Welded steel pipe and method for producing welded steel pipe

Abstract

To provide a welded steel pipe with excellent LME resistance, even when the tensile strength of a base material portion is high such as 800 MPa or higher.SOLUTION: The welded steel pipe disclosed herein comprises a base material portion and a welded portion. The carbon content of the base material portion is more than 0.30% by mass and up to 0.55% by mass, and the tensile strength of the base material portion is 800 MPa or more. In a cross-section perpendicular to the pipe axis in the region near the welded portion, which is within ±10 mm in the circumferential direction from the central position in the width direction of the welded portion, the microstructure consists of bainite and / or tempered martensite containing a plurality of laths and a plurality of cementite. This includes a cementite-containing hard structure with an area fraction of 30.0% or more and fresh martensite with an area fraction of 0.0 to less than 20.0%. In the cementite-containing hard structure, the total area fraction of cementite having an area of 0.008 μm2 or more is 5.0% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a welded steel pipe and a method for manufacturing the welded steel pipe, and more particularly to a welded steel pipe having a base metal portion and a weld portion extending in the pipe axis direction, and a method for manufacturing the welded steel pipe. [Background technology]

[0002] Overhead poles are used to support electric wires, power lines, telephone lines, etc. Overhead poles are installed outdoors, so their surfaces are plated.

[0003] Furthermore, overhead line poles are required to have improved earthquake resistance against large shocks such as earthquakes. Welded steel pipes are used as the material for overhead line poles. Increasing the strength of the welded steel pipes will increase the strength of the overhead line poles, and improve the earthquake resistance of the poles. For this reason, there is a demand for higher strength welded steel pipes, which are the material for overhead line poles. For overhead line pole applications, it is believed that sufficient earthquake resistance can be achieved if the tensile strength of the base material of the welded steel pipe is 800 MPa or more.

[0004] When welded steel pipes are plated, they are prone to LME (Liquid Metal Embrittlement) cracking in the area near the weld. LME cracking occurs when liquefied zinc penetrates into grain boundaries. Increasing the strength of welded steel pipes increases their susceptibility to LME cracking. Therefore, welded steel pipes used for overhead line poles must have both excellent strength and LME resistance.

[0005] A technique for suppressing LME cracking in welded portions is proposed in Japanese Patent Laid-Open Publication No. 2004-211158 (Patent Document 1).

[0006] Patent Document 1 discloses a zinc-based alloy-plated steel material for welding that has a zinc-based alloy plating layer on its surface, and the chemical composition of the base metal of the steel material contains, in mass %, 0.01-0.3% C, 0.01-2.0% Si, 0.1-3.0% Mn, 0.015% or less S, 0.001-0.5% Al, 3-40 ppm B, and 0.0005-0.006% N, with the balance consisting of Fe and unavoidable impurities. In this zinc-based alloy-plated steel material for welding, the base metal contains 0.01-0.3% C and 3-40 ppm B. Patent Document 1 states that this ensures toughness in the welded zone of the zinc-based alloy-plated steel material for welding while suppressing LME cracking. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-211158 Summary of the Invention [Problem to be solved by the invention]

[0008] However, Patent Document 1 does not consider achieving both high strength, with a tensile strength of 800 MPa or more, and excellent LME resistance in a welded steel pipe.

[0009] An object of the present disclosure is to provide a welded steel pipe that exhibits excellent LME resistance even when the tensile strength of the base metal portion is high, at least 800 MPa, and a method for manufacturing such a welded steel pipe. [Means for solving the problem]

[0010] The welded steel pipe of the present disclosure is A base material portion; a welded portion extending in the axial direction of the welded steel pipe, The C content of the base material is greater than 0.30% and less than 0.55% by mass, The tensile strength of the base material portion is 800 MPa or more, The microstructure in a cross section perpendicular to the pipe axis direction in the region near the weld, which is a region within ±10 mm in the circumferential direction of the welded steel pipe from the center position in the width direction of the weld, is A cementite-containing hard structure consisting of bainite and / or tempered martensite containing a plurality of laths and a plurality of cementites, and having an area ratio of 30.0% or more; and fresh martensite having an area ratio of 0.0 to less than 20.0%, 0.008 μm in the cementite-containing hard structure 2 The total area ratio of cementite having an area of ​​at least 1000 nm is 5.0% or less.

[0011] The method for manufacturing a welded steel pipe of the present disclosure includes: A forming step of bending a steel plate having a C content of more than 0.30 to 0.55% by mass and a tensile strength of 800 MPa or more into a cylindrical open pipe; a welding step of welding the open pipe to form an intermediate welded steel pipe having a welded portion extending in the pipe axial direction; a seam heat treatment process for normalizing the welded portion of the intermediate welded steel pipe, In the seam heat treatment step, a heating and holding step of heating the welded portion to a normalizing temperature and holding the welded portion at the normalizing temperature; a first cooling step of cooling the welded portion maintained at the normalizing temperature to a forced cooling stop temperature at a first cooling rate; a second cooling step of cooling from the forced cooling stop temperature at a second cooling rate, The forced cooling stop temperature is 650 to 300°C, the first cooling rate is 35°C / sec or more; The second cooling rate is less than 35° C. / second. [Effects of the Invention]

[0012] The welded steel pipe of the present disclosure exhibits excellent LME resistance even when the base metal portion has a high tensile strength of 800 MPa or more. The manufacturing method for the welded steel pipe of the present disclosure can manufacture the above-mentioned welded steel pipe. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a scanning electron microscope (SEM) image of cementite-containing hard structure in the region near the weld of the welded steel pipe of this embodiment. [Figure 2] Figure 2 is an SEM image of cementite-containing hard structures in the vicinity of the weld of a welded steel pipe, different from that shown in Figure 1. [Figure 3] FIG. 3 is an enlarged view of a cross section perpendicular to the pipe axis direction of the welded steel pipe of this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present inventors have investigated and studied welded steel pipes that can achieve excellent LME resistance during plating treatment even when the tensile strength of the base material is high, at least 800 MPa. Here, increasing the C content of the base material makes it easier to achieve a tensile strength of at least 800 MPa. Therefore, the present inventors have set the C content of the base material to more than 0.30 to 0.55% by mass, thereby achieving a tensile strength of at least 800 MPa.

[0015] In this specification, the weld and the base metal portion adjacent to the weld are referred to as the "near-weld region." The near-weld region is affected by the heat input during welding. Therefore, the near-weld region has a different microstructure from the rest of the base metal. Therefore, the inventors first manufactured welded steel pipes having base metal portions with a tensile strength of 800 MPa or more, and investigated and examined the occurrence of LME cracking in the near-weld region. As a result, the inventors obtained the following findings.

[0016] The microstructure in the vicinity of the weld affects LME resistance. In the microstructure in the vicinity of the weld, the area fraction of fresh martensite is set to less than 20.0%, and the area fraction of cementite-containing hard structure, which is a structure consisting of bainite and / or tempered martensite and containing multiple laths and multiple cementite, is set to 30.0% or more. The hardness of fresh martensite is higher than that of cementite-containing hard structure. Therefore, fresh martensite has low toughness and high LME cracking susceptibility. On the other hand, the LME cracking susceptibility of cementite-containing hard structure is lower than that of fresh martensite. Therefore, in the vicinity of the weld, the area fraction of fresh martensite is set low and the area fraction of cementite-containing hard structure is set higher than that of fresh martensite. Specifically, the area fraction of fresh martensite is set to 0.0 to less than 20.0%, and the area fraction of cementite-containing hard structure is set to 30.0% or more. In this case, LME resistance in the vicinity of the weld is improved.

[0017] However, even when the area ratio of fresh martensite in the microstructure in the vicinity of the weld was set to 0.0 to less than 20.0% and the area ratio of cementite-containing hard structure was set to 30.0% or more, LME cracking still occurred in some cases. Therefore, the present inventors conducted further studies.

[0018] Here, the inventors focused on the precipitation state of cementite in the cementite-containing hard structure in the vicinity of the weld. As described above, the cementite-containing hard structure contains a plurality of laths and a plurality of cementite. Note that, since the cementite-containing hard structure is bainite and / or tempered martensite, prior austenite grain boundaries exist in the cementite-containing hard structure.

[0019] If cementite is present at prior austenite grain boundaries or lath boundaries, the cementite becomes a propagation path for LME cracking. Therefore, in this case, even if the area ratio of the cementite-containing hard structure is increased in the region near the weld, the LME cracking susceptibility of the cementite-containing hard structure itself remains high. On the other hand, if cementite is present within prior austenite grains and laths, the cementite is less likely to become a propagation path for LME cracking. In this case, the LME cracking susceptibility of the cementite-containing hard structure itself decreases. Therefore, it is believed that the LME resistance of the cementite-containing hard structure can be improved by suppressing the formation of cementite within the cementite-containing hard structure at prior austenite grain boundaries and lath boundaries and instead allowing cementite to form within the prior austenite grains and laths.

[0020] Therefore, the present inventors further investigated the morphology of cementite formed at prior austenite grain boundaries or lath boundaries, and the morphology of cementite formed within prior austenite grains and laths. Figures 1 and 2 are scanning electron microscope (SEM) images of a portion of cementite-containing hard structure in the region near the weld of a welded steel pipe. The symbol θ in Figures 1 and 2 represents cementite. As a result of the investigation, it was found that in Figure 1, cementite was mainly present within prior austenite grains and laths, and in Figure 2, cementite was mainly present at prior austenite grain boundaries or lath boundaries.

[0021] Here, the inventors focused on the size of cementite. The size of cementite θ in Fig. 1 is smaller than the size of cementite θ in Fig. 2. As a result of further investigation, the inventors found that in the cementite-containing hard structure, the area of ​​0.008 μm 2 Fine cementite less than 0.008 μm in area is likely to exist within prior austenite grains and laths. 2 It was found that the above-mentioned coarse cementite is likely to exist at prior austenite grain boundaries or lath boundaries.

[0022] Based on the above findings, the inventors have determined that the area of ​​the cementite-containing hard structure in the vicinity of the weld is 0.008 μm 2When the area ratio of the above-mentioned coarse cementite is low, fine cementite is present in the prior austenite grains and laths, and therefore the cementite is unlikely to become a propagation path for LME cracks, and as a result, the LME resistance of the welded steel pipe is thought to be improved. 2 The present inventors have found that, if the total area ratio of cementite having the above area is 5.0% or less, excellent LME resistance can be obtained even if the tensile strength of the base material is as high as 800 MPa or more.

[0023] The welded steel pipe of this embodiment has been completed based on the above technical concept and has the following configuration.

[0024] The first configuration of welded steel pipe is A base material portion; a welded portion extending in the axial direction of the welded steel pipe, The C content of the base material is greater than 0.30% and less than 0.55% by mass, The tensile strength of the base material portion is 800 MPa or more, The microstructure in a cross section perpendicular to the pipe axis direction in the region near the weld, which is a region within ±10 mm in the circumferential direction of the welded steel pipe from the center position in the width direction of the weld, is A cementite-containing hard structure consisting of bainite and / or tempered martensite containing a plurality of laths and a plurality of cementites, and having an area ratio of 30.0% or more; and fresh martensite having an area ratio of 0.0 to less than 20.0%, 0.008 μm in the cementite-containing hard structure 2 The total area ratio of cementite having an area of ​​at least 1000 nm is 5.0% or less.

[0025] The second configuration of welded steel pipe is A welded steel pipe of a first configuration, The chemical composition of the base material is, in mass%, C: more than 0.30~0.55%, Si: 0.03 to 0.40% Mn: 0.50 to 2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005 to 0.500%, Nb: 0.010 to 0.060%, and N: 0.0003 to 0.0060%, The balance consists of Fe and impurities.

[0026] The third configuration of welded steel pipe is A welded steel pipe of a first configuration, The chemical composition of the base material is, in mass%, C: more than 0.30~0.55%, Si: 0.03 to 0.40% Mn: 0.50 to 2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005 to 0.500%, Nb: 0.010 to 0.060%, and N: 0.0003 to 0.0060%, Further, it contains one or more selected from the group consisting of Group 1 and Group 2, The balance consists of Fe and impurities. [Group 1] Ti: 1.000% or less, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 0.50% or less V: 0.20% or less, B: 0.0100% or less, and W: 0.10% or less, one or more selected from the group consisting of [Group 2] Ca: 0.0200% or less, Mg: 0.0200% or less, Zr: 0.0200% or less, and Rare earth elements (REM): 0.0200% or less, one or more selected from the group consisting of

[0027] The fourth configuration of welded steel pipe is A welded steel pipe of a third configuration, The chemical composition includes the first group.

[0028] The fifth configuration of welded steel pipe is A welded steel pipe of the third or fourth configuration, The chemical composition includes the second group.

[0029] The sixth configuration of welded steel pipe is A welded steel pipe having any one of the first to fifth configurations, The width of the weld is 0.3 mm or less.

[0030] The seventh configuration of welded steel pipe is A welded steel pipe having any one of the first to sixth configurations, The welded steel pipe has a plating layer formed on at least a portion of the surface thereof.

[0031] The eighth configuration of welded steel pipe is A welded steel pipe of the seventh configuration, The plating layer is a zinc-based plating layer.

[0032] The first method for manufacturing welded steel pipes is as follows: A method for producing a welded steel pipe having any one of the first to eighth configurations, A forming step of bending a steel plate having a C content of more than 0.30 to 0.55% by mass and a tensile strength of 800 MPa or more into a cylindrical open pipe; a welding step of welding the open pipe to form an intermediate welded steel pipe having a welded portion extending in the pipe axial direction; a seam heat treatment process for normalizing the welded portion of the intermediate welded steel pipe, In the seam heat treatment step, a heating and holding step of heating the welded portion to a normalizing temperature and holding the welded portion at the normalizing temperature; a first cooling step of cooling the welded portion maintained at the normalizing temperature to a forced cooling stop temperature at a first cooling rate; a second cooling step of cooling from the forced cooling stop temperature at a second cooling rate, The forced cooling stop temperature is 650 to 300°C, the first cooling rate is 35°C / sec or more; The second cooling rate is less than 35° C. / second.

[0033] The second method for manufacturing welded steel pipe is as follows: A first method for producing a welded steel pipe, further comprising: The method includes a tempering step in which the welded portion is tempered at a tempering temperature of 300 to 500°C after the seam heat treatment step.

[0034] The third method for manufacturing welded steel pipe is as follows: A method for producing a welded steel pipe having any one of the first to eighth configurations, A forming step of bending a steel plate having a C content of more than 0.30 to 0.55% by mass and a tensile strength of 800 MPa or more into a cylindrical open pipe; a welding step of welding the open pipe to form an intermediate welded steel pipe having a welded portion extending in the pipe axial direction; a seam heat treatment process for normalizing the welded portion of the intermediate welded steel pipe, In the seam heat treatment step, a heating and holding step of heating the welded portion to a normalizing temperature and holding the welded portion at the normalizing temperature; a first cooling step of cooling the welded portion maintained at the normalizing temperature to a forced cooling stop temperature at a first cooling rate; a second cooling step of cooling from the forced cooling stop temperature at a second cooling rate, The forced cooling stop temperature is less than 300°C, the first cooling rate is 35°C / sec or more; The manufacturing method further comprises: The method includes a tempering step in which the welded portion is tempered at a tempering temperature of 300 to 500°C after the seam heat treatment step.

[0035] The fourth method for manufacturing welded steel pipe is as follows: Any one of the first to third methods for producing a welded steel pipe, In the welding step, Electric resistance welding is performed on the open pipe.

[0036] The fifth method for manufacturing welded steel pipe is as follows: Any one of the first to fourth methods for producing a welded steel pipe, further comprising: The method includes a plating process for forming a plating layer on at least a portion of the surface of the intermediate welded steel pipe after the seam heat treatment process.

[0037] The welded steel pipe of this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0038] [Configuration of the welded steel pipe of this embodiment] Fig. 3 is an enlarged view of a cross section perpendicular to the pipe axis direction of a welded steel pipe of this embodiment. Referring to Fig. 3, a welded steel pipe 1 of this embodiment includes a base material portion 2 and a welded portion 3. The base material portion 2 is cylindrical. The base material portion 2 is formed by bending a steel plate in a manufacturing process described below. The welded portion 3 extends in the pipe axis direction of the welded steel pipe 1. The welded portion 3 is formed by butt-welding opposing ends of a cylindrical steel plate formed by bending.

[0039] In a cross section (FIG. 3) of the welded steel pipe 1 perpendicular to the pipe axis, the region ranging from the widthwise center position P0 on the outer peripheral surface of the welded portion 3 to ±10 mm in the circumferential direction of the welded steel pipe 1 (CL=P0-10 mm, CR=P0+10 mm in FIG. 3) is defined as the weld vicinity region 10. In other words, the weld vicinity region 10 is a region that has a width of 20 mm in the circumferential direction of the welded steel pipe 1, centered on the widthwise center position of the outer peripheral surface of the welded portion 3, and extends in the pipe axis direction. In FIG. 3, the right direction from the widthwise center position P0 is designated as plus (+), and the left direction is designated as minus (-). The weld vicinity region 10 includes the welded portion 3 and a portion of the heat-affected zone (HAZ) of the base metal portion 2.

[0040] The welded steel pipe 1 of this embodiment satisfies the following features 1 to 3. (Feature 1) The C content of the base material 2 is, in mass %, more than 0.30 to 0.55%, and the tensile strength of the base material 2 is 800 MPa or more. (Feature 2) The microstructure of the cross section perpendicular to the pipe axis direction in the weld vicinity region 10 consists of bainite and / or tempered martensite containing multiple laths and multiple cementite, and contains cementite-containing hard structure with an area ratio of 30.0% or more and fresh martensite with an area ratio of 0.0 to less than 20.0%. (Feature 3) 0.008 μm in cementite-containing hard tissue 2 The total area ratio of cementite having an area of ​​at least 1000 nm is 5.0% or less. Features 1 to 3 will be explained below.

[0041] [(Feature 1) C content and tensile strength of base material 2] In the welded steel pipe 1 of this embodiment, the C content in the chemical composition of the base material portion 2 is, in mass %, more than 0.30 to 0.55%, and the tensile strength of the base material portion 2 is 800 MPa or more. In the welded steel pipe 1 of this embodiment, the C content of the base material portion 2 is set to more than 0.30% to 0.55%, which makes it easier to make the tensile strength of the base material portion 2 800 MPa or more.

[0042] The lower limit of the tensile strength of the base material portion 2 is preferably 850 MPa, more preferably 900 MPa, and even more preferably 950 MPa. There is no particular limitation on the preferred upper limit of the tensile strength of the base material portion 2. The preferred upper limit of the tensile strength of the base material portion 2 is, for example, 1350 MPa, more preferably 1300 MPa, and even more preferably 1200 MPa.

[0043] [Method for measuring the C content of base material 2] The C content (mass %) of the base material portion 2 is measured by the following method. Chips are collected using a drill from the center of the wall thickness of the base material 2 at a position 180° shifted in the circumferential direction of the welded steel pipe 1 from the center of the width direction of the welded portion 3 (i.e., a portion of the base material 2 other than the HAZ).The carbon content (mass%) of the collected chips is determined using the well-known high-frequency combustion method (combustion-infrared absorption method).

[0044] [Method for measuring tensile strength] The tensile strength (MPa) of the base material portion 2 is determined by the following method. A JIS No. 12 tensile test piece is taken from a position 180° circumferentially shifted from the widthwise center position of the welded portion 3 of the welded steel pipe 1. The longitudinal direction of the tensile test piece is parallel to the pipe axis direction of the welded steel pipe 1. Using the taken tensile test piece, a tensile test is carried out in air at room temperature in accordance with JIS Z 2241:2022 to determine the tensile strength (MPa). The obtained tensile strength corresponds to the tensile strength of the base material portion 2 in the pipe axis direction.

[0045] [(Feature 2) Microstructure of the cross section perpendicular to the pipe axis direction in the welded area 10] The microstructure of the weld vicinity region 10 of the welded steel pipe 1 of this embodiment contains a cementite-containing hard structure with an area ratio of 30.0% or more and fresh martensite with an area ratio of 0.0 to less than 20.0%.

[0046] The microstructure of the weld vicinity region 10 may be a structure consisting of cementite-containing hard structure and fresh martensite, or may be a structure consisting of cementite-containing hard structure and fresh martensite and a remaining structure other than the cementite-containing hard structure and fresh martensite. When the microstructure of the weld vicinity region 10 contains a remaining structure other than the cementite-containing hard structure and fresh martensite, the remaining structure consists of one or more types selected from the group consisting of ferrite, pearlite, and retained austenite. As mentioned above, the area ratio of the remaining structure may be 0%.

[0047] In other words, the microstructure of the weld vicinity region 10 consists of a cementite-containing hard structure with an area ratio of 30.0% or more, fresh martensite with an area ratio of 0.0 to less than 20.0%, and a remaining structure of 0% or more, and the remaining structure consists of one or more types selected from the group consisting of ferrite, pearlite, and retained austenite.

[0048] Here, the "cementite-containing hard structure" refers to a structure containing multiple laths and multiple cementites, and is composed of bainite and / or tempered martensite. Here, bainite also includes tempered bainite. The area fraction (%) of the cementite-containing hard structure refers to the total area fraction (%) of bainite and tempered martensite.

[0049] Fresh martensite means martensite formed by quenching, consisting of multiple laths, and containing no cementite.

[0050] If the area ratio of the cementite-containing hard structure in the weld vicinity region 10 is less than 30.0%, coarse cementite is likely to form in the weld vicinity region 10. In this case, even if the area ratio of fresh martensite is less than 0.0 to 20.0%, the LME resistance of the welded steel pipe 1 decreases. If the area ratio of the cementite-containing hard structure in the weld vicinity region is 30.0% or more, the LME resistance of the welded steel pipe 1 can be improved, provided that the area ratio of fresh martensite is less than 0.0 to 20.0%.

[0051] Furthermore, fresh martensite has a high hardness and is therefore highly susceptible to LME cracking. Therefore, if the area fraction of fresh martensite in the region near the weld is 20.0% or more, the LME resistance of the welded steel pipe decreases. If the area fraction of fresh martensite is 0.0 to less than 20.0%, the LME resistance of the welded steel pipe can be improved, provided that the area fraction of cementite-containing hard structure is 30.0% or more.

[0052] The lower limit of the area ratio of cementite-containing hard structure in the region near the weld is preferably 35.0%, more preferably 40.0%, even more preferably 45.0%, even more preferably 50.0%, and even more preferably 55.0%. The upper limit of the area ratio of cementite-containing hard structure in the region near the weld is preferably 95.0%, more preferably 90.0%, even more preferably 85.0%, and still more preferably 80.0%.

[0053] The upper limit of the area ratio of fresh martensite in the region near the weld is preferably 19.5%, more preferably 19.0%, and even more preferably 18.0%. As described above, the area ratio of fresh martensite may be 0.0%.

[0054] [Method for observing the microstructure of a cross section perpendicular to the pipe axis direction in the region 10 near the weld] The area ratio of cementite-containing hard structure, the area ratio of fresh martensite, and the area ratio of the remaining structure (ferrite, pearlite, and retained austenite) in the microstructure of a cross section perpendicular to the pipe axis direction in the weld vicinity region 10 are determined by the following method.

[0055] A test piece including a region 10 near the weld is taken from a cross section perpendicular to the axial direction of the welded steel pipe 1. Of the surfaces of the taken test piece, a cross section perpendicular to the axial direction of the welded steel pipe 1 is used as the observation surface. The observation surface is mirror-polished. After mirror polishing, the observation surface is etched using a 3% nital solution to reveal the microstructure.

[0056] 3, on the observation surface after etching, the observation field V1 is a rectangular region having a center point X1 (see FIG. 3), which is the center of the width direction of the weld vicinity region 10 and the center of the wall thickness, and a width of 200 μm in the wall thickness direction and a width of 200 μm in the direction perpendicular to the wall thickness direction (approximately the circumferential direction) passing through the center point X1. The observation field V2 is a rectangular region having a center point X2 (see FIG. 3), which is +5.0 mm away from the center point X1 in the circumferential direction of the welded steel pipe 1, and a width of 200 μm in the wall thickness direction and a width of 200 μm in the direction perpendicular to the wall thickness direction (approximately the circumferential direction) passing through the center point X2. The observation field V3 is a rectangular region having a center point X3 (see FIG. 3), which is -5.0 mm away from the center point X1 in the circumferential direction of the welded steel pipe 1, and a width of 200 μm in the wall thickness direction and a width of 200 μm in the direction perpendicular to the wall thickness direction (approximately the circumferential direction) passing through the center point X3.

[0057] Each of the observation fields V1 to V3 is observed at 500x magnification using an SEM-EBSD device to generate an SEM image (secondary electron image). Using the obtained SEM image, cementite-containing hard structure, fresh martensite, ferrite, pearlite, and retained austenite are identified based on contrast and morphology as follows:

[0058] Cementite-containing hard structures and fresh martensite can be distinguished from other remaining structures (ferrite, pearlite, and retained austenite) based on contrast. Specifically, a structure with a lamellar structure can be identified as pearlite. Areas that are brighter than pearlite and in which no lath structure is observed are recognized as ferrite and retained austenite. Note that retained austenite is lower in brightness than ferrite, so it is also possible to distinguish between retained austenite and ferrite. Structures that are brighter than ferrite, pearlite, and retained austenite and in which multiple laths are observed are identified as cementite-containing hard structures and fresh martensite.

[0059] Cementite-containing hard structures and fresh martensite are distinguished in the following way. EBSD measurements are performed on the cementite-containing hard structure and fresh martensite regions (hereinafter referred to as "specific regions") identified in the SEM images of each observation field V1 to V3 (200 μm × 200 μm). EBSD measurements are performed at a magnification of 500x and an acceleration voltage of 20 kV. The specific regions are divided into regular hexagonal pixel units (measurement points). Each pixel has a side length of 0.15 μm. One regular hexagonal pixel in the observation field is selected as the center pixel. The crystal orientation difference between each pixel is calculated based on the crystal orientation of the selected center pixel and the crystal orientations of the six pixels adjacent to the center pixel. Based on the obtained crystal orientation difference, the boundary between the cementite-containing hard structure and fresh martensite is determined as follows. Specifically, the region containing cementite, surrounded by pixels with a crystal orientation difference of 15° or more, is identified as the "cementite-containing hard structure." On the other hand, a region surrounded by pixels where the crystal orientation difference is 15° or more and does not contain cementite is identified as "fresh martensite."

[0060] The total area of ​​the cementite-containing hard structure and the total area of ​​fresh martensite identified in the three observation fields V1 to V3 are determined. The area ratio (%) of the cementite-containing hard structure is determined based on the total area of ​​the observation fields V1 to V3 and the total area of ​​the cementite-containing hard structure. The area ratio (%) of the fresh martensite is determined based on the total area of ​​the observation fields V1 to V3 and the total area of ​​fresh martensite. The area ratios of the cementite-containing hard structure and the area ratios of fresh martensite are determined by rounding the obtained values ​​(%) to one decimal place. The value obtained by subtracting the area ratio of the cementite-containing hard structure and the area ratio of fresh martensite from 100.0% is defined as the total area ratio (%) of the remaining structure (ferrite, pearlite, and retained austenite).

[0061] [(Feature 3) Total area ratio of coarse cementite in cementite-containing hard structures] In the welded steel pipe 1 of this embodiment, the cementite-containing hard structure in the vicinity of the weld 10 is further 2 The total area ratio of cementite having an area of ​​0.008 μm or more is 5.0% or less. 2 "Cementite having an area of ​​1000 or more" is also called "coarse cementite."

[0062] If the total area ratio of coarse cementite in the cementite-containing hard structure in the weld vicinity region 10 exceeds 5.0%, excessive coarse cementite is formed at the prior austenite grain boundaries or lath interfaces of the cementite-containing hard structure. In this case, the coarse cementite becomes a propagation path for LME cracking, and the LME cracking susceptibility in the weld vicinity region 10 becomes excessively high. As a result, the LME resistance of the welded steel pipe 1 decreases.

[0063] If the total area ratio of coarse cementite is 5.0% or less, in the cementite-containing hard structure, the prior austenite grains and laths are not present at the prior austenite grain boundaries and lath boundaries, but at the prior austenite grain boundaries and laths. 2 Fine cementite with an area less than 100 μm is formed. Because the fine cementite is present within the prior austenite grains and laths, it is difficult for it to form a propagation path for LME cracks. This reduces the LME cracking susceptibility of the cementite-containing hard structure in the weld vicinity region 10, and as a result, the LME resistance of the welded steel pipe 1 is improved.

[0064] The upper limit of the total area ratio of coarse cementite is preferably 4.5%, more preferably 4.0%, even more preferably 3.5%, even more preferably 3.0%, and even more preferably 2.5%.

[0065] The total area ratio of coarse cementite is preferably as low as possible. However, excessive reduction in the total area ratio of coarse cementite excessively increases the production cost. Therefore, the preferred lower limit of the total area ratio of coarse cementite is 0.1%, more preferably 0.3%, and even more preferably 0.5%.

[0066] [Method for measuring the total area ratio of coarse cementite] The total area ratio (%) of coarse cementite in the cementite-containing hard structure of the weld vicinity region 10 is determined by the following method. In each of the observation fields V1, V2, and V3 obtained by a method conforming to the [Method for observing the microstructure of a cross section perpendicular to the pipe axis direction in the region 10 near the weld], a region of cementite-containing hard structure is identified.

[0067] A part of the cementite-containing hard structure identified in each observation field, Vc, is observed at a magnification of 50,000 times using an SEM to generate an SEM image. The area of ​​the observation field Vc at this time is 3.50 μm 2 Based on the contrast, cementite is identified within the observation field Vc. Figures 1 and 2 are examples of SEM images of the observation field Vc. In Figures 1 and 2, the granular or linear regions θ that are brighter than the lath, which is the parent phase, are cementite. Based on the contrast (brightness), cementite can be easily identified within the observation field Vc.

[0068] The area of ​​each identified cementite (μm 2 The identification of cementite based on brightness and the area of ​​the cementite can be determined using a general-purpose image processing application. An example of a general-purpose image processing application is the product name: imageJ.

[0069] Of the identified cementite, the area is 0.008 μm 2 The total area (μm 2 ) is calculated. Based on the obtained total area of ​​the coarse cementite and the area of ​​the observation field Vc, the total area ratio (%) of the coarse cementite in the cementite-containing hard structure is calculated. The total area ratio of the coarse cementite is calculated by rounding off the value to one decimal place.

[0070] The arithmetic mean value of the total area ratio of coarse cementite determined for each of the observation fields V1 to V3 (a total of three observation fields Vc) is defined as the total area ratio (%) of coarse cementite in the cementite-containing hard structure in the region near the weld. The arithmetic mean value of the total area ratio of coarse cementite is rounded to one decimal place.

[0071] [Effects of the welded steel pipe 1 of this embodiment] The welded steel pipe 1 of this embodiment satisfies Features 1 to 3. Therefore, the welded steel pipe 1 of this embodiment has high strength and excellent LME resistance even after being subjected to a plating treatment.

[0072] [Chemical composition of base material 2 of welded steel pipe 1] The chemical composition of the base material 2 of the welded steel pipe 1 of this embodiment is not particularly limited as long as the C content of the base material 2 is greater than 0.30 and less than 0.55% by mass and the tensile strength is 800 MPa or more.

[0073] Preferably, the base material portion 2 has the following chemical composition: The chemical composition of the base metal portion 2 is, in mass%, C: over 0.30 to 0.55%, Si: 0.03 to 0.40%, Mn: 0.50 to 2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005 to 0.500%, Nb: 0.010 to 0.060%, N: 0.0003 to 0.0060%, Ti: 0.000 to 1.000%, Cu: 0.00 to 1.00%, Ni: 0.00 to 1.00%, It contains Cr: 0.00 to 1.00%, Mo: 0.00 to 0.50%, V: 0.00 to 0.20%, B: 0.0000 to 0.0100%, W: 0.00 to 0.10%, Ca: 0.0000 to 0.0200%, Mg: 0.0000 to 0.0200%, Zr: 0.0000 to 0.0200%, and rare earth elements (REM): 0.0000 to 0.0200%, with the remainder being Fe and impurities. Each element in the preferred chemical composition of the base material portion 2 will be described below.

[0074] C: More than 0.30~0.55% Carbon (C) increases the strength of steel. If the C content is more than 0.30%, the above effect can be sufficiently obtained. On the other hand, if the C content exceeds 0.55%, an excess of fresh martensite is formed in the region near the weld. Furthermore, an excess of coarse cementite is formed in the cementite-containing hard structure. In this case, even if the contents of other elements are within the ranges of this embodiment, the LME resistance of the welded steel pipe 1 decreases. Therefore, the C content is more than 0.30% to 0.55%. The lower limit of the C content is preferably 0.35%, more preferably 0.37%, even more preferably 0.40%, and still more preferably 0.42%. The upper limit of the C content is preferably 0.52%, more preferably 0.50%, and even more preferably 0.47%.

[0075] Si: 0.03 to 0.40% Silicon (Si) deoxidizes steel. If the Si content is less than 0.03%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.40%, excessive Si oxides are formed, and in this case, cracks may occur originating from the Si oxides during bending in the manufacturing process of the welded steel pipe 1, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.03 to 0.40%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%, and even more preferably 0.12%. The upper limit of the Si content is preferably 0.35%, more preferably 0.33%, even more preferably 0.30%, and still more preferably 0.28%.

[0076] Mn: 0.50 to 2.00% Manganese (Mn) improves the hardenability of steel and increases its strength. If the Mn content is less than 0.50%, these effects cannot be fully achieved. On the other hand, if the Mn content exceeds 2.00%, the strength of the steel becomes excessively high, and in this case, the formability of the welded steel pipe 1 decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.50 to 2.00%. The lower limit of the Mn content is preferably 0.60%, more preferably 0.70%, even more preferably 0.80%, and still more preferably 0.90%. The upper limit of the Mn content is preferably 1.70%, more preferably 1.50%, even more preferably 1.30%, and still more preferably 1.20%.

[0077] P:0.030% or less Phosphorus (P) is an unavoidable impurity. That is, the P content is greater than 0.000%. If the P content exceeds 0.030%, P segregates at the grain boundaries and embrittles the grain boundaries. Therefore, even if the contents of other elements are within the ranges of this embodiment, the grain boundaries may be embrittled, and cracks may occur in the base material 2 during the manufacturing process of the welded steel pipe 1. Therefore, the P content is 0.030% or less. The lower the P content, the better. However, excessive reduction in the P content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the P content is preferably 0.001%, and more preferably 0.002%. The upper limit of the P content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0078] S: 0.010% or less Sulfur (S) is an unavoidable impurity. In other words, the S content is greater than 0.000%. S combines with Mn to form Mn sulfides. If the S content exceeds 0.010%, excessive Mn sulfides are formed. Mn sulfides act as starting points for cracks. Therefore, even if the contents of other elements are within the ranges of this embodiment, cracks may occur in the base metal portion 2 during the manufacturing process of the welded steel pipe 1. Therefore, the S content is 0.010% or less. The S content is preferably as low as possible. However, excessive reduction of the S content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the S content is preferably 0.001%, and more preferably 0.002%. The upper limit of the S content is preferably 0.008%, more preferably 0.006%, and even more preferably 0.005%.

[0079] Al: 0.005 to 0.500% Aluminum (Al) combines with nitrogen to form AlN. AlN has a pinning effect that prevents austenite grains from coarsening during the seam heat treatment process in the manufacturing process of the welded steel pipe 1. This reduces the circle-equivalent diameter of the packet in the weld vicinity region 10, improving the LME resistance of the welded steel pipe 1. If the Al content is less than 0.005%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.500%, the Al nitrides become coarse. The coarse Al nitrides become the starting points for cracks. Therefore, even if the contents of other elements are within the ranges of this embodiment, cracks may occur in the base material 2 during the manufacturing process of the welded steel pipe 1. Therefore, the Al content is 0.005 to 0.500%. The lower limit of the Al content is preferably 0.010%, more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Al content is preferably 0.400%, more preferably 0.300%, even more preferably 0.200%, even more preferably 0.100%, even more preferably 0.080%, and even more preferably 0.060%.

[0080] Nb: 0.010 to 0.060% Niobium (Nb) combines with C or N in the steel to form Nb precipitates. The Nb precipitates have a pinning effect and suppress coarsening of austenite grains during the seam heat treatment process in the manufacturing process of the welded steel pipe 1. This reduces the circle-equivalent diameter of the packet in the weld vicinity region 10, improving the LME resistance of the welded steel pipe 1. If the Nb content is less than 0.010%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Nb content exceeds 0.060%, Nb precipitates become coarse even if the contents of other elements are within the ranges of this embodiment. The coarse Nb precipitates become the starting points for cracks. Therefore, even if the contents of other elements are within the ranges of this embodiment, cracks may occur in the base material 2 during the manufacturing process of the welded steel pipe 1. Therefore, the Nb content is 0.010 to 0.060%. The lower limit of the Nb content is preferably 0.013%, more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Nb content is preferably 0.055%, more preferably 0.050%, even more preferably 0.045%, and still more preferably 0.040%.

[0081] N: 0.0003 to 0.0060% Nitrogen (N) combines with Al to form AlN, which refines the grains in the weld vicinity region 10. If the N content is less than 0.0003%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.0060%, AlN becomes coarse. The coarse AlN becomes the starting point for cracks. Therefore, even if the contents of other elements are within the ranges of this embodiment, cracks may occur in the base material portion 2 during the manufacturing process of the welded steel pipe 1. Therefore, the N content is 0.0003 to 0.0060%. The lower limit of the N content is preferably 0.0008%, more preferably 0.0010%, and even more preferably 0.0013%. The upper limit of the N content is preferably 0.0055%, more preferably 0.0050%, and even more preferably 0.0047%.

[0082] The remainder of the chemical composition of the base material 2 of the steel plate and welded steel pipe 1 of this embodiment consists of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when the steel plate that is the raw material for the base material 2 is industrially manufactured, and are acceptable within a range that does not adversely affect the welded steel pipe 1 of this embodiment.

[0083] [About optional elements] The chemical composition of the base material portion 2 of the welded steel pipe 1 of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of a first group and a second group. [Group 1] Ti: 1.000% or less, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 0.50% or less V: 0.20% or less, B: 0.0100% or less, and W: 0.10% or less, one or more selected from the group consisting of [Group 2] Ca: 0.0200% or less, Mg: 0.0200% or less, Zr: 0.0200% or less, and Rare earth elements (REM): 0.0200% or less, one or more selected from the group consisting of The elements of the first and second groups will be explained below.

[0084] [Group 1: Ti, Cu, Ni, Cr, Mo, V, B and W] The chemical composition of the base material 2 of the welded steel pipe 1 of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti, Cu, Ni, Cr, Mo, V, B, and W. These elements are optional elements, and all of them increase the strength of the steel. Each element will be described below.

[0085] Ti: 1.000% or less Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0.000%. When contained, that is, when the Ti content exceeds 0.000%, Ti forms Ti precipitates to increase the strength of the steel. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content exceeds 1.000%, excessive coarse Ti precipitates are formed. These coarse Ti precipitates become the starting points for cracks. Therefore, even if the contents of other elements are within the ranges of this embodiment, cracks may occur in the base material 2 during the manufacturing process of the welded steel pipe 1. Therefore, the Ti content is 0.000 to 1.000%, and if contained, it is 1.000% or less. The lower limit of the Ti content is preferably 0.001%, more preferably 0.010%, even more preferably 0.020%, and still more preferably 0.030%. The upper limit of the Ti content is preferably 0.900%, more preferably 0.800%, even more preferably 0.700%, even more preferably 0.600%, even more preferably 0.500%, and even more preferably 0.400%.

[0086] Cu:1.00% or less Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0.00%. When contained, that is, when the Cu content exceeds 0.00%, Cu improves the hardenability of the steel and increases the strength of the steel. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 1.00%, the strength of the steel increases excessively, and in this case, the formability of the welded steel pipe 1 decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.00 to 1.00%, and when Cu is contained, the Cu content is 1.00% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and still more preferably 0.15%. The upper limit of the Cu content is preferably 0.98%, more preferably 0.90%, and even more preferably 0.85%.

[0087] Ni: 1.00% or less Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0.00%. When contained, that is, when the Ni content exceeds 0.00%, Ni improves the hardenability of the steel and increases the strength of the steel. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, if the Ni content exceeds 1.00%, the strength of the steel increases excessively, and in this case, the formability of the welded steel pipe 1 decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0.00 to 1.00%, and when Ni is contained, the Ni content is 1.00% or less. The lower limit of the Ni content is preferably 0.01%, more preferably 0.03%, even more preferably 0.05%, and still more preferably 0.10%. The upper limit of the Ni content is preferably 0.98%, more preferably 0.95%, even more preferably 0.92%, and still more preferably 0.85%.

[0088] Cr:1.00% or less Chromium (Cr) is an optional element and may not be contained, that is, the Cr content may be 0.00%. When contained, that is, when the Cr content exceeds 0.00%, Cr improves the hardenability of the steel and increases the strength of the steel. Even if even a small amount of Cr is contained, these effects can be obtained to some extent. However, if the Cr content exceeds 1.00%, the strength of the steel increases excessively, and in this case, the formability of the welded steel pipe 1 decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0.00 to 1.00%, and when Cr is contained, the Cr content is 1.00% or less. The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and still more preferably 0.20%. The upper limit of the Cr content is preferably 0.97%, more preferably 0.94%, and even more preferably 0.90%.

[0089] Mo: 0.50% or less Molybdenum (Mo) is an optional element and may not be contained, that is, the Mo content may be 0.00%. When contained, that is, when the Mo content exceeds 0.00%, Mo improves the hardenability of steel and increases its strength. Mo also forms Mo precipitates, which increase the strength of steel through precipitation strengthening. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. However, if the Mo content exceeds 0.50%, the strength of the steel becomes excessively high, and therefore the formability of the welded steel pipe 1 decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0.00 to 0.50%, and when Mo is contained, the Mo content is 0.50% or less. The lower limit of the Mo content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Mo content is preferably 0.48%, more preferably 0.40%, even more preferably 0.36%, and still more preferably 0.30%.

[0090] V:0.20% or less Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0.00%. When V is contained, that is, when the V content exceeds 0.00%, V forms V precipitates. The V precipitates increase the strength of the steel through precipitation strengthening. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.20%, the strength of the steel becomes excessively high, and therefore the formability of the welded steel pipe 1 decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0.00 to 0.20%, and when V is contained, the V content is 0.20% or less. The lower limit of the V content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the V content is preferably 0.19%, more preferably 0.17%, and even more preferably 0.15%.

[0091] B: 0.0100% or less Boron (B) is an optional element and may not be contained, that is, the B content may be 0.0000%. When B is contained, that is, when the B content exceeds 0.0000%, B improves the hardenability of the steel and increases the strength of the steel. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.0100%, not only does the effect saturate, but the manufacturing cost also increases. Therefore, the B content is 0.0000 to 0.0100%, and when B is contained, the B content is 0.0100% or less. The lower limit of the B content is preferably 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the B content is preferably 0.0095%, more preferably 0.0090%, and even more preferably 0.0085%.

[0092] W: 0.10% or less Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0.00%. When W is contained, that is, when the W content exceeds 0.00%, W dissolves in the steel to increase the strength of the steel. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content exceeds 0.10%, the strength of the steel becomes excessively high, and therefore the formability of the welded steel pipe 1 decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0.00 to 0.10%, and if W is contained, it is 0.10% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the W content is preferably 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0093] [Group 2: Ca, Mg, Zr and rare earth elements (REM)] The chemical composition of the base metal of the welded steel pipe of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ca, Mg, Zr, and rare earth elements (REM). These elements are optional elements, and each of them controls the morphology of inclusions and suppresses the occurrence of cracks originating from inclusions. Each element will be explained below.

[0094] Ca:0.0200% or less Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0.0000%. When contained, that is, when the Ca content exceeds 0.0000%, Ca controls the morphology of inclusions, making them spheroidized and refined. Therefore, the occurrence of cracks originating from inclusions is suppressed. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content exceeds 0.0200%, coarse oxides are formed, which become the starting points for cracks. Therefore, the Ca content is 0 to 0.0200%, and when Ca is contained, the Ca content is 0.0200% or less. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0015%, even more preferably 0.0030%, and still more preferably 0.0045%. The upper limit of the Ca content is preferably 0.0190%, more preferably 0.0180%, and even more preferably 0.0170%.

[0095] Mg: 0.0200% or less Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0.0000%. When Mg is contained, that is, when the Mg content exceeds 0.0000%, Mg controls the morphology of inclusions, making them spheroidized and refined. Therefore, the occurrence of cracks originating from inclusions is suppressed. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content exceeds 0.0200%, the above effect becomes saturated. Therefore, the Mg content is 0.0000 to 0.0200%, and if contained, it is 0.0200% or less. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, and still more preferably 0.0030%. The upper limit of the Mg content is preferably 0.0190%, more preferably 0.0180%, and even more preferably 0.0170%.

[0096] Zr: 0.0200% or less Zirconium (Zr) is an optional element and may not be contained, that is, the Zr content may be 0%. When contained, that is, when the Zr content exceeds 0.0000%, Zr controls the morphology of inclusions, making them spheroidized and refined. Therefore, the occurrence of cracks originating from inclusions is suppressed. Even if even a small amount of Zr is contained, the above effects can be obtained to some extent. However, if the Zr content exceeds 0.0200%, the above effect becomes saturated. Therefore, the Zr content is 0.0000 to 0.0200%, and when Zr is contained, the Zr content is 0.0200% or less. The lower limit of the Zr content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0030%, and even more preferably 0.0040%. The upper limit of the Zr content is preferably 0.0190%, more preferably 0.0170%, and even more preferably 0.0150%.

[0097] Rare earth elements (REM): 0.0200% or less Rare earth elements (REM) are optional elements and may not be contained, that is, the REM content may be 0%. When REM is contained, that is, when the REM content exceeds 0.0000%, REM controls the morphology of inclusions, making them spheroidized and refined. This suppresses the occurrence of cracks originating from inclusions. Even if even a small amount of REM is contained, the above effects can be obtained to some extent. However, if the REM content exceeds 0.0200%, coarse oxides are formed, which can become crack initiation sites. Therefore, the REM content is 0 to 0.0200%, and if contained, it is 0.0200% or less. The lower limit of the REM content is preferably 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, and still more preferably 0.0030%. The upper limit of the REM content is preferably 0.0190%, more preferably 0.0180%, and even more preferably 0.0170%.

[0098] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. In addition, the REM content in this specification refers to the total content (mass%) of these elements.

[0099] [Method for measuring the chemical composition of base material 2] The chemical composition of the base material 2 is measured by a known elemental analysis method. For example, chips are collected using a drill from the center of the wall thickness of the base material 2, which is 180° circumferentially offset from the center of the weld 3 of the welded steel pipe 1 in the width direction. The collected chips are dissolved in acid to obtain a solution. The solution is subjected to ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) for elemental analysis of the chemical composition. The carbon and sulfur contents are determined using the well-known high-frequency combustion method (combustion-infrared absorption method). The nitrogen content is determined using the well-known inert gas fusion-thermal conductivity method.

[0100] [Other preferred configurations of welded steel pipe 1] [Width of welded part 3] The width of the welded portion 3 of the welded steel pipe 1 (the circumferential length of the welded steel pipe 1 at the welded portion 3) is not particularly limited. However, the smaller the width of the welded portion 3, the smaller the influence of welding on the welded steel pipe 1. This increases the LME resistance. Therefore, the smaller the width of the welded portion 3, the better. A preferred upper limit of the width of the welded portion 3 is 0.3 mm. When the welded steel pipe 1 is an electric resistance welded steel pipe, the width of the welded portion 3 is 0.3 mm or less.

[0101] [Method for measuring the width of weld 3] The width of the weld 3 can be measured by the following method. A test piece including the weld vicinity region 10 of the welded steel pipe 1 is taken. The test piece includes a cross section perpendicular to the pipe axis direction of the weld vicinity region 10. This cross section is used as the observation surface. The observation surface is mirror-polished. After mirror-polishing, the observation surface is subjected to metal flow etching using 5% (mass%) picric acid to reveal the microstructure. By performing the etching, the microstructure of the weld 3 can be clearly distinguished from the microstructure of the base material portion 2. On the observation surface, the width of the weld is measured at five locations at 1 mm intervals in the depth direction from the outer surface. However, if the thickness of the weld 3 is 5 mm or less, the width of the weld 3 is measured at five locations at 0.5 mm intervals in the thickness direction from the outer surface. The arithmetic mean of the five widths obtained is used as the width of the weld (mm). The width of the weld 3 is the value obtained by rounding the obtained arithmetic mean value to one decimal place (i.e., the value to one decimal place).

[0102] [Regarding welded steel pipes with plated layers 1] The welded steel pipe 1 of this embodiment may have a plating layer on at least a portion of its surface. The plating layer may be formed on a portion of the outer peripheral surface of the welded steel pipe 1, or may be formed on the entire outer peripheral surface. As described above, the welded steel pipe 1 of this embodiment has excellent LME resistance even when subjected to a plating treatment.

[0103] The type of the plating layer is not particularly limited, and may be a well-known plating layer. The plating layer may be a hot-dip plating layer or an electroplating layer. The plating layer is, for example, a zinc-based plating layer. Here, the zinc-based plating layer means a plating layer in which the Zn content in the plating is 50% by mass or more. The zinc-based plating layer is, for example, one or more selected from an electrogalvanized layer, a hot-dip galvanized layer, a hot-dip zinc alloy plating layer, a hot-dip Zn-Al alloy plating layer, a Zn-Ni alloy plating layer, a Zn-Al-Mg alloy plating layer, and a Zn-Al-Mg-Si alloy plating layer.

[0104] [Use of the welded steel pipe 1 of this embodiment] The welded steel pipe 1 of this embodiment has high strength and exhibits excellent LME resistance even after plating. Therefore, it can be widely used in applications where these properties are required. The welded steel pipe 1 of this embodiment is particularly suitable for use as an overhead line pole that supports electric wires. The welded steel pipe 1 of this embodiment is not particularly limited as long as it has a welded portion 3 extending in the pipe axial direction. The welded steel pipe 1 may be an electric resistance welded steel pipe or a welded steel pipe other than an electric resistance welded steel pipe. Preferably, the welded steel pipe 1 is an electric resistance welded steel pipe.

[0105] [Manufacturing method] A method for manufacturing a welded steel pipe 1 according to this embodiment will be described. Note that the manufacturing method described below is an example, and the manufacturing method for the welded steel pipe 1 according to this embodiment is not limited to this. In other words, as long as a welded steel pipe 1 having the above-described configuration can be manufactured, the manufacturing method is not limited to the manufacturing method described below. However, the manufacturing method described below is a suitable method for manufacturing the welded steel pipe 1 according to this embodiment.

[0106] [First manufacturing method example] The first example of the method for producing a welded steel pipe of this embodiment includes the following steps. (Process 1) Molding process (Process 2) Welding process (Process 3) Seam heat treatment process Each step will be described below.

[0107] [(Process 1) Molding process] In the forming process, a steel plate having a C content of more than 0.30 to 0.55% by mass and a tensile strength of 800 MPa or more is bent into a cylindrical open pipe. Specifically, first, a steel plate is prepared. Methods for manufacturing steel plate having a C content of more than 0.30 to 0.55% by mass and a tensile strength of 800 MPa or more are well known. A slab having the chemical composition of the base material portion 2 described above is manufactured by continuous casting. The manufactured slab is subjected to hot rolling (rough rolling) to manufacture a rough bar. The rough bar is further hot rolled (finish rolling) to manufacture a steel plate. By adjusting the temperature during hot rolling and the cooling conditions after finish rolling, a steel plate having a C content of more than 0.30 to 0.55% by mass and a tensile strength of 800 MPa or more can be manufactured. The manufactured steel plate is wound into a coil. The prepared steel sheet is unwound and bent by roll forming to produce a cylindrical open pipe. The forming process may be carried out by a known method.

[0108] [(Process 2) Welding process] In the welding process, welding is performed on a cylindrical open pipe to form a welded portion extending in the pipe axis direction. Specifically, opposing ends of the open pipe are butted together, and welding is performed on the butt joint. In this way, a mid-section welded steel pipe having a welded portion extending in the pipe axis direction is manufactured.

[0109] The welding method may be arc welding or electric resistance welding. Preferably, electric resistance welding is performed in the welding step. In this case, the width of the welded portion 3 is 0.3 mm or less. When electric resistance welding is performed, the ends of the open pipes are butted together and brought into contact, and electric resistance welding is performed by high-frequency induction heating while applying pressure to the butted portion.

[0110] [(Process 3) Seam heat treatment process] In the seam heat treatment process, normalizing is performed on the welded portion of the manufactured intermediate welded steel pipe. Specifically, a predetermined range (for example, ±30 to ±40 mm in the circumferential direction of the intermediate welded steel pipe) from the widthwise center position of the welded portion of the intermediate welded steel pipe is heated. A heating device having an inductor (heating coil) is used for heating. The heating device is placed above the region 10 near the welded portion, and the welded portion is heated. This performs normalizing on the welded portion.

[0111] More specifically, the seam heat treatment process includes a heating and holding process, a first cooling process, and a second cooling process. In the heating and holding process, the weld is heated to a normalizing temperature and held at the normalizing temperature. The normalizing temperature is, for example, 900 to 1100°C. In the first cooling process, the weld held at the normalizing temperature is cooled to a forced cooling stop temperature T1 (°C) at a first cooling rate CR1 (°C / sec). In the second cooling process, the weld is cooled from the forced cooling stop temperature T1 (°C) at a second cooling rate CR2 (°C / sec). In other words, the seam heat treatment process involves two stages of cooling.

[0112] The seam heat treatment process satisfies the following conditions: (Condition 1) The forced cooling stop temperature T1 is set to 650 to 300°C. (Condition 2) The first cooling rate CR1 is set to 35° C. / sec or more. (Condition 3) The second cooling rate CR2 is set to less than 35° C. / sec. Each condition will be explained below.

[0113] [(Condition 1) Forced cooling stop temperature T1] The forced cooling stop temperature T1 refers to the temperature (°C) at which cooling is started from the normalizing temperature at a first cooling rate CR1 after the heating and holding step, and at which cooling at the first cooling rate CR1 is stopped. If the forced cooling stop temperature T1 is higher than 650°C, cooling in the first cooling step is insufficient. In this case, the area ratio of the cementite-containing hard structure is less than 30.0%. Furthermore, the area ratio of coarse cementite in the cementite-containing hard structure exceeds 5.0%. On the other hand, if the forced cooling stop temperature T1 is less than 300°C, excessive cooling occurs in the first cooling step. In this case, if the first cooling rate CR1 is 35°C / sec or more, the area fraction of fresh martensite becomes 20.0% or more. Furthermore, the cementite-containing hard structure becomes less than 30.0%. Therefore, the forced cooling stop temperature T1 is set to 650 to 300°C. The upper limit of the forced cooling stop temperature T1 is preferably 600°C, more preferably 550°C, and even more preferably 500°C. The lower limit of the forced cooling stop temperature T1 is preferably 330°C, more preferably 350°C, and even more preferably 400°C.

[0114] The forced cooling stop temperature T1 (°C) is the surface temperature (°C) of the weld vicinity region 10 at the time when the first cooling step is completed. The surface temperature of the weld vicinity region 10 can be measured using a well-known thermometer, such as a thermograph.

[0115] [(Condition 2) First cooling rate CR1] The average cooling rate in the temperature range from the normalizing temperature to the forced cooling stop temperature T1 is defined as the first cooling rate CR1 (°C / sec). The first cooling rate CR1 can be calculated based on the normalizing temperature, the forced cooling stop temperature T1, and the time from the start to the end of the first cooling step.

[0116] If the first cooling rate CR1 is less than 35°C / sec, the cooling in the first cooling step is too slow. In this case, the area ratio of the cementite-containing hard structure will be less than 30.0%. Furthermore, the area ratio of coarse cementite in the cementite-containing hard structure will exceed 5.0%. Therefore, the first cooling rate CR1 is set to 35°C / sec or more.

[0117] A preferred lower limit of the first cooling rate CR1 is 40° C. / sec, more preferably 50° C. / sec, even more preferably 60° C. / sec, and even more preferably 70° C. / sec. The upper limit of the first cooling rate CR1 is not particularly limited. However, if the first cooling rate CR1 is set too fast, the equipment capacity may be exceeded or the manufacturing cost may increase. Therefore, the upper limit of the first cooling rate CR1 is preferably 180°C / sec, more preferably 170°C / sec, and even more preferably 160°C / sec.

[0118] [(Condition 3) Second cooling rate CR2] The average cooling rate in the temperature range from the forced cooling stop temperature T1 to 100°C is defined as the second cooling rate CR2 (°C / sec). The second cooling rate CR2 can be determined based on the time it takes for the surface of the weld vicinity region 10 to cool from the forced cooling stop temperature T1 to 100°C. The microstructure of the weld vicinity region 10 is formed by cooling down to 100°C. Therefore, there are no particular restrictions on the cooling rate in the temperature range below 100°C.

[0119] If the second cooling rate CR2 is 35°C / sec or more, the cooling in the second cooling step is too fast, and in this case, the area ratio of fresh martensite becomes excessive. Therefore, the second cooling rate CR2 is set to less than 35°C / sec.

[0120] A preferred upper limit of the second cooling rate CR2 is 30°C / sec, more preferably 28°C / sec, even more preferably 26°C / sec, even more preferably 24°C / sec, even more preferably 22°C / sec, and even more preferably 20°C / sec. The lower limit of the second cooling rate CR2 is not particularly limited, and the lower limit of the second cooling rate CR2 is preferably 2°C / sec, more preferably 4°C / sec, and even more preferably 6°C / sec.

[0121] [Another embodiment of the first manufacturing method] The first example of the manufacturing method of the welded steel pipe of this embodiment may further include a tempering step.

[0122] [(Process 4) Tempering process] The tempering process is carried out after the seam heat treatment process. In the tempering process, the welded portion of the intermediate welded steel pipe after the seam heat treatment process is tempered. Specifically, at least a predetermined range from the widthwise center position of the welded portion of the intermediate welded steel pipe (for example, ±30 to ±40 mm in the circumferential direction of the intermediate welded steel pipe) is heated and held at a tempering temperature T2 (°C) for a predetermined time.

[0123] The tempering process satisfies the following conditions: (Condition 4) The tempering temperature T2 is set to 300 to 500°C.

[0124] In this case, the fresh martensite in the weld vicinity region 10 can be converted into a cementite-containing hard structure (tempered martensite in this case), thereby further reducing the area ratio of fresh martensite in the weld vicinity region 10.

[0125] The lower limit of the tempering temperature T2 is preferably 320°C, and more preferably 340°C. The upper limit of the tempering temperature T2 is preferably 480°C, and more preferably 460°C.

[0126] In the tempering step, the holding time at the tempering temperature T2 is, for example, 30 to 3600 seconds.

[0127] The tempering process may be carried out using a heat treatment furnace or a high-frequency heating device. In tempering using a heat treatment furnace, the entire intermediate-welded steel pipe is loaded into the heat treatment furnace. Therefore, tempering is carried out on the entire intermediate-welded steel pipe. In tempering using a high-frequency heating device, the entire intermediate-welded steel pipe may be heated to temper the entire intermediate-welded steel pipe, or only the region near the weld of the intermediate-welded steel pipe may be heated to temper only the region near the weld.

[0128] Furthermore, when the tempering process is performed, mobile dislocations in the intermediate welded steel pipe are fixed, which causes an upper yield point and a lower yield point to appear in the stress-strain curve of the manufactured welded steel pipe.

[0129] [Second manufacturing method example] The second example of the manufacturing method for a welded steel pipe of this embodiment includes steps 1 to 4, and furthermore, in steps 3 and 4, conditions 1 to 3 are set as follows. (Condition 1) The forced cooling stop temperature T1 is set to less than 300°C. (Condition 2) The first cooling rate CR1 is set to 35° C. / sec or more. (Condition 3) The tempering temperature T2 is set to 300 to 500°C.

[0130] In this manufacturing method example, the forced cooling stop temperature T1 in Condition 1 is set to less than 300°C. In this case, an excess of fresh martensite is generated in the region near the weld. However, in the subsequent tempering process, the fresh martensite becomes a cementite-containing hard structure (tempered martensite in this case). Therefore, in the microstructure in the region near the weld, the area ratio of the cementite-containing hard structure can be set to 30.0% or more, and the area ratio of fresh martensite can be set to 0.0 to less than 20.0%.

[0131] In the second manufacturing method example, a tempering step is carried out, and therefore an upper yield point and a lower yield point appear in the stress-strain curve of the manufactured welded steel pipe.

[0132] [Other steps in the method for manufacturing a welded steel pipe according to this embodiment] The above-described manufacturing method may further include a plating process.

[0133] [(Process 5) Plating process] The plating process is an optional process and does not have to be performed. If performed, the plating process is performed after the seam heat treatment process, or after the tempering process if a tempering process is performed. In the plating process, the intermediate welded steel pipe is plated to form a plating layer on at least a portion of the surface of the intermediate welded steel pipe.

[0134] The plating process may be carried out by a known method. For example, a zinc-plated layer is formed by hot-dip plating or electroplating. The zinc-based plating layer can also be formed by a known plating process. For example, the intermediate welded steel pipe is subjected to known solvent degreasing, pickling, and flux treatment. Thereafter, a known hot-dip galvanizing process is carried out. By the above steps, a welded steel pipe having a zinc-plated layer formed on its surface can be manufactured. [Example]

[0135] The effects of the welded steel pipe of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the welded steel pipe of this embodiment. Therefore, the welded steel pipe of this embodiment is not limited to this one example of conditions.

[0136] Welded steel pipes having base metal parts with the chemical compositions shown in Table 1 (Tables 1A and 1B) were manufactured.

[0137] [Table 1A]

[0138] [Table 1B]

[0139] The welded steel pipes of each test number were manufactured by the following method. A steel plate was prepared and bent to form a cylindrical open pipe (forming process). The open pipe was subjected to electric resistance welding to manufacture an intermediate welded steel pipe (welding process). The intermediate welded steel pipe was normalized (seam heat treatment process). The normalizing temperature in the seam heat treatment process was 1000°C for all test numbers. Furthermore, the first cooling rate CR1 (°C / sec), forced cooling stop temperature T1 (°C), and second cooling rate CR2 (°C / sec) in the seam heat treatment process were as shown in Table 2. Note that for test numbers 11 to 14 and 32, a tempering process was carried out after the seam heat treatment process. The tempering temperature T2 (°C) in the tempering process was as shown in Table 2. The holding time at the tempering temperature T2 was 300 seconds for all test numbers.

[0140] [Table 2]

[0141] Welded steel pipes (electric resistance welded steel pipes) of each test number were manufactured using the above manufacturing process. The width of the welded portion of each welded steel pipe was measured using the method described above in [Method for measuring the width of welded portion 3]. As a result, the width of the welded portion was 0.3 mm or less for all test numbers.

[0142] [Evaluation test] The following evaluation tests were carried out on the welded steel pipes with each test number. (Test 1) Tensile strength measurement test (Test 2) Microstructure observation test of the area near the weld (Test 3) Measurement of the total area ratio of coarse cementite in the cementite-containing hard structure in the area near the weld (Test 4) LME resistance evaluation test for welded steel pipes Tests 1 to 4 will be explained below.

[0143] [(Test 1) Tensile strength measurement test] The tensile strength (MPa) of the base metal of the welded steel pipe of each test number was determined based on the method described above in [Method for measuring tensile strength]. The results are shown in the "Tensile strength (MPa)" column of Table 3.

[0144] [Table 3]

[0145] [(Test 2) Microstructure observation test of the area near the weld] Based on the method described above in [Method for Observing Microstructures in Cross Sections Perpendicular to the Pipe Axis Direction of the Near-Weld Region 10], the microstructures of cross sections perpendicular to the pipe axis direction in the near-weld region of the welded steel pipes of each test number were observed, and the area fraction (%) of cementite-containing hard structure and the area fraction (%) of fresh martensite were determined to identify the remaining structure other than the cementite-containing hard structure and fresh martensite in the microstructure. The area fraction (%) of cementite-containing hard structure is shown in the "Cementite-containing Hard Structure Area Fraction A1 (%)" column in Table 3. The area fraction (%) of fresh martensite is shown in the "Fresh Martensite Area Fraction A2 (%)" column in Table 3. The remaining structure is shown in the "Remaining Structure" column in Table 3. In the "Remaining Structure" column, "α" means ferrite, "p" means pearlite, and "γ" means retained austenite. For example, when "α, p" is listed in the "Remaining Structure" column, it means that the remainder was ferrite and pearlite.

[0146] [(Test 3) Measurement of the total area ratio of coarse cementite in the cementite-containing hard structure in the area near the weld] Based on the method described in the above [Method for measuring the total area ratio of coarse cementite], the total area ratio (%) of coarse cementite in the cementite-containing hard structure in the region near the weld of the welded steel pipe of each test number was determined. The obtained total area ratio (%) of coarse cementite is shown in the "Total area ratio (%) of coarse cementite" column in Table 3.

[0147] [(Test 4) LME resistance evaluation test for welded steel pipe] The LME resistance of the welded steel pipes of each test number was evaluated by the following method. First, a plating process was performed on the welded steel pipe. Specifically, the welded steel pipe of each test number was subjected to solvent degreasing under the same conditions, and then flux treatment was performed under the same conditions. The welded steel pipe was then immersed in a hot-dip galvanizing bath for a hot-dip galvanizing process. The plating bath temperature was set to 443°C, and the pipe was immersed for 3 minutes. The lifting speed after immersion was set to 100 mm / min. Through the above plating process, a welded steel pipe was manufactured in which a galvanized layer (Zn content: 99.95 mass%, Al content: 0.005 mass%) was formed on the entire outer surface.

[0148] The LME crack length of the welded steel pipe after plating treatment was evaluated by the following method. The presence or absence of LME cracks was visually inspected along the longitudinal (axial) direction of the galvanized welded steel pipe in the area near the weld on its surface. If LME cracks were observed, the observation field was the surface layer near the weld in a cross section perpendicular to the longitudinal (axial) direction of the galvanized welded steel pipe where the LME cracks occurred. Observation was performed using an optical microscope at 500x magnification to determine the maximum length of the LME cracks. The obtained maximum LME crack lengths are shown in the "LME crack length (μm)" column in Table 3. If the maximum LME crack length was 10 μm or less, it was determined that excellent LME resistance was obtained. On the other hand, if the maximum LME crack length exceeded 10 μm, it was determined that LME resistance was poor. If no LME cracks were visually observed, a "0" was entered in the "LME crack length (μm)" column in Table 3.

[0149] [Evaluation results] Referring to Tables 1 to 3, the welded steel pipes of test numbers 1 to 17 satisfied characteristics 1 to 3. Therefore, excellent LME resistance was obtained while having a tensile strength of 800 MPa or more.

[0150] On the other hand, in test number 18, the C content was too low, and therefore the tensile strength was less than 800 MPa.

[0151] In test number 19, the C content was too high. As a result, the area fraction of fresh martensite exceeded 20.0% and the area fraction of coarse cementite in the cementite-containing hard structure also exceeded 5.0%. As a result, the LME resistance was low.

[0152] In test numbers 20 and 21, the first cooling rate CR1 in the seam heat treatment process was too slow. As a result, the area ratio of the cementite-containing hard structure was less than 30.0%, and the area ratio of coarse cementite in the cementite-containing hard structure also exceeded 5.0%. As a result, the LME resistance was low.

[0153] In Test Nos. 22 and 23, the first cooling rate CR1 in the seam heat treatment process was too slow. Furthermore, the forced cooling stop temperature T1 was too low. As a result, the area ratio of the cementite-containing hard structure was less than 30.0%, and the area ratio of coarse cementite in the cementite-containing hard structure also exceeded 5.0%. As a result, the LME resistance was low.

[0154] In test numbers 24 and 25, the forced cooling stop temperature T1 in the seam heat treatment process was too low. As a result, the area ratio of cementite-containing hard structure was less than 30.0%, and the area ratio of fresh martensite was 20.0% or more. As a result, the LME resistance was low.

[0155] In Test Nos. 26 and 27, the forced cooling stop temperature T1 in the seam heat treatment process was too high. As a result, the area ratio of the cementite-containing hard structure was less than 30.0%, and the area ratio of coarse cementite in the cementite-containing hard structure also exceeded 5.0%. As a result, the LME resistance was low.

[0156] In test numbers 28 and 29, the second cooling rate CR2 in the seam heat treatment process was too fast. As a result, the area fraction of fresh martensite was 20.0% or more. As a result, the LME resistance was low.

[0157] In test number 30, the first cooling rate CR1 and the second cooling rate CR2 were both 20°C / sec in the seam heat treatment process. Therefore, the area ratio of the cementite-containing hard structure was less than 30.0%, and the area ratio of coarse cementite in the cementite-containing hard structure also exceeded 5.0%. As a result, the LME resistance was low.

[0158] In test number 31, the first cooling rate CR1 and the second cooling rate CR2 were both 210°C / sec in the seam heat treatment process. Therefore, the area fraction of cementite-containing hard structure was less than 30.0%, and the area fraction of fresh martensite was 20.0% or more. As a result, the LME resistance was low.

[0159] In test number 32, although a tempering process was performed after the seam heat treatment process, the forced cooling stop temperature T1 during the seam heat treatment process was too low, and the tempering temperature T2 was also too low. As a result, the area fraction of fresh martensite was 20.0% or more. As a result, the LME resistance was low.

[0160] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A welded steel pipe, A base material portion; a welded portion extending in the axial direction of the welded steel pipe, The C content of the base material is greater than 0.30% to 0.55% by mass, The tensile strength of the base material portion is 800 MPa or more, The microstructure in a cross section perpendicular to the pipe axis direction in the vicinity of the weld zone, which is a region within ±10 mm in the circumferential direction of the welded steel pipe from the width direction center position of the weld zone, is a cementite-containing hard structure consisting of bainite and / or tempered martensite containing a plurality of laths and a plurality of cementites, the cementite-containing hard structure having an area ratio of 30.0% or more; and an area ratio of fresh martensite of 0.0 to less than 20.0%, 0.008 μm in the cementite-containing hard structure ２ The total area ratio of cementite having an area of ​​5.0% or less is 5.0% or less. Welded steel pipe.

2. The welded steel pipe according to claim 1, The chemical composition of the base material portion is, in mass%, C: more than 0.30 to 0.55%, Si: 0.03-0.40%, Mn: 0.50-2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005-0.500%, Nb: 0.010 to 0.060%, and N: 0.0003 to 0.0060%, The balance consists of Fe and impurities. Welded steel pipe.

3. The welded steel pipe according to claim 1, The chemical composition of the base material portion is, in mass%, C: more than 0.30 to 0.55%, Si: 0.03-0.40%, Mn: 0.50-2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005-0.500%, Nb: 0.010 to 0.060%, and N: 0.0003 to 0.0060%, Further, the composition contains one or more selected from the group consisting of Group 1 and Group 2, The balance consists of Fe and impurities. Welded steel pipe. [Group 1] Ti: 1.000% or less, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 0.50% or less, V: 0.20% or less, B: 0.0100% or less, and W: 0.10% or less, one or more selected from the group consisting of [Group 2] Ca: 0.0200% or less, Mg: 0.0200% or less, Zr: 0.0200% or less, and Rare earth elements (REM): 0.0200% or less, one or more selected from the group consisting of

4. The welded steel pipe according to claim 3, the chemical composition contains the first group; Welded steel pipe.

5. The welded steel pipe according to claim 3, The chemical composition contains the second group. Welded steel pipe.

6. The welded steel pipe according to claim 1, The width of the weld is 0.3 mm or less. Welded steel pipe.

7. The welded steel pipe according to any one of claims 1 to 6, further comprising: A plating layer is formed on at least a portion of the surface of the welded steel pipe. Welded steel pipe.

8. The welded steel pipe according to claim 7, The plating layer is a zinc-based plating layer. Welded steel pipe.

9. The method for manufacturing a welded steel pipe according to claim 1, a forming step of bending a steel plate having a C content of more than 0.30 to 0.55% by mass and a tensile strength of 800 MPa or more into a cylindrical open pipe; a welding step of welding the open pipe to form an intermediate welded steel pipe having a welded portion extending in the pipe axial direction; a seam heat treatment process for normalizing the welded portion of the intermediate welded steel pipe, In the seam heat treatment step, a heating and holding step of heating the welded portion to a normalizing temperature and holding the welded portion at the normalizing temperature; a first cooling step of cooling the welded portion held at the normalizing temperature to a forced cooling stop temperature at a first cooling rate; a second cooling step of cooling from the forced cooling stop temperature at a second cooling rate, The forced cooling stop temperature is 650 to 300°C, the first cooling rate is 35°C / sec or more; the second cooling rate is less than 35°C / sec; Manufacturing method for welded steel pipe.

10. The method for producing a welded steel pipe according to claim 9, further comprising: A tempering process is provided in which the welded portion is tempered at a tempering temperature of 300 to 500 ° C. after the seam heat treatment process. Manufacturing method for welded steel pipe.

11. The method for manufacturing a welded steel pipe according to claim 1, a forming step of bending a steel plate having a C content of more than 0.30 to 0.55% by mass and a tensile strength of 800 MPa or more into a cylindrical open pipe; a welding step of welding the open pipe to form an intermediate welded steel pipe having a welded portion extending in the pipe axial direction; a seam heat treatment process for normalizing the welded portion of the intermediate welded steel pipe, In the seam heat treatment step, a heating and holding step of heating the welded portion to a normalizing temperature and holding the welded portion at the normalizing temperature; a first cooling step of cooling the welded portion held at the normalizing temperature to a forced cooling stop temperature at a first cooling rate; a second cooling step of cooling from the forced cooling stop temperature at a second cooling rate, The forced cooling stop temperature is less than 300°C, the first cooling rate is 35°C / sec or more; The manufacturing method further comprises: A tempering process is provided in which the welded portion is tempered at a tempering temperature of 300 to 500 ° C. after the seam heat treatment process. Manufacturing method for welded steel pipe.

12. The method for manufacturing a welded steel pipe according to any one of claims 9 to 11, In the welding step, Electric resistance welding is performed on the open pipe. Manufacturing method for welded steel pipe.

13. The method for producing a welded steel pipe according to any one of claims 9 to 11, further comprising: A plating process is provided to form a plating layer on at least a part of the surface of the intermediate welded steel pipe after the seam heat treatment process. Manufacturing method for welded steel pipe.

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