Method for nitriding steel member

Abstract

To provide a nitriding method capable of excellently depositing a γ' phase in a nitrided compound layer in nitriding performed at a temperature of 500-590°C to achieve high pitching resistance and bending fatigue strength.SOLUTION: A method for nitriding a steel member, comprising a nitriding process consisting of at least two steps comprises: a first nitriding step of nitriding the steel member in a nitriding gas atmosphere having a first nitriding potential; and a second nitriding step of nitriding the steel member in a nitriding gas atmosphere having a second nitriding potential lower than the first nitriding potential after the first nitriding step. The first and second nitriding steps are performed at a temperature of 500-590°C; the first nitriding potential has a value of 0.300-10.000; and the second nitriding potential has a value of 0.253-0.600.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a nitriding method for steel members having at least a two-stage nitriding treatment step.

Background Art

[0002] Steel members such as gears used in automotive transmissions are required to have high anti-pitting properties and bending fatigue strength. In order to meet such requirements, carburizing treatment and nitriding treatment are known as methods for strengthening steel members such as gears.

[0003] For example, Patent Document 1 discloses that in order to improve the anti-pitting properties and bending fatigue strength of steel members, it is effective to form an iron nitride compound layer mainly composed of γ' phase on the surface of the steel members by nitriding treatment.

[0004] Further, Patent Document 2 discloses a first nitriding treatment step of nitriding the steel member in a nitriding gas atmosphere with a nitriding potential in which a nitriding compound layer of γ' phase or ε phase is formed in order to suppress variations during mass production, and then, a second nitriding treatment step of nitriding the steel member in a nitriding gas atmosphere with a nitriding potential lower than that of the first nitriding treatment step, thereby precipitating γ' phase in the nitriding compound layer. Specifically, a gas nitriding treatment carried out at a temperature of 600°C using two types of gases, NH3 gas and H2 gas, is described as an example. More specifically, at a temperature of 600°C, a range of 0.6 to 1.51 is adopted for the nitriding potential in the first nitriding treatment step, and a range of 0.16 to 0.25 is adopted for the nitriding potential in the second nitriding treatment step.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In further studies on the nitriding treatment method disclosed in Patent Document 2, the present inventor found that in the temperature range of 500°C to 590°C, increasing the nitriding potential in the second nitriding treatment step to be greater than 0.25 has a higher effect of precipitating the γ' phase in the nitrided compound layer.

[0007] According to the present inventor, the action (reaction) of precipitating the γ' phase in the nitrided compound layer is affected by both the nitriding potential and the furnace temperature. In the temperature range of 500°C to 590°C, if the nitriding potential in the second nitriding treatment step is set to 0.25 or less, the α phase with a hardness lower than that of the γ' phase will also precipitate, resulting in insufficient pitting resistance and bending fatigue strength.

[0008] The present invention was conceived based on the above findings. The object of the present invention is to provide a nitriding treatment method capable of favorably precipitating the γ' phase in the nitrided compound layer in a nitriding treatment carried out in the temperature range of 500°C to 590°C, and thus realizing high pitting resistance and bending fatigue strength.

Means for Solving the Problems

[0009] The present invention is a nitriding treatment method for a steel member having at least two-stage nitriding treatment steps, a first nitriding treatment step of nitriding the steel member in a nitriding gas atmosphere with a first nitriding potential, a second nitriding treatment step of further nitriding the steel member in a nitriding gas atmosphere with a second nitriding potential lower than the first nitriding potential after the first nitriding treatment step, comprising the first nitriding treatment step is carried out at a temperature of 500°C to 590°C, the second nitriding treatment step is also carried out at a temperature of 500°C to 590°C, The first nitriding potential is a value within the range of 0.300 to 10.000, The second nitriding potential is a value within the range of 0.253 to 0.600, In the first nitriding treatment step, a nitrided compound layer of γ' phase, ε phase, or a mixture of γ' phase and ε phase is formed, In the second nitriding treatment step, γ' phase is precipitated on the nitrided compound layer A nitriding treatment method characterized by being as follows.

[0010] According to the present invention, in the second nitriding treatment step carried out at a temperature of 500°C to 590°C, by setting the second nitriding potential to a value within the range of 0.253 to 0.600, precipitation of the α phase having a hardness lower than that of the γ' phase can be suppressed, and the γ' phase can be favorably precipitated on the nitrided compound layer, thereby realizing high anti-pitting property and bending fatigue strength.

[0011] In the present invention, the first nitriding treatment step and the second nitriding treatment step are sequentially carried out, for example, in the same batch-type heat treatment furnace. In the first nitriding treatment step, three types of gases, namely NH3 gas, AX gas, and N2 gas, are used, and by changing the introduction amounts of NH3 gas and AX gas while keeping their total flow rate constant, the nitriding potential during the first nitriding treatment step is controlled to be the first nitriding potential. In the second nitriding treatment step, two types of gases, namely NH3 gas and AX gas, are used, and by changing their introduction amounts while keeping their total flow rate constant, the nitriding potential during the second nitriding treatment step is controlled to be the second nitriding potential.

[0012] In such a control mode, the first nitriding treatment step is carried out at a temperature of 500°C to 590°C, the second nitriding treatment step is also carried out at a temperature of 500°C to 590°C, the first nitriding potential is a value within the range of 0.300 to 10.000, and the second nitriding potential is lower than the first nitriding potential and is a value within the range of 0.253 to 0.600, thereby demonstrating the effectiveness of the present invention.

[0013] Alternatively, in the present invention, the first nitriding treatment step and the second nitriding treatment step are, for example, sequentially performed in the same single-chamber heat treatment furnace. In the first nitriding treatment step, three types of gases, namely NH3 gas, AX gas, and N2 gas, are used. By changing the introduction amounts of the NH3 gas and the AX gas while keeping their total flow rate constant, the nitriding potential during the first nitriding treatment step is controlled to be the first nitriding potential. In the second nitriding treatment step, two types of gases, namely NH3 gas and AX gas, are used. By changing their introduction amounts while keeping their total flow rate constant, the nitriding potential during the second nitriding treatment step is controlled to be the second nitriding potential.

[0014] Even in such a control mode, the first nitriding treatment step is performed at a temperature of 500°C to 590°C, the second nitriding treatment step is also performed at a temperature of 500°C to 590°C, the first nitriding potential is a value within the range of 0.300 to 10.000, and the second nitriding potential is lower than the first nitriding potential and is a value within the range of 0.253 to 0.600. The effectiveness of the present invention was thus demonstrated.

[0015] Alternatively, in the present invention, the first nitriding treatment step and the second nitriding treatment step are, for example, sequentially performed in the same batch-type heat treatment furnace. In the first nitriding treatment step, two types of gases, namely NH3 gas and AX gas, are used. By changing their introduction amounts while keeping their total flow rate constant, the nitriding potential during the first nitriding treatment step is controlled to be the first nitriding potential. Also in the second nitriding treatment step, two types of gases, namely NH3 gas and AX gas, are used. By changing their introduction amounts while keeping their total flow rate constant, the nitriding potential during the second nitriding treatment step is controlled to be the second nitriding potential.

[0016] Even in such a control mode, the first nitriding treatment step is carried out at a temperature of 500°C to 590°C, the second nitriding treatment step is also carried out at a temperature of 500°C to 590°C, the first nitriding potential is a value within the range of 0.300 to 10.000, and the second nitriding potential is lower than the first nitriding potential and is a value within the range of 0.253 to 0.600, and the effectiveness of the present invention is demonstrated.

[0017] Alternatively, in the present invention, the first nitriding treatment step and the second nitriding treatment step are sequentially carried out, for example, in the same one-chamber type heat treatment furnace. In the first nitriding treatment step, two types of gases, NH3 gas and AX gas, are used, and by changing the introduction amount of each of them while keeping their total flow rate constant, the nitriding potential during the first nitriding treatment step is controlled to be the first nitriding potential. Also in the second nitriding treatment step, two types of gases, NH3 gas and AX gas, are used, and by changing the introduction amount of each of them while keeping their total flow rate constant, the nitriding potential during the second nitriding treatment step is controlled to be the second nitriding potential.

[0018] Even in such a control mode, the first nitriding treatment step is carried out at a temperature of 500°C to 590°C, the second nitriding treatment step is also carried out at a temperature of 500°C to 590°C, the first nitriding potential is a value within the range of 0.300 to 10.000, and the second nitriding potential is lower than the first nitriding potential and is a value within the range of 0.253 to 0.600, and the effectiveness of the present invention is demonstrated.

[0019] Alternatively, in the present invention, the first nitriding treatment step and the second nitriding treatment step are, for example, sequentially performed in the same single-chamber heat treatment furnace. In the first nitriding treatment step, two types of gases, NH3 gas and AX gas, are used. By changing the introduction amount of one of them while keeping the introduction amount of the other constant, the nitriding potential during the first nitriding treatment step is controlled to be the first nitriding potential. Also in the second nitriding treatment step, two types of gases, NH3 gas and AX gas, are used. By changing the introduction amount of one of them while keeping the introduction amount of the other constant, the nitriding potential during the second nitriding treatment step is controlled to be the second nitriding potential.

[0020] Even in such a control mode, the first nitriding treatment step is performed at a temperature of 500°C to 590°C, the second nitriding treatment step is also performed at a temperature of 500°C to 590°C, the first nitriding potential is a value within the range of 0.300 to 10.000, and the second nitriding potential is lower than the first nitriding potential and is a value within the range of 0.253 to 0.600, which demonstrates the effectiveness of the present invention.

[0021] Alternatively, in the present invention, the first nitriding treatment step and the second nitriding treatment step are, for example, sequentially performed in the same single-chamber heat treatment furnace. In the first nitriding treatment step, three types of gases, NH3 gas, AX gas, and N2 gas, are used. By changing the introduction amount of one of NH3 gas and AX gas while keeping the introduction amount of the other constant, the nitriding potential during the first nitriding treatment step is controlled to be the first nitriding potential. Also in the second nitriding treatment step, two types of gases, NH3 gas and AX gas, are used. By changing the introduction amount of one of them while keeping the introduction amount of the other constant, the nitriding potential during the second nitriding treatment step is controlled to be the second nitriding potential.

[0022] Even in such a control mode, the first nitriding treatment step is carried out at a temperature of 500°C to 590°C, the second nitriding treatment step is also carried out at a temperature of 500°C to 590°C, the first nitriding potential is a value within the range of 0.300 to 10.000, and the second nitriding potential is lower than the first nitriding potential and is a value within the range of 0.253 to 0.600. The effectiveness of the present invention was demonstrated.

[0023] Note that a single-chamber heat treatment furnace is a heat treatment furnace that does not have a cooling chamber separate from the heating chamber like a batch-type heat treatment furnace (see Fig. 1), and heating and cooling are performed in only one chamber. Pit furnaces (see Fig. 3) and horizontal furnaces (see Fig. 5) are common.

[0024] Also, in each of the above inventions, it is preferable that the time of the first nitriding treatment step is longer than the time of the second nitriding treatment step. According to the findings of the present inventor, by carrying out the first nitriding treatment step longer than the second nitriding treatment step, the thickness of the compound layer after nitriding treatment can be adjusted to an arbitrary thickness.

Effects of the Invention

[0025] According to the present invention, in the second nitriding treatment step carried out at a temperature of 500°C to 590°C, when the second nitriding potential is set to a value within the range of 0.253 to 0.600, the precipitation of the α phase with a hardness lower than that of the γ' phase can be suppressed, and the γ' phase can be favorably precipitated in the nitrided compound layer, thereby realizing high pitting resistance and bending fatigue strength.

Brief Description of the Drawings

[0026]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Mode for Carrying Out the Invention

[0027] [Examples of the object to be treated (workpiece)] The object to be treated (workpiece) is a steel member. Specifically, it is a steel member made of carbon steel for mechanical structures or alloy steel for mechanical structures such as gears used in automatic transmissions. For example, a cylindrical ring gear or a bottomed cylindrical ring gear is mounted on a multi-stage jig and nitriding-treated in a state of being placed flat in a case (described later).

[0028] It is preferable that pre-cleaning for removing dirt and oil is performed on the steel member before nitriding treatment. The pre-cleaning is preferably, for example, vacuum cleaning for degreasing and drying by dissolving and replacing oil with a hydrocarbon-based cleaning liquid and evaporating it, or alkaline cleaning for degreasing with an alkaline-based cleaning liquid.

[0029] [Configuration example of a batch-type heat treatment furnace] FIG. 1 is a schematic configuration diagram of a batch-type heat treatment furnace 1 used in the nitriding treatment method of the present invention.

[0030] As shown in FIG. 1, a batch-type heat treatment furnace 1 includes a loading section 10, a heating chamber 11, a transfer chamber 12, and an unloading conveyor 13. In the loading section 10, a case 20 is placed, and steel members as workpieces to be processed are stored in the case 20. The maximum processing weight is 700 kg gross.

[0031] An inlet hood 22 having an openable and closable door 21 is attached to the inlet side (left side in FIG. 1) of the heating chamber 11. The heating chamber 11 has a retort structure, and the temperature inside the furnace is controlled to a predetermined temperature by heating the outer periphery of the retort with a heater (not shown). Then, a plurality of types of gases for nitriding treatment are introduced into the heating chamber 11 while being controlled as described later.

[0032] Also, a fan 26 for stirring the gas introduced into the heating chamber 11 to equalize the heating temperature of the steel members is mounted on the ceiling of the heating chamber 11. An intermediate door 27 that can be opened and closed is attached to the outlet side (right side in FIG. 1) of the heating chamber 11.

[0033] An elevator 30 for raising and lowering the case 20 in which the steel members are stored is provided in the transfer chamber 12. A cooling chamber (oil tank) 32 storing cooling oil 31 is provided below the transfer chamber 12. An outlet hood 36 having an openable and closable door 35 is attached to the outlet side (right side in FIG. 1) of the transfer chamber 12.

[0034] Note that a configuration may be adopted in which the heating chamber 11 and the transfer chamber 12 are a processing chamber in the same space, and the heat-treated steel members are air-cooled by gas. Further, the heating chamber 11 may be divided into two, and the two-stage nitriding treatment process described later may be performed in each heating chamber.

[0035] [Operating Example of Batch-Type Heat Treatment Furnace] In the heat treatment furnace 1 configured as described above, the case 20 containing the steel member is carried into the heating chamber 11 from the loading section 10 by a pusher or the like. After the case 20 (containing the steel member) is carried into the heating chamber 11, a processing gas is introduced into the heating chamber 11, and the processing gas is heated to a predetermined temperature by a heater and further stirred by a fan 26 (for example, rotating at 1500 rpm), and then nitriding treatment of the steel member carried into the heating chamber 11 is performed.

[0036] FIG. 2 is a process diagram of an embodiment of the nitriding treatment method of the present invention using the heat treatment furnace 1 of FIG. 1.

[0037] In the example of FIG. 2, before the steel member (workpiece) is loaded, the inside of the heating chamber 11 is preheated to 550°C. Further, during this heating process, N2 gas is introduced at a constant flow rate of 70 (L / min), and NH3 gas is introduced at a constant flow rate of 90 (L / min). The total flow rate is 70 + 90 = 160 (L / min).

[0038] Next, the steel member (workpiece) is loaded into the heating chamber 11. At this time, when the door 21 is opened, as shown in FIG. 2, the temperature inside the heating chamber 11 temporarily drops. Then, the door 21 is closed, and the temperature inside the heating chamber 11 is heated again to 550°C.

[0039] Even during such loading of the steel member, in the example of FIG. 2, N2 gas is introduced at a constant flow rate of 70 (L / min), and NH3 gas is introduced at a constant flow rate of 90 (L / min), and the total flow rate is 70 + 90 = 160 (L / min).

[0040] Thereafter, a two-stage nitriding treatment process is performed. Specifically, first, a value of, for example, 1.500 (0.300 to 10.000) is adopted as the first nitriding potential, and the first nitriding treatment process is performed at a temperature of 550°C.

[0041] Nitriding potential K N is known to be expressed by the following formula using the partial pressure P(NH3) of NH3 gas and the partial pressure P(H2) of H2 gas. K N = P(NH3) / P(H2) 3 / 2

[0042] In the first nitriding treatment step, the partial pressure P(NH3) of NH3 gas or the partial pressure P(H2) of H2 gas in the heating chamber 11 is measured, and the introduction amount of the treatment gas is feedback-controlled so that the value of the nitriding potential calculated from the measured value is within the vicinity range of the target first nitriding potential.

[0043] In the example of FIG. 2, the partial pressure P(H2) of H2 gas in the heating chamber 11 is measured by a thermal conductivity type H2 sensor (not shown), and the introduction amount of the treatment gas is feedback-controlled while analyzing the measured value online (while calculating the nitriding potential from the measured value). Specifically, N2 gas is introduced at a constant flow rate of 70 (L / min), while NH3 gas and AX gas are increased or decreased respectively under the condition that the total flow rate is 90 (L / min). The total flow rate is maintained at 70 + 90 = 160 (L / min).

[0044] In the example of FIG. 2, such a first nitriding treatment step is carried out for 240 minutes. Thereby, a nitriding compound layer of γ' phase, ε phase, or a mixture of γ' phase and ε phase is formed on the steel member.

[0045] Subsequently, a value of, for example, 0.300 (0.253 to 0.600) is adopted as the second nitriding potential, and the second nitriding treatment step is carried out at a temperature of 550°C.

[0046] Also in the second nitriding treatment step, the partial pressure P(NH3) of NH3 gas or the partial pressure P(H2) of H2 gas in the heating chamber 11 is measured, and the introduction amount of the treatment gas is feedback-controlled so that the value of the nitriding potential calculated from the measured value is within the vicinity range of the target second nitriding potential.

[0047] In the example of FIG. 2, the partial pressure P(H2) of H2 gas in the heating chamber 11 is measured by a heat conductivity type H2 sensor (not shown), and while analyzing the measured value online (while calculating the nitriding potential from the measured value), the introduction amount of the processing gas is feedback-controlled. Specifically, the NH3 gas and the AX gas are increased or decreased respectively under the condition that the total flow rate is 160 (L / min).

[0048] In the example of FIG. 2, such a second nitriding treatment step is carried out for 60 minutes. As a result, a γ' phase is precipitated in the nitrided compound layer.

[0049] When the second nitriding treatment step is completed, a cooling step is carried out. In the example of FIG. 2, the cooling step is carried out for 15 minutes (it is an oil bath with a stirrer and is held in oil at 100 °C for 15 minutes). When the cooling step is completed, the case 20 containing the steel member is carried out onto the carry-out conveyor 13.

[0050] [Configuration example of a pit-type heat treatment furnace] FIG. 3 is a schematic configuration diagram of a pit-type heat treatment furnace 201 used in the nitriding treatment method of the present invention.

[0051] As shown in FIG. 3, the pit-type heat treatment furnace 201 includes a bottomed cylindrical furnace wall 211 and a furnace lid 212.

[0052] A fan 213 is provided on the lower side (inside) of the furnace lid 212, and the rotation shaft of the fan 213 penetrates the furnace lid 212 and is connected to a fan motor 214 provided on the upper side (outside) of the furnace lid 212.

[0053] A retort 221 is provided inside the furnace wall 211, and a gas guide cylinder 222 is provided further inside the retort 221. The outer peripheral portion of the retort 221 is heated by a heater (not shown) so that the temperature inside the furnace (inside the retort 221) is controlled to a predetermined temperature. And the case 20 is placed inside the gas guide cylinder 222, and a steel member as a workpiece to be processed is stored inside the case 20. The processing weight is at most 700 kg gross.

[0054] Also, a plurality of types of gases for nitriding treatment are introduced into the retort 221 while being controlled as described later. Further, the outer peripheral portion of the retort 221 has a function of cooling by a blower (not shown), and during cooling, the temperature of the retort 221 itself is lowered to cool the workpiece in the furnace (furnace cooling).

[0055] [Operation Example of Pit-Type Heat Treatment Furnace] In the heat treatment furnace 201 having the above-described configuration, the furnace lid 212 is opened, and the case 20 containing the steel member is carried into the gas guide cylinder 222. Then, after the steel member (the case 20 containing the same) is carried into the gas guide cylinder 222, the processing gas is introduced into the gas guide cylinder 222, the processing gas is heated to a predetermined temperature by a heater, and further, while being stirred by the fan 213 (for example, rotating at 1500 rpm), the nitriding treatment of the steel member carried into the gas guide cylinder 222 is performed.

[0056] FIG. 4 is a process diagram of an embodiment of the nitriding treatment method of the present invention using the heat treatment furnace 201 of FIG. 3.

[0057] In the example of FIG. 4, after the steel member (workpiece) is loaded into the gas guide cylinder 222, the inside of the retort 221 is heated to 550°C. In the first half of this heating process, N2 gas is introduced at a constant flow rate of 40 (L / min), and in the second half of this heating process, NH3 gas is introduced at a constant flow rate of 40 (L / min).

[0058] Thereafter, a two-stage nitriding treatment process is performed. Specifically, first, a value of, for example, 1.500 (0.300 to 10.000) is adopted as the first nitriding potential, and the first nitriding treatment process is performed at a temperature of 550°C.

[0059] As described above, the nitriding potential K N is known to be expressed by the following formula in terms of the partial pressure P(NH3) of NH3 gas and the partial pressure P(H2) of H2 gas. K N = P(NH3) / P(H2)3 / 2

[0060] In the first nitriding treatment step, the partial pressure P(NH3) of NH3 gas or the partial pressure P(H2) of H2 gas in the gas guide cylinder 222 is measured (the partial pressure P(NH3) of NH3 gas or the partial pressure P(H2) of H2 gas in the exhaust gas may also be measured), and the introduction amount of the treatment gas is feedback-controlled so that the value of the nitriding potential calculated from the measured value is within the vicinity range of the target first nitriding potential.

[0061] In the example of FIG. 4, the partial pressure P(H2) of H2 gas in the gas guide cylinder 222 is measured by a thermal conductivity type H2 sensor (not shown), and the introduction amount of the treatment gas is feedback-controlled while analyzing the measured value online (while calculating the nitriding potential from the measured value). Specifically, the AX gas is introduced at a constant flow rate of 20 (L / min), while the NH3 gas is increased or decreased. The total flow rate will also vary.

[0062] In the example of FIG. 4, such a first nitriding treatment step is carried out for 240 minutes. Thereby, a nitrided compound layer of γ' phase, ε phase, or a mixture of γ' phase and ε phase is formed on the steel member.

[0063] Subsequently, a value of, for example, 0.300 (0.253 to 0.600) is adopted as the second nitriding potential, and the second nitriding treatment step is carried out at a temperature of 550°C.

[0064] Also in the second nitriding treatment step, the partial pressure P(NH3) of NH3 gas or the partial pressure P(H2) of H2 gas in the gas guide cylinder 222 is measured, and the introduction amount of the treatment gas is feedback-controlled so that the value of the nitriding potential calculated from the measured value is within the vicinity range of the target second nitriding potential.

[0065] In the example of FIG. 4, the partial pressure P(H2) of H2 gas in the gas guide cylinder 222 is measured by a thermal conductivity type H2 sensor (not shown), and while analyzing the measured value online (while calculating the nitride potential from the measured value), the introduction amount of the processing gas is feedback-controlled. Specifically, while the AX gas is introduced at a constant flow rate of 30 (L / min), the NH3 gas is increased or decreased. The total flow rate will also vary.

[0066] In the example of FIG. 4, such a second nitriding treatment step is carried out for 60 minutes. As a result, a γ' phase is precipitated in the nitride compound layer.

[0067] When the second nitriding treatment step is completed, a cooling step is carried out. In the example of FIG. 4, in the first half of the cooling step (up to about 400°C), the introduction amount of the processing gas is controlled in the same manner as in the second nitriding treatment step. That is, while the AX gas is introduced at a constant flow rate of 30 (L / min), the NH3 gas is increased or decreased. In the second half of the cooling step (400°C to about 100°C), N2 gas is introduced at a constant flow rate of 20 (L / min). When the cooling step is completed, the furnace lid 212 is opened, and the case 20 containing the steel member is carried out from the gas guide cylinder 222.

[0068] [Configuration example of horizontal heat treatment furnace] FIG. 5 is a schematic configuration diagram of a horizontal heat treatment furnace used in the nitriding treatment method of the present invention.

[0069] The horizontal heat treatment furnace is basically a furnace with a pit type heat treatment furnace placed horizontally. However, as shown in FIG. 5, a configuration in which the fan 213 and the fan motor 214 are provided on the wall surface of the furnace wall 211 facing the furnace lid 212 instead of the furnace lid 212 can also be adopted.

[0070] Other configurations of the horizontal heat treatment furnace are substantially the same as the configurations of the pit type heat treatment furnace described with reference to FIG. 3.

[0071] [Operation example of horizontal heat treatment furnace] Even in a horizontal heat treatment furnace, the furnace lid 212 is opened, and the case 20 containing the steel member is carried into the gas guide cylinder 222. Then, after the case 20 (containing the steel member) is carried into the gas guide cylinder 222, the processing gas is introduced into the retort 211, the processing gas is heated to a predetermined temperature by a heater, and while being stirred by a fan 213 (rotating at, for example, 1500 rpm), the nitriding treatment of the steel member carried into the gas guide cylinder 222 is performed.

[0072] The process diagram of FIG. 4 is effective even when a horizontal heat treatment furnace is used. Specifically, a heating process (with different gas introduction modes in the first half and the second half), a first nitriding treatment process, a second nitriding treatment process, and a cooling process can be performed. When the cooling process is completed, the furnace lid 212 is opened, and the case 20 containing the steel member is carried out from the gas guide cylinder 222.

[0073] [Summary of Effects] According to the embodiments of the present invention as described above, a nitrided steel member having an iron nitride compound layer mainly composed of γ'-phase on the surface can be obtained whether a batch-type heat treatment furnace or a single-chamber heat treatment furnace is used.

[0074] The steel member obtained by each embodiment is strengthened by forming a nitrogen diffusion layer and nitrides inside, and a γ'-phase-rich iron nitride compound layer is formed on the surface, so that sufficient pitting resistance and bending fatigue strength can be realized.

[0075] Also, compared with carburizing and carbonitriding treatments, the nitriding treatment of the present invention is a treatment at a temperature below the austenite transformation temperature, so the amount of strain is small. In addition, since the quenching process, which is an essential process in carburizing and carbonitriding treatments, can be omitted, the amount of strain variation is also small. As a result, a high-strength and low-strain nitrided steel member can be obtained.

[0076] [Supplement Regarding the Temperature Range of the Present Invention] In the present invention, the temperature of each nitriding treatment step is set to 500°C to 590°C. It is said that the productivity is better when the temperature of the nitriding treatment is higher. However, according to the verification by the inventor of this case, if it is higher than 590°C, the amount of hardening decreases, and an austenite layer is formed on the surface. Therefore, it is preferable to set 590°C as the upper limit. On the other hand, according to the verification by the inventor of this case, if the nitriding treatment temperature is lower than 500°C, the formation rate of the nitrided compound layer becomes slow, which is not preferable in terms of cost. Therefore, it is preferable to set 500°C as the lower limit.

[0077] Also, the smaller the difference between the temperature of the first nitriding treatment step and the temperature of the second nitriding treatment step, the smaller the variation in the temperature of the steel member (workpiece) can be made, and the variation in the nitriding quality of the steel member (workpiece) can be suppressed. Specifically, the temperature difference between the two nitriding treatment steps is preferably controlled within 50°C, and more preferably within 30°C.

[0078] [Examples 1-1 to 1-9, Comparative Examples 1-1 to 1-4] For a plurality of cylindrical ring gears (the steel types may be different), using a batch-type heat treatment furnace 1, two-stage nitriding treatment was carried out according to the conditions shown in Table 1 in FIG. 6.

[0079] In Examples 1-1 to 1-9 and Comparative Examples 1-1 to 1-4, the first nitriding treatment step and the second nitriding treatment step were sequentially carried out in the same batch-type heat treatment furnace 1.

[0080] Also, in the first nitriding treatment step of Examples 1-1 to 1-9 and Comparative Examples 1-1 to 1-4, three types of gases, namely NH3 gas, AX gas, and N2 gas, were used. By changing the introduction amounts of NH3 gas and AX gas while keeping their total flow rate constant, the nitriding potential during the first nitriding treatment step was controlled to be the target first nitriding potential (K N ).

[0081] In addition, in the second nitriding treatment steps of Examples 1-1 to 1-9 and Comparative Examples 1-1 to 1-4, two types of gases, NH3 gas and AX gas, were used, and by changing the introduction amounts of each of them while keeping their total flow rate constant, the nitriding potential during the second nitriding treatment step was controlled to be the target second nitriding potential (K N ).

[0082] In Examples 1-1 to 1-9 and Comparative Examples 1-1 to 1-4, before and after the first nitriding treatment step and the second nitriding treatment step, the respective steps described with reference to FIG. 2 were carried out.

[0083] In Table 1, the phase identification method was carried out based on the X-ray diffraction pattern obtained by performing X-ray diffraction measurement (Rigaku MiniFlex600, Cu tube, 40 kV - 15 mA) by the 2θ-θ scanning method from the surface of the steel material.

[0084] In addition, in Table 1, the compound layer thickness was measured by cutting the nitrided steel material in the depth direction and measuring the thickness of the surface compound layer from the cross-sectional microstructure observation results. (It is preferable that the thickness of the γ'-phase rich compound layer is 4 to 16 μm. If it is less than 4 μm, it is too thin and the improvement of fatigue strength is not sufficient, while if it exceeds 16 μm, the porous layer of the compound layer serving as the starting point of fatigue cracks becomes thick and the fatigue strength decreases.)

[0085] As can be seen from the results shown in Table 1, according to Examples 1-1 to 1-9, in the control mode using the above three types of gases in a batch furnace, the first nitriding treatment step is carried out at a temperature of 500°C to 590°C, the second nitriding treatment step is also carried out at a temperature of 500°C to 590°C, the first nitriding potential is a value within the range of 0.300 to 10.000, and the second nitriding potential is lower than the first nitriding potential and is a value within the range of 0.253 to 0.600, thereby demonstrating the effectiveness of the present invention.

[0086] On the other hand, according to Comparative Examples 1-1 to 1-4, in the temperature range of 500°C to 590°C, when the nitriding potential in the second nitriding treatment step is set to 0.25 or less, an α-phase with a hardness lower than that of the γ'-phase also precipitates, resulting in insufficient pitting resistance and bending fatigue strength.

[0087] [Examples 2-1 to 2-9, Comparative Examples 2-1 to 2-4] For a plurality of cylindrical ring gears (the steel types may be different), using a pit-type heat treatment furnace 201, two-stage nitriding treatment was carried out according to the conditions shown in Table 2 in FIG. 7.

[0088] In Examples 2-1 to 2-9 and Comparative Examples 2-1 to 2-4, the first nitriding treatment step and the second nitriding treatment step were sequentially carried out in the same pit-type heat treatment furnace 201.

[0089] In addition, in the first nitriding treatment step of Examples 2-1 to 2-9 and Comparative Examples 2-1 to 2-4, three types of gases, namely NH3 gas, AX gas, and N2 gas, were used. By changing the introduction amounts of NH3 gas and AX gas while keeping their total flow rate constant, the nitriding potential during the first nitriding treatment step became the target first nitriding potential (K N ).

[0090] In addition, in the second nitriding treatment step of Examples 2-1 to 2-9 and Comparative Examples 2-1 to 2-4, two types of gases, namely NH3 gas and AX gas, were used. By changing their introduction amounts while keeping their total flow rate constant, the nitriding potential during the second nitriding treatment step became the target second nitriding potential (K N ).

[0091] In Examples 2-1 to 2-9 and Comparative Examples 2-1 to 2-4, before and after the first nitriding treatment step and the second nitriding treatment step, the respective steps described with reference to FIG. 4 were carried out.

[0092] The phase identification method and the compound layer thickness in Table 2 were determined in the same manner as those in Table 1.

[0093] As can be seen from the results shown in Table 2, in the control modes using the above three types of gases in the pit furnace according to Examples 2-1 to 2-9, the first nitriding treatment step was carried out at a temperature of 500°C to 590°C, the second nitriding treatment step was also carried out at a temperature of 500°C to 590°C, the first nitriding potential was a value within the range of 0.300 to 10.000, and the second nitriding potential was lower than the first nitriding potential and was a value within the range of 0.253 to 0.600. The effectiveness of the present invention was thus demonstrated.

[0094] On the other hand, according to Comparative Examples 2-1 to 2-4, in the temperature range of 500°C to 590°C, when the nitriding potential in the second nitriding treatment step was set to 0.25 or less, an α-phase with a hardness lower than that of the γ'-phase was also precipitated, demonstrating that the pitching resistance and bending fatigue strength became insufficient.

[0095] [Examples 3-1 to 3-9, Comparative Examples 3-1 to 3-4] For a plurality of cylindrical ring gears (the steel types may be different), two-stage nitriding treatment was carried out using a batch-type heat treatment furnace 1 according to the conditions shown in Table 3 as shown in FIG. 8.

[0096] In Examples 3-1 to 3-9 and Comparative Examples 3-1 to 3-4, the first nitriding treatment step and the second nitriding treatment step were sequentially carried out in the same batch-type heat treatment furnace 1.

[0097] In addition, in the first nitriding treatment steps of Examples 3-1 to 3-9 and Comparative Examples 3-1 to 3-4, two types of gases, NH3 gas and AX gas, were used. By changing the introduction amounts of each of them while keeping their total flow rate constant, the nitriding potential during the first nitriding treatment step was controlled to be the target first nitriding potential (K N ).

[0098] Also, in the second nitriding treatment steps of Examples 3-1 to 3-9 and Comparative Examples 3-1 to 3-4, two types of gases, NH3 gas and AX gas, were used, and by changing the introduction amounts of each of them while keeping their total flow rate constant, the nitriding potential during the second nitriding treatment step was adjusted to the target second nitriding potential (K N ).

[0099] In Examples 3-1 to 3-9 and Comparative Examples 3-1 to 3-4, the respective steps described with reference to FIG. 2 were carried out before and after the first nitriding treatment step and the second nitriding treatment step.

[0100] The phase identification method and the compound layer thickness in Table 3 were determined in the same manner as those in Tables 1 and 2.

[0101] As can be seen from the results shown in Table 3, in Examples 3-1 to 3-9, in the control mode using the two types of gases in a batch furnace, the first nitriding treatment step was carried out at a temperature of 500°C to 590°C, the second nitriding treatment step was also carried out at a temperature of 500°C to 590°C, the first nitriding potential was a value within the range of 0.300 to 10.000, and the second nitriding potential was lower than the first nitriding potential and was a value within the range of 0.253 to 0.600, thus demonstrating the effectiveness of the present invention.

[0102] On the other hand, in Comparative Examples 3-1 to 3-4, in the temperature range of 500°C to 590°C, when the nitriding potential in the second nitriding treatment step was made 0.25 or less, an α phase having a hardness lower than that of the γ' phase was also precipitated, and it was demonstrated that the pitching resistance and the bending fatigue strength became insufficient.

[0103] [Examples 4-1 to 4-9, Comparative Examples 4-1 to 4-4] For a plurality of cylindrical ring gears (the steel types may be different), two-stage nitriding treatment was carried out using a pit-type heat treatment furnace 201 according to the conditions shown in Table 4 shown in FIG. 9.

[0104] In Examples 4-1 to 4-9 and Comparative Examples 4-1 to 4-4, the first nitriding treatment step and the second nitriding treatment step were sequentially carried out in the same pit-type heat treatment furnace 201.

[0105] Also, in the first nitriding treatment step of Examples 4-1 to 4-9 and Comparative Examples 4-1 to 4-4, two types of gases, NH3 gas and AX gas, were used, and by changing the introduction amounts of each of them while keeping their total flow rate constant, the nitriding potential during the first nitriding treatment step was made to be the target first nitriding potential (K N ).

[0106] Also, in the second nitriding treatment step of Examples 4-1 to 4-9 and Comparative Examples 4-1 to 4-4, two types of gases, NH3 gas and AX gas, were used, and by changing the introduction amounts of each of them while keeping their total flow rate constant, the nitriding potential during the second nitriding treatment step was made to be the target second nitriding potential (K N ).

[0107] In Examples 4-1 to 4-9 and Comparative Examples 4-1 to 4-4, before and after the first nitriding treatment step and the second nitriding treatment step, the respective steps described with reference to FIG. 4 were carried out.

[0108] The phase identification method and the compound layer thickness in Table 4 were determined in the same manner as those in Tables 1 to 3.

[0109] As can be seen from the results shown in Table 4, according to Examples 4-1 to 4-9, in the control mode using the above two types of gases in the pit furnace, the first nitriding treatment step was carried out at a temperature of 500°C to 590°C, the second nitriding treatment step was also carried out at a temperature of 500°C to 590°C, the first nitriding potential was a value within the range of 0.300 to 10.000, and the second nitriding potential was lower than the first nitriding potential and was a value within the range of 0.253 to 0.600, thereby demonstrating the effectiveness of the present invention.

[0110] On the other hand, according to Comparative Examples 4-1 to 4-4, in the temperature range of 500°C to 590°C, when the nitriding potential in the second nitriding treatment step is 0.25 or less, an α-phase with a hardness lower than that of the γ'-phase also precipitates, resulting in insufficient pitting resistance and bending fatigue strength.

[0111] [Examples 5-1 to 5-9, Comparative Examples 5-1 to 5-4] For a plurality of cylindrical ring gears (the steel types can be different), using a pit-type heat treatment furnace 201, two-stage nitriding treatment was carried out according to the conditions shown in Table 5 in FIG. 10.

[0112] In Examples 5-1 to 5-9 and Comparative Examples 5-1 to 5-4, the first nitriding treatment step and the second nitriding treatment step were sequentially carried out in the same pit-type heat treatment furnace 201.

[0113] In the first nitriding treatment steps of Examples 5-1 to 5-9 and Comparative Examples 5-1 to 5-4, two types of gases, NH3 gas and AX gas, were used. By changing the introduction amount of one of them while keeping the introduction amount of the other constant, the nitriding potential during the first nitriding treatment step became the target first nitriding potential (K N ) and was controlled accordingly.

[0114] In the second nitriding treatment steps of Examples 5-1 to 5-9 and Comparative Examples 5-1 to 5-4, two types of gases, NH3 gas and AX gas, were also used. By changing the introduction amount of one of them while keeping the introduction amount of the other constant, the nitriding potential during the second nitriding treatment step became the target second nitriding potential (K N ) and was controlled accordingly.

[0115] In Examples 5-1 to 5-9 and Comparative Examples 5-1 to 5-4, before and after the first nitriding treatment step and the second nitriding treatment step, the respective steps described with reference to FIG. 4 were carried out.

[0116] The phase identification method and the compound layer thickness in Table 5 were determined in the same manner as those in Tables 1 to 4.

[0117] As can be seen from the results shown in Table 5, according to Examples 5-1 to 5-9, in the control mode using the two types of gases in the pit furnace, the first nitriding treatment step is carried out at a temperature of 500°C to 590°C, the second nitriding treatment step is also carried out at a temperature of 500°C to 590°C, the first nitriding potential is a value within the range of 0.300 to 10.000, and the second nitriding potential is lower than the first nitriding potential and is a value within the range of 0.253 to 0.600. The effectiveness of the present invention was demonstrated.

[0118] On the other hand, according to Comparative Examples 5-1 to 5-4, in the temperature range of 500°C to 590°C, when the nitriding potential in the second nitriding treatment step is set to 0.25 or less, an α-phase with a hardness lower than that of the γ'-phase is also precipitated, and it was demonstrated that the pitching resistance and the bending fatigue strength become insufficient.

[0119] [Examples 6-1 to 6-9, Comparative Examples 6-1 to 6-4] For a plurality of cylindrical ring gears (the steel types can be different), using a pit-type heat treatment furnace 201, two-stage nitriding treatment was carried out according to the conditions shown in Table 6 as shown in FIG. 11.

[0120] In Examples 6-1 to 6-9 and Comparative Examples 6-1 to 6-4, the first nitriding treatment step and the second nitriding treatment step were sequentially carried out in the same pit-type heat treatment furnace 201.

[0121] In addition, in the first nitriding treatment step of Examples 6-1 to 6-9 and Comparative Examples 6-1 to 6-4, three types of gases, namely NH3 gas, AX gas, and N2 gas, were used. By changing the introduction amount of one of the NH3 gas and the AX gas while keeping the introduction amount of the other constant, the nitriding potential during the first nitriding treatment step becomes the target first nitriding potential (K N ) and was controlled accordingly.

[0122] Also, in the second nitriding treatment steps of Examples 6-1 to 6-9 and Comparative Examples 6-1 to 6-4, three types of gases, namely NH3 gas, AX gas, and N2 gas, were used. By changing the introduction amount of one of the NH3 gas and the AX gas while keeping the introduction amount of the other constant, the nitriding potential during the second nitriding treatment step was controlled to be the target second nitriding potential (K N ).

[0123] In Examples 6-1 to 6-9 and Comparative Examples 6-1 to 6-4, each step described with reference to FIG. 4 was performed before and after the first nitriding treatment step and the second nitriding treatment step.

[0124] The phase identification method and the compound layer thickness in Table 6 were determined in the same manner as those in Tables 1 to 5.

[0125] As can be seen from the results shown in Table 6, in Examples 6-1 to 6-9, in the control mode using the three types of gases in the pit furnace, the first nitriding treatment step was carried out at a temperature of 500°C to 590°C, and the second nitriding treatment step was also carried out at a temperature of 500°C to 590°C. The first nitriding potential was a value within the range of 0.300 to 10.000, and the second nitriding potential was lower than the first nitriding potential and was a value within the range of 0.253 to 0.600, demonstrating the effectiveness of the present invention.

[0126] On the other hand, in Comparative Examples 6-1 to 6-4, in the temperature range of 500°C to 590°C, when the nitriding potential in the second nitriding treatment step was set to 0.25 or less, an α phase having a hardness lower than that of the γ' phase was also precipitated, demonstrating that the pitting resistance and the bending fatigue strength became insufficient.

Explanation of Reference Numerals

[0127] 1 Heat treatment furnace 10 Loading section 11 Heating chamber 12 Transfer chamber 13 Unloading conveyor 20 Case 21 Door 22 Inlet hood 26 fans 27 intermediate door 30 elevator 31 cooling chamber (oil tank) 35 door 36 outlet hood 201 heat treatment furnace 211 furnace wall 212 furnace lid 213 fan 214 fan motor 221 retort 222 gas guide cylinder

Claims

1. A nitriding treatment method for a steel member having at least a two-stage nitriding treatment process, comprising: a first nitriding treatment step of nitriding the steel member in a nitriding gas atmosphere with a first nitriding potential; after the first nitriding treatment step, a second nitriding treatment step of further nitriding the steel member in a nitriding gas atmosphere with a second nitriding potential lower than the first nitriding potential; and the first nitriding treatment step is carried out at a temperature of 500°C to 590°C; the second nitriding treatment step is also carried out at a temperature of 500°C to 590°C; the first nitriding potential is a value within the range of 0.300 to 10.000; the second nitriding potential is a value within the range of 0.253 to 0.600; in the first nitriding treatment step, a nitrided compound layer of γ' phase, ε phase, or a mixture of γ' phase and ε phase is formed; in the second nitriding treatment step, γ' phase is precipitated on the nitrided compound layer A nitriding treatment method characterized by the above.

2. The first nitriding treatment step and the second nitriding treatment step are sequentially carried out in the same batch-type heat treatment furnace, In the first nitriding treatment step, NH 3 gas, AX gas, and N 2 gas are used, and while keeping the total flow rate of these three gases constant, by changing the introduction amounts of each of the NH 3 gas and the AX gas, the nitriding potential during the first nitriding treatment step is controlled to be the first nitriding potential. In the second nitriding treatment step, NH 3 Two types of gases, namely an NH gas and an AX gas, are used. By changing the introduction amounts of these gases while keeping their total flow rate constant, the nitriding potential during the second nitriding treatment step is controlled to be the second nitriding potential. The nitriding treatment method according to claim 1, characterized by the above.

3. The first nitriding treatment step and the second nitriding treatment step are sequentially carried out in the same single-chamber type heat treatment furnace, In the first nitriding process, NH 3 gas, AX gas, and N 2 gas are used. While keeping the total flow rate of these three gases constant, by changing the introduction amounts of NH 3 gas and AX gas respectively, the nitriding potential during the first nitriding process is controlled to be the first nitriding potential. In the second nitriding treatment step, NH 3 Two types of gases, namely an NH gas and an AX gas, are used. By changing the introduction amounts of these gases while keeping their total flow rate constant, the nitriding potential during the second nitriding treatment step is controlled to be the second nitriding potential. The nitriding treatment method according to claim 1, characterized by the above.

4. The first nitriding treatment step and the second nitriding treatment step are sequentially carried out in the same batch-type heat treatment furnace, In the first nitriding treatment step, NH 3 Two types of gases, an NH gas and an AX gas, are used, and by changing the introduction amount of each of them while keeping the total flow rate constant, the nitriding potential during the first nitriding treatment step is controlled to be the first nitriding potential. Even in the second nitriding treatment step, NH 3 Two types of gases, an NH gas and an AX gas, are used, and by changing the introduction amount of each of them while keeping the total flow rate constant, the nitriding potential during the second nitriding treatment step is controlled to be the second nitriding potential. The nitriding treatment method according to claim 1, characterized by the above.

5. The first nitriding treatment step and the second nitriding treatment step are sequentially carried out in the same single-chamber type heat treatment furnace, In the first nitriding treatment step, NH 3 Two types of gases, an NH gas and an AX gas, are used, and by changing the introduction amount of each of them while keeping the total flow rate constant, the nitriding potential during the first nitriding treatment step is controlled to be the first nitriding potential. Even in the second nitriding treatment step, NH 3 Two types of gases, an NH gas and an AX gas, are used, and by changing the introduction amounts of these gases while keeping their total flow rate constant, the nitriding potential during the second nitriding treatment step is controlled to be the second nitriding potential. The nitriding treatment method according to claim 1, characterized by the above.

6. The first nitriding treatment step and the second nitriding treatment step are sequentially carried out in the same single-chamber type heat treatment furnace, In the first nitriding treatment step, two types of gases, an NH 3 gas and an AX gas, are used. By changing the introduction amount of one of them while keeping the introduction amount of the other constant, the nitriding potential during the first nitriding treatment step is controlled to be the first nitriding potential. Even in the second nitriding treatment step, NH 3 Two types of gases, an NH gas and an AX gas, are used. By changing the introduction amount of one of them while keeping the introduction amount of the other constant, the nitriding potential during the second nitriding treatment step is controlled to be the second nitriding potential. The nitriding treatment method according to claim 1, characterized by the above.

7. The first nitriding treatment step and the second nitriding treatment step are sequentially carried out in the same single-chamber type heat treatment furnace, In the first nitriding treatment step, NH 3 gas, AX gas, and N 2 gas are used. By changing the introduction amount of one of the NH 3 gas and the AX gas while keeping the introduction amount of the other constant, the nitriding potential during the first nitriding treatment step is controlled to be the first nitriding potential. Even in the second nitriding treatment step, NH 3 Two types of gases, NH 3 gas and AX gas, are used. While keeping the introduction amount of one of the NH gas and the AX gas constant, the other introduction amount is changed so that the nitriding potential during the second nitriding treatment step becomes the second nitriding potential. The nitriding treatment method according to claim 1, characterized by the above.

8. The time of the first nitriding treatment step is longer than the time of the second nitriding treatment step The nitriding treatment method according to any one of claims 1 to 7, characterized by the above.

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