JP2024034953A - Steel materials and steel parts - Google Patents

Abstract

The present invention provides a steel material and a steel part having excellent surface fatigue strength and bending fatigue strength. The steel material has a chemical composition, in mass%, of C: 0.28-0.45%, Si: 0.05-0.30%, Mn: 0.75-1.50%, P: 0.030% or less, S: 0.100% or less, Cr: 1.30-1.90%, V: 0.02-0.40%, Al: 0.070% or less, and N: 0.0250% or less, in which Si, Mn, Cr, and V satisfy the following formula (1), Al and N satisfy the following formula (2), and the balance is Fe and impurities. 25.0≦(Mn+3Cr+2V)/Si ... formula (1) 1.20≦Al/N≦3.00 ... formula (2) However, each element symbol in the above formula (1) and formula (2) indicates the content of the corresponding element in mass %. [Selected figure] None

Description

The present disclosure relates to steel materials and steel parts.

Steel parts used in automobiles and various industrial machines, such as power transmission parts such as transmission gears, CVT (Continuously Variable Transmission) pulleys, and bearings, have high surface fatigue strength, bending fatigue strength, and wear resistance. required. Therefore, machine structural alloy steels such as JIS standard SCr420, SCM420, and SNCM420 are used as raw materials for these parts, and after being processed into a predetermined shape, a hardened layer is applied to the surface layer of the parts through carburizing and quenching, which reduces fatigue. Efforts have been made to improve the strength.

In recent years, in order to reduce CO ₂ emissions, the automobile industry has been promoting electrification of power instead of internal combustion engines. Therefore, the characteristics required of steel parts such as gears, especially power transmission parts, may change dramatically in the future. For example, in electric vehicles (EVs), there are many examples in which a reduction gear is installed directly below a motor that is a power source to ensure torque. The miniaturization of these units leads to lighter vehicle bodies and greater design freedom, so it is thought that demand will continue to grow in the future. However, for example, considering both the need to downsize the motor and increase its rotation speed to ensure output, and the downsizing of the gears, EV reduction gears are more likely to have close contact between gears than gasoline car transmissions. There is a concern about frictional heat generation on the tooth surface due to contact. Therefore, it is suggested that securing fatigue strength under high-temperature environments will become an issue for steel parts such as gears in the future.

Carburizing and quenching is a method of heating steel parts to the austenite region to increase the C (carbon) concentration on the surface layer of the part and then rapidly cooling it to generate hard martensite on the surface layer of the part, resulting in high surface fatigue. Provides strength. On the other hand, carburizing and quenching is a heat treatment that hardens down to the core of the component, and the distortion after the treatment (heat treatment distortion) tends to increase, which increases the grinding cost in the post-process. In addition, when a part hardened with a martensitic structure with a high C concentration is used in a high-temperature environment, cementite precipitates and the C concentration in the martensite structure decreases, resulting in a decrease in the hardness of the part. Therefore, with the emergence of the above-mentioned problems regarding frictional heat, it may become difficult to ensure fatigue strength, especially surface fatigue strength, in carburized parts.

Against this background, in recent years, nitriding and induction hardening, which are surface hardening heat treatments that produce smaller heat treatment distortions than carburizing and quenching, have been attracting attention.

Nitriding is a method in which steel parts are heated to a ferrite temperature range below the A1 point, and nitrogen compounds are precipitated by increasing the N (nitrogen) concentration in the part surface layer to a high concentration. Hardness can be increased. Since nitriding does not involve phase transformation, heat treatment distortion can be reduced. Furthermore, since nitriding is a process that utilizes the precipitation of nitrogen compounds in a temperature range of about 450 to 600°C, the hardened layer has higher thermal stability than the hardened layer formed by carburizing and quenching. The hardened layer produced by nitriding is composed of a nitrogen diffusion layer and a nitrogen compound layer with a thickness of several μm to several tens of μm, which is generated on the part surface side relative to the diffusion layer. The diffusion layer is a layer hardened by the solid solution strengthening mechanism of nitrogen and carbon that invades, and the particle dispersion strengthening mechanism of nitride. The nitride compound layer is mainly composed of iron nitrides of Fe ₂ N, Fe ₃ N (ε phase), and Fe ₄ N (γ' phase), and has a lower hardness than the parent phase (non-nitrided layer). Extremely high.
However, since nitriding is heat treated at a relatively low temperature, the depth of the hardened layer is small and the surface fatigue strength is inferior to that of carburizing and quenching. For this reason, it is difficult to apply nitriding to parts that require high fatigue strength. Further, the surface fatigue strength of nitrided parts tends to decrease due to the presence of a brittle compound layer and voids formed near the surface of the compound layer.

Induction hardening is a heat treatment in which steel parts are hardened by rapidly heating them to the austenite region in a short period of time, and the hardening distortion is smaller than that in carburizing and hardening. However, unlike carburizing and quenching, induction hardening is not a heat treatment method that increases the C concentration in the surface layer of parts, and therefore is not suitable for parts that require high surface fatigue strength.

From the above, it has been found that simply applying nitriding or induction hardening to steel parts used in automobiles and various industrial machines will not increase the surface fatigue strength enough to make the steel parts lighter, smaller, and able to withstand high stress loads. No improvement can be made. Therefore, recently, a composite heat treatment combining nitriding and induction hardening has been attempted as a method for increasing fatigue strength, especially surface fatigue strength, which is a drawback of nitriding and induction hardening.

Patent Document 1 discloses improving the properties of nitriding steel used for mechanical structural parts such as gears and rolling parts used in automobiles, construction machines, industrial machines, etc.

Patent Document 2 proposes a method for manufacturing mechanical structural parts with excellent nitrocarburizing properties and machinability.

Japanese Patent Application Publication No. 11-106865 Japanese Unexamined Patent Publication No. 59-190321

In the steel for nitriding disclosed in Patent Document 1, since the contents of Mn, Cr, and V are low relative to the amount of Si, it is difficult to stably increase the amount of N penetrating in the surface layer during nitriding. Therefore, it is difficult to maintain a stable amount of residual γ in the surface layer after induction hardening, and the deformation-induced transformation of residual γ due to surface hardening cannot be stably utilized, making it impossible to obtain high bending fatigue strength. Sometimes.

In the steel material disclosed in Patent Document 2, since the content ratio of N to Al is low, Al-based inclusions are likely to be formed, and there is room for improvement in improving the bending fatigue strength.

The present disclosure has been made to solve the above problems, and an object of the present disclosure is to provide steel materials and steel parts that have excellent surface fatigue strength and bending fatigue strength.

In order to solve the above-mentioned problems, the inventors of the present disclosure have independently changed various alloy components in steel for nitriding and induction hardening, and each element has been found to increase the hardness, N concentration, and The effects on fatigue strength and bending fatigue strength were investigated, and the following findings (a) to (i) were obtained.

(a) As mentioned above, there is a concern that fatigue strength of power transmission parts may decrease in high-temperature environments. In particular, in sliding parts such as gears used in automobiles, the surface of the part may rise to around 300°C due to frictional heat. For example, in order to ensure the cruising range and output of EVs, sliding parts are required to be smaller and rotate at higher speeds, so there are concerns about frictional heat generation of over 300 degrees Celsius on the surfaces of the parts. Therefore, in order to provide gear parts suitable for gasoline vehicles and EVs, it is necessary to suppress softening of the part surface layer due to frictional heat generation and maintain hardness at high temperatures (high temperature hardness). Compared to parts that are made of carbon steel and made into a martensitic structure by quenching, parts that are made of carbon steel and made into a martensitic structure with a high N concentration by nitriding and quenching to form a solid solution of N in the surface layer. Fatigue strength is often high.

(b) The major reason for this is the high resistance to temper softening that occurs in the martensitic structure with a high N concentration. Generally, the surface layer of the sliding part of a component after surface fatigue strength evaluation is softened due to frictional heat generation. However, compared to a martensitic structure whose surface layer has a high C concentration, a martensitic structure whose surface layer has a high N concentration ensures hardness at temperatures exceeding 300° C. due to the precipitation of iron nitrides.

(c) N is an element that more easily stabilizes austenite than C, and residual γ tends to exist in a martensitic structure with a high N concentration. On the other hand, by subjecting this structure to a surface hardening treatment such as shot peening and causing the residual γ to undergo deformation-induced transformation, the compressive residual stress in the surface layer can be effectively increased, improving the surface fatigue strength and bending fatigue strength of the part. .

(d) In addition, in the surface layer of parts made of carbon steel that have been nitrided to form a solid solution of N in the surface layer and then quenched to form a martensitic structure, precipitation of alloy nitrides such as CrN and VN occurs at temperatures exceeding 300°C. hardness is often high. Therefore, increasing the Cr content and V content of the material is effective in improving the softening resistance, that is, the surface fatigue strength when frictional heat generation becomes larger.

(e) Mn, Cr, and V are all nitride-forming elements, and have the function of increasing the N concentration in the surface layer by lowering the N activity of the steel during nitriding treatment. On the other hand, Si has the function of lowering the N concentration in the surface layer by increasing the N activity of the steel during nitriding treatment. That is, by balancing these elements, it is possible to stably increase the N concentration in the surface layer during nitriding, and it is possible to stably increase the N concentration in the martensitic structure after induction hardening in the subsequent process. That is, since the amount of residual γ can be stably ensured, deformation-induced transformation by surface hardening treatment such as shot peening can be stably utilized.

(f) However, even if the content of each element in the chemical composition is within the above range, if Al inclusions are excessively present in the steel material, the Al inclusions can become the starting point of cracks. In other words, if too many Al inclusions remain in the steel material, the bending fatigue strength of the component may decrease. Al is an element that is more likely to form nitrides (strong nitride-forming element), so if the Al content is excessively high relative to the amount of N in the steel material, Al inclusions are likely to exist, and the parts Bending fatigue strength tends to decrease. On the other hand, if the N content is excessively large with respect to the Al content in the steel material, coarse nitrides, typified by AlN, are generated. Coarse nitrides reduce the bending fatigue strength of steel parts. Therefore, it is necessary to determine the appropriate content of Al and N.

(g) Although the shear stress caused by Hertzian contact between parts varies depending on their shape and load, it often reaches its maximum at a depth of approximately 0.1 mm from the part surface, and near this depth position, Most vulnerable to pitching occurrence. Therefore, it is important to ensure hardness at a depth of 0.10 mm from the component surface.

(h) Furthermore, in order to develop softening resistance at higher temperatures due to the precipitation of nitrides mentioned above, it is necessary to ensure a certain level of N concentration in the region from the part surface to a depth of 0.10 mm. .

(i) In addition, in a region where the amount of N intrusion is small, if the C concentration of the base material (component material) is low, sufficient hardness cannot be obtained after induction hardening, and high surface fatigue strength cannot be obtained. Therefore, in order to further increase the surface fatigue strength of a nitrided induction hardened part, it is preferable to utilize the formation of a martensitic structure by C in the base material to ensure hardness in areas where the amount of N penetration is low.

The present disclosure was completed based on the above findings, and the gist thereof is as follows.

<1>
In mass%,
C: 0.28-0.45%,
Si: 0.05-0.30%,
Mn: 0.75-1.50%,
P: 0.030% or less,
S: 0.100% or less,
Cr: 1.30-1.90%,
V: 0.02-0.40%,
Al: 0.070% or less, and N: 0.0250% or less,
A steel material having a chemical composition in which Si, Mn, Cr, and V satisfy the following formula (1), Al and N satisfy the following formula (2), and the balance is Fe and impurities.
25.0≦(Mn+3Cr+2V)/Si... Formula (1)
1.20≦Al/N≦3.00... Formula (2)
However, each element symbol in the above formula (1) and the above formula (2) is the content in mass % of the element.
<2>
In mass%,
C: 0.28-0.45%,
Si: 0.05-0.30%,
Mn: 0.75-1.50%,
P: 0.030% or less,
S: 0.100% or less,
Cr: 1.30-1.90%,
V: 0.02-0.40%,
Al: 0.070% or less, and N: 0.0250% or less,
Si, Mn, Cr, and V satisfy the following formula (1), and Al and N satisfy the following formula (2),
A steel material having a chemical composition further containing one or more selected from the group consisting of Group A to Group E below, with the balance being Fe and impurities.
[Group A] Bi: 0.100% or less,
Pb: 0.09% or less,
Sn: 0.100% or less,
One or more selected from the group consisting of Sb: 0.0100% or less, and Te: 0.100% or less [Group B] Mo: 0.30% or less,
One or more selected from the group consisting of W: 0.50% or less, and B: 0.0100% or less [Group C] Cu: 0.50% or less,
One or more selected from the group consisting of Ni: 0.50% or less, and Co: 0.100% or less [Group D] Ti: 0.100% or less, and Nb: 0.030% or less One or two selected from the group [Group E] Ca: 0.0100% or less,
One or more selected from the group consisting of Mg: 0.0100% or less, and REM: 0.020% or less 25.0≦(Mn+3Cr+2V)/Si...Formula (1)
1.20≦Al/N≦3.00... Formula (2)
However, each element symbol in the above formula (1) and the above formula (2) is the content in mass % of the element.
<3>
The steel material according to <2>, wherein the chemical composition contains the A group.
<4>
The steel material according to <2>, wherein the chemical composition contains the B group.
<5>
The steel material according to <2>, wherein the chemical composition contains the C group.
<6>
The steel material according to <2>, wherein the chemical composition contains the D group.
<7>
The steel material according to <2>, wherein the chemical composition contains the E group.
<8>
The core is mass%,
C: 0.28-0.45%,
Si: 0.05-0.30%,
Mn: 0.75-1.50%,
P: 0.030% or less,
S: 0.100% or less,
Cr: 1.30-1.90%,
V: 0.02-0.40%,
Al: 0.070% or less, and N: 0.0250% or less,
It has a chemical composition in which Si, Mn, Cr, and V satisfy the following formula (1), and Al and N satisfy the following formula (2), and the remainder is Fe and impurities,
The thickness of the nitrogen compound layer containing at least Fe and N formed on the surface layer is 5 μm or less,
Vickers hardness at a depth of 0.10mm from the surface is 700HV or more,
A steel part having an average N concentration of 2.00 atomic % or more in a region from the surface to a depth of 0.10 mm.
25.0≦(Mn+3Cr+2V)/Si... Formula (1)
1.20≦Al/N≦3.00... Formula (2)
However, each element symbol in the above formula (1) and the above formula (2) is the content in mass % of the element.
<9>
The core is mass%,
C: 0.28-0.45%,
Si: 0.05-0.30%,
Mn: 0.75-1.50%,
P: 0.030% or less,
S: 0.100% or less,
Cr: 1.30-1.90%,
V: 0.02-0.40%,
Al: 0.070% or less, and N: 0.0250% or less,
Si, Mn, Cr, and V satisfy the following formula (1), and Al and N satisfy the following formula (2),
Furthermore, it has a chemical composition containing one or more selected from the group consisting of the following Group A to the following Group E, with the remainder being Fe and impurities,
The thickness of the nitrogen compound layer containing at least Fe and N formed on the surface layer is 5 μm or less,
Vickers hardness at a depth of 0.10mm from the surface is 700HV or more,
A steel component having an average N concentration of 2.00 atomic % or more in a region from the surface to a depth of 0.10 mm.
[Group A] Bi: 0.100% or less,
Pb: 0.09% or less,
Sn: 0.100% or less,
One or more selected from the group consisting of Sb: 0.0100% or less, and Te: 0.100% or less [Group B] Mo: 0.30% or less,
One or more selected from the group consisting of W: 0.50% or less, and B: 0.0100% or less [Group C] Cu: 0.50% or less,
One or more selected from the group consisting of Ni: 0.50% or less, and Co: 0.100% or less [Group D] Ti: 0.100% or less, and Nb: 0.030% or less One or two selected from the group [Group E] Ca: 0.0100% or less,
One or more types selected from the group consisting of Mg: 0.0100% or less, and REM: 0.020% or less 25.0≦(Mn+3Cr+2V)/Si...Formula (1)
1.20≦Al/N≦3.00... Formula (2)
However, each element symbol in the above formula (1) and the above formula (2) is the content in mass % of the element.
<10>
The steel component according to <9>, wherein the chemical composition of the core contains the A group.
<11>
The steel component according to <9>, wherein the chemical composition of the core contains the B group.
<12>
The steel component according to <9>, wherein the chemical composition of the core contains the C group.
<13>
The steel component according to <9>, wherein the chemical composition of the core contains the D group.
<14>
The steel component according to <9>, wherein the chemical composition of the core contains the E group.

According to the present disclosure, it is possible to provide steel materials and steel parts that have excellent surface fatigue strength and bending fatigue strength. Therefore, the steel materials and steel parts of the present disclosure are suitable as materials for, for example, automobiles and industrial machines, particularly gears of machines powered by electric motors.

It is a diagram showing an example of a small roller for a roller pitching test. FIG. 3 is a diagram showing an example of a test piece for the Ono rotary bending fatigue test. It is a figure which shows an example of the large roller for a roller pitching test. It is a schematic diagram of a roller pitching test.

Hereinafter, steel materials and steel parts according to an embodiment of the present disclosure will be described.

In this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower limit and upper limit. However, a numerical range in which "more than" or "less than" is attached to the numerical value written before and after "~" means a range that does not include these numerical values as the lower limit or upper limit.
The content of an element in a chemical composition may be expressed by adding "amount" to the element symbol (for example, the amount of C, the amount of Si, etc.).
Regarding the content of elements in the chemical composition, "%" means "% by mass".
When the content of an element in a chemical composition is described as "0~", it means that the element does not need to be included.
Furthermore, in the numerical ranges described in stages in this specification, the upper limit of one numerical range may be replaced with the upper limit of another numerical range described in stages, and You may substitute the values shown in the example. Further, the lower limit value of a certain stepwise numerical range may be replaced with the lower limit value of another stepwise described numerical range, or may be replaced with the value shown in the examples.
Furthermore, the term "process" is used not only to refer to an independent process but also to include a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

Although the use of the steel material according to the present disclosure is not particularly limited, it is preferably used for nitriding induction hardening. Hereinafter, as representative examples (one embodiment) of steel materials and steel parts according to the present disclosure, steel materials used for nitriding induction hardening (hereinafter sometimes referred to as "steel for nitriding induction hardening" or simply "steel materials") and nitriding Steel parts subjected to induction hardening (hereinafter sometimes referred to as "nitriding induction hardened parts" or simply "components") will be explained.
Note that the steel component according to the present disclosure is, for example, a component that has been subjected to nitriding induction hardening treatment (nitriding treatment and induction hardening treatment) on the steel material of the present disclosure, and has a core portion that is a central region in the depth direction of the component. (Hereinafter, it may be simply referred to as a “core portion”), and a hardened layer located closer to the surface of the component than the core portion.
Here, the core portion refers to a portion to which nitrogen has not penetrated due to the nitriding treatment. In other words, the core is a region in which there is no change in chemical composition, or the change is negligibly small, despite undergoing nitriding and induction hardening, and which is the base material of the part for nitriding and induction hardening. It is a part that has a chemical composition (sometimes simply referred to as "composition") equivalent to that of steel. Note that the composition of the core can also be said to be, for example, the composition at a depth of 2.0 mm or more from the surface of the component.
The hardened layer refers to a portion into which nitrogen has penetrated due to nitriding treatment and a portion where the structure has been transformed to martensite due to induction hardening treatment. That is, the hardened layer is a region in which the chemical composition or metal structure has changed due to the effects of nitriding treatment and induction hardening treatment. For example, it refers to a region up to a depth of approximately 1.0 mm from the surface of the component.

First, the steel material of the present disclosure, which is suitable for use in nitriding induction hardening, will be described.

[Chemical composition]
The chemical composition (component elements) of the steel material of the present disclosure will be explained. Usually, the core of a nitrided induction hardened part also has the same composition as the steel, so unless otherwise specified, the composition of the steel and the core of the part are the same. Note that "%" in the content of each component element in steel means "% by mass" unless otherwise specified.

[C: 0.28-0.45%]
C (carbon) is an element necessary to ensure hardening ability of the surface layer and core strength in induction hardening. If the C content is less than 0.28%, the above effects are insufficient. Moreover, when the content of C exceeds 0.45%, the hardness of steel materials such as steel bars and wire rods used as parts materials becomes too high, resulting in a significant decrease in machinability. Therefore, the C content of the steel material is set to 0.28 to 0.45%.
Preferably, the C content may be 0.30% or more, or 0.32% or more. Similarly, the C content may preferably be 0.43% or less, 0.41% or less.

[Si: 0.05-0.30%]
Si has the effect of improving hardenability. Furthermore, Si has the effect of suppressing agglomeration and coarsening of carbides and nitrides during tempering at about 400° C. and increasing resistance to temper softening. In order to obtain these effects, Si is contained in an amount of 0.05% or more. On the other hand, if the Si content is excessively high, the hardness of steel materials such as steel bars and wire rods used as component materials becomes too high, resulting in a significant decrease in machinability. Therefore, the Si content of the steel material is set to 0.05 to 0.30%.
Preferably, the Si content may be 0.10% or more, or 0.15% or more. Similarly, the Si content may preferably be 0.25% or less, or 0.20% or less.

[Mn: 0.75-1.50%]
Since Mn has the effect of increasing hardenability, it is an effective element for increasing surface fatigue strength. Further, during tempering at a temperature exceeding 300° C., it is an element that combines with N in a martensitic structure with a high N concentration to precipitate fine Mn ₃ N ₂ to develop softening resistance. However, if the content is less than 0.75%, the above effects are insufficient. On the other hand, if the Mn content exceeds 1.50%, not only will the effect of increasing surface fatigue strength become saturated, but the hardness of the steel materials such as steel bars and wire rods used as part materials will become too high, making it difficult to cut the parts. performance is greatly reduced. Therefore, the Mn content of the steel material is set to 0.75% to 1.50%.
Preferably, the Mn content may be 0.80% or more, or 0.85% or more. Similarly, the Mn content may preferably be 1.45% or less, or 1.40% or less.

[P: 0.030% or less]
P is an impurity and is an element that segregates at grain boundaries and makes parts brittle. Excessive inclusion may reduce surface fatigue strength, so the P content is set to 0.030% or less. It is preferable that the P content is as low as possible. A preferable upper limit of the P content is 0.025%, 0.020%, 0.015%, or 0.010%. Although the content of P may desirably be 0%, reducing P too much leads to an increase in cost for removing P. Therefore, considering the economic efficiency of refining, the P content may be set to 0.001% or more, 0.005% or more, or 0.008% or more.

[S: 0.100% or less]
S is an element that combines with Mn to form MnS and improves machinability. However, when the S content is high, coarse MnS is likely to be produced, and the surface fatigue strength is greatly reduced. Therefore, the S content is set to 0.100% or less. A preferable upper limit of the S content is 0.080%, 0.060%, or 0.040%. Although the S content may be 0%, reducing S excessively leads to an increase in cost for removing S. Therefore, in consideration of the economical efficiency of refining, the S content may be set to 0.001% or more, 0.002% or more, or 0.005% or more.

[Cr: 1.30-1.90%]
In addition to having the effect of increasing hardenability, Cr combines with N in the martensite structure with a high N concentration during tempering at temperatures above 300°C, precipitates fine CrN, exhibits high softening resistance, and increases surface fatigue strength. It is an element that improves. However, if the content is less than 1.30%, the above effects are insufficient. On the other hand, if the Cr content exceeds 1.90%, not only will the effect of increasing surface fatigue strength become saturated, but the hardness of the steel materials such as steel bars and wire rods used as part materials will become too high, resulting in poor machinability. Significant decline. Therefore, the Cr content of the steel material is set to 1.30 to 1.90%.
Preferably, the Cr content may be 1.35% or more, or 1.40% or more. Similarly, the Cr content may preferably be 1.85% or less, or 1.80% or less.

[V:0.02-0.40%]
In addition to having the effect of increasing hardenability, V combines with N in the martensite structure with a high N concentration during tempering at over 300°C, precipitates fine VN, develops high softening resistance, and increases surface fatigue strength. It is an element that improves. However, if the content is less than 0.02%, the above effects are insufficient. On the other hand, if the V content exceeds 0.40%, not only will the effect of increasing surface fatigue strength become saturated, but the hardness of steel materials such as steel bars and wire rods used as part materials will become too high, resulting in poor machinability. Significant decline. Therefore, the V content of the steel material is set to 0.02% to 0.40%.
Preferably, the V content may be 0.10% or more, or 0.15% or more. Similarly, the V content may be set to 0.35% or less, or 0.30% or less.

[Al: 0.070% or less]
Al is a deoxidizing element. In addition, it combines with N to form AlN, which has the effect of refining the structure of the base material before nitriding due to the pinning effect of austenite grains, thereby reducing variations in the mechanical properties of nitrided parts. In order to obtain this effect, the Al content is preferably 0.001% or more, more preferably 0.002% or more, 0.003% or more, 0.004% or more, or 0.005%. It is better to make it more than that.
On the other hand, Al is an element that tends to form hard oxide-based inclusions, and if the Al content is high, there is a risk that the surface fatigue strength will be significantly lowered. Furthermore, if the Al content is too high, the desired surface fatigue strength may not be obtained even if other requirements are met, so the Al content is preferably 0.070% or less. In order to prevent a decrease in surface fatigue strength, the preferable upper limit of the Al content is 0.060% or 0.050%.

[N: 0.0250% or less]
N (nitrogen) combines with each of Mn, Cr, V, and Al to form Mn ₃ N ₂ , CrN, VN, and AlN. In particular, AlN is bonded to the base material before nitriding due to the pinning effect of austenite grains. This has the effect of making the structure finer and reducing variations in the mechanical properties of nitrided parts. From the viewpoint of obtaining this effect, the N content is set to 0.0010% or more, 0.0015% or more, 0.0020% or more, 0.0025% or more, 0.0030% or more, 0.0035% or more, or 0. It may be 0.040% or more.
On the other hand, if the N content is high, coarse AlN is likely to be formed, and there is a possibility that the surface fatigue strength will be significantly lowered due to the coarse AlN. Furthermore, if the N content is too high, the desired surface fatigue strength may not be obtained even if other requirements are met, so the N content is preferably 0.0250% or less. Preferably, the N content may be 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[25.0≦(Mn+3Cr+2V)/Si...Formula (1)]
Each element symbol in the above formula (1) represents the content in mass % of the element.
As mentioned above, Mn, Cr, and V are all nitride-forming elements, and have the function of increasing the N concentration in the surface layer by lowering the N activity of the steel in the nitriding treatment. On the other hand, Si has the function of lowering the N concentration in the surface layer by increasing the N activity of the steel during nitriding treatment. That is, by balancing these elements, it is possible to stably increase the N concentration in the surface layer during nitriding, and it is possible to stably increase the N concentration in the martensitic structure after induction hardening in the subsequent process. That is, since the amount of residual γ can be stably ensured, deformation-induced transformation by surface hardening treatment such as shot peening can be stably utilized. In the present disclosure, the composition of the steel material is specified to satisfy the above formula (1) regarding Mn, Cr, and V. Specifically, if (Mn+3Cr+2V)/Si in the above formula (1) is 25.0 or more, surface fatigue strength and bending fatigue strength can be stably increased. If (Mn+3Cr+2V)/Si is less than 25.0, surface fatigue strength and bending fatigue strength cannot be ensured. Preferably, the lower limit of (Mn+3Cr+2V)/Si may be set to 27.0, and more preferably, the lower limit may be set to 29.0. Although there is no regulation regarding the upper limit of (Mn+3Cr+2V)/Si, from the viewpoint of alloy cost reduction, it may be preferably set to 120.0, and more preferably set to 90.0.

[1.20≦Al/N≦3.00...Formula (2)]
Each element symbol in the above formula (2) represents the content in mass % of the element.
Even if the Al content and N content in the chemical composition are within the above-mentioned ranges, coarse Al inclusions and AlN exist in the steel material, and these may become starting points for cracks during bending fatigue.
Al is an element that is more likely to form nitrides (strong nitride-forming element), so if the Al content is excessively high relative to the amount of N in the steel material, Al inclusions are likely to exist, and the parts Bending fatigue strength tends to decrease. On the other hand, if the N content is excessively large with respect to the Al content in the steel material, coarse nitrides, typified by AlN, are generated. Coarse nitrides also reduce the bending fatigue strength of the part. Therefore, it is necessary to determine the appropriate content of Al and N. In the present disclosure, the composition of the steel material is specified to satisfy the above formula (2) regarding Al and N. Specifically, if Al/N in the above formula (2) is 1.20 or more and 3.00 or less, the bending fatigue strength can be stably increased. If Al/N is less than 1.20 or more than 3.00, the bending fatigue strength of the component cannot be ensured. Preferably, the lower limit of Al/N may be 1.30 and the upper limit may be 2.90, and more preferably the lower limit may be 1.40 and the upper limit may be 2.80.

In the chemical composition of the steel material of the present disclosure, the remainder of the above elements basically consists of Fe and impurities. Impurities are components contained in raw materials or components mixed in during the manufacturing process, and include elements that are not intentionally contained, within a range that does not impair the characteristics of the steel materials and steel parts related to the present disclosure. It is acceptable if it is.

The following optional elements may be mentioned as further components that the steel material of the present disclosure, that is, the base material of the component may contain. That is, the steel material of the present disclosure can contain the following elements in place of a part of Fe. However, the steel materials and parts according to the present disclosure can solve the problem without containing the elements exemplified below. Therefore, the lower limit of the content of the elements exemplified below is 0%.

[Cu: 0.50% or less]
Cu is an element that can ensure hardenability and increase core hardness through solid solution strengthening. In order to reliably obtain this effect, the Cu content may be set to 0.01% or more. On the other hand, if the Cu content is high, the hardness of the base material after hot working becomes too high, and the machinability of the base material decreases significantly, so the Cu content is preferably 0.45% or less.
When Cu is contained, the Cu content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the Cu content is preferably 0.40% or less, 0.35% or less, 0.30% or less, or 0.25% or less.

[Ni: 0.50% or less]
Ni is an element that can ensure hardenability and increase core hardness through solid solution strengthening. In order to reliably obtain this effect, the Ni content may be set to 0.01% or more. On the other hand, if the Ni content is high, the hardness of the base material after hot working becomes too high, and the machinability of the base material decreases significantly, so the Ni content is preferably 0.45% or less.
When Ni is contained, the Ni content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the Ni content is preferably 0.40% or less, 0.35% or less, 0.30% or less, or 0.25% or less.

[Mo: 0.30% or less]
In addition to having the effect of increasing hardenability, Mo combines with N in a martensitic structure with a high N concentration during tempering at high temperatures (for example, around 400°C), precipitates fine Mo ₂ N, and softens. It is an element that causes resistance. In order to obtain these effects, the Mo content may be set to 0.01% or more. On the other hand, if the Mo content is high, the cost of the steel material will increase, and the hardness of the base material after hot working will become too high, which will greatly reduce the machinability of the base material. It is best to keep it below 25%.
Note that when Mo is contained, the Mo content is preferably 0.03% or more, or 0.05% or more. Similarly, the Mo content is preferably 0.25% or less, or 0.20% or less.

[W: 0.50% or less]
W has the effect of increasing hardenability and has the effect of improving the strength of steel. In order to reliably obtain the effect of W, the W content is preferably 0.01% or more. On the other hand, if the W content is high, the hardness of the base material after hot working becomes too high, and the machinability of the base material decreases significantly, so the W content is preferably 0.50% or less.
When containing W, the W content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the W content is preferably 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[Bi: 0.100% or less]
Bi has the effect of lowering cutting resistance and extending tool life. In order to reliably obtain this effect, the Bi content is preferably set to 0.001% or more. On the other hand, if the Bi content is high, cracks and flaws are likely to occur during hot working, so the Bi content is preferably 0.100% or less.
When Bi is contained, the Bi content is preferably 0.005% or more, 0.010% or more, or 0.015% or more. Similarly, the Bi content may preferably be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[Co: 0.100% or less]
Co is an element that increases core hardness through solid solution strengthening. In order to reliably obtain this effect, the Co content is preferably set to 0.001% or more. On the other hand, if the Co content is high, the hardness of the base metal after hot working will become too high, which will greatly reduce the machinability of the base metal, so it is better to keep the Co content below 0.100%. .
When containing Co, the Co content is preferably 0.005% or more, 0.010% or more, or 0.015% or more. Similarly, the Co content may preferably be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[Ti: 0.100% or less]
Ti combines with N that penetrates into the surface layer of the base material during nitriding and with C in the base material to form fine nitrides (TiN) and carbides (TiC), improving surface hardness and core hardness. be able to. In order to reliably obtain this effect, the Ti content is preferably set to 0.001% or more. On the other hand, if the Ti content is high, coarse nitrides and carbides are likely to be formed, so the Ti content is preferably 0.100% or less.
When containing Ti, the Ti content is preferably 0.005% or more, or 0.010% or more. Similarly, the Ti content is preferably 0.090% or less, or 0.080% or less.

[Nb: 0.030% or less]
Nb combines with N that penetrates into the surface layer of steel during nitriding and C in the matrix, forming fine nitrides (NbN) and carbides (NbC), which can improve surface hardness and core hardness. It has the effect of In order to reliably obtain this effect, the Nb content is preferably set to 0.005% or more. On the other hand, if the Nb content is high, coarse nitrides and carbonitrides are likely to be generated, so the Nb content is preferably set to 0.030% or less.
When containing Nb, the Nb content is preferably 0.010% or more, or 0.015%. Similarly, the Nb content is preferably 0.025% or less, or 0.020% or less.

[B: 0.0100% or less]
In addition to the effect of increasing hardenability, solid solution B has the effect of suppressing grain boundary segregation of P and improving toughness. Furthermore, BN which is precipitated by combining with N improves machinability. In order to reliably obtain these effects, the B content is preferably 0.0005% (5 ppm) or more. On the other hand, if the B content is high, segregation of a large amount of BN is promoted, which may lead to cracking of the steel material, so the B content is preferably set to 0.0100% or less.
When B is contained, the B content is preferably 0.0010% or more. Similarly, the B content is preferably 0.0050% or less.

[Ca: 0.0100% or less]
Ca has the function of making MnS finer and improving surface fatigue strength. In order to reliably obtain this effect of Ca, the Ca content is preferably 0.0010% or more. On the other hand, if the Ca content is high, the effect will be saturated and the economic efficiency will be impaired, so the Ca content is preferably 0.0100% or less.
When Ca is contained, the Ca content is preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more. Similarly, the Ca content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[Mg: 0.0100% or less]
Mg has the effect of making MnS finer and improving surface fatigue strength. In order to reliably obtain the effect of Mg, the Mg content is preferably 0.0010% or more. On the other hand, if the Mg content is high, the effect will be saturated and the economical efficiency will be impaired, so the Mg content is preferably set to 0.0100% or less.
When containing Mg, the Mg content is preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more. Similarly, the Mg content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[Te: 0.100% or less]
Te has the effect of reducing cutting resistance and extending tool life. In order to reliably obtain the effect of Te, the Te content is preferably 0.010% or more. On the other hand, if the Te content is high, the effect will be saturated and the economical efficiency will be impaired, so the Te content is preferably set to 0.100% or less.
When containing Te, the Te content is preferably 0.020% or more, 0.030% or more, or 0.040% or more. Similarly, the Te content is preferably 0.090% or less, 0.080% or less, or 0.070% or less.

[Pb: 0.09% or less]
Pb has the effect of reducing cutting force and extending tool life, but if the Pb content is increased, the effect will become saturated and economic efficiency will be impaired, and this will also lead to environmental burden, so Pb content should be reduced to 0. It is best to keep it below .09%.
When containing Pb, the Pb content is preferably 0.08% or less, 0.07% or less, 0.06% or less, or 0.05% or less from the viewpoint of suppressing environmental load.

[Sn: 0.100% or less]
Sn has the effect of reducing cutting resistance and extending tool life. In order to reliably obtain this effect, the Sn content is preferably set to 0.001% or more. On the other hand, if the Sn content is high, the effect will be saturated and the economic efficiency will be impaired, so the Sn content is preferably 0.100% or less.
When Sn is contained, the Sn content is preferably 0.005% or more, or 0.010% or more. Similarly, the Sn content is preferably 0.090% or less, 0.080% or less.

[Sb: 0.0100% or less]
Sb has the effect of reducing cutting resistance and extending tool life. In order to reliably obtain this effect, the Sb content is preferably set to 0.0010% or more. On the other hand, if the Sb content is high, the effect will be saturated and the economical efficiency will be impaired, so it is preferable to set the Sb content to 0.0100% or less.
When Sb is contained, the Sb content is preferably 0.0050% or more, or 0.0060% or more. Similarly, the Sb content is preferably 0.0090% or less, 0.0080% or less.

[REM: 0.020% or less]
REM (rare earth element) refers to a total of 17 elements consisting of Sc, Y, and lanthanoids. Moreover, "REM" as used in the present disclosure is composed of one or more types selected from these rare earth elements, and the REM content means the total content of these 17 elements. When using lanthanoids as REM, industrially REM is added in the form of mischmetal.

REM has the function of making MnS finer and improving surface fatigue strength. In order to reliably obtain the effect of REM, the REM content is preferably 0.001% or more. On the other hand, if the REM content is high, the effect will be saturated and the economical efficiency will be impaired, so the REM content is preferably set to 0.020% or less.
When containing REM, the REM content is preferably 0.002% or more, 0.003% or more, or 0.004% or more. Similarly, the REM content is preferably 0.018% or less, 0.016% or less, or 0.014% or less.

The above arbitrary elements can be divided into the following groups A to E from the viewpoint of action and effect. The steel material of the present disclosure may contain one or more selected from these groups.
[Group A] Longer tool life Bi: 0.100% or less,
Pb: 0.09% or less,
Sn: 0.100% or less,
Sb: 0.0100% or less, Te: 0.100% or less [Group B] Hardenability improvement, high strength Mo: 0.30% or less,
W: 0.50% or less, and B: 0.0100% or less [Group C] Improved core hardness by solid solution strengthening Cu: 0.50% or less,
Ni: 0.50% or less, Co: 0.100% or less [Group D] Improvement of surface hardness by compound formation Ti: 0.100% or less, and Nb: 0.030% or less [Group E] MnS Improved surface fatigue strength through refinement Ca: 0.0100% or less,
Mg: 0.0100% or less, and REM: 0.020% or less

[Shape of steel material]
The shape of the steel material is not particularly limited, and examples thereof include steel bars, wire rods, steel plates, and the like.

Next, a nitrided induction hardened part that is an example of the steel part of the present disclosure will be described.

[Nitrogen compound layer after nitriding induction hardening]
In the component surface layer, the thickness of the nitrogen compound layer in which a nitrogen compound containing at least Fe and N is present is 5 μm or less. The "nitrogen compound layer" here is a compound layer whose main constituent phases are Fe ₃ N and Fe ₄ N, and which also contains a nitrogen compound consisting of Fe, N, and alloy elements of the base material. It's okay.
When steel is nitrided, nitrogen enters from the surface of the steel, forming a nitrogen compound layer on the outermost layer, and a diffusion layer (nitrogen diffuses into the steel matrix) inside the nitrogen compound layer (on the core side). layer) is formed. The nitride compound layer formed on the surface of the steel during nitriding decreases in thickness and in some cases disappears while acting as a source of nitrogen to the matrix during heating in the induction hardening process. However, if a thick nitrogen compound layer remains even after induction hardening, this becomes a starting point for cracks and reduces the surface fatigue strength of the part.
If the nitrogen compound layer after induction hardening is 5 μm or less, good surface fatigue strength can be obtained. The nitrogen compound layer after induction hardening is preferably 3 μm or less. Note that there is no problem even if the part after induction hardening does not have a nitrogen compound layer, so the thickness of the nitrogen compound layer may be 0 μm.

[Vickers hardness (surface layer hardness) in the surface layer after nitriding induction hardening]
In the part, the Vickers hardness (surface hardness) at a depth of 0.10 mm from the surface is 700 HV or more. This surface hardness affects the surface fatigue strength and bending fatigue strength of the component. If the surface hardness is 700HV or more, surface fatigue strength and bending fatigue strength are good. On the other hand, if the surface hardness is less than 700 HV, the surface fatigue strength and bending fatigue strength of the part will be low. Therefore, the surface hardness of the nitrided induction hardened parts is set to 700 HV or more. The lower limit of the surface hardness is preferably 710HV, more preferably 730HV. Note that Vickers hardness in the present disclosure refers to Vickers hardness (HV) based on JIS Z 2244:2009 "Vickers hardness test - Test method". Furthermore, in the present disclosure, "a position at a depth of 0.10 mm from the surface" is located within the hardened layer.

Surface hardness is measured by cutting the part perpendicular to the main axis or longitudinal direction after nitriding induction hardening, mirror-polishing the cross section that appears, and then cutting the part at a depth of 0.10 mm (100 μm) from the part surface. It refers to the average value of Vickers hardness calculated by measuring arbitrary 10 points at vertical positions) with a test force of 1.96N.

[N concentration (Ns) in the surface layer after nitriding induction hardening]
In the part, the average N concentration (atomic %) in a region from the surface to a depth of 0.10 mm is 2.00% or more.
The N concentration in the surface layer of a part after nitriding induction hardening is determined by collecting chips from a region up to a depth of 0.10 mm from the surface of the part after nitriding induction hardening by turning with a lathe, etc., and chemically analyzing it. For example, it refers to the N content (atomic %) measured by EPMA (electron beam microanalyzer).

This N concentration affects the high temperature hardness and surface fatigue strength of the part. If Ns is 2.00 atomic % or more, the N concentration in the martensitic structure in the surface layer of the component will be sufficiently high, resulting in high hardness after quenching and high surface fatigue strength. In addition, when the surface layer of the component is tempered at about 400° C., alloy nitrides are precipitated, so that softening due to tempering is suppressed. Therefore, the content of Ns is preferably 2.00 atomic % or more. A preferable lower limit of Ns is 2.20 atom %, more preferably 2.40 atom %. The upper limit of Ns is not particularly limited, and may be 5.00 atomic %. In addition, when a part surface layer has a nitrogen compound layer, Ns also includes the N content in the nitrogen compound layer.

[Manufacturing method of steel materials]
One aspect of the method for manufacturing steel materials according to the present disclosure will be described. However, the method for manufacturing steel materials according to the present disclosure is not limited to this embodiment.

An example of the method for manufacturing a steel material according to the present disclosure includes a step of preparing a material (material preparation step) and a step of hot working the material to manufacture the steel material (hot working step). Each step will be explained below.

[Material preparation process]
In the material preparation step, a material for the steel material of the present disclosure is prepared. Specifically, molten steel is produced in which the content of each element in the chemical composition is within the range of the present disclosure and satisfies formulas (1) and (2). The refining method is not particularly limited, and any known method may be used. For example, molten metal produced by a well-known method is subjected to refining (primary refining) in a converter. Well-known secondary refining is performed on the molten steel tapped from the converter. In secondary refining, alloying elements are added for component adjustment to produce molten steel whose content of each element is within the range of this disclosure and has a chemical composition that satisfies formulas (1) and (2). do.

Using the molten steel produced by the above-described refining method, a steel material is produced by a well-known casting method. For example, an ingot is manufactured using molten steel by an ingot forming method. Alternatively, the bloom may be manufactured by a continuous casting method using molten steel. A steel material (ingot or bloom) is manufactured by the above method.

[Hot processing process]
In the hot working step, hot working is performed on the material (ingot or bloom) prepared in the material preparation step to produce the steel material (eg, steel bar) of the present disclosure. The hot working method may be hot forging or hot rolling. In the following description, a 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.

(Bulking rolling process)
In the blooming process, a billet is manufactured by hot rolling a material. Specifically, in the blooming process, a billet is manufactured by hot rolling (blending) the material using a blooming mill. When a continuous rolling mill is installed downstream of the blooming mill, the billet after blooming is further hot-rolled using the continuous mill to produce a billet with a smaller size. It's okay. The heating temperature in the blooming process may be within a known range. The heating temperature is, for example, 1000 to 1300°C.

(Final 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 material (for example, a steel bar). The heating temperature in the finish rolling process may be a well-known temperature. The heating temperature is, for example, 900 to 1250°C. The steel material after hot rolling is cooled to room temperature. Although the cooling method is not particularly limited, for example, it may be left to cool.

The steel material of the present disclosure is manufactured by the above manufacturing method. Note that the above-described manufacturing method is an example of a manufacturing method for manufacturing the steel material of the present disclosure. Therefore, the steel material of the present disclosure may be manufactured by a method other than the manufacturing method described above. That is, the manufacturing method is not limited as long as the content of each element in the chemical composition is within the range of the present disclosure and satisfies formulas (1) and (2).

In one example of the above-described manufacturing method, the hot working step is performed after the material preparation step. However, in the method for manufacturing a steel material of the present disclosure, the hot working step may not be performed after the material preparation step is performed. That is, the steel material of the present disclosure may be a cast material (ingot, bloom, or billet).

Further, the steel material after the material preparation step or the steel material after the hot working step may be subjected to known normalizing treatment and/or known spheroidizing annealing. In the spheroidizing annealing, for example, the annealing temperature is 700 to 780°C, and the holding time at the annealing temperature is 2 to 8 hours. Furthermore, the cooling time from the annealing temperature to 650° C. is set to 4 hours or more (preferably 8 hours or less). Then let it cool.
The structure after hot working or normalizing and annealing is preferably a ferrite+pearlite or a mixed structure of ferrite+pearlite+bainite, and the average Vickers hardness is preferably less than 260 HV.

[Manufacturing method of steel parts]
An example of the method for manufacturing a steel component according to the present disclosure will be described. The method for manufacturing steel parts described below is an example for manufacturing steel parts using the steel material of the present disclosure as a raw material. Therefore, the steel component having the above-described 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 manufacturing method of the steel component of the present disclosure.

The method for manufacturing steel parts includes a hot working process, a machining process, and a heat treatment process.

[Hot processing process]
When a hot working step is performed, hot working is performed on the steel material of the present disclosure. 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. After hot working, the steel material is left to cool (air-cooled).

[Machining process]
After the hot process, the intermediate product is further machined into a part shape. When a machining process is performed, the intermediate product is machined to form a predetermined shape. By performing machining, it is possible to give steel parts a precise shape that is difficult to achieve with just a hot working process. Machining is, for example, cutting or drilling. If the part is a gear, it is processed, for example, by broaching.

[Nitriding treatment]
Parts processed into a predetermined shape by hot processing are subjected to nitriding treatment.
The nitriding method in the present disclosure is not particularly limited, and well-known gas nitriding, gas soft nitriding, salt bath soft nitriding, plasma nitriding, etc. can be applied. The gas used for the nitriding process may be only NH ₃ or may be a mixed gas containing N ₂ , H ₂ , and CO ₂ in addition to NH ₃ . In addition, the parts are heat treated (for example, nitriding and quenching) at a temperature far exceeding the A1 point (approximately 590°C) in the Fe-N binary system, and after N is penetrated into the surface layer, induction hardening is performed as described below. Good too. The treatment time (holding time) in the nitriding treatment is also not particularly limited, and may be, for example, 0.5 hours to 10.0 hours. Further, as a pre-treatment or post-treatment of the nitriding treatment, chemical treatment such as film removal or oxidation treatment may be performed as long as the temperature does not exceed the nitriding treatment temperature.

In addition, during gas nitriding or nitriding and quenching, in order to suppress voids in the surface layer, the nitriding potential K _N (atm ^-1/2 ) determined by the following formula (3) should be 0.15 or more and 0.40 or less. It is best to apply within the range of .
K _N =P _NH3 /P _H2 ^3/2 ... Formula (3)
Here, P _NH3 in formula (3) is the NH ₃ partial pressure [atm] of the atmospheric gas, and P _H2 is the H ₂ partial pressure [atm] of the atmospheric gas.

[Induction hardening treatment]
As a post-nitriding process, induction hardening may be performed for the purpose of increasing surface fatigue strength. By performing induction hardening, the surface layer becomes a hardened layer consisting of a martensitic structure with a high N concentration in which nitride-forming elements are dissolved. Therefore, when the temperature of parts rises (approximately 200 to 400 degrees Celsius) due to contact friction during surface fatigue, hardness is less likely to decrease as Cr forms nitride clusters, and furthermore, solid solution Si causes agglomeration of carbonitrides. By suppressing coarsening, high surface fatigue strength can be obtained. In addition, by deepening the hardened layer, internally initiated fractures can be suppressed. In order to obtain these effects, it is preferable to perform induction hardening to a depth of 100 μm or more from the surface of the component. Further, the heating temperature of the induction hardening treatment is preferably 1000° C. or more and 1200° C. or less, and the time required to raise the temperature from room temperature to the heating temperature is preferably 4 seconds or less. The time period during which the steel material is maintained in the temperature range of 1000° C. or more and 1200° C. or less is preferably 0.2 seconds or more and 10 seconds or less.

[Tempering treatment]
The steel component according to the present disclosure may be manufactured by performing a tempering treatment after an induction hardening treatment. When tempering is performed, it is preferred that the tempering be maintained at a temperature of 200° C. or lower for 60 to 150 minutes. Tempering at a temperature exceeding 400° C. is not preferable because surface hardness is lost. Further, the tempering treatment is not essential, and the product can be used as a component without going through this.

[Other processes]
The method for manufacturing a steel component of the present disclosure may further include a shot peening process and a finish grinding process. These steps are optional steps.

(shot peening process)
The shot peening process is an optional process and may not be performed. When carried out, in the shot peening process, the shot peening process is performed on the intermediate product after the heat treatment process. By performing the shot peening treatment, residual austenite in the hardened layer of the steel component undergoes deformation-induced transformation and becomes martensite. As a result, the volume fraction of retained austenite in the hardened layer can be reduced. In the shot peening treatment, for example, it is preferable to use a cut wire or shot grains with a diameter of 1.0 mm or less, an arc height of 0.3 mm or more, and a coverage of 300% or more.

(Final grinding process)
The finish grinding process is an optional process and does not need to be performed. When carried out, in the finish grinding process, the finish cutting process is performed on the intermediate product after the heat treatment process or the shot peening process to improve the surface properties.

Above, the steel for nitriding induction hardening and the nitriding induction hardening parts of the present disclosure have been mainly described as representative examples (one embodiment) of the steel materials and steel parts of the present disclosure, but according to the present disclosure, surface fatigue strength and bending We can provide steel materials and steel parts with excellent fatigue strength. Therefore, the steel material of the present disclosure is suitable as a material for, for example, automobiles and industrial machines, particularly gears of machines powered by electric motors.

Hereinafter, the effects of the steel material and steel parts of the present disclosure will be explained in more detail using Examples. The conditions in the following examples are examples of conditions adopted to confirm the feasibility and effects of the steel materials and steel parts of the present disclosure. Therefore, the steel materials and steel parts of the present disclosure are not limited to this one example condition.

[Manufacture of steel materials]
Molten steel having the chemical composition shown in Table 1 was manufactured. In the table, the remainder of the chemical components represent Fe and impurities. A blank column indicates that alloying elements are not intentionally added. Underlining indicates outside the scope of this disclosure.

An ingot was manufactured using the molten steel shown in Table 1 by an ingot-forming method. The cross section perpendicular to the longitudinal direction of the ingot was a rectangle of 180 mm x 180 mm. The manufactured ingot was allowed to cool to room temperature. Note that the underline in Table 1 indicates a composition outside the scope of the present disclosure. A blank column means that the corresponding element content is 0% in significant figures (values to the smallest digit) as specified in the present disclosure.

The obtained ingot was heated at 1200°C for 2 hours. The heated ingot was subjected to hot working (hot forging and stretching) to produce a steel material (steel bar) with a diameter of 40 mm and a length of 1000 mm. The steel material after hot working was allowed to cool to room temperature. Normalizing treatment was performed on the steel material after it was allowed to cool. The treatment temperature in the normalizing treatment 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 during cooling was 0.3 to 0.9°C/sec. Spheroidizing annealing was performed on the steel material after normalizing treatment. The processing temperature in the spheroidizing annealing was 760°C, and after heating for 30 minutes, the processing temperature was raised to 700°C and held for 2 hours. Thereafter, the steel materials (steel bars) of each test number were produced by slowly cooling the material to 650° C. in a furnace and then allowing it to cool.

[Manufacture of steel component test pieces]
The following three types of steel component test pieces (a small roller test piece, a rotary bending fatigue test piece, and a hardened layer investigation test piece) were prepared.

(1) Small roller test piece Figure 1 shows a side view of the small roller test piece produced in this example. The numbers in FIG. 1 indicate dimensions (unit: mm). "φ" in FIG. 1 means the diameter. The inverted triangular symbol in FIG. 1 means a "finishing symbol" indicating the surface roughness described in Explanation Table 1 of JIS B 0601:1982. The "G" attached to the finishing symbol means the abbreviation of the processing method that indicates 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.

Specifically, first, steel materials of each test number were machined to produce rough test pieces having the rough shape of small roller test pieces. The center axis of the rough test piece was coaxial with the center axis of the steel bar. The crude test piece was subjected to gas nitriding, gas soft nitriding, plasma nitriding, and nitriding and quenching. Table 2 shows each heat treatment condition. In both heat treatments, the temperature was 550 to 850°C, and the nitriding time (holding time) was 1 hour to 10 hours. The test piece after the nitriding treatment was oil-cooled using 80°C oil.

Among the nitriding treatments, for gas nitriding treatment and nitriding quenching, the test piece is charged into a gas nitriding furnace, NH ₃ , H ₂ , and N ₂ gases are introduced into the furnace, and gas soft nitriding treatment is performed. In addition to these gases, CO ₂ gas was introduced at a volume ratio of 3%.

The H ₂ partial pressure in the atmosphere was measured using a thermally conductive H ₂ sensor attached directly to the gas nitriding furnace body. The difference in thermal conductivity between the standard gas and the measurement gas was converted into gas concentration and measured. The _H2 partial pressure was continuously measured during the gas nitriding process.

Further, the NH ₃ partial pressure was measured using an infrared absorption type NH ₃ analyzer installed outside the furnace. _NH3 partial pressure was continuously measured during the gas nitriding process. Regarding test number 5, which was conducted under an atmosphere of CO ₂ gas mixture, (NH ₄ ) ₂ CO ₃ was deposited in the infrared absorption type NH ₃ analyzer, which could cause the device to malfunction, so a glass tube type NH 3 analyzer was used. The NH ₃ partial pressure was measured every 10 minutes using a NH ₃ analyzer.

The average nitriding potential K _N (atm ^−1/2 ) of the gas nitriding treatment, nitriding quenching, and gas nitrocarburizing treatment is defined by the following equation (3).
K _N =P _NH3 /P _H2 ^3/2 ... Formula (3)
Here, P _NH3 in formula (3) is the NH ₃ partial pressure [atm] of the atmospheric gas, and P _H2 is the H ₂ partial pressure [atm] of the atmospheric gas.

The NH ₃ flow rate and the N ₂ flow rate were controlled so that the nitriding potential K _N calculated within the apparatus converged to the target value. The nitriding potential K _N was recorded every 10 minutes, and the average value of K _N measured during the treatment time was calculated.
For plasma nitriding treatment, the test piece is loaded into plasma nitriding equipment, and H ₂ and N ₂ gases are introduced into the furnace, so that the partial pressure ratio of H ₂ gas and N ₂ gas is constant at 3:1. The gas flow rate was controlled accordingly.

These nitrided test pieces were subjected to induction hardening. In both treatments, the heating temperature was 1000 to 1200°C, the time required to raise the temperature from room temperature to the heating temperature was 3 seconds, the time to hold the nitrided specimen was 1 to 10 seconds, and immediately after induction hardening, the temperature was at room temperature. quenched with water. Thereafter, tempering was performed at 170° C. for 1.5 hours.

After the heat treatment, the central cylindrical portion of the rough test piece was ground to produce a cylindrical portion (test surface portion) with a diameter of 26 mm as shown in FIG. At this time, the surface of the cylindrical part with a diameter of 26 mm was adjusted 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. I finished it. The grinding depth was approximately 10 μm. In an actual roller pitting test using a small roller test piece, a cylindrical part (test surface part) with a diameter of 26 mm is brought into contact with a large roller, and rotated after applying a predetermined surface pressure.

Furthermore, shot peening was performed on the rough test pieces of some of the test numbers ("Yes" was written in the "Shot peening" column in Table 2). In the shot peening, a commercially available round cut wire with a diameter of 0.6 mm was used as the shot material. Further, the arc height was set to 0.4 mm, and the coverage was set to 300%. Shot peening was performed on the outer circumferential surface of a cylindrical portion having a diameter of 26 mm shown in FIG. Shot peening was not performed on the remaining rough test pieces with test numbers ("None" was written in the "Shot peening" column in Table 2). A small roller test piece was produced by the above manufacturing process.

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

Specifically, first, steel materials of each test number were machined to produce rough test pieces for rotary bending fatigue test pieces. The center axis of the rough test piece was coaxial with the center axis of the steel bar. The rough test piece was subjected to the same heat treatment as the small roller test piece.

Furthermore, for some of the test numbers, shot peening treatment was performed on the rough test pieces after tempering (described as "Yes" in the "Shot Peening" column in Table 2). In the shot peening treatment, a commercially available round cut wire with a diameter of 0.6 mm was used as the shot material. Further, the arc height was set to 0.4 mm, and the coverage was set to 300%. The shot peening treatment was performed on a location corresponding to the notch of the rotary bending fatigue test piece. The shot peening treatment was not performed on the remaining rough test pieces of the test numbers ("None" was written in the "Shot peening" column in Table 2).

Cutting was performed on the surface of the rough test piece after heat treatment to produce a rotary bending fatigue test piece having the dimensions shown in FIG. 2. Note that cutting to improve the surface texture was not performed on the notch formed at the longitudinal center of the rotary bending fatigue test piece. A rotary bending fatigue test piece was produced through the above manufacturing process.

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

Specifically, first, the steel materials of each test number were machined to produce two cylindrical rough test pieces with a diameter of 26 mm and a length of 100 mm. The center axis of the rough test piece was coaxial with the center axis of the steel bar. The rough test piece was subjected to the same heat treatment as the small roller test piece. Thereafter, similarly to the small roller test piece, the outer peripheral surface of the rough test piece was ground to finish the outer peripheral surface. At this time, the outer periphery of the rough test piece with a diameter of 26 mm was adjusted so that the arithmetic mean roughness Ra based on JIS B 0601:2001 was 0.6 to 0.8 μm and the maximum height Rz was 2.0 to 4.0 μm. Finished the surface. The grinding depth was approximately 10 μm. Through the above manufacturing process, a test piece for investigating the hardened layer was produced.

Furthermore, shot peening treatment was performed on the rough test pieces of some of the test numbers ("Yes" was written in the "Shot peening" column in Table 2). In the shot peening treatment, a commercially available round cut wire with a diameter of 0.6 mm was used as the shot material. Further, the arc height was set to 0.4 mm, and the coverage was set to 300%. The shot peening treatment was performed on the outer peripheral surface of the rough test piece. The shot peening treatment was not performed on the remaining rough test pieces of the test numbers ("None" was written in the "Shot peening" column in Table 2). Through the above manufacturing process, a test piece for investigating the hardened layer was produced.

(Manufacture of large roller test piece used for two-cylinder rolling fatigue test)
Furthermore, a large roller test piece used in a two-cylinder rolling fatigue test for measuring surface fatigue strength was manufactured by the following method.
A rough test piece of a large roller test piece having the shape shown in FIG. 3 was cut from a cylindrical material having a diameter of 140 mm and having a chemical composition corresponding to SUJ2 specified in JIS G 4805:2008. Numerical values in FIG. 3 indicate dimensions (unit: mm). "φ" in FIG. 3 means the diameter. Further, the inverted triangular symbol in FIG. 3 means a "finishing symbol" indicating the surface roughness described in Explanation Table 1 of JIS B 0601:1982. The "G" attached to the finishing symbol means the abbreviation of the processing method that indicates grinding specified in JIS B 0122:1978.

The cut out rough test piece was quenched. The quenching temperature was 870°C, and the holding time at the quenching temperature was 90 minutes. After the holding time had elapsed, it was rapidly cooled with oil at 60°C. After quenching, the outer peripheral surface of the rough test piece was 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. A large roller test piece was produced through the above manufacturing process.

[Measurement of compound layer thickness in surface layer]
The cross section of the small roller subjected to the induction hardening treatment in the direction perpendicular to the longitudinal direction was mirror polished and etched. The etched cross section was observed using a scanning electron microscope (SEM), and the thickness of the nitrogen compound layer was measured. Etching was performed with a 3% nital solution for 20-30 seconds.

The nitrogen compound layer can be seen as a white, uncorroded layer on the surface. The nitrogen compound layer was observed from 10 fields of view (field area: 6.6×10 ² μm ² ) of the structure taken at 4000 times magnification, and the thickness of the compound layer was measured at three points every 10 μm. The average value of the 30 measured points was defined as the nitrogen compound thickness (μm).

[Measurement of N concentration]
The surface layer, which is an area up to a depth of 1.5 mm from the surface of the test part (φ26 mm) of each small roller that has been subjected to the nitriding induction hardening treatment, is subjected to lathe processing and chips are collected at a pitch of 0.05 mm. The N and C contents (atomic %) were determined by chemical analysis. Then, the average N concentration in the region from the surface of the test section to a depth of 0.10 mm was calculated. Similarly, at a depth where the N concentration is 0.3 Ns, the average N concentration at 0.05 mm before and after in the depth direction was calculated.

[Measurement of surface hardness]
A sample (thickness: 10 mm) having a cross section perpendicular to the longitudinal direction was taken from the test portion (φ26 mm) of each small roller subjected to the nitriding induction hardening treatment, and the cut surface was mirror-polished. Thereafter, the Vickers hardness at arbitrary 10 points at a depth of 0.10 mm (100 μm) from the cross section (polished surface) was measured using a micro Vickers hardness meter (manufactured by Shimadzu Corporation; HMV-G31-FA) with a test force of 1. Measurement was performed under the condition of 96N. The average value of these 10 points was defined as surface hardness.

[Evaluation test]
The following evaluation tests were conducted using the above various test pieces. In this example, assuming application to gear parts, the reference values serving as evaluation criteria for surface fatigue strength and rotational bending fatigue strength were set as follows. For surface fatigue strength and rotary bending fatigue strength evaluation, steel that meets the SCr420 standard of JIS G 4053:2016 is used, and the general manufacturing process is ``normalizing → test piece processing → eutectoid carburizing in a gas carburizing furnace → A test piece was prepared by the process of "low-temperature tempering". Next, the above-mentioned roller pitting test and rotary bending fatigue test were conducted using the test piece, and the obtained fatigue limits were used as reference values for the surface fatigue strength and rotary bending fatigue strength in this example.

(Surface fatigue strength measurement test (two cylinder rolling fatigue test))
A two-cylinder rolling fatigue test was conducted using a small roller test piece and a large roller test piece, and the surface fatigue strength was determined as follows. Note that a roller pitching tester "RP201" manufactured by Komatsu Engineering Co., Ltd. was used as the tester.

As shown in FIG. 4, the cylindrical portion of the small roller test piece 10 with a diameter of 26 mm and the center position of the outer peripheral surface of the large roller test piece 20 (the outer peripheral part with a diameter of 130 mm) were brought into contact with each other while rolling. Table 3 shows the test conditions. The surface pressure during contact was 2000 to 3500 MPa in Hertzian surface pressure. The rotation speed of the small roller test piece 10 was set to 2000 rpm. The circumferential speed of the small roller test piece 10 was 163 m/min, and the circumferential speed of the large roller test piece 10 was 229 m/min. During the test, lubricating oil was supplied to the contact area between the small roller test piece and the large roller test piece. The lubricating oil was an automatic oil, the oil temperature was 100° C., and the oil amount was 1.0 L/min. The slip rate was set to -40%.

The number of repetitions of discontinuation in the test was 2.0×10 ⁷ times, which is the fatigue limit of general steel. The maximum surface pressure (MPa) that reached 2.0×10 ⁷ times without pitching in the small roller test piece was defined as the fatigue limit of the small roller test piece.

The occurrence of pitching was detected using a vibration meter installed in the test machine. After the vibration occurred, the rotation of both the small roller test piece and the large roller test piece was stopped, and the occurrence of pitching and the number of rotations were confirmed.

In this example, assuming application to gear parts, the fatigue limit of a small roller test piece made of a steel material (reference steel material) that meets the above-mentioned SCr420 standard was used as a reference value. When the fatigue limit was 1.00 times or more that of the reference steel material, it was judged that the surface fatigue strength was excellent ("○" in the "surface fatigue strength determination" column in Table 2). On the other hand, when the fatigue limit was less than 1.00 times that of the reference steel material, the surface fatigue strength was determined to be low ("x" in the "surface fatigue strength determination" column in Table 2).

(Rotating bending strength measurement test (rotating bending fatigue test))
A rotating bending fatigue test was conducted using a rotating bending fatigue test piece in accordance with the "Rotating bending fatigue test method for metal materials" specified in JIS Z 2274:1978. The test was carried out at room temperature and in an air atmosphere, and the rotation speed was 3000 rpm. The maximum stress that did not cause rupture after ¹⁰⁷ stress loading cycles was defined as the bending fatigue strength (MPa). If the obtained bending fatigue strength is 1.20 times or more the bending fatigue strength of a rotating bending fatigue test piece of a steel material meeting the standard steel SCr420 standard (standard steel material, test number 35 corresponds), the bending fatigue strength is determined. It was judged to be excellent ("○" in the "bending fatigue strength evaluation" 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 test number 35, which is the reference steel material, the bending fatigue strength was judged to be low ("Bending fatigue strength determination" in Table 2) (×) in the field.

[Test results]
The results are shown in Table 2. Underlining in Table 2 indicates that it is outside the scope of the present disclosure. In the table, underlining means outside the scope of the present disclosure. "-" means that it has not been implemented.

In test numbers 1 to 18, the steel components were within the range of the present disclosure, the hardness and N concentration in the surface layer after nitriding induction quenching were high, and excellent results were obtained in surface fatigue strength and bending fatigue strength. .

On the other hand, in test numbers 19 to 29, the steel components were outside the range of the present disclosure, and the desired surface fatigue strength and bending fatigue strength were not achieved after nitriding induction hardening. In Test No. 30, although the steel components were within the range of the present disclosure, the desired surface fatigue strength and bending fatigue strength were not achieved because induction hardening treatment was not performed. In Test No. 31, although the steel components were within the range of the present disclosure, the desired surface fatigue strength and bending fatigue strength were not achieved because the nitriding treatment was not performed.

The embodiments and examples of the present disclosure have been described above. However, the embodiments and examples described above are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the embodiments and examples described above, and can be implemented with appropriate changes within the scope without departing from the spirit thereof.

The present disclosure can be used in a wide range of industrial fields as a material for automobiles and industrial machines, particularly gears of machines powered by electric motors.

10 Small roller (evaluation material)
20 Large roller (mating material)

Claims (14)

In mass%,
C: 0.28-0.45%,
Si: 0.05-0.30%,
Mn: 0.75-1.50%,
P: 0.030% or less,
S: 0.100% or less,
Cr: 1.30-1.90%,
V: 0.02-0.40%,
Al: 0.070% or less, and N: 0.0250% or less,
A steel material having a chemical composition in which Si, Mn, Cr, and V satisfy the following formula (1), Al and N satisfy the following formula (2), and the balance is Fe and impurities.
25.0≦(Mn+3Cr+2V)/Si... Formula (1)
1.20≦Al/N≦3.00... Formula (2)
However, each element symbol in the above formula (1) and the above formula (2) is the content in mass % of the element. In mass%,
C: 0.28-0.45%,
Si: 0.05-0.30%,
Mn: 0.75-1.50%,
P: 0.030% or less,
S: 0.100% or less,
Cr: 1.30-1.90%,
V: 0.02-0.40%,
Al: 0.070% or less, and N: 0.0250% or less,
Si, Mn, Cr, and V satisfy the following formula (1), and Al and N satisfy the following formula (2),
A steel material having a chemical composition further containing one or more selected from the group consisting of Group A to Group E below, with the balance being Fe and impurities.
[Group A] Bi: 0.100% or less,
Pb: 0.09% or less,
Sn: 0.100% or less,
One or more selected from the group consisting of Sb: 0.0100% or less, and Te: 0.100% or less [Group B] Mo: 0.30% or less,
One or more selected from the group consisting of W: 0.50% or less, and B: 0.0100% or less [Group C] Cu: 0.50% or less,
One or more selected from the group consisting of Ni: 0.50% or less, and Co: 0.100% or less [Group D] Ti: 0.100% or less, and Nb: 0.030% or less One or two selected from the group [Group E] Ca: 0.0100% or less,
One or more selected from the group consisting of Mg: 0.0100% or less, and REM: 0.020% or less 25.0≦(Mn+3Cr+2V)/Si...Formula (1)
1.20≦Al/N≦3.00... Formula (2)
However, each element symbol in the above formula (1) and the above formula (2) is the content in mass % of the element. The steel material according to claim 2, wherein the chemical composition contains the A group. The steel material according to claim 2, wherein the chemical composition contains the B group. The steel material according to claim 2, wherein the chemical composition contains the C group. The steel material according to claim 2, wherein the chemical composition contains the D group. The steel material according to claim 2, wherein the chemical composition contains the E group. The core is mass%,
C: 0.28-0.45%,
Si: 0.05-0.30%,
Mn: 0.75-1.50%,
P: 0.030% or less,
S: 0.100% or less,
Cr: 1.30-1.90%,
V: 0.02-0.40%,
Al: 0.070% or less, and N: 0.0250% or less,
It has a chemical composition in which Si, Mn, Cr, and V satisfy the following formula (1), and Al and N satisfy the following formula (2), and the remainder is Fe and impurities,
The thickness of the nitrogen compound layer containing at least Fe and N formed on the surface layer is 5 μm or less,
Vickers hardness at a depth of 0.10mm from the surface is 700HV or more,
A steel part having an average N concentration of 2.00 atomic % or more in a region from the surface to a depth of 0.10 mm.
25.0≦(Mn+3Cr+2V)/Si... Formula (1)
1.20≦Al/N≦3.00... Formula (2)
However, each element symbol in the above formula (1) and the above formula (2) is the content in mass % of the element. The core is mass%,
C: 0.28-0.45%,
Si: 0.05-0.30%,
Mn: 0.75-1.50%,
P: 0.030% or less,
S: 0.100% or less,
Cr: 1.30-1.90%,
V: 0.02-0.40%,
Al: 0.070% or less, and N: 0.0250% or less,
Si, Mn, Cr, and V satisfy the following formula (1), and Al and N satisfy the following formula (2),
Furthermore, it has a chemical composition containing one or more selected from the group consisting of the following Group A to the following Group E, with the remainder being Fe and impurities,
The thickness of the nitrogen compound layer containing at least Fe and N formed on the surface layer is 5 μm or less,
Vickers hardness at a depth of 0.10mm from the surface is 700HV or more,
A steel part having an average N concentration of 2.00 atomic % or more in a region from the surface to a depth of 0.10 mm.
[Group A] Bi: 0.100% or less,
Pb: 0.09% or less,
Sn: 0.100% or less,
One or more selected from the group consisting of Sb: 0.0100% or less, and Te: 0.100% or less [Group B] Mo: 0.30% or less,
One or more selected from the group consisting of W: 0.50% or less, and B: 0.0100% or less [Group C] Cu: 0.50% or less,
One or more selected from the group consisting of Ni: 0.50% or less, and Co: 0.100% or less [Group D] Ti: 0.100% or less, and Nb: 0.030% or less One or two selected from the group [Group E] Ca: 0.0100% or less,
One or more selected from the group consisting of Mg: 0.0100% or less, and REM: 0.020% or less 25.0≦(Mn+3Cr+2V)/Si...Formula (1)
1.20≦Al/N≦3.00... Formula (2)
However, each element symbol in the above formula (1) and the above formula (2) is the content in mass % of the element. The steel component according to claim 9, wherein the chemical composition of the core contains the A group. The steel component according to claim 9, wherein the chemical composition of the core contains the B group. The steel component according to claim 9, wherein the chemical composition of the core contains the C group. The steel component according to claim 9, wherein the chemical composition of the core contains the D group. The steel component according to claim 9, wherein the chemical composition of the core contains the E group.