JP6260757B1 - AZROLL ERW Steel Pipe and Hot Rolled Steel Sheet for Line Pipe - Google Patents

Abstract

The base material part is mass%, C: 0.030 to 0.120%, Si: 0.05 to 0.30%, Mn: 0.50 to 2.00%, Al: 0.010 to 0. 0. 035%, N: 0.0010 to 0.0080%, Nb: 0.010 to 0.080%, Ti: 0.005 to 0.030%, Ni: 0.001 to 0.20%, and Mo: 0.10 to 0.20%, the remainder contains Fe and impurities, the following F1 is 0.300 to 0.350, and in the metal structure of the thickness center of the base material, polygonal ferrite An as-roll electric-welded steel pipe for line pipes having a fraction of 60 to 90%, an average crystal grain size of 15 μm or less, and a coarse crystal grain size of 20% or less, which is the area ratio of crystal grains having a crystal grain size of 20 μm or more . F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3

Description

  The present disclosure relates to an as-roll electric-welded steel pipe for a line pipe and a hot-rolled steel sheet.

Conventionally, various studies have been made on steel pipes for line pipes used in the manufacture of pipelines and hot-rolled steel sheets used in the manufacture of steel pipes for line pipes.
For example, in Patent Document 1, as a high-strength hot-rolled steel sheet for spiral line pipes with excellent low-temperature toughness, C = 0.02 to 0.08%, Si = 0.05 to 0.5 in mass%. %, Mn = 1 to 2%, Nb = 0.03 to 0.12%, Ti = 0.005 to 0.05%, with the balance being Fe and inevitable impurity elements. In the microstructure at a depth of 1/2 the thickness from the surface of the steel sheet, the proeutectoid ferrite fraction is 3% or more and 20% or less, the other is a low temperature transformation phase and 1% or less pearlite, The overall number average crystal grain size is 1 μm or more and 2.5 μm or less, the area average grain size is 3 μm or more and 9 μm or less, and the standard deviation of the area average grain size is 0.8 μm or more and 2.3 μm or less. Steel plate at a depth of 1/2 to the wall thickness Hot-rolled steel sheet is disclosed which not less than 1.1 {211} direction and a {111} direction of the reflected X-ray intensity ratio {211} / {111} against a plane parallel to the plane.
Patent Document 1 describes that the hot-rolled steel sheet described in the same document can be used for the production of an electric resistance welded steel pipe or a spiral steel pipe.

  Patent Document 1: International Publication No. 2012/002481

  Among steel pipes for line pipes, welded steel pipes are manufactured using UOE steel pipes manufactured using thick plates (for example, thick plates having a thickness of 30 mm or more) or hot coils made of hot-rolled steel plates. ERW steel pipes or spiral steel pipes are used.

  Steel pipes for line pipes may be required to have low temperature toughness (hereinafter also simply referred to as “low temperature toughness”) evaluated by DWTT (Drop Weight Tear Test). Specifically, the lower the DWTT guaranteed temperature, which is the lowest temperature at which the ductile fracture surface ratio is 85% or more, the lower the low temperature toughness.

Low temperature toughness generally tends to be required for steel pipes for line pipes having a large wall thickness. A thick steel pipe for line pipe is advantageous in terms of strength but disadvantageous in terms of low temperature toughness.
Therefore, low temperature toughness has been attracting attention in the field of UOE steel pipes having a relatively thick wall.
On the other hand, little attention has been paid to low temperature toughness in the field of ERW steel pipes, which are relatively thin.

Further, the reason why low temperature toughness has attracted attention in the field of UOE steel pipe and low temperature toughness has received little attention in the field of ERW steel pipe has the following manufacturing reasons.
A thick plate process for manufacturing a thick plate that is a material of a UOE steel pipe has a relatively high degree of freedom in manufacturing conditions. For example, in the thick plate process, it is easy to perform low temperature rolling, and it is easy to perform complex control cooling for cooling after rolling. Therefore, in the field of UOE steel pipes, in order to improve the low temperature toughness of UOE steel pipes, it has been generally performed to finely adjust the metal structure by cold rolling, complicated control cooling, etc. in the thick plate process. .
On the other hand, the hot rolling process for manufacturing hot coils (specifically, hot rolled steel sheets in the form of hot coils), which is the material of ERW steel pipes, Compared with the process, the manufacturing conditions are less flexible. For example, in the hot rolling process, the hot rolled steel sheet after rolling is cooled to a winding temperature (CT) of, for example, about 400 to 600 ° C. and then wound in a coil shape. Due to this limitation, the hot rolling process is difficult to perform low-temperature rolling and complicated control cooling after rolling, as compared to the thick plate process. Under such circumstances, in the field of ERW steel pipes, it has been difficult to achieve the idea of performing fine adjustment of the metal structure in the hot rolling process in order to improve the low temperature toughness of the ERW steel pipe.

Moreover, even if the chemical composition is the same between the thick steel plate and its final product UOE steel pipe and the hot-rolled steel plate and its final product ERW steel pipe, the metal structure and / or strength are often quite different. . Under such circumstances, the problem (that is, low-temperature toughness) that has attracted attention in the UOE steel pipe is not necessarily noticed in the ERW steel pipe as well.
For example, in the thick plate process, the thick plate after cooling stop is air-cooled from both sides in the state of a single thick plate (not wound), so that the cooling rate at the time of air cooling is relatively fast. On the other hand, in the hot rolling process, the hot-rolled steel sheet after the cooling stop is air-cooled in the form of a wound hot coil, so that the cooling rate during air-cooling is relatively slow. In the hot rolling process, since the cooling rate when air-cooled in the form of a hot coil is slow, the metal structure may be substantially tempered while air-cooled in the form of a hot coil.

As described above, conventionally, low temperature toughness has attracted attention in the field of UOE steel pipes for line pipes, but little attention has been paid to low temperature toughness for ERW steel pipes for line pipes.
However, in recent years, the pipeline installation environment has become more severe; the progress of ERW steel pipe manufacturing technology has made it possible to manufacture thick ERW steel pipes; Low temperature toughness may also be required for ERW steel pipes for pipes.

The aforementioned Patent Document 1 is one of the few documents that focus on the low-temperature toughness of hot-rolled steel sheets that may be used in the production of ERW steel pipes.
However, the technique disclosed in Patent Document 1 may be required to further improve the low temperature toughness.

The present disclosure has been made in view of the above circumstances.
An object of the present disclosure is to provide an as-roll electric-welded steel pipe for line pipes excellent in low-temperature toughness evaluated by DWTT, and a hot-rolled steel sheet suitable for manufacturing the as-roll electric-welded steel pipe for line pipes.

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: 0.030 to 0.120%,
Si: 0.05-0.30%,
Mn: 0.50 to 2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010 to 0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005 to 0.030%,
Ni: 0.001 to 0.20%,
Mo: 0.10 to 0.20%,
V: 0 to 0.010%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
Cr: 0 to 0.30%,
Cu: 0 to 0.30%,
Mg: 0 to 0.0050%,
REM: 0 to 0.0100%, balance: Fe and impurities, F1 defined by the following formula (1) is 0.300 to 0.350,
In the metal structure of the central portion of the base metal part, the polygonal ferrite fraction is 60 to 90%, the average crystal grain size is 15 μm or less, and the crystal grain area ratio is 20 μm or more. A certain coarse crystal grain ratio is 20% or less,
An as-roll ERW steel pipe for line pipes with a yield ratio in the pipe axis direction of 80 to 95%.

F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 Formula (1)
[In Formula (1), C, Si, Mn, Ni, Cr, Mo, V, and Nb represent the mass% of each element, respectively. ]

<2> The chemical composition of the base material part is mass%,
V: more than 0% and 0.010% or less,
Ca: more than 0% and 0.0030% or less,
Cr: more than 0% and 0.30% or less,
Cu: more than 0% and 0.30% or less,
The as-rolled electric-welded steel pipe for line pipes according to <1>, containing one or more selected from the group consisting of Mg: more than 0% and 0.0050% or less and REM: more than 0% and 0.0100% or less.
<3> The as-roll electric-welded steel pipe for line pipes according to <1> or <2>, wherein the yield strength in the tube axis direction is 450 to 540 MPa and the tensile strength in the tube axis direction is 510 to 625 MPa.
<4> The as-roll electric-welded steel pipe for line pipes according to any one of <1> to <3>, having a wall thickness of 12 to 25 mm and an outer diameter of 304.8 to 660.4 mm.
<5> The as-roll electric-welded steel pipe for line pipes according to any one of <1> to <4>, wherein a yield ratio in the pipe axis direction is 80 to 93%.

<6> A hot-rolled steel sheet used for producing an as-roll electric-welded steel pipe for line pipes according to any one of <1> to <5>,
Chemical composition is mass%,
C: 0.030 to 0.120%,
Si: 0.05-0.30%,
Mn: 0.50 to 2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010 to 0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005 to 0.030%,
Ni: 0.001 to 0.20%,
Mo: 0.10 to 0.20%,
V: 0 to 0.010%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
Cr: 0 to 0.30%,
Cu: 0 to 0.30%,
Mg: 0 to 0.0050%,
REM: 0 to 0.0100%, balance: Fe and impurities, F1 defined by the above formula (1) is 0.300 to 0.350,
In the metal structure at the center of the wall thickness, the fraction of polygonal ferrite is 60 to 90%, the average crystal grain size is 15 μm or less, and the coarse crystal grain ratio is the area ratio of crystal grains having a crystal grain size of 20 μm or more. Is a hot-rolled steel sheet with 20% or less.
<7> The hot rolled steel sheet according to <6>, wherein the yield strength in the rolling direction is 450 to 500 MPa, and the tensile strength in the rolling direction is 510 to 580 MPa.

  According to the present disclosure, an as-roll electric-welded steel pipe for line pipes excellent in low-temperature toughness evaluated by DWTT and a hot-rolled steel sheet suitable for manufacturing the as-roll electric-welded steel pipe for line pipes are provided.

In an example of the metal structure of the base material part in this indication, it is a KAM map used for measurement of polygonal ferrite fraction. In an example of the metal structure of the base material part in this indication, it is a 15 degrees large angle grain boundary map used for measurement of an average crystal grain size and a coarse crystal grain rate. It is a scanning electron micrograph (SEM photograph; magnification 500 times) which shows an example of the metal structure of the base material part in this indication. It is a schematic front view of the tensile test piece in this indication. It is a continuous cooling transformation figure (CCT diagram) at the time of manufacturing the hot-rolled steel plate concerning an example of this indication. It is a schematic front view of the DWTT test piece in this indication.

In this 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) in the base material part may be referred to as “C content”. The content of other elements in the base material part may be described 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.

In the present specification, “azurole electric pipes for line pipes” may be simply referred to as “electric rolls” or “azurole electric pipes”.
In this specification, an as-rolled electric resistance welded steel pipe refers to an electric resistance steel pipe that has not been subjected to heat treatment other than seam heat treatment after pipe making.
In this specification, “pipe making” refers to a process from roll forming a hot-rolled steel sheet into an open pipe, and the process of forming the electro-welded section by electro-welding the butt portion of the obtained open pipe. Point to.
In this specification, “roll forming” refers to bending a hot-rolled steel sheet into an open tubular shape.

[AZROLL ERW Steel Pipe for Line Pipe]
The electric resistance welded steel pipe of the present disclosure (that is, an as-roll electric resistance welded steel pipe for line pipes) includes a base material portion and an electric resistance welded portion, and the chemical composition of the base material portion is C: 0.030-0. 120%, Si: 0.05 to 0.30%, Mn: 0.50 to 2.00%, P: 0 to 0.030%, S: 0 to 0.0100%, Al: 0.010 to 0 0.035%, N: 0.0010 to 0.0080%, Nb: 0.010 to 0.080%, Ti: 0.005 to 0.030%, Ni: 0.001 to 0.20%, Mo: 0.10 to 0.20%, V: 0 to 0.010%, O: 0 to 0.0030%, Ca: 0 to 0.0050%, Cr: 0 to 0.30%, Cu: 0 to 0 .30%, Mg: 0 to 0.0050%, REM: 0 to 0.0100%, and the balance: Fe and impurities, defined by the following formula (1) F1 is 0.300 to 0.350, and in the metal structure in the central portion of the base metal portion, the polygonal ferrite fraction is 60 to 90%, the average crystal grain size is 15 μm or less, The coarse crystal grain ratio, which is the area ratio of crystal grains having a grain size of 20 μm or more, is 20% or less, and the yield ratio in the tube axis direction is 80 to 95%.

F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 Formula (1)
[In Formula (1), C, Si, Mn, Ni, Cr, Mo, V, and Nb represent the mass% of each element, respectively. ]

In the electric resistance welded steel pipe of the present disclosure, the base metal portion refers to a portion of the electric resistance welded pipe other than the electric resistance welded portion and the heat affected zone.
Here, the heat affected zone (hereinafter also referred to as “HAZ”) is the influence of heat by electric resistance welding (however, when performing seam heat treatment after electric resistance welding, electric resistance welding and seam heat treatment). The part affected by the heat).

The electric resistance welded steel pipe of the present disclosure is excellent in low temperature toughness (that is, low temperature toughness evaluated by DWTT).
Such effects include the above-described chemical composition of the base material part (including that F1 is 0.300 to 0.350) and the above-described metal structure of the base material part (in summary, the crystal grains are refined). Is achieved by the metallographic structure).
The metal structure of the base metal part is achieved by the chemical composition and production conditions of the hot-rolled steel sheet as the material. The chemical composition of the base material part, the metal structure of the base material part, and preferable production conditions for the hot-rolled steel sheet will be described later.

The electric resistance welded steel pipe of the present disclosure is excellent in low temperature toughness as described above.
For this reason, the ERW steel pipe of this indication is suitable as one member for forming the submarine pipeline which receives the distortion repeatedly by waves, or one member for forming the line pipe for cold districts, for example.

Moreover, the electric resistance steel pipe of this indication has a yield ratio of 80-95% of a pipe axis direction.
When the yield ratio of the ERW steel pipe is 95% or less, a plastic deformation allowance required as a steel pipe for a line pipe is secured. Moreover, when the yield ratio of the ERW steel pipe is 95% or less, buckling when laying a pipeline formed using the ERW steel pipe by a reeling method or the like is further suppressed.
When the yield ratio of the ERW steel pipe is 80% or more, the suitability for manufacturing the ERW steel pipe is excellent.

<Chemical composition of base material part>
The chemical composition of the base material part in the present disclosure will be described.
Hereinafter, the chemical composition of the base material part in the present disclosure (including that F1 is 0.300 to 0.350) is also referred to as “chemical composition in the present disclosure”.

C: 0.030 to 0.120%
C increases the strength of the steel. If the C content is too low, this effect cannot be obtained. Therefore, the C content is 0.030% or more. The C content is preferably 0.035% or more, and more preferably 0.045% or more.
On the other hand, if the C content is too high, carbides are generated and the low temperature toughness and ductility of the steel are reduced. Therefore, the C content is 0.120% or less. The C content is preferably 0.110% or less.

  In the present specification, the term “strength” simply means tensile strength (hereinafter also referred to as “TS”) and / or yield strength (hereinafter also referred to as “YS”).

Si: 0.05-0.30%
Si deoxidizes steel. If the Si content is too low, this effect cannot be obtained. Accordingly, the Si content is 0.05% or more. The Si content is preferably 0.10% or more, and more preferably 0.15% or more.
On the other hand, if the Si content is too high, the low temperature toughness of the steel decreases. Accordingly, the Si content is 0.30% or less. Si content becomes like this. Preferably it is 0.25% or less, More preferably, it is 0.21% or less.

Mn: 0.50 to 2.00%
Mn increases the hardenability of the steel and increases the strength of the steel. If the Mn content is too low, this effect cannot be obtained. Therefore, the Mn content is 0.50% or more. The Mn content is preferably 0.80% or more, more preferably 1.00% or more.
On the other hand, if the Mn content is too high, the strength of the steel becomes too high, and the low temperature toughness of the steel decreases. Therefore, the Mn content is 2.00% or less. The Mn content is preferably 1.80% or less, more preferably 1.50% or less.

P: 0 to 0.030%
P is an impurity. P decreases the low temperature toughness of the steel. Therefore, it is preferable that the P content is small. Specifically, the P content is 0.030% or less. The P content is preferably 0.020% or less, and more preferably 0.015% or less.
On the other hand, the P content may be 0%. From the viewpoint of reducing the dephosphorization cost, the P content may be more than 0%, 0.001% or more, or 0.005% or more.

S: 0 to 0.0100%
S is an impurity. S combines with Mn to form a Mn-based sulfide. Therefore, when there is too much S content, the low temperature toughness and sour resistance of steel will fall. Accordingly, the S content is 0.0100% or less. S content becomes like this. Preferably it is 0.0080% or less, More preferably, it is 0.0050% or less.
On the other hand, the S content may be 0%. From the viewpoint of reducing the desulfurization cost, the S content may be more than 0%, 0.0001% or more, 0.0010% or more, or 0.0020% or more. Also good.

Al: 0.010-0.035%
Al deoxidizes steel. If the Al content is too low, this effect cannot be obtained. Therefore, the Al content is 0.010% or more. The Al content is preferably 0.015% or more, and more preferably 0.020% or more.
On the other hand, if the Al content is too high, the Al oxide becomes coarse, and the low temperature toughness of the steel decreases. Therefore, the Al content is 0.050% or less. Al content becomes like this. Preferably it is 0.040% or less, More preferably, it is 0.035% or less, More preferably, it is 0.030% or less.
In addition, Al content in this specification means content of all the Al in steel.

N: 0.0010 to 0.0080%
N forms nitrides and suppresses coarsening of austenite grains during the heating process. In this case, the austenite grains become finer in the rolling process, and the crystal grains after transformation become finer. As a result, the low temperature toughness of the steel is increased. N further increases the strength of the steel by solid solution strengthening. If the N content is too low, this effect cannot be obtained. Therefore, the N content is 0.0010% or more. The N content is preferably 0.0020% or more, and more preferably 0.0025% or more.
On the other hand, if the N content is too high, the carbonitrides become coarse and the low temperature toughness of the steel decreases. Accordingly, the N content is 0.0080% or less. N content becomes like this. Preferably it is 0.0070% or less, More preferably, it is 0.0060% or less, More preferably, it is 0.0050% or less.

Nb: 0.010 to 0.080%
Nb combines with C and N in steel to form fine Nb carbonitride. Nb carbonitride suppresses the coarsening of crystal grains and reduces the average crystal grain size. Therefore, the low temperature toughness of steel increases. Furthermore, fine Nb carbonitride increases the strength of the steel by dispersion strengthening. If the Nb content is too low, this effect cannot be obtained. Therefore, the Nb content is 0.010% or more. The Nb content is preferably 0.015% or more.
On the other hand, if the Nb content is too high, the Nb carbonitride becomes coarse and the low-temperature toughness of the steel decreases. Therefore, the Nb content is 0.050% or less. The Nb content is preferably 0.040% or less, and more preferably 0.030% or less.

Ti: 0.005 to 0.030%,
Ti combines with N in the steel to form TiN, and suppresses a decrease in the low temperature toughness of the steel due to the dissolved N. Furthermore, coarse TiN is suppressed by the fine TiN being dispersed and precipitated. This increases the low temperature toughness of the steel. If the Ti content is too low, this effect cannot be obtained. Accordingly, the Ti content is 0.005% or more. The Ti content is preferably 0.007% or more, and more preferably 0.010% or more.
On the other hand, if the Ti content is too high, TiN becomes coarse or coarse TiC is generated. In this case, the low temperature toughness of the steel decreases. Therefore, the Ti content is 0.030% or less. The Ti content is preferably 0.020% or less, more preferably 0.017% or less.

Ni: 0.001 to 0.20%
Ni increases the hardenability of the steel and increases the strength of the steel. If the Ni content is too low, this effect cannot be obtained. Therefore, the Ni content is 0.001% or more. Ni content becomes like this. Preferably it is 0.01% or more, More preferably, it is 0.05% or more, More preferably, it is 0.07% or more.
On the other hand, if the Ni content is too high, the above effect is saturated. Therefore, the Ni content is 0.20% or less. The Ni content is preferably 0.15% or less.

Mo: 0.10 to 0.20%,
Mo increases the hardenability of the steel and increases the strength of the steel. Mo further refines austenite grains and enhances the low temperature toughness of the steel. If the Mo content is too low, these effects cannot be obtained. Therefore, the Mo content is 0.10% or more. The Mo content is preferably 0.15% or more.
On the other hand, if the Mo content is too high, the on-site weldability of the steel decreases. Therefore, the Mo content is 0.20% or less. Mo content becomes like this. Preferably it is 0.19% or less, More preferably, it is 0.18% or less.

V: 0 to 0.010%
V is an arbitrary element. Therefore, the V content may be 0%.
V combines with C and N in the steel in the winding process to form fine carbonitrides and increases the strength of the steel. The fine V carbonitride further suppresses the coarsening of crystal grains and increases the low temperature toughness of the steel. From the viewpoint of these effects, the V content may be more than 0%, 0.001% or more, or 0.002% or more.
On the other hand, if the V content exceeds 0.010%, the low temperature toughness deteriorates due to the coarsening of the V carbonitride. Therefore, the V content is 0.010% or less.

O: 0 to 0.0030%
O is an impurity. O forms an oxide and lowers the hydrogen induced cracking resistance (hereinafter also referred to as “HIC resistance”) of the steel. O further reduces the low temperature toughness of the steel. Therefore, the O content is 0.0030% or less. The O content is preferably 0.0025% or less. The O content is preferably as low as possible.
On the other hand, the O content may be 0%. From the viewpoint of reducing the deoxidation cost, the O content may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, and may be 0.0015% or more. It may be 0.0020% or more.

Ca: 0 to 0.0050%
Ca is an arbitrary element. Therefore, the Ca content may be 0%.
Ca spheroidizes by controlling the form of MnS, thereby improving the low temperature toughness of the steel. From the viewpoint of such effects, the Ca content may be greater than 0%, may be 0.0001% or more, may be 0.0010% or more, and may be 0.0015% or more. It may be 0.0020% or more.
On the other hand, when the Ca content exceeds 0.0050%, coarse oxide inclusions are formed. Therefore, the Ca content is 0.0050% or less. The Ca content is preferably 0.0045% or less.

Cr: 0 to 0.30%
Cr is an arbitrary element. Therefore, the Cr content may be 0%.
Cr is an element that improves hardenability and increases the strength of steel. From the viewpoint of this effect, the Cr content may be more than 0% or 0.01% or more.
On the other hand, if the Cr content exceeds 0.30%, the hardenability becomes too high and the low temperature toughness of the steel decreases. Therefore, the Cr content is 0.30% or less. The Cr content is preferably 0.20% or less, more preferably 0.10% or less, and still more preferably 0.05% or less.

Cu: 0 to 0.30%
Cu is an arbitrary element. Therefore, the Cu content may be 0%.
Cu increases the hardenability of the steel and increases the strength of the steel. From the viewpoint of this effect, the Cu content may be more than 0%, 0.01% or more, 0.05% or more, or 0.10% or more. Also good.
On the other hand, if the Cu content is too high, the hardenability becomes too high and the low-temperature toughness of the steel decreases. Therefore, the Cu content is 0.30% or less. The Cu content is preferably 0.25% or less, and more preferably 0.20% or less.

Mg: 0 to 0.0050%
Mg is an arbitrary element and may not be contained. That is, the Mg content may be 0%.
When Mg is contained, Mg functions as a deoxidizing agent and a desulfurizing agent. Moreover, a fine oxide is produced and it contributes also to the improvement of the toughness of HAZ. From the viewpoint of these effects, the Mg content is preferably more than 0%, more preferably 0.0001% or more, and further preferably 0.0010% or more.
On the other hand, if the Mg content is too high, the oxide tends to agglomerate or coarsen, and as a result, the HIC resistance may be lowered, or the toughness of the base material or HAZ may be lowered. Therefore, the Mg content is 0.0050% or less. The Mg content is preferably 0.0030% or less.

REM: 0 to 0.0100%,
REM is an arbitrary element and may not be contained. That is, 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.
When REM is contained, REM functions as a deoxidizer and a desulfurizer. From the viewpoint of such effects, the REM content is preferably more than 0%, more preferably 0.0001% or more, and further preferably 0.0010% or more.
On the other hand, if the REM is too high, coarse oxides are formed, and as a result, the HIC resistance may be lowered, or the toughness of the base material or HAZ may be lowered. Therefore, the REM content is 0.0100% or less. The REM content is preferably 0.0070% or less, more preferably 0.0050% or less.

The chemical composition of the base metal part is as follows: V: more than 0% to 0.010% or less, Ca: more than 0% to 0.0030% or less, Cr: more than 0% to 0.30% or less, Cu: more than 0% to 0.30% Hereinafter, one or more selected from the group consisting of Mg: more than 0% and 0.0050% or less and REM: more than 0% and 0.0100% or less may be contained.
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 raw materials (for example, ore, scrap, etc.) or a component mixed in a manufacturing process and not intentionally contained in 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 impurities include B, Sb, Sn, W, Co, As, Pb, Bi, and H.
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.

F1: 0.300 to 0.350
In the chemical composition of the base material part, F1 defined by the following formula (1) is 0.300 to 0.350.

F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 Formula (1)
[In Formula (1), C, Si, Mn, Ni, Cr, Mo, V, and Nb represent the mass% of each element, respectively. ]

  Needless to say, when the chemical composition does not include any element corresponding to the element symbol in the formula (1), “0” is assigned to the corresponding element symbol in the formula (1).

F1 has a correlation with the metal structure (particularly the crystal grain size) of the base metal part.
If F1 is less than 0.300, polygonal ferrite grains (hereinafter also simply referred to as “ferrite grains”) may become coarse, resulting in an increase in the average crystal grain size and a mixed grain structure. As a result, the coarse grain ratio may increase. As a result, the low temperature toughness may deteriorate. On the other hand, if F1 is less than 0.300, the hardenability is lowered and sufficient strength may not be obtained. Therefore, F1 is 0.300 or more. F1 is preferably 0.305 or more.
On the other hand, if F1 is more than 0.350, the average ferrite grain size and / or coarse crystal grain fraction may be excessive due to the polygonal ferrite fraction being excessively small. As a result, the low temperature toughness may deteriorate. Therefore, F1 is 0.350 or less. F1 is preferably 0.345 or less, and more preferably 0.340 or less.

From the viewpoint of easily achieving that F1 is 0.300 to 0.350, F2 defined by the following formula (2) is 0.230 to 0.300 in the chemical composition of the base material part. Is more preferable, and 0.230 to 0.290 is more preferable.
When F2 is 0.230 or more, it is easier to achieve that F1 is 0.300 or more.
When F2 is 0.300 or less, it is easier to achieve that F1 is 0.350 or less.

F2 = Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 Formula (2)
[In Formula (2), Si, Mn, Ni, Cr, Mo, V, and Nb represent the mass% of each element, respectively. ]

  Needless to say, when the chemical composition does not include any element corresponding to the element symbol in the formula (2), “0” is assigned to the corresponding element symbol in the formula (2).

<Metallic structure of the central part of the base metal part>
Hereinafter, the metal structure of the central portion of the base material portion (hereinafter, also referred to as “metal structure of the base material portion”) will be described.
In the metal structure of the thickness center part of the base metal part, the polygonal ferrite fraction (hereinafter also simply referred to as “ferrite fraction”) is 60 to 90%, the average crystal grain size is 15 μm or less, The coarse crystal grain ratio, which is the area ratio of crystal grains having a diameter of 20 μm or more, is 20% or less.

Ferrite fraction: 60-90%
In the metal structure of the thickness center part of the base material part, the ferrite fraction (that is, the polygonal ferrite fraction) is 60 to 90%. That is, the metal structure at the center of the thickness of the base material part is a metal structure mainly composed of ferrite (ie, polygonal ferrite).
When the ferrite fraction is less than 60%, the average crystal grain size and / or the coarse crystal grain ratio becomes too large, and as a result, the low temperature toughness may deteriorate. When the ferrite fraction is 60% or more, the crystal grains are refined (specifically, the average crystal grain size and the coarse crystal grain ratio are reduced), and as a result, the low temperature toughness is increased. Therefore, the ferrite fraction is 60% or more. The ferrite fraction is preferably 65% or more, and more preferably 70% or more.
On the other hand, under the chemical composition in the present disclosure containing C, a metal structure having a ferrite fraction of 90% or less is easily formed. Therefore, the ferrite fraction in the metal structure at the center of the thickness of the base material is 90% or less. The ferrite fraction is preferably 85% or less.

Average crystal grain size: 15 μm or less The average crystal grain size is 15 μm or less in the metal structure at the center of the thickness of the base metal part.
When the average crystal grain size exceeds 15 μm, the low temperature toughness deteriorates. Therefore, the average crystal grain size is 15 μm or less, preferably 12 μm or less.
From the viewpoint of low temperature toughness, the lower limit of the average crystal grain size is not particularly limited. From the viewpoint of steel production suitability, the average crystal grain size is preferably 3 μm or more, more preferably 5 μm or more, and still more preferably 8 μm or more.

Coarse crystal grain ratio: 20% or less The coarse crystal grain ratio is 20% or less in the metal structure in the center of the thickness of the base metal part.
In this specification, the coarse crystal grain ratio means the area ratio of crystal grains having a crystal grain size of 20 μm or more as described above.
When the coarse crystal grain size ratio exceeds 20%, the low temperature toughness deteriorates. Accordingly, the coarse crystal grain size ratio is 20%. The coarse crystal grain size ratio is preferably 18% or less, and more preferably 15% or less.
From the viewpoint of low temperature toughness, the lower limit of the coarse crystal grain size rate is not particularly limited. From the viewpoint of steel production suitability, the coarse crystal grain size ratio is preferably 3% or more, more preferably 5% or more, and still more preferably 8% or more.

  In the present specification, the ferrite fraction (that is, polygonal ferrite fraction) means the area ratio of ferrite (that is, polygonal ferrite).

In this specification, confirmation of the metal structure of the thickness center part of a base material part is performed by confirming the metal structure of the thickness center part of the L cross section in the base material 90 degree position of an ERW steel pipe.
The base material 90 ° position refers to a position shifted by 90 ° in the pipe circumferential direction from the ERW weld.
The L cross section refers to a cross section parallel to the tube axis direction and the thickness direction.

The ferrite fraction is measured by the following method.
A sample for observing the central portion of the L cross section at the 90 ° position of the base material is taken from the ERW steel pipe. The observation surface of the collected sample is polished with a colloidal silica abrasive for 30 to 60 minutes. The polished observation surface was analyzed using EBSD-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy), and 200 μm (tube axis centered on the center of the L section at the 90 ° position of the base material) Direction) × 500 μm (thickness direction) area ratio of polygonal ferrite in the visual field range is determined and used as the ferrite fraction.
The field magnification (observation magnification) of EBSD-OIM is 400 times, and the measurement step is 0.3 μm.

Specifically, the ferrite fraction is obtained by a KAM (Kernel Average Misorientation) method equipped in the EBSD-OIM.
More specifically, first, the visual field range is divided into regular hexagonal pixels, and one regular hexagonal pixel in the visual field range is selected as the central pixel. A selected center pixel, six pixels outside the center pixel, twelve pixels further outside the six pixels, and eighteen pixels further outside the twelve pixels. , The orientation difference between each pixel in all 37 pixels. The average value of the obtained orientation differences is obtained and set as the KAM value of the center pixel. Similarly, a KAM value is obtained for each pixel included in the visual field range. These KAM value calculation methods are sometimes referred to as “third approximation”.
Based on the above results, a KAM map indicating the KAM value of each pixel included in the visual field range is created.
Based on the obtained KAM map, an area fraction of pixels having a KAM value of 1 ° or less with respect to the entire area of the visual field range is obtained as a ferrite fraction.
Here, the structure of a pixel having a KAM value of 1 ° or less is polygonal ferrite, and the structure of a pixel having a KAM value exceeding 1 ° is at least one of bainite and pearlite.

FIG. 1 is a KAM map used for measurement of the ferrite fraction in an electric resistance steel pipe according to an example of the present disclosure.
In FIG. 1, the KAM map is displayed in gray scale, but the KAM map is normally displayed in color.
In FIG. 1 displayed in gray scale, the black portion is polygonal ferrite. In this example, the area ratio of the black portion (polygonal ferrite) occupying the entire FIG. 1 (entire metal structure) is the polygonal ferrite fraction.

In this specification, the average crystal grain size and the coarse crystal grain ratio are measured by the EBSD-OIM method as follows.
In the same manner as the measurement of the ferrite fraction described above, a sample for observing the center of thickness of the L cross section at the base material 90 ° position is collected from the ERW steel pipe, and the observation surface of the sample collected is colloidal silica polished. Polish with an agent for 30-60 minutes.
The observation surface of the polished sample is analyzed by EBSD-OIM, and in the field of view of 200 μm (in the tube axis direction) × 500 μm (in the thickness direction) centered on the center of the L section at the 90 ° position of the base material The average crystal grain size is determined as the area average grain size at.
Further, the area ratio of the crystal grains having a crystal grain size of 20 μm or more (that is, coarse crystal grains) to the entire visual field range is obtained as the coarse crystal grain ratio.
The field magnification (observation magnification) of EBSD-OIM is 400 times, and the measurement step is 0.3 μm.

More specifically, in the measurement of the average crystal grain size, the orientation measurement is performed for each measurement step of 0.3 μm, and the position where the orientation difference between adjacent measurement points exceeds 15 ° is 15 ° large. Create a tilt boundary map. Here, 15 ° is a threshold of a high angle grain boundary, and is generally recognized as a crystal grain boundary.
Based on the created 15 ° large-angle grain boundary map, the region surrounded by the crystal grain boundary is used as the crystal grain, and the grain size and area of each crystal grain are obtained. Here, the grain size of each crystal grain is the equivalent circle diameter of each crystal grain.
The area average particle size is obtained based on the particle size and area of each crystal grain, and the obtained area average particle size is defined as the average crystal particle size.
Further, the area ratio of crystal grains having a crystal grain size of 20 μm or more with respect to the entire visual field range (that is, coarse crystal grains) is obtained, and the obtained area ratio is defined as the coarse crystal grain ratio.

FIG. 2 is a 15 ° large-angle grain boundary map used for measuring the average crystal grain size and the coarse crystal grain ratio in the ERW steel pipe according to an example of the present disclosure.
FIG. 2 shows a metal structure at the same location as in FIG.
In FIG. 2, fine crystal grains (that is, a small area) are ferrite grains, and crystal grains having a large area are bainite grains or pearlite grains.

In the electric resistance welded steel pipe of the present disclosure, it is preferable that the remaining portion (that is, the remaining portion other than polygonal ferrite) in the metal structure of the base material portion is composed of at least one of bainite and pearlite. Thereby, low temperature toughness improves compared with the case where a remainder contains a martensite, for example.
The concept of “bainite” in this specification includes bainitic ferrite, upper bainite, and lower bainite. In addition, the concept of “bainite” in the present specification further includes tempered bainite that is generated during air cooling after winding a hot-rolled steel sheet (that is, during air cooling in the form of a hot coil).
The concept of “perlite” in this specification includes pseudo pearlite.

The electric resistance welded steel pipe of the present disclosure is an as-roll electric resistance welded steel pipe (that is, an electric resistance welded steel pipe that has not been subjected to heat treatment other than seam heat treatment after pipe making). For this reason, the remainder tends to be at least one of bainite and pearlite.
Unlike ERW steel pipes (azurole ERW steel pipes) of the present disclosure, in ERW steel pipes formed by heat treatment other than seam heat treatment after pipe forming, martensite is formed as the metal structure of the base metal part. There is a case. The ERW steel pipe in this case tends to be inferior in low temperature toughness.

  FIG. 3 is a scanning electron micrograph (SEM photograph; magnification 500 times) showing an example of the metal structure of the base material part in the present disclosure.

Specifically, the SEM photograph shown in FIG. 3 was measured as follows.
From the ERW steel pipe according to an example of the present disclosure, a test piece for observing the central thickness portion in the L cross section at the base material 90 ° position was collected. The L section of the collected test piece was subjected to nital etching, and a photograph of the metal structure after the nital etching (hereinafter, also referred to as “metal structure photograph”) was obtained at a magnification of 500 times using a scanning electron microscope (SEM). Taken with

  FIG. 3 shows that the metal structure according to this example is a metal structure mainly composed of ferrite (that is, polygonal ferrite).

In addition, it can confirm that it is an as-roll ERW steel pipe when yield elongation is not observed when a pipe axial direction tensile test is done.
No yield elongation is observed in the as-rolled electric resistance welded steel pipe when a pipe axial direction tensile test is performed.
On the other hand, the yield elongation of the ERW steel pipe that has been subjected to heat treatment other than seam heat treatment (for example, tempering) after pipe forming is observed when a pipe axial direction tensile test is performed.

<Yield strength in the tube axis direction (YS)>
The electric resistance welded steel pipe of the present disclosure preferably has a yield strength (YS) in the pipe axis direction of 450 to 540 MPa.
When YS is 450 MPa or more, it is easier to satisfy the strength required for an electric resistance welded steel pipe for line pipes. YS is preferably 460 MPa or more, more preferably 480 MPa or more.
On the other hand, when YS is 540 MPa or less, it is advantageous in terms of bending deformability or buckling suppression when laying a pipeline formed using an electric resistance steel pipe for line pipe. YS is preferably 530 MPa or less, more preferably 520 MPa or less.

<Tensile strength (TS) in the tube axis direction>
The electric resistance welded steel pipe of the present disclosure preferably has a tensile strength (TS) in the pipe axis direction of 510 to 625 MPa.
When TS is 510 MPa or more, it is easier to satisfy the strength required for an electric resistance welded steel pipe for line pipes. TS is preferably 530 MPa or more, more preferably 540 MPa or more, and further preferably 545 MPa or more.
On the other hand, when TS is 625 MPa or less, it is advantageous in terms of bending deformability or buckling suppression when laying a pipeline formed using an electric resistance welded steel pipe. TS is preferably 620 MPa or less, more preferably 600 MPa or less, further preferably 590 MPa or less, and further preferably 575 MPa or less.

YS and TS are measured by the following method.
A full thickness tensile test piece is taken from the 90 ° position of the base material of the ERW steel pipe. Specifically, in the tensile test piece, the longitudinal direction of the tensile test piece is parallel to the tube axis direction of the ERW steel pipe, and the cross section of the tensile test piece (that is, the width direction and thickness of the tensile test piece). Samples are taken so that the shape of the cross section parallel to the direction is an arc.

FIG. 4 is a schematic front view of a tensile test piece used for the tensile test.
The unit of the numerical values in FIG. 4 is mm.
As shown in FIG. 4, the length of the parallel part of the tensile test piece is 50.8 mm, and the width of the parallel part is 38.1 mm.

In the present disclosure, a tensile test (that is, a tensile test in the tube axis direction) is performed at room temperature in accordance with the API standard 5CT, using the tensile test piece.
Based on the test results, YS and TS are obtained.

<Yield ratio in the tube axis direction (YR)>
As described above, the electric resistance welded steel pipe of the present disclosure has a yield ratio (YR = (YS / TS) × 100) in the pipe axis direction of 80 to 95%.
From the viewpoint of more effectively suppressing buckling when laying a pipeline formed using an electric-welded steel pipe for line pipes, YR is preferably 93% or less.
Further, from the viewpoint of further improving the production suitability of the electric resistance welded steel pipe, YR is preferably 84% or more.

<Wall thickness of ERW steel pipe>
The wall thickness of the electric resistance steel pipe of the present disclosure is preferably 12 to 25 mm.
When the thickness of the electric resistance steel pipe of the present disclosure is 12 mm or more, the strength of the electric resistance steel pipe is improved.
In general, as the wall thickness increases, brittle fracture tends to occur (that is, toughness decreases). However, the electric resistance welded steel pipe of the present disclosure exhibits excellent low temperature toughness even when the wall thickness is 12 mm or more.
Therefore, when the thickness of the electric resistance welded steel pipe of the present disclosure is 12 mm or more, the strength and the low temperature toughness are compatible at a higher level.
The wall thickness of the electric resistance welded steel pipe of the present disclosure is more preferably 14 mm or more, and further preferably 16 mm or more.

On the other hand, when the wall thickness is 25 mm or less, it is advantageous in terms of the suitability for producing an electric resistance welded steel pipe (specifically, the formability when roll forming a hot-rolled steel sheet as a material).
The wall thickness is preferably less than 25 mm, more preferably 22 mm or less, and even 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 304.8 to 660.4 mm (i.e., 12 to 26 inches).
When the outer diameter is 304.8 mm (that is, 12 inches) or more, the transport efficiency of fluid (for example, natural gas) is excellent. The outer diameter is preferably 355.6 mm (ie, 14 inches) or more, and more preferably 406.4 mm (ie, 16 inches) or more.
On the other hand, when the outer diameter is 609.6 mm (that is, 24 inches) or less, the suitability for producing an electric resistance welded steel pipe is excellent. The outer diameter is more preferably 508 mm (that is, 20 inches) or less.

[Hot rolled steel sheet]
Next, a hot-rolled steel sheet (hereinafter, also referred to as “hot-rolled steel sheet of the present disclosure”) suitable as a material for the electric-welded steel pipe of the present disclosure will be described.
The hot-rolled steel sheet of the present disclosure has the chemical composition in the present disclosure described above, the polygonal ferrite fraction is 60 to 90% in the metal structure at the center of the thickness, and the average crystal grain size is 15 μm. The coarse crystal grain ratio, which is the area ratio of crystal grains having a crystal grain diameter of 20 μm or more, is 20% or less.

  The preferable aspect of the chemical composition in the hot-rolled steel sheet of the present disclosure is the same as the preferable aspect of the chemical composition in the present disclosure described above (that is, the chemical composition in the base material part of the electric resistance welded steel pipe of the present disclosure). In the hot rolled steel sheet of the present disclosure, each preferred aspect of the polygonal ferrite fraction, the average crystal grain size, and the coarse crystal grain ratio is the polygonal ferrite fraction, the average crystal grain size, And it is the same as that of each preferable aspect of coarse grain ratio.

The form of the hot-rolled steel sheet of the present disclosure is preferably a hot coil wound in a coil shape.
The preferable range of the thickness (that is, the plate thickness) of the hot-rolled steel sheet of the present disclosure is the same as the preferable range of the thickness of the ERW steel pipe of the present disclosure.

The hot-rolled steel sheet of the present disclosure preferably has a yield strength (YS) in the rolling direction of 450 to 500 MPa and a tensile strength (TS) in the rolling direction of 510 to 580 MPa.
Here, the rolling direction in the hot rolled steel sheet coincides with the longitudinal direction in the hot rolled steel sheet unwound from the hot coil.
The measurement of YS and TS of the hot-rolled steel sheet is performed in the same manner as the measurement of TS and YS of the ERW steel pipe.

The YS of the hot rolled steel sheet is preferably 465 to 495 MPa.
The TS of the hot-rolled steel sheet is preferably 531 to 565 MPa.
The YR of the hot-rolled steel sheet is preferably 82 to 92%.

  When manufacturing the ERW steel pipe of the present disclosure using the hot rolled steel sheet of the present disclosure, YS and TS (particularly YS) are increased by roll forming the hot rolled steel sheet of the present disclosure.

[Example of manufacturing method of hot-rolled steel sheet]
Next, manufacturing method A of a hot-rolled steel sheet, which is an example of a preferable method for producing the hot-rolled steel sheet of the present disclosure, will be described.
The manufacturing method A of hot-rolled steel sheet is
Preparing a slab having a chemical composition in the present disclosure; and
Heating the prepared slab to a temperature of 1060 to 1200 ° C., hot rolling the heated slab to obtain a hot-rolled steel sheet; and
The hot-rolled steel sheet that has been hot-rolled is 580 to 680 at a cooling rate V1 of 5 ° C./s or more, with the time from the end of hot rolling (in detail, the end of finish rolling) to the start of strong cooling within 20 seconds. Strong cooling is performed until the strong cooling stop temperature T1 is 1, and then the slow cooling stop temperature T2 is 550 to 670 ° C. at a cooling rate V2 of 2 to 4 ° C./s (provided that T1> T2 is satisfied) ) Cooling step of gradually cooling until
Winding the gradually cooled hot-rolled steel sheet at a coiling temperature CT of 500 to 600 ° C. (provided that T2> CT is satisfied) to obtain a hot-rolled steel sheet in the form of a hot coil; ,
including.

In the manufacturing method A, the heating temperature of the slab means the surface temperature of the slab.
In the manufacturing method A, the temperature (FT, T1, T2, CT) of a hot-rolled steel sheet means the surface temperature of a hot-rolled steel sheet.
In production method A, the cooling rates (V1, V2) mean the cooling rate at the central portion of the wall thickness. The cooling rate (V1, V2) is obtained by heat conduction calculation.

  The chemical composition of the hot-rolled steel sheet in the form of a hot coil manufactured by the manufacturing method A can be regarded as the same as the chemical composition of the slab that is a raw material. The reason is that each process in the manufacturing method A does not affect the chemical composition of steel.

According to the manufacturing method A, a metal structure mainly composed of ferrite and having fine crystal grains can be formed.
Therefore, according to the manufacturing method A, the present disclosure in which the ferrite fraction is 60 to 90%, the average crystal grain size is 15 μm or less, and the coarse crystal grain ratio is 20% or less in the metal structure in the central portion of the thickness. Can be manufactured.

The reason why the metal structure mainly composed of ferrite and having a refined crystal grain can be formed by the manufacturing method A is as follows.
In the manufacturing method A, the heating temperature in the hot rolling step is set to 1200 ° C. or less, so that coarsening of crystal grains (specifically, austenite grains in the heated stage) is suppressed.
Furthermore, the hot-rolled steel sheet formed in the hot-rolling process is cooled at 5 ° C./s or more in the cooling process with the time from the end of hot rolling (in detail, finishing rolling finished) to the start of strong cooling within 20 seconds. A large number of nucleation sites are generated in the unrecrystallized structure of the hot-rolled steel sheet by vigorously cooling at the speed V1 until the strong cooling stop temperature T1 is 580 to 680 ° C.
By slowly cooling the hot-rolled steel sheet that has been strongly cooled under the above conditions, and then winding it under the above conditions, fine ferrite grains are generated from each of a large number of nucleation sites generated during strong cooling, and polygonal ferrite. A main metal structure is formed.
For the above reasons, it is considered that according to the production method A, a metal structure mainly composed of ferrite and having a refined crystal grain (specifically, ferrite grains) can be formed.

  On the other hand, when the metal structure is mainly bainite, laths (elongated structures) are generated in the crystal grains that inherited the prior austenite grains as they are, but the orientation of these laths is aligned for each block, and each block is It becomes substantially one crystal grain. Therefore, the size of the crystal grains in the bainite-based metal structure is determined by the size of the prior austenite grains. Therefore, when the metal structure is mainly bainite, the crystal grains are likely to be coarsened.

  Next, the reason why fine ferrite grains are formed by the manufacturing method A will be described in more detail using a continuous cooling transformation diagram (CCT diagram) of a hot-rolled steel sheet.

FIG. 5 is a continuous cooling transformation diagram (CCT diagram) of a hot-rolled steel sheet in production method A.
In FIG. 5, F represents a ferrite region, P represents a pearlite region, B represents a bainite region, Ar ₃ represents an Ar ₃ transformation temperature, and Ms represents a temperature at which martensite begins to be generated.
As shown in FIG. 5, the ferrite region exists at a position where the temperature is higher than that of the pearlite region and the bainite region.
In this example, the finish rolling temperature (that is, finish rolling end temperature) is a temperature equal to or higher than the Ar ₃ transformation temperature.
The hot-rolled steel sheet after finish rolling is cooled from a temperature equal to or higher than the Ar ₃ transformation temperature.

A broken line C1 in FIG. 5 is a cooling curve when the hot-rolled steel sheet is cooled under conventional cooling conditions.
Under conventional cooling conditions, all of the ferrite region, pearlite region, and bainite region are routed. For this reason, the ferrite fraction in a metal structure becomes low. For example, it becomes a metal structure mainly composed of bainite.

In contrast to the conventional cooling conditions, in the cooling process in manufacturing method A, the hot-rolled steel sheet is cooled along a cooling curve indicated by a broken line C2.
Specifically, in the cooling process in the manufacturing method A, the cooling rate V1 of 5 ° C./s or higher is set for a hot-rolled steel sheet within 20 seconds from the end of hot rolling (more specifically, finishing rolling) to the start of strong cooling. Then, strong cooling is performed until a strong cooling stop temperature T1 of 580 to 680 ° C. is reached (S31 in FIG. 5). The strong cooling stop temperature T1 is located in the vicinity of the ferrite nose. When the steel is cooled rapidly by strong cooling, numerous strains are created in the steel, resulting in numerous nucleation sites in the unrecrystallized structure.
After the strong cooling, the hot-rolled steel sheet is gradually cooled until it reaches a slow cooling stop temperature T2 of 550 to 670 ° C. (however, T1> T2 is satisfied) (S32 in FIG. 5). By setting the slow cooling stop temperature T2 to the above temperature, the temperature of the steel is held in the ferrite region in FIG. Thereby, fine ferrite grains are generated from each of a large number of nucleation sites generated during strong cooling.
As a result, a metal structure mainly composed of fine ferrite grains (specifically, a metal structure having a high ferrite fraction and fine crystal grains) is formed.

Here, F1 defined by the above-mentioned formula (1) affects the position of the S curve of each phase of ferrite, pearlite, and bainite in the CCT diagram.
As for the chemical composition in this indication, F1 is 0.300-0.350 as above-mentioned.
Thereby, as shown in FIG. 5, the S curve of each phase is arrange | positioned in a moderate position in a CCT diagram. For this reason, the hot-rolled steel sheet is cooled mainly through the ferrite region as indicated by a cooling curve C2 in FIG.
As a result, the ferrite fraction in the structure is increased and the crystal grains (that is, ferrite grains) are refined.

When F1 is less than 0.300, the S curve of each phase is shifted too much to the left. In this case, the temperature of the steel enters the ferrite region before the nucleation sites are sufficiently generated in the cooling process. As a result, the ferrite grains become coarse and the average crystal grain size increases. Furthermore, since it tends to be a mixed grain structure, the coarse crystal grain ratio increases.
On the other hand, when F1 exceeds 0.350, the S curve of each phase is shifted to the right side. In this case, the cooling curve C2 becomes difficult to pass through the ferrite region. As a result, the generation amount of structures other than ferrite (pearlite, bainite, etc.) increases, and the ferrite fraction in the structure decreases.

  Hereinafter, each process of the manufacturing method A is demonstrated.

<Preparation process>
The preparation process in the manufacturing method A is a process of preparing the slab which has the chemical composition in this indication.
The step of preparing the slab may be a step of manufacturing a slab, or a step of simply preparing a slab that has been manufactured in advance.
When manufacturing a slab, the molten steel which has the above-mentioned chemical composition is manufactured, for example, and a slab is manufactured using the manufactured molten steel. At this time, the slab may be manufactured by a continuous casting method, or the ingot may be manufactured using molten steel, and the ingot may be subjected to partial rolling to manufacture the slab.
The chemical composition of the slab can be regarded as the same as the chemical composition of the molten steel that is the raw material. The reason is that the process of manufacturing the slab does not affect the chemical composition of the steel.

<Hot rolling process>
The hot rolling step in production method A is a step of heating the slab to a temperature of 1060 to 1200 ° C. and hot rolling the heated slab to obtain a hot rolled steel sheet.

When the temperature for heating the slab (hereinafter also referred to as “heating temperature”) is 1200 ° C. or less, the austenite grains can be refined. The heating temperature is preferably 1180 ° C. or lower.
Moreover, refinement | miniaturization of the crystal grain during rolling is realizable because heating temperature is 1060 degreeC or more. Moreover, when the heating temperature is 1060 ° C. or higher, precipitation strengthening after rolling can be realized, and as a result, the strength of the hot-rolled steel sheet can be improved. From the viewpoint of these effects, the heating temperature is preferably 1100 ° C. or higher.

In the manufacturing method A, the heating temperature of the slab means the surface temperature of the slab.
In the manufacturing method A, the temperature (FT, T1, T2, CT) of a hot-rolled steel sheet means the surface temperature of a hot-rolled steel sheet.
In the production method A, the cooling rates (V1, V2) mean the cooling rate at the central portion of the thickness, which is obtained by heat conduction calculation.

Hot rolling is performed by subjecting the slab heated to the heating temperature to rough rolling and finish rolling in this order.
Rough rolling and finish rolling are performed using a rough rolling mill and a finish rolling mill, respectively. Both the rough rolling mill and the finish rolling mill include a plurality of rolling stands arranged in a row, and each rolling stand includes a roll pair.
The following finish rolling temperature FT (that is, finish rolling finish temperature) is the surface temperature of the hot-rolled steel sheet on the exit side of the final stand of the finish rolling mill.

The finish rolling temperature FT (° C.) is preferably not less than the Ar ₃ transformation temperature from the viewpoint of reducing the rolling resistance and improving the productivity. When the finish rolling temperature (° C.) is equal to or higher than the Ar ₃ transformation temperature, the phenomenon of rolling in the two-phase region of ferrite and austenite is suppressed, and the formation of lamellar structure and the deterioration of mechanical properties accompanying this phenomenon can be suppressed. .
With the chemical composition in the present disclosure, the Ar ₃ transformation temperature can be 750 or higher.

  In hot rolling, the rolling reduction in the austenite non-recrystallization temperature region is preferably 60 to 80%. In this case, the unrecrystallized structure is refined.

<Cooling process>
The cooling step in the production method A is performed at a temperature of 5 ° C./s or more with the hot-rolled steel sheet obtained in the hot-rolling step being within 20 seconds from the end of hot rolling (more specifically, finishing rolling) to the start of strong cooling. At the cooling rate V1, strong cooling is performed until the strong cooling stop temperature T1 is 580 to 680 ° C., and then the slow cooling stop temperature T2 is 550 to 670 ° C. (however, T1> T2 is satisfied) This is a step of slow cooling.
The cooling process in the manufacturing method A is performed on a ROT (runout table).
Hereinafter, the cooling step in the production method A may be referred to as “ROT cooling”.

The surface temperature of the steel plate immediately before strong cooling is not particularly limited, but is preferably not less than the Ar ₃ transformation temperature. If the surface temperature of the steel sheet immediately before the strong cooling is equal to or higher than the Ar ₃ transformation temperature, the crystal grains can be prevented from becoming coarse and the strength from being lowered.

  Strong cooling starts within 20 seconds (more preferably within 10 seconds) from the end of hot rolling (more specifically, finishing rolling).

Strong cooling is performed at a cooling rate V1 of 5 ° C./s or higher.
Here, the cooling rate V1 is a cooling rate at the central portion of the wall thickness. The cooling rate V1 is a value calculated by heat conduction.
When the cooling rate V1 is 5 ° C./s or more, the degree of supercooling due to cooling becomes sufficient, and as a result, sufficient ferrite nucleation sites are obtained.
The cooling rate V1 is preferably 7 ° C./s or higher, more preferably 8 ° C./s or higher.

The strong cooling is performed until a strong cooling stop temperature T1 of 580 to 680 ° C. is reached.
Since the strong cooling stop temperature T1 is 580 ° C. or higher, in the CCT diagram, the phenomenon that the temperature of the hot-rolled steel sheet passes through the ferrite region and reaches the pearlite region and / or the bainite region can be suppressed. It is easy to achieve 60% or more. The strong cooling stop temperature T1 is preferably 600 ° C. or higher, more preferably 610 ° C. or higher.
Moreover, when the strong cooling stop temperature T1 is 680 ° C. or less, the phenomenon that the precipitation of Nb strengthening the pro-eutectoid ferrite becomes over-aged can be suppressed, and as a result, the strength reduction of the hot-rolled steel sheet can be suppressed. The strong cooling stop temperature T1 is preferably 670 ° C. or lower, more preferably 655 ° C. or lower.

Strong cooling is preferably performed by water cooling.
The strong cooling is performed by, for example, using a water cooling device and increasing the water flow density in the water cooling device higher than the normal condition.

  In other words, the strong cooling stop temperature T1 is the slow cooling start temperature.

  In the cooling step, the strongly cooled hot-rolled steel sheet is gradually cooled until it reaches a slow cooling stop temperature T2 of 550 to 670 ° C. (however, T1> T2 is satisfied).

The slow cooling is preferably performed at a cooling rate V2 of 2 to 4 ° C./s.
When the cooling rate V2 is 2 ° C./s or more, the slow cooling stop temperature T2 and the coiling temperature CT can be further lowered, so that crystal grain coarsening can be suppressed.
When the cooling rate V2 is 4 ° C./s or less, the phenomenon that the temperature of the hot-rolled steel sheet passes through the ferrite region and reaches the pearlite region and / or the bainite region in the CCT diagram can be suppressed. % Or more is easy to achieve.

The gradual cooling is performed until a gradual cooling stop temperature T2 of 550 to 670 ° C. (however, T1> T2 is satisfied).
When the slow cooling stop temperature T2 is 550 ° C. or higher, in the CCT diagram, the phenomenon that the temperature of the hot-rolled steel sheet passes through the ferrite region and reaches the pearlite region and / or the bainite region can be suppressed. % Or more is easy to achieve. The slow cooling stop temperature T2 is preferably 580 ° C. or higher, more preferably 590 ° C. or higher.
When the slow cooling stop temperature T2 is 670 ° C. or lower, the coarsening of crystal grains can be suppressed. The slow cooling stop temperature T2 is preferably 650 ° C. or lower, more preferably 635 ° C. or lower, and further preferably 620 ° C. or lower.

The slow cooling is preferably performed by water cooling.
The slow cooling is performed, for example, by using a water cooling device and setting the water flow density in the water cooling device to be lower than the water flow density at the time of strong cooling.

<Winding process>
The winding process in the manufacturing method A is the form of a hot coil by winding the hot-rolled steel sheet cooled in the cooling process at a winding temperature CT of 500 to 600 ° C. (provided T2> CT is satisfied). It is the process of obtaining the hot-rolled steel plate.

  The cooling rate at the time of cooling from the slow cooling stop temperature T2 to the coiling temperature CT is preferably 0.1 to 1.5 ° C / s, more preferably 0.3 to 1.5 ° C / s. Yes, more preferably 0.5 to 1.5 ° C./s.

The winding temperature CT is 500 to 600 ° C.
When the coiling temperature CT is 500 ° C. or higher, in the CCT diagram, the phenomenon that the temperature of the hot-rolled steel sheet passes through the ferrite region and reaches the pearlite region and / or the bainite region can be suppressed, so the ferrite fraction is 60%. It is easy to achieve the above. Thereby, it is easy to achieve an average crystal grain size of 15 μm or less and a coarse crystal grain ratio of 20% or less. The winding temperature CT is preferably 510 ° C. or higher, more preferably 520 ° C. or higher.
When the coiling temperature CT is 580 ° C. or less, the coarsening of ferrite grains can be suppressed. Thereby, it is easy to achieve an average crystal grain size of 15 μm or less and a coarse crystal grain ratio of 20% or less. The winding temperature CT is preferably 590 ° C. or lower, and more preferably 580 ° C. or lower.

[Example of manufacturing method of ERW steel pipe]
Next, as an example of a preferred method for producing the electric resistance welded steel pipe of the present disclosure, a method X for producing the electric resistance welded steel pipe will be described.
The manufacturing method X of ERW steel pipe is
The step of preparing the hot-rolled steel sheet of the present disclosure described above (hereinafter, also referred to as “hot-rolled steel sheet preparation step”),
A process of obtaining an electric-welded steel pipe by forming an electric-welded welded portion by electro-welding the butt portion of the obtained open pipe by roll forming the hot-rolled steel sheet (hereinafter referred to as “pipe-forming process”). ")
including.

  The tube forming process in the production method X does not affect the chemical composition, the polygonal ferrite fraction, the average crystal grain size, and the coarse crystal grain ratio. Therefore, the electric resistance welded steel pipe of the present disclosure is manufactured by the manufacturing method X using the hot-rolled steel sheet of the present disclosure.

It is preferable that a hot-rolled steel plate preparation process is a process of preparing the hot-rolled steel plate of this indication which is a form of a hot coil.
In this case, in the tube forming step, the hot rolled steel sheet of the present disclosure is unwound from a hot coil, and the unrolled hot rolled steel sheet of the present disclosure is roll-formed.

The hot-rolled steel sheet preparation step may be a process of manufacturing the hot-rolled steel sheet of the present disclosure (preferably, the hot-rolled steel sheet of the present disclosure in the form of a hot coil), or the hot-rolled steel sheet of the present disclosure that has been manufactured in advance. It may be a step of merely preparing a steel plate (preferably, a hot-rolled steel plate of the present disclosure in the form of a hot coil).
In any case, the hot-rolled steel sheet of the present disclosure in the form of a hot coil is preferably manufactured according to the manufacturing method A described above.

  There is no restriction | limiting in particular in each operation in a pipe making process, According to a well-known method, it can carry out.

The manufacturing method X of an electric resistance steel pipe may include other steps as necessary.
Examples of the other steps include a step of performing seam heat treatment on the ERW welded portion of the ERW steel pipe after the pipe making step, a step of adjusting the shape of the ERW steel pipe with a sizing roll after the pipe making step, and the like.

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

[Examples 1 to 13 and Comparative Examples 1 to 8]
<Manufacture of slabs and hot coils>
Slabs were produced by continuously casting molten steel having the chemical compositions of Steel A to Steel O shown in Table 1.
Specifically, REM in steel J is Ce.

The slab was heated in a heating furnace.
The heating temperature (° C.) of the slab was as shown in Table 2. The heated slab was rolled using a roughing mill and allowed to cool to 920 ° C.
Thereafter, finish rolling was performed with a finish rolling mill. The rolling reduction in the non-recrystallization temperature range was 60 to 80% in all examples and comparative examples. The finish rolling temperature was Ar ₃ or higher (specifically, 750 ° C. or higher) in any of the examples and comparative examples.

ROT cooling (that is, cooling process) was performed on the steel sheet after finish rolling.
ROT cooling was performed by sequentially applying strong cooling and gradual cooling to the hot-rolled steel sheet obtained by finish rolling.
The time from the end of finish rolling to the start of strong cooling was within 10 seconds.
Both strong cooling and gradual cooling were performed using a water cooling device. The cooling rate V1 in the strong cooling and the cooling rate V2 in the slow cooling were both adjusted by adjusting the water flow density in the water cooling device.
The cooling rate V1 (° C./s), the strong cooling stop temperature T1 (° C.), and the slow cooling stop temperature T2 (° C.) in the strong cooling were as shown in Table 2.
The cooling rate V2 (° C./s) in the slow cooling was in the range of 2 to 4 ° C./s in all examples.

The hot rolled steel sheet after ROT cooling was allowed to cool and wound at the winding temperature CT shown in Table 2 to obtain a hot coil (that is, a hot rolled steel sheet in the form of a hot coil).
The cooling rate in cooling from the slow cooling stop temperature T2 (° C.) to the coiling temperature CT was estimated to be 0.5 to 1.5 ° C./s in any of the examples and comparative examples.

<Manufacture of ERW steel pipe>
The hot rolled steel sheet is unwound from the hot coil, the rolled hot rolled steel sheet is formed into an open pipe, and the butt portion of the obtained open pipe is electro-welded to form an electro-welded weld. An ERW steel pipe (hereinafter, also referred to as “ERW pipe before shape adjustment”) was obtained.
Next, the ERW welded portion of the ERW steel pipe before the shape adjustment is subjected to seam heat treatment, and then the shape is adjusted by a sizing roll, whereby an ERW steel pipe having an outer diameter of 406.4 mm and a wall thickness of 17 mm (that is, an as-roll ERW steel pipe). )

  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 molten steel as a raw material.

<YS, TS and YR of hot-rolled steel sheet>
The hot-rolled steel sheet was unwound from the hot coil, and the rolled hot-rolled steel sheet was subjected to a tensile test in the rolling direction, thereby measuring YS in the rolling direction and TS in the rolling direction. Furthermore, YR (%) in the rolling direction was calculated based on YS in the rolling direction and TS in the rolling direction.
The results are shown in Table 2.

  Here, the tensile test piece of full thickness used for the measurement of YS and TS is a position where the distance from one end in the plate width direction of the hot-rolled steel plate is 1/4 of the plate width (that is, the base material 90 ° in the ERW steel pipe). From the position corresponding to the position).

<Measurement and evaluation of ERW steel pipe>
The following measurements and evaluations were performed on the ERW steel pipe after shape adjustment by the sizing roll.
The results are shown in Table 2.

(YS, TS and YR)
About the electric resistance welded steel pipe after shape adjustment by a sizing roll, the tensile test of the pipe axis direction was performed, and YS of the pipe axis direction and TS of the pipe axis direction were measured. The detailed measurement method is as described above. Further, YR (%) in the rolling direction was calculated based on YS in the tube axis direction and TS in the tube axis direction.

  In the tensile test in the tube axis direction in the measurement of YS and TS, no yield elongation was observed in any of the examples and comparative examples. That is, it was confirmed that the electric resistance welded steel pipes of any of the examples and the comparative examples were as roll electric resistance welded steel pipes.

(Ferrite fraction, average crystal grain size, and coarse crystal grain ratio)
About ERW steel pipe after shape adjustment by sizing roll, ferrite fraction, average crystal grain size, and coarse grain ratio in metal structure of thick central part of base metal part using EBSD-OIM and by the method described above Was measured respectively.
As analysis software in EBSD-OIM, “TSL OIM Analysis 7” manufactured by TSL Solutions was used.

Further, in the measurement of the ferrite fraction, the type of the remainder (that is, the structure other than polygonal ferrite) in the metal structure at the center of the thickness of the base metal part was also confirmed.
In Table 2, the notation “B, P” means at least one of bainite and pearlite.

(Average crystal grain size, coarse crystal grain ratio)
By the method described above, the average crystal grain size and the coarse crystal grain ratio of the central portion of the base metal part in the ERW steel pipe after the shape adjustment by the sizing roll were measured.

(Evaluation of low temperature toughness (DWTT guaranteed temperature measurement))
A full-thickness DWTT test piece was prepared by collecting an arc-shaped member from the ERW steel pipe after shape adjustment by a sizing roll and processing the collected arc-shaped member into a flat plate shape.

FIG. 6 is a schematic front view of the produced DWTT test piece.
The unit of the numerical values in FIG. 6 is mm.
Here, the longitudinal direction (direction of length 300 mm) of the DWTT test piece corresponds to the pipe circumferential direction of the ERW steel pipe. The center part in the longitudinal direction of the DWTT test corresponds to the 90 ° position of the base material of the ERW steel pipe.
As shown in FIG. 6, the DWTT test piece was provided with a notch having a depth of 5 mm at the center in the longitudinal direction.

Using the above DWTT test piece, a DWTT test was performed in accordance with the provisions of ASTM E 436, and a DWTT guaranteed temperature, which is the lowest temperature at which the ductile fracture surface ratio becomes 85% or more, was determined.
The lower the DWTT guaranteed temperature, the better the low temperature toughness.

As shown in Table 1 and Table 2, it satisfies the chemical composition of the base material part (including that F1 is 0.300 to 0.350) in the present disclosure, and the metal structure of the central part of the base material part. The ERW steel pipe of each example having an F fraction of 60 to 90%, an average crystal grain size of 15 μm or less, and a coarse crystal grain ratio of 20% or less has a low DWTT guaranteed temperature and low temperature toughness It was excellent.
Moreover, it was confirmed that the ERW steel pipe of each Example has YR in the range of 80 to 95%, and a plastic deformation allowance required as a steel pipe for line pipes is secured.

  In each of Examples, in Comparative Examples 1 and 7 in which F1 was less than 0.300, the average crystal grain size was excessive and the DWTT guaranteed temperature was too high (that is, the low temperature toughness was inferior). The reason why the average crystal grain size was excessive is considered that the ferrite grains were coarsened because F1 was less than 0.300.

Further, in Comparative Examples 2 and 8 in which F1 was more than 0.350, the F fraction was too low and the DWTT guaranteed temperature was too high (that is, the low temperature toughness was inferior).
In these Comparative Examples 2 and 8, the average crystal grain size and the coarse crystal grain ratio were excessive. The reason for this is considered to be that the F fraction is too low because F1 exceeds 0.350.

Further, in Comparative Example 3, the average crystal grain size and the coarse crystal grain ratio were excessive, and the DWTT guaranteed temperature was too high (that is, the low temperature toughness was inferior).
The reason why the average crystal grain size and the coarse crystal grain ratio were excessive in Comparative Example 3 is considered that the austenite grains became coarse during the heating of the slab because the heating temperature of the slab was too high.

In Comparative Example 4, the average crystal grain size and the coarse crystal grain ratio were excessive, and the DWTT guaranteed temperature was too high (that is, the low temperature toughness was inferior).
The reason why the average crystal grain size and the coarse crystal grain ratio were excessive in Comparative Example 4 is considered to be because the effect of refining crystal grains by rolling was insufficient because the heating temperature of the slab was too low.

Moreover, in the comparative example 5, F fraction was too low and DWTT guarantee temperature was too high (namely, it was inferior to low temperature toughness).
The reason why the F fraction was too low in Comparative Example 5 is considered to be that the strong cooling stop temperature T1, the slow cooling stop temperature T2, and the winding temperature CT were too low.

In Comparative Example 6, the average crystal grain size and the coarse crystal grain ratio were excessive, and the DWTT guaranteed temperature was too high (that is, the low temperature toughness was inferior).
The reason why the average crystal grain size and the coarse crystal grain ratio were excessive in Comparative Example 6 was that the cooling rate V1 in the strong cooling was too low, and thus the strong cooling stop temperature T1 and the winding temperature CT were too high. This is probably because coarse ferrite grains were generated.

Claims (7)

Including the base metal part and the ERW welded part,
The chemical composition of the base material part is mass%,
C: 0.030 to 0.120%,
Si: 0.05-0.30%,
Mn: 0.50 to 2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010 to 0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005 to 0.030%,
Ni: 0.001 to 0.20%,
Mo: 0.10 to 0.20%,
V: 0 to 0.010%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
Cr: 0 to 0.30%,
Cu: 0 to 0.30%,
Mg: 0 to 0.0050%,
REM: 0 to 0.0100%, balance: Fe and impurities, F1 defined by the following formula (1) is 0.300 to 0.350,
In the metal structure of the central portion of the base metal part, the polygonal ferrite fraction is 60 to 90%, the average crystal grain size is 15 μm or less, and the crystal grain area ratio is 20 μm or more. A certain coarse crystal grain ratio is 20% or less,
An as-roll ERW steel pipe for line pipes with a yield ratio in the pipe axis direction of 80 to 95%.
F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 Formula (1)
[In Formula (1), C, Si, Mn, Ni, Cr, Mo, V, and Nb represent the mass% of each element, respectively. ] The chemical composition of the base material part is mass%,
V: more than 0% and 0.010% or less,
Ca: more than 0% and 0.0030% or less,
Cr: more than 0% and 0.30% or less,
Cu: more than 0% and 0.30% or less,
The as-rolled electric-welded steel pipe for line pipes according to claim 1, containing one or more selected from the group consisting of Mg: more than 0% and 0.0050% or less, and REM: more than 0% and 0.0100% or less.   The as-roll electric-welded steel pipe for line pipes according to claim 1 or 2, wherein the yield strength in the tube axis direction is 450 to 540 MPa, and the tensile strength in the tube axis direction is 510 to 625 MPa.   The as-rolled electric-welded steel pipe for a line pipe according to any one of claims 1 to 3, wherein the wall thickness is 12 to 25 mm and the outer diameter is 304.8 to 660.4 mm.   The as-roll electric-welded steel pipe for a line pipe according to any one of claims 1 to 4, wherein a yield ratio in the pipe axis direction is 80 to 93%. A hot-rolled steel sheet used in the manufacture of an as-rolled electric-welded steel pipe for a line pipe according to any one of claims 1 to 5,
Chemical composition is mass%,
C: 0.030 to 0.120%,
Si: 0.05-0.30%,
Mn: 0.50 to 2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010 to 0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005 to 0.030%,
Ni: 0.001 to 0.20%,
Mo: 0.10 to 0.20%,
V: 0 to 0.010%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
Cr: 0 to 0.30%,
Cu: 0 to 0.30%,
Mg: 0 to 0.0050%,
REM: 0 to 0.0100%, balance: Fe and impurities, F1 defined by the above formula (1) is 0.300 to 0.350,
In the metal structure at the center of the wall thickness, the fraction of polygonal ferrite is 60 to 90%, the average crystal grain size is 15 μm or less, and the coarse crystal grain ratio is the area ratio of crystal grains having a crystal grain size of 20 μm or more. Is a hot-rolled steel sheet with 20% or less.   The hot rolled steel sheet according to claim 6, wherein the yield strength in the rolling direction is 450 to 500 MPa, and the tensile strength in the rolling direction is 510 to 580 MPa.