JP2002194484A - Steel for machine structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for a machine structure having excellent surface hardening treatment characteristics including carburizing and quenching properties, cold workability, machinability and fatigue resistance. SOLUTION: (1) The steel for a machine structure has a composition containing 0.10 to <0.30% C, 0.05 to 1.0% Si, 0.30 to 2.0% Mn, 0.005 to 0.05% S, 0.05 to 0.2% Ti and Cu, Cr, Ni, Mo, B, Nb, V and Al, and the balance Fe with impurities, and in which the value of Ti-3S-3.4N is >=0. Further, the maximum diameter of the equivalent circle of inclusions expressed by (πLW/4)0.5 when the cumulative distribution function estimated by extremum statistical treatment with the length of the inclusions in the longitudinal cross-sectional face in the longitudinal direction as L (μm), and the breadth as W (μm) is 99% is <=30 μm. (2) It is possible that the content of Ti is <=0.2%, the content of Zr is <=0.2%, and the value of Ti+Zr-3S-3.4N is >=0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a steel material for a machine structure. More specifically, the present invention relates to a steel material for a machine structure suitable as a material of a part to be subjected to a surface hardening treatment such as carburizing and quenching or carbonitriding treatment, such as an automobile gear.

[0002]

2. Description of the Related Art For the purpose of increasing the output of a power machine and reducing its weight for improving fuel efficiency, it is required to increase the strength of a power transmission component. Above all, there is a great demand for high strength gears of automobiles.

[0003] In many cases, automobile gears and the like are manufactured by cold forging having high dimensional accuracy after forging and capable of reducing the amount of cutting, and then subjected to a surface hardening treatment such as carburizing and quenching. Therefore, a steel material as a raw material is required to have good surface hardening treatment properties including carburizing and quenching properties.
In recent years, for further cost reduction and simplification of the manufacturing process, there is an increasing demand for steel materials having good surface hardening properties and having better machinability and cold workability. . Further, in the case of the above-described automobile gear, bending stress repeatedly acts on the tooth root. Therefore, in order to increase the fatigue strength at the root of the tooth and facilitate the weight reduction of the gear, there is an increasing demand for a steel material having more excellent fatigue resistance.

[0004] In order to enhance the cold workability, conventionally, a method of performing spheroidizing annealing on a steel material or improving the cleanliness of the steel material has been adopted. However, in such a method, the machinability when cutting the steel material is reduced. It tends to decrease. On the other hand, in order to enhance machinability, free-cutting elements such as Pb, Te, Bi, and Ca (elements for improving machinability) are added to steel. May be reduced.

[0005] Japanese Patent Application Laid-Open No. 11-1743 proposes a technique for improving machinability by controlling the size and amount of Ti sulfide as a "high-strength, high-toughness tempered steel excellent in machinability". Have been. However, when steel is cold-worked, not only sulfides but also non-metallic inclusions such as oxides and nitrides (hereinafter simply referred to as inclusions) may cause a reduction in cold workability. Simply defining the size and amount of Ti sulfide may not achieve both machinability and cold workability.

As described above, the cold workability and the machinability are often contradictory, and it is difficult to achieve both.

On the other hand, in order to increase the fatigue strength at the tooth root portion of an automotive gear, a technique of performing shot peening after surface hardening treatment such as carburizing and quenching has been proposed. this is,
A residual compressive stress is introduced into the surface hardened layer by shot peening, and the residual stress is intended to relieve the tensile stress acting on the surface layer portion by a repeated load.
Therefore, various shot peening methods have been proposed in order to obtain higher compressive residual stress.

[0008] However, compressive residual stress is introduced by shot peening at most 0.2 mm from the surface.
It is up to a portion having a depth of about 0.3 mm, and at a portion deeper than that, the effect of reducing the repeated load stress due to the compressive residual stress is lost. For this reason, the above-mentioned 0.2-0.
If a defect such as an inclusion is present at a portion deeper than about 3 mm, a fatigue crack is generated from the defect, which often causes early destruction.

A technique for reducing the inclusions and improving the fatigue resistance is disclosed in, for example, Japanese Patent Application Laid-Open No. 2-270935. The technique proposed in the above publication aims to improve fatigue characteristics by minimizing oxides and nitrides.

However, the stress applied to the actual parts, that is, the bending stress at the root of the gear of an automobile often acts perpendicular to the rolling direction or the forging axis, so that the rolling direction is higher than that of oxides or nitrides. And coarse MnS extending in the direction of the forging axis are the starting points of fatigue fracture. That is, in gears of automobiles and the like, MnS causes a decrease in fatigue strength more than oxides and nitrides. Therefore, in order to improve fatigue strength, not only oxides and nitrides but also all intervening materials including MnS are included. You need to control things finely.

As a technique for miniaturizing sulfide-based inclusions, especially MnS, Japanese Patent Application Laid-Open No. H11-1743 mentioned above can be cited. In the above-mentioned publication, all inclusions are examined. However, it is not necessarily sufficient to increase the fatigue strength when a stress perpendicular to the rolling direction or the wrought axis acts.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide excellent surface hardening properties such as carburizing and hardening properties, excellent cold workability and machinability, and An object of the present invention is to provide a steel material for machine structural use which is excellent in fatigue resistance characteristics, especially excellent in fatigue resistance characteristics when a stress perpendicular to the rolling direction or the wrought axis is applied, and is suitable as a material of a part subjected to surface hardening treatment. .

[0013]

SUMMARY OF THE INVENTION The gist of the present invention resides in steel materials for machine structures shown in the following (1) and (2).

(1) Chemical composition in mass%, C: 0.10
% To less than 0.30%, Si: 0.05 to 1.0%,
Mn: 0.30 to 2.0%, S: 0.005 to 0.05
%, Ti: 0.05 to 0.2%, Cu: 0 to 0.50
%, Cr: 0 to 2.0%, Ni: 0 to 3.5%, Mo:
0 to 1.0%, B: 0 to 0.005%, Nb: 0 to 0.
1%, V: 0 to 0.3%, Al: 0.01% or less, the balance being Fe and impurities, P in the impurities is 0.03% or less, N is 0.008% or less, O (oxygen) is 0.0025% or less, and fn represented by the following formula (1)
The value of 1 is 0 or more, and the cumulative distribution function predicted by the extreme value statistical processing with the major axis of the nonmetallic inclusion in the longitudinal longitudinal section being L (μm) and the minor axis being W (μm) is 9
A steel material for machine structure in which the maximum equivalent circular diameter fn2 of the nonmetallic inclusion represented by the following formula (2) at 9% is 30 μm or less. fn1 = Ti (%)-3S (%)-3.4N (%) (1) fn2 = (πLW / 4) ^0.5 (2) (2) Chemical composition is% by mass and C: 0.1% or more.
Less than 30%, Si: 0.05 to 1.0%, Mn: 0.3
0 to 2.0%, S: 0.005 to 0.05%, Ti:
0.2% or less, Zr: 0.2% or less, and Ti
(%) + Zr (%): 0.05 to 0.2%, Cu: 0 to 0
0.50%, Cr: 0 to 2.0%, Ni: 0 to 3.5
%, Mo: 0 to 1.0%, B: 0 to 0.005%, N
b: 0 to 0.1%, V: 0 to 0.3%, Al: 0.01
%, The balance consists of Fe and impurities, P in the impurities is 0.03% or less, N is 0.008% or less, and O
(Oxygen) is 0.0025% or less, the value of fn3 represented by the following formula (3) is 0 or more, and the major axis of the nonmetallic inclusion in the longitudinal longitudinal section is L (μm), Diameter W
A steel material for machine structural use in which the maximum equivalent circular diameter fn2 of nonmetallic inclusions represented by the following formula (2) when the cumulative distribution function predicted by the extreme value statistical processing as (μm) is 99% is 30 μm or less. fn3 = Ti (%) + Zr (%)-3S (%)-3.4N (%) (3) fn2 = (πLW / 4) ^0.5 (2) "Longitudinal longitudinal section" (hereinafter,
The term “L section” means a plane cut through the center line of the steel material in a direction parallel to the rolling direction or the forging axis. In the L-section, when the cumulative distribution function predicted by the extreme value statistical processing is 99%, the maximum equivalent circular diameter fn2 of the inclusion represented by the above equation (2) indicates the value obtained as follows. .

After the L section of a test piece taken from a steel material is mirror-polished, the polished surface is used as a surface to be measured, and the magnification of an optical microscope is set to 400 times. 50 fields in accordance with “5. Microscopic test method by dot calculation method” in “Microscopic test method”, and the major axis of each inclusion is L (μm) and the minor axis is W (μm). (πLW / 4) Find the one with the maximum value of ^0.5 .

The above-obtained 50 (πLW / 4)
^Rearrange the values of ^0.5 in ascending order and assign
LW / 4) ^0.5 _j (where j = 1 to 50), and the cumulative distribution function F _j = 100 (j / 51) for each j
Calculate (%).

The scaling variable y _j = −log _e (−log
_e (j / 51)) on the vertical axis and (πLW / 4) ^0.5 on the horizontal axis.
_Draw a graph with _j taken and find the approximate straight line by the least squares method.

From the straight line obtained above, when the cumulative distribution function F _j becomes 99% (that is, when the standardized variable y _j ≒
The value of (πLW / 4) ^0.5 _j at the time of 4.6) is read, and this is read as the maximum equivalent circular diameter fn2 = (πLW / 4) ^0.5
And Hereinafter, those described in the above (1) and (2) are referred to as the invention of (1) and the invention of (2), respectively.

The present inventors have conducted various studies to solve the above-mentioned problems, and have obtained the following findings.

(A) In the L section, when the cumulative distribution function predicted by the extreme value statistical processing of all inclusions is 99%, the maximum equivalent circular diameter fn2 of the inclusion represented by the above equation (2) is 3
When it is 0 μm or less, it is possible to prevent a decrease in cold workability due to inclusion starting points, and also to prevent a decrease in fatigue strength when a stress perpendicular to the rolling direction or the wrought axis acts. Then, as a result of further study, the following matters became clear. (B) Maximum equivalent circular diameter fn of the inclusion in (a) above
In order to make 2 smaller than or equal to 30 μm, first, it is only necessary to prevent the generation of coarse MnS.

(C) The content of Al is 0.01% by mass.
Ti, which forms sulfide more stably than Mn
And Zr are added, and if the value of fn1 represented by the formula (1) or the value of fn3 represented by the formula (3) is set to 0 or more, Ti sulfide or Zr sulfide is formed. Coarse Mn
The formation of S can be prevented, and the generated Ti sulfide and Z
r Sulphide is fine. The Ti sulfide and Zr sulfide referred to in the present invention include not only mere sulfides of Ti and Zr but also “carbosulfides”.

(D) Al forms an oxide in steel and easily forms clusters.
By setting the content to 1% or less and adding an appropriate amount of Ti or Zr, formation of an oxide cluster can also be prevented. (E) When the content of N is 0.008% or less by mass%, formation of coarse TiN or ZrN can be prevented.

The present invention has been completed based on the above findings.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, "%" of the content of each element means "% by mass". (A) Chemical composition of steel C: C enhances the strength of the entire surface-hardened part,
It is an effective element for securing the fatigue strength. However, if the content is less than 0.10%, the effect of addition is poor. On the other hand, when C is contained in an amount of 0.30% or more, carburizing and nitrocarburizing treatment (hereinafter, may be collectively referred to as “carburizing treatment”).
In some cases, the toughness of the entire part may be reduced when the method is applied.
Therefore, when the content of C is 0.10% or more and 0.30%
Less than. The content of C is 0.15 to 0.25%.
It is preferable that

Si: Si is an element effective for improving the rolling fatigue life of a part that has undergone steel deoxidation and surface hardening treatment.
However, if the content is less than 0.05%, the effect of addition is poor. On the other hand, if it exceeds 1.0%, the carburizing property is lowered. Therefore, the content of Si is set to 0.05 to 1.0%. Note that the content of Si is preferably set to 0.1 to 0.5%.

Mn: Mn has the effect of improving the hardenability of steel. However, when the C content is 0.10% or more, 0.30%
In the case of the steel material according to the present invention, which is lower and lower, the effect is difficult to be obtained if the Mn content is less than 0.30%. On the other hand, M
n is an element that tends to segregate, and if its content exceeds 2.0%, the mechanical properties of the parts subjected to the surface hardening treatment vary. Therefore, the content of Mn is 0.30
2.0%. In addition, the content of Mn is 0.5 to 1.5.
% Is preferable. S: S combines with Ti, Zr and Mn to form a sulfide, and has an effect of enhancing the machinability of steel. However, if the content is less than 0.005%, the above effects cannot be obtained.
On the other hand, when a large amount of S is contained, the amount of sulfide-based inclusions becomes too large and the cold workability decreases, and in particular, when the S content exceeds 0.05%, the cold workability decreases. It becomes remarkable. Therefore, the content of S is 0.005 to 0.05%
And The S content is preferably set to 0.005 to 0.03%.

Ti, Zr: Ti and Zr are important alloy elements for preventing the formation of coarse MnS in the present invention. Further, Ti and Zr have an effect of deoxidizing and denitrifying steel.

In the case where Ti is added alone, the above-mentioned effects are surely obtained when the content is 0.05% or more.
Further, machinability is improved by Ti sulfide in which Ti is combined with S. As described above, the Ti sulfide in the present invention includes not only a simple sulfide of Ti but also a “carbosulfide”. However, even if a large amount of Ti is contained in excess of 0.2%, the effect of preventing the formation of coarse MnS due to the formation of Ti sulfide is saturated, and the deoxidizing and denitrifying effects of steel are also saturated. Get out. Therefore, in the invention of (1), the content of Ti is set to 0.05 to 0.2%. In the case of the invention (1), the content of Ti is preferably set to 0.05 to 0.15%.

On the other hand, the above effect can be reliably obtained even when the value of Ti (%) + Zr (%) is 0.05% or more with respect to the contents of Ti and Zr. However, Ti (%) + Zr
Even when Ti and Zr exceeding 0.2% (%) are contained, the effect of preventing the formation of coarse MnS due to the formation of Ti sulfide and Zr sulfide is saturated, and the deoxidation and denitrification of steel are effected. Is saturated, so that the cost increases. In addition, Ti (%) + Z
Since it is sufficient that the value of r (%) is 0.05 to 0.2%, it is not always necessary to contain Ti and Zr in a composite manner. In the case where Zr is not added, the invention of the above-mentioned (1) is obtained. In this case, if the content of Ti exceeds 0.2%, the fatigue strength may be significantly deteriorated. When Ti is not added, that is, when Zr is added alone, even if Zr is contained in a large amount exceeding 0.2%, the effect of preventing the formation of coarse MnS due to the formation of Zr sulfide saturates, and Since the deoxidizing and denitrifying effects are saturated, the cost increases.
Therefore, in the invention of (2), the contents of Ti and Zr are both 0.2% or less, and Ti (%) + Zr
The value of (%) was set to 0.05 to 0.2%. (2)
In the case of the invention of (1), the value of Ti (%) + Zr (%) is 0.05
It is preferably set to 0.15%. As described above, the Zr sulfide referred to in the present invention includes not only a simple sulfide of Zr but also a “carbosulfide”. Cu: Cu need not be added. If added, it has the effect of increasing the hardenability. To ensure this effect, C
It is preferable that the content of u is 0.05% or more,
On the other hand, Cu deteriorates hot workability, and particularly when the Cu content exceeds 0.50%, the hot workability may be significantly deteriorated. Therefore, the content of Cu is set to 0 to 0.50%.

Cr: Cr need not be added. If added, it has the effect of increasing the hardenability. To ensure this effect, it is preferable that the content of Cr be 0.05% or more, but on the other hand, Cr increases the deformation resistance and deteriorates the cold workability. Is 0.10%
In the case of the steel material according to the present invention having a low content of less than 0.30% as described above, when the content of Cr exceeds 2.0%, the cold workability significantly decreases. Therefore, if the content of Cr is 0 to
2.0%. The upper limit of the Cr content is preferably 1.5%.

When Cu and Cr are added for the purpose of improving hardenability, one or more of these elements may be added.

Ni: Ni may not be added. If added, it has the effect of increasing the fatigue strength of components that have been subjected to surface hardening, especially carburizing. To ensure this effect, the content of Ni is preferably set to 0.05% or more. However, when the content exceeds 3.5%, the machinability is significantly reduced. Therefore, the content of Ni is 0 to
3.5%. The upper limit of the Ni content is preferably 2.5%. Mo: Mo may not be added. If added, it has the effect of increasing the fatigue strength of components that have been subjected to surface hardening, especially carburizing. To ensure this effect, it is preferable that the content of Mo be 0.05% or more. However, when the content exceeds 1.0%, the machinability is significantly reduced. Therefore, the content of Mo is set to 0 to 1.0%. The upper limit of the Mo content is preferably set to 0.8%. B: B may not be added. If added, it has the effect of increasing the fatigue strength of components that have been subjected to surface hardening, especially carburizing. To ensure this effect, B should be 0.0
The content is preferably 010% or more. But,
Even if the content exceeds 0.005%, the above effect is saturated and the cost is increased. Therefore, the content of B is set to 0 to 0.005%. The upper limit of the B content is 0.1.
004% is preferable.

When Ni, Mo, or B is added for the purpose of increasing the fatigue strength of a part that has been subjected to a surface hardening treatment, especially a carburizing treatment, one or more of these elements may be added. Nb: Nb may not be added. If added, it has the effect of forming carbonitrides to refine the austenite grains and increasing the fatigue strength. In order to surely obtain this effect, the content of Nb is preferably set to 0.01% or more. However, if the content exceeds 0.1%, the above effect is saturated and the cost is increased, and the machinability is also reduced. Therefore, the content of Nb was set to 0 to 0.1%. The upper limit of the Nb content is preferably set to 0.06%. V: V may not be added. If added, it has the effect of forming carbonitrides to refine the austenite grains and increasing the fatigue strength. To ensure this effect, V should be 0.0
The content is preferably 3% or more. However, 0.
If the content exceeds 3%, the above effect is saturated and the cost is increased, and the machinability is also reduced. Therefore, the content of V is set to 0 to 0.3%. The upper limit of the V content is preferably set to 0.2%. When V and Nb are added for the purpose of refining austenite grains and increasing fatigue strength, one or more of these elements may be added.

Al: Al may not be added. Even when Al is added for the purpose of deoxidation or the like, in the present invention, it is extremely important to suppress the Al content to 0.01% or less. That is, the content of Al is suppressed to 0.01% or less, the content of Ti, Zr, and S is set in the range described above. Further, the value of fn1 represented by the above formula (1) and the value of (3)
By setting the value of fn3 represented by the formula to 0 or more, Ti sulfide and Zr sulfide become finer, formation of coarse MnS is prevented, and formation of oxide clusters is also prevented. is there. If the Al content exceeds 0.01%, fn
Since the value of 2 may exceed 30 μm, the fatigue strength and the cold workability may be significantly reduced even if other conditions are satisfied. Therefore, the content of Al is set to 0.01
% Or less. The upper limit of the Al content is 0.008%
It is preferable that

In the present invention, the contents of P, N and O (oxygen) as impurity elements are limited as follows.

P: P segregates at the grain boundaries and lowers toughness. In particular, when the content of P exceeds 0.03%, the toughness is significantly reduced. Therefore, P as an impurity element
Was 0.03% or less. Note that the content of P as an impurity element is preferably set to 0.02% or less.

N: N increases the deformation resistance and lowers the cold workability, and combines with Ti and Zr to form coarse TiN and Z.
The generation of rN lowers the fatigue strength. In particular, when the content of N exceeds 0.008%, even if the value of fn1 or fn3 is 0 or more, the value of fn2 may exceed 30 μm. May be significant. Therefore, the content of N as an impurity element is set to 0.008% or less. Note that the content of N as an impurity element is preferably set to 0.006% or less.

O (oxygen): O (oxygen) forms an oxide and is present in steel, and reduces fatigue strength and cold workability.
In particular, when the O content exceeds 0.0025%, fn1
Alternatively, even when the value of fn3 is 0 or more, the value of fn2 may exceed 30 μm, so that the fatigue strength and the cold workability may significantly decrease. Therefore,
The content of O as an impurity element was set to 0.0025% or less. The content of O as an impurity element is 0.00
It is preferable that the content be 15% or less.

Fn1, fn3: In the invention of (1),
The contents of Ti, S, and Al are set in the ranges described above.
By setting the value of fn1 represented by the formula (1) to 0 or more, the Ti sulfide becomes fine, the formation of coarse MnS is prevented, and the formation of oxide clusters can be prevented. In addition, excellent machinability and cold workability can be ensured, and a decrease in fatigue resistance can be prevented. f
When the value of n1 is less than 0, generation of coarse MnS cannot be suppressed.
The maximum equivalent circular diameter fn2 of the inclusion represented by the equation (2) is 30
Since the thickness cannot be reduced to μm or less, a significant decrease in fatigue strength occurs. Further, the cold workability is also reduced. For this reason, the value of fn1 in the invention of (1) is defined as 0 or more. It should be noted that under the contents of Ti, S, Al, N and O in the range already described, the refinement of Ti sulfide and the coarse MnS
In order to stably secure the actions of preventing the formation of oxides and the formation of oxide clusters, as well as ensuring better machinability and cold workability, and preventing deterioration in fatigue resistance,
The value of fn1 is preferably 0.01 or more.
In the invention of (2), the contents of Ti, Zr, S, and Al are set to the above-mentioned ranges, and further, f is expressed by the above formula (3).
By setting the value of n3 to 0 or more, Ti sulfide and Zr sulfide become finer, coarse MnS is prevented from being formed, and the formation of oxide clusters can be prevented. Cold workability can be ensured, and a decrease in fatigue resistance can be prevented. The value of fn3 is 0
If it is less than 30, the generation of coarse MnS cannot be suppressed, and therefore, in the L cross section, the maximum equivalent circular diameter fn2 of the inclusion represented by the formula (2) cannot be reduced to 30 μm or less. Significant reduction in fatigue strength occurs.
Further, the cold workability is also reduced. For this reason, the value of fn3 in the invention of (2) is defined as 0 or more. It should be noted that, under the contents of Ti, Zr, S, Al, N and O in the range already described, the refinement of Ti sulfide and Zr sulfide and the coarse M
In order to stably secure the actions of preventing the generation of nS and the formation of oxide clusters, as well as to ensure better machinability and cold workability, and to prevent a decrease in fatigue resistance, fn3 Is preferably 0.01 or more. (B) Inclusions In addition to the chemical components described in the above (A), good surface hardening treatment is applied to steel for machine structural use only by controlling the size of all inclusions such as sulfides, oxides, and nitrides. Properties, cold workability and machinability, and also imparts good fatigue resistance properties, especially good fatigue resistance properties when stress is applied perpendicular to the rolling direction or the forging axis. can do. That is, in the L section, the cumulative distribution function predicted by the extreme value statistical processing of all inclusions is 99.
% Maximum equivalent circular diameter f of the inclusion represented by the formula (2)
When n2 is 30 μm or less, a decrease in cold workability due to the inclusion starting point is prevented, and a decrease in fatigue strength when stress perpendicular to the rolling direction or the wrought axis acts is also prevented.

When the maximum equivalent circular diameter fn2 of the inclusion represented by the above-mentioned formula (2) exceeds 30 μm in the L section, the starting point of the inclusion is destroyed and the cold workability (particularly the critical upsetting ratio) is increased. ) Is reduced, and the fatigue strength is also significantly reduced.

Therefore, in the L section, the cumulative distribution function predicted by the extreme value statistical processing of all inclusions is 99%
Maximum equivalent circular diameter fn of the inclusion represented by the above equation (2)
2 was defined as 30 μm or less. It is preferable that the maximum equivalent circular diameter fn2 of the inclusion represented by the formula (2) is 20 μm or less.

It should be noted that even if the chemical composition range described in the above item (A) and the value of fn1 represented by the formula (1) and the value of fn3 represented by the formula (3) are satisfied, the oxides may not be satisfied. And the like, the maximum equivalent circular diameter fn2 of the aforementioned inclusions
In some cases, for example, after melting the converter, it is sufficiently deoxidized with an element having a deoxidizing effect such as Si, and then subjected to out-of-pile refining by a usual method.
It is desirable to adopt a steelmaking method in which i or Zr is added.

Hereinafter, the present invention will be described with reference to examples.

[0044]

EXAMPLES (Example 1) Steels having the chemical compositions shown in Tables 1 to 3 were melted using a test melting furnace. In Tables 1 to 3, fn1 and fn3 are described as values calculated by the equation (3) as fn.

Steels A1 to A27 and Steel B in Tables 1 to 3
24 to 26 are steels whose components satisfy the conditions specified in the present invention,
Steel B1 to B23, Steel B27 and Steel B in Tables 2 and 3
Reference numeral 28 denotes a steel of a comparative example in which any of the components deviated from the conditions specified in the present invention.

Of the above steels, steels A1 to A27, steel B4,
Steel B6 and steel B11 were melted using a vacuum melting furnace.
Among these steels, steels other than steel B4 were produced by deoxidizing with Al, Si or the like, and then adding at least one or more of Ti and Zr. After steel B4 is deoxidized with Al and Si, Ti
No Zr was added.

On the other hand, steel B1 to B3, steel B5, steel B7 to B
10 and steels B12 to B28 were melted using an atmospheric melting furnace. Among these steels, steels excluding steel B1, steel B2, steel B12, steel B13, steel B18, steel B22 and steel B23, after deoxidizing with Al, Si, etc., at least one or more of Ti, Zr. Manufactured with addition. Steel B1, Steel B2, Steel B
12, steel B13, steel B18, steel B22 and steel B23 are A
It was manufactured without adding Ti or Zr after deoxidation with 1, Si or the like.

[0048]

[Table 1]

[Table 2]

[Table 3] Next, these steels were made into billets by a usual method, and then heated to 1200 ° C, and then from 1200 ° C to 950 ° C.
At 60 ° C. to obtain a round bar having a diameter of 60 mm.

Each round bar having a diameter of 60 mm obtained as described above is divided, one of the bars is subjected to spheroidizing annealing by a usual method, and a 30 mm-long test piece is cut out from each round bar to obtain "D".
/ 4 part (D is the diameter of a round bar, that is, 60 mm) "was measured for Rockwell B hardness (hereinafter referred to as HRB hardness).

Further, the above-mentioned diameter of 60 m subjected to spheroidizing annealing
A test piece for inclusion measurement is cut out from each round bar of m in accordance with FIG. 1 of JIS G 0555, and the value of the maximum equivalent circular diameter fn2 of the inclusion represented by the formula (2) is determined by the method described above. I asked.

That is, the L of the test piece taken from the steel material
After mirror-polishing the cross section, the polished surface was used as the surface to be inspected, and the magnification of the optical microscope was set to 400 times, and “5. Point” in “Microscope test method for non-metallic inclusions in steel” specified in JIS G 0555. 50 field of view measurement according to "Microscopic test method by arithmetic",
Assuming that the major axis of each inclusion is L (μm) and the minor axis is W (μm), the one that maximizes the value of (πLW / 4) ^0.5 in each field of view is determined. πL
W / 4) The values of ^0.5 are rearranged in ascending order to obtain (πLW / 4) ^0.5 _j (where j = 1 to 50), and the cumulative distribution function F _j = 100 for each j
(J / 51) (%) was calculated. Further, the scaling variable y
Write a graph with _j = -log _e (-log _e (j / 51)) on the vertical axis and (πLW / 4) ^0.5 _j on the horizontal axis,
An approximate straight line is obtained by the least squares method. Finally, from the straight line obtained above, when the cumulative distribution function F _j becomes 99% (that is, when the normalized variable y _j ≒ 4.6), (π
(LW / 4) ^0.5 _j was read, and this was taken as the maximum equivalent circular diameter fn2 = (πLW / 4) ^0.5 .

As an example of obtaining the maximum equivalent circular diameter fn2, FIG. 1 shows the case of steel A5 and steel B3.

Further, the cold workability was investigated as follows. That is, a notched cold-working test piece having a diameter of 15 mm and a length of 22.5 mm was prepared from a round bar having a diameter of 30 mm subjected to spheroidizing annealing, using a 500-t high-speed press machine in a usual manner. A cold restraint type upsetting test was performed, and the upsetting rate at which cracking occurred was measured to investigate cold workability.

The upsetting test was performed five times under each condition until the upsetting ratio reached 75%, and the minimum upsetting ratio at which three or more cracks occurred was evaluated as the limit upsetting ratio.

The machinability was also investigated by a drilling test. That is, a diameter of 60 m after the spheroidizing annealing was performed.
m round bar having a length of 25 mm was used, and “R / 2 parts (R is the radius of the round bar, ie, 30 m
m)), a through-hole was made in the length direction, and the number of through-holes when drilling became impossible due to abrasion of the cutting edge was counted to evaluate the machinability.

The drilling conditions were JIS high speed tool steel SKH5.
Using a 5 mm diameter straight shank drill, a water-soluble lubricant was used at a feed rate of 0.15 mm / rev and a rotation speed of 980 rpm.

Tables 4 and 5 collectively show the results of the above tests.

[0058]

[Table 4]

[Table 5] From Tables 4 and 5, it can be seen that as the hardness increases, the critical upsetting ratio as cold workability decreases and the number of through holes as machinability decreases, but the maximum equivalent of the chemical composition of steel and inclusions. In the case of test numbers 1 to 27 of the examples of the present invention in which the circular diameter fn2 satisfies the conditions specified in the present invention, test numbers 28 to 5 as comparative examples
As compared with No. 5, it is clear that the critical upsetting ratio at which cracks as deformability occur is high, the cold workability is excellent, and the machinability is also good.

(Example 2) The remaining one of the round bars having a diameter of 60 mm divided in Example 1 was heated to 925 ° C. and maintained for 1 hour, and allowed to cool in the air.

A test piece for measuring inclusions is cut out from each of the round bars having a diameter of 60 mm that has been subjected to the above-described processing in accordance with FIG. 3 of JIS G 0555, and is represented by the above-mentioned formula (2) by the method described above. The value of the maximum equivalent circular diameter fn2 of the inclusion was determined. In addition, a smooth Ono-type rotating bending fatigue test piece (parallel portion having a diameter of 6 mm and a length of 25 mm) was prepared, and the fatigue resistance was investigated.

FIG. 2 shows a method of cutting out a sample from each round bar. As shown in FIG. 2, for each round bar, “R / 2” in a direction parallel to the training axis (hereinafter, referred to as “L direction”).
Samples were taken from the center of the “section” and the direction perpendicular to the forging axis (hereinafter referred to as “T direction”). As shown in FIG. 3, both ends of each sample taken from the T direction were joined by electron beam welding, and together with the sample taken from the L direction, finished to the size of a predetermined smooth Ono-type rotating bending fatigue test piece. .

The smooth Ono-type rotating bending fatigue test piece obtained as described above was subjected to carburizing and tempering under the conditions shown in FIG. In FIG. 4, "Cp" means carbon potential, "OQ" means oil quenching, and "AC" means air cooling. Next, the chuck portion of the smooth Ono-type rotating bending fatigue test piece obtained as described above was masked, and the parallel portion and the R portion of the test piece were shot peened.

In the shot peening process, the hardness was set to Rockwell C hardness of 60 in an impeller type machine.
The projection speed was 90 m / sec, the projection density was 540 kgf / m ³ , the arc height was 0.79 mm, and the garbage was 300%, using a projection material (trade name: SB-6Pm).

[0064] Ono rotating bending fatigue test expression conducted in air at room temperature was measured 10 ⁷ times fatigue strength of each sample (fatigue limit). Tables 6 and 7 show the maximum equivalent circular diameter fn2 and the results of the fatigue test.

[0065]

[Table 6]

[Table 7] From Tables 6 and 7, from the test numbers 56 to 82 of the examples of the present invention in which the chemical composition of steel and the maximum equivalent circular diameter fn2 of the inclusion satisfy the conditions specified in the present invention, the fatigue strength exceeding 719 MPa also in the T direction. In addition, the decrease in the fatigue strength in the T direction relative to the fatigue strength in the L direction is small. Therefore, it is clear that the fatigue strength when the stress perpendicular to the forged axis acts is hard to decrease.

On the other hand, Test Nos. 83 to 1 of Comparative Examples
In the case of 10, the fatigue strength in the T direction is significantly lower than the fatigue strength in the L direction. In the case of Test Nos. 106 to 108, although the chemical components of the steels 24 to 26 satisfy the conditions specified in the present invention, the values of fn2 deviate from the conditions specified in the present invention. The fatigue strength in the T direction is significantly lower than the fatigue strength in the L direction as compared with the case of (1).

[0067]

The steel material for machine structural use of the present invention is excellent in surface hardening properties such as carburizing and quenching properties, cold workability and machinability, as well as fatigue resistance properties, especially rolling direction and forged shaft. It also has excellent fatigue resistance characteristics when a vertical stress is applied, and prevents deterioration of the fatigue resistance characteristics when a vertical stress is applied to the rolling direction and the wrought shaft. It can be used as a material for parts to be processed.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of a maximum equivalent circular diameter fn2 of a steel material according to an embodiment.

FIG. 2 is a diagram illustrating a method of cutting out a sample of a smooth Ono-type rotating bending fatigue test piece from a direction parallel to the wrought axis (L direction) and a direction perpendicular to the wrought axis (T direction).

FIG. 3 is a diagram for explaining that both ends of a sample cut out from a direction perpendicular to a wrought axis are joined by electron beam welding to complete a predetermined smooth Ono-type rotating bending fatigue test piece.

FIG. 4 is a view showing a heat pattern of carburizing quenching and tempering applied to a smooth Ono type rotating bending fatigue test piece in Example 2.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masayuki Horimoto 1 Konomi-cho, Kokurakita-ku, Kitakyushu-shi, Fukuoka Prefecture (72) Inventor Takefumi Sato 1 Konomi-cho, Kitakyushu-shi, Fukuoka Sumitomo Metals Kokurauchi (72) Inventor Daisuke Suzuki 1 Konomi-cho, Kokurakita-ku, Kitakyushu-shi, Fukuoka Prefecture Sumitomo Metals Kokuranai

Claims (2)

[Claims] 1. A chemical composition in mass%, C: 0.10% or more and less than 0.30%, Si: 0.05 to 1.0%, Mn:
0.30 to 2.0%, S: 0.005 to 0.05%, T
i: 0.05 to 0.2%, Cu: 0 to 0.50%, C
r: 0 to 2.0%, Ni: 0 to 3.5%, Mo: 0 to 0%
1.0%, B: 0 to 0.005%, Nb: 0 to 0.1
%, V: 0 to 0.3%, Al: 0.01% or less, the balance being Fe and impurities, P in the impurities is 0.03% or less, N is 0.008% or less, and O (Oxygen) is 0.0025% or less, and fn represented by the following formula (1)
The value of 1 is 0 or more, and the cumulative distribution function predicted by the extreme value statistical processing with the major axis of the nonmetallic inclusion in the longitudinal longitudinal section being L (μm) and the minor axis being W (μm) is 9
A steel material for machine structure in which the maximum equivalent circular diameter fn2 of the nonmetallic inclusion represented by the following formula (2) at 9% is 30 μm or less. fn1 = Ti (%)-3S (%)-3.4N (%) (1) fn2 = (πLW / 4) ^0.5 (2) 2. Chemical composition in mass%, C: 0.10% or more and less than 0.30%, Si: 0.05-1.0%, Mn:
0.30 to 2.0%, S: 0.005 to 0.05%, T
i: 0.2% or less, Zr: 0.2% or less, and Ti
(%) + Zr (%): 0.05 to 0.2%, Cu: 0 to 0
0.50%, Cr: 0 to 2.0%, Ni: 0 to 3.5
%, Mo: 0 to 1.0%, B: 0 to 0.005%, N
b: 0 to 0.1%, V: 0 to 0.3%, Al: 0.01
%, The balance consists of Fe and impurities, P in the impurities is 0.03% or less, N is 0.008% or less, and O
(Oxygen) is 0.0025% or less, the value of fn3 represented by the following formula (3) is 0 or more, and the major axis of the nonmetallic inclusion in the longitudinal vertical section is L (μm), Diameter W
A steel material for machine structural use in which the maximum equivalent circular diameter fn2 of nonmetallic inclusions represented by the following formula (2) when the cumulative distribution function predicted by the extreme value statistical processing as (μm) is 99% is 30 μm or less. fn3 = Ti (%) + Zr (%)-3S (%)-3.4N (%) (3) fn2 = (πLW / 4) ^0.5 (2)