WO2023157975A1

WO2023157975A1 - Steel pipe, component for vehicles, method for producing steel pipe and method for producing component for vehicles

Info

Publication number: WO2023157975A1
Application number: PCT/JP2023/006173
Authority: WO
Inventors: 守早川; 光洋濱石; 真也坂本
Original assignee: 日本製鉄株式会社
Priority date: 2022-02-21
Filing date: 2023-02-21
Publication date: 2023-08-24
Also published as: WO2023157297A1

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

Steel pipe, vehicle part, method for manufacturing steel pipe, and method for manufacturing vehicle part

The present disclosure relates to steel pipes, vehicle parts, methods of manufacturing steel pipes, and methods of manufacturing vehicle parts.

Vehicles such as automobiles are equipped with vehicle parts. Vehicle parts are, for example, stabilizers, inner tie rods, drive shafts and upper arms. Vehicle components are subjected to repetitive stress due to vibrations that occur during vehicle travel. Therefore, vehicle parts are required to have excellent fatigue strength.

In recent years, hollow vehicle parts have been used for the purpose of reducing the weight of the vehicle body. For example, stabilizers include solid stabilizers made from steel bars and the like, and hollow stabilizers made from steel pipes and the like. In recent years, the use of hollow stabilizers has increased.

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

The electric resistance welded steel pipe for a hollow stabilizer disclosed in Patent Document 1 has C: 0.20 to 0.40%, Si: 0.1 to 1.0%, and Mn: 0.1 to 2.0% by mass. %, 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%, the balance being Fe and unavoidable It has a chemical composition consisting of organic impurities, and the total decarburized layer depth on the inner and outer surfaces is 100 μm or less.

In Patent Document 1, 0.010% or more Sn is contained in an electric resistance welded steel pipe for a hollow stabilizer. This suppresses the formation of a decarburized layer and increases the fatigue strength.

The hollow stabilizer disclosed in Patent Document 2 has, as chemical components, C: 0.26 to 0.30%, Si: 0.05 to 0.35%, and Mn: 0.5 to 1.0% by mass. %, Cr: 0.05 to 1.0%, Ti: 0.005 to 0.05%, B: 0.0005 to 0.005%, Ca: 0.0005 to 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, the balance being Fe and unavoidable impurities, Mn It has a component composition in which the product of the content and the S content is 0.0025 or less, and the critical cooling rate Vc90 represented by (Equation 1) is 40° C./s or less. The metallographic structure of the hollow stabilizer consists of tempered martensite. The length of the stretched MnS present in the thickness central portion of the hollow stabilizer is 150 μm or less. The hollow stabilizer has a Rockwell C scale hardness (HRC) of 40 to 50, a wall thickness/outer diameter ratio of 0.14 or more, and a decarburized layer depth of 20 μm or less from the inner surface. .
logVc90=2.94−0.75β (Formula 1)
However, β=2.7C+0.4Si+Mn+0.8Cr.

In Patent Document 2, the formation of stretched MnS is suppressed to control the Rockwell C scale hardness, the thickness/outer diameter ratio, and the depth of the decarburized layer on the inner surface. This increases the fatigue strength of the hollow stabilizer.

WO2020/230795 WO2013/175821

The technologies disclosed in

Patent Documents

1 and 2 can increase the fatigue strength of vehicle parts. However, a vehicle component having excellent fatigue strength may be obtained by means different from those disclosed in

Patent Documents

1 and 2.

An object of the present disclosure is to provide a vehicle part with excellent fatigue strength, a steel pipe from which the vehicle part with excellent fatigue strength can be manufactured, a method for manufacturing the steel pipe, and a method for manufacturing the vehicle component.

The steel pipe of the present disclosure, in mass %,
C: 0.23 to 0.50%,
Si: 0.01 to 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 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
a chemical composition with the balance being Fe and impurities;
A base material having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite in area ratio;
on the base material,
When the sum of the X-ray diffraction peak intensities _of Fe ₃ O ₄ , Fe ₂ O ₃ and FeO is taken as 100%, the peak intensity ratio of the X-ray diffraction is ₇₀ % or more, and 20% or more. of Fe ₂ O ₃ , up to 10% FeO, and the balance consisting of impurities,
and 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.

The vehicle component of the present disclosure, in mass %,
C: 0.23 to 0.50%,
Si: 0.01 to 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 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
a chemical composition with the balance being Fe and impurities;
and 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 sum of the X-ray diffraction peak intensities of Fe ₃ O ₄ , FeO, and Fe ₂ O ₃ is taken as 100%, the peak intensity ratio of the X-ray diffraction _is 80% or more _, and 15% FeO below, Fe ₂ O ₃ below 5%, and the balance consisting of impurities,
and an oxide film having a thickness of 3.50 μm or less.

The steel pipe manufacturing method of the present disclosure includes:
in % by mass,
C: 0.23 to 0.50%,
Si: 0.01 to 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 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
a chemical composition with the balance being Fe and impurities;
preparing a steel sheet having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite in area ratio;
a step of heat-treating the steel plate at 450-600° C. for 0.5-3.0 minutes;
and a step of manufacturing a steel pipe by electric resistance welding the steel plate after the heat treatment.

A method for manufacturing a vehicle component of the present disclosure includes:
a step of preparing the steel pipe as described above;
a step of bending the steel pipe;
a step of holding the bent steel pipe at Ac ₃ +50° C. or higher and 1150° C. or lower for 10 seconds or more and then rapidly cooling the steel pipe;
and tempering the steel pipe after the quenching by holding it at 150 to 350° C. for 10 minutes or more.

The vehicle component of the present disclosure has excellent fatigue strength. The steel pipe of the present disclosure can be used to manufacture vehicle parts with excellent fatigue strength. The vehicle component manufacturing method of the present disclosure can manufacture a vehicle component having excellent fatigue strength. The method for manufacturing a steel pipe according to the present disclosure can manufacture a steel pipe from which vehicle parts having excellent fatigue strength can be manufactured.

FIG. 1 is a perspective view of the end of the steel pipe of this embodiment. FIG. 2 is a perspective view of an end portion of the vehicle component of this embodiment. FIG. 3 is a front view of a torsional fatigue test piece. FIG. 4 is a longitudinal side view of the torsional fatigue test piece.

The inventors have studied vehicle parts with excellent fatigue strength, and steel pipes from which vehicle parts with excellent fatigue strength can be manufactured. As a result, the following findings were obtained.

For example, hollow vehicle parts are manufactured by cold bending a steel pipe and then quenching and tempering it.

In Patent Document 1, "In particular, surface decarburization is considered to be an important factor among surface properties. If surface decarburization occurs during the heating stage of quenching, it is possible to improve the surface hardness even if quenching is performed. As a result, sufficient fatigue properties cannot be obtained” (paragraph [0005] of Patent Document 1). Patent Literature 2 states, "Fatigue fracture may occur from the inner surface of the hollow stabilizer, which does not exist in the solid stabilizer. This is because even if the fatigue strength of the outer surface is improved by increasing the strength of the steel pipe, This is because the decarburized layer becomes the starting point of fatigue fracture" (paragraph [0005] of Patent Document 2). It is known that the formation of a decarburized layer in this way lowers the fatigue strength of a vehicle component including a hollow stabilizer.

In Patent Document 1, "The surface decarburization reaction when steel is heated progresses by outward diffusion of carbon atoms in the steel toward the surface and reaction with oxygen. This outward diffusion of carbon It is effective to increase the lattice constant of iron to suppress the "(paragraph [0017] of Patent Document 1). Therefore, in Patent Document 1, 0.010% or more of Sn, which is effective for increasing the lattice constant of iron, is contained to suppress the formation of a decarburized layer.

Patent Document 2 states that "a decarburized layer is likely to be formed on the inner surface of the hollow stabilizer steel pipe when it is cooled from a high temperature at which the metal structure becomes an austenitic single phase and passes through a two-phase temperature range." (Paragraph [0053] of Patent Document 2). Patent Document 2 describes that the formation of a decarburized layer can be suppressed by increasing the cooling rate when passing through the two-phase temperature (paragraph [0054] of Patent Document 2).

The inventors have studied means for suppressing the formation of decarburized layers other than the means disclosed in

Patent Documents

1 and 2.

The decarburized layer is formed by quenching during manufacturing of vehicle parts. During heating and/or cooling for quenching, carbon in the steel out-diffuses to the surface and reacts with oxygen. This forms a decarburized layer. The present inventors paid attention to the behavior of oxygen during quenching. The present inventors thought that the formation of a decarburized layer could be suppressed by suppressing contact between the surface of a steel pipe for manufacturing vehicle parts and oxygen during quenching.

Therefore, the inventors investigated means for suppressing contact between the steel pipe surface and oxygen during quenching. The present inventors paid attention to 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 surface of the steel pipe and oxygen during quenching, thereby suppressing the formation of a decarburized layer.

As a result of diligent studies by the present inventors, when the sum of the X-ray diffraction peak intensities of Fe ₃ O ₄ , Fe ₂ O ₃ and FeO is taken as 100%, the X-ray diffraction peak intensity ratio is 70% or more. Fe ₃ O ₄ , 20% or more Fe ₂ O ₃ , 10% or less FeO, and the balance consisting of impurities, the thickness is 0.80 to 2.50 μm, and the standard deviation of the thickness is 0.90 μm or less It has been found that the formation of an oxide film extends the rupture life of steel pipes after quenching.

The reason for this is not clear, but the inventors believe as follows. Heating during quenching thermally expands steel pipes for manufacturing vehicle parts. If the difference between the coefficient of linear expansion of the steel pipe and the coefficient of linear expansion of the oxide film on the surface of the steel pipe is small, the difference between the amount of change in the surface area of the steel pipe due to heating during quenching and the amount of change in volume of the oxide film is small. In this case, the oxide film is less likely to separate from the steel pipe surface, and the oxide film tends 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 conceivable that the coefficient of linear expansion of the oxide film having the composition described above is close to the coefficient of linear expansion of the steel pipe. Since the oxide film remains on the steel pipe surface until just before quenching, contact between the steel pipe surface and oxygen is suppressed. Therefore, formation of a decarburized layer is suppressed. As a result, the fatigue strength of the steel pipe is increased and the rupture life is extended.

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

Furthermore, if there is a large deviation in the thickness of the oxide film, contact between the steel pipe surface and oxygen cannot be sufficiently suppressed at the portion where the oxide film is thin. In this case, intergranular oxidation occurs locally on the surface of the steel pipe. In the part where the grain boundary oxidation occurs, a dent occurs locally. The fatigue strength of the vehicle component decreases due to the concentration of stress on the recess. Therefore, it is considered that local intergranular oxidation of the steel pipe surface can be suppressed by reducing the deviation of the thickness of the oxide film.

Furthermore, the inventors have found the following matter. When a vehicle part is manufactured by quenching the steel pipe provided with the oxide film described above, when the sum of the X-ray diffraction peak intensities of Fe ₃ O ₄ , FeO, and Fe ₂ O ₃ is taken as 100%, the X-ray Formation of an oxide film having a diffraction peak intensity ratio of 80% or _more , FeO of 15% or less, Fe ₂ O ₃ of 5 _% or less, and the balance being impurities, and having a thickness of 3.50 μm or less. It turns out that it will be. This vehicle component has a long rupture life and excellent fatigue strength.

The steel pipe, steel pipe manufacturing method, vehicle component, and vehicle component manufacturing method of the present embodiment completed based on the above findings have the following configurations.

[1]
in % by mass,
C: 0.23 to 0.50%,
Si: 0.01 to 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 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
a chemical composition with the balance being Fe and impurities;
A base material having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite in area ratio;
on the base material,
When the sum of the X-ray diffraction peak intensities _of Fe ₃ O ₄ , Fe ₂ O ₃ and FeO is taken as 100%, the peak intensity ratio of the X-ray diffraction is ₇₀ % or more, and 20% or more. of Fe ₂ O ₃ , up to 10% FeO, and the balance consisting of impurities,
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;
steel pipe.

[2]
The steel pipe according to [1],
The chemical composition, in mass %,
Sol. Al: 0.001 to 0.080%,
Cr: 0.01 to 1.50%,
Mo: 0.01 to 1.00%,
Ni: 0.01 to 1.00%,
Cu: 0.01 to 1.00%,
Ti: 0.001 to 0.100%,
Nb: 0.001 to 0.100%,
V: 0.001 to 0.100%,
B: 0.0001 to 0.0050%, and
Ca: 0.0001 to 0.0050%,
containing one or more elements selected from the group consisting of
steel pipe.

[3]
in % by mass,
C: 0.23 to 0.50%,
Si: 0.01 to 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 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
a chemical composition with the balance being Fe and impurities;
and 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 sum of the X-ray diffraction peak intensities of Fe ₃ O ₄ , FeO, and Fe ₂ O ₃ is taken as 100%, the peak intensity ratio of the X-ray diffraction _is 80% or more _, and 15% FeO below, Fe ₂ O ₃ below 5%, and the balance consisting of impurities,
an oxide film having a thickness of 3.50 μm or less,
vehicle parts.

[4]
The vehicle component according to [3],
The chemical composition, in mass %,
Sol. Al: 0.001 to 0.080%,
Cr: 0.01 to 1.50%,
Mo: 0.01 to 1.00%,
Ni: 0.01 to 1.00%,
Cu: 0.01 to 1.00%,
Ti: 0.001 to 0.100%,
Nb: 0.001 to 0.100%,
V: 0.001 to 0.100%,
B: 0.0001 to 0.0050%, and
Ca: 0.0001 to 0.0050%,
containing one or more elements selected from the group consisting of
vehicle parts.

[5]
in % by mass,
C: 0.23 to 0.50%,
Si: 0.01 to 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 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
a chemical composition with the balance being Fe and impurities;
preparing a steel sheet having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite in area ratio;
a step of heat-treating the steel plate at 450-600° C. for 0.5-3.0 minutes;
A step of manufacturing a steel pipe by electric resistance welding the steel plate after the heat treatment.
A method of manufacturing steel pipes.

[6]
A method for manufacturing a vehicle part, comprising:
A step of preparing the steel pipe according to [1] or [2];
a step of bending the steel pipe;
a step of holding the bent steel pipe at a temperature of Ac ₃ +50° C. or higher and 1150° C. or lower for 10 seconds or longer and then quenching;
a step of holding and tempering the steel pipe after the quenching at 150 to 350° C. for 10 minutes or longer;
A method for manufacturing vehicle parts.

The steel pipe, the vehicle part, the method of manufacturing the steel pipe, and the method of manufacturing the vehicle part of this embodiment will be described in detail below. In addition, "%" regarding an element means the mass % unless there is particular notice.

[Structure of steel pipe of the present embodiment]
FIG. 1 is a perspective view of the end of the steel pipe of this embodiment. Referring to FIG. 1, steel pipe 1 includes base material 2 and oxide film 3 on base material 2 . 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, may be formed only on the inner surface 5, or may be formed on both the outer surface 4 and the inner surface 5. good. The decarburized layer on the outer surface of the vehicle component can be removed by shot peening, for example. On the other hand, the decarburized layer on the inner surface of the vehicle component may be difficult to remove. Steel pipe 1 is therefore preferably provided with an oxide layer 3 on at least the inner surface 5 .

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. Although the outer diameter of the steel pipe 1 is not particularly limited, it is, for example, 10 to 100 mm. Although the thickness of the steel pipe 1 is not particularly limited, it is, for example, 2 to 10 mm.

[Characteristics of the steel pipe of the present embodiment]
The steel pipe 1 of this embodiment has the following features.
(Feature 1)
The chemical composition of the base material 2 is, in 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 balance consists of Fe and impurities.
(Feature 2)
The microstructure of the base material 2 consists of 20% to 60% ferrite and 40% to 80% pearlite in area ratio.
(Feature 3)
Fe ₃ O ₄ having an X-ray diffraction peak intensity ratio of 70% or _more on the base material 2 when the sum of the X-ray diffraction peak intensities of Fe ₃ O 4 , Fe ₂ O ₃ and FeO is 100%. , 20% or more of Fe ₂ O ₃ , 10% or less of FeO, and the balance being impurities.
(Feature 4)
The oxide film 3 has a thickness of 0.80 to 2.50 μm.
(Feature 5)
The standard deviation of the thickness of oxide film 3 is 0.90 μm or less.
Features 1 to 5 will be described below.

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

C: 0.23-0.50%
Carbon (C) enhances the hardenability of steel. C further dissolves in steel. Thereby, C increases the strength of steel. If the C content is less than 0.23%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the C content exceeds 0.50%, the hot workability of the steel deteriorates even if the content of other elements is within the range of the present embodiment. If the C content exceeds 0.50%, the toughness of the vehicle component after quenching is further reduced even if the content of other elements is within the range of the present embodiment. Therefore, the C content is 0.23-0.50%.
The lower limit of the C content is preferably 0.25%, more preferably 0.27%, still more preferably 0.30%, still more preferably 0.33%, still more preferably 0.33%. It is 35%, more preferably 0.38%, and still more preferably 0.40%.
The upper limit of the C content is preferably 0.48%, more preferably 0.46%, still more preferably 0.44%, still more preferably 0.42%, still more preferably 0.42%. 40%, more preferably 0.38%.

Si: 0.01-0.50%
Silicon (Si) deoxidizes steel. Si also forms a solid solution in steel to increase the strength of the steel. If the Si content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content exceeds 0.50%, the ductility and toughness of the steel pipe 1 are lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Si content is 0.01-0.50%.
The lower limit of the Si content is preferably 0.05%, more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20%, still more preferably 0 .25%.
The upper limit of the Si content is preferably 0.45%, more preferably 0.40%, still more preferably 0.35%, still more preferably 0.30%.

Mn: 0.50-2.50%
Manganese (Mn) increases the hardenability of steel. Mn further dissolves in steel. Thereby, Mn increases the strength of steel. If the Mn content is less than 0.50%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 2.50%, the toughness and ductility of the vehicle component after quenching decrease even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Mn content is 0.50-2.50%.
The lower limit of the Mn content is preferably 0.60%, more preferably 0.70%, still more preferably 0.75%, still more preferably 0.80%, still more preferably 0.80%. It is 90%, more preferably 1.00%, more preferably 1.10%.
The upper limit of the Mn content is preferably 2.40%, more preferably 2.30%, still more preferably 2.20%, still more preferably 2.10%, still more preferably 2.10%. 00%, more preferably 1.90%, more preferably 1.80%, still more preferably 1.70%, still more preferably 1.60%, still more preferably 1.0%. 50%.

P: 0.050% or less Phosphorus (P) is an impurity. Therefore, the P content is over 0%. If the P content exceeds 0.050%, P segregates at the grain boundaries and reduces the ductility of the steel even if the content of other elements is within the range of the present embodiment. Therefore, the P content is 0.050% or less.
The lower the P content, the better. However, drastic reduction of the P content greatly increases manufacturing costs. Therefore, when considering industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, still more preferably 0.003%, still more preferably 0.005 %.
The upper limit of the P content is preferably 0.040%, more preferably 0.030%, still more preferably 0.020%, still more preferably 0.010%.

S: 0.0100% or less Sulfur (S) is an impurity. Therefore, the S content is over 0%. If the S content exceeds 0.0100%, the hot workability, toughness and fatigue strength of the steel are lowered even if the contents of other elements are within the range of the present embodiment. Therefore, the S content is 0.0100% or less.
The lower the S content, the better. However, drastic reduction of the S content greatly increases manufacturing costs. Therefore, when considering industrial production, the lower limit of the S content is preferably 0.0001%, more preferably 0.0002%, still more preferably 0.0003%, still more preferably 0.0005 %.
The upper limit of the S content is preferably 0.0080%, more preferably 0.0070%, still more preferably 0.0060%, still more preferably 0.0050%, still more preferably 0.0050%. 0040%.

N: 0.0100% or less Nitrogen (N) is an impurity. Therefore, the N content is over 0%. If the N content exceeds 0.0100%, the toughness of the steel is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the N content is 0.0100% or less. On the one hand, N forms nitrides and/or carbonitrides to increase the strength of steel.
The preferred lower limit of the N content is 0.0001%, more preferably 0.0002%, still more preferably 0.0003%, still more preferably 0.0005%, still more preferably 0.0010 %, more preferably 0.0020%, more preferably 0.0030%.
The upper limit of the N content is preferably 0.0080%, more preferably 0.0070%, still more preferably 0.0060%, still more preferably 0.0050%, still more preferably 0.0050%. 0040%.

O: 0.0100% or less Oxygen (O) is an impurity. Therefore, the O content is over 0%. If the O content exceeds 0.0100%, the toughness of the steel is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the O content is 0.0100% or less.
The lower the O content, the better. However, drastic reduction of O content greatly increases manufacturing cost. Therefore, when considering industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0002%, still more preferably 0.0003%, still more preferably 0.0005% is.
The upper limit of the O content is preferably 0.0080%, more preferably 0.0070%, still more preferably 0.0060%, still more preferably 0.0050%, still more preferably 0.0050%. 0040%, more preferably 0.0030%.

The rest of the chemical composition of the steel pipe 1 of this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when the steel pipe 1 is industrially manufactured, and are not intentionally included. , means a permissible range that does not adversely affect the steel pipe 1 of the present embodiment.

[Optional Elements]
In the chemical composition of the base material 2 of the steel pipe 1 of the present embodiment, instead of part of Fe,
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%, and
Ca: 0 to 0.0050%,
It may contain one or more elements selected from the group consisting of
These arbitrary elements are described below.

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

Sol. Al: 0-0.080%
Aluminum (Al) is an optional element and may not be contained. That is, the Al content may be 0%. When Al is contained, that is, when the Al content is greater than 0%, Al deoxidizes the steel. Al further combines with nitrogen (N) to produce AlN. AlN suppresses coarsening of crystal grains during quenching. If even a small amount of Al is contained, 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 the present embodiment, Al combines with oxygen (O) to excessively generate inclusions. This reduces the fatigue strength of the vehicle component. Therefore, the Al content is 0-0.080%.
The lower limit of the Al content is preferably more than 0%, more preferably 0.001%, still more preferably 0.005%, still more preferably 0.010%, still more preferably 0.015 %.
The upper limit of the Al content is preferably 0.070%, more preferably 0.060%, still more preferably 0.050%, still more preferably 0.040%, still more preferably 0.040%. 030%.

[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 contain one or more elements selected from the group consisting of Cr, Mo, Ni and Cu instead of part of Fe. All of these elements are optional elements and may not be contained. Both of these elements, when included, increase the strength of the steel.

Cr: 0-1.50%
Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When Cr is included, that is, when the Cr content is greater than 0%, Cr increases the strength of the steel. If even a little Cr is contained, 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 is lowered even if the contents of other elements are within the range of the present embodiment. Therefore, the Cr content is 0-1.50%.
The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%, still more preferably 0.10%, still more preferably 0.20%, still more preferably 0.20%. 30%.
The upper limit of the Cr content is preferably 1.20%, more preferably 1.00%, still more preferably 0.80%, still more preferably 0.60%, still more preferably 0.60%. 40%.

Mo: 0-1.00%
Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When Mo is contained, that is, when the Mo content is over 0%, Mo enhances the strength of the steel. If even a little Mo is contained, 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 is lowered even if the contents of other elements are within the range of the present embodiment. Therefore, the Mo content is 0-1.00%.
The lower limit of the Mo content is preferably 0.01%, more preferably 0.02%, still more preferably 0.03%, still more preferably 0.04%, still more preferably 0.04%. 05%.
The upper limit of the Mo content is preferably 0.80%, more preferably 0.60%, still more preferably 0.40%, still more preferably 0.20%, still more preferably 0.20%. 10%.

Ni: 0-1.00%
Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When Ni is contained, that is, when the Ni content is over 0%, Ni increases the strength of the steel. If Ni is contained even in a small amount, 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 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Ni content is 0-1.00%.
The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, still more preferably 0.05%, still more preferably 0.10%, still more preferably 0.05%. 15%.
The upper limit of the Ni content is preferably 0.80%, more preferably 0.60%, still more preferably 0.40%, still more preferably 0.20%.

Cu: 0-1.00%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When Cu is contained, that is, when the Cu content is greater than 0%, Cu increases the strength of the steel. If even a small amount of Cu is contained, the above effects can be obtained to some extent. On the other hand, if the Cu content exceeds 1.00%, the ductility of the steel is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Cu content is 0-1.00%.
The lower limit of the Cu content is preferably 0.01%, more preferably 0.02%, still more preferably 0.03%, still more preferably 0.04%, still more preferably 0.04%. 05%.
The upper limit of the Cu content is preferably 0.80%, more preferably 0.60%, still more preferably 0.40%, still more preferably 0.20%.

[Third group: Ti, Nb and V]
The chemical composition of the base material 2 of the steel pipe 1 according to the present embodiment may further contain one or more elements selected from the group consisting of Ti, Nb and V instead of part of Fe. All of these elements are optional elements and may not be contained. Both of these elements, when included, increase the strength and workability of the steel.

Ti: 0-0.100%
Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When Ti is contained, ie when the Ti content is greater than 0%, Ti forms carbides, nitrides and/or carbonitrides. This increases the strength and workability of the steel. If even a small amount of Ti is contained, 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 is lowered even if the contents of other elements are within the range of the present embodiment. Therefore, the Ti content is 0-0.100%.
The lower limit of the Ti content is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%, still more preferably 0.020%, still more preferably 0.020%. 030%.
The upper limit of the Ti content is preferably 0.090%, more preferably 0.080%, still more preferably 0.070%, still more preferably 0.060%.

Nb: 0-0.100%
Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. If Nb is included, ie if the Nb content is greater than 0%, Nb forms carbides, nitrides and/or carbonitrides. This increases the strength and workability of the steel. If even a small amount of Nb is contained, 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 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Nb content is 0-0.100%.
The lower limit of the Nb content is preferably 0.001%, more preferably 0.002%, still more preferably 0.005%, still more preferably 0.010%, still more preferably 0.01%. 015%.
The upper limit of the Nb content is preferably 0.090%, more preferably 0.070%, still more preferably 0.050%, still more preferably 0.030%, still more preferably 0.030%. 020%.

V: 0-0.100%
Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When V is included, ie when the V content is greater than 0%, V forms carbides, nitrides and/or carbonitrides. This increases the strength and workability of the steel. If even a small amount of V is contained, 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 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the V content is 0-0.100%.
The lower limit of the V content is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%, still more preferably 0.015%, still more preferably 0.01%. 020%.
The upper limit of the V content is preferably 0.090%, more preferably 0.080%, still more preferably 0.070%, still more preferably 0.060%, still more preferably 0.060%. 050%, more preferably 0.040%.

[Fourth group: B]
The chemical composition of the base material 2 of the steel pipe 1 according to this embodiment may further contain B instead of part of Fe.

B: 0 to 0.0050%
Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When B is contained, that is, when the B content is over 0%, B enhances the hardenability of the steel. If even a small amount of B is contained, the above effect can be obtained to some extent. On the other hand, if the B content exceeds 0.0050%, the steel tends to embrittle even if the content of other elements is within the range of the present embodiment. Therefore, the B content is 0-0.0050%.
The lower limit of the B content is preferably 0.0001%, more preferably 0.0002%, still more preferably 0.0003%, still more preferably 0.0005%, still more preferably 0.0005%. 0010%.
The upper limit of the B content is preferably 0.0040%, more preferably 0.0030%, still more preferably 0.0020%.

[Group 5: Ca]
The chemical composition of the base material 2 of the steel pipe 1 according to this embodiment may further contain Ca instead of part of Fe.

Ca: 0-0.0050%
Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When Ca is contained, that is, when the Ca content is over 0%, Ca enhances the hot workability of the steel. If even a little Ca is contained, 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 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Ca content is 0-0.0050%.
The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, still more preferably 0.0003%, still more preferably 0.0005%, still more preferably 0.0005%. 0010%, more preferably 0.0015%.
The upper limit of the Ca content is preferably 0.0040%, more preferably 0.0030%, still more preferably 0.0025%.

[Method for measuring chemical composition of steel pipe base material]
The chemical composition of the base material 2 of the steel pipe 1 of this embodiment can be measured by a known component analysis method. A steel pipe 1 is cut into lengths of 10 cm in the axial direction of the steel pipe 1 . The oxide film 3 on the outer surface 4 and the 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 pulverized 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 obtained by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using the well-known inert gas fusion-thermal conductivity method. The O content is determined using a well-known inert gas fusion-nondispersive infrared absorption method.

It should be noted that each element content, based on the significant digits defined in this embodiment, rounded off the measured numerical value, the numerical value up to the minimum digit of each element content defined in this embodiment do. For example, the C content of the steel pipe 1 of the present embodiment is specified by a numerical value up to the second decimal place. Therefore, the C content is a numerical value to the second decimal place obtained by rounding the measured numerical value to the third decimal place.

Similarly, the contents of elements other than the C content of the steel pipe 1 of the present embodiment are obtained by rounding the measured values to the minimum digit specified in the present embodiment. The value is taken as the elemental content. Rounding means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[(Feature 2) Microstructure of base material of steel pipe]
The base material 2 of the steel pipe 1 of this embodiment has a microstructure composed of 20% to 60% ferrite and 40% to 80% pearlite in area ratio. As described above, for example, a vehicle component can be manufactured by quenching and tempering the steel pipe 1 after cold bending. 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 in area ratio, the steel pipe 1 has excellent workability.

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

[Method for measuring area ratio of ferrite and pearlite]
The area ratios of ferrite and pearlite in the base material 2 of the steel pipe 1 are obtained by the following method. A test piece having a length of 10 cm in the axial direction of the steel pipe 1 including the thickness central portion of the cross section perpendicular to the axial direction of the steel pipe 1 is taken at any three locations of the steel pipe 1 . That is, three specimens are taken. Among the surfaces of the test piece, the surface corresponding to the cross section perpendicular to the axial direction of the steel pipe 1 is used as the observation surface. The viewing surface of each specimen is mirror-polished. Etching is performed on the mirror-polished observation surface using 3% nitric acid alcohol (nital etchant). Let the thickness center part of the steel pipe 1 be an observation field among the etched observation surfaces. The size of the observation field is 200 μm×200 μm. The field of view is observed with a 500x optical microscope.

In the observation field, each structure such as pearlite and ferrite can be easily distinguished by contrast. For example, ferrite is observed as white areas. Pearlite is observed as a region with a lamellar texture that is less bright than ferrite. Identify each tissue in the field of view. Then, the area ratio (%) of ferrite is obtained based on the area of ferrite in the observation field of view and the total area of the observation field of view. Based on the pearlite area in the observation field of view and the total area of the observation field of view, the pearlite area ratio (%) is determined. The arithmetic mean value of the values obtained from the three test pieces is used as the area ratio of ferrite and the area ratio of pearlite.

[(Feature 3) Composition of Oxide Film of Steel Pipe]
A steel pipe 1 of this embodiment has an oxide film 3 on a base material 2 . The oxide film 3 _has an X-ray diffraction peak intensity ratio of 70% or more when the sum of the X-ray diffraction peak intensities _of Fe ₃ O ₄ , Fe ₂ O ₃ and FeO is 100%. 20% or more of Fe ₂ O ₃ , 10% or less of FeO, and the balance consists of impurities. An oxide film 3 having the composition described above is formed on a steel pipe 1 before quenching. This increases the fatigue strength of the vehicle component.

The lower limit of the peak intensity ratio of Fe ₃ O ₄ is preferably 72%, more preferably 74%, still more preferably 75%. Although the upper limit of the peak intensity ratio of Fe ₃ O ₄ is not particularly limited, it is, for example, 80%. The upper limit of the peak intensity ratio of Fe ₃ O ₄ is preferably 78%, more preferably 76%, still more preferably 75%.

The lower limit of the peak intensity ratio of Fe ₂ O ₃ is preferably 21%, more preferably 22%, still more preferably 23%, still more preferably 25%. Although the upper limit of the peak intensity ratio of Fe ₂ O ₃ is not particularly limited, it is, for example, 30%. The upper limit of the peak intensity ratio of Fe ₂ O ₃ is preferably 29%, more preferably 28%, still more preferably 27%.

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

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

From the obtained X-ray diffraction profile, the X-ray diffraction peak intensities of Fe ₃ O ₄ , Fe ₂ O ₃ and FeO are determined. The sum of the X-ray diffraction peak intensities of Fe ₃ O ₄ , Fe ₂ O ₃ and FeO is taken as 100%. The peak intensity ratio of Fe ₃ O ₄ , Fe ₂ O ₃ and FeO is determined from the sum of the peak intensities and the X-ray diffraction peak intensities of Fe ₃ O ₄ , Fe ₂ O ₃ and FeO. Let the arithmetic mean value of the numerical value of three places be each peak intensity ratio. Note that the intensity ratio of each peak does not necessarily match the area ratio of each peak or the mass % determined by quantitative analysis.

[(Feature 4) Thickness of oxide film on steel pipe]
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 exceeds 2.50 μm, the adhesion of the oxide film 3 is reduced, and the oxide film 3 is peeled off from the

surfaces

4 and 5 of the steel pipe 1 . Therefore, oxide film 3 has a thickness of 0.80 to 2.50 μm.
The lower limit of the thickness of oxide film 3 is preferably 0.84 μm, more preferably 0.88 μm, still more preferably 1.00 μm, still 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, still more preferably 2.20 μm, still more preferably 2.00 μm, still more preferably 1.80 μm. is.

[Method for measuring thickness of oxide film on steel pipe]
The thickness of the oxide film 3 of the steel pipe 1 is obtained by the following method. A test piece is taken by cutting the steel pipe 1 perpendicularly to the axial direction. Three test pieces are taken in the axial direction of the steel pipe 1 at a pitch of 100 mm. In each test piece, the cut surface perpendicular to the axial direction of the steel pipe 1 is the observation surface. Fill with resin so that the observation surface can be observed. After filling with resin, the observation surface is polished. A scanning electron microscope (SEM)-energy dispersive X-ray spectrometer (EDS) is used on the observation surface after polishing to generate a secondary electron image of the observation field including the oxide film 3 of the observation surface. do. The size of the observation field is 50 μm×40 μm. Here, the observation field of view is 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.

In the secondary electron image, the base material 2 and the oxide film 3 can be easily distinguished by contrast. Note that the base material 2 and the oxide film 3 may be distinguished from each other by performing elemental mapping of oxygen (O) in the observation field using an EDS device attached to the SEM. In elemental mapping of oxygen (O) by EDS, a region with a high oxygen concentration corresponds to the oxide film 3 and a region with a low oxygen concentration corresponds to the base material 2 . Since a region with a high oxygen concentration and a region with a low oxygen concentration are clearly separated, the oxide film 3 can be easily distinguished.

After identifying the oxide film 3, the thickness of the identified oxide film 3 is measured at 10 locations in the C direction at a pitch of 3 μm. The arithmetic mean value of the thickness of the oxide film 3 at the measurement points (30 points in total) of the three test pieces is taken as the thickness of the oxide film 3 .

[(Feature 5) Standard deviation of oxide film thickness of steel pipe]
If the standard deviation of the thickness of the oxide film 3 exceeds 0.90 μm, the contact between the

surfaces

4 and 5 of the steel pipe 1 and oxygen cannot be suppressed in the portion where the oxide film 3 is thin. In this case, intergranular oxidation occurs locally on the

surfaces

4 and 5 of the steel pipe 1 . In the part where the grain boundary oxidation occurs, a dent occurs locally. The fatigue strength of the vehicle component decreases due to the concentration of stress on the recess. Therefore, the standard deviation of the thickness of oxide film 3 is 0.90 μm or less. The lower limit of the standard deviation of the thickness of 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, More preferably, it is 0.05 μm. The upper limit of the thickness of the oxide film 3 is preferably 0.80 μm, more preferably 0.70 μm, still more preferably 0.60 μm, still more preferably 0.50 μm.

[Method for measuring standard deviation of oxide film thickness of steel pipe]
The method for measuring the standard deviation of the oxide film thickness of steel pipes is obtained by the following method. The thickness of the oxide film 3 is measured by the above-described [Method for measuring thickness of oxide film of steel pipe]. Let the standard deviation of the thickness of the oxide film 3 at 30 locations be the standard deviation of the thickness of the oxide film 3 . In the present disclosure, standard deviation is sample standard deviation (JIS Z8101-1:2015).

[Manufacturing method of steel pipe 1]
An example of a method for manufacturing the steel pipe 1 of this embodiment will be described. The following example describes a method of manufacturing an electric resistance welded steel pipe. The method for manufacturing the steel pipe 1 described below is an example for manufacturing the steel pipe 1 of the present embodiment. Therefore, the steel pipe 1 having the structure described above 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 the manufacturing method of the steel pipe 1 of this embodiment.

An example of the method for manufacturing the steel pipe 1 of this embodiment includes the following steps.
(Step 1) Steel plate preparation step (Step 2) Low temperature heat treatment step (Step 3) Pipe making step Each step will be described below.

[(Step 1) Steel plate preparation step]
In the steel plate preparation step, a steel plate for manufacturing the steel pipe 1 of the present embodiment is prepared. The steel plate may be obtained from a third party or manufactured. When producing, molten steel is produced in which the content of each element in the chemical composition is within the range of the present embodiment. A refining method is not particularly limited, and a well-known method may be used. Using molten steel, a material is manufactured by a well-known casting method. For example, an ingot may be manufactured by an ingot casting method using molten steel. Moreover, you may manufacture a bloom by the continuous casting method using molten steel. A raw material (ingot or bloom) is manufactured by the above method. The raw material is heated and subjected to rough rolling and finish rolling by well-known methods. The coiling temperature of the steel sheet is, for example, over 600 to 700°C. Through the above manufacturing process, a steel sheet having a microstructure composed of 20% to 60% ferrite and 40% to 80% pearlite in area ratio is manufactured.

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

The manufactured hot-rolled steel sheet is in a state of being wound into a coil. In the low-temperature heat treatment step, the steel sheet is unwound and subjected to low-temperature heat treatment while the surface of the steel sheet is exposed to the atmosphere. The low temperature heat treatment conditions are as described above. By low-temperature heat treatment, the surface of the steel sheet has an X-ray diffraction peak intensity ratio of 70% or more Fe ₃ O ₄ , 20% or more Fe ₂ O ₃ , 10% or less FeO, and the balance is impurities, and the thickness is An oxide film 3 having a thickness of 0.80 to 2.50 μm and a standard deviation of thickness of 0.90 μm or less is formed. Formation of the oxide film described above increases the fatigue strength of the vehicle component.

The preferred lower limit of the heat treatment temperature is over 450°C, more preferably 460°C, and even more preferably 470°C.

[(Step 3) Pipe making step]
Electric resistance welded steel pipes are manufactured using hot-rolled steel sheets subjected to low-temperature heat treatment. In the pipe-making process, a forming roll is used to form a hot-rolled steel sheet into a cylindrical blank pipe (open pipe). The formed blank tube is formed so that the width direction of the hot-rolled steel sheet coincides with the circumferential direction of the blank tube. Electric resistance welding is performed on butt portions extending in the longitudinal direction of the tube. An electric resistance welded steel pipe is manufactured by the pipe manufacturing process described above.

The steel pipe 1 of the present embodiment can be manufactured through the above steps.

The method for manufacturing the steel pipe 1 of this embodiment may further include other steps. Another process is, for example, a diameter-reducing rolling process. In the diameter-reducing rolling step, for example, diameter-reducing rolling may be performed under well-known conditions.

[Use of the steel pipe of the present embodiment]
The steel pipe 1 of the present disclosure is used as a material for vehicle parts. Vehicle parts are, for example, stabilizers, inner tie rods, drive shafts, upper arms, and the like. The steel pipe 1 is suitable for stabilizer applications.

[Effects of the steel pipe of the present embodiment]
The steel pipe 1 of this embodiment has the following features.
(Feature 1)
The chemical composition of the base material 2 is, in 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 balance consists of Fe and impurities.
(Feature 2)
The microstructure of the base material 2 consists of 20% to 60% ferrite and 40% to 80% pearlite in area ratio.
(Feature 3)
Fe ₃ O ₄ having an X-ray diffraction peak intensity ratio of 70% or _more on the base material 2 when the sum of the X-ray diffraction peak intensities of Fe ₃ O 4 , Fe ₂ O ₃ and FeO is 100%. , 20% or more of Fe ₂ O ₃ , 10% or less of FeO, and the balance of impurities.
(Feature 4)
The oxide film 3 has a thickness of 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. That is, with the steel pipe 1 of the present embodiment, excellent fatigue strength can be obtained in a vehicle component manufactured using the steel pipe 1 as a raw material.

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

The vehicle parts 10 are, for example, stabilizers, inner tie rods, drive shafts, upper arms, and the like.

[Characteristics of the vehicle component of the present embodiment]
The vehicle component 10 of this embodiment has the following features.
(Feature 6)
The chemical composition of the base material 20 is, in 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 balance consists of Fe and impurities.
(Feature 7) The microstructure of the base material 20 consists of tempered martensite and has a Vickers hardness of 400 to 550 HV according to JIS Z 2244:2020.
(Feature 8)
Fe ₃ O 4 having an X-ray diffraction peak intensity ratio of 80% or _more when the sum of the X-ray diffraction peak intensities of Fe ₃ O ₄ , FeO, and Fe ₂ O ₃ on the base material 20 is 100%. , 15% or less of FeO, 5% or less of Fe ₂ O ₃ , and the balance being impurities.
(Feature 9)
The thickness of oxide film 30 is 3.50 μm or less.
Characteristics 6 to 9 will be described below.

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

C: 0.23-0.50%
Carbon (C) enhances the hardenability of steel. C further dissolves in steel. Thereby, C increases the strength of steel. If the C content is less than 0.23%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the C content exceeds 0.50%, the hot workability of the steel deteriorates even if the content of other elements is within the range of the present embodiment. If the C content exceeds 0.50%, the toughness of the vehicle component 10 after quenching is further reduced even if the content of other elements is within the range of the present embodiment. Therefore, the C content is 0.23-0.50%.
The lower limit of the C content is preferably 0.25%, more preferably 0.27%, still more preferably 0.30%, still more preferably 0.33%, still more preferably 0.33%. It is 35%, more preferably 0.38%, and still more preferably 0.40%.
The upper limit of the C content is preferably 0.48%, more preferably 0.46%, still more preferably 0.44%, still more preferably 0.42%, still more preferably 0.42%. 40%, more preferably 0.38%.

Si: 0.01-0.50%
Silicon (Si) deoxidizes steel. Si also forms a solid solution in steel to increase the strength of the steel. If the Si content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content exceeds 0.50%, the ductility and toughness of the vehicle component 10 are lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Si content is 0.01-0.50%.
The lower limit of the Si content is preferably 0.05%, more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20%, still more preferably 0 .25%.
The upper limit of the Si content is preferably 0.45%, more preferably 0.40%, still more preferably 0.35%, still more preferably 0.30%.

Mn: 0.50-2.50%
Manganese (Mn) increases the hardenability of steel. Mn further dissolves in steel. Thereby, Mn increases the strength of steel. If the Mn content is less than 0.50%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 2.50%, the toughness and ductility of the vehicle component 10 after quenching decrease even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Mn content is 0.50-2.50%.
The lower limit of the Mn content is preferably 0.60%, more preferably 0.70%, still more preferably 0.75%, still more preferably 0.80%, still more preferably 0.80%. It is 90%, more preferably 1.00%, more preferably 1.10%.
The upper limit of the Mn content is preferably 2.40%, more preferably 2.30%, still more preferably 2.20%, still more preferably 2.10%, still more preferably 2.10%. 00%, more preferably 1.90%, more preferably 1.80%, still more preferably 1.70%, still more preferably 1.60%, still more preferably 1.0%. 50%.

The rest of the chemical composition of the base material 20 of the vehicle component 10 of the present embodiment consists of Fe and impurities. Here, the impurities in the chemical composition are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when the base material 20 of the vehicle component 10 is industrially manufactured. It does not mean that it is contained, but that it is permissible within a range that does not adversely affect the base material 20 of the vehicle component 10 of the present embodiment.

[Optional Elements]
In the chemical composition of the base material 20 of the vehicle component 10 of the present embodiment, instead of part of Fe,
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%, and
Ca: 0 to 0.0050%,
It may contain one or more elements selected from the group consisting of
These arbitrary elements are described below.

[Group 1: Al]
The chemical composition of the base material 20 of the vehicle component 10 according to the present embodiment may further contain Al instead of part of Fe.

Sol. Al: 0-0.080%
Aluminum (Al) is an optional element and may not be contained. That is, the Al content may be 0%. When Al is contained, that is, when the Al content is greater than 0%, Al deoxidizes the steel. Al further combines with nitrogen (N) to produce AlN. AlN suppresses coarsening of crystal grains during quenching. If even a small amount of Al is contained, 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 the present embodiment, Al combines with oxygen (O) to excessively generate inclusions. This reduces the fatigue strength of the vehicle component 10 . Therefore, the Al content is 0-0.080%.
The lower limit of the Al content is preferably more than 0%, more preferably 0.001%, still more preferably 0.005%, still more preferably 0.010%, still more preferably 0.015 %.
The upper limit of the Al content is preferably 0.070%, more preferably 0.060%, still more preferably 0.050%, still more preferably 0.040%, still more preferably 0.040%. 030%.

[Group 2: Cr, Mo, Ni and Cu]
The chemical composition of the base material 20 of the vehicle component 10 according to this embodiment may further contain one or more elements selected from the group consisting of Cr, Mo, Ni and Cu instead of part of Fe. All of these elements are optional elements and may not be contained. When included, both of these elements increase the strength of vehicle component 10 .

Cr: 0-1.50%
Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When Cr is contained, that is, when the Cr content is over 0%, Cr enhances the strength of the vehicle component 10 . If even a little Cr is contained, 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 component 10 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Cr content is 0-1.50%.
The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%, still more preferably 0.10%, still more preferably 0.20%, still more preferably 0.20%. 30%.
The upper limit of the Cr content is preferably 1.20%, more preferably 1.00%, still more preferably 0.80%, still more preferably 0.60%, still more preferably 0.60%. 40%.

Mo: 0-1.00%
Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When Mo is contained, that is, when the Mo content is over 0%, Mo enhances the strength of the vehicle component 10 . If even a little Mo is contained, 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 component 10 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Mo content is 0-1.00%.
The lower limit of the Mo content is preferably 0.01%, more preferably 0.02%, still more preferably 0.03%, still more preferably 0.04%, still more preferably 0.04%. 05%.
The upper limit of the Mo content is preferably 0.80%, more preferably 0.60%, still more preferably 0.40%, still more preferably 0.20%, still more preferably 0.20%. 10%.

Ni: 0-1.00%
Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When Ni is contained, that is, when the Ni content is over 0%, Ni enhances the strength of the vehicle component 10 . If Ni is contained even in a small amount, 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 component 10 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Ni content is 0-1.00%.
The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, still more preferably 0.05%, still more preferably 0.10%, still more preferably 0.05%. 15%.
The upper limit of the Ni content is preferably 0.80%, more preferably 0.60%, still more preferably 0.40%, still more preferably 0.20%.

Cu: 0-1.00%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When Cu is contained, that is, when the Cu content is over 0%, Cu enhances the strength of the vehicle component 10 . If even a small amount of Cu is contained, the above effects can be obtained to some extent. On the other hand, if the Cu content exceeds 1.00%, the ductility of the vehicle component 10 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Cu content is 0-1.00%.
The lower limit of the Cu content is preferably 0.01%, more preferably 0.02%, still more preferably 0.03%, still more preferably 0.04%, still more preferably 0.04%. 05%.
The upper limit of the Cu content is preferably 0.80%, more preferably 0.60%, still more preferably 0.40%, still more preferably 0.20%.

[Third group: Ti, Nb and V]
The chemical composition of the base material 20 of the vehicle component 10 according to this embodiment may further contain one or more elements selected from the group consisting of Ti, Nb and V instead of part of Fe. All of these elements are optional elements and may not be contained. When included, both of these elements enhance the strength and workability of the vehicle component 10 .

Ti: 0-0.100%
Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When Ti is contained, ie when the Ti content is greater than 0%, Ti forms carbides, nitrides and/or carbonitrides. This enhances the strength and workability of the vehicle component 10 . If even a small amount of Ti is contained, 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 component 10 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Ti content is 0-0.100%.
The lower limit of the Ti content is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%, still more preferably 0.020%, still more preferably 0.020%. 030%.
The upper limit of the Ti content is preferably 0.090%, more preferably 0.080%, still more preferably 0.070%, still more preferably 0.060%.

Nb: 0-0.100%
Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. If Nb is included, ie if the Nb content is greater than 0%, Nb forms carbides, nitrides and/or carbonitrides. This enhances the strength and workability of the vehicle component 10 . If even a small amount of Nb is contained, 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 component 10 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Nb content is 0-0.100%.
The lower limit of the Nb content is preferably 0.001%, more preferably 0.002%, still more preferably 0.005%, still more preferably 0.010%, still more preferably 0.01%. 015%.
The upper limit of the Nb content is preferably 0.090%, more preferably 0.070%, still more preferably 0.050%, still more preferably 0.030%, still more preferably 0.030%. 020%.

V: 0-0.100%
Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When V is included, ie when the V content is greater than 0%, V forms carbides, nitrides and/or carbonitrides. This enhances the strength and workability of the vehicle component 10 . If even a small amount of V is contained, 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 component 10 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the V content is 0-0.100%.
The lower limit of the V content is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%, still more preferably 0.015%, still more preferably 0.01%. 020%.
The upper limit of the V content is preferably 0.090%, more preferably 0.080%, still more preferably 0.070%, still more preferably 0.060%, still more preferably 0.060%. 050%, more preferably 0.040%.

[Fourth group: B]
The chemical composition of the base material 20 of the vehicle component 10 according to this embodiment may further contain B instead of part of Fe.

B: 0 to 0.0050%
Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When B is contained, that is, when the B content is over 0%, B enhances the hardenability of the steel. If even a small amount of B is contained, the above effect can be obtained to some extent. On the other hand, if the B content exceeds 0.0050%, the vehicle component 10 tends to become embrittled even if the content of other elements is within the range of the present embodiment. Therefore, the B content is 0-0.0050%.
The lower limit of the B content is preferably 0.0001%, more preferably 0.0002%, still more preferably 0.0003%, still more preferably 0.0005%, still more preferably 0.0005%. 0010%.
The upper limit of the B content is preferably 0.0040%, more preferably 0.0030%, still more preferably 0.0020%.

[Group 5: Ca]
The chemical composition of the base material 20 of the vehicle component 10 according to this embodiment may further contain Ca instead of part of Fe.

Ca: 0-0.0050%
Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When Ca is contained, that is, when the Ca content is over 0%, Ca enhances the hot workability of the steel. If even a little Ca is contained, 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 component 10 is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Ca content is 0-0.0050%.
The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, still more preferably 0.0003%, still more preferably 0.0005%, still more preferably 0.0005%. 0010%, more preferably 0.0015%.
The upper limit of the Ca content is preferably 0.0040%, more preferably 0.0030%, still more preferably 0.0025%.

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

As with the content of each element in the base material 2 of the steel pipe 1, the content of each element is defined in this embodiment by rounding off the fractions of the measured numerical values based on the significant digits defined in this embodiment. It is a numerical value to the lowest digit of each element content.

[(Feature 7) Microstructure and Vickers Hardness of Base Material of Vehicle Parts]
The base material 20 of the vehicle component 10 of this embodiment has a microstructure composed of tempered martensite, and has a Vickers hardness of 400 to 550 HV according to JIS Z 2244:2020.

[Regarding the microstructure of vehicle parts]
The microstructure of the base material 20 of the vehicle component 10 of this embodiment consists of tempered martensite. In the microstructure of the base material 20 of the vehicle component 10, the area ratio of phases other than tempered martensite is so small that it can be ignored.

[Method for measuring area ratio of tempered martensite]
The area ratio of tempered martensite in the base material 20 of the vehicle component 10 is obtained by the following method. A test piece having a length of 10 cm in the longitudinal direction of the vehicle component 10 and including a thickness central portion of a cross section perpendicular to the longitudinal direction of the vehicle component 10 is sampled at any three locations of the vehicle component 10. . That is, three specimens are taken. Among the surfaces of the test piece, the surface corresponding to the cross section perpendicular to the longitudinal direction of the vehicle component 10 is used as the observation surface. The viewing surface of each specimen is mirror-polished. Etching is performed on the mirror-polished observation surface using 3% nitric acid alcohol (nital etchant). Let the thickness center part of the vehicle component 10 be an observation field among the etched observation surfaces. The size of the observation field is 200 μm×200 μm. The field of view is observed with a 500x optical microscope.

In the observation field, tempered martensite and other structures (pearlite, ferrite, etc.) can be easily distinguished by contrast. Tempered martensite is observed as gray with low brightness and fine structure. Ferrite is observed as a brighter white region than tempered martensite and pearlite. Pearlite is observed as a phase with a lamellar structure that is less bright than ferrite. Based on the area of tempered martensite in the observation field of view and the total area of the observation field of view, the area ratio (%) of tempered martensite is determined. Let the arithmetic average value of the value obtained with three test pieces be the area ratio of tempered martensite.

[Vickers hardness]
The base material 20 of the vehicle component 10 of this embodiment has a Vickers hardness of 400 to 550 HV according to JIS Z 2244:2020.
A preferable lower limit of the Vickers hardness is 405HV, more preferably 410HV, still more preferably 415HV, still more preferably 420HV.
A preferable upper limit of the Vickers hardness is 545HV, more preferably 540HV, still more preferably 535HV, still more preferably 530HV, still more preferably 525HV.

[Method for measuring Vickers hardness]
The Vickers hardness of the base material 20 of the vehicle component 10 of this embodiment is measured by the following method.
A test piece having a longitudinal section parallel to the longitudinal direction of the vehicle component 10 as a measurement surface is taken. Polish the measuring surface of the specimen. JIS Z 2244: 2020 at arbitrary three points of the base material 20 at a depth of 20 μm into the base material 20 from the boundary (interface) between the oxide film 30 and the base material 20 on the measurement surface after polishing. Measure the Vickers hardness (HV) according to. The test force at the time of measurement shall be 0.098N. Let the arithmetic mean value of the obtained value be Vickers hardness (HV). The determination of the oxide film 30 and the base material 20 is based on the difference in brightness with a microscope.

[(Feature 8) Composition of Oxide Film of Vehicle Parts]
The vehicle component 10 of this embodiment has an oxide film 30 on the base material 20 . The oxide film 30 is Fe ₃ O _{4 having an X-ray diffraction peak intensity ratio of 80% or more when the sum of the X-ray diffraction peak intensities of Fe 3 O 4} _, FeO, and Fe ₂ _O ₃ is 100%. , up to 15% FeO, up to 5% Fe ₂ O ₃ , and the balance consisting of impurities.

The lower limit of the peak intensity ratio of Fe ₃ O ₄ is preferably 81%, more preferably 85%, still more preferably 90%. The upper limit of the peak intensity ratio of Fe ₃ O ₄ may be 100%. The upper limit of the peak intensity ratio of Fe ₃ O ₄ is preferably 99%, more preferably 98%, still more preferably 97%, still more preferably 96%.

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

The lower limit of the peak intensity ratio of Fe ₂ O ₃ may be 0%. The lower limit of the peak intensity ratio of Fe ₂ O ₃ is preferably 0.1%, more preferably 0.2%. The upper limit of the peak intensity ratio of Fe ₂ O ₃ is preferably 4%, more preferably 3%.

[Method for measuring composition of oxide film of vehicle parts]
The composition of the oxide film 30 of the vehicle component 10 is obtained 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 three arbitrary points on the surface of the oxide film 30 . The measurement conditions for the X-ray diffraction measurement are the same as those in the above-mentioned [Method for Measuring Composition of Oxide Film of Steel Pipe]. From the obtained X-ray diffraction profile, the X-ray diffraction peak intensities of Fe ₃ O ₄ , FeO and Fe ₂ O ₃ are obtained. The sum of X-ray diffraction peak intensities of Fe ₃ O ₄ , FeO and Fe ₂ O ₃ is taken as 100%. The peak intensity ratio of Fe ₃ O ₄ , FeO and Fe ₂ O ₃ is determined from the sum of the peak intensities and the X-ray diffraction peak intensities of Fe ₃ O ₄ , FeO and Fe ₂ O ₃ . Let the arithmetic mean value of the numerical value of three places be each peak intensity ratio. Note that the intensity ratio of each peak does not necessarily match the area ratio of each peak or the mass % determined by quantitative analysis.

[(Feature 9) Thickness of oxide film of vehicle parts]
If the thickness of oxide film 30 exceeds 3.50 μm, the adhesion of oxide film 30 is reduced. Therefore, oxide film 30 has a thickness of 3.50 μm or less.
Although the lower limit of the thickness of the oxide film 30 is not particularly limited, it is, for example, 0.01 μm. The lower limit of the thickness of the oxide film 30 is preferably 0.50 μm, more preferably 1.00 μm, still more preferably 1.50 μm, still 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, still more preferably 3.00 μm, still more preferably 2.90 μm, still more preferably 2.80 μm. and more preferably 2.50 μm.

[Method for measuring thickness of oxide film of vehicle parts]
A method for measuring the thickness of the oxide film 30 of the vehicle component 10 is obtained by the following method. A test piece is obtained by cutting the vehicle component 10 perpendicularly to the longitudinal direction. Three test pieces are sampled at a pitch of 100 mm in the longitudinal direction of the vehicle component 10 . Let the cut surface be the observation surface. Fill with resin so that the observation surface can be observed. After filling with resin, the observation surface is polished. Using the SEM-EDS, a secondary electron image of the observation field including the oxide film 3 is generated on the observation surface after polishing. The size of the observation field is 50 μm×40 μm. Here, the observation field of view is 50 μm in the radial direction of the

vehicle component

10 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.

In the secondary electron image, the base material 20 and the oxide film 30 can be easily distinguished from each other by contrast. Note that the base material 20 and the oxide film 30 may be distinguished from each other by performing elemental mapping of oxygen (O) in the observation field using an EDS device attached to the SEM. In elemental mapping of oxygen (O) by EDS, a region with a high oxygen concentration corresponds to the oxide film 30 and a region with a low oxygen concentration corresponds to the base material 2 . Since a region with a high oxygen concentration and a region with a low oxygen concentration are clearly separated, the oxide film 30 can be easily distinguished.
Since the oxide film 30 is thicker than the oxide film 3, it may be peeled off during polishing. In this case, elemental mapping of oxygen (O) by SEM-EDS may not identify the oxide film. However, in this case, the gap between the resin and the base material 20 after the oxide film 30 is peeled off may be regarded as the oxide film 30 .

After identifying the oxide film 30, the thickness of the identified oxide film 30 is measured at 10 locations in the C direction at a pitch of 3 μm. The thickness of the oxide film 30 is defined as the arithmetic mean value of the thicknesses of the oxide film 30 at the measurement points (30 points in total) of the three test pieces.

[Method for manufacturing vehicle parts]
An example of a method for manufacturing the vehicle component 10 of this embodiment will be described. The following example describes how to manufacture a stabilizer. The manufacturing method of the vehicle component 10 described below is an example for manufacturing the vehicle component 10 of the present embodiment. Therefore, the vehicle component 10 having the configuration described above 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 the manufacturing method of the vehicle component 10 of the present embodiment.

An example of a method for manufacturing the vehicle component 10 of this embodiment includes the following steps.
(Step 4) Steel pipe preparation step (Step 5) Bending step (Step 6) Quenching step (Step 7) Tempering step Each step will be described below.

[(Step 4) Steel pipe preparation step]
In the steel pipe preparing step, the steel pipe 1 for manufacturing the vehicle component 10 of this embodiment is prepared.

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

[(Step 6) Quenching Step]
In the quenching step, the bent steel pipe 1 is heated under the following conditions and then quenched. The cooling method is a well-known cooling method.
(Manufacturing condition 3)
Quenching temperature: A _c3 +50° C. or higher and 1150° C. or lower The A _c3 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)
The content of the corresponding element in mass % is substituted for each element symbol in the formula (A).

[(Step 7) Tempering step]
In the tempering step, the quenched steel pipe 1 is tempered under the following conditions.
(Manufacturing condition 4)
Tempering temperature: 150-350°C
(Manufacturing condition 5)
Holding time: 10 minutes or longer

The vehicle component 10 of the present embodiment can be manufactured through the above steps.

The method for manufacturing the vehicle component 10 of this embodiment may further include other steps. Another process is, for example, a surface treatment process. In the surface treatment step, for example, shot peening may be performed on the outer surface 40 of the obtained vehicle component 10 . The outer surface 40 of the obtained vehicle component 10 may be subjected to dustproof treatment.

[Applications of the vehicle component of the present embodiment]
The vehicle component 10 of the present disclosure is suitable as a stabilizer. However, the application of the vehicle component 10 of the present disclosure is not limited to stabilizers. The vehicle component 10 of the present disclosure can be used for inner tie rods, drive shafts, and upper arms, for example.

[Effect of the vehicle component of the present embodiment]
The vehicle component 10 of this embodiment has the following features.
(Feature 6)
The chemical composition of the base material 20 is, in 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 balance consists of Fe and impurities.
(Feature 7) The microstructure of the base material 20 consists of tempered martensite and has a Vickers hardness of 400 to 550 HV according to JIS Z 2244:2020.
(Feature 8)
Fe ₃ O 4 having an X-ray diffraction peak intensity ratio of 80% or _more when the sum of the X-ray diffraction peak intensities of Fe ₃ O ₄ , FeO, and Fe ₂ O ₃ on the base material 20 is 100%. , 15% or less of FeO, 5% or less of Fe ₂ O ₃ , and the balance being impurities.
(Feature 9)
The thickness of oxide film 30 is 3.50 μm or less.
The vehicle component 10 of this embodiment having features 6 to 9 is excellent in fatigue strength.

The effects of the steel pipe 1 and the vehicle component 10 of this embodiment will be explained more specifically by way of examples. The conditions in the following examples are examples of conditions adopted for confirming the feasibility and effects of the steel pipe 1 and the vehicle component 10 of this embodiment. Therefore, the steel pipe 1 and the vehicle component 10 of this embodiment are not limited to this one condition example.

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

"-" in Table 1B indicates that the corresponding element content is below the impurity level.

A slab was prepared from the molten steel of each test number. The slab was subjected to rough rolling and finish rolling to prepare a steel plate having a length of 1000 cm, a width of 300 cm and a thickness of 4 mm. The steel sheets of each test number were subjected to low-temperature heat treatment at the heat treatment temperature and heat treatment time shown in Table 2 using a heat treatment furnace in an air atmosphere. A test material (steel plate) simulating a steel pipe was manufactured through the manufacturing process described above.

[Measurement test of composition of oxide film before quenching]
The composition of the oxide film formed on the base material (steel plate) of the test material was measured based on the above-described [Method for measuring composition of oxide film on steel pipe]. In the measurement, the surface of the oxide film formed on the surface of the steel sheet was subjected to X-ray diffraction measurement to obtain an X-ray diffraction profile. Table 2 shows the results.

[Thickness of oxide film before quenching and measurement test of standard deviation of thickness]
The thickness of the oxide film formed on the base material (steel plate) of the test material and the standard deviation of the thickness are calculated according to the above [Method for measuring the thickness of the oxide film on the steel pipe] and [Measurement of the thickness of the oxide film on the steel pipe]. Standard deviation measurement method]. In addition, in the measurement, three test pieces were collected at a pitch of 100 mm in the rolling direction of the steel plate as the test material.

[Measurement test of ferrite and pearlite area ratio of steel plate (base material) before quenching]
The area ratios of ferrite and pearlite in the base material (steel plate) of the test material were measured based on the above-described [Method for measuring area ratio of ferrite and pearlite]. Among the surfaces of the test piece, the surface corresponding to the cross section perpendicular to the rolling direction of the steel plate as the test material was used as the observation surface. As a result, the base material (steel plate) of the test material of any test number had a microstructure composed of 20% to 60% ferrite and 40% to 80% pearlite in terms of area ratio.

[Manufacture of simulated parts simulating vehicle parts]
Hardening was performed at the hardening temperature shown in Table 3 for the test material of each test number. After quenching, tempering was performed at a tempering temperature of 200° C. for 60 minutes. A simulated part simulating a vehicle part was manufactured by the manufacturing process described above.

[Measurement test of area ratio of tempered martensite of simulated parts]
The area ratio of tempered martensite in the base material of the simulated part was obtained based on the above-described [Method for measuring area ratio of tempered martensite]. Among the surfaces of the test piece, the surface corresponding to the cross section perpendicular to the longitudinal direction of the simulated part was used as the observation surface. As a result of the measurement, the microstructures of the base metals of the steel sheets of all test numbers were also composed of tempered martensite.

The Vickers hardness of the base material of the simulated part was measured according to the above [Method for measuring Vickers hardness]. Table 3 shows the results obtained.

[Measurement Test of Composition of Oxide Film of Simulated Part]
The composition of the oxide film of the simulated part was measured based on the above-mentioned [Method for measuring composition of oxide film of vehicle part]. Table 3 shows the results.

[Measurement test of oxide film thickness of simulated parts]
The thickness of the oxide film formed on the steel plate after quenching was measured based on the above-described [Method for measuring thickness of oxide film of vehicle parts]. Observation and measurement were performed on three steel plates for each test number. The arithmetic average value of the oxide film thicknesses of the three steel sheets was taken as the oxide film thickness for each test number. Table 3 shows the results.

[Fatigue test]
The rupture life of the simulated parts was measured. A plate-like torsional fatigue test piece having a thickness of 2 mm, a width of 8 mm, a length of 60 mm, and a parallel portion of 9.5 mm was obtained from the simulated part (steel plate) of each test number. FIG. 3 is a front view of a torsional fatigue test piece. FIG. 4 is a longitudinal side view of the torsional fatigue test piece. The torsional fatigue test piece had a concave curved groove a with a bottom depth of 0.1 mm and a curvature radius of 8.7 mm, simulating the inner surface of a steel pipe, in the central portion of the surface in the width direction. A torsional fatigue test was performed in air at a set stress of 400 MPa. In the torsional fatigue test, an electromagnetic force type torsional fatigue tester is used, test waveform: sine wave, test speed: 15 Hz, test environment: room temperature, in the atmosphere, stress ratio: -1 (double swing) Cycle fatigue test. The number of times until the torsional fatigue test piece fractured was measured. When the test piece did not break at 5.0×10 ⁶ cycles, it was judged that excellent fatigue strength was obtained. Table 3 shows the results.

[Evaluation test]
With reference to Tables 1 to 3, in Test Nos. 1 to 16, the composition, thickness, and standard deviation of the oxide film of the test material were appropriate. In addition, the oxide layer composition and thickness of the simulated parts with these test numbers were appropriate. As a result, excellent fatigue strength was obtained in these test numbers.

On the other hand, in test number 17, the oxide film of the test material was too thick. Therefore, the oxide film on the simulated part was too thick. As a result, excellent fatigue strength was not obtained.

In test number 18, the oxide film of the test material was too thin. Therefore, the oxide film on the simulated part was too thick. As a result, excellent fatigue strength was not obtained.

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

In Test Nos. 20, 21 and 24, the composition of the oxide film of the test material was inappropriate, the oxide film was too thick, and the standard deviation of the thickness was too large. Therefore, the composition of the oxide film of the simulated part was inappropriate, and the oxide film was too thick. As a result, excellent fatigue strength was not obtained.

In test numbers 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). Therefore, the oxide film after quenching was too thick. As a result, excellent fatigue strength was not obtained.

In test number 25, the C content of the base material was too low. Therefore, the Vickers hardness of the simulated parts was too low. As a result, excellent fatigue strength was not obtained.

In test number 26, the C content in the base material was too high. Therefore, the Vickers hardness of the simulated parts was too high. As a result, excellent fatigue strength was not obtained.

The embodiment of the present disclosure has been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be modified as appropriate without departing from the scope of the present disclosure.

REFERENCE SIGNS LIST 1 steel pipe 2 base material 3 oxide film 10 vehicle component 20 base material 30 oxide film

Claims

in % by mass,
C: 0.23 to 0.50%,
Si: 0.01 to 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 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
a chemical composition with the balance being Fe and impurities;
A base material having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite in area ratio;
on the base material,
When the sum of the X-ray diffraction peak intensities of Fe 3 O 4 , Fe 2 O 3 and FeO is taken as 100%, the peak intensity ratio of the X-ray diffraction is 70 % or more, and 20% or more. of Fe 2 O 3 , up to 10% FeO, and the balance consisting of impurities,
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;
steel pipe.
The steel pipe according to claim 1,
The chemical composition, in mass %,
Sol. Al: 0.001 to 0.080%,
Cr: 0.01 to 1.50%,
Mo: 0.01 to 1.00%,
Ni: 0.01 to 1.00%,
Cu: 0.01 to 1.00%,
Ti: 0.001 to 0.100%,
Nb: 0.001 to 0.100%,
V: 0.001 to 0.100%,
B: 0.0001 to 0.0050%, and
Ca: 0.0001 to 0.0050%,
containing one or more elements selected from the group consisting of
steel pipe.
in % by mass,
C: 0.23 to 0.50%,
Si: 0.01 to 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 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
a chemical composition with the balance being Fe and impurities;
and 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 sum of the X-ray diffraction peak intensities of Fe 3 O 4 , FeO, and Fe 2 O 3 is taken as 100%, the peak intensity ratio of the X-ray diffraction is 80% or more , and 15% FeO below, Fe 2 O 3 below 5%, and the balance consisting of impurities,
an oxide film having a thickness of 3.50 μm or less,
vehicle parts.
The vehicle component according to claim 3,
The chemical composition, in mass %,
Sol. Al: 0.001 to 0.080%,
Cr: 0.01 to 1.50%,
Mo: 0.01 to 1.00%,
Ni: 0.01 to 1.00%,
Cu: 0.01 to 1.00%,
Ti: 0.001 to 0.100%,
Nb: 0.001 to 0.100%,
V: 0.001 to 0.100%,
B: 0.0001 to 0.0050%, and
Ca: 0.0001 to 0.0050%,
containing one or more elements selected from the group consisting of
vehicle parts.
in % by mass,
C: 0.23 to 0.50%,
Si: 0.01 to 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 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
a chemical composition with the balance being Fe and impurities;
preparing a steel sheet having a microstructure consisting of 20% to 60% ferrite and 40% to 80% pearlite in area ratio;
a step of heat-treating the steel plate at 450-600° C. for 0.5-3.0 minutes;
A step of manufacturing a steel pipe by electric resistance welding the steel plate after the heat treatment.
A method of manufacturing steel pipes.
A method for manufacturing a vehicle part, comprising:
A step of preparing the steel pipe according to claim 1 or claim 2;
a step of bending the steel pipe;
a step of holding the bent steel pipe at a temperature of Ac 3 +50° C. or higher and 1150° C. or lower for 10 seconds or longer and then quenching;
a step of holding and tempering the steel pipe after the quenching at 150 to 350° C. for 10 minutes or longer;
A method for manufacturing vehicle parts.