WO2023152906A1

WO2023152906A1 - Electric resistance welded steel pipe for automobile component, and method for manufacturing automobile component

Info

Publication number: WO2023152906A1
Application number: PCT/JP2022/005451
Authority: WO
Inventors: 優輝茂手木; 幸伸永田; 文彦新山; 康浩篠原; 敬久小川; 侑正上田
Original assignee: 日本製鉄株式会社
Priority date: 2022-02-10
Filing date: 2022-02-10
Publication date: 2023-08-17
Also published as: JP7188649B1; JPWO2023152906A1

Abstract

The present invention is an electric resistance welded steel pipe for an automobile component including a base material part and an electric resistance welded part. The chemical composition of the base material part includes, by mass, 0.42-0.48% of C, 0.01-0.20% of Si, 0.10-0.70% of Mn, 0-0.030% of P, 0-0.030% of S, 0.005-0.050% of Al, 0.005-0.040% of Ti, and 0.0005-0.0050% of B, with the remainder made up by Fe and impurities. The steel structure of the base material part is a ferrite-pearlite mixed structure, and the hardness of the base material part is 110-150 Hv.

Description

Electric resistance welded steel pipe for automobile parts, and method for manufacturing automobile parts

The present disclosure relates to an electric resistance welded steel pipe for automobile parts and a method for manufacturing automobile parts.

2. Description of the Related Art Conventionally, techniques for manufacturing automobile parts such as rack bars using steel pipes as materials have been known (see, for example, Patent Documents 1 to 5).
Patent Documents 6 and 7 disclose electric resistance welded steel pipes for automobile parts or machine structures, which are for cold working.
Patent Document 8 discloses, as a high-carbon steel pipe for automobile parts, a high-carbon steel pipe having a structure in which cementite particles are finely dispersed in a matrix phase that is a ferrite phase.

Patent Document 1: JP-A-2003-094140 Patent Document 2: JP-A-2007-253190 Patent Document 3: JP-A-2008-044533 Patent Document 4: Japanese Patent No. 4798674 Patent Document 5: JP-A-2004- Patent Document 6: Japanese Patent No. 3999915 Patent Document 7: Japanese Patent No. 4486516 Patent Document 8: Japanese Patent No. 5679115

Assuming that electric resistance welded steel pipes for automobile parts are cold-worked using a mold and then quenched to manufacture automobile parts (for example, rack bars), the cold workability of electric resistance welded steel pipes for automobile parts is And there are cases where it is required to be excellent in mold life.
Furthermore, from the viewpoint of ensuring the hardness and machinability of the manufactured automobile parts, the electric resistance welded steel pipes for automobile parts are required to have an appropriate hardness after quenching (that is, neither too high nor too low). may be

Here, "excellent mold life" means that the life of the mold used when cold working electric resistance welded steel pipes for automobile parts can be extended.

An object of the present disclosure is to provide electric resistance welded steel pipes for automotive parts that are excellent in cold workability and mold life and have moderate hardness after quenching, and to manufacture automobile parts using the electric resistance welded steel pipes for automotive parts. To provide a method for manufacturing automobile parts.

Means for solving the above problems include the following aspects.
<1> Including the base metal portion and the electric resistance welded portion,
The chemical composition of the base material portion is, in mass %,
C: 0.42 to 0.48%,
Si: 0.01 to 0.20%,
Mn: 0.10-0.70%,
P: 0 to 0.030%,
S: 0 to 0.030%,
Al: 0.005 to 0.050%,
Ti: 0.005 to 0.040%,
B: 0.0005 to 0.0050%,
N: 0 to 0.005%,
O: 0 to 0.005%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
REM: 0 to 0.0050%,
Cr: 0 to 0.50%,
Ni: 0 to 0.50%,
Cu: 0-0.50%,
Nb: 0 to 0.10%,
Mo: 0-0.50%,
V: 0 to 0.20%, and
Balance: Fe and impurities,
The steel structure of the base material portion is a ferrite-pearlite mixed structure,
The hardness of the base material portion is 110 to 150 Hv,
Electric resistance welded steel pipes for automobile parts.
<2> The chemical composition of the base material portion is, in mass %,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005-0.0050%,
REM: 0.0005 to 0.0050%,
Cr: 0.01 to 0.50%,
Ni: 0.01 to 0.50%,
Cu: 0.01-0.50%,
Nb: 0.01 to 0.10%,
Mo: 0.01 to 0.50%, and
V: 0.01-0.20%
containing one or more of
The electric resistance welded steel pipe for automobile parts according to <1>.
<3> The heating temperature is 900 ° C., the heating time is 1 minute, the cooling rate after heating is 30 ° C./s, and the cooling temperature at the cooling rate is within the range of 50 ° C. to 0 ° C. The hardness when quenching under certain conditions is 650 to 800 Hv,
The electric resistance welded steel pipe for automobile parts according to <1> or <2>.
<4> A method for manufacturing an automobile part, comprising a step of subjecting the electric resistance welded steel pipe for automobile parts according to any one of <1> to <3> to cold working and quenching in this order to obtain the automobile part. .

According to the present disclosure, an electric resistance welded steel pipe for automobile parts that is excellent in cold workability and mold life and has an appropriate hardness after quenching, and an electric resistance welded steel pipe for automobile parts is used to manufacture automobile parts. A method for manufacturing an automotive component is provided.

In the present disclosure, a numerical range represented using "to" means a range including the numerical values described before and after "to" as lower and upper limits.
In the present disclosure, "%" indicating the content of a component (element) means "% by mass".
In the present disclosure, the content of C (carbon) may be referred to as "C content". Contents of other elements may also be expressed similarly.
In the present disclosure, the term "step" includes not only independent steps, but also if the intended purpose of the step is achieved even if it cannot be clearly distinguished from other steps. .

In the present disclosure, the base metal portion refers to a portion of an electric resistance welded steel pipe other than the electric resistance welded portion and the heat affected zone. Here, the heat affected zone (sometimes referred to as "HAZ") is the area near the electric resistance welded part that is affected by heat due to electric resistance welding and seam heat treatment. Point.

In the present disclosure, "as-rolled electric resistance welded steel pipe" refers to an electric resistance welded steel pipe that has not been subjected to heat treatment other than seam heat treatment after pipe making.
“Tube-making” refers to the process of forming an open pipe by roll-forming a hot-rolled steel sheet unwound from a hot coil, and forming an electric-resistance welded portion by electric resistance welding of the butt portions of the obtained open pipe. refers to the process
"Hot coil" means a hot-rolled steel sheet produced using a hot strip mill and wound into a coil.
“Roll forming” refers to continuously bending a hot-rolled steel sheet unwound from a hot coil into an open tubular shape.

Hot-rolled steel sheet manufactured using a hot strip mill is a continuous steel sheet. It differs from the steel plate that is produced.
Steel plate cannot be used for roll forming, which is a continuous bending process, because it is not a continuous steel sheet.
Electric resistance welded steel pipes are clearly distinguished from welded steel pipes (for example, UOE steel pipes) manufactured using thick steel plates in the above points.

[ERW steel pipes for automobile parts]
The electric resistance welded steel pipe for automobile parts of the present disclosure (hereinafter also simply referred to as "electric resistance welded steel pipe") is
Including the base material part and the electric resistance welding part,
The chemical composition of the base material is % by mass,
C: 0.42 to 0.48%,
Si: 0.01 to 0.20%,
Mn: 0.10-0.70%,
P: 0 to 0.030%,
S: 0 to 0.030%,
Al: 0.005 to 0.050%,
Ti: 0.005 to 0.040%,
B: 0.0005 to 0.0050%,
N: 0 to 0.005%,
O: 0 to 0.005%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
REM: 0 to 0.0050%,
Cr: 0 to 0.50%,
Ni: 0 to 0.50%,
Cu: 0-0.50%,
Nb: 0 to 0.10%,
Mo: 0-0.50%,
V: 0 to 0.20%, and
Balance: Fe and impurities,
The steel structure of the base material is a ferrite-pearlite mixed structure,
The hardness of the base material portion (hereinafter also referred to as “hardness before quenching”) is 110 to 150 Hv.

The electric resistance welded steel pipe of the present disclosure is excellent in cold workability and mold life, and has an appropriate hardness after quenching (that is, neither too high nor too low).

The effects of cold workability and mold life described above are contributed by the fact that the hardness of the base material (that is, the hardness before quenching) is not too hard, specifically 150 Hv or less. .

The effect that the hardness after quenching is appropriate includes the chemical composition of the base material (for example, the C content is 0.42% or more to make the hardness of the base material appropriate, and B content is 0.0005% or more in order to reduce the hardness difference ΔHv in the thickness direction described later), and the hardness of the base material (that is, the hardness before quenching) is 110 to 150 Hv is contributing.
When the electric resistance welded steel pipe of the present disclosure is quenched, the preferred range of hardness of the quenched electric resistance welded steel pipe will be described later.

In the electric resistance welded steel pipe of the present disclosure, the fact that the hardness of the base material is 110 to 150 Hv means that the chemical composition of the base material has a Si content of 0.20% or less and a Mn content of 0.70%. Below, it is realized by heat-treating the as-rolled electric resistance welded steel pipe in which B is appropriately reduced and B is appropriately contained under predetermined conditions without performing general normalizing (for example, (see manufacturing method X).

<Chemical Composition of Base Material>
The content of each element in the chemical composition of the base metal portion of the electric resistance welded steel pipe of the present disclosure (hereinafter also referred to as the “chemical composition of the present disclosure”) will be described below.

C: 0.42-0.48%
C is an effective element for enhancing the cold workability of the steel structure and ensuring the hardness after quenching required for automobile parts.
If the C content is less than 0.42%, the hardness after quenching may be insufficient. Therefore, the C content is 0.42% or more. The C content is preferably 0.43% or more.
On the other hand, when the C content exceeds 0.48%, the cold workability may deteriorate. In addition, the hardness after quenching may become too hard and the toughness may decrease. Therefore, the C content is 0.48% or less. The C content is preferably 0.47% or less.

Si: 0.01-0.20%
Si is an element that contributes to the improvement of hardness and controls the precipitation of carbides to contribute to the improvement of cold workability.

If the Si content is less than 0.01%, the refining cost will increase significantly, and the addition effect may not be sufficiently exhibited. Therefore, the Si content is 0.01% or more. The Si content is preferably 0.05% or more.
On the other hand, if the Si content exceeds 0.20%, the hardness may become too high and the cold workability may deteriorate. Therefore, the Si content is 0.20% or less. The Si content is preferably 0.15% or less.

Mn: 0.10-0.70%
Mn is an element that contributes to the improvement of hardness and can contribute to the improvement of cold workability by controlling the precipitation of carbides.

If the Mn content is less than 0.10%, the effect of addition may not be sufficiently exhibited. Therefore, Mn is 0.10% or more. The Mn content is preferably 0.25% or more.
On the other hand, if the Mn content exceeds 0.70%, the hardness may become too high and the cold workability may deteriorate. Therefore, the Mn content is 0.70% or less. The Mn content is preferably 0.55 or less.

P: 0-0.030%
P is an element that segregates at grain boundaries and can impair cold workability and toughness.
If the P content exceeds 0.030%, cold workability and toughness may deteriorate. Therefore, the P content is 0.030% or less. The P content is preferably 0.020% or less.

The P content may be 0% or more than 0%.
From the viewpoint of manufacturing cost, the P content may be 0.0001% or more.

S: 0-0.030%
S is an element that forms MnS and can impair cold workability and toughness.
When the S content exceeds 0.030%, cold workability and toughness may deteriorate. Therefore, the S content is 0.030% or less. The S content is preferably 0.020% or less.

The S content may be 0% or more than 0%.
From the viewpoint of manufacturing cost, the S content may be 0.0001% or more.

Al: 0.005-0.050%
Al is an element effective for deoxidation.
If the Al content is less than 0.005%, the effect of addition may not be sufficiently exhibited. Therefore, the Al content is 0.005% or more. The Al content is preferably 0.03% or more.
On the other hand, when the Al content exceeds 0.050%, coarse Al oxides are formed, which may deteriorate cold workability and toughness. Therefore, the Al content is 0.050% or less. The Al content is preferably 0.040% or less.

Ti: 0.005-0.040%
Ti is an element that forms TiN to contribute to the refinement of the steel structure, and suppresses the formation of BN to ensure the effect of improving the hardenability of B.

If the Ti content is less than 0.005%, the effect of addition may not be sufficiently exhibited. Therefore, the Ti content is 0.005% or more. The Ti content is preferably 0.008% or more.
On the other hand, when the Ti content exceeds 0.040%, coarse Ti compounds are formed, which may deteriorate cold workability and toughness. Therefore, the Ti content is 0.040% or less. The Ti content is preferably 0.003% or less.

B: 0.0005 to 0.0050%
B is an element that can improve hardenability and contribute to an improvement in hardness.
If the B content is less than 0.0005%, the effect of addition may not be sufficiently exhibited, and quenching unevenness may occur. As a result, a hardness difference ΔHv in the thickness direction, which will be described later, may increase.
Therefore, the B content is 0.0005% or more. The B content is preferably 0.0008% or more.

On the other hand, if the B content exceeds 0.0050%, the B compound may precipitate at the grain boundaries, degrading cold workability and toughness. Therefore, the B content is 0.0050% or less. The B content is preferably 0.040% or less.

N: 0-0.005%
N is an element that forms fine nitrides and can contribute to refinement of the steel structure.
When the N content exceeds 0.005%, coarse nitrides are formed, which may deteriorate cold workability and toughness. Therefore, the N content is 0.005% or less. The N content is preferably 0.003% or less.

The N content may be 0% or greater than 0%.
From the viewpoint of manufacturing cost, the N content may be 0.0001% or more.

O: 0 to 0.005%
O is an element that can form oxide-based inclusions and impair cold workability and toughness. When the O content exceeds 0.005%, coarse oxide-based inclusions are formed, which may deteriorate cold workability and toughness. Therefore, the O content is 0.005% or less. The O content is preferably 0.003% or less.

The O content may be 0% or greater than 0%.
From the viewpoint of production cost, the O content may be 0.0001% or more.

Ca: 0-0.0050%
Ca is an optional element.
Therefore, the Ca content may be 0% or greater than 0%.
Ca is an element that can control the shape of inclusions, homogenize the steel structure, and contribute to the improvement of the cold workability of the steel structure. From the viewpoint of such effects, the Ca content is preferably 0.0005% or more, more preferably 0.0008% or more.
On the other hand, when the Ca content exceeds 0.0050%, inclusions may be excessively formed and the cold workability may deteriorate. Therefore, the Ca content is preferably 0.0050% or less, more preferably 0.0035% or less.

Mg: 0-0.0050%
Mg is an optional element.
Therefore, the Mg content may be 0% or greater than 0%.
Mg is an element that can control the shape of inclusions, homogenize the steel structure, and contribute to the improvement of the cold workability of the steel structure. From the viewpoint of such effects, the Mg content is preferably 0.0005% or more, more preferably 0.0008% or more.
On the other hand, if the Mg content exceeds 0.005%, inclusions may be excessively formed and the cold workability of the steel structure may deteriorate. Therefore, the Mg content is preferably 0.0050% or less, more preferably 0.0035% or less.

REM: 0-0.0050%
REM is an optional element.
Therefore, the REM content may be 0% or greater than 0%.
wherein REM is selected from the group consisting of the rare earth elements, i.e. Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu means at least one element Also, the REM content means the total content of rare earth elements.
REM is an element that can control the shape of inclusions, homogenize the steel structure, and contribute to the improvement of the cold workability of the annealed structure. From the viewpoint of such effects, the REM content is preferably 0.0005% or more, more preferably 0.0008% or more.
On the other hand, if the REM content exceeds 0.0050%, inclusions may be excessively formed and the cold workability of the steel structure may deteriorate. Therefore, the REM content is preferably 0.0050% or less, more preferably 0.0035% or less.

Cr: 0-0.50%
Cr is an optional element.
Therefore, the Cr content may be 0% or greater than 0%.
Cr is an element that can contribute to improving the strength of the steel structure. From the viewpoint of such effects, the Cr content is preferably 0.01% or more, more preferably 0.03% or more.
On the other hand, when the Cr content exceeds 0.50%, coarse Cr compounds are formed, and the homogeneity of the steel structure may deteriorate, resulting in deterioration of cold workability and toughness. Therefore, the Cr content is preferably 0.50% or less, more preferably 0.10% or less, and still more preferably 0.07% or less.

Ni: 0-0.50%
Ni is an optional element.
Therefore, the Ni content may be 0% or more than 0%.
Ni is an element that can contribute to improving the strength of the steel structure. From the viewpoint of such effects, the Ni content is preferably 0.01% or more, more preferably 0.03% or more.
On the other hand, if the Ni content exceeds 0.50%, it may segregate at grain boundaries and the homogeneity of the steel structure may deteriorate, resulting in deterioration of cold workability and toughness. Therefore, the Ni content is preferably 0.50% or less, more preferably 0.10% or less, and even more preferably 0.07% or less.

Cu: 0-0.50%
Cu is an optional element.
Therefore, the Cu content may be 0% or greater than 0%.
Cu is an element that can contribute to improving the strength of the steel structure. From the viewpoint of such effects, the Cu content is preferably 0.01% or more, more preferably 0.03% or more.
On the other hand, if the Cu content exceeds 0.50%, the weldability of the steel may deteriorate, and the mechanical properties of the electric resistance weld may deteriorate. Therefore, the Cu content is preferably 0.50% or less, more preferably 0.10% or less, and even more preferably 0.07% or less.

Nb: 0-0.10%
Nb is an arbitrary element.
Therefore, the Nb content may be 0%, may be greater than 0%, or may be greater than 0%.
Nb is an element that can contribute to refinement of the steel structure and improvement of strength. From the viewpoint of such effects, the Nb content is preferably 0.01% or more, more preferably 0.03% or more.
On the other hand, when the Nb content exceeds 0.10%, coarse Nb compounds are formed, and the homogeneity of the steel structure may deteriorate, resulting in deterioration of cold workability and toughness. Therefore, the Nb content is preferably 0.10% or less, more preferably 0.05% or less, and still more preferably 0.03% or less.

Mo: 0-0.50%
Mo is an optional element.
Therefore, the Mo content may be 0% or greater than 0%.
Mo is an element that can contribute to improving the strength of the steel structure. From the viewpoint of such effects, the Mo content is preferably 0.01% or more, more preferably 0.03% or more.
On the other hand, if the Mo content exceeds 0.50%, the weldability of the steel may deteriorate, and the mechanical properties of the electric resistance welded joint may deteriorate. Therefore, the Mo content is preferably 0.50% or less, more preferably 0.35% or less.

V: 0-0.20%
V is an arbitrary element.
Therefore, the V content may be 0% or greater than 0%.
V is an element that can contribute to refining the steel structure and improving strength. From the viewpoint of such effects, the V content is preferably 0.01% or more, more preferably 0.03% or more.
On the other hand, if the V content exceeds 0.20%, coarse V compounds are formed, and the homogeneity of the steel structure may deteriorate, resulting in deterioration of cold workability and toughness. Therefore, the V content is preferably 0.20% or less, more preferably 0.10% or less.

Balance: Fe and Impurities In the chemical composition of the base material portion, the balance excluding the above elements is Fe and impurities.
Here, impurities refer to components contained in raw materials (e.g., ore, scrap, etc.) or components mixed in during the manufacturing process and not intentionally included in steel.
Impurities include all elements other than those mentioned above. Only one element or two or more elements may be used as impurities.
Impurities include, for example, Sb, Sn, W, Co, As, Pb, Bi, H, and the like.
Among the above elements, for example, Sb, Sn, Co, and As are mixed in a content of 0.1% or less, and Pb and Bi are mixed in a content of 0.005% or less, for example. With respect to, for example, contamination with a content of 0.0004% or less is possible.
There is no particular need to control the contents of other elements as long as they are within normal ranges.

From the viewpoint of the effect of the following elements, the chemical composition of the base material is
Ca: 0.0005 to 0.0050%,
Mg: 0.0005-0.0050%,
REM: 0.0005 to 0.0050%,
Cr: 0.01 to 0.50%,
Ni: 0.01 to 0.50%,
Cu: 0.01-0.50%,
Nb: 0.01 to 0.10%,
Mo: 0.01 to 0.50%, and
V: 0.01-0.20%
You may contain 1 type, or 2 or more types.
More preferable ranges for the contents of these elements are as described above.

<Steel structure>
In the electric resistance welded steel pipe of the present disclosure, the steel structure of the base material portion is a ferrite-pearlite mixed structure.
The steel structure of the base material is confirmed as follows.
The center of the wall thickness at the 180° position of the base material in the C cross section of the electric resistance welded steel pipe (that is, the position shifted by 180° in the pipe circumferential direction from the electric resistance welded part) is used as an observation surface, and this observation surface is observed with an optical microscope at a magnification of 100 times. Observe at and confirm the steel structure.
In the electric resistance welded steel pipe of the present disclosure, the cementite in the pearlite in the ferrite-pearlite mixed structure may be lamellar cementite or cementite obtained by dividing lamellar cementite.

<Hardness of Base Material>
The hardness of the base metal portion of the electric resistance welded steel pipe of the present disclosure (that is, the hardness before quenching) is 110 to 150 Hv.
If the hardness before quenching is less than 110 Hv, the hardness required for automotive parts may not be obtained after quenching. Therefore, the hardness of the base material portion before quenching is 110 Hv or more, preferably 120 Hv or more.
On the other hand, if the hardness before quenching exceeds 150 Hv, the cold workability of the steel structure may be impaired, and the life of the mold may be shortened. Therefore, the hardness of the base material before quenching is 150 Hv or less, preferably 140 Hv or less.

In the present disclosure, hardness (Hv) means Vickers hardness measured according to JIS Z 2244 (2009) with a test force of 0.98N.

The hardness (that is, Vickers hardness) of the base metal portion of the electric resistance welded steel pipe of the present disclosure is obtained as follows.
In the C section of the electric resistance welded steel pipe (that is, the cross section perpendicular to the pipe axis direction), the electric resistance welded portion is set at 0°, and clockwise, 90°, 180°, and 270° in the circumferential direction from the electric resistance welded portion. ° position (that is, base material 90 ° position, base material 180 ° position, and base material 270 ° position), a position of 0.5 mm deep from the inner surface (three places in total) and a depth from the outer surface The position of 0.5 mm (three places in total) and the position of measurement (six places in total).
A Vickers hardness test according to JIS Z 2244 (2009) is performed at each of the six measurement positions to obtain Vickers hardness (Hv). The test force shall be 0.98N.
The arithmetic mean value of the obtained six Vickers hardnesses (measured values) is defined as "hardness of base material".

<Size of ERW steel pipe>
There is no particular limitation on the size of the electric resistance welded steel pipe of the present disclosure.
The outer diameter of the electric resistance welded steel pipe of the present disclosure is, for example, 19.0 to 114.3 mm.
In the electric resistance welded steel pipe of the present disclosure, the value (t/D value) obtained by dividing the wall thickness (t) of the base material portion by the outer diameter (D) of the electric resistance welded steel pipe is, for example, 0.02 to 0.30.
In the electric resistance welded steel pipe of the present disclosure, the thickness of the base material portion is, for example, 2.0 to 10.0 mm.

<Preferred hardness of electric resistance welded steel pipe after quenching>
As described above, the electric resistance welded steel pipe of the present disclosure has moderate hardness after quenching.
Hereinafter, a preferable range of hardness of the quenched electric resistance welded steel pipe of the present disclosure will be described.

The electric resistance welded steel pipe of the present disclosure has a heating temperature of 900 ° C., a heating time of 1 minute, a cooling rate after heating of 30 ° C./s, and a cooling ultimate temperature at the cooling rate of 50 ° C. to 0 It is preferable that the hardness (that is, the hardness after quenching) is 650 to 800 Hv when quenching is performed under conditions within the range of °C.
When the hardness after quenching is 650 Hv or more, the hardness and wear resistance of automobile parts manufactured by subjecting the electric resistance welded steel pipe of the present disclosure to cold working and quenching can be ensured. The hardness after quenching is preferably 700 Hv or more.
When the hardness after quenching is 800 Hv or less, it is advantageous in terms of machinability when further machining automobile parts manufactured by subjecting the electric resistance welded steel pipe of the present disclosure to cold working and quenching. . The hardness after quenching is preferably 770 Hv or less.

The hardness of the electric resistance welded steel pipe of the present disclosure after quenching is obtained in the same manner as the hardness of the base metal portion (that is, the hardness before quenching) of the electric resistance welded steel pipe of the present disclosure described above.

In addition, in the electric resistance welded steel pipe after quenching, from the viewpoint of further suppressing distortion in the thickness direction, the hardness at the position of 1/4 thickness from the outer surface of the electric resistance welded steel pipe at the base material 180 ° position The absolute value of the difference between the maximum and minimum values of the hardness at the position of 1/2 thickness from the outer surface of the electric resistance welded steel pipe and the hardness at the position of 3/4 thickness from the outer surface of the electric resistance welded steel pipe (hereinafter referred to as the thickness The hardness difference ΔHv in the thickness direction) is preferably 50 Hv or less.

<Example of manufacturing method of electric resistance welded steel pipe (manufacturing method X)>
An example of a manufacturing method (hereinafter referred to as “manufacturing method X”) for manufacturing the electric resistance welded steel pipe of the present disclosure will be described below.
The following manufacturing method X is a manufacturing method of an electric resistance welded steel pipe of an example described later.

Manufacturing method X is
Including the base material portion A and the electric resistance welded portion A, the chemical composition of the base material portion A is the “chemical composition in the present disclosure” described above, and the steel structure of the base material portion A is a ferrite-pearlite mixed structure. A preparatory step of preparing an as-rolled electric resistance welded steel pipe;
A heat treatment step of subjecting the as-rolled electric resistance welded steel pipe to heat treatment under the conditions described below;
including.

According to manufacturing method X, the "chemical composition in the present disclosure" in which the Si content is reduced to 0.20% or less and the Mn content is reduced to 0.70% or less, and the heat treatment under the conditions described later. By combining, the electric resistance welded steel pipe of the present disclosure having a hardness of the base material portion of 110 to 150 Hv can be produced.

Each step that can be included in manufacturing method X will be described below.

(Azroll ERW steel pipe preparation process)
The as-rolled electric resistance welded steel pipe preparation step in manufacturing method X is a step of preparing the above-described as-rolled electric resistance welded steel pipe.
This step may be a step of simply preparing the pre-manufactured as-rolled electric resistance welded steel pipe, or may be a step of manufacturing the above-described as-rolled electric resistance welded steel pipe.

Azroll ERW steel pipe
Prepare a hot coil made of a hot-rolled steel sheet having the chemical composition in the present disclosure,
A hot-rolled steel sheet is unwound from the hot coil, and the hot-rolled steel sheet unwound from the hot coil is roll-formed to form an open pipe,
It can be produced by forming an electric resistance welded portion A by electric resistance welding the butt portion of the obtained open pipe.
In the preparation step, the seam heat treatment may be applied to the electric resistance welded portion after the electric resistance welded portion is formed.

(Heat treatment process)
Manufacturing method X includes a heat treatment step of heat treating the as-rolled electric resistance welded steel pipe under the following heat treatment conditions.

-Heat treatment conditions in the heat treatment step in manufacturing method X-
・Heating temperature: 720-780℃
・Holding time at heating temperature: 1 to 20 minutes ・Cooling rate when cooling from heating temperature to “heating temperature -100 ° C”: 1.0 ° C/s or less

By setting the heat treatment temperature (heating temperature) to 720°C or higher, it can be realized that the hardness of the base metal portion is 150 Hv or lower in the manufactured electric resistance welded steel pipe.

By setting the heat treatment temperature (heating temperature) to 780°C or less, the hardness of the base metal portion of the manufactured electric resistance welded steel pipe can be realized to be 150 Hv or less.

By setting the heat treatment time (holding time at the heating temperature) to 20 minutes or less, the hardness of the base metal portion of the manufactured electric resistance welded steel pipe can be realized to be 150 Hv or less.
The heat treatment time (holding time at the heating temperature) is preferably 10 minutes or less.

By setting the heat treatment time (holding time at the heating temperature) to 1 minute or more, the hardness of the base metal portion of the manufactured electric resistance welded steel pipe can be realized to be 150 Hv or less.
The heat treatment time (holding time at the heating temperature) is preferably 5 minutes or longer.

The hardness of the base metal portion of the manufactured electric resistance welded steel pipe is 150 Hv or less by cooling at a cooling rate of 1.0° C./s or less when cooling from the heat treatment temperature to “heat treatment temperature −100° C.” can be realized.

The lower limit of the cooling rate when cooling from the heat treatment temperature to "heat treatment temperature -100°C" is not particularly limited, but the lower limit is, for example, 0.01°C/s.

Cooling methods for cooling from the heat treatment temperature to "heat treatment temperature -100°C" include, for example, air cooling and furnace cooling.

In the heat treatment process, there is no particular limitation on the cooling rate after reaching the "heat treatment temperature -100°C" by cooling, for example air cooling.

(Other processes)
Manufacturing method X may include other steps than the steps described above.
For example, a pipe drawing step of drawing an electric resistance welded steel pipe may be included between the preparation step and the heat treatment step.
Conditions for tube drawing in the tube drawing step include, for example, a condition that the area reduction rate is 20 to 50%.

[Manufacturing method of automobile parts]
The manufacturing method of the automobile part of the present disclosure includes the step of subjecting the electric resistance welded steel pipe of the present disclosure described above to cold working and quenching in this order to obtain the automobile part.
According to the manufacturing method of the automobile component of the present disclosure, the same effects as those of the electric resistance welded steel pipe of the present disclosure are exhibited.

In the method for manufacturing an automobile component of the present disclosure, tempering may be further performed after quenching.
In the method for manufacturing an automobile part of the present disclosure, cold working is performed using a dedicated mold.
The manufacturing method of the automobile part of the present disclosure uses the electric resistance welded steel pipe of the present disclosure to produce the automobile part, so that the life of the mold can be extended.
Automotive parts include, for example, rack bars, drive shafts, transmission shaft stators, and the like.

Examples of the present disclosure are shown below, but the present disclosure is not limited to the following examples.
Underlines in Tables 1 and 2 indicate outside the scope of the present disclosure or outside the range of preferred manufacturing conditions.

[Examples 1 to 9, Comparative Examples 1 to 14]
An electric resistance welded steel pipe for automobile parts was manufactured by the manufacturing method X described above.
Details are shown below.

<Azroll ERW steel pipe preparation process>
Hot coils made of hot-rolled steel sheets (thickness 2.0 to 10.0 mm) having the chemical compositions of steel grades A to N shown in Table 1 were prepared.
Specifically, REM in steel types B, D, G, and J is Ce.

The hot-rolled steel sheet is unwound from the hot coil, the hot-rolled steel sheet unwound from the hot coil is roll-formed to form an open pipe, and the butt portions of the obtained open pipe are electric resistance welded to form an electric resistance welded portion A. As-rolled electric resistance welded steel pipes having the wall thicknesses and outer diameters shown in Table 2 were manufactured by forming.

<Pipe drawing process>
In Table 2, in the examples described as "yes" in the stretched column, the azurol electric resistance welded steel pipe was stretched so that the area reduction rate was 23%, and the azurol electric resistance welded steel pipe was stretched. was subjected to heat treatment in the following heat treatment step.
In other examples and comparative examples, the as-rolled electric resistance welded steel pipes were not subjected to this drawing but were subjected to heat treatment in the following heat treatment step.

<Heat treatment process>
Heat treatment under the conditions shown in Table 2 was performed on the as-rolled electric resistance welded steel pipe.
However, in Comparative Example 11, no heat treatment was applied to the as-rolled electric resistance welded steel pipe. This Comparative Example 11 is an example imitating the steel pipe described in the aforementioned Patent Document 8 (that is, Japanese Patent No. 5679115).
In Comparative Example 13, the as-rolled electric resistance welded steel pipe was subjected to heating at 900°C for 10 minutes, air cooling, 23% drawing, heating at 720°C for 10 minutes, and air cooling. Comparative Examples 13 and 12 (heating temperature: 900° C. in both cases) are examples imitating the steel pipe described in the aforementioned Patent Document 5 (that is, JP-A-2004-190086).
As described above, an electric resistance welded steel pipe for automobile parts was obtained.

When the steel structure of the base metal portion of the electric resistance welded steel pipe for automobile parts was confirmed by the method described above, the steel structure was a ferrite-pearlite mixed structure in all of the examples and comparative examples.

<Hardness of base material of electric resistance welded steel pipe for automobile parts>
The hardness (hardness before quenching) of the base metal portion of the electric resistance welded steel pipe for automobile parts was measured by the method described above.
Table 2 shows the results.

<Quenching of electric resistance welded steel pipes for automobile parts>
For electric resistance welded steel pipes for automobile parts, a heating temperature of 900 ° C., a heating time of 1 minute, a cooling rate (A) after heating of 30 ° C./s, and a cooling ultimate temperature of 50 ° C. to 0 ° C. at the above cooling rate (A) Quenching conditions were applied.

<Hardness after quenching>
The hardness after quenching and the hardness difference in the wall thickness direction of the quenched electric resistance welded steel pipe for automobile parts were measured by the method described above.
Table 2 shows the results.

As shown in Tables 1 and 2, the chemical composition of the base metal portion is the chemical composition in the present disclosure, the steel structure of the base metal portion is a ferrite-pearlite mixed structure, and the hardness of the base metal portion is 110. The electric resistance welded steel pipes for automobile parts of each example having a hardness of up to 150 Hv had an appropriate hardness after quenching.
The electric resistance welded steel pipe for automotive parts of each example has a hardness of 150 Hv or less in the base material portion, so it is considered to be excellent in cold workability and also excellent in the life of the mold used for cold working.

On the other hand, the results of the comparative examples were as follows.
In Comparative Example 1, in which the C content was too small in the chemical composition of the base material portion, the hardness after quenching was insufficient.
In Comparative Example 2 in which the B content was too small in the chemical composition of the base material portion, the hardness difference ΔHv in the thickness direction was excessive.
In Comparative Example 3 in which the Si content and Mn content were too high in the chemical composition of the base material, the hardness of the base material exceeded 150 Hv.
In Comparative Example 4 in which the C content was too high in the chemical composition of the base material, the hardness of the base material exceeded 150 Hv.
Although the chemical composition of the base material is the chemical composition in the present disclosure, the hardness of the base material exceeded 150 Hv in Comparative Examples 5 to 7 in which the heating temperature in the heat treatment step was too low.
In Comparative Example 8 in which the chemical composition of the base material contained too much Si, the hardness of the base material exceeded 150 Hv.
In Comparative Example 9 in which the Mn content was too high in the chemical composition of the base material, the hardness of the base material exceeded 150 Hv.
In Comparative Example 10, in which the Ti content was too low in the chemical composition of the base material portion, the hardness after quenching was insufficient.
Although the chemical composition of the base material is the chemical composition in the present disclosure, in Comparative Example 11 in which the heat treatment process was not performed, the hardness of the base material exceeded 150 Hv.
Although the chemical composition of the base material is the chemical composition in the present disclosure, in Comparative Example 12 in which the heating temperature in the heat treatment step was too high, the hardness of the base material exceeded 150 Hv.
The chemical composition of the base metal part is the chemical composition in the present disclosure, and the as-rolled electric resistance welded steel pipe was subjected to heating at 900 ° C. for 10 minutes, air cooling, 23% drawing, heating at 720 ° C. for 10 minutes, and air cooling. In Comparative Example 13, the hardness of the base material portion was over 150 Hv.
Although the chemical composition of the base material is the chemical composition in the present disclosure, in Comparative Example 14 in which the heating time in the heat treatment step was too long, the hardness of the base material exceeded 150 Hv.

Claims

Including the base material part and the electric resistance welding part,
The chemical composition of the base material portion is, in mass %,
C: 0.42 to 0.48%,
Si: 0.01 to 0.20%,
Mn: 0.10-0.70%,
P: 0 to 0.030%,
S: 0 to 0.030%,
Al: 0.005 to 0.050%,
Ti: 0.005 to 0.040%,
B: 0.0005 to 0.0050%,
N: 0 to 0.005%,
O: 0 to 0.005%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
REM: 0 to 0.0050%,
Cr: 0 to 0.50%,
Ni: 0 to 0.50%,
Cu: 0-0.50%,
Nb: 0 to 0.10%,
Mo: 0-0.50%,
V: 0 to 0.20%, and
Balance: Fe and impurities,
The steel structure of the base material portion is a ferrite-pearlite mixed structure,
The hardness of the base material portion is 110 to 150 Hv,
Electric resistance welded steel pipes for automobile parts.
The chemical composition of the base material portion is, in mass %,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005-0.0050%,
REM: 0.0005 to 0.0050%,
Cr: 0.01 to 0.50%,
Ni: 0.01 to 0.50%,
Cu: 0.01-0.50%,
Nb: 0.01 to 0.10%,
Mo: 0.01 to 0.50%, and
V: 0.01-0.20%
containing one or more of
The electric resistance welded steel pipe for automobile parts according to claim 1.
The heating temperature is 900 ° C., the heating time is 1 minute, the cooling rate after heating is 30 ° C./s, and the cooling temperature at the cooling rate is within the range of 50 ° C. to 0 ° C. The hardness when quenched is 650 to 800Hv,
The electric resistance welded steel pipe for automobile parts according to claim 1 or 2.
A method for manufacturing automobile parts, comprising the step of subjecting the electric resistance welded steel pipe for automobile parts according to any one of claims 1 to 3 to cold working and quenching in this order to obtain automobile parts.