Steel pipes, vehicle parts, method for manufacturing steel pipes, and method for manufacturing vehicle parts

Abstract

The present invention provides: a component for vehicles having excellent fatigue strength; a steel pipe which enables the production of a component for vehicles having excellent fatigue strength; a method for producing this steel pipe; and a method for producing this component for vehicles. A steel pipe according to the present disclosure comprises a base material and an oxide coating film on the base material. The base material has a chemical structure that contains, in mass%, 0.23% to 0.50% of C, 0.01% to 0.50% of Si, 0.50% to 2.50% of Mn, 0.050% or less of P, 0.0100% or less of S, 0.0100% or less of N and 0.0100% or less of O, with the balance being made up of Fe and impurities; and the base material has a microstructure that is composed, in area ratios, of 20% to 60% of ferrite and 40% to 80% of pearlite. The oxide coating film is composed, in terms of the X-ray diffraction peak intensity ratios, of 70% or more of Fe3O4, 20% or more of Fe2O3 and 10% or less of FeO, with the balance being made up of impurities, if the sum of the X-ray diffraction peak intensities of Fe3O4, Fe2O3 and FeO is taken as 100%; the oxide coating film has a thickness of 0.80 µm to 2.50 µm; and the standard deviation of the thickness is 0.90 µm or less.

Description

Technical Field

[0001] The present disclosure relates to steel pipes, vehicle parts, a method for manufacturing steel pipes, and a method for manufacturing vehicle parts.

Background Art

[0002] Vehicles such as automobiles are equipped with vehicle parts. Vehicle parts are, for example, stabilizers, inner tie rods, drive shafts, and upper arms. Repeated stress is applied to vehicle parts due to vibrations generated during vehicle travel. Therefore, excellent fatigue strength is required for vehicle parts.

[0003] In recent years, hollow vehicle parts have been used for the purpose of reducing the weight of vehicle bodies. For example, stabilizers include solid stabilizers manufactured from round bars or the like, and hollow stabilizers manufactured from steel pipes or the like. In recent years, the use of hollow stabilizers has been increasing.

[0004] Techniques for increasing the fatigue strength of vehicle parts typified by hollow stabilizers are disclosed in International Publication No. 2020 / 230795 (Patent Document 1) and International Publication No. 2013 / 175821 (Patent Document 2).

[0005] The electric resistance welded steel pipe for a hollow stabilizer disclosed in Patent Document 1 has a component composition consisting of, in mass%, C: 0.20 to 0.40%, Si: 0.1 to 1.0%, Mn: 0.1 to 2.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.10%, Cr: 0.01 to 0.50%, Ti: 0.010 to 0.050%, B: 0.0005 to 0.0050%, Ca: 0.0001 to 0.0050%, N: 0.0050% or less, and Sn: 0.010 to 0.050%, with the balance being Fe and unavoidable impurities, and the total decarburized layer depth on the inner and outer surfaces is 100 μm or less.

[0006] In Patent Document 1, Sn is contained in the electric resistance welded steel pipe for a hollow stabilizer at 0.010% or more. Thereby, the formation of the decarburized layer is suppressed and the fatigue strength is increased.

[0007] The hollow stabilizer disclosed in Patent Document 2 has a chemical composition consisting of, by mass%, C: 0.26-0.30%, Si: 0.05-0.35%, Mn: 0.5-1.0%, Cr: 0.05-1.0%, Ti: 0.005-0.05%, B: 0.0005-0.005%, Ca: 0.0005-0.005%, Al: 0.08% or less, P: 0.05% or less, S: less than 0.0030%, N: 0.006% or less, O: 0.004% or less, with the remainder being Fe and unavoidable impurities, the product of the Mn content and S content being 0.0025 or less, and a component composition such that the critical cooling rate Vc90 represented by (Equation 1) is 40°C / s or less. The metal structure of the hollow stabilizer consists of tempered martensite. The length of the stretched MnS present in the center of the wall thickness of the hollow stabilizer is 150 μm or less. The Rockwell C scale hardness (HRC) of the hollow stabilizer is 40-50, the wall thickness / outer diameter ratio is 0.14 or more, and the depth of the decarburized layer on the inner surface is 20 μm or less from the inner surface. logVc90=2.94-0.75β···(Formula 1) However, β = 2.7C + 0.4Si + Mn + 0.8Cr.

[0008] Patent Document 2 describes how to suppress the formation of stretched MnS and control the Rockwell C-scale hardness, wall thickness / outer diameter ratio, and the depth of the decarburized layer on the inner surface. This increases the fatigue strength of the hollow stabilizer. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] International Publication No. 2020 / 230795 [Patent Document 2] International Publication No. 2013 / 175821 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] The technologies disclosed in Patent Documents 1 and 2 can increase the fatigue strength of vehicle components. However, vehicle components with superior fatigue strength may be obtained by means other than those disclosed in Patent Documents 1 and 2.

[0011] The purpose of this disclosure is to provide vehicle parts with excellent fatigue strength, steel pipes capable of manufacturing vehicle parts with excellent fatigue strength, a method for manufacturing such steel pipes, and a method for manufacturing vehicle parts. [Means for solving the problem]

[0012] The steel pipes of this disclosure are, by mass%, C: 0.23~0.50%, Si: 0.01~0.50%, Mn: 0.50~2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol.Al: 0~0.080%, Cr: 0~1.50%, Mo: 0~1.00%, Ni: 0~1.00%, Cu: 0~1.00%, Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.100%, B: 0~0.0050%, Ca: 0 ~ 0.0050%, and, The remainder has a chemical composition consisting of Fe and impurities, A base material having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite in terms of area ratio, On the aforementioned base material, When the sum of the peak intensities of X-ray diffraction for Fe3O4, Fe2O3, and FeO is taken as 100%, the peak intensity ratio of the X-ray diffraction consists of 70% or more Fe3O4, 20% or more Fe2O3, 10% or less FeO, and the remainder being impurities. It is provided with an oxide film having a thickness of 0.80 to 2.50 μm and a standard deviation of the thickness of 0.90 μm or less.

[0013] The vehicle part of the present disclosure contains, by mass%, C: 0.23 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol.Al: 0 to 0.080%, Cr: 0 to 1.50%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.100%, B: 0 to 0.0050%, Ca: 0 to 0.0050%, and the balance consists of Fe and impurities, and has a microstructure composed of tempered martensite, a hollow base material having a Vickers hardness of 400 to 550 HV in accordance with JIS Z 2244:2020, on the base material, when the total peak intensity of X-ray diffraction of Fe3O4, FeO, and Fe2O3 is 100%, it consists of Fe3O4 with a peak intensity ratio of 80% or more, FeO with 15% or less, Fe2O3 with 5% or less, and the balance consists of impurities in the X-ray diffraction peak intensity ratio, and is provided with an oxide film having a thickness of 3.50 μm or less.

[0014] The manufacturing method of the steel pipe of the present disclosure contains, by mass%, C: 0.23 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol.Al: 0~0.080%, Cr: 0~1.50%, Mo: 0~1.00%, Ni: 0~1.00%, Cu: 0~1.00%, Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.100%, B: 0~0.0050%, Ca: 0 ~ 0.0050%, and, The remainder has a chemical composition consisting of Fe and impurities, A process for preparing a steel sheet having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite by area ratio, The process involves heat-treating the steel plate at 450-600°C for 0.5-3.0 minutes, The process includes a step of manufacturing a steel pipe by electric resistance welding the heat-treated steel plate.

[0015] The method for manufacturing vehicle parts described herein is: The process of preparing the steel pipes as described above, A process of bending the steel pipe, The process involves holding the steel pipe after bending at an Ac3+50°C to 1150°C for 10 seconds or more, and then rapidly cooling it. The process includes a step of tempering the steel pipe after rapid cooling by holding it at 150-350°C for 10 minutes or more. [Effects of the Invention]

[0016] The vehicle components of this disclosure have excellent fatigue strength. The steel pipes of this disclosure can be used to manufacture vehicle components with excellent fatigue strength. The manufacturing method for vehicle components of this disclosure can be used to manufacture vehicle components with excellent fatigue strength. The manufacturing method for steel pipes of this disclosure can be used to manufacture steel pipes capable of producing vehicle components with excellent fatigue strength. [Brief explanation of the drawing]

[0017] [Figure 1] Figure 1 is a perspective view of the end of the steel pipe in this embodiment. [Figure 2] Figure 2 is a perspective view of the end portion of the vehicle component according to this embodiment. [Figure 3] Figure 3 is a front view of a torsional fatigue test specimen. [Figure 4] Figure 4 is a side view of a torsional fatigue test specimen, viewed from the longitudinal direction. [Modes for carrying out the invention]

[0018] The inventors of this invention conducted research on vehicle parts with excellent fatigue strength and steel pipes capable of manufacturing vehicle parts with excellent fatigue strength. As a result, they obtained the following findings.

[0019] Hollow vehicle parts are manufactured, for example, by cold bending steel pipes and then quenching and tempering them.

[0020] Patent Document 1 states, "Surface decarburization, in particular, is considered an important factor in surface properties. If surface decarburization occurs during the quenching heating stage, the surface hardness cannot be improved even if quenching is performed, and as a result, sufficient fatigue properties cannot be obtained" (Paragraph

[0005] of Patent Document 1). Patent Document 2 states, "In hollow stabilizers, fatigue failure may occur from the inner surface, which is not present in solid stabilizers. This is because even if the fatigue strength of the outer surface is improved by increasing the strength of the steel pipe, the decarburized layer on the inner surface becomes the starting point for fatigue failure" (Paragraph

[0005] of Patent Document 2). Thus, it is known that the formation of a decarburized layer reduces the fatigue strength of vehicle parts, including hollow stabilizers.

[0021] Patent Document 1 states that "the surface decarburization reaction when steel is heated proceeds as carbon atoms in the steel diffuse outward toward the surface and react with oxygen. Increasing the lattice constant of iron is effective in suppressing this outward diffusion of carbon" (paragraph

[0017] of Patent Document 1). Therefore, Patent Document 1 suppresses the formation of a decarburized layer by including 0.010% or more of Sn, which is effective in increasing the lattice constant of iron.

[0022] Patent Document 2 states that "when the metal structure is cooled from a high temperature where it is a single austenite phase and passes through a two-phase temperature range, a decarburized layer is likely to form on the inner surface of the steel pipe for the hollow stabilizer" (Patent Document 2, paragraph

[0053] ). Patent Document 2 also states that the formation of a decarburized layer can be suppressed by increasing the cooling rate when passing through the two-phase temperature range (Patent Document 2, paragraph

[0054] ).

[0023] The present inventors investigated means other than those disclosed in Patent Documents 1 and 2 for suppressing the formation of a decarburized layer.

[0024] The decarburized layer is formed during the quenching process when manufacturing vehicle parts. During heating and / or cooling in the quenching process, carbon in the steel diffuses outward to the surface and reacts with oxygen. This forms the decarburized layer. The inventors focused on the behavior of oxygen during quenching. They hypothesized that if contact between the surface of the steel pipe used to manufacture vehicle parts and oxygen is suppressed during quenching, the formation of the decarburized layer can be suppressed.

[0025] Therefore, the inventors investigated means to suppress contact between the steel pipe surface and oxygen during quenching. The inventors focused on the oxide film before quenching. An oxide film that suppresses contact between the steel pipe surface and oxygen is formed before quenching. It is believed that this suppresses contact between the steel pipe surface and oxygen during quenching, thereby suppressing the formation of a decarburized layer.

[0026] As a result of diligent research by the inventors, it was found that if an oxide film is formed in which, when the sum of the X-ray diffraction peak intensities of Fe3O4, Fe2O3, and FeO is taken as 100%, Fe3O4 accounts for 70% or more of the X-ray diffraction peak intensity ratio, Fe2O3 accounts for 20% or more, FeO accounts for 10% or less, and the remainder is impurities, and the thickness is 0.80 to 2.50 μm with a standard deviation of thickness of 0.90 μm or less, the fracture life of the steel pipe after quenching can be extended.

[0027] The reason for this is not entirely clear, but the inventors believe it to be as follows: During quenching, the steel pipes used to manufacture vehicle parts undergo thermal expansion due to heating. If the difference between the linear expansion coefficient of the steel pipe and the linear expansion coefficient of the oxide film on the steel pipe surface is small, then the difference between the change in surface area of ​​the steel pipe and the change in volume of the oxide film due to heating during quenching becomes small. In this case, the oxide film is less likely to peel off the steel pipe surface and is more likely to remain on the steel pipe surface until just before quenching. In this case, contact between the steel pipe surface and oxygen is suppressed. It is possible that the linear expansion coefficient of the oxide film having the above composition is close to that of the steel pipe. Because the oxide film remains on the steel pipe surface until just before quenching, contact between the steel pipe surface and oxygen is suppressed. Therefore, the formation of a decarburized layer is suppressed. As a result, the fatigue strength of the steel pipe is increased and the fracture life is extended.

[0028] Furthermore, in order to achieve the effect of suppressing contact between the steel pipe surface and oxygen, the oxide film needs to have a certain thickness or more. On the other hand, if the oxide film is too thick, it will easily peel off. Therefore, it is necessary to control the thickness of the oxide film within a certain range.

[0029] Furthermore, if the variation in the thickness of the oxide film is large, contact between the steel pipe surface and oxygen cannot be sufficiently suppressed in areas where the oxide film is thin. In this case, localized intergranular oxidation occurs on the steel pipe surface. Local depressions occur in the areas where intergranular oxidation occurs. Stress concentrates in these depressions, reducing the fatigue strength of the vehicle parts. Therefore, it is believed that localized intergranular oxidation on the steel pipe surface can be suppressed by reducing the variation in the thickness of the oxide film.

[0030] Furthermore, the inventors have found the following: When a vehicle part is manufactured by quenching a steel pipe having the above-mentioned oxide film, it was found that when the sum of the X-ray diffraction peak intensities of Fe3O4, FeO, and Fe2O3 is taken as 100%, an oxide film is formed in which the X-ray diffraction peak intensity ratio consists of 80% or more Fe3O4, 15% or less FeO, 5% or less Fe2O3, and the remainder being impurities, with a thickness of 3.50 μm or less. This vehicle part has a long fracture life and excellent fatigue strength.

[0031] Based on the above findings, the steel pipe, steel pipe manufacturing method, vehicle component, and vehicle component manufacturing method of this embodiment have the following configuration.

[0032] [1] In mass%, C: 0.23~0.50%, Si: 0.01~0.50%, Mn: 0.50~2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol.Al: 0~0.080%, Cr: 0~1.50%, Mo: 0~1.00%, Ni: 0~1.00%, Cu: 0~1.00%, Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.100%, B: 0~0.0050%, Ca: 0 ~ 0.0050%, and, The remainder has a chemical composition consisting of Fe and impurities, A base material having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite in terms of area ratio, On the aforementioned base material, When the sum of the peak intensities of X-ray diffraction for Fe3O4, Fe2O3, and FeO is taken as 100%, the peak intensity ratio of the X-ray diffraction consists of 70% or more Fe3O4, 20% or more Fe2O3, 10% or less FeO, and the remainder being impurities. It comprises an oxide film having a thickness of 0.80 to 2.50 μm and a standard deviation of the said thickness of 0.90 μm or less. Steel pipe.

[0033] [2] [1] The steel pipe described above, The aforementioned chemical composition is expressed in mass%, Sol.Al: 0.001~0.080%, Cr: 0.01~1.50%, Mo: 0.01~1.00%, Ni: 0.01~1.00%, Cu: 0.01~1.00%, Ti: 0.001~0.100%, Nb: 0.001~0.100%, V: 0.001~0.100%, B: 0.0001~0.0050%, and, Ca: 0.0001~0.0050%, It contains one or more elements selected from the group consisting of, Steel pipe.

[0034] [3] In mass%, C: 0.23~0.50%, Si: 0.01~0.50%, Mn: 0.50~2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol.Al: 0~0.080%, Cr: 0~1.50%, Mo: 0~1.00%, Ni: 0~1.00%, Cu: 0~1.00%, Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.100%, B: 0~0.0050%, Ca: 0 ~ 0.0050%, and, The remainder has a chemical composition consisting of Fe and impurities, It has a microstructure consisting of tempered martensite, A hollow base material with a Vickers hardness of 400-550 HV in accordance with JIS Z 2244:2020, On the aforementioned base material, When the sum of the peak intensities of X-ray diffraction of Fe3O4, FeO, and Fe2O3 is taken as 100%, the peak intensity ratio of the X-ray diffraction consists of 80% or more Fe3O4, 15% or less FeO, 5% or less Fe2O3, and the remainder being impurities. It comprises an oxide film with a thickness of 3.50 μm or less. Vehicle parts.

[0035] [4] [3] Vehicle parts as described above, The aforementioned chemical composition is expressed in mass%, Sol.Al: 0.001~0.080%, Cr: 0.01~1.50%, Mo: 0.01~1.00%, Ni: 0.01~1.00%, Cu: 0.01~1.00%, Ti: 0.001~0.100%, Nb: 0.001~0.100%, V: 0.001~0.100%, B: 0.0001~0.0050%, and, Ca: 0.0001~0.0050%, It contains one or more elements selected from the group consisting of, Vehicle parts.

[0036] [5] In mass%, C: 0.23~0.50%, Si: 0.01~0.50%, Mn: 0.50~2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol.Al: 0~0.080%, Cr: 0~1.50%, Mo: 0~1.00%, Ni: 0~1.00%, Cu: 0~1.00%, Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.100%, B: 0~0.0050%, Ca: 0 ~ 0.0050%, and, The remainder has a chemical composition consisting of Fe and impurities, A process for preparing a steel sheet having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite by area ratio, The process involves heat-treating the steel plate at 450-600°C for 0.5-3.0 minutes, The process includes the step of manufacturing a steel pipe by electric resistance welding the steel plate after the heat treatment. A method for manufacturing steel pipes.

[0037] [6] A method for manufacturing vehicle parts, The process of preparing the steel pipe described in [1] or [2], A process of bending the steel pipe, The process involves holding the steel pipe after bending at an Ac3+50°C to 1150°C for 10 seconds or more, and then rapidly cooling it. The process includes a step of tempering the steel pipe after rapid cooling by holding it at 150-350°C for 10 minutes or more. A method for manufacturing vehicle parts.

[0038] The steel pipe, vehicle parts, manufacturing method of the steel pipe, and manufacturing method of the vehicle parts of this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass percent.

[0039] [Structure of the steel pipe in this embodiment] Figure 1 is a perspective view of the end of a steel pipe according to this embodiment. Referring to Figure 1, the steel pipe 1 comprises a base material 2 and an oxide film 3 on the base material 2. The steel pipe 1 includes an outer surface 4 and an inner surface 5. The oxide film 3 may be formed only on the outer surface 4 of the steel pipe 1, or only on the inner surface 5, or on both the outer surface 4 and the inner surface 5. The decarburized layer on the outer surface of a vehicle part can be removed, for example, by shot peening. On the other hand, the decarburized layer on the inner surface of a vehicle part can be difficult to remove. Therefore, preferably, the steel pipe 1 has an oxide film 3 on at least the inner surface 5.

[0040] The steel pipe 1 may be a seamless steel pipe or an electric resistance welded steel pipe. Preferably, the steel pipe 1 is an electric resistance welded steel pipe. The outer diameter of the steel pipe 1 is not particularly limited, but is, for example, 10 to 100 mm. The wall thickness of the steel pipe 1 is not particularly limited, but is, for example, 2 to 10 mm.

[0041] [Features of the steel pipe of this embodiment] The steel pipe 1 of this embodiment has the following features. (Feature 1) The chemical composition of base material 2 is as follows (by mass%): C: 0.23-0.50%, Si: 0.01-0.50%, Mn: 0.50-2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol.Al: 0-0.080%, Cr: 0-1.50%, Mo: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, Ti: 0-0.100%, Nb: 0-0.100%, V: 0-0.100%, B: 0-0.0050%, Ca: 0-0.0050%, and the remainder consists of Fe and impurities. (Feature 2) The microstructure of base material 2 consists of 20% to 60% ferrite and 40% to 80% pearlite by area ratio. (Feature 3) On the base material 2, an oxide film 3 is disposed, consisting of Fe3O4 at 70% or more, Fe2O3 at 20% or more, FeO at 10% or less, and the remainder being impurities, based on the X-ray diffraction peak intensity ratio of Fe3O4, Fe2O3, and FeO, when the sum of the X-ray diffraction peak intensities of Fe3O4, Fe2O3, and FeO is taken as 100%. (Feature 4) The thickness of oxide film 3 is 0.80 to 2.50 μm. (Feature 5) The standard deviation of the thickness of oxide film 3 is 0.90 μm or less. The following explains each of the five features.

[0042] [(Feature 1) Chemical composition of the steel pipe base material] The chemical composition of the base material 2 of the steel pipe 1 in this embodiment contains the following elements.

[0043] C: 0.23~0.50% Carbon (C) enhances the hardenability of steel. Furthermore, C dissolves in the steel. As a result, C increases the strength of the steel. If the C content is less than 0.23%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the C content exceeds 0.50%, the hot workability of the steel decreases, even if the content of other elements is within the range of this embodiment. If the C content exceeds 0.50%, the toughness of the vehicle parts after quenching decreases, even if the content of other elements is within the range of this embodiment. Therefore, the C content is 0.23 to 0.50%. The lower limit of the C content is preferably 0.25%, more preferably 0.27%, more preferably 0.30%, more preferably 0.33%, more preferably 0.35%, more preferably 0.38%, and more preferably 0.40%. The upper limit of the C content is preferably 0.48%, more preferably 0.46%, more preferably 0.44%, more preferably 0.42%, more preferably 0.40%, and more preferably 0.38%.

[0044] Si: 0.01~0.50% Silicon (Si) deoxidizes steel. Furthermore, Si dissolves in the steel, increasing its strength. If the Si content is less than 0.01%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 0.50%, the ductility and toughness of the steel pipe 1 will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.01 to 0.50%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%, more preferably 0.15%, more preferably 0.20%, and more preferably 0.25%. The upper limit of the Si content is preferably 0.45%, more preferably 0.40%, more preferably 0.35%, and still more preferably 0.30%.

[0045] Mn: 0.50~2.50% Manganese (Mn) enhances the hardenability of steel. Furthermore, Mn dissolves in the steel. As a result, Mn increases the strength of the steel. If the Mn content is less than 0.50%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 2.50%, the toughness and ductility of the vehicle parts after quenching will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Mn content is 0.50 to 2.50%. The lower limit of the Mn content is preferably 0.60%, more preferably 0.70%, more preferably 0.75%, more preferably 0.80%, more preferably 0.90%, more preferably 1.00%, and more preferably 1.10%. The upper limit of the Mn content is preferably 2.40%, more preferably 2.30%, more preferably 2.20%, more preferably 2.10%, more preferably 2.00%, more preferably 1.90%, more preferably 1.80%, more preferably 1.70%, more preferably 1.60%, and more preferably 1.50%.

[0046] P:0.050% or less Phosphorus (P) is an impurity. Therefore, the P content is greater than 0%. If the P content exceeds 0.050%, even if the content of other elements is within the range of this embodiment, P will segregate at the grain boundaries, reducing the ductility of the steel. Therefore, the P content is 0.050% or less. A lower P content is preferable. However, an extreme reduction in P content significantly increases manufacturing costs. Therefore, considering industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, more preferably 0.003%, and still more preferably 0.005%. The upper limit of the P content is preferably 0.040%, more preferably 0.030%, even more preferably 0.020%, and even more preferably 0.010%.

[0047] S: 0.0100% or less Sulfur (S) is an impurity. Therefore, the S content is greater than 0%. If the S content exceeds 0.0100%, the hot workability, toughness, and fatigue strength of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the S content is 0.0100% or less. A lower sulfur content is preferable. However, an extreme reduction in sulfur content significantly increases manufacturing costs. Therefore, considering industrial production, the lower limit of the sulfur content is preferably 0.0001%, more preferably 0.0002%, more preferably 0.0003%, and more preferably 0.0005%. The upper limit of the S content is preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0048] N: 0.0100% or less Nitrogen (N) is an impurity. Therefore, the N content is greater than 0%. If the N content exceeds 0.0100%, the toughness of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the N content is 0.0100% or less. On the other hand, N forms nitrides and / or carbonitrides, which increase the strength of the steel. The preferred lower limit of the N content is 0.0001%, more preferably 0.0002%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0010%, more preferably 0.0020%, and more preferably 0.0030%. The upper limit of the N content is preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0049] O: 0.0100% or less Oxygen (O) is an impurity. Therefore, the O content is greater than 0%. If the O content exceeds 0.0100%, the toughness of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the O content is 0.0100% or less. A lower oxygen content is preferable. However, an extreme reduction in oxygen content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of oxygen content is 0.0001%, more preferably 0.0002%, more preferably 0.0003%, and more preferably 0.0005%. The upper limit of the O content is preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, more preferably 0.0040%, and more preferably 0.0030%.

[0050] The remainder of the chemical composition of the steel pipe 1 in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the steel pipe 1, and are not intentionally included, but are acceptable within a range that does not adversely affect the steel pipe 1 in this embodiment.

[0051] [Optional Elements] The chemical composition of the base material 2 of the steel pipe 1 in this embodiment is further modified by substituting a portion of the Fe with: Sol.Al: 0~0.080%, Cr: 0~1.50%, Mo: 0~1.00%, Ni: 0~1.00%, Cu: 0~1.00%, Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.100%, B: 0~0.0050%, and, Ca: 0~0.0050%, It may contain one or more elements selected from the group consisting of the following. The following describes these arbitrary elements.

[0052] [Group 1: Al] The chemical composition of the base material 2 of the steel pipe 1 according to this embodiment may further include Al instead of some of the Fe.

[0053] Sol.Al: 0~0.080% Aluminum (Al) is an optional element and may not be present at all. In other words, the Al content may be 0%. If Al is present, that is, if the Al content is greater than 0%, Al deoxidizes the steel. Al further combines with nitrogen (N) to produce AlN. AlN suppresses grain coarsening during quenching. Even if only a small amount of Al is present, the above effect can be obtained to some extent. On the other hand, if the Al content exceeds 0.080%, even if the content of other elements is within the range of this embodiment, Al will combine with oxygen (O) to produce an excessive amount of inclusions. This reduces the fatigue strength of the vehicle parts. Therefore, the Al content is 0 to 0.080%. The lower limit of the Al content is preferably greater than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%. The upper limit of the Al content is preferably 0.070%, more preferably 0.060%, more preferably 0.050%, more preferably 0.040%, and still more preferably 0.030%.

[0054] [Group 2: Cr, Mo, Ni, and Cu] The chemical composition of the base material 2 of the steel pipe 1 according to this embodiment may further include one or more elements selected from the group consisting of Cr, Mo, Ni, and Cu in place of a portion of Fe. Any of these elements are optional and may not be included. If included, any of these elements will increase the strength of the steel.

[0055] Cr: 0~1.50% Chromium (Cr) is an optional element and may not be present. In other words, the Cr content may be 0%. When Cr is present, that is, when the Cr content is greater than 0%, Cr increases the strength of the steel. Even if only a small amount of Cr is present, the above effect can be obtained to some extent. On the other hand, if the Cr content exceeds 1.50%, the ductility of the steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Cr content is 0 to 1.50%. The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.20%, and more preferably 0.30%. The upper limit of the Cr content is preferably 1.20%, more preferably 1.00%, more preferably 0.80%, more preferably 0.60%, and more preferably 0.40%.

[0056] Mo: 0~1.00% Molybdenum (Mo) is an optional element and may not be present. In other words, the Mo content may be 0%. When Mo is present, that is, when the Mo content is greater than 0%, Mo increases the strength of the steel. Even if only a small amount of Mo is present, the above effect can be obtained to some extent. On the other hand, if the Mo content exceeds 1.00%, the ductility of the steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Mo content is 0 to 1.00%. The lower limit of the Mo content is preferably 0.01%, more preferably 0.02%, more preferably 0.03%, more preferably 0.04%, and more preferably 0.05%. The upper limit of the Mo content is preferably 0.80%, more preferably 0.60%, more preferably 0.40%, more preferably 0.20%, and more preferably 0.10%.

[0057] Ni: 0~1.00% Nickel (Ni) is an optional element and may not be present. In other words, the Ni content may be 0%. When Ni is present, that is, when the Ni content is greater than 0%, Ni increases the strength of the steel. Even if only a small amount of Ni is present, the above effect can be obtained to some extent. On the other hand, if the Ni content exceeds 1.00%, the ductility of the steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Ni content is 0 to 1.00%. The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, more preferably 0.05%, more preferably 0.10%, and more preferably 0.15%. The upper limit of the Ni content is preferably 0.80%, more preferably 0.60%, more preferably 0.40%, and still more preferably 0.20%.

[0058] Cu: 0~1.00% Copper (Cu) is an optional element and may not be present. In other words, the Cu content may be 0%. When Cu is present, that is, when the Cu content is greater than 0%, Cu increases the strength of the steel. Even if only a small amount of Cu is present, the above effect can be obtained to some extent. On the other hand, if the Cu content exceeds 1.00%, the ductility of the steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Cu content is 0 to 1.00%. The lower limit of the Cu content is preferably 0.01%, more preferably 0.02%, more preferably 0.03%, more preferably 0.04%, and more preferably 0.05%. The upper limit of the Cu content is preferably 0.80%, more preferably 0.60%, more preferably 0.40%, and still more preferably 0.20%.

[0059] [Group 3: Ti, Nb, and V] The chemical composition of the base material 2 of the steel pipe 1 according to this embodiment may further include one or more elements selected from the group consisting of Ti, Nb, and V in place of a portion of Fe. These elements are all optional and may not be included. If included, these elements enhance the strength and workability of the steel.

[0060] Ti: 0~0.100% Titanium (Ti) is an optional element and may not be present. In other words, the Ti content may be 0%. If Ti is present, that is, if the Ti content is greater than 0%, Ti forms carbides, nitrides and / or carbonitrides. This increases the strength and workability of the steel. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. On the other hand, if the Ti content exceeds 0.100%, the ductility of the steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Ti content is 0 to 0.100%. The lower limit of the Ti content is preferably 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.020%, and more preferably 0.030%. The upper limit of the Ti content is preferably 0.090%, more preferably 0.080%, more preferably 0.070%, and still more preferably 0.060%.

[0061] Nb: 0~0.100% Niobium (Nb) is an optional element and may not be present. In other words, the Nb content may be 0%. If Nb is present, that is, if the Nb content is greater than 0%, the Nb forms carbides, nitrides and / or carbonitrides. This increases the strength and workability of the steel. Even if only a small amount of Nb is present, the above effect can be obtained to some extent. On the other hand, if the Nb content exceeds 0.100%, the ductility of the steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Nb content is 0 to 0.100%. The lower limit of the Nb content is preferably 0.001%, more preferably 0.002%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%. The upper limit of the Nb content is preferably 0.090%, more preferably 0.070%, more preferably 0.050%, more preferably 0.030%, and more preferably 0.020%.

[0062] V: 0~0.100% Vanadium (V) is an optional element and may not be present. In other words, the V content may be 0%. If V is present, that is, if the V content is greater than 0%, V forms carbides, nitrides and / or carbonitrides. This increases the strength and workability of the steel. Even if only a small amount of V is present, the above effect can be obtained to some extent. On the other hand, if the V content exceeds 0.100%, the ductility of the steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the V content is 0 to 0.100%. The lower limit of the V content is preferably 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.015%, and more preferably 0.020%. The upper limit of the V content is preferably 0.090%, more preferably 0.080%, more preferably 0.070%, more preferably 0.060%, more preferably 0.050%, and more preferably 0.040%.

[0063] [Group 4:B] The chemical composition of the base material 2 of the steel pipe 1 according to this embodiment may further contain B instead of some of the Fe.

[0064] B: 0~0.0050% Boron (B) is an optional element and may not be present. In other words, the B content may be 0%. When B is present, that is, when the B content is greater than 0%, B improves the hardenability of the steel. Even if only a small amount of B is present, the above effect can be obtained to some extent. On the other hand, if the B content exceeds 0.0050%, the steel becomes more brittle, even if the content of other elements is within the range of this embodiment. Therefore, the B content is 0 to 0.0050%. The lower limit of the B content is preferably 0.0001%, more preferably 0.0002%, more preferably 0.0003%, more preferably 0.0005%, and more preferably 0.0010%. The upper limit of the B content is preferably 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0065] [Group 5: Ca] The chemical composition of the base material 2 of the steel pipe 1 according to this embodiment may further include Ca in place of some of the Fe.

[0066] Ca: 0~0.0050% Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. If Ca is present, that is, if the Ca content is greater than 0%, Ca improves the hot workability of the steel. Even if only a small amount of Ca is present, the above effect can be obtained to some extent. On the other hand, if the Ca content exceeds 0.0050%, the toughness of the steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Ca content is 0 to 0.0050%. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0010%, and more preferably 0.0015%. The upper limit of the Ca content is preferably 0.0040%, more preferably 0.0030%, and even more preferably 0.0025%.

[0067] [Method for measuring the chemical composition of the base material of steel pipes] The chemical composition of the base material 2 of the steel pipe 1 in this embodiment can be measured by a well-known component analysis method. The steel pipe 1 is cut to a length of 10 cm in the axial direction of the steel pipe 1. The oxide film 3 on the outer surface 4 and inner surface 5 of the cut steel pipe 1 is removed by cutting. The steel pipe 1 from which the oxide film 3 has been removed is finely crushed and dissolved in acid to obtain a solution. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a well-known inert gas melting-thermal conductivity method. The O content is determined using a well-known inert gas melting-non-dispersive infrared absorption method.

[0068] Furthermore, the content of each element is rounded to the minimum digit of the element content specified in this embodiment, based on the significant figures defined in this embodiment. For example, the carbon content of steel pipe 1 in this embodiment is defined to two decimal places. Therefore, the carbon content is the value obtained by rounding the third decimal place of the measured value to two decimal places.

[0069] Similarly, for the elemental content of steel pipe 1 in this embodiment, other than the C content, the value obtained by rounding the measured value to the minimum digit specified in this embodiment is considered the elemental content. Rounding means truncating if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0070] [(Feature 2) Microstructure of the base material of steel pipes] The base material 2 of the steel pipe 1 in this embodiment has a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite by area ratio. As described above, for example, vehicle parts can be manufactured by cold bending the steel pipe 1 and then quenching and tempering it. Therefore, the steel pipe 1 is required to have excellent workability. When the microstructure of the base material 2 consists of 20% to 60% ferrite and 40% to 80% pearlite by area ratio, the steel pipe 1 has excellent workability.

[0071] The lower limit of the ferrite area ratio is preferably 25%, more preferably 30%, more preferably 35%, and still more preferably 40%. The upper limit of the ferrite area ratio is preferably 55%, more preferably 50%, and still more preferably 45%. The lower limit of the area ratio of perlite is preferably 45%, more preferably 50%, and still more preferably 55%. The upper limit of the area ratio of perlite is preferably 75%, more preferably 70%, more preferably 65%, and still more preferably 60%.

[0072] [Method for measuring the area ratio of ferrite and pearlite] The area ratio of ferrite and pearlite in the base material 2 of steel pipe 1 is determined by the following method: Three test specimens are taken from any three locations on steel pipe 1, each containing the central part of the wall thickness in a cross section perpendicular to the axial direction of steel pipe 1, and having a length of 10 cm in the axial direction of steel pipe 1. In other words, three test specimens are taken. The surface of each test specimen corresponding to the cross section perpendicular to the axial direction of steel pipe 1 is designated as the observation surface. The observation surface of each test specimen is mirror-polished. Etching is performed on the mirror-polished observation surface using 3% nitric acid alcohol (Nital etching solution). The central part of the wall thickness of steel pipe 1 within the etched observation surface is designated as the observation field of view. The size of the observation field of view is 200 μm × 200 μm. The observation field of view is observed with a 500x optical microscope.

[0073] In the observation field, each tissue, such as pearlite and ferrite, can be easily distinguished by contrast. For example, ferrite is observed as a white area. Perlite is observed as an area with a lamellar structure that is less bright than ferrite. Identify each tissue in the observation field. Then, calculate the area percentage (%) of ferrite based on the area of ​​ferrite in the observation field and the total area of ​​the observation field. Calculate the area percentage (%) of pearlite based on the area of ​​pearlite in the observation field and the total area of ​​the observation field. The arithmetic mean of the values ​​obtained from three test specimens is taken as the area percentage of ferrite and the area percentage of pearlite.

[0074] [(Feature 3) Composition of the oxide film on steel pipes] The steel pipe 1 of this embodiment is provided with an oxide film 3 on the base material 2. The oxide film 3 consists of Fe3O4 at 70% or more, Fe2O3 at 20% or more, FeO at 10% or less, and the remainder being impurities, when the sum of the X-ray diffraction peak intensities of Fe3O4, Fe2O3, and FeO is taken as 100%. An oxide film 3 with the above composition is formed on the steel pipe 1 before quenching. This increases the fatigue strength of the vehicle parts.

[0075] The lower limit of the peak intensity ratio for Fe3O4 is preferably 72%, more preferably 74%, and still more preferably 75%. The upper limit of the peak intensity ratio for Fe3O4 is not particularly limited, but for example, it is 80%. The upper limit of the peak intensity ratio for Fe3O4 is preferably 78%, more preferably 76%, and still more preferably 75%.

[0076] The lower limit of the Fe2O3 peak intensity ratio is preferably 21%, more preferably 22%, more preferably 23%, and still more preferably 25%. The upper limit of the Fe2O3 peak intensity ratio is not particularly limited, but for example, it is 30%. The upper limit of the Fe2O3 peak intensity ratio is preferably 29%, more preferably 28%, and still more preferably 27%.

[0077] The lower limit of the FeO peak intensity ratio is not particularly limited and may be 0%. The upper limit of the FeO peak intensity ratio is preferably 8%, more preferably 6%, even more preferably 4%, and even more preferably 2%.

[0078] [Method for measuring the composition of the oxide film on steel pipes] The composition of the oxide film 3 on the steel pipe 1 is determined by the following method: X-ray diffraction measurements are performed on the surface of the oxide film 3 to obtain an X-ray diffraction profile. The measurements are performed at three arbitrary locations on the surface of the oxide film 3. The measurement conditions for the X-ray diffraction measurements are as follows: X-ray tube: Cu-Kα rays (converted to Cu-Kα1 rays using a monochromator) X-ray output: 45kV 200mA measurement range: 2θ = 10~120° Scanning method: Continuous scan Continuous scan speed: 2.0° / min

[0079] From the obtained X-ray diffraction profile, the peak intensities of Fe3O4, Fe2O3, and FeO are determined. The sum of the peak intensities of Fe3O4, Fe2O3, and FeO is set to 100%. The peak intensity ratios of Fe3O4, Fe2O3, and FeO are calculated from the sum of the peak intensities and the peak intensities of Fe3O4, Fe2O3, and FeO. The arithmetic mean of the three values ​​is taken as the peak intensity ratio for each peak. Note that the intensity ratio of each peak does not necessarily coincide with the area ratio of each peak or the mass percentage obtained by quantitative analysis.

[0080] [(Feature 4) Thickness of the oxide film on steel pipes] If the thickness of the oxide film 3 is less than 0.80 μm, the effect of suppressing contact between the surfaces 4 and 5 of the steel pipe 1 and oxygen cannot be obtained. On the other hand, if the thickness of the oxide film 3 is greater than 2.50 μm, the adhesion strength of the oxide film 3 decreases, and the oxide film 3 peels off from the surfaces 4 and 5 of the steel pipe 1. Therefore, the thickness of the oxide film 3 is between 0.80 and 2.50 μm. The lower limit of the thickness of the oxide film 3 is preferably 0.84 μm, more preferably 0.88 μm, even more preferably 1.00 μm, and even more preferably 1.20 μm. The upper limit of the thickness of the oxide film 3 is preferably 2.40 μm, more preferably 2.30 μm, more preferably 2.20 μm, more preferably 2.00 μm, and still more preferably 1.80 μm.

[0081] [Method for measuring the thickness of the oxide film on steel pipes] The thickness of the oxide film 3 on the steel pipe 1 is determined by the following method. The steel pipe 1 is cut perpendicular to the axial direction to obtain test specimens. Three test specimens are taken at 100 mm intervals along the axial direction of the steel pipe 1. For each test specimen, the cut surface perpendicular to the axial direction of the steel pipe 1 is used as the observation surface. The observation surface is embedded in resin so that it can be observed. After resin embedding, the observation surface is polished. A scanning electron microscope (SEM)-energy dispersive X-ray spectrometer (EDS) is used to generate a secondary electron image of the observation field of view, including the oxide film 3, on the polished observation surface. The size of the observation field of view is 50 μm × 40 μm. Here, the observation field of view is defined as 50 μm in the radial direction of the steel pipe 1 and 40 μm in the direction perpendicular to the radial direction (corresponding to the circumferential direction; hereinafter referred to as the C direction) on the observation surface.

[0082] In the secondary electron image, the base material 2 and the oxide film 3 can be easily distinguished by contrast. Alternatively, the base material 2 and the oxide film 3 can be distinguished by performing elemental mapping of oxygen (O) on the observation field using the EDS device attached to the SEM. In the elemental mapping of oxygen (O) by EDS, regions with high oxygen concentration correspond to the oxide film 3, and regions with low oxygen concentration correspond to the base material 2. Since the regions with high oxygen concentration and regions with low oxygen concentration are clearly separated, the oxide film 3 can be easily distinguished.

[0083] After identifying oxide film 3, the thickness of the identified oxide film 3 is measured at 10 locations in the C direction at 3 μm intervals. The arithmetic mean of the thicknesses of oxide film 3 at the measurement locations (30 locations in total) of the three test specimens is taken as the thickness of oxide film 3.

[0084] [(Feature 5) Standard deviation of the thickness of the oxide film on steel pipes] If the standard deviation of the thickness of the oxide film 3 exceeds 0.90 μm, contact between the surface 4 and 5 of the steel pipe 1 and oxygen cannot be suppressed in areas where the oxide film 3 is thin. In this case, localized intergranular oxidation occurs on the surface 4 and 5 of the steel pipe 1. Local depressions occur in the areas where intergranular oxidation occurs. Stress concentrates in these depressions, reducing the fatigue strength of the vehicle parts. Therefore, the standard deviation of the thickness of the oxide film 3 is 0.90 μm or less. The lower limit of the standard deviation of the thickness of the oxide film 3 may be 0 μm. However, since the thickness of the oxide film 3 may be uneven during the manufacturing process, the lower limit of the standard deviation of the thickness of the oxide film 3 is preferably 0.01 μm, more preferably 0.03 μm, and still more preferably 0.05 μm. The upper limit of the thickness of the oxide film 3 is preferably 0.80 μm, more preferably 0.70 μm, more preferably 0.60 μm, and still more preferably 0.50 μm.

[0085] [Method for measuring the standard deviation of the thickness of the oxide film on steel pipes] The standard deviation of the thickness of the oxide film on steel pipes is determined by the following method: The thickness of the oxide film 3 is measured according to the method described above in [Method for measuring the thickness of the oxide film on steel pipes]. The standard deviation of the thickness of the oxide film 3 at 30 locations is taken as the standard deviation of the thickness of the oxide film 3. In this disclosure, the standard deviation refers to the sample standard deviation (JIS Z8101-1:2015).

[0086] [Manufacturing method for steel pipe 1] An example of a manufacturing method for the steel pipe 1 of this embodiment will be described. The following example describes a method for manufacturing an electric resistance welded steel pipe. The manufacturing method for the steel pipe 1 described below is an example for manufacturing the steel pipe 1 of this embodiment. Therefore, the steel pipe 1 having the above configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a manufacturing method for the steel pipe 1 of this embodiment.

[0087] An example of a method for manufacturing the steel pipe 1 of this embodiment includes the following steps. (Process 1) Steel plate preparation process (Process 2) Low-temperature heat treatment process (Process 3) Pipe manufacturing process The following describes each step.

[0088] [(Process 1) Steel plate preparation process] In the steel sheet preparation process, a steel sheet for manufacturing the steel pipe 1 of this embodiment is prepared. The steel sheet may be obtained from a third party or manufactured. If manufactured, molten steel is produced in which the content of each element in the chemical composition is within the range of this embodiment. The refining method is not particularly limited, and any well-known method may be used. Using the molten steel, the raw material is manufactured by a well-known casting method. For example, an ingot may be manufactured using the ingot-making method with the molten steel. Alternatively, a bloom may be manufactured using the continuous casting method with the molten steel. The raw material (ingot or bloom) is manufactured by the above method. The raw material is heated and rough rolling and finish rolling are performed by a well-known method. The winding temperature of the steel sheet is, for example, over 600 to 700°C. Through the above manufacturing process, a steel sheet having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite by area ratio is manufactured.

[0089] [(Step 2) Low-temperature heat treatment process] In the low-temperature heat treatment process, the steel sheet is subjected to low-temperature heat treatment under the following conditions. (Manufacturing condition 1) Heat treatment temperature: 450~600℃ (Manufacturing condition 2) Heat treatment time: 0.5 to 3.0 minutes

[0090] The manufactured hot-rolled steel sheet is wound into a coil. In the low-temperature heat treatment process, the steel sheet is unwound and the low-temperature heat treatment is performed with the surface of the steel sheet exposed to the atmosphere. The low-temperature heat treatment conditions are as described above. The low-temperature heat treatment forms an oxide film 3 on the surface of the steel sheet, consisting of 70% or more Fe3O4, 20% or more Fe2O3, 10% or less FeO, and the remainder being impurities, with a thickness of 0.80 to 2.50 μm and a standard deviation of thickness of 0.90 μm or less, as measured by the peak intensity ratio of X-ray diffraction. The formation of the above oxide film increases the fatigue strength of the vehicle parts.

[0091] Furthermore, the preferred lower limit of the heat treatment temperature is above 450°C, more preferably 460°C, and even more preferably 470°C.

[0092] [(Process 3) Pipe manufacturing process] Electric resistance welded (ERW) steel pipes are manufactured using hot-rolled steel sheets that have undergone low-temperature heat treatment. In the pipe manufacturing process, the hot-rolled steel sheets are formed into cylindrical open pipes using forming rolls. In the formed open pipes, the width direction of the hot-rolled steel sheet is aligned with the circumferential direction of the open pipe. The butt joints extending in the longitudinal direction of the open pipes are welded using electric resistance welding. Electric resistance welded steel pipes are manufactured through the above pipe manufacturing process.

[0093] By following the above steps, the steel pipe 1 of this embodiment can be manufactured.

[0094] The manufacturing method of the steel pipe 1 of this embodiment may further include other steps. These other steps may include, for example, a diameter reduction rolling step. In the diameter reduction rolling step, for example, diameter reduction rolling may be carried out under well-known conditions.

[0095] [Applications of the steel pipe of this embodiment] The steel pipe 1 of this disclosure is used as a material for vehicle parts. Examples of vehicle parts include stabilizers, inner tie rods, drive shafts, and upper arms. The steel pipe 1 is suitable for stabilizer applications.

[0096] [Effects of the steel pipe in this embodiment] The steel pipe 1 of this embodiment has the following features. (Feature 1) The chemical composition of base material 2 is as follows (by mass%): C: 0.23-0.50%, Si: 0.01-0.50%, Mn: 0.50-2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol.Al: 0-0.080%, Cr: 0-1.50%, Mo: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, Ti: 0-0.100%, Nb: 0-0.100%, V: 0-0.100%, B: 0-0.0050%, Ca: 0-0.0050%, and the remainder consists of Fe and impurities. (Feature 2) The microstructure of base material 2 consists of 20% to 60% ferrite and 40% to 80% pearlite by area ratio. (Feature 3) On the base material 2, an oxide film 3 is disposed, consisting of Fe3O4 at 70% or more, Fe2O3 at 20% or more, FeO at 10% or less, and the remainder being impurities, based on the X-ray diffraction peak intensity ratio of Fe3O4, Fe2O3, and FeO, when the sum of the X-ray diffraction peak intensities of Fe3O4, Fe2O3, and FeO is taken as 100%. (Feature 4) The thickness of oxide film 3 is 0.80 to 2.50 μm. (Feature 5) The standard deviation of the thickness of oxide film 3 is 0.90 μm or less. The steel pipe 1 of this embodiment, having features 1 to 5, can be used to manufacture vehicle parts with excellent fatigue strength. In other words, with the steel pipe 1 of this embodiment, excellent fatigue strength can be obtained in vehicle parts manufactured using the steel pipe 1 as a material.

[0097] [Configuration of vehicle components in this embodiment] Figure 2 is a perspective view of the end of a vehicle part according to this embodiment. Referring to Figure 2, the vehicle part 10 comprises a hollow base material 20 and an oxide film 30 on the base material 20. The vehicle part 10 includes an outer surface 40 and an inner surface 50. The oxide film 30 may be formed only on the outer surface 40 of the vehicle part 10, or only on the inner surface 50, or on both the outer surface 40 and the inner surface 50. The decarburized layer on the outer surface 40 of the vehicle part 10 can be removed, for example, by shot peening. On the other hand, the decarburized layer on the inner surface 50 of the vehicle part 10 may be difficult to remove. Therefore, preferably, the vehicle part 10 has an oxide film 30 on at least the inner surface 50.

[0098] Vehicle parts 10 include, for example, stabilizers, inner tie rods, drive shafts, and upper arms.

[0099] [Features of the vehicle parts of this embodiment] The vehicle component 10 of this embodiment has the following features. (Feature 6) The chemical composition of the base material 20 is as follows (by mass%): C: 0.23-0.50%, Si: 0.01-0.50%, Mn: 0.50-2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol.Al: 0-0.080%, Cr: 0-1.50%, Mo: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, Ti: 0-0.100%, Nb: 0-0.100%, V: 0-0.100%, B: 0-0.0050%, Ca: 0-0.0050%, and the remainder consists of Fe and impurities. (Feature 7) The microstructure of the base material 20 consists of tempered martensite, and its Vickers hardness is 400-550 HV in accordance with JIS Z 2244:2020. (Feature 8) On the base material 20, an oxide film 30 is arranged, consisting of Fe3O4 at 80% or more, FeO at 15% or less, Fe2O3 at 5% or less, and the remainder being impurities, based on the X-ray diffraction peak intensity ratio of Fe3O4, FeO, and Fe2O3, when the sum of the X-ray diffraction peak intensities of Fe3O4, FeO, and Fe2O3 is taken as 100%. (Feature 9) The thickness of the oxide film 30 is 3.50 μm or less. Features 6-9 are explained below.

[0100] [(Feature 6) Chemical composition of the base material of vehicle parts] The chemical composition of the base material 20 of the vehicle part 10 in this embodiment contains the following elements.

[0101] C: 0.23~0.50% Carbon (C) enhances the hardenability of steel. Furthermore, C dissolves in the steel. As a result, C increases the strength of the steel. If the C content is less than 0.23%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the C content exceeds 0.50%, the hot workability of the steel decreases, even if the content of other elements is within the range of this embodiment. If the C content exceeds 0.50%, the toughness of the vehicle part 10 after quenching decreases, even if the content of other elements is within the range of this embodiment. Therefore, the C content is 0.23 to 0.50%. The lower limit of the C content is preferably 0.25%, more preferably 0.27%, more preferably 0.30%, more preferably 0.33%, more preferably 0.35%, more preferably 0.38%, and more preferably 0.40%. The upper limit of the C content is preferably 0.48%, more preferably 0.46%, more preferably 0.44%, more preferably 0.42%, more preferably 0.40%, and more preferably 0.38%.

[0102] Si: 0.01~0.50% Silicon (Si) deoxidizes steel. Furthermore, Si dissolves in the steel, increasing its strength. If the Si content is less than 0.01%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 0.50%, the ductility and toughness of the vehicle part 10 will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.01 to 0.50%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%, more preferably 0.15%, more preferably 0.20%, and more preferably 0.25%. The upper limit of the Si content is preferably 0.45%, more preferably 0.40%, more preferably 0.35%, and still more preferably 0.30%.

[0103] Mn: 0.50~2.50% Manganese (Mn) enhances the hardenability of steel. Furthermore, Mn dissolves in the steel. As a result, Mn increases the strength of the steel. If the Mn content is less than 0.50%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 2.50%, the toughness and ductility of the vehicle part 10 after quenching will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Mn content is 0.50 to 2.50%. The lower limit of the Mn content is preferably 0.60%, more preferably 0.70%, more preferably 0.75%, more preferably 0.80%, more preferably 0.90%, more preferably 1.00%, and more preferably 1.10%. The upper limit of the Mn content is preferably 2.40%, more preferably 2.30%, more preferably 2.20%, more preferably 2.10%, more preferably 2.00%, more preferably 1.90%, more preferably 1.80%, more preferably 1.70%, more preferably 1.60%, and more preferably 1.50%.

[0104] P:0.050% or less Phosphorus (P) is an impurity. Therefore, the P content is greater than 0%. If the P content exceeds 0.050%, even if the content of other elements is within the range of this embodiment, P will segregate at the grain boundaries, reducing the ductility of the steel. Therefore, the P content is 0.050% or less. A lower P content is preferable. However, an extreme reduction in P content significantly increases manufacturing costs. Therefore, considering industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, more preferably 0.003%, and still more preferably 0.005%. The upper limit of the P content is preferably 0.040%, more preferably 0.030%, even more preferably 0.020%, and even more preferably 0.010%.

[0105] S: 0.0100% or less Sulfur (S) is an impurity. Therefore, the S content is greater than 0%. If the S content exceeds 0.0100%, the hot workability, toughness, and fatigue strength of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the S content is 0.0100% or less. A lower sulfur content is preferable. However, an extreme reduction in sulfur content significantly increases manufacturing costs. Therefore, considering industrial production, the lower limit of the sulfur content is preferably 0.0001%, more preferably 0.0002%, more preferably 0.0003%, and more preferably 0.0005%. The upper limit of the S content is preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0106] N: 0.0100% or less Nitrogen (N) is an impurity. Therefore, the N content is greater than 0%. If the N content exceeds 0.0100%, the toughness of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the N content is 0.0100% or less. On the other hand, N forms nitrides and / or carbonitrides, which increase the strength of the steel. The preferred lower limit of the N content is 0.0001%, more preferably 0.0002%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0010%, more preferably 0.0020%, and more preferably 0.0030%. The upper limit of the N content is preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0107] O: 0.0100% or less Oxygen (O) is an impurity. Therefore, the O content is greater than 0%. If the O content exceeds 0.0100%, the toughness of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the O content is 0.0100% or less. A lower oxygen content is preferable. However, an extreme reduction in oxygen content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of oxygen content is 0.0001%, more preferably 0.0002%, more preferably 0.0003%, and more preferably 0.0005%. The upper limit of the O content is preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, more preferably 0.0040%, and more preferably 0.0030%.

[0108] The remainder of the chemical composition of the base material 20 of the vehicle part 10 in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are introduced during the industrial manufacturing of the base material 20 of the vehicle part 10 from raw materials such as ore, scrap, or the manufacturing environment, and are not intentionally included, but are acceptable within a range that does not adversely affect the base material 20 of the vehicle part 10 in this embodiment.

[0109] [Optional Elements] The chemical composition of the base material 20 of the vehicle part 10 in this embodiment is further modified by replacing a portion of the Fe with: Sol.Al: 0~0.080%, Cr: 0~1.50%, Mo: 0~1.00%, Ni: 0~1.00%, Cu: 0~1.00%, Ti: 0~0.100%, Nb: 0~0.100%, V: 0~0.100%, B: 0~0.0050%, and, Ca: 0~0.0050%, It may contain one or more elements selected from the group consisting of the following. The following describes these arbitrary elements.

[0110] [Group 1: Al] The chemical composition of the base material 20 of the vehicle part 10 according to this embodiment may further include Al in place of some of the Fe.

[0111] Sol.Al: 0~0.080% Aluminum (Al) is an optional element and may not be present. In other words, the Al content may be 0%. If Al is present, that is, if the Al content is greater than 0%, Al deoxidizes the steel. Al further combines with nitrogen (N) to produce AlN. AlN suppresses grain coarsening during quenching. Even if only a small amount of Al is present, the above effect can be obtained to some extent. On the other hand, if the Al content exceeds 0.080%, even if the content of other elements is within the range of this embodiment, Al will combine with oxygen (O) to produce an excessive amount of inclusions. This reduces the fatigue strength of the vehicle part 10. Therefore, the Al content is 0 to 0.080%. The lower limit of the Al content is preferably greater than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%. The upper limit of the Al content is preferably 0.070%, more preferably 0.060%, more preferably 0.050%, more preferably 0.040%, and still more preferably 0.030%.

[0112] [Group 2: Cr, Mo, Ni, and Cu] The chemical composition of the base material 20 of the vehicle part 10 according to this embodiment may further include one or more elements selected from the group consisting of Cr, Mo, Ni, and Cu in place of a portion of Fe. Any of these elements are optional and may not be included. If included, any of these elements will increase the strength of the vehicle part 10.

[0113] Cr: 0~1.50% Chromium (Cr) is an optional element and may not be present. In other words, the Cr content may be 0%. If Cr is present, that is, if the Cr content is greater than 0%, Cr increases the strength of the vehicle part 10. Even if only a small amount of Cr is present, the above effect can be obtained to some extent. On the other hand, if the Cr content exceeds 1.50%, the ductility of the vehicle part 10 decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Cr content is 0 to 1.50%. The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.20%, and more preferably 0.30%. The upper limit of the Cr content is preferably 1.20%, more preferably 1.00%, more preferably 0.80%, more preferably 0.60%, and more preferably 0.40%.

[0114] Mo: 0~1.00% Molybdenum (Mo) is an optional element and may not be included. In other words, the Mo content may be 0%. If Mo is included, that is, if the Mo content is greater than 0%, Mo increases the strength of the vehicle part 10. Even if only a small amount of Mo is included, the above effect can be obtained to some extent. On the other hand, if the Mo content exceeds 1.00%, the ductility of the vehicle part 10 decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Mo content is 0 to 1.00%. The lower limit of the Mo content is preferably 0.01%, more preferably 0.02%, more preferably 0.03%, more preferably 0.04%, and more preferably 0.05%. The upper limit of the Mo content is preferably 0.80%, more preferably 0.60%, more preferably 0.40%, more preferably 0.20%, and more preferably 0.10%.

[0115] Ni: 0~1.00% Nickel (Ni) is an optional element and may not be included. In other words, the Ni content may be 0%. If Ni is included, that is, if the Ni content is greater than 0%, Ni increases the strength of the vehicle part 10. Even if only a small amount of Ni is included, the above effect can be obtained to some extent. On the other hand, if the Ni content exceeds 1.00%, the ductility of the vehicle part 10 decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Ni content is 0 to 1.00%. The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, more preferably 0.05%, more preferably 0.10%, and more preferably 0.15%. The upper limit of the Ni content is preferably 0.80%, more preferably 0.60%, more preferably 0.40%, and still more preferably 0.20%.

[0116] Cu: 0~1.00% Copper (Cu) is an optional element and may not be included. In other words, the Cu content may be 0%. If Cu is included, that is, if the Cu content is greater than 0%, Cu increases the strength of the vehicle part 10. Even if only a small amount of Cu is included, the above effect can be obtained to some extent. On the other hand, if the Cu content exceeds 1.00%, the ductility of the vehicle part 10 decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Cu content is 0 to 1.00%. The lower limit of the Cu content is preferably 0.01%, more preferably 0.02%, more preferably 0.03%, more preferably 0.04%, and more preferably 0.05%. The upper limit of the Cu content is preferably 0.80%, more preferably 0.60%, more preferably 0.40%, and still more preferably 0.20%.

[0117] [Group 3: Ti, Nb, and V] The chemical composition of the base material 20 of the vehicle part 10 according to this embodiment may further include one or more elements selected from the group consisting of Ti, Nb, and V in place of a portion of Fe. Any of these elements are optional and may not be included. If included, any of these elements enhance the strength and workability of the vehicle part 10.

[0118] Ti: 0~0.100% Titanium (Ti) is an optional element and may not be present. In other words, the Ti content may be 0%. If Ti is present, that is, if the Ti content is greater than 0%, Ti will form carbides, nitrides and / or carbonitrides. This increases the strength and workability of the vehicle part 10. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. On the other hand, if the Ti content exceeds 0.100%, the ductility of the vehicle part 10 will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Ti content is 0 to 0.100%. The lower limit of the Ti content is preferably 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.020%, and more preferably 0.030%. The upper limit of the Ti content is preferably 0.090%, more preferably 0.080%, more preferably 0.070%, and still more preferably 0.060%.

[0119] Nb: 0~0.100% Niobium (Nb) is an optional element and may not be present. In other words, the Nb content may be 0%. If Nb is present, that is, if the Nb content is greater than 0%, the Nb forms carbides, nitrides and / or carbonitrides. This increases the strength and workability of the vehicle part 10. Even if only a small amount of Nb is present, the above effect can be obtained to some extent. On the other hand, if the Nb content exceeds 0.100%, the ductility of the vehicle part 10 will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Nb content is 0 to 0.100%. The lower limit of the Nb content is preferably 0.001%, more preferably 0.002%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%. The upper limit of the Nb content is preferably 0.090%, more preferably 0.070%, more preferably 0.050%, more preferably 0.030%, and more preferably 0.020%.

[0120] V: 0~0.100% Vanadium (V) is an optional element and may not be present. In other words, the V content may be 0%. If V is present, that is, if the V content is greater than 0%, V forms carbides, nitrides and / or carbonitrides. This increases the strength and workability of the vehicle part 10. Even if only a small amount of V is present, the above effect can be obtained to some extent. On the other hand, if the V content exceeds 0.100%, the ductility of the vehicle part 10 decreases, even if the content of other elements is within the range of this embodiment. Therefore, the V content is 0 to 0.100%. The lower limit of the V content is preferably 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.015%, and more preferably 0.020%. The upper limit of the V content is preferably 0.090%, more preferably 0.080%, more preferably 0.070%, more preferably 0.060%, more preferably 0.050%, and more preferably 0.040%.

[0121] [Group 4:B] The chemical composition of the base material 20 of the vehicle part 10 according to this embodiment may further include B in place of some of the Fe.

[0122] B: 0~0.0050% Boron (B) is an optional element and may not be present. In other words, the B content may be 0%. When B is present, that is, when the B content is greater than 0%, B improves the hardenability of the steel. Even if only a small amount of B is present, the above effect can be obtained to some extent. On the other hand, if the B content exceeds 0.0050%, the vehicle part 10 becomes more brittle, even if the content of other elements is within the range of this embodiment. Therefore, the B content is 0 to 0.0050%. The lower limit of the B content is preferably 0.0001%, more preferably 0.0002%, more preferably 0.0003%, more preferably 0.0005%, and more preferably 0.0010%. The upper limit of the B content is preferably 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0123] [Group 5: Ca] The chemical composition of the base material 20 of the vehicle part 10 according to this embodiment may further include Ca in place of some of the Fe.

[0124] Ca: 0~0.0050% Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. If Ca is present, that is, if the Ca content is greater than 0%, Ca improves the hot workability of the steel. Even if only a small amount of Ca is present, the above effect can be obtained to some extent. On the other hand, if the Ca content exceeds 0.0050%, the toughness of the vehicle part 10 will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Ca content is 0 to 0.0050%. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0010%, and more preferably 0.0015%. The upper limit of the Ca content is preferably 0.0040%, more preferably 0.0030%, and even more preferably 0.0025%.

[0125] [Method for measuring the chemical composition of base materials for vehicle parts] The chemical composition of the base material 20 of the vehicle part 10 in this embodiment is determined by the same method as the chemical composition of the base material 2 of the steel pipe 1. The vehicle part 10 is cut to a length of 10 cm in the axial direction of the vehicle part 10. The oxide film 30 on the outer surface 40 and inner surface 50 of the cut vehicle part 10 is removed by cutting. The vehicle part 10 from which the oxide film 30 has been removed is finely crushed and dissolved in acid to obtain a solution. Elemental analysis of the chemical composition is performed on the solution by ICP-AES. The C content and S content are determined by the well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined by the well-known inert gas melting-thermal conductivity method. The O content is determined by the well-known inert gas melting-non-dispersive infrared absorption method.

[0126] Similar to the elemental content of the base material 2 of the steel pipe 1, the elemental content of each element is rounded to the minimum digit of the elemental content specified in this embodiment, based on the significant figures defined in this embodiment.

[0127] [(Feature 7) Microstructure and Vickers hardness of base materials for vehicle parts] The base material 20 of the vehicle part 10 in this embodiment has a microstructure consisting of tempered martensite and has a Vickers hardness of 400 to 550 HV in accordance with JIS Z 2244:2020.

[0128] [Regarding the microstructure of vehicle parts] The microstructure of the base material 20 of the vehicle part 10 in this embodiment consists of tempered martensite. In the microstructure of the base material 20 of the vehicle part 10, the area ratio of phases other than tempered martensite is negligibly small.

[0129] [Method for measuring the area ratio of tempered martensite] The area ratio of tempered martensite in the base material 20 of the vehicle part 10 is determined by the following method: Three test specimens are taken from any three locations on the vehicle part 10, including the center of the thickness of the cross section perpendicular to the longitudinal direction of the vehicle part 10, and having a length of 10 cm in the longitudinal direction of the vehicle part 10. In other words, three test specimens are taken. Of the surfaces of the test specimens, the surface corresponding to the cross section perpendicular to the longitudinal direction of the vehicle part 10 is designated as the observation surface. The observation surface of each test specimen is mirror-polished. Etching is performed on the mirror-polished observation surface using 3% nitric acid alcohol (Nital etching solution). Of the etched observation surface, the center of the thickness of the vehicle part 10 is designated as the observation field of view. The size of the observation field of view is 200 μm × 200 μm. The observation field of view is observed with a 500x optical microscope.

[0130] In the observation field, tempered martensite and other structures (pearlite, ferrite, etc.) can be easily distinguished by contrast. Tempered martensite is observed as a low-brightness gray with a fine structure. Ferrite is observed as a white area with higher brightness than tempered martensite and pearlite. Pearlite is observed as a phase with a lamellar structure that is lower in brightness than ferrite. The area percentage (%) of tempered martensite is determined based on the area of ​​tempered martensite in the observation field and the total area of ​​the observation field. The arithmetic mean of the values ​​obtained from three test specimens is taken as the area percentage of tempered martensite.

[0131] [Vickers hardness] The base material 20 of the vehicle part 10 in this embodiment has a Vickers hardness of 400 to 550 HV in accordance with JIS Z 2244:2020. The preferred lower limit of the Vickers hardness is 405 HV, more preferably 410 HV, even more preferably 415 HV, and even more preferably 420 HV. The preferred upper limit of the Vickers hardness is 545 HV, more preferably 540 HV, more preferably 535 HV, more preferably 530 HV, and more preferably 525 HV.

[0132] [Method for measuring Vickers hardness] The Vickers hardness of the base material 20 of the vehicle part 10 in this embodiment is measured by the following method. A test specimen is taken from the vehicle part 10, with the measurement surface being the longitudinal section parallel to the longitudinal direction. The measurement surface of the test specimen is polished. On the polished measurement surface, the Vickers hardness (HV) is measured in accordance with JIS Z 2244:2020 at three arbitrary locations in the base material 20, 20 μm deep from the boundary (interface) between the oxide film 30 and the base material 20. The test force during measurement is 0.098 N. The arithmetic mean of the obtained values ​​is taken as the Vickers hardness (HV). The oxide film 30 and the base material 20 are distinguished by the difference in brightness observed with a microscope.

[0133] [(Feature 8) Composition of oxide film on vehicle parts] The vehicle component 10 of this embodiment is provided with an oxide film 30 on a base material 20. The oxide film 30 consists of Fe3O4 at 80% or more, FeO at 15% or less, Fe2O3 at 5% or less, and the remainder being impurities, when the sum of the X-ray diffraction peak intensities of Fe3O4, FeO, and Fe2O3 is taken as 100%.

[0134] The lower limit of the peak intensity ratio for Fe3O4 is preferably 81%, more preferably 85%, and even more preferably 90%. The upper limit of the peak intensity ratio for Fe3O4 may be 100%. The upper limit of the peak intensity ratio for Fe3O4 is preferably 99%, more preferably 98%, even more preferably 97%, and even more preferably 96%.

[0135] The lower limit of the FeO peak intensity ratio may be 0%. Preferably, the lower limit of the FeO peak intensity ratio is 1%, more preferably 2%, and still more preferably 3%. Preferably, the upper limit of the FeO peak intensity ratio is 14%, more preferably 13%, more preferably 12%, and still more preferably 11%.

[0136] The lower limit of the Fe2O3 peak intensity ratio may be 0%. Preferably, the lower limit of the Fe2O3 peak intensity ratio is 0.1%, and more preferably 0.2%. Preferably, the upper limit of the Fe2O3 peak intensity ratio is 4%, and more preferably 3%.

[0137] [Method for measuring the composition of oxide film on vehicle parts] The composition of the oxide film 30 on the vehicle part 10 is determined by the following method: X-ray diffraction measurement is performed on the surface of the oxide film 30 to obtain an X-ray diffraction profile. The measurement is performed at any three locations on the surface of the oxide film 30. The measurement conditions for the X-ray diffraction measurement are the same as those in the measurement conditions for [Method for measuring the composition of oxide film on steel pipes] described above. The peak intensities of Fe3O4, FeO, and Fe2O3 X-ray diffraction are determined from the obtained X-ray diffraction profile. The sum of the peak intensities of Fe3O4, FeO, and Fe2O3 X-ray diffraction is set to 100%. The peak intensity ratio of Fe3O4, FeO, and Fe2O3 is determined from the sum of the peak intensities and the peak intensities of Fe3O4, FeO, and Fe2O3 X-ray diffraction. The arithmetic mean of the values ​​at the three locations is taken as the peak intensity ratio for each peak. Note that the intensity ratio of each peak does not necessarily coincide with the area ratio of each peak or the mass % obtained by quantitative analysis.

[0138] [(Feature 9) Thickness of the oxide film on vehicle parts] If the thickness of the oxide film 30 exceeds 3.50 μm, the adhesion strength of the oxide film 30 decreases. Therefore, the thickness of the oxide film 30 should be 3.50 μm or less. The lower limit of the thickness of the oxide film 30 is not particularly limited, but is, for example, 0.01 μm. Preferably, the lower limit of the thickness of the oxide film 30 is 0.50 μm, more preferably 1.00 μm, even more preferably 1.50 μm, and even more preferably 2.00 μm. The upper limit of the thickness of the oxide film 30 is preferably 3.40 μm, more preferably 3.20 μm, more preferably 3.00 μm, more preferably 2.90 μm, more preferably 2.80 μm, and still more preferably 2.50 μm.

[0139] [Method for measuring the thickness of oxide film on vehicle parts] The thickness of the oxide film 30 on the vehicle part 10 is determined by the following method: The vehicle part 10 is cut perpendicular to its longitudinal direction to obtain test specimens. Three test specimens are taken at 100 mm intervals along the longitudinal direction of the vehicle part 10. The cut surface is used as the observation surface. The observation surface is embedded in resin so that it can be observed. After resin embedding, the observation surface is polished. A secondary electron image of the observation field, including the oxide film 3, is generated on the polished observation surface using SEM-EDS. The size of the observation field is 50 μm × 40 μm. Here, the observation field is defined as 50 μm in the radial direction of the vehicle part 10 on the observation surface, and 40 μm in the direction perpendicular to the radial direction (corresponding to the circumferential direction; hereinafter referred to as the C direction).

[0140] In the secondary electron image, the base material 20 and the oxide film 30 can be easily distinguished by contrast. Alternatively, the base material 20 and the oxide film 30 can be distinguished by performing elemental mapping of oxygen (O) on the observation field using the EDS device attached to the SEM. In the elemental mapping of oxygen (O) by EDS, the region with a high oxygen concentration corresponds to the oxide film 30, and the region with a low oxygen concentration corresponds to the base material 2. Since the region with a high oxygen concentration and the region with a low oxygen concentration are clearly separated, the oxide film 30 can be easily distinguished. Furthermore, since the oxide film 30 is thicker than the oxide film 3, it may peel off during polishing. In this case, the oxide film may not be identifiable by elemental mapping of oxygen (O) using SEM-EDS. However, in this case, the gap between the resin and the base material 20 after the oxide film 30 has peeled off may be considered as the oxide film 30.

[0141] After identifying the oxide film 30, the thickness of the identified oxide film 30 is measured at 10 locations in the C direction at 3 μm intervals. The arithmetic mean of the thicknesses of the oxide film 30 at the measurement locations (30 locations in total) of the three test specimens is taken as the thickness of the oxide film 30.

[0142] [Manufacturing method for vehicle parts] An example of a manufacturing method for the vehicle component 10 of this embodiment will be described. The following example describes a manufacturing method for a stabilizer. The manufacturing method for the vehicle component 10 described below is an example for manufacturing the vehicle component 10 of this embodiment. Therefore, the vehicle component 10 having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a manufacturing method for the vehicle component 10 of this embodiment.

[0143] An example of a method for manufacturing the vehicle part 10 of this embodiment includes the following steps. (Process 4) Steel pipe preparation process (Process 5) Bending process (Step 6) Hardening process (Process 7) Tempering process The following describes each step.

[0144] [(Process 4) Steel pipe preparation process] In the steel pipe preparation process, steel pipes 1 for manufacturing the vehicle parts 10 of this embodiment are prepared.

[0145] [(Process 5) Bending Process] In the bending process, the steel pipe 1 is cut to a predetermined length. The cut steel pipe 1 is then cold-bent into a predetermined shape.

[0146] [(Step 6) Hardening process] In the quenching process, the bent steel pipe 1 is heated under the following conditions and then rapidly cooled. The cooling method is a well-known method. (Manufacturing condition 3) Hardening temperature: A c3 More than +50℃ and less than 1150℃ Note A c3 A point (°C) is defined by the following formula: A c3 =910-203×(√C)-15.2×Ni+44.7×Si+104×V+31.5×Mo (A) In formula (A), each element symbol is substituted with the mass percentage content of the corresponding element.

[0147] [(Process 7) Tempering Process] In the tempering process, the steel pipe 1 after quenching is tempered under the following conditions. (Manufacturing Condition 4) Tempering temperature: 150 - 350 °C (Manufacturing Condition 5) Holding time: 10 minutes or more

[0148] <了 Through the above processes, the vehicle part 10 of the present embodiment can be manufactured.

[0149] The manufacturing method of the vehicle part 10 of the present embodiment may further include other processes. Examples of other processes include a surface treatment process. In the surface treatment process, for example, shot peening may be performed on the outer surface 40 of the obtained vehicle part 10. Dustproof treatment may be performed on the outer surface 40 of the obtained vehicle part 10.

[0150] [Use of the Vehicle Part of the Present Embodiment] The vehicle part 10 of the present disclosure is suitable as a stabilizer. However, the use of the vehicle part 10 of the present disclosure is not limited to a stabilizer. The vehicle part 10 of the present disclosure can be used, for example, for inner tie rods, drive shafts, and upper arms.

[0151] [Effect of the Vehicle Part of the Present Embodiment] The vehicle part 10 of the present embodiment has the following characteristics. (Characteristic 6) The chemical composition of the base material 20 is, by mass, C: 0.23 - 0.50%, Si: 0.01 - 0.50%, Mn: 0.50 - / 2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol.Al: 0 - 0.080%, Cr: 0 - 1.50%, Mo: 0 - 1.00%, Ni: 0 - 1.00%, Cu: 0 - 1.00%, Ti: 0 - 0.100%, Nb: 0 - 0.100%, V: 0 - 0.100%, B: 0 - 0.0050%, Ca: 0 - 0.0050%, and the balance consists of Fe and impurities. (Feature 7) The microstructure of the base material 20 consists of tempered martensite, and its Vickers hardness is 400-550 HV in accordance with JIS Z 2244:2020. (Feature 8) On the base material 20, an oxide film 30 is arranged, consisting of Fe3O4 at 80% or more, FeO at 15% or less, Fe2O3 at 5% or less, and the remainder being impurities, based on the X-ray diffraction peak intensity ratio of Fe3O4, FeO, and Fe2O3, when the sum of the X-ray diffraction peak intensities of Fe3O4, FeO, and Fe2O3 is taken as 100%. (Feature 9) The thickness of the oxide film 30 is 3.50 μm or less. The vehicle component 10 of this embodiment, having features 6 to 9, exhibits excellent fatigue strength. [Examples]

[0152] The effects of the steel pipe 1 and vehicle component 10 of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effectiveness of the steel pipe 1 and vehicle component 10 of this embodiment. Therefore, the steel pipe 1 and vehicle component 10 of this embodiment are not limited to this one example of conditions.

[0153] Test materials (steel plates simulating steel pipes) having the chemical compositions shown in Tables 1A and 1B were manufactured.

[0154] [Table 1A]

[0155] [Table 1B]

[0156] In Table 1B, "-" indicates that the corresponding element content is below the impurity level.

[0157] Slabs were prepared from the molten steel of each test number. Rough rolling and finish rolling were performed on the slabs to create steel plates with a length of 1000 cm, a width of 300 cm, and a thickness of 4 mm. Low-temperature heat treatment was performed on each steel plate of each test number using a heat treatment furnace in an atmospheric environment at the heat treatment temperature and heat treatment time shown in Table 2. Through the above manufacturing process, test materials (steel plates) simulating steel pipes were produced.

[0158] [Table 2]

[0159] [Measurement test of the composition of the oxide film before quenching] The composition of the oxide film formed on the base material (steel plate) of the test specimen was measured based on the [Method for measuring the composition of the oxide film on steel pipes] described above. For the measurement, X-ray diffraction was performed on the surface of the oxide film formed on the surface of the steel plate to obtain the X-ray diffraction profile. The results are shown in Table 2.

[0160] [Measurement test of the thickness of the oxide film before quenching and the standard deviation of the thickness] The thickness and standard deviation of the oxide film formed on the base material (steel plate) of the test specimen were measured based on the above-described methods for measuring the thickness of the oxide film on steel pipes and the method for measuring the standard deviation of the oxide film thickness on steel pipes. For the measurement, three test specimens were taken at 100 mm intervals in the rolling direction of the steel plate used as the test specimen.

[0161] [Measurement test of ferrite and pearlite area ratio of steel plate (base material) before hardening] The area ratios of ferrite and pearlite in the base material (steel sheet) of the test specimens were measured based on the [Method for Measuring the Area Ratio of Ferrite and Pearlite] described above. The surface of the test specimen corresponding to the cross-section perpendicular to the rolling direction of the steel sheet was used as the observation surface. As a result, the base material (steel sheet) of all test specimens had a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite in terms of area ratio.

[0162] [Manufacturing of simulated parts that mimic vehicle components] For the test specimens of each test number, quenching was carried out at the quenching temperature shown in Table 3. After quenching, tempering was carried out by holding at a tempering temperature of 200 °C for 60 minutes. By the above manufacturing process, simulated parts simulating vehicle parts were manufactured.

[0163] [Measurement Test of the Area Ratio of Tempered Martensite in Simulated Parts] The area ratio of tempered martensite in the base material of the simulated parts was determined based on the above [Measurement Method of the Area Ratio of Tempered Martensite]. Among the surfaces of the test pieces, the surface corresponding to the cross-section perpendicular to the longitudinal direction of the simulated parts was taken as the observation surface. As a result of the measurement, the microstructure of the base material of the steel plates of all test numbers was a structure consisting of tempered martensite.

[0164] The Vickers hardness of the base material of the simulated parts was measured in accordance with the above [Measurement Method of Vickers Hardness]. The obtained results are shown in Table 3.

[0165] [Measurement Test of the Composition of the Oxide Film on Simulated Parts] The composition of the oxide film on the simulated parts was measured based on the above [Measurement Method of the Composition of the Oxide Film on Vehicle Parts]. The results are shown in Table 3.

[0166]

Table 3

[0167] [Measurement Test of the Thickness of the Oxide Film on Simulated Parts] The thickness of the oxide film formed on the steel plate after quenching was measured based on the above [Measurement Method of the Thickness of the Oxide Film on Vehicle Parts]. Observation and measurement were carried out on three steel plates for each test number. The arithmetic mean value of the thicknesses of the oxide films of the three steel plates was taken as the thickness of the oxide film for each test number. The results are shown in Table 3.

[0168] [Fatigue Test] The fracture life of the simulated parts was measured. From the simulated parts (steel plates) of each test number, plate-shaped torsional fatigue test specimens with a thickness of 2 mm, a width of 8 mm, a length of 60 mm, and a parallel section length of 9.5 mm were obtained. Figure 3 is a front view of the torsional fatigue test specimen. Figure 4 is a side view of the torsional fatigue test specimen viewed from the longitudinal direction. The torsional fatigue test specimen had a groove a in the center of its width direction that simulated the inner surface of a steel pipe, with a concave curve, a bottom depth of 0.1 mm, and a radius of curvature of 8.7 mm. Torsional fatigue tests were conducted in air at a set stress of 400 MPa. In the torsional fatigue tests, an electromagnetic torsional fatigue testing machine was used, and cycle fatigue tests were performed under the conditions of test waveform: sinusoidal waveform, test speed: 15 Hz, test environment: room temperature, air, stress ratio: -1 (double swing), and the number of cycles until the torsional fatigue test specimen fractured was measured. 5.0 × 10 6 If the test specimen did not fracture in one test, it was determined that excellent fatigue strength had been achieved. The results are shown in Table 3.

[0169] [Evaluation Test] Referring to Tables 1 to 3, the composition, thickness, and standard deviation of the oxide film on the test material were appropriate in test numbers 1 to 16. Furthermore, the composition and thickness of the oxide film on the simulated parts for these test numbers were also appropriate. As a result, excellent fatigue strength was obtained in these test numbers.

[0170] On the other hand, in test number 17, the oxide film on the test material was too thick. Consequently, the oxide film on the simulated part was also too thick. As a result, excellent fatigue strength could not be obtained.

[0171] In test number 18, the oxide film on the test material was too thin. Consequently, the oxide film on the simulated part was too thick. As a result, excellent fatigue strength could not be obtained.

[0172] In test number 19, the standard deviation of the oxide film thickness on the test material was too large. As a result, the oxide film on the simulated part was too thick. Consequently, excellent fatigue strength could not be obtained.

[0173] In tests 20, 21, and 24, the oxide film composition of the test material was inappropriate, and furthermore, the oxide film was too thick, with an excessively large standard deviation of thickness. Consequently, the oxide film composition of the simulated part was inappropriate, and furthermore, the oxide film was too thick. As a result, excellent fatigue strength could not be obtained.

[0174] In tests 22 and 23, the oxide film on the test material was too thin. Therefore, it was difficult to analyze the composition of the oxide film (indicated by "-" in the "Composition" column of "Oxide Film of Test Material" in Table 2). Consequently, the oxide film after quenching was too thick. As a result, excellent fatigue strength could not be obtained.

[0175] In test number 25, the carbon content of the base material was too low. As a result, the Vickers hardness of the simulated part was too low. Consequently, excellent fatigue strength could not be obtained.

[0176] In test number 26, the carbon content of the base material was too high. As a result, the Vickers hardness of the simulated part was too high. Consequently, excellent fatigue strength could not be obtained.

[0177] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure. [Explanation of Symbols]

[0178] 1 Steel pipe 2 Base material 3 Oxide film 10 Vehicle parts 20 Base material 30 Oxide film

Claims

1. In mass percent, C: 0.23-0.50%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol. Al: 0-0.080%, Cr: 0-1.50%, Mo: 0-1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0-0.100%, B: 0 to 0.0050%, Ca: 0-0.0050%, and, The remainder has a chemical composition consisting of Fe and impurities, A base material having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite in terms of area ratio, On the aforementioned base material, Fe ３ O ４ Fe ２ O ３ And when the sum of the peak intensities of the X-ray diffraction of FeO is taken as 100%, Fe has a peak intensity ratio of 70% or more in the X-ray diffraction. ３ O ４ , 20% or more Fe ２ O ３ It consists of less than 10% FeO, and the remainder is impurities. It comprises an oxide film having a thickness of 0.80 to 2.50 μm and a standard deviation of the said thickness of 0.90 μm or less. Steel pipe.

2. A steel pipe according to claim 1, The aforementioned chemical composition is expressed in mass%, Sol. Al: 0.001–0.080%, Cr: 0.01-1.50%, Mo: 0.01-1.00%, Ni: 0.01-1.00%, Cu: 0.01 to 1.00%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.001-0.100%, B: 0.0001 to 0.0050%, and, Ca: 0.0001-0.0050%, It contains one or more elements selected from the group consisting of, Steel pipe.

3. In mass percent, C: 0.23-0.50%, Si: 0.01-0.50%, Mn: 0.50 to 2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol. Al: 0-0.080%, Cr: 0-1.50%, Mo: 0-1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0-0.100%, B: 0 to 0.0050%, Ca: 0-0.0050%, and, The remainder has a chemical composition consisting of Fe and impurities, It has a microstructure consisting of tempered martensite, A hollow base material with a Vickers hardness of 400-550 HV in accordance with JIS Z 2244:2020, On the aforementioned base material, Fe ３ O ４ , FeO, and Fe ２ O ３ When the total peak intensity of the X-ray diffraction of ３ ３ O ４ is taken as 100%, Fe ４ ２ O ３ with a peak intensity ratio of 80% or more in the X-ray diffraction, FeO of 15% or less, Fe ３ O ３ of 5% or less, and the balance consists of impurities, It comprises an oxide film with a thickness of 3.50 μm or less. Vehicle parts.

4. A vehicle part according to claim 3, The aforementioned chemical composition is expressed in mass%, Sol. Al: 0.001–0.080%, Cr: 0.01-1.50%, Mo: 0.01-1.00%, Ni: 0.01-1.00%, Cu: 0.01 to 1.00%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.001-0.100%, B: 0.0001 to 0.0050%, and, Ca: 0.0001-0.0050%, It contains one or more elements selected from the group consisting of, Vehicle parts.

5. A method for manufacturing a steel pipe according to Claim 1 or Claim 2, In mass percent, C: 0.23-0.50%, Si: 0.01-0.50%, Mn: 0.50 to 2.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Sol. Al: 0-0.080%, Cr: 0-1.50%, Mo: 0-1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0-0.100%, B: 0 to 0.0050%, Ca: 0-0.0050%, and, The remaining portion is a process of preparing a hot-rolled steel sheet having a chemical composition consisting of Fe and impurities, The process involves heat-treating the hot-rolled steel sheet at 450 to 550°C for 0.5 to 3.0 minutes. The process includes the step of manufacturing the steel pipe by electric resistance welding the hot-rolled steel sheet after heat treatment. A method for manufacturing steel pipes.

6. A method for manufacturing a vehicle part according to claim 3 or claim 4, A step of preparing a steel pipe according to claim 1 or claim 2, A process of bending the steel pipe, The steel pipe after bending is, Ac ３ The process involves holding the temperature at +50°C to 1150°C for 10 seconds or more, followed by rapid cooling. The process includes a step of tempering the steel pipe after rapid cooling by holding it at 150 to 350°C for 10 minutes or more. A method for manufacturing vehicle parts.

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