WO2017163987A1

WO2017163987A1 - Electric resistance welded steel tube for line pipe

Info

Publication number: WO2017163987A1
Application number: PCT/JP2017/010023
Authority: WO
Inventors: 健介長井; 雅和尾▲崎▼; 長谷川　昇
Original assignee: 新日鐵住金株式会社
Priority date: 2016-03-22
Filing date: 2017-03-13
Publication date: 2017-09-28
Also published as: JP6213703B1; EP3375900A1; EP3375900A4; JPWO2017163987A1; CN108368582A; KR20180077259A

Abstract

Provided is an electric resistance welded steel tube for line pipes in which: the chemical composition of the base material part comprises, in terms of mass%, C: 0.080-0.120%, Mn: 0.30-1.00%, Ti: 0.005-0.050%, Nb: 0.010-0.100%, N: 0.001-0.020 %, Si: 0.010-0.450 %, and Al: 0.001-0.100 %, the balance being Fe and impurities; CMeq expressed by equation (1) is 0.170-0.300; the Mn/Si ratio is at least 2.0; LR expressed by equation (2) is at least 0.210; and when the metallographic structure of the base material part is observed using SEM with magnification factor of 1,000, the ferrite area ratio is 60-98% and the balance comprises tempered bainite. Equation (1): CMeq = C+Mn/6+Cr/5 + (Ni+Cu)/15 + Nb + Mo/3 + V Equation (2): LR = (2.1×C+Nb)/Mn

Description

ERW steel pipe for line pipe

This disclosure relates to ERW steel pipes for line pipes.

In recent years, the importance of line pipes, which are one of the means of transporting crude oil or natural gas, has been increasing.
In some cases, it is required to lower the yield ratio of the ERW steel pipe in the axial direction of the ERW steel pipe used as the line pipe (that is, the ERW steel pipe for line pipe).
For example, in Patent Document 1, before forming pipes, the axial direction of the ERW steel pipe obtained by inducing a Bausinger effect by applying repeated strain by, for example, bending and unbending treatment to the steel strip as a raw material. A technique for lowering the yield ratio is disclosed.
In Patent Document 2, the metal structure of a hot-rolled steel sheet for producing an ERW steel pipe is a microstructure made of ferrite and martensite having an area ratio of 1 to 20%, so that the direction of the axis of the ERW steel pipe is increased. Techniques for lowering the yield ratio have been proposed.
Moreover, in patent document 3, as for the manufacturing method of the ERW steel pipe excellent in the distortion aging resistance which suppressed the raise of the yield ratio by coating heating and improved the deformation | transformation characteristic, Nb amount is 0.003% or more and 0.02 A method for manufacturing an ERW steel pipe using a steel slab of less than% is proposed. In paragraph 0019 of Patent Document 3, in the conventional ERW steel pipe with a large amount of Nb, precipitation of Nb carbide proceeds due to processing strain introduced at the time of pipe making, and yield strength and tensile strength increase. It has been clarified that the yield strength is significantly increased by the strong precipitation strengthening, and as a result, the yield ratio is increased. "

Patent Document 1: Japanese Patent No. 4466320 Patent Document 2: Japanese Patent Laid-Open No. 10-176239 Patent Document 3: International Publication No. 2012/133558

However, in the technique of Patent Document 1, since a process of applying strain to the steel strip is necessary, the number of processes increases, and as a result, the manufacturing cost of the ERW steel pipe may increase.
Moreover, with respect to the technique of Patent Document 2, it may be required to further improve the toughness of the base material portion of the ERW steel pipe.
Moreover, it may be calculated | required with respect to the technique of patent document 3 to reduce YR of an ERW steel pipe by methods other than the method of reducing the amount of Nb.

An object of the present disclosure is to provide an ERW steel pipe for a line pipe that has a certain degree of tensile strength and yield strength, has a reduced yield ratio, and is excellent in toughness of a base material portion and an ERW weld portion.

Means for solving the above problems include the following aspects.
<1> Including the base metal part and the ERW welded part,
The chemical composition of the base material part is mass%,
C: 0.080 to 0.120%,
Mn: 0.30 to 1.00%
Ti: 0.005 to 0.050%,
Nb: 0.010 to 0.100%,
N: 0.001 to 0.020%,
Si: 0.010 to 0.450%,
Al: 0.001 to 0.100%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Mo: 0 to 0.50%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Cr: 0 to 1.00%,
V: 0 to 0.100%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
REM: 0 to 0.0100% and the balance: Fe and impurities,
CMeq represented by the following formula (1) is 0.170 to 0.300,
The ratio of mass% of Mn to mass% of Si is 2.0 or more,
LR represented by the following formula (2) is 0.210 or more,
When the metal structure of the base material part is observed at a magnification of 1000 times using a scanning electron microscope, the area ratio of the first phase made of ferrite is 60 to 98%, and the remaining second phase is tempered. Including bainite,
Yield strength in the tube axis direction is 390 to 562 MPa,
The tensile strength in the tube axis direction is 520 to 690 MPa,
The yield ratio in the tube axis direction is 90% or less,
The Charpy absorbed energy in the pipe circumferential direction in the base material part is 100 J or more at 0 ° C.,
An electric resistance welded steel pipe for line pipes, wherein Charpy absorbed energy in the pipe circumferential direction in the electric resistance welded portion is 80 J or more at 0 ° C.
CMeq = C + Mn / 6 + Cr / 5 + (Ni + Cu) / 15 + Nb + Mo / 3 + V Formula (1)
LR = (2.1 × C + Nb) / Mn Formula (2)
[In Formula (1) and Formula (2), C, Mn, Cr, Ni, Cu, Nb, Mo, and V represent the mass% of each element, respectively. ]
<2> The chemical composition of the base material part is mass%,
Mo: more than 0% and 0.50% or less,
Cu: more than 0% and 1.00% or less,
Ni: more than 0% and 1.00% or less,
Cr: more than 0% and 1.00% or less,
V: more than 0% and 0.100% or less,
Ca: more than 0% and 0.0100% or less,
The electric-welded steel pipe for line pipes according to <1>, which contains one or more of Mg: more than 0% and not more than 0.0100% and REM: more than 0% and not more than 0.0100%.
<3> When the metal structure of the base material portion is observed at a magnification of 100,000 using a transmission electron microscope, the area ratio of precipitates having an equivalent circle diameter of 100 nm or less is 0.10 to 1.00%. The electric resistance welded steel pipe for line pipes according to <1> or <2>.
<4> The ERW steel pipe for line pipes according to any one of <1> to <3>, wherein the Nb content in the chemical composition of the base material part is 0.020% or more by mass%.
<5> The electric-welded steel pipe for line pipes according to any one of <1> to <4>, wherein the wall thickness is 10 to 25 mm and the outer diameter is 114.3 to 609.6 mm.

According to the present disclosure, there is provided an ERW steel pipe for a line pipe that has a certain degree of tensile strength and yield strength, has a reduced yield ratio, and is excellent in toughness of a base material portion and an ERW weld portion.

It is a scanning electron micrograph which shows an example of the metal structure of the base material part in this indication.

In the present specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
In this specification, “%” indicating the content of a component (element) means “% by mass”.
In the present specification, the content of C (carbon) may be expressed as “C amount”. The content of other elements may be expressed in the same manner.
In this specification, the term “process” is not limited to an independent process, and is included in this term if the intended purpose of the process is achieved even when it cannot be clearly distinguished from other processes. It is.

The ERW steel pipe for line pipes of the present disclosure (hereinafter, also simply referred to as “ERW steel pipe”) includes a base metal part and an ERW weld part, and the base metal part has a chemical composition of mass%, C: 0.00. 080 to 0.120%, Mn: 0.30 to 1.00%, Ti: 0.005 to 0.050%, Nb: 0.010 to 0.100%, N: 0.001 to 0.020% , Si: 0.010 to 0.450%, Al: 0.001 to 0.100%, P: 0 to 0.030%, S: 0 to 0.0100%, Mo: 0 to 0.50%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Cr: 0 to 1.00%, V: 0 to 0.100%, Ca: 0 to 0.0100%, Mg: 0 to 0 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impurities, and CMeq represented by the following formula (1) is 0.170 to 0.3 The ratio of mass% of Mn to mass% of Si (hereinafter also referred to as “Mn / Si ratio”) is 2.0 or more, and LR represented by the following formula (2) is 0.210 or more. When the metal structure of the base material is observed at a magnification of 1000 times using a scanning electron microscope, the area ratio of the first phase made of ferrite (hereinafter also referred to as “ferrite fraction”) is 60 to 98. %, The remaining second phase includes tempered bainite, the yield strength in the tube axis direction (hereinafter also referred to as “YS”) is 390 to 562 MPa, and the tensile strength in the tube axis direction (hereinafter referred to as “TS”). ) Is 520 to 690 MPa, the yield ratio in the tube axis direction (hereinafter also referred to as “YR”) is 90% or less, and the Charpy absorbed energy in the tube circumferential direction in the base material portion is 100 J or more at 0 ° C. In the ERW weld Charpy absorbed energy takes the tube circumferential direction, is 80J or more at 0 ° C..
CMeq = C + Mn / 6 + Cr / 5 + (Ni + Cu) / 15 + Nb + Mo / 3 + V Formula (1)
LR = (2.1 × C + Nb) / Mn Formula (2)
[In Formula (1) and Formula (2), C, Mn, Cr, Ni, Cu, Nb, Mo, and V represent the mass% of each element, respectively. ]

The electric resistance steel pipe of this indication contains a base material part and an electric resistance welding part.
ERW steel pipes are generally made into open pipes by forming hot-rolled steel sheets into tubes (hereinafter also referred to as “roll forming”). It is manufactured by forming an electric resistance welded portion and then, if necessary, seam heat treating the electro-welded weld.
In the ERW steel pipe of the present disclosure, the base metal portion refers to a portion other than the ERW weld and the heat affected zone.
Here, the heat affected zone (hereinafter also referred to as “HAZ”) is the effect of heat by electric resistance welding (when seam heat treatment is performed after electric resistance welding, heat generated by electric resistance welding and seam heat treatment). The part affected.
In this specification, the electric resistance welded portion may be simply referred to as a “welded portion”.

The electric resistance welded steel pipe of the present disclosure includes a certain amount of YS and TS (that is, YS and TS in the above-described range), YR is reduced to 90% or less, and is excellent in toughness of the base material portion and the electric resistance welded portion.
In the present disclosure, being excellent in toughness means that Charpy absorbed energy (J) (hereinafter also referred to as “vE”) in the pipe circumferential direction at 0 ° C. is large.
Specifically, in the ERW steel pipe of the present disclosure, the vE in the base metal part is 100 J or more, and the vE in the ERW weld part is 80 J or more.

Since the ERW steel pipe of this indication has low YR, the effect which can control buckling of an ERW steel pipe is expected.
As an example of the case where buckling suppression of a steel pipe is required, there is a case where a steel pipe for a submarine line pipe is laid by reeling. In reeling laying, steel pipes are manufactured in advance on land, and the manufactured steel pipes are wound on a barge ship spool. The rolled steel pipe is laid on the seabed while unwinding at sea. In this reeling laying, the steel pipe may be buckled because plastic bending is applied to the steel pipe at the time of winding or unwinding the steel pipe. When the buckling of the steel pipe occurs, the laying work must be stopped and the damage is enormous.
The buckling of the steel pipe can be suppressed by reducing the YR of the steel pipe.
Therefore, according to the ERW steel pipe of the present disclosure, for example, it is expected that the buckling at the time of laying laying when used as an ERW steel pipe for a submarine line pipe can be suppressed.

Furthermore, since the ERW steel pipe of the present disclosure is excellent in the toughness of the base metal part and the ERW welded part, when used as an ERW steel pipe for a line pipe, it is expected to have an excellent effect of stopping crack propagation during bursting. Is done.

The above-mentioned YS, TS, YR, vE of the base metal part, and vE of the ERW welded part are the above chemical composition (including CMeq, Mn / Si ratio, and LR) in the ERW steel pipe, and the above metal structure. Achieved by a combination of

[Chemical composition of base material]
Hereinafter, regarding the chemical composition of the base material part, first, each component in the chemical composition will be described, and subsequently, CMeq, Mn / Si ratio, and LR will be described.

C: 0.080 to 0.120%
C is an element necessary for forming the second phase.
When the C content is 0.080% or more, it is easy to achieve a ferrite fraction of 98% or less, and it is easy to achieve that the LR is 0.210 or more. As a result, it is easy to achieve YR 90% or less. Therefore, the C amount is 0.080% or more. The amount of C is preferably 0.085% or more.
On the other hand, when the C content is 0.120% or less, it is easy to achieve a ferrite fraction of 60% or more. As a result, it is easy to achieve YR 90% or less. Therefore, the amount of C in the present disclosure is 0.120% or less. The amount of C is preferably 0.115% or less, more preferably 0.110% or less.

Mn: 0.30 to 1.00%
Mn is an element that enhances the hardenability of steel. Mn is an essential element for detoxification of S.
When the amount of Mn is less than 0.30%, embrittlement due to S occurs, and the toughness of the base material and the ERW weld may deteriorate. Therefore, the amount of Mn is 0.30% or more. The amount of Mn is preferably 0.40% or more, more preferably 0.50% or more.
On the other hand, if the amount of Mn exceeds 1.00%, coarse MnS may be generated at the center of the plate thickness, and the toughness of the base metal and the ERW weld may be impaired. Moreover, when the amount of Mn exceeds 1.00%, LR0.210 or more may not be achieved, and as a result, YR90% or less may not be achieved. Therefore, the amount of Mn is 1.00% or less. The amount of Mn is preferably 0.90% or less, more preferably 0.85% or less.

Ti: 0.005 to 0.050%
Ti is an element that forms carbonitrides and contributes to refinement of the crystal grain size.
From the viewpoint of ensuring the toughness of the base metal and the ERW weld, the Ti content is 0.005% or more.
On the other hand, if the amount of Ti exceeds 0.050%, coarse TiN is generated, and the toughness of the base material and the ERW weld may deteriorate. Therefore, the Ti amount is 0.050% or less. The amount of Ti is preferably 0.040% or less, more preferably 0.030 or less, and particularly preferably 0.025%.

Nb: 0.010 to 0.100%
Nb is an element that contributes to toughness improvement and YR reduction.
In order to improve toughness by non-recrystallization rolling, the Nb content is 0.010% or more. The Nb amount is preferably 0.020% or more.
On the other hand, if the Nb content exceeds 0.100%, the toughness deteriorates due to coarse carbides. For this reason, the amount of Nb is 0.100% or less. The Nb amount is preferably 0.090% or less.

N: 0.001 to 0.020%
N is an element that suppresses the coarsening of crystal grains by forming nitrides, and as a result, improves the toughness of the base metal part and the ERW weld part. From the viewpoint of this effect, the N content is 0.001% or more. The amount of N is preferably 0.003% or more.
On the other hand, if the N content exceeds 0.020%, the amount of alloy carbide produced increases, and the toughness of the base metal part and the ERW weld part deteriorates. Therefore, the N amount is 0.020% or less. The N content is preferably 0.015% or less, more preferably 0.010% or less, and particularly preferably 0.008% or less.

Si: 0.010 to 0.450%
Si is an element that functions as a deoxidizer for steel. More specifically, when the Si amount is 0.010% or more, the generation of coarse oxides in the base material portion and the ERW weld portion is suppressed, and as a result, the base material portion and the ERW weld portion are suppressed. Toughness is improved. Therefore, the amount of Si is 0.010% or more. The amount of Si is preferably 0.015% or more, and more preferably 0.020% or more.
On the other hand, if the amount of Si exceeds 0.450%, inclusions are generated in the ERW weld, and Charpy absorbed energy may be reduced and toughness may be deteriorated. Therefore, the amount of Si is 0.450% or less. The amount of Si is preferably 0.400% or less, more preferably 0.350% or less, and particularly preferably 0.300% or less.

Al: 0.001 to 0.100%
Al, like Si, is an element that functions as a deoxidizer. More specifically, when the Al amount is 0.001% or more, generation of coarse oxides in the base metal part and the ERW weld part is suppressed, and as a result, the base metal part and the ERW weld part are suppressed. Toughness is improved. Therefore, the Al content is 0.001% or more. The amount of Al is preferably 0.010% or more, and more preferably 0.015% or more.
On the other hand, if the Al content exceeds 0.100%, the toughness of the welded portion may deteriorate with the generation of the Al-based oxide during ERW welding. Therefore, the Al content is 0.100% or less. The amount of Al is preferably 0.090% or less.

P: 0 to 0.030%
P is an impurity element. When the amount of P exceeds 0.030%, the toughness may be impaired due to segregation at the grain boundaries. Therefore, the amount of P is 0.030% or less. The amount of P is preferably 0.025% or less, more preferably 0.020% or less, further preferably 0.015% or less, and more preferably 0.010% or less.
The amount of P may be 0%. From the viewpoint of reducing the dephosphorization cost, the amount of P may be more than 0% or 0.001% or more.

S: 0 to 0.0100%
S is an impurity element. If the amount of S exceeds 0.0100%, the toughness may be impaired. Therefore, the amount of S is 0.0100% or less. The amount of S is preferably 0.0070 or less, more preferably 0.0050% or less, and more preferably 0.0030% or less.
The amount of S may be 0%. From the viewpoint of reducing the desulfurization cost, the S amount may be more than 0%, may be 0.0001% or more, and may be 0.0003% or more.

Mo: 0 to 0.50%
Mo is an arbitrary element. Therefore, the Mo amount may be 0%.
Mo is an element that improves the hardenability of the steel material and contributes to the high strength of the steel material. From the viewpoint of this effect, the Mo amount may be more than 0%, 0.01% or more, or 0.03% or more.
On the other hand, if the amount of Mo exceeds 0.50%, the toughness may be reduced due to the formation of Mo carbonitride. Therefore, the Mo amount is 0.50% or less. The amount of Mo is preferably 0.40% or less, more preferably 0.30% or less, still more preferably 0.20% or less, and particularly preferably 0.10% or less.

Cu: 0 to 1.00%
Cu is an arbitrary element. Therefore, the amount of Cu may be 0%.
Cu is an element effective for improving the strength of the base material. From the viewpoint of this effect, the amount of Cu may be more than 0%, 0.01% or more, or 0.03% or more.
On the other hand, when the amount of Cu exceeds 1.00%, fine Cu particles are generated, and the toughness may be remarkably deteriorated. Therefore, the amount of Cu is 1.00% or less. The amount of Cu is preferably 0.80% or less, more preferably 0.70% or less, still more preferably 0.60% or less, and particularly preferably 0.50% or less.

Ni: 0 to 1.00%
Ni is an arbitrary element. Therefore, the Ni amount may be 0%.
Ni is an element that contributes to improvement in strength and toughness. From the viewpoint of this effect, the Ni content may be greater than 0%, 0.01% or more, or 0.05% or more.
On the other hand, if the Ni content exceeds 1.00%, the strength may be too high. Therefore, the Ni content is 1.00% or less. The amount of Ni is preferably 0.80% or less, more preferably 0.70% or less, and still more preferably 0.60% or less.

Cr: 0 to 1.00%
Cr is an arbitrary element. Therefore, the Cr amount may be 0%.
Cr is an element that improves hardenability. From the viewpoint of this effect, the Cr content may be more than 0%, 0.01% or more, or 0.05% or more.
On the other hand, if the Cr content exceeds 1.00%, the toughness of the welded portion may be deteriorated by the Cr-based inclusions generated in the ERW welded portion. Therefore, the Cr content is 1.00% or less. The amount of Cr is preferably 0.80% or less, more preferably 0.70% or less, still more preferably 0.50% or less, and particularly preferably 0.30% or less.

V: 0 to 0.100%
V is an arbitrary element. Therefore, the V amount may be 0%.
V is an element that contributes to improved toughness and YR reduction. From the viewpoint of this effect, the V amount may be greater than 0%, may be 0.005% or more, and may be 0.010% or more.
On the other hand, if the amount of V exceeds 0.100%, the toughness may be deteriorated by the V carbonitride. Therefore, the V amount is 0.100% or less. The amount of V is preferably 0.080% or less, more preferably 0.070% or less, still more preferably 0.050% or less, and particularly preferably 0.030% or less.

Ca: 0 to 0.0100%
Ca is an arbitrary element. Therefore, the Ca content may be 0%.
Ca is an element that controls the form of sulfide inclusions and improves low-temperature toughness. From the viewpoint of such an effect, the Ca content may be greater than 0%, may be 0.0001% or more, may be 0.0010% or more, or may be 0.0030% or more. It may be 0.0050% or more.
On the other hand, if the Ca content exceeds 0.0100%, large clusters or large inclusions made of CaO—CaS are generated, which may adversely affect toughness. Therefore, the Ca content is 0.0100% or less. The Ca content is preferably 0.0090% or less, more preferably 0.0080% or less, and particularly preferably 0.0060% or less.

Mg: 0 to 0.0100%
Mg is an arbitrary element. Therefore, the amount of Mg may be 0%.
Mg is an effective element as a deoxidizing agent and a desulfurizing agent. In particular, Mg is an element that generates fine oxides and contributes to improvement in toughness of HAZ (Heat affected zone). From the viewpoint of such an effect, the amount of Mg may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, or may be 0.0020% or more. It may be 0.0030% or more, or 0.0050% or more.
On the other hand, if the amount of Mg exceeds 0.0100%, the oxide tends to agglomerate or coarsen, resulting in a decrease in HIC resistance (Hydrogen-Induced Cracking Resistance) or a decrease in the toughness of the base material or HAZ. There is a risk. Therefore, the amount of Mg is 0.0100% or less. The amount of Mg is preferably 0.0060% or less.

REM: 0 to 0.0100%
REM is an arbitrary element. Therefore, the REM amount may be 0%.
Here, “REM” is a rare earth element, that is, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. It refers to at least one element selected.
REM is an element effective as a deoxidizer and a desulfurizer. From the viewpoint of this effect, the amount of REM may be greater than 0%, may be 0.0001% or more, and may be 0.0010% or more.
On the other hand, when the amount of REM exceeds 0.0100%, a coarse oxide is formed, and as a result, there is a possibility that the HIC resistance is lowered or the toughness of the base material or HAZ is lowered. Therefore, the amount of REM is 0.0100% or less. The amount of REM is preferably 0.0070% or less, and more preferably 0.0050% or less.

The chemical composition of the base metal part is Mo: more than 0% to 0.50% or less, Cu: more than 0% to 1.00% or less, Ni: more than 0% to 1.00, from the viewpoint of obtaining the effect of any element described above. %: Cr: more than 0% and 1.00% or less, V: more than 0% and 0.100% or less, Ca: more than 0% and 0.0100% or less, Mg: more than 0% and 0.0100% or less, and REM: You may contain 1 type or 2 types or more of more than 0% and 0.0100% or less.
The more preferable amount of each arbitrary element is as described above.

Remainder: Fe and impurities In the chemical composition of the base metal part, the remainder excluding the above-described elements 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.
Examples of impurities include all elements other than the elements described above. The element as the impurity may be only one type or two or more types.
Examples of the impurities include O, B, Sb, Sn, W, Co, As, Pb, Bi, and H.
Among the elements described above, O is preferably controlled so that the content is 0.006% or less.
As for other elements, Sb, Sn, W, Co, and As are usually mixed with a content of 0.1% or less, and Pb and Bi are mixed with a content of 0.005% or less. Can be mixed with a content of 0.0003% or less, and H can be mixed with a content of 0.0004% or less, but the content of other elements is controlled within the normal range. There is no need.

CMeq: 0.170-0.300
In the chemical composition of the base material part, CMeq represented by the following formula (1) is 0.170 to 0.300.
CMeq = C + Mn / 6 + Cr / 5 + (Ni + Cu) / 15 + Nb + Mo / 3 + V Formula (1)
[In Formula (1), C, Mn, Cr, Ni, Cu, Nb, Mo, and V represent the mass% of each element, respectively. ]

CMeq has a positive correlation with the yield strength.
From the viewpoint of easily achieving a yield strength of 390 MPa or more, CMeq is 0.170 or more. CMeq is preferably 0.180 or more, more preferably 0.200 or more, and further preferably 0.230 or more.
On the other hand, from the viewpoint of easily achieving a yield strength of 562 MPa or less, CMeq is 0.300 or less. CMeq is preferably 0.290 or less, and more preferably 0.275 or less.

LR: 0.210 or more In the chemical composition of the base material part, LR represented by the following formula (2) is 0.210 or more.
In the electric resistance welded steel pipe of the present disclosure, when LR is 0.210 or more, YR of 90% or less can be achieved.
When LR is less than 0.210, YR may exceed 90%. The reason for this is considered to be that the amount of precipitates in the steel is reduced and the work hardening ability is lowered (ie, TS is lowered).

LR = (2.1 × C + Nb) / Mn Formula (2)
[In Formula (2), C, Nb, and Mn represent the mass% of each element, respectively. ]

The technical meaning of formula (2) is as follows.
In the formula (2), the reason why the C amount and the Nb amount are arranged in the molecule is that C and Nb form precipitates, thereby improving the work hardening ability of the steel (that is, TS increases), and as a result. This is because YR of steel is considered to be reduced.
The reason for multiplying the amount of C by “2.1” is that the effect of improving the work hardening ability due to the above-described precipitate formation is considered to be about 2.1 times the effect of containing Cb compared to the effect of containing Nb. It is.
In the formula (2), the reason for arranging the amount of Mn in the denominator is that although the steel can be transformed at a relatively low temperature by the inclusion of Mn, the work hardening ability of the steel itself is impaired by the inclusion of Mn (that is, , TS decreases), and as a result, the YR of steel increases.

As described above, LR has a positive correlation with the Nb amount and the C amount, and has a negative correlation with the Mn amount.
In the electric resistance welded steel pipe of the present disclosure, when the LR is 0.210 or more, and the Nb amount is relatively large, for example, the Nb amount (0 in International Publication No. 2012/133558) (0 Even if it is more than 0.003% and less than 0.02%), LR may be 0.210 or more depending on the amount of C and the amount of Mn. In this case, YR 90% or less can be achieved.
In the ERW steel pipe of the present disclosure, even when the Nb amount is less than 0.02%, YR of 90% or less is achieved by satisfying conditions other than LR and LR of 0.210 or more. obtain.

LR is preferably 0.230 or more and more preferably 0.270 or more from the viewpoint of easily achieving 90% or less of YR.
There is no particular limitation on the upper limit of LR. LR is preferably 0.500 or less, more preferably 0.450 or less, and particularly preferably 0.400 or less, from the viewpoint of the suitability for manufacturing an electric resistance welded steel pipe.

Mn / Si ratio: 2.0 or more In the chemical composition of the base material part, the Mn / Si ratio (that is, the ratio Mn / Si ratio of Mn to Si mass%) is 2.0 or more.
In the ERW steel pipe of the present disclosure, the Mn / Si ratio is 2.0 or more, whereby the toughness of the ERW welded portion is improved, and vE in the ERW welded portion (that is, Charpy absorption in the pipe circumferential direction at 0 ° C.). Energy) is 80 J or more.
When the Mn / Si ratio is less than 2.0, vE may be less than 80J. The reason for this is considered to be that when the Mn / Si ratio is less than 2.0, the toughness deteriorates due to the MnSi inclusions being the starting point of brittle fracture in the ERW weld.
The Mn / Si ratio is preferably 2.1 or more from the viewpoint of further improving the toughness of the ERW weld.
There is no restriction | limiting in particular in the upper limit of Mn / Si ratio. The Mn / Si ratio is preferably 50 or less, more preferably 40 or less, and particularly preferably 30 or less, from the viewpoint of further improving the toughness of the ERW weld and the toughness of the base material.

[Metal structure of base material]
In the electric resistance welded steel pipe of the present disclosure, the metal structure of the base metal part is a ferrite fraction (that is, the first phase composed of ferrite) when the metal structure is observed at a magnification of 1000 times using a scanning electron microscope. Area ratio) is 60 to 98%, and the remaining second phase is at least one of tempered bainite and pearlite.

In the electric resistance welded steel pipe of the present disclosure, YR of 90% or less can be achieved when the ferrite fraction is 60% or more. The ferrite fraction is preferably 65% or more, and more preferably 70% or more.
Moreover, in the electric-resistance-welded steel pipe of this indication, TS520MPa or more can be achieved because a ferrite fraction is 98% or less as mentioned above. The ferrite fraction is preferably 95% or less, more preferably 92% or less.

In the electric resistance welded steel pipe of the present disclosure, the remaining second phase includes tempered bainite.
The fact that the second phase contains tempered bainite means that the ERW steel pipe of the present disclosure is tempered after pipe forming (that is, after ERW welding (after seam heat treatment if seam heat treatment is applied after ERW welding)). Means an electric resistance welded steel pipe.

</ RTI> The ERW steel pipe of the present disclosure is an ERW steel pipe that has been tempered after pipe making, whereby YR of 90% or less can be achieved. The reason is considered that YR is lowered by tempering after pipe making. The reason why YR decreases due to tempering after pipe forming is that YS decreases as the dislocation density decreases, and that work hardening increases due to precipitation of cementite on the dislocations (ie, TS increases). Conceivable.

In the present specification, tempered bainite is distinguished from bainite that is not tempered bainite in that it contains granular cementite in its structure.
The concept of “bainite” in this specification includes bainitic ferrite, granular bainite, upper bainite, and lower bainite.

The second phase only needs to contain tempered bainite, may be a phase composed only of tempered bainite, or may contain a structure other than tempered bainite.
Examples of the structure other than tempered bainite include pearlite.
In the present specification, the concept of “perlite” includes pseudo pearlite.

In the metal structure of the base metal part, the ferrite fraction measurement and the identification of the second phase are performed by performing a nital etching on the metal structure at the thickness 1/4 position in the L cross section of the base material 90 ° position, and after the nital etching. A metal structure photograph (hereinafter, also referred to as “metal structure photograph”) is observed by observing at a magnification of 1000 times using a scanning electron microscope (SEM). Here, the metal structure photograph is taken for 10 fields of view at a magnification of 1000 times (0.12 mm ² minutes as the actual area of the cross section). Image processing is performed on the photographed metal structure photograph to measure the ferrite fraction and identify the second phase. The image processing is performed using, for example, a small general-purpose image analyzer LUZEX AP manufactured by Nireco Corporation.

In this specification, the “base material 90 ° position” refers to a position shifted by 90 ° in the pipe circumferential direction from the ERW weld, and the “L cross section” is parallel to the pipe axis direction and the thickness direction. A “thickness ¼ position” indicates a position where the distance from the outer peripheral surface of the ERW steel pipe is ¼ of the thickness.
In this specification, the tube axis direction may be referred to as the “L direction”.

FIG. 1 is a scanning electron micrograph (SEM photograph; magnification 1000 times) showing an example of the metal structure of the base material part in the present disclosure.
The SEM photograph in FIG. 1 is one (one field of view) of the SEM photographs used for the measurement of the ferrite fraction and the identification of the second phase in Example 17 described later.
As shown in FIG. 1, a first phase composed of ferrite and a second phase containing tempered bainite can be confirmed. In particular, the presence of white spots (cementite) indicates that the second phase contains tempered bainite.

The metal structure of the base metal part is the area of a precipitate having an equivalent circle diameter of 100 nm or less (hereinafter also referred to as “specific precipitate”) when the metal structure is observed at a magnification of 100,000 using a transmission electron microscope. The ratio (hereinafter also referred to as “specific precipitate area ratio”) is preferably 0.10 to 1.00%.
When the specific precipitate area ratio is 0.10% or more, it is easier to achieve that YR is 90% or less. The reason for this is considered to be that the specific precipitate (that is, the precipitate having an equivalent circle diameter of 100 nm or less) contributes to the improvement of work-hardening properties (that is, the increase in TS), and as a result, YR decreases.
On the other hand, when the specific precipitate area ratio is 1.00% or less, brittle fracture is suppressed (that is, excellent toughness of the base material portion). The specific precipitate area ratio is preferably 0.80% or less, and more preferably 0.70% or less.

The specific precipitate area ratio of 0.10 to 1.00% can be achieved by tempering at a temperature of 400 ° C. or higher and Ac1 point or lower after pipe forming.

In the present disclosure, the area ratio of precipitates (that is, the area ratio of precipitates having a circle-equivalent diameter of 100 nm or less) is determined by using a transmission electron microscope ( TEM) and observing at a magnification of 100,000 times.
More specifically, based on a sample taken from a thickness 1/4 position in the L cross section of the base material at 90 °, it is composed of 10% by volume of acetylacetone, 1% by volume of tetramethylammonium chloride, and 89% by volume of methyl alcohol. A replica for TEM observation is produced by the SPEED method using an electrolytic solution. The obtained TEM observation replica is observed at a magnification of 100000 times using a TEM, thereby acquiring 10 TEM images of a 1 μm square field size. The area ratio of precipitates having an equivalent circle diameter of 100 nm or less with respect to the total area of the acquired TEM image is calculated, and the obtained result is defined as a specific precipitate area ratio (%).
The etching conditions in the SPEED method are such that a saturated gypsum electrode is used as a reference electrode and a charge of 10 coulomb is applied at a voltage of −200 mV to a surface area of about 80 square millimeters.

The specific precipitate (that is, the precipitate having an equivalent circle diameter of 100 nm or less) is specifically composed of a carbide of a metal other than Fe, a nitride of a metal other than Fe, and a carbonitride of a metal other than Fe. It is considered to be at least one selected from the group.
Ti and Nb can be considered as metals other than Fe here. Moreover, when a chemical composition contains at least 1 sort (s) of V, Mo, and Cr, at least 1 sort (s) of this V, Mo, and Cr is also considered as metals other than the said Fe.

[Yield strength in the pipe axis direction (YS)]
The electric resistance welded steel pipe of the present disclosure has a yield strength (YS) in the pipe axis direction of 390 to 562 MPa.
YS in the tube axis direction is preferably 430 MPa or more, more preferably 450 MPa or more, more preferably 470 MPa or more, and particularly preferably 500 MPa or more.
YS in the tube axis direction is preferably 550 MPa or less, more preferably 540 MPa or less, and particularly preferably 530 MPa or less.

The fact that the YS in the pipe axis direction is 562 MPa or less can be achieved by tempering after pipe making. The reason for this is considered to be that the tube-forming strain is relaxed by tempering after tube-making, and the dislocation density is lowered.

[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 520 to 690 MPa.
TS in the tube axis direction is preferably 550 MPa or more, and more preferably 580 MPa or more.
TS in the tube axis direction is preferably 680 MPa or less, more preferably 660 MPa or less, and particularly preferably 650 MPa or less.

[Yield ratio in the tube axis direction]
The electric resistance welded steel pipe of the present disclosure has a yield ratio (YR = (YS / TS) × 100) in the pipe axis direction of 90% or less.
Thereby, buckling of the ERW steel pipe at the time of laying etc. is controlled.

The fact that the YR in the pipe axis direction is 90% or less can be achieved by tempering after pipe making. This is presumably because YS decreases as the dislocation density decreases, and work hardening increases (ie, TS increases) due to fine precipitation of cementite on the dislocations.

[Wall thickness of ERW steel pipe]
The wall thickness of the electric resistance welded steel pipe of the present disclosure is preferably 10 to 25 mm.
When the wall thickness is 10 mm or more, it is advantageous in that the YR is easily lowered by utilizing strain when the hot-rolled steel sheet is formed into a tubular shape. The wall thickness is more preferably 12 mm or more.
If the wall thickness is 25 mm or less, it is advantageous in terms of the suitability for producing an electric resistance steel pipe (specifically, the formability when forming a hot-rolled steel sheet into a tubular shape). The wall thickness is more preferably 20 mm or less.

[Outer shape of ERW steel pipe]
The outer diameter of the electric resistance welded steel pipe of the present disclosure is preferably 114.3 to 609.6 mm (ie, 4.5 to 24 inches).
When the outer diameter is 114.3 mm or more, it is more suitable as an ERW steel pipe for line pipes. The outer diameter is preferably 139.7 mm (ie 5.5 inches) or more, more preferably 177.8 mm (ie 7 inches) or more.
When the outer diameter is 609.6 mm or less, it is advantageous in that the YR can be easily reduced by utilizing strain when the hot-rolled steel sheet is formed into a tubular shape. The outer diameter is preferably 406.4 mm (ie 16 inches) or less, more preferably 304.8 mm (ie 12 inches) or less.

[Example of production method]
The following manufacturing method A is mentioned as an example of the manufacturing method of the ERW steel pipe of this indication.
Process A is
A step of producing an azurol ERW pipe using the hot-rolled steel sheet having the chemical composition described above;
A tempering step of obtaining an ERW steel pipe by tempering the as-roll ERW steel pipe,
Have

According to the above production method A, it is easy to produce an ERW steel pipe having a YR of 90% or less due to the above-mentioned reason by having a tempering step.

The tempering temperature (that is, the holding temperature in tempering) is preferably 400 ° C. or higher and Ac1 point or lower.
When the tempering temperature is 400 ° C. or higher, cementite and specific precipitates (precipitates having a circle-equivalent diameter of 100 nm or less) are more likely to be precipitated, so that it is easier to achieve YR of 90% or less. The tempering temperature is more preferably 420 ° C. or higher.
It can suppress more that a precipitate becomes coarse as tempering temperature is below Ac1 point. The tempering temperature is also preferably 720 ° C. or lower, preferably 710 ° C. or lower, and preferably 700 ° C. or lower, although it depends on the Ac1 point of the steel.

Here, Ac1 point means the temperature at which transformation to austenite is started when the temperature of the steel is raised.
The Ac1 point is calculated by the following formula.
Ac1 point (° C) = 750.8-26.6C + 17.6Si-11.6Mn-22.9Cu-23Ni + 24.1Cr + 22.5Mo-39.7V-5.7Ti + 232.4Nb-169.4Al
[Here, C, Si, Mn, Ni, Cu, Cr, Mo, V, Ti, Nb, and Al are mass% of each element, respectively. Ni, Cu, Cr, Mo, and V are arbitrary elements, and among these optional elements, the Ac1 point is calculated as 0% by mass for an element that is not contained in the steel slab. ]

The tempering time in the tempering step (that is, the holding time at the tempering temperature) is preferably 5 minutes or more from the viewpoint of easily reducing YR.

In production method A, an as-roll electric-welded steel pipe is an electric-welded steel pipe manufactured by roll-forming (that is, tubular forming) a hot-rolled steel sheet, and is not subjected to heat treatment other than seam heat treatment after roll-forming. ERW steel pipe.
The preferable aspect of the process which manufactures the as-roll electric-resistance-welded steel pipe in the manufacturing method A is mentioned later.

In the manufacturing method A, the amount of change in roundness before and after adjustment by the sizer (hereinafter, “sizer roundness”) It is preferable to have a sizer process that is adjusted under the condition that the amount of change (also referred to as “change amount (%)” is 1.0% or more.
When production method A includes a sizer process, it is easier to produce the above-mentioned electric resistance welded steel pipe having a specific precipitate area ratio of 0.10 to 1.00%.
The reason for this is that the sizer roundness change amount of 1.0% or more introduces a certain amount or more of dislocations into the inside of the as-rolled electric-welded steel pipe, and then into the as-rolled electric-welded steel pipe. On the other hand, it is considered that fine specific precipitates are easily deposited on dislocations by tempering at a temperature of 400 ° C. or more and Ac1 point or less.

Here, the roundness of the as-roll ERW pipe is obtained as follows.
First, four measured values are obtained by measuring the outer diameter of an as-roll ERW steel pipe at a 45 ° pitch in the pipe circumferential direction. A maximum value, a minimum value, and an average value are obtained from the four obtained measurement values. Based on the maximum value, the minimum value, and the average value, the roundness of the as-rolled electric resistance welded steel pipe is obtained by the following formula.
Roundness of AZROLL ERW pipe = (maximum value-minimum value) / average value

The change in size (%) of the sizer roundness is based on the roundness of the as-rolled ERW pipe before shape adjustment by the sizer and the roundness of the Azuroll-ERW pipe after shape adjustment by the sizer. Ask.
Amount of change in roundness of sizer (%) = (| Roundness of as-roll ERW pipe after shape adjustment by sizer-Roundness of Azuroll ERW pipe before shape adjustment by sizer | / Azroll before shape adjustment by sizer Roundness of ERW pipe) x 100

The process for producing an as-roll ERW steel pipe in production method A is as follows:
A hot-rolling step of obtaining a hot-rolled steel sheet by heating a steel slab (slab) having the above-described chemical composition and hot-rolling the heated steel slab,
A cooling step for cooling the hot-rolled steel sheet obtained in the hot rolling step;
By winding the hot rolled steel sheet cooled in the cooling process, a winding process for obtaining a hot coil made of the hot rolled steel sheet,
By unwinding the hot-rolled steel sheet from the hot coil, roll-forming the unrolled hot-rolled steel sheet to form an open pipe, and forming the ERW welded part by electro-welding the butt portion of the obtained open pipe A pipe making process for obtaining an as-roll ERW steel pipe,
It is preferable to have.
In the pipe making process, seam heat treatment may be applied to the ERW welded portion as necessary after ERW welding.

In the hot rolling step, it is preferable to heat a steel slab (slab) having the above-described chemical composition to a temperature of 1150 ° C to 1350 ° C.
The toughness of the base material part of an electric-resistance-welded steel pipe can be improved more as the temperature which heats a steel piece is 1150 degreeC or more. The reason for this is considered to be that when the temperature at which the steel slab is heated is 1150 ° C. or higher, the formation of undissolved Nb carbide can be suppressed.
The toughness of the base material part of an ERW steel pipe can be improved more as the temperature which a steel piece heats is 1350 degrees C or less. The reason for this is considered that the coarsening of the metal structure can be suppressed when the heating temperature of the steel slab is 1350 ° C. or less.

In the hot rolling step, for example, a steel slab heated to a temperature of 1150 ° C. to 1350 ° C. is preferably hot rolled at a temperature of Ar 3 point + 100 ° C. or higher. Thereby, the hardenability of a hot-rolled steel sheet can be improved.

Here, the Ar3 point is determined by the following formula from the chemical composition of the base material part.
Ar3 (° C.) = 910-310C-80Mn-55Ni-20Cu-15Cr-80Mo
[Here, C, Mn, Ni, Cu, Cr, and Mo are the mass% of each element, respectively. Ni, Cu, Cr, and Mo are arbitrary elements, and among these optional elements, an Ar3 point is calculated as 0% by mass for an element that is not contained in the steel slab. ]

In the cooling step, it is preferable to cool the hot-rolled steel sheet obtained in the hot rolling step with a cooling start temperature of Ar3 point or higher. Thereby, the intensity | strength and toughness of a base material part can be improved more. The reason for this is considered to be that the formation of coarse polygonal ferrite is suppressed by setting the cooling start temperature to the Ar3 point or higher.

In the cooling step, it is preferable to cool the hot-rolled steel sheet obtained in the hot rolling step at a cooling rate of 5 ° C./s to 80 ° C./s.
When the cooling rate is 5 ° C./s or more, the toughness deterioration of the base material portion is further suppressed. The reason for this is considered to be that the formation of coarse ferrite is suppressed when the cooling rate in the cooling step is 5 ° C./s or more.
When the cooling rate is 80 ° C./s or less, the toughness deterioration of the base material portion is suppressed. This is because, when the cooling rate in the cooling step is 80 ° C./s or less, the second phase fraction is suppressed from being excessive (that is, the ferrite fraction is less than 60%). Conceivable.

In the winding process, it is preferable to wind the hot-rolled steel sheet cooled in the cooling process at a winding temperature of 450 to 650 ° C.
When the winding temperature is 450 ° C. or higher, the toughness deterioration of the base material portion is suppressed. The reason for this is considered to be that when the winding temperature is 450 ° C. or higher, the formation of martensite is suppressed.
When the winding temperature is 650 ° C. or lower, the increase in YR can be suppressed. The reason for this is considered to be that when the winding temperature is 650 ° C. or lower, excessive generation of Nb carbonitride is suppressed, and as a result, an increase in YS is suppressed.

Hereinafter, examples of the present disclosure will be described, but the present disclosure is not limited to the following examples.

[Examples 1 to 17, Comparative Examples 1 to 26]
<Manufacture of hot coils>
The steel slab having the chemical composition shown in Table 1 and Table 2 is heated to a temperature of 1250 ° C., the heated steel slab is hot-rolled to form a hot-rolled steel sheet, and the obtained hot-rolled steel sheet is cooled to a cooling start temperature. Was cooled at a cooling rate of 50 ° C./s, and the cooled hot-rolled steel sheet was wound at a winding temperature of 550 ° C. to obtain a hot coil made of a hot-rolled steel sheet.

In each example and each comparative example, the remainder excluding the elements shown in Tables 1 and 2 is Fe and impurities.
In Tables 1 and 2, REM in Example 11 is Ce, REM in Example 16 is Nd, and REM in Example 17 is La.
In Tables 1 to 3, the numbers underlined are values outside the scope of the present disclosure.

<Manufacture of AZROLL ERW pipe>
The hot-rolled steel sheet is unwound from the hot coil, and the unrolled hot-rolled steel sheet is roll-formed to form an open pipe. The butt portion of the obtained open pipe is electro-welded and electro-welded (hereinafter referred to as “ As welded part "was formed, and then the welded part was subjected to seam heat treatment to obtain an as-rolled electric resistance welded steel pipe.

<Manufacturing of electric resistance steel pipe (sizer and tempering)>
The shape of the as-rolled electric resistance welded steel pipe was adjusted by a sizer under the condition that the amount of change in circularity (%) shown in Table 3 was obtained.
The as-rolled electric resistance welded steel pipe after shape adjustment was tempered at the tempering temperature and tempering time shown in Table 3 to obtain an electric resistance welded steel pipe.
The outer diameter of the obtained ERW steel pipe was 219 mm, and the thickness of this ERW steel pipe was 15.9 mm.
In addition, the above manufacturing process does not affect the chemical composition of steel. Therefore, it can be considered that the chemical composition of the base material part of the obtained ERW steel pipe is the same as the chemical composition of the steel slab which is a raw material.

<Measurement>
The following measurements were performed on the obtained ERW steel pipe.
The results are shown in Table 3.

(Measurement of ferrite fraction and confirmation of second phase structure)
By the method described above, the ferrite fraction ("F fraction" in Table 3) was measured to confirm the type of the second phase.
In Table 3, TB means tempered bainite, P means pearlite, and MA means island martensite.

(Measurement of YS, TS, and YR)
From the 90 ° position of the base material of the ERW steel pipe, the test piece for the tensile test is oriented so that the test direction (tensile direction) of the tensile test is the pipe axis direction of the ERW steel pipe (hereinafter also referred to as “L direction”). Collected. Here, the shape of the test piece was a flat plate conforming to the American Petroleum Institute Standard API 5L (hereinafter simply referred to as “API 5L”).
Using the sampled specimens, at room temperature, in accordance with API 5L, a tensile test is performed in which the test direction is the L direction of the ERW steel pipe, the L direction TS of the ERW steel pipe, and the L direction of the ERW steel pipe YS of each was measured.
Further, the YR (%) in the L direction of the ERW steel pipe was obtained by the calculation formula “(YS / TS) × 100”.

(Measurement of vE (J) of base metal part (Charpy absorbed energy at 0 ° C.))
A full-size test piece with V-notch (test piece for Charpy impact test) was taken from the base metal position of the electric resistance steel pipe at 90 ° C. A full-size test piece with a V-notch was collected such that the test direction was the pipe circumferential direction (C direction). The collected full-size test piece with V notch was subjected to a Charpy impact test in accordance with API 5L under a temperature condition of 0 ° C., and vE (J) was measured.
The above measurement was performed 5 times per ERW steel pipe, and the average value of the 5 measurements was taken as vE (J) of the base material part of the ERW steel pipe.

(Measurement of vE (J) (Charpy absorbed energy at 0 ° C.) of weld)
The same operation as the measurement of vE (J) of the base metal part was performed except that the position where the V-notched full-size test piece was collected was changed to the welded part of the ERW steel pipe.

(Measurement of specific precipitate area ratio)
The specific precipitate area ratio (that is, the area ratio of precipitates having an equivalent circle diameter of 100 nm or less; simply expressed as “precipitate area ratio (%)” in Table 3) was measured by the method described above.

As shown in Tables 1 to 3, the ERW steel pipe of each example satisfied TS, YS, YR, vE (base material part), and vE (welded part) in the present disclosure. That is, it was shown that the ERW steel pipe of each example has a certain degree of tensile strength and yield strength, the yield ratio is reduced, and the toughness of the base metal part and the welded part is excellent.

In Comparative Example 1 in which the amount of C exceeded the upper limit, the F fraction decreased and YR increased.
In Comparative Example 2 in which the Si amount exceeded the upper limit, the toughness of the welded portion deteriorated.
In Comparative Example 3 in which the Si amount was below the lower limit, the toughness of the base metal part and the welded part deteriorated. This is probably because deoxidation was insufficient and a coarse oxide was generated.
In Comparative Example 4 where the amount of Mn was below the lower limit, the toughness of the base metal part and the welded part deteriorated. The reason is considered to be that embrittlement due to S occurred.
In Comparative Example 5 in which the amount of Mn exceeded the upper limit, the toughness of the base metal part and the welded part deteriorated. The reason for this is considered to be that cracks due to MnS are likely to occur.
In Comparative Example 6 where Ti was below the lower limit, the toughness of the base metal part and the welded part deteriorated. The reason for this is considered that the crystal grains became coarse.
In Comparative Example 7 where Ti exceeded the upper limit, the toughness of the base metal part and the welded part deteriorated. The reason for this is considered to be that coarse TiN was generated.
In Comparative Example 8 where Nb was below the lower limit, the toughness of the base material portion was deteriorated. The reason for this is considered to be that non-recrystallization rolling has become insufficient.
In Comparative Example 9 where Nb exceeded the upper limit, the toughness of the base metal part and the welded part deteriorated. The reason for this is considered to be that coarse Nb carbonitride was produced.
In Comparative Example 10 where Al was below the lower limit, the toughness of the base metal part and the welded part deteriorated. The reason for this is considered that deoxidation is insufficient.
In Comparative Example 11 in which Al exceeded the upper limit, the toughness of the welded portion deteriorated. The reason for this is considered that a large amount of Al-based inclusions were generated.
In Comparative Example 12 where CMeq exceeded the upper limit, YS and TS exceeded the upper limit.
In Comparative Example 13 in which CMeq was below the lower limit, the F fraction exceeded the upper limit, and TS was below the lower limit.

In Comparative Example 14 where LR was less than 0.210, the yield ratio exceeded 90%.
In Comparative Example 15, YS and YR exceeded the upper limit. This is because the tempering temperature was too low, so the effect of relaxing pipe forming strain by tempering (that is, the effect of reducing dislocation density by tempering) was insufficient, and precipitation on dislocations was insufficient. it is conceivable that.
In Comparative Example 16, the toughness of the base metal part and the welded part was reduced (that is, the vE of the base metal part and the welded part was below the lower limit). The reason for this is that since the tempering temperature is too high, transformation into austenite occurs in some areas, and the concentration of C increases in the above-mentioned some areas. As a result, the island-like martensite ( This is probably because MA) was generated.
In Comparative Example 17, YR exceeded 90%. The reason for this is thought to be that introduction of dislocations and precipitation on dislocations were insufficient because the change in sizer roundness was small.
In Comparative Example 18, YR exceeded 90%. The reason for this is thought to be that precipitation on dislocations was insufficient because the tempering time was short.
In Comparative Example 19 in which the N amount was below the lower limit, the toughness of the base metal part and the welded part deteriorated. The reason for this is considered to be that the crystal grains became coarse.
In Comparative Example 20 in which the N amount exceeded the upper limit, the toughness of the base metal part and the welded part deteriorated. The reason for this is thought to be that the carbide became coarse.
In Comparative Examples 21 to 23, YR exceeded 90%. The reason for this is thought to be that introduction of dislocations and precipitation on dislocations were insufficient because the change in sizer roundness was small.
In Comparative Example 24 where the Mn / Si ratio was 2.0 or less, the toughness of the welded portion deteriorated.
In Comparative Examples 25 and 26 in which the LR was below 0.210, the yield ratio exceeded 90%.

The disclosure of Japanese Patent Application No. 2016-056858 is incorporated herein by reference in its entirety.
All documents, patent applications, and technical standards mentioned in this specification are to the same extent as if each individual document, patent application, and technical standard were specifically and individually described to be incorporated by reference, Incorporated herein by reference.

Claims

Including the base metal part and the ERW welded part,
The chemical composition of the base material part is mass%,
C: 0.080 to 0.120%,
Mn: 0.30 to 1.00%
Ti: 0.005 to 0.050%,
Nb: 0.010 to 0.100%,
N: 0.001 to 0.020%,
Si: 0.010 to 0.450%,
Al: 0.001 to 0.100%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Mo: 0 to 0.50%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Cr: 0 to 1.00%,
V: 0 to 0.100%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
REM: 0 to 0.0100% and the balance: Fe and impurities,
CMeq represented by the following formula (1) is 0.170 to 0.300,
The ratio of mass% of Mn to mass% of Si is 2.0 or more,
LR represented by the following formula (2) is 0.210 or more,
When the metal structure of the base material part is observed at a magnification of 1000 times using a scanning electron microscope, the area ratio of the first phase made of ferrite is 60 to 98%, and the remaining second phase is tempered. Including bainite,
Yield strength in the tube axis direction is 390 to 562 MPa,
The tensile strength in the tube axis direction is 520 to 690 MPa,
The yield ratio in the tube axis direction is 90% or less,
The Charpy absorbed energy in the pipe circumferential direction in the base material part is 100 J or more at 0 ° C.,
An electric resistance welded steel pipe for line pipes, wherein Charpy absorbed energy in the pipe circumferential direction in the electric resistance welded portion is 80 J or more at 0 ° C.
CMeq = C + Mn / 6 + Cr / 5 + (Ni + Cu) / 15 + Nb + Mo / 3 + V Formula (1)
LR = (2.1 × C + Nb) / Mn Formula (2)
[In Formula (1) and Formula (2), C, Mn, Cr, Ni, Cu, Nb, Mo, and V represent the mass% of each element, respectively. ]
The chemical composition of the base material part is mass%,
Mo: more than 0% and 0.50% or less,
Cu: more than 0% and 1.00% or less,
Ni: more than 0% and 1.00% or less,
Cr: more than 0% and 1.00% or less,
V: more than 0% and 0.100% or less,
Ca: more than 0% and 0.0100% or less,
The ERW steel pipe for line pipes according to claim 1, comprising one or more of Mg: more than 0% and 0.0100% or less and REM: more than 0% and 0.0100% or less.
2. The area ratio of precipitates having a circle-equivalent diameter of 100 nm or less is 0.10 to 1.00% when the metal structure of the base material part is observed at a magnification of 100000 times using a transmission electron microscope. Or the electric-resistance-welded steel pipe for line pipes of Claim 2.
The ERW steel pipe for a line pipe according to any one of claims 1 to 3, wherein the Nb content in the chemical composition of the base material part is 0.020% or more by mass%.
The ERW steel pipe for a line pipe according to any one of claims 1 to 4, wherein the wall thickness is 10 to 25 mm and the outer diameter is 114.3 to 609.6 mm.