JP2023163968A - Bar steel and carburized component - Google Patents

Abstract

To provide a bar steel that is superior in low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength when subjected to carburizing, and a carburized component.SOLUTION: A bar steel includes, in mass%, C: 0.10-0.25%, Si: 0.60-1.20%, Mn: 0.86-1.20%, P: 0.030% or less, S: 0.005-0.030%, Cr: 1.20-1.75%, Al: 0.010-0.060%, N: 0.003-0.020%, and O: 0.0020% or less and satisfies the following formulae (1) and (2), with the balance being Fe and impurities. In the formulae (1) and (2), each element symbol denotes the content of each element in mass%. 76-28×Si+37×Mn+3×Cr≤90.0 (1) and 1.0≤Al/N≤3.0 (2).SELECTED DRAWING: None

Description

The present disclosure relates to steel bars and carburized and hardened parts.

In recent years, from the perspective of improving the fuel efficiency of automobiles, there has been an increasing demand for smaller and lighter mechanical structural parts such as gear parts, and there is a need for improved strength of parts. Particularly, parts such as differential gears and transmission gears used in automobile gear parts are often subjected to impact loads of tens to thousands of loads when the car suddenly starts or stops, or runs over a bump in the road surface. It may fail due to low cycle bending fatigue, which can lead to failure after a very small number of repetitions. Therefore, parts used in these applications are required to have improved strength against low-cycle bending fatigue failure.

Many of the above-mentioned parts are manufactured by machining a steel material into a predetermined shape and then performing a carburizing and quenching process. In this case, most of the steel materials used are alloy steel materials for machine structures specified in JIS G 4053:2008, and by using case-hardened steels such as SCr420 and SCM420, the toughness of the core can be ensured. By carburizing and quenching and low-temperature tempering at around 180°C, the surface becomes a tempered martensitic structure with C: around 0.8%, increasing bending fatigue strength.
Furthermore, the tooth surfaces of gears slide against each other in short cycles. Therefore, pitching must be suppressed on the tooth surface. In other words, mechanical parts such as gears used in automobiles and construction vehicles require not only bending fatigue strength but also surface fatigue strength (pitting characteristics). Carburizing and quenching is very effective in improving the surface fatigue strength of mechanical parts.

Various gear steels, gear parts, and methods for manufacturing the same have been proposed for the purpose of improving low-cycle bending fatigue strength and surface fatigue strength.
For example, Patent Document 1 proposes a method for manufacturing carburized parts that have improved grain boundary strength by adding Ti and B and have excellent low cycle bending fatigue strength. Specifically, the chemical components are C: 0.10 to 0.60%, Si: 0.01 to 1.5%, Mn: 0.3 to 2.0%, Cr: 0. 1 to 3.0%, Ti: 0.02 to 0.2%, preferably 0.05 to 0.2%, B: 0.0002 to 0.005%, P: 0.02% or less, S: Contains 0.001 to 0.15%, N: 0.001 to 0.03%, Al: 0.001 to 0.06%, O: 0.005% or less, and contains C, Si, Mn, Quenching obtained by formula (1) in which the Cr content is (0.04+0.35C)×(1.00+0.70Si)×(0.70+3.96Mn)×(1.00+2.16Cr)≧1.10 A steel is used that satisfies the index and the remainder is substantially composed of Fe and unavoidable impurities. Steel with such chemical composition is subjected to carburizing treatment consisting of a carburizing temperature of 880°C to 950°C and a total carburizing time and diffusion time of 2 to 7 hours, followed by oil quenching, and then tempering at 150°C to 200°C. After holding for 1 to 3 hours, it is air cooled to form a steel material. Then, after making a notch with a radius of 1.5 mm and a depth of 3 mm on a square bar test piece made of this steel material with a cross section of 13 mm x 13 mm, the distance between the fulcrums on the notch side was 80 mm, and the fulcrum on the opposite side of the notch was In a low cycle 4-point bending fatigue test in which repeated bending loads are applied at low cycles with a spacing of 20 mm, a component having a bending fatigue strength of 17 kN or more at 100 cycles and 15 kN or more at 500 cycles is formed from this steel material.

Further, Patent Document 2 proposes a case hardening steel that has excellent low cycle fatigue strength accompanied by macroscopic strain by improving plastic deformation resistance and grain boundary strength. Specifically, a steel has been proposed in which C is limited to 0.15 to 0.30%, Si is limited to 0.50% or less, and Mo is added in an amount of more than 0.45 to 1.0%.

Patent Document 3 and Patent Document 4 propose carburized parts with excellent low cycle impact fatigue properties. Specifically, in order to increase the toughness of the surface layer of carburized parts, the surface layer C concentration was adjusted to 0.50 to 0.70% C, and the effective hardened layer depth (550 HV position from the surface layer) was adjusted to 0.30 to 0.70% C. Carburized parts have been proposed that have a shallower depth of 0.60 mm to extend the life of the process from crack initiation to fracture.

Patent Document 5 and Patent Document 6 propose case hardening steel that is excellent in surface pressure fatigue strength, impact strength, and bending fatigue strength. Specifically, in mass %, C: 0.10 to 0.35%, Si: 0.40 to 1.50%, Mn: 0.10 to 1.50%, P: 0.030% or less, Contains S: 0.030% or less, Cr: 0.50-3.0%, Al: 0.02-0.05%, N: 0.01-0.03%, and 7Si+3Cr+Mn≧7.0. A case hardening steel has been proposed that satisfies the above requirements, the remainder consists of Fe and unavoidable impurities, and the carburized abnormal layer depth during gas carburizing is 10 μm or less.

JP2011-208225A Japanese Patent Application Publication No. 10-259450 JP 2017-218608 Publication JP2018-199838A Japanese Patent Application Publication No. 2009-068064 Japanese Patent Application Publication No. 2009-068065

As mentioned above, proposals have been made regarding methods for improving low cycle bending fatigue strength. Patent Document 1 discloses a manufacturing method for manufacturing parts with excellent low cycle fatigue strength, but there is still room to explore a range of components that exhibit better properties for the steel material used as the raw material.
In the steel disclosed in Patent Document 2, it is essential to add 0.45% or more of Mo in order to increase the hardenability of the steel and improve the grain boundary strength, which inevitably increases the alloy cost.
Patent Documents 3 and 4 propose carburized parts with a surface C concentration of 0.50 to 0.70%, especially for the purpose of improving low-cycle impact fatigue properties, but 10 ² to 10 ³ If a low cycle fatigue strength of about 10 7 cycles and a high cycle bending fatigue strength of about 10 ⁷ cycles are to be achieved at the same time, a higher surface layer C concentration is desired.
In Patent Documents 5 and 6, the influence of Si, Cr, and Mn on the carburized abnormal layer depth is studied, and by satisfying 7Si+3Cr+Mn≧7.0, the carburized abnormal layer depth during gas carburizing is 10 μm or less. Case hardening steel has been proposed that has excellent surface pressure strength, impact strength, and bending fatigue strength. However, the Charpy impact test is used to evaluate impact strength, which is insufficient to evaluate low cycle bending fatigue strength. Further, as a result of studies conducted by the inventors of the present disclosure, the case hardening steels disclosed in Patent Document 5 and Patent Document 6 have insufficient improvement in low cycle bending fatigue strength.

In this way, even from the perspective of conventional technologies such as Patent Documents 1 to 6, it is possible to achieve a higher level of low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength without using expensive metal elements. It is desirable to use a steel material that

The object of the present disclosure is to provide a steel bar having excellent low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength when subjected to carburizing and quenching, as well as excellent low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength. An object of the present invention is to provide a carburized and quenched part having surface fatigue strength.

The gist of the present disclosure is as follows.
<1> The chemical composition is in mass%,
C: 0.10-0.25%,
Si: 0.60-1.20%,
Mn: 0.86-1.20%,
P: 0.030% or less,
S: 0.005-0.030%,
Cr: 1.20-1.75%,
Al: 0.010-0.060%,
Contains N: 0.003 to 0.020% and O: 0.0020% or less,
Satisfies (1) and (2) below,
A steel bar with the balance consisting of Fe and impurities.
76-28×Si+37×Mn+3×Cr≦90.0...(1)
1.0≦Al/N≦3.0...(2)
However, the element symbols in formulas (1) and (2) indicate the content in mass % of each element.
<2> The chemical composition is in mass%,
C: 0.10-0.25%,
Si: 0.60-1.20%,
Mn: 0.86-1.20%,
P: 0.030% or less,
S: 0.005-0.030%,
Cr: 1.20-1.75%,
Al: 0.010-0.060%,
Contains N: 0.003 to 0.020%, and O: 0.0020% or less, and further,
Mo: 0.25% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
W: 0.50% or less,
V: 0.25% or less,
Bi: 0.50% or less,
Co: 0.50% or less,
Nb: 0.10% or less,
Ti: 0.20% or less,
Ca: 0.0015% or less,
Pb: 0.09% or less,
Containing one or more selected from the group consisting of Sn: 0.10% or less, and B: 0.0070% or less,
Satisfies (1) and (2) below,
A steel bar with the balance consisting of Fe and impurities.
76-28×Si+37×Mn+3×Cr≦90.0...(1)
1.0≦Al/N≦3.0...(2)
However, the element symbols in formulas (1) and (2) indicate the content in mass % of each element.
<3> Includes a hardened layer that is a carburized layer and a core portion inside the hardened layer,
The chemical composition of the core is in mass%,
C: 0.10-0.25%,
Si: 0.60-1.20%,
Mn: 0.86-1.20%,
P: 0.030% or less,
S: 0.005-0.030%,
Cr: 1.20-1.75%,
Al: 0.010-0.060%,
Contains N: 0.003 to 0.020% and O: 0.0020% or less,
Satisfies (1) and (2) below,
The remainder consists of Fe and impurities,
The average C concentration in the region from the surface to a depth of 50 μm is 0.65% or more,
A carburized and quenched part having an average surface hardness of 600 HV or more at a depth of 50 μm from the surface.
76-28×Si+37×Mn+3×Cr≦90.0...(1)
1.0≦Al/N≦3.0...(2)
However, the element symbols in formulas (1) and (2) indicate the content in mass % of each element.
<4> Includes a hardened layer that is a carburized layer and a core portion inside the hardened layer,
The chemical composition of the core is in mass%,
C: 0.10-0.25%,
Si: 0.60-1.20%,
Mn: 0.86-1.20%,
P: 0.030% or less,
S: 0.005-0.030%,
Cr: 1.20-1.75%,
Al: 0.010-0.060%,
Contains N: 0.003 to 0.020%, and O: 0.0020% or less, and further,
Mo: 0.25% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
W: 0.50% or less,
V: 0.25% or less,
Bi: 0.50% or less,
Co: 0.50% or less,
Nb: 0.10% or less,
Ti: 0.20% or less,
Ca: 0.0015% or less,
Pb: 0.09% or less,
Containing one or more selected from the group consisting of Sn: 0.10% or less, and B: 0.0070% or less,
Satisfies (1) and (2) below,
The remainder consists of Fe and impurities,
The average C concentration in the region from the surface to a depth of 50 μm is 0.65% or more,
A carburized and quenched part having an average surface hardness of 600 HV or more at a depth of 50 μm from the surface.
76-28×Si+37×Mn+3×Cr≦90.0...(1)
1.0≦Al/N≦3.0...(2)
However, the element symbols in formulas (1) and (2) indicate the content in mass % of each element.

According to the present disclosure, there is provided a steel bar having excellent low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength, and excellent low cycle bending fatigue strength and high cycle bending fatigue strength when subjected to carburizing and quenching. A carburized and quenched part having surface fatigue strength and surface fatigue strength is provided.

It is a figure which shows an example of the heat pattern in a carburizing process and a quenching process in carburizing process. It is (A) a side view and (B) a front view of the low cycle bending fatigue test piece produced in the Example. FIG. 2 is a side view of an Ono-type rotary bending fatigue test piece prepared in an example. FIG. 2 is a side view of a small roller test piece produced in an example. FIG. 2 is a front view of a large roller test piece prepared in an example. FIG. 2 is a schematic diagram of a two-cylinder rolling fatigue test in an example.

An embodiment that is an example of the present disclosure will be described.
In the present disclosure, 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.
In addition, in the numerical ranges described stepwise in this specification, the upper limit of one stepwise numerical range may be replaced with the upper limit of another stepwise numerical range, or the example You may substitute the values shown in .
In the numerical ranges described step by step in this specification, the lower limit value of one stepwise numerical range may be replaced by the lower limit value of the numerical range described step by step, or in the examples. You may substitute the values shown.
Further, when a preferable lower limit value and an upper limit value are separately described for the content of each element, a numerical range obtained by arbitrarily combining the lower limit value and the upper limit value may be used as the preferable content of the element.
The term "process" is included not only in an independent process but also in the case where the intended purpose of the process is achieved even if the process cannot be clearly distinguished from other processes.

Hereinafter, the steel bar and carburized and quenched parts according to the present disclosure will be explained. Note that the unit "%" for the content of each component element means "mass %".

The inventors of the present disclosure conducted extensive studies to solve the above problems. Specifically, we analyzed in detail the chemical composition of the steel before carburizing and the abnormal carburized structure that forms on the carburized surface layer of the steel after carburizing, that is, the carburized surface layer of the carburized and quenched parts, and analyzed the low cycle bending fatigue strength, incompletely quenched structure, and grain We investigated the relationship between the field oxidation depth (internal oxidation) and the surface oxidation layer (external oxidation), and also investigated the chemical composition of the steel before carburizing, the surface hardness, surface carbon concentration, and core hardness of carburized and quenched parts. The following findings were obtained by examining the relationship between

First, the inventors of the present disclosure have studied steel that can provide excellent low cycle bending fatigue strength when carburized. Such steel is subjected to, for example, gas carburizing treatment or vacuum carburizing treatment in the manufacturing process of carburized and quenched parts. In gas carburizing treatment and vacuum carburizing treatment, the metal structure (microstructure) of the steel transforms into austenite because the steel is heated above the A _c3 transformation point temperature. Therefore, the influence of the structure before starting the gas carburizing process and the vacuum carburizing process does not remain after the gas carburizing process and the vacuum carburizing process. Therefore, the inventors of the present disclosure did not consider a means to increase the low cycle bending fatigue strength after carburizing from the perspective of the microstructure of the steel before carburizing, but instead changed the method even after carburizing. This study was conducted from the viewpoint of the chemical composition of steel, which has never been considered.

As a result, the inventors of the present disclosure found that in order to improve low cycle bending fatigue strength, it is important to reduce the depth of grain boundary oxidation in the carburized surface layer, and that the grain boundary oxidation depth in the carburized surface layer is In addition, a large amount of grain boundary S segregation occurs, and it has been found that low cycle bending fatigue strength can be improved by reducing the segregation depth of the large amount of grain boundary S segregation.
Therefore, in order to reduce the depth of grain boundary oxidation and the depth of grain boundary S segregation in the carburized surface layer, the appropriate range of the amount of Si was investigated. As a result, it was found that when 0.60% or more of Si was added to steel, the depth of the carburized abnormal layer was reduced.

(A) The inventors of the present disclosure focused on Mn and Cr when the amount of Si is 0.60% or more, and diligently investigated their appropriate ranges.
As a result, when the value of the following formula F1 satisfies 90.0 or less, the grain boundary oxidation depth is reduced and the grain boundary S segregation depth is also reduced, thereby improving low cycle bending fatigue strength. That's what I found out.
F1=76-28×Si+37×Mn+3×Cr
When the value of F1 exceeds 90.0, the depth of grain boundary oxidation (internal oxidation) or the depth of grain boundary S segregation is not reduced, and sufficient low cycle bending fatigue strength cannot be obtained.

(B) The inventors of the present disclosure also investigated an appropriate range in which carburization inhibition does not occur when further gas carburizing treatment is performed within this range and the high cycle bending fatigue strength is excellent.
The inventors of the present disclosure have discovered that when the Si content exceeds 1.20% or the Cr content exceeds 1.75%, an external oxide layer is generated over the entire carburized part during gas carburizing. It was found that carburization inhibition occurs, the carbon concentration in the surface layer decreases, and the high cycle bending fatigue strength decreases.

As a result, the inventors of the present disclosure have investigated the chemical composition of steel that has excellent low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength when carburized. In mass%, C: 0.10 to 0.25%, Si: 0.60 to 1.20%, Mn: 0.86 to 1.20%, P: 0.030% or less, S: 0.005 ~0.030%, Cr: 1.20~1.75%, Al: 0.010~0.060%, N: 0.003~0.020%, O: 0.0020% or less, Optionally, Mo: 0.25% or less, Cu: 0.50% or less, Ni: 0.50% or less, W: 0.50% or less, V: 0.25% or less, Bi: 0.50. % or less, Co: 0.50% or less, Nb: 0.10% or less, Ti: 0.20% or less, Ca: 0.0015% or less, Pb: 0.09% or less, Sn: 0.10% or less and B: steel that contains 0.0070% or less, the respective contents of Si, Mn, and Cr satisfy the relationship of 76-28 x Si + 37 x Mn + 3 x Cr≦90.0, and the balance is Fe and impurities. For example, it was thought that excellent low-cycle bending fatigue strength, high-cycle bending fatigue strength, and surface fatigue strength could be obtained after carburizing.

However, even if the steel has a chemical composition in which the content of each element is within the above-mentioned range, coarse graining may occur and sufficient low-cycle bending fatigue strength may not be obtained. Therefore, the inventors of the present disclosure conducted further investigation and study. As a result, the inventors of the present disclosure obtained the following knowledge.

(C) 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, the Al inclusions can become the starting point of cracks. Furthermore, Al inclusions in steel remain in carburized and quenched parts after carburizing. Therefore, if too many Al inclusions remain in the steel, the bending fatigue strength after carburizing may decrease. Moreover, Al in steel can be precipitated as a precipitate (AlN). Coarse AlN (precipitates) can become a starting point for cracks, similar to Al inclusions. Therefore, if excessively large amounts of coarse AlN (precipitates) are present in the steel, the bending fatigue strength after carburizing may decrease.

Here, F2=Al/N is defined. "Al" is the Al content in mass % and "N" is the N content in mass %. In the chemical composition of the steel bar according to the present disclosure, assuming that the content of each element is within the range of the present disclosure, if the F2 value is less than 1.0, the amount of AlN (precipitate) precipitated is small. , coarsening occurs due to lack of pinning particles. Therefore, the bending fatigue strength after carburizing treatment decreases.
On the other hand, in the chemical composition of the steel bar according to the present disclosure, assuming that the content of each element is within the range of the present disclosure, if the F2 value exceeds 3.0, the Al content is higher than the N content. becomes excessive. In this case, not only an excessive amount of coarse AlN (precipitates) is formed in the steel, but also an excessive amount of Al inclusions (oxide-based inclusions) are generated. Therefore, also in this case, the bending fatigue strength after the carburizing treatment decreases.

In the chemical composition of the steel bar according to the present disclosure, assuming that the content of each element is within the range of the present disclosure, if the value of F2 is 1.0 to 3.0, that is, the formula ( If 2) is satisfied, the formation of Al inclusions in the steel can be sufficiently suppressed, and the formation of coarse AlN (precipitates) can be sufficiently suppressed. Therefore, on the premise that the following formula (2) is satisfied, sufficient bending fatigue strength can be obtained after carburizing treatment.
1.0≦Al/N≦3.0...(2)

<Steel bar>
[Chemical composition]
The specific structure of the steel bar according to the present disclosure obtained based on the above findings will be described in detail below. The chemical composition of the steel bar according to the present disclosure contains elements within the following range. Note that when the steel bar according to the present disclosure is carburized, the C content in the surface layer is higher than the C content in the core, but the content of elements other than C, which will be explained below, is the content in the entire steel bar. is the same as

C: 0.10-0.25%
Carbon (C) improves the hardenability of steel and increases the hardness of steel. Therefore, C increases the bending fatigue strength after carburizing treatment. 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 range of the present disclosure. On the other hand, if the C content exceeds 0.25%, even if the contents of other elements are within the ranges of the present disclosure, cracks generated due to low cycle bending fatigue tend to propagate and break. Therefore, the C content is 0.10-0.25%. The preferable lower limit of the C content is 0.11%, more preferably 0.12%, and still more preferably 0.13%. A preferable upper limit of the C content is 0.24%, more preferably 0.23%, and still more preferably 0.22%.

Si: 0.60% to 1.20%
Silicon (Si) is an element that promotes grain boundary oxidation (internal oxidation), and reduces the depth of grain boundary oxidation (internal oxidation) when F1≦90.0 is satisfied. Therefore, low cycle bending fatigue strength is increased.
If the Si content is less than 0.60%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of the present disclosure. On the other hand, if the Si content exceeds 1.20%, even if the content of other elements is within the range of the present disclosure, carburization will be inhibited during gas carburizing, the carbon concentration in the surface layer will be insufficient, and fatigue will result. Strength decreases. Therefore, the Si content is 0.60-1.20%. The lower limit of the Si content is preferably 0.62%, more preferably 0.65%, even more preferably 0.68%, and still more preferably 0.70%. A preferable upper limit of the Si content is 1.20%, more preferably 1.15%, still more preferably 1.10%, and still more preferably 1.05%.

Mn: 0.86-1.20%
Manganese (Mn) improves the temper softening resistance of steel. Therefore, Mn increases the surface fatigue strength after carburizing treatment. If the Mn content is less than 0.86%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Mn content exceeds 1.20%, a large amount of retained austenite will be generated, the hardness after carburization will decrease, and sufficient fatigue strength will not be obtained. Therefore, the Mn content is 0.86-1.20%. The preferable lower limit of the Mn content is 0.87%, more preferably 0.88%, even more preferably 0.89%, and still more preferably 0.90%. A preferable upper limit of the Mn content is 1.18%, more preferably 1.16%, even more preferably 1.14%, still more preferably 1.12%, and still more preferably 1.10%. %.

P: 0.030% or less Phosphorus (P) is an impurity. P segregates at austenite grain boundaries during carburizing and reduces the bending fatigue strength after carburizing. If the P content exceeds 0.030%, even if the contents of other elements are within the ranges of the present disclosure, the bending fatigue strength after carburizing treatment will decrease significantly. Therefore, the P content is 0.030% or less. A preferable upper limit of the P content is 0.029%, more preferably 0.028%, and still more preferably 0.025%. The P content is preferably as low as possible, and may be, for example, 0%. However, in order to suppress refining costs, the P content may be set to 0.001% or more, or 0.002% or more.

S: 0.005% to 0.030%
Sulfur (S) is an impurity. S combines with Mn to form MnS and improves the machinability of steel. However, if the S content exceeds 0.030%, sulfides become coarse even if the contents of other elements are within the ranges of the present disclosure. In this case, the bending fatigue strength after carburizing is reduced. Therefore, the S content is 0.030% or less. The preferable lower limit of the S content is 0.005%. A preferable upper limit of the S content is 0.029%, more preferably 0.028%, still more preferably 0.027%, and still more preferably 0.026%.

Cr: 1.20-1.75%
Chromium (Cr) promotes the formation of external and internal oxidation. Therefore, when Cr is added in excess, carburization is inhibited and the high cycle bending fatigue strength is reduced. On the other hand, Cr improves the hardenability of steel and increases the hardness of steel. Therefore, Cr increases the low cycle bending fatigue strength after carburizing treatment. That is, Cr has the effect of improving hardness and low cycle bending fatigue strength inside the steel, but on the other hand, in the carburized part, it has the effect of inhibiting carburization and impairing high cycle bending fatigue strength. For this reason, it is necessary to set the Cr content within a predetermined range and then set the relationship between the Si content and the Cr content within a predetermined range so as to satisfy the above-mentioned formula (1).

If the Cr content is less than 1.20%, even if the contents of other elements are within the ranges of the present disclosure, a sufficient strength improvement effect cannot be obtained when gas carburizing is performed. On the other hand, if the Cr content exceeds 1.75%, carburization may be inhibited even if the contents of other elements are within the range of the present disclosure. In this case, high cycle bending fatigue strength decreases. Therefore, the Cr content is 1.20-1.75%. The lower limit of the Cr content is preferably 1.23%, more preferably 1.25%, even more preferably 1.30%, and still more preferably 1.35%. A preferable upper limit of the Cr content is 1.73%, more preferably 1.70%, 1.68%, or 1.65%.

Al: 0.010% to 0.060%
Aluminum (Al) deoxidizes steel. However, if the Al content exceeds 0.060%, coarse Al inclusions (oxide-based inclusions) will be generated even if the contents of other elements are within the range of the present disclosure. Coarse Al inclusions reduce the bending fatigue strength after carburizing. Therefore, the Al content is 0.060% or less. The lower limit of the Al content is 0.010%, preferably 0.011%, more preferably 0.012%, and even more preferably 0.013%. A preferable upper limit of the Al content is 0.58%, more preferably 0.55%, still more preferably 0.50%, and still more preferably 0.48%.

N: 0.003-0.020%
Nitrogen (N) forms nitrides such as AlN, acts as pinning particles, and suppresses coarsening of crystal grains. When the N content is low, the amount of pinning particles precipitated is small, and the fatigue strength is reduced due to coarsening of crystal grains. Therefore, the lower limit of N content is 0.003%. The lower limit of the N content is preferably 0.004%, more preferably 0.005%. If the N content exceeds 0.020%, even if the content of other elements is within the range of the present disclosure, the pinning particles represented by AlN will become coarser and the pinning effect of crystal grains will be reduced. This reduces the bending fatigue strength after carburizing. Therefore, the N content is 0.020% or less. A preferable upper limit of the N content is 0.019%, more preferably 0.018%.

O: 0.0020% or less Oxygen (O) is contained in steel as an impurity and segregates at grain boundaries, making it easier to cause grain boundary embrittlement. It is an element that tends to form inclusions. In order to prevent grain boundary embrittlement and brittle fracture, the content of O is 0.0020% or less. The lower limit of O is not particularly limited, and may be, for example, 0%. On the other hand, in order to suppress refining costs, the lower limit of the O content may be set to 0.0001% or 0.0005%.

Balance: Fe and Impurities The balance of the chemical composition of the steel bar according to the present disclosure consists of Fe and impurities. Here, impurities are those that are mixed in from ores used as raw materials, scrap, or the manufacturing environment when steel is manufactured industrially, and are allowed to the extent that they do not adversely affect the steel bar according to the present disclosure. means something that is done.

[Optional element]
The steel bar according to the present disclosure further includes one type selected from the group consisting of Mo, Cu, V, Ni, W, Bi, Co, Nb, Ti, Ca, Pb, Sn, and B in place of a part of Fe. Or it may contain two or more kinds. These elements are optional elements.

Mo: 0-0.25%
Molybdenum (Mo) is an optional element and may not be included. That is, the Mo content may be 0%. On the other hand, Mo improves the hardenability of steel, but since it is an expensive element, containing Mo increases the cost of steel materials. Therefore, the Mo content is 0-0.25%. The lower limit of the Mo content is preferably 0.01%, more preferably 0.02%. A preferable upper limit of the Mo content is 0.23%, more preferably 0.21%.

Cu: 0-0.50%
Copper (Cu) is an optional element and may not be included. That is, the Cu content may be 0%. On the other hand, when Cu is contained, since Cu has the effect of increasing the hardenability of steel, the core hardness can be increased and the bending fatigue strength after carburizing treatment can be increased. This effect can be obtained if even a small amount of Cu is contained. On the other hand, if the Cu content exceeds 0.50%, the hot workability of the steel will decrease. Therefore, the Cu content is 0-0.50%. The preferable lower limit of the Cu content to stably obtain the above effects is 0.01%. A preferable upper limit of the Cu content is 0.08%.

Ni: 0-0.50%
Nickel (Ni) is an optional element and may not be included. When Ni is contained, the hardness of the steel can be increased because Ni has the effect of increasing the hardenability of the steel. Thereby, the bending fatigue strength characteristics after carburizing treatment can be improved. Ni also has the effect of increasing the toughness of the carburized layer. These effects can be obtained if even a small amount of Ni is contained. However, if the Ni content exceeds 0.50%, the amount of retained austenite increases, the surface hardness decreases, and the bending fatigue strength after carburizing treatment decreases. Therefore, the Ni content is 0 to 0.50%. The preferable lower limit of the Ni content in order to stably obtain the above effects is 0.01%. A preferable upper limit of the Ni content is 0.45%.

W: 0-0.50%
Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. When W 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 steel. As a result, the bending fatigue strength after carburizing treatment increases. If even a small amount of W is contained, the above effects can be obtained to some extent. However, if the W content exceeds 0.50%, the strength of the steel will become excessively high even if the contents of other elements are within the ranges of the present disclosure. In this case, the machinability of the steel decreases. Therefore, the W content is 0 to 0.50%, and if contained, it is 0.50% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.05%, and still more preferably 0.08%. The upper limit of the W content is preferably 0.40%, more preferably 0.30%.

V: 0-0.25%
Vanadium (V) is an optional element and may not be included. That is, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms precipitates (carbides, nitrides, carbonitrides, etc.), and the pinning effect causes the formation of steel grains during carburizing. Suppress coarsening. As a result, the bending fatigue strength after carburizing treatment is increased. If even a small amount of V is contained, the above effects can be obtained to some extent. However, if the V content exceeds 0.25%, the hardness of the steel becomes excessively high even if the other element contents are within the ranges of the present disclosure. In this case, the machinability of the steel decreases. Therefore, the V content is 0-0.25%. The lower limit of the V content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%. A preferable upper limit of the V content is 0.23%, more preferably 0.21%.

Nb: 0-0.10%
Niobium (Nb) is an optional element and may not be included. That is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms precipitates (carbides, carbonitrides, etc.), and has a pinning effect that causes coarsening of the steel grains during carburizing. suppress. As a result, the bending fatigue strength after carburizing treatment is increased. If even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.030%, even if the contents of other elements are within the ranges of the present disclosure, the Nb precipitates become coarse and the pinning effect cannot be obtained. Therefore, the Nb content is 0 to 0.10%, and if it is contained, it is 0.10% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.005%. A preferable upper limit of the Nb content is 0.08%, more preferably 0.06%, and still more preferably 0.05%.

Ti: 0-0.20%
Titanium (Ti) is an optional element and may not be included. That is, the Ti content may be 0%. When Ti is contained, that is, when the Ti content exceeds 0%, Ti forms precipitates (carbides, nitrides, carbonitrides, etc.), and the pinning effect causes the formation of steel grains during carburizing. Suppress coarsening. As a result, the bending fatigue strength after carburizing treatment is increased. If even a small amount of Ti is contained, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.20%, even if the contents of other elements are within the ranges of the present disclosure, the Ti precipitates become coarse and the pinning effect cannot be obtained. Therefore, the Ti content is 0 to 0.20%, and if contained, it is 0.20% or less. The lower limit of the Ti content is preferably 0.001%, more preferably 0.005%, and still more preferably 0.010%. A preferable upper limit of the Ti content is 0.18%, more preferably 0.16%, even more preferably 0.14%, still more preferably 0.12%, and still more preferably 0.10%. %.

Co: 0-0.50%
Cobalt (Co) is an optional element and may not be included. That is, the Co content may be 0%. When contained, that is, when the Co content exceeds 0%, Co increases the hardenability of the steel and increases the hardness of the steel. As a result, the bending fatigue strength after carburizing treatment increases. If even a small amount of Co is contained, the above effects can be obtained to some extent. However, if the Co content exceeds 0.50%, the strength of the steel will become excessively high even if the contents of other elements are within the ranges of the present disclosure. In this case, the machinability of the steel decreases. Therefore, the Co content is 0 to 0.50%, and if contained, it is 0.50% or less. The preferable lower limit of the Co content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. A preferable upper limit of the Co content is 0.40%, more preferably 0.30%.

Bi: 0~0.50%
Pb: 0-0.09%
Bismuth (Bi) and lead (Pb) are both optional elements and may not be included. That is, one or both of the Bi content and Pb content may be 0%. On the other hand, Bi and Pb improve the machinability of steel. Specifically, Bi and Pb dissolve or become brittle during cutting, improving the machinability of steel. If one or more of these elements is contained in the steel, the above effects can be obtained. On the other hand, if these elements are contained excessively, the forgeability and machinability of the steel will decrease. Therefore, the Bi content is 0.50% or less, and the Pb content is 0.09% or less. The lower limit of the preferable Bi content is 0.01%. The lower limit of the preferable Pb content is 0.01%.

Ca: 0-0.0015%
Calcium (Ca) is an optional element and may not be included. That is, the Ca content may be 0%. These elements control the morphology of inclusions and improve the machinability of steel. If it contains even a small amount of one or more of these elements. The above effects can be obtained. On the other hand, if Ca is contained in excess, oxides of Ca will be produced in excess. These oxides are the starting point for bending fatigue and surface fatigue. Therefore, the bending fatigue strength decreases. Therefore, the Ca content is 0.0015% or less. The lower limit of the preferable Ca content is 0.0001%.

Sn: 0-0.10%
Tin (Sn) is an optional element and may not be included. That is, the Sn content may be 0%. When contained, that is, when the Sn content is more than 0%, Sn improves machinability. If even a small amount of Sn is contained, the above effects can be obtained to some extent. However, if the Sn content exceeds 0.10%, hot forging cracks will occur even if the contents of other elements are within the ranges of the present disclosure. Therefore, the Sn content is 0 to 0.10%, and when contained, it is 0.10% or less, that is, more than 0 to 0.10%. A preferable lower limit of the Sn content in order to more effectively obtain the above effects is 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. A preferable upper limit of the Sn content is 0.09%, more preferably 0.08%, still more preferably 0.07%, and still more preferably 0.06%.

B: 0-0.0070%
Boron (B) is an optional element and may not be included. That is, the B content may be 0%. When B 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 steel. As a result, the bending fatigue strength after carburizing treatment increases. If even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.0070%, these effects are saturated. Therefore, the B content is 0 to 0.0070%, and if it is contained, it is 0.0070% or less. The preferable lower limit of the B content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%. A preferable upper limit of the B content is 0.0060%, more preferably 0.0050%.

Note that examples of elements that can be mixed into steel as impurities include Te, Sb, REM, Zr, and Mg. Even if these elements are contained, their contents are respectively Te: 0.100% or less, Sb: 0.002% or less, REM: 0.010% or less, and Zr: 0.002% or less. , Mg: If it is 0.002% or less, there is no problem.

[Formula (1) and (2)]
The chemical composition of the steel bar according to the present disclosure further satisfies formulas (1) and (2) on the premise that the content of each element is within the above range.
76-28×Si+37×Mn+3×Cr≦90.0...(1)
1.0≦Al/N≦3.0...(2)
Equation (1) is an equation corresponding to the value of F1 mentioned above. Sufficient bending fatigue strength can be obtained after the carburizing treatment by making the Si, Mn, and Cr contents satisfy the relationship expressed by Equation (1), and the Al and Ni contents satisfying the relationship expressed by Equation (2).
From the viewpoint of bending fatigue strength after carburizing treatment, the contents of Si, Mn, and Cr preferably satisfy the following relationship (1A), and more preferably satisfy the relationship (1B).
76-28×Si+37×Mn+3×Cr≦88.0...(1A)
76-28×Si+37×Mn+3×Cr≦85.0...(1B)
Further, from the viewpoint of bending fatigue strength after carburizing treatment, the contents of Al and N preferably satisfy the following relationship (2A), and more preferably satisfy the relationship (2B).
1.2≦Al/N≦2.8...(2A)
1.5≦Al/N≦2.5 (2B)

[Metal structure]
The metal structure (microstructure) of the steel bar according to the present disclosure is not particularly limited. The objective of the steel bar according to the present disclosure is to obtain high bending fatigue strength and high surface fatigue strength after carburizing treatment. In the carburizing process, the steel is heated to a temperature higher than the A _c3 transformation temperature, so that the microstructure of the steel is reset. Therefore, it is considered that the microstructure of the steel bar before carburizing has almost no effect on the mechanical properties after carburizing. Therefore, the microstructure of the steel bar according to the present disclosure is not particularly limited.
For example, when the steel bar according to the present disclosure is a mechanical part, in order to increase the strength of the mechanical part, it is preferable that sufficiently high strength is obtained not only in the hardened layer but also in the core after carburizing and quenching. . Note that the steel bar according to the present disclosure has the content of each element in the chemical composition within the above-mentioned range, and further satisfies formulas (1) and (2), so the steel bar according to the present disclosure is carburized as a raw material. When the treatment is carried out to produce carburized and hardened parts, high low cycle bending fatigue strength, high high cycle bending fatigue strength and high surface fatigue strength are obtained in the carburized and hardened parts.

[Shape, size]
The shape and size of the steel bar according to the present disclosure are not particularly limited, and can be appropriately selected depending on its use. Note that the length and cross-sectional shape of the steel bar are not particularly limited, and may be, for example, a round bar or a square bar. The diameter of the round bar is, for example, 15 to 120 mm. The cross-sectional shape of the square rod is, for example, 50 mm to 85 mm square. Further, the steel bar according to the present disclosure is not limited to a straight steel bar, and includes, as an example, a bar-in-coil that is a rolled steel bar.

Furthermore, the steel bar according to the present disclosure may be subjected to various surface treatments. For example, the steel bar according to the present disclosure may be used for carbonitriding and quenching, and the carbonitriding treatment may be applied thereto. The carbonitriding treatment may be a vacuum carbonitriding treatment or a gas carbonitriding treatment.
Plating, chemical conversion treatment, painting, etc. may be applied to the steel bar according to the present disclosure.
A steel bar subjected to such surface treatment also falls under the steel bar according to the present disclosure, as long as the chemical composition of its core satisfies the above-mentioned requirements. For example, the carburized and quenched parts described below are considered to be an example of the steel bar according to the present disclosure because the chemical composition of the core thereof is the same as the steel bar according to the present disclosure.

[Application]
Although the use of the steel bar according to the present disclosure is not particularly limited, the steel bar according to the present disclosure is suitable as a material for carburized and quenched parts manufactured by performing carburizing treatment. In particular, it is suitable as a material for carburized and quenched parts that require low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength, such as gears used in mechanical products such as automobiles and construction vehicles, especially differential gears. be.

[Hardness]
The hardness of the steel bar according to the present disclosure is not particularly limited. When the steel bar according to the present disclosure is a mechanical part, the higher the hardness of the steel, the better the strength of the mechanical part.

<Method for manufacturing steel bars>
The method for manufacturing a steel bar according to the present disclosure is not particularly limited. The steel bar according to the present disclosure has a unique chemical composition (chemical composition) not found in conventional steel bars, but the method for obtaining this chemical composition is not particularly limited. Known refining methods can be used as methods for controlling chemical components in the present disclosure.
When manufacturing the steel bar according to the present disclosure, any hot working may be performed on a slab whose chemical composition is within a predetermined range. The steel bar obtained by hot working may be subjected to heat treatment such as spheroidization treatment.

[Material preparation process]
In the material preparation step, a material for the steel bar according to the present disclosure is prepared. Specifically, molten steel is produced in which the content of each element in the chemical composition is within the above range and satisfies formulas (1) and (2). The refining method is not particularly limited, and any known method may be used. For example, hot metal produced by a known method is subjected to refining in a converter, that is, primary refining. The molten steel tapped from the converter is subjected to a known secondary refining process. Component adjustment is performed in the secondary refining to produce molten steel having a chemical composition in which the content of each element is within the range of the present disclosure and satisfies formulas (1) and (2).

Molten steel produced by the above-mentioned refining method is cast by a known casting method to produce a raw material. For example, an ingot may be manufactured from molten steel by an ingot forming method. Alternatively, bloom may be manufactured from molten steel by a continuous casting method. A 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 manufacture the 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 is 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. The heating temperature in the finish rolling step is 900 to 1250°C. The hot rolled steel bar is cooled to room temperature. Although the cooling method is not particularly limited, for example, it may be left to cool.

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 bar of the present disclosure, the hot working step does not need to be performed after the material preparation step. That is, the steel material for obtaining the steel bar according to the present disclosure may be a cast material (ingot, bloom, or billet). Moreover, when manufacturing the steel bar according to the present disclosure, a hot forging process may be performed as the hot working process instead of the hot rolling process. Furthermore, as the hot working process, a hot rolling process and a hot forging process may be performed after the hot rolling process.

[Spheroidization heat treatment process]
The steel bar after the finish rolling process may be further subjected to a spheroidizing heat treatment process to obtain a steel bar according to the present disclosure.
The spheroidization heat treatment may be performed by a known method. The spheroidizing heat treatment is carried out, for example, by the following method. After heating the steel material after the above finish rolling process to a temperature just below or just above the A _c1 point (the temperature at which austenite begins to form during heating) (for example, within A _c1 point + 50 ° C.) and holding it for a predetermined time, Cool slowly. Alternatively, the steel material after the finish rolling process is heated to a temperature just above the A _c1 point, and then cooled to a temperature just below the A _r1 point (the temperature at which austenite completes transformation into ferrite, ferrite, or cementite during cooling). may be repeated several times. Alternatively, the steel material after the above finish rolling process may be quenched once, and then tempered at a temperature range of 600 to 700° C. for 3 to 100 hours. Note that the method for the spheroidizing heat treatment is not particularly limited, and any known annealing or spheroidizing heat treatment method as described above may be applied.
Through the above manufacturing process, the steel bar according to the present disclosure can be manufactured.

<Carburized and quenched parts>
Next, the carburized and quenched parts according to the present disclosure will be specifically described. The carburized and quenched part according to the present disclosure is manufactured by using the above-described steel bar according to the present disclosure as a material and subjecting it to carburizing treatment (carburizing treatment or carbonitriding treatment). Carburized and quenched parts are, for example, mechanical parts used in automobiles, construction vehicles, etc., and are, for example, gears.

A carburized and quenched component according to the present disclosure includes a hardened layer and a core located inside the hardened layer. The hardened layer is a layer in which carbon penetrates and diffuses into the surface of the steel bar by carburizing and hardens. Specifically, when a steel bar is subjected to carburizing treatment, the hardened layer corresponds to a carburized layer, and when a steel bar is subjected to carbonitriding treatment, the hardened layer corresponds to a carbonitrided layer. Note that in the present disclosure, a carbonitrided layer is included in a carburized layer.
The core is a region inside the hardened layer and is not affected by C intrusion and diffusion due to carburization. According to general carburizing treatment or carbonitriding treatment, the hardened layer has a thickness of 100 μm or more even when it is shallow, but it has a thickness of 2.0 mm or less even when it is deep. That is, in the carburized and quenched part according to the present disclosure, a position 5.0 mm below the surface (and a position deeper than that) belongs to the core, and a position 50 μm below the surface belongs to the hardened layer.

[Core]
The chemical composition of the core of the carburized and quenched part according to the present disclosure is the same as the chemical composition of the steel bar according to the present disclosure described above. Specifically, the chemical composition of the core of the carburized and quenched part according to the present disclosure is, in mass %, C: 0.10 to 0.25%, Si: 0.60 to 1.20%, Mn: 0.86. ~1.20%, P: 0.030% or less, S: 0.005-0.030%, Cr: 1.20-1.75%, Al: 0.010-0.060%, N: 0 .003 to 0.020%, and O: 0.0020% or less, optionally Mo: 0.25% or less, Cu: 0.50% or less, Ni: 0.50% or less, W. : 0.50% or less, V: 0.25% or less, Bi: 0.50% or less, Co: 0.50% or less, Nb: 0.10% or less, Ti: 0.20% or less, Ca: 0 .0015% or less, Pb: 0.09% or less, Sn: 0.10% or less, and B: 0.0070% or less, with the remainder consisting of Fe and impurities, and the following formula (1) and formula (2) ) is satisfied.
76-28×Si+37×Mn+3×Cr≦90.0...(1)
1.0≦Al/N≦3.0...(2)
Here, each element symbol in formula (1) and formula (2) is substituted with the content in mass % of the corresponding element. Regarding the chemical composition of the carburized and quenched part according to the present disclosure, the preferable upper and lower limits of the content of each element and the preferable upper and lower limits of F1 and F2 are based on the chemical composition of the steel bar according to the present disclosure described above.

[Hardened layer]
The composition of the hardened layer is as follows.
(1) The average C concentration in a region from the surface of the carburized and quenched part to a depth of 50 μm is 0.65% or more in mass %.
(2) The hardness at a depth of 50 μm from the surface of the carburized and quenched part is 600 HV or more.
Each configuration will be explained below. Hereinafter, the region up to a depth of 50 μm from the surface may be referred to as a “surface layer region”, and the hardness at a position 50 μm deep from the surface may be simply referred to as “surface layer hardness”.

[Average C concentration in surface region]
The region (surface layer region) from the surface of the carburized and quenched part to a depth of 50 μm is included in the hardened layer. The average C concentration in the surface layer region is 0.65% or more by mass. The average C concentration in the surface region of the carburized and quenched part is higher than the C concentration in the core. If the C concentration in the surface layer region is 0.65% or more in mass %, the hardness of the hardened layer is sufficiently hard. Therefore, sufficient bending fatigue strength can be obtained in carburized and quenched parts.

A preferable lower limit of the average C concentration in the surface layer region is 0.67%, more preferably 0.70%, and still more preferably 0.75%. The upper limit of the average C concentration in the surface layer region is not particularly limited, but from the viewpoint of reducing surface layer hardness due to an increase in residual γ, it is preferably, for example, 1.30%, more preferably 1.20%, More preferably, it is 1.10%.

[Method for measuring C concentration in surface layer region]
The average C concentration in the surface region can be measured using EPMA. Cut the carburized and quenched part perpendicular to its surface (perpendicular to the longitudinal direction if it is rod-shaped). Next, polish the cut surface. Then, the C concentration is continuously measured by continuously irradiating an electron beam along the depth direction from the surface to a depth of 50 μm. That is, the surface layer region is subjected to line analysis along the depth direction regarding the C concentration. The measurement interval in the line analysis is 5 μm, and the number of measurement points is 10. The average value of the C concentration at the ten measurement points thus obtained is taken as the average C concentration in the region from the surface to a depth of 50 μm.

[Average hardness in surface area]
A position 50 μm deep from the surface of the carburized and quenched part is included in the hardened layer. The hardness at a depth of 50 μm from the surface (surface layer hardness) is 600 HV or more. Thereby, bending fatigue strength and surface fatigue strength of the carburized and quenched parts can be ensured.
The average surface hardness is preferably 650 HV or more, more preferably 680 HV or more, or more preferably 700 HV or more.

[Method of measuring average surface hardness]
Surface hardness is measured by the following method. Cut the carburized and quenched part perpendicular to its surface (perpendicular to the longitudinal direction if it is rod-shaped). Next, polish the cut surface. Then, the hardness of the cut surface at a depth of 50 μm from the surface is measured using a Vickers hardness tester at a measurement load of 300 gf in accordance with JIS Z 2244:2009. Taking into account variations in hardness, the hardness at three measurement points at a depth of 50 μm is measured, and the average value thereof is calculated. The average value of hardness is regarded as the hardness at a depth of 50 μm from the surface.

In the carburized and quenched part having the above configuration, the content of each element in the chemical composition of the core is within the above range and satisfies formulas (1) and (2). Furthermore, the C concentration in a region (surface layer region) up to a depth of 50 μm from the surface of the carburized and quenched part is 0.65% or more, and the surface hardness at a depth of 50 μm from the surface of the carburized and quenched part is 600 HV or more. . Therefore, the carburized and quenched part according to the present disclosure has high low cycle bending fatigue strength and high high cycle bending fatigue strength. In addition, in the component system of the carburized and quenched part according to the present disclosure, if the surface hardness after carburizing and quenching is less than 600 HV, an incompletely quenched structure will occur in the surface layer, or the carburizing process will be incomplete, such as insufficient penetration of carbon. This means that the carburized and quenched parts cannot exhibit sufficient properties.

<Manufacturing method of carburized and quenched parts>
An example of a method for manufacturing a carburized and quenched component according to the present disclosure will be described. The method for manufacturing a carburized and quenched component described below is a preferred example of a method for manufacturing a carburized and quenched component according to the present disclosure, and the carburized and quenched component according to the present disclosure may be manufactured by other manufacturing methods.

The method for manufacturing a carburized and quenched part includes, for example, a hot working step or a cold working step, a cutting step, and a heat treatment step. Either one or both of the hot working process and the cold working process may be performed.

[Hot working process] and/or [cold working process]
When the hot working step is performed, the hot working is performed on the steel bar according to the present disclosure. The hot working is, for example, known hot forging. After hot working, the steel is left to cool (air cool). When a cold working step is performed, the steel bar according to the present disclosure may be subjected to cold working as is, or the steel bar may be subjected to known spheroidizing annealing or annealing treatment (IA treatment), or both. After that, cold working may be performed. The conditions for cold working are not particularly limited. Furthermore, normalizing treatment may be performed after hot working or cold working.

[Cutting process]
In the cutting process, cutting is performed on the steel after the hot working and/or cold working process to produce an intermediate product having a predetermined shape. By performing the cutting process, it is possible to give the carburized and quenched part a precise shape that is difficult to achieve only by hot working and/or cold working.

[Heat treatment process]
Carburizing heat treatment is performed on the intermediate product after the cutting process. Here, "carburizing heat treatment" includes a known carburizing process, a known quenching process, and a known tempering process. Further, the carburizing heat treatment may be gas carburizing heat treatment or vacuum carburizing heat treatment. In the gas carburizing process and the vacuum carburizing process, it is a technical matter known to those skilled in the art to adjust the C concentration and microstructure of the hardened layer of the carburized and quenched part by appropriately adjusting known conditions. Hereinafter, a known carburizing process, quenching process, and tempering process will be described with reference to FIG.

[Gas carburizing process]
The gas carburizing process includes a gas carburizing process and a quenching (quenching) process. The gas carburizing process and the quenching process will be explained below.

(Gas carburizing process)
FIG. 1 is a diagram showing an example of a heat pattern in the gas carburizing step S10 and the quenching step S20. The vertical axis of FIG. 1 is the treatment temperature (° C.) during gas carburizing treatment, and the horizontal axis is time (minutes). As shown in FIG. 1, the gas carburizing step S10 includes a heating step S0, a gas carburizing step S1, and a diffusion step S2.

In the heating step S0, the intermediate product charged into the furnace is heated to a carburizing temperature Tc. The carburizing temperature Tc in the heating step S0 is, for example, 900 to 1100°C.

In the gas carburizing step S1, the intermediate product is held at the carburizing temperature Tc for a predetermined time (holding time t1) in an atmosphere of a predetermined carbon potential CP1, and gas carburizing treatment is performed. The carbon potential CP1 in the gas carburizing step S1 is, for example, 0.6 to 1.3%, and the holding time t1 at the carburizing temperature Tc is, for example, 60 minutes or more.

In the diffusion step S2, the carburizing temperature Tc is maintained for a predetermined time (holding time t2) in an atmosphere with a predetermined carbon potential CP2. Here, the carbon potential CP2 in the diffusion step S2 is, for example, 0.6 to 1.3%, and the holding time t2 at the carburizing temperature Tc is, for example, 30 minutes or more.

(quenching process)
A quenching step S20 is performed on the intermediate product after the gas carburizing step S10. In the quenching step S20, the intermediate product after the gas carburizing step S10 is held at a quenching temperature Ts of _Ar3 or higher in an atmosphere of a predetermined carbon potential CP3 (S3), and then the intermediate product is rapidly cooled and quenched. Here, the carbon potential CP3 in the quenching step S20 is, for example, 0.6 to 1.3%, and 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 maintained at the quenching temperature is immersed in a cooling bath containing oil or water as a cooling medium to rapidly cool it.

[Tempering process]
A known tempering process is performed on the intermediate product after the quenching process in the vacuum carburizing process or the gas carburizing process is completed. The tempering temperature and holding time are not limited and can be appropriately selected so as to obtain mechanical properties suitable for the use of the carburized and quenched part.

[Other processes]
The method for manufacturing a carburized and quenched part according to the present disclosure may further include a shot peening process and a finish grinding process. These steps are optional steps. Alternatively, a nitriding process may be performed in parallel with the gas carburizing process S1 or the diffusion process S2, or subsequent to the diffusion process S2, in which a nitriding process is performed by introducing a nitrogen-containing gas into the furnace.

[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 carburized and quenched part undergoes deformation-induced transformation and becomes martensite. As a result, the volume fraction of retained austenite in the hardened layer decreases.

[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.

Through the above manufacturing process, a carburized and quenched component according to the present disclosure can be manufactured. Note that the above-mentioned manufacturing method is an example of a manufacturing method for manufacturing a carburized and quenched component according to the present disclosure. Therefore, the carburized and quenched component according to the present disclosure may be manufactured by other methods than the above-mentioned manufacturing method. That is, as long as the carburized and quenched part according to the present disclosure can be obtained, the method for manufacturing the carburized and quenched part is not particularly limited.

Next, an example of the present disclosure will be described. The conditions in the example are examples of conditions adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is based on this example of conditions. It is not limited. The present disclosure may adopt various conditions as long as the objectives of the present disclosure are achieved without departing from the gist of the present disclosure.

<Manufacture of steel bars (test pieces)>
An ingot was manufactured by an ingot-forming method using molten steel having the chemical composition shown in Table 1. The cross section perpendicular to the length direction of the ingot was a rectangle of 180 mm x 180 mm. The manufactured ingot was allowed to cool to room temperature.

In Table 1, the remainder of the chemical components (*1) represents Fe and impurities. Underlining indicates outside the scope of this disclosure.
A blank space in Table 1 means that the content of the corresponding element was below the detection limit. In other words, blank areas mean that the corresponding element content was below the detection limit in the smallest digit. For example, in the case of Mo content in Table 1, the minimum digit is the second decimal place. Therefore, the Mo content of Test No. 1 was not detected in the number of digits up to the second decimal place (meaning that the content in the number of significant figures up to the second decimal place was 0%).

The ingot was heated at 1250°C for 2 hours. The ingot after heating was subjected to hot working (hot forging and stretching) to produce steel (steel bar) with a diameter of 40 mm and a length of 1000 mm. The hot-worked steel bar was allowed to cool to room temperature. After cooling, the steel was subjected to normalizing treatment. The treatment temperature in the normalizing treatment was 925° C., and the holding time at the treatment temperature was 90 minutes.
Through the above steps, steels (steel bars) of each test number were manufactured.
The steel material with test number 17 had a chemical composition equivalent to SCM420 specified in JIS G 4805:2019, and the test piece obtained by carburizing the steel material with test number 17 was used as the "standard test piece."

<Evaluation test>
[Evaluation test of carburized and quenched parts test pieces]
(Low cycle bending fatigue test)
The low cycle fatigue strength of carburized and quenched parts was evaluated using the following method.
Machining (cutting) was performed on the steel materials of each test number to produce a low cycle bending fatigue test piece (cantilever fatigue test piece) having the shape shown in FIG. The shape of the test piece shown in FIG. 2 simulated the root R portion of a real gear. The numbers in FIG. 2 indicate dimensions (unit: mm). "R=2" indicates that the radius of curvature of the R portion is 2 mm. The thickness of the test piece is 10 mm. Multiple low cycle bending fatigue test pieces were prepared for each test number.

A gas carburizing treatment process (carburizing quenching and tempering) was performed on the test piece. The following three patterns were set as the gas carburizing process, and one of them was selected depending on the components of each test piece.
First pattern: Specifically, the test piece was held at 950° C. for 120 minutes in an atmosphere with a carbon potential CP of 1.1%. Thereafter, the test piece was held at 950° C. for 40 minutes in an atmosphere with a carbon potential CP of 1.0%. Thereafter, the temperature was lowered to 870°C and held at 870°C for 30 minutes in an atmosphere with a carbon potential of 0.8%. After the holding time had elapsed, the sample was cooled with oil at 60°C. After the quenching process, a tempering process was performed. In the tempering step, the temperature was 180° C. and the holding time was 120 minutes.
Second pattern: Specifically, the test piece was held at 950° C. for 120 minutes in an atmosphere with a carbon potential CP of 1.1%. Thereafter, the test piece was held at 950° C. for 40 minutes in an atmosphere with a carbon potential CP of 1.0%. Thereafter, the temperature was lowered to 870°C and held at 870°C for 30 minutes in an atmosphere with a carbon potential of 0.9%. After the holding time had elapsed, the sample was cooled with oil at 60°C. After the quenching process, a tempering process was performed. In the tempering step, the temperature was 180° C. and the holding time was 120 minutes.
Third pattern: Specifically, the test piece was held at 950° C. for 120 minutes in an atmosphere with a carbon potential CP of 1.1%. Thereafter, the test piece was held at 950° C. for 40 minutes in an atmosphere with a carbon potential CP of 1.0%. Thereafter, the temperature was lowered to 870°C and held at 870°C for 30 minutes in an atmosphere with a carbon potential of 1.0%. After the holding time had elapsed, the sample was cooled with oil at 60°C. After the quenching process, a tempering process was performed. In the tempering step, the temperature was 180° C. and the holding time was 120 minutes.

After carburizing, it was subjected to a low cycle bending fatigue test without finishing.
The low cycle bending fatigue test was conducted using an axial force type fatigue testing machine. Specifically, the cantilever fatigue test piece is 20×14(10)×210 mm as shown in FIG. 2, and has a notch of R2.0 at a position of 110 mm from the end. Moreover, the point of effort is 10 mm from the end.
The central axis of the cantilever fatigue specimen was coaxial with the central axis of the hot-forged part. Using the above cantilever fatigue test pieces, the fatigue tests shown in Table 2 were conducted at room temperature and in an air atmosphere. The test was conducted under oscillation load control, and the number of breaks was measured. Measurements were performed five times while changing the load, and the load at which the number of breaks occurred 100 times (100 times breaking strength) was determined from a line plotting the number of breaks and the load.

In this example, assuming application to gear parts, we used steel that met the SCM420 standard of JIS G 4053:2016 and followed the general manufacturing process, that is, "normalizing → test piece processing → gas carburizing furnace. The 100-cycle rupture strength of a cantilever fatigue test piece prepared by the process of "eutectoid carburizing → low-temperature tempering" is used as the standard (test number 17), and in the column of "Low cycle bending (100-cycle strength)" in Table 3, the target When the fatigue limit of the test number exceeded this by 20% or more, the target was achieved (marked with a circle), and when it was less than 20%, the target was not achieved (marked with an x).

(High cycle bending fatigue test)
The high cycle bending fatigue strength of carburized and quenched parts was evaluated using the following method.
Machining (cutting) was performed on the steel materials of each test number to produce rough test pieces for rotary bending fatigue test pieces. The above-mentioned carburizing treatment was performed on the rough test piece.

Cutting was performed on the surface of the rough test piece after the carburizing treatment to produce a rotary bending fatigue test piece having the dimensions shown in FIG. 3. The numerical values in FIG. 3 mean dimensions (mm). 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.

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).
The bending fatigue limit obtained by carburizing steel (standard test piece) whose bending fatigue strength met the SCM420 standard of test number 17 was taken as the evaluation standard value. If the fatigue limit is 1.20 times or more that of the standard steel, it is judged that the bending fatigue strength is excellent, and it is marked with a circle in the "High cycle bending ( ¹⁰⁷ times strength)" column in Table 3. On the other hand, if the obtained bending fatigue strength is less than 1.20 times the bending fatigue strength of test number 17, which is the reference steel, the bending fatigue strength is judged to be low, and the "high cycle bending (10 ⁷ times strength)" column was marked with an x.

(Roller pitting test (surface fatigue strength evaluation test))
The surface fatigue strength of carburized and quenched parts was evaluated using the following method.

・Manufacture of small roller test piece used in two-cylinder rolling fatigue test FIG. 4 shows a side view of the small roller test piece produced in this example. The numbers in FIG. 4 indicate dimensions (unit: mm). "φ" in FIG. 4 means the diameter. The inverted triangular symbol in FIG. 4 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.

Machining (cutting) was performed on the steel materials of each test number to produce rough test pieces (rough test pieces of small roller test pieces) for the roller pitting test. The cylindrical part at the center of the rough test piece was finished into a cylindrical part with a diameter of 26 mm as shown in FIG. At this time, 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. Finished the surface. The grinding depth was approximately 10 μm.
The above-mentioned carburizing treatment was performed on the above-mentioned rough test piece. After the carburizing heat treatment, the cylindrical portion of the grip portion of the rough test piece was ground to produce a grip portion with a diameter of 24 mm as shown in FIG. 4.

・Manufacture of a large roller test piece used in a two-cylinder rolling fatigue test A large roller test piece used in a two-cylinder rolling fatigue test for measuring surface fatigue strength was manufactured in the following manner. A rough test piece of a large roller test piece having the shape shown in FIG. 5 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. 5 indicate dimensions (unit: mm). Further, the inverted triangular symbol in FIG. 5 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.
Carburizing and quenching was performed on the cut out rough test piece. 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.

・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. As a test machine, RP201 manufactured by Nikko Create Co., Ltd. was used. As shown in FIG. 6, 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. The surface pressure during contact was 1800 to 3500 MPa in Hertzian surface pressure. The rotation speed of the small roller test piece 10 was set to 1500 rpm. The circumferential speed of the small roller test piece 10 was 123 m/min, and the circumferential speed of the large roller test piece 10 was 172 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 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 (standard test piece) made of carburized steel meeting the SCM420 standard of test number 17 was used as the evaluation standard value. When the fatigue limit was 1.20 times or more that of the standard steel, it was judged that the surface fatigue strength was excellent, and the "surface fatigue" column in Table 3 was marked with a circle. On the other hand, if the fatigue limit was less than 1.20 times that of the reference steel, it was determined that the surface fatigue strength was low, and the "surface fatigue" column in Table 3 was marked with an x.

[C concentration measurement test of hardened layer of carburized and quenched parts test piece]
Carburized and quenched parts (low-cycle bending fatigue test pieces) of each test number are cut in a direction perpendicular to the length direction, the cut surfaces are embedded with a micromount, polished, and test pieces are taken with the cut surfaces as measurement surfaces. did. Then, the C concentration in a region up to a depth of 50 μm from the surface of the carburized and quenched part was subjected to line analysis using an electron beam microanalyzer. The measurement interval in the line analysis was 5 μm, and the number of measurement points was 10. The average value of the C concentration at the ten measurement points thus obtained is listed in Table 3 as the average C concentration (surface layer carbon concentration) in the region from the surface to a depth of 50 μm.

[Measurement of hardness of hardened layer and core]
The hardness of the hardened layer is measured by the following method. Carburized and quenched parts (low cycle bending fatigue test pieces) of each test number were cut in a direction perpendicular to the length direction, and test pieces were taken with the cut surface as the measurement surface. Then, the hardness of the cut surface at a depth of 50 μm from the surface of the carburized and quenched part was measured using a Vickers hardness tester at a measurement load of 300 gf in accordance with JIS Z 2244:2009. Taking into account variations in hardness, the hardness was measured at three measurement points at a depth of 50 μm, and the average value was calculated. The average value of the hardness is shown in Table 3 as the hardness at a depth of 50 μm from the surface (surface layer hardness). Regarding core hardness (hardness at 5 mm position), the hardness at three measurement points at a depth of 5 mm from the surface was similarly measured, and the average value thereof was calculated and listed in Table 3.

<Evaluation results>
The test results are shown in Table 3.

As shown in Table 1, the content of each element in the chemical composition of the steels of test numbers 1 to 16 is within the range of the present disclosure, and furthermore, F1 and F2 satisfy formulas (1) and (2). Filled.

In addition, in the carburized and quenched parts manufactured by carburizing the steel of test numbers 1 to 16, the external oxidation layer did not cover the entire surface layer, and the depth of grain boundary oxidation (internal oxidation) and the depth of grain boundary S segregation were reduced. The C concentration in a region up to a depth of 50 μm from the surface of the carburized and quenched part was 0.65% or more by mass, and the surface hardness at a depth of 50 μm from the surface of the carburized and quenched part was 600 HV or more. As a result, these carburized and quenched parts showed excellent fatigue strength.

Test number 17 is a steel equivalent to a conventional product, and was adopted as a reference material in the test in the present disclosure. In test number 17, the Si content was lower and the Mn content was lower than that of the present disclosure. Therefore, the value of F1 was below the upper limit of formula (1), and the depth of grain boundary oxidation (internal oxidation) was not reduced. When the fatigue strength of the gas carburized material of test number 17 was taken as the reference value, all the gas carburized and quenched parts of test numbers 1 to 16 described above showed fatigue strength superior to test number 17.

In test number 18, although the content of each element in the chemical composition of the steel was appropriate, F1 exceeded the upper limit of formula (1). As a result, low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength were all insufficient.

In test number 19, the amount of Al was high and F2 exceeded the upper limit of formula (2). As a result, test number 19 was insufficient in low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength.

In test number 20, the C content of the steel was too high. As a result, during the low-cycle bending fatigue test, the steel broke at the same time as cracking occurred, resulting in insufficient low-cycle bending fatigue strength.

In test number 21, the Cr content of the steel was low. As a result, Test No. 21 had insufficient hardenability in the core, and when gas carburized, low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength were all insufficient.

In test number 22, the Cr content of the steel was too high. As a result, in test number 22, an external oxidation layer was formed on the entire test piece, carburization was inhibited, the surface layer carbon concentration was low, and the high cycle bending fatigue strength and surface fatigue strength were insufficient.

In test number 23, the Si content of the steel was too low, and F1 exceeded the upper limit of formula (1). As a result, in Test No. 23, the depth of grain boundary oxidation (internal oxidation) was not reduced, and low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength were all insufficient.

In test number 24, the amount of Si in the steel was too high. As a result, in test number 24, an external oxidation layer was formed on the entire specimen during gas carburization, inhibiting carburization, and the surface layer carbon concentration was low, the surface layer hardness was insufficient, and the high cycle bending fatigue strength and surface fatigue strength were There was a shortage.

In test number 25, the Mn content of the steel was too high, and F1 exceeded the upper limit of formula (1). As a result, in Test No. 25, the surface hardness decreased due to a large amount of retained austenite, and the high cycle bending fatigue strength and surface fatigue strength were insufficient.

In test number 26, the amount of P in the steel was too high. As a result, test number 26 was insufficient in low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength.

In test number 27, the S content of the steel was too high. As a result, test number 27 was insufficient in low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength due to coarsening of sulfides.

In test number 28, the Al content of the steel was too high. As a result, test number 28 was insufficient in low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength due to coarsening of inclusions.

In test number 29, the N content of the steel was too high. As a result, test number 29 was insufficient in low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength due to coarsening of inclusions.

As mentioned above, by using the carburized steel parts of the present disclosure that have excellent low cycle bending fatigue strength, high cycle bending fatigue strength, and surface fatigue strength, gears such as automobile differential gears and transmission gears can be significantly downsized. , it is possible to reduce the weight, and as a result, it becomes possible to increase the fuel efficiency of the automobile and reduce CO ₂ emissions. Therefore, the effects of the present disclosure are extremely significant, and the present disclosure has great industrial applicability.

Claims (4)

The chemical composition is in mass%,
C: 0.10-0.25%,
Si: 0.60-1.20%,
Mn: 0.86-1.20%,
P: 0.030% or less,
S: 0.005-0.030%,
Cr: 1.20-1.75%,
Al: 0.010-0.060%,
Contains N: 0.003 to 0.020% and O: 0.0020% or less,
Satisfies (1) and (2) below,
A steel bar with the balance consisting of Fe and impurities.
76-28×Si+37×Mn+3×Cr≦90.0...(1)
1.0≦Al/N≦3.0...(2)
However, the element symbols in formulas (1) and (2) indicate the content in mass % of each element. The chemical composition is in mass%,
C: 0.10-0.25%,
Si: 0.60-1.20%,
Mn: 0.86-1.20%,
P: 0.030% or less,
S: 0.005-0.030%,
Cr: 1.20-1.75%,
Al: 0.010-0.060%,
Contains N: 0.003 to 0.020%, and O: 0.0020% or less, and further,
Mo: 0.25% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
W: 0.50% or less,
V: 0.25% or less,
Bi: 0.50% or less,
Co: 0.50% or less,
Nb: 0.10% or less,
Ti: 0.20% or less,
Ca: 0.0015% or less,
Pb: 0.09% or less,
Containing one or more selected from the group consisting of Sn: 0.10% or less, and B: 0.0070% or less,
Satisfies (1) and (2) below,
A steel bar with the balance consisting of Fe and impurities.
76-28×Si+37×Mn+3×Cr≦90.0...(1)
1.0≦Al/N≦3.0...(2)
However, the element symbols in formulas (1) and (2) indicate the content in mass % of each element. including a hardened layer that is a carburized layer and a core inside the hardened layer,
The chemical composition of the core is in mass%,
C: 0.10-0.25%,
Si: 0.60-1.20%,
Mn: 0.86-1.20%,
P: 0.030% or less,
S: 0.005-0.030%,
Cr: 1.20-1.75%,
Al: 0.010-0.060%,
Contains N: 0.003 to 0.020% and O: 0.0020% or less,
Satisfies (1) and (2) below,
The remainder consists of Fe and impurities,
The average C concentration in the region from the surface to a depth of 50 μm is 0.65% or more,
A carburized and quenched part having an average surface hardness of 600 HV or more at a depth of 50 μm from the surface.
76-28×Si+37×Mn+3×Cr≦90.0...(1)
1.0≦Al/N≦3.0...(2)
However, the element symbols in formulas (1) and (2) indicate the content in mass % of each element. including a hardened layer that is a carburized layer and a core inside the hardened layer,
The chemical composition of the core is in mass%,
C: 0.10-0.25%,
Si: 0.60-1.20%,
Mn: 0.86-1.20%,
P: 0.030% or less,
S: 0.005-0.030%,
Cr: 1.20-1.75%,
Al: 0.010-0.060%,
Contains N: 0.003 to 0.020%, and O: 0.0020% or less, and further,
Mo: 0.25% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
W: 0.50% or less,
V: 0.25% or less,
Bi: 0.50% or less,
Co: 0.50% or less,
Nb: 0.10% or less,
Ti: 0.20% or less,
Ca: 0.0015% or less,
Pb: 0.09% or less,
Containing one or more selected from the group consisting of Sn: 0.10% or less, and B: 0.0070% or less,
Satisfies (1) and (2) below,
The remainder consists of Fe and impurities,
The average C concentration in the region from the surface to a depth of 50 μm is 0.65% or more,
A carburized and quenched part having an average surface hardness of 600 HV or more at a depth of 50 μm from the surface.
76-28×Si+37×Mn+3×Cr≦90.0...(1)
1.0≦Al/N≦3.0...(2)
However, the element symbols in formulas (1) and (2) indicate the content in mass % of each element.