JP2989766B2 - Case hardened steel with excellent fatigue properties and machinability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technology for modifying case hardened steel used as a material for machine parts subjected to surface hardening by carburizing and quenching or carburizing / nitriding quenching, and particularly to gears and shafts for automobiles and the like. The present invention relates to a case hardened steel which exhibits excellent fatigue characteristics as a mechanical component such as a constant velocity joint and has excellent machinability.

[0002]

2. Description of the Related Art Case hardening steel is applied to, for example, parts requiring high fatigue characteristics among mechanical parts such as automobiles and construction machines, and is used after being subjected to a surface hardening treatment such as carburizing treatment or carburizing / nitriding treatment. Can be Among them, a gear is a representative one. In recent years, in order to increase the weight and output of an automobile, a further increase in strength is required.

By the way, in order to achieve high strength as a gear, practical use as a steel for a gear requires bending fatigue characteristics, pitting life, and impact fatigue characteristics. Among them, the life of a gear is particularly limited. Since the rate is often determined by the bending fatigue characteristics, application of high-strength case hardening steel (Japanese Patent Application Laid-Open No. 1-306545) and surface hardening treatment by shot peening after carburizing (Japanese Patent Application Laid-Open No. 1-306521)
No.) etc., a method of improving the bending fatigue characteristics has been adopted.

[0004] On the other hand, as the strength of case hardening steel has increased, deterioration of fatigue life due to coarse inclusions present in the steel has become noticeable.
In Japanese Patent Publication No. 35, there is also proposed a technique for improving fatigue properties by suppressing a large amount of oxide-based or nitride-based inclusions that adversely affect fatigue properties. However, for gears, etc., simply specifying the size of oxide-based inclusions or nitride-based inclusions does not always provide satisfactory fatigue characteristics, and depends on the form of oxide-based inclusions and sulfide-based inclusions. Has a remarkable change in fatigue properties.

[0005] In addition, in an actual case hardening part, an external force may act in an inclined direction of about 20 to 40 ° with respect to the rolling direction of steel, such as a helical gear, so that both vertical and horizontal eyes may be used. Excellent fatigue strength is required. In particular, the fatigue strength of the horizontal grain is more significantly affected by the oxide-based and sulfide-based composite inclusions than by hard granular inclusions such as Al ₂ O ₃ . Furthermore, when manufacturing gears, after forging, machining is performed by carbide turning, hobbing, etc., but after forging or normalizing as the amount of alloying elements is increased and the material strength is increased. Since the later hardness increases, the problem of machinability arises.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and an object thereof is to provide an excellent bending fatigue characteristic in both longitudinal and transverse directions, and to realize carbide turning. An object of the present invention is to provide a case hardened steel which is excellent in machinability such as hobbing and hobbing.

[0007]

The case hardening steel according to the present invention, which can solve the above problems, comprises: C: 0.10 to 0.30% Mn: 0.30 to 2.0% Si: 1. 0% or less (including 0%) S: 0.003 to 0.070% Al: 0.01 to 0.06% N: 0.003 to 0.03% O: 0.002% or less (0% The remainder: made of steel material that satisfies the requirements of Fe and inevitable impurities, and in a vertical section passing through the axis of the linear or rod-shaped rolled material, is parallel to the axis and 1/4 · D from the axis ( D represents the diameter of the rolled material) There are no more than 20 composite inclusions of 10 mm or more in diameter consisting of oxides and sulfides existing in a test area 100 mm ² including the imaginary distant line as the center line. And sulfide-based inclusions having a diameter of 3 μm or more and less than 10 μm existing in the same test area as described above Has a gist where there are 50 or more.

[0008] In the case hardening steel of the present invention, Ni: 0.20 to 4.5% as another element.
Cr: 0.20 to 2.5%, Mo: 0.05 to 1.0
%, Cu: at least one selected from the group consisting of 0.20 to 1.0%, or B:
0.0003-0.0050% and / or Ti:
0.003 to 0.05%, further V: 0.03 to 1.5
% And / or Nb: 0.005 to 1.5%, Ca: 0.0005 to 0.01%, Pb: 0.2% or less (excluding 0%), Te: 0.1% or less (Not including 0%), Zr: 0.1% or less (not including 0%), by further containing at least one selected from the group consisting of: it can.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reason for defining the chemical composition of the steel used in the present invention will be described. C: 0.10 to 0.30% C is an indispensable element for securing core hardness as a strengthening element, and must be contained at least 0.10% or more. However, if the content exceeds 0.30%, the toughness and machinability deteriorate, so the upper limit is set to 0.30%. A more preferred content of C is 0.1
The range is 0 to 0.25%.

Si: 1.0% or less (including 0%) Si not only functions effectively as a deoxidizing agent at the time of melting, but also acts as a strengthening element and contributes to improvement of core hardness. If it is too large, it impairs carburizing properties and promotes grain boundary oxidation to adversely affect bending fatigue properties. Therefore, the content should be kept at most 1.0% or less, preferably 0.30% or less.

Mn: 0.30 to 2.0% Mn is an element which effectively acts as a deoxidizing agent at the time of melting, and also enhances hardenability to increase the hardness of the surface layer and the core and contributes to the improvement of fatigue strength. In order to exert their effects effectively, the content must be 0.30% or more. But,
If the content is too large, the material becomes too hard and the workability and machinability deteriorate, so the content should be suppressed to 2.0% or less. The more preferred content of Mn is in the range of 0.30 to 1.6%.

S: 0.003 to 0.070% S is an element necessary as a machinability improving component, and must be contained at least 0.003% or more. However, if it is contained too much, the amount of sulfide inclusions and the formation of coarse composite inclusions of oxides and sulfides will be too large, and adversely affect the bending fatigue properties and impact properties of the cross-section in particular. Therefore, it should be suppressed to 0.070% or less, more preferably 0.035% or less.

Al: 0.01 to 0.06% Al also effectively acts as a deoxidizing component at the time of smelting, has the effect of suppressing the amount of composite inclusions formed, and also suppresses the growth of austenite crystal grains during heat treatment. It also has the effect of suppressing and increasing the toughness, and such an effect is effectively exhibited by containing 0.01% or more. However, their effects saturate at about 0.06%, and above that, the austenite crystal grains are rather coarsened and the toughness is deteriorated, so they must be suppressed to 0.06% or less. The more preferred content of Al is in the range of 0.015 to 0.040%.

N: 0.003 to 0.03% N combines with Al to form AlN and refines austenite crystal grains and contributes to improvement in toughness.
Below 3%, such effects cannot be expected, while 0.03%
%, It is easy to cause cracking during casting or hot working. In view of such advantages and disadvantages, the more preferable content of N is in the range of 0.003 to 0.02%.

O: 0.002% or less (including 0%) O becomes a source of oxide-based inclusions such as Al ₂ O ₃ and SiO ₂ and adversely affects fatigue characteristics and the life of a cemented carbide tool during cutting. In addition, the oxides serving as nuclei of the composite inclusions are coarsened to increase the size of the composite inclusions, and particularly to degrade the fatigue characteristics of the grain, so that at most 0.002% or less, more preferably 0.1% or less. It should be kept below 0015%.

The remaining components of the case hardening steel according to the present invention are Fe and unavoidable impurities. However, if necessary, the following elements may be added in appropriate amounts to further improve the properties of the case hardening steel. It is possible to improve.

Ni: 0.20 to 4.5%, Cr: 0.2
0 to 2.5%, Mo: 0.05 to 1.0%, Cu: 0.
At least one element selected from the group consisting of 20% to 1.0% These elements can be said to be the same in that they contribute to the improvement of hardenability, and their actions are described in detail below. . That is, Cu is an element that secures good hardenability and effectively acts to improve toughness, and also effectively acts to improve corrosion resistance. The effect is effectively exhibited by containing about 0.20% or more. However, if the content is too large, hot cracking is likely to occur and hot workability is impaired, so that the content is 1.0% or less, more preferably 0.60%.
It should be kept below. Ni is an element that secures the hardenability of the carburized portion, suppresses the formation of an incompletely quenched structure, and effectively acts to improve the toughness. Such an effect is effectively achieved by containing about 0.20% or more. The effect is saturated, but the effect is saturated at 4.5%, so that further addition is economically useless. The more preferred content of Ni is in the range of 0.20 to 2.0%.

[0018] When Cr is also contained in an amount of about 0.20% or more, it effectively acts to improve the hardenability. However, if it is too much, it not only inhibits carburization, but also generates a large amount of carbides, thereby generating cracks originating from the carbides. Therefore, it should be suppressed to 2.5% or less because it promotes the development of the steel sheet and adversely affects the bending fatigue properties and toughness. The more preferred content of Cr is 0.30.
２．０2.0%. Mo exists in a solid solution state without forming an oxide on the surface layer of the carburized layer, and enhances the hardenability of the carburized layer to reduce the incompletely quenched structure. It is an element that effectively acts on the improvement of toughness, and these actions are effectively exhibited by containing about 0.05% or more. However, if the content is too large, the machinability will be adversely affected, so it must be suppressed to 1.0% or less. The more preferred content of Mo is in the range of 0.05 to 0.50%.

V: 0.03-1.5% and / or N
b: 0.005 to 1.5% These elements contribute to the improvement of toughness by refining the crystal grains, and also increase the surface hardness by precipitating carbides and carbonitrides during carburizing or carbonitriding to increase the bending fatigue properties. It is the same element in that it has an effect of improving the pitting resistance. That is, V generates carbides and carbonitrides to refine the crystal grains and contributes to the improvement of toughness, and also increases the amount of carburizing or generates fine carbides and carbonitrides by nitriding to improve tempering softening resistance. In addition, it has an effect of improving bending fatigue characteristics and pitting resistance, and these effects can be effectively exhibited by containing about 0.03% or more, preferably 0.2% or more. However, when the V content becomes too large, A ₃ ,
The A ₁ transformation point is greatly reduced, and the core is not sufficiently gamma-converted.
It should be kept below 5%, more preferably below 1.0%. Nb is also a carbide and carbonitride forming element, like V, and contributes to the improvement of toughness by refining crystal grains, and is an element effective for improving bending fatigue properties and pitting resistance by improving surface hardness. Yes, the effect is effectively exerted by containing about 0.005% or more, more preferably 0.01% or more. However, the effect saturates at 1.5%, and if added more than that, it is only economically disadvantageous. Therefore, the effect should be suppressed to 1.5% or less, more preferably 1.0% or less. .

B: 0.0003% to 0.0050% and / or Ti: 0.003% to 0.05% B is an element effective for increasing the hardenability and increasing the grain boundary strength. 0.0003% or more,
More preferably, it is effectively exhibited by containing 0.0005% or more. But their effect is 0.00
Since it saturates at 50%, further addition is useless,
Preferably, it is desirable to suppress the content to about 0.0035% or less. Further, Ti effectively acts on deoxidation and denitrification of steel, and also has an effect of stably and effectively exerting the effect of improving the hardenability by adding B. Such an effect is 0.0%.
Effectively exhibited by containing at least 03%.
However, if the content is too large, a large amount of hard inclusions such as TiN are generated, which tends to deteriorate toughness and bending fatigue properties. Therefore, the content is preferably suppressed to 0.05% or less, more preferably 0.04% or less. .

Ca: 0.0005-0.01%, Pb:
0.2% or less (excluding 0%), Te: 0.1% or less (excluding 0%), Zr: 0.1% or less (excluding 0%), at least selected from the group consisting of: Kind One of these elements is the same in that it contributes to improving machinability. In addition, Ca, Te, and Zr also have an effect of spheroidizing inclusions to improve anisotropy. That is, Ca
Has an effect of generating sulfide-based inclusions together with Mn, spheroidizing the inclusions, improving anisotropy, and enhancing machinability without deteriorating toughness or bending fatigue properties. The effect is about 0.0005% or more, more preferably 0.000%.
Effectively exhibited by containing at least 8%. However, if it is too large, a large amount of coarse composite inclusions in which sulfide-based inclusions are bound around coarse CaS or oxide-based inclusions and deteriorates the bending fatigue properties. Preferably, it should be suppressed to 0.005% or less. Pb also effectively acts as a machinability improving element, but if it is too much, the bending fatigue characteristics and the pitting life are significantly deteriorated. Therefore, it should be suppressed to 0.2% or less, more preferably 0.1% or less.

Te forms Mn-Te and coexists around MnS, suppresses deformation of MnS during hot rolling, promotes spheroidization of MnS, and does not deteriorate the toughness and bending fatigue characteristics of steel. Has the effect of enhancing machinability,
If it exceeds 1%, the bending fatigue characteristics are rather deteriorated due to an increase in the amount of nonmetallic inclusions, so that it must be suppressed to 0.1% or less. Zr is also MnS during hot rolling.
In addition to contributing to the spheroidization of MnS by suppressing deformation and effectively acting to improve anisotropy, it also has the effect of increasing machinability without deteriorating toughness or bending fatigue properties, but too much ZrO
_{Since the} amount of non-metallic inclusions such as ₂ increases to deteriorate the bending fatigue properties in reverse, it must be suppressed to 0.1% or less.

Next, the reasons for limiting the size and number of inclusions, which are important components in the present invention, will be described in detail. The case hardened steel of the present invention is a rolled material obtained by rolling a steel material satisfying the requirements for the component composition into a wire or a rod, in a vertical cross section passing through the axis, being parallel to the axis and 1 / from the axis. 4 ・ D (D
Represents the diameter of the rolled material) Of the inclusions present per 100 mm ^{2 of the} test area including the imaginary line as a center line, there are 20 composite inclusions of oxide and sulfide with a diameter of 10 μm or more. Or less, and a diameter of 3 μm or more and 10
A major feature is that there are 50 or more sulfide-based inclusions of less than μm.

The above requirement is a requirement reached as a result of repeated studies on the influence of inclusions on the fatigue characteristics of case hardened steel from various angles. It is important to control the number of composite inclusions of a specific size of oxide and sulfide among non-metallic inclusions in order to ensure satisfactory performance with respect to characteristics. As described above, in a longitudinal section passing through the axis of the rolled material, the area exists in a test area 100 mm ^{2 which} includes, as a center line, an imaginary line parallel to the axis and 1/4 · D away from the axis. It is based on the new finding that, if the number of composite inclusions having a diameter of 10 μm or more is reduced to 20 or less, excellent characteristics, particularly in fatigue characteristics of the eyes, will be stably exhibited.

FIG. 1 shows that the number of composite inclusions having a diameter of 10 μm or more in the test area of 100 mm ² was determined by the fatigue strength of the vertical and horizontal lines from among many experimental data including the examples described later. As is clear from this figure, the number of composite inclusions of the above-mentioned size is less than 20, especially 15 or less, showing excellent fatigue strength. On the other hand, it can be confirmed that when the number exceeds 20, the fatigue strength becomes extremely poor.

It has been conventionally known that even in case hardened steel, the inclusions present therein have an adverse effect on the fatigue strength, but this has been confirmed as a qualitative and sensory tendency. However, no specific and quantitative tendency has been confirmed as to what kind of inclusions among various inclusions adversely affect the fatigue strength of the eyes. As a result, as described above, the conventional case-hardened steel has a particularly unstable fatigue strength at the side edges,
For example, when practically used as a helical gear, insufficient fatigue life often occurred, but according to the present invention, the number of composite inclusions of a specific size existing per specific test area as described above was evaluated. By using as a reference, it is possible to reliably obtain case hardened steel that exhibits excellent fatigue strength stably not only in the longitudinal direction but also in the lateral direction.

Next, the above requirements are defined as requirements for enhancing machinability and extending the life of the cutting tool without lowering the impact strength and bending fatigue strength of the cross section.
That is, to improve machinability, it is desirable to increase the number of sulfide-based inclusions.
Coarse ones exceeding μm significantly degrade the impact strength and bending fatigue strength, especially of grain, while fine sulfide-based inclusions less than 3 μm in diameter have little adverse effect on fatigue strength, but improve machinability. Hardly contributes to However, it is based on a new finding that sulfide-based inclusions having a diameter of 3 μm or more and less than 10 μm exhibit an excellent machinability improving effect with almost no adverse effect on the impact strength or bending fatigue strength of the grain. It is. However, in order to stably exert the excellent machinability improving effect as intended in the present invention, the absolute amount of the sulfide-based inclusion having the appropriate size is also important, and the amount of the inclusion is also quantified. As a result of further study, it was found that a specimen having the above-specified size of sulfide-based inclusions of 50 or more in the test area of 100 mm ² specified above can stably exhibit good machinability. confirmed.

From the viewpoint of improving machinability, the number of the sulfide-based inclusions is preferably as large as possible. However, if the number is too large, even if the sulfide-based inclusions have an appropriate size, the impact characteristics and the fatigue characteristics are not improved. Therefore, it is desirable that the number in the test area be suppressed to about 250 or less. A more preferable number is in the range of 50 to 250 in consideration of both machinability and physical properties such as fatigue characteristics (see FIG. 2).

Thus, according to the present invention, by specifying the component composition of the steel material and specifying the number of complex oxide and sulfide-based inclusions of a characteristic size having a specific cross section as a surface to be inspected, It becomes possible to obtain a case hardened steel satisfying both the impact characteristics, fatigue characteristics and machinability of the horizontal grain.

When manufacturing parts such as gears using the case hardened steel of the present invention, after processing into a part shape in accordance with a conventional method, carburizing or carburizing / nitriding (carburizing or carburizing by gas, vacuum, plasma, etc.) (Nitriding) or nitrocarburizing treatment and, if necessary, shot peening or the like to harden the surface.

[0031]

EXAMPLES Next, the structure and operation and effect of the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples, and can be adapted to the spirit of the preceding and following examples. Of course, the present invention can be implemented with modifications within the scope, and all of them are included in the technical scope of the present invention.

Example 1 A steel material having the composition shown in Table 1 was used, and the size and amount of composite inclusions were changed by changing the cooling rate during casting.
In the example of the present invention, it was melted and cast using a 150 kg vacuum melting furnace, and then forged into a round bar having a diameter of 65 mm.
Rolled into round bars.

[0033]

[Table 1]

Thereafter, in order to examine the non-metallic inclusions in each round bar, as shown in FIG. 3, in a longitudinal section including the axis of each round bar, the diameter (vertical section) is parallel to the axis and from the axis. (Width of the surface) A sample having a width of 20 mm × 30 mm is cut out from a position including a virtual line 1/4 · D apart from 65 mm as a center line, and the composition, size, The number was checked. The measurement was performed in a continuous automatic operation at a magnification of 400 times, and the composition, size, and number of all nonmetallic inclusions existing per 100 mm ^{2 of the} test area were measured, and the average particle diameter [(major axis + minor axis) was obtained. / 2] is 10
The number of oxide-based and sulfide-based composite inclusions of μm or more was determined. As a result, the number of composite inclusions of 10 μm or more is
It was confirmed that the number was 11 in the present invention and 28 in the comparative example.

For each forged / rolled material having a diameter of 65 mm,
1200 ° C x 2 hours → solution treatment with air cooling and 900 ° C x
After 2 hours of air-cooling normalizing treatment, a sample of a cross section perpendicular to the rolling direction was cut out from the center of each round bar as shown in FIG. 4, and a smooth Ono-type rotary bending fatigue having the dimensions and shape shown in FIG. Test pieces were prepared. Both ends of each specimen were friction bonded,
Finished. Thereafter, carburizing and quenching and tempering were performed under the conditions shown in FIG. 6, and then a rotating bending fatigue test was performed.

FIG. 7 is a graph showing the relationship between the results of inclusion measurement obtained above and the results of a rotary bending fatigue test. As is clear from this graph, the number of composite inclusions having a size of 10 μm or more was 20 μm. In the present invention example in which the number is
It can be seen that the bending fatigue strength of the side grain is remarkably excellent and the variation is small as compared with the comparative examples exceeding the number.

Example 2 A steel material having the chemical composition shown in Table 2 was melted and cast and forged (or rolled) by the following methods. That is, steel No. 1
Nos. To 14, 16 to 18, and 20 to 23 are 150 kg vacuum melting furnaces. No. 24 was melted in a 10 kg vacuum furnace.
No. 15 was melted in an atmospheric furnace and then forged into a round bar having a diameter of 65 mm. 19 was cast with a 7-ton ingot and then rolled into a round bar having a diameter of 65 mm.

[0038]

[Table 2]

Thereafter, in order to examine non-metallic inclusions in each round bar, as shown in FIG. 3, in a vertical section including the axis of each round bar, an imaginary line 1 / 4.D away from the axis is shown. A sample having a width of 20 mm × 30 mm was cut out from a position including as a center line, and the composition, size, and number of nonmetallic inclusions present in the cross section were examined by EPMA in the same manner as in Example 1. The measurement was performed in a continuous automatic operation at a magnification of 400 times, and the composition, size, and number of all nonmetallic inclusions existing per 100 mm ^{2 of the} test area were measured, and the average particle diameter [(major axis + minor axis) was obtained. / 2] is 10 μm or more, and the number of oxide- and sulfide-based composite inclusions and the number of sulfide-based inclusions having an average particle diameter of 3 μm or more and less than 10 μm were determined.

Each forged / rolled material having a diameter of 65 mm is
After performing a solution treatment of 1200 ° C. × 2 hours → air cooling and a normalizing process of 900 ° C. × 2 hours → air cooling in the same manner as in Example 1, as shown in FIG. From the center of each round bar.
A longitudinal sample was cut out from a position 1 / 4.D away in parallel with the rolling direction to produce a smooth Ono-type rotating bending fatigue test piece. Regarding the test piece on the side, the both ends of the test piece were friction-welded and finished. Thereafter, carburizing and quenching and tempering were performed under the conditions shown in FIG. 6, and then a rotary bending fatigue test was performed.

Table 3 shows the results of inclusion measurement and bending fatigue test. The rotational bending fatigue test results were compared with the fatigue strength of 10 ⁷ times. Further, a cutting test was performed on each of the round bars subjected to the solution treatment and the normalizing process under the conditions shown in Table 4, and the results are also shown in Table 3.

[0042]

[Table 3]

[0043]

[Table 4]

From Tables 3 and 4, it can be considered as follows. Steel No. Examples 1 to 11 satisfy all of the requirements of the present invention. The composition of the steel material is appropriate, and the number of 10 μm composite inclusions is 10 or less and 3 μm.
Since there are 50 or more sulfide-based inclusions of m or more and less than 10 μm, excellent results are obtained in both bending fatigue strength and machinability (hard tool life).

On the other hand, steel No. Nos. 12 to 24 are comparative examples lacking any of the requirements specified in the present invention, and have a problem in either fatigue strength or machinability (carbide tool life) as described below.

Steel No. In No. 12, the hardness after normalizing is too high because the carbon content of the steel material is too large, the machinability is poor, and the life of the carbide tool is short. Steel No. Sample No. 13 has a low core hardness due to an insufficient carbon content of the steel material, and has low bending fatigue strength in longitudinal and transverse directions. Steel No. In No. 14, since the S content of the steel material is too large, the number of the coarse composite inclusions becomes too large, and the bending fatigue strength of the side grain is low.

Steel No. In No. 15, since the amount of oxygen in the steel material is too large, the size of the oxide-based inclusions increases, the number of coarse composite inclusions increases, and the bending fatigue strength in the longitudinal and transverse directions is low. Steel No. In Nos. 16 and 17, since the amount of Si or Cr in the steel material is too large, the carburization of the surface layer is inhibited, and the bending fatigue strength in the longitudinal and transverse directions is low.

Steel No. In No. 18, since the amount of Ca in the steel material was too large, the number of coarse composite inclusions increased, and the bending fatigue strength of the longitudinal and transverse eyes decreased. Steel No. In No. 19, since the S content in the steel material is insufficient and the casting speed is low, the number of sulfide-based inclusions is insufficient, and the machinability is deteriorated, so that a satisfactory carbide tool life cannot be obtained.

[0049]

The present invention is configured as described above, and specifies the chemical composition of the steel material and specifies the number of composite inclusions and sulfide-based inclusions of a characteristic size having a specific cross section as a test surface. By doing so, it is possible to provide a case hardened steel that satisfies both the impact characteristics, fatigue characteristics, and machinability of vertical and horizontal eyes.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the number of composite inclusions (10 μm or more) and bending fatigue strength.

FIG. 2 is a graph showing the relationship between the number of sulfide-based inclusions (3 to 10 μm) and the life of a carbide tool and the bending fatigue strength.

FIG. 3 is an explanatory view showing a sampling position for observing inclusions.

FIG. 4 is an explanatory view showing a sampling position of a bending fatigue test piece.

FIG. 5 is a view showing the dimensions and shape of a bending fatigue test piece.

FIG. 6 is a diagram showing a heat pattern of a carburizing and quenching process.

FIG. 7 is a graph showing the relationship between inclusion measurement results and rotational bending fatigue test results.

Continuation of the front page (56) References JP-A-4-350113 (JP, A) JP-A-6-299287 (JP, A)

Claims (5)

(57) [Claims] 1. C: 0.10 to 0.30% (hereinafter, mass% unless otherwise specified) Mn: 0.30 to 2.0% Si: 1.0% or less (including 0%) S: 0.003 to 0.070% Al: 0.01 to 0.06% N: 0.003 to 0.03% O: 0.002% or less (including 0%) Balance: Fe and inevitable impurities In a longitudinal section passing through the axis of a linear or rod-shaped rolled material, which is parallel to the axis and 1 / 4.D from the axis (D represents the diameter of the rolled material) No more than 20 composite inclusions of 10 μm or more in diameter consisting of oxides and sulfides are present in the test area 100 mm ² including the imaginary line as a center line, and the same test area as described above. Characterized in that there are 50 or more sulfide-based inclusions with a diameter of 3 µm or more and less than 10 µm Fatigue properties and machinability in excellent hardened steel to. 2. The steel material contains Ni: 0.20 as another element.
-4.5%, Cr: 0.20-2.5%, Mo: 0.0
The case hardening steel according to claim 1, which contains at least one selected from the group consisting of 5 to 1.0% and Cu: 0.20 to 1.0%. 3. The steel material further contains B: 0.0 as another element.
003 to 0.0050% and / or Ti: 0.00
3. The composition according to claim 1, which contains 3 to 0.05%.
Case hardening steel according to the above. 4. The steel material further comprises V: 0.0 as another element.
3 to 1.5% and / or Nb: 0.005 to 1.5
%. The case hardened steel according to any one of claims 1 to 3, which contains%. 5. The steel according to claim 1, wherein the other element is Ca: 0.0
005 to 0.01%, Pb: 0.2% or less (excluding 0%), Te: 0.1% or less (excluding 0%), Z
r: containing at least one selected from the group consisting of 0.1% or less (excluding 0%).
5. The case hardened steel according to any one of items 1 to 4.