JP2019104972A - Carburized component - Google Patents

Abstract

To provide a carburized component having excellent hydrogen embrittlement resistance and sufficient wear resistance and fatigue strength.SOLUTION: The carburized component is provided in which a core part contains, by mass%, C:0.10 to 0.30%, Si:less than 0.50%, Mn:0.30 to 1.40%, P:less than 0.030%, S:less than 0.030%, Cr:0.50 to 2.00%, Al:0.010 to 0.100%, N:0.001 to 0.030% and the balance composed of Fe with impurities. The surface of the carburized component has a C concentration of 0.50 o 0.70%, and the carburized component satisfies expressions: 1.0<fn1=3×r×D<60.0, 0.010<r/D, 580+fn1≤HV≤730+fn1, 430+fn1≤HV≤630+fn1, -900<(HV-HV)/0.25<-280, where maximum outer diameter of notch is defined as D (mm), curvature radius of notch bottom is defined as r (mm), Vickers hardness at 0.15 mm depth and 0.40 mm depth from a surface are defined as HVand HV, respectively in a shaft part.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a carburized part.

  In mechanical components used for power sources and power transmission mechanisms, such as engines of automobiles and industrial machines, wear occurs due to an impact or a slide applied by impact. In addition, they are subjected to bending stresses during use. Therefore, machine parts for these applications are required to have wear resistance and high bending fatigue strength. In order to obtain sufficient wear resistance and high bending fatigue strength, carburized parts are often used in machine parts used for these applications. A carburized part is obtained by carburizing a steel material. The carburizing treatment forms a hardened layer on the surface of the carburized part. The hardened layer provides wear resistance and high bending fatigue strength.

  By the way, in the carburizing process which manufactures a carburized part, hydrocarbon gas is usually used as atmosphere gas. During the carburizing process, hydrocarbon gas is decomposed to generate hydrogen. Hydrogen generated during carburization may be absorbed into the carburized component. Therefore, in the carburized part, hydrogen embrittlement cracking may occur due to hydrogen absorbed in the steel during carburizing treatment. Therefore, not only wear resistance and high bending fatigue strength, but also excellent hydrogen embrittlement resistance is required of carburized parts.

  Techniques for enhancing the resistance to hydrogen embrittlement of carburized parts are disclosed in JP-A-9-217148 (PTD 1), JP-A 2014-19920 (PTC 2) and JP-A 2012-36475 (PTC 3). Has been proposed.

The high-strength case-hardened steel described in Patent Document 1 is, by weight%, C: 0.15 to 0.25%, Si: 0.1% or less, Mn: 0.30 to 1.00%, P: 0.030% or less, S: 0.005 to 0.020%, Ni: 0.30 to 2.00%, Cr: 0.40 to 1.50%, Mo: 0.30 to 0.45%, al: 0.015~0.030%, N: 0.0100~0.0180 %, O T: containing 0.0015% or less, the balance being Fe and inevitable impurity elements, and the annealing by high frequency heating And hardness Hv 420 or less.

  The steel material for carburizing or carbonitriding parts described in Patent Document 2 is, by mass%, C: 0.15 to 0.40%, Si: 0.15 to 0.40%, Mn: 0.5 to 1. 5%, S: 0.003 to 0.050%, Cr: 0.7 to 1.5%, Cu: 0.30 to 0.80%, Ni: 0.15 to 1.0%, N: 0 It contains .003 to 0.020% and Al: 0.005 to 0.050%, and the balance consists of Fe and impurities. P and O in the impurities are respectively P: 0.025% or less, O: 0.0020% or less, Formula (1): 2Ni-Cu ≧ 0, and Formula (2): 6C-7.5Si + 1 .6 Mn + 4 Cr-Cu-1.6 Ni 4.0 4.0 is satisfied. As a result, during use of the carburized or carbonitrided component, the amount of hydrogen generated from the lubricant and the externally mixed water can be suppressed to a low level into the steel, and the susceptibility to hydrogen embrittlement can be suppressed. Patent Document 2 describes this.

  The rolling parts or gears described in Patent Document 3 are, by mass%, C: 0.10 to 0.45%, Si: 0.01 to 1.0%, Mn: 0.10 to 2.0% , P: 0.030% or less, S: 0.035% or less, Cr: 1.30 to 3.50%, Al: 0.003 to 0.10%, N: 0.004 to 0.050% It is made of steel containing and the balance being Fe and unavoidable impurities. In patent document 3, the amount of (C + N) in the steel surface layer surface of this rolling part or gear is made 0.50 to 0.75% by carburizing or carbonitriding treatment.

JP-A-9-217148 JP, 2014-19920, A JP 2012-36475 A

  However, in the carburized component manufactured using the high-strength case-hardened steel of Patent Document 1, annealing at a high temperature is performed. Therefore, the bending fatigue strength of the carburized part may be low.

  Patent documents 2 and 3 disclose the art which controls the embrittlement to hydrogen which invades during use of a carburized part. However, as described above, hydrogen is absorbed to some extent by steel during carburizing treatment. Therefore, it is preferable to improve the resistance to hydrogen embrittlement also for hydrogen absorbed during carburizing treatment. Patent Document 2 does not disclose this point.

  In Patent Document 3, by setting the carbon concentration and the nitrogen concentration of the surface to 0.50 to 0.75%, the hydrogen embrittlement resistance in the smooth portion is enhanced. However, in carburized parts, hydrogen embrittlement tends to occur at stress concentration sites including notches. In patent document 3, there is no indication regarding the point which improves the hydrogen embrittlement resistance characteristic in a stress concentration part.

  An object of the present invention is to provide a carburized part having excellent resistance to hydrogen embrittlement and having sufficient wear resistance and flexural fatigue strength.

A carburized component according to an embodiment of the present invention includes a sliding portion and a shaft portion. The sliding portion has a sliding surface in contact with other members. The shaft portion is connected to the sliding portion. The shank comprises a stress concentrator. The stress concentration portion includes one or more notches extending in the circumferential direction of the shaft portion.
Among the shaft parts, the chemical composition of the core part excluding the hardened layer is, in mass%, C: 0.10 to 0.30%, Si: less than 0.50%, Mn: 0.30 to 1.40%, P: less than 0.030%, S: less than 0.030%, Cr: 0.50 to 2.00%, Al: 0.010 to 0.100%, N: 0.001 to 0.030%, Mo : 0 to 0.80%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, Ti: 0 to 0.10%, and Nb: 0 to 0.10%, the balance Is composed of Fe and impurities.
The C concentration of the surface of the shaft portion is 0.50 to 0.70%.
In the shaft, the maximum outer diameter of the notch is D (mm), the radius of curvature of the notch bottom is r (mm), the Vickers hardness at a depth of 0.15 mm from the surface is HV _0.15 , 0 from the surface When the Vickers hardness at a 40 mm depth position is defined as HV _0.40 , the equations (1) to (5) are satisfied.
1.0 <fn1 = 3 × r × D <60.0 (1)
0.010 <r / D (2)
580 + fn1 ≦ HV _0.15 ≦ 730 + fn1 (3)
430 + fn 1 HV HV _0.40 630 630 + fn 1 (4)
−900 <(HV _0.40 −HV _0.15 ) /0.25 <−280 (5)

  The carburized component of this embodiment has excellent resistance to hydrogen embrittlement and has sufficient wear resistance and flexural fatigue strength.

FIG. 1 is a side view showing an example of a carburized component according to the present embodiment. FIG. 2 is a view schematically showing residual stress distribution in a cross section of a carburized part. FIG. 3 is a view schematically showing a residual stress distribution in a cross section of a carburized component, which is different from FIG. FIG. 4 is a view showing an example of heat patterns in the gas carburizing step and the quenching step. FIG. 5 is a diagram showing an example of a heat pattern in the gas carburizing step and the quenching step, which is different from FIG. 4. FIG. 6 is a cross-sectional view of a test piece for evaluating the resistance to hydrogen embrittlement resistance produced in the example. FIG. 7 is a plan view of the roller pitching test piece produced in the example. FIG. 8 is a schematic view of a four-point bending test piece manufactured in the example. FIG. 9 is a schematic view of a roller pitching test.

  The carburized parts according to the present embodiment will be described below.

[About the carburized part of this embodiment]
First, a carburized part to be a target of the present embodiment will be described. FIG. 1 is a side view showing an example of a carburized component 1 according to the present embodiment. The carburized component 1 is manufactured by carburizing a steel material having a chemical composition described later. The carburized component 1 of the present embodiment includes a sliding portion 2 and a shaft portion 3.

  The sliding portion 2 has a sliding surface. The sliding surface is a surface in contact with another member and the other member slides relative to the sliding surface. Other members are, for example, machine parts. The mechanical component which is another component may be the carburized component 1. The sliding surface is, for example, a tooth surface of a gear such as a bevel gear or a pinion, or a contact surface with a belt in a pulley.

  The shaft portion 3 is connected to the sliding portion 2. The shaft 3 has a bar-like shape extending in the longitudinal direction. The shaft portion 3 includes a stress concentration portion 4. The stress concentration portion 4 includes one or more notches 5 extending in the circumferential direction of the shaft portion 3. The notches 5 may extend parallel to the circumferential direction of the shaft portion 3 or may be inclined within a range of ± 10 ° with respect to the circumferential direction. The notch 5 may be formed in a ring shape or may be formed in a spiral shape.

  Such a carburized part 1 is used, for example, as a drive source and transmission mechanism of an engine of a car or an industrial machine. The carburized component 1 is, for example, a pulley, a pinion gear or the like. When the carburized component 1 is a pinion gear, the sliding portion 2 corresponds to a pinion portion, and the shaft portion 3 corresponds to a pinion shaft portion.

The carburized component 1 of the present embodiment having the above-described configuration includes a hardened layer and a core. The hardened layer is formed on the surface layer including the surface of the carburized part 1. The core is an area inside the hardened layer. A 0.15 mm depth position from the surface of the carburized part 1 corresponds to a hardened layer. The Vickers hardness at a depth of 0.15 mm from the surface of the carburized part 1 is defined as the Vickers hardness HV _0.15 . The 0.40 mm depth position from the surface of the carburized part 1 corresponds to the core. The Vickers hardness at a 0.40 mm depth position from the surface of the carburized part 1 is defined as a Vickers hardness HV _0.40 . In the carburized part 1 of the present embodiment, the Vickers hardness HV _0.15 is higher than the Vickers hardness HV _0.40 .

[Technical thought up to considering the carburized part 1 of the present embodiment]
In general, in order to increase the strength of machine parts, it is effective to improve the hardenability and to increase the hardness of the core.

  However, in the carburized part 1 including the stress concentration portion 4 as described above, when the hardness of the core portion is high, the hydrogen embrittlement resistance in the stress concentration portion 4 is lowered. Therefore, the present inventors examined a method of enhancing the resistance to hydrogen embrittlement of the stress concentration portion 4 while having the wear resistance and the high bending fatigue strength necessary as a mechanical component for power transmission. As a result, the following findings were obtained.

  When the carburizing part 1 is manufactured by carburizing the steel material, compressive residual stress is generated in the surface layer of the carburized part 1. The reason is as follows. When the carburizing treatment is performed, the C concentration in the surface layer of the steel material becomes higher than the C concentration in the core portion. Therefore, the martensitic transformation start temperature (Ms point) of the surface layer is lower than the Ms point of the core portion. As a result, during the carburizing treatment, the core starts martensitic transformation earlier than the surface layer. Since the surface layer portion undergoes martensitic transformation later than the core portion, compressive residual stress occurs in the surface layer.

  The present inventors paid attention to the compressive residual stress generated by the above-mentioned carburizing treatment. Specifically, it was considered that if the compressive residual stress on the surface of the stress concentration portion 4 of the carburized part 1 is increased, the resistance to hydrogen embrittlement is enhanced.

  Therefore, the present inventors further investigated a method of increasing the compressive residual stress on the surface. FIG. 2 is a view schematically showing the residual stress distribution in the cross section of the carburized part 1. The horizontal axis of FIG. 2 shows the distance from the surface in the cross section of the carburized part 1, and the left end and the right end of the horizontal axis of FIG. 2 correspond to the surface. The vertical axis in FIG. 2 indicates the residual stress (MPa). Plus (+) means tensile residual stress and minus (-) means compressive residual stress.

  The curve C1 of FIG. 2 shows the residual stress distribution in the cross section of the carburized part 1. Referring to FIG. 2, in the carburized part 1, compressive residual stress is applied in a region from both surfaces to a predetermined depth H1, and compressive residual stress YC1 is applied on the surface. Then, in the region inside the depth H1, tensile residual stress is applied. Since residual stresses in the carburized part 1 are balanced, the area of the tensile residual stress area A1 corresponds to the sum of the area of the compressive residual stress area A2 and the area of the compressive residual stress area A3 in the curve C1. Do.

  The area to which compressive residual stress is applied (the area from the surface to the depth H1 in the depth direction) depends on the depth of the hardened layer. Therefore, as shown in FIG. 3, it is assumed that the carburizing process is adjusted to make the hardened layer shallower than that in FIG. 2, and the depth to which the compressive residual stress is applied is H2 shallower than the depth H1.

  In this case, in order to match the area of the tensile residual stress area A1 with the total area of the compressive residual stress area A2 and the area A3, the residual stress distribution has a curve C2. Specifically, in the area A2 and the area A3 of the compressive residual stress, the gradient (residual stress gradient) of the residual stress in the depth direction from the surface becomes steep. As a result, the compressive residual stress at the surface becomes YC2 larger than YC1.

  As described above, in order to increase the compressive residual stress on the surface in order to suppress the occurrence of cracks on the surface at the stress concentration portion 4 of the carburized part 1, the area to which the compressive stress is applied in the depth direction is narrowed. (Depth H1 → H2). If the compressive residual stress on the surface of the stress concentration portion 4 of the carburized part 1 is increased, the tensile residual stress generated at the bottom of the notch is relaxed and a crack on the surface is less likely to occur. As a result, the resistance to hydrogen embrittlement in the stress concentrated portion 4 of the carburized part 1 is enhanced.

  Based on the above thinking, the present inventors examined the shape of the stress concentration portion 4, the hardness of the surface layer portion of the stress concentration portion 4, and the hardness of the core portion.

  As shown in FIG. 1, the maximum outer diameter of the notch 5 of the stress concentration portion 4 is defined as D (mm). Also, the radius of curvature of the notch bottom of the notch 5 is defined as r (mm). When a plurality of notches 5 are arranged in the axial direction of the stress concentration portion 4, the average of the maximum outer diameter of each notch 5 is defined as D (mm), and the average of the radius of curvature of the notch bottom of each notch 5 is r ( Define as mm).

Furthermore, as described above, the Vickers hardness at a position 0.15 mm deep from any surface of the shaft 3 is defined as HV _0.15, and at a depth 0.40 mm from any surface of the shaft 3 The Vickers hardness is defined as HV _0.40 .

From the surface of carburized part 1, provided that the maximum outer diameter D, radius of curvature r, Vickers hardness HV _0.15 and HV _0.40 defined above satisfy the following equations (1) to (5) In the depth direction, the area to which compressive residual stress is applied is sufficiently narrowed, and sufficient compressive residual stress can be applied to the surface. As a result, it was found that sufficient hydrogen embrittlement resistance was obtained.
1.0 <fn1 = 3 × r × D <60.0 (1)
0.010 <r / D (2)
580 + fn1 ≦ HV _0.15 ≦ 730 + fn1 (3)
430 + fn 1 HV HV _0.40 630 630 + fn 1 (4)
−900 <(HV _0.40 −HV _0.15 ) /0.25 <−280 (5)

The carburized component 1 according to the embodiment of the present invention completed based on the above findings includes the sliding portion 2 and the shaft portion 3. The sliding portion 2 has a sliding surface in contact with other members. The shaft portion 3 is connected to the sliding portion 2. The shaft portion 3 includes a stress concentration portion 4. The stress concentration portion 4 includes one or more notches 5 extending in the circumferential direction of the shaft portion 3.
The chemical composition of the core part excluding the hardened layer in the shaft part 3 is, by mass%, C: 0.10 to 0.30%, Si: less than 0.50%, Mn: 0.30 to 1.40% P: less than 0.030% S: less than 0.030% Cr: 0.50 to 2.00% Al: 0.010 to 0.100% N: 0.001 to 0.030% Mo: 0 to 0.80%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, Ti: 0 to 0.10%, and Nb: 0 to 0.10%, The balance consists of Fe and impurities.
The C concentration of the surface of the shaft portion 3 is 0.50 to 0.70%.
In the shaft portion 3, the maximum outer diameter of the notch 5 is D (mm), the radius of curvature of the notch bottom of the notch 5 is r (mm), and the Vickers hardness at a depth of 0.15 mm from the surface is HV _0.15 , Expressions (1) to (5) are satisfied when the Vickers hardness at a depth of 0.40 mm from the surface is defined as HV _0.40 .
1.0 <fn1 = 3 × r × D <60.0 (1)
0.010 <r / D (2)
580 + fn1 ≦ HV _0.15 ≦ 730 + fn1 (3)
430 + fn 1 HV HV _0.40 630 630 + fn 1 (4)
−900 <(HV _0.40 −HV _0.15 ) /0.25 <−280 (5)

  The chemical composition contains one or more selected from the group consisting of Mo: 0.01 to 0.80%, Ni: 0.05 to 0.50%, and Cu: 0.10 to 0.50%. You may

  The chemical composition may contain one or more selected from the group consisting of Ti: 0.05 to 0.10% and Nb: 0.01 to 0.10%.

  Hereinafter, the carburized component 1 according to the embodiment of the present invention will be described in detail.

[Chemical composition of core portion of shaft portion 3 of carburized part 1]
The carburized part 1 by this embodiment is provided with the sliding part 2 and the axial part 3 as shown in FIG. The chemical composition of the core portion excluding the hardened layer in the shaft portion 3 of the carburized part 1 contains the following elements. Here, the chemical composition of the core refers to a product according to JIS G 0321 (2010) in the sample taken at a position deeper than 2.0 mm deep from the surface in the shaft 3 of the carburized part 1 It means the chemical composition obtained by analysis. Hereinafter, “%” relating to the content of the element means mass%.

C: 0.10 to 0.30%
Carbon (C) enhances the hardenability of the steel. Thereby, the hardness of the carburized part 1 is increased and the wear resistance is improved. If the C content is less than 0.10%, this effect can not be obtained. On the other hand, if the C content exceeds 0.30%, the machinability and cold forgeability of the steel may be reduced. Therefore, the C content is 0.10 to 0.30%. The preferable lower limit of the C content is 0.15%, and more preferably 0.18%. The upper limit of the C content is preferably 0.25%, more preferably 0.23%.

Si: less than 0.50% Silicon (Si) is inevitably contained. That is, the Si content is more than 0%. Si deoxidizes the steel. Si further enhances the hardenability of the steel and further enhances the softening resistance. As a result, the wear resistance of the carburized part 1 is enhanced. However, if the Si content is 0.50% or more, the cold workability of the steel may be reduced. Therefore, the Si content is less than 0.50%. The preferable lower limit of the Si content is 0.02%, and more preferably 0.05%. The upper limit of the Si content is preferably 0.20%, more preferably 0.15%.

Mn: 0.30 to 1.40%
Manganese (Mn) deoxidizes the steel. Mn further enhances the hardenability and strength of the steel. Mn further enhances the softening resistance of the steel. As a result, the wear resistance of the carburized part 1 is enhanced. If the Mn content is less than 0.30%, these effects can not be obtained. On the other hand, if the Mn content exceeds 1.40%, cosegregation with P becomes remarkable, and grain boundary embrittlement occurs. As a result, the hydrogen embrittlement resistance of the carburized part 1 is reduced. Therefore, the Mn content is 0.30 to 1.40%. The preferable lower limit of the Mn content is 0.50%, more preferably 0.70%. The upper limit of the Mn content is preferably 1.20%, more preferably 1.00%.

P: less than 0.030% Phosphorus (P) is an unavoidable impurity. That is, the P content is more than 0%. P segregates at austenite grain boundaries during carburizing, and reduces the grain boundary strength of the carburized layer. If the grain boundary strength of the carburized layer is reduced, the hydrogen embrittlement resistance is reduced. When the P content is less than 0.030%, the P content of not only the core but also the surface layer is low. For this reason, the toughness of the surface layer is enhanced, and the occurrence of intergranular cracks is suppressed. As a result, the resistance to hydrogen embrittlement is enhanced. Therefore, the P content is less than 0.030%. The preferable upper limit of P content is 0.015%. The P content is preferably as low as possible. However, if the P content is extremely reduced in the steelmaking process, the manufacturing cost is increased and the productivity is also reduced. Therefore, the lower limit of preferable P content is 0.0001%, and more preferably 0.0005%.

S: less than 0.030% Sulfur (S) is an unavoidable impurity. That is, the S content is more than 0%. S remains at grain boundaries to lower the grain boundary strength of the carburized layer. S further forms coarse MnS at grain boundaries, which is a point at which residual stress is concentrated. As a result, the hydrogen embrittlement resistance of the carburized part 1 is reduced. Therefore, the S content is less than 0.030%. The preferred upper limit of the S content is 0.015%. The S content is preferably as low as possible. However, if the S content is extremely reduced in the steel making process, the manufacturing cost is increased and the productivity is also reduced. Therefore, the lower limit of the preferable S content is 0.0001%, more preferably 0.0005%.

Cr: 0.50 to 2.00%
Chromium (Cr) enhances the hardenability of the steel and increases the hardness of the core. Cr further enhances the temper softening resistance. As a result, the wear resistance of the carburized part 1 is enhanced. If the Cr content is less than 0.50%, this effect can not be sufficiently obtained. On the other hand, if the Cr content exceeds 2.00%, the cold workability is reduced. Therefore, the Cr content is 0.50 to 2.00%. The preferable lower limit of the Cr content is 0.60%, and more preferably 0.80%. The upper limit of the Cr content is preferably 1.85%, more preferably 1.70%.

Al: 0.010 to 0.100%
Aluminum (Al) deoxidizes the steel. Furthermore, Al combines with N in the steel to form AlN, and suppresses the coarsening of austenite grains during carburization. As a result, the hydrogen embrittlement resistance of the carburized part 1 is enhanced. If the Al content is less than 0.010%, this effect can not be obtained. On the other hand, if the Al content exceeds 0.100%, the above effect is saturated. Therefore, the Al content is 0.010 to 0.100%. The preferable lower limit of the Al content is 0.020%, and more preferably 0.025%. The upper limit of the Al content is preferably 0.080%, more preferably 0.065%. In the chemical composition of the core portion of the carburized part 1 of the present embodiment, the Al content means the total amount of Al contained in the steel material.

N: 0.001 to 0.030%
Nitrogen (N) combines with Ti, Al, V and Nb in steel to form nitrides and carbonitrides, and suppresses coarsening of austenite grains during carburization. Thereby, the hydrogen embrittlement resistance of the carburized part 1 is enhanced. If the N content is less than 0.001%, a sufficient coarsening suppression effect can not be obtained. On the other hand, if the N content exceeds 0.030%, the above effect is saturated. Therefore, the N content is 0.001 to 0.030%. The preferable lower limit of the N content is 0.005%, and more preferably 0.008%. The upper limit of the N content is preferably 0.025%, more preferably 0.020%.

  The balance of the chemical composition of the core of the carburized part 1 according to the present embodiment is made of Fe and impurities. Here, when manufacturing the carburized part 1 industrially, impurities are mixed from the ore as a raw material, scrap, or a manufacturing environment etc., and have a bad influence on the carburized part 1 of this embodiment. It means something that is acceptable without.

[About any element]
The chemical composition of the core portion of the shaft portion 3 of the carburized part 1 of the present embodiment further contains one or more selected from the group consisting of Mo, Ni and Cu instead of a part of Fe It is also good. These elements are optional elements, and all enhance the hardenability of the steel.

Mo: 0 to 0.80%
Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When it is contained, Mo enhances the temper softening resistance of the carburized part 1 and enhances the wear resistance of the carburized part 1. These effects can be obtained if any Mo is contained. However, if the Mo content exceeds 0.80%, these effects are saturated and the raw material cost becomes high. Therefore, the Mo content is 0 to 0.80%. The preferable lower limit of the Mo content for stably obtaining the above effect is 0.01%, and more preferably 0.10%. The preferable upper limit of the Mo content is 0.60%, and more preferably 0.40%.

Ni: 0 to 0.50%
Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When it is contained, Ni enhances the hardenability of the steel and enhances the strength of the carburized part 1. These effects can be obtained if any Ni is contained. However, if the Ni content exceeds 0.50%, the amount of retained austenite increases and the workability decreases. Therefore, the Ni content is 0 to 0.50%. The preferable lower limit of the Ni content to stably obtain the above effect is 0.05%, and more preferably 0.10%. The preferable upper limit of the Ni content is 0.45%, and more preferably 0.40%.

Cu: 0 to 0.50%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When it is contained, Cu enhances the hardenability of the steel and enhances the strength of the carburized part 1. This effect is obtained if any Cu is contained. On the other hand, if the Cu content exceeds 0.50%, the hot workability is reduced. Therefore, the Cu content is 0 to 0.50%. The preferable lower limit of the Cu content to stably obtain the above effect is 0.10%, and more preferably 0.15%. The upper limit of the Cu content is preferably 0.35%, more preferably 0.25%.

  The core of the carburized part 1 of the present embodiment may further contain one or two selected from the group consisting of Ti and Nb, instead of part of Fe. These elements are optional elements, and all suppress the coarsening of crystal grains.

Ti: 0 to 0.10%
Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When it is contained, Ti combines with C and S in the steel to form fine TiC and TiS, and suppresses the coarsening of austenite grains during carburization. Thereby, the hydrogen embrittlement resistance of the carburized part 1 is enhanced. This effect is obtained if any Ti is contained. However, if the Ti content exceeds 0.10%, TiC is coarsened to lower the toughness of the steel. In this case, the hydrogen embrittlement resistance of the carburized part 1 is reduced. Therefore, the Ti content is 0 to 0.10%. The preferable lower limit of the Ti content for stably obtaining the above effect is 0.05%. The upper limit of the Ti content is preferably 0.08%, more preferably 0.07%.

Nb: 0 to 0.10%
Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When it is contained, Nb combines with C and N in the steel to form Nb carbonitride (Nb (CN)), and suppresses the coarsening of austenite grains during carburization. Thereby, the hydrogen embrittlement resistance of the carburized part 1 is enhanced. This effect can be obtained if any Nb is contained. However, if the Nb content exceeds 0.10%, the carburizing property decreases. Therefore, the Nb content is 0 to 0.10%. The preferable lower limit of the Nb content to stably obtain the above effect is 0.01%, and more preferably 0.02%. The preferred upper limit of the Nb content is 0.07%, and more preferably 0.05%.

[C Concentration of Surface of Shaft 3 of Carburized Part 1]
C concentration of the surface of the shaft portion 3 of the carburized part 1: 0.50 to 0.70%
The C concentration (hereinafter, surface C concentration) of the surface of the shaft portion 3 of the carburized part 1 is 0.50 to 0.70% in mass%. If the surface C concentration is less than 0.50%, the surface hardness of the carburized part 1 is too low, and the wear resistance is reduced. On the other hand, if the surface C concentration exceeds 0.70%, the toughness of the carburized layer becomes low, so the resistance to hydrogen embrittlement deteriorates. Therefore, the surface C concentration is 0.50 to 0.70%. The preferable lower limit of the surface C concentration is 0.54%, more preferably 0.56%. The upper limit of the surface C concentration is preferably 0.66%, more preferably 0.64%.

  The surface C concentration of the carburized part 1 is measured by the following method. Of the surfaces of the carburized component 1, arbitrary five measurement positions are selected. The C concentration (mass%) at the selected measurement position is analyzed by EPMA (electron beam microanalyzer). The average of the five C concentrations obtained by EPMA is defined as the surface C concentration (mass%) of the carburized part 1.

[Regarding Formula (1) and Formula (2)]
As described above, in the stress concentration portion 4 of the shaft portion 3 of the carburized part 1, when the maximum outer diameter of the notch 5 is defined as D (mm) and the radius of curvature of the notch bottom of the notch 5 is r (mm) The carburized part 1 of a form satisfy | fills Formula (1) and Formula (2).
1.0 <fn1 = 3 × r × D <60.0 (1)
0.010 <r / D (2)
When the plurality of notches 5 are arranged in the axial direction in the stress concentration portion 4, the average value of the maximum outer diameters of the notches 5 is defined as D (mm). The average value of the radius of curvature is defined as r (mm).

  In the equation (1), if fn1 (the product of the curvature radius r of the notch bottom and the maximum outer diameter D of the notch 5) is too small, stress (tensile stress) is concentrated on the surface of the stress concentration portion 4 of the shaft 3 It becomes easy to crack and it becomes easy to do. Therefore, the resistance to hydrogen embrittlement is reduced. On the other hand, the upper limit of fn1 does not exist in relation to the hydrogen embrittlement resistance, but the upper limit of fn1 is less than 60.0 as the shape of the carburized part 1. Therefore, fn1 is more than 1.0 and less than 60.0. The preferred lower limit of fn1 is 1.5, more preferably 4.0, and still more preferably 8.0. The upper limit of f1 is preferably 55.0, more preferably 50.0, still more preferably 45.0.

  In formula (2), it defines as fn2 = r / D. If fn2 is too large, stress (tensile stress) tends to be concentrated at the bottom of the notch when a load is applied. In this case, the hydrogen embrittlement resistance is reduced. Therefore, fn2 is greater than 0.010. The preferable lower limit of fn2 is 0.015, and more preferably 0.018. The upper limit of fn2 is not particularly limited, and is, for example, 0.200.

[Regarding Formula (3) and Formula (4)]
In the above-described stress concentration portion 4, the Vickers hardness HV _0.15 at a depth of 0.15 mm from the surface and the Vickers hardness HV _0.40 at a depth of 0.40 mm from the surface are represented by the formula (3 And Equation (4) is satisfied.
580 + fn1 ≦ HV _0.15 ≦ 730 + fn1 (3)
430 + fn 1 HV HV _0.40 630 630 + fn 1 (4)

Vickers hardness HV _0.15 is an index of hardness in a hardened layer, and Vickers hardness HV _0.40 is an index of hardness in a core. Referring to Formula (3), if the hardness of the hardened layer represented by Vickers hardness HV _0.15 is too low, the wear resistance of the carburized part 1 is reduced. On the other hand, if the hardness of the hardened layer is too high, cracking is likely to occur particularly at the notch bottom, and the hydrogen embrittlement resistance is lowered. When the HV _0.15 satisfies the formula (3), the above-mentioned chemical composition is excellent in the abrasion resistance and the excellent abrasion resistance on the premise that the formulas (1), (2), (4) and (5) are satisfied. Hydrogen embrittlement resistance is obtained.
fn3 ₌ is defined as _HV 0.15 _-fn1. The preferable lower limit of fn3 is 600, more preferably 620. The preferable upper limit of fn3 is 710, more preferably 690.

Referring to equation (4), if the hardness of the core represented by Vickers hardness HV _0.40 is too low, plastic deformation occurs when a bending load is applied, and the stress on the surface increases, and the machine Bending fatigue strength as a part becomes low. On the other hand, if the hardness of the core portion is too high, cracking tends to occur particularly at the notch bottom, and the hydrogen embrittlement resistance is lowered. If the HV _0.40 satisfies the formula (4), excellent fatigue strength and resistance can be obtained on the premise that the formulas (1), (2), (3) and (5) are satisfied in the above chemical composition. Hydrogen embrittlement properties are obtained.
fn4 ₌ is defined as _HV 0.40 _-fn1. The preferred lower limit of fn4 is 450, more preferably 470. The preferred upper limit of fn3 is 610, more preferably 590.

[About formula (5)]
As shown in FIG. 2, the compressive residual stress applied to the surface can be increased by narrowing the regions (A2 and A3) to which the compressive residual stress is applied in the depth direction from the surface of the carburized part 1. In order to narrow the area (A2 and A3) to which the compressive residual stress is applied, the hardness difference between the surface layer hardness and the core portion hardness may be increased, and the hardness gradient between the surface layer and the core portion may be sharpened. do it.

Fn5 ₌ is defined as _{(HV 0.40} -HV 0.15) /0.25. fn5 means the hardness gradient between the surface at a depth of 0.15 mm and the surface at a depth of 0.40 mm. If fn5 is -900 or less, the hardness gradient between the surface hardness and the core hardness is too large. In this case, the hardness of the core portion becomes too low, and the bending fatigue strength as a part becomes low. On the other hand, when fn5 is -280 or more, since the hardness gradient of the hardness of surface layer and the hardness of a core part is too small, sufficient compressive residual stress is not obtained on the surface. In this case, the hydrogen embrittlement resistance of the carburized part 1 is reduced.

  If fn5 is greater than -900 and less than -280, that is, if fn5 satisfies the equation (5), the above-mentioned chemical composition is excellent in fatigue strength on the premise that the equations (1) to (4) are satisfied. And hydrogen embrittlement resistant properties are obtained. The preferred lower limit of fn5 is -850, and more preferably -800. The preferred upper limit of fn5 is -330, and more preferably -380.

[Measuring method of Vickers hardness HV _0.15 and HV _0.40 ]
The Vickers hardnesses HV _0.15 and HV _0.40 can be measured by the following method. Vickers according to JIS Z 2244 (2009) at the shaft 5 of the carburized part 1 at any five points at a depth of 0.15 mm from the surface and at any five points at a depth of 0.40 mm from the surface Conduct hardness test. The test force is 0.98N. The average of the values obtained at any five points is defined as Vickers hardness HV _0.15 and HV _0.40 , respectively.

[Manufacturing process]
An example of the manufacturing method of the carburized component 1 by this embodiment is demonstrated.

  Prepare a steel material meeting the above-mentioned chemical composition. Steel materials are manufactured and prepared by the following method, for example. A molten steel of the above chemical composition is manufactured, and a slab (slab or bloom) is manufactured by the continuous casting method using the molten steel. You may manufacture an ingot (steel ingot) by the ingot method using molten steel. The slab or ingot is hot worked to produce a billet. The billet is hot worked to produce a bar or wire. The hot working may be hot rolling or hot forging.

  Forging is performed on the manufactured steel material. The forging method may be hot or cold. In the case of hot forging, the heating temperature of the steel material at the time of hot forging is not particularly limited, but is, for example, 1000 to 1300 ° C. A machining process represented by cutting is performed on the forged steel material as necessary to manufacture an intermediate product including the sliding portion 2 and the shaft portion 3.

  Carburizing and quenching treatment is performed on the manufactured intermediate product, and tempering is performed on the intermediate product after carburizing and quenching processing. The intermediate product after quenching may be further machined (cutting, etc.). The carburized part 1 is manufactured by the above manufacturing process.

  An example of the conditions of the carburizing and quenching treatment is as follows.

[Carburizing and quenching treatment]
The carburizing and quenching treatment includes a gas carburizing step S10 and a quenching (quenching) step S20. The carburizing process implemented by the carburizing component 1 of the present embodiment is a gas carburizing process. Hereinafter, the gas carburizing step S10 and the quenching step S20 will be described.

[Gas carburizing step S10]
FIG. 4 is a view showing an example of heat patterns in the gas carburizing step S10 and the quenching step S20. The vertical axis in FIG. 4 is the treatment temperature (° C.) at the time of carburizing treatment, and the horizontal axis is time. Referring to FIG. 4, gas carburizing step S10 includes heating step S0, carburizing step S1, diffusion step S2, and heat holding step S3.

  In the heating step S0, the intermediate charged in the furnace is heated to the carburizing temperature Tc. In the carburizing step S1, in the atmosphere of a predetermined carbon potential Cp1, the intermediate product is held at a carburizing temperature Tc for a predetermined time (treatment time t1) to carry out a carburizing process. In the diffusion step S2, in the atmosphere of the carbon potential Cp2 lower than the carbon potential Cp1 in the carburizing step S1 and substantially the same as the surface C concentration of the carburized component 1, the carburizing temperature Tc is maintained for a predetermined time (treatment time t2) Do. The soaking step S3 is a step aimed at equalizing the entire intermediate product to a predetermined hardening temperature. In the soaking step S3, soaking is performed at a soaking temperature Ts lower than the carburizing temperature Tc for a predetermined time (holding time t3). However, the soaking step S3 may be omitted.

  The type of carburizing gas used in the carburizing step S1 can be a known one used for gas carburizing treatment. The carburizing gas is, for example, a hydrocarbon gas such as acetylene, propane and ethylene.

  The preferred carburizing temperature Tc is 900 to 1100 ° C., more preferably 920 to 1050 ° C. If the carburizing temperature Tc is 900 ° C. or more, a carburized part 1 having a predetermined carbon concentration can be obtained in a short time. If the carburizing temperature Tc is 1100 ° C. or less, the crystal grains are unlikely to be coarsened.

  The treatment time t1 in the carburizing step S1 and the treatment time t2 in the diffusion step S2 are 120 minutes or less in total (t1 + t2). However, the condition is that the carbon potentials Cp1 and Cp2 are within the preferable range described later. If the carburizing time is too long, the core will be baked and the hardness of the core will increase. In this case, the hardness gradient fn5 between the surface layer portion and the core portion decreases. As a result, the hydrogen embrittlement resistance of the carburized part 1 is reduced.

The lower limit of the treatment time t1 in the carburizing step S1 is 45 minutes, provided that the carbon potentials Cp1 and Cp2 are in the preferable range described later. If the treatment time t1 is too short, the hardness of the core portion represented by Vickers hardness HV _0.40 is reduced. A further preferable lower limit of the treatment time t1 in the carburizing step S1 is 50 minutes. The preferable upper limit of the treatment time t1 is 80 minutes, more preferably 70 minutes.

The preferable lower limit of the treatment time t2 in the diffusion step S2 is more than 25 minutes, provided that the carbon potentials Cp1 and Cp2 are in the preferable range described later. If the treatment time t2 is too short, the diffusion time is not sufficient, so the diffusion of carbon becomes closer to the surface. That is, the gradient of the carbon concentration on the surface and the core becomes large. As a result, the hardness of the surface represented by HV _0.15 is increased and HV _0.40 is decreased. The lower limit of the treatment time t2 in the diffusion step S2 is more preferably 30 minutes. The preferred upper limit of the treatment time t2 is 50 minutes, more preferably 45 minutes.

  The preferable upper limit of the total of the treatment times t1 and t2 is 110 minutes, more preferably 100 minutes.

  The carbon potential Cp1 in the carburizing step S1 is preferably 0.60 to 0.90. The carbon potential Cp2 in the diffusion step S2 is preferably 0.45 to 0.70. If the carbon potentials Cp1 and Cp2 are too high, the core portion is burned to increase the hardness of the core portion. In this case, the hardness gradient between the hardness of the surface layer and the hardness of the core decreases. As a result, the hydrogen embrittlement resistance is reduced. The preferable lower limit of the carbon potential Cp1 is 0.65, 0.70, and more preferably 0.75. The upper limit of carbon potential Cp1 is preferably 0.85, and more preferably 0.80. The preferable lower limit of carbon potential Cp2 is 0.50, more preferably 0.55. The preferable upper limit of carbon potential Cp2 is 0.65, and more preferably 0.60.

  The soaking step S3 may or may not be performed. When the soaking step S3 is performed, the upper limit of the preferable soaking temperature Ts is the carburizing temperature Tc-10 ° C, and more preferably Tc-50 ° C. The lower limit of the soaking temperature Ts is preferably Tc-100 ° C, more preferably Tc-70 ° C.

[Hardening process (S20)]
Quenching step S20 is performed on the intermediate product after gas carburizing step S10. In the quenching step S20, the intermediate product of the gas carburizing step degree is quenched and quenched. In the present embodiment, for example, as shown in FIG. 4, in the soaking step S3 in the gas carburizing step S10, the quenching may be carried out after the soaking temperature Ts is set as the quenching temperature and soaking for a predetermined time t3. . Further, as shown in FIG. 5, the intermediate product after the gas carburizing step S10 may be cooled to normal temperature and then the quenching step S20 may be performed. In this case, in the quenching step S20, first, the intermediate product is heated to the quenching temperature, held for a predetermined time, and then rapidly cooled. In FIG. 5, the soaking step S3 in the gas carburizing step S10 may be omitted.

  The preferable hardening temperature in hardening process S20 is 800-900 degreeC. A further preferable lower limit of the quenching temperature is 820 ° C. The preferred upper limit of the quenching temperature is 880 ° C.

The cooling method in the quenching process is oil cooling. Specifically, the intermediate held at the quenching temperature is immersed and quenched in a cooling bath containing oil, which is a cooling medium. The temperature (oil temperature) of the oil that is the cooling medium is 120 to 140 ° C. If the oil temperature is less than 120 ° C., the core will be baked and the core hardness will be too high. In this case, the hardness gradient between the surface layer and the core decreases, and fn5 does not satisfy the equation (5). As a result, the hydrogen embrittlement resistance is reduced. Further, when the oil temperature exceeds 140 ° C., the surface hardness _{HV 0.15} is too low, abrasion resistance is lowered. Therefore, the oil temperature at the time of quenching is 120 to 140 ° C. At the time of oil cooling, the oil in the bath is sufficiently stirred by a known method. The characteristic temperature of the oil as the cooling medium is not particularly limited, but is, for example, 500 to 650 ° C.

  The carburized component 1 according to the present embodiment is manufactured by the above steps. In addition, you may implement a tempering process by a well-known method after hardening treatment.

  Molten steel having steels 1 to 32 having the chemical compositions shown in Table 1 was produced. The ingot was manufactured using the manufactured molten steel. The ingot was hot forged to produce a 50 mm diameter steel bar. The following evaluation test was implemented using the manufactured steel bar.

[Production of test piece]
At first, normalizing treatment was performed on each bar. The treatment temperature in the normalizing treatment was 925 ° C., and the holding time was 1 hour. The steel bar after holding time was allowed to cool in the air. Three types of test pieces were produced from the bar steel after normalizing treatment.

[Hydrogen Embrittlement Characteristics Evaluation Test Piece]
Machining was performed on a 50 mm diameter steel bar after normalizing treatment, and a plurality of hydrogen embrittlement resistance evaluation test pieces shown in FIG. 6 were produced for each steel bar of each test number. 1.00 mm in FIG. 6 shows that the depth of the V notch of a test piece is 1.00 mm. “D” in FIG. 6 indicates the maximum outer diameter (the outer diameter of the shaft) (mm) of the V notch. “60 °” in the drawing indicates that the V notch angle is 60 °. "R" in the figure means the radius of curvature (mm) of the V notch bottom.

  The dimensions of the maximum outer diameter D and the radius of curvature r of each test number were as follows. In Test No. 32, the maximum outer diameter D was 20 mm, and the curvature radius r was 0.9 mm. In Test No. 33, the maximum outer diameter D was 5 mm, and the radius of curvature r was 0.072 mm. In Test No. 34, the maximum outer diameter D was 20 mm, and the radius of curvature r was 1.1 mm. In Test No. 35, the maximum outer diameter D was 4 mm, and the curvature radius r was 0.072 mm. In Test No. 36, the maximum outer diameter D was 20 mm, and the curvature radius r was 0.3 mm. In Test No. 37, the maximum outer diameter D was 20 mm, and the radius of curvature r was 0.15 mm. In all the test numbers other than the above, the maximum outer diameter D was 20 mm, and the curvature radius r was 0.36 mm.

[Roller pitching test piece]
Machining was performed on a 50 mm diameter steel bar after normalizing treatment to produce a roller pitching test piece having the shape shown in FIG. The numerical values in FIG. 7 indicate dimensions, and the unit is mm. The roller pitching test piece was cylindrical and had a parallel portion with a diameter of 26 mm at the center. The diameter other than the parallel part of the roller pitching test piece was 22 mm. The roller pitching test piece was the small roller 200 in the roller pitching test described later.

[Preparation of 4-point bending fatigue test piece]
The steel bar having a diameter of 50 mm after the normalizing treatment was machined to obtain a plurality of four-point bending fatigue test pieces having the shape shown in FIG. The four-point bending fatigue test piece had a height and a width of 13 mm and a length of 100 mm. A notch 5 having a semicircular cross-sectional shape was formed at the longitudinal center of the four-point bending fatigue test piece. The semicircular notch 5 extends in the width direction of the four-point bending fatigue test piece. The radius of curvature r of the semicircular notch 5 was 2 mm.

  Carburized parts of test No. 1 to test No. 55 are subjected to carburizing treatment and hardening treatment on hydrogen embrittlement resistance evaluation test pieces, roller pitting test pieces and 4-point bending fatigue test pieces of steel 1 to steel 32 1 was manufactured. In addition, about the roller pitting test and the 4 point | piece bending fatigue test of the test number 32-the test number 37, it estimated that it would become the same evaluation as the test number 7, and was abbreviate | omitted.

  Specifically, the test pieces were heated to a carburizing temperature Tc of 930 ° C. in a furnace for the hydrogen embrittlement resistance evaluation test pieces of each test number, the roller pitting test pieces and the four-point bending fatigue test pieces. An acetylene gas was introduced into the furnace to carry out a carburizing step S1 of carburizing the test piece. The carbon potential Cp1 and the treatment time t1 in the carburizing step S1 were as shown in Table 2.

  Next, a diffusion step S2 was performed to diffuse the carbon which has intruded into the test piece into the steel material. The carbon potential Cp2 and the treatment time t2 in the diffusion step S2 were as shown in Table 2.

  The total time (t1 + t2) of the treatment time t1 in the carburizing step S1 and the treatment time t2 in the diffusion step S2 was as shown in Table 2.

  Thereafter, the test piece was cooled, and the cooling was stopped at a quenching temperature of 870 ° C. After soaking for 10 minutes at a quenching temperature (870 ° C.), oil quenching was performed.

  The oil temperature at the time of oil quenching was as shown in Table 2. The characteristic temperature of the oil was 600 ° C. in any of the test numbers.

  Each oil-quenched test piece was subjected to tempering at a temperature of 180 ° C. and a holding time of 120 minutes at the tempering temperature.

  According to the above manufacturing process, carburized parts 1 of test numbers 1 to 55 (hydrogen embrittlement resistance evaluation test pieces, roller pitting test pieces and 4-point bending fatigue test pieces) were produced. In the carburized part 1, samples were taken from a position deeper than 2.0 mm deep from the surface, and product analysis was performed according to JIS G 0 321 (2010). It is a chemical composition shown.

[Evaluation test]
The following evaluation tests were performed on the carburized parts 1 of each of the manufactured test numbers.

[Surface C concentration measurement]
The surface C concentration (%) was measured by the method described above for the hydrogen embrittlement resistance evaluation test pieces before the test. The results are shown in Table 3.

[Measurement of hardness of surface layer and core portion]
The hydrogen embrittlement resistance evaluation test pieces before the test were cut in the direction orthogonal to the length direction. The Vickers hardness test according to JIS Z 2244 (2009) was conducted with the cut surface as the measurement surface, and the Vickers hardness HV _0.15 and HV _0.40 were measured based on the method described above. The test force in the Vickers hardness test was 0.98 N. The obtained Vickers hardnesses HV _0.15 and HV _0.40 are shown in Table 3. Further, fn 5 obtained from Vickers hardnesses of HV _0.15 and HV _0.40 is shown in Table 3.

[Hydrogen embrittlement characteristics evaluation test]
Various concentrations of hydrogen were introduced to the test piece for each test number using a continuous charge constant load (constant potential) test method. The continuous charge constant load test method was performed as follows. The test piece was immersed in a 3% aqueous sodium chloride solution. With the test piece immersed, a cathode potential was generated on the surface of the test piece at a potential of 1000 mA to introduce hydrogen into the test piece.

  After hydrogen was introduced into the test piece, a zinc-plated film was formed on the surface of the test piece to prevent dissipation of hydrogen in the test piece. Subsequently, a constant load test was performed in which a constant load was applied to the V-notch cross section of the test piece. The maximum applied stress (MPa) that did not break even after 200 hours was determined.

Furthermore, a steel having a chemical composition corresponding to SCr 420 specified in JIS G 4053 (2016) was prepared as a standard product for hydrogen embrittlement resistance evaluation test. SCr 420 is a commonly used carburizing steel. Test pieces were produced in the same manner as in Test No. 1 with respect to the reference product. Carburizing treatment was performed on the produced test pieces under normal carburizing conditions. Specifically, the carburizing conditions were as follows.
Carburizing temperature Tc in the carburizing step S1: 930 ° C.
Treatment time t1: 180 minutes in carburizing step S1 Carbon potential Cp 1: 0.8 in carburizing step S1
Soaking temperature in soaking process S3: 870 ° C.
Holding time t3: 30 minutes in the soaking process S3 Carbon potential Cp3 in the soaking process S3: 0.8
The conditions for quenching and tempering were the same as in Test No. 1.

The C concentration of the surface of the shaft 3 of the reference product was 0.8%. The HV _0.15 of the reference product was 750 and the HV _0.40 was 700. In the reference product, the maximum applied stress (MPa) (reference endurance stress) that did not break even after 200 hours was determined.

  In the test result of each test number, when the ratio of the maximum applied stress (MPa) to the standard endurance stress was 1.6 or more, it was regarded as evaluation "A". When the ratio of the maximum applied stress (MPa) to the standard endurance stress was less than 1.4 to 1.6, it was evaluated as "B". When the ratio of the maximum applied stress (MPa) to the standard endurance stress was 1.2 to less than 1.4, it was regarded as "C". When the ratio of the maximum applied stress (MPa) to the reference endurance stress was less than 1.2, it was evaluated as "x". In the case of evaluations A to C, it was judged that the hydrogen embrittlement resistance was excellent. In the case of evaluation x, it was judged that the hydrogen embrittlement resistance was low. The results are shown in Table 3.

[Abrasion resistance evaluation test (roller pitching test)]
In order to evaluate the abrasion resistance, a roller pitting test was conducted. FIG. 9 is a schematic view of a roller pitching test. Specifically, tests were conducted under the following conditions using a roller pitching fatigue strength tester manufactured by Komatsu Engineering Co., Ltd.
Slip ratio: -40%
Lubricant: Automatic oil Lubricant temperature: 90 ° C
Lubricant flow rate: 2 L / min Speed: 1500 rpm
Surface pressure: 2000MPa

As shown in FIG. 9, the small roller 200 was rotated while pressing the large roller 100 against the small roller 200 with the surface pressure. The small roller 200 was a roller pitching test piece produced in the production of the above test piece. The large roller 100 uses a steel having a chemical composition corresponding to SCM 420 defined in JIS G 4053 (2016), which has been subjected to eutectoid carburization at low temperature and then surface-polished. The radius of the large roller 100 was 130 mm. The wear depth Dw of each test piece was measured at a rotational speed of 1 × 10 ⁶ times. A stylus type surface roughness meter was used to measure the wear depth Dw. The measuring length was 24 mm, and the stylus was scanned in the axial direction of each test piece to obtain a cross-sectional curve. In each test piece, measurement was carried out at two points every 180 ° in the circumferential direction to obtain cross-sectional curves. From the obtained cross-sectional curves, in each test piece, the average height of the cross-sectional curvilinear elements in the portion not in contact with the large roller 100 and the average height of the cross-sectional curvilinear elements in the portion worn by the large roller 100 in contact Were calculated respectively. Then, the difference in height between the portion where the large roller 100 was not in contact and the portion where the large roller 100 was in contact was calculated. The average value of the differences in height obtained at the measurement points was taken as the wear depth Dw (μm) of each test number.

  A test piece in which the wear depth Dw was less than 40 μm was evaluated as “A”, a test piece which was less than 40 to 70 μm was evaluated “B”, a test piece which was less than 70 to 100 μm was evaluated “C”, 100 The test piece which was less than 150 μm was evaluated as “D”, and the test piece which was 150 μm or more was evaluated as “x”. In the case of evaluations A to D, it was judged that the wear resistance was excellent. In the case of evaluation x, it was judged that the abrasion resistance was low. The results are shown in Table 3.

[Bending fatigue strength evaluation test (4 point bending fatigue test)]
A four-point bending fatigue test was performed using four-point bending fatigue test pieces of each test number. A servo fatigue tester was used for the test. The distance between the fulcrums of the four-point bending fatigue test piece was 45 mm. The maximum applied stress was 1150 MPa, and the stress ratio between the maximum applied stress and the minimum applied stress was 0.1. The frequency was 10 Hz. The breaking strength at 1 × 10 ⁴ stress load repetitions was defined as 4-point bending fatigue strength (MPa).

  A test piece with a 4-point bending fatigue strength of 850 MPa or more is regarded as evaluation “A”, a test piece which is less than 750-850 MPa is evaluated “B”, a test piece which is less than 650-750 MPa is evaluated “C”, The test piece which was less than 650 MPa was evaluated as "x". In the case of evaluations A to C, it was judged that the bending fatigue strength was excellent. In the case of evaluation x, it was judged that the bending fatigue strength was low. The results are shown in Table 3.

[Evaluation results]
Table 3 shows the evaluation results.

  Referring to Tables 1 to 3, test numbers 1 to 4, 6 to 10, 13, 14, 16, 17, 19, 20, 22, 23, 25, 26, 28, 29, 32, 33, 36, In 38, 39, 42, 43, 46, 47 and 50 to 54, any test number had the chemical composition defined in this embodiment. Furthermore, the surface C concentration was 0.50 to 0.70%. Formulas (1) to (5) are satisfied. As a result, it showed excellent resistance to hydrogen failure and had sufficient wear resistance and flexural fatigue strength.

  On the other hand, in test No. 5, the C content was too high. Therefore, the hydrogen embrittlement resistance was low.

  In Test No. 11, the Mn content was too high. Therefore, the hydrogen embrittlement resistance was low.

  In Test No. 12, the Mn content was too low. Therefore, the wear resistance was low.

  In Test No. 15, the P content was too high. Therefore, the hydrogen embrittlement resistance was low.

  In Test No. 18, the S content was too high. Therefore, the hydrogen embrittlement resistance was low.

  In Test No. 21, the Al content was too low. Therefore, the hydrogen embrittlement resistance was low.

  In Test No. 24, the N content was too low. Therefore, the hydrogen embrittlement resistance was low.

  In Test No. 27, the Cr content was too low. Therefore, the wear resistance was low.

In Test No. 30, the surface C concentration was too high. Therefore, HV _0.15 was too high. As a result, the hydrogen embrittlement resistance was low.

  In Test No. 31, the surface C concentration was too low. Therefore, the wear resistance was low.

  In Test No. 34, fn1 was too high. Therefore, the hydrogen embrittlement resistance was low.

  In Test No. 35, fn1 was too low. Therefore, the hydrogen embrittlement resistance was low.

  In Test No. 37, fn2 was too low. Therefore, the hydrogen embrittlement resistance was low.

For test number 40, HV _0.15 was too high. As a result, the hydrogen embrittlement resistance was low. Since the oil temperature at the time of oil quenching was less than 120 ° C., the hardness of the carburized part 1 was overall increased, and the treatment time t 1 was longer, so it is considered that the surface C concentration was higher. .

For test number 41, HV _0.15 was too low. Therefore, the wear resistance was low. It is considered that the oil temperature exceeded 140 ° C.

For test number 44, HV _0.40 was too high. Therefore, the hardness gradient fn5 was small. As a result, the hydrogen embrittlement resistance was low. It is considered that the sum (t1 + t2) of the treatment time t1 in the carburizing step S1 and the treatment time t2 in the diffusion step S2 is too long.

In Test No. 45, HV _0.40 was too low. Therefore, bending fatigue strength was low. It is considered that the treatment time t1 in the carburizing step S1 is too short.

  In the test numbers 48 and 55, the oil temperature was less than 120 ° C. Therefore, the hardness gradient fn5 was too small. Therefore, the hydrogen embrittlement resistance was low.

  In the test No. 49, the hardness gradient fn5 was too large. Therefore, bending fatigue strength was low. It is considered that the processing time t2 in the diffusion step S2 is too short.

  The embodiment of the present invention has been described above. However, the embodiments described above are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the scope of the invention.

1 carburizing part 2 sliding part 3 axis part 4 stress concentration part 5 notches 100 large roller 200 small roller

Claims (3)

A sliding portion having a sliding surface in contact with another member;
And a shaft connected to the sliding portion,
The shaft portion includes a stress concentration portion including one or more notches extending in the circumferential direction of the shaft portion,
The chemical composition of the core portion excluding the hardened layer in the shaft portion is in mass%,
C: 0.10 to 0.30%,
Si: less than 0.50%,
Mn: 0.30 to 1.40%,
P: less than 0.030%,
S: less than 0.030%,
Cr: 0.50 to 2.00%,
Al: 0.010 to 0.100%,
N: 0.001 to 0.030%,
Mo: 0 to 0.80%,
Ni: 0 to 0.50%,
Cu: 0 to 0.50%,
Ti: 0 to 0.10%, and
Nb: containing 0 to 0.10%, the balance being Fe and impurities,
The C concentration of the surface of the shaft portion is 0.50 to 0.70%,
In the shaft portion, the maximum outer diameter of the notch is D (mm), the radius of curvature of the notch bottom of the notch is r (mm), and the Vickers hardness at a depth of 0.15 mm from the surface is HV _0.15 , When the Vickers hardness at a depth of 0.40 mm from the surface is defined as HV _0.40 , the formulas (1) to (5) are satisfied.
Carburized parts.
1.0 <fn1 = 3 × r × D <60.0 (1)
0.010 <r / D (2)
580 + fn1 ≦ HV _0.15 ≦ 730 + fn1 (3)
430 + fn 1 HV HV _0.40 630 630 + fn 1 (4)
−900 <(HV _0.40 −HV _0.15 ) /0.25 <−280 (5) A carburized part according to claim 1, wherein
The chemical composition is
Mo: 0.01 to 0.80%,
Ni: 0.05 to 0.50%, and
Cu: The carburized part characterized by containing 1 type, or 2 or more types selected from the group which consists of 0.10 to 0.50%. A carburized part according to claim 1 or claim 2, wherein
The chemical composition is
Ti: 0.05 to 0.10%, and
A carburized part characterized in that it contains one or two selected from the group consisting of Nb: 0.01 to 0.10%.

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