JP7606077B2 - Steel and carburized steel parts - Google Patents

Description

This disclosure relates to steel materials suitable for use as materials for carburized steel parts, and to carburized steel parts.

As power units such as engines and motors have become more powerful and smaller in recent years, mechanical parts used in and around power units are required to have excellent bending fatigue strength. Among these mechanical parts, gears used in automobiles, construction vehicles, etc. have tooth surfaces that slide against each other at short intervals. For this reason, pitting must be suppressed on the tooth surfaces. In other words, mechanical parts such as gears used in automobiles, construction vehicles, etc. are required to have not only bending fatigue strength but also surface fatigue strength (pitting characteristics).

Carburizing is known as a method for increasing the bending fatigue strength and surface fatigue strength of mechanical parts. In this specification, carburizing includes not only carburizing but also carbonitriding. In carburizing, a hardened layer (carburized layer or carbonitriding layer) is formed on the surface of the mechanical part. It is known that this hardened layer increases the bending fatigue strength and surface fatigue strength of the mechanical part. Therefore, it is required that the steel material that is the material for carburized steel parts can further increase the bending fatigue strength and surface fatigue strength when the mechanical part (carburized steel part) is made by carburizing.

Steel materials that can increase bending fatigue strength and surface fatigue strength when carburized into carburized steel parts are proposed in JP 2010-185123 A (Patent Document 1) and JP 2017-214642 A (Patent Document 2).

The steel material disclosed in Patent Document 1 contains, by mass%, C: 0.15 to 0.25%, Si: 0.40 to 0.80%, Mn: 0.20 to 1.0%, P: 0.030% or less, S: 0.10% or less, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.8 to 1.8%, Mo: 0.60% or less, Al: 0.02 to 0.10%, N: 0.005 to 0.03%, O: 0.003% or less, with the balance being Fe and unavoidable impurities, and satisfies the following formulas (1) and (2).
1.8≦2×[Si]+[Cr]≦3.5 (1)
114×[Si]+2×[Cr]+68×[Mo]≧50 (2)

The steel material disclosed in Patent Document 2 contains, in mass%, C: 0.15 to 0.30%, Si: 0.80% to 2.00%, Mn: 0.20 to 0.80%, P: 0.003 to 0.030%, S: 0.005 to 0.050%, Cr: 1.00 to less than 1.80%, Mo: 0.03 to 0.30%, Al: 0.020 to 0.060%, N: 0.0060 to 0.0300%, and O: 0.0003 to 0.0025%, with the balance being Fe and unavoidable impurities, and satisfies the following formulas (1) to (3).
[%Si]+([%Mn]+[%Cr]+[%Mo])/3≧1.5 (1)
180-45[%Mn]-14[%Cr]-51[%Mo]+5[%Si]≧125 (2)
√I≦80 (3)
Here, I indicates the area (μm ² ) of oxide-based inclusions located at the center of the fisheye on the fracture surface after the steel material is carburized, quenched and tempered, and then subjected to a rotating bending fatigue test.

JP 2010-185123 A JP 2017-214642 A

However, there may be a steel material that can obtain excellent bending fatigue strength and excellent surface fatigue strength when carburized into a carburized steel part by a means different from that proposed in Patent Documents 1 and 2. There are also gas carburizing and vacuum carburizing. In vacuum carburizing, carburizing is performed in a vacuum or under reduced pressure. Therefore, vacuum carburizing can suppress the formation of a grain boundary oxide layer on the surface, which is a problem in gas carburizing. Therefore, in steel materials used for vacuum carburizing, the content of elements that easily form oxides, such as Si, Cr, and Mn, which promote the formation of grain boundary oxide layers, can be increased in gas carburizing. Therefore, there is a demand for a steel material that can be used as a material for carburized steel parts and is suitable for vacuum carburizing.

The objective of this disclosure is to provide a steel material and a carburized steel part that, when subjected to vacuum carburization treatment to produce a carburized steel part, exhibit excellent bending fatigue strength and excellent surface fatigue strength (pitting characteristics).

The steel material according to the present disclosure is
Chemical composition in mass percent:
C: 0.10-0.35%,
Si: 0.60-1.50%,
Mn: 0.20-1.30%,
P: 0.030% or less,
S: 0.050% or less,
Ni: 0.01 to 0.20%,
Cr: 0.65-1.50%,
Mo: 0.01 to 0.60%,
Al: 0.045% or less, and
N: 0.0250% or less,
and the balance being Fe and impurities, and satisfying formulas (1) to (4).
2.5≦(2Mn+5Cr+Mo)/Si≦7.5 (1)
Mn×Ni≦0.05 (2)
1.7≦Al/N≦2.4 (3)
22≦Mn/S≦65 (4)
Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (4).

The carburized steel component according to the present disclosure comprises:
A hardened layer;
A core portion is provided inside the hardened layer,
The chemical composition of the core is, in mass%,
C: 0.10-0.35%,
Si: 0.60-1.50%,
Mn: 0.20-1.30%,
P: 0.030% or less,
S: 0.050% or less,
Ni: 0.01 to 0.20%,
Cr: 0.65-1.50%,
Mo: 0.01 to 0.60%,
Al: 0.045% or less, and
N: 0.0250% or less,
and the balance being Fe and impurities, and satisfying formulas (1) to (4),
The carbon concentration in a region from the surface of the carburized steel part to a depth of 50 μm is 0.60% or more by mass%,
The microstructure of the carburized steel part at a depth of 20 μm from the surface is made of martensite or made of martensite and retained austenite, and the volume fraction of the retained austenite is 0 to 40%.
2.5≦(2Mn+5Cr+Mo)/Si≦7.5 (1)
Mn×Ni≦0.05 (2)
1.7≦Al/N≦2.4 (3)
22≦Mn/S≦65 (4)
Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (4).

When the steel material according to the present disclosure is subjected to vacuum carburization to form a carburized steel part, excellent bending fatigue strength and excellent surface fatigue strength (pitting characteristics) are obtained. The carburized steel part according to the present disclosure has excellent bending fatigue strength and excellent surface fatigue strength.

FIG. 1 is a diagram showing an example of a heat pattern in the vacuum carburizing process and the quenching process. FIG. 2 is a side view of a small roller test piece prepared in the example. FIG. 3 is a side view of a rotating bending fatigue test piece prepared in the example. FIG. 4 is a front view of a large roller test piece prepared in the example. FIG. 5 is a schematic diagram of a twin-roller rolling fatigue test in the examples.

The present inventors have studied steel materials that can obtain excellent bending fatigue strength and excellent surface fatigue strength (pitting characteristics) when they are subjected to vacuum carburization (vacuum carburization or vacuum carbonitriding) to produce carburized steel parts. Such steel materials are subjected to vacuum carburization, for example, as described above, in the manufacturing process of carburized steel parts. In vacuum carburization, the steel material is heated to the A _c3 transformation point temperature or higher, so that the microstructure of the steel material is transformed into austenite, and the influence of the previous structure is eliminated. Therefore, the present inventors have studied means for increasing the bending fatigue strength and surface fatigue strength of carburized steel parts not from the viewpoint of the microstructure of the steel material, but from the viewpoint of the chemical composition that does not change even when vacuum carburization is performed. As a result, the present inventors have considered that it is effective to increase the contents of Si, Cr, Mn, and Mo, which increase temper softening resistance, in order to increase the surface fatigue strength (pitting characteristics). As a result of examining the compatibility between bending fatigue strength and surface fatigue strength from the viewpoint of chemical composition, the following composition was determined by mass %: C: 0.10-0.35%, Si: 0.60-1.50%, Mn: 0.20-1.30%, P: 0.030% or less, S: 0.050% or less, Ni: 0.01-0.20%, Cr: 0.65-1.50%, Mo: 0.01-0.60%, Al: 0.045% or less, N: 0.0250% or less, V: 0-0.50%, Nb: 0-0.030%, Ti: 0-0.200%, Cu: 0-0.50%, W: 0-0.50%, ... It was considered that if a steel material had a chemical composition consisting of O: 0-0.50%, B: 0-0.0010%, Ca: 0-0.0015%, Mg: 0-0.010%, rare earth elements (REM): 0-0.0100%, Te: 0-0.100%, Bi: 0-0.500%, Pb: 0-0.09%, Sn: 0-0.015%, and Sb: 0-0.006%, with the balance being Fe and impurities, then when a carburized steel part is manufactured by carrying out a vacuum carburizing treatment, the carburized steel part would have excellent bending fatigue strength and excellent surface fatigue strength.

However, even when the content of each element in the chemical composition of a steel material falls within the above-mentioned range, there are cases in which sufficient bending fatigue strength and surface fatigue strength cannot be obtained when the steel is subjected to vacuum carburizing to form a carburized steel part. Therefore, the inventors of the present invention have further investigated and studied the matter. As a result, the inventors of the present invention have come to the following findings.

(A) In the vacuum carburizing process, the carburizing process and the diffusion process are performed once each in a vacuum or under reduced pressure, or the carburizing process and the diffusion process are alternately repeated multiple times. In the carburizing process, a hydrocarbon-based gas is introduced at low pressure to form an appropriate amount of cementite on the surface layer of the steel material. Then, in the diffusion process, the introduction of the hydrocarbon-based gas is stopped. In this case, the cementite decomposes in the diffusion process, and the carbon concentration in the surface layer of the steel material increases due to the decomposition of the cementite. As a result, in the diffusion process of the vacuum carburizing process, the gradient of the carbon concentration in the austenite in the surface layer is larger than in the diffusion process of the gas carburizing process, and the amount of C penetrating into the inside of the steel material can be increased. In this way, in the diffusion process of the vacuum carburizing process, no hydrocarbon-based gas is introduced, and the cementite formed on the surface layer of the steel material in the previous carburizing process is used as a carbon (C) supply source to diffuse and penetrate C into the inside of the steel material. As a result, the vacuum carburizing process can form a hardened layer in a short time compared to the gas carburizing process.

As mentioned above, Si, Cr, Mn and Mo are all elements that increase the temper softening resistance of steel and increase surface fatigue strength (pitting characteristics). Here, Mn, Cr and Mo all have a high affinity with C, and tend to form cementite and alloy carbides (hereinafter referred to as cementite, etc.) on the surface layer of steel during the carburizing process of vacuum carburizing treatment. On the other hand, Si has a low affinity with C, and tends to form cementite, etc. on the surface layer of steel during the carburizing process of vacuum carburizing treatment.

If the Si content is too high relative to the Mn, Cr, and Mo contents in the temperature range in which the vacuum carburizing process is performed, cementite and other elements are unlikely to form on the surface of the steel during the carburizing process of the vacuum carburizing process. In this case, there is a shortage of cementite and other elements that serve as a C supply source during the diffusion process. As a result, the gradient of C concentration between the surface and interior of the steel is small. This results in insufficient penetration and diffusion of C into the interior of the steel. As a result, it becomes difficult for a hardened layer to form, and the surface fatigue strength of the carburized steel part decreases.

On the other hand, if the Cr, Mn and Mo contents are too high relative to the Si content, excessive cementite etc. will be formed during the carburizing process of the vacuum carburizing treatment. In this case, although a certain amount of cementite etc. will be decomposed in the diffusion process, some of the cementite etc. will not be decomposed and will remain in the surface layer. Cementite etc. will be the starting point of cracks during bending fatigue. Therefore, if an excessive amount of cementite etc. remains in the surface layer, the bending fatigue strength of the carburized steel part will decrease.

By setting the ratio of the Cr, Mn and Mo contents to the Si content within an appropriate range, the C concentration that penetrates into the steel material during the carburizing process of the vacuum carburizing treatment will be within an appropriate range. As a result, a hardened layer with a sufficient C concentration can be stably formed on the surface of the carburized steel part. As a result, the bending fatigue strength and surface fatigue strength of the carburized steel part can be increased.

It is defined as F1 = (2Mn + 5Cr + Mo) / Si. If the content of each element in the chemical composition is within the above-mentioned range and F1 is 2.5 to 7.5, that is, if the following formula (1) is satisfied, the Si content relative to the contents of Mn, Cr, and Mo is in an appropriate range. Therefore, an appropriate amount of C penetrates and diffuses into the steel material during vacuum carburization. As a result, carburized steel parts manufactured using the steel material as a material can obtain excellent bending fatigue strength and excellent surface fatigue strength, provided that the content of each element in the chemical composition is within the above-mentioned range and satisfies the formulas (2) to (4) described below.
2.5≦(2Mn+5Cr+Mo)/Si≦7.5 (1)

(B) Even if an appropriate amount of C penetrates and diffuses into the steel material to satisfy formula (1) in the temperature range in which vacuum carburizing is performed, if the amount of retained austenite in the surface layer of the steel material after vacuum carburizing is excessively high, sufficient hardness may not be obtained and bending fatigue strength may decrease. As a result of further investigation, the inventors of the present invention have come to the following conclusions.

Mn and Ni are austenite stabilizing elements, and both increase the amount of retained austenite in the surface layer of steel after vacuum carburization. Therefore, even if the content of each element in the chemical composition is within the above-mentioned range and satisfies formula (1), if the Mn content and Ni content are not appropriately adjusted, excessive retained austenite will be present in the surface layer of the steel after vacuum carburization. In this case, sufficient hardness and sufficient bending fatigue strength cannot be obtained in the carburized steel part.

F2 is defined as Mn x Ni. If F2 is 0.05 or less, that is, if the following formula (2) is satisfied, the amount of retained austenite in the surface layer of the carburized steel part can be set to an appropriate range, provided that the content of each element in the chemical composition is within the above-mentioned range and formulas (1), (3) and (4) are satisfied. Therefore, a hardened layer with sufficient surface hardness is formed. As a result, the carburized steel part can obtain sufficient bending fatigue strength.
Mn×Ni≦0.05 (2)

(C) Even if the content of each element in the chemical composition is within the above-mentioned range, if there is an excess of Al inclusions in the steel, the Al inclusions can become the starting point of cracks. Therefore, if there are excessive Al inclusions remaining in the steel, the bending fatigue strength of the carburized steel part may decrease. Furthermore, Al in the steel may precipitate as precipitates (AlN). Coarse AlN (precipitates) can become the starting point of cracks, just like Al inclusions. Therefore, if there is an excessive amount of coarse AlN (precipitates) remaining in the steel, the bending fatigue strength of the carburized steel part may decrease.

F3 is defined as Al/N. In the chemical composition of the steel material of this embodiment, assuming that the content of each element is within the range of this embodiment, if F3 is less than 1.7, the Al content is excessively high compared to the N content. In this case, excessive amounts of Al inclusions (oxidation-based inclusions) that have not bonded with N are formed. As a result, the bending fatigue strength of the carburized steel part decreases.

On the other hand, in the chemical composition of the steel material of this embodiment, assuming that the content of each element is within the range of this embodiment, if F3 exceeds 2.4, the N content is excessively higher than the Al content. In this case, an excessive amount of coarse AlN (precipitates) is formed in the steel material. Therefore, in this case as well, the bending fatigue strength of the carburized steel part is reduced.

In the chemical composition of the steel material of this embodiment, assuming that the content of each element is within the range of this embodiment, if F3 is 1.7 to 2.4, that is, if the following formula (3) is satisfied, the formation of Al inclusions in the steel material can be sufficiently suppressed, and the formation of coarse AlN (precipitates) can be sufficiently suppressed. Therefore, assuming that formulas (1), (2), and (4) are satisfied, sufficient bending fatigue strength can be obtained in the carburized steel part.
1.7≦Al/N≦2.4 (3)

(D) When the content of each element in the chemical composition is within the above-mentioned range, Mn combines with S to form MnS. MnS improves the machinability of steel. However, coarse MnS can be the starting point of cracks during bending fatigue of carburized steel parts. Therefore, if excessive amounts of coarse MnS are formed, the bending fatigue strength of the carburized steel parts decreases. Therefore, if the ratio of Mn content to S content can be appropriately adjusted to suppress the formation of coarse MnS, the bending fatigue strength of the carburized steel parts can be increased.

F4 is defined as Mn/S. If F4 is 22 to 65, that is, if the following formula (4) is satisfied, the formation of coarse MnS can be effectively suppressed. As a result, on the premise that the content of each element in the chemical composition is within the above-mentioned range and formulas (1) to (3) are satisfied, sufficient surface fatigue strength and sufficient bending fatigue strength can be obtained in the carburized steel part.
22≦Mn/S≦65 (4)

The steel material and carburized steel parts of this embodiment were completed based on the above technical concepts and have the following configuration:

[1]
Chemical composition in mass percent:
C: 0.10-0.35%,
Si: 0.60-1.50%,
Mn: 0.20-1.30%,
P: 0.030% or less,
S: 0.050% or less,
Ni: 0.01 to 0.20%,
Cr: 0.65-1.50%,
Mo: 0.01 to 0.60%,
Al: 0.045% or less, and
N: 0.0250% or less,
The balance is Fe and impurities, and satisfies formulas (1) to (4).
Steel.
2.5≦(2Mn+5Cr+Mo)/Si≦7.5 (1)
Mn×Ni≦0.05 (2)
1.7≦Al/N≦2.4 (3)
22≦Mn/S≦65 (4)
Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (4).

[2]
The steel material according to [1],
The chemical composition is, in place of a part of Fe,
V: 0.50% or less,
Nb: 0.030% or less,
Ti: 0.200% or less,
Cu: 0.50% or less,
W: 0.50% or less,
Co: 0.50% or less, and
B: 0.0010% or less,
Contains one or more selected from the group consisting of
Steel.

[3]
The steel material according to [1] or [2],
The chemical composition is, in place of a part of Fe,
Ca: 0.0015% or less,
Mg: 0.010% or less, and
Rare earth elements: 0.0100% or less,
Contains one or more selected from the group consisting of
Steel.

[4]
The steel material according to any one of [1] to [3],
The chemical composition is, in place of a part of Fe,
Te: 0.100% or less,
Bi: 0.500% or less,
Pb: 0.09% or less,
Sn: 0.015% or less, and
Sb: 0.006% or less,
Contains one or more selected from the group consisting of
Steel.

[5]
A carburized steel part,
A hardened layer;
A core portion is provided inside the hardened layer,
The chemical composition of the core is, in mass%,
C: 0.10-0.35%,
Si: 0.60-1.50%,
Mn: 0.20-1.30%,
P: 0.030% or less,
S: 0.050% or less,
Ni: 0.01 to 0.20%,
Cr: 0.65-1.50%,
Mo: 0.01 to 0.60%,
Al: 0.045% or less, and
N: 0.0250% or less,
and the balance being Fe and impurities, and satisfying formulas (1) to (4),
The carbon concentration in a region from the surface of the carburized steel part to a depth of 50 μm is 0.60% or more by mass%,
The microstructure at a depth of 20 μm from the surface of the carburized steel part is made of martensite or made of martensite and retained austenite, and the volume fraction of the retained austenite is 0 to 40%.
Carburized steel parts.
2.5≦(2Mn+5Cr+Mo)/Si≦7.5 (1)
Mn×Ni≦0.05 (2)
1.7≦Al/N≦2.4 (3)
22≦Mn/S≦65 (4)
Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (4).

[6]
A carburized steel part according to [5],
The chemical composition of the core portion is, in place of a part of Fe,
V: 0.50% or less,
Nb: 0.030% or less,
Ti: 0.200% or less,
Cu: 0.50% or less,
W: 0.50% or less,
Co: 0.50% or less, and
B: 0.0010% or less,
Contains one or more selected from the group consisting of
Carburized steel parts.

[7]
A carburized steel part according to [5] or [6],
The chemical composition of the core portion is, in place of a part of Fe,
Ca: 0.0015% or less,
Mg: 0.0100% or less, and
Rare earth elements: 0.0100% or less,
Contains one or more selected from the group consisting of
Carburized steel parts.

[8]
A carburized steel part according to any one of [5] to [7],
The chemical composition of the core portion is, in place of a part of Fe,
Te: 0.100% or less,
Bi: 0.500% or less,
Pb: 0.09% or less,
Sn: 0.015% or less, and
Sb: 0.006% or less,
Contains one or more selected from the group consisting of
Carburized steel parts.

The steel material of this embodiment and the carburized steel parts manufactured using this steel material are described in detail below. Unless otherwise specified, the "%" for the content of each element means "mass %."

[Steel applications]
The steel material of this embodiment is suitable as a material for carburized steel parts. Specifically, the steel material of this embodiment is suitable as a material for carburized steel parts manufactured by carrying out a vacuum carburizing process. In this specification, the vacuum carburizing process also includes a vacuum carbonitriding process.

[Chemical composition of steel]
The chemical composition of the steel material of this embodiment contains the following elements.

C: 0.10-0.35%
Carbon (C) improves the hardenability of steel and increases the hardness of the core of carburized steel parts manufactured from steel materials. Therefore, C increases the bending fatigue strength of carburized steel parts. C content If the C content is less than 0.10%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. Even if the contents of other elements are within the ranges of this embodiment, the machinability of the steel material decreases. Therefore, the C content is 0.10 to 0.35%. The preferred lower limit of the C content is The upper limit of the C content is preferably 0.32%, more preferably 0.30%. , and more preferably 0.28%.

Si: 0.60-1.50%
Silicon (Si) increases the temper softening resistance of steel, and as a result, increases the surface fatigue strength (pitting characteristics) of carburized steel parts manufactured using the steel material. If the Si content is less than 0.60%, On the other hand, if the Si content exceeds 1.50%, the above-mentioned effect cannot be sufficiently obtained even if the contents of the other elements are within the range of this embodiment. Even if the content of C in the steel is within the above range, it inhibits the formation of cementite and the like (cementite and alloy carbides) in the temperature range of the vacuum carburization treatment. In this case, the penetration of C into the steel material during the vacuum carburization treatment is suppressed. Therefore, the Si content is 0.60 to 1.50%. The preferred lower limit of the Si content is 0. 65%, more preferably 0.70%, more preferably 0.75%, more preferably 0.80%, more preferably 0.85%, and even more preferably 0. The answer is 90%. The upper limit of the Si content is preferably 1.45%, more preferably 1.40%, further preferably 1.35%, and further preferably 1.30%.

Mn: 0.20-1.30%
Manganese (Mn) increases the temper softening resistance of steel, and as a result, increases the surface fatigue strength (pitting properties) of carburized steel parts manufactured using the steel material. If the Mn content is less than 0.20%, On the other hand, if the Mn content exceeds 1.30%, the above-mentioned effects cannot be sufficiently obtained even if the contents of the other elements are within the ranges of this embodiment. Even if the thickness is within the range, the amount of retained austenite in the surface layer of the carburized steel part after vacuum carburizing is too high. In this case, the hardness of the carburized steel part is insufficient, and the bending fatigue strength of the carburized steel part is low. Therefore, the Mn content is 0.20 to 1.30%. The lower limit of the Mn content is preferably 0.25%, more preferably 0.30%, and even more preferably 0. .35%, more preferably 0.40%, and even more preferably 0.45%. The upper limit of the Mn content is preferably 1.25%, more preferably 1.20%, still more preferably 1.15%, still more preferably 1.10%, and still more preferably 1.05%. %, and more preferably 1.00%.

P: 0.030% or less Phosphorus (P) is an impurity. In a vacuum carburizing process when manufacturing a carburized steel part using a steel material as a base material, P segregates at the austenite grain boundary, reducing the bending fatigue strength of the carburized steel part. If the P content exceeds 0.030%, the bending fatigue strength of the carburized steel part will be significantly reduced even if the contents of other elements are within the range of this embodiment. Therefore, the P content is 0.030% or less. The preferred upper limit of the P content is 0.029%, more preferably 0.028%, and even more preferably 0.025%. The P content is preferably as low as possible. However, excessive reduction of the P content increases the manufacturing cost. Therefore, in consideration of normal industrial production, the preferred lower limit of the P content is more than 0%, more preferably 0.001%, and even more preferably 0.002%.

S: 0.050% or less Sulfur (S) is an impurity. S combines with Mn to form MnS, which enhances the machinability of the steel material. However, if the S content exceeds 0.050%, the sulfides will become coarse even if the contents of other elements are within the range of this embodiment. In this case, the bending fatigue strength of the carburized steel part will decrease. Therefore, the S content is 0.050% or less. The preferred lower limit of the S content is more than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.005%, and more preferably 0.007%. The preferred upper limit of the S content is 0.045%, more preferably 0.040%, more preferably 0.035%, and more preferably 0.030%.

Ni: 0.01~0.20%
Nickel (Ni) improves the hardenability of steel and increases the hardness of the core of carburized steel parts manufactured from steel materials. As a result, the bending fatigue strength of carburized steel parts is increased. If the Ni content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. Even if the content of Ni is within the range of this embodiment, the volume fraction of the retained austenite in the surface layer of the carburized steel part is excessively large. As a result, the bending fatigue strength of the carburized steel part is reduced. The Ni content is 0.01 to 0.20%. The lower limit of the Ni content is preferably 0.02%, more preferably 0.03%, and even more preferably 0.04%. The preferred upper limit of the amount is 0.17%, more preferably 0.14%, and even more preferably 0.11%.

Cr: 0.65-1.50%
Chromium (Cr) increases the temper softening resistance of steel, and as a result, increases the surface fatigue strength (pitting characteristics) of carburized steel parts manufactured using the steel material. If the Cr content is less than 0.65%, On the other hand, if the Cr content exceeds 1.50%, the above-mentioned effect cannot be sufficiently obtained even if the contents of the other elements are within the range of this embodiment. Even if the amount of carbon in the steel is within the range, excessive carbon penetrates and diffuses into the steel during vacuum carburization. As a result, excessive coarse cementite is formed on the surface layer of the steel. In this case, As a result, cracks are more likely to occur starting from the coarse cementite in the carburized steel component, and the bending fatigue strength of the carburized steel component decreases. , the Cr content is 0.65 to 1.50%. The lower limit of the Cr content is preferably 0.70%, more preferably 0.75%, and even more preferably 0.80%. More preferably, it is 0.85%. The upper limit of the Cr content is preferably 1.45%, more preferably 1.40%, further preferably 1.35%, and further preferably 1.30%.

Mo: 0.01~0.60%
Molybdenum (Mo) increases the temper softening resistance of steel, and as a result, increases the surface fatigue strength (pitting properties) of carburized steel parts manufactured using the steel material. If the Mo content is less than 0.01%, Even if the contents of the other elements are within the ranges of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Mo content exceeds 0.60%, the contents of the other elements are not as effective as those of this embodiment. Even if the amount of carbon in the steel is within the range, excessive carbon penetrates and diffuses into the steel during vacuum carburization. As a result, excessive coarse cementite is formed on the surface layer of the steel. In this case, As a result, cracks are more likely to occur starting from the coarse cementite in the carburized steel component, and the bending fatigue strength of the carburized steel component decreases. The Mo content is 0.01 to 0.60%. The lower limit of the Mo content is preferably 0.02%, more preferably 0.03%, and even more preferably 0.04%. More preferably, it is 0.05%. The upper limit of the Mo content is preferably 0.55%, more preferably 0.50%, and further preferably 0.45%.

Al: 0.045% or less Aluminum (Al) deoxidizes steel. However, if the Al content exceeds 0.045%, coarse Al inclusions (oxide-based inclusions) are generated even if the contents of other elements are within the range of this embodiment. The coarse Al inclusions reduce the bending fatigue strength of the carburized steel parts. Therefore, the Al content is 0.045% or less. The preferred lower limit of the Al content is more than 0%, more preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Al content is 0.042%, more preferably 0.039%, more preferably 0.036%, and even more preferably 0.033%.

N: 0.0250% or less Nitrogen (N) is an impurity. If the N content exceeds 0.0250%, coarse nitrides, such as AlN, will be generated even if the contents of other elements are within the range of this embodiment. The coarse nitrides reduce the bending fatigue strength of the carburized steel parts. Therefore, the N content is 0.0250% or less. The preferred upper limit of the N content is 0.0230%, more preferably 0.0210%, more preferably 0.0200%, more preferably 0.0190%, and more preferably 0.0180%. It is preferable that the N content is as low as possible. However, if the N content is reduced excessively, the manufacturing cost will increase. Therefore, in consideration of normal industrial production, the preferred lower limit of the N content is more than 0%, more preferably 0.0001%, and more preferably 0.0005%.

The remainder of the chemical composition of the steel material according to this embodiment is composed of Fe and impurities. Here, impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when the steel material is industrially manufactured, and are acceptable within a range that does not adversely affect the steel material according to this embodiment.

[Optional elements]
The steel material of the present embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of V, Nb, Ti, Cu, W, Co, and B. All of these elements are optional elements, and increase the bending fatigue strength of a carburized steel part manufactured using the steel material as a material.

V: 0.50% or less Vanadium (V) is an optional element and may not be contained. In other words, the V content may be 0%. When vanadium (V) is contained, that is, when the V content is more than 0%, V forms precipitates (carbides, nitrides, carbonitrides, etc.) and suppresses the coarsening of the crystal grains of the steel during vacuum carburization treatment due to the pinning effect. As a result, the bending fatigue strength of the carburized steel parts manufactured using the steel as a material is increased. If even a small amount of V is contained, the above effect can be obtained to a certain extent. However, if the V content exceeds 0.50%, the hardness of the steel becomes excessively high even if the contents of other elements are within the range of this embodiment. In this case, the machinability of the steel decreases. Therefore, the V content is 0 to 0.50%, and when contained, it is 0.50% or less. The preferable lower limit of the V content is 0.01%, more preferably 0.05%, and more preferably 0.10%. The preferable upper limit of the V content is 0.40%, and more preferably 0.30%.

Nb: 0.030% or less Niobium (Nb) is an optional element and may not be contained. In other words, the Nb content may be 0%. When it is contained, that is, when the Nb content is more than 0%, Nb forms precipitates (carbides, carbonitrides, etc.) and suppresses the coarsening of the crystal grains of the steel material during vacuum carburization treatment due to the pinning effect. As a result, the bending fatigue strength of the carburized steel parts manufactured using the steel material as a material is increased. If even a small amount of Nb is contained, the above effect can be obtained to a certain extent. However, if the Nb content exceeds 0.030%, even if the contents of other elements are within the range of this embodiment, the Nb precipitates will become coarse and the pinning effect will not be obtained. Therefore, the Nb content is 0 to 0.030%, and when it is contained, it is 0.030% or less. The preferable lower limit of the Nb content is 0.001%, and more preferably 0.005%. The upper limit of the Nb content is preferably 0.027%, more preferably 0.025%, and further preferably 0.020%.

Ti: 0.200% or less Titanium (Ti) is an optional element and may not be contained. In other words, the Ti content may be 0%. When titanium (Ti) is contained, that is, when the Ti content is more than 0%, Ti forms precipitates (carbides, nitrides, carbonitrides, etc.) and suppresses the coarsening of crystal grains of steel during vacuum carburization treatment due to the pinning effect. As a result, the bending fatigue strength of carburized steel parts manufactured using steel as a material is increased. If even a small amount of Ti is contained, the above effect can be obtained to a certain extent. However, if the Ti content exceeds 0.200%, even if the contents of other elements are within the range of this embodiment, the Ti precipitates become coarse and the pinning effect cannot be obtained. Therefore, the Ti content is 0 to 0.200%, and when contained, it is 0.200% or less. The preferable lower limit of the Ti content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Ti content is preferably 0.180%, more preferably 0.160%, further preferably 0.140%, further preferably 0.120%, and further preferably 0.100%.

Cu: 0.50% or less Copper (Cu) is an optional element and may not be contained. In other words, the Cu content may be 0%. When it is contained, that is, when the Cu content is more than 0%, Cu increases the hardenability of the steel and increases the hardness of the core of the carburized steel part manufactured using the steel as a material. As a result, the bending fatigue strength of the carburized steel part is increased. If even a small amount of Cu is contained, the above effect can be obtained to a certain extent. However, if the Cu content exceeds 0.50%, the hot workability of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Cu content is 0 to 0.50%, and if it is contained, it is 0.50% or less. The preferable lower limit of the Cu content is 0.01%, more preferably 0.05%, more preferably 0.08%, and more preferably 0.10%. The preferable upper limit of the Cu content is 0.48%, and more preferably 0.46%.

W: 0.50% or less Tungsten (W) is an optional element and may not be contained. In other words, the W content may be 0%. When it is contained, that is, when the W content is more than 0%, W increases the hardenability of the steel and increases the hardness of the core of the carburized steel part manufactured using the steel as a material. As a result, the bending fatigue strength of the carburized steel part is increased. If even a small amount of W is contained, the above effect can be obtained to a certain extent. However, if the W content exceeds 0.50%, the strength of the steel becomes excessively high even if the contents of other elements are within the range of this embodiment. In this case, the machinability of the steel decreases. Therefore, the W content is 0 to 0.50%, and when it is contained, it is 0.50% or less. The preferable lower limit of the W content is 0.01%, more preferably 0.05%, and more preferably 0.08%. The preferable upper limit of the W content is 0.40%, and more preferably 0.30%.

Co: 0.50% or less Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When it is contained, that is, when the Co content is more than 0%, Co increases the hardenability of the steel and increases the hardness of the core of the carburized steel part manufactured using the steel as a material. As a result, the bending fatigue strength of the carburized steel part is increased. If even a small amount of Co is contained, the above effect can be obtained to a certain extent. However, if the Co content exceeds 0.50%, the strength of the steel becomes excessively high even if the contents of other elements are within the range of this embodiment. In this case, the machinability of the steel decreases. Therefore, the Co content is 0 to 0.50%, and when it is contained, it is 0.50% or less. The preferred lower limit of the Co content is 0.01%, more preferably 0.05%, and more preferably 0.08%. The preferred upper limit of the Co content is 0.40%, and more preferably 0.30%.

B: 0.0010% or less Boron (B) is an optional element and may not be contained. In other words, the B content may be 0%. When it is contained, that is, when the B content is more than 0%, B increases the hardenability of the steel and increases the hardness of the core of the carburized steel part manufactured using the steel material as a material. As a result, the bending fatigue strength of the carburized steel part is increased. If even a small amount of B is contained, the above effect can be obtained to a certain extent. However, if the B content exceeds 0.0010%, the effect is saturated. Therefore, the B content is 0 to 0.0010%, and if it is contained, it is 0.0010% or less. The preferable lower limit of the B content is 0.0001%, more preferably 0.0002%, and more preferably 0.0003%. The preferable upper limit of the B content is 0.0009%, and more preferably 0.0008%.

The steel material of this embodiment may further contain one or more elements selected from the group consisting of Ca, Mg, and rare earth elements (REM) in place of a portion of the Fe. All of these elements are optional elements, and increase the bending fatigue strength of carburized steel parts manufactured using the steel material as a material.

Ca: 0.0015% or less Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When it is contained, that is, when the Ca content is more than 0%, Ca modifies the sulfides in the steel material and inhibits the sulfides from elongating during hot working. As a result, the bending fatigue strength of the carburized steel part is increased. If even a small amount of Ca is contained, the above effect can be obtained to a certain extent. However, if the Ca content exceeds 0.0015%, the above effect is saturated. Therefore, the Ca content is 0 to 0.0015%, and if it is contained, it is 0.0015% or less. The preferable lower limit of the Ca content is 0.0001%, and more preferably 0.0002%. The preferable upper limit of the Ca content is 0.0012%, and more preferably 0.0010%.

Mg: 0.010% or less Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When it is contained, that is, when the Mg content is more than 0%, Mg modifies the sulfides in the steel material and inhibits the sulfides from elongating during hot working. As a result, the bending fatigue strength of the carburized steel part is increased. If even a small amount of Mg is contained, the above effect can be obtained to a certain extent. However, if the Mg content exceeds 0.010%, the above effect is saturated. Therefore, the Mg content is 0 to 0.010%, and if it is contained, it is 0.010% or less. The preferable lower limit of the Mg content is 0.001%, and more preferably 0.002%. The preferable upper limit of the Mg content is 0.009%, and more preferably 0.008%.

Rare earth element: 0.0100% or less Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, that is, when the REM content is more than 0%, REM modifies the sulfides in the steel material and inhibits the sulfides from elongating during hot working. As a result, the bending fatigue strength of the carburized steel part is increased. If even a small amount of REM is contained, the above effect can be obtained to a certain extent. However, if the REM content exceeds 0.0100%, the formation of coarse oxides is promoted even if the contents of other elements are within the range of this embodiment. In this case, the bending fatigue strength of the carburized steel part is reduced. Therefore, the REM content is 0 to 0.0100%, and when contained, it is 0.0100% or less. The preferable lower limit of the REM content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the REM content is preferably 0.0098%, and more preferably 0.0097%.

The steel material of this embodiment may further contain one or more elements selected from the group consisting of Te, Bi, Pb, Sn, and Sb in place of a portion of Fe. These elements are optional elements, and all of them improve the machinability of the steel material.

Te: 0.100% or less Tellurium (Te) is an optional element and may not be contained. That is, the Te content may be 0%. When it is contained, that is, when the Te content is more than 0%, Te enhances the machinability of the steel material. If even a small amount of Te is contained, the above effect can be obtained to some extent. However, if the Te content exceeds 0.100%, the hot workability of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Te content is 0 to 0.100%, and when it is contained, it is 0.100% or less. The preferred lower limit of the Te content is 0.001%, and more preferably 0.002%. The preferred upper limit of the Te content is 0.095%, and more preferably 0.090%.

Bi: 0.500% or less Bismuth (Bi) is an optional element and may not be contained. That is, the Bi content may be 0%. When contained, that is, when the Bi content is more than 0%, Bi enhances the machinability of the steel material. If even a small amount of Bi is contained, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.500%, the hot workability of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Bi content is 0 to 0.500%, and when contained, it is 0.500% or less. The preferred lower limit of the Bi content is 0.001%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the Bi content is 0.450%, more preferably 0.400%, more preferably 0.350%, and more preferably 0.300%.

Pb: 0.09% or less Lead (Pb) is an optional element and may not be contained. In other words, the Pb content may be 0%. When it is contained, that is, when the Pb content is more than 0%, Pb enhances the machinability of the steel material. If even a small amount of Pb is contained, the above effect can be obtained to a certain extent. However, if the Pb content exceeds 0.09%, the hot workability of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Pb content is 0 to 0.09%, and when it is contained, it is 0.09% or less. The preferable lower limit of the Pb content is 0.01%, and more preferably 0.02%. The preferable upper limit of the Pb content is 0.08%, and more preferably 0.07%.

Sn: 0.015% or less Tin (Sn) is an optional element and may not be contained. In other words, the Sn content may be 0%. When it is contained, that is, when the Sn content is more than 0%, Sn enhances the machinability of the steel material. If even a small amount of Sn is contained, the above effect can be obtained to a certain extent. However, if the Sn content exceeds 0.015%, the hot workability of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Sn content is 0 to 0.015%, and when it is contained, it is 0.015% or less. The preferable lower limit of the Sn content is 0.001%, and more preferably 0.005%. The preferable upper limit of the Sn content is 0.013%, and more preferably 0.010%.

Sb: 0.006% or less Antimony (Sb) is an optional element and may not be contained. In other words, the Sb content may be 0%. When it is contained, that is, when the Sb content is more than 0%, Sb enhances the machinability of the steel material. If even a small amount of Sb is contained, the above effect can be obtained to a certain extent. However, if the Sb content exceeds 0.006%, the hot workability of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Sb content is 0 to 0.006%, and when it is contained, it is 0.006% or less. The preferable lower limit of the Sb content is 0.001%, and more preferably 0.002%. The preferable upper limit of the Sb content is 0.005%.

[Regarding formulas (1) to (4)]
The chemical composition of the steel material of this embodiment further satisfies formulas (1) to (4), on the premise that the content of each element is within the range of this embodiment described above.
2.5≦(2Mn+5Cr+Mo)/Si≦7.5 (1)
Mn×Ni≦0.05 (2)
1.7≦Al/N≦2.4 (3)
22≦Mn/S≦65 (4)
Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (4).
Each formula will be explained below.

[Regarding formula (1)]
F1 is defined as (2Mn+5Cr+Mo)/Si. F1 is an index showing the ability of cementite to be formed in the surface layer of a steel material during a vacuum carburizing process in a manufacturing process for producing carburized steel parts using steel material as a raw material.

In the vacuum carburizing process, a carburizing step and a diffusion step are performed once each in a vacuum or under reduced pressure, or the carburizing step and the diffusion step are alternately repeated multiple times. In the carburizing step, a hydrocarbon-based gas is introduced to form an appropriate amount of cementite on the surface layer of the steel material. Then, in the diffusion step, the introduction of the hydrocarbon-based gas is stopped. Therefore, in the diffusion step, the cementite is decomposed, and the carbon concentration in the surface layer of the steel material increases due to the decomposition of the cementite. As a result, in the diffusion step of the vacuum carburizing process, the gradient of the carbon concentration in the austenite in the surface layer is larger than in the diffusion step of the gas carburizing process, and the amount of C penetrating into the inside of the steel material can be increased. As described above, in the diffusion step of the vacuum carburizing process, no hydrocarbon-based gas is introduced, and the cementite formed on the surface layer of the steel material in the previous carburizing step is used as a carbon (C) supply source to diffuse and penetrate C into the inside of the steel material. As a result, the vacuum carburizing process can form a hardened layer in a short time compared to the gas carburizing process. The vacuum carburizing process is further performed under vacuum or reduced pressure. Therefore, a grain boundary oxide layer is less likely to form on the surface of the steel material. Therefore, the content of elements that easily form oxides, such as Si, Cr, and Mn, can be increased in the chemical composition of the steel material.

In the chemical composition of the steel material of this embodiment, Mn, Cr, and Mo all have a high affinity with C, and are likely to form cementite and alloy carbides (hereinafter, cementite, etc.) on the surface layer of the steel material in the carburizing process of the vacuum carburizing treatment. On the other hand, Si has a low affinity with C, and is unlikely to form cementite, etc. on the surface layer of the steel material in the carburizing process of the vacuum carburizing treatment. Therefore, in the chemical composition of the steel material of this embodiment, the amount of cementite, etc. on the surface layer of the steel material in the carburizing process of the vacuum carburizing treatment can be adjusted to an appropriate range by appropriately adjusting the contents of Mn, Cr, Mo, and Si, assuming that the content of each element is within the range of this embodiment.

In the chemical composition of the steel material of this embodiment, assuming that the content of each element is within the range of this embodiment, if F1 is less than 2.5, the Si content is excessively high relative to the contents of Mn, Cr, and Mo. In this case, cementite and the like are unlikely to form in the surface layer of the steel material in the carburizing process of the vacuum carburizing treatment. Therefore, the gradient of the carbon concentration in the austenite becomes small in the diffusion process. As a result, the penetration and diffusion of C into the interior of the steel material is insufficient in the vacuum carburizing treatment. As a result, the hardness of the surface layer of the carburized steel part decreases, and the surface fatigue strength (pitting characteristics) decreases.

On the other hand, in the chemical composition of the steel material of this embodiment, assuming that the content of each element is within the range of this embodiment, when F1 exceeds 7.5, the content of Mn, Cr, and Mo is excessively high relative to the Si content. In this case, cementite, etc. is formed in an excessively large amount on the surface layer of the steel material in the carburizing process of the vacuum carburizing treatment. As a result, although the cementite, etc. is decomposed to a certain extent during the diffusion process, some of the coarse cementite, etc. remains without being decomposed. Therefore, coarse cementite, etc. may remain in the carburized steel part. In this case, the coarse cementite, etc. becomes the starting point of cracks, and the bending fatigue strength of the carburized steel part decreases.

If F1 is 2.5 to 7.5, the Si content relative to the Mn, Cr, and Mo contents is in the appropriate range. Therefore, an appropriate amount of C penetrates into the steel material during the carburizing process of the vacuum carburizing treatment. As a result, carburized steel parts manufactured using the steel material as a material have high bending fatigue strength and high surface fatigue strength, provided that the content of each element in the chemical composition is within the range of this embodiment and formulas (2), (3), and (4) are satisfied.

The preferred lower limit of F1 is 2.6, more preferably 2.8, more preferably 3.0, more preferably 3.5, and more preferably 4.0. The preferred upper limit of F1 is 7.4, more preferably 7.3, more preferably 7.2, more preferably 7.0, and more preferably 6.8. F1 is a value obtained by rounding the value obtained by calculation to one decimal place.

[Regarding formula (2)]
F2 is defined as Mn×Ni. F2 is an index related to the amount of retained austenite in the surface layer of a steel material. Mn and Ni are austenite stabilizing elements, and both of them excessively increase the amount of retained austenite in the surface layer of a steel material after carburizing. Therefore, if the content of these elements is excessively high, sufficient bending fatigue strength cannot be obtained in the carburized steel part.

In the chemical composition of the steel material of this embodiment, if F2 exceeds 0.05, the amount of retained austenite in the steel surface layer increases excessively after carburizing, assuming that the content of each element is within the range of this embodiment. In this case, sufficient bending fatigue strength is not obtained in the carburized steel part. If F2 is 0.05 or less, the Mn content and Ni content are kept within appropriate ranges. As a result, assuming that the content of each element in the chemical composition is within the range of this embodiment and satisfies formulas (1), (3), and (4), a carburized steel part manufactured using the steel material as a material will have high bending fatigue strength.

The preferred upper limit of F2 is 0.04, more preferably 0.03, more preferably 0.02, and even more preferably 0.01. There is no particular limit to the lower limit of F2. F2 may be 0. Note that F2 is a value obtained by rounding off the value obtained by calculation to two decimal places.

[Regarding formula (3)]
F3 is defined as F3=Al/N. F3 is an index related to Al inclusions and AlN precipitates that affect bending fatigue strength. Al inclusions become the starting points of cracks during bending fatigue of carburized steel parts. Furthermore, coarse AlN (precipitates) also become the starting points of cracks during bending fatigue of carburized steel parts. Therefore, it is preferable to suppress the formation of Al inclusions and coarse AlN (precipitates) as much as possible.

In the chemical composition of the steel material of this embodiment, assuming that the content of each element is within the range of this embodiment, if F3 is less than 1.7, the Al content is excessively high relative to the N content. In this case, there will be an excessive amount of Al inclusions in the steel material. As a result, the bending fatigue strength of the carburized steel part will decrease.

On the other hand, in the chemical composition of the steel material of this embodiment, assuming that the content of each element is within the range of this embodiment, if F3 exceeds 2.4, the N content is excessively higher than the Al content. In this case, the amount of coarse AlN (precipitates) in the steel material becomes excessively large. Therefore, in this case as well, the bending fatigue strength of the carburized steel part decreases.

In the chemical composition of the steel material of this embodiment, assuming that the content of each element is within the range of this embodiment, if F3 is 1.7 to 2.4, the formation of Al inclusions in the steel material can be sufficiently suppressed, and the formation of coarse AlN (precipitates) can also be sufficiently suppressed. Therefore, assuming that formulas (1), (2), and (4) are satisfied, the bending fatigue strength of the carburized steel part can be increased.

The preferred lower limit of F3 is 1.8, more preferably 1.9, and even more preferably 2.0. The preferred upper limit of F3 is 2.3, more preferably 2.2, and even more preferably 2.1. Note that F3 is a value obtained by rounding off the calculated value to one decimal place.

[Regarding formula (4)]
F4 is defined as Mn/S. F4 is an index related to MnS, which affects bending fatigue strength. MnS improves the machinability of steel materials. However, coarse MnS can be the starting point of cracks during bending fatigue of carburized steel parts. Therefore, if excessive coarse MnS is formed, the bending fatigue strength of the carburized steel parts will decrease.

In the chemical composition of the steel material of this embodiment, assuming that the content of each element is within the range of this embodiment, if F4 is less than 22, the S content is excessively high relative to the Mn content in the steel material. In this case, excessively large coarse MnS is formed. As a result, the bending fatigue strength of carburized steel parts manufactured using the steel material as a material decreases. Furthermore, excessive FeS is formed at the grain boundaries. As a result, the hot workability of the steel material decreases.

On the other hand, in the chemical composition of the steel material of this embodiment, assuming that the content of each element is within the range of this embodiment, if F4 exceeds 65, the Mn content is excessively high relative to the S content in the steel material. In this case, Mn segregates near the central axis of the steel material. Therefore, in this central segregation area, an excessive amount of coarse MnS is formed. As a result, the bending fatigue strength of the carburized steel part manufactured using the steel material as a material decreases.

In the chemical composition of the steel material of this embodiment, assuming that the content of each element is within the range of this embodiment, if F4 is 22 to 65, the relationship between the Mn content and the S content is appropriate. Therefore, the excessive generation of coarse MnS can be suppressed. As a result, sufficient bending fatigue strength is obtained in carburized steel parts manufactured using the steel material as a material.

The preferred lower limit of F4 is 24, more preferably 26, more preferably 28, more preferably 30, more preferably 32, and more preferably 34. The preferred upper limit of F4 is 63, more preferably 61, more preferably 59, more preferably 57, more preferably 55, and more preferably 53. Note that F4 is a value obtained by rounding off the calculated value to one decimal place.

[About the microstructure of steel materials]
The microstructure of the steel material of this embodiment is not particularly limited. The objective of the steel material of this embodiment is to obtain high bending fatigue strength and high surface fatigue strength in a carburized steel part manufactured using the steel material as a material. In the manufacturing process for manufacturing a carburized steel part using the steel material as a material, for example, a vacuum carburizing treatment is performed on the steel material as described below. In the vacuum carburizing treatment, the steel material is heated to the A _c3 transformation point temperature or higher, so that the microstructure of the steel material is reset. Therefore, the microstructure of the steel material that is the material for the carburized steel part is not particularly limited. The steel material of this embodiment has each element content in the chemical composition in the above-mentioned range, and further satisfies formulas (1) to (4). Therefore, when a carburized steel part is manufactured using the steel material of this embodiment as a material and a vacuum carburizing treatment is performed, the carburized steel part can obtain high bending fatigue strength and high surface fatigue strength (pitting characteristics).

[Use of the steel material according to this embodiment]
The steel material of this embodiment is suitable as a material for carburized steel parts manufactured by vacuum carburizing. In particular, it is suitable as a material for carburized steel parts that require bending fatigue strength and surface fatigue strength (pitting characteristics), such as gears used in mechanical products such as automobiles and construction vehicles. The steel material of this embodiment can also be used as a material for carburized steel parts manufactured by gas carburizing.

[Method of manufacturing steel material according to this embodiment]
An example of a method for manufacturing the steel material of this embodiment will be described. The method for manufacturing the steel material described below is one example for manufacturing the steel material of this embodiment. Therefore, the steel material having the above-mentioned configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the method for manufacturing the steel material of this embodiment.

An example of a method for manufacturing a steel material according to this embodiment includes a process for preparing a raw material (raw material preparation process) and a process for hot working the raw material to manufacture a steel material (hot working process). Each process will be described below.

[Material preparation process]
In the material preparation step, a material for the steel material of this embodiment is prepared. Specifically, molten steel is produced in which the content of each element in the chemical composition is within the range of this embodiment and satisfies formulas (1) to (4). The refining method is not particularly limited, and any known method may be used. For example, molten iron produced by a known method is refined in a converter (primary refining). The molten steel tapped from the converter is subjected to known secondary refining. In the secondary refining, alloy elements are added to adjust the composition, and molten steel is produced in which the content of each element is within the range of this embodiment and has a chemical composition that satisfies formulas (1) to (4).

The molten steel produced by the above-mentioned refining method is used to manufacture materials using known casting methods. For example, the molten steel is used to manufacture ingots using an ingot casting method. The molten steel may also be used to manufacture blooms using a continuous casting method. Materials (ingots or blooms) are manufactured using the above methods.

[Hot processing process]
In the hot working process, hot working is performed on the material (ingot or bloom) prepared in the material preparation process to produce the steel material (steel bar) of this embodiment. The hot working method may be hot forging or hot rolling. In the following description, the case where the hot working is hot rolling will be described. In this case, the hot working process includes, for example, a blooming process and a finish rolling process.

[Slabbing rolling process]
In the blooming process, the material is hot rolled to produce a billet. Specifically, in the blooming process, the material is hot rolled (blooming) by a blooming mill to produce a billet. If a continuous rolling mill is disposed downstream of the blooming mill, the billet after blooming may be further hot rolled by the continuous rolling mill to produce a smaller billet. The heating temperature in the blooming process may be within a known range. The heating temperature is, for example, 1000 to 1300°C.

[Finish rolling process]
In the finish rolling process, the billet produced in the blooming process is hot-rolled using a continuous rolling mill to produce a steel bar. A known heating temperature is sufficient for the finish rolling process. The heating temperature is, for example, 900 to 1250°C. The steel after hot rolling is cooled to room temperature. The cooling method is not particularly limited, but for example, it is natural cooling.

The steel material of this embodiment is manufactured by the above manufacturing method. Note that the above manufacturing method is one example of a manufacturing method for manufacturing the steel material of this embodiment. Therefore, the steel material of this embodiment may be manufactured by a method other than the above manufacturing method. In other words, as long as the content of each element in the chemical composition is within the range of this embodiment and the steel material satisfies formulas (1) to (4), the manufacturing method is not limited.

In the example of the manufacturing method described above, a hot working process is carried out after the material preparation process. However, in the manufacturing method of the steel material of this embodiment, it is not necessary to carry out the hot working process after carrying out the material preparation process. In other words, the steel material of this embodiment may be a cast material (ingot, bloom, or billet).

In addition, the steel material after the material preparation process or the steel material after the hot working process may be subjected to a well-known normalizing process and/or a well-known spheroidizing annealing process. In spheroidizing annealing, for example, the annealing temperature is set to 720 to 780°C, and the holding time at the annealing temperature is set to 3 to 8 hours. Furthermore, the cooling time from the annealing temperature to 600°C is set to 4 hours or more (8 hours or less). Then, the steel material is allowed to cool.

[Composition of carburized steel parts]
The carburized steel part of this embodiment is manufactured by subjecting the steel material of this embodiment described above to a vacuum carburizing process (vacuum carburizing process or vacuum carbonitriding process). The carburized steel part is a machine part, such as a gear, used in, for example, automobiles and construction vehicles.

The carburized steel part of this embodiment has a hardened layer and a core part inside the hardened layer. The hardened layer is a layer hardened by the penetration and diffusion of C by vacuum carburization. Specifically, when vacuum carburization is performed, the hardened layer corresponds to the carburized layer, and when vacuum carbonitriding is performed, the hardened layer corresponds to the carbonitrided layer. The core part is a part inside the hardened layer, and is an area that is not affected by the penetration and diffusion of C by vacuum carburization. It is a technical matter well known to those skilled in the art that the hardened layer and the core part can be distinguished by well-known microstructural observation.

[About the core]
The chemical composition of the core of the carburized steel part of this embodiment is the same as that of the steel material of this embodiment described above. Specifically, the chemical composition of the core of the carburized steel part of this embodiment contains, in mass%, C: 0.10-0.35%, Si: 0.60-1.50%, Mn: 0.20-1.30%, P: 0.030% or less, S: 0.050% or less, Ni: 0.01-0.20%, Cr: 0.65-1.50%, Mo: 0.01-0.60%, Al: 0.045% or less, and N: 0.0250% or less, with the balance being Fe and impurities, and satisfies formulas (1) to (4).
2.5≦(2Mn+5Cr+Mo)/Si≦7.5 (1)
Mn×Ni≦0.05 (2)
1.7≦Al/N≦2.4 (3)
22≦Mn/S≦65 (4)
Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (4).

The chemical composition of the core may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of V: 0.50% or less, Nb: 0.030% or less, Ti: 0.200% or less, Cu: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, and B: 0.0010% or less.

The chemical composition of the core may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ca: 0.0015% or less, Mg: 0.010% or less, and rare earth elements: 0.0100% or less.

The chemical composition of the core may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Te: 0.100% or less, Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.015% or less, and Sb: 0.006% or less.

[About the hardened layer]
The composition of the hardened layer is as follows:
(1) The carbon concentration in the region from the surface of the carburized steel component to a depth of 50 μm is 0.60% or more by mass.
(2) The microstructure at a depth of 20 μm from the surface of the carburized steel part consists of martensite or of martensite and retained austenite, and the volume fraction of the retained austenite is 0 to 40%.
Each component will be described below.

[C concentration in the surface layer]
The area from the surface of the carburized steel part to a depth of 50 μm (hereinafter referred to as the surface region) is included in the hardened layer. The C concentration in the surface region is 0.60% or more by mass. The C concentration in the hardened layer is higher than the C concentration in the core. If the C concentration in the surface region is 0.60% or more by mass, the hardness of the hardened layer is sufficiently high. Therefore, the carburized steel part can obtain sufficient surface fatigue strength and sufficient bending fatigue strength.

The preferred lower limit of the C concentration in the surface region is 0.65%, more preferably 0.70%, and even more preferably 0.75%. The upper limit of the C concentration in the surface region is not particularly limited. The preferred upper limit of the C concentration in the surface region is, for example, 1.30%, more preferably 1.20%, and even more preferably 1.10%.

[Method for measuring C concentration in surface layer]
The carbon concentration in the surface region can be measured by the following method. Cutting is performed from the surface of the carburized steel part to a depth of 50 μm, and chips from the surface region to a depth of 50 μm are collected. Chemical analysis is performed using the collected chips. Specifically, the well-known high-frequency combustion method (combustion-infrared absorption method) is performed on the collected chips to obtain the carbon concentration. Specifically, the above-mentioned chips are burned by high-frequency induction heating in an oxygen stream, and the generated carbon dioxide and carbon monoxide are detected to obtain the carbon concentration (mass %). The obtained carbon concentration (mass %) is defined as the carbon concentration (mass %) in the region (surface region) from the surface of the carburized steel part to a depth of 50 μm.

[Microstructure of the hardened layer]
The position 20 μm deep from the surface of the carburized steel part is included in the hardened layer. The microstructure at the position 20 μm deep from the surface of the carburized steel part is made of martensite or made of martensite and retained austenite, and the volume fraction of the retained austenite is 0 to 40%. In other words, when the microstructure is made of martensite and retained austenite, the volume fraction of the retained austenite is 40% or less. It is technically common knowledge that the microstructure in the hardened layer is mainly martensite.

If the volume fraction of the retained austenite at a depth of 20 μm from the surface of the carburized steel part exceeds 40%, the hardness of the hardened layer is too low. In this case, the bending fatigue strength of the carburized steel part is low. If the volume fraction of the retained austenite at a depth of 20 μm from the surface of the carburized steel part is 0-40%, the hardened layer has sufficient hardness. Therefore, the bending fatigue strength of the carburized steel part is increased. The preferred upper limit of the volume fraction of the retained austenite at a depth of 20 μm from the surface of the carburized steel part is 35%, more preferably 30%, and even more preferably 28%. The lower the volume fraction of the retained austenite, the better. However, it is difficult to reduce the retained austenite to 0%, and the manufacturing cost will be high. Therefore, when considering industrial production, the preferred lower limit of the retained austenite is more than 0%, and even more preferably 1%.

[Method for measuring volume fraction of retained austenite]
The volume fraction of retained austenite is measured by the following method. The surface of the carburized steel part is polished by electrolytic polishing to a depth of 20 μm, and the portion 20 μm deep from the surface is exposed. An X-ray diffraction device is used to irradiate an arbitrary position on the exposed surface with X-rays to measure the volume fraction (%) of retained austenite. The volume fraction of retained austenite is calculated from the ratio (integral intensity ratio) of the integrated intensity of the (211) bcc diffraction peak and the integrated intensity of the (220) fcc diffraction peak obtained by X-ray diffraction. Specifically, the volume fraction (%) of retained austenite can be calculated from the following formula, where Iα is the integrated intensity of (211) bcc (α phase) and Iγ is the integrated intensity of (220) fcc (γ phase).
Volume fraction of retained austenite = Iγ / (RIα + Iγ)
Here, R = 0.36746.

The carburized steel part having the above configuration has the content of each element in the chemical composition of the core within the above-mentioned range, and satisfies formulas (1) to (4). Furthermore, the C concentration in the region from the surface of the carburized steel part to a depth of 50 μm (surface region) is 0.60% or more, and the microstructure at a depth of 20 μm from the surface of the carburized steel part consists of martensite or martensite and retained austenite, with the volume fraction of retained austenite being 0 to 40%. Therefore, the carburized steel part of this embodiment has high bending fatigue strength and high surface fatigue strength.

[Manufacturing method of carburized steel parts]
An example of a manufacturing method for the carburized steel part of this embodiment will be described. The manufacturing method for the carburized steel part described below is one example for manufacturing the carburized steel part of this embodiment. Therefore, the carburized steel part having the above-mentioned configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a manufacturing method for the carburized steel part of this embodiment.

Carburized steel parts are processed through a hot or cold working process, a cutting process, and a heat treatment process. Either the hot working process or the cold working process is performed.

[Hot processing process]
When the hot working step is carried out, hot working is carried out on the steel material of this embodiment. The hot working is, for example, the well-known hot forging. The heating temperature in the hot working step is, for example, 1000 to 1300°C. The steel material after the hot working is allowed to cool (air-cooled).

[Cold working process]
When the cold working step is performed, the steel material of the present embodiment is subjected to a well-known spheroidizing annealing, and then cold working is performed. The conditions of the cold working are not particularly limited.

[Cutting process]
In the cutting process, the steel material after the hot working process is cut to produce an intermediate product of a predetermined shape. By performing cutting, it is possible to give the carburized steel part a precise shape that would be difficult to achieve by the hot working process or cold working process alone.

[Heat treatment process]
The intermediate product after the cutting process is subjected to a heat treatment. Here, "heat treatment" includes a well-known vacuum carburizing process, a well-known quenching process, and a well-known tempering process. In the vacuum carburizing process, it is a well-known technical matter for those skilled in the art to adjust the C concentration and microstructure of the hardened layer of the carburized steel part by appropriately adjusting well-known conditions. The well-known vacuum carburizing process, quenching process, and tempering process will be described below.

[Vacuum carburizing process]
Fig. 1 is a diagram showing an example of a heat pattern of the vacuum carburizing process S10 and the quenching process S20. The vacuum carburizing process S10 includes a heating step S0, a carburizing process S1, and a diffusion step S2. In the heat pattern of Fig. 1, the carburizing process S1 is followed by the diffusion step S2, and the carburizing process S1 and the diffusion step S2 are then repeatedly performed. In this way, in the vacuum carburizing process S10, the carburizing process S1 and the diffusion step S2 may be repeated multiple times, or the carburizing process S1 and the diffusion step S2 may be performed once each. The carburizing process S1 and the diffusion step S2 may be repeated three or more times.

In the heating step S0, the intermediate product loaded into the furnace is heated to the carburizing temperature Tc. The carburizing temperature Tc in the heating step S0 is, for example, 900 to 1100°C. In the heating step S0, the furnace is further evacuated or depressurized. For example, the furnace is depressurized to 1 kPa or less.

In the carburizing process S1, a hydrocarbon gas is introduced into the furnace in a vacuum or under reduced pressure, and the intermediate product is held at the carburizing temperature Tc for a specified time (holding time t1) to carry out the carburizing process. The gas introduced in the carburizing process S1 is not particularly limited as long as it is a hydrocarbon gas, but for example, acetylene or propane is used. The holding time t1 at the carburizing temperature Tc is not particularly limited, but is, for example, 5 to 120 minutes. By carrying out carburizing in a vacuum or under reduced pressure, the C concentration penetrating into the steel surface can be increased compared to gas carburizing processing.

In the diffusion step S2, the carburizing temperature Tc is maintained for a predetermined time (retention time t2) without introducing any hydrocarbon gas into the furnace. The pressure in the furnace in the diffusion step may be the same as that in the carburizing step S1, or may be reduced in pressure (e.g., 100 Pa or less) compared to the carburizing step S1 in order to remove any residual gas from the carburizing step S1. The retention time t2 at the carburizing temperature Tc is not particularly limited, but is, for example, 5 to 120 minutes.

In the vacuum carburizing process S10, in the carburizing process S1, C penetrates the surface layer of the steel material, forming cementite and the like in the surface layer. Then, in the diffusion process S2, the cementite and the like in the surface layer are decomposed and the C in the surface layer is diffused inward. By repeating the combination of the carburizing process S1 and the diffusion process S2 once or multiple times under vacuum or reduced pressure, a large amount of C can penetrate and diffuse into the steel material in a short time compared to gas carburizing.

[Quenching process]
The intermediate product after the vacuum carburizing process S10 is subjected to a quenching process S20. In the quenching process S20, the intermediate product after the vacuum carburizing process S10 is held at a quenching temperature equal to or higher than the _Ar3 point, and then the intermediate product is quenched. The holding time t3 at the quenching temperature Ts is not particularly limited, but is, for example, 15 to 60 minutes. The quenching temperature Ts is preferably lower than the carburizing temperature Tc. The cooling method in the quenching process is oil cooling or water cooling. Specifically, the intermediate product held at the quenching temperature is immersed in a cooling bath containing oil or water as a cooling medium to be quenched.

[Tempering process]
The intermediate product after the quenching process is subjected to a known tempering process. The tempering temperature is, for example, 100 to 200° C. The holding time at the tempering temperature is, for example, 60 to 150 minutes.

[Other steps]
The method for manufacturing the carburized steel part of this embodiment may further include a shot peening step and a finish grinding step, which are optional steps.

[Shot peening process]
The shot peening process is an optional process and does not have to be performed. If performed, the shot peening process is performed on the intermediate product after the heat treatment process. By performing the shot peening process, the retained austenite in the hardened layer of the carburized steel part undergoes processing-induced transformation to martensite. As a result, the volume fraction of the retained austenite in the hardened layer decreases. For example, the shot peening process preferably uses a cut wire or shot grains having a diameter of 1.0 mm or less, has an arc height of 0.3 mmA or more, and has a coverage of 300% or more.

[Finish grinding process]
The finish grinding process is an optional process and does not have to be performed. If performed, the finish grinding process is performed on the intermediate product after the heat treatment process or the shot peening process to adjust the surface properties.

The above manufacturing process allows the carburized steel part of this embodiment to be manufactured. The above manufacturing method is one example of a manufacturing method for manufacturing the carburized steel part of this embodiment. Therefore, the carburized steel part of this embodiment may be manufactured by a method other than the above manufacturing method. In other words, the manufacturing method of the carburized steel part is not particularly limited as long as the content of each element in the chemical composition of the core of the carburized steel part is within the range of this embodiment, formulas (1) to (4) are satisfied, the C concentration in the region from the surface of the carburized steel part to a depth of 50 μm is 0.60% or more, the microstructure at a depth of 20 μm from the surface of the carburized steel part consists of martensite or consists of martensite and retained austenite, and the volume fraction of the retained austenite is 0 to 40%.

The effects of the steel material and carburized steel parts of this embodiment will be explained in more detail below using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel material and carburized steel parts of this embodiment. Therefore, the steel material and carburized steel parts of this embodiment are not limited to this one example of conditions.

[Steel manufacturing]
Molten steel having the chemical composition shown in Table 1 was produced.

Ingots were produced by the ingot casting method using the molten steel in Table 1. The cross section perpendicular to the longitudinal direction of the ingot was a rectangle measuring 180 mm x 180 mm. The produced ingot was allowed to cool to room temperature.

The ingot was heated at 1200°C for 2 hours. The heated ingot was then hot worked (hot forging) to produce steel material (steel bar) with a diameter of 40 mm and a length of 1000 mm. The hot worked steel material was allowed to cool to room temperature. Normalizing was performed on the steel material after cooling. The normalizing temperature was 925°C, and the holding time at the treatment temperature was 90 minutes. After the holding time had elapsed, the steel material was allowed to cool. The cooling rate of the steel material when allowed to cool was 0.3 to 0.9°C/second. Through the above process, steel materials (steel bars) of each test number were produced.

The steel material with test number 46 had a chemical composition equivalent to SCr420 as specified in JIS G 4805 (2019). The steel material with test number 46 was designated as the "reference steel material."

[Evaluation test]
[Manufacture of carburized steel part test pieces]
The steel materials produced with the respective test numbers were used to prepare the following three types of carburized steel part test pieces for each test number.

(1) Small roller test piece Figure 2 shows a side view of the small roller test piece produced in this example. The numbers in Figure 2 indicate dimensions (unit: mm). "φ" in Figure 2 means diameter. The inverted triangle symbol in Figure 2 means the "finishing symbol" indicating the surface roughness described in Table 1 of JIS B 0601 (1982). The "G" attached to the finishing symbol means the abbreviation of the processing method indicating the grinding specified in JIS B 0122 (1978). The small roller test piece is a test piece for measuring surface fatigue strength. Multiple small roller test pieces were prepared for each test number.

(2) Rotating bending fatigue test specimen Figure 3 shows a side view of the rotating bending fatigue test specimen prepared in this example. The numbers in Figure 3 indicate dimensions (unit: mm). "φ" in Figure 3 means diameter. "R" in Figure 3 means radius of curvature. The rotating bending fatigue test specimen is a test specimen for measuring rotating bending fatigue strength.

(3) Test pieces for investigating the hardened layer Two test pieces for investigating the hardened layer were prepared for each test number. The test pieces for investigating the hardened layer were cylindrical test pieces with a diameter of 26 mm and a length of 100 mm.

Test pieces for each carburized steel part were prepared using the following method.

[Small roller test piece]
The steel material of each test number was machined to produce a rough test piece having the rough shape of the small roller test piece. The rough test piece was subjected to one of the following heat treatments, Pattern 1 to Pattern 3. The heat treatment patterns (1 to 3) performed for each test number are shown in the "Heat Treatment Pattern" column of Table 2. Note that "-" in the "Heat Treatment Pattern" column means that no heat treatment was performed.

(Pattern 1)
The following vacuum carburizing process was carried out. A carburizing process was carried out in which acetylene gas was introduced at a furnace pressure of 100 Pa or less. The temperature in the carburizing process was 930°C, and the holding time was 80 minutes. After the carburizing process, a diffusion process was carried out. In the diffusion process, the introduction of acetylene gas was stopped, and the furnace pressure was set to 10 Pa or less. The temperature in the diffusion process was 930°C, and the holding time was 20 minutes. After the diffusion process, a quenching process was carried out. In the quenching process, the temperature was 900°C, and the holding time was 30 minutes. After the holding time had elapsed, oil cooling was performed using oil at 60°C. After the quenching process, a tempering process was carried out. In the tempering process, the temperature was 180°C, and the holding time was 120 minutes.

(Pattern 2)
The following vacuum carburizing process was carried out. A carburizing process was carried out in which acetylene gas was introduced at a furnace pressure of 100 Pa or less. The temperature in the carburizing process was set to 950°C, and the holding time was set to 50 minutes. After the carburizing process, a diffusion process was carried out. In the diffusion process, the introduction of acetylene gas was stopped, and the furnace pressure was set to 10 Pa or less. The temperature in the diffusion process was set to 950°C, and the holding time was set to 15 minutes. After the diffusion process, a quenching process was carried out. In the quenching process, the temperature was set to 900°C, and the holding time was set to 30 minutes. After the holding time had elapsed, the specimen was oil-cooled using oil at 60°C. After the quenching process, a tempering process was carried out. In the tempering process, the temperature was set to 180°C, and the holding time was set to 120 minutes.

(Pattern 3)
The following vacuum carburizing process was carried out. A carburizing process was carried out in which acetylene gas was introduced at a furnace pressure of 100 Pa or less. The temperature in the carburizing process was 970°C, and the holding time was 15 minutes. After the carburizing process, a diffusion process was carried out. In the diffusion process, the introduction of acetylene gas was stopped, and the furnace pressure was set to 10 Pa or less. The temperature in the diffusion process was 970°C, and the holding time was 40 minutes. After the diffusion process, a quenching process was carried out. In the quenching process, the temperature was 900°C, and the holding time was 30 minutes. After the holding time had elapsed, oil cooling was performed using oil at 60°C. After the quenching process, a tempering process was carried out. In the tempering process, the temperature was 180°C, and the holding time was 120 minutes.

For test number 46 (reference steel), the following reference steel heat treatment pattern (well-known gas carburizing process) was performed on the rough test piece. Specifically, the test piece was held at 930°C for 180 minutes in an atmosphere with a carbon potential CP of 1.0% (carburizing process). Then, the carbon potential CP was set to 0.8% and the test piece was held at 930°C for 120 minutes (diffusion process). The temperature was then lowered to 870°C, and the test piece was held at 870°C for 30 minutes, after which it was oil-cooled in 60°C oil (quenching process). The rough test piece after oil cooling was subjected to a tempering process. The tempering temperature was 180°C, and the holding time at the tempering temperature was 120 minutes.

After the heat treatment, the cylindrical part in the center of the rough test piece was ground to produce a cylindrical part with a diameter of 26 mm as shown in Figure 2. At this time, the surface of the cylindrical part with a diameter of 26 mm was finished so that the arithmetic mean roughness Ra was 0.6 to 0.8 μm and the maximum height Rz was 2.0 to 4.0 μm, in accordance with JIS B 0601 (2001). The grinding depth was approximately 10 μm.

In addition, shot peening was performed on the rough test pieces of some of the test numbers (indicated as "Yes" in the "Shot Peening" column in Table 2). For the shot peening, a commercially available round cut wire with a diameter of 0.6 mm was used as the projectile. Furthermore, the arc height was set to 0.4 mmA, and the coverage was set to 300%. Shot peening was performed on the outer circumferential surface of the cylindrical part with a diameter of 26 mm shown in Figure 2. Shot peening was not performed on the rough test pieces of the remaining test numbers (indicated as "No" in the "Shot Peening" column in Table 2). Small roller test pieces of each test number were produced by the above manufacturing process.

[Rotating bending fatigue test piece]
The steel material of each test number was machined to produce a rough test piece for a rotating bending fatigue test piece. The rough test pieces were subjected to any one of the heat treatments of the above-mentioned patterns 1 to 3. The heat treatment pattern performed on the rough test pieces of each test number was as shown in Table 2. The rough test piece of test number 46 was subjected to the heat treatment of the above-mentioned reference steel heat treatment pattern. The rough test pieces after heat treatment of some test numbers were subjected to a shot peening treatment (shown as "Yes" in the "Shot Peening" column in Table 2). In the shot peening, commercially available high-hardness particles with a diameter of 0.5 mm and a hardness of 900 HV were used as the projectile. Furthermore, the arc height was set to 0.4 mmA, and the coverage was set to 300%. The rough test pieces of the remaining test numbers were not subjected to a shot peening treatment (shown as "No" in the "Shot Peening" column in Table 2).

After shot peening or heat treatment, the surface of the rough test piece was machined to produce a rotating bending fatigue test piece with the dimensions shown in Figure 3. Note that no machining was performed to adjust the surface properties of the notch formed in the longitudinal center of the rotating bending fatigue test piece. The rotating bending fatigue test piece was produced by the above manufacturing process.

[Test piece for hardened layer investigation]
The steel material of each test number was machined to prepare two cylindrical rough test pieces with a diameter of 26 mm and a length of 100 mm. The rough test pieces were subjected to the heat treatment of any one of the above-mentioned patterns 1 to 3. The heat treatment pattern performed on the rough test pieces of each test number was as shown in Table 2. Thereafter, the outer peripheral surface of the rough test pieces was ground to finish the outer peripheral surface, similar to the small roller test pieces. At this time, the outer peripheral surface of the rough test pieces with a diameter of 26 mm was finished so that the arithmetic mean roughness Ra according to JIS B 0601 (2001) was 0.6 to 0.8 μm and the maximum height Rz was 2.0 to 4.0 μm. The grinding depth was about 10 μm.

Furthermore, shot peening was performed on the rough test pieces of some of the test numbers (indicated as "Yes" in the "Shot Peening" column in Table 2). For shot peening, commercially available high-hardness particles with a diameter of 0.5 mm and a hardness of 900 HV were used as the projectile. Furthermore, the arc height was set to 0.4 mmA, and the coverage was set to 300%. Shot peening was not performed on the rough test pieces of the remaining test numbers (indicated as "No" in the "Shot Peening" column in Table 2). Test pieces for investigating the hardened layer were produced by the above manufacturing process.

[Manufacture of large roller test pieces for twin cylinder rolling fatigue testing]
A large roller test piece used in a twin-cylinder rolling fatigue test to measure surface fatigue strength was manufactured by the following method. A rough test piece of the large roller test piece having the shape shown in FIG. 4 was cut out from a cylindrical material with a diameter of 140 mm having a chemical composition equivalent to SUJ2 specified in JIS G 4805 (2008). The numbers in FIG. 4 indicate dimensions (unit: mm). The inverted triangle symbol in FIG. 4 indicates the "finishing symbol" indicating the surface roughness described in the explanatory table 1 of JIS B 0601 (1982). The "G" attached to the finishing symbol indicates the abbreviation of the processing method indicating grinding specified in JIS B 0122 (1978).

The cut rough test pieces were then quenched. The quenching temperature was 870°C, and the holding time at the quenching temperature was 90 minutes. After the holding time had elapsed, the pieces were quenched in oil at 60°C. The outer peripheral surface of the quenched rough test pieces was then finished by cutting. The outer peripheral surface was finished so that the arithmetic mean roughness Ra was 0.6 to 0.8 μm, and the maximum height Rz was 2.0 to 4.0 μm. Using the above manufacturing process, large roller test pieces were produced.

[C concentration measurement test of hardened layer of carburized steel part test piece]
Using the test pieces for hardened layer investigation of each test number, the C concentration in the region from the surface to a depth of 50 μm was measured by the following method. Turning was performed from the surface of the test pieces for hardened layer investigation to a depth of 50 μm, and cutting chips were collected. Chemical analysis was performed using the collected cutting chips. Specifically, the collected cutting chips were dissolved in acid to obtain a solution. The well-known high-frequency combustion method (combustion-infrared absorption method) was performed on the obtained solution to obtain the C concentration. Specifically, the above-mentioned solution was burned by high-frequency induction heating in an oxygen stream, and the generated carbon dioxide was detected to obtain the C concentration (mass %). The obtained C concentration (mass %) was defined as the C concentration (mass %) in the region from the surface of the carburized steel part to a depth of 50 μm (surface region).

[Measurement test of retained austenite volume fraction in hardened layer of carburized steel part test piece]
Using the test pieces for hardened layer investigation of each test number, the volume fraction of the retained austenite at a depth of 20 μm from the surface of the carburized steel part was obtained by the following method. The carburized steel part was polished by electrolytic polishing to a depth of 20 μm from the surface, and the depth of 20 μm from the surface was exposed. Using an X-ray diffraction device, an arbitrary position on the exposed surface was irradiated with X-rays to measure the volume fraction (%) of the retained austenite. The volume fraction of the retained austenite was calculated from the ratio (integral intensity ratio) of the integrated intensity of the diffraction peak of (211) bcc obtained by X-ray diffraction to the integrated intensity of the diffraction peak of (220) fcc. Specifically, the volume fraction (%) of the retained austenite was calculated from the following formula, where the integrated intensity of (211) bcc (α phase) is Iα and the integrated intensity of (220) fcc (γ phase) is Iγ.
Volume fraction of retained austenite = Iγ / (RIα + Iγ)
Here, R = 0.36746.

[Surface fatigue strength measurement test (two-cylinder rolling fatigue test)]
A two-cylinder rolling fatigue test was carried out using the small roller test piece and the large roller test piece to determine the surface fatigue strength as follows. A RP201 manufactured by Komatsu Engineering Co., Ltd. was used as the test machine. As shown in FIG. 5, the cylindrical portion of the small roller test piece 10 with a diameter of 26 mm and the central position of the outer peripheral surface of the large roller test piece 20 (the outer peripheral portion with a diameter of 130 mm) were rolled while being in contact with each other. The contact pressure was 1800 to 3500 MPa in Hertzian pressure. The rotation speed of the small roller test piece 10 was 1500 rpm. The peripheral speed of the small roller test piece 10 was 123 m/min, and the peripheral speed of the large roller test piece 10 was 172 m/min. During the test, lubricating oil was supplied to the contact portion between the small roller test piece and the large roller test piece. The lubricating oil was automatic transmission oil, the oil temperature was 100° C., and the oil amount was 1.0 L/min. The number of repeated cutoffs in the test was 2.0 x ¹⁰⁷ times, which indicates the fatigue limit of general steel. The maximum surface pressure (MPa) at which the small roller test piece reached 2.0 x ¹⁰⁷ times without pitting was taken as the fatigue limit of the small roller test piece. The occurrence of pitting was detected by a vibration meter attached to the testing machine. After vibration occurred, the rotation of both the small roller test piece and the large roller test piece was stopped, and the occurrence of pitting and the number of rotations were confirmed. In this embodiment, assuming application to gear parts, the fatigue limit of the small roller test piece of steel (reference steel) satisfying the SCr420 standard of test number 46 was taken as the reference value. When the fatigue limit was 1.20 times or more that of the reference steel, it was judged to have excellent surface fatigue strength ("○" in the "surface fatigue strength" column in Table 2). On the other hand, when the fatigue limit was less than 1.20 times that of the reference steel, it was judged to have low surface fatigue strength ("X" in the "surface fatigue strength" column in Table 2).

[Rotating bending strength measurement test (rotating bending fatigue test)]
Rotating bending fatigue tests were carried out using rotating bending fatigue test pieces in accordance with the "Rotating bending fatigue test method for metallic materials" specified in JIS Z 2274 (1978). The test was carried out at room temperature in an air atmosphere, and the rotation speed was 3000 rpm. The maximum stress at which the specimen did not break after ¹⁰⁷ cycles of repeated stress loading was taken as the bending fatigue strength (MPa). If the obtained bending fatigue strength was 1.20 times or more the bending fatigue strength of the reference steel material, test number 46, it was judged to have excellent bending fatigue strength ("○" in the "Bending fatigue strength" column in Table 2). On the other hand, if the obtained bending fatigue strength was less than 1.20 times the bending fatigue strength of the reference steel material, test number 46, it was judged to have low bending fatigue strength ("X" in the "Bending fatigue strength" column in Table 2).

[Evaluation Results]
The test results are shown in Table 2.

Referring to Table 2, the content of each element in the chemical composition of the steel materials of test numbers 1 to 26 was appropriate, and furthermore, F1 to F4 satisfied formulas (1) to (4). Therefore, in the carburized steel parts manufactured by vacuum carburizing, the C concentration in the region from the surface of the carburized steel part to a depth of 50 μm was 0.60% by mass or more, and the microstructure at a depth of 20 μm from the surface of the carburized steel part consisted of martensite and retained austenite, with the volume fraction of retained austenite being 40% or less. As a result, excellent bending fatigue strength and excellent surface fatigue strength were obtained.

On the other hand, test number 27 had a low C content. As a result, the bending fatigue strength was low.

In test number 28, the Si content was too high. As a result, the C content in the surface layer was too low, resulting in low surface fatigue strength.

In test number 29, the Si content was too low. As a result, the surface fatigue strength was low.

In test number 30, the Mn content was too high. As a result, the bending fatigue strength was low. In addition, because the Mn content was too high, the volume fraction of retained austenite exceeded 40% in the microstructure at a depth of 20 μm from the surface of the carburized steel part.

In test number 31, the Mn content was too low. As a result, the surface fatigue strength was low.

In test number 32, the P content was too high. As a result, the bending fatigue strength was low.

In test number 33, the S content was too high. As a result, the bending fatigue strength was low.

In test number 34, the Cr content was too high. As a result, the bending fatigue strength was low.

In test number 35, the Cr content was too low. As a result, the surface fatigue strength was low.

In test number 36, the Al content was too high. As a result, the bending fatigue strength was low.

In test number 37, the N content was too high. As a result, the bending fatigue strength was low.

In test number 38, the content of each element in the chemical composition of the steel was appropriate, but F1 exceeded the upper limit of formula (1). As a result, the bending fatigue strength was low.

In test number 39, the content of each element in the chemical composition of the steel was appropriate, but F1 was below the lower limit of formula (1). As a result, the surface fatigue strength was low.

In test number 40, the content of each element in the chemical composition of the steel was appropriate, but F3 exceeded the upper limit of formula (3). As a result, the bending fatigue strength was low.

In test number 41, the content of each element in the chemical composition of the steel was appropriate, but F3 was below the lower limit of formula (3). As a result, the bending fatigue strength was low.

In test number 42, the content of each element in the chemical composition of the steel was appropriate, but F4 exceeded the upper limit of formula (4). As a result, the bending fatigue strength was low.

In test number 43, the content of each element in the chemical composition of the steel was appropriate, but F4 was below the lower limit of formula (4). As a result, cracks occurred during hot forging. Therefore, the above evaluation tests were not performed on test number 43.

In test number 44, the Mo content was too low. As a result, the surface fatigue strength was too low.

In test number 45, the content of each element in the chemical composition of the steel was appropriate, but F2 exceeded the upper limit of formula (2). As a result, the volume fraction of retained austenite in the surface layer of the carburized steel part exceeded 40%, and the bending fatigue strength was low.

The above describes an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be modified as appropriate within the scope of the spirit of the present invention.

Claims (8)

Chemical composition in mass percent:
C: 0.10-0.35%,
Si: 0.60-1.50%,
Mn: 0.20-1.30%,
P: 0.030% or less,
S: 0.050% or less,
Ni: 0.01 to 0.20%,
Cr: 0.65-1.50%,
Mo: 0.01-0.60%,
Al: 0.045% or less ,
N: 0.0250% or less,
V: 0-0.50%,
Nb: 0 to 0.030%,
Ti: 0-0.200%,
Cu: 0 to 0.50%,
W: 0-0.50%,
Co: 0 to 0.50%,
B: 0 to 0.0050%,
Ca: 0-0.0015%,
Mg: 0 to 0.010%,
Rare earth elements: 0 to 0.0100%,
Te: 0 to 0.100%,
Bi: 0-0.500%,
Pb: 0 to 0.09%,
Sn: 0 to 0.015%, and
Sb: 0 to 0.006%,
The balance is Fe and impurities, and satisfies formulas (1) to (4).
Steel.
2.5≦(2Mn+5Cr+Mo)/Si≦7.5 (1)
Mn×Ni≦ 0.03 (2)
1.9 ≦Al/N≦ 2.1 (3)
24 ≦Mn/S≦65 (4)
Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (4). The steel material according to claim 1,
The chemical composition is, in place of a part of Fe,
V: 0.01~ 0.50 %,
Nb: 0.001-0.030 %,
Ti: 0.001-0.200 %,
Cu: 0.01-0.50 %,
W: 0.01~ 0.50 %,
Co: 0.01 to 0.50 %, and
B: 0.0001 to 0.0010 %,
Contains one or more selected from the group consisting of
Steel. The steel material according to claim 1 or 2,
The chemical composition is, in place of a part of Fe,
Ca: 0.0001-0.0015 %,
Mg: 0.001 to 0.010 %, and
Rare earth elements: 0.0001-0.0100 %,
Contains one or more selected from the group consisting of
Steel. The steel material according to any one of claims 1 to 3,
The chemical composition is, in place of a part of Fe,
Te: 0.001~ 0.100 %,
Bi: 0.001~ 0.500 %,
Pb: 0.01-0.09 %,
Sn: 0.001 to 0.015 %, and
Sb: 0.001-0.006 %,
Contains one or more selected from the group consisting of
Steel. A carburized steel part,
A hardened layer;
A core portion is provided inside the hardened layer,
The chemical composition of the core is, in mass%,
C: 0.10-0.35%,
Si: 0.60 to 1.50%,
Mn: 0.20-1.30%,
P: 0.030% or less,
S: 0.050% or less,
Ni: 0.01 to 0.20%,
Cr: 0.65-1.50%,
Mo: 0.01-0.60%,
Al: 0.045% or less ,
N: 0.0250% or less,
V: 0-0.50%,
Nb: 0 to 0.030%,
Ti: 0-0.200%,
Cu: 0 to 0.50%,
W: 0-0.50%,
Co: 0 to 0.50%,
B: 0 to 0.0050%,
Ca: 0-0.0015%,
Mg: 0 to 0.010%,
Rare earth elements: 0 to 0.0100%,
Te: 0 to 0.100%,
Bi: 0-0.500%,
Pb: 0 to 0.09%,
Sn: 0 to 0.015%, and
Sb: 0 to 0.006%,
and the balance being Fe and impurities, and satisfying formulas (1) to (4),
The carbon concentration in a region from the surface of the carburized steel part to a depth of 50 μm is 0.60% or more by mass%,
The microstructure at a depth of 20 μm from the surface of the carburized steel part is made of martensite or made of martensite and retained austenite, and the volume fraction of the retained austenite is 0 to 40%.
Carburized steel parts.
2.5≦(2Mn+5Cr+Mo)/Si≦7.5 (1)
Mn×Ni≦ 0.03 (2)
1.9 ≦Al/N≦ 2.1 (3)
24 ≦Mn/S≦65 (4)
Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (4). 6. A carburized steel component according to claim 5,
The chemical composition of the core portion is, in place of a part of Fe,
V: 0.01~ 0.50 %,
Nb: 0.001-0.030 %,
Ti: 0.001-0.200 %,
Cu: 0.01-0.50 %,
W: 0.01~ 0.50 %,
Co: 0.01 to 0.50 %, and
B: 0.0001 to 0.0010 %,
Contains one or more selected from the group consisting of
Carburized steel parts. A carburized steel part according to claim 5 or claim 6,
The chemical composition of the core portion is, in place of a part of Fe,
Ca: 0.0001-0.0015 %,
Mg: 0.001 to 0.010 %, and
Rare earth elements: 0.0001-0.0100 %,
Contains one or more selected from the group consisting of
Carburized steel parts. A carburized steel part according to any one of claims 5 to 7,
The chemical composition of the core portion is, in place of a part of Fe,
Te: 0.001~ 0.100 %,
Bi: 0.001~ 0.500 %,
Pb: 0.01-0.09 %,
Sn: 0.001 to 0.015 %, and
Sb: 0.001-0.006 %,
Contains one or more selected from the group consisting of
Carburized steel parts.

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