mm, and the number of grids including the position of the center of gravity is measured. The variation in the number between the 25 grids is calculated as the standard deviation σ, and this standard deviation σ is calculated as the average value X of the number.
_The value (σ / X ₂ ) normalized by ₂ (the average value of the number of sulfide particles per unit area) was defined as the sulfide particle distribution index F2.
This was measured for five visual fields and the average value was determined.

Table 1 below shows the chemical composition of the test pieces used at this time, and Table 2 shows the measurement results of the distribution and shape of these sulfide inclusions, the cutting test results and the impact test results. It is shown. FIG. 1 shows N in Table 2.
o. Based on the results of 1 to 15, the size of the sulfide (evaluated by the average area here) gives the chip breaking property [FIG. 1 (a)] and the mechanical properties [FIG. 1 (b)] as in the conventional case. FIG. 2 is a graph showing the influence, and FIG. 2 also shows the effect of the aspect ratio (major axis / minor axis) of the sulfide on the chip breaking property [FIG. 2 (a)] and the mechanical properties [FIG. 2 (b)]. It is a graph shown.

On the other hand, FIG. 4 is a graph showing the influence of the sulfide inclusion distribution index F1 on the chip breaking property [FIG. 3 (a)] and the mechanical properties [FIG. 3 (b)] based on the results of 1 to 8. Also, FIG. 9
Based on the results of １５15, the sulfide inclusion distribution index F2 is
Chip breaking [Fig. 4 (a)] and mechanical properties [Fig.
(B)] is a graph showing the effect on [b].

[0045]

[Table 1]

[0046]

[Table 2] From these results, the following can be considered. First, FIGS.
As is clear from the above, it is understood that excellent characteristics are not necessarily obtained only by specifying the size and aspect ratio of the sulfide. On the other hand, as is apparent from FIG.
When the sulfide particle distribution index F1 becomes 0.5 or less (that is, the distribution state of the sulfide is non-uniform), it can be seen that the cutting and cutting properties are improved. Also, at this time, the lateral impact value does not decrease, indicating that it is at a level sufficient for practical use. As is clear from FIG. 4, when the sulfide particle distribution index F2 is 1.5 or more, good results are obtained. However, when this F2 is less than 1.5, cluster formation is insufficient. As a result, it can be seen that the chip breaking ability is reduced.

Note that FIG. Based on the results of Nos. 16 to 23, the sulfide particle distribution index F1 was made substantially constant, and the aspect ratio of the sulfide inclusions was changed to the chip breaking property [FIG. 5 (a)] and the mechanical properties [FIG. 5 (b)]. Is a graph showing the influence of the sulfide-based inclusions on average, when the aspect ratio of the sulfide-based inclusions is less than 1.5, the chip breaking property is reduced, and conversely, 8.
It can be seen that when the value is larger than 0, the lateral impact value decreases.

Example 2 Using a high-frequency vacuum melting furnace, various steel materials adjusted to the chemical composition shown in Table 3 below were melted. The obtained ingot was hot forged to a diameter of 140 mm → 80 mm, cut into appropriate dimensions, and quenched and tempered to adjust the Vickers hardness to 270 ± 10. Then, materials for the cutting test piece and the Charpy impact test piece were produced. At this time, the impact test piece was cut out in a direction perpendicular to the longitudinal direction of the round bar. Using the test piece thus obtained, a cutting test and an impact test were performed in the same manner as in Example 1, and the morphology of sulfide was measured. Table 4 below shows the measurement results of the distribution and shape of the sulfide inclusions, the cutting test results, and the impact test results at this time.

[0049]

[Table 3]

[0050]

[Table 4] From these results, the following can be considered. First, no. 2
4 to 28 show the effect of adding Ti. Further, FIG. 6 shows that, based on these results, the addition of Ti makes the chip breaking property [FIG. 6 (a)] and the mechanical properties [FIG.
(B)] is a graph of the effect on [b]. As is clear from these, the Ti content is 0.005 to 0.2.
%, It can be seen that both the chip breaking property and the mechanical properties (transverse impact value) are good.

No. 29 to 35 show the effect of adding a rare earth element (REM). FIG. 7 is a graph showing the influence of the addition of REM on the chip breaking property [FIG. 7A] and the mechanical properties [FIG. 7B] based on these results. As is clear from these, the REM content is 0.0002 to 0.2 in total.
%, It can be seen that both the chip breaking property and the mechanical properties (transverse impact value) are good.

No. 36 to 39 show the effect of adding Bi. FIG. 8 is a graph showing the influence of the addition of Bi on the chip breaking property [FIG. 8A] and the mechanical properties [FIG. 8B] based on the results. As is evident from these results, No. 2 containing an appropriate amount of Bi was added. In the case of Nos. 37 and 38, No.
No. 36 had better chip breaking properties and mechanical properties than No. 36. It can be seen that in the case of No. 39, the Bi content was too large, so that the lateral impact value was lowered.

[0053]

The present invention is constituted as described above. By appropriately defining the distribution of sulfide-based inclusions, excellent chip control performance comparable to that of conventional Pb-added steel even in Pb-free steel. And a steel for machine structural use that can exhibit mechanical properties stably and reliably.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of sulfide size on chip breaking and mechanical properties.

FIG. 2 is a graph showing the effect of the sulfide aspect ratio on chip breaking and mechanical properties.

FIG. 3 is a graph showing the effect of the sulfide inclusion distribution index F1 on chip breaking and mechanical properties.

FIG. 4 is a graph showing the effect of a sulfide inclusion distribution index F2 on chip breaking and mechanical properties.

FIG. 5 is a graph showing the effect of the aspect ratio of sulfide inclusions on chip breaking and mechanical properties.

FIG. 6 is a graph showing the effect of the addition of Ti on chip breaking and mechanical properties.

FIG. 7 is a graph showing the effect of the addition of REM on chip breaking and mechanical properties.

FIG. 8 is a graph showing the effect of the addition of Bi on chip breaking properties and mechanical properties.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Katsuhiko Ozaki 1-5-5 Takatsukadai, Nishi-ku, Kobe City Inside Kobe Research Institute, Kobe Steel Ltd. (72) Inventor Takahiro Kudo 1-chome, Takatsukadai, Nishi-ku, Kobe-shi No. 5-5 Kobe Steel, Ltd.Kobe Research Institute (72) Inventor Moriga Kanamaru 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi Kobe Steel Research Institute, Kobe Research Institute (72) Inventor Masami 2 Nadahama-Higashi-cho, Nada-ku, Kobe Kobe Steel Works, Ltd.Kobe Works

Claims (6)

[Claims] 1. A chip for machine structural use in which sulfide-based inclusions are present, wherein a sulfide particle distribution index F1 defined by the following formula (1) is 0.5 or less. And mechanical structural steel with excellent mechanical properties. F1 = X ₁ / (A / n) ^1/2 (1) where X ₁ is the distance between each particle in the observation field and another particle closest to the particle, and A: Observed area (mm ² ) n: Number of sulfide particles observed within the above-mentioned observed area (μm) 2. A chip treating property in a steel for machine structural use in which sulfide-based inclusions are present, wherein a sulfide particle distribution index F2 defined by the following formula (2) is 1.5 or more. And mechanical structural steel with excellent mechanical properties. F2 = σ / X ₂ (2) where σ: Standard deviation of the number of sulfide particles per unit area X ₂ : Average value of the number of sulfide particles per unit area 3. The steel for machine structural use according to claim 1, wherein a ratio (L1 / L2) of the major axis L1 and the minor axis L2 of the sulfide-based inclusions is 1.5 to 8.0. 4. C: 0.01 to 0.7% (meaning by mass%, the same applies hereinafter), Si: 0.01 to 2.5%, Mn:
0.1-3%, S: 0.01-0.2%, P: 0.05
% Or less (including 0%), Al: 0.1% or less (including 0%), and N: 0.002 to 0.02%, respectively. Machine structural steel. 5. The composition according to claim 4, further comprising at least one selected from the group consisting of 0.002 to 0.2% of Ti and 0.0002 to 0.2% of rare earth elements in total.
The steel for machine structural use according to claim 1. 6. The steel for machine structural use according to claim 4, further comprising Bi: 0.3% or less (excluding 0%).