BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The present invention relates to a method of manufacturing a ferrous metal component in which a metal surface is hardened using a carburizing treatment and particularly to a technique in which a brittle grain boundary oxidation layer formed on the metal surface is suitably reduced during the carburizing treatment.

2. Description of Related Art

[0002] For example, in a ferrous metal component such as a steel component containing Fe as a major component, a technique of embedding carbon into a metal surface through, for example, a gas carburizing treatment and then enhancing the hardness of the metal surface by quenching is known. For example, Japanese Patent Application Publication No. 05-171348 (JP 05-171348 A) discloses such a ferrous metal component.

[0003] However, in JP 05-171348 A, it is known that oxygen contained in carburizing gas during the gas carburizing treatment infiltrates into a grain boundary of a material surface of the ferrous metal component and forms a brittle grain boundary oxidation layer by binding to an element such as Si, Mn, or Cr contained in the material surface. To deal with this, in JP 05-171348 A, the formation of the grain boundary oxidation layer is reduced by reducing the content of Si, Mn, or Cr contained in the ferrous metal component. SUMMARY OF THE INVENTION

[0004] The ferrous metal component disclosed in JP 05-171348 A has a limit in reducing the content of the element such as Si, Mn, or Cr and thus has a problem in that the fatigue strength of the ferrous metal component decreases due to the grain boundary oxidation layer formed by the element such as Si, Mn, or Cr contained in the surface of the ferrous metal component.

[0005] The invention provides a method of manufacturing a ferrous metal component in which the fatigue strength can be improved by suitably reducing a grain boundary oxidation layer which is formed during a carburizing treatment.

[0006] As a result of various kinds of analysis and investigation, the present inventors have found the following facts. That is, it was found that an element such as Mn, Si, or Cr is evaporated, that is, an element removal phenomenon occurs from a surface of a workpiece formed of a ferrous metal material under a condition of a higher temperature and a lower pressure than in a carburizing treatment. In addition, typically, this element removal phenomenon has a negative image as in the case of "decarburization". However, it was found that, conversely, by causing this element removal phenomenon to occur before a carburizing treatment on the basis of the technical knowledge relating to carburizing, the formation of a grain boundary oxidation layer can be suitably inhibited in a subsequent carburizing treatment. The invention has been made based on the above-described findings.

[0007] A method of manufacturing a ferrous metal component according to an aspect of the invention includes: performing an element removal treatment on a workpiece formed of a ferrous metal material; and performing a surface hardening treatment on the workpiece through a carburizmg treatment after the element removal treatment. In this method, the element removal treatment is performed under a condition of a higher temperature and a lower pressure than in the carburizing treatment.

[0008] In the method of manufacturing a ferrous metal component according to the aspect, the element removal treatment is performed under a condition of a higher temperature and a lower pressure than in the carburizing treatment. Therefore, before the carburizmg treatment, an element which causes an oxide to be formed during the carburizing treatment is evaporated from the surface of the workpiece. Accordingly, a grain boundary oxidation layer which is formed on the surface of the workpiece during the carburizing treatment can be suitably removed, and the fatigue strength of the ferrous metal component can be improved. [0009] According to the aspect, in the element removal treatment, before the carburizing treatment, an element which forms an oxide on a surface of the workpiece during the carburizing treatment may be evaporated from the surface of the workpiece. Accordingly, a grain boundary oxidation layer which is formed on the surface of the workpiece during the carburizing treatment can be suitably removed.

[0010] According to the aspect, in the element removal treatment, the element may be evaporated from the surface of the workpiece in a vacuum. Accordingly, a grain boundary oxidation layer which is formed on the surface of the workpiece during the carburizing treatment can be suitably removed.

[0011] According to the aspect, the element may be at least one of Mn, Si, and Cr.

Therefore, in the element removal treatment, at least one of Mn Si, and Sr having a relatively high vapor pressure is evaporated from the surface of the workpiece. Accordingly, a grain boundary oxidation layer which is formed on the surface of the workpiece during the carburizing treatment can be suitably removed.

[0012] According to the aspect, after the element removal treatment, the higher temperature of the element removal treatment may be decreased to a temperature of the carburizing treatment to perform the carburizing treatment. Accordingly, after the element removal treatment, the carburizing treatment can be continuously performed suitably.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG 1 is a diagram illustrating a shaft which is manufactured using a method of manufacturing a ferrous metal component according to the invention and illustrating a configuration of an apparatus used for manufacturing the shaft;

FIG 2 is a flow chart illustrating manufacturing processes of the shaft of FIG 1; FIG. 3 is a diagram illustrating temperature conditions and pressure conditions in a desiliconizing and demanganizing process and a gas carburizing process illustrated in FIG 2;

FIG 4 is a diagram illustrating the results of Experiment I in which, by using test pieces formed of the same material as the shaft of FIG 1, the contents (mass ) of Si, Mn, and Cr contained in surfaces of the test pieces were measured under different conditions of a temperature (°C), a pressure (Pa), and a holding time (min) in the desiliconizing and demanganizing process of FIG 2;

FIG 5 is a diagram illustrating the contents of Mn and Si contained in surfaces of test pieces formed of the same material as the shaft of FIG 1, the test pieces including a test piece (a shaft according to Example 1) which was manufactured through the desiliconizing and demanganizing process and the gas carburizing process of FIG 2 and a test piece (a shaft according to Comparative Example 1) which was manufactured through only the gas carburizing process of FIG 2;

FIG. 6' is a diagram illustrating the thicknesses of grain boundary oxidation layers which were formed in the test piece (the shaft according to Example 1) and the test piece (the shaft according to Comparative Example 1) of FIG 5;

FIG 7 is a diagram illustrating a part of the surface of the test piece (the shaft according to Comparative Example 1) of FIG 5;

FIG 8 is a diagram illustrating a part of the surface of the test piece (the shaft according to Example 1) of FIG 5;

FIG 9 is a diagram illustrating the fatigue strengths of the test piece (the shaft according to Example 1) and the test piece (the shaft according to Comparative Example 1) of FIG 5; and

FIG 10 is a diagram corresponding to FIG 1 illustrating a method of manufacturing a ferrous metal component according to another example of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0014] Hereinafter, Example 1 of the invention will be described with reference to the drawings. In the drawings of the following Example 1, each part is appropriately simplified and modified, and the dimension, shape, and the like thereof may be not accurately illustrated.

[0015] FIG 1 is a diagram illustrating a ferrous metal component to which the invention is suitably applied, that is, a shaft 10 which is included in, for example, a belt-type continuously variable transmission (CVT) used in a vehicle. The shaft 10 is formed of a ferrous metal material containing Fe as a major component, for example, formed of low carbon steel such as steel or case-hardened steel having a C content of 0.02% to 2.14% (wt%). A surface of the shaft 10 is carburized by a gas carburizing apparatus 12 of FIG. 1; as a result, the surface is hardened.

[0016] Here, the gas carburizing apparatus 12 will be described. As illustrated in FIG 1, the gas carburizing apparatus 12 includes: a heat treatment chamber 16 that is formed of a heat insulating material 14 and accommodates the shaft 10; a jig 18 that fixes and supports the shaft 10 in the heat treatment chamber 16; a heater 20 that heats the inside of the heat treatment chamber 16; a mass flow controller 24 that measures and controls a flow rate of nitrogen gas flowing from a supply device 22, which supplies, for example, nitrogen gas, into the heat treatment chamber 16; and a pressure-reducing pump 26 that evacuates the inside of the heat treatment chamber 16 to reduce an internal pressure of the heat treatment chamber 16. Therefore, in the gas carburizing apparatus 12, the shaft 10 can be held in the heat treatment chamber 16 under a condition of a relatively high temperature and a relatively low pressure by the heater 20 and the pressure-reducing pump 26 which are included in the gas carburizing apparatus 12. In addition, the temperature of the shaft 10 is decreased by nitrogen gas which is a cooling gas supplied from the supply device 22, and thus the shaft 10 is cooled. In addition, the gas carburizing apparatus 12 is provided with a carburizing gas supply device (not illustrated) which supplies carburizing gas into the heat treatment chamber 16. During a gas carburizing treatment, the carburizing gas is supplied from the carburizing gas supply device continuously. The carburizing gas is prepared by, for example, mixing a source gas such as propane gas, town gas, natural gas, or charcoal gas with air at a predetermined ratio and heating the mixture to be decomposed. [0017] In addition, here, a method of manufacturing the shaft 10 according to the Example 1, that is, manufacturing processes PI to P5 will be described using FIG 2.

[0018] As illustrated in FIG 2, first, in a forging process PI, a workpiece formed of a ferrous metal material (steel material) of, for example, SCR420 which is case-hardened steel is formed into a predetermined shape by, for example, forging.

[0019] Next, in a preheating (annealing) process P2, the workpiece formed in the forging process PI is annealed to be softened.

[0020] Next, in a machining process P3, the workpiece softened in the preheating process P2 is cut into the same shape as the shaft 10 by machining.

[0021] Next, in a desiliconizing and demanganizing (element removal) process P4, the shaft 10 which is the workpiece cut in the machining process P3 is arranged in the gas carburizing apparatus 12 and is held under a condition of a higher temperature and a lower pressure than in a gas carburizing process (carburizing process) P5 described below, for example, under a condition of an internal temperature T (°C) of the heat treatment chamber 16 of 1000°C to 1300°C and a vacuum, that is, an internal pressure P (Pa) of the heat treatment chamber 16 of 100 Pa to 1000 Pa for a predetermined time t (min) of, for example, 5 minutes to 30 minutes. As a result, an element such as Mn, Si, or Cr having a relatively high vapor pressure which is contained in the surface of the shaft 10 is evaporated. In the desiliconizing and demanganizing process P4, a vacuum represents a pressure being sufficiently lower than the atmospheric pressure, for example, about 100 Pa to 1000 Pa. The pressure P (100 Pa to 1000 Pa) of the desiliconizing and demanganizing process P4 is sufficiently lower than a pressure condition (higher than 1 kPa and 10 kPa or lower) of, for example, a vacuum carburizing treatment of the related art.

[0022] Next, in the gas carburizing process P5, carbon is embedded into the surface of the shaft 10 having the surface, from which the element such as Mn, Si, or Cr is evaporated in the desiliconizing and demanganizing process P4, by carburizing gas at a gas carburizing temperature of about 930° C as illustrated in FIG 3. Next, the carburized shaft 10 is rapidly cooled and quenched. As a result, the shaft 10 in which the fatigue strength is improved by the surface being hardened is manufactured. In the gas carburizing apparatus 12, as illustrated in FIG 3, after the desiliconizing and demanganizing process P4, a temperature is decreased to, a gas carburizing temperature of for example, about 930°C to perform the gas carburizing process P5. In addition, the gas carburizing process P5 is performed under an internal pressure of the heat treatment chamber 16 of about 1.0x10 ^s Pa, that is, the atmospheric pressure as illustrated in FIG. 3.

[0023] The gas carburizing apparatus 12 includes a mechanism of holding the inside of the heat treatment chamber 16 at a high temperature and a low pressure (vacuum) before carburizing, in addition to a mechanism of carburizing and quenching the shaft 10. Therefore, when the manufacturing processes PI to P5 of the Example 1 are performed, that is, when the desiliconizing and demanganizing process P4 and the gas carburizing process P5 are performed, it is not necessary that a new device which holds the shaft 10 under a condition of a high temperature and a low pressure, for example, in the desiliconizing and demanganizing process P4 be added in addition to a gas carburizing apparatus of the related art which carburizes and quenches the shaft 10. Therefore, the manufacturing cost can be significantly reduced.

[0024] [Experiment I]

Here, Experiment I which was performed by the present inventors will be described. Experiment I was performed in order to verify the fact that the amounts of Si, Mn, and Cr evaporated from the surface of the shaft 10 can be suitably increased, that is, the contents of Si, Mn, and Cr contained in the surface of the shaft 10 can be suitably reduced by changing conditions of the temperature T (°C), the pressure P (Pa), and the holding time t (min) in the desiliconizing and demanganizing process P4.

[0025] In Experiment I, the desiliconizing and demanganizing process P4 was performed under 16 kinds of conditions, that is, under Condition 1 to Condition 16, in which: test pieces formed of the same material as the shaft 10, that is, SCR 420 and having a predetermined shape (for example, φ 18 mmx 50 mm) were used; the temperature T (°C) was changed in a range of 1000°C to 1300°C, that is, was 1000°C, 1100°C, 1200°C, or 1300°C as illustrated in FIG 4; the pressure P (Pa) was changed in a range of 100 Pa to 1000 Pa, that is, was 100 Pa, 200 Pa, 500 Pa, or 1000 Pa; and the holding time t (min) was changed in a range of 5 minutes to 30 minutes, that is, was 5 minutes, 10 minutes, 15 minutes, or 30 minutes. The contents of Si, Mn, and Cr in the surfaces of the test pieces corresponding to the shafts 10 on which the desiliconizing and demanganizing process P4 was performed under Condition 1 to Condition 16 were measured.

[0026] In Experiment I, as illustrated in FIG 5, the sum (lOSi+Mn+Cr) of 10 times of the Si content (mass ), the Mn content (mass%), and the Cr content (mass%) per unit mass at a depth of 6 μιη from the surface of the test piece corresponding to the shaft 10 is represented by the content y (mass%) of Si, Mn, and Cr in the surface of the test piece corresponding to the shaft 10. In addition, the content (mass%) of Si, Mn, and Cr per unit mass at a depth of 6 μπι from the surface of the test piece was measured by glow discharge optical emission spectroscopy.

[0027] Hereinafter, the results of Experiment I will be described using FIG 4. As illustrated in FIG 4, the content y (mass%) of Si, Mn, and Cr of the test piece was 2 (mass ) or less under Condition 8, Condition 9, and Condition 13, which was relatively small. Therefore, it is considered that, by performing the desiliconizing and demanganizing process P4 under Condition 8, Condition 9, and Condition 13, the content y (mass%) of Si, Mn, and Cr contained in the surface of the shaft 10 can be suitably reduced.

[0028] In addition, by performing multiple regression analysis using the experimental results of Condition 1 to Condition 16 illustrated in FIG 4, a relational expression (1) between the temperature T (°C), the pressure P (Pa), and the holding time t (min) in the desiliconizing and demanganizing process P4 and the content y (mass%) of Si, Mn, and Cr in the surface of the test piece corresponding to the shaft 10 is obtained, y (mass%)=-0.0018xT (°C)+0.0001xP (Pa)-0.0242xt (min)+6.47677 ... (1)

[0029] It can be considered from the relational expression (1) that, in the desiliconizing and demanganizing process P4, the element such as Si, Mn, or Cr is suitably evaporated from the surface of the shaft 10 by increasing the temperature T (°C), the element such as Si, Mn, or Cr is suitably evaporated from the surface of the shaft 10 by reducing the pressure P (Pa), and the element such as Si, Mn, or Cr is suitably evaporated from the surface of the shaft 10 by increasing the holding time t (min). Typically, when the content y (mass%) of Si, Mn, and Cr in the surface of the shaft 10 is 2 (mass%) or less, the thickness of a grain boundary oxidation layer A (refer to FIG 7) which is formed on the surface by the carburizing treatment can be inhibited to be 6.0 um or less, and thus a decrease in fatigue strength can be inhibited. Therefore, it is considered that, for example, by setting the temperature T (°C), the pressure P (Pa), and the holding time t (min) in the desiliconizing and demanganizing process P4 such that the content y (mass ) of Si, Mn, and Cr in the surface of the shaft 10 is 2 (mass%) or less, the grain boundary oxidation layer A which is formed on the surface of the shaft 10 in the gas carburizing process P5 can be suitably inhibited.

[0030] [Experiment IT)

Here, Experiment II which was performed by the present inventors will be described. Experiment Π was performed in order to verify the effect of the desiliconizing and demanganizing process P4 on the shaft 10 in the manufacturing processes PI to P5 of FIG 2, that is, the effect of the desiliconizing and demanganizing process P4 on the grain boundary oxidation layer A which is formed on the surface of the shaft 10. In Experiment II, the effect of the grain boundary oxidation layer A, which is formed on the shaft 10, on the fatigue strength of the shaft 10 was also verified.

[0031] In Experiment II, the thicknesses (μπι) of the grain boundary oxidation layers A, which were formed on test pieces formed of the same material as the shaft 10, that is, formed of SCR420 and having a predetermined shape (for example, φ 18 mmx 50 mm), were measured, the test pieces including: a test piece (desiliconizing and demanganizing+gas carburizing) corresponding to the shaft 10 according to Example 1 on which the desiliconizing and demanganizing process P4 and the gas carburizing process P5 were performed; and a test piece (only gas carburizing) corresponding to the shaft 10 according to Comparative Example 1 on which only the gas carburizing process P5 was performed without performing the desiliconizing and demanganizing process P4. In addition, the fatigue strengths, that is, the nominal stresses σ (MPa) of the test piece corresponding to the shaft 10 according Example 1 and the test piece corresponding to the shaft 10 according to Comparative Example 1 were measured. In the desiliconizing and demanganizing process P4, the element removal treatment was performed under, for example, the above-described Condition 8. In addition, in Experiment Π, a test piece formed of the same material as the shaft 10, that is SCR 420 and having a predetermined shape (for example, φ 18 mmx 50 mm) was prepared, only the gas carburizing process P5 was performed thereon without performing desiliconizing and demanganizing process P4, and a finishing process of removing the surface of the test piece, that is, the grain boundary oxidation layer A by machining was performed thereon. As a result, a test piece (gas carburizmg+finishing) corresponding to the shaft 10 according to Comparative Example 2 was prepared. Using this test piece corresponding to the shaft 10 according to Comparative Example 2, the fatigue strength was measured.

[0032] Hereinafter, the results of Experiment II will be described using FIGS. 5 and 9. As illustrated in FIG 5, in the test piece (the shaft 10 according to Example 1), the contents (mass%) of Si and Mn contained in the surface of the test piece were suitably reduced as compared to the test piece (the shaft 10 according to Comparative Example 1). FIG 5 illustrates the results of the above-described measurement using glow discharge optical emission spectroscopy.

[0033] In addition, as illustrated in FIG 6, the thickness of the grain boundary oxidation layer A which was formed in the test piece (the shaft 10 according to Example 1) was 4 um, and the thickness of the grain boundary oxidation layer A which was formed in the test piece (the shaft 10 according to Comparative Example 1) was 20 um. In the test piece (the shaft 10 according to Example 1), the grain boundary oxidation layer A was suitably reduced as compared to the test piece (the shaft 10 according to Comparative Example 1). The measured values of the thicknesses of the grain boundary oxidation layers A of FIG 6 were obtained by measuring, for example, the grain boundary oxidation layers of the surface of the test piece (the shaft 10 according to Example 1) and the surface of the test piece (the shaft 10 according to Comparative Example 1) illustrated in FIGS. 7 and 8 using an optical microscope. The thickness of the grain boundary oxidation layer A is defined as the depth from the surface of the test piece at which a grain boundary is observed. In FIG. 8, the grain boundary oxidation layer A was not observed. [0034] In addition, as illustrated in FIG 9, when the number of repetitions Nf was about 10 ⁷ , the nominal stress σ of the test piece (the shaft 10 according to Example 1) was about 580 MPa, the nominal stress σ of the test piece (the shaft 10 according to Comparative Example 2) was about 575 MPa, and the nominal stress σ of the test piece (the shaft 10 according to Comparative Example 1) was about 515 MPa. Therefore, the fatigue strength of the test piece corresponding to the shaft 10 according to Example 1 was suitably higher than that of the test piece corresponding to the shaft 10 according to Comparative Example 1. The measurement results of FIG 9 were obtained using, for example, the Ono-type rotary bending fatigue test apparatus.

[0035] According to the results of Experiment II, as illustrated in the measurement results of FIG. 5, in the test piece corresponding to the shaft 10 according to Example 1 on which the desiliconizing and demanganizing process P4 was performed, the contents (mass%) of Si and Mn contained in the surface of the test piece was suitably reduced as compared to the test piece corresponding to the shaft 10 according to Comparative Example 1 on which the desiliconizing and demanganizing process P4 was not performed. Therefore, it is considered that Si and Mn, which cause oxides (SiO, MnO) to be formed on the surface of the shaft 10 in the gas carburizing process P5, was evaporated from the surface of the shaft 10 through the desiliconizing and demanganizing process P4.

[0036] In addition, according to the results of Experiment Π, as illustrated in the measurement results of FIGS. 5 and 6, in the test piece corresponding to the shaft 10 according to Example 1 on which the desiliconizing and demanganizing process P4 was performed, the contents of Si and Mn contained in the surface of the test piece was suitably reduced, and the thickness of the grain boundary oxidation layer A formed in the test piece was suitably reduced, as compared to the test piece corresponding to the shaft 10 according to Comparative Example 1 on which the desiliconizing and demanganizing process P4 was not performed. Therefore, it is considered that, by evaporating Si and Mn from the surface of the shaft 10 in the desiliconizing and demanganizing process P4, the contents of Si and Mn in the surface of the shaft 10 were suitably reduced, the amount of O, which was contained in carburizing gas, binding to Si and Mn in the subsequent gas carburizing process P5 was reduced, and thus the grain boundary oxidation layer A formed on the shaft 10 was reduced.

[0037] In addition, according to the results of Experiment Π, as illustrated in measurement results of FIGS. 6 and 9, the fatigue strength of the test piece corresponding to the shaft 10 according to Example 1 in which the thickness of the grain boundary oxidation layer A was relatively thin (4 μπι) was higher than that of the test piece corresponding to the shaft 10 according to Comparative Example 1 in which the thickness of the grain boundary oxidation layer A was relatively thick (20 um). In addition, the fatigue strength of the test piece corresponding to the shaft 10 according to Comparative Example 2 in which the desiliconizing and demanganizing process P4 was not performed and the grain boundary oxidation layer A was removed by the finishing process was higher than that of the test piece corresponding to the shaft 10 according to Comparative Example 1. Therefore, it is considered that, by reducing the thickness of the grain boundary oxidation layer A formed on the shaft 10, the fatigue strength of the shaft 10 was improved. In addition, it is considered that, in the manufacturing processes of the shaft 10 according to Example 1, in the finishing process, which was performed on the test piece corresponding to the shaft 10 according to Comparative Example 2, of removing the grain boundary oxidation layer A by machining, the manufacturing cost was higher than that in the desiliconizing and demanganizing process P4. Therefore, it is considered that, in the manufacturing processes PI to P5 of the shaft 10 according to Example 1, the manufacturing cost was suitably suppressed as compared to the manufacturing processes of the shaft 10 according to Comparative Example 2 in which the desiliconizing and demanganizing process P4 was not performed and the finishing process was performed.

[0038] In the manufacturing processes PI to P5 of the shaft 10 according to

Example 1, before the gas carburizing process P5, the desiliconizing and demanganizing process P4 is performed under a condition of a higher temperature and a lower pressure than in the gas carburizing process P5. Therefore, before the gas carburizing process P5, the element such as Si, Mn, or Cr which causes an oxide to be formed during the gas carburizing process P5 is evaporated from the surface of the shaft 10. Accordingly, the grain boundary oxidation layer A formed on the surface of the shaft 10 during the gas carburizing process P5 can be suitably reduced, and the fatigue strength of the shaft 10 can be improved.

[0039] In addition, in the manufacturing processes PI to P5 of the shaft 10 according to Example 1, in the desiliconizing and demanganizing process P4, before the gas carburizing process P5, the element such as Si, Mn, or Cr which causes an oxide to be formed on the surface of the shaft 10 during the gas carburizing process P5 is evaporated from the surface of the shaft 10. Accordingly, the grain boundary oxidation layer A which is formed on the surface of the shaft 10 during the gas carburizing process P5 can be suitably reduced.

[0040] In addition, in the manufacturing processes PI to P5 of the shaft 10 according to Example 1, in the desiliconizing and demanganizing process P4, the element such as Si, Mn, or Cr which causes an oxide to be formed on the surface of the shaft 10 during the gas carburizing process P5 is evaporated from the surface of the shaft 10 in a vacuum in which the pressure was sufficiently lower than the atmospheric pressure, that is, under a pressure of 100 Pa to 1000 Pa. Accordingly, the grain boundary oxidation layer A which is formed on the surface of the shaft 10 during the gas carburizing process P5 can be suitably reduced, and the fatigue strength of the shaft 10 can be improved.

[0041] In addition, in the manufacturing processes PI to P5 of the shaft 10 according to Example 1, the element which causes an oxide to be formed during the gas carburizing process P5 is Mn, Si, or Cr. Therefore, in the desiliconizing and demanganizing process P4, the element such as Mn, Si, or Cr having a relatively high vapor pressure is evaporated from the surface of the shaft 10. Accordingly, the grain boundary oxidation layer A which is formed on the surface of the shaft 10 during the gas carburizing process P5 can be suitably reduced.

[0042] In addition, in the manufacturing processes PI to P5 of the shaft 10 according to Example 1, after the desiliconizing and demanganizing process P4, a temperature is decreased to a temperature of the gas carburizing process P5 of about 930°C to perform the gas carburizing process P5. Accordingly, after the desiliconizing and demanganizing process P4, the gas carburizing process P5 can be continuously performed suitably.

[0043] Next, another example of the invention will be described. In the following description, the same components as in the above-described Example 1 are represented by the same reference numerals, and the description thereof will not be repeated.

[0044] Manufacturing processes of a ferrous metal component according to this example are substantially the same as the manufacturing processes PI to P5 of the shaft 10 according to Example 1, except that a gear 28 which is a driving component used in, for example, a vehicle is manufactured instead of the shaft 10 according to Example 1. A gas carburizing apparatus 12 according to the example illustrated in FIG 10 has a slightly different shape from the gas carburizing apparatus 12 according to Example 1 illustrated in FIG 1. For example, a heater 20 and a jig 18 according to the example have different shapes from Example 1 but are functionally the same as in Example 1.

[0045] In the manufacturing processes of the gear 28 according to the example, similarly to the effects of the above-described Example 1, in the desiliconizing and demanganizing process P4, a grain boundary oxidation layer A which is formed on a surface of the gear 28 during the gas carburizing process P5 can be suitably reduced, and thus the fatigue strength of the gear 28 can be improved. In addition, in order to manufacture the gear 28, typically, a shot peening process for improving the fatigue strength is performed. However, in the manufacturing processes of the gear 28 according to the example, since the fatigue strength can be suitably improved, the shot spinning process is not necessary. Accordingly, the manufacturing cost of the gear 28 can be significantly reduced.

[0046] Hereinabove, the examples of the invention have been described with reference to the drawings, but the invention is also applicable to other embodiments.

[0047] In the manufacturing processes PI to P5 of the shaft 10 according to the examples, in the desiliconizing and demanganizing process P4, the element such as Si, Mn, or Cr having a high vapor pressure is suitably evaporated from the surface of the shaft 10. However, other elements may be evaporated from the surface of the shaft 10. In addition, by evaporating at least one element of Mn, Si, and Cr from the surface of the shaft 10, the formation of the grain boundary oxidation layer A can be inhibited, and thus the fatigue strength of the shaft 10 can be improved.

[0048] In addition, in the manufacturing processes PI to P5 of the shaft 10 according to the examples, i the desiliconizing and demanganizing process P4, by performing multiple regression analysis using the results of Experiment I illustrated in FIG 4, the relational expression (1) between the temperature T (°C), the pressure P (Pa), and the holding time t (min) in the desiliconizing and demanganizing process P4 and the content y (mass%) of Si, Mn, and Cr in the surface of the shaft 10 is obtained. However, for example, when the material of the shaft 10 is changed from SCR420 to another one, a new relational expression may be obtained by multiple regression analysis after performing Experiment I with the same method as the example.

[0049] In addition, in the above-described examples, the shaft 10 and the gear 28 used in a vehicle are used as examples of a ferrous metal component. However, the invention is suitably applicable to other ferrous metal components. That is, the invention is suitably applicable to any ferrous metal component on which a carburizing treatment is performed. In addition, in the above-described examples, the shaft 10, that is, the ferrous metal component is formed of the ferrous metal material containing Fe as a major component, for example, formed of a steel material having a C content of 0.02% to 2.14% (wt%). However, the ferrous metal component may be formed of pure iron having a C content of 0.02% (wt%) or less.

[0050] The above-described examples are merely exemplary, and various modifications and improvements may be added to the invention based on the knowledge of a person skilled in the art.