DE112020006870T5 - GAS SOFT NITRIDING-TREATED COMPONENT AND MANUFACTURING METHOD THEREOF - Google Patents

Abstract

A gas-soft nitriding-treated member according to an aspect of the present invention includes a steel core, a compound layer, and a nitrogen diffusion layer present between the steel core and the compound layer, wherein the steel core contains as composition % by mass: C: 0.05% to 0.60% %, Si: 0.05% to 1.50%, Mn: 0.20% to 2.50%, P: 0.025% or less, S: 0.050% or less, Cr: 0.50% to 2.50% %, V: 0.05% to 1.30%, Al: 0.050% or less, N: 0.0250% or less, and a balance containing Fe and an impurity, the amounts of C, Mn, Cr , V and Mo in the composition of the steel core 0.00 ≤ -2.1 × C + 0.04 × Mn + 0.5 × Cr + 1.8 × V - 1.5 × Mo ≤ 0. 50, the thickness of the compound layer is 3 to 20 µm, the compound layer contains more than 50% of an ε phase in terms of an area ratio and a balance of a γ' phase, and the porosity area ratio in a range from the surface of the compound layer to a depth of 3 µm is less than 12%.

Description

[Technical Field of the Invention]

The present invention relates to a steel member subjected to gas soft nitriding treatment and a method of manufacturing the same.

[Related Prior Art]

Some steel members used in automobiles, various industrial machines or the like require surface fatigue strength. So z. B. CVT pulleys or camshafts in transmissions must be wear-resistant or resistant to bending, and gears must have fatigue properties such as surface and reverse bending strength. It is believed that improving surface hardness helps improve these properties. Therefore, nitriding and soft nitriding treatments are currently used for these steel components. Nitriding treatments and soft nitriding treatments have the advantages that high surface hardness can be obtained and stress from the heat treatment is small.

Nitriding is a surface hardening heat treatment in which nitrogen is infiltrated into the steel surface, and soft nitriding is a surface hardening heat treatment in which nitrogen and carbon are infiltrated into the steel surface. Gas, salt bath, plasma and the like are used as media for the nitriding and the soft nitriding, for example. The gas nitriding process and the gas soft nitriding process, which are characterized by high productivity, are mainly used for motor vehicle transmission components.

As hardened layers formed by gas nitriding and soft gas nitriding, a nitrogen diffusion layer (hereinafter abbreviated as diffusion layer in some cases) and a compound layer several to several tens of microns thick are formed on the surface side of the diffusion layer.

The diffusion layer is a layer hardened by a solid solution strengthening mechanism by nitrogen and carbon penetrating and a particle dispersion strengthening mechanism of a nitride. It is known that improving the hardness and depth of the diffusion layer improves the reverse bending strength or the surface fatigue strength of components. A number of studies have hitherto been made to improve the hardness or depth of the diffusion layer.

The compound layer is mainly formed of iron nitrides of Fe ₂ N to Fe ₃ N (ε phase) and Fe ₄ N (γ' phase), and is extremely harder than the primary phases. Therefore, the composite layer is an effective means of improving wear resistance. Compared to the γ' phase, the ε phase has a large C solid solution area and a fast growth rate. For this reason, soft nitriding in which carburizing gases are mixed is likely to form a compound layer mainly containing the ε phase. Therefore, with soft nitriding, it is possible to obtain thick composite layers in a short time compared with nitriding, regardless of the type of steel of the components. Therefore, soft nitriding has long been used to improve the wear resistance of components.

Conventional understandings of the relationship between composite layers and wear resistance and fatigue strength are described below.

Patent Document 1 discloses a nitrided or soft nitrided member in which the thickness of an ε-single-phase compound layer is 8 to 30 μm, the Vickers hardness is 680 HV or more, and the volume ratio of porosities in the compound layer is less than 10%.

Furthermore, Patent Document 2 discloses a blade for a rotary compressor, in which the thickness of a compound layer after nitriding is 1 to 5 µm, the surface roughness after nitriding is Rz1.6 or less, the compound layer has a γ' phase or a mixed phase of a γ' phase and an ε phase, and the porosity ratio is 5% or less.

Patent Document 3 discloses that the amounts of the steel compositions, particularly C, Mn, Cr, V, and Mo, are adjusted depending on the properties aimed at, and a nitrided member is manufactured while controlling the nitriding potential.

Patent Document 4 discloses a nitriding-treated member excellent not only in surface fatigue strength but also in rotational flex life, using a steel material having a predetermined chemical composition as a material, a compound layer containing iron, nitrogen and carbon, and a thickness of 3 µm or more and less than 20 µm is formed on a surface of the steel material, the phase structure in the compound layer in a region from the surface to a depth of 5 µm contains 50% or more of a γ' phase in terms of an area ratio contains, the porosity area ratio in a range from the surface to a depth of 3 µm is less than 1%, and the compressive residual stress at the surface of the composite layer is 500 MPa or more.

Patent Document 5 discloses a gas-soft nitriding treatment method of low-alloy steel, in which the low-alloy steel is heated to 550° C. to 620° C. when gas-soft nitriding treatment is performed with treatment time A set to 1.5 to 10 hours, the nitriding potentials Knx are 0.10 to 0.10 1.00, the average value Kn _{Xave is} 0.20 to 0.55, after a high Kn value treatment with a treatment time fixed at X hours, the nitration potentials Kn _y are in a range from 0.01 to 0.20 , the mean value Kny _ave is 0.02 to 0.15, a low Kn value treatment is performed with a treatment time fixed at Y hours, and from the mean value Kn _Xave , the mean value Kn _Yave , the treatment time A and the treatment times X and Y of the high Kn treatment and the low Kn treatment, the average value Kn _Xave , which is 0.05 to 0.20 nitriding potentials of the soft nitriding treatment.

[Prior Art Document]

[patent document]

• [Patent Document 1] PCT International Publication No. WO 2016/153009
• [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2005-16386
• [Patent Document 3] PCT International Publication No. WO 2019/098340
• [Patent Document 4] PCT International Publication No. WO 2018/066666
• [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. 2015-175009

[Disclosure of the Invention]

[Problems to be Solved by the Invention]

In the nitrided member or the soft nitrided member of Patent Document 1, the proportion of porosity is suppressed from the outermost surface to the lowermost surface (the interface between the compound layer and a diffusion layer) of the compound layer. In practice, however, the porosities are often concentrated in an area from the steel surface to about 3 µm. If there are a number of porosities in this area, favorable reverse flex life cannot be obtained. When evaluating the nitriding-treated member of Patent Document 3, the inventors found that voids were formed intensively in a compound layer in a surface layer portion of the member, and the volume fraction of voids in this surface layer portion was over 40%. Therefore, with respect to the steel compositions and a method of controlling nitriding to suppress the proportion of porosity from the surface to 3 μm in the method of Patent Document 1, there is room for improvement.

In the nitrided member of Patent Document 2, the compound layer is as extremely thin as 1 μm minimum, and the phase structure mainly includes a γ′ phase that is lower in hardness than an ε phase. Therefore, according to the technique of Patent Document 2, there is a possibility that favorable wear resistance cannot be obtained.

In the nitrided members of Patent Document 3 and Patent Document 4, the structure of the compound layer mainly contains a γ' phase with low hardness. In the method described in Patent Document 5, formation of a compound layer is intended to be suppressed, and for this purpose, the nitriding potential is set to a lower value than usual. Therefore, in the nitrided members of Patent Documents 3 to 5, it is considered that the wear resistance can still be improved.

It is the object of the present invention to provide a component which not only has favorable wear resistance but also outstanding rotational flexural fatigue strength, and a method for its manufacture.

[Means to Solve the Problem]

• (1) Ein gasweichnitrierbehandeltes Bauteil gemäß einem Aspekt der vorliegenden Erfindung umfasst einen Stahlkern, eine Verbundschicht und eine Stickstoffdiffusionsschicht, die sich zwischen dem Stahlkern und der Verbundschicht befindet, wobei der Stahlkern als Zusammensetzung, in Massenprozent, enthält: C: 0,05% bis 0,60%, Si: 0,05% bis 1,50%, Mn: 0,20% bis 2,50%, P: 0,025% oder weniger, S: 0,050% oder weniger, Cr: 0,50% bis 2,50%, V: 0,05% bis 1,30%, Al: 0,050% oder weniger, N: 0. 0250% oder weniger und einen Rest, der Fe und eine Verunreinigung aufweist, die Mengen an C, Mn, Cr, V und Mo in der Zusammensetzung des Stahlkerns der Formel 1 genügen, eine Dicke der Verbundschicht 3 bis 20 µm beträgt, die Verbundschicht mehr als 50% einer ε-Phase bezogen auf ein Flächenverhältnis und einen Rest einer γ'-Phase enthält, und ein Porositätsflächenanteil (Flächenanteil von Porositäten) in einem Bereich von einer Oberfläche der Verbundschicht bis zu einer Tiefe von 3 µm weniger als 12% beträgt. $$\mathrm{0,00}\le -\mathrm{2,1}\times \text{C}+\text{0}\text{,04}\times \text{Mn}+\text{0}\text{,5}\times \text{Cr}+\text{1}\text{,8}\times \text{V}-\text{1}\text{,5}\times \text{Mo}\le \text{0}\text{,50}$$
  
  Hier geben die Elementsymbole in der Formel 1 die Mengen (Masse-%) der entsprechenden Elemente an, und Null wird zugewiesen, wenn die Elemente nicht enthalten sind.
• (2) In dem gasweichnitrierbehandelten Bauteil nach (1) kann der Stahlkern als Zusammensetzung in Masse-% enthalten: C: 0,05% bis 0,60%, Si: 0,05% bis 1,50%, Mn: 0,20% bis 2,50%, P: 0,025% oder weniger, S: 0,050% oder weniger, Cr: 0,50% bis 2,50%, V: 0,05% bis 1,30%, Al: 0,050% oder weniger, N: 0,0250% oder weniger, Mo: 0% bis 1,50%, Cu: 0% bis 1,00%, Ni: 0% bis 1,00%, W: 0% bis 0,50%, Co: 0% bis 0,50%, Nb: 0% bis 0,300%, Ti: 0% bis 0,250%, B: 0% bis 0,0100%, Ca: 0% bis 0,010%, Mg: 0% bis 0,010%, REM: 0% bis 0,010%, und der Rest besteht aus Fe und einer Verunreinigung.
• (3) In dem gasweichnitrierbehandelten Bauteil gemäß den obigen (1) oder (2) kann C in der Zusammensetzung des Stahlkerns 0,05% bis 0,35% betragen.
• (4) Bei dem gasweichnitrierbehandelten Bauteil nach einem der obigen (1) bis (3) enthält die Verbundschicht 90% oder weniger der ε-Phase, bezogen auf das Flächenverhältnis.
• (5) Bei dem gasweichnitrierbehandelten Bauteil nach einem der obigen (1) bis (4) kann die Dicke der Verbundschicht 6 µm oder mehr betragen.
• (6) Ein Verfahren zur Herstellung eines gasweichnitrierbehandelten Bauteils gemäß einem weiteren Aspekt der vorliegenden Erfindung ist ein Verfahren zur Herstellung des gasweichnitrierbehandelten Bauteils gemäß einem der obigen (1) bis (5), bei dem ein Stahlmaterial mit der Zusammensetzung des Stahlkerns gemäß einem der obigen (1) bis (3) in eine vorbestimmte Form bearbeitet wird, um einen Rohformstahl zu erhalten, und bei dem an dem Rohformstahl eine Gasweichnitrierbehandlung durchgeführt wird, wobei die Gasweichnitrierungsbehandlung in einer Gasatmosphäre durchgeführt wird, die mindestens eines von CO₂, CO und Kohlenwasserstoffgas in einem Aufkohlungsgas-Injektionsanteil, der durch eine Formel 2 dargestellt wird, von 2 Vol.-% oder mehr und weniger als 10 Vol.-% und einen Rest von NH₃, H₂, N₂ und einem Verunreinigungsgas, durch Halten einer Temperatur bei 550°C oder höher und 630°C oder niedriger für 1,0 Stunden oder länger und 7,0 Stunden oder kürzer, in der Gasatmosphäre ein durch Formel 3 erhaltenes Nitrierungspotential K_N innerhalb eines Bereichs von 0,15 oder mehr und 0,40 oder weniger liegt, indem die Gasweichnitrierungsbehandlung durchgeführt wird, und, und ein Durchschnittswert K_Nave der Nitrierungspotentiale K_N 0,18 oder mehr und weniger als 0,30 beträgt. $$\begin{array}{l}\text{Aufkohlunhsgas}-\text{Injektionsanteil}\left(\text{Vol}\text{.}-\text{\%}\right)=\text{Gesamt}-{\text{Injektionsdurchsatz von CO}}_2,\text{CO}\\ \text{und Kohlenwasserstoffgas }\left(1/\text{min}\right)/\text{Gesamt}-\text{Injektionsdurchsatz von atmosph}\ddot{\text{a}}\text{rischem Gas}\\ \left(1/\text{min}\right)\times 100\end{array}$$
  
  $${\text{K}}_{\text{N}}=\left({\text{NH}}_3-\text{Partialdruck}\right)/\left[{\left({\text{H}}_2-\text{Partialdruck}\right)}^{3/2}\right]\left({\text{atm}}^{-1/2}\right)$$
  

The outline of the present invention is described below.

• (1) A gas-soft nitriding-treated component according to an aspect of the present invention comprises a steel core, a compound layer, and a nitrogen diffusion layer located between the steel core and the compound layer, the steel core containing as a composition, in percentage by mass: C: 0.05% to 0.60%, Si: 0.05% to 1.50%, Mn: 0.20% to 2.50%, P: 0.025% or less, S: 0.050% or less, Cr: 0.50% to 2.50%, V: 0.05% to 1.30%, Al: 0.050% or less, N: 0.0250% or less and a balance comprising Fe and an impurity, the amounts of C, Mn, Cr, V and Mo in the composition of the steel core satisfy formula 1, a thickness of the compound layer is 3 to 20 µm, the compound layer contains more than 50% of an ε-phase in terms of an area ratio and a balance of a γ'-phase, and an area ratio of porosity (area ratio of porosities) in a range from a surface of the composite layer to a depth of 3 µm is less than 12%. $$0.00\le -2.1\times \text{C}+\text{0}\text{,04}\times \text{Mn}+\text{0}\text{,5}\times \text{Cr}+\text{1}\text{,8th}\times \text{V}-\text{1}\text{,5}\times \text{Mon}\le \text{0}\text{,50}$$
  
  Here, the element symbols in Formula 1 indicate the amounts (mass%) of the respective elements, and zero is assigned when the elements are not included.
• (2) In the gas-soft nitriding-treated component according to (1), the steel core may contain, as a composition in % by mass: C: 0.05% to 0.60%, Si: 0.05% to 1.50%, Mn: 0, 20% to 2.50%, P: 0.025% or less, S: 0.050% or less, Cr: 0.50% to 2.50%, V: 0.05% to 1.30%, Al: 0.050% or less, N: 0.0250% or less, Mo: 0% to 1.50%, Cu: 0% to 1.00%, Ni: 0% to 1.00%, W: 0% to 0.50 %, Co: 0% to 0.50%, Nb: 0% to 0.300%, Ti: 0% to 0.250%, B: 0% to 0.0100%, Ca: 0% to 0.010%, Mg: 0% to 0.010%, REM: 0% to 0.010%, and the balance is Fe and an impurity.
• (3) In the gas-soft nitriding-treated member according to the above (1) or (2), C in the composition of the steel core may be 0.05% to 0.35%.
• (4) In the gas-soft nitriding-treated member according to any one of the above (1) to (3), the compound layer contains 90% or less of the ε phase in terms of area ratio.
• (5) In the gas-soft nitriding-treated member according to any one of the above (1) to (4), the thickness of the composite layer may be 6 µm or more.
• (6) A method for manufacturing a gas-softened nitriding-treated member according to another aspect of the present invention is a method for manufacturing the gas-softening-treated member according to any one of the above (1) to (5), in which a steel material having the composition of the steel core according to any one of the above (1) to (3) is worked into a predetermined shape to obtain a raw shape steel, and in which a gaseous soft nitriding treatment is performed on the raw shape steel, the gaseous soft nitriding treatment being performed in a gas atmosphere containing at least one of CO ₂ , CO and hydrocarbon gas in a carburizing gas injection rate represented by a formula 2 of 2% by volume or more and less than 10% by volume and a balance of NH ₃ , H ₂ , N ₂ and an impurity gas, by maintaining a temperature at 550°C or higher and 630°C or lower for 1.0 hour or longer and 7.0 hours or shorter in the gas atmosphere nitriding potential KN obtained by Formula 3 is within a range of 0.15 or more and 0.40 or less by performing the gas soft nitriding treatment, and, and an average value K _Nave of the nitriding _{potentials KN} is 0.18 or more and less than 0 is .30. $$\begin{array}{l}\text{carburizing gas}-\text{injection proportion}\left(\text{volume}\text{.}-\text{\%}\right)=\text{In total}-{\text{injection throughput of CO}}_2,\text{CO}\\ \text{and hydrocarbon gas}\left(1/\text{at least}\right)/\text{In total}-\text{injection throughput of atmosph}\ddot{\text{a}}\text{ric gas}\\ \left(1/\text{at least}\right)\times 100\end{array}$$
  
  $${\text{K}}_{\text{N}}=\left({\text{NH}}_3-\text{partial pressure}\right)/\left[{\left({\text{H}}_2-\text{partial pressure}\right)}^{3/2}\right]\left({\text{atm}}^{-1/2}\right)$$
  

[Effects of the invention]

According to the present invention, it is possible to obtain a soft nitrided member excellent not only in wear resistance but also in rotational flex life.

character list

• 1 14 is an enlarged view of a surface of a gas nitriding treated member according to the present embodiment.
• 2 14 is a flow chart showing a method for manufacturing a gas soft nitriding treated member according to the present embodiment.
• 3 Fig. 14 is a view showing the shape of a small roller for a roller pitting test. The units of measurement in the drawing are "mm".
• 4 Fig. 14 is a view showing the shape of a large roll for the roll pitting test. The units of measurement in the drawing are "mm".
• 5 14 is a view showing the shape of a cylindrical test piece for a rotational flexural fatigue test. The units of measurement in the drawing are "mm".

[Embodiments of the invention]

The present inventors have studied the shape of a compound layer formed by soft nitriding on a surface of a steel material and studied the relationship between the shape of the compound layer and fatigue strength.

As a result, it was found that by soft nitriding a steel material having adjusted compositions in a specific atmosphere while controlling the nitriding potential, porosities formed on the surface side of the compound layer can be suppressed. In addition, it has been found that when the thickness of the compound layer is set within a certain range and the hardness of the compound layer is set to a certain value or more, a soft nitrided member excellent in wear resistance and rotational flex life can be manufactured.

A gas-soft nitriding-treated component according to the present embodiment obtained from the above knowledge will be described in detail. As in 1 As shown, a gas-soft nitriding-treated component 1 according to the present embodiment (hereinafter simply referred to as “component” in some cases) comprises a steel core 11 (hereinafter simply referred to as “steel” in some cases), a compound layer 12 and a nitrogen diffusion layer 13 between the steel core 11 and the composite layer 12 (hereinafter referred to simply as “diffusion layer” in some cases).

[composition of the steel]

The composition of the steel is described below. Unless specifically described otherwise, the amount of each composition element described below is based on the composition of the steel. The compositions of the compound layer and the diffusion layer are not limited to the ranges to be described below. In addition, "%" in terms of the amount of each composition element in the steel and the concentration of an element on the surface of the member means "% by mass" unless expressly described otherwise.

[C: 0.05% to 0.60%]

C stabilizes the formation of an ε phase. Therefore, C is an effective element for improving the wear resistance of the member by increasing the thickness of the compound layer after nitriding and the volume fraction of the ε phase. In addition, C is a necessary element to ensure the hardness of the core portion of the member. In order to obtain these effects, the C content must be 0.05% or more. On the other hand, if the C content exceeds 0.60%, the hardness of a steel bar, wire rod and the like serving as a material becomes too high after hot forging. Therefore, the workability of the material deteriorates significantly. The C content can be set to 0.08% or more, 0.10% or more, or 0.15% or more. The C content can be set to 0.55% or less, 0.50% or less, 0.40% or less, or 0.35% or less.

[Si: 0.05% to 1.50%]

Si is an element that increases the hardness of the core portion by solid solution strengthening. In addition, since Si increases resistance to high-temperature softening, it increases wear resistance when the temperature of the member becomes high in a contact friction environment. In order to obtain these effects, the Si content must be 0.05% or more. However, if the Si content exceeds 1.50%, the hardness of the steel bar, wire rod and the like serving as the material becomes too high after hot forging. Therefore, the workability of the material deteriorates significantly. The Si content can be set to 0.08% or more, 0.10% or more, or 0.20% or more. The Si content can be set to 1.30% or less, 1.10% or less, or 1.00% or less.

[Mn: 0.20% to 2.50%]

Mn is an element that forms a fine soft nitride (Mn ₃ N ₂ ) in the compound layer or the diffusion layer by soft nitriding treatment and increases wear resistance or flex life. In addition, Mn increases the hardness of the core portion through solid solution strengthening. In order to obtain these effects, the Mn content must be 0.20% or more. However, if the Mn content exceeds 2.50%, not only the effect of improving wear resistance or reverse bending fatigue strength is saturated, but also the hardness of the steel bar, wire rod and the like serving as the material becomes too high after hot forging. Therefore, the machinability of the material deteriorates significantly. The Mn content can be set to 0.40% or more, 0.60% or more, or 1.00% or more. The Mn content can be set to 2.30% or less, 2.00% or less, or 1.80% or less.

[P: 0.025% or less]

P is an impurity and embrittles the component through edge segregation. Therefore, the P content is preferably as small as possible. With a P content of more than 0.025%, wear resistance or flex fatigue strength may deteriorate. A preferred upper limit of the P content to avoid deterioration in wear resistance or flex fatigue strength is 0.018%, 0.015% or 0.010%. The P content may be 0%, but it may be set to 0.001% or more, 0.005% or more, or 0.008% or more in consideration of refining economy.

[S: 0.050% or less]

S is an element that combines with Mn to form MnS and improves machinability. However, when the S content exceeds 0.050%, coarse MnS is likely to be formed, and wear resistance or flex fatigue strength deteriorates significantly. A preferred upper limit of the S content to avoid deterioration in wear resistance or flex fatigue strength is 0.030%, 0.025% or 0.020%. The S content may be 0%, but it may be set to 0.001% or more, 0.002% or more, or 0.005% or more in consideration of refining economy.

[Cr: 0.50% to 2.50%]

Cr is an element that forms a fine soft nitride (CrN) in the compound layer or in the diffusion layer by soft nitriding treatment and increases wear resistance or flex life. In order to obtain these effects, the Cr content must be 0.50% or more. On the other hand, when the Cr content exceeds 2.50%, not only the effect of improving wear resistance or reverse bending fatigue strength is saturated, but also the hardness of the steel bar and wire rod serving as the material becomes too high after hot forging. As a result, the machinability of the material deteriorates significantly. The Cr content can be set to 0.70% or more, 0.80% or more, or 1.00% or more. The Cr content can be set to 2.20% or less, 2.00% or less, or 1.80% or less.

[V: 0.05% to 1.30%]

V is an element that forms fine soft nitride (VN) in the compound layer or the diffusion layer by soft nitriding treatment and increases wear resistance or flex life. In order to obtain these effects, the V content must be 0.05% or more. On the other hand, when the V content exceeds 1.30%, not only the effect of improving wear resistance or reverse bending fatigue strength is saturated, but also the hardness of the steel bar and wire rod serving as the material becomes too high after hot forging. As a result, the machinability of the material deteriorates significantly. The V content can be set to 0.10% or more, 0.20% or more, or 0.40% or more. The V content can be set to 1.10% or less, 1.00% or less, or 0.80% or less.

[Al: 0.050% or less]

Al is not essential in the components according to the present embodiment. On the other hand, Al is a deoxidizing element. In addition, Al combines with N to form AlN, and has effects on refinement of the structure of the steel material before soft nitriding treatment due to the pinning effect for austenite grains and reduction of variation in mechanical properties of the soft nitriding treated member. In order to obtain these effects, the Al content must be 0.005% or more. On the other hand, Al is likely to form a hard oxide-based inclusion. If the Al content exceeds 0.050%, the flex fatigue strength deteriorates significantly, and it becomes impossible to obtain the desired flex fatigue strength even when other requirements are satisfied. A preferable upper limit of the Al content to avoid deterioration of the flex fatigue strength is 0.040%, 0.030% or 0.020%. The Al content can be 0%; however, in order to obtain the effects described above, the Al content may be set to 0.001% or more, 0.002% or more, or 0.005% or more.

[N: 0.0250% or less]

N is not essential in the device according to the present embodiment, and the N content may be 0%. On the other hand, N combines with Mn, Cr, Al and V to form Mn ₃ N ₂ , CrN, AlN and VN. Among these nitriding elements, Al and V, which have a high nitriding tendency, have effects on refinement of the structure of the steel material before the soft nitriding treatment due to the pinning effect for austenite grains and reduction of variations in the mechanical properties of the soft nitriding-treated member. In order to obtain the effects described above, the N content may be set to 0.0030% or more, 0.0035% or more, or 0.0040% or more. On the other hand, when the N content exceeds 0.0250%, coarse AlN or VN is likely to be formed, making it difficult to obtain the effects described above. The N content can be set to 0.0220% or less, 0.0200% or less, or 0.0100% or less.

The chemical composition of the steel core of the gas-soft nitriding-treated component according to the present embodiment contains the elements described above and a balance containing Fe and an impurity. The impurity is a composition contained in a raw material, or a composition introduced in the manufacturing process, or the like, and a composition that does not impair the properties of the device according to the present embodiment. For example, the impurity is 0.0040% or less O (oxygen).

However, in the gas-soft nitriding-treated member according to the present embodiment, the part that mainly contributes to solving the problem is the compound layer. As long as the composite layer is configured as described below, it is not excluded that the steel core contains compositions other than those described above. As the compositions which can be further contained in the steel core, the following elements can be exemplified. However, the device according to the present embodiment is able to solve the problem without including the elements exemplified below. Therefore, the lower limits on the amounts of the elements exemplified below are 0%.

[Mon: 0% to 1.50%]

Mo stabilizes the ε-phase in the compound layer. In addition, Mo forms a fine nitride (Mo ₂ N) in the compound layer or the diffusion layer and increases the hardness. Therefore, Mo is an effective element for improving wear resistance or flex fatigue strength. In order to reliably measure these effects obtained, 0.01% or more Mo is preferably contained. On the other hand, when the Mo content is set to 1.50% or less, the hardness of the steel bar and wire rod serving as the material is suppressed after hot forging, and the machinability of the materials can be secured, which is preferable. A preferred lower limit for the Mo content is 0.05% or 0.10%. A preferred upper limit of the Mo content is less than 1.20%, 1.10% or 1.00%.

[Cu: 0% to 1.00%]

Cu, as a solid solution strengthening element, improves the hardness of the core portion of the device and the hardness of the nitrogen diffusion layer. In order to reliably exhibit the solid solution strengthening effect of Cu, 0.01% or more of Cu is preferably contained. On the other hand, when the Cu content is set to 1.00% or less, the hardness of the steel bar and wire rod serving as the material is suppressed after hot forging, and the machinability of the materials can be secured, which is preferable. In addition, when the Cu content is set to 1.00% or less, the hot ductility of the material is improved, whereby generation of surface scratches in hot rolling and hot forging can be further suppressed, which is preferable. The Cu content can be set to 0.05% or more, 0.10% or more, or 0.20% or more. The content can be set to 0.90% or less, 0.80% or less, or 0.60% or less.

[Ni: 0% to 1.00%]

Ni improves the hardness of the core portion and the surface hardness through solid solution strengthening. In order to reliably exhibit the solid solution strengthening effect of Ni, 0.01% or more of Ni is preferably contained. On the other hand, when the Ni content is set to 1.00% or less, the hardness of the steel bar and wire rod after hot forging is suppressed, and the workability of the materials can be further improved, which is preferable. The Ni content can be set to 0.05% or more, 0.10% or more, or 0.20% or more. The Ni content can be set to less than 0.90%, 0.80% or less, or 0.70% or less.

[W: 0% to 0.50%]

W improves core portion hardness and surface hardness by solid solution strengthening or precipitation of a carbide (WC or W ₂ C). In order to exhibit the effect of W reliably, 0.01% or more of W is preferably contained. On the other hand, when the W content is set to 0.50% or less, the hardness of the steel bar and wire rod after hot forging is suppressed and workability can be secured, which is preferable. The W content can be set to 0.05% or more, 0.10% or more, or 0.20% or more. The W content can be 0.40% or less, 0.35% or less, or 0.30% or less.

[Co: 0% to 0.50%]

Co improves core portion hardness and surface hardness through solid solution strengthening. In order to reliably exhibit the effect of Co, it is preferable to contain 0.01% or more of Co. On the other hand, when the Co content is set to 0.50% or less, the hardness of the steel bar and wire rod after hot forging is suppressed and workability can be secured, which is preferable. The Co content can be set to 0.05% or more, 0.10% or more, or 0.20% or more. The Co content can be 0.40% or less, 0.35% or less, or 0.30% or less.

[NB: 0% to 0.300%]

Nb bonds to N, which has entered the surface layer of the steel during nitriding, and combines with C in the primary phase to form a fine nitride or carbonitride. Thereby, Nb improves the surface hardness or the hardness of the core portion. In order to exhibit this effect reliably, 0.010% or more of Nb is preferably contained. On the other hand, the Nb content is set to 0.300% or less, whereby the formation of a coarse nitride or carbonitride can be suppressed, which is preferable. The Nb content can be set to 0.015% or more, 0.020% or more, or 0.050% or more. The Nb content can be set to less than 0.250%, 0.200% or less, or 0.180% or less.

[Ti: 0% to 0.250%]

Ti combines with N, which has penetrated into the surface layer of the steel during nitriding, and C in the primary phase to form a fine nitride or carbonitride. Thereby, Ti improves the surface hardness or the hardness of the core portion. In order to exhibit this effect reliably, 0.005% or more of Ti is preferably contained. On the other hand, the Ti content is set to 0.250% or less, whereby the formation of a coarse nitride or carbonitride can be suppressed, which is preferable. The Ti content can be set to 0.007% or more, 0.010% or more, or 0.020% or more. The Ti content can be set to 0.200% or less, 0.150% or less, or 0.100% or less.

[B: 0% to 0.0100%]

A B solid solution has an effect of suppressing the boundary segregation of P and improving toughness. In addition, BN, which precipitates after binding B to N, improves machinability. In order to reliably obtain these effects, B is preferably set to 0.0005% (5 ppm) or more. On the other hand, when the B content is set to 0.0100% or less, segregation of a large amount of BN is suppressed, whereby cracking of the steel material can be suppressed, which is preferable. The B content can be set to 0.0008% or more, 0.0010% or more, or 0.0020% or more. The content can be set to 0.0080% or less, 0.0070% or less, or 0.0060% or less.

[Approx: 0% to 0.010%]

Ca has the effect of refining MnS to improve surface fatigue strength. In order to reliably exhibit the effect of Ca, it is preferable to contain 0.001% or more of Ca. On the other hand, when the Ca content is set to 0.010% or less, the effect of Ca can be effectively exhibited without impairing the economy. The Ca content can be set to 0.002% or more, 0.003% or more, or 0.004% or more. The Ca content can be set to 0.009% or less, 0.008% or less, or 0.007% or less.

[Mg: 0% to 0.010%]

Mg has the effect of refining MnS to improve surface fatigue strength. In order to reliably exhibit the effect of Mg, it is preferable to contain 0.001% or more of Mg. On the other hand, when the Mg content is set to 0.010% or less, the effect of Mg can be effectively exhibited without impairing the economy. The Mg content can be set to 0.002% or more, 0.003% or more, or 0.004% or more. The Mg content can be set to 0.009% or less, 0.008% or less, or 0.007% or less.

[REM: 0% to 0.010%]

The term "REM" refers to a total of 17 elements made up of Sc, Y and lanthanides, and REM content denotes the total content of these 17 elements. When lanthanides are used as REM, REM is industrially added in the form of misch metals.

REM has the effect of refining MnS to improve surface fatigue strength. In order to reliably exhibit the effect of REM, 0.001% or more of REM is preferably contained. On the other hand, when the REM content is set to 0.010% or less, the effect of REM can be effectively exhibited without impairing the economy. The REM content can be set to 0.002% or more, 0.003% or more, or 0.004% or more. The REM content can be specified as 0.009% or less, 0.008% or less, or 0.007% or less.

$$\left[0.00\le -2.1\times \text{cb}+\text{0}\text{,04}\times \text{Mn}+\text{0}\text{,5}\times \text{Cr}+\text{1}\text{,8th}\times \text{V-1}\text{,5}\times \text{Mon}\le \text{0}\text{,50}\right]$$

In addition, in the compositions of the steel core of the gas-soft nitriding-treated member according to the present embodiment, the amounts (mass %) of C, Mn, Cr, V, and Mo satisfy the following formula (1). $$0.00\le -2.1\times \text{C}+\text{0}\text{,04}\times \text{Mn}+\text{0}\text{,5}\times \text{Cr}+\text{1}\text{,8th}\times \text{V}-\text{1}\text{,5}\times \text{Mon}\le \text{0}\text{,50}$$

The element symbols in the formula (1) indicate the amounts (mass %) of the respective elements, and the value zero is indicated in the case of containing no elements.

C, Mn, Cr, V and Mo are elements affecting the thickness of the compound layer. C and Mo affect the stabilization of the ε-phase and increase the thickness. On the other hand, Mn, Cr and V affect the thinning of the composite layer. Therefore, if the amounts of these elements are controlled within a certain range, it is possible to stably control the thickness of the compound layer and improve wear resistance and flex life.

In order to obtain these effects, the value of {-2.1 × C + 0.04 × Mn + 0.5 × Cr + 1.8 × V - 1.5 × Mo} in the formula (1) is preferably 0 .00 or more. On the other hand, when the value of {-2.1 × C + 0.04 × Mn + 0.5 × Cr + 1.8 × V - 1.5 × Mo} exceeds 0.50, the composite layer becomes thin and there is a case where the desired surface strength and reverse bending strength cannot be obtained. A preferred lower limit of the value of {-2.1xC + 0.04xMn + 0.5xCr + 1.8xV - 1.5xMo} is 0.03%, 0.05% or 0.10%. A preferred upper limit of the value of {-2.1xC + 0.04xMn + 0.5xCr + 1.8xV - 1.5xMo} is 0.45%, 0.40% or 0.30%.

A measuring part of the chemical composition of the steel core is a part that is 5.0 mm or deeper from the surface of the component. The reason for this is as described below.

It is difficult to precisely determine the boundary between the nitrogen diffusion layer into which nitrogen has penetrated by the nitriding treatment and the steel core into which nitrogen has not penetrated. Under the nitriding conditions of the present embodiment, penetration of nitrogen by the nitriding treatment to a depth of 5.0 mm or more from the surface has almost no influence. Therefore, the chemical composition of the steel core can be measured without obtaining the influence of nitriding on the chemical composition by measuring the chemical composition at a position 5.0 mm or deeper from the surface of the component.

The compound layer of the gas-soft nitriding-treated member according to the present embodiment will be described below.

The soft nitriding treated component according to the present embodiment is manufactured by forming the steel material into a raw form steel and then subjecting it to a soft nitriding treatment under predetermined conditions. As described above, the gas nitriding treated member 1 according to the present embodiment includes the steel core 11, the nitrogen diffusion layer 13 formed on the steel core 11, and the compound layer 12 formed on the nitrogen diffusion layer 13. That is, the gas nitriding treated member 1 according to the present embodiment has a structure , in which the compound layer 12 is present on the surface, the nitrogen diffusion layer 13 is present inside the compound layer 12, and the steel core 11 is present inside the nitrogen diffusion layer 13.

The composite layer is a layer containing, as a main composition, an iron nitride formed by bonding nitrogen atoms penetrated into the raw mold steel by the nitriding treatment and iron atoms contained in the raw mold steel. The compound layer consists mainly of the iron nitride; however, in addition to iron and nitrogen, oxygen taken in from the outside air and each element contained in the raw shape steel (ie, each element contained in the steel core) are also contained in the compound layer. In general, 90% or more (mass%) of the elements contained in the compound layer are composed of nitrogen and iron. The iron nitride contained in the compound layer consists mainly of Fe _{2 to 3} N (ε phase) or Fe ₄ N (γ' phase).

[Thickness of compound layer: 3 µm or more and 20 µm or less]

The thickness of the compound layer affects the wear resistance or the flexural strength of the gas-soft nitrided component. Compared to the diffusion layer 13, the compound layer 12 has a lower deformability. Therefore, when the compound layer is too thick, the compound layer is likely to act as a cause of breakage for the deflection. In addition, if the compound layer is too thin, a part of the component may have a surface without a compound layer, thereby deteriorating wear resistance or flexural strength. In the gas-soft nitriding treated member according to the present embodiment, the thickness of the compound layer is set to 3 μm or more and 20 μm or less from the viewpoint of wear resistance or flexural strength. The composite layer thickness can be set to 5 µm or more, 6 µm or more, or 8 µm or more. The Ver Bond layer thickness can be set to 15 µm or less, 14 µm or less, or 12 µm or less.

The thickness of the compound layer can be measured from a secondary electron image of a scanning electron microscope (SEM). A cross section perpendicular to the surface of the soft nitrided part is polished and etched with a 3% Nital solution for 20 to 30 seconds. The compound layer 12 is observed on the surface layer of the member 1 as a non-corroded layer having no unevenness, and the diffusion layer 13 directly under the compound layer is observed as a corroded layer. The composite layers 12 in 10 fields of view (area per field of view: 6.6×10 ² μm ² ) on structural photographs taken at 4000× magnification are indicated. In addition, on each photograph, the thicknesses of the composite layers 12 are measured at three points 10 µm apart in the horizontal direction. The 10 fields of view are selected so that they do not overlap. In addition, the mean value of the thicknesses of the compound layers 12 at the measured 30 points is defined as the thickness (µm) of the compound layer of the gas-soft nitriding-treated member.

[Area ratio of ε phase in compound layer: more than 50%]

The structural phases of the composite layer influence the wear resistance or flexural strength of the gas-soft nitrided component. The ε-phase has an hcp structure and has low ductility compared to the γ'-phase with fcc structure. On the other hand, the ε-phase has broader N and C mixed crystal regions and higher hardness compared to the γ'-phase. When the area ratio of the ε phase is small, the hardness of the compound layer is likely to become small and the wear resistance tends to deteriorate. In the gas-soft nitriding-treated member according to the present embodiment, the area ratio of the ε-phase in the compound layer is set to be more than 50%. A preferred range of the area fraction of the ε-phase is more than 70%, 75% or more, or 80% or more. The area fraction of the ε phase can be 100%. The upper limit of the area ratio of the ε phase is not particularly limited; however, the area ratio of the ε phase may be set to 95% or less, 92% or less, 90% or less, or 88% or less, for example, from the viewpoint of further increasing the rotational flex life.

The area fraction of the ε phase can be 100%, so the rest should not be present. When the remainder of the composite layer is present, the remainder consists mainly of the γ' phase. There is a case where a peculiar phase belonging to neither ε-phase nor γ'-phase is included. However, the composite layer in which the area ratio of the ε phase is more than 50% and the total value of the area ratios of the ε phase and the γ' phase is more than 95% is regarded as the composite layer having more than 50% of the ε- phase in terms of area ratio and has a residue of γ' phase.

The area ratio of the ε phase is obtained by image processing of structure photographs. Specifically, γ'-phases and ε-phases in the composite layers are distinguished on 10 structural photographs of a cross section perpendicular to the surface of the nitrided member taken by electron backscatter diffraction (EBSD) at a magnification of 4000x. In addition, the area proportions of the ε-phases in the composite layers are obtained by binarizing the structural images using image processing. In addition, the average of the area ratios of the ε phases in the 10 measured visual fields is defined as the area ratio (%) of the ε phase.

[Porosity area ratio in the range from the surface of the compound layer to a depth of 3 µm : Less than 12%]

When porosities exist in a region from the surface of the composite layer to a depth of 3 µm, stress concentration occurs and wear resistance deteriorates, or the porosities act as causes of flex fatigue fracture. Therefore, the area ratio of porosities (porosity area ratio) in the range from the surface of the compound layer to a depth of 3 µm needs to be set to be less than 12%.

The porosity area fraction can be measured by SEM. First, the nitriding treated component is plated with Ni. The nitrided component is then cut perpendicularly to the surface and this cross section is polished. The Ni plating is applied before polishing to prevent deformation of the composite layer during polishing. Also, in the polished cross section, a secondary electron image of a rectangular portion (area: 90 µm ² ) showing the product of a depth of 3 µm from the outermost surface and a length of 30 µm along the outermost surface. Black portions on the secondary electron image in this area can be regarded as porosities. In the secondary electron image, there is a case where the outermost surface of the device is uneven. In this case, the integral average of the outermost surface is regarded as the outermost surface.

In addition, the ratio of the total area of porosities in the photograph of each secondary electron image (porosity area ratio, unit: %) is obtained with an image processing application. In addition, the mean value of the porosity area percentage in the 10 measured fields of view is defined as the porosity area percentage (%) of the component. Even if the thickness of the composite layer is less than 3 µm, an area from the surface to a depth of 3 µm is also considered as a measurement object. The 10 fields of view are selected so that they do not overlap. As for the size of porosities to be measured, porosities having an equivalent circular diameter of 0.3 µm or more in terms of area are regarded as objects. This means that when measuring the porosity area fractions, porosities with an equivalent circular diameter of less than 0.3 µm are ignored. Normally, the equivalent circular diameters of porosities are at most 1 µm.

The porosity area fraction is preferably less than 11%, less than 10%, less than 9%, less than 7% or less than 3% and could be 0%. The lower limit of the porosity area ratio is not particularly limited, and the porosity area ratio can be set to, for example, 0% or more, 1% or more, 2% or more, or 4% or more.

[Hardness of compound layer: preferably 740 HV or more]

When the hardness of the composite layer is high, the wear resistance or rotational flex life of the member improves. The hardness of the compound layer can be increased by increasing the area ratio of ε-phase, by precipitating a nitride such as CrN and VN in the compound layer, or by forming a mixed crystal of a substitutional element in the compound layer. In addition, the hardness changes depending on the nitriding temperature. The gas-soft nitriding treated member according to the present embodiment exhibits excellent wear resistance or rotational flex life when the hardness of the compound layer is set to 740 HV or more, which is preferable. The hardness of the compound layer is preferably 770 HV or more. When the compound layer is controlled to be configured as described above, it is possible to set the hardness of the compound layer to 740 HV or more.

An example of a method for manufacturing a gas-soft nitriding-treated member according to the present embodiment will be described below.

As in 2 1, in the method for manufacturing a gas-soft nitriding-treated member according to the present embodiment, first, a steel material having the above-described steel core compositions is formed into a predetermined shape by working such as hot forging, and machining or grinding is performed as necessary , whereby a rough shape steel is obtained. In addition, a gas-soft nitriding treatment is performed on the raw form steel, whereby a gas-soft nitriding-treated component is obtained.

[Gas Soft Nitriding Treatment]

The soft gas nitration treatment is carried out in a gas atmosphere containing 99% by volume or more in total of CO ₂ , CO or a hydrocarbon gas such as CH ₄ or C ₃ H ₈ under a condition where the nitration potential is controlled to, in addition to NH ₃ , H ₂ and N ₂ to cause C to penetrate into the steel surface. The remainder may contain an impurity gas such as O ₂ . Preferably, the total proportion of NH ₃ , H ₂ , N ₂ , CO ₂ , CO, CH ₄ and C ₃ H ₈ must be 99.5% by volume or more. The gas added for the purpose of penetrating C (CO ₂ , CO or a hydrocarbon gas such as CH ₄ or C ₃ H ₈ ) is hereinafter referred to as carburizing gas.

[Treatment temperature: 550°C to 630°C]

The temperature of the gas soft nitriding treatment mainly correlates with the diffusion rate of nitrogen and affects the surface hardness and the depth of a hardened layer. If the treatment temperature is too low, the diffusion rate of nitrogen is slow, and the thickness of the Composite layer or the depth of the hardened layer becomes shallow. On the other hand, if the soft nitriding treatment temperature is too high, porosity is likely to be generated on the surface of the compound layer, and also the hardness of the compound layer decreases. In addition, when the treatment temperature exceeds the A _Cl point, austenite (γ-phase), which has a slower nitrogen diffusion rate than ferrite (a-phase), is formed at the interface between the compound layer and the diffusion layer, and the depth of the hardened layer becomes shallow. Therefore, in the present embodiment, the soft nitriding treatment temperature is 550° C. to 630° C. near the ferrite temperature range. In this case, it is possible to suppress a decrease in hardness of the compound layer and prevent the depth of the hardened layer from becoming too shallow.

[Treatment Time (Residence Time) of Entire Gas Nitriding Treatment]

The duration of the entire soft gas nitriding treatment, i. H. the time from the start to the end of the soft nitriding treatment (dwell time), correlates with the formation and decomposition of the compound layer and the diffusion and permeation of nitrogen, and affects the surface hardness and the depth of the hardened layer. If the treating time is too short, the thickness of the bonded layer or the depth of the cured layer becomes too small. On the other hand, if the treatment time is too long, the porosity area ratio in the surface of the composite layer increases and the flex fatigue strength decreases. If the treatment time is too long, the manufacturing cost becomes high. Therefore, the treatment time (residence time) of the soft gas nitriding treatment is preferably 1.0 hour or longer and 7.0 hours or shorter. The lower limit of the residence time is preferably set at 1.5 hours, and more preferably at 2.0 hours.

[Carburizing Gas Injection Rate in Soft Gas Nitriding Treatment]

In the soft gas nitriding treatment in the present invention, a single or mixed gas containing at least one of CO ₂ , CO or a hydrocarbon gas such as CH ₄ or C ₃ H ₈ is handled in a carburizing gas injection proportion (vol%) determined by a formula (2) is represented. $$\begin{array}{l}\text{carburizing gas}-\text{injection proportion}\left(\text{volume}\text{.}-\text{\%}\right)=\text{In total}-{\text{injection throughput of CO}}_2,\text{CO}\\ \text{and hydrocarbon gas}\left(1/\text{at least}\right)/\text{In total}-\text{injection throughput of atmosph}\ddot{\text{a}}\text{ric gas}\\ \left(1/\text{at least}\right)\times 100\end{array}$$

When the carburizing gas injection ratio is less than 2% by volume, ε phase is not formed uniformly and wear resistance deteriorates. On the other hand, when the carburizing gas injection ratio is 10% by volume or more, the partial pressure of a nitriding reaction gas such as NH ₃ or H ₂ becomes relatively low, whereby the formation rate of the compound layer becomes slow and the compound layer becomes thin or a variation in the thickness of the compound layer becomes large, deteriorating wear resistance or flex fatigue strength. Therefore, in the method according to the present embodiment, the carburizing gas injection rate is set to be 2% by volume or more and less than 10% by volume. The carburizing gas injection ratio may be set to 3% by volume or more, or 4% by volume or more. The carburizing gas injection rate may be set to 9% by volume or less or less than 8% by volume.

[Nitriding Potential in Soft Gas Nitriding Treatment]

In the method for manufacturing a gas soft nitrided device according to the present embodiment, the nitriding potential is controlled. When soft nitriding is performed on the above-described raw form steel under the following conditions, it is possible to obtain a gas-soft nitriding-treated member in which there is a compound layer with a thickness of 3 to 20 µm and the porosity area ratio ranges from the surface of the compound layer to to a depth of 3 µm is less than 12%.

The nitriding potential K _N of the soft gas nitriding treatment is defined by the following formula (3). $${\text{K}}_{\text{N}}=\left({\text{NH}}_3-\text{partial pressure}\right)/\left[{\left({\text{H}}_2-\text{partial pressure}\right)}^{3/2}\right]\left({\text{atm}}^{-1/2}\right)$$

In addition, the average value K _Nave of the nitriding potentials _KN is the average value of the above-described nitriding potentials _KN measured every 10 minutes from the start to the end of the soft gas nitriding treatment are recorded. As shown in the formula, values in the unit (atm) are used as the partial pressures of NH ₃ and H ₂ .

The partial pressures of NH ₃ and H ₂ in the atmosphere of the soft gas nitriding treatment can be controlled by adjusting the flow amount of the gas.

The investigations of the present inventors have revealed that the nitriding potential of the soft gas nitriding treatment affects the thickness of the compound layer and the porosity area ratio. It has been found that at the optimum nitriding potential, the nitriding potential KN obtained by the formula (3) is maintained in a range of 0.15 or more and 0.40 or less during a soft gas nitriding treatment, and that the average _{value KNave} of the nitriding potentials _KN becomes 0.18 or more and less than 0.30 during the gas soft nitriding treatment.

When soft nitriding is performed on steel with the composition system of the present invention under such conditions, it is possible to produce a gas-soft nitriding-treated member having a compound layer in which the thickness of the compound layer is stably 3 to 20 µm and the porosity area ratio ranges from the surface to to a depth of 3 µm is less than 12%.

[Examples]

Steels a to ab having chemical compositions shown in Table 1 were melted in a 50 kg vacuum melting furnace to produce molten steels, and the molten steels were cast to produce ingots. a to t in Table 2-1 and Table 2-2 are steels having the chemical composition specified in the present invention. On the other hand, Steels u to ab are comparative example steels in which at least one or more elements differ from the chemical composition specified in the present invention. In Table 1, "X" denotes the value of "-2.1 × C + 0.04 × Mn + 0.5 × Cr + 1.8 × V - 1.5 × Mo". In addition, underlining indicates compositions not falling within the scope of the present invention and blank cells indicate that the corresponding alloying elements were not intentionally added. In addition, the compositions of the steels a to ab listed in Table 1 contain, in addition to the composition (balance) shown in Table 1, Fe and an impurity. All steels contained about 10 ppm O as an impurity.

The ingot of each of steels a to ab was hot forged to produce a round bar having a diameter of 40 mm. Then, each round bar was annealed and cut to form a small roller for a pitting test for wear resistance evaluation as described in 3 shown, and a large roller, as in 4 shown to produce. In addition, a cylindrical test piece was used to evaluate the in 5 shown rotational bending fatigue strength produced.

Soft gas nitriding treatment was performed on the collected test pieces under the following conditions. The test piece was loaded into a gas soft nitriding furnace, each gas of NH ₃ , H ₂ , N ₂ and CO ₂ was introduced into the furnace, and the soft nitriding treatment was performed under the conditions shown in Table 2-1 and Table 2-2. The total injection flow rate of the NH ₃ , H ₂ and N ₂ gases and the injection flow rate of the CO ₂ gas were set to remain unchanged during the treatment so that the injection proportion of the CO ₂ gas became constant. After the gas soft nitriding treatment, the test pieces were cooled with an oil of 80°C.

The H ₂ partial pressure in the atmosphere was measured with a heat conduction type H ₂ sensor installed directly in the furnace body of the soft gas nitriding furnace. The measurement is made by converting the difference in thermal conductivity between a standard gas and the measured gas into a gas concentration. The H ₂ partial pressure was continuously measured during the gas soft nitration treatment.

In addition, the NH ₃ partial pressure was measured every 10 minutes with a glass tube type NH ₃ analyzer attached to the outside of the furnace.

The NH ₃ flow rate, H ₂ flow rate, and _{N 2} flow rate were adjusted so that the nitration potential KN converged to a target value. The nitration potential _KN was recorded every 10 minutes, and the minimum, maximum and mean values during the treatment were derived. [TABLE 2-1] test number steel soft gas nitriding treatment temperature time Nitration potential K _N CO ₂ volume fraction minimum maximum Average (ºC) (H) (atm ^-1/2 ) (%) 1 a 590 3.0 0.18 0.31 0.24 4 2 a 600 4.0 0.19 0.36 0.29 5 3 a 570 3.0 0.16 0.28 0.20 3 4 a 610 3.0 0.17 0.35 0.28 6 5 a 560 4.0 0.20 0.36 0.25 3 6 a 590 3.0 0.17 0.27 0.23 9 7 a 590 3.0 0.22 0.38 0.28 2 8th b 570 5.0 0.22 0.38 0.29 5 9 c 610 3.0 0.16 0.26 0.18 6 10 i.e 570 3.0 0.21 0.40 0.27 7 11 e 590 3.0 0.16 0.32 0.21 8th 12 f 610 1.0 0.18 0.25 0.20 5 13 9 550 7.0 0.20 0.36 0.27 4 14 H 630 3.0 0.16 0.23 0.19 3 15 i 600 3.0 0.18 0.26 0.23 4 16 j 590 4.0 0.20 0. 35 0.26 5 17 k 590 3.0 0.17 0.32 0.24 4 18 l 590 3.0 0.18 0.34 0.26 3 19 m 590 3.0 0.16 0.33 0.20 6 20 n 590 3.0 0.20 0.35 0.25 5 21 o 590 3.0 0.16 0.31 0.21 4 22 p 590 3.0 0.18 0.33 0.25 5 23 q 590 3.0 0.17 0.35 0.21 5 24 right 570 6.0 0.16 0.30 0.20 6 25 s 590 3.0 0.20 0.34 0.28 8th 26 t 590 3.0 0.19 0.33 0.25 5 27 a 640 5.0 0.18 0.37 0.28 4 28 a 540 2.0 0.20 0.35 0.25 5 29 a 600 11.0 0.19 0.34 0.28 6 30 a 550 0.5 0.16 0.28 0.20 7 31 a 630 3.0 0.15 0.41 0.29 5 32 a 560 1.0 0.14 0.30 0.18 8th 33 a 620 10.0 0.23 0.39 0.29 9 34 a 570 2.0 0.15 0.21 0.17 2 35 b 560 1.5 0.15 0.25 0.18 10 36 b 590 6.5 0.17 0.36 0.25 0 37 and 570 2.0 0.18 0.32 0.22 2 38 v 610 4.0 0.20 0.38 0.25 5 39 w 560 5.0 0.17 0.26 0.20 2 40 x 580 2.0 0.18 0.30 0.22 3 41 y 580 2.0 0.17 0.32 0.23 4 42 e.g 570 2.0 0.17 0.35 0.23 5 43 ah 600 1.5 0.17 0.36 0.24 4 44 away 610 1.0 0.16 0.38 0.26 8th 45 i 590 5.0 0.95 1.70 1.28 3
Underlining means that the corresponding values do not fall within the scope of the present invention or do not meet the passing criteria.

[Measurement of compound layer thickness and porosity area ratio]

A cross section of the small roller after the gas soft nitriding treatment in a direction perpendicular to the longitudinal direction was mirror polished and etched. The etched cross section was observed with a scanning electron microscope (SEM, manufactured by JEOL Ltd.; JSM-7100F), the thickness of the compound layer was measured, and the presence or absence of porosities in the surface layer of Ver bund layer was confirmed. Etching was performed with a 3% Nital solution for 20 to 30 seconds.

The compound layer can be confirmed as a non-corroded layer on the surface layer of the steel. The composite layer was observed in 10 fields of view (1 field of view area: 30 µm in width × 22 µm in length = 6.6 × 10 ² µm ² ) in a structure photograph taken at 4000× magnification, and the thicknesses of the composite layer were measured at 3 Points measured every 10 µm in each field of view. In addition, the average value of the 30 measured points was defined as the thickness of the composite layer (µm).

In addition, from the same structure photograph, the ratio of the total area of porosities in an area of 90 µm ² in a range from the outermost surface to a depth of 3 µm (porous area ratio, unit: %) was calculated with an image processing program (manufacturer: JEOL Ltd.; Analysis station) received. In addition, the average value of the 10 measured fields of view was defined as a porosity area ratio (%). Also, even when the thickness of the compound layer was less than 3 µm, an area from the surface to a depth of 3 µm was considered as a measurement object.

[hardness of compound layer]

Der Durchschnittswert der 50 gemessenen Punkte wurde als Härte (HV) der Verbundschicht definiert.The hardness of the compound layer was measured with a nanoindentation device (manufactured by Hysitron Inc.; TI950) by the following method. The Vickers hardness HV was measured from a force-displacement curve obtained by accidentally pressing a position near the center in the thickness direction of the composite layer at 50 points with an indenter under a compressive load of 10 mN. The indenter was in the shape of a triangular pyramid (Berkovic), and the hardness was determined based on ISO 14577-1 by converting the nanoindentation hardness H _IT to the Vickers hardness HV using the following formula. $$\text{HV}=0.0924\times {\text{H}}_{\text{IT}}\left(\text{MPa}\right)$$

The average value of the 50 measured points was defined as the hardness (HV) of the compound layer.

[Wear Resistance Evaluation Test]

The wear resistance was evaluated by the following method using a roller pitting tester (manufactured by Komatsu-Setsubi; RP102). In the small rollers for the roller pitting test, the grip portions were reworked to eliminate the heating stress, and then the small rollers were each used as a roller pitting test piece. The shape after finishing is in 3 shown.

The pitting test was carried out under the conditions shown in Table 3 with a combination of the above-described small roller for roller pitting test and large roller for roller pitting test with the in 4 performed in the form shown.

The units of measurement 3 and 4 are "mm". The large roller for the roller pitting test was a roller made through ordinary manufacturing steps, ie, "normalizing → processing the test piece → eutectic carburizing with a gas carburizing furnace → low-temperature annealing → polishing" using steel conforming to the SCM420 standard of JIS G 4053 (2016) and provided with fine unevenness on the surface by shot peening treatment at a projection pressure of 0.2 MPa using a steel ball with a particle size of 0.8 mm, Vickers hardness HV at a position of 0.05 mm from the surface, that is, at a position at a depth of 0.05 mm was 740 to 760, and the depth having a Vickers hardness Hv of 550 or more was in a range of 0.8 to 1.0 mm.

Table 3 shows the test conditions under which wear resistance was evaluated. The test was terminated after the number of repetitions reached 5×10 ⁶ , and the depth of wear of the small roller after the test was measured. The worn portion of the small roll after the test was scanned along the axial main direction with a surface roughness shape measuring machine (manufactured by Tokyo Seimitsu Co., Ltd.; SURFCOM FLEX), a cross-sectional shape profile was detected, and a difference between the maximum depth (worn portion) and the minimum depth (non-worn portion) in the detected cross-sectional shape profile was measured as the maximum wear depth. For the same test piece (small roller), the maximum wear depths were measured at five measurement positions and averaged, thereby calculating the wear depth value. In the component of the present invention, a wear depth of 10 µm or less was aimed for.

[TABLE 3] tester Roller Pitting Tester test piece size Small roller: diameter of 26mm Large roller: diameter of 130mm Contact area 300mmR surface pressure 1700MPa number of tests 5 hatch rate 0% Number of rotations of the small roller 1500rpm circulation speed Small roller: 123 m/minute Large roller: 123 m/minute lubricant Type: Automatic oil Oil temperature: 80°C

[Rotational flex fatigue strength evaluation test]

An Ono-type rotational flexural fatigue test was conducted based on JIS Z 2274 (1978) on a cylindrical test piece subjected to gas soft nitriding treatment. The number of rotations was 3000 rpm. The number of test failures was set at 1 × 10 ⁷ which indicates the fatigue limit of ordinary steel. The maximum stress reached in the test 1 × 10 ⁷ times without breaking in a rotational flexing test piece was regarded as the fatigue limit of the rotational flexing test piece.

In the component of the present invention, a maximum fatigue limit stress of 500 MPa or more was aimed for.

[test results]

The results are shown in Table 2-1 and Table 2-2. In Tests Nos. 1 to 26, the steel compositions and the conditions of the gas soft nitriding treatments were within the scope of the present invention, the bonded layer thicknesses were 3 µm or more and 20 µm or less, and the bonded layer porosity area ratios were less than 12%. As a result, favorable results of wear depths of less than 10 µm and rotational flex fatigue strengths of 500 MPa or more were obtained.

In Tests Nos. 27 to 45, the steel compositions and some of the conditions for the gas soft nitriding treatments were outside the scope of the present invention, and one of the thicknesses of the compound layer, the area ratio of the ε phase and the porosity area ratio, or a variety of properties did not reach the target values of the present invention. As a result, the wear resistance or the rotational flex life did not meet the objectives of the present invention.

Test No. 27 is a comparative example in which the temperature of the gas soft nitriding treatment was too high at the time of manufacture. As a result, in Test No. 27, the porosity area ratio became too large, and wear resistance and rotational flex life were insufficient.

Test No. 28 is a comparative example in which the temperature of the gas soft nitriding treatment was too low at the time of manufacture. As a result, in Test No. 28, the thickness of the composite layer was insufficient, and the wear resistance and rotational flex life were insufficient.

Test No. 29 is a comparative example in which the gas soft nitriding treatment time at the time of manufacture was too long. As a result, in Test No. 29, the thickness of the compound layer increased large, the porosity area ratio became too large, and the wear resistance and rotational flex life were insufficient.

Test No. 30 is a comparative example in which the gas soft nitriding treatment time at the time of manufacture was too short. As a result, in Test No. 30, the thickness of the composite layer was insufficient, and the wear resistance and rotational flex life were insufficient.

Test No. 31 is a comparative example in which the nitriding potential _KN is too high at the time of manufacture. As a result, in Test No. 31, the porosity area ratio became too large, and wear resistance and rotational flex life were insufficient.

Test No. 32 is a comparative example in which the nitriding potential _KN becomes too low at the time of manufacture. As a result, in Test No. 32, the thickness of the composite layer was insufficient, and the wear resistance and rotational flex life were insufficient.

Test No. 33 is a comparative example in which the gas soft nitriding treatment time was too long at the time of manufacture. As a result, in Test No. 33, the thickness of the compound layer became too large, the porosity area ratio became too large, and the wear resistance was insufficient.

Test No. 34 is a comparative example in which the average nitriding potential K _Nave was too low at the time of manufacture. As a result, in Test No. 34, the thickness of the composite layer was insufficient, the ε area ratio was insufficient, and the wear resistance and rotational flex life were insufficient.

Test No. 35 is a comparative example in which the carburizing gas injection rate was too high at the time of manufacture. As a result, in Test No. 35, the thickness of the compound layer was insufficient, and the wear resistance and rotational flex life were insufficient.

Test No. 36 is a comparative sample in which the carburizing gas injection rate was too low at the time of manufacture (i.e., nitriding treatment was not performed but soft nitriding was performed). As a result, in Test No. 36, the ε area ratio was insufficient and the wear resistance was insufficient.

Preparation No. 37 (Steel u) is a comparative example in which the chemical composition of the steel core is -2.1 × C + 0.04 × Mn + 0.5 × Cr + 1.8 × V - 1.5 × Mo was too high. As a result, in Test No. 37, the thickness of the composite layer was insufficient, and the wear resistance and rotational flex life were insufficient.

Preparation No. 38 (Steel v) is a comparative example in which the chemical composition of the steel core is -2.1 × C + 0.04 × Mn + 0.5 × Cr + 1.8 × V - 1.5 × Mo was too low. As a result, in Test No. 38, the thickness of the composite layer became too large and the rotational flex life was insufficient.

Preparation No. 39 (steel w) is a comparative example in which the C content of the steel core was too low. As a result, in Test No. 39, wear resistance was insufficient.

Production No. 40 (Steel x) is a comparative example in which the Mn content in the steel core was too low. As a result, in Test No. 40, wear resistance and rotational flex life were insufficient.

Production No. 41 (Steel y) is a comparative example in which the Cr content of the steel core was too low. As a result, in Test No. 41, wear resistance and rotational flex life were insufficient.

Production No. 42 (Steel z) is a comparative example in which the V content in the steel core was too low. As a result, in Test No. 42, wear resistance and rotational flex life were insufficient.

Production No. 43 (steel aa) is a comparative example in which the content of P in the steel core was too high. As a result, in Test No. 43, wear resistance and rotational flex life were insufficient.

Preparation No. 44 (steel ab) is a comparative example in which the content of S in the steel core was too high. As a result, in Test No. 44, wear resistance and rotational flex life were insufficient.

Preparation No. 45 is a comparative example in which the nitriding potential _KN and the average nitriding potential K _Nave were too high at the time of preparation. As a result, in Test No. 45, the thickness of the composite layer became too large, the porosity area ratio became too large, and wear resistance and rotational flex life were insufficient.

Up to this point, the embodiment of the present invention has been described. However, the embodiment described above is only an exemplary example for carrying out the present invention. Therefore, the present invention is not limited to the embodiment described above, and the embodiment described above can be suitably modified and performed within the scope of the gist of the present invention.

[Industrial Applicability]

According to the present invention, it is possible to provide a soft nitrided component excellent not only in wear resistance but also in rotational flex life and a method of manufacturing the same, and to provide a continuously variable transmission (CVT), a camshaft component and the like which are excellent in terms of wear resistance and reverse bending strength are particularly excellent.

Reference List

1

Gas-soft nitrided component (component)

11

steel core (steel)

12

composite layer

13

Nitrogen diffusion layer (diffusion layer)

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• WO 2016153009 [0012]
• WO 2019098340 [0012]
• WO 2018066666 [0012]

Claims (6)

A gas-soft nitriding-treated member comprising: a steel core; a composite layer; and a nitrogen diffusion layer located between the steel core and the composite layer, wherein the steel core contains, as a composition in % by mass, C: 0.05% to 0.60%; Si: 0.05% to 1.50%; Mn: 0.20% to 2.50%; P: 0.025% or less; S: 0.050% or less; Cr: 0.50% to 2.50%; V: 0.05% to 1.30%; Al: 0.050% or less; N: 0.0250% or less; and a balance comprising Fe and an impurity, the amounts of C, Mn, Cr, V and Mo in the composition of the steel core satisfy the formula (1), a thickness of the compound layer is 3 to 20 µm, the compound layer is more than 50 % of a ε phase in terms of an area ratio and a balance of a γ' phase, and a porosity area ratio in a range from a surface of the composite layer to a depth of 3 µm is less than 12%, $$0.00\le -2.1\times \text{C}+\text{0}\text{,04}\times \text{Mn}+\text{0}\text{,5}\times \text{Cr}+\text{1}\text{,8th}\times \text{V}-\text{1}\text{,5}\times \text{Mon}\le \text{0}\text{,50}$$

here, the element symbols in formula (1) indicate the amounts (mass%) of the respective elements, and zero is assigned when the elements are not included. Component treated with gas soft nitriding, according to claim 1 , wherein the steel core contains as a composition in mass %: C: 0.05% to 0.60%; Si: 0.05% to 1.50%; Mn: 0.20% to 2.50%; P: 0.025% or less; S: 0.050% or less; Cr: 0.50% to 2.50%; V: 0.05% to 1.30%; Al: 0.050% or less; N: 0.0250% or less; Mon: 0% to 1.50%; Cu: 0% to 1.00%; Ni: 0% to 1.00%; W: 0% to 0.50%; Co: 0% to 0.50%; Nb: 0% to 0.300%; Ti: 0% to 0.250%; B: 0% to 0.0100%; Ca: 0% to 0.010%; Mg: 0% to 0.010%; REM: 0% to 0.010%; and the balance consisting of Fe and an impurity. Component treated with gas soft nitriding claim 1 or 2 , where in the composition of the steel core, C is 0.05% to 0.35%. Gas soft nitrided component according to one of Claims 1 until 3 , wherein the composite layer contains 90% or less of the ε phase in terms of area ratio. Gas soft nitrided component according to one of Claims 1 until 4 , wherein the thickness of the compound layer is 6 µm or more. Manufacturing process of the gas-soft nitrided component according to one of Claims 1 until 5 comprising: processing a steel material having the composition of the steel core according to any one of Claims 1 until 3 into a predetermined shape to obtain a rough shape steel; and performing a gas soft nitriding treatment on the raw shape steel, wherein the gas soft nitriding treatment is performed in a gas atmosphere containing at least one of CO ₂ , CO and hydrocarbon gas in a carburizing gas injection proportion represented by a formula (2) of 2% by volume or _more and _less than ₁₀ vol .0 hours or shorter, in the gas atmosphere, a nitriding potential K _N obtained by formula (3) is within a range of 0.15 or more and 0.40 or less by performing the gas soft nitriding treatment, and an average value K _Nave of the nitriding potentials K _N is 0.18 or more and less than 0.30, $$\begin{array}{l}\text{carburizing gas}-\text{injection proportion}\left(\text{volume}\text{.}-\text{\%}\right)=\text{In total}-{\text{injection throughput of CO}}_2,\text{CO}\\ \text{and hydrocarbon gas}\left(1/\text{at least}\right)/\text{In total}-\text{injection flow rate of atmosph}\ddot{\text{a}}\text{ric gas}\\ \left(1/\text{at least}\right)\times 100\end{array}$$

$${\text{K}}_{\text{N}}=\left({\text{NH}}_3-\text{partial pressure}\right)/\left[{\left({\text{H}}_2-\text{partial pressure}\right)}^{3/2}\right]\left({\text{atm}}^{-1/2}\right)$$

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