Carburizing steel materials and carburizing steel parts

Abstract

To provide a steel that can be vacuum-carburized into a carburized steel part, which gives excellent cold forgeability, bending fatigue strength and surface fatigue strength (pitching properties).SOLUTION: A steel comprises, in mass%, C: 0.07% to 0.30%, Si: 0.30% or more to less than 0.90%, Mn: 0.10% to 0.50%, P: 0.030% or less, S: 0.100% or less, Cr: more than 0.80% to less than 2.00%, Al: 0.045% or less, Nb: 0.001% to 0.080%, and N: 0.0250% or less, with the balance being Fe and impurities, satisfying formula (1) and formula (2): 2Si+Mn+3Cr≤6.7 (1) and (Al+2Nb) / N≤6.71 (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] Carburizing is known as a method for increasing the bending fatigue strength and surface fatigue strength of machine parts. The term "carburizing" as used here includes not only carburizing but also carbonitriding. In carburizing, a hardened layer (carburized layer or carbonitrided layer) is formed on the surface of a mechanical part. It is known that this hardened layer increases 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 performance that can further increase the bending fatigue strength and surface fatigue strength when carburized to make the mechanical part (carburized steel part).

[0004] In recent years, there has also been a growing trend to switch from hot to cold forging methods in order to reduce CO2 emissions from hot forging.

[0005] Patent Documents 1 and 2 propose steel materials that can increase bending fatigue strength and surface fatigue strength when carburized to form carburized steel parts.

[0006] 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)

[0007] 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. The steel material disclosed in Patent Document 3 contains, by mass%, C: 0.15 to 0.26%, Si: 0.05 to 1.0%, Mn: 0.1 to 0.6%, P: 0.030% or less, S: 0.030% or less, Cr: 1.80 to 2.50%, Al: 0.005 to 0.050%, N: 0.030% or less, the balance being Fe and unavoidable impurities, and has a minor axis of 50 nm or more and an aspect ratio of 3 This steel for machine structural use has excellent cold workability and resistance to grain coarsening, characterized in that the area ratio of ferrite grains in which the following carbides are precipitated intragranularly is 90% or more of the entire structure. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-185123 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-214642 [Patent Document 3] Japanese Patent Publication No. 2020-023728 Summary of the Invention [Problem to be solved by the invention]

[0009] Carburizing processes include gas carburizing and vacuum carburizing. Vacuum carburizing, which is performed under vacuum or reduced pressure, can suppress the formation of intergranular oxide layers at the surface, a problem associated with gas carburizing. Therefore, in steels used for vacuum carburizing, the content of elements that readily form oxides, such as Si, Cr, and Mn, which promote the formation of intergranular oxide layers in gas carburizing, can be increased, while still allowing the fatigue strength-improving effects of Si, Cr, and Mn to be fully utilized. On the other hand, excessively increasing the content of these elements can lead to poor cold forgeability of machine parts. Furthermore, the high dislocation density in the surface layer of cold-forged parts leads to the formation of fine austenite from the high-dislocation-density parent phase (ferrite) during vacuum carburizing, promoting abnormal grain growth. Machine parts with structures with abnormal grain growth are prone to reduced surface fatigue strength and bending fatigue strength.

[0010] To address this problem, the above-mentioned prior art techniques have not been sufficient in suppressing abnormal grain growth, and have not achieved any significant improvement in the contact fatigue strength and bending fatigue strength.

[0011] 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 cold forgeability, bending fatigue strength, and surface fatigue strength (pitting characteristics), and a carburized steel part that has excellent cold forgeability, bending fatigue strength, and surface fatigue strength. [Means for solving the problem]

[0012] 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: 0.07 to 0.30%, Si: 0.30 to less than 0.90% Mn: 0.10 to 0.50% P: 0.030% or less, S: 0.100% or less, Cr: over 0.80 to less than 2.00% Al: 0.045% or less, Nb: 0.001 to 0.080%, and N: 0.0250% down, and the balance is Fe and impurities, and satisfies formula (1) and formula (2). 2Si+Mn+3Cr≦6.7 (1) (Al + 2Nb) / N≦6.71 (2) However, the element symbols in the above 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: 0.20% or less, Ni: 0.20% or less, Mo: less than 0.100% V: 0.50% or less, W: 0.50% or less, Co: 0.50% 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: 0.0100% or less, 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: 0.0100% or less, Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.015% or less, and Sb: 0.015% or less, It may contain one or more selected from the group consisting of:

[0013] (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: 0.07 to 0.30%, Si: 0.30 to 0.90% Mn: 0.10 to 0.50% P: 0.030% or less, S: 0.100% or less, Cr: over 0.80 to less than 2.00% Al: 0.045% or less, Nb: 0.001 to 0.080%, and N: 0.0250% down, and the balance being Fe and impurities, satisfying formula (1) and formula (2), The carbon concentration in a region from the surface of the carburized steel part to a depth of 50 μm is 0.60% or more by mass%, In a region from the surface of the carburized steel part to a depth of 2 mm, the grain size of prior austenite is 6.0 or more. 2Si+Mn+3Cr≦6.7 (1) (Al + 2Nb) / N≦6.71 (2) However, the element symbols in the 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: 0.20% or less, Ni: 0.20% or less, Mo: less than 0.100% V: 0.50% or less, W: 0.50% or less, Co: 0.50% 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: 0.0100% or less, 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: 0.0100% or less, Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.015% or less, and Sb: 0.015% or less, It may contain one or more selected from the group consisting of: [Effects of the Invention]

[0014] According to the present invention, a steel material having excellent cold forgeability, bending fatigue strength, and surface fatigue strength (pitting characteristics) can be provided. Also, according to the present invention, a carburized steel part having excellent cold forgeability, bending fatigue strength, and surface fatigue strength can be provided. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram showing an example of a heat pattern in the vacuum carburizing process and the quenching process. [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

[0016] The present inventors have conducted research into steel materials that, when subjected to vacuum carburizing (including vacuum carbonitriding) to form carburized steel parts, can provide excellent bending fatigue strength and excellent surface fatigue strength (pitting characteristics).

[0017] In vacuum carburizing, steel parts formed into the part shape are c3 Because the steel part is heated above its transformation temperature, the microstructure of the steel part transforms to austenite. Therefore, the structure of the resulting carburized steel part is no longer 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, which remains unchanged even when vacuum carburizing is performed.

[0018] As a result, the inventors of the present invention have concluded that increasing the contents of Si, Mn, and Cr, which increase temper softening resistance, is effective in improving contact fatigue strength (pitting characteristics). Furthermore, after examining the compatibility of bending fatigue strength and contact fatigue strength from the perspective of chemical composition, they concluded that a carburized steel part manufactured by vacuum carburizing can have excellent bending fatigue strength and excellent contact fatigue strength if the steel has a chemical composition consisting, by mass%, of C: 0.07 to 0.30%, Si: 0.30% or more but less than 0.90%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.100% or less, Cr: more than 0.80% but less than 2.00%, Al: 0.045% or less, Nb: 0.001 to 0.080%, and N: 0.0250% or less.

[0019] However, even when the content of each element in the chemical composition of a steel material is within the above-mentioned range, when the steel material is subjected to vacuum carburizing to produce a carburized steel part, abnormal grain growth may not be sufficiently suppressed, and furthermore, sufficient cold forgeability, bending fatigue strength, and surface fatigue strength may not be obtained. Therefore, the present inventors have conducted further research and studies. As a result, the present inventors have obtained the following findings (A) and (B).

[0020] (A) Si, Mn, and Cr are all elements that improve the hardenability of parts. Si is effective in improving softening resistance, while Mn and Cr are effective in improving carburization resistance. Therefore, Si, Mn, and Cr are all essential for improving surface fatigue strength and bending fatigue strength. However, excessive addition of these elements deteriorates cold forgeability, so it is necessary to determine the appropriate upper limit range. Specifically, if the content of these elements is defined as "F1 = 2Si + Mn + 3Cr," and the content of each element in the chemical composition is within the above range and F1 is 6.7 or less (i.e., if formula (1) below is satisfied), excellent cold forging is achieved. Furthermore, if formula (2) below is satisfied, carburized steel parts manufactured from the steel material will have excellent bending fatigue strength and excellent surface fatigue strength. 2Si+Mn+3Cr≦6.7 (1)

[0021] (B) Vacuum carburizing is a process in which a carburizing step and a diffusion step are performed once each in a vacuum or under reduced pressure, or a process in which the carburizing step and the diffusion step are alternately repeated multiple times. In the carburizing step, a hydrocarbon-based gas is introduced at low pressure to form an appropriate amount of cementite on the surface layer of the steel. Then, in the diffusion step, the introduction of the hydrocarbon-based gas is stopped. In this case, cementite decomposes in the diffusion step, which increases the carbon concentration in the surface layer of the steel. As a result, in the diffusion step of vacuum carburizing, the carbon concentration gradient in the austenite in the surface layer is larger than in the diffusion step of gas carburizing, and the amount of carbon penetrated into the steel can be increased. Thus, in the diffusion step of vacuum carburizing, no hydrocarbon-based gas is introduced, and cementite formed on the surface layer of the steel in the previous carburizing step serves as a carbon (C) supply source, allowing carbon to diffuse and penetrate into the steel. As a result, vacuum carburizing can form a hardened layer in a shorter time than gas carburizing. In addition, increasing the temperature of vacuum carburizing increases the diffusion rate of C, allowing the hardened layer to be formed in an even shorter time.

[0022] On the other hand, when vacuum carburizing is applied to cold-forged steel parts, abnormal grain growth can occur on the surface of the part. This is due to the high dislocation density on the surface of cold-forged parts. When cold-forged parts are subjected to vacuum carburizing, fine austenite is formed from the parent phase (ferrite) with a high dislocation density, which promotes abnormal grain growth. Parts with a structure that has abnormal grain growth tend to have poor surface fatigue and bending fatigue.

[0023] To suppress abnormal grain growth, it is effective to disperse precipitates that are stable even at carburizing temperatures, such as AlN and Nb-based precipitates such as Nb(C,N), NbC, and NbN, in the matrix to pin the growth of γ grains.

[0024] However, 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 for cracks. In other words, if excessive Al inclusions remain in the steel, the bending fatigue strength of the carburized steel part may decrease. Furthermore, both Al and Nb are elements that are more likely to form nitrides (strong nitride-forming elements). If their content is excessively high relative to the N content in the steel, Al inclusions are more likely to exist, which can reduce the bending fatigue strength of the carburized steel part. Therefore, it is necessary to determine the appropriate content of Al, Nb, and N. Specifically, if the content of these elements is defined as "F2 = (Al + 2Nb) / 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 can be suppressed if F2 is 6.71 or less (i.e., if the following formula (2) is satisfied). On the other hand, if F2 exceeds 6.71, the Al and Nb contents become excessively high relative to the N content, and in this case, an excessive amount of Al inclusions (oxide inclusions) that do not bond with N are formed, resulting in a decrease in the bending fatigue strength and surface fatigue strength of the carburized steel part. (Al + 2Nb) / N≦6.71 (2)

[0025] The steel material and carburized steel part of this embodiment have been completed based on the above technical concept.

[0026] 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.

[0027] 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 vacuum carburizing. In other words, the core is a region where, despite having undergone vacuum 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.

[0028] [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.

[0029] C: 0.07 to 0.30% Carbon (C) improves the hardenability of steel and increases the core hardness of carburized steel parts manufactured using the steel material. Therefore, C increases the bending fatigue strength of carburized steel parts. If the C content is less than 0.07%, 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 0.30%, the machinability and cold forgeability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.07 to 0.30%. The preferred lower limit of the C content is 0.09% or more, more preferably 0.11% or more, and even more preferably 0.13% or more. The preferred upper limit of the C content is 0.28% or less, more preferably 0.25% or less, and even more preferably 0.22% or less.

[0030] Si: 0.30% or more, 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 material. If the Si content is less than 0.30%, 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 excessively high, the cold forgeability of the steel material decreases. Furthermore, if the Si content is 0.90% or more, the formation of cementite and other compounds (cementite and alloy carbides) is inhibited in the temperature range of the vacuum carburizing treatment, even if the contents of other elements are within the ranges of this embodiment. In this case, the penetration of C into the steel material during the vacuum 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. Therefore, the Si content is 0.30% or more but less than 0.90%. The lower limit of the Si content is preferably 0.35% or more, more preferably 0.40% or more, and even more preferably 0.45% or more. The upper limit of the Si content is preferably 0.85% or less, more preferably 0.80% or less, even more preferably 0.75% or less, and even more preferably 0.70% or less.

[0031] Mn: 0.10 to 0.50% Manganese (Mn) increases the temper softening resistance of steel, and as a result, has the effect of increasing the surface fatigue strength (pitting resistance) of carburized steel parts manufactured using the steel as a raw material. If the Mn content is less than 0.10%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is excessively high, the cold forgeability of the steel decreases. Therefore, the Mn content is 0.10 to 0.50%. The lower limit of the Mn content is preferably 0.15% or more, and more preferably 0.20% or more. The upper limit of the Mn content is preferably 0.45% or less, and more preferably 0.40% or less.

[0032] P:0.030% or less Phosphorus (P) is an impurity. During vacuum carburizing when manufacturing carburized steel parts using steel as a base 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% or more.

[0033] 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 0.080% or less, more preferably 0.070% or less, even more preferably 0.060% or less, and even more preferably 0.050% or less. On the other hand, S also bonds with Mn to form MnS, which improves 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, even more preferably 0.005% or more, and even more preferably 0.007% or more.

[0034] Cr: Over 0.80% to less than 2.00% 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 material. If the Cr content is less than 0.80%, 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 Cr content is 2.00% or more, the cold forgeability of the steel material is reduced. Furthermore, if the Cr content is excessively high, even if the contents of other elements are within the ranges of this embodiment, excessive C penetrates and diffuses into the steel material during vacuum carburizing. As a result, excessive coarse cementite is formed in the surface layer of the steel material. In this case, some coarse cementite remains without decomposition even during the diffusion process of the vacuum carburizing treatment, making it more likely that cracks will initiate from the coarse cementite in the carburized steel part, resulting in a decrease in the bending fatigue strength of the carburized steel part. Therefore, the Cr content is 0.80 to 2.00%. The lower limit of the Cr content is preferably 0.85% or more, more preferably 0.90% or more, even more preferably 0.95% or more, and even more preferably 1.00% or more. The upper limit of the Cr content is preferably 1.95% or less, more preferably 1.90% or less, even more preferably 1.85% or less, and even more preferably 1.80% or less.

[0035] 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.

[0036] Nb: 0.001 to 0.080% Niobium (Nb) forms precipitates (carbides, carbonitrides, etc.) and, due to its pinning effect, has the effect of suppressing the coarsening of steel grains during vacuum carburization. This can improve the bending fatigue strength and surface fatigue strength of carburized steel parts manufactured using the steel as a raw material. The above effect can be achieved if the Nb content is 0.001% or more. However, if the Nb content exceeds 0.080%, 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 0.001 to 0.080%. The lower limit of the Nb content is preferably 0.005% or more, and more preferably 0.010% or more. The upper limit of the Nb content is preferably 0.070% or less, more preferably 0.060% or less, and even more preferably 0.050% or less.

[0037] N: 0.0250% or less Nitrogen (N) is an impurity. If the N content exceeds 0.0250%, coarse nitrides, such as AlN, will form, even if the contents of other elements are within the ranges specified in this embodiment. The coarse nitrides reduce the bending fatigue strength of carburized steel parts. Therefore, the N content is 0.0250% or less. The upper limit of the N content is preferably 0.0230% or less, more preferably 0.0210% or less, even more preferably 0.0200% or less, even more preferably 0.0190% or less, and even more preferably 0.0180% or less. The N content is preferably as low as possible. However, excessive reduction of the N content increases production costs. Therefore, considering normal industrial production, the lower limit of the N content is preferably greater than 0%, more preferably 0.0001%, and even more preferably 0.0005%.

[0038] 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.

[0039] [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, Mo, V, W, Co, 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.

[0040] Cu:0.20% or less Copper (Cu) is an optional element and may not be contained. That is, 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. However, if the Cu content exceeds 0.20%, 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 0.20%, and if contained, it is 0.20% 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% or more. The upper limit of the Cu content is preferably 0.18% or less, and even more preferably 0.16% or less.

[0041] Ni: 0.20% or less Nickel (Ni) is an optional element and may not be contained. In other words, the Ni content may be 0%. When 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 material. As a result, the bending fatigue strength of the carburized steel part is improved. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. On the other hand, if the Ni content exceeds 0.20%, even if the contents of other elements are within the ranges of this embodiment, the volume fraction of retained austenite in the surface layer of the carburized steel part will be excessively high. As a result, the bending fatigue strength of the carburized steel part will decrease. Therefore, the Ni content is 0 to 0.20%. The lower limit of the Ni content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more. The upper limit of the Ni content is preferably 0.18% or less, more preferably 0.16% or less, and even more preferably 0.14% or less.

[0042] Mo: Less than 0.100% Molybdenum (Mo) is an optional element and may not be included. In other words, the Mo content may be 0%. When Mo is included, that is, when the Mo content exceeds 0%, Mo increases the temper softening resistance of the steel, thereby improving the surface fatigue strength (pitting resistance) of carburized steel parts manufactured using the steel material. Even if even a small amount of Mo is included, the above effects can be achieved to some extent. On the other hand, if the Mo content is 0.100% or more, even if the contents of other elements are within the ranges of this embodiment, excessive C penetrates and diffuses into the steel material during the vacuum carburizing treatment. As a result, excessive coarse cementite is formed in the surface layer of the steel material. In this case, some coarse cementite remains without decomposition even during the diffusion process of the vacuum carburizing treatment, making it more likely that cracks will initiate from the coarse cementite in the carburized steel part, resulting in a decrease in the bending fatigue strength of the carburized steel part. Therefore, the Mo content is 0 to less than 0.100%, and if contained, it is less than 0.100%. The lower limit of the Mo content is preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.030% or more. The upper limit of the Mo content is preferably 0.090% or less, more preferably 0.080% or less, and even more preferably 0.070% or less.

[0043] 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 vacuum carburization. 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.

[0044] 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 material. As a result, the bending fatigue strength of the carburized steel part is increased. Even if even a small amount of W is contained, the above effects can be obtained to some extent. However, if the W content exceeds 0.50%, the strength of the steel material 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 material 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.

[0045] Co:0.50% or less Cobalt (Co) is an optional element and may not be contained. In other words, the Co content may be 0%. When 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 parts is increased. Even if even a small amount of Co is contained, the above effects can be obtained to some extent. However, 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.

[0046] 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 vacuum carburization. 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 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 preferred lower limit of the Ti content is 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 0.080% or less, more preferably 0.065% or less, even more preferably 0.050% or less, and even more preferably 0.035% or less.

[0047] 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 parts is increased. Even if even a small amount of B is contained, the above effects can be obtained to a certain extent. However, if the B content exceeds 0.0010%, the effect saturates. Therefore, the B content is 0 to 0.0010%, and if contained, it is 0.0010% or less. The lower limit of the B content is preferably 0.0001% or more, more preferably 0.0002% or more, and even more preferably 0.0003% or more. The upper limit of the B content is preferably 0.0009% or less, and even more preferably 0.0008% or less.

[0048] 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. 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.

[0049] 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.

[0050] Mg: 0.0100% or less 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 inhibits 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 effects can be obtained to some extent. However, when the Mg content exceeds 0.0100%, the above effects saturate. Therefore, the Mg content is 0 to 0.0100%, and when contained, it is 0.0100% or less. The lower limit of the Mg content is preferably 0.0010% or more, more preferably 0.0020% or more. The upper limit of the Mg content is preferably 0.0075% or less, more preferably 0.0050% or less.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] Te: 0.0100% or less Tellurium (Te) is an optional element and may not be contained. In other words, the Te content may be 0%. When contained, that is, when the Te content exceeds 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 exceeds 0.0100%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Te content is 0 to 0.0100%, and when contained, it is 0.0100% or less. The lower limit of the Te content is preferably 0.00010% or more, and more preferably 0.00020% or more. The upper limit of the Te content is preferably 0.0095% or less, and more preferably 0.0090% or less.

[0055] Bi:0.500% or less Bismuth (Bi) is an optional element and does not necessarily need to 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 material decreases 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 0.450% or less, more preferably 0.400% or less, even more preferably 0.350% or less, and even more preferably 0.300% or less.

[0056] 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.

[0057] 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.

[0058] 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 0.001% or more, and more preferably 0.002% or more. The upper limit of the Sb content is preferably 0.013% or less, and more preferably 0.010% or less.

[0059] 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%.

[0060] [Regarding formulas (1) and (2)] From the viewpoint of achieving both cold forgeability, 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 further satisfies formulas (1) to (2), provided that the content of each element is within the range of this embodiment described above.

[0061] 2Si+Mn+3Cr≦6.7 (1) (Al + 2Nb) / N≦6.71 (2) However, the element symbols in formulas (1) and (2) indicate the content of the element in mass %. Each formula will be explained below.

[0062] [Regarding formula (1)] Regarding the left side of formula (1), F1 is defined as 2Si+Mn+3Cr. F1 is an index indicating the cold forgeability of a steel material. Si, Mn, and Cr all improve the hardenability of parts, and Si improves softening resistance, while Mn and Cr improve carburization. Therefore, these elements are essential for improving surface fatigue strength and bending fatigue strength. The above effects can be achieved if the contents of these elements are within the above ranges. However, because each of these elements dissolves in the matrix, increasing hardness and reducing the cold forgeability of the steel, cold forgeability may not be ensured even if the contents of each element are within the above ranges. In light of these circumstances, the inventors carefully investigated the sum of the element amounts that ensures cold forgeability. As a result, they found that if F1 is 6.7 or less, that is, if the above formula (1) is satisfied, excellent cold forgeability can be ensured. Furthermore, if the formula (2) described below is satisfied, carburized steel parts manufactured using the steel material can achieve excellent bending fatigue strength and excellent surface fatigue strength.

[0063] The upper limit of F1 is preferably 6.5 or less, more preferably 6.3 or less, even more preferably 6.1 or less, even more preferably 5.9 or less, and even more preferably 5.7 or less. F1 is a value obtained by rounding the calculated value to one decimal place.

[0064] [Regarding formula (2)] Regarding the left side of equation (2), F2 = (Al + 2Nb) / N is defined. F2 is an index related to Al inclusions (oxide-based inclusions) that affect bending fatigue strength. Al inclusions become the starting point for cracks during bending fatigue in carburized steel parts. Therefore, it is preferable to suppress the formation of Al inclusions as much as possible. To achieve this, it is effective to include N in the steel material and precipitate solute Al as AlN. Note that Nb, which is an essential element in this embodiment, is a strong nitride element like Al. Therefore, Nb combines with N in the steel material to precipitate Nb carbonitrides that are stable at high temperatures. In other words, if the Al and Nb contents in the steel material are excessively high compared to the N content in the steel material, Al inclusions are more likely to be present, and the bending fatigue strength of the carburized steel part is likely to decrease.

[0065] In the chemical composition of the steel material of this embodiment, provided that the content of each element is within the range of this embodiment, if F2 exceeds 6.71, the Al and Nb contents are high relative to the N content. In this case, excessive amounts of Al inclusions (oxide-based inclusions) that do not bond with N are formed. As a result, the bending fatigue strength of the carburized steel part decreases. On the other hand, in the chemical composition of the steel material of this embodiment, provided that the content of each element is within the range of this embodiment, if F2 is 6.71 or less, that is, if the following formula (2) is satisfied, the formation of Al inclusions in the steel material can be sufficiently suppressed. As a result, provided that formula (1) is satisfied, the bending fatigue strength of the carburized steel part can be increased.

[0066] The upper limit of F2 is preferably 6.35 or less, more preferably 6.00 or less, and even more preferably 5.65 or less. F2 is a value obtained by rounding off the calculated value to two decimal places.

[0067] [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, a vacuum carburizing treatment is performed on the steel material, as will be described later. In the vacuum carburizing treatment, the steel material is subjected to A c3 The microstructure of the steel is reset by heating it to a temperature above the transformation point. Therefore, the microstructure of the steel that is the raw material for the carburized steel part is not particularly limited. The microstructure of the steel may be, for example, a structure (or a mixed structure) consisting of one or more of ferrite, pearlite, and bainite. 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 performing a vacuum carburizing treatment using the steel material of this embodiment as a raw material, the carburized steel part can achieve high bending fatigue strength and high surface fatigue strength (pitting resistance).

[0068] [Steel use] The steel material of this embodiment is suitable as a material for carburized steel parts manufactured by vacuum carburizing. In particular, it is suitable as a material for carburized steel parts that require bending fatigue strength and surface fatigue strength (pitting characteristics), such as gears used in mechanical products such as automobiles and construction vehicles. The steel material of this embodiment can also be used as a material for carburized steel parts manufactured by gas carburizing.

[0069] [Carburized steel parts] The carburized steel part of this embodiment is manufactured by subjecting the steel material of this embodiment described above to vacuum carburizing (vacuum carburizing or vacuum carbonitriding). The carburized steel part is a machine part, such as a gear, used in, for example, automobiles and construction vehicles.

[0070] 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 and diffusion of C by vacuum carburizing. Specifically, when vacuum carburizing is performed, the hardened layer corresponds to the carburized layer, and when vacuum 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 vacuum 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.

[0071] [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%, 0.07 to 0.30% C, 0.30% or more but less than 0.90% Si, 0.10 to 0.50% Mn, 0.030% or less P, 0.100% or less S, 0.80% to less than 2.00% Cr, 0.045% or less Al, 0.001 to 0.080% Nb, and 0.0250% or less N, with the balance being Fe and impurities, and satisfies formulas (1) and (2).

[0072] 2Si+Mn+3Cr≦6.7 (1) (Al + 2Nb) / N≦6.71 (2) However, the element symbols in the formulas (1) and (2) indicate the content of the element in mass %.

[0073] [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 50 μm of the carburized steel part is 0.60% or more by mass. (ii) In the region from the surface of the carburized steel part to a depth of 2 mm, the prior austenite grain size is 6.0 or more. Each component will be described below.

[0074] [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 50 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.

[0075] 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.

[0076] [Method for measuring surface carbon concentration] The carbon concentration in the surface layer can be measured by the following method. Cutting is performed from the surface of the carburized steel part (the surface of the hardened layer) to a depth of 50 μm, and chips from the surface to a depth of 50 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 50 μm (surface region).

[0077] [Regarding prior austenite grain size] The region from the surface to a depth of 2 mm of a carburized steel part is primarily subjected to high bending stress and contact stress. Cold forging introduces a large number of dislocations into these regions, causing abnormal grain growth of austenite during carburizing. These coarse grains become the starting point for crack initiation in bending fatigue and surface fatigue, significantly reducing fatigue strength. On the other hand, if the region from the surface to a depth of 2 mm of a carburized steel part does not contain coarse-grained prior austenite with a grain size of 5.9 or less, i.e., if the prior austenite in this region is fine-grained with a grain size of 6.0 or more, the carburized steel part will achieve sufficient surface fatigue strength and sufficient bending fatigue strength. The lower limit of the prior austenite grain size is preferably 7.0 or more, and more preferably 8.0 or more.

[0078] [Method for measuring prior austenite grain size] The grain size of the prior austenite is measured by the following method. The carburized steel part is cut in a direction perpendicular to the compression direction during forging, embedded in resin so that the cut surface serves as the observation surface, and mirror-polished. Note that the "direction perpendicular to the compression direction" here refers to the direction perpendicular to the face width direction (axial direction) in the case of a gear manufactured by compressing a cylindrical steel material in the height direction, for example. The polished surface is then corroded with picric acid (a solution of 10 g of picric acid added to 500 ml of water) to reveal the crystal grains on the polished surface. Next, in a region 2 mm deep from the surface of the part, the prior austenite grain size is measured in five fields using an optical microscope (100x magnification) in accordance with JIS G 0551 (2020). Abnormal grain growth is defined as not occurring if the measured prior austenite grain size is 6.0 or greater in all five fields.

[0079] In a carburized steel part having the above configuration, the content of each element in the chemical composition of the core is within the above-mentioned range, 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 50 μm (surface region) is 0.60% or more, and the grain size of the prior austenite grains in the region from the surface of the carburized steel part to a depth of 2 mm is 6.0 or more. Therefore, the carburized steel part of this embodiment has high cold forgeability, bending fatigue strength, and surface fatigue strength.

[0080] [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.

[0081] 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.

[0082] [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).

[0083] 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.

[0084] [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.

[0085] (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.

[0086] (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.

[0087] 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).

[0088] 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).

[0089] 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.

[0090] [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.

[0091] The method for manufacturing a carburized steel part comprises cold working, machining and heat treatment steps, and may include a hot working step prior to the cold working step.

[0092] [Hot processing process] When a hot working step is performed, the steel material of this embodiment is subjected to hot working. The hot working is, for example, well-known 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).

[0093] [Cold working process] After the steel material of this embodiment is subjected to well-known normalizing or spheroidizing annealing (see above), or after the above-mentioned hot working step, it is cold worked to be formed into a predetermined shape to produce an intermediate product. The cold working is, for example, cold forging. The conditions for the cold working are not particularly limited.

[0094] [Machining process] The intermediate product after the cold working process may be further machined. When a machining process is performed, the intermediate product after the cold working process is machined to form a predetermined shape. By performing machining, it is possible to give the carburized steel part a precise shape that would be difficult to achieve by the hot working process or cold working process alone. Machining is, for example, cutting or drilling. If the part is a gear, it is machined by, for example, broaching.

[0095] [Heat treatment process] After the cold working process or the machining process, the intermediate product is subjected to a heat treatment. Here, "heat treatment" includes a vacuum carburizing process, a quenching process, and a tempering process. The vacuum carburizing process, the quenching process, and the tempering process will be described below.

[0096] [Vacuum carburizing process] FIG. 1 shows an example of a heat pattern for the vacuum carburizing process S10 and the quenching process S20. The vacuum carburizing process S10 includes a heating process S0, a carburizing process S1, and a diffusion process S2. In the heat pattern of FIG. 1, the carburizing process S1 is followed by the diffusion process S2, and the carburizing process S1 and the diffusion process S2 are then repeated. In this way, in the vacuum carburizing process S10, the carburizing process S1 and the diffusion process S2 may be repeated multiple times, or the carburizing process S1 and the diffusion process S2 may each be performed once. The carburizing process S1 and the diffusion process S2 may also be repeated three or more times.

[0097] In the heating step S0, the intermediate product loaded into the furnace is heated to the carburizing temperature Tc. The carburizing temperature Tc in the heating step S0 is, for example, 900 to 1100°C. In the heating step S0, the furnace is further evacuated or depressurized. For example, the pressure in the furnace is depressurized to 1 kPa or less.

[0098] In the carburizing step S1, a hydrocarbon gas is introduced into a furnace under vacuum or reduced pressure, and the intermediate product is held at the carburizing temperature Tc for a predetermined time (holding time t1) to carry out the carburizing treatment. The gas introduced in the carburizing step S1 is not particularly limited as long as it is a hydrocarbon gas, and examples of such gases include acetylene and propane. The holding time t1 at the carburizing temperature Tc is not particularly limited, and is, for example, 5 to 120 minutes. Carburizing under vacuum or reduced pressure allows for a higher concentration of C penetrating into the surface layer of the steel compared to gas carburizing.

[0099] In the diffusion step S2, the furnace is maintained at the carburizing temperature Tc for a predetermined time (maintenance time t2) without introducing any hydrocarbon gas into the furnace, i.e., with the introduction of hydrocarbon gas into the furnace stopped. The pressure in the furnace during the diffusion step may be the same as that during the carburizing step S1, or may be reduced below that during the carburizing step S1 (for example, 100 Pa or less) in order to remove any residual gas from the carburizing step S1. The maintenance time t2 at the carburizing temperature Tc is not particularly limited, but is, for example, 5 to 120 minutes.

[0100] In the vacuum carburizing process S10, C penetrates into the surface layer of the steel material in the carburizing step S1, forming cementite and the like in the surface layer. Then, in the diffusion step S2, the cementite and the like in the surface layer are decomposed and the C in the surface layer is diffused into the interior. By repeating the combination of the carburizing step S1 and the diffusion step S2 once or multiple times under vacuum or reduced pressure, a larger amount of C can penetrate and diffuse into the steel material in a shorter time than with gas carburizing.

[0101] [Quenching process] The intermediate product after the vacuum carburizing process S10 is subjected to the quenching process S20. In the quenching process S20, the intermediate product after the vacuum 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.

[0102] [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, 60 to 150 minutes.

[0103] [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.

[0104] (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.

[0105] (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.

[0106] 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 using methods other than the above-described manufacturing method. In other words, there are no particular limitations on the manufacturing method for the carburized steel part, 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, satisfying formulas (1) and (2), the C concentration in the region from the surface of the carburized steel part to a depth of 50 μm (surface layer region) is 0.60% or more, and the grain size of the prior austenite grains in the region from the surface of the carburized steel part to a depth of 2 mm is 6.0 or more. [Example]

[0107] 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.

[0108] [Steel manufacturing] Molten steel having the chemical composition shown in Table 1 was produced.

[0109] [Table 1]

[0110] 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 underlined values ​​in Table 1 indicate compositions outside the scope of the present invention. Blank spaces indicate that the content of the corresponding element is 0% in significant figures (numbers down to the least significant digit) specified in the embodiment. However, test numbers 2, 8 and 12 were used as reference examples.

[0111] 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 normalizing treatment. The normalizing treatment temperature 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. The normalized steel materials were subjected to spheroidizing annealing. The spheroidizing annealing temperature was 760°C, and after heating for 30 minutes, the temperature was raised to 700°C and held at that temperature for 2 hours. The steel materials (steel bars) for each test number were then produced by slowly cooling to 650°C in a furnace and then allowed to cool.

[0112] The chemical composition of the steel material of test number 35 corresponds to that of SCr420 specified in JIS G 4805 (2019). In this example, the steel material of test number 35 was evaluated as a "reference steel material" in the various tests described below.

[0113] [Manufacturing carburized steel part test pieces] (1) Cylindrical test piece Using the steel material (steel bar) manufactured for each test number, multiple cylindrical test pieces (cylindrical test pieces) with a diameter of 14 mm and a height (length) of 21 mm were taken. The central axis of the cylindrical test pieces was coaxial with the central axis of the steel bar.

[0114] In addition to the cylindrical test pieces, the following three types of carburized steel part test pieces (small roller test pieces, rotating bending fatigue test pieces, and test pieces for investigating hardened layers) were also prepared.

[0115] (2) 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.

[0116] 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 the following vacuum carburizing treatment process. A carburizing process was carried out in which acetylene gas was introduced at a furnace pressure of 100 Pa or less. The temperature in the carburizing process was set to 930°C and the holding time was 80 minutes. After the carburizing process, a diffusion process was carried out. In the diffusion process, the introduction of acetylene gas was stopped and the furnace pressure was set to 10 Pa or less. The temperature in the diffusion process was set to 930°C and the holding time was 20 minutes. After the diffusion process, a quenching process was carried out. In the quenching process, the temperature was set to 900°C and the holding time was 30 minutes. After the holding time had elapsed, the material was oil-cooled using 60°C oil. After the quenching process, a tempering process was carried out. In the tempering process, the temperature was set to 180°C and the holding time was 120 minutes.

[0117] Furthermore, for test number 35 (reference steel), the following reference steel heat treatment pattern was applied to the rough test piece. Specifically, the 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 test pieces were held at 930°C for 120 minutes (diffusion process). The temperature was then lowered 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.

[0118] After heat treatment, 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 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, in accordance with JIS B 0601 (2001). 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] (3) 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.

[0120] Specifically, the steel material of each test number was first 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 the same heat treatment as the small roller test piece. Note that the rough test piece of test number 46 was subjected to the heat treatment according to the above-mentioned reference steel heat treatment pattern.

[0121] The surface of the heat-treated 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 longitudinal center of the rotating bending fatigue test piece. The rotating bending fatigue test piece was prepared by the above manufacturing process.

[0122] (4) 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.

[0123] 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 the same heat treatment as the small roller test pieces. Then, similar to the small roller test pieces, the outer circumferential surface of the rough test pieces was ground to finish the outer circumferential surface. The outer circumferential surface of the 26 mm diameter rough test pieces was finished 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 in accordance with JIS B 0601 (2001). The grinding depth was approximately 10 μm. Test pieces for hardened layer investigation were prepared using the above manufacturing process.

[0124] (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).

[0125] 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.

[0126] [Evaluation test] The following evaluation tests were carried out using the above-mentioned various test pieces.

[0127] (Cold forgeability evaluation test) Using cylindrical test pieces, a compression test was carried out at room temperature (25°C) to simulate cold forging, as shown below, and cold forgeability was evaluated.

[0128] In the compression test, the cylindrical test piece was compressed until its length (height) reached 10 mm, and the forging load at this time and the presence or absence of cracks in the cylindrical test piece after the test were visually confirmed. Regarding the forging load, if the values ​​obtained by compressing five cylindrical test pieces for each test number were within 1.5 times the forging load obtained using the SCr420 standard for test number 46, the forging load was determined to be within the allowable range.

[0129] Furthermore, the presence or absence of cracks after the compression test was confirmed as follows. For each test number, five cylindrical specimens were inspected for cracks using a 5x magnifying glass. If no fine cracks (0.5 to 1.0 mm in length) were observed in any of the five cylindrical specimens, it was determined that no cracks had occurred. In Table 2, if the forging load was within 1.5 times the forging load obtained under the SCr420 standard and no cracks were observed, the cold forgeability was judged to be excellent and marked with "○", and if the forging load was more than 1.5 times the forging load obtained under the SCr420 standard or if cracks were observed, the cold forgeability was judged to be poor and marked with "×".

[0130] (Resistant to coarsening) After the compression test, the following vacuum carburizing process was carried out. First, a carburizing process was carried out in which acetylene gas was introduced at a furnace pressure of 100 Pa or less. The temperature in the carburizing process was set to 930°C and the holding time was 80 minutes. After the carburizing process, a diffusion process was carried out. In the diffusion process, the introduction of acetylene gas was stopped and the furnace pressure was set to 10 Pa or less. The temperature in the diffusion process was set to 930°C and the holding time was 20 minutes. After the diffusion process, a quenching process was carried out. In the quenching process, the quenching temperature was set to 900°C and the holding time was 30 minutes. After the holding time had elapsed, the material was oil-cooled using 60°C oil. After the quenching process, a tempering process was carried out. In the tempering process, the temperature was set to 180°C and the holding time was 120 minutes.

[0131] The following coarsening resistance evaluation was carried out using cylindrical test pieces (corresponding to carburized steel parts) after vacuum carburizing treatment.

[0132] Specifically, the cylindrical test piece after vacuum carburization was cut in the vertical direction, embedded in resin so that the cut surface became the observation surface, and mirror-polished. The polished surface was then corroded with picric acid (a solution of 10 g of picric acid added to 500 ml of water), revealing the crystal grains on the polished surface. Next, grain size measurements were performed in five fields of view at a depth of 2 mm from the polished surface using an optical microscope (100x magnification) in accordance with JIS G 0551 (2020). If no austenite grains with a grain size of 5 or less were present in any of the five fields of view, it was determined that abnormal grain growth had not occurred and coarsening had been suppressed. In Table 2, cases in which coarsening had been suppressed are marked with an "O" and cases in which it was not suppressed are marked with an "X."

[0133] (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 50 μm was measured by the following method. First, turning was performed on the test piece for hardened layer investigation to a depth of 50 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 50 μm (surface region).

[0134] (Prior austenite grain size) Using the test pieces after the compression test in the cold forgeability evaluation test, the crystal grain size of prior austenite in a region from the side surface of the test piece (corresponding to the surface of the part) to a depth of 2 mm was measured by the following method. First, the test specimens after the compression test were cut perpendicular to the compression direction, embedded in resin, and mirror-polished so that the cut surface served as the observation surface. The polished surface was then corroded with picric acid (a solution of 10 g of picric acid added to 500 ml of water) to reveal crystal grains on the polished surface. Next, the prior austenite grain size was measured in five fields of view using an optical microscope (100x magnification) in accordance with JIS G 0551 (2020) in a region extending 2 mm from the side of the test specimen. If the measured prior austenite grain size was 6.0 or greater in all five fields of view, it was evaluated as having no abnormal grain growth and marked with a "Good" in Table 2. In Table 2, if even one of the five fields of view had a prior austenite grain size of 5.9 or less, it was evaluated as having abnormal grain growth and marked with a "Poor" (Poor).

[0135] (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.

[0136] 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%.

[0137] The test cutoff number of repetitions 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.

[0138] 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.

[0139] 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 35 was used as the reference value. If the fatigue limit was 1.10 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 1.10 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).

[0140] (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. 7The 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 35, 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 35, the specimen was judged to have low bending fatigue strength (marked "×" in the "Bending fatigue strength" column in Table 2).

[0141] [Evaluation results] The test results are shown in Table 2.

[0142] [Table 2]

[0143] Referring to Table 2, the contents of each element in the chemical composition of the steels of test numbers 1 to 18 were appropriate, and furthermore, F1 and F2 satisfied formulas (1) and (2). Therefore, in the carburized steel parts manufactured by vacuum carburizing, the carbon concentration in the region from the surface to a depth of 50 μm of the carburized steel part was 0.60% by mass or more, and the grain size of the prior austenite grains in the region from the surface to a depth of 2 mm of the carburized steel part was 6.0 or more. As a result, excellent cold forgeability, bending fatigue strength, and excellent surface fatigue strength were obtained. On the other hand, the steel materials of test numbers 19 to 35 either had element contents in their chemical compositions outside the ranges of the present invention, or F1 or F2 did not satisfy formula (1) or formula (2). As a result, the cold forgeability of the steel materials of test numbers 19 to 21, 24, and 27 was poor, so further processing was not performed. Furthermore, the steel materials of test numbers 22, 23, 25, 26, and 28 to 35 had good cold forgeability, but low surface fatigue strength and bending fatigue strength.

[0144] 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: 0.07-0.25%, Si: 0.30% or more and less than 0.90%; Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.100% or less, Cr: more than 0.80% to less than 2.00%; Al: 0.045% or less, Nb: 0.001 to 0.080%, and N: 0.0250% or less, and the balance consisting of Fe and impurities, and satisfying formulas (1) and (2). 2Si+Mn+3Cr≦6.7...(1) (Al+2Nb) / N≦6.45 (2) However, the element symbols in the formulas (1) and (2) indicate the content of the element in mass %.

2. Furthermore, in mass%, Cu: 0.20% or less, Ni: 0.20% or less, Mo: less than 0.100% V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Ti: 0.100% or less, and B: 0.0010% or less, The steel material for carburizing according to claim 1, which contains one or more selected from the group consisting of:

3. Furthermore, in mass%, Ca: 0.0100% or less, Mg: 0.0100% or less, 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: 0.0100% or less, Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.015% or less, and Sb: 0.015% or less, 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 that is carburized using the carburizing steel material according to claim 1, A hardened layer; a core portion located inside the hardened layer, The composition of the core is, in mass %, C: 0.07-0.25%, Si: 0.30% or more and less than 0.90%; Mn: 0.10-0.50%, P: 0.030% or less, S: 0.100% or less, Cr: more than 0.80% to less than 2.00%; Al: 0.045% or less, Nb: 0.001 to 0.080%, and N: 0.0250% below, and the balance being Fe and impurities, satisfying formula (1) and formula (2), the C concentration in a region from the surface of the hardened layer to a depth of 50 μm is 0.60% by mass or more, A carburized steel part, characterized in that the grain size of prior austenite is 6.0 or more in a region from the surface of the carburized steel part to a depth of 2 mm. 2Si+Mn+3Cr≦6.7...(1) (Al+2Nb) / N≦6.45 (2) However, the element symbols in the formulas (1) and (2) indicate the content of the element in mass %.

6. Furthermore, the composition of the core is, in mass %, Cu: 0.20% or less, Ni: 0.20% or less, Mo: less than 0.100% V: 0.50% or less, W: 0.50% or less, Co: 0.50% 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: 0.0100% or less, 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: 0.0100% or less, Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.015% or less, and Sb: 0.015% or less, 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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