Carburization member, and, method for producing carburization member

Abstract

To provide a technique for enhancing pitching resistance while suppressing deterioration of fatigue resistance of a carburization member.SOLUTION: A carburization member has a chemical composition consisting of mass percentages of: C: 0.15% or more and 0.40% or less, Si: 1.00% or more and 2.00% or less, Mn: 0.30% or more and 1.50% or less, Cu: 0.20% or less, Ni: 0.20% or less, Cr: 0.90% or less, Mo: 0.40% or more and 1.00% or less, with the remainder being Fe and unavoidable impurities. When the mass content rates of Si, Mn, Cr, and Mo be denoted as [Si], [Mn], [Cr], and [Mo], respectively, and the mass content rate of carbon in the surface layer be denoted as [surface C amount], the relationship 73×[Si]- 12×[Mn]+10×[Cr]+58×[Mo]+165×[surface C amount]+478≥720 is satisfied, with a tempering hardness at 300°C being greater than 700 HV and the size of the carbide within a depth of 0.2 mm from the surface being less than 2.0 μm.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a carburized member and a method for manufacturing the carburized member.

Background Art

[0002] A carburized member is a steel material that has been subjected to a carburizing treatment in which carbon (C) is introduced into the surface layer of low-carbon steel and then hardened by quenching. Carburized members are often used in mechanical parts of drive systems that require high strength, such as automotive parts such as gears and pulleys. Conventionally, various techniques for increasing the strength of carburized members have been proposed.

[0003] For example, Patent Document 1 below discloses a technique for making the hardness after tempering at 300°C for 3 hours 650 HV or more by adjusting the surface C concentration of the carburized and quenched layer and the contents of silicon (Si) and chromium (Cr). Further, Patent Document 2 below discloses a technique for achieving a desired strength of a carburized member by adjusting the contents of C, Si, Cr, and molybdenum (Mo) and defining the amount of retained austenite after quenching.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] Among the carburized members used for the mechanical parts as described above, there has been a problem that pitching may occur due to surface pressure and the member may be damaged. In order to improve the pitching resistance of the carburized member, for example, as disclosed in Patent Document 1, it is known that it is effective to increase the C content contained in the surface layer and increase the hardness after tempering at 300 °C.

[0006] However, when the C content in the surface layer of the carburized member is increased, the amount of retained austenite after quenching may increase, and the effect of improving the hardness by quenching may be reduced. In addition, when the C content in the surface layer of the carburized member is increased, there is a high possibility that coarse carbides are generated in the surface layer, which may cause a decrease in the bending fatigue strength of the carburized member.

[0007] An object of the present invention is to provide a technique capable of improving the pitching resistance while suppressing a decrease in the fatigue strength of a carburized member by a method different from the conventional method.

Means for Solving the Problems

[0008] The present invention can be realized, for example, in the following forms.

[0009] [First Embodiment] The first embodiment is provided as a carburized member. The carburized member of the first embodiment has a chemical composition consisting of, in mass%, C: 0.15% or more and 0.40% or less, Si: 1.00% or more and 2.00% or less, Mn: 0.30% or more and 1.50% or less, Cu: 0.20% or less, Ni: 0.20% or less, Cr: 0.90% or less, and Mo: 0.40% or more and 1.00% or less, with the balance being Fe and unavoidable impurities. When the mass content ratio of Si is [Si], the mass content ratio of Mn is [Mn], the mass content ratio of Cr is [Cr], the mass content ratio of Mo is [Mo], and the mass content ratio of carbon in the surface layer is [surface layer C amount], the relationship of 73×[Si] - 12×[Mn] + 10×[Cr] + 58×[Mo] + 165×[surface layer C amount] + 478 ≧ 720 is satisfied. The size of the carbides in the range from the surface to a depth of 0.2 mm is less than 2.0 μm, and the tempering hardness at 300 °C is 720 HV or more.

[0010] [Second form] A carburized member according to the first form, having a chemical composition further containing at least one of V: 0.10% or less and Nb: 0.10% or less in mass%.

[0011] [Third form] A carburized member according to any one of the first form or the second form, in which the retained austenite amount is 12.00% by volume or less.

[0012] [Fourth form] A carburized member according to any one of the first form, the second form, and the third form, in which [surface C amount] is 0.700% or more.

[0013] [Fifth form] The fifth form is provided as a method for manufacturing a carburized member. The manufacturing method of the fifth form includes a step of carburizing the skin-burned steel, a step of performing sub-zero treatment on the skin-burned steel after the carburizing, and a step of performing tempering on the skin-burned steel after the sub-zero treatment. The carburized member obtained in the above steps has a chemical composition of C: 0.15% or more and 0.40% or less, Si: 1.00% or more and 2.00% or less, Mn: 0.30% or more and 1.50% or less, Cu: 0.20% or less, Ni: 0.20% or less, Cr: 0.90% or less, and Mo: 0.40% or more and 1.00% or less in mass%, and the balance contains Fe and inevitable impurities. When the mass content rate of Si is [Si], the mass content rate of Mn is [Mn], the mass content rate of Cr is [Cr], the mass content rate of Mo is [Mo], and the mass content rate of carbon in the surface layer is [surface C amount], 73 × [Si] - 12 × [Mn] + 10 × [Cr] + 58 × [Mo] + 165 × [surface C amount] + 478 ≥ 720 satisfies the relationship. [Advantages of the Invention]

[0014] According to the present invention, a carburized member having high anti-pitting property and high fatigue strength can be obtained.

[0015] The present invention can be realized in various forms other than carburized members and their manufacturing methods. For example, it can also be realized in the form of a carburizing method for steel materials, mechanical parts such as gears and pulleys composed of carburized members, machines, devices, instruments, etc. using such mechanical parts.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0017] Hereinafter, embodiments of the carburized member and its manufacturing method according to the present invention will be described.

[0018] 1. Chemical Composition of the Carburized Member: The carburized member of the present embodiment consists of at least carbon (C), silicon (Si), manganese (Mn), copper (Cu), nickel (Ni), chromium (Cr), and molybdenum (Mo), and has a chemical composition containing iron (Fe) and inevitable impurities in the balance.

[0019] The range of the content rate of each chemical component in the carburized member of the present embodiment and the reasons for preferring the range of the content rate will be detailed below. In the following description, unless otherwise specified, the content rate is the mass content rate, and “%” means “mass %”. Also, in this specification, “inevitable impurities” means elements mixed from ores, scraps used as raw materials for steel, or the environment of the manufacturing process, etc.

[0020] ·C: 0.15% or more and 0.40% or less C is an element effective for increasing the hardness of the core part inside the carburized member. The carburized member of this embodiment contains 0.15% or more of C. However, if the C content exceeds 0.40%, it will cause a decrease in the machinability of the carburized member and a decrease in the internal toughness. Therefore, in the carburized member of this embodiment, the C content is set to 0.40% or less.

[0021] · Si: 1.00% or more and 2.00% or less Si is an element effective for increasing the softening resistance. The carburized member of this embodiment contains 1.00% or more of Si. However, in the carburized member of this embodiment, in order to suppress a decrease in workability, the Si content is set to 2.00% or less.

[0022] · Mn: 0.30% or more and 1.50% or less Mn is an element effective for improving hardenability. The carburized member of this embodiment contains 0.30% or more of Mn. However, in the carburized member of this embodiment, in order to suppress a decrease in workability, the Mn content is set to 1.50% or less.

[0023] · Cu: 0.20% or less Cu is an element mixed from the raw material of the carburized member. In the carburized member of this embodiment, in order to suppress a decrease in hot workability, the Cu content is set to 0.20% or less.

[0024] · Ni: 0.20% or less Ni is an element mixed from the raw material of the carburized member. The mixing of Ni causes an increase in the amount of retained austenite. An increase in the amount of retained austenite reduces the tempering hardness at 300°C and causes a decrease in surface fatigue strength. Therefore, in the carburized member of this embodiment, in order to suppress a decrease in surface fatigue strength, the Ni content is set to 0.20% or less. In order to more reliably suppress a decrease in surface fatigue strength, it is more preferable that the Ni content is 0.05% or less. The Ni content can be reduced by using a raw material with a lower Ni content. Since iron scrap contains a large amount of stainless steel containing Ni, care is required when manufacturing a carburized member using iron scrap as a raw material.

[0025] ·Cr: Below 0.90% Cr is an element effective for improving the tempering hardness at 300°C. However, if Cr is added in excess, carbides become stabilized, so the C concentration at the Ag point described later increases, and coarse carbides described later are likely to precipitate on the surface layer of the carburized member. The coarse carbides on the surface layer of the carburized member cause a decrease in the bending fatigue strength. Therefore, in the carburized member of the present embodiment, the Cr content is set to 0.90% or less.

[0026] ·Mo: 0.40% or more and 1.00% or less Mo is an element effective for improving the tempering hardness at 300°C. In the carburized member of the present embodiment, Mo is contained at 0.40% or more. However, if Mo is contained in excess, the workability of the carburized member may decrease. Therefore, in the carburized member of the present embodiment, the Mo content is set to 1.00% or less.

[0027] In the carburized member of the present embodiment, in addition to the above elements, other elements such as aluminum (Al) may be added. Further, the carburized member of the present embodiment may contain, as inevitable impurities, for example, phosphorus (P) and sulfur (S).

[0028] In the carburized member of the present embodiment, it is preferable that at least one of vanadium (V) and niobium (Nb) is contained within the following content ranges.

[0029] ·V: 0.10% or less By adding V, the tempering hardness at 300°C can be further improved. However, if the carburized member contains an excessive amount of V, the core hardness may decrease. Therefore, the V content is preferably 0.10% or less.

[0030] ·Nb: 0.10% or less When Nb is added, niobium carbide (NbC) is formed, and fine crystal grains can be obtained, further improving the strength. However, if the carburized member contains an excessive amount of Nb, the workability of the carburized member may decrease. Therefore, the Nb content is preferably 0.10% or less.

[0031] 2. Component reference value: In the carburized member of this embodiment, the relationship of the following inequality (1) is satisfied. Hereinafter, the value on the left side of this inequality (1) is also referred to as the "component reference value".

[0032] 73×[Si] - 12×[Mn] + 10×[Cr] + 58×[Mo] + 165×[Surface C content] + 478 ≥ 720 …(1)

[0033] [Si], [Mn], [Cr], and [Mo] respectively represent the mass content ratios of Si, Mn, Cr, and Mo in the carburized member.

[0034] [Surface C content] corresponds to the amount of carbon present on the surface of the carburized member and is represented by the mass content ratio of C on the surface of the carburized member. As [Surface C content], the carbon concentration measured by an Electron Probe Micro Analyzer (EPMA) can be used. The carbon concentration here is the mass content ratio.

[0035] FIG. 1 shows an example of a graph of the measurement results of the carbon concentration of the carburized member obtained by EPMA. The carbon concentration measured by EPMA takes the maximum value C 0 at the surface of the carburized member, decreases while drawing a smooth curve from the surface, and reaches the minimum value Cb corresponding to the carbon concentration of the base material at a depth of approximately 2 to 3 mm from the surface. [Surface C content] in this embodiment is the carbon concentration C 0 at the surface of the carburized member.

[0036] Referring to FIG. 2, the reason for deriving the above inequality (1) will be explained. FIG. 2 shows a scatter diagram created based on the experimental results conducted by the inventor of the present invention in order to derive the above inequality (1). The vertical axis of the scatter diagram in FIG. 2 is the component reference value, and the horizontal axis is the tempering hardness at 300°C. In this specification, "tempering hardness" is represented by Vickers hardness, and the unit is HV.

[0037] The inventor of the present invention measured the tempering hardness at 300°C for various carburized member specimens with different element contents, and by performing regression analysis on the measurement results, obtained a regression equation for the left side of the above inequality (1) that has a proportional relationship with the tempering hardness at 300°C. As shown in the scatter diagram of FIG. 2, if the component reference value obtained by the regression equation is 720 or more, the tempering hardness at 300°C of any carburized member becomes greater than 700 HV.

[0038] For a carburized member having the above-described chemical composition and adjusted to satisfy the relationship of the above inequality (1), the tempering hardness at 300°C can be increased to more than 700 HV. In order to improve the anti-pitting property in the carburized member, the higher the tempering hardness at 300°C, the more preferable. Therefore, it is more preferable that the tempering hardness at 300°C is 720 HV or more.

[0039] In order to increase the tempering hardness at 300°C, the [surface layer C amount] is preferably 0.7% or more and 0.9% or less. If the [surface layer C amount] is 0.7% or more, the tempering hardness at 300°C can be increased by the C present in the surface layer. On the other hand, if the [surface layer C amount] is 0.9% or less, it is possible to suppress the excessive amount of C present in the surface layer from generating coarse carbides, which will be described later.

[0040] 3. Coarse Carbides: In the carburized member of the present embodiment, most of the sizes of the carbides in the surface layer in the range from the surface to a depth of 0.2 mm are less than 2.0 μm. That is, in the carburized member of the present embodiment, there are almost no coarse carbides with a size of 2.0 μm or more in the range from the surface to a depth of 0.2 mm. The method for measuring the size of the carbides in the surface layer is as follows.

[0041] Cut the carburized member with a cross-section perpendicular to the surface, mirror-polish it, and then etch the cut surface with picral. Using a scanning electron microscope, observe and photograph the cut surface, and measure the diameter of the carbides present on the cut surface. The "diameter of the carbides" is the equivalent circle diameter obtained from the area of the carbides on the cut surface of the carburized member.

[0042] As shown in FIG. 1, in the surface layer region from the surface of the carburized member to a depth of 0.2 mm, the C content tends to increase due to the C that penetrates during carburization. Therefore, coarse carbides are likely to be generated in that surface layer region. According to the carburized member of the present embodiment, since there are no coarse carbides as described above in the surface layer region, a decrease in the bending fatigue strength due to the presence of coarse carbides is suppressed. Although details will be described later, in order to suppress the generation of coarse carbides in the surface layer of the carburized member, it is preferable to perform sub-zero treatment after carburization and before tempering.

[0043] 4. Ag point: FIG. 3 is an explanatory diagram schematically showing the relationship between the phase diagram of general skin-burned steel and the Ag point. In FIG. 3, α represents ferrite, γ represents austenite, and θ represents cementite. The Ag point is the point at which graphite begins to precipitate. According to the findings of the inventors of the present invention, the coarse carbides near the surface of the above-described carburized member are likely to be generated when the C content at the Ag point is high and the time in the diffusion period during carburization described later is long compared to the time in the carburization period. In order to suppress the generation of coarse carbides, the C content at the Ag point is preferably 2.00 wt% or less, and more preferably 1.60 wt% or less.

[0044] 5. Retained austenite amount: As described above, an increase in the amount of retained austenite in the carburized member causes a decrease in the tempering hardness at 300 °C, leading to a decrease in surface fatigue strength. Therefore, the amount of retained austenite in the carburized member is preferably 12.00% by volume or less. The amount of retained austenite is calculated from the peak intensity ratios obtained by determining the peak intensities of (200) and (211) of the ferrite phase and (200), (220), and (311) of the austenite phase obtained by X-ray diffraction measurement.

[0045] 6. Manufacturing method of carburized member: The carburized member of the present embodiment is manufactured by performing the carburizing treatment described below on the skin-burned steel that is its base material. The skin-burned steel is obtained by melting steel based on the chemical composition of the above-described carburized member in a vacuum induction melting furnace, high-frequency induction furnace, etc., forming the steel ingot by hot rolling or hot forging, and then performing a normalizing (annealing) treatment on the formed steel material.

[0046] FIG. 4 shows a process flow diagram of the carburizing treatment of the present embodiment. Inside the blocks showing each of the steps P1 to P3 in FIG. 4, graphs schematically showing the time change of the temperature of the skin-burned steel in each of the steps P1 to P3 are shown.

[0047] In step P1, carburizing is performed to infiltrate carbon into the surface layer of the skin-burned steel. In the present embodiment, vacuum carburizing is performed. In the carburizing in step P1, the skin-burned steel is heated in a vacuum furnace, and a carburizing gas is introduced while the skin-burned steel is held at a predetermined carburizing temperature Tc for a predetermined carburizing time tc. The carburizing temperature tc is, for example, about 900 to 1000 °C. The carburizing time t is, for example, 0.5 to 10 hours.

[0048] The carburizing time tc is divided into a carburizing period Pc in which a carburizing gas is supplied into the furnace to infiltrate C into the case-hardened steel, and a diffusion period Pd in which the supply of the carburizing gas into the furnace is stopped and the C infiltrated into the case-hardened steel is diffused within the case-hardened steel. When the amount of C at the Ag point is high, if the diffusion period Pd becomes longer than the carburizing period Pc, the above-mentioned coarse carbides are likely to be generated on the surface layer of the carburized member. Therefore, the larger the carburizing time ratio Pc / Pd, which is the ratio of the diffusion period Pd to the carburizing period Pc, the more preferable. The carburizing time ratio Pc / Pd is preferably 0.40 or more.

[0049] In step P2, a sub-zero treatment is performed in which the case-hardened steel after carburizing is rapidly cooled to a predetermined sub-zero temperature Tz below room temperature and held for a predetermined sub-zero time tz. The sub-zero temperature Tz may be, for example, -50 to -100 °C, or may be -80 °C. The sub-zero time tz is, for example, about 1 hour.

[0050] According to the findings obtained by the inventor of the present invention through repeated experiments, for the case-hardened steel having a chemical composition adjusted to satisfy the relationship of the above-mentioned inequality (1) after the carburizing treatment, by performing the above-mentioned sub-zero treatment after carburizing and before tempering, the generation of coarse carbides on the surface layer is suppressed. Therefore, it is possible to suppress a decrease in the bending fatigue strength of the carburized member due to the presence of coarse carbides on the surface layer of the carburized member.

[0051] In step P3, tempering is performed on the case-hardened steel after the sub-zero treatment. In tempering, the case-hardened steel is heated and held at a predetermined tempering temperature Tt for a predetermined tempering time tt. The tempering temperature Tt is, for example, 100 to 200 °C. The tempering time tt is, for example, about 2 hours.

[0052] By the carburizing treatment having the above steps, the carburized member of the present embodiment can be obtained. Note that the carburizing in step P1 is not limited to vacuum carburizing. In other embodiments, in step P1, instead of vacuum carburizing, for example, gas carburizing treatment, gas carburizing nitriding treatment, vacuum carburizing nitriding treatment, etc. may be performed.

[0053] 7. Summary of the embodiment: According to the carburized member of the present embodiment, the tempering hardness at 300 ° C can be made 700 HV or more, so that high pitching resistance can be obtained. Further, since there are almost no coarse carbides in the surface layer having a depth of 0.2 mm from the surface, high fatigue strength can be obtained. According to the manufacturing method of the carburized member of the present embodiment, since the sub-zero treatment is performed on the skin-burned steel after carburizing and before tempering, generation of coarse carbides in the surface layer of the carburized member can be suppressed. Therefore, a carburized member having high pitching resistance and fatigue strength can be efficiently manufactured.

Example

[0054] Next, examples of the carburized member and the manufacturing method thereof according to the present invention will be described.

[0055] A. Chemical composition of carburized member: As examples and comparative examples of the carburized member according to the present invention, test materials having the chemical compositions shown in Table 1 below were manufactured. In Table 1, test materials within the range of the chemical composition of the above-described embodiment are shown as Examples 1 to 4, and test materials outside the range of the chemical composition are shown as Comparative Examples 1 to 7.

[0056]

Table 1

[0057] All of the carburized members of Examples 1 to 4 contained C, Si, Mn, Cu, Ni, Cr, and Mo at the content rates within the preferable ranges described in the above embodiment. In all of the carburized members of Examples 1 to 5, the content rate of Ni was 0.05% or less. Further, the carburized members of Examples 1 and 3 contained V and Nb at the content rates within the preferable ranges described in the above embodiment, the carburized members of Examples 2 and 4 did not contain V, and contained Nb at the content rate within the preferable range described in the above embodiment.

[0058] In Comparative Examples 1 to 6, the Mo content deviated from the preferred range described in the above embodiment. Also, in Comparative Examples 2, 4, 5, 6, and 7, the Mo content deviated from the preferred range described in the above embodiment. In Comparative Example 7, the Ni content deviated from the preferred range described in the above embodiment.

[0059] B. Various parameters of the carburized member, etc.: Table 2 below shows, as the configuration indicating the respective configurations of Examples 1 to 4 and Comparative Examples 1 to 7, the measured values of [surface C amount], component reference values, C concentration at the Ag point at the carburizing temperature, and the amount of retained austenite (γ). Table 2 also shows, as the items indicating the respective manufacturing conditions of Examples 1 to 4 and Comparative Examples 1 to 7, the carburizing temperature Tc, the carburizing time ratio Pc / Pd, and the presence or absence of cryogenic treatment. Further, Table 2 shows, as the items indicating the respective characteristics of Examples 1 to 4 and Comparative Examples 1 to 7, the presence or absence of coarse carbides and the measured value of the tempering hardness at 300°C.

[0060]

Table 2

[0061] [Surface C amount] is the measured value of the carbon concentration on the surface obtained when the test piece of the carburized member is cut in a cross-section perpendicular to the carburizing direction, which is the direction perpendicular to the surface, and the surface is polished, and then the carbon concentration distribution is measured by EPMA.

[0062] The component reference value is the value calculated by the formula on the left side of the above inequality (1) using the Si, Mn, Cr, and Mo content shown in Table 1 and the [surface C amount] in Table 2.

[0063] The C concentration at the Ag point is the value calculated using the database TCFE9 by Thermo-Calc2021b, which is the thermodynamics software manufactured by Thermo-Calc Software.

[0064] The residual γ amount is a value calculated from the peak intensity ratios of (200) and (211) of the ferrite phase and (200), (220), and (311) of the austenite phase obtained by X-ray diffraction measurement.

[0065] C. Manufacturing methods of Examples and Comparative Examples: Test specimens of carburized members of Examples 1 to 5 and Comparative Examples 1 to 7 were manufactured as follows.

[0066] [1] Preparation of skin-burned steel: A 30-kg ingot was melted by vacuum induction melting and formed into a round bar with a diameter of 30 mm by hot forging. The skin-burned steel obtained by subjecting the formed steel material to annealing treatment was machined into a coin shape with a diameter of 25 mm and a length of 5 mm.

[0067] [2] Carburizing treatment: Vacuum carburizing was performed on the coin-shaped skin-burned steels of Examples 1 to 5 and Comparative Examples 1 to 7 according to the carburizing temperature Tc and the carburizing time ratio Pc / Pd shown in Table 2. The carburizing time tc was in the range of 0.5 to 4 hours.

[0068] For the skin-burned steels of Examples 1 to 5 and Comparative Examples 3, 4, and 7, sub-zero treatment was carried out at a sub-zero temperature Tz of about -80°C and a sub-zero time tz of about 1 hour. For the remaining skin-burned steels of Comparative Examples 1, 2, 5, and 6, sub-zero treatment was not carried out.

[0069] Finally, tempering was performed on all the skin-burned steels of Examples 1 to 5 and Comparative Examples 1 to 7 at a tempering temperature Tt of about 300°C and a tempering time tt of about 3 hours.

[0070] D. Evaluation of Examples and Comparative Examples: (i) Coarse carbides: For each of the test materials of Examples 1 to 5 and Comparative Examples 1 to 7, the test material was cut along a cross-section perpendicular to the upper surface of the coin, the cut surface was mirror-polished, and etched with picral. Then, the cut surface was photographed with a scanning electron microscope, and the presence or absence of coarse carbides in the surface layer region at a depth of 0.2 mm from the surface was verified. The item "coarse carbides" in Table 2 shows the verification results.

[0071] (ii) Tempering hardness: After cutting and polishing the test material of the carburized member in a cross-section perpendicular to the carburizing direction, the tempering hardness at 300 °C was measured by a Vickers hardness test at a position 50 μm from the carburized surface on the cut surface. The item "tempering hardness at 300 °C" in Table 2 shows the measurement results.

[0072] (iii) Evaluation: All of the carburized members of Examples 1 to 5 had a component reference value of 720 or more and satisfied the relationship of the above inequality (1). Also, the tempering hardness at 300 °C was greater than 700 HV and 720 HV or more, and all exceeded 730 HV. Thus, high pitting resistance was obtained for all of the carburized members of Examples 1 to 5. Also, in any of the carburized members of Examples 1 to 5, there were no coarse carbides in the surface layer, and a decrease in fatigue strength due to coarse carbides in the surface layer was suppressed.

[0073] All of the carburized members of Examples 1 to 5 had been subjected to sub-zero treatment and there were no coarse carbides in the surface layer. This indicates that having a chemical composition with a suitable content rate as described in the above embodiment, satisfying the relationship of the above inequality (1), and carrying out sub-zero treatment in the carburizing treatment suppresses the generation of coarse carbides in the surface layer.

[0074] All of the carburized members of Examples 1 to 5 contained at least one of V and Nb at a suitable content rate, whereas none of Comparative Examples 1 to 7 contained either V or Nb. From this, it can be seen that V and Nb contribute to the improvement of the pitting resistance and strength of the carburized member.

[0075] In addition, all of the carburized members of Examples 1 to 5 had a Ni content of 0.050% or less and a retained austenite amount of 12.0 or less. As described above, a tempering hardness at 300°C as high as 720 HV or more was ensured. This indicates that in the carburized member described in the above embodiment, when the Ni content is 0.050% or less, a decrease in surface fatigue strength is further suppressed.

[0076] In all of the carburized members of Examples 1 to 5, the retained austenite amount was 12.0 or less, and as described above, a tempering hardness at 300°C as high as 720 HV or more was ensured. In all of the carburized members of Examples 1 to 5, a decrease in surface fatigue strength was suppressed.

[0077] In all of the carburized members of Examples 1 to 5, the [surface layer C amount] was 0.700% or more, and as described above, a tempering hardness at 300°C of 720 HV or more was obtained. In addition, in all of the carburized members of Examples 1 to 5, the [surface layer C amount] was 0.900% or less, and no coarse carbides were formed on the surface layer.

[0078] All of the carburized members of Comparative Examples 1 to 5 had a component reference value smaller than 720 and did not satisfy the relationship of the above inequality (1), and the tempering hardness at 300°C was also less than 700 HV. In the carburized member of Comparative Example 6, since the C concentration at the Ag point was high and the carburizing time ratio Pc / Pd was large, the formation of coarse carbides on the surface layer was not suppressed. In the carburized member of Comparative Example 6, as a result of Cr being out of the suitable range, coarse carbides on the surface layer were observed. In the carburized member of Comparative Example 7, since the contents of Ni and Cr were out of the suitable range, the tempering hardness at 300°C was less than 720 HV and did not reach that of Examples 1 to 5.

[0079] As described above, the results of the examples and comparative examples indicate that according to the carburized member and the method for manufacturing the same according to the present invention described in the above embodiment, a carburized member having high anti-pitting property and high fatigue strength can be obtained.

[0080] The present invention is not limited to the above-described embodiments and examples, and various modifications can be made without departing from the spirit of the present invention.

Claims

1. By mass percentage, C: 0.15% or more and 0.40% or less, Si: 1.00% or more and 2.00% or less, Mn: 0.30% or more and 1.50% or less, Cu: 0.20% or less, Ni: 0.20% or less, Cr: 0.90% or less, and, Mo: 0.40% or more and 1.00% or less, comprising a chemical composition containing Fe and inevitable impurities in the balance, When the mass content rate of Si is [Si], the mass content rate of Mn is [Mn], the mass content rate of Cr is [Cr], the mass content rate of Mo is [Mo], and the mass content rate of carbon in the surface layer is [surface layer C content], 73×[Si] - 12×[Mn] + 10×[Cr] + 58×[Mo] + 165×[surface layer C content] + 478 ≥ 720 satisfies the relationship of, the tempering hardness at 300 °C is greater than 700 HV, a carburized member in which the size of carbides in the range from the surface to a depth of 0.2 mm is less than 2.0 μm.

2. By mass percentage, V: 0.10% or less, and, Nb: 0.10% or less, The carburized member according to claim 1, having a chemical composition further containing at least one of them.

3. The carburized member according to claim 1, wherein the retained austenite amount is 12.00% by volume or less.

4. The carburized member according to any one of claims 1 to 3, wherein [surface layer C content] is 0.70% or more and 0.90% or less.

5. A step of carburizing the skin-burned steel which is the base material of the carburized member, A step of performing sub-zero treatment on the skin-burned steel after the carburizing, A step of performing tempering on the skin-burned steel after the sub-zero treatment, comprising, By mass percentage, C: 0.15% or more and 0.40% or less, Si: 1.00% or more and 2.00% or less, Mn: 0.30% or more and 1.50% or less, Cu: 0.20% or less, Ni: 0.20% or less, Cr: 0.90% or less, and, Mo: 0.40% or more and 1.00% or less, comprising a chemical composition containing Fe and inevitable impurities in the balance, When the mass content rate of Si is [Si], the mass content rate of Mn is [Mn], the mass content rate of Cr is [Cr], the mass content rate of Mo is [Mo], and the mass content rate of carbon in the surface layer is [surface layer C content], 73×[Si] - 12×[Mn] + 10×[Cr] + 58×[Mo] + 165×[surface layer C content] + 478 ≥ 720 A method for manufacturing a carburized member, for obtaining a carburized member that satisfies the relationship.

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