Electric welded steel pipe

Abstract

To provide an electroseamed steel pipe having excellent low-temperature toughness and HIC resistance.SOLUTION: A matrix part of an electroseamed steel pipe includes, by mass%, 0.010 to 0.080% C, 0.05 to 0.40% Si, 0.60 to 1.60% Mn, 0 to 0.020% P, 0 to 0.0010% S, 0.010 to 0.040% Al, 0.0010 to 0.0050% N, 0.001 to 0.080% Nb, 0.001 to 0.020% Ti, 0 to 0.0030% O, and 0.0015 to 0.0035% Ca, and has a chemical composition satisfying an expression (1) stipulated in an embodiment. In the matrix part, a ferrite fraction is 50 to 90%, and an effective crystal grain diameter is 15.0 μm or less. In an electroseamed weld part, number density of a specific inclusion containing Ca, Al and O is 2.5 pcs / mm2 or less, and an average aspect ratio of the specific inclusion is 4.5 or less.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to an electric resistance welded steel pipe, and more particularly to an electric resistance welded steel pipe used in pipelines. [Background technology]

[0002] A pipeline laid on the seabed is composed of multiple steel pipes (line pipes). Subsea pipelines are subjected to high pressure from 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.

[0003] JP 2020-128577 A (Patent Document 1) proposes an electric resistance welded steel pipe for pipeline use having excellent low-temperature toughness.

[0004] The electric resistance welded steel pipe disclosed in Patent Document 1 contains, 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 / mm 2 The thickness of the heat-affected zone is t S When the heat-affected zone is S In the microstructure of the heat-affected zone, the area ratio of ferrite is 0 to 40%, the remainder is tempered bainite, and the average grain size is 15 μm or less. S In the microstructure of the / 2 part, the area ratio of ferrite is 0 to 50%, the remainder is tempered bainite, and the average grain size is 15 μm or less. B When the t of the base material is B / 4 Part and base material t B In the microstructure of the heat-affected zone / 2, the area ratio of ferrite is 0-50%, the remainder is bainite, and the average crystal grain size is 15 μm or less. In this electric resistance welded steel pipe, the microstructures of the heat-affected zone and the base material are mainly made of tempered bainite, and further, the number density of specific inclusions in specific regions of the heat-affected zone is suppressed. This results in high strength and excellent low-temperature toughness, as described in Patent Document 1. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2020-128577 A Summary of the Invention [Problem to be solved by the invention]

[0006] 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 this case, the electric resistance welded steel pipe is required to have not only excellent low-temperature toughness but also excellent HIC resistance. In the electric resistance welded steel pipe disclosed in Patent Document 1, no consideration has been given to achieving both low-temperature toughness and HIC resistance.

[0007] An object of the present disclosure is to provide an electric resistance welded steel pipe having excellent low temperature toughness and excellent HIC resistance. [Means for solving the problem]

[0008] 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 to 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 to 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: 0~0.30%, Cu: 0-0.30%, 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 / mm 2 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.

[0009] 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 crystal grain, for crystal 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 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 crystal grain size. Effect of the Invention

[0010] The electric resistance welded steel pipe according to the present disclosure has excellent low-temperature toughness and excellent HIC resistance. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a continuous cooling transformation curve diagram of an electric resistance welded steel pipe according to the present embodiment. [Diagram 2]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. [Diagram 3] FIG. 3 is a schematic diagram for explaining the method for measuring the average aspect ratio of specific inclusions according to this embodiment. [Figure 4] 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. [Diagram 5] 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. [Figure 6] FIG. 6 is a flow diagram showing an example of a manufacturing process for an electric resistance welded steel pipe according to this embodiment. [Figure 7] FIG. 7 is a front view and a side view of the DWTT test piece used in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present inventors have investigated and examined the low temperature toughness and HIC resistance of electric resistance welded steel pipes for pipeline use. First, the present inventors investigated and examined the HIC resistance of electric resistance welded steel pipes and obtained the following findings.

[0013] 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 cracking. Therefore, it is preferable to suppress the generation of sulfides and the elongation of sulfides as much as possible. By suppressing the S content, the generation of sulfides can be suppressed. In addition, Ca modifies sulfides and suppresses the elongation of sulfides. 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.

[0014] From the above viewpoints, the chemical composition of electric resistance welded steel pipe was examined. As a result, the base metal of electric resistance welded steel pipe was found to have the following composition by 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 excellent HIC resistance would be obtained with a chemical composition consisting of: Ca: 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: Fe and impurities.

[0015] Therefore, further investigation was conducted into means for improving the low-temperature toughness of an electric resistance welded steel pipe having a base metal part with the above-mentioned chemical composition.

[0016] Normally, the strength (or hardness) and toughness of steel are in conflict with each other. 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.

[0017] Therefore, in order to obtain not only excellent HIC resistance but also excellent low-temperature toughness, the present 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, it has been 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 %.

[0018] 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.

[0019] Fig. 1 is a continuous cooling transformation curve (CCT diagram) of an electric resistance welded steel pipe according to this embodiment. That is, Fig. 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 metal is within the above-mentioned range. In Fig. 1, F indicates ferrite nose, P indicates pearlite nose, and B indicates bainite nose.

[0020] Referring to Fig. 1, if F1, which is composed of C, Si, Mn, Ni, Cr, Mo, V, and Nb, is too low, the S curve in Fig. 1 shifts too far to the left. In this case, the low-temperature toughness of the steel material decreases. The reason for this is as follows.

[0021] 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.

[0022] Therefore, 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 ferrite grains generated by phase transformation tend 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 ferrite grains generated by phase transformation tend to become fine.

[0023] If F1 is too low and the S curve shifts too far to the left in Figure 1, the cooling curve C1 enters the ferrite region while the steel temperature is high. Therefore, as mentioned above, the ferrite grains formed by the phase transformation become coarse. As a result, the low-temperature toughness of the steel decreases.

[0024] 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.

[0025] If F1 is 0.20 to 0.29, the S curves of each phase (ferrite, pearlite, bainite) in Figure 1 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. Therefore, 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 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.

[0026] 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 the low-temperature toughness are improved.

[0027] However, it was found that even an electric resistance welded steel pipe having the above-mentioned configuration may still not have a sufficiently high low-temperature toughness. The present inventors therefore investigated the cause of the insufficient low-temperature toughness of an electric resistance welded steel pipe having the above-mentioned configuration. As a result, the present inventors newly discovered that, in an electric resistance welded steel pipe having the above-mentioned configuration, the low-temperature toughness of the base material is sufficiently high, but the low-temperature toughness of the electric resistance welded portion may not be sufficiently high. The present inventors therefore investigated means for further increasing the low-temperature toughness of the electric resistance welded portion. As a result, the present inventors obtained the following findings.

[0028] When the content of each element in the chemical composition of the base metal falls within the above-mentioned range and satisfies formula (1), the electric resistance welded portion contains inclusions containing Ca, Al and O. In this specification, these inclusions are referred to as "specific inclusions."

[0029] When the number density of specific inclusions is high in an electric resistance weld, cracks are likely to occur originating from the specific inclusions. In this case, the low-temperature toughness of the electric resistance weld may decrease. However, assuming 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, if the number density of specific inclusions in the electric resistance weld is 2.5 pieces / mm 2 It has been found that the low temperature toughness of the electric resistance welded portion can be improved if the temperature is less than or equal to the above range.

[0030] However, the number density of a particular inclusion is simply set to 2.5 pieces / mm 2 There are cases where the low temperature toughness cannot be sufficiently improved by merely suppressing the temperature to below the range. The reasons are as follows.

[0031] In the electric resistance welded steel pipe according to this 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 also likely to be contained in the inclusions, and the above-mentioned specific inclusions are generated. The specific inclusions are soft because they contain Ca. Therefore, there are cases where the specific inclusions in the electric resistance weld are elongated and exist in a state with a large aspect ratio. When the number density of the specific inclusions is 2.5 pieces / mm 2Even if the aspect ratio is less than this, if certain inclusions with large aspect ratios are present, these inclusions will become the starting points of cracks, resulting in a decrease in the low temperature toughness of the electric resistance welded joint.

[0032] Therefore, the inventors have developed a method for manufacturing a steel sheet having a chemical composition in which the content of each element in the base metal is within the above-mentioned range and satisfies formula (1), and further the number density of specific inclusions in the electric resistance welded portion is 2.5 pieces / mm 2 The relationship between the average aspect ratio of specific inclusions in the electric resistance welded zone and the low temperature toughness was investigated for the following electric resistance welded steel pipes, and Figure 2 was obtained.

[0033] Referring to FIG. 2, the content of each element in the chemical composition of the base metal is within the above-mentioned range, formula (1) is satisfied, and the number density of the specific inclusions in the electric resistance welded portion is 2.5 pieces / mm 2 In the electric resistance welded steel pipe described below, when the average aspect ratio of the specific inclusions was greater than 4.5, the absorbed energy at -20°C did not increase significantly, being less than 100 J, even if the average aspect ratio was reduced. On the other hand, when the average aspect ratio was 4.5 or less, the absorbed energy at -20°C was 100 J or more, and further, as the average aspect ratio decreased, the absorbed energy at -20°C increased significantly. In other words, by setting the average aspect ratio to 4.5 or less, the low temperature toughness increased significantly.

[0034] Based on the above findings, the electric resistance welded steel pipe of the present embodiment has been completed. The electric resistance welded steel pipe of the present embodiment has the following configuration.

[0035] [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 to 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 to 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: 0~0.30%, Cu: 0-0.30%, 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 / mm 2 and the average aspect ratio of the specific inclusions is 4.5 or less. ERW 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.

[0036] [2] An electric resistance welded steel pipe according to [1], The base material portion comprises, in mass%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.50%, V: 0.001 to 0.100%, Cr: 0.01 to 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 ERW steel pipe.

[0037] [3] An electric resistance welded steel pipe according to [1] or [2], The thickness is 10.0~25.4mm. ERW steel pipe.

[0038] The electric resistance welded steel pipe according to the present embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0039] [Structure 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.

[0040] [Chemical composition] The chemical composition of the base material of the electric resistance welded steel pipe according to this embodiment contains the following elements.

[0041] C: 0.010~0.080% Carbon (C) increases the strength of steel material. If the C 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 C content exceeds 0.080%, C forms excessive carbides. In this case, even if the contents of other elements 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 more preferably 0.030%. The preferred upper limit of the C content is 0.075%, more preferably 0.070%, more preferably 0.065%, and more preferably 0.060%.

[0042] Silicon: 0.05 to 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%.

[0043] 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 preferable 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 preferable 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%.

[0044] 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 P content is preferably as low as possible. However, excessive reduction of 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%.

[0045] S: 0 to 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 preferable 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, in consideration of normal industrial production, a preferable lower limit of the S content is more than 0%, more preferably 0.0001%, and even more preferably 0.0002%.

[0046] 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%.

[0047] 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 effects 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%.

[0048] Nb: 0.001 to 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%.

[0049] Ti: 0.001 to 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 contents of other elements are within the range of this embodiment. On the other hand, if the Ti content exceeds 0.020%, the Ti 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 Ti content is 0.001 to 0.020%. The preferable lower limit of the Ti content is 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 further preferably 0.017%.

[0050] O: 0 to 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 preferable 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, in consideration of normal industrial production, a preferable lower limit of the O content is more than 0%, more preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%.

[0051] 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 more preferably 0.0018%. The preferred upper limit of the Ca content is 0.0034%, more preferably 0.0033%, and more preferably 0.0032%.

[0052] The balance 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, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the electric resistance welded steel pipe, and are permissible within a range that does not adversely affect the electric resistance welded steel pipe of this embodiment.

[0053] [About 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.

[0054] Ni: 0 to 0.50% Nickel (Ni) is an optional element and may not be contained. In other words, the Ni content may be 0%. When it is contained, that is, when the Ni content is more than 0%, Ni improves the hardenability of the steel material and increases 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 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 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%.

[0055] Mo: 0 to 0.50% Molybdenum (Mo) is an optional element and may not be contained. In other words, the Mo content may be 0%. When it is contained, that is, when the Mo content is more than 0%, Mo improves 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.). The fine Mo carbonitrides, etc. increase the strength of the electric resistance welded steel pipe by dispersion strengthening. The fine Mo carbonitrides, etc. further suppress the coarsening of austenite grains. Therefore, the effective grain size of ferrite is reduced. In other words, 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 more preferably 0.08%. The preferred upper limit of the Mo content is 0.40%, more preferably 0.30%, more preferably 0.20%, and more preferably 0.10%.

[0056] V: 0~0.100% Vanadium (V) is an optional element and may not be contained. In other words, the V content may be 0%. When it is contained, that is, when the V content is more than 0%, V bonds with C and / or N in the steel material to form fine carbides, nitrides, or carbonitrides (hereinafter referred to as V carbonitrides, etc.). The fine V carbonitrides, etc. increase the strength of the electric resistance welded steel pipe by dispersion strengthening. The fine V carbonitrides, etc. further suppress the coarsening of austenite crystal grains. Therefore, the effective crystal grain size of ferrite is reduced. In other words, 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 some 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%.

[0057] 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 improves the hardenability of the steel material and increases 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 a certain 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%.

[0058] 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 some 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%.

[0059] 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 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.

[0060] Magnesium: 0 to 0.0050% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When it is 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, when the Mg content exceeds 0.0050%, oxides 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 are reduced. 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%.

[0061] 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%.

[0062] 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.

[0063] [Regarding formula (1)] In the base material, 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.

[0064] As described above, among the elements in the chemical composition of the base metal of the electric resistance welded steel pipe according to this embodiment, C, Si, Mn, Ni, Cr, Mo, V, and Nb affect the S curve in the CCT diagram (see FIG. 1).

[0065] 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.

[0066] In the cooling process after hot working (described later) in the manufacturing process, the magnitude of the driving force (driving force of phase transformation) for the transformation from austenite to ferrite correlates with the steel temperature. In the CCT diagram shown in Figure 1, when the steel temperature is high and the cooling curve C1 enters the ferrite nose (ferrite region), the driving force of 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, the ferrite grains are likely to become coarse. On the other hand, when the steel temperature is low and the cooling curve C1 enters the ferrite nose (ferrite region), the driving force of 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, the ferrite grains are likely to become fine.

[0067] 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 FIG. 1 shift too far to the left (short time side). In this case, during cooling, the cooling curve C1 enters the ferrite region when the steel temperature is high. Therefore, the ferrite grains become coarse, and the effective grain size of the base material exceeds 15.0 μm. As a result, the low temperature toughness of the base material decreases.

[0068] 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 likely to be 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 likely to be 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.

[0069] 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 Fig. 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.

[0070] The lower limit of F1 is preferably 0.21, more preferably 0.22, and even more preferably 0.23. The upper limit of F1 is preferably 0.28, more preferably 0.27, and even more preferably 0.26.

[0071] [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. Note that the "microstructure of the base material" in this specification means the microstructure of the base material excluding the heat-affected zone.

[0072] 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 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 grain size is 15.0 μm or less, and the number density of the specific inclusions is 2.5 pieces / mm 2 On the premise that the average aspect ratio of the specific inclusions is 4.5 or less, the low temperature toughness of the base material of the electric resistance welded steel pipe is improved.

[0073] 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 grain size is 15.0 μm or less, and the number density of the specific inclusions is 2.5 pieces / mm 2 Even if the average aspect ratio of 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%.

[0074] The ferrite fraction is measured by the following method. A sample is taken from the base material of the electric resistance welded steel pipe, in 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, using the following method. The observation field when measuring the ferrite fraction by the KAM method is 200 μm × 500 μm. The observation magnification is 400 times, and the measurement step is 0.3 μm.

[0075] 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 6 pixels adjacent to this central pixel, or the second approximation (total of 19 pixels) using 12 pixels further outside these 6 pixels, or the third approximation (total of 37 pixels) using 18 pixels further outside these 12 pixels. The calculated orientation differences are averaged, and the obtained arithmetic average value is set 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. Those with an orientation difference of more than 1° in the third approximation are considered to be structures other than ferrite, such as bainite.

[0076] [Effective grain size] Further, in this embodiment, the base material has an effective crystal grain size of 15.0 μm or less. If the effective crystal grain size is 15.0 μm or less, the microstructure of the base material is sufficiently fine. In this case, 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%, and the number density of the specific inclusions is 2.5 pieces / mm 2On the premise that the average aspect ratio of the specific inclusions is 4.5 or less, the low temperature toughness of the base material of the electric resistance welded steel pipe is improved.

[0077] If the effective grain size of the base material exceeds 15.0 μm, 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%, and the number density of the specific inclusions is 2.5 pieces / mm 2 Even if the average aspect ratio of the specific inclusions is 4.5 or less, the low-temperature toughness of the base material of the electric resistance welded steel pipe decreases.

[0078] The upper limit of the effective grain size is preferably 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.

[0079] 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 at a position 90° circumferentially shifted from the electric resistance welded part of the base material of the electric resistance welded steel pipe (i.e., the base material part excluding the heat-affected zone). 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 measurement is 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 crystal grain size and area fraction of each crystal grain, which are the targets of the crystal grain size of each crystal grain having a crystal grain size of 0.5 μm or more. Specifically, when the total area of ​​the crystal grains having a crystal grain size of 0.5 μm or more in the field of view is taken as 100%, the sum of the crystal grain size and 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 × 500 μm, centered on the center of the wall thickness. The observation magnification is 400 times, and the measurement step is 0.3 μm.

[0080] [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 / mm 2 or less, and the average aspect ratio of the specific inclusions is 4.5 or less.

[0081] [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 Al2O3.

[0082] In this embodiment, the specific inclusion has an equivalent circle diameter of, for example, 0.5 μm or more. Here, the equivalent circle diameter means a diameter (μm) when the area of ​​the specific inclusion obtained by observing the structure using SEM (Scanning Electron Microscope)-EDX (Energy Dispersive X-ray Spectrometry) described later is assumed to be the area of ​​a circle.

[0083] In this embodiment, the EDX beam diameter used to identify the inclusions is 0.5 μm. In this case, the accuracy of elemental analysis by EDX cannot be improved for inclusions with a circle-equivalent diameter of less than 0.5 μm. Inclusions with a circle-equivalent 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 a circle-equivalent diameter of 0.5 μm or more are measured. The upper limit of the circle-equivalent diameter of the specific inclusions is not particularly limited, but is, for example, 200 μm.

[0084] [Number density of specific inclusions] The number density of specific inclusions in electric resistance welds is 2.5 pieces / mm 2 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 are likely to occur in the electric resistance welded joint starting from the specific inclusions. Furthermore, the propagation of cracks along the interface between the parent phase and the specific inclusions is likely to be accelerated. As a result, the low temperature toughness of the electric resistance welded steel pipe decreases. The preferred upper limit of the number density of the specific inclusions is 2.4 pieces / mm 2 and more preferably 2.3 pieces / mm 2 and more preferably 2.2 pieces / mm 2It 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, 0 pieces / mm 2 It's super.

[0085] The number density of the specific inclusions in the electric resistance welded portion is measured using SEM-EDX. Specifically, in a cross section perpendicular to the axial direction of the electric resistance welded steel pipe, 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 wall thickness of the electric resistance welded portion and the center of the width of the electric resistance welded portion in the circumferential direction of the electric resistance welded steel pipe. 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 welded portion to be observed. For example, the size of the observation area is 3.0 mm in height in the wall thickness direction and 5.0 mm in width in the direction perpendicular to the wall thickness direction. The specific inclusions containing Ca, Al, and O are identified using SEM-EDX. 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 inclusions are identified as specific inclusions. 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 are excluded from the scope of the test.

[0086] The observation area is observed at a magnification of 500 to 850 times to determine the number of specific inclusions. Based on the determined number of specific inclusions and the area of ​​the observation area, the number density (pieces / mm 2 ) is required.

[0087] [Average aspect ratio of specific inclusions] Furthermore, the specific inclusions in the electric resistance welded portion have an average aspect ratio of 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 to 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 progression of cracks. Therefore, the number density of the specific inclusions in the electric resistance welded portion is 2.5 pieces / mm 2 Even if the ratio of the plate-like specific inclusions is below this range, if the ratio is too high, the low-temperature toughness of the electric resistance welded portion decreases.

[0088] 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 On the other hand, if the average aspect ratio of the specific inclusions in the electric resistance welded portion exceeds 4.5, 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 Even if the aspect ratio is less than 1.0, cracks are likely to occur starting from the specific inclusions, and the plate-like specific inclusions with a large aspect ratio promote the propagation 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.

[0089] The average aspect ratio of the specific inclusions in the electric resistance welded portion 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 is measured. As described above, inclusions containing Ca, Al, and O (specific inclusions) are identified using SEM-EDX. 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.

[0090] The major axis and minor axis of each specific inclusion with a circle equivalent diameter of 0.5 μm or more identified as a measurement target 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.

[0091] 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. A portion of the straight line SL1 that overlaps with the specific inclusion SI is defined as a line segment S1. Similarly, 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). The aspect ratio is calculated using the obtained major axis L and minor axis W by the following formula. The arithmetic average value of the aspect ratios of all the measured specific inclusions is calculated and used as the average aspect ratio. Aspect ratio = L / W

[0092] [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.

[0093] [Manufacturing 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.

[0094] FIG. 6 is a flow diagram showing an example of a manufacturing process of an electric resistance welded steel pipe according to this embodiment. With reference to FIG. 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 a raw material for an electric resistance welded steel pipe, is manufactured.

[0095] 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 mother pipe (open pipe) using forming rolls. In the formed mother pipe, the plate width direction of the hot rolled steel sheet is formed to be the circumferential direction of the mother pipe. The butt joints extending in the longitudinal direction of the mother pipe are electric resistance welded (welding process). An electric resistance welded steel pipe is manufactured by the above pipe making process. Each process will be described in detail below.

[0096] [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.

[0097] [Hot rolling process (S1)] In the hot rolling process (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 process (S1) includes a heating process (S11), a rough rolling process (S12), and a finish rolling process (S13).

[0098] [Heating process (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.

[0099] [Rough rolling process (S12)] In the rough rolling step (S12), the heated slab is subjected to rough rolling 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.

[0100] [Finishing rolling process (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). Rolling may also be performed using a reversing type rolling mill having a pair of work rolls.

[0101] In this embodiment, the finish rolling conditions are as follows. Cumulative rolling reduction rate below 950℃: 60~80% Finish rolling temperature: 850℃ or less The cumulative reduction at 950°C or less means the cumulative reduction at a steel sheet in a state where the steel sheet temperature is 950°C or less during finish rolling.

[0102] In the finish rolling process (S13), the surface temperature of the steel plate at the exit side of the stand of the finish rolling mill that performs the final reduction is defined as the finish rolling temperature (°C). As described above, the finish rolling temperature (°C) is 850°C or lower. 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 lower, coarsening of grains can be suppressed, providing that other manufacturing conditions are satisfied, and the effective grain size is 15.0 μm or less. The lower limit of the finish rolling temperature (°C) is equal to or higher than the Ar3 transformation point.

[0103] [ROT cooling process (S2)] In the ROT (run-out table) cooling process (S2), the steel plate produced in the hot rolling process (S1) is cooled.

[0104] First, the steel plate is cooled until the surface temperature of the steel plate 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 increases. The cooling is, for example, water cooling using a water cooling device.

[0105] [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.

[0106] After the ROT cooling step (S2) is completed, the steel sheet is air-cooled and coiled. The surface temperature of the steel sheet during coiling (hereinafter referred to as 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.

[0107] Through the above manufacturing steps, a hot-rolled steel sheet that is the raw material for the electric resistance welded steel pipe according to this embodiment is manufactured.

[0108] [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. Next, 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).

[0109] 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 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 of hot-rolled steel plate L0 (mm)) - (Outer circumference of steel pipe after welding L1 (mm))

[0110] If the upset amount exceeds 1.20t (mm), the pressure applied to the butt joint of the open pipe will be excessively high even if other manufacturing conditions are met. This makes it easier for the specific inclusions in the electric resistance weld to become plate-like. This increases the average aspect ratio of the specific inclusions. As a result, the low-temperature toughness of the electric resistance welded steel pipe will decrease. 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 will be insufficient even if other manufacturing conditions are met. In this case, the specific inclusions cannot be sufficiently expelled from the inner or outer surface of the steel pipe. This makes it difficult for the number density of the specific inclusions in the electric resistance weld to exceed 2.5 pieces / mm. 2As a result, the low-temperature toughness of the electric resistance welded steel pipe decreases. Therefore, the upset amount is more than 0.76t and less than 1.20t (mm). If the upset amount is more than 0.76t and less than 1.20t (mm), the low-temperature toughness of the electric resistance welded steel pipe increases, provided that other manufacturing conditions are satisfied.

[0111] Using the steel pipe after the welding process, a known seam heat treatment is performed on the electric resistance welded portion as necessary. Through the above steps, the electric resistance welded steel pipe according to this embodiment is manufactured.

[0112] In the electric resistance welded steel pipe manufactured by the above manufacturing process, the ferrite fraction of the base material is 50 to 90%, and the effective grain size is 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 / mm 2 The average aspect ratio of the specific inclusions is 4.5 or less, and therefore it is possible to achieve both excellent HIC resistance and low-temperature toughness. EXAMPLES

[0113] Molten steels of steel grades A1 to A18 and steel grades B1 to B6 shown in Table 1 were continuously cast to produce slabs.

[0114] [Table 1]

[0115] 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 balance 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 with steel grade numbers A1 to A18 and steel grade numbers B1 to B6.

[0116] [Table 2]

[0117] 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), the cumulative reduction rate (%) below 950°C in the finish rolling process, and the finish rolling temperature (°C) were as shown in Table 2.

[0118] The steel sheet 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 sheet was manufactured by the above manufacturing process. The obtained steel sheet was coiled at the coiling temperature T2 (°C) shown in Table 2 (with the proviso that T1>T2 is satisfied) to manufacture a hot-rolled steel sheet. The sheet thickness t (mm) of the hot-rolled steel sheet was as shown in Table 2.

[0119] The hot-rolled steel sheet was bent using a series of forming rolls to form an open pipe. The butt joint of the open pipe was welded using an electric resistance welding method. 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 sheet shown in Table 2.

[0120] [Evaluation test] For the electric 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 welded parts (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 welded parts, and HIC resistance evaluation tests were conducted.

[0121] [Microstructure observation of base material] The ferrite fraction and effective grain size of the base material of the electric resistance welded steel pipe were measured using EBSP-OIM.

[0122] [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.

[0123] [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 area surrounded by the grain boundary was defined as a crystal grain, and the grain size and surface area of ​​the crystal grain were obtained. The obtained crystal grain size of less than 0.5 μm was excluded because it may be noise. In other words, the target was the crystal grain size of 0.5 μm or more. 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 crystal grain size, the area average grain size was defined as the sum of the crystal grain size and area fraction of each crystal grain for each crystal grain with a crystal grain size of 0.5 μm or more. Specifically, the total area of ​​crystal grains with a crystal grain size of 0.5 μm or more in the field of view was taken as 100%, and the sum of the products of the crystal grain size and the 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 crystal grain size (μm). The field of view was 200 μm × 500 μm, centered on the center of the wall thickness. The observation magnification was 400 times, and the measurement step was 0.3 μm.

[0124] [Microstructure observation of electric resistance welded parts] For the electric resistance welded zone 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.

[0125] [Number density of specific inclusions] In a cross section perpendicular to the axial direction of the electric resistance welded steel pipe, a test piece was taken 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 of 3.0 mm in the thickness direction and a width of 5.0 mm in the direction perpendicular to the thickness direction, with the center position being the center of the thickness of the electric resistance welded part and the width of the electric resistance welded part in the circumferential direction of the electric resistance welded steel pipe. Using SEM-EDX, specific inclusions containing Ca, Al and O were identified. Specifically, in the elemental analysis results of the identified inclusions, if 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.

[0126] The observation area was observed at a magnification of 700 to 850 times to determine the number of specific inclusions. Based on the determined number of specific inclusions and the area of ​​the observation area, the number density (pieces / mm 2 ) was sought.

[0127] [Average aspect ratio of specific inclusions] Test pieces were taken in the same manner as in the measurement of the number density of specific inclusions. Using SEM-EDX, specific inclusions containing Ca, Al, and O were identified. In this case, specific inclusions with a circle equivalent diameter of 0.5 μm or more were used to calculate the average aspect ratio. In other words, specific inclusions with a circle equivalent diameter of less than 0.5 μm were excluded from the calculation.

[0128] The major axis and minor axis of each specific inclusion with a circle equivalent diameter of 0.5 μm or more that was specified as a measurement target were measured as follows. With reference to Fig. 3, a minimum circumscribing rectangle 100 was defined, which 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.

[0129] 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. The length of the maximum line segment among the line segments S1 to S4 was defined as the major axis L of the specific inclusion SI. The length of the minimum line segment among the line segments S1 to S4 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

[0130] The arithmetic average value of the aspect ratios of the specific inclusions was calculated and defined as the average aspect ratio.

[0131] [Low temperature toughness test of base material] DWTT test pieces were taken from the center of the wall thickness at a position 90° circumferentially shifted from the electric welded part of the electric welded steel pipe of each test number. The circular arc-shaped member taken in the circumferential direction of the pipe from the taking position was unfolded 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. If the ductile fracture surface ratio at -20°C was 85% or more, it was determined that the low-temperature toughness of the base material was high.

[0132] [Low temperature toughness test of electric resistance welded joints] V-notch test specimens were prepared from the electric resistance welds of the electric resistance welded steel pipes of each test number in accordance with the standard test specimen of JIS Z2242 (2018). The V-notch test specimens were prepared so that the longitudinal direction of the test specimen was perpendicular to the longitudinal direction and wall thickness direction of the electric resistance welded steel pipe (i.e., the circumferential direction), and the electric resistance weld was located at the longitudinal center position of the test specimen. A V-notch was prepared in the part of the test specimen corresponding to the electric resistance weld so that the propagation direction of the crack in the Charpy impact test was the longitudinal direction of the electric resistance welded steel pipe. The cross section of the V-notch test specimen was 10 mm x 10 mm, and the depth of the V-notch was 2 mm. Using the V-notch test specimens, a Charpy impact test in accordance with 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 resistance weld was high.

[0133] [HIC resistance evaluation test] HIC test pieces were taken from the base metal at a position 90° circumferentially shifted from the electric resistance welded part 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 unfolded to 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 was carried out in accordance with NACE-TM0284. Specifically, the HIC test piece was immersed for 96 hours in a test liquid in which 100% H2S 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 occurrence of HIC using an ultrasonic flaw detector. Based on the measurement results, the crack length ratio CLR (%) 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(%)

[0134] [Test Results] The test results are shown in Table 3.

[0135] [Table 3]

[0136] 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 zone, the number density of specific inclusions was 2.5 pieces / mm 2 The average aspect ratio of the specific inclusions was 4.5 or less. Therefore, the DWTT ductile fracture surface area ratio of the base material 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 material and the electric resistance welded part was high. Furthermore, the CLR of the base material was 15% or less, and the HIC resistance was excellent.

[0137] On the other hand, in test number 19, F1 exceeded 0.29. Therefore, the ferrite fraction of the base material was less than 50%, and the effective grain size exceeded 15.0 μm. Therefore, 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.

[0138] In test number 20, the Ca content was too high. As a result, the number density of specific inclusions in the electric resistance welded joint was 2.5 pieces / mm 2 Therefore, the absorbed energy at -20℃ in the Charpy impact test was less than 100J. As a result, the low-temperature toughness of the electric resistance welded joint was low.

[0139] 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.

[0140] In test number 22, the Al content was too high. As a result, the number density of specific inclusions in the electric resistance welded joint was 2.5 pieces / mm 2 Therefore, the absorbed energy at -20℃ in the Charpy impact test was less than 100J. As a result, the low-temperature toughness of the electric resistance welded joint was low.

[0141] In test number 23, the O content was too high. As a result, the number density of specific inclusions in the electric resistance welded joint was 2.5 pieces / mm 2 Therefore, the absorbed energy at -20℃ in the Charpy impact test was less than 100J. As a result, the low-temperature toughness of the electric resistance welded joint was low.

[0142] 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 area ratio of the base material at -20°C was less than 85%, and the low-temperature toughness of the base material was low.

[0143] In test number 25, the finish rolling temperature was too high. Therefore, the effective grain size in the base material exceeded 15.0 μm. Therefore, 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.

[0144] In test numbers 26 to 28, the amount of upset was too high. Therefore, the average aspect ratio of the specific inclusions in the electric resistance welded joint exceeded 4.5. 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.

[0145] In test numbers 29 and 30, the amount of upset was too low. As a result, the number density of specific inclusions in the electric resistance welded joint was 2.5 pieces / mm 2 Therefore, the absorbed energy at -20℃ in the Charpy impact test was less than 100J. As a result, the low-temperature toughness of the electric resistance welded joint was low.

[0146] The embodiments of the present disclosure have been described above. 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 appropriately modifying the above-described embodiments without departing from the spirit of the present disclosure.

Claims

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 / mm ２ and the average aspect ratio of the specific inclusions is 4.5 or less. ERW 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. 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 ERW steel pipe.

3. The electric welded steel pipe according to claim 1 or 2, The thickness is 10.0 to 25.4 mm. ERW steel pipe.

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