JP6922415B2 - Carburized parts - Google Patents

Description

The present invention relates to a carburized part, which is a carburized part.

With the recent increase in engine output and miniaturization of parts, it is required to increase the strength of parts. In particular, transfer gear parts and the like used for gear parts of automobiles are subjected to a shocking load at the time of extreme shifting, sudden start and stop of the car, and when the car rides on a step on the road surface. For this reason, these parts may be destroyed by an impact (low cycle impact fatigue) with a very small number of repetitions of several tens to several hundreds. Therefore, the parts used for these applications are required to have resistance to impact fatigue fracture in a low cycle (hereinafter referred to as low cycle impact fatigue characteristics).

Most transfer gear parts are manufactured by machining a steel material into a predetermined shape and then performing a carburizing and quenching treatment. In this case, most of the steel materials used are alloy steel materials for machine structure specified in JIS G 4053 (2008), such as SCr420 and SCM420. Therefore, it is required to manufacture carburized parts using steel materials having a chemical composition similar to those of alloy steel materials for machine structural use to improve low-cycle impact fatigue characteristics.

By the way, in the conventional carburized parts, the C concentration on the surface is generally set to about 0.8% as disclosed in Japanese Patent Application Laid-Open No. 10-8199 (Patent Document 1). This is because it is considered that if the C concentration on the surface is less than 0.8%, the hardness of the surface layer of the carburized parts is lowered and sufficient fatigue strength cannot be obtained. The evaluation of such fatigue strength, fatigue strength test at 10 ⁷ times higher cycle is utilized. For this reason, not be sufficient verification for 10 to 10 ⁴ times of low cycle impact fatigue characteristics.

Techniques for improving the low-cycle impact fatigue characteristics of carburized parts are described in JP-A-2007-332438 (Patent Document 2), JP-A-2011-63886 (Patent Document 3), and International Publication No. 2010/137607 (Patent Document 2). It is proposed in Document 4).

The carburized and hardened steel material described in Patent Document 2 has C: 0.1 to 0.4%, Si: 0.02 to 1.3%, Mn: 0.3 to 1.8%, Al in mass%. : 0.001 to 0.05% and N: 0.003 to 0.020%, the balance is composed of iron and unavoidable impurities, and the hardness of the projection core defined by the following equation (1) It is characterized in that Hp-core is HV390 or more.
Hp-core = Hcore / (1-t / r) ・ ・ ・ (1)
However, Hcore; core hardness, t; effective hardened layer depth, r; radius of the broken part or half the wall thickness of the broken part.
It is described in Patent Document 2 that this makes it possible to stably improve low-cycle impact fatigue characteristics in carburized and hardened steel materials and carburized and hardened parts.

The carburized and hardened steel material described in Patent Document 3 has C: 0.1 to 0.4%, Si: 0.02 to 1.3%, Mn: 0.3 to 1.8%, Al in mass%. : 0.001 to 0.05%, N: 0.003 to 0.020%, and Cr: 1.8% or less, the balance is composed of iron and unavoidable impurities, from the hardened end in the Jomini test. The hardness at the position of 13 mm is 60 × C ^0.5-5 (HRC) or more, and A defined by the following equation (2) and B defined by the following equation (3) are A-0.00000293. It is characterized by having a relationship of × B ≧ -14.
A = Mo + 0.227Ni + 190B-0.087Si-17.2P-2.74V-7.18Cs-0.00955Hs + 0.0344Nγ ... (2)
However, Cs; carburized concentration (mass%) of the surface layer, Hs; surface hardness (HV), Nγ; former austenite crystal grain size of the carburized layer.
B = t × (Hcore) ² ... (3)
However, Hcore; core hardness, t; effective hardened layer depth.
It is described in Patent Document 3 that this makes it possible to stably improve low-cycle impact fatigue characteristics in carburized and hardened steel materials and carburized and hardened parts.

In the steel carburized parts described in Patent Document 4, the base steel is mass%, C: 0.15 to 0.25%, Si: 0.03 to 0.50%, Mn: 0.60. It contains more than% and 1.5% or less, Cr: 0.05 to 2.0%, Al: 0.10%, N: 0.03% or less and O: 0.0020% or less, and the balance is Fe. It is a steel having a chemical composition composed of and impurities, and is characterized in that the surface-hardened layer portion satisfies the following conditions (a) to (c).
(A) C (ave): 0.35 to 0.60% by mass,
(B) Surface roughness Rz: 15 μm or less, and
(C) σr (0): −800 MPa or less, σr (100): −800 MPa or less, and residual stress strength index Ir: 80,000 or more.
However,
C (ave): Average carbon concentration from the outermost surface to a depth of 0.2 mm,
σr (0): compressive residual stress on the outermost surface of the part,
σr (100): compressive residual stress at a position 100 μm from the outermost surface of the part,
Residual stress strength index Ir: The depth from the outermost surface at a position from the outermost surface to a depth of 100 μm is yμm, and the residual stress at that site is σr (y) [Ir = ∫ | σr (y) | dy] Refers to the value represented by.
It is described in Patent Document 4 that this makes it possible to obtain a carburized part in which the fatigue strength in the "low to medium cycle range" is significantly improved as compared with the conventional carburized-quenched-tempered part.

Japanese Unexamined Patent Publication No. 10-8199 Japanese Unexamined Patent Publication No. 2007-332438 Japanese Unexamined Patent Publication No. 2011-63886 International Publication No. 2010/137607

By the way, the tooth surfaces of these parts are constantly sliding. Therefore, the surface of these parts may be partially peeled off due to repeated friction, resulting in fracture called pitching. Therefore, in addition to the above-mentioned low-cycle impact fatigue characteristics, these parts are required to have resistance to pitching (hereinafter referred to as wear resistance).

As mentioned above, the low cycle impact fatigue characteristics have not been sufficiently verified. Furthermore, the technology for improving both low-cycle impact fatigue characteristics and wear resistance has not been sufficiently verified. Therefore, even if the above technique is used, the low cycle impact fatigue characteristics and wear resistance of the carburized parts may not be sufficiently obtained.

An object of the present invention is to provide a carburized component having excellent low cycle impact fatigue characteristics and wear resistance.

In the carbonized parts according to the present embodiment, the chemical composition of the core portion is mass%, C: 0.10 to 0.30%, Si: 0.50 to 1.50%, Mn: 0.30 to 1.40%. , P: less than 0.030%, S: less than 0.030%, Cr: 0.50 to 2.00%, Al: 0.010 to 0.100%, N: 0.001 to 0.030%, Mo: 0 to 0.80%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, Ti: 0 to 0.10%, and Nb: 0 to 0.10%. The balance is composed of Fe and impurities, satisfies the formula (1), has a C concentration of 0.50 to 0.70% on the surface, and is the distance from the surface to a position where the critical hardness is 550 HV in Vickers hardness. The effective cured layer depth is 0.30 to 0.60 mm.
X + Y + Z ≦ 26 (1)
However,
^{X = -15.7 × Si 4 + 74.4} × Si 3 -118.9 × Si 2 + 61.6 × Si + 5.8 (2)
Y = -0.3 x Mn ³ -3 x Mn ² + 14.2 x Mn-3.4 (3)
^{Z = -0.2 × Cr 4 + 4.4} × Cr 3 -16.6 × Cr 2 + 17 × Cr + 3.1 (4)
Here, the content (mass%) of the corresponding element is substituted for the element symbol of the formulas (2) to (4).

The carburized parts according to this embodiment have excellent low cycle impact fatigue characteristics and wear resistance.

FIG. 1 is a diagram showing an example of a heat pattern of gas carburizing and quenching treatment. FIG. 2 is a diagram showing an example of a heat pattern of vacuum carburizing and quenching treatment. FIG. 3 is a side view of the drop weight test piece produced in the example. FIG. 4 is a plan view of the roller pitching test piece produced in the examples. FIG. 5 is a schematic view of a roller pitching test.

The present inventors investigated and investigated the low-cycle impact fatigue characteristics and wear resistance of carburized parts. As a result, the present inventors obtained the following findings. In this specification, the low cycle impact fatigue refers fatigue strength when given a shocking load exceeding the yield stress of the material 10 to 10 ⁴ times.

Repeated use of carburized parts causes cracks on the surface of the carburized parts (crack generation process). The crack grows and eventually the carburized part breaks (breaking process). That is, the low cycle impact fatigue characteristic is divided into the life of the carburized part for the crack generation process (crack generation life) and the life of the carburized part for the fracture process (fracture life). The characteristics required for longer life differ between the crack generation life and the fracture life.

In the crack generation process, cracks are generated on the surface layer of the carburized part. In order to suppress the occurrence of cracks, it is effective to reduce the C concentration on the surface of the carburized part to increase the toughness of the surface layer of the carburized part. Low cycle impact fatigue is fatigue accompanied by plastic deformation. If the toughness of the surface layer of the carburized part is high, the amount of plastic deformation that can be tolerated before the occurrence of cracks is large. Therefore, the occurrence of cracks on the surface layer of the carburized parts can be suppressed. Specifically, when the C concentration on the surface of the carburized part is 0.50 to 0.70%, the C concentration on the surface is sufficiently low, so that the generation of cracks in the crack generation step can be suppressed. In this case, the low cycle impact fatigue characteristic is enhanced.

The generation of cracks in the crack generation step can also be suppressed by increasing the effective hardened layer depth (Effective Case Depth: hereinafter, ECD) of the carburized parts.

The generation of cracks in the crack generation step can be suppressed by the above-mentioned method. However, the fracture process has a greater effect on the low-cycle impact fatigue characteristics than the crack generation process.

In the low cycle impact fatigue test, when an initial crack occurs, the initial crack depth is approximately equal to the effective hardened layer depth (ECD). The shallower the ECD, the shallower the initial crack depth. In this case, the crack growth rate is suppressed. Therefore, it is effective to make the ECD shallow for the breaking step. If the ECD is shallow, the fracture process, which accounts for most of the low-cycle impact fatigue characteristics, is extended, and the low-cycle impact fatigue characteristics of the carburized parts are enhanced.

Therefore, in order to enhance the low cycle impact fatigue characteristics, there is an optimum ECD range. Specifically, if the ECD of the carburized part is 0.30 to 0.60 mm, which is shallower than the conventional one, excellent low cycle impact fatigue characteristics can be obtained.

On the other hand, in order to increase the wear resistance of the carburized parts, it is effective to increase the surface hardness of the carburized parts. Specifically, the Si content of the steel material is set to 0.50 to 1.50% to increase the temper softening resistance. As a result, even if heat is generated due to friction during use, a decrease in surface hardness of the carburized part is suppressed. As a result, the wear resistance of the carburized parts can be improved.

The higher the Si content, the higher the wear resistance of the carburized parts. However, Si is oxidized to form an intergranular oxide layer. Therefore, the higher the Si content, the thicker the intergranular oxide layer. In this case, the low cycle impact fatigue characteristics and wear resistance of the carburized parts are deteriorated.

Therefore, the chemical composition of the core of the carburized part is set to the chemical composition in which the intergranular oxide layer becomes thin. Specifically, the chemical composition of the core of the carburized part satisfies the formula (1).
X + Y + Z ≦ 26 (1)
However,
^{X = -15.7 × Si 4 + 74.4} × Si 3 -118.9 × Si 2 + 61.6 × Si + 5.8 (2)
Y = -0.3 x Mn ³ -3 x Mn ² + 14.2 x Mn-3.4 (3)
^{Z = -0.2 × Cr 4 + 4.4} × Cr 3 -16.6 × Cr 2 + 17 × Cr + 3.1 (4)
Here, the content (mass%) of the corresponding element is substituted for the element symbol of the formulas (2) to (4).

If the chemical composition of the core of the carburized part satisfies the formula (1), the intergranular oxide layer of the carburized part becomes thin even though it contains a large amount of Si. This enhances the low cycle impact fatigue characteristics and wear resistance of the carburized parts.

The carbonized parts of the present embodiment completed based on the above findings have a core chemical composition of mass%, C: 0.10 to 0.30%, Si: 0.50 to 1.50%, Mn: 0.30 to 1.40%, P: less than 0.030%, S: less than 0.030%, Cr: 0.50 to 2.00%, Al: 0.010 to 0.100%, N: 0 .001 to 0.030%, Mo: 0 to 0.80%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, Ti: 0 to 0.10%, and Nb: 0 to 0 It contains 0.10%, the balance is composed of Fe and impurities, satisfies the formula (1), the C concentration on the surface is 0.50 to 0.70%, and the limit hardness from the surface is 550 HV in Vickers hardness. The effective cured layer depth, which is the distance to the position, is 0.30 to 0.60 mm.
X + Y + Z ≦ 26 (1)
However,
^{X = -15.7 × Si 4 + 74.4} × Si 3 -118.9 × Si 2 + 61.6 × Si + 5.8 (2)
Y = -0.3 x Mn ³ -3 x Mn ² + 14.2 x Mn-3.4 (3)
^{Z = -0.2 × Cr 4 + 4.4} × Cr 3 -16.6 × Cr 2 + 17 × Cr + 3.1 (4)
Here, the content (mass%) of the corresponding element is substituted for the element symbol of the formulas (2) to (4).

The carburized parts of this embodiment are excellent in low cycle impact fatigue characteristics and wear resistance.

The chemical composition of the core of the carburized parts is selected from the group consisting of Mo: 0.01 to 0.80%, Ni: 0.05 to 0.50%, and Cu: 0.10 to 0.50%. It may contain one kind or two or more kinds. The chemical composition of the core of the carburized part also contains one or two selected from the group consisting of Ti: 0.05 to 0.10% and Nb: 0.01 to 0.10%. You may.

Hereinafter, the carburized parts of the present embodiment will be described in detail. The "%" of the content of each element means "mass%".

[Core and surface of carburized parts]
The carburized parts according to the present embodiment include a core portion and a surface layer. The core portion means a portion of the carburized part inside the surface layer. More specifically, an internal portion deeper than 2.0 mm from the surface of the carburized part is defined as a core portion. The portion within 2.0 mm from the surface of the carburized part is defined as the surface layer.

[Chemical composition of the core of carburized parts]
The chemical composition of the core of the carburized parts contains the following elements.

C: 0.10 to 0.30%
Carbon (C) enhances the hardenability of steel and enhances the hardness of the core. This enhances the low cycle impact fatigue characteristics of carburized parts. If the C content is less than 0.10%, this effect cannot be obtained. On the other hand, if the C content exceeds 0.30%, the machinability and cold forging property of the steel may decrease. Therefore, the C content is 0.10 to 0.30%. The lower limit of the C content is preferably 0.15%, more preferably 0.18%. The preferred upper limit of the C content is 0.25%, more preferably 0.23%.

Si: 0.50 to 1.50%
Silicon (Si) deoxidizes steel. Si further enhances the hardenability of steel and further enhances the strength of steel by solid solution strengthening. Therefore, the hardness of the core portion is increased, and the low-cycle impact fatigue characteristics of the carburized parts are enhanced. Si also increases the temper softening resistance of steel. This enhances the wear resistance of the carburized parts. However, if the Si content is less than 0.50%, the above effect cannot be sufficiently obtained, and the wear resistance of the carburized parts is lowered. On the other hand, if the Si content exceeds 1.50%, carburizing of steel is inhibited. In this case, the low cycle impact fatigue characteristics of the carburized parts are reduced. Therefore, the Si content is 0.50 to 1.50%. The lower limit of the Si content is preferably 0.52%, more preferably 0.80%. The preferred upper limit of the Si content is 1.30%, more preferably 1.20%.

Mn: 0.30-1.40%
Manganese (Mn) deoxidizes steel. Mn further enhances the hardenability and strength of steel and enhances the low cycle impact fatigue properties of carburized parts. If the Mn content is less than 0.30%, this effect cannot be obtained. On the other hand, if the Mn content exceeds 1.40%, the amount of retained austenite becomes excessive, the surface hardness decreases, and the wear resistance of the carburized parts decreases. Therefore, the Mn content is 0.30 to 1.40%. The preferred lower limit of the Mn content is 0.50%, more preferably 0.70%. The preferred upper limit of the Mn content is 1.20%, more preferably 1.00%.

P: Less than 0.030% Phosphorus (P) is an impurity. P segregates at the austenite grain boundaries during carburizing, and lowers the grain boundary strength of the carburized layer. If the grain boundary strength of the carburized layer decreases, the low-cycle impact fatigue characteristics decrease. When the P content is less than 0.030%, not only the core portion but also the surface layer has a low P content. Therefore, the toughness of the surface layer is increased, and the occurrence of grain boundary cracks is suppressed. As a result, the low cycle impact fatigue characteristics are enhanced. Therefore, the P content is less than 0.030%. The preferred upper limit of the P content is 0.015%. The P content is preferably as low as possible.

S: Less than 0.030% Sulfur (S) is an impurity. S remains at the grain boundaries and lowers the grain boundary strength of the carburized layer. S further forms coarse MnS at the grain boundaries to reduce low cycle impact fatigue characteristics. Therefore, the S content is less than 0.030%. The preferred upper limit of the S content is 0.015%. The S content is preferably as low as possible.

Cr: 0.50 to 2.00%
Chromium (Cr) enhances hardenability of steel, increases core hardness, and enhances low-cycle impact fatigue characteristics. If the Cr content is less than 0.50%, this effect cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 2.00%, the intergranular oxide layer becomes thicker and the low-cycle impact fatigue characteristics deteriorate. Therefore, the Cr content is 0.50 to 2.00%. The lower limit of the Cr content is preferably 0.60%, more preferably 0.80%. The preferred upper limit of the Cr content is 1.85%, more preferably 1.70%.

Al: 0.010 to 0.100%
Aluminum (Al) deoxidizes steel. Al further combines with N in the steel to form AlN, which suppresses the coarsening of austenite grains during carburizing. If the coarsening of austenite grains is suppressed, the grain boundary area per volume of steel is maintained high. Cracks in the crack generation process start from grain boundaries. Therefore, if the grain boundary area per volume of steel is high, the load in the low cycle impact fatigue test is dispersed. Therefore, the generation of cracks in the crack generation process is suppressed, and the low-cycle impact fatigue characteristics of the carburized parts are enhanced. If the Al content is less than 0.010%, this effect cannot be obtained. On the other hand, if the Al content exceeds 0.100%, the above effect is saturated. Therefore, the Al content is 0.010 to 0.100%. The lower limit of the Al content is preferably 0.020%, more preferably 0.025%. The preferred upper limit of the Al content is 0.080%, more preferably 0.065%. In the chemical composition of the core of the carburized part of the present embodiment, the Al content means the total amount of Al contained in the steel material.

N: 0.001 to 0.030%
Nitrogen (N) combines with Ti, Al, V and Nb in steel to form nitrides and carbonitrides, and suppresses coarsening of austenite grains during carburizing. This enhances the low cycle impact fatigue characteristics of carburized parts. If the N content is less than 0.001%, a sufficient effect of suppressing coarsening cannot be obtained. On the other hand, if the N content exceeds 0.030%, the above effect is saturated. Therefore, the N content is 0.001 to 0.030%. The preferred lower limit of the N content is 0.005%, more preferably 0.008%. The preferred upper limit of the N content is 0.025%, more preferably 0.020%.

The rest of the chemical composition of the core of the carburized parts according to this embodiment consists of Fe and impurities. Here, the impurities are those that are mixed in from ore, scrap, or the manufacturing environment as a raw material when the carburized parts are industrially manufactured, and are in a range that does not adversely affect the carburized parts of the present embodiment. Means what is allowed in.

[About arbitrary elements]
The core of the carburized component of the present embodiment may further contain one or more selected from the group consisting of Mo, Ni and Cu instead of a part of Fe. These elements are arbitrary elements, and all of them enhance the hardenability of steel.

Mo: 0-0.80%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo enhances the hardenability of steel, increases the hardness of the core, and enhances the low cycle impact fatigue characteristics of carburized parts. Mo further enhances the toughness of the carburized layer. Mo also increases the temper softening resistance of carburized parts. These effects can be obtained if even a small amount of Mo is contained. However, if the Mo content exceeds 0.80%, these effects are saturated and the raw material cost becomes high. Therefore, the Mo content is 0 to 0.80%. The preferable lower limit of the Mo content for stably obtaining the above effect is 0.01%, and more preferably 0.10%. The preferred upper limit of the Mo content is 0.60%, more preferably 0.40%.

Ni: 0 to 0.50%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni enhances the hardenability of steel, increases the core height, and enhances the low cycle impact fatigue characteristics of carburized parts. Ni further shallows the intergranular oxide layer. This enhances the low cycle impact fatigue characteristics of carburized parts. Ni further enhances 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 and the workability decreases. Therefore, the Ni content is 0 to 0.50%. The preferable lower limit of the Ni content for stably obtaining the above effect is 0.05%, and more preferably 0.10%. The preferred upper limit of the Ni content is 0.45%, more preferably 0.40%.

Cu: 0 to 0.50%
Copper (Cu) is an optional element and may not be contained. When contained, Cu enhances the hardenability of steel, increases the hardness of the core, and enhances the low cycle impact fatigue characteristics of carburized parts. 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 is lowered. Therefore, the Cu content is 0 to 0.50%. The preferable lower limit of the Cu content for stably obtaining the above effect is 0.10%, and more preferably 0.15%. The preferred upper limit of the Cu content is 0.35%, more preferably 0.25%.

The core of the carburized component of the present embodiment may further contain one or two selected from the group consisting of Ti and Nb instead of a part of Fe. These elements are arbitrary elements, and all of them suppress the coarsening of crystal grains.

Ti: 0 to 0.10%
Titanium (Ti) is an optional element and may not be contained. When contained, Ti combines with C and S in the steel to form fine TiC and TiS, which suppresses the coarsening of austenite grains during carburizing. This enhances the low cycle impact fatigue characteristics of carburized parts. This effect can be obtained if even a small amount of Ti is contained. However, if the Ti content exceeds 0.10%, TiC becomes coarse and the toughness of the steel decreases. In this case, the low cycle impact fatigue characteristics of the carburized parts are reduced. Therefore, the Ti content is 0 to 0.10%. The preferable lower limit of the Ti content for stably obtaining the above effect is 0.05%, and more preferably 0.06%. The preferred upper limit of the Ti content is 0.08%, more preferably 0.07%.

Nb: 0 to 0.10%
Niobium (Nb) is an optional element and may not be contained. When contained, Nb combines with C and N in steel to form Nb carbonitrides (Nb (CN)) and suppresses coarsening of austenite grains during carburizing. This enhances the low cycle impact fatigue characteristics of carburized parts. This effect can be obtained if even a small amount of Nb is contained. However, if the Nb content exceeds 0.10%, the carburizing property is lowered. Therefore, the Nb content is 0 to 0.10%. The preferable lower limit of the Nb content for stably obtaining the above effect is 0.01%, more preferably 0.02%. The preferred upper limit of the Nb content is 0.07%, more preferably 0.05%.

[About the formula]
The chemical composition of the core of the carburized component of the present embodiment further satisfies the formula (1).
X + Y + Z ≦ 26 (1)
However,
^{X = -15.7 × Si 4 + 74.4} × Si 3 -118.9 × Si 2 + 61.6 × Si + 5.8 (2)
Y = -0.3 x Mn ³ -3 x Mn ² + 14.2 x Mn-3.4 (3)
^{Z = -0.2 × Cr 4 + 4.4} × Cr 3 -16.6 × Cr 2 + 17 × Cr + 3.1 (4)
Here, the content (mass%) of the corresponding element is substituted for the element symbol of the formulas (2) to (4).

Here, fn = X + Y + Z is defined. The larger fn, the thicker the intergranular oxide layer of the carburized parts. When fn is larger than 26, the intergranular oxide layer becomes too thick, and the low-cycle impact fatigue characteristics and wear resistance of the carburized parts deteriorate. Therefore, fn ≦ 26. The chemical composition of the core of the carburized component of the present embodiment satisfies the formula (1). Therefore, the intergranular oxide layer of the carburized part becomes thin even though it contains a large amount of Si. This enhances the low cycle impact fatigue characteristics of carburized parts. The preferred upper limit of fn is 21, and the more preferred upper limit is 19.

[C concentration on the surface of carburized parts]
C concentration on the surface of carburized parts: 0.50 to 0.70%
The C concentration on the surface of the carburized part (hereinafter referred to as the surface C concentration) is 0.50 to 0.70% in mass%. If the surface C concentration exceeds 0.70%, the toughness of the carburized layer is lowered, so that the crack generation life in the low cycle impact fatigue test is shortened. On the other hand, if the surface C concentration is less than 0.50%, the surface hardness of the carburized part is too low and the plastic deformation resistance is lowered. In this case, the amount of deformation of the carburized parts per load in the low cycle impact fatigue test becomes large. Therefore, the low-cycle impact fatigue characteristics of the carburized parts are reduced. If the surface hardness of the carburized part is low, the wear resistance is further lowered. Therefore, the surface C concentration is 0.50 to 0.70%. The preferred lower limit of the surface C concentration is 0.54%, more preferably 0.56%. The preferred upper limit of the surface C concentration is 0.66%, more preferably 0.64%.

The C concentration on the surface of the carburized part is measured by the following method. Select any 5 measurement positions on the surface of the carburized parts. The C concentration (mass%) at the selected measurement position is analyzed by EPMA (electron probe microanalyzer). The average of the C concentrations at the five locations obtained by EPMA is defined as the C concentration (mass%) on the surface of the carburized parts.

[Effective curing layer depth]
Effective cured layer depth: 0.30 to 0.60 mm
In the low cycle impact fatigue test, when an initial crack occurs, the initial crack depth is approximately equal to the effective hardened layer depth (ECD). Here, the effective cured layer depth (ECD) referred to in the present specification is the effective cured layer depth defined in JIS G 0557 (2006), and is as-quenched or at a temperature not exceeding 200 ° C. It means the distance (depth) from the surface of the tempered hardened layer to the position where the limit hardness is 550 HV.

The shallower the ECD, the shallower the initial crack depth. In this case, the crack growth rate is suppressed and the rupture process, which accounts for most of the low cycle impact fatigue characteristics, is extended. As a result, the fracture life is improved and the low cycle impact fatigue characteristics of carburized parts are enhanced. If the ECD exceeds 0.60 mm, the above effect cannot be obtained. On the other hand, if the ECD is less than 0.30 mm, the plastic deformation resistance of the carburized part is lowered. Therefore, the ECD is 0.30 to 0.60 mm. The preferable lower limit of ECD is 0.31 mm, more preferably 0.33 mm, still more preferably 0.35 mm, still more preferably 0.39 mm. The preferred upper limit of ECD is 0.57 mm, more preferably 0.55 mm, still more preferably 0.50 mm.

ECD depends on the carburizing time. The longer the carburizing time, the deeper the ECD. The longer the carburizing treatment time, the deeper the intergranular oxide layer is formed.

ECD can be measured by the following method. Ten carburized parts are prepared, and a sample including an observation surface is collected from two places for one carburized part. The observation surface has a cross section perpendicular to the surface of the carburized part and includes a portion near the surface of the carburized part. After surface polishing the observation surface of the sample, a Vickers hardness test conforming to JIS Z 2244 (2009) is performed at a pitch of 0.1 mm in the depth direction from the surface. The test force is 0.98N. The obtained hardness at each position is continuously connected to create a hardness profile near the surface layer including the hardened layer. Based on the created hardness profile, the distance from the surface of the carburized part to the position where the critical hardness is 550 HV is determined. The average of the results obtained in each measurement is defined as ECD.

[Thickness of intergranular oxide layer]
For carburized parts having a chemical composition, surface C concentration and ECD that satisfy the above formula, the thickness of the intergranular oxide layer is 15 μm or less. Since the thickness of the intergranular oxide layer is 15 μm or less, the occurrence of cracks is suppressed. As a result, the low cycle impact fatigue characteristics are enhanced.

The thickness of the intergranular oxide layer can be measured by the following method. A sample is taken that has a cross section perpendicular to the surface of the carburized part and has an observation surface including a portion near the surface of the carburized part. After polishing the observation surface of the sample, a photographic image near the surface of the sample (carburized part) is prepared with a 1000x optical microscope. The depth of the intergranular oxide layer (μm) is determined using a photographic image. Specifically, in the photographic image, the contrast between the base material and the intergranular oxide layer is different. Therefore, the intergranular oxide layer can be easily identified. The depth of the intergranular oxide layer at 10 points of the photographic image is obtained by image processing, and the average thereof is defined as the intergranular oxide layer depth (μm).

[Manufacturing process]
An example of a method for manufacturing a carburized part according to the present embodiment will be described.

A steel material satisfying the above chemical composition is produced. For example, molten steel having the above chemical composition is produced, and slabs (slabs or blooms) are produced using the molten steel by a continuous casting method. An ingot (steel ingot) may be produced by an ingot method using molten steel. A billet (steel piece) is manufactured by hot-working a slab or an ingot. The billet is hot-worked to produce steel bars or wires. The hot working may be hot rolling or hot forging. The manufactured steel bars or wires are cold forged or machined to produce intermediate products of a predetermined shape. Machining is, for example, cutting or drilling. The shape of the intermediate product is formed by a well-known method.

Carburize and quench the manufactured intermediate products. Further, after the carburizing and quenching treatment, the intermediate product is tempered to manufacture carburized parts. Carburized parts may be manufactured by further machining (cutting, etc.) on the intermediate product after quenching.

By carrying out the carburizing and quenching treatment and the tempering treatment, the C concentration on the surface of the carburized parts can be adjusted to 0.50 to 0.70%, and the ECD can be adjusted to 0.30 to 0.60 mm. An example of the conditions for carburizing and quenching is as follows.

[Carburizing and quenching]
The carburizing treatment performed on the carburized parts of the present embodiment may be a gas carburizing treatment or a vacuum carburizing treatment. By appropriately adjusting the conditions of the carburizing treatment, the C concentration on the surface of the carburized part can be adjusted to 0.50 to 0.70%. Hereinafter, the gas carburizing treatment will be described as an example.

FIG. 1 is a diagram showing an example of a heat pattern of a gas carburizing and quenching treatment. The vertical axis of FIG. 1 is the processing temperature (° C.), and the horizontal axis is time. With reference to FIG. 1, the gas carburizing treatment includes a heating step S0, a carburizing step S1, a diffusion step S2, and a heat soaking step S3.

In the heating step S0, the intermediate product charged in the furnace is heated to the carburizing temperature. In the carburizing step S1, the carburizing treatment is carried out by holding the intermediate product at t1 for a predetermined time at the carburizing temperature Tc in the atmosphere of the predetermined carbon potential Cp1. In the diffusion step S2, t2 is held at the carburizing temperature Tc for a predetermined time in an atmosphere of carbon potential Cp2 lower than the carbon potential Cp1 in the carburizing step and having a carbon potential Cp2 equal to or higher than the C concentration on the surface of the carburized component. The heat equalizing step S3 is a step for the purpose of soaking the entire intermediate product to a predetermined quenching temperature. In the heat equalizing step S3, heat is equalized to t3 for a predetermined time at a temperature Ts lower than the carburizing temperature Tc. However, the heat equalizing step S3 may be omitted.

After the diffusion step S2 or the soaking step S3, the intermediate product is rapidly cooled and hardened to manufacture carburized parts. Quenching may be water quenching or oil quenching.

The preferred conditions for each step are as follows.
Carburizing temperature Tc: Ac ₃ points to 1100 ° C
Holding time in carburizing step S1 t1: 1.0 to 3.5 hours Holding time in diffusion step S2 t2: 0.5 to 2.4 hours Carbon potential in carburizing step S1 Cp1: 0.80 to 1.00
Carbon potential Cp2 in diffusion step S2: 0.60 to 0.85

However, even if any of the above conditions is out of the above range, by adjusting the other conditions, the C concentration on the surface of the carburized part is set to 0.50 to 0.70%, and the ECD is set to 0.30 to 0. It may be 60 mm.

As described above, in the heat pattern of FIG. 1, the heat equalizing step S3 may be omitted. Further, the vacuum carburizing and quenching treatment shown in FIG. 2 may be carried out. In FIG. 2, the soaking step S4 is performed after the heating step S0 and before the carburizing step S1. In this case, in the heat equalizing step S4, the heat is equalized to t4 for a predetermined time at the carburizing temperature Tc. Further, in the vacuum carburizing and quenching treatment of FIG. 2, the pressure in the furnace is set to 0.1 kPa or less.

A well-known tempering process is carried out on the intermediate product that has been carburized and quenched. The tempering temperature is, for example, less than 200 ° C.

By the above steps, the carburized parts of the present embodiment can be manufactured.

A plurality of carburized parts were manufactured under various chemical compositions and manufacturing conditions, and the low cycle impact fatigue characteristics, the thickness of the intergranular oxide layer, and the wear resistance were investigated.

[Manufacturing of steel materials for carburized parts]
A molten steel having the chemical composition shown in Table 1 was produced. In Table 1, "fn" is the total value of X, Y and Z (fn = X + Y + Z). ^{However, X = -15.7 × Si 4 +} 74.4 × Si 3 -118.9 × Si 2 + 61.6 × Si + 5.8 (2), Y = -0.3 × Mn 3 -3 × Mn 2 +14 .2 × Mn-3.4 (3) , a ^{Z = -0.2 × Cr 4 + 4.4} × Cr 3 -16.6 × Cr 2 + 17 × Cr + 3.1 (4), equation (2) - The content (mass%) of the corresponding element is substituted for the element symbol of the formula (4).

The steel of each steel number was melted in a 180 kg vacuum melting furnace and then ingot to produce an ingot.

The ingot was heated at 1250 ° C. for 8 hours. Then, the ingot was hot forged to produce a steel bar having a diameter of 50 mm.

The following steps were carried out on each steel bar to manufacture carburized parts.

[Preparation of test piece]
First, each steel bar was subjected to normalizing treatment. The processing temperature in the normalizing treatment was 925 ° C., and the holding time was 1 hour. After the holding time had elapsed, the steel bars were allowed to cool in the air. Two types of test pieces were prepared from the steel bars after the normalizing treatment.

[Drop test piece]
Machining was performed on a steel bar having a diameter of 50 mm after the normalizing treatment to prepare a drop weight test piece having the shape shown in FIG. FIG. 3 is a side view of the drop weight test piece as viewed from the horizontal direction. The shape of the drop weight test piece in FIG. 3 simulated the tooth root R portion of the actual gear. The numerical values in FIG. 3 indicate the dimensions, and the unit is mm. “R = 2” indicates that the radius of curvature of the R portion is 2 mm. The width of the drop weight test piece was 20 mm.

[Roller pitching test piece]
Machining was performed on a steel bar having a diameter of 50 mm after the normalizing treatment to prepare a roller pitching test piece having the shape shown in FIG. The numerical values in FIG. 4 indicate the dimensions, and the unit is mm. The roller pitching test piece was columnar and had a parallel portion having a diameter of 26 mm in the center. The diameter of the roller pitching test piece other than the parallel portion was 22 mm. The roller pitching test piece was a small roller in the roller pitching test described later.

[Carburizing and quenching]
The drop weight test piece and the roller pitching test piece having the test numbers shown in Table 2 were carburized under the heat patterns shown in FIGS. 1 and 2 and the conditions shown in Table 3. The carbon potential in the soaking step S3 was the same as the carbon potential Cp2. Further, the pressure inside the furnace in the pattern 11 which is vacuum carburizing was 0.05 kPa.

[Tempering]
Tempering was performed on the falling weight test piece and the roller pitching test piece after carburizing and quenching. The tempering temperature was 180 ° C. and the holding time was 120 minutes.

By the above manufacturing process, carburized parts (drop weight test piece and roller pitching test piece) of test numbers 1 to 46 were produced.

[Evaluation test]
[Low cycle impact fatigue test]
A low-cycle impact fatigue test was performed on the drop weight test pieces of each test number using a drop weight type impact fatigue tester. Specifically, a weight of 61 kg is freely dropped from a height within a predetermined range (20 to 80 mm) and collided with a drop weight test piece at a position 10 mm from the end on the thickness 10 mm side, resulting in a shocking stress load. Gave. In FIG. 3, the arrows indicate the collision direction of the weight. This collision was repeated, and the stress at which the falling weight test piece broke at the 100th stress load (referred to as the 100th breaking strength) was determined. The test piece having a 100-fold breaking strength of 3000 MPa or more was evaluated as A, the test piece having a breaking strength of 2800 MPa to less than 3000 MPa was evaluated as B, and the test piece having a breaking strength of 2600 to less than 2800 MPa was evaluated as C, which was less than 2400 to 2600 MPa. The test piece was evaluated as D, and the test piece having a value of less than 2400 MPa was evaluated as x. In the case of evaluations A to D, it was judged that the low cycle impact fatigue characteristics were excellent. When the evaluation was ×, it was judged that the low cycle impact fatigue characteristics were low. The results are shown in Table 2.

[Roller pitching test]
FIG. 5 is a schematic view of a roller pitching test. A roller pitching test was performed on the roller pitching test pieces of each test number. Specifically, the test was carried out under the following conditions using a roller pitching fatigue strength tester manufactured by Komatsu Engineering Co., Ltd. Sliding rate: -40%, Lubricant: Automatic oil, Lubricant temperature: 90 ° C., Lubricant flow rate: 2 L / min, Rotation speed: 1500 rpm and Surface pressure: 2000 MPa. As shown in FIG. 5, the small roller 2 was rotated while pressing the large roller 1 against the small roller 2 with the above surface pressure. The small roller 2 was a roller pitching test piece produced by producing the above test piece. For the large roller 1, a steel satisfying the SCM420 standard of JIS G 4053 (2016) was used, and after co-deposit carburizing, low-temperature tempering and surface polishing were used. The radius of the large roller 1 was 130 mm. The wear depth Dw of each test piece was measured at a rotation speed of 1 × 10 ^{6 times.} A stylus type surface roughness meter was used to measure the wear depth Dw. The measurement length was 24 mm, and the stylus was scanned in the axial direction of each test piece to obtain a cross-sectional curve. Each test piece was measured at two points every 180 ° in the circumferential direction to obtain a cross-sectional curve. From the obtained cross-sectional curve, in each test piece, the average height of the cross-sectional curve element in the portion where the large roller 1 is not in contact and the average height of the cross-sectional curve element in the portion where the large roller 1 is in contact and worn. Were calculated respectively. Then, the difference in height between the portion not in contact with the large roller 1 and the portion in contact with the large roller 1 was calculated. The average value of the obtained height differences at the above-mentioned measurement points was defined as the wear depth Dw (μm) of each test number. The test piece having a wear depth Dw of less than 20 μm was evaluated as A, the test piece having a wear depth of less than 20 to 40 μm was evaluated as B, the test piece having a wear depth of less than 40 to 60 μm was evaluated as C, and the wear depth was less than 60 to 80 μm. The test piece was evaluated as D, and the test piece having a size of 80 μm or more was evaluated as ×. In the case of evaluations A to D, it was judged that the wear resistance was excellent. When the evaluation was ×, it was judged that the wear resistance was low. The results are shown in Table 2.

[Surface C concentration measurement]
The surface C concentration of the dropped weight test piece before the test was measured by the above method. The results are shown in Table 2.

[Measurement of effective cured layer depth (ECD)]
The Vickers hardness test was carried out by the above-mentioned method at any two points of the R portion of the drop weight test piece before the test to determine the effective hardened layer depth ECD (mm). The results are shown in Table 2.

[Measurement of grain boundary oxide layer thickness]
A sample including the surface of the R portion of the drop weight test piece before the test and having a cross section perpendicular to the surface of the R portion (hereinafter referred to as an observation surface) was collected. The thickness of the intergranular oxide layer was measured with respect to the obtained sample by the above-mentioned method. The results are shown in Table 2.

[Test results]
The test results are shown in Table 2. With reference to Table 2, the carburized parts of Test Nos. 1-31 had an appropriate chemical composition and an appropriate surface C concentration and ECD. Therefore, the carburized parts of these test numbers were excellent in low cycle impact fatigue characteristics and wear resistance.

On the other hand, the carburized part of test number 32 had too low C content. Therefore, the low cycle impact fatigue characteristics were low.

The carburized part of test number 33 had too high a Si content. Therefore, the low cycle impact fatigue characteristics were low.

The carburized part of test number 34 had too low a Si content. Therefore, the wear resistance was low.

The carburized part of test number 35 had too high a Mn content. Therefore, the wear resistance was low.

The carburized part of test number 36 had too low a Mn content. Therefore, the low cycle impact fatigue characteristics were low.

The carburized part of test number 37 had too high a P content. Therefore, the low cycle impact fatigue characteristics were low.

The carburized part of test number 38 had too high an S content. Therefore, the low cycle impact fatigue characteristics were low.

The carburized part of test number 39 had too low Al content. Therefore, the low cycle impact fatigue characteristics were low.

The carburized part of test number 40 had too high a Cr content. Therefore, the low cycle impact fatigue characteristics were low.

The carburized part of test number 41 had too low a Cr content. Therefore, the low cycle impact fatigue characteristics were low.

The surface C concentration of the carburized part of test number 42 was too high because the carbon potential Cp2 in the diffusion step S2 was too high. Therefore, the low cycle impact fatigue characteristics were low.

The surface C concentration of the carburized part of test number 43 was too low because the carbon potential Cp2 in the diffusion step S2 was too low. Therefore, the low cycle impact fatigue characteristics and wear resistance were low.

The carburized part of test number 44 had an ECD too deep because the holding time t2 in the diffusion step S2 was too long. Therefore, the low cycle impact fatigue characteristics were low.

The carburized part of test number 45 had an ECD too shallow because the holding time t1 in the carburizing step S1 was too short. Therefore, the low cycle impact fatigue characteristics were low.

The chemical composition of the core of the carburized part of Test No. 46 was fn = 28, and did not satisfy the formulas (1) to (4). Therefore, the low cycle impact fatigue characteristics and wear resistance were low.

The embodiments of the present invention have been described above. However, the embodiments described above 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.

The carburized parts of the present embodiment can be widely used for parts, and are particularly suitable for power transmission parts for automobiles, construction machines, industrial machines, and shaft parts.

1 large roller 2 small roller

Claims (3)

The chemical composition of the core is mass%,
C: 0.10 to 0.30%,
Si: 0.50-1.50%,
Mn: 0.30-1.40%,
P: Less than 0.030%,
S: Less than 0.030%,
Cr: 0.50 to 2.00%,
Al: 0.010 to 0.100%,
N: 0.001 to 0.030%,
Mo: 0-0.80%,
Ni: 0 to 0.50%,
Cu: 0-0.50%,
Ti: 0 to 0.10% and
Nb: contains 0 to 0.10%, the balance is composed of Fe and impurities, and satisfies the formula (1).
The C concentration on the surface is 0.50 to 0.70%,
A carburized part having an effective hardened layer depth of 0.30 to 0.60 mm, which is the distance from the surface to a position where the critical hardness is 550 HV in Vickers hardness.
X + Y + Z ≤16 or 18≤X + Y + Z ≤26 (1 )
However,
^{X = -15.7 × Si 4 + 74.4} × Si 3 -118.9 × Si 2 + 61.6 × Si + 5.8 (2)
Y = -0.3 x Mn ³ -3 x Mn ² + 14.2 x Mn-3.4 (3)
^{Z = -0.2 × Cr 4 + 4.4} × Cr 3 -16.6 × Cr 2 + 17 × Cr + 3.1 (4)
Here, the content (mass%) of the corresponding element is substituted for the element symbol of the formulas (2) to (4). The carburized part according to claim 1.
The chemical composition of the core is
Mo: 0.01 to 0.80%, and
Ni: 0.05 to 0.50%, and
Cu: A carburized component containing one or more selected from the group consisting of 0.10 to 0.50%. The carburized part according to claim 1 or 2.
Ti: 0.05 to 0.10%, and
Nb: A carburized component comprising one or two selected from the group consisting of 0.01 to 0.10%.

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