Steel, steel component, and method for producing steel

Abstract

Provided are steel having excellent fatigue strength, a method for producing the steel, and a steel component. The steel has a given composition which satisfies the following relationships (1) and (2), the remainder comprising Fe and unavoidable impurities. The core portion contains a precipitate of one or more of Cr, Mo, V, and Nb. The steel has a surface hardness of 700 HV or greater and a surface compressive residual stress of 750 MPa or greater. (1): ([Mo] / 96+[V] / 50.9+[Nb] / 92.9)×1000≥5.5 (2): ([C] / 12+[N] / 14)×1000≥3.5 In the relationships, [X] is the content (mass%) of element X.

Description

Steel, steel parts and steel manufacturing method

[0001] The present invention relates to a steel, a steel part, and a method for manufacturing a steel. More specifically, the present invention relates to a steel, a method for manufacturing a steel, and a steel part that have excellent fatigue properties due to a surface hardened layer formed by nitriding or nitrocarburizing. The steel and steel part of the present invention can be suitably used in the steel, steel part, and steel manufacturing method, particularly for parts used in automobiles and construction machinery.

[0002] Steel mechanical structural parts, such as automotive gears, require excellent fatigue properties, so they are usually subjected to surface hardening treatments. Well-known surface hardening treatments include carburizing, induction hardening, and nitriding. Recently, there are also cases where wear resistance is required.

[0003] Among these, carburizing is effective in improving the fatigue properties and wear resistance of parts because it penetrates and diffuses carbon into steel in the high-temperature austenite region, resulting in a deep hardening depth. However, because carburizing generates heat treatment distortion in the steel, it has been difficult to apply to parts that require strict dimensional accuracy from the perspective of quietness, etc.

[0004] Furthermore, because induction hardening is a process in which the surface layer of a part is hardened by high-frequency induction heating, distortion during heat treatment also occurs, and there are problems with dimensional accuracy, just like with carburizing.

[0005] On the other hand, nitriding treatment is ｃ１ This is a process in which N penetrates and diffuses into steel at a relatively low temperature below the transformation point, forming a hardened layer near the surface and increasing surface hardness. Therefore, the heat treatment distortion of the steel described above is small, and in recent years this process has been widely used for machine structural parts, etc. In addition, soft nitriding, in which C penetrates and diffuses into steel at the same time as N, is also widely used. Soft nitriding has the characteristic of forming a hardened layer more quickly.

[0006] Here, the hardened layer will be explained. In nitriding or nitrocarburizing, N in the atmosphere of these treatments diffuses into the steel, forming a hardened layer. In the outermost surface layer of the steel, Fe, which is the main component of the steel subjected to nitriding or nitrocarburizing, and other contained components combine with N to form nitrides, and a compound layer composed of these nitrides is formed. In the case of nitrocarburizing, C in the nitrocarburizing atmosphere also diffuses into the compound layer. Inside this compound layer (i.e., toward the center of the steel), there is a diffusion layer where N diffuses into the steel and the N concentration becomes higher than before the nitriding or nitrocarburizing, and this diffusion layer is formed adjacent to the inside of the compound layer. The hardened layer consists of the compound layer and diffusion layer described above.

[0007] In recent years, the application of shot peening has been considered to meet the need for improving the fatigue strength and wear resistance of parts due to demands for miniaturization of machine structural parts. However, even if shot peening introduces a stronger hardened layer onto the surface of a part to suppress the occurrence of surface cracks, unless the internal hardness of the part is also ensured, cracks will occur from the inside, and fatigue strength will not be improved. Therefore, to improve the fatigue strength of parts, it is necessary to ensure the hardness of the core part of the part (the part of the steel excluding the hardened layer), which is not affected by the hardening caused by the above-mentioned nitriding treatment, soft nitriding treatment, and shot peening, i.e., to ensure the hardness of the material.

[0008] To solve the above problems, Patent Documents 1 and 2 propose soft nitrided steels that contain about 0.20% C, Mn, Mo, and V together with Si to ensure core hardness, and Pb and / or Ca to ensure machinability.

[0009] Japanese Patent Laid-Open No. 9-227992 Japanese Patent Laid-Open No. 2007-332421

[0010] However, the addition of Pb as described in Patent Document 1 not only places a heavy burden on the environment, but also raises concerns about adverse effects on fatigue properties. The addition of Ca as described in Patent Document 2 also raises concerns about adverse effects on fatigue properties and increases alloy costs. Furthermore, the steel described in Patent Document 2 has poor machinability if Pb or Ca is not added. Furthermore, the steel described in Patent Document 2 has a surface hardness of 600 or more on the Vickers hardness scale after soft nitriding or soft nitriding treatment to improve wear resistance, but its wear resistance is inferior to that of carburized materials.

[0011] The present invention advantageously solves the above problems and aims to provide a steel, a steel part and a method for manufacturing a steel having excellent fatigue strength.

[0012] To solve the above problems, the inventors conducted extensive research into the effects of the chemical composition and structure of steel on machinability and fatigue properties. As a result, they discovered the novel finding that excellent machinability, i.e., machinability, can be achieved by incorporating appropriate amounts of Cr, Mo, V, and Nb into the chemical composition of steel and by making the steel structure bainite at an area ratio of more than 50%. The inventors also discovered the novel finding that steel with excellent fatigue properties can be obtained by suppressing internal fatigue crack initiation by finely dispersing and precipitating precipitates containing at least one of Cr, Mo, V, and Nb in the core of the steel. The present invention was completed based on the above findings and after further research. The gist of the present invention is as follows:

[0013] 1. A steel comprising: a hardened layer consisting of a compound layer and a diffusion layer disposed inside the compound layer; and a core which is the portion of the steel excluding the hardened layer, wherein the core contains, in mass %, C: 0.100% or less, Si: 1.00% or less, Mn: 0.50% or more but 3.00% or less, P: 0.020% or less, S: 0.060% or less, Cr: 0.30% or more but 3.00% or less, Mo: 0.005% or more but 0.400% or less, V: 0.02% or more but 0.50% or less, Nb: 0.003% or more but 0.150% or less, Al: 0.005% or more but 0.200% or less, N: 0.0200% or less, and A steel containing Sb: 0.0005% or more and 0.0200% or less, satisfying the following formulas (1) and (2), with the balance being Fe and unavoidable impurities, wherein the core contains precipitates of one or more of Cr, Mo, V and Nb, and wherein the hardened layer has a surface hardness of 700 HV or more and a surface compressive residual stress of 750 MPa or more: ([Mo] / 96+[V] / 50.9+[Nb] / 92.9)×1000≧5.5 (1) ([C] / 12+[N] / 14)×1000≧3.5 (2) where [X] is the content (mass%) of element X.

[0014] 2. The steel according to 1 above, wherein the chemical composition further contains, in mass%, one or more elements selected from W: 0.30% or less, Co: 0.30% or less, Hf: 0.20% or less, Zr: 0.20% or less, B: 0.0100% or less, Cu: 0.30% or less, Ni: 0.30% or less, Pb: 0.20% or less, Bi: 0.20% or less, Zn: 0.20% or less, Sn: 0.20% or less, and Ti: 0.10% or less.

[0015] 3. The number density of precipitates of one or more of Cr, Mo, V, and Nb in the compound layer is 0.00005 / nm ３ 3. The steel according to 1 or 2 above.

[0016] 4. The steel according to 3 above, wherein the hardened layer contains dispersed precipitates containing one or more of Cr, Mo, V, and Nb.

[0017] 5. A steel part made from the steel according to any one of 1 to 4 above.

[0018] 6. In mass%, C: 0.100% or less, Si: 1.00% or less, Mn: 0.50% or more, 3.00% or less, P: 0.020% or less, S: 0.060% or less, Cr: 0.30% or more, 3.00% or less, Mo: 0.005% or more, 0.400% or less, V: 0.02% or more, 0.50% or less, Nb: 0.003% or more, 0.150% or less, Al: 0.005% or more, 0.200% or less, N: 0.0200% or less, and A method for producing steel, comprising the steps of: ([Mo] / 96+[V] / 50.9+[Nb] / 92.9)×1000≧5.5 (1) ([C] / 12+[N] / 14)×1000≧3.5 (2) A method for producing steel, comprising the steps of: ([Mo] / 96+[V] / 50.9+[Nb] / 92.9)×1000≧5.5 (1) ([C] / 12+[N] / 14)×1000≧3.5 (2) A method for producing steel, comprising the steps of: ([Mo] / 96+[V] / 50.9+[Nb] / 92.9)×1000≧5.5 (1) ([C] / 12+[N] / 14)×1000≧3.5 (2) where [X] is the content (mass%) of element X.

[0019] 7. The method for producing steel according to 6 above, wherein the chemical composition further contains, in mass%, one or more elements selected from W: 0.30% or less, Co: 0.30% or less, Hf: 0.20% or less, Zr: 0.20% or less, B: 0.0100% or less, Cu: 0.30% or less, Ni: 0.30% or less, Pb: 0.20% or less, Bi: 0.20% or less, Zn: 0.20% or less, Sn: 0.20% or less, and Ti: 0.10% or less.

[0020] 8. The method for producing steel according to 6 or 7 above, wherein the hot working is hot forging performed after hot rolling.

[0021] According to the present invention, it is possible to provide a steel having excellent fatigue strength, a method for manufacturing the steel, and a steel part. Therefore, the steel of the present invention is extremely useful as a material for mechanical structural parts of automobiles, etc. Furthermore, the steel part of the present invention is extremely useful when applied to mechanical structural parts of automobiles, etc.

[0022] FIG. 1 is a schematic diagram showing a rotating bending fatigue test piece; FIG. 2 is a graph showing the effect of surface hardness on rotating bending fatigue strength; FIG. 3 is a graph showing the effect of compressive residual stress on rotating bending fatigue strength; FIG. 4 is a graph showing the effect of the value of formula (1) on rotating bending fatigue strength; FIG. 5 is a graph showing the effect of the value of formula (2) on rotating bending fatigue strength; FIG. 6 is a graph showing the effect of precipitate number density on specific wear rate; and FIG. 7 is a diagram showing a typical manufacturing process for nitrided or soft nitrided parts.

[0023] The present invention will be described in detail below. Note that the following description is of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below. The steel according to the present invention comprises a hardened layer and a core, which is the portion of the steel excluding the hardened layer. The hardened layer is formed by nitriding, soft nitriding, and shot peening. First, in nitriding or soft nitriding, the hardened layer is formed by the diffusion of N in the atmosphere of these treatments into the steel. The outermost surface of the hardened layer is a compound layer formed by N-combining Fe, the main component of the steel subjected to nitriding or soft nitriding, and other components to form nitrides. In the case of soft nitriding, C from the soft nitriding atmosphere also diffuses into the compound layer. Inside this compound layer (i.e., toward the center of the steel), a diffusion layer is located adjacent to the compound layer, where N diffuses into the steel, resulting in a higher N concentration than before the nitriding or soft nitriding treatment. The hardened layer consists of the above-mentioned compound layer and diffusion layer. On the other hand, the core is a portion inside the hardened layer where no diffusion of N has occurred, and has the following composition: The hardened layer has a higher N content than the core.

[0024] In the present invention, the hardened layer is further strengthened by shot peening after nitriding or soft nitriding, as will be described later.

[0025] The hardened layer and the core can be distinguished by their hardness. When the hardness is measured at regular intervals from the surface to the center of a cross section in the thickness direction, the hardness decreases with increasing distance from the surface and eventually reaches a constant value. The part with a constant hardness corresponds to the core, and the part where a significant increase in hardness is observed compared to the core corresponds to the hardened layer. The significant increase in hardness means an increase in hardness that exceeds the measurement error. The measurement error depends on the measuring device and the person performing the measurement, but the normal measurement error is within ±5%.

[0026] The composition of the steel will be described below, but the composition refers to the composition of the core. Note that "%" representing the composition below means "mass %" unless otherwise specified.

[0027] C: 0.100% or less C is necessary to ensure hardness through solid solution strengthening and precipitation strengthening. For this purpose, the C content is preferably 0.010% or more. On the other hand, if the C content exceeds 0.100%, the hardness after hot rolling or hot forging increases too much, and machinability (also called machinability) decreases. Therefore, the C content is set to 0.100% or less. The C content is preferably 0.080% or less.

[0028] Si: 1.00% or less Si is effective in ensuring strength through solid solution strengthening. For this purpose, the Si content is preferably 0.01% or more. On the other hand, if the Si content exceeds 1.00%, machinability deteriorates. Therefore, the Si content is set to 1.00% or less. The Si content is preferably 0.50% or less.

[0029] Mn: 0.50% or more, 3.00% or less Mn improves the hardenability of steel and stabilizes the formation of bainite. It also improves machinability by forming MnS with S. If the Mn content is less than 0.50%, the amount of MnS formed is insufficient, resulting in reduced machinability. Therefore, the Mn content is set to 0.50% or more. The Mn content is preferably set to 1.50% or more. On the other hand, if the Mn content exceeds 3.00%, the steel becomes excessively hard and deteriorates the machine. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably set to 2.50% or less, more preferably 2.00% or less.

[0030] P: 0.020% or less P is an element that is mixed into steel as an impurity and is known to embrittle grain boundaries, thereby reducing fatigue strength. Therefore, it is desirable to minimize the P content, but a P content of up to 0.020% is acceptable. However, since reducing the P content to less than 0.001% requires high costs, the P content is preferably 0.001% or more. Industrially, the P content can be reduced to 0.020%.

[0031] S: 0.060% or less S is an element that is mixed into steel as an impurity, but it also has the effect of improving machinability. That is, if the S content is less than 0.010%, the amount of MnS produced in the steel decreases, resulting in a decrease in machinability. Therefore, the S content is preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, if the S content exceeds 0.060%, not only will the above effect saturate, but excessive precipitation of MnS will reduce fatigue strength. Therefore, the S content is set to 0.060% or less. The S content is preferably 0.040% or less.

[0032] Cr: 0.30% or more, 3.00% or less Cr forms CrN together with N diffused from the steel surface during nitriding or nitrocarburizing, thereby strengthening the hardened layer through precipitation. The hardened layer can be formed by nitriding or nitrocarburizing, as described above, and is composed of a compound layer made of nitrides and a diffusion layer where N has diffused into the steel. The core, which is the portion other than the hardened layer, maintains the chemical composition of the steel material before nitriding or nitrocarburizing, except for N and C. It also improves the hardenability of the steel and facilitates the formation of bainite. If the Cr content is less than 0.30%, the amount of CrN precipitated in the hardened layer during nitriding is insufficient, making it difficult to ensure strength. Furthermore, if the number density of precipitates in the compound layer is less than 0.00005 / nm ３ The Cr content is 0.30% or less, which deteriorates the wear resistance. Therefore, the Cr content is set to 0.30% or more. The Cr content is preferably 0.50% or more. On the other hand, if the Cr content exceeds 3.00%, the hardness increases and the machinability is impaired. Therefore, the Cr content is set to 3.00% or less. The Cr content is preferably 1.50% or less.

[0033] Mo: 0.005% or more, 0.400% or less Mo forms nitrides with N diffused from the steel surface during nitriding or nitrocarburizing, contributing to an increase in the hardness of the surface layer. Mo also facilitates the formation of bainite, contributing to an improvement in machinability. To achieve these effects, the Mo content must be 0.005% or more. On the other hand, excessive addition of Mo excessively improves the hardenability of the steel, generates martensite, and excessively increases hardness, resulting in a decrease in machinability. Therefore, the Mo content is set to 0.400% or less. The Mo content is preferably 0.150% or less.

[0034] V: 0.02% or more, 0.50% or less V forms nitrides with N diffused from the steel surface during nitriding or soft nitriding, contributing to an increase in the hardness of the surface layer. If the V content is less than 0.02%, the above effect cannot be obtained, and furthermore, the number density of precipitates precipitated in the compound layer during nitriding is 0.00005 / nm ３The V content is 0.02% or less, which deteriorates the wear resistance. Therefore, the V content is set to 0.02% or more. The V content is preferably 0.05% or more. On the other hand, if excessive V is added, the precipitates become coarse and the effect of increasing hardness becomes saturated. Furthermore, the hardness after hot forging increases and the machinability decreases. Therefore, the V content is set to 0.50% or less. The V content is preferably 0.40% or less.

[0035] Nb: 0.003% or more, 0.150% or less Nb forms nitrides with N diffused from the steel surface during nitriding or nitrocarburizing, contributing to an increase in the hardness of the surface layer. If the Nb content is less than 0.003%, the above effect cannot be obtained. Therefore, the Nb content is set to 0.003% or more. The Nb content is preferably 0.020% or more. On the other hand, if Nb is added in excess, precipitates become coarse and the strength improvement effect saturates. Furthermore, the hardness after hot forging increases and machinability decreases. Therefore, the Nb content is set to 0.150% or less. The Nb content is preferably 0.120% or less.

[0036] Al: 0.005% or more, 0.200% or less Al is an element useful for improving surface hardness after nitriding or nitrocarburizing. If the Al content is less than 0.005%, the above effect cannot be obtained. Therefore, the Al content is set to 0.005% or more. The Al content is preferably 0.010% or more. On the other hand, if the Al content exceeds 0.200%, excessive precipitates are formed, the surface becomes embrittled, and fatigue strength decreases. Therefore, the Al content is set to 0.200% or less. The Al content is preferably 0.100% or less, more preferably 0.050% or less.

[0037] N: 0.0200% or less N is an element that forms carbonitrides in steel and has the effect of improving strength. However, if the N content exceeds 0.0200%, the hardness after hot forging increases and machinability decreases. Therefore, the N content is set to 0.0200% or less. On the other hand, although there is no particular lower limit for the N content, from the viewpoint of improving strength, the N content is preferably 0.0020% or more.

[0038] Sb: 0.0005% or more, 0.0200% or less Sb is an element that has the effect of promoting the formation of bainite and increasing hardness. To obtain this effect, the Sb content needs to be 0.0005% or more. The Sb content is preferably 0.0010% or more. On the other hand, even if the Sb content exceeds 0.0200%, the above effect saturates, leading not only to an increase in component costs but also to a decrease in base material toughness due to segregation. Therefore, the Sb content is set to 0.0200% or less. The Sb content is preferably 0.0100% or less.

[0039] ([Mo] / 96+[V] / 50.9+[Nb] / 92.9)×1000≧5.5 (1) ([C] / 12+[N] / 14)×1000≧3.5 (2) As will be described later, when nitrided or nitrocarburized steel satisfies the surface hardness and compressive residual stress specified in the present invention, fatigue fracture will initiate not from the surface but from the core. Therefore, in the present invention, after nitriding or nitrocarburizing, not only in the hardened layer but also in the core other than the hardened layer, precipitates of one or more of Cr, Mo, V, and Nb, i.e., individual precipitates of Cr, Mo, V, and Nb and composite precipitates of two or more of Cr, Mo, V, and Nb (hereinafter collectively referred to as Cr, Mo, V, and Nb precipitates), are dispersed and precipitated. This suppresses the accumulation of dislocations that cause fatigue cracks and improves fatigue properties after nitriding or nitrocarburizing. In particular, Mo, V, and Nb are elements that precipitate as nuclei, and must be contained so that the value of formula (1) is 5.5 or more. The value of formula (1) is preferably 6.5 or more. Furthermore, since Mo, V, and Nb in the core combine with C and N in the steel to precipitate as carbonitrides, it is necessary to contain C and N corresponding to Mo, V, and Nb. Therefore, C and N must be contained so that the value of formula (2) is 3.5 or more. The value of formula (2) is preferably 4.0 or more. The experiments that led to the identification of formulas (1) and (2) will be described later.

[0040] The grain size of Cr, Mo, V and Nb precipitates is preferably less than 20 nm, and the grain size is preferably less than 1 μm ２It is preferable to have 100 or more dispersed precipitates per particle in order to contribute to precipitation strengthening after nitriding or nitrocarburizing. The measurement limit for the particle size of precipitates, i.e., the smallest measurable particle size, is 1 nm. Incidentally, in parts obtained by nitriding or nitrocarburizing, the hardened layer (portions other than the core) has a composition with a higher N content than the composition of the core.

[0041] Furthermore, in addition to the above basic components, the steel of the present invention may contain one or more elements selected from W: 0.30% or less, Co: 0.30% or less, Hf: 0.20% or less, Zr: 0.20% or less, B: 0.0100% or less, Cu: 0.30% or less, Ni: 0.30% or less, Pb: 0.20% or less, Bi: 0.20% or less, Zn: 0.20% or less, Sn: 0.20% or less, and Ti: 0.10% or less. Optional elements and their contents are described below.

[0042] W: 0.30% or less W is an element effective in further improving the strength of steel. However, if the W content exceeds 0.30%, the toughness of the steel decreases. Therefore, when W is added, the W content is set to 0.30% or less, preferably 0.25% or less. On the other hand, although there is no particular lower limit for the W content, from the viewpoint of improving strength, the W content is preferably set to 0.01% or more.

[0043] Co: 0.30% or less Co is an element effective in further improving the strength of steel. However, if the Co content exceeds 0.30%, the toughness of the steel decreases. Therefore, when Co is added, the Co content is set to 0.30% or less, preferably 0.25% or less. On the other hand, although there is no particular lower limit for the Co content, from the viewpoint of improving strength, the Co content is preferably set to 0.01% or more.

[0044] Hf: 0.20% or less Hf is an element effective in further improving the strength of steel. However, if the Hf content exceeds 0.20%, the toughness of the steel decreases. Therefore, when Hf is added, the Hf content is set to 0.20% or less, preferably 0.15% or less. On the other hand, although there is no particular lower limit for the Hf content, from the viewpoint of improving strength, the Hf content is preferably set to 0.01% or more.

[0045] Zr: 0.20% or less Zr is an element effective in further improving the strength of steel. However, if the Zr content exceeds 0.20%, the toughness of the steel decreases. Therefore, when Zr is added, the Zr content is set to 0.20% or less, preferably 0.15% or less. On the other hand, although there is no particular lower limit for the Zr content, from the viewpoint of improving strength, the Hf content is preferably set to 0.01% or more.

[0046] B: 0.0100% or less B is an element that improves hardenability and promotes the formation of bainite. However, if the B content exceeds 0.0100%, B precipitates as BN, which not only saturates the hardenability improvement effect but also increases the cost of the component. Therefore, when B is added, the B content is set to 0.0100% or less, preferably 0.0080% or less. On the other hand, although there is no particular lower limit for the B content, from the viewpoint of improving hardenability, the B content is preferably set to 0.0003% or more, more preferably 0.0005% or more.

[0047] Cu: 0.30% or less Cu is an element that forms intermetallic compounds with Fe and Ni during nitriding or soft nitriding, and has the effect of improving the strength of steel obtained through nitriding or soft nitriding by precipitation strengthening. Cu also contributes to the formation of bainite. However, if the Cu content exceeds 0.30%, hot workability deteriorates. Therefore, when Cu is added, the Cu content is set to 0.30% or less, preferably 0.25% or less. On the other hand, although there is no particular lower limit for the Cu content, from the viewpoint of improving strength, the Cu content is preferably set to 0.05% or more.

[0048] Ni: 0.30% or less Ni is an element that has the effect of increasing hardenability and suppressing low-temperature brittleness. However, if the Ni content exceeds 0.30%, the hardness increases, which not only adversely affects machinability but also becomes cost-inefficient. Therefore, when Ni is added, the Ni content is set to 0.30% or less, preferably 0.25% or less. On the other hand, although there is no particular lower limit for the Ni content, from the viewpoint of suppressing low-temperature brittleness, the Ni content is preferably set to 0.05% or more.

[0049] Pb: 0.20% or less Pb is an element that improves the machinability of steel. However, if the Pb content exceeds 0.20%, toughness decreases. Furthermore, excessive addition of Pb may impair the fatigue properties of steel. Pb is also an element that places a large burden on the environment and is cost-inefficient. Therefore, when Pb is added, the Pb content is set to 0.20% or less. The Pb content is preferably set to 0.10% or less. While there is no particular lower limit for the Pb content, the Pb content can be set to 0% (no Pb) from the perspectives of improving fatigue properties, reducing environmental impact, and cost. From the perspective of improving machinability, the Pb content is preferably set to 0.02% or more.

[0050] Bi: 0.20% or less Bi is an element that has the effect of improving the machinability of steel. However, if the Bi content exceeds 0.20%, toughness decreases. Therefore, when Bi is added, the Bi content is set to 0.20% or less, preferably 0.1% or less. On the other hand, although there is no particular lower limit for the Bi content, from the viewpoint of improving machinability, the Bi content is preferably set to 0.02% or more.

[0051] Zn: 0.20% or less Zn is an element that has the effect of improving the machinability of steel. However, if the Zn content exceeds 0.20%, toughness decreases. Therefore, when Zn is added, the Zn content is set to 0.20% or less, preferably 0.10% or less. On the other hand, although there is no particular lower limit for the Zn content, from the viewpoint of improving machinability, the Bi content is preferably set to 0.02% or more.

[0052] Sn: 0.20% or less Sn is an element that has the effect of improving the machinability of steel. However, if the Sn content exceeds 0.20%, toughness decreases. Therefore, when Sn is added, the Sn content is set to 0.20% or less, preferably 0.10% or less. On the other hand, although there is no particular lower limit for the Sn content, from the viewpoint of improving machinability, the Zn content is preferably set to 0.02% or more.

[0053] Ti: 0.10% or less Ti is an element effective in further improving the strength of steel. However, if the Ti content exceeds 0.10%, the toughness of the steel decreases. Therefore, when Ti is added, the Ti content is set to 0.10% or less. The Ti content is preferably set to 0.01% or less. On the other hand, although there is no particular lower limit for the Ti content, from the viewpoint of improving strength, the Ti content is preferably set to 0.003% or more.

[0054] The composition of the core of the steel of the present invention is the balance of Fe and inevitable impurities other than the elements described above. Incidentally, inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, manufacturing equipment, etc., and are allowed to be included to the extent that they do not impair the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap. Examples of inevitable impurities include O, H, and As.

[0055] The steel of the present invention has a surface hardness of 700 HV or more, a surface compressive residual stress of 750 MPa or more, a value of formula (1) of 5.5 or more, and a value of formula (2) of 3.5 or more. The experimental results that led to limiting the hardness and compressive residual stress of the steel of the present invention to the above ranges are described below.

[0056] Specifically, rotating bending fatigue test pieces shown in FIG. 1 were prepared using various materials and processing methods described in the Examples below, with different surface hardnesses and surface compressive residual stresses, and the fatigue properties of each were evaluated. The results of the rotating bending fatigue test (rotating bending fatigue strength) are summarized in FIGS. 2 to 5, showing the relationship between the surface hardness, surface compressive residual stress, and the values ​​of Equations (1) and (2). The methods for measuring the surface hardness and surface compressive residual stress, as well as the rotating bending fatigue strength, were the same as those used in the Examples below. The acceptable value for rotating bending fatigue strength was set at 700 MPa.

[0057] [Surface hardness: 700 HV or more] First, the relationship between rotating bending fatigue strength (fatigue limit) and surface hardness is shown in Figure 2. In the experiment whose results are shown in Figure 2, the effect of surface hardness on rotating bending fatigue strength was investigated under the conditions that the surface compressive residual stress was 750 MPa or more, the value of formula (1) was 5.5 or more, and the value of formula (2) was 3.5 or more. The measurement positions for surface hardness and the surface compressive residual stress shown below are positions corresponding to the hardened layer.

[0058] From Figure 2, it can be seen that when the surface hardness is 700 HV or more, the fatigue limit is significantly increased, and the rotating bending fatigue strength is particularly excellent. When the fracture surface after the fatigue test was observed, it was estimated that the fracture originated from the inside of the test piece in the test piece with a surface hardness of 700 HV or more, but the fracture originated from the surface in the test piece in the test piece with a surface hardness of less than 700 HV. In other words, it is thought that when the surface hardness of the steel is high, the initiation or propagation of cracks from the surface is suppressed, thereby improving the fatigue strength. The surface hardness is usually 1000 HV or less.

[0059] [Surface compressive residual stress: 750 MPa or more] The relationship between rotating bending fatigue strength (fatigue limit) and surface compressive residual stress is shown in Figure 3. In the experiment whose results are shown in Figure 3, the effect of surface compressive residual stress on rotating bending fatigue strength was investigated under the conditions that the surface hardness was 700 HV or more, the value of formula (1) was 5.5 or more, and the value of formula (2) was 3.5 or more.

[0060] From Figure 3, it can be seen that when the surface compressive residual stress is 750 MPa or more, the fatigue limit is significantly increased, and the rotating bending fatigue strength is particularly excellent. Observation of the fracture surface after fatigue testing revealed that in test specimens with a surface compressive residual stress of 750 MPa or more, fracture originated from the interior of the test specimen, whereas in test specimens with a surface compressive residual stress of less than 750 MPa, fracture originated from the surface of the test specimen. In other words, when the surface compressive residual stress of the steel is sufficiently high, at 750 MPa or more, crack initiation or propagation from the surface is suppressed, which is thought to improve fatigue strength. Furthermore, the surface compressive residual stress is usually 1800 MPa or less.

[0061] [Value of Equation (1): 5.5 or More] Next, the relationship between the rotating bending fatigue strength and the value of Equation (1) is shown in Fig. 4. In the experiment whose results are shown in Fig. 4, the effect of the value of Equation (1) on the rotating bending fatigue strength was investigated under the conditions that the surface hardness was 700 HV or more, the surface compressive residual stress was 750 MPa or more, and the value of Equation (2) was 3.5 or more.

[0062] 4, it can be seen that when the value of formula (1) is 5.5 or more, the rotating bending fatigue strength is particularly excellent. As mentioned above, when the surface hardness is 700 HV or more and the compressive residual stress is 750 MPa or more, fatigue fracture occurs from the inside. Therefore, it is thought that the higher the value of formula (1), the more the movement of dislocations due to precipitates is suppressed, making it less likely that fatigue cracks will initiate from the inside, resulting in improved fatigue strength.

[0063] [Value of Equation (2): 3.5 or More] Next, the relationship between the rotating bending fatigue strength and the value of Equation (2) is shown in Fig. 5. In the experiment whose results are shown in Fig. 5, the effect of the value of Equation (2) on the rotating bending fatigue strength was investigated under the conditions that the surface hardness was 700 HV or more, the surface compressive residual stress was 750 MPa or more, and the value of Equation (1) was 5.5 or more.

[0064] 5, it can be seen that when the value of formula (2) is 3.5 or higher, the rotating bending fatigue strength is particularly excellent. As mentioned above, when the surface hardness is 700 HV or higher and the compressive residual stress is 750 MPa or higher, fatigue fracture occurs from the inside. Therefore, it is thought that the higher the value of formula (2), the more the movement of dislocations due to precipitates is suppressed, making it less likely that fatigue cracks will initiate from the inside, resulting in improved fatigue strength.

[0065] [The number density of precipitates in the compound layer is 0.00005 / nm ３ In the present invention, the number density of precipitates of one or more of Cr, Mo, V and Nb in the compound layer is 0.00005 nm or more. ３ The relationship between the specific wear rate and the surface hardness is shown in Figure 6. From this, it can be seen that the number density of precipitates is 0.00005 / nm ３ The specific wear rate of the above test pieces was 1.0 × 10 －９ mm ３ / kgf·mm or less, and excellent wear resistance was obtained.

[0066] [Area Ratio of Bainite Exceeds 50% Before Nitriding or Nitrocarburizing] When obtaining the steel according to the present invention, it is extremely important that the area ratio of bainite to the entire structure is greater than 50% in the steel material to be subjected to nitriding or nitrocarburizing.

[0067] As described above, the present invention aims to improve fatigue properties after nitriding or nitrocarburizing by dispersing and precipitating Cr, Mo, V, and Nb not only in the hardened layer but also in the core portion other than the hardened layer after nitriding or nitrocarburizing. The presence of Cr, Mo, V, and Nb precipitates before nitriding or nitrocarburizing is disadvantageous from the perspective of machinability during cutting, which is typically performed before nitriding or nitrocarburizing. Furthermore, the presence of Cr, Mo, V, and Nb precipitates reduces the dispersed precipitation of precipitates during the nitriding or nitrocarburizing process, which is disadvantageous from the perspective of fatigue strength. In this regard, Cr, Mo, V, and Nb precipitates are less likely to form in the matrix during the bainite transformation process than during the ferrite-pearlite transformation process. Therefore, the steel structure before nitriding or nitrocarburizing is primarily bainite. Specifically, the area ratio of bainite to the entire structure is set to be greater than 50%. The area ratio of bainite is preferably greater than 60%, more preferably greater than 80%, and may even be 100%. Note that possible structures other than bainite include ferrite, pearlite, etc., but it goes without saying that the less of these structures there are, the better. Note that this bainite refers to a structure in which cementite is dispersed and precipitated in a ferrite matrix, but in the process of precipitation of Cr, Mo, V, and Nb in the core by nitriding or soft nitriding, they steal C contained in the cementite in the bainite and precipitate as carbides, so the amount of cementite contained in the bainite decreases or disappears. As a result, a structure that was bainite before nitriding or soft nitriding may become a structure different from bainite after nitriding or soft nitriding, such as a structure consisting of bainitic ferrite and precipitates such as VC.

[0068] The area ratio of each structure can be determined as follows. Specifically, a test specimen is taken from a steel obtained by the manufacturing method described below, and a vertical cross section (L cross section) parallel to the rolling direction is polished and etched with nital. The type of structure is identified by observing the cross-sectional structure (optical microscope structure observation at 200x magnification) using an optical microscope or a scanning electron microscope (SEM), and the area ratio of each structure is determined. Furthermore, in the steel part of the present invention, a steel having a chemical composition according to the present invention is subjected to nitriding or nitrocarburizing treatment, so that precipitates containing Cr, Mo, V, and Nb, such as compounds of (Cr, Mo, Nb, V) and (C, N), are dispersed and precipitated in bainite.

[0069] Next, a steel part according to the present invention will be described. The steel part according to the present invention is formed by giving the steel according to the present invention the shape of various parts, preferably machine structural parts. Here, it is particularly preferable that the steel part according to the present invention is a toothed part such as a gear. The teeth of a toothed part such as a gear are required to have excellent bending fatigue strength because bending stress occurs at the tooth root. The hard layer described above is formed in this tooth portion, satisfies formulas (1) and (2), contains precipitates of one or more of Cr, Mo, V, and Nb, has a surface hardness of 700 HV or more, and has a surface compressive residual stress of 750 MPa or more, which leads to ensuring the durability of the toothed part.

[0070] In addition, even for steel parts other than toothed parts that are repeatedly subjected to bending stress, bending fatigue in this area is important for ensuring durability of the part, and therefore, by forming a hardened layer consisting of the above-mentioned compound layer and diffusion layer in such an area, the effect of improving durability can be obtained. Therefore, the steel part of the present invention is not limited to toothed parts.

[0071] Next, a method for manufacturing steel and steel parts according to the present invention will be described. Figure 7 shows a typical method for manufacturing steel parts using the steel (e.g., steel bar) according to the present invention. Here, S1 is a manufacturing process for manufacturing steel bars from raw steel, S2 is a transport process for the steel bars, and S3 is a finishing process for nitriding or soft-nitriding steel and steel parts.

[0072] First, in the steel bar manufacturing process (S1), a steel ingot having a predetermined chemical composition is hot-rolled to form a steel bar, which is then inspected for quality and shipped. After transportation (S2), the steel bar is cut to desired dimensions and subjected to hot or cold forging in the finishing process (S3) for nitriding or nitrocarburizing steel and steel parts. Thereafter, if necessary, the bar is formed into a desired shape (e.g., a gear product or a shaft part) by cutting processes such as drilling or turning, and then subjected to nitriding or nitrocarburizing treatment to form the final product.

[0073] In some cases, hot-rolled material is directly finished into the desired shape by cutting processes such as turning or drilling, and then nitrided or nitrocarburized to produce the finished product. In the case of hot forging, cold straightening may be performed after hot forging. In other cases, the final product may be coated with paint or plating.

[0074] In the method for producing steel according to the present invention, in the hot working step immediately before nitriding or nitrocarburizing, the heating temperature and working temperature during hot working are set to specific conditions, thereby forming a structure mainly composed of bainite as described above and suppressing the formation of Cr, V and Nb precipitates.

[0075] The above-mentioned hot working mainly means hot rolling or hot forging, but hot forging may be further carried out after hot rolling. Needless to say, cold forging may also be carried out after hot rolling.

[0076] Here, when the hot working step immediately before the nitriding or nitrocarburizing treatment is a hot rolling step, that is, when hot forging is not performed after the hot rolling, the following conditions are satisfied in the hot rolling step.

[0077] [Rolling heating temperature: 950°C or higher and 1250°C or lower] In the hot rolling process, carbides remaining from melting are dissolved to prevent fine precipitates from precipitating in the rolled material (steel bar that serves as the raw material for steel parts to be subjected to cold forging and / or cutting) and impairing forgeability. That is, if the rolling heating temperature is lower than 950°C, the carbides remaining from melting are less likely to dissolve. On the other hand, if the rolling finish temperature exceeds 1250°C, the crystal grains become coarse, which tends to deteriorate forgeability. For this reason, the rolling heating temperature is set to a range of 950°C or higher and 1250°C or lower.

[0078] [Finish rolling temperature: 800°C or higher] If the finish rolling temperature is lower than 800°C, ferrite will be generated, which is disadvantageous in generating bainite that satisfies an area ratio of more than 50% relative to the entire structure before nitriding or soft nitriding. In addition, the rolling load will also increase. Therefore, the finish rolling temperature is set to 800°C or higher. The upper limit of the finish rolling temperature is preferably set to about 1100°C.

[0079] [Average cooling rate after rolling in a temperature range of at least 700°C to 550°C: more than 0.4°C / s] In order to prevent the precipitation of fine precipitates before the steel is finished into a desired shape in cutting, which would impair machinability, the average cooling rate after rolling is set to a rate that exceeds 0.4°C / s, which is the critical cooling rate at which fine precipitates are obtained, in a temperature range of at least 700°C to 550°C, which is the precipitation temperature range of fine precipitates. The upper limit of the average cooling rate after rolling is preferably about 200°C / s.

[0080] Furthermore, when the hot working step immediately before the nitriding or nitrocarburizing treatment is a hot forging step, that is, when only hot forging is performed or when hot forging is performed after hot rolling, the following conditions are satisfied in the hot forging step. Note that when hot rolling is performed before hot forging, the hot rolling conditions do not necessarily have to satisfy the above-mentioned conditions.

[0081] [Hot forging conditions] In this hot forging, in order to make bainite exceed 50% in area ratio relative to the entire structure, and to prevent fine precipitates from precipitating before cold straightening and cutting from the viewpoint of machinability in cold straightening and cutting after hot forging, the heating temperature during hot forging is set to 950°C or higher and 1250°C or lower, the forging finish temperature is set to 800°C or higher, and the average cooling rate after forging is set to exceed 0.4°C / s in the temperature range of at least 700°C or lower and 550°C or higher. All of these conditions are set for the same reasons as in the case of hot rolling described above.

[0082] The obtained rolled or forged material is then subjected to cutting or other processes to form a part shape, and then is subjected to nitriding or soft nitriding under the following conditions: By performing this nitriding or soft nitriding, a compound layer is formed on the surface, which is made up of compounds formed by the bonding of N in the atmosphere with the component elements of the steel, and a hardened layer made up of a diffusion layer of N is formed inside the compound layer.

[0083] In this series of manufacturing steps, to obtain the steel or steel part of the present invention, a steel having the above-mentioned component composition for the core is used as the steel to be subjected to hot rolling and / or hot forging in the steel bar manufacturing step S1 shown in Figure 7. In order to obtain the above-mentioned hardness conditions for the hardened layer, the nitriding or soft-nitriding temperature in the nitriding or soft-nitriding step is preferably 550°C or higher and 590°C or lower, and the nitriding or soft-nitriding time is preferably 3 hours or longer. On the other hand, when the nitriding or soft-nitriding time is this long, N is formed in the outermost layer of the compound layer. ２ It is necessary to prevent excessive gas generation, which leads to excessive growth of the porous layer and a decrease in fatigue strength. It is generally known that lowering the nitriding potential of the atmosphere during nitriding or soft nitriding reduces the thickness of the porous layer. Therefore, it is necessary to determine in advance the relationship between the nitriding potential and the porous layer thickness for each standard or composition of the steel material, and to adopt a nitriding potential that can achieve the desired porous layer thickness. From the perspective of fatigue strength, it is desirable that the region where pores occur be 5 μm or less from the surface.

[0084] By carrying out the above-mentioned nitriding or soft nitriding treatment in two stages, the number density of precipitates in the compound layer increases, and the specific wear rate can be reduced. When carrying out two-stage nitriding or soft nitriding treatment, it is preferable to carry out the treatment under the following conditions.

[0085] By setting the first stage nitriding or soft nitriding temperature in the low temperature range of 500°C or more and 550°C or less, the number density of precipitates in the compound layer is reduced to 0.00005 / nm ３ If the first stage nitriding or soft nitriding temperature is less than 500°C, no precipitates are formed, and if it exceeds 550°C, the number of precipitates is small, and the number density is 0.00005 / nm３ It cannot be more than that.

[0086] The range of diffusion of N can be deepened by setting the second-stage nitriding or soft-nitriding temperature in a high temperature range of 550° C. or higher and 590° C. or lower. If the second-stage nitriding or soft-nitriding temperature is lower than 550° C., the depth of diffusion of N is shallow, and if it exceeds 590° C., it will transform into austenite, and the advantage of nitriding treatment, that is, hardening without undergoing deformation due to phase transformation, cannot be obtained.

[0087] In the nitriding process, N penetrates and diffuses into the steel, so NH ３ YaN ２ A mixed atmosphere of nitrogenous gases such as NH ３ :N ２ Nitriding can be performed in an atmosphere with a nitriding ratio of 50:50. In soft nitriding, N and C penetrate into the steel at the same time, forming a compound layer in which C is dissolved, and then N is diffused into the base steel. ３ YaN ２ Nitrogen gases such as CO ２ A mixture of carburizing gases such as NH ３ :N ２ :CO ２ Nitrocarburizing may be carried out in an atmosphere of SiO 2 = 50:45:5.

[0088] The obtained nitrided or nitrocarburized material is subjected to shot peening under the following conditions. By performing this shot peening, the surface layer is plastically deformed, and compressive residual stress is generated. That is, to obtain the desired compressive residual stress, shot material having a hardness greater than the hardness of the nitrided or nitrocarburized material surface can be collided at an appropriate injection pressure so that the surface of the nitrided or nitrocarburized material is sufficiently plastically deformed. For example, shot material such as cast steel having a hardness of about HV 600 or more can be projected at an injection pressure of 0.2 MPa or more. If the shot material is too large, it will be difficult to collide it at the desired position, so it is recommended to use shot material with a diameter of 2 mm or less. Shot peening may be performed two or more times to improve the deterioration of surface roughness caused by the initial shot peening, for example.

[0089] The steel according to the present invention or a steel part made from this steel can be obtained by the above manufacturing process.

[0090] Examples of the present invention will be described in detail below. Molten steel (steel types 1 to 46) having the chemical compositions shown in Table 1 was cast in a continuous casting machine to produce a slab with a cross section of 300 mm x 400 mm. This slab was soaked at 1250°C for 30 minutes and then hot rolled to produce a slab with a rectangular cross section of 140 mm on a side. This slab was then hot rolled according to the conditions shown in Table 2 to produce an 80 mm diameter steel bar (as-hot-rolled material). This steel bar was then hot forged according to the conditions shown in Table 2 to produce a smaller diameter steel bar (hot-forged material) of 35 mm diameter. Some of the as-hot-rolled material was not hot forged and was used as the as-hot-rolled material.

[0091]

[0092]

[0093] The machinability (tool life) of the hot-rolled and hot-forged materials thus obtained was evaluated by peripheral turning tests. The test materials were as-hot-rolled or hot-forged materials cut to 200 mm lengths. The cutting tools used were a Mitsubishi Materials Corporation CSBNR 2020 folder and a Mitsubishi Materials Corporation SNGN 120408 UTi20 high-speed tool steel insert. The peripheral turning test conditions were a depth of cut of 1.0 mm, a feed rate of 0.25 mm / rev, and a cutting speed of 200 m / min, and Yushiroken lubricant was used. The tool life was evaluated as the time until the tool wear (flank wear) reached 0.2 mm. The pass value was 1200 seconds (s).

[0094] In addition, the hardness of the above-mentioned hot-rolled material and hot-forged material was measured. Test specimens for evaluation were taken from the center of the hot-rolled material or hot-forged material. The hardness was measured using a Vickers hardness tester in accordance with JIS Z2244, measuring the hardness at five points at 1 / 4 radial positions with a test load of 2.94 N (300 gf), and the average value was taken as the hardness HV.

[0095] Furthermore, the structure was identified and the area ratio was measured for the as-hot-rolled material and the hot-forged material. That is, test pieces were taken from the as-hot-rolled material and the hot-forged material, and the cross sections (L cross sections) parallel to the rolling direction were etched with nital after surface polishing, and the type of structure was identified by cross-sectional structure observation using an optical microscope (optical microscope structure observation at 200 magnification), and the area ratio of each structure was determined.

[0096]

[0097] Furthermore, for the above-mentioned as-hot-rolled material and hot-forged material, rotary bending fatigue test specimens shown in Figure 1 were taken parallel to the longitudinal direction, and these test specimens were subjected to nitriding or soft-nitriding. That is, to obtain the desired hardness distribution, the nitriding temperature, time, and nitriding potential (atmospheric gas composition) were appropriately adjusted. Next, the nitrided or soft-nitrided test specimens were subjected to shot peening. The shot material and injection pressure were appropriately adjusted to obtain the desired compressive residual stress.

[0098] The test pieces thus obtained after shot peening were subjected to measurements of surface hardness, surface wear resistance, observation of precipitates, and surface compressive residual stress, as well as evaluation of fatigue properties.

[0099] Here, the surface hardness was measured at five points 50 μm from the surface on the cross section of the nitrided material, and the average value was taken as the surface hardness (HV). It was confirmed that this position 50 μm from the surface was included in the hardened layer (surface layer portion) according to the above-mentioned method for confirming the hardened layer (surface layer portion) and core portion. The effective hardened layer depth (HV550) was determined by measuring hardness at 0.1 mm intervals from the surface toward the axial center and interpolating the depth at which HV550 was achieved. All hardness measurements were performed using a Vickers hardness tester in accordance with JIS Z2244, with a test load of 2.94 N (300 gf).

[0100] For observation of precipitates, samples for transmission electron microscope observation were prepared from a radial quarter position of the parallel part of the soft nitrided material and the carburized, quenched, and tempered material by electrolytic polishing using the twin jet method, and the obtained samples were observed using a transmission electron microscope with an accelerating voltage of 200 V. Furthermore, the composition of the observed precipitates was determined by energy dispersive X-ray spectroscopy (EDX).

[0101] A three-dimensional atom probe (3DAP) was used to analyze the precipitates in the compound layer. Needle-shaped samples for the 3D atom probe were prepared from the compound layers of the soft-nitrided and carburized, quenched, and tempered materials using a focused ion beam device, and measurements were performed. From the three-dimensional composition images obtained by 3DAP, an isoconcentration surface with a 10% Cr concentration was created, and the area surrounded by the isoconcentration surface was considered to be the precipitate, thereby calculating the precipitate density in the compound layer.

[0102] Wear resistance was measured using a sliding wear tester, paraffin-based 60 spindle oil as the lubricant, and a contact pressure of 147 N / cm ２ The wear rate was 3.40 m / sec. The specific wear amount was calculated by dividing the wear volume by the load and the friction distance after the wear test.

[0103] The compressive residual stress was measured by electrolytically polishing the notch bottom of the test specimen approximately 50 μm from the surface, and measuring the axial stress using the sin2ψ method with an AutoMATE manufactured by Rigaku Corporation. Measurements were made using α-Fe 211 diffraction (unstrained 2θ = 156.40°) and calculations were performed using a stress constant of -318 MPa / deg.

[0104] For the fatigue property evaluation, among the rotating bending fatigue test pieces (see Figure 1) after shot peening treatment, those that had not undergone any of structure observation, hardness measurement, and precipitate observation were used, and fatigue limit strength was determined by creating an S-N diagram using an Ono-type rotating bending fatigue tester. The fatigue limit strength was calculated by 10 for two or more test pieces. ７ The maximum stress at which the test continued for more than 1000 times was used. The rotation speed was 3500 rpm.

[0105] The above measurement results and evaluation results are shown in Tables 3 and 4. In Table 3, Nos. 1 to 26 are inventive examples according to the present invention, and Nos. 27 to 65 are comparative examples. Note that Nos. 63 to 65 are comparative examples without shot peening. Furthermore, in Table 4, Nos. 1 to 26 and 66 to 91 are examples according to the present invention, and Nos. 27 to 61 are comparative examples. Note that Nos. 63 to 65 are comparative examples without shot peening.

[0106]

[0107] As is clear from Tables 3 and 4, all of Inventive Examples Nos. 1 to 26 and Nos. 66 to 91 exhibited excellent fatigue strength and machinability. On the other hand, Comparative Examples Nos. 27 to 65 exhibited inferior fatigue properties or machinability because either the chemical composition, the resulting steel structure, the hardness, or the compressive residual stress was outside the range of the present invention.

[0108] According to the present invention, it is possible to provide a steel having excellent fatigue strength, a method for manufacturing the steel, and a steel part.

Claims

1. It is made of steel, The material comprises a hardened layer consisting of a compound layer and a diffusion layer disposed inside the compound layer, and a core portion which is the portion of the steel from which the hardened layer has been removed. The aforementioned core portion is, by mass %, C: 0.100% or less, Si: 1.00% or less, Mn: 0.50% or more, 3.00% or less, P: 0.020% or less, S: 0.060% or less, Cr: 0.30% or more, 3.00% or less, Mo: 0.005% or more, 0.400% or less, V: 0.02% or more, 0.50% or less, Nb: 0.003% or more, 0.150% or less, Al: 0.005% or more, 0.200% or less, N: 0.0200% or less and Sb: 0.0005% or more, 0.0200% or less It contains and satisfies the following formulas (1) and (2), with the remainder having a component composition of Fe and unavoidable impurities. The core portion contains one or more precipitates of Cr, Mo, V, and Nb. A steel having a surface hardness of 700 HV or more in the hardened layer and a compressive residual stress of 750 MPa or more on the surface. ([Mo] / 96+[V] / 50.9+[Nb] / 92.9)×1000≧5.5 (1) ([C] / 12+[N] / 14)×1000≧3.5 (2) Here, [X] is the mass %) of element X.

2. The aforementioned component composition is further expressed in mass%, W: 0.30% or less, Co: 0.30% or less, Hf: 0.20% or less, Zr: 0.20% or less, B: 0.0100% or less, Cu: 0.30% or less, Ni: 0.30% or less, Pb: 0.20% or less, Bi: 0.20% or less, Zn: 0.20% or less, Sn: 0.20% or less and Ti: 0.10% or less The steel according to claim 1, comprising one or more selected from among.

3. The number density of one or more precipitates of Cr, Mo, V, and Nb in the compound layer is 0.00005 / nm ３ The steel according to claim 1 or 2.

4. The steel according to claim 3, wherein precipitates containing one or more precipitates of Cr, Mo, V, and Nb are dispersed and precipitated in the hardened layer.

5. A steel part made of the steel described in claim 1 or 2.

6. In mass percent, C: 0.100% or less, Si: 1.00% or less, Mn: 0.50% or more, 3.00% or less, P: 0.020% or less, S: 0.060% or less, Cr: 0.30% or more, 3.00% or less, Mo: 0.005% or more, 0.400% or less, V: 0.02% or more, 0.50% or less, Nb: 0.003% or more, 0.150% or less, Al: 0.005% or more, 0.200% or less, N: 0.0200% or less and Sb: 0.0005% or more, 0.0200% or less A method for manufacturing steel, comprising: using a steel material containing and satisfying the following formulas (1) and (2), with the remainder being Fe and unavoidable impurities, hot working the material at a heating temperature of 950°C to 1250°C and a finishing temperature of 800°C or higher to obtain a hot-worked material; then cooling the hot-worked material at a temperature range of at least 700°C to 550°C with an average cooling rate of more than 0.4°C / s, followed by nitriding or soft nitriding, and then shot peening. ([Mo] / 96+[V] / 50.9+[Nb] / 92.9)×1000≧5.5 (1) ([C] / 12+[N] / 14)×1000≧3.5 (2) Here, [X] is the mass %) of element X.

7. The aforementioned component composition is further expressed in mass%, W: 0.30% or less, Co: 0.30% or less, Hf: 0.20% or less, Zr: 0.20% or less, B: 0.0100% or less, Cu: 0.30% or less, Ni: 0.30% or less, Pb: 0.20% or less, Bi: 0.20% or less, Zn: 0.20% or less, Sn: 0.20% or less and Ti: 0.10% or less A method for producing steel according to claim 6, comprising one or more selected from among them.

8. The method for manufacturing steel according to claim 6 or 7, wherein the hot working is hot forging performed after hot rolling.