JP7626934B2 - Electric resistance welded steel pipe - Google Patents

Description

This disclosure relates to electric-resistance welded steel pipes, and more specifically to electric-resistance welded steel pipes used in pipelines.

Pipelines laid on the seabed are composed of multiple steel pipes (line pipes). Subsea pipelines are subjected to high pressure from the production fluids, such as natural gas and crude oil, passing through the inside of the pipeline. Pipelines are also subjected to repeated strain from waves and seawater pressure from the outside. Electric resistance welded steel pipes are sometimes used as steel pipes (line pipes) for subsea pipelines. For the reasons mentioned above, electric resistance welded steel pipes used as line pipes are required to have excellent low-temperature toughness.

JP 2020-128577 A (Patent Document 1) proposes an electric resistance welded steel pipe for pipeline applications that has excellent low-temperature toughness.

The electric resistance welded steel pipe disclosed in Patent Document 1 has, in mass %, C: 0.030 to 0.100%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, P: 0.050% or less, S: 0.0050% or less, Al: 0.040% or less, Ti: 0.003 to 0.030%, Nb: 0.003 to 0.200%, N: 0.0080% or less, O: 0.005 The alloy contains 0% or less, Cu: 0-1.00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-1.00%, V: 0-0.10%, B: 0-0.0050%, Ca: 0-0.0008%, and rare earth elements (REM): 0-0.0050%, with the balance being Fe and impurities, and has a chemical composition that satisfies formulas (1) and (2). Here, formula (1) is 0.20≦C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5≦0.53. Formula (2) is 0.120≦C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5×B≦0.220. In a specific region of the heat-affected zone of the seam heat treatment, the average Vickers hardness is 200 to 240. Furthermore, in the microstructure of the specific region, the number density of inclusions containing one or more selected from the group consisting of Ca, Al, O, and Ti is 12.0 pieces/mm2 or ^less . When the thickness of the heat-affected zone is _tS , in the microstructure of the _tS /4 part of the heat-affected zone, the area ratio of ferrite is 0 to 40%, the remainder is tempered bainite, and the average crystal grain size is 15 μm or less. In the microstructure of the _tS /2 part of the heat-affected zone, the area ratio of ferrite is 0 to 50%, the remainder is tempered bainite, and the average crystal grain size is 15 μm or less. When the thickness of the base material is _tB , in the microstructures of the _tB /4 part of the base material and the _tB /2 part of the base material, the area ratio of ferrite is 0 to 50%, the remainder is bainite, and the average crystal grain size is 15 μm or less. In this electric resistance welded steel pipe, the microstructure of the heat-affected zone and the base material is made mainly of tempered bainite, and further, the number density of specific inclusions in specific regions of the heat-affected zone is suppressed, which results in high strength and excellent low-temperature toughness, as described in Patent Document 1.

JP 2020-128577 A

Incidentally, the production fluid flowing inside the electric resistance welded steel pipe used in the pipeline may contain corrosive gases such as hydrogen sulfide. In such cases, the electric resistance welded steel pipe is required to have not only excellent low-temperature toughness but also excellent HIC resistance. The electric resistance welded steel pipe disclosed in Patent Document 1 does not consider the compatibility of low-temperature toughness and HIC resistance.

The objective of this disclosure is to provide an electric resistance welded steel pipe that has excellent low-temperature toughness and excellent HIC resistance.

The electric resistance welded steel pipe according to the present disclosure is
An electric resistance welded steel pipe having a base material portion and an electric resistance welded portion,
The base material portion comprises, in mass%,
C: 0.010-0.080%,
Si: 0.05-0.40%,
Mn: 0.60-1.60%,
P: 0-0.020%,
S: 0 to 0.0010%,
Al: 0.010-0.040%,
N: 0.0010 to 0.0050%,
Nb: 0.001-0.080%,
Ti: 0.001 to 0.020%,
O: 0 to 0.0030%,
Ca: 0.0015-0.0035%,
Ni: 0 to 0.50%,
Mo: 0 to 0.50%,
V: 0-0.100%,
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 is Fe and impurities.
and satisfying formula (1),
The ferrite fraction of the base material is 50 to 90%, and the effective grain size is 15.0 μm or less,
In the electric resistance welded portion, the number density of specific inclusions containing Ca, Al and O is 2.5 pieces/ ^mm2 or less, and the average aspect ratio of the specific inclusions is 4.5 or less.
0.20≦C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≦0.29 (1)
Here, the content of each element in formula (1) is substituted in mass % for the corresponding element.

In this disclosure, the effective grain size means the area average grain size, which is the sum of the grain size and area fraction of each grain, for grains with a grain size of 0.5 μm or more, when the position where the orientation difference between adjacent measurement points (pixels) exceeds 15° is defined as a grain boundary, the area surrounded by the grain boundary is defined as a grain, and the equivalent circle diameter of the grain is defined as the grain size.

The electric resistance welded steel pipe disclosed herein has excellent low-temperature toughness and excellent HIC resistance.

FIG. 1 is a continuous cooling transformation curve diagram of an electric resistance welded steel pipe according to this embodiment. FIG. 2 is a graph showing the relationship between the average aspect ratio of specific inclusions (inclusions containing Ca, Al and O) in an electric resistance weld and the absorbed energy (J) at −20° C. in a Charpy impact test. FIG. 3 is a schematic diagram for explaining the method for measuring the average aspect ratio of specific inclusions according to this embodiment. FIG. 4 is a schematic diagram for explaining a method for measuring the average aspect ratio of specific inclusions according to this embodiment, which is related to FIG. FIG. 5 is a schematic diagram for explaining a method for measuring the average aspect ratio of specific inclusions according to this embodiment, which is related to FIG. 3 and FIG. FIG. 6 is a flow diagram showing an example of a manufacturing process for an electric resistance welded steel pipe according to this embodiment. FIG. 7 is a front view and a side view of the DWTT test piece used in the examples.

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

Among the inclusions in electric resistance welded steel pipes, sulfides such as MnS tend to elongate during rolling in the manufacturing process. Hydrogen tends to accumulate in elongated sulfides. Sulfides with accumulated hydrogen tend to become the starting point for cracks. Therefore, it is preferable to suppress the generation of sulfides and the elongation of sulfides as much as possible. By suppressing the S content, it is possible to suppress the generation of sulfides. In addition, Ca modifies sulfides and suppresses their elongation. Therefore, the accumulation of hydrogen in sulfides is suppressed, and the occurrence of cracks is suppressed. As a result, the HIC resistance of the electric resistance welded steel pipe is improved. Therefore, when considering the improvement of the HIC resistance of electric resistance welded steel pipes, it is desirable to suppress the S content as low as possible and to contain Ca from the viewpoint of chemical composition.

From the above perspectives, the chemical composition of electric resistance welded steel pipes was examined. As a result, the base metal of the electric resistance welded steel pipes contained, in mass%, C: 0.010-0.080%, Si: 0.05-0.40%, Mn: 0.60-1.60%, P: 0-0.020%, S: 0-0.0010%, Al: 0.010-0.040%, N: 0.0010-0.0050%, Nb: 0.001-0.080%, Ti: 0.001-0.020%, O: 0- It was believed that a chemical composition consisting of 0.0030%, Ca: 0.0015-0.0035%, Ni: 0-0.50%, Mo: 0-0.50%, V: 0-0.100%, Cr: 0-0.30%, Cu: 0-0.30%, Mg: 0-0.0050%, rare earth elements: 0-0.0100%, and the balance being Fe and impurities, would provide excellent HIC resistance.

Therefore, we further investigated ways to improve low-temperature toughness in electric resistance welded steel pipes having a base material with the above-mentioned chemical composition.

Normally, the strength (or hardness) and toughness of steel are in conflict. Ferrite is a soft structure. Therefore, if the structure of the base material is mainly ferrite, the low-temperature toughness of the base material of the electric resistance welded steel pipe is increased. Furthermore, if the ferrite crystal grains are fine, the low-temperature toughness is increased.

Therefore, in order to obtain not only excellent HIC resistance but also excellent low-temperature toughness, the inventors have investigated means for making the microstructure of the base material of an electric resistance welded steel pipe having the above-mentioned chemical composition into a fine ferrite-based structure from the viewpoint of chemical composition. As a result, they have found that, on the premise that the content of each element in the chemical composition of the base material is within the above-mentioned range, and further, when C, Si, Mn, Ni, Cr, Mo, V, and Nb satisfy the following formula (1), a fine ferrite-based structure is formed, and as a result, the low-temperature toughness of the electric resistance welded steel pipe is improved.
0.20≦C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≦0.29 (1)
Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %.

F1 is defined as C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/3 + Nb/3. F1 is an index related to the microstructure and crystal grains of the steel. Of the elements in the chemical composition of the base metal of an electric resistance welded steel pipe, C, Si, Mn, Ni, Cr, Mo, V, and Nb all affect the S curve (ferrite region, pearlite region, and bainite region) of the Continuous Cooling Transformation Diagram (CCT diagram) of an electric resistance welded steel pipe whose base metal has the above-mentioned chemical composition.

Figure 1 is a continuous cooling transformation curve diagram (CCT diagram) of an electric resistance welded steel pipe according to this embodiment. In other words, Figure 1 is a CCT diagram of an electric resistance welded steel pipe in which the content of each element in the chemical composition of the base material is within the above-mentioned range. In Figure 1, F indicates ferrite nose, P indicates pearlite nose, and B indicates bainite nose.

Referring to Figure 1, if F1, which is composed of C, Si, Mn, Ni, Cr, Mo, V, and Nb, is too low, the S curve in Figure 1 will shift too far to the left. In this case, the low-temperature toughness of the steel will decrease. The reasons for this are as follows.

When steel is cooled, the magnitude of the driving force for the transformation from austenite to ferrite (driving force for phase transformation) correlates with the steel temperature. When austenite transforms to ferrite when the steel temperature is high, the driving force for phase transformation is small. Therefore, ferrite transformation nuclei are not easily generated. Furthermore, because the steel temperature is high, ferrite grains grow quickly. As a result, the generated ferrite grains become coarse. On the other hand, when austenite transforms to ferrite when the steel temperature is low, the driving force for phase transformation is large. Therefore, ferrite transformation nuclei are easily generated. Furthermore, because the steel temperature is low, the generated ferrite grains grow slowly. As a result, the ferrite grains become fine.

Therefore, in the CCT diagram shown in Figure 1, if the cooling curve C1 enters the ferrite nose (ferrite region) when the steel temperature is high, the ferrite grains generated by phase transformation tend to become coarse. On the other hand, if the cooling curve C1 enters the ferrite nose (ferrite region) when the steel temperature is low, the ferrite grains generated by phase transformation tend to become fine.

If F1 is too low and the S curve shifts too far to the left in Figure 1, the cooling curve C1 will enter the ferrite region while the steel temperature is high. Therefore, as mentioned above, the ferrite grains formed by phase transformation will become coarse. As a result, the low-temperature toughness of the steel will decrease.

On the other hand, if F1 is too high, the S curve will shift too far to the right in Figure 1. In this case, the temperature at which the cooling curve C1 enters the ferrite nose (ferrite region) will be lower. As a result, the amount of hard structures such as bainite and martensite produced will increase, and the ferrite fraction in the structure will decrease. As a result, the low-temperature toughness of the steel will decrease.

If F1 is 0.20 to 0.29, the S-curves of each phase in Figure 1 (ferrite, pearlite, bainite) are positioned at appropriate positions on the CCT diagram, assuming that the content of each element in the base material is within the above-mentioned range. In this case, as shown in Figure 1, the steel can be cooled while the cooling curve C1 passes mainly through the ferrite region. As a result, a fine ferrite-based structure can be generated in the base material of the electric resistance welded steel pipe, and high low-temperature toughness can be obtained. Specifically, the ferrite fraction in the base material can be set to 50 to 90%, and the effective grain size can be set to 15.0 μm or less.

As described above, the inventors have discovered that, assuming that the content of each element in the chemical composition of the base material is within the above-mentioned range, and further satisfying formula (1), not only the HIC resistance of the electric resistance welded steel pipe but also its low-temperature toughness is improved.

However, it was found that even electric resistance welded steel pipes having the above-mentioned configuration may still not have sufficiently high low-temperature toughness. The inventors therefore investigated the cause of the insufficient low-temperature toughness of electric resistance welded steel pipes having the above-mentioned configuration. As a result, the inventors newly discovered that, in electric resistance welded steel pipes having the above-mentioned configuration, although the low-temperature toughness of the base material is sufficiently high, the low-temperature toughness of the electric resistance welded parts may not be sufficiently high. The inventors therefore investigated means for further increasing the low-temperature toughness of the electric resistance welded parts. As a result, the inventors obtained the following findings.

When the content of each element in the chemical composition of the base material is within the above-mentioned range and formula (1) is satisfied, the electric resistance welded portion contains inclusions that contain Ca, Al, and O. In this specification, these inclusions are referred to as "specific inclusions."

When the number density of the specific inclusions is high in the electric resistance welded portion, cracks originating from the specific inclusions are likely to occur. In this case, the low-temperature toughness of the electric resistance welded portion may be reduced. However, it has been found that the low-temperature toughness of the electric resistance welded portion can be improved if the number density of the specific inclusions in the electric resistance welded portion is 2.5 pieces/mm2 or ^less , provided that the content of each element in the chemical composition of the base material is within the above-mentioned range and formula (1) is satisfied.

However, there are cases where the low temperature toughness cannot be sufficiently improved simply by suppressing the number density of specific inclusions to 2.5 pieces/mm2 ^or less. The reasons are as follows.

In the electric resistance welded steel pipe according to the present embodiment, as described above, in order to enhance HIC resistance, the content of each element in the chemical composition is set within the above-mentioned range, and the Ca content is 0.0015% or more. In this case, Ca is easily contained in the inclusions, and the above-mentioned specific inclusions are generated. The specific inclusions are soft because they contain Ca. Therefore, the specific inclusions in the electric resistance welded portion may be elongated and exist in a state with a large aspect ratio. Even if the number density of the specific inclusions is 2.5 pieces/ ^mm2 or less, if there are specific inclusions with a large aspect ratio, the specific inclusions with a large aspect ratio become the starting point of cracks. As a result, the low-temperature toughness of the electric resistance welded portion decreases.

Therefore, the inventors investigated the relationship between the average aspect ratio of specific inclusions in an electric resistance weld and low temperature toughness in an electric resistance welded steel pipe in which the content of each element in the chemical composition of the base material is within the above-mentioned range, formula (1) is satisfied, and the number density of specific inclusions in the electric resistance weld is 2.5 pieces/mm2 or ^less , and obtained Figure 2.

² , in an electric resistance welded steel pipe in which the content of each element in the chemical composition of the base metal falls within the above-mentioned range, formula (1) is satisfied, and the number density of specific inclusions in the electric resistance welded portion is 2.5 pieces/mm2 or less, when the average aspect ratio of the specific inclusions is greater than 4.5, even if the average aspect ratio is reduced, the absorbed energy at -20°C does not increase significantly and is less than 100 J. On the other hand, when the average aspect ratio is 4.5 or less, the absorbed energy at -20°C is 100 J or more, and further, as the average aspect ratio is reduced, the absorbed energy at -20°C increases significantly. In other words, by setting the average aspect ratio to 4.5 or less, the low temperature toughness is increased significantly.

Based on the above findings, the electric resistance welded steel pipe of this embodiment has been completed. The electric resistance welded steel pipe of this embodiment has the following configuration.

[1]
An electric resistance welded steel pipe having a base material portion and an electric resistance welded portion,
The base material portion comprises, in mass%,
C: 0.010-0.080%,
Si: 0.05-0.40%,
Mn: 0.60-1.60%,
P: 0-0.020%,
S: 0 to 0.0010%,
Al: 0.010-0.040%,
N: 0.0010 to 0.0050%,
Nb: 0.001-0.080%,
Ti: 0.001 to 0.020%,
O: 0 to 0.0030%,
Ca: 0.0015-0.0035%,
Ni: 0 to 0.50%,
Mo: 0 to 0.50%,
V: 0-0.100%,
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 is Fe and impurities.
and satisfying formula (1),
The ferrite fraction of the base material is 50 to 90%, and the effective grain size is 15.0 μm or less,
In the electric resistance welded portion, the number density of specific inclusions containing Ca, Al, and O is 2.5 pieces/ ^mm2 or less, and the average aspect ratio of the specific inclusions is 4.5 or less.
Electric resistance welded steel pipe.
0.20≦C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≦0.29 (1)
Here, the content of each element in formula (1) is substituted in mass % for the corresponding element.

[2]
An electric welded steel pipe according to [1],
The base material portion comprises, in mass%,
Ni: 0.01-0.50%,
Mo: 0.01-0.50%,
V: 0.001-0.100%,
Cr: 0.01-0.30%,
Cu: 0.01-0.30%,
Mg: 0.0010 to 0.0050%, and
Rare earth elements: 0.0010 to 0.0100%, containing one or more selected from the group consisting of
Electric resistance welded steel pipe.

[3]
An electric welded steel pipe according to [1] or [2],
The thickness is 10.0 to 25.4 mm.
Electric resistance welded steel pipe.

The electric resistance welded steel pipe according to this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[Configuration of electric resistance welded steel pipe]
The electric resistance welded steel pipe according to the present embodiment has a base material portion and an electric resistance welded portion. The base material portion is cylindrical. The electric resistance welded portion extends in the longitudinal direction of the electric resistance welded steel pipe.

[Chemical composition]
The chemical composition of the base material of the electric resistance welded steel pipe according to this embodiment contains the following elements.

C: 0.010-0.080%
Carbon (C) increases the strength of the steel material. If the C content is less than 0.010%, the above effect cannot be sufficiently obtained even if the other element contents are within the range of this embodiment. On the other hand, if the C content exceeds 0.080%, C forms an excessive amount of carbides. In this case, even if the other element contents are within the range of this embodiment, the low-temperature toughness of the electric resistance welded steel pipe decreases. If the C content exceeds 0.080%, the weldability of the steel material further decreases. Therefore, the C content is 0.010 to 0.080%. The preferred lower limit of the C content is 0.015%, more preferably 0.020%, more preferably 0.025%, and even more preferably 0.030%. The preferred upper limit of the C content is 0.075%, more preferably 0.070%, more preferably 0.065%, and even more preferably 0.060%.

Si: 0.05-0.40%
Silicon (Si) deoxidizes steel. If the Si content is less than 0.05%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Si content exceeds 0.40%, the strength of the electric resistance welded steel pipe increases excessively even if the contents of other elements are within the range of this embodiment. As a result, the low temperature toughness of the electric resistance welded steel pipe decreases. Therefore, the Si content is 0.05 to 0.40%. The preferred lower limit of the Si content is 0.08%, more preferably 0.10%, and even more preferably 0.13%. The preferred upper limit of the Si content is 0.38%, more preferably 0.35%, more preferably 0.30%, and even more preferably 0.27%.

Mn: 0.60-1.60%
Manganese (Mn) improves the hardenability of steel material and increases the strength of electric resistance welded steel pipe. If the Mn content is less than 0.60%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Mn content exceeds 1.60%, the strength of the electric resistance welded steel pipe increases excessively even if the contents of other elements are within the range of this embodiment. As a result, the low temperature toughness of the electric resistance welded steel pipe decreases. Therefore, the Mn content is 0.60 to 1.60%. The preferred lower limit of the Mn content is 0.65%, more preferably 0.70%, more preferably 0.75%, more preferably 0.80%, more preferably 0.85%, and more preferably 0.90%. The preferred upper limit of the Mn content is 1.59%, more preferably 1.58%, more preferably 1.56%, more preferably 1.55%, more preferably 1.54%, and more preferably 1.53%.

P: 0-0.020%
Phosphorus (P) is an impurity. P segregates at grain boundaries and reduces the low-temperature toughness of electric resistance welded steel pipes. Therefore, the P content is 0 to 0.020%. A preferred upper limit of the P content is 0.016%, more preferably 0.013%, and even more preferably 0.010%. The lower the P content, the better. However, excessive reduction in the P content increases manufacturing costs. Therefore, taking into consideration normal industrial production, a preferred lower limit of the P content is more than 0%, more preferably 0.001%, and even more preferably 0.002%.

S: 0~0.0010%
Sulfur (S) is an impurity. S combines with Mn to form MnS. MnS reduces the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe. Therefore, the S content is 0 to 0.0010%. A preferred upper limit of the S content is 0.0009%, more preferably 0.0008%, and even more preferably 0.0007%. The S content is preferably as low as possible. However, excessive reduction of the S content increases the manufacturing cost. Therefore, taking into consideration normal industrial production, a preferred lower limit of the S content is more than 0%, more preferably 0.0001%, and even more preferably 0.0002%.

Al: 0.010-0.040%
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.010%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Al content exceeds 0.040%, Al nitrides become coarse even if the contents of other elements are within the range of this embodiment. As a result, the low-temperature toughness of the electric resistance welded steel pipe decreases. Therefore, the Al content is 0.010 to 0.040%. The preferred lower limit of the Al content is 0.011%, more preferably 0.012%, more preferably 0.013%, and even more preferably 0.014%. The preferred upper limit of the Al content is 0.038%, more preferably 0.035%, more preferably 0.033%, and even more preferably 0.030%.

N: 0.0010-0.0050%
Nitrogen (N) forms nitrides and carbonitrides to suppress the coarsening of austenite grains during the heating process. This reduces the effective grain size of ferrite. In other words, ferrite is refined. As a result, the low-temperature toughness of the electric resistance welded steel pipe is increased. N also increases the strength of the electric resistance welded steel pipe by solid solution strengthening. If the N content is less than 0.0010%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the N content exceeds 0.0050%, the nitrides and carbonitrides coarsen even if the contents of other elements are within the range of this embodiment. As a result, the low-temperature toughness of the electric resistance welded steel pipe is reduced. Therefore, the N content is 0.0010 to 0.0050%. The preferable lower limit of the N content is 0.0013%, more preferably 0.0015%, and even more preferably 0.0017%. The upper limit of the N content is preferably 0.0049%, more preferably 0.0047%, and further preferably 0.0046%.

Nb: 0.001-0.080%
Niobium (Nb) combines with C and/or N in the steel material to form fine carbides, nitrides, or carbonitrides (hereinafter referred to as Nb carbonitrides, etc.). Fine Nb carbonitrides, etc. suppress the coarsening of austenite grains. Therefore, the effective grain size of ferrite is reduced. In other words, ferrite is refined. As a result, the low-temperature toughness of the electric resistance welded steel pipe is increased. Fine Nb carbonitrides, etc. further increase the strength of the electric resistance welded steel pipe by dispersion strengthening. If the Nb content is less than 0.001%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Nb content exceeds 0.080%, the Nb carbonitrides, etc. become coarse even if the contents of other elements are within the range of this embodiment. As a result, the low-temperature toughness of the electric resistance welded steel pipe is reduced. Therefore, the Nb content is 0.001 to 0.080%. The lower limit of the Nb content is preferably 0.005%, more preferably 0.010%, more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Nb content is preferably 0.075%, more preferably 0.070%, more preferably 0.065%, more preferably 0.060%, more preferably 0.055%, and even more preferably 0.050%.

Ti: 0.001-0.020%
Titanium (Ti) combines with C and/or N in the steel material to form fine carbides, nitrides, or carbonitrides (hereinafter referred to as Ti carbonitrides, etc.). Fine Ti carbonitrides, etc. suppress the coarsening of austenite grains. Therefore, the effective grain size of ferrite is reduced. In other words, ferrite is refined. As a result, the low-temperature toughness of the electric resistance welded steel pipe is increased. Fine Ti carbonitrides, etc. further increase the strength of the electric resistance welded steel pipe by dispersion strengthening. If the Ti content is less than 0.001%, the above effect cannot be sufficiently obtained even if the other element contents are within the range of this embodiment. On the other hand, if the Ti content exceeds 0.020%, the Ti carbonitrides, etc. coarse even if the other element contents are within the range of this embodiment. As a result, the low-temperature toughness of the electric resistance welded steel pipe is reduced. Therefore, the Ti content is 0.001 to 0.020%. The lower limit of the Ti content is preferably 0.003%, more preferably 0.005%, and even more preferably 0.007%. The upper limit of the Ti content is preferably 0.019%, more preferably 0.018%, and even more preferably 0.017%.

O: 0-0.0030%
Oxygen (O) is an impurity. O forms oxides and reduces the hydrogen-induced cracking resistance (HIC resistance) and low-temperature toughness of the electric resistance welded steel pipe. Therefore, the O content is 0 to 0.0030%. A preferred upper limit of the O content is 0.0028%, more preferably 0.0025%, and even more preferably 0.0023%. The O content is preferably as low as possible. However, excessive reduction of the O content increases the manufacturing cost. Therefore, taking into consideration normal industrial production, a preferred lower limit of the O content is more than 0%, more preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%.

Ca: 0.0015-0.0035%
Calcium (Ca) controls the morphology of MnS and makes MnS spheroidal. In this case, the HIC resistance of the electric resistance welded steel pipe is improved. If the Ca content is less than 0.0015%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Ca content exceeds 0.0035%, coarse Ca oxides are formed even if the contents of other elements are within the range of this embodiment. In this case, the low temperature toughness of the electric resistance welded steel pipe is reduced. Therefore, the Ca content is 0.0015 to 0.0035%. The preferred lower limit of the Ca content is 0.0016%, more preferably 0.0017%, and even more preferably 0.0018%. The preferred upper limit of the Ca content is 0.0034%, more preferably 0.0033%, and even more preferably 0.0032%.

The remainder of the chemical composition of the base material of the electric resistance welded steel pipe according to this embodiment is composed of Fe and impurities. Here, impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when the electric resistance welded steel pipe is industrially manufactured, and are acceptable to the extent that they do not adversely affect the electric resistance welded steel pipe according to this embodiment.

[Regarding optional elements]
The chemical composition of the base metal of the electric resistance welded steel pipe according to this embodiment may further contain one or more elements selected from the group consisting of Ni, Mo, V, Cr and Cu in place of a portion of Fe. These elements are optional elements, and all of them increase the strength of the electric resistance welded steel pipe.

Ni: 0-0.50%
Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When it is contained, that is, when the Ni content is more than 0%, Ni enhances the hardenability of the steel material and enhances the strength of the electric resistance welded steel pipe. If even a small amount of Ni is contained, the above effect can be obtained to a certain extent. However, if the Ni content exceeds 0.50%, the strength of the electric resistance welded steel pipe will be excessively increased even if the contents of other elements are within the range of this embodiment. As a result, the low temperature toughness of the electric resistance welded steel pipe will decrease. Therefore, the Ni content is 0 to 0.50%. The preferred lower limit of the Ni content is more than 0%, more preferably 0.01%, more preferably 0.03%, more preferably 0.05%, and more preferably 0.08%. The preferred upper limit of the Ni content is 0.40%, more preferably 0.30%, and more preferably 0.20%.

Mo: 0~0.50%
Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When it is contained, that is, when the Mo content is more than 0%, Mo increases the hardenability of the steel material and increases the strength of the electric resistance welded steel pipe. Mo further combines with C and/or N in the steel material to form fine carbides, nitrides, or carbonitrides (hereinafter referred to as Mo carbonitrides, etc.). Fine Mo carbonitrides, etc. increase the strength of the electric resistance welded steel pipe by dispersion strengthening. Fine Mo carbonitrides, etc. further suppress the coarsening of austenite grains. Therefore, the effective grain size of ferrite is reduced. That is, the ferrite is refined. As a result, the low-temperature toughness of the electric resistance welded steel pipe is increased. If even a small amount of Mo is contained, the above effect can be obtained to some extent. However, if the Mo content exceeds 0.50%, the strength of the electric resistance welded steel pipe is excessively increased even if the contents of other elements are within the range of this embodiment. As a result, the low-temperature toughness of the electric resistance welded steel pipe is reduced. If the Mo content exceeds 0.50%, the Mo carbonitrides and the like will become coarse. As a result, the low-temperature toughness of the electric resistance welded steel pipe will decrease. Therefore, the Mo content is 0 to 0.50%. The preferred lower limit of the Mo content is more than 0%, more preferably 0.01%, more preferably 0.03%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the Mo content is 0.40%, more preferably 0.30%, more preferably 0.20%, and even more preferably 0.10%.

V: 0~0.100%
Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When it is contained, that is, when the V content is more than 0%, V combines with C and/or N in the steel material to form fine carbides, nitrides, or carbonitrides (hereinafter referred to as V carbonitrides, etc.). Fine V carbonitrides, etc. increase the strength of the electric resistance welded steel pipe by dispersion strengthening. Fine V carbonitrides, etc. further suppress the coarsening of austenite grains. Therefore, the effective grain size of ferrite is reduced. That is, the ferrite is refined. As a result, the low-temperature toughness of the electric resistance welded steel pipe is improved. If even a small amount of V is contained, the above effect can be obtained to a certain extent. However, if the V content exceeds 0.100%, the V carbonitrides, etc. become coarse even if the contents of other elements are within the range of this embodiment. Therefore, the low-temperature toughness of the electric resistance welded steel pipe is reduced. Therefore, the V content is 0 to 0.100%. The lower limit of the V content is preferably more than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%. The upper limit of the V content is preferably 0.090%, more preferably 0.070%, more preferably 0.050%, and more preferably 0.030%.

Cr: 0-0.30%
Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When contained, that is, when the Cr content is more than 0%, Cr enhances the hardenability of the steel material and enhances the strength of the electric resistance welded steel pipe. If even a small amount of Cr is contained, the above effect can be obtained to some extent. However, if the Cr content exceeds 0.30%, the strength of the electric resistance welded steel pipe will be excessively increased even if the contents of other elements are within the range of this embodiment. As a result, the low temperature toughness of the electric resistance welded steel pipe will decrease. Therefore, the Cr content is 0 to 0.30%. The preferred lower limit of the Cr content is more than 0%, more preferably 0.01%, more preferably 0.05%, more preferably 0.10%, and more preferably 0.15%. The preferred upper limit of the Cr content is 0.28%, more preferably 0.25%, and more preferably 0.23%.

Cu: 0-0.30%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When it is contained, that is, when the Cu content is more than 0%, Cu enhances the hardenability of the steel material and enhances the strength of the electric resistance welded steel pipe. If even a small amount of Cu is contained, the above effect can be obtained to a certain extent. However, if the Cu content exceeds 0.30%, the strength of the electric resistance welded steel pipe will be excessively increased even if the contents of other elements are within the range of this embodiment. As a result, the low temperature toughness of the electric resistance welded steel pipe will decrease. Therefore, the Cu content is 0 to 0.30%. The preferred lower limit of the Cu content is more than 0%, more preferably 0.01%, more preferably 0.05%, more preferably 0.10%, and more preferably 0.15%. The preferred upper limit of the Cu content is 0.28%, more preferably 0.26%, and more preferably 0.25%.

The chemical composition of the base material of the electric resistance welded steel pipe according to this embodiment may further contain one or more elements selected from the group consisting of Mg and rare earth elements in place of a portion of Fe. These elements are optional elements, and all of them deoxidize and desulfurize the steel.

Mg: 0-0.0050%
Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, that is, when the Mg content is more than 0%, Mg deoxidizes and desulfurizes the steel. If even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0050%, oxides will aggregate or coarsen even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe will decrease. Therefore, the Mg content is 0 to 0.0050%. The preferable lower limit of the Mg content is more than 0%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0007%, and more preferably 0.0010%. The preferable upper limit of the Mg content is 0.0040%, more preferably 0.0030%, and more preferably 0.0020%.

Rare earth elements: 0-0.0100%
Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, that is, when the REM content is more than 0%, REM deoxidizes and desulfurizes the steel. However, when the REM content exceeds 0.0100%, coarse oxides are formed even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe are reduced. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is more than 0%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0007%, and more preferably 0.0010%. The preferred upper limit of the REM content is 0.0040%, more preferably 0.0030%, and more preferably 0.0020%.

In this embodiment, REM is a collective term for 17 elements in the periodic table, ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, plus yttrium (Y) and scandium (Sc). The content of REM means the total content of one or more of these elements.

[Regarding formula (1)]
In the base material portion, on the premise that the content of each element in the chemical composition is within the range of this embodiment, formula (1) is further satisfied.
0.20≦C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≦0.29 (1)
Here, the content of each element in formula (1) is substituted in mass % for the corresponding element.

As described above, among the elements in the chemical composition of the base material of the electric resistance welded steel pipe according to this embodiment, C, Si, Mn, Ni, Cr, Mo, V, and Nb affect the S curve of the CCT diagram (see Figure 1).

F1 is defined as 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 Figure 1 will shift too far to the left in Figure 1. In this case, the low-temperature toughness of the base material of the electric resistance welded steel pipe will decrease. The reasons for this are as follows.

During the cooling process after hot working (described later) in the manufacturing process, the magnitude of the driving force (driving force for phase transformation) for the transformation from austenite to ferrite correlates with the steel temperature. In the CCT diagram shown in Figure 1, when the cooling curve C1 enters the ferrite nose (ferrite region) when the steel temperature is high, the driving force for phase transformation is small. Therefore, ferrite transformation nuclei are not easily generated. In addition, because the steel temperature is high, the growth rate of ferrite grains is fast. Therefore, ferrite grains are likely to become coarse. On the other hand, when the cooling curve C1 enters the ferrite nose (ferrite region) when the steel temperature is low, the driving force for phase transformation is large. Therefore, ferrite transformation nuclei are likely to be generated. In addition, because the steel temperature is low, the growth rate of ferrite grains is slow. Therefore, ferrite grains are likely to become fine.

Assuming that the content of each element in the chemical composition of the base material of the electric resistance welded steel pipe according to this embodiment is within the range of this embodiment, if F1 is less than 0.20, F1 is too low. In this case, the S curves of ferrite, pearlite, and bainite in Figure 1 shift too far to the left (toward the short time). In this case, during cooling, the cooling curve C1 enters the ferrite region when the steel temperature is high. As a result, the ferrite grains become coarse, and the effective crystal grain size of the base material exceeds 15.0 μm. As a result, the low-temperature toughness of the base material decreases.

On the other hand, assuming that the content of each element in the chemical composition of the base material of the electric resistance welded steel pipe according to this embodiment is within the range of this embodiment, if F1 exceeds 0.29, F1 is too high. In this case, the S curves of ferrite, pearlite, and bainite in FIG. 1 shift to the right (longer time side). In this case, the cooling curve C1 does not pass through the ferrite nose. Therefore, hard structures such as bainite and martensite are easily generated. As a result, the ferrite fraction in the microstructure of the base material is less than 50%. As a result, the low-temperature toughness of the base material of the electric resistance welded steel pipe is reduced. Furthermore, the crystal orientation of the hard structure is more easily aligned than the crystal orientation of ferrite. Therefore, if F1 is too high and the ferrite fraction is less than 50%, the effective crystal grain size of the base material exceeds 15.0 μm. As a result, the low-temperature toughness of the base material is reduced.

Assuming that the content of each element in the chemical composition of the base material of the electric resistance welded steel pipe according to this embodiment is within the range of this embodiment, if F1 is 0.20 to 0.29, the S-curves of ferrite, pearlite, and bainite are located at appropriate positions in Figure 1. Therefore, when the steel material (steel plate) is cooled along the cooling curve C1 in the manufacturing process, the ferrite fraction of the base material can be set to 50 to 90%, and the effective grain size can be set to 15.0 μm or less. As a result, the low-temperature toughness of the base material of the electric resistance welded steel pipe can be improved.

The preferred lower limit of F1 is 0.21, more preferably 0.22, and even more preferably 0.23. The preferred upper limit of F1 is 0.28, more preferably 0.27, and even more preferably 0.26.

[Microstructure of the base material]
The microstructure of the base material of the electric resistance welded steel pipe according to this embodiment is a structure mainly composed of ferrite. Specifically, in the microstructure of the base material, the ferrite fraction is 50 to 90%. The microstructure of the base material contains pearlite and/or a hard phase (bainite and/or martensite) as a phase other than ferrite. In this specification, the "microstructure of the base material" refers to the microstructure of the base material excluding the heat-affected zone.

As described above, if the microstructure of the base material of the electric-resistance welded steel pipe is a structure mainly composed of ferrite, that is, a structure with a ferrite fraction of 50 to 90%, the low-temperature toughness of the base material of the electric-resistance welded steel pipe will be improved, provided that the content of each element in the chemical composition of the base material is within the range of this embodiment, the above formula (1) is satisfied, the effective crystal grain size is 15.0 μm or less, the number density of the specific inclusions is 2.5 pieces/ ^mm2 or less, and the average aspect ratio of the specific inclusions is 4.5 or less.

Even if the content of each element in the chemical composition of the base material is within the range of this embodiment, satisfies the above formula (1), the effective grain size is 15.0 μm or less, the number density of the specific inclusions is 2.5 pieces/ ^mm2 or less, and the average aspect ratio of the specific inclusions is 4.5 or less, if the ferrite fraction is less than 50%, the low-temperature toughness of the base material of the electric resistance welded steel pipe decreases. In the chemical composition of the base material of the electric resistance welded steel pipe according to this embodiment, the upper limit of the ferrite fraction of the base material is 90%. Therefore, the ferrite fraction of the base material is 50 to 90%. The preferred lower limit of the ferrite fraction of the base material is 53%, more preferably 55%, and even more preferably 60%. The preferred upper limit of the ferrite fraction is 88%, more preferably 86%, and even more preferably 84%.

The ferrite fraction is measured by the following method. A sample is taken from the base material of the electric resistance welded steel pipe, from the center of the wall thickness at a position 90° circumferentially shifted from the electric resistance weld (i.e., the base material portion excluding the heat-affected zone). The observation surface of the taken sample is polished with 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 observation field when measuring the ferrite fraction by the KAM method is 200 μm x 500 μm. The observation magnification is 400 times, and the measurement step is 0.3 μm.

In the KAM method, an arbitrary regular hexagonal pixel in the measurement data is set as the central pixel. The orientation difference between each pixel is calculated for the first approximation (total of 7 pixels) using the 6 pixels adjacent to this central pixel, or the second approximation (total of 19 pixels) using the 12 pixels further outside these 6 pixels, or the third approximation (total of 37 pixels) using the 18 pixels further outside these 12 pixels. The calculated orientation differences are averaged, and the obtained arithmetic mean value is used as the value of the central pixel. This operation is performed for all pixels. Pixels with an orientation difference between adjacent pixels of 5° or less by the third approximation are displayed on the map. In this embodiment, the area fraction of pixels calculated to have an orientation difference of 1° or less in the third approximation relative to the total area of the field of view is defined as the ferrite fraction. Pixels with an orientation difference of more than 1° in the third approximation are considered to be structures other than ferrite, such as bainite.

[Effective grain size]
Further, in this embodiment, the effective grain size of the base material is 15.0 μm or less. If the effective grain size is 15.0 μm or less, the microstructure of the base material is sufficiently fine. In this case, the low-temperature toughness of the base material of the electric resistance welded steel pipe is improved, provided that the content of each element in the chemical composition of the base material is within the range of this embodiment, the above formula (1) is satisfied, the ferrite fraction is 50 to 90%, the number density of the specific inclusions is 2.5 pieces/ ^mm2 or less, and the average aspect ratio of the specific inclusions is 4.5 or less.

If the effective crystal grain size of the base material exceeds 15.0 μm, the low-temperature toughness of the base material of the electric resistance welded steel pipe decreases even if the content of each element in the chemical composition of the base material is within the range of this embodiment, the above formula (1) is satisfied, the ferrite fraction is 50 to 90%, the number density of the specific inclusions is 2.5 pieces/ ^mm2 or less, and the average aspect ratio of the specific inclusions is 4.5 or less.

The preferred upper limit of the effective grain size is 14.5 μm, more preferably 14.0 μm, and even more preferably 13.5 μm. The lower limit of the effective grain size is not particularly limited, but is, for example, 7.0 μm.

The effective grain size is measured using EBSP-OIM. Specifically, a sample is taken and polished in the same manner as in the measurement of the ferrite fraction. Specifically, a sample is taken from the center of the wall thickness (i.e., the base material portion excluding the heat-affected zone) at a position 90° circumferentially shifted from the electric welded weld in the base material of the electric welded steel pipe. The observation surface of the taken sample is polished with colloidal silica abrasive for 30 to 60 minutes. The polished sample is analyzed using EBSP-OIM. Orientation measurements are performed for each measurement step (0.3 μm), and the position where the orientation difference between adjacent measurement points exceeds 15° is considered to be a grain boundary. 15° is the threshold value for high-angle grain boundaries and is generally recognized as a grain boundary. The area surrounded by the grain boundary is considered to be a grain, and the grain size and surface area of the grain are calculated. Those with a grain size of less than 0.5 μm are excluded because they may be noise. In other words, those with a grain size of 0.5 μm or more are the target. The area average grain size is calculated from the grain size and surface area obtained. Specifically, when the region surrounded by the grain boundary is defined as a crystal grain and the circle equivalent diameter of the crystal grain is defined as the grain size, the area average grain size is defined as the sum of the products of the crystal grain size and the area fraction of each crystal grain with a crystal grain size of 0.5 μm or more. Specifically, when the total area of the crystal grains with a crystal grain size of 0.5 μm or more in the field of view is taken as 100%, the sum of the products of the crystal grain size and the area fraction of each crystal grain is calculated. The resulting sum is defined as the area average diameter. In this specification, the calculated area average grain size is the effective grain size (μm). The field of view is 200 μm x 500 μm, centered on the center of the wall thickness. The observation magnification is 400 times, and the measurement step is 0.3 μm.

[Inclusions in electric resistance welds]
As described above, in this specification, inclusions containing Ca, Al, and O are referred to as specific inclusions. In the electric resistance welded portion of the electric resistance welded steel pipe according to this embodiment, the number density of the specific inclusions is 2.5 pieces/ ^mm2 or less, and the average aspect ratio of the specific inclusions is 4.5 or less.

[Regarding specific inclusions]
As described above, the specific inclusions are inclusions containing Ca, Al, and O. For example, the specific inclusions are composite inclusions of CaO and Al ₂ O ₃ .

In this embodiment, the circle-equivalent diameter of the specific inclusion is, for example, 0.5 μm or more. Here, the circle-equivalent diameter means the diameter (μm) when the area of the specific inclusion obtained by observing the structure using a SEM (scanning electron microscope)-EDX (energy dispersive X-ray spectrometry) described below is assumed to be the area of a circle.

In this embodiment, the EDX beam diameter used to identify inclusions is 0.5 μm. In this case, for inclusions with an equivalent circle diameter of less than 0.5 μm, the accuracy of elemental analysis by EDX cannot be improved. Inclusions with an equivalent circle diameter of less than 0.5 μm are even less likely to become the starting point of cracks. Therefore, in this embodiment, specific inclusions with an equivalent circle diameter of 0.5 μm or more are measured. The upper limit of the equivalent circle diameter of the specific inclusions is not particularly limited, but is, for example, 200 μm.

[Regarding the number density of specific inclusions]
If the number density of the specific inclusions in the electric resistance welded portion exceeds 2.5 pieces/ ^mm2 , even if the content of each element in the chemical composition of the base material is within the range of this embodiment, the above formula (1) is satisfied, the ferrite fraction is 50 to 90%, the effective grain size is 15.0 μm or less, and the average aspect ratio of the specific inclusions is 4.5 or less, cracks originating from the specific inclusions are likely to occur in the electric resistance welded portion. Furthermore, the progression of cracks along the interface between the parent phase and the specific inclusions is likely to be promoted. As a result, the low temperature toughness of the electric resistance welded steel pipe is reduced. The preferred upper limit of the number density of the specific inclusions is 2.4 pieces/ ^mm2 , more preferably 2.3 pieces/ ^mm2 , and even more preferably 2.2 pieces/ ^mm2 . It is preferable that the number density of the specific inclusions is as low as possible. However, since Al and Ca are essential elements, the specific inclusions are generated to a certain extent. Therefore, the lower limit of the number density of the specific inclusions is, for example, more than 0 pieces/ ^mm2 .

The number density of specific inclusions in the electric resistance weld is measured using SEM-EDX. Specifically, in a cross section perpendicular to the axial direction of the electric resistance weld, a test piece is taken having a cross section (hereinafter also referred to as an observation surface) including an observation area centered on the center position, where the center position is the center of the thickness of the electric resistance weld and the center of the width of the electric resistance weld in the circumferential direction of the electric resistance weld. The size of the observation area is not particularly limited as long as it is a size that allows the specific inclusions in the electric resistance weld to be observed. For example, the size of the observation area is 3.0 mm high in the thickness direction and 5.0 mm wide in the direction perpendicular to the thickness direction. Using SEM-EDX, specific inclusions containing Ca, Al, and O are identified. Specifically, when the elemental analysis results of the identified inclusions show that the Ca content is 5% or more, the Al content is 5% or more, and the O content is 5% or more, the inclusion is identified as a specific inclusion. In addition, specific inclusions with a circle equivalent diameter of 0.5 μm or more are likely to become the starting point of cracks. Therefore, inclusions with a circle equivalent diameter of less than 0.5 μm are excluded from the scope.

The number of specific inclusions in the above observation region is determined by observing at a magnification of 500 to 850. The number density (pieces/mm ² ) of the specific inclusions is calculated based on the determined number of specific inclusions and the area of the observation region.

[Average aspect ratio of specific inclusions]
Furthermore, the average aspect ratio of the specific inclusions in the electric resistance welded portion is 4.5 or less. Here, the aspect ratio means the major axis L/minor axis W of the specific inclusions obtained by the method described later. Among the specific inclusions in the electric resistance welded portion, the specific inclusions are soft because they contain Ca. Therefore, when pressure is applied in the plate width direction (upset) during electric resistance welding in the manufacturing process described later, the specific inclusions containing Ca tend to elongate in the thickness direction and become plate-like. Compared with spherical inclusions, the plate-like specific inclusions elongated in the thickness direction are more likely to become the starting point of cracks and further promote the progress of cracks. Therefore, even if the number density of the specific inclusions in the electric resistance welded portion is 2.5 pieces/ ^mm2 or less, if the proportion of the plate-like specific inclusions is too high, the low-temperature toughness of the electric resistance welded portion decreases.

In the electric resistance welded portion, if the average aspect ratio of the specific inclusions is 4.5 or less, the content of each element in the chemical composition of the base material is within the range of this embodiment, the above formula (1) is satisfied, the ferrite fraction of the base material is 50 to 90%, the effective grain size is 15.0 μm or less, and the number density of the specific inclusions in the electric resistance welded portion is 2.5 pieces / mm ² or less, and on the premise that, excellent low-temperature toughness is obtained. On the other hand, if the average aspect ratio of the specific inclusions in the electric resistance welded portion exceeds 4.5, even if the content of each element in the chemical composition of the base material is within the range of this embodiment, the above formula (1) is satisfied, the ferrite fraction of the base material is 50 to 90%, the effective grain size is 15.0 μm or less, and the number density of the specific inclusions in the electric resistance welded portion is 2.5 pieces / mm ² or less, cracks are likely to occur starting from the specific inclusions, and the plate-shaped specific inclusions with a large aspect ratio promote the progression of the cracks. As a result, the low-temperature toughness of the electric resistance welded steel pipe is reduced. The upper limit of the average aspect ratio of the specific inclusions is preferably 4.4, more preferably 4.3, and even more preferably 4.2. The lower limit of the average aspect ratio of the specific inclusions is not particularly limited, but is, for example, 1.0.

The average aspect ratio of specific inclusions in electric resistance welds is measured using SEM-EDX. Specifically, the average aspect ratio of the specific inclusions is determined using the observation area where the number density of the specific inclusions described above was measured. As described above, SEM-EDX is used to identify inclusions containing Ca, Al, and O (specific inclusions). At this time, specific inclusions with a circle equivalent diameter of 0.5 μm or more are subject to calculation of the average aspect ratio. In other words, specific inclusions with a circle equivalent diameter of less than 0.5 μm are excluded from the calculation.

The major and minor axes of each specific inclusion identified as a measurement target and having a circle equivalent diameter of 0.5 μm or more are measured as follows. With reference to FIG. 3, a minimum circumscribing rectangle 100 is defined, which has two sides extending in the TD direction and two sides extending in the CD direction, is circumscribing the outer edge of the specific inclusion SI, and has the smallest area. The center of gravity of the defined minimum circumscribing rectangle 100 (the intersection of the diagonals of the minimum circumscribing rectangle 100) is defined as the center point CP of the specific inclusion SI.

Referring to Fig. 4, a straight line SL1 is placed that passes through the center point CP of the specific inclusion SI and is parallel to the TD direction. Furthermore, straight lines SL2 to SL4 that pass through the center point CP are placed at 45° intervals from the straight line SL1. The portion of the straight line SL1 that overlaps with the specific inclusion SI is defined as a line segment S1. Similarly, the portions of the straight lines SL2 to SL4 that overlap with the specific inclusion SI are defined as line segments S2 to S4. The length of the largest line segment among the line segments S1 to S4 is defined as the major axis L of the specific inclusion SI (in the case of Fig. 5, the length of the line segment S2 corresponds to the major axis L). The length of the smallest line segment among the line segments S1 to S4 is defined as the minor axis W of the specific inclusion SI (in the case of Fig. 5, the length of the line segment S4 corresponds to the minor axis W). Using the obtained major axis L and minor axis W, the aspect ratio is calculated by the following formula. The arithmetic average value of the aspect ratios of all the specific inclusions measured is calculated and used as the average aspect ratio.
Aspect ratio = L/W

[Uses of electric resistance welded steel pipes]
The electric resistance welded steel pipe according to this embodiment can be used for pipeline applications, and is particularly suitable for pipeline applications in sour environments. The wall thickness of the electric resistance welded steel pipe according to this embodiment is not particularly limited. The electric resistance welded steel pipe according to this embodiment can achieve both excellent HIC resistance and excellent low-temperature toughness even if it is a thick-walled electric resistance welded steel pipe having a wall thickness of 10.0 mm or more. The lower limit of the wall thickness of the electric resistance welded steel pipe according to this embodiment is, for example, 10.0 mm. The upper limit of the wall thickness of the electric resistance welded steel pipe according to this embodiment is, for example, 25.4 mm.

[Production method]
A method for manufacturing an electric-resistance welded steel pipe according to this embodiment will be described. Note that the manufacturing method described below is one example, and the manufacturing method for an electric-resistance welded steel pipe according to this embodiment is not limited to this. In other words, as long as an electric-resistance welded steel pipe having the above-mentioned configuration can be manufactured, the manufacturing method is not limited to the manufacturing method described below. However, the manufacturing method described below is a suitable manufacturing method for an electric-resistance welded steel pipe according to this embodiment.

Figure 6 is a flow diagram showing an example of the manufacturing process of electric resistance welded steel pipe according to this embodiment. With reference to Figure 6, in this manufacturing method, a slab, which is a raw material, is manufactured using molten steel satisfying the above-mentioned chemical composition (raw material preparation process: S0). The manufactured slab is hot rolled (hot rolling process: S1). In the hot rolling process, the slab is heated in a heating furnace (heating process: S11). The heated slab is rough rolled to manufacture a rough bar (rough rolling process: S12). Furthermore, the rough bar is finish rolled by a finishing rolling machine to manufacture a steel plate (finish rolling process: S13). The manufactured steel plate is cooled using a run out table (ROT: Run Out Table) (ROT cooling process: S2). The steel plate after the ROT cooling process (S2) is coiled (coil process: S3). Through the above manufacturing process, a hot rolled steel plate, which is the raw material for electric resistance welded steel pipe, is manufactured.

Furthermore, an electric resistance welded steel pipe is manufactured using the hot rolled steel sheet (pipe making process: S4). In the pipe making process, the hot rolled steel sheet is formed into a cylindrical blank pipe (open pipe) using forming rolls. In the blank pipe thus formed, the width direction of the hot rolled steel sheet is formed to be the circumferential direction of the blank pipe. The butt joints extending in the longitudinal direction of the blank pipe are electric resistance welded (welding process). The electric resistance welded steel pipe is manufactured by the above pipe making process. Each process will be described in detail below.

[Material preparation process (S0)]
A material having the above-mentioned chemical composition is prepared. Specifically, molten steel having the above-mentioned chemical composition is produced. A material (slab) is produced using the molten steel. A cast piece may be produced by a continuous casting method. An ingot may be produced using the molten steel, and the material (slab) may be produced by blooming the ingot.

[Hot rolling step (S1)]
In the hot rolling step (S1), the slab is heated in a heating furnace and hot rolled using a rough rolling mill and a finish rolling mill to produce a steel plate. The hot rolling step (S1) includes a heating step (S11), a rough rolling step (S12), and a finish rolling step (S13).

[Heating step (S11)]
In the heating step (S11), the manufactured slab is heated in a heating furnace. The heating temperature of the slab in the heating furnace may be a known temperature, for example, 1000 to 1300°C.

[Rough rolling process (S12)]
In the rough rolling step (S12), rough rolling is performed on the heated slab to produce a rough rolled plate (rough bar). In the rough rolling step, rolling may be performed using a tandem rolling mill including a plurality of rolling stands arranged in a row (each rolling stand having a pair of work rolls). Also, rolling may be performed using a reverse rolling mill having a pair of work rolls.

[Finish rolling step (S13)]
In the finish rolling step (S13), finish rolling is performed by a finish rolling mill to produce a steel sheet. Rolling may be performed using a tandem type finish rolling 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 reversing type rolling mill having a pair of work rolls.

In this embodiment, the finish rolling conditions are as follows.
Cumulative rolling reduction rate below 950°C: 60-80%
Finish rolling temperature: 850°C or less. The cumulative reduction at 950°C or less means the cumulative reduction at a steel plate in a state where the steel plate temperature is 950°C or less during finish rolling.

In the finish rolling step (S13), the surface temperature of the steel plate at the exit side of the stand that performs the final rolling in the finish rolling mill is defined as the finish rolling temperature (°C). As described above, the finish rolling temperature (°C) is 850°C or less. If the finish rolling temperature exceeds 850°C, the effective grain size of the base material of the electric resistance welded steel pipe exceeds 15.0 μm. If the finish rolling temperature is 850°C or less, coarsening of grains can be suppressed, and the effective grain size is 15.0 μm or less, provided that other manufacturing conditions are satisfied. The lower limit of the finish rolling temperature (°C) is the _Ar3 transformation point or higher.

[ROT cooling process (S2)]
In the ROT (run-out table) cooling step (S2), the steel plate produced in the hot rolling step (S1) is cooled.

First, the steel plate is cooled until its surface temperature reaches the cooling stop temperature T1. The cooling stop temperature T1 is, for example, 500 to 670°C. In this case, the ferrite fraction increases, and the low-temperature toughness of the electric resistance welded steel pipe is improved. The cooling is, for example, water-cooled using a water-cooling device.

[Winding process (S3)]
In the coiling step (S3), the steel sheet cooled in the ROT cooling step (S2) is coiled to produce a coiled hot-rolled steel sheet.

After the ROT cooling process (S2) is completed, the steel plate is air-cooled and coiled. The surface temperature of the steel plate during coiling (hereinafter referred to as the coiling temperature) T2 is, for example, 430 to 620°C (where T1>T2). In this case, the low-temperature toughness of the electric resistance welded steel pipe is improved.

The above manufacturing process produces hot-rolled steel sheet, which is the raw material for the electric resistance welded steel pipe according to this embodiment.

[Pipe making process (S4)]
In the pipe making process (S4), electric resistance welded steel pipes are manufactured while unwinding the coiled hot rolled steel sheet. Specifically, the hot rolled steel sheet is bent into a cylindrical shape (open pipe) by a series of forming rolls. Then, the widthwise ends (butt joints) of the open pipe are brought into contact with each other, and electric resistance welding is performed by high frequency induction heating while applying pressure (welding process).

In the above welding process, when the wall thickness of the open pipe (i.e., the plate thickness of the hot-rolled steel plate) is t (mm), the upset amount in the electric resistance welding is set to more than 0.76t to 1.20t (mm). Here, the upset amount is calculated by the following formula.
Upset amount (mm)=(width L ₀ (mm) of hot rolled steel plate)−(outer circumference L ₁ (mm) of welded steel pipe)

If the upset amount exceeds 1.20t (mm), the pressure applied to the butt joint of the open pipe becomes excessively high even if other manufacturing conditions are met. Therefore, the specific inclusions in the electric resistance welded portion are likely to become plate-shaped. Therefore, the average aspect ratio of the specific inclusions becomes large. As a result, the low-temperature toughness of the electric resistance welded steel pipe decreases. On the other hand, if the upset amount is 0.76t (mm) or less, the pressure applied to the butt joint of the open pipe becomes insufficient even if other manufacturing conditions are met. In this case, the specific inclusions cannot be sufficiently discharged to the inner or outer surface of the steel pipe. Therefore, the number density of the specific inclusions in the electric resistance welded portion exceeds 2.5 pieces/ ^mm2 . As a result, the low-temperature toughness of the electric resistance welded steel pipe decreases. Therefore, the upset amount is more than 0.76t to 1.20t (mm). If the upset amount is more than 0.76t to 1.20t (mm), the low-temperature toughness of the electric resistance welded steel pipe increases, provided that other manufacturing conditions are met.

If necessary, the steel pipe after the welding process is subjected to a well-known seam heat treatment on the electric resistance welded portion. Through the above process, the electric resistance welded steel pipe according to this embodiment is manufactured.

The electric resistance welded steel pipe manufactured by the above manufacturing process has a ferrite fraction of 50 to 90% in the base material and an effective grain size of 15.0 μm or less. Furthermore, in the electric resistance welded part, the number density of specific inclusions containing Ca, Al, and O is 2.5 pieces/mm2 ^or less, and the average aspect ratio of the specific inclusions is 4.5 or less. Therefore, it is possible to achieve both excellent HIC resistance and low temperature toughness.

Slabs were produced by continuous casting of molten steels with steel grade numbers A1 to A18 and steel grade numbers B1 to B6 shown in Table 1.

In Table 1, "-" indicates that the content of the corresponding element was below the detection limit. In other words, it means that the corresponding element was not contained. For example, the Ni content of steel grade number A1 was "0"% when rounded off to two decimal places. The remainder other than the elements listed in Table 1 was Fe and impurities. Electric resistance welded steel pipes with test numbers 1 to 30 shown in Table 2 were manufactured using multiple slabs of steel grade numbers A1 to A18 and steel grade numbers B1 to B6.

Specifically, the slab was heated in a heating furnace to the temperature shown in Table 2. The heated slab was subjected to rough rolling. The heating temperature (°C), cumulative reduction rate (%) below 950°C in the finish rolling process, and finish rolling temperature (°C) were as shown in Table 2.

The steel plate after the finish rolling process was subjected to ROT cooling. It was cooled to the cooling stop temperature T1 (°C) shown in Table 2. The steel plate was manufactured by the above manufacturing process. The obtained steel plate was coiled at the coiling temperature T2 (°C) shown in Table 2 (where T1 > T2) to manufacture a hot-rolled steel plate. The plate thickness t (mm) of the hot-rolled steel plate was as shown in Table 2.

The above hot-rolled steel plate was bent using a series of forming rolls to form an open pipe. The butt joint of the open pipe was welded using electric resistance welding. The upset amount (mm) in the welding process was as shown in Table 2. An electric resistance welded steel pipe was manufactured using the above manufacturing process. The wall thickness of the electric resistance welded steel pipe was the same as the wall thickness t (mm) of the hot-rolled steel plate shown in Table 2.

[Evaluation test]
For the electric resistance welded steel pipes of test numbers 1 to 30, microstructural observations of the base material (ferrite fraction, effective crystal grain size), microstructural observations of the electric resistance welds (number density of specific inclusions, average aspect ratio of specific inclusions), low-temperature toughness tests of the base material, low-temperature toughness tests of the electric resistance welds, and HIC resistance evaluation tests were performed.

[Microstructure observation of base material]
For the electric resistance welded steel pipe, the ferrite fraction and effective crystal grain size of the base metal were measured using EBSP-OIM.

[Ferrite fraction]
A sample was taken from the center of the wall thickness at a position 90° circumferentially shifted from the electric resistance welded part in the base material of the electric resistance welded steel pipe. The observation surface of the taken sample was polished with colloidal silica abrasive for 30 to 60 minutes. The ferrite fraction (%) of the polished sample was determined by the above-mentioned method using the KAM method with EBSP-OIM. The observation field when measuring the ferrite fraction by the KAM method was 200 μm × 500 μm. The observation magnification was 400 times, and the measurement step was 0.3 μm.

[Effective grain size]
Samples were taken and polished in the same manner as in the measurement of the ferrite fraction. The polished samples were analyzed using EBSP-OIM. More specifically, the position where the orientation difference between adjacent measurement points exceeded 15° in the orientation measurement for each measurement step (0.3 μm) was defined as the grain boundary. The grain size and surface area of the grain were determined by defining the area surrounded by the grain boundary as a crystal grain. The grain size of the obtained crystal grain of less than 0.5 μm was excluded because it may be noise. In other words, the grain size of the obtained crystal grain of 0.5 μm or more was targeted. The area average grain size was calculated from the obtained grain size and surface area. Specifically, when the area surrounded by the grain boundary was defined as a crystal grain and the circle equivalent diameter of the crystal grain was defined as the grain size, the grain size of each crystal grain of 0.5 μm or more was targeted, and the sum of the grain size of each crystal grain multiplied by the area fraction was defined as the area average grain size. Specifically, the total area of the crystal grains in the field of view with a crystal grain size of 0.5 μm or more was taken as 100%, and the sum of the crystal grain size and area fraction of each crystal grain was calculated. The resulting sum was defined as the area average diameter. The calculated area average grain size was taken as the effective grain size (μm). The field of view was 200 μm x 500 μm, centered on the central part of the wall thickness. The observation magnification was 400 times, and the measurement step was 0.3 μm.

[Microstructure observation of electric resistance welded part]
For the electric resistance welded portion of the electric resistance welded steel pipe, the number density of specific inclusions containing Ca, Al and O and the average aspect ratio of the specific inclusions were measured using SEM-EDX.

[Number density of specific inclusions]
In a cross section perpendicular to the axial direction of the electric resistance welded steel pipe, a test piece having a cross section (hereinafter also referred to as an observation surface) including a region (hereinafter also referred to as an observation region) having a height in the thickness direction of 3.0 mm and a width in the direction perpendicular to the thickness direction of 5.0 mm, centered on the center position, when the position at the center of the thickness of the electric resistance welded part and the width center of the electric resistance welded part in the circumferential direction of the electric resistance welded steel pipe is taken as the center position. Using SEM-EDX, specific inclusions containing Ca, Al and O were identified. Specifically, in the elemental analysis results of the identified inclusions, when the Ca content was 5% or more, the Al content was 5% or more, and the O content was 5% or more, the inclusion was identified as a specific inclusion. Note that specific inclusions with a circle equivalent diameter of 0.5 μm or more are likely to be the starting point of cracks. Therefore, inclusions with a circle equivalent diameter of less than 0.5 μm were excluded from the target.

The number of specific inclusions in the above observation region was determined by observing at a magnification of 700 to 850. Based on the determined number of specific inclusions and the area of the observation region, the number density (pieces/mm ² ) of the specific inclusions was calculated.

[Average aspect ratio of specific inclusions]
Test pieces were taken in the same manner as in the measurement of the number density of the specific inclusions. The specific inclusions containing Ca, Al and O were identified using SEM-EDX. In this case, the specific inclusions having a circle equivalent diameter of 0.5 μm or more were used as the object of calculation of the average aspect ratio. In other words, the specific inclusions having a circle equivalent diameter of less than 0.5 μm were excluded from the calculation.

The major and minor axes of each specific inclusion with a circle equivalent diameter of 0.5 μm or more that was identified as the measurement target were measured as follows. With reference to FIG. 3, a minimum circumscribing rectangle 100 was defined that has two sides extending in the TD direction and two sides extending in the CD direction, is a circumscribing rectangle that is in contact with the outer edge of the specific inclusion SI, and has the smallest area. The center of gravity of the defined minimum circumscribing rectangle 100 (the intersection of the diagonals of the minimum circumscribing rectangle 100) was defined as the center point CP of the specific inclusion SI.

Referring to FIG. 4, a straight line SL1 was placed that passed through the center point CP of the specific inclusion SI and was parallel to the TD direction. Furthermore, straight lines SL2 to SL4 that passed through the center point CP were placed at 45° intervals from the straight line SL1 around the center point CP. In the straight line SL1, a portion that overlaps with the specific inclusion SI was defined as a line segment S1. Similarly, in each of the straight lines SL2 to SL4, a portion that overlaps with the specific inclusion SI was defined as a line segment S2 to S4. Of the line segments S1 to S4, the length of the maximum line segment was defined as the major axis L of the specific inclusion SI. Of the line segments S1 to S4, the length of the minimum line segment was defined as the minor axis W of the specific inclusion SI. Using the obtained major axis L and minor axis W, the aspect ratio was calculated by the following formula.
Aspect ratio = L/W

The arithmetic mean value of the aspect ratio of a specific inclusion was calculated and defined as the average aspect ratio.

[Low temperature toughness test of base material]
A DWTT test piece was taken from the center of the wall thickness at a position 90° circumferentially shifted from the electric welded portion of the electric welded steel pipe of each test number. The circular arc-shaped member taken in the pipe circumferential direction from the taking position was developed into a flat plate, and a notch was machined at the 90° position. The size of the DWTT test piece was as shown in Figure 7. The numbers in Figure 7 indicate the dimensions (unit: mm) of the corresponding part of the test piece. t1 indicates the wall thickness (unit: mm). The longitudinal direction of the DWTT test piece corresponded to the circumferential direction of the electric welded steel pipe. The DWTT test was performed on the DWTT test piece in accordance with the provisions of ASTM E 436. The ductile fracture surface ratio at -20 ° C was obtained. When the ductile fracture surface ratio at -20 ° C was 85% or more, it was determined that the low temperature toughness of the base material part was high.

[Low temperature toughness test of electric resistance welded joint]
A V-notch test piece conforming to the standard test piece of JIS Z2242 (2018) was prepared from the electric welded portion of the electric welded steel pipe of each test number. The V-notch test piece was prepared so that the longitudinal direction of the test piece was perpendicular to the longitudinal direction and wall thickness direction of the electric welded steel pipe (i.e., the circumferential direction), and the electric welded portion was located at the longitudinal center position of the test piece. A V-notch was prepared in the part of the test piece corresponding to the electric welded portion so that the propagation direction of the crack in the Charpy impact test was the longitudinal direction of the electric welded steel pipe. The cross section of the V-notch test piece was 10 mm x 10 mm, and the depth of the V-notch was 2 mm. Using the V-notch test piece, a Charpy impact test conforming to JIS Z2242 (2018) was performed at -20 ° C., and the absorbed energy at -20 ° C. was obtained. If the absorbed energy at -20 ° C. was 100 J or more, it was determined that the low-temperature toughness of the electric welded portion was high.

[HIC Resistance Evaluation Test]
HIC test pieces were taken from the base metal at a position 90° circumferentially shifted from the electric resistance welded portion of the electric resistance welded steel pipe of each test number. The circular arc-shaped member taken from the taking position in the pipe circumferential direction was developed into a flat plate. The size of the HIC test piece was 20 mm wide x 100 mm long x wall thickness (mm). Using the obtained HIC test piece, an HIC test in accordance with NACE-TM0284 was carried out. Specifically, the HIC test piece was immersed for 96 hours in a test solution in which 100% H ₂ S gas was saturated in Solution A liquid (5 mass% NaCl + 0.5 mass% glacial acetic acid aqueous solution). The test piece after immersion for 96 hours was measured for the presence or absence of HIC using an ultrasonic flaw detector. Based on this measurement result, the crack length ratio CLR (Crack Length Ratio) (%) was calculated using the following formula. If the CLR was 15% or less, it was determined that the HIC resistance was excellent.
CLR (%) = (total crack length/test piece length) x 100 (%)

[Test Results]
The test results are shown in Table 3.

With reference to Tables 1 to 3, the electric resistance welded steel pipes of test numbers 1 to 18 had appropriate chemical compositions, satisfied formula (1), and were manufactured under appropriate conditions. Therefore, in the base metal of the electric resistance welded steel pipes of each test number, the ferrite fraction was 50 to 90%, and the effective grain size was 15.0 μm or less. Furthermore, in the electric resistance welded part, the number density of the specific inclusions was 2.5 pieces/mm ² or less, and the average aspect ratio of the specific inclusions was 4.5 or less. Therefore, the DWTT ductile fracture surface ratio of the base metal at −20° C. was 85% or more, and the absorbed energy of the electric resistance welded part at −20° C. was 100 J or more. Therefore, the low-temperature toughness of the base metal and the electric resistance welded part was high. Furthermore, the CLR of the base metal was 15% or less, and the HIC resistance was excellent.

On the other hand, in test number 19, F1 exceeded 0.29. As a result, the ferrite fraction of the base material was less than 50%, and the effective grain size exceeded 15.0 μm. As a result, the DWTT ductile fracture surface area rate of the base material at -20°C was less than 85%, and the low-temperature toughness of the base material was low.

In test number 20, the Ca content was too high. Therefore, the number density of specific inclusions in the electric resistance welded portion exceeded 2.5 pieces/ ^mm2 . Therefore, the absorbed energy at -20°C in the Charpy impact test was less than 100 J. As a result, the low temperature toughness of the electric resistance welded portion was low.

In test number 21, the Ca content was too low. As a result, the CLR of the base material was over 15%, and the HIC resistance was low.

In test number 22, the Al content was too high. Therefore, the number density of specific inclusions in the electric resistance welded joint exceeded 2.5 pieces/ ^mm2 . Therefore, the absorbed energy at -20°C in the Charpy impact test was less than 100 J. As a result, the low temperature toughness of the electric resistance welded joint was low.

In test number 23, the O content was too high. Therefore, the number density of specific inclusions in the electric resistance welded joint exceeded 2.5 pieces/ ^mm2 . Therefore, the absorbed energy at -20°C in the Charpy impact test was less than 100 J. As a result, the low temperature toughness of the electric resistance welded joint was low.

In test number 24, F1 was less than 0.20. Therefore, the effective grain size in the base material exceeded 15.0 μm. Therefore, the DWTT ductile fracture surface ratio of the base material at -20°C was less than 85%, and the low-temperature toughness of the base material was low.

In test number 25, the finish rolling temperature was too high. As a result, the effective grain size in the base material exceeded 15.0 μm. As a result, the DWTT ductile fracture surface area ratio of the base material at -20°C was less than 85%, and the low-temperature toughness of the base material was low.

In test numbers 26 to 28, the amount of upset was too high. As a result, the average aspect ratio of specific inclusions in the electric resistance welds exceeded 4.5. As a result, the absorbed energy at -20°C in the Charpy impact test was less than 100 J. As a result, the low-temperature toughness of the electric resistance welds was low.

In test numbers 29 and 30, the amount of upset was too low. Therefore, the number density of specific inclusions in the electric resistance welded joint exceeded 2.5 pieces/ ^mm2 . Therefore, the absorbed energy at -20°C in the Charpy impact test was less than 100 J. As a result, the low temperature toughness of the electric resistance welded joint was low.

The above describes the embodiments of the present disclosure. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by modifying the above-described embodiments as appropriate within the scope of the spirit of the present disclosure.

Claims (3)

An electric resistance welded steel pipe having a base material portion and an electric resistance welded portion,
The base material portion comprises, in mass%,
C: 0.010-0.080%,
Si: 0.05-0.40%,
Mn: 0.60-1.60%,
P: 0-0.020%,
S: 0 to 0.0010%,
Al: 0.010-0.040%,
N: 0.0010 to 0.0050%,
Nb: 0.001-0.080%,
Ti: 0.001 to 0.020%,
O: 0 to 0.0030%,
Ca: 0.0015-0.0035%,
Ni: 0 to 0.50%,
Mo: 0 to 0.50%,
V: 0-0.100%,
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,
and satisfying formula (1),
The ferrite fraction of the base material is 50 to 90%, and the effective grain size is 15.0 μm or less,
In the electric resistance welded portion, the number density of specific inclusions containing Ca, Al, and O is 2.5 pieces/ ^mm2 or less, and the average aspect ratio of the specific inclusions is 4.5 or less.
Electric resistance welded steel pipe.
0.20≦C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≦0.29 (1)
Here, the content of each element in formula (1) is substituted in mass % for the corresponding element. The electric resistance welded steel pipe according to claim 1,
The base material portion comprises, in mass%,
Ni: 0.01-0.50%,
Mo: 0.01-0.50%,
V: 0.001-0.100%,
Cr: 0.01-0.30%,
Cu: 0.01-0.30%,
Mg: 0.0010 to 0.0050%, and
Rare earth elements: 0.0010 to 0.0100%, containing one or more selected from the group consisting of
Electric resistance welded steel pipe. The electric welded steel pipe according to claim 1 or 2,
The thickness is 10.0 to 25.4 mm.
Electric resistance welded steel pipe.

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