JP6693610B1 - ERW steel pipe for line pipe - Google Patents

Abstract

The base material portion is% by mass, C: 0.03% or more and less than 0.10%, Mn: 0.30% to 1.00%, Nb: 0.010 to 0.100%, and Si: 0. 010 to 0.500%, the balance contains Fe and impurities, CNeq is 0.12 to 0.25, Mn / Si ratio is 1.8 or more, LR is 0.25 or more, The metal structure of the base material has a ferrite ratio of 80 to 98%, the balance structure includes pearlite and / or bainite, and the hardness difference [balance structure-ferrite] is 50 to 100 Hv, YS 360 MPa or higher, TS 465 MPa or higher, And YR 0.90 or less, and the Charpy absorbed energies at 0 ° C. of the base material portion and the electric resistance welded portion are 100 J or more, respectively, for an electric resistance welded steel pipe for a line pipe. CNeq = C + Mn / 6 + Cr / 5 + (Ni + Cu) / 15 + Nb + Mo + VLR = (2.1C + Nb) / Mn

Description

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

  In recent years, the importance of a pipeline, which is one of the means for transporting mainly crude oil or natural gas, and a line pipe used to form this pipeline has been increasing.

In some cases, it is required to lower the yield ratio of the electric resistance welded steel pipe in the axial direction of the electric resistance welded steel pipe for line pipes.
For example, in Patent Document 1, for example, a tube shaft of an electric resistance welded steel pipe obtained by applying a repeated strain due to bending-bending back treatment to a band steel as a raw material to induce the Bauschinger effect before pipe forming. A technique for lowering the yield ratio in the direction is disclosed.
Further, in Patent Document 2, the metal structure of the hot-rolled steel sheet for producing an electric resistance welded steel pipe has a microstructure composed of a ferrite structure and martensite having an area ratio of 1 to 20%, whereby the axial direction of the electric resistance welded steel pipe A technique for lowering the yield ratio has been proposed.

Further, in Patent Document 3, as a line pipe electric resistance welded steel pipe having a certain degree of tensile strength and yield strength, a reduced yield ratio, and excellent toughness of a base material portion and an electric resistance welded portion, a chemical treatment of the base material portion is performed. The composition is% by 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%, and Al: 0.001 to 0.100%, the balance contains Fe and impurities, and the following CMeq is 0. 170-0.300, Mn / Si ratio is 2.0 or more, LR below is 0.210 or more, area ratio of ferrite is 60-98%, and the balance contains tempered bainite. The yield strength in the axial direction is 390 to 562 MPa, and the tensile force in the axial direction of the pipe is Degree is 520 to 690 MPa, the yield ratio in the pipe axis direction is 90% or less, the Charpy absorbed energy in the pipe circumferential direction at the base metal portion is 100 J or more at 0 ° C., and the pipe circumferential direction at the electric resistance welded portion is Of the electric resistance welded steel pipe for a line pipe having a Charpy absorbed energy of 80 J or more at 0 ° C.
CMeq = C + Mn / 6 + Cr / 5 + (Ni + Cu) / 15 + Nb + Mo / 3 + V
LR = (2.1 × C + Nb) / Mn

Further, in Patent Document 4, as a thick-walled electric resistance welded steel tube having both a low yield ratio of 95% or less, preferably 92% or less and low temperature toughness, a base material steel sheet formed into a tubular shape is electric resistance welded. A thick-walled electric resistance welded steel pipe having a wall thickness / outer diameter ratio of 4.0 to 7.0%, in which the base steel sheet is C: 0.06 to 0.15% and Mn: 1 by mass%. 0.000 to 1.65%, Ti: 0.005 to 0.020%, Nb: 0.005 to 0.030%, N: 0.001 to 0.006%, P: 0.02% or less , S: 0.005% or less, Si: 0.45% or less, Al: 0.08% or less, Mo: less than 0.20%, Cu: 0.50% or less, Ni: 0.50% Below, Cr: 1.00% or less, V: 0.10% or less, Ca: 0.0050% or less, REM: 0.0050% or less, and the following Ceq is 0. 2 to 0.43 with the balance being Fe and unavoidable impurities, and the metal structure of the base steel sheet contains polygonal ferrite in an area ratio of 50 to 92%. Is 15 μm or less, the hardness of the electric resistance welded portion is Hv160 to 240, and the structure of the electric resistance welded portion is bainite, fine grain ferrite and pearlite, or fine grain ferrite and bainite. A meat ERW steel pipe is disclosed.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15

Further, in Patent Document 5, as an as-roll electric resistance welded steel pipe for a line pipe which is excellent in low temperature toughness evaluated by DWTT, a base material part is C: 0.030 to 0.120%, Si: 0. 05-0.30%, Mn: 0.50-2.00%, Al: 0.010-0.035%, N: 0.0010-0.0080%, Nb: 0.010-0.080% , Ti: 0.005 to 0.030%, Ni: 0.001 to 0.20%, and Mo: 0.10 to 0.20%, the balance contains Fe and impurities, and the following F1 is 0.300 to 0.350, and in the metal structure of the central portion of the wall thickness of the base material, the polygonal ferrite fraction is 60 to 90%, the average crystal grain size is 15 μm or less, and the crystal grain size is 20 μm. The coarse crystal grain size, which is the area ratio of the above crystal grains, is 20% or less, The as-roll electric resistance welded steel pipe for line pipes having a yield ratio of 80 to 95% is disclosed.
F1 = Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3

Patent Document 1: Patent No. 4466320 Patent Document 2: Japanese Patent Application Laid-Open No. 10-176239 Patent Document 3: International Publication No. 2017/163987 Patent Document 4: International Publication No. 2013/027779 Patent Document 5: Patent No. 6260757

However, in the technique of Patent Document 1, the number of steps is increased because a step of applying strain to the strip steel is required, and as a result, the manufacturing cost of the steel pipe may be increased.
Further, in some cases, it is required to improve the toughness of the base material portion of the steel pipe with respect to the technique of Patent Document 2.
Further, in all of the techniques of Patent Document 3, the entire electric resistance welded steel pipe (that is, the as-roll electric resistance welded steel pipe) as-made is subjected to a heat treatment of 400 ° C. or higher and Ac1 point or lower after pipe forming, so that the electric resistance The yield ratio of sewn steel pipe is reduced. However, the yield ratio of the electric resistance welded steel pipe is reduced and the toughness of the base material portion and the electric resistance welded portion is not limited to the technique of Patent Document 3 (especially, without performing heat treatment after pipe making). It may be required to secure.
Further, in paragraph 0013 of Patent Document 4, “Nb carbonitride is obtained by reducing the content of Nb as compared with the conventional one, optimizing hot rolling conditions, and performing two-stage accelerated cooling after hot rolling. It has been found that it is possible to suppress the precipitation of No. 2 and form a multiphase structure, and as a result, a low Y / T can be secured. ” However, it is not limited to the technique (reduction of Nb amount) described in Patent Document 4, and it is required to reduce the yield ratio of the electric resistance welded steel pipe and to secure the toughness of the base material portion and the electric resistance welded portion. May be
Further, in Patent Document 5, by limiting the chemical composition of the base material part to a specific range and controlling the conditions of the hot rolling process, “the average crystal grain size is 15 μm or less and the crystal grain size is 20 μm”. It is disclosed that the coarse crystal grain size, which is the area ratio of the crystal grains described above, is 20% or less. ”, Thereby improving the low temperature toughness evaluated by DWTT. However, without being limited to the technique described in Patent Document 5, it may be required to reduce the yield ratio of the electric resistance welded steel pipe and to secure the toughness of the base material portion and the electric resistance welded portion.

  An object of the present disclosure is to provide an electric resistance welded steel pipe for a line pipe which 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 electric resistance welded portion.

Means for solving the above problems include the following aspects.
<1> Including a base material portion and an electric resistance welded portion,
The chemical composition of the base material part is mass%,
C: 0.03% or more and less than 0.10%,
Mn: 0.30 to 1.00%,
Ti: 0.005 to 0.050%,
Nb: 0.010 to 0.100%,
N: 0.001-0.020%,
Si: 0.010 to 0.500%,
Al: 0.001 to 0.100%,
P: 0 to 0.030%,
S: 0 to 0.010%,
Mo: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Cr: 0 to 0.50%,
V: 0 to 0.10%,
Ca: 0 to 0.0100%,
REM: 0 to 0.0100%, and
The balance: Fe and impurities, CNeq represented by formula (1) is 0.12 to 0.25, the ratio of Mn content to Si content is 1.8 or more, and formula (2) Has an LR of 0.25 or more,
In the metal structure of the base metal part, the area ratio of the first phase made of ferrite is 80 to 98%, the second phase which is the rest contains at least one of pearlite and bainite, and the whole second phase. The area ratio of martensite is less than 1%, the value obtained by subtracting the hardness of the first phase from the hardness of the second phase is 50 to 100 Hv,
The yield strength in the pipe axis direction is 360 to 600 MPa,
The tensile strength in the tube axis direction is 465 to 760 MPa,
The yield ratio in the tube axis direction is 0.90 or less,
Charpy absorbed energy at 0 ° C. of the base material portion and the electric resistance welded portion is 100 J or more,
ERW steel pipe for line pipe, which has a yield elongation of less than 0.2% in a tensile test in the pipe axis direction.
CNeq = C + Mn / 6 + Cr / 5 + (Ni + Cu) / 15 + Nb + Mo + V ... Formula (1)
LR = (2.1C + Nb) / Mn ... Formula (2)
[In the formulas (1) and (2), each element symbol means mass% of each element. ]

<2> The chemical composition of the base material part is% by mass,
Mo: more than 0% and 0.50% or less,
Cu: more than 0% and 0.50% or less,
Ni: more than 0% and 0.50% or less,
Cr: more than 0% and 0.50% or less,
V: more than 0% and 0.10% or less,
Ca: more than 0% and 0.0100% or less, and
REM: The electric resistance welded steel pipe for a line pipe according to <1>, containing at least one selected from the group consisting of more than 0% and 0.0100% or less.
<3> The electric resistance welded steel pipe for line pipe according to the contract <1> or <2>, which has a wall thickness of 10 to 25.4 mm and an outer diameter of 254.0 to 660.4 mm.

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

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

ERW steel pipe for line pipe of the present disclosure (hereinafter, also simply referred to as "ERW steel pipe"),
Including base material and ERW welded part,
The chemical composition of the base material portion is% by mass, C: 0.03% or more and less than 0.10%, Mn: 0.30 to 1.00%, Ti: 0.005 to 0.050%, Nb: 0. 0.010 to 0.100%, N: 0.001 to 0.020%, Si: 0.010 to 0.500%, Al: 0.001 to 0.100%, P: 0 to 0.030%, S: 0 to 0.010%, Mo: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, V: 0 to 0 10%, Ca: 0 to 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impurities, and CNeq represented by the formula (1) is 0.12 to 0.25. , The ratio of the content of Mn to the content of Si is 1.8 or more, and the LR represented by the formula (2) is 0.25 or more,
In the metal structure of the base metal part, the area ratio of the first phase composed of ferrite is 80 to 98%, the second phase which is the rest contains at least one of pearlite and bainite, and is a martensite based on the whole second phase. The area ratio of the site is less than 1%, the value obtained by subtracting the hardness of the first phase from the hardness of the second phase is 50 to 100 Hv,
The yield strength in the pipe axis direction is 360 to 600 MPa,
The tensile strength in the tube axis direction is 465 to 760 MPa,
The yield ratio in the tube axis direction is 0.90 or less,
Charpy absorbed energy at 0 ° C of the base metal part and the electric resistance welded part is 100 J or more,
Yield elongation is less than 0.2% in the tensile test in the tube axis direction.
CNeq = C + Mn / 6 + Cr / 5 + (Ni + Cu) / 15 + Nb + Mo + V ... Formula (1)
LR = (2.1C + Nb) / Mn ... Formula (2)
[In the formulas (1) and (2), each element symbol means mass% of each element. ]

  In the present disclosure, the above-described chemical composition of the base material portion (including the ratio of the content of Mn to the content of Ceq and Si, and each of LR includes satisfying the above conditions) is the chemistry of the present disclosure. Also called composition.

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

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

The electric resistance welded steel pipe of the present disclosure has a yield elongation of less than 0.2% in a tensile test in the pipe axis direction. Here, the yield elongation being less than 0.2% means that the yield elongation is not substantially observed.
That the yield elongation is less than 0.2% means that the electric resistance welded steel pipe of the present disclosure is not subjected to heat treatment other than seam heat treatment after pipe forming (that is, the electric resistance welded steel pipe as-made ( It is also referred to as "Azroll ERW steel pipe")).

The electric resistance welded steel pipe of the present disclosure has a certain degree of tensile strength and yield strength even though it is an as-fabricated electric resistance welded steel pipe (specifically, the yield strength in the pipe axis direction is 360 to 600 MPa, and Has a tensile strength of 465 to 760 MPa), a yield ratio is reduced (specifically, a yield ratio in the pipe axis direction is 0.90 or less), and excellent toughness (specifically, a base metal part and The Charpy absorbed energy at 0 ° C of the electric resistance welded portion is 100 J or more, respectively).
Hereinafter, the tensile strength in the pipe axis direction is also referred to as “TS”, the yield strength in the pipe axis direction is also referred to as “YS”, the yield ratio in the pipe axis direction is also referred to as “YR”, and the YR of the electric resistance welded steel pipe is reduced. Is also referred to as "reduction in YR".

As described above, the electric resistance welded steel pipe of the present disclosure is an electric resistance welded steel pipe that is still manufactured, and exhibits the above effects.
The above effect is achieved by the combination of the chemical composition of the base material portion and the metal structure of the base material portion.
For example, the LR represented by the formula (2) being 0.25 or more contributes to the low YR (that is, YR of 0.90 or less). The reason for this is that when LR is 0.25 or more, improvement of work hardening characteristics by C and improvement of work hardening characteristics by Nb are effectively realized, and as a result, low YR is achieved. It is thought to be because.
Further, the value obtained by subtracting the hardness of the first phase from the hardness of the second phase (hereinafter, also referred to as “hardness difference”) is 50 Hv or more contributes to low YR (that is, YR of 0.90 or less). To do. The reason for this is considered to be that when the hardness difference is 50 Hv or more, non-uniform deformation occurs due to processing strain during pipe forming, and the anisotropic hardening characteristics of the steel are exhibited. As a result, it is considered that low YR (that is, YR of 0.90 or less) is achieved even though the ERW steel pipe is as-made. Supplementally, as a means for realizing a low YR, a method of heat-treating an electric resistance welded steel pipe as-made can be considered (see, for example, Patent Document 3 described above). However, in the electric resistance welded steel pipe of the present disclosure, Low YR can be achieved even though it is an electric resistance welded steel pipe.
Further, the hardness difference of 100 Hv or less contributes to the improvement of the toughness of the base material portion. It is considered that this is because when the hardness difference is 100 Hv or less, the internal stress in the metal structure is reduced.
The metallographic structure of the base material portion is mainly created in the process of manufacturing a hot-rolled steel sheet which is a material of the electric resistance welded steel pipe. An example of the method for manufacturing the electric resistance welded steel pipe, including the preferable manufacturing conditions for the hot-rolled steel sheet, will be described later.

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

  Furthermore, since the electric resistance welded steel pipe of the present disclosure is excellent in toughness of the base metal portion and the electric resistance welded welded portion, when used as an electric resistance welded steel pipe for a line pipe, it is expected to have an effect of excellent crack propagation stopping characteristics at the time of burst. To be done.

<Chemical composition of base metal part>
Hereinafter, the chemical composition of the base material portion (that is, the chemical composition in the present disclosure) will be described.

C: 0.03% or more and less than 0.10% C is an element necessary for forming at least one of pearlite and bainite and improving work-hardening characteristics important for low YR. From the viewpoint of obtaining such effects, the amount of C is 0.03% or more.
When the amount of C is less than 0.03%, the area ratio of the ferrite structure becomes too high and the work hardening characteristics are not improved, and as a result, low YR may not be achieved.
On the other hand, when the C content is 0.10% or more, the toughness of the base material portion and the electric resistance welded portion may be deteriorated due to a large amount of cementite generated. Therefore, the amount of C is less than 0.10%. The C content is preferably 0.09% or less, more preferably less than 0.08%, and further preferably 0.07% or less.

Mn: 0.30% to 1.00%
Mn is an element that enhances the hardenability of steel. Further, Mn is an essential element for making S harmless. From the viewpoint of obtaining these effects, the amount of Mn is 0.30% or more.
If the Mn content is less than 0.30%, embrittlement due to S may occur, and the toughness of the base material part may decrease.
On the other hand, when the Mn amount is excessive, the segregated portion of Mn in the central portion of the wall thickness becomes prominent, MnS is generated, and a coarse martensite or bainite hardening phase is generated, resulting in the base metal portion and the electric resistance welding. The toughness of the part may be impaired. Further, if the Mn amount is excessive, CNeq may exceed 0.25, and as a result, the strength becomes too high (specifically, at least one of YS 600 MPa or less and TS 760 MPa or less cannot be achieved). There is. For these reasons, the Mn content is 1.00% or less. The Mn content is preferably less than 1.00%, more preferably 0.90% or less, still more preferably 0.80% or less, still more preferably 0.70% or less.

Ti: 0.005-0.050%
Ti is an element that forms carbonitrides and contributes to the refinement of the crystal grain size, and is an element necessary to secure the toughness of the base material portion and the electric resistance welded portion. From the viewpoint of obtaining such effects, the amount of Ti is 0.005% or more. The amount of Ti is preferably 0.010% or more.
On the other hand, if the amount of Ti exceeds 0.050%, coarse TiN may be generated, and the toughness of a base material part and an electric resistance welded part may fall. Therefore, the Ti amount is 0.050% or less.

Nb: 0.010 to 0.100%
Nb has the effect of increasing the toughness by rolling in the non-recrystallized region at high temperature. Nb is also an element that improves work hardening characteristics by precipitation strengthening (that is, an element that contributes to low YR). From the viewpoint of these effects, the amount of Nb is 0.010% or more, preferably 0.020% or more, and more preferably 0.030% or more.
On the other hand, when the amount of Nb exceeds 0.100%, coarse Nb carbide may reduce the toughness. Therefore, the amount of Nb is 0.100% or less. The Nb amount is preferably 0.080% or less, more preferably 0.060% or less.

N: 0.001-0.020%
N is an element that suppresses the coarsening of crystal grains by forming a metal nitride and improves the toughness of the base material portion and the electric resistance welded portion. From the viewpoint of obtaining such effects, the N content is 0.001% or more.
On the other hand, when the N content exceeds 0.020%, the amount of alloy carbide produced increases, and the toughness of the base material portion and the electric resistance welded portion may decrease. Therefore, the N content is 0.020% or less. The N content is preferably 0.010% or less, more preferably 0.006% or less.

Si: 0.010 to 0.500%
Si is an element used as a deoxidizing agent for steel, and is an element that suppresses the formation of a coarse oxide in the base material portion and the electric resistance welded portion, thereby improving the toughness. From the viewpoint of obtaining such effects, the amount of Si is 0.010% or more. The amount of Si is preferably 0.030% or more.
On the other hand, if the Si content exceeds 0.500%, inclusions may be generated in the electric resistance welded portion, the Charpy absorbed energy may be reduced, and the toughness may be reduced. Therefore, the amount of Si is 0.500% or less. The amount of Si is preferably 0.400% or less, more preferably 0.350% or less.

Al: 0.001 to 0.100%
Al, like Si, is an element used as a deoxidizer. From the viewpoint of improving the toughness of the base material and preventing cracks due to free oxygen, the Al amount is 0.001% or more. The amount of Al is preferably 0.005% or more, more preferably 0.010% or more.
On the other hand, if the Al content exceeds 0.100%, the toughness of the electric resistance welded portion may be reduced due to the formation of an Al-based oxide during electric resistance welding. Therefore, the Al amount is 0.100% or less. The amount of Al is preferably 0.090% or less.

P: 0 to 0.030%
P is an element that can be present as an impurity in steel.
If the P amount exceeds 0.030%, segregation at grain boundaries may impair the toughness of the base material portion and the electric resistance welded portion. Therefore, the P amount is 0.030% or less.
On the other hand, the P amount may be 0%. From the viewpoint of reducing the dephosphorization cost, the P amount may be more than 0% or may be 0.001% or more.

S: 0 to 0.010%
S is an element that can be present as an impurity in steel.
If the S content exceeds 0.030%, the toughness of the base material portion and the electric resistance welded portion may be impaired. Therefore, the S amount is 0.030% or less. The S amount is preferably 0.020% or less, more preferably 0.010% or less.
On the other hand, the S amount may be 0%. From the viewpoint of reducing the desulfurization cost, the S amount may be more than 0% or may be 0.001% or more.

Mo: 0 to 0.50%
Mo is an arbitrary element. That is, the amount of Mo may be 0% or may be more than 0%.
Mo is an element that has the effect of improving the hardenability of steel and the strength of steel. From the viewpoint of obtaining such effects, the Mo amount may be 0.01% or more.
On the other hand, if the Mo content exceeds 0.50%, the toughness may be reduced due to the formation of Mo carbonitrides. Therefore, the amount of Mo is 0.50% or less. The Mo content may be 0.30% or less, or 0.10% or less.

Cu: 0 to 0.50%
Cu is an arbitrary element. That is, the amount of Cu may be 0% or may be more than 0%.
Cu is an element effective for improving the strength of the base material portion. From the viewpoint of obtaining such effects, the amount of Cu may be 0.05% or more.
On the other hand, if the Cu content exceeds 0.50%, fine Cu particles may be generated, and the toughness may be significantly reduced. Therefore, the amount of Cu is 0.50% or less. The amount of Cu may be 0.40% or less, or may be 0.30% or less.

Ni: 0 to 0.50%
Ni is an arbitrary element. That is, the amount of Ni may be 0% or may be more than 0%.
Ni is an element that contributes to the improvement of strength and toughness. From the viewpoint of obtaining such an effect, the amount of Ni may be 0.05% or more.
On the other hand, if the Ni content exceeds 0.50%, the strength may be too high (specifically, at least one of YS 600 MPa or less and TS 760 MPa or less cannot be achieved). Therefore, the amount of Ni is 0.50% or less.

Cr: 0 to 0.50%
Cr is an arbitrary element. That is, the Cr amount may be 0% or may be more than 0%.
Cr is an element that improves hardenability. From the viewpoint of obtaining such an effect, the Cr amount may be 0.05% or more.
On the other hand, when the Cr content exceeds 0.50%, the toughness of the electric resistance welded portion may be deteriorated due to the Cr-based inclusions generated in the electric resistance welded portion. Therefore, the Cr content is 0.50% or less. The Cr amount may be 0.40% or less.

V: 0 to 0.10%
V is an arbitrary element. That is, the V amount may be 0% or may be more than 0%.
V has almost the same effect as Nb. From the viewpoint of obtaining such an effect, the V amount may be 0.010% or more.
On the other hand, if the V content exceeds 0.10%, the toughness may decrease due to the V carbonitride. Therefore, the V amount is 0.10% or less.

Ca: 0 to 0.0100%
Ca is an arbitrary element. That is, the amount of Ca may be 0% or may be more than 0%.
Ca is an element that controls the morphology of sulfide inclusions and improves low temperature toughness. From the viewpoint of obtaining such an effect, the amount of Ca may be 0.0001% or more, and may be 0.0002% or more.
On the other hand, when the Ca content exceeds 0.0100%, CaO-CaS becomes large clusters or inclusions, which may adversely affect the toughness. Therefore, the amount of Ca is 0.0100% or less. The amount of Ca may be 0.0080% or less, or 0.0060% or less.

REM: 0 to 0.0100%
REM is an arbitrary element. That is, the REM amount may be 0%.
Here, “REM” is a rare earth element, that is, from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Refers to at least one element selected.
REM has an effect as a deoxidizing agent and a desulfurizing agent. From the viewpoint of obtaining such effects, the REM amount may be 0.0001% or more.
On the other hand, if the REM content is more than 0.0100%, coarse oxides are generated, and the HIC resistance (hydrogen cracking resistance during electric resistance welding) and the toughness of the base material and HAZ are reduced. There is. Therefore, the REM amount is 0.0100% or less.

The chemical composition of the base material is Mo: more than 0% and not more than 0.50%, Cu: more than 0% and not more than 0.50%, Ni: more than 0% and not more than 0.50%, Cr: more than 0% and not more than 0.50%. Hereinafter, at least one selected from the group consisting of V: more than 0% and 0.10% or less, Ca: more than 0% and 0.0100% or less, and REM: more than 0% and 0.0100% or less is contained. Good.
More preferable amounts of each of these optional elements are as described above.

Remainder: Fe and impurities In the chemical composition of the base material part, the balance excluding the above-mentioned elements is Fe and impurities.
Here, the impurities refer to components contained in raw materials (for example, ores, scraps, etc.) or components mixed in the manufacturing process and not intentionally contained in steel.
The impurities include all elements other than the above-mentioned elements. The element as an impurity may be only one kind or two or more kinds.
Examples of the impurities include B, Sb, Sn, W, Co, As, Pb, Bi, and H.
Regarding other elements, usually, Sb, Sn, W, Co, and As are mixed with a content of 0.1% or less, and Pb and Bi are mixed with a content of 0.005% or less, for example. B may be mixed with a content of 0.0003% or less, and H may be mixed with a content of 0.0004% or less, but the content of other elements is within the normal range. , No need to control.

CNeq: 0.12-0.25
In the chemical composition of the base material portion, CNeq represented by the formula (1) is 0.12 to 0.25.

CNeq = C + Mn / 6 + Cr / 5 + (Ni + Cu) / 15 + Nb + Mo + V ... Formula (1)
[In the formula (1), each element symbol means mass% of each element. ]

CNeq has a positive correlation with the strength of the electric resistance welded steel pipe.
When CNeq is less than 0.12, at least one of YS of 360 MPa or more and TS of 465 MPa or more may not be achieved. Therefore, CNeq is 0.12 or more. From the viewpoint of further improving at least one of YS and TS of the electric resistance welded steel pipe, CNeq is preferably 0.15 or more.
On the other hand, when CNeq is more than 0.25, the strength may be too high (specifically, at least one of YS 600 MPa or less and TS 760 MPa or less cannot be achieved). Therefore, CNeq is 0.25 or less.

Mn / Si ratio: 1.8 or more In the chemical composition of the base material part, the Mn / Si ratio (that is, the ratio of the content of Mn to the content of Si; hereinafter also referred to as “Mn / Si”) is 1. 8 or more.
If the Mn / Si ratio is less than 1.8, the toughness of the electric resistance welded portion may decrease due to MnSi-based inclusions. From the viewpoint of further improving the toughness of the electric resistance welded portion, Mn / Si is preferably 1.9 or more, more preferably 2.0 or more.
There is no particular upper limit for Mn / Si. From the viewpoint of further improving the toughness of the base material portion and the electric resistance welded portion, Mn / Si is preferably 50 or less, preferably 30 or less, and more preferably 20 or less.

LR: 0.25 or more In the chemical composition of the base material part, the LR represented by the formula (2) is 0.25 or more.
When LR is less than 0.25, there are cases where low YR cannot be achieved (that is, YR exceeds 0.90).
Although the upper limit of LR is not particularly limited, examples of the upper limit of LR include 0.90 and 0.85.

LR = (2.1C + Nb) / Mn ... Formula (2)
[In the formula (2), each element symbol means mass% of each element. ]

  In the formula (2), C and Nb, which improve the work hardening characteristics and contribute to YR reduction, are arranged in the numerator, and Mn, which may lower the work hardening characteristics and increase YR, are arranged in the denominator. There is.

  The upper limit of LR is not particularly limited, but examples of the upper limit of LR include 0.90 and 0.80.

<Metal structure of base material>
Hereinafter, the metal structure of the base material portion will be described.

(Area ratio of the first phase (Ferrite fraction))
In the metal structure of the base material portion, the area ratio of the first phase made of ferrite (hereinafter, also referred to as “ferrite fraction”) is 80 to 98%.
When the ferrite fraction is less than 80%, the degree of carbon concentration in the second phase becomes insufficient, and as a result, the hardness difference between the first phase and the second phase becomes too small, and the hardness difference becomes less than 50 Hv. There is. Therefore, the ferrite fraction is 80% or more, preferably 82% or more.
On the other hand, when the ferrite fraction exceeds 98%, the degree of carbon concentration in the second phase becomes excessive, and as a result, the hardness of the second phase becomes too high, and the hardness difference may exceed 100 Hv. Therefore, the ferrite fraction is 98% or less, preferably 95% or less, and more preferably 90% or less.

(Phase 2)
In the metal structure of the base metal part, the second phase which is the balance (that is, the balance except the first phase from the metal structure of the base metal part) contains at least one of pearlite and bainite, and with respect to the entire second phase. The area ratio of martensite is less than 1%.
When the second phase satisfies the above condition, it is easy to achieve that the hardness difference (that is, the value obtained by subtracting the hardness of the first phase from the hardness of the second phase) is 100 Hv or less.
On the other hand, the area ratio of martensite with respect to the entire second phase being less than 1% means that the second phase does not substantially contain martensite. When the second phase substantially contains martensite (that is, when it contains 1% or more martensite with respect to the entire second phase), the strength becomes too high (specifically, YS 600 MPa or less and There is a case where at least one of TS760 MPa or less cannot be achieved).

The concept of "bainite" in the present disclosure includes bainitic ferrite, granular bainite, upper bainite and lower bainite.
The concept of “perlite” in the present disclosure also includes a pseudo perlite structure.

(Ferrite fraction measurement method and second phase identification method)
In the metal structure of the base metal part, the ferrite fraction was measured and the second phase was identified by metallizing the metallographic structure of the central part of the wall thickness in the L cross section at the 90 ° position of the base metal with nital etching, and after the nital etching. (Hereinafter, also referred to as "metallographic photograph") is observed by using a scanning electron microscope (SEM) at a magnification of 1000 times. Here, as for the metallographic structure, 10 fields of view (the actual area of the cross section is 0.12 mm ² minutes) are taken at a field of view of 1000 times. The ferrite fraction is measured and the second phase is specified by image-processing the photographed metallographic structure. The image processing is performed using, for example, a small general-purpose image analyzer LUZEX AP manufactured by Nireco Corporation.

  In the present disclosure, the “base material 90 ° position” refers to a position deviated from the welded portion by 90 ° in the pipe circumferential direction, and the “L cross section” refers to a cross section parallel to the pipe axial direction and the wall thickness direction. The "thickness center portion" means a position of 1/2 of the wall thickness.

(Hardness difference)
In the metal structure of the base material part, the hardness difference (that is, the value obtained by subtracting the hardness of the first phase from the hardness of the second phase) is 50 to 100 Hv.
When the difference in hardness is less than 50 Hv, low YR may not be achieved. Therefore, the hardness difference is 50 Hv or more, and preferably 52 Hv or more, from the viewpoint of low YR.
On the other hand, if the hardness difference is more than 100 Hv, the internal stress of the metal structure becomes too large, and the toughness of the base material part may decrease. Therefore, the hardness difference is 100 Hv or less, preferably 96 Hv or less, and more preferably 90 Hv or less.

(Method of measuring hardness difference)
The hardness difference is measured as follows.
The hardness of the first phase and the second phase in the metallographic structure of the central part of the wall thickness (that is, the metallographic structure of the central part of the wall thickness in the L cross section at the 90 ° position of the base metal) are obtained, respectively, The value obtained by subtracting the hardness of the phase is defined as the hardness difference.
Here, the hardness of the first phase is obtained as follows.
50 measurement points are randomly selected from the first phase, and the micro Vickers hardness test is performed on each of the 50 selected points by a micro Vickers hardness test under a load of 10 gf. Each selected measurement point may straddle the crystal grain boundary. From the obtained 50 measured values, the measured values other than the obviously too high measured value (specifically, the measured value exceeding 350 Hv) are selected, and the arithmetic mean value is calculated for the selected measured values. The obtained arithmetic mean value is taken as the hardness of the first phase.
The hardness of the second phase is also determined in the same manner as the hardness of the first phase.

<Yield strength (YS) in the pipe axis direction>
The electric resistance welded steel pipe of the present disclosure has a yield strength (YS) in the pipe axis direction of 360 to 600 MPa.
When YS is 360 MPa or more, the strength required as a steel pipe for a line pipe is secured. YS is preferably 380 MPa or more, more preferably 400 MPa or more.
When YS is 600 MPa or less, bending deformability (that is, ease of bending) when laying the ERW steel pipe for a line pipe is secured, and buckling of the ERW steel pipe for a line pipe is suppressed. YS is preferably 590 MPa or less.

  Note that YS in the electric resistance welded steel pipe of the present disclosure means 0.5% underload proof stress.

<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 465 to 760 MPa.
When TS is 465 MPa or more, the strength required as a steel pipe for a line pipe is secured. TS is preferably 470 MPa or more.
When TS is 760 MPa or less, bending deformability (that is, bending easiness) when laying the ERW steel pipe for a line pipe is secured, and buckling of the ERW steel pipe for a line pipe is suppressed. Further, deterioration of the toughness of the base material portion is further suppressed. TS is preferably 700 MPa or less, more preferably 680 MPa or less.

<Yield ratio (YR) in the pipe axis direction>
In the electric resistance welded steel pipe of the present disclosure, the yield ratio (YR = (YS / TS)) in the pipe axis direction is 0.90 or less.
Thereby, buckling of the electric resistance welded steel pipe at the time of laying is suppressed.
The lower limit of YR is not particularly limited, but examples of the lower limit include 0.80 and 0.82.

<Method of measuring TS, YS and YR>
In the present disclosure, TS, YS, and YR mean values measured as follows, respectively.
From the base material 90 ° position of the ERW steel pipe, a test piece for a tensile test is sampled in a direction in which the test direction of the tensile test (pulling direction) is the pipe axis direction of the ERW steel pipe. Here, the shape of the test piece is a flat plate shape conforming to the American Petroleum Institute standard API 5L (hereinafter, simply referred to as “API 5L”).
Using the collected test piece, at room temperature, in accordance with API 5L, a tensile test in the pipe axis direction (that is, a tensile test in which the test direction is the pipe axis direction of the electric resistance welded steel pipe) is performed, and TS and YS are measured respectively. To do. Here, YS is 0.5% underload proof stress as described above.
Based on the obtained TS and YS, YR is calculated by the calculation formula “YR = (YS / TS)”.

<Yield elongation>
The electric resistance welded steel pipe of the present disclosure has a yield elongation of less than 0.2% (that is, substantially no yield elongation is observed) in a pipe axial tensile test.
This yield elongation is confirmed by the above-mentioned tensile test in the tube axis direction for determining TS, YS and YR.

As described above, the yield elongation of less than 0.2% means that the electric resistance welded steel pipe of the present disclosure is an electric resistance welded steel pipe as it is.
In contrast to the electric resistance welded steel pipe of the present disclosure, the electric resistance welded steel pipe (for example, the electric resistance welded steel pipe of Patent Document 3) in which the entire pipe is heat-treated after pipe forming is substantially subjected to a tensile test in the pipe axial direction. Yield elongation (ie 0.2% or more yield elongation) is observed.

<Charpy absorbed energy>
In the electric resistance welded steel pipe of the present disclosure, the toughness of the base material portion and the electric resistance welded portion is ensured.
Specifically, the Charpy absorbed energy (hereinafter, also referred to as “vE”) at 0 ° C. of the base material portion and the electric resistance welded portion is 100 J or more, respectively.
Hereinafter, the Charpy absorbed energy at 0 ° C. is also referred to as “vE”.

The vE of the base material portion is preferably 150 J or more, more preferably 200 J or more, still more preferably 250 J or more.
On the other hand, vE of the base material portion is preferably 400 J or less.

The vE of the electric resistance welded portion is preferably 150 J or more, more preferably 190 J or more, and further preferably 200 J or more.
On the other hand, vE of the electric resistance welded portion is preferably 400 J or less, more preferably 350 J or less.

VE of the base material portion (that is, Charpy absorbed energy at 0 ° C.) means a value measured as follows.
A full size test piece with a V notch (a test piece for a Charpy impact test) is taken from the base material 90 ° position of the electric resistance welded steel pipe. The V-notch full-size test piece is sampled so that the test direction is the tube circumferential direction (C direction). The full-size test piece with a V-notch sampled is subjected to a Charpy impact test according to API 5L under a temperature condition of 0 ° C. to measure vE.
The above measurement is performed 5 times for each electric resistance welded steel pipe, and the average value of the 5 measured values is taken as vE of the base material portion of the electric resistance welded steel pipe.

VE of the electric resistance welded portion means a value measured as follows.
Except for changing the position where the V-notch full-size test piece is sampled to the ERW welded portion of the ERW steel pipe, the same operation as in measuring vE of the base metal portion is performed to perform the ERW welded portion of the ERW welded pipe. VE (ie, the average of 5 measurements) is obtained.

<Thickness of ERW steel pipe>
The wall thickness of the electric resistance welded steel pipe of the present disclosure is preferably 10 to 25.4 mm.
When the wall thickness is 10 mm or more, it is advantageous in that YR is likely to be lowered by utilizing the strain when the hot rolled steel sheet is formed into a tubular shape. The wall thickness is more preferably 12 mm or more.
When the wall thickness is 25.4 mm or less, it is advantageous in terms of suitability for manufacturing an electric resistance welded steel pipe (specifically, formability when forming a hot-rolled steel sheet into a tube). The wall thickness is more preferably 20 mm or less.

<Outer diameter of ERW steel pipe>
The outer diameter of the electric resistance welded steel pipe of the present disclosure is preferably 254.0 to 660.4 mm (ie, 10 to 26 inches).
When the outer diameter is 254.0 mm (that is, 10 inches) or more, it is more suitable as an electric resistance welded steel pipe for a line pipe. The outer diameter is preferably 304.8 mm (ie, 12 inches) or greater.
When the outer diameter is 660.4 mm (that is, 26 inches) or less, it is advantageous in that YR is likely to be lowered by utilizing the strain when the hot-rolled steel sheet is formed into a tubular shape. The outer diameter is more preferably 508.4 mm (ie, 20 inches) or less.

<Example of method for manufacturing ERW steel pipe (manufacturing method A)>
Hereinafter, an example of a manufacturing method for manufacturing the electric resistance welded steel pipe of the present disclosure (hereinafter, referred to as “manufacturing method A”) will be described.
Hereinafter, the temperature and the cooling rate mean the temperature and the cooling rate of the surface of the steel material (that is, the slab or the hot rolled steel sheet), respectively, unless otherwise specified.
This manufacturing method A is a method for manufacturing an electric resistance welded steel pipe of an example described later.

The manufacturing method A is
A slab preparation step of preparing a slab having a chemical composition according to the present disclosure;
The prepared slab is heated to a slab heating temperature of 1100 ° C. to 1350 ° C., the heated slab is roughly rolled, and the rough rolled slab has a finish rolling start temperature of 950 ° C. or less and a finish rolling end temperature. A hot rolling step of obtaining a hot rolled steel sheet by hot rolling under the condition of 820 ° C. or less and a cumulative rolling ratio in finish rolling of 2.5 or more;
For the hot-rolled steel sheet, the time from the end of finish rolling to the start of the first cooling is set to within 20 s (seconds), and the first cooling rate of 10 ° C / s to 80 ° C / s is set to 600 ° C to 700 ° C. A first cooling step of performing a first cooling until the temperature reaches 1 cooling end temperature;
The hot-rolled steel sheet subjected to the first cooling has a coiling temperature of 450 ° C. to 700 ° C. (that is, a second cooling end temperature) at a second cooling rate of 5 ° C./s to 30 ° C./s. A second cooling step of performing second cooling up to
A winding step of obtaining a hot coil made of the hot-rolled steel sheet by winding the hot-rolled steel sheet subjected to the second cooling at the above-mentioned winding temperature;
By unrolling the hot-rolled steel sheet from the hot coil and roll-forming the unrolled hot-rolled steel sheet to form an open pipe, the butt portion of the obtained open pipe is electric resistance welded to form an electric resistance welded portion. , A pipe forming process for obtaining an electric resistance welded steel pipe,
including.

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

(Slab preparation process)
The slab preparation step in manufacturing method A is a step of preparing a slab having the chemical composition according to the present disclosure.
The step of preparing the slab may be a step of manufacturing the slab, or may be a step of simply preparing a pre-manufactured slab.
When manufacturing a slab, for example, a molten steel having the chemical composition according to the present disclosure is manufactured, and the manufactured molten steel is used to manufacture a slab. At this time, the slab may be manufactured by a continuous casting method, or an ingot may be manufactured using molten steel and the ingot may be slab-rolled to manufacture the slab.

(Hot rolling process)
In the hot rolling step in production method A, the slab prepared above is heated to a slab heating temperature of 1100 ° C. to 1350 ° C., the heated slab is roughly rolled, and the roughly rolled slab has a finish rolling start temperature of 950. It is a step of obtaining hot-rolled steel sheet by hot rolling under conditions that the finish rolling temperature is 820 ° C. or lower, and the cumulative reduction ratio in finish rolling is 2.5 or higher.
When the slab heating temperature is 1100 ° C. or higher, generation of undissolved Nb carbide is suppressed, and as a result, deterioration of toughness is suppressed.
When the slab heating temperature is 1350 ° C. or less, coarsening of crystal grains is suppressed, and as a result, deterioration of toughness of the finally obtained electric resistance welded steel pipe can be suppressed.
When the finish rolling start temperature is 950 ° C. or less, coarsening of crystal grains is suppressed, and as a result, deterioration of toughness of the finally obtained electric resistance welded steel pipe can be suppressed.
When the finish rolling end temperature is 820 ° C. or less, coarsening of crystal grains is suppressed, and as a result, deterioration of toughness of the finally obtained electric resistance welded steel pipe can be suppressed.
When the cumulative reduction ratio is 2.5 or more, coarsening of crystal grains is suppressed, and as a result, deterioration of toughness of the finally obtained electric resistance welded steel pipe can be suppressed.

(First cooling step)
In the first cooling step in the production method A, with respect to the hot rolled steel sheet obtained in the hot rolling step, the time from the end of finish rolling to the start of the first cooling is within 20 s (seconds), and 10 ° C / s to 80 ° C. This is a step of performing the first cooling at the first cooling rate of / s until the first cooling end temperature of 600 ° C. to 700 ° C. is reached.
When the time from the end of finish rolling to the start of the first cooling is within 20 s, coarsening of crystal grains is suppressed, and as a result, deterioration of toughness of the finally obtained electric resistance welded steel pipe can be suppressed.
When the first cooling rate is 10 ° C./s or more, excessive generation of ferrite is suppressed, and excessive C concentration (carbon concentration) in the second phase is suppressed, and as a result, In the finally obtained electric resistance welded steel pipe, a ferrite fraction of 98% or less and a hardness difference of 100 Hv or less can be achieved.
When the first cooling rate is 80 ° C./s or less, the generation of ferrite is promoted, and the C concentration (carbon concentration) in the second phase proceeds to some extent, and as a result, the finally obtained electric charge is obtained. In the sewn steel pipe, a ferrite fraction of 80% or more and a hardness difference of 50 Hv or more can be achieved.

The first cooling may be water cooling or air cooling.
When the first cooling is water cooling, the first cooling rate is controlled by adjusting the water flow density of the cooling water.
When the first cooling is air cooling, the first cooling rate is controlled by adjusting the amount of cooling air.

(Second cooling step)
The second cooling step in the manufacturing method A is performed on the hot-rolled steel sheet subjected to the first cooling at a second cooling rate of 5 ° C./s to 30 ° C./s at a coiling temperature of 450 ° C. to 700 ° C. , The second cooling end temperature).
When the second cooling rate is 5 ° C./s or more, the hardness of the second phase increases to some extent, and as a result, a hardness difference of 50 Hv or more can be achieved.
When the second cooling rate is 30 ° C./s or less, an excessive increase in hardness of the second phase is suppressed, and as a result, a hardness difference of 100 Hv or less can be achieved.
When the coiling temperature is 450 ° C or higher, the production of martensite is suppressed, and as a result, in the finally obtained electric resistance welded steel pipe, increase in YS, increase in YR, and decrease in toughness of the base metal part are suppressed. To be done.
When the coiling temperature is 700 ° C. or lower, excessive formation of ferrite is suppressed, and excessive C concentration (carbon concentration) in the second phase is suppressed, and as a result, finally. In the obtained electric resistance welded steel pipe, a ferrite fraction of 98% or less and a hardness difference of 100 Hv or less can be achieved.

The second cooling may be water cooling or air cooling.
When the second cooling is water cooling, the second cooling rate is controlled by adjusting the water flow density of the cooling water.
When the second cooling is air cooling, the second cooling rate is controlled by adjusting the amount of cooling air.

(Winding process)
The winding step in the manufacturing method A is a step of obtaining a hot coil made of a hot rolled steel sheet by winding the hot rolled steel sheet subjected to the second cooling at the above winding temperature.
The winding step may be performed according to known conditions and is not particularly limited.

(Pipe making process)
In the pipe making process, hot-rolled steel sheet is unwound from the hot coil, and the unrolled hot-rolled steel sheet is roll-formed into an open tube. Is a step of obtaining an electric resistance welded steel pipe.
The pipe making step may be performed according to known conditions and is not particularly limited.

In addition, the pipe making process, if necessary,
Seam heat treatment of ERW welds;
Adjusting the shape of the electric resistance welded steel pipe with a sizer after the formation of the electric resistance welded portion (after the seam heat treatment when the seam heat treatment is performed as described above);
Etc. may be included.

In the manufacturing method A, heat treatment (heat treatment other than seam heat treatment) is not performed after pipe forming.
If heat treatment is performed after pipe making, the yielded elongation (yield elongation of 0.2% or more) is substantially increased in the finally obtained electric resistance welded steel pipe when a tensile test of the pipe axis method is performed. To be observed.
Further, when the heat treatment is performed after the pipe making, the difference in hardness becomes small, and the effect of lowering YR due to the anisotropic hardening characteristics disappears.

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

Examples of the present disclosure will be shown below, but the present disclosure is not limited to these examples.
Hereinafter, No. 1-No. No. 31 is an example within the scope of the present disclosure, and 32-No. 58 is a comparative example that is outside the scope of the present disclosure.

<Manufacture of ERW pipe>
According to the above-mentioned manufacturing method A, No. 1-No. 31 (Example) electric resistance welded steel pipes were obtained.
In addition, the chemical composition and / or the manufacturing conditions in the electric resistance welded steel pipes of the examples were changed, and No. 32-No. 58 (comparative examples) ERW steel pipes were obtained.
The details will be described below.

  Slabs having the chemical composition shown in Table 1 were produced (slab preparation step).

In Table 1, the numerical value shown in the column of each element is mass% of each element.
In Table 1, the blank column means that the corresponding element is not contained.
The balance excluding the elements shown in Table 1 is Fe and impurities.
In Table 1, No. The REM at 31 is Ce.
In Table 1, CNeq is the CNeq represented by the above formula (1), is the ratio of the Mn amount to the Si amount, and LR is the LR represented by the above formula (2).
The underline in Tables 1 and 2 indicates that it is outside the scope of the present disclosure or outside the range of the conditions of the production method A.

The slab obtained above was heated to the slab heating temperature shown in Table 2, and the heated slab was subjected to the hot rolling conditions shown in Table 2 (specifically, finish rolling start temperature, finish rolling end temperature, And a reduction ratio) to obtain a hot rolled steel sheet (hot rolling step).
Here, the reduction ratio means a cumulative reduction ratio in finish rolling.
For the hot-rolled steel sheet obtained in the hot-rolling step, the time (seconds) from the end of finish rolling to the start of cooling was adjusted as shown in the "time to cooling (s)" column in Table 2 and shown in Table 2. The first cooling was started at the first cooling rate shown.
The first cooling was performed until the first cooling end temperature of 600 ° C. to 700 ° C. was reached.
Immediately after the completion of the first cooling, the second cooling is performed at the second cooling rate shown in Table 2 until the winding temperature (CT; Coiling Temperature) shown in Table 2 is reached, and the winding is performed at this winding temperature. Thus, a hot coil made of a hot-rolled steel sheet having a plate thickness of 17.5 mm was obtained (first cooling step, second cooling step, and winding step).
Both the first cooling and the second cooling were water cooling, and the first cooling rate and the second cooling rate were both adjusted by adjusting the water flow density of the cooling water.
The hot rolling step, the first cooling step, the second cooling step, and the winding step described above were performed using a hot strip mill.

  Next, the hot rolled steel plate is unwound from the hot coil, and the unrolled hot rolled steel plate is roll-formed into an open pipe, and the butt portion of the obtained open pipe is electric resistance welded to form an electric resistance welded portion. After being formed, the seam heat treatment was applied to the electric resistance welded portion, and then the shape was adjusted using a sizer to obtain an as-roll electric resistance welded steel pipe having an outer diameter of 406 mm and a wall thickness of 17.5 mm. Pipe process).

<Measurement and confirmation>
The following measurements and confirmations were performed on the electric resistance welded steel pipe obtained above.

(Measurement of ferrite fraction and confirmation of second phase species)
The ferrite fraction (that is, the area ratio of the first phase to the entire metallographic structure) was measured and the second phase species (that is, the kind of the second phase) was confirmed by the above-described method.
The results are shown in Table 2.

  In Table 2, the notation with “P, B” in the second phase type column indicates that the second phase contains at least one of pearlite and bainite and does not substantially contain martensite (that is, relative to the entire second phase). The area ratio of martensite is less than 1%), and the expression "B + M" means a mixed structure of bainite and martensite (that is, the area ratio of martensite to the entire second phase is 1% or more). Means that.

(Measurement of hardness difference)
The hardness difference (that is, the value obtained by subtracting the hardness of the first phase from the hardness of the second phase) was measured by the method described above.
The results are shown in Table 2.

(Confirmation of TS, YS, YR, and yield elongation)
The TS, YS, YR, and yield elongation were confirmed by the method described above.
In the yield elongation column of Table 2, "N" means that the yield elongation was not substantially observed (that is, the yield elongation was less than 0.2%).
The results are shown in Table 2.

(VE of the base metal part, vE of the electric resistance welded part)
By the method described above, the vE (Charpy absorbed energy at 0 ° C.) of the base material portion and the electric resistance welded portion was measured.
The results are shown in Table 2.

  As shown in Tables 1 and 2, the electric resistance welded steel pipes of the respective examples (No. 1 to No. 31) are YS of 360 to 600 MPa and TS of 465 to 760 MPa even though they are as-made ERW steel pipes. , 0.90 or less, and excellent toughness of the base material portion and the electric resistance welded portion. Regarding the toughness, specifically, it was satisfied that the vE of the base material portion and the vE of the electric resistance welded portion were each 100 J or more.

The results of the comparative examples were as follows for each example.
No. In No. 32, the C content and LR were below the lower limits, so the work-hardening characteristics were low and YR was high.
No. In No. 33, since the C content exceeded the upper limit, a large amount of cementite was generated, and the toughness of the base material portion and the electric resistance welded portion deteriorated.
No. In No. 34, the Mn content was below the lower limit, so embrittlement due to S occurred, and the toughness of the base material portion deteriorated.
No. In No. 35, since the Mn content exceeded the upper limit, segregation of Mn in the central portion of the wall thickness and accompanying generation of MnS and a hardening phase deteriorated the toughness of the base material portion. Furthermore, since CNeq exceeded the upper limit, YS exceeded the upper limit.
No. In No. 36, the amount of Ti was less than the lower limit, so the crystal grain size became coarse, and the toughness of the base material portion and the electric resistance welded portion deteriorated.
No. In No. 37, since the Ti amount exceeded the upper limit, coarse TiN was generated, and the toughness of the base material portion and the electric resistance welded portion deteriorated.
No. In No. 38, since the Nb amount and the LR were below the lower limits, the toughness of the base material portion deteriorated.
No. In No. 39, since the Nb amount exceeded the upper limit, the toughness of the base material portion deteriorated due to the formation of coarse Nb carbide.
No. In No. 40, the Si content was below the lower limit, so deoxidation was insufficient, cracking due to free oxygen occurred, and the toughness of the base material portion and the electric resistance welded portion deteriorated.
No. In No. 41, the amount of Al was less than the lower limit, so cracking due to free oxygen occurred, and the toughness of the base material portion deteriorated.
No. In No. 42, the Al amount exceeded the upper limit, so Al-based oxide was generated in the electric resistance welded portion, and the toughness of the electric resistance welded portion deteriorated.
No. In No. 43, the P content exceeded the upper limit, so the grain boundary segregation of P deteriorated the toughness of the base material portion and the electric resistance welded portion.
No. In No. 44, since the S content exceeded the upper limit, coarse inclusions were generated, and the toughness of the base material portion and the electric resistance welded portion deteriorated.
No. In No. 45, since Mn / Si was below the lower limit, MnSi-based oxide was generated in the welded portion, and the toughness of the electric resistance welded portion was deteriorated.
No. In No. 46, since LR was below the lower limit, YR was high.
No. As for 47, YS exceeded the upper limit because CNeq exceeded the upper limit.
No. In No. 48, since CNeq was below the lower limit, YS and TS were insufficient.

No. In No. 49, the first cooling rate was lower than 10 ° C./s, the ferrite fraction exceeded the upper limit, and as a result, the carbon concentration in the second phase became excessive and the hardness difference exceeded the upper limit. For this reason, the toughness of the base material portion deteriorated.
No. In No. 50, since the first cooling rate exceeded 80 ° C./s, the ferrite fraction fell below the lower limit, and as a result, carbon concentration in the second phase was insufficient and the hardness difference fell below the lower limit. Therefore, YR has increased.
No. In No. 51, the second cooling rate was less than 5 ° C./s, the hardness of the second phase was insufficient, and the hardness difference was less than the lower limit. As a result, YR rose.
No. In No. 52, the second cooling rate exceeded 30 ° C./s, so the hardness of the second phase became too high, and the hardness difference exceeded the upper limit. As a result, the toughness of the base metal part deteriorated.
No. In No. 53, the toughness of the base material was deteriorated. This No. In No. 53, since it took a long time to start cooling, the crystal grains were coarsened, and as a result, the toughness of the base material portion was considered to be deteriorated.
No. In No. 54, the toughness of the base material was deteriorated. This No. It is considered that in No. 54, since the finish rolling start temperature was too high, the crystal grains were coarsened, and as a result, the toughness of the base material portion was deteriorated.
No. In No. 55, the toughness of the base material part was deteriorated. This No. In No. 55, since the finish rolling finish temperature was too high, the crystal grains were coarsened, and as a result, the toughness of the base material part was considered to be deteriorated.
No. In No. 56, the toughness of the base material was deteriorated. This No. In No. 56, it is considered that the grain ratio was coarsened because the reduction ratio was too low, and as a result, the toughness of the base material portion was deteriorated.
No. In No. 57, the winding temperature (CT) was too low, martensite was generated, and as a result, YS and TS exceeded the upper limits, and the toughness of the base material portion deteriorated.
No. In No. 58, the coiling temperature (CT) was too high, so the ferrite fraction exceeded the upper limit, and as a result, the carbon concentration in the second phase became excessive and the hardness difference exceeded the upper limit. For this reason, the toughness of the base material portion deteriorated.

Claims (3)

Including base material and ERW welded part,
The chemical composition of the base material part is mass%,
C: 0.03% or more and less than 0.10%,
Mn: 0.30 to 1.00%,
Ti: 0.005 to 0.050%,
Nb: 0.010 to 0.100%,
N: 0.001-0.020%,
Si: 0.010 to 0.500%,
Al: 0.001 to 0.100%,
P: 0 to 0.030%,
S: 0 to 0.010%,
Mo: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Cr: 0 to 0.50%,
V: 0 to 0.10%,
Ca: 0 to 0.0100%,
REM: 0 to 0.0100%, and
The balance: Fe and impurities, CNeq represented by formula (1) is 0.12 to 0.25, the ratio of Mn content to Si content is 1.8 or more, and formula (2) Has an LR of 0.25 or more,
In the metal structure of the base metal part, the area ratio of the first phase made of ferrite is 80 to 98%, the second phase which is the rest contains at least one of pearlite and bainite, and the whole second phase. The area ratio of martensite is less than 1%, the value obtained by subtracting the hardness of the first phase from the hardness of the second phase is 50 to 100 Hv,
The yield strength in the pipe axis direction is 360 to 600 MPa,
The tensile strength in the tube axis direction is 465 to 760 MPa,
The yield ratio in the tube axis direction is 0.90 or less,
Charpy absorbed energy at 0 ° C. of the base material portion and the electric resistance welded portion is 100 J or more,
ERW steel pipe for line pipe, which has a yield elongation of less than 0.2% in a tensile test in the pipe axis direction.
CNeq = C + Mn / 6 + Cr / 5 + (Ni + Cu) / 15 + Nb + Mo + V ... Formula (1)
LR = (2.1C + Nb) / Mn ... Formula (2)
[In the formulas (1) and (2), each element symbol means mass% of each element. ] The chemical composition of the base material part is mass%,
Mo: more than 0% and 0.50% or less,
Cu: more than 0% and 0.50% or less,
Ni: more than 0% and 0.50% or less,
Cr: more than 0% and 0.50% or less,
V: more than 0% and 0.10% or less,
Ca: more than 0% and 0.0100% or less, and
REM: The electric resistance welded steel pipe for a line pipe according to claim 1, containing at least one selected from the group consisting of more than 0% and 0.0100% or less.   The electric resistance welded steel pipe for a line pipe according to claim 1 or 2, having a wall thickness of 10 to 25.4 mm and an outer diameter of 254.0 to 660.4 mm.