Immersion method

Abstract

This invention provides a nitrification method that allows for highly accurate estimation of the concentration of nitrogen atoms in a metal material, such as iron or an iron-based alloy, and the setting of nitrification conditions, without the need to conduct an actual nitrification test. [Solution] When nitriding a metal material made of iron or an iron-based alloy in an atmosphere containing NH3, the concentration distribution of N atoms in the depth direction of the metal material is estimated by considering surface reactions including the nitriding reaction due to the decomposition of NH3, the denitrification reaction accompanied by the removal of NH3, and the denitrification reaction accompanied by the removal of N2, as well as the diffusion of N atoms inside the metal material and the movement of the phase interface between the α phase and the γ phase inside the metal material. The nitriding conditions, including the partial pressure of NH3 in the atmosphere, temperature, and nitriding time, are then set to obtain the desired concentration distribution of N atoms.

Description

[Technical Field]

[0001] The present invention relates to a nitrification method, and more particularly to a method for performing nitrification on a metal material made of iron or an iron-based alloy, by setting conditions to obtain a desired concentration distribution of N atoms. [Background technology]

[0002] In some cases, nitrogen atoms are incorporated into metal materials made of iron or iron-based alloys for purposes such as improving magnetic properties. A widely used method for incorporating nitrogen atoms into metal materials is nitrogen immersion, which involves heating and holding the metal material in an atmosphere containing NH3. Methods for incorporating nitrogen atoms into soft magnetic materials using nitrogen immersion with NH3 are disclosed, for example, in Patent Documents 1 to 3. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-168753 [Patent Document 2] Japanese Patent Publication No. 2001-176715 [Patent Document 3] Japanese Patent Publication No. 2021-102799 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] When nitriding a metal material, the concentration of N atoms in the metal material greatly depends on nitriding conditions such as temperature and the partial pressure of NH3 in the atmosphere. Also, the concentration of N atoms at each position in the metal material greatly affects the properties of the metal material. For example, increasing the concentration of N atoms contained in a metal material that takes the α-phase (ferrite phase) causes an α-γ phase transition and generates the γ-phase (austenite phase). The generation of the γ-phase contributes to the improvement of magnetic properties. On the other hand, if the N atom concentration is too high, the mechanical properties of the metal material, such as a decrease in toughness, are likely to deteriorate. Therefore, from the viewpoint of obtaining a metal material having desired magnetic and mechanical properties, it is desirable to control the nitriding conditions to obtain a desired N atom concentration.

[0005] As a method of controlling nitriding conditions, control of the nitriding potential is known. The nitriding potential Kn is expressed using the partial pressure P NH3 of NH3 and the partial pressure P H2 of H2 as Kn = P NH3 / P H2 3 / 2 and, by using the nitriding potential as an index and controlling the composition of the atmosphere, the concentration of N atoms incorporated into the metal material by the equilibrium reaction of the following formula (A) can be controlled.

Chemical formula

[0006] Although the control by nitridation potential can easily control the amount of N atoms introduced from the atmosphere into the metal material, it cannot fully consider the behavior of N atoms inside the metal material such as diffusion. Also, as for the exchange of N atoms between the metal material and the atmosphere, in addition to the decomposition of NH3 by the forward reaction of formula (A) and the introduction of N atoms into the metal material, and the desorption (denitrification) of N atoms in the form of NH3 molecules by the reverse reaction of formula (A), denitrification in the form of N2 molecules can occur. However, in the control of nitridation potential, the contribution of denitrification in the form of N2 molecules cannot be considered. Therefore, in the control of nitridation potential, there are cases where the concentration of N atoms in the metal material cannot be controlled with sufficient accuracy. In particular, when the metal material is made of pure iron or contains a large amount of elements that inhibit nitriding such as Ni and Co, in the control of nitridation potential, the accuracy of controlling the N atom concentration tends to be low. This is considered to be because the nitrogen activity in the metal material increases, and the contribution of denitrification in the form of N2 molecules becomes large.

[0007] As described above, only controlling the nitridation potential may result in a situation where the concentration of N atoms in the metal material cannot be sufficiently controlled. In such a case, in order to obtain a metal material having a desired N atom concentration by nitriding, it may be necessary to actually conduct nitriding tests under many conditions and set the nitriding conditions by trial and error. Then, it will take a lot of time and labor to determine the nitriding conditions. If it is possible to estimate the N atom concentration obtained under predetermined nitriding conditions through desk studies rather than actual nitriding tests and set the nitriding conditions based on the results, it is expected that the time, labor, and cost required to determine the nitriding conditions can be reduced.

[0008] The problem to be solved by the present invention is to provide a nitriding method that can estimate the concentration of N atoms in a metal material with high accuracy and set nitriding conditions without conducting an actual nitriding test when nitriding a metal material made of iron or an iron-based alloy.

Means for Solving the Problem

[0009] To solve the above problems, the nitriding method according to the present invention has the following configuration. [1] In the nitriding method according to the present invention, when nitriding a metal material made of iron or an iron-based alloy in an atmosphere containing NH3, considering the surface reaction including the nitriding reaction due to the decomposition of NH3, the denitriding reaction accompanied by the desorption of NH3, and the denitriding reaction accompanied by the desorption of N2, the diffusion of N atoms inside the metal material, and the movement of the phase boundary between the α-phase and the γ-phase inside the metal material, estimate the concentration distribution of N atoms in the depth direction of the metal material, and set the nitriding conditions including the NH3 partial pressure, temperature, and nitriding time in the atmosphere so as to obtain the desired concentration distribution of N atoms.

[0010] [2] In the aspect of [1] above, regarding the surface reaction, using the rate J1 of the nitriding reaction due to the decomposition of NH3, the rate J2 of the denitriding reaction accompanied by the desorption of NH3, and the rate J3 of the denitriding reaction accompanied by the desorption of N2 according to the following formulas, regarding the diffusion, using the diffusion fluxes J α , J γ respectively, and regarding the movement of the phase boundary, considering the interface movement speed J i , it is advisable to estimate the N atom concentration at each position where the metal material is subdivided by simulation using a kinetics model.

Equation

[0011] [3] In the embodiment of [1] or [2] above, the nitrification conditions should be set such that the N atom concentration is equal to or greater than the reference concentration throughout the entire area of ​​the metal material.

[0012] [4] Alternatively, in the embodiment of [1] or [2] above, the nitrification conditions may be set such that, with respect to the depth position from the surface of the metal material, the N atom concentration in the region on the surface side of the reference position is equal to or greater than the first reference concentration, and in the region inside the reference position, the N atom concentration is equal to or less than the second reference concentration, which is set to a value equal to or less than the first reference concentration.

[0013] [5] Alternatively, in the embodiment of [1] or [2] above, the nitriding conditions may be set such that, with respect to the depth position from the surface of the metal material, the N atom concentration in the region inside the reference position is equal to or greater than the first reference concentration, and in the region on the surface side of the reference position, the N atom concentration is equal to or less than the second reference concentration, which is set to a value equal to or less than the first reference concentration. [Effects of the Invention]

[0014] In the nitrification method according to the present invention having the configuration described in [1] above, not only the nitrification reaction by decomposition of NH3 and the denitrification reaction accompanied by the desorption of NH3, but also the denitrification reaction accompanied by the desorption of N2 are considered as processes of N atoms entering and leaving the atmosphere and the surface of the metal material. Furthermore, in addition to the entry and exit of N atoms on the surface of the metal material, the diffusion of N atoms and the movement of the phase interface between the α phase and the γ phase are considered as phenomena within the metal material. After considering all of these elements, the concentration distribution of N atoms in the depth direction of the metal material is estimated, and the nitrification conditions are set so that the desired distribution is obtained. In this way, by considering various phenomena on the surface and inside the metal material, the concentration of N atoms at each position in the depth direction of the metal material can be estimated with high accuracy, and it becomes possible to manufacture a metal material having the desired N atom concentration distribution, or a distribution close to it, without going through excessive trial and error in actual nitrification tests.

[0015] In the embodiment described in [2] above, each phenomenon included in the nitriding treatment is incorporated by the above equations, and a simulation using a kinetic model is performed to estimate the N atom concentration at each position in the metal material. In this case, the N atom concentration at each position in the metal material can be estimated with high accuracy while incorporating each phenomenon included in the nitriding treatment in a simplified form.

[0016] In the embodiment described in [3] above, the nitriding conditions are set so that the N atom concentration is above a standard value throughout the entire metal material. If the partial pressure of NH3 in the atmosphere is not sufficiently high, or if the nitriding time is not sufficiently long, it is difficult to sufficiently increase the N atom concentration to a deep position in the metal material. However, by using the method described in [1] or [2] above and setting the nitriding conditions so that the N atom concentration is sufficiently high throughout the entire depth of the metal material, it is possible to obtain a high level of effect from the inclusion of N atoms, such as improved magnetic properties, for the entire metal material, and to manufacture a metal material with excellent properties and high homogeneity. In particular, if nitriding is performed at a temperature in the γ single-phase region so as to sufficiently increase the N atom concentration throughout the entire metal material, a high effect on improving magnetic properties can be obtained.

[0017] In the embodiment described in [4] above, the nitriding conditions are set such that, with respect to the depth of the metal material, the N atom concentration is equal to or greater than the first reference concentration in the region on the surface side of the reference position, and equal to or less than the second reference concentration in the region inside the reference position. Setting the nitriding conditions so that a non-uniform distribution with different N atom concentrations is formed at the surface side and the inside of a predetermined reference position can also be done using the methods described in [1] or [2] above. If the N atom concentration of the entire metal material is increased too much, the toughness of the metal material may decrease, but by leaving the interior of the metal material in a state with a low N atom concentration in this way, it is possible to achieve both improved magnetic properties due to the contribution of the surface side region with a high N atom concentration and the assurance of mechanical properties such as toughness.

[0018] In the embodiment described in [5] above, the nitriding conditions are set such that, with respect to the depth of the metal material, the N atom concentration in the region inside the reference position is equal to or greater than the first reference concentration, and the N atom concentration in the region on the surface side of the reference position is equal to or less than the second reference concentration. Normally, in nitriding treatment, it is difficult to increase the N atom concentration inside the metal material compared to the surface, as N atoms are introduced from the surface of the metal material. However, by using the method described in [1] or [2] above, for example, by setting nitriding conditions such as changing the composition of the atmosphere after a certain amount of time has elapsed at a predetermined temperature, it is possible to form a unique concentration distribution in which the N atom concentration is higher inside a predetermined reference position. If the N atom concentration on the surface of the metal material is increased too much, the aggressiveness of the metal material surface will increase. However, by increasing the N atom concentration inside the metal material while keeping the N atom concentration in the region near the surface from becoming too high, it is possible to achieve both reduced aggressiveness and improved magnetic properties. [Brief explanation of the drawing]

[0019] [Figure 1] This is a schematic diagram illustrating several phenomena that occur during the nitriding process of metal materials. [Figure 2] This is a phase diagram of the Fe-20Co alloy when the nitrogen concentration is varied. [Figure 3] This figure compares the distribution of nitrogen atom concentration obtained from actual nitrification tests with the distribution obtained from simulations, for a temperature of 925°C. As shown in the figure, it illustrates multiple cases with different metal composition and nitrification time. [Figure 4] This figure compares the distribution of nitrogen atom concentration obtained from actual nitriding treatment with the distribution obtained from simulation, for a temperature of 875°C. As shown in the figure, it illustrates multiple cases with different nitriding potential (Kn), metal composition, and nitriding time. [Figure 5] The simulation results show the relationship between nitrification conditions and the distribution of N atom concentration, with (a) showing the case where uniform nitrification of the entire area is aimed at, (b) showing the case where surface nitrification is aimed at, and (c) showing the case where internal nitrification is aimed at. [Modes for carrying out the invention]

[0020] A nitrification method according to one embodiment of the present invention will be described in detail below with reference to the drawings.

[0021] [Outline of the nitrification method] In the nitriding method according to this embodiment, a metal material made of iron or an iron-based alloy is subjected to nitriding. Nitriding is carried out by holding the metal material in an atmosphere containing NH3 at a predetermined temperature for a predetermined time. The specific component composition and manufacturing method of the metal material are not particularly limited, but the entire material is in the α phase before nitriding. The metal material may be in the form of a rod, plate, or other processed material, or in the form of a machine part processed into a predetermined component shape, but in the following, the plate form will be mainly assumed.

[0022] In the nitriding method according to this embodiment, the concentration distribution of N atoms in the depth direction is estimated for a metal material obtained by nitriding under predetermined nitriding conditions. Then, based on the estimation results, the nitriding conditions to be adopted in the actual nitriding treatment are set so that a desired concentration distribution of N atoms, or a distribution close to it, can be obtained. The nitriding conditions are a set of various parameters related to nitriding, and include at least the partial pressure of NH3 in the atmosphere, temperature, and nitriding time (length of time for nitriding). Note that temperature refers to the temperature of the atmosphere and the metal material, and due to the heating of the metal material by the atmosphere, both become the same temperature.

[0023] The concentration distribution of nitrogen atoms is estimated by considering multiple phenomena occurring on the surface and within the metal material. In this embodiment, the following phenomena are considered. Figure 1 schematically shows these phenomena to be considered. The symbols assigned to each phenomenon in the description below correspond to those in Figure 1. (A) Formation of atmosphere (B1)~(B3) Chemical reactions on the surface of metal materials (C) Diffusion of N atoms within a metal material (D) Movement of the phase interface within the metal material

[0024] The concentration distribution of nitrogen atoms can be suitably estimated by simulation using a kinetic model that takes each phenomenon into account. The metal material can be subdivided into a grid along the depth direction, and the concentration of nitrogen atoms at each of these subdivided locations can be estimated using a kinetic model. As will be explained in detail below, the kinetic model can be constructed by combining rate equations that represent each of the phenomena listed above, and the time change in the concentration of nitrogen atoms due to the inflow and outflow of nitrogen atoms into the grid region at each location can be estimated according to these rate equations.

[0025] [Details of the N atom concentration estimation] As listed above, the estimation of the N atom concentration in the metal material in the nitriding method according to this embodiment takes into account the phenomena (A) to (D). The handling of each phenomenon will be explained in order below. The various constants appearing in the following equations can be obtained from databases or the results of preliminary tests.

[0026] (A) Formation of atmosphere As will be explained later regarding surface reactions, nitrification proceeds via an equilibrium reaction between the atmosphere and the metal material; therefore, the composition of the atmosphere greatly influences the change in the N atom concentration in the metal material due to nitrification. As shown in Figure 1, in a nitrification furnace that contains the metal material 1 and performs nitrification, the composition of the internal atmosphere 2 can be controlled by the supply of an atmosphere gas 3 containing NH3 gas from the outside. Therefore, by considering the rate of material exchange between atmosphere 2 and the metal material 1, and the rate of gas supply 3 from the outside, the composition of atmosphere 2 and its changes at each time during nitrification can be expressed. Since the partial pressure of NH3 in atmosphere 2 makes an important contribution in nitrification, we will focus here on the partial pressure of NH3 and its changes.

[0027] The specific method for expressing the NH3 partial pressure can be determined appropriately based on the actual nitrification equipment configuration and the specific procedure for nitrification, but for example, it can be expressed as shown in equation (0) below.

number

[0028] (B) Surface reaction As shown in Figure 1, on the surface 11 of the metal material 1, the chemical reactions that occur between the metal material 1 and the atmosphere 2 are considered to be (B1) nitrification reaction by decomposition of NH3, (B2) denitrification reaction accompanied by detachment of NH3, and (B3) denitrification reaction accompanied by detachment of N2. Each is represented by the following reaction equations. Reaction B2 is the reverse reaction of reaction B1, and as shown in equation (A) above, equilibrium is established between the metal material 1 and the atmosphere 2 by both reactions. Note that the reverse reaction of reaction B3 does not substantially occur. [ka] Here, in the chemical reaction equation, [N] represents the N atom incorporated into the metal material.

[0029] If the reaction rates of reactions B1, B2, and B3 described above are denoted as J1, J2, and J3, respectively, then their reaction rates can be expressed as follows.

number

[0030] Furthermore, the nitrogenation rate under the conditions in which reactions B1 to B3 proceed, that is, the rate at which N atoms incorporated into the metal surface increase, can be expressed as shown in Jr below.

number

[0031] (C) Diffusion of N atoms As shown in Figure 1, the N atoms incorporated into the metal material 1 from the surface 11 by the reaction of equation (B1) above diffuse within the metal material 1. The N atoms diffuse within the metal material 1 at a diffusion rate determined by the diffusion equation. The metal material 1 can take the form of α phase 12 and γ phase 13, and the diffusion rates of N atoms in these phases are different from each other. The diffusion flux in the α phase and γ phase is J α , J γ Therefore, J α , J γ It is written as follows:

number

[0032] (D) Movement of the phase interface As shown in Figure 2, which illustrates the phase diagram using Fe-20Co alloy as an example, in regions with low N content, metal materials consisting of Fe or Fe-based alloys take on the α single phase. However, as the N content increases, a phase transition occurs from the α phase to the γ phase, and after passing through the α+γ phase, it takes on the γ single phase. In the process of introducing N atoms to the surface of the metal material through nitrification and diffusing N atoms into the interior of the metal material, the N atom concentration gradually increases at each position from the surface to the interior of the metal material. Then, at a certain depth, when the N atom concentration becomes sufficiently high, a phase transition occurs from the α phase to the γ phase, and the γ phase is formed in the surface region. On the other hand, in the initial stages of nitrification, the N atom concentration inside the metal material is low, so the regions at deeper positions inside the metal material (positions far from the surface) maintain the α phase. Therefore, as shown in Figure 1, a phase interface 14 between the α phase 12 and the γ phase 13 is formed at a predetermined depth from the surface 11 of the metal material 1. N atoms can diffuse beyond the phase interface 14, and as nitridation progresses, the concentration of N atoms gradually increases even in deeper interior locations. As the concentration of N atoms increases, the phase interface 14 moves away from the surface 11 and inward towards deeper areas. When the interfacial movement from both sides of the metal material 1 reaches the center of the thickness of the metal material 1, the entire metal material 1 becomes the γ phase 13.

[0033] Thus, a phase interface is formed between the α and γ phases, and N atoms can diffuse across this interface, but local equilibrium is established on both sides of the phase interface. Based on this local equilibrium, the migration velocity J at the phase interface is determined. i This can be expressed as follows:

number

[0034] [Setting nitrification conditions based on estimation of N atom concentration distribution] As described above, by considering the rates of each step—atmosphere formation as shown in equation (0), nitrification due to surface reactions expressed by equations (4) to (7), diffusion of N atoms in the α and γ phases, and migration at the phase interface—and evaluating the change in N atom concentration at each depth position of the metal material, the concentration distribution of N atoms in the metal material can be estimated. Based on the estimation results, the nitrification conditions should be set so that the desired N atom concentration and its spatial distribution are obtained. For example, a simulation corresponding to a certain nitrification condition can be performed to estimate the distribution of N atom concentration, and if the distribution matches or is sufficiently close to the desired distribution, that nitrification condition should be adopted. On the other hand, if the distribution deviates from the desired one, a simulation with a different nitrification condition can be performed, and the process of comparing the obtained N atom concentration distribution with the desired distribution can be repeated. Alternatively, the distributions of N atom concentration obtained from simulations corresponding to multiple nitrification conditions can be compared, and the nitrification condition that yields the distribution closest to the desired one can be adopted. The nitrification conditions include parameters such as the partial pressure of NH3 in the atmosphere, temperature, and nitrification time. By changing the values ​​of these parameters, the distribution of N atom concentration obtained in the simulation can be varied in many ways. The values ​​of each parameter can be kept constant throughout the entire nitrification process, or they can be changed midway through.

[0035] In the nitriding method according to this embodiment, as described above, a kinetic model is used that incorporates not only nitriding / denitrification by surface reactions, but also the diffusion and interfacial migration of N atoms within the metal material to estimate the N atom concentration at each depth position of the metal material, and the nitriding conditions are set based on the results of this estimation. Therefore, the distribution of N atom concentration can be estimated with high accuracy and reflected in the setting of the nitriding conditions. Furthermore, as a denitrification process at the surface, by considering not only denitrification in the form of NH3 incorporated into the nitriding potential (reaction B2) but also denitrification in the form of N2 (reaction B3), the distribution of N atom concentration can be estimated and controlled with high accuracy even when it is difficult to control the N atom concentration with high accuracy by controlling only the nitriding potential, such as when the metal material is pure iron or contains many elements that inhibit nitriding, such as Ni and Cr. In this way, by performing high-precision studies on the nitriding conditions through computer simulations and other theoretical studies, it is possible to manufacture nitrided materials with the desired N atom concentration and spatial distribution without requiring excessive trial and error through actual nitriding tests.

[0036] The actual nitriding conditions can be arbitrarily determined according to the desired concentration and spatial distribution of N atoms, but a typical example is a form that aims for uniform nitriding throughout the entire material, that is, a form in which nitriding is performed so that the N atom concentration throughout the entire depth of the metal material is equal to or greater than the standard concentration. By including N atoms at a sufficient concentration throughout the entire metal material, the effects of N atom addition, such as improved magnetic properties, can be obtained with high homogeneity throughout the entire material. As will be shown in later examples, by raising the temperature sufficiently, setting the NH3 partial pressure in the atmosphere to an appropriate range according to the desired N atom concentration, and extending the nitriding time makes it easier to achieve uniform nitriding throughout the entire material. In particular, if the entire metal material becomes the γ phase, that is, if the N atom concentration throughout the entire metal material is N γ Setting the nitriding conditions as described above will yield a significant improvement in magnetic properties. It is preferable to set the nitriding conditions so that a sufficient concentration of N atoms is distributed throughout the entire metal material at a temperature in the γ single-phase region.

[0037] On the other hand, depending on the application of the metal material, it may be necessary to create a non-uniform distribution of N atom concentration. For example, there may be a form that aims for surface nitriding, that is, a form in which the N atom concentration on the surface of the metal material is higher than that inside. Specifically, an arbitrary reference position is set in the depth position of the metal material, and the N atom concentration in the region on the surface side of the reference position is set to be equal to or higher than the first reference concentration, and the N atom concentration in the region inside the reference position is set to be equal to or lower than the second reference concentration. The second reference concentration is set to be equal to or lower than the first reference concentration. Such surface nitriding is easily achievable by not raising the temperature and NH3 partial pressure too high, and by setting an appropriate nitriding time according to the desired reference position, as will be shown in later examples. As described above, increasing the concentration of N atoms in a metal material has a high effect on improving magnetic properties, but if the N atom concentration is increased too high, the mechanical properties of the metal material may decrease. Therefore, by performing surface nitriding and increasing the N atom concentration on the surface of the metal material while leaving the interior in a state of low N atom concentration, it is possible to achieve both improved magnetic properties due to the contribution of the surface region and the assurance of mechanical properties such as toughness.

[0038] Another example of a non-uniform distribution of nitrogen (N) atom concentration is a form that aims for internal nitrogen filtration, that is, a form that aims for a distribution in which the nitrogen atom concentration is higher inside the metal material than on the surface. Specifically, an arbitrary reference position is set at the depth of the metal material, and in the region inside the reference position, the nitrogen atom concentration is set to be equal to or greater than the first reference concentration, and in the region on the surface side of the reference position, the nitrogen atom concentration is set to be equal to or less than the second reference concentration. The second reference concentration is set to be equal to or less than the first reference concentration. Note that the reference position and the two reference concentrations are unrelated to the values ​​in the surface nitrogen filtration described above. Since nitrogen atoms are introduced from the surface of the metal material during the filtration process, it is difficult to increase the nitrogen atom concentration inside the material compared to the surface using the usual filtration method that keeps the NH3 partial pressure and temperature constant. However, it is possible to achieve this by, for example, lowering the NH3 partial pressure midway through while keeping the temperature high, thereby denitrifying some of the nitrogen atoms incorporated into the metal material. As described above, increasing the nitrogen (N) atom concentration in a metal material can improve its properties, such as magnetic properties. However, if the concentration of N atoms on the surface is too high, it may increase aggressiveness (the property of damaging other materials when in contact with them). Therefore, by suppressing the N atom concentration near the surface of the metal material while increasing the N atom concentration inside, it is possible to fully obtain the effects of N atom content, such as improved magnetic properties, while suppressing aggressiveness. [Examples]

[0039] The present invention will be described in more detail below using examples.

[0040] [1] Comparison of actual measurements and simulations First, in order to confirm the accuracy of estimating the N atom concentration in the nitridation method according to the embodiment of the present invention, an experiment was conducted to compare the distribution of N atom concentration using simulation and actual measurements.

[0041] [Test Method] In the experimental measurement, a metal material with Co added to Fe was subjected to nitridation. Nitriding was carried out by holding the metal material in a nitriding furnace in an atmosphere controlled to a predetermined nitriding potential Kn and nitriding temperature for a predetermined nitriding time. The metal materials and nitriding conditions used are shown in the graphs of Figures 3 and 4. The metal materials used were Fe-5Co, Fe-10Co, and Fe-18Co, which contain 5 mass%, 10 mass%, and 18 mass% of Co, respectively, with the remainder being Fe and unavoidable impurities. The metal materials were prepared as plates with a thickness of 0.2 mm. The nitriding potential Kn was 0.025 atm -1 / 2 or 0.050 atm -1 / 2 The immersion temperature was selected from 925°C or 875°C, and the immersion time was selected from 15 minutes, 30 minutes, or 60 minutes.

[0042] The depth distribution of nitrogen atom concentration was measured for nitrogen-treated materials obtained after nitrogen immersion under various conditions. The measurements were performed using an electron probe microanalyzer (EPMA).

[0043] The simulation was performed using a kinetic model that considered the rates of each step: atmosphere formation (0), nitrification by surface reactions expressed by equations (4) to (7), diffusion of N atoms, and movement at the phase interface, as described in detail above regarding the nitrification method according to the embodiment of the present invention. The N atom concentration at each depth position of the metal material was estimated in accordance with the nitrification conditions used in the above-mentioned experimental test. The representative parameters used in the simulation are as follows. The reaction rate constant is defined as the amount of substance moved per unit time and per unit area on the surface of the metal material when the reaction rate is defined. • Diffusion coefficient (D α , Dγ): 1 × 10 -5 mm 2 / s (@925℃), 1×10 -6 mm 2 / s(@875℃) • Reaction rate constant k for reaction B1 a :0.02mm / s (@925℃, 875℃) • Reaction rate constant k for reaction B3 c:0.00015mm / s(@925℃), 0.00010mm / s(@875℃),

[0044] [Test Results] Figures 3 and 4 show the depth distribution of nitrogen atom concentration obtained from both experimental and simulated measurements for several cases with different metal material compositions and nitriding conditions. The horizontal axis represents the depth position of the metal material relative to the surface (x in Figure 1), and the vertical axis represents the nitrogen atom concentration. The continuous line represents the experimental results, and the discrete points represent the simulation results.

[0045] As shown in Figures 3 and 4, the measured N atom concentration distribution is well reproduced by the simulation for all metal materials and nitriding conditions. The increase in N atom concentration due to longer nitriding time, lower Co concentration, and higher nitriding potential Kn is also reproduced. In particular, for the case where the nitriding temperature is 925°C shown in Figure 3, the measured N atom concentration distribution is quantitatively reproduced with high accuracy by the simulation for all metal materials and nitriding times. Also, for the case where the nitriding temperature is 875°C shown in Figure 4, the N atom concentration distribution in the figure is well reproduced. γ The corresponding N atom concentration is shown with a dashed line, but when Fe-10Co is used, the N γ At this boundary, a phase transition occurs, causing a rapid change in nitrogen (N) concentration between the surface region, where the N concentration is high, and the inner region, where the N concentration is low. This rapid change observed in actual measurements is reproduced with high accuracy in the simulation.

[0046] As described above, by comparing the measured results with those obtained, it is confirmed that the distribution of N atom concentration obtained by nitrification treatment can be reproduced by simulation using a kinetic model. Therefore, it is shown that this simulation method can predict the distribution of N atom concentration with high accuracy.

[0047] [2] Examination of nitrification conditions that give various nitrification forms Next, simulations were used to investigate the nitrification conditions required to achieve three types of nitrification morphologies in which the N atom concentration exhibits a predetermined distribution.

[0048] [Test Method] The simulation was performed in the same manner as the test described in [1] above. The simulation assumed three types of nitriding: uniform nitriding throughout the entire area, surface nitriding, and internal nitriding. As explained below, the desired nitriding type was verified by changing the values ​​of the nitriding parameters. A 0.2 mm thick plate made of Fe-Co alloy was assumed as the metal material.

[0049] [Test Results] (1) Uniform nitrification throughout the entire area To achieve uniform nitrogen nitriding throughout the entire depth of the metal material, we explored nitriding conditions that would result in a nitrogen atom concentration of 0.5 ± 0.1 mass%. Here, we focused on Fe-20Co alloy, and as shown in Table 1 below, we varied the nitriding time from 300 seconds to 1200 seconds at 950°C, corresponding to the γ single-phase region. The NH3 partial pressure was adjusted for each nitriding time so that the total nitrogen atom concentration throughout the depth was 0.5 mass%, as shown in Table 1.

[0050] [Table 1]

[0051] Figure 5(a) shows the distribution of N atom concentration obtained from simulations for each nitrification time. As shown in Figure 5(a), under all nitrification conditions, a distribution is observed where the N atom concentration increases with depth within the metal material. However, as the NH3 partial pressure decreases and the nitrification time increases, the change in N atom concentration with respect to depth becomes smaller, and a flat concentration distribution is obtained. Furthermore, under conditions where the nitrification time is 900 seconds or longer, the N atom concentration remains within the range of 0.5 ± 0.1 mass% across the entire depth range. The change in the N atom concentration distribution with respect to the change in nitrification time also becomes smaller.

[0052] These results show that by lowering the NH3 partial pressure and increasing the nitrification time, nitrification can proceed more gently, resulting in highly homogeneous nitrification across the entire depth range. Furthermore, by proceeding with nitrification at a nitrification temperature of 950°C, with an NH3 partial pressure of 0.012 atm or less and a nitrification time of 900 seconds or more, a state that can be considered uniform nitrification throughout the entire range can be achieved. Note that when the nitrification time reaches 1200 seconds, the efficiency of the nitrification process deteriorates, so among the simulations, the most preferable conditions are an NH3 partial pressure of 0.012 atm and a nitrification time of 900 seconds.

[0053] (2) Surface nitriding For surface nitriding, a reference position was set at a depth of 0.05 mm (50 μm) from the surface. Nitrogen atom concentrations were searched for conditions that would result in a first reference concentration of 0.5 mass% or higher in the region closer to the reference position, and a second reference concentration of 0.1 mass% or lower in the region further inside the reference position. Here, Fe-10Co alloy was used as the target, and as shown in Table 2 below, the nitriding conditions were varied from 1200 seconds to 3000 seconds at 815°C, where the α-γ transformation point is located at a concentration of 0.4 mass%, slightly lower than 0.5 mass%. The NH3 partial pressure was fixed at 0.015 atm.

[0054] [Table 2]

[0055] Figure 5(b) shows the distribution of N atom concentration obtained from simulations for each nitriding time. According to Figure 5(b), in all nitriding times, the N atom concentration is high on both surfaces of the metal material, but drops sharply at a depth of approximately 50 μm from both surfaces, resulting in a low N atom concentration in the interior. In all nitriding times, the N atom concentration in the interior is approximately 0 mass%. This distribution, in which N atoms are concentrated on the surface side of the metal material, is formed because the N atoms introduced from the surface do not sufficiently reach the interior of the metal material. Furthermore, according to Figure 5(b), the longer the nitriding time, the greater the depth of the drop in N atom concentration, that is, the greater the difference in N atom concentration between the high-concentration area on the surface and the low-concentration area inside. Also, the depth at which this drop occurs becomes deeper. Furthermore, when the immersion time is 2400 seconds, the N atom concentration is 0.5 mass% or higher at a depth of 50 μm, which is set as the reference position, closer to the surface, and 0.1 mass% or lower at a depth of 50 μm, which is set as the first reference concentration, while further inward, separated by a very thin transition region (a region in which the N atom concentration changes in a gradual manner), it is 0.1 mass% or lower, which is set as the second reference concentration.

[0056] These results show that by keeping the nitrification temperature below a temperature that does not enter the γ single-phase region and ensuring a sufficient nitrification time for N atoms to diffuse to the reference position, it is possible to increase the N atom concentration in the region near the surface while keeping the N atom concentration in the inner region low. Furthermore, by proceeding with nitrification under conditions of a nitrification temperature of 815°C, an NH3 partial pressure of 0.015 atm, and a nitrification time of approximately 2400 seconds, a state that can be considered surface nitrification can be achieved with a reference position depth of 0.05 mm, a first reference concentration of 0.5 mass%, and a second reference concentration of 0.1 mass%.

[0057] (3) Internal nitriding For internal nitridation, a reference position was set at a depth of 0.05 mm (50 μm) from the surface. Nitrogen atom concentrations were searched for that could be set to a first reference concentration of 0.5 mass% or higher in the region inside the reference position, and a second reference concentration of 0.1 mass% or lower in the region on the surface side of the reference position. Here, Fe-10Co alloy was used as the target, and as shown in Table 3 below, the temperature was set to 815°C, where the α-γ transformation point is located at a concentration of 0.4 mass%, slightly lower than 0.5 mass%. At that temperature, the NH3 partial pressure was maintained at 0.015 atm for 8000 seconds as a nitridation time (see the nitridation column in Table 3). After that, the NH3 partial pressure was set to zero, and the temperature was maintained at the same temperature as during nitridation for a predetermined holding time (see the holding time after nitridation column in Table 3). The holding times were 0 seconds (no holding after nitridation), 1000 seconds, 2000 seconds, and 3000 seconds, as shown in Table 3. A separate test was also conducted with an immersion time of 6000 seconds.

[0058] [Table 3]

[0059] Figure 5(c) shows the distribution of N atom concentration obtained from simulations for each retention time. According to Figure 5(c), regardless of the retention time taken (1000 to 3000 seconds), the N atom concentration is low at depths near the surface on both sides of the metal material, and high in the interior. In all of these retention times, the N atom concentration in the interior is close to 0.5 mass%. This non-uniform distribution with low N atom concentration on the surface side of the metal material is formed by the denitrification of N atoms introduced into the metal material by nitriding during the retention time. Furthermore, according to Figure 5(c), the width of the low-concentration region on the surface widens as the retention time increases. Also, the rise of the boundary where the low-concentration region on the surface changes to the low-concentration region inside becomes steeper. Furthermore, when the nitriding time is 2000 seconds or longer, the nitrogen atom concentration is divided at a depth of 50 μm, which is set as the reference position. At positions closer to the surface, it falls below 0.1 mass%, which is set as the second reference concentration, while at positions further inward, it reaches a high value close to 0.5 mass%, which is also set as the second reference concentration, separated by a very thin transition region. Separately, when the nitriding time was set to 6000 seconds, it was not possible to sufficiently increase the nitrogen atom concentration in the center of the metal material.

[0060] These results show that by keeping the nitrification temperature below the γ single-phase range, ensuring sufficient NH3 partial pressure and nitrification time to distribute N atoms to the interior of the metal material at a predetermined high concentration, and then heating with a sufficient holding time while the NH3 partial pressure in the atmosphere is reduced to induce denitrification from the region near the surface, it is possible to obtain a high N atom concentration inside the metal material while keeping the N atom concentration in the region near the surface low. Furthermore, by maintaining the temperature at 815°C, ensuring an 8000-second nitrification time at an NH3 partial pressure of 0.015 atm, then setting the NH3 partial pressure to zero and holding for 2000 seconds or more, a state that can be considered internal nitrification with a reference depth of 0.05 mm, a first reference concentration of 0.5 mass%, and a second reference concentration of 0.1 mass% can be achieved.

[0061] The embodiments of the present invention have been described above. The present invention is not particularly limited to these embodiments, and various modifications are possible. [Explanation of Symbols]

[0062] 1 Metal material 11 Surface 12 α phase 13 γ phase 14 Phase interface 2 Atmosphere 3. Gas supply

Claims

1. NH ３ When nitriding a metal material consisting of iron or an iron-based alloy in an atmosphere containing, NH ３ Decomposition of nitrification reaction, NH ３ Denitrification reaction involving the elimination of N ２ Surface reactions including denitrification reactions involving the elimination of, The diffusion of N atoms within the aforementioned metal material, Movement of the phase interface between the α phase and the γ phase within the aforementioned metal material, Taking this into consideration, we estimate the concentration distribution of N atoms in the depth direction of the metal material. To obtain the desired concentration distribution of N atoms, the NH in the atmosphere ３ A nitrification method that involves setting nitrification conditions, including partial pressure, temperature, and nitrification time.

2. According to the following equations, for the surface reaction, the rate J of the nitriding reaction by the decomposition of NH ３ , the rate J of the denitriding reaction involving the desorption of NH １ , the rate J of the denitriding reaction involving the desorption of N ３ are used. For the diffusion, the diffusion fluxes J ２ and J ２ in the α-phase and γ-phase are used respectively. For the movement of the phase interface, considering the interface movement rate J ３ ,​​​​​​ The nitriding method according to claim 1, wherein the concentration of N atoms at each subdivided position of the metal material is estimated by simulation using a kinetic model. [Math 1] Here, P ＮＨ３ and P Ｈ２ Each of these is NH in the aforementioned atmosphere. ３ and H ２ The partial pressure, [N], is the concentration of N atoms in the metal material. ａ , k ｂ , k ｃ These are the reaction rate constants, and D α and D γ These are the diffusion coefficients of N atoms in the α and γ phases, respectively. α and N γ The values ​​of x and x represent the boundary lines between the α phase and the α+γ phase, and the N atom concentration at the boundary line between the α+γ phase and the γ phase, respectively, in the Fe-N phase diagram. x represents the depth position from the surface of the metal material, and x γ This indicates the depth position reached by the γ phase.

3. The nitrification method according to claim 1 or claim 2, wherein the nitrification conditions are set such that the N atom concentration is equal to or greater than a reference concentration throughout the entire area of ​​the metal material.

4. Regarding the depth position from the surface of the aforementioned metal material, in the region closer to the surface than the reference position, the N atom concentration is equal to or greater than the first reference concentration. The nitrification method according to claim 1 or 2, wherein the nitrification conditions are set such that the N atom concentration in the region inside the reference position is less than or equal to a second reference concentration, which is set to a value less than or equal to the first reference concentration.

5. Regarding the depth position from the surface of the aforementioned metal material, in the region inside the reference position, the N atom concentration becomes equal to or greater than the first reference concentration. The nitrification method according to claim 1 or 2, wherein the nitrification conditions are set such that the N atom concentration in the region on the surface side of the reference position is less than or equal to a second reference concentration, which is set to a value less than or equal to the first reference concentration.

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