Electric seam steel pipe

Abstract

An electric resistance welded steel pipe (1) of the present disclosure has a base material (2) and an electric resistance welded portion (3) extending in the axial direction of the electric resistance welded steel pipe (1). In the base material (2), the ferrite area fraction is 50% or more, the pearlite area fraction is 10% or more, and the sum of the ferrite area fraction and the pearlite area fraction is 95% or more. The Vickers hardness H at a depth of 0.1 mm from the inner surface is 100% or more. B In the electric resistance welded portion (3), the ferrite area ratio is 50% or more, the pearlite area ratio is 10% or more, the sum of the ferrite area ratio and the pearlite area ratio is 95% or more, and the average circle equivalent diameter D W is 10.0 μm or less, and the Vickers hardness H W is 190HV or more and satisfies formula (1). 0.80≦H W / H B ≦1.20 (1)

Description

[Technical Field]

[0001] The present disclosure relates to an electric resistance welded steel pipe. [Background technology]

[0002] Hydraulic and pneumatic cylinders used in industrial and construction machinery include cylinder tubes. Electrically welded steel pipes are used for the cylinder tubes. When an electric resistance welded steel pipe is used as a cylinder tube, a mechanical part such as a piston is inserted into the electric resistance welded steel pipe, and the inserted mechanical part slides in the axial direction of the pipe while contacting the inner surface of the electric resistance welded steel pipe. This sliding action of the mechanical part causes wear on the inner surface of the electric resistance welded steel pipe. When wear generates wear debris, the wear debris impedes the sliding of the mechanical part. As a result, the operability of the cylinder is reduced. Therefore, when an electric resistance welded steel pipe is used as a cylinder tube, the electric resistance welded steel pipe is required to have excellent wear resistance on its inner surface.

[0003] Electric resistance welded steel pipes with improved wear resistance have been proposed in Japanese Patent Laid-Open Nos. 8-225853 (Patent Document 1) and 6-306459 (Patent Document 2). In the electric resistance welded steel pipes disclosed in Patent Documents 1 and 2, the microstructure is a matrix of ferrite or bainite, etc., with island martensite dispersed in the matrix. Patent Documents 1 and 2 state that this improves wear resistance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 8-225853 [Patent Document 2] Japanese Patent Application Publication No. 6-306459 Summary of the Invention [Problem to be solved by the invention]

[0005] However, even if an electric resistance welded steel pipe having the above-mentioned microstructure is used for a cylinder tube, sufficient wear resistance may not be obtained on the inner surface.

[0006] An object of the present disclosure is to provide an electric resistance welded steel pipe having excellent wear resistance on the inner surface. [Means for solving the problem]

[0007] The electric resistance welded steel pipe of the present disclosure comprises a base metal portion and an electric resistance weld portion extending in the axial direction of the electric resistance welded steel pipe. The chemical composition of the electric resistance welded steel pipe is, in mass%, C: 0.10-0.30%, Si: 0.03-1.20%, Mn: 1.00-2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005-0.500%, Nb: 0.010-0.060%, N: 0.0003-0.0060%, Ti: 0-0.200%, and Cu: 0-1. The alloy contains 0.00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-0.50%, V: 0-0.20%, B: 0-0.0100%, W: 0-0.10%, Ca: 0-0.0200%, Mg: 0-0.0200%, Zr: 0-0.0200%, and rare earth elements: 0-0.0200%, with the balance being Fe and impurities. In the base metal, the ferrite area fraction is 50% or more, the pearlite area fraction is 10% or more, and the sum of the ferrite area fraction and the pearlite area fraction is 95% or more. The Vickers hardness H at a depth of 0.1 mm from the inner surface of the base metal is 0.000. B In the electric resistance welded portion, the ferrite area ratio is 50% or more, the pearlite area ratio is 10% or more, the sum of the ferrite area ratio and the pearlite area ratio is 95% or more, and the average circle equivalent diameter D of the pearlite region consisting of one or a plurality of continuously connected pearlite blocks is W is 10.0 μm or less, and the Vickers hardness H W is 190HV or more and satisfies formula (1). 0.80≦H W / H B ≦1.20 (1) [Effects of the Invention]

[0008] The electric resistance welded steel pipe of the present disclosure has excellent wear resistance on the inner surface. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is an enlarged view of the vicinity of an electric resistance weld in a cross section perpendicular to the pipe axis direction of an electric resistance welded steel pipe of this embodiment. [Figure 2] FIG. 2 is a schematic diagram showing an example of a microstructure photograph image of an electric resistance weld for explaining the pearlite region. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present inventors first investigated electric resistance welded steel pipes that provide excellent wear resistance on the inner surface, and as a result, the present inventors discovered the following.

[0011] To improve wear resistance, it is preferable to increase the hardness of steel. Therefore, from the viewpoint of increasing the hardness of steel and improving wear resistance, the chemical composition of electric resistance welded steel pipe was investigated. As a result, the composition was found to be, in mass%, C: 0.10-0.30%, Si: 0.03-1.20%, Mn: 1.00-2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005-0.500%, Nb: 0.010-0.060%, N: 0.0003-0.0060%, Ti: 0-0.200%, Cu: 0-1.00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0- The inventors considered that an electric resistance welded steel pipe having a chemical composition containing 0.50%, V: 0-0.20%, B: 0-0.0100%, W: 0-0.10%, Ca: 0-0.0200%, Mg: 0-0.0200%, Zr: 0-0.0200%, and rare earth elements: 0-0.0200%, with the balance being Fe and impurities, would be able to increase the hardness of the inner surface and improve the wear resistance.

[0012] The present inventors further investigated means for improving the wear resistance of electric resistance welded steel pipes having the above-mentioned chemical composition from the viewpoint of the microstructure. As described above, the microstructures of the electric resistance welded steel pipes of Patent Documents 1 and 2 have improved wear resistance by dispersing hard structures such as island martensite in a ferrite or bainite matrix. However, the present inventors' investigations revealed that the wear resistance of the above-mentioned microstructures is not necessarily sufficient for sliding motion of mechanical parts in the axial direction of the pipe, such as a cylinder tube.

[0013] The inventors of the present invention have the following idea about the cause of this problem. When a hard structure such as bainite or island martensite is used in the microstructure of an electric resistance welded steel pipe, the hardness of the inner surface increases. Therefore, it would seem that the wear resistance would also increase. However, when a mechanical part such as a piston repeatedly slides on the inner surface of the electric resistance welded steel pipe in the axial direction, the hardness of the martensite or bainite does not increase easily even with repeated sliding. Instead, repeated sliding causes cracks to initiate and propagate. As a result, the amount of wear increases with the number of sliding movements.

[0014] Based on the above considerations, the inventors of the present invention thought that the wear resistance of the inner surface of an electric resistance welded steel pipe could be improved if the microstructure of the electric resistance welded steel pipe was made to have a structure in which hardness increases with repeated sliding action, rather than being made a hard structure (bainite or island martensite) in advance. Therefore, the inventors of the present invention investigated a structure in which hardness increases with repeated sliding action. As a result, the inventors of the present invention found that the wear resistance of the inner surface of an electric resistance welded steel pipe could be significantly improved if the microstructure and hardness of the base material of the electric resistance welded steel pipe and the microstructure and hardness of the electric resistance welded part satisfy the following characteristics: (I) In the base material, the ferrite area ratio is 50% or more, and the pearlite area ratio is 10% or more, and the sum of the ferrite area ratio and the pearlite area ratio is 95% or more, and the Vickers hardness H at a depth of 0.1 mm from the inner surface of the base material B is 190 to 250HV. (II) In the electric resistance weld, the ferrite area ratio is 50% or more, and the pearlite area ratio is 10% or more, and the sum of the ferrite area ratio and the pearlite area ratio is 95% or more, and the Vickers hardness H at a depth of 0.1 mm from the inner surface of the electric resistance weld W is 190HV or more and satisfies formula (1). 0.80≦H W / H B ≦1.20 (1)

[0015] In the electric resistance welded steel pipe of this embodiment, as shown in (I) and (II) above, the base material and the electric resistance welded zone contain pearlite in an area ratio of 10% or more, and the hardness of the inner surface of the base material and the electric resistance welded zone are substantially equal. The reason why the wear resistance of the inner surface of the electric resistance welded steel pipe is significantly improved by the base material and the electric resistance welded zone satisfying these characteristics is not clear, but the following reason is thought to be the reason.

[0016] When a mechanical part such as a piston repeatedly slides against the inner surface of an electric resistance welded steel pipe, the inner surface of the electric resistance welded steel pipe is subjected to an external force from the mechanical part. This external force pulverizes the pearlite on the surface layer of the inner surface. As the pearlite is pulverized, the cementite in the pearlite is pulverized, and fine cementite is dispersed throughout the surface layer. This dispersion of cementite dispersion strengthens the surface layer, increasing its hardness. Furthermore, carbon in the crushed fine cementite dissolves in the ferrite in the pearlite in a supersaturated state. In this case, solid solution strengthening further increases the hardness of the surface layer of the inner surface. Due to the above mechanism, the hardness of the inner surface of the electric resistance welded steel pipe increases significantly with repeated sliding. As a result, it is believed that the wear resistance of the inner surface increases significantly.

[0017] However, even if the base material and the electric resistance welded joint satisfy (I) and (II), there are still cases where sufficient wear resistance is not obtained. Therefore, the present inventors conducted further studies and obtained the following findings.

[0018] In the manufacturing process of electric resistance welded steel pipes, electric resistance welds are formed by melting both widthwise ends of a steel plate serving as a base material during electric resistance welding, then upsetting the melted portions and crimping the both ends while squeezing out the melted portions. Through these steps, both widthwise ends of the steel plate are joined to form an electric resistance weld. Because electric resistance welds are formed by heating to high temperatures, the microstructure of the electric resistance weld may differ from that of the base material. In this case, wear may be accelerated in the electric resistance weld compared to the base material. Therefore, the present inventors have considered that further adjusting the microstructure of the electric resistance weld would be effective in improving the wear resistance of electric resistance welded steel pipes.

[0019] As a result of further investigation based on the above considerations, it was found that in the electric resistance weld, the average circle equivalent diameter D W The present inventors have found that if the grain size is 10.0 μm or less, excellent wear resistance can be obtained in the electric resistance welded steel pipe.

[0020] Average circle equivalent diameter D of the pearlite region of the electric resistance weld WAlthough the reason why excellent wear resistance can be obtained by setting the diameter to 10.0 μm or less is unclear, the following reasons are thought to be the case. When the pearlite regions in an electric resistance weld are coarse, the number of pearlite regions per unit area is smaller than when the pearlite regions are fine. As a result, when the pearlite regions are coarse, multiple ferrite grains, which are softer than the pearlite regions, are likely to be arranged continuously. In this case, the ferrite regions consisting of continuously arranged ferrite grains have lower wear resistance than the pearlite regions. As a result, sufficient wear resistance is not obtained. On the other hand, when the pearlite regions are fine, the pearlite regions and ferrite regions are appropriately dispersed, and the size of the ferrite regions consisting of multiple ferrite grains tends to be small. Therefore, wear resistance against repeated sliding is likely to be improved. Furthermore, as described above, pearlite contains cementite, which serves as a source of dispersion strengthening and solid solution strengthening. Therefore, when pearlite regions are sufficiently dispersed, there are more sources of dispersion strengthening and solid solution strengthening. As a result, wear resistance against repeated sliding is significantly improved.

[0021] The above mechanism is speculative, and it is possible that excellent wear resistance is obtained by other mechanisms. However, the examples described below prove that an electric resistance welded steel pipe in which the base material and electric resistance welded joint satisfy the above characteristics can achieve excellent wear resistance.

[0022] The electric resistance welded steel pipe of this embodiment has been completed based on the above technical concept and has the following configuration.

[0023] A first embodiment of the electric resistance welded steel pipe comprises a base metal portion and an electric resistance weld portion extending in the axial direction of the electric resistance welded steel pipe. The chemical composition of the electric resistance welded steel pipe is, in mass %, C: 0.10-0.30%, Si: 0.03-1.20%, Mn: 1.00-2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005-0.500%, Nb: 0.010-0.060%, N: 0.0003-0.0060%, Ti: 0-0.200%, and Cu: 0-1. The alloy contains 0.00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-0.50%, V: 0-0.20%, B: 0-0.0100%, W: 0-0.10%, Ca: 0-0.0200%, Mg: 0-0.0200%, Zr: 0-0.0200%, and rare earth elements: 0-0.0200%, with the balance being Fe and impurities. In the base metal, the ferrite area fraction is 50% or more, the pearlite area fraction is 10% or more, and the sum of the ferrite area fraction and the pearlite area fraction is 95% or more. The Vickers hardness H at a depth of 0.1 mm from the inner surface of the base metal is 0.000. B In the electric resistance welded portion, the ferrite area ratio is 50% or more, the pearlite area ratio is 10% or more, the sum of the ferrite area ratio and the pearlite area ratio is 95% or more, and the average circle equivalent diameter D of the pearlite region consisting of one or a plurality of continuously connected pearlite blocks is W is 10.0 μm or less, and the Vickers hardness H W is 190HV or more and satisfies formula (1). 0.80≦H W / H B ≦1.20 (1)

[0024] The electric resistance welded steel pipe of the second embodiment is the electric resistance welded steel pipe of the first embodiment, and has a Vickers hardness H W is the Vickers hardness H B Higher than.

[0025] The electric-resistance welded steel pipe of the third embodiment is the electric-resistance welded steel pipe of the first or second embodiment, and its chemical composition contains, in mass %, one or more elements selected from the group consisting of Ti: 0.001-0.200%, Cu: 0.01-1.00%, Ni: 0.01-1.00%, Cr: 0.01-1.00%, Mo: 0.01-0.50%, V: 0.01-0.20%, B: 0.0001-0.0100%, W: 0.01-0.10%, Ca: 0.0001-0.0200%, Mg: 0.0001-0.0200%, Zr: 0.0001-0.0200%, and rare earth elements: 0.0001-0.0200%.

[0026] The electric resistance welded steel pipe of the fourth embodiment is the electric resistance welded steel pipe of any one of the first to third embodiments, and has an outer diameter of 100 to 400 mm and a wall thickness of 5.0 to 18.0 mm.

[0027] The electric resistance welded steel pipe of this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0028] [Configuration of the electric resistance welded steel pipe of this embodiment] 1 is an enlarged view of the vicinity of an electric resistance welded portion in a cross section (C cross section) perpendicular to the pipe axis direction of an electric resistance welded steel pipe of this embodiment. Referring to FIG. 1, the electric resistance welded steel pipe 1 of this embodiment includes a base material portion 2 and an electric resistance welded portion 3. The base material portion 2 is cylindrical. The base material portion 2 is formed by bending a steel plate in a manufacturing process described below. The electric resistance welded portion 3 extends in the axial direction of the electric resistance welded steel pipe 1. The electric resistance welded portion 3 is formed by butting together opposing ends of a cylindrical steel plate formed by bending and electric resistance welding (upset welding). The width of the electric resistance welded portion 3 (the length of the electric resistance welded portion 3 in the circumferential direction of the electric resistance welded steel pipe 1) is not particularly limited, but is, for example, about 0.1 to 0.3 mm.

[0029] The electric resistance welded steel pipe 1 of this embodiment satisfies the following characteristics. (Feature 1) The chemical composition, in mass%, is C: 0.10-0.30%, Si: 0.03-1.20%, Mn: 1.00-2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005-0.500%, Nb: 0.010-0.060%, N: 0.0003-0.0060%, Ti: 0-0.200%, Cu: 0-1.00 %, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-0.50%, V: 0-0.20%, B: 0-0.0100%, W: 0-0.10%, Ca: 0-0.0200%, Mg: 0-0.0200%, Zr: 0-0.0200%, and rare earth elements: 0-0.0200%, with the remainder consisting of Fe and impurities. (Feature 2) In the base material portion 2, the ferrite area ratio is 50% or more, the pearlite area ratio is 10% or more, and the sum of the ferrite area ratio and the pearlite area ratio is 95% or more. (Feature 3) Vickers hardness H at a depth of 0.1 mm from the inner surface of the base material 2 B is 190 to 250HV. (Feature 4) In the electric resistance welded portion 3, the ferrite area ratio is 50% or more, the pearlite area ratio is 10% or more, and the sum of the ferrite area ratio and the pearlite area ratio is 95% or more. (Feature 5) In the electric resistance welded portion 3, the average circle equivalent diameter D of the pearlite region consisting of one or a plurality of continuously connected pearlite blocks W is 10.0 μm or less. (Feature 6) Vickers hardness H at a depth of 0.1 mm from the inner surface of the electric resistance weld 3 W is 190HV or more and satisfies formula (1). 0.80≦H W / H B ≦1.20 (1) Each feature will be explained below.

[0030] [(Feature 1) Chemical composition of electric resistance welded steel pipe 1] The chemical composition of the electric resistance welded steel pipe 1 contains the following elements.

[0031] C: 0.10 to 0.30% Carbon (C) increases the strength of steel and increases the hardness of the inner surface of the electric resistance welded steel pipe 1. If the C content is less than 0.10%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.30%, the microstructure of the electric resistance welded steel pipe 1 (base material 2 and electric resistance welded portion 3) tends to be a structure mainly composed of bainite. In this case, even if the contents of other elements are within the ranges of this embodiment, the wear resistance of the inner surface of the electric resistance welded steel pipe decreases. Therefore, the C content is 0.10 to 0.30%. The lower limit of the C content is preferably 0.12%, more preferably 0.14%, and even more preferably 0.16%. The upper limit of the C content is preferably 0.28%, more preferably 0.26%, and even more preferably 0.24%.

[0032] Si: 0.03 to 1.20% Silicon (Si) deoxidizes steel. Si also increases the strength of steel through solid solution strengthening. If the Si content is less than 0.03%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 1.20%, excessive Si oxides are formed, and in this case, cracks are likely to occur originating from the Si oxides during bending in the manufacturing process of the electric resistance welded steel pipe 1, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.03 to 1.20%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%, and even more preferably 0.15%. The upper limit of the Si content is preferably 1.10%, more preferably 1.00%, even more preferably 0.90%, and still more preferably 0.80%.

[0033] Mn: 1.00 to 2.00% Manganese (Mn) improves the hardenability of steel and increases its strength. If the Mn content is less than 1.00%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 2.00%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel becomes excessively high, and in this case, cold formability such as bending deteriorates. Therefore, the Mn content is 1.00 to 2.00%. The lower limit of the Mn content is preferably 1.05%, more preferably 1.10%, and even more preferably 1.15%. The upper limit of the Mn content is preferably 1.90%, more preferably 1.80%, and even more preferably 1.70%.

[0034] P:0.030% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content exceeds 0.030%, P segregates excessively at grain boundaries. In this case, even if the contents of other elements are within the ranges of this embodiment, cold formability such as bending deteriorates. Therefore, the P content is 0.030% or less. The lower the P content, the better. However, excessive reduction in the P content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.005%. The upper limit of the P content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0035] S: 0.010% or less Sulfur (S) is an unavoidable impurity. In other words, the S content is greater than 0%. S combines with Mn to form Mn sulfides. If the S content exceeds 0.010%, excessive Mn sulfides are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, cracks are likely to occur originating from the Mn sulfides. Therefore, the S content is 0.010% or less. The S content is preferably as low as possible. However, excessive reduction of the S content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the S content is preferably 0.001%, and more preferably 0.002%. The upper limit of the S content is preferably 0.008%, more preferably 0.006%, and even more preferably 0.005%.

[0036] Al: 0.005 to 0.500% Aluminum (Al) combines with nitrogen to form AlN. AlN has a pinning effect that prevents austenite grains from coarsening during the seam heat treatment process in the manufacturing process of the electric resistance welded steel pipe 1. If the Al content is less than 0.005%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.500%, coarse AlN is generated. In this case, even if the contents of other elements are within the ranges of this embodiment, cracks are likely to occur originating from the coarse AlN during bending in the manufacturing process of the electric resistance welded steel pipe 1. Therefore, the Al content is 0.005 to 0.500%. The lower limit of the Al content is preferably 0.010%, more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Al content is preferably 0.400%, more preferably 0.200%, even more preferably 0.100%, even more preferably 0.050%, even more preferably 0.040%, and even more preferably 0.030%.

[0037] Nb: 0.010 to 0.060% Niobium (Nb) combines with C or N in the steel material to form Nb precipitates. The Nb precipitates have a pinning effect and suppress coarsening of austenite grains in the seam heat treatment step in the manufacturing process of the electric resistance welded steel pipe 1. If the Nb content is less than 0.010%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Nb content exceeds 0.060%, coarse Nb precipitates are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, cracks are likely to occur originating from the coarse Nb precipitates during bending in the manufacturing process of the electric resistance welded steel pipe 1. Therefore, the Nb content is 0.010 to 0.060%. The lower limit of the Nb content is preferably 0.013%, more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Nb content is preferably 0.055%, more preferably 0.050%, even more preferably 0.045%, and still more preferably 0.040%.

[0038] N: 0.0003 to 0.0060% Nitrogen (N) combines with Al to form AlN. AlN has a pinning effect that prevents austenite grains from coarsening during the seam heat treatment process in the manufacturing process of the electric resistance welded steel pipe 1. If the N content is less than 0.0003%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.0060%, coarse AlN is generated. In this case, even if the contents of other elements are within the ranges of this embodiment, cracks are likely to occur originating from the coarse AlN during bending in the manufacturing process of the electric resistance welded steel pipe 1. Therefore, the N content is 0.0003 to 0.0060%. The lower limit of the N content is preferably 0.0008%, more preferably 0.0010%, and even more preferably 0.0013%. The upper limit of the N content is preferably 0.0055%, more preferably 0.0050%, and even more preferably 0.0047%.

[0039] The balance of the chemical composition of the electric resistance welded steel pipe 1 of this embodiment is composed of Fe and impurities. Here, impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during industrial production of the steel plate that serves as the raw material for the electric resistance welded steel pipe 1, and are acceptable within a range that does not adversely affect the electric resistance welded steel pipe 1 of this embodiment. The impurities are, for example, one or more selected from the group consisting of Sn: 0-0.2% and Pb: 0-0.2%.

[0040] [About optional elements] The chemical composition of the electric resistance welded steel pipe 1 of this embodiment may further contain, in mass %, one or more elements selected from the group consisting of Ti: 0-0.200%, Cu: 0-1.00%, Ni: 0-1.00%, Cr: 0-1.00%, Mo: 0-0.50%, V: 0-0.20%, B: 0-0.0100%, W: 0-0.10%, Ca: 0-0.0200%, Mg: 0-0.0200%, Zr: 0-0.0200%, and rare earth elements: 0-0.0200% or less, in place of a portion of Fe. All of these elements are optional elements. Each optional element will be described below.

[0041] [Group 1: Ti, Cu, Ni, Cr, Mo, V, B and W] The chemical composition of the electric resistance welded steel pipe 1 of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti, Cu, Ni, Cr, Mo, V, B, and W. These elements are optional elements, and all of them increase the strength of the steel. Each element will be described below.

[0042] Ti: 0 to 0.200% Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When contained, that is, when the Ti content is more than 0%, Ti forms Ti precipitates to increase the strength of the steel. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.200%, coarse Ti precipitates are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, cracks are likely to occur originating from the coarse Ti precipitates during bending in the manufacturing process of the electric resistance welded steel pipe 1. Therefore, the Ti content is 0 to 0.200%. The lower limit of the Ti content is preferably 0.001%, more preferably 0.010%, even more preferably 0.020%, and still more preferably 0.030%. The upper limit of the Ti content is preferably 0.190%, more preferably 0.180%, even more preferably 0.170%, even more preferably 0.150%, and still more preferably 0.100%.

[0043] Cu: 0 to 1.00% Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0%. When contained, that is, when the Cu content is more than 0%, Cu improves the hardenability of the steel and increases the strength of the steel. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 1.00%, the strength of the steel will be excessively high even if the contents of other elements are within the ranges of this embodiment, and in this case, sufficient cold formability for bending and other processes will not be obtained. Therefore, the Cu content is 0 to 1.00%. The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and still more preferably 0.15%. The upper limit of the Cu content is preferably 0.98%, more preferably 0.90%, and even more preferably 0.85%.

[0044] Ni: 0 to 1.00% Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%. When contained, that is, when the Ni content is more than 0%, Ni improves the hardenability of the steel and increases the strength of the steel. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, if the Ni content exceeds 1.00%, the strength of the steel will be excessively high even if the contents of other elements are within the ranges of this embodiment, and in this case, sufficient cold formability for bending and other processes will not be obtained. Therefore, the Ni content is 0 to 1.00%. The lower limit of the Ni content is preferably 0.01%, more preferably 0.03%, even more preferably 0.05%, and still more preferably 0.10%. The upper limit of the Ni content is preferably 0.98%, more preferably 0.95%, even more preferably 0.92%, and still more preferably 0.85%.

[0045] Cr: 0 to 1.00% Chromium (Cr) is an optional element and may not be contained, that is, the Cr content may be 0%. When contained, that is, when the Cr content is more than 0%, Cr improves the hardenability of the steel and increases the strength of the steel. Even if even a small amount of Cr is contained, these effects can be obtained to some extent. However, if the Cr content exceeds 1.00%, the strength of the steel will be excessively high even if the contents of other elements are within the ranges of this embodiment, and in this case, sufficient cold formability for bending and other processes will not be obtained. Therefore, the Cr content is 0 to 1.00%. The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and still more preferably 0.20%. The upper limit of the Cr content is preferably 0.97%, more preferably 0.94%, and even more preferably 0.90%.

[0046] Mo: 0 to 0.50% Molybdenum (Mo) is an optional element and may not be contained, that is, the Mo content may be 0%. When contained, that is, when the Mo content is more than 0%, Mo improves the hardenability of steel and increases the strength of steel. Mo also forms Mo precipitates, which increase the strength of steel through precipitation strengthening. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. However, if the Mo content exceeds 0.50%, the strength of the steel will be excessively high even if the contents of other elements are within the ranges of this embodiment, and in this case, sufficient cold formability for bending and other processes will not be obtained. Therefore, the Mo content is 0 to 0.50%. The lower limit of the Mo content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Mo content is preferably 0.48%, more preferably 0.40%, even more preferably 0.36%, and still more preferably 0.30%.

[0047] V: 0 to 0.20% Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms V precipitates. The V precipitates increase the strength of steel through precipitation strengthening. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.20%, the strength of the steel will be excessively high even if the contents of other elements are within the ranges of this embodiment, and in this case, sufficient cold formability for bending and other processes will not be obtained. Therefore, the V content is 0 to 0.20%. The lower limit of the V content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the V content is preferably 0.19%, more preferably 0.17%, and even more preferably 0.15%.

[0048] B: 0 to 0.0100% Boron (B) is an optional element and may not be contained, that is, the B content may be 0%. When B is contained, that is, when the B content exceeds 0%, B improves the hardenability of the steel and increases the strength of the steel. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.0100%, coarse precipitates containing B will precipitate at grain boundaries, and therefore the low-temperature toughness of the steel will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0 to 0.0100%. The lower limit of the B content is preferably 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the B content is preferably 0.0095%, more preferably 0.0090%, and even more preferably 0.0085%.

[0049] W: 0 to 0.10% Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W dissolves in the steel and increases the strength of the steel. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content exceeds 0.10%, the strength of the steel will be excessively high even if the contents of other elements are within the ranges of this embodiment, and in this case, sufficient cold formability for bending and other processes will not be obtained. Therefore, the W content is 0 to 0.10%. The lower limit of the W content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the W content is preferably 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0050] [Group 2: Ca, Mg, Zr and rare earth elements (REM)] The chemical composition of the electric resistance welded steel pipe 1 of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ca, Mg, Zr, and rare earth elements (REM). These elements are optional elements, and each of them controls the morphology of inclusions and suppresses the occurrence of cracks originating from the inclusions. Each element will be explained below.

[0051] Ca: 0 to 0.0200% Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0%. When contained, that is, when the Ca content is more than 0%, Ca controls the morphology of inclusions, making them spheroidized and refined. Therefore, the occurrence of cracks originating from inclusions is suppressed. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content exceeds 0.0200%, coarse oxides are formed, and in this case, sufficient cold formability cannot be obtained even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0 to 0.0200%. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0015%, even more preferably 0.0030%, and still more preferably 0.0045%. The upper limit of the Ca content is preferably 0.0190%, more preferably 0.0180%, and even more preferably 0.0170%.

[0052] Mg: 0 to 0.0200% Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%. When Mg is contained, that is, when the Mg content is more than 0%, Mg controls the morphology of inclusions, making them spheroidized and refined. Therefore, the occurrence of cracks originating from inclusions is suppressed. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content exceeds 0.0200%, excessive Mg oxides are produced, which reduces the toughness and wear resistance of the steel even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mg content is 0 to 0.0200%. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, and still more preferably 0.0030%. The upper limit of the Mg content is preferably 0.0190%, more preferably 0.0180%, and even more preferably 0.0170%.

[0053] Zr: 0 to 0.0200% Zirconium (Zr) is an optional element and may not be contained, that is, the Zr content may be 0%. When contained, that is, when the Zr content is more than 0%, Zr controls the morphology of inclusions, making them spheroidized and refined. Therefore, the occurrence of cracks originating from inclusions is suppressed. Even if even a small amount of Zr is contained, the above effects can be obtained to some extent. However, if the Zr content exceeds 0.0200%, Zr oxides are produced in excess, which reduces the toughness and wear resistance of the steel even if the contents of other elements are within the ranges of this embodiment. Therefore, the Zr content is 0 to 0.0200%. The lower limit of the Zr content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0030%, and even more preferably 0.0040%. The upper limit of the Zr content is preferably 0.0190%, more preferably 0.0170%, and even more preferably 0.0150%.

[0054] Rare earth elements (REM): 0~0.0200% Rare earth elements (REM) are optional elements and may not be contained, i.e., the REM content may be 0%. When REM is contained, that is, when the REM content is more than 0%, REM controls the morphology of inclusions, making them spheroidized and refined. This suppresses the occurrence of cracks originating from inclusions. Even if even a small amount of REM is contained, the above effects can be obtained to some extent. However, if the REM content exceeds 0.0200%, coarse oxides are formed, and in this case, sufficient cold formability cannot be obtained even if the contents of other elements are within the ranges of this embodiment. Therefore, the REM content is 0 to 0.0200%. The lower limit of the REM content is preferably 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, and still more preferably 0.0030%. The upper limit of the REM content is preferably 0.0190%, more preferably 0.0180%, and even more preferably 0.0170%.

[0055] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. In addition, the REM content in this specification refers to the total content (mass%) of these elements. [(Feature 2) Microstructure of Base Material 2] In the base material portion 2 of the electric resistance welded steel pipe 1 of this embodiment, the ferrite area fraction is 50% or more, the pearlite area fraction is 10% or more, and the sum of the ferrite area fraction and the pearlite area fraction is 95% or more. In the microstructure of the base material portion 2, the remainder other than ferrite and pearlite is not particularly limited. The remainder may be, for example, one or more selected from the group consisting of bainite, martensite, and retained austenite. In the following description, bainite and martensite are also referred to as hard structures.

[0056] If the microstructure of the base material portion 2 has a ferrite area ratio of 50% or more, a pearlite area ratio of 10% or more, and the sum of the ferrite area ratio and pearlite area ratio is 95% or more, the wear resistance of the inner surface of the electric resistance welded steel pipe 1 is enhanced. When a mechanical component repeatedly slides against the inner surface of the electric resistance welded steel pipe 1, pearlite serves as a supply source for cementite. Specifically, the sliding pulverizes the pearlite, and fine cementite is dispersed in the surface layer of the inner surface. The dispersed fine cementite dispersion strengthens the inner surface layer. Furthermore, carbon in the crushed cementite supersaturates and dissolves in the ferrite, solid-solution strengthening the inner surface layer. As a result, the hardness of the inner surface layer of the electric resistance welded steel pipe 1 is increased, improving the wear resistance of the electric resistance welded steel pipe 1.

[0057] The lower limit of the ferrite area ratio of the base material portion 2 is preferably 55%, more preferably 60%, and even more preferably 65%. The upper limit of the ferrite area ratio of the base material portion 2 is preferably 90%, more preferably 85%, and even more preferably 70% or less.

[0058] The lower limit of the pearlite area ratio of the base material portion 2 is preferably 12%, more preferably 16%, and even more preferably 20%. The upper limit of the pearlite area ratio of the base material portion 2 is preferably 50%, more preferably 40%, and even more preferably 35%.

[0059] [Method for measuring the microstructure of base material 2] The ferrite area ratio (%) and the pearlite area ratio (%) in the microstructure of the base material portion 2 are measured by the following method.

[0060] A test piece is taken from a position on the base material 2 that is 180° circumferentially shifted around the center axis of the electric resistance welded pipe 1 from the width center position of the electric resistance welded portion 3 of the electric resistance welded steel pipe 1. The observation surface is a cross section (L cross section) on the surface of the test piece that includes the pipe axis direction and wall thickness direction of the electric resistance welded steel pipe 1. The test piece is taken so that one side of the observation surface of the test piece is the inner surface of the base material 2.

[0061] The observation surface of the test piece is mirror-polished. After mirror polishing, the observation surface is etched using a 3% nital solution to reveal the microstructure. Five observation fields are selected from the etched observation surface, from a region extending 0.3 mm (300 μm) deep from the inner surface of the base material 2. The size of each observation field is a rectangle measuring 200 μm in the tube axis direction and 200 μm in the wall thickness direction. The five observation fields are arranged in a continuous row in the tube axis direction on the observation surface.

[0062] Each observation field is observed at 500x magnification using a scanning electron microscope (SEM) to generate an SEM image (secondary electron image). Using the obtained SEM image, ferrite, pearlite, bainite, martensite, and retained austenite are identified based on their contrast and morphology as follows:

[0063] A striped structure in which regions of high brightness (ferrite) and regions of low brightness (cementite) are mixed is identified as pearlite. A region that is brighter than pearlite and in which a substructure such as a lath structure is not observed is identified as ferrite. A region that is brighter than pearlite but lower than ferrite and in which a substructure such as a lath structure is not observed is identified as retained austenite. A structure that is brighter than pearlite, ferrite, and retained austenite and in which a lath structure is observed is identified as a hard structure (bainite and / or martensite). In this embodiment, there is no need to clearly distinguish between bainite and martensite, so in microstructure observation, bainite and martensite are not distinguished and are identified as a hard structure.

[0064] The ferrite area ratio (%) of the base material 2 is calculated based on the total area of ​​ferrite identified in the five observation fields and the total area of ​​the five observation fields. The ferrite area ratio (%) is an integer value obtained by rounding off the first decimal place of the obtained value. The pearlite area ratio (%) of the base material 2 is calculated based on the total area of ​​pearlite identified in the five observation fields and the total area of ​​the five observation fields. The pearlite area ratio (%) is an integer value obtained by rounding off the first decimal place of the obtained value.

[0065] (Feature 3) Vickers hardness H at a depth of 0.1 mm from the inner surface of the base material 2 B About In the electric resistance welded steel pipe 1 of this embodiment, the Vickers hardness H B is 190 to 250HV.

[0066] Vickers hardness H B If the Vickers hardness H is less than 190 HV, sufficient wear resistance cannot be obtained even if other characteristics are satisfied. B If the hardness exceeds 250 HV, the area ratio of pearlite in the inner surface layer of the base material portion 2 is less than 10%, and the area ratio of hard structures such as bainite and / or martensite is high, so sufficient wear resistance cannot be obtained. Therefore, the Vickers hardness H at a depth of 0.1 mm from the inner surface of the base material 2 B is 190 to 250HV.

[0067] Vickers hardness H B The preferred lower limit is 195HV, more preferably 200HV, even more preferably 205HV, even more preferably 210HV, even more preferably 215HV, and even more preferably 220HV. Vickers hardness H B The upper limit of the hardness is preferably 245 HV, more preferably 240 HV, even more preferably 235 HV, and even more preferably 230 HV.

[0068] [Vickers hardness H at a depth of 0.1 mm from the inner surface of the base material 2 B Measurement method] Vickers hardness H at a depth of 0.1 mm from the inner surface of the base material 2 B is calculated in the following way: A test piece having a cross section (L cross section) including the pipe axial direction and wall thickness direction is taken from the base material portion 2. This cross section is used as the observation surface. When the wall thickness of the electric resistance welded steel pipe 1 is t mm, the size of the observation surface is 20.0 mm in the axial direction of the electric resistance welded steel pipe 1 and t mm in the wall thickness direction.

[0069] The observation surface is mirror-polished. On the mirror-polished observation surface, 25 measurement points are selected at a depth of 0.1 mm in the wall thickness direction from the inner surface of the base material 2. The distance between adjacent measurement points (axial distance of the electric resistance welded steel pipe 1) is 0.5 mm.

[0070] At each measurement point, a Vickers hardness test in accordance with JIS Z 2241-1 (2020) is carried out to obtain the Vickers hardness (HV). At this time, the test force is set to 0.49 N. The arithmetic mean value of the obtained 25 Vickers hardness values ​​is defined as the Vickers hardness HV of the inner surface layer of the base material portion 2. B (HV) Vickers hardness H B (HV) is the integer value obtained by rounding off the obtained arithmetic mean value to the nearest whole number.

[0071] [(Feature 4) Microstructure of Electric Resistance Weld 3] In the electric resistance welded portion 3, the ferrite area ratio is 50% or more, the pearlite area ratio is 10% or more, and the sum of the ferrite area ratio and the pearlite area ratio is 95% or more. Note that, in the microstructure of the electric resistance welded portion 3, the remainder other than ferrite and pearlite is not particularly limited. The remainder may be, for example, one or more types selected from the group consisting of a hard structure (bainite and / or martensite) and retained austenite.

[0072] As with the microstructure of the base material portion 2, if the microstructure of the electric resistance welded portion 3 has a ferrite area fraction of 50% or more, a pearlite area fraction of 10% or more, and the sum of the ferrite area fraction and pearlite area fraction is 95% or more, the wear resistance of the inner surface of the electric resistance welded steel pipe 1 will be improved. Specifically, when the inner surface of the electric resistance welded steel pipe 1 is subjected to repeated sliding, the pearlite in the microstructure serves as a supply source of fine cementite, which dispersion strengthens and solid solution strengthens the inner surface of the electric resistance welded portion 3. This improves the wear resistance of the inner surface of the electric resistance welded steel pipe 1.

[0073] The lower limit of the ferrite area ratio of the electric resistance welded portion 3 is preferably 55%, more preferably 60%, and even more preferably 65%. The upper limit of the ferrite area ratio of the electric resistance welded portion 3 is preferably 90%, more preferably 85%, even more preferably 80%, even more preferably 77%, and even more preferably 75%.

[0074] The lower limit of the pearlite area ratio of the electric resistance welded portion 3 is preferably 12%, more preferably 15%, and even more preferably 20%. The upper limit of the pearlite area ratio of the electric resistance welded portion 3 is preferably 50%, more preferably 40%, and even more preferably 35%.

[0075] [Method for measuring the microstructure of electric resistance weld 3] The ferrite area ratio (%) and pearlite area ratio (%) in the microstructure of the electric resistance welded portion 3 are measured by the following method.

[0076] A test piece having a cross section (L-section) including the pipe axial direction and wall thickness direction at the width center position of the electric resistance welded portion 3 of the electric resistance welded steel pipe 1 is taken. This cross section is used as the observation surface. The test piece is taken so that one side of the observation surface of the test piece is the inner surface of the electric resistance welded portion 3.

[0077] The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is then etched using a 3% nital solution to reveal the microstructure. Five observation fields are selected from the etched observation surface, extending from the inner surface of the electric resistance weld 3 to a depth of 0.3 mm (300 μm). The size of each observation field is a rectangle measuring 200 μm in the tube axis direction and 200 μm in the wall thickness direction. The five observation fields are arranged in a continuous row in the tube axis direction on the observation surface.

[0078] Each observation field is observed at 500x magnification using an SEM to generate an SEM image (secondary electron image). Using the obtained SEM image, pearlite, ferrite, hard structure (bainite and / or martensite), and retained austenite are identified based on contrast and morphology using the same method as described in [Method for measuring the microstructure of the base material portion 2].

[0079] The ferrite area ratio (%) of the electric resistance weld 3 is calculated based on the total area of ​​ferrite identified in the five observation fields and the total area of ​​the five observation fields. The ferrite area ratio (%) is an integer value obtained by rounding off the first decimal place of the obtained value. The pearlite area ratio (%) of the electric resistance weld 3 is calculated based on the total area of ​​pearlite identified in the five observation fields and the total area of ​​the five observation fields. The pearlite area ratio (%) is an integer value obtained by rounding off the first decimal place of the obtained value.

[0080] [(Feature 5) Average circle equivalent diameter D of the pearlite region of the electric resistance welded part 3 W About In the electric resistance welded portion 3, the average circle equivalent diameter D W is 10.0 μm or less.

[0081] Here, the pearlite region is a region consisting of one or a plurality of continuously connected pearlite blocks. FIG. 2 is a schematic diagram showing an example of a microstructure photograph image (secondary electron image) of an electric resistance welded portion 3 to explain the pearlite region. Referring to FIG. 2, the high-brightness (white) region without a substructure is ferrite 10. The region in which ferrite 21 (white region) and low-brightness (black) pearlite 22 are arranged in layers is pearlite. Here, a region of pearlite in which the ferrite 21 has the same crystal orientation is referred to as a pearlite block 20B. In this case, a region consisting of one or a plurality of continuously connected pearlite blocks 20B is defined as a pearlite region. For example, in FIG. 2, pearlite block 20B1 is a single pearlite block that is not continuously connected to other pearlite blocks 20B. Therefore, pearlite block 20B1 is identified as a pearlite region. Furthermore, pearlite blocks 20B2 to 20B5 are continuously connected. Therefore, the pearlite blocks 20B2 to 20B5 are identified as one pearlite region.

[0082] If the pearlite regions in the electric resistance weld 3 are excessively coarse, the number of pearlite regions dispersed per unit area of ​​the microstructure will be smaller than when the pearlite regions are fine. As described above, pearlite contains cementite, which serves as a source of dispersion strengthening and solid solution strengthening. Therefore, if the number of pearlite regions dispersed in the microstructure is small, it will be difficult to achieve dispersion strengthening and solid solution strengthening of the inner surface layer of the electric resistance weld 3 when sliding is repeated. Therefore, it is preferable that the size of each pearlite region is small.

[0083] Average circle equivalent diameter D of pearlite region W If the pearlite area ratio is 10.0 μm or less, the pearlite region is sufficiently fine. Therefore, if the pearlite area ratio is 10% or more, a sufficient amount of pearlite region is dispersed in the electric resistance welded portion 3. Therefore, when sliding is repeated, there are sufficient sources of dispersion strengthening and solid solution strengthening in the inner surface layer of the electric resistance welded portion 3. As a result, excellent wear resistance is obtained on the inner surface of the electric resistance welded portion 3.

[0084] Average circle equivalent diameter D of pearlite region W The preferred upper limit is 9.5 μm, more preferably 9.0 μm, even more preferably 8.5 μm, even more preferably 8.0 μm, even more preferably 7.5 μm, even more preferably 7.0 μm, even more preferably 6.5 μm, even more preferably 6.0 μm, and even more preferably 5.5 μm. Average circle equivalent diameter D of pearlite region W However, the average equivalent circle diameter D of the pearlite region is W Excessive reduction of the average circle equivalent diameter D of the pearlite region increases the manufacturing cost. W The preferred lower limit is 1.0 μm, more preferably 1.5 μm, and even more preferably 2.0 μm.

[0085] [Average circle equivalent diameter D of pearlite region of electric resistance weld 3 W Measurement method] The average circle equivalent diameter D of the pearlite region of the electric resistance weld 3 W is measured in the following way: In the above-mentioned [Method for Measuring the Microstructure of the Electric Resistance Welded Zone 3], pearlite regions are identified in the SEM images of each of the five observation fields by the above-mentioned method. 2 Furthermore, of the multiple identified pearlite regions, pearlite regions that come into contact with each side of the 200 μm × 200 μm rectangular observation field are also excluded from the measurement.

[0086] The average equivalent circle diameter (μm) of each pearlite region that has been identified as the measurement target is determined. The equivalent circle diameter is the value obtained by rounding off the first decimal place to the nearest tenth. The arithmetic mean value of the equivalent circle diameters of all the pearlite regions that are being measured is determined, and the average equivalent circle diameter D W (μm) Equivalent circle diameter D Wis the value obtained by rounding off the first decimal place to the nearest tenth. The circle equivalent diameter can be determined, for example, by using well-known image analysis software.

[0087] (Feature 6) Vickers hardness H at a depth of 0.1 mm from the inner surface of the electric resistance welded part 3 W About Furthermore, in the electric resistance welded part 3, the Vickers hardness H W is 190HV or more and satisfies formula (1). 0.80≦H W / H B ≦1.20 (1) Here, H in equation (1) W The Vickers hardness H of the electric resistance welded part 3 is W (HV) is substituted, and H B The Vickers hardness H of the base material 2 is B (HV) is substituted.

[0088] The Vickers hardness H at a depth of 0.1 mm from the inner surface of the base material 2 is the same as the hardness at a depth of 0.1 mm from the inner surface of the electric resistance welded portion 3. W If the Vickers hardness H is less than 190 HV, sufficient wear resistance cannot be obtained even if other characteristics are satisfied. W is 190HV or more.

[0089] Vickers hardness H W further satisfies formula (1). If the hardness at a position 0.1 mm deep from the inner surface of the electric resistance welded portion 3 is significantly different from the hardness at a position 0.1 mm deep from the inner surface of the base material 2, the inner surface of the electric resistance welded portion 3 or the base material 2, whichever has the lower hardness, will suffer localized excessive wear. If the inner surface hardness of the electric resistance welded portion 3 is similar to the inner surface hardness of the base material 2, localized wear can be suppressed.

[0090] Define F1 as follows: F1=H W / H B If F1 is within the range of 0.80 to 1.20, the inner surface hardness of the electric resistance welded portion 3 is approximately the same as the inner surface hardness of the base material portion 2. In this case, excellent wear resistance can be obtained, provided that the electric resistance welded steel pipe 1 satisfies other characteristics.

[0091] Vickers hardness H W The preferred lower limit is 195HV, more preferably 200HV, even more preferably 205HV, even more preferably 210HV, even more preferably 215HV, and even more preferably 220HV. Vickers hardness H W The upper limit of Vickers hardness H is not particularly limited as long as it satisfies formula (1). W The upper limit is, for example, 260HV, for example, 255HV, for example, 250HV.

[0092] The lower limit of F1 is preferably 0.85, more preferably 0.90, and even more preferably 0.95. The upper limit of F1 is preferably 1.15, more preferably 1.10, and even more preferably 1.05.

[0093] More preferably, the Vickers hardness H W is the Vickers hardness H of the base material 2 B The proportion of the inner surface of the electric resistance welded portion 3 to the inner surface of the electric resistance welded steel pipe 1 is significantly smaller than the proportion of the inner surface of the base material portion 2. The Vickers hardness H W The Vickers hardness H of the base material 2 B By making it higher than , it is possible to suppress localized wear on the inner surface of the electric resistance welded portion 3, which accounts for a small proportion of the inner surface of the electric resistance welded pipe. As a result, the electric resistance welded steel pipe 1 can achieve even better wear resistance.

[0094] [Vickers hardness H at a depth of 0.1 mm from the inner surface of the electric resistance weld 3 W Measurement method] Vickers hardness H at a depth of 0.1 mm from the inner surface of the electric resistance weld 3 W is calculated in the following way: A test piece having a cross section (L cross section) including the pipe axial direction and wall thickness direction at the width center position of the electric resistance welded portion 3 of the electric resistance welded steel pipe 1 is taken. This cross section is used as the observation surface. When the wall thickness of the electric resistance welded steel pipe 1 is t mm, the size of the observation surface is 20.0 mm in the axial direction of the electric resistance welded steel pipe 1 and t mm in the wall thickness direction.

[0095] The observation surface is mirror-polished. On the mirror-polished observation surface, 25 measurement points are selected at a depth of 0.1 mm in the wall thickness direction from the inner surface of the base material 2. The distance between adjacent measurement points (axial distance of the electric resistance welded steel pipe 1) is 0.5 mm.

[0096] At each measurement point, a Vickers hardness test in accordance with JIS Z 2241-1 (2020) is carried out to obtain the Vickers hardness (HV). The test force is set to 0.49 N. The arithmetic mean value of the obtained 25 Vickers hardness values ​​is defined as the Vickers hardness HV of the inner surface of the electric resistance welded portion 3. W (HV) Vickers hardness H W is the integer value obtained by rounding off the obtained arithmetic mean value to the nearest whole number.

[0097] [Effects of the electric resistance welded steel pipe 1 of this embodiment] The electric resistance welded steel pipe 1 of this embodiment satisfies Features 1 to 6. Therefore, the electric resistance welded steel pipe 1 of this embodiment has excellent wear resistance on the inner surface.

[0098] [Use of the electric resistance welded steel pipe 1 of this embodiment] The electric resistance welded steel pipe 1 of this embodiment has excellent wear resistance on the inner surface. Therefore, it can be widely used in applications where the above properties are required. The electric resistance welded steel pipe 1 of this embodiment is particularly suitable for use in hydraulic cylinders, such as arm cylinders for construction machinery.

[0099] [Shape of the electric resistance welded steel pipe 1 of this embodiment] There are no particular limitations on the shape and dimensions of the electric resistance welded steel pipe 1 of this embodiment. Preferably, the electric resistance welded steel pipe 1 has an outer diameter of 100 to 400 mm and a wall thickness of 5.0 to 18.0 mm. The lower limit of the outer diameter is preferably 110 mm, more preferably 120 mm, and even more preferably 130 mm. The upper limit of the outer diameter is preferably 380 mm, more preferably 360 mm, and even more preferably 340 mm. The lower limit of the wall thickness is preferably 6.0 mm, more preferably 7.0 mm, and even more preferably 8.0 mm. The upper limit of the wall thickness is preferably 17.0 mm, more preferably 16.0 mm, and even more preferably 15.0 mm.

[0100] [Manufacturing method] A method for manufacturing an electric-resistance welded steel pipe 1 according to this embodiment will be described. Note that the manufacturing method described below is an example, and the manufacturing method for the electric-resistance welded steel pipe 1 according to this embodiment is not limited to this. In other words, as long as an electric-resistance welded steel pipe 1 having the above-described configuration can be manufactured, the manufacturing method is not limited to the manufacturing method described below. However, the manufacturing method described below is a suitable method for manufacturing the electric-resistance welded steel pipe 1 according to this embodiment.

[0101] An example of a method for manufacturing an electric resistance welded steel pipe according to this embodiment includes the following steps. (Process 1) Molding process (Process 2) Welding process (Process 3) Seam heat treatment process (Process 4) Sizing process Each step will be described below.

[0102] [(Process 1) Molding process] In the forming process, a steel sheet is bent into a cylindrical open pipe. Specifically, a steel sheet having a chemical composition that satisfies Feature 1 is prepared. The steel sheet is formed into an open pipe shape (tubular shape) so that both ends of the steel sheet face each other. Specifically, after the steel sheet is unwound from a coil, a group of breakdown rolls in a forming device bend both ends of the steel sheet upward, curving the entire steel sheet into an arc. Furthermore, a group of fin pass rolls arranged downstream of the group of breakdown rolls apply finish forming to the steel sheet so that the steel sheet has a substantially circular cross section. Through the above steps, a steel sheet is formed into an open pipe shape with both ends of the steel sheet facing each other and spaced apart in the circumferential direction.

[0103] The steel plate used in the forming process is manufactured by the following method. A slab satisfying Feature 1 is produced by casting using a known method. The slab is heated. The heating temperature is not particularly limited, but is, for example, 1000 to 1300°C. The heated slab is rough rolled to produce a rough bar (intermediate steel plate). Rough rolling is performed using a reversing rolling mill. A tandem rolling mill is used to perform finish rolling on the rough bar to produce a hot-rolled steel plate. Accelerated cooling is performed on the hot-rolled steel plate after finish rolling. The cooling rate in the accelerated cooling is, for example, 10°C / second or more. The accelerated cooling is stopped at a cooling stop temperature T0, and the plate is then allowed to cool naturally.

[0104] Here, the finish rolling (hot rolling) satisfies the following conditions: (Condition 1) The cooling stop temperature T0 is set to 650 to 500°C.

[0105] [(Condition 1) Cooling stop temperature T0] If the cooling stop temperature T0 is too high, the Vickers hardness H B On the other hand, if the cooling stop temperature T0 is too low, the area ratio of pearlite decreases and the area ratio of hard structure (bainite and / or martensite) increases in the microstructure of the base material portion 2. Therefore, the cooling stop temperature T0 is set to 650 to 500°C.

[0106] [(Process 2) Welding process] In the welding process, both ends of an open-pipe steel plate are heated and melted, and then squeeze rolls are used to press the steel plate together (upset) and weld the ends. As both ends of the steel plate pass through the squeeze rolls, they are naturally cooled and solidified and joined as they are transported downstream. This results in the steel plate becoming an electric-resistance welded steel pipe intermediate with an electric-resistance weld extending in the longitudinal direction. Immediately after the welding process, weld beads are formed on the inner and outer surfaces of the electric-resistance welded steel pipe intermediate. The weld beads are cut and removed by a bead cutting device located downstream of the squeeze rolls.

[0107] [(Process 3) Seam heat treatment process] In the seam heat treatment step, normalizing is performed on the electric resistance welded portion of the manufactured electric resistance welded steel pipe intermediate body. Specifically, a predetermined range from the width center position of the electric resistance welded portion of the electric resistance welded steel pipe intermediate body (specifically, 60 to 80 mm in the circumferential direction of the electric resistance welded steel pipe intermediate body, centered on the width center position of the electric resistance welded portion) is heated. A heating device with an inductor (heating coil) is used for heating. The heating device is placed above the electric resistance welded portion, and the electric resistance welded portion is heated. This performs normalizing on the electric resistance welded portion.

[0108] The seam heat treatment process includes a heating and holding process, a first cooling process, and a second cooling process. In the heating and holding process, the electric resistance weld is heated to a normalizing temperature T1 (°C) and held at the normalizing temperature T1 (°C). In the first cooling process, the electric resistance weld held at the normalizing temperature T1 (°C) is cooled to a quenching stop temperature T2 (°C) at a first cooling rate CR1 (°C / sec). In the second cooling process, the electric resistance weld is cooled from the quenching stop temperature T2 (°C) at a second cooling rate CR2 (°C / sec). In other words, the seam heat treatment process involves two stages of cooling.

[0109] The seam heat treatment process satisfies the following conditions: (Condition 2) Normalizing temperature T1 is A of the steel plate c3 Transformation point + relative temperature T N If the relative temperature T N is set to 10 to 100°C. (Condition 3) The quenching stop temperature T2 is set to 500 to 300°C. (Condition 4) The first cooling rate CR1 is set to 100 to 200° C. / sec. (Condition 5) The second cooling rate CR2 is set to 20° C. / sec or less. Each condition will be explained below.

[0110] [(Condition 2) Normalizing temperature T1] The normalizing temperature T1 (℃) is the A c3 Higher than the transformation point, A c3 The temperature range is near the transformation point. Specifically, the normalizing temperature T1 is the A c3 Transformation point +10℃~A c3 The transformation point is +100°C. Here, the normalizing temperature T1 is the A c3 Transformation point + T N (℃), the relative temperature T N is 10 to 100°C.

[0111] Relative temperature T N When the temperature is less than 10°C, the microstructure of the electric resistance weld is maintained in the temperature range close to the two-phase region (ferrite and austenite) or in the two-phase region. In this case, a part of the structure is not transformed during the normalizing treatment and becomes coarse. As a result, the average circle equivalent diameter D of the pearlite region of the electric resistance weld 3 of the manufactured electric resistance welded steel pipe 1 W may become excessively large. On the other hand, the relative temperature T N If the temperature exceeds 100°C, the austenite grains in the electric resistance weld become coarse. As a result, the average circle equivalent diameter D W becomes excessively large. Therefore, the relative temperature T N is set to 10 to 100°C.

[0112] [(Condition 3) Quenching stop temperature T2] The quenching stop temperature T2 means the temperature (°C) at which cooling is started from the normalizing temperature T1 (°C) at a first cooling rate CR1 (°C / sec) after the heating and holding step, and at which cooling at the first cooling rate CR1 is stopped. If the quenching stop temperature T2 is higher than 500°C, the transformation temperature to pearlite becomes high. As a result, the average equivalent circle diameter D of the pearlite region becomes W becomes excessively large. On the other hand, if the quenching stop temperature T2 is less than 300°C, the cooling in the first cooling step will be excessive. In this case, the pearlite area ratio in the electric resistance welded portion 3 of the manufactured electric resistance welded steel pipe 1 will be less than 10%, and the remaining portion will have a microstructure mainly composed of hard structures (bainite and martensite). Therefore, the Vickers hardness H W becomes excessively high. Therefore, the quenching stop temperature T2 is set to 500 to 300°C.

[0113] The quenching stop temperature T2 (°C) is the surface temperature (°C) of the electric resistance welded portion at the time when the first cooling step is completed. The surface temperature of the electric resistance welded portion can be measured using a well-known thermometer, such as a thermograph.

[0114] [(Condition 4) First cooling rate CR1] The average cooling rate in the temperature range from the normalizing temperature T1 (°C) to the quenching stop temperature T2 (°C) is defined as the first cooling rate CR1 (°C / sec). The first cooling rate CR1 can be calculated based on the normalizing temperature T1, the quenching stop temperature T2, and the time from the start to the end of the first cooling step.

[0115] If the first cooling rate CR1 is less than 100°C / sec, the cooling in the first cooling step is too slow. In this case, the pearlite transformation start temperature becomes high. As a result, the average circle equivalent diameter D W becomes excessively large. On the other hand, if the first cooling rate CR1 exceeds 200°C / sec, the cooling in the first cooling step will be excessive, and in this case, the area fraction of pearlite in the microstructure of the electric resistance weld will be less than 10%, with the remainder being a microstructure mainly composed of martensite and / or bainite. Therefore, the first cooling rate CR1 is set to 100 to 200° C. / sec.

[0116] [(Condition 5) Second cooling rate CR2] The average cooling rate in the temperature range from the quenching stop temperature T2 to 50°C is defined as the second cooling rate CR2 (°C / sec). The second cooling rate CR2 can be determined based on the time it takes for the surface of the electric resistance weld to cool from the quenching stop temperature T2 (°C) to 50°C.

[0117] If the second cooling rate CR2 exceeds 20°C / sec, the cooling in the second cooling step is too fast, resulting in an excessively small area fraction of pearlite and an excessively large area fraction of bainite, etc. Therefore, the second cooling rate CR2 is set to 20° C. / sec or less.

[0118] [(Process 4) Sizing Process] In the sizing process, the electric-resistance welded steel pipe intermediates after the seam heat treatment process are continuously fed to a shaping machine, where they are shaped into their final shape. Specifically, the electric-resistance welded steel pipe intermediates are subjected to a slight drawing process as they pass through multiple roll stands included in the shaping machine. This finishes the electric-resistance welded steel pipe intermediates so that their cross sections are perfectly circular and their outer diameters are within dimensional tolerances.

[0119] Through the above manufacturing steps, the electric resistance welded steel pipe 1 is manufactured.

[0120] [Another embodiment of the manufacturing method of the electric resistance welded steel pipe of this embodiment] The method for producing an electric resistance welded steel pipe according to this embodiment may further include a tempering step.

[0121] The tempering process does not have to be performed. If it is performed, the tempering process is performed on the electric resistance welded portion of the electric resistance welded steel pipe intermediate body after the seam heat treatment process and before the sizing process. Specifically, an area within a circumferential direction of 60 to 80 mm centered on the width center position of the electric resistance welded portion of the intermediate electric resistance welded steel pipe is heated and held at a tempering temperature (°C) for a predetermined time. The tempering temperature is, for example, 200 to 400°C. [Example]

[0122] The effects of the electric resistance welded steel pipe of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the electric resistance welded steel pipe of this embodiment. Therefore, the electric resistance welded steel pipe of this embodiment is not limited to this one example of conditions.

[0123] Electric resistance welded steel pipes having base metal parts with the chemical compositions shown in Table 1 (Tables 1A and 1B) were manufactured.

[0124] [Table 1A]

[0125] [Table 1B]

[0126] The electric resistance welded steel pipes of each test number were manufactured by the following method. First, the steel plate was manufactured using the following manufacturing process. After heating a slab at 1000 to 1300°C, rough rolling was performed to produce a rough bar (intermediate steel plate). Using a tandem rolling mill, the rough bar was subjected to finish rolling to produce a hot-rolled steel plate. The hot-rolled steel plate after finish rolling was subjected to accelerated cooling. The cooling rate in the accelerated cooling was 30°C / sec. The accelerated cooling was stopped at the cooling stop temperature T0 shown in Table 2, and then the plate was allowed to cool to room temperature. Through the above manufacturing process, a steel plate having the chemical composition shown in Table 1 (Table 1A and Table 1B) was manufactured.

[0127] [Table 2]

[0128] The manufactured steel plate was bent to form a cylindrical open pipe (forming process). The open pipe was subjected to electric resistance welding to manufacture an electric resistance welded steel pipe intermediate (welding process). The electric resistance welded steel pipe intermediate was subjected to a seam heat treatment process (seam heat treatment process). The relative temperature T N The cooling time (°C), first cooling rate CR1 (°C / sec), quenching stop temperature T2 (°C), and second cooling rate CR2 (°C / sec) were as shown in Table 2. A sizing process was carried out on the electric resistance welded steel pipe intermediate product after the seam heat treatment process.

[0129] Using the above manufacturing process, electric resistance welded steel pipes of each test number were manufactured. Each electric resistance welded steel pipe of each test number had an outer diameter of 100 mm and a wall thickness of 10.0 mm.

[0130] [Evaluation test] The electric resistance welded steel pipes with each test number were subjected to the following evaluation tests. (Test 1) Microstructure observation test of base material (Test 2) Vickers hardness test of the inner surface of the base material (Test 3) Microstructure observation test of electric resistance welded joint (Test 4) Average circle equivalent diameter D of the pearlite region of the electric resistance weld W Measurement Test (Test 5) Vickers hardness test of the inner surface of the electric resistance weld (Test 6) Abrasion resistance evaluation test Each test will be explained below.

[0131] [(Test 1) Microstructure observation test of base material] Based on the method described in the above-mentioned [Method for measuring the microstructure of base material 2], the ferrite area ratio (%) and pearlite area ratio (%) of the base material of the electric resistance welded steel pipe of each test number were determined. Furthermore, the remaining structure other than ferrite and pearlite was identified. The obtained ferrite area ratio (%) and pearlite area ratio (%) are shown in Table 3. Furthermore, the "Remaining structure" column in the "Base material" column in Table 3 shows the remaining structure other than ferrite and pearlite in the microstructure of the base material. "Hard structure" means that the remaining structure was a hard structure consisting of bainite and / or martensite. "-" means that the remaining structure was not confirmed.

[0132] [(Test 2) Vickers hardness test of the inner surface of the base material] The above-mentioned [Vickers hardness H at a depth of 0.1 mm from the inner surface of the base material 2] B Based on the method described in [Measurement method of], the Vickers hardness H of the inner surface of the base metal of the welded steel pipe of each test number was measured. B The Vickers hardness (HV) was calculated. B (HV) is shown in Table 3.

[0133] [(Test 3) Microstructure observation test of electric resistance welded joint] Based on the method described in the above [Method for measuring the microstructure of electric resistance weld 3], the ferrite area ratio (%) and pearlite area ratio (%) of the electric resistance weld of the electric resistance welded steel pipe of each test number were determined. Furthermore, the remaining structure other than ferrite and pearlite was identified. The obtained ferrite area ratio (%) and pearlite area ratio (%) are shown in Table 3. Furthermore, the "Remaining structure" column in the "Electric resistance weld" column in Table 3 shows the remaining structure other than ferrite and pearlite in the microstructure of the base material. "Hard structure" means that the remaining structure was a hard structure consisting of bainite and / or martensite. "-" means that the remaining structure was not confirmed.

[0134] [(Test 4) Average circle equivalent diameter D of pearlite region of electric resistance weld W Measurement Test] The above-mentioned [average circle equivalent diameter D of the pearlite region of the electric resistance welded portion 3] WBased on the method described in [Measuring Method of Electric Resistance Welding], the average circle equivalent diameter D of the pearlite region of the electric resistance weld of the electric resistance welded steel pipe of each test number W The average equivalent circle diameter D (μm) was calculated. W (μm) are shown in Table 3.

[0135] [(Test 5) Vickers hardness test of the inner surface of the electric resistance welded part] The Vickers hardness H at a depth of 0.1 mm from the inner surface of the electric resistance welded portion 3 is W Based on the method described in [Measuring Method of Electric Resistance Welding], the Vickers hardness H of the electric resistance welded part of the electric resistance welded steel pipe of each test number was measured. W The Vickers hardness (HV) was calculated. W (HV) is shown in Table 3.

[0136] [(Test 6) Wear resistance evaluation test] The wear resistance of the inner surface of the electric resistance welded steel pipe of each test number was evaluated by the following sliding test. Cylindrical intermediate test specimens measuring 200 mm in the axial direction were collected from each ERW steel pipe. Semi-cylindrical test specimens (hereinafter referred to as arc-shaped test specimens) were prepared by cutting the intermediate test specimens axially at a cross section including the central axis of the ERW steel pipe. When viewed axially, the arc-shaped test specimens had a convex arc shape, with the ERW weld located at the apex of the arc shape. In the following description, the convex cylindrical surface of the arc-shaped test specimens will be referred to as the "outer surface," and the concave cylindrical surface located opposite the outer surface will be referred to as the "inner surface." The inner surfaces of the arc-shaped test specimens were finish-polished using 1000-grit sandpaper. Note that the hardness of the inner surface did not change before and after polishing, even after finish-polishing using 1000-grit sandpaper. The arc-shaped test specimens were then degreased with alkali, and their masses (g) were measured.

[0137] A semi-cylindrical jig with an outer surface identical to the inner surface of the arc-shaped test specimen was prepared. The radius of the jig's arc was 40 mm, and the jig's length was 100 mm. The jig's material corresponded to SUJ2 as specified in JIS G 4805 (2019). The following sliding test was performed on the arc-shaped test specimen using the jig. The outer surface of the jig was brought into contact with the inner surface of the arc-shaped test specimen under a pressure load of 10 kg, and the jig was reciprocated 5,000 times in the longitudinal direction over the entire length (200 mm) of the arc-shaped test specimen. The sliding speed during reciprocation was 100 mm / s. After the sliding test, the arc-shaped test specimen was subjected to alkaline degreasing to remove any dirt, such as wear particles, adhering to the surface of the arc-shaped test specimen. The mass (g) of the arc-shaped test specimen after alkaline degreasing was measured.

[0138] The wear amount (g) in the sliding test was calculated by subtracting the mass of the arc-shaped test piece after the sliding test from the mass of the arc-shaped test piece before the sliding test. The calculated wear amount is shown in the "Wear amount (g)" column under "Wear resistance" in Table 3.

[0139] [Table 3]

[0140] [Evaluation results] Referring to Tables 1 to 3, Test Nos. 1 to 20 satisfied Features 1 to 6. Therefore, the amount of wear in the wear resistance evaluation test was 10 g or less, and excellent wear resistance was obtained.

[0141] Furthermore, among test numbers 1 to 20, test numbers 1, 2, 5, 7, and 10 to 20 show that the Vickers hardness H W is the Vickers hardness H of the base material B As a result, the amount of wear in the wear resistance evaluation test was 5 g or less, and even better wear resistance was achieved.

[0142] On the other hand, in test numbers 21 and 22, the cooling stop temperature T0 in the manufacturing process (finish rolling) of the hot-rolled steel sheet was too low, and the pearlite area ratio of the base material was less than 10%. As a result, the wear amount in the wear resistance evaluation test exceeded 10 g, and sufficient wear resistance was not obtained.

[0143] In test numbers 23 and 24, the cooling stop temperature T0 in the manufacturing process of the hot-rolled steel sheet was too high, and the Vickers hardness H B As a result, the amount of wear exceeded 10 g in the wear resistance evaluation test, and sufficient wear resistance was not achieved.

[0144] In test numbers 25 and 26, the relative temperature T N Therefore, the average circle equivalent diameter D of the pearlite region of the electric resistance weld W As a result, the amount of wear exceeded 10 g in the wear resistance evaluation test, and sufficient wear resistance was not achieved.

[0145] In test numbers 27 and 28, the relative temperature T N Therefore, the average circle equivalent diameter D of the pearlite region of the electric resistance weld W As a result, the amount of wear exceeded 10 g in the wear resistance evaluation test, and sufficient wear resistance was not achieved.

[0146] In test numbers 29 and 30, the first cooling rate CR1 was slow. Therefore, the average circular equivalent diameter D W As a result, the amount of wear exceeded 10 g in the wear resistance evaluation test, and sufficient wear resistance was not achieved.

[0147] In test numbers 31 and 32, the quenching stop temperature T2 was too low. As a result, the pearlite area ratio of the electric resistance weld was too low, and excessive hard structure was generated. As a result, the wear amount in the wear resistance evaluation test exceeded 10 g, and sufficient wear resistance was not obtained.

[0148] In test numbers 33 and 34, the quenching stop temperature T2 was too high. Therefore, the average circle equivalent diameter D of the pearlite region of the electric resistance weld W As a result, the amount of wear exceeded 10 g in the wear resistance evaluation test, and sufficient wear resistance was not achieved.

[0149] In test number 35, the C content in the base material was too low. Therefore, the pearlite area ratio in the base material was too low, and the Vickers hardness H B Furthermore, the pearlite area ratio of the electric resistance welded part was too low, and the Vickers hardness H W As a result, the amount of wear exceeded 10 g in the wear resistance evaluation test, and sufficient wear resistance was not achieved.

[0150] In test number 36, the C content in the base material was too high. Therefore, the pearlite area ratio in the base material was too low, and the Vickers hardness H B The surface area ratio of pearlite in the electric resistance welded joint was too low. As a result, the wear amount in the wear resistance evaluation test exceeded 10g, and sufficient wear resistance was not achieved.

[0151] In test numbers 37 and 38, the cooling stop temperature T0 was low. Therefore, the pearlite area ratio of the base material was too low, and H W / H B As a result, the amount of wear exceeded 10 g in the wear resistance evaluation test, and sufficient wear resistance was not achieved.

[0152] In test numbers 39 and 40, the cooling stop temperature T0 was high. Therefore, the Vickers hardness H B is too low, H W / H B As a result, the amount of wear exceeded 10 g in the wear resistance evaluation test, and sufficient wear resistance was not achieved.

[0153] In test numbers 41 and 42, one-stage cooling was performed instead of two-stage cooling during the seam heat treatment process. As a result, the pearlite area ratio of the electric resistance weld was too low. As a result, the wear amount in the wear resistance evaluation test exceeded 10 g, and sufficient wear resistance was not obtained.

[0154] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure. [Explanation of symbols]

[0155] 1 ERW steel pipe 2 Base material part 3 Electric resistance welded section

Claims

1. An electric resistance welded steel pipe, A base material portion; an electric resistance welded portion extending in the axial direction of the electric resistance welded steel pipe, The chemical composition of the electric resistance welded steel pipe is, in mass%, C: 0.10-0.30%, Si: 0.03-1.20%, Mn: 1.00-2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005-0.500%, Nb: 0.010-0.060%, N: 0.0003 to 0.0060%, Ti: 0-0.200%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Cr: 0-1.00%, Mo: 0 to 0.50%, V: 0-0.20%, B: 0 to 0.0100%, W: 0-0.10%, Ca: 0-0.0200%, Mg: 0 to 0.0200%, Zr: 0 to 0.0200%, and Rare earth elements: 0 to 0.0200%; The balance is Fe and impurities. In the base material portion, The ferrite area ratio is 50% or more, and the pearlite area ratio is 10% or more, and the sum of the ferrite area ratio and the pearlite area ratio is 95% or more, Vickers hardness H at a depth of 0.1 mm from the inner surface of the base material Ｂ is 190 to 250 HV, In the electric resistance welded portion, The ferrite area ratio is 50% or more, and the pearlite area ratio is 10% or more, and the sum of the ferrite area ratio and the pearlite area ratio is 95% or more, Average circle equivalent diameter D of a pearlite region consisting of one or a plurality of continuously connected pearlite blocks Ｗ is 10.0 μm or less, Vickers hardness H at a depth of 0.1 mm from the inner surface of the electric resistance weld Ｗ is 190HV or more and satisfies formula (1); ERW steel pipe. 0.80≦H Ｗ / H Ｂ ≦1.20 (1)

2. The electric resistance welded steel pipe according to claim 1, The Vickers hardness H Ｗ is the Vickers hardness H Ｂ Higher than ERW steel pipe.

3. The electric resistance welded steel pipe according to claim 1, The chemical composition is, in mass %, Ti: 0.001 to 0.200%, Cu: 0.01 to 1.00%, Ni: 0.01-1.00%, Cr: 0.01-1.00%, Mo: 0.01-0.50%, V: 0.01-0.20%, B: 0.0001 to 0.0100%, W: 0.01-0.10%, Ca: 0.0001-0.0200%, Mg: 0.0001 to 0.0200%, Zr: 0.0001 to 0.0200%, and Rare earth elements: 0.0001 to 0.0200%, containing one or more selected from the group consisting of ERW steel pipe.

4. The electric resistance welded steel pipe according to claim 1, An outer diameter of 100 to 400 mm; A wall thickness of 5.0 to 18.0 mm. ERW steel pipe.

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