US20240200179A1

US20240200179A1 - Steel material for carbonitriding and carbonitrided steel material

Info

Publication number: US20240200179A1
Application number: US18/287,521
Authority: US
Inventors: Yukihiro Hayashi; Kenta TSUJII
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 2021-04-20
Filing date: 2022-04-08
Publication date: 2024-06-20
Also published as: CN117222771A; WO2022224849A1; JP2022165628A

Abstract

The present invention relates to a steel material for carbonitriding treatment, containing: C, Si, Mn, P, S, Cu, Ni, Cr, Mo, Al, and N with a balance being Fe and inevitable impurities, satisfying Expression 1 of 0.6 to 1.4 and Expression 2 of >560 when subjected to carbonitriding, Expression 1: Surface C concentration (mass %)+12/14×Surface N concentration (mass %), and Expression 2: 129.7805×[Cr (mass %)]−76.9797×[Cr (mass %)]2+339.3375×[Surface N concentration (mass %)]−539.345×[Surface N concentration (mass %)]2+181.4983×[Cr (mass %)]×[Surface N concentration (mass %)]+437.6799, and providing a carbonitrided steel material having a hardness of 560 HV or more at a portion from a surface thereof to a depth of 0.05 mm when subjected to a tempering treatment at 500° C. after the carbonitriding.

Description

TECHNICAL FIELD

The present invention relates to a steel material for carbonitriding treatment and a carbonitrided steel material.

BACKGROUND ART

For example, a high surface fatigue strength is required for steel parts such as gears and shafts of automobiles and industrial machines. Therefore, in the related art, a carbonitrided part is used as the part. It is widely known in the related art that when a steel material is carbonitrided, surface fatigue strength thereof is improved.
Related methods in the related art include those disclosed in Patent Literatures 1 to 5.
For example, Patent Literatures 1 and 2 disclose a steel carbonitrided part. In the steel carbonitrided part, a material is a steel material having a specific composition, and in a region from a surface to a depth of 0.1 mm, an average C concentration Cs is 0.60% to 0.90%, an average N concentration Ns is 0.15% to 0.35%, Cs+Ns is 0.80% to 1.10%, and each component satisfies a specific relational expression. It is disclosed that, with such a carbonitrided part, it is possible to provide a steel carbonitrided part having significantly excellent bending fatigue strength and surface fatigue strength as compared with the case of being manufactured by the most typical carburizing and quenching as a surface curing treatment, without significantly increasing the cost, and capable of meeting demands for weight reduction, size reduction, and stress load increase of the part.
Patent Literature 3 discloses a steel material for carbonitriding that has a specific composition and each component satisfies a specific relational expression. It is disclosed that, with such a carbonitrided steel material, it is possible to provide a steel material excellent in workability to a part shape and excellent in pitting life even in the case where a polishing process after a carbonitriding treatment is omitted, and provide a part using the steel material.
Patent Literature 4 discloses a steel part. In the steel part, a base is a steel material having a specific composition, a ratio of voids is less than 10% in a region from a surface to a depth of 5 μm, and an average C concentration Cave is 0.005% to 0.80%, an average N concentration Nave is 0.30% to 0.70%, and Cave+Nave is 0.50% to 1.40% in a region from the surface to a depth of 100 μm. It is disclosed that, with such a steel part, it is possible to provide a steel part that is excellent in surface fatigue strength and wear resistance and that can be used for parts such as gears, crank shafts, and camshafts of an automobile or an industrial machine.
Patent Literature 5 discloses a carbonitrided part including: a surface layer portion including a surface that includes a flat portion and an edge portion; and a core portion inside the surface layer portion. The core portion has a specific composition. In a region from the flat portion to a depth of 0.05 mm, a carbon concentration CP1 is 0.70% to 0.89%, and a nitrogen concentration is 0.10% to 0.80%. In a region from the edge portion to a depth of 0.05 mm, a carbon concentration CP2 is higher than the carbon concentration CP1 and 1.20% or less. A Vickers hardness at a depth of 0.3 mm from the flat portion is HV650 or more. A grain boundary oxidation layer depth in the surface layer portion is less than 3.0 μm. A Vickers hardness of the core portion is HV260 or more. It is disclosed that, with such a carbonitrided part, it is possible to provide a carbonitrided part that includes a surface including a flat portion and an edge portion and has excellent bending fatigue strength and pitting strength.

CITATION LIST

Patent Literature

- Patent Literature 1: JP2010-70827A
- Patent Literature 2: JP2010-70831A
- Patent Literature 3: JP2016-186120A
- Patent Literature 4: JP2017-171951A
- Patent Literature 5: JP2017-171970A

SUMMARY OF INVENTION

Problems to be Solved by Invention

In recent years, the automobiles have been electrically powered and there is a possibility that a sliding temperature of some parts may become higher (for example, about 300° C. to 500° C.) than in the related art accompanying the electrification. Therefore, some parts are required to have a sufficient hardness even in the case of being exposed to a high temperature, in other words, high-temperature tempering hardness.
On the other hand, the inventors of the present invention found that generation of a CrN cluster contributes greatly as a surface fatigue strength improvement factor of carbonitriding and that it is necessary to add Cr to a certain extent in order to sufficiently improve the surface fatigue strength. Specifically, they found that addition of N is effective up to 300° C., and addition of two elements of Cr and N is effective in a high-temperature range higher than 300° C.
Further, the inventors of the present invention found that tempering hardness at a high temperature (about 300° C. to 500° C.) decreases depending on addition amounts of Cr and N.
That is, they found that it is difficult to improve both the surface fatigue strength and the high-temperature tempering hardness, which are in a trade-off relationship.
An object of the present invention is to solve the above problems.
That is, an object of the present invention is to provide a steel material for carbonitriding treatment having a high surface fatigue strength and a sufficiently high high-temperature tempering hardness, and a carbonitrided steel material obtained by carbonitriding the steel material.

Solution to Problems

The inventors of the present invention have conducted intensive studies to solve the above problems, and completed the present invention.
The present invention is the following (1) to (7).

- (1) A steel material for carbonitriding treatment, containing:
  - C at 0.1 mass % to 0.3 mass %;
  - Si at 0.3 mass % or less;
  - Mn at 0.4 mass % to 2.0 mass %;
  - P at 0.03 mass % or less;
  - S at 0.03 mass % or less;
  - Cu at 0.3 mass % or les;
  - Ni at 2.5 mass % or less;
  - Cr at 0.5 mass % to 3.0 mass %;
  - Mo at 0.001 mass % to 1.0 mass %;
  - Al at 0.01 mass % to 0.08 mass %; and
  - N at 0.005 mass % to 0.03 mass %;
  - with a balance being Fe and inevitable impurities,
- satisfying the following Expression 1 of 0.6 to 1.4 and Expression 2 of >560 when subjected to carbonitriding,

Surface C concentration(mass %)+12/14×Surface N concentration(mass %), Expression 1:
129.7805×[Cr(mass %)]−76.9797×[Cr(mass %)]²+339.3375×[Surface N concentration(mass %)]−539.345×[Surface N concentration (mass %)]2+181.4983×[Cr(mass %)]×[Surface N concentration(mass %)]+437.6799, and Expression 2:

- providing a carbonitrided steel material having a hardness of 560 HV or more at a portion from a surface thereof to a depth of 0.05 mm when subjected to a tempering treatment at 500° C. after the carbonitriding.
- (2) The steel material for carbonitriding treatment according to (1), further containing at least one selected from the group consisting of:
- Nb at 0.001 mass % to 0.08 mass %;
- V at 0.5% mass % or less;
- Ti at 0.05% mass % or less; and
- B at 0.0005 mass % to 0.003 mass %.
- (3) The steel material for carbonitriding treatment according to (1) or (2), containing:
  - C at 0.15 mass % to 0.25 mass %;
  - Si at 0.01 mass % to 0.24 mass %;
  - Mn at 0.5 mass % to 1.8 mass %;
  - Cu at 0.001 mass % to 0.3 mass %;
  - Ni at 0.01 mass % to 0.6 mass %;
  - Cr at 0.6 mass % to 1.8 mass %;
  - Mo at 0.01 mass % to 0.8 mass %;
  - Al at 0.02 mass % to 0.05 mass %;
  - N at 0.01 mass % to 0.025 mass %; and
  - at least one selected from the group consisting of:
    - Nb at 0.0015 mass % to 0.06 mass %;
    - V at 0.25 mass % or less;
    - Ti at 0.012 mass % to 0.04 mass %; and
    - B at 0.0006 mass % to 0.0025 mass %,
  - with a balance being Fe and inevitable impurities, and
- providing, by performing the carbonitriding, a carbonitrided steel material that satisfies the following Expression 1 being 0.7 to 1.2:

Surface C concentration(mass %)+12/14×Surface N concentration(mass %). Expression 1:

- (4) The steel material for carbonitriding treatment according to any one of (1) to (3), containing:
- Cu at 0.03 mass % to 0.3 mass %.
- (5) The steel material for carbonitriding treatment according to any one of (1) to (4), containing:
- Si at 0.01 mass % to 0.17% mass %.
- (6) The steel material for carbonitriding treatment according to any one of (1) to (5), containing:
- Cu at 0.03 mass % to 0.25 mass %.
- (7) A carbonitrided steel material obtained by carbonitriding the steel material for carbonitriding treatment according to any one of (1) to (6),
- satisfying the following Expression 1 of 0.6 to 1.4 and Expression 2 of >560,

Surface C concentration(mass %)+12/14×Surface N concentration(mass %), and Expression 1:
129.7805×[Cr(mass %)]−76.9797×[Cr(mass %)]²+339.3375×[Surface N concentration(mass %)]−539.345×[Surface N concentration (mass %)]²+181.4983×[Cr(mass %)]×[Surface N concentration(mass %)]+437.6799, and Expression 2:

- having a hardness of 560 HV or more at a portion from a surface thereof to a depth of 0.05 mm when subjected to a tempering treatment at 500° C.

Effects of the Invention

According to the present invention, it is possible to provide a steel material for carbonitriding treatment having a high surface fatigue strength and a sufficiently high high-temperature tempering hardness, and a carbonitrided steel material obtained by carbonitriding the steel material.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side view of a small roller used in Examples.

FIG. 2 is a graph showing a relationship between a tempering hardness at 500° C. and a fatigue strength life ratio in Examples and Comparative Examples.

FIG. 3 is a graph showing a relationship between a tempering hardness at 500° C. and a seizing limit load ratio in Examples and Comparative Examples.

DESCRIPTION OF EMBODIMENTS

The present invention will be described.
A steel material for carbonitriding treatment according to the present invention is a steel material for carbonitriding treatment, containing: C at 0.1 mass % to 0.3 mass %; Si at 0.3 mass % or less; Mn at 0.4 mass % to 2.0 mass %; P at 0.03 mass % or less; S at 0.03 mass % or less; Cu at 0.3 mass % or less; Ni at 2.5 mass % or less; Cr at 0.5 mass % to 3.0 mass %; Mo at 0.001 mass % to 1.0 mass %; Al at 0.01 mass % to 0.08 mass %; and N at 0.005 mass % to 0.03 mass %, with a balance being Fe and inevitable impurities. The steel material for carbonitriding treatment satisfies Expression 1 of 0.6 to 1.4 and Expression 2 of >560 when subjected to carbonitriding, Expression 1 being Surface C concentration (mass %)+12/14×Surface N concentration (mass %), and Expression 2 being 129.7805×[Cr (mass %)]−76.9797×[Cr (mass %)]²+339.3375×[Surface N concentration (mass %)]−539.345×[Surface N concentration (mass %)]²+181.4983×[Cr (mass %)]×[Surface N concentration (mass %)]+437.6799. The steel material for carbonitriding treatment provides a carbonitrided steel material having a hardness of 560 HV or more in a portion from a surface thereof to a depth of 0.05 mm when subjected to a tempering treatment at 500° C. after the carbonitriding.
Further, a carbonitrided steel material according to the present invention is a carbonitrided steel material obtained by carbonitriding the steel material for carbonitriding treatment according to the present invention. The carbonitrided steel material satisfies Expression 1 of 0.6 to 1.4 and Expression 2 of >560, Expression 1 being Surface C concentration (mass %)+12/14×Surface N concentration (mass %), and Expression 2 being 129.7805×[Cr (mass %)]−76.9797×[Cr (mass %)]²+339.3375×[Surface N concentration (mass %)]−539.345×[Surface N concentration (mass %)]²+181.4983×[Cr (mass %)]×[Surface N concentration (mass %)]+437.6799. The carbonitrided steal material has a hardness of 560 HV or more in a portion from a surface thereof to a depth of 0.05 mm when subjected to a tempering treatment at 500° C.
The composition of the steel material for carbonitriding treatment according to the present invention will be described.
<C>
The C content in the steel material for carbonitriding treatment according to the present invention is 0.1 mass % to 0.3 mass %, and preferably 0.15 mass % to 0.25 mass %.
In the case of such a C content, quenching property of the steel material for carbonitriding treatment according to the present invention is improved, and hardness of a surface portion and a core portion is secured. In the case where the C content is too high, toughness and hot workability may degrade.

<Si>

The Si content in the steel for carbonitriding treatment according to the present invention is 0.3 mass % or less, preferably 0.01 mass % to 0.24 mass %, more preferably 0.01 mass % to 0.17 mass %, and still more preferably 0.01 mass % to 0.16 mass %.
In the case of such a Si content, precipitation of nitrides (Si₃N₄, SiMnN₂, etc.) in the steel material for carbonitriding treatment according to the present invention is suppressed, and a decrease in fatigue strength can be prevented.

<Mn>

The Mn content in the steel material for carbonitriding treatment according to the present invention is 0.4 mass % to 2.0 mass %, preferably 0.5 mass % to 2.0 mass %, more preferably 0.53 mass % to 2.0 mass %, or preferably 0.5 mass % to 1.8 mass %, more preferably 0.5 mass % to 1.4 mass %, or preferably 0.53 mass % to 1.5 mass %.
In the case of such a Mn content, since the quenching property is improved, hardness of the core portion is improved and fatigue strength is improved. In addition, cutting property, quenching property, and manufacturability are improved. In the case where the Mn content is too low, a quenching property improving effect cannot be obtained. In the case where the Mn content is too high, manufacturability may be impaired.

<P>

The P content in the steel material for carbonitriding treatment according to the present invention is 0.03 mass % or less, and preferably 0.020 mass % or less. P is an impurity contained in steel, and segregates in a crystal grain boundary to make the steel brittle. In particular, in the case where the P content exceeds 0.030 mass %, the degree of embrittlement may be great. Accordingly, the P content in the steel material for carbonitriding treatment according to the present invention is 0.03 mass % or less.

<S>

The S content in the steel material for carbonitriding treatment according to the present invention is 0.03 mass % or less, and preferably 0.020 mass % or less.
In the case of such an S content, MnS is formed to improve cutting property. On the other hand, in the case where the S content exceeds 0.030 mass %, coarse MnS tends to be formed, and hot forgeability and bending fatigue strength tend to degrade. Therefore, the S content is preferably 0.005 mass % to 0.030 mass %. When the hot forgeability and the bending fatigue strength are more emphasized, the S content is preferably 0.020 mass % or less.

<Cu>

The Cu content in the steel material for carbonitriding treatment according to the present invention is 0.3 mass % or less, preferably 0.001 mass % to 0.3 mass %, more preferably 0.03 mass % to 0.3 mass %, and still more preferably 0.03 mass % to 0.25 mass %.
In the case of such a Cu content, formation of a carbide is suppressed, and quenching property is improved. In the case where the Cu content is too high, hot workability may degrade.

<Ni>

The Ni content in the steel material for carbonitriding treatment according to the present invention is 2.5 mass % or less, preferably 0.01 mass % to 0.6 mass %, and more preferably 0.05 mass % to 0.6 mass %.
In the case of such a Ni content, quenching property is improved and toughness is improved. In addition, since Ni is a non-oxidizing element, a steel surface can be strengthened without increasing the depth of the grain boundary oxide layer during carburization.

<Cr>

The Cr content in the steel material for carbonitriding treatment according to the present invention is 0.5 mass % to 3.0 mass %, preferably 0.5 mass % to 2.5 mass %, and more preferably 0.6 mass % to 1.8 mass %.
In the case of such a Cr content, quenching property is improved, cutting property is also secured, pitting fatigue strength is improved, and toughness is also improved.
In the case where the Cr content is too high, hardness increases and cutting property degrades, and when a coarse Cr carbide is generated during carburization or a coarse CrN is generated along a crystal grain boundary during carbonitriding, bending strength may degrade.

<Mo>

The Mo content in the steel material for carbonitriding treatment according to the present invention is 0.001 mass % to 1.0 mass %, preferably 0.01 mass % to 0.8 mass %, and more preferably 0.05 mass % to 0.6 mass %.
In the case of such a Mo content, since quenching property is improved, hardness of a core portion of a part subjected to a quenching treatment is improved, and fatigue strength is improved. In addition, surface hardness and hardness of a quench-hardened layer are improved.
In the case where the Mo content is too low, strength after hot forging is high, and machinability may be processed. In addition, in the case where the Mo content is too high, a precipitation nucleation site tends to occur to promote generation of precipitates such as carbonitrides. In addition, a coarse carbonitride and the like in a non-solid solution state may remain in steel, the coarse carbonitride may further grow and be coarsened during the carbonitriding quenching, and thus fatigue strength may degrade.

<Al>

The Al content in the steel material for carbonitriding treatment according to the present invention is 0.01 mass % to 0.08 mass %, and preferably 0.02 mass % to 0.05 mass %.
In the case of such an Al content, Al easily combines with N to form AlN, and an effect of enhancing steel by miniaturizing crystal grains is exhibited.
In the case where the Al content is too high, there is a possibility that cutting property is degraded due to formation of hard and coarse Al₂O₃. In addition, there is a possibility that Al₂O₃as a large and hard inclusion serves a starting point of fatigue fracture, and causes a decrease in bending fatigue strength and pitting strength.

<N>

The N content in the steel material for carbonitriding treatment according to the present invention is 0.005 mass % to 0.03 mass %, preferably 0.01 mass % to 0.025 mass %, more preferably 0.01 mass % to 0.020 mass %, and still more preferably 0.01 mass % to 0.015 mass %.
In the case of such an N content, crystal grains are miniaturized due to formation of nitrides, and bending fatigue strength is improved.
In the case where the N content is too high, toughness may degrade due to formation of coarse nitrides.

<Nb>

The steel material for carbonitriding treatment according to the present invention may contain Nb.
The Nb content in the steel material for carbonitriding treatment according to the present invention is preferably 0.001 mass % to 0.08 mass %, and more preferably 0.0015 mass % to 0.06 mass %.
In the case of such a Nb content, a fine precipitate (NbC) is generated, and thus crystal grains during carburization are less likely to be coarsened.

<V>

The steel material for carbonitriding treatment according to the present invention may contain V.
The V content in the steel material for carbonitriding treatment according to the present invention is preferably 0.5% mass % or less, and more preferably 0.25% mass % or less. In the case of such a V content, a V precipitate appears dispersedly, and fracture characteristics are improved.

<Ti>

The steel material for carbonitriding treatment according to the present invention may contain Ti.
The Ti content of the steel material for carbonitriding treatment according to the present invention is preferably 0.05% mass % or less, more preferably 0.08% mass % or less, and still more preferably 0.012 mass % to 0.04% mass %.
In the case of such a Ti content, a fine precipitate (TiC) is generated, and thus crystal grains during carburization are less likely to be coarsened.

<B>

The steel material for carbonitriding treatment according to the present invention may contain B.
The B content in the steel material for carbonitriding treatment according to the present invention is preferably 0.0005 mass % to 0.003 mass %, and more preferably 0.0006 mass % to 0.0025 mass %.
In the case of such a B content, quenching property is significantly improved, and cracking workability is improved.
In the case where the B content is too high, BN is formed, and the effect of improving quenching property at a deep portion is reduced.
The steel material for carbonitriding treatment according to the present invention may contain C, Si, Mn, P, S, Cu, Ni, Cr, Mo, Al, and N at contents as described above, and may further contain at least one selected from the group consisting of Nb, V, Ti, and B as an optional component at a specific content. The balance is Fe and inevitable impurities.
Here, the inevitable impurities refer to components that may be mixed from raw materials or manufacturing processes even if they are not intentionally added. Specific examples of the inevitable impurities include O and As.
The content of each component contained in the steel material for carbonitriding treatment according to the present invention means a value obtained by measurement using the following method.
The contents of Si, Mn, P, Cu, Ni, Cr, Mo, V, Ti, and Nb mean values obtained by an X-ray fluorescence analysis method, the content of Al means a value obtained by emission spectral analysis, the content of O means a value obtained by an inert gas fusion-infrared absorption method, and the contents of C and S mean values obtained by a combustion-infrared absorption method. In addition, the content of N means a value obtained by an inert gas fusion-thermal conductivity method, and the content of B means a value obtained by emission spectral analysis.
A method for producing the steel material for carbonitriding treatment according to the present invention is not particularly limited. For example, the steel material for carbonitriding treatment according to the present invention can be produced by a known method in the related art.
The steel material for carbonitriding treatment according to the present invention having such a composition as described above can be subjected to a carbonitriding treatment to provide a carbonitrided steel material according to the present invention.
Here, the carbonitriding treatment is not particularly limited, and is sufficient to be a carbonitriding treatment through which the carbonitrided steel material according to the present disclosure can be obtained from the steel material for carbonitriding treatment according to the present invention. The carbonitriding treatment may be, for example, a carbonitriding treatment X as described below.
The carbonitriding treatment X may be any of gas carbonitriding and vacuum carbonitriding. Conditions (carburization temperature, type of a carburizing gas, carburizing gas pressure, treatment time in a carburizing process, treatment time in a diffusion process, cooling rate in a cooling process, nitriding gas pressure in a nitriding process, ammonia gas amount, treatment time, quenching temperature, and the like) for the carbonitriding treatment can be appropriately determined according to the hardness and tempering hardness of a surface layer portion required in a carbonitrided part, and are not particularly limited. For example, in a gas carbonitriding treatment, surface N concentration is controlled by controlling the normal CP to 0.5 to 1.0, using ammonia as a nitriding gas, and adjusting ammonia flow rate, in-oven ammonia concentration, diffusion time, and quenching temperature, to thereby perform quenching. Thereafter, heating is performed at 100° C. to 300° C. for 1 to 3 hours, to thereby perform tempering.
The carbonitrided steel material according to the present invention will be described. The carbonitrided steel material according to the present invention can be obtained by performing a carbonitriding treatment (for example, the above-described carbonitriding treatment X) on the steel material for carbonitriding treatment according to the present invention as described above.

The carbonitrided steel material according to the present invention preferably has a surface C concentration of 0.4 mass % to 0.8 mass %, and more preferably 0.45 mass % to 0.70 mass %.
Here, the surface C concentration means a C concentration obtained by shaving the surface of the carbonitrided steel material according to the present invention to a depth of 100 μm and applying a combustion-infrared absorption method to the obtained chips (machining chips).

The carbonitrided steel material according to the present invention preferably has a surface N concentration of 0.25 mass % to 0.8 mass %, and more preferably 0.30 mass % to 0.70 mass %.
Here, the surface N concentration means an N concentration obtained by shaving the surface of the carbonitrided steel material according to the present invention to a depth of 100 μm and applying a fusion-thermal conductivity measurement to the obtained chips (machining chips).
In the carbonitrided steel material according to the present disclosure, the surface C concentration and the surface N concentration described above satisfy the following Expression 1 of 0.6 to 1.4.
Surface C concentration(mass %)+12/14×Surface N concentration (mass %) Expression 1:
The calculation result of Expression 1 is preferably 0.7 to 1.2.
In the carbonitrided steel material according to the present disclosure, the Cr content, the surface C concentration, and the surface N concentration as described above satisfy the following Expression 2 of >560.
129.7805×[Cr(mass %)]−76.9797×[Cr(mass %)]²+339.3375×[Surface N concentration(mass %)]−539.345×[Surface N concentration(mass %)]²+181.4983×[Cr(mass %)]×[Surface N concentration(mass %)]+437.6799 Expression 2:
The carbonitrided steel material according to the present invention that is obtained by performing the carbonitriding treatment X described above on the steel material for carbonitriding treatment according to the present invention preferably has a hardness of 600 HV or more in a portion from a surface thereof to a depth of 0.05 mm.
The carbonitrided steel material according to the present invention that is obtained by performing the carbonitriding treatment X described above on the steel material for carbonitriding treatment according to the present invention has a hardness of 560 HV or more in a portion from a surface thereof to a depth of 0.05 mm when subjected to a tempering treatment at 500° C.

EXAMPLES

Examples of the present invention will be described below.
For each of Examples 1 to 31 and Comparative Examples 1 to 19 shown in Table 1, raw materials were mixed so as to achieve the respective composition shown in Tables 1 and 2 (unit: mass %, balance: Fe and inevitable impurities), and were melted by using a 150 kg high-frequency induction furnace and cast to obtain a steel ingot A.
Next, the steel ingot A was subjected to hot rolling and hot forging to obtain a round bar having a cross-sectional diameter of 125 mm, and further hot forging was performed thereon to obtain a round bar having a cross-sectional diameter of 32 mm. Further, a normalizing treatment was performed (925° C.×1 HrAC), and a round bar (having a length of 210 mm) having a cross-sectional diameter of 15 mm was cut out from the obtained round bar.
Next, the round bar was subjected to a carburizing treatment and a nitriding treatment to obtain a test piece. However, in some Comparative Examples, only the carburizing treatment was performed, and the nitriding treatment was not performed.
Here, the carburizing treatment is as follows.
The round bar was placed in a gas carbonitriding furnace, a carburizing gas (using propane gas as an enriched gas) was introduced at a temperature of 930° C., and partial pressures of carbon monoxide and carbon dioxide was adjusted to control CP (carbon potential) to 0.7, and carburization was performed.
The nitriding treatment is as follows.
The round bar subjected to the carburizing treatment was cooled to 850° C., and ammonia gas was introduced as a nitriding gas in the state where CP was kept constant, and the nitriding treatment was performed. After the nitriding treatment was performed, quenching was performed with a semi-hot quenching oil having a temperature of 120° C. Further, as a subsequent treatment, a tempering treatment was performed in which the round bar after the quenching was placed in a furnace, which was adjusted to 160° C., was heated for 2 hours therein, then was taken out from the furnace, and was allowed to cool indoors.

The test piece subjected to the carburizing treatment and the nitriding treatment (some were a test piece which was subjected to only the carburizing treatment) was shaved from a surface to a depth of 100 μm, and the C concentration and the N concentration of the obtained chips (machining chips) were measured. For the measurement of the C concentration, a combustion-infrared absorption method was used, and for the measurement of the N concentration, a fusion-thermal conductivity measurement was used.
The results are shown in Table 1.

The surface of the test piece subjected to the carburizing treatment and the nitriding treatment (some were a test piece which was subjected to only the carburizing treatment) was mirror-polished, and a hardness at a position of 0.05 mm from the surface was measured at a load of 2.94 N based on JIS Z 2244.
The results are shown in Tables 1 and 2.

A tempering treatment was performed in which the test piece subjected to the carburizing treatment and the nitriding treatment (some were a test piece which was subjected to only the carburizing treatment) was placed in a furnace, which was adjusted to 500° C., was heated for 3 hours therein, then was taken out from the furnace, and was allowed to cool indoors. Thereafter, the surface was mirror-polished, and a hardness at a position of 0.05 mm from the surface was measured at a load of 2.94 N based on JIS Z 2244.
The results are shown in Tables 1 and 2.

A round bar was produced from the steel ingot A in the same processes, and was machined to obtain a small rotor. A small roller 1 includes, as illustrated in FIG. 1 , a contact portion 2 having a diameter of 26 mm and a width of 28 mm and a small-diameter portion 4 having a diameter of 22 mm disposed on both sides of the contact portion 2. The small rotor was subjected to the carburizing treatment and the nitriding treatment to obtain a test piece.
Next, a large roller mating to the test piece was prepared. The large roller was subjected to a quenching and tempering treatment so that the material became SUJ2 with HRC61. A radius of curvature of the large roller was 150R.
Then, a roller pitting test was performed. In the roller pitting test, the test piece and the mating large roller were brought into contact with each other at a rotational speed of 3000 rpm at various surface pressures of 2.0 GPa to 4.0 GPa. The test piece and the mating large roller were rotated at a slip ratio of −100% by using a roller pitting tester, and a load stress with which pitting did not occur until 10⁷cycles was taken as a surface fatigue strength (pitting fatigue strength). The surface fatigue strength with respect to a vacuum carburized material of JIS SCR420 was determined for each test piece. That is, the fatigue strength life ratio means (the surface fatigue strength of the test piece/surface fatigue strength of the vacuum carburized material of JIS SCR420).
The results are shown in Tables 1 and 2.

The steel ingot A was machined to obtain two test pieces of a load roller side and a test roller side.
The test pieces had a diameter of 78 mm and a width of 18 mm, and the test piece on the load roller side had a radius of curvature of 700 R.
Regarding the test pieces, the carburizing treatment and the nitriding treatment were performed to obtain test samples.
Then, a seizing test was performed by using a roller pitting tester.
In the seizing test, a load on the two test pieces produced above was increased stepwise by 0.05 GPa every 60 seconds under a certain slip speed (2.0 m/s to 20.0 m/s).
Determination of seizing was performed at a time point at which a torque of a torque meter installed on the load side rapidly increased, and a load at that time was taken as a seizing load.
The surface fatigue strength with respect to the vacuum carburized material of JIS SCR420 was determined for each test piece.
That is, the seizing limit load ratio means “a seizing load of the test piece/a seizing load of the vacuum carburized material of JIS SCR420”.
The seizing limit load was measured for each of Examples 1 to 31 and Comparative Examples 1 to 19.
The results are shown in Tables 3 and 4.

	TABLE 1

	Composition

	C	Si	Mn	P	S	Cu	Ni	Cr	Mo	Al	N	Nb	V	Ti	B

Ex. 1	0.21	0.14	1.10	0.010	0.010	0.15	0.10	0.52	0.39	0.028	0.0132
Ex. 2	0.20	0.16	1.72	0.010	0.010	0.12	0.09	0.62	0.79	0.032	0.0138
Ex. 3	0.21	0.14	1.10	0.010	0.010	0.15	0.10	0.70	0.40	0.028	0.0147
Ex. 4	0.20	0.09	0.80	0.009	0.010	0.14	0.09	0.99	0.40	0.031	0.0134
Ex. 5	0.21	0.14	1.10	0.010	0.010	0.15	0.10	1.23	0.10	0.028	0.0125
Ex. 6	0.20	0.02	0.91	0.010	0.010	0.13	0.10	1.38	0.25	0.030	0.0150
Ex. 7	0.20	0.10	0.80	0.010	0.010	0.16	0.09	1.41	0.40	0.031	0.0151
Ex. 8	0.21	0.08	0.96	0.010	0.010	0.14	0.10	1.50	0.19	0.028	0.0149
Ex. 9	0.20	0.17	0.78	0.010	0.010	0.14	0.11	1.50	0.34	0.029	0.0132
Ex. 10	0.18	0.13	1.0	0.010	0.010	0.15	0.09	1.54	0.10	0.030	0.0162
Ex. 11	0.19	0.09	0.42	0.010	0.010	0.16	0.12	1.52	0.55	0.029	0.0161
Ex. 12	0.21	0.13	0.65	0.011	0.010	0.13	0.08	1.61	0.44	0.031	0.0155
Ex. 13	0.21	0.06	1.81	0.011	0.010	0.15	0.12	1.67	0.09	0.029	0.0162
Ex. 14	0.20	0.04	0.74	0.011	0.010	0.16	0.11	1.76	0.17	0.033	0.0147
Ex. 15	0.19	0.15	0.43	0.011	0.010	0.13	0.08	1.37	0.31	0.026	0.0146
Ex. 16	0.20	0.10	0.78	0.011	0.010	0.14	0.08	1.92	0.03	0.032	0.0153
Ex. 17	0.19	0.11	0.52	0.011	0.010	0.13	0.11	2.03	0.13	0.026	0.0135
Ex. 18	0.19	0.03	0.59	0.011	0.010	0.13	0.10	2.37	0.61	0.026	0.0138
Ex. 19	0.19	0.06	0.61	0.011	0.010	0.14	0.11	2.59	0.07	0.025	0.0166
Ex. 20	0.21	0.14	1.10	0.010	0.010	0.15	0.12	1.11	0.82	0.028	0.0162
Ex. 21	0.21	0.14	0.90	0.010	0.010	0.15	0.12	1.11	0.91	0.028	0.0162
Ex. 22	0.21	0.14	1.10	0.010	0.010	0.15	0.68	0.70	0.40	0.028	0.0134
Ex. 23	0.21	0.14	1.10	0.010	0.010	0.15	2.10	1.02	0.40	0.028	0.0134
Ex. 24	0.21	0.14	1.10	0.010	0.010	0.15	0.10	0.70	0.40	0.028	0.0134	0.0100
Ex. 25	0.21	0.14	1.10	0.010	0.010	0.15	0.10	1.23	0.10	0.028	0.0125	0.0100
Ex. 26	0.21	0.14	1.10	0.010	0.010	0.15	0.10	0.70	0.40	0.028	0.0134		0.2000
Ex. 27	0.21	0.14	1.10	0.010	0.010	0.15	0.10	1.23	0.10	0.028	0.0125		0.2000
Ex. 28	0.21	0.14	1.10	0.010	0.010	0.15	0.10	0.70	0.40	0.028	0.0134	0.0100		0.0500	0.0020
Ex. 29	0.21	0.14	1.10	0.010	0.010	0.15	0.10	1.23	0.10	0.028	0.0125	0.0100		0.0350	0.0020
Ex. 30	0.21	0.14	1.10	0.010	0.010	0.15	0.10	0.70	0.40	0.028	0.0134	0.0100	0.2000	0.0500	0.0020
Ex. 31	0.21	0.14	1.10	0.010	0.010	0.15	0.10	1.23	0.10	0.028	0.0125	0.0100	0.2000	0.0350	0.0020

		Hardness from
		Surface to Depth	Fatigue
Expr. 1	Expr. 2	of 0.05 mm (HV)	Strength

		Surface C	Surface N	Calc.	Calc.	Normal		Life
	Treatment condition	Conc. (%)	Conc. (%)	Result	Result	Temp.	500° C.	Ratio

Ex. 1	carburizing + nitriding	0.53	0.26	0.753	560.7	707	562	3.1
Ex. 2	carburizing + nitriding	0.58	0.43	0.950	583.1	726	650	3.9
Ex. 3	carburizing + nitriding	0.57	0.36	0.879	588.8	717	602	3.8
Ex. 4	carburizing + nitriding	0.60	0.51	1.037	615.1	709	621	3.7
Ex. 5	carburizing + nitriding	0.57	0.45	0.956	624.8	717	643	5.2
Ex. 6	carburizing + nitriding	0.53	0.51	0.959	630.5	702	660	6.0
Ex. 7	carburizing + nitriding	0.60	0.50	1.029	630.4	723	641	3.7
Ex. 8	carburizing + nitriding	0.56	0.33	0.841	602.2	707	599	4.1
Ex. 9	carburizing + nitriding	0.53	0.59	1.034	632.2	731	640	4.1
Ex. 10	carburizing + nitriding	0.50	0.52	0.941	630.9	710	652	4.5
Ex. 11	carburizing + nitriding	0.62	0.45	1.010	625.3	700	634	4.4
Ex. 12	carburizing + nitriding	0.41	0.67	0.934	628.1	730	660	5.6
Ex. 13	carburizing + nitriding	0.43	0.50	0.858	625.6	690	630	6.3
Ex. 14	carburizing + nitriding	0.71	0.54	1.177	626.8	721	636	6.1
Ex. 15	carburizing + nitriding	0.51	0.42	0.872	601.0	701	643	4.9
Ex. 16	carburizing + nitriding	0.57	0.56	1.056	619.4	705	659	5.9
Ex. 17	carburizing + nitriding	0.45	0.56	0.927	610.9	699	654	5.9
Ex. 18	carburizing + nitriding	0.45	0.59	0.953	579.1	700	632	6.5
Ex. 19	carburizing + nitriding	0.52	0.71	1.130	560.2	680	601	6.2
Ex. 20	carburizing + nitriding	0.57	0.53	1.025	622.0	738	667	5.5
Ex. 21	carburizing + nitriding	0.57	0.53	1.025	622.0	738	644	4.5
Ex. 22	carburizing + nitriding	0.57	0.59	1.076	578.2	724	610	4.2
Ex. 23	carburizing + nitriding	0.57	0.59	1.076	611.7	680	630	4.7
Ex. 24	carburizing + nitriding	0.57	0.36	0.879	588.8	724	612	4.0
Ex. 25	carburizing + nitriding	0.57	0.45	0.956	624.8	717	653	5.7
Ex. 26	carburizing + nitriding	0.57	0.36	0.879	588.8	724	602	3.8
Ex. 27	carburizing + nitriding	0.57	0.45	0.956	624.8	717	643	5.5
Ex. 28	carburizing + nitriding	0.57	0.36	0.879	588.8	724	612	4.5
Ex. 29	carburizing + nitriding	0.57	0.45	0.956	624.8	717	653	6.2
Ex. 30	carburizing + nitriding	0.57	0.36	0.879	588.8	724	612	5.0
Ex. 31	carburizing + nitriding	0.57	0.45	0.956	624.8	717	663	6.8

	TABLE 2

	Composition

	C	Si	Mn	P	S	Cu	Ni	Cr	Mo	Al	N	Nb	V	Ti	B

Comp.	0.21	0.33	0.85	0.017	0.015	0.18	0.07	1.22	0.04	0.026	0.0152	—	—	—	—
Ex. 1
Comp.	0.20	1.01	0.80	0.010	0.008	0.15	0.10	1.00	0.40	0.028	0.0133
Ex. 2
Comp.	0.20	0.49	0.80	0.010	0.008	0.15	0.10	0.99	0.40	0.028	0.0133	—	—	—	—
Ex. 3
Comp.	0.20	1.01	0.80	0.010	0.008	0.15	0.10	1.00	0.40	0.028	0.0133	—	—	—	—
Ex. 4
Comp.	0.23	0.07	2.10	0.021	0.011	0.15	0.11	1.67	0.11	0.030	0.0156	—	—	—	—
Ex.5
Comp.	0.21	0.04	0.37	0.022	0.016	0.14	0.10	1.67	0.20	0.031	0.0132	—	—	—	—
Ex. 6
Comp.	0.18	0.12	1.18	0.014	0.015	0.14	0.09	1.39	1.20	0.031	0.0159	—	—	—	—
Ex. 7
Comp.	0.21	0.05	0.68	0.016	0.021	0.16	0.09	3.09	0.14	0.028	0.0144	—	—	—	—
Ex. 8
Comp.	0.20	0.10	0.80	0.010	0.009	0.15	0.10	0.20	0.40	0.026	0.0153	—	—	—	—
Ex. 9
Comp.	0.18	0.09	1.01	0.014	0.011	0.15	0.11	1.69	0.38	0.031	0.0164	—	—		—
Ex. 10
Comp.	0.20	0.10	0.80	0.010	0.010	0.15	0.10	1.41	0.40	0.026	0.0142	—	—	—	—
Ex. 11
Comp.	0.20	0.10	0.80	0.010	0.010	0.15	0.10	1.41	0.40	0.029	0.0161	—	—	—	—
Ex. 12
Comp.	0.21	0.07	0.88	0.014	0.020	0.14	0.09	1.59	0.17	0.029	0.0143	—	—	—	—
Ex. 13
Comp.	0.24	0.11	0.95	0.015	0.017	0.15	0.12	1.53	0.14	0.027	0.0139	—	—	—	—
Ex. 14
Comp.	0.08	0.04	1.20	0.015	0.013	0.14	0.09	1.37	0.11	0.028	0.0159	—	—	—	—
Ex. 15
Comp.	0.33	0.10	0.82	0.013	0.017	0.13	0.12	1.70	0.31	0.025	0.0136	—	—	—	—
Ex. 16
Comp.	0.18	0.03	0.61	0.018	0.017	0.14	0.11	1.54	0.10	0.027	0.0142	—	—	—	—
Ex. 17
Comp.	0.19	0.07	1.24	0.014	0.013	0.13	0.09	1.57	0.21	0.026	0.0164	—	—	—	—
Ex. 18
Comp.	0.20	0.25	0.79	0.010	0.009	0.15	0.10	1.19	0.30	0.150	0.1000	0.0300	0		—
Ex. 19

		Hardness from
		Surface to Depth	Fatigue
Expr. 1	Exp. 2	of 0.05 mm (HV)	Strength

		Surface C	Conc. (%)	Calc.	Calc.	Normal		Life
	Treatment Condition	Conc. (%)	Surface N	Result	Result	Temp.	500° C.	Ratio

Comp.	carburizing + nitriding	0.78	0.34	1.072	610.3	673	597	2.90
Ex. 1
Comp.	carburizing + nitriding	0.52	0.57	1.004	612.3	737	625	2.50
Ex. 2
Comp.	carburizing + nitriding	0.55	0.49	0.970	615.5	744	619	2.80
Ex. 3
Comp.	carburizing	0.73	0.02	0.743	500.7	729	531	2.06
Ex. 4
Comp.	carburizing + nitriding	0.49	0.54	0.955	629.6	680	632	—
Ex.5
Comp.	carburizing + nitriding	0.44	0.56	0.921	630.1	580	555	2.90
Ex. 6
Comp.	carburizing + nitriding	0.47	0.54	0.930	631.5	668	660	—
Ex. 7
Comp.	carburizing + nitriding	0.45	0.59	0.957	448.1	683	512	2.80
Ex. 8
Comp.	carburizing + nitriding	0.56	0.36	0.869	525.9	741	532	1.41
Ex. 9
Comp.	carburizing + nitriding	0.42	0.90	1.191	581.6	850	636	2.00
Ex. 10
Comp.	carburizing + nitriding	0.53	0.11	0.624	526.6	725	532	2.23
Ex. 11
Comp.	carburizing	0.81	0.02	0.825	476.4	682	509	2.24
Ex. 12
Comp.	carburizing + nitriding	0.88	0.37	1.197	608.0	708	556	2.70
Ex. 13
Comp.	carburizing + nitriding	0.32	0.44	0.700	623.6	680	610	—
Ex. 14
Comp.	carburizing + nitriding	0.53	0.47	0.938	628.3	711	655	2.80
Ex. 15
Comp.	carburizing + nitriding	0.51	0.51	0.953	626.3	694	630	5.00
Ex. 16
Comp.	carburizing + nitriding	0.79	0.75	1.433	615.8	632	611	2.30
Ex. 17
Comp.	carburizing + nitriding	0.35	0.25	0.564	574.2	702	559	2.30
Ex. 18
Comp.	carburizing	0.81	0.00	0.810	483.1	727	459	1.00
Ex. 19

	TABLE 3

	Seizing Limit Load Ratio

	Example 1	1.15
	Example 2	1.22
	Example 3	1.11
	Example 4	1.27
	Example 5	1.19
	Example 6	1.30
	Example 7	1.17
	Example 8	1.14
	Example 9	1.24
	Example 10	1.25
	Example 11	1.31
	Example 12	1.38
	Example 13	1.14
	Example 14	1.17
	Example 15	1.39
	Example 16	1.30
	Example 17	1.50
	Example 18	1.11
	Example 19	1.20
	Example 20	1.25
	Example 21	1.13
	Example 22	1.12
	Example 23	1.13
	Example 24	1.13
	Example 25	1.20
	Example 26	1.11
	Example 27	1.19
	Example 28	1.13
	Example 29	1.20
	Example 30	1.13
	Example 31	1.20

	TABLE 4

	Seizing Limit Load Ratio

	Comparative Example 1	0.98
	Comparative Example 2	1.06
	Comparative Example 3	1.01
	Comparative Example 4	0.98
	Comparative Example 5	1.00
	Comparative Example 6	0.93
	Comparative Example 7	0.97
	Comparative Example 8	0.99
	Comparative Example 9	0.98
	Comparative Example 10	1.03
	Comparative Example 11	1.09
	Comparative Example 12	0.95
	Comparative Example 13	1.01
	Comparative Example 14	1.04
	Comparative Example 15	—
	Comparative Example 16	0.98
	Comparative Example 17	1.09
	Comparative Example 18	1.03
	Comparative Example 19	1.00

FIG. 2 illustrates a relationship between the surface hardness in the case where the tempering treatment was performed at 500° C. as shown in Tables 1 and 2 and the fatigue strength life ratio (life ratio under a high speed roller pitting condition).
From FIG. 2 , it can be confirmed that all of the Examples were improved with respect to the Comparative Examples.
FIG. 3 illustrates a relationship between the surface hardness in the case where the tempering treatment was performed at 500° C. as shown in Tables 1 and 2 and the seizing limit load ratio shown in Tables 3 and 4.
From FIG. 3 , it can be confirmed that all of the Examples were improved with respect to the Comparative Examples.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a steel material for carbonitriding treatment having a high surface fatigue strength and a sufficiently high high-temperature tempering hardness, and a carbonitrided steel material obtained by carbonitriding the steel material.
The present application is based on Japanese patent application No. 2021-071046 filed on Apr. 20, 2021, and the contents thereof are incorporated herein as reference.

REFERENCE SIGNS LIST

- 1 small roller
- 2 contact portion
- 4 small-diameter portion

Claims

1. A steel material for carbonitriding treatment, comprising:

C at 0.1 mass % to 0.3 mass %;

Si at 0.3 mass % or less;

Mn at 0.4 mass % to 2.0 mass %;

P at 0.03 mass % or less;

S at 0.03 mass % or less;

Cu at 0.3 mass % or les;

Ni at 2.5 mass % or less;

Cr at 0.5 mass % to 3.0 mass %;

Mo at 0.001 mass % to 1.0 mass %;

Al at 0.01 mass % to 0.08 mass %; and

N at 0.005 mass % to 0.03 mass %;

with a balance being Fe and inevitable impurities,

satisfying the following Expression 1 of 0.6 to 1.4 and Expression 2 of >560 when subjected to carbonitriding,

Surface C concentration(mass %)+12/14×Surface N concentration(mass %), Expression 1:

129.7805×[Cr(mass %)]−76.9797×[Cr(mass %)]²+339.3375×[Surface N concentration(mass %)]−539.345×[Surface N concentration (mass %)]²+181.4983×[Cr(mass %)]×[Surface N concentration(mass %)]+437.6799, and Expression 2:

providing a carbonitrided steel material having a hardness of 560 HV or more at a portion from a surface thereof to a depth of 0.05 mm when subjected to a tempering treatment at 500° C. after the carbonitriding.

2. The steel material for carbonitriding treatment according to claim 1, further comprising at least one selected from the group consisting of:

Nb at 0.001 mass % to 0.08 mass %;

V at 0.5% mass % or less;

Ti at 0.05% mass % or less; and

B at 0.0005 mass % to 0.003 mass %.

3. The steel material for carbonitriding treatment according to claim 1, comprising:

C at 0.15 mass % to 0.25 mass %;

Si at 0.01 mass % to 0.24 mass %;

Mn at 0.5 mass % to 1.8 mass %;

Cu at 0.001 mass % to 0.3 mass %;

Ni at 0.01 mass % to 0.6 mass %;

Cr at 0.6 mass % to 1.8 mass %;

Mo at 0.01 mass % to 0.8 mass %;

Al at 0.02 mass % to 0.05 mass %;

N at 0.01 mass % to 0.025 mass %; and

at least one selected from the group consisting of:

Nb at 0.0015 mass % to 0.06 mass %;

V at 0.25 mass % or less;

Ti at 0.012 mass % to 0.04 mass %; and

B at 0.0006 mass % to 0.0025 mass %,

with a balance being Fe and inevitable impurities, and

providing, by performing the carbonitriding, a carbonitrided steel material that satisfies the following Expression 1 being 0.7 to 1.2:

4. The steel material for carbonitriding treatment according to claim 1, comprising:

Cu at 0.03 mass % to 0.3 mass %.

5. The steel material for carbonitriding treatment according to claim 1, comprising:

Si at 0.01 mass % to 0.17% mass %.

6. The steel material for carbonitriding treatment according to claim 1, comprising:

Cu at 0.03 mass % to 0.25 mass %.

7. A carbonitrided steel material obtained by carbonitriding the steel material for carbonitriding treatment according to claim 1,

satisfying the following Expression 1 of 0.6 to 1.4 and Expression 2 of >560,

129.7805×[Cr(mass %)]−76.9797×[Cr(mass %)]²+339.3375×[Surface N concentration(mass %)]−539.345×[Surface N concentration (mass %)]2+181.4983×[Cr(mass %)]×[Surface N concentration(mass %)]+437.6799, and Expression 2:

having a hardness of 560 HV or more at a portion from a surface thereof to a depth of 0.05 mm when subjected to a tempering treatment at 500° C.