Electric resistance welded steel pipe, line pipe, and method of manufacturing electric resistance welded steel pipe

Abstract

The objective of the present invention is to provide an electric resistance welded steel pipe and a line pipe that excel in bending resistance, and a method of manufacturing the electric resistance welded steel pipe.　This electric resistance welded steel pipe has a parent-material section and an electric resistance seam-welded section. In a flatness test performed by pressure-sandwiching between two flat plates a flat test piece sampled from the electric resistance welded steel pipe, at the point of time when a flatness value h / D becomes 0.25, the ratio of the total length of cracks having a length of 0.50 mm or greater in the pipe axis direction and observed on the outer surface of the flatness test piece, to the total length of the flatness test piece in the pipe axis direction, is 0.050 or less, and the maximum crack length in the pipe axis direction among the cracks is 2.0 mm or less. Herein, h represents a distance (mm) between the flat plates in the flatness test, and D represents an outer diameter (mm) of the electric resistance welded steel pipe. It should be noted that at the start of the flatness test, the flatness value h / D is 1.

Description

Electric-resistance welded steel pipe, line pipe, and method for manufacturing electric-resistance welded steel pipe

[0001] The present invention relates to an electric resistance welded steel pipe suitable for use as a line pipe, and to a method for manufacturing the line pipe and the electric resistance welded steel pipe.

[0002] Pipelines that transport oil, natural gas, etc. may be subject to bending deformation due to earthquakes, etc. In order to prevent leakage of the fluid inside at such times, the internal pressure, dimensions, and materials of the line pipe must be designed to accommodate the expected amount of deformation.

[0003] Conventionally, UOE steel pipes have been used for line pipes for such applications, but in recent years, cheaper electric resistance welded steel pipes have been used in some cases. However, if defects exist in the electric resistance welds of electric resistance welded steel pipes, cracks may occur from those defects, increasing the risk of internal fluid leakage. Therefore, there has been a demand for electric resistance welded steel pipes with fewer defects in the electric resistance welds.

[0004] The main defects in electric resistance welds are inclusions such as oxides that are generated during electric resistance welding.

[0005] Patent Documents 1 and 2 propose electric resistance welded steel pipes in which the composition and number density of inclusions in electric resistance welds are controlled to improve toughness and HIC resistance.

[0006] Japanese Patent No. 5534111 Japanese Patent Application Laid-Open No. 2022-168987

[0007] However, in the electric resistance welded steel pipes described in Patent Documents 1 and 2, cracking of the electric resistance welded portion during bending deformation cannot be sufficiently suppressed.

[0008] The present invention has been made in view of the above circumstances, and has as its object to provide an electric resistance welded steel pipe, a line pipe, and a method for manufacturing an electric resistance welded steel pipe that are excellent in bending resistance.

[0009] In this invention, "excellent bending resistance" means that the cross section parallel to the pipe circumference does not fracture at the electric resistance weld, even when the cross section is flattened. Specifically, as will be described later, this means that when the flattening value h / D in a flattening test reaches 0.25, the ratio of the total length of cracks with an axial length of 0.50 mm or more observed on the outer surface of the flattening test piece to the total length of the flattening test piece in the axial direction is 0.050 or less, and the maximum crack length in the axial direction of the cracks is 2.0 mm or less, where h is the distance (mm) between the flattened plates in the flattening test, and D is the outer diameter (mm) of the electric resistance welded steel pipe.

[0010] As a result of extensive research, the inventors have found that when a steel pipe is bent, the circumferential cross section of the pipe is flattened, and tensile stress is generated in the circumferential direction on the outer surface of the pipe in areas with high curvature. If an electric resistance weld is present in this area, cracks may occur in the electric resistance weld.

[0011] Furthermore, they discovered that by homogenizing the steel structure of the electric resistance weld, it is possible to suppress the concentration of deformation around inclusions and at the interface between the hard and soft structures, thereby suppressing cracking in the electric resistance weld.

[0012] The present invention was completed based on the above findings and is summarized as follows: [1] An electric-resistance welded steel pipe having a base metal portion and an electric-resistance welded portion, wherein, in a flattening test in which a flattening test specimen taken from the electric-resistance welded steel pipe is clamped between two flat plates, when the flattening value h / D reaches 0.25, the ratio of the total length of cracks having an axial length of 0.50 mm or more observed on the outer surface of the flattening test specimen to the total axial length of the flattening test specimen is 0.050 or less, and the maximum crack length in the axial direction of the cracks is 2.0 mm or less, where h: distance between the flattening test plates (mm), D: outer diameter of the electric-resistance welded steel pipe (mm). Note that the flattening value h / D is 1 at the start of the flattening test. [2] The composition of the base metal portion contains, in mass%, C: 0.020% or more and 0.200% or less, Si: 0.40% or less, Mn: 0.50% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less, and optionally Nb: 0.080% or less, V: 0.080% or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Sn: 0.100% or less, and the balance consisting of Fe and unavoidable impurities. [3] The electric resistance welded steel pipe according to [1] or [2], wherein the steel structure at the center of the wall thickness of the electric resistance weld has an average KAM (Kernel Average Misorientation) value, which is the average of the KAM values, of 2.0° or more and 4.0° or less, and the standard deviation of the KAM value distribution is 1.5° or less. [4] A line pipe using the electric resistance welded steel pipe according to any of [1] to [3]. [5] A method for manufacturing an electric resistance welded steel pipe according to any one of [1] to [3], wherein in an electric resistance welding step in which a hot rolled steel plate that is a raw material for the electric resistance welded steel pipe is used to butt an end portion in the width direction of the hot rolled steel plate and an end portion in the other width direction of the hot rolled steel plate and electric resistance welding the end portion, the angle formed by the opposing butt surfaces is set to 0° or more and 15° or less, and the cross-sectional area (mm ２) is 0.5 to 2.0 times the plate thickness (mm) of the hot-rolled steel plate, and the upset amount (mm) is 20% to 100% of the plate thickness (mm). [6] The method for producing an electric-resistance welded steel pipe according to [5], which includes, after the electric-resistance welding step, a heat treatment step of heating at a heating temperature of 900°C to 1150°C in the center of the wall thickness of the electric-resistance welded portion, and cooling under conditions where the average cooling rate from 950 to 850°C is 2°C / s to 10°C / s, the average cooling rate from 800 to 650°C is 12°C / s or more, and the average cooling rate from 600 to 200°C is 15°C / s or less. [7] The method for producing an electric-resistance welded steel pipe according to [6], which includes, after the heat treatment step, a tempering step of holding at a holding temperature of 500°C to 700°C in the center of the wall thickness of the electric-resistance welded portion.

[0013] According to the present invention, it is possible to provide an electric resistance welded steel pipe, a line pipe, and a method for manufacturing an electric resistance welded steel pipe, which have excellent bending resistance.

[0014] Fig. 1 is a diagram showing a side view of a flattening test of an electric resistance welded steel pipe. Fig. 2 is a diagram showing an electric resistance welding process in which an end portion in the width direction of a hot-rolled steel plate and another end portion in the width direction are butted together and electric resistance welded. Fig. 3 is a diagram for explaining the spacing of fin pass rolls and the angle of the butt surfaces in electric resistance welding. Fig. 4 is a schematic diagram explaining a method for calculating the cross-sectional area of ​​molten steel discharged onto the outer surface. Fig. 5 is a schematic diagram of a cross section parallel to the circumferential direction of the pipe (cross section perpendicular to the axial direction of the pipe) including an electric resistance weld.

[0015] The electric resistance welded steel pipe of the present invention will be described below.

[0016] FIG. 1 shows a side view of a flattening test of an electric resistance welded steel pipe. In the flattening test, a flattening test piece (test piece) 8 is clamped between two flat plates. The test piece 8 is placed so that the electric resistance weld 3 of the test piece 8 is perpendicular to the compression direction 12, with the line connecting the center 10 of the test piece 8 and the electric resistance weld 3 perpendicular to the compression direction 12. The electric resistance welded steel pipe of the present invention has a base material and an electric resistance weld. In a flattening test performed by clamping a flattening test piece 8 taken from the electric resistance welded steel pipe between two flat plates 11, when the flattening value h / D reaches 0.25, the ratio of the total length of cracks 0.50 mm or longer in the axial direction observed on the outer surface of the flattening test piece 8 to the total length in the axial direction of the flattening test piece is 0.050 or less, and the maximum crack length in the axial direction of the cracks is 2.0 mm or less. Note that the flattening value h / D is 1 at the start of the flattening test. The outer surface of the flattened test piece corresponds to the outer surface of the electric resistance welded steel pipe.

[0017] In this specification, unless otherwise specified, "%" indicating the steel composition means "mass %".

[0018] The ratio of the total length of cracks 0.50 mm or more in the axial direction observed on the outer surface of the flattened test specimen to the entire axial length of the flattened test specimen is 0.050 or less. When cracks occur during bending deformation of a steel pipe, stress concentrates at the crack tip, causing the crack to propagate in the wall thickness direction and fracture, resulting in a decrease in load capacity. If the ratio of the total length of cracks at this point exceeds 0.050, the proportion of fractures in the electric resistance welded joint increases, preventing sufficient bending resistance. Therefore, the ratio of the total length of cracks 0.50 mm or more in the axial direction observed on the outer surface of the flattened test specimen to the entire axial length of the flattened test specimen is set to 0.050 or less. The smaller the ratio of the total length of cracks at this point, the better, preferably 0.040 or less, more preferably 0.035 or less, even more preferably 0.030 or less, most preferably 0.020 or less, and most preferably 0. Note that the maximum value determined for each test specimen is used as the ratio of the total length of cracks to the entire length of the test specimen. The cracks referred to here are visually observable cracks with a length of 0.50 mm or more. The cracks referred to here are cracks observed in electric resistance welds. The cracks observed on the outer surface of the flattened test specimen with a length of 0.50 mm or more in the axial direction include not only cracks parallel to the axial direction but also cracks whose longitudinal direction is at an angle to the axial direction. The crack length of the angled cracks is defined as the length resolved into the axial direction components. The upper limit of the crack length is not particularly limited, but may be 5.00 mm or less.

[0019] Maximum crack length: 2.0 mm or less Portions with a large crack length in the axial direction of the tube are often deep cracks in the wall thickness direction, and therefore fracture is more likely to occur earlier. If the maximum crack length at this point exceeds 2.0 mm, sufficient bending resistance performance cannot be obtained. The smaller the maximum crack length at this point, the better, preferably 1.5 mm or less, more preferably 1.0 mm or less, and most preferably 0 mm. Note that the maximum crack length used is the maximum value found for each test piece.

[0020] The flattening test is performed in accordance with the provisions of JIS G 3445 (2021). The test specimen is a tubular specimen with a length in the axial direction of the pipe of 100 mm. Five specimens are tested. The electric resistance weld 3 on the outer surface of the pipe (the outer surface side of the flattening test specimen) is polished to a metallic luster, and the compression speed is set to 10 mm / min. The compression is stopped when the flattening value h / D reaches 0.25, at which point the length of each crack is measured. The flattening value is calculated using the formula h / D, where h is the distance (mm) between the flattened plates in the flattening test, and D is the outer diameter (mm) of the electric resistance welded steel pipe.

[0021] The base metal of the electric resistance welded steel pipe of the present invention has a chemical composition, in mass%, of C: 0.020% or more and 0.200% or less, Si: 0.40% or less, Mn: 0.50% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less, and optionally Nb: 0.080% or less, V: 0.080% or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, It is preferable that the alloy contains one or more elements selected from the following: Sn: 0.100% or less, with the balance being Fe and unavoidable impurities.

[0022] In this specification, unless otherwise specified, "%" indicating the steel composition means "mass %".

[0023] C: 0.020% or more and 0.200% or less. C is an element that increases the strength of steel through solid solution strengthening. Furthermore, C refines crystal grains by lowering the transformation start temperature, thereby increasing the strength of steel. Furthermore, a low C content tends to form a soft structure in the electric resistance weld, resulting in a low average KAM (kernel average misorientation) value of the electric resistance weld. To achieve this effect, it is preferable to contain 0.020% or more of C. More preferably, it is 0.025% or more, and even more preferably, it is 0.030% or more. Most preferably, it is 0.034% or more. However, excessive C content reduces ductility and toughness, resulting in a decrease in bending resistance. Furthermore, a hard structure tends to form in the electric resistance weld, resulting in a high average KAM value of the electric resistance weld and a large standard deviation in the KAM value distribution. Therefore, the C content is preferably 0.200% or less. The C content is more preferably 0.180% or less, further preferably 0.170% or less, and most preferably 0.166% or less.

[0024] Si: 0.40% or less Si is an element that increases the strength of steel through solid solution strengthening. To achieve this effect, it is preferable to contain 0.02% or more of Si. The Si content is more preferably 0.05% or more, even more preferably 0.08% or more, and most preferably 0.10% or more. However, excessive Si content reduces ductility and toughness, and bending resistance performance decreases. In addition, a large amount of Si-based oxides is generated in the electric resistance weld, further reducing the ductility and toughness of the electric resistance weld. Therefore, the Si content is preferably 0.40% or less. The Si content is more preferably 0.35% or less, even more preferably 0.30% or less, and most preferably 0.26% or less.

[0025] Mn: 0.50% or more and 2.50% or less Mn is an element that increases the strength of steel through solid solution strengthening. Furthermore, Mn refines crystal grains by lowering the transformation start temperature, thereby increasing the strength of steel. To achieve this effect, it is preferable to contain 0.50% or more of Mn. The Mn content is more preferably 0.60% or more, even more preferably 0.70% or more, and most preferably 0.80% or more. Furthermore, a low Mn content tends to form a soft structure in the electric resistance weld, resulting in a low average KAM value of the electric resistance weld. However, excessive Mn content reduces ductility and toughness, resulting in a decrease in bending resistance. Furthermore, a hard structure tends to form in the electric resistance weld, resulting in a high average KAM value of the electric resistance weld and a large standard deviation in the KAM value distribution. Furthermore, a large amount of Mn-based oxides is formed in the electric resistance weld, further reducing the ductility and toughness of the electric resistance weld. Therefore, the Mn content is preferably 2.50% or less, more preferably 2.30% or less, further preferably 2.00% or less, and most preferably 1.80% or less.

[0026] P: 0.050% or less Since P segregates at grain boundaries and reduces toughness, it is preferable to reduce its content as an unavoidable impurity as much as possible, and the P content is preferably in the range of 0.050% or less. The P content is more preferably 0.040% or less, and even more preferably 0.030% or less. Most preferably 0.020% or less. Although there is no particular lower limit for P, excessive reduction leads to an increase in smelting costs, so the P content is preferably 0.001% or more. It is more preferably 0.002% or more, and even more preferably 0.003% or more.

[0027] S: 0.0200% or less S is usually present in steel as MnS, which is thinly drawn during the hot rolling process and has a negative effect on ductility and toughness. For this reason, in the present invention, it is preferable to reduce S as much as possible, and the S content is preferably 0.0200% or less. The S content is more preferably 0.0100% or less, and even more preferably 0.0050% or less. Most preferably 0.0040% or less. Although there is no particular lower limit for S, excessive reduction leads to an increase in smelting costs, so the S content is preferably 0.0001% or more. It is more preferably 0.0002% or more, and even more preferably 0.0003% or more.

[0028] Al: 0.005% or more and 0.100% or less Al is an element that acts as a powerful deoxidizer. To obtain this effect, it is preferable to contain 0.005% or more of Al. The Al content is more preferably 0.010% or more, even more preferably 0.015% or more, and most preferably 0.020% or more. However, excessive Al content deteriorates weldability and increases alumina-based inclusions, deteriorating surface properties. For this reason, the Al content is preferably 0.100% or less. The Al content is more preferably 0.080% or less, even more preferably 0.070% or less, and most preferably 0.060% or less.

[0029] N: 0.0100% or less N is an inevitable impurity and an element that acts to reduce ductility and toughness by firmly fixing dislocation motion. In the present invention, it is desirable to reduce N as an impurity as much as possible, but an N content of up to 0.0100% is acceptable. Therefore, the N content is preferably 0.0100% or less. The N content is more preferably 0.0080% or less. Further preferably, it is 0.0060% or less. Most preferably, it is 0.0050% or less. There is no particular lower limit, but it is preferably 0.0010% or more. It is more preferably 0.0015% or more.

[0030] The above elements are the basic components of the present invention, and further elements that can be optionally contained will be described below.

[0031] Nb: 0.080% or less Nb is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel and also contributes to refining the structure by suppressing coarsening of the structure during hot rolling, thereby increasing the strength of the steel. To achieve the above-mentioned effects, it is preferable to contain 0.002% or more of Nb. The Nb content is more preferably 0.005% or more, even more preferably 0.008% or more, and most preferably 0.010% or more. However, excessive Nb content reduces ductility and toughness, and bending resistance performance decreases. Therefore, when Nb is contained, the Nb content is set to 0.080% or less. The Nb content is preferably 0.075% or less, more preferably 0.070% or less, even more preferably 0.065% or less, and most preferably 0.060% or less.

[0032] V: 0.080% or less V is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. To achieve the above-mentioned effects, it is preferable to contain 0.002% or more of V. The V content is more preferably 0.005% or more, even more preferably 0.008% or more, and most preferably 0.010% or more. However, excessive V content reduces ductility and toughness, and bending resistance performance. Therefore, when V is contained, the V content is set to 0.080% or less. The V content is preferably 0.075% or less, more preferably 0.070% or less, even more preferably 0.065% or less, and most preferably 0.060% or less.

[0033] Ti: 0.080% or less Ti is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. It also contributes to reducing the amount of dissolved N in the steel due to its high affinity with N. To achieve the above-mentioned effects, it is preferable to contain 0.002% or more of Ti. The Ti content is more preferably 0.005% or more, even more preferably 0.008% or more, and most preferably 0.010% or more. However, excessive Ti content reduces ductility and toughness, resulting in a decrease in bending resistance. Therefore, when Ti is contained, the Ti content is set to 0.080% or less. The Ti content is preferably 0.070% or less, more preferably 0.060% or less, even more preferably 0.050% or less, and most preferably 0.040% or less.

[0034] Cu: 0.50% or less Cu is an element that increases the strength of steel through solid solution strengthening. It also contributes to microstructural refinement by lowering the transformation start temperature, thereby increasing the strength of steel. To achieve the above-mentioned effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.02% or more, even more preferably 0.05% or more, and most preferably 0.10% or more. Furthermore, a low Cu content tends to form a soft microstructure in the electric resistance weld, thereby reducing the average KAM value of the electric resistance weld. However, excessive Cu content reduces ductility and toughness, resulting in reduced bending resistance. Furthermore, a hard microstructure tends to form in the electric resistance weld, thereby increasing the average KAM value of the electric resistance weld and increasing the standard deviation of the KAM value distribution. Therefore, when Cu is contained, the Cu content is set to 0.50% or less. The Cu content is preferably 0.45% or less, and more preferably 0.40% or less. The content is more preferably 0.35% or less, and most preferably 0.30% or less.

[0035] Ni: 0.50% or less Ni is an element that increases the strength of steel through solid solution strengthening. It also contributes to refining the structure by lowering the transformation start temperature, thereby increasing the strength of steel. To achieve the above-mentioned effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.02% or more, even more preferably 0.05% or more, and most preferably 0.10% or more. Furthermore, a low Ni content tends to lead to the formation of soft structures in the electric resistance welds, thereby reducing the average KAM value of the electric resistance welds. However, excessive Ni content reduces ductility and toughness, resulting in reduced bending resistance. Furthermore, a hard structure tends to form in the electric resistance welds, thereby increasing the average KAM value of the electric resistance welds and increasing the standard deviation of the KAM value distribution. Therefore, when Ni is contained, the Ni content is set to 0.50% or less. The Ni content is preferably 0.40% or less, and more preferably 0.30% or less. More preferably, it is 0.25% or less, and most preferably, it is 0.20% or less.

[0036] Cr: 0.50% or less Cr is an element that contributes to microstructural refinement by lowering the transformation start temperature and thereby increases the strength of the steel. To achieve the above-mentioned effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.02% or more, even more preferably 0.05% or more, and most preferably 0.08% or more. Furthermore, a low Cr content tends to form a soft structure in the electric resistance weld, resulting in a low average KAM value of the electric resistance weld. However, excessive Cr content reduces ductility and toughness, resulting in a decrease in bending resistance. Furthermore, a hard structure tends to form in the electric resistance weld, resulting in a high average KAM value of the electric resistance weld and a large standard deviation in the KAM value distribution. Furthermore, a large amount of Cr-based oxides is formed in the electric resistance weld, further reducing the ductility and toughness of the electric resistance weld. Therefore, when Cr is contained, the Cr content is set to 0.50% or less. The Cr content is preferably 0.45% or less, more preferably 0.40% or less, further preferably 0.35% or less, and most preferably 0.30% or less.

[0037] Mo: 0.50% or less Mo is an element that contributes to microstructural refinement by lowering the transformation start temperature and thereby increases the strength of the steel. To achieve the above-mentioned effects, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.02% or more, even more preferably 0.05% or more, and most preferably 0.08% or more. Furthermore, a low Mo content tends to form a soft structure in the electric resistance weld, resulting in a low average KAM value of the electric resistance weld. However, excessive Mo content reduces ductility and toughness, resulting in a decrease in bending resistance. Furthermore, a hard structure tends to form in the electric resistance weld, resulting in a high average KAM value of the electric resistance weld and a large standard deviation in the KAM value distribution. Therefore, when Mo is contained, the Mo content is set to 0.50% or less. The Mo content is preferably 0.45% or less, and more preferably 0.40% or less. The content is more preferably 0.35% or less, and most preferably 0.30% or less.

[0038] Ca: 0.0050% or less Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn during the hot rolling process. To achieve the above-mentioned effects, it is preferable to contain 0.0002% or more of Ca. The Ca content is more preferably 0.0005% or more, even more preferably 0.0008% or more, and most preferably 0.0010% or more. However, excessive Ca content can cause the formation of Ca oxide clusters in the steel, reducing ductility and toughness, and thus reducing bending resistance. Therefore, when Ca is contained, the Ca content is set to 0.0050% or less. Preferably, it is 0.0045% or less. More preferably, it is 0.0040% or less. Even more preferably, it is 0.0035% or less, and most preferably, it is 0.0030% or less.

[0039] B: 0.0050% or less B is an element that contributes to microstructural refinement by lowering the transformation start temperature and thereby increases the strength of the steel. To achieve the above-mentioned effects, it is preferable to contain 0.0002% or more of B. The B content is more preferably 0.0005% or more, even more preferably 0.0008% or more, and most preferably 0.0010% or more. Furthermore, a low B content tends to lead to the formation of soft structures in the electric resistance welds, thereby reducing the average KAM value of the electric resistance welds. However, excessive B content reduces ductility and toughness, resulting in reduced bending resistance. Furthermore, a hard structure tends to form in the electric resistance welds, thereby increasing the average KAM value of the electric resistance welds and increasing the standard deviation of the KAM value distribution. Therefore, when B is contained, the B content is set to 0.0050% or less. The B content is preferably 0.0040% or less, and more preferably 0.0035% or less. The content is more preferably 0.0030% or less, and most preferably 0.0025% or less.

[0040] Sn: 0.100% or less Sn is an element that suppresses decarburization caused by nitriding or oxidation of the steel surface and suppresses a decrease in strength. To achieve the above-mentioned effects, it is preferable to contain 0.001% or more of Sn. The Sn content is more preferably 0.002% or more, even more preferably 0.005% or more, and most preferably 0.008% or more. However, excessive Sn content reduces the ductility and toughness of the steel and the bending resistance of the electric resistance welded joint. Therefore, when Sn is contained, the Sn content is set to 0.100% or less. The Sn content is preferably 0.060% or less, more preferably 0.050% or less, even more preferably 0.040% or less, and most preferably 0.030% or less.

[0041] The balance is Fe and unavoidable impurities. Examples of the unavoidable impurities in the balance include Mg, Zr, REM, As, Sb, Bi, Co, Pb, Zn, O, Ta, W, Te, Hf, Ge, Sr, and Cs. However, as long as the effects of the present invention are not impaired, Mg, Zr, and REM may each be contained in an amount of 0.020% or less, As, Sb, Co, Ta, W, Te, Hf, Ge, Sr, and Cs in an amount of 0.10% or less, and Bi, Pb, Zn, and O in an amount of 0.005% or less. Here, REM is a collective term for 17 elements, including Sc, Y, and lanthanoid elements. One or more of these 17 elements may be contained as unavoidable impurities, and the REM content refers to the total content of these elements.

[0042] Furthermore, the steel structure at the center of the wall thickness of the electric resistance welded portion of the electric resistance welded steel pipe of the present invention preferably has an average KAM value of 2.0° or more and 4.0° or less, and the standard deviation of the KAM value distribution is 1.5° or less.

[0043] Average KAM value: 2.0° or more and 4.0° or less The KAM (Kernel Average Misorientation) value represents the local misorientation. The higher the KAM value, the higher the dislocation density at that measurement point and the higher the hardness tends to be. If the average KAM value is less than 2.0°, there is a lot of soft ferrite with low dislocation density, and stress concentrates at the interface with the surrounding hard phase, becoming the starting point for cracks, resulting in a decrease in bending resistance. Therefore, it is preferable that the average KAM value is 2.0° or more. The average KAM value is more preferably 2.2° or more, even more preferably 2.3° or more. Most preferably 2.4° or more. On the other hand, if the average KAM value exceeds 4.0°, there will be a lot of hard, low-ductility martensite and work-hardened structures, resulting in a decrease in bending resistance. Therefore, it is preferable that the average KAM value at the center of the wall thickness of the electric resistance weld be 4.0° or less. At this time, the steel structure becomes mainly bainite, and the work-hardened structure is reduced, improving bending resistance. The average KAM value is more preferably 3.8° or less, even more preferably 3.7° or less, and most preferably 3.6° or less.

[0044] Standard deviation of KAM value distribution: 1.5° or less. If the variation in KAM values ​​is large, soft and hard parts are mixed and the difference in hardness between them becomes large, so stress concentrates at the interface between them, becoming the starting point for cracks and reducing bending resistance. Therefore, the standard deviation of the KAM value distribution at the center of the wall thickness of the electric resistance weld is preferably 1.5° or less. The standard deviation of the KAM value distribution is more preferably 1.3° or less, even more preferably 1.2° or less. Most preferably 1.1° or less. The smaller the standard deviation of the KAM value distribution, the better, but excessive reduction leads to increased manufacturing costs and manufacturing load, so the standard deviation of the KAM value distribution is preferably 0.5° or more. It is more preferably 0.6° or more, and even more preferably 0.7° or more.

[0045] The average KAM value and the standard deviation of the KAM value distribution are measured using the SEM / EBSD method. The measurement area is 400 μm × 400 μm, the measurement step size is 0.1 μm, and the measured values ​​from five fields of view are averaged. Based on the obtained EBSD data, a distribution image of KAM values ​​(KAM map) is obtained using crystal orientation analysis software OIM Analysis (trademark). Here, the KAM value is calculated using the following method. At each measurement point (a regular hexagonal pixel), the orientation difference between each pixel is calculated using the pixels up to three adjacent pixels (37 pixels in total) from the center, and the average of the calculated orientation differences is used as the KAM value of the central pixel. This operation is performed for all pixels in the field of view to obtain a KAM map. From the obtained KAM value distribution, the average KAM value and the standard deviation of the KAM value distribution are calculated using equations (1) and (2), respectively.

[0046]

[0047]

[0048] Furthermore, in order for the electric resistance welded steel pipe of the present invention to withstand the internal pressure of the fluid being transported, the base material preferably has a yield strength of 400 MPa or more. More preferably, it is 420 MPa or more, even more preferably 440 MPa or more, and most preferably 450 MPa or more. On the other hand, as the yield strength increases, the ductility decreases and the bending resistance performance decreases. Therefore, the yield strength is preferably 900 MPa or less, more preferably 800 MPa or less, even more preferably 750 MPa or less, and most preferably 700 MPa or less.

[0049] The yield strength can be measured by a tensile test. The tensile test is performed in accordance with the provisions of JIS Z 2241 (2022). The yield strength (MPa) is defined as the flow stress at a nominal strain of 0.5%. A JIS No. 5 tensile test specimen is used for the tensile test. The test specimen is taken from the base material so that the tensile direction (longitudinal direction of the test specimen) is parallel to the axial direction of the pipe. The base material is located 90 degrees (°) away from the electric resistance weld in the circumferential direction. The 90 degrees (°) away position may be either clockwise or counterclockwise.

[0050] The electric resistance welded steel pipe described above can also be applied to line pipes.

[0051] Next, a method for manufacturing an electric resistance welded steel pipe according to one embodiment of the present invention will be described.

[0052] The electric resistance welded steel pipe of the present invention is produced, for example, by heating and hot rolling a steel material having the above-mentioned chemical composition, cooling it, and then winding it into a coil to form a hot-rolled steel sheet, and then forming the hot-rolled steel sheet into a cylindrical shape by cold rolling, and butting together one end of the hot-rolled steel sheet in the width direction and the other end of the hot-rolled steel sheet in the width direction and electric resistance welding it.

[0053] In the following description of the manufacturing method, unless otherwise specified, the temperature indicated in "°C" refers to the surface temperature of the hot-rolled steel sheet. These surface temperatures can be measured using a radiation thermometer or the like.

[0054] In the present invention, the method for producing the steel material (steel slab) is not particularly limited, and any of the known methods such as converter, electric furnace, and vacuum melting furnace are suitable. The casting method is also not particularly limited, and the slab can be produced to the desired dimensions by a known casting method such as continuous casting. However, there is no problem if an ingot-making and blooming rolling method is used instead of the continuous casting method. The molten steel may further be subjected to secondary refining such as ladle refining.

[0055] Next, the obtained steel material (steel slab) is heated and hot-rolled, cooled, and then wound into a coil to form a hot-rolled steel sheet.

[0056] The hot-rolled steel sheet is then cold rolled into a cylindrical open pipe, which is then subjected to electric resistance welding. The heating method used for electric resistance welding may be either resistance heating or induction heating.

[0057] Figure 2 is a schematic diagram showing an electric resistance welding process in which one widthwise end of a hot-rolled steel sheet is butted against another widthwise end and electric resistance welded. Figure 3 is a diagram for explaining the spacing of the fin pass rolls and the angle of the butt surfaces in electric resistance welding. In Figure 2, the open pipe is indicated by reference numeral 20, the butt point is indicated by reference numeral 5A, the fin pass roll is indicated by reference numeral 21, the squeeze roll is indicated by reference numeral 22, the top roll is indicated by reference numeral 23, the contact tip is indicated by reference numeral 24, the high-frequency oscillator is indicated by reference numeral 25, the laser shapemeter is indicated by reference numeral 26, the welding direction (pipe-making direction) is indicated by reference numeral 27, the angle formed by the butt surfaces in Figure 3 is indicated by reference numeral 30, and the spacing of the fin pass rolls is indicated by reference numeral 31. In electric resistance welding, the spacing 31 of the fin pass rolls 21 and the spacing 31 of the squeeze rolls 22 are adjusted so that the angle 30 formed by the opposing butt surfaces is between 0° and 15°. The angle 30 between the opposing butt surfaces is determined from the end face shape measured by a goniometer or laser shapemeter 26 at a position three times the outer diameter of the electric resistance welded steel pipe, on the opposite side of the butt point 5A in the pipe-making direction 27. The smaller the spacing 31 between the fin pass rolls 21, the stronger the contact between the widthwise end (butt surface) of the hot-rolled steel sheet with the fin portion of the fin pass roll 21, resulting in a smaller angle 30 between the opposing butt surfaces. However, if the spacing 31 between the fin pass rolls is too small, defects are more likely to occur between the rolls. Furthermore, excessive roll loads result, increasing the operational load. The smaller the spacing between the squeeze rolls 22, the smaller the spacing between the butt surfaces, resulting in a smaller angle 30 between the butt surfaces. However, if the spacing between the squeeze rolls 22 is too small, the amount of upset becomes excessive. Furthermore, excessive roll loads result, increasing the operational load.

[0058] In addition to the above, the cross-sectional area of ​​the molten steel discharged to the outer surface (mm ２The welding power, pipe-making speed, and the gap between the squeeze rolls 22 are adjusted so that the height of the bead 4 is 0.5 to 2.0 times the thickness (mm) of the hot-rolled steel sheet. Figure 4 shows a schematic diagram illustrating a method for calculating the cross-sectional area of ​​the molten steel discharged onto the outer surface. Specifically, as shown in Figure 4, the height of the bead 4 is measured at 0.2 mm intervals in an area twice the plate thickness on both sides of the pipe circumferential direction around the butt joint 5, and the heights are integrated to determine the cross-sectional area. When the welding power is reduced, the amount of melting at the butt joint surfaces (butt joint surfaces before joining) decreases, thereby reducing the cross-sectional area of ​​the molten steel discharged onto the outer surface. When the welding power is increased, the amount of melting at the butt joint surfaces increases, thereby increasing the cross-sectional area of ​​the molten steel discharged onto the outer surface. When the pipe-making speed is reduced, the heat input per unit time increases, thereby increasing the cross-sectional area of ​​the molten steel discharged onto the outer surface. When the pipe-making speed is increased, the heat input per unit time decreases, thereby reducing the cross-sectional area of ​​the molten steel discharged onto the outer surface. When the gap between the squeeze rolls 22 becomes smaller, the amount of upset increases, and therefore the cross-sectional area of ​​the molten steel discharged onto the outer surface increases. When the gap between the squeeze rolls 22 becomes larger, the amount of upset decreases, and therefore the cross-sectional area of ​​the molten steel discharged onto the outer surface decreases. Note that the pipe-making speed refers to the electric resistance welding speed.

[0059] The gap between the squeeze rolls 22 is adjusted so that the upset amount (mm) is 20% or more and 100% or less of the plate thickness (mm). The upset amount (mm) can be calculated by subtracting the circumference (mm) of the open pipe immediately before electric resistance welding from the circumference (mm) of the electric resistance welded steel pipe immediately after electric resistance welding.

[0060] If the angle 30 between the butt surfaces is large, molten steel accumulates near either the inner or outer surface of the steel pipe, preventing sufficient drainage. As a result, oxides generated in the welded portion remain in the electric resistance weld and become the starting point for fracture, reducing the ductility and toughness of the electric resistance weld and the bending resistance. Furthermore, the difference between the amount of molten steel drained from the outer surface and the amount of molten steel drained from the inner surface becomes large, making it difficult to control the welding conditions based solely on the amount of molten steel drained from the outer surface. The smaller the angle 30 between the butt surfaces, the better. Therefore, the angle 30 between the butt surfaces is set to 15° or less. It is preferably set to 13° or less, more preferably 10° or less. It is even more preferably set to 9° or less, and most preferably 8° or less. Furthermore, the smaller the angle 30 between the butt surfaces, the better. Therefore, the angle 30 between the butt surfaces is set to 0° or greater. The angle between the butt surfaces is preferably set to 0.5° or greater, more preferably 1° or greater.

[0061] If the cross-sectional area of ​​the molten steel discharged to the outer surface is large, the welded portion is excessively melted, and oxides generated in the welded portion are not sufficiently discharged when the weld is butted, and remain in the electric resistance welded portion 3, which reduces the ductility and toughness of the electric resistance welded portion 3 and reduces the bending resistance. ２ ) is set to 2.0 times or less the thickness (mm) of the open pipe 20. As mentioned above, the thickness (mm) of the open pipe 20 refers to the thickness (mm) of the hot-rolled steel plate that is the raw material for the electric resistance welded steel pipe. It is preferably 1.9 times or less. More preferably, it is 1.8 times or less, even more preferably, it is 1.7 times or less, and most preferably, it is 1.6 times or less. On the other hand, if the cross-sectional area of ​​the molten steel discharged to the outer surface is small, the upset becomes excessive, resulting in increased work hardening of the electric resistance welded portion 3, a higher average KAM value, and a larger standard deviation of the KAM value distribution. Furthermore, since the electric resistance welded portion 3 is not sufficiently melted, oxides generated in the electric resistance welded portion 3 are not sufficiently discharged and remain in the electric resistance welded portion 3. As a result, the ductility and toughness of the electric resistance welded portion 3 decrease, and bending resistance performance decreases. Therefore, the cross-sectional area (mm ２ ) is 0.5 times or more, preferably 0.6 times or more, more preferably 0.7 times or more, even more preferably 0.8 times or more, and most preferably 0.9 times or more of the thickness (mm) of the hot-rolled steel sheet.

[0062] If the upset amount is large, the work hardening of the electric resistance welded portion 3 increases, the average KAM value increases, and the standard deviation of the KAM value distribution increases. Furthermore, the central segregation region rises steeply and becomes the starting point of cracks on the outer surface (outer surface of the electric resistance welded steel pipe), thereby reducing the ductility and toughness of the electric resistance welded portion 3 and reducing bending resistance. Therefore, the upset amount is set to 100% or less of the thickness of the hot-rolled steel sheet. It is preferably 90% or less. It is more preferably 85% or less, even more preferably 80% or less, and most preferably 75% or less. On the other hand, if the upset amount is small, oxides generated in the electric resistance welded portion 3 are not sufficiently expelled and remain in the electric resistance welded portion, reducing the ductility and toughness of the electric resistance welded portion 3 and reducing bending resistance. Therefore, the upset amount is set to 20% or more of the thickness of the sheet. It is preferably 30% or more. It is more preferably 35% or more, even more preferably 40% or more, and most preferably 45% or more.

[0063] After the electric resistance welding step, heat treatment can be carried out as appropriate to adjust the hardness of the electric resistance welded portion 3. In this case, the entire electric resistance welded steel pipe may be heat treated, or only the electric resistance welded portion 3 may be heat treated. Heating methods for heat treatment include, for example, induction heating, but are not limited to this method, and a heating furnace may also be used.

[0064] The heating temperature for the heat treatment is preferably 900°C or higher and 1150°C or lower. If the heating temperature is lower than 900°C, austenite transformation occurs partially, and C concentrates there, resulting in a hard structure, increasing the standard deviation of the KAM value distribution. Therefore, the heating temperature is preferably 900°C or higher. More preferably, it is 950°C or higher. Even more preferably, it is 960°C or higher, and most preferably, it is 970°C or higher. On the other hand, if the heating temperature exceeds 1150°C, the crystal grains become coarse, the variation in the KAM values ​​within the crystal grains increases, and the standard deviation of the KAM value distribution increases. Therefore, the heating temperature for the heat treatment of the electric resistance welded portion 3 is preferably 1150°C or lower. The heating temperature is more preferably 1100°C or lower. Even more preferably, it is 1090°C or lower, and most preferably, it is 1080°C or lower.

[0065] In the cooling after heating, it is preferable that the average cooling rate at the center of the wall thickness from 950 to 850°C is 2°C / s or more and 10°C / s or less, the average cooling rate from 800 to 650°C is 12°C / s or more, and the average cooling rate from 600 to 200°C is 15°C / s or less.

[0066] If the average cooling rate from 950 to 850°C is high, austenite becomes finer and ferrite is more likely to form between 800 and 650°C. As a result, the average KAM value becomes lower and the standard deviation of the KAM value distribution becomes larger. Therefore, the average cooling rate from 950 to 850°C is preferably 10°C / s or less. More preferably, it is 9°C / s or less. Even more preferably, it is 8°C / s or less, and most preferably, it is 7°C / s or less. On the other hand, if the average cooling rate from 950 to 850°C is low, the crystal grains become coarse, the variation in KAM values ​​within the crystal grains increases, and the standard deviation of the KAM value distribution becomes larger. Therefore, the average cooling rate from 950 to 850°C is preferably 2°C / s or more. More preferably, it is 3°C / s or more. Even more preferably, it is 4°C / s or more, and most preferably, it is 5°C / s or more.

[0067] If the average cooling rate from 800 to 650°C is low, ferrite will be generated, the average KAM value will be low, and the standard deviation of the KAM value distribution will be large. Therefore, the average cooling rate from 800 to 650°C is preferably 12°C / s or more. More preferably, it is 15°C / s or more. Even more preferably, it is 17°C / s or more, and most preferably, it is 20°C / s or more. The higher the average cooling rate from 800 to 650°C, the better, but from the viewpoint of equipment load, it is preferably 100°C / s or less. The average cooling rate from 800 to 650°C is more preferably 50°C / s or less. Even more preferably, it is 45°C / s or less, and most preferably, it is 40°C / s or less.

[0068] If the average cooling rate from 600 to 200°C is high, the dislocation density of bainite and martensite will increase, resulting in a high average KAM value. Therefore, the average cooling rate from 600 to 200°C is preferably 15°C / s or less. More preferably, it is 14°C / s or less. Even more preferably, it is 13°C / s or less, and most preferably, it is 12°C / s or less. The lower the average cooling rate from 600 to 200°C, the better, but from the perspective of manufacturing efficiency, it is preferably 2°C / s or more. The average cooling rate from 600 to 200°C is more preferably 3°C / s or more. Even more preferably, it is 4°C / s or more, and most preferably, it is 5°C / s or more.

[0069] The above-mentioned average cooling rates are all calculated by dividing the temperature difference that defines each average cooling rate by the time required for that cooling.

[0070] The heat treatment process after electric welding is a process including the heating and cooling.

[0071] After the heat treatment process, tempering can be performed to further adjust the hardness. In the tempering process, the holding temperature is preferably 500°C or higher and 700°C or lower. If the holding temperature is lower than 500°C, the hardness reduction effect is not obtained, the average KAM value becomes higher, and the standard deviation of the KAM value distribution becomes larger. In addition, low-temperature tempering embrittlement may occur, reducing ductility and toughness, and reducing bending resistance. Therefore, the holding temperature is preferably 500°C or higher. The holding temperature is more preferably 550°C or higher. Even more preferably, it is 560°C or higher, and most preferably, it is 570°C or higher. On the other hand, if the holding temperature exceeds 700°C, partial austenite transformation occurs, and C concentrates there, forming a hard structure, thereby increasing the standard deviation of the KAM value distribution. Therefore, the holding temperature in the tempering process is preferably 700°C or lower. More preferably, it is 670°C or lower. Even more preferably, it is 660°C or lower, and most preferably, it is 650°C or lower.

[0072] Whether a steel pipe is an electric resistance welded steel pipe can be determined by cutting the electric resistance welded steel pipe perpendicular to the pipe axis direction, polishing and corroding the cut surface including the welded portion (electric resistance welded portion), and observing it with an optical microscope. If the width of the molten solidified portion of the welded portion (electric resistance welded portion) in the circumferential direction of the pipe is 1.0 μm or more and 1000 μm or less throughout the entire pipe thickness, it is an electric resistance welded steel pipe. That is, the width of the molten solidified portion of the welded portion (electric resistance welded portion) in the circumferential direction of the pipe is preferably 1.0 μm or more throughout the entire pipe thickness, and more preferably 2.0 μm or more. Moreover, it is preferably 1000 μm or less, and more preferably 800 μm or less.

[0073] Here, an appropriate etching solution may be selected depending on the steel composition and the type of electric resistance welded steel pipe. Furthermore, the molten solidified portion can be visually recognized as an electric resistance welded portion 3, which is a region having a different structural morphology and contrast from the base material portion 1 and the heat-affected zone 2, as shown in the schematic diagram of the cross section after corrosion in FIG. 5. For example, the molten solidified portion of an electric resistance welded steel pipe made of carbon steel or low alloy steel can be identified as a white region observed under an optical microscope in the cross section corroded with nital. Furthermore, the molten solidified portion of a UOE steel pipe made of carbon steel or low alloy steel can be identified as a region containing a cellular or dendritic solidification structure under an optical microscope in the cross section corroded with nital.

[0074] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0075] Molten steel having the chemical composition shown in Table 1 was melted to prepare a slab (steel material). The obtained slab was heated and hot-rolled, cooled, and then wound into a coil to obtain a hot-rolled steel sheet having a thickness (mm) shown in Table 2. The hot-rolled steel sheet was then formed into a cylindrical shape by cold rolling and electric resistance welded under the conditions shown in Table 2 to obtain an electric resistance welded steel pipe having an outer diameter (mm) and wall thickness (mm) shown in Table 2.

[0076] Test pieces were taken from the obtained electric resistance welded steel pipes and subjected to the KAM value measurement, tensile test, and flattening test described below. Note that the "base material" below refers to the base material located 90° away from the electric resistance weld in the circumferential direction of the pipe.

[0077] [KAM Value Measurement] The average KAM value and the standard deviation of the KAM value distribution were measured using the SEM / EBSD method. The measurement area was 400 μm × 400 μm, the measurement step size was 0.1 μm, and the measured values ​​of five fields were averaged. Based on the obtained EBSD data, a distribution image of the KAM values ​​(KAM map) was obtained using crystal orientation analysis software OIM Analysis (trademark). Here, the KAM value was calculated using the following method. At each measurement point (a regular hexagonal pixel), the orientation difference between each pixel was calculated using the center and the three neighboring pixels (37 pixels in total), and the average of the calculated orientation differences was used as the KAM value of the central pixel. This operation was performed for all pixels in the field of view to obtain a KAM map. From the obtained KAM value distribution, the average KAM value and the standard deviation of the KAM value distribution were calculated using equations (1) and (2), respectively.

[0078]

[0079]

[0080] [Tensile test] JIS No. 5 tensile test specimens were taken from the base material of the electric resistance welded steel pipe so that the tensile direction was parallel to the pipe axis. The tensile test was carried out in accordance with the provisions of JIS Z 2241 (2022). The yield strength (MPa) was defined as the flow stress at a nominal strain of 0.5%.

[0081] [Flattening Test] The flattening test was performed in accordance with the provisions of JIS G 3445 (2021). As described above, it was performed using the method shown in Figure 1. In the flattening test, the test specimen was clamped between two flat plates. The test specimen was placed so that the line connecting the center of the test specimen and the electric resistance weld was perpendicular to the compression direction. The test specimen was a tubular test specimen with an axial length of 100 mm. Five specimens were tested. The outer surface of the weld was polished to a metallic luster. The compression speed was 10 mm / min. Compression was stopped when cracks were visually confirmed. The distance h between the flat plates and the length of each crack were measured. The flattening value was calculated using the formula h / D, where D is the outer diameter of the steel pipe.

[0082] The results obtained are shown in Table 3.

[0083] In Table 3, electric resistance welded steel pipes Nos. 1, 3, 5, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, and 22 are examples of the present invention, and electric resistance welded steel pipes Nos. 2, 4, 6, 7, and 9 are comparative examples.

[0084] In all of the electric resistance welded steel pipes of the present invention, when the flattening value h / D in the flattening test reached 0.25, the ratio of the total crack length to the total length of the test piece was 0.050 or less, and the maximum crack length was 2.0 mm or less.

[0085] On the other hand, the electric resistance welded steel pipe No. 2 of the comparative example did not achieve the desired bending resistance because the angle of the butt surfaces in the electric resistance weld was large.

[0086] The electric resistance welded steel pipe of Comparative Example No. 4 did not achieve the desired bending resistance because the cross-sectional area of ​​the molten steel discharged onto the outer surface during electric resistance welding was large.

[0087] The electric resistance welded steel pipe No. 6 of the comparative example did not achieve the desired bending resistance because the cross-sectional area of ​​the molten steel discharged onto the outer surface during electric resistance welding was small.

[0088] The electric resistance welded steel pipe No. 7 of the comparative example did not achieve the desired bending resistance because the amount of upset in the electric resistance welding was large.

[0089] The electric resistance welded steel pipe No. 9 of the comparative example did not achieve the desired bending resistance because the amount of upset in the electric resistance welding was small.

[0090]

[0091]

[0092]

[0093] REFERENCE SIGNS LIST 1 base material portion 2 heat-affected zone 3 electric resistance welded portion 4 bead 5A butt point 5 butt portion 8 flattened test piece (test piece) 10 center of flattened test piece (test piece) 11 flat plate 12 compression direction 20 open pipe 21 fin pass roll 22 squeeze roll 23 top roll 24 contact tip 25 high-frequency oscillator 26 laser shape meter 27 welding direction (pipe-making direction) 30 angle of butt surface 31 spacing of fin pass rolls

Claims

1. An electric resistance welded steel pipe having a base metal portion and an electric resistance welded portion, wherein, in a flattening test in which a flattening test piece taken from the electric resistance welded steel pipe is clamped between two flat plates, when the flattening value h / D reaches 0.25, the ratio of the total length of cracks 0.50 mm or longer in the axial direction observed on the outer surface of the flattening test piece to the total length of the flattening test piece in the axial direction is 0.050 or less, and the maximum crack length in the axial direction of the cracks is 2.0 mm or less, where h: distance between the flattening test plates (mm), D: outer diameter of the electric resistance welded steel pipe (mm). Note that the flattening value h / D is 1 at the start of the flattening test.

2. The composition of the base material portion contains, in mass%, C: 0.020% or more and 0.200% or less, Si: 0.40% or less, Mn: 0.50% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less, and optionally Nb: 0.080% or less, V: 0.080% or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, The electric resistance welded steel pipe according to claim 1, containing one or more elements selected from the group consisting of Sn: 0.100% or less, and the balance consisting of Fe and unavoidable impurities.

3. An electric resistance welded steel pipe according to claim 1 or 2, wherein the steel structure at the center of the wall thickness of the electric resistance weld has an average KAM (Kernel Average Misorientation) value, which is the average of the KAM values, of 2.0° or more and 4.0° or less, and the standard deviation of the KAM value distribution is 1.5° or less.

4. A line pipe using the electric resistance welded steel pipe according to any one of claims 1 to 3.

5. A method for manufacturing an electric resistance welded steel pipe according to any one of claims 1 to 3, wherein in the electric resistance welding process in which a hot rolled steel plate that is the raw material for the electric resistance welded steel pipe is used and one end in the width direction of the hot rolled steel plate is butted against another end in the width direction and electric resistance welded, the angle formed by the opposing butt surfaces is set to between 0° and 15°, and the cross-sectional area (mm ２ and the upset amount (mm) is 20% or more and 100% or less of the plate thickness (mm).

6. A method for producing an electric resistance welded steel pipe according to claim 5, further comprising, after the electric resistance welding step, a heat treatment step of heating the center of the wall thickness of the electric resistance welded portion to a heating temperature of 900°C or higher and 1150°C or lower, and cooling under conditions where the average cooling rate from 950 to 850°C is 2°C / s or higher and 10°C / s or lower, the average cooling rate from 800 to 650°C is 12°C / s or higher, and the average cooling rate from 600 to 200°C is 15°C / s or lower.

7. A method for producing an electric resistance welded steel pipe according to claim 6, further comprising, after the heat treatment step, a tempering step in which the holding temperature is 500°C or higher and 700°C or lower at the center of the wall thickness of the electric resistance welded portion.

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