WO2020178943A1

WO2020178943A1 - Electric resistance welded steel pipe for line pipes

Info

Publication number: WO2020178943A1
Application number: PCT/JP2019/008380
Authority: WO
Inventors: 孝聡福士; 長谷川　昇; 雅和尾▲崎▼; 祐輔市瀬; 健介長井
Original assignee: 日本製鉄株式会社
Priority date: 2019-03-04
Filing date: 2019-03-04
Publication date: 2020-09-10
Also published as: JP6575734B1; JPWO2020178943A1

Abstract

An electric resistance welded steel pipe for line pipes, in which a matrix part has a chemical composition comprising, in % by mass, 0.03 to 0.10% of C, 0.03 to 0.60% of Si, 0.30 to 1.60% of Mn, 0.010 to 0.200% of Ti, 0.005 to 0.500% of Al, 0.010 to 0.050% of Nb, 0.0001 to 0.0200% of Ca, and a remainder made up by Fe and impurities, wherein the area ratio of ferrite is 60% or more, the average ferrite grain diameter is 10 μm or less and the ratio of the largest ferrite grain diameter to the average ferrite grain diameter is 3.0 or less in the metal structure of a center part of the matrix part as observed in the thickness direction of the matrix part, the TS is 400 to 700 MPa, the YS is 300 to 650 MPa, the YR is 95% or less, and yield elongation is observed when a tensile test in the tube axis direction is carried out.

Description

Electric resistance sewn steel pipe for line pipe

The present disclosure relates to an electric resistance welded steel pipe for a line pipe.

A line pipe is widely known as one of transportation means for crude oil, natural gas and the like.
In recent years, various studies have been made on electric resistance sewn steel pipes used as line pipes (that is, electric sewn steel pipes for line pipes).

For example, Patent Document 1 describes a hot-rolled steel plate for high-strength line pipes having excellent HIC resistance, which is suitable as a material for high-strength electric sewing pipe line pipes of API X70 or higher, and has a specific chemical composition. The metal structure of the segregation part is a bainitic ferrite structure with an area fraction of 95% or more, the average particle size of the bainitic ferrite structure is 8.0 μm or less, and a heat for high strength line pipe with a tensile strength of 540 MPa or more. A rolled steel sheet is disclosed.

Further, in Patent Document 2, an electric resistance welded steel pipe for a line pipe having excellent sour resistance, having a certain degree of tensile strength and yield strength, a reduced yield ratio, and excellent toughness of a base material portion and an electric resistance welded portion. Has a specific chemical composition, the area ratio of the first phase made of ferrite is 40 to 80%, the remaining second phase contains tempered bainite, and the yield strength in the tube axial direction is 390 to 562 MPa. The tensile strength in the pipe axis direction is 520 to 690 MPa, the yield ratio in the pipe axis direction is 93% or less, and the Charpy absorbed energy in the pipe circumferential direction in the base material is 100 J or more at 0° C. An electrosewn steel pipe for a line pipe having a charpy absorption energy in the pipe circumferential direction at a sewing welded portion of 80 J or more at 0 ° C. is disclosed.

Further, in Patent Document 3, in order to suppress an increase in Y / T during pipe making of a thick-walled electric resistance steel pipe, the structure of a hot-rolled steel sheet serving as a base steel sheet is controlled to bend and bend back. As a thick-walled ERW steel pipe of API X60 to X70 grade, which has low Y/T so that buckling does not occur and has excellent low temperature toughness, it has a specific chemical composition and the metal structure of the base steel sheet is , The polygonal ferrite having an area ratio of 50 to 92%, the average particle diameter of the polygonal ferrite is 15 μm or less, the hardness of the electric resistance welded portion is Hv160 to 240, and the structure of the electric resistance welded portion is There are disclosed thick-walled welded steel pipes which are bainite, fine-grained ferrite and pearlite, or fine-grained ferrite and bainite.

Japanese Patent No. 5884201 International Publication No. 2018/008194 Japanese Patent No. 5293903

By the way, it may be required to further improve the sour resistance (that is, the resistance to sour gas) after strain is applied to the electrosewn steel pipe for line pipes.
An object of the present disclosure is to provide an electric resistance welded steel pipe for a line pipe that has excellent sour resistance after strain is applied.

Means for solving the above problems include the following aspects.
<1> Including a base material portion and an electric resistance welded portion,
The chemical composition of the base material part is mass%,
C: 0.03 to 0.10%,
Si: 0.03 to 0.60%,
Mn: 0.30 to 1.60%,
P: 0 to 0.030%,
S: 0 to 0.0015%,
Ti: 0.010 to 0.200%,
Al: 0.005 to 0.500%,
Nb: 0.010 to 0.050%,
N: 0 to 0.006%,
O: 0 to 0.004%,
Ca: 0.0001 to 0.0200%,
Cu: 0 to 1.000%,
Ni: 0 to 1.000%,
Cr: 0 to 1.00%,
Mo: 0 to 0.50%,
V: 0 to 0.200%,
W: 0 to 0.100%,
B: 0 to 0.0050%,
Mg: 0-0.0200%,
Zr: 0 to 0.0200%,
REM: 0 to 0.0200%, and
The balance: Fe and impurities,
Ceq represented by the following formula (1) is 0.10 to 0.50,
The ESSP represented by the following formula (2) is 0 to 10.00,
In the metal structure of the central part of the wall thickness of the base material, the area ratio of ferrite is 60 to 90%, the balance contains at least one selected from the group consisting of tempered bainite and pearlite, and the balance The total area ratio of martensite and tempered martensite is less than 1% with respect to the entire balance,
In the metal structure at the center of the wall thickness of the base metal portion, the average ferrite grain size is 10 μm or less, and the ratio of the maximum ferrite grain size to the average ferrite grain size is 3.0 or less.
The tensile strength in the tube axis direction is 400 to 700 MPa,
The yield strength in the tube axis direction is 300 to 650 MPa,
The yield ratio in the tube axis direction is 95% or less,
ERW steel pipe for line pipe whose yield elongation is observed when a tensile test is conducted in the pipe axial direction.
Ceq=C+Mn/6+Cr/5+(Ni+Cu)/15+Nb+Mo+V... Formula (1)
EESP=Ca×(1-124O)/1.25S... Formula (2)
[In Formula (1) and Formula (2), each element symbol represents the mass% of each element. ]
<2> The chemical composition of the base material part is% by mass,
Cu: more than 0% and 1.000% or less,
Ni: more than 0% and 1.000% or less,
Cr: more than 0% and 1.00% or less,
Mo: more than 0% and 0.5% or less,
V: more than 0% and 0.200% or less,
W: more than 0% and 0.100% or less,
B: more than 0% and 0.0050% or less,
Mg: more than 0% and 0.0200% or less,
Zr: more than 0% and 0.0200% or less, and
REM: The electrosewn steel pipe for a line pipe according to <1>, which contains at least one selected from the group consisting of more than 0% and 0.0200% or less.
<3> The electric resistance welded steel pipe for a line pipe according to <1> or <2>, wherein the dislocation density in the central portion of the thickness of the base material is 2.0×10 ¹⁵ m ⁻² or less.
<4> The electric resistance welded steel pipe for a line pipe according to any one of <1> to <3>, having a wall thickness of 5 to 20 mm and an outer diameter of 100 to 400 mm.

According to the present disclosure, an electric resistance welded steel pipe for a line pipe having excellent sour resistance after being subjected to strain is provided.

In the present disclosure, the numerical range represented by using "-" means a range including the numerical values before and after "-" as the lower limit value and the upper limit value.
In the present disclosure, “%” indicating the content of the component (element) means “mass %”.
In the present disclosure, the content of C (carbon) may be referred to as “C content”. The contents of other elements may be expressed in the same manner.
In the present disclosure, the term “process” is included in this term as long as the intended purpose of the process is achieved not only as an independent process but also when it cannot be clearly distinguished from other processes. ..

The electric-sewn steel pipe for line pipes (hereinafter, also simply referred to as “electrically-sewn steel pipe”) of the present disclosure includes a base material portion and an electric sewing welded portion, and the chemical composition of the base metal portion is mass% and C: 0. 03-0.10%, Si: 0.03-0.60%, Mn: 0.30-1.60%, P:0-0.030%, S:0-0.0015%, Ti:0 0.010 to 0.200%, Al: 0.005 to 0.500%, Nb: 0.010 to 0.050%, N: 0 to 0.006%, O: 0 to 0.004%, Ca: 0.0001 to 0.0200%, Cu: 0 to 1.000%, Ni: 0 to 1.000%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, V: 0 to 0 200%, W:0 to 0.100%, B:0 to 0.0050%, Mg:0 to 0.0200%, Zr:0 to 0.0200%, REM:0 to 0.0200%, and The balance: Fe and impurities, Ceq represented by the following formula (1) is 0.10 to 0.50, ESSP represented by the following formula (2) is 0 to 10.00, and mother In the metallographic structure of the central part of the thickness of the material, the area ratio of ferrite is 60 to 90%, the balance contains at least one selected from the group consisting of tempered bainite and pearlite, and the martensite in the balance And the total area ratio of tempered martensite is less than 1% with respect to the rest, the average ferrite grain size is 10 μm or less in the metal structure of the central portion of the wall thickness of the base material, and the maximum ferrite with respect to the average ferrite grain size. The particle size ratio is 3.0 or less, the tensile strength in the pipe axis direction is 400 to 700 MPa, the yield strength in the pipe axis direction is 300 to 650 MPa, and the yield ratio in the pipe axis direction is 95% or less. This is a bainite steel pipe in which yield elongation is observed when a tensile test in the axial direction of the pipe is performed.

Ceq=C+Mn/6+Cr/5+(Ni+Cu)/15+Nb+Mo+V... Formula (1)
EESP=Ca×(1-124O)/1.25S... Formula (2)
[In Formula (1) and Formula (2), each element symbol represents the mass% of each element. ]

In the present disclosure, the chemical composition of the base metal portion described above (including a Ceq of 0.10 to 0.50 and an ESSP of 0 to 10.00) is also referred to as the chemical composition in the present disclosure. Say.

The electric resistance welded steel pipe of the present disclosure includes a base material portion and an electric resistance welded portion.
ERW steel pipe is generally formed by forming a hot-rolled steel sheet into a tube (hereinafter, also referred to as "roll forming") to form an open pipe, and the resulting open pipe is welded by electric resistance welding. It is manufactured by forming a portion (electric resistance welded portion) (hereinafter, the process up to here is also referred to as “pipe making”), and then subjecting the electric resistance welded portion to seam heat treatment, if necessary.
In the electric resistance welded steel pipe of the present disclosure, the base metal portion refers to a portion other than the electric resistance welded portion and the heat affected zone.
Here, the heat affected zone (hereinafter, also referred to as “HAZ”) refers to the influence of heat due to electric resistance welding (when seam heat treatment is performed after electric resistance welding, heat due to electric resistance welding and seam heat treatment). (Influenced by) refers to the affected part.

The hot-rolled steel plate, which is a material for ERW steel pipe, is manufactured using a hot strip mill. Specifically, a hot strip mill manufactures a long hot-rolled steel sheet (hereinafter, also referred to as a hot coil) wound into a coil.
The hot-rolled steel plate, which is a material of the electric resistance welded pipe, is different from a steel plate manufactured by using a plate mill in that it is a long steel plate.
Since a steel plate is not a continuous steel sheet, it cannot be used for roll forming, which is a continuous bending process.
The electrosewn steel pipe is clearly distinguished from the welded steel pipe (for example, UOE steel pipe) manufactured by using the thick steel plate in that it is manufactured by using the hot-rolled steel plate described above.

In the electric resistance welded steel pipe of the present disclosure, yield elongation is observed when a tensile test in the pipe axial direction is performed.
The fact that the yield elongation is observed when performing the pipe axial tensile test indicates that the electric resistance welded steel pipe of the present disclosure is the electric resistance welded steel pipe manufactured by subjecting it to heat treatment after pipe forming. ..
The electric resistance welded steel pipe of the present disclosure is heat-treated before pipe making (that is, a hot-rolled steel sheet that is a raw material) and is not heat treated after pipe making, and before pipe making In addition, in the electrosewn steel pipes that have not been heat-treated even after the pipes are made, no yield elongation is observed when the pipe axial tensile test is performed.

More specifically, in the electric resistance welded steel pipe of the present disclosure,
Yield elongation is observed when performing a pipe axial tensile test,
The tensile strength in the pipe axis direction is 700 MPa or less,
The area ratio of ferrite is 60 to 90% in the metal structure of the central portion of the wall thickness of the base material, and
The fact that the total area ratio of martensite and tempered martensite in the balance is less than 1% with respect to the entire balance means that the electric resistance welded steel pipe of the present disclosure is tempered without being quenched after pipe forming (hereinafter, "pipe forming"). It is indicated that this is an electric resistance welded steel pipe manufactured by performing "tempering".
With respect to the electric resistance welded steel pipe of the present disclosure, in the electric resistance welded steel pipe manufactured by quenching and tempering after pipe manufacturing, the area ratio of the ferrite is less than 60%, and the total area ratio of martensite and tempered martensite in the balance is Is more than 1%, and the tensile strength in the tube axis direction exceeds 700 MPa.

The electric resistance welded steel pipe of the present disclosure has a tensile strength in the above range, a yield strength in the above range, and a yield ratio in the above range. This ensures the performance as a line pipe. Under this premise, the electric resistance welded steel pipe of the present disclosure has excellent sour resistance after being subjected to strain.
The "sour resistance" in the present disclosure means resistance to hydrogen-induced cracking (HIC) (hereinafter, also referred to as "HIC resistance").

As described above, it may be required for the electric resistance welded steel pipe for a line pipe to further improve the sour resistance after the strain is applied. The electric resistance welded steel pipe of the present disclosure satisfies such requirements.
For example, in recent years, from the viewpoint of reducing the laying cost of the electric resistance welded steel pipe for line pipes, the reeling method has been often adopted as a method of laying the electric resistance welded steel pipe for line pipes. Here, the reeling method is to first wind the ERW steel pipe for a line pipe around a drum on land, then carry the ERW steel pipe for a line pipe around the drum to the sea, and then the line around the drum. It is a method of laying on the seabed while unwinding the ERW steel pipe for pipes on the sea. In the reeling method, strain is applied to the ERW steel pipe for a line pipe when the ERW steel pipe for a line pipe is wound around a drum and when the ERW steel pipe for a line pipe is unwound. Therefore, for example, in an electrosewn steel pipe for a line pipe used in a reeling method, it may be required to further improve sour resistance after strain is applied.

The effect of excellent sour resistance after strain is applied is mainly due to the effect.
(1) The chemical composition of the base material part is the chemical composition according to the present disclosure,
(2) The area ratio of ferrite is 60 to 90% in the metal structure at the center of the wall thickness of the base metal part (generally speaking, the metal structure is mainly ferrite).
(3) In the metal structure at the center of the wall thickness of the base metal portion, the average ferrite grain size is 10 μm or less (generally speaking, the ferrite grains are miniaturized).
(4) In the metal structure at the center of the wall thickness of the base metal, the ratio of the maximum ferrite grain size to the average ferrite grain size is 3.0 or less (generally speaking, the ferrite grains are sized. Being) and
(5) Yield elongation was observed when a tensile test in the pipe axis direction was performed, the tensile strength in the pipe axis direction was 700 MPa or less, and the area ratio of ferrite was 60 to 60 in the metal structure of the central portion of the wall thickness of the base material. 90%, and the total area ratio of martensite and tempered martensite in the balance is less than 1% with respect to the entire balance (that is, it is an electric resistance welded steel pipe manufactured by tempering after pipe forming). thing)
However, it is considered that they have contributed.

The reason why (1) above contributes to the above effect will be described in the chemical composition description below.

The reason why the above (2) contributes to the above effect is presumed as follows.
When there is a locally high dislocation density part in the structure of steel, that part becomes a hydrogen trap site, and as a result, hydrogen induced cracking (HIC) is likely to occur. Therefore, in order to improve the sour resistance (that is, the HIC resistance), it is considered effective to suppress the occurrence of locally high dislocation density portions.
When the structure of steel is bainite or martensite having a lath structure, when strain is applied by the Reeling method etc., the lath wall becomes a barrier, and crystal grains that are difficult to deform with respect to the maximum principal stress and easily deform It is considered that there are crystal grains, and as a result, a portion having a large deformation and a portion having a small deformation are generated. Then, it is considered that excessive dislocations are introduced at the site where the deformation is large, and the dislocation density at that site increases locally.
On the other hand, when the structure of steel is mainly ferrite, dislocations can move freely in 12 slip systems when strain is applied by the reeling method, etc., so that it is almost uniform in each crystal grain. It is considered that dislocations are dispersed in. As a result, it is considered that the occurrence of locally high dislocation density is suppressed and the sour resistance is improved.
Further, since ferrite is generated by diffusion transformation, it is advantageous from the viewpoint of reducing the initial dislocation density (that is, the dislocation density before strain is applied) as compared with bainite or martensite generated by shear deformation. is there.

The reason why the above (3) contributes to the above effect is presumed as follows.
Dislocations introduced into the structure of steel when strain is applied are likely to collect at grain boundaries. Therefore, it is considered that when strain is applied, the dislocation density in the grain boundaries becomes high and the dislocation density in the grains becomes low.
When the above (3) is satisfied (that is, when the ferrite grains are refined), the region within the grains with a low dislocation density becomes narrower, so when viewed as the entire steel structure, dislocations locally occur. It is considered that the generation of high density areas is suppressed. As a result, sour resistance is considered to be improved.

The reason why the above (4) contributes to the above effect is presumed as follows.
When coarse crystal grains are present in the structure of steel, the dislocation density within the grains of the crystal grains is low, and the dislocation density of the grain boundaries surrounding the crystal grains is high. Specifically, it is considered that grain boundaries surrounding coarse crystal grains) are generated.
When the above (4) is satisfied (that is, the ferrite grains are sized and the generation of coarse crystal grains is suppressed), the occurrence of locally high dislocation density is suppressed. Therefore, it is considered that the sour resistance is improved.

The reason why the above (5) contributes to the above effect is presumed as follows.
When the above (5) is satisfied (that is, in the case of an electric resistance welded steel pipe manufactured by being subjected to tempering after pipe forming), the initial (ie, strain is imparted by the tempering after pipe forming. Since the dislocation density (before) is reduced, it is believed that the dislocation density in the steel after strain is also reduced. As a result, hydrogen trap sites (locations where the dislocation density is locally high) are reduced, and it is considered that hydrogen-induced cracking (HIC) is suppressed (that is, sour resistance is improved).
The specific upper limit of the initial dislocation density is not particularly limited, but the upper limit is, for example, 2.0 × 10 ¹⁵ m- ² .

[Evaluation method of sour resistance after strain is applied]
In the present disclosure, the sour resistance (that is, HIC resistance) after strain is applied is evaluated as follows.
A test piece specified in NACE (National Association of Corrosion and Engineer) TM0284-2003 is collected from the 180 ° position of the base metal of the electric resistance pipe. The longitudinal direction of this test piece is the pipe axis direction of the electric resistance welded steel pipe.
A tensile tester is used to apply a tensile strain (strain amount 5%) in the longitudinal direction of the test piece (that is, the tube axis direction) to the test piece.
Using the test piece to which tensile strain is applied, the HIC resistance test is carried out in accordance with NACE TM0284-2003 as follows.
As the test liquid, a test liquid (pH 2.7) obtained by saturating Solution A liquid (5 mass% NaCl+0.5 mass% glacial acetic acid aqueous solution) with 1 atm of H ₂ S gas is used.
The test piece to which the above tensile strain is applied is immersed in this test solution for 96 hours. The total area of cracks on the upper surface (the surface with the largest area) of the test piece after immersion for 96 hours is measured by an ultrasonic flaw detector.
Based on the measurement result, the ratio (%) of the total area of cracks to the area of the upper surface is obtained, and the obtained ratio (%) is defined as CAR (crack area ratio) (%).
The smaller the CAR (%), the better the sour resistance after the strain is applied.

[Chemical composition of base metal part]
Hereinafter, regarding the chemical composition of the base material portion (that is, the chemical composition in the present disclosure), first, each component in the chemical composition will be described, and then Ceq and EESP will be described.

C: 0.03 to 0.10%
C is an element that improves the strength of steel. From the viewpoint of obtaining such effects, the C content is 0.03% or more. The C content is preferably 0.04% or more, more preferably 0.05% or more.
On the other hand, if the C content exceeds 0.10%, the formation of carbides may be promoted and the sour resistance may deteriorate. Therefore, the C content is 0.10% or less. The C content is 0.09% or less, more preferably 0.08% or less.

Si: 0.03 to 0.60%
Si is a deoxidizing element. From the viewpoint of obtaining such effects, the Si content is 0.03% or more. The Si content is preferably 0.10% or more, more preferably 0.20% or more.
On the other hand, if the Si content exceeds 0.60%, a large amount of Si oxide is generated, which may deteriorate the sour resistance. Therefore, the Si content is 0.60% or less. The Si content is preferably 0.50% or less, more preferably 0.40% or less.

Mn: 0.30 to 1.60%
Mn is an element that improves the strength of steel. From the viewpoint of obtaining such an effect, the Mn content is 0.30% or more. The Mn content is preferably 0.40% or more, and more preferably 0.50% or more.
On the other hand, if the Mn content exceeds 1.60%, coarse MnS may be generated and the sour resistance may deteriorate. Therefore, the Mn content is 1.60% or less. The Mn content is preferably 1.40% or less, more preferably 1.30% or less.

P: 0 to 0.030%
P is an impurity element, and the smaller the amount, the better. If the P content exceeds 0.030%, the sour resistance 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%. From the viewpoint of reducing the dephosphorization cost, the P content may be more than 0%, may be 0.001% or more, and may be 0.010% or more.

S: 0 to 0.0015%
S is an impurity element, and the smaller the amount, the better. If the S content exceeds 0.0015%, coarse MnS elongated in the rolling direction may occur, and the sour resistance may deteriorate. Therefore, the S content is 0.0015% or less. The S content is preferably 0.0014% or less.
The S content may be 0%. From the viewpoint of reducing the desulfurization cost, the S content may be more than 0%, may be 0.0001% or more, and may be 0.0005% or more.

Ti: 0.010 to 0.200%
Ti is an element necessary for refining the γ grain size by the pinning effect and consequently refining the ferrite grain size. From the viewpoint of obtaining such effects, the Ti content is 0.010% or more. The Ti content is preferably 0.020% or more, more preferably 0.030% or more.
On the other hand, when the Ti content exceeds 0.200%, coarse inclusions such as TiN may occur and the sour resistance may deteriorate. Therefore, the Ti content is 0.200% or less. The Ti content is preferably 0.150% or less, more preferably 0.110% or less, and further preferably 0.100% or less.

Al: 0.005 to 0.500%
Al is a deoxidizing element. From the viewpoint of obtaining such effects, the Al content is 0.005% or more. The Al content is preferably 0.010% or more, more preferably 0.020% or more.
On the other hand, if the Al content exceeds 0.500%, a large number of coarse Al—Ca-based inclusions are generated, and the sour resistance deteriorates. Therefore, the Al content is 0.500% or less. The Al content is preferably 0.200% or less, more preferably 0.100% or less, still more preferably 0.050% or less.

Nb: 0.010 to 0.050%
Nb is an element necessary for refining the γ grain size due to the pinning effect, and as a result, refining the ferrite grain size. From the viewpoint of obtaining such effects, the Nb content is 0.010% or more. The Nb content is preferably 0.020% or more.
On the other hand, if the Nb content exceeds 0.050%, coarse Nb carbonitrides may be generated, and the sour resistance may deteriorate. Therefore, the Nb content is 0.050% or less. The Nb content is preferably 0.040% or less.

N: 0 to 0.006%
N is an impurity element, and a small amount is preferable. If the N content exceeds 0.006%, coarse nitrides (for example, TiN, NbN, etc.) are generated, and sour resistance may deteriorate. Therefore, the N content is 0.006% or less. The N content is preferably 0.005% or less.
The N content may be 0%. From the viewpoint of reducing the denitrification cost, the N content may be more than 0%, may be 0.001% or more, and may be 0.003% or more.

O: 0 to 0.004%
O is an impurity element, and the smaller the amount, the better. If the O content exceeds 0.004%, the formation of CaO may impair the effect of Ca described later and deteriorate the sour resistance. Therefore, the O content is 0.004% or less. The O content is preferably 0.003% or less.
The O content may be 0%. From the viewpoint of reducing the deoxidation cost, the O content may be more than 0% or may be 0.001% or more.

Ca: 0.0001 to 0.0200%
Ca is an element that forms composite inclusions together with MnS and is finely dispersed in the form of composite inclusions to improve sour resistance.
From the viewpoint of obtaining such effects, the Ca content is 0.0001% or more. The Ca content is preferably 0.0005% or more.
On the other hand, when the Ca content exceeds 0.0200%, coarse Al—Ca-based inclusions are generated, and sour resistance may deteriorate. Therefore, the Ca content is 0.0200% or less. The Ca content is preferably 0.0150% or less, more preferably 0.0100% or less.

Cu: 0 to 1.000%
Cu is an arbitrary element. That is, the Cu content may be 0%.
Cu is an element that can contribute to improving the strength of steel. From the viewpoint of obtaining such effects, the Cu content may be more than 0%, may be 0.001% or more, and may be 0.010% or more.
On the other hand, if the Cu content exceeds 1.000%, the effect may be saturated and the cost may be increased. Therefore, the Cu content is 1.000% or less. The Cu content is preferably 0.500% or less, more preferably 0.200% or less, and further preferably 0.100% or less.

Ni: 0 to 1.000%
Ni is an arbitrary element. That is, the Ni content may be 0%.
Ni is an element that can contribute to improving the strength of steel. From the viewpoint of obtaining such effects, the Ni content may be more than 0%, may be 0.001% or more, and may be 0.010% or more.
On the other hand, if the Ni content exceeds 1.000%, the effect may be saturated and the cost may increase. Therefore, the Ni content is 1.000% or less. The Ni content is preferably 0.500% or less, more preferably 0.200% or less.

Cr: 0 to 1.00%
Cr is an arbitrary element. That is, the Cr content may be 0%.
Cr is an element that can contribute to improving the strength of steel. From the viewpoint of obtaining such an effect, the Cr content may be more than 0%, 0.01% or more, or 0.05% or more.
On the other hand, when the Cr content exceeds 1.00%, the effect is saturated and the cost may be increased. Therefore, the Cr content is 1.00% or less. The Cr content is preferably 0.50% or less, more preferably 0.20% or less.

Mo: 0 to 0.50%
Mo is an arbitrary element. That is, the Mo content may be 0%.
Mo is an element that can contribute to improving the strength of steel. From the viewpoint of obtaining such effects, the Mo content may be more than 0%, 0.01% or more, and may be 0.05% or more.
On the other hand, if the Mo content exceeds 0.50%, the effect may be saturated and the cost may increase. Therefore, the Mo content is 0.50% or less. The Mo content is preferably 0.30% or less, more preferably 0.20% or less.

V: 0 to 0.200%
V is an arbitrary element. That is, the V content may be 0%.
V is an element that can contribute to improving the strength of steel. From the viewpoint of obtaining such effects, the V content may be more than 0%, may be 0.001% or more, and may be 0.005% or more.
On the other hand, if the V content exceeds 0.200%, the effect may be saturated and the cost may increase. Therefore, the V content is 0.200% or less. The V content is preferably 0.100% or less, more preferably 0.050% or less, and further preferably 0.020% or less.

W: 0 to 0.100%
W is an arbitrary element. That is, the W content may be 0%.
W is an element that can contribute to improving the strength of steel. From the viewpoint of obtaining such effects, the W content may be more than 0%, may be 0.001% or more, and may be 0.005% or more.
On the other hand, when the W content exceeds 0.100%, the effect is saturated and the cost may be increased. Therefore, the W content is 0.100% or less. The W content is preferably 0.050% or less, more preferably 0.020% or less.

B: 0 to 0.0050%
B is an arbitrary element. That is, the B content may be 0%.
B is an element that can contribute to the strength improvement of steel. From the viewpoint of obtaining such effects, the B content may be more than 0%, may be 0.0001% or more, and may be 0.0005% or more.
On the other hand, if the B content exceeds 0.0050%, the effect may be saturated and the cost may increase. Therefore, the B content is 0.0050% or less. The B content is preferably 0.0040% or less, more preferably 0.0020% or less.

Mg: 0-0.0200%
Mg is an optional element. That is, the Mg content may be 0%.
Mg is an element that can contribute to the strength improvement of steel. From the viewpoint of obtaining such an effect, the Mg content may be more than 0% or 0.0001% or more.
On the other hand, if the Mg content exceeds 0.0200%, the effect may be saturated and the cost may increase. Therefore, the Mg content is 0.0200% or less. The Mg content is preferably 0.0040% or less, more preferably 0.0020% or less.

Zr: 0 to 0.0200%
Zr is an arbitrary element. That is, the Zr content may be 0%.
Zr is an element that can contribute to improving the strength of steel. From the viewpoint of obtaining such an effect, the Zr content may be more than 0% or may be 0.0001% or more.
On the other hand, if the Zr content exceeds 0.0200%, the effect is saturated and the cost may increase. Therefore, the Zr content is 0.0200% or less. The Zr content is preferably 0.0040% or less, more preferably 0.0020% or less.

REM: 0-0.0200%
REM is an arbitrary element. Therefore, the REM content may be 0%.
Here, "REM" is a rare earth element, that is, a group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Refers to at least one element selected.
REM is an element that controls inclusions in steel and can contribute to improving the strength of steel. From the viewpoint of obtaining such effects, the REM content may be more than 0%, may be 0.0001% or more, and may be 0.0010% or more.
On the other hand, if the REM content exceeds 0.0100%, the number of coarse inclusions may increase and the sour resistance may deteriorate. Therefore, the REM content is 0.0200% or less. The REM content is preferably 0.0100% or less, more preferably 0.0050% or less.

The chemical composition of the base material portion is Cu: more than 0% and not more than 1.000%, Ni: more than 0% and not more than 1.000%, Cr: more than 0% and not more than 1.00% from the viewpoint of obtaining the effect of the above-mentioned arbitrary element. Below, Mo: more than 0% and 0.5% or less, V: more than 0% and 0.200% or less, W: more than 0% and 0.100% or less, B: more than 0% and 0.0050% or less, Mg: 0% It may contain at least one selected from the group consisting of super 0.0200% or less, Zr: 0% and 0.0200% or less, and REM: 0% and 0.0200% or less.
The more preferable content of each arbitrary element is as described above.

Remaining part: Fe and impurities In the chemical composition of the base metal part, the balance excluding each element described above is Fe and impurities.
Here, the impurity refers to a component contained in the raw material or a component mixed in the manufacturing process and not intentionally contained in the steel.
The impurities include all elements other than the above-mentioned elements. The element as an impurity may be only one kind or two or more kinds.
Examples of the impurities include Sb, Sn, Co, As, Pb, Bi, and H.
Of the above-mentioned elements, for example, Sb, Sn, Co, and As are mixed with a content of 0.1% or less, Pb and Bi are mixed with a content of 0.005% or less, and H are contained with a content of, for example. There can be mixed amounts of 0.0004% or less, but the content of other elements does not need to be controlled within the normal range.

Ceq: 0.10 to 0.50
In the chemical composition of the base material portion, Ceq represented by the following formula (1) is 0.10 to 0.50.
Ceq=C+Mn/6+Cr/5+(Ni+Cu)/15+Nb+Mo+V... Formula (1)
[In the formula (1), each element symbol represents mass% of each element. ]

When Ceq is less than 0.10, the strength of steel may be insufficient. Therefore, Ceq is 0.10. Ceq is preferably 0.12 or more, more preferably 0.14 or more.
If Ceq is more than 0.50, sour resistance may decrease. Therefore, Ceq is 0.50 or less. Ceq is preferably 0.45 or less, more preferably 0.42 or less.

EESP: 0 to 10.00
In the chemical composition of the base material portion, EESP represented by the following formula (2) is 0 to 10.00.
EESP=Ca×(1-124O)/1.25S... Formula (2)
[In the formula (2), each element symbol represents mass% of each element. ]

When the EESP is more than 10.00, the sour resistance may decrease. Therefore, EESP is 10.00 or less. EESP is preferably 9.50 or less, and more preferably 9.26 or less.
EESP may be 0, may be more than 0, may be 0.01 or more, and may be 0.06 or more.

[Metal structure of base metal]
In the electric resistance welded steel pipe of the present disclosure, the area ratio of ferrite (hereinafter, also referred to as “ferrite fraction”) is 60 to 90% in the metal structure of the central portion of the wall thickness of the base material, and the balance is tempered bainite and It contains at least one selected from the group consisting of pearlite, and the total area ratio of martensite and tempered martensite in the balance is less than 1% of the total balance.
Furthermore, in the electric resistance welded steel pipe of the present disclosure, the average ferrite grain size is 10 μm or less in the metal structure of the central portion of the wall thickness of the base metal portion, and the ratio of the maximum ferrite grain size to the average ferrite grain size is 3.0 or less. is there.

The above-mentioned features in the metal structure of the central part of the wall thickness of the base material include the chemical composition of the base material and the manufacturing conditions of the electrosewn steel pipe (the manufacturing conditions of the hot-rolled steel sheet as the material and the tempering conditions after the pipe making). For example, each of the following conditions in the production method A) can be realized.

In the present disclosure, the metal structure of the central portion of the base metal portion is confirmed by observing the metal structure of the central portion of the wall thickness in the L cross section at the position of 180 ° of the base metal.
Here, the central portion of the wall thickness in the L cross section at the 180 ° position of the base metal is only a position selected as a representative position of the base metal portion. Therefore, in the electric resistance welded steel pipe of the present disclosure, the metal structure at a position other than the central portion of the wall thickness in the L section at the base material 180° position in the base material portion may have the above characteristics.

In the present disclosure, the "base metal 180 ° position" means a position displaced by 180 ° in the pipe circumferential direction from the electric sewing welded portion. Here, the base material 180° position is a position selected as a representative position of the base material portion.
In the present disclosure, the “L cross section” refers to a cross section parallel to the tube axis direction and the wall thickness direction.

<Ferrite area ratio (Ferrite fraction)>
The area ratio of ferrite (ferrite fraction) in the metal structure of the central portion of the wall thickness of the base material is 60 to 90%.
The fact that the ferrite fraction is 60% or more (that is, the metal structure in the central portion of the base metal portion is mainly ferrite) contributes to the improvement of sour resistance. The ferrite fraction is preferably 65% or more.
The ferrite fraction of 90% or less contributes to the improvement of the tensile strength in the axial direction of the electric resistance welded steel pipe. The ferrite fraction is preferably 85% or less.

In the present disclosure, the ferrite fraction is measured according to JIS G 0551:2013.
Specifically, first, the central portion of the wall thickness of the E-welded steel pipe in the L section at the 180° position of the base material is polished, and then etched with a nital corrosive solution. A photograph of the etched metal structure of the central portion of the wall thickness is taken with an optical microscope at a magnification of 1000 times. The metal structure photograph is taken for three visual fields (one visual field is in the range of 100 μm×100 μm). The obtained metal structure photographs of the three visual fields are image-processed to obtain the ferrite fraction (that is, the area ratio of ferrite in the entire metal structure). The image processing is performed using, for example, a small general-purpose image analyzer LUZEX AP manufactured by Nireco Corporation.

<Remainder>
In the metal structure of the central portion of the thickness of the base metal part, the rest (that is, the portion other than ferrite) contains at least one selected from the group consisting of tempered bainite and pearlite, and the remaining martensite and tempered martensite. The total area ratio of the site is less than 1% of the rest.

In the present disclosure, the remaining part is confirmed based on the metallographic photographs of the three fields of view used for measuring the ferrite fraction.

The concept of "bainite" in the present disclosure includes both an upper bainite in which the morphology of bainitic ferrite is lath-shaped and a lower bainite in which the morphology of bainitic ferrite is platy [eg, Japan Metallurgical Society "Materia" Vol.46 (2007), No.5, pp.321-326].
“Tempered bainite” in the present disclosure is distinguished from bainite that is not tempered bainite in that it contains granular cementite in its structure.
The concept of "pearlite" in the present disclosure includes not only typical pearlite containing layered cementite, but also pseudo pearlite, tempered pearlite, tempered pseudo pearlite and the like.

Further, in the balance, the total area ratio of martensite and tempered martensite is less than 1% with respect to the whole balance, that the metal structure of the base metal part does not substantially contain martensite and tempered martensite. Means
Since the metal structure of the base material is substantially free of martensite having a lath structure and tempered martensite, local concentration of dislocations is suppressed (that is, dislocations are dispersed), and as a result, sour resistance is increased. The property is improved. In addition, since the metal structure of the base material does not substantially contain martensite and tempered martensite, it is possible to prevent the tensile strength of the electric resistance welded steel pipe in the pipe axis direction from becoming excessive.
The total area ratio of martensite and tempered martensite in the balance may be 0%.

<Average ferrite grain size>
In the metal structure of the central portion of the wall thickness of the base material, the average ferrite grain size is 10 μm or less.
As described above, the average ferrite grain size of 10 μm or less contributes to the improvement of sour resistance.
There is no particular lower limit to the average ferrite grain size. From the viewpoint of suitability for manufacturing steel, the average ferrite grain size is preferably 3 μm or more, more preferably 5 μm or more.

In the present disclosure, the average ferrite grain size is measured based on the metallographic photographs of the three fields of view used for the measurement of the ferrite fraction.
From these metallographic photographs, 100 ferrite grains are arbitrarily extracted, and the equivalent circle diameter of each ferrite grain is determined as the particle size. The obtained 100 measured values (particle size) are arithmetically averaged, and the obtained arithmetic mean value is taken as the average ferrite particle size.

<Ratio of maximum ferrite grain size to average ferrite grain size>
The ratio of the maximum ferrite grain size to the average ferrite grain size (hereinafter, also referred to as "maximum ferrite grain size / average ferrite grain size ratio") is 3.0 or less in the metal structure at the center of the wall thickness of the base metal portion. ..
As described above, the maximum ferrite grain size/average ferrite grain size ratio of 3.0 or less contributes to the improvement of sour resistance.
The lower limit of the maximum ferrite grain size/average ferrite grain size ratio is ideally 1.0. From the viewpoint of suitability for manufacturing steel, the lower limit of the maximum ferrite grain size/average ferrite grain size ratio may be 1.5 or 1.8.

In the present disclosure, the measurement of the maximum ferrite grain size/average ferrite grain size ratio is performed based on the metallographic photographs of the three visual fields used for the measurement of the ferrite fraction.
From these metallographic photographs, ferrite grains with the largest equivalent circle diameter are selected. The circle equivalent diameter of the selected ferrite grains is the maximum ferrite grain size.
The maximum ferrite grain size/average ferrite grain size ratio is determined by dividing the obtained maximum ferrite grain size by the above-mentioned average ferrite grain size.

<Dislocation density>
In the electric resistance welded steel pipe of the present disclosure, there is no particular limitation on the specific upper limit of the dislocation density at the central portion of the wall thickness of the base material. The upper limit of the dislocation density in the central portion of the thickness of the base material is, for example, 2.0×10 ¹⁵ m ⁻² .
Here, the dislocation density at the center of the wall thickness of the base metal portion is the dislocation density (that is, the initial dislocation density) before strain is applied by reeling laying or the like. The lower the initial dislocation density, the lower the dislocation density after strain is applied, and as a result, the sour resistance after strain is applied can be further improved.
Therefore, when the dislocation density in the central portion of the thickness of the base material is 2.0×10 ¹⁵ m ⁻² or less, the sour resistance after the strain is applied can be further improved.

Here, the dislocation density in the central portion of the thickness of the base material is determined as follows.
The half-widths of the (110) plane, the (211) plane, and the (220) plane were measured by X-ray diffraction for the central portion of the wall thickness in the L cross section at the base material 180° position, and based on the measured values, Williamson The dislocation density is calculated according to the -Hall method (specifically, the method described in ACTA METALLURGICA Vol.1, JAN. 1953, pp.22-31).
The above measurement and calculation are performed at three points in the central portion of the wall thickness, and the arithmetic mean value of the obtained three calculated values is defined as the dislocation density in the present disclosure.

The conditions of X-ray diffraction are as follows. As the X-ray diffraction device used for X-ray diffraction, for example, "RINT2200" manufactured by Rigaku Corporation is used.
Tube: Mo tube (tube using Mo as a target)
Target output: 50KV, 40mA
Slit: Divergence 1/2°, scattering 1°, received light 0.15mm
Sampling width: 0.010 °
Measuring range (2θ): 34.2° to 36.2°
Maximum count: 3000 or more

[Tensile strength in the tube axis direction (TS)]
The electric resistance welded steel pipe of the present disclosure has a tensile strength (TS) in the pipe axis direction of 400 to 700 MPa.
When TS in the pipe axis direction is 400 MPa or more, the strength of the electric resistance welded steel pipe for a line pipe is secured. The TS in the tube axis direction is preferably 450 MPa or more, more preferably 490 MPa or more.
When TS in the pipe axis direction is 700 MPa or less, bending deformability (that is, bending ease) when laying the electric resistance welded steel pipe for line pipe is further improved, and the buckling of the electric resistance welded steel pipe for line pipe is further improved. Is more suppressed. TS in the tube axis direction is preferably 650 MPa or less.

[Yield strength (YS) in the pipe axis direction]
The electric resistance welded steel pipe of the present disclosure has a yield strength (YS) in the pipe axis direction of 300 to 650 MPa.
When YS in the pipe axis direction is 300 MPa or more, the strength of the electric resistance welded steel pipe for a line pipe is secured. The YS in the tube axis direction is preferably 350 MPa or more, more preferably 360 MPa or more.
When YS in the pipe axis direction is 650 MPa or less, bending deformability (that is, easiness of bending) when laying an electric resistance welded steel pipe for a line pipe is further improved, and buckling of the electric resistance welded steel pipe for a line pipe is further improved. Is more suppressed. TS in the tube axis direction is preferably 620 MPa or less.

[Yield ratio in the axial direction (YR)]
The yield ratio (YR = (YS / TS) × 100) in the pipe axis direction of the electrosewn steel pipe of the present disclosure is 95% or less.
Thereby, bending deformability (that is, easiness of bending) when laying the electric resistance welded steel pipe for a line pipe is further improved, and buckling of the electric resistance welded steel pipe for a line pipe is further suppressed.
YR in the tube axis direction is preferably 91% or less.
The lower limit of YR in the pipe axis direction is preferably 80% or more, more preferably 85% or more, still more preferably 88% or more, from the viewpoint of suitability for manufacturing electric resistance welded steel pipe.

A YR of 95% or less in the pipe axis direction can be achieved by tempering after pipe making. It is considered that the reason for this is that the dislocation density decreases and YS decreases due to tempering after tube formation, and work hardening increases (that is, TS increases) due to the precipitation of cementite on the dislocations.

[Method of measuring TS, YS and YR]
In the present disclosure, TS in the tube axis direction, YS in the tube axis direction, and YR in the tube axis direction mean values measured as follows, respectively.
From the 180 ° position of the base metal of the electric resistance pipe, a test piece for a tensile test is collected in a direction in which the test direction (tensile direction) of the tensile test is the pipe axial direction of the electric resistance pipe. The shape of the test piece is a flat plate shape conforming to the American Petroleum Institute standard API 5L (hereinafter, simply referred to as “API 5L”).
Using the collected test piece, at room temperature, in accordance with API 5L, a tensile test (that is, a pipe axial direction tensile test) in which the test direction is the pipe axial direction of the electric resistance welded steel pipe is performed, and the result of the pipe axial direction tensile test Based on the above, TS in the pipe axial direction of the electric resistance pipe and YS in the pipe axial direction of the electric resistance pipe are obtained.
YR(%) in the axial direction of the electric resistance welded steel pipe is calculated by the calculation formula “YR(%)=(YS/TS)×100” based on TS and YR calculated above.

The TS in the above range, the YS in the above range, and the YR in the above range are the manufacturing conditions (materials) of the electric resistance pipe including the chemical composition of the base material and the manufacturing method of the hot-rolled steel sheet which is the raw material. Including the manufacturing conditions of the hot rolled steel sheet and the tempering conditions after pipe making;

[Wall thickness and outer diameter of ERW steel pipe]
There is no particular limitation on the wall thickness of the electric resistance welded steel pipe of the present disclosure.
The wall thickness is, for example, 5 to 20 mm, preferably 10 to 20 mm.
There is no particular limitation on the outer diameter of the electric resistance welded steel pipe of the present disclosure.
The outer diameter is preferably 100 to 400 mm, more preferably 150 to 400 mm, and further preferably 200 to 400 mm.

[Yield elongation]
Yield elongation is observed in the electrosewn steel pipe of the present disclosure when a tensile test is performed in the axial direction of the pipe.
In the present disclosure, "the yield elongation is observed when a pipe axial tensile test is performed" means that a substantial yield elongation (specifically, in the pipe axial tensile test described above for determining TS, YS, and YR). This means that a yield elongation of 1% or more) is observed.
As described above, in the electrosewn steel pipe manufactured by tempering after the pipe is made, the yield elongation is observed when the pipe axial tensile test is performed.

[One Example of Manufacturing Method of ERW Steel Pipe (Manufacturing Method A)]
Hereinafter, an example of a manufacturing method for manufacturing the electric resistance welded steel pipe of the present disclosure (hereinafter, referred to as “manufacturing method A”) will be described.
This manufacturing method A is a manufacturing method of an electrosewn steel pipe according to an embodiment described later.

Manufacturing method A is
A slab preparation step of preparing a slab having a chemical composition according to the present disclosure;
A hot-rolling process in which the prepared slab is hot-rolled under the conditions described below to obtain a hot-rolled steel sheet,
A cooling process in which the hot-rolled steel sheet is cooled until the temperature of the outer surface of the hot-rolled steel sheet reaches a winding temperature of 450 to 650 ° C.
A winding process of obtaining a hot coil made of a hot-rolled steel sheet by winding a hot-rolled steel sheet cooled to the above-mentioned winding temperature at the above-mentioned winding temperature.
By unrolling the hot-rolled steel sheet from the hot coil and rolling the unrolled hot-rolled steel sheet into an open pipe, the butt portion of the obtained open pipe is electric resistance welded to form an electric resistance welded portion. , A pipe-making process to obtain Azroll ERW steel pipe,
A post-pipe forming tempering process for tempering an Azroll electric resistance welded pipe under conditions of a tempering temperature of 500 to 700° C. and a tempering time of 1 to 120 minutes;
including.

Here, as-roll ERW steel pipe means ERW steel pipe that has not undergone heat treatment other than seam heat treatment after pipe making.

According to the manufacturing method A, the electric resistance welded steel pipe of the present disclosure can be manufactured.
Hereinafter, each step in the manufacturing method A will be described.

<Slab preparation process>
The slab preparation step in production method A is a step of preparing a slab having the chemical composition according to the present disclosure.
The step of preparing the slab may be a step of manufacturing the slab, or may be a step of simply preparing a slab that has been manufactured in advance.
When manufacturing a slab, for example, molten steel having the chemical composition according to the present disclosure is manufactured, and the manufactured molten steel is used to manufacture the slab. At this time, a slab may be produced by a continuous casting method, or an ingot may be produced using molten steel, and the ingot may be lump-rolled to produce a slab.

<Hot rolling process>
The hot-rolling step in the manufacturing method A is a step of heating the slab prepared above and hot-rolling the heated slab to obtain a hot-rolled steel sheet.
The slab heating temperature when heating the slab is 1100 ° C to 1240 ° C.
In the hot rolling step in the production method A, the slab heating temperature is 1100° C. or higher, so that the remaining undissolved precipitate is further suppressed.
In the hot rolling step in the production method A, the slab heating temperature is 1240° C. or lower, so that the coarsening of the γ grains (that is, austenite grains) is suppressed, and as a result, the finally obtained electric resistance welded steel pipe (ie, the formed steel pipe) In an electric resistance welded steel pipe obtained through a post-tube tempering step (the same applies hereinafter), the average ferrite grain size is 10 μm or less, and the maximum ferrite grain size/average ferrite grain size ratio is 3.0 or less. Easy to achieve.
The slab heating temperature is preferably 1220° C. or lower.

In the hot rolling step in the production method A, the slab heated to the slab heating temperature has a rough rolling finish temperature of 950° C. or higher, a finish rolling start temperature of 850° C. to 950° C., and a finish rolling finish temperature of 800° C. The temperature is 890 ° C., and the ratio of the unrecrystallized area rolling rate to the recrystallized area rolling rate (hereinafter, also referred to as “unrecrystallized area rolling rate / recrystallized area rolling rate ratio”) is 0.8 to 1.6. Hot rolling is performed under the conditions to obtain a hot rolled steel sheet.

In the hot rolling step in the manufacturing method A, when the rough rolling end temperature is 950 ° C. or higher, recrystallization is sufficiently performed and the particle size of the γ grains can be reduced. Therefore, in the finally obtained electric resistance welded steel pipe, it is easy to achieve that the average ferrite grain size is 10 μm or less and the maximum ferrite grain size / average ferrite grain size ratio is 3.0 or less.

In the hot rolling step in the production method A, the finish rolling start temperature of 850° C. or higher suppresses the generation of ferrite grains and strain-induced grain growth during rolling, and as a result, the coarsening of ferrite grains is suppressed. Therefore, in the finally obtained electric resistance welded steel pipe, it is easy to achieve that the average ferrite grain size is 10 μm or less and the maximum ferrite grain size / average ferrite grain size ratio is 3.0 or less.
In the hot rolling step in the production method A, since the finish rolling start temperature is 950 ° C. or lower, the shear band in the γ grains increases, so that the coarsening of the ferrite grains is suppressed. Therefore, in the finally obtained electric resistance welded steel pipe, it is easy to achieve that the average ferrite grain size is 10 μm or less and the maximum ferrite grain size / average ferrite grain size ratio is 3.0 or less.

In the hot rolling step in the production method A, the finish rolling end temperature is 800° C. or higher, so that the generation of ferrite grains and the strain-induced grain growth during rolling are suppressed, and as a result, the coarsening of ferrite grains is suppressed. Therefore, in the finally obtained electric resistance welded steel pipe, it is easy to achieve that the average ferrite grain size is 10 μm or less and the maximum ferrite grain size / average ferrite grain size ratio is 3.0 or less.
In the hot rolling step in the production method A, since the finish rolling end temperature is 890 ° C. or lower, the shear band in the γ grains increases, so that the coarsening of the ferrite grains is suppressed. Therefore, in the finally obtained electric resistance welded steel pipe, it is easy to achieve that the average ferrite grain size is 10 μm or less and the maximum ferrite grain size / average ferrite grain size ratio is 3.0 or less.

The hot rolling in the hot rolling step is performed under the condition that the ratio of reduction ratio of non-recrystallization region/reduction ratio of recrystallization region is 0.8 to 1.6.
In the present disclosure, the reduction ratio in the non-recrystallization region means the reduction ratio in the temperature region of 950°C or lower, and the reduction ratio in the recrystallization region means the reduction ratio in the temperature region of 950°C or higher.
In the hot spreading step in the production method A, the nucleation site of ferrite increases because the ratio of the unrecrystallized region reduction rate / the recrystallized region reduction ratio is 0.8 or more. As a result, in the finally obtained electric resistance welded steel pipe, the ferrite fraction is 60% or more, the average ferrite grain size is 10 μm or less, and the maximum ferrite grain size/average ferrite grain size ratio is 3. It is easy to achieve 0 or less.
In the hot rolling step in the production method A, since the unrecrystallized region reduction ratio/recrystallization region reduction ratio is 1.6 or less, the γ grains are refined in the recrystallization region, so that the ferrite grains are refined. easy. As a result, it is easy to achieve that the average ferrite grain size is 10 μm or less and the maximum ferrite grain size / average ferrite grain size ratio is 3.0 or less in the finally obtained electric resistance welded steel pipe.

<Cooling process, winding process>
The cooling step in production method A is a step of cooling the hot rolled steel sheet obtained in the hot rolling step until the temperature of the outer surface of the hot rolled steel sheet reaches a coiling temperature of 450 to 650°C.
The winding step in the production method A is a step of obtaining a hot coil made of a hot rolled steel sheet by winding the hot rolled steel sheet cooled to the above winding temperature at the above winding temperature.
In the production method A, since the winding temperature is 450° C. or higher, the formation of bainite or martensite is suppressed. As a result, in the finally obtained electric resistance welded steel pipe, it is easy to achieve that the ferrite fraction is 60% or more.
In the production method A, since the winding temperature is 650° C. or lower, the growth (that is, coarsening) of ferrite grains is suppressed. As a result, it is easy to achieve that the average ferrite grain size is 10 μm or less and the maximum ferrite grain size / average ferrite grain size ratio is 3.0 or less in the finally obtained electric resistance welded steel pipe.

<Pipe making process>
In the pipe making step in the manufacturing method A, a hot-rolled steel sheet is unwound from a hot coil, and the unrolled hot-rolled steel sheet is roll-formed into an open pipe. It is a step of obtaining a s-roll electric resistance welded steel pipe by forming a sewn welded portion.
Each operation in the pipe making step is performed according to a known method.
In addition, the pipe making process, if necessary,
Seam heat treatment of ERW welds;
After the formation of the electrosewn weld (after the seam heat treatment if the seam heat treatment is performed as described above), the shape of the azurol electric resistance pipe is adjusted by a sizer;
Etc. may be included.

<Tempering process after pipe making>
The post-pipe-making tempering step in the production method A is a step of subjecting the as-roll electric resistance welded steel tube to a tempering temperature of 500 to 700° C. and a tempering time of 1 to 120 minutes.
Since the manufacturing method A has a tempering step after pipe making, an electrosewn steel pipe in which yield elongation is observed when a tensile test in the pipe axial direction is performed can be obtained.
In the tempering process after pipe forming, the tempering temperature is 500° C. or higher, so that distortion due to pipe forming is reduced. As a result, in the finally obtained electric resistance welded steel pipe, it is easy to achieve that YR in the pipe axis direction is 95% or less.
In the post-pipe tempering step, the tempering temperature is 700 ° C. or lower, so that the formation of bainite or martensite is suppressed. As a result, it is easy to achieve that the ferrite fraction is 60% or more and the TS in the pipe axial direction is 700 MPa or less in the finally obtained electric resistance steel pipe.
In the tempering process after pipe forming, the tempering time is 1 minute or more, so that strain due to pipe forming is reduced. As a result, in the finally obtained electric resistance welded steel pipe, it is easy to achieve that YR in the pipe axis direction is 95% or less.
In the tempering process after pipe making, the tempering time of 120 minutes or less is advantageous from the viewpoint of productivity (manufacturing cost).

Each step of the above-mentioned production method A does not affect the chemical composition of steel.
Therefore, the chemical composition of the base material of the electric resistance welded steel pipe manufactured by the manufacturing method A can be regarded as the same as the chemical composition of the raw material (molten steel or slab).

Hereinafter, examples of the present disclosure will be shown, but the present disclosure is not limited to these examples.
Hereinafter, the test No. 1-Test No. No. 10 is an example within the scope of the present disclosure, and the test No. 11-Test No. 47 is a comparative example that is outside the scope of the present disclosure.

<Manufacture of ERW steel pipe>
According to the manufacturing method A described above, the test No. ERW steel pipes of 1 to 10 (Examples) were obtained.
Moreover, the chemical composition and/or manufacturing conditions of the electric resistance welded steel pipes of the examples were changed, and the test No. 11 to 47 (comparative examples) ERW steel pipes were obtained.
The details will be described below.

A molten steel having the chemical compositions (steel A to steel J, steel AA, steel AB, steel AC, and steel AD) shown in Table 1 was melted in a furnace, and then a slab having a thickness of 250 mm was prepared by casting (slab preparation). Process).

In Table 1, the numerical value shown in the column of each element is the mass% of each element.
In Table 1, the blank column means that the corresponding element is not contained.
The balance excluding the elements shown in Table 1 is Fe and impurities.
In Table 1, REM in Steel H is La.
In Table 1, Ceq is a Ceq represented by the above-mentioned formula (1), and EESP is an EESP represented by the above-mentioned formula (2).
Underlines in Tables 1 to 3 indicate outside the scope of the present disclosure.

The slab obtained above was heated to the slab heating temperature shown in Table 2, and the hot rolling conditions shown in Table 2 were applied to the heated slab (specifically, rough rolling end temperature, finish rolling start temperature, And hot rolling at the finish rolling end temperature) to obtain a hot rolled steel sheet (hot rolling step).
The hot rolled steel sheet obtained in the hot rolling step is cooled to the winding temperature shown in Table 2 and wound at this winding temperature to obtain a hot coil made of a hot rolled steel sheet having a thickness of 15 mm. Obtained (cooling step and winding step).
The hot rolling process, cooling process, and winding process described above were performed using a hot strip mill.

Test No. 1-Test No. 42 and Test No. 44-Test No. In No. 47, a hot-rolled steel sheet is unwound from the hot coil, the unwound hot-rolled steel sheet is roll-formed to form an open pipe, and the butt portion of the obtained open pipe is electrosewn to form an electrosewn welded portion. After forming, seam heat treatment was applied to the electrosewn welded portion, and then the shape was adjusted using a sizer to obtain an azurol electrosewn steel pipe having an outer diameter of 300 mm and a wall thickness of 15 mm (pipe making process). ).

Test No. In No. 43, a hot-rolled steel sheet was unwound from the hot coil, and the unwound hot-rolled steel sheet was subjected to pre-pipe-making tempering under the conditions (tempering temperature and tempering time) shown in Table 2 and then again hot-rolled steel sheet. Wound up. The wound hot-rolled steel sheet was unwound again, and the unwound hot-rolled steel sheet was used to obtain Test No. In the same manner as in No. 1, an azurol electric resistance pipe having an outer diameter of 300 mm and a wall thickness of 15 mm was obtained.

Test No. 1-Test No. 41 and the test No. 45-Test No. In No. 47, as-rolled electric-resistance welded steel pipes having an outer diameter of 300 mm and a wall thickness of 15 mm were subjected to post-pipe-making tempering under the conditions (tempering temperature and tempering time) shown in Table 2 and then air-cooled. A steel pipe was obtained (tempering process after pipe making).
Test No. 42 and test No. In No. 43, the Azroll electric resistance welded steel pipe was not tempered after pipe making.
Test No. In No. 44, the Azroll electric resistance welded steel pipe is subjected to quenching with a quenching temperature of 950° C., a quenching time of 5 minutes, a water cooling cooling method, a tempering temperature of 600° C., and a quenching time of 60 minutes. By performing tempering and then air cooling (the above operation is referred to as “950° C. tempering and tempering” in Table 2), an electric resistance welded steel pipe having an outer diameter of 300 mm and a wall thickness of 15 mm was obtained.

<Observation and various measurements of the central part of the wall thickness in the L cross section at the base material 180° position>
Regarding the electric resistance welded steel pipes (Azroll electric resistance welded steel pipes in Test No. 42 and Test No. 43; the same applies hereinafter), the center portion of the wall thickness at the L cross section at the base material 180° position was observed. Various measurements were made.

(Measurement of ferrite fraction and confirmation of the rest)
The ferrite fraction was measured and the balance was confirmed by the method described above.
The results are shown in Table 3.

In Table 3, "TB, P" includes at least one of tempered bainite and pearlite, and the balance is substantially free of martensite and tempered martensite (that is, the total area ratio of martensite and tempered martensite in the balance). However, it is less than 1% of the total balance), and "TB + TM" means that it contains both tempered baynite and tempered martensite.

(Average ferrite grain size, maximum ferrite grain size/average ferrite grain size ratio, and dislocation density)
The average ferrite grain size, the maximum ferrite grain size / average ferrite grain size ratio, and the dislocation density were measured by the methods described above. As the X-ray diffractometer for measuring the dislocation density, "RINT2200" manufactured by Rigaku Corporation was used.
The results are shown in Table 3.

<Measurement of TS in the tube axis direction, YS in the tube axis direction, and YR in the tube axis direction>
By the method described above, TS in the tube axis direction, YS in the tube axis direction, and YR in the tube axis direction were measured.
The results are shown in Table 3.

<Confirmation of yield elongation>
The presence or absence of yield elongation was confirmed by the method described above.
The results are shown in Table 3.
In Table 3, "Y" means that the yield elongation was observed, and "N" means that the yield elongation was not observed.

<Sour resistance after strain is applied (HIC test CAR)>
The sour resistance after strain was applied was evaluated by the method described above.
The CAR (%) of the HIC resistance test in this evaluation is shown in Table 3.
The smaller the value of CAR (%), the better the sour resistance after strain is applied.

In the metal structure of the central part of the base metal, the ferrite content is 60 to 90%, the balance contains at least one selected from the group consisting of tempered bainite and pearlite, and martensite in the balance. And the total area ratio of tempered martensite is less than 1% with respect to the entire balance, the average ferrite particle size is 10 μm or less in the metal structure at the center of the wall thickness of the base metal part, and the maximum ferrite particle size / average ferrite. The particle size ratio is 3.0 or less, the TS in the tube axis direction is 400 to 700 MPa, the YS in the tube axis direction is 300 to 650 MPa, the YR in the tube axis direction is 95% or less, and the tube axis direction. The tempered steel pipes of Examples (Test No. 1 to Test No. 10) in which yield elongation is observed when a tensile test is performed have a sour resistance (CAR of the HIC test) after strain is applied. It was excellent.

The results of the comparative examples (Test No. 11 to Test No. 47) were as follows with respect to these Examples.

Test No. with too low C content. At 11, TS was insufficient.
Test No. with too much C content. In No. 12, tempered bainite and tempered martensite were generated, TS and YS became excessive, and sour resistance (CAR in HIC resistance test) after strain was deteriorated.
Test No. with too low Si content. 13 and Test No. with too much Si content. In all 14, the sour resistance (CAR of the HIC resistance test) after the strain was applied deteriorated.
Test No. with too low Mn content At 15, TS was insufficient.
Test No. with too much Mn content. In No. 16, tempered bainite and tempered martensite were generated, TS and YS became excessive, and sour resistance (CAR in HIC resistance test) after strain was deteriorated.
Test No. with too much P content. In No. 17, the sour resistance (CAR of the HIC resistance test) after the strain was applied deteriorated.
Test No. with too much S content. At No. 18, the sour resistance (CAR of the HIC resistance test) after the strain was applied deteriorated.
Test No. with too low Ti content. In No. 19, the average ferrite grain size was more than 10 μm, the maximum ferrite grain size/average ferrite grain size ratio was more than 3.0, and the sour resistance (CAR in the HIC resistance test) after strain was deteriorated.
Test No. with too much Ti content. In No. 20, sour resistance (CAR in HIC resistance test) after the strain was applied deteriorated.
Test No. with too little Al content. In No. 21, sour resistance (CAR in HIC resistance test) after the strain was applied deteriorated.
Test No. with too much Al content. In No. 22, sour resistance (CAR in HIC resistance test) after the strain was applied deteriorated.
Test No. with too low Nb content. In No. 23, the average ferrite grain size was more than 10 μm, the maximum ferrite grain size/average ferrite grain size ratio was more than 3.0, and the sour resistance (CAR in the HIC resistance test) after strain was deteriorated.
Test No. with too much Nb content. In No. 24, the sour resistance (CAR in the HIC resistance test) after the strain was applied deteriorated.

Test No. with too much N content At No. 25, the sour resistance (CAR of the HIC resistance test) after the strain was applied deteriorated.
Test No. containing no Ca At No. 26, the sour resistance (CAR of the HIC resistance test) after the strain was applied deteriorated.
Test No. with too much Ca content. In No. 27, the sour resistance (CAR of the HIC resistance test) after the strain was applied deteriorated.

Each element is within the specified range, but Ceq is too large. In No. 40, the ferrite fraction became too low, and tempered bainite and tempered martensite were present in the balance, TS and YS became excessive, and sour resistance (CAR in HIC resistance test) after strain was imparted deteriorated. ..
Each element is within the specified range, but ESSP is too large. In No. 41, the sour resistance (CAR of the HIC resistance test) after the strain was applied deteriorated.

Test No. which satisfies the chemical composition of the present disclosure but the slab heating temperature is too high. In No. 28, the average ferrite grain size was more than 10 μm, the maximum ferrite grain size/average ferrite grain size ratio was more than 3.0, and the sour resistance (CAR in the HIC resistance test) after strain was deteriorated.
Although the chemical composition of the present disclosure was satisfied, the rough rolling finish temperature was too low. In No. 29, the average ferrite grain size was more than 10 μm, the maximum ferrite grain size/average ferrite grain size ratio was more than 3.0, and the sour resistance (CAR in the HIC resistance test) after strain was deteriorated.
Test No. which satisfies the chemical composition of the present disclosure, but the finish rolling start temperature is too high. In No. 30, the average ferrite grain size was more than 10 μm, the maximum ferrite grain size/average ferrite grain size ratio was more than 3.0, and the sour resistance (CAR in HIC resistance test) after strain was deteriorated.
Test No. which satisfies the chemical composition of the present disclosure, but the finish rolling end temperature is too low. In No. 31, the average ferrite grain size was more than 10 μm, the maximum ferrite grain size/average ferrite grain size ratio was more than 3.0, and the sour resistance (CAR in the HIC resistance test) after strain was deteriorated.
Test No. which satisfies the chemical composition of the present disclosure, but the finish rolling end temperature is too high. In No. 32, the average ferrite grain size was more than 10 μm, the maximum ferrite grain size/average ferrite grain size ratio was more than 3.0, and the sour resistance (CAR in the HIC resistance test) after strain was deteriorated.
Although the chemical composition of the present disclosure was satisfied, the winding temperature was too low. In No. 33, the ferrite fraction was less than 60%, and the sour resistance (CAR in the HIC resistance test) after strain was deteriorated.
Although the chemical composition of the present disclosure is satisfied, the winding temperature was too high. In No. 34, the average ferrite grain size was more than 10 μm, the maximum ferrite grain size/average ferrite grain size ratio was more than 3.0, and the sour resistance (CAR in the HIC resistance test) after strain was deteriorated.

Test No. which satisfies the chemical composition of the present disclosure, but the unrecrystallized region reduction ratio / recrystallized region reduction ratio was too small. In No. 35, although the average ferrite grain size of 10 μm or less was satisfied, the ferrite fraction was less than 60%, the maximum ferrite grain size/average ferrite grain size ratio was over 3.0, and the sour resistance after strain was applied. The property (CAR of HIC resistance test) deteriorated.
Test No. which satisfies the chemical composition of the present disclosure, but the unrecrystallized region reduction ratio / recrystallized region reduction ratio was too small. In No. 45, the ferrite fraction was less than 60%, the average ferrite grain size was more than 10 μm, and the maximum ferrite grain size/average ferrite grain size ratio was more than 3.0, sour resistance (strain resistance after strain was given). CAR of HIC test) deteriorated.
Test No. which satisfies the chemical composition of the present disclosure, but the ratio of unrecrystallized area reduction rate / recrystallized area reduction rate is too large. 36 and test No. In No. 46, the average ferrite grain size was more than 10 μm, the maximum ferrite grain size/average ferrite grain size ratio was more than 3.0, and the sour resistance (CAR in the HIC resistance test) after strain was deteriorated.

Test No. which satisfies the chemical composition of the present disclosure, but the tempering temperature in tempering after tube making was too low. In No. 37, YR became excessive, and further, sour resistance (CAR in HIC resistance test) after the strain was applied deteriorated.
Test No. which satisfies the chemical composition of the present disclosure, but the tempering temperature in tempering after tube making was too high. In No. 38, the average ferrite grain size was more than 10 μm, the maximum ferrite grain size/average ferrite grain size ratio was more than 3.0, and the sour resistance (CAR in the HIC resistance test) after strain was deteriorated.
Test No. which satisfies the chemical composition of the present disclosure, but the tempering time in tempering after tube making was too short. In No. 39, YR was excessively large, and the sour resistance (CAR in HIC resistance test) after the strain was applied was deteriorated.

Test No. which satisfied the chemical composition of the present disclosure but was not tempered after tube formation. In No. 42, the yield elongation was not observed, YR became excessive, and further, the sour resistance (CAR in the HIC resistance test) after the strain was applied deteriorated.
Test No. that satisfies the chemical composition of the present disclosure, but was subjected to tempering before pipe forming (that is, tempering for hot-rolled steel sheet) and no tempering after pipe forming. In No. 43, the yield elongation was not observed, YR was excessive, and the sour resistance (CAR in the HIC resistance test) after the strain was applied was deteriorated.

Test No., which satisfies the chemical composition of the present disclosure, is subjected to quenching at 950 ° C. after tube formation and then tempering. In No. 44, the ferrite fraction was less than 60%, tempered bainite and tempered martensite were generated, TS and YS were excessive, and sour resistance (CAR in HIC resistance test) after strain was deteriorated.
Test No. with too much O content. At 47, the sour resistance (CAR of the HIC resistance test) after the strain was applied deteriorated.

Claims

Including base material and ERW welded part,
The chemical composition of the base material part is mass%,
C: 0.03 to 0.10%,
Si: 0.03 to 0.60%,
Mn: 0.30 to 1.60%,
P: 0 to 0.030%,
S: 0 to 0.0015%,
Ti: 0.010 to 0.200%,
Al: 0.005 to 0.500%,
Nb: 0.010 to 0.050%,
N: 0 to 0.006%,
O: 0 to 0.004%,
Ca: 0.0001-0.0200%,
Cu: 0 to 1.000%,
Ni: 0 to 1.000%,
Cr: 0 to 1.00%,
Mo: 0 to 0.50%,
V: 0 to 0.200%,
W: 0 to 0.100%,
B: 0 to 0.0050%,
Mg: 0-0.0200%,
Zr: 0-0.0200%,
REM: 0 to 0.0200%, and
Remaining: Consists of Fe and impurities
Ceq represented by the following formula (1) is 0.10 to 0.50,
The ESSP represented by the following formula (2) is 0 to 10.00,
In the metal structure of the central part of the wall thickness of the base material, the area ratio of ferrite is 60 to 90%, the balance contains at least one selected from the group consisting of tempered bainite and pearlite, and the balance The total area ratio of martensite and tempered martensite is less than 1% with respect to the entire balance,
In the metal structure at the center of the wall thickness of the base metal portion, the average ferrite grain size is 10 μm or less, and the ratio of the maximum ferrite grain size to the average ferrite grain size is 3.0 or less.
The tensile strength in the tube axis direction is 400 to 700 MPa,
The yield strength in the tube axis direction is 300 to 650 MPa,
The yield ratio in the tube axis direction is 95% or less,
ERW steel pipe for line pipe whose yield elongation is observed when a tensile test is conducted in the pipe axial direction.
Ceq=C+Mn/6+Cr/5+(Ni+Cu)/15+Nb+Mo+V... Formula (1)
EESP=Ca×(1-124O)/1.25S... Formula (2)
[In Formula (1) and Formula (2), each element symbol represents the mass% of each element. ]
The chemical composition of the base material part is mass%,
Cu: more than 0% and 1.000% or less,
Ni: more than 0% and 1.000% or less,
Cr: more than 0% and 1.00% or less,
Mo: More than 0% and less than 0.5%,
V: more than 0% and 0.200% or less,
W: more than 0% and 0.100% or less,
B: more than 0% and 0.0050% or less,
Mg: more than 0% and 0.0200% or less,
Zr: more than 0% and 0.0200% or less, and
REM: The electric pipe for line pipe according to claim 1, which contains at least one selected from the group consisting of more than 0% and 0.0200% or less.
The electric resistance welded steel pipe for a line pipe according to claim 1 or 2, wherein a dislocation density at a central portion of the wall thickness of the base material portion is 2.0×10 15 m -2 or less.
The electrosewn steel pipe for a line pipe according to any one of claims 1 to 3, wherein the wall thickness is 5 to 20 mm and the outer diameter is 100 to 400 mm.