Electric resistance welded steel pipes, piping, and methods for manufacturing electric resistance welded steel pipes.

Abstract

To provide electric resistance welded steel pipes with excellent resistance to groove corrosion. [Solution] An electric resistance welded steel pipe having a base material portion and an electric resistance welded portion, The difference in hardness between the base material and the electric resistance welded portion is 60 HV or less. The hardness gradient in the circumferential direction of the electric resistance welded joint is 30 HV / mm or less. An electric resistance welded steel pipe in which the KAM value gradient in the circumferential direction of the electric resistance welded joint is 4.0° / μm or less.

Description

Technical Field

[0001] The present invention relates to electric welded steel pipes, and particularly to electric welded steel pipes that can be suitably used for pipes such as water supply pipes, industrial water pipes, and seawater pipes. Further, the present invention relates to a pipe using the above electric welded steel pipe and a method for manufacturing the above electric welded steel pipe.

Background Art

[0002] Pipes used for transporting water supply, industrial water, seawater, etc. are in contact with water, so when the material is a steel pipe, corrosion is likely to occur on the inner surface of the pipe. In particular, in electric welded steel pipes, V-shaped selective corrosion called groove corrosion occurs at the electric weld joint, and as a result, the internal fluid may leak.

[0003] Therefore, various methods have been proposed to improve the groove corrosion resistance of electric welded steel pipes.

[0004] For example, in Patent Document 1, an electric welded steel pipe has been proposed in which the form of inclusions at the electric weld joint is controlled to improve the groove corrosion resistance.

[0005] Also, in Patent Document 2, an electric welded steel pipe has been proposed in which the aspect ratio of crystal grains at the electric weld joint is controlled to improve the groove corrosion resistance.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, in conventional electric welded steel pipes as proposed in Patent Documents 1 and 2, sufficient groove corrosion resistance may not be obtained.

[0008] The present invention has been made in view of the above circumstances and aims to provide an electric resistance welded steel pipe with excellent resistance to groove corrosion, and piping using the electric resistance welded steel pipe.

[0009] In this invention, "excellent resistance to groove corrosion" means that, in the groove corrosion test described later, when the amount of wall thinning in the electric resistance welded section is h1 (mm) and the amount of wall thinning in the base material section is h2 (mm), (h1 / h2) is 1.50 or less, and the ratio of the depth (mm) to the width (mm) of the groove corrosion section (depth / width) is 1.00 or less. [Means for solving the problem]

[0010] As a result of diligent research to solve the above problems, the inventors obtained the following findings regarding the shape of groove-shaped corrosion areas and groove-shaped corrosion properties.

[0011] (1) The greater the (depth / width) of the grooved corrosion area that occurs in the grooved corrosion test, that is, the sharper the shape of the grooved corrosion area, the more Fe is produced in the grooved corrosion area by anodic dissolution. 2+ Cl tends to accumulate. Therefore, in order to maintain electrical neutrality in the groove-shaped corrosion area, - The molecules migrate toward the groove-shaped corrosion area, and the Cl of the groove-shaped corrosion area - The concentration increases. - As the concentration increases, the oxide film on the surface is destroyed, resulting in an acceleration of groove corrosion.

[0012] (2) Groove corrosion occurs when localized galvanic cells are formed due to the difference in material properties between the base material and the electric resistance welded joint. This difference in material properties can be expressed as a difference in hardness. The electric resistance welded joint tends to be harder than the base material because it is work-hardened by upsetting (pressing the joint surfaces together) during welding and is rapidly cooled after welding. In the electric resistance welded joint, the smaller the change in hardness per unit length in the circumferential direction of the pipe (hardness gradient), the smaller the (depth / width) of the groove corrosion area becomes, and the improved resistance to groove corrosion.

[0013] The present invention has been completed based on the above findings, and its main configuration is as follows.

[0014] 1. An electric resistance welded steel pipe having a base material portion and an electric resistance welded portion, where the hardness difference between the base material portion and the electric resistance welded portion is 60 HV or less, the hardness gradient in the circumferential direction of the pipe in the electric resistance welded portion is 30 HV / mm or less, and the KAM value gradient in the circumferential direction of the pipe in the electric resistance welded portion is 4.0° / μm or less.

[0015] 2. The base material portion contains, in mass%, C: 0.020 to 0.200%, Si: 0.40% or less, Mn: 0.20 to 2.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, and N: 0.0100% or less, the balance consists of Fe and inevitable impurities, and the electric resistance welded steel pipe according to 1 above, having a component composition in which Ceq represented by the following formula (1) is 0.10 to 0.40%. Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 4…(1) Here, the element symbols in the above formula (1) represent the content (mass%) of each element in the component composition of the base material portion, and when the element is not contained, it is taken as zero.

[0016] 3. The component composition of the base material portion contains, in mass%, Nb: 0.080% or less, V: 0.080% or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, and The electric resistance welded steel pipe according to 2 above, further containing at least one selected from the group consisting of Sn: 0.100% or less.

[0017] 4. A pipe using the electric resistance welded steel pipe according to any one of 1 to 3 above.

[0018] 5. A method for manufacturing an electric resistance welded steel pipe according to any one of 1 to 3 above, wherein a hot-rolled steel sheet with a thickness t (mm) is formed into a tubular body, and both circumferential ends of the tubular body are butt-welded by electric resistance welding to obtain an electric resistance welded steel pipe, During the electric resistance welding, the temperature gradients in the pipe axis direction in the regions at a depth of 1 mm from the outer surface, at the 1 / 2t position, and at a depth of 1 mm from the inner surface of the butt joint surface are each set to (50 / t) °C / mm or more in the temperature range of 900 to 1300 °C, The width of the bead on the outer surface of the pipe is set to 0.20t to 0.80t (mm), and The upset amount is set to 0.20t to t (mm). A method for manufacturing an electric resistance welded steel pipe.

Effect of the Invention

[0019] According to the present invention, it is possible to provide an electric resistance welded steel pipe excellent in groove corrosion resistance and a pipe using the electric resistance welded steel pipe.

Brief Description of the Drawings

[0020] [Figure 1] It is a schematic diagram showing the amount of wall thickness reduction h1 of the electric resistance welded part and the amount of wall thickness reduction h2 of the base material part in the groove corrosion test. [Figure 2] It is a schematic diagram showing the depth and width of the groove corrosion part. [Figure 3] It is a diagram showing the electric resistance welding process of butting the end in the width direction of the hot-rolled steel sheet and the other end in the width direction for electric resistance welding. [Figure 4] It is a schematic diagram of an open pipe in electric resistance welding. [Figure 5] It is a diagram explaining a method for obtaining the temperature gradient in the pipe axis direction at the center of the plate thickness of the butt joint surface in electric resistance welding. [Figure 6]This diagram illustrates the method for measuring the bead width of an electric resistance welded joint. [Figure 7] This diagram illustrates the spacing of the fin pass rolls and the angle of the butt joint in electric resistance welding. [Figure 8] This is a schematic diagram of a cross-section parallel to the circumferential direction of the pipe (a cross-section perpendicular to the pipe axis direction) including the electric resistance welded joint. [Figure 9] This diagram illustrates a method for measuring the hardness gradient in the circumferential direction of a pipe in an electric resistance welded joint. [Modes for carrying out the invention]

[0021] The present invention will be described in detail below. The following description is an example of a preferred embodiment of the present invention, and the present invention is not limited to the embodiments described below.

[0022] [ERW steel pipe] An electric resistance welded (ERW) steel pipe in one embodiment of the present invention has a base material portion and an ERW welded portion, and (1) the difference in Vickers hardness between the base material portion and the ERW welded portion, (2) the Vickers hardness gradient of the ERW welded portion, and (3) the KAM value gradient of the ERW welded portion each satisfy specific conditions. Each of these conditions will be described in detail below.

[0023] • Hardness difference: 60HV or less Groove corrosion occurs when localized galvanic cells are formed due to the difference in material properties between the base material and the electric resistance welded joint. This difference in material properties can be expressed as a difference in hardness. If the difference in hardness between the base material and the electric resistance welded joint exceeds 60 HV, the difference in material properties between the base material and the electric resistance welded joint becomes large, and sufficient resistance to groove corrosion cannot be obtained. Therefore, the difference in hardness should be 60 HV or less, preferably 50 HV or less, and more preferably 40 HV or less. On the other hand, from the viewpoint of resistance to groove corrosion, the smaller the difference in hardness, the better, so the lower limit of the difference in hardness is not particularly limited and may be 0 HV. However, from the viewpoint of ease of industrial manufacture, the difference in hardness may be 3 HV or more, 5 HV or more, or 10 HV or more.

[0024] The difference in hardness is determined by a Vickers test. Specifically, the Vickers hardness of the base material and the electric resistance welded joint are measured using a Vickers test, and the difference between the two is calculated. The Vickers test is performed on both the electric resistance welded joint and the base material with a cross section perpendicular to the pipe axis as the measurement surface, using the method described in JIS Z 2244 (2020) with a load of 1 kgf. For the electric resistance welded joint, measurements are taken at 1 mm intervals from the outer surface to the inner surface in the thickness direction, and the average value of these measurements is taken as the Vickers hardness of the electric resistance welded joint. For the base material, measurements are taken at 90° circumferentially from the electric resistance welded joint, at 1 mm intervals from the outer surface to the inner surface in the thickness direction, and the average value of these measurements is taken as the Vickers hardness of the base material.

[0025] • Hardness gradient: 30HV / mm or less The smaller the hardness gradient, i.e., the change in hardness per unit length (1 mm) in the circumferential direction of the pipe in the electric resistance welded joint, the smaller the material difference per unit length in the circumferential direction of the pipe. As a result, groove corrosion is less likely to occur, and even if groove corrosion occurs, the width of the groove corrosion area will be larger. If the hardness gradient exceeds 30 HV / mm, sufficient resistance to groove corrosion cannot be obtained. Therefore, the hardness gradient should be 30 HV / mm or less, preferably 25 HV / mm or less, and more preferably 20 HV / mm or less. On the other hand, from the viewpoint of resistance to groove corrosion, the smaller the hardness gradient, the better, so the lower limit of the hardness gradient is not particularly limited and may be 0 HV / mm. However, from the viewpoint of ease of industrial manufacture, the hardness gradient may be 5 HV / mm or more, and may be 7 HV / mm or more.

[0026] The circumferential hardness gradient in an electric resistance welded (ERW) joint is determined by a Vickers test. Specifically, as shown in Figure 9, the Vickers hardness is measured at a depth of 1 mm from the outer surface, at the center of the wall thickness, and at a depth of 1 mm from the inner surface of the ERW weld. In the measurement, for each of the three positions, the Vickers hardness is measured at 41 points at 1 mm intervals within a range of 20 mm to the left and right in the circumferential direction centered on the ERW weld. Next, for each of the 41 points, the difference in Vickers hardness between adjacent measurement points (i.e., the change in hardness per 1 mm) is determined. The maximum value of the 120 values ​​obtained in this way is defined as the circumferential hardness gradient in the ERW weld. The Vickers test is performed with a load of 1 kgf using the method described in JIS Z 2244 (2020), with the measurement surface being a cross section perpendicular to the axial direction of the pipe including the ERW weld.

[0027] • KAM value gradient: 4.0° / μm or less The KAM (Kernel Average Misorientation) value represents the local orientation difference. A higher KAM value indicates a higher dislocation density and greater hardness at the measurement point. A larger KAM gradient results in a larger hardness gradient in minute regions within the crystal grains, which can become the starting point for groove corrosion. If the KAM gradient in an electric resistance weld exceeds 4.0° / μm, sufficient resistance to groove corrosion cannot be obtained. Therefore, the KAM gradient should be 4.0° / μm or less, preferably 3.5° / μm or less, and more preferably 3.0° / μm or less. On the other hand, from the viewpoint of resistance to groove corrosion, a smaller KAM gradient is preferable, so the lower limit of the KAM gradient is not particularly limited and may be 0° / μm. However, from the viewpoint of ease of industrial manufacture, the hardness gradient may be 1° / μm or more, or even 2° / μm or more.

[0028] The KAM value in the electric resistance welded joint is measured using the SEM / EBSD method. Specifically, the measurement surface is a cross section perpendicular to the pipe axis direction including the electric resistance welded joint, and the KAM value is measured at a depth of 1 mm from the outer surface, at the center of the wall thickness, and at a depth of 1 mm from the inner surface of the electric resistance welded joint. The measurement is performed for five fields of view, with each field of view (measurement area) having a size of 100 μm in the wall thickness direction and 400 μm in the circumferential direction. The measurement step size is 1 μm.

[0029] Based on the obtained EBSD data, a distribution map of KAM values ​​(KAM map) is obtained using the crystal orientation analysis software OIM Analysis (trademark). Here, the KAM values ​​are determined by the following method: At each measurement point (regular hexagonal pixel), the orientation difference between each pixel is calculated using the three adjacent pixels (37 pixels in total) centered on the measurement point, and the average of the calculated orientation differences is taken as the KAM value of the central pixel. This operation is performed for all pixels in the field of view to obtain a KAM map. A line profile of KAM values ​​with a length of 400 μm in the horizontal direction is created at the vertical center of the obtained KAM map and at positions 20 μm above and below it. In each obtained line profile, the difference in KAM values ​​between all adjacent measurement points is calculated, and the largest value among them is taken as the KAM value gradient in the circumferential direction of the tube. However, if the KAM value of any or both adjacent measurement points is 5° or more, the grain boundary is included between the measurement points, and this is excluded from the calculation of the KAM value gradient.

[0030] [Component composition of the base material] The component composition of the base material is not particularly limited and may be any component composition. However, if the base material is C: 0.020~0.200%, Si: 0.40% or less, Mn: 0.20~2.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.005~0.100%, and N: Contains 0.0100% or less. The remainder consists of Fe and unavoidable impurities, It is preferable to have a component composition in which the Ceq represented by the following formula (1) is 0.10 to 0.40%. The reason for this is explained below.

[0031] C: 0.020~0.200% 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 onset temperature. To obtain the above effects, the carbon content is preferably 0.020% or more, more preferably 0.025% or more, and even more preferably 0.030% or more. On the other hand, if the carbon content is 0.200% or less, the formation of a hard structure in the electric resistance welded joint can be suppressed. As a result, the hardness gradient and KAM value gradient can be further reduced, and groove corrosion resistance can be further improved. Therefore, the carbon content is preferably 0.200% or less, more preferably 0.180% or less, and even more preferably 0.170% or less.

[0032] Si:0.40% or less Si is an element that increases the strength of steel through solid solution strengthening. However, if it is present in excessive amounts, a large amount of Si-based oxides are generated in the electric resistance welded joint, which becomes the starting point for groove corrosion, thus reducing groove corrosion resistance. Therefore, in order to further improve groove corrosion resistance, it is preferable to have a Si content of 0.40% or less, more preferably 0.35% or less, and even more preferably 0.30% or less. On the other hand, from the viewpoint of enhancing the strength-improving effect, it is preferable to have a Si content of 0.02% or more, more preferably 0.05% or more, and even more preferably 0.08% or more.

[0033] Mn: 0.20~2.00% 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 onset temperature. To obtain the above effects, the Mn content is preferably 0.20% or more, more preferably 0.25% or more, and even more preferably 0.30% or more. On the other hand, if the Mn content is 2.00% or less, the formation of a hard structure in the electric resistance welded joint can be suppressed. As a result, the hardness gradient and KAM value gradient can be further reduced, and groove corrosion resistance can be further improved. Therefore, the Mn content is preferably 2.00% or less, more preferably 1.80% or less, and even more preferably 1.60% or less.

[0034] P:0.050% or less P is an element that is inevitably present in steel as an impurity. Since P segregates at grain boundaries and reduces toughness, it is preferable to reduce it as much as possible. For this reason, the P content is preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less. On the other hand, there is no lower limit to the P content, but excessive reduction leads to a surge in smelting costs, so it is preferable that the P content be 0.001% or more.

[0035] S: 0.0100% or less S usually exists in steel as MnS, and a region with a high concentration of solid-solution S forms around the MnS. Because this region with a high S concentration has a low potential, it becomes the starting point for groove corrosion. To further improve resistance to groove corrosion, it is desirable to reduce the S content as much as possible. Specifically, it is preferable to have an S content of 0.0100% or less, more preferably 0.0080% or less, and even more preferably 0.0050% or less. On the other hand, there is no particular lower limit to the S content, but excessive reduction will lead to a surge in smelting costs, so it is preferable to have an S content of 0.0001% or more.

[0036] Al: 0.005~0.100% Al is an element that acts as a strong deoxidizing agent. To obtain the above effect, the Al content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more. On the other hand, if it is included in excess, the weldability deteriorates, the amount of alumina-based inclusions increases, and the surface properties deteriorate. For this reason, the Al content is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.070% or less.

[0037] N: 0.0100% or less N is an element that is inevitably contained in steel as an impurity, and it has the effect of reducing ductility and toughness by firmly fixing the movement of dislocations. It is desirable to reduce the N content as much as possible, specifically, it is preferable to have an N content of 0.0100% or less, more preferably 0.0080% or less, and even more preferably 0.0060% or less. On the other hand, there is no particular lower limit to the N content, but excessive reduction will lead to a surge in smelting costs, so it is preferable to have an N content of 0.0010% or more.

[0038] The component composition of the base material in one embodiment of the present invention may contain the above components, with the remainder being Fe and unavoidable impurities. Examples of such unavoidable impurities include Mg, Zr, REM, As, Sb, Bi, Co, Pb, Zn, O, Ta, W, Te, Hf, Ge, Sr, and Cs. Here, REM is a collective term for 17 elements in total, including Sc, Y, and lanthanide elements. One or more of these 17 elements can be included as unavoidable impurities, and the REM content refers to the total content of these elements.

[0039] The component composition of the above-mentioned base material may further optionally contain at least one of the following components.

[0040] Nb: 0.080% or less Nb is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. Furthermore, Nb contributes to refining the microstructure by suppressing coarsening during hot rolling, thereby increasing the strength of the steel. However, excessive Nb content increases the hardness of the electric resistance welded joint, leading to a larger hardness gradient and KAM value gradient, and a decrease in groove corrosion resistance. Therefore, the Nb content is preferably 0.080% or less, more preferably 0.070% or less, and even more preferably 0.060% or less. On the other hand, there is no particular lower limit to the Nb content, but when Nb is added, in order to obtain the above effects, the Nb content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

[0041] 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. However, if V is present in excess, the hardness of the electric resistance welded joint increases, and the hardness gradient and KAM value gradient become larger, thus reducing resistance to groove corrosion. For this reason, the V content is preferably 0.080% or less, more preferably 0.070% or less, and even more preferably 0.060% or less. On the other hand, there is no particular lower limit to the V content, but when V is added, in order to obtain the above effect, the V content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

[0042] 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. However, excessive Ti content increases the hardness of the electric resistance welded joint, leading to a larger hardness gradient and KAM value gradient, thus reducing resistance to groove corrosion. Therefore, the Ti content is preferably 0.080% or less, more preferably 0.070% or less, and even more preferably 0.060% or less. On the other hand, there is no particular lower limit to the Ti content, but when adding Ti, to obtain the above effects, the Ti content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

[0043] Cu: 0.50% or less Cu is an element that increases the strength of steel through solid solution strengthening. Furthermore, Cu contributes to microstructure refinement by lowering the transformation initiation temperature, thus increasing the strength of the steel. However, excessive Cu content leads to the formation of a hard microstructure in the electric resistance welded joint, increasing its hardness. As a result, the hardness gradient and KAM value gradient increase, reducing resistance to groove corrosion. Therefore, the Cu content is preferably 0.50% or less, more preferably 0.45% or less, and even more preferably 0.40% or less. On the other hand, there is no particular lower limit to the Cu content, but when adding Cu, to obtain the aforementioned effects, the Cu content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0044] Ni: 0.50% 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. However, excessive Ni content leads to the formation of a hard microstructure in the electric resistance welded joint, increasing its hardness. As a result, the hardness gradient and KAM value gradient increase, reducing resistance to groove corrosion. Therefore, the Ni content is preferably 0.50% or less, more preferably 0.40% or less, and even more preferably 0.30% or less. On the other hand, there is no particular lower limit to the Ni content, but when Ni is added, to obtain the aforementioned effects, the Ni content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0045] Cr:0.50% or less Cr is an element that contributes to microstructure refinement by lowering the transformation initiation temperature, thereby increasing the strength of steel. However, excessive Cr content leads to the formation of a hard microstructure in the electric resistance welded joint, increasing its hardness. As a result, the hardness gradient and KAM value gradient increase, reducing resistance to groove corrosion. Therefore, the Cr content is preferably 0.50% or less, more preferably 0.40% or less, and even more preferably 0.30% or less. On the other hand, there is no particular lower limit to the Cr content, but when Cr is added, it is preferable to have a Cr content of 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more in order to obtain the above effects.

[0046] Mo: 0.50% or less Mo is an element that contributes to microstructure refinement by lowering the transformation initiation temperature, thereby increasing the strength of steel. However, excessive Mo content leads to the formation of a hard microstructure in the electric resistance welded joint, increasing the hardness of the welded joint. As a result, the hardness gradient and KAM value gradient increase, reducing resistance to groove corrosion. Therefore, the Mo content is preferably 0.50% or less, more preferably 0.40% or less, and even more preferably 0.30% or less. On the other hand, there is no particular lower limit to the Mo content, but when Mo is added, in order to obtain the above effect, the Mo content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0047] Ca: 0.0050% or less Ca is an element that contributes to further improvement of groove corrosion resistance by refining MnS and suppressing its formation. However, if Ca is present in excess, a large amount of Ca-based oxides are generated, which become the starting point for groove corrosion, thus reducing groove corrosion resistance. For this reason, the Ca content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0035% or less. On the other hand, there is no particular lower limit to the Ca content, but when Ca is added, in order to obtain the above effect, the Ca content is preferably 0.0002% or more, more preferably 0.0005% or more, and even more preferably 0.0008% or more.

[0048] 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 steel. However, if B is present in excess, a hard microstructure is formed in the electric resistance welded joint, increasing the hardness of the welded joint. As a result, the hardness gradient and KAM value gradient become larger, and groove corrosion resistance decreases. Therefore, the B content is preferably 0.0050% or less, more preferably 0.0040% or less, and more preferably 0.0030% or less. On the other hand, there is no particular lower limit to the B content, but when B is added, in order to obtain the above effect, the B content is preferably 0.0002% or more, more preferably 0.0005% or more, and even more preferably 0.0008% or more.

[0049] Sn: 0.100% or less Sn is an element that suppresses decarburization caused by nitriding or oxidation of the steel surface, thereby suppressing the decrease in strength. However, if Sn is present in excess, the hardness of the electric resistance welded joint increases. As a result, the hardness gradient and KAM value gradient increase, and groove corrosion resistance decreases. Therefore, the Sn content is preferably 0.100% or less, more preferably 0.070% or less, and even more preferably 0.040% or less. On the other hand, there is no particular lower limit to the Sn content, but when Sn is added, in order to obtain the above effect, the Sn content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0050] Ceq: 0.10~0.40% Ceq (carbon equivalent) is a parameter that represents the hardenability of steel. The higher the Ceq, the harder the microstructure formed, resulting in higher hardness in both the base metal and the electric resistance welded joint. Ceq has a particularly strong correlation with the hardness of the electric resistance welded joint, and when Ceq exceeds 0.40%, the hardness of the electric resistance welded joint increases significantly. Therefore, from the viewpoint of reducing the hardness gradient and KAM value gradient and further improving resistance to groove corrosion, it is preferable to set Ceq to 0.40% or less, more preferably to 0.35% or less, and even more preferably to 0.30% or less. On the other hand, if Ceq is excessively low, the strength decreases. Therefore, from the viewpoint of improving strength, it is preferable to set Ceq to 0.10% or more, more preferably to 0.15% or more, and even more preferably to 0.20% or more.

[0051] [Yield strength] The strength of the electric resistance welded steel pipe of the present invention is not particularly limited and may be any value. However, in order to withstand the internal pressure of the fluid being transported when used as piping, the yield strength of the base material is preferably 200 MPa or more, more preferably 220 MPa or more, and even more preferably 240 MPa or more. On the other hand, as the yield strength of the base material increases, the hardness of the electric resistance weld increases. Therefore, from the viewpoint of reducing the hardness gradient and KAM value gradient and further improving resistance to groove corrosion, the yield strength of the base material is preferably 900 MPa or less, more preferably 800 MPa or less, and even more preferably 700 MPa or less.

[0052] 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 (2022). The yield strength (MPa) shall be determined by the offset method, with the plastic elongation value set to 0.2%. The test specimen for the tensile test shall be a JIS No. 5 tensile test specimen. The test specimen shall be taken from the base material so that the tensile direction (longitudinal direction of the test specimen) is parallel to the axial direction of the pipe. The base material shall be located 90° circumferentially away from the electric resistance weld. The 90° distance may be either clockwise or counterclockwise.

[0053] [Resistant to groove corrosion] As described above, by controlling the difference in hardness between the base material and the electric resistance welded portion, the hardness gradient in the circumferential direction of the electric resistance welded portion, and the KAM value gradient in the circumferential direction of the electric resistance welded portion, an electric resistance welded steel pipe with excellent resistance to groove corrosion can be obtained. Therefore, the electric resistance welded steel pipe of the present invention can be suitably used for piping.

[0054] The groove corrosion resistance of the electric resistance welded steel pipe of the present invention is not particularly limited, but for example, in the accelerated corrosion test (groove corrosion test) described in Japanese Patent Publication No. 56-98451 and Japanese Patent Publication No. 56-213349, when the amount of wall thinning in the electric resistance welded part is h1 (mm) and the amount of wall thinning in the base material is h2 (mm), (h1 / h2) is 1.50 or less, and the ratio of the depth (mm) to the width (mm) of the groove corrosion part (depth / width) is 1.00 or less. The positions of the wall thinning amount h1 in the electric resistance welded part and the wall thinning amount h2 in the base material are as shown in Figure 1. Similarly, the positions of the width and depth of the groove corrosion part are as shown in Figure 2, and the width of the groove corrosion part is the width at the 1 / 2 depth position.

[0055] In the groove corrosion test described above, a tile-shaped test specimen with a total thickness × width of 40 mm × length of 30 mm is used. The test specimen is taken so that the width direction is parallel to the circumferential direction of the pipe, the length direction is parallel to the axial direction of the pipe, and the electric resistance welded portion is located in the center of the width. After polishing the inner surface of the pipe side of the test specimen, the parts other than the polished portion are covered with resin and subjected to the groove corrosion test.

[0056] h1, h2, and the depth and width (mm) of the groove-like corrosion area are determined by dividing the test specimen lengthwise into four equal parts and averaging the values ​​measured at three locations on the resulting cross-sections. The amount of material thinning in the base metal, h2, is measured at a position separated from the electric resistance weld by the thickness of the pipe in the circumferential direction (width direction of the test specimen).

[0057] [Manufacturing method] Next, a method for manufacturing electric resistance welded steel pipes according to one embodiment of the present invention will be described.

[0058] The electric resistance welded steel pipe of the present invention can be formed by any method without particular limitation, but typically it can be manufactured by forming a hot-rolled steel sheet into a tubular shape and then joining the circumferential ends of the tubular shape together and welding them together using electric resistance. Preferred manufacturing conditions will be described below. In the following description, temperature (°C) refers to the surface temperature of the hot-rolled steel sheet unless otherwise specified. The surface temperature can be measured using a radiation thermometer or the like.

[0059] Any hot-rolled steel sheet can be used as the material for manufacturing electric resistance welded steel pipes, without any particular limitations. The hot-rolled steel sheet can be manufactured according to conventional methods. That is, it can be manufactured by hot-rolling a steel material. Typically, a hot-rolled steel sheet can be manufactured by heating a steel slab as the steel material, hot-rolling it, cooling it, and then winding it into a coil. The thickness of the hot-rolled steel sheet is denoted as t (mm).

[0060] It is preferable to use a steel material having the same component composition as the base material described above.

[0061] The method for melting the aforementioned steel material is not particularly limited, and any known melting method such as a converter, electric furnace, or vacuum melting furnace is suitable. The casting method is also not particularly limited, but it can be manufactured to the desired dimensions by a known casting method such as continuous casting. There is no problem in applying the ingot-parting rolling method instead of continuous casting. The molten steel may be further refined, such as ladle refining.

[0062] The hot-rolled steel sheet is formed into a tubular body (open pipe). The forming method is not particularly limited, but typically the hot-rolled steel sheet can be formed into a cylindrical shape by cold roll forming. Then, the circumferential ends of the tubular body are butted together and electric resistance welded to form an electric resistance welded steel pipe. The heating method during electric resistance welding may be either electric heating or induction heating.

[0063] As an example, Figure 3 shows a schematic diagram illustrating an electric resistance welding process in which two ends of a hot-rolled steel sheet are joined together in the width direction and welded. After heating the butt joint surface with a high-frequency oscillator 15 via a contact tip 14, the temperature distribution of the butt joint surface is measured with a thermometer 16. As mentioned above, induction heating with a coil may be used instead of electric heating with a contact tip.

[0064] • Temperature gradient of butt joint surface: (50 / t)℃ / mm or more During the aforementioned electric resistance welding, the temperature gradient in the axial direction of the pipe at a depth of 1 mm from the outer surface, at a 1 / 2t position, and at a depth of 1 mm from the inner surface, in the temperature range of 900 to 1300°C, should be (50 / t)°C / mm or more. The reason for this is explained below.

[0065] Figure 4 shows a schematic diagram of an open pipe in electric resistance welding, and Figure 5 shows a diagram illustrating the method for determining the temperature gradient in the axial direction of the pipe at the center (1 / 2t) of the plate thickness of the butt joint surface in electric resistance welding.

[0066] The temperature of the butt joint surface 21 continues to rise from the heating start position 25 toward the pipe-forming direction 17 until it reaches the melting point, and then reaches the melting start position 28 just before the V convergence point 24. In electric resistance welding, the current concentrates on the outer or inner surface that forms a corner, so when the butt joint surfaces are parallel, the outer or inner surface becomes hotter than 1 / 2t at the same pipe axis position.

[0067] Here, let x1 be the axial coordinate of the pipe at position 26 where the temperature is 900°C, and x2 be the axial coordinate of the pipe at position 27 where the temperature is 1300°C. The average temperature gradient between these two points (°C / mm) is calculated as (1300-900) / (x2-x1). Similarly, the temperature gradients at positions 1 mm from the outer surface and 1 mm from the inner surface of the butt joint are also calculated.

[0068] When the aforementioned temperature gradient decreases, the heating, especially at 1 / 2t, weakens, resulting in a lower temperature during upsetting. This increases the amount of work hardening in the electric resistance welded (ERW) weld and a larger difference in hardness between the base metal and the ERW weld. The hardness gradient and KAM value gradient in the circumferential direction of the pipe also increase. Furthermore, the longer contact time between the butt joint and the atmosphere leads to the formation of a large amount of oxides in the ERW weld, which become the starting point for groove corrosion, reducing groove corrosion resistance. Additionally, if only one of the temperature gradients on the inner or outer surface decreases, the butt joint becomes non-parallel, and the heating on the side with the smaller temperature gradient weakens. This results in a lower temperature during upsetting on the side with the smaller temperature gradient, increasing the amount of work hardening in the ERW weld and a larger difference in hardness between the base metal and the ERW weld. The hardness gradient and KAM value gradient in the circumferential direction of the pipe also increase.

[0069] Therefore, the temperature gradient in the axial direction of the pipe at a depth of 1 mm from the outer surface, at a 1 / 2t position, and at a depth of 1 mm from the inner surface, in the temperature range of 900 to 1300°C, should be (50 / t)°C / mm or more. Preferably, the temperature gradient is (60 / t)°C / mm or more, and more preferably (70 / t)°C / mm or more. On the other hand, since a larger temperature gradient increases the load on the welding power supply, it is preferable that the temperature gradient be (500 / t)°C / mm or less.

[0070] The method for controlling the temperature gradient is not particularly limited, but for example, the temperature gradient can be adjusted by adjusting at least one of the welding power, the pipe-making speed, and the spacing of the fin pass rolls 11.

[0071] Bead width: 0.20t~0.80t (mm) When the bead width on the outer surface of the pipe becomes smaller, the heating of the outer surface in particular becomes weaker, resulting in a lower temperature during upsetting, a greater amount of work hardening in the electric resistance welded joint, and a larger difference in hardness between the base material and the electric resistance welded joint. In addition, the hardness gradient and KAM value gradient in the circumferential direction of the pipe also increase. For this reason, the bead width on the outer surface of the pipe should be 0.20t (mm) or more, preferably 0.25t (mm) or more, and more preferably 0.30t (mm) or more.

[0072] On the other hand, as the bead width increases, the heating becomes stronger, resulting in a higher temperature during upsetting. Consequently, the electric resistance welded joint softens, and the difference in hardness between the base material and the electric resistance welded joint increases. The hardness gradient and KAM value gradient in the circumferential direction of the pipe also increase. Therefore, the bead width on the outer surface of the pipe should be 0.80t (mm) or less, preferably 0.75t (mm) or less, and more preferably 0.70t (mm) or less.

[0073] Figure 6 is a view of the open pipe shown in Figure 4, seen from the outer surface of the pipe at a position including the electric resistance weld. The bead width 43 in the section from the V convergence point 24 to three times the plate thickness t in the pipe-forming direction is measured using the high-speed camera 18 shown in Figure 3, and the maximum and minimum values ​​are determined. In this invention, it is assumed that both the maximum and minimum values ​​satisfy the above conditions.

[0074] The width of the bead can be adjusted by controlling at least one of the welding power, pipe-making speed, the spacing of the fin pass rolls 11, and the spacing of the squeeze rolls 12.

[0075] • Upset amount: 0.20t~t (mm) Furthermore, a smaller upset amount results in less work hardening of the electric resistance welded joint, increasing the hardness difference between the base material and the welded joint. The hardness gradient and KAM value gradient in the circumferential direction of the pipe also increase. The bead width also decreases. In addition, oxides generated at the butt joint are not sufficiently discharged with the bead and remain in the electric resistance welded joint, becoming the starting point for groove corrosion and reducing groove corrosion resistance. For this reason, the upset amount should be 0.20t (mm) or more, preferably 0.25t (mm) or more, and more preferably 0.30t (mm) or more.

[0076] On the other hand, as the upset amount increases, the work hardening of the electric resistance weld increases, and the difference in hardness between the base material and the electric resistance weld increases. Also, the hardness gradient and KAM value gradient in the circumferential direction of the pipe increase. In addition, the bead width increases. For this reason, the upset amount should be t (mm) or less, preferably 0.95t (mm) or less, and more preferably 0.90t (mm) or less.

[0077] The upset amount (mm) can be adjusted, for example, by controlling the spacing of the squeeze rolls 12. The upset amount can be calculated as (circumference of the open pipe immediately before electric resistance welding (mm)) - (circumference of the electric resistance welded steel pipe immediately after electric resistance welding (mm)).

[0078] As mentioned above, these parameters can be adjusted by controlling the welding power, pipe-making speed, fin pass roll spacing, squeeze roll spacing, and so on.

[0079] For example, increasing the welding power increases the heat input, thus increasing the temperature gradient. Also, the amount of melting at the butt joint increases, resulting in a wider weld bead. Decreasing the welding power decreases the heat input, thus decreasing the temperature gradient. Also, the amount of melting at the butt joint decreases, resulting in a narrower weld bead.

[0080] Increasing the pipe-making speed reduces the heat input per unit time, thus decreasing the temperature gradient and reducing the bead width. Decreasing the pipe-making speed increases the heat input per unit time, thus increasing the temperature gradient and increasing the bead width. Note that pipe-making speed refers to the electric resistance welding speed.

[0081] Increasing the spacing between the fin pass rolls increases the angle of the abutting surfaces, as shown in Figure 7, and the spacing between the abutting surfaces becomes larger on the outer surface than on the inner surface. As a result, current concentrates more on the inner surface than on the outer surface, the heating of the outer surface weakens, the temperature gradient on the outer surface decreases, and the bead width decreases. Decreasing the spacing between the fin pass rolls reduces the angle of the abutting surfaces, but if the spacing is reduced too much, the spacing between the abutting surfaces becomes larger on the inner surface than on the outer surface. As a result, current concentrates more on the outer surface than on the inner surface, the heating of the inner surface weakens, and the temperature gradient on the inner surface decreases.

[0082] Increasing the spacing between the squeeze rolls reduces the amount of upset and narrows the bead width. Decreasing the spacing between the squeeze rolls increases the amount of upset and widens the bead width.

[0083] After electric resistance welding (ERW), heat treatment may be optionally applied to adjust the hardness of the ERW welded joint 3. When heat treatment is applied, the entire ERW steel pipe may be heat-treated, or only the ERW welded joint 3 may be heat-treated. The heating method in the heat treatment is not particularly limited; for example, induction heating may be used, or a heating furnace may be used.

[0084] Furthermore, whether a steel pipe is an electric resistance welded (ERW) pipe can be determined by cutting the steel pipe perpendicular to the pipe axis and observing the cut surface, including the weld (ERW weld), with an optical microscope. This observation is performed after polishing the cut surface and then corroding it with an etching solution. If the width of the molten and solidified area in the circumferential direction of the pipe at the weld (ERW weld) is 1.0 μm or more and 1000 μm or less across the entire thickness of the pipe, then it is an ERW pipe.

[0085] Here, the corrosive solution should be selected appropriately according to the type and composition of the steel pipe being observed. In the cross-section after corrosion, the molten and solidified area can be seen as an electric resistance welded area 3, which has a different microstructure and contrast from the base material 1 and the heat-affected zone 2, as schematically shown in Figure 8. For example, the molten and solidified area of ​​electric resistance welded steel pipes made of carbon steel and low-alloy steel can be identified as a white area observed under an optical microscope in the cross-section corroded with nital. In addition, the molten and solidified area of ​​UOE steel pipes made of carbon steel and low-alloy steel can be identified as a region containing a cellular or dendritic solidification structure under an optical microscope in the cross-section corroded with nital. [Examples]

[0086] The present invention will be described in more detail below based on the following examples. However, the present invention is not limited to the following examples.

[0087] Molten steel having the component composition shown in Table 1 was produced to form a steel slab (steel material). The obtained slab was heated and hot-rolled, cooled, and then wound into a coil to obtain a hot-rolled steel sheet with the thickness (mm) shown in Table 2. Subsequently, the hot-rolled steel sheet was formed into a tubular shape by cold roll forming and then electric resistance welded (ERW) under the conditions shown in Table 2 to obtain an ERW steel pipe. The outer diameter (mm) and wall thickness (mm) of the obtained ERW steel pipe are as shown in Table 2.

[0088] Test specimens were taken from the obtained electric resistance welded steel pipes, and the following hardness measurements, KAM value measurements, tensile tests, and groove corrosion tests were performed. Note that, unless otherwise specified, "base material" below refers to the base material located 90° from the electric resistance weld in the circumferential direction of the pipe.

[0089] (Vickers test) The hardness of the base material and the electric resistance welded joint, as well as the hardness gradient in the circumferential direction of the electric resistance welded joint, were determined by Vickers testing. For both the electric resistance welded joint and the base material, the Vickers test was performed with a load of 1 kgf, using the method described in JIS Z 2244 (2020), with the measurement surface being a cross section perpendicular to the pipe axis. For the electric resistance welded joint, measurements were taken at 1 mm intervals from the outer surface to the inner surface in the thickness direction, and the average value was taken as the hardness of the electric resistance welded joint. For the base material, measurements were taken at 90° circumferentially from the electric resistance welded joint, at 1 mm intervals from the outer surface to the inner surface in the thickness direction, and the average value was taken as the hardness of the base material. The difference in hardness between the base material and the electric resistance welded joint was calculated using the obtained Vickers hardness values.

[0090] To determine the circumferential hardness gradient in electric resistance welded joints, as shown in Figure 9, tests were conducted at 21 points at 1 mm intervals, within a 20 mm range to the left and right of the weld in the circumferential direction, centered on the electric resistance welded joint, at positions 1 mm from the outer surface, the center of the wall thickness, and 1 mm from the inner surface. At each position, the difference in hardness between all adjacent measurement points was determined, and the largest value among these was defined as the circumferential hardness gradient.

[0091] (KAM value measurement) KAM values ​​were measured using the SEM / EBSD method. First, using a cross section perpendicular to the pipe axis direction including the electric resistance weld as the measurement surface, measurements were taken at 1 mm from the outer surface of the electric resistance weld, at the center of the wall thickness, and at 1 mm from the inner surface. The wall thickness direction was defined as the longitudinal direction and the circumferential direction as the transverse direction, with a measurement area of ​​100 μm vertically × 400 μm horizontally and a measurement step size of 1 μm, and five fields of view were measured. Based on the obtained EBSD data, a distribution image of KAM values ​​(KAM map) was obtained using the crystal orientation analysis software OIM Analysis (trademark).

[0092] Here, the KAM value was determined by the following method. At each measurement point (regular hexagonal pixel), the orientation difference between each pixel was calculated using the three adjacent pixels (37 pixels in total) centered on the measurement point. The average of these orientation differences was taken as the KAM value of the central pixel. This operation was performed for all pixels in the field of view to obtain a KAM map. A line profile of the KAM value with a length of 400 μm in the horizontal direction was created at the vertical center of the obtained KAM map and at positions 20 μm above and below it. In each obtained line profile, the difference in KAM values ​​between all adjacent measurement points was calculated, and the largest of these differences was taken as the KAM value gradient in the circumferential direction of the tube. However, if the KAM value of any or both adjacent measurement points was 5° or greater, it was excluded from the calculation of the KAM value gradient because a grain boundary was included between the measurement points.

[0093] (Tensile test) Tensile tests were conducted to determine the yield strength of the obtained electric resistance welded steel pipes. The tensile tests were carried out in accordance with the provisions of JIS Z 2241 (2022). From the results of the tensile tests, the 0.2% strength obtained by the offset method was taken as the yield strength. JIS No. 5 tensile test specimens were used for the tensile tests. The specimens were taken from the base material so that the tensile direction (longitudinal direction of the specimen) was parallel to the axial direction of the pipe. The base material was located 90° circumferentially away from the electric resistance weld.

[0094] (Grove corrosion test) Next, a groove corrosion test was conducted to evaluate the groove corrosion resistance of the electric resistance welded steel pipe. The groove corrosion test was carried out according to the method described in Japanese Patent Publication No. 56-98451 and Japanese Patent Publication No. 56-213349. The test specimens for the groove corrosion test were tile-shaped with a total thickness × width of 40 mm × length of 30 mm, with the width direction parallel to the circumferential direction of the pipe and the length direction parallel to the axial direction of the pipe, and the electric resistance welded portion located in the center of the width. After polishing the inner surface of the test specimen, the parts other than the polished portion were covered with resin and subjected to the groove corrosion test. h1 (mm), h2 (mm), the depth (mm) and width (mm) of the groove corrosion portion were obtained by dividing the test specimen lengthwise into four equal parts and averaging the values ​​measured at three locations on the resulting cross-sections. The amount of wall thickness reduction h2 of the base material was measured at a position separated from the electric resistance welded portion by the wall thickness in the circumferential direction of the pipe (width direction of the test specimen).

[0095] The results obtained are shown in Table 3. In Table 3, electric resistance welded steel pipes No. 1, 3, 5, 6, 9, 10, 12, and 13 are examples of the present invention, while electric resistance welded steel pipes No. 2, 4, 7, 8, and 11 are comparative examples.

[0096] In all of the electric resistance welded (ERW) steel pipes of the present invention, the difference in hardness between the base material and the ERW welded portion was 60 HV or less, the hardness gradient in the circumferential direction of the ERW welded portion at 1 mm from the outer surface, the center of the wall thickness, and 1 mm from the inner surface was 30 HV / mm or less, and the steel structure of the ERW welded portion had a KAM value gradient in the circumferential direction of the ERW welded portion of 4.0° / μm or less at 1 mm from the outer surface, the center of the wall thickness, and 1 mm from the inner surface.

[0097] On the other hand, in comparative example No. 2, the electric resistance welded steel pipe had a large difference in hardness between the base material and the welded section, as well as a large KAM value gradient, which prevented it from achieving the groove-like corrosion resistance required in the present invention. Comparative Example No. 4, an electric resistance welded steel pipe, had a large KAM value gradient, and therefore failed to achieve the groove-like corrosion resistance required by the present invention. Comparative Example No. 7, an electric resistance welded steel pipe, had a large hardness gradient and a large KAM value gradient, and therefore failed to achieve the groove-like corrosion resistance required by the present invention. Comparative Example No. 8, an electric resistance welded steel pipe, had a large hardness gradient, and therefore failed to achieve the groove-like corrosion resistance required by the present invention.

[0098] [Table 1]

[0099] [Table 2]

[0100] [Table 3] [Explanation of Symbols]

[0101] 1 Base metal part 2. Heat-affected zone 3. Electric resistance welded joint 4. Groove-shaped corrosion 5. Inner surface of the pipe before corrosion 6. Inner surface of pipe after corrosion 10 open tubes 11 Fin Pass Roll 12 squeeze rolls 13 Top Roll 14 Contact Tips 15. High-frequency oscillator 16 Thermometer 17. Welding direction (pipe manufacturing direction, pipe axis direction) 18 high-speed cameras 21 Butt joint 22 Outer surface of open tube 23. Inner surface of an open tube 24 V convergence point 25 Heating start position at the center of the plate thickness 26. The temperature at the center of the plate thickness is 900°C. 27. The temperature at the center of the plate thickness is 1300℃. 28. Melting start position at the center of the plate thickness 30 Angle of the butt joint 31 Fin pass roll interval 40 Molten steel 41 ERW steel pipe 42 Outer surface of electric resistance welded joint 43 Bead width

Claims

1. An electric resistance welded steel pipe having a base material portion and an electric resistance welded portion, The difference in hardness between the base material and the electric resistance welded portion is 60 HV or less. The hardness gradient in the circumferential direction of the electric resistance welded joint is 30 HV / mm or less. An electric resistance welded steel pipe in which the KAM value gradient in the circumferential direction of the electric resistance welded joint is 4.0° / μm or less.

2. The aforementioned base material portion is, by mass%, C: 0.020-0.200%, Si: 0.40% or less, Mn: 0.20-2.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.005–0.100%, and N: Contains 0.0100% or less, The remainder consists of Fe and unavoidable impurities, The electric resistance welded steel pipe according to claim 1, having a component composition in which the Ceq represented by the following formula (1) is 0.10 to 0.40%. Ceq (%)=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 4...(1) Here, the element symbols in equation (1) above represent the content (mass%) of each element in the component composition of the base material, and zero is used if the element is not present.

3. The component composition of the aforementioned base material is, in mass%, Nb: 0.080% or less, V: 0.080% or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less Ca: 0.0050% or less, B: 0.0050% or less, and The electric resistance welded steel pipe according to claim 2, further comprising at least one selected from the group consisting of Sn: 0.100% or less.

4. Piping using electric resistance welded steel pipes according to any one of claims 1 to 3.

5. A method for manufacturing an electric resistance welded steel pipe according to any one of claims 1 to 3, comprising forming a hot-rolled steel sheet with a thickness t (mm) into a tubular body, and then butting the circumferential ends of the tubular body together and performing electric resistance welding to form an electric resistance welded steel pipe, During the aforementioned electric resistance welding, the temperature gradient in the axial direction of the pipe at a depth of 1 mm from the outer surface, at a 1 / 2t position, and at a depth of 1 mm from the inner surface, in the temperature range of 900 to 1300°C, shall be (50 / t)°C / mm or more. The width of the bead on the outer surface of the pipe shall be 0.20t to 0.80t (mm), and, A method for manufacturing electric resistance welded steel pipes, wherein the upset amount is 0.20 t to t (mm).

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