JP6636829B2 - Nitrided steel member and method of manufacturing nitrided steel - Google Patents

Description

  The present invention relates to a nitrided steel member and a method for producing a nitrided steel member, and more particularly, to abrasion resistance whose surface is nitrided by gas nitriding, which is useful for gears and crankshafts for automobiles and transmissions. The present invention relates to a nitrided steel member having a sufficiently thick compound layer having excellent fatigue resistance and a method for producing a nitrided steel member.

Among the surface hardening treatments of steel, there is a great need for nitriding treatment, which is a low strain treatment. In recent years, there has been an increasing interest in gas nitriding atmosphere control technology. This is due to the fact that the gas nitriding treatment improves the carburizing or carbonitriding treatment involving quenching of machine parts and the distortion caused by induction hardening. The basis of the atmosphere control by the gas nitriding treatment is to control the nitriding potential (K _N ) in the furnace, whereby the γ ′ phase (Fe ₄ N) and the ε phase (Fe ₄ N) in the compound layer formed on the steel material surface are formed. A wide range of nitriding qualities can be obtained, such as control of the volume fraction of Fe _2-3 N) or treatment without formation of a compound layer, and various proposals have been made. For example, in Patent Literature 1, bending fatigue strength and surface fatigue are improved by selecting the γ 'phase, and carburization is substituted.

On the other hand, formation of a thick compound layer is desired from the viewpoints of wear resistance and fatigue resistance. However, in the conventional nitridation mainly composed of ε-phase, the compound layer is easily peeled off, and a thick film cannot be formed (Patent Document 2-4). Specifically, the technology of Patent Document 2 relates to a nitrocarburized gear for the purpose of reducing gear noise, but specifies that the thickness of the compound layer and the thickness of the porous layer are 12 μm or less. Further, in the technique of Patent Document 3, stress concentration due to unevenness is avoided by reducing the thickness of the compound layer to 5 μm or less. In the technique of Patent Document 4, bending correction is performed at a compound layer thickness of 5 μm or less. Cracks are suppressed. Further, even in the technique of Patent Document 1 described above, since the γ ′ phase is processed with a low K _N , if the γ ′ phase is to be made thicker, it takes a long time for the nitriding treatment, and as a result, the diffusion layer Since the softening and the reduction of the compressive residual stress occur, it is difficult to increase the film thickness.

On the other hand, as a method for uniformly forming the γ 'phase-based compound layer, the amount and flow rate of the material to be treated, and first, nitriding is performed in an atmosphere where the NH ₃ partial pressure is high in the furnace, and then the furnace is heated. A two-stage nitriding method in which the atmosphere has a low NH ₃ partial pressure has been performed (see Patent Document 5). Using this method, the atmosphere in the first stage is set to a higher K _N, 'be selected to K _N of the phase field, gamma' gamma the second stage compound layer of a phase mainly can be thickened It is. Patent Document 5 discloses that the thickness of the compound layer is preferably 4 to 16 μm. However, Patent Document 5 describes that if the thickness exceeds 16 μm, the proportion of the ε phase increases and the material becomes brittle, so that improvement in fatigue strength cannot be expected. In Comparative Example 2, the thickness of the compound layer is reduced. Although there is a description that the thickness varies from lot to lot, there is no description about the thickness of the compound layer in the examples, and it is estimated that the thickness of the compound layer that can be put to practical use by this technique has not reached 16 μm. Further, in the technique described in Patent Document 5, the nitriding time in the second-stage gas atmosphere is not controlled, and according to the embodiment, the nitriding time is long. According to the study of the present inventors, under such conditions, the crystal grains of the formed γ ′ phase are large, which causes a decrease in mechanical properties such as abrasion resistance. is there.

JP 2013-221203 A JP-A-11-72159 JP 2009-30134 A JP 2014-129607 A WO 2015/046593

Although there is a problem that it is difficult to increase the film thickness as described above, conventionally, when a steel member is subjected to gas nitriding or gas nitrocarburizing treatment, wear resistance and fatigue strength are improved as compared with an untreated material,
These processes have been used. In order to improve wear resistance and fatigue resistance, it is desired that the compound layer be thick.However, since increasing the thickness of the compound layer lowers the fatigue strength, as described above, the thickness of the compound These problems are addressed by optimizing the thickness according to the requirements (see Patent Documents 2 to 4). On the other hand, recently, as can be seen from the descriptions of Patent Documents 1 and 5, fatigue strength can be improved by using a compound layer mainly composed of a γ ′ phase rather than a compound layer mainly composed of an ε phase that has been conventionally used. It is clear.

However, since the formation of the compound layer mainly composed of the γ ′ phase is performed at a low K _N , it is necessary to increase the processing temperature or perform the treatment for a long time to increase the thickness of the compound layer. . On the other hand, the higher temperature and longer time of the gas nitriding process cause a decrease in compressive residual stress and hardness formed in the nitrided diffusion layer, and increase the load on production and the environment. The construction method could not be realized industrially and could not be realized. Further, it is possible to increase the thickness of the compound layer mainly composed of γ ′ by a two-stage gas nitriding treatment as described in Patent Document 5, but with this technique, the ratio of the ε phase increases at 16 μm or more. Therefore, improvement in fatigue strength cannot be expected, and further increase in film thickness cannot be expected. Further, in the technique described in Patent Document 5, the time of the second-stage gas nitriding treatment is not optimized, and as described above, the nitriding time is long. There is a problem that crystal grains become large and mechanical properties such as abrasion resistance deteriorate.

  That is, the compound layer mainly composed of γ ′ formed by the gas nitriding treatment can improve the fatigue strength without lowering the fatigue strength, and can have a thickness satisfying the desired wear resistance. There is no conventional technology that can be realized, and if it can be realized, its industrial value is extremely large.

  Therefore, an object of the present invention is to provide a compound layer mainly composed of a γ ′ phase to have a thickness of 13 μm or more, preferably 15 μm or more, even during a normal gas nitriding treatment time. Developing a technology that can provide a nitrided member that can increase the fatigue strength by increasing the thickness of the layer and further improve the wear resistance while having higher fatigue strength than the conventional technology It is to be.

  The present inventors have conducted intensive studies to achieve the above object, and as a result, the volume ratio of the γ ′ phase and the ε phase in the iron-nitrogen compound layer is specific, and in addition, the thickness of the compound layer is 13 μm or more. The above object is achieved when the ratio of the thickness CLT [μm] of the compound layer after gas nitriding to the practical hardening depth DLT [μm] of the nitrided diffusion layer after gas nitriding satisfies a specific relationship. The inventors have found that the present invention can be performed, and have completed the present invention.

That is, the present invention is a nitrided steel member in which an iron-nitrogen compound layer is formed on the surface of a steel member made of a steel material component in which a γ′-phase-based compound layer is generated, and γ ′ occupying the iron-nitrogen compound layer. When the volume ratio of the ε phase to the ε phase is Vγ ′ and Vε, and the proportion of the γ ′ phase is represented by the ratio defined by Vγ ′ / (Vγ ′ + Vε), the value is γ of 0.5 or more. 'The thickness of the phase-based compound layer is 13 μm to 30 μm, and the iron-nitrogen compound layer represents the value of the thickness [μm] of the compound layer after gas nitriding as CLT, and Provided is a nitrided steel member that satisfies the following expression (1) when the value of the practical hardening depth [μm] of the nitrided diffusion layer is represented by DLT.
CLT ÷ DLT ≧ 0.04 (1)

  In the nitrided steel member of the present invention, the thickness of the γ′-phase-based compound layer is preferably 15 μm to 30 μm, and can be suitably applied to a gear or a crankshaft.

According to another aspect of the present invention, the nitriding member according to claim 1, wherein the material to be treated in the furnace is subjected to gas nitriding while adjusting the nitriding potential (K _N ) in the furnace. Wherein the gas introduced into the gas nitriding furnace is a mixture of only two kinds of ammonia and ammonia decomposition gas, or a plurality of mixed gases containing ammonia and ammonia decomposition gas. As a gas, perform continuous detection of the hydrogen concentration in the vicinity of the material to be processed, estimate the ammonia partial pressure in the furnace based on the detection result, and automatically control the atmosphere to control to a set nitriding potential. At the time of control, the nitriding potential K _N = p _NH3 / p _H2 ^1.5 in the atmosphere during the gas nitriding treatment is first set to K _N1, and then, if necessary, the nitriding potential is set to K _{N2 to} K _Nx-1 , K _Nx. As well However, the final K _Nx is controlled so as to satisfy the following equations (2) and (3) simultaneously, and the nitriding time at the final K _Nx controlled so as to satisfy the above requirements is 5 to 5. A method for producing a nitrided steel member, wherein the method is performed for 60 minutes, is provided.
K _{N1 to} K _Nx-1 > K _Nx (2)
126.7034−5.68797 × 10 ⁻¹ × T + 8.64682 × 10 ⁻⁴ × T ² −4.43596 × 10 ^-7 × T ³ > K _Nx > 22.2265-1.15 × 10 ^-1 × T + 2.03 × 10 ^-4 × T ² − 1.21466 × 10 ^-7 × T ³ ... (3)
(However, p _NH3 and p _H2 are the partial pressures of NH ₃ and H ₂ in the nitriding furnace, and T in Expression (3) is the temperature [° C.].)
In the method for manufacturing a nitrided steel member of the present invention, it is preferable that the K _N1 is 1.0 to 2.0.

According to the present invention, the compound layer mainly composed of the γ ′ phase can be formed into a thick film in the same manner as the compound layer mainly composed of the ε phase even during the normal gas nitriding treatment time. Even when the film thickness is increased, the fatigue strength can be improved without lowering the bending fatigue strength as compared with the compound layer mainly composed of the γ ′ phase formed at a low K _N , and It is possible to provide an unprecedented new nitriding member capable of improving abrasion. Note that, as defined in the present invention, the “compound layer mainly composed of the γ ′ phase” in the present invention refers to an existing ratio of the γ ′ phase in the iron-nitrogen compound layer formed on the surface of the steel member. , Vγ ′ / (Vγ ′ + Vε) means a portion whose value is 0.5 or more.

It is a Lehrer diagram of iron (E. Lehrer: Zeitschrift fur Elektrochemie, 36, p.383 (1930).). It is a schematic diagram of a pit type gas nitriding furnace. It is a heat pattern (pit furnace). It is a schematic diagram of a batch type gas nitriding furnace. It is a heat pattern (batch furnace). It is an Ono-type rotating bending fatigue test piece. It is explanatory drawing of the outline | summary of an SRV testing machine. It is an explanatory view of an oscillation method. It is a figure showing an example (black part: analysis impossible) of an EBSD analysis result.

  Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. First, the premise of the technology of the present invention will be described. The present invention is a nitrided steel member in which a γ ′ phase-based iron-nitrogen compound layer is formed on the surface of a steel member, wherein a specific volume ratio of the γ ′ phase and the ε phase in the iron-nitrogen compound layer is specified. Therefore, the steel material forming the compound layer needs to be a component that forms a compound layer mainly composed of a γ ′ phase. Further, since the present invention aims at providing a nitrided steel member that can be effectively applied to gears and crankshafts for automobiles and transmissions, the present invention is required for functions such as machinability and manufacturability of steel materials. Some elements are indispensable and some are necessarily present as impurity elements. In consideration of these points, the “steel comprising a steel material component that forms the γ′-phase-based compound layer” constituting the present invention is preferably a steel type satisfying the following component range. In addition, the following% is based on mass.

[Steel component as base material]
<Carbon (C)>
C can be said to be an indispensable element for securing the strength of the nitrided component, and requires a content of 0.05% or more. On the other hand, when the content of C is increased to exceed 0.5%, the hardness before nitriding is increased and the machinability is reduced. Therefore, the content of C is 0.05 to 0.5%. It is preferred that

<Silicon (Si)>
Si has a deoxidizing effect. To obtain this effect, a Si content of 0.10% or more is required. However, when the content of Si is increased to exceed 0.90%, the hardness before nitriding is increased and the machinability is reduced, which is not preferable. Therefore, the content of Si is preferably 0.10 to 0.90%.

<Manganese (Mn)>
Mn has a deoxidizing effect. To obtain this effect, a Mn content of 0.3% or more is required. However, when the content of Mn is increased to exceed 1.65%, the hardness before nitriding becomes too high, and the machinability is undesirably reduced. Therefore, the content of Mn is preferably from 0.3 to 1.65%.

<Nickel (Ni)>
Ni does not necessarily have to be contained. However, Ni is a component contributing to the improvement of hardenability and toughness, and may be contained from this viewpoint. However, if the content is too large, the above effect only reaches saturation and not only raises the cost but also lowers the machinability, which is not preferable. Therefore, its content is preferably limited to 2.10% or less.

<Chrome (Cr)>
Cr does not necessarily have to be contained. However, when the content of Cr is increased to exceed 2.7%, the thickness of the nitride compound layer is reduced, and the effect of the compound layer mainly composed of the γ ′ phase constituting the present invention cannot be sufficiently obtained. Therefore, the content of Cr is preferably 0 to 2.7%.

<Molybdenum (Mo)>
Mo does not necessarily have to be contained. However, Mo combines with C in the steel at the nitriding temperature to form carbides, thereby improving the core hardness after nitriding, and is an element necessary for some mechanical parts. However, when the content of Mo is increased to exceed 0.50%, not only is the raw material cost increased, but also the hardness before nitriding is increased and the machinability is reduced, which is not preferable. Therefore, the content of Mo is preferably 0.50% or less.

<Vanadium (V)>
V does not necessarily have to be contained. However, when V is contained, as in Mo, it combines with C in the steel at the nitriding temperature to form carbides and has the effect of improving the core hardness after nitriding, and has the effect of penetrating from the surface during nitriding. -It combines with N and C which diffuse to form nitrides and carbonitrides, has an effect of improving surface hardness, and is an element required in some cases. However, when the content of V is increased to exceed 0.40%, the hardness before nitriding becomes too high and the machinability is reduced, and the V content in the matrix by hot forging or subsequent normalizing is increased. Does not form a solid solution, so that the above effect is saturated. Therefore, the content of V is preferably 0 to 0.30%.

<Aluminum (Al)>
Al does not necessarily have to be contained. However, when the content of Al is increased to exceed 1.1%, the formation amount of the nitride compound layer is reduced, and the effect of the compound layer mainly composed of the γ ′ phase of the present invention cannot be sufficiently obtained. Therefore, the content of Al is preferably 0 to 1.1%.

<Sulfur (S)>
S combines with Mn to form MnS and has an effect of improving machinability. However, when the S content exceeds 0.030%, coarse MnS is formed, and the hot forgeability and the bending fatigue strength decrease. Therefore, the content of S is preferably 0.002 to 0.030%. In order to secure the machinability more stably, the content of S is preferably 0.010% or more. Further, in the case of a member applied to an application in which hot forgeability and bending fatigue strength are more important, the S content is preferably 0.025% or less.

<Phosphorus (P)>
P is an impurity contained in steel and segregates at grain boundaries to embrittle the steel. In particular, if the content exceeds 0.030%, the degree of embrittlement may become significant. Therefore, in the present invention, the content of P in the impurities needs to be 0.030% or less. Note that the content of P in the impurities is preferably 0.020% or less.

<Other elements>
The steel member used in the present invention has a chemical composition consisting of Fe and other impurities in addition to the above elements. In addition, “Fe and impurities” as the balance refers to, for example, copper (Cu) or titanium (Ti), which is inevitably mixed from ore as a raw material when a steel material is manufactured industrially, or in a manufacturing environment. For example, it means that components other than Fe, such as O (oxygen), which are inevitably mixed in from the material.

(Nitrided steel members)
The nitrided steel member of the present invention is characterized in that an iron-nitrogen compound layer having a specific configuration is formed on the surface of a steel member made of the above-described steel material components in which a γ ′ phase-based compound layer is generated. I do. That is, in the iron-nitrogen compound layer that characterizes the nitrided steel member of the present invention, the relationship between the volume ratio Vγ ′ and Vε of the γ ′ phase and the ε phase in the layer is defined by Vγ ′ / (Vγ ′ + Vε). When the ratio is expressed by the following ratio, the value is 0.5 or more (the existence ratio of the γ ′ phase is 0.5 or more), the thickness of the γ ′ phase-based compound layer is 13 μm to 30 μm, and , Wherein the iron-nitrogen compound layer satisfies the relationship of the following formula (1).
CLT ÷ DLT ≧ 0.04 (1)
(However, CLT in the formula (1) indicates the value of the thickness [μm] of the compound layer after the gas nitriding treatment, and DLT indicates the value of the practical hardening depth [μm] of the nitrided diffusion layer after the gas nitriding treatment. Value.)
Hereinafter, these will be described.

<Nitrogen compound layer>
In the present invention, the “iron-nitrogen compound layer” (sometimes called a nitrogen compound layer or a compound layer) refers to a γ ′ phase (Fe ₄ N) or an ε phase (Fe ₄ N) on a steel member surface formed by gas nitrocarburizing. Fe _2-3 N) is a layer composed of a nitrogen compound of iron. However, the steel material contains carbon in the base material, and since a part of this carbon content is also contained in the compound layer, it is strictly a carbonitride. As shown in FIG. 9, the nitrogen compound layer is deposited as a layer on the surface. In the present invention, a nitride compound layer composed of the γ ′ phase and the ε phase is formed on the surface of the steel member (base material) in a thickness of 13 to 30 μm. Here, the thickness means the thickness of the compound layer mainly composed of the γ 'phase. In the nitrided steel member of the present invention in which the iron-nitrogen compound layer is formed on the surface of the steel member, first, the relationship between the volume ratio Vγ ′ and Vε of the γ ′ phase and the ε phase in the nitrogen compound layer is Vγ ′ / The ratio defined by (Vγ ′ + Vε) needs to be 0.5 or more. That is, in the nitrided steel member of the present invention, the iron-nitrogen compound layer is configured mainly of the γ ′ phase in this way, thereby improving the fatigue strength and wear resistance.

(About improvement of mechanical properties)
The reason why the nitrided steel member of the present invention having the above configuration is excellent in fatigue strength and wear resistance is considered as follows. The crystal structure of the γ 'phase is FCC (face-centered cubic) and has 12 slip systems, and therefore the crystal structure itself is rich in toughness. Further, it is considered that the crystal structure of the γ ′ phase forms a fine equiaxed structure, so that the fatigue strength is improved. On the other hand, the crystal structure of the ε phase is HCP (Hexagonal closest packing), and since the bottom surface slip is prioritized, it is considered that the crystal structure itself has the property of being hardly deformed and brittle. For this reason, in the present invention, it is considered that the fatigue strength was able to be improved by employing a configuration mainly composed of the γ 'phase.

(Volume ratio of γ 'phase and ε phase)
In the iron-nitride compound layer formed on the surface of the nitrided steel member of the present invention, the relationship between the volume ratio Vγ ′ and Vε of the γ ′ phase and the ε phase in the iron-nitrogen compound layer is Vγ ′ / (Vγ ′ + Vε). It is necessary that the ratio defined by the above is 0.5 or more, and this point will be described. As described above, the “nitrogen compound layer” is a layer composed of an ε phase (Fe _2-3 N) or a γ ′ phase (Fe ₄ N) having the above-described characteristics. Is determined from the results (volume ratio) of analyzing the phase distribution of the γ 'phase and the ε phase in the cross section in the depth direction of the compound layer by EBSD (Electron BackScatter Diffraction) in a visual field of 100 μm × 3 fields. According to the study of the present inventors, when the volume ratio is 0.5 or more, the fatigue strength of the nitrided steel member is excellent. The intensity ratio is preferably 0.7 or more, and more preferably 0.8 or more.

(Compound layer thickness)
The thickness of the iron-nitrogen compound layer (hereinafter, also simply referred to as a compound layer) formed on the surface of the nitrided steel member of the present invention is defined as 13 to 30 μm. This will be described. Since the rotational bending fatigue strength tends to increase as the thickness of the compound layer increases, the thicker the compound layer, the better. On the other hand, in the conventional two-stage gas nitriding method, as described in Patent Document 5, the compound layer mainly composed of the γ 'phase could not be made 16 μm or more. On the other hand, by employing the gas nitriding method defined in the method for manufacturing a nitrided steel member of the present invention, a compound layer mainly composed of a γ ′ phase of 16 μm or more can also be formed. For this reason, simply speaking in comparison with respect to the thickness of the compound layer, the present invention is a technique that has made it possible to obtain a compound layer having a thickness of more than 16 μm, which cannot be obtained by the technique described in Patent Document 5. However, the superiority of the present invention over the technique described in Patent Document 5 is not only in the thickness of the compound layer, but also as described later, the compound layer satisfies the relationship of the formula (1) defined in the present invention. They differ in that they can have excellent functionality. Further, while the steel targeted in Patent Document 5 is a steel for machine structural use containing about 1.2% of Cr, the present invention provides a steel containing a steel component in which a compound layer mainly composed of a γ 'phase is formed. If it is a `` member '', it can be applied to any of them, and it is a highly versatile technology that can reliably form a useful compound layer with excellent functionality at a desired thickness on various steel surfaces. different. Specifically, for example, in nitrided steels such as JIS-SACM645 steel and DIN-31CrMoV9 steel, the thickness of the compound layer is slightly thinner than the steel for machine structural use which is the target of the technology described in Patent Document 5. Tend to be. In such a nitrided steel, it is often sufficient that the thickness of the compound layer is about 13 μm. For this reason, the present invention specifies that the thickness of the compound layer having excellent functionality satisfying the relationship of the formula (1) specified in the present invention is 13 μm or more. Further, regarding the upper limit of the thickness of the compound layer having excellent functionality satisfying the relationship of the formula (1) defined in the present invention, the compound layer is liable to become the thickest in a practical nitriding time, and the carbon layer is made of carbon steel. Taking into account that the steel type can be reached, the thickness is specified as 30 μm.

(Nitriding time with K _Nx 5 to 60 minutes)
In the method for manufacturing a nitrided steel member of the present invention, the nitriding time with K _Nx is specified to be 5 to 60 minutes, and this point will be described below. K _{1 to} K _Nx-1 specified in the method for manufacturing a nitrided steel member of the present invention are regions containing an ε phase, and the ε phase formed at this time is changed to γ ′ by changing the atmosphere to K _Nx . Transform into phase. The rate of transformation at this time is determined by the rate at which nitrogen in the compound layer is denitrified into the atmosphere. According to the study of the present inventors, the retention time for nitriding under the control of K _Nx required to obtain a sufficient γ ′ phase up to the inside of the compound layer varies depending on the temperature. For example, when the temperature is 580 ° C., the holding time is as short as 5 to 20 minutes, but when the temperature is 500 ° C., it takes 20 to 60 minutes. If the holding time is short, a sufficient γ ′ phase tends not to be obtained, and if the holding time is long, the crystal grains of the γ ′ phase may become large, in which case the mechanical properties are reduced. Therefore, in the method for producing a nitrided steel member of the present invention, the lower limit of the retention time at which the sufficient γ 'phase is obtained at 580 ° C. or higher for a short time is specified as 5 minutes. In addition, the upper limit of the holding time for nitriding under the control of K _Nx is defined as 60 minutes at which a sufficient γ ′ phase can be obtained at 500 ° C., which is the lower limit of the practical nitriding temperature. More preferably, it is about 5 to 30 minutes. Within the above range, even at the nitriding temperature of 580 ° C. or higher where the holding time can be shortened, the crystal grains in the γ ′ phase do not become large, and there is no possibility that the mechanical properties such as wear resistance are lowered.

The iron-nitrogen compound layer formed on the surface of the nitrided steel member according to the present invention is such that the volume ratio of the γ ′ phase and the ε phase satisfies the specific relationship and also satisfies the following expression (1). Features.
CLT ÷ DLT ≧ 0.04 (1)
(However, CLT in the formula (1) indicates the value of the thickness [μm] of the compound layer after the gas nitriding treatment, and DLT indicates the value of the practical hardening depth [μm] of the nitrided diffusion layer after the gas nitriding treatment. Value.)

The above formula (1) is an index indicating the ratio between the thickness of the compound layer after the nitriding treatment and the depth of the nitrided diffusion layer (hereinafter, sometimes simply referred to as a diffusion layer). Method of forming a compound layer of gamma 'phase mainly described in Patent Document 1 mentioned above is a typical gas nitriding conditions, and Lehrer diagram shown in FIG. 1 (K _N and temperature axis are implemented by a combination of the equilibrium phase diagram), the temperature of the gamma 'phase field and K _N. The difference between the prior art and the present invention is that even in the case of a compound layer mainly composed of a γ ′ phase, the layer thickness becomes as thick as that of a compound layer mainly composed of an ε phase. Is defined. Usually, since the compound layer mainly composed of the ε phase is brittle, mechanical properties such as fatigue strength tend to be deteriorated. Generally, the treatment for selecting this phase is not performed. Since the width is large, the growth rate is higher than that of the γ ′ phase. In the present invention, by utilizing the growth rate of the ε phase, a compound layer mainly composed of the γ ′ phase is formed thickly, and the compound layer mainly composed of the γ ′ phase is mentioned above even during the normal gas nitriding time. As described in Patent Document 5, the thickness is increased to 16 μm or more, which cannot be achieved by the conventional two-stage nitriding method. Further, the present invention, 'even if the thickening of the phase mainly of the compound layer, gamma and formed of a low K _N' gamma compared with a compound layer consisting mainly of phase, without reducing the bending fatigue strength, In addition, it is possible to realize the provision of a novel nitridation member which has not been able to improve the wear resistance.

Here, the practical hardening depth indicates the hardening depth at the position of the core hardness +50 HV in the hardness distribution in the nitrided layer (JIS-0563). The practical curing depth indicates the sum of the compound layer thickness and the diffusion layer depth. Generally, the compound layer thickness is 1/10 or less of the diffusion layer thickness. Layer depth. The growth of the nitrided diffusion layer proceeds by the diffusion of N atoms supplied from the compound layer into the steel material. Since the growth rate of this layer is determined by temperature and time irrespective of the growth rate of the compound layer, the growth rate of this diffusion layer is divided by the thickness of the compound layer, and the temperature on the right side of the equation (1) gives the temperature. And the growth rate of the compound layer with respect to time. The present inventors have conducted intensive studies to achieve the above object of the present invention, and as a result, the compound layer formed is a compound layer mainly composed of a γ 'phase, and the compound layer is rapidly grown. It is possible to improve the abrasion resistance and fatigue resistance by increasing the thickness of the compound layer, achieve the effect of shortening the nitriding time, and reduce the ε phase without reducing the compressive residual stress and hardness in the diffusion layer. The inventors of the present invention have found that the nitrided steel member having a higher fatigue strength than the main compound layer has a value on the right side of the above formula (1) of 0.04 or more, and has completed the present invention. The total nitriding time is preferably set to 6 hours or less. Further, in the conventional gas nitriding treatment with low K _N , even if a compound layer mainly composed of a γ ′ phase can be formed, its growth rate is less than 0.040 defined by the formula (1). Does not result in a nitrided steel member capable of achieving the above-mentioned effect.

(Production method of nitrided steel member)
The method for producing a nitrided steel member of the present invention is for obtaining a nitrided steel member having the above-described configuration, and heats a material to be processed in a processing furnace while flowing a nitriding gas into the furnace. It is characterized in that the processing conditions at the time of gas nitriding are configured as follows. That is, in the method for manufacturing a nitrided steel member of the present invention, the gas introduced into the gas nitriding furnace is a mixed gas of only two types of ammonia and ammonia decomposition gas, or a plurality of gases including ammonia and ammonia decomposition gas. As a mixed gas, continuously detect the hydrogen concentration in the vicinity of the material to be treated in the furnace, estimate the ammonia partial pressure in the furnace based on the detection result, and control the atmosphere to control the nitriding potential to a set value. Automatic control is performed, and at the time of control, the nitriding potential K _N = p _NH3 / p _H2 ^1.5 in the atmosphere during the gas nitriding treatment is first set to K _N1, and then, if necessary, the nitriding potential is set to K _{N2 to} K _{Nx−. 1} , K _Nx may be used, but what is important in the present invention is that the final K _Nx is controlled so as to simultaneously satisfy the following equations (2) and (3), and that these requirements are satisfied. Controlled so that the final The nitriding time with K _Nx is set to 5 to 60 minutes, more preferably 5 to 30 minutes.
K _{N1 to} K _Nx-1 > K _Nx (2)
126.7034−5.68797 × 10 ⁻¹ × T + 8.64682 × 10 ⁻⁴ × T ² −4.43596 × 10 ^-7 × T ³ > K _Nx > 22.2265-1.15 × 10 ^-1 × T + 2.03 × 10 ^-4 × T ² − 1.21466 × 10 ^-7 × T ³ ... (3)
(However, p _NH3 and p _H2 are the partial pressures of NH ₃ and H ₂ in the nitriding furnace, and T in Expression (3) is the temperature [° C.].)

The phase structure (γ ′ phase or ε phase) of the surface nitrogen compound layer formed during the gas nitriding treatment is determined by the temperature and the nitriding potential K _N from the iron Lehrer diagram shown in FIG. In general, K _N during the gas nitriding treatment is kept constant. On the other hand, according to the study of the present inventors, in the present invention, the final K _Nx during the gas nitriding process governs the final phase structure of the compound layer regardless of the atmosphere control process. In that case, the final K _Nx simultaneously satisfies the above formulas (2) and (3), and the nitriding time at the final K _Nx controlled to satisfy these requirements is 5 It has been found that by setting the time to 分 60 minutes, a nitrided steel member having a phase structure can achieve the object of the present invention.

In the present invention, the K _N1 ~K _Nx-1 of the first and intermediate stage, the relationship between the final K _Nx is, in addition to satisfying the _{K N1 ~K Nx-1> K} Nx of formula (2), the final _Needs to be limited to “in the γ′-phase region” defined by the equation (3). In other words, K _{N1 to} K _{Nx−1 at} the first and intermediate stages are regions outside the “ _γ′- phase region”. On the other hand, in the prior art, in order to make the nitrided compound layer have a γ ′ phase, the treatment was consistently performed at a low K _N in the γ ′ phase region, and thus the compound layer could not be thickened. By utilizing the above-described effects, in the method of the present invention, the setting of K _N from the beginning to the point where K _Nx is reached can be freely set. It is effective to further increase K _N1 . Further, the duration of the nitriding treatment during the final K _Nx holding is determined by the diffusion rate of nitrogen in the compound layer flowing into the atmosphere. According to the study of the present inventors, the thickness of the compound layer, the type of steel, and the change of the atmosphere are changed. It takes 10 to 60 minutes including time.

(Differences from conventional two-stage nitriding)
Hereinafter, the difference between the present invention and a conventional two-stage nitriding method in which the nitriding potential K _N is changed in the middle will be described. For example, conventional two-stage nitriding method disclosed in Patent Document 5 mentioned above is carried out nitriding at first the NH ₃ partial pressure in the furnace high atmosphere, then the atmosphere in the furnace, NH ₃ minutes A two-stage nitriding method for reducing the pressure is performed. The purpose of the technique described in Patent Document 5 is to produce a compound layer mainly composed of γ ′ at each position of a part to be processed uniformly and in large quantities, without being restricted by the wind speed. Not intended to be. Further, according to the technique described in Patent Document 5, the thickness of a compound layer capable of forming a γ ′ phase is limited to 4 μm to 16 μm. It says that improvement in strength cannot be expected. In addition, the time of the second-stage gas nitriding treatment is not optimized, and the nitriding time is long, so that the crystal grains in the γ 'phase become large and the mechanical properties such as abrasion resistance deteriorate. On the other hand, the method defined in the present invention can also enable the formation of a γ ′ phase thicker than 16 μm, which cannot be achieved by the conventional technique, as described above. This is a two-stage nitriding method in which the crystal grains are fine and the hardness and the compressive residual stress of the diffusion layer are not reduced. Since the purpose of the method defined in the present invention is different from that of the conventional method, the final nitridation atmosphere (K _Nx ) in the second stage is reduced, but the region is limited to the γ ′ phase region, and The time is also 5 to 60 minutes, which is shorter than the nitriding time of the first step. In the second step, the compound layer containing the ε phase formed in the first step is converted to the γ ′ phase. It is characterized by being transformed into

(About the automatic control of the atmosphere to control the setting nitriding potential)
In Patent Document 5, NH ₃ gas introduced into the furnace is inserted at a constant flow rate, and the NH ₃ concentration in the furnace is continuously measured by an infrared absorption method, and the H ₂ concentration in the furnace is continuously measured by a high corrosion resistance thermal conductivity method. By performing the analysis, the nitriding potential required in the furnace is adjusted by changing the introduction flow rate of the H ₂ gas. However, in the measurement of NH ₃ concentration in the furnace by the infrared absorption method, the analysis is performed on the furnace gas drawn outside the furnace, and the response speed is slower than that of the thermal conductivity type hydrogen sensor. In addition, there are some technical problems such as generally causing a problem in measurement accuracy due to blockage of the sample line due to precipitation of ammonium carbonate and loss due to adsorption of NH ₃ due to contamination of the sample line. It is still left (see Masahiko Fujiwara, Heat Treatment, Vol. 45, No. 5, p. 311, "A continuous measurement apparatus for ammonia concentration in nitriding treatment"). On the other hand, a thermal conductivity type hydrogen sensor has good responsiveness and is generally used to measure the furnace air of a nitriding furnace. However, the thermal conductivity type hydrogen sensor can measure the furnace gas drawn out of the furnace in the same way as NH _3, and the sensor can be directly inserted into the furnace to measure the furnace gas near the material to be treated. Can be measured in any case, and the measurement is possible in any case. Therefore, the accuracy differs depending on where the furnace gas is selected. From the above, the method of simultaneously performing the NH ₃ concentration analysis and the H ₂ concentration analysis and controlling the furnace gas while referring to these values has an adverse effect on the accuracy of the nitriding potential control, and cannot be said to be the best method. .

On the other hand, the method for manufacturing a nitrided steel member of the present invention is based on only continuous detection of the H ₂ concentration in the vicinity of the material to be treated, and specifically, only with a thermal conductivity type H ₂ sensor. The furnace gas is controlled by continuously measuring the H ₂ concentration in the vicinity. That is, in the method for manufacturing a nitrided steel member of the present invention, since the NH ₃ decomposition gas is used instead of the H ₂ gas as the gas used for lowering the nitriding potential, the NH ₃ concentration in the furnace can be reduced even with the hydrogen sensor alone. You can know exactly. On the other hand, in the control method described in Patent Document 5, since the amount of H ₂ gas inserted into the furnace fluctuates, it is difficult to accurately grasp the partial pressure of NH ₃ in the furnace using only the H ₂ sensor. Precise control is difficult. As a result, in the production method of the present invention, even if a thick compound layer having a thickness of 16 μm or more, which is considered to be impossible with the technique described in Patent Document 5, is formed without increasing the ε phase, It is believed that the compound layer was formed.

The processing furnace for performing the gas nitriding treatment performed by the production method of the present invention may be a pit type or a batch type, regardless of the shape of the furnace, and as described above, the processing result is the temperature and the temperature in the processing furnace. time, also determined in an atmosphere of K _N. This is commonly referred to as Lehrer diagram, can be known from the state diagram to the axis of the K _N and temperature shown in FIG. 1 (Source: DIETER Ried Ke addition "nitride and nitrocarburizing of iron" Agne Technology Center p.131). The nitriding method using the pit type and batch type processing furnaces performed by the production method of the present invention is described below.

(Example of pit type)
FIG. 2 shows a schematic diagram of the pit furnace, and FIG. 3 shows an example of processing conditions in the pit furnace. As shown in FIG. 2, in order to control K _N in the furnace, a H ₂ sensor 21 and a PLC + K _N regulator 22 are set, and a mass flow meter (MFC) 23 is set for each process gas. is there. The processed product 24 to be subjected to the gas nitriding treatment is set in the center of the furnace in advance, sealed in the furnace, and the inside of the furnace is evacuated. Then, the inside of the furnace is restored with N ₂ gas. Heating is started while flowing a flow of N ₂ gas into the furnace. For heating, the retort 25 is heated from the outside by a heater (not shown) set on the outer periphery, and the temperature is adjusted by a temperature controller to a desired temperature based on the temperature measured by the in-furnace thermocouple 26. Is done. 27 in FIG. 2 is a stirrer.

In the case of processing in the pit furnace having the above-described configuration, for example, as shown in FIG. 3, after the inside of the furnace reaches about 400 ° C., the process gas is changed from N ₂ gas to NH ₃ + AX gas (NH ₃ decomposition gas). H ₂ + N ₂ = 3: 1), and the atmosphere control is started so as to obtain a desired K _N value (= K _N1 ). Generally, in order to form a compound layer mainly composed of a γ 'phase, K _{N1 is set} to a low K _N constant (for example, K _N = 0.3) in the soaking zone, and this K _{N1 is} maintained until about 400 ° C. during cooling. The processing is performed while performing. On the other hand, in a preferred embodiment of the present invention, K _{N1 is set} to a high K _N (for example, K _N = 1.3) and changed to K _Nx (for example, K _N = 0.3) 20 minutes before cooling. is there. The nitrided compound layer formed at a high K _N is a compound layer containing an ε phase, and has a higher growth rate than the γ ′ phase. In the present invention, the high growth rate at a high K _N is used to form the compound layer. The technique devised in the present invention is to increase the film thickness, then change the atmosphere to K _Nx , and transform from the ε phase to the γ ′ phase. Then, after cooling to about 400 ° C. while keeping K _Nx , the atmosphere is replaced with N ₂ gas again, and the furnace is cooled to 100 ° C. or less. During cooling, the heater is stopped, and the inside of the furnace is cooled while cooling the outer periphery of the retort with a fan.

(For batch type)
FIG. 4 shows a schematic diagram of a straight-through type batch furnace, and FIG. 5 shows an example of processing conditions in the batch furnace. As shown in FIG. 4, the H ₂ sensor 41 and the PLC + K _N regulator 42 for controlling the K _N in the furnace, and the MFC 43 for each process gas are the same as in the pit furnace. is there. In the case of a batch furnace, the processed product 44 is inserted into the furnace by opening the door 48 into the furnace previously heated to 580 ° C. in an NH ₃ gas atmosphere. In the case of processing in a batch furnace having the above-described configuration, for example, as shown in FIG. 5, after the processed product 44 is inserted into the furnace, the process gas is switched to NH ₃ + AX gas, and the desired K _N value ( = K _N1 ). As in the case of the pit furnace described above, after performing gas nitriding for 2 hours at a high K _N , the atmosphere is changed to K _Nx and held for 20 minutes, so that a thick γ ′ phase mainly Can be formed. In FIG. 4, 46 is a thermocouple, 47 is a stirrer, and 49 is a cooling tank.

  Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the present invention is not limited to these examples.

  Table 1 shows the steel components used in the examples and comparative examples. The balance is iron (Fe). As shown below, each steel material is composed of a steel material component capable of increasing the thickness of the γ 'phase-based compound layer, and is a steel type that can be satisfied from the viewpoint of the purpose of use such as machinability and manufacturability of the steel material. is there.

(Examples 1 to 15)
Table 2 shows the gas nitriding conditions for each. Using the batch-type or pit-type furnace described above, each was performed in a two-stage process in which the nitriding potential K _{N of the} atmosphere during the gas nitriding process was different between K _N1 and K _Nx at the beginning. The nitriding potential K _{N1 in} the first stage of the embodiment is different from that in the Lehrer diagram of each alloy steel [FIG. 1 is pure iron and different from that (Hiraoka Yasushi, Watanabe Yoichi, heat treatment, vol. 55, No. 1, p.7, “Controlling the Nitriding Potential in Gas Nitriding and Controlling the Layer Structure of the Compound in Low-Alloy Steel Using Thermodynamic Calculation Method”)]). The thickness of the compound layer is increased by utilizing the fast growth rate of the phase. Subsequently, the nitriding potential K _{Nx in} the second stage was all performed in the γ ′ phase region, and the nitriding time was set between 15 and 50 minutes according to the thickness of the compound layer to be formed and the characteristics of the furnace.

  For the nitrided steel member obtained after the gas nitriding treatment performed under each condition, the thickness CLT of the iron-nitrogen compound layer formed on the surface and the practical hardening depth DLT of the diffusion layer were measured, and the value of CLTＣＬDLT was obtained. Was calculated, and the results are shown in Table 3. In the relationship between the γ ′ phase and the ε phase in the iron-nitrogen compound layer, the existence ratio of the γ ′ phase was determined by a method described later, and the results are shown in Table 3. Further, each of the obtained nitrided steel members was subjected to a rotating bending fatigue test and a friction wear test described below, and mechanical properties were examined and evaluated. Table 3 summarizes the results.

<Evaluation>
(1) Ono-type rotating bending fatigue test After using a notched test piece having the shape shown in Fig. 6 and performing gas nitriding under the conditions of the examples, an Ono-type rotating bending fatigue test (JIS Z 2274) was performed. did. As the test load, one of two levels of 30 kgf or 50 kgf was selected according to the steel material component, and the rotation speed was common to 3000 rpm. The evaluation of the test result was evaluated as "O" when passing 10 ⁷ rotations, and "Fail" when it did not.

(2) Friction and Wear Test A friction and wear test (evaluation of sliding characteristics) was measured using an SRV (Schwingungs Reihung and Vershceiss) test machine shown in FIG. 7 as follows. A sample of ２５25 × 8 mm size subjected to nitriding treatment was used as a fixed piece, and a ball made of JIS-SUJ2 steel of ｍｍ10 mm was used as a slider. The measurement was performed by measuring the coefficient of friction when reciprocating sliding (amplitude 2 mm, 20 Hz) while applying a load (FIGS. 7 and 8). The sliding distance was 100 m. The friction coefficient is determined by applying a load (600 N) from above the slider to the slider by vibrating the slider with an electromagnetic servo, detecting the sliding resistance generated between the ball and the sample with a load cell, The friction coefficient was calculated from the values of the dynamic resistance and the applied load. The judgment of the test result was evaluated as ○ when the coefficient of friction during the test (after sliding for 5 m or more) was 0.15 or less, and evaluated as × when the coefficient of friction was more than 0.15.

(3) Measurement method of nitrogen compound layer Using a coin-shaped test piece of φ20 × 5 mm, gas nitriding treatment was performed under the conditions of the examples, and the plane part of the test piece after gas nitriding was cut perpendicular to the plane part. Then, the thickness of the compound layer in the cross section was measured according to JIS-0562.

(4) Phase Distribution Analysis Method by EBSD A backscattered electron diffraction (EBSD) device (EBSD) mounted on a scanning electron microscope (Sirion manufactured by FEI), after mechanically mirror-polishing the cross section of a steel material subjected to nitriding treatment, Phase Map was measured using Oxford Instruments (Inca Crystal). The Phase Map is obtained by color-coding a phase determined by matching an actually measured electron diffraction pattern with an electron diffraction pattern of a candidate phase. FIG. 9 shows an example of an EBSD analysis result having a compound layer mainly composed of a γ ′ phase and a compound layer mainly composed of an ε phase. The upper part of FIG. 9 is a micrograph of the γ′-phase main compound layer, and the lower part is a micrograph of the ε-phase main compound layer. Each right column is a Phase Map, and a gray-colored part is a part of the γ 'phase. In the present invention, using this Phase Map, a ratio defined by Vγ ′ / (Vγ ′ + Vε) was determined, and thereby the ratios of the γ ′ phase and the ε phase occupying in the iron-nitrogen compound layer were compared.

(Comparative Examples 1 to 3)
In Comparative Examples 1 to 3, as shown in Table 2, the same steel materials as those used in Examples 1, 5, and 9 were used, and the low K _N (= 0 .25), the gas nitriding treatment of only one stage was performed, and the obtained nitrided steel material was subjected to the same test as in the example. As a result, as shown in Table 3, although there was no problem with the rotating bending fatigue strength, the result of the friction and wear test was rejected.

(Comparative Examples 4, 5, 6)
In Comparative Examples 4, 5, and 6, the same two-stage nitriding treatment was performed using the same steel materials as used in Examples 3, 5, and 7, respectively. However, the second stage nitriding time was set to 60 minutes or more. As a result, as shown in Table 3, since the nitridation time of the second stage held at K _Nx was 60 minutes or more as specified in the present invention, the crystal grains in the γ ′ phase became large, and As a result of performing the same test on the nitrided steel material as in the example, as shown in Table 3, not only the fatigue strength (50 kgf) but also the wear resistance was rejected.

(Comparative Examples 7, 8, 9)
In Comparative Examples 7, 8, and 9, the same steel materials as those used in Examples 3, 5, and 7 were used, and K _N was set to 0.25, which forms the γ ′ phase, as in the conventional technique. Processed in stages. In order to increase the thickness of the compound layer to 13 μm or more as specified in the present invention, the nitriding time was set to a longer time in Comparative Examples 7 and 8, and in Comparative Example 9, the nitriding temperature was increased to shorten the time. did. Specifically, in Comparative Example 7, the nitriding time was 10 hours, in Comparative Example 8 was 12 hours, and in Comparative Example 9, the nitriding temperature was increased to 610 ° C. and the nitriding time was reduced to 3 hours. As a result, the thickness of the iron-nitrogen compound layer of the obtained nitrided steel material satisfied 13 μm or more. However, as shown in Table 3, the compound layer did not satisfy the formula (1) defined in the present invention. As a result of performing the same test as in the examples on the obtained nitrided steel materials, as shown in Table 3, the fatigue strength was rejected (50 kgf) in all cases.

According to the present invention, the compound layer mainly composed of the γ ′ phase can be formed into a thick film in the same manner as the compound layer mainly composed of the ε phase even during the normal gas nitriding treatment time. Even if the film thickness is increased, compared with a compound layer mainly composed of a γ ′ phase formed at a low K _N , it is possible to improve the abrasion resistance without lowering the bending fatigue strength. It is possible to provide a steel member, and the nitrided steel member provided by the present invention is expected to be used for, for example, gears and crankshafts for automobiles and transmissions.

1: Oscillation block head plate 2a: Torsion sensor 2b: Fixture for test piece 3: Arm for amplitude motion of upper test piece 3a: Upper test piece holder 4: Vertical load axis

Claims (4)

While adjusting the gas nitriding potential nitriding furnace (K _N), a method for manufacturing a nitrided member performs gas nitriding process on a target material in the furnace,
The gas introduced into the gas nitriding furnace may be a mixed gas of only two kinds of ammonia and ammonia decomposition gas, or a plurality of mixed gases containing ammonia and ammonia decomposition gas, and the hydrogen concentration in the vicinity of the material to be treated Is continuously detected, the ammonia partial pressure in the furnace is estimated based on the detection result, and the atmosphere for controlling to the set nitriding potential is automatically controlled. nitriding potential _{K N = p NH3 / p H2} 1.5 of first and K _N1, followed by adjusting the nitriding potential and the final K _Nx, the following equation (2) the following formula (3) and at the same time A method for producing a nitrided steel member, characterized in that the nitriding time at K _Nx controlled so as to satisfy the above-mentioned requirements is 5 to 60 minutes.
K _N1 > K _Nx ... (2)
126.7034-5.68797 × 10 ^-1 × T + 8.64682 × 10 ^-4 × T ² -4.43596 × 10 ^-7 × T ³ > K _Nx > 22.2265-1.15 × 10 ^-1 × T + 2.03 × 10 ^-4 × T ² − 1.21466 × 10 ^-7 × T ³ ... (3)
(However, p _NH3 and p _H2 are the partial pressures of NH ₃ and H ₂ in the nitriding furnace, and T in Expression (3) is the temperature [° C.].) Wherein K _N1 method of producing a nitride steel member according to claim 1 is 1.0 to 2.0. The first K _N1 And the final K _Nx And the nitriding potential in the gas nitriding furnace is K _Nx-1 Adjust so that K _Nx-1 > K _Nx 3. The method for producing a nitrided steel member according to claim 1, wherein the control is performed so as to simultaneously satisfy the formula (3) and the formula (3). 4. An iron-nitrogen compound layer is formed on a surface of a steel member obtained by the method for manufacturing a nitrided steel member according to any one of claims 1 to 3 and comprising a steel component in which a compound layer mainly composed of a γ 'phase is formed. A nitrided steel member comprising:
When the volume ratio of the γ ′ phase and the ε phase in the iron-nitrogen compound layer is Vγ ′ and Vε, and the existence ratio of the γ ′ phase is represented by a ratio defined by Vγ ′ / (Vγ ′ + Vε), The thickness of the compound layer mainly composed of the γ ′ phase having a value of 0.5 or more is 13 μm to 30 μm, and the iron nitride compound layer has a value of the thickness [μm] of the compound layer after the gas nitriding treatment. Is represented by CLT, and when the value of the practical hardening depth [μm] of the nitrided diffusion layer after the gas nitriding treatment is represented by DLT, a relation of the following formula (1) is satisfied.
CLT ÷ DLT ≧ 0.04 (1)