Electric resistance welded steel pipe

Abstract

Provided is an electric resistance welded steel pipe having excellent wear resistance on the inner surface thereof. An electric resistance welded steel pipe (1) according to the present disclosure comprises: a base material part (2); and an electric resistance welded part (3) that extends in the pipe axis direction of the electric resistance welded steel pipe (1). In the base material part (2), the ferrite area ratio is at least 50%, the pearlite area ratio is at least 10%, the total of the ferrite area ratio and the pearlite area ratio is at least 95%, and the Vickers hardness HB at a depth position of 0.1 mm from the inner surface is 190-250 HV. In the electric resistance welded part (3), the ferrite area ratio is at least 50%, the pearlite area ratio is at least 10%, the total of the ferrite area ratio and the pearlite area ratio is at least 95%, the average equivalent circle diameter DW in a pearlite region is at most 10.0 μm, the Vickers hardness HW at a depth position of 0.1 mm from the inner surface is at least 190 HV, and expression (1) is satisfied.　Expression (1): 0.80≤HW / HB≤1.20

Description

ERW steel pipe

[0001] This disclosure relates to electric resistance welded steel pipes.

[0002] Hydraulic and pneumatic cylinders used in industrial and construction machinery are equipped with cylinder tubes. Electric resistance welded (ERW) steel pipes are used for the cylinder tubes. When ERW steel pipes are used as cylinder tubes, mechanical parts such as pistons are inserted into the ERW steel pipe, and these inserted mechanical parts slide in the axial direction of the pipe while in contact with the inner surface of the ERW steel pipe. This sliding motion of the mechanical parts causes wear on the inner surface of the ERW steel pipe. When wear particles are generated due to wear, these particles hinder the sliding of the mechanical parts. As a result, the operability of the cylinder decreases. Therefore, when ERW steel pipes are used as cylinder tubes, the ERW steel pipes are required to have excellent wear resistance on their inner surface.

[0003] Electric resistance welded (ERW) steel pipes with enhanced wear resistance are proposed in Japanese Patent Publication No. 8-225853 (Patent Document 1) and Japanese Patent Publication No. 6-306459 (Patent Document 2). In the ERW steel pipes disclosed in Patent Documents 1 and 2, the microstructure is made of a matrix such as ferrite or bainite, and island-like martensite is dispersed in the matrix. Patent Documents 1 and 2 state that this increases wear resistance.

[0004] JP-A No. 8-225853 JP-A No. 6-306459

[0005] However, even when electric resistance welded steel pipes with the above-described microstructure are used for cylinder tubes, sufficient wear resistance on the inner surface may not be obtained.

[0006] The purpose of this disclosure is to provide electric resistance welded steel pipes that have excellent wear resistance on the inner surface.

[0007] The electric resistance welded (ERW) steel pipe of this disclosure comprises a base material portion and an ERW weld portion extending in the axial direction of the ERW steel pipe. The chemical composition of the ERW steel pipe is, in mass%, C: 0.10 to 0.30%, Si: 0.03 to 1.20%, Mn: 1.00 to 2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005 to 0.500%, Nb: 0.010 to 0.060%, N: 0.0003 to 0.0060%, Ti: 0 to 0.200%, Cu: 0 to 1%. It contains 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 being Fe and impurities. 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 pearlite area ratio is 95% or more, and the Vickers hardness at a depth of 0.1 mm from the inner surface of the base material is H Ｂ The voltage is 190-250 HV. In the electric resistance welded joint, 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 equivalent circle diameter D of the pearlite region consisting of one or more continuously connected pearlite blocks is 1 or more. Ｗ The thickness is 10.0 μm or less, and the Vickers hardness H is measured at a depth of 0.1 mm from the inner surface of the electric resistance welded joint. Ｗ The voltage is 190 HV or higher, and equation (1) is satisfied. 0.80 ≤ H Ｗ / H Ｂ ≤ 1.20 (1)

[0008] The electric resistance welded steel pipe of this disclosure provides excellent wear resistance on the inner surface.

[0009] Figure 1 is an enlarged view of the vicinity of the electric resistance welded joint in a cross-section perpendicular to the pipe axis direction of the electric resistance welded steel pipe of this embodiment. Figure 2 is a schematic diagram showing an example of a microstructural photograph of the electric resistance welded joint to explain the pearlite region.

[0010] The inventors first investigated electric resistance welded steel pipes that could obtain excellent wear resistance on the inner surface. As a result, the inventors obtained the following findings.

[0011] To improve wear resistance, it is preferable to increase the hardness of the steel. Therefore, from the viewpoint of increasing the hardness of the steel and improving wear resistance, the chemical composition of electric resistance welded steel pipes was investigated. As a result, in mass percent, 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 believe 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 remainder being Fe and impurities, can increase the hardness of its inner surface and improve its wear resistance.

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

[0013] The inventors considered the cause of this problem as follows: When hard structures such as bainite and island martensite are used in the microstructure of electric resistance welded steel pipes, the hardness of the inner surface increases. Therefore, it might seem that wear resistance would increase. However, when mechanical parts such as pistons repeatedly slide against the inner surface of the electric resistance welded steel pipe in the axial direction, the hardness of the martensite and bainite does not increase easily even with repeated sliding. Rather, repeated sliding causes cracks to occur and propagate. As a result, the amount of wear increases with the number of sliding cycles.

[0014] Based on the above considerations, the present inventors considered that if the microstructure of an electric resistance welded steel pipe is not formed of hard structures (bainite, island martensite) in advance, but is formed into a structure whose hardness increases with repeated sliding motion, the wear resistance on the inner surface of the electric resistance welded steel pipe will be improved. Accordingly, the present inventors conducted studies on a structure whose hardness increases with repeated sliding. As a result, the present inventors found that when the microstructure and hardness of the base metal portion and the microstructure and hardness of the electric resistance welded portion of the electric resistance welded steel pipe satisfy the following features, the wear resistance on the inner surface of the electric resistance welded steel pipe is remarkably improved. (I) In the base metal portion, the area fraction of ferrite is 50% or more, the area fraction of pearlite is 10% or more, the total of the area fraction of ferrite and the area fraction of pearlite is 95% or more, and the Vickers hardness H at a position 0.1 mm deep from the inner surface of the base metal portion Ｂ is 190 to 250 HV. (II) In the electric resistance welded portion, the area fraction of ferrite is 50% or more, the area fraction of pearlite is 10% or more, the total of the area fraction of ferrite and the area fraction of pearlite is 95% or more, and the Vickers hardness H at a position 0.1 mm deep from the inner surface of the electric resistance welded portion Ｗ is 190 HV or more, and satisfies the formula (1). 0.80≦H Ｗ / H Ｂ ≦1.20 (1)

[0015] In the electric resistance welded steel pipe of the present embodiment, as shown in (I) and (II) above, the base metal portion and the electric resistance welded portion contain pearlite in an area fraction of 10% or more, and the hardness of the inner surfaces of the base metal portion and the electric resistance welded portion is substantially equal. Although the reason why the wear resistance on the inner surface of the electric resistance welded steel pipe is remarkably improved when the base metal portion and the electric resistance welded portion satisfy such features is not clear, the following reason is considered.

[0016] When mechanical parts such as pistons repeatedly slide against the inner surface of an electric resistance welded (ERW) steel pipe, the inner surface of the ERW steel pipe is subjected to external forces from the mechanical parts. These external forces cause the pearlite on the inner surface to pulverize. As the pearlite is pulverized, the cementite within the pearlite is also pulverized, and fine cementite particles are dispersed throughout the surface. This dispersion of cementite strengthens the surface, increasing its hardness. Furthermore, the carbon in the crushed fine cementite particles forms a supersaturated solid solution with the ferrite in the pearlite. In this case, solid solution strengthening further increases the hardness of the inner surface. Through the above mechanism, the hardness of the inner surface of the ERW steel pipe increases significantly with repeated sliding. As a result, the wear resistance of the inner surface is expected to increase significantly.

[0017] However, even when the base material and the electric resistance welded joint satisfied conditions (I) and (II), sufficient wear resistance was still sometimes not obtained. Therefore, the inventors conducted further investigations. As a result, they obtained the following findings.

[0018] In electric resistance welded (ERW) steel pipes, the ERW welded section is formed during the ERW welding process in the manufacturing of the ERW steel pipe. This process involves melting both ends of the steel plate in the width direction, then upsetting the plate, squeezing out the molten portion, and pressing the ends together. Through these steps, both ends of the steel plate in the width direction are joined, forming the ERW welded section. Because the ERW welded section is formed by heating to high temperatures, its microstructure may differ from that of the base material. In this case, wear may be accelerated in the ERW welded section compared to the base material. Therefore, the inventors believe that further adjusting the microstructure of the ERW welded section is effective in improving the wear resistance of the ERW steel pipe.

[0019] Based on the above considerations, further investigation revealed that in an electric resistance welded joint, the average equivalent circle diameter D of the pearlite region, which consists of one or more continuously connected pearlite blocks, is... Ｗ The inventors have found that if the thickness is 10.0 μm or less, excellent wear resistance can be obtained in electric resistance welded steel pipes.

[0020] Average circular equivalent diameter D of the perlite region in the electric resistance welded joint ＷThe reason why excellent wear resistance can be obtained by making the pearlite region 10.0 μm or less is not entirely clear, but the following reasons are possible. When the pearlite region in an electric resistance weld is coarse, the number of pearlite regions per unit area is less compared to when the pearlite region is fine. As a result, when the pearlite region is coarse, multiple ferrite grains, which are softer than the pearlite region, tend to be arranged in a continuous manner. In this case, the ferrite region consisting of continuously arranged ferrite grains has lower wear resistance than the pearlite region. As a result, sufficient wear resistance cannot be obtained. On the other hand, when the pearlite region is fine, the pearlite region and ferrite region are moderately dispersed, and the size of the ferrite region consisting of multiple ferrite grains tends to be small. Therefore, wear resistance to repeated sliding tends to be higher. Furthermore, as mentioned above, pearlite contains cementite, which is a source of dispersion strengthening and solid solution strengthening. Therefore, when the pearlite region is sufficiently dispersed, there are more sources of dispersion strengthening and solid solution strengthening. As a result, wear resistance to repeated sliding is significantly increased.

[0021] The above mechanism is a hypothesis, and it is possible that superior wear resistance is achieved by other mechanisms. However, as demonstrated in the examples described later, if the base material and the electric resistance welded joint of the electric resistance welded steel pipe satisfy the above characteristics, superior wear resistance can be obtained.

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

[0023] The first embodiment of the electric resistance welded (ERW) steel pipe comprises a base material portion and an ERW weld portion extending in the axial direction of the ERW steel pipe. The chemical composition of the ERW 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%, Cu: 0-1%. It contains 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 being Fe and impurities. 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 pearlite area ratio is 95% or more, and the Vickers hardness at a depth of 0.1 mm from the inner surface of the base material is H Ｂ The voltage is 190-250 HV. In the electric resistance welded joint, 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 equivalent circle diameter D of the pearlite region consisting of one or more continuously connected pearlite blocks is 1 or more. Ｗ The thickness is 10.0 μm or less, and the Vickers hardness H is measured at a depth of 0.1 mm from the inner surface of the electric resistance welded joint. Ｗ The voltage is 190 HV or higher, and equation (1) is satisfied. 0.80 ≤ H Ｗ / H Ｂ ≤ 1.20 (1)

[0024] The second form of the electric resistance welded steel pipe is the same as the first form of the electric resistance welded steel pipe, with a Vickers hardness H Ｗ It has a Vickers hardness of H Ｂ It's higher than that.

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

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

[0027] The following describes the electric resistance welded steel pipe of this embodiment in detail. Unless otherwise specified, "%" in relation to elements refers to mass percentage.

[0028] [Configuration of the electric resistance welded steel pipe of this embodiment] Figure 1 is an enlarged view of the vicinity of the electric resistance welded portion in a cross section (C section) perpendicular to the pipe axis direction of the electric resistance welded steel pipe of this embodiment. Referring to Figure 1, the electric resistance welded steel pipe 1 of this embodiment comprises 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 later. The electric resistance welded portion 3 extends in the pipe axis direction of the electric resistance welded steel pipe 1. The electric resistance welded portion 3 is formed by butting the opposing ends of a cylindrical steel plate formed by bending together and performing electric resistance welding (upset welding). The width of the electric resistance welded portion 3 (the circumferential length of the electric resistance welded portion 3 on 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: (Characteristic 1) The chemical composition is, in mass%, C: 0.10 to 0.30%, Si: 0.03 to 1.20%, Mn: 1.00 to 2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005 to 0.500%, Nb: 0.010 to 0.060%, N: 0.0003 to 0.0060%, Ti: 0 to 0.200%, Cu: 0 to 1.00% It contains %, 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 being Fe and impurities. (Feature 2) In the base material 2, 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 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 The voltage is 190 to 250 HV. (Feature 4) In the electric resistance welded joint 3, 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. (Feature 5) In the electric resistance welded joint 3, the average equivalent circle diameter D of the pearlite region consisting of one or a plurality of continuously connected pearlite blocks. Ｗ The thickness 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 welded joint 3 Ｗ The voltage is 190 HV or higher, and equation (1) is satisfied. 0.80 ≤ H Ｗ / H Ｂ ≤ 1.20 (1) The following describes each characteristic.

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

[0031] C: 0.10-0.30% Carbon (C) increases the strength of the steel and 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 fully obtained, even if the content of other elements is within the range 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 part 2 and electric resistance welded part 3) tends to become a bainite-dominant structure. In this case, even if the content of other elements is within the range of this embodiment, the wear resistance of the inner surface of the electric resistance welded steel pipe decreases. Therefore, the C content is 0.10-0.30%. The preferred lower limit of the C content is 0.12%, more preferably 0.14%, and still more preferably 0.16%. The preferred upper limit of the C content is 0.28%, more preferably 0.26%, and still more preferably 0.24%.

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

[0033] Mn: 1.00 to 2.00% Manganese (Mn) enhances the hardenability and strength of steel. If the Mn content is less than 1.00%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 2.00%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. In this case, the cold formability, such as bending, decreases. Therefore, the Mn content is 1.00 to 2.00%. The preferred lower limit of the Mn content is 1.05%, more preferably 1.10%, and even more preferably 1.15%. The preferred upper limit of the Mn content is 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 will excessively segregate at the grain boundaries. In this case, even if the content of other elements is within the range of this embodiment, the cold formability such as bending will decrease. Therefore, the P content is 0.030% or less. It is preferable that the P content be as low as possible. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the P content is 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 sulfide. If the S content exceeds 0.010%, Mn sulfide will be produced in excess. In this case, even if the content of other elements is within the range of this embodiment, cracking starting from Mn sulfide is likely to occur. Therefore, the S content is 0.010% or less. It is preferable that the S content be as low as possible. However, excessive reduction of the S content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is 0.001%, and more preferably 0.002%. The preferred upper limit of the S content is 0.008%, more preferably 0.006%, and still more preferably 0.005%.

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

[0037] Nb: 0.010 to 0.060% Niobium (Nb) combines with C or N in the steel to form Nb precipitates. The Nb precipitates suppress the coarsening of austenite grains during the seam heat treatment process in the manufacturing process of the electric resistance welded steel pipe 1 by a pinning effect. If the Nb content is less than 0.010%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Nb content exceeds 0.060%, coarse Nb precipitates are generated. In this case, even if the content of other elements is within the range of this embodiment, cracks are more likely to occur starting from the coarse Nb precipitates during the bending process in the manufacturing process of the electric resistance welded steel pipe 1. Therefore, the Nb content is 0.010 to 0.060%. The preferred lower limit of the Nb content is 0.013%, more preferably 0.015%, and even more preferably 0.020%. The preferred upper limit for the Nb content is 0.055%, more preferably 0.050%, 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 suppresses the coarsening of austenite grains during the seam heat treatment process in the manufacturing process of the electric resistance welded steel pipe 1 by its pinning effect. If the N content is less than 0.0003%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the N content exceeds 0.0060%, coarse AlN is generated. In this case, even if the content of other elements is within the range of this embodiment, cracks originating from coarse AlN are more likely to occur 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 preferred lower limit of the N content is 0.0008%, more preferably 0.0010%, and even more preferably 0.0013%. The preferred upper limit for the N content is 0.0055%, more preferably 0.0050%, and even more preferably 0.0047%.

[0039] The remainder of the chemical composition of the electric resistance welded steel pipe 1 in this embodiment consists of Fe and impurities. Here, impurities are substances that are mixed in from the raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the steel plate that forms the 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 in this embodiment. For example, the impurities are one or more selected from the group consisting of Sn: 0 to 0.2% and Pb: 0 to 0.2%.

[0040] [Regarding Optional Elements] 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: 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, selected by mass%, from the group. 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 one or more elements selected from the group consisting of Ti, Cu, Ni, Cr, Mo, V, B, and W in place of a portion of Fe. These elements are arbitrary 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 included. In other words, the Ti content may be 0%. If Ti is included, that is, if the Ti content is greater than 0%, Ti forms Ti precipitates, increasing the strength of the steel. Even if only a small amount of Ti is included, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.200%, coarse Ti precipitates will be formed. In this case, even if the content of other elements is within the range of this embodiment, cracks are more likely to occur during bending in the manufacturing process of the electric resistance welded steel pipe 1, starting from the coarse Ti precipitates. Therefore, the Ti content is 0 to 0.200%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.010%, even more preferably 0.020%, and even more preferably 0.030%. The preferred upper limit for the Ti content is 0.190%, more preferably 0.180%, more preferably 0.170%, more preferably 0.150%, and still more preferably 0.100%.

[0043] Cu: 0-1.00% Copper (Cu) is an optional element and may not be present. In other words, the Cu content may be 0%. If Cu is present, that is, if the Cu content is greater than 0%, Cu increases the hardenability and strength of the steel. Even if only a small amount of Cu is present, 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 content of other elements is within the range of this embodiment. In this case, sufficient cold formability such as bending cannot be obtained. Therefore, the Cu content is 0-1.00%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.05%, more preferably 0.10%, and still more preferably 0.15%. The preferred upper limit of the Cu content is 0.98%, more preferably 0.90%, and still more preferably 0.85%.

[0044] Ni: 0-1.00% Nickel (Ni) is an optional element and may not be included. In other words, the Ni content may be 0%. If Ni is included, that is, if the Ni content is greater than 0%, Ni increases the hardenability and strength of the steel. Even if only a small amount of Ni is included, 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 content of other elements is within the range of this embodiment. In this case, sufficient cold formability such as bending cannot be obtained. Therefore, the Ni content is 0-1.00%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.03%, more preferably 0.05%, and still more preferably 0.10%. The preferred upper limit of the Ni content is 0.98%, more preferably 0.95%, more preferably 0.92%, and still more preferably 0.85%.

[0045] Cr: 0-1.00% Chromium (Cr) is an optional element and may not be present. In other words, the Cr content may be 0%. If Cr is present, that is, if the Cr content is greater than 0%, Cr increases the hardenability and strength of the steel. Even if only a small amount of Cr is present, the above 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 content of other elements is within the range of this embodiment. In this case, sufficient cold formability such as bending cannot be obtained. Therefore, the Cr content is 0-1.00%. The preferred lower limit of the Cr content is 0.01%, more preferably 0.05%, more preferably 0.10%, and still more preferably 0.20%. The preferred upper limit of the Cr content is 0.97%, more preferably 0.94%, and still more preferably 0.90%.

[0046] Mo: 0-0.50% Molybdenum (Mo) is an optional element and may not be present. In other words, the Mo content may be 0%. When present, i.e., when the Mo content is greater than 0%, Mo enhances the hardenability and strength of the steel. Mo further forms Mo precipitates. Mo precipitates enhance the strength of the steel through precipitation strengthening. Even if only a small amount of Mo is present, 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 content of other elements is within the range of this embodiment. In this case, sufficient cold formability for bending and other processes cannot be obtained. Therefore, the Mo content is 0-0.50%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the Mo content is 0.48%, more preferably 0.40%, even more preferably 0.36%, and even more preferably 0.30%.

[0047] V: 0-0.20% Vanadium (V) is an optional element and may not be present. In other words, the V content may be 0%. If V is present, that is, if the V content is greater than 0%, V forms V precipitates. V precipitates increase the strength of the steel through precipitation strengthening. Even if only a small amount of V is present, 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 content of other elements is within the range of this embodiment. In this case, sufficient cold formability such as bending cannot be obtained. Therefore, the V content is 0-0.20%. The preferred lower limit of the V content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the V content is 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 present. In other words, the B content may be 0%. If it is present, that is, if the B content is greater than 0%, B increases the hardenability and strength of the steel. Even if only a small amount of B is present, the above effects can be obtained to some extent. However, if the B content exceeds 0.0100%, coarse precipitates containing B will precipitate at the grain boundaries. Therefore, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the steel will decrease. Accordingly, the B content is 0 to 0.0100%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The preferred upper limit of the B content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0085%.

[0049] W: 0-0.10% Tungsten (W) is an optional element and may not be present. In other words, the W content may be 0%. If it is present, that is, if the W content is greater than 0%, W dissolves in the steel and increases the strength of the steel. Even if only a small amount of W is present, 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 content of other elements is within the range of this embodiment. In this case, sufficient cold formability such as bending cannot be obtained. Therefore, the W content is 0-0.10%. The preferred lower limit of the W content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the W content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0050] [Second Group: 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 one or more elements selected from the group consisting of Ca, Mg, Zr, and rare earth elements (REM) in place of a portion of Fe. These elements are arbitrary elements, and each controls the morphology of inclusions and suppresses the occurrence of cracks originating from inclusions. The following describes each element.

[0051] Ca: 0 to 0.0200% Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. If Ca is present, that is, if the Ca content is greater than 0%, Ca controls the morphology of inclusions, causing them to spheroidize and refine. As a result, the occurrence of cracks originating from inclusions is suppressed. Even if only a small amount of Ca is present, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0200%, coarse oxides will be formed. In this case, even if the content of other elements is within the range of this embodiment, sufficient cold formability cannot be obtained. Therefore, the Ca content is 0 to 0.0200%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0015%, even more preferably 0.0030%, and even more preferably 0.0045%. The preferred upper limit for the Ca content is 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 present. In other words, the Mg content may be 0%. If Mg is present, that is, if the Mg content is greater than 0%, Mg controls the morphology of inclusions, causing them to spheroidize and refine. As a result, the occurrence of cracks originating from inclusions is suppressed. Even if only a small amount of Mg is present, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0200%, Mg oxide will be produced in excess. In this case, even if the content of other elements is within the range of this embodiment, the toughness and wear resistance of the steel will decrease. Therefore, the Mg content is 0 to 0.0200%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0030%. The preferred upper limit for the Mg content is 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 present. In other words, the Zr content may be 0%. If it is present, that is, if the Zr content is greater than 0%, Zr controls the morphology of inclusions, causing them to spheroidize and refine. As a result, the occurrence of cracks originating from inclusions is suppressed. Even if only a small amount of Zr is present, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.0200%, excessive Zr oxide is produced. In this case, even if the content of other elements is within the range of this embodiment, the toughness and wear resistance of the steel will decrease. Therefore, the Zr content is 0 to 0.0200%. The preferred lower limit of the Zr content is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0030%, and even more preferably 0.0040%. The preferred upper limit for the Zr content is 0.0190%, more preferably 0.0170%, and even more preferably 0.0150%.

[0054] Rare Earth Elements (REM): 0 to 0.0200% Rare earth elements (REM) are optional elements and do not need to be included. In other words, the REM content may be 0%. If REM is included, that is, if the REM content is greater than 0%, REM controls the morphology of inclusions, causing them to spheroidize and refine. As a result, the occurrence of cracks originating from inclusions is suppressed. Even if only a small amount of REM is included, the above effect can be obtained to some extent. However, if the REM content exceeds 0.0200%, coarse oxides will be formed. In this case, even if the content of other elements is within the range of this embodiment, sufficient cold formability cannot be obtained. Therefore, the REM content is 0 to 0.0200%. The preferred lower limit of the REM content is 0.0001%, more preferably 0.0010%, more preferably 0.0020%, and still more preferably 0.0030%. The preferred upper limit for the REM content is 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) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this specification, REM content refers to the total content (mass%) of these elements. [(Feature 2) Microstructure of the base material 2] In the base material 2 of the electric resistance welded steel pipe 1 of this embodiment, 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 pearlite area ratio is 95% or more. In the microstructure of the base material 2, the remainder other than ferrite and pearlite is not particularly limited. The remainder consists of, for example, one or more elements selected from the group consisting of bainite, martensite, and retained austenite. In the following description, bainite and martensite will also be referred to as hard structures.

[0056] If the microstructure of the base material 2 has a ferrite area ratio of 50% or more and 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 will be increased. When machine parts repeatedly slide against the inner surface of the electric resistance welded steel pipe 1, pearlite becomes a source of cementite. Specifically, the pearlite is crushed by the sliding, and fine cementite is dispersed in the surface layer of the inner surface. The dispersed fine cementite strengthens the inner surface layer by dispersion. Furthermore, the carbon in the crushed cementite supersaturates and solid-solves 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 increases, and the wear resistance of the electric resistance welded steel pipe 1 is increased.

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

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

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

[0060] A test specimen is taken from the base material 2, which is offset 180° circumferentially around the central axis of the electric resistance welded steel pipe 1 from the center of the width of the electric resistance welded joint 3 of the electric resistance welded steel pipe 1. The surface of the test specimen is observed from the cross section (L section) that includes the pipe axis direction and wall thickness direction of the electric resistance welded steel pipe 1. The test specimen is taken so that one side of the observation surface of the test specimen is on the inner surface of the base material 2.

[0061] The observation surface of the test specimen is polished to a mirror finish. After mirror polishing, the observation surface is etched using a 3% Nital solution to reveal the microstructure. From the etched observation surface, five observation fields are selected from the region up to a depth of 0.3 mm (300 μm) from the inner surface of the base material 2. Each observation field is a rectangle measuring 200 μm in the axial direction of the tube and 200 μm in the thickness direction. The five observation fields are arranged in a continuous line along the axial direction of the tube 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 images, ferrite, pearlite, bainite, martensite, and retained austenite are identified as follows based on contrast and morphology.

[0063] A striped structure containing both high-brightness regions (ferrite) and low-brightness regions (cementite) is identified as pearlite. Regions that are brighter than pearlite and where no underlying structures such as lath are observed are identified as ferrite. Regions that are brighter than pearlite but darker than ferrite and where no underlying structures such as lath are observed are identified as retained austenite. A structure that is darker than pearlite, ferrite, and retained austenite and where a lath structure is observed is identified as hard tissue (bainite and / or martensite). In this embodiment, since it is not necessary to clearly distinguish between bainite and martensite, they are not distinguished in microstructural observation and are identified as hard tissue.

[0064] The ferrite area ratio (%) of the base material 2 is determined 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 the obtained value to the first decimal place. The pearlite area ratio (%) of the base material 2 is determined 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 the obtained value to the first decimal place.

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

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

[0067] Vickers hardness H Ｂ The preferred lower limit is 195 HV, more preferably 200 HV, more preferably 205 HV, more preferably 210 HV, more preferably 215 HV, and more preferably 220 HV. Vickers hardness H Ｂ A preferred upper limit is 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 This is determined by the following method: A test piece having a cross-section (L-section) that includes the pipe axis direction and the wall thickness direction is taken from the base material 2. This cross-section is to be 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 polished to a mirror finish. On the mirror-polished observation surface, 25 measurement points are selected at a depth of 0.1 mm in the 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 is performed in accordance with JIS Z 2241-1 (2020) to obtain the Vickers hardness (HV). The test force is set to 0.49 N. The arithmetic mean of the 25 obtained Vickers hardness values ​​is used to determine the Vickers hardness H of the inner surface layer of the base material 2. Ｂ (HV) Vickers hardness H Ｂ (HV) is the integer value obtained by rounding the first decimal place of the obtained arithmetic mean.

[0071] [(Feature 4) Microstructure of the electric resistance welded joint 3] In the electric resistance welded joint 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. The remainder of the microstructure of the electric resistance welded joint 3, other than ferrite and pearlite, is not particularly limited. The remainder consists of, for example, one or more materials selected from the group consisting of hard structures (bainite and / or martensite) and retained austenite.

[0072] In the microstructure of the electric resistance welded joint 3, similar to the microstructure of the base material 2, if 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 pearlite area ratio is 95% or more, the wear resistance of the inner surface of the electric resistance welded steel pipe 1 will be increased. Specifically, when repeated sliding occurs on the inner surface of the electric resistance welded steel pipe 1, the pearlite in the microstructure becomes a source of fine cementite, dispersing and solid-solution strengthening the inner surface of the electric resistance welded joint 3. This increases the wear resistance of the inner surface of the electric resistance welded steel pipe 1.

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

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

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

[0076] A test specimen is taken from the electric resistance welded steel pipe 1, having a cross-section (L-section) that includes the pipe axis direction and wall thickness direction at the center of the width of the electric resistance welded joint 3. This cross-section will be the observation surface. The test specimen is taken so that one side of the observation surface is on the inner surface of the electric resistance welded joint 3.

[0077] The observation surface of the test specimen is polished to a mirror finish. After mirror polishing, the observation surface is etched using a 3% Nital solution to reveal the microstructure. From the etched observation surface, five observation fields are selected from the region up to a depth of 0.3 mm (300 μm) from the inner surface of the electric resistance weld 3. Each observation field is a rectangle measuring 200 μm in the axial direction of the pipe and 200 μm in the wall thickness direction. The five observation fields are arranged in a continuous line in the axial direction of the pipe on the observation surface.

[0078] Each observation field is observed at a magnification of 500x using a SEM to generate an SEM image (secondary electron image). Using the obtained SEM image, pearlite, ferrite, hard tissue (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 2].

[0079] The ferrite area ratio (%) of the electric resistance welded joint 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 the obtained value to the first decimal place. The pearlite area ratio (%) of the electric resistance welded joint 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 the obtained value to the first decimal place.

[0080] [(Feature 5) Average circular equivalent diameter D of the pearlite region of the electric resistance welded joint 3] Ｗ Regarding the electric resistance welded joint 3, the average circular equivalent diameter D of the pearlite region is also Ｗ The particle size is 10.0 μm or less.

[0081] Here, a pearlite region is a region consisting of one or more continuously connected pearlite blocks. Figure 2 is a schematic diagram showing an example of a microstructural photograph (secondary electron image) of an electric resistance welded joint 3 to explain the pearlite region. Referring to Figure 2, the bright (white) region without a substructure is ferrite 10. The region in which ferrite 21 (white region) and dark (black) pearlite 22 are arranged in layers is pearlite. Here, among the pearlite, the region in which the crystal orientation of ferrite 21 is the same is defined as a pearlite block 20B. In this case, a region consisting of one or more continuously connected pearlite blocks 20B is defined as a pearlite region. For example, in Figure 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. Also, pearlite blocks 20B2 to 20B5 are continuously connected. Therefore, the perlite blocks 20B2 to 20B5 are identified as a single perlite region.

[0082] In the electric resistance welded joint 3, if the pearlite regions are excessively coarse, the number of pearlite regions dispersed per unit area of ​​the microstructure will be less compared to when the pearlite regions are fine. As mentioned above, pearlite contains cementite, which is 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 disperse and solid-solution strengthen the inner surface layer of the electric resistance welded joint 3 when sliding is repeated. For this reason, it is preferable for the size of each pearlite region to be small.

[0083] Average circular equivalent diameter D of the perlite region Ｗ If the area 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 joint 3. As a result, when sliding is repeated, there is a sufficient source of dispersion strengthening and solid solution strengthening on the inner surface layer of the electric resistance welded joint 3. As a result, excellent wear resistance is obtained on the inner surface of the electric resistance welded joint 3.

[0084] Average circular equivalent diameter D of the perlite region Ｗ The preferred upper limit is 9.5 μm, more preferably 9.0 μm, more preferably 8.5 μm, more preferably 8.0 μm, more preferably 7.5 μm, more preferably 7.0 μm, more preferably 6.5 μm, more preferably 6.0 μm, and more preferably 5.5 μm. Average equivalent circle diameter D of the pearlite region Ｗ It is preferable that it be as small as possible. However, the average equivalent diameter D of the pearlite region Ｗ Excessive reduction of increases manufacturing costs. Therefore, the average equivalent circle diameter D of the pearlite region. Ｗ The preferred lower limit is 1.0 μm, more preferably 1.5 μm, and even more preferably 2.0 μm.

[0085] [Average circular equivalent diameter D of the pearlite region in the electric resistance welded joint 3] Ｗ [Measurement Method] Average circular equivalent diameter D of the pearlite region of the electric resistance welded joint 3 Ｗ The measurement is performed using the following method. In the above-described [Method for measuring the microstructure of the electric resistance welded joint 3], the pearlite region is identified in the SEM image of each of the five observation fields using the method described above. Among the multiple pearlite regions identified, the area of ​​0.75 μm ２ Perlite regions smaller than a certain size are excluded from measurement. Additionally, among the identified multiple perlite regions, those that come into contact with each side of a 200 μm x 200 μm rectangular observation field are also excluded from measurement.

[0086] The average equivalent circle diameter (μm) of each pearlite region identified as the target of measurement is determined. The equivalent circle diameter is rounded to one decimal place by rounding the second decimal place of the obtained value. The arithmetic mean of the equivalent circle diameters of all pearlite regions targeted for measurement is calculated to determine the average equivalent circle diameter D of the pearlite regions. Ｗ Let (μm) be the equivalent diameter of the circle D. Ｗ The value obtained is rounded to one decimal place, with the second decimal place being rounded off. The equivalent circle diameter can be determined, for example, 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 joint 3] Ｗ Regarding the electric resistance welded joint 3, the Vickers hardness H at a depth of 0.1 mm from the inner surface is further measured. Ｗ The voltage is 190 HV or higher, and equation (1) is satisfied. 0.80 ≤ H Ｗ / H Ｂ ≤ 1.20 (1) Here, H in equation (1) Ｗ The Vickers hardness of the electric resistance welded joint 3 is H Ｗ (HV) is substituted, H Ｂ The Vickers hardness H of the base material 2 is Ｂ (HV) is substituted.

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

[0089] Vickers hardness H Ｗ Furthermore, equation (1) is satisfied. If the hardness at a depth of 0.1 mm from the inner surface of the electric resistance welded joint 3 differs significantly from the hardness at a depth of 0.1 mm from the inner surface of the base material 2, the inner surface of the electric resistance welded joint 3 or the base material 2 with the lower hardness will be locally excessively worn. If the inner surface hardness of the electric resistance welded joint 3 is about the same as the inner surface hardness of the base material 2, localized wear can be suppressed.

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

[0091] Vickers hardness H ＷThe preferred lower limit is 195 HV, more preferably 200 HV, more preferably 205 HV, more preferably 210 HV, more preferably 215 HV, and more preferably 220 HV. Vickers hardness H Ｗ The upper limit is not particularly limited as long as equation (1) is satisfied. Vickers hardness H Ｗ The upper limits are, for example, 260HV, for example, 255HV, and for example, 250HV.

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

[0093] More preferably, the Vickers hardness H of the electric resistance welded joint 3 Ｗ The Vickers hardness of the base material 2 is H. Ｂ It is higher than [the other value]. The proportion of the inner surface of the electric resistance welded section 3 within the inner surface of the electric resistance welded steel pipe 1 is significantly less than the proportion of the inner surface of the base material section 2. The Vickers hardness of the electric resistance welded section 3 is H. Ｗ The Vickers hardness of the base material 2 is H Ｂ By increasing the value, localized wear on the inner surface of the electric resistance welded section 3, which accounts for a small proportion of the inner surface of the electric resistance welded pipe, can be suppressed. As a result, even better wear resistance can be obtained in the electric resistance welded steel pipe 1.

[0094] [Vickers hardness H at a depth of 0.1 mm from the inner surface of the electric resistance welded joint 3] Ｗ [Measurement Method] Vickers hardness H at a depth of 0.1 mm from the inner surface of the electric resistance welded joint 3. Ｗ This is determined by the following method: A test piece is taken from the electric resistance welded steel pipe 1, having a cross-section (L-section) that includes the pipe axis direction and the wall thickness direction at the center of the width of the electric resistance welded joint 3. This cross-section is to be 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 and t mm in the wall thickness direction of the electric resistance welded steel pipe 1.

[0095] The observation surface is polished to a mirror finish. On the mirror-polished observation surface, 25 measurement points are selected at a depth of 0.1 mm in the 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 is performed in accordance with JIS Z 2241-1 (2020) to obtain the Vickers hardness (HV). The test force is set to 0.49 N. The arithmetic mean of the 25 obtained Vickers hardness values ​​is used to determine the Vickers hardness HV of the inner surface of the electric resistance welded joint 3. Ｗ (HV) Vickers hardness H Ｗ This is the integer value obtained by rounding the first decimal place of the obtained arithmetic mean.

[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 provides excellent wear resistance on the inner surface.

[0098] [Applications of the electric resistance welded steel pipe 1 of this embodiment] The electric resistance welded steel pipe 1 of this embodiment provides excellent wear resistance on its inner surface. Therefore, it can be widely applied to applications where the above characteristics are required. The electric resistance welded steel pipe 1 of this embodiment is particularly suitable for hydraulic cylinder applications, such as the arm cylinders of construction machinery.

[0099] [Shape of the electric resistance welded steel pipe 1 in this embodiment] The shape and dimensions of the electric resistance welded steel pipe 1 in this embodiment are not particularly limited. 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 preferred lower limit of the outer diameter is 110 mm, more preferably 120 mm, and even more preferably 130 mm. The preferred upper limit of the outer diameter is 380 mm, more preferably 360 mm, and even more preferably 340 mm. The preferred lower limit of the wall thickness is 6.0 mm, more preferably 7.0 mm, and even more preferably 8.0 mm. The preferred upper limit of the wall thickness is 17.0 mm, more preferably 16.0 mm, and even more preferably 15.0 mm.

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

[0101] An example of the manufacturing method for electric resistance welded steel pipes according to this embodiment includes the following steps: (Step 1) Forming step (Step 2) Welding step (Step 3) Seam heat treatment step (Step 4) Sizing step Each step will be described below.

[0102] [(Process 1) Forming Process] In the forming process, the steel sheet is bent to form 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 face each other. Specifically, after the steel sheet is unwound from the coil, the breakdown rolls in the forming apparatus bend both ends of the steel sheet upward, causing the entire steel sheet to curve into an arc shape. Furthermore, the fin pass rolls positioned downstream of the breakdown rolls perform finish forming on the steel sheet so that the steel sheet has a substantially circular cross-section. Through the above process, an open pipe-shaped steel sheet is formed in which both ends of the steel sheet are separated in the circumferential direction and face each other.

[0103] The steel sheets used in the forming process are manufactured by the following method: A slab satisfying characteristic 1 is manufactured by casting using a well-known method. The slab is heated. The heating temperature is not particularly limited, but for example, it is 1000 to 1300°C. The heated slab is roughly rolled to manufacture a rough bar (intermediate steel sheet). Rough rolling is carried out using a reverse rolling mill. Finish rolling is performed on the rough bar using a tandem rolling mill to manufacture a hot-rolled steel sheet. Accelerated cooling is performed on the hot-rolled steel sheet after finish rolling. The cooling rate in accelerated cooling is, for example, 10°C / second or more. Accelerated cooling is stopped at the cooling stop temperature T0, and then the sheet is allowed to cool naturally.

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

[0105] [Regarding (Condition 1) Cooling stop temperature T0] If the cooling stop temperature T0 is too high, the Vickers hardness H of the inner surface of the base material 2 of the manufactured electric resistance welded steel pipe 1 will be Ｂ This becomes lower. On the other hand, if the cooling stop temperature T0 is too low, the pearlite area ratio decreases and the hard structure (bainite and / or martensite) area ratio increases in the microstructure of the base material 2. Therefore, the cooling stop temperature T0 should be set to 650 to 500°C.

[0106] [(Process 2) Welding Process] In the welding process, both ends of the open pipe-shaped steel plate are heated and melted, and then a squeeze roll is used to press the ends of the steel plate together (upset) to weld them together. As the ends of the steel plate pass through the squeeze roll are transported downstream, they solidify and are joined by natural cooling. As a result, the steel plate becomes an electric resistance welded steel pipe intermediate having an electric resistance welded section that extends in the longitudinal direction. Weld beads are formed on the inner and outer surfaces of the electric resistance welded steel pipe intermediate immediately after the welding process. The weld beads are cut off by a bead cutting device located downstream of the squeeze roll.

[0107] [(Step 3) Seam Heat Treatment Process] In the seam heat treatment process, the electric resistance welded (ERW) welded section of the manufactured ERW steel pipe intermediate is subjected to pre-heating. Specifically, a predetermined range (specifically, 60 to 80 mm in the circumferential direction of the ERW steel pipe intermediate, centered on the width of the ERW welded section) is heated from the width of the ERW welded section of the ERW steel pipe intermediate. A heating device with an induction element (heating coil) is used for heating. The heating device is placed above the ERW welded section, and the ERW welded section is heated. This pre-heats the ERW welded section.

[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 welded joint is heated to a normalizing temperature T1 (°C) and held at that temperature. In the first cooling process, the electric resistance welded joint, held at the normalizing temperature T1 (°C), is cooled to a rapid cooling stop temperature T2 (°C) at a first cooling rate CR1 (°C / second). In the second cooling process, the joint is cooled from the rapid cooling stop temperature T2 (°C) at a second cooling rate CR2 (°C / second). In other words, the seam heat treatment process involves two stages of cooling.

[0109] In the seam heat treatment process, the following conditions must be met: (Condition 2) The normalizing temperature T1 is set to the A of the steel sheet. ｃ３ Transformation point + relative temperature T Ｎ In that case, the relative temperature T Ｎ The temperature should be 10 to 100°C. (Condition 3) The rapid cooling stop temperature T2 should be 500 to 300°C. (Condition 4) The first cooling rate CR1 should be 100 to 200°C / second. (Condition 5) The second cooling rate CR2 should be 20°C / second or less. The following explains each condition.

[0110] [Regarding (Condition 2) Normalizing temperature T1] The normalizing temperature T1 (°C) is the temperature of the steel plate A ｃ３ Higher than the transformation point, A ｃ３ The temperature range is defined as the temperature near the transformation point. Specifically, the normalizing temperature T1 is the temperature of the steel sheet A ｃ３ Transformation point +10°C to A ｃ３ Let the transformation point be +100°C. Here, the normalizing temperature T1 is the A of the steel plate. ｃ３ Perversion point + T Ｎ When expressed in (°C), the relative temperature T Ｎ The temperature range is 10 to 100°C.

[0111] Relative temperature T Ｎ If the temperature is below 10°C, the microstructure of the electric resistance welded joint is maintained in a temperature range near the two-phase region (ferrite and austenite) or within the two-phase region. In this case, during the normalizing process, a portion of the microstructure becomes coarser without transformation. As a result, the average equivalent circle diameter D of the pearlite region of the electric resistance welded joint 3 of the manufactured electric resistance welded steel pipe 1 Ｗ In some cases, it can become excessively large. On the other hand, relative temperature T Ｎ If the temperature exceeds 100°C, the austenite grains in the electric resistance welded area become coarser. As a result, the average equivalent circle diameter D of the pearlite region decreases.Ｗ It becomes excessively large. Therefore, the relative temperature T Ｎ The temperature should be between 10 and 100°C.

[0112] [Regarding (Condition 3) Rapid Cooling Stop Temperature T2] Rapid cooling stop temperature T2 refers to the temperature (°C) at which cooling starts from the normalizing temperature T1 (°C) after the heating and holding process, and stops at the first cooling rate CR1 (°C / sec). If the rapid cooling stop temperature T2 is higher than 500°C, the transformation temperature to pearlite will be higher. As a result, the average equivalent circle diameter D of the pearlite region will be higher. Ｗ This becomes excessively large. On the other hand, if the rapid cooling stop temperature T2 is less than 300°C, the cooling in the first cooling process becomes excessive. In this case, the pearlite area ratio in the electric resistance welded joint 3 of the manufactured electric resistance welded steel pipe 1 becomes less than 10%, and the remainder becomes a microstructure mainly composed of hard structures (bainite and martensite). Therefore, the Vickers hardness H of the electric resistance welded joint 3 becomes excessive. Ｗ The temperature becomes excessively high. Therefore, the rapid cooling stop temperature T2 should be set to 500-300°C.

[0113] The rapid cooling stop temperature T2 (°C) is the surface temperature (°C) of the electric resistance welded joint at the end of the first cooling process. The surface temperature of the electric resistance welded joint can be measured using a well-known thermometer, such as a thermograph.

[0114] [Condition 4: Regarding the first cooling rate CR1] The average cooling rate in the temperature range from the normalizing temperature T1 (°C) to the rapid cooling stop temperature T2 (°C) is defined as the first cooling rate CR1 (°C / second). The first cooling rate CR1 can be determined based on the normalizing temperature T1, the rapid cooling stop temperature T2, and the time from the start time to the end time of the first cooling process.

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

[0116] [Condition 5: Regarding the second cooling rate CR2] The average cooling rate in the temperature range from the rapid cooling stop temperature T2 to 50°C is defined as the second cooling rate CR2 (°C / second). The second cooling rate CR2 can be determined based on the time it takes for the surface of the electric resistance welded joint to reach 50°C from the rapid cooling stop temperature T2 (°C).

[0117] If the second cooling rate CR2 exceeds 20°C / second, the cooling in the second cooling process is too fast. In this case, the area ratio of pearlite becomes excessively low, and the area ratio of bainite and other materials becomes excessively high. Therefore, the second cooling rate CR2 should be kept below 20°C / second.

[0118] [(Process 4) Sizing Process] In the sizing process, the electric resistance welded steel pipe intermediates after the seam heat treatment process are continuously supplied to the shaping machine to form the final shape. Specifically, the electric resistance welded steel pipe intermediates pass through multiple roll stands included in the shaping machine, which applies a slight drawing process to them. As a result, the electric resistance welded steel pipe intermediates are finished so that their cross-section is perfectly circular and their outer diameter is within the dimensional tolerance.

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

[0120] [Other embodiments of the electric resistance welded steel pipe manufacturing method of this embodiment] The electric resistance welded steel pipe manufacturing method of this embodiment may further include a tempering step.

[0121] The tempering step may be omitted. When implemented, in the tempering step, tempering is performed on the electric resistance welded seam of the intermediate electric resistance welded steel pipe after the seam heat treatment step and before the sizing step. Specifically, the area within 60 to 80 mm in the circumferential direction centered on the width center position of the electric resistance welded seam of the intermediate electric resistance welded steel pipe is heated and held at a tempering temperature (°C) for a predetermined period of time. The tempering temperature is, for example, 200 to 400°C.

[0122] The effects of the electric resistance welded steel pipe according to the present embodiment will be further specifically described with reference to working examples. The conditions used in the following working examples are examples of conditions adopted for confirming the feasibility and effects of the electric resistance welded steel pipe according to the present embodiment. Therefore, the electric resistance welded steel pipe according to the present embodiment is not limited to these example conditions.

[0123] Electric resistance welded steel pipes each having a base material portion with chemical compositions shown in Table 1 (Table 1A and Table 1B) were manufactured.

[0124]

[0125]

[0126] Electric resistance welded steel pipes with respective test numbers were manufactured by the following method. First, steel sheets were manufactured by the following manufacturing process. After heating a slab at 1000 to 1300°C, rough rolling was performed to produce a rough bar (intermediate steel sheet). Using a tandem rolling mill, finish rolling was performed on the rough bar to produce a hot-rolled steel sheet. Accelerated cooling was performed on the hot-rolled steel sheet after finish rolling. 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 steel sheet was left to cool to normal temperature. Through the above manufacturing process, steel sheets having the chemical compositions shown in Table 1 (Table 1A and Table 1B) were manufactured.

[0127]

[0128] The manufactured steel sheet was bent to form a cylindrical open pipe (forming step). Electric resistance welding was performed on the open pipe to produce an intermediate electric resistance welded steel pipe (welding step). A seam heat treatment step was performed on the intermediate electric resistance welded steel pipe (seam heat treatment step). The relative temperature T in the seam heat treatment step ＮThe temperature (°C), first cooling rate CR1 (°C / sec), rapid cooling stop temperature T2 (°C), and second cooling rate CR2 (°C / sec) are as shown in Table 2. A sizing process was performed on the electric resistance welded steel pipe intermediate after the seam heat treatment process.

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

[0130] [Evaluation Tests] The following evaluation tests were performed on each electric resistance welded steel pipe with the specified test number: (Test 1) Microstructure observation test of the base material (Test 2) Vickers hardness test of the inner surface layer of the base material (Test 3) Microstructure observation test of the electric resistance welded area (Test 4) Average equivalent circle diameter D of the pearlite region of the electric resistance welded area Ｗ Measurement Tests (Test 5) Vickers hardness test of the inner surface layer of the electric resistance welded joint (Test 6) Abrasion resistance evaluation test The following describes each test.

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

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

[0133] [(Test 3) Microstructure observation test of electric resistance welded portion] Based on the method described in the above [Method for measuring microstructure of electric resistance welded portion 3], the ferrite area ratio (%) and pearlite area ratio (%) of the electric resistance welded portion of the electric resistance welded steel pipe of each test number were obtained. Further, the remaining structure other than ferrite and pearlite was identified. The obtained ferrite area ratio (%) and pearlite area ratio (%) are shown in Table 3. Further, in the "Remaining structure" column of the "Electric resistance welded portion" column in Table 3, the remaining structure other than ferrite and pearlite among the microstructures of the base material portion is shown. "Hard structure" means that the remaining structure was a hard structure composed of bainite and / or martensite. "-" means that no remaining structure was confirmed.

[0134] [(Test 4) Average equivalent circle diameter D of pearlite regions in electric resistance welded portion Ｗ Measurement test] Based on the method described in the above [Method for measuring average equivalent circle diameter D of pearlite regions in electric resistance welded portion 3 Ｗ , the average equivalent circle diameter D of pearlite regions in the electric resistance welded portion of the electric resistance welded steel pipe of each test number Ｗ (μm) was obtained. The obtained average equivalent circle diameter D Ｗ (μm) is shown in Table 3.

[0135] [(Test 5) Vickers hardness test on inner surface layer of electric resistance welded portion] Based on the method described in the above [Method for measuring Vickers hardness H at a position 0.1 mm deep from the inner surface of electric resistance welded portion 3 Ｗ , the Vickers hardness H of the electric resistance welded portion of the electric resistance welded steel pipe of each test number Ｗ (HV) was obtained. The obtained Vickers hardness H Ｗ (HV) is shown in Table 3.

[0136] [(Test 6) Abrasion Resistance Evaluation Test] The abrasion resistance of the inner surface of each electric resistance welded steel pipe was evaluated by the following sliding test. A cylindrical intermediate test piece with a length of 200 mm in the pipe axis direction was taken from each electric resistance welded steel pipe of each test number. A semi-cylindrical test piece (hereinafter referred to as an arc-shaped test piece) was prepared by cutting the intermediate test piece in the pipe axis direction at a cross section including the central axis of the electric resistance welded steel pipe. The arc-shaped test piece was prepared such that when viewed in the pipe axis direction, the test piece has a convex arc shape, and the electric resistance welded part is positioned at the apex of the arc shape. In the following description, the convex cylindrical surface of the arc-shaped test piece will be referred to as the "outer surface," and the concave cylindrical surface located on the opposite side of the outer surface will be referred to as the "inner surface." The inner surface of the arc-shaped test piece was finished by sanding with 1000 grit sandpaper. Furthermore, even after finishing with 1000-grit sandpaper, the hardness of the inner surface did not change before and after polishing. After further alkaline degreasing of the arc-shaped test specimen, the mass (g) of the arc-shaped test specimen was measured.

[0137] A semi-cylindrical jig was prepared, having the same outer surface shape as the inner surface shape of the arc-shaped test specimen. The radius of the jig's arc was 40 mm, and the length of the jig was 100 mm. The material of the jig corresponded to SUJ2 as specified in JIS G 4805 (2019). Using the jig, the following sliding test was performed on the arc-shaped test specimen. With a pressing load of 10 kg, the outer surface of the jig was brought into contact with the inner surface of the arc-shaped test specimen, and the jig was moved back and forth 5000 times in the longitudinal direction along the entire length of the arc-shaped test specimen (200 mm). The sliding speed during the reciprocating motion was 100 mm / second. After the sliding test, the arc-shaped test specimen was subjected to alkaline degreasing to remove wear particles and other contaminants adhering to its surface. The mass (g) of the arc-shaped test specimen after alkaline degreasing was measured.

[0138] The amount of wear (g) during the sliding test was determined 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 determined amount of wear is shown in the "Amount of Wear (g)" column of the "Abrasion Resistance" column in Table 3.

[0139]

[0140] [Evaluation Results] Referring to Tables 1 to 3, test numbers 1 to 20 satisfied features 1 to 6. Therefore, the amount of wear in the abrasion resistance evaluation test was 10 g or less, indicating excellent abrasion resistance.

[0141] Furthermore, in test numbers 1 through 20, test numbers 1, 2, 5, 7, and 10 through 20 showed a Vickers hardness of H in the electric resistance welded joint. Ｗ The Vickers hardness H of the base material Ｂ It was higher than that. As a result, the amount of wear in the abrasion resistance evaluation test was 5g or less, and even better abrasion resistance was achieved.

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

[0143] In tests 23 and 24, the cooling stop temperature T0 in the manufacturing process of the hot-rolled steel sheet was too high, resulting in a Vickers hardness H of the base material. Ｂ The value was too low. As a result, the amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[0144] In tests 25 and 26, the relative temperature T during the seam heat treatment process was Ｎ The value was too low. Therefore, the average equivalent circle diameter D of the pearlite region in the electric resistance welded area. Ｗ The amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[0145] In tests 27 and 28, the relative temperature T during the seam heat treatment process was Ｎ The value was too high. Therefore, the average equivalent circle diameter D of the pearlite region in the electric resistance welded area was too high. Ｗ The amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[0146] In tests 29 and 30, the first cooling rate CR1 was slow. Therefore, the average equivalent circle diameter D of the pearlite region in the electric resistance welded joint was slow. ＷThe amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[0147] In tests 31 and 32, the rapid cooling stop temperature T2 was too low. As a result, the pearlite area ratio in the electric resistance welded joint was too low, and excessive hard structure was generated. Consequently, the amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[0148] In tests 33 and 34, the rapid cooling stop temperature T2 was too high. As a result, the average equivalent circle diameter D of the pearlite region in the electric resistance welded joint was too high. Ｗ The amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[0149] In test number 35, the carbon content of the base material was too low. As a result, the perlite area ratio of the base material was too low, resulting in a Vickers hardness of H. Ｂ The value was too low. Furthermore, the pearlite area ratio of the electric resistance weld was too low, and the Vickers hardness was H Ｗ The value was too low. As a result, the amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[0150] In test number 36, the carbon content in the base material was too high. As a result, the perlite area ratio in the base material was too low, resulting in a Vickers hardness of H. Ｂ The pressure was too high. Furthermore, the pearlite area ratio in the electric resistance welded joint was too low. As a result, the amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[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, H Ｗ / H Ｂ The value was too low. As a result, the amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[0152] In tests 39 and 40, the cooling stop temperature T0 was high. Therefore, the Vickers hardness H Ｂ It's too low, H Ｗ / H Ｂ The value was too high. As a result, the amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[0153] In tests 41 and 42, the seam heat treatment process involved only one cooling stage instead of two. As a result, the pearlite area ratio in the electric resistance welded joint was too low. Consequently, the amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[0154] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

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

Claims

1. An electric resistance welded steel pipe comprising a base material portion and an electric resistance welded portion extending in the axial direction of the electric resistance welded steel pipe, wherein the chemical composition of the electric resistance welded steel pipe is, in mass%, C: 0.10 to 0.30%, Si: 0.03 to 1.20%, Mn: 1.00 to 2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.005 to 0.500%, Nb: 0.010 to 0.060%, N: 0.0003 to 0.0060%, Ti: 0 to 0.200%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, V: 0 to 0.20%, It contains 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 being Fe and impurities, the base material having 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 the pearlite area ratio being 95% or more, and the Vickers hardness H at a depth of 0.1 mm from the inner surface of the base material. Ｂ The temperature is 190 to 250 HV, and in the electric resistance welded joint, 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 average equivalent circle diameter D of the pearlite region consisting of one or a plurality of continuously connected pearlite blocks is Ｗ The thickness is 10.0 μm or less, and the Vickers hardness H at a depth of 0.1 mm from the inner surface of the electric resistance welded joint is Ｗ Electric resistance welded steel pipe having a voltage of 190 HV or higher and satisfying equation (1). 0.80 ≤ H Ｗ / H Ｂ ≤ 1.20 (1) 2. The electric resistance welded steel pipe according to claim 1, wherein the Vickers hardness H Ｗ The Vickers hardness H Ｂ Higher than electric resistance welded steel pipes.

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

4. An electric resistance welded steel pipe according to claim 1, having an outer diameter of 100 to 400 mm and a wall thickness of 5.0 to 18.0 mm.

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