JP2019183211A - Carburization component - Google Patents

Abstract

To provide a carburization component excellent hydrogen embrittlement resistance, and sufficient abrasion resistance and fatigue strength.SOLUTION: A chemical composition of a core part of a carburization component contains C:0.10 to 0.30%, Si:0.50% to 1.50%, 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 has surface C concentration of 0.50 to 0.70%. Following formulae are satisfied when maximum outer diameter of notch of a shank is D (mm), curvature radius of notch bottom is r (mm), Vickers hardness at 0.15 mm depth and 0.40 mm depth from the surface are HVand HV. 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.SELECTED DRAWING: Figure 2

Description

  The present invention relates to carburized parts.

  Wear is generated in a mechanical power source such as an engine of an automobile or an industrial machine and a mechanical component used in a power transmission mechanism due to a load or sliding applied impactively. Furthermore, it is subjected to bending stress during use. Accordingly, wear resistance and high bending fatigue strength are required for mechanical parts for these applications. In order to obtain sufficient wear resistance and high bending fatigue strength, carburized parts are often used as machine parts used in these applications. The carburized part is obtained by carburizing a steel material. By the carburizing process, a hardened layer is formed on the surface of the carburized component. This hardened layer provides wear resistance and high bending fatigue strength.

  By the way, in the carburizing process for manufacturing carburized parts, hydrocarbon gas is usually used as the atmospheric gas. During the carburizing process, the hydrocarbon gas is decomposed to generate hydrogen. Hydrogen generated during the carburizing process may be absorbed into the carburized parts. Therefore, in the carburized parts, hydrogen embrittlement cracks due to hydrogen absorbed in the steel during the carburizing process may occur. Therefore, carburized parts are required to have not only wear resistance and high bending fatigue strength but also excellent hydrogen embrittlement resistance.

  Japanese Patent Application Laid-Open No. 9-217148 (Patent Document 1), Japanese Patent Application Laid-Open No. 2014-19920 (Patent Document 2) and Japanese Patent Application Laid-Open No. 2012-36475 (Patent Document 3) ) Is proposed.

The high-strength case-hardened steel described in Patent Document 1 is in weight percent, C: 0.15-0.25%, Si: 0.1% or less, Mn: 0.30-1.00%, 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%, It contains Al: 0.015-0.030%, N: 0.0100-0.0180%, O _T : 0.0015% or less, and consists of the balance Fe and inevitable impurity elements, and is annealed by high-frequency heating. And has a hardness of Hv420 or less.

  The steel material for carburized or carbonitrided parts described in Patent Document 2 is 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 0.003 to 0.020% and Al: 0.005 to 0.050%, with the balance being 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 .6Mn + 4Cr—Cu—1.6Ni ≧ 4.0 is satisfied. Thereby, during use of carburized or carbonitrided parts, the amount of hydrogen generated from the lubricant and moisture mixed from the outside can be kept low, and the sensitivity to hydrogen embrittlement can be suppressed. This is described in Patent Document 2.

  The rolling parts or gears described in Patent Document 3 are in 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% Contained, the balance is made of steel with Fe and inevitable impurities. In Patent Literature 3, the amount of (C + N) in the steel material surface of the rolling component or gear is set to 0.50 to 0.75% by carburizing or carbonitriding.

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

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

  Patent Documents 2 and 3 disclose techniques for suppressing embrittlement against hydrogen that enters during use of carburized parts. However, as described above, hydrogen is absorbed by the steel material to some extent during the carburizing process. Therefore, it is preferable to improve the hydrogen embrittlement resistance against hydrogen absorbed during the carburizing process. Patent Document 2 does not disclose this point.

  According to Patent Document 3, the hydrogen embrittlement resistance in the smooth portion is enhanced by setting the surface carbon concentration and nitrogen concentration to 0.50 to 0.75%. However, in a carburized part, hydrogen embrittlement is likely to occur at a stress concentration portion including a notch. In patent document 3, there is no disclosure regarding the point which improves the hydrogen embrittlement resistance in a stress concentration part.

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

The carburized component according to the embodiment of the present invention includes a sliding portion and a shaft portion. A sliding part has a sliding surface which contacts another member. The shaft portion is connected to the sliding portion. The shaft portion includes a stress concentration portion. The stress concentration portion includes one or a plurality of 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 mass%, C: 0.10 to 0.30%, Si: 0.50 to 1.50%, 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 on the surface of the shaft 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 of the notch is r (mm), the Vickers hardness at a depth of 0.15 mm from the surface is HV _0.15 , and 0 from the surface. When the Vickers hardness at the 40 mm depth position is defined as HV _0.40 , the expressions (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 + fn1 ≦ HV _0.40 ≦ 630 + fn1 (4)
−900 <(HV _0.40 −HV _0.15 ) /0.25 <−280 (5)

  The carburized part of the present embodiment has excellent hydrogen embrittlement resistance and sufficient wear resistance and bending fatigue strength.

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

  Hereinafter, the carburized component according to the present embodiment will be described.

[Carburized parts of this embodiment]
First, a carburized component that is 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 according to the present embodiment includes a sliding portion 2 and a shaft portion 3.

  The sliding part 2 has a sliding surface. The sliding surface is a surface that comes into contact with another member and the other member slides with respect to the sliding surface. The other member is, for example, a machine part. The machine part which is another member may be a carburized part. 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 portion 3 has a rod 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 notch 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 component 1 is used for a drive source and a transmission mechanism such as an engine of an automobile or an industrial machine, for example. The carburized component 1 is, for example, a pulley or a pinion gear. 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 portion. The hardened layer is formed on the surface layer including the surface of the carburized component. The core part is an area inside the hardened layer. The depth position of 0.15 mm from the surface of the carburized component 1 corresponds to the hardened layer. The Vickers hardness at a depth of 0.15 mm from the surface of the carburized part 1 is defined as Vickers hardness HV _0.15 . The 0.40 mm depth position from the surface of the carburized component 1 corresponds to the core portion. The Vickers hardness at a position 0.40 mm deep from the surface of the carburized part 1 is defined as Vickers hardness HV _0.40 . In the carburized part of this embodiment, the Vickers hardness HV _0.15 is higher than the Vickers hardness HV _0.40 .

[Technical concept up to the idea of carburized parts of this embodiment]
Usually, in order to increase the strength of mechanical parts, it is effective to increase the hardenability and increase the hardness of the core.

  However, in the carburized component including the stress concentration portion as described above, when the core portion has a high hardness, the hydrogen embrittlement resistance at the stress concentration portion becomes low. For this reason, the present inventors have studied a method for improving the hydrogen embrittlement resistance of the stress concentration portion while having the wear resistance and high bending fatigue strength required as a mechanical component for power transmission. As a result, the following knowledge was obtained.

  When carburized parts are manufactured by performing carburizing treatment on steel materials, compressive residual stress is generated on the surface layer of the carburized parts. The reason for this is as follows. When the carburizing process is performed, the C concentration of the surface layer of the steel material becomes higher than the C concentration of the core portion. Therefore, the martensitic transformation start temperature (Ms point) of the surface layer is lower than the Ms point of the core part. As a result, during the carburizing process, the core starts martensitic transformation before the surface layer. Since the surface layer part undergoes martensitic transformation after the core part, compressive residual stress is generated in the surface layer.

  The inventors paid attention to the compressive residual stress generated by the carburizing process. Specifically, it was considered that the hydrogen embrittlement resistance would be improved if the compressive residual stress on the surface of the stress concentration part of the carburized part was increased.

  Therefore, the present inventors further examined a method for increasing the compressive residual stress on the surface. FIG. 2 is a diagram schematically showing a residual stress distribution in a cross section of the carburized component. The horizontal axis in FIG. 2 indicates the distance from the surface in the cross section of the carburized component, and the left end and right end of the horizontal axis in FIG. 2 correspond to the surface. The vertical axis in FIG. 2 indicates the residual stress. Plus (+) means tensile residual stress, and minus (−) means compressive residual stress.

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

  The region where the compressive residual stress is applied (the region 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 so that the hardened layer is shallower than that in FIG. 2, and the depth at which the compressive residual stress is applied is H2 which is shallower than the depth H1.

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

  As described above, in order to suppress the occurrence of cracks on the surface at the stress concentration portion of the carburized part, in order to increase the compressive residual stress on the surface, it is only necessary to narrow the region where the compressive stress is applied in the depth direction. (Depth H1 → H2). If the compressive residual stress on the surface of the stress concentration portion of the carburized part is increased, the tensile residual stress generated at the notch bottom is relieved and cracking on the surface is less likely to occur. As a result, the hydrogen embrittlement resistance at the stress concentration part of the carburized part is enhanced.

  Based on the above concept, the present inventors have studied the shape of the stress concentration portion, the hardness of the stress concentration portion at the surface layer portion, 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). Further, 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 diameters of the notches 5 is defined as D (mm), and the average radius of curvature of the notch bottom of each notch 5 is r ( mm).

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

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

The carburized component according to the embodiment of the present invention completed based on the above knowledge includes a sliding portion and a shaft portion. A sliding part has a sliding surface which contacts another member. The shaft portion is connected to the sliding portion. The shaft portion includes a stress concentration portion. The stress concentration portion includes one or a plurality of 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 mass%, C: 0.10 to 0.30%, Si: 0.50 to 1.50%, 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 on the surface of the shaft 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 of the notch is r (mm), the Vickers hardness at a depth of 0.15 mm from the surface is HV _0.15 , and 0 from the surface. When the Vickers hardness at the 40 mm depth position is defined as HV _0.40 , the expressions (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 + fn1 ≦ HV _0.40 ≦ 630 + fn1 (4)
−900 <(HV _0.40 −HV _0.15 ) /0.25 <−280 (5)

  The chemical composition contains at least one 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%. May be.

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

  Hereinafter, carburized parts according to embodiments of the present invention will be described in detail.

[Chemical composition of the core of the shaft of carburized parts]
The carburized component 1 according to the present embodiment includes a sliding portion 2 and a shaft portion 3 as shown in FIG. Of the shaft part 3 of the carburized part, the chemical composition of the core part excluding the hardened layer contains the following elements. Here, the chemical composition of the core is determined by product analysis in accordance with JIS G 0321 (2010) in a sample collected at a position deeper than a depth of 2.0 mm from the surface in the shaft portion of the carburized part. It means the chemical composition obtained. Hereinafter, “%” regarding the element content means mass%.

C: 0.10 to 0.30%
Carbon (C) increases the hardenability of the steel. Thereby, the hardness of a carburized component increases and wear resistance increases. If the C content is less than 0.10%, this effect cannot be obtained. On the other hand, if the C content exceeds 0.30%, the machinability and cold forgeability of steel may be reduced. Therefore, the C content is 0.10 to 0.30%. The minimum with preferable C content is 0.15%, More preferably, it is 0.18%. The upper limit with preferable C content is 0.25%, More preferably, it is 0.23%.

Si: 0.50 to 1.50%
Silicon (Si) deoxidizes steel. Si further increases the temper softening resistance of the steel. This increases the wear resistance of the carburized parts. However, if the Si content is less than 0.50%, the above effect cannot be obtained sufficiently, and the wear resistance of the carburized parts is lowered. On the other hand, if the Si content exceeds 1.50%, the carburization of steel is inhibited. In this case, the wear resistance of the carburized parts decreases. Therefore, the Si content is 0.50 to 1.50%. The minimum with preferable Si content is 0.52%, More preferably, it is 0.60%, More preferably, it is 0.80%. The upper limit with preferable Si content is 1.30%, More preferably, it is 1.20%.

Mn: 0.30 to 1.40%
Manganese (Mn) deoxidizes steel. Mn further increases the hardenability and strength of the steel. Mn further increases the softening resistance of the steel. As a result, the wear resistance of the carburized parts is increased. If the Mn content is less than 0.30%, these effects cannot be obtained. On the other hand, if the Mn content exceeds 1.40%, co-segregation with P becomes remarkable and grain boundary embrittlement occurs. As a result, the hydrogen embrittlement resistance of the carburized parts decreases. Therefore, the Mn content is 0.30 to 1.40%. The minimum with preferable Mn content is 0.50%, More preferably, it is 0.70%. The upper limit with preferable Mn content is 1.20%, More preferably, it is 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 the austenite grain boundaries during carburizing and lowers the grain boundary strength of the carburized layer. If the grain boundary strength of the carburized layer decreases, the hydrogen embrittlement resistance decreases. If the P content is less than 0.030%, not only the core but also the P content of the surface layer is low. For this reason, the toughness of the surface layer is increased and the occurrence of grain boundary cracks is suppressed. As a result, the hydrogen embrittlement resistance is enhanced. Accordingly, the P content is less than 0.030%. The upper limit with preferable 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 lowered. Therefore, the lower limit of the preferable P content is 0.0001%, 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 the crystal grain boundary and lowers the grain boundary strength of the carburized layer. Further, S is a point where coarse MnS is formed at the grain boundary and the residual stress is concentrated. As a result, the hydrogen embrittlement resistance of the carburized parts decreases. Therefore, the S content is less than 0.030%. The upper limit with preferable S content is 0.015%. The S content is preferably as low as possible. However, if the S content is extremely reduced in the steelmaking process, manufacturing costs are increased and 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) increases the hardenability of the steel and increases the hardness of the core. Cr further increases temper softening resistance. As a result, the wear resistance of the carburized parts is increased. If the Cr content is less than 0.50%, this effect cannot be obtained sufficiently. On the other hand, if the Cr content exceeds 2.00%, the cold workability deteriorates. Therefore, the Cr content is 0.50 to 2.00%. The minimum with preferable Cr content is 0.60%, More preferably, it is 0.80%. The upper limit with preferable Cr content is 1.85%, More preferably, it is 1.70%.

Al: 0.010 to 0.100%
Aluminum (Al) deoxidizes steel. Further, Al combines with N in the steel to form AlN and suppresses austenite grain coarsening during carburizing. As a result, the hydrogen embrittlement resistance of carburized parts is enhanced. If the Al content is less than 0.010%, this effect cannot 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 minimum with preferable Al content is 0.020%, More preferably, it is 0.025%. The upper limit with preferable Al content is 0.080%, More preferably, it is 0.065%. In the chemical composition of the core part of the carburized part 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 austenite grain coarsening during carburization. Thereby, the hydrogen embrittlement resistance characteristics of the carburized parts are enhanced. If the N content is less than 0.001%, a sufficient coarsening suppressing effect cannot be obtained. On the other hand, if N content exceeds 0.030%, the said effect will be saturated. Therefore, the N content is 0.001 to 0.030%. The minimum with preferable N content is 0.005%, More preferably, it is 0.008%. The upper limit with preferable N content is 0.025%, More preferably, it is 0.020%.

  The balance of the chemical composition of the core of the carburized component according to the present embodiment is composed of Fe and impurities. Here, the impurities are mixed from ore as a raw material, scrap, or a manufacturing environment when industrially producing the carburized parts, and do not adversely affect the carburized parts of the present embodiment. Means what is allowed.

[Arbitrary elements]
The chemical composition of the core part of the shaft part of the carburized component of this embodiment may further include one or more selected from the group consisting of Mo, Ni, and Cu instead of a part of Fe. . These elements are arbitrary elements, and all enhance the hardenability of 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 contained, Mo increases the temper softening resistance of the carburized part and increases the wear resistance of the carburized part. These effects can be obtained if even a small amount of Mo is contained. However, if the Mo content exceeds 0.80%, these effects are saturated and the raw material costs are increased. Therefore, the Mo content is 0 to 0.80%. The minimum with preferable Mo content for acquiring the said effect stably is 0.01%, More preferably, it is 0.10%. The upper limit with preferable Mo content is 0.60%, More preferably, it is 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 contained, Ni increases the hardenability of the steel and increases the strength of the carburized component. If Ni is contained even a little, these effects can be obtained. 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 minimum with preferable Ni content for acquiring the said effect stably is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Ni content is 0.45%, More preferably, it is 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 contained, Cu increases the hardenability of the steel and increases the strength of the carburized part. This effect can be obtained if even a small amount of Cu is contained. On the other hand, if Cu content exceeds 0.50%, hot workability will fall. Therefore, the Cu content is 0 to 0.50%. The minimum with preferable Cu content for acquiring the said effect stably is 0.10%, More preferably, it is 0.15%. The upper limit with preferable Cu content is 0.35%, More preferably, it is 0.25%.

  The core part of the carburized component of the present embodiment may further contain one or two selected from the group consisting of Ti and Nb instead of a part of Fe. These elements are arbitrary 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 contained, Ti combines with C and S in the steel to form fine TiC and TiS, and suppresses austenite grain coarsening during carburization. Thereby, the hydrogen embrittlement resistance characteristics of the carburized parts are enhanced. This effect can be obtained if Ti is contained even a little. However, if the Ti content exceeds 0.10%, TiC becomes coarse and the toughness of the steel decreases. In this case, the hydrogen embrittlement resistance of the carburized parts decreases. Therefore, the Ti content is 0 to 0.10%. The minimum with preferable Ti content for acquiring the said effect stably is 0.05%. The upper limit with preferable Ti content is 0.08%, More preferably, it is 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 contained, Nb combines with C and N in the steel to form Nb carbonitride (Nb (CN)), and suppresses austenite grain coarsening during carburizing. Thereby, the hydrogen embrittlement resistance characteristics of the carburized parts are enhanced. This effect can be obtained if Nb is contained even a little. However, if the Nb content exceeds 0.10%, the carburizing property is lowered. Therefore, the Nb content is 0 to 0.10%. The minimum with preferable Nb content for acquiring the said effect stably is 0.01%, More preferably, it is 0.02%. The upper limit with preferable Nb content is 0.07%, More preferably, it is 0.05%.

[C concentration on the surface of the shaft of the carburized part: 0.50 to 0.70%]
The C concentration on the surface of the shaft portion of the carburized component (hereinafter, “surface C concentration”) is 0.50 to 0.70% in mass%. If the surface C concentration is less than 0.50%, the surface hardness of the carburized component is too low and the wear resistance decreases. On the other hand, if the surface C concentration exceeds 0.70%, the toughness of the carburized layer is lowered, so that the hydrogen embrittlement resistance is deteriorated. Therefore, the surface C concentration is 0.50 to 0.70%. A preferable lower limit of the surface C concentration is 0.54%, and more preferably 0.56%. The upper limit with preferable surface C density | concentration is 0.66%, More preferably, it is 0.64%.

  The surface C concentration of the carburized part is measured by the following method. Select any five measurement positions on the surface of the carburized parts. 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.

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

  In Formula (1), if fn1 (the product of the curvature radius r of the notch bottom and the maximum outer diameter D of the notch) is too small, stress (tensile stress) tends to concentrate on the surface of the stress concentration portion of the shaft portion. , Cracking is likely to occur. For this reason, the hydrogen embrittlement resistance is reduced. On the other hand, there is no upper limit of fn1 in relation to hydrogen embrittlement resistance, but the upper limit of fn1 is less than 60.0 as the shape of the carburized part. Therefore, fn1 is more than 1.0 and less than 60.0. A preferred lower limit of fn1 is 1.5, more preferably 4.0, and even more preferably 8.0. The upper limit with preferable fn1 is 55.0, More preferably, it is 50.0, More preferably, it is 45.0.

  In the formula (2), it is defined as fn2 = r / D. When fn2 is too small, stress (tensile stress) tends to concentrate on the notch bottom when a load is applied. In this case, the hydrogen embrittlement resistance decreases. Therefore, fn2 is greater than 0.010. The minimum with preferable fn2 is 0.015, More preferably, it is 0.018. Although the upper limit of fn2 is not specifically limited, For example, it is 0.200.

[Regarding Formula (3) and Formula (4)]
In the stress concentration portion, 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 expressed by the formula (3). And Expression (4) is satisfied.
580 + fn1 ≦ HV _0.15 ≦ 730 + fn1 (3)
430 + fn1 ≦ HV _0.40 ≦ 630 + fn1 (4)

Vickers hardness HV _0.15 is an index of hardness in the cured layer, and Vickers hardness HV _0.40 is an index of hardness in the 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 component is lowered. On the other hand, if the hardness of the hardened layer is too high, cracks are likely to occur, particularly at the notch bottom, and the hydrogen embrittlement resistance is reduced. If HV _0.15 satisfies the formula (3), the above chemical composition has excellent wear resistance and the premise that the formula (1), the formula (2), the formula (4) and the formula (5) are satisfied. Hydrogen embrittlement resistance is obtained.

It is defined as fn3 = HV _0.15− fn1. The preferable lower limit of fn3 is 600, and more preferably 620. The upper limit with preferable fn3 is 710, More preferably, it is 690.

Referring to equation (4), if the core part represented by Vickers hardness HV _0.40 is too low, it will be plastically deformed when a bending load is applied, and the stress on the surface will increase. Bending fatigue strength as a part is lowered. On the other hand, if the hardness of the core is too high, cracks are likely to occur particularly at the notch bottom, and the hydrogen embrittlement resistance is reduced. If HV _0.40 satisfies the formula (4), excellent fatigue strength and anti-resistance are provided on the premise that the chemical composition satisfies the formula (1), the formula (2), the formula (3), and the formula (5). Hydrogen embrittlement characteristics are obtained.

It is defined as fn4 = HV _0.40- fn1. The preferable lower limit of fn4 is 450, more preferably 470. The upper limit with preferable fn3 is 610, More preferably, it is 590.

[Regarding Formula (5)]
As shown in FIG. 2, the compressive residual stress applied to the surface can be increased by narrowing the region (A2 and A3) where the compressive residual stress is applied in the depth direction from the surface of the carburized component. In order to narrow the area (A2 and A3) where the compressive residual stress is applied, the hardness difference between the hardness of the surface layer and the hardness of the core portion can be increased, and the hardness gradient between the surface layer and the core portion can be made steep. do it.

It is defined as fn5 = (HV _0.40 −HV _0.15 ) /0.25. fn5 means a hardness gradient between a 0.15 mm depth position from the surface and a 0.40 mm depth position from the surface. When fn5 is −900 or less, the hardness gradient between the hardness of the surface layer and the hardness of the core is too large. In this case, the hardness of the core portion becomes too low, and the bending fatigue strength as a part is lowered. On the other hand, when fn5 is −280 or more, since the hardness gradient between the hardness of the surface layer and the hardness of the core is too small, sufficient compressive residual stress cannot be obtained on the surface. In this case, the hydrogen embrittlement resistance of the carburized parts decreases.

  If fn5 is more than −900 to less than −280, that is, if fn5 satisfies the formula (5), excellent fatigue strength is obtained on the premise that the chemical composition satisfies the formula (1) to the formula (4). And hydrogen embrittlement resistance can be obtained. A preferable lower limit of fn5 is −850, and more preferably −800. The upper limit with preferable fn5 is -330, More preferably, it is -380.

[Measurement method of Vickers hardness HV _0.15 and HV _0.40 ]
Vickers hardness HV _0.15 and HV _0.40 can be measured by the following method. Vickers hardness according to JIS Z 2244 (2009) 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 in the shaft part of the carburized part Conduct the test. The test force is 0.98N. The average of the values obtained at any five locations is defined as Vickers hardness HV _0.15 and HV _0.40 , respectively.

[Manufacturing process]
An example of a method for manufacturing a carburized component according to the present embodiment will be described.

  A steel material satisfying the above chemical composition is prepared. For example, the steel material is manufactured and prepared by the following method. Molten steel having the above chemical composition is produced, and a slab (slab or bloom) is produced by continuous casting using the molten steel. You may manufacture an ingot (steel ingot) by the ingot-making method using molten steel. A billet (steel piece) is manufactured by hot working a slab or an ingot. The billet is hot worked to produce a steel bar or wire. The hot working may be hot rolling or hot forging.

  Forging is performed on the manufactured steel. The forging method may be hot or cold. In the case of hot forging, the heating temperature of the steel material during hot forging is not particularly limited, but is, for example, 1000 to 1300 ° C. An intermediate product including a sliding portion and a shaft portion is manufactured by performing machining such as cutting as necessary on the steel material after forging.

  Carburizing and quenching is performed on the manufactured intermediate product, and further, tempering is performed on the intermediate product after the carburizing and quenching processing. Machining (cutting or the like) may be further performed on the intermediate product after quenching. Carburized parts are manufactured by the above manufacturing process.

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

[Carburizing and quenching]
The carburizing and quenching process includes a gas carburizing process and a quenching (rapid cooling) process. The carburizing process performed on the carburized component of the present embodiment is a gas carburizing process. Hereinafter, the gas carburizing process and the quenching process will be described.

[Gas carburizing step S10]
FIG. 4 is a diagram illustrating an example of a heat pattern in the gas carburizing step S10 and the quenching step S20. The vertical axis in FIG. 4 is the processing temperature (° C.) during the carburizing process, and the horizontal axis is the time. Referring to FIG. 4, gas carburizing step S10 includes a heating step S0, a carburizing step S1, a diffusion step S2, and a soaking step S3.

  In the heating step S0, the intermediate product charged in the furnace is heated to the carburizing temperature Tc. In the carburizing step S1, the carburizing process is performed by holding the intermediate product at a carburizing temperature Tc for a predetermined time (processing time t1) in an atmosphere of a predetermined carbon potential Cp1. In the diffusion step S2, the carburizing temperature Tc is maintained for a predetermined time (processing time t2) in an atmosphere having a carbon potential Cp2 lower than the carbon potential Cp1 in the carburizing step S1 and similar to the surface C concentration of the carburized component. . The soaking step S3 is a step aimed at soaking the entire intermediate product to a predetermined quenching temperature. In the soaking step S3, soaking is performed for a predetermined time (holding time t3) at a soaking temperature Ts lower than the carburizing temperature Tc. However, the soaking step S3 may be omitted.

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

  A preferable carburizing temperature Tc is 900 to 1100 ° C, and more preferably 920 to 1050 ° C. When the carburizing temperature Tc is 900 ° C. or higher, carburized parts 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 difficult to coarsen.

  The processing time t1 in the carburizing step S1 and the processing time t2 in the diffusion step S2 are 110 minutes or less in total (t1 + t2). However, it is a condition that the carbon potentials Cp1 and Cp2 are in a preferable range described later. If the carburizing time is too long, the core is baked and the hardness of the core is increased. In this case, the hardness gradient fn5 between the surface layer portion and the core portion is reduced. As a result, the hydrogen embrittlement resistance of the carburized parts decreases.

On the condition that the carbon potentials Cp1 and Cp2 are in a preferable range described later, the lower limit of the treatment time t1 in the carburizing step S1 is 45 minutes. If processing time t1 is too short, the hardness of the core part represented by Vickers hardness HV _0.40 will fall. A more preferable lower limit of the processing time t1 in the carburizing step S1 is 50 minutes. The upper limit with preferable processing time t1 is 80 minutes, More preferably, it is 70 minutes.

On the condition that the carbon potentials Cp1 and Cp2 are in a preferable range described later, the preferable lower limit of the processing time t2 in the diffusion step S2 is more than 25 minutes. If the treatment time t2 is too short, the diffusion time is not sufficient, and the carbon diffusion becomes closer to the surface. That is, the gradient of the carbon concentration between the surface and the core is increased. As a result, the hardness of the surface represented by HV _0.15 increases and HV _0.40 decreases. The lower limit of the processing time t2 in the diffusion step S2 is more preferably 30 minutes. The upper limit with preferable processing time t2 is 50 minutes, More preferably, it is 45 minutes.

  The upper limit of the total of processing time t1 and t2 is 110 minutes, More preferably, it is 100 minutes.

  The carbon potential Cp1 in the carburizing step S1 is preferably 0.80 to 1.10. The carbon potential Cp2 in the diffusion step S2 is preferably 0.55 to 0.80. If the carbon potentials Cp1 and Cp2 are too high, the core portion is baked to increase the hardness of the core portion. In this case, the hardness gradient between the hardness of the surface layer portion and the hardness of the core portion becomes small. As a result, the hydrogen embrittlement resistance is reduced. A preferable lower limit of the carbon potential Cp1 is 0.85, and more preferably 0.90. The upper limit with preferable carbon potential Cp1 is 1.05, More preferably, it is 1.00. The minimum with preferable carbon potential Cp2 is 0.60, More preferably, it is 0.65. The upper limit with preferable carbon potential Cp2 is 0.75, More preferably, it is 0.70.

  The soaking step S3 may or may not be performed. When performing soaking step S3, the upper limit of preferable soaking temperature Ts is carburizing temperature Tc-10 degreeC, More preferably, it is Tc-50 degreeC. The minimum of preferable soaking temperature Ts is Tc-100 degreeC, More preferably, it is Tc-70 degreeC.

[Quenching step (S20)]
A quenching step S20 is performed on the intermediate product after the gas carburizing step S10. In the quenching step S20, the intermediate product after the gas carburizing step 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 soaking temperature Ts may be set as the quenching temperature, and after soaking for a predetermined time t3, rapid cooling may be performed. . Moreover, as shown in FIG. 5, the intermediate product after gas carburizing process S10 may be cooled to normal temperature, and hardening process S20 may be implemented after that. 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.

  A preferable quenching temperature in the quenching step S20 is 800 to 900 ° C. A more preferred lower limit of the quenching temperature is 820 ° C. A preferable upper limit of the quenching temperature is 880 ° C.

The cooling method in the quenching process is oil cooling. Specifically, the intermediate product maintained at the quenching temperature is immersed in a cooling bath containing oil as a cooling medium and rapidly cooled. 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 portion becomes small, and fn5 does not satisfy Expression (5). As a result, the hydrogen embrittlement resistance is reduced. On the other hand, when the oil temperature exceeds 140 ° C., the hardness HV _{0.15 of the} surface layer becomes too low, and the wear resistance decreases. Therefore, the oil temperature during quenching is 120 to 140 ° C. During oil cooling, the oil in the bath is sufficiently stirred by a well-known method. In addition, the characteristic temperature of the oil which is a cooling medium is not specifically limited, For example, it is 500-650 degreeC.

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

  Molten steel having the chemical composition shown in Table 1 and Table 2 was manufactured. Ingots were produced using the produced molten steel. The ingot was hot forged to produce a 50 mm diameter round bar. The following evaluation test was carried out using the manufactured round bar.

[Preparation of test piece]
First, a normalizing process was performed on each steel bar. The treatment temperature in the normalization treatment was 925 ° C., and the holding time was 1 hour. The steel bar after the holding time had elapsed was allowed to cool in the atmosphere. Three types of test pieces were prepared from the steel bars after the normalizing treatment.

[Test specimen for evaluating hydrogen embrittlement resistance]
Machining was performed on the steel bar having a diameter of 50 mm after the normalizing treatment, and a plurality of hydrogen embrittlement resistance evaluation test pieces shown in FIG. In FIG. 6, 1.00 mm indicates that the V-notch depth of the test piece is 1.00 mm. “D” in FIG. 6 indicates the maximum outer diameter of the V notch (the outer diameter of the shaft) (mm). “60 °” in the figure 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 number 35, the maximum outer diameter D was 20 mm, and the curvature radius r was 0.9 mm. In Test No. 36, the maximum outer diameter D was 5 mm and the curvature radius r was 0.072 mm. In Test No. 37, the maximum outer diameter D was 20 mm, and the curvature radius r was 1.1 mm. In Test No. 38, the maximum outer diameter D was 4 mm, and the curvature radius r was 0.072 mm. In Test No. 39, the maximum outer diameter D was 20 mm and the curvature radius r was 0.3 mm. In test number 40, the maximum outer diameter D was 20 mm, and the curvature radius r was 0.15 mm. In all other test numbers, the maximum outer diameter D was 20 mm and the curvature radius r was 0.36 mm.

[Roller pitching specimen]
Machine processing was performed on the steel bar having a diameter of 50 mm after the normalizing treatment, and a roller pitching test piece having a shape shown in FIG. 7 was produced. Numerical values in FIG. 7 indicate dimensions. The roller pitching specimen was cylindrical and had a parallel part with a diameter of 26 mm in the center. The diameter of the roller pitching test piece other than the parallel portion was 22 mm. The roller pitching test piece was a small roller in a roller pitching test described later.

[Fabrication of 4-point bending fatigue test piece]
Machining was performed on the steel bar having a diameter of 50 mm after the normalizing treatment, and a plurality of 4-point bending fatigue test pieces having the shape shown in FIG. 8 were collected. The four-point bending fatigue test piece was 13 mm in height and width, and 100 mm in length. A notch having a semicircular cross-sectional shape was formed at the center in the length direction of the four-point bending fatigue test piece. The semicircle notch extended in the width direction of the 4-point bending fatigue test piece. The radius of curvature r of the semicircular notch was 2 mm.

  Carburizing treatment and quenching treatment were performed on the hydrogen embrittlement resistance evaluation test piece, the roller pitching test piece, and the 4-point bending fatigue test piece, and carburized parts of Test No. 1 to Test No. 59 were manufactured. In addition, about the roller pitching test and the 4-point bending fatigue test of the test numbers 35-40, it estimated that it might become the same evaluation as the test number 7, and abbreviate | omitted.

  Specifically, the test piece was heated to a carburizing temperature Tc of 930 ° C. in the furnace with respect to the hydrogen embrittlement resistance evaluation test piece, the roller pitching test piece, and the 4-point bending fatigue test piece of each test number. Carburization process S1 which introduce | transduces acetylene gas in a furnace and carburizes a test piece was implemented. The carbon potential Cp1 and the treatment time t1 in the carburizing step S1 are as shown in Tables 3 and 4.

  Next, a diffusion step S2 for diffusing carbon that has entered the test piece into the steel material was performed. The carbon potential Cp2 and the treatment time t2 in the diffusion step S2 are as shown in Tables 3 and 4.

  The total time (t1 + t2) of the processing time t1 in the carburizing step S1 and the processing time t2 in the diffusion step S2 was as shown in Table 3 and Table 4.

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

  The oil temperature at the time of oil quenching was as shown in Tables 3 and 4. In addition, the characteristic temperature of oil was 600 degreeC also in any test number.

  Each test piece after oil quenching was tempered with a tempering temperature of 180 ° C. and a holding time of 120 minutes at the tempering temperature.

  Carburized parts (hydrogen embrittlement resistance evaluation test piece, roller pitching test piece, and 4-point bending fatigue test piece) of Test Nos. 1 to 59 were produced by the above manufacturing process. In addition, in carburized parts, a sample was taken from a position deeper than the depth of 2.0 mm from the surface, and a product analysis based on JIS G 0321 (2010) was performed. The chemical composition.

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

[Surface C concentration measurement]
Surface C concentration (%) was measured by the above-mentioned method with respect to the hydrogen embrittlement resistance evaluation test piece before the test. The results are shown in Tables 3 and 4.

[Measurement of hardness of surface layer and core]
A test piece for evaluating hydrogen embrittlement resistance before the test was cut in a direction perpendicular to the length direction. Using the cut surface as a measurement surface, a Vickers hardness test in accordance with JIS Z 2244 (2009) was performed, and 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 0.98N. The obtained Vickers hardness HV _0.15 and HV _0.40 are shown in Tables 5 and 6. Moreover, fn5 obtained from Vickers hardness HV _0.15 and HV _0.40 is shown in Table 5 and Table 6.

[Hydrogen embrittlement resistance evaluation test]
Using the continuous charge constant load (constant potential) test method, various concentrations of hydrogen were introduced into the test piece for each test number. 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, hydrogen was taken into the test piece by generating a cathode potential at a potential of 1000 mA on the surface of the test piece.

  After introducing hydrogen into the test piece, a galvanized film was formed on the surface of the test piece to prevent hydrogen dissipation in the test piece. Then, the constant load test which loads a fixed load with respect to the V notch cross section of a test piece was implemented. The maximum load stress (MPa) that did not break after 200 hours was determined.

Furthermore, steel having a chemical composition corresponding to SCr420 defined in JIS G 4053 (2016) was prepared as a reference product for the hydrogen embrittlement resistance evaluation test. SCr420 is a commonly used carburizing steel. A test piece was prepared in the same manner as test number 1 for the reference product. The produced test piece was subjected to carburizing treatment under normal carburizing conditions. Specifically, the following carburizing conditions were used.
Carburizing temperature Tc in the carburizing step S1: 930 °
Processing time t1: 180 minutes in the carburizing step S1 Carbon potential Cp1: 0.8 in the carburizing step S1
Soaking temperature in soaking step S3: 870 ° C
Holding time t3 in the soaking step S3: 30 minutes Carbon potential Cp3 in the soaking step S3: 0.8
The conditions for quenching and tempering were the same as in test number 1.

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

  In the test results of each test number, when the ratio of the maximum load stress (MPa) to the reference durability stress was 1.6 or more, the evaluation was “A”. When the ratio of the maximum load stress (MPa) to the reference durability stress was 1.4 to less than 1.6, the evaluation was “B”. When the ratio of the maximum load stress (MPa) to the reference durability stress was 1.2 to less than 1.4, the evaluation was “C”. When the ratio of the maximum load stress (MPa) to the reference durability stress was less than 1.2, the evaluation was “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, the hydrogen embrittlement resistance was judged to be low.

[Abrasion resistance evaluation test (roller pitching test)]
A roller pitching test was performed to evaluate the wear resistance. FIG. 9 is a schematic diagram of a roller pitching test. Specifically, the test was carried out under the following conditions using a roller pitching fatigue strength tester manufactured by Komatsu Engineering.
Slip rate: -40%
Lubricant: Automatic oil Lubricant temperature: 90 ° C
Lubricant flow rate: 2 L / min Rotation speed: 1500 rpm
Surface pressure: 2000 MPa

As shown in FIG. 9, the small roller 200 was rotated while pressing the large roller 100 against the small roller 200 with the above surface pressure. The small roller 200 was a roller pitching test piece produced by producing the test piece. As the large roller 100, steel having a chemical composition corresponding to SCM420 defined in JIS G 4053 (2016) was used which was subjected to surface polishing by low temperature tempering after eutectoid carburizing. The radius of the large roller 100 was 130 mm. The wear depth Dw of each test piece at a rotation number of 1 × 10 ⁶ times was measured. A stylus type surface roughness meter was used to measure the wear depth Dw. The measurement length was 24 mm, and the cross section curve was obtained by scanning the stylus in the axial direction of each test piece. In each test piece, measurement was performed at two locations every 180 ° in the circumferential direction to obtain a cross-sectional curve. From the obtained cross-sectional curve, in each test piece, the average height of the cross-sectional curve element in the part where the large roller 100 is not in contact, and the average height of the cross-sectional curve element in the part where the large roller 100 is in contact and worn out Was calculated respectively. And the difference of the height of the part which the large roller 100 was not contacting and the part which the large roller 100 was contacting was computed. The average value of the obtained height differences at the measurement points was defined as the wear depth Dw (μm) of each test number.

  The test piece whose wear depth Dw was less than 20 μm was evaluated as “A”, the test piece that was less than 20 to 40 μm was evaluated as “B”, the test piece that was less than 40 to 60 μm was evaluated as “C”, 60 The test piece that was less than ˜80 μm was evaluated as “D”, and the test piece that was 80 μm or more was evaluated as “x”. In the case of evaluations A to D, it was judged that the abrasion resistance was excellent. In the case of evaluation x, it was judged that the wear resistance was low.

[Bending fatigue strength evaluation test (4-point bending fatigue test)]
A four-point bending fatigue test was performed using a four-point bending fatigue test piece of each test number. A servo type fatigue tester was used for the test. The 4-point bending fatigue test piece was installed so that the surface having the notch was the lower surface. On the 4-point bending fatigue test piece, the distance between the upper fulcrums was 45 mm, and the distance between the lower fulcrums was 80 mm. The maximum load stress was 1150 MPa, and the stress ratio between the maximum load stress and the minimum load stress was 0.1. The frequency was 10 Hz. The breaking strength when the number of stress load repetitions was 1 × 10 ⁴ was defined as 4-point bending fatigue strength (MPa).

  A test piece having a four-point bending fatigue strength of 850 MPa or more was evaluated as “A”, a test piece that was less than 750 to 850 MPa was evaluated as “B”, and a test piece that was less than 650 to 750 MPa was evaluated as “C”. The test piece that was less than 650 MPa was evaluated as “x”. In the case of evaluation AC, it was judged that it was excellent in bending fatigue strength. In the case of evaluation x, it was judged that the bending fatigue strength was low.

[Evaluation results]
Tables 5 and 6 show the evaluation results.

  Referring to Tables 1 to 6, test numbers 1 to 4, 6 to 9, 12, 13, 16, 17, 19, 20, 22, 23, 25, 26, 28, 29, 31, 32, 35, 36, 39, 41, 42, 45, 46, 49, 50, and 53 to 57 had the chemical composition defined in this embodiment in any test number. Furthermore, the surface C concentration was 0.50 to 0.70%. Expressions (1) to (5) were satisfied. As a result, it showed excellent hydrogen resistance performance characteristics and sufficient wear resistance and bending fatigue strength.

  On the other hand, in test number 5, the C content was too high. Therefore, the hydrogen embrittlement resistance was low.

  In test number 10, the Si content was too high. Therefore, the wear resistance was low. It is considered that carburization was inhibited and the surface C concentration was lowered.

  In test number 11, the Si content was too low. Therefore, the wear resistance was low.

  In test number 14, the Mn content was too high. Therefore, the hydrogen embrittlement resistance was low.

  In test number 15, the Mn content was too low. Therefore, the wear resistance was low.

  In test number 18, the P content was too high. Therefore, the hydrogen embrittlement resistance was low.

  In test number 21, the S content was too high. Therefore, the hydrogen embrittlement resistance was low.

  In test number 24, the Al content was too low. Therefore, the hydrogen embrittlement resistance was low.

  In test number 27, the N content was too low. Therefore, the hydrogen embrittlement resistance was low.

  In test number 30, the Cr content was too low. Therefore, the wear resistance was low.

In test number 33, the surface C concentration was too high. Therefore, HV _0.15 was too high. As a result, the hydrogen embrittlement resistance was low. This is probably because Cp2 was high.

  In test number 34, the surface C concentration was too low. Therefore, the wear resistance was low. This is probably because Cp2 was low.

  In test number 37, fn1 was too high. Therefore, the hydrogen embrittlement resistance was low.

  In test number 38, fn1 was too low. Therefore, the hydrogen embrittlement resistance was low.

  In test number 40, fn2 was too low. Therefore, the hydrogen embrittlement resistance was low.

In test number 43, 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 parts was increased overall, and the treatment time t1 was longer, so the surface C concentration was increased.

In test number 44, HV _0.15 was too low. Therefore, the wear resistance was low. This is probably because the oil temperature exceeded 140 ° C.

In test number 47, HV _0.40 was too high. As a result, the hydrogen embrittlement resistance was low. It is considered that the total (t1 + t2) of the processing time t1 in the carburizing step S1 and the processing time t2 in the diffusion step S2 is too long.

In test number 48, HV _0.40 was too low. Therefore, the bending fatigue strength was low. This is probably because the processing time t1 in the carburizing step S1 was too short.

  In test numbers 51 and 58, the oil temperature was less than 120 ° C. Therefore, the hardness gradient fn5 was too small. Therefore, the hydrogen embrittlement resistance was low.

  In test number 52, the hardness gradient fn5 was too large. Therefore, the bending fatigue strength was low. This is probably because the processing time t2 in the diffusion step S2 is too short.

In test number 59, HV _0.15 and HV _0.40 were too low. Therefore, the wear resistance was low. It is considered that the total (t1 + t2) of the processing time t1 in the carburizing step S1 and the processing time t2 in the diffusion step S2 is too long.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

DESCRIPTION OF SYMBOLS 1 Carburized part 2 Sliding part 3 Shaft part 4 Stress concentration part 5 Notch 100 Large roller 200 Small roller

Claims (3)

A sliding part having a sliding surface in contact with another member;
A shaft portion connected to the sliding portion,
The shaft portion includes a stress concentration portion including one or a plurality of notches extending in the circumferential direction of the shaft portion,
Of the shaft part, the chemical composition of the core part excluding the cured layer is mass%,
C: 0.10 to 0.30%,
Si: 0.50 to 1.50%,
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% is contained, the balance consists of Fe and impurities,
C concentration on 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), the Vickers hardness at a depth position 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 expressions (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 + fn1 ≦ HV _0.40 ≦ 630 + fn1 (4)
−900 <(HV _0.40 −HV _0.15 ) /0.25 <−280 (5) The carburized part according to claim 1,
The chemical composition is
Mo: 0.01 to 0.80%,
Ni: 0.05 to 0.50%, and
Cu: Carburized parts characterized by containing one or more selected from the group consisting of 0.10 to 0.50%. The carburized component according to claim 1 or claim 2,
The chemical composition is
Ti: 0.05-0.10%, and
Nb: Carburized parts containing 1 type or 2 types selected from the group which consists of 0.01 to 0.10%.

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