JP7549286B1 - Electric resistance welded steel pipe for line pipe - Google Patents

Abstract

This electric resistance welded steel pipe for line pipe has a base material portion and an electric resistance welded portion, the base material portion has a predetermined chemical composition, in the microstructure of a 1/2×tB portion of the base material portion, the area ratio of ferrite is 40 to 80% and the average crystal grain size is 35 μm or less, in the microstructure of a 1/4×tS portion of the electric resistance welded portion, the area ratio of ferrite is 40 to 70%, the area ratio of island martensite is 0.2 to 10.0%, the average crystal grain size is 35 μm or less, the yield stress is 360 MPa or more, and the tensile strength is 465 MPa or more.

Description

The present invention relates to steel pipes, and more specifically to electric resistance welded steel pipes for line pipes.

Pipelines are one of the means of transporting crude oil and natural gas, but in recent years, as natural resource drilling areas have become more severe, the environment in which line pipes are laid has also become more severe. One consequence of this is an increase in the number of submarine pipelines laid on the seabed. There are various methods for laying submarine pipelines. One method, known as the reel method, involves first making the pipe on land, welding the pipes together, and winding the resulting long pipe onto a spool on a barge, after which the pipe is unwound while being laid on the seabed at sea. This method allows for efficient laying of submarine line pipes.

However, when winding and unwinding steel pipes, the steel pipes are subjected to plastic bending. At this time, compressive force is applied to the inside of the bend, which can cause buckling on the inside of the bend and the steel pipe to buckle. If steel pipe buckling occurs, installation work must be stopped, and the damage is enormous. It is known that buckling of steel pipes can be prevented by lowering the yield ratio (calculated as yield strength (YS)/tensile strength (TS), denoted as YR) in the longitudinal direction of the steel pipe in the base material and electric resistance welded parts of the steel pipe. It is also known that buckling of steel pipes can be prevented by increasing the wall thickness of the steel pipe. Against this background, in addition to steel pipes with sufficient strength to withstand high internal pressure, steel pipes with low axial YR and thick wall thickness are now required.

In addition, if brittle fracture occurs in a line pipe, oil or natural gas will leak out, leading to enormous damage, so excellent toughness is required. Specifically, it is essential to ensure the characteristics in the Charpy test for the base material and in the CTOD test for the electric resistance welded part. In particular, the steel pipe for line pipe used in the submarine pipeline of the present application has a thick wall thickness to prevent buckling, so it is not easy to ensure excellent toughness in the base material and the electric resistance welded part. Specifically, if the base material is thick, the finishing reduction rate in hot rolling is insufficient, and the cooling rate in the center of the wall thickness cannot be increased, and the microstructure becomes coarse, so it is not easy to ensure toughness. In addition, if the electric resistance welded part is thick, the reheating temperature of the outer surface increases and the microstructure becomes coarse, so that the required temperature is reached to the inner surface by heating from the outer surface after electric resistance welding, making it difficult to ensure toughness.

Therefore, electric resistance welded steel pipes used in the reel method require high strength, low YR and excellent toughness in the base material and electric resistance welds.

For example, Patent Document 1 describes an electric-resistance welded steel pipe that has excellent toughness in the base material and electric-resistance welded parts. In the electric-resistance welded steel pipe described in Patent Document 1, the chemical composition of the base material contains, in mass %, C: 0.04 to 0.12%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, Ti: 0.005 to 0.030%, Nb: 0.005 to 0.050%, and N: 0.001 to 0.008%, with the balance containing Fe and impurities. When the thickness of the base material is tB and the thickness of the electric-resistance weld is tS, the hardness of outer surface layer B, which is located at a depth of 1 mm from the outer surface of the base material, minus the hardness of ½tB portion is 30 HV10 or less, and the hardness of outer surface layer S, which is located at a depth of 1 mm from the outer surface of the electric-resistance weld, minus the hardness of ½tS portion is 0 HV10 or more and 30 HV10 or less. Patent Document 1 describes that this provides excellent low-temperature toughness.

Patent Document 2 describes an electric-resistance welded steel pipe with excellent toughness in the base metal and electric-resistance welded parts. The electric-resistance welded steel pipe described in Patent Document 2 contains, by mass%, C: 0.02-0.10%, Si: 0.05-0.30%, Mn: 0.80-2.00%, and Nb: 0.010-0.100%, has a composition in which the carbon equivalent Ceq satisfies 0.25-0.50, and has a structure consisting of a bainitic ferrite phase and/or a bainite phase, and has high strength with a yield strength of 52 ksi or more and high toughness with a fracture transition temperature vTrs of -45°C or less. The material is a thick hot-rolled steel plate, and the electric resistance welded portion is subjected to heat treatment in which the electric resistance welded portion is induction heated at a minimum temperature of 830°C or more and a maximum temperature of 1150°C or less, and cooled at an average cooling rate of 10 to 70°C/s at each position in the thickness direction and at a cooling stop temperature of 550°C or less, so that the structure is made of a bainitic ferrite phase and/or a bainite phase, and the ratio of the average grain size at the coarsest grain position to the average grain size at the finest grain position at each position in the thickness direction is 2.0 or less. Patent Document 2 describes that this provides excellent low-temperature toughness.

International Publication No. 2020/170333 International Publication No. 2015/004901

Through the investigations of the present inventors, it has been found that if the chemical composition or manufacturing method is not appropriate, the area ratio of island martensite (MA), a hard phase, in the electric resistance welded joint may fall outside the appropriate range, which may result in the toughness of the electric resistance welded joint being incompatible with a low YR.

In Patent Documents 1 and 2, no consideration is given to controlling the above-mentioned MA in the electric resistance welded portion. Therefore, there is a risk that the low-temperature toughness and low YR of the electric resistance welded portion of the electric resistance welded steel pipes described in Patent Documents 1 and 2 may not necessarily be sufficient.

The present invention, made in consideration of the above-mentioned circumstances, aims to provide an electric-welded steel pipe for line pipe having high strength, low YR and excellent low-temperature toughness in the base material and electric-welded welded parts.

The gist of the present invention is as follows.
(1) An electric resistance welded steel pipe for line pipe according to one aspect of the present invention has a base material portion and an electric resistance welded portion,
The chemical composition of the base material is, in mass%,
C: 0.060-0.120%,
Si: 0.01-0.50%,
Mn: 0.5-2.0%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.080% or less,
Ti: 0.003 to 0.030%,
Nb: 0.003 to 0.046%,
N: 0.0010-0.0080%,
O: 0.005% or less,
Cu: 0-0.500%,
Ni: 0 to 0.500%,
Cr: 0-0.500%,
Mo: 0-0.500%,
V: 0 to 0.100%,
W: 0 to 0.500%,
Ca: 0-0.0040%,
REM: 0 to 0.0050%, and
The balance is composed of Fe and impurities.
Ceq represented by the following formula (i) is 0.16 to 0.53 mass%,
M represented by the following formula (ii) is 0.06 to 0.25 mass %,
When the thickness of the base material portion is tB and the thickness of the electric resistance welded portion is tS,
The tB and the tS are 15.0 to 25.4 mm,
The outer diameter is 304.8 to 660.4 mm,
In the microstructure of the 1/2 × tB portion of the base material,
The area ratio of ferrite is 40 to 80%, and the average crystal grain size is 35 μm or less.
In the microstructure of the 1/4×tS portion of the electric resistance welded portion,
The area ratio of ferrite is 40 to 70%, the area ratio of island martensite is 0.2 to 10.0%, and the average crystal grain size is 35 μm or less;
The yield stress is 360 MPa or more, and the tensile strength is 465 MPa or more.
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5...(i)
M=C/3+5×Nb...(ii)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formulas (i) and (ii), and 0 is substituted when the corresponding element is not contained.
(2) The electric resistance welded steel pipe for line pipe described in (1) above is
The chemical composition of the base material is, in mass%,
Cu: more than 0% and less than 0.500%,
Ni: more than 0% and less than 0.500%,
Cr: more than 0% and less than 0.500%,
Mo: more than 0% and less than 0.500%,
V: more than 0% and less than 0.100%,
W: more than 0% and less than 0.500%,
The steel may contain one or more selected from the group consisting of Ca: more than 0% and 0.0040% or less, and REM: more than 0% and 0.0050% or less.

According to the above aspect of the present invention, an electric resistance welded steel pipe for line pipe can be obtained having high strength, low YR and excellent low temperature toughness in the base material and electric resistance welded parts.

FIG. 13 is a diagram showing the relationship between the MA area ratio of an electric resistance welded portion and δc. FIG. 13 is a diagram showing the relationship between the MA area ratio of an electric resistance weld and YR.

The inventors have investigated a method for obtaining electric-resistance welded steel pipe for line pipe which has high strength, low YR and excellent low-temperature toughness in both the base material and the electric-resistance welded portion, and have obtained the following findings.

(I) In the base material, in addition to adjusting the chemical composition of the electric resistance welded steel pipe, it is important to control the microstructure by controlling the hot rolling conditions.

(II) In electric resistance welded steel pipes, if they are left as they are after electric resistance welding, the microstructure of the weld becomes a hardened martensite structure and becomes very hard, which significantly deteriorates the toughness. Therefore, in order to improve toughness, electric resistance welded steel pipes are subjected to heat treatment in which the electric resistance weld is reheated from the outer surface side by induction heating or the like after electric resistance welding and then water-cooled. Since water cooling is performed on the outer surface, the outer surface side is cooled rapidly, which increases the hardness of the outer surface side, and this can deteriorate the toughness of the outer surface side. Therefore, it is important to optimize the water cooling conditions after reheating and reduce the hardness of the outer surface side. Specifically, it is important to optimize the water cooling conditions so as to generate ferrite, a soft structure, in the microstructure on the outer surface side.

(III) When carbon concentrates in untransformed austenite during water cooling after reheating, island martensite (MA) is formed. MA is hard, so if there is a large amount of MA, toughness deteriorates. On the other hand, MA increases the work hardening ability of the material, so if there is a large amount of MA, tensile strength increases. In order to achieve both low-temperature toughness and low YR of electric resistance welds, it is important to properly control the amount of MA.

(IV) From the standpoint of chemical composition, the amount of MA in electric welds is affected by the C and Nb contents. C forms MA by concentrating in untransformed austenite during water cooling after reheating. Therefore, controlling the C content is important for optimizing the amount of MA. On the other hand, Nb has the effect of arresting transformation. The lower the Nb content, the less carbon is concentrated in untransformed austenite, thereby suppressing the generation of MA. For the above reasons, controlling the C and Nb contents is important for optimizing the amount of MA in electric welds.

(V) The amount of MA in electric welded parts is affected by the water cooling stop temperature after reheating. When the water cooling stop temperature is high, the concentration of carbon in untransformed austenite is promoted, and the MA area ratio increases. On the other hand, when the water cooling stop temperature is low, the concentration of carbon in untransformed austenite is suppressed, and the MA area ratio decreases.

The amount of MA in the weld is also affected by the segregation state of the electric resistance weld. When the alloy concentration of the segregation zone is high, carbon enrichment in the untransformed austenite is promoted, and the MA area ratio increases. On the other hand, when the alloy concentration of the segregation zone is low, carbon enrichment in the untransformed austenite is suppressed, and the MA area ratio decreases. The electric resistance weld corresponds to the end of the slab (coil), and the segregation zone in the electric resistance weld is not a central (macro) segregation that exists in the center of the plate thickness in the center of the width of the slab (coil), but a microsegregation, so it is significantly affected by the heating conditions during the production of hot-rolled steel sheets. Therefore, in order to optimize the segregation state of the electric resistance weld, it is important to optimize the slab heating conditions during the production of hot-rolled steel sheets, which are the raw material for electric resistance welded steel pipes.

Hereinafter, an electric-resistance welded steel pipe for line pipe according to one embodiment of the present invention, which has been made based on the above findings (electric-resistance welded steel pipe for line pipe according to this embodiment (sometimes simply referred to as electric-resistance welded steel pipe)) will be described.
However, the present invention is not limited to the configurations disclosed in the present embodiment, and various modifications are possible without departing from the spirit of the present invention.

The electric resistance welded steel pipe for line pipe according to this embodiment has a base material portion and an electric resistance welded portion, and the base material portion has a predetermined chemical composition. When the thickness of the base material portion is tB and the thickness of the electric resistance welded portion is tS, the tB and the tS are 15.0 to 25.4 mm, the outer diameter is 304.8 to 660.4 mm, in the microstructure of a 1/2×tB portion of the base material portion, the area ratio of ferrite is 40 to 80% and the average grain size is 35 μm or less, in the microstructure of a 1/4×tS portion of the electric resistance welded portion, the area ratio of ferrite is 40 to 70%, the area ratio of island martensite (MA) is 0.2 to 10.0%, the average grain size is 35 μm or less, the yield stress is 360 MPa or more, and the tensile strength is 465 MPa or more.
Hereinafter, each requirement of the electric resistance welded steel pipe for line pipe according to this embodiment will be described in detail.

1. Chemical composition of the base material The reasons for limiting each element are as follows. The numerical ranges described below with "to" include the lower and upper limits. Numerical values indicated as "less than" or "greater than" are not included in the numerical range. In the following explanation, "%" in the chemical composition means "mass %" unless otherwise specified.

The electric resistance welded steel pipe for line pipe according to this embodiment has a steel plate that serves as a base material, and a welded portion (electric resistance welded portion) that is provided at the butt joint of the steel plate and extends in the longitudinal direction of the steel plate. In the electric resistance welded steel pipe according to this embodiment, no welding material is used when electric resistance welding the steel plate to form the electric resistance welded steel pipe, so the chemical composition of the base material and the electric resistance welded portion are essentially the same.

The electric resistance welded steel pipe for line pipe according to this embodiment has a chemical composition of the base material, in mass%, of C: 0.060 to 0.120%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.080% or less, Ti: 0.003 to 0.030%, Nb: 0.003 to 0.046%, N: 0.0010 to 0.0080%, O: 0.005% or less, and the balance: Fe and impurities, with Ceq represented by formula (i) described below being 0.16 to 0.53 mass%, and M represented by formula (ii) described below being 0.06 to 0.25 mass%.
Each element will be described in detail below.

C: 0.060-0.120%
C is an essential element for improving hardenability and obtaining high strength. It also affects the YR of steel. If the C content is too low, the base material of electric resistance welded steel pipe for line pipes may be damaged. In order to obtain a desired YR in an electric resistance welded steel pipe for line pipe, the C content is set to 0.060% or more. The C content is preferably 0.070% or more. That's all.
On the other hand, if the C content is too high, the center segregation hardens and the low-temperature toughness of the base material deteriorates. In order to ensure the low-temperature toughness of the base material, the C content is set to 0.120% or less. The C content is preferably 0.100% or less.

Si: 0.01~0.50%
Silicon is an effective element for deoxidizing steel. If the silicon content is too low, silicon will not be able to deoxidize steel (it will prevent defects such as blowholes from occurring in steel). In order to fully utilize this effect and obtain excellent low-temperature toughness in the base metal, the Si content is set to 0.01% or more, and preferably 0.10% or more.
On the other hand, if the Si content exceeds 0.50%, oxides are formed in the electric resistance welded portion, and the low temperature toughness of the electric resistance welded portion deteriorates. Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.40% or less.

Mn: 0.5-2.0%
Mn is an element that is effective in improving hardenability and ensuring the strength of the base material. In order to obtain a desired strength in the base material, the Mn content is set to 0.5% or more. The Mn content is as follows: It is preferably 0.7% or more.
On the other hand, if the Mn content exceeds 2.0%, a hardened phase is generated in the central segregation portion, and the low-temperature toughness of the base material is significantly deteriorated. Therefore, the Mn content is set to 2.0% or less. The content is preferably 1.6% or less.

P: 0.030% or less P is an impurity element that affects the low-temperature toughness of steel. If the P content exceeds 0.030%, grain boundary embrittlement is caused in the base material and electric resistance welded parts, and the low-temperature toughness is significantly deteriorated. Therefore, the P content is set to 0.030% or less.
The lower the P content, the better, and it may be 0%. However, since the substantial lower limit of the P content in mass-produced steel is 0.002%, the P content may be 0.002% or more.

S: 0.0050% or less S is an impurity element that affects the low-temperature toughness of steel. If the S content exceeds 0.0050%, coarse sulfides are generated, and the low-temperature toughness of the base material and electric resistance welded parts deteriorates. Therefore, the S content is set to 0.0050% or less.
The smaller the S content, the better, and it may be 0%. However, since the substantial lower limit of the S content in mass-produced steel is 0.0003%, the S content may be 0.0003% or more.

Al: 0.080% or less Al is an effective element as a deoxidizer. However, if the Al content exceeds 0.080%, a large amount of Al oxide is generated, and the toughness of the base material and the electric resistance welded part deteriorates. Therefore, the Al content is set to 0.080% or less. The Al content is preferably 0.050% or less.
Since deoxidation is possible with Si or Ti, the Al content may be 0%. However, in order to obtain a sufficient deoxidation effect, the Al content is preferably 0.010% or more.

Ti: 0.003~0.030%
Ti is a nitride-forming element, and contributes to the refinement of crystal grains by forming nitrides. In order to obtain this effect and ensure the low-temperature toughness of the base material, the Ti content is set to 0. The Ti content is preferably 0.010% or more.
On the other hand, if the Ti content exceeds 0.030%, the low temperature toughness of the base metal is significantly deteriorated due to the formation of coarse carbonitrides. Therefore, the Ti content is set to 0.030% or less. , preferably 0.025% or less.

Nb: 0.003-0.046%
Nb is an element that forms carbides, nitrides and/or carbonitrides and contributes to improving the strength of steel. In addition, Nb expands the pre-recrystallization rolling temperature range, thereby improving the strength of steel for line pipes. Nb is an element that has the effect of improving the low-temperature toughness of the base metal of the welded steel pipe. In addition, Nb has the effect of halting the transformation, and is an effective element for optimizing MA. The Nb content is set to 0.003% or more, and preferably 0.010% or more.
On the other hand, if the Nb content exceeds 0.046%, a large amount of Nb-based carbonitrides is formed, and the low-temperature toughness of the base metal is deteriorated. Therefore, the Nb content is set to 0.046% or less. The content is preferably 0.030% or less.

N: 0.0010-0.0080%
N is an element that forms nitrides, refines the crystal grains of steel, and improves the low-temperature toughness of the base material. The amount should be at least 0.0010%.
On the other hand, if the N content exceeds 0.0080%, a large amount of nitrides is formed, which deteriorates the low-temperature toughness of the base metal. Therefore, the N content is set to 0.0080% or less.

O: 0.005% or less O is an element that affects the low-temperature toughness of steel. If the O content exceeds 0.005%, a large amount of oxide is generated, and the low-temperature toughness of the base material and electric resistance welded parts is significantly deteriorated. Therefore, the O content is set to 0.005% or less.
The lower the O content, the better, and it may be 0%. However, since the substantial lower limit of the O content in mass-produced steel is 0.001%, the O content may be 0.001% or more.

The electric resistance welded steel pipe for line pipe according to this embodiment basically has a chemical composition containing the above elements with the balance being Fe and impurities. However, in order to improve strength, low temperature toughness or other properties, the following optional elements may be further contained within the ranges described below. However, since the inclusion of these elements is not essential, the lower limit for each is 0%.

In addition, in this embodiment, "impurities" refer to components that are mixed in from raw materials such as ores and scraps, or due to various factors in the manufacturing process, when steel is industrially produced, and which are acceptable within a range that does not adversely affect the characteristics of the electric-welded steel pipe for line pipe according to this embodiment.

Cu: 0-0.500%
Cu is an element effective for increasing strength without deteriorating low-temperature toughness. Therefore, Cu may be contained as necessary. To obtain the above effect, the Cu content should be set to more than 0%. It is preferable to set the content of C to 0.010% or more, and more preferable to set the content of C to 0.010% or more.
On the other hand, if the Cu content exceeds 0.500%, cracks are likely to occur during heating and electric resistance welding of the steel billet. Therefore, even if Cu is contained, the Cu content is set to 0.500% or less. .

Ni: 0~0.500%
Ni is an element effective in improving low-temperature toughness and strength. Therefore, Ni may be contained as necessary. In order to obtain the above effects, it is preferable that the Ni content is more than 0%, and 0% is preferable. It is more preferable to set it to 0.010% or more.
On the other hand, if the Ni content exceeds 0.500%, the electric resistance weldability deteriorates. Therefore, even if Ni is contained, the Ni content is set to 0.500% or less.

Cr: 0~0.500%
Cr is an element that improves the strength of steel by precipitation strengthening. Therefore, Cr may be contained as necessary. To obtain this effect, the Cr content is preferably more than 0%, and more preferably 0%. It is more preferable to set it to 0.010% or more.
On the other hand, if the Cr content exceeds 0.500%, the hardenability increases, the proportion of bainite in the structure becomes too high, and the low-temperature toughness deteriorates. . 500% or less.

Mo: 0~0.500%
Mo is an element that improves hardenability and at the same time forms carbonitrides, thereby contributing to improving the strength of steel. Therefore, Mo may be contained as necessary. In order to obtain the above effects, The Mo content is preferably more than 0%, and more preferably 0.010% or more.
On the other hand, if the Mo content exceeds 0.500%, the strength of the steel becomes higher than necessary and the low-temperature toughness deteriorates. Therefore, even if Mo is contained, the Mo content is set to 0.500% or less. .

V: 0~0.100%
V is an element that forms carbides and/or nitrides and contributes to improving the strength of steel. Therefore, V may be contained as necessary. To obtain the above effect, the V content is set to 0. %, and more preferably 0.001% or more.
On the other hand, if the V content exceeds 0.100%, the amount of precipitates increases and the low-temperature toughness deteriorates. Therefore, even if V is contained, the V content is set to 0.100% or less.

W: 0~0.500%
W is an element that forms carbides and contributes to improving the strength of steel. Therefore, W may be contained as necessary. To obtain the above effect, the W content should be more than 0%. It is preferable that the content of C is 0.100% or more.
On the other hand, if the W content exceeds 0.500%, the amount of carbides increases and the low temperature toughness deteriorates. Therefore, even if W is contained, the W content is set to 0.500% or less.

Ca: 0-0.0040%
Ca is an element that suppresses the formation of elongated MnS by forming sulfides and contributes to improving low-temperature toughness and lamellar tear resistance, so Ca may be contained as necessary. In order to obtain the above-mentioned effects, the Ca content is preferably more than 0%, and more preferably 0.0003% or more.
On the other hand, if the Ca content exceeds 0.0040%, a large amount of CaO is generated in the electric resistance welded portion, and the low temperature toughness of the electric resistance welded portion deteriorates. Therefore, even if Ca is contained, the Ca content is The content shall be 0.0040% or less.

REM: 0~0.0050%
Like Ca, REM is an element that suppresses the formation of elongated MnS by forming sulfides, and contributes to improving low-temperature toughness and lamellar tear resistance. In order to obtain the above-mentioned effects, the REM content is preferably more than 0%, and is preferably 0.0010% or more.
On the other hand, if the REM content exceeds 0.0050%, the number of REM oxides increases and low-temperature toughness deteriorates. Therefore, even if REM is contained, the REM content is set to 0.0050% or less. .
Here, REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the REM content means the total content of these elements.

As described above, the electric resistance welded steel pipe for line pipe according to this embodiment has a chemical composition in the base material and the electric resistance welded portion that contains essential elements, optional elements as required, and the balance being Fe and impurities.

In the electric welded steel pipe of this embodiment, the content of each element must be controlled as described above, and furthermore, Ceq and M, which are determined by the content of each element, must be within a specified range as follows.

Ceq: 0.16 to 0.53 mass%
Ceq is a value that serves as an index of hardenability and is represented by the following formula (i): If Ceq is less than 0.16 mass %, the desired strength cannot be obtained in the base material and the electric resistance welded portion. Therefore, Ceq is set to 0.16 mass% or more, preferably 0.25 mass% or more, and more preferably 0.30 mass% or more.
On the other hand, if Ceq exceeds 0.53 mass%, the low temperature toughness of the base material and the electric resistance welded portion deteriorates. Therefore, Ceq is set to 0.53 mass% or less. Ceq is preferably set to 0.45 mass% or less. % or less, and more preferably 0.40% or less by mass.

Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5...(i)
In the above formula (i), each element symbol is substituted with the content (mass%) of the corresponding element, and 0 is substituted when the corresponding element is not contained.

M: 0.06 to 0.25% by mass
M is a value that is an index of carbon concentration in untransformed austenite and is expressed by the following (ii). If M is less than 0.06 mass%, the desired amount of MA cannot be obtained in the electric resistance welded portion. Therefore, M is set to 0.06% by mass or more, preferably 0.10% by mass or more, and more preferably 0.12% by mass or more.
On the other hand, if M exceeds 0.25 mass%, the MA area ratio increases and the low-temperature toughness of the electric resistance welded portion deteriorates. Therefore, M is set to 0.25 mass% or less. M is preferably 0. . 20% by mass or less.

M=C/3+5×Nb...(ii)
In the above formula (ii), each element symbol is substituted with the content (mass%) of the corresponding element, and 0 is substituted when the corresponding element is not contained.

As described above, in order to improve the strength and low-temperature toughness of electric resistance welded steel pipe for line pipe, it is important to control the microstructures of the base material and the electric resistance welded part. The microstructures of the base material and the electric resistance welded part are described in detail below.
In this embodiment, the thickness of the base material is denoted as tB, and the thickness of the electric resistance weld is denoted as tS. In addition, in this embodiment, the electric resistance weld indicates the range from the butt surface of the electric resistance weld to a position 600 μm away in the circumferential direction of the base material (i.e., a range of 1200 μm in total centered on the butt surface).

<Base material part>
[In the microstructure of the 1/2×tB portion of the base material, the area ratio of ferrite is 40 to 80%]
In order to ensure the strength and low-temperature toughness of electric resistance welded steel pipe for line pipe, it is important to control the microstructure of the base material. Specifically, it is necessary that the microstructure of the base material contains 40 to 80% ferrite in terms of area ratio. If the area ratio of ferrite contained in the base material is less than 40%, the low-temperature toughness of the base material will deteriorate. Therefore, the area ratio of ferrite in the base material is set to 40% or more. The area ratio of ferrite is preferably 45% or more, and more preferably 50% or more.
On the other hand, if the area ratio of ferrite in the base material exceeds 80%, sufficient strength cannot be obtained in the base material. Therefore, the area ratio of ferrite in the base material is set to 80% or less. The area ratio of ferrite is preferably 75% or less, and more preferably 70% or less.

In this embodiment, the concept of "ferrite" includes polygonal ferrite and pseudo-polygonal ferrite. The microstructure of the base material may include one or more of pearlite (P), bainite (B), and retained austenite (γ) as residual structures. The concept of "bainite" includes granular bainitic ferrite and bainitic ferrite. The concept of "pearlite" includes pseudo-pearlite in which the lamellar cementite is not completely shaped. The area ratio of these residual structures may be 20 to 60% in relation to the area ratio of ferrite.

In this embodiment, the "1/2 x tB portion of the base material" refers to a position that is (1/2) x tB from the outer surface of the base material in the thickness direction.
The reason for limiting the microstructure at the position (1/2)×tB from the outer surface of the base material is that the structure at this position affects the low temperature toughness of the base material. In this embodiment, when simply referring to the surface of an electric resistance welded steel pipe for line pipe, it means the outer surface, not the inner surface.

Regarding the microstructure of the base material, the proportion (area ratio) of ferrite is measured by the following method.
For the base material of the electric resistance welded steel pipe for line pipe, a sample for microstructure observation is taken from a position at 90° in the circumferential direction from the electric resistance welded part so that the cross section parallel to the pipe axis direction (longitudinal direction) and the thickness direction becomes the observation surface. The electric resistance welded part can be easily distinguished from the base material part because the bead generated by electric resistance welding is cut. The taken sample for microstructure observation is polished for 30 to 60 minutes using a colloidal silica abrasive. The polished sample is analyzed using EBSP-OIM (trademark) (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) to determine the area ratio of ferrite. The field of view is a 200 μm range centered on (1/2) × tB from the outer surface in the thickness direction, and a 500 μm range at any position in the pipe axis direction. The observation magnification is 400 times, the acceleration voltage is 20 kV, and the measurement step is 0.3 μm. The measurement device used is an EBSD device composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (high-speed operation Hikari detector manufactured by TSL).

Specifically, the area ratio of ferrite is determined by the KAM (Kernel Average Misorientation) method equipped in the EBSP-OIM (trademark).
In the KAM method, an arbitrary regular hexagonal pixel in the measurement data is set as the center pixel. The orientation difference between each pixel is calculated for the first approximation (total of 7 pixels) using the 6 pixels adjacent to this center pixel, the second approximation (total of 19 pixels) using the 12 pixels further outside these 6 pixels, or the third approximation (total of 37 pixels) using the 18 pixels further outside these 12 pixels. The calculated orientation differences are averaged, and the obtained average value is used as the value of the center pixel. This operation is performed for all pixels.

In this embodiment, the area ratio of pixels calculated to have an orientation difference of 1° or less to the third approximation relative to the total area of the field of view is defined as the area ratio of ferrite. Those with an orientation difference of more than 1° to the third approximation are defined as structures other than ferrite, such as bainite. Retained austenite is defined as the fcc phase measured by EBSP-OIM, and pearlite is identified by observation with an optical microscope at a magnification of 400x after nital etching.

[In the microstructure of the 1/2×tB portion of the base material, the average grain size is 35 μm or less]
In the electric resistance welded steel pipe for line pipe according to the present embodiment, in order to ensure good toughness of the base material, the average crystal grain size in the microstructure of the 1/2×tB portion of the base material is set to 35 μm or less. If the average crystal grain size exceeds 35 μm, sufficient toughness is deteriorated in the base material. The average crystal grain size is preferably set to 30 μm or less, and more preferably set to 20 μm or less.
The lower limit of the average crystal grain size in the microstructure of the 1/2×tB portion of the base material is not particularly limited, but may be 1 μm or more, 3 μm or more, or 5 μm or more.

The average crystal grain size in the microstructure of the 1/2×tB portion of the base material is measured by the following method.
The average grain size is obtained by analyzing the microstructure at 1/2×tB of the base material using the same sample as the sample for which the area ratio of ferrite was measured, using EBSP-OIM. The field of view is a 200 μm range centered on (1/2)×tB from the outer surface in the thickness direction, and a 500 μm range at any position in the tube axis direction. The observation magnification is 400 times, and the measurement step is 0.3 μm. From the data obtained by the measurement, the region surrounded by high-angle grain boundaries with an inclination angle of 15° or more is regarded as a grain, and the circle-equivalent diameter of the grain is regarded as the grain size. The average grain size is calculated from the obtained grain size using the AREA FRACTION method.
However, the region of 0.25 μm or less in equivalent circle diameter is excluded from the calculation of the average crystal grain size, because the region of 0.25 μm or less in equivalent circle diameter cannot be evaluated correctly due to the measurement limit.

<Electric resistance welded section>
[In the microstructure of the 1/4×tS part of the electric resistance weld, the area ratio of ferrite is 40 to 70%]
The microstructure of the electric resistance weld can be controlled by reheating the electric resistance weld and then water cooling from the outer surface side. In the electric resistance weld, the hardness on the outer surface side increases, which deteriorates the low temperature toughness. Therefore, in the electric resistance weld steel pipe for line pipe according to this embodiment, it is necessary for the microstructure of the 1/4×tS part of the electric resistance weld to contain soft ferrite in order to ensure the low temperature toughness of the electric resistance weld.

Specifically, in the microstructure of the 1/4×tS portion of the electric resistance weld, the ferrite area ratio must be 40 to 70%. If the ferrite area ratio is less than 40%, the hardness of the outer surface side of the electric resistance weld increases, and the low temperature toughness deteriorates. The ferrite area ratio is preferably 45% or more, and more preferably 50% or more.
Furthermore, if the area ratio of ferrite in the microstructure of the 1/4×tS portion of the electric resistance welded portion exceeds 70%, the strength of the electric resistance welded portion decreases. The area ratio of ferrite is preferably 65% or less, and more preferably 60% or less.

[In the microstructure of the 1/4×tS part of the electric resistance weld, the area ratio of island martensite (MA) is 0.2 to 10.0%]
Island martensite (MA) also affects the low-temperature toughness of electric resistance welds. Since MA increases the starting point of fracture and hardness, the lower the area ratio of MA, the higher the low-temperature toughness of electric resistance welds. Figure 1 shows the relationship between the MA area ratio of electric resistance welds and δc. δc indicates the limit opening displacement in a CTOD test at -20°C conducted by the method described below. As shown in Figure 1, the limit opening displacement δc in a CTOD test at -20°C of electric resistance welds increases with a decrease in the MA area ratio of electric resistance welds, and it can be seen that if the MA area ratio is 10.0% or less, δc is 0.15 mm or more.

MA also increases the work hardening capacity, so it also affects the yield ratio (YR). The higher the area ratio of MA, the better the work hardening capacity and the lower the YR. Figure 2 shows the relationship between the MA area ratio of electric resistance welds and the YR obtained by conducting a tensile test using the method described below. As shown in Figure 2, as the MA area ratio of electric resistance welds increases, the YR of the electric resistance welds decreases, and it can be seen that if the MA area ratio is 0.2% or more, the YR of the electric resistance welds is 90% or less. Unlike the base material, electric resistance welds do not contain hard structures caused by central segregation, so control of the MA area ratio is essential to reduce the YR of electric resistance welds.

Therefore, in the electric resistance welded steel pipe for line pipe according to the present embodiment, in order to achieve both excellent low temperature toughness and low YR in the electric resistance welded part, the area ratio of MA is set to 0.2 to 10.0% in the microstructure of the 1/4×tS part of the electric resistance welded part. If the area ratio of MA is less than 0.2%, the work hardening ability decreases, so that the YR increases. The area ratio of MA is preferably 1.0% or more, more preferably 2.0% or more, and even more preferably 3.0% or more.
Furthermore, in the microstructure of the 1/4×tS portion of the electric resistance weld, if the area ratio of MA exceeds 10.0%, the low temperature toughness of the electric resistance weld deteriorates. The area ratio of MA is preferably 8.0% or less, more preferably 7.0% or less, and even more preferably 6.0% or less.

In this embodiment, the "1/4×tS portion of the electric resistance weld" refers to a position that is (1/4)×tS from the outer surface of the electric resistance weld in the thickness direction.
The reason why the microstructure at the position (1/4)×tS from the outer surface of the electric resistance weld in the thickness direction is limited is because the structure at this position affects the low temperature toughness and YR of the electric resistance weld.

The microstructure of electric resistance welds may contain one or more of pearlite (P), bainite (B), and retained austenite (γ) as residual structures. The concept of "bainite" includes granular bainitic ferrite and bainitic ferrite. The area ratio of these residual structures may be 20.0 to 59.8% in relation to the area ratios of ferrite and MA.

The area ratio of ferrite in the microstructure of the electric resistance welded portion is determined by the following method.
A sample for microstructure observation is taken from an electric resistance welded steel pipe for line pipe so that the cross section perpendicular to the pipe axis direction including the electric resistance weld is the observation surface. As described above, in this embodiment, the electric resistance weld indicates the range from the butt surface of the electric resistance weld to a position 600 μm away from the base metal in the circumferential direction (i.e., a total range of 1200 μm centered on the butt surface). After the observation surface is mirror-finished by wet polishing, the area ratio of ferrite is measured using EBSD in the same manner as the base metal. The measurement position is a 200 μm range centered on (1/4) × tS in the thickness direction from the outer surface in the thickness direction, and a 200 μm range centered on a position 400 μm away from the butt surface of the electric resistance weld in the circumferential direction.
The butt surfaces of the electric resistance welded parts can be etched with nital to distinguish them from the base material.

The area ratio of MA in the microstructure of the electric resistance welded portion is determined by the following method.
A sample is taken in the same manner as when the area ratio of ferrite in the electric resistance welded portion was measured. After LePera etching of the observation surface, a microstructure photograph is taken using an optical microscope at 400 times magnification. In the obtained microstructure photograph, the part observed as white can be identified as island martensite, and the area ratio of island martensite (MA) is calculated by performing image analysis. The field of view is a 200 μm range centered on (1/4) × tS in the thickness direction from the outer surface in the thickness direction, and a 200 μm range centered on a position 400 μm away from the butt surface of the electric resistance welded portion in the circumferential direction.
However, regions with a circle equivalent diameter of 1 μm or less are excluded from the area ratio of MA, because MA with a circle equivalent diameter of 1 μm or less does not affect low temperature toughness and YR.

[In the microstructure at the 1/4×tS part of the electric resistance welded part, the average crystal grain size is 35 μm or less]
In order to ensure good low-temperature toughness in the electric resistance welded portion, it is important to control the area ratio of ferrite and MA as described above, and to refine the microstructure. In the electric resistance welded steel pipe for line pipe according to this embodiment, in order to ensure low-temperature toughness of the electric resistance welded portion, the average crystal grain size is controlled to 35 μm or less in the microstructure in the 1/4×tS part of the electric resistance welded portion. If the average crystal grain size exceeds 35 μm, the low-temperature toughness of the electric resistance welded portion deteriorates. The average crystal grain size is preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 15 μm or less.
The lower limit of the average crystal grain size is not particularly limited, but may be 1 μm or more, 3 μm or more, or 5 μm or more.

The average grain size in the electric resistance welded portion is determined in the same manner as in the base material portion.
The viewing range is a 200 μm range centered on (1/4)×tS from the outer surface in the thickness direction, and a 200 μm range centered on a position 400 μm away from the butt surface of the electric resistance weld in the circumferential direction. The area with a circle equivalent diameter of 0.25 μm or less is excluded from the calculation of the average grain size. This is because the area with a circle equivalent diameter of 0.25 μm or less does not adversely affect the low temperature toughness of the electric resistance weld.

3. Mechanical properties (base material and electric resistance welded parts)
Yield stress (YS): 360 MPa or more Tensile strength (TS): 465 MPa or more Yield ratio (YR): 90% or less Since the electric resistance welded steel pipe for line pipe according to this embodiment is intended to be used as a line pipe, the yield stress (YS) measured in both the base material and the electric resistance welded part is 360 MPa or more, and the tensile strength (TS) is 465 MPa or more. The yield stress is preferably 400 MPa or more or 450 MPa or more. The yield stress may be 600 MPa or less or 550 MPa or less. The tensile strength is preferably 500 MPa or more or 550 MPa or more. The tensile strength may also be 700 MPa or less or 650 MPa or less.
The yield ratio (YR) is preferably 90% or less. The yield ratio may be 80% or more, or 85% or more. The yield ratio can be calculated by dividing the yield ratio by the tensile strength (YS/TS).

(Base material)
Charpy impact absorption energy at −20° C.: 150 J or more In the electric resistance welded steel pipe according to this embodiment, the base material preferably has a Charpy impact absorption energy of 150 J or more at −20° C. If the base material has a Charpy impact absorption energy of 150 J or more at −20° C., sufficient toughness can be ensured even when used in cold regions, etc.
The Charpy impact absorption energy at -20°C may be 400J or less, or 350J or less.

(Electric resistance welded section)
Critical opening displacement δc in CTOD test at −20° C.: 0.15 mm or more In the electric resistance welded steel pipe for line pipe according to this embodiment, the critical opening displacement δc in the CTOD test at −20° C. is preferably 0.15 mm or more. If the critical opening displacement δc in the CTOD test at −20° C. is 0.15 mm or more in the electric resistance welded part, it can be determined that the low temperature toughness is excellent.
The critical opening displacement δc in the CTOD test at −20° C. may be 1.00 mm or less, or 0.80 mm or less.

Tensile tests are conducted on the base material using a full-thickness test piece in the longitudinal direction of the electric-resistance welded steel pipe for line pipe as the tensile test piece. Yield strength and tensile strength are measured based on the tensile test results. Here, the tensile test piece of the base material is taken from a portion corresponding to a position 90° circumferentially from the seam of the electric-resistance welded steel pipe. The tensile test is conducted in accordance with DNV-ST-F101 (2021 edition).

For the tensile test of the electric resistance welded part, a round bar test piece in the longitudinal direction of the electric resistance welded steel pipe for line pipe is taken from the 1/2 x tS part of the welded part of the electric resistance welded steel pipe and a tensile test is performed. The round bar tensile test piece is an ISO6892 (2019 edition) proportional test piece with a parallel part diameter of φ12.7 mm and a gauge length of 65 mm. Yield strength and tensile strength are measured based on the tensile test results. The tensile test is performed in accordance with ISO6892.

In the Charpy test to evaluate the low-temperature toughness of the base material, a V-notch Charpy test specimen is taken from the center of the wall thickness of the base material of the electric resistance welded steel pipe for line pipe (the part corresponding to a position 90° circumferentially from the butt surface of the electric resistance welded part) so that the longitudinal direction of the test specimen is the circumferential direction of the electric resistance welded steel pipe for line pipe. In this case, the depth direction of the V-notch is the longitudinal direction of the steel pipe. The V-notch Charpy test is performed at a test temperature of -20°C, and the impact absorption energy at -20°C is measured. The Charpy test is performed in accordance with DNV-ST-F101 (2021 edition).

In the CTOD (Crack Tip Opening Displacement) test to evaluate the low temperature toughness of the electric resistance welded part, the electric resistance welded steel pipe for line pipe is cut to a length of 300 mm in the longitudinal direction and 300 mm in the circumferential direction, including the electric resistance welded part, and a CTOD test piece including the electric resistance welded part is taken. The CTOD test piece is processed so that the fatigue pre-crack is on the butt surface of the electric resistance weld and the depth direction of the fatigue pre-crack is in the longitudinal direction of the steel pipe. The CTOD test piece was subjected to a CTOD test at a test temperature of -20°C in accordance with the provisions of BS7448-1:1991, and the limit opening displacement δc (mm) at -20°C was measured. If a CTOD test piece cannot be taken because it is a circular arc, the sleeve of the test piece may be welded together and the test may be performed.

4. Thickness The wall thickness tB of the base metal part and the wall thickness tS of the electric resistance welded part of the electric resistance welded steel pipe for line pipe according to this embodiment are set to 15.0 mm or more from the viewpoint of buckling resistance performance when used as a line pipe. The wall thickness tB and the wall thickness tS are preferably 17.0 mm or more. On the other hand, the wall thickness tB and the wall thickness tS of the electric resistance welded steel pipe for line pipe are generally set to an upper limit of 25.4 mm.

5. Outer Diameter The outer diameter of the electric resistance welded steel pipe for line pipe according to the present embodiment is set to 304.8 mm or more from the viewpoint of improving the transport efficiency of the fluid passing through the pipe when used as a line pipe. On the other hand, the outer diameter of the electric resistance welded steel pipe for line pipe is generally set to an upper limit of 660.4 mm.

The electric resistance welded steel pipe for line pipe according to the present embodiment can obtain the above-mentioned effects regardless of the manufacturing method. The electric resistance welded steel pipe for line pipe according to the present embodiment can be manufactured, for example, by a manufacturing method including the following steps.
(a) a casting process for producing a slab having a predetermined chemical composition; (b) a heating process for heating the slab; (c) a hot rolling process for hot rolling the heated slab to produce a hot rolled steel sheet; (d) a winding process for cooling and winding the hot rolled steel sheet after the hot rolling process; (e) an electric resistance welding process for uncoiling the hot rolled steel sheet after the winding process, roll-forming it into a tubular shape and electric resistance welding it to produce an electric resistance welded steel pipe; (f) a heat treatment process for heat treating the electric resistance welded portion of the electric resistance welded steel pipe; and (g) a heat treatment process for heat treating the electric resistance welded portion of the electric resistance welded steel pipe. Furthermore, if necessary, sizing may be performed to improve roundness. Preferred conditions for each process are described below.

<Casting process>
In the casting process, the steel having the above-mentioned chemical composition is melted in a furnace and then cast into a slab. The casting method is not particularly limited, and may be any of ordinary continuous casting, casting by the ingot method, thin slab casting, etc.

<Heating process>
In the heating step, the manufactured slab is heated in a heating furnace. The heating temperature T (° C.) of the slab in the heating furnace is preferably 1100 to 1170° C. The residence time t (min) in the furnace is preferably 100 to 450 min.
In this embodiment, the residence time t (minutes) is the time from when the slab is charged into the heating furnace to when the slab is removed from the heating furnace.

In the heating process, it is further preferable that F1, defined by the following formula (iii), is 2800 to 3700.

F1=(T+273.15)×log(t)...(iii)
In the formula (iii), T is the heating temperature (° C.) in the heating step, and t is the time spent in the furnace (minutes).

If the heating conditions are not appropriate, the austenite grain size will become coarse during heating, and accordingly the average crystal grain size in the 1/2×tB portion of the base material will also become coarse, which may result in a deterioration in the low-temperature toughness of the base material.
On the other hand, the segregation state at the slab (coil) end corresponding to the welded portion of the electric resistance welded steel pipe is affected by the slab heating conditions, which ultimately affects the generation of MA in the electric resistance welded portion.
In the electric resistance welded portion, a segregation zone caused by the slab (coil) exists. The segregation zone has a higher alloy concentration of C and Mn than the non-segregation zone, so that the transformation is relatively difficult to start compared to the non-segregation zone, and the alloy is concentrated from the non-segregation zone that has been transformed first, and MA is generated along the segregation zone. When the alloy concentration of the segregation zone is high, the area ratio of MA present in the electric resistance welded portion increases, and the low-temperature toughness of the electric resistance welded portion may deteriorate. When the alloy concentration of the segregation zone of the electric resistance welded portion is low, the area ratio of MA present in the electric resistance welded portion decreases, and the YR increases. The electric resistance welded portion corresponds to the end of the slab (coil), and the segregation zone in this portion is not a central (macro) segregation that exists in the center of the plate thickness of the width center part of the slab (coil), but a microsegregation, so it is significantly affected by the heating conditions during the production of hot-rolled steel sheets.

Therefore, in this embodiment, by appropriately heating the slab under heating conditions that take into account the heating temperature and furnace time, coarsening of the heated austenite grain size is suppressed in the slab before hot rolling, and atoms are uniformly diffused to control the segregation zone at the width end of the hot-rolled steel sheet, i.e., the electric resistance welded part after electric resistance welding. Specifically, it is preferable to control the heating temperature and furnace time so that F1 represented by formula (iii) is 2800 to 3700.

Assuming that the content of each element in the chemical composition of the slab is within the range of this embodiment and that formulas (i) and (ii) are satisfied, if F1 is less than 2,800, even if other manufacturing conditions are satisfied, the MA area ratio of the electric welded portion will increase, and the low-temperature toughness of the electric welded portion may deteriorate.
Furthermore, when F1 exceeds 3700, the austenite grain size during heating becomes coarse, and the average crystal grain size of the base material becomes coarse, which may deteriorate the low-temperature toughness of the base material. Furthermore, when F1 exceeds 3700, the MA area ratio of the electric resistance welded part decreases, and the YR of the electric resistance welded part may increase.

<Hot rolling process>
In the hot rolling process, it is preferable that the reduction ratio in the recrystallized region is 2.0 or more and the reduction ratio in the non-recrystallized region is 2.0 or more. In particular, by setting the reduction ratio in the non-recrystallized region to 2.0 or more, it is possible to set the average crystal grain size of the base material to 20 μm or less. The boundary between the recrystallized region and the non-recrystallized region is about 900 to 950° C., depending on the composition of the steel.

The finish rolling start temperature is preferably 900 to 950° C. in order to ensure low temperature toughness by rolling in the unrecrystallized region.
The hot rolling end temperature (finish rolling end temperature) is preferably 770° C. or higher. If the hot rolling end temperature is less than 770° C., the rolling will be in a two-phase region, and the toughness of the base material will deteriorate.

<Winding process>
In the coiling process, the steel sheet after the hot rolling process is cooled to a surface temperature range of 500 to 650°C so that the average cooling rate at the center of the plate thickness is in the range of 5 to 80°C/sec, and coiled at that temperature range. The average cooling rate at the center of the plate thickness can be calculated by heat transfer calculation from the temperature history of the outer surface.

In order to control the microstructure of the base material of the electric resistance welded steel pipe for line pipe according to the present embodiment so as to have a predetermined structure, it is particularly important to control the cooling rate. If the average cooling rate is less than 5° C./sec, ferrite transformation may proceed and the ferrite area ratio may exceed 80%.
On the other hand, when the average cooling rate exceeds 80° C./sec, the cooling rate is too fast, so that ferrite transformation does not occur and the ferrite area ratio may become less than 40%.

In addition, if the cooling stop temperature exceeds 650°C, ferrite transformation occurs after coiling, and the ferrite area ratio may exceed 80%. If the cooling stop temperature (coiling temperature) is less than 500°C, the temperature variation during cooling becomes large, resulting in strength variation, and the electric resistance welded steel pipe for line pipe according to this embodiment cannot be stably produced.

<Electric resistance welding process>
Electric resistance welded steel pipe for line pipe is manufactured while uncoiling the coiled hot rolled steel sheet. Specifically, the hot rolled steel sheet is processed into an open pipe by bending using a continuous forming roll. Then, the joint of the open pipe, that is, both end faces in the width direction of the hot rolled steel sheet, are welded by electric resistance welding to manufacture the electric resistance welded steel pipe for line pipe.

<Heat treatment process>
In the heat treatment step, the electric resistance welded portion formed in the electric resistance welding step is heated from the outer surface and then water-cooled from the outer surface side. Heating can be performed by induction heating, for example.

Specifically, the heating involves heating the electric resistance weld to a temperature range of 870-1070°C, and then cooling with water to a surface temperature of 500°C or less so that the average cooling rate of the 1/4×tS part is in the range of 5-30°C/s. The average cooling rate of the 1/4×tS part can be calculated by heat transfer calculations from the temperature history of the outer surface. This heat treatment (heating and cooling) makes it possible to control the microstructure of the electric resistance weld (fraction of each structure, average crystal grain size) within the above-mentioned range.

If the heating temperature is below 870°C, regions that do not transform into austenite during heat treatment will remain, causing the microstructure to coarsen and the average crystal grain size to coarsen, which may result in a deterioration in the low-temperature toughness of the electric resistance welded joint.
Furthermore, if the heating temperature exceeds 1070°C, coarse austenite is generated during heat treatment, which may result in a coarse microstructure after cooling and a deterioration in the low-temperature toughness of the electric resistance welded portion.

If the average cooling rate is below 5°C/s, the ferrite area ratio increases and the strength of the electric resistance weld may decrease. If the average cooling rate is above 80°C/s, the ferrite area ratio falls below 40%, and the low-temperature toughness of the electric resistance weld may deteriorate.

If the cooling stop temperature exceeds 500°C, the area ratio of MA in the electric resistance welded joint may increase, resulting in deterioration of low-temperature toughness.

The effects of one aspect of the present invention will be explained in more detail below using an example. However, the conditions in the example are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Steel types A1 to A47 having the chemical compositions shown in Tables 1A and 1B (the balance being Fe and impurities) were produced. These steel types A1 to A47 were heated, hot rolled, cooled, and coiled under the conditions shown in Tables 2A and 2B to obtain hot-rolled steel sheets.

The obtained hot-rolled steel sheet was subjected to bending using a forming roll and electric resistance welding, and the welded portion was heat-treated (heated and water-cooled) under specified conditions as shown in Tables 2A and 2B to produce an electric resistance welded steel pipe for line pipe.
The wall thickness tB and wall thickness tS of the produced electric resistance welded steel pipes were in the range of 15.0 to 25.4 mm, and the outside diameter was in the range of 304.8 to 660.4 mm.

For the obtained electric-resistance welded steel pipe, the area ratio of ferrite and the average crystal grain size in the microstructure of the 1/2×tB portion of the base material, and the area ratio of ferrite, the area ratio of island martensite (MA) and the average crystal grain size in the microstructure of the 1/4×tS portion of the electric-resistance welded portion were evaluated using the methods described above.
The results obtained are shown in Tables 3A and 3B.
In the metal structure of the base material, the remainder other than ferrite was bainite, pearlite, and retained austenite. In the metal structure of the electric resistance welded joint, the remainder other than ferrite was bainite, pearlite, and retained austenite.

In addition, the tensile test, the Charpy test at -20°C, and the CTOD test at -20°C were performed by the above-mentioned method, and the strength (yield stress, tensile strength), the yield ratio (YR), and the low-temperature toughness (impact absorption energy at -20°C, the critical opening displacement δc at -20°C) were evaluated.
The results obtained are shown in Tables 4A and 4B.

In both the base material and the electric resistance welded parts, if the yield stress (YS) was 360 MPa or more and the tensile strength (TS) was 465 MPa or more, the electric resistance welded steel pipe for line pipes was judged to have passed the test as having high strength in the base material and the electric resistance welded parts. On the other hand, if the yield stress (YS) was less than 360 MPa or the tensile strength (TS) was less than 465 MPa, the electric resistance welded steel pipe for line pipes was judged to have failed the test as it did not have high strength in the base material and the electric resistance welded parts.

When the yield ratio (YR=YS/TS) was 90% or less in both the base material and the electric resistance welded portion, the electric resistance welded steel pipe for line pipe was judged to be acceptable as having a low YR in the base material and the electric resistance welded portion.
On the other hand, when the yield ratio was more than 90%, the electric resistance welded steel pipe for line pipe was judged to be unacceptable since it did not have a low YR in the base metal and electric resistance welded parts.

When the Charpy impact absorption energy at -20°C of the base material was 150 J or more, the electric resistance welded steel pipe for line pipes was judged to have excellent low-temperature toughness in the base material and to have passed the test. On the other hand, when the Charpy impact absorption energy at -20°C was less than 150 J, the electric resistance welded steel pipe for line pipes was judged to have poor low-temperature toughness in the base material and to have passed the test.

In the case where the critical opening displacement δc in the CTOD test at -20°C was 0.15 mm or more in the electric resistance welded joint, the electric resistance welded steel pipe for line pipe use was judged to have excellent low-temperature toughness and passed the test. On the other hand, in the case where the critical opening displacement δc in the CTOD test at -20°C was less than 0.15 mm, the electric resistance welded steel pipe for line pipe use was judged to have excellent low-temperature toughness and failed the test.

As shown in Tables 1A to 4B, for Test Nos. 1 to 34, the chemical composition and microstructure of the base material were within the range of the present invention, and the microstructure of the electric resistance welded portion was also within the range of the present invention. As a result, it can be seen that electric resistance welded steel pipes for line pipes having high strength, low YR, and excellent low temperature toughness were obtained.
On the other hand, the characteristics of Test Nos. 35 to 61, which are comparative examples, did not satisfy the pass criteria for the reasons described below.

In test No. 35, the C content exceeded the upper limit of the range of the present invention, and the central segregation hardened. As a result, the low-temperature toughness of the base material deteriorated.

In Test No. 36, the C content was below the lower limit of the range of the present invention. As a result, the YR increased in the base material.

In test No. 37, the Si content exceeded the upper limit of the range of the present invention, and oxides increased in the electric resistance welded joint. As a result, the low temperature toughness of the electric resistance welded joint deteriorated.

In Test No. 38, the Si content was below the lower limit of the range of the present invention, and deoxidation was insufficient. As a result, the base material toughness was deteriorated.

In Test No. 39, the Mn content exceeded the upper limit of the range of the present invention, and the central segregation of the base material hardened. As a result, the low-temperature toughness of the base material deteriorated.

In Test No. 40, the Mn content was below the lower limit of the range of the present invention. As a result, sufficient strength was not obtained in the base material and the electric resistance welded part.

In test No. 41, the Ti content exceeded the upper limit of the range of the present invention, and coarse inclusions were formed. As a result, the low-temperature toughness of the base material was deteriorated.

In Test No. 42, the Ti content was below the lower limit of the range of the present invention. As a result, the average crystal grain size of the base material became coarse and the low-temperature toughness deteriorated.

In test No. 43, the Nb content and M exceeded the upper limit of the range of the present invention, and coarse inclusions were generated. As a result, the low-temperature toughness of the base material deteriorated. In addition, the MA area ratio in the electric resistance weld increased, and the low-temperature toughness of the electric resistance weld deteriorated.

In test No. 44, the Nb content and M were below the lower limit of the range of the present invention. As a result, the average crystal grain size of the base material became coarse, and the low-temperature toughness of the base material deteriorated. In addition, sufficient strength was not obtained in the base material. In addition, the MA area ratio in the electric resistance welded part decreased, and the YR increased.

In test No. 45, the N content exceeded the upper limit of the range of the present invention, and coarse inclusions were formed. As a result, the low-temperature toughness of the base material was deteriorated.

In Test No. 46, the N content was below the lower limit of the range of the present invention. As a result, the average crystal grain size of the base material became coarse, and the low-temperature toughness of the base material deteriorated.

In test No. 47, Ceq exceeded the upper limit of the range of the present invention, and the ferrite fraction decreased. As a result, the low-temperature toughness of the base material and the electric resistance welded part deteriorated.

In Test No. 48, Ceq was below the lower limit of the range of the present invention, and the ferrite fraction increased. As a result, sufficient strength was not obtained in the base material and the electric resistance welded part.

In Test No. 49, M exceeded the upper limit of the range of the present invention. As a result, the MA area ratio in the electric resistance weld increased, and the low-temperature toughness of the electric resistance weld deteriorated.

In Test No. 50, M was below the lower limit of the range of the present invention. As a result, the MA area ratio in the electric resistance welded part decreased and YR increased.

In test No. 51, F1 exceeded the upper limit, so the average grain size of the base material became coarse. As a result, the low-temperature toughness of the base material deteriorated. In addition, the alloy concentration of the segregation zone of the electric resistance weld became too low, the MA area ratio in the electric resistance weld decreased, and the YR increased.

In test No. 52, F1 was below the lower limit, so the alloy concentration in the segregation zone of the electric resistance weld was high and the area ratio of MA increased. As a result, the low-temperature toughness of the electric resistance weld was deteriorated.

In test No. 53, the reduction ratio in the non-recrystallized region was below the lower limit, so the average grain size of the base material became large. As a result, the low-temperature toughness of the base material deteriorated.

In test No. 54, the cooling rate after hot rolling was high, and the ferrite area ratio of the base material decreased. As a result, the low-temperature toughness of the base material deteriorated.

In test No. 55, the cooling rate after hot rolling was low, and the ferrite area ratio in the base material increased. As a result, sufficient strength was not obtained in the base material.

In Test No. 56, the cooling stop temperature and coiling temperature after hot rolling were high, and the ferrite area ratio of the base material increased. As a result, sufficient strength was not obtained in the base material.

In Test No. 57, the heating temperature of the electric resistance weld was high, and the average grain size of the electric resistance weld became large. As a result, the low temperature toughness of the weld deteriorated.

In Test No. 58, the heating temperature of the electric resistance weld was low, and the average grain size of the electric resistance weld became large. As a result, the low-temperature toughness of the electric resistance weld deteriorated.

In Test No. 59, the cooling rate of the electric resistance weld was high, and the ferrite area ratio decreased. As a result, the low-temperature toughness of the electric resistance weld deteriorated.

In Test No. 60, the cooling rate of the electric resistance weld was low, and the ferrite area ratio increased. As a result, sufficient strength was not obtained in the electric resistance weld.

In Test No. 61, the cooling stop temperature of the weld was high, and the area ratio of MA increased. As a result, the low-temperature toughness of the electric resistance weld deteriorated.

According to the above aspect of the present invention, it is possible to obtain an electric resistance welded steel pipe for line pipes having high strength, low YR and excellent low temperature toughness in the base material and electric resistance welded parts. Therefore, it has high industrial applicability.

Claims (2)

A base material portion and an electric resistance welded portion are included.
The chemical composition of the base material is, in mass%,
C: 0.060-0.120%,
Si: 0.01-0.50%,
Mn: 0.5-2.0%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.080% or less,
Ti: 0.003 to 0.030%,
Nb: 0.003-0.046%,
N: 0.0010-0.0080%,
O: 0.005% or less,
Cu: 0-0.500%,
Ni: 0 to 0.500%,
Cr: 0-0.500%,
Mo: 0-0.500%,
V: 0 to 0.100%,
W: 0 to 0.500%,
Ca: 0-0.0040%,
REM: 0 to 0.0050%, and
The balance is composed of Fe and impurities.
Ceq represented by the following formula (i) is 0.16 to 0.53 mass%,
M represented by the following formula (ii) is 0.06 to 0.25 mass %,
When the thickness of the base material portion is tB and the thickness of the electric resistance welded portion is tS,
The tB and the tS are 15.0 to 25.4 mm,
The outer diameter is 304.8 to 660.4 mm,
In the microstructure of the 1/2 × tB portion of the base material,
The area ratio of ferrite is 40 to 80%, and the average crystal grain size is 35 μm or less.
In the microstructure of the 1/4×tS portion of the electric resistance welded portion,
The area ratio of ferrite is 40 to 70%, the area ratio of island martensite is 0.2 to 10.0%, and the average crystal grain size is 35 μm or less;
An electric resistance welded steel pipe for line pipe having a yield stress of 360 MPa or more and a tensile strength of 465 MPa or more.
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5...(i)
M=C/3+5×Nb...(ii)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formulas (i) and (ii), and 0 is substituted when the corresponding element is not contained. The chemical composition of the base material is, in mass%,
Cu: more than 0% and less than 0.500%,
Ni: more than 0% and less than 0.500%,
Cr: more than 0% and less than 0.500%,
Mo: more than 0% and less than 0.500%,
V: more than 0% and less than 0.100%,
W: more than 0% and less than 0.500%,
2. The electric resistance welded steel pipe for line pipe according to claim 1, further comprising at least one selected from the group consisting of Ca: more than 0% and not more than 0.0040%, and REM: more than 0% and not more than 0.0050%.

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