JPH10259450A - Case hardening steel excellent in low cycle fatigue strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a case hardening steel excellent in low cycle fatigue strength accompanying macroscopic strains by preparing a case hardening steel in which the chemical componental compsn. is specified and the relation among alloy elements is also strictly prescribed by formulae and inequalities to improve the plastic deformation resistance and intergranular strength. SOLUTION: A case hardening steel having a compsn. contg., by mass, 0.15 to 0.30% C, 0.5 to 3.0% Mn, <=0.50% Si, <=0.010% P, <=0.035% S, 0.3 to 2.0% Cr, >0.45 to 1.0% Mo, 0.003 to 1.0% Nb, 0.015 to 0.060% Al and 0.003 to 0.030% N, in which the content of O is regulated to <=20 ppm, and the balance Fe with inevitable impurities is prepd. At this time, the plastic deformation resistance HVav and intergranular strength Iequ1 and Iequ2 prescribed by the formulae, I, II and III satisfy the inequalities IV and V. Furthermore, in the formulae, [C], [Si], [Mn], [P], [S], [Mo], [Nb], and [Cr] respectively denote the mass % of each element.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface hardening treatment such as carburizing treatment, carburizing / nitriding treatment or the like for the surface layer of steel materials used for various structural parts, in particular to improve wear resistance and fatigue resistance. (Including gas carburization, solid carburization, liquid carburization, salt bath carburization, plasma carburization, vacuum carburization, etc.), parts that need to increase the hardness of the surface layer, such as gears used in transmissions and differentials of automobiles And a case hardening steel used as a part such as a shaft. In the following, an example of application to gears will be described. However, the application of the case hardening steel of the present invention is not limited to gears, and it is a part manufactured by surface hardening treatment, particularly low cycle fatigue strength. Is used for all mechanical structural parts that are regarded as important.

[0002]

2. Description of the Related Art In response to the trend toward higher engine output and lighter parts, such as car gears used in transmissions and differentials of automobiles, carburizing, carburizing / nitriding, etc. There is a strong demand for improved fatigue strength of components manufactured by performing a surface hardening treatment or the like.

[0003] The gear Upon increasing the strength of, the past bending fatigue strength at high cycle range exceeding 10 ⁵ times (hereinafter, referred to as "high-cycle fatigue strength") have been emphasized. Various such techniques have been proposed so far. For example, Japanese Patent Application Laid-Open No. 51-90918 discloses a technique of reducing the grain boundary oxide layer in a carburized layer by reducing the amount of Si, thereby achieving high cycle fatigue strength. Steels with improved steel have been proposed. Further, there has been proposed a steel in which the amount of Mo added is increased to reduce the incompletely quenched layer of the carburized layer, thereby improving the high cycle fatigue strength (JP-A-61-253346).

On the other hand, irrespective of the number of repetitions, there is also a demand for an improvement in fatigue strength without macroscopic distortion due to a shock input with a high strain rate under load.
For example, Japanese Patent Application Laid-Open No. 1-247561 proposes a steel in which the impact characteristics are improved by reducing P and S and adding Mo and V. Japanese Patent Application Laid-Open No. 62-1843 discloses that the core structure after carburizing by adding Mo and Si is made into a (ferrite + martensite) two-phase structure of fine grains having a grain size number of 9 or more. The grain size number of the carburized layer is also 9
By increasing the number, steels with improved impact strength have been proposed.

[0005] However, there has been a demand for further increasing the output of the engine and reducing the weight of parts and reducing the number of parts of gears used in differential machines for the purpose of reducing manufacturing costs. The fact is that the phenomenon of fatigue fracture in the low cycle region is occurring. For these reasons, improvement in fatigue strength in a low cycle region accompanied by macroscopic strain (hereinafter referred to as “low cycle fatigue strength”) is desired. However, most techniques for improving low cycle fatigue strength have so far. It has not been proposed yet.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its object is to appropriately adjust the composition of chemical components and strictly define the relationship between alloying elements. Another object of the present invention is to provide a case hardening steel excellent in low cycle fatigue strength accompanied by macroscopic strain by improving plastic deformation resistance and grain boundary strength.

[0007]

The case hardening steel according to the present invention, which can solve the above-mentioned problems, comprises: C: 0.15 to 0.30% Mn: 0.5 to 3.0% Si: 0. 50% or less P: 0.010% or less S: 0.035% or less Cr: 0.3 to 2.0% Mo: more than 0.45 to 1.0% Nb: 0.003 to 0.1% Al: It is a steel containing 0.015 to 0.060% N: 0.003 to 0.030%, and O: 20 ppm or less, with the balance being Fe and unavoidable impurities, and the following ( The gist lies in that the plastic deformation resistance HVav and the grain boundary strengths Ieq1 and Ieq2 defined by the respective equations (1) to (3) satisfy the following equations (4) and (5). HVav = [C] + 0.08 [Si] + 0.14 [Mn] + 1.74 [P]-0.83 [S] + 0.16 [Cr] + 0.22 [Mo] ... (1) Ieq1 = 0.20 [Mo] + [Nb] −0.10 [Si] − [P] ^1/2 (when Cr ≦ 1.6%) (2) Ieq2 = 0.20 ([Mo] − [Cr]) + [Nb] −0.10 [Si]-[P] ^1/2 (however, when Cr> 1.6%) (3) where [C], [Si], [Mn], [P], [S], [M]
o], [Nb] and [Cr] are C, Si, Mn, P,
The content (% by mass) of S, Mo, Nb and Cr is shown. HVav + 0.13 (Ieq1 or Ieq2) −0.63 ≧ 0 (4) HVav <0.80 (5)

In the case hardening steel according to the present invention, as other elements, Ca: 0.08% or less, Zr: 0.08%
% And at least one selected from the group consisting of Pb: 0.3% or less.

Further, in the case hardening steel of the present invention, (a) As: 0.008% or less (including 0%) and Sb: 0.008% or less (including 0%) in the unavoidable impurities. And (b) the average particle size of Nb carbonitride is 1 to 50 nm and 100 μm ²
It is also effective to satisfy the requirement that the number of Nb carbonitrides precipitated in the steel is 20 or more, whereby the properties of case hardening steel can be further improved.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied various factors for improving the low cycle fatigue strength from various angles. As a result, it was found that in order to improve the low cycle fatigue strength, it is only necessary to improve the plastic deformation resistance HVav and the grain boundary strength Ieq of the steel. Based on this finding, the inventors further studied specific means for improving the low cycle fatigue strength, and found that the object of the present invention could be achieved satisfactorily by adopting the above-described structure, and completed the present invention. . The operation of the present invention will be described along the history of completion of the present invention.

In the case of a gear, the plastic deformation resistance HVav of the member
If it is low, macroscopic plastic deformation occurs at low stress during operation, tensile stress is generated in the surface layer of the tooth surface, and the fatigue life is reached by grain boundary fracture starting from the surface. When the plastic resistance of the member is high, the low cycle fatigue strength is governed by the grain boundary strength Ieq of the member. As described above, the low cycle fatigue strength associated with macroscopic plastic strain is determined by a balance between the plastic resistance HVav of the member and the grain boundary strength Ieq.

As concrete means for increasing the low cycle fatigue strength, (a) both the plastic deformation resistance HVav and the grain boundary strength Ieq of the member are increased, and (b) even if the grain boundary strength Ieq of the member is low. And (c) increasing the grain boundary strength Ieq even if the plastic deformation resistance HVav of the member is low. The macroscopic plastic strain is
For example, it means a distortion such that a gear tooth thickness during operation is accompanied by plastic deformation of about 1% or more.

The present inventors have found that the plastic deformation resistance HVav and the grain boundary strength Ieq (Ieq1, 2) can be expressed as a function of the alloy component [Equations (1) to (3) below]. In addition, they have found that it is necessary to satisfy the following equation (4) in order to secure low cycle fatigue strength accompanied by macroscopic strain determined by the balance between the plastic deformation resistance HVav of the member and the grain boundary strength Ieq. HVav = [C] + 0.08 [Si] + 0.14 [Mn] + 1.74 [P]-0.83 [S] + 0.16 [Cr] + 0.22 [Mo] ... (1) Ieq1 = 0.20 [Mo] + [Nb] −0.10 [Si] − [P] ^1/2 (when Cr ≦ 1.6%) (2) Ieq2 = 0.20 ([Mo] − [Cr]) + [Nb] −0.10 [Si]-[P] ^1/2 (however, when Cr> 1.6%) (3) where [C], [Si], [Mn], [P], [S], [M]
o], [Nb] and [Cr] are C, Si, Mn, P,
The content (% by mass) of S, Mo, Nb and Cr is shown. HVav + 0.13 (Ieq1 or Ieq2) −0.63 ≧ 0 (4)

In order to increase the plastic deformation resistance HVav of the member, it is necessary to add a large amount of alloying elements such as C, Si and Mn. However, since these elements increase the hardness of the member and significantly reduce the machinability, in order to increase the plastic deformation resistance HVav of the steel without lowering the machinability, the following equation (5) is used. Needs to be satisfied. HVav <0.80… (5)

The plastic deformation resistance HVav is, for example, about 1 to 5 in an atmosphere having a carbon potential of 0.7 to 1.2%.
There is a positive correlation with the average sectional hardness expressed by the following equation (6) when the time carburizing and quenching treatment is performed. The average cross-sectional hardness ^{= (1 / πa 2) ∫} a 0 HV (r) 2πrdr ... (6) ( where, HV (r): Hardness, a: radius, r: distance from center)

On the other hand, in order to increase the grain boundary strength Ieq, it is necessary to reduce the alloying elements Si, P, and Cr that make the grain boundary strength Ieq brittle, and to reduce the embrittlement of the grain boundary strength Ieq. The addition is effective. From these facts, the grain boundary strength I
The upper limit of eq is 0.30 indicating the highest value of the grain boundary strength obtained from the chemical component composition of the present invention, and the lower limit indicates the lowest value of the grain boundary strength obtained from the chemical component composition of the present invention.
0.39.

The above results are shown in FIG. That is,
FIG. 1 is a graph showing the influence of the plastic deformation resistance HVav and the grain boundary strength Ieq on the fatigue strength and the like, and the hatched portion is the range defined in the present invention. In FIG. 1, "●
The meanings of "symbol", "o" and "*" are as shown below, respectively. ● mark: Low cycle fatigue strength ≧ 15 Ton ○ mark: Low cycle fatigue strength <15 Ton * mark: Cannot be machined

In order to suppress the decrease in the grain boundary strength Ieq, it is necessary to prevent the crystal grains from becoming coarse. When the crystal grains become coarse, the grain boundary embrittlement element is concentrated at the grain boundaries, and the grain boundary strength I
This is because eq decreases. In order to suppress such crystal grain coarsening, the average particle size of the Nb carbonitride of the member is 1 to 50.
It is effective to satisfy the requirement that the precipitation number of Nb carbonitrides in nm and 100 μm ² is 20 or more. The average particle size of Nb carbonitride is less than 1 nm or 50 nm
In the case where it exceeds, the crystal grains become coarse and the grain boundary strength Ieq
Will decrease. Also, when the number of precipitated Nb carbonitrides in 100 μm ² is less than 20, the crystal grains are coarsened and the grain boundary strength Ieq is reduced.

Next, the reasons for limiting the chemical components of the case hardening steel of the present invention will be described. C: 0.15 to 0.30% C is an element effective for increasing the core hardness of the member, and as a result, improves the plastic deformation resistance HVav.
In order to exhibit such an effect, it is necessary to contain C at least 0.15% or more. However, if the content exceeds 0.30%, the toughness, machinability and cold forgeability deteriorate, so the upper limit is set to 0.30%. In addition, the more preferable content of C is 0.20-0.
It is about 26%.

Si: 0.50% or less Si is an element effective as a deoxidizing element and for improving the hardenability of steel to improve the plastic deformation resistance HVav. However, if it is contained excessively, it promotes grain boundary oxidation and lowers the grain boundary strength Ieq, and also lowers the cold forgeability, so the upper limit was set to 0.50%. From the viewpoint of improving the grain boundary strength Ieq, a preferred content is
It should be less than 0.35%, more preferably less than 0.15%.

Mn: 0.5-3.0% Mn is an element which is effective for deoxidizing at the time of smelting and is also necessary for improving the quenchability of steel and improving the plastic deformation resistance HVav. In order to exhibit this effect, it is necessary to contain 0.5% or more. However, if it is contained excessively, it causes a decrease in machinability and cold workability, and also promotes segregation of P to the grain boundary, lowering the grain boundary strength Ieq, and consequently lowering low cycle fatigue strength. Therefore, it needs to be 3.0% or less. Note that a more preferable content of Mn is
It is about 0.5 to 2.0%.

P: 0.010% or less P is an element necessary for improving the plastic deformation resistance HVav. However, since P segregates at the grain boundary and lowers the grain boundary strength Ieq, P should be set to 0.010% or less. From the viewpoint of improving the grain boundary strength, the content should preferably be 0.008% or less.

S: 0.035% or less S is an element that lowers the plastic deformation resistance HVav and deteriorates the cold forgeability. However, S combines with Mn to form MnS and improve machinability. It is an effective element. On the other hand, when applied to gears, not only the impact strength in the vertical direction but also the impact intensity in the horizontal direction becomes important, but in order to improve the impact intensity in the horizontal direction, it is necessary to reduce the anisotropy. Become. For that purpose, the S content needs to be 0.035% or less. The preferable range of the S content is 0.030% or less, and more preferably, about 0.005 to 0.020%.

Cr: 0.3 to 2.0% Cr is an element effective for improving the hardenability of steel and improving the plastic deformation resistance HVav. In order to exhibit such effects, it is necessary to contain 0.3% or more. However, if it is excessively contained in excess of 2.0%, Cr segregates at the grain boundaries, and forms carbides to lower the grain boundary strength Ieq, thereby lowering the low cycle fatigue strength.

Mo: more than 0.45% to 1.0% Mo is an element effective for improving the hardenability of steel to improve the plastic deformation resistance HVav and the grain boundary strength Ieq. In order to exert such effects, it is necessary to contain more than 0.45%. However, even if the content exceeds 1.0%, the above effect is saturated, so the upper limit is set to 1.0%.

Nb: 0.003-0.1% Nb combines with C and N in steel to form carbonitride,
It is an element effective for suppressing the coarsening of crystal grains. In order to exert such effects, it is necessary to contain 0.003% or more. However, if it is contained excessively, machinability and cold workability are reduced, so the content should be 0.1% or less.

Al: 0.015 to 0.060% Al effectively acts as a deoxidizing agent at the time of smelting, and combines with N in steel to form AlN to prevent coarsening of crystal grains. It is also an effective element. In order to exert such effects, it is necessary to contain 0.015% or more.
Since the effect saturates when it exceeds 0.060%, the upper limit is set to 0.060% or less.

N: 0.003 to 0.030% N combines with Al, V, Ti, Nb and the like in steel to form nitrides, and has an effect of suppressing the coarsening of crystal grains. In order to exhibit this effect, it is necessary to contain 0.003% or more, but since this effect eventually reaches saturation, the upper limit is made 0.030%.

O: 20 pm or less O is a harmful element that forms hard oxide-based inclusions such as Al ₂ O ₃ and SiO ₂ by combining with Al and Si. Since the bending fatigue strength and machinability are reduced by the formation of inclusions, the upper limit was set to 20 ppm.

The case hardening steel according to the present invention comprises the above components as basic components, and the balance is composed of Fe and unavoidable impurities. The object of the present invention is achieved by such chemical composition. However, it is also effective to further include at least one element selected from the group consisting of Ca: 0.08% or less, Zr: 0.08% or less, and Pb: 0.3% or less as other elements.

Ca: 0.08% or less, Zr: 0.08%
And at least one element selected from the group consisting of Pb: 0.3% or less. These elements are effective elements for improving machinability, but Ca and Zr exceed 0.08%. If they are contained, these effects are saturated. In addition, if Pb is excessively contained in excess of 0.3%, the pitting life and the fatigue strength are reduced. Therefore, it is necessary to reduce Pb to 0.3% or less.

Incidentally, in steel, in addition to P, S, N and O,
Various trace components such as As and Sb exist, and these trace components have been almost neglected in the past. However, since these elements are segregated at grain boundaries, it is necessary to obtain excellent low cycle fatigue strength. Is something that cannot be ignored.
That is, each element such as As and Sb segregates at the grain boundary and lowers the low cycle fatigue strength.
It is preferable to suppress the following.

The average particle size of Nb carbonitride is 1 to 50 n.
m and the precipitation number of Nb carbonitride in 100 μm ² is 2
Satisfying the requirement of 0 or more is preferable because the temperature of crystal grain coarsening can be increased.
The steel satisfying these requirements can suppress the coarsening of the crystal grains even when carburizing after annealing or cold working, and thus can secure good grain boundary strength Ieq. Note that Nb
The average particle size and the number of precipitated carbonitrides can be measured by transmission electron microscope observation using an extraction replica.
For example, in the case of observation with a transmission electron microscope, the magnification is 15,000.
0, after taking 5 fields of view with a test area ratio of 0.5 μm ² , counting the number of Nb carbonitrides by the photograph and calculating the number of Nb carbonitrides precipitated in 100 μm ² by multiplying the number by 40 times. can do.

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

[0035]

【Example】

Example 1 Steel No. shown in Table 1 below. Using a steel material with a chemical composition of 1 to 16, 150kg vacuum smelting → hot forging into a round bar with a diameter of 20mm → normalizing treatment (annealing treatment), then the effective hardened layer depth and core thickness Diameter: 1 to match the ratio with the gear
A test piece of 0 mm was prepared. Thereafter, a notch having a radius of 0.5 mm and a width of 1 mm was provided in parallel with the axial direction. The test piece was carburized at 930 ° C. for 2.5 hours (carbon potential: 0.9%), and then subjected to a blast cooling treatment. Further, in order to make the core hardness uniform with the gear, the outer diameter: 20
A test piece was placed in a half jig having a size of 10 mm and an inner diameter of 10 mm, reheated at 820 ° C., and subjected to an oil cooling treatment to obtain a test piece for a fatigue test. After that, using this test piece, P +
A low cycle fatigue test (compression fatigue test) was performed under the condition of min / Pmax = 0.

FIG. 2 shows a representative example of the low cycle fatigue test results.
Shown in Table 2 below shows the values of the plastic deformation resistance HVav, the grain boundary strength Ieq, and the calculated value of (HVav + 0.13Ieq-0.63) together with the values obtained by measuring the fatigue strength at 100 times from the compression fatigue test.

[0037]

[Table 1]

[0038]

[Table 2]

From these results, the following can be considered.
First, steel No. 1 to 10 are steels of the present invention satisfying all the requirements specified in the present invention, and it can be seen that the fatigue strength of each steel is 15 Ton or more.

On the other hand, steel No. 11 to 13 are comparative steels whose chemical composition satisfies the requirements specified in the present invention, but whose relationship between the plastic deformation resistance HVav and the grain boundary strength Ieq does not satisfy the requirements specified in the present invention. Therefore, the fatigue strength is less than 15 Ton. Steel No. No. 14 is a comparative steel in which the content of P is larger than the range specified in the present invention, and the relationship between the plastic deformation resistance HVav and the grain boundary strength Ieq does not satisfy the requirements specified in the present invention, and the fatigue strength is It is less than 15 Ton. Steel No. 15 is C
Was higher than the range specified in the present invention, and as a result, the value of the plastic deformation resistance HVav was high, and a test piece could not be produced by machining. Also steel N
o. In the case of No. 16, the Mn content was larger than the range specified in the present invention, and as a result, the value of the plastic deformation resistance HVav was high, and a test piece could not be produced by machining.

Example 2 Steel No. 1 shown in Table 1 was used. Steels having chemical composition compositions of 4, 5, and 8, and steels having chemical composition compositions shown in Table 3 below (steel No.
17) Using 150kg vacuum melting → hot forging into a round bar with a diameter of 20mm → after normalizing treatment, a test with a diameter of 10mm to make the ratio of the effective hardened layer depth and the core thickness equal to the gear Pieces were made. Thereafter, a notch having a radius of 0.5 mm and a width of 1 mm was provided in parallel with the axial direction. The test piece was carburized at 930 ° C. for 2.5 hours (carbon potential: 0.9%), and then subjected to a blast cooling treatment.
Furthermore, in order to make the core hardness equal to the gear, the outer diameter: 20 mm x
Inner diameter: Put the test piece in a 10 mm half jig,
The specimen was reheated to 0 ° C. and subjected to an oil cooling treatment to obtain a test piece for fatigue test.

The steel No. shown in Table 3 below. Using steel with chemical composition of 18, 150kg vacuum melting → diameter: 20m
After hot forging into a round bar of m → normalizing treatment, the diameter of the effective hardened layer depth and the thickness of the core part are adjusted to 10g in order to make the gear ratio.
m test pieces were prepared. Thereafter, a notch having a radius of 0.5 mm and a width of 1 mm was provided in parallel with the axial direction. Subsequently, the test piece was subjected to a solution treatment (1000 ° C. × 1 hour → A)
C) → annealing treatment (750 ° C. × 5 hours → FC). The test piece was carburized at 930 ° C. for 2.5 hours (carbon potential: 0.9%), and then subjected to a blast cooling treatment. Furthermore, in order to make the core hardness equal to the gear, the test piece was placed in a half jig having an outer diameter of 20 mm and an inner diameter of 10 mm, reheated at 820 ° C., and subjected to oil cooling to cause fatigue. A test specimen was used.

Further, the steel No. shown in Table 3 below was used. Using steel with chemical composition of 19, vacuum melting 150 kg → diameter: 20
After hot forging → normalizing to a round bar with a diameter of 10 mm, the diameter of the effective hardened layer and the thickness of the core are adjusted to 10
mm test pieces were prepared. Thereafter, a notch having a radius of 0.5 mm and a width of 1 mm was provided in parallel with the axial direction. Subsequently, the test piece was subjected to a solution treatment (1000 ° C. × 1 hour →
AC) → annealing treatment (750 ° C. × 24 hours → FC). The test piece was carburized at 930 ° C. for 2.5 hours (carbon potential: 0.9%), and then subjected to a blast cooling treatment. Furthermore, in order to make the core hardness equal to the gear, the test piece was placed in a half jig having an outer diameter of 20 mm and an inner diameter of 10 mm, reheated at 820 ° C., and subjected to oil cooling to cause fatigue. A test specimen was used.

Using each of the test pieces obtained above, ± 60
Pmin / P by Ton hydraulic servo type fatigue tester
A low cycle fatigue test (compression fatigue test) was performed under the condition of max = 0. The particle size and the number of precipitates of Nb carbonitride before carburizing were measured by transmission electron microscope observation using an extraction replica.

The results of the low cycle fatigue test and the average grain size and the number of precipitates of Nb carbonitride were calculated by calculating the values of plastic deformation resistance HVav, grain boundary strength Ieq, and (HVav + 0.13Ieq-0.63),
Table 4 below shows the values obtained by measuring the fatigue strength at 100 times from the compression fatigue test.

[0046]

[Table 3]

[0047]

[Table 4]

From these results, the following can be considered.
First, steel No. 4, 5, and 8 satisfy the requirements defined in the present invention, and the particle size and the number of Nb carbonitrides also satisfies a preferable range.
It can be seen that Ton or more is achieved.

On the other hand, steel No. In No. 17, the relationship between the plastic deformation resistance HVav and the grain boundary strength Ieq satisfies the requirements specified in the present invention, but the Nb content is smaller than the range specified in the present invention, and Nb carbonitride Since the number of precipitates is less than the range specified in the present invention, coarsening of crystal grains occurs during carburizing treatment, and the fatigue strength is less than 15 Ton.
In addition, steel No. 18 and 19, the chemical composition and the relationship between the plastic deformation resistance HVav and the grain boundary strength Ieq satisfy the requirements specified in the present invention, but the number of precipitated Nb carbonitrides is specified in the present invention. The range is smaller than the range, the crystal grains become coarse during the carburizing treatment, and the fatigue strength is less than 15 Ton.

Example 3 Steel No. 1 shown in Table 1 was used. Using steel with chemical composition of 4, 5, 8, 11, and 13, 150kg vacuum melting → diameter: 8
Hot forging into 0mm round bar → normalizing, then hot forging →
A differential gear having an outer diameter of 62 mm and 10 teeth and a differential gear having an outer diameter of 99 mm and 18 teeth were produced by machining. Thereafter, carburizing quenching and tempering were performed according to the heat treatment patterns shown in FIGS. 3A and 3B, and a low cycle fatigue test was performed using a differential strength test machine that absorbed energy with a hydraulic motor.

The results are shown in FIG. 4,
The specimens Nos. 5 and 8 satisfy the requirements defined in the present invention, and thus have a fatigue strength of 1500 kgf-m or more. On the other hand, steel No. 11 and 13, the chemical composition is within the range specified by the present invention, but since the relationship between the plastic deformation resistance HVav and the grain boundary strength Ieq is out of the range specified by the present invention, the fatigue strength is 1500 kgf. -M.

[0052]

The present invention is constituted as described above, and aims to improve the plastic deformation resistance and the grain boundary strength by appropriately adjusting the chemical composition and strictly defining the relationship between alloying elements. As a result, a case hardened steel having excellent low cycle fatigue strength was realized.

[Brief description of the drawings]

FIG. 1 is a graph showing the effects of plastic deformation resistance Heq and grain boundary strength Ieq on low cycle fatigue strength and the like.

FIG. 2 is a graph showing a typical example of a low cycle fatigue test result in Example 1.

FIG. 3 is a diagram showing an example of a heat pattern of carburizing and quenching and tempering.

FIG. 4 is a graph showing a representative example of a low cycle fatigue test result in Example 4.

 ──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Shinichi Yasugi 2 Nadahama-Higashi-cho, Nada-ku, Kobe Kobe Steel Co., Ltd. Inside Kobe Steel Works (72) Inventor Yoshitake Matsushima 2nd Nadahama-Higashi-cho, Nada-ku, Kobe City Kobe Steel, Ltd. Inside Kobe Works (72) Inventor Jun Inada 2nd Nadahama-Higashi-cho, Nada-ku, Kobe Kobe Steel Works Kobe Works

Claims (4)

[Claims] 1. C: 0.15 to 0.30% (meaning by mass%, the same applies hereinafter) Mn: 0.5 to 3.0% Si: 0.50% or less P: 0.010% or less S: 0.035% or less Cr: 0.3 to 2.0% Mo: More than 0.45 to 1.0% Nb: 0.003 to 0.1% Al: 0.015 to 0.060% N: 0. 003-0.030%, and O: 20 ppm or less, the balance being Fe and unavoidable impurities, and defined by the following formulas (1) to (3). A case hardening steel excellent in low cycle fatigue strength, wherein the plastic deformation resistance HVav and the grain boundary strengths Ieq1 and Ieq2 satisfy the following equations (4) and (5). HVav = [C] + 0.08 [Si] + 0.14 [Mn] + 1.74 [P]-0.83 [S] + 0.16 [Cr] + 0.22 [Mo] ... (1) Ieq1 = 0.20 [Mo] + [Nb] −0.10 [Si] − [P] ^1/2 (when Cr ≦ 1.6%) (2) Ieq2 = 0.20 ([Mo] − [Cr]) + [Nb] −0.10 [Si]-[P] ^1/2 (however, when Cr> 1.6%) (3) where [C], [Si], [Mn], [P], [S], [M]
o], [Nb] and [Cr] are C, Si, Mn, P,
The content (% by mass) of S, Mo, Nb and Cr is shown. HVav + 0.13 (Ieq1 or Ieq2) −0.63 ≧ 0 (4) HVav <0.80 (5) 2. As another element, Ca: 0.08%
The case hardening steel according to claim 1, wherein the case hardening steel contains at least one selected from the group consisting of Zr: 0.08% or less and Pb: 0.3% or less. 3. As in inevitable impurities: 0.008%
Or less (including 0%) and Sb: 0.008% or less (0%
%.) The case hardening steel according to claim 1 or 2, wherein the content thereof is suppressed so as to obtain the case hardening steel. 4. An Nb carbonitride having an average particle size of 1 to 50 nm.
The case hardening steel according to any one of claims 1 to 3, wherein the precipitation number of Nb carbonitride in 100 µm ² is 20 or more.