Steel component

Abstract

To provide a steel component having a carburized hardened layer with a sufficiently reduced surface friction coefficient.SOLUTION: A steel component comprises a core with a chemical composition, by mass%, containing C: 0.10-0.30%, Si: 0.15-1.00%, Mn: 0.30-1.00%, P: 0.030% or less, S: 0.025% or less, Cr: 0.90-2.00%, Al: 0.005-0.100%, N: 0.0250% or less, O: 0.0050% or less, Cu: 0.10-0.50%, and Ni: 0.05-2.00%, with the remainder comprising Fe and impurities. In a depth direction elemental concentration profile obtained by glow discharge optical emission spectrometry from the surface of a carburized hardened layer toward the carburized hardened layer depth direction, the surface CuNi concentration Ts, defined as the sum of Cu and Ni concentrations from the surface of the carburized hardened layer to a depth of 10 nm, and the core CuNi concentration Tc, defined as the sum of Cu and Ni concentrations in the core, satisfy the following expression: Ts / Tc≥0.60 (1).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to steel parts, and more particularly to steel parts that have been subjected to a carburizing process and have a carburized case layer. [Background technology]

[0002] Steel parts, such as gears, are used in the power transmission mechanisms (power units and peripheral parts of the power units) of automobiles, etc. Steel parts for such applications require high surface fatigue strength.

[0003] Carburizing is a known method for increasing the surface fatigue strength of steel parts. In carburizing, a carburized hardened layer is formed on the surface of the steel part. This carburized hardened layer increases the surface fatigue strength of the steel part.

[0004] In order to reduce friction loss in power transmission mechanisms, steel parts are also required to have a reduced coefficient of friction on the surface of the carburized hardened layer that forms the sliding surface. Japanese Patent Laid-Open No. 2013-083322 (Patent Document 1) and Japanese Patent Laid-Open No. 2021-167395 (Patent Document 2) propose steel parts that can reduce the coefficient of friction.

[0005] In the gear, a steel part proposed in Patent Document 1, the friction coefficient is reduced by forming a plateau structure surface on the sliding surface of the gear.In Patent Document 2, the friction coefficient is reduced by forming a tribofilm with a specific chemical composition on the gear, a steel part that has been carburized. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-083322 [Patent Document 2] Patent Publication No. 2021-167395 Summary of the Invention [Problem to be solved by the invention]

[0007] In Patent Document 1, the surface shape of the steel part is adjusted to reduce the friction coefficient of the surface of the steel part, and in Patent Document 2, the chemical composition of the tribofilm is adjusted to reduce the friction coefficient of the surface of the steel part. However, the friction coefficient of the surface of the steel part may also be reduced by other means.

[0008] An object of the present disclosure is to provide a steel part having a carburized hardened layer, in which the coefficient of friction of the surface of the carburized hardened layer is sufficiently suppressed. [Means for solving the problem]

[0009] The steel component according to the present disclosure comprises: A carburized hardened layer, The core part is deeper than the carburized hardened layer. Equipped with The core is The chemical composition is, in mass%, C: 0.10~0.30%, Si: 0.15 to 1.00%, Mn: 0.30 to 1.00%, P:0.030% or less, S: 0.025% or less, Cr: 0.90~2.00%, Al: 0.005 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Cu: 0.10 to 0.50%, and Ni: 0.05 to 2.00%, The balance is Fe and impurities. The carbon concentration in the surface layer of the carburized hardened layer is 0.60 to 1.00% by mass, In a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the carburized hard layer in the depth direction of the carburized hard layer, the surface CuNi concentration Ts is the sum of the Cu concentration and Ni concentration in mass % in the region from the surface to a depth of 10 nm of the carburized hard layer, and The core CuNi concentration Tc, which is the sum of the Cu concentration and Ni concentration in mass % in the core, satisfies formula (1). Surface layer CuNi concentration Ts / core CuNi concentration Tc≧0.60 (1)

[0010] The steel component according to the present disclosure comprises: A carburized hardened layer, The core part is deeper than the carburized hardened layer. Equipped with The core is The chemical composition is, in mass%, C: 0.10~0.30%, Si: 0.15 to 1.00%, Mn: 0.30 to 1.00%, P:0.030% or less, S: 0.025% or less, Cr: 0.90~2.00%, Al: 0.005 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Cu: 0.10 to 0.50%, and Ni: 0.05 to 2.00%, Further, it contains one or more selected from the group consisting of Groups 1 to 4, The balance is Fe and impurities. The carbon concentration in the surface layer of the carburized hardened layer is 0.60 to 1.00% by mass, In a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the carburized hard layer in the depth direction of the carburized hard layer, the surface CuNi concentration Ts is the sum of the Cu concentration and Ni concentration in mass % in the region from the surface to a depth of 10 nm of the carburized hard layer, and The core CuNi concentration Tc, which is the sum of the Cu concentration and Ni concentration in mass % in the core, satisfies formula (1). [Group 1] Mo: 0.60% or less, and B: 0.0050% or less, one or more selected from the group consisting of [Group 2] Ti: 0.050% or less, Nb: 0.050% or less, and V: 0.150% or less, one or more selected from the group consisting of [Group 3] Sn: 0.10% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of Surface layer CuNi concentration Ts / core CuNi concentration Tc≧0.60 (1) [Effects of the Invention]

[0011] In the steel part according to the present disclosure, the coefficient of friction of the surface of the carburized hardened layer is sufficiently suppressed. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram of a block-on-ring test according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present inventors have investigated steel parts with a reduced coefficient of friction on the surface of a carburized hardened layer that serves as a sliding surface. First, the inventors investigated a chemical composition suitable for steel parts with a carburized hardened layer. As a result, they found that the chemical composition of the core of the steel part is, in mass %, C: 0.10 to 0.30%, Si: 0.15 to 1.00%, Mn: 0.30 to 1.00%, P: 0.030% or less, S: 0.025% or less, Cr: 0.90 to 2.00%, Al: 0.005 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Cu: 0.10 to 0.50%, Ni: It was considered that a chemical composition consisting of the following: 0.05-2.00%, Mo: 0-0.60%, B: 0-0.0050%, Ti: 0-0.050%, Nb: 0-0.050%, V: 0-0.150%, Sn: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, and the balance being Fe and impurities would be appropriate for steel parts with a carburized hardened layer.

[0014] The present inventors further investigated means for reducing the friction coefficient of the surface of the carburized hardened layer of steel parts whose core chemical composition is within the above-mentioned range. Conventionally, the friction coefficient of the surface (sliding surface) of the carburized hardened layer of steel parts has been thought to be affected by the surface shape and the characteristics of the lubricating oil, as shown in Patent Documents 1 and 2. However, the present inventors believed that the chemical composition of the outermost layer of the steel part affects the friction coefficient. Therefore, they investigated the element concentrations in the outermost layer of the steel part.

[0015] As a result of investigation, the inventors found that, among the above-mentioned chemical compositions, Cu and Ni in particular contribute to reducing the coefficient of friction. Further investigation revealed that when the surface CuNi concentration Ts, which is the sum of the Cu concentration and Ni concentration in the outermost layer of the carburized hardened layer of a steel part, is lower than the core CuNi concentration Tc, which is the sum of the Cu concentration and Ni concentration in the core, the coefficient of friction is not sufficiently reduced.

[0016] Therefore, the inventors investigated and examined the relationship between the surface CuNi concentration Ts, the core CuNi concentration Tc, and the friction coefficient, and found that the friction coefficient is sufficiently reduced when the surface CuNi concentration Ts and the core CuNi concentration Tc satisfy the formula (1). Surface layer CuNi concentration Ts / core CuNi concentration Tc≧0.60 (1)

[0017] The steel part of this embodiment has been completed based on the above-mentioned technical concept, and has the following configuration.

[0018] The steel components of the first configuration are: A carburized hardened layer, a core portion located inside the carburized hardened layer; Equipped with The core portion is The chemical composition is, in mass%, C: 0.10~0.30%, Si: 0.15 to 1.00%, Mn: 0.30 to 1.00%, P:0.030% or less, S: 0.025% or less, Cr: 0.90~2.00%, Al: 0.005 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Cu: 0.10 to 0.50%, and Ni: 0.05 to 2.00%, The balance is Fe and impurities. The carbon concentration in the surface layer of the carburized hardened layer is 0.60 to 1.00% by mass, a surface CuNi concentration Ts, which is the sum of the Cu concentration and Ni concentration in mass % in a region from the surface of the carburized hard layer to a depth of 10 nm in a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the carburized hard layer in the depth direction of the carburized hard layer; The core CuNi concentration Tc, which is the sum of the Cu concentration and Ni concentration in mass % in the core, satisfies formula (1). Surface layer CuNi concentration Ts / core CuNi concentration Tc≧0.60 (1)

[0019] The steel components of the second configuration are: A carburized hardened layer, a core portion located inside the carburized hardened layer; Equipped with The core portion is The chemical composition is, in mass%, C: 0.10~0.30%, Si: 0.15 to 1.00%, Mn: 0.30 to 1.00%, P:0.030% or less, S: 0.025% or less, Cr: 0.90~2.00%, Al: 0.005 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Cu: 0.10 to 0.50%, and Ni: 0.05 to 2.00%, Further, it contains one or more selected from the group consisting of Groups 1 to 4, The balance is Fe and impurities. The carbon concentration in the surface layer of the carburized hardened layer is 0.60 to 1.00% by mass, a surface CuNi concentration Ts, which is the sum of the Cu concentration and Ni concentration in mass % in a region from the surface of the carburized hard layer to a depth of 10 nm in a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the carburized hard layer in the depth direction of the carburized hard layer; The core CuNi concentration Tc, which is the sum of the Cu concentration and Ni concentration in mass % in the core, satisfies formula (1). [Group 1] Mo: 0.60% or less, and B: 0.0050% or less, one or more selected from the group consisting of [Group 2] Ti: 0.050% or less, Nb: 0.050% or less, and V: 0.150% or less, one or more selected from the group consisting of [Group 3] Sn: 0.10% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of Surface layer CuNi concentration Ts / core CuNi concentration Tc≧0.60 (1)

[0020] The third component is steel. A steel component of a second configuration, The chemical composition includes the first group.

[0021] The fourth steel component is A steel part of the second or third configuration, The chemical composition includes the second group.

[0022] The fifth structural steel component is A steel part having any one of the second to fourth configurations, The chemical composition includes the third group.

[0023] The sixth steel component is A steel part having any one of the second to fifth configurations, The chemical composition includes the fourth group.

[0024] The steel part according to this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0025] [Steel parts configuration] The steel part of this embodiment is a steel part that has been carburized. In this specification, carburizing includes carburizing and carbonitriding. Therefore, in this specification, the term "carburized hard layer" is a concept that also includes a carbonitriding hard layer. Note that carburizing refers to gas carburizing.

[0026] The steel part comprises a carburized hardened layer and a core. The carburized hardened layer is a hardened layer formed on the surface by carburizing the raw steel material. The core is the part of the steel part that is deeper than the carburized hardened layer. The carbon concentration of the core is lower than that of the carburized hardened layer. In a steel part, the core corresponds to the part at least 5 mm deep from the surface.

[0027] It is well known to those skilled in the art that the carburized hardened layer and the core can be distinguished by performing well-known microstructural observations. The microstructure of the carburized hardened layer is primarily composed of tempered martensite. Here, "primarily" means that the area ratio of tempered martensite is 85% or more. The area ratio of tempered martensite in the microstructure of the carburized hardened layer may be 100%. When a structure other than tempered martensite is present, the remaining structure other than the tempered martensite is composed of one or more types selected from the group consisting of bainite, retained austenite, ferrite, and pearlite. The microstructure of the core portion contains tempered martensite, and may contain one or more types selected from the group consisting of bainite, retained austenite, ferrite, and pearlite as the remainder other than the tempered martensite. In the microstructure of the core portion, the area ratio of tempered martensite is larger than the total area ratio of the other structures (bainite, retained austenite, ferrite, and pearlite). The area ratio of tempered martensite may be 100%.

[0028] [Features of the steel part of the embodiment] The steel part of this embodiment includes the following features. (Feature 1) The chemical composition of the core is, in mass%, C: 0.10-0.30%, Si: 0.15-1.00%, Mn: 0.30-1.00%, P: 0.030% or less, S: 0.025% or less, Cr: 0.90-2.00%, Al: 0.005-0.100%, N: 0.0250% or less, O: 0.0050% or less, Cu: 0. 10-0.50%, Ni: 0.05-2.00%, Mo: 0-0.60%, B: 0-0.0050%, Ti: 0-0.050%, Nb: 0-0.050%, V: 0-0.150%, Sn: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, and the balance is Fe and impurities. (Feature 2) The carbon concentration in the surface layer of the carburized hardened layer is 0.60 to 1.00% by mass. (Feature 3) In the depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the carburized hard layer in the depth direction of the carburized hard layer, the surface CuNi concentration Ts, which is the sum of the Cu concentration and Ni concentration in mass% in the region from the surface to a depth of 10 nm of the carburized hard layer, and the core CuNi concentration Tc, which is the sum of the Cu concentration and Ni concentration in mass% in the core, satisfy formula (1). Surface layer CuNi concentration Ts / core CuNi concentration Tc≧0.60 (1) Features 1 to 3 will be explained below.

[0029] [(Feature 1) Chemical composition] The chemical composition of the core of the steel part of this embodiment contains the following elements.

[0030] C: 0.10 to 0.30% Carbon (C) improves the hardenability of the steel material used to make steel parts and increases the core hardness of the steel part. Therefore, C increases the surface fatigue strength of the steel part. If the C content is less than 0.10%, the above effects cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.30%, the workability of the steel material decreases. Therefore, the C content is 0.10 to 0.30%. The lower limit of the C content is preferably 0.11%, more preferably 0.12%, and even more preferably 0.13%. The upper limit of the C content is preferably 0.29%, more preferably 0.28%, and even more preferably 0.27%.

[0031] Si: 0.15 to 1.00% Silicon (Si) increases the temper softening resistance of the steel material used to make steel parts and improves the surface fatigue strength of the steel parts. If the Si content is less than 0.15%, the above effects cannot be fully achieved. On the other hand, if the Si content exceeds 1.00%, the chemical potential of C in the surface layer of the steel material in the carburizing temperature range becomes excessively high. In this case, the penetration of C into the steel material during the carburizing treatment is suppressed. As a result, the carburized hardened layer of the steel part does not form sufficiently deep. As a result, the surface fatigue strength of the steel part decreases. Therefore, the Si content is 0.15 to 1.00%. The lower limit of the Si content is preferably 0.18%, more preferably 0.20%, even more preferably 0.50%, and still more preferably 0.70%. The upper limit of the Si content is preferably 0.98%, more preferably 0.95%, and even more preferably 0.90%.

[0032] Mn: 0.30 to 1.00% Manganese (Mn) improves the hardenability of steel materials used to make steel parts and increases the core hardness of the steel parts. As a result, Mn increases the surface fatigue strength of steel parts. If the Mn content is less than 0.30%, the above effects cannot be sufficiently obtained. On the other hand, if the Mn content exceeds 1.00%, the workability of the steel material decreases. Therefore, the Mn content is 0.30 to 1.00%. The lower limit of the Mn content is preferably 0.32%, more preferably 0.35%, and even more preferably 0.50%. The upper limit of the Mn content is preferably 0.98%, more preferably 0.95%, and even more preferably 0.90%.

[0033] P:0.030% or less Phosphorus (P) is an impurity. If the P content exceeds 0.030%, it segregates at the austenite grain boundaries during the gas carburizing process used to manufacture steel parts, reducing the surface fatigue strength of the steel parts. Therefore, the P content should be 0.030% or less. The P content is preferably as low as possible. However, excessive reduction in the P content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the P content is preferably 0.029%, more preferably 0.028%, and even more preferably 0.025%.

[0034] S: 0.025% or less Sulfur (S) is an impurity. S combines with Mn to form MnS, which improves the machinability of steel materials used in steel parts. However, if the S content exceeds 0.025%, the S not fixed by Mn forms as FeS at grain boundaries. In this case, the hot workability of the steel material is reduced. Furthermore, the excessive MnS reduces the wear resistance and cold workability. Therefore, the S content should be 0.025% or less. The S content is preferably as low as possible. However, excessive reduction of the S content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The upper limit of the S content is preferably 0.020%, more preferably 0.015%, even more preferably 0.010%, and still more preferably 0.007%.

[0035] Cr: 0.90~2.00% Chromium (Cr) increases the temper softening resistance of the steel material used to make steel parts, and increases the surface fatigue strength of the steel parts. If the Cr content is less than 0.90%, the above effects cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 2.00%, the workability of the steel material decreases. Therefore, the Cr content is 0.90 to 2.00%. The lower limit of the Cr content is preferably 0.92%, more preferably 0.95%, even more preferably 1.00%, even more preferably 1.10%, and even more preferably 1.50%. The upper limit of the Cr content is preferably 1.98%, more preferably 1.95%, even more preferably 1.90%, and still more preferably 1.85%.

[0036] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. Furthermore, during heat treatment of the steel material that is the raw material for steel parts, Al combines with N in the steel to form AlN, which suppresses the coarsening of austenite grains. This increases the toughness of the steel part. If the Al content is less than 0.005%, the above effect cannot be fully achieved. On the other hand, if the Al content exceeds 0.100%, excessive oxide-based inclusions are formed, reducing the surface fatigue strength of the steel part. Therefore, the Al content is 0.005 to 0.100%. The lower limit of the Al content is preferably 0.006%, more preferably 0.008%, even more preferably 0.012%, and still more preferably 0.025%. The upper limit of the Al content is preferably 0.095%, more preferably 0.080%, even more preferably 0.070%, and still more preferably 0.060%.

[0037] N: 0.0250% or less Nitrogen (N) is an impurity. If the N content exceeds 0.0250%, coarse nitrides are formed. These coarse nitrides reduce the surface fatigue strength of steel parts. Therefore, the N content should be 0.0250% or less. The N content is preferably as low as possible. However, excessive reduction in the N content increases the production cost. Therefore, taking into consideration normal industrial production, the lower limit of the N content is preferably more than 0%, more preferably 0.0001%, and even more preferably 0.0005%. The upper limit of the N content is preferably 0.0230%, more preferably 0.0210%, even more preferably 0.0200%, even more preferably 0.0190%, and even more preferably 0.0180%.

[0038] O: 0.0050% or less Oxygen (O) is an impurity. If the O content exceeds 0.0050%, O will combine with other elements in the steel material used to make steel parts to form coarse oxide inclusions. These coarse oxide inclusions reduce the surface fatigue strength of the steel parts. Therefore, the O content must be 0.0050% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases the production cost. Therefore, in consideration of normal industrial production, the lower limit of the O content is preferably more than 0%, more preferably 0.0001%, and even more preferably 0.0005%. The upper limit of the O content is preferably 0.0040%, and more preferably 0.0030%.

[0039] Cu: 0.10 to 0.50% Copper (Cu) reduces the coefficient of friction on the surface of the carburized hardened layer of steel parts. Cu also improves the hardenability of the steel material from which steel parts are made, increasing the hardness of the core of the steel part. As a result, the bending fatigue strength of the steel part is increased. If the Cu content is less than 0.10%, the above effects cannot be fully achieved. On the other hand, if the Cu content exceeds 0.50%, the hardness of the steel material becomes excessively high, reducing the machinability of the steel material. Therefore, the Cu content is 0.10 to 0.50%. The lower limit of the Cu content is preferably 0.11%, more preferably 0.12%, and even more preferably 0.13%. The upper limit of the Cu content is preferably 0.49%, more preferably 0.48%, and even more preferably 0.47%.

[0040] Ni: 0.05 to 2.00% Nickel (Ni) reduces the coefficient of friction on the surface of the carburized hardened layer of steel parts. Ni also improves the hardenability of the steel material used to make steel parts, increasing the hardness of the core of the steel part. As a result, the bending fatigue strength of the steel part is increased. If the Ni content is less than 0.05%, the above effects cannot be fully achieved. On the other hand, if the Ni content exceeds 2.00%, the hardness of the steel material becomes excessively high, reducing the machinability of the steel material. Therefore, the Ni content is 0.05 to 2.00%. The lower limit of the Ni content is preferably 0.06%, more preferably 0.07%, even more preferably 0.08%, even more preferably 0.10%, even more preferably 0.13%, and even more preferably 0.15%. The upper limit of the Ni content is preferably 1.98%, more preferably 1.95%, even more preferably 1.90%, and still more preferably 1.85%.

[0041] The remainder of the chemical composition of the core of the steel part according to this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore or scrap or the manufacturing environment during industrial production of steel material that is the raw material for the steel part, and are acceptable within a range that does not adversely affect the steel part according to this embodiment.

[0042] [About optional elements] The chemical composition of the core of the steel part of this embodiment may further contain one or more elements selected from the group consisting of first to fourth groups, in place of a portion of Fe. [Group 1] Mo: 0.60% or less, and B: 0.0050% or less, one or more selected from the group consisting of [Group 2] Ti: 0.050% or less, Nb: 0.050% or less, and V: 0.150% or less, one or more selected from the group consisting of [Group 3] Sn: 0.10% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of These elements are optional and may not be contained. These optional elements will be described below.

[0043] [Group 1: Mo and B] The chemical composition of the core of the steel part of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo and B. These elements are optional elements, and all of them improve the hardenability of the steel material that is the raw material for the steel part.

[0044] Mo: 0.60% or less Molybdenum (Mo) is an optional element and does not necessarily need to be contained. In other words, the Mo content may be 0%. When contained, that is, when the Mo content exceeds 0%, Mo improves the hardenability of the steel material and increases the core hardness of the steel part. As a result, the bending fatigue strength of the steel part is increased. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. However, if the Mo content exceeds 0.60%, the workability of the steel material decreases. Therefore, the Mo content is 0 to 0.60%, and if Mo is contained, it is 0.60% or less. The lower limit of the Mo content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Mo content is preferably 0.58%, more preferably 0.52%, and even more preferably 0.40%.

[0045] B: 0.0050% or less Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B improves the hardenability of the steel material and increases the core hardness of the steel part. As a result, the bending fatigue strength of the steel part is increased. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.0050%, this effect becomes saturated. Therefore, the B content is 0 to 0.0050%, and if B is contained, it is 0.0050% or less. The lower limit of the B content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%. The upper limit of the B content is preferably 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0046] [Group 2: Ti, Nb and V] The chemical composition of the core of the steel part of this embodiment may further contain one or more elements selected from the group consisting of Ti, Nb, and V, in place of a portion of Fe. These elements are optional elements, and all of them form precipitates and have a pinning effect that suppresses coarsening of crystal grains in the steel material that is the raw material for the steel part during carburizing. As a result, the bending fatigue strength of the steel part is increased.

[0047] Ti: 0.050% or less Titanium (Ti) is an optional element and does not need to be contained. In other words, the Ti content may be 0%. When titanium is contained, that is, when the Ti content exceeds 0%, Ti forms Ti precipitates such as Ti carbides and Ti carbonitrides. The Ti precipitates have a pinning effect, which suppresses the coarsening of crystal grains in the steel material during carburizing. As a result, the bending fatigue strength of the steel part is increased. Even if even a small amount of Ti is contained, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.050%, the Ti precipitates become coarse. In this case, the coarsening of crystal grains during carburizing cannot be sufficiently suppressed. Therefore, the Ti content is 0 to 0.050%, and if Ti is contained, it is 0.050% or less. The lower limit of the Ti content is preferably 0.001%, more preferably 0.010%, even more preferably 0.015%, and still more preferably 0.018%. The upper limit of the Ti content is preferably 0.048%, more preferably 0.045%, even more preferably 0.043%, and still more preferably 0.041%.

[0048] Nb: 0.050% or less Niobium (Nb) is an optional element and does not need to be contained. In other words, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb forms Nb precipitates such as Nb carbides and Nb carbonitrides, and the pinning effect suppresses the coarsening of crystal grains in the steel material during carburizing. As a result, the bending fatigue strength of the steel part is improved. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.050%, the Nb precipitates become coarse. In this case, the coarsening of crystal grains during carburizing cannot be sufficiently suppressed. As a result, the fatigue strength of the steel part decreases. Therefore, the Nb content is 0 to 0.050%, and if Nb is contained, it is 0.050% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.010%, even more preferably 0.015%, and still more preferably 0.018%. The upper limit of the Nb content is preferably 0.048%, more preferably 0.045%, even more preferably 0.043%, and still more preferably 0.041%.

[0049] V:0.150% or less Vanadium (V) is an optional element and does not necessarily need to be contained. In other words, the V content may be 0%. When vanadium is contained, that is, when the V content exceeds 0%, V forms V precipitates such as V carbides and V carbonitrides, and the pinning effect suppresses the coarsening of grains in the steel during carburizing. As a result, the bending fatigue strength of the steel part is increased. Even if even a small amount of V is contained, the above effects can be obtained to some extent. However, if the V content exceeds 0.150%, the hardness of the steel becomes excessively high, which reduces the machinability of the steel. Therefore, the V content is 0 to 0.150%, and if V is contained, the V content is 0.150% or less. The lower limit of the V content is preferably 0.001%, more preferably 0.003%, even more preferably 0.005%, and still more preferably 0.010%. The upper limit of the V content is preferably 0.140%, more preferably 0.130%, even more preferably 0.120%, and still more preferably 0.100%.

[0050] [Group 3: Sn] The chemical composition of the core of the steel part of this embodiment may further contain Sn in place of a portion of Fe.

[0051] Sn: 0.10% or less Tin (Sn) is an optional element and does not need to be contained. In other words, the Sn content may be 0%. When it is contained, that is, when the Sn content is more than 0%, Sn reduces the cutting resistance of the steel material that is the raw material for steel parts and improves tool life. Even if even a small amount of Sn is contained, the above effects can be obtained to some extent. However, if the Sn content exceeds 0.10%, the hot workability of the steel material decreases. Therefore, the Sn content is 0 to 0.10%, and if Sn is contained, the Sn content is 0.10% or less. The lower limit of the Sn content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Sn content is preferably 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0052] [Group 4: Ca and Mg] The chemical composition of the steel part of this embodiment may further contain one or more elements selected from the group consisting of Ca and Mg in place of a portion of Fe. These elements are optional elements, and each of them refines and spheroidizes sulfides in the steel material that is the raw material for the steel part, thereby increasing the bending fatigue strength of the steel part.

[0053] Ca:0.0050% or less Calcium (Ca) is an optional element and does not need to be contained. In other words, the Ca content may be 0%. When contained, that is, when the Ca content is more than 0%, Ca refines the sulfides in the steel. Furthermore, Ca promotes the spheroidization of the sulfides in the steel. Therefore, the bending fatigue strength of the steel part is increased. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content exceeds 0.0050%, the formation of coarse Ca oxides in the steel material is promoted, which reduces the bending fatigue strength of the steel part. Therefore, the Ca content is 0 to 0.0050%, and if contained, it is 0.0050% or less. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and still more preferably 0.0015%. The upper limit of the Ca content is preferably 0.0048%, more preferably 0.0040%, and even more preferably 0.0030%.

[0054] Mg: 0.0050% or less Magnesium (Mg) is an optional element and does not need to be contained. In other words, the Mg content may be 0%. When magnesium is contained, that is, when the Mg content is more than 0%, Mg refines the sulfides in the steel. Furthermore, Mg promotes the spheroidization of the sulfides in the steel. This increases the bending fatigue strength of the steel part. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content exceeds 0.0050%, the formation of coarse Mg oxides in the steel material is promoted, which reduces the bending fatigue strength of the steel part. Therefore, the Mg content is 0 to 0.0050%, and if Mg is contained, it is 0.0050% or less. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and still more preferably 0.0015%. The upper limit of the Mg content is preferably 0.0048%, more preferably 0.0040%, and even more preferably 0.0030%.

[0055] [Method for measuring the chemical composition of the core of steel parts] The chemical composition of the core of the steel part of this embodiment can be measured by a known elemental analysis method in accordance with JIS G0321:2017. Specifically, chips are collected from the core of the steel part using a drill. The collected chips are dissolved in acid to obtain a solution. The solution is subjected to ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined by a known inert gas fusion-thermal conductivity method. The O content is determined by a known inert gas fusion-infrared absorption method.

[0056] [(Feature 2) C concentration in the surface of the carburized hardened layer] In the steel part of this embodiment, the carbon concentration in the region from the surface to a depth of 50 μm is 0.60 to 1.00% by mass. Hereinafter, the region from the surface to a depth of 50 μm of the carburized hardened layer is also referred to as the "surface layer." The carbon concentration in the surface layer of the carburized hardened layer is higher than the carbon concentration in the core. If the carbon concentration in the surface layer of the carburized hardened layer is 0.60% by mass or more, the hardness of the carburized hardened layer is sufficiently high. Therefore, the steel part can obtain sufficient surface fatigue strength. On the other hand, if the carbon concentration in the surface layer exceeds 1.00%, lenticular martensite, which has poor toughness, may form in the carburized hardened layer, or excessive retained austenite may remain in the carburized hardened layer. In this case, the hardness of the carburized hardened layer may decrease. Therefore, the carbon concentration in the surface layer of the carburized hardened layer is 0.60 to 1.00%.

[0057] The lower limit of the carbon concentration in the surface layer of the carburized hardened layer is preferably 0.65%, more preferably 0.70%, and even more preferably 0.75%. The upper limit of the carbon concentration in the surface layer of the carburized hardened layer is preferably 0.95%, and more preferably 0.90%.

[0058] [Method for measuring carbon concentration in the surface of carburized hardened layer] The carbon concentration in the surface layer of the carburized hardened layer can be measured by the following method. The steel part is cut perpendicular to the surface of the carburized hardened layer. The cut surface is mirror-polished. Point analysis is then performed on the mirror-polished cut surface using an electron probe micro analyzer (EPMA). Specifically, point analysis is performed at 1,000 measurement points within the surface region of the steel part, extending 50 μm from the surface, to obtain the carbon concentration (mass%) at each measurement point. The measurement points are spaced at 1 μm intervals in the depth direction and in directions parallel to the surface of the steel part. The arithmetic mean value of the obtained carbon concentrations is defined as the carbon concentration (mass%) in the surface layer of the carburized hardened layer. Note that the EPMA point analysis is performed using an acceleration voltage of 15 kV, a probe current of 30 nA, and an electron beam diameter of 1 μm.

[0059] [(Feature 3) Regarding Formula (1)] In the steel part of this embodiment, the surface layer CuNi concentration Ts (mass %) and the core CuNi concentration Tc (mass %) are defined as follows. Surface CuNi concentration Ts: The sum of the Cu and Ni concentrations in mass% in the region from the surface of the carburized hard layer to a depth of 10 nm in the depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the carburized hard layer in the depth direction of the carburized hard layer. Core CuNi concentration Tc: Sum of core Cu and Ni concentrations Here, the region from the surface of the carburized hard layer to a depth of 10 nm is also referred to as the "outermost layer." At this time, the surface CuNi concentration Ts and the core CuNi concentration Tc satisfy the formula (1). Surface layer CuNi concentration Ts / core CuNi concentration Tc≧0.60 (1) The formula (1) will be explained below.

[0060] Fn1 is defined as follows: Fn1=Surface CuNi concentration Ts / core CuNi concentration Tc Fn1 corresponds to the left side of equation (1). Fn1 is an index related to the coefficient of friction at the surface of the carburized hardened layer in a steel part that satisfies Feature 1. If Fn1 is less than 0.60, the sum of the Cu and Ni concentrations in the outermost layer of the steel part is excessively low compared to the sum of the Cu and Ni concentrations in the core. In this case, the coefficient of friction at the surface of the carburized hardened layer cannot be sufficiently suppressed. If Fn1 is 0.60 or greater, in a steel part that satisfies Feature 1, the CuNi concentration Ts in the surface layer is sufficiently high compared to the CuNi concentration Tc in the core. Therefore, the coefficient of friction at the surface of the carburized hardened layer can be sufficiently suppressed.

[0061] The lower limit of Fn1 is preferably 0.70, more preferably 0.80, and even more preferably 0.90. The upper limit of Fn1 is not particularly specified. However, if the steel part satisfies Feature 1, the upper limit of Fn1 is 1.00.

[0062] [Method for measuring surface CuNi concentration Ts] The CuNi concentration Ts in the surface layer of a steel part can be measured by the following method. Glow discharge optical emission spectrometry (GD-OES) is used to perform elemental analysis in the depth direction on the surface of the carburized hard layer of a steel part, obtaining a depth profile of elemental concentration. The GD-OES measurement conditions are as follows: For example, a Marcus-type high-frequency glow discharge optical emission spectrometry (GD-Profiler2) manufactured by Horiba, Ltd. is used for the GD-OES. The measurement mode is pulse sputtering mode, and high-purity argon (Ar) gas is used as the sputtering gas. Furthermore, a discharge range of 4 mm diameter, RF power of 30 W, measurement interval of 0.025 seconds, and Ar pressure of 650 Pa are used to obtain depth profiles of Cu and Ni concentrations in mass percent from three arbitrary measurement points on the surface of the carburized hard layer. Using the depth direction element concentration profiles obtained from the three measurement points, the arithmetic mean value of the Cu concentration from the surface of the carburized hard layer to a depth of 10 nm is calculated. Similarly, the arithmetic mean value of the Ni concentration from the surface of the carburized hard layer to a depth of 10 nm is calculated. The sum of the obtained arithmetic mean values ​​of the Cu concentration and the arithmetic mean values ​​of the Ni concentration is defined as the surface layer CuNi concentration Ts (mass%).

[0063] [Method for measuring core CuNi concentration Tc] The core CuNi concentration Tc of a steel part can be measured by the following method. The Cu content (mass%) and Ni content (mass%) obtained by the above-mentioned "Method for measuring the chemical composition of the core of a steel part" are defined as the Cu concentration (mass%) and Ni concentration (mass%) in the core. The sum of the obtained Cu concentration and Ni concentration is defined as the core CuNi concentration Tc (mass%).

[0064] [Effects of the steel part of this embodiment] The steel part of this embodiment satisfies Features 1 to 3. Therefore, the steel part of this embodiment has a carburized hardened layer on the surface of the steel part, and the friction coefficient of the surface of the carburized hardened layer is sufficiently suppressed, thereby reducing friction loss of the steel part.

[0065] [Use of the steel part of this embodiment] The steel part of this embodiment can be widely applied to mechanical parts that require a carburized hardened layer on the surface of the steel part and a reduced friction coefficient, such as gear parts used in power transmission mechanisms (power units and peripheral parts of the power unit) of automobiles.

[0066] [Method of manufacturing steel parts] An example of a method for manufacturing a steel part of this embodiment will be described. The method for manufacturing a steel part described below is one example for manufacturing a steel part of this embodiment. Therefore, a steel part satisfying Features 1 to 3 may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel part of this embodiment.

[0067] The method for manufacturing a steel part according to this embodiment includes the following steps. (Process 1) Steel material preparation process (Process 2) Standardizing process (Process 3) Rough processing process (Step 4) Carburizing process (Process 5) Finishing process Each step will be described below.

[0068] [(Process 1) Steel material preparation process] In the steel preparation step, a steel having a chemical composition that satisfies Feature 1 is prepared. The method for manufacturing the steel is not particularly limited. For example, the steel is manufactured by hot working a cast material (bloom). The hot working is, for example, hot forging or hot rolling.

[0069] [(Process 2) Standard baking process] The normalizing step is an optional step. In other words, the normalizing step does not have to be performed. For example, when cold working is performed in the rough working step, the normalizing step is performed before the rough working. The normalizing step is performed under well-known conditions. In the normalizing step, for example, the normalizing temperature is set to 880 to 950°C.

[0070] [(Process 3) Rough processing process] In the rough processing step, the steel material is roughly processed to produce an intermediate preform. In the rough processing step, for example, one or more of a hot processing step, a cold processing step, and a rough cutting processing step are carried out. Each step will be described below.

[0071] [Hot processing process] When the hot working step is carried out, hot working is carried out on the steel material. The hot working is, for example, the well-known hot forging. The heating temperature in the hot working is, for example, 1000 to 1300°C. After the hot working, the steel material is allowed to cool (air-cooled).

[0072] [Cold working process] When a cold working step is performed, the steel material of this embodiment is subjected to the above-mentioned normalizing step and then cold working. The cold working is, for example, cold forging. The conditions of the cold working are not particularly limited.

[0073] [Rough cutting process] In the rough cutting step, the hot-worked or cold-worked steel material is subjected to rough cutting as necessary to produce an intermediate preform. The conditions for rough cutting are not particularly limited.

[0074] [(Process 4) Carburizing process] In the carburizing process, the intermediate blank after the rough machining process is carburized. The carburizing process includes the well-known gas carburizing process and the well-known tempering process. The gas carburizing process includes the gas carburizing process and the quenching process. In the gas carburizing process, adjusting the C concentration in the surface layer of the carburized hardened layer of the steel part by appropriately adjusting the well-known conditions is a technical matter well-known to those skilled in the art. The well-known gas carburizing process will be described below.

[0075] [Gas carburizing treatment] The gas carburizing process includes a gas carburizing step and a quenching (quenching) step. The gas carburizing step and the quenching step will be described below.

[0076] [Gas carburizing process] The gas carburizing process includes a heating step, a carburizing step, and a diffusion step. In the heating step, the intermediate shaped material placed in the furnace is heated to a carburizing temperature of, for example, 850 to 1050°C.

[0077] In the carburizing process, the intermediate material is held at the carburizing temperature for a predetermined time in an atmosphere of a predetermined carbon potential CP1, for example, 0.60 to 1.20%, and the holding time t1 at the carburizing temperature is, for example, 60 to 600 minutes.

[0078] In the diffusion process, the material is held at the carburizing temperature for a predetermined time in an atmosphere of a predetermined carbon potential CP2. Here, the carbon potential CP2 in the diffusion process is, for example, 0.60 to 1.00%, and the holding time t2 at the carburizing temperature is, for example, 30 to 300 minutes. It is preferable that the carbon potential CP2 in the diffusion process be lower than the carbon potential CP1 in the carburizing process.

[0079] [Quenching process] The intermediate material after the gas carburizing process is subjected to the quenching process. c3 After being held at a quenching temperature equal to or higher than the quenching temperature, the intermediate preform is rapidly cooled and quenched. The holding time t3 at the quenching temperature is not particularly limited, but is, for example, 15 to 60 minutes. The quenching temperature is preferably lower than the carburizing temperature. The cooling method in the quenching process is oil cooling or water cooling. Specifically, the intermediate preform held at the quenching temperature is immersed in a cooling bath containing oil or water as a cooling medium to be rapidly cooled.

[0080] [Tempering process] The intermediate preform after gas carburizing is subjected to a known tempering process. The tempering temperature is, for example, 100 to 200° C. The holding time at the tempering temperature is, for example, 30 to 120 minutes.

[0081] Through the above steps, an intermediate preform having a carburized hardened layer is manufactured. If necessary, the intermediate preform may be subjected to well-known shot peening after the carburizing treatment. Shot peening is an optional step.

[0082] [(Process 5) Finishing process] In the finish machining process, the intermediate material after the carburizing process is subjected to finish machining to manufacture steel parts. Specifically, finish grinding that satisfies the following conditions is performed. (Condition 1) CBN is used as the grinding material for the grinding wheel. (Condition 2) The grinding wheel has a hardness of L or less as specified in JIS R 6242:2023. (Condition 3) The peripheral speed during grinding is set to 800 to 1000 m / min. Each condition will be explained below.

[0083] [Conditions 1 to 3] CBN is used as the grinding material for the grinding wheel, and the bond of the grinding wheel is set to a hardness of L or less as specified in JIS R 6242:2023. A grinding wheel satisfying these conditions is used to grind the carburized hardened layer of the intermediate forged material at a peripheral speed of 800 to 1000 m / min. This prevents a decrease in the Cu and Ni concentrations in the outermost surface layer of the steel part. As a result, the CuNi concentration Ts in the surface layer and the CuNi concentration Tc in the core satisfy formula (1).

[0084] Although the reason for the above is not clear, the following is thought to be the reason. When conditions 1 to 3 are satisfied, the temperature rise due to processing heat in the surface layer of the carburized hardened layer of the intermediate preform during grinding is suppressed. This is thought to suppress the decrease in the average Cu concentration and average Ni concentration in the outermost layer of the steel part. It is also possible that the CuNi concentration Ts in the outermost layer and the CuNi concentration Tc in the core of the steel part satisfy formula (1) through a mechanism different from this. However, it has been proven in the examples described below that the decrease in the average Cu concentration and average Ni concentration in the outermost layer of the steel part is suppressed and Fn1 becomes 0.60 or more by satisfying conditions 1 to 3.

[0085] By the above manufacturing process, the steel part of this embodiment that satisfies Features 1 to 3 can be manufactured. [Example]

[0086] The effects of the steel part of this embodiment will be explained in more detail below using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel part of this embodiment. Therefore, the steel part of this embodiment is not limited to this one example of conditions.

[0087] [Manufacturing of steel parts] Steel materials were prepared having the chemical compositions shown in Tables 1A and 1B. In the following description, Tables 1A and 1B will be collectively referred to as "Table 1."

[0088] [Table 1A]

[0089] [Table 1B]

[0090] The steel materials of test numbers 26 and 27 had chemical compositions equivalent to SCr420 specified in JIS G 4053:2023. Hot working (hot forging) was performed on the steel materials of each test number under the same conditions. The steel materials after hot working were cooled to room temperature. A normalizing process was performed on the steel materials after they had been allowed to cool. The normalizing temperature was 920°C, and the holding time at the normalizing temperature was 1 hour. A rough machining process was performed on the steel materials after the normalizing process. Specifically, the steel materials were turned on a lathe to produce intermediate blanks having a circular cross section perpendicular to the axial direction with a diameter of 35.29 mm.

[0091] The intermediate preforms of each test number were subjected to carburizing. Specifically, the test pieces were held at 930°C for 600 minutes (holding time t1) in an atmosphere with a carbon potential CP1 of 1.00% (gas carburizing process). Then, the carbon potential CP2 was set to 0.80% and the pieces were held at 930°C for 60 minutes (holding time t2) (diffusion process). The temperature was then lowered to 870°C, held at 870°C for 30 minutes (holding time t3), and then oil-quenched in 80°C oil (quenching process). The intermediate preforms after oil-quenching were then tempered. The tempering temperature was 150°C and the holding time at the tempering temperature was 60 minutes.

[0092] The surface of the tempered intermediate material was subjected to finishing (grinding). The abrasive material of the grinding wheel used in the grinding (Condition 1), the bond strength of the grinding wheel (Condition 2), and the peripheral speed (m / min) during grinding (Condition 3) were as shown in Table 2.

[0093] [Table 2]

[0094] Through the above manufacturing process, steel parts (round steel bars) with a diameter of 34.99 mm and each test number were manufactured.

[0095] The following tests were carried out on the steel parts with each test number. (Test 1) Core chemical composition measurement test (Test 2) C concentration measurement test for the surface of the carburized hardened layer (Test 3) Measurement of average Cu and Ni concentrations in the outermost layer (Test 4) Friction coefficient evaluation test Tests 1 to 4 will be explained below.

[0096] [(Test 1) Core Chemical Composition Measurement Test] The chemical composition of the core of the steel part of each test number was measured based on the method described in the above-mentioned "Method for measuring the chemical composition of the core of a steel part." As a result, the chemical composition of the core of the steel part of each test number was as shown in Table 1 (Table 1A and Table 1B). Furthermore, the sum Tc (mass%) of the Cu concentration and Ni concentration in the core of the steel part of each test number was calculated. The obtained Tc was used to derive Fn1 in the later-described "(Test 3) Measurement test of the average Cu concentration and average Ni concentration in the outermost layer."

[0097] [(Test 2) Measurement of carbon concentration in the surface of the carburized hardened layer] The carbon concentration (mass%) in the surface layer of the carburized hardened layer of the steel part with each test number was measured based on the method described in "Method for measuring carbon concentration in the surface layer of the carburized hardened layer" above. The obtained carbon concentrations (mass%) are shown in the "C concentration (mass%) in surface layer" column in Table 2.

[0098] [(Test 3) Measurement of average Cu concentration and average Ni concentration in the outermost layer] Based on the method described in the above [Method for measuring CuNi concentration Ts in surface layer], the sum Ts (mass%) of the average Cu concentration (mass%) and the average Ni concentration (mass%) in the outermost layer of the steel part for each test number was determined. Fn1 was determined based on Ts and Tc obtained in [(Test 1) Core chemical composition measurement test]. The obtained CuNi concentration Ts in the surface layer, the internal CuNi concentration Tc, and Fn1 are shown in the "Ts (mass%)," "Tc (mass%)," and "Fn1" columns in Table 2.

[0099] [(Test 4) Friction Coefficient Evaluation Test] A block-on-ring test was carried out on the steel part with each test number to determine the dynamic friction coefficient.

[0100] Fig. 1 is a schematic diagram of a block-on-ring test. Referring to Fig. 1, a block-on-ring tester 200 includes a bath 201 containing lubricating oil 202 and a ring test piece 203. The lubricating oil 202 is a lubricating oil having a kinematic viscosity of 5.8 mm at 100°C. 2A commercially available automatic transmission fluid was used. The automatic transmission fluid was Ultra ATF DW-1 (Honda Motor Co., Ltd.).

[0101] A ring test piece 203 was prepared from a steel part (round steel) having a diameter of 34.99 mm. The peripheral surface of the ring test piece 203 was left as the steel part (round steel) and was not machined (cut), so that the properties of the steel part (round steel) were maintained. In other words, the diameter D of the ring test piece 203 was 34.99 mm. The width W of the ring test piece 203 was 8.74 mm.

[0102] The material of the block test piece 300 was SAEO1, an industrial standard defined by the Society of Automotive Engineers. The size of the block test piece 300 was 15.75 mm × 10.16 mm × 6.35 mm. The 15.75 mm × 6.35 mm surface of the block test piece 300 was the surface that came into contact with the ring test piece 203. Hereinafter, this surface will be referred to as the contact surface.

[0103] As shown in Fig. 1, the lower part of the ring test piece 203 was immersed in lubricating oil 202 in a bath 201. Then, a block test piece 300 was placed above the ring test piece 203. At this time, the block test piece 300 was placed so that the contact surface of the block test piece 300 faced the circumferential surface of the ring test piece 203. Furthermore, the contact surface of the block test piece 300 was placed so that the 6.35 mm side of the block test piece 300 and the width W of the ring test piece 203 were parallel to each other.

[0104] After the above preparations were made, the following steps 1 to 3 were carried out. Step 1: The ring test piece 203 started to rotate at a sliding speed of 1.0 m / sec (550 rpm). Step 2: The block test piece 300 was pressed downward from above onto the circumferential surface of the ring test piece 203 with a load P of 200 N. Step 3: After rotating for 300 seconds, the load P was removed, and then the rotation of the ring test piece 203 was stopped.

[0105] During the steps 1 to 3, the friction force F applied to the block test piece 300 was measured with a load cell. Then, the friction coefficient μ(-) was calculated using the following formula. F=μP The arithmetic mean value of the friction coefficient μ obtained during 300 seconds of rotation, excluding the initial peak, was taken as the dynamic friction coefficient (-) for each test number. If the dynamic friction coefficient (-) was less than 0.120, it was determined that the friction coefficient was sufficiently suppressed. On the other hand, if the dynamic friction coefficient (-) was 0.120 or more, it was determined that the friction coefficient was not sufficiently suppressed. The evaluation results of the dynamic friction coefficient (-) are shown in Table 2.

[0106] [Evaluation results] Referring to Table 1A, Table 1B and Table 2, in test numbers 1 to 25, the steel parts satisfied features 1 to 3. Therefore, the dynamic friction coefficient was less than 0.120, and the friction coefficient was sufficiently suppressed.

[0107] On the other hand, in test numbers 26 and 27, the Cu content and Ni content were too low, and therefore the dynamic friction coefficient was 0.120 or more, and the friction coefficient could not be sufficiently suppressed.

[0108] In test numbers 28 and 29, the finishing process was performed, but an alumina abrasive was used as the grinding stone abrasive instead of a CBN abrasive. Therefore, Fn1 was less than 0.60. As a result, the dynamic friction coefficient was 0.120 or more, and the friction coefficient could not be sufficiently suppressed.

[0109] In test numbers 30 and 31, although a finishing process was performed, the binding strength of the grinding stone exceeded L as specified in JIS R 6242:2023. Therefore, Fn1 was less than 0.60. As a result, the dynamic friction coefficient was 0.120 or more, and the friction coefficient could not be sufficiently suppressed.

[0110] In test numbers 32 and 33, although a finishing process was performed, the peripheral speed was high. Therefore, Fn1 was less than 0.60. As a result, the dynamic friction coefficient was 0.120 or more, and the friction coefficient could not be sufficiently suppressed.

[0111] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A steel part, A carburized hardened layer, a core portion located inside the carburized hardened layer; Equipped with The core portion is The chemical composition, in mass%, is C: 0.10-0.30%, Si: 0.15-1.00%, Mn: 0.30-1.00%, P: 0.030% or less, S: 0.025% or less, Cr: 0.90-2.00%, Al: 0.005-0.100%, N: 0.0250% or less, O: 0.0050% or less, Cu: 0.10 to 0.50%, and Ni: 0.05 to 2.00%; The balance is Fe and impurities. the carbon concentration in the surface layer of the carburized hardened layer is 0.60 to 1.00% by mass, a surface CuNi concentration Ts, which is the sum of the Cu concentration and Ni concentration in mass % in a region from the surface of the carburized hard layer to a depth of 10 nm in a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the carburized hard layer in the depth direction of the carburized hard layer; The core CuNi concentration Tc, which is the sum of the Cu concentration and Ni concentration in mass% in the core, satisfies formula (1), Steel parts. Surface layer CuNi concentration Ts / core CuNi concentration Tc≧0.60 (1)

2. A steel part, A carburized hardened layer, a core portion located inside the carburized hardened layer; Equipped with The core portion is The chemical composition, in mass%, is C: 0.10-0.30%, Si: 0.15-1.00%, Mn: 0.30-1.00%, P: 0.030% or less, S: 0.025% or less, Cr: 0.90-2.00%, Al: 0.005-0.100%, N: 0.0250% or less, O: 0.0050% or less, Cu: 0.10 to 0.50%, and Ni: 0.05 to 2.00%; Further, it contains one or more selected from the group consisting of Groups 1 to 4, The balance is Fe and impurities. the carbon concentration in the surface layer of the carburized hardened layer is 0.60 to 1.00% by mass, a surface CuNi concentration Ts, which is the sum of the Cu concentration and Ni concentration in mass % in a region from the surface of the carburized hard layer to a depth of 10 nm in a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the carburized hard layer in the depth direction of the carburized hard layer; The core CuNi concentration Tc, which is the sum of the Cu concentration and Ni concentration in mass% in the core, satisfies formula (1), Steel parts. [Group 1] Mo: 0.60% or less, and B: 0.0050% or less, one or more selected from the group consisting of [Group 2] Ti: 0.050% or less, Nb: 0.050% or less, and V: 0.150% or less, and one or more selected from the group consisting of [Group 3] Sn: 0.10% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of Surface layer CuNi concentration Ts / core CuNi concentration Tc≧0.60 (1)

3. 3. The steel part according to claim 2, The chemical composition comprises a first group, Steel parts.

4. 3. The steel part according to claim 2, The chemical composition contains a second group, Steel parts.

5. 3. The steel part according to claim 2, The chemical composition contains a third group. Steel parts.

6. 3. The steel part according to claim 2, The chemical composition contains a fourth group. Steel parts.

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