JP7315834B2 - Electric resistance welded steel pipes for line pipes and hot-rolled steel plates for line pipes - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electric resistance welded steel pipe for line pipes and a hot-rolled steel plate for line pipes used for manufacturing the electric resistance welded steel pipes for line pipes.

A pipeline laid on the seabed is composed of a plurality of line pipes. A pipeline laid under the sea receives high pressure from a high-pressure fluid such as natural gas or crude oil passing through the pipeline. Pipelines are also externally subjected to cyclic strain due to waves and sea pressure. Therefore, electric resistance welded steel pipes for line pipes used in submarine pipelines are required to have high low-temperature toughness.

As an index of low temperature toughness, there is a DWTT (Drop Weight Tear Test) guaranteed temperature. The DWTT guarantee temperature means the temperature at which the DWTT test has a ductile fracture rate of 85% or more. The lower the DWTT guaranteed temperature, the higher the low temperature toughness. In recent years, electric resistance welded steel pipes for line pipes are required to have better low-temperature toughness than ever before.

International Publication No. 2012/002481 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2008-240151 (Patent Document 2) propose hot-rolled steel sheets for line pipes having excellent low-temperature toughness.

The hot-rolled steel sheet for line pipe use disclosed in Patent Document 1 contains, in % by mass, C: 0.02 to 0.08%, Si: 0.05 to 0.5%, Mn: 1 to 2%, It contains Nb: 0.03 to 0.12%, Ti: 0.005 to 0.05%, and the balance consists of Fe and unavoidable impurity elements. The hot-rolled steel sheet further has a proeutectoid ferrite fraction of 3% or more and 20% or less in the microstructure at a depth of 1/2 of the plate thickness from the steel plate surface, and other phases are a low-temperature transformation phase and 1% or less. of perlite. The number average grain size of the entire microstructure 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. be. In addition, at a depth of 1/2 the plate thickness from the steel plate surface, the reflected X-ray intensity ratio {211}/{111} in the {211} direction and the {111} direction with respect to the plane parallel to the steel plate surface is 1.1. That's it. Patent Document 1 describes that in this hot-rolled steel sheet, excellent strength and low-temperature toughness can be obtained by controlling the proeutectoid ferrite fraction in the central portion of the thickness, the average grain size, and the texture. ing.

The high-strength hot-rolled steel sheet for line pipe disclosed in Patent Document 2 contains C: 0.01 to 0.1%, Si: 0.05 to 0.5%, and Mn: 1 to 2% by mass%. , P ≤ 0.03%, S ≤ 0.005%, O ≤ 0.003%, Al: 0.005-0.05%, N: 0.0015-0.006%, Nb: 0.005- 0.08%, Ti: 0.005 to 0.02%, and N-14/48 × Ti>0%, Nb-93/14 × (N-14/48 × Ti)>0.005%, and the balance consists of Fe and unavoidable impurities. The microstructure is a continuous cooling transformation structure, and the reflected X-ray intensity ratio {211}/{111} of the {211} plane parallel to the plate surface and the {111} plane parallel to the plate surface in the texture at the center of the plate thickness is 1.1. That's it. The intragranular precipitate density of the carbonitride precipitates of Nb and/or Ti is 10 ¹⁷ to 10 ¹⁸ /cm ³ . Patent Document 2 describes that the high-strength hot-rolled steel sheet for line pipes has excellent low-temperature toughness by controlling the texture at the central portion of the sheet thickness.

WO2012/002481 JP 2008-240151 A

However, in the hot-rolled steel sheet disclosed in Patent Document 1, the heating temperature before the rolling process is high, and the austenite grains may become coarse. In this case, the crystal grains may become coarse and the low temperature toughness may deteriorate. Patent Document 2 does not pay attention to the size of intragranular precipitates.

An object of the present disclosure is to provide an electric resistance welded steel pipe for line pipes having excellent low-temperature toughness, and a hot-rolled steel plate for line pipes used for manufacturing the electric resistance welded steel pipe for line pipes.

The electric resistance welded steel pipe for line pipe according to the present disclosure is
An electric resistance welded steel pipe for a line pipe comprising a base material portion and an electric resistance welded portion,
The chemical composition of the base material portion is, in mass %,
C: 0.010 to 0.060%,
Si: 0.05 to 0.30%,
Mn: 0.50-2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010-0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005 to 0.030%,
Mo: 0.05-0.30%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
Ni: 0 to 0.50%,
V: 0 to 0.10%,
Cr: 0 to 0.30%,
Cu: 0-0.30%,
Mg: 0-0.0050%,
Rare earth elements: 0 to 0.0100% and
Balance: Fe and impurities, satisfying formula (1),
Crystals having a ferrite fraction of 60 to 90%, an effective crystal grain size of 15.0 μm or less, and a crystal grain size of 20 μm or more in the thickness central portion of the base material portion of the electric resistance welded steel pipe for line pipe. The coarse crystal grain ratio, which is the grain area ratio, is 20% or less,
The number density of the specific coarse particles having a maximum diameter of 3 μm or more and a total content of Ti and Nb of 10% by mass or more is 50 particles/100 mm ² or less,
When the surface perpendicular to the rolling direction of the electric resistance welded steel pipe for line pipe is defined as the RD surface, the rolled surface is defined as the ND surface, and the RD surface and a surface perpendicular to the ND surface are defined as the TD surface, the angle formed with the RD surface is 45° and the angle formed with the TD plane is 45°, the degree of integration of {100} planes is 1.85 to 3.50.
0.30≤C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≤0.38 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1).

In the present disclosure, the effective crystal grain size means that the orientation difference between adjacent measurement points (pixels) exceeds 15° in the EBSP-OIM (trademark, the same shall apply hereinafter) (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method. When the position is defined as the grain boundary, the area surrounded by the grain boundary is defined as the crystal grain, and the equivalent circle diameter of the crystal grain is defined as the crystal grain size, the average area of the grain size is calculated in the number distribution for each grain size. It means an area-average particle size, which is the average value (particle size corresponding to the average area) when taking the distribution of the multiplied values.

The hot-rolled steel sheet for line pipe according to the present disclosure is
A hot-rolled steel sheet for line pipes,
The chemical composition, in mass %,
C: 0.010 to 0.060%,
Si: 0.05 to 0.30%,
Mn: 0.50-2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010-0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005 to 0.030%,
Mo: 0.05-0.30%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
Ni: 0 to 0.50%,
V: 0 to 0.10%,
Cr: 0 to 0.30%,
Cu: 0-0.30%,
Mg: 0-0.0050%,
Rare earth elements: 0 to 0.0100% and
Balance: Fe and impurities, satisfying formula (1),
In the central part of the thickness of the hot-rolled steel sheet for line pipe, the ferrite fraction is 60 to 90%, the effective grain size is 15.0 μm or less, and the area ratio of grains with a grain size of 20 μm or more is A certain coarse grain ratio is 20% or less,
The number density of the specific coarse particles having a maximum diameter of 3 μm or more and a total content of Ti and Nb of 10% by mass or more is 50 particles/100 mm ² or less,
When the surface perpendicular to the rolling direction of the hot-rolled steel sheet for line pipe is defined as the RD surface, the rolled surface is defined as the ND surface, and the RD surface and a surface perpendicular to the ND surface are defined as the TD surface, the angle formed with the RD surface is 45° and the angle formed with the TD plane is 45°, the degree of integration of {100} planes is 1.85 to 3.50.
0.30≤C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≤0.38 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1).

The electric resistance welded steel pipe for line pipe according to the present disclosure has excellent low temperature toughness. The hot-rolled steel sheet for line pipe according to the present disclosure has excellent low temperature toughness when produced into an electric resistance welded steel pipe for line pipe.

FIG. 1 is a continuous cooling transformation curve diagram of an electric resistance welded steel pipe for line pipes and a hot-rolled steel plate for line pipes of the present disclosure. FIG. 2 is a schematic diagram of a minute area in the thickness central portion of the base material portion of the electric resistance welded steel pipe for line pipes, or the thickness central portion of the hot-rolled steel plate for line pipes for manufacturing the electric resistance welded steel pipe for line pipes. is. In FIG. 3, when the surface perpendicular to the rolling direction is defined as the RD surface, the rolled surface is defined as the ND surface, and the RD surface and a surface perpendicular to the ND surface are defined as the TD surface, the angle formed with the RD surface is 45 °, and , the degree of accumulation of the {100} plane ({100} degree of accumulation) in a specific plane having an angle of 45 ° with the TD plane, and the ductile fracture surface rate (%) when the DWTT test is performed at -30 ° C. is a diagram showing the relationship of FIG. 4 is a flow chart showing an example of a manufacturing process of a hot-rolled steel sheet for line pipes and an electric resistance welded steel pipe for line pipes according to the present disclosure. FIG. 5 is a plan view of a tensile test piece. In addition, the numerical value in a figure shows a dimension (unit: mm). FIG. 6 is a front view and a side view of a DWTT specimen. Numerical values and t in the figure indicate dimensions (unit: mm).

The present inventors have investigated and studied the low-temperature toughness of electric resistance welded steel pipes for line pipes, and obtained the following findings. First, the present inventors investigated and examined the low temperature toughness of steel materials for line pipes, and obtained the following findings.

Generally, strength (or hardness) and toughness of steel are mutually exclusive. Ferrite is a soft tissue. Therefore, if the structure of the steel is mainly ferrite, the low temperature toughness of the steel increases.

If the ferrite grains are fine, the low temperature toughness of the steel is further enhanced. For example, in order to make ferrite grains finer, the heating temperature during rolling is set to 1200° C. or less to suppress coarsening of crystal grains. Furthermore, cooling is controlled so that a large number of nucleation sites are generated in the non-recrystallized structure after rolling (structure during the finish rolling process) and a large number of new ferrite grains are generated. In this case, the final ferrite grains become finer, resulting in an increase in the low temperature toughness of the steel.

On the other hand, if the structure of the steel is mainly bainite, although laths (elongated structures) are generated in the crystal grains inherited from the previous austenite grains, the orientation of the laths is aligned for each block, and each block is substantially uniform. It becomes one crystal grain. Therefore, the size of crystal grains in bainite is determined by the size of prior austenite grains. Therefore, crystal grains tend to coarsen, and low-temperature toughness tends to decrease. Therefore, in the electric resistance welded steel pipe for line pipe of the present disclosure, the structure is mainly ferrite. The chemical composition is mass %, C: 0.010 to 0.060%, Si: 0.05 to 0.30%, Mn: 0.50 to 2.00%, P: 0 to 0.030%, S: 0-0.0010%, Al: 0.010-0.035%, N: 0.0010-0.0080%, Nb: 0.010-0.080%, Ti: 0.005-0. 030%, Mo: 0.05 to 0.30%, O: 0 to 0.0030%, Ca: 0 to 0.0050%, Ni: 0 to 0.50%, V: 0 to 0.10%, Cr: 0 to 0.30%, Cu: 0 to 0.30%, Mg: 0 to 0.0050%, rare earth elements: 0 to 0.0100%, and the balance: Fe and impurities. A ferrite-based microstructure can be obtained by carrying out the manufacturing process of .

All of C, Si, Mn, Ni, Cr, Mo, V, and Nb are S curves (ferrite regions, pearlite area and bainite area).

Define F1=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3. If F1 is too low, the S-curve of the CCT diagram will shift too far to the left. In this case, the low temperature toughness of the steel is reduced. The reason for this is as follows.

The magnitude of the driving force (driving force for phase transformation) for transforming from austenite to ferrite correlates with the temperature of the steel material. When the steel material temperature is high, the driving force for phase transformation is small. Therefore, ferrite transformation nuclei are less likely to be generated. Furthermore, since the temperature of the steel material is high, the growth of ferrite grains is rapid. As a result, ferrite grains become coarse. On the other hand, when the steel material temperature is low, the driving force for phase transformation is large. Therefore, ferrite transformation nuclei are likely to be generated. Furthermore, due to the low steel material temperature, the growth of ferrite grains is slow. As a result, ferrite grains are refined. Therefore, in the CCT diagram, if the ferrite nose (ferrite region) is entered while the temperature of the steel material is high, the ferrite grains tend to coarsen. On the other hand, if the ferrite nose (ferrite region) is entered while the temperature of the steel material is low to some extent, the ferrite grains are likely to be fine.

When F1 is too low and the S curve of the CCT diagram shifts too far to the left, the steel material temperature enters the ferrite region with a high temperature. Therefore, as described above, the ferrite grains are coarsened and the effective crystal grain size is increased. Furthermore, since it tends to form a mixed grain structure, the ratio of coarse grains increases. In this case, the low temperature toughness of the steel is reduced. If F1 is too low, the hardenability is further lowered and sufficient strength cannot be obtained.

On the other hand, if F1 is too high, the S-curve will shift too far to the right. In this case, the cooling curve is less likely to overlap the ferrite nose. As a result, the amount of hard structures such as bainite and martensite generated increases, and the ferrite fraction in the structure decreases. As a result, the low temperature toughness of the steel is reduced.

FIG. 1 is a continuous cooling transformation curve diagram (CCT diagram) of an electric resistance welded steel pipe for line pipes and a hot-rolled steel plate for line pipes according to the present disclosure. FIG. 1 shows the chemical composition in mass %, C: 0.010 to 0.060%, Si: 0.05 to 0.30%, Mn: 0.50 to 2.00%, P: 0 to 0 0.030%, S: 0-0.0100%, Al: 0.010-0.035%, N: 0.0010-0.0080%, Nb: 0.010-0.080%, Ti: 0.010% 005-0.030%, Mo: 0.05-0.30%, O: 0-0.0030%, Ca: 0-0.0050%, Ni: 0-0.50%, V: 0-0 .10%, Cr: 0 to 0.30%, Cu: 0 to 0.30%, Mg: 0 to 0.0050%, rare earth elements: 0 to 0.0100%, and the balance: Fe and impurities, It is an example of a CCT diagram in a steel material. In FIG. 1, F indicates ferrite nose, P indicates pearlite nose, and B indicates bainite nose.

In FIG. 1, when F1 is 0.30 to 0.35, the S curves of each phase (ferrite, pearlite, bainite) are arranged at appropriate positions in the CCT diagram. In this case, it is possible to cool mainly through the ferrite region as shown by the cooling curve C1 in FIG. Therefore, a ferrite-based structure can be generated, and high low-temperature toughness can be obtained. Therefore, if the chemical composition described above satisfies the formula (1), the microstructure can be made mainly of ferrite, the ferrite grains can be refined, and the ratio of coarse ferrite grains can be suppressed. Thought.
0.30≤C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≤0.38 (1)

The inventors of the present invention further considered that it is possible to further improve the low-temperature toughness by controlling the microstructure as well as adjusting the chemical composition as described above. Accordingly, the present inventors have investigated further improvement of low-temperature toughness in the electric resistance welded steel pipe for line pipe having the chemical composition described above that satisfies the formula (1). As a result, the inventors obtained the following knowledge.

As described above, the inside of the line pipe is filled with a fluid (gas or liquid) such as natural gas or crude oil at high pressure, and the maximum stress acts in the circumferential direction of the steel pipe. Therefore, when a crack occurs, it tends to propagate along the axial direction of the electric resistance welded steel pipe for line pipe. That is, in the electric resistance welded steel pipe for line pipes, cracks are likely to occur along the axial direction of the electric resistance welded steel pipe.

By the way, electric resistance welded steel pipes for line pipes are manufactured by the following method. A hot-rolled steel sheet for line pipe is formed into a cylindrical blank pipe (open pipe) using forming rolls. The formed blank tube is formed so that the plate width direction (hereinafter also referred to as the TD direction) of the hot-rolled steel sheet coincides with the circumferential direction of the blank tube. Electric resistance welding is performed on the butt portions of the base pipe to manufacture electric resistance welded steel pipes for line pipes. That is, in the electric resistance welded steel pipe for line pipes, the axial direction is the rolling direction (hereinafter also referred to as the RD direction), and the circumferential direction of the electric resistance welded steel pipe corresponds to the TD direction of the hot rolled steel plate. As described above, in the electric resistance welded steel pipe for line pipes, cracks tend to propagate in the RD direction, that is, in the axial direction of the electric resistance welded steel pipe for line pipes.

Therefore, the present inventors considered that in order to increase the low-temperature toughness of electric resistance welded steel pipes for line pipes, it is necessary to suppress the propagation of cracks in the RD direction, which corresponds to the axial direction of electric resistance welded steel pipes for line pipes. . Therefore, the present inventors investigated a method for suppressing crack propagation in the RD direction.

The present inventors focused on the texture of steel as a method for suppressing crack propagation.

As described above, the microstructure of the electric resistance welded steel pipe for line pipes of the present disclosure, which has the above-described chemical composition that satisfies formula (1), is mainly ferrite. Ferrite has a body-centered cubic structure. Therefore, in the texture of steel, cracks are more likely to propagate along the {100} planes than the {110} and {111} planes, and delamination is more likely to occur along the {100} planes.

From the above study results, the present inventors found that when the normal to the {100} plane of the ferrite crystal grain is perpendicular to the RD direction, that is, when the {100} plane extends along the RD direction, It was thought that the crack propagated easily. They also thought that if the normal to the {100} plane is inclined by 45° from the direction perpendicular to the RD direction, crack propagation can be suppressed. When the angle formed by the normal to the {100} plane and the RD direction is 45°, the extending direction of the {100} plane can be most inclined (separated) from the RD direction, which is the direction in which cracks tend to propagate. Because you can. This point will be described more specifically below.

FIG. 2 is a schematic diagram of a minute area in the thickness central portion of the base material portion of the electric resistance welded steel pipe for line pipes, or the thickness central portion of the hot-rolled steel plate for line pipes for manufacturing the electric resistance welded steel pipe for line pipes. is. With reference to FIG. 2, the plane perpendicular to the rolling direction is defined as the RD plane, the rolled plane as the ND plane, and the RD plane and a plane perpendicular to the ND plane as the TD plane. Also, the normal to the RD plane is defined as the RD direction, the normal to the ND plane is defined as the ND direction, and the normal to the TD plane is defined as the TD direction. The RD direction corresponds to the rolling direction. The TD direction corresponds to the width direction of hot-rolled steel sheets, and corresponds to the circumferential direction of electric resistance welded steel pipes for line pipes. The ND direction corresponds to the plate thickness direction and the thickness direction.

In the case of an electric resistance welded steel pipe for line pipes, the circumferential direction is curved, but in the above-described minute regions, the circumferential direction is substantially straight and substantially coincides with the straight TD direction. Therefore, the specific surface shown in FIG. 2 is similarly shown in the electric resistance welded steel pipe for line pipe.

In the minute region of FIG. 2, when the plane perpendicular to the rolling direction is defined as the RD plane, the rolled plane is defined as the ND plane, and the RD plane and the plane perpendicular to the ND plane are defined as the TD plane, the angle formed with the RD plane is 45°. A specific plane is defined as a plane which is present and forms an angle of 45° with the TD plane, that is, a plane whose normal is 45° with respect to the RD direction. FIG. 2 shows, as an example of a specific plane, a specific plane in which the angle formed by the normal line and the RD direction is 45° and the angle formed by the normal line and the TD direction is 45°. In the specific plane at 45°, if the degree of accumulation of the {100} plane is increased, that is, in the specific plane, the proportion of crystal grains in which the {100} plane is parallel to the specific plane (hereinafter referred to as {100} crystal grains) As the number increases, the ratio of crystal grains whose {100} faces are along the RD direction decreases. As a result, the present inventors thought that the propagation of cracks along the RD direction could be suppressed.

Based on the above idea, the present inventors investigated the relationship between the degree of accumulation of {100} planes in a specific plane and the ductile fracture surface rate (%) when a DWTT test was performed at -30 ° C. rice field.

In FIG. 3, when the surface perpendicular to the rolling direction is defined as the RD surface, the rolled surface is defined as the ND surface, and the RD surface and a surface perpendicular to the ND surface are defined as the TD surface, the angle formed with the RD surface is 45 °, and , is a diagram showing the relationship between the degree of accumulation of {100} planes in a specific plane having an angle of 45° with the TD plane and the ductile fracture surface ratio (%) when a DWTT test is performed at −30° C. . FIG. 3 was obtained by the following method.

Electric resistance welded steel pipes for line pipes were manufactured using slabs having the composition of Steel A shown in Table 1 in Examples described later. Specifically, the slab was heated to a temperature of 1060-1200° C. in a heating furnace. Rough rolling was performed on the heated slab. The temperature T ₀ (° C.) at the final stand delivery side of rough rolling was 900 to 985° C., and the time t ₀ (s) from immediately after the end of rough rolling to the start of finish rolling was 50 to 230 s. After rough rolling, finish rolling was performed at 760 to 800° C. to produce a hot rolled steel sheet. The hot-rolled steel sheet after the finish rolling was cooled on a run out table (ROT). A hot-rolled steel sheet for line pipes was manufactured through the above-described manufacturing process. Using hot-rolled steel sheets for line pipes, pipes were manufactured by the method described above to produce electric resistance welded steel pipes for line pipes having an outer diameter of 304.8 to 660.4 mm and a wall thickness of 12 to 25 mm.

Based on the method described later, the EBSD (Electron Back Scatter Diffraction Patterns) method was used to measure the degree of integration of {100} planes on a specific plane. The measurement condition for the EBSD method was a magnification of 400 times. Also, the visual field area was set to 200 μm×500 μm, and the measurement step was set to 0.3 μm.

Furthermore, a low-temperature toughness test was performed on the electric resistance welded steel pipe for line pipes. Specifically, a DWTT test was performed at −30° C. in accordance with API 5L3 to determine the ductile fracture surface ratio. FIG. 3 was created using the degree of accumulation of the {100} plane in the specific plane and the obtained ductile fracture surface ratio.

Referring to FIG. 3, the ductile fracture rate remarkably increases as the degree of {100} plane integration in a specific plane increases. When the {100} accumulation degree exceeds 1.50, the ductile fracture surface ratio becomes 85% or more. When the {100} accumulation degree was 1.50 or more, the ductile fracture surface ratio did not change significantly even when the {100} accumulation degree increased. That is, the ductile fracture surface ratio with respect to the {100} accumulation degree on the specific plane had an inflection point near the {100} accumulation degree=1.50. The inventors of the present invention have found that when the degree of {100} plane accumulation in the specific plane is 1.50 or more, the ductile fracture surface ratio is 85% or more, and an electric resistance welded steel pipe for line pipes having excellent low temperature toughness can be obtained. found out for the first time.

Therefore, the inventors aimed to further lower the DWTT guarantee temperature. Specifically, the present inventors conducted various investigations and studies on methods for making the DWTT guarantee temperature of electric resistance welded steel pipes for line pipes -40°C or lower.

During the manufacturing process of steel pipes for line pipes, specific coarse particles having a maximum diameter of 3 μm or more and a total Ti and Nb content of 10% by mass or more (hereinafter simply referred to as coarse specific particles) are introduced into the steel. May exist. The specific coarse particles are specific particles having a total content of Ti and Nb of 10% by mass or more and having a maximum diameter of 3 μm or more. In the present specification, the maximum diameter is the longest line segment having both ends on the outer periphery of each specific particle when observed with an SEM at a magnification of 400 times and a visual field area of 200 μm × 500 μm. means the length of a line segment.

Coarse specific particles are considered to be generated, for example, as follows. In the steelmaking stage, TiN is produced as inclusions. Coarse specific particles are generated by bonding Nb to TiN during solidification in continuous casting. The generation process of the coarse specific particles is not limited to this. The coarse specific particles may be Nb carbonitrides, Ti carbonitrides, etc., and/or composite particles thereof. In the present specification, "carbonitrides, etc." is a generic term for carbides, nitrides, and carbonitrides. The specific coarse particles are inclusions or precipitates having a total content of Ti and Nb of 10% by mass or more. In the coarse specific particles, the Ti content may be 0% by mass, and the Nb content may be 0% by mass, as long as the total content of Ti and Nb is 10% by mass or more.

When the specific coarse particles are present inside the steel, the specific coarse particles act as starting points for cracks. As a result, the low temperature toughness is lowered. Therefore, in order to further lower the DWTT guarantee temperature, it is necessary to reduce the coarse specific particles present inside the steel.

The present inventors obtained the following findings as a result of various studies. If the number density of the coarse specific particles can be suppressed, solid solution Nb and solid solution Ti increase. Solid-solution Nb and solid-solution Ti suppress the recrystallization of austenite, so that the rolling texture tends to develop. As a result, it was found that the degree of accumulation of {100} planes in a specific plane at 45° can be dramatically increased in the metallographic structure after transformation.

Specifically, by setting the number density of the specific coarse particles to 50/100 mm ² or less, the degree of accumulation of the {100} plane can be increased to 1.85 to 3.50 in the specific plane at 45°. can. As a result, the guaranteed DWTT temperature of the electric resistance welded steel pipe for line pipe can be -40°C or lower.

The gist of the electric resistance welded steel pipe for line pipe according to the present disclosure completed based on the above knowledge is as follows.

The electric resistance welded steel pipe for line pipe of [1] is
An electric resistance welded steel pipe for a line pipe comprising a base material portion and an electric resistance welded portion,
The chemical composition of the base material portion is, in mass %,
C: 0.010 to 0.060%,
Si: 0.05 to 0.30%,
Mn: 0.50-2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010-0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005 to 0.030%,
Mo: 0.05-0.30%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
Ni: 0 to 0.50%,
V: 0 to 0.10%,
Cr: 0 to 0.30%,
Cu: 0-0.30%,
Mg: 0-0.0050%,
Rare earth elements: 0 to 0.0100% and
Balance: Fe and impurities, satisfying formula (1),
Crystals having a ferrite fraction of 60 to 90%, an effective crystal grain size of 15.0 μm or less, and a crystal grain size of 20 μm or more in the thickness central portion of the base material portion of the electric resistance welded steel pipe for line pipe. The coarse crystal grain ratio, which is the grain area ratio, is 20% or less,
The number density of the specific coarse particles having a maximum diameter of 3 μm or more and a total content of Ti and Nb of 10% by mass or more is 50 particles/100 mm ² or less,
When the surface perpendicular to the rolling direction of the electric resistance welded steel pipe for line pipe is defined as the RD surface, the rolled surface is defined as the ND surface, and the RD surface and a surface perpendicular to the ND surface are defined as the TD surface, the angle formed with the RD surface is 45° and the angle formed with the TD plane is 45°, the density of the {100} planes being 1.85 to 3.50.
0.30≤C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≤0.38 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1).

The electric resistance welded steel pipe for line pipe of [2] is
The electric resistance welded steel pipe for line pipe according to [1],
The chemical composition, in mass %,
Contains Ca: more than 0 to 0.0050%.

The electric resistance welded steel pipe for line pipe of [3] is
The electric resistance welded steel pipe for line pipe according to [1] or [2],
The chemical composition, in mass %,
Ni: more than 0 to 0.50%,
V: more than 0 to 0.10%,
Cr: more than 0 to 0.30%, and
Cu: more than 0 to 0.30%, contains one or more selected from the group consisting of.

The electric resistance welded steel pipe for line pipe of [4] is
The electric resistance welded steel pipe for line pipe according to any one of [1] to [3],
The chemical composition, in mass %,
Mg: more than 0 to 0.0050%, and
Rare earth element: more than 0 to 0.0100%, containing one or more selected from the group consisting of.

The electric resistance welded steel pipe for line pipe of [5] is
The electric resistance welded steel pipe for line pipe according to any one of [1] to [4],
It has an axial yield strength of 450-540 MPa and an axial tensile strength of 510-625 MPa.

The electric resistance welded steel pipe for line pipe of [6] is
The electric resistance welded steel pipe for line pipe according to any one of [1] to [5],
It has a wall thickness of 12 to 25 mm and an outer diameter of 304.8 to 660.4 mm.

The hot-rolled steel sheet for line pipes of [7] is
A hot-rolled steel sheet for line pipes,
The chemical composition, in mass %,
C: 0.010 to 0.060%,
Si: 0.05 to 0.30%,
Mn: 0.50-2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010-0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005 to 0.030%,
Mo: 0.05-0.30%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
Ni: 0 to 0.50%,
V: 0 to 0.10%,
Cr: 0 to 0.30%,
Cu: 0-0.30%,
Mg: 0-0.0050%,
Rare earth elements: 0 to 0.0100% and
Balance: Fe and impurities, satisfying formula (1),
In the central part of the thickness of the hot-rolled steel sheet for line pipe, the ferrite fraction is 60 to 90%, the effective grain size is 15.0 μm or less, and the area ratio of grains with a grain size of 20 μm or more is A certain coarse grain ratio is 20% or less,
The number density of the specific coarse particles having a maximum diameter of 3 μm or more and a total content of Ti and Nb of 10% by mass or more is 50 particles/100 mm ² or less,
When the surface perpendicular to the rolling direction of the hot-rolled steel sheet for line pipe is defined as the RD surface, the rolled surface is defined as the ND surface, and the RD surface and a surface perpendicular to the ND surface are defined as the TD surface, the angle formed with the RD surface is 45° and the angle formed with the TD plane is 45°, and the degree of accumulation of {100} planes is 1.85 to 3.50.
0.30≤C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≤0.38 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1).

The hot-rolled steel sheet for line pipes of [8] is
The hot-rolled steel sheet for line pipes according to [7],
The chemical composition, in mass %,
Contains Ca: more than 0 to 0.0050%.

The hot-rolled steel sheet for line pipes of [9] is
The hot-rolled steel sheet for line pipes according to [7] or [8],
The chemical composition, in mass %,
Ni: more than 0 to 0.50%,
V: more than 0 to 0.10%,
Cr: more than 0 to 0.30%, and
Cu: more than 0 to 0.30%, contains one or more selected from the group consisting of.

The hot-rolled steel sheet for line pipes of [10] is
The hot-rolled steel sheet for line pipes according to any one of [7] to [9],
The chemical composition, in mass %,
Mg: more than 0 to 0.0050%, and
Rare earth element: more than 0 to 0.0100%, containing one or more selected from the group consisting of.

Hereinafter, the electric resistance welded steel pipe for line pipes and the hot rolled steel plate for line pipes of the present disclosure will be described in detail. Here, the hot-rolled steel sheet for line pipes is a hot-rolled steel sheet used for manufacturing electric resistance welded steel pipes for line pipes, and corresponds to the raw material for electric resistance welded steel pipes for line pipes. In addition, in this specification, "%" regarding an element means the mass %, unless there is particular notice.

The electric resistance welded steel pipe of the present disclosure has a base material portion and an electric resistance welded portion. The base material portion is cylindrical, and the electric resistance welded portion extends in a direction parallel to the axial direction of the electric resistance welded steel pipe. The base metal portion is the portion that is not affected by heat during electric resistance welding.

[Chemical composition]
Both the chemical composition of the base metal portion of the electric resistance welded steel pipe for line pipes of the present disclosure and the chemical composition of the hot-rolled steel plate for line pipes contain the following elements.

C: 0.010-0.060%
Carbon (C) increases the strength of steel. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, carbides are formed and the low temperature toughness and ductility of the steel are reduced. Too high a C content further reduces weldability. Therefore, the C content is 0.010-0.060%. A preferred lower limit for the C content is 0.025%, more preferably 0.030%. A preferable upper limit of the C content is 0.058%.

Si: 0.05-0.30%
Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the low temperature toughness of the steel will decrease. Therefore, the Si content is 0.05-0.30%. A preferable lower limit of the Si content is 0.10%, more preferably 0.15%. A preferred upper limit for the Si content is 0.25%, more preferably 0.21%.

Mn: 0.50-2.00%
Manganese (Mn) increases the hardenability of steel and increases the strength of steel. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, the strength of the steel will be too high and the low temperature toughness of the steel will be reduced. Therefore, the Mn content is 0.50-2.00%. A preferred lower limit for the Mn content is 0.80%, more preferably 1.00%. A preferable upper limit of the Mn content is 1.80%, more preferably 1.50%.

P: 0-0.030%
Phosphorus (P) is an impurity. P lowers the low temperature toughness of steel. Therefore, it is preferable that the P content is as low as possible. Specifically, the P content is 0 to 0.030%. A preferable upper limit of the P content is 0.020%, more preferably 0.015%. On the other hand, the P content may be 0%. However, from the viewpoint of dephosphorization cost reduction, the P content may be more than 0%, may be 0.001% or more, or may be 0.005% or more.

S: 0-0.0100%
Sulfur (S) is an impurity. S combines with Mn to form Mn-based sulfides. Therefore, the low temperature toughness and SSC resistance of the steel are lowered. Therefore, it is preferable that the S content is as low as possible. Specifically, the S content is 0 to 0.0100%. A preferable upper limit of the S content is 0.0080%, more preferably 0.0050%. On the other hand, the S content may be 0%. However, from the viewpoint of desulfurization cost reduction, the S content may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, or may be 0.0020% or more. There may be.

Al: 0.010-0.035%
Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, Al nitrides will coarsen and the low temperature toughness of the steel will deteriorate. Therefore, the Al content is 0.010-0.035%. A preferable lower limit of the Al content is 0.015%, more preferably 0.020%. A preferable upper limit of the Al content is 0.030%. As used herein, Al content means the total Al content in the steel.

N: 0.0010 to 0.0080%
Nitrogen (N) forms nitrides to suppress coarsening of austenite grains during the heating process. In this case, the austenite grains are refined in the rolling process, and the crystal grains after transformation are refined. As a result, the low temperature toughness of the steel is enhanced. N further increases the strength of steel through solid-solution strengthening. If the N content is too low, these effects cannot be obtained. On the other hand, if the N content is too high, the carbonitrides are coarsened and the low temperature toughness of the steel is lowered. Therefore, the N content is 0.0010-0.0080%. A preferable lower limit of the N content is 0.0020%, more preferably 0.0025%. A preferable upper limit of the N content is 0.0060%, more preferably 0.0050%.

Nb: 0.010-0.080%
Niobium (Nb) combines with C and N in steel to form fine Nb carbonitrides and the like. In the present specification, "carbonitrides, etc." is a generic term for carbides, nitrides, and carbonitrides. Fine Nb carbonitrides and the like suppress coarsening of crystal grains and reduce the effective crystal grain size. Fine Nb carbonitrides and the like further suppress recrystallization of austenite, so that rolling texture develops. This increases the degree of accumulation of {100} planes in the specific plane at 45° in the metallographic structure after transformation. As a result, the low temperature toughness of the steel is enhanced. Furthermore, fine Nb carbonitrides and the like increase the strength of steel through dispersion strengthening. If the Nb content is too low, these effects cannot be obtained. Therefore, the Nb content is 0.010% or more. On the other hand, if the Nb content is too high, Nb carbonitrides and the like coarsen, and the low-temperature toughness of the steel deteriorates. Therefore, the Nb content is 0.080% or less. From the above, the Nb content is 0.010 to 0.080%. A preferred lower limit for the Nb content is 0.015%. A preferable upper limit of the Nb content is 0.040%, more preferably 0.030%.

Ti: 0.005-0.030%
Titanium (Ti) combines with C and N in steel to form fine Ti carbonitrides and the like. By forming TiN, the deterioration of the low-temperature toughness of the steel due to dissolved N is suppressed. Further, fine TiN is dispersed and precipitated, thereby suppressing coarsening of crystal grains. Furthermore, since fine Ti carbonitrides and the like suppress recrystallization of austenite, the rolling texture develops. This increases the degree of accumulation of {100} planes in the specific plane at 45° in the metallographic structure after transformation. As a result, the low temperature toughness of the steel is enhanced. If the Ti content is too low, these effects cannot be obtained. Therefore, the Ti content is 0.005% or more. On the other hand, if the Ti content is too high, TiN becomes coarse and coarse TiC is produced. In this case, the low temperature toughness of the steel is reduced. Therefore, the Ti content is 0.030% or less. From the above, the Ti content is 0.005 to 0.030%. A preferable lower limit of the Ti content is 0.007%, more preferably 0.010%. A preferable upper limit of the Ti content is 0.020%, more preferably 0.017%.

Mo: 0.05-0.30%
Molybdenum (Mo) increases the hardenability of steel and increases the strength of steel. Mo further refines austenite grains and increases the low temperature toughness of steel. If the Mo content is too low, these effects cannot be obtained. On the other hand, if the Mo content is too high, the field weldability of the steel deteriorates. Therefore, the Mo content is 0.05-0.30%. A preferred lower limit for the Mo content is 0.15%. A preferred upper limit for the Mo content is 0.20%, more preferably 0.18%.

O: 0 to 0.0030%
Oxygen (O) is an impurity. O forms oxides and lowers the resistance to hydrogen-induced cracking of steel. O further reduces the low temperature toughness of steel. Therefore, it is preferable that the O content is as low as possible. Specifically, the O content is 0 to 0.0030%. A preferable upper limit of the O content is 0.0025%. On the other hand, the O content may be 0%. However, 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, or may be 0.0015% or more. or 0.0020% or more.

The balance of the chemical composition of the line pipe steel according to the present disclosure consists of Fe and impurities. Here, the impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when industrially manufacturing the steel material for line pipes, and have an adverse effect on the steel material for line pipes of the present disclosure. It means what is permissible within the range not given.

[Regarding arbitrary elements]
The chemical composition of the steel material for line pipe described above may further contain Ca instead of part of Fe.

Ca: 0 to 0.0050%,
Calcium (Ca) is an optional element and may not be contained. When included, Ca controls the morphology of MnS to spheroidize. In this case, the low temperature toughness of the steel is enhanced. However, if the Ca content is too high, coarse oxide inclusions are formed. Therefore, the Ca content is 0-0.0050%. The Ca content may be 0%. A preferable lower limit of the Ca content is more than 0%, more preferably 0.0001%, still more preferably 0.0010%, still more preferably 0.0015%. A preferred upper limit for the Ca content is 0.0045%.

The chemical composition of the steel material for line pipe described above may further contain one or more selected from the group consisting of Ni, V, Cr and Cu in place of part of Fe. These elements increase the strength of steel.

Ni: 0-0.50%
Nickel (Ni) is an optional element and may not be contained. When included, Ni enhances the hardenability of the steel and enhances the strength of the steel. However, if the Ni content is too high, this effect saturates. Therefore, the Ni content is 0-0.50%. The Ni content may be 0%. A preferable lower limit of the Ni content is more than 0%, more preferably 0.01%, still more preferably 0.05%, still more preferably 0.07%. A preferable upper limit of the Ni content is 0.20%.

V: 0-0.10%
Vanadium (V) is an optional element and may not be contained. When contained, V combines with C and N in the steel during the winding process to form fine carbonitrides and increases the strength of the steel. Fine V carbonitrides further suppress coarsening of grains and enhance the low temperature toughness of steel. However, if the V content is too high, V carbonitrides coarsen and the low temperature toughness of the steel decreases. Therefore, the V content is 0-0.10%. The V content may be 0%. A preferable lower limit of the V content is more than 0%, more preferably 0.001%, and still more preferably 0.002%. A preferable upper limit of the V content is 0.08%, more preferably 0.06%.

Cr: 0-0.30%
Chromium (Cr) is an optional element and may not be contained. When included, Cr increases the hardenability of the steel and increases the strength of the steel. However, if the Cr content is too high, the hardenability will be too high and the low temperature toughness of the steel will be reduced. Therefore, the Cr content is 0-0.30%. The Cr content may be 0%. A preferable lower limit of the Cr content is over 0%, more preferably 0.01%. A preferable upper limit of the Cr content is 0.20%, more preferably 0.10%, and still more preferably 0.05%.

Cu: 0-0.30%
Copper (Cu) is an optional element and may not be contained. When included, Cu enhances the hardenability of the steel and enhances the strength of the steel. However, if the Cu content is too high, the hardenability will be too high and the low temperature toughness of the steel will be reduced. Too high a Cu content also causes surface cracking during casting and hot rolling due to liquid metal embrittlement. Therefore, the Cu content is 0-0.30%. The Cu content may be 0%. A preferable lower limit of the Cu content is more than 0%, more preferably 0.01%, more preferably 0.05%, still more preferably 0.10%. A preferable upper limit of the Cu content is 0.25%, more preferably 0.20%.

The chemical composition of the steel material for line pipe described above may further contain one or more selected from the group consisting of Mg and rare earth elements in place of part of Fe. These elements function as deoxidizers and desulfurizers.

Mg: 0-0.0050%
Magnesium (Mg) is an optional element and may not be contained. When included, Mg functions as a deoxidizing and desulfurizing agent. In addition, it produces fine oxides and contributes to the improvement of HAZ toughness. However, if the Mg content is too high, the oxide tends to aggregate or coarsen. As a result, the HIC resistance may be lowered, or the toughness of the base metal portion or HAZ may be lowered. Therefore, the Mg content is 0-0.0050%. The Mg content may be 0%. A preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, and still more preferably 0.0010%. A preferred upper limit for the Mg content is 0.0030%.

Rare earth elements: 0 to 0.0100%
Rare earth elements (REM) are optional elements and need not be included. When included, REM functions as a deoxidizing and desulfurizing agent. However, if the REM content is too high, coarse oxides are produced. As a result, the HIC resistance may be lowered, or the toughness of the base metal portion or HAZ may be lowered. Therefore, the REM content is 0-0.0100%. The REM content may be 0%. A preferable lower limit of the REM content is more than 0%, more preferably 0.0001%, and more preferably 0.0010%. A preferred upper limit for the REM content is 0.0070%, more preferably 0.0050%.

Here, REM is yttrium (Y) with an atomic number of 39, lanthanide (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71, which is a lanthanoid, and actinium with an atomic number of 89, which is an actinide. It is one or more elements selected from the group consisting of (Ac) to No. 103 lawrencium (Lr).

[Regarding formula (1)]
The above chemical composition further satisfies formula (1).
0.30≤C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≤0.38 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1). Further, when the element corresponding to the element symbol in formula (1) is not contained, "0" is substituted for the corresponding element symbol in formula (1).

As mentioned above, in the chemical compositions of the present disclosure, the C, Si, Mn, Ni, Cr, Mo, V, and Nb contents enhance the hardenability of the steel.

Define F1=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3. If F1 is too low, the S-curves of ferrite, pearlite and bainite in the CCT diagram shown in FIG. 1 shift too far to the left in the diagram. In this case, the low temperature toughness of the steel is reduced. The reason for this is as follows.

In the cooling process, the magnitude of the driving force (driving force for phase transformation) for transforming from austenite to ferrite correlates with the steel material temperature. In the CCT diagram, when the steel material enters the ferrite nose (ferrite region) with a high temperature, the driving force for phase transformation is small. Therefore, ferrite transformation nuclei are less likely to be generated. However, since the steel material temperature is high, the growth of ferrite grains is rapid. Therefore, ferrite grains are coarsened. On the other hand, when the steel material enters the ferrite nose (ferrite region) at a low temperature, the driving force for phase transformation is large. Therefore, ferrite transformation nuclei are likely to be generated. Furthermore, due to the low steel material temperature, the growth of ferrite grains is slow. Therefore, ferrite grains are refined.

When F1 is too low and the S-curves of ferrite, pearlite and bainite in the CCT diagram shift too far to the left, the temperature of the steel material enters the ferrite region with a high temperature. In this case, the ferrite grains are coarsened and the effective crystal grain size is increased. Furthermore, since it tends to form a mixed grain structure, the ratio of coarse grains increases. As a result, the low temperature toughness of the steel is reduced. If F1 is too low, the hardenability is further lowered and sufficient strength cannot be obtained. If F1 is too low, the degree of integration of {100} planes will also decrease.

On the other hand, if F1 is too high, the S-curves of ferrite, pearlite and bainite in the CCT diagram shift to the right (longer time side). In this case, hard structures such as bainite and martensite are likely to form, and the ferrite fraction in the structure decreases. In this case, the low temperature toughness of the steel is reduced. If F1 is too high, the hardenability of the steel will be too high and the low temperature toughness of the steel will be reduced. If F1 is too high, the coarse crystal grain ratio will further increase.

When F1 is 0.30 to 0.38, the S curves of ferrite, pearlite and bainite are positioned appropriately when the cooling curve C1 shown in FIG. 1 is performed. Therefore, when the steel plate is cooled along the cooling curve C1 in the manufacturing process, the ferrite fraction is 60 ∼90%, and the low temperature toughness of the steel can be enhanced.

[Regarding the ferrite fraction]
The microstructure at the thickness center of the base metal portion of the electric resistance welded steel pipe for line pipe according to the present disclosure and the microstructure at the thickness center of the hot-rolled steel plate for line pipe consist of ferrite, bainite, and pearlite, The balance is precipitates and/or inclusions. Here, the thickness center portion means a range of ±20% in the thickness direction from the thickness center position when the thickness is defined as t mm. Further, the plate thickness center portion means a range of ±20% in the plate thickness direction from the plate thickness center position when the plate thickness is defined as t mm.

As described above, if the ferrite fraction in the structure at the center of the thickness of the steel is 60% or more, the effective grain size is 15.0 μm or less and the coarse grain ratio is 20% or less. , the low temperature toughness of the steel increases. If the ferrite fraction is less than 60%, the low temperature toughness is lowered. In addition, in the chemical composition of the present disclosure containing C, the upper limit of the ferrite fraction is 90%. Therefore, in the microstructure at the thickness central portion of the base metal portion of the electric resistance welded steel pipe for line pipes according to the present disclosure and the microstructure at the thickness central portion of the hot-rolled steel plate for line pipes according to the present disclosure, the ferrite fraction is 60. ~90%. A preferable lower limit of the ferrite fraction is 65%, more preferably 70%, and still more preferably 75%.

A ferrite fraction is measured by the following method. In the case of an electric resistance welded steel pipe for line pipes, a sample is taken from the thickness central portion of the base material portion of the electric resistance welded steel pipe for line pipes at a position displaced by 90° in the circumferential direction from the electric resistance welded portion. In the case of the hot-rolled steel sheet for line pipes, a sample is taken from the central portion of the plate thickness of the hot-rolled steel sheet for line pipes.

The observed surface of the collected sample is polished with a colloidal silica abrasive for 30 to 60 minutes. The ferrite fraction (%) of the polished sample is determined by the KAM (Kernel Average Misorientation) method using EBSP-OIM as follows. The viewing field plane for measuring the ferrite fraction by the KAM method is a rectangle of 200 μm in the RD direction and 500 μm in the ND direction. The observation magnification is 400 times and the measurement step is 0.3 μm.

In the KAM method, any one regular hexagonal pixel in the measurement data is taken as the central pixel. A first approximation using 6 pixels adjacent to this center pixel (7 pixels in total), or a second approximation using 12 pixels further outside of these 6 pixels (19 pixels in total). ), or a third approximation using 18 pixels outside of these 12 pixels (total of 37 pixels), obtain the orientation difference between each pixel. The obtained orientation differences are averaged, and the obtained average value is taken as the value of the central pixel. This operation is performed for all pixels. In this disclosure, a third approximation will display those that result in an orientation difference of 5° or less between adjacent pixels. In the present disclosure, the ferrite fraction is defined as the area fraction of pixels with a misorientation difference of 1° or less calculated to the third approximation with respect to the total area of the viewing range. A structure other than ferrite such as bainite has a misorientation exceeding 1° in the third approximation.

[Effective grain size]
Further, in the present disclosure, the microstructure at the thickness central portion of the base metal portion of the electric resistance welded steel pipe for line pipes and the microstructure at the thickness central portion of the hot-rolled steel plate for line pipes have an effective grain size of 15.5. 0 μm or less. If the effective grain size is too large, the low temperature toughness of the steel will decrease. In the present disclosure, since the effective grain size is 15.0 μm or less, excellent low temperature toughness can be obtained. A preferable upper limit of the effective crystal grain size is 14.0 μm, more preferably 13.0 μm.

Effective grain size is measured using EBSP-OIM. Specifically, a sample is taken and polished in the same manner as the measurement of the ferrite fraction. Polished samples are analyzed using EBSP-OIM. More specifically, in orientation measurement at each measurement step (0.3 μm), the grain boundary is defined as a position where the orientation difference between adjacent measurement points exceeds 15°. 15° is the threshold value of the large tilt angle grain boundary, which is generally recognized as the grain boundary. The grain size and the surface area of the crystal grains are obtained by using the region surrounded by the grain boundaries as the crystal grains. The area-average particle size is determined from the obtained particle size and surface area. Specifically, when the region surrounded by grain boundaries is defined as a crystal grain, and the circle-equivalent diameter of a crystal grain is defined as a crystal grain size, the number distribution for each crystal grain size is multiplied by the average area of the grain size. The average value (particle size corresponding to the average area) of the distribution of the grains is defined as the area average particle size. In this specification, the determined area-average grain size is defined as the effective crystal grain size. The visual field range is 200 μm (RD direction)×500 μm (ND direction) centered on the center of the thickness. The observation magnification is 400 times and the measurement step is 0.3 μm.

[Regarding coarse grain ratio]
In the present disclosure, the crystal grain size is 20 μm or more in the microstructure at the thickness central portion of the base material portion of the electric resistance welded steel pipe for line pipes and the microstructure at the plate thickness central portion of the hot-rolled steel plate for line pipes. The "coarse crystal grain ratio", which is the area ratio of ferrite crystal grains, is 20% or less. If the grains are coarse, the low temperature toughness of the steel is reduced. If the coarse grain ratio is 20% or less, excellent low temperature toughness can be obtained. A preferable upper limit of the coarse grain ratio is 18%, more preferably 15%. The lower the coarse grain ratio, the better.

The coarse grain ratio is measured using EBSP-OIM. Samples are taken and polished as in the ferrite fraction measurement. Polished samples are analyzed using EBSP-OIM. The visual field range is 200 μm (RD direction)×500 μm (ND direction) centered on the center of the thickness. The observation magnification is 400 times and the measurement step is 0.3 μm. It can be obtained by substituting N for the area of the visual field range of the measurement object observed in the EBSP-OIM measurement and n for the area of the coarse crystal grains into the formula (3).
Coarse grain ratio (%) = (n/N) x 100 (3)

[Regarding coarse specific particles]
In the present disclosure, the maximum diameter is 3 μm or more in the microstructure at the thickness central portion of the base material portion of the electric resistance welded steel pipe for line pipes and the microstructure at the plate thickness central portion of the hot-rolled steel plate for line pipes, The number density of the specific coarse particles having a total Ti and Nb content of 10% by mass or more is 50 particles/100 mm ² or less. In the present specification, the maximum diameter is the longest line segment having both ends on the outer periphery of each specific particle when observed with an SEM at a magnification of 400 times and a visual field area of 200 μm × 500 μm. means the length of a line segment.

The coarse specific particles may be Nb carbonitrides, Ti carbonitrides, etc., and/or composite particles thereof. In the present specification, "carbonitrides, etc." is a generic term for carbides, nitrides, and carbonitrides. The specific coarse particles are inclusions or precipitates having a total content of Ti and Nb of 10% by mass or more. In the coarse specific particles, the Ti content may be 0% by mass, and the Nb content may be 0% by mass, as long as the total content of Ti and Nb is 10% by mass or more.

When the specific coarse particles are present inside the steel, the specific coarse particles act as starting points for cracks. As a result, the low temperature toughness is lowered. Therefore, in order to further lower the DWTT guarantee temperature, it is necessary to reduce the coarse specific particles present inside the steel.

Suppressing the number density of the specific coarse particles increases solid solution Nb and solid solution Ti. Solid-solution Nb and solid-solution Ti suppress the recrystallization of austenite, so that the rolling texture tends to develop. As a result, in the metallographic structure after transformation, the degree of accumulation of {100} planes in the specific plane at 45° can be dramatically increased.

When the number density of the specific coarse particles exceeds 50/100 mm ² , the specific coarse particles become starting points of cracks. When the number density of the specific coarse grains exceeds 50/100 mm ² , the degree of accumulation of the {100} plane is further lowered in the specific plane at 45°. As a result, the low temperature toughness is lowered. Therefore, the number density of the specific coarse particles is 50 particles/100 mm ² or less. It is preferable that the number density of the coarse specific particles is as low as possible.

The number density of coarse specific particles is measured using a scanning electron microscope (SEM). Here, the central portion of the steel is a portion where the cooling rate is slower than the vicinity of the surface of the steel material. That is, the center of the steel is the last part to solidify. Therefore, the coarse specific particles tend to concentrate in the center of the steel. In the case of hot-rolled steel sheets for line pipes, the central portion of the steel is the plate thickness central portion. When the electric resistance welded steel pipe for line pipe is manufactured, the central portion of the steel becomes the thickness central portion of the electric resistance welded steel pipe for line pipe. Therefore, in the case of an electric resistance welded steel pipe for line pipes, a sample is taken from the thickness central portion at a position shifted by 90° in the circumferential direction from the electric resistance welded portion. In the case of the hot-rolled steel sheet for line pipes, a sample is taken from the central portion of the plate thickness of the hot-rolled steel sheet for line pipes. The observed surface of the collected sample is polished with a colloidal silica abrasive for 30 to 60 minutes. Observe the polished sample using an SEM. The observation range is set to 10 mm (RD direction)×5 mm (ND direction) centering on the thickness central portion (or plate thickness central portion). The observation magnification is 400 times. In the SEM image, particles with a maximum diameter of 3 μm or more and a total content of Ti and Nb of 10% by mass or more are identified as coarse by quantitative analysis using an energy dispersive X-ray fluorescence spectrometer (EDS). Identify as particles. The number density of the specific coarse particles is obtained by counting the number of the specific coarse particles existing in the observation range and dividing by the area of the observation range.

[The degree of integration of the {100} plane in the specific plane: 1.85 to 3.50]
In the present disclosure, the surface perpendicular to the rolling direction is the RD surface and the rolled surface is the ND surface in the thickness center portion of the base material portion of the electric resistance welded steel pipe for line pipes and the thickness center portion of the hot-rolled steel plate for line pipes. When a plane perpendicular to the plane, the RD plane and the ND plane is defined as the TD plane, in a specific plane having an angle of 45° with the RD plane and an angle of 45° with the TD plane, {100 } planes have a degree of integration of 1.85 to 3.50. In the present disclosure, since the number density of the specific coarse particles is 50/100 mm ² or less, the degree of accumulation of the {100} plane in the specific plane at 45° can be 1.85 to 3.50. . In the specific plane at 45°, the {100} plane accumulation degree is 1.85 to 3.50, so that the propagation of cracks in the RD direction is suppressed, and the electric resistance welded steel pipe for line pipe and line pipe The low-temperature toughness of the hot-rolled steel sheet for steel is further enhanced. Specifically, the guaranteed DWTT temperature of the electric resistance welded steel pipe for line pipe can be set to −40° C. or lower.

When the {100} plane density in the specific plane is less than 1.85, the low temperature toughness is rapidly degraded. On the other hand, when the {100} plane accumulation degree in the specific plane is 1.85 or more, the ductile fracture rate can be maintained at 85% or more in the DWTT test at −40° C., exhibiting excellent low temperature toughness. Although the upper limit of the degree of integration of the {100} plane is not particularly limited, 3.50 or less at the DWTT guaranteed temperature of -40°C is sufficient. Therefore, the degree of integration of {100} planes in the specific plane is 1.85 to 3.50. A preferable lower limit of the degree of {100} plane accumulation in the specific plane is 1.90, more preferably 1.95, still more preferably 2.00, still more preferably 2.50. The preferred upper limit of the degree of integration of {100} planes is 3.40.

[Method for measuring degree of integration of {100} plane in specific plane]
The degree of integration of {100} planes in a specific plane is measured using EBSP-OIM. Specifically, in the case of an electric resistance welded steel pipe for line pipes, a sample is taken from the thickness central portion of the base material portion of the electric resistance welded steel pipe for line pipes at a position displaced by 90° in the circumferential direction from the electric resistance welded portion. . In the case of the hot-rolled steel sheet for line pipes, a sample is taken from the central portion of the plate thickness of the hot-rolled steel sheet for line pipes. The sample taken is polished with a colloidal silica abrasive for 30-60 minutes. The polished samples are analyzed using the EBSD method of EBSP-OIM. The measurement conditions for the EBSD method are magnification: 400 times, visual field area: 200 μm×500 μm, and measurement step: 0.3 μm. Texture analysis of an inverse pole figure in a direction perpendicular to a specific plane having an angle of 45° with the RD surface and an angle of 45° with the TD surface using the EBSD measurement using the spherical harmonics method is carried out to obtain the degree of integration of {100} planes in a specific plane having an angle of 45° with the RD plane and an angle of 45° with the TD plane.

Microscopically, the circumferential direction of the electric resistance welded steel pipe for line pipes coincides with the TD direction of the hot-rolled steel plate for line pipes. Therefore, the specific surface shown in FIG. 1 is similarly shown in the electric resistance welded steel pipe for line pipe. Therefore, regardless of whether the steel material for line pipes is an electric resistance welded steel pipe for line pipes or a hot-rolled steel plate for line pipes, the degree of accumulation of {100} planes is similarly measured.

The ferrite fraction is 60 to 90% in the microstructure at the thickness central portion of the base metal portion of the electric resistance welded steel pipe for line pipes, or the microstructure at the plate thickness central portion of the hot-rolled steel plate for line pipes, and the ferrite crystals The effective crystal grain size of the grains is 15.0 μm or less, the coarse crystal grain ratio, which is the area ratio of ferrite crystal grains having a crystal grain size of 20 μm or more, is 20% or less, and the number density of the coarse specific grains is 50/ 100 mm ² or less, and when the surface perpendicular to the rolling direction is defined as the RD surface, the rolled surface as the ND surface, and the RD surface and a surface perpendicular to the ND surface as the TD surface, the angle formed with the RD surface is 45°. and if the degree of integration of {100} planes on a specific plane with an angle of 45° with the TD plane is 1.85 to 3.50, the guaranteed DWTT temperature should be −40° C. or less. can be obtained, and excellent low temperature toughness can be obtained.

[Axial yield strength YS of electric resistance welded steel pipe for line pipe]
In the electric resistance welded steel pipe for line pipe of the present disclosure, the preferred yield strength YS in the RD direction of the base material portion, that is, in the axial direction of the electric resistance welded steel pipe is 450 to 540 MPa. If the yield strength YS in the axial direction is 450 MPa or more, sufficient strength for use as a line pipe is obtained. Further, if the yield strength YS in the axial direction is 540 MPa or less, it is advantageous in suppressing bending deformation or buckling when laying a pipeline made of an electric resistance welded steel pipe for line pipe on the seabed or the like. A more preferable lower limit of the axial yield strength YS is 460 MPa, more preferably 480 MPa. A more preferable upper limit of the yield strength YS in the axial direction is 530 MPa, more preferably 520 MPa.

[Axial tensile strength TS of electric resistance welded steel pipe for line pipe]
In the electric resistance welded steel pipe for line pipe of the present disclosure, the preferred tensile strength TS in the RD direction of the base material portion, that is, in the axial direction of the electric resistance welded steel pipe is 510 to 625 MPa. If the tensile strength TS in the axial direction is 510 MPa or more, sufficient strength for line pipe applications is obtained. Further, if the tensile strength TS in the axial direction is 625 MPa or less, it is advantageous in suppressing bending deformation or buckling when laying a pipeline made of an electric resistance welded steel pipe for line pipe on the seabed or the like. A preferable lower limit of the tensile strength TS in the axial direction is 530 MPa, more preferably 540 MPa, further preferably 545 MPa. A preferable upper limit of the tensile strength TS in the axial direction is 620 MPa, more preferably 600 MPa.

Yield strength YS and tensile strength TS in the axial direction can be measured by the following methods. A full-thickness tensile test piece is taken from a base metal portion of an electric resistance welded steel pipe for line pipe, which is displaced by 90° in the pipe circumferential direction from the electric resistance welded portion. The longitudinal direction of the tensile test piece shall be parallel to the axial direction of the electric resistance welded steel pipe for line pipe. The cross section of the tensile test piece (the cross section parallel to the width direction and thickness direction of the tensile test piece) has an arc shape. The parallel portion of the tensile test piece has a length of 50.8 mm and a width of 38.1 mm. In the present disclosure, the above tensile test piece is used to perform a tensile test at room temperature in accordance with the API standard 5CT. Normal temperature is, for example, 24°C. Axial yield strength YS (MPa) and tensile strength TS (MPa) are determined based on the results of the tensile test.

[Size of electric resistance welded steel pipe for line pipe]
The size of the electric resistance welded steel pipe for line pipe of the present disclosure is not particularly limited. For example, the electric resistance welded steel pipe for line pipe of the present disclosure has a wall thickness of 12 to 25 mm and an outer diameter of 304.8 to 660.4 mm.

[Production method]
An example of a method for manufacturing the hot-rolled steel sheet for line pipe and the electric resistance welded steel pipe for line pipe will be described. FIG. 4 is a flowchart showing an example of the manufacturing process.

Referring to FIG. 4, in this manufacturing method, a slab as a raw material is manufactured using molten steel satisfying the chemical composition described above (material preparation step: S0). The manufactured slab is heated in a heating furnace (heating step: S1). The heated slab is rolled using a rough rolling mill and a finishing rolling mill to produce a steel plate (rolling step: S2). In the rolling step (S2), the slab is subjected to rough rolling to manufacture a rough bar (rough rolling step: S21). Further, the rough bar is subjected to finish rolling by a finish rolling mill to manufacture a steel plate (finish rolling step: S22). The manufactured steel plate is cooled using a run out table (ROT: Run Out Table) (ROT cooling step: S3). In the ROT cooling step (S3), first, the steel sheet is strongly cooled by a water cooling device (strong cooling step S31). After intense cooling, the steel plate is slowly cooled (slow cooling step: S32). The steel sheet after the ROT cooling process is wound up (winding process: S4). A hot-rolled steel sheet for line pipes is manufactured through the above-described manufacturing process.

Furthermore, an electric resistance welded steel pipe for line pipe is manufactured using the hot-rolled steel plate for line pipe (pipe making step: S5). In the pipe-making process, a forming roll is used to form a hot-rolled steel sheet for a line pipe into a cylindrical base pipe (open pipe). The formed blank tube is formed so that the plate width direction (hereinafter also referred to as the TD direction) of the hot-rolled steel sheet coincides with the circumferential direction of the blank tube. Electric resistance welding is performed on butt portions extending in the longitudinal direction of the tube. Further, if necessary, the electric resistance welded portion is subjected to well-known seam heat treatment. An electric resistance welded steel pipe for line pipes is manufactured by the pipe manufacturing process described above. Each step will be described in detail below.

[Material preparation step (S0)]
A material having the chemical composition described above is prepared. Specifically, molten steel having the chemical composition described above is produced. A raw material (slab) is manufactured using molten steel. You may manufacture a slab by a continuous casting method. An ingot may be manufactured using molten steel, and the ingot may be bloomed to manufacture a raw material (slab).

[Heating step (S1)]
In the heating step (S1), the manufactured slab is heated in a heating furnace. The heating temperature of the slab in the heating furnace is preferably 1060-1200°C. If the heating temperature is 1060° C. or higher, precipitation strengthening after rolling can be obtained, and appropriate strength can be obtained. If the heating temperature is 1200° C. or less, coarsening of crystal grains (austenite grains) can be suppressed. If the heating temperature is 1200° C. or less, the temperature T ₀ (° C.) at the exit side of the final stand in the next step of rough rolling can be maintained appropriately. Therefore, the heating temperature is 1060-1200°C. A preferable lower limit of the heating temperature is 1100°C. A preferable upper limit of the heating temperature is 1160°C.

Further, in the heating step (S1), the time in the furnace is preferably 150 to 400 minutes. If the time in the furnace is 150 minutes or longer, the coarse specific particles crystallized or precipitated during solidification after continuous casting are solid-dissolved. Therefore, the number density of the coarse specific particles can be 50 particles/100 mm ² or less. If the number density of the specific coarse particles is 50 particles/100 mm ² or less, the solid solution Nb and the solid solution Ti increase in the subsequent rolling step. Solid-solution Nb and solid-solution Ti suppress the recrystallization of austenite, so that the rolling texture tends to develop. As a result, in the metallographic structure after transformation, the degree of accumulation of {100} planes in the specific plane at 45° increases dramatically. Therefore, the time in the furnace is 150 to 400 minutes. A preferable lower limit of the time in the furnace is 200 minutes. A preferable upper limit of the time in the furnace is 350 minutes.

[Rolling step (S2)]
In the rolling step (S2), the slab heated in the heating step (S1) is hot-rolled using a rough rolling mill and a finishing rolling mill to form a steel sheet. The rolling process (S2) includes a rough rolling process (S21) and a finish rolling process (S22).

[Rough rolling step (S21)]
In the rough rolling step (S21), the prepared slab is subjected to rough rolling to produce a rough rolled plate (rough bar). The roughing mill may be a reverse type or a tandem type having a plurality of rolling stands arranged in a row. The cumulative rolling reduction in rough rolling is not particularly limited as long as the effects of the present disclosure can be obtained, but is preferably 60 to 75%.

The time from immediately after the end of rough rolling to the start of finish rolling in the next step is controlled according to the temperature at the delivery side of the final stand for rough rolling. Note that "immediately after completion of rough rolling" means within 5 m from the exit side of the final stand for rough rolling.

The shorter the time from immediately after the end of rough rolling to the start of finish rolling, the better. If the time from the end of rough rolling to the start of finish rolling is short, it becomes difficult to recrystallize the steel after rough rolling and before finish rolling. In this case, the finish rolling can be performed while maintaining the shape of the crystal grains flattened by the rough rolling. As a result, the degree of integration of the {100} plane in the specific plane tends to increase.

The time from immediately after the end of rough rolling to the start of finish rolling can be changed according to the temperature on the delivery side of the final stand for rough rolling. More specifically, if the temperature on the delivery side of the final stand in rough rolling is low, it becomes easier to maintain the shape of flattened crystal grains. In this case, the time from immediately after the end of rough rolling to the start of finish rolling can be lengthened. If the temperature on the delivery side of the final stand in rough rolling is high, it becomes difficult to maintain the shape of the flattened crystal grains. In this case, it is necessary to shorten the time from immediately after the end of rough rolling to the start of finish rolling.

The temperature T ₀ (° C.) at the delivery side of the final stand for rough rolling and the time t ₀ (s) from immediately after the end of rough rolling to the start of finish rolling satisfy the following equation (2).
t ₀ (s) ≤ -3.7 T ₀ (°C) + 3686 (2)
Define F2=-3.7T ₀ (°C)+3686. When the heating temperature is within the above range and t ₀ (s) is F2 or less, recrystallization does not occur and the shape of crystal grains flattened by rough rolling can be easily maintained. As a result, the {100} planes tend to accumulate on the specific plane. On the other hand, if t ₀ (s) exceeds F2, recrystallization occurs and the shape of crystal grains flattened by rough rolling cannot be maintained. As a result, {100} planes are less likely to accumulate on a specific plane.

[Finish rolling step (S22)]
In the finish rolling step, the obtained rough rolled plate is subjected to finish rolling by a finish rolling mill to produce a steel plate.

In the finish rolling step, rolling may be performed using a tandem finishing mill including a plurality of rolling stands arranged in a row (each rolling stand having a pair of work rolls). Alternatively, rolling may be performed using a reverse rolling mill having a pair of work rolls.

In the finish rolling process, the surface temperature of the steel sheet on the delivery side of the final stand of the finish rolling mill is defined as the finish rolling temperature (°C). The finish rolling temperature (°C) is preferably low. A low temperature is, specifically, 800° C. or lower. If the finish rolling temperature is 800° C. or less, the rolling texture and its transformation texture develop. Thereby, the degree of integration of the {100} plane can be increased.

However, the finish rolling temperature (°C) is preferably equal to or higher than the Ar ₃ transformation temperature. If the finish rolling temperature is equal to or higher than the Ar ₃ transformation temperature, the rolling resistance of the steel sheet can be reduced, and productivity increases. If the finish rolling temperature is equal to or higher than the Ar ₃ transformation temperature, it is possible to further prevent the steel sheet from being rolled in the two-phase region of ferrite and austenite. In this case, it is possible to suppress the microstructure of the steel sheet from forming a layered structure, and the mechanical properties are enhanced. Therefore, the finish rolling temperature is preferably equal to or higher than the _Ar3 transformation temperature. In the steel material for line pipe of the present disclosure having the chemical composition described above, the Ar ₃ transformation temperature is 700-750°C.

It is preferable that the cumulative rolling reduction in finish rolling is 60 to 80%. In this case, the degree of integration of the {100} plane on the specific plane is further increased. The cumulative rolling reduction (%) in the finish rolling process can be obtained by the following formula.
Cumulative rolling reduction (%) = {1-(thickness of steel plate after finish rolling/thickness of rough bar before finish rolling)} × 100

From the above, the heating temperature is set to 1060 to 1200° C., the time in the furnace is set to 150 to 400 minutes, the time t ₀ (s) from immediately after the end of rough rolling to the start of finish rolling is F2 or less, and the finish rolling temperature is Ar ₃ . If the temperature is higher than the transformation point and lower than 800° C., the degree of integration of {100} planes in the specific plane can be 1.85 or higher.

The thickness of the steel sheet after the finish rolling process is not particularly limited, but is, for example, 12 to 25 mm. By using the manufacturing method of the present disclosure, excellent low temperature toughness can be obtained even if the plate thickness is 12 mm or more.

[ROT cooling step (S3)]
In the ROT (run out table) cooling step (S3), the steel plate manufactured in the rolling step (S2) is cooled. The ROT cooling step (S3) preferably includes a strong cooling step (S31) and a slow cooling step (S32). As a result, the ferrite fraction is increased in the thickness central portion of the hot-rolled steel sheet for line pipe and the thickness central portion of the electric resistance welded steel pipe for line pipe, and the low-temperature toughness of the steel is enhanced. This point will be described in detail below.

FIG. 1 is an example of a CCT diagram of a line pipe steel material according to the present disclosure. In FIG. 1, F indicates ferrite nose, P indicates pearlite nose, and B indicates bainite nose.

Referring to the CCT diagram of FIG. 1, the ferrite nose exists at a higher position than the pearlite nose and the bainite nose. A broken-line cooling curve C2 in FIG. 1 indicates a cooling curve according to a conventional cooling process. The dashed cooling curve C2 may pass through the pearlite nose. Conventional cooling methods pass through all of the ferrite nose, pearlite nose and/or bainite nose at a uniform rate during the cooling process. Therefore, a large amount of pearlite and/or bainite is generated in the structure, and the ferrite fraction in the structure is reduced.

Therefore, in the present disclosure, cooling is performed, for example, along the dashed cooling curve C1. Specifically, in the early stages of cooling, strong cooling is performed up to the vicinity of the ferrite nose (S31). When the steel is cooled rapidly by intense cooling, it creates multiple strains in the steel, resulting in multiple nucleation sites in the unrecrystallized structure. After strong cooling, slow cooling is performed (S32). At this time, the temperature of the steel is kept within the ferrite region in FIG. As a result, fine ferrite is generated from numerous nucleation sites generated during intense cooling. As a result, the ferrite fraction in the structure is increased and the crystal grains are refined. Therefore, the low temperature toughness of the steel is enhanced. The dashed cooling curve C1 may pass through the pearlite nose.

[Strong cooling step (S31)]
First, the steel sheet is strongly cooled. Intensive cooling is, for example, water cooling with a water cooling device. Although the surface temperature of the steel sheet immediately before water cooling is not particularly limited, it is preferably the Ar ₃ transformation temperature or higher. If the surface temperature of the steel sheet immediately before water cooling is equal to or higher than the Ar ₃ transformation temperature, it is possible to prevent reduction in strength due to coarsening of crystal grains due to grain growth.

Let the cooling rate in the strong cooling step (S31) be V1 (°C/s). V1 is calculated by heat conduction. V1 is preferably 5.0° C./s or more at the plate thickness central portion. If the cooling rate V1 is less than 5.0° C./s, the degree of supercooling due to cooling is insufficient, and sufficient ferrite nucleation sites cannot be obtained. In this case, since the amount of ferrite grains produced is reduced, the ferrite grains are coarsened and the low-temperature toughness of the steel is lowered. Therefore, the cooling rate V1 is 5.0° C./s or more. A preferable lower limit of the cooling rate V1 is 7.0° C./s, more preferably 8.0° C./s. Although the upper limit of the cooling rate V1 is not particularly limited, it is preferably 40.0° C./s, more preferably 30.0° C./s, still more preferably 20.0° C./s.

In the strong cooling step (S31), the steel plate is cooled until the surface temperature of the steel plate reaches 580 to 680°C. In other words, the strong cooling stop temperature T1 is 580-680.degree. If the strong cooling stop temperature T1 is too low, the steel sheet temperature passes through the ferrite region and reaches the pearlite region and/or the bainite region in the CCT diagram. In this case, the ferrite fraction decreases and the low temperature toughness of the steel decreases. On the other hand, if the strong cooling stop temperature T1 is too high, the precipitation of Nb, which strengthens the pro-eutectoid ferrite, is overaged, and the strength of the steel is lowered. If the strong cooling stop temperature T1 is set to 580 to 680° C., slow cooling in the subsequent slow cooling step (S32) can increase the ferrite fraction to 60% or more, thereby enhancing the low temperature toughness of the steel. The strong cooling stop temperature T1 is preferably 600 to 670°C, more preferably 610 to 670°C.

[Slow cooling step (S32)]
Slow cooling is performed with respect to the steel plate hard-cooled by the strong-cooling process (S31). Let the cooling rate in the slow cooling step (S32) be V2 (°C/s). V2 is calculated by heat conduction. The cooling rate V2 is preferably 2.0 to 4.0° C./s at the plate thickness central portion. If the cooling rate V2 is too slow, the crystal grains become coarse and the low temperature toughness of the steel deteriorates. If the cooling rate V2 is too fast, the steel sheet temperature passes through the ferrite region and reaches the pearlite region and/or the bainite region in the CCT diagram. In this case, the ferrite fraction decreases and the low temperature toughness of the steel decreases. Therefore, the cooling rate V2 is 2.0-4.0° C./s.

In the slow cooling step (S32), the steel plate is cooled until the surface temperature of the steel plate reaches 500 to 670°C. In other words, the slow cooling stop temperature T2 is 500 to 670° C. (where T1>T2). If the slow cooling stop temperature T2 is too low, the steel sheet temperature passes through the ferrite region and reaches the pearlite region and/or the bainite region in the CCT diagram. In this case, the ferrite fraction decreases and the low temperature toughness of the steel decreases. If the slow cooling stop temperature T2 is too high, the strength of steel will decrease. Therefore, the slow cooling stop temperature T2 is 500 to 670°C. A preferable lower limit of the slow cooling stop temperature T2 is 580°C, more preferably 590°C. A preferable upper limit of the slow cooling stop temperature T2 is 650°C, more preferably 635°C, and still more preferably 620°C.

[Winding step (S4)]
In the coiling process (S4), the steel sheet cooled in the ROT cooling process (S3) is coiled to manufacture a coil-shaped hot-rolled steel sheet for line pipes.

After the slow cooling step, the coiled hot-rolled steel sheet for line pipe is air-cooled and then coiled. The surface temperature T3 of the steel sheet during winding (hereinafter referred to as winding temperature) is 500 to 650° C. (where T2>T3). If the coiling temperature T3 is too low, the coarse grain ratio increases and the low temperature toughness of the steel decreases. On the other hand, if the coiling temperature T3 is too high, the crystal grains become coarse and the low temperature toughness of the steel deteriorates. Therefore, the winding temperature T3 is 500-650.degree. T3 is preferably 510-600°C, more preferably 520-560°C.

The hot-rolled steel sheet for line pipes of the present disclosure is manufactured by the manufacturing process described above.

[Pipe manufacturing step (S5)]
An electric resistance welded steel pipe for line pipe is manufactured by a well-known method while unwinding the coiled hot-rolled steel plate for line pipe. Specifically, a hot-rolled steel sheet for line pipe is bent into a tubular shape (open pipe) by continuous forming rolls. Subsequently, the open pipe butt portion is welded by an electric resistance welding method. If necessary, well-known seam heat treatment is performed on the electric resistance welded portion. Through the above steps, the electric resistance welded steel pipe for line pipe of the present disclosure is manufactured.

In the hot-rolled steel sheet for line pipes and the electric resistance welded steel pipe for line pipes manufactured by the above manufacturing process, the ferrite fraction is 60 to 90% in the microstructure at the center of the plate thickness or at the center of the wall thickness. The ferrite crystal grains have an effective crystal grain size of 15.0 μm or less, a coarse crystal grain ratio, which is the area ratio of crystal grains having a crystal grain size of 20 μm or more, is 20% or less, and the number density of the coarse specific grains is 50. /100 mm ² or less. Furthermore, the integration degree of the {100} plane in the specific plane is 1.85 to 3.50. As a result, the DWTT guarantee temperature becomes −40° C. or lower, and excellent low temperature toughness is obtained. The axial yield strength of the electric resistance welded steel pipe for line pipe can be adjusted to 450 to 540 MPa, and the tensile strength can be adjusted to 510 to 625 MPa.

Slabs were manufactured by continuously casting the molten steels of Steel A to Steel O shown in Table 1.

A blank in Table 1 indicates that the corresponding elemental content was below the detection limit. Using a plurality of slabs of Steel A to Steel O, electric resistance welded steel pipes for line pipes of Test Nos. 1 to 29 shown in Table 2 were manufactured.

Specifically, the slab was heated to the heating temperature shown in Table 2 in a heating furnace. Rough rolling was performed on the heated slab. The time in the furnace (minutes), the temperature T ₀ (°C) at the exit side of the final stand for rough rolling, the time t ₀ (s) from immediately after the end of rough rolling to the start of finish rolling, and F2 were as shown in Table 2. . The cumulative rolling reduction in the rough rolling process was 60 to 75% for all test numbers. After the rough rolling, finish rolling was performed at the finish rolling temperature shown in Table 2 to produce steel sheets. The cumulative rolling reduction in the finish rolling process was 60 to 80% for all test numbers. The finish rolling temperature was above the Ar ₃ transformation temperature.

ROT cooling was implemented with respect to the steel plate after finish rolling. The time from the end of finish rolling to the start of strong cooling was set within 20 seconds. In the ROT cooling step, except for Test No. 25, the cooling rate V1 was 5°C/s or more, and strong cooling was performed until the strong cooling stop temperature T1 of 580 to 680°C was reached. Next, except for test numbers 26 and 27, the slow cooling stop temperature T2 (which satisfies T1>T2) is 500 to 670 ° C. at a cooling rate V2 of 2.0 to 4.0 ° C./s. Slowly cooled to

A steel plate was manufactured by the manufacturing process described above. The obtained steel sheet was coiled at a coiling temperature T3 shown in Table 2 (where T2>T3 was satisfied) to obtain a hot-rolled steel sheet for line pipe in the form of a hot coil.

Using hot-rolled steel sheets for line pipes, pipes were manufactured by the well-known method described above to produce electric resistance welded steel pipes for line pipes having an outer diameter of 304.8 to 660.4 mm and a wall thickness of 12 to 25 mm.

[Test method]
[Strength test]
Tensile test pieces were taken from electric resistance welded steel pipes for line pipes of each test number. Specifically, when the electric resistance welded steel pipe for line pipes is viewed in the axial direction, the position of 90° from the welded portion of the electric resistance welded steel pipe for line pipes (position shifted by 90° in the pipe circumferential direction from the electric resistance welded steel pipe) Axial tensile specimens were taken. The cross section of the tensile test piece was arc-shaped, and the longitudinal direction of the tensile test piece was parallel to the longitudinal direction of the steel pipe. The size of the tensile test piece was as shown in FIG. 5, the length of the parallel portion was 50.8 mm, and the width of the parallel portion was 38.1 mm. Numerical values in FIG. 5 indicate dimensions (unit: mm) of corresponding portions of the test piece. A tensile test was performed at room temperature (24° C.) using the tensile test piece in accordance with the API standard 5CT. Yield strength YS (MPa) and tensile strength TS (MPa) of electric resistance welded steel pipes for line pipes were obtained based on the test results.

[Microstructure]
Regarding the electric resistance welded steel pipe for line pipe, the ferrite fraction, effective grain size, and coarse grain ratio at the thickness central portion of the base metal portion were measured using EBSP-OIM based on the above method. The measurement conditions of EBSP-OIM in effective crystal grain size measurement were magnification: 400 times, visual field area: 200 μm×500 μm, measurement step: 0.3 μm. As analysis software for EBSP-OIM, "TSL OIM Analysis 7" manufactured by TSL Solutions was used. In addition, in the measurement of the ferrite fraction, the type of the remainder (that is, the structure other than ferrite) in the metal structure of the thickness central portion of the base material portion was also confirmed. The ferrite fraction, effective crystal grain size, and coarse grain ratio at the plate thickness central portion of the hot-rolled steel sheet for line pipe were also measured in the same manner as for the electric resistance welded steel pipe for line pipe. As a result, the ferrite fraction, effective grain size, and coarse grain ratio of the hot-rolled steel sheet for line pipe were all the same as those of the corresponding electric resistance welded steel pipe for line pipe.

[Number density of coarse specific particles]
Regarding the electric resistance welded steel pipes for line pipes, the number density of the coarse specific particles in the thickness central portion of the base material portion was measured based on the method described above. More specifically, a sample was taken from the thickness central portion of the base metal portion of the electric resistance welded steel pipe for line pipe at a position shifted by 90° in the circumferential direction from the electric resistance welded portion. The measurement conditions of the scanning electron microscope (SEM) were magnification: 400 times, observation range: 10 mm (RD direction) x 5 mm (ND direction). In the SEM image, the maximum diameter is 3 μm or more, and the total content of Ti and Nb is 10% by mass or more by quantitative analysis using an energy dispersive X-ray fluorescence spectrometer (EDS). identified as particles. The number density of the specific coarse particles was obtained by counting the number of the specific coarse particles present in the observation range and dividing by the area of the observation range.

[The degree of integration of the {100} plane in the specific plane]
Regarding the electric resistance welded steel pipe for line pipe, the degree of integration of the {100} plane was measured using EBSP-OIM based on the method described above. The measurement conditions for EBSP-OIM were magnification: 400 times, visual field area: 200 μm×500 μm, and measurement step: 0.3 μm. In addition, the degree of accumulation of {100} planes on a specific plane in the thickness central portion of the hot-rolled steel sheet for line pipe was obtained by the method described above. As a result, the density of the {100} planes on the specific surface of the electric resistance welded steel pipe for line pipe was the same as that of the corresponding electric resistance welded steel pipe for line pipe.

[Low temperature toughness test]
A DWTT test piece was taken from the center of the wall thickness at a position shifted by 90° in the circumferential direction from the electric resistance welded portion of the electric resistance welded steel pipe for line pipe of each test number. An arc-shaped member sampled in the pipe circumferential direction from the sampling position was developed into a flat plate, and a notch was processed at a 90° position. The size of the DWTT specimen was as shown in FIG. Numerical values in FIG. 6 indicate dimensions (unit: mm) of corresponding portions of the test piece. t indicates the thickness (unit: mm). The longitudinal direction of the DWTT test piece corresponded to the circumferential direction of the electric resistance welded steel pipe for line pipe. A DWTT test was performed on the DWTT specimens in accordance with the ASTM E 436 specification. The lowest temperature (DWTT guarantee temperature) at which the ductile fracture surface ratio is 85% was obtained. When the DWTT guarantee temperature was −40° C. or less, it was evaluated that the low temperature toughness was high.

[Test results]
Table 3 shows the test results. In Table 3, "P, B" means at least one of pearlite and bainite.

With reference to Tables 1 to 3, the chemical compositions of the steels of Test Nos. 1 to 13 were appropriate and satisfied formula (1). Furthermore, the manufacturing conditions for any test number were appropriate. Therefore, in Test Nos. 1 to 13, the ferrite fraction was 60 to 90%, the effective grain size was 15.0 μm or less, and the coarse grain ratio was 20% or less. Furthermore, since the number density of the coarse specific particles was 50 particles/100 mm ² or less, the degree of accumulation of {100} planes was 1.85 to 3.50. As a result, the DWTT guaranteed temperature was −40° C. or lower, indicating excellent low-temperature toughness. Furthermore, the axial yield strength YS of the electric resistance welded steel pipes for line pipes was 450 to 540 MPa, and the tensile strength TS was 510 to 625 MPa.

On the other hand, in Test Nos. 14 and 15, F1 was less than the lower limit of formula (1), although the manufacturing conditions were appropriate. Therefore, the crystal grains became coarse, and the effective crystal grain size exceeded 15.0 μm. Furthermore, the degree of integration of {100} planes was less than 1.85. Therefore, the DWTT guaranteed temperature was higher than -40°C and the low temperature toughness was low.

In Test No. 16 and Test No. 17, although the manufacturing conditions were appropriate, F1 exceeded the upper limit of formula (1). As a result, the ferrite fraction was less than 60%, resulting in a bainite-based structure. Because of the bainite-based structure, the {100} plane accumulation degree was 1.85 or more, but the coarse grain ratio exceeded 20%. Therefore, the DWTT guarantee temperature was higher than -40°C and the low temperature toughness was low. Furthermore, the yield strength YS of the electric resistance welded steel pipe for line pipe exceeded 540 MPa, and the tensile strength TS exceeded 625 MPa, which were too high.

In test number 18, the heating temperature exceeded 1200°C. Therefore, the temperature T ₀ (° C.) at the exit side of the final stand in rough rolling became too high, and the time t ₀ (s) from immediately after the end of rough rolling to the start of finish rolling exceeded F2. Therefore, the crystal grains became coarse, the effective crystal grain size exceeded 15.0 μm, and the coarse crystal grain ratio exceeded 20%. Furthermore, the degree of integration of {100} planes was less than 1.85. As a result, the DWTT guarantee temperature was higher than -40°C and the low temperature toughness was low.

In Test No. 19, the heating temperature was less than 1060°C. Therefore, the crystal grains became coarse, the effective crystal grain size exceeded 15.0 μm, and the coarse crystal grain ratio exceeded 20%. Furthermore, the degree of integration of {100} planes was less than 1.85. As a result, the DWTT guarantee temperature was higher than -40°C and the low temperature toughness was low. In addition, the strength was low due to insufficient precipitation strengthening of Nb.

In Test No. 20 and Test No. 21, the time in the furnace was less than 150 minutes. Therefore, the number density of the coarse specific particles exceeded 50/100 mm ² . Therefore, the degree of integration of the {100} plane was less than 1.85. As a result, the DWTT guarantee temperature was higher than -40°C and the low temperature toughness was low.

In Test Nos. 22 and 23, the time t ₀ (s) from immediately after the end of rough rolling to the start of finish rolling exceeded F2. Therefore, the degree of integration of the {100} plane was less than 1.85. As a result, the DWTT guarantee temperature was higher than -40°C and the low temperature toughness was low.

In test number 24, the finish rolling temperature was too high. Therefore, the coarse crystal grain ratio exceeded 20%. Furthermore, the degree of integration of {100} planes was less than 1.85. As a result, the DWTT guarantee temperature was higher than -40°C and the low temperature toughness was low.

In test number 25, V1 was less than 5.0°C/s. Therefore, the crystal grains became coarse, the effective crystal grain size exceeded 15.0 μm, and the coarse crystal grain ratio exceeded 20%. As a result, the DWTT guarantee temperature was higher than -40°C and the low temperature toughness was low.

In Test No. 26, V2 was less than 2.0°C/s. Therefore, the crystal grains became coarse, the effective crystal grain size exceeded 15.0 μm, and the coarse crystal grain ratio exceeded 20%. As a result, the DWTT guarantee temperature was higher than -40°C and the low temperature toughness was low.

In Test No. 27, V2 exceeded 4.0°C/s. Therefore, the ferrite fraction became less than 60%. As a result, the DWTT guarantee temperature was higher than -40°C and the low temperature toughness was low.

In Test No. 28, T3 was less than 500°C. Therefore, the coarse crystal grain ratio exceeded 20%. As a result, the DWTT guarantee temperature was higher than -40°C and the low temperature toughness was low.

In test number 29, T3 exceeded 650°C. Therefore, the crystal grains became coarse, the effective crystal grain size exceeded 15.0 μm, and the coarse crystal grain ratio exceeded 20%. As a result, the DWTT guarantee temperature was higher than -40°C and the low temperature toughness was low.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the present invention.

Claims (10)

An electric resistance welded steel pipe for a line pipe comprising a base material portion and an electric resistance welded portion,
The chemical composition of the base material portion is, in mass %,
C: 0.010 to 0.060%,
Si: 0.05 to 0.30%,
Mn: 0.50-2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010-0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005 to 0.030%,
Mo: 0.05-0.30%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
Ni: 0 to 0.50%,
V: 0 to 0.10%,
Cr: 0 to 0.30%,
Cu: 0-0.30%,
Mg: 0-0.0050%,
Rare earth elements: 0 to 0.0100% and
Balance: Fe and impurities, satisfying formula (1),
Crystals having a ferrite fraction of 60 to 90%, an effective crystal grain size of 15.0 μm or less, and a crystal grain size of 20 μm or more in the thickness central portion of the base material portion of the electric resistance welded steel pipe for line pipe. The coarse crystal grain ratio, which is the grain area ratio, is 20% or less,
The number density of the specific coarse particles having a maximum diameter of 3 μm or more and a total content of Ti and Nb of 10% by mass or more is 50 particles/100 mm ² or less,
When the surface perpendicular to the rolling direction of the electric resistance welded steel pipe for line pipe is defined as the RD surface, the rolled surface is defined as the ND surface, and the RD surface and a surface perpendicular to the ND surface are defined as the TD surface, the angle formed with the RD surface is 45° and the angle formed with the TD plane is 45°, the density of the {100} planes being 1.85 to 3.50.
0.30≤C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≤0.38 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1). The electric resistance welded steel pipe for line pipe according to claim 1,
The chemical composition, in mass %,
An electric resistance welded steel pipe for line pipes, containing Ca: greater than 0 to 0.0050%. The electric resistance welded steel pipe for line pipe according to claim 1 or claim 2,
The chemical composition, in mass %,
Ni: more than 0 to 0.50%,
V: more than 0 to 0.10%,
Cr: more than 0 to 0.30%, and
An electric resistance welded steel pipe for line pipe, containing one or more selected from the group consisting of Cu: more than 0 to 0.30%. The electric resistance welded steel pipe for line pipe according to any one of claims 1 to 3,
The chemical composition, in mass %,
Mg: more than 0 to 0.0050%, and
Rare earth element: more than 0 to 0.0100%, an electric resistance welded steel pipe for line pipe containing one or more selected from the group consisting of. The electric resistance welded steel pipe for line pipe according to any one of claims 1 to 4,
An electric resistance welded steel pipe for line pipe, having an axial yield strength of 450 to 540 MPa and an axial tensile strength of 510 to 625 MPa. The electric resistance welded steel pipe for line pipe according to any one of claims 1 to 5,
An electric resistance welded steel pipe for line pipe, having a wall thickness of 12 to 25 mm and an outer diameter of 304.8 to 660.4 mm. A hot-rolled steel sheet for line pipes,
The chemical composition, in mass %,
C: 0.010 to 0.060%,
Si: 0.05 to 0.30%,
Mn: 0.50-2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010-0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005 to 0.030%,
Mo: 0.05-0.30%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
Ni: 0 to 0.50%,
V: 0 to 0.10%,
Cr: 0 to 0.30%,
Cu: 0-0.30%,
Mg: 0-0.0050%,
Rare earth elements: 0 to 0.0100% and
Balance: Fe and impurities, satisfying formula (1),
In the central part of the thickness of the hot-rolled steel sheet for line pipe, the ferrite fraction is 60 to 90%, the effective grain size is 15.0 μm or less, and the area ratio of grains with a grain size of 20 μm or more is A certain coarse grain ratio is 20% or less,
The number density of the specific coarse particles having a maximum diameter of 3 μm or more and a total content of Ti and Nb of 10% by mass or more is 50 particles/100 mm ² or less,
When the surface perpendicular to the rolling direction of the hot-rolled steel sheet for line pipe is defined as the RD surface, the rolled surface is defined as the ND surface, and the RD surface and a surface perpendicular to the ND surface are defined as the TD surface, the angle formed with the RD surface is 45° and the angle formed with the TD plane is 45°, and the degree of accumulation of {100} planes is 1.85 to 3.50.
0.30≤C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≤0.38 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1). The hot-rolled steel sheet for line pipes according to claim 7,
The chemical composition, in mass %,
A hot-rolled steel sheet for line pipes containing Ca: more than 0 to 0.0050%. The hot-rolled steel sheet for line pipes according to claim 7 or claim 8,
The chemical composition, in mass %,
Ni: more than 0 to 0.50%,
V: more than 0 to 0.10%,
Cr: more than 0 to 0.30%, and
A hot-rolled steel sheet for line pipes, containing one or more selected from the group consisting of Cu: more than 0 to 0.30%. The hot-rolled steel sheet for line pipes according to any one of claims 7 to 9,
The chemical composition, in mass %,
Mg: more than 0 to 0.0050%, and
A hot-rolled steel sheet for line pipes, containing one or more selected from the group consisting of rare earth elements: more than 0 to 0.0100%.

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