Low pressure carburizing method

Abstract

According to the present invention, a low pressure carburizing treatment includes a carburizing step in which a carburization gas is supplied into a heat treatment chamber into which a workpiece is charged, and a diffusion step which is performed subsequent to the carburization step and in which the carburization gas is not supplied into the heat treatment chamber, the low pressure carburizing treatment being performed in a state in which the temperature of the carburizing step is lower than the temperature of the diffusion step by 10°C or more.

Description

Low-pressure carburizing method

[0001] The present invention relates to a low-pressure carburizing method.

[0002] Conventionally, as a heat treatment for improving the durability of automobile parts, mechanical parts and the like made of steel materials, carburizing treatment has been performed in which carbon is solid-dissolved in the surface of a workpiece (part surface) to harden the workpiece surface. Further, as a carburizing treatment capable of reducing the amount of carbon dioxide discharged from a carburizing furnace in which carburizing is performed, vacuum carburizing treatment in which carburizing is performed under a vacuum atmosphere is known.

[0003] As a conventional technique related to vacuum carburizing, Patent Document 1 discloses a vacuum carburizing method characterized in that a gaseous chain unsaturated hydrocarbon such as acetylene is used as a carburizing gas, and carburizing is performed with the inside of a heating chamber set to a vacuum state of 1 kPa or less.

[0004] Japanese Patent Application Publication No. 2007-224357

[0005] After being supplied into the heat treatment chamber, a thermal decomposition reaction occurs in the carburizing gas before it reaches the surface of the workpiece, and part of the carbon components in the carburizing gas supplied into the heat treatment chamber does not contribute to the carburizing reaction on the workpiece surface. Due to the progress of such a thermal decomposition reaction, the amount of carbon components in the carburizing gas that reaches the workpiece surface located at a position far from the gas inlet tends to be smaller than the amount of carbon components in the carburizing gas that reaches the workpiece surface located at a position close to the gas inlet. As a result, depending on the distance from the gas inlet, the uniformity of carburizing quality may decrease at the arrangement position of the workpiece in the heat treatment chamber and at each part of the workpiece. In particular, when the carburizing temperature is high, or when a chain unsaturated hydrocarbon gas such as ethylene gas or acetylene gas, which is chemically active and easily decomposes in a short time, is used, the uniformity of carburizing quality tends to decrease.

[0006] To suppress the decrease in uniformity of carburizing quality, one method can be employed to increase the flow rate of carburizing gas supplied to the heat treatment chamber, thereby increasing the amount of carburizing gas that reaches parts of the workpiece that are relatively far from the gas inlet. Furthermore, by increasing the flow rate of carburizing gas and raising the carburizing temperature, it is possible to shorten the processing time required to obtain the desired amount of carburization, thereby improving productivity.

[0007] On the other hand, if the supply flow rate of carburizing gas is excessive, some of the carbon produced by the thermal decomposition of the carburizing gas will accumulate in the heat treatment chamber, making it easier for problems such as blockage of the gas inlet and outlet, heater short circuits, and contamination of the vacuum pump to occur. When such problems occur, it will lead to a reduction in continuous operating time due to maintenance work and a decrease in equipment lifespan.

[0008] This invention has been made in view of the above circumstances, and aims to achieve both an extension of equipment lifespan and an improvement in productivity in vacuum carburizing treatment.

[0009] The inventors discovered that by suppressing the thermal decomposition of the carburizing gas by lowering the temperature during the carburizing phase of the vacuum carburizing process, it is possible to reduce variations in carburizing quality depending on the distance from the gas inlet without supplying an excessive amount of carburizing gas. In addition, they found that by setting the temperature during the diffusion phase higher than the temperature during the carburizing phase, the diffusion rate of carbon from the workpiece surface to the interior of the workpiece can be increased, making it possible to shorten the overall time of the vacuum carburizing process.

[0010] In other words, the inventors of the present invention have found that the above problems can be solved by controlling the temperature while considering the behavior of the carburizing gas during the carburizing and diffusion phases, the carburizing reaction of the workpiece, and the behavior of carbon atoms inside the workpiece, and have completed the present invention.

[0011] One such embodiment of the present invention is a vacuum carburizing treatment method comprising: a carburizing step in which a carburizing gas is supplied to a heat treatment chamber in which a workpiece is placed; and a diffusion step performed following the carburizing step in which the carburizing gas is not supplied to the heat treatment chamber, wherein the temperature of the carburizing step is 10°C or more lower than the temperature of the diffusion step.

[0012] According to the present invention, it is possible to achieve both an extension of equipment lifespan and an improvement in productivity in vacuum carburizing treatment.

[0013] This figure shows an example of the schematic configuration of a vacuum carburizing furnace according to an embodiment of the present invention. This figure shows an example of the supply of carburizing gas and control of the temperature inside the heat treatment chamber during vacuum carburizing treatment. This figure shows an example of the supply of carburizing gas and control of the temperature inside the heat treatment chamber during vacuum carburizing treatment. This figure shows an example of the supply of carburizing gas and control of the temperature inside the heat treatment chamber during vacuum carburizing treatment.

[0014] Embodiments of the present invention will be described below with reference to the drawings. In this specification and the drawings, elements having substantially the same functional configuration are denoted by the same reference numerals, and redundant explanations will be omitted.

[0015] <Reduced Pressure Carburizing Furnace> Figure 1 shows an example of the schematic configuration of the reduced pressure carburizing furnace 1 according to this embodiment. Reduced pressure carburizing is carburizing performed under a pressure lower than atmospheric pressure and is also called vacuum carburizing.

[0016] The vacuum carburizing furnace 1 is equipped with a heat treatment chamber 2 in which the workpieces W to be processed are contained. In Figure 1, the workpieces W are shown as a single block, but the workpieces W are actually placed in multiples in a container such as a tray or basket, and the number of workpieces W to be carburized in one lot is, for example, between 10 and 100,000. The workpieces W are processed parts such as automobile parts and machine parts made of steel material.

[0017] Heaters 3 for heating the workpiece W are installed on the ceiling and bottom surfaces of the heat treatment chamber 2, and thermocouples 4, which serve as temperature measuring units for measuring the temperature inside the heat treatment chamber 2, are provided on the side walls of the heat treatment chamber 2. The set temperature of the heaters 3 is controlled by a control device (not shown), such as a PLC (Programmable Logic Controller), according to the temperature inside the heat treatment chamber 2 measured by the thermocouples 4.

[0018] A gas inlet 5 is provided on the side wall of the heat treatment chamber 2 to which carburizing gas is supplied. The gas inlet 5 is connected to a carburizing gas supply source 6, such as a gas cylinder, and a mass flow controller 7 is provided between the gas inlet 5 and the carburizing gas supply source 6 as a flow rate adjustment unit. The mass flow controller 7 can switch between supplying the carburizing gas into the heat treatment chamber 2 and stopping the supply by adjusting the supply flow rate of the carburizing gas based on a control signal from a control device (not shown).

[0019] Furthermore, the gas inlet 5 is connected to a source of inert gas (not shown), such as nitrogen gas or argon gas. The type of gas supplied from the gas inlet 5 can be switched as needed, and either an inert gas or a carburizing gas is supplied from the gas inlet 5 to create a desired atmosphere inside the heat treatment chamber 2.

[0020] The carburizing gas supplied into the heat treatment chamber 2 is, for example, a chain-type unsaturated hydrocarbon gas such as acetylene gas or ethylene gas, or a chain-type saturated hydrocarbon gas such as propane gas or butane gas, or a mixture of these gases. The vacuum carburizing treatment method described later is particularly useful when using a chain-type unsaturated hydrocarbon gas that is chemically active and easily decomposes in a short time as the carburizing gas; therefore, it is preferable that the carburizing gas is a chain-type unsaturated hydrocarbon gas.

[0021] A gas outlet 8 is provided on the side wall of the heat treatment chamber 2 facing the gas inlet 5, and the atmosphere inside the heat treatment chamber 2 is discharged from there. The gas outlet 8 is connected to a vacuum pump 9.

[0022] The general configuration of the vacuum carburizing furnace 1 according to this embodiment has been described above. Note that the configuration of the vacuum carburizing furnace is not limited to the configuration described in this embodiment; any configuration that can realize the vacuum carburizing treatment described later is acceptable.

[0023] <Reduced Pressure Carburizing Treatment Method> Next, an example of a reduced pressure carburizing treatment method for the workpiece W according to this embodiment will be described.

[0024] (Atmosphere adjustment and workpiece loading) First, before the workpiece W is loaded into the heat treatment chamber 2, an inert gas (for example, nitrogen gas) is supplied into the heat treatment chamber 2 from the gas inlet 5, and the atmosphere inside the heat treatment chamber 2 is replaced with the inert gas. At this time, the pressure inside the heat treatment chamber 2 is maintained at, for example, 10 to 150 kPa, and the temperature inside the heat treatment chamber 2 is maintained at, for example, 750 to 1000°C. After the atmosphere inside the heat treatment chamber 2 has been adjusted in this way, the workpiece W is loaded into the heat treatment chamber 2.

[0025] (Heating process) After the workpiece W is placed in the heat treatment chamber 2, the chamber is evacuated until the pressure inside the heat treatment chamber 2 is, for example, less than 1.0 kPa. Then, an inert gas is supplied into the heat treatment chamber 2, and the temperature inside the heat treatment chamber 2 is raised to a predetermined temperature suitable for carburizing under an inert gas atmosphere.

[0026] (Primary soaking process) In the primary soaking process, after the temperature inside the heat treatment chamber 2 is raised to a predetermined temperature suitable for carburizing, the pressure inside the heat treatment chamber 2 is maintained at, for example, less than 1.0 kPa, and the workpiece W is soaked.

[0027] (Carburizing Process and Diffusion Process) The carburizing process is a process in which carburizing gas is decomposed on the surface of the workpiece W, and carbon is dissolved in the surface of the workpiece W. In this specification, the carburizing process is defined as the state in which carburizing gas is supplied into a heat treatment chamber 2 that is maintained at a temperature suitable for carburizing. For example, even if the temperature inside the heat treatment chamber 2 is suitable for carburizing, if carburizing gas is not supplied into the heat treatment chamber 2, or even if carburizing gas is supplied into the heat treatment chamber 2, if the temperature inside the heat treatment chamber 2 is too low to be suitable for carburizing, this is not considered a carburizing process in this specification.

[0028] The temperature of the carburizing process (hereinafter sometimes referred to as "carburizing temperature") is set to, for example, 850°C or higher. From the viewpoint of achieving both an extension of equipment life and an improvement in productivity, as described later, there is no particular upper limit to the carburizing temperature. However, if the carburizing temperature exceeds 950°C, the shape distortion of the workpiece W may exceed the allowable range, so it is preferable that the carburizing temperature be 950°C or lower. The carburizing temperature may also be 880°C or higher, or 900°C or higher. The temperature inside the heat treatment chamber 2 may be varied during the carburizing process rather than kept constant. In this specification, "carburizing process temperature" is defined as the temperature at the end of the carburizing process.

[0029] The carburizing process is completed when a predetermined time has elapsed while the supply of carburizing gas and vacuum evacuation are continued, after which the diffusion process begins. The duration of the carburizing process (hereinafter sometimes referred to as "carburizing time") is set appropriately based on experience, prior tests, or numerical simulations, taking into account other processing conditions such as carburizing temperature and the supply flow rate of carburizing gas, in order to obtain the desired carburizing quality, such as surface carbon concentration and effective hardened layer depth.

[0030] The diffusion process is a process of diffusing carbon into the interior of the workpiece W. In this specification, the diffusion process is defined as a process that follows the carburizing process and in which no carburizing gas is supplied to the heat treatment chamber 2, which is adjusted to a temperature range suitable for carburizing. For example, a state in which carburizing gas is supplied to the heat treatment chamber 2, or a state in which the heater 3 is stopped and cooling is in progress even if no carburizing gas is supplied to the heat treatment chamber 2, is not considered a diffusion process in this specification.

[0031] The temperature of the diffusion process (hereinafter sometimes referred to as "diffusion temperature") is set to, for example, 860°C or higher. From the viewpoint of achieving both an extended equipment lifespan and improved productivity, as described later, there is no particular upper limit to the diffusion temperature. However, if the diffusion temperature exceeds 1000°C, the thermal load on the components of the vacuum carburizing furnace 1 increases, so it is preferable that the diffusion temperature be 1000°C or lower. The diffusion temperature may also be 890°C or higher, 900°C or higher, or 940°C. The temperature inside the heat treatment chamber 2 may be varied during the diffusion process rather than kept constant. In this specification, "diffusion process temperature" is defined as the average temperature during the diffusion process time.

[0032] Figure 2 shows an example of the supply of carburizing gas and control of the heat treatment chamber temperature in the vacuum carburizing treatment according to this embodiment. In the figure, "ON" for "carburizing gas" indicates that carburizing gas is being supplied into the heat treatment chamber 2, and "OFF" indicates that the supply of carburizing gas to the heat treatment chamber 2 has been stopped. In Figure 2, the state where the carburizing gas is ON corresponds to the carburizing process, and the state where the carburizing gas is OFF corresponds to the diffusion process.

[0033] In conventional vacuum carburizing processes, the temperature of the carburizing process was appropriately set to obtain desired carburizing qualities such as surface carbon concentration and effective hardened layer depth, and this temperature was the same as or higher than that of the diffusion process. On the other hand, as shown in Figure 2, the carburizing process according to this embodiment is carried out at a lower temperature than the diffusion process. Specifically, the carburizing process is carried out at a temperature that is 10°C or more lower than that of the diffusion process.

[0034] By keeping the temperature of the carburizing process lower than before, the rate of the thermal decomposition reaction of the carburizing gas supplied into the heat treatment chamber 2 can be reduced. This reduces the difference in the progress of the carburizing reaction between workpieces located close to the gas inlet 5 and those located farther away. In other words, even if the supply flow rate of the carburizing gas is reduced to some extent, the carburizing process can be performed without increasing the variation in carburizing quality that occurs depending on the distance to the gas inlet 5. By performing the carburizing process in this manner, the total amount of carburizing gas supplied can be reduced, thereby suppressing the adhesion and accumulation of carbon on the inner surface of the heat treatment chamber 2 and the deterioration of the components of the heat treatment chamber 2 caused by carbon that did not contribute to the carburizing of the workpiece W. As a result, the lifespan of the equipment can be extended.

[0035] Furthermore, in the vacuum carburizing treatment according to this embodiment, even if the temperature of the carburizing process is kept lower than before, the diffusion process is performed without lowering the temperature of the diffusion process to match the temperature of the carburizing process. This allows the diffusion process to be performed without reducing the diffusion rate of carbon from the surface to the interior of the workpiece W. As a result, the time required for the diffusion process can be shortened and productivity can be improved compared to vacuum carburizing treatment in which the temperature of the diffusion process is lowered to match the temperature of the carburizing process.

[0036] Therefore, by performing a reduced-pressure carburizing treatment in which the temperature of the carburizing process is 10°C or more lower than the temperature of the diffusion process, as in this embodiment, it is possible to achieve both an extension of equipment life and an improvement in productivity.

[0037] The diffusion process ends when the supply of carburizing gas is stopped and the vacuum is evacuated for a predetermined period of time. The duration of the diffusion process (hereinafter sometimes referred to as "diffusion time") is set appropriately based on experience, prior tests, or numerical simulations, taking into account the conditions of the carburizing process and other processing conditions such as the diffusion temperature, so as to obtain the desired carburizing quality, such as the surface carbon concentration and effective hardened layer depth.

[0038] After the diffusion process is complete, an inert gas is supplied to the heat treatment chamber 2, and the heater 3 stops operating. This state is maintained for a certain period of time, allowing the heat treatment chamber 2 to cool and the workpiece W to undergo secondary soaking treatment. Subsequently, the workpiece W is quenched, completing the carburizing and quenching treatment for one lot of workpiece W.

[0039] In this embodiment, the carburizing process and the diffusion process were performed once each, as shown in Figure 2. However, pulsed carburizing treatment may be performed, in which each of the carburizing process and the diffusion process is repeated two or more times, as described below. By using pulsed carburizing, in which the supply of carburizing gas is intermittent, the total amount of carburizing gas supplied can be further reduced while performing the carburizing treatment.

[0040] Figure 3 shows an example of performing four carburizing and four diffusion processes by switching between supplying and stopping the carburizing gas. In this example, the temperature of each of the first to fourth carburizing processes is at least 10°C lower than the temperature of the fourth diffusion process. Even when the supply of carburizing gas and the temperature inside the heat treatment chamber 2 are controlled in this way, the rate of the thermal decomposition reaction of the carburizing gas can be reduced compared to when each carburizing and diffusion process is performed at the same temperature. This allows for a reduction in the supply flow rate of carburizing gas without increasing the variation in carburizing quality, and the equipment life can be extended by reducing the total amount of carburizing gas supplied.

[0041] Furthermore, in the example shown in Figure 3, since the temperature of the fourth diffusion step is not lowered to match the temperature of the carburizing step, the carbon diffusion rate into the workpiece during the fourth diffusion step is greater than when the temperature of all diffusion steps is lowered to match the carburizing step. As a result, the total processing time until all carburizing and diffusion steps are completed can be shortened. This improves productivity. Therefore, even when performing pulsed carburizing as shown in Figure 3, it is possible to achieve both extended equipment life and improved productivity.

[0042] FIG. 4 is a diagram showing another example of temperature control for pulse carburizing. In the example of FIG. 4, the temperature in the heat treatment chamber 2 is lowered only in the third carburizing step, so that the aforementioned effect of suppressing the thermal decomposition reaction of the carburizing gas to reduce the total supply amount of the carburizing gas and the effects accompanying the same can be obtained. Further, in the example of FIG. 4, the temperature of the third diffusion step is not lowered in accordance with the temperature of the third carburizing step, so the carbon diffusion rate is higher than that in the case where the third diffusion step is performed at the same temperature as the third carburizing step, thereby improving productivity.

[0043] Therefore, when the temperature of at least one carburizing step is 10°C or more lower than the temperature of the diffusion carburizing step performed immediately after the carburizing step, in other words, when the temperature of the n-th (n: integer) carburizing step is 10°C or more lower than the temperature of the n-th diffusion step, the effect of achieving both extension of equipment life and improvement of productivity can be obtained.

[0044] In pulse carburizing treatment in which the carburizing step and the diffusion step are repeated two or more times, it is preferable that, as in the temperature control pattern illustrated in FIG. 3, the temperature at the end of the last carburizing step (the fourth carburizing step in the example of FIG. 3) is set to a temperature 10°C or more lower than the temperature of the diffusion step performed immediately after the carburizing step.

[0045] Although the embodiments of the present invention have been exemplified above, the present invention is not limited to such examples. It is obvious to those skilled in the art that various changes and modifications can be conceived within the scope of the technical idea described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention.

[0046] For example, the constituent features of the above embodiments can be arbitrarily combined. From such arbitrary combinations, the functions and effects of each constituent feature involved in the combination can be naturally obtained, and other functions and effects obvious to those skilled in the art from the description of the present specification can also be obtained.

[0047] Vacuum carburizing treatment was performed on a plurality of test pieces made of the same type of steel, and the total supply amount of carburizing gas and the total treatment time required for carburizing diffusion treatment were evaluated. The results are shown in Table 1 below.

[0048]

[0049] In this example, as shown in Table 1, vacuum carburizing was performed under the conditions of "Examples 1 to 5". Vacuum carburizing was also performed under the conditions of "Comparative Examples 1-1, 1-2" which are comparative objects for Example 1, "Comparative Examples 2-1, 2-2" which are comparative objects for Example 2, "Comparative Examples 3-1, 3-2" which are comparative objects for Example 3, "Comparative Examples 4-1, 4-2" which are comparative objects for Example 4, and "Comparative Examples 5-1, 5-2" which are comparative objects for Example 5.

[0050] The vacuum carburizing treatment in each example was all carried out in a vacuum carburizing furnace with the same structure, and the plurality of test pieces to be treated were placed on one container and subjected to the vacuum carburizing treatment. It has been confirmed that the variation in the surface carbon concentration Cs and the effective hardened layer depth ECD of each test piece, which varies depending on the distance from the gas inlet, is within the range allowable for products. Furthermore, all treatment conditions other than the carburizing step and diffusion step in Examples 1 to 5 and all treatment conditions other than the carburizing step and diffusion step in each comparative example corresponding to Examples 1 to 5 are the same.

[0051] The treatment "without pulse carburizing" in Table 1 is a treatment in which carburizing gas supply and temperature adjustment were performed according to the control pattern shown in Figure 2, and the treatment "with pulse carburizing" is a treatment in which carburizing gas supply and temperature adjustment were performed according to the control pattern shown in Figure 3. The carburizing time in Example 5 where pulse carburizing was performed, Comparative Example 5-1, and Comparative Example 5-2 is the total treatment time of four carburizing steps, and the diffusion time is the total treatment time of four diffusion steps.

[0052] The "Temperature Pattern" in Table 1 is included for convenience to make it easier to distinguish between high and low carburizing temperatures and diffusion temperatures. It shows which of the two temperature settings is used for each example and its two corresponding comparative examples. Note that in cases where the temperature pattern is "low temperature / high temperature" and the diffusion temperature is higher than the carburizing temperature, the "Diffusion Time" includes the heating time required to reach the target diffusion temperature from the carburizing temperature. The length of this heating time varies depending on the volume of the vacuum carburizing furnace and the equipment specifications, but it is a very short time relative to the total processing time and does not affect productivity.

[0053] The "temperature difference" in Table 1 is the temperature obtained by subtracting the diffusion temperature from the carburizing temperature. The "total processing time" in Table 1 is the sum of the carburizing time and the diffusion time.

[0054] As shown in Table 1, in all examples where vacuum carburizing was performed using the "low temperature / high temperature" pattern, the total supply of carburizing gas was reduced compared to the comparative example using the "high temperature / high temperature" pattern because the carburizing temperature was 10°C or more lower than the diffusion temperature. Furthermore, in all examples where vacuum carburizing was performed using the "low temperature / high temperature" pattern, the total time of carburizing and diffusion was shortened compared to the comparative example using the "low temperature / low temperature" pattern. In other words, productivity can be improved compared to the comparative example using the "low temperature / low temperature" pattern by not lowering the diffusion temperature in accordance with the carburizing temperature.

[0055] Therefore, by performing vacuum carburizing treatment where the carburizing temperature is 10°C or more lower than the diffusion temperature, it is possible to achieve both an extended equipment lifespan and improved productivity.

[0056] Furthermore, according to the results of Examples 2 to 4 and the comparative examples corresponding to each of those examples, the larger the temperature difference, the greater the effect of reducing the total amount of carburizing gas supplied compared to the comparative example of the "high temperature, high temperature" pattern, and the greater the effect of shortening the total processing time compared to the comparative example of the "low temperature, low temperature" pattern.

[0057] Examples of the vacuum carburizing treatment according to the present invention have been described above. The effects described herein are merely descriptive or illustrative and not limiting. In other words, the technology relating to this disclosure may produce other effects that will be apparent to those skilled in the art from the description herein, in addition to or in lieu of the effects described herein.

[0058] The following configuration examples also fall within the technical scope of this disclosure: (1) A vacuum carburizing treatment method comprising: a carburizing step in which a carburizing gas is supplied to a heat treatment chamber in which a workpiece is placed; and a diffusion step performed following the carburizing step in which the carburizing gas is not supplied to the heat treatment chamber, wherein the temperature of the carburizing step is 10°C or more lower than the temperature of the diffusion step. (2) The vacuum carburizing treatment method according to (1), wherein the carburizing step and the diffusion step are repeated two or more times, wherein the temperature of at least one of the carburizing steps is 10°C or more lower than the temperature of a diffusion carburizing step performed immediately after the carburizing step. (3) The vacuum carburizing treatment method according to (1) or (2), wherein the temperature of the carburizing step is 850 to 950°C. (4) The vacuum carburizing treatment method according to any one of (1) to (3), wherein the temperature of the diffusion step is 860 to 1000°C.

[0059] 1. Reduced pressure carburizing furnace 2. Heat treatment chamber 3. Heater 4. Thermocouple 5. Gas inlet 6. Carburizing gas supply source 7. Mass flow controller 8. Gas outlet 9. Vacuum pump W Work

Claims

1. A vacuum carburizing method comprising: a carburizing step in which a carburizing gas is supplied to a heat treatment chamber in which a workpiece is placed; and a diffusion step performed following the carburizing step in which the carburizing gas is not supplied to the heat treatment chamber, wherein the temperature of the carburizing step is 10°C or more lower than the temperature of the diffusion step.

2. The vacuum carburizing method according to claim 1, wherein the carburizing step and the diffusion step are repeated two or more times, and the temperature of at least one of the carburizing steps is 10°C or more lower than the temperature of the diffusion carburizing step performed immediately after the carburizing step.

3. The vacuum carburizing method according to claim 1 or 2, wherein the temperature of the carburizing step is 850 to 950°C.

4. The vacuum carburizing treatment method according to claim 1 or 2, wherein the temperature of the diffusion step is 860 to 1000°C.

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