Nitriding treatment method of steel material

Abstract

To provide a technique capable of effectively performing nitriding treatment.SOLUTION: A nitriding treatment method of a steel material includes a nitriding process of alternatively repeating a first step of lowering a surface temperature of the steel material to a first temperature suitable for a nitrogen to immerse from a surface of the steel material to the inside of the steel material to bring a nitriding gas into contact with the surface of the steel material and a second step of raising the surface temperature to a second temperature that is higher than the first temperature as a temperature suitable for the nitrogen immersed from the surface of the steel material to diffuse to the inside of the steel material to bring the nitriding gas into contact with the surface of the steel material.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a method for nitriding steel materials. [Background technology]

[0002] Nitriding treatment is known, in which nitrogen is dissolved in the surface of a steel material. Patent Document 1 describes a technique that can shorten the nitriding treatment time by maintaining the atmospheric pressure at 72,000 Pa or more at a temperature in the range of 900 to 970°C in a furnace to which ammonia has been introduced. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6344495 Summary of the Invention [Problem to be solved by the invention]

[0004] However, ammonia gas, the nitrogen source, decomposes at high temperatures, increasing the partial pressure of hydrogen, and therefore a large amount of ammonia gas must be introduced. Therefore, there was a need for a technology that could perform nitriding treatment efficiently. [Means for solving the problem]

[0005] The present disclosure has been made to solve the above-mentioned problems, and can be realized in the following forms.

[0006] (1) According to an embodiment of the present disclosure, there is provided a nitriding method for steel material, which includes a nitriding step that alternately repeats a first step of lowering the surface temperature of the steel material to a first temperature suitable for nitrogen to penetrate from the surface of the steel material into the steel material, and bringing a nitriding gas into contact with the surface of the steel material, and a second step of raising the surface temperature to a second temperature higher than the first temperature suitable for the nitrogen that has penetrated from the surface of the steel material to diffuse into the steel material, and bringing the nitriding gas into contact with the surface of the steel material. According to this form of nitriding method, the surface temperature of the steel material can be raised to the second temperature without raising the temperature inside the entire furnace, so that, for example, decomposition of ammonia as a nitrogen source is small and a large amount of ammonia gas as a nitriding gas is not required. Also, the surface temperature can be lowered to the first temperature in a short time. Therefore, the nitriding treatment can be carried out efficiently. (2) The nitriding method of the above aspect may include, after the nitriding step, a cooling step of cooling the steel material to reduce the surface temperature to a third temperature or lower, which is a temperature at which austenite formed on the surface of the steel material transforms into a structure containing martensite or ferrite and is lower than the first temperature, and a quenching step of raising the surface temperature to a fourth temperature, which is a temperature at which a structure containing martensite or ferrite transforms into austenite, and then reducing the surface temperature to a fourth temperature or lower, after the cooling step. According to this form of nitriding method, the steel material cooled in the cooling step is reheated in the quenching step, so that the crystal grains of the steel material can be prevented from becoming coarse, thereby improving the strength of the steel material. (3) In the nitriding method of the above embodiment, the quenching step may include a carburizing treatment for dissolving carbon in the steel material. According to this form of nitriding method, carbon is made inherent in the steel material, and therefore the strength of the steel material can be improved. (4) In the nitriding method of the above aspect, the surface temperature may be the temperature of a portion of the surface of the steel material where nitrogen is to be dissolved. According to this form of nitriding method, it is only necessary to heat the surface of the portion of the steel material where nitrogen is to be dissolved, and therefore the heating energy required for the nitriding treatment can be reduced. (5) In the nitriding treatment method of the above embodiment, the second nitriding step may be carried out after the first step, which is carried out last. According to this form of nitriding method, nitrogen that penetrates into the steel material from the surface in the first step diffuses into the interior in the second step, thereby improving the strength of the steel material.

[0007] The present disclosure can be realized in various forms, for example, as a nitriding apparatus, a control method for a nitriding apparatus, a nitriding-carbonization treatment method, and the like. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is an explanatory diagram showing the configuration of a nitriding apparatus. [Figure 2] 1 is a flowchart showing an example of a nitriding treatment. [Figure 3] FIG. 2 is a diagram showing an example of the surface temperature during nitriding treatment. [Figure 4] 10 is a flowchart showing an example of nitriding treatment in the third embodiment. [Figure 5] FIG. 10 is a diagram showing an example of the surface temperature during the nitriding treatment in the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] A. First embodiment: FIG. 1 is an explanatory diagram showing the configuration of a nitriding apparatus 100 according to this embodiment. The nitriding apparatus 100 includes a vacuum nitriding furnace 110, a first gas supply device 120, a first vacuum pump 130, a first heating device 140, a first control device 200, a general-purpose furnace 310, a second gas supply device 320, a second vacuum pump 330, a second heating device 340, and a second control device 400. The nitriding apparatus 100 is used in a nitriding process in which a workpiece, such as steel, is placed in the vacuum nitriding furnace 110 under reduced pressure and heating, and a nitriding gas is brought into contact with the surface of the workpiece, thereby allowing nitrogen from the nitriding gas to penetrate and diffuse through the surface of the workpiece. The nitriding apparatus 100 is also used in a quenching process in a nitriding process in which the workpiece is placed in the general-purpose furnace 310 and heated.

[0010] The first gas supply device 120 is a device that supplies nitriding gas and cooling gas into the vacuum nitriding furnace 110. The nitriding gas is, for example, ammonia gas, and the cooling gas is, for example, nitrogen gas.

[0011] The first vacuum pump 130 sucks in the air inside the vacuum nitriding furnace 110 and the gas supplied by the first gas supply device 120 during the nitriding process and discharges them to the outside, thereby reducing the pressure inside the vacuum nitriding furnace 110. The first vacuum pump 130 can be, for example, a mechanical booster pump.

[0012] The first heating device 140 is a device that heats the steel material to be processed that has been placed into the vacuum nitriding furnace 110. It is preferable that the first heating device 140 be able to uniformly heat the portion of the steel material surface where nitrogen is to be dissolved. In this embodiment, the first heating device 140 is an infrared lamp and is able to heat the steel material to 1000 degrees or more. The first heating device 140 may be provided inside the vacuum nitriding furnace 110 or outside the vacuum nitriding furnace 110. When the first heating device 140 is provided outside the vacuum nitriding furnace 110, at least a portion of the vacuum nitriding furnace 110 is made of quartz glass. The first heating device 140 heats the steel material through the quartz glass.

[0013] The first control device 200 includes a first memory 210 and a first CPU 220. The first CPU 220 controls the operations of the first gas supply device 120, the first vacuum pump 130, and the first heating device 140 by executing a program pre-installed in the first memory 210. However, some or all of the functions of these components may be realized by a hardware circuit.

[0014] The second gas supply device 320 is a device that supplies a cooling gas into the general-purpose furnace 310 .

[0015] The second vacuum pump 330 sucks in the air inside the general-purpose furnace 310 and the gas supplied by the second gas supply device 320 during the nitriding treatment and discharges them to the outside, thereby reducing the pressure inside the general-purpose furnace 310. The second vacuum pump 330 can be, for example, a mechanical booster pump.

[0016] The second heating device 340 is a device that heats the steel material to be processed that has been placed in the general-purpose furnace 310. It is preferable that the second heating device 340 can uniformly heat the portion of the surface of the steel material where nitrogen is to be dissolved. In this embodiment, the second heating device 340 is a heater provided in the general-purpose furnace 310, and can heat the steel material to 1000°C or higher.

[0017] The second control device 400 includes a second memory 410 and a second CPU 420. The second CPU 420 controls the operations of the second gas supply device 320, the second vacuum pump 330, and the second heating device 340 by executing a program pre-installed in the second memory 410. However, some or all of the functions of these components may be realized by a hardware circuit.

[0018] Fig. 2 is a flow chart showing an example of nitriding treatment. Fig. 3 is a diagram showing an example of surface temperature in nitriding treatment. In the graph shown in Fig. 3, the horizontal axis represents time and the vertical axis represents surface temperature. The nitriding treatment is performed in a state where the steel material to be processed is placed in a vacuum nitriding furnace 110.

[0019] In step S100 (see FIG. 2), the first control device 200 executes a "nitriding process" that alternately repeats a "first process" in which the surface temperature of the steel material is lowered to a first temperature T1 and the nitriding gas is brought into contact with the surface of the steel material, and a "second process" in which the surface temperature of the steel material is raised to a second temperature T2 and the nitriding gas is brought into contact with the surface of the steel material. In this embodiment, the first control device 200 controls the first gas supply device 120 to supply the nitriding gas into the vacuum nitriding furnace 110, and then controls the first heating device 140 to alternately lower the surface temperature of the steel material to the first temperature T1 and raise the surface temperature of the steel material to the second temperature T2. In addition, in this embodiment, the first control device 200 controls the first vacuum pump 130 to reduce the pressure inside the vacuum nitriding furnace 110 from atmospheric pressure to 0.1 Pa, and then controls the first gas supply device 120 to introduce vacuum gas into the vacuum nitriding furnace 110 to reach 5000 Pa, thereby performing the nitriding process.

[0020] In this embodiment, the surface temperature is the temperature of the portion on the surface of the steel material where nitrogen is to be dissolved. The portion on the surface of the steel material where nitrogen is to be dissolved is the portion of the steel material where it is desired to increase the strength, for example, the gear portion of a gear component. The first temperature T1 is a temperature suitable for nitrogen to penetrate from the surface of the steel material into the interior of the steel material. In this embodiment, the first temperature T1 is 900°C. The second temperature T2 is a temperature suitable for nitrogen that has penetrated from the surface of the steel material to diffuse into the interior of the steel material, and is a temperature higher than the first temperature T1. In this embodiment, the second temperature T2 is 1000°C.

[0021] In this embodiment, the first control device 200 performs the second step three times and the first step twice in the nitriding process in the order of the second step, the first step, the second step, the first step, and the second step.

[0022] More specifically, the first control device 200 controls the first vacuum pump 130 to reduce the pressure inside the vacuum nitriding furnace 110 to 0.1 Pa or less, and then, as shown in Fig. 3, in the first second step, controls the first heating device 140 to maintain the surface temperature at the second temperature T2 from time t1 when the surface temperature rises to the second temperature T2 until a predetermined maintenance time D1 has elapsed. Note that, at time t1, the first control device 200 controls the first gas supply device 120 to introduce ammonia gas until the pressure inside the vacuum nitriding furnace 110 reaches 5000 Pa, and then stops the introduction of ammonia gas when the pressure inside the vacuum nitriding furnace 110 reaches 5000 Pa. As a method of controlling the pressure inside the vacuum nitriding furnace 110, the first control device 200 may control the first gas supply device 120 and the first vacuum pump 130 to maintain the pressure inside the vacuum nitriding furnace 110 at 5000 Pa while flowing a small amount of ammonia gas.

[0023] Next, as a first step of the first cycle, the first control device 200 controls the first heating device 140 to lower the surface temperature to the first temperature T1 at timing t2, which is when the holding time D1 has elapsed since the surface temperature rose to the second temperature T2. More specifically, the first control device 200 controls the first heating device 140 to lower its output. The first control device 200 controls the first heating device 140 to maintain the surface temperature at the first temperature T1 from timing t3, when the surface temperature has decreased to the first temperature T1, until a predetermined holding time D2 has elapsed.

[0024] Next, in the second step of the second run, the first control device 200 controls the first heating device 140 to raise the surface temperature to the second temperature T2 at timing t4, which is when the holding time D2 has elapsed since the surface temperature dropped to the first temperature T1. The first control device 200 controls the first heating device 140 to maintain the surface temperature at the second temperature T2 from timing t5, when the surface temperature rose to the second temperature T2, until a predetermined holding time D3 has elapsed.

[0025] Next, in the second first step, the first control device 200 controls the first heating device 140 to lower the surface temperature to the first temperature T1 at timing t6, when the holding time D3 has elapsed since the surface temperature rose to the second temperature T2. The first control device 200 controls the first heating device 140 to maintain the surface temperature at the first temperature T1 from timing t7, when the surface temperature has decreased to the first temperature T1, until a predetermined holding time D4 has elapsed.

[0026] Finally, in the third second step, the first control device 200 controls the first heating device 140 to raise the surface temperature to the second temperature T2 at timing t8, which is the elapse of a holding time D4 after the surface temperature has dropped to the first temperature T1. The first control device 200 controls the first heating device 140 to maintain the surface temperature at the second temperature T2 from timing t9, when the surface temperature has risen to the second temperature T2, until a predetermined holding time D5 has elapsed.

[0027] The holding times D1 to D5 are times determined experimentally and empirically, and are preferably times that do not cause voids in the steel material. In this embodiment, the holding times D1 to D5 are all 2 minutes.

[0028] In step S110 (see FIG. 2 ), the first control device 200 executes a “cooling step” in which the steel material is cooled to lower the surface temperature to a third temperature T3 or lower, which is a temperature at which austenite formed on the surface of the steel material transforms into a structure containing martensite or ferrite and is lower than the first temperature T1. In this embodiment, the third temperature T3 is 200° C. In the cooling step, the surface temperature may be lowered by rapidly cooling the steel material, or by slowly cooling the steel material.

[0029] More specifically, as shown in FIG. 3 , from time t9 when the surface temperature rises to the second temperature T2 to time t10 when the surface temperature has been maintained at the second temperature T2 for a holding time D5, the first control device 200 stops the first heating device 140 to lower the surface temperature, controls the first vacuum pump 130 to evacuate the gas in the vacuum nitriding furnace 110 and reduce the pressure in the vacuum nitriding furnace 110 to 0.1 Pa, and then controls the first gas supply device 120 to supply cooling gas into the vacuum nitriding furnace 110. The surface temperature is lowered to a third temperature T3 at time t11. The cooling gas is, for example, nitrogen gas. After the surface temperature has lowered to the third temperature T3, the steel is placed in the general-purpose furnace 310.

[0030] In step S120 (see FIG. 2), the second control device 400 executes a "quenching process" in which the surface temperature is raised to a fourth temperature T4, which is the temperature at which a structure containing martensite or ferrite transforms into austenite, and then lowered to the third temperature T3 or lower. In this embodiment, the fourth temperature T4 is 950°C. The fourth temperature T4 is preferably equal to or lower than the critical temperature at which the crystal grains of the steel material become coarse, and may be equal to or lower than the first temperature T1.

[0031] More specifically, as shown in FIG. 3 , the second control device 400 controls the second heating device 340 to raise the surface temperature to a fourth temperature T4 at timing t12, which follows timing t11. In this embodiment, the second control device 400 maintains the surface temperature at the fourth temperature T4 for a predetermined time, and then stops the second heating device 340 and controls the second gas supply device 320 to supply cooling gas into the general-purpose furnace 310 at timing t13 to lower the surface temperature to a third temperature T3. The surface temperature then drops to the third temperature T3 at timing t14. The length of time from timing t11 to timing t12 can be determined arbitrarily. That is, the second control device 400 may start the quenching process at timing t11, when the surface temperature drops to the third temperature T3.

[0032] According to the nitriding method of this embodiment described above, the surface temperature of the steel material can be raised to the second temperature T2 without raising the temperature inside the vacuum nitriding furnace 110 as a whole. Therefore, for example, decomposition of ammonia as a nitrogen source is small, and a large amount of ammonia gas as a nitriding gas is not required. Furthermore, the surface temperature of the steel material can be lowered to the first temperature T1 in a short time. Therefore, the nitriding treatment can be carried out efficiently.

[0033] Furthermore, since the steel material cooled in the cooling step is reheated in the quenching step, it is possible to prevent the crystal grains of the steel material from becoming coarse, thereby improving the strength of the steel material.

[0034] Furthermore, according to this form of nitriding method, it is only necessary to heat the surface of the portion of the steel material where nitrogen is to be dissolved, and therefore the heating energy required for the nitriding treatment can be reduced.

[0035] In addition, in the nitriding process, the third step, Step 2, is carried out after the second, final step, Step 1. Nitrogen that penetrates the steel material from the surface in Step 1 is quickly diffused into the interior in Step 2, allowing for efficient nitriding in a short time.

[0036] B. Second embodiment: The nitriding method of the second embodiment differs from the nitriding method of the first embodiment in that a carburizing treatment is performed in the quenching step (step S120 in FIG. 2) to dissolve carbon in the steel material, but the other steps are the same as those of the first embodiment. The configuration of the nitriding apparatus 100 of the second embodiment differs from that of the nitriding apparatus 100 of the first embodiment in that the second gas supply device 320 can supply carburizing gas separately from the cooling gas, but the other configurations are the same as those of the first embodiment.

[0037] Carburizing is a process in which the carburizing gas is brought into contact with the surface of the steel material, causing carbon in the carburizing gas to penetrate and diffuse from the surface of the steel material. In this embodiment, the second control device 400 controls the second gas supply device 320 to supply the carburizing gas during the quenching process. At timing t12 (see FIG. 3) when the quenching process starts, the second control device 400 controls the second vacuum pump 330 to reduce the pressure inside the vacuum nitriding furnace 110.

[0038] According to the nitriding method of the second embodiment described above, carbon is made inherent in the steel material, and therefore the strength of the steel material can be improved.

[0039] C. Third embodiment: Fig. 4 is a flowchart showing an example of nitriding treatment in the third embodiment. Fig. 5 is a diagram showing an example of surface temperature in the nitriding treatment in the third embodiment. In the graph shown in Fig. 5, the horizontal axis represents time and the vertical axis represents surface temperature. The nitriding treatment method in the third embodiment differs from the nitriding treatment method in the second embodiment in that a tempering step is performed after the quenching step, but the other steps are the same as those in the first embodiment. The configuration of the nitriding apparatus 100 in the third embodiment is the same as that of the nitriding apparatus 100 in the second embodiment, so a description of the configuration of the nitriding apparatus 100 will be omitted.

[0040] In step S130 (see FIG. 4), the first control device 200 stops the first heating device 140 and controls the first gas supply device 120 to supply nitriding gas into the vacuum nitriding furnace 110 so as to raise the surface temperature of the steel material to a predetermined fifth temperature T5 and then lower it. The fifth temperature T5 is equal to or lower than the fourth temperature T4 and is a temperature suitable for reducing the hardness of the steel material and increasing its toughness and tenacity. In this embodiment, the fifth temperature T5 is 150°C.

[0041] More specifically, as shown in FIG. 5 , the second control device 400 starts the tempering process at timing t15. The second control device 400 controls the second heating device 340 to raise the surface temperature to a fifth temperature T5 at timing t15, which is after timing t14 when the surface temperature has dropped to the third temperature T3. In this embodiment, the second control device 400 maintains the surface temperature at the fifth temperature T5 for a predetermined time, and controls the second heating device 340 to lower the surface temperature to the third temperature T3 at timing t16. Note that the length of time from timing t14 to timing t15 can be determined arbitrarily. That is, the second control device 400 may start the tempering process at timing t14 when the surface temperature has dropped to the third temperature T3.

[0042] According to the nitriding method of the third embodiment described above, the steel material is reheated in the tempering process after the carburizing process in the quenching process, so that the hardness of the steel material can be reduced and the tenacity and toughness can be increased, thereby further improving the strength of the steel material.

[0043] D. Other Embodiments: (D1) The quenching process in the above-described embodiments and the tempering process in the third embodiment are performed using the vacuum nitriding furnace 110. However, all steps in the nitriding treatment may be performed using the vacuum nitriding furnace 110. In this case, the quenching process and the tempering process in the third embodiment are performed with the steel material placed in the vacuum nitriding furnace 110. The first control device 200 performs the same control as the second control device 400 described above on the first gas supply device 120, the first vacuum pump 130, and the first heating device 140. The first heating device 140 may include an infrared lamp and a heater.

[0044] (D2) In the nitriding step in the above-described embodiment, the first control device 200 alternately controls the first heating device 140 to lower the surface temperature of the steel material to the first temperature T1 and to raise the surface temperature of the steel material to the second temperature T2, while controlling the first gas supply device 120 to supply the nitriding gas into the vacuum nitriding furnace 110. This is not limiting, and the first control device 200 may control the first gas supply device 120 to supply the nitriding gas into the vacuum nitriding furnace 110 after the surface temperature of the steel material has lowered to the first temperature T1. Alternatively, the first control device 200 may control the first gas supply device 120 to supply the nitriding gas into the vacuum nitriding furnace 110 after the surface temperature of the steel material has risen to the second temperature T2.

[0045] (D3) In the first step in the above-described embodiment, the first control device 200 controls the first heating device 140 to reduce its output, thereby lowering the surface temperature of the steel material to the first temperature T1. Without being limited to this, for example, the first control device 200 may control the first gas supply device 120 to supply nitriding gas into the vacuum nitriding furnace 110, thereby lowering the temperature inside the vacuum nitriding furnace 110, and thereby lowering the surface temperature of the steel material to the first temperature T1.

[0046] (D4) In the above-described embodiment, the nitriding process includes two first steps and three second steps. That is, the second step is performed after the last first step. However, the nitriding process is not limited to this, as long as it includes two or more first steps. More specifically, the nitriding process may include a first step of lowering the surface temperature from a temperature higher than or equal to a first temperature T1 to the first temperature T1, a second step of raising the surface temperature to a second temperature T2, and a first step of lowering the surface temperature from the second temperature T2 to the first temperature T1, all of which are performed in this order.

[0047] (D5) In the nitriding process in the above-described embodiment, the first control device 200 controls the first vacuum pump 130 to reduce the pressure inside the vacuum nitriding furnace 110. In addition, in the quenching process in the second embodiment, the second control device 400 controls the second vacuum pump 330 to reduce the pressure inside the general-purpose furnace 310. However, the present invention is not limited to this. The first control device 200 does not have to control the first vacuum pump 130 to reduce the pressure inside the vacuum nitriding furnace 110 in the nitriding process. In addition, the second control device 400 does not have to control the second vacuum pump 330 to reduce the pressure inside the general-purpose furnace 310 in the tempering process.

[0048] (D6) In the cooling step in the above-described embodiment, the first control device 200 stops the first heating device 140 and controls the first gas supply device 120 to supply cooling gas into the vacuum nitriding furnace 110, thereby lowering the surface temperature of the steel material. Also, in the quenching step and the tempering step in the third embodiment, the second control device 400 stops the second heating device 340 and controls the second gas supply device 320 to supply cooling gas into the general-purpose furnace 310, thereby lowering the surface temperature of the steel material. However, without being limited to this, the steel material may be quenched by immersing it in a quenching coolant such as water or oil, for example.

[0049] (D7) In the above-described embodiment, the surface temperature refers to the temperature of the portion on the surface of the steel material where nitrogen is to be dissolved. This is not limiting, and the surface temperature may refer to the temperature of the entire surface of the steel material. In this case, the first heating device 140 and the second heating device 340 may heat the entire steel material or the entire furnace interior, rather than uniformly heating the portion on the surface of the steel material where nitrogen is to be dissolved.

[0050] (D8) In the first embodiment described above, the cooling step and the quenching step may be omitted. Furthermore, induction hardening may be performed instead of the quenching step. Induction hardening is a process in which, for example, electromagnetic induction is caused in the steel material by high-frequency electromagnetic waves outside the vacuum nitriding furnace 110, thereby overheating the surface of the steel material.

[0051] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0052] 100...nitriding apparatus, 110...vacuum nitriding furnace, 120...gas supply device, 130...vacuum pump, 140...heating device, 200...control device, 210...memory, 220...CPU, 100...nitriding apparatus, 110...vacuum nitriding furnace, 120...first gas supply device, 130...first vacuum pump, 140...first heating device, 200...first control device, 210...first memory, 220...first CPU, 310...general-purpose furnace, 320...second gas supply device, 330...second vacuum pump, 340...second heating device, 400...second control device, 410...second CPU, 420...second memory

Claims

1. A method for nitriding steel materials, comprising: a first step of lowering the surface temperature of the steel material to a first temperature suitable for nitrogen to penetrate from the surface of the steel material into the steel material, and bringing a nitriding gas into contact with the surface of the steel material; and a second step of raising the surface temperature to a second temperature which is higher than the first temperature and which is suitable for diffusing the nitrogen which has penetrated from the surface of the steel material into the interior of the steel material, and bringing the nitriding gas into contact with the surface of the steel material, and

2. The nitriding treatment method according to claim 1, further comprising: a cooling step of cooling the steel material after the nitriding step to reduce the surface temperature to a third temperature or lower, which is a temperature at which austenite formed on the surface of the steel material transforms into a structure containing martensite or ferrite, and which is lower than the first temperature; and a quenching step, after the cooling step, of raising the surface temperature to a fourth temperature at which a structure containing martensite or ferrite transforms into austenite, and then lowering the surface temperature to not more than the third temperature.

3. The nitriding treatment method according to claim 2, a nitriding treatment method in which, in the quenching step, a carburizing treatment is carried out to dissolve carbon in the steel material;

4. The nitriding treatment method according to any one of claims 1 to 3, The nitriding treatment method, wherein the surface temperature is the temperature of a portion of the surface of the steel material into which nitrogen is to be dissolved.

5. The nitriding treatment method according to any one of claims 1 to 3, The nitriding treatment method, wherein the second step is carried out after the first step, which is carried out last.

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