Carburizing steel materials and carburizing steel parts

Abstract

To provide a steel that can be gas-carburized into a carburized steel part, which gives excellent bending fatigue strength and surface fatigue strength (pitching properties).SOLUTION: A steel comprises, in mass%, C: 0.09% to 0.27%, Si: more than 0.35% to less than 0.90%, Mn: 0.25% to less than 0.60%, P: 0.030% or less, S: 0.100% or less, Cr: more than 1.45% to 2.90%, Mo: 0.010% to 0.150% or less, Al: 0.045% or less, and N: 0.0250% or less, with the balance being Fe and impurities, satisfying formula (1) and formula (2): 21≤(2Si+Mn+3Cr) / 3Mo≤248 (1) and 1.6≤Al / N≤2.8 (2), where each element symbol in (1) and (2) denotes the content of the corresponding element in mass percentage.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention is suitable for the material of carburized steel parts. For carburizing This invention relates to steel materials and carburized steel parts. [Background technology]

[0002] In recent years, with the increasing output and miniaturization of power units such as engines and motors, excellent bending fatigue strength is required for power units and mechanical parts used in and around the power units. Among these mechanical parts, gears used in automobiles, construction vehicles, etc. have tooth surfaces that slide against each other at short intervals. Therefore, pitting suppression is required for the tooth surfaces. In other words, mechanical parts such as gears used in automobiles, construction vehicles, etc. are required to have not only bending fatigue strength but also surface fatigue strength (pitting characteristics).

[0003] Gas carburizing is known as a method for increasing the bending fatigue strength and surface fatigue strength of machine parts. The term "gas carburizing" as used here includes not only gas carburizing but also gas carbonitriding. Gas carburizing forms a hardened layer (carburized layer or carbonitrided layer) on the surface of a mechanical part. This hardened layer is known to increase the bending fatigue strength and surface fatigue strength of the mechanical part. Therefore, the steel material used to make carburized steel parts is required to have the performance to further increase the bending fatigue strength and surface fatigue strength when gas carburized to make the mechanical part (carburized steel part).

[0004] Patent Documents 1 and 2 propose steel materials that can improve bending fatigue strength and surface fatigue strength when gas carburized to form carburized steel parts.

[0005] The steel material disclosed in Patent Document 1 contains, in mass%, C: 0.15 to 0.25%, Si: 0.40 to 0.80%, Mn: 0.20 to 1.0%, P: 0.030% or less, S: 0.10% or less, Cu: 0.30% or less, Ni: 0.30% or less, Cr: 0.8 to 1.8%, Mo: 0.60% or less, Al: 0.02 to 0.10%, N: 0.005 to 0.03%, O: 0.003% or less, with the balance being Fe and inevitable impurities, and satisfies the following formulas (1) and (2). 1.8≦2×[Si]+[Cr]≦3.5 (1) 114×[Si]+2×[Cr]+68×[Mo]≧50 (2)

[0006] The steel material disclosed in Patent Document 2 contains, in mass%, C: 0.15 to 0.30%, Si: 0.80% to 2.00%, Mn: 0.20 to 0.80%, P: 0.003 to 0.030%, S: 0.005 to 0.050%, Cr: 1.00 to less than 1.80%, Mo: 0.03 to 0.30%, Al: 0.020 to 0.060%, N: 0.0060 to 0.0300%, and O: 0.0003 to 0.0025%, with the remainder being Fe and inevitable impurities, and satisfies the following formulas (1) to (3). [%Si]+([%Mn]+[%Cr]+[%Mo]) / 3≧1.5 ··· (1) 180-45〔%Mn〕-14〔%Cr〕-51〔%Mo〕+5〔%Si〕≧125... (2) √I≦80 (3) Here, I is the area (μm ) of oxide inclusions located at the center of fisheyes on the fracture surface after carburizing, quenching, and tempering the steel material and then performing a rotating bending fatigue test. 2 ) is shown. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-185123 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-214642 Summary of the Invention [Problem to be solved by the invention]

[0008] In carburized and quenched parts, a grain boundary oxide layer forms on the outermost surface of the part during carburizing due to the Si, Mn, and Cr in the steel and the O2 in the atmosphere, and an incompletely quenched structure forms nearby due to a deficiency of alloying elements. Because the incompletely quenched structure is soft, it reduces the surface fatigue strength and bending fatigue strength of the part. Therefore, a technology that can sufficiently suppress the formation of soft, incompletely quenched structure is desired.

[0009] To address this problem, the above-mentioned prior art techniques may not be able to sufficiently suppress the formation of an incompletely hardened structure, and have not achieved significant improvements in contact fatigue strength and bending fatigue strength.

[0010] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a steel material that can obtain excellent bending fatigue strength and excellent surface fatigue strength (pitting characteristics), and a carburized steel part that has excellent bending fatigue strength and surface fatigue strength. [Means for solving the problem]

[0011] The gist of the present invention is as follows. (1) A steel material according to one aspect of the present invention is Chemical composition in mass %: C: 0.09~0.27%, Si: over 0.35% to less than 0.90% Mn: 0.25% or more, less than 0.60% P: 0.030% or less, S: 0.100% or less, Cr: more than 1.45%, less than 2.90%, Mo: 0.010~0.150%, Al: 0.045% or less, and N: 0.0250% down, The balance is Fe and impurities, and satisfies the following formulas (1) and (2). 21≦(2Si+Mn+3Cr) / 3Mo≦248 (1) 1.6≦Al / N≦2.8 (2) However, the element symbols in the formulas (1) and (2) indicate the content of the element in mass %. (2) The steel material described in (1) above further comprises, in mass%, Cu: 0.50% or less, Ni: less than 0.05% V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.100% or less, Ti: 0.100% or less, and B: 0.0010% or less, It may contain one or more selected from the group consisting of: (3) The steel material according to (1) or (2) above further comprises, in mass%, Ca: 0.0100% or less, Mg: less than 0.0015%, and Rare earth elements: 0.0100% or less, It may contain one or more selected from the group consisting of: (4) The steel material according to any one of (1) to (3) above further comprises, in mass%, Te: less than 0.0080% Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.015% or less, Sb: 0.015% or less, and It may contain one or more selected from the group consisting of:

[0012] (5) A carburized steel part according to one aspect of the present invention is A hardened layer; a core portion located inside the hardened layer, The chemical composition of the core is, in mass%, C: 0.09~0.27%, Si: more than 0.35% and less than 0.90% Mn: 0.25% or more, less than 0.60% P: 0.030% or less, S: 0.100% or less, Cr: more than 1.45%, less than 2.90%, Mo: 0.010~0.150%, Al: 0.045% or less, and N: 0.0250% down, and the balance being Fe and impurities, and satisfying the following formulas (1) and (2): The carbon concentration in a region from the surface of the hardened layer to a depth of 30 μm is 0.60% by mass or more, the microstructure at a depth of 30 μm from the surface of the carburized steel part is a structure consisting of martensite or a structure consisting of martensite and retained austenite, The volume fraction of the retained austenite is 0 to 50%. 21≦(2Si+Mn+3Cr) / 3Mo≦248 (1) 1.6≦Al / N≦2.8 (2) However, the element symbols in the above formulas (1) and (2) indicate the content of the element in mass %. (6) The carburized steel part according to (5) above further comprises a core having a composition, in mass%, of: Cu: 0.50% or less, Ni: less than 0.05% V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.100% or less, Ti: 0.100% or less, and B: 0.0010% or less, It may contain one or more selected from the group consisting of: (7) The carburized steel part according to (5) or (6) above further comprises a core having a composition, in mass%, of: Ca: 0.0100% or less, Mg: less than 0.0015%, and Rare earth elements: 0.0100% or less, It may contain one or more selected from the group consisting of: (8) The carburized steel part according to any one of (5) to (7) above, further comprising a core having a composition, in mass%, of: Te: less than 0.0080% Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.015% or less, Sb: 0.015% or less, and It may contain one or more selected from the group consisting of: [Effects of the Invention]

[0013] According to the present invention, a steel material that can obtain excellent bending fatigue strength and excellent surface fatigue strength (pitting characteristics) can be provided. Also, according to the present invention, a carburized steel part that has excellent bending fatigue strength and excellent surface fatigue strength can be provided. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram showing an example of a heat pattern in gas carburizing treatment (gas carburizing step and quenching step). [Figure 2] FIG. 2 is a side view of a small roller test piece prepared in the example. [Figure 3] FIG. 3 is a side view of a rotating bending fatigue test piece prepared in the example. [Figure 4] FIG. 4 is a front view of a large roller test piece prepared in the example. [Figure 5] FIG. 5 is a schematic diagram of a two-roller rolling fatigue test in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have conducted research into steel materials that, when subjected to gas carburizing treatment (including gas carbonitriding treatment) to form carburized steel parts, can provide excellent bending fatigue strength and excellent surface fatigue strength (pitting characteristics).

[0016] In gas carburizing, steel parts formed into the part shape are c3Because the steel part is heated to above its transformation temperature, the microstructure of the steel part transforms to austenite. Therefore, the structure of the resulting carburized steel part is not affected by the structure of the raw steel material. Therefore, the inventors investigated ways to improve the bending fatigue strength and surface fatigue strength of carburized steel parts not from the perspective of the microstructure of the steel material, but from the perspective of the chemical composition that remains unchanged even when gas carburizing is performed.

[0017] As a result, the inventors of the present invention have concluded that in order to improve contact fatigue strength (pitting characteristics), it is effective to increase the contents of Si, Cr, Mn, and Mo, which increase temper softening resistance. Furthermore, after examining the compatibility of bending fatigue strength and contact fatigue strength from the viewpoint of chemical composition, they concluded that a steel material having a chemical composition consisting of, in mass%, C: 0.09 to 0.27%, Si: more than 0.35% but less than 0.90%, Mn: 0.25% or more but less than 0.60%, P: 0.030% or less, S: 0.100% or less, Cr: more than 1.45% but 2.90% or less, Mo: 0.010 to 0.150%, Al: 0.045% or less, and N: 0.0250% or less will provide excellent bending fatigue strength and excellent contact fatigue strength when a carburized steel part is manufactured by gas carburizing.

[0018] However, even when the content of each element in the chemical composition of a steel material falls within the above-mentioned range, there are cases in which sufficient bending fatigue strength and surface fatigue strength are not obtained when the steel material is gas carburized to form a carburized steel part. Therefore, the present inventors have conducted further research and studies. As a result, the present inventors have obtained the following findings (A) and (B).

[0019] (A) In carburized and quenched gear parts, a grain boundary oxide layer is formed on the outermost surface of the steel during carburizing due to the Si, Mn, and Cr in the steel and the O2 in the atmosphere, and an incompletely quenched structure is formed in the vicinity of this layer due to a deficiency of alloying elements. Because the incompletely quenched structure is soft, it reduces the bending fatigue strength of the steel part. As such, since Si, Mn, and Cr are all oxide-forming elements, their inclusion in steel is undesirable from the viewpoint of grain boundary oxide layer formation.

[0020] However, all of these elements improve hardenability, and Si is effective in improving softening resistance, while Mn and Cr are effective in improving carburization resistance, so they are essential for improving surface fatigue strength and bending fatigue strength. On the other hand, although Mo is much more expensive to alloy than these elements, it is an element that significantly improves hardenability without forming oxides.

[0021] We have found that optimizing the ratio of oxide-forming elements Si, Mn, and Cr to Mo, which improves hardenability without forming oxides and effectively suppresses the formation of incompletely hardened structures, can improve the softening resistance and carburization properties of carburized parts while consistently suppressing the formation of incompletely hardened layers. Specifically, if the contents of these elements are defined as "F1 = (2Si + Mn + 3Cr) / 3Mo," then if F1 is 21 to 248 (i.e., if the following formula (1) is satisfied), the Mo content relative to the Si, Mn, and Cr contents is in the appropriate range, thereby suppressing the formation of incompletely hardened layers during gas carburizing. As a result, carburized steel parts manufactured from steel materials with the contents of each element within the above ranges and satisfying the formula (2) below can achieve excellent bending fatigue strength and excellent surface fatigue strength. 21≦(2Si+Mn+3Cr) / 3Mo≦248 (1)

[0022] (B) Even if the content of each element in the chemical composition is within the above-mentioned range, if there are excessive Al inclusions in the steel, the Al inclusions can become the initiation point of cracks. In other words, if excessive Al inclusions remain in the steel, the bending fatigue strength of the carburized steel part may decrease. Furthermore, Al in the steel can precipitate as precipitates (AlN). Coarse AlN (precipitates) can become the initiation point of cracks, just like Al inclusions. Therefore, if there is an excessive amount of coarse AlN (precipitates) in the steel, the bending fatigue strength of the carburized steel part may decrease. Therefore, it is necessary to determine the appropriate content of Al and N. Specifically, if the content of these elements is defined as "F2 = Al / N," and the content of each element in the chemical composition of the steel is within the above-mentioned range, the excessive formation of Al inclusions and coarse AlN can be suppressed if F2 is 1.6 to 2.8 (i.e., if the following formula (2) is satisfied). On the other hand, assuming that the content of each element in the chemical composition of the steel is within the above-mentioned range, if F2 is less than 1.6, the composition will have an excessively high N content relative to the Al content, and in this case, excessively large amounts of coarse AlN (precipitates) will form in the steel. As a result, the bending fatigue strength of the carburized steel part will be reduced. On the other hand, assuming that the content of each element in the chemical composition of the steel is within the above-mentioned range, if F2 exceeds 2.8, the composition will have an excessively high Al content relative to the N content, and in this case, excessive amounts of Al inclusions (oxide inclusions) that do not bond with N will be formed. Therefore, in this case as well, the bending fatigue strength of the carburized steel part will be reduced. 1.6≦Al / N≦2.8 (2)

[0023] The steel material and carburized steel part of this embodiment have been completed based on the above technical concept.

[0024] The steel material of this embodiment and the carburized steel part manufactured using this steel material will be described in detail below. The "%" for the content of each element means "mass %" unless otherwise specified.

[0025] The carburized steel part according to this embodiment has a core part (hereinafter sometimes simply referred to as "core part") that is the center part in the depth direction of the part, and a hardened layer located on the surface layer of the part. Here, the core refers to the portion that was not penetrated by carbon during gas carburizing. In other words, the core is a region where, despite having undergone gas carburizing, there is no change in chemical composition or metal structure, or the change is so small that it can be ignored, and has a component composition equivalent to that of the base material of the part. Note that the composition of the core can also be said to be the composition at a depth of 2.0 mm from the surface of the part. Note that, in this embodiment, "vacuum carburizing" also includes vacuum carbonitriding.

[0026] [Steel] [Chemical composition] The chemical composition of the steel material of this embodiment will be described. Note that the composition of the core of a carburized steel part is usually the same as the composition of the material (steel material) of the part. In other words, the chemical composition described below can also be said to be the chemical composition of the core.

[0027] C: 0.09 to 0.27% Carbon (C) improves the hardenability of steel and increases the core hardness of carburized steel parts manufactured using the steel material. Therefore, it increases the bending fatigue strength of the carburized steel part. If the C content is less than 0.09%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.27%, the machinability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.09 to 0.27%. The lower limit of the C content is preferably 0.12% or more, more preferably 0.15% or more, and even more preferably 0.18% or more. The upper limit of the C content is preferably 0.26% or less, more preferably 0.25% or less, and even more preferably 0.24% or less.

[0028] Si: over 0.35% and less than 0.90% Silicon (Si) increases the temper softening resistance of steel, thereby improving the surface fatigue strength (pitting resistance) of carburized steel parts manufactured using the steel as a base material. If the Si content is 0.35% or less, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content is 0.90% or more, the chemical potential of C in the surface layer of the steel at the temperature range of the gas carburizing treatment is excessively increased, even if the contents of other elements are within the ranges of this embodiment. In this case, the penetration of C into the steel during the gas carburizing treatment is suppressed. As a result, the depth of the surface layer (hardened layer) of the carburized steel part decreases, and the surface fatigue strength decreases. Furthermore, because Si is an oxide-forming element, if the Si content is excessively high, Si oxides are formed at the grain boundaries on the part surface due to an oxidation reaction with O2 in the atmosphere during gas carburizing. The lack of alloying elements in the vicinity of these oxides reduces hardenability. As a result, a soft, incompletely hardened layer is formed, reducing the surface fatigue strength and bending fatigue strength of the part. Therefore, the Si content is more than 0.35% and less than 0.90%. The lower limit of the Si content is preferably 0.40% or more, more preferably 0.45% or more, and even more preferably 0.50% or more. The upper limit of the Si content is preferably 0.85% or less, more preferably 0.80% or less, and even more preferably 0.75% or less.

[0029] Mn: 0.25% or more, less than 0.60% Manganese (Mn) improves the hardenability of steel and increases the core hardness of carburized steel parts manufactured using the steel. This increases the bending fatigue strength of the carburized steel part. Mn also increases the temper softening resistance of steel, thereby improving the surface fatigue strength (pitting resistance) of carburized steel parts manufactured using the steel. If the Mn content is less than 0.25%, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges specified in this embodiment. On the other hand, if the Mn content is 0.60% or more, Mn oxides are formed at the grain boundaries on the part surface due to an oxidation reaction with O2 in the atmosphere during gas carburizing. The lack of alloying elements in the vicinity of these oxides reduces the hardenability. As a result, a soft, incompletely hardened layer is formed, reducing the bending fatigue strength of the part. Therefore, the Mn content is set to 0.25% or more but less than 0.60%. The lower limit of the Mn content is preferably 0.30% or more, more preferably 0.35% or more, and even more preferably 0.40% or more. The upper limit of the Mn content is preferably 0.57% or less, more preferably 0.55% or less, and even more preferably 0.53% or less.

[0030] P:0.030% or less Phosphorus (P) is an impurity. During the gas carburizing process when manufacturing carburized steel parts using steel as a raw material, P segregates at austenite grain boundaries and reduces the bending fatigue strength of the carburized steel part. If the P content exceeds 0.030%, the bending fatigue strength of the carburized steel part will be significantly reduced, even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.030% or less. The preferred upper limit of the P content is 0.029% or less, more preferably 0.028% or less, and even more preferably 0.025% or less. The P content is preferably as low as possible. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is more than 0%, more preferably 0.001% or more, and even more preferably 0.002%.

[0031] S: 0.100% or less Sulfur (S) is an impurity. If the S content exceeds 0.100%, sulfides will coarsen even if the contents of other elements are within the ranges of this embodiment. In this case, the bending fatigue strength of the carburized steel part will decrease. Therefore, the S content is 0.100% or less. The upper limit of the S content is preferably 0.050% or less, more preferably 0.025% or less, even more preferably 0.020% or less, and even more preferably 0.015% or less. On the other hand, S combines with Mn to form MnS, improving the machinability of the steel material. Therefore, S may be contained within a range that does not impair the effects of the present invention. The lower limit of the S content is preferably greater than 0%, more preferably 0.001% or more, even more preferably 0.002% or more, and even more preferably 0.005% or more.

[0032] Cr: more than 1.45%, less than 2.90% Chromium (Cr) increases the temper softening resistance of steel, thereby improving the surface fatigue strength (pitting resistance) of carburized steel parts manufactured using the steel. If the Cr content is 1.45% or less, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges specified in this embodiment. On the other hand, if the Cr content exceeds 2.90%, even if the contents of other elements are within the ranges specified in this embodiment, the chemical potential of C in the surface layer of the steel at the temperature range of gas carburizing treatment is excessively reduced, resulting in excessive penetration and diffusion of C into the steel during gas carburizing treatment. As a result, a large amount of soft retained austenite is formed in the surface layer of the carburized steel part, reducing the bending fatigue strength of the carburized steel part. Furthermore, Cr oxides are formed at the grain boundaries on the part surface due to an oxidation reaction with O2 in the atmosphere during gas carburizing. The lack of alloying elements near these oxides reduces hardenability, resulting in the formation of a soft, incompletely hardened layer, which ultimately reduces the bending fatigue strength of the part. Therefore, the Cr content is greater than 1.45% and less than 2.90%. The lower limit of the Cr content is preferably 1.50% or more, more preferably 1.55% or more, and even more preferably 1.60% or more. The upper limit of the Cr content is preferably 2.80% or less, more preferably 2.70% or less, even more preferably 2.60% or less, still more preferably 2.50% or less, and even more preferably 2.40% or less.

[0033] Mo: 0.010 to 0.150% Molybdenum (Mo) significantly improves the hardenability of steel and does not form oxides. Therefore, even if a grain boundary oxide layer forms on the surface of a part during gas carburizing and Si and Cr are depleted around it, the formation of an incompletely hardened structure can be suppressed, thereby improving the bending fatigue strength and surface fatigue strength of the part. If the Mo content is less than 0.010%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content exceeds 0.150%, not only will the above effects saturate, but the machinability of the steel material will decrease and the alloy cost will increase. Therefore, the Mo content is 0.010 to 0.150%. The preferred lower limit of the Mo content is 0.020% or more, more preferably 0.030% or more, and even more preferably 0.040% or more. The preferred upper limit of the Mo content is 0.140% or less, more preferably 0.130% or less, and even more preferably 0.120% or less.

[0034] Al: 0.045% or less Aluminum (Al) has the effect of deoxidizing steel. However, if the Al content exceeds 0.045%, coarse Al inclusions (oxide-based inclusions) are formed even if the contents of other elements are within the ranges of this embodiment. Coarse Al inclusions reduce the bending fatigue strength of carburized steel parts. Therefore, the Al content is 0.045% or less. The preferred upper limit of the Al content is 0.042% or less, more preferably 0.039% or less, even more preferably 0.036% or less, and even more preferably 0.033% or less. The Al content is preferably as low as possible. There is no particular restriction on the lower limit of the Al content, but it may be greater than 0% to obtain the deoxidizing effect. The preferred lower limit of the Al content is 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

[0035] N: 0.0250% or less Nitrogen (N) is an impurity. If the N content exceeds 0.0250%, coarse nitrides will form even if the contents of other elements are within the ranges of this embodiment. The coarse nitrides reduce the bending fatigue strength of carburized steel parts. Therefore, the N content is 0.0250% or less. The preferred upper limit of the N content is 0.0230% or less, more preferably 0.0210% or less, and even more preferably 0.0200% or less. The N content is preferably as low as possible. However, excessive reduction of the N content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the N content is more than 0%, more preferably 0.0001% or more, and even more preferably 0.0005% or more.

[0036] The chemical composition of the core of the steel material and carburized steel part of this embodiment contains the above elements, with the remainder consisting of Fe and impurities. Here, impurities refer to elements that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment during industrial production of steel, and also include elements that are not intentionally added. Furthermore, the term "impurities" as used herein refers to elements that are acceptable within a range that does not adversely affect the steel material and carburized steel part of this embodiment.

[0037] [Optional element] The steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu, Ni, V, W, Co, Nb, Ti, and B. All of these elements are optional elements, and have the effect of increasing the bending fatigue strength of carburized steel parts manufactured using the steel material as a raw material.

[0038] Cu: 0.50% or less Copper (Cu) is an optional element and may not be contained. In other words, the Cu content may be 0%. When copper is contained, that is, when the Cu content exceeds 0%, Cu improves the hardenability of the steel and increases the core hardness of carburized steel parts manufactured using the steel material. As a result, the bending fatigue strength of the carburized steel part is increased. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. On the other hand, if the Cu content exceeds 0.50%, the hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0 to 0.50%, and if contained, it is 0.50% or less. The lower limit of the Cu content is preferably 0.01% or more, more preferably 0.05% or more, even more preferably 0.08% or more, and even more preferably 0.10%. The upper limit of the Cu content is preferably 0.48% or less, and even more preferably 0.46% or less.

[0039] Ni: Less than 0.05% Nickel (Ni) is an optional element and may not be contained. In other words, the Ni content may be 0%. When nickel is contained, that is, when the Ni content exceeds 0%, Ni improves the hardenability of the steel and increases the core hardness of carburized steel parts manufactured using the steel. As a result, the bending fatigue strength of the carburized steel parts is increased. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, if the Ni content is 0.05% or more, the strength of the steel will be excessively high even if the contents of other elements are within the ranges of this embodiment. In this case, the machinability of the steel will be reduced. Therefore, the Ni content is 0 to less than 0.05%, and if contained, it is less than 0.05%. The lower limit of the Ni content is preferably 0.01% or more, more preferably 0.02% or more. The upper limit of the Ni content is preferably 0.04% or less, more preferably 0.03% or less.

[0040] V: 0.50% or less Vanadium (V) is an optional element and may not be contained. In other words, the V content may be 0%. When vanadium (V) is contained, that is, when the V content exceeds 0%, V forms precipitates (carbides, nitrides, carbonitrides, etc.) and, through a pinning effect, suppresses coarsening of steel grains during gas carburizing. As a result, the bending fatigue strength of carburized steel parts manufactured using the steel is improved. Even if even a small amount of V is contained, the above effect can be achieved to some extent. However, if the V content exceeds 0.50%, the hardness of the steel becomes excessively high even if the contents of other elements are within the ranges of this embodiment. In this case, the machinability of the steel decreases. Therefore, the V content is 0 to 0.50%, and when contained, it is 0.50% or less. The lower limit of the V content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more. The upper limit of the V content is preferably 0.40% or less, and even more preferably 0.30% or less.

[0041] W: 0.50% or less Tungsten (W) is an optional element and may not be contained. In other words, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W improves the hardenability of the steel and increases the core hardness of carburized steel parts manufactured using the steel. As a result, the bending fatigue strength of the carburized steel parts is increased. Even if even a small amount of W is contained, the above effects can be achieved to some extent. On the other hand, if the W content exceeds 0.50%, the strength of the steel will be excessively high even if the contents of other elements are within the ranges of this embodiment. In this case, the machinability of the steel will be reduced. Therefore, the W content is 0 to 0.50%, and when contained, it is 0.50% or less. The lower limit of the W content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.08% or more. The upper limit of the W content is preferably 0.40% or less, and even more preferably 0.30% or less.

[0042] Co:0.50% or less Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When it is contained, that is, when the Co content exceeds 0%, Co improves the hardenability of the steel and increases the core hardness of carburized steel parts manufactured using the steel. As a result, the bending fatigue strength of the carburized steel part is increased. Even if even a small amount of Co is contained, the above effects can be obtained to some extent. On the other hand, if the Co content exceeds 0.50%, the strength of the steel will be excessively high even if the contents of other elements are within the ranges of this embodiment. In this case, the machinability of the steel will be reduced. Therefore, the Co content is 0 to 0.50%, and when contained, it is 0.50% or less. The lower limit of the Co content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.08% or more. The upper limit of the Co content is preferably 0.40% or less, and even more preferably 0.30% or less.

[0043] Nb: 0.100% or less Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb forms precipitates (carbides, carbonitrides, etc.) and suppresses the coarsening of steel grains during gas carburizing due to its pinning effect. As a result, the bending fatigue strength of carburized steel parts manufactured using the steel as a raw material is improved. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the Nb precipitates will coarsen and the pinning effect will not be obtained. Therefore, the Nb content is 0 to 0.100%, and when contained, it is 0.100% or less. The lower limit of the Nb content is preferably 0.001% or more, and more preferably 0.005% or more. The upper limit of the Nb content is preferably 0.060% or less, more preferably 0.050% or less, even more preferably 0.040% or less, and even more preferably 0.030% or less.

[0044] Ti:0.100% or less Titanium (Ti) is an optional element and may not be contained. In other words, the Ti content may be 0%. When titanium is contained, that is, when the Ti content exceeds 0%, Ti forms precipitates (carbides, nitrides, carbonitrides, etc.) and, through a pinning effect, suppresses the coarsening of steel grains during gas carburizing. As a result, the bending fatigue strength of carburized steel parts manufactured using the steel is improved. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the Ti precipitates will coarsen and the pinning effect will not be obtained. Therefore, the Ti content is 0 to 0.100%, and when contained, it is 0.100% or less. The lower limit of the Ti content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. The upper limit of the Ti content is preferably 0.075% or less, and even more preferably 0.050% or less.

[0045] B: 0.0010% or less Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B improves the hardenability of the steel and increases the core hardness of carburized steel parts manufactured using the steel material. As a result, the bending fatigue strength of the carburized steel part is improved. Even if even a small amount of B is contained, the above effects can be obtained to a certain extent. On the other hand, when the B content exceeds 0.0010%, the effect saturates. Therefore, the B content is 0 to 0.0010%, and when contained, it is 0.0010% or less. The preferred lower limit of the B content is 0.0001% or more, more preferably 0.0002% or more, and even more preferably 0.0003% or more. The preferred upper limit of the B content is 0.0009% or less, more preferably 0.0008% or less, and even more preferably 0.0007% or less.

[0046] The steel material of this embodiment may further contain one or more elements selected from the group consisting of Ca, Mg, and rare earth elements (REM) in place of a portion of Fe. These elements are optional elements, and all have the effect of increasing the bending fatigue strength of the carburized steel part.

[0047] Ca:0.0100% or less Calcium (Ca) is an optional element and does not necessarily need to be contained. In other words, the Ca content may be 0%. When contained, that is, when the Ca content is more than 0%, Ca modifies sulfides in the steel material and suppresses the elongation of the sulfides during hot working. As a result, the bending fatigue strength of the carburized steel part is improved. Even if even a small amount of Ca is contained, the above effect can be obtained to a certain extent. However, when the Ca content exceeds 0.0100%, the above effect saturates. Therefore, the Ca content is 0 to 0.0100%, and when contained, it is 0.0100% or less. The lower limit of the Ca content is preferably 0.0001% or more, more preferably 0.0002% or more. The upper limit of the Ca content is preferably 0.0075% or less, more preferably 0.0050% or less.

[0048] Mg: Less than 0.0015% Magnesium (Mg) is an optional element and does not necessarily need to be contained. In other words, the Mg content may be 0%. When magnesium is contained, that is, when the Mg content is greater than 0%, Mg modifies sulfides in the steel material and suppresses the elongation of the sulfides during hot working. As a result, the bending fatigue strength of the carburized steel part is improved. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content is 0.0015% or more, the above effect saturates. Therefore, the Mg content is 0% or more and less than 0.0015%, and if contained, it is less than 0.0015%. The lower limit of the Mg content is preferably 0.001% or more, more preferably 0.002% or more. The upper limit of the Mg content is preferably 0.0013% or less, more preferably 0.0011% or less.

[0049] Rare earth elements: 0.0100% or less Rare earth elements (REM) are optional elements and may not be included. That is, the REM content may be 0%. When included, that is, when the REM content is greater than 0%, REM modifies sulfides in the steel material and inhibits the elongation of sulfides during hot working. As a result, the bending fatigue strength of the carburized steel part is improved. Even if even a small amount of REM is included, the above effect can be achieved to some extent. However, if the REM content exceeds 0.0100%, the formation of coarse oxides is promoted even if the contents of other elements are within the ranges of this embodiment. In this case, the bending fatigue strength of the carburized steel part is reduced. Therefore, the REM content is 0 to 0.0100%, and when included, it is 0.0100% or less. The lower limit of the REM content is preferably 0.0001% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more. The upper limit of the REM content is preferably 0.0098% or less, and even more preferably 0.0097% or less.

[0050] The term "REM" as used herein refers collectively to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). The REM of this embodiment may be composed of one or more elements selected from these rare earth elements. The REM content of this embodiment refers to the total amount of rare earth elements.

[0051] The steel material of this embodiment may further contain one or more elements selected from the group consisting of Te, Bi, Pb, Sn, and Sb in place of a portion of Fe. These elements are optional elements, and all of them improve the machinability of the steel material.

[0052] Te: Less than 0.0080% Tellurium (Te) is an optional element and does not necessarily need to be contained. In other words, the Te content may be 0%. When contained, that is, when the Te content is greater than 0%, Te improves the machinability of the steel material. Even if even a small amount of Te is contained, the above effect can be obtained to some extent. However, if the Te content is 0.0080% or more, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Te content is 0% or more and less than 0.0080%, and if contained, it is less than 0.0080%. The lower limit of the Te content is preferably 0.001% or more, more preferably 0.002% or more. The upper limit of the Te content is preferably 0.0075% or less, more preferably 0.0070% or less.

[0053] Bi:0.500% or less Bismuth (Bi) is an optional element and may not be contained. In other words, the Bi content may be 0%. When contained, that is, when the Bi content is greater than 0%, Bi improves the machinability of the steel. Even if even a small amount of Bi is contained, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.500%, the hot workability of the steel deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Bi content is 0 to 0.500%, and if contained, it is 0.500% or less. The lower limit of the Bi content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. The upper limit of the Bi content is preferably 0.400% or less, more preferably 0.300% or less, even more preferably 0.200% or less, even more preferably 0.100% or less, even more preferably 0.080% or less, and even more preferably 0.070% or less.

[0054] Pb: 0.09% or less Lead (Pb) is an optional element and does not necessarily need to be contained. In other words, the Pb content may be 0%. When contained, that is, when the Pb content is more than 0%, Pb improves the machinability of the steel material. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.09%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Pb content is 0 to 0.09%, and when contained, it is 0.09% or less. The lower limit of the Pb content is preferably 0.01% or more, and more preferably 0.02% or more. The upper limit of the Pb content is preferably 0.08% or less, and more preferably 0.07% or less.

[0055] Sn: 0.015% or less Tin (Sn) is an optional element and does not necessarily need to be contained. In other words, the Sn content may be 0%. When contained, that is, when the Sn content exceeds 0%, Sn improves the machinability of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.015%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.015%, and when contained, it is 0.015% or less. The lower limit of the Sn content is preferably 0.001% or more, and more preferably 0.005% or more. The upper limit of the Sn content is preferably 0.013% or less, and more preferably 0.010% or less.

[0056] Sb: 0.015% or less Antimony (Sb) is an optional element and does not necessarily need to be contained. In other words, the Sb content may be 0%. When contained, that is, when the Sb content exceeds 0%, Sb improves the machinability of the steel material. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.015%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.015%, and when contained, it is 0.015% or less. The lower limit of the Sb content is preferably 0.0001% or more, and more preferably 0.0005% or more. The upper limit of the Sb content is preferably 0.013% or less, and more preferably 0.010% or less.

[0057] The chemical compositions of the steel material and carburized steel part of this embodiment have been described above, but the carburized steel part of this embodiment can solve the problem without containing the optional elements exemplified above. Therefore, the lower limit of the content of the optional elements exemplified above is 0%.

[0058] [Regarding formulas (1) and (2)] From the viewpoint of achieving both bending fatigue strength and surface fatigue strength, it is important that the chemical composition of the core of the steel material and carburized steel part of this embodiment satisfies the following formulas (1) and (2). In other words, the chemical composition of the core of the steel material and carburized steel part of this embodiment satisfies formulas (1) to (2), provided that the content of each element is within the range of this embodiment described above.

[0059] 21≦(2Si+Mn+3Cr) / 3Mo≦248 (1) 1.6≦Al / N≦2.8 (2) However, the element symbols in the formulas (1) and (2) indicate the content of the element in mass %. Each formula will be explained below.

[0060] [Regarding formula (1)] Regarding the left side of formula (1), F1 is defined as (2Si+Mn+3Cr) / 3Mo. F1 is an index indicating the likelihood of the formation of a soft, incompletely hardened layer on the surface of steel during gas carburizing, a process used to manufacture carburized steel parts from steel. Because Si, Mn, and Cr are all oxide-forming elements, these elements dissolved in the steel and the O2 in the atmosphere form intergranular oxide layers during gas carburizing. Furthermore, incompletely hardened structures are formed in the vicinity of these layers due to the lack of alloying elements. Because the incompletely hardened structures are soft, they reduce the bending fatigue strength of steel parts. Therefore, from the perspective of intergranular oxide layer formation, the inclusion of Si, Mn, and Cr in steel is undesirable. However, these elements are effective in improving hardenability and softening resistance, making them essential for increasing surface fatigue strength. On the other hand, Mo, while significantly more expensive than these elements, significantly improves hardenability without forming oxides. Therefore, by optimizing the content ratio of Si, Mn, and Cr, which are oxide-forming elements, to Mo, which does not form oxides but improves hardenability and is effective in suppressing incompletely hardened structures, it is possible to improve the softening resistance and carburization properties of carburized parts while stably suppressing the formation of incompletely hardened layers.

[0061] In the chemical composition of the steel material of this embodiment, provided that the contents of each element are within the above-mentioned ranges, if F1 is 21 to 248, that is, if the above formula (1) is satisfied, the Mo content relative to the Si, Mn, and Cr contents will be in an appropriate range, and the formation of an incompletely hardened layer during gas carburizing can be suppressed. As a result, provided that the contents of each element in the chemical composition are within the above-mentioned ranges and the formula (2) described below is satisfied, carburized steel parts manufactured using this steel material will have excellent surface fatigue strength and bending fatigue strength.

[0062] When F1 is less than 21, the contents of Si, Mn, and Cr are extremely low relative to the content of Mo. In this case, not only is the effect of suppressing the formation of incompletely hardened structures saturated, but sufficient tempering softening resistance is difficult to obtain. As a result, the surface fatigue strength of the carburized steel part decreases. On the other hand, when F1 exceeds 248, the contents of Si, Mn, and Cr are excessively high relative to the content of Mo. In this case, a thick intergranular oxide layer forms on the surface of the steel during gas carburizing, which easily leads to the formation of an incompletely hardened structure layer. As a result, the bending fatigue strength of the carburized steel part decreases. The lower limit of F1 is preferably 25 or more, more preferably 30 or more, and even more preferably 35 or more. The upper limit of F1 is preferably 240 or less, more preferably 230 or less, even more preferably 220 or less, and even more preferably 210 or less. F1 is a value obtained by rounding off the calculated value to one decimal place.

[0063] [Regarding formula (2)] Regarding the left side of equation (2), F2=Al / N is defined. F2 is an index related to Al inclusions (oxide-based inclusions) and AlN precipitates, which affect bending fatigue strength. Al inclusions become the starting point for cracks during bending fatigue of carburized steel parts. Coarse AlN (precipitates) also become the starting point for cracks during bending fatigue of carburized steel parts. Therefore, it is preferable to suppress the formation of Al inclusions and coarse AlN (precipitates) as much as possible. To achieve this, it is important to optimize the balance between the N and Al contents in the steel.

[0064] In the chemical composition of the steel material of this embodiment, assuming that the contents of each element are within the above ranges, if F2 is less than 1.6, the composition will have an excessively high N content relative to the Al content, and in this case, the steel material will contain excessively large coarse AlN (precipitates), which will reduce the bending fatigue strength of the carburized steel part.

[0065] On the other hand, in the chemical composition of the steel material of this embodiment, assuming that the contents of each element are within the above-mentioned ranges, if F2 exceeds 2.8, the composition will have an Al content that is excessively higher than the Nl content, and in this case, there will be an excessive amount of Al inclusions (oxide-based inclusions) that have not bonded with N. Therefore, in this case too, the bending fatigue strength of the carburized steel part will decrease.

[0066] In the chemical composition of the steel material of this embodiment, provided that the content of each element is within the above range, if F2 is 1.6 to 2.8, the formation of Al inclusions and coarse AlN (precipitates) in the steel material can be sufficiently suppressed. Therefore, provided that formula (1) is satisfied, the bending fatigue strength of the carburized steel part can be increased.

[0067] The lower limit of F2 is preferably 1.7 or more, more preferably 1.8 or more, and even more preferably 1.9 or more. The upper limit of F2 is preferably 2.7 or less, more preferably 2.6 or less, and even more preferably 2.5 or less. F2 is a value obtained by rounding off the calculated value to one decimal place.

[0068] [Steel microstructure] The microstructure of the steel material of this embodiment is not particularly limited. The objective of the steel material of this embodiment is to obtain high bending fatigue strength and high surface fatigue strength when a carburized steel part is manufactured using the steel material as a raw material. In the manufacturing process for manufacturing a carburized steel part using the steel material as a raw material, for example, gas carburizing is performed on the steel material, as will be described later. In the gas carburizing process, a steel part formed into a part shape is subjected to A c3 Because the steel part is heated above its transformation temperature, the microstructure of the steel part transforms to austenite. As a result, the structure of the resulting carburized steel part is not affected by the structure of the steel material from which the carburized steel part is made. Therefore, the microstructure of the steel material from which the carburized steel part is made does not need to be particularly limited. The microstructure of the steel material may be, for example, a structure consisting of one or more of ferrite, pearlite, and bainite (or a mixed structure). It is important that the steel material of this embodiment has the content of each element in the chemical composition within the above-mentioned range, and furthermore, satisfies formulas (1) and (2). As a result, when a carburized steel part is manufactured by carrying out gas carburizing treatment using the steel material of this embodiment as a raw material, the carburized steel part can obtain high bending fatigue strength and surface fatigue strength (pitting resistance).

[0069] [Steel use] The steel material of this embodiment is suitable as a material for carburized steel parts manufactured by gas carburizing, particularly for carburized steel parts that require high bending fatigue strength and surface fatigue strength (pitting characteristics), such as gears used in machinery products such as automobiles and construction vehicles.

[0070] [Carburized steel parts] The carburized steel part of this embodiment is manufactured by gas carburizing (gas carburizing or gas carbonitriding) using the steel material of this embodiment as a raw material. The carburized steel part is a machine part, such as a gear, used in, for example, automobiles and construction vehicles.

[0071] The carburized steel part of this embodiment comprises a hardened layer and a core portion located inside the hardened layer. The hardened layer is a layer hardened by the penetration of C by gas carburizing. Specifically, when gas carburizing is performed, the hardened layer corresponds to the carburized layer, and when gas carbonitriding is performed, the hardened layer corresponds to the carbonitrided layer. The core portion is located inside the hardened layer and is a region that is not affected by the penetration and diffusion of C by gas carburizing. It is a technical matter well known to those skilled in the art that the hardened layer and the core portion can be distinguished by well-known microstructure observation. The hardened layer is, for example, a region from the surface of the part to a depth of approximately 1.5 mm.

[0072] [About the core] The chemical composition of the core of the carburized steel part of this embodiment is the same as that of the steel material of this embodiment described above. Specifically, the chemical composition of the core of the carburized steel part of this embodiment contains, in mass%, C: 0.09 to 0.27%, Si: more than 0.35% but less than 0.90%, Mn: 0.25% or more but less than 0.60%, P: 0.030% or less, S: 0.100% or less, Cr: more than 1.45% but less than 2.90%, Mo: 0.010 to 0.150%, Al: 0.045% or less, and N: 0.0250% or less, with the balance being Fe and impurities, and satisfies formulas (1) and (2).

[0073] 21≦(2Si+Mn+3Cr) / 3Mo≦248 (1) 1.6≦Al / N≦2.8 (2) However, the element symbols in the formulas (1) and (2) indicate the content of the element in mass %.

[0074] [About the hardened layer] The hardened layer has the following structure: (i) The carbon concentration in the region from the surface to a depth of 30 μm of the carburized steel part is 0.60% or more by mass. (ii) The microstructure at a depth of 30 μm from the surface of the carburized steel part is a structure consisting of martensite or a structure consisting of martensite and retained austenite, and the volume fraction of retained austenite is 0 to 50%. Each component will be described below.

[0075] [C concentration in the surface layer] The surface of the carburized steel part, that is, the region from the surface of the hardened layer to a depth of 30 μm (hereinafter referred to as the surface region), is included in the hardened layer. The carbon concentration in the surface region is 0.60% or more by mass. The carbon concentration in the hardened layer is higher than the carbon concentration in the core. If the carbon concentration in the surface region is 0.60% or more by mass, the hardened layer is sufficiently hard. Therefore, the carburized steel part can achieve sufficient surface fatigue strength and sufficient bending fatigue strength.

[0076] The lower limit of the C concentration in the surface region is preferably 0.65% or more, more preferably 0.70% or more, and even more preferably 0.75% or more. The upper limit of the C concentration in the surface region is not particularly limited. For example, the upper limit of the C concentration in the surface region is preferably 1.30% or less, more preferably 1.20% or less, and even more preferably 1.10% or less.

[0077] [Method for measuring surface carbon concentration] The carbon concentration in the surface layer can be measured by the following method. Cutting is performed to a depth of 30 μm from the surface of the hardened layer, and chips from the surface to a depth of 30 μm are collected. Chemical analysis is performed using the collected chips. Specifically, the well-known high-frequency combustion method (combustion-infrared absorption method) is performed on the collected chips to obtain the carbon concentration. More specifically, the above-mentioned chips are burned by high-frequency induction heating in an oxygen stream, and the generated carbon dioxide and carbon monoxide are detected to determine the carbon concentration (mass %). The obtained carbon concentration (mass %) is defined as the carbon concentration (mass %) in the region from the surface of the carburized steel part to a depth of 30 μm (surface region).

[0078] [Microstructure of hardened layer] The carburized steel part, i.e., the hardened layer, is included at a depth of 30 μm from the surface. The microstructure at a depth of 30 μm from the surface of the carburized steel part is a structure consisting of martensite or a structure consisting of martensite and retained austenite, and the volume fraction of retained austenite is 0 to 50%. In other words, when the microstructure consists of martensite and retained austenite, the volume fraction of retained austenite is 50% or less.

[0079] If the volume fraction of retained austenite at a depth of 30 μm from the surface of a carburized steel part exceeds 50%, the hardness of the hardened layer decreases. In this case, the bending fatigue strength of the carburized steel part decreases. On the other hand, if the volume fraction of retained austenite at a depth of 30 μm from the surface of a carburized steel part is 50% or less, the hardened layer has sufficient hardness. Therefore, the bending fatigue strength of the carburized steel part increases. The upper limit of the volume fraction of retained austenite at a depth of 30 μm from the surface of a carburized steel part is preferably 40% or less, more preferably 35% or less, and even more preferably 30% or less. A lower volume fraction of retained austenite is preferable. However, it is difficult to achieve 0% retained austenite, and this increases manufacturing costs. Therefore, considering industrial production, the lower limit of retained austenite is preferably greater than 0%, and even more preferably 1% or more. Although the region shallower than 30 μm deep in a carburized steel part is more strongly affected by carburization than the region at 30 μm deep, the region is closer to the part surface and therefore has a weaker restraining force from the surrounding structure of the retained austenite. This region is also a region where an incompletely hardened structure may occur due to a deficiency of alloying elements caused by the grain boundary oxide layer. For these reasons, the amount of retained austenite in this region is smaller than that at 30 μm deep. Therefore, if the volume fraction of retained austenite at a depth of 30 μm from the surface of a carburized steel part is 50% or less, the volume fraction of retained austenite in the region shallower than 30 μm deep is also 50% or less.

[0080] [Method for measuring the volume fraction of retained austenite] The volume fraction of retained austenite is measured by the following method. A carburized steel component is electropolished to a depth of 30 μm from the surface, exposing a portion 30 μm from the surface. An X-ray diffractometer is then used to irradiate X-rays at any point on the exposed surface to measure the volume fraction (%) of retained austenite. The volume fraction of retained austenite is calculated from the ratio (integral intensity ratio) of the integrated intensity of the (211) bcc diffraction peak obtained by X-ray diffraction to the integrated intensity of the (220) fcc diffraction peak. Specifically, the volume fraction (%) of retained austenite can be calculated using the following formula, where Iα is the integrated intensity of the (211) bcc (α phase) and Iγ is the integrated intensity of the (220) fcc (γ phase). Volume fraction of retained austenite = Iγ / (RIα+Iγ) Here, R = 0.36746.

[0081] A carburized steel part having the above configuration has the content of each element in the chemical composition of the core within the above-mentioned ranges and satisfies formulas (1) and (2). Furthermore, the carbon concentration in the region from the surface of the carburized steel part to a depth of 30 μm (surface region) is 0.60% or more, and the microstructure at a depth of 30 μm from the surface of the carburized steel part is a structure consisting of martensite or a structure consisting of martensite and retained austenite, with the volume fraction of retained austenite being 0 to 50%. Therefore, the carburized steel part of this embodiment has high bending fatigue strength and high surface fatigue strength.

[0082] [Steel manufacturing method] An example of a method for manufacturing a steel material according to this embodiment will be described. The method for manufacturing a steel material described below is one example for manufacturing the steel material according to this embodiment. Therefore, a steel material having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel material according to this embodiment.

[0083] An example of the method for manufacturing a steel material according to this embodiment includes a step of preparing a material (material preparation step) and a step of manufacturing a steel material by hot working the material (hot working step). Each step will be described below.

[0084] [Material preparation process] In the material preparation step, a material for the steel material of this embodiment is prepared. Specifically, molten steel is produced in which the content of each element in the chemical composition is within the range of this embodiment and satisfies formulas (1) and (2). The refining method is not particularly limited, and a well-known method may be used. For example, molten pig iron produced by a well-known method is subjected to refining (primary refining) in a converter. The molten steel tapped from the converter is subjected to well-known secondary refining. In the secondary refining, alloy elements are added to adjust the composition, and molten steel is produced in which the content of each element is within the range of this embodiment and which has a chemical composition that satisfies formulas (1) and (2).

[0085] The molten steel produced by the above-described refining method is used to produce raw steel materials by a well-known casting method. For example, the molten steel is used to produce ingots by an ingot casting method. Alternatively, the molten steel may be used to produce blooms by a continuous casting method. By the above-described methods, raw steel materials (ingots or blooms) are produced.

[0086] [Hot processing process] In the hot working step, the material (ingot or bloom) prepared in the material preparation step is hot worked to produce the steel material (e.g., steel bar) of this embodiment. The hot working method may be hot forging or hot rolling. In the following explanation, the case where the hot working is hot rolling will be described. In this case, the hot working step includes, for example, a blooming step and a finish rolling step.

[0087] (Bulking rolling process) In the blooming process, a material is hot rolled to produce billets. Specifically, in the blooming process, a billet is produced by hot rolling the material using a blooming mill. If a continuous rolling mill is located downstream of the blooming mill, the billet after blooming may be further hot rolled using the continuous rolling mill to produce a smaller billet. The heating temperature in the blooming process may be within a known range. The heating temperature is, for example, 1000 to 1300°C.

[0088] (Finishing rolling process) In the finish rolling process, the billet produced in the blooming process is hot rolled using a continuous rolling mill to produce a steel material (e.g., a steel bar). A known temperature is sufficient for the heating temperature in the finish rolling process. The heating temperature is, for example, 900 to 1250°C. After hot rolling, the steel material is cooled to room temperature. The cooling method is not particularly limited, but may be, for example, natural cooling.

[0089] The steel material of this embodiment is manufactured by the above manufacturing method. Note that the above manufacturing method is one example of a manufacturing method for manufacturing the steel material of this embodiment. Therefore, the steel material of this embodiment may be manufactured by a method other than the above manufacturing method. In other words, the manufacturing method is not limited as long as the content of each element in the chemical composition is within the range of this embodiment and the steel material satisfies formulas (1) and (2).

[0090] In the example of the manufacturing method described above, the hot working step is carried out after the material preparation step. However, in the manufacturing method of the steel material of this embodiment, the hot working step does not have to be carried out after the material preparation step. In other words, the steel material of this embodiment may be a cast material (ingot, bloom, or billet).

[0091] Furthermore, the steel material after the material preparation step or the steel material after the hot working step may be subjected to a known normalizing treatment and / or a known spheroidizing annealing. In the spheroidizing annealing, for example, the annealing temperature is set to 720 to 780°C, and the holding time at the annealing temperature is set to 3 to 8 hours. Furthermore, the cooling time from the annealing temperature to 600°C is set to 4 hours or more (8 hours or less). Thereafter, the steel material is allowed to cool naturally.

[0092] [Manufacturing method for carburized steel parts] An example of a method for manufacturing a carburized steel part according to this embodiment will now be described. The method for manufacturing a carburized steel part described below is one example for manufacturing a carburized steel part using the steel material according to this embodiment as a raw material. Therefore, a carburized steel part having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a carburized steel part according to this embodiment.

[0093] The method for manufacturing a carburized steel part includes a hot working step or a cold working step, a machining step, and a heat treatment step. Either the hot working step or the cold working step is performed.

[0094] [Hot processing process] When a hot working step is performed, the steel material of this embodiment is hot worked and formed into a predetermined shape to produce an intermediate product. The hot working is, for example, hot forging. The heating temperature in the hot working step is, for example, 1000 to 1300°C. After the hot working, the steel material is allowed to cool (air-cooled).

[0095] [Cold working process] When a cold working step is performed, the steel material of this embodiment is subjected to well-known normalizing or spheroidizing annealing (see above), and then cold working is performed to form it into a predetermined shape to produce an intermediate product. The cold working is, for example, cold forging. The conditions for cold working are not particularly limited.

[0096] [Machining process] The intermediate product after the hot working process or cold working process may be further machined. In the machining process, the intermediate product is machined to produce an intermediate product of a predetermined shape. By performing machining, it is possible to give the carburized steel part a precise shape that would be difficult to achieve using only the hot working process or cold working process. Machining includes, for example, cutting and drilling. If the part is a gear, it is machined by, for example, broaching.

[0097] [Heat treatment process] The obtained intermediate product is subjected to a heat treatment. Here, "heat treatment" includes a gas carburizing treatment process and a tempering process. The gas carburizing treatment process includes a gas carburizing process and a quenching process. The gas carburizing treatment process will be described below as an example of a heat treatment process.

[0098] [Gas carburizing process] The gas carburizing process includes a gas carburizing process and a quenching (quenching) process. The gas carburizing process and the quenching process will be described below.

[0099] (Gas carburizing process) Figure 1 shows an example of a heat pattern for the gas carburizing step S10 and the quenching step S20. The vertical axis of Figure 1 represents the treatment temperature (°C) during the gas carburizing treatment, and the horizontal axis represents time (minutes). The gas carburizing step S10 includes a heating step S0, a carburizing step S1, and a diffusion step S2.

[0100] In the heating step S0, the intermediate product placed in the furnace is heated to a carburizing temperature Tc, which is, for example, 900 to 1100°C.

[0101] In the carburizing step S1, the intermediate product is held at the carburizing temperature Tc for a predetermined time (holding time t1) in an atmosphere of a predetermined carbon potential CP1, for example, 0.6 to 1.3%, and the holding time t1 at the carburizing temperature Tc is, for example, 60 minutes or more.

[0102] In the diffusion step S2, the carburizing temperature Tc is maintained for a predetermined time (maintenance time t2) in an atmosphere of a predetermined carbon potential CP2. Here, the carbon potential CP2 in the diffusion step S2 is, for example, 0.6 to 1.3%, and the maintenance time t2 at the carburizing temperature Tc is, for example, 30 minutes or more. It is preferable that the carbon potential CP2 in the diffusion step S2 is lower than the carbon potential CP1 in the carburizing step S1.

[0103] (Quenching process) The intermediate product after the gas carburizing process S10 is subjected to the quenching process S20. In the quenching process S20, the intermediate product after the gas carburizing process S10 is r3 After being held at a quenching temperature Ts equal to or higher than the quenching temperature Ts, the intermediate product is quenched and quenched. The holding time t3 at the quenching temperature Ts is not particularly limited, but is, for example, 15 to 60 minutes. The quenching temperature Ts is preferably lower than the carburizing temperature Tc. The cooling method in the quenching process is oil cooling or water cooling. Specifically, the intermediate product held at the quenching temperature is immersed in a cooling bath containing oil or water as a cooling medium to be quenched.

[0104] [Tempering process] The intermediate product after the quenching process is subjected to a known tempering process. The tempering temperature is, for example, 100 to 200° C. The holding time at the tempering temperature is, for example, 90 to 150 minutes.

[0105] [Other processes] The method for manufacturing a carburized steel part of this embodiment may further include a shot peening step and a finish grinding step, which are optional steps.

[0106] (Shot peening process) The shot peening process is an optional process and does not necessarily have to be performed. If performed, the shot peening process is performed on an intermediate product after the heat treatment process. By performing the shot peening process, the retained austenite in the hardened layer of the carburized steel part undergoes a deformation-induced transformation to martensite. As a result, the volume fraction of retained austenite in the hardened layer can be reduced. For example, the shot peening process preferably uses a cut wire or shot particles with a diameter of 1.0 mm or less, an arc height of 0.3 mm or more, and a coverage of 300% or more.

[0107] (finish grinding process) The finish grinding process is an optional process and does not have to be performed. If performed, the finish grinding process is a finish cutting process performed on the intermediate product after the heat treatment process or the shot peening process to adjust the surface texture.

[0108] The carburized steel part of this embodiment can be manufactured using the above manufacturing steps. The above-described manufacturing method is one example of a manufacturing method for manufacturing the carburized steel part of this embodiment. Therefore, the carburized steel part of this embodiment may also be manufactured by a method other than the above-described manufacturing method. That is, the manufacturing method for the carburized steel part is not particularly limited as long as the element contents in the chemical composition of the core of the carburized steel part are within the ranges of this embodiment and satisfy formulas (1) and (2), the C concentration in the region from the surface to a depth of 30 μm of the carburized steel part is 0.60% or more, the microstructure at a depth of 30 μm from the surface of the carburized steel part consists of martensite or martensite and retained austenite, and the volume fraction of retained austenite is 0 to 50%. [Example]

[0109] The effects of the steel material and carburized steel part of this embodiment will be explained in more detail below using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel material and carburized steel part of this embodiment. Therefore, the steel material and carburized steel part of this embodiment are not limited to this one example of conditions.

[0110] [Steel manufacturing] Molten steel having the chemical composition shown in Table 1 was produced.

[0111] [Table 1]

[0112] Ingots were produced by ingot casting using the molten steel shown in Table 1. The cross section of the ingot perpendicular to the longitudinal direction was a rectangle measuring 180 mm x 180 mm. The produced ingots were allowed to cool to room temperature. Note that underlines in Table 1 indicate compositions outside the scope of the present invention, and blank spaces mean that the corresponding element content is 0% in significant figures (numbers down to the least significant digit) as specified in the embodiment. Also, an "*" in the "F1" column in Table 1 means that the Mo content was 0% in significant figures in other examples, and therefore formula (1) could not be calculated. However, test numbers 3, 11, 12 and 15 were used as reference examples.

[0113] The obtained ingots were heated at 1200°C for 2 hours. The heated ingots were subjected to hot working (hot forging) to produce steel materials (steel bars) with a diameter of 40 mm and a length of 1000 mm. The hot-worked steel materials were allowed to cool to room temperature. The steel materials after cooling were subjected to a normalizing treatment. The treatment temperature in the normalizing treatment was 925°C, and the holding time at the treatment temperature was 90 minutes. After the holding time had elapsed, the steel materials were allowed to cool. The cooling rate of the steel materials during cooling was 0.3 to 0.9°C / second. Steel materials (steel bars) of each test number were produced by the above steps.

[0114] The chemical composition of the steel material of test number 36 corresponds to that of SCr420 specified in JIS G 4805 (2019). In this example, the steel material of test number 36 was evaluated as a "reference steel material" in the various tests described below.

[0115] [Manufacturing carburized steel part test pieces] The steel materials manufactured for each test number were used to prepare the following three types of carburized steel part test pieces for each test number.

[0116] (1) Small roller test piece Figure 2 shows a side view of the small roller test piece produced in this example. The numbers in Figure 2 indicate dimensions (unit: mm). "φ" in Figure 2 refers to diameter. The inverted triangle symbol in Figure 2 refers to the "finish code" indicating surface roughness listed in Table 1 of JIS B 0601 (1982). The "G" attached to the finish code refers to the abbreviation for the processing method indicating grinding specified in JIS B 0122 (1978). The small roller test piece is a test piece for measuring surface fatigue strength. Multiple small roller test pieces were prepared for each test number.

[0117] Specifically, first, the steel material of each test number was machined to produce a rough test piece having the rough shape of the small roller test piece. The central axis of the rough test piece was coaxial with the central axis of the steel bar. The rough test piece was subjected to gas carburizing treatment (gas carburizing quenching and tempering). Specifically, the rough test piece was held at 930°C for 180 minutes in an atmosphere with a carbon potential CP of 1.0% (carburizing process). Then, the carbon potential CP was set to 0.8%, and the rough test piece was held at 930°C for 120 minutes (diffusion process). Then, the rough test piece was cooled to 870°C, held at 870°C for 30 minutes, and then oil-quenched in 60°C oil (quenching process). The rough test piece after oil-quenching was subjected to tempering treatment. The tempering temperature was 180°C, and the holding time at the tempering temperature was 120 minutes.

[0118] After tempering, the cylindrical portion at the center of the rough test piece was ground to produce the 26 mm diameter cylindrical portion (test surface) shown in Figure 2. The surface of the 26 mm diameter cylindrical portion was finished in accordance with JIS B 0601 (2001) so that the arithmetic mean roughness Ra was 0.6 to 0.8 μm and the maximum height Rz was 2.0 to 4.0 μm. The grinding depth was approximately 10 μm. In the actual roller pitting test using the small roller test piece, the 26 mm diameter cylindrical portion (test surface) was brought into contact with a large roller and rotated under a specified surface pressure.

[0119] Furthermore, shot peening was performed on the rough test pieces of some of the test numbers (indicated as "Yes" in the "Shot Peening" column in Table 2). For shot peening, a commercially available round cut wire with a diameter of 0.6 mm was used as the shot material. Furthermore, the arc height was set to 0.4 mm, and the coverage was set to 300%. Shot peening was performed on the outer circumferential surface of the cylindrical part with a diameter of 26 mm shown in Figure 2. Shot peening was not performed on the rough test pieces of the remaining test numbers (indicated as "No" in the "Shot Peening" column in Table 2). Small roller test pieces were produced using the above manufacturing process.

[0120] (2) Rotating bending fatigue test specimen Figure 3 shows a side view of the rotating bending fatigue test specimen prepared in this example. The numbers in Figure 3 indicate dimensions (unit: mm). "φ" in Figure 3 means diameter. "R" in Figure 3 means radius of curvature. The rotating bending fatigue test specimen is a test specimen for measuring rotating bending fatigue strength.

[0121] Specifically, first, the steel material of each test number was machined to produce a rough test piece for the rotating bending fatigue test. The central axis of the rough test piece was coaxial with the central axis of the steel bar. The rough test piece was subjected to gas carburizing (gas carburizing, quenching, and tempering). Specifically, the rough test piece was held at 930°C for 180 minutes in an atmosphere with a carbon potential CP of 1.0% (carburizing process). Then, the carbon potential CP was set to 0.8%, and the rough test piece was held at 930°C for 120 minutes (diffusion process). Then, the rough test piece was cooled to 870°C, held at 870°C for 30 minutes, and then oil-quenched in 60°C oil (quenching process). The rough test piece after oil-quenching was subjected to tempering. The tempering temperature was 180°C, and the holding time at the tempering temperature was 120 minutes.

[0122] Furthermore, for some test numbers, shot peening was performed on the rough test pieces after tempering (indicated as "Yes" in the "Shot Peening" column in Table 2). For the shot peening, a commercially available round cut wire with a diameter of 0.6 mm was used as the shot material. Furthermore, the arc height was set to 0.4 mm, and the coverage was set to 300%. The shot peening was performed on the area corresponding to the notch in the rotating bending fatigue test piece. For the rough test pieces of the remaining test numbers, shot peening was not performed (indicated as "No" in the "Shot Peening" column in Table 2).

[0123] After shot peening or tempering, the surface of the rough test piece was machined to prepare a rotating bending fatigue test piece with the dimensions shown in Figure 3. Note that no machining to adjust the surface texture was performed on the notch formed in the center of the longitudinal direction of the rotating bending fatigue test piece. The rotating bending fatigue test piece was prepared by the above manufacturing process.

[0124] (3) Test piece for hardened layer investigation Two test pieces for each test number were prepared for the hardened layer investigation. The test pieces for the hardened layer investigation were cylindrical test pieces with a diameter of 26 mm and a length of 100 mm.

[0125] Specifically, the steel material of each test number was first machined to prepare two cylindrical rough test pieces with a diameter of 26 mm and a length of 100 mm. The central axis of the rough test pieces was coaxial with the central axis of the steel bar. The rough test pieces were subjected to gas carburizing (gas carburizing, quenching, and tempering). Specifically, the rough test pieces were held at 930°C for 180 minutes in an atmosphere with a carbon potential CP of 1.0% (carburizing process). Then, the carbon potential CP was set to 0.8%, and the rough test pieces were held at 930°C for 120 minutes (diffusion process). The rough test pieces were then cooled to 870°C, held at 870°C for 30 minutes, and then oil-quenched in 60°C oil (quenching process). The rough test pieces after oil-quenching were then tempered. The tempering temperature was 180°C, and the holding time at the tempering temperature was 120 minutes. The outer peripheral surface of the coarse test piece was then ground in the same manner as the small roller test piece to finish the outer peripheral surface. In accordance with JIS B 0601 (2001), the outer peripheral surface of the coarse test piece with a diameter of 26 mm was ground to an arithmetic mean roughness Ra of 0.6 to 0.8 μm and a maximum height Rz of 2.0 to 4.0 μm. The grinding depth was approximately 10 μm.

[0126] Furthermore, shot peening was performed on the rough test pieces of some test numbers (indicated as "Yes" in the "Shot Peening" column in Table 2). In the shot peening, a commercially available round cut wire with a diameter of 0.6 mm was used as the shot material. Furthermore, the arc height was set to 0.4 mm, and the coverage was set to 300%. The shot peening was performed on the outer periphery of the rough test pieces. No shot peening was performed on the rough test pieces of the remaining test numbers (indicated as "No" in the "Shot Peening" column in Table 2). Test pieces for investigating the hardened layer were prepared using the above manufacturing process.

[0127] [Manufacturing large roller test pieces for two-roller rolling fatigue tests] Furthermore, large roller test pieces to be used in a two-cylinder rolling fatigue test for measuring surface fatigue strength were prepared by the following method. A rough specimen for the large roller test piece, with the shape shown in Figure 4, was cut from a cylindrical material with a diameter of 140 mm and a chemical composition equivalent to SUJ2 as specified in JIS G 4805 (2008). The numbers in Figure 4 indicate dimensions (unit: mm). The inverted triangle symbol in Figure 4 represents the "finishing symbol" indicating the surface roughness listed in Table 1 of JIS B 0601 (1982). The "G" attached to the finishing symbol represents the abbreviation for the processing method, indicating grinding as specified in JIS B 0122 (1978).

[0128] The cut-out rough test pieces were quenched. The quenching temperature was 870°C, and the holding time at the quenching temperature was 90 minutes. After the holding time had elapsed, the pieces were quenched in oil at 60°C. The outer peripheral surfaces of the quenched rough test pieces were then finished by cutting. The outer peripheral surfaces were finished so that the arithmetic mean roughness Ra was 0.6 to 0.8 μm and the maximum height Rz was 2.0 to 4.0 μm. Large roller test pieces were produced using the above manufacturing process.

[0129] [Evaluation test] The following evaluation tests were carried out using the above-mentioned various test pieces.

[0130] (C concentration measurement test for hardened layer) Using the test pieces for hardened layer investigation of each test number, the carbon concentration in the region from the surface of the test piece to a depth of 30 μm was measured by the following method. First, turning was performed on the test piece for hardened layer investigation to a depth of 30 μm from the surface, and chips were collected. Chemical analysis was performed using the collected chips. Specifically, the collected chips were dissolved in acid to obtain a solution. The obtained solution was subjected to the well-known high-frequency combustion method (combustion-infrared absorption method) to obtain the carbon concentration. More specifically, the above solution was combusted by high-frequency induction heating in an oxygen stream, and the generated carbon dioxide was detected to determine the carbon concentration (mass%). The obtained carbon concentration (mass%) was defined as the carbon concentration (mass%) in the region from the surface of the carburized steel part to a depth of 30 μm (surface region).

[0131] (Test to measure the volume fraction of retained austenite in the hardened layer) Using the test pieces for hardened layer investigation of each test number, the volume fraction of retained austenite at a depth of 20 μm from the surface of the carburized steel part was determined by the following method. First, the surface of a carburized steel component was electropolished to a depth of 20 μm, exposing a portion 20 μm from the surface. Next, an X-ray diffractometer was used to irradiate X-rays three times at random positions on the exposed surface to measure the average volume fraction (%) of retained austenite. The volume fraction of retained austenite was calculated from the ratio of the integrated intensity of the (211) bcc diffraction peak to the integrated intensity of the (220) fcc diffraction peak (integral intensity ratio) obtained by X-ray diffraction. Specifically, the volume fraction (%) of retained austenite was calculated using the following equation, where Iα is the integrated intensity of the (211) bcc (α phase) and Iγ is the integrated intensity of the (220) fcc (γ phase). Volume fraction of retained austenite = Iγ / (RIα+Iγ) Here, R = 0.36746.

[0132] (Surface fatigue strength measurement test (two-cylinder rolling fatigue test)) A two-cylinder rolling fatigue test was carried out using the small roller test piece and the large roller test piece to determine the surface fatigue strength as follows. The test machine used was a roller pitting tester "RP201" manufactured by Komatsu Engineering Co., Ltd.

[0133] As shown in Figure 5, the small roller test piece 10 was rolled while in contact with the central position of the outer circumferential surface (the outer circumferential portion of 130 mm diameter) of the large roller test piece 20. The Hertzian contact pressure during contact was 1800 to 3500 MPa. The rotation speed of the small roller test piece 10 was 1500 rpm. The peripheral speed of the small roller test piece 10 was 123 m / min, and the peripheral speed of the large roller test piece 10 was 172 m / min. During the test, lubricating oil was supplied to the contact area between the small roller test piece and the large roller test piece. The lubricating oil was automatic transmission oil, and the oil temperature was 100°C and the oil flow rate was 1.0 L / min. The slide-to-roll ratio was -40%.

[0134] The number of repeated cycles in the test was 2.0 × 10, which indicates the fatigue limit of general steel. 7 The small roller test piece was tested for 2.0 × 10 7 The maximum surface pressure (MPa) reached at this time was taken as the fatigue limit of the small roller test piece.

[0135] The occurrence of pitting was detected using a vibration meter attached to the testing machine. After vibration was generated, the rotation of both the small roller test piece and the large roller test piece was stopped, and the occurrence of pitting and the number of rotations were confirmed.

[0136] In this example, assuming application to gear components, the fatigue limit of a small roller test piece made of steel (reference steel) meeting the SCr420 standard for test number 36 was used as the reference value. If the fatigue limit was 1.15 times or more that of the reference steel, the test piece was judged to have excellent contact fatigue strength (marked "○" in the "contact fatigue strength" column in Table 2). On the other hand, if the fatigue limit was less than 1.15 times that of the reference steel, the test piece was judged to have low contact fatigue strength (marked "×" in the "contact fatigue strength" column in Table 2).

[0137] (Rotating bending strength measurement test (rotating bending fatigue test)) Rotating bending fatigue tests were conducted using rotating bending fatigue test specimens in accordance with the "Rotating bending fatigue test method for metallic materials" specified in JIS Z 2274 (1978). The tests were conducted at room temperature in an air atmosphere, with the rotation speed set to 3000 rpm. The number of repeated stress loads was 10. 7 The maximum stress at which the specimen did not fracture after the cycles was defined as the bending fatigue strength (MPa). If the obtained bending fatigue strength was 1.10 times or more the bending fatigue strength of the reference steel material, test number 37, the specimen was judged to have excellent bending fatigue strength (marked "○" in the "Bending fatigue strength" column in Table 2). On the other hand, if the obtained bending fatigue strength was less than 1.10 times the bending fatigue strength of the reference steel material, test number 37, the specimen was judged to have low bending fatigue strength (marked "×" in the "Bending fatigue strength" column in Table 2).

[0138] [Evaluation results] The test results are shown in Table 2.

[0139] [Table 2]

[0140] Referring to Table 2, the contents of each element in the chemical composition of the steels of test numbers 1 to 19 were appropriate, and furthermore, F1 and F2 satisfied formulas (1) and (2). Therefore, in the carburized steel parts manufactured by gas carburizing, the C concentration in the region from the surface to a depth of 30 μm of the carburized steel part was 0.60% by mass or more, and the microstructure at a depth of 30 μm from the surface of the carburized steel part consisted of martensite and retained austenite, with the volume fraction of retained austenite being 50% or less. As a result, excellent bending fatigue strength and excellent surface fatigue strength were obtained.

[0141] On the other hand, in the steel materials of test numbers 20 to 35, the content of each element in the chemical composition was outside the range of the present invention, or F1 or F2 did not satisfy formula (1) or formula (2), and therefore the contact fatigue strength and bending fatigue strength were low.

[0142] The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

Claims

1. In mass%, C: 0.09-0.27%, Si: more than 0.35%, less than 0.85%, Mn: 0.30% or more and less than 0.60%; P: 0.030% or less, S: 0.100% or less, Cr: more than 1.45%, less than 2.90%, Mo: 0.010-0.150%, Al: 0.045% or less, and N: 0.0250% below, and the balance being Fe and impurities, and satisfying the following formulas (1) and (2): 21≦(2Si+Mn+3Cr) / 3Mo≦248...(1) 1.6≦Al / N≦2.5 (2) However, the element symbols in the formulas (1) and (2) indicate the content of the element in mass %.

2. Furthermore, in mass%, Cu: 0.50% or less, Ni: less than 0.05% V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.100% or less, Ti: 0.100% or less, and B: 0.0010% or less, 2. The steel material for carburizing according to claim 1, comprising one or more selected from the group consisting of:

3. Furthermore, in mass%, Ca: 0.0100% or less, Mg: less than 0.0015%, and Rare earth elements: 0.0100% or less, 3. The steel material for carburizing according to claim 1, which contains one or more selected from the group consisting of:

4. Furthermore, in mass%, Te: less than 0.0080% Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.015% or less, Sb: 0.015% or less, and The steel material for carburizing according to any one of claims 1 to 3, comprising one or more selected from the group consisting of:

5. A carburized steel part, A hardened layer; a core portion located inside the hardened layer, The composition of the core is, in mass %, C: 0.09-0.27%, Si: more than 0.35%, less than 0.85%, Mn: 0.30% or more and less than 0.60%; P: 0.030% or less, S: 0.100% or less, Cr: more than 1.45%, less than 2.90%, Mo: 0.010-0.150%, Al: 0.045% or less, and N: 0.0250% below, and the balance being Fe and impurities, and satisfying the following formulas (1) and (2): the C concentration in a region from the surface of the hardened layer to a depth of 30 μm is 0.60% by mass or more, the microstructure at a depth of 30 μm from the surface of the carburized steel part is a structure consisting of martensite or a structure consisting of martensite and retained austenite, A carburized steel part, characterized in that the volume fraction of the retained austenite is 0 to 50%. 21≦(2Si+Mn+3Cr) / 3Mo≦248...(1) 1.6≦Al / N≦2.5 (2) However, the element symbols in the above formulas (1) and (2) indicate the content of the element in mass %.

6. Furthermore, the composition of the core is, in mass %, Cu: 0.50% or less, Ni: less than 0.05% V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.100% or less, Ti: 0.100% or less, and B: 0.0010% or less, The carburized steel part according to claim 5, containing one or more selected from the group consisting of:

7. Furthermore, the composition of the core is, in mass %, Ca: 0.0100% or less, Mg: less than 0.0015%, and Rare earth elements: 0.0100% or less, 7. The carburized steel part according to claim 5, further comprising one or more selected from the group consisting of:

8. Furthermore, the composition of the core is, in mass %, Te: less than 0.0080% Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.015% or less, Sb: 0.015% or less, and The carburized steel part according to any one of claims 5 to 7, containing one or more selected from the group consisting of:

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