Steel and carburized / hardened parts

Abstract

To provide a steel having excellent bending fatigue strength and excellent surface fatigue strength when subjected to carburizing, and a carburized component having excellent bending fatigue strength and excellent surface fatigue strength.SOLUTION: A steel according to one embodiment has a chemical composition consisting of, in mass%, C: 0.10-0.30%, Si: 0.76-2.20%, Mn: 0.50-1.40%, P: 0.030% or less, S: 0.005-0.020%, Cr: 0.10-0.35%, Al: 0.010-0.100%, N: 0.0121-0.0300%, and O: 0.0020% or less, Si and Cr satisfying 1.7≤Si / Cr≤7.5, Mn and Si satisfying Si / Mn≤2.0, and Al and N satisfying 1.0≤Al / N≤3.0, with the balance being Fe and impurities.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to steel and carburized and quenched parts.

Background Art

[0002] In recent years, due to the miniaturization of drive units accompanying the electrification of automobiles, construction vehicles, etc., further strengthening of mechanical drive parts such as gears has been demanded. Generally, mechanical parts such as gears are often subjected to carburizing and quenching.

[0003] In gears, a bending load is applied to the teeth when they mesh. Therefore, bending fatigue strength is required for the teeth of the gears. Furthermore, the tooth surfaces slide against each other in a short cycle in gears. Therefore, pitch suppression is required on the tooth surfaces. That is, in mechanical parts typified by gears used in automobiles, construction vehicles, etc., not only bending fatigue strength but also surface fatigue strength (pitching characteristics) is required. Carburizing and quenching is very effective in improving the surface fatigue strength of mechanical parts.

[0004] As a carburizing treatment method, instead of the gas carburizing treatment that has been widely used in the past, vacuum carburizing treatment with reduced CO2 emissions has come to be used. Gas carburizing treatment is a surface treatment carried out by heating an object to the carburizing temperature in a carburizing gas. Vacuum carburizing treatment is a surface treatment carried out by heating an object to the carburizing temperature under vacuum, and then providing one or more carburizing periods in which a carburizing gas is introduced into the atmosphere at a low pressure and a diffusion period in which the object is held in a vacuum atmosphere to diffuse carbon. Vacuum carburizing treatment has the following effects compared to gas carburizing treatment.

[0005] In vacuum carburizing treatment, a hydrocarbon-based gas is used as a carbon supply source. Therefore, vacuum carburizing treatment does not generate CO2 in the carburizing reaction. Therefore, vacuum carburizing treatment can manufacture parts in an environmentally friendly process.

[0006] Furthermore, vacuum carburizing is a heat treatment performed under vacuum or reduced pressure. Therefore, vacuum carburizing can suppress the formation of grain boundary oxide layers on the surface of parts, which is a problem in gas carburizing. In steel subjected to gas carburizing, it was difficult to increase the content of elements that readily form oxides, such as Si, Cr, and Mn, which promote the formation of grain boundary oxide layers. However, in steel subjected to vacuum carburizing, it is possible to increase the content of elements that readily form oxides. In addition, since Si is an element with excellent tempering softening resistance, it has the effect of improving surface fatigue strength. Therefore, there is a need for steel suitable for vacuum carburizing and used as material for carburized and quenched parts.

[0007] However, when manufacturing carburized and hardened parts using vacuum carburizing, excessive carburizing tends to occur at corners such as the cutting edge and tooth root. This is because overlapping areas of carbon diffusion fields occur at corners. Figure 7 shows a schematic cross-sectional view of a corner of a carburized and hardened part where excessive carburizing has occurred. The carburized area X, which is the carbon diffusion field, is formed at a substantially uniform depth along the surface of the carburized and hardened part. The carbon concentration in the carburized area X is generally uniform along the direction parallel to the surface. However, near the corner A, the carbon diffusion fields overlap, so an excessive carburized area Y is formed with a higher carbon concentration than the carburized area X.

[0008] In areas with excessive carburization (Y), coarse cementite, which serves as a fracture initiation point, is easily formed. As a result, bending fatigue strength and surface fatigue strength may be insufficient in areas with excessive carburization (Y). Furthermore, during processing of parts after vacuum carburization (such as gear cutting processes like grinding or shot peening), there is a risk of cracking and chipping originating from the coarse cementite. In particular, in carburized and hardened parts with surface shapes that have many corners, such as gears and CVT pulleys, deterioration of bending fatigue strength due to excessive carburization of corners during vacuum carburization has been a problem.

[0009] In order to solve the problem of excessive carburization at the corners of carburized and quenched parts subjected to the vacuum carburizing treatment described above, various countermeasures have been proposed to date.

[0010] For example, there is a method of performing vacuum carburizing under conditions that result in a low carbon concentration on the surface of a carburized and quenched part. Specifically, Patent Document 1 describes a method for manufacturing a steel member in which a reduced-pressure carburizing process is performed under conditions that the surface carburization concentration on the tooth surface or tooth root of the tooth profile, where the carbon diffusion rate is fast, is within the range of 0.65 ± 0.1 mass%.

[0011] Patent Document 2 states that by reducing the maximum Cr concentration in cementite in the steel sheet before carburizing to 23% or less, the precipitation of cementite after carburizing and quenching can be suppressed even in parts with sharp angles. Specifically, the high-carbon hot-rolled steel sheet for vacuum carburizing described in Patent Document 2 contains, by mass%, C: 0.10% to 0.30%, Si: 0.20% to 0.80%, Mn: 0.25% to 1.00%, P: 0.03% or less, S: 0.010% or less, sol.Al: 0.10% or less, N: 0.01% or less, Cr: 0.05% to 0.80%, and B: 0.0005% to 0.0050%, with the remainder being Fe and unavoidable impurities, and has a microstructure containing ferrite and cementite, wherein the ferrite area ratio is 80% or more, the average particle size of the ferrite is 5 μm to 25 μm, the maximum Cr concentration in the total cementite is 23% by mass or less, and the average spacing of the total cementite is 1.0 μm or more.

[0012] Patent Document 3 describes a carburized part in which coarse cementite formed at the edge during vacuum carburizing has been rendered harmless. Specifically, in the technology of Patent Document 3, during the cooling process after vacuum carburizing, the carburized part is cooled to below 500°C, and then reheated to 800-900°C for quenching. If the microstructure of the edge surface is pearlite-bainite, the cementite in the steel begins to dissolve upon reheating, and the cementite is broken up. As a result, fine cementite is formed at the edge. In this case, the chemical composition of the carburized member is C: 0.10% to 0.30%, Si: 0.05% to 0.80%, Mn: 0.30% to 2.00%, P: 0.06% or less, S: 0.006-0.10%, sol.Al: 0.010%-0.1%, N: 0.025%, Cr: 0.5% to 3.0%.

[0013] Patent Document 4 proposes suppressing the formation of coarse cementite at the edges by increasing the Si, Ni, and Cu content and decreasing the Cr concentration. Specifically, the technology of Patent Document 4 involves forming a carburizing steel into a part shape and carburizing it by vacuum carburizing. The alloy composition contains, by weight %, C: 0.1~0.3%, Si: 0.5~3.0%, Mn: 0.3~3.0%, P: 0.03% or less, S: 0.03% or less, Cu: 0.01~1.00%, Ni: 0.01~3.00%, and Cr: 0.3~1.0%, Mo: 2.0% or less, Al: 0.20% or less, and N: 0.05% or less, with the remainder being unavoidable impurities and Fe, and satisfying the condition [Si%]+[Ni%]+[Cu%]-[Cr%]>0.5.

[0014] Patent Document 5 describes a vacuum carburizing steel that suppresses excessive carburizing even when manufacturing carburized parts with sharp edges by increasing the Si content and decreasing the Cr content, and also achieves high core hardness by increasing the Mn content. Specifically, the vacuum carburizing steel of Patent Document 5 contains, by mass%, C: 0.10-0.30%, Si: 1.41-2.50%, Mn: 1.40-3.00%, P: 0.030% or less, S: 0.060% or less, Cr: 0.01-0.59%, Al: 0.010-0.100%, N: 0.003-0.030%, Mo: 0-0.20%, Cu: 0-0.20%, Ni: 0-0.40%, Nb: 0-0.10%, Ti: 0-0.100%, and B: 0-0.0030%, with the remainder being Fe and impurities, and fn1 as defined by formula (1) is 0.90 or more, and fn2 as defined by formula (2) is 0.50 or less. fn1 = Si - Cr (1) fn² = Si - 0.8 × Mn (2) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equations (1) and (2). [Prior art documents] [Patent Documents]

[0015] [Patent Document 1] International Publication No. 2009 / 131202 [Patent Document 2] Japanese Patent Publication No. 2021-21105 [Patent Document 3] International Publication No. 2014 / 034150 [Patent Document 4] Patent No. 4254816 [Patent Document 5] Patent No. 6838508 [Overview of the project] [Problems that the invention aims to solve]

[0016] As mentioned above, several proposals have been made to suppress excessive carburization at the corners of carburized and quenched parts. However, the conventional techniques described above have the following problems.

[0017] Patent Document 1 proposes a component in which the surface carburization concentration of the tooth surface or tooth root is within the range of 0.65 ± 0.1 mass%. However, if the carbon concentration of the surface layer of the carburized part is adjusted to be low during vacuum carburizing in order to suppress excessive carburization at sharp corners, the carbon concentration becomes insufficient in parts other than the corners (flat parts) of the carburized part. In this case, the surface hardness of the carburized and quenched part decreases, and the bending fatigue strength decreases.

[0018] Patent Document 2 states that cementite precipitation can be suppressed even in parts with sharp angles by reducing the maximum Cr concentration of cementite in the steel sheet before carburizing to 23% or less. However, Patent Document 2 is a document that assumes parts with a surface carbon concentration of about 0.6%. It is not intended for application to parts that require a surface carbon concentration higher than this, i.e., parts that are harder and require higher surface fatigue strength and rotational bending fatigue strength. Considering that the carburizing conditions are controlled in the examples so that the surface carbon concentration is 0.6%C, it is thought that the technology described in Patent Document 2 cannot suppress cementite formation at corners unless the surface carbon concentration is reduced or the temperature and time of the carburizing and diffusion phases are strictly controlled.

[0019] In Patent Document 3, in the cooling process after vacuum carburizing heat treatment, after cooling to 500°C or lower, reheating to 800 to 900°C and quenching are performed to break up coarse cementite and precipitate fine cementite, and a carburized and quenched component is described. However, in order to realize such manufacturing conditions in actual operation, it may be necessary to newly introduce equipment for the reheating process. In addition, due to reheating, the manufacturing cost and manufacturing time increase, and according to the technology of Patent Document 3, the manufacturing process load increases. Therefore, it is also necessary to provide steel and members for vacuum carburizing that do not require reheating as described in Patent Document 3.

[0020] In Patent Document 4, a carburized and quenched member that suppresses the generation of coarse cementite at corners by increasing the Si concentration, Ni concentration, and Cu concentration and decreasing the Cr concentration is proposed. According to this carburized and quenched member, it is said that it is not necessary to lower the carbon concentration of the flat part or perform reheating in the carburizing process. However, in the technology of Patent Document 4, since Cu and Ni are actively contained, it is difficult to avoid an increase in the steel material cost.

[0021] In Patent Document 5, even when manufacturing a carburized and quenched component having an acute-angled corner by increasing the Si content and decreasing the Cr content without lowering the carbon concentration of the flat part or performing reheating in the carburizing process, overcarburization is suppressed, and by adding 1.4% or more of Mn, a steel for vacuum carburizing with a high core hardness is described. However, when both the Si content and the Mn content are high, the rolling hardness and the hardness after normalizing become very high, and productivity and workability become problems.

[0022] As described above, in Patent Documents 1, 2, and 3, in order to suppress cementite at the corners, the carbon concentration of the flat part is lowered or post-heat treatment is performed. Further, in Patent Documents 4 and 5, carburized quenched members and carburizing steels having chemical components capable of suppressing the formation of coarse cementite at the corners have been proposed. However, in the technologies of Patent Documents 1 to 3 regarding carburizing conditions, a decrease in the surface fatigue strength of the flat part and complication of the manufacturing process become problems. In the technologies of Patent Documents 4 and 5 regarding the chemical components of carburizing steels, an increase in alloy cost and over-hardening due to alloy addition become problems. In order to solve these problems, there is a need for a steel that can obtain excellent bending fatigue strength and excellent surface fatigue strength when carburizing treatment is performed by means different from those of Patent Documents 1 to 5 to obtain carburized quenched parts.

[0023] An object of the present invention is to provide a steel having excellent bending fatigue strength and excellent surface fatigue strength when subjected to carburizing quenching, and a carburized quenched part having excellent bending fatigue strength and excellent surface fatigue strength.

Means for Solving the Problems

[0024] The inventor of the present invention conducted intensive studies to solve the above problems. That is, the relationship between the component composition of the steel before vacuum carburizing treatment and the formation of coarse cementite at the corners of the steel after vacuum carburizing treatment, that is, the carburized quenched part, and the relationship between the component composition of the steel before vacuum carburizing treatment and the surface hardness, subcritical hardness, and core hardness of the carburized quenched part were studied, and the following findings were obtained.

[0025] The gist of the present invention is as follows.

[0026] (1) A steel according to one aspect of the present invention has a chemical composition in mass% of C: 0.10-0.30%, Si: 0.76-2.20%, Mn: 0.50-1.40%, P: 0.030% or less, S: 0.005-0.020%, Cr: 0.10-0.35%, Al: 0.010-0.100%, N: 0.0121-0.0300%, and O: 0.0020% or less, wherein Si and Cr satisfy formula (A) below, Mn and Si satisfy formula (B) below, Al and N satisfy formula (C) below, and the remainder is Fe and impurities. 1.7 ≤ Si / Cr ≤ 7.5···(A) Si / Mn ≤ 2.0 ···(B) 1.0 ≤ Al / N ≤ 3.0 ···(C) Here, the mass percentage content of each element is substituted for each element symbol in equations (A) to (C). (2) The steel described in (1) above may further contain, in mass%, one or more of the following chemical compositions: Mo: 0.40% or less, Cu: 0.10% or less, V: 0.50% or less, Ni: 0.40% or less, W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Nb: 0.10% or less, Ti: 0.20% or less, Ca: 0.0015% or less, Pb: 0.09% or less, Zr: 0.020% or less, Mg: 0.020% or less, and B: 0.007% or less.

[0027] (3) A carburized and quenched part according to another aspect of the present invention has a core chemical composition in mass% of C: 0.10~0.30%, Si: 0.76~2.20%, Mn: 0.50~1.40%, P: 0.030% or less, S: 0.005~0.020%, Cr: 0.10~0.35%, Al: 0.010~0.100%, N: 0.0121~0.0300%, and O: 0.0020% or less, wherein Si and Cr satisfy formula (A) below, Mn and Si satisfy formula (B) below, Al and N satisfy formula (C) below, and the remainder consists of Fe and impurities, the average C concentration in the region from the surface to a depth of 50 μm in the flat portion is 0.65 mass% or more, and the hardness at a position 50 μm from the surface in the flat portion is 600 HV or more. 1.7 ≤ Si / Cr ≤ 7.5···(A) Si / Mn ≤ 2.0 ···(B) 1.0 ≤ Al / N ≤ 3.0 ···(C) Here, the mass percentage content of each element is substituted for each element symbol in equations (A) to (C). (4) In the carburized and quenched part described in (3) above, the chemical composition of the core may further contain one or more of the following in mass%, in the following amounts: Mo: 0.40% or less, Cu: 0.10% or less, V: 0.50% or less, Ni: 0.40% or less, W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Nb: 0.10% or less, Ti: 0.20% or less, Ca: 0.0015% or less, Pb: 0.09% or less, Zr: 0.020% or less, Mg: 0.020% or less, and B: 0.007% or less. [Effects of the Invention]

[0028] According to the steel of the present invention, it is possible to obtain steel having excellent bending fatigue strength and excellent surface fatigue strength when subjected to carburizing and quenching, and carburized and quenched parts having excellent bending fatigue strength and excellent surface fatigue strength. [Brief explanation of the drawing]

[0029] [Figure 1] Figure 1 shows an example of a heat pattern during the vacuum carburizing and quenching processes. [Figure 2] Figure 2 is a front view of the corner microscopic test specimen prepared in the example. [Figure 3] Figure 3 is a side view of the small roller test specimen prepared in the example. [Figure 4] Figure 4 is a side view of the rotational bending fatigue test specimen prepared in the example. [Figure 5] Figure 5 is a front view of the large roller test specimen prepared in the example. [Figure 6] Figure 6 is a schematic diagram of the two-cylinder rolling fatigue test in the embodiment. [Figure 7] Figure 7 is a schematic cross-sectional view of the corner of a carburized and quenched part where excessive carburization occurred. [Modes for carrying out the invention]

[0030] The steel and carburized / quenched parts according to this embodiment will be described below. Note that the unit "%" for the content of each component element means "mass%".

[0031] The inventors of the present invention investigated steels that exhibit excellent bending fatigue strength and excellent surface fatigue strength (pitting characteristics) when subjected to vacuum carburizing (vacuum carburizing or vacuum carbonitriding). For such steels, vacuum carburizing is performed, for example, in the manufacturing process of carburized and quenched parts, as described above. In vacuum carburizing, the steel is heated to a temperature above the Ac3 transformation point, so the microstructure of the steel transforms into austenite. Therefore, the influence of the microstructure before the start of vacuum carburizing does not remain after the vacuum carburizing. For this reason, the inventors investigated means to increase bending fatigue strength and surface fatigue strength after carburizing not from the perspective of the microstructure of the steel before carburizing, but from the perspective of the chemical composition of the steel, which is not changed by vacuum carburizing.

[0032] As a result, the inventors focused on (1) Cr, an element that promotes the precipitation of coarse cementite, and (2) Si, an element that suppresses the precipitation of coarse cementite, in order to suppress the precipitation of coarse cementite at the corners without lowering the surface carbon concentration after carburizing or performing reheating and quenching after the vacuum carburizing process is completed and cooling has been carried out, and investigated their appropriate ranges.

[0033] As a result, we found that increasing the amount of Si in proportion to the amount of Cr in the chemical composition of steel before vacuum carburizing treatment can suppress the precipitation of coarse cementite due to excessive carburizing.

[0034] As a result of investigating the balance between bending fatigue strength and surface fatigue strength from the perspective of chemical composition, the inventors have found that a chemical composition of mass% is C: 0.10~0.30%, Si: 0.76~2.20%, Mn: 0.50~1.40%, P: 0.030% or less, S: 0.005~0.020%, Cr: 0.10~0.35%, Al: 0.010~0.100%, N: 0.0121~0.0300%, and O: 0.0020% or less, Mo: 0.40% or less, Cu: 0.10% or less, V: It was thought that if a steel containing 0.50% or less of the following elements, Ni: 0.40% or less, W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Nb: 0.10% or less, Ti: 0.20% or less, Ca: 0.0015% or less, Pb: 0.09% or less, Zr: 0.020% or less, Mg: 0.020% or less, and B: 0.007% or less, with the remainder being Fe and impurities, then vacuum carburizing treatment could be performed to obtain excellent bending fatigue strength and excellent surface fatigue strength.

[0035] However, even with steels whose chemical composition contained elements within the above-mentioned ranges, coarse cementite remained at the corners after vacuum carburizing, resulting in insufficient bending fatigue strength and surface fatigue strength. In addition, the steel had extremely high hardness before hardening, making it difficult to process into parts, and abnormal grain growth occurred, resulting in insufficient bending fatigue strength. Therefore, the inventors conducted further investigations and studies. As a result, the inventors obtained the following findings.

[0036] (A) Vacuum carburizing is performed by carrying out the carburizing process and the diffusion process once each, or by repeating the carburizing process and the diffusion process alternately multiple times, under vacuum or reduced pressure. In the carburizing process, a hydrocarbon gas is introduced at low pressure to form an appropriate amount of cementite on the surface of the steel. Then, in the diffusion process, the introduction of the hydrocarbon gas is stopped. In this case, the cementite decomposes in the diffusion process, and the carbon concentration on the surface of the steel increases due to the decomposition of cementite. As a result, in the diffusion process of vacuum carburizing, the carbon concentration gradient in the austenite on the surface of the steel becomes larger compared to the diffusion process of gas carburizing, and the amount of carbon that penetrates into the interior of the steel can be increased. Thus, in the diffusion process of vacuum carburizing, hydrocarbon gas is not introduced. In the diffusion process, the cementite formed on the surface of the steel in the preceding carburizing process is used as a carbon (C) supply source to diffuse and penetrate into the interior of the steel. As a result, vacuum carburizing can form a hardened layer in a shorter time compared to gas carburizing.

[0037] However, if the diffusion process time for decomposing cementite is insufficient, the cementite does not decompose, and coarse cementite remains in the steel after the vacuum carburizing treatment, resulting in a decrease in bending fatigue strength.

[0038] The inventors investigated a method to suppress the formation of coarse cementite even when the diffusion time for decomposing cementite is relatively short, or when complex carburizing methods such as pulsed carburizing are applied, in which the diffusion and carburizing phases are repeated multiple times, and the formation and decomposition of cementite are repeated. The inventors focused on (1) Cr, an element that promotes the precipitation of cementite, and (2) Si, an element that suppresses the precipitation of cementite, and diligently investigated their appropriate ranges.

[0039] As a result, it was found that if the Si content is 1.7 times or more relative to the Cr content, cementite does not form during the carburizing process of vacuum carburizing, and the C concentration that penetrates into the steel is within an appropriate range. Even under conditions with relatively short diffusion times, or with complex carburizing methods such as pulse carburizing which repeats the diffusion and carburizing phases multiple times, if cementite is not formed during the carburizing phase, coarse cementite is not formed during the diffusion phase either, and sufficient bending fatigue strength and surface fatigue strength can be obtained after carburizing.

[0040] Furthermore, if the ratio of Si content to Cr content is less than 1.7, cementite is formed during the carburizing process of vacuum carburizing. In this case, although some cementite is decomposed during the diffusion process, if the diffusion time is short or if the steel has corners, some cementite remains undecomposed. Cementite can become the starting point for cracks in environments where bending fatigue is applied. Therefore, if an excessive amount of coarse cementite remains on the surface of the steel, the bending fatigue strength after carburizing will decrease.

[0041] On the other hand, when the ratio of Si content to Cr content is 7.5 or higher, although cementite is not formed on the surface of the steel during the carburizing process of vacuum carburizing, the hardness of the steel increases significantly due to the excessive addition of Si. As a result, the shear cutting properties of the steel deteriorate. Furthermore, the hardness of the steel when forging or cutting it into part shapes also increases significantly, degrading the machinability and forgeability of the steel.

[0042] By maintaining an appropriate ratio of Cr content to Si content, cementite formation is suppressed during the carburizing process of vacuum carburizing, and the carbon concentration penetrating the steel remains within an appropriate range. As a result, even when the diffusion time is relatively short, or when pulse carburizing is performed under complex carburizing conditions where the diffusion and carburizing phases are repeated multiple times, causing repeated formation and decomposition of cementite, the formation of coarse cementite can be suppressed, and the bending fatigue strength and surface fatigue strength after carburizing can be increased.

[0043] F1 is defined as Si / Cr, where "Si" is the Si content per unit mass percent and "Cr" is the Cr content per unit mass percent. If the content of each element in the chemical composition is within the above range and F1 is between 1.7 and 7.5, that is, if equation (1) below is satisfied, then the Si content relative to the Cr content is within an appropriate range. In this case, during the vacuum carburizing treatment, an appropriate amount of C penetrates and diffuses into the steel. As a result, assuming that the content of each element in the chemical composition is within the above range and equations (2) and (3) described below are satisfied, excellent bending fatigue strength and excellent surface fatigue strength can be obtained after the carburizing treatment. 1.7 ≤ Si / Cr ≤ 7.5 (1)

[0044] (B) However, even if the chemical composition of the steel satisfies equation (1) and an appropriate amount of C penetrates and diffuses into the steel during the carburizing process, the bending fatigue strength sometimes decreased after the carburizing process. It was estimated that this was because the A3 point increased with increasing Si content. The A3 point is the temperature at which the transformation from austenite to ferrite begins. When the A3 point of the steel increases, vacuum carburizing is performed at a two-phase temperature of ferrite + austenite. That is, the metal structure during carburizing becomes a two-phase structure of ferrite + austenite. In this case, it was thought that the surface carbon concentration becomes non-uniform for each structure, and sufficient surface hardness cannot be obtained. In order to avoid carburizing at a two-phase temperature and perform carburizing in a single-phase austenite state, it is necessary to increase the normalizing temperature and the carburizing temperature. However, increasing the heat treatment temperature is undesirable from the perspective of the energy cost of the heat treatment furnace. It is preferable to improve the bending fatigue strength through the chemical composition of the steel rather than the carburizing conditions. Therefore, the inventors conducted further investigations and obtained the following findings.

[0045] Si and Cr are ferrite-stabilizing elements and increase the A3 point. On the other hand, Mn is an austenite-stabilizing element and decreases the A3 point. Therefore, in addition to ensuring that the content of each element in the chemical composition is within the above range and that equation (1) is satisfied, the Mn content must also be appropriately adjusted. If the Mn content is inappropriate, the increase in the A3 point will cause carburizing to occur in the ferrite + austenite two-phase region temperature during vacuum carburizing. As a result, the surface carbon concentration during carburizing will be non-uniform for each microstructure, and sufficient surface hardness will not be obtained after the completion of the carburizing treatment, leading to a decrease in bending fatigue strength.

[0046] F2 is defined as Si / Mn, where "Si" is the Si content per unit mass percent and "Mn" is the Mn content per unit mass percent. If F2 is 2.0 or less, that is, if equation (2) shown below is satisfied, then, assuming that the content of each element in the chemical composition is within the above range and equations (1) and (3) are satisfied, a significant increase in the A3 point of the steel can be suppressed and kept within an appropriate range. As a result, a hardened layer with sufficient surface hardness is formed after carburizing. Consequently, sufficient bending fatigue strength is obtained after carburizing. Si / Mn≦2.0 (2)

[0047] (C) Even if the content of each element in the chemical composition is within the above range, if there is an excess of Al inclusions in the steel, the Al inclusions can become the starting point for cracks. In addition, Al inclusions in the steel remain in the carburized and quenched parts after the carburizing treatment. Therefore, if there is an excess of Al inclusions remaining in the steel, the bending fatigue strength after carburizing treatment may decrease. In addition, Al in the steel can precipitate as precipitates (AlN). Coarse AlN (precipitates), like Al inclusions, can become the starting point for cracks. Therefore, if there is an excess of coarse AlN (precipitates) in the steel, the bending fatigue strength after carburizing treatment may decrease.

[0048] F3 is defined as Al / N, where "Al" is the Al content per unit mass and "N" is the N content per unit mass. In the chemical composition of the steel of this embodiment, assuming that the content of each element is within the range of this embodiment, if F3 is less than 1.0, the Al content is in excess of the N content. In this case, the Al that does not bond with N becomes an excess amount of Al inclusions (oxide inclusions). Therefore, the bending fatigue strength after carburizing treatment decreases.

[0049] On the other hand, assuming that the content of each element in the chemical composition of the steel of this embodiment is within the range of this embodiment, if F3 exceeds 3.0, the N content will be in excess of the Al content. In this case, an excessive amount of coarse AlN (precipitates) will be formed in the steel. Therefore, in this case as well, the bending fatigue strength after carburizing treatment will decrease.

[0050] In the chemical composition of the steel of this embodiment, assuming that the content of each element is within the range of this embodiment, if F3 is between 1.0 and 3.0, that is, if the following equation (3) is satisfied, the formation of Al inclusions in the steel can be sufficiently suppressed, and the formation of coarse AlN (precipitates) can also be sufficiently suppressed. Therefore, assuming that equations (1) and (2) are satisfied, sufficient bending fatigue strength can be obtained after carburizing treatment. 1.0 ≤ Al / N ≤ 3.0 (3)

[0051] [Chemical composition of steel] Based on the above findings, the specific composition of the steel according to this embodiment will be described in detail below. The chemical composition of the steel in this embodiment contains the following elements.

[0052] C: 0.10~0.30% Carbon (C) enhances the hardenability and hardness of steel. Therefore, C increases the bending fatigue strength after carburizing. If the C content is less than 0.10%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the C content exceeds 0.30%, the machinability of the steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the C content is 0.10 to 0.30%. The preferred lower limit of the C content is 0.11%, more preferably 0.12%, and even more preferably 0.13%. The preferred upper limit of the C content is 0.28%, more preferably 0.27%, and even more preferably 0.25%.

[0053] Si: 0.76%~2.20% Silicon (Si) suppresses cementite formation. Therefore, it suppresses the precipitation of coarse cementite in corners, which are areas prone to excessive carburization during vacuum carburizing. As a result, Si increases the bending fatigue strength and surface fatigue strength after carburizing. If the Si content is less than 0.76%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 2.20%, even if the content of other elements is within the range of this embodiment, the rolling hardness and tempering hardness of the steel increase, and the cuttability, machinability, and manufacturability of the steel decrease significantly. Therefore, the Si content is 0.76 to 2.20%. The preferred lower limit of the Si content is 0.78%, more preferably 0.80%, more preferably 0.85%, more preferably 0.90%, more preferably 1.00%, and more preferably 1.20%. The preferred upper limit for the Si content is 2.18%, more preferably 2.15%, even more preferably 2.10%, and even more preferably 2.05%.

[0054] Mn: 0.50~1.40% Manganese (Mn) enhances the hardenability and hardness of steel. Therefore, Mn increases the bending fatigue strength after carburizing. If the Mn content is less than 0.50%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 1.40%, even if the content of other elements is within the range of this embodiment, the amount of retained austenite on the surface after carburizing becomes too high. In this case, sufficient surface hardness after carburizing cannot be obtained, and the bending fatigue strength after carburizing decreases. Therefore, the Mn content is 0.50 to 1.40%. The preferred lower limit of the Mn content is 0.55%, more preferably 0.60%, more preferably 0.65%, more preferably 0.70%, and still more preferably 0.75%. The preferred upper limit for the Mn content is 1.39%, more preferably 1.38%, more preferably 1.36%, more preferably 1.34%, more preferably 1.32%, and more preferably 1.30%.

[0055] P:0.030% or less Phosphorus (P) is an impurity. During vacuum carburizing, P segregates at austenite grain boundaries, reducing the bending fatigue strength after carburizing. If the P content exceeds 0.030%, the bending fatigue strength after carburizing decreases significantly, even if the content of other elements is within the range of this embodiment. Therefore, the P content is 0.030% or less. The preferred upper limit for the P content is 0.029%, more preferably 0.028%, and even more preferably 0.025%. It is preferable to have as low a P content as possible, for example, it may be 0%. However, in order to suppress refining costs, the P content may be 0.001% or more, or 0.002% or more.

[0056] S: 0.005%~0.020% Sulfur (S) is an impurity. S combines with Mn to form MnS, which improves the machinability of steel. However, if the S content exceeds 0.020%, the sulfides become coarser, even if the content of other elements is within the range of this embodiment. In this case, the bending fatigue strength after carburizing decreases. Therefore, the S content is 0.020% or less. The preferred lower limit of the S content is 0.005%, more preferably 0.006%, and even more preferably 0.007%. The preferred upper limit of the S content is 0.019%, more preferably 0.018%, even more preferably 0.017%, and even more preferably 0.016%. On the other hand, in order to suppress refining costs, the lower limit of the S content is set to 0.005%.

[0057] Cr: 0.10~0.35% Chromium (Cr) promotes the precipitation of cementite. Therefore, if Cr is added in excess, it promotes the precipitation of coarse cementite in corners, which are areas where excessive carburization is likely to occur during vacuum carburizing, and reduces the bending fatigue strength and surface fatigue strength after carburizing. On the other hand, Cr increases the hardenability and hardness of steel. Therefore, Cr increases the bending fatigue strength after carburizing. In other words, Cr has the effect of improving hardness and bending fatigue strength within the steel, but in the carburized area, it has the effect of impairing bending fatigue strength by precipitation of coarse cementite. For this reason, it is necessary to keep the Cr content within a predetermined range and to keep the relationship between the Si content and Cr content within a predetermined range so as to satisfy the above-mentioned equation (1).

[0058] If the Cr content is less than 0.10%, even if the content of other elements is within the range of this embodiment, the strength improvement effect cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 0.35%, even if the content of other elements is within the range of this embodiment, cementite may be generated during the carburizing stage depending on the conditions of the vacuum carburizing treatment. In this case, some coarse cementite remains without decomposing even in the diffusion step of the vacuum carburizing treatment. As a result, cracks originating from coarse cementite are more likely to occur after the carburizing treatment, and the bending fatigue strength decreases. Therefore, the Cr content is 0.10 to 0.35%. The preferred lower limit of the Cr content is 0.12%, more preferably 0.14%, more preferably 0.15%, and still more preferably 0.18%. The preferred upper limit of the Cr content is 0.33%, and more preferably 0.30%, 0.28%, 0.25%, or 0.20%.

[0059] Al: 0.010%~0.100% Aluminum (Al) deoxidizes steel. However, if the Al content exceeds 0.100%, coarse Al inclusions (oxide inclusions) will form, even if the content of other elements is within the range of this embodiment. Coarse Al inclusions reduce the bending fatigue strength after carburizing. Therefore, the Al content should be 0.100% or less. The lower limit of the Al content is 0.010%, preferably 0.011%, more preferably 0.012%, and still more preferably 0.013%. The preferred upper limit of the Al content is 0.095%, more preferably 0.090%, more preferably 0.085%, and still more preferably 0.080%.

[0060] N: 0.0121~0.0300% Nitrogen (N) is an impurity. If the N content exceeds 0.0300%, coarse nitrides, such as AlN, will be formed, even if the content of other elements is within the range of this embodiment. Coarse nitrides reduce the bending fatigue strength after carburizing. Therefore, the N content should be 0.0300% or less. The preferred upper limit for the N content is 0.0280%, more preferably 0.0260%, more preferably 0.0250%, more preferably 0.0230%, and more preferably 0.0200%. It is preferable to have as low an N content as possible. However, excessively reducing the N content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit for the N content is 0.0121%, more preferably 0.0123%, and more preferably 0.0125%.

[0061] O: 0.0020% or less Oxygen (O) is present in steel as an impurity, and it is an element that easily segregates at grain boundaries, causing grain boundary embrittlement, and also readily forms hard oxide inclusions in steel that cause brittle fracture. To prevent grain boundary embrittlement and brittle fracture, the O content should be 0.0020% or less. The lower limit of O is not particularly limited and may be 0%, for example. On the other hand, in order to suppress refining costs, the lower limit of O content may be 0.0001% or 0.0005%.

[0062] The remainder of the chemical composition of the steel according to this embodiment consists of Fe and impurities. Here, impurities refer to substances that are introduced during the industrial production of steel from raw materials such as ore, scrap, or the manufacturing environment, and are acceptable as long as they do not adversely affect the steel of this embodiment.

[0063] [Regarding arbitrary elements] The steel of this embodiment may further contain, in place of some of the Fe, one or more elements selected from the group consisting of Mo, Cu, V, Ni, W, Bi, Co, Nb, Ti, Ca, Pb, Zr, Mg, and B. These elements are arbitrary elements.

[0064] V: 0~0.50% Vanadium (V) is an optional element and may not be present. In other words, the V content may be 0%. If V is present, i.e., if the V content is greater than 0%, V forms precipitates (carbides, nitrides, carbonitrides, etc.) and suppresses grain coarsening of the steel during vacuum carburizing treatment through a pinning effect. As a result, the bending fatigue strength after carburizing treatment is increased. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.50%, the hardness of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. In this case, the machinability of the steel decreases. Therefore, the V content is 0 to 0.50%, and if present, it is 0.50% or less. The preferred lower limit of the V content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the V content is 0.40%, and even more preferably 0.30%.

[0065] Nb: 0~0.10% Niobium (Nb) is an optional element and may not be present. In other words, the Nb content may be 0%. If Nb is present, that is, if the Nb content is greater than 0%, Nb forms precipitates (carbides, carbonitrides, etc.), and the pinning effect suppresses the coarsening of the steel crystal grains during vacuum carburizing. As a result, the bending fatigue strength after carburizing is increased. Even if only a small amount of Nb is present, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.030%, even if the content of other elements is within the range of this embodiment, the Nb precipitates will coarse, and the pinning effect will not be obtained. Therefore, the Nb content is 0 to 0.10%, and if present, it is 0.10% or less. The preferred lower limit of the Nb content is 0.001%, and more preferably 0.005%. The preferred upper limit of the Nb content is 0.08%, more preferably 0.06%, and still more preferably 0.05%.

[0066] Ti: 0~0.20% Titanium (Ti) is an optional element and may not be present. In other words, the Ti content may be 0%. If Ti is present, that is, if the Ti content is greater than 0%, Ti forms precipitates (carbides, nitrides, carbonitrides, etc.) and suppresses grain coarsening of the steel during vacuum carburizing treatment through a pinning effect. As a result, the bending fatigue strength after carburizing treatment is increased. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.200%, even if the content of other elements is within the range of this embodiment, the Ti precipitates will coarse and the pinning effect will not be obtained. Therefore, the Ti content is 0 to 0.20%, and if present, it is 0.20% or less. The preferred lower limit of the Ti content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Ti content is 0.18%, more preferably 0.16%, more preferably 0.14%, more preferably 0.12%, and more preferably 0.10%.

[0067] Mo: 0~0.40% Molybdenum (Mo) is an optional element and may not be present. In other words, the Mo content may be 0%. On the other hand, Mo increases the tempering softening resistance of steel, and as a result, increases the surface fatigue strength (pitting characteristics) after carburizing. If the Mo content exceeds 0.50%, even if the content of other elements is within the range of this embodiment, carbon will penetrate and diffuse excessively into the steel during the vacuum carburizing process. As a result, coarse cementite will be excessively generated on the surface of the steel. In this case, some of the coarse cementite will remain without decomposing even in the diffusion process of the vacuum carburizing process. As a result, cracks originating from the coarse cementite are more likely to occur after carburizing, and the bending fatigue strength will decrease. Therefore, the Mo content is 0 to 0.40%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.02%, more preferably 0.03%, and still more preferably 0.04%. The preferred upper limit of the Mo content is 0.35%, more preferably 0.30%, and still more preferably 0.25%.

[0068] Cu: 0~0.10% Copper (Cu) is an optional element and does not need to be included. In other words, the Cu content may be 0%. On the other hand, if Cu is included, it has the effect of increasing the hardenability of steel, thereby increasing the core hardness and improving the bending fatigue strength after carburizing. This effect can be obtained even if only a small amount of Cu is included. On the other hand, if the Cu content exceeds 0.10%, the hot workability of the steel decreases. Therefore, the Cu content is between 0 and 0.10%. The preferred lower limit of the Cu content for stably obtaining the above effect is 0.01%. The preferred upper limit of the Cu content is 0.08%.

[0069] Ni: 0~0.40% Nickel (Ni) is an optional element and may not be present. When Ni is present, it enhances the hardenability of the steel, thereby increasing its hardness. This improves the bending fatigue strength characteristics after carburizing. Ni also enhances the toughness of the carburized layer. These effects can be obtained even with a small amount of Ni present. However, if the Ni content exceeds 0.40%, the amount of retained austenite increases, reducing surface hardness and decreasing bending fatigue strength after carburizing. Therefore, the Ni content is between 0 and 0.40%. The preferred lower limit of the Ni content for stably obtaining the above effects is 0.01%. The preferred upper limit of the Ni content is 0.35%.

[0070] W: 0~0.50% Tungsten (W) is an optional element and may not be present. In other words, the W content may be 0%. When W is present, that is, when the W content is greater than 0%, W increases the hardenability of the steel and thus the hardness of the steel. As a result, the bending fatigue strength after carburizing treatment increases. Even if only a small amount of W is present, the above effect can be obtained to some extent. However, if the W content exceeds 0.50%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. In this case, the machinability of the steel decreases. Therefore, the W content is 0 to 0.50%, and if present, it is 0.50% or less. The preferred lower limit of the W content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the W content is 0.40%, and even more preferably 0.30%.

[0071] Co: 0~0.50% Cobalt (Co) is an optional element and may not be present. In other words, the Co content may be 0%. If Co is present, that is, if the Co content is greater than 0%, Co increases the hardenability of the steel and thus increases its hardness. As a result, the bending fatigue strength after carburizing treatment increases. Even if only a small amount of Co is present, the above effect can be obtained to some extent. However, if the Co content exceeds 0.50%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. In this case, the machinability of the steel decreases. Therefore, the Co content is 0 to 0.50%, and if present, it is 0.50% or less. The preferred lower limit of the Co content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the Co content is 0.40%, and even more preferably 0.30%.

[0072] Bi: 0~0.50% Pb: 0~0.09% Zr: 0~0.020% Bismuth (Bi), lead (Pb), and zirconium (Zr) are all optional elements and do not need to be included. In other words, the Bi, Pb, and Zr content may be 0%. On the other hand, Bi, Pb, and Zr improve the machinability of steel. Specifically, Bi and Pb dissolve or become brittle during cutting, improving the machinability of the steel. Zr forms an oxide, improving the machinability of the steel. The above effects can be obtained if one or more of these elements are included in the steel. On the other hand, if these elements are included in excess, the forgeability and machinability of the steel will decrease. Therefore, the Bi content is 0.50% or less, the Pb content is 0.09% or less, and the Zr content is 0.020% or less. The lower limit of the preferred Bi content is 0.01%. The lower limit of the preferred Pb content is 0.01%. The lower limit of the preferred Zr content is 0.001%.

[0073] Ca: 0~0.0015% Mg: 0~0.020% Calcium (Ca) and magnesium (Mg) are optional elements and do not need to be present. In other words, the Ca and Mg content may be 0%. These elements control the morphology of inclusions and improve the machinability of the steel. The above effect can be obtained if even a small amount of one or more of these elements is present. On the other hand, if Ca and / or Mg are present in excess, excess oxides of Ca and / or Mg will be produced. These oxides become the starting point for bending fatigue and surface fatigue. Therefore, the bending fatigue strength and surface fatigue strength will decrease. Accordingly, the Ca content should be 0.0015% or less, and the Mg content should be 0.020% or less. The preferred lower limit for Ca content is 0.0001%, and the lower limit for Mg content is 0.001%.

[0074] B: 0~0.007% Boron (B) is an optional element and may not be present. In other words, the B content may be 0%. If it is present, that is, if the B content is greater than 0%, B increases the hardenability of the steel and increases its hardness. As a result, the bending fatigue strength after carburizing treatment increases. Even if only a small amount of B is present, the above effects can be obtained to some extent. However, if the B content exceeds 0.007%, these effects saturate. Therefore, the B content is 0 to 0.007%, and if present, it is 0.007% or less. The preferred lower limit of the B content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit of the B content is 0.006%, and even more preferably 0.005%.

[0075] Examples of elements that may be mixed into the steel as impurities include Te, Sn, Sb, and REM. Even if these elements are present, the present invention can be carried out without any problems as long as their content is Te: 0.100% or less, Sn: 0.015% or less, Sb: 0.002% or less, and REM: 0.010% or less, respectively.

[0076] [Regarding equations (1) to (3)] The chemical composition of the steel in this embodiment satisfies formulas (1) to (3), provided that the content of each element is within the range described above for this embodiment. 1.7 ≤ Si / Cr ≤ 7.5 ···(1) Si / Mn ≤ 2.0 ···(2) 1.0 ≤ Al / N ≤ 3.0 ···(3) Here, the elemental symbols in equations (1) to (3) are substituted with the mass percentage content of the corresponding element. The following explains each equation.

[0077] [Regarding equation (1)] As stated above, F1 is defined as Si / Cr. F1 is an index that represents the ease with which coarse cementite is formed on the surface layer of steel during the carburizing process in vacuum carburizing treatment.

[0078] Vacuum carburizing involves performing a carburizing process and a diffusion process once each, or alternating between the two processes multiple times, under vacuum or reduced pressure. In typical vacuum carburizing of steel, the carburizing process involves introducing a hydrocarbon gas at low pressure to form an appropriate amount of cementite on the steel's surface. Then, the introduction of the hydrocarbon gas is stopped during the diffusion process. In this case, the cementite decomposes during the diffusion process, increasing the carbon concentration on the steel's surface. As a result, the diffusion process in vacuum carburizing creates a larger carbon concentration gradient in the austenite on the surface compared to the diffusion process in gas carburizing, increasing the amount of carbon (C) penetrating into the steel. Thus, in the diffusion process of vacuum carburizing, without introducing a hydrocarbon gas, the cementite formed on the steel's surface in the preceding carburizing process is used as a carbon (C) source to diffuse and penetrate into the steel's interior. As a result, vacuum carburizing allows for the formation of a hardened layer in a shorter time compared to gas carburizing.

[0079] However, if the diffusion process time for decomposing cementite is insufficient, or if complex carburizing treatments such as pulsed carburizing are performed, in which the diffusion and carburizing phases are repeated multiple times, resulting in repeated formation and decomposition of cementite, coarse cementite may be formed during the carburizing phase and remain even during the diffusion phase. This coarse cementite deteriorated the bending fatigue strength after carburizing. To suppress the formation of coarse cementite during the carburizing phase, the inventors focused on (1) Cr, an element that promotes cementite precipitation, and (2) Si, an element that suppresses cementite precipitation, and diligently investigated their appropriate ranges.

[0080] As a result, it was found that when F1 was 1.7 or higher, almost no cementite was generated during the carburizing process of the vacuum carburizing treatment, while the concentration of carbon that penetrated the steel during the subsequent diffusion process remained within an appropriate range. Even when applying conditions with relatively short diffusion times, or when performing complex carburizing treatments such as pulsed carburizing which involves repeating the diffusion and carburizing phases multiple times, if cementite was not generated during the carburizing phase, coarse cementite was not generated during the diffusion phase either, and sufficient bending fatigue strength and surface fatigue strength could be obtained after the carburizing treatment.

[0081] On the other hand, assuming that the content of each element in the chemical composition of the steel of this embodiment is within the range of this embodiment, if F1 is less than 1.7, cementite is formed during the carburizing process of the vacuum carburizing treatment. In this case, although some cementite is decomposed during the diffusion process, if the diffusion time is short or if the steel being carburized has corners, some cementite remains without being decomposed. Cementite can become the starting point for cracks in environments where bending fatigue is applied. Therefore, if an excessive amount of cementite remains on the surface after carburizing, the bending fatigue strength after carburizing will decrease.

[0082] In the chemical composition of the steel of this embodiment, assuming that the content of each element is within the range of this embodiment, if F1 is greater than 7.5, although cementite is not formed on the surface of the steel during the carburizing process of the vacuum carburizing treatment, the hardness of the steel increases significantly due to the excess amount of Si. As a result, the shear cutting performance of the steel is poor, and the hardness when forging and cutting the steel into part shapes also increases significantly, degrading the machinability and forgeability of the steel.

[0083] In the chemical composition of the steel of this embodiment, assuming that the content of each element is within the range of this embodiment, if F1 is between 1.7 and 7.5, the Si content relative to the Cr content is within an appropriate range. Therefore, during the vacuum carburizing process, coarse cementite is not formed during the carburizing stage, and an appropriate amount of C penetrates and diffuses into the steel. As a result, high bending fatigue strength and high surface fatigue strength are obtained after the carburizing process.

[0084] The preferred lower limit of F1 is 1.8, more preferably 2.0, and even more preferably 2.5. The preferred upper limit of F1 is 7.4, more preferably 7.3, even more preferably 7.2, even more preferably 7.0, and even more preferably 6.8. F1 is the value obtained by rounding the calculated value to the second decimal place.

[0085] [Regarding equation (2)] As stated above, F2 is defined as F2 = Si / Mn. F2 is an index related to the A3 point in steel (the temperature at which the transformation from austenite to ferrite begins). Si and Cr are ferrite-stabilizing elements and increase the A3 point of steel. Mn is an austenite-stabilizing element and decreases the A3 point. Therefore, when the Si content is very high relative to the Mn content, the A3 point of steel increases, and when the steel is subjected to vacuum carburizing treatment, the steel is carburized at the ferrite + austenite two-phase temperature. As a result, the surface carbon concentration becomes non-uniform for each microstructure, and sufficient surface hardness cannot be obtained after carburizing treatment, leading to a decrease in bending fatigue strength and surface fatigue strength.

[0086] In the chemical composition of the steel of this embodiment, assuming that the content of each element is within the range of this embodiment, if F2 exceeds 2.0, the surface hardness after carburizing treatment will decrease. If F2 is 2.0 or less, a significant increase in the A3 point of the steel can be suppressed and kept within an appropriate range. As a result, a hardened layer with sufficient surface hardness is formed. Consequently, sufficient bending fatigue strength and surface fatigue strength can be obtained after carburizing treatment. Si / Mn≦2.0 (2)

[0087] The preferred upper limit of F2 is 1.9, more preferably 1.8, even more preferably 1.7, and even more preferably 1.6. The lower limit of F2 is not particularly limited. F2 may be 0. Note that F2 is a value obtained by rounding the value obtained by calculation to the third decimal place.

[0088] [Regarding equation (3)] As stated above, F3 is defined as F3 = Al / N. F3 is an index related to Al inclusions and AlN precipitates, which affect bending fatigue strength. Al inclusions serve as crack initiation points in environments subjected to bending fatigue. Coarse AlN (precipitates) also serve as crack initiation points in environments subjected to bending fatigue. Therefore, it is preferable to minimize the formation of Al inclusions and coarse AlN (precipitates).

[0089] In the chemical composition of the steel of this embodiment, assuming that the content of each element is within the range of this embodiment, if F3 is less than 1.0, the Al content is excessive relative to the N content. In this case, there is an excess of Al inclusions in the steel. Therefore, the bending fatigue strength decreases.

[0090] On the other hand, assuming that the content of each element in the chemical composition of the steel of this embodiment is within the range of this embodiment, if F3 exceeds 3.0, the N content is in excess of the Al content. In this case, there is an excess of coarse AlN (precipitates) in the steel. Therefore, in this case as well, the bending fatigue strength after carburizing treatment decreases.

[0091] The preferred lower limit of F3 is 1.8, more preferably 1.9, and even more preferably 2.0. The preferred upper limit of F3 is 2.3, more preferably 2.2, and even more preferably 2.1. Note that F3 is the value obtained by rounding the calculated value to the second decimal place.

[0092] [About the microstructure of steel] The microstructure of the steel in this embodiment is not particularly limited. The objective of the steel in this embodiment is to obtain high bending fatigue strength and high surface fatigue strength after carburizing. In the carburizing process, the steel is heated to a temperature above the Ac3 transformation point, which resets the microstructure of the steel. Therefore, the microstructure of the steel before carburizing is considered to have little effect on the mechanical properties after carburizing. For this reason, the microstructure of the steel is not particularly limited. For example, if the steel is a machine part, it is preferable that the microstructure consists of a hard structure in order to increase the strength of the machine part. On the other hand, if the steel is a material for a steel part, i.e., a steel material, it is preferable that the microstructure consists of a soft structure in order to improve the workability of the steel material. The steel in this embodiment has the above-mentioned range for the content of each element in its chemical composition, and further satisfies equations (1) to (3). Therefore, when a carburized and quenched part is manufactured by vacuum carburizing using the steel in this embodiment as the base material, high bending fatigue strength and high surface fatigue strength (pitting characteristics) can be obtained in the carburized and quenched part.

[0093] [Regarding the shape, size, and application of the steel in this embodiment] The shape and size of the steel in this embodiment are not particularly limited and can be appropriately selected according to their application. When using the steel according to this embodiment as the material for steel parts, i.e., the steel material, it is preferable to use the steel as, for example, a steel bar. Steel bars include, for example, round bars and square bars. The diameter of a round bar is, for example, 25 to 60 mm. The cross-sectional shape of a square bar is, for example, 50 mm to 85 mm square. On the other hand, the shape of the steel according to this embodiment may be any. Steel parts obtained by processing and heat-treating the steel material also fall under the category of the steel of this embodiment, as long as their chemical composition satisfies the above requirements.

[0094] Furthermore, various surface treatments may be applied to the steel of this embodiment. For example, the steel according to this embodiment may be used as steel for carburizing and quenching, and a carburizing treatment may be applied to it. Vacuum carburizing is preferred for the carburizing treatment, but gas carburizing may also be used. Plating, chemical conversion treatment, and painting may also be applied to the steel according to this embodiment. Steel with such surface treatments also falls under the category of the steel of this embodiment, as long as the chemical composition of its core satisfies the above requirements. For example, the carburized and quenched parts described later have the same chemical composition of their core as the steel of this embodiment, and are therefore considered an example of the steel according to this embodiment.

[0095] The steel of this embodiment is suitable as a base material for carburized and quenched parts manufactured by vacuum carburizing. In particular, it is suitable as a base material for carburized and quenched parts that require bending fatigue strength and surface fatigue strength (pitting characteristics), such as gears used in mechanical products like automobiles and construction vehicles. When the steel of this embodiment is used as a gear material, the shape of the steel is preferably a round bar.

[0096] [Method for manufacturing steel according to this embodiment] The method for manufacturing the steel of this embodiment is not particularly limited. The steel according to this embodiment has a unique chemical composition not found in conventional steel, but the method for obtaining this chemical composition is not particularly limited. Known refining methods can be used as appropriate as methods for controlling the chemical composition. When the steel according to this embodiment is made into a steel bar, any hot working can be performed on a slab in which the chemical composition is within a predetermined range. The steel bar obtained by hot working may be subjected to heat treatment such as spheroidizing treatment.

[0097] [Structure of carburized and hardened parts] Next, the carburized and quenched parts according to this embodiment will be described in detail. The carburized and quenched parts of this embodiment are manufactured by using the steel of this embodiment as the material and subjecting it to a vacuum carburizing treatment (vacuum carburizing treatment or vacuum carbonitriding treatment). Carburized and quenched parts are mechanical parts used in automobiles and construction vehicles, for example, gears.

[0098] The carburized and quenched 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 carbon into the surface of the steel material through vacuum carburizing. Specifically, when vacuum carburizing is performed on the steel material, the hardened layer corresponds to the carburized layer, and when vacuum carbonitriding is performed on the steel material, the hardened layer corresponds to the carbonitriding layer. The core portion is the part located inside the hardened layer and is an area unaffected by the penetration and diffusion of carbon due to vacuum carburizing. It is a well-known technical matter to those skilled in the art that the hardened layer and the core portion can be distinguished by well-known microstructural observations.

[0099] [About the core] The chemical composition of the core of the carburized and quenched part in this embodiment is the same as the chemical composition of the steel in this embodiment described above. Specifically, the chemical composition of the core of the carburized and quenched part in this embodiment is as follows (in mass%): C: 0.10~0.30%, Si: 0.76~2.20%, Mn: 0.50~1.40%, P: 0.030% or less, S: 0.005~0.020%, Cr: 0.10~0.35%, Al: 0.010~0.100%, N: 0.0121~0.0300%, O: 0.0020% or less, Mo: 0.40% or less, Cu: 0.1 It contains 0% or less of the following elements: V: 0.50% or less, Ni: 0.40% or less, W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Nb: 0.10% or less, Ti: 0.20% or less, Ca: 0.0015% or less, Pb: 0.09% or less, Zr: 0.020% or less, Mg: 0.020% or less, and B: 0.007% or less, with the remainder being Fe and impurities, satisfying formulas (1) to (3). 1.7 ≤ Si / Cr ≤ 7.5 ···(1) Si / Mn ≤ 2.0 ···(2) 1.0 ≤ Al / N ≤ 3.0 ···(3) Here, the elemental symbols in equations (1) to (3) are substituted with the mass percentage content of the corresponding element. Regarding the chemical composition of the carburized and quenched part according to this embodiment, the preferred upper and lower limits for the content of each element, and the preferred upper and lower limits for F1 to F3, are the same as those for the steel according to this embodiment described above.

[0100] [About the hardened layer] The composition of the hardened layer is as follows: (1) The average carbon concentration in the flat portion of the carburized and quenched part, in the region from the surface to a depth of 50 μm, is 0.65% or more by mass. (2) The hardness of the flat portion of the carburized and quenched part at a depth of 50 μm from the surface is 600 HV. The following describes each component. Here, the hardness at a depth of 50 μm from the surface is sometimes simply referred to as surface hardness. Note that the C concentration and surface hardness are defined in the flat portion of the carburized and quenched part. A flat portion is a part where there is no overlap in the C diffusion fields. Figure 7 shows a schematic cross-sectional view of a corner of a carburized and quenched part. Figure 7 shows the region where the C diffusion field formed by carbon diffused from a plane extending transversely to the plane of the paper and the C diffusion field formed by carbon diffused from a plane extending vertically to the plane of the paper overlap, i.e., the excess carburized portion Y. The substantially flat region that does not include this excess carburized portion Y is the flat portion. The shape of the excess carburized portion Y changes depending on the shape of the corner and the carburizing depth, but it can be easily identified by cross-sectional observation. In this embodiment, the portion where the C diffusion fields overlap is referred to as the corner. However, the shape of the carburized and quenched part according to this embodiment is not particularly limited, and therefore it does not have to have a corner. The surface carbon concentration measured in a flat area will inevitably be lower than the surface carbon concentration measured in a corner.

[0101] [Regarding the average C concentration in the surface region] The region from the surface of the flat part of the carburized and quenched part to a depth of 50 μm (hereinafter referred to as the surface region) is included in the hardened layer. The average carbon concentration in the surface region of the flat part is 0.65% or more by mass. The average carbon concentration in the hardened layer of the flat part is higher than the carbon concentration in the core. If the carbon concentration in the surface region of the flat part is 0.65% or more by mass, the hardness of the hardened layer is sufficiently hard. Therefore, sufficient surface fatigue strength and sufficient bending fatigue strength can be obtained in the carburized and quenched part.

[0102] The preferred lower limit for the average C concentration in the surface region of the flat portion is 0.67%, more preferably 0.70%, and still more preferably 0.75%. The upper limit for the average C concentration in the surface region of the flat portion is not particularly limited. For example, a preferred upper limit for the average C concentration in the surface region of the flat portion is 1.30%, more preferably 1.20%, and still more preferably 1.10%. If the carburized and quenched part has corners, the C concentration in the corners is not particularly limited. If the chemical composition of the core and the C concentration of the flat portion satisfy the above requirements, the corners will naturally also have a good C concentration.

[0103] [Method for measuring the carbon concentration in the surface layer] The average carbon concentration in the surface region of a flat area can be measured using EPMA. A carburized and quenched part is cut perpendicular to its surface. Next, the cut surface is polished. Then, the carbon concentration in the flat area is continuously measured by irradiating the region from the surface to a depth of 50 μm with an electron beam along the depth direction. That is, the surface region of the flat area is linearly analyzed in terms of carbon concentration along the depth direction. The measurement interval in the linear analysis is 5 μm, and there are 10 measurement points. The average value of the carbon concentration at these 10 measurement points is taken as the average carbon concentration in the region from the surface to a depth of 50 μm.

[0104] [About the microstructure of the hardened layer] The hardened layer extends 50 μm from the surface of the flat portion of the carburized and quenched part. The hardness at this depth of 50 μm from the surface of the flat portion should be 600 HV or higher. This ensures the bending fatigue strength and surface fatigue strength of the carburized and quenched part.

[0105] [Method for measuring the hardness of the hardened layer] The hardness of the hardened layer is measured by the following method: The carburized and quenched part is cut perpendicular to its surface. The cut surface is then polished. The hardness of the cut surface at a depth of 50 μm from the surface of the flat section is measured using a Vickers hardness tester with a measurement load of 300 gf, in accordance with JIS Z 2244 (2009). To account for variations in hardness, the hardness of three measurement points at a depth of 50 μm is measured, and the average value of these values ​​is calculated. The average hardness is considered to be the hardness at a depth of 50 μm from the surface of the flat section.

[0106] The carburized and quenched part having the above configuration has the content of each element in the chemical composition of the core within the above range and satisfies equations (1) to (3). Furthermore, the C concentration in the region from the surface of the carburized and quenched part to a depth of 50 μm (surface region) is 0.65% or more, the microstructure at a depth of 20 μm from the surface of the carburized and quenched part consists of martensite, or martensite and retained austenite, and the hardness of the surface region is 650 HV or more. Therefore, the carburized and quenched part of this embodiment has high bending fatigue strength and high surface fatigue strength.

[0107] [Manufacturing method for carburized and hardened parts] An example of a manufacturing method for the carburized and quenched parts of this embodiment will be described. The manufacturing method for carburized and quenched parts described below is just one example for manufacturing the carburized and quenched parts of this embodiment. Therefore, carburized and quenched parts having the above-described configuration may be manufactured by other manufacturing methods other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a manufacturing method for the carburized and quenched parts of this embodiment.

[0108] A method for manufacturing carburized and quenched parts includes, for example, a hot working step or a cold working step, a cutting step, and a heat treatment step. Either one of the hot working step or the cold working step may be performed, or both may be performed.

[0109] [Hot working process] [Cold working process] If a hot working process is performed, the steel of this embodiment is subjected to hot working. Hot working is, for example, well-known hot forging. After hot working, the steel is allowed to cool (air-cooled). If a cold working process is performed, the steel of this embodiment is subjected to well-known spheroidizing annealing, and then cold working is performed. The conditions for cold working are not particularly limited.

[0110] [Cutting process] In the machining process, intermediate parts of a predetermined shape are manufactured by machining steel after hot working and / or cold working. By performing machining, it is possible to impart precise shapes to carburized and quenched parts that would be difficult to achieve through hot working or cold working alone.

[0111] [Heat treatment process] Heat treatment is performed on the intermediate product after the machining process. Here, "heat treatment" includes a well-known vacuum carburizing process, a well-known quenching process, and a well-known tempering process. In the vacuum carburizing process, it is a well-known technical matter to those skilled in the art to adjust the carbon concentration and microstructure of the hardened layer of the carburized and quenched part by appropriately adjusting the known conditions. The well-known vacuum carburizing process, quenching process, and tempering process will be described below.

[0112] [Vacuum Carburizing Process] Figure 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 Figure 1, the diffusion process S2 is performed after the carburizing process S1, and then the carburizing process S1 and the diffusion process S2 are repeated. Thus, 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 be performed once each. The carburizing process S1 and the diffusion process S2 may also be repeated three or more times.

[0113] In heating step S0, the intermediate material charged into the furnace is heated to the carburizing temperature Tc. In heating step S0, the furnace is further evacuated or depressurized.

[0114] In the carburizing process S1, a hydrocarbon gas is introduced into the 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 perform the carburizing treatment. The gas introduced in the carburizing process S1 is not particularly limited as long as it is a hydrocarbon gas, but for example, acetylene or propane can be used. The holding time t1 at the carburizing temperature Tc is not particularly limited. By performing carburizing under vacuum or reduced pressure, the concentration of carbon that penetrates the steel surface can be increased compared to the case of gas carburizing treatment.

[0115] In the diffusion step S2, the mixture is held at the carburizing temperature Tc for a predetermined time (holding time t2) without introducing a hydrocarbon gas into the furnace. The pressure inside the furnace during the diffusion step may be the same as in the carburizing step S1, or it may be reduced to a lower pressure than in the carburizing step S1 in order to remove residual gas from the carburizing step S1. The holding time t2 at the carburizing temperature Tc is not particularly limited.

[0116] In the vacuum carburizing process S10, carbon (C) is introduced into the steel surface layer in the carburizing process S1, forming cementite and the like on the surface. Then, in the diffusion process S2, the cementite and the like in the surface layer are decomposed, and the carbon from the surface layer is diffused into the interior. By repeating the combination of the carburizing process S1 and the diffusion process S2 once or multiple times under vacuum or reduced pressure, a large amount of carbon can be introduced and diffused into the steel in a shorter time compared to gas carburizing. Furthermore, since the composition of the steel used as the material for the carburized and quenched parts according to this embodiment is within the above-mentioned range, coarse cementite is extremely unlikely to form. Therefore, a variety of vacuum carburizing conditions can be applied to the manufacturing method of the carburized and quenched parts according to this embodiment.

[0117] [Heat treatment process] A quenching process S20 is performed on the intermediate product after the vacuum carburizing process S10. In the quenching process S20, the intermediate product after the vacuum carburizing process S10 is held at a quenching temperature of Ar3 or higher, and then rapidly cooled to quench. The holding time t3 at the quenching temperature Ts is not particularly limited. It is preferable that the quenching temperature Ts is 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 rapidly cooled by immersing it in a cooling bath containing oil or water as a cooling medium.

[0118] [Tempering process] A well-known tempering process is performed on the intermediate product after the quenching process. The tempering temperature and holding time are not limited and can be appropriately selected to obtain the mechanical properties appropriate for the intended use of the carburized and quenched part.

[0119] [Other processes] The manufacturing method for the carburized and quenched parts of this embodiment may further include a shot peening process and a finish grinding process. These processes are optional.

[0120] [Shot Peening Process] Shot peening is an optional process and does not have to be performed. If performed, the shot peening process is carried out on the intermediate product after the heat treatment process. By performing the shot peening process, the retained austenite in the hardened layer of the carburized and quenched part undergoes a work-induced transformation into martensite. As a result, the volume fraction of retained austenite in the hardened layer decreases.

[0121] [Finishing grinding process] The finish grinding process is optional and does not need to be performed. If performed, the finish grinding process is carried out on intermediate products after the heat treatment or shot peening process to improve the surface texture.

[0122] The carburized and quenched parts of this embodiment can be manufactured through the above manufacturing process. Note that the above-described manufacturing method is merely one example of a manufacturing method for producing the carburized and quenched parts of this embodiment. Therefore, the carburized and quenched parts of this embodiment may be manufactured by methods other than those described above. In other words, the manufacturing method for the carburized and quenched parts is not particularly limited, as long as the carburized and quenched parts of this embodiment can be obtained. [Examples]

[0123] Next, embodiments of the present invention will be described. The conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0124] Ingots were manufactured using the ingot-forming method with the molten steel shown in Tables 1A and 1B. The ingot had a rectangular cross-section perpendicular to its length, measuring 180 mm x 180 mm. The manufactured ingots were allowed to cool to room temperature.

[0125] The ingot was heated at 1200°C for 2 hours. After heating, the ingot was subjected to hot working (hot forging) to produce steel (bar) with a diameter of 40 mm and a length of 1000 mm. The steel after hot working was allowed to cool to room temperature. After cooling, the steel was subjected to normalizing treatment. The treatment temperature for normalizing was 925°C, and the holding time at the treatment temperature was 90 minutes. In addition, some test pieces underwent IA treatment (high-temperature annealing) after normalizing. For IA treatment, the steel was held at 925°C for 60 minutes, then slowly cooled to 650°C, and held at 650°C for 60 minutes. After the time had elapsed, the steel was allowed to cool. The cooling rate of the steel during cooling was 0.3 to 0.9°C / second. The steel (bar) for each test number was produced by the above process.

[0126] Furthermore, the steel specimen with test number 20 had a chemical composition equivalent to SCr420 as specified in JIS G 4805 (2019). Test number 20 was designated as the "reference specimen."

[0127] [Evaluation Test] [Manufacturing of carburized and quenched component test pieces] Using the steel produced for each test number, the following three types of carburized and quenched component test pieces were prepared for each test number.

[0128] (1) Small roller test piece Figure 2 shows a side view of the small roller test specimen prepared in this embodiment. The numbers in Figure 2 indicate dimensions (in mm). "φ" in Figure 2 means diameter. The inverted triangle symbol in Figure 2 represents the "finishing symbol" indicating surface roughness as described in Explanatory Table 1 of JIS B 0601 (1982). The "G" attached to the finishing symbol is an abbreviation for the grinding method specified in JIS B 0122 (1978). The small roller test specimen is a test specimen used to measure surface fatigue strength. Multiple small roller test specimens were prepared for each test number.

[0129] (2) Rotational bending fatigue test specimen Figure 3 shows a side view of the rotary bending fatigue test specimen prepared in this embodiment. The numbers in Figure 3 indicate dimensions (in mm). "φ" in Figure 3 means diameter. "R" in Figure 3 means radius of curvature. The rotary bending fatigue test specimen is a test specimen used to measure the rotary bending fatigue strength.

[0130] (3) Test specimens for microscopic examination of corners Two test specimens were prepared for each test number for investigating the hardened layer. For micro-investigation of the corners, triangular specimens with a diameter of 26 mm and a length of 100 mm were used. Each carburized and quenched component test piece was prepared using the following method.

[0131] [Small roller test piece] The steel for each test number was machined to produce rough test specimens having the rough shape of small roller test specimens. One of the following heat treatments, patterns 1 to 3, was performed on the rough test specimens. The carburizing patterns (1 to 3) performed for each test number are shown in the "Carburizing Pattern" column of Table 2A.

[0132] (Pattern 1) The following vacuum carburizing process was performed. A carburizing process was carried out by introducing acetylene gas at a furnace pressure of 100 Pa or less. The temperature for the carburizing process was set to 950°C and the holding time was 70 minutes. After the carburizing process, a diffusion process was carried out. During the diffusion process, the introduction of acetylene gas was stopped and the furnace pressure was set to 10 Pa or less. The temperature for the diffusion process was set to 950°C and the holding time was 100 minutes. After the diffusion process, a quenching process was carried out. During the quenching process, the temperature was set to 900°C and the holding time was 30 minutes. After the holding time, oil cooling was performed using 60°C oil. After the quenching process, a tempering process was carried out. During the tempering process, the temperature was set to 180°C and the holding time was 120 minutes.

[0133] (Pattern 2) The following vacuum carburizing process was performed. A carburizing process was carried out by introducing acetylene gas at a furnace pressure of 100 Pa or less. The temperature for the carburizing process was set to 950°C and the holding time was 95 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 for the diffusion process was set to 950°C and the holding time was 90 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, oil cooling was performed using oil at 60°C. 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.

[0134] (Pattern 3) The following vacuum carburizing process was performed. A carburizing process was carried out by introducing acetylene gas at a furnace pressure of 100 Pa or less. The temperature for the carburizing process was set to 950°C and the holding time was 120 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 for the diffusion process was set to 950°C and the holding time was 80 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, oil cooling was performed 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.

[0135] After heat treatment, the central cylindrical portion of the rough test specimen was ground to create a cylindrical portion with a diameter of 26 mm, as shown in Figure 2. At this time, the surface of the 26 mm diameter cylindrical portion was finished to have 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.

[0136] [Rotational bending fatigue test specimen] The steel for each test number was machined to produce rough specimens for rotary bending fatigue testing. One of the heat treatments described in patterns 1 to 3 above was performed on the rough specimens. The heat treatment patterns performed on the rough specimens for each test number are shown in Table 2A. For the rough specimen for test number 36, the heat treatment described in the standard steel heat treatment pattern above was performed.

[0137] After heat treatment, the surface of the rough test specimen was machined to produce a rotary bending fatigue test specimen with the dimensions shown in Figure 3. However, no machining was performed on the notch formed at the longitudinal center of the rotary bending fatigue test specimen to improve its surface texture. The rotary bending fatigue test specimen was produced using the above manufacturing process.

[0138] [Test specimen for microscopic examination of corners] Two triangular test specimens, each 26 mm in diameter and 100 mm in length, were prepared by machining the steel for each test number. One of the heat treatments described in patterns 1 to 3 was then applied to each test specimen. The heat treatment patterns applied to the rough test specimens for each test number are shown in Table 2A.

[0139] [Manufacturing of large roller test specimens used in two-cylinder rolling fatigue tests] Large roller test specimens used in a two-cylinder rolling fatigue test to measure surface fatigue strength were manufactured by the following method. Rough test specimens of large roller test specimens with the shape shown in Figure 4 were cut from a 140 mm diameter cylindrical element having a chemical composition equivalent to SUJ2 as specified in JIS G 4805 (2008). The numbers in Figure 4 indicate dimensions (in mm). The inverted triangle symbol in Figure 4 represents the "finishing symbol" indicating surface roughness as described in Explanatory Table 1 of JIS B 0601 (1982). The "G" attached to the finishing symbol is an abbreviation for the grinding method specified in JIS B 0122 (1978).

[0140] The cut-out rough test specimens were subjected to quenching. The quenching temperature was set to 870°C, and the holding time at the quenching temperature was 90 minutes. After the holding time, they were rapidly cooled in 60°C oil. The outer surface of the quenched rough test specimens was finished by machining. The outer surface was 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 specimens were manufactured using the above process.

[0141] [Hardness measurement test of steel bars] Each rolled steel bar with a test number was cut perpendicular to its length, and a test specimen was taken with the cut surface as the measurement surface. The hardness of the cut surface at a position D / 4 (where D represents the diameter) from the surface of the roll was measured using a Vickers hardness tester with a measurement load of 300 gf, in accordance with JIS Z 2244 (2009).

[0142] [Measurement test of coarse cementite precipitation at the corners of carburized and quenched component test pieces] For each test number, the carburized and quenched parts (test specimens for micro-investigation of corners) were cut to a thickness of t / 2 (where t is the thickness), and test specimens were taken with the cut surface as the observation surface. The observation positions of the microstructure at the corners were defined as 30° corners, 60° corners, and 90° corners, respectively, and the microstructure was investigated and examined. Regardless of the angle, if the formation of coarse cementite was observed at the corners, it was considered "present" at the corners. "Coarse cementite" refers to cementite that is visible when observed at a magnification of 1000x with an optical microscope. Normally, the coarse cementite that forms at the corners of carburized and quenched parts is formed in a network along the grain boundaries. Therefore, the presence or absence of coarse cementite can be easily determined.

[0143] [Measurement test of carbon concentration in the hardened layer of carburized and quenched component test pieces] For each test number, the carburized and quenched parts (Ono-type rotary bending fatigue test specimens) were cut perpendicular to their length, the cut surfaces were embedded in a micromount and polished, and test specimens were taken with the cut surface as the measurement surface. The carbon concentration in the region from the surface to a depth of 50 μm of the carburized and quenched parts was then analyzed using an electron beam microanalyzer. The measurement interval for the line analysis was 5 μm, and 10 measurement points were used. The average value of the carbon concentration at these 10 measurement points was recorded in the table as the average carbon concentration (flat portion carbon concentration) in the region from the surface to a depth of 50 μm.

[0144] [Method for measuring the hardness of the hardened layer] The hardness of the hardened layer was measured by the following method. Carburized and quenched parts (Ono-type rotary bending fatigue test specimens) numbered 1 to 35 were cut perpendicular to the lengthwise direction, and test specimens were taken with the cut surface as the measurement surface. The hardness of the cut surface at a depth of 50 μm from the surface of the carburized and quenched part was measured using a Vickers hardness tester with a measurement load of 300 gf, in accordance with JIS Z 2244 (2009). To account for variations in hardness, the hardness of three measurement points at a depth of 50 μm was measured, and the average value was calculated. The average hardness value is recorded in the table as the hardness at a depth of 50 μm from the surface in the flat section (surface hardness).

[0145] [Surface fatigue strength measurement test (two-cylinder rolling fatigue test)] A two-cylinder rolling fatigue test was conducted using a small roller specimen and a large roller specimen to determine the surface fatigue strength as follows. The RP201 test machine manufactured by Nikko Create Co., Ltd. was used. As shown in Figure 5, the cylindrical portion of the small roller specimen 10 (26 mm in diameter) was rolled in contact with the center of the outer circumferential surface of the large roller specimen 20 (the outer circumferential portion with a diameter of 130 mm). The contact surface pressure was set to 1800-3500 MPa in Hertzian pressure. The rotation speed of the small roller specimen 10 was set to 1500 rpm. The peripheral speed of the small roller specimen 10 was set to 123 m / min, and the peripheral speed of the large roller specimen 10 was set to 172 m / min. During the test, lubricating oil was supplied to the contact area between the small and large roller specimens. Automatic transmission oil was used as the lubricating oil, with an oil temperature of 100°C and an oil flow rate of 1.0 L / min. The number of cycles to be terminated in the test was 2.0 × 10⁻⁶, which represents the fatigue limit for typical steel. 7 The number of times was set to 2.0 × 10 in the small roller test specimen without pitting occurring. 7 The maximum surface pressure (MPa) reached was defined as the fatigue limit of the small roller test specimen. Pitching was detected using a vibration meter installed in the testing machine. After vibration occurred, the rotation of both the small and large roller test specimens was stopped, and the occurrence of pitting and the rotation speed were confirmed. In this embodiment, assuming application to gear parts, the fatigue limit of a small roller test specimen (reference specimen) made of steel meeting the SCr420 standard (test number 20) and vacuum carburized was used as the reference value. If the fatigue limit was 1.20 times or more that of the reference steel, it was judged to have excellent surface fatigue strength (indicated as "Pass" in the "Surface Fatigue Strength" column in Table 2B). On the other hand, if the fatigue limit was less than 1.20 times that of the reference steel, it was judged to have low surface fatigue strength (indicated as "Fail" in the "Surface Fatigue Strength" column in Table 2B).

[0146] [Rotational Bending Strength Measurement Test (Rotational Bending Fatigue Test)] Rotary bending fatigue tests were conducted using rotary bending fatigue test specimens in accordance with the "Rotational Bending Fatigue Test Method for Metallic Materials" specified in JIS Z 2274 (1978). The tests were conducted at room temperature in an atmospheric environment with a rotation speed of 3000 rpm. The number of stress load cycles was 10 7The maximum stress at which fracture did not occur after the cycle was defined as the bending fatigue strength (MPa). If the obtained bending fatigue strength was 1.20 times or more the bending fatigue strength of the reference test specimen (test number 20), it was judged to have superior bending fatigue strength (indicated as "Pass" in the "Bending Fatigue Strength" column in Table 2B). On the other hand, if the obtained bending fatigue strength was less than 1.20 times the bending fatigue strength of the reference steel (test number 20), it was judged to have low bending fatigue strength (indicated as "Fail" in the "Bending Fatigue Strength" column in Table 2B).

[0147] [Evaluation Results] The test results are shown in Tables 2A and 2B.

[0148] [Table 1A]

[0149] [Table 1B]

[0150] [Table 2A]

[0151] [Table 2B]

[0152] (Test results) Referring to Table 2, the elemental content in the chemical composition of steels from test numbers 1 to 19 was appropriate, and furthermore, F1 to F3 satisfied equations (1) to (3).

[0153] Furthermore, in the carburized and quenched parts manufactured by vacuum carburizing, no coarse cementite was observed at the corners, the carbon concentration in the region from the surface to a depth of 50 μm was 0.65% or more by mass, and the surface hardness from the surface to a depth of 50 μm was 600 HV or more. As a result, these carburized and quenched parts exhibited excellent bending fatigue strength and excellent surface fatigue strength.

[0154] On the other hand, in test number 20, the Si content was low and the Cr content was high. As a result, the F1 value fell below the lower limit of equation (1), and cementite precipitated at the corners. The fatigue strength of test number 20 was used as the baseline value. All of the carburized and quenched parts from test numbers 1 to 19 mentioned above showed superior fatigue strength compared to test number 20.

[0155] In test number 21, although the elemental content in the steel's chemical composition was appropriate, F1 exceeded the upper limit of equation (1). As a result, cracking occurred during hot forging. Therefore, none of the above evaluation tests were performed on test number 21.

[0156] In test number 22, although the content of each element in the chemical composition of the steel was appropriate, F2 exceeded the upper limit of equation (2). As a result, test number 22 had insufficient bending fatigue strength.

[0157] In test number 23, although the elemental content in the chemical composition of the steel was appropriate, F3 exceeded the upper limit of equation (3). As a result, test number 23 lacked sufficient bending fatigue strength.

[0158] In test number 24, the carbon content of the steel was too high. As a result, cracking occurred during the processing of the test specimen. Therefore, none of the strength evaluation tests described above were performed on test number 24.

[0159] In test number 25, the Si and Cr content of the steel was low. As a result, test number 25 exhibited insufficient surface fatigue strength and bending fatigue strength.

[0160] In test number 26, the Si content of the steel was low, and the Cr content was too high. As a result, cementite precipitated at the corners of test number 26, resulting in insufficient bending fatigue strength and surface fatigue strength.

[0161] In test number 27, the Si content of the steel was too low. As a result, cementite precipitated at the corners of test number 27, resulting in insufficient bending fatigue strength and surface fatigue strength.

[0162] In test number 28, the Si content of the steel was too high. As a result, cracks occurred during the processing of the test specimen in test number 28. Therefore, none of the strength evaluation tests described above were performed on test number 28.

[0163] In test number 29, the Mn content of the steel was too low. As a result, test number 29 exhibited insufficient bending fatigue strength and surface fatigue strength.

[0164] In test number 30, the Mn content of the steel was too high. As a result, test number 30 lacked sufficient surface hardness, bending fatigue strength, and face fatigue strength.

[0165] In test number 31, the amount of phosphorus (P) in the steel was too high. As a result, test number 31 exhibited insufficient bending fatigue strength.

[0166] In test number 32, the amount of sulfur in the steel was too high. As a result, test number 32 exhibited insufficient bending fatigue strength.

[0167] In test number 33, the aluminum content of the steel was too high. As a result, test number 33 exhibited insufficient bending fatigue strength and surface fatigue strength.

[0168] In test number 34, the nitrogen content of the steel was too low. As a result, test number 34 exhibited low bending fatigue strength.

[0169] In test number 35, the nitrogen content of the steel was too high. As a result, test number 35 had low bending fatigue strength.

[0170] Embodiments of the present invention have been described above. However, the embodiments described above are merely illustrative examples for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and the embodiments described above can be appropriately modified and implemented without departing from the spirit of the invention. [Industrial applicability]

[0171] As described above, by using the steel bars of the present invention, it is possible to significantly reduce the size and weight of angular parts such as automotive gears, thereby improving the fuel efficiency of automobiles and reducing CO2 emissions. Furthermore, it is not necessary to lower the surface carbon concentration after carburizing, nor is it necessary to reheat and harden in vacuum heat treatment, and even in pulse carburizing, which involves repeated carburizing and diffusion, it is possible to perform the carburizing treatment without generating coarse cementite, resulting in significant cost reductions. Therefore, the effects of the present invention are extremely remarkable, and the present invention has great industrial applicability.

Claims

1. The chemical composition is expressed in mass percent. C: 0.10-0.30%, Si: 0.76-2.20%, Mn: 0.50-1.40%, P: 0.030% or less, S: 0.005-0.020%, Cr: 0.10-0.35%, Al: 0.010-0.100%, N: 0.0121 to 0.0300%, and O: 0.0020% or less It contains, Si and Cr satisfy the following equation (1), Mn and Si satisfy the following equation (2), Al and N satisfy equation (3) below, A steel characterized by having Fe and impurities as the remainder. 4.0 ≤ Si / Cr ≤ 7.5 ... (1) Si / Mn≦2.0 (2) 1.0 ≤ Al / N ≤ 3.0 ... (3) Here, the mass percentage content of the corresponding element is substituted for each element symbol in equations (1) to (3).

2. Furthermore, the chemical composition is, in mass%, Mo: 0.40% or less Cu: 0.10% or less, V: 0.50% or less, Ni: 0.40% or less, W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Nb: 0.10% or less, Ti: 0.20% or less, Ca: 0.0015% or less, Pb: 0.09% or less, Zr: 0.020% or less, Mg: 0.020% or less, B: 0.007% or less, The steel according to claim 1, characterized in that it contains one or more of the following.

3. The chemical composition of the core is, in mass percent, C: 0.10-0.30%, Si: 0.76-2.20%, Mn: 0.50-1.40%, P: 0.030% or less, S: 0.005-0.020%, Cr: 0.10-0.35%, Al: 0.010-0.100%, N: 0.0121-0.0300%, O: 0.0020% or less It contains, Si and Cr satisfy the following equation (1), Mn and Si satisfy the following equation (2), Al and N satisfy equation (3) below, The remainder consists of Fe and impurities. In the flat portion, the average C concentration in the region from the surface to a depth of 50 μm is 0.65% by mass or more. A carburized and quenched part characterized in that the hardness at a depth of 50 μm from the surface in the flat portion is 600 HV or more. 4.0 ≤ Si / Cr ≤ 7.5 ... (1) Si / Mn≦2.0 (2) 1.0 ≤ Al / N ≤ 3.0 ... (3) Here, the mass percentage content of the corresponding element is substituted for each element symbol in equations (1) to (3).

4. Furthermore, the chemical composition of the core portion is, in mass%, Mo: 0.40% or less Cu: 0.10% or less, V: 0.50% or less, Ni: 0.40% or less, W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Nb: 0.10% or less, Ti: 0.20% or less, Ca: 0.0015% or less, Pb: 0.09% or less, Zr: 0.020% or less, Mg: 0.020% or less, B: 0.007% or less, The carburized and quenched part according to claim 3, characterized in that it contains one or more of the following.

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