Steel component

Abstract

To provide a steel component with a sufficiently reduced friction coefficient.SOLUTION: A steel component comprises a surface quench-hardened layer, with a chemical composition, by mass%, containing C: 0.30-0.70%, 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 the chemical composition of the steel component, the internal CuNi concentration Tc, defined as the sum of Cu and Ni contents, and the surface CuNi concentration Ts, defined as the sum of Cu and Ni concentrations in the region from the surface to a depth of 10 nm, in a depth direction elemental concentration profile obtained by glow discharge optical emission spectrometry from the surface of the steel component toward the depth direction of the steel component, 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 hardening process. [Background technology]

[0002] Steel parts, such as gears, are used in the power transmission mechanisms (power units and peripheral parts of the power unit) of automobiles, etc. Steel parts used in such applications require a low coefficient of friction on the sliding surfaces to reduce friction loss in the power transmission mechanism.

[0003] Japanese Patent Application Laid-Open No. 2013-083322 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2021-167395 (Patent Document 2) propose steel parts that can reduce the coefficient of friction. In the gear, which is a steel part proposed in Patent Document 1, the coefficient of friction is reduced by forming a plateau structure surface on the sliding surface of the gear. In Patent Document 2, the coefficient of friction is reduced by forming a tribofilm with a specific chemical composition on a gear, which is a carburized steel part. [Prior art documents] [Patent documents]

[0004] [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]

[0005] 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.

[0006] An object of the present disclosure is to provide a steel part with a well-controlled coefficient of friction. [Means for solving the problem]

[0007] The steel component according to the present disclosure comprises: At least the surface has a quench-hardened layer, The chemical composition is, in mass%, C: 0.30~0.70%, 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. an internal CuNi concentration Tc, which is the sum of the Cu content and Ni content in mass% in the chemical composition of the steel part; In a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of a steel part in the depth direction of the steel part, the surface layer CuNi concentration Ts, which is the sum of the Cu concentration and Ni concentration in mass %, in a region from the surface of the steel part to a depth of 10 nm, satisfies formula (1). Surface layer CuNi concentration Ts / internal CuNi concentration Tc≧0.60 (1)

[0008] The steel component according to the present disclosure comprises: At least the surface has a quench-hardened layer, The chemical composition is, in mass%, C: 0.30~0.70%, 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. an internal CuNi concentration Tc, which is the sum of the Cu content and Ni content in mass% in the chemical composition of the steel part; In a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of a steel part in the depth direction of the steel part, the surface layer CuNi concentration Ts, which is the sum of the Cu concentration and Ni concentration in mass %, in a region from the surface of the steel part to a depth of 10 nm, 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 / internal CuNi concentration Tc≧0.60 (1) [Effects of the Invention]

[0009] In the steel parts according to the invention, the coefficient of friction is significantly reduced. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram of a block-on-ring test according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] The inventors first investigated steel parts applicable to power transmission mechanisms, and found that the chemical composition, in mass %, was: C: 0.30-0.70%, 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. We believe that steel parts consisting of the following components can be used in power transmission mechanisms: 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 remainder being Fe and impurities.

[0012] The present inventors further investigated means for reducing the friction coefficient of the surface of a steel part having the above-mentioned chemical composition. Conventionally, the friction coefficient of the surface (sliding surface) of a steel part has been thought to be affected by the surface shape and the properties of the lubricating oil, as shown in Patent Documents 1 and 2. However, the present inventors believed that the chemical composition of the outermost surface layer of a steel part affects the friction coefficient. Therefore, they investigated the element concentrations in the outermost surface layer of a steel part.

[0013] As a result of investigation, the inventors found that, among the above-mentioned chemical compositions, Cu and Ni in particular contribute to a reduction in the friction coefficient. 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 a steel part, is lower than the internal CuNi concentration Tc, which is the sum of the Cu concentration and Ni concentration in the interior of the steel part, the friction coefficient is not sufficiently reduced. Therefore, the inventors investigated and examined the relationship between the surface layer CuNi concentration Ts, the internal CuNi concentration Tc, and the friction coefficient, and found that the friction coefficient is sufficiently reduced when the surface layer CuNi concentration Ts and the internal CuNi concentration Tc satisfy the formula (1). Surface layer CuNi concentration Ts / internal CuNi concentration Tc≧0.60 (1)

[0014] The steel part of this embodiment has been completed based on the above-mentioned technical concept, and has the following configuration.

[0015] The steel components of the first configuration are: At least the surface has a quench-hardened layer, The chemical composition is, in mass%, C: 0.30~0.70%, 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. an internal CuNi concentration Tc, which is the sum of the Cu content and Ni content in mass% in the chemical composition of the steel part; In a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the steel part in the depth direction of the steel part, the surface layer CuNi concentration Ts, which is the sum of the Cu concentration and Ni concentration in mass %, in a region from the surface of the steel part to a depth of 10 nm, satisfies formula (1). Surface layer CuNi concentration Ts / internal CuNi concentration Tc≧0.60 (1)

[0016] The steel components of the second configuration are: At least the surface has a quench-hardened layer, The chemical composition is, in mass%, C: 0.30~0.70%, 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. an internal CuNi concentration Tc, which is the sum of the Cu content and Ni content in mass% in the chemical composition of the steel part; In a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the steel part in the depth direction of the steel part, the surface layer CuNi concentration Ts, which is the sum of the Cu concentration and Ni concentration in mass %, in a region from the surface of the steel part to a depth of 10 nm, 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 / internal CuNi concentration Tc≧0.60 (1)

[0017] The third component is steel. A steel component of a second configuration, The chemical composition includes the first group.

[0018] The fourth steel component is A steel part of the second or third configuration, The chemical composition includes the second group.

[0019] 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.

[0020] The sixth steel component is A steel part having any one of the second to fifth configurations, The chemical composition includes the fourth group.

[0021] The steel part according to this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0022] [Steel parts configuration] The steel part of this embodiment is a steel part that has been hardened. In this specification, the term "hardening" includes not only hardening in which the steel part is inserted into a heat treatment furnace, heated, and then rapidly cooled, but also high-frequency hardening in which the surface layer of the steel part is heated using a high-frequency induction heating device and then rapidly cooled. The steel part of this embodiment includes a quench-hardened layer at least on the surface. The microstructure of the quench-hardened layer is mainly composed of tempered martensite. Here, "mainly" means that the area ratio of tempered martensite is 80% or more. It is common technical knowledge well known to those skilled in the art that whether or not a part is a quench-hardened layer can be determined by performing well-known microstructure observation. The steel part of this embodiment may include a quench-hardened layer on the surface, or the entire steel part may have a structure consisting of a quench-hardened layer (i.e., a structure consisting of tempered martensite). In other words, the steel part of this embodiment includes a quench-hardened layer at least on the surface.

[0023] [Features of the steel part of the embodiment] The steel part of this embodiment includes the following features. (Feature 1) The chemical composition, in mass%, is C: 0.30-0.70%, 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: 0.1 0-0.50%, 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) An internal CuNi concentration Tc, which is the sum of the Cu content and Ni content in mass % in the chemical composition of a steel part, and 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 a steel part 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 steel part in the depth direction of the steel part, satisfy formula (1). Surface layer CuNi concentration Ts / internal CuNi concentration Tc≧0.60 (1) Feature 1 and Feature 2 will be explained below.

[0024] [(Feature 1) Chemical composition] The chemical composition of the steel part of this embodiment contains the following elements.

[0025] C: 0.30 to 0.70% Carbon (C) improves the hardenability of the steel material used to make steel parts, and increases the hardness of the steel parts. This reduces the friction coefficient of the steel parts. If the C content is less than 0.30%, the above effects cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.70%, the workability of the steel material decreases. Therefore, the C content is 0.30 to 0.70%. The lower limit of the C content is preferably 0.31%, more preferably 0.32%, and even more preferably 0.33%. The upper limit of the C content is preferably 0.69%, more preferably 0.68%, and even more preferably 0.67%.

[0026] Si: 0.15 to 1.00% Silicon (Si) increases the temper softening resistance of steel materials used to make steel parts, and increases the fatigue strength of the steel parts. If the Si content is less than 0.15%, the above effects cannot be sufficiently obtained. On the other hand, if the Si content exceeds 1.00%, the workability of the steel materials used to make steel parts 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%.

[0027] Mn: 0.30 to 1.00% Manganese (Mn) improves the hardenability of the steel material used to make steel parts, thereby increasing the hardness of the 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 even if the contents of other elements are within the ranges of this embodiment. 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%.

[0028] 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 hardening in the production of steel parts, reducing the 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%.

[0029] S: 0.025% or less Sulfur (S) is an impurity. S combines with Mn to form MnS, which improves the machinability of steel used in steel parts. However, if the S content exceeds 0.025%, the S not fixed by Mn forms FeS at grain boundaries. In this case, the hot workability of the steel deteriorates. Furthermore, the excessive MnS reduces the wear resistance and cold workability of the steel. 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%.

[0030] Cr: 0.90~2.00% Chromium (Cr) improves the hardenability of steel materials used to make steel parts, thereby increasing the hardness 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%.

[0031] 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 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%.

[0032] 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 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%.

[0033] 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 fatigue strength of 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%.

[0034] Cu: 0.10 to 0.50% Copper (Cu) reduces the coefficient of friction on the surface of steel parts. Cu also improves the hardenability of the steel material from which the steel parts are made, increasing the hardness of the steel parts. As a result, the fatigue strength of the steel parts is increased. If the Cu content is less than 0.10%, the above effects cannot be sufficiently obtained. 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%.

[0035] Ni: 0.05 to 2.00% Nickel (Ni) reduces the coefficient of friction on the surface of steel parts. Ni also improves the hardenability of the steel material from which steel parts are made, increasing the hardness of the steel parts. As a result, the fatigue strength of the steel parts is increased. If the Ni content is less than 0.05%, the above effects cannot be sufficiently obtained. On the other hand, if the Ni content exceeds 2.00%, the hardness of the steel material becomes excessively high, and the machinability of the steel material decreases. 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 0.18%. 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%.

[0036] The balance of the chemical composition of the steel part according to the present embodiment is 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 the steel part, and are acceptable to the extent that they do not adversely affect the steel part according to the present embodiment.

[0037] [About optional elements] The chemical composition 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.

[0038] [Group 1: Mo and B] The chemical composition 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.

[0039] 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 hardness of the steel part. As a result, the 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%.

[0040] B: 0.0050% or less Boron (B) is an optional element and does not 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 hardness of the steel part. As a result, the 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%.

[0041] [Group 2: Ti, Nb and V] The chemical composition 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 Ti, Nb, and V. 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 quenching. As a result, the bending fatigue strength of the steel part is increased.

[0042] Ti: 0.050% or less Titanium (Ti) is an optional element and does not necessarily 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 inhibits the coarsening of crystal grains in the steel material during quenching. As a result, the 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 quenching cannot be sufficiently suppressed. As a result, the fatigue strength of the steel part decreases. 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%.

[0043] 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 quenching. As a result, the 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 quenching 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%.

[0044] 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 quenching. As a result, the fatigue strength of the steel part is improved. 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%.

[0045] [Group 3: Sn] The chemical composition of the steel part of this embodiment may further contain Sn in place of a portion of Fe.

[0046] 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%.

[0047] [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 all of them refine and spheroidize sulfides in the steel material that is the raw material for the steel part, thereby increasing the fatigue strength of the steel part.

[0048] 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 material. Furthermore, Ca promotes the spheroidization of the sulfides in the steel material. Therefore, the 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 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%.

[0049] 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 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 oxides in the steel material is promoted, which reduces the 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%.

[0050] [Method for measuring the chemical composition of steel parts] The chemical composition 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 interior of the steel part to a depth of 1 mm or more from the surface 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 using a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a known inert gas fusion-thermal conductivity method. The O content is determined using a known inert gas fusion-infrared absorption method.

[0051] [About the quenched hardened layer] As described above, the steel part of this embodiment includes a quench-hardened layer at least in the surface layer. In the quench-hardened layer, the area ratio of the hard structure made of tempered martensite is 80% or more. In the quench-hardened layer, the area ratio of the hard structure made of tempered martensite may be 100%. Here, the region from the surface of the steel part to a depth of 0.2 mm is defined as the "surface layer" of the steel part. Note that when the area ratio of the hard structure in the surface layer of the steel part is 80% or more, the structure other than the hard structure in the microstructure of the surface layer is one or more types selected from the group consisting of bainite, retained austenite, ferrite, and pearlite.

[0052] [Method for measuring the area ratio of hard tissue in the surface layer] The area ratio of the hard structure in the surface layer of a steel part can be determined by the following method. A sample is taken that includes the center position in the depth direction of the surface layer of the steel part. The surface of the sample (referred to as the observation surface) that includes the center position is designated as the observation surface. The observation surface is polished to a mirror finish and then immersed in a nital etching solution to reveal the structure by etching. The etched observation surface is observed under an optical microscope at 1000x magnification, and photographs are generated in three arbitrary fields of view corresponding to the center position. Each field of view has an area of ​​130 μm × 260 μm. In each field of view, the structures of tempered martensite, retained austenite, ferrite, and pearlite can be distinguished by their contrast. Specifically, when the observation surface is etched with a nital etching solution, a phase with a lamellar structure can be identified as pearlite. Furthermore, areas brighter than pearlite (white areas) can be identified as ferrite. Furthermore, areas darker than ferrite and pearlite (dark areas) can be identified as tempered martensite or retained austenite. Furthermore, in the areas identified as tempered martensite or retained austenite, areas with high brightness (white areas) can be identified as retained austenite. The area of ​​tempered martensite in each field of view is calculated. The area ratio (%) of the hard structure in the surface layer is defined based on the total area of ​​tempered martensite in the three fields of view and the total area of ​​each field of view.

[0053] [(Feature 2) Regarding Formula (1)] In the steel part of this embodiment, the internal CuNi concentration Tc (mass %) and the surface layer CuNi concentration Ts (mass %) are defined as follows. Internal CuNi concentration Tc: The sum of the Cu content and Ni content in mass% in the chemical composition of the above steel part Surface CuNi concentration Ts: The sum of the Cu concentration and Ni concentration in mass% in the region from the surface of the steel part 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 steel part in the depth direction of the steel part At this time, the internal CuNi concentration Tc and the surface layer CuNi concentration Ts satisfy the formula (1). Hereinafter, the region from the surface of the quench-hardened layer to a depth of 10 nm is also referred to as the "outermost layer." Surface layer CuNi concentration Ts / internal CuNi concentration Tc≧0.60 (1)

[0054] Fn1 is defined as follows: Fn1=surface CuNi concentration Ts / internal CuNi concentration Tc Fn1 corresponds to the left side of equation (1). Fn1 is an index related to the coefficient of friction on the surface of a steel part that satisfies Feature 1. If Fn1 is less than 0.60, the sum of the Cu concentration and Ni concentration in the outermost layer of the steel part is insufficient compared to the sum of the Cu concentration and Ni concentration inside the steel part. In this case, the coefficient of friction on the surface of the steel part cannot be sufficiently suppressed. If Fn1 is 0.60 or more, in a steel part that satisfies Feature 1, the surface CuNi concentration Ts is sufficiently high relative to the internal CuNi concentration Tc. Therefore, the coefficient of friction on the surface of the steel part can be sufficiently suppressed.

[0055] 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.

[0056] [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 from the surface 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 GD-OES. In this case, 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 mmφ, RF output 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 % from the surface of the steel part to the depth direction. The obtained depth profile of elemental concentration is used to calculate the arithmetic mean value of Cu concentration from the surface of the steel part to a depth of 10 nm. Similarly, the arithmetic mean value of Ni concentration from the surface of the steel part to a depth of 10 nm is calculated. The sum of the obtained arithmetic mean values ​​of the Cu concentration and the Ni concentration is defined as the surface layer CuNi concentration Ts (mass %).

[0057] [Method for measuring internal CuNi concentration Tc] The internal 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 a steel part" are defined as the Cu concentration (mass%) and Ni concentration (mass%) inside the steel part. The sum of the obtained Cu concentration and Ni concentration is defined as the internal CuNi concentration Tc (mass%).

[0058] [Effects of the steel part of this embodiment] The steel part of this embodiment satisfies Features 1 and 2. Therefore, the steel part of this embodiment has a sufficiently suppressed coefficient of friction.

[0059] [Use of the steel part of this embodiment] The steel part of this embodiment can be widely applied to mechanical parts that require a reduced coefficient of friction, such as gear parts used in power transmission mechanisms (power units and peripheral parts of power units) of automobiles.

[0060] [Method of manufacturing steel parts] An example of a manufacturing method for a steel part of this embodiment will be described below. The manufacturing method for a steel part described below is one example for manufacturing a steel part of this embodiment. Therefore, a steel part satisfying Features 1 and 2 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 manufacturing method for a steel part of this embodiment.

[0061] 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) Quenching process (Process 5) Finishing process Each step will be described below.

[0062] [(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.

[0063] [(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. 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.

[0064] [(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 step are performed. Each step will be described below.

[0065] [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).

[0066] [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.

[0067] [Rough cutting process] In the rough cutting step, the hot-worked or cold-worked steel material is subjected to rough cutting to produce an intermediate preform. The conditions for rough cutting are not particularly limited.

[0068] [(Process 4) Quenching process] In the quenching process, the intermediate blank after the rough machining process is quenched and tempered. The quenching may be performed using a heat treatment furnace or may be high-frequency quenching using a high-frequency induction heating device. Furthermore, the quenching may be vacuum quenching using a vacuum furnace. The quenching temperature is, for example, 850 to 1050°C.

[0069] The intermediate preform after quenching 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.

[0070] Through the above steps, an intermediate blank is manufactured.

[0071] If necessary, the intermediate material may be subjected to a well-known shot peening process after the quenching process.

[0072] [(Process 5) Finishing process] In the finishing process, the intermediate blank after the quenching process is subjected to finishing to manufacture a steel part. Specifically, a finish grinding process that satisfies the following conditions is carried out. (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.

[0073] [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. The surface layer of the intermediate preform is ground using a grinding wheel that satisfies these conditions at a peripheral speed of 800 to 1000 m / min. This prevents a decrease in the average Cu concentration and average Ni concentration in the outermost surface layer of the steel part. As a result, the surface CuNi concentration Ts and the internal CuNi concentration Tc satisfy formula (1).

[0074] Although the reason for the above is unclear, 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 quench-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 decrease in the average Cu concentration and average Ni concentration in the outermost layer of the steel part can be suppressed by 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 can be suppressed and Fn1 becomes 0.60 or more by satisfying conditions 1 to 3.

[0075] By the above manufacturing process, the steel part of this embodiment that satisfies Features 1 and 2 can be manufactured. [Example]

[0076] 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.

[0077] [Manufacturing of steel parts] Steel materials having the chemical compositions shown in Tables 1A and 1B were prepared. In the following explanation, Tables 1A and 1B will be collectively referred to as "Table 1." In Table 1, "-" indicates that the content of the corresponding element is below the impurity level.

[0078] [Table 1A]

[0079] [Table 1B]

[0080] A normalizing process was carried out on the steel material of each test number. The normalizing temperature was 920°C, and the holding time at the normalizing temperature was 1 hour. The steel material after the normalizing process was subjected to a rough machining process. Specifically, the steel material was turned on a lathe to produce an intermediate blank having a circular cross section perpendicular to the axial direction with a diameter of 35.29 mm.

[0081] The intermediate preforms of each test number were subjected to a quenching process. Specifically, the intermediate preforms were subjected to vacuum quenching. The vacuum quenching was performed by holding the preform at 920°C for 45 minutes. The preforms were then oil-cooled in 80°C oil (quenching). After oil cooling, the intermediate preforms were tempered. The tempering temperature was 150°C, and the holding time at the tempering temperature was 60 minutes.

[0082] 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.

[0083] [Table 2]

[0084] Using the above manufacturing process, steel parts (round steel bars) with a diameter of 34.99 mm were manufactured for each test number. The area ratio of the hard structure in the surface layer of the steel parts with each test number was measured based on the [Method for measuring the area ratio of hard structure in the surface layer]. As a result, for all test numbers, the area ratio of the hard structure in the surface layer was 80% or more, and at least the surface layer was a quench-hardened layer.

[0085] The following tests were carried out on the steel parts with each test number. (Test 1) Chemical composition measurement test for steel parts (Test 2) Measurement of average Cu and Ni concentrations in the outermost layer (Test 3) Friction coefficient evaluation test Tests 1 to 3 will be explained below.

[0086] [(Test 1) Chemical composition measurement test for steel parts] The chemical composition of the steel parts of each test number was measured based on the method described above in [Method for measuring the chemical composition of steel parts]. As a result, the chemical compositions of the steel parts of each test number were as shown in Tables 1A and 1B.

[0087] [(Test 2) Measurement of average Cu and Ni concentrations in the outermost layer] Based on the method described in the above-mentioned [Method for measuring surface layer CuNi concentration Ts], the surface layer CuNi concentration Ts (mass%), which is the sum of the average Cu concentration (mass%) and average Ni concentration (mass%) in the outermost surface layer of the steel part for each test number, was determined. Fn1 was determined based on the surface layer CuNi concentration Ts and the internal CuNi concentration Tc (mass%) obtained in [(Test 1) Steel part chemical composition measurement test]. The obtained surface layer CuNi concentration Ts, internal CuNi concentration Tc, and Fn1 are shown in the "Ts (mass%)," "Tc (mass%)," and "Fn1" columns in Table 2.

[0088] [(Test 3) 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.

[0089] 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. 2 A commercially available automatic transmission fluid of 1 / s was used. As the automatic transmission fluid, for example, Ultra ATF DW-1 (Honda Motor Co., Ltd.) can be used.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] [Evaluation results] Referring to Table 1A, Table 1B and Table 2, Test Nos. 1 to 25 satisfied Features 1 and 2. Therefore, the dynamic friction coefficient was less than 0.120, and the friction coefficient was sufficiently suppressed.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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, At least the surface has a quench-hardened layer, The chemical composition, in mass%, is C: 0.30-0.70%, 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. an internal CuNi concentration Tc, which is the sum of the Cu content and the Ni content in mass% in the chemical composition of the steel part; In a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the steel part in the depth direction of the steel part, a surface layer CuNi concentration Ts, which is the sum of the Cu concentration and the Ni concentration in mass% in a region from the surface of the steel part to a depth of 10 nm, satisfies formula (1). Steel parts. Surface layer CuNi concentration Ts / internal CuNi concentration Tc≧0.60 (1)

2. A steel part, At least the surface has a quench-hardened layer, The chemical composition, in mass%, is C: 0.30-0.70%, 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. an internal CuNi concentration Tc, which is the sum of the Cu content and the Ni content in mass% in the chemical composition of the steel part; In a depth direction element concentration profile obtained by performing glow discharge optical emission spectroscopy from the surface of the steel part in the depth direction of the steel part, a surface layer CuNi concentration Ts, which is the sum of the Cu concentration and the Ni concentration in mass% in a region from the surface of the steel part to a depth of 10 nm, 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 [Second group] 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 / internal 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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