Electric resistance welded steel pipes for line pipes

Abstract

To provide electric resistance welded steel pipes for line pipes that have high strength, low YR, and excellent low-temperature toughness in the base material and electric resistance welded joints. [Solution] The material has a base material portion and an electric resistance welded portion, and in the microstructure of the 1 / 2 × tB portion of the base material, the area ratio of ferrite is 40-80%, the average grain size is 35 μm or less, and the dislocation density is 1.0 × 10 13 ~5.0×10 15 m -2 The electric resistance welded steel pipes used for line pipes are characterized in that, in the microstructure of the 1 / 4 × tS portion of the electric resistance welded joint, the area ratio of ferrite is 40-70%, the area ratio of island martensite is 0.2-10.0%, the average grain size is 35 μm or less, the maximum Vickers hardness is 180-220 HV, the yield stress is 360 MPa or more, and the tensile strength is 465 MPa or more.

Description

[Technical Field]

[0001] The present invention relates to steel pipes, and more particularly to electric resistance welded steel pipes for line pipes. [Background technology]

[0002] Pipelines are one means of transporting crude oil and natural gas, but with the increasing harshness of natural resource drilling areas in recent years, the environment for laying pipelines is also becoming more challenging. One consequence of this is the increase in the number of subsea pipelines being laid on the seabed. There are various methods for laying subsea pipelines. Among them, the S-Lay method involves welding the ends of pipes together at sea and then laying them on the seabed. This method allows for long-distance laying operations.

[0003] However, since plastic bending is applied to the steel pipes during installation, buckling can occur. When steel pipe buckling occurs, installation work must be stopped, resulting in enormous damage. It is known that steel pipe buckling can be prevented by lowering the yield ratio in the longitudinal direction of the steel pipe (determined by yield strength (YS) / tensile strength (TS), and denoted as YR) in the base material and electric resistance welded joints of the steel pipe. Furthermore, it is also known that steel pipe buckling can be prevented by increasing the wall thickness of the steel pipe. For these reasons, in addition to steel pipe strength that can sufficiently withstand high internal pressure, there is a growing demand for steel pipes with low axial YR and thick walls.

[0004] Furthermore, S-Lay pipelines are often laid in deep waters several hundred meters deep, and if brittle fracture occurs, repairing the pipeline becomes difficult, and the brittle fracture can propagate over long distances. Long-distance propagation of brittle fracture causes enormous damage from the perspective of oil and natural gas spills. Therefore, in order to strictly prevent the occurrence and propagation of brittle fracture, superior toughness is required in environments lower than the laying environment. Specifically, in the base material, in addition to the Charpy test, it is essential to ensure properties at a test temperature of -30°C in the DWTT test from the perspective of stopping the propagation of brittle fracture, and in the electric resistance welded joint, it is essential to ensure properties at a test temperature of -30°C in the CTOD test. In particular, since the steel pipes used for line pipes have thick walls to prevent buckling, it is not easy to ensure excellent toughness in the base material and electric resistance welded joints. Specifically, when the base material has a thick wall, the finishing reduction ratio in hot rolling is insufficient, and the microstructure becomes coarse, making it difficult to ensure toughness. Furthermore, when the wall thickness is thick, the strain during pipe formation increases, and in particular, at a 90° position circumferentially from the butt joint surface of the electric resistance welded section to the base material (hereinafter referred to as the 90° position of the base material), the strain due to pipe formation is concentrated, making it more difficult to ensure toughness. In addition, when the wall thickness is thick in the electric resistance welded section, the reheating temperature of the outer surface rises in order to reach the required temperature on the inner surface through heating from the outer surface after electric resistance welding, causing the microstructure to coarseen, and the hardness increases due to subsequent cooling from the outer surface, making it difficult to ensure toughness.

[0005] Therefore, electric resistance welded steel pipes used in the S-Lay method require high strength, low YR, and excellent low-temperature toughness in both the base material and the welded joint.

[0006] For example, Patent Document 1 describes an electric resistance welded steel pipe that has excellent toughness in both the base material and the electric resistance welded portion. In the electric resistance welded steel pipe described in Patent Document 1, the chemical composition of the base material is, by mass%, C: 0.04~0.12%, Si: 0.01~0.50%, Mn: 0.5~2.0%, Ti: 0.005~0.030%, Nb: 0.005~0.050%, and N: 0.001~0.008%, with the remainder being Fe and impurities. When the thickness of the base material is tB and the thickness of the electric resistance welded section is tS, the value obtained by subtracting the hardness of 1 / 2tB from the hardness of the outer surface layer B, which is 1 mm deep from the outer surface of the base material, is 30HV10 or less, and the value obtained by subtracting the hardness of 1 / 2tS from the hardness of the outer surface layer S, which is 1 mm deep from the outer surface of the electric resistance welded section, is 0HV10 or more and 30HV10 or less. Patent Document 1 states that this provides excellent low-temperature toughness.

[0007] Patent Document 2 describes an electric resistance welded (ERW) steel pipe with excellent toughness in both the base material and the ERW welded portion. The ERW steel pipe described in Patent Document 2 has a composition containing, by mass%, C: 0.02~0.10%, Si: 0.05~0.30%, Mn: 0.80~2.00%, and Nb: 0.010~0.100%, satisfying a carbon equivalent Ceq of 0.25~0.50, and a structure consisting of a bainite ferrite phase and / or a bainite phase, possessing high strength with a yield strength of 52ksi or higher and high toughness with a fracture transition temperature vTrs of -45°C or lower. The material is a thick hot-rolled steel sheet, and the electric resistance welded (ERW) section is subjected to induction heating with a minimum temperature of 830°C or higher and a maximum temperature of 1150°C or lower, and is cooled at an average cooling rate of 10 to 70°C / s at each position in the thickness direction, with a cooling stop temperature of 550°C or lower. The resulting structure consists of a bainitic ferrite phase and / or bainite phase, and the ratio of the average grain size at the coarsest grain position to the average grain size at the finest grain position at each position in the thickness direction is 2.0 or less. Patent Document 2 states that this results in excellent low-temperature toughness. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] International Publication No. 2020 / 170333 [Patent Document 2] International Publication No. 2015 / 004901 [Summary of the Invention] [Problems to be Solved by the Invention]

[0009] As a result of the studies by the present inventors, when the chemical composition and manufacturing method are not appropriate, in the electric seam welded part, the area ratio of the island-like martensite (MA), which is a hard phase, deviates from the appropriate range, and thus the low-temperature toughness and low YR of the electric seam welded part may not be compatible. Also, in the electric seam welded part, when the maximum value of the Vickers hardness deviates from the appropriate range, it has been found that it becomes difficult to ensure the low-temperature toughness of the welded part. Furthermore, in the base metal part, when the dislocation density introduced during pipe manufacturing deviates from the appropriate range, it has been found that it becomes difficult to ensure the low-temperature toughness of the base metal part. Also, when the C content deviates from the appropriate range, it has been found that the characteristics may not be satisfied in the DWTT test of the base metal part.

[0010] In Patent Documents 1 and 2, no consideration is given at all to controlling the above-described MA and dislocation density. Therefore, there is a possibility that the electric resistance welded steel pipes described in Patent Documents 1 and 2 may not necessarily have sufficient low-temperature toughness of the base metal part, low-temperature toughness of the electric seam welded part, and low YR.

[0011] In view of the above circumstances, the present invention aims to provide an electric resistance welded steel pipe for a line pipe having high strength, low YR, and excellent low-temperature toughness in the base metal part and the electric seam welded part. [Means for Solving the Problems]

[0012] The gist of the present invention is as follows. [1] The electric resistance welded steel pipe for a line pipe according to one aspect of the present invention has a base metal part and an electric seam welded part, where the chemical composition of the base metal part is, in mass%, C: 0.010 to 0.059%, Si: 0.02 to 0.50%, Mn: 0.5 to 2.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.080% or less, Ti: 0.003 - 0.030%, Nb: 0.008 - 0.048%, N: 0.0010 - 0.0080%, O: 0.005% or less, Cu: 0 - 0.500%, Ni: 0 - 0.500%, Cr: 0 - 0.500%, Mo: 0 - 0.500%, V: 0 - 0.100%, W: 0 - 0.500%, Ca: 0 - 0.0040%, B: 0 - 0.0050%, Mg: 0 - 0.0200%, Zr: 0 - 0.0200%, REM: 0 - 0.0050%, and the balance consists of Fe and impurities, Ceq represented by the following formula (i) is 0.16 - 0.53 mass%, M represented by the following formula (ii) is 0.06 - 0.25 mass%, When the thickness of the base material part is tB and the thickness of the electric seam welded part is tS, the tB and the tS are 15.0 - 25.4 mm, the outer diameter is 304.8 - 660.4 mm, in the microstructure of the 1 / 2×tB part of the base material part, the ferrite area ratio is 40 - 80%, the average crystal grain size is 35 μm or less, and the dislocation density is 1.0×10 13 ~5.0×10 15 m ―2 and in the microstructure of the 1 / 4×tS part of the electric seam welded part, the ferrite area ratio is 40 - 70%, the area ratio of island - shaped martensite is 0.2 - 10.0%, the average crystal grain size is 35 μm or less, and the maximum value of Vickers hardness is 180 - 220 The yield strength is 360 MPa or higher, and the tensile strength is 465 MPa or higher. Ceq=C+Mn / 6+(Ni+Cu) / 15+(Cr+Mo+V) / 5 (i) M = C / 3 + 5 × Nb (ii) Here, the elemental symbols in equations (i) and (ii) above are substituted with the content (mass%) of the corresponding element, and 0 is substituted if the element is not present. [2] The electric resistance welded steel pipe for line pipes described in [1] above is The chemical composition of the aforementioned base material is, in mass%, Cu: more than 0% and less than 0.500%, Ni: more than 0% and less than 0.500%, Cr: more than 0% and less than 0.500%, Mo: more than 0% and less than 0.500%, V: more than 0% and less than 0.100%, W: more than 0% and less than 0.500%, Ca: more than 0% and less than 0.0040% B: More than 0% 0.0050%, Mg: over 0% 0.0200%, Zr: greater than 0% and 0.0200%, and REM: May contain one or more substances in amounts greater than 0% and less than or equal to 0.0050%. [Effects of the Invention]

[0013] According to the above embodiment of the present invention, an electric resistance welded steel pipe for line pipes can be obtained that has high strength, low YR, and excellent low-temperature toughness in the base material portion and the electric resistance welded portion. [Brief explanation of the drawing]

[0014] [Figure 1] This figure shows the relationship between the dislocation density in the base material and the absorbed energy. [Figure 2] This figure shows the relationship between the MA area ratio and δc in electric resistance welded joints. [Figure 3] This figure shows the relationship between the MA area ratio and YR in electric resistance welded joints. [Modes for carrying out the invention]

[0015] The inventors of the present invention have investigated a method for obtaining electric resistance welded steel pipes for line pipes that have high strength and low YR and excellent low-temperature toughness in both the base material and the electric resistance welded joint, and have obtained the following findings.

[0016] In the base material, it is important to control the microstructure by adjusting the chemical composition of the electric resistance welded steel pipe and by controlling the hot rolling conditions.

[0017] In electric resistance welded (ERW) steel pipes, if left as is, the microstructure of the weld becomes a quenched martensitic structure, resulting in high hardness and a significant deterioration of toughness. Therefore, in ERW steel pipes, from the perspective of improving toughness, the ERW weld is reheated from the outer surface by induction heating after electric resistance welding, water-cooled, and then tempered by induction heating from the outer surface. Since water cooling is performed on the outer surface, the outer surface cools rapidly, creating a hard structure on the outer surface and increasing hardness, which can deteriorate the toughness of the outer surface. Therefore, it is important to optimize the water cooling conditions after reheating to create a soft ferrite. Furthermore, it is important to optimize the tempering conditions to reduce the hardness of the outer surface.

[0018] During water cooling after reheating, if carbon becomes concentrated in untransformed austenite, island-like martensite (MA) is formed. Since MA is hard, its amount degrades toughness. On the other hand, MA enhances the work hardening ability of the material, so its amount increases tensile strength. To achieve both low-temperature toughness and low YR in electric resistance welded joints, it is important to appropriately control the amount of MA.

[0019] From a chemical composition standpoint, the amount of MA in electric resistance welded joints is influenced by the carbon (C) and nitrogen (Nb) content. Carbon (C) contributes to MA formation by concentrating in untransformed austenite during water cooling after reheating. Therefore, controlling the carbon content is crucial for optimizing the MA amount. On the other hand, nitrogen (Nb) has the effect of preventing transformation. The lower the nitrogen content, the less carbon is concentrated in the untransformed austenite, thus suppressing MA formation. For these reasons, controlling the carbon and nitrogen content is important for optimizing the MA amount in electric resistance welded joints.

[0020] The amount of MA (major alloy) in an electric resistance welded joint is affected by the water cooling stop temperature after reheating. A higher water cooling stop temperature promotes carbon concentration in the untransformed austenite, increasing the MA area ratio. Conversely, a lower water cooling stop temperature suppresses carbon concentration in the untransformed austenite, decreasing the MA area ratio.

[0021] The amount of MA in an electric resistance welded (ERW) joint is also affected by the segregation state of the ERW joint. When the alloy concentration in the segregation zone is high, the concentration of carbon in the untransformed austenite is promoted, and the amount of MA increases. On the other hand, when the alloy concentration in the segregation zone is low, the concentration of carbon in the untransformed austenite is suppressed, and the amount of MA decreases. The ERW joint corresponds to the end of the slab (coil), and the segregation zone present in the ERW joint is not central (macro) segregation, such as that which is located at the center of the plate thickness in the middle of the width of the slab (coil), but rather microsegregation, and is therefore significantly affected by the heating conditions during the manufacture of the hot-rolled steel sheet. Accordingly, optimizing the segregation state of the ERW joint requires optimizing the slab heating conditions during the manufacture of the hot-rolled steel sheet that will become the material for the ERW steel pipe.

[0022] Tempering after water cooling affects not only the low-temperature toughness of the electric resistance welded joint but also the low-temperature toughness of the base metal. In electric resistance welded joints, tempering reduces the hardness of the outer surface, thus improving toughness. Furthermore, when the tempering temperature and pipe-making speed are appropriate, the 90° position of the base metal is also tempered by heat conduction, reducing the dislocation density and improving the low-temperature toughness of the base metal.

[0023] The following describes an electric resistance welded steel pipe for line pipes according to one embodiment of the present invention based on the above findings (an electric resistance welded steel pipe for line pipes according to this embodiment (sometimes simply referred to as an electric resistance welded steel pipe)). However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention.

[0024] The electric resistance welded steel pipe for line pipes according to this embodiment has a base material portion and an electric resistance welded portion, the base material portion has a predetermined chemical composition, when the wall thickness of the base material portion is tB and the wall thickness of the electric resistance welded portion is tS, then tB and tS are 15.0 to 25.4 mm, the outer diameter is 304.8 to 660.4 mm, the microstructure of the 1 / 2 × tB portion of the base material portion has a ferrite area ratio of 40 to 80%, the average grain size is 35 μm or less, and the dislocation density is 1.0 × 10⁻⁶ 13 ~5.0×10 15 m ―2 In the microstructure of the 1 / 4 × tS portion of the electric resistance welded joint, the area ratio of ferrite is 40-70%, the area ratio of island martensite (MA) is 0.2-10.0%, the average grain size is 35 μm or less, the maximum Vickers hardness is 180-220 HV, the yield stress is 360 MPa or more, and the tensile strength is 465 MPa or more. The following describes in detail the requirements for electric resistance welded steel pipes for line pipes according to this embodiment.

[0025] 1. Chemical composition of the base material The reasons for the limitations on each element are as follows. The numerical limits indicated below, separated by a "~", include both a lower and upper limit. Numbers indicated as "less than" or "greater than" do not include that value in the numerical range. In the following explanation, "%" in relation to chemical composition refers to "mass%" unless otherwise specified.

[0026] In the electric resistance welded (ERW) steel pipe for line pipes according to this embodiment, there is a steel plate that serves as the base material and a welded portion (ERW welded portion) provided at the butt joint of the steel plate and extending in the longitudinal direction of the steel plate. In the electric resistance welded steel pipe according to this embodiment, no welding material is used when the steel plate is electro-welded to form the ERW steel pipe, so the chemical composition of the base material and the ERW welded portion is substantially the same.

[0027] The electric resistance welded steel pipe for line pipes according to this embodiment has a base material chemical composition of, in mass%, C: 0.010~0.059%, Si: 0.02~0.50%, Mn: 0.5~2.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.080% or less, Ti: 0.003~0.030%, Nb: 0.008~0.048%, N: 0.0010~0.0080%, O: 0.005% or less, with the remainder being Fe and impurities, and the Ceq represented by formula (i) described later is 0.16~0.53 mass%, and the M represented by formula (ii) described later is 0.06~0.25 mass%. The following provides a detailed explanation of each element.

[0028] C: 0.010~0.059% Carbon (C) is an effective element for increasing the strength of steel and also affects the low-temperature toughness of the base material. If the carbon content is too low, the desired strength cannot be obtained in electric resistance welded steel pipes for line pipes. To obtain the desired strength in electric resistance welded steel pipes for line pipes, the carbon content should be 0.010% or more. Preferably, the carbon content is 0.020% or more. On the other hand, if the carbon content becomes too high, the central segregation hardens, and the low-temperature toughness of the base material, especially the DWTT properties, deteriorates. To ensure the low-temperature toughness of the base material, the carbon content should be 0.059% or less. Preferably, the carbon content is 0.050% or less.

[0029] Si: 0.02~0.50% Si is an effective element for deoxidizing steel. If the Si content is too low, Si cannot sufficiently improve the integrity of the steel through deoxidation (suppressing the formation of defects such as blowholes in the steel). To obtain this effect sufficiently and to obtain excellent low-temperature toughness in the base material, the Si content should be 0.02% or higher. Preferably, it should be 0.10% or higher. On the other hand, if the Si content exceeds 0.50%, oxides will form in the electric resistance welded joint, degrading the low-temperature toughness of the welded joint. Therefore, the Si content should be 0.50% or less. Preferably, the Si content is 0.40% or less.

[0030] Mn: 0.5~2.0% Mn is an effective element for improving hardenability and ensuring the strength of the base material. To obtain the desired strength in the base material, the Mn content should be 0.5% or more. Preferably, the Mn content is 0.7% or more. On the other hand, if the Mn content exceeds 2.0%, a hardened phase is formed in the central segregation area, and the low-temperature toughness of the base material deteriorates significantly. Therefore, the Mn content should be 2.0% or less. Preferably, the Mn content is 1.6% or less.

[0031] P:0.030% or less P affects the low-temperature toughness of steel. If the P content exceeds 0.030%, grain boundary embrittlement occurs in the base metal and electric resistance welded joints, significantly degrading the low-temperature toughness. Therefore, the P content should be 0.030% or less. A lower P content is preferable, and it may even be 0%. However, since the practical lower limit of P content in mass-produced steel is 0.002%, the P content may be 0.002% or higher.

[0032] S: 0.0050% or less S (S) affects the low-temperature toughness of steel. If the S content exceeds 0.0050%, coarse sulfides are formed, and the low-temperature toughness deteriorates in the base metal and electric resistance welded joints. Therefore, the S content should be 0.0050% or less. A low sulfur content is preferable, and it may even be 0%. However, since the practical lower limit of sulfur content in mass-produced steel is 0.0003%, the sulfur content may be 0.0003% or higher.

[0033] Al: 0.080% or less Al is an effective element as a deoxidizing agent. However, if the Al content exceeds 0.080%, a large amount of Al oxide is generated, degrading the low-temperature toughness of the base material and the electric resistance welded joint. Therefore, the Al content should be 0.080% or less. Preferably, the Al content is 0.050% or less. Deoxidation is possible with Si and Ti as well, so an Al content of 0% is acceptable. However, to obtain a sufficient deoxidation effect, an Al content of 0.010% or more is preferable.

[0034] Ti: 0.003~0.030% Ti is a nitride-forming element that contributes to the refinement of crystal grains by forming nitrides. To obtain this effect and ensure low-temperature toughness of the base material, the Ti content should be 0.003% or more. Preferably, the Ti content is 0.010% or more. On the other hand, if the Ti content exceeds 0.030%, the low-temperature toughness of the base material deteriorates significantly due to the formation of coarse carbonitrides. Therefore, the Ti content should be 0.030% or less. Preferably, the Ti content is 0.025% or less.

[0035] Nb: 0.008~0.048% Nb is an element that forms carbides, nitrides, and / or carbonitrides, contributing to the improvement of steel strength. Furthermore, Nb has the effect of improving the low-temperature toughness of the base material of electric resistance welded steel pipes for line pipes by expanding the unrecrystallized rolling temperature range. Nb also has the effect of stabilizing transformations, making it an effective element for optimizing MA (metamorphosis ratio). To obtain these effects, the Nb content should be 0.010% or higher. Preferably, the Nb content is 0.011% or higher. On the other hand, if the Nb content exceeds 0.048%, a large amount of Nb-based carbonitride is generated, and the low-temperature toughness of the base material deteriorates. Therefore, the Nb content should be 0.048% or less. Preferably, the Nb content is 0.030% or less.

[0036] N: 0.0010~0.0080% N is an element that forms nitrides, which refines the crystal grains of steel and improves the low-temperature toughness of the base material. To obtain this effect and ensure low-temperature toughness of the base material, the N content should be 0.0010% or more. On the other hand, if the N content exceeds 0.0080%, a large amount of nitride is generated, which deteriorates the low-temperature toughness of the base material. Therefore, the N content should be 0.0080% or less.

[0037] O: 0.005% or less Oxygen (O) is an element that affects the low-temperature toughness of steel. If the Oxygen content exceeds 0.005%, a large amount of oxide is formed, and the low-temperature toughness of the base metal and electric resistance welded joint deteriorates significantly. Therefore, the Oxygen content should be 0.005% or less. A lower oxygen content is preferable, and it may even be 0%. However, since the practical lower limit of oxygen content in mass-produced steel is 0.001%, the oxygen content may be 0.001% or higher.

[0038] The electric resistance welded steel pipe for line pipes according to this embodiment has a chemical composition that contains the above-mentioned elements, with the remainder being Fe and impurities. However, in order to improve strength, low-temperature toughness, or other properties, the following optional elements may be further included within the range described later. However, the inclusion of these elements is not essential, so the lower limit for each is 0%.

[0039] Furthermore, in this embodiment, "impurities" refer to components that are mixed in during the industrial production of steel from raw materials such as ore and scrap, or due to various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the properties of the electric resistance welded steel pipe for line pipes according to this embodiment. Examples of impurities include, for example, Sn, As, Sb, Bi, Co, Pb, and Zn, and it is not prohibited to include any of these in amounts of 0.05% or less.

[0040] Cu: 0~0.500% Cu is an effective element for increasing strength without degrading low-temperature toughness. Therefore, Cu may be included as needed. To obtain the above effect, it is preferable that the Cu content be greater than 0%, and more preferably 0.010% or more. On the other hand, if the copper content exceeds 0.500%, cracks are more likely to occur during heating and electric resistance welding of the steel billet. Therefore, even when copper is included, the copper content should be 0.500% or less.

[0041] Ni: 0~0.500% Ni is an effective element for improving low-temperature toughness and strength. Therefore, Ni may be included as needed. To obtain the above effects, it is preferable that the Ni content be greater than 0%, and more preferably 0.010% or more. On the other hand, if the nickel content exceeds 0.500%, the electric resistance weldability deteriorates. Therefore, even when nickel is included, the nickel content should be 0.500% or less.

[0042] Cr: 0~0.500% Cr is an element that improves the strength of steel through precipitation strengthening. Therefore, Cr may be included as needed. To obtain this effect, it is preferable that the Cr content be greater than 0%, and more preferably 0.010% or more. On the other hand, if the chromium content exceeds 0.500%, the hardenability increases, the proportion of bainite in the microstructure becomes too high, and the low-temperature toughness deteriorates. Therefore, even when chromium is included, the chromium content should be kept below 0.500%.

[0043] Mo: 0~0.500% Mo is an element that improves hardenability and simultaneously forms carbonitrides, contributing to an improvement in steel strength. Therefore, Mo may be included as needed. To obtain the above effects, it is preferable to have a Mo content of more than 0%, and more preferably 0.010% or more. On the other hand, if the Mo content exceeds 0.500%, the strength of the steel becomes excessively high, and its low-temperature toughness deteriorates. Therefore, even when Mo is included, the Mo content should be kept below 0.500%.

[0044] V: 0~0.100% V is an element that forms carbides and / or nitrides, contributing to an improvement in the strength of steel. Therefore, V may be included as needed. To obtain the above effect, it is preferable that the V content be greater than 0%, and more preferably 0.001% or more. On the other hand, if the V content exceeds 0.100%, precipitates increase, and low-temperature toughness deteriorates. Therefore, even when V is included, the V content should be 0.100% or less.

[0045] W: 0~0.500% W is an element that forms carbides and contributes to improving the strength of steel. Therefore, W may be included as needed. To obtain the above effect, it is preferable that the W content be greater than 0%, and preferably 0.100% or more. On the other hand, if the W content exceeds 0.500%, the amount of carbides increases, and the low-temperature toughness deteriorates. Therefore, even when W is included, the W content should be 0.500% or less.

[0046] Ca: 0~0.0040% Ca is an element that contributes to improving low-temperature toughness and lamellar tear resistance by suppressing the formation of elongated MnS through the generation of sulfides. Therefore, Ca may be included as needed. To obtain the above effects, it is preferable to have a Ca content of more than 0%, and more preferably 0.0003% or more. On the other hand, if the Ca content exceeds 0.0040%, a large amount of CaO is generated in the electric resistance welded joint, degrading the low-temperature toughness of the welded joint. Therefore, even when Ca is included, the Ca content should be 0.0040% or less.

[0047] B: 0~0.0050% B is an element that contributes to increased strength by improving hardenability. Therefore, B may be included as needed. To obtain the above effect, it is preferable that the B content be greater than 0%, and more preferably 0.0005% or more. On the other hand, if the B content exceeds 0.0050%, the strength becomes too high. Therefore, even if B is included, the B content should be 0.0050% or less.

[0048] Mg: 0~0.0200% Mg is an element that contributes to improving the low-temperature toughness of the base material through grain refinement. Therefore, Mg may be included as needed. To obtain the above effect, it is preferable that the Mg content be greater than 0%, and more preferably 0.0005% or more. On the other hand, if the Mg content exceeds 0.0200%, coarse oxides are formed, and the low-temperature toughness deteriorates. Therefore, even when Mg is included, the Mg content should be 0.0200% or less.

[0049] Zr: 0~0.0200% Zr is an element that contributes to improving the low-temperature toughness of the base material through grain refinement. Therefore, Zr may be included as needed. To obtain the above effect, it is preferable to have a Zr content of more than 0%, and more preferably 0.0005% or more. On the other hand, if the Zr content exceeds 0.0200%, coarse oxides are formed, and the low-temperature toughness deteriorates. Therefore, even when Zr is included, the Zr content should be 0.0200% or less.

[0050] REM: 0~0.0050% REM, like Ca, is an element that contributes to improving low-temperature toughness and lamellar tear resistance by suppressing the formation of elongated MnS through the generation of sulfides. Therefore, REM may be included as needed. To obtain the above effects, it is preferable that the REM content be greater than 0%, and preferably 0.0010% or more. On the other hand, if the REM content exceeds 0.0050%, the number of REM oxides increases, and the low-temperature toughness deteriorates. Therefore, even when REM is included, the REM content should be kept below 0.0050%. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content means the total content of these elements.

[0051] As described above, the electric resistance welded steel pipe for line pipes according to this embodiment has a chemical composition in the base material and the electric resistance welded portion in which essential elements are included, optional elements are included as needed, and the remainder is Fe and impurities.

[0052] In this embodiment, the electric resistance welded steel pipe has the content of each element controlled as described above, and furthermore, the Ceq and M determined by the content of each element must be within a predetermined range, as described below.

[0053] Ceq:0.16~0.53 mass% Ceq is an index value for hardenability and is expressed by the following formula (i). If Ceq is less than 0.16 mass%, the desired strength cannot be obtained in the base material and the electric resistance welded joint. Therefore, Ceq should be 0.16 mass% or higher. Ceq is preferably 0.25 mass% or higher, and more preferably 0.30 mass% or higher. On the other hand, if Ceq exceeds 0.53 mass%, the low-temperature toughness deteriorates in the base material and the electric resistance welded joint. Therefore, Ceq should be 0.53 mass% or less. Preferably, Ceq is 0.45 mass% or less, and more preferably 0.40 mass% or less.

[0054] Ceq=C+Mn / 6+(Ni+Cu) / 15+(Cr+Mo+V) / 5 (i) However, in formula (i) above, the content (mass%) of the corresponding element is substituted for each element symbol, and 0 is substituted if the element is not present.

[0055] M:0.06~0.25% by mass M is an indicator of carbon enrichment in untransformed austenite and is expressed as (ii) below. If M is less than 0.06 mass%, the desired amount of MA cannot be obtained in the electric resistance welded joint, and YR cannot be reduced. For this reason, M should be 0.06 mass% or more. M is preferably 0.10 mass% or more, and more preferably 0.12 mass% or more. On the other hand, if M exceeds 0.25 mass%, the MA area ratio increases, and the low-temperature toughness of the electric resistance welded joint deteriorates. Therefore, M should be 0.25 mass% or less. Preferably, M is 0.20 mass% or less.

[0056] M = C / 3 + 5 × Nb (ii) However, in formula (ii) above, the content (mass%) of the corresponding element is substituted for each element symbol, and 0 is substituted if the element is not present.

[0057] 2. Microorganisms As mentioned above, controlling the microstructure of the base material and the welded section is crucial for improving the strength and low-temperature toughness of electric resistance welded steel pipes for line pipes. The microstructures of the base material and the welded section are described in detail below. In this embodiment, the thickness of the base material is denoted as tB, and the thickness of the electric resistance welded portion is denoted as tS. Furthermore, in this embodiment, the electric resistance welded portion refers to the area from the butt joint of the electric resistance welded portion to a position 600 μm away from the base material in the circumferential direction (i.e., an area of ​​1200 μm in total, centered on the butt joint).

[0058] <Base material> [In the microstructure of the 1 / 2 × tB portion of the base material, the area ratio of ferrite is 40-80%.] To ensure the strength and low-temperature toughness of electric resistance welded steel pipes for line pipes, controlling the microstructure of the base material is crucial. Specifically, the microstructure of the base material must contain 40-80% ferrite by area. If the area ratio of ferrite in the base material is less than 40%, the low-temperature toughness of the base material will deteriorate. Therefore, the area ratio of ferrite in the base material should be 40% or more. Preferably, the area ratio of ferrite is 45% or more, and more preferably 50% or more. On the other hand, if the ferrite area ratio in the base material exceeds 80%, sufficient strength cannot be obtained in the base material. Therefore, the ferrite area ratio in the base material should be 80% or less. Preferably, the ferrite area ratio is 75% or less, and more preferably 70% or less.

[0059] In this embodiment, the concept of "ferrite" includes polygonal ferrite and pseudo-polygonal ferrite. Furthermore, the microstructure of the matrix material may include one or more of the following as residual structures: pearlite (P), bainite (B), and retained austenite (γ). Note that the concept of "bainite" includes granular bainite and bainite. Also, the concept of "pearlite" includes pseudo-pearlite in which the lamellar cementite shape is not complete. The area ratio of these residual structures may be 20-60%, depending on the relationship with the area ratio of ferrite.

[0060] In this embodiment, the "1 / 2 × tB portion of the base material" refers to a position (1 / 2) × tB in the thickness direction from the outer surface of the base material. The reason for limiting the microstructure at a position (1 / 2) × tB from the outer surface of the base material is that the structure at this position affects the low-temperature toughness of the base material. In this embodiment, when simply referring to the surface of an electric resistance welded steel pipe for line pipes, it means the outer surface, not the inner surface.

[0061] The proportion (area ratio) of ferrite in the base material's microstructure is measured using the following method. For the base material of electric resistance welded (ERW) steel pipes used for line pipes, a sample for microstructural observation is taken from a position 90° circumferentially from the ERW weld, such that the cross-section parallel to the pipe axis (longitudinal direction) and the thickness direction serves as the observation surface. The ERW weld can be easily distinguished from the base material because the weld bead generated by ERW welding has been machined. The collected sample for microstructural observation is polished for 30 to 60 minutes using colloidal silica polishing compound. The polished sample is analyzed using EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) to determine the area ratio of ferrite. The field of view is defined as a 200 μm range centered on (1 / 2) × tB in the thickness direction from the outer surface, and a 500 μm range at an arbitrary position in the pipe axis direction. The observation magnification is 400x, the acceleration voltage is 20 kV, and the measurement step is 0.3 μm. The measurement system uses an EBSD (Electron Beam Scanning Electron) system consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL high-speed Hikari detector).

[0062] Specifically, the ferrite area ratio is determined using the KAM (Kernel Average Misorientation) method equipped in EBSP-OIM (trademark). In the KAM method, any one regular hexagonal pixel in the measurement data is designated as the center pixel. The orientation difference between each pixel is calculated for the first approximation (7 pixels in total) using the 6 pixels adjacent to this center pixel, the second approximation (19 pixels in total) using the 12 pixels further outside these 6 pixels, and the third approximation (37 pixels in total) using the 18 pixels further outside these 12 pixels. The calculated orientation differences are averaged, and the resulting average value is taken as the value for the center pixel. This operation is performed for all pixels.

[0063] In this embodiment, the area ratio of pixels with an orientation difference of 1° or less (third approximation) relative to the total area of ​​the field of view is defined as the area ratio of ferrite. Those with an orientation difference exceeding 1° (third approximation) are defined as structures other than ferrite, such as bainite. Retained austenite is defined as the fcc phase measured by EBSP-OIM, and pearlite is identified by optical microscopy observation at 400x magnification after nital etching.

[0064] [In the microstructure of the 1 / 2 × tB portion of the base material, the average grain size is 35 μm or less.] In the electric resistance welded steel pipe for line pipes according to this embodiment, in order to ensure good toughness of the base material, the average grain size in the microstructure of the 1 / 2 × tB portion of the base material is set to 35 μm or less. If the average grain size exceeds 35 μm, the toughness of the base material deteriorates. The average grain size is preferably 30 μm or less, and more preferably 20 μm or less. There is no particular limit to the average grain size in the microstructure of the 1 / 2 × tB portion of the base material, but it may be 1 μm or more, 3 μm or more, or 5 μm or more.

[0065] The average grain size in the microstructure of the 1 / 2 × tB portion of the base material is measured by the following method. Using the same sample from which the ferrite area fraction was measured, the average grain size is determined by analyzing the microstructure of the 1 / 2 × tB portion of the base material using EBSP-OIM. The field of view is defined as a 200 μm range centered on (1 / 2) × tB in the thickness direction from the outer surface, and a 500 μm range at any position in the tube axis direction. The observation magnification is 400x, and the measurement step is 0.3 μm. From the data obtained from the measurement, the region enclosed by large-angle grain boundaries with an inclination angle of 15° or more is considered a grain, and the equivalent circle diameter of that grain is considered the grain size. The average grain size is calculated from the obtained grain sizes using the AREA FRACTION method. However, regions with an equivalent circle diameter of 0.25 μm or less are excluded from the calculation of the average crystal grain size. This is because regions with an equivalent circle diameter of 0.25 μm or less are within the measurement limit and cannot be properly evaluated.

[0066] [In the microstructure of the 1 / 2×tB part of the base material part, the dislocation density is 1.0×10 13 ~5.0×10 15 m -2 The low-temperature toughness of the base material part is also affected by the dislocation density. The lower the dislocation density, the lower the stress required for plastic deformation, and the less likely brittle fracture occurs. Therefore, the low-temperature toughness, that is, the Charpy absorption energy and the ductile fracture surface rate in the DWTT test increase. Here, Fig. 1 shows the relationship between the dislocation density of the base material part and the absorption energy obtained by performing a Charpy impact test by the method described later. As shown in Fig. 1, as the dislocation density decreases, the absorption energy increases, and when the dislocation density is 5.0×10 15 m -2 or less, it can be seen that the Charpy impact absorption energy is 200 J or more.

[0067] Therefore, in the electric resistance welded steel pipe for line pipe according to the present embodiment, in order to ensure good low-temperature toughness of the base material part, the dislocation density in the microstructure of the 1 / 2×tB part of the base material part is set to 1.0×10 13 ~5.0×10 15 m -2 . If the dislocation density is less than 1.0×10 13 m -2 , the strength of the base material is insufficient. Therefore, the dislocation density is 1.0×10 13 m -2 or more. The dislocation density is preferably 4.0×10 13 m -2 or more, and more preferably 4.0×10 14 m -2 or more. On the other hand, if the dislocation density exceeds 5.0×10 15 m -2 , the low-temperature toughness of the base material part deteriorates. Therefore, the dislocation density is 5.0×10 15 m -2 or less. The dislocation density is preferably 1.0×10 15 m -2 or less, and more preferably 5.0×10 14 m -2 or less.

[0068] ​ The dislocation density of the base material is measured by the following method. A test specimen is taken from the center of the wall thickness at a 90° position in the base material, so that the cross-section parallel to the axial direction (longitudinal direction) and the thickness direction becomes the observation surface. The size of the test specimen is 10 mm in the longitudinal direction × 10 mm in the thickness direction × 2 mm in the circumferential direction. After mirror polishing the observation surface, the surface treatment layer is removed by electropolishing to a depth of 100 μm. The full width at half maximum ΔK of the peaks of the (110), (211), and (220) planes of the body-centered cubic structure (iron) is determined by X-ray diffraction on the observation surface after electropolishing.

[0069] In X-ray diffraction, the full width at half maximum (FWHM) ΔK is measured using a CoKα radiation source, a tube voltage of 30kV, and a tube current of 100mA. Furthermore, LaB6 (lanthanum hexaboride) powder is used for calibrating the device.

[0070] The non-uniform strain ε of the specimen is determined from the full width at half maximum ΔK obtained by the method described above and the Williamson-Hall equation (equation (1) below). (ΔK×cosθ) / λ=0.9 / D+2ε×(sinθ / λ) (1) Here, in equation (1), θ is the diffraction angle, λ is the wavelength of the X-ray, and D is the crystallite size.

[0071] Furthermore, using the obtained heterogeneous strain ε and the following equation (2), the dislocation density ρ(m -2 ) ρ = 14.4 × (ε / b) 2 (2) Here, in equation (2), b is the Burgers vector for the body-centered cubic structure (iron) (b = 0.248 (nm)).

[0072] <Electric resistance welded section> [In the microstructure of the 1 / 4 × tS portion of the electric resistance welded joint, the area ratio of ferrite is 40-70%.] The microstructure of the electric resistance welded joint can be controlled by reheating the welded joint and then water-cooling it from the outer surface side. In electric resistance welded joints, the low-temperature toughness deteriorates as the hardness of the outer surface increases. Therefore, in the electric resistance welded steel pipe for line pipes according to this embodiment, from the viewpoint of ensuring low-temperature toughness in the electric resistance welded joint, it is necessary to include soft ferrite in the microstructure of the 1 / 4 × tS portion of the electric resistance welded joint.

[0073] Specifically, the ferrite area ratio in the 1 / 4 × tS portion of the electric resistance welded joint needs to be 40-70%. If the ferrite area ratio is less than 40%, the hardness of the outer surface of the electric resistance welded joint increases, leading to a deterioration of low-temperature toughness. The ferrite area ratio is preferably 45% or more, and more preferably 50% or more. Furthermore, if the ferrite area ratio exceeds 70% in the microstructure of the 1 / 4 × tS portion of the electric resistance welded joint, the strength of the electric resistance welded joint decreases. The ferrite area ratio is preferably 65% ​​or less, and more preferably 60% or less.

[0074] [In the microstructure of the 1 / 4 × tS portion of the electric resistance welded joint, the area ratio of island martensite (MA) is 0.2-10.0%.] Island martensite (MA) also affects the low-temperature toughness of electric resistance welds. Since MA increases the fracture initiation point and hardness, the low-temperature toughness of electric resistance welds improves as the area ratio of MA decreases. Figure 2 shows the relationship between the MA area ratio and δc in electric resistance welds. δc represents the critical opening displacement in the CTOD test at -30°C performed using the method described later. As shown in Figure 2, the critical opening displacement δc in the CTOD test at -30°C increases with decreasing MA area ratio in electric resistance welds, and it can be seen that δc is 0.15 mm or more when the MA area ratio is 10.0% or less.

[0075] Furthermore, MA (matrix oxide) increases work hardening ability and therefore also affects the yield ratio (YR). The higher the area ratio of MA, the better the work hardening ability and the lower the YR. Figure 3 shows the relationship between the MA area ratio of the electric resistance welded (ERW) weld and the YR obtained by tensile testing using the method described later. As shown in Figure 3, the YR of the ERW weld decreases with increasing MA area ratio, and it can be seen that if the MA area ratio is 0.2% or higher, the YR of the ERW weld will be 93% or lower. Unlike the base metal, the ERW weld does not contain a hard structure caused by central segregation, so controlling the MA area ratio is essential to lower the YR of the ERW weld.

[0076] Therefore, in the electric resistance welded steel pipe for line pipes according to this embodiment, in order to achieve both excellent low-temperature toughness and low YR of the electric resistance welded joint, the area ratio of MA in the microstructure of the 1 / 4 × tS portion of the electric resistance welded joint is set to 0.2 to 10.0%. If the area ratio of MA is less than 0.2%, the work hardening ability decreases, and YR increases. The area ratio of MA is preferably 1.0% or more, more preferably 2.0% or more, and even more preferably 3.0% or more. Furthermore, in the microstructure of the 1 / 4 × tS portion of the electric resistance welded joint, if the area ratio of MA exceeds 10.0%, the low-temperature toughness of the electric resistance welded joint deteriorates. The area ratio of MA is preferably 8.0% or less, more preferably 7.0% or less, and even more preferably 6.0% or less.

[0077] In this embodiment, "1 / 4 × tS portion of the electric resistance welded area" refers to a position (1 / 4) × tS in the thickness direction from the outer surface of the electric resistance welded area. The reason for limiting the microstructure at a position (1 / 4) × tS in the thickness direction from the outer surface of the electric resistance weld is that the structure at this position affects the low-temperature toughness and YR of the electric resistance weld.

[0078] Furthermore, the microstructure of electric resistance welded joints may contain one or more of the following residual structures: pearlite (P), bainite (B), and retained austenite (γ). The concept of "bainite" includes granular bainitic ferrite and bainitic ferrite. The area ratio of these residual structures may be between 20.0% and 59.8%, based on their relationship to the area ratios of ferrite and MA.

[0079] The area ratio of ferrite in the microstructure of an electric resistance weld is determined by the following method. A sample for microstructural observation is taken from an electric resistance welded (ERW) steel pipe for line pipes, such that the cross section perpendicular to the pipe axis direction, including the ERW weld, becomes the observation surface. As described above, in this embodiment, the ERW weld refers to the area from the butt joint of the ERW weld to a position 600 μm away from the base material in the circumferential direction (i.e., a total area of ​​1200 μm centered on the butt joint). After finishing the observation surface to a mirror finish by wet polishing, the area ratio of ferrite is measured using EBSD in the same manner as the base material. The measurement position is set to a 200 μm area centered on (1 / 4) × tS in the thickness direction from the outer surface in the thickness direction, and to a 200 μm area centered on a position 400 μm away from the butt joint of the ERW weld in the circumferential direction. Furthermore, the butt joint surface of the electric resistance weld can be distinguished and identified from the base material by etching it with Nital.

[0080] The area ratio of MA in the microstructure of an electric resistance weld is determined by the following method. Samples are taken using the same method as when measuring the ferrite area ratio of the electric resistance weld. After LePera etching of the observation surface, microscopic images are taken using a 400x optical microscope. From the obtained microscopic images, areas observed as white can be identified as island martensite, and the area ratio of island martensite (MA) is calculated by image analysis. The field of view is defined as a 200 μm area centered on (1 / 4) × tS in the thickness direction from the outer surface, and a 200 μm area centered on a position 400 μm away from the butt joint surface of the electric resistance weld in the circumferential direction. However, regions with an equivalent circle diameter of 1 μm or less are excluded from the calculation of the MA area ratio. This is because MA with an equivalent circle diameter of 1 μm or less does not affect low-temperature toughness and YR.

[0081] [In the microstructure of the 1 / 4 × tS portion of the electric resistance welded joint, the average grain size is 35 μm or less.] To ensure good low-temperature toughness in electric resistance welded joints, it is important to control the area ratio of ferrite and MA as described above, as well as to refine the microstructure. In the electric resistance welded steel pipe for line pipes according to this embodiment, in order to ensure low-temperature toughness of the electric resistance welded joint, the average grain size in the microstructure of the 1 / 4 × tS portion of the electric resistance welded joint is controlled to 35 μm or less. If the average grain size exceeds 35 μm, the low-temperature toughness of the electric resistance welded joint deteriorates. The average grain size is preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 15 μm or less. There is no particular lower limit to the average crystal grain size, but it may be set to 1 μm or more, 3 μm or more, or 5 μm or more.

[0082] The average grain size in the electric resistance welded joint is determined in the same manner as in the base metal. The field of view is defined as a 200 μm area centered on (1 / 4) × tS in the thickness direction from the outer surface, and a 200 μm area centered on a point 400 μm away from the butt joint surface of the electric resistance weld in the circumferential direction. Furthermore, areas with an equivalent circle diameter of 0.25 μm or less are excluded from the calculation of the average grain size. This is because areas with an equivalent circle diameter of 0.25 μm or less do not adversely affect the low-temperature toughness of the electric resistance weld.

[0083] [In the 1 / 4 × tS section of the electric resistance welded joint, the maximum Vickers hardness is 180-220 HV] After electric resistance welding, the electric resistance welded (ERW) section is reheated and then water-cooled from the outer surface side. This increases the hardness of the outer surface, which degrades the low-temperature toughness. Therefore, in the electric resistance welded steel pipe for line pipes according to this embodiment, it is necessary to suppress the maximum value of the Vickers hardness of the 1 / 4 × tS portion of the ERW welded section from the viewpoint of ensuring low-temperature toughness in the ERW welded section.

[0084] Specifically, the maximum Vickers hardness of the 1 / 4 × tS portion of the electric resistance welded joint must be between 180 and 220. If the maximum Vickers hardness is less than 180 HV, the strength of the electric resistance welded joint will decrease. The maximum Vickers hardness is preferably 185 HV or higher, and more preferably 188 HV or higher. Furthermore, if the maximum Vickers hardness exceeds 220 HV, the low-temperature toughness of the electric resistance welded joint deteriorates. The maximum Vickers hardness is preferably 215 HV or less, and more preferably 210 HV or less.

[0085] Limiting the Vickers hardness at a position (1 / 4) × tS in the thickness direction from the outer surface of the electric resistance welded joint is necessary because, in addition to the increase in Vickers hardness at the (1 / 4) × tS position, stress concentration during the CTOD test of the electric resistance welded joint (described later) is superimposed, making it prone to brittle fracture. Therefore, controlling the maximum value of Vickers hardness at the (1 / 4) × tS position is important for ensuring the low-temperature toughness of the electric resistance welded joint.

[0086] The maximum Vickers hardness of the 1 / 4 × tS portion of the electric resistance welded joint is determined by the following method. A sample for hardness testing is taken from an electric resistance welded (ERW) steel pipe used for line pipes, such that the cross section perpendicular to the pipe axis, including the ERW weld, is the observation surface. The sample should include the area from the butt joint of the ERW weld to a point 5 mm away in the circumferential direction of the base material (i.e., a total area of ​​10 mm centered on the butt joint). A Vickers hardness test will be performed on the observation surface using a method compliant with JIS Z 2244-1:2024. The test force will be 9.8 N. Specifically, hardness measurements will be taken at a total of six locations in the 1 / 4 × tS section of the electric resistance welded joint, at positions 1 mm, 2 mm, and 3 mm from the butt joint surface in the circumferential direction of the base material on both sides. The maximum value of the hardness measured at these six locations will be used to determine the maximum Vickers hardness.

[0087] 3. Mechanical properties (Base material and electric resistance welded section) Yield strength (YS): 360 MPa or higher Tensile strength (TS): 465 MPa or higher Yield ratio (YR): 93% or less The electric resistance welded steel pipe for line pipes according to this embodiment is intended for use as a line pipe, so the yield stress (YS) measured in both the base material and the electric resistance welded section is 360 MPa or higher, and the tensile strength (TS) is 465 MPa or higher. A yield stress of 400 MPa or higher or 450 MPa or higher is preferable. A yield stress of 600 MPa or lower or 550 MPa or lower is also acceptable. A tensile strength of 500 MPa or higher or 550 MPa or higher is preferable. Furthermore, a tensile strength of 700 MPa or lower or 650 MPa or lower is also acceptable. Furthermore, the yield ratio (YR) is preferably 93% or less. The yield ratio may be 80% or more, or 85% or more. The yield ratio can be calculated by dividing the yield ratio by the tensile strength (YS / TS).

[0088] (base material) Charpy impact absorption energy at -30°C: 200J or more Ductile fracture surface ratio (SA) of 85% or higher in DWTT test at -30°C In this embodiment, it is preferable that the Charpy impact absorption energy at -30°C in the base material of the electric resistance welded (ERW) steel pipe is 200 J or more. Furthermore, in this embodiment, it is preferable that the ductile fracture surface ratio SA in the DWTT test at -30°C in the base material of the ERWW steel pipe for line pipes is 85% or more. If the Charpy impact absorption energy at -30°C in the base material is 200 J or more, and the ductile fracture surface ratio SA in the DWTT test at -30°C is 85% or more, it can be determined that the pipe has excellent low-temperature toughness. The Charpy impact absorption energy at -30°C may be 400 J or less, or 350 J or less. The ductile fracture surface ratio SA in the DWTT test at -30°C may be 100% or less.

[0089] (Electric resistance welded section) Limiting aperture displacement δc: 0.15 mm or more in CTOD testing at -30°C In this embodiment, the electric resistance welded steel pipe for line pipes preferably has a critical opening displacement δc of 0.15 mm or more in the electric resistance welded joint during a CTOD test at -30°C. If the critical opening displacement δc in the electric resistance welded joint during a CTOD test at -30°C is 0.15 mm or more, it can be determined that the pipe has excellent low-temperature toughness. The critical opening displacement δc in the CTOD test at -30℃ may be 1.00 mm or less, or 0.80 mm or less.

[0090] For the tensile test of the base material, a full-thickness test specimen in the longitudinal direction of the electric resistance welded (ERW) steel pipe for line pipes will be used as the tensile test specimen, and the tensile test will be performed. Based on the tensile test results, the yield strength and tensile strength will be measured. Here, the tensile test specimen of the base material will be taken from a point corresponding to a 90° position in the circumferential direction from the seam of the ERW steel pipe. The tensile test will be conducted in accordance with DNV-ST-F101 (2021 edition).

[0091] Tensile testing of electric resistance welded joints is performed using round bar specimens from the longitudinal direction of electric resistance welded steel pipes for line pipes. These specimens are taken from the 1 / 2 × tS portion of the weld joint of the electric resistance welded steel pipe, and the tensile test is performed. The round bar tensile test specimens are ISO 6892 (2019 edition) proportional specimens with a parallel section diameter of φ12.7 mm and a gauge length of 65 mm. Based on the tensile test results, the yield strength and tensile strength are measured. The tensile test is conducted in accordance with ISO 6892.

[0092] In the Charpy test to evaluate the low-temperature toughness of the base material, a V-notch Charpy test specimen is taken from the center of the wall thickness of the base material of the electric resistance welded (ERW) steel pipe for line pipes (the portion corresponding to a position 90° circumferentially from the butt joint surface of the ERW weld), such that the longitudinal direction of the specimen is the circumferential direction of the ERW steel pipe for line pipes. In this case, the depth direction of the V-notch is in the longitudinal direction of the steel pipe. The V-notch Charpy test is performed at a test temperature of -30°C, and the impact absorption energy at -30°C is measured. The Charpy test is performed in accordance with DNV-ST-F101 (2021 edition).

[0093] In the Drop Weight Tear Test (DWTT) test, used to evaluate the low-temperature toughness of the base material, a circular arc-shaped member is taken from the base material of an electric resistance welded (ERW) steel pipe for line pipes (the portion corresponding to a 90° angle circumferentially from the butt joint surface of the ERW weld). This member is unfolded into a flat plate, and a V-notch is machined at the 90° position. The size of the DWTT specimen is 75 mm in the longitudinal direction of the steel pipe, 300 mm in the circumferential direction, and the thickness direction is the thickness of the steel pipe. The depth of the V-notch is 5 mm. The depth direction of the V-notch is in the longitudinal direction of the steel pipe. The DWTT test is performed on the DWTT specimen in accordance with ASTM E 436, and the ductile fracture surface ratio at -30°C is measured.

[0094] In the CTOD (Crack Tip Opening Displacement) test to evaluate the low-temperature toughness of electric resistance welded joints, a CTOD test specimen is taken from an electric resistance welded steel pipe for line pipes, cut to a length of 300 mm longitudinally and 300 mm circumferentially, including the electric resistance weld. The CTOD test specimen is processed so that the fatigue precrack is located on the butt joint surface of the electric resistance weld, with the depth of the fatigue precrack oriented along the longitudinal direction of the steel pipe. The CTOD test is then performed on this CTOD test specimen at a test temperature of -30°C in accordance with the provisions of BS7448-1:1991, and the critical opening displacement δc (mm) at -30°C is measured. If a CTOD test specimen cannot be taken due to its arc shape, the sleeve portion of the specimen may be joined by welding and the test may be performed.

[0095] 4. Thick In this embodiment, the wall thickness tB of the base material and the wall thickness tS of the electric resistance welded portion of the electric resistance welded steel pipe for line pipes shall be 15.0 mm or more from the viewpoint of buckling resistance when used as a line pipe. Preferably, the wall thickness tB and wall thickness tS are 17.0 mm or more. On the other hand, the upper limit for the wall thickness tB and wall thickness tS of electric resistance welded steel pipe for line pipes is generally 25.4 mm.

[0096] 5. Outer diameter In this embodiment, the outer diameter of the electric resistance welded steel pipe for line pipes is set to 304.8 mm or more, from the viewpoint of improving the transport efficiency of the fluid passing through the pipe when used as a line pipe. On the other hand, the upper limit for the outer diameter of electric resistance welded steel pipes for line pipes is generally 660.4 mm.

[0097] 6. Manufacturing method The electric resistance welded steel pipe for line pipes according to this embodiment can achieve the above-described effects regardless of the manufacturing method, as long as it possesses the above-described characteristics. The electric resistance welded steel pipe for line pipes according to this embodiment can be manufactured, for example, by a manufacturing method including the following steps. (a) Casting process for producing a slab having a predetermined chemical composition (b) Heating process for heating the slab (c) Hot rolling process after heating the slab to produce hot-rolled steel sheet. (d) A winding process in which the hot-rolled steel sheet is cooled and wound after the hot-rolling process. (e) After the winding process, the hot-rolled steel sheet is unwound, roll-formed into a tubular shape, and then electric resistance welded to form an electric resistance welded steel pipe. (f) Heat treatment process for heat treatment of the electric resistance welded joint of electric resistance welded steel pipes (g) Tempering process for treating electric resistance welded joints (h) Furthermore, sizing may be performed as needed to improve roundness. The following describes the preferred conditions for each process.

[0098] <Casting Process> In the casting process, steel having the above-mentioned chemical composition is melted in a furnace, and then slabs are produced by casting. The casting method is not particularly limited and may be any of the following methods: conventional continuous casting, ingot casting, or thin slab casting.

[0099] <Heating process> In the heating process, the manufactured slab is heated in a heating furnace. The heating temperature T (°C) of the slab in the heating furnace is preferably 1100 to 1170°C. The heating time t (minutes) is preferably 100 to 450 minutes. In this embodiment, the furnace time t (minutes) is the time from when the slab is loaded into the heating furnace until it is removed from the heating furnace.

[0100] In the heating process, it is preferable to further ensure that F1, defined by the following equation (iii), is between 2800 and 3700.

[0101] F1 = (T + 273.15) × log(t) (iii) In equation (iii), T is the heating temperature (°C) during the heating process, and t is the time spent in the furnace (minutes).

[0102] If the heating conditions are not appropriate, the austenite grain size may coarse during heating, and consequently, the average grain size in the 1 / 2 × tB portion of the base material may also coarse, potentially degrading the low-temperature toughness of the base material. On the other hand, the segregation state at the slab (coil) end corresponding to the weld of an electric resistance welded (ERW) steel pipe is affected by the slab heating conditions, which in turn affects the formation of MA in the ERW weld. In the ERW weld, there is a segregation zone originating from the slab (coil). Because the segregation zone has a higher concentration of alloys such as C and Mn compared to the non-segregation zone, transformation is relatively less likely to begin compared to the non-segregation zone. The alloy becomes concentrated in the non-segregation zone, which has already transformed, and MA is formed along the segregation zone. If the alloy concentration in the segregation zone is high, the area ratio of MA present in the ERW weld increases, which may degrade the low-temperature toughness of the ERW weld. If the alloy concentration in the segregation zone of the ERW weld is low, the area ratio of MA present in the ERW weld decreases, which increases YR. The ERW weld corresponds to the end of the slab (coil), and the segregation zone in this area is microsegregation, not central (macro) segregation that exists at the center of the plate thickness in the middle of the width of the slab (coil). Therefore, it is significantly affected by the heating conditions during the manufacturing of the hot-rolled steel sheet.

[0103] Therefore, in this embodiment, by appropriately heating the slab under heating conditions that take into account the heating temperature and time spent in the furnace, it is possible to suppress the coarsening of the heated austenite grain size in the slab before hot rolling, and to uniformly diffuse atoms to control the segregation zone at the width edge of the hot-rolled steel sheet, i.e., the electric resistance welded area after electric resistance welding. Specifically, it is preferable to control the heating temperature and time spent in the furnace so that F1, expressed by formula (iii), is between 2800 and 3700.

[0104] Assuming that the content of each element in the chemical composition of the slab is within the range of this embodiment and that formulas (i) and (ii) are satisfied, if F1 is less than 2800, even if other manufacturing conditions are met, the MA area ratio of the electric resistance welded joint may increase, and the low-temperature toughness of the electric resistance welded joint may deteriorate. Furthermore, if F1 exceeds 3700, the austenite grain size may coarse during heating, and the average grain size of the base material may coarse, potentially degrading the low-temperature toughness of the base material. Also, if F1 exceeds 3700, the MA area ratio of the electric resistance welded joint may decrease, potentially increasing the YR of the electric resistance welded joint.

[0105] <Hot rolling process> In the hot rolling process, it is preferable to set the reduction ratio in the recrystallized region to 2.0 or higher, and the reduction ratio in the non-recrystallized region to 2.0 or higher. In particular, by setting the reduction ratio in the non-recrystallized region to 2.0 or higher, it becomes possible to reduce the average grain size of the base material to 20 μm or less. The boundary between the recrystallized region and the non-recrystallized region depends on the composition of the steel, but is approximately 900 to 950°C.

[0106] The starting temperature for finish rolling is preferably 900 to 950°C in order to ensure low-temperature toughness by rolling in the unrecrystallized region. The hot rolling completion temperature (finish rolling completion temperature) is preferably 770°C or higher. If the hot rolling completion temperature is below 770°C, two-phase rolling occurs, and the low-temperature toughness of the base material deteriorates.

[0107] <Winding process> In the winding process, the steel sheet after the hot rolling process is cooled to a surface temperature range of 500 to 650°C so that the average cooling rate at the center of the sheet thickness is in the range of 5 to 80°C / second, and then wound up within that temperature range. The average cooling rate at the center of the sheet thickness can be calculated from the temperature history of the outer surface using heat transfer calculations.

[0108] In order to control the microstructure of the base material of the electric resistance welded steel pipe for line pipes according to this embodiment so that it has a predetermined structure, controlling the cooling rate is particularly important. If the average cooling rate is less than 5°C / second, ferrite transformation will proceed, and the ferrite area ratio may exceed 80%. On the other hand, if the average cooling rate exceeds 80°C / second, the cooling rate is too fast, and ferrite transformation may not occur, resulting in a ferrite area ratio of less than 40%.

[0109] Furthermore, if the cooling stop temperature exceeds 650°C, ferrite transformation occurs after winding, which may result in a ferrite area ratio exceeding 80%. If the cooling stop temperature (winding temperature) falls below 500°C, temperature variations during cooling become large, leading to strength variations, and stable production of electric resistance welded steel pipes for line pipes according to this embodiment becomes impossible.

[0110] <Electric resistance welding process> Electric resistance welded (ERW) steel pipes for line pipes are manufactured by unwinding hot-rolled steel sheets that have been rolled into coils. Specifically, the hot-rolled steel sheets are processed into open pipes by bending using a continuous forming roll. Subsequently, the joints of the open pipes, that is, both ends in the width direction of the hot-rolled steel sheets, are welded by electric resistance welding to produce ERW steel pipes for line pipes.

[0111] <Heat treatment process> In the heat treatment process, the electric resistance welded joint formed in the electric resistance welding process is heated from the outer surface and then water-cooled from the outer surface side. Heating can be performed, for example, by induction heating.

[0112] Specifically, the electric resistance welded section is heated to a temperature range of 870-1070°C, and then cooled by water cooling to a surface temperature range of 500°C or less, so that the average cooling rate of the 1 / 4 × tS section is in the range of 5-30°C / s. The average cooling rate of the 1 / 4 × tS section can be calculated by heat transfer calculations from the temperature history of the outer surface. This heat treatment (heating and cooling) makes it possible to control the microstructure (fraction of each structure, average grain size) of the electric resistance welded section to the range described above.

[0113] If the heating temperature falls below 870°C, regions that do not undergo austenite transformation during heat treatment may remain, leading to coarsening of the microstructure and an increase in the average grain size, which can degrade the low-temperature toughness of the electric resistance welded joint. Furthermore, if the heating temperature exceeds 1070°C, coarse austenite may be formed during heat treatment, resulting in a coarser microstructure after cooling and potentially degrading the low-temperature toughness of the electric resistance welded joint.

[0114] If the average cooling rate falls below 5°C / s, the ferrite area ratio increases, which may reduce the strength of the electric resistance welded joint. Conversely, if the average cooling rate exceeds 80°C / s, the ferrite area ratio may fall below 40%, which may degrade the low-temperature toughness of the electric resistance welded joint.

[0115] If the cooling stop temperature exceeds 500°C, the area ratio of MA in the electric resistance welded joint may increase, potentially degrading its low-temperature toughness.

[0116] <Tempering process> In the tempering process, after heating and water cooling as described above, the electric resistance welded joint is tempered online from the outer surface side. Tempering can be performed, for example, by induction heating.

[0117] In the tempering process, the electric resistance welded joint is heated from the outer surface. The tempering temperature is the temperature of the outer surface of the electric resistance welded joint at the exit of the induction heating device. The tempering temperature is preferably 510 to 700°C.

[0118] In the tempering process, it is preferable to further set T1, as defined by the following equation (iv), to be between 20,000 and 32,000.

[0119] T1=(T+273.15)×(500×(L / V×1 / 60)+20.836) (iv) In equation (iv), T is the tempering temperature (°C), L is the line length from the inlet to the outlet of the induction heating device (m), and V is the pipe-making speed (m / min).

[0120] If the tempering conditions are not appropriate, the maximum Vickers hardness and / or MA area ratio of the 1 / 4 × tS portion of the electric resistance weld may increase, and the low-temperature toughness of the electric resistance weld may deteriorate. Furthermore, although the electric resistance weld is heated by tempering, heat conduction occurs in the circumferential direction of the base metal, affecting the dislocation density at the 90° position of the base metal. In this embodiment, by appropriately controlling the tempering temperature and pipe-making speed according to the line length of the induction heating device, hardening of the 1 / 4 × tS portion of the electric resistance weld is suppressed, and the dislocation density at the 90° position of the base material is reduced by heat conduction. Specifically, it is preferable to control the tempering temperature and pipe-making speed according to the line length so that T1, expressed by equation (iv), is between 20,000 and 32,000.

[0121] Assuming that the content of each element in the chemical composition of the slab is within the range of this embodiment and that formulas (i) and (ii) are satisfied, if T1 is less than 20,000, even if other manufacturing conditions are met, the dislocation density of the base material and the maximum value of the Vickers hardness of the 1 / 4 × tS portion of the electric resistance weld may increase, and the low-temperature toughness of the base material and the electric resistance weld may deteriorate. Furthermore, if T1 exceeds 32000, the dislocation density in the base material decreases, the strength of the base material deteriorates, and the Vickers hardness of the electric resistance welded joint also decreases, potentially leading to a deterioration in strength. [Examples]

[0122] The effects of one aspect of the present invention will be described in more detail below with reference to examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0123] Steel grades A1 to A50 having the chemical compositions shown in Tables 1A to 1D (the remainder being Fe and impurities) were melted. Hot-rolled steel sheets were obtained by heating, hot-rolling, cooling, and coiling these steel grades A1 to A50 under the conditions shown in Tables 2A to 3B.

[0124] The obtained hot-rolled steel sheets were subjected to bending using forming rolls and electric resistance welded welding. As shown in Tables 3A and 3B, the welded areas were heat-treated and tempered under predetermined conditions to produce electric resistance welded steel pipes for line pipes.

[0125] For the obtained electric resistance welded steel pipes, the area ratio of ferrite, average grain size, and dislocation density in the microstructure of the 1 / 2 × tB portion of the base material, and the area ratio of ferrite, area ratio of island martensite (MA), average grain size, and maximum Vickers hardness in the microstructure of the 1 / 4 × tS portion of the electric resistance weld were evaluated using the method described above. The results obtained are shown in Tables 4A and 4B.

[0126] Furthermore, tensile tests, Charpy tests at -30°C, DWTT tests at -30°C, and CTOD tests at -30°C were performed using the methods described above to evaluate strength (yield stress, tensile strength), yield ratio (YR), and low-temperature toughness (impact absorption energy at -30°C, ductile fracture surface ratio SA at -30°C, and critical opening displacement δc at -30°C). The results obtained are shown in Tables 5A and 5B. Underlined text in the tables indicates that the results are outside the scope of the present invention, the manufacturing conditions are undesirable, or the characteristic values ​​are undesirable.

[0127] If the yield strength (YS) was 360 MPa or higher and the tensile strength (TS) was 465 MPa or higher in both the base material and the electric resistance welded section, the pipe was deemed to be a high-strength electric resistance welded steel pipe for line pipes and was judged to be acceptable. On the other hand, if the yield strength (YS) was less than 360 MPa or the tensile strength (TS) was less than 465 MPa, the pipe was deemed not to be a high-strength electric resistance welded steel pipe for line pipes and was judged to be unacceptable.

[0128] If the yield ratio (YR = YS / TS) was 93% or less in both the base material and the electric resistance welded section, the electric resistance welded steel pipe for line pipes was deemed acceptable as having a low YR in both the base material and the electric resistance welded section. On the other hand, if the yield ratio was over 93%, it was judged as not being an electric resistance welded steel pipe for line pipes with a low YR in the base material and electric resistance welded joint, and was therefore deemed unacceptable.

[0129] If the Charpy impact absorption energy at -30°C in the base material is 200 J or more, and the ductile fracture surface ratio SA in the DWTT test at -30°C in the base material is 85% or more, the electric resistance welded steel pipe for line pipes is judged to be acceptable as having excellent low-temperature toughness in the base material. On the other hand, if the Charpy impact absorption energy at -30°C is less than 200 J and / or the ductile fracture surface ratio SA in the DWTT test at -30°C is less than 85%, the electric resistance welded steel pipe for line pipes is judged to be unacceptable as it does not have excellent low-temperature toughness in the base material.

[0130] In electric resistance welded (ERW) joints, if the critical opening displacement δc in the CTOD test at -30°C was 0.15 mm or more, the ERW steel pipe for line pipes was judged to be acceptable as it possessed excellent low-temperature toughness in the ERW welded joint. On the other hand, if the critical opening displacement δc in the CTOD test at -30°C was less than 0.15 mm, the ERW steel pipe for line pipes was judged to be unacceptable as it did not possess excellent low-temperature toughness in the ERW welded joint.

[0131] As shown in Tables 1A to 5B, for Tests No. 1 to 34, the chemical composition and microstructure of the base material were within the scope of the present invention, and the microstructure of the electric resistance welded joint was also within the scope of the present invention. As a result, it can be seen that electric resistance welded steel pipes for line pipes with high strength, low YR, and excellent low-temperature toughness were obtained. On the other hand, comparative examples No. 35 to 65 failed to meet the pass criteria for the following reasons.

[0132] In Test No. 35, the C content exceeded the upper limit of the present invention, and the central segregation hardened. As a result, the Charpy and DWTT properties of the base material deteriorated.

[0133] In test No. 36, the carbon content fell below the lower limit of the range specified in the present invention. As a result, sufficient strength could not be obtained in the base material and the electric resistance welded joint.

[0134] In Test No. 37, the Si content exceeded the upper limit of the present invention, resulting in an increase in oxides in the electric resistance welded joint. Consequently, the low-temperature toughness of the electric resistance welded joint deteriorated.

[0135] In test No. 38, the Si content fell below the lower limit of the present invention range, indicating insufficient deoxidation. As a result, the toughness of the base material deteriorated.

[0136] In test No. 39, the Mn content exceeded the upper limit of the present invention, and the central segregation portion of the base material hardened. As a result, the low-temperature toughness of the base material deteriorated.

[0137] In test No. 40, the Mn content fell below the lower limit of the range specified in the present invention. As a result, sufficient strength could not be obtained in the base material and the electric resistance welded joint.

[0138] In test No. 41, the Ti content exceeded the upper limit of the present invention, resulting in the formation of coarse inclusions. Consequently, the low-temperature toughness of the base material deteriorated.

[0139] In test No. 42, the Ti content fell below the lower limit of the present invention range. As a result, the average grain size of the base material became coarser, and the low-temperature toughness deteriorated.

[0140] In test No. 43, the Nb content exceeded the upper limit of the present invention, resulting in the formation of coarse inclusions. Consequently, the low-temperature toughness of the base material deteriorated.

[0141] In Test No. 44, the Nb content and M fell below the lower limit of the present invention range. As a result, the average grain size of the base material became coarser, and the low-temperature toughness of the base material deteriorated. Furthermore, sufficient strength could not be obtained in the base material. In addition, the MA area ratio in the electric resistance weld decreased, and the YR increased.

[0142] In test No. 45, the N content exceeded the upper limit of the present invention, resulting in the formation of coarse inclusions. As a result, the low-temperature toughness of the base material deteriorated.

[0143] In test No. 46, the N content fell below the lower limit of the present invention range. As a result, the average grain size of the base material became coarser, and the low-temperature toughness of the base material deteriorated.

[0144] In test No. 47, the Ceq value exceeded the upper limit of the present invention, and the ferrite fraction decreased. As a result, the low-temperature toughness of the base material and the electric resistance welded joint deteriorated.

[0145] In test No. 48, the Ceq fell below the lower limit of the present invention range, and the ferrite fraction increased. As a result, sufficient strength could not be obtained in the base material and the electric resistance welded joint.

[0146] In test No. 49, M exceeded the upper limit of the present invention. As a result, the MA area ratio in the electric resistance welded joint increased, and the low-temperature toughness of the electric resistance welded joint deteriorated.

[0147] In test No. 50, M fell below the lower limit of the present invention range. As a result, the MA area ratio in the electric resistance welded joint decreased, and YR increased.

[0148] In test No. 51, F1 exceeded the upper limit, resulting in a coarser average grain size in the base material. Consequently, the low-temperature toughness of the base material deteriorated. Furthermore, the alloy concentration in the segregation zone of the electric resistance weld became too low, leading to a decrease in the MA area ratio in the electric resistance weld and an increase in YR.

[0149] In test No. 52, F1 fell below the lower limit, resulting in a high alloy concentration in the segregation zone of the electric resistance welded joint and an increased area ratio of MA. Consequently, the low-temperature toughness of the electric resistance welded joint deteriorated.

[0150] In test No. 53, the reduction ratio in the non-recrystallized region fell below the lower limit, resulting in an increase in the average grain size of the base material. Consequently, the low-temperature toughness of the base material deteriorated.

[0151] In Test No. 54, the cooling rate after hot rolling was high, resulting in a decrease in the ferrite area ratio of the base material. Consequently, the low-temperature toughness of the base material deteriorated.

[0152] In test No. 55, the cooling rate after hot rolling was low, which increased the ferrite area ratio of the base material. As a result, sufficient strength could not be obtained in the base material.

[0153] In Test No. 56, the cooling stop temperature and winding temperature after hot rolling were high, resulting in an increase in the ferrite area ratio of the base material. Consequently, sufficient strength could not be obtained in the base material.

[0154] In Test No. 57, the heating temperature of the electric resistance welded joint was high, resulting in a larger average grain size. Consequently, the low-temperature toughness of the weld deteriorated.

[0155] In Test No. 58, the heating temperature of the electric resistance welded (ERW) joint was low, resulting in a larger average grain size. Consequently, the low-temperature toughness of the ERW welded joint deteriorated.

[0156] In Test No. 59, the cooling rate of the electric resistance welded joint was high, resulting in a decrease in the ferrite area ratio. Consequently, the low-temperature toughness of the electric resistance welded joint deteriorated.

[0157] In Test No. 60, the cooling rate of the electric resistance welded joint was low, and the ferrite area ratio increased. As a result, sufficient strength could not be obtained in the electric resistance welded joint.

[0158] In Test No. 61, the cooling stop temperature of the weld was high, and the MA area ratio increased. As a result, the low-temperature toughness of the electric resistance welded joint deteriorated.

[0159] In Test No. 62, the tempering temperature of the weld was high, causing austenite to form due to reverse transformation. This led to increased carbon concentration in the untransformed austenite, resulting in an increase in the MA area ratio. Consequently, the low-temperature toughness of the electric resistance welded joint deteriorated.

[0160] In Test No. 63, the tempering temperature of the weld was low, and the hardness of the outer surface of the electric resistance weld increased. As a result, the low-temperature toughness of the weld deteriorated.

[0161] In test No. 64, T1 was high, resulting in decreased dislocation density in the base metal and reduced hardness on the outer surface of the electric resistance weld. Consequently, sufficient strength could not be obtained in both the base metal and the weld.

[0162] In test No. 65, T1 was low, and the hardness and dislocation density on the outer surface of the electric resistance welded joint increased. As a result, the low-temperature toughness of both the weld and the base material deteriorated.

[0163] [Table 1A]

[0164] [Table 1B]

[0165] [Table 1C]

[0166] [Table 1D]

[0167] [Table 2A]

[0168] [Table 2B]

[0169] [Table 3A]

[0170] [Table 3B]

[0171] [Table 4A]

[0172] [Table 4B]

[0173] [Table 5A]

[0174] [Table 5B] [Industrial applicability]

[0175] According to the above embodiment of the present invention, electric resistance welded steel pipes for line pipes can be obtained that have high strength, low YR, and excellent low-temperature toughness in the base material and electric resistance welded joints. Therefore, they have high potential for industrial use.

Claims

1. It has a base material section and an electric resistance welded section, The chemical composition of the aforementioned base material is, in mass%, C: 0.010-0.059%, Si: 0.02-0.50%, Mn: 0.5-2.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.080% or less, Ti: 0.003 to 0.030%, Nb: 0.008-0.048%, N: 0.0010-0.0080%, O: 0.005% or less, Cu: 0-0.500%, Ni: 0 to 0.500%, Cr: 0-0.500%, Mo: 0-0.500%, V: 0 to 0.100%, W: 0 to 0.500%, Ca: 0-0.0040%, B: 0 to 0.0050%, Mg: 0 to 0.0200%, Zr: 0 to 0.0200%, REM: 0-0.0050%, and, The remainder consists of Fe and impurities. The Ceq expressed by the following formula (i) is 0.16 to 0.53 mass%, The M expressed by the following formula (ii) is 0.06 to 0.25 mass%, When the thickness of the base material is tB and the thickness of the electric resistance welded portion is tS, The aforementioned tB and tS are 15.0 to 25.4 mm. The outer diameter is 304.8 to 660.4 mm. In the microstructure of the 1 / 2 × tB portion of the base material, The ferrite area fraction is 40-80%, the average grain size is 35 μm or less, and the dislocation density is 1.0 × 10⁻⁶. １３ ~5.0 x 10 １５ I understand ―２ And, In the microstructure of the 1 / 4 × tS portion of the electric resistance welded joint, The ferrite area ratio is 40-70%, the island martensite area ratio is 0.2-10.0%, the average grain size is 35 μm or less, and the maximum Vickers hardness is 180-220 HV. Electric resistance welded steel pipe for line pipes, having a yield strength of 360 MPa or more and a tensile strength of 465 MPa or more. Ceq=C+Mn / 6+(Ni+Cu) / 15+(Cr+Mo+V) / 5 (i) M=C / 3+5×Nb (ii) Here, the elemental symbols in formulas (i) and (ii) above are substituted with the content (mass%) of the corresponding element, and 0 is substituted if the element is not present.

2. The chemical composition of the aforementioned base material is, in mass%, Cu: more than 0% and less than 0.500%, Ni: more than 0% and less than 0.500%, Cr: more than 0% and less than 0.500%, Mo: more than 0% and less than 0.500%, V: more than 0% and less than 0.100%, W: more than 0% and less than 0.500%, Ca: more than 0% and less than 0.0040%, B: More than 0% 0.0050%, Mg: more than 0% 0.0200%, Zr: greater than 0% and 0.0200%, and REM: The electric resistance welded steel pipe for line pipes according to claim 1, containing one or more of these in an amount greater than 0% and less than or equal to 0.0050%.

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