TWI833179B - Nitriding treatment method for steel components - Google Patents

Abstract

The object of the present invention is to provide a nitriding treatment method that can successfully precipitate the γ' phase in the nitride compound layer during the nitriding treatment carried out in the temperature range of 500°C to 590°C, thereby achieving higher Pitting corrosion resistance and bending fatigue strength. The present invention is a nitriding treatment method for steel components with at least three nitriding treatment steps. The first to third nitriding treatment steps are carried out at a temperature of 500°C to 590°C. The first nitriding potential in the first nitriding step is a value in the range of 0.10 to 1.00, and the second nitriding potential in the second nitriding step is higher than the first nitriding potential and is in the range of 0.30 to 10.00. The value of , the third nitriding potential of the third nitriding treatment step is lower than the second nitriding potential, and is a value in the range of 0.26 to 0.60.

Description

Nitriding treatment method for steel components

The present invention relates to a nitriding treatment method for steel components having at least three stages of nitriding treatment steps.

Steel components such as gears used in automobile transmissions require high pitting corrosion resistance and bending fatigue strength. In response to such demands, carburizing treatment or nitriding treatment is known as a method of strengthening steel members such as gears.

For example, Patent Document 1 discloses that in order to improve the pitting corrosion resistance or bending fatigue strength of steel members, it is effective to form an iron-nitrogen compound layer containing the γ' phase as the main component on the surface of the steel member through nitriding treatment.

Furthermore, Patent Document 2 discloses a nitriding treatment method for forming a nitride compound layer with a high γ' ratio (0.7 or more). Specifically, a three-stage gas nitriding process using two gases, NH ₃ gas and AX gas, at a temperature of 570° C. to 600° C. is described. More specifically, at a temperature of 570°C to 600°C, the nitriding potential in the first nitriding step is 0.1 to 0.25, the nitriding potential in the second nitriding step is 1.0 to 2.0, and the nitriding potential in the third nitriding step is 1.0 to 2.0. The nitridation potential in the chemical treatment step is 0.25. [Prior art documents] [Patent documents]

[Patent Document 1] Japanese Patent Application No. 2012-095035 [Patent Document 2] Japanese Patent No. 6755106

[The problem that the invention aims to solve]

The inventor of this case conducted repeated and in-depth research on the nitriding treatment method disclosed in Patent Document 2 and found that in the temperature range of 500°C to 590°C, making the nitriding potential in the third nitriding step greater than 0.25 makes γ 'The effect of phase precipitation in the nitride compound layer is high.

According to the inventor of this case, the effect (reaction) of γ' phase precipitation in the nitrided compound layer is affected by both the nitriding potential and the temperature in the furnace. In the temperature range of 500°C to 590°C, if the third nitriding treatment is If the nitriding potential in the step is set to 0.25 or less, an α phase with a lower hardness than the γ' phase may also precipitate, resulting in insufficient pitting corrosion resistance or bending fatigue strength.

The present invention was invented based on the above findings. The object of the present invention is to provide a nitriding treatment method, which can enable the γ' phase to be well precipitated in the nitride compound layer during the nitriding treatment carried out in the temperature range of 500°C to 590°C, thereby achieving higher Pitting corrosion resistance and bending fatigue strength. [Technical means to solve problems]

The invention is a nitriding treatment method, which is characterized by: It is a nitriding treatment method for steel components with at least three stages of nitriding treatment steps, including: The first nitriding treatment step involves nitriding the steel component in a nitriding gas atmosphere of the first nitriding potential; a second nitriding treatment step, which is to further nitriding the steel member in a nitriding gas atmosphere with a second nitriding potential higher than the first nitriding potential after the above-mentioned first nitriding step; and a third nitriding step, which is to further nitriding the steel member in a nitriding gas atmosphere with a third nitriding potential lower than the second nitriding potential after the above-mentioned second nitriding step; The above-mentioned first nitriding step is carried out at a temperature of 500°C to 590°C. The above-mentioned second nitriding step is also carried out at a temperature of 500°C to 590°C. The above-mentioned third nitriding step is also carried out at a temperature of 500°C to 590°C. The above-mentioned first nitriding potential is a value in the range of 0.10 to 1.00, The above-mentioned second nitriding potential is a value in the range of 0.30 to 10.00, The above third nitriding potential is a value in the range of 0.26 to 0.60, In the above second nitriding step, a nitrided compound layer in which the γ' phase, the ε phase, or the γ' phase and the ε phase are mixed is generated, and In the above-mentioned third nitriding treatment step, the γ′ phase is precipitated in the above-mentioned nitride compound layer.

According to the present invention, in the third nitriding step performed at a temperature of 500°C to 590°C, by setting the third nitriding potential to a value in the range of 0.26 to 0.60, the hardness can be suppressed from being lower than γ' The precipitation of the α phase allows the γ' phase to be well precipitated in the nitride compound layer, which can achieve higher pitting corrosion resistance and bending fatigue strength.

In the present invention, the above-mentioned first nitriding step, the second nitriding step and the above-mentioned third nitriding step are performed sequentially in the same batch-type heat treatment furnace, for example. In the above-mentioned first nitriding step, Two gases, NH ₃ gas and AX gas, are used, and the total flow rate of them is fixed while changing the introduction amount of each of them, thereby controlling so that the nitriding potential in the first nitriding step becomes In the above-mentioned first nitriding process, in the above-mentioned second nitriding treatment step, three gases, namely NH ₃ gas, AX gas and N ₂ gas, are used, and the total flow rate of the gases is fixed while changing the NH ₃ gas and AX gas. The introduction amount of each gas is controlled so that the nitriding potential in the second nitriding step becomes the above-mentioned second nitriding potential. In the above-mentioned third nitriding step, NH ₃ gas and AX gas are used. The two gases are controlled so that the nitriding potential in the third nitriding treatment step becomes the third nitriding potential by setting the total flow rate of the two gases to be fixed and changing the introduction amounts of the respective gases.

In this control mode, the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is also carried out at a temperature of 500°C to 590°C. When implemented at a temperature of 500°C to 590°C, the first nitriding potential is in the range of 0.10 to 1.00, the second nitriding potential is higher than the first nitriding potential, and is in the range of 0.30 to 10.00, and the third The nitriding potential is lower than the second nitriding potential and is a value in the range of 0.26 to 0.60. The effectiveness of the present invention characterized by this was confirmed.

Alternatively, in the present invention, the above-mentioned first nitriding step, the second nitriding step and the above-mentioned third nitriding step are performed sequentially in the same single-chamber heat treatment furnace, for example, in the above-mentioned first nitriding step In this method, two gases, NH ₃ gas and AX gas, are used, and the total flow rate of the gases is fixed while changing the respective introduction amounts thereof, thereby controlling the nitriding in the first nitriding treatment step. The potential becomes the above-mentioned first nitriding potential. In the above-mentioned second nitriding treatment step, three gases, namely NH ₃ gas, AX gas and N ₂ gas, are used, and the total flow rate of them is fixed while changing the NH ₃ gas. and AX gas, thereby controlling the nitriding potential in the second nitriding step to become the above-mentioned second nitriding potential. In the above-mentioned third nitriding step, NH ₃ gas and AX are used These two gases are controlled so that the nitriding potential in the third nitriding treatment step becomes the above-mentioned third nitriding potential by setting the total flow rate of the two gases to be fixed and changing the introduction amounts of the respective gases. .

In this control mode, similarly, the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is carried out at a temperature of 500°C to 590°C. The step is also carried out at a temperature of 500°C to 590°C. The first nitriding potential is a value in the range of 0.10 to 1.00. The second nitriding potential is higher than the first nitriding potential and is a value in the range of 0.30 to 10.00. , the third nitriding potential is lower than the second nitriding potential and is a value in the range of 0.26 to 0.60. The effectiveness of the present invention characterized by this is confirmed.

Alternatively, in the present invention, the above-mentioned first nitriding step, the second nitriding step and the above-mentioned third nitriding step are performed sequentially in the same batch-type heat treatment furnace, for example, in the above-mentioned first nitriding step In this method, two gases, NH ₃ gas and AX gas, are used, and the total flow rate of the gases is fixed while changing the respective introduction amounts thereof, thereby controlling the nitriding in the first nitriding treatment step. The potential becomes the above-mentioned first nitriding potential. In the above-mentioned second nitriding treatment step, two gases, NH ₃ gas and AX gas, are also used. The total flow rate of them is fixed and the respective introduction amounts of them are changed. , thereby controlling the nitriding potential in the second nitriding step to become the above-mentioned second nitriding potential. In the above-mentioned third nitriding step, two gases, NH ₃ gas and AX gas, are also used, The total flow rate is fixed and the respective introduction amounts are changed, thereby controlling the nitriding potential in the third nitriding treatment step to become the third nitriding potential.

In this control mode, similarly, the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is carried out at a temperature of 500°C to 590°C. The step is also carried out at a temperature of 500°C to 590°C. The first nitriding potential is a value in the range of 0.10 to 1.00. The second nitriding potential is higher than the first nitriding potential and is a value in the range of 0.30 to 10.00. , the third nitriding potential is lower than the second nitriding potential and is a value in the range of 0.26 to 0.60. The effectiveness of the present invention characterized by this is confirmed.

Alternatively, in the present invention, the above-mentioned first nitriding step, the second nitriding step and the above-mentioned third nitriding step are performed sequentially in the same single-chamber heat treatment furnace, for example, in the above-mentioned first nitriding step In this method, two gases, NH ₃ gas and AX gas, are used, and the total flow rate of the gases is fixed while changing the respective introduction amounts thereof, thereby controlling the nitriding in the first nitriding treatment step. The potential becomes the above-mentioned first nitriding potential. In the above-mentioned second nitriding treatment step, two gases, NH ₃ gas and AX gas, are also used. The total flow rate of them is fixed and the respective introduction amounts of them are changed. , thereby controlling the nitriding potential in the second nitriding step to become the above-mentioned second nitriding potential. In the above-mentioned third nitriding step, two gases, NH ₃ gas and AX gas, are also used, The total flow rate is fixed and the respective introduction amounts are changed, thereby controlling the nitriding potential in the third nitriding treatment step to become the third nitriding potential.

In this control mode, similarly, the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is carried out at a temperature of 500°C to 590°C. The step is also carried out at a temperature of 500°C to 590°C. The first nitriding potential is a value in the range of 0.10 to 1.00. The second nitriding potential is higher than the first nitriding potential and is a value in the range of 0.30 to 10.00. , the third nitriding potential is lower than the second nitriding potential and is a value in the range of 0.26 to 0.60. The effectiveness of the present invention characterized by this is confirmed.

Alternatively, in the present invention, the above-mentioned first nitriding step, the second nitriding step and the above-mentioned third nitriding step are performed sequentially in the same single-chamber heat treatment furnace, for example, in the above-mentioned first nitriding step In this method, two gases, NH ₃ gas and AX gas, are used, and the amount of introduction of one of them is fixed while changing the amount of introduction of the other, thereby controlling so that the amount of introduction in the first nitriding treatment step is The nitriding potential becomes the above-mentioned first nitriding potential. In the above-mentioned second nitriding treatment step, two gases, NH ₃ gas and AX gas, are also used. The amount of introduction of one of them is fixed while changing the amount of the other. The introduction amount of the gas is controlled so that the nitriding potential in the second nitriding step becomes the above-mentioned second nitriding potential. In the above-mentioned third nitriding step, NH ₃ gas and AX gas are also used. The two gases are controlled so that the nitriding potential in the third nitriding treatment step becomes the above-mentioned third nitriding potential by setting the introduction amount of one of them to be fixed and changing the introduction amount of the other. .

In this control mode, similarly, the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is carried out at a temperature of 500°C to 590°C. The step is also carried out at a temperature of 500°C to 590°C. The first nitriding potential is a value in the range of 0.10 to 1.00. The second nitriding potential is higher than the first nitriding potential and is a value in the range of 0.30 to 10.00. , the third nitriding potential is lower than the second nitriding potential and is a value in the range of 0.26 to 0.60. The effectiveness of the present invention characterized by this is confirmed.

Alternatively, in the present invention, the above-mentioned first nitriding step, the second nitriding step and the above-mentioned third nitriding step are performed sequentially in the same single-chamber heat treatment furnace, for example, in the above-mentioned first nitriding step In this method, two gases, NH ₃ gas and AX gas, are used, and the introduction amount of one of the NH ₃ gas and AX gas is fixed while changing the introduction amount of the other, thereby controlling so that the first nitriding The nitriding potential in the treatment step becomes the first nitriding potential. In the second nitriding treatment step, three gases, namely NH ₃ gas, AX gas and N ₂ gas, are used. One of the NH ₃ gas and the AX gas is The amount of introduction of one is fixed and the amount of introduction of the other is changed while controlling it so that the nitriding potential in the second nitriding step becomes the above-mentioned second nitriding potential, and in the above-mentioned third nitriding step In this method, two gases, NH ₃ gas and AX gas, are used, and the amount of introduction of one of NH ₃ gas and AX gas is fixed while changing the amount of introduction of the other, thereby controlling so that the third nitriding The nitriding potential in the processing step becomes the above-mentioned third nitriding potential.

In this control mode, similarly, the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is carried out at a temperature of 500°C to 590°C. The step is also carried out at a temperature of 500°C to 590°C. The first nitriding potential is a value in the range of 0.10 to 1.00. The second nitriding potential is higher than the first nitriding potential and is a value in the range of 0.30 to 10.00. , the third nitriding potential is lower than the second nitriding potential and is a value in the range of 0.26 to 0.60. The effectiveness of the present invention characterized by this is confirmed.

Furthermore, the so-called single-chamber heat treatment furnace does not have a cooling chamber that is different from the heating chamber like the batch-type heat treatment furnace (see Figure 1). It only performs heating and cooling in one chamber. It is generally a pit furnace ( (Refer to Figure 3) or horizontal furnace (Refer to Figure 5).

Furthermore, in each of the above inventions, the time of the third nitriding step is preferably 60 minutes or more. According to the inventor's opinion, by setting the time of the third nitriding step to more than 60 minutes, the conversion of the gas atmosphere in the furnace accompanying the change of the nitriding potential will be fully achieved without impairing the improvement of the γ' rate. Effect. [Effects of the invention]

According to the present invention, in the third nitriding step performed at a temperature of 500°C to 590°C, by setting the third nitriding potential to a value in the range of 0.26 to 0.60, the hardness can be suppressed from being lower than γ' The precipitation of the α phase allows the γ' phase to be well precipitated in the nitride compound layer, which can achieve higher pitting corrosion resistance and bending fatigue strength.

[Example of object to be processed (workpiece)] The object to be processed (workpiece) is a steel member. Specifically, they are steel members including carbon steel for mechanical structure or alloy steel for mechanical structure, such as gears used in automatic transmissions. For example, a cylindrical ring gear or a bottomed cylindrical ring gear is nitrided in a state where it is mounted on a plurality of jig and laid flat in a box (described later).

For steel members, it is preferable to perform pre-washing to remove dirt or oil before nitriding. For example, pre-washing is preferably vacuum cleaning in which oil and the like are dissolved, replaced, and evaporated using a hydrocarbon-based cleaning solution to perform degreasing and drying, and alkali washing in which degreasing treatment is performed using an alkali-based cleaning solution.

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

As shown in FIG. 1 , the batch-type heat treatment furnace 1 includes a carry-in part 10 , a heating chamber 11 , a transfer chamber 12 , and a carry-out conveyor 13 . A box 20 is placed on the loading part 10, and a steel member serving as an object to be processed (workpiece) is accommodated in the box 20. Handling weight up to 700 kg total.

An entrance cover 22 having an openable and closable door 21 is installed on the entrance side of the heating chamber 11 (left side in FIG. 1 ). The heating chamber 11 has a retort structure, and the furnace temperature is controlled to a specific temperature by heating the outer peripheral portion of the retort with a heater (not shown). Furthermore, a plurality of gases used for the nitriding process are controlled in a manner described below and introduced into the heating chamber 11 .

In addition, a fan 26 is installed on the ceiling of the heating chamber 11 to stir the gas introduced into the heating chamber 11 to uniformize the heating temperature of the steel member. Furthermore, a freely openable and closable intermediate door 27 is installed on the exit side of the heating chamber 11 (right side in FIG. 1 ).

The transfer chamber 12 is provided with an elevator 30 for raising and lowering the box 20 housing the steel members. A cooling chamber (oil tank) 32 for storing cooling oil 31 is provided in the lower part of the transfer chamber 12 . Furthermore, an exit cover 36 having an openable and closable door 35 is installed on the exit side of the transfer chamber 12 (right side in FIG. 1 ).

Furthermore, the heating chamber 11 and the transfer chamber 12 may be configured as a treatment chamber in the same space, and the steel member after heat treatment may be air-cooled with gas. Alternatively, the heating chamber 11 may be divided into two or three heating chambers, and the three-stage nitriding process described below may be performed in the two or three heating chambers.

[Operation example of batch type heat treatment furnace] In the heat treatment furnace 1 configured as described above, the box 20 accommodating the steel member is carried into the heating chamber 11 from the carrying part 10 using a propeller or the like. Then, after the steel member (the box 20 containing it) is carried into the heating chamber 11, the processing gas is introduced into the heating chamber 11, and the processing gas is heated to a specific temperature with a heater, while the fan 26 ( For example, rotating at 1500 rpm) while stirring, the steel member carried into the heating chamber 11 is nitrided.

FIG. 2 is a step diagram of one embodiment of the nitriding treatment method of the present invention using the heat treatment furnace 1 of FIG. 1 .

In the example of FIG. 2 , before installing the steel member (workpiece), the heating chamber 11 is preheated to 550°C. Moreover, during this heating step, N ₂ gas was introduced at a fixed flow rate of 70 (L/min), and NH ₃ gas was introduced at a fixed flow rate of 90 (L/min). The total flow rate is 70+90=160(L/min).

Next, the steel member (workpiece) is loaded into the heating chamber 11 . At this time, the door 21 is opened, thereby temporarily lowering the temperature in the heating chamber 11 as shown in FIG. 2 . Thereafter, the door 21 is closed, and the temperature in the heating chamber 11 is heated to 550°C again.

In the installation of such steel members, in the example of Figure 2, NH ₃ gas is introduced at a fixed flow rate of 160 (L/min). The total flow rate is also 160 (L/min).

Thereafter, a three-stage nitriding treatment step was performed (Examples 1-7 to be described later). Specifically, first, for example, a value of 0.30 (0.10 to 1.00) is used as the first nitriding potential, and the first nitriding step is performed at a temperature of 550°C.

It is known that the nitriding potential K _N is expressed by the following formula using the partial pressure P(NH ₃ ) of the NH ₃ gas and the partial pressure P(H ₂ ) of the H ₂ gas. K _N =P(NH ₃ )/P(H ₂ ) ^3/2

In the first nitriding step, the introduction amount of the processing gas is feedback controlled in the following manner: the partial pressure P (NH ₃ ) of the NH ₃ gas or the partial pressure P (H ₂ ) of the H ₂ gas in the heating chamber 11 is measured. , so that the value of the nitriding potential calculated based on the measured value is within the vicinity of the target first nitriding potential.

In the example of FIG. 2 , a thermal conductivity type H ₂ sensor (not shown) is used to measure the partial pressure P (H ₂ ) of the H ₂ gas in the heating chamber 11, and the measured value is analyzed on a line (according to the measured value Calculate the nitriding potential) and perform feedback control on the introduction amount of the processing gas. Specifically, the NH ₃ gas and the AX gas were increased or decreased respectively under the condition that the total flow rate was 160 (L/min).

In the example of Figure 2, this first nitriding step is performed for 60 minutes. Thereby, no compound layer or a compound layer that is the main body of the ε phase is formed on the surface of the steel component, thereby reducing the amount of surface carbon that is not conducive to the formation of the γ' phase. On the other hand, nitrogen can be efficiently diffused into the interior.

Next, for example, a value of 2.00 (0.30-10.00) is used as the second nitriding potential, and the second nitriding step is performed at a temperature of 550°C.

In the second nitriding step, the introduction amount of the processing gas is also feedback controlled in the following manner: measuring the partial pressure P (NH ₃ ) of the NH ₃ gas or the partial pressure P (H ₂ ) of the H ₂ gas in the heating chamber 11 ), so that the value of the nitriding potential calculated based on the measured value is within the vicinity of the target second nitriding potential.

In the example of FIG. 2 , a thermal conductivity type H ₂ sensor (not shown) is used to measure the partial pressure P (H ₂ ) of the H ₂ gas in the heating chamber 11, and the measured value is analyzed on a line (according to the measured value Calculate the nitriding potential) and perform feedback control on the introduction amount of the processing gas. Specifically, N ₂ gas was introduced at a fixed flow rate of 70 (L/min), while NH ₃ gas and AX gas were each increased or decreased under the condition of a total flow rate of 90 (L/min). The total flow rate is maintained at 70+90=160 (L/min).

In the example of Figure 2, this second nitriding step is performed for 200 minutes. Thereby, a nitride compound layer in which the γ' phase, the ε phase, or the γ' phase and the ε phase are mixed is generated on the steel member.

Next, for example, a value of 0.30 (0.26-0.60) is used as the third nitriding potential, and the third nitriding step is performed at a temperature of 550°C.

In the third nitriding step, the introduction amount of the processing gas is also feedback controlled in the following manner: measuring the partial pressure P (NH ₃ ) of the NH ₃ gas or the partial pressure P (H ₂ ) of the H ₂ gas in the heating chamber 11 ), so that the value of the nitriding potential calculated based on the measured value is within the vicinity of the target third nitriding potential.

In the example of FIG. 2 , a thermal conductivity type H ₂ sensor (not shown) is used to measure the partial pressure P (H ₂ ) of the H ₂ gas in the heating chamber 11, and the measured value is analyzed on a line (according to the measured value Calculate the nitriding potential) and perform feedback control on the introduction amount of the processing gas. Specifically, the NH ₃ gas and the AX gas were increased or decreased respectively under the condition that the total flow rate was 160 (L/min).

In the example of Figure 2, this third nitriding step is performed for 60 minutes. Thereby, the γ' phase is precipitated in the nitride compound layer.

After the third nitriding step is completed, a cooling step is performed. In the example of Figure 2, the cooling step is carried out for 15 minutes (oil tank with stirrer, kept in oil (100°C) for 15 minutes). After the cooling step is completed, the box 20 containing the steel members is carried out to the unloading conveyor 13 .

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

As shown in FIG. 3 , the pit heat treatment furnace 201 includes a bottomed cylindrical furnace wall 211 and a furnace cover 212 .

A fan 213 is provided on the lower side (inside) of the furnace cover 212. The rotating shaft of the fan 213 passes through the furnace cover 212 and is connected to the fan motor 214 provided on the upper side (outside) of the furnace cover 212.

A distillation tank 221 is provided inside the furnace wall 211, and a gas guide tube 222 is provided further inside the distillation tank 221. The temperature in the furnace (inside the distillation tank 221 ) is controlled to a specific temperature by heating the outer peripheral portion of the distillation tank 221 with a heater (not shown). Furthermore, a box body 20 is placed in the gas guide tube 222, and a steel member serving as a target object (workpiece) to be processed is accommodated in the box body 20. Handling weight up to 700 kg total.

In addition, a plurality of gases used for the nitriding process are introduced into the distillation tank 221 while being controlled in a manner described below. Furthermore, the outer peripheral portion of the retort 221 also has a cooling function using a blower (not shown). During cooling, the temperature of the retort 221 itself is lowered to cool the workpiece in the furnace (in-furnace cooling).

[Operation example of pit type heat treatment furnace] In the heat treatment furnace 201 configured as above, the furnace lid 212 is opened, and the box 20 housing the steel member is moved into the gas guide tube 222 . Then, after the steel member (the box 20 containing it) is carried into the gas guide tube 222, the processing gas is introduced into the distillation tank 221, and the processing gas is heated to a specific temperature with a heater, and the fan 213 is used to heat the processing gas. While stirring (for example, rotating at 1500 rpm), the steel member carried into the gas guide tube 222 is nitrided.

FIG. 4 is a step diagram of one embodiment of the nitriding treatment method of the present invention using the heat treatment furnace 201 of FIG. 3 .

In the example of FIG. 4 , after the steel member (workpiece) is installed into the gas guide tube 222 , the inside of the distillation tank 221 is heated to 550°C. In the first half of the heating step, N ₂ gas was introduced at a fixed flow rate of 40 (L/min), and in the second half of the heating step, NH ₃ gas was introduced at a fixed flow rate of 40 (L/min).

Thereafter, a three-stage nitriding treatment step was performed (Examples 5-7 to be described later). Specifically, first, for example, a value of 0.30 (0.10 to 1.00) is used as the first nitriding potential, and the first nitriding step is performed at a temperature of 550°C.

As mentioned above, it is known that the nitriding potential K _N is expressed by the following formula using the partial pressure P(NH ₃ ) of the NH ₃ gas and the partial pressure P(H ₂ ) of the H ₂ gas. K _N =P(NH ₃ )/P(H ₂ ) ^3/2

In the first nitriding treatment step, feedback control is performed on the introduction amount of the processing gas in the following manner: measuring the partial pressure P (NH ₃ ) of the NH ₃ gas or the partial pressure P (H _{2 )} of the H ₂ gas in the gas guide tube 222 ) (You can also measure the partial pressure P (NH ₃ ) of the NH ₃ gas or the partial pressure P (H ₂ ) of the H ₂ gas in the exhaust gas, so that the value of the nitriding potential calculated based on the measured value is within the target. Within the vicinity of the first nitriding potential.

In the example of FIG. 4 , a thermal conductivity H ₂ sensor (not shown) is used to measure the partial pressure P (H ₂ ) of the H ₂ gas in the gas guide tube 222 , and the measured value is analyzed on a line (according to this measurement The value calculation nitriding potential) performs feedback control on the introduction amount of the processing gas. Specifically, AX gas was introduced at a fixed flow rate of 50 (L/min), and NH ₃ gas was increased or decreased. Total traffic will also change.

In the example of Figure 4, this first nitriding step is performed for 60 minutes. Thereby, no compound layer or a compound layer that is the main body of the ε phase is formed on the surface of the steel component, thereby reducing the amount of surface carbon that is not conducive to the formation of the γ' phase. On the other hand, nitrogen can be efficiently diffused into the interior.

Next, for example, a value of 0.50 (0.30-10.00) is used as the second nitriding potential, and the second nitriding step is performed at a temperature of 550°C.

In the second nitriding step, the introduction amount of the processing gas is also feedback controlled in the following manner: measuring the partial pressure P( _NH3 ) of the NH ₃ gas or the partial pressure P(H) of the H ₂ gas in the gas guide tube 222 ₂ ), so that the value of the nitriding potential calculated based on the measured value is within the vicinity of the target second nitriding potential.

In the example of FIG. 4 , a thermal conductivity H ₂ sensor (not shown) is used to measure the partial pressure P (H ₂ ) of the H ₂ gas in the gas guide tube 222 , and the measured value is analyzed on a line (according to this measurement The value calculation nitriding potential) performs feedback control on the introduction amount of the processing gas. Specifically, AX gas was introduced at a fixed flow rate of 50 (L/min), and NH ₃ gas was increased or decreased. Total traffic will also change.

In the example of Figure 4, this second nitriding step is performed for 200 minutes. Thereby, a nitride compound layer in which the γ' phase, the ε phase, or the γ' phase and the ε phase are mixed is generated on the steel member.

Next, for example, a value of 0.30 (0.26-0.60) is used as the third nitriding potential, and the third nitriding step is performed at a temperature of 550°C.

In the third nitriding step, the introduction amount of the processing gas is also feedback controlled in the following manner: measuring the partial pressure P (NH ₃ ) of the NH ₃ gas or the partial pressure P (H ₂ ) of the H ₂ gas in the heating chamber 11 ), so that the value of the nitriding potential calculated based on the measured value is within the vicinity of the target third nitriding potential.

In the example of FIG. 4 , a thermal conductivity H ₂ sensor (not shown) is used to measure the partial pressure P (H ₂ ) of the H ₂ gas in the gas guide tube 222 , and the measured value is analyzed on a line (according to this measurement The value calculation nitriding potential) performs feedback control on the introduction amount of the processing gas. Specifically, AX gas was introduced at a fixed flow rate of 50 (L/min), and NH ₃ gas was increased or decreased. Total traffic will also change.

In the example of Figure 4, this third nitriding step is performed for 60 minutes. Thereby, the γ' phase is precipitated in the nitride compound layer.

After the third nitriding step is completed, a cooling step is performed. In the example of FIG. 4 , the same process gas introduction amount control as in the third nitriding step is performed in the first half of the cooling step (until approximately 400° C.). That is, AX gas is introduced at a fixed flow rate of 50 (L/min), and NH ₃ gas is increased or decreased. In the second half of the cooling step (around 400°C to 100°C), N ₂ gas was introduced at a fixed flow rate of 20 (L/min). After the cooling step is completed, the furnace cover 212 is opened, and the box 20 containing the steel components is moved out of the gas guide tube 222 .

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

The horizontal heat treatment furnace is basically a pit-type heat treatment furnace placed horizontally, but it can also adopt the following structure: as shown in Figure 5, the fan 213 and the fan motor 214 are arranged on the furnace wall opposite to the furnace cover 212. 211 on the wall, not on the furnace cover 212.

The other structures of the horizontal heat treatment furnace are substantially the same as those of the pit heat treatment furnace explained using Figure 3 .

[Operation example of horizontal heat treatment furnace] In the horizontal heat treatment furnace, the furnace cover 212 is also opened, and the box 20 containing the steel components is moved into the gas guide tube 222. Then, after the steel member (the box 20 containing it) is carried into the gas guide tube 222 , the processing gas is introduced into the retort 211 , and the processing gas is heated to a specific temperature with a heater, while the fan 213 is used to heat the processing gas. While stirring (for example, rotating at 1500 rpm), the steel member carried into the gas guide tube 222 is nitrided.

The step diagram in Figure 4 is also valid when using a horizontal heat treatment furnace. Specifically, a heating step (the form of gas introduction is different in the first half and the second half), a first nitriding step, a second nitriding step, a third nitriding step, and a cooling step (in the first half) can be performed. The shape of gas introduction is different between the first stage and the second half stage). After the cooling step is completed, the furnace cover 212 is opened, and the box 20 containing the steel components is moved out of the gas guide tube 222 .

[Summary of effects] According to the embodiment of the present invention as described above, it is possible to obtain a nitrided steel member having an iron nitride compound layer containing the γ' phase as the main component on the surface using either a batch-type heat treatment furnace or a single-chamber heat treatment furnace.

The steel member obtained by each embodiment is strengthened by the formation of a nitrogen diffusion layer and nitride inside, and a γ' phase-rich iron nitride compound layer is formed on the surface, so that sufficient pitting corrosion resistance can be achieved and bending fatigue strength.

In addition, compared with carburizing or carbonitriding, the nitriding treatment of the present invention is a treatment below the austenite transformation temperature, so the amount of strain is smaller. In addition, since the quenching step, which is a necessary step in carburizing or carbonitriding, can be omitted, the amount of strain unevenness is also small. As a result, a high-strength and low-strain nitrided steel member can be obtained.

[Supplement to the temperature range of the present invention] In the present invention, the temperature of each nitriding step is set to 500°C to 590°C. It is said that the higher the temperature in the nitriding treatment, the better the productivity. However, according to the inspection of 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, so it is better to set 590°C as the upper limit. On the other hand, according to the examination of the present inventor, if the nitriding treatment temperature is lower than 500°C, the formation speed of the nitride compound layer becomes slow, which is unfavorable in terms of cost. Therefore, it is better to set 500°C as the lower limit.

In addition, the temperature difference between the second nitriding step and the third nitriding step is smaller, which can reduce the temperature unevenness of the steel member (workpiece), thereby suppressing the nitriding quality of the steel member (workpiece). all. Specifically, the temperature difference between the two nitriding steps is preferably controlled within 50°C, and further preferably, controlled within 30°C.

[Examples 1-1 to 1-14, Comparative Examples 1-1 to 1-8] For a plurality of cylindrical ring gears (the steel types may be different), a batch-type heat treatment furnace 1 is used, and three-stage nitriding treatment is performed according to the conditions in Table 1 shown in FIG. 6 .

In Examples 1-1 to 1-14 and Comparative Examples 1-1 to 1-8, the first to third nitriding steps are performed sequentially in the same batch-type heat treatment furnace 1 .

Furthermore, in the first nitriding treatment steps of Examples 1-1 to 1-14 and Comparative Examples 1-1 to 1-8, two gases, NH ₃ gas and AX gas, were used, and the total flow rate was set to By fixing and changing their respective introduction amounts, control is performed so that the nitriding potential in the first nitriding treatment step becomes the target first nitriding potential (K _N ).

In addition, in the second nitriding treatment step of Examples 1-1 to 1-14 and Comparative Examples 1-1 to 1-8, three gases, namely NH ₃ gas, AX gas and N ₂ gas, were used. The total flow rate is fixed and the introduction amounts of NH ₃ gas and AX gas are changed while controlling so that the nitriding potential in the first nitriding treatment step becomes the target first nitriding potential (K _N ).

Moreover, in the third nitriding treatment step of Examples 1-1 to 1-14 and Comparative Examples 1-1 to 1-8, two gases, NH ₃ gas and AX gas, were used, and the total flow rate was set to By fixing and changing their respective introduction amounts, control is performed so that the nitriding potential in the second nitriding treatment step becomes the target second nitriding potential (K _N ).

In Examples 1-1 to 1-14 and Comparative Examples 1-1 to 1-8, each step described using FIG. 2 was performed before and after the first nitriding step to the third nitriding step.

In Table 1, the phase identification method is based on the X-ray diffraction pattern measured from the steel surface using the 2θ-θ scanning method (MiniFlex600 manufactured by Rigaku, Cu tube, 40 kV-15 mA).

Moreover, in Table 1, the thickness of the compound layer is measured by cutting the nitrided steel material in the depth direction and measuring the thickness of the surface compound layer based on the observation results of the structure of the cross section. The thickness of the γ' phase-rich compound layer is preferably 2 to 20 μm. If it is less than 2 μm, it is too thin and the improvement in fatigue strength is insufficient. On the other hand, if it exceeds 20 μm, the porous layer of the compound layer that is the starting point of fatigue cracks becomes thicker and the fatigue strength decreases.

From the results shown in Table 1, it can be seen that the effectiveness of the present invention has been confirmed through Examples 1-1 to 1-14. The feature of the present invention is that in the control form of using the above three gases in a batch furnace , the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is also carried out at a temperature of 500°C to 590°C. When implemented at a certain temperature, the first nitriding potential is in the range of 0.10 to 1.00, the second nitriding potential is higher than the first nitriding potential and is in the range of 0.30 to 10.00, and the third nitriding potential is lower than the first nitriding potential. 2Nitridation potential is a value in the range of 0.26 to 0.60.

On the other hand, it was confirmed from Comparative Examples 1-1 to 1-8 that if the nitriding potential in the third nitriding step is set to 0.25 or less in the temperature range of 500°C to 590°C, precipitation will also occur. The hardness of the α phase is lower than that of the γ' phase, resulting in insufficient pitting corrosion resistance or bending fatigue strength.

[Examples 2-1 to 2-14, Comparative Examples 2-1 to 2-8] For a plurality of cylindrical ring gears (the steel types may be different), a pit type heat treatment furnace 201 is used, and three stages of nitriding treatment are performed according to the conditions in Table 2 shown in FIG. 7 .

In Examples 2-1 to 2-14 and Comparative Examples 2-1 to 2-8, the first to third nitriding steps are performed sequentially in the same pit heat treatment furnace 201 .

In addition, in the first nitriding treatment steps of Examples 2-1 to 2-14 and Comparative Examples 2-1 to 2-8, two gases, NH ₃ gas and AX gas, were used, and the total flow rate was set to By fixing and changing their respective introduction amounts, control is performed so that the nitriding potential in the first nitriding treatment step becomes the target first nitriding potential (K _N ).

In addition, in the second nitriding treatment step of Examples 2-1 to 2-14 and Comparative Examples 2-1 to 2-8, three gases, namely NH ₃ gas, AX gas and N ₂ gas, were used. The total flow rate is fixed and the introduction amounts of NH ₃ gas and AX gas are changed while controlling so that the nitriding potential in the first nitriding treatment step becomes the target first nitriding potential (K _N ).

Furthermore, in the third nitriding treatment step of Examples 2-1 to 2-14 and Comparative Examples 2-1 to 2-8, two gases, NH ₃ gas and AX gas, were used, and the total flow rate was set to By fixing and changing their respective introduction amounts, control is performed so that the nitriding potential in the second nitriding treatment step becomes the target second nitriding potential (K _N ).

In Examples 2-1 to 2-14 and Comparative Examples 2-1 to 2-8, each step described using FIG. 4 was performed before and after the first nitriding step to the third nitriding step.

The phase identification methods and compound layer thicknesses in Table 2 were determined in the same manner as those in Table 1.

From the results shown in Table 2, it can be seen that the effectiveness of the present invention has been confirmed through Examples 2-1 to 2-14. The feature of the present invention is that in the control form of using the above three gases in a pit furnace , the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is also carried out at a temperature of 500°C to 590°C. When implemented at a certain temperature, the first nitriding potential is in the range of 0.10 to 1.00, the second nitriding potential is higher than the first nitriding potential and is in the range of 0.30 to 10.00, and the third nitriding potential is lower than the first nitriding potential. 2Nitridation potential is a value in the range of 0.26 to 0.60.

On the other hand, it was confirmed from Comparative Examples 2-1 to 2-8 that if the nitriding potential in the second nitriding step is set to 0.25 or less in the temperature range of 500°C to 590°C, precipitation will also occur. The hardness of the α phase is lower than that of the γ' phase, resulting in insufficient pitting corrosion resistance or bending fatigue strength.

[Examples 3-1 to 3-14, Comparative Examples 3-1 to 3-8] For a plurality of cylindrical ring gears (the steel types may be different), a batch-type heat treatment furnace 1 is used, and three-stage nitriding treatment is performed according to the conditions in Table 3 shown in FIG. 8 .

In Examples 3-1 to 3-14 and Comparative Examples 3-1 to 3-8, the first nitriding step and the second nitriding step were performed sequentially in the same batch-type heat treatment furnace 1 .

Furthermore, in the first nitriding treatment steps of Examples 3-1 to 3-14 and Comparative Examples 3-1 to 3-8, two gases, NH ₃ gas and AX gas, were used, and the total flow rate was set to By fixing and changing their respective introduction amounts, control is performed so that the nitriding potential in the first nitriding treatment step becomes the target first nitriding potential (K _N ).

In addition, in the second nitriding treatment step of Examples 3-1 to 3-14 and Comparative Examples 3-1 to 3-8, two gases, NH ₃ gas and AX gas, were also used, and their total flow rates were equal to By setting the respective introduction amounts to be fixed and varying, the nitriding potential in the first nitriding treatment step is controlled so that the nitriding potential becomes the target first nitriding potential (K _N ).

In addition, in the third nitriding treatment step of Examples 3-1 to 3-14 and Comparative Examples 3-1 to 3-8, two gases, NH ₃ gas and AX gas, were also used, and their total flow rates were equal to By setting them as fixed and changing their respective introduction amounts, the nitriding potential in the second nitriding treatment step is controlled so that the nitriding potential becomes the target second nitriding potential (K _N ).

In Examples 3-1 to 3-14 and Comparative Examples 3-1 to 3-8, each step described using FIG. 2 was performed before and after the first nitriding step to the third nitriding step.

The phase identification methods and compound layer thicknesses in Table 3 were determined in the same manner as those in Tables 1 and 2.

From the results shown in Table 3, it can be seen that the effectiveness of the present invention has been confirmed through Examples 3-1 to 3-14. The feature of the present invention is that in the control form of using the above two gases in a batch furnace , the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is also carried out at a temperature of 500°C to 590°C. When implemented at a certain temperature, the first nitriding potential is in the range of 0.10 to 1.00, the second nitriding potential is higher than the first nitriding potential and is in the range of 0.30 to 10.00, and the third nitriding potential is lower than the first nitriding potential. 2Nitridation potential is a value in the range of 0.26 to 0.60.

On the other hand, it was confirmed from Comparative Examples 3-1 to 3-8 that if the nitriding potential in the second nitriding step is set to 0.25 or less in the temperature range of 500°C to 590°C, precipitation will also occur. The hardness of the α phase is lower than that of the γ' phase, resulting in insufficient pitting corrosion resistance or bending fatigue strength.

[Examples 4-1 to 4-14, Comparative Examples 4-1 to 4-8] For a plurality of cylindrical ring gears (the steel types may be different), a pit-type heat treatment furnace 201 is used, and three stages of nitriding treatment are performed according to the conditions in Table 4 shown in FIG. 9 .

In Examples 4-1 to 4-14 and Comparative Examples 4-1 to 4-8, the first to third nitriding steps are performed sequentially in the same pit heat treatment furnace 201.

Furthermore, in the first nitriding treatment steps of Examples 4-1 to 4-14 and Comparative Examples 4-1 to 4-8, two gases, NH ₃ gas and AX gas, were used, and the total flow rate was set to By fixing and changing their respective introduction amounts, control is performed so that the nitriding potential in the first nitriding treatment step becomes the target first nitriding potential (K _N ).

In addition, in the second nitriding treatment step of Examples 4-1 to 4-14 and Comparative Examples 4-1 to 4-8, two gases, NH ₃ gas and AX gas, were also used, and their total flow rates were equal to By setting the respective introduction amounts to be fixed and varying, the nitriding potential in the first nitriding treatment step is controlled so that the nitriding potential becomes the target first nitriding potential (K _N ).

In addition, in the third nitriding treatment step of Examples 4-1 to 4-14 and Comparative Examples 4-1 to 4-8, two gases, NH ₃ gas and AX gas, were also used, and their total flow rates were equal to By setting them as fixed and changing their respective introduction amounts, the nitriding potential in the second nitriding treatment step is controlled so that the nitriding potential becomes the target second nitriding potential (K _N ).

In Examples 4-1 to 4-14 and Comparative Examples 4-1 to 4-8, each step described using FIG. 4 was performed before and after the first nitriding step to the third nitriding step.

The phase identification methods and compound layer thicknesses in Table 4 were determined in the same manner as those in Tables 1 to 3.

From the results shown in Table 4, it can be seen that the effectiveness of the present invention has been confirmed through Examples 4-1 to 4-14. The feature of the present invention is that in the control form of using the above two gases in a pit furnace , the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is also carried out at a temperature of 500°C to 590°C. When implemented at a certain temperature, the first nitriding potential is in the range of 0.10 to 1.00, the second nitriding potential is higher than the first nitriding potential and is in the range of 0.30 to 10.00, and the third nitriding potential is lower than the first nitriding potential. 2Nitridation potential is a value in the range of 0.26 to 0.60.

On the other hand, it was confirmed from Comparative Examples 4-1 to 4-8 that if the nitriding potential in the second nitriding step is set to 0.25 or less in the temperature range of 500°C to 590°C, precipitation will also occur. The hardness of the α phase is lower than that of the γ' phase, resulting in insufficient pitting corrosion resistance or bending fatigue strength.

[Examples 5-1 to 5-14, Comparative Examples 5-1 to 5-8] For a plurality of cylindrical ring gears (the steel types may be different), a pit type heat treatment furnace 201 is used, and three stages of nitriding treatment are performed according to the conditions in Table 5 shown in FIG. 10 .

In Examples 5-1 to 5-14 and Comparative Examples 5-1 to 5-8, the first to third nitriding steps are performed sequentially in the same pit heat treatment furnace 201.

Furthermore, in the first nitriding treatment step of Examples 5-1 to 5-14 and Comparative Examples 5-1 to 5-8, two gases, NH ₃ gas and AX gas, were used, and one of them was introduced. While the amount is fixed and the other introduced amount is changed, control is performed so that the nitriding potential in the first nitriding treatment step becomes the target first nitriding potential (K _N ).

In addition, in the second nitriding treatment step of Examples 5-1 to 5-14 and Comparative Examples 5-1 to 5-8, two gases, NH ₃ gas and AX gas, were also used, and one of them was used. The introduction amount is fixed and the other introduction amount is changed, thereby controlling so that the nitriding potential in the first nitriding treatment step becomes the target first nitriding potential (K _N ).

In addition, in the third nitriding treatment step of Examples 5-1 to 5-14 and Comparative Examples 5-1 to 5-8, two gases, NH ₃ gas and AX gas, were also used, and one of them was used. The introduction amount is fixed and the other introduction amount is changed, thereby controlling so that the nitriding potential in the second nitriding treatment step becomes the target second nitriding potential (K _N ).

In Examples 5-1 to 5-14 and Comparative Examples 5-1 to 5-8, each step explained using FIG. 4 was performed before and after the first nitriding step and the second nitriding step.

The phase identification methods and compound layer thicknesses in Table 5 were determined in the same manner as those in Tables 1 to 4.

From the results shown in Table 5, it can be seen that the effectiveness of the present invention has been confirmed through Examples 5-1 to 5-14. The feature of the present invention is that in the control form of using the above two gases in a pit furnace , the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is also carried out at a temperature of 500°C to 590°C. When implemented at a certain temperature, the first nitriding potential is in the range of 0.10 to 1.00, the second nitriding potential is higher than the first nitriding potential and is in the range of 0.30 to 10.00, and the third nitriding potential is lower than the first nitriding potential. 2Nitridation potential is a value in the range of 0.26 to 0.60.

On the other hand, it was confirmed from Comparative Examples 5-1 to 5-8 that if the nitriding potential in the second nitriding step is set to 0.25 or less in the temperature range of 500°C to 590°C, precipitation will also occur. The hardness of the α phase is lower than that of the γ' phase, resulting in insufficient pitting corrosion resistance or bending fatigue strength.

[Examples 6-1 to 6-14, Comparative Examples 6-1 to 6-8] For a plurality of cylindrical ring gears (the steel types may be different), a pit-type heat treatment furnace 201 is used to perform a two-stage nitriding treatment according to the conditions in Table 6 shown in FIG. 11 .

In Examples 6-1 to 6-14 and Comparative Examples 6-1 to 6-8, the first to third nitriding steps are performed sequentially in the same pit heat treatment furnace 201.

Furthermore, in the first nitriding treatment steps of Examples 6-1 to 6-14 and Comparative Examples 6-1 to 6-8, two gases, NH ₃ gas and AX gas, were used, and the NH ₃ gas and AX gas were One of the introduced amounts is fixed and the other introduced amount is changed, thereby controlling so that the nitriding potential in the first nitriding treatment step becomes the target first nitriding potential (K _N ).

Furthermore, in the second nitriding step of Examples 6-1 to 6-14 and Comparative Examples 6-1 to 6-8, three gases, namely NH ₃ gas, AX gas and N ₂ gas, were used to convert NH ₃ The introduction amount of one of the gas and AX gas is fixed while changing the introduction amount of the other, thereby controlling so that the nitriding potential in the second nitriding treatment step becomes the target second nitriding potential (K _N ).

Moreover, in the third nitriding treatment step of Examples 6-1 to 6-14 and Comparative Examples 6-1 to 6-8, two gases, NH ₃ gas and AX gas, were used, and the NH ₃ gas and AX gas were One of the introduced amounts is fixed and the other introduced amount is changed, thereby controlling so that the nitriding potential in the first nitriding treatment step becomes the target first nitriding potential (K _N ).

In Examples 6-1 to 6-14 and Comparative Examples 6-1 to 6-8, each step explained using FIG. 4 was performed before and after the first nitriding step to the third nitriding step.

The phase identification methods and compound layer thicknesses in Table 6 were determined in the same manner as those in Tables 1 to 5.

From the results shown in Table 6, it can be seen that the effectiveness of the present invention was confirmed through Examples 6-1 to 6-14. The feature of the present invention is that in the control form of using the above three gases in a pit furnace , the first nitriding step is carried out at a temperature of 500°C to 590°C, the second nitriding step is also carried out at a temperature of 500°C to 590°C, and the third nitriding step is also carried out at a temperature of 500°C to 590°C. When implemented at a certain temperature, the first nitriding potential is in the range of 0.10 to 1.00, the second nitriding potential is higher than the first nitriding potential and is in the range of 0.30 to 10.00, and the third nitriding potential is lower than the first nitriding potential. 2Nitridation potential is a value in the range of 0.26 to 0.60.

On the other hand, it was confirmed from Comparative Examples 6-1 to 6-8 that if the nitriding potential in the second nitriding step is set to 0.25 or less in the temperature range of 500°C to 590°C, precipitation will also occur. The hardness of the α phase is lower than that of the γ' phase, resulting in insufficient pitting corrosion resistance or bending fatigue strength.

1:Heat treatment furnace 10: Move-in department 11:Heating chamber 12:Transportation room 13: Move out the conveyor 20:Box body 21:door 22: Put on the mask 26:Fan 27:Middle door 30: Lift 31: Cooling room (oil tank) 35:door 36:Take out the mask 201:Heat treatment furnace 211:furnace wall 212:furnace cover 213:Fan 214:Fan motor 221:Distillation tank 222:Gas guide tube

Figure 1 is a schematic diagram of the structure of a batch-type heat treatment furnace used in the nitriding treatment method of the present invention. FIG. 2 is a step diagram of one embodiment of the nitriding treatment method of the present invention using the heat treatment furnace of FIG. 1 . Figure 3 is a schematic diagram of the structure of a pit-type (single-chamber type) heat treatment furnace used in the nitriding treatment method of the present invention. FIG. 4 is a step diagram of one embodiment of the nitriding treatment method of the present invention using the heat treatment furnace of FIG. 3 . Figure 5 is a schematic diagram of the structure of a horizontal (single chamber type) heat treatment furnace used in the nitriding treatment method of 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. FIG. 11 is a table showing nitriding conditions and treatment results of Examples and Comparative Examples of the present invention.

Claims (8)

A nitriding treatment method, characterized in that it is a nitriding treatment method for a steel member having at least three nitriding treatment steps, including: a first nitriding step, which is nitrogen at a first nitriding potential. The steel component is nitrided in a nitriding gas atmosphere; the second nitriding step is performed after the above-mentioned first nitriding step in a nitriding gas with a second nitriding potential higher than the above-mentioned first nitriding potential. Further nitriding the above-mentioned steel member in an atmosphere; and a third nitriding step, which is nitriding at a third nitriding potential lower than the above-mentioned second nitriding potential after the above-mentioned second nitriding step. The above-mentioned steel component is further nitrided in a gas atmosphere; the above-mentioned first nitriding step is carried out at a temperature of 500°C to 590°C, and the above-mentioned second nitriding step is also carried out at a temperature of 500°C to 590°C. The above-mentioned third nitriding step is also carried out at a temperature of 500°C to 590°C. The above-mentioned first nitriding potential is a value in the range of 0.10-1.00, and the above-mentioned second nitriding potential is a value in the range of 0.30-10.00. , the above-mentioned third nitriding potential is a value in the range of 0.26~0.60 (except for the case where the above-mentioned third nitriding potential is 0.30), in the above-mentioned second nitriding step, γ' phase, ε phase, Or a nitride compound layer in which the γ' phase and the ε phase are mixed, and in the above-mentioned third nitridation treatment step, the γ' phase precipitates out of the above-mentioned nitride compound layer. The nitriding treatment method of claim 1, wherein the above-mentioned first nitriding step, the second nitriding step and the above-mentioned third nitriding step are performed sequentially in the same batch-type heat treatment furnace, and in the above-mentioned first nitriding step In the nitriding treatment step, two gases, NH ₃ gas and AX gas, are used, and the total flow rate of the gases is fixed while changing the respective introduction amounts thereof, thereby controlling the first nitriding treatment step. The nitriding potential in the process becomes the first nitriding potential. In the second nitriding step, three gases, namely NH ₃ gas, AX gas and N ₂ gas, are used, and the total flow rate is set to a fixed level. By changing the introduction amounts of NH ₃ gas and AX gas, the nitriding potential in the second nitriding step becomes the second nitriding potential. In the third nitriding step, NH is used. The two gases, ₃ gas and AX gas, are controlled so that the nitriding potential in the third nitriding treatment step becomes the above-mentioned third nitriding treatment step by setting the total flow rate of the gases to be fixed and changing the introduction amounts of the respective gases. 3 Nitriding potential. The nitriding treatment method of claim 1, wherein the first nitriding step, the second nitriding step and the third nitriding step are performed sequentially in the same single-chamber heat treatment furnace, and in the above-mentioned first nitriding step In the nitriding treatment step, two gases, NH ₃ gas and AX gas, are used, and the total flow rate of the gases is fixed while changing the respective introduction amounts thereof, thereby controlling the first nitriding treatment step. The nitriding potential in the process becomes the first nitriding potential. In the second nitriding step, three gases, namely NH ₃ gas, AX gas and N ₂ gas, are used, and the total flow rate is set to a fixed level. By changing the introduction amounts of NH ₃ gas and AX gas, the nitriding potential in the second nitriding step becomes the second nitriding potential. In the third nitriding step, NH is used. The two gases, ₃ gas and AX gas, are controlled so that the nitriding potential in the third nitriding treatment step becomes the above-mentioned third nitriding treatment step by setting the total flow rate of the gases to be fixed and changing the introduction amounts of the respective gases. 3 Nitriding potential. The nitriding treatment method of claim 1, wherein the above-mentioned first nitriding step, the second nitriding step and the above-mentioned third nitriding step are performed sequentially in the same batch-type heat treatment furnace, and in the above-mentioned first nitriding step In the nitriding treatment step, two gases, NH ₃ gas and AX gas, are used, and the total flow rate of the gases is fixed while changing the respective introduction amounts thereof, thereby controlling the first nitriding treatment step. The nitriding potential becomes the first nitriding potential. In the second nitriding step, two gases, NH ₃ gas and AX gas, are also used, and the total flow rate of them is set to be fixed while changing them. The amount of each introduced is controlled so that the nitriding potential in the second nitriding step becomes the above-mentioned second nitriding potential. In the above-mentioned third nitriding step, NH ₃ gas and AX gas are also used. The two gases are controlled so that the nitriding potential in the third nitriding treatment step becomes the third nitriding potential by setting the total flow rate of the two gases to be fixed and changing the introduction amounts of the respective gases. The nitriding treatment method of claim 1, wherein the first nitriding step, the second nitriding step and the third nitriding step are performed sequentially in the same single-chamber heat treatment furnace, and in the above-mentioned first nitriding step In the nitriding treatment step, two gases, NH ₃ gas and AX gas, are used, and the total flow rate of the gases is fixed while changing the respective introduction amounts thereof, thereby controlling the first nitriding treatment step. The nitriding potential becomes the first nitriding potential. In the second nitriding step, two gases, NH ₃ gas and AX gas, are also used, and the total flow rate of them is set to be fixed while changing them. The amount of each introduced is controlled so that the nitriding potential in the second nitriding step becomes the above-mentioned second nitriding potential. In the above-mentioned third nitriding step, NH ₃ gas and AX gas are also used. The two gases are controlled so that the nitriding potential in the third nitriding treatment step becomes the third nitriding potential by setting the total flow rate of the two gases to be fixed and changing the introduction amounts of the respective gases. The nitriding treatment method of claim 1, wherein the first nitriding step, the second nitriding step and the third nitriding step are performed sequentially in the same single-chamber heat treatment furnace, and in the above-mentioned first nitriding step In the nitriding treatment step, two gases, NH ₃ gas and AX gas, are used, and the introduction amount of one of them is fixed while changing the introduction amount of the other, thereby controlling so that the first nitridation The nitriding potential in the treatment step becomes the first nitriding potential. In the second nitriding treatment step, two gases, NH ₃ gas and AX gas, are also used, and the introduction amount of one of them is fixed. While changing the introduction amount of the other, the nitriding potential in the second nitriding step is controlled to become the above-mentioned second nitriding potential. In the above-mentioned third nitriding step, NH ₃ gas is also used. and AX gas, by setting the introduction amount of one of them to be fixed while changing the introduction amount of the other, thereby controlling the nitriding potential in the third nitriding step to become the above-mentioned third nitriding step. 3 Nitriding potential. The nitriding treatment method of claim 1, wherein the first nitriding step, the second nitriding step and the third nitriding step are performed sequentially in the same single-chamber heat treatment furnace, and in the above-mentioned first nitriding step In the nitriding treatment step, two gases, NH ₃ gas and AX gas, are used. The introduction amount of one of the NH ₃ gas and the AX gas is fixed while changing the introduction amount of the other, thereby controlling so that the amount of the other gas is introduced. The nitriding potential in the first nitriding step becomes the above-mentioned first nitriding potential. In the above-mentioned second nitriding step, three gases of NH ₃ gas, AX gas and N ₂ gas are used, and NH ₃ gas and The introduction amount of one of the AX gases is fixed while changing the introduction amount of the other gas, thereby controlling the nitriding potential in the second nitriding step to become the second nitriding potential, and in the third step In the nitriding treatment step, two gases, NH ₃ gas and AX gas, are used. The introduction amount of one of the NH ₃ gas and the AX gas is fixed while changing the introduction amount of the other, thereby controlling so that the amount of the other gas is introduced. The nitriding potential in the third nitriding treatment step becomes the above-mentioned third nitriding potential. The nitriding treatment method of any one of claims 1 to 7, wherein the time of the above-mentioned third nitriding step is more than 60 minutes.