JP2021105203A - Steel for carburized steel components - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material for a carburized steel part, which has excellent cold forging property and can obtain excellent surface fatigue strength and excellent hydrogen embrittlement resistance when it is used as a carburized steel part. SOLUTION: The steel material for carbonized steel parts of the present disclosure has a chemical composition of mass%, C: 0.05 to less than 0.10%, Si: 0.50 to 0.75%, Mn: 0.20. ~ 0.55%, S: 0.005 to 0.050%, Cr: 1.30 to less than 2.00%, Mo: 0.25 to 0.40%, B: 0.0005 to 0.0100% , Al: 0.100 to 0.150%, Ca: 0.0002 to 0.0030%, Ti: less than 0.0200%, N: 0.0080% or less, P: 0.050% or less, and O : Contains 0.0030% or less, the balance is composed of Fe and impurities, and satisfies the formulas (1) to (4) described in the specification. [Selection diagram] None

Description

The present invention relates to a steel material for carburized steel parts, which is a steel material used for carburized steel parts.

Machine structural parts are used in automobiles, construction machinery, mining machinery, etc. Mechanical structural parts are, for example, gears, shafts, pulleys and the like. These mechanical structural parts are required to have high bending fatigue strength and high surface fatigue strength. In order to obtain high bending fatigue strength and high surface fatigue strength, carburized steel parts are often used as mechanical structural parts.

As described above, mechanical structural parts are required to have high bending fatigue strength and high surface fatigue strength. In addition, mechanical structural parts may come into contact with engine oil, lubricating oil, or the like. Hydrogen may enter the machine structural parts from these oils. Therefore, carburized steel parts used as machine structural parts may be required to have not only high surface fatigue strength but also high hydrogen embrittlement resistance.

By the way, the carburized steel parts are manufactured by the following steps. Cold forging is performed on the steel material for carburized steel parts to manufacture intermediate members. Carburize the intermediate members. Carburized steel parts are manufactured by the above steps. The carburized steel component includes a carburized layer formed on the surface layer and a core portion inside the carburized layer. As described above, in the manufacturing process of carburized steel parts, cold forging is carried out on the steel material for carburized steel parts. Therefore, the steel material for carburized steel parts is required to have excellent cold forging property.

Techniques related to carburized steel parts are disclosed in International Publication No. 2012/108460 (Patent Document 1), International Publication No. 2012/108461 (Patent Document 2), and Japanese Patent Application Laid-Open No. 2006-307273 (Patent Document 3). ing.

The carbonized steel disclosed in Patent Document 1 has a chemical composition of mass%, C: 0.07% to 0.13%, Si: 0.0001% to 0.50%, Mn: 0.0001%. ~ 0.80%, S: 0.0001% ~ 0.100%, Cr: Over 1.30% ~ 5.00%, B: 0.0005% ~ 0.0100%, Al: 0.0001% ~ It contains 1.0%, Ti: 0.010% to 0.10%, N: 0.0080% or less, P: 0.050% or less, O: 0.0030% or less, and the balance is Fe. And unavoidable impurities, and the content represented by mass% of each element in the chemical component satisfies the formulas (1) to (3). Here, the equations (1) to (3) are as follows. Equation (1): 0.10 <C + 0.194 × Si + 0.065 × Mn + 0.012 × Cr + 0.078 × Al <0.235, Equation (2): 7.5 <(0.7 × Si + 1) × (5) .1 x Mn + 1) x (2.16 x Cr + 1) <44, formula (3): 0.004 <Ti-N x (48/14) <0.030. By having the above-mentioned chemical composition, this carburizing steel can increase the limit processing rate at the time of cold forging, and further, after the carburizing treatment, a hardened layer and core hardness equivalent to those of the conventional steel can be obtained. It is described in Patent Document 1.

The carbonizing steel disclosed in Patent Document 2 has a chemical composition of mass%, C: 0.07% to 0.13%, Si: 0.0001% to 0.50%, Mn: 0.0001%. ~ 0.80%, S: 0.0001% ~ 0.100%, Cr: Over 1.30% ~ 5.00%, B: 0.0005% ~ 0.0100%, Al: 0.070% ~ It contains 0.200%, N: 0.0030% to 0.0100%, is limited to Ti: 0.020% or less, P: 0.050% or less, O: 0.0030% or less, and the balance is Fe. And unavoidable impurities, satisfying equations (1) to (3). Here, the equations (1) to (3) are as follows. Equation (1): 0.10 <C + 0.194 × Si + 0.065 × Mn + 0.012 × Cr + 0.078 × Al <0.235, Equation (2): 7.5 <(0.7 × Si + 1) × (5) .1 x Mn + 1) x (2.16 x Cr + 1) <44, formula (3) 0.0003 <Al x (N-Ti x (14/48)) <0.0011. By having the above-mentioned chemical composition, this carburizing steel can increase the limit processing rate at the time of cold forging, and further, after the carburizing treatment, a hardened layer and core hardness equivalent to those of the conventional steel can be obtained. It is described in Patent Document 2.

The skin-baking steel disclosed in Patent Document 3 has a mass% of C: 0.05 to 0.30%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, S: 0.002-0.2%, N: 0.003 to 0.030%, Al: 0.01 to 0.12%, Nb: 0.01 to 0.20%, Ti: 0.005 to It contains 0.12%, the balance is made of steel substantially composed of Fe, the average value of Vickers hardness in the cross section is 180 or less, and the maximum value of the standard deviation of Vickers hardness is 5 or less. .. Patent Document 3 describes that this skin-baking steel is excellent in grain coarsening resistance and cold workability.

International Publication No. 2012/108460 International Publication No. 2012/108461 Japanese Unexamined Patent Publication No. 2006-307273

By the way, as described above, carburized steel parts are required to have high bending fatigue strength and surface fatigue strength. It is known that bending fatigue strength can be replaced by core hardness. The higher the core hardness of the carburized steel part, the higher the bending fatigue strength. Carburized steel parts may also be required to have hydrogen embrittlement resistance as well as high core hardness and surface fatigue strength. In Patent Documents 1 to 3, patent documents 1 to 3 have excellent cold forging properties, high core hardness when carburized steel parts are used, excellent surface fatigue strength, and excellent hydrogen embrittlement resistance. Steel materials that can obtain brittle properties have not been studied.

An object of the present disclosure is to have excellent cold forging property, high core hardness when used as a carburized steel part, excellent surface fatigue strength, and excellent hydrogen embrittlement resistance. Is to provide steel materials for carburized steel parts.

The steel materials for carburized steel parts according to the present disclosure are
The chemical composition is mass%,
C: 0.05 to less than 0.10%,
Si: 0.50 to 0.75%,
Mn: 0.25 to 0.55%,
S: 0.005 to 0.050%,
Cr: 1.30 to less than 2.00%,
Mo: 0.25 to 0.40%,
B: 0.0005 to 0.0100%,
Al: 0.1000 to 0.150%,
Ca: 0.0002% to 0.0030%,
Ti: less than 0.0200%,
N: 0.0080% or less,
P: 0.050% or less, and
O: Contains 0.0030% or less, and the balance is composed of Fe and impurities, and satisfies the formulas (1) to (4).
0.195 <C + 0.194 x Si + 0.065 x Mn + 0.012 x Cr + 0.033 x Mo + 0.067 x Ni + 0.097 x Cu + 0.078 x Al <0.235 (1)
0.0003 <Al × (N-Ti × (14/48)) <0.0011 (2)
Si / Mn> 1.00 (3)
0.070 <C / Si <0.180 (4)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formulas (1) to (4).

The steel material for carburized steel parts according to the present disclosure has excellent cold forging properties, has a high core hardness when used as a carburized steel part, has excellent surface fatigue strength, and has excellent hydrogen embrittlement resistance. Chemical characteristics can be obtained.

FIG. 1 is a side view of a small roller test piece used in the roller pitching test in the examples. FIG. 2 is a heat pattern diagram of the carburizing treatment performed on the small roller test piece. FIG. 3 is a front view of the large roller used in the roller pitching test in the embodiment. FIG. 4 is a side view of the annular V-notch test piece used in the surface fatigue strength test.

The present inventors have studied a steel material for carburized steel parts, which has excellent cold forging properties and can obtain excellent surface fatigue strength and excellent hydrogen embrittlement resistance when used as carburized steel parts. .. As a result, the present inventors obtained the following findings.

[Chemical composition]
The lower the C content, the higher the cold forging property of the steel material for carburized steel parts. However, if the C content is too low, the core hardness of the carburized steel parts manufactured by performing cold forging and carburizing treatment will decrease. Further, B dissolves in the steel material to improve the hardenability of the steel material and enhances the hardness of the core portion of the carburized steel part. Further, Si increases the hardness of the core of the carburized steel part, increases the temper softening resistance, and enhances the surface fatigue strength. Therefore, the chemical composition of the steel material for carburized steel parts is determined by mass%, C: 0.05 to less than 0.10%, Si: 0.50 to 0.75%, Mn: 0.25 to 0.55%. S: 0.005 to 0.050%, Cr: 1.30 to less than 2.00%, Mo: 0.25 to 0.40%, B: 0.0005 to 0.0100%, Al: 0.100 ~ 0.150%, Ca: 0.0002% ~ 0.0030%, Ti: less than 0.0200%, N: 0.0080% or less, P: 0.050% or less, and O: 0.0030% If the chemical composition contains the following and the balance is composed of Fe and impurities, sufficient cold forging property can be obtained, and in the case of carburized steel parts, high core hardness can be obtained and a high surface can be obtained. Fatigue strength may be obtained.

[About equation (1)]
However, even if the content of each element in the chemical composition is within the above range, the cold forging property may still be low or the core hardness may be low. Therefore, as a result of further studies by the present inventors, if the above-mentioned chemical composition further satisfies the following formula (1), it is sufficient for the carburized steel parts after the carburizing treatment while suppressing the deterioration of the cold forging property. It was found that a good core hardness can be obtained.
0.195 <C + 0.194 x Si + 0.065 x Mn + 0.012 x Cr + 0.033 x Mo + 0.067 x Ni + 0.097 x Cu + 0.078 x Al <0.235 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (1).

[About equation (2)]
In order to increase the surface fatigue strength, it is further effective to suppress the coarsening of austenite crystal grains during heating of the carburizing treatment. If AlN is finely dispersed and precipitated in the steel material, coarsening of austenite crystal grains during heating in the carburizing treatment can be sufficiently suppressed due to the pinning effect. Therefore, the present inventors have investigated a method for effectively suppressing the coarsening of austenite crystal grains from a stoichiometric point of view, considering Ti and Al, which are elements that bind to N. As a result, it was found that the surface fatigue strength is further increased if the Al content, N content and Ti content in the steel material for carburized steel parts satisfy the formula (2).
0.0003 <Al × (N-Ti × (14/48)) <0.0011 (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (2).

If the Al content, Ti content and N content in the chemical composition of the steel material for carburized steel parts satisfy the formula (2), the surface fatigue strength of the carburized steel parts is increased. The following reasons can be considered as the reason. When the formula (2) is satisfied, sufficient AlN is finely dispersed, and the abnormal grain growth of austenite crystal grains during heating of the carburizing treatment is suppressed by the pinning effect. Therefore, sufficient hardness can be obtained at the core of the carburized steel part. As a result, sufficient surface fatigue strength can be obtained for carburized steel parts.

[About equation (3)]
The steel material for carburized steel parts of the present embodiment is also required to have excellent hydrogen embrittlement resistance in carburized steel parts. Therefore, the present inventors have studied a method for further enhancing the hydrogen embrittlement resistance property in the above chemical composition. In order to enhance the hydrogen embrittlement resistance, it is effective to miniaturize the inclusions in the steel material. Hydrogen that has entered from the outside of the steel material is easily trapped by inclusions in the steel material. If the inclusions are coarse, the amount of hydrogen trapped will be small. As a result, hydrogen embrittlement cracks are likely to occur. Miniaturizing inclusions increases the amount of hydrogen trapped. As a result, the hydrogen embrittlement resistance is enhanced.

Therefore, the present inventors have investigated a method for refining inclusions from the viewpoint of chemical composition. As a result, it was found that in the above chemical composition, if the ratio of the Si content to the Mn content (= Si / Mn) satisfies the formula (3), the hydrogen embrittlement resistance property of the carburized steel part is enhanced.
Si / Mn> 1.00 (3)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (3).

The reason why the hydrogen embrittlement resistance of carburized steel parts is enhanced if the formula (3) is satisfied is not clear, but the following reasons can be considered. If the formula (3) is satisfied, MnO-SiO ₂ which is a composite inclusion is easily generated as an inclusion in the steel material. MnO-SiO ₂ is soft and is divided and refined during hot working. As the inclusions become finer, the number of hydrogen trap sites increases, and the amount of hydrogen trapped increases. As a result, the hydrogen embrittlement resistance of carburized steel parts is enhanced.

[About equation (4)]
As described above, in the steel material for carburized steel parts of the present embodiment, the C content is suppressed to less than 0.05 to 0.10% to improve the cold forging property, and the Si content is 0.50 to 0.75. Increase to% to increase the surface fatigue strength of carburized steel parts. However, even if the steel material for carburized steel parts has a chemical composition satisfying the formulas (1) to (3), the surface fatigue strength may still be lowered or sufficient cold forging property may not be obtained. there were. Therefore, the present inventors further investigated. As a result, if the chemical composition satisfies the formulas (1) to (3) and the C content and the Si content satisfy the formula (4), excellent cold forging property can be obtained and the cold forging property can be obtained. , It was found that excellent surface fatigue strength and excellent hydrogen embrittlement resistance can be obtained when carburized steel parts are used.
0.070 <C / Si <0.180 (4)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formula (4).

The steel material for carburized steel parts according to the present embodiment completed based on the above findings has the following constitution.

[1]
The chemical composition is mass%,
C: 0.05 to less than 0.10%,
Si: 0.50 to 0.75%,
Mn: 0.25 to 0.55%,
S: 0.005 to 0.050%,
Cr: 1.30 to less than 2.00%,
Mo: 0.25 to 0.40%,
B: 0.0005 to 0.0100%,
Al: 0.1000 to 0.150%,
Ca: 0.0002% to 0.0030%,
Ti: less than 0.0200%,
N: 0.0080% or less,
P: 0.050% or less, and
O: Contains 0.0030% or less, the balance is composed of Fe and impurities, and satisfies the formulas (1) to (4).
Steel material for carburized steel parts.
0.195 <C + 0.194 x Si + 0.065 x Mn + 0.012 x Cr + 0.033 x Mo + 0.067 x Ni + 0.097 x Cu + 0.078 x Al <0.235 (1)
0.0003 <Al × (N-Ti × (14/48)) <0.0011 (2)
Si / Mn> 1.00 (3)
0.070 <C / Si <0.180 (4)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formulas (1) to (4).

[2]
The steel material for carburized steel parts according to [1].
The chemical composition is
Nb: 0.100% or less,
V: 0.200% or less,
Ni: 0.500% or less and
Cu: 0.500% or less,
Contains one or more elements selected from the group consisting of
Steel material for carburized steel parts.

Hereinafter, the details of the steel material for carburized steel parts of the present embodiment will be described. In the present specification, "%" for an element means mass% unless otherwise specified.

[Chemical composition of steel materials for carburized steel parts]
The chemical composition of the steel material for carburized steel parts of the present embodiment contains the following elements.

C: 0.05 to less than 0.10% Carbon (C) increases the hardness of the core of the carburized steel part and increases the bending fatigue strength of the carburized steel part. When the C content is less than 0.05%, the hardness of the core of the carburized steel part decreases and the bending fatigue of the carburized steel part decreases even if the other element content is within the range of this embodiment. The strength decreases. On the other hand, when the C content is 0.10% or more, the cold forging property of the steel material for carburized steel parts is lowered. Therefore, the C content is less than 0.05 to 0.10%. The lower limit of the C content is preferably 0.06%, more preferably 0.07%. The preferred upper limit of the C content is 0.09%, more preferably 0.08%.

Si: 0.50 to 0.75%
Silicon (Si) increases the hardness of the core of carburized steel parts. Si also increases temper softening resistance and enhances surface fatigue strength of carburized steel parts. Specifically, when a carburized steel part in use comes into contact with another part, heat is generated on the surface layer of the carburized steel part. Since Si increases temper softening resistance, it suppresses the decrease in hardness of the surface layer of carburized steel parts due to heat. Therefore, the surface fatigue strength of the carburized steel parts is increased. If the Si content is less than 0.50%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content exceeds 0.75%, the cold forging property of the steel material for carburized steel parts is lowered even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.50 to 0.75%. The lower limit of the Si content is preferably 0.51%, more preferably 0.52%, still more preferably 0.53%. The preferred upper limit of the Si content is 0.73%, more preferably 0.71%, still more preferably 0.69%.

Mn: 0.25 to 0.55%
Manganese (Mn) enhances the hardenability of steel and enhances the core hardness of carburized steel parts. This increases the bending fatigue strength of the carburized steel parts. If the Mn content is less than 0.20%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 0.55%, the hydrogen embrittlement resistance property of the steel material for carburized steel parts deteriorates even if the content of other elements is within the range of this embodiment. Therefore, the Mn content is 0.25 to 0.55%. The preferable lower limit of the Mn content is 0.21%, more preferably 0.25%, still more preferably 0.28%. The preferred upper limit of the Mn content is 0.53%, more preferably 0.51%, still more preferably 0.49%, still more preferably 0.46%, still more preferably 0.44. %.

S: 0.005 to 0.050%
Sulfur (S) combines with Mn in steel to form MnS and enhances machinability of steel materials for carburized steel parts. If the S content is less than 0.005%, the above effect cannot be sufficiently obtained even if the other element content is within the range of the present embodiment. On the other hand, if the S content exceeds 0.050%, MnS is excessively generated even if the other element content is within the range of the present embodiment. In this case, the cold forging property of the steel material for carburized steel parts is lowered. Therefore, the S content is 0.005 to 0.050%. The lower limit of the S content is preferably 0.006%, more preferably 0.008%, still more preferably 0.010%. The preferred upper limit of the S content is 0.040%, more preferably 0.030%, still more preferably 0.025%.

Cr: 1.30 to less than 2.00% Chromium (Cr) enhances hardenability of steel and enhances core hardness of carburized steel parts. This increases the bending fatigue strength of the carburized steel parts. If the Cr content is less than 1.30%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, when the Cr content is 2.00% or more, the cold forging property of the steel material for carburized steel parts is lowered even if the content of other elements is within the range of this embodiment. Therefore, the Cr content is less than 1.30 to 2.00%. The lower limit of the Cr content is preferably 1.31%, more preferably 1.35%, still more preferably 1.40%, still more preferably 1.45%, still more preferably 1.50. %. The preferred upper limit of the Cr content is 1.98%, more preferably 1.95%, still more preferably 1.90%.

Mo: 0.25 to 0.40%
Molybdenum (Mo) enhances the hardenability of steel and enhances the core hardness of carburized steel parts. This increases the bending fatigue strength of the carburized steel parts. If the Mo content is less than 0.20%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mo content exceeds 0.40%, the cold forging property of the steel material for carburized steel parts is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Mo content is 0.25 to 0.40%. The lower limit of the Mo content is preferably 0.21%, more preferably 0.22%, still more preferably 0.23%, still more preferably 0.24%. The preferred upper limit of the Mo content is 0.38%, more preferably 0.36%, and even more preferably 0.34%.

B: 0.0005 to 0.0100%
Boron (B), when dissolved in austenite, significantly enhances the hardenability of steel, even in trace amounts. Therefore, the hardness of the core of the carburized steel part is increased, and the bending fatigue strength of the carburized steel part is increased. Further, since B exerts the above effect by containing a small amount of B, the hardness of ferrite in the steel material for carburized steel parts is unlikely to increase. That is, it is possible to improve the hardenability while maintaining the cold forging property of the steel material for carburized steel parts. If the B content is less than 0.0005%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the B content exceeds 0.0100%, the above effect is saturated. Therefore, the B content is 0.0005 to 0.0100%. The lower limit of the B content is preferably 0.0007%, more preferably 0.0010%, still more preferably 0.0015%, still more preferably 0.0020%. The preferred upper limit of the B content is 0.0080%, more preferably 0.0050%, still more preferably 0.0040%, still more preferably 0.0030%.

Al: 0.100 to 0.150%
Aluminum (Al) deoxidizes steel. Al further combines with N to finely disperse fine AlN. The pinning effect of the fine AlN suppresses the coarsening of austenite crystal grains during heating of the carburizing treatment. As a result, the hardness of the core portion of the carburized steel part is increased, and the surface fatigue strength of the carburized steel part is increased. If the Al content is less than 0.100%, these effects cannot be sufficiently obtained even if the other element content is within the range of the present embodiment. On the other hand, if the Al content exceeds 0.150%, coarse oxides are formed in the steel, and the surface fatigue strength of the carburized steel parts is rather lowered. Therefore, the Al content is 0.1000 to 0.150%. The lower limit of the Al content is preferably 0.101%, more preferably 0.104%, still more preferably 0.110%, still more preferably 0.120%. The preferred upper limit of the Al content is 0.145%, more preferably 0.140%, still more preferably 0.138%.

Ca: 0.0002 to 0.0030%
Calcium (Ca) dissolves in sulfide in steel to make the sulfide fine and spheroidal. As a result, the cold forging property of the steel material for carburized steel parts is enhanced. If the Ca content is less than 0.0002%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ca content exceeds 0.0030%, coarse oxides are formed in the steel even if the content of other elements is within the range of the present embodiment. In this case, the cold forging property of the steel material for carburized steel parts is rather lowered. Therefore, the Ca content is 0.0002 to 0.0030%. The preferable lower limit of the Ca content is 0.0003%, more preferably 0.0004%, still more preferably 0.0005%. The preferred upper limit of the Ca content is 0.0025%, more preferably 0.0020%.

Ti: Less than 0.0200% Titanium (Ti) is inevitably contained. That is, the Ti content is more than 0%. For Ti, N in the steel is fixed as TiN. As a result, the formation of BN is further suppressed, and the solid solution B can be further secured. When a steel material for carburized steel parts is manufactured by an electric furnace, it may be difficult to adjust the N content in the steel. Therefore, if even a small amount of Ti is contained, the above effect can be obtained to some extent. On the other hand, if the Ti content exceeds 0.0200%, the manufacturing cost becomes high. Therefore, the Ti content is less than 0.0200%. The preferred lower limit of the Ti content is more than 0%, more preferably 0.0001%, still more preferably 0.0002%. The preferred upper limit of the Ti content is 0.0180%, more preferably 0.0160%, still more preferably 0.0120%.

N: 0.0080% or less Nitrogen (N) is an impurity that is inevitably contained. That is, the N content is more than 0%. N combines with B to form BN and reduces the amount of solid solution B. If the N content exceeds 0.0080%, even if the Ti content in the steel material for carburized steel parts is within the range of this embodiment, Ti cannot sufficiently fix N, and BN becomes excessive. To generate. As a result, the hardenability of the steel material for carburized steel parts is lowered. If the N content exceeds 0.0080%, coarse TiN is further generated, and the coarse TiN becomes the starting point of cracking during cold forging. Therefore, the cold forging property of the steel material for carburized steel parts is lowered. Therefore, the N content is 0.0080% or less. The preferred upper limit of the N content is 0.0078%, more preferably 0.0076%, still more preferably 0.0075%, still more preferably 0.0065%. The N content is preferably as low as possible. However, excessive reduction of N content increases manufacturing costs. Therefore, when considering normal industrial production, the preferable lower limit of the N content is 0.0001%, more preferably 0.0005%, still more preferably 0.0010%, still more preferably 0. It is 0020%.

P: 0.050% or less Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is more than 0%. P lowers the surface fatigue strength of the steel material. Therefore, the P content is 0.050% or less. The preferred upper limit of the P content is 0.035%, more preferably 0.020%, still more preferably 0.010%. The P content is preferably as low as possible. However, excessive reduction of P content increases manufacturing costs. Therefore, when considering normal industrial production, the preferable lower limit of the P content is 0.001%.

O: 0.0030% or less Oxygen (O) is an impurity inevitably contained. That is, the O content is more than 0%. O forms an oxide, lowers the cold forging property of the steel material for carburized steel parts, and lowers the surface fatigue strength of the carburized steel parts. Therefore, the O content is 0.0030% or less. The preferred upper limit of the O content is 0.0028%, more preferably 0.0026%, still more preferably 0.0024%. The O content is preferably as low as possible. However, excessive reduction of O content increases manufacturing costs. Therefore, when considering normal industrial production, the preferable lower limit of the O content is 0.0001%, more preferably 0.0005%, still more preferably 0.0007%.

The balance of the chemical composition of the steel material for carburized steel parts according to this embodiment is composed of Fe and impurities. Here, the impurities are those mixed from ore, scrap, manufacturing environment, etc. as a raw material when the steel material for carburized steel parts is industrially manufactured, and the steel material for carburized steel parts of the present embodiment. It means something that is acceptable as long as it does not adversely affect.

[About optional elements]
The chemical composition of the steel material for carburized steel parts of the present embodiment may further contain one or more elements selected from the group consisting of Nb, V, Ni and Cu instead of a part of Fe. All of these elements are optional elements and increase the surface fatigue strength of steel materials for carburized steel parts.

Nb: 0.100% or less Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb combines with C and N to form carbides and / or carbonitrides, and the pinning effect suppresses the coarsening of austenite grains during heating of the carburizing treatment. As a result, the surface fatigue strength of the carburized steel parts is increased. If even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.100%, coarse carbides and / or carbonitrides are generated, and the cold forging property of the steel material for carburized steel parts is lowered. Therefore, the Nb content is 0.100% or less. The preferable lower limit of the Nb content is more than 0%, more preferably 0.001%, further preferably 0.002%, still more preferably 0.004%, still more preferably 0.010%. Is. The preferred upper limit of the Nb content is 0.080%, more preferably 0.060%, still more preferably 0.050%.

V: 0.200% or less Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, V combines with C and N to form carbides and / or carbonitrides and suppresses coarsening of austenite grains during heating during carburizing due to the pinning effect. As a result, the surface fatigue strength of the carburized steel parts is increased. If even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.200%, coarse carbides and / or carbonitrides are generated, and the cold forging property of the steel material for carburized steel parts is lowered. Therefore, the V content is 0.200% or less. The preferable lower limit of the V content is more than 0%, more preferably 0.001%, further preferably 0.002%, still more preferably 0.010%, still more preferably 0.030%. Is. The preferred upper limit of the V content is 0.150%, more preferably 0.120%, still more preferably 0.110%.

Ni: 0.500% or less Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, Ni enhances hardenability of steel and enhances core hardness of carburized steel parts. This increases the surface fatigue strength of the carburized steel parts. Furthermore, when the carburizing treatment by gas carburizing is carried out, Ni does not generate oxides and nitrides during the carburizing treatment. Therefore, Ni suppresses the formation of an oxide layer, a nitride layer and an abnormal carburizing layer in the carburized layer. If even a small amount of Ni is contained, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.500%, the cold forging property of the steel material for carburized steel parts deteriorates. Therefore, the Ni content is 0.500% or less. The preferable lower limit of the Ni content is more than 0%, more preferably 0.005%, further preferably 0.010%, further preferably 0.020%, still more preferably 0.040%. Is. The preferred upper limit of the Ni content is 0.400%, more preferably 0.300%, and even more preferably 0.250%.

Cu: 0.500% or less Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu enhances hardenability of steel and enhances core hardness of carburized steel parts. This increases the surface fatigue strength of the carburized steel parts. Further, when the carburizing treatment by gas carburizing is carried out, Cu does not generate oxides and nitrides during the carburizing treatment. Therefore, Cu suppresses the formation of an oxide layer, a nitride layer, and an abnormal carburizing layer on the surface of the carburized layer. If even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.500%, the hardness of the steel material for carburized steel parts becomes excessively high, and the critical processing rate decreases. Therefore, the Cu content is 0 to 0.500%. The lower limit of the Cu content is more than 0%, more preferably 0.001%, still more preferably 0.005%, still more preferably 0.010%, still more preferably 0.020%. It is more preferably 0.030%. The preferred upper limit of the Cu content is 0.400%, more preferably 0.300%, and even more preferably 0.250%.

[About equations (1) to (4)]
The chemical composition of the steel material for carburized steel parts of the present embodiment further satisfies the formulas (1) to (4).
0.195 <C + 0.194 x Si + 0.065 x Mn + 0.012 x Cr + 0.033 x Mo + 0.067 x Ni + 0.097 x Cu + 0.078 x Al <0.235 (1)
0.0003 <Al × (N-Ti × (14/48)) <0.0011 (2)
Si / Mn> 1.00 (3)
0.070 <C / Si <0.180 (4)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formulas (1) to (4). If the corresponding element is an arbitrary element and is not contained, "0" is substituted for the element symbol. Hereinafter, each equation will be described.

[About equation (1)]
It is defined as F1 = C + 0.194 × Si + 0.065 × Mn + 0.012 × Cr + 0.033 × Mo + 0.067 × Ni + 0.097 × Cu + 0.078 × Al. F1 is an index of the hardness of the carburized steel parts and the carburized steel parts.

F1 indicates the contribution of each alloying element to the solid solution strengthening of ferrite in the steel material for carburized steel parts. If F1 is 0.235 or more, the hardness of the steel material for carburized steel parts is too high. In this case, the cold forging property of the steel material for carburized steel parts is lowered. On the other hand, if F1 is 0.195 or less, the hardness of the core portion as a carburized steel part is too low. In this case, the surface fatigue strength of the carburized steel parts decreases. When F1 is more than 0.195 to less than 0.235, the core hardness of the carburized steel part can be increased while maintaining the cold forging property of the carburized steel part, and the surface fatigue of the carburized steel part can be increased. The strength can be increased. The preferred lower limit of F1 is 0.200, more preferably 0.202, and even more preferably 0.204. The preferred upper limit of F1 is 0.234, more preferably 0.232, and even more preferably 0.230. The F1 value is a value obtained by rounding off the fourth decimal place of the calculated value.

[About equation (2)]
It is defined as F2 = Al × (N—Ti × (14/48)). F2 is an index regarding the amount of AlN precipitation. When Ti is stoichiometrically excessive with respect to N, all N is fixed as TiN. That is, (N—Ti × (14/48)) in F2 indicates the amount of N in the steel in a form other than TiN. That is, (N—Ti × (14/48)) indicates the amount of N that is not bonded to Ti in the steel. In F2, "14" represents the atomic weight of N, and "48" represents the atomic weight of Ti.

If F2 is 0.0003 or less, the amount of Al bound to N is insufficient. In this case, the dispersion of fine AlN is insufficient. Therefore, the pinning effect does not work effectively, and coarse grains are generated in the austenite crystal grains during the heating of the carburizing treatment. As a result, the surface fatigue strength of the carburized steel parts is reduced. On the other hand, when F3 is 0.0011 or more, the AlN precipitates are coarsened without being finely dispersed, so that the limit processing rate of the steel material for carburized steel parts is lowered. Therefore, F2 is greater than 0.0003 and less than 0.0011. The preferred lower limit of F2 is 0.0004, more preferably 0.0005. The preferred upper limit of F2 is 0.0010, more preferably 0.0009. The F2 value is a value obtained by rounding off the fifth decimal place of the calculated value.

[About equation (3)]
It is defined as F3 = Si / Mn. F3 is an index of hydrogen embrittlement resistance. _{Si and Mn form MnO-SiO 2} which is a composite inclusion in the refining process in the steelmaking process. MnO-SiO ₂ has a low melting point and is a soft inclusion. Therefore, MnO-SiO ₂ is divided and refined during hot working. The miniaturization of inclusions increases hydrogen trap sites. As a result, the hydrogen embrittlement resistance of carburized steel parts is enhanced.

If F3 is 1.00 or less, the Si content is too small with respect to the Mn content. In this case, MnO-SiO ₂ is not sufficiently formed in the refining step. As a result, coarse inclusions remain in the steel material for carburized steel parts. With coarse inclusions, the amount of hydrogen trap is insufficient. Therefore, the hydrogen embrittlement resistance of the carburized bearing component is lowered. Therefore, F3 is more than 1.00. The preferred lower limit of F3 is 1.03, more preferably 1.05, and even more preferably 1.10. The upper limit of F3 is not particularly limited. In the case of the chemical composition of this embodiment, the upper limit of F3 is 3.75. The F3 value is a value obtained by rounding off the third decimal place of the calculated value.

[About equation (4)]
It is defined as F4 = C / Si. F4 is an index of surface fatigue strength and cold forging property on the premise that the content of each element in the chemical composition is within the above range. As described above, in the present embodiment, the C content is suppressed to improve the cold forging property, and the Si content is increased to increase the surface fatigue strength. However, if F4 is 0.070 or less, the Si content is too high with respect to the C content. In this case, even if the content of each element in the chemical composition is within the above range, the cold forging property of the steel material for carburized steel parts is low. On the other hand, if F4 is 0.180 or more, the Si content is too low with respect to the C content. In this case, even if the content of each element in the chemical composition is within the above range, the surface fatigue strength of the carburized steel part is lowered. When F4 is more than 0.070 to less than 0.180, sufficient cold forging property can be obtained with the steel material for carburized steel parts on the premise that the content of each element of the chemical composition is within the above range. , Sufficient surface fatigue strength can be obtained with carburized bearing parts. The preferred lower limit of F4 is 0.071, more preferably 0.072, and even more preferably 0.075. The preferred upper limit of F4 is 0.178, more preferably 0.175, even more preferably 0.172, still more preferably 0.170. The F4 value is a value obtained by rounding off the fourth decimal place of the calculated value.

[Microstructure of steel for carburized steel parts]
The microstructure of steel for carburized steel parts mainly consists of ferrite and pearlite. Here, "mainly composed of ferrite and pearlite" means that the total area ratio of ferrite and pearlite in the microstructure is 85.0 to 100.0%. In the matrix, the phases other than ferrite and pearlite are, for example, bainite, martensite, cementite, retained austenite, precipitates other than cementite, inclusions and the like.

[Measurement method of ferrite and pearlite area ratio]
The total area ratio (%) of ferrite and pearlite in the microstructure of the steel material for carburized steel parts of the present embodiment is measured by the following method. When the steel material for carburized steel parts is steel bar, the center position of the radius R connecting the surface and the central axis of the cross section perpendicular to the longitudinal direction (axial direction) of the steel material for carburized steel parts (hereinafter referred to as the cross section) (hereinafter referred to as the cross section). Take a sample from the R / 2 position). Of the surfaces of the collected samples, the surface corresponding to the cross section is used as the observation surface. After mirror polishing the observation surface, the observation surface is etched with 2% alcohol nitrate (Nital corrosive liquid). The etched observation surface is observed using a 500x optical microscope to generate an arbitrary 20-field photographic image. The size of each field of view is 100 μm × 100 μm.

In each field of view, the contrast of each phase of ferrite, pearlite, etc. is different for each phase. Therefore, each phase is identified based on the contrast. Of the identified phase, the total area ([mu] m ²⁾ of the ferrite in each field, and determines the total area of perlite (μm ^2). The ratio of the total area of the total area of ferrite and the total area of pearlite in all fields of view to the total area of all fields of view is defined as the total area ratio (%) of ferrite and pearlite.

The steel material for carburized steel parts of the present embodiment having the above configuration is excellent in cold forging property. Further, the carburized steel parts produced by using the steel material for carburized steel parts of the present embodiment have excellent surface fatigue strength and excellent hydrogen embrittlement resistance.

[About carburized steel parts]
The carburized steel parts of the present embodiment are manufactured by using the above-mentioned steel material for carburized steel parts of the present embodiment. Specifically, it is manufactured by performing a carburizing treatment on a steel material for carburized steel parts after cold forging. The method for manufacturing carburized steel parts will be described later.

The carburized steel component includes a carburized layer and a core portion. The carburized layer is formed on the surface layer of the carburized steel part. The carburized layer is a region where the depth from the surface of the carburized steel part is 0.4 mm to less than 2.0 mm. In the present embodiment, the carburized layer means a region where the Vickers hardness according to JIS Z 2244 (2009) is 550 HV or more. The core portion corresponds to a region inside the carburized steel component rather than the carburized layer. The chemical composition of the core is the same as the chemical composition of the steel material for carburized steel parts described above. That is, the elements in the chemical composition of the core portion are within the above numerical range and satisfy the formulas (1) to (4).

In the carburized steel part, the position at a depth of 50 μm from the surface of the carburized steel part corresponds to the carburized layer. The Vickers hardness according to JIS Z 2244 (2009) at a depth of 50 μm from the surface of the carburized steel component is preferably 650 to 1000 HV. That is, the Vickers hardness of the carburized layer at the above position is preferably 650 to 1000 HV.

In the carburized steel part having the above structure, the position at a depth of 10.0 mm from the surface of the carburized steel part corresponds to the core portion. The Vickers hardness according to JIS Z 2244 (2009) at a depth of 10.0 mm from the surface of the carburized steel part is preferably 300 to 500 HV. That is, the Vickers hardness of the core portion at the above position is preferably 300 to 500 HV.

The carburized layer is formed by carburizing. The Vickers hardness of the carburized layer is higher than that of the steel material for the carburized steel part (that is, the core of the carburized steel part) which is the material.

The Vickers hardness of carburized steel parts is measured by the following method. The measurement surface is a cross section perpendicular to any surface of the carburized steel part. On the measurement surface, the Vickers hardness at a depth of 50 μm from the surface and the Vickers hardness at a depth of 0.4 mm from the surface were tested for Vickers hardness in accordance with JIS Z 2244 (2009) using a micro Vickers hardness tester. To be calculated by. The test force is 0.49N. The Vickers hardness HV at 10 points is measured at a depth position of 50 μm, and the average value thereof is taken as the Vickers hardness HV at a depth position of 50 μm. Further, the Vickers hardness HV at 10 points is measured at a depth position of 0.4 mm, and the average value thereof is taken as the Vickers hardness HV at a depth position of 0.4 mm. If the Vickers hardness at the 0.4 mm depth position is 550 HV or more, it is determined that the carburized layer depth is at least 0.4 mm or more. Further, on the measurement surface, the Vickers hardness at a depth of 10.0 mm from the surface is determined by a Vickers hardness test based on JIS Z 2244 (2009) using a Vickers hardness tester. The test force is 0.49N. The Vickers hardness HV at 10 points is measured at the 10.0 mm depth position, and the average value is defined as the Vickers hardness HV at the 10.0 mm depth position.

Carburized steel parts are applied, for example, as mechanical structural parts used in mining machines, construction machines, automobiles and the like. Mechanical structural parts are, for example, gears, shafts, pulleys and the like.

[Manufacturing method of steel materials for carburized steel parts]
An example of a method for manufacturing a steel material for carburized steel parts according to this embodiment will be described. If the steel material for carburized steel parts of the present embodiment has the above configuration, the manufacturing method is not limited to the following manufacturing method. However, the manufacturing method described below is a preferable example of manufacturing the steel material for carburized steel parts of the present embodiment.

An example of the method for producing a steel material for carburized steel parts of the present embodiment includes a material preparation step and a hot working step. Hereinafter, each step will be described.

[Material preparation process]
In the material preparation step, a material having a chemical composition satisfying the above formulas (1) to (4) is prepared. The material is produced, for example, by the following method. A molten steel having a chemical composition satisfying the above formulas (1) to (4) is produced. A material (slab or ingot) is manufactured by a casting method using the molten steel. For example, a slab (bloom) is produced by a well-known continuous casting method using the molten steel. Alternatively, the ingot is manufactured by a well-known ingot forming method using the molten steel.

[Hot working process]
In the hot working process, the material (bloom or ingot) prepared in the material preparation process is hot-worked to produce a steel material for carburized steel parts. The shape of the steel material for carburized steel parts is not particularly limited. The steel material for carburized steel parts is, for example, steel bar or wire rod. In the following description, as an example, a case where the steel material for carburized steel parts is bar steel will be described. However, even if the steel material for carburized steel parts has a shape other than bar steel (wire rod, etc.), it can be manufactured by the same hot working process.

The hot working step includes a rough rolling step and a finish rolling step. In the rough rolling process, the material is hot-rolled to produce billets. For the rough rolling process, for example, a bulk rolling mill is used. Billets are manufactured by performing slab rolling on the material with a slab rolling mill. When a continuous rolling mill is installed downstream of the ingot rolling mill, hot rolling is further performed on the billet after the ingot rolling using the continuous rolling mill to produce a smaller billet. You may. In a continuous rolling mill, horizontal stands having a pair of horizontal rolls and vertical stands having a pair of vertical rolls are alternately arranged in a row. Through the above steps, the material is manufactured into billets in the rough rolling step. The heating temperature in the heating furnace in the rough rolling step is not particularly limited, but is, for example, 1100 to 1300 ° C.

In the finish rolling process, the billet is first heated using a heating furnace. The billets after heating are hot-rolled using a continuous rolling machine to produce steel bars, which are steel materials for carburized steel parts. The heating temperature in the heating furnace in the finish rolling step is not particularly limited, but is, for example, 1000 to 1250 ° C. Further, in finish rolling, the temperature of the steel material on the outlet side of the rolling stand where the final rolling is performed is defined as the finish temperature. At this time, the finishing temperature is, for example, 800 to 1000 ° C. The finishing temperature is measured, for example, by a temperature gauge installed on the outlet side of the rolling stand where the final rolling was performed.

The steel material after the finish rolling process is cooled at a cooling rate equal to or lower than the cooling rate to produce the steel material for carburized steel parts of the present embodiment. Specifically, the average cooling rate in the temperature range where the steel material temperature is 800 to 500 ° C. is preferably more than 0 to 1.3 ° C./sec. If the average cooling rate in the temperature range where the steel material temperature is 800 ° C to 500 ° C is more than 0 to 1.3 ° C / sec, the total area ratio of ferrite and pearlite in the microstructure is 85.0 to 100.0%. It becomes.

The average cooling rate is measured by the following method. The steel material after finish rolling is transported downstream on a transfer line. A plurality of temperature gauges are arranged along the transport line on the transport line, and it is possible to measure the temperature of the steel material at each position of the transport line. Based on a plurality of temperature gauges arranged along the transport line, the time until the steel material temperature reaches 800 ° C. to 500 ° C. is obtained, and the average cooling rate (° C./sec) is obtained. For example, the average cooling rate can be adjusted by arranging a plurality of slow cooling covers on the transport line at intervals.

Through the above manufacturing process, the steel material for carburized steel parts of the present embodiment having the above configuration can be manufactured.

[Manufacturing method of carburized steel parts]
Next, an example of a method for manufacturing carburized steel parts according to the present embodiment will be described. This manufacturing method includes a cold forging step of cold forging the above-mentioned steel material for carburized steel parts to manufacture an intermediate member, a cutting process of cutting the intermediate member as necessary, and an intermediate member. On the other hand, the carburizing treatment step of carrying out the carburizing treatment or the carburizing nitriding treatment and the tempering step are included.

[Cold forging process]
In the cold forging step, an intermediate member is manufactured by performing cold forging as cold working on the steel material for carburized steel parts manufactured by the above-mentioned manufacturing method. The plastic working conditions such as the working rate and the strain rate in the cold forging step are not particularly limited, and suitable conditions may be appropriately selected.

[Cutting process]
The cutting process is carried out as necessary. That is, the cutting process does not have to be performed. When it is carried out, in the cutting process, the cutting process is performed on the intermediate member after the cold forging process and before the carburizing process described later. By performing the cutting process, it is possible to impart a precise shape to the carburized steel part, which is difficult only by the cold forging process.

[Carburizing process]
In the carburizing process, the intermediate member after the cutting process is carburized. Here, in the present embodiment, the carburizing treatment includes not only the carburizing treatment but also the carburizing nitriding treatment. In the carburizing process, a well-known carburizing process is carried out. The carburizing step includes a carburizing step, a diffusion step, and a quenching step.

The carburizing treatment conditions in the carburizing step and the diffusion step may be appropriately adjusted. The carburizing temperature in the carburizing step and the diffusion step is, for example, 830 ° C to 1100 ° C. The carbon potential in the carburizing step and the diffusion step is, for example, 0.5% to 1.2%. The holding time of the carburizing step is, for example, 60 minutes or more, and the holding time of the diffusion step is, for example, 30 minutes or more. It is preferable that the carbon potential in the diffusion step is lower than the carbon potential in the carburizing step. However, the carburizing treatment conditions are not limited to this.

After the diffusion step, a well-known quenching step is carried out. In the quenching step, the intermediate member after the diffusion step is held at a quenching temperature _{equal to or higher than the Ar3 transformation point.} The holding time at the quenching temperature is not particularly limited, but is, for example, 30 to 60 minutes. Preferably, the quenching temperature is lower than the carburizing temperature. The temperature of the quenching medium is preferably room temperature to 250 ° C. The quenching medium is, for example, water or oil. Further, if necessary, subzero treatment may be carried out after quenching.

[Tempering process]
A well-known tempering process is carried out on the intermediate member after the carburizing process. The tempering temperature is, for example, 100 to 250 ° C. The holding time at the tempering temperature is, for example, 30 to 150 minutes.

[Other processes]
If necessary, the carburized steel part after the finish heat treatment step may be further subjected to a grinding process or a shot peening process. By performing the grinding process, a precise shape can be imparted to the carburized steel part. Further, by performing the shot peening treatment, compressive residual stress is introduced into the surface layer portion of the carburized steel part. Compressive residual stress suppresses the generation and growth of fatigue cracks. Therefore, the fatigue strength of carburized steel parts is increased. For example, when the carburized steel part is a gear, the fatigue strength of the tooth root and the tooth surface of the carburized steel part can be improved. The shot peening process may be carried out by a well-known method.

The effects of one aspect of the present invention will be described more specifically by way of examples. The conditions in the following examples are one condition example adopted for confirming the feasibility and effect of the steel material for carburized steel parts of the present embodiment. Therefore, the present invention is not limited to this one-condition example. The present invention may adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

Molten steel having the chemical composition shown in Table 1 was prepared.

Blanks in Table 1 mean that the content of the corresponding element was below the detection limit. That is, it means that the corresponding element content was 0%. A slab was produced by a continuous casting method using the molten steel. After heating this slab, bulk rolling, which is a rough rolling step, and subsequent rolling by a continuous rolling mill were carried out to produce billets having a cross section of 162 mm × 162 mm perpendicular to the longitudinal direction. The heating temperature in the ingot rolling was 1200 to 1250 ° C.

Using the manufactured billets, a finish rolling process was carried out to produce steel bars (steel materials for carburized steel parts) having a diameter of 80 mm. The heating temperature in the heating furnace of each test number in the finish rolling step was 103 to 1150 ° C. The holding time in the heating furnace was 1.5 to 3.0 hours in all the test numbers. The finishing temperature of each test number was 825 to 860 ° C. The average cooling rate in the steel material temperature range of 800 to 500 ° C. was 0.8 to 1.3 ° C./sec. Through the above manufacturing process, steel materials (steel bars) for carburized steel parts of each test number were manufactured.

[Evaluation test]
[Microstructure observation test]
A sample for microstructure observation was taken from the R / 2 position of the steel material for carburized steel parts of each test number. Of the surfaces of the sample, the surface corresponding to the cross section perpendicular to the longitudinal direction of the steel bar was used as the observation surface. After mirror polishing the observation surface, the observation surface was etched with 2% alcohol nitrate (Nital corrosive liquid). The etched observation surface was observed using a 500x optical microscope to generate a photographic image of any 20 fields of view. The size of each field of view was 100 μm × 100 μm. Each phase of ferrite, pearlite, etc. has a different contrast for each phase. Therefore, each phase was identified based on the contrast. Of the identified phase, the total area of the ferrite in the field of view ([mu] m ^2), and to determine the total area of perlite (μm ^2). The ratio of the total area of the total area of ferrite and the total area of pearlite in all fields of view to the total area of all fields of view was defined as the total area ratio (%) of ferrite and pearlite. As a result of the measurement, the ferrite and pearlite area ratios of each test number were 85.0% or more.

[Limited compression test]
A limit compression test was carried out as an evaluation test of the cold forging property of steel materials for carburized steel parts. Specifically, a plurality of limit compression test pieces were collected from the steel material (steel bar) for carburized steel parts of each test number. The limit compression test piece had a diameter of 6 mm and a length of 9 mm. The longitudinal direction of the critical compression test piece was parallel to the longitudinal direction of the steel bars of each test number. Further, the central axis of the critical compression test piece corresponded to the R / 2 position of the steel bar of each test number. A notch was formed in the circumferential direction at the center position in the longitudinal direction of the test piece. The notch angle was 30 degrees, the notch depth was 0.8 mm, and the radius of curvature of the notch tip was 0.15 mm.

A 500 ton hydraulic press was used for the limit compression test. The limit compression test piece was subjected to the limit compression test by the following method. Each test piece was cold compressed using a restraint die at a rate of 10 mm / min. Compression was stopped when microcracks of 0.5 mm or more occurred in the vicinity of the notch, and the compression rate (%) at that time was calculated. This measurement was performed a total of 10 times to obtain a compression rate (%) at which the cumulative failure probability was 50%, and the compression rate was defined as the limit compression rate (%). Table 2 shows the critical compression ratio (%) of each test number. Since the limit compression rate of the conventional steel material for carburized steel parts is about 65%, it is judged that the limit processing rate is excellent when it is 68% or more, which can be regarded as a value clearly higher than this value. For test numbers with a critical compression ratio of less than 68%, the evaluation test and fatigue test of carburized steel parts were not carried out.

[Carburized steel parts evaluation test]
Carburized steel parts were manufactured from the steel materials for carburized steel parts of each test number by the following method. A test piece having a diameter of 26 mm and a length of 150 mm was collected from the steel bars of each test number. The center of the test piece almost coincided with the center of the steel bar of each test number. The collected test pieces were carburized by the metamorphic furnace gas method (gas carburizing treatment). As shown in FIG. 2, in the gas carburizing treatment, the carbon potential was set to 0.8%, and the carburizing step was held at 950 ° C. for 5 hours (the carburizing step was carried out at 950 ° C. for 240 minutes, and the diffusion step was held at 950 ° C. for 60 minutes). Subsequently, it was held at a quenching temperature of 850 ° C. for 30 minutes. After the above steps, the test piece was immersed in an oil tank at 130 ° C. and oil-quenched. The hardened test piece was tempered at 150 ° C. for 90 minutes to produce a carburized steel part.

The following measurements were carried out for the carburized layer and core of the carburized steel parts of each test number. Specifically, on the cut surface perpendicular to the longitudinal direction of the carburized steel part of each test number, the Vickers hardness at a depth of 50 μm from the surface and the Vickers hardness at a depth of 0.4 mm from the surface are defined as the Micro Vickers hardness. It was determined by a Vickers hardness test according to JIS Z 2244 (2009) using a meter. The test force was 0.49N. The Vickers hardness HV at 10 points at a depth of 50 μm was measured, and the average value was taken as the Vickers hardness HV at a depth of 50 μm. Further, the Vickers hardness HV at 10 points at a depth of 0.4 mm was measured, and the average value was taken as the Vickers hardness HV at a depth of 0.4 mm.

If the hardness at a depth of 0.4 mm from the surface is 550 HV or more, it is determined that the carburized layer exists up to at least 0.4 mm from the surface. Further, when the Vickers hardness at a depth of 50 μm from the surface was 650 HV or more, it was determined that the hardness of the carburized layer of the carburized steel part was sufficient. The measurement results are shown in Table 2.

The Vickers hardness and chemical composition of the core of the carburized steel part were measured by the following methods. The Vickers hardness at a depth of 10.0 mm from the surface of the cut surface perpendicular to the longitudinal direction of the carburized steel part was determined by a Vickers hardness test in accordance with JIS Z 2244 (2009) using a Vickers hardness tester. .. The test force was 0.49N. The measurement was performed 10 times at the 10.0 mm depth position, and the average value was taken as the Vickers hardness (HV) at the 10.0 mm depth position from the surface. The obtained Vickers hardness is shown in Table 2. When the Vickers hardness at the 10.0 mm depth position was 300 to 500 HV, it was determined that the core hardness was sufficiently high.

In addition, the chemical composition at a depth of 10.0 mm from the surface was quantitatively analyzed for elements having an atomic number of 5 or more using EPMA (electron probe microanalyzer). As a result, the chemical composition at a depth of 10.0 mm from the surface was substantially the same as the chemical composition shown in Table 1 for all test numbers.

[Surface fatigue strength test]
A steel bar having a diameter of 80 mm for each test number was machined to prepare a roller pitching small roller test piece (the unit of dimension in the figure is mm, hereinafter simply referred to as a small roller test piece) shown in FIG. The small roller test piece shown in FIG. 1 was provided with a test portion (cylindrical portion having a diameter of 26 mm and a width of 28 mm) in the center.

Carburizing treatment conditions were carried out for each of the produced test pieces using a gas carburizing furnace under the conditions shown in FIG. The hardened test piece was tempered at 150 ° C. for 90 minutes to prepare a test piece simulating a carburized steel part.

[Surface fatigue strength test]
In the roller pitching test, a small roller test piece having the shape shown in FIG. 1 and a large roller having the shape shown in FIG. 3 (the unit of dimension in the figure is mm) were combined. The large roller shown in FIG. 3 is made of steel that meets the JIS standard SCM420, and is a general manufacturing process, that is, normalizing, test piece processing, eutectoid carburizing by a gas carburizing furnace, low temperature tempering and polishing. Made by.

A roller pitching test using a small roller test piece and a large roller was performed under the conditions shown in Table 3.

As shown in Table 3, the rotation speed of the small roller test piece is 1000 rpm, the slip ratio is -40%, the contact surface pressure (maximum surface pressure) between the large roller and the small roller test piece under test is 4000 MPa, and the number of repetitions is set. It was 2.0 × 10 ⁷ times. When the rotation speed of the large roller was V1 (m / sec) and the rotation speed of the small roller test piece was V2 (m / sec), the slip ratio (%) was calculated by the following formula.
Slip rate = (V2-V1) / V2 × 100

During the test, a lubricant (commercially available oil for automatic transmission) was sprayed on the contact portion (surface of the test portion) between the large roller and the small roller test piece from the direction opposite to the rotation direction under the condition of an oil temperature of 90 ° C. A roller pitching test was carried out under the above conditions to evaluate the surface fatigue strength.

For each steel number, the number of tests in the roller pitching test was 6. After the test, an SN diagram was created in which the vertical axis represents the surface pressure and the horizontal axis represents the number of repetitions until the occurrence of pitching. Among those in which pitching did not occur up to 2.0 × 10 ⁷ repetitions, the highest surface pressure was defined as the surface fatigue strength of the steel number. Among the damaged parts on the surface of the small roller test piece, the case where the area of the largest one was 1 mm ² or more was defined as pitching occurrence.

Table 2 shows the surface fatigue strength obtained by the test. Regarding the surface fatigue strength in Table 2, surface fatigue of a steel material having a chemical composition satisfying the SCM420 standard of JIS G4053 (2008) was carburized and hardened under the carburizing treatment conditions and quenching conditions shown in FIG. 2 (test number 28). The strength was set as a reference value (100%). Then, the surface fatigue strength of each test number was shown as a ratio (%) with respect to the reference value. When the surface fatigue strength ratio was 120% or more, it was judged that excellent surface fatigue strength was obtained.

[Hydrogen embrittlement resistance evaluation test]
An annular V-notch test piece shown in FIG. 4 was produced by machining a steel bar having a diameter of 80 mm for each test number. The numerical value in which the unit is not shown in FIG. 4 indicates the dimension (unit is mm) of the corresponding portion of the test piece. The "φ value" in the figure indicates the diameter (mm) of the designated part. “60 °” indicates that the V notch angle is 60 °. "0.175R" indicates that the V-notch bottom radius is 0.175 mm. The longitudinal direction of the annular V-notch test piece was parallel to the longitudinal direction of the steel bar. Further, the central axis of the annular V-notch test piece substantially coincided with the R / 2 position of the steel bar.

The produced annular V-notch test piece was carburized under the conditions shown in FIG. 2 using a gas carburizing furnace. The hardened test piece was tempered at 150 ° C. for 90 minutes to prepare a test piece simulating a carburized steel part.

Using the electrolytic charging method, hydrogen of various concentrations was introduced into the test piece for each test number. The electrolytic charge method was carried out as follows. The test piece was immersed in an aqueous solution of ammonium thiocyanate. With the test piece immersed, hydrogen was taken into the test piece by generating an anode potential on the surface of the test piece.

After introducing hydrogen into the test piece, a zinc-plated film was formed on the surface of the test piece to prevent the dissipation of hydrogen in the test piece. Subsequently, a constant load test was carried out in which a constant load was applied so that a tensile stress of a nominal stress of 1080 MPa (90% of the tensile strength) was applied to the V-notch cross section of the test piece. The amount of hydrogen in the test piece was measured by performing a temperature rise analysis method using a gas chromatograph on the test piece that broke during the test and the test piece that did not break. After the measurement, in each test number, the maximum amount of hydrogen in the unbroken test piece was defined as the critical diffusible hydrogen amount Hc.

Further, the limit diffusible hydrogen amount in the steel material (test number 28) obtained by carburizing a steel material having a chemical composition satisfying the SCM420 standard of JIS G4053 (2008) was used as the standard (Href) for the limit diffusible hydrogen amount ratio HR. .. The limit diffusible hydrogen amount ratio HR was determined using the formula (A) with the limit diffusible hydrogen amount Href as a reference.
HR = Hc / Href (A)
When the critical diffusible hydrogen amount ratio HR was 1.10 or more, it was judged that the hydrogen embrittlement resistance was excellent.

[Test results]
With reference to Tables 1 and 2, the chemical composition of the steel materials for carburized steel parts of test numbers 1 to 16 was within the range of the chemical composition of the present embodiment, and the formulas (1) to (4) were satisfied. .. As a result, the limit compressibility was 68% or more, showing a sufficient limit processing rate. Further, the surface fatigue strength ratio of the steel material after the carburizing treatment was 120% or more, and the steel material had excellent surface fatigue strength. Further, the critical diffusible hydrogen content ratio HR of the steel material after the carburizing treatment was 1.10 or more, showing excellent hydrogen embrittlement resistance. In the carburized steel parts, the carburized layer had a depth of at least 0.4 mm or more. The Vickers hardness of the carburized layer at a depth of 50 μm is 650 to 1000 HV, and the Vickers hardness of the core at a depth of 10.0 mm is 300 to 500 HV, and both the carburized layer and the core are sufficient. Hardness.

On the other hand, in test number 17, the C content was too low. As a result, the hardness of the core of the carburized steel part was too low.

In test number 18, the C content was too high. Therefore, the limit processing rate of the steel material for carburized steel parts was less than 68%, and the cold forging property was low.

In test number 19, the Si content was too low. Therefore, the surface fatigue strength ratio was less than 120%.

In test number 20, the Si content was too high. Therefore, the limit processing rate of the steel material for carburized steel parts was less than 68%, and the cold forging property was low.

In test number 21, the Mn content was too low. As a result, the hardness of the core of the carburized steel part was too low.

In test number 22, the Mn content was too high. As a result, the critical diffusible hydrogen amount ratio HR was less than 1.10, and the hydrogen embrittlement resistance was low.

In test number 23, F3 was below the lower limit of equation (3). As a result, the critical diffusible hydrogen amount ratio HR was less than 1.10, and the hydrogen embrittlement resistance was low.

In test number 24, F2 was below the lower limit of equation (2). As a result, the surface fatigue strength ratio was less than 120%.

In test number 25, F2 exceeded the upper limit of equation (2). As a result, the limit processing rate of the steel material for carburized steel parts was less than 68%, and the cold forging property was low.

In test number 26, F4 was below the lower limit of equation (4). As a result, the limit processing rate of the steel material for carburized steel parts was less than 68%, and the cold forging property was low.

In test number 27, F4 exceeded the upper limit of equation (4). As a result, the surface fatigue strength ratio was less than 120%.

In test number 29, F1 was below the lower limit of equation (1). As a result, the hardness of the core of the carburized steel part was too low.

In test number 30, F1 exceeded the upper limit of equation (1). As a result, the limit processing rate of the steel material for carburized steel parts was less than 68%, and the cold forging property was low.

Although the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.

Claims (2)

The chemical composition is mass%,
C: 0.05 to less than 0.10%,
Si: 0.50 to 0.75%,
Mn: 0.25 to 0.55%,
S: 0.005 to 0.050%,
Cr: 1.30 to less than 2.00%,
Mo: 0.25 to 0.40%,
B: 0.0005 to 0.0100%,
Al: 0.1000 to 0.150%,
Ca: 0.0002 to 0.0030%,
Ti: less than 0.0200%,
N: 0.0080% or less,
P: 0.050% or less, and
O: Contains 0.0030% or less, the balance is composed of Fe and impurities, and satisfies the formulas (1) to (4).
Steel material for carburized steel parts.
0.195 <C + 0.194 x Si + 0.065 x Mn + 0.012 x Cr + 0.033 x Mo + 0.067 x Ni + 0.097 x Cu + 0.078 x Al <0.235 (1)
0.0003 <Al × (N-Ti × (14/48)) <0.0011 (2)
Si / Mn> 1.00 (3)
0.070 <C / Si <0.180 (4)
Here, the content (mass%) of the corresponding element is substituted for each element symbol of the formulas (1) to (4). The steel material for carburized steel parts according to claim 1.
The chemical composition is
Nb: 0.100% or less,
V: 0.200% or less,
Ni: 0.500% or less and
Cu: 0.500% or less,
Contains one or more elements selected from the group consisting of
Steel material for carburized steel parts.

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