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

Abstract

Provided is an electric-resistance-welded steel pipe having excellent corrosion resistance. The electric-resistance-welded steel pipe comprises a base and an electric-resistance-welded joint by which one end of the base and the other end of the base are connected to each other. The base and the electric-resistance-welded joint of the electric-resistance-welded steel pipe contain at least one of Cr, Mo, and Cu. The concentrations of Cr, Mo, and Cu in the base and the concentrations of Cr, Mo, and Cu in the electric-resistance-welded joint satisfy a given relationship. The electric-resistance-welded steel pipe of the present invention has excellent corrosion resistance.

Description

[Technical Field]

[0001] The present invention relates to electric resistance welded steel pipes, line pipes, and methods for manufacturing electric resistance welded steel pipes, which are suitably used as line pipes constituting pipelines. [Background technology]

[0002] In recent years, Carbon dioxide Capture and Storage (CCS), a technology that captures CO2 released into the atmosphere and stores it underground, has attracted attention as a measure against global warming. In CCS, CO2 generated mainly at industrial facilities such as power plants and factories is captured. The captured CO2 is transported to storage sites via pipelines. This CO2 contains SO2, which is generated when coal and natural gas are burned. X NO X These substances contain sulfuric acid and nitric acid, which are strong acids when dissolved in water. Therefore, pipelines, which are the transport routes for CO2 emitted from industrial facilities, are exposed to a harsh corrosive environment.

[0003] Efforts are being made to improve the corrosion resistance of steel materials and steel pipes used as line pipes in such pipelines. For example, Patent Documents 1 and 2 disclose the addition of elements such as Cr and Cu to steel plates, etc. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2000-199029 [Patent Document 2] Patent No. 4499949 [Overview of the project] [Problems that the invention aims to solve]

[0005] Here, as an example of a steel pipe used as a line pipe, mention may be made of an electric resistance welded steel pipe in which one end side and the other end side of a base material are connected by electric resistance welding. In an electric resistance welded steel pipe, the electric resistance welded portion that connects the ends of the base material portions tends to be inferior in corrosion resistance to the base material portions. Therefore, even when an electric resistance welded steel pipe is manufactured using the steel materials described in Patent Documents 1 and 2, there remains room for improvement in the corrosion resistance of the electric resistance welded portion.

[0006] The present invention has been made in view of the above problems, and an object of the present invention is to provide an electric resistance welded steel pipe or the like excellent in corrosion resistance. [Means for Solving the Problem]

[0007] In order to solve the above problems, the present invention has the following features. [1] An electric resistance welded steel pipe comprising: a base material portion; and an electric resistance welded portion connecting one end of the base material portion and the other end of the base material portion, wherein the base material portion and the electric resistance welded portion contain at least one of Cr, Cu and Mo, An electric resistance welded steel pipe wherein the relationship between the concentrations of Cr, Cu and Mo in the base material portion and the concentrations of Cr, Cu and Mo in the electric resistance welded portion satisfies the following formula. [Cr] B +[Cu] B +[Mo] B ≧0.02 Provided that when the base material portion contains 0.02 mass% or more of Cr, it satisfies formula (1), when it contains 0.02 mass% or more of Cu, it satisfies formula (2), and when it contains 0.02 mass% or more of Mo, it satisfies formula (3). [Cr] W / [Cr] B ≦1.50 (1) [Cu] W / [Cu] B ≦1.50 (2) [Mo]w / [Mo] B ≦1.50 (3) Here, [Cr] B is the Cr concentration (mass%) in the base material portion. [Cr] WThis is the concentration (mass%) of Cr in the electric resistance welded area. [Cu] B This is the concentration (mass%) of Cu in the base material. [Cu] W This is the concentration (mass%) of Cu in the electric resistance welded portion. [Mo] B This is the concentration (mass%) of Mo in the base material. [Mo] W This is the concentration (mass%) of Mo in the electric resistance welded area. [2] The component composition of the aforementioned base material is, in mass%, C: 0.020% or more and 0.200% or less, Si: 0.50% or less, Mn: 0.50% or more and 2.00% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% to 0.100%, and N: including 0.0100% or less, moreover, Cr:5.00% or less, Cu: 2.00% or less, Mo: 2.00% or less It includes at least one of the following: The electric resistance welded steel pipe according to [1], wherein the remainder consists of Fe and unavoidable impurities. [3] The component composition of the aforementioned base material is further expressed in mass%, Nb: 0.080% or less, V: 0.080% or less, Ti: 0.080% or less, Ni: 2.00% or less, Ca: 0.0050% or less, B: 0.0050% or less, The electric resistance welded steel pipe described in [2], comprising at least one selected from among those with Sn: 0.100% or less. [4] The electric resistance welded steel pipe according to any one of [1] to [3], wherein the absolute value of the difference between the Vickers hardness at a depth of 1 mm from the inner surface of the electric resistance welded joint and the Vickers hardness at a depth of 1 mm from the inner surface of the base material is 100 HV or less. [5] A line pipe using electric resistance welded steel pipe as described in any of [1] to [4]. [6] A method for manufacturing an electric resistance welded (ERW) steel pipe, comprising: a hot-rolled steel sheet production step of heating and hot-rolling a steel material to produce a hot-rolled steel sheet; a pipe forming step of forming the hot-rolled steel sheet into an open pipe; and a welding step of electric resistance welding one end and the other end of the open pipe, In the aforementioned hot-rolled steel sheet production process, a steel material containing Cu, Cr, and Mo in a total amount of 0.02% by mass or more is heated and hot-rolled to form a hot-rolled steel sheet. A method for manufacturing electric resistance welded steel pipes, wherein the welding process is carried out under the conditions that the time for which the center of the part of the open pipe being welded in the thickness direction reaches 1000°C or higher is 1.0 second or less, the oxygen concentration at the part of the open pipe being welded is 0.10% by mass or less, and the amount of molten steel discharged is 0.2 times or more the thickness of the open pipe. [Effects of the Invention]

[0008] According to the electric resistance welded steel pipe of the present invention, the base material and the electric resistance welded portion contain at least one of Cr, Cu, and Mo. Furthermore, the concentrations of Cr, Cu, and Mo in the base material and the concentrations of Cr, Cu, and Mo in the electric resistance welded portion satisfy a predetermined relationship. This makes it possible to provide electric resistance welded steel pipes with excellent corrosion resistance. [Brief explanation of the drawing]

[0009] [Figure 1] This is an explanatory diagram showing the cross-sectional configuration of an electric resistance welded steel pipe. [Figure 2] This is a flow chart showing the manufacturing method for electric resistance welded steel pipes. [Figure 3] Figure 2 is an explanatory diagram showing the welding process. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 shows a cross-section of the electric resistance welded steel pipe 100 perpendicular to the axial direction. The electric resistance welded steel pipe 100 has a base material portion 10 and an electric resistance welded portion 20 connecting one end 11 of the base material portion 10 and the other end 12 of the base material portion 10. The electric resistance welded steel pipe 100 is not particularly limited, but in this embodiment it is formed in a cylindrical shape.

[0011] The base material portion 10 has a C-shaped cross-section perpendicular to the axial direction of the electric resistance welded steel pipe 100. The electric resistance welded portion 20 is formed by melting the base material portion 10 by heating, and is also called a bead. The electric resistance welded portion 20 is located in the circumferential direction of the electric resistance welded steel pipe 100 between one end 11 and the other end 12 of the base material portion 10. In the circumferential direction, a heat-affected zone 30 is located between the base material portion 10 and the electric resistance welded portion 20.

[0012] The base material 10 and the electric resistance welded portion 20 contain at least one of Cr, Cu, and Mo. In other words, the base material 10 and the electric resistance welded portion 20 may contain "0 mass%" of the other components as long as they contain one of these three components. Here, the concentration (mass%) of Cr in the base material 10 is [Cr] B The concentration of Cr (mass%) in the electric resistance welded joint 20 is defined as [Cr] W It shall be expressed as follows. Also, the concentration of Cu (mass%) in the base material 10 shall be expressed as [Cu] B The concentration of Cu (mass%) in the electric resistance welded joint 20 is defined as [Cu] W It shall be expressed as follows: The concentration of Mo (mass%) in the base material 10 is [Mo] B The concentration of Mo (mass%) in the electric resistance welded joint 20 is defined as [Mo] W It will be written as follows.

[0013] The relationship between the concentrations (mass%) of Cr, Cu, and Mo in the base material 10 and the concentrations (mass%) of Cr, Cu, and Mo in the electric resistance welded section 20 satisfies the following equation. Hereafter, "mass%" representing the concentrations of Cr, Cu, and Mo will be simply denoted as "%". [Cr] B +[Cu] B+[Mo] B ≥0.02

[0014] However, if the base material 10 contains 0.02% by mass or more of Cr, it satisfies equation (1); if it contains 0.02% by mass or more of Cu, it satisfies equation (2); and if it contains 0.02% by mass or more of Mo, it satisfies equation (3). [Cr] W / [Cr] B ≤1.50 (1) [Cu] W / [Cu] B ≤1.50 (2) [Mo]w / [Mo] B ≤1.50 (3)

[0015] The sum of the concentrations of Cr, Mo, and Cu in the base material 10 is preferably 0.05% or more, and more preferably 0.10% or more. If the sum of the concentrations of Cr, Mo, and Cu exceeds 7.00%, the ductility and toughness of the electric resistance welded joint 20 will decrease, and cracks will be more likely to occur in the electric resistance welded joint 20. Therefore, the sum of the concentrations of Cr, Mo, and Cu in the base material 10 is preferably 7.00% or less, and more preferably 6.00% or less.

[0016] Cr readily oxidizes in the atmosphere at high temperatures. Therefore, the electric resistance welded joint 20 contains Cr oxide. The concentration of Cr decreases around the Cr oxide, resulting in reduced corrosion resistance.

[0017] Here, "corrosion resistance" can be evaluated by the corrosion rate and pitting growth rate in the corrosion test described later. Corrosion resistance is evaluated as sufficient or good if the corrosion rate in the corrosion test is, for example, 0.100 mm / year or less and the pitting growth rate is 0.100 mm / year or less.

[0018] If Cr oxide remains in the electric resistance welded joint 20, the concentration of Cr in the electric resistance welded joint 20 will be higher than that in the base metal 10. As the concentration of Cr in the area where the Cr oxide has been removed becomes lower, the corrosion resistance of the electric resistance welded joint 20 will decrease.

[0019] If the base material 10 contains 0.02% or more Cr, the concentration of Cr in the electric resistance welded portion 20 relative to the base material 10 is, i.e., [Cr] W / [Cr] B It is 1.50 or less. [Cr] W / [Cr] B By keeping the value below 1.50, it becomes possible to ensure corrosion resistance in the electric resistance welded joint 20. [Cr] W / [Cr] B It is preferably 1.40 or less, and more preferably 1.30 or less.

[0020] If the Cr concentration in the electric resistance welded joint 20 becomes too low compared to the base material 10, the corrosion resistance of the electric resistance welded joint 20 may decrease. The electric resistance welded steel pipe 100 is [Cr] W / [Cr] B It is preferable that the value is 0.50 or higher. [Cr] W / [Cr] B It is more preferable that the value be 0.60 or higher, and even more preferable that it be 0.70 or higher.

[0021] Cu is less susceptible to oxidation at high temperatures than Fe. Therefore, in the electric resistance welded joint 20, Fe oxidizes preferentially, and Cu becomes concentrated. If the concentrated Cu remains in the electric resistance welded joint 20, the concentration of Cu in the electric resistance welded joint 20 becomes higher than that in the base material 10. The difference in Cu concentration between the base material 10 and the electric resistance welded joint 20 can form a local galvanic cell, which may reduce the corrosion resistance of the heat-affected zone 30.

[0022] If the base material 10 contains 0.02% or more of Cu, the concentration of Cu in the electric resistance welded portion 20 relative to the base material 10 is, i.e., [Cu] W / [Cu] B It is 1.50 or less. [Cu] W / [Cu] B By keeping the value below 1.50, it becomes possible to ensure corrosion resistance in the electric resistance welded joint 20.

[0023] [Cu] W / [Cu] BThe concentration is preferably 1.40 or less, and more preferably 1.30 or less. If the Cu concentration in the electric resistance welded joint 20 becomes too low compared to the base material 10, the corrosion resistance of the electric resistance welded joint 20 may decrease. [Cu] W / [Cu] B It is preferably 0.50 or higher, more preferably 0.60 or higher, and even more preferably 0.70 or higher.

[0024] Mo is an element that improves the corrosion resistance of steel. By making the total concentration of Cr, Cu, and Mo in the base material 10 0.02% or more, sufficient corrosion resistance can be obtained in the base material 10.

[0025] If the base material 10 contains 0.02% or more of Mo, [Mo] W / [Mo] B It is 1.50 or less, preferably 1.40 or less, and more preferably 1.30 or less. Also, [Mo] W / [Mo] B It is best to set it to 0.50 or higher, [Mo] W / [Mo] B It is preferable that the value be 0.60 or higher, and more preferably 0.70 or higher.

[0026] [Mo] W / [Mo] B By keeping the ratio below 1.50, the corrosion resistance of the heat-affected zone 30 can be ensured. Specifically, Mo in steel is less prone to oxidation at high temperatures compared to Fe. Therefore, in the electric resistance welded joint 20, Fe oxidizes preferentially, and Mo becomes concentrated. If the concentrated Mo remains in the electric resistance welded joint 20, the concentration of Mo in the electric resistance welded joint 20 becomes higher than that in the base metal 10. In such a state, a local galvanic cell may form due to the difference in Mo concentration between the base metal 10 and the electric resistance welded joint 20, which may reduce the corrosion resistance of the heat-affected zone 30. W / [Mo] B By setting this to 0.50 or higher, the corrosion resistance of the electric resistance welded joint 20 can be further improved.

[0027] [Cr]W [Cr] B [Cu] W [Cu] B [Mo] W [Mo] B This can be measured using the EPMA (Electron Probe Microanalyzer) method. The concentrations of the base material 10 and the electric resistance welded joint 20 can both be measured using a cross-section perpendicular to the axial direction of the electric resistance welded steel pipe 100 as the measurement surface. In the EPMA method, the beam diameter can be set to 1 μm.

[0028] Here, the concentration of the components in the base material 10 was measured at a position 90 degrees circumferentially away from the electric resistance weld 20. The concentration of the components in the electric resistance weld 20 was measured at its center.

[0029] More specifically, [Cr] B [Cu] B And, [Mo] B This can be determined by taking measurements at a total of nine points at 1 μm intervals in the circumferential direction, centered at a position 90 degrees circumferentially away from the electric resistance welded joint 20, at a depth of 1 mm from the inner surface, and averaging these values.

[0030] [Cr] W [Cu] W And, [Mo] W This can be determined by taking measurements at a total of nine points at 1 μm intervals in the circumferential direction, centered on the electric resistance welded joint 20, at a depth of 1 mm from the inner surface, and averaging these values.

[0031] The component composition of the base material 10 is, in mass%, C: 0.020% or more and 0.200% or less, Si: 0.50% or less, Mn: 0.50% or more and 2.00% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% to 0.100%, and N: 0.0100% or less, Includes, moreover, Cr:5.00% or less, Cu: 2.00% or less, Mo: 2.00% or less It includes at least one of the following: The remainder preferably consists of Fe and unavoidable impurities. In this specification, unless otherwise specified, "%" indicating steel composition refers to "mass%".

[0032] C: 0.020% or more and 0.200% or less Carbon (C) is an element that increases the strength of steel through solid solution strengthening. Furthermore, carbon also increases the strength of steel by refining the crystal grains by lowering the transformation initiation temperature. These effects can be achieved by the base material 10 containing 0.020% or more carbon.

[0033] If the base material 10 contains an excessive amount of carbon, an excessive amount of hard tissue will be generated in the electric resistance welded joint 20. As a result, the hardness of the electric resistance welded joint 20 will increase, and its ductility and toughness will decrease. In addition, due to the difference in material properties between the base material 10 and the electric resistance welded joint 20, a local galvanic cell may be formed, which may reduce the corrosion resistance of the heat-affected zone 30.

[0034] The carbon content in the base material 10 is preferably 0.200% or less, more preferably 0.180% or less, and more preferably 0.170% or less. The carbon content in the base material 10 is preferably 0.020% or more, more preferably 0.025% or more, and even more preferably 0.030% or more. The carbon content in the base material 10 is preferably 0.025% or more and 0.180% or less, and more preferably 0.030% or more and 0.170% or less.

[0035] Si:0.50% or less Si is an element that increases the strength of steel through solid solution strengthening. This effect can be obtained by the base material 10 containing 0.02% or more Si. If the base material 10 contains an excessive amount of Si, the hardness of the electric resistance welded joint 20 will increase. As a result, the ductility and toughness of the electric resistance welded joint 20 will decrease. In addition, a local galvanic cell may form due to the difference in material properties between the base material 10 and the electric resistance welded joint 20, which may reduce the corrosion resistance of the heat-affected zone 30.

[0036] The base material 10 preferably contains 0.50% or less of Si, more preferably 0.40% or less, and more preferably 0.30% or less. The base material 10 preferably contains 0.05% to 0.40% of Si, and more preferably 0.08% to 0.30%.

[0037] Mn: 0.50% or more and 2.00% or less Mn is an element that increases the strength of steel through solid solution strengthening. Furthermore, Mn also increases the strength of steel by refining the crystal grains by lowering the transformation initiation temperature. These effects can be achieved by the base material 10 containing 0.50% or more Mn.

[0038] If the base material 10 contains an excessive amount of Mn, an excessive amount of hard tissue will be generated in the electric resistance welded joint 20. As a result, the hardness of the electric resistance welded joint 20 will increase, and its ductility and toughness will decrease. In addition, due to the difference in material properties between the base material 10 and the electric resistance welded joint 20, a local galvanic cell may be formed, which may reduce the corrosion resistance of the heat-affected zone 30.

[0039] The base material 10 preferably contains 0.05% or more of Mn, more preferably 0.60% or more, and even more preferably 0.70% or more. Furthermore, the base material 10 preferably contains 2.00% or less of Mn, more preferably 1.90% or less, and even more preferably 1.80% or less. The base material 10 preferably contains 2.00% or less of Mn, more preferably 0.60% to 1.90%, and even more preferably 0.70% to 1.80%.

[0040] P:0.050% or less P is an element that segregates at grain boundaries and reduces toughness. P is considered an unavoidable impurity. The base material 10 should preferably contain P in the lowest possible amount, preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less. There is no particular lower limit for the P content, but excessive reduction will lead to increased smelting costs. For this reason, it is preferable that the P content be 0.001% or more.

[0041] S: 0.0200% or less S usually exists as MnS in steel. Before the base material 10 is formed, the steel is thinned in a hot rolling process. MnS adversely affects the ductility and toughness of the steel during the hot rolling process.

[0042] The base material 10 preferably has the lowest possible sulfur content. The base material 10 should preferably have an sulfur content of 0.0200% or less, more preferably 0.0100% or less, and more preferably 0.0050% or less. There is no specific lower limit for the sulfur content, but excessive reduction will lead to increased smelting costs. For this reason, it is preferable that the sulfur content be 0.0001% or more.

[0043] Al: 0.005% or more and 0.100% or less Al is an element that acts as a strong deoxidizing agent. This effect can be obtained by the base material 10 containing 0.005% or more Al. If the base material 10 contains an excessive amount of Al, the weldability deteriorates, and the amount of alumina-based inclusions increases, worsening the surface properties of the base material 10.

[0044] The base material 10 preferably contains 0.100% or less Al, more preferably 0.080% or less, and more preferably 0.070% or less. The base material 10 preferably contains 0.010% to 0.080% Al, and more preferably 0.015% to 0.070% Al.

[0045] N: 0.0100% or less N is an unavoidable impurity and is an element that reduces the ductility and toughness of the base material 10 by firmly fixing the movement of dislocations. It is preferable that the base material 10 contains N in the lowest possible amount. The N content is acceptable up to 0.0100%. The N content of the base material 10 should preferably be 0.0100% or less, preferably 0.0080% or less, and even more preferably 0.0060% or less.

[0046] Cr:5.00% or less Cr is an element that improves corrosion resistance. Furthermore, Cr contributes to microstructure refinement by lowering the transformation initiation temperature, thereby increasing the strength of the steel. These effects can be achieved by containing 0.01% or more Cr in the base material 10. However, excessive Cr content in the base material 10 reduces its ductility and toughness.

[0047] Furthermore, if the base material 10 contains an excessive amount of Cr, the hardness of the electric resistance welded joint 20 may increase, and the corrosion resistance of the heat-affected zone 30 may decrease. If the base material 10 contains an excessive amount of Cr, a large amount of Cr oxide may be generated in the electric resistance welded joint 20, and the corrosion resistance of the electric resistance welded joint 20 may decrease.

[0048] The base material 10 preferably contains 5.00% or less of Cr, more preferably 3.00% or less, and even more preferably 2.00% or less. The base material 10 preferably contains 0.02% or more of Cr, more preferably 0.05% or more. The base material 10 preferably contains 0.05% to 2.00% of Cr.

[0049] Cu:2.00% or less Cu is an element that improves corrosion resistance. Furthermore, Cu increases the strength of steel through solid solution strengthening. In addition, Cu contributes to microstructure refinement by lowering the transformation initiation temperature, further increasing the strength of steel.

[0050] The base material 10 can achieve these effects by containing 0.01% or more Cu. If the base material 10 contains an excessive amount of Cu, its ductility and toughness will decrease. Also, if the base material 10 contains an excessive amount of Cu, the hardness of the electric resistance welded joint 20 may increase, and the corrosion resistance of the heat-affected zone 30 may decrease. Furthermore, even if an amount of Cu exceeding the solid solubility limit is added to the base material 10, Cu precipitates will form, and the amount of solid solubility of Cu in the steel will not increase. Therefore, the corrosion resistance improvement effect relative to the amount added will not be obtained. The base material 10 preferably contains 2.00% or less Cu, more preferably 1.80% or less, and more preferably 1.60% or less. The base material 10 preferably contains 0.02% or more Cu, and more preferably 0.05% or more. The base material portion 10 preferably contains 0.02% to 1.80% Cu, and more preferably 0.05% to 1.60% Cu.

[0051] Mo: 2.00% or less Mo is an element that contributes to microstructure refinement by lowering the transformation initiation temperature, thereby increasing the strength of steel. These effects can be obtained by containing 0.01% or more of Mo in the base material 10. If the base material 10 contains an excessive amount of Mo, the ductility and toughness will decrease. Also, if the base material 10 contains an excessive amount of Mo, the hardness of the electric resistance welded joint 20 may increase, and the corrosion resistance of the heat-affected zone 30 may decrease. The base material 10 preferably contains 2.00% or less of Mo, more preferably 1.80% or less, and more preferably 1.60% or less. The base material 10 preferably contains 0.02% or more of Mo, more preferably 0.05% or more. The base material 10 preferably contains 0.02% to 1.80% of Mo, and more preferably 0.05% to 1.60%.

[0052] The base material portion 10 contains Fe and unavoidable impurities as the remainder. Hereinafter, the electric resistance welded steel pipe according to one embodiment of the present invention contains only the above components and the remainder, and the remainder may be Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes or manufacturing equipment, etc., and are permitted to be included in a range that does not hinder the purpose of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of unavoidable impurities in the remainder include Mg, Zr, REM, Sn, As, Sb, Bi, Co, Pb, Zn, O, Ta, W, Te, Hf, Ge, Sr, Cs, etc.Hereinafter, REM is a collective term for Sc, Y, and a total of 17 lanthanide elements.The base material portion 10 may contain at least one of these 17 elements as an unavoidable impurity.The REM content in the base material portion 10 is the total content of these elements.

[0053] The component composition of the base material 10 is further expressed in mass %, Nb: 0.080% or less, V: 0.080% or less, Ti: 0.080% or less, Ni: 2.00% or less, Ca: 0.0050% or less, B: 0.0050% or less, The base material may contain at least one element selected from Sn: 0.100% or less. Nb, V, Ni, Ca, B, and Sn are elements that can be optionally included in the base material 10. Therefore, the content of these components in the base material 10 may be "0% by mass".

[0054] Nb: 0.080% or less Nb contributes to improving the strength of steel by forming fine carbides and nitrides within the steel. Furthermore, Nb is an element that increases the strength of steel by suppressing the coarsening of the microstructure during hot rolling, thereby contributing to the refinement of the microstructure.

[0055] The base material 10 can achieve these effects by containing 0.002% or more of Nb. If the base material 10 contains an excessive amount of Nb, its ductility and toughness will decrease.

[0056] The base material 10 preferably contains 0.080% or less of Nb, more preferably 0.070% or less, and more preferably 0.060% or less. The base material 10 preferably contains 0.005% or more of Nb, more preferably 0.010% or more. The base material 10 preferably contains 0.005% to 0.070% of Nb, and more preferably 0.010% to 0.060%.

[0057] V:0.080% or less V is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. This effect can be obtained by the base material 10 containing 0.002% or more of V.

[0058] If the base material 10 contains an excessive amount of V, its ductility and toughness will decrease. The base material 10 preferably contains 0.080% or less of V, more preferably 0.070% or less, and more preferably 0.060% or less. The base material 10 preferably contains 0.005% or more of V, more preferably 0.010% or more. The base material 10 preferably contains 0.005% to 0.070% of V, and more preferably 0.010% to 0.060%.

[0059] Ti: 0.080% or less Ti is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. Furthermore, because Ti has a high affinity for N, it also contributes to reducing the amount of dissolved N in the steel. It is preferable that the base material 10 contains 0.002% or more Ti.

[0060] If the base material 10 contains an excessive amount of Ti, its ductility and toughness will decrease. When the base material 10 contains Ti, it is preferable that it contains 0.080% or less, more preferably 0.070% or less, and more preferably 0.060% or less. The base material 10 preferably contains 0.005% or more of Ti, and more preferably 0.010% or more. The base material 10 preferably contains 0.005% to 0.070% of Ti, and more preferably 0.010% to 0.060%.

[0061] Ni: 2.00% or less Ni is an element that increases the strength of steel through solid solution strengthening. Furthermore, Ni contributes to microstructure refinement by lowering the transformation onset temperature, thus increasing the strength of the steel. These effects can be obtained by containing 0.01% or more Ni in the base material 10. However, excessive Ni content in the base material 10 reduces its ductility and toughness. Additionally, excessive Ni content in the base material 10 may increase the hardness of the electric resistance welded joint 20 and reduce the corrosion resistance of the heat-affected zone 30. The base material 10 preferably contains 2.00% or less Ni, more preferably 1.80% or less, and more preferably 1.60% or less. The base material 10 preferably contains 0.02% or more Ni, more preferably 0.05% or more. The base material 10 preferably contains 0.02% to 1.80% Ni, and more preferably 0.05% to 1.60% Ni.

[0062] Ca:0.0050% or less Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS, which are thinned during the hot rolling process. These effects can be obtained by containing 0.0005% or more Ca in the base material 10. If the base material 10 contains an excessive amount of Ca, Ca oxide clusters are formed in the steel, and the toughness deteriorates. When the base material 10 contains Ca, it is preferable to contain 0.0050% or less, more preferably 0.0040% or less, and more preferably 0.0035% or less. The base material 10 preferably contains 0.0008% or more Ca, more preferably 0.0010% or more. The base material 10 preferably contains 0.0008% to 0.0040% Ca, and more preferably 0.0010% to 0.0035%.

[0063] B: 0.0050% or less B is an element that contributes to microstructure refinement by lowering the transformation initiation temperature, thereby increasing the strength of the steel. The base material 10 can achieve this effect by containing 0.0002% or more of B. If the base material 10 contains an excessive amount of B, its ductility and toughness will decrease. In addition, if the base material 10 contains an excessive amount of B, the hardness of the electric resistance welded joint 20 may increase, and the corrosion resistance of the heat-affected zone 30 may decrease. For this reason, when the base material 10 contains B, it is preferable to contain 0.0050% or less of B, more preferably 0.0005% to 0.0040%, and more preferably 0.0008% to 0.0030%.

[0064] Sn: 0.100% or less Sn is an element that suppresses decarburization caused by nitriding or oxidation of the steel surface, thereby suppressing a decrease in strength. The base material 10 can achieve this effect by containing 0.001% or more of Sn. If the base material 10 contains an excessive amount of Sn, its ductility and toughness will decrease. Therefore, if the base material 10 contains Sn, it is preferable that it contains 0.100% or less of Sn. Furthermore, it is preferable that the base material 10 contains 0.080% or less of Sn, and more preferably 0.060% or less. The base material 10 is good if it contains 0.001% or more of Sn, preferably 0.002% or more, and more preferably 0.005% or more.

[0065] Here, the Vickers hardness at a depth of 1 mm from the inner surface of the base material 10 is defined as the base material hardness HB, and the Vickers hardness at a depth of 1 mm from the inner surface of the electric resistance welded joint 20 is defined as the electric resistance welded joint hardness HW. The absolute value of the difference between the base material hardness HB and the electric resistance welded joint hardness HW is 100 HV or less, i.e., |HW - ​​HB| ≤ 100 HV, and preferably 80 HV or less.

[0066] If the difference between the hardness of the base material (HB) and the hardness of the electric resistance welded joint (HW) is large, a local galvanic cell may form due to the material difference between the base material 10 and the electric resistance welded joint 20, which may reduce the corrosion resistance of the heat-affected zone 30. By setting |HW-HB| to 100HV or less, the corrosion resistance of the heat-affected zone 30 can be made sufficient.

[0067] Furthermore, while a smaller |HW-HB| is preferable, excessive reduction leads to increased manufacturing costs and workload. Therefore, |HW-HB| is often 2HV or higher, and preferably 5HV or higher.

[0068] The hardness of the base material (HB) and the hardness of the electric resistance welded joint (HW) can be measured by the Vickers test. This test can be performed according to the method described in JIS Z 2244 (2020). In this test, the base material 10 and the electric resistance welded joint 20 should both be measured on a cross section perpendicular to the axial direction of the electric resistance welded steel pipe 100, and the test should be performed with a load of 1 kgf. The hardness of the base material (HB) and the hardness of the electric resistance welded joint (HW) can be determined, for example, by measuring the hardness at three points in the test area and averaging the values ​​obtained. The hardness of the base material (HB) can be measured at a position in the base material 10 90 degrees circumferentially from the electric resistance welded joint 20. The hardness of the electric resistance welded joint (HW) can be measured at the electric resistance welded joint 20.

[0069] Furthermore, the electric resistance welded steel pipe 100 preferably has a yield strength of 400 MPa or more, and more preferably 450 MPa or more. Having a yield strength of 400 MPa or more allows the electric resistance welded steel pipe 100 to have sufficient resistance to internal pressure.

[0070] If the yield strength of the electric resistance welded (ERW) steel pipe 100 is too high, its corrosion resistance may decrease. Therefore, the yield strength should ideally be 1100 MPa or less, and preferably 1000 MPa or less. The yield strength can be measured by a tensile test. The tensile test shall be conducted in accordance with the provisions of JIS Z 2241. The yield strength (MPa) shall be the flow stress at a nominal strain of 0.5%. A JIS No. 5 tensile test specimen shall be used for the tensile test. The specimen shall be taken from the base material 10 so that the tensile direction is parallel to the axial direction of the ERW steel pipe 100.

[0071] It is preferable that both the base material portion 10 and the electric resistance welded portion 20 of the electric resistance welded steel pipe 100 have a predetermined corrosion rate and pitting growth rate. The corrosion rate and pitting growth rate can be measured by the following corrosion test.

[0072] Test specimens are taken from the base material 10 and the electric resistance welded joint 20. The dimensions of the test specimens are 3 mm thick x 10 mm wide x 25 mm long. The width direction of the test specimen is parallel to the circumferential direction of the electric resistance welded steel pipe 100. The length direction of the test specimen is parallel to the axial direction of the electric resistance welded steel pipe 100. The test specimens are taken so that the center of their thickness is 2.5 mm deep from the inner surface. The test specimen from the electric resistance welded joint 20 is taken from the center of its width.

[0073] The test specimens used for the corrosion test are mirror-polished on all surfaces and then degreased with acetone. The corrosion test is conducted by exposing the test specimens to an environment heated and pressurized to 50°C and 10 MPa with a mixture of SO2, NO2, O2, H2O, and CO2. The test duration is 72 hours (3 days).

[0074] After testing, the test specimens are de-rusted in accordance with ASTM standard G1-03, and the corrosion rate is determined. The pitting corrosion growth rate (unit: mm / year) is calculated by measuring the maximum pitting corrosion depth (unit: mm) on the surface of the test specimen and multiplying that value by 365 / 3. The maximum pitting corrosion depth is determined by microscopy in accordance with ASTM standard G46-94.

[0075] The base material portion 10 and the electric resistance welded portion 20 have a corrosion rate of 0.100 mm / year or less in the above corrosion test, and preferably 0.090 mm / year or less. The base material portion 10 and the electric resistance welded portion 20 have a pitting growth rate of 0.100 mm / year or less in the above corrosion test, and preferably 0.090 mm / year or less.

[0076] Next, a method for manufacturing an electric resistance welded (ERW) steel pipe according to one embodiment of the present invention will be described. Figure 2 shows the manufacturing flow of an ERW steel pipe. As shown in Figure 2, the method for manufacturing an ERW steel pipe includes a hot-rolled steel sheet production step (step S01) in which a steel material is hot-rolled to produce a hot-rolled steel sheet, and a pipe forming step (step S02) in which the hot-rolled steel sheet is formed into an open pipe. The method for manufacturing an ERW steel pipe also includes a welding step (step S03) in which one end and the other end of the open pipe are welded using electric resistance welding.

[0077] Furthermore, the surface temperature of hot-rolled steel sheets can be measured using a radiation thermometer or similar device.

[0078] In the hot-rolled steel sheet production process of step S01, for example, a steel material having the above-described component composition is heated and hot-rolled. Specifically, a steel material containing a total amount of Cu, Cr, and Mo of 0.02% by mass or more is hot-rolled. After the hot-rolled steel material is cooled, it is wound into a coil to form a hot-rolled steel sheet.

[0079] Steel material is also called steel slab. The method of melting the steel material is not particularly limited, and known melting methods such as converters, electric furnaces, and vacuum melting furnaces may be used. The method of casting the steel material is also not particularly limited, but known casting methods such as continuous casting may be used. The steel material is manufactured to predetermined dimensions by casting. Alternatively, the ingot-forming rolling method may be applied instead of the continuous casting method. The molten steel may be further refined, such as ladle refining.

[0080] In the tube forming process of step S02, the tube is formed into a cylindrical shape, for example, by cold roll forming. The open tube formed in the tube forming process has a C-shaped cross-section perpendicular to the axial direction of the tube. That is, the open tube is formed with a part of it open in the circumferential direction. The opening of the open tube is formed along the axial direction of the tube.

[0081] The heating method for electric resistance welding performed in step S03 can be, for example, electric current heating, induction heating, etc. The welding process in step S03 is performed under the following conditions (Condition 1) to (Condition 3).

[0082] (Condition 1) The time during which the center of the thickness direction of the area where the open pipe is welded (hereinafter also referred to as the welded area) reaches 1000°C or higher is 1.0 second or less. (Condition 2) The oxygen concentration of the welded area shall be 0.10% by mass or less. (Condition 3) The amount of molten steel discharged must be at least 0.2 times the thickness of the open pipe plate.

[0083] Condition 1 can be met, for example, by adjusting at least one of the following: the pipe-making speed, the heating start position, and the spacing of the squeeze rolls.

[0084] If the temperature at the center of the welded portion in the thickness direction remains above 1000°C for an extended period, the amount of Cr oxide generated in the welded portion increases. As a result, Cr oxide remains in the electric resistance welded joint 20, increasing the concentration of Cr in the electric resistance welded joint 20, which may reduce the corrosion resistance of the electric resistance welded joint 20. Furthermore, if the temperature at the center of the welded portion in the thickness direction remains above 1000°C for an extended period, the amount of Cu and Mo concentrated in the welded portion increases. As a result, increasing the concentration of Cu and Mo in the electric resistance welded joint 20 may reduce the corrosion resistance of the heat-affected zone 30. Therefore, it is desirable that the time at which the temperature at the center of the welded portion in the thickness direction remains above 1000°C is 1.0 second or less, and preferably 0.8 seconds or less. The temperature at the center of the welded portion in the thickness direction can be measured using a two-color thermometer. An example of a suitable location for measuring the temperature at the center of the welded portion in the thickness direction is 20 mm away from the welding start point P1, in the opposite direction to the pipe-making direction D1.

[0085] Condition 2 can be addressed, for example, by adjusting the composition of the air in the atmosphere surrounding the workpiece. Specifically, by blowing nitrogen gas or argon gas onto the workpiece, the oxygen concentration in the surrounding atmosphere decreases. This reduces the oxygen concentration at the workpiece. To facilitate control of the oxygen concentration in the atmosphere, an enclosure may be provided around the workpiece. The oxygen concentration at the workpiece can be calculated by measuring the oxygen concentration of the atmosphere using an oxygen concentration meter and analyzing the measurement data.

[0086] High oxygen concentration in the welded area increases the amount of chromium oxide produced. Chromium oxide remains in the electric resistance welded (ERW) weld, increasing the chromium concentration and potentially reducing the corrosion resistance of the weld. Furthermore, high oxygen concentration in the welded area increases the amount of Fe oxidation, leading to increased concentration of Cu and Mo in the weld. This increases the Cu and Mo concentrations in the ERW weld, potentially reducing the corrosion resistance of the heat-affected zone. Therefore, the oxygen concentration in the welded area should be 0.10% by mass or less, preferably 0.08% by mass or less.

[0087] Condition 3 is met by adjusting the welding power, pipe-making speed, and squeeze roll spacing so that the amount of molten steel discharged is 0.2 times or more the thickness of the plate. Figure 3 shows an embodiment of the welding process. As shown in Figure 3, when electric resistance welding is started, molten steel 13 is formed. Electric resistance welding is performed from the welding start point P1 toward the pipe-making direction D1.

[0088] The amount of molten steel discharged is expressed in length (mm) by pre-setting the length per pixel (mm) when continuously photographing the weld area with a high-speed camera and calculating the length (mm) on the screen. The amount of molten steel discharged is determined by continuously photographing the weld area with a high-speed camera. Specifically, the molten steel width W1 is photographed with a high-speed camera for 1 second at a condition of 2000fps. The average of the values ​​of the 2000 captured points is taken as the amount of molten steel discharged. The amount of molten steel discharged can also be determined, for example, by installing a 2D image sensor camera equipped with a high-speed shutter above the weld point and capturing still images of the area around the weld point. Then, by binarizing the captured images, the molten steel portion is identified, and the amount corresponding to the molten steel portion is calculated.

[0089] The molten steel width W1 should preferably be the one measured at a distance L1 downstream from the welding start point P1 in the pipe-making direction. The welding start point P1 can be determined, for example, as follows: The butt end faces corresponding to the pipe edges of the open pipe are detected as a point cloud by image processing. Two straight lines corresponding to the butt end faces are calculated from the detected point cloud using the least squares method. The intersection point of the two calculated straight lines is found, and this intersection point can be used as the welding start point P1.

[0090] The molten steel width W1 can be measured as the distance from the axis AX of the electric resistance welded steel pipe 100 to the base metal portion 10. The distance L1 should ideally be twice the plate thickness. Measuring the molten steel width W1 at this position reduces variations in the molten steel width W1 and appropriately reflects the influence of welding conditions.

[0091] If the amount of molten steel discharged is small, Cr oxide may not be sufficiently discharged and may remain in the electric resistance welded joint 20, which may reduce the corrosion resistance of the electric resistance welded joint 20. Also, if the amount of molten steel discharged is small, concentrated Cu and Mo may not be sufficiently discharged and may remain in the electric resistance welded joint 20, which may reduce the corrosion resistance of the heat-affected zone 30. For this reason, the amount of molten steel discharged should be at least 0.2 times the thickness of the plate, and preferably at least 0.3 times.

[0092] If the amount of molten steel discharged is large, the welded area will be excessively melted, making it impossible to achieve sufficient upset. As a result, Cr oxide may not be sufficiently discharged and may remain in the electric resistance welded joint 20, potentially reducing the corrosion resistance of the electric resistance welded joint 20. In addition, concentrated Cu and Mo may not be sufficiently discharged and may remain in the electric resistance welded joint 20, potentially reducing the corrosion resistance of the heat-affected zone 30. For this reason, it is preferable to discharge molten steel at 3.0 times the thickness of the plate or less. More preferably, it should be 2.0 times the thickness of the plate or less.

[0093] After electric resistance welding (ERW), heat treatment can be applied to adjust the hardness of the ERW welded joint 20. In this case, the entire ERW steel pipe 100 may be heat-treated, or only the ERW welded joint 20 may be heat-treated. The heating method in the heat treatment is, for example, induction heating, but is not limited to this method, and a heating furnace may also be used.

[0094] Furthermore, whether a steel pipe is an electric resistance welded (ERW) steel pipe can be determined by measuring the width of the ERW weld 20 on the cut surface of the steel pipe cut in a direction perpendicular to the axial direction. For example, when the cut surface of the steel pipe is corroded, the ERW weld 20 can be seen as an area with a different microstructure and contrast from the base material 10 and the heat-affected zone 30.

[0095] Specifically, when the electric resistance welded joint 20 of electric resistance welded (ERW) steel pipes 100 made of carbon steel and low-alloy steel is corroded with Nital and observed under an optical microscope, it appears whiter than the base metal 10 and the heat-affected zone 30. UOE steel pipes are an example of electric resistance welded (ERW) steel pipes made of carbon steel and low-alloy steel. When the electric resistance welded joint 20 of a UOE steel pipe is corroded with Nital and observed under an optical microscope, a cellular or dendritic solidification structure is visible. Furthermore, the corrosion solution used for corroding steel pipes should be selected appropriately according to the steel composition and type of steel pipe.

[0096] If the width of the electric resistance welded joint 20 in the circumferential direction of the pipe, as observed in this manner, is between 1.0 μm and 1000 μm across the entire thickness of the pipe, it can be identified as an electric resistance welded steel pipe.

[0097] The electric resistance welded steel pipe 100 described above can be used, for example, as a line pipe to constitute a pipeline connecting one point to another. When a pipeline is composed of multiple line pipes, using the electric resistance welded steel pipe 100 described above in at least one line pipe makes it possible to improve the corrosion resistance of the location where the line pipe is installed. [Examples]

[0098] The present invention will be described in more detail below based on examples. However, the present invention is not limited to the following examples.

[0099] Molten steel having the component composition shown in Table 1 was produced, and a slab was obtained as the steel material. The obtained slab was heated and hot-rolled, cooled, and then wound into a coil to obtain a hot-rolled steel sheet with the thickness shown in Table 2 (indicated as plate thickness in Table 2) (mm). Subsequently, the hot-rolled steel sheet was formed into a cylindrical shape by cold roll forming, and electric resistance welded under the conditions shown in Table 2 to obtain an electric resistance welded steel pipe with the outer diameter (mm) and wall thickness (mm) shown in Table 2.

[0100] [Table 1]

[0101] [Table 2]

[0102] Test specimens were taken from the obtained electric resistance welded steel pipes, and the following elemental concentration measurements, tensile tests, Vickers tests, and corrosion tests were performed.

[0103] [molten steel emissions] The molten steel discharge volume (mm) is determined by continuously photographing the welded area with a high-speed camera. Specifically, the length per pixel (mm) is set in advance, and the length on the screen (mm) is calculated. The molten steel width W1 is photographed with a high-speed camera for 1 second at a rate of 2000fps. The average of the 2000 captured values ​​is used as the molten steel discharge volume.

[0104] The molten steel width W1 was measured at a distance L1 downstream from the welding start point P1 in the pipe-making direction. The welding start point P1 was set as follows: In the image of the open pipe, the butt end faces corresponding to the pipe edges were detected as a point cloud by image processing. Two straight lines corresponding to the butt end faces were calculated from the detected point cloud using the least squares method. The intersection point of the two calculated straight lines was found, and this intersection point was set as the welding start point P1. The molten steel width W1 was measured as the distance from the axis AX of the electric resistance welded steel pipe 100 to the base material portion 10. The distance L1 should be twice the plate thickness. The nominal plate thickness of the hot-rolled plate was used for the plate thickness.

[0105] [Element concentration measurement] The elemental concentrations of the base material and the electric resistance welded joint were measured using the EPMA method. For both the EPMA welded joint and the base material, the measurement surface was a cross section perpendicular to the pipe axis. In the EPMA measurement, the beam diameter was set to 1 μm.

[0106] [Cr] B [Cu] B And, [Mo] B The values ​​were measured at nine points at 1 μm intervals in the circumferential direction, centered at a position 90 degrees circumferentially away from the electric resistance weld 20, and at a depth of 1 mm from the inner surface. The average of these values ​​was then calculated. [Cr] W [Cu] W And, [Mo] W The value was obtained by taking measurements at a total of nine points at a depth of 1 mm from the inner surface of the electric resistance welded joint, with 1 μm intervals in the circumferential direction, and averaging these values.

[0107] [Tensile test] Tensile tests were conducted in accordance with the provisions of JIS Z 2241. The yield strength (MPa) was defined as the flow stress at a nominal strain of 0.5%. Test specimens conforming to JIS No. 5 were used for the tensile tests. These specimens were taken from the base material so that the tensile direction was parallel to the axial direction of the electric resistance welded steel pipe.

[0108] [Vickers test] Vickers hardness was measured using the Vickers hardness test. For both the electric resistance welded joint and the base metal, the measurement surface was a cross section perpendicular to the pipe axis direction, and the test was conducted at a depth of 1 mm from the inner surface with a load of 1 kgf according to the method described in JIS Z 2244 (2020). For both the electric resistance welded joint and the base metal, three test points were obtained, and the values ​​obtained were averaged to determine HW and HB, respectively.

[0109] [Corrosion Test] Test specimens were taken from the base material and the electric resistance welded section. The dimensions of the test specimens were 3 mm thick x 10 mm wide x 25 mm long. The width direction of the test specimen was parallel to the circumferential direction of the electric resistance welded steel pipe. The length direction of the test specimen was parallel to the axial direction of the electric resistance welded steel pipe. The test specimens were taken so that the center of their thickness was 2.5 mm deep from the inner surface. In addition, for the electric resistance welded section test specimens, the electric resistance weld was positioned at the center of the width.

[0110] The test specimens used for the corrosion test were mirror-polished on all surfaces and then degreased with acetone. The corrosion test was conducted by exposing the test specimens to an environment heated and pressurized to 50°C and 10 MPa with a gas mixture of SO2, NO2, O2, H2O, and CO2. The test duration was 72 hours (3 days). The gas used was CO2 containing 50 ppm SO2, 300 ppm NO2, 100 ppm O2, and 600 ppm H2O by volume fraction.

[0111] After the test, the specimens were de-rusted in accordance with ASTM standard G1-03, and the corrosion rate was determined. The pitting corrosion growth rate (unit: mm / year) was calculated by measuring the maximum pitting corrosion depth (unit: mm) on the surface of the specimen and multiplying that value by 365 / 3. The maximum pitting corrosion depth was determined by microscopy in accordance with ASTM standard G46-94. The obtained results are shown in Table 3.

[0112] [Table 3]

[0113] In Table 3, the electric resistance welded steel pipes of Nos. 1, 2, 8, 9, and 10 to 19 are examples of the present invention, and the electric resistance welded steel pipes of Nos. 3, 4, 5, 6, and 7 are comparative examples.

[0114] All of the electric resistance welded steel pipes according to examples of the present invention satisfy [Cr] B +[Mo] B +[Cu] B ≧0.02, when [Cr] B ≧0.20, [Cr] W / [Cr] B ≦1.50, when [Mo] B ≧0.20, [Mo] W / [Mo] B ≦1.50, when [Cu] B ≧0.20, [Cu] W / [Cu] B ≦1.50.

[0115] In the electric resistance welded steel pipe of No. 3 as a comparative example, [Cr] B +[Mo] B +[Cu] B was below the scope of the present invention, so desired corrosion resistance could not be obtained.

[0116] In the electric resistance welded steel pipe of No. 4 as a comparative example, the time period during which the temperature at the central portion in the thickness direction of the welded part when electric resistance welding was performed was 1000°C or more was long, so [Mo] W / [Mo] B and [Cu] W / [Cu] B exceeded 1.50. As a result, desired corrosion resistance could not be obtained.

[0117] In the electric resistance welded steel pipe of No. 5 as a comparative example, the oxygen concentration in the welded part during electric resistance welding was high, so [Cr] W / [Cr] B and [Cu] W / [Cu] B exceeded 1.50. As a result, desired corrosion resistance could not be obtained.

[0118] In the electric resistance welded steel pipe of No. 6 as a comparative example, the discharge amount of molten steel during electric resistance welding was large, so [Mo] W / [Mo]B And, [Cu] W / [Cu] B The value exceeded 1.50. As a result, the desired corrosion resistance was not achieved.

[0119] In comparative example No. 7, the electric resistance welded steel pipe had a small amount of molten steel discharged during electric resistance welding, therefore [Cu] W / [Cu] B The value exceeded 1.50. As a result, the desired corrosion resistance was not achieved. [Explanation of Symbols]

[0120] 100 ERW steel pipe 10 Base material part 20 Electric Resistance Welded Sections 30 Heat-affected zone

Claims

1. An electric resistance welded steel pipe having a base material portion and an electric resistance welded portion connecting one end of the base material portion and the other end of the base material portion, The base material portion and the electric resistance welded portion contain at least one of Cr, Cu, and Mo. The relationship between the concentrations of Cr, Cu, and Mo in the base material and the concentrations of Cr, Cu, and Mo in the electric resistance welded portion satisfies the following equation: The component composition of the aforementioned base material is, in mass%, C: 0.020% or more and 0.200% or less, Si: 0.50% or less, Mn: 0.50% or more and 2.00% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less Includes, moreover, Cr: 5.00% or less, Cu: 2.00% or less, Mo: 2.00% or less It includes at least one of the following: Electric resistance welded steel pipe, the remainder consisting of Fe and unavoidable impurities. [Cre Ｂ +[Cu] Ｂ +[[] Ｂ ≧0.02 However, if the base material contains 0.02% by mass or more of Cr, it satisfies formula (1); if it contains 0.02% by mass or more of Cu, it satisfies formula (2); and if it contains 0.02% by mass or more of Mo, it satisfies formula (3). [Cr] Ｗ / [Cr] Ｂ ≦1.50 (1) [C] Ｗ / [C] Ｂ ≦1.50 (2) [Mo] Ｗ / [Mo] Ｂ ≦150 (3) In addition, [Cr] Ｂ This is the concentration (mass%) of Cr in the base material. [Cr] Ｗ This is the concentration (mass%) of Cr in the electric resistance welded portion. [Cu] Ｂ This is the concentration (mass%) of Cu in the base material. [Cu] Ｗ This is the concentration (mass%) of Cu in the electric resistance welded portion. [Mo] Ｂ This is the concentration (mass%) of Mo in the base material. [Mo] Ｗ This is the concentration (mass%) of Mo in the electric resistance welded area.

2. The component composition of the aforementioned base material is further expressed in mass%, Nb: 0.080% or less, V: 0.080% or less, Ti: 0.080% or less, Ni: 2.00% or less, Ca: 0.0050% or less, B: 0.0050% or less, The electric resistance welded steel pipe according to claim 1, comprising at least one selected from among those with Sn: 0.100% or less.

3. The electric resistance welded steel pipe according to claim 1 or 2, wherein the absolute value of the difference between the Vickers hardness at a depth of 1 mm from the inner surface of the electric resistance welded portion and the Vickers hardness at a depth of 1 mm from the inner surface of the base material portion is 100 HV or less.

4. A line pipe using electric resistance welded steel pipe according to claim 1 or 2.

5. A line pipe using electric resistance welded steel pipe as described in Claim 3.

6. A method for manufacturing an electric resistance welded (ERW) steel pipe, comprising: a hot-rolled steel sheet production step of heating and hot-rolling a steel material to produce a hot-rolled steel sheet; a pipe forming step of forming the hot-rolled steel sheet into an open pipe; and a welding step of electric resistance welding one end and the other end of the open pipe, In the hot-rolled steel sheet production process, the total amount of Cu, Cr, and Mo is 0.02% by mass or more, The component composition is expressed in mass percent. C: 0.020% or more and 0.200% or less, Si: 0.50% or less, Mn: 0.50% or more and 2.00% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less Includes, moreover, Cr: 5.00% or less, Cu: 2.00% or less, Mo: 2.00% or less It includes at least one of the following: A steel material consisting of Fe and unavoidable impurities is heated and hot-rolled to form a hot-rolled steel sheet. A method for manufacturing electric resistance welded steel pipes, wherein the welding process is carried out under the conditions that the time for which the center of the part of the open pipe being welded in the thickness direction reaches 1000°C or higher is 1.0 second or less, the oxygen concentration at the part of the open pipe being welded is 0.10% by mass or less, and the amount of molten steel discharged is 0.2 times or more the thickness of the open pipe.

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