EP4317480A1

EP4317480A1 - Nitriding treatment method for steel member

Info

Publication number: EP4317480A1
Application number: EP22781088.4A
Authority: EP
Inventors: Yasushi Hiraoka
Original assignee: Parker Netsushori Kogyo KK
Current assignee: Parker Netsushori Kogyo KK
Priority date: 2021-03-31
Filing date: 2022-03-30
Publication date: 2024-02-07
Also published as: TWI833179B; US20240301541A1; JPWO2022210878A1; KR20230160357A; WO2022210878A1; CN117043384A; MX2023010011A; TW202246545A; EP4317480A4

Abstract

The invention is a nitriding treatment method for a steel component including at least three nitriding treatment steps. The first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed at a temperature within a range of 500 °C to 590 °C. The first nitriding potential for the first nitriding treatment step is a value within a range of 0.10 to 1.00. The second nitriding potential for the second nitriding treatment step is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00. The third nitriding potential for the third nitriding treatment step is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60.

Description

Technical Field

0001 The present invention relates to a nitriding treatment method for a steel component which comprises at least three nitriding treatment steps.

Background Art

0002 For a steel component such as a gear used in a transmission for an automobile, a high pitting resistance and a high bending fatigue strength are required. In order to meet such requirements, a carburizing treatment and/or a nitriding treatment are known as a technique for reinforcing such a steel component such as a gear.
0003 For example, JP-A-2013-221203 ( JP application number 2012-095035 ) (Patent Document 1) has disclosed that, in order to improve a pitting resistance and/or flexural fatigue strength of a steel component, it is effective to produce an iron nitride compound layer having a γ' phase as a main component on a surface of the steel component by a nitriding treatment.
0004 In addition, JP-B-6755106 (Patent Document 2) has disclosed a nitriding treatment method in which a nitride compound layer having a high ratio of a γ' phase (0.7 or more) is generated. Specifically, a gas nitriding treatment having three steps performed at a temperature within a range of 570 °C to 600 °C using two types of gases, which are an NH₃ gas and an AX gas, has been described. More specifically, at a temperature within a range of 570 °C to 600 °C, a range of 0.1 to 0.25 is adopted for the nitriding potential in the first nitriding treatment step, a range of 1.0 to 2.0 is adopted for the nitriding potential in the second nitriding treatment step and, 0.25 is adopted for the nitriding potential in the third nitriding treatment step.

Prior Document

0005 The Patent Document 1 cited in the present specification is JP-A-2013-221203 ( JP application number 2012-095035 ).
The Patent Document 2 cited in the present specification is JP-B-6755106 .

Summary of Invention

Technical Problem

0006 The present inventor has further studied the nitriding treatment method disclosed in JP-B-6755106 (Patent Document 2), and has found that, in a temperature range of 500 °C to 590 °C, setting the nitriding potential at the third nitriding treatment step to be higher than 0.25 is more effective in depositing the γ' phase in the nitride compound layer.
0007 According to the present inventor, the action (reaction) in which the γ' phase is deposited in the nitride compound layer is affected by both the nitriding potential and the furnace temperature. In a temperature range of 500 °C to 590 °C, if the nitriding potential at the third nitriding treatment step is set to be 0.25 or less, an α phase which is lower in hardness than the γ' phase is also deposited. This may result in an insufficient pitting resistance and/or an insufficient bending fatigue strength.
0008 The present invention has been made based on the above findings. It is an object of the present invention to provide a nitriding treatment method to be performed in a temperature range of 500 °C to 590 °C, which enables a γ' phase to be deposited in a nitride compound layer in a suitable manner and thus achieves a high pitting resistance and a high bending fatigue strength.

Solution to Problem

0009 The present invention is a nitriding treatment method for a steel component, including at least three nitriding treatment steps, i.e., a first nitriding treatment step in which a nitriding treatment is performed to a steel component under a nitriding gas atmosphere of a first nitriding potential, a second nitriding treatment step in which another nitriding treatment is performed to the steel component under another nitriding gas atmosphere of a second nitriding potential higher than the first nitriding potential, after the first nitriding treatment step, and a third nitriding treatment step in which a further another nitriding treatment is performed to the steel component under a further another nitriding gas atmosphere of a third nitriding potential lower than the second nitriding potential, after the second nitriding treatment step, wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, the first nitriding potential is a value within a range of 0.10 to 1.00, the second nitriding potential is a value within a range of 0.30 to 10.00, the third nitriding potential is a value within a range of 0.26 to 0.60, a nitride compound layer consisting of a γ' phase, or an ε phase, or mixture of a γ' phase and an ε phase, is generated during the second nitriding treatment step, and a γ' phase is deposited in the nitride compound layer during the third nitriding treatment step.
0010 According to the present invention, since the third nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C and the third nitriding potential is a value within a range of 0.26 to 0.60, the γ' phase can be deposited in the nitride compound layer in a suitable manner while an α phase, which is lower in hardness than the γ' phase, is inhibited to be deposited therein. Thus, a high pitting resistance and a high bending fatigue strength can be achieved.
0011 In the present invention, for example, the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a batch type of thermal processing furnace; two types of gases, which are an NH₃ gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant; three types of gases, which are an NH₃ gas, an AX gas and an N₂ gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the three types of gases constant; two types of gases, which are an NH₃ gas and an AX gas, may be used in the third nitriding treatment step; and a nitriding potential during the third nitriding treatment step may be controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0012 In the above control manner, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
0013 Alternatively, in the present invention, for example, the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace; two types of gases, which are an NH₃ gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant; three types of gases, which are an NH₃ gas, an AX gas and an N₂ gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the three types of gases constant; two types of gases, which are an NH₃ gas and an AX gas, may be used in the third nitriding treatment step; and a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0014 In the above control manner as well, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
0015 Alternatively, in the present invention, for example, the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a batch type of thermal processing furnace; two types of gases, which are an NH₃ gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the three types of gases constant; two types of gases, which are an NH₃ gas and an AX gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant; two types of gases, which are an NH₃ gas and an AX gas, may be used in the third nitriding treatment step; and a nitriding potential during the third nitriding treatment step may be controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0016 In the above control manner as well, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
0017 Alternatively, in the present invention, for example, the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace; two types of gases, which are an NH₃ gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the three types of gases constant; two types of gases, which are an NH₃ gas and an AX gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant; two types of gases, which are an NH₃ gas and an AX gas, may be used in the third nitriding treatment step; and a nitriding potential during the third nitriding treatment step may be controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0018 In the above control manner as well, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
0019 Alternatively, in the present invention, for example, the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace; two types of gases, which are an NH₃ gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant; two types of gases, which are an NH₃ gas and an AX gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant; two types of gases, which are an NH₃ gas and an AX gas, may be used in the third nitriding treatment step; and a nitriding potential during the third nitriding treatment step may be controlled to be close to the third nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant.
0020 In the above control manner as well, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
0021 Alternatively, in the present invention, for example, the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step may be performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace; two types of gases, which are an NH₃ gas and an AX gas, may be used in the first nitriding treatment step; a nitriding potential during the first nitriding treatment step may be controlled to be close to the first nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant; three types of gases, which are an NH₃ gas, an AX gas and an N₂ gas, may be used in the second nitriding treatment step; a nitriding potential during the second nitriding treatment step may be controlled to be close to the second nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant; two types of gases, which are an NH₃ gas and an AX gas, may be sed in the third nitriding treatment step; and a nitriding potential during the third nitriding treatment step may be controlled to be close to the third nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant.
0022 In the above control manner as well, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, has been proved.
0023 Herein, the one-chamber type of thermal processing furnace means a thermal processing furnace which does not include a chamber for a cooling step separately from a chamber for a heating step like in the batch type of thermal processing furnace (see Fig. 1) and in which both the heating step and the cooling step are conducted to only one chamber. A pit type furnace (see Fig. 3) and a horizontal type furnace (see Fig. 5) are common examples.
0024 In addition, in the above respective inventions, it is preferable that a time for which the third nitriding treatment step is performed is 60 minutes or longer. According to the inventor's finding, by setting 60 minutes or longer as the time for which the third nitriding treatment step is performed, replacement of the gas atmosphere in the thermal processing furnace (which is caused by the change of the nitriding potential) is so sufficiently achieved that the effects of increasing a ratio of the γ' phase are not undermined.

Advantageous Effects of Invention

0025 According to the present invention, since the third nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C and the third nitriding potential is a value within a range of 0.26 to 0.60, the γ' phase can be deposited in the nitride compound layer in a suitable manner while an α phase, which is lower in hardness than the γ' phase, is inhibited to be deposited therein. Thus, a high pitting resistance and a high bending fatigue strength can be achieved.

Brief Description of Drawings

0026

Fig. 1 is a schematic view showing a structure of a batch type of thermal processing furnace to be used for a nitriding treatment method according to the present invention;
Fig. 2 is a process diagram of an embodiment of the nitriding treatment method according to the present invention in a case wherein the thermal processing furnace shown in Fig. 1 is used;
Fig. 3 is a schematic view showing a structure of a pit type thermal processing furnace (one-chamber type of thermal processing furnace) to be used for a nitriding treatment method according to the present invention;
Fig. 4 is a process diagram of an embodiment of the nitriding treatment method according to the present invention in a case wherein the thermal processing furnace shown in Fig. 3 is used;
Fig. 5 is a schematic view showing a structure of a horizontal type thermal processing furnace (one-chamber type of thermal processing furnace) to be used for a nitriding treatment method according to the present invention;
Fig. 6 is a table showing nitriding conditions and treatment results of examples and comparative examples of the present invention;
Fig. 7 is a table showing nitriding conditions and treatment results of examples and comparative examples of the present invention;
Fig. 8 is a table showing nitriding conditions and treatment results of examples and comparative examples of the present invention;
Fig. 9 is a table showing nitriding conditions and treatment results of examples and comparative examples of the present invention;
Fig. 10 is a table showing nitriding conditions and treatment results of examples and comparative examples of the present invention; and
Fig. 11 is a table showing nitriding conditions and treatment results of examples and comparative examples of the present invention.

Description of Embodiments

0027

(Example of Object to be processed (Work))

An object to be processed (a work) is a steel component. Specifically, it is a steel component consisting of a carbon steel component used for a machine structure or an alloy steel component used for a machine structure, such as a gear used for an automatic transmission. For example, a plurality of cylindrical ring gears or a plurality of bottomed cylindrical ring gears are mounted on a plurality of stages of jigs, placed in a flat state in a case (described below), and subjected to a nitriding treatment.
0028 Preferably, the steel component is pre-cleaned to remove dirt and oil before being subjected to the nitriding treatment. The pre-cleaning process is preferably, for example, a vacuum cleaning process for degreasing and drying by dissolving and replacing oil or the like with a hydrocarbon-based cleaning liquid and evaporating it, an alkali cleaning process for degreasing with an alkaline-based cleaning liquid, or the like.
0029

(Structural Example of Batch Type of Thermal Processing Furnace)

Fig. 1 is a schematic view showing a structure of a batch type of thermal processing furnace 1 to be used for a nitriding treatment method according to the present invention.
0030 As shown in Fig. 1, the batch type of thermal processing furnace 1 includes a loading section 10, a heating chamber 11, a transfer chamber 12, and an unloading conveyor 13. A case 20 is configured to be loaded into the loading section 10. A steel component as an object to be processed (work) is configured to be contained in the case 20. The maximum gross weight to be processed is 700 kg.
0031 An inlet hood 22 having an openable and closable door 21 is attached to an inlet side (left side in Fig. 1) of the heating chamber 11. The heating chamber 11 has a retort structure, and an outer periphery of the retort structure is configured to be heated by a heater (not shown) so that a temperature in the furnace (chamber) is controlled to a predetermined temperature. A plurality of types of gases for the nitriding treatment are configured to be introduced into the heating chamber 11 while being controlled as described below.
0032 A fan 26 is mounted on a ceiling of the heating chamber 11 to stir the gases introduced into the heating chamber 11 so that a heating temperature for the steel component is made uniform therein. An openable and closable intermediate door 27 is attached to an exit side (right side in Fig. 1) of the heating chamber 11.
0033 The transfer chamber 12 is provided with an elevator 30 for raising and lowering the case 20 that contains the steel component. A lower part of the transfer chamber 12 is provided with a cooling chamber (oil tank) 32 in which a cooling oil 31 is stored. An outlet hood 36 having an openable and closable door 35 is attached to an outlet side (right side in Fig. 1) of the transfer chamber 12
0034 The heating chamber 11 and the transfer chamber 12 may be the same processing space, and a configuration for air-cooling the thermally-processed steel component with a gas may be employed. Alternatively, the heating chamber 11 may be divided into two or three chambers, and a three-stage nitriding treatment as described below may be performed in the two or three chambers.
0035

(Operational Example of Batch Type of Thermal Processing Furnace)

In the thermal processing furnace 1 as described above, the case 20 that contains the steel component is loaded into the heating chamber 11 from the loading section 10 by a pusher or the like. After the steel component (the case 20 that contains the steel component) is loaded into the heating chamber 11, the plurality of types of process gases are introduced into the heating chamber 11, and the process gases are heated to a predetermined temperature by the heater and stirred by the fan 26 (for example, rotating at 1500 rpm) so that the steel component loaded into the heating chamber 11 is subjected to the nitriding treatment.
0036 Fig. 2 is a process diagram of an embodiment of the nitriding treatment method according to the present invention in a case wherein the thermal processing furnace 1 shown in Fig. 1 is used.
0037 In the example shown in Fig. 2, before a steel component (work) is loaded into the heating chamber 11, the heating chamber 11 is pre-heated to 550 °C in advance. During this pre-heating step, an N₂ gas is introduced at a constant flow rate of 70 (L/min), and an NH₃ gas is introduced at a constant flow rate of 90 (L/min). That is to say, the total amount (flow rate) thereof is 70 + 90 = 160 (L/min).
0038 Then, a steel component (work) is loaded into the heating chamber 11. At this time, the door 21 is opened, and thus the temperature in the heating chamber 11 is temporarily lowered as shown in Fig. 2. Thereafter, the door 21 is closed and the temperature in the heating chamber 11 is heated again to 550 °C.
0039 During this loading step, in the example shown in Fig. 2, the NH₃ gas is introduced at a constant flow rate of 160 (L/min). The total amount (flow rate) is also 160 (L/min).
0040 Thereafter, a three-stage nitriding treatment is performed (Example 1-7, which will be described hereinafter). Specifically, at first, for example, a value of 0.30 (an example of a value within a range of 0.10 to 1.00) is employed as a first nitriding potential, and a first nitriding treatment step is performed at a temperature of 550 °C.
0041 It is known that a nitriding potential K_N is represented by the following formula using P(NH₃) which is a partial pressure of the NH₃ gas and P(H₂) which is a partial pressure of the H₂ gas. $K_{N} = P (NH) (_{3}) / P (H) {(_{2})}^{3 / 2}$
0042 In the first nitriding treatment step, P(NH₃) i.e. a partial pressure of the NH₃ gas in the heating chamber 11 or P(H₂) i.e. a partial pressure of the H₂ gas in the heating chamber 11 is measured. Then, the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the first nitriding potential, which is a target nitriding potential.
0043 In the example shown in Fig. 2, P(H ₂) i.e. a partial pressure of the H₂ gas in the heating chamber 11 is measured by a heat conduction type H₂ sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control. Specifically, the introduction amounts (flow rates) of the NH₃ gas and an AX gas are respectively increased or decreased while keeping the sum (total) amount of the two gases to be 160 (L/min).
0044 In the example shown in Fig. 2, the first nitriding treatment step is performed for 60 minutes. Thereby, a surface carbon content (which has an adverse effect on generation of a γ' phase) is reduced since a nitride compound layer is not generated on a surface of the steel component or since a nitride compound layer mainly consisting of an ε phase is not generated on a surface of the steel component. On the other hand, thereby, nitrogen can be effectively diffused inside the steel component.
0045 Subsequently, for example, a value of 2.00 (an example of a value within a range of 0.30 to 10.00) is employed as a second nitriding potential, and a second nitriding treatment step is performed at a temperature of 550 °C.
0046 In the second nitriding treatment step as well, P(NH₃) i.e. a partial pressure of the NH₃ gas in the heating chamber 11 or P(H₂) i.e. a partial pressure of the H₂ gas in the heating chamber 11 is measured. Then, the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the second nitriding potential, which is a target nitriding potential.
0047 In the example shown in Fig. 2, P(H ₂) i.e. a partial pressure of the H₂ gas in the heating chamber 11 is measured by the heat conduction type H₂ sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control. Specifically, the N₂ gas is introduced at a constant flow rate of 70 (L/min), and the introduction amounts (flow rates) of the NH₃ gas and the AX gas are respectively increased or decreased while keeping the sum amount of the two gases to be 90 (L/min). That is to say, the total amount of the three gases continues to be 70 + 90 = 160 (L/min).
0048 In the example shown in Fig. 2, the second nitriding treatment step is performed for 240 minutes. Thereby, a nitride compound layer consisting of a γ' phase, or an ε phase, or mixture of a γ' phase and an ε phase, is generated in the steel component.
0049 Subsequently, for example, a value of 0.30 (an example of a value within a range of 0.26 to 0.60) is employed as a third nitriding potential, and a third nitriding treatment step is performed at a temperature of 550 °C.
0050 In the third nitriding treatment step as well, P(NH₃) i.e. a partial pressure of the NH₃ gas in the heating chamber 11 or P(H₂) i.e. a partial pressure of the H₂ gas in the heating chamber 11 is measured. Then, the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the third nitriding potential, which is a target nitriding potential.
0051 In the example shown in Fig. 2, P(H ₂) i.e. the partial pressure of the H₂ gas in the heating chamber 11 is measured by the heat conduction type H₂ sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control. Specifically, the introduction amounts (flow rates) of the NH₃ gas and the AX gas are respectively increased or decreased while keeping the sum (total) amount of the two gases to be 160 (L/min).
0052 In the example shown in Fig. 2, the third nitriding treatment step is performed for 60 minutes. Thereby, a γ' phase is deposited in the nitride compound layer.
0053 After the third nitriding treatment step has been completed, a cooling step is performed. In the example shown in Fig. 2, the cooling step is performed for 15 minutes (the case 20 is held in the oil bath (100 °C) for 15 minutes, the oil bath being provided with a stirrer). After the cooling step has been completed, the case 20 that contains the steel component is unloaded onto the unloading conveyor 13.
0054

(Structural Example of Pit Type Thermal Processing Furnace)

Fig. 3 is a schematic view showing a structure of a pit type thermal processing furnace 201 to be used for a nitriding treatment method according to the present invention.
0055 As shown in Fig. 3, the pit type thermal processing furnace 201 includes a bottomed cylindrical furnace wall 211 and a furnace lid 212.
0056 A fan 213 is provided on a lower (inner) side of the furnace lid 212. A rotation shaft of the fan 213 passes through the furnace lid 212, and is connected to a fan motor 214, which is provided on an upper (outer) side of the furnace lid 212.
0057 A retort 221 is provided inside the furnace wall 211. A gas guide tube 222 is provided further inside the retort 221. An outer periphery of the retort 221 is configured to be heated by a heater (not shown) so that a temperature in the furnace (in the retort 221) is controlled to a predetermined temperature. A case 20 is configured to be placed into the gas guide tube 222. A steel component as an object to be processed (work) is configured to be contained in the case 20. The maximum gross weight to be processed is 700 kg.
0058 A plurality of types of gases for the nitriding treatment are configured to be introduced into the retort 221 while being controlled as described below. In addition, the outer periphery of the retort 221 has a cooling function by a blower (not shown). When cooled, a temperature of the retort 221 itself is lowered, and thus the temperature in the furnace (in the retort 221) is lowered (furnace cooling).
0059

(Operational Example of Pit Type Thermal Processing Furnace)

In the thermal processing furnace 201 as described above, the furnace lid 212 is opened, and the case 20 that contains the steel component is loaded into the gas guide tube 222. After the steel component (the case 20 that contains the steel component) has been loaded into the gas guide tube 222, the plurality of types of process gases are introduced into the retort 221, and the process gases are heated to a predetermined temperature by the heater and stirred by the fan 213 (for example, rotating at 1500 rpm) so that the steel component loaded into the gas guide tube 222 is subjected to the nitriding treatment.
0060 Fig. 4 is a process diagram of an embodiment of the nitriding treatment method according to the present invention in a case wherein the thermal processing furnace 201 shown in Fig. 3 is used.
0061 In the example shown in Fig. 4, after a steel component (work) has been loaded into the gas guide tube 222, the inside of the retort 221 is heated to 550 °C. During a former half of this heating step, an N₂ gas is introduced at a constant flow rate of 40 (L/min). During a latter half of this heating step, an NH₃ gas is introduced at a constant flow rate of 40 (L/min).
0062 Thereafter, a three-stage nitriding treatment is performed (Example 5-7, which will be described hereinafter). Specifically, at first, for example, a value of 0.30 (an example of a value within a range of 0.10 to 1.00) is employed as a first nitriding potential, and a first nitriding treatment step is performed at a temperature of 550 °C.
0063 As described above, it is known that a nitriding potential K_N is represented by the following formula using P(NH₃) which is a partial pressure of the NH₃ gas and P(H₂) which is a partial pressure of the H₂ gas. $K_{N} = P (NH) (_{3}) / P (H) {(_{2})}^{3 / 2}$
0064 In the first nitriding treatment step, P(NH₃) i.e. a partial pressure of the NH₃ gas in the gas guide tube 222 or P(H₂) i.e. a partial pressure of the H₂ gas in the gas guide tube 222 is measured (alternatively, a partial pressure of the NH₃ gas in the exhaust gas or a partial pressure of the H₂ gas in the exhaust gas may be measured). Then, the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the first nitriding potential, which is a target nitriding potential.
0065 In the example shown in Fig. 4, P(H ₂) i.e. a partial pressure of the H₂ gas in the gas guide tube 222 is measured by a heat conduction type H₂ sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control. Specifically, the introduction amount (flow rate) of the NH₃ gas is increased or decreased while an AX gas is introduced at a constant flow rate of 50 (L/min). In this case, the total amount of the two gases is also increased or decreased.
0066 In the example shown in Fig. 4, the first nitriding treatment step is performed for 60 minutes. Thereby, a surface carbon content (which has an adverse effect on generation of a γ' phase) is reduced since a nitride compound layer is not generated on a surface of the steel component or since a nitride compound layer mainly consisting of an ε phase is not generated on a surface of the steel component. On the other hand, thereby, nitrogen can be effectively diffused inside the steel component.
0067 Subsequently, for example, a value of 0.50 (an example of a value within a range of 0.30 to 10.00) is employed as a second nitriding potential, and a second nitriding treatment step is performed at a temperature of 550 °C.
0068 In the second nitriding treatment step as well, P(NH₃) i.e. a partial pressure of the NH₃ gas in the gas guide tube 222 or P(H₂) i.e. a partial pressure of the H₂ gas in the gas guide tube 222 is measured. Then, the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the second nitriding potential, which is a target nitriding potential.
0069 In the example shown in Fig. 4, P(H ₂) i.e. a partial pressure of the H₂ gas in the gas guide tube 222 is measured by the heat conduction type H₂ sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control. Specifically, the introduction amount (flow rate) of the NH₃ gas is increased or decreased while the AX gas is introduced at a constant flow rate of 50 (L/min). In this case, the total amount of the two gases is also increased or decreased.
0070 In the example shown in Fig. 4, the second nitriding treatment step is performed for 240 minutes. Thereby, a nitride compound layer consisting of a γ' phase, or an ε phase, or mixture of a γ' phase and an ε phase, is generated in the steel component.
0071 Subsequently, for example, a value of 0.30 (an example of a value within a range of 0.26 to 0.60) is employed as a third nitriding potential, and a third nitriding treatment step is performed at a temperature of 550 °C.
0072 In the third nitriding treatment step as well, P(NH₃) i.e. a partial pressure of the NH₃ gas in the heating chamber 11 or P(H₂) i.e. a partial pressure of the H₂ gas in the heating chamber 11 is measured. Then, the introduction amounts (flow rates) of the process gases are subjected to a feedback control in such a manner that a nitriding potential calculated from the measured value is brought into the vicinity of the third nitriding potential, which is a target nitriding potential.
0073 In the example shown in Fig. 4, P(H ₂) i.e. the partial pressure of the H₂ gas in the gas guide tube 222 is measured by the heat conduction type H₂ sensor (not shown), the measured value is analyzed online (so that a nitriding potential is calculated from the measured value), and the introduction amounts (flow rates) of the process gases are subjected to a feedback control. Specifically, the introduction amount (flow rate) of the NH₃ gas is increased or decreased while the AX gas is introduced at a constant flow rate of 50 (L/min). In this case, the total amount of the two gases is also increased or decreased.
0074 In the example shown in Fig. 4, the third nitriding treatment step is performed for 60 minutes. Thereby, a γ' phase is deposited in the nitride compound layer.
0075 After the third nitriding treatment step has been completed, a cooling step is performed. In the example shown in Fig. 4, during a former half of the cooling step (until about 400 °C), the same control as in the third nitriding treatment step is performed for the introduction amounts of the process gases. That is to say, the introduction amount (flow rate) of the NH₃ gas is increased or decreased while the AX gas is introduced at a constant flow rate of 50 (L/min). During a latter half of the cooling step (about 400 °C to 100 °C), the N₂ gas is introduced at a constant flow rate of 20 (L/min). After the cooling step has been completed, the furnace lid 212 is opened, and the case 20 that contains the steel component is unloaded from the gas guide tube 222.
0076

(Structural Example of Horizontal Type Thermal Processing Furnace)

Fig. 5 is a schematic view showing a structure of a horizontal type thermal processing furnace to be used for a nitriding treatment method according to the present invention.
0077 A horizontal type thermal processing furnace is basically a furnace in which a pit type thermal processing furnace is oriented horizontally. However, as shown in Fig. 5, the fan 213 and the fan motor 214 may be provided on a wall surface of the furnace wall 211 facing the furnace lid 212, instead of on the furnace lid 212.
0078 The other structure of the horizontal type thermal processing furnace is substantially the same as the pit type thermal processing furnace explained with reference to Fig. 3.
0079

(Operational Example of Horizontal Type Thermal Processing Furnace)

In the horizontal type thermal processing furnace as well, the furnace lid 212 is opened, and the case 20 that contains the steel component is loaded into the gas guide tube 222. After the steel component (the case 20 that contains the steel component) has been loaded into the gas guide tube 222, the plurality of types of process gases are introduced into the gas guide tube 222, and the process gases are heated to a predetermined temperature by the heater and stirred by the fan 213 (for example, rotating at 1500 rpm) so that the steel component loaded into the gas guide tube 222 is subjected to the nitriding treatment.
0080 The process diagram shown in Fig. 4 is also applicable in a case wherein the horizontal type thermal processing furnace is used. Specifically, the heating step (introduction manners of the process gases are different between a former half thereof and a latter half thereof), the first nitriding treatment step, the second nitriding treatment step, the third nitriding treatment step and the cooling step (introduction manners of the process gases are different between a former half thereof and a latter half thereof) may be performed. After the cooling step has been completed, the furnace lid 212 is opened, and the case 20 that contains the steel component is unloaded from the gas guide tube 222.
0081

(Summary of Effects)

According to the embodiments of the present invention as described above, it is possible to obtain a nitrided steel component including an iron nitride compound layer which has a γ' phase as a main element (main component) on a surface thereof, regardless of whether a batch type of thermal processing furnace is used or a one-chamber type of thermal processing furnace is used.
0082 The steel component obtained by the respective embodiments can achieve a sufficient pitting resistance and a sufficient bending fatigue strength because a nitrogen diffusion layer and a nitride are formed in the inside thereof to reinforce the same and an iron nitride compound layer rich in a γ' phase is formed on the surface thereof.
0083 In addition, compared with a carburizing treatment and a nitrocarburizing treatment, the nitriding treatment according to the present invention is performed at a temperature not higher than the austenite transformation temperature, so that an amount of strain is small. In addition, a quenching step, which is necessary for a carburizing treatment or a nitrocarburizing treatment, can be omitted, so that an amount of strain variation is small. As a result, a high-strength and low-strain nitrided steel member can be obtained.
0084

(Supplementary Information about Temperature Range of Present Invention)

In the present invention, the temperature of each nitriding treatment step is 500 °C to 590 °C. It is said that, when the temperature of a nitriding treatment step is higher, the productivity thereof is better. However, according to the inventor's verification, if the temperature of a nitriding treatment step is higher than 590 °C, an amount (degree) of hardening is reduced and an austenite layer is formed on the surface. Thus, it is preferable that 590°C is the upper limit. On the other hand, according to the inventor's verification, if the temperature of a nitriding treatment step is lower than 500 °C, a formation speed of the nitride compound layer is slow, which is not cost effective. Thus, it is preferable that 500°C is the lower limit.
0085 In addition, when the difference between the temperature of the second nitriding treatment step and the temperature of the third nitriding treatment step is smaller, variation in temperature of the steel component (work) is also smaller, which can inhibit variation in nitriding quality of the steel component (work). Specifically, the temperature difference between both the nitriding treatment steps is controlled to be preferably 50 °C or less, more preferably 30 °C or less.
0086

(Examples 1-1 to 1-14 and Comparative Examples 1-1 to 1-8)

For a plurality of cylindrical ring gears (whose steel types may be different), using a batch type of thermal processing furnace 1, a three-stage nitriding treatment was performed according to the conditions of Table 1 shown in Fig. 6.
0087 In examples 1-1 to 1-14 and comparative examples 1-1 to 1-8, a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same batch type of thermal processing furnace 1.
0088 In addition, in the first nitriding treatment step of each of the examples 1-1 to 1-14 and the comparative examples 1-1 to 1-8, two types of gases, which are an NH₃ gas and an AX gas, were used, and a nitriding potential during the first nitriding treatment step was controlled to be close to the first nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0089 In addition, in the second nitriding treatment step of each of the examples 1-1 to 1-14 and the comparative examples 1-1 to 1-8, three types of gases, which are an NH₃ gas, an AX gas and an N₂ gas, were used, and a nitriding potential during the second nitriding treatment step was controlled to be close to the second nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the three types of gases constant.
0090 In addition, in the third nitriding treatment step of each of the examples 1-1 to 1-14 and the comparative examples 1-1 to 1-8, two types of gases, which are an NH₃ gas and an AX gas, were used, and a nitriding potential during the third nitriding treatment step was controlled to be close to the third nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0091 In the examples 1-1 to 1-14 and the comparative examples 1-1 to 1-8, the respective steps explained with reference to Fig. 2 were performed before and after the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step.
0092 In Table 1, an identification method for a phase was performed based on an X-ray diffraction pattern obtained by an X-ray diffraction measurement from a surface of a steel component in accordance with a 2θ-θ scanning method (MiniFlex 600 made by Rigalku, Cu tube, 40 kV-15 mA).
0093 In addition, in Table 1, a thickness of the compound layer was measured as a thickness of a surface compound layer from a tissue observation result of a cross section which was cut in a depth direction of the nitrided steel component. It is preferable that a thickness of the compound layer rich in the γ' phase is 2 µm to 20 µm. When it is less than 2 µm, i.e., too thin, the fatigue strength is not sufficiently improved. When it is more than 20 µm, a porous layer in the compound layer which may be an origin of fatigue crack is too thick, which deteriorates the fatigue strength.
0094 As seen from the result of Table 1, in the control manner using the three types of gases in the batch type of thermal processing furnace, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 1-1 to 1-14.
0095 On the other hand, at a temperature within a range of 500 °C to 590 °C, when the nitriding potential in the third nitriding treatment step is not higher than 0.25, the comparative examples 1-1 to 1-8 have proved that an α phase which is lower in hardness than the γ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.
0096

(Examples 2-1 to 2-14 and Comparative Examples 2-1 to 2-8)

For a plurality of cylindrical ring gears (whose steel types may be different), using a pit type thermal processing furnace 201, a three-stage nitriding treatment was performed according to the conditions of Table 2 shown in Fig. 7.
0097 In examples 2-1 to 2-14 and comparative examples 2-1 to 2-8, a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same pit type thermal processing furnace 201.
0098 In addition, in the first nitriding treatment step of each of the examples 2-1 to 2-14 and the comparative examples 2-1 to 2-8, two types of gases, which are an NH₃ gas and an AX gas, were used, and a nitriding potential during the first nitriding treatment step was controlled to be close to the first nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0099 In addition, in the second nitriding treatment step of each of the examples 2-1 to 2-14 and the comparative examples 2-1 to 2-8, three types of gases, which are an NH₃ gas, an AX gas and an N₂ gas, were used, and a nitriding potential during the second nitriding treatment step was controlled to be close to the second nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the three types of gases constant.
0100 In addition, in the third nitriding treatment step of each of the examples 2-1 to 2-14 and the comparative examples 2-1 to 2-8, two types of gases, which are an NH₃ gas and an AX gas, were used, and a nitriding potential during the third nitriding treatment step was controlled to be close to the third nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0101 In the examples 2-1 to 2-14 and the comparative examples 2-1 to 2-8, the respective steps explained with reference to Fig. 4 were performed before and after the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step.
0102 In Table 2, an identification of a phase and a thickness of the compound layer were judged in the same manner as those in Table 1.
0103 As seen from the result of Table 2, in the control manner using the three types of gases in the pit type thermal processing furnace, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 2-1 to 2-14.
0104 On the other hand, at a temperature within a range of 500 °C to 590 °C, when the nitriding potential in the third nitriding treatment step is not higher than 0.25, the comparative examples 2-1 to 2-8 have proved that an α phase which is lower in hardness than the γ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.
0105

(Examples 3-1 to 3-14 and Comparative Examples 3-1 to 3-8)

For a plurality of cylindrical ring gears (whose steel types may be different), using a batch type of thermal processing furnace 1, a three-stage nitriding treatment was performed according to the conditions of Table 3 shown in Fig. 8.
0106 In examples 3-1 to 3-14 and comparative examples 3-1 to 3-8, a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same batch type of thermal processing furnace 1.
0107 In addition, in the first nitriding treatment step of each of the examples 3-1 to 3-14 and the comparative examples 3-1 to 3-8, two types of gases, which are an NH₃ gas and an AX gas, were used, and a nitriding potential during the first nitriding treatment step was controlled to be close to the first nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0108 In addition, in the second nitriding treatment step of each of the examples 1-1 to 1-14 and the comparative examples 1-1 to 1-8 as well, the two types of gases, which are the NH₃ gas and the AX gas, were used, and a nitriding potential during the second nitriding treatment step was controlled to be close to the second nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0109 In addition, in the third nitriding treatment step of each of the examples 3-1 to 3-14 and the comparative examples 3-1 to 3-8 as well, the two types of gases, which are the NH₃ gas and the AX gas, were used, and a nitriding potential during the third nitriding treatment step was controlled to be close to the third nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0110 In the examples 3-1 to 3-14 and the comparative examples 3-1 to 3-8, the respective steps explained with reference to Fig. 2 were performed before and after the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step.
0111 In Table 3, an identification of a phase and a thickness of the compound layer were judged in the same manner as those in Tables 1 and 2.
0112 As seen from the result of Table 3, in the control manner using the two types of gases in the batch type of thermal processing furnace, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 3-1 to 3-14.
0113 On the other hand, at a temperature within a range of 500 °C to 590 °C, when the nitriding potential in the third nitriding treatment step is not higher than 0.25, the comparative examples 3-1 to 3-8 have proved that an α phase which is lower in hardness than the γ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.
0114

(Examples 4-1 to 4-14 and Comparative Examples 4-1 to 4-8)

For a plurality of cylindrical ring gears (whose steel types may be different), using a pit type thermal processing furnace 201, a three-stage nitriding treatment was performed according to the conditions of Table 4 shown in Fig. 9.
0115 In examples 4-1 to 4-14 and comparative examples 4-1 to 4-8, a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same pit type thermal processing furnace 201.
0116 In addition, in the first nitriding treatment step of each of the examples 4-1 to 4-14 and the comparative examples 4-1 to 4-8, two types of gases, which are an NH₃ gas and an AX gas, were used, and a nitriding potential during the first nitriding treatment step was controlled to be close to the first nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0117 In addition, in the second nitriding treatment step of each of the examples 4-1 to 4-14 and the comparative examples 4-1 to 4-8 as well, the two types of gases, which are the NH₃ gas and the AX gas, were used, and a nitriding potential during the second nitriding treatment step was controlled to be close to the second nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0118 In addition, in the third nitriding treatment step of each of the examples 4-1 to 4-14 and the comparative examples 4-1 to 4-8 as well, the two types of gases, which are the NH₃ gas and the AX gas, were used, and a nitriding potential during the third nitriding treatment step was controlled to be close to the third nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
0119 In the examples 4-1 to 4-14 and the comparative examples 4-1 to 4-8, the respective steps explained with reference to Fig. 4 were performed before and after the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step.
0120 In Table 4, an identification of a phase and a thickness of the compound layer were judged in the same manner as those in Tables 1 to 3.
0121 As seen from the result of Table 4, in the control manner using the two types of gases in the pit type thermal processing furnace, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 4-1 to 4-14.
0122 On the other hand, at a temperature within a range of 500 °C to 590 °C, when the nitriding potential in the third nitriding treatment step is not higher than 0.25, the comparative examples 4-1 to 4-8 have proved that an α phase which is lower in hardness than the γ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.
0123

(Examples 5-1 to 5-14 and Comparative Examples 5-1 to 5-8)

For a plurality of cylindrical ring gears (whose steel types may be different), using a pit type thermal processing furnace 201, a three-stage nitriding treatment was performed according to the conditions of Table 5 shown in Fig. 10.
0124 In examples 5-1 to 5-14 and comparative examples 5-1 to 5-8, a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same pit type thermal processing furnace 201.
0125 In addition, in the first nitriding treatment step of each of the examples 5-1 to 5-14 and the comparative examples 5-1 to 5-8, two types of gases, which are an NH₃ gas and an AX gas, were used, and a nitriding potential during the first nitriding treatment step was controlled to be close to the first nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant.
0126 In addition, in the second nitriding treatment step of each of the examples 5-1 to 5-14 and the comparative examples 5-1 to 5-8 as well, the two types of gases, which are the NH₃ gas and the AX gas, were used, and a nitriding potential during the second nitriding treatment step was controlled to be close to the second nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant.
0127 In addition, in the third nitriding treatment step of each of the examples 5-1 to 5-14 and the comparative examples 5-1 to 5-8 as well, the two types of gases, which are the NH₃ gas and the AX gas, were used, and a nitriding potential during the third nitriding treatment step was controlled to be close to the third nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant.
0128 In the examples 5-1 to 5-14 and the comparative examples 5-1 to 5-8, the respective steps explained with reference to Fig. 4 were performed before and after the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step.
0129 In Table 5, an identification of a phase and a thickness of the compound layer were judged in the same manner as those in Tables 1 to 4.
0130 As seen from the result of Table 5, in the control manner using the two types of gases in the pit type thermal processing furnace, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 5-1 to 5-14.
0131 On the other hand, at a temperature within a range of 500 °C to 590 °C, when the nitriding potential in the third nitriding treatment step is not higher than 0.25, the comparative examples 5-1 to 5-8 have proved that an α phase which is lower in hardness than the γ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.
0132

(Examples 6-1 to 6-14 and Comparative Examples 6-1 to 6-8)

For a plurality of cylindrical ring gears (whose steel types may be different), using a pit type thermal processing furnace 201, a three-stage nitriding treatment was performed according to the conditions of Table 6 shown in Fig. 11.
0133 In examples 6-1 to 6-14 and comparative examples 6-1 to 6-8, a first nitriding treatment step, a second nitriding treatment step and a third nitriding treatment step were performed in sequence in the same pit type thermal processing furnace 201.
0134 In addition, in the first nitriding treatment step of each of the examples 6-1 to 6-14 and the comparative examples 6-1 to 6-8, two types of gases, which are an NH₃ gas and an AX gas, were used, and a nitriding potential during the first nitriding treatment step was controlled to be close to the first nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant.
0135 In addition, in the second nitriding treatment step of each of the examples 6-1 to 6-9 and the comparative examples 6-1 to 6-4, three types of gases, which are the NH₃ gas, the AX gas and an N₂ gas, were used, and a nitriding potential during the second nitriding treatment step was controlled to be close to the second nitriding potential (K_N), which is a target nitriding potential, by changing the introduction amount of the one of the NH₃ gas and the AX gas while keeping the introduction amount of the other of the NH₃ gas and the AX gas constant.
0136 In addition, in the third nitriding treatment step of each of the examples 6-1 to 6-14 and the comparative examples 6-1 to 6-8, the two types of gases, which are the NH₃ gas and the AX gas, were used, and a nitriding potential during the third nitriding treatment step was controlled to be close to the third nitriding potential (K_N), which is a target nitriding potential, by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant.
0137 In the examples 6-1 to 6-14 and the comparative examples 6-1 to 6-8, the respective steps explained with reference to Fig. 4 were performed before and after the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step.
0138 In Table 6, an identification of a phase and a thickness of the compound layer were judged in the same manner as those in Tables 1 to 5.
0139 As seen from the result of Table 6, in the control manner using the three types of gases in the pit type thermal processing furnace, the effectiveness of the present invention wherein the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C, wherein the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C, wherein the first nitriding potential is a value within a range of 0.10 to 1.00, wherein the second nitriding potential is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00, and wherein the third nitriding potential is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60, was proved by the examples 6-1 to 6-14.
0140 On the other hand, at a temperature within a range of 500 °C to 590 °C, when the nitriding potential in the third nitriding treatment step is not higher than 0.25, the comparative examples 6-1 to 6-8 have proved that an α phase which is lower in hardness than the γ' phase was deposited, resulting in an insufficient pitting resistance and an insufficient bending fatigue strength.

Description of Reference Signs

0141

1: Thermal Processing Furnace
10: Loading Section
11: Heating Chamber
12: Transfer Chamber
13: Unloading Conveyor
20: Case
21: Door
22: Inlet Hood
26: Fan
27: Intermediate Door
30: Elevator
32: Cooling Chamber (Oil Tank)
35: Door
36: Outlet Hood
201: Thermal Processing Furnace
211: Furnace Wall
212: Furnace Lid
213: Fan
214: Fan Motor
221: Retort
222: Gas Guide Tube

Claims

A nitriding treatment method for a steel component, comprising at least three nitriding treatment steps, i.e.,
a first nitriding treatment step in which a nitriding treatment is performed to a steel component under a nitriding gas atmosphere of a first nitriding potential,

a second nitriding treatment step in which another nitriding treatment is performed to the steel component under another nitriding gas atmosphere of a second nitriding potential higher than the first nitriding potential, after the first nitriding treatment step, and

a third nitriding treatment step in which a further another nitriding treatment is performed to the steel component under a further another nitriding gas atmosphere of a third nitriding potential lower than the second nitriding potential, after the second nitriding treatment step,

wherein

the first nitriding treatment step is performed at a temperature within a range of 500 °C to 590 °C,

the second nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C,

the third nitriding treatment step is also performed at a temperature within a range of 500 °C to 590 °C,

the first nitriding potential is a value within a range of 0.10 to 1.00,

the second nitriding potential is a value within a range of 0.30 to 10.00,

the third nitriding potential is a value within a range of 0.26 to 0.60,

a nitride compound layer consisting of a γ' phase, or an ε phase, or mixture of a γ' phase and an ε phase, is generated during the second nitriding treatment step, and

a γ' phase is deposited in the nitride compound layer during the third nitriding treatment step.
The nitriding treatment method according to claim 1,
the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a batch type of thermal processing furnace,

two types of gases, which are an NH₃ gas and an AX gas, are used in the first nitriding treatment step,

a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant,

three types of gases, which are an NH₃ gas, an AX gas and an N₂ gas, are used in the second nitriding treatment step,

a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the three types of gases constant,

two types of gases, which are an NH₃ gas and an AX gas, are used in the third nitriding treatment step, and

a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
The nitriding treatment method according to claim 1,
the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace,

two types of gases, which are an NH₃ gas and an AX gas, are used in the first nitriding treatment step,

a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant,

three types of gases, which are an NH₃ gas, an AX gas and an N₂ gas, are used in the second nitriding treatment step,

a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the three types of gases constant,

two types of gases, which are an NH₃ gas and an AX gas, are used in the third nitriding treatment step, and

a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
The nitriding treatment method according to claim 1,
the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a batch type of thermal processing furnace,

two types of gases, which are an NH₃ gas and an AX gas, are used in the first nitriding treatment step,

a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the three types of gases constant,

two types of gases, which are an NH₃ gas and an AX gas, are used in the second nitriding treatment step,

a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant,

two types of gases, which are an NH₃ gas and an AX gas, are used in the third nitriding treatment step, and

a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
The nitriding treatment method according to claim 1,
the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace,

two types of gases, which are an NH₃ gas and an AX gas, are used in the first nitriding treatment step,

a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the three types of gases constant,

two types of gases, which are an NH₃ gas and an AX gas, are used in the second nitriding treatment step,

a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant,

two types of gases, which are an NH₃ gas and an AX gas, are used in the third nitriding treatment step, and

a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH₃ gas and the AX gas while keeping a total introduction amount of the two types of gases constant.
The nitriding treatment method according to claim 1,
the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace,

two types of gases, which are an NH₃ gas and an AX gas, are used in the first nitriding treatment step,

a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant,

two types of gases, which are an NH₃ gas and an AX gas, are used in the second nitriding treatment step,

a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant,

two types of gases, which are an NH₃ gas and an AX gas, are used in the third nitriding treatment step, and

a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant.
The nitriding treatment method according to claim 1,
the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace,

two types of gases, which are an NH₃ gas and an AX gas, are used in the first nitriding treatment step,

a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant,

three types of gases, which are an NH₃ gas, an AX gas and an N₂ gas, are used in the second nitriding treatment step,

a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant,

two types of gases, which are an NH₃ gas and an AX gas, are used in the third nitriding treatment step, and

a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of one of the NH₃ gas and the AX gas while keeping an introduction amount of the other of the NH₃ gas and the AX gas constant.
The nitriding treatment method according to any of claims 1 to 7,
a time for which the third nitriding treatment step is performed is 60 minutes or longer.