JP7768341B2 - Manufacturing method of steel members - Google Patents

Description

The present invention relates to a method for manufacturing a steel member.

Conventionally, there has been known a method for manufacturing a steel member that includes a carburizing step of infiltrating carbon into an austenitic steel member. Such a method for manufacturing a steel member is disclosed, for example, in JP 2019-127624 A.

Japanese Patent Laid-Open Publication No. 2019-127624 discloses a method for manufacturing a steel member that includes a carburizing step of infiltrating carbon into a steel member in an austenitized state. In the method for manufacturing a steel member described in Japanese Patent Laid-Open Publication No. 2019-127624, in the carburizing step, carbon is infiltrated into the steel member in an austenitized state at a carbon concentration such that the steel member and carbon form a hyper-eutectoid composition.

Japanese Patent Application Laid-Open No. 2019-127624

However, in the manufacturing of steel members described in JP 2019-127624 A, carbon is introduced into an austenitized steel member at a carbon concentration that results in a hypereutectoid composition between the steel member and carbon, which tends to result in an excessively high carbon concentration being introduced into the steel member. Here, the hardness of a steel member is generally approximately proportional to the carbon concentration of the surface (carburized layer) of the steel member. Furthermore, the hardness of a steel member is generally approximately inversely proportional to the toughness of the steel member. That is, in the manufacturing of steel members described in JP 2019-127624 A, the hardness of the steel member tends to be excessively high and the toughness of the steel member tends to be excessively low. When the toughness of a steel member decreases, the steel member becomes vulnerable to impact. Therefore, a manufacturing method of a steel member that can improve the hardness and toughness of a steel member in a balanced manner is desired.

The present invention has been made to solve the above-mentioned problems, and one object of the present invention is to provide a method for manufacturing a steel member that can improve the hardness and toughness of the steel member in a balanced manner.

In order to achieve the above object, a method for manufacturing a steel member in one aspect of the present invention includes: a carburizing step in which a steel member is heated to a temperature equal to or higher than an austenitic transformation completion temperature and then carbon is introduced into the steel member in an austenitized state, at a carbon concentration such that the steel member and carbon form a hypoeutectoid composition, and the carbon-infiltrated steel member is slowly cooled; a quenching step in which, after the carburizing step, the steel member is heated again to a temperature equal to or higher than the austenitic transformation completion temperature and then the heated steel member is rapidly cooled; and a nitriding step in which, after the carburizing step and before the quenching step, nitrogen is introduced into the steel member in a state in which the steel member has been heated to a temperature below the austenitic transformation completion temperature .

In one aspect of the present invention, in the method for manufacturing a steel member, as described above, in the carburizing step, carbon is introduced into an austenitized steel member at a carbon concentration such that the steel member and carbon form a hypo-eutectoid composition. This prevents the carbon concentration introduced into the steel member from becoming excessively high, unlike when carburizing is performed on an austenitized steel member at a carbon concentration such that the steel member and carbon form a hyper-eutectoid composition. This prevents the hardness of the steel member from becoming excessively high, and also prevents the toughness of the steel member from becoming excessively low. As a result, the hardness and toughness of the steel member can be improved in a balanced manner. This improves bending fatigue strength and the like by improving hardness, and improves impact strength by improving toughness.

According to the present invention, as described above, it is possible to provide a method for manufacturing a steel member that can improve the hardness and toughness of the steel member in a well-balanced manner.

FIG. 2 is a diagram showing a manufacturing flow of a steel member according to an embodiment of the present invention. FIG. 4 is a diagram showing a temperature change of a steel member in a method for manufacturing a steel member according to an embodiment of the present invention. FIG. 1 is a state diagram of S20C as an example of a steel member. FIG. 10 is a diagram showing temperature changes of a steel member in a method for manufacturing a steel member according to Comparative Example 1. FIG. 10 is a diagram showing temperature changes of a steel member in a method for manufacturing a steel member according to Comparative Example 2. FIG. 1 is a diagram showing the bending fatigue strength and toughness of a steel member in a method for manufacturing a steel member according to one embodiment of the present invention, a steel member in a method for manufacturing a steel member according to the first variant, a steel member in a method for manufacturing a steel member according to the second variant, and a steel member in a method for manufacturing a steel member according to comparative example 1. FIG. 10 is a diagram showing the bending fatigue strength of a steel member produced by a method for producing a steel member according to an embodiment of the present invention, a steel member produced by a method for producing a steel member according to a first modified example, and a steel member produced by a method for producing a steel member according to comparative example 2. FIG. 10 is a diagram showing the torsional fatigue strength of a steel member produced by a method for producing a steel member according to an embodiment of the present invention, a steel member produced by a method for producing a steel member according to a first modified example, and a steel member produced by a method for producing a steel member according to comparative example 3. FIG. 1 is a diagram showing the surface hardness, surface hardened layer depth, surface carbon concentration, etc. of a steel member in a method for manufacturing a steel member according to an embodiment of the present invention, a steel member in a method for manufacturing a steel member according to a first modified example, and a steel member in a method for manufacturing a steel member according to comparative example 2. FIG. 10 is a diagram showing the grain boundary oxidation depth and the average crystal grain size of a steel member produced by a method for producing a steel member according to an embodiment of the present invention and a steel member produced by a method for producing a steel member according to Comparative Example 2.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[Method of manufacturing steel members]
A method for manufacturing a steel member according to an embodiment of the present invention will be described with reference to Figures 1 to 3. The method for manufacturing a steel member according to an embodiment of the present invention can be applied to steel members such as gears, bearings, and shafts.

(Cold forging process)
First, as shown in Fig. 1, a cold forging process is performed in step S1. The cold forging process (S1) is a process in which a steel member is forged at room temperature into a desired shape (for example, the shape of a gear, a bearing, a shaft, etc.). For the steel member, case-hardened steel (for example, SCM420) is used, the surface of which is subjected to heat treatment or the like to produce a hardened layer.

(carburizing process)
Next, in step S2, a carburizing process is performed. As shown in FIG. 2 , the carburizing process (S2) involves heating a steel member to a temperature T1 (e.g., approximately 1000°C) equal to or higher than the austenitic transformation completion temperature A3, infiltrating carbon into the austenitized steel member, and slowly cooling the carbon-infiltrated steel member. As shown in FIG. 3 , the austenitized steel member is a steel member made of austenite (γ-iron). The austenitic transformation completion temperature A3 is the temperature at which the steel member is heated to complete the austenitic transformation of the steel member. Specifically, as shown in FIG. 2 , the steel member is placed in a first heat treatment chamber. The steel member is then heated to temperature T1 so that the steel member is austenitized. Then, while the steel member is maintained at temperature T1 (i.e., the steel member is in an austenitized state), a carburizing gas (e.g., C ₂ H ₂ ) is introduced into the first heat treatment chamber. The carburizing gas then decomposes on the surface of the austenitized steel member, generating carbon. The generated carbon then diffuses from the surface of the steel member toward the interior, forming a carburized layer on the surface layer of the steel member. The steel member is then slowly cooled so that it transforms into pearlite. The slow cooling of the steel member is performed at a cooling rate that transforms the steel member into pearlite. In other words, the slow cooling of the steel member is performed at a cooling rate that does not transform the steel member into martensite. Furthermore, the slow cooling of the steel member is performed in an inert gas (e.g., Ar, _N2 , He, etc.) atmosphere to prevent the formation of an oxide film on the steel member. The carburizing step (S2) is performed while the interior of the first heat treatment chamber is decompressed by a vacuum pump.

As shown in Figure 3, when the carbon concentration of the steel member and carbon is less than about 0.77%, the steel member is composed of ferrite (α-iron) and pearlite at temperatures lower than the austenitic transformation start temperature A1. Also, when the carbon concentration is less than the carbon concentration at which the steel member and carbon form a hypo-eutectoid composition, the steel member is composed of austenite (γ-iron) and ferrite (α-iron) at temperatures higher than the austenitic transformation start temperature A1 and lower than the austenitic transformation completion temperature A3. Also, when the carbon concentration is less than the carbon concentration at which the steel member and carbon form a hypo-eutectoid composition, the steel member is composed of austenite (γ-iron) at temperatures higher than the austenitic transformation completion temperature A3.

As shown in FIG. 2 , in the carburizing step (S2), carbon is introduced into the austenitized steel member at a carbon concentration such that the steel member and carbon form a hypoeutectoid composition. Specifically, the carbon concentration of the surface (carburized layer) of the steel member increases as carburizing gas is continuously introduced into the first heat treatment chamber. The time for introducing the carburizing gas is then adjusted so that the carbon concentration of the surface (carburized layer) of the steel member is less than the carbon concentration at which the steel member and carbon form a eutectoid composition (e.g., approximately 0.77% when the steel member is case-hardened steel SCM420). Here, the hardness of a steel member is generally approximately proportional to the carbon concentration of the surface (carburized layer) of the steel member. Furthermore, the hardness of a steel member is generally approximately inversely proportional to the toughness of the steel member. This prevents the carbon concentration penetrating the steel member from becoming excessively high, unlike when carburizing is performed on an austenitized steel member at a carbon concentration (a concentration of about 0.77% or more) at which the steel member and carbon form a hyper-eutectoid composition. This prevents the hardness of the steel member from becoming excessively high, and also prevents the toughness of the steel member from becoming excessively low. As a result, the hardness and toughness of the steel member can be improved in a balanced manner. This improves bending fatigue strength and the like by improving hardness, and improves impact strength by improving toughness.

(Nitriding process)
Next, as shown in FIG. 1 , a nitriding step is performed in step S3. As shown in FIG. 2 , the nitriding step (S3) is a step of infiltrating nitrogen into a steel member in a state in which the steel member has been heated to a temperature T2 (e.g., about 810°C) that is lower than the austenitic transformation completion temperature A3. Specifically, the steel member is placed in a second heat treatment chamber. Then, the steel member in the pearlitic state in the carburizing step (S2) is heated to temperature T2. At this time, the steel member is in a state in which ferrite (α-iron) and austenite (γ-iron) are mixed. Then, while the steel member is maintained at temperature T2 (i.e., the steel member is in a state in which ferrite (α-iron) and austenite (γ-iron) are mixed), a nitriding gas (e.g., NH ₃ ) is introduced into the second heat treatment chamber at a predetermined rate (e.g., 0.8 m ³ /hour). The nitriding gas is decomposed on the surface of the steel member to generate nitrogen. The generated nitrogen then diffuses from the surface of the steel member toward the interior, causing the nitrogen to penetrate into the carburized layer at the surface layer of the steel member, forming a carbo-nitrided layer. The nitriding step (S3) is performed while adjusting the carbon concentration inside the second heat treatment chamber so as not to decrease the carbon concentration of the carburized layer. The amount of nitrogen penetrating into the surface layer of the steel member is generally inversely proportional to the temperature of the steel member at temperatures equal to or higher than the austenitic transformation start temperature A1 (described below). Furthermore, when nitrogen penetrates into the carburized layer at the surface layer of the steel member, the higher the carbon concentration of the carburized layer, the more likely voids are to be generated in the steel member. Furthermore, the higher the temperature of the steel member, the more likely a grain boundary oxide layer is to be formed in the steel member. Furthermore, the greater the amount of nitrogen penetrating into the surface layer of the steel member, the greater the hardness of the steel member. This allows a larger amount of nitrogen to penetrate into the steel member, and also makes it possible to suppress the generation of voids in the steel member and the formation of an intergranular oxide layer, compared to when nitrogen is penetrated into a steel member that has been heated to a temperature T3 (e.g., about 850°C) that is equal to or higher than the austenitic transformation completion temperature A3. As a result, the hardness of the steel member can be effectively improved, and therefore sufficient hardness of the steel member can be ensured even if the hardness of the steel member decreases by the amount that the toughness of the steel member is improved in the carburizing step (S2).

In the nitriding step (S3), the time for introducing the nitriding gas and the amount of the nitriding gas are adjusted so that the nitrogen concentration on the surface (carbonitrided layer) of the steel member reaches a predetermined concentration. That is, the nitriding step (S3) is a step of infiltrating nitrogen into the steel member so that the nitrogen concentration on the surface of the steel member reaches a predetermined concentration. The predetermined concentration is at least about 0.5% or less. The predetermined concentration is preferably about 0.05% or more and 0.35% or less. This allows nitriding to be performed so as to improve the hardness and toughness of the steel member in a balanced manner.

The nitriding step (S3) is a step of infiltrating nitrogen into the steel member at a temperature T2 (e.g., about 810°C) that is equal to or higher than the austenitic transformation start temperature A1 and lower than the austenitic transformation finish temperature A3. The austenitic transformation start temperature A1 is the temperature at which the austenitic transformation of the steel member starts. When nitriding is performed at a temperature lower than the austenitic transformation start temperature A1, the amount of nitrogen that penetrates into the steel member is generally significantly smaller than when nitriding is performed at a temperature higher than the austenitic transformation start temperature A1. This allows the amount of nitrogen that penetrates into the steel member to be greater than when nitriding is performed at a temperature lower than the austenitic transformation start temperature A1.

As described above, the cold forging process is performed in step S1. That is, the carburizing process (S2) is performed on the cold-forged steel member. In this way, the steel member is heated and slowly cooled in the carburizing process (S2), thereby removing residual stresses generated in the steel member by cold forging. As a result, coarsening of the grains of the steel member due to residual stresses can be suppressed in subsequent processes (such as the quenching process (S4) described later). That is, a decrease in the strength of the steel member due to coarsening of the grains can be suppressed. Furthermore, by suppressing grain coarsening, dimensional changes at each location of the steel member can be suppressed in subsequent processes. Furthermore, case-hardened steels (e.g., SCM420) commonly used for steel members can be used without using special steel members containing Nb, Ti, V, etc., to suppress grain coarsening in the cold-forged steel member.

(Quenching process)
Next, as shown in FIG. 1 , a quenching process is performed in step S4. As shown in FIG. 2 , the quenching process (S4) involves reheating the steel member to a temperature T3 (e.g., approximately 850°C) equal to or higher than the austenitic transformation completion temperature A3, and then quenching the heated steel member. Specifically, the steel member is placed in a second heat treatment chamber. The steel member is then reheated to temperature T3 so as to transform into austenite. After the steel member is maintained at temperature T3 (i.e., the austenitic state) for a period of time, the steel member is transported from the heat treatment chamber into a cooling device, where it is quenched so as to transform into martensite. The steel member is quenched at a cooling rate that transforms the steel member into martensite. In other words, the steel member is quenched at a cooling rate that does not transform the steel member into pearlite. The steel member is also quenched using water or oil.

The quenching step (S4) is a step of heating the steel member from a temperature T2 (e.g., about 810°C) below the austenitic transformation completion temperature A3 to a temperature T3 (e.g., about 850°C) equal to or higher than the austenitic transformation completion temperature A3 immediately after the nitriding step (S3), and then quenching the heated steel member. That is, the nitriding step (S3) and the quenching step (S4) are performed consecutively in this order. As a result, it is only necessary to heat the steel member, which has been heated to a temperature T2 (e.g., about 810°C) below the austenitic transformation completion temperature A3 in the nitriding step (S3), from temperature T2 (e.g., about 810°C) to a temperature T3 (e.g., about 850°C) equal to or higher than the austenitic transformation completion temperature A3. Therefore, energy consumption in heating the steel member can be reduced compared to when the quenching step (S4) is not performed immediately after the nitriding step (S3).

(Tempering process)
Next, as shown in Fig. 1, a tempering process is performed in step S5. The tempering process (S5) is a process of reheating and cooling the martensite-transformed steel member. As a result, the steel member, which has temporarily become excessively hard and excessively reduced in toughness due to the martensite transformation, is tempered to have appropriate fatigue strength and toughness.

[Example]
With reference to FIGS. 4 to 10, examples of the method for manufacturing a steel member according to the above embodiment will be described in comparison with methods for manufacturing a steel member according to Comparative Example 1 and Comparative Example 2.

(Manufacturing of steel members according to the above embodiment)
Based on the manufacturing method of the steel member according to the above embodiment, a steel member was fabricated as a gear (Example 1). Specifically, first, a cold forging process was performed on the steel member. The steel member was made of case-hardened steel SCM420. The steel member was then heated to a temperature T1 (1000°C) equal to or higher than the austenitic transformation completion temperature A3, and carbon was introduced into the austenitized steel member. The carburizing process involved slowly cooling the carbon-infused steel member. In the carburizing process, carbon was introduced into the austenitized steel member at a carbon concentration (less than 0.77%) that resulted in a hypoeutectoid composition between the steel member and carbon. The carburizing process was performed while the pressure inside the first heat treatment chamber was reduced using a vacuum pump. The steel member was then heated to a temperature T2 (810°C) lower than the austenitic transformation completion temperature A3, and a nitriding process was performed in which nitrogen was introduced into the steel member. Immediately after the nitriding step, the steel members were reheated to a temperature T3 (850°C) equal to or higher than the austenitic transformation completion temperature A3, and the heated steel members were rapidly cooled in a quenching step.The martensite-transformed steel members were then reheated and cooled in a tempering step.

(Manufacturing of steel members according to first and second modified examples)
As a first modification of the above embodiment, a steel member was produced by omitting the nitriding step from the method for producing a steel member according to the above embodiment (Example 2). Also, as a second modification of the above embodiment, a steel member was produced by omitting the nitriding step from the method for producing a steel member according to the above embodiment and replacing the case-hardened steel with a sintered material (Example 3).

(Manufacturing of another steel member according to the above embodiment and first modification)
A steel member was fabricated using the steel member as a shaft based on the manufacturing method of the steel member according to the above embodiment (Example 4). Also, a steel member was fabricated using the steel member as a shaft based on the manufacturing method of the steel member according to the first modified example (Example 5).

(Production of steel member according to comparative example 1)
Based on the steel member manufacturing method according to Comparative Example 1 shown in FIG. 4 , a steel member was fabricated as a gear. Specifically, a cold forging process was first performed, similar to the steel member manufacturing method according to the above embodiment. The steel member was made of case-hardened steel SCM420, similar to the steel member manufacturing method according to the above embodiment. Then, similar to the steel member manufacturing method according to the above embodiment, the steel member was heated to a temperature T1 (1000°C) equal to or higher than the austenitic transformation completion temperature A3 to infiltrate carbon into the austenitized steel member, and a carburizing process was performed to slowly cool the carbon-infiltrated steel member. Unlike the steel member manufacturing method according to the above embodiment, in the carburizing process, the steel member was maintained at a temperature T4 (710°C) lower than the austenitic transformation start temperature A1 for a predetermined time during the slow cooling so that only a portion of the austenite was transformed into pearlite. In other words, the time during which the steel member was maintained at temperature T4 (710°C) extended the high-temperature state (not at room temperature) compared to the steel member manufacturing method according to the above embodiment. Furthermore, unlike the method for manufacturing a steel member according to the above embodiment, in the carburizing step, carbon was introduced into the austenitized steel member at a carbon concentration (0.77% or higher) such that the steel member and carbon constituted a hypereutectoid composition. The carburizing step was carried out while the interior of the heat treatment chamber was depressurized using a vacuum pump, as in the method for manufacturing a steel member according to the above embodiment. The steel member was then heated to a temperature T3 (850°C) equal to or higher than the austenitization transformation completion temperature A3. After the steel member was maintained at temperature T3 (850°C) for a period of time, a quenching step was carried out to rapidly cool the steel member. During the quenching step, a nitriding step was also carried out to introduce nitrogen into the steel member while the steel member was maintained at temperature T3 (850°C) for a period of time (i.e., the austenitized state of the steel member). Then, a tempering step was carried out, as in the method for manufacturing a steel member according to the above embodiment.

(Production of steel member according to comparative example 2)
Based on the steel member manufacturing method according to Comparative Example 2 shown in FIG. 5 , a steel member was fabricated as a gear. Specifically, a cold forging process was first performed, similar to the steel member manufacturing method according to the above embodiment. The steel member was made of case-hardened steel SCM420, similar to the steel member manufacturing method according to the above embodiment. Then, similar to the steel member manufacturing method according to the above embodiment, the steel member was heated to a temperature T1 (1000°C) equal to or higher than the austenitic transformation completion temperature A3 to infiltrate carbon into the austenitized steel member, and the carburizing process was performed to slowly cool the carbon-infiltrated steel member. The slow cooling in the carburizing process was performed until the temperature of the steel member reached a temperature T3 (850°C) for the quenching process. Unlike the steel member manufacturing method according to the above embodiment, the carburizing process involved infiltrating carbon into the austenitized steel member at a carbon concentration (0.77%) such that the steel member and carbon form a eutectoid composition. Unlike the steel member manufacturing method according to the above embodiment, the carburizing process was performed without reducing the pressure inside the heat treatment chamber. Then, after the steel member was maintained at a temperature T3 (850°C) equal to or higher than the austenitic transformation completion temperature A3 for a while, a quenching step was performed to rapidly cool the steel member. That is, unlike the method for manufacturing a steel member according to the above embodiment, the steel member was not pearlitized in the carburizing step , and the quenching step was performed immediately after the carburizing step. Note that, unlike the method for manufacturing a steel member according to the above embodiment, a nitriding step was not performed. Then, a tempering step was performed in the same manner as in the method for manufacturing a steel member according to the above embodiment.

(Production of steel member according to Comparative Example 3)
As Comparative Example 3, a steel member was produced by the method for producing a steel member according to Comparative Example 2, except that the gear was replaced with a shaft.

(Test results of steel member performance)
Below, we will explain the test results of the performance of steel members manufactured using the manufacturing method for steel members according to the above embodiments (steel members of Example 1 and Example 4), steel members manufactured using the manufacturing method for steel members according to the above first modified example (steel members of Example 2 and Example 5), steel members manufactured using the manufacturing method for steel members according to the above second modified example (steel members of Example 3), steel members manufactured using the manufacturing method for steel members according to Comparative Example 1 (steel members of Comparative Example 1), steel members manufactured using the manufacturing method for steel members according to Comparative Example 2 (steel members of Comparative Example 2), and steel members manufactured using the manufacturing method for steel members according to Comparative Example 3 (steel members of Comparative Example 3).

As shown in Figure 6, the crack initiation energy Ei (J/cm 2 ) during impact testing and bending fatigue strength σ (MPa) of the steel member of Example 1 (SCM 420 (with nitriding) in the figure), the steel member of Example ² (SCM 420 (without nitriding) in the figure), and the steel member of Example 3 (sintered material (without nitriding) in the figure) were all greater than those of the steel member of Comparative Example 1 (SCM420 (Comparative Example 1) in the figure). Specifically, the crack initiation energy Ei of the steel member of Example 1 was greater than that of the steel member of Comparative Example 1. The crack initiation energy Ei of the steel member of Example 2 was greater than that of the steel member of Example 1. The crack initiation energy Ei of the steel member of Example 3 was greater than that of the steel member of Example 2. The bending fatigue strength σ of the steel member of Example 3 was greater than that of the steel member of Comparative Example 1. The bending fatigue strength σ of the steel member of Example 2 was greater than the bending fatigue strength σ of the steel member of Example 3. The bending fatigue strength σ of the steel member of Example 1 was greater than the bending fatigue strength σ of the steel member of Example 2. The magnitude of the crack initiation energy Ei during the impact test is approximately proportional to the magnitude of toughness. Furthermore, the bending fatigue strength σ means the magnitude of strength against bending. Generally, the bending fatigue strength σ is approximately proportional to hardness. The crack initiation energy Ei during the impact test was measured using a Charpy impact test. Furthermore, the bending fatigue strength σ was measured using an Ono-type rotating bending fatigue test.

As shown in FIG. 7, the steel member of Example 1 (SCM 420 (with nitriding) in the figure) and the steel member of Example 2 (SCM 420 (without nitriding) in the figure) both had a bending fatigue strength σ (MPa) greater than that of the steel member of Comparative Example 2 (SCM420 (Comparative Example 2) in the figure).

As shown in Figure 8, the steel member of Example 4 (SCM 420 (with nitriding) in the figure) and the steel member of Example 5 (SCM 420 (without nitriding) in the figure) both had a torque amplitude Ta (N m) greater than that of the steel member of Comparative Example 3 (SCM 420 (Comparative Example 3) in the figure). The torque amplitude Ta is an index representing torsional fatigue strength. The torque amplitude Ta was measured using a torsional fatigue test.

As shown in Figure 9, the surface hardness of the steel member of Comparative Example 2 (SCM420 (Comparative Example 2) in the figure) was 760 HV, whereas the surface hardness of the steel member of Example 1 ( SCM420 (with nitriding) in the figure) was 770 HV, and the surface hardness of the steel member of Example 2 ( SCM420 (without nitriding) in the figure) was 771 HV. In other words, the steel members of Examples 1 and 2 had greater surface hardness than the steel member of Comparative Example 2. The surface hardness was measured using a Vickers hardness test in accordance with JIS Z 2244.

The surface-hardened layer depth of the steel member of Comparative Example 2 (SCM420 (Comparative Example 2) in the figure) was 0.77 mm, whereas the surface-hardened layer depth of the steel member of Example 1 ( SCM420 (nitrided) in the figure) was 0.82 mm, and the surface-hardened layer depth of the steel member of Example 2 ( SCM420 (no nitriding) in the figure) was 0.80 mm. That is, the surface-hardened layer depths of the steel members of Examples 1 and 2 were greater than that of the steel member of Comparative Example 2.

The surface carbon concentration of the steel member of Comparative Example 2 (SCM420 (Comparative Example 2) in the figure) was 0.70 wt%, whereas the surface carbon concentration of the steel member of Example 1 ( SCM420 (nitrided) in the figure) was 0.54 wt%, and the surface carbon concentration of the steel member of Example 2 ( SCM420 (no nitriding) in the figure) was 0.57 wt%. In other words, the surface carbon concentrations of the steel members of Examples 1 and 2 were significantly lower than that of the steel member of Comparative Example 2. This is thought to be because, in the carburizing process, for the steel member of Comparative Example 2, carbon was introduced into the austenitized steel member at a carbon concentration (0.77%) such that the steel member and carbon form a eutectoid composition, whereas, in the carburizing process, for the steel members of Examples 1 and 2, carbon was introduced into the austenitized steel member at a carbon concentration (less than 0.77%) such that the steel member and carbon form a hypoeutectoid composition.

The total carburized depth of the steel member of Example 1 (SCM 420 (with nitriding) in the figure), the total carburized depth of the steel member of Example 2 (SCM 420 (without nitriding) in the figure), and the total carburized depth of the steel member of Comparative Example 2 (SCM420 (Comparative Example 2) in the figure) were all 1.4 mm. This is thought to be because the steel members of Example 1, Example 2, and Comparative Example 2 were all steel members that had been heated to a temperature T1 (1000°C) and austenitized, and then carbon was allowed to penetrate into them.

The surface nitrogen concentration of the steel member of Example 1 (SCM 420 (nitrided) in the figure) was 0.34 wt %. That is, the surface nitrogen concentration of the steel member of Example 1 was lower than the surface carbon concentration. Furthermore, the total nitriding depth of the steel member of Example 1 (SCM 420 (nitrided) in the figure) was 0.3 mm. That is, the total nitriding depth of the steel member of Example 1 was smaller than the total carburization depth.

As shown in Fig. 10, the grain boundary oxidation depth of the steel member of Comparative Example 2 (SCM420 (Comparative Example 2) in the figure) was 14 µm, whereas the grain boundary oxidation depth of the steel member of Example 1 ( SCM420 (nitrided) in the figure) was 5 µm. That is, the grain boundary oxidation depth of the steel member of Example 1 was smaller than that of the steel member of Comparative Example 2. Therefore, the formation of a grain boundary oxide layer during the manufacturing process of the steel member of Example 1 was significantly suppressed.

The minimum value of the crystal grain size of the steel member of Example 1 (MIN in the figure) and the minimum value of the crystal grain size of the steel member of Comparative Example 2 were both 10 μm. On the other hand, the maximum value of the crystal grain size of the steel member of Comparative Example 2 (MAX in the figure) was 170 μm, while the maximum value of the crystal grain size of the steel member of Example 1 was 22 μm. Furthermore, the average crystal grain size of the steel member of Comparative Example 2 was 38 μm, while the average crystal grain size of the steel member of Example 1 was 14 μm. In other words, the average crystal grain size of the steel member of Example 1 was smaller than that of the steel member of Comparative Example 2. Therefore, the steel member of Example 1 was significantly prevented from experiencing coarsening of the crystal grain size during the manufacturing process.

As described above, it was shown that the steel members of the examples (Examples 1 to 5) had relatively high toughness, bending fatigue strength, surface hardness, surface-hardened layer depth, etc. Therefore, the method for manufacturing a steel member according to the above embodiment, the method for manufacturing a steel member according to the first modified example of the above embodiment, and the method for manufacturing a steel member according to the second modified example of the above embodiment are suitable as methods for manufacturing steel members (for example, gears, bearings, shafts, etc.) that require both high hardness and high toughness.

[Modification]
It should be noted that the embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is defined by the claims, not by the description of the above embodiments, and further includes all modifications (variations) within the meaning and scope of the claims.

For example, in the above embodiment, an example was shown in which the carburizing step (S2) was performed on a cold-forged steel member, but the present invention is not limited to this. In the present invention, the carburizing step may be performed on a hot-forged steel member.

In the above embodiment, the nitriding step (S3) is a step of infiltrating nitrogen into the steel member at a temperature T2 that is equal to or higher than the austenitic transformation start temperature A1 and lower than the austenitic transformation finish temperature A3, but the present invention is not limited to this. In the present invention, the nitriding step may be a step of infiltrating nitrogen into the steel member at a temperature lower than the austenitic transformation start temperature.

Furthermore, in the above embodiment, an example was shown in which the nitriding step (S3) is a step of infiltrating nitrogen into a steel member in a state in which the steel member has been heated to a temperature T2 that is lower than the austenitic transformation completion temperature A3, but the present invention is not limited to this. In the present invention, as in Comparative Example 1, the nitriding step may be a step of infiltrating nitrogen into a steel member in a state in which the steel member has been heated to a temperature equal to or higher than the austenitic transformation completion temperature. In this case, as in Comparative Example 1, the nitriding step may be performed simultaneously with the heating in the quenching step.

In the above embodiment, an example was shown in which the nitriding step (S3) of infiltrating nitrogen into the steel members was performed, but the present invention is not limited to this. In the present invention, the nitriding step of infiltrating nitrogen into the steel members does not necessarily have to be performed, as in the first and second modifications described above.

In the above embodiment, the steel members are made of case-hardened steel, but the present invention is not limited to this. In the present invention, the steel members may be made of sintered material, as in the second modified example described above.

Austenitic transformation start temperature... A1, austenitic transformation completion temperature... A3, temperature (above austenitic transformation completion temperature)... T1, T3, temperature (below austenitic transformation completion temperature)... T2

Claims (5)

a carburizing step of heating a steel member to a temperature equal to or higher than an austenitic transformation completion temperature, infiltrating carbon into the steel member in an austenitized state at a carbon concentration such that the steel member and carbon form a hypoeutectoid composition, and slowly cooling the steel member into which the carbon has been infiltrated;
a quenching step of reheating the steel member to a temperature equal to or higher than the austenitizing transformation completion temperature after the carburizing step and quenching the heated steel member;
a nitriding step of infiltrating nitrogen into the steel member in a state in which the steel member is heated to a temperature lower than the austenitizing transformation completion temperature, after the carburizing step and before the quenching step. The method for manufacturing a steel member according to claim 1, wherein the quenching step is a step of heating the steel member from a temperature below the austenitizing transformation completion temperature to a temperature equal to or higher than the austenitizing transformation completion temperature immediately after the nitriding step, and then quenching the heated steel member. The method for manufacturing a steel member according to claim 1, wherein the nitriding process is a process of infiltrating nitrogen into the steel member at a temperature equal to or higher than the austenitic transformation start temperature and lower than the austenitic transformation completion temperature. The method for manufacturing a steel member according to claim 1, wherein the nitriding process is a process of infiltrating nitrogen into the steel member so that the nitrogen concentration on the surface of the steel member is 0.5% or less. The method for manufacturing a steel component according to claim 1, wherein the carburizing process is performed on the cold-forged steel component.