Cylinder member

Abstract

Provided is a cylinder member achieving excellent wear resistance. A base (11) and an electric-resistance weld (12) of the cylinder member according to the present disclosure have an areal ferrite content of 50% or higher and an areal pearlite content of 10% or higher, with the total of the areal ferrite content and the areal pearlite content being 95% or higher. In the electric-resistance weld (12), a pearlite region comprising one pearlite block or composed of a plurality of pearlite blocks connected consecutively has an average equivalent-circle diameter DW of 10.0 μm or less. The pipe body (1) further comprises a hardened layer region (21) formed as an inner surface layer and a substrate region (22). In the base (11) and the electric-resistance weld (12), the hardened layer region (21) has a nanoindentation hardness of 5.0 GPa or greater on the inner surface and the substrate region (22) located at a depth of 10.0 μm from the inner surface has a nanoindentation hardness of 3.0-4.0 GPa.

Description

Cylinder member

[0001] This disclosure relates to tubular cylinder members, such as cylinder tubes and cylinder rods in cylinder devices.

[0002] Cylinder devices, such as hydraulic and pneumatic cylinders, are widely used in industrial and construction machinery. Cylinder devices consist of cylindrical components, such as cylinder tubes.

[0003] When a cylinder component is incorporated into a cylinder device as a cylinder tube, the inner surface (inner circumferential surface) of the cylinder component will wear down. Therefore, excellent wear resistance is required for the cylinder component.

[0004] Patent documents 1 (Japanese Patent Publication No. 8-225853) and 2 (Japanese Patent Publication No. 6-306459) propose techniques for improving the wear resistance of steel pipes used as the material for cylinder members. In the 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 improves wear resistance.

[0005] JP-A No. 8-225853 JP-A No. 6-306459

[0006] However, when the steel pipes disclosed in Patent Documents 1 and 2 are applied to cylinder members, sufficient wear resistance may not be obtained.

[0007] The purpose of this disclosure is to provide a cylinder member having excellent wear resistance on its inner surface.

[0008] The cylinder member of this disclosure comprises a tube body including a base material portion and an electric resistance welded portion. The chemical composition of the tube body 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 (REM): 0-0.0200%, with the remainder being Fe and impurities. At the center of the thickness of the base material, 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. At the center of the wall thickness of 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 diameter D of the pearlite region consisting of one or more continuously connected pearlite blocks is... Ｗ The nanoindentation hardness of the hardened layer is 10.0 μm or less. The pipe body further includes a hardened layer region and a base material region. The hardened layer region is formed on the surface of the inner surface of the pipe body. The base material region is the region other than the hardened layer region and has a lower hardness than the hardened layer region. In the base material, the nanoindentation hardness of the hardened layer region on the inner surface is 5.0 GPa or more, and the nanoindentation hardness of the base material region at a depth of 10.0 μm from the inner surface is 3.0 to 4.0 GPa. In the electric resistance welded section, the nanoindentation hardness of the hardened layer region on the inner surface is 5.0 GPa or more, and the nanoindentation hardness of the base material region at a depth of 10.0 μm from the inner surface is 3.0 to 4.0 GPa.

[0009] The cylinder member of this disclosure provides excellent wear resistance.

[0010] Figure 1 is an enlarged view of the cylinder member of this embodiment, perpendicular to the pipe axis direction. Figure 2A is a schematic diagram of the pipe body in Figure 1 viewed from the inside. Figure 2B is a schematic diagram of the pipe body in Figure 1 viewed from the inside, different from Figure 2A. Figure 3 is a schematic diagram showing an example of a microstructural photograph (secondary electron image) of an electric resistance welded joint to explain the pearlite region. Figure 4 is a schematic diagram of a device for performing shear strain application (shear strain application device).

[0011] The inventors first investigated cylinder members that could obtain excellent wear resistance. As a result, the inventors obtained the following findings: To improve the wear resistance of the inner surface of the cylinder member, it is preferable to increase the hardness of the electric resistance welded steel pipe that constitutes the pipe body of the cylinder member. Therefore, from the viewpoint of increasing the hardness of the steel and improving wear resistance, the inventors investigated the chemical composition of the pipe body made of electric resistance welded steel pipe. 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 a tube body 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 wear resistance.

[0012] The inventors further investigated means of improving wear resistance in a pipe body (electric resistance welded steel pipe) having the above-described 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 against wear caused by sliding motion in the axial direction of the pipe, such as in cylinder members.

[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 in the axial direction on the inner surface of the pipe body made of electric resistance welded steel pipe, the hardness of 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 inventors hypothesized that if a remarkably hardened layer region is formed in the extremely thin outermost region of the inner surface layer, wear resistance will be enhanced. Furthermore, they hypothesized that if the hardened layer region is worn away due to repeated sliding motion, a new hardened layer region will be generated and the hardened layer region will be repaired by the microstructure of the substrate region inside the hardened layer region and the shear strain applied to the microstructure by the sliding motion, then even with continuous use of the cylinder member, the hardened layer region of the outermost inner surface will be automatically repaired, resulting in excellent wear resistance.

[0015] Based on the above findings, further investigation was conducted into the microstructure of the tube body of the cylinder member made of electric resistance welded steel pipe. As a result, it was found that excellent wear resistance can be obtained in the cylinder member if the tube body includes the following configuration: (I) An extremely thin hardened layer region of less than 10.0 μm in thickness is formed on the inner surface layer of the tube body made of electric resistance welded steel pipe having a base material and an electric resistance welded joint. In the following explanation, the region other than the hardened layer region and having a lower hardness than the hardened layer region is called the base material region. (II) In the base material region of the base material, 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. (III) In the base material region of the electric resistance welded joint, 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. Furthermore, the average equivalent circle diameter D of the pearlite region consisting of one or a plurality of continuously connected pearlite blocks. Ｗ (IV) In the base material, the nanoindentation hardness of the hardened layer region on the inner surface shall be 5.0 GPa or higher, and the nanoindentation hardness of the base material region at a depth of 10.0 μm from the inner surface shall be 3.0 to 4.0 GPa. (V) In the electric resistance welded section, the nanoindentation hardness of the hardened layer region on the inner surface shall be 5.0 GPa or higher, and the nanoindentation hardness of the base material region at a depth of 10.0 μm from the inner surface shall be 3.0 to 4.0 GPa.

[0016] The reason why satisfying the above configurations (I) to (V) significantly improves the wear resistance on the inner surface of the cylinder member is not entirely clear, but the following reasons are possible.

[0017] When mechanical parts such as pistons repeatedly slide against the inner surface of a cylinder member's tube, the extremely thin hardened layer formed on the inner surface initially exhibits excellent wear resistance. However, as repeated sliding continues, the hardened layer gradually wears away.

[0018] On the other hand, due to repeated sliding motion, the inner surface of the pipe body is constantly subjected to shear strain from the machine parts. This shear strain causes the pearlite in the base material region of the inner surface layer of the base material and the electric resistance welded joint to pulverize. As the pearlite is pulverized, the cementite within the pearlite is also pulverized, and fine cementite particles are dispersed in the outermost layer of the inner surface. This dispersion of cementite strengthens the outermost layer, increasing its hardness. Furthermore, the carbon in the crushed fine cementite supersaturates and dissolves into the ferrite in the pearlite. Therefore, the hardness of the outermost layer of the inner surface is further increased by solid solution strengthening. Through the above mechanism, as repeated sliding motion occurs, a new hardened layer region is generated in the area where the hardened layer region of the inner surface outermost layer of the electric resistance welded steel pipe has been worn away. In this way, it is believed that the wear resistance of the inner surface is significantly increased by the self-repair of the hardened layer region during the use of the cylinder member.

[0019] The above mechanism is a hypothesis, and it is possible that superior wear resistance is achieved by other mechanisms. However, the fact that superior wear resistance is obtained when the tube body of the cylinder member satisfies the above-mentioned configurations (I) to (V) has been demonstrated in the embodiments described later.

[0020] Based on the above findings, the cylinder member of this embodiment has the following configuration.

[0021] The cylinder member according to the first aspect includes a pipe main body including a base material portion and an electric resistance welded portion. The chemical composition of the pipe main body, in mass%, includes 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%, B: 0 to 0.0100%, W: 0 to 0.10%, Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, Zr: 0 to 0.0200%, and rare earth elements (REM): 0 to 0.0200%, with the balance being Fe and impurities. At the thickness center position of the base material portion, the area fraction of ferrite is 50% or more, the area fraction of pearlite is 10% or more, and the total of the area fraction of ferrite and the area fraction of pearlite is 95% or more. At the thickness center position of 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 average equivalent circle diameter D of a pearlite region composed of one pearlite block or a plurality of continuously connected pearlite blocks Ｗ is 10.0 μm or less. The pipe main body further includes a hardened layer region and a base material region. The hardened layer region is formed in a surface layer of an inner surface of the pipe main body. The base material region is a region other than the hardened layer region, and has a lower hardness than the hardened layer region. In the base material portion, the nanoindentation hardness of the hardened layer region on the inner surface is 5.0 GPa or more, and the nanoindentation hardness of the base material region at a position 10.0 μm deep from the inner surface is 3.0 to 4.0 GPa. In the electric resistance welded portion, the nanoindentation hardness of the hardened layer region on the inner surface is 5.0 GPa or more, and the nanoindentation hardness of the base material region at a position 10.0 μm deep from the inner surface is 3.0 to 4.0 GPa.

[0022] The cylinder member according to the second embodiment is the cylinder member according to the first embodiment, wherein the chemical composition of the pipe body contains, by mass%, one or more 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 (REM): 0.0001 to 0.0200%.

[0023] Hereinafter, the cylinder member of the present embodiment will be described in detail. Note that "%" regarding elements means mass% unless otherwise specified.

[0024] [Configuration of Cylinder Member of the Present Embodiment] FIG. 1 is an enlarged view of a cross section perpendicular to the pipe axis direction of the cylinder member of the present embodiment. Referring to FIG. 1, the cylinder member of the present embodiment includes a pipe body 1. The pipe body 1 is tubular. The pipe body 1 has a cylindrical shape and a through hole extending in the pipe axis direction.

[0025] The pipe body 1 includes a base material portion 11 and an electric resistance welded portion 12 extending in the pipe axis direction. The base material portion 11 is cylindrical, and is formed by bending a steel plate in the manufacturing process described later. The electric resistance welded portion 12 extends in the pipe axis direction of the pipe body 1. The electric resistance welded portion 12 is formed by butting opposite ends of a cylindrical steel plate formed by bending and performing electric resistance welding (upset welding). As described above, the pipe body 1 is formed of an electric resistance welded steel pipe.

[0026] The pipe body 1 further includes a hardened layer region 21 and a base material region 22. The hardened layer region 21 is formed on the surface of the inner surface of the pipe body 1. The hardened layer region 21 only needs to be formed on at least a part of the inner surface of the pipe body 1. Therefore, the hardened layer region 21 may be formed on a part of the inner surface of the pipe body 1, or it may be formed on the entire inner surface. Figures 2A and 2B are schematic diagrams of the pipe body 1 as seen from the inner surface. In Figures 2A and 2B, the L direction means the pipe axis direction. As shown in Figure 2A, the hardened layer region 21 may be formed on the entire inner surface. Also, as shown in Figure 2B, the hardened layer region 21 may be formed on a part of the inner surface. As shown in Figure 2B, on the inner surface, the areas other than the area where the hardened layer region 21 is formed correspond to the base material region 22. For example, when a piston slides in the axial direction of the pipe inside a cylinder member, a hardened layer region 21 may be formed in the sliding region between the piston and the inner surface of the cylinder member, and the hardened layer region 21 may not be formed in other regions other than the sliding region. Alternatively, a hardened layer region 21 may be formed in a predetermined region including the sliding region, and the hardened layer region 21 may not be formed in other regions. The hardened layer region 21 may extend over the entire circumferential direction of the inner surface of the pipe body 1, and may also extend in a part in the axial direction of the pipe.

[0027] The cylinder member of this disclosure further satisfies the following features: (Feature 1) The chemical composition of the tube body 1 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 (REM): 0-0.0200%, with the remainder being Fe and impurities. (Feature 2) In the microstructure at the center of the thickness of the base material 11, 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. (Feature 3) In the microstructure at the center of the wall thickness of the electric resistance welded joint 12, 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. (Feature 4) The average equivalent circle diameter D of the pearlite area in the base material region 22 of the electric resistance welded joint 12. Ｗ The thickness is 10.0 μm or less. (Feature 5) In the base material portion 11, the nanoindentation hardness of the hardened layer region 21 on the inner surface is 5.0 GPa or more, and the nanoindentation hardness of the base material region 22 at a depth of 10.0 μm from the inner surface is 3.0 to 4.0 GPa. (Feature 6) In the electric resistance welded portion 12, the nanoindentation hardness of the hardened layer region 21 on the inner surface is 5.0 GPa or more, and the nanoindentation hardness of the base material region 22 at a depth of 10.0 μm from the inner surface is 3.0 to 4.0 GPa. Features 1 to 6 will be described below.

[0028] [(Feature 1) Chemical composition of the tube body 1] The chemical composition of the tube body 1 contains the following elements.

[0029] C: 0.10-0.30% Carbon (C) increases the strength of steel. If the C content is less than 0.10%, the above effect 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 pipe body 1 (base material part 11 and electric resistance welded part 12) 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 pipe body 1 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%.

[0030] 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 constituting the pipe body 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 0.12%. 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%.

[0031] 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%.

[0032] 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%.

[0033] 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%.

[0034] Al: 0.005 to 0.500% Aluminum (Al) combines with nitrogen to form AlN. AlN, through its pinning effect, suppresses the coarsening of austenite grains during the seam heat treatment process in the manufacturing process of the electric resistance welded steel pipe that constitutes the pipe body 1. 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 the coarse AlN are more likely to occur during bending in the manufacturing process of the electric resistance welded steel pipe. 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%.

[0035] 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 electric resistance welded steel pipes 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 bending in the manufacturing process of electric resistance welded steel pipes. 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%.

[0036] 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 electric resistance welded steel pipes 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 the coarse AlN are more likely to occur during bending in the manufacturing process of electric resistance welded steel pipes. 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%.

[0037] The remainder of the chemical composition of the pipe body 1 in this embodiment consists of Fe and impurities. Here, impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the steel plate that forms the material for the electric resistance welded steel pipe constituting the pipe body 1, and are acceptable within a range that does not adversely affect the pipe body 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%.

[0038] [Regarding Optional Elements] The chemical composition of the tube body 1 of this embodiment may further contain, in place of a portion of Fe, one or more elements selected by mass% 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. All of these elements are optional elements. Each optional element will be described below.

[0039] [Group 1: Ti, Cu, Ni, Cr, Mo, V, B, and W] The chemical composition of the tube body 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.

[0040] 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 electric resistance welded steel pipes, 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%, more preferably 0.020%, and still 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%.

[0041] 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%.

[0042] 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%.

[0043] 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%.

[0044] 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%.

[0045] 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%.

[0046] 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%.

[0047] 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%.

[0048] [Group 2: Ca, Mg, Zr, and rare earth elements (REM)] The chemical composition of the tube body 1 in 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.

[0049] 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%.

[0050] 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%.

[0051] 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%.

[0052] 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%.

[0053] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and lanthanides, lanthanum (La), atomic number 57 to lutetium (Lu), atomic number 71. In this specification, REM content refers to the total content (mass%) of these elements.

[0054] [(Feature 2) Microstructure of the base material 11] In the microstructure of the base material 11 of the pipe body 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 the pearlite area ratio is 95% or more. The remainder of the microstructure of the base material 11 other than ferrite and pearlite is not particularly limited. The remainder consists of, for example, one or more 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.

[0055] If the microstructure of the base material 11 has a ferrite area ratio of 50% or more, a pearlite area ratio of 10% or more, and the sum of the ferrite area ratio and pearlite area ratio is 95% or more, then a hardened layer region 21 satisfying feature 5 can be formed on the inner surface of the pipe body 1. In other words, the above-described microstructure of the base material 11 is a prerequisite for forming a hardened layer region 21 on the inner surface of the base material 11. The formation of the hardened layer region 21 increases the wear resistance of the inner surface of the pipe body 1.

[0056] The preferred lower limit for the ferrite area ratio of the base material 11 is 55%, more preferably 60%, and even more preferably 65%. The preferred upper limit for the ferrite area ratio of the base material 11 is 90%, more preferably 85%, and even more preferably 70% or less.

[0057] The preferred lower limit for the perlite area ratio of the base material 11 is 12%, more preferably 16%, and even more preferably 20%. The preferred upper limit for the perlite area ratio of the base material 11 is 50%, more preferably 40%, and even more preferably 35%.

[0058] [Method for measuring the microstructure of the base material 11] The ferrite area ratio (%) and pearlite area ratio (%) in the microstructure of the base material 11 are measured by the following method.

[0059] A test specimen is taken from a position on the base material 11 that is offset 180° circumferentially around the central axis of the electric resistance welded steel pipe, from the center of the width of the electric resistance welded joint 12 of the electric resistance welded steel pipe that constitutes the pipe body 1. The surface of the test specimen is observed from the cross section (L section) that includes the pipe axis direction and the wall thickness direction of the electric resistance welded steel pipe. The test specimen is taken so that the center of the observation surface is at the center of the wall thickness of the base material 11.

[0060] 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 center of the thickness of the base material 11. Each observation field is a rectangle measuring 200 μm in the axial direction of the tube and 200 μm in the thickness direction. The observation fields are selected so that the center of each observation field in the thickness direction is the center of the thickness of the base material 11. The five observation fields are arranged in a continuous line in the axial direction of the tube on the observation surface.

[0061] 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.

[0062] 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.

[0063] The ferrite area ratio (%) of the base material 11 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 11 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.

[0064] [(Feature 3) Microstructure of the electric resistance welded joint 12] In the microstructure of the electric resistance welded joint 12 of the pipe body 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 the pearlite area ratio is 95% or more. The remainder of the microstructure of the electric resistance welded joint 12 other than ferrite and pearlite is not particularly limited. The remainder consists of, for example, one or more selected from the group consisting of hard structures (bainite and / or martensite) and retained austenite.

[0065] In the microstructure of the electric resistance welded joint 12, similar to the microstructure of the base material 11, 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, then a hardened layer region 21 satisfying feature 6 can be formed on the inner surface of the electric resistance welded joint 12. In other words, the above-described microstructure of the electric resistance welded joint 12 is a prerequisite for forming a hardened layer region 21 on the inner surface of the electric resistance welded joint 12. The formation of the hardened layer region 21 increases the wear resistance of the inner surface of the pipe body 1.

[0066] The preferred lower limit for the ferrite area ratio of the electric resistance welded joint 12 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 12 is 90%, more preferably 85%, more preferably 80%, more preferably 77%, and still more preferably 75%.

[0067] The preferred lower limit for the pearlite area ratio of the electric resistance welded joint 12 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 12 is 50%, more preferably 40%, and still more preferably 35%.

[0068] [Method for measuring the microstructure of the electric resistance welded joint 12] The ferrite area ratio (%) and pearlite area ratio (%) in the microstructure of the electric resistance welded joint 12 are measured by the following method.

[0069] A test specimen is taken from the center of the width of the electric resistance welded joint 12 of the electric resistance welded steel pipe that constitutes the pipe body 1, and the specimen has a cross-section (L-section) that includes the pipe axis direction and the wall thickness direction. This cross-section is to be used as the observation surface. The test specimen is taken so that the center of the observation surface is at the center of the wall thickness of the electric resistance welded joint 12.

[0070] 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 center of the wall thickness of the electric resistance welded joint 12. 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.

[0071] Each observation field is observed at a magnification of 500x using an 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 11].

[0072] The ferrite area ratio (%) of the electric resistance welded joint 12 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 12 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.

[0073] [(Feature 4) Average circular equivalent diameter D of the perlite region of the electric resistance welded joint 12 Ｗ Regarding the electric resistance welded joint 12 of the pipe body 1, the average circular equivalent diameter D of the pearlite region is further... Ｗ The particle size is 10.0 μm or less.

[0074] Here, a pearlite region is a region consisting of one or more continuously connected pearlite blocks. Figure 3 is a schematic diagram showing an example of a microstructural photograph (secondary electron image) of an electric resistance welded joint 12 to explain the pearlite region. Referring to Figure 3, the bright (white) region without a substructure is ferrite 30. The region in which ferrite 41 (white region) and low-brightness (black) cementite 42 are arranged in layers is pearlite. Here, among the pearlite, regions where the crystal orientation of ferrite 41 is the same are defined as pearlite blocks 40B. In this case, a region consisting of one or more continuously connected pearlite blocks 40B is defined as a pearlite region. For example, in Figure 3, pearlite block 40B1 is a single pearlite block that is not continuously connected to other pearlite blocks 40B. Therefore, pearlite block 40B1 is identified as a pearlite region. Also, pearlite blocks 40B2 to 40B5 are continuously connected. Therefore, the perlite blocks 40B2 to 40B5 are identified as a single perlite region.

[0075] In the electric resistance welded joint 12, 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 becomes difficult to form a hardened layer region 21 on the inner surface of the electric resistance welded joint 12. For this reason, it is preferable for each pearlite region to be small.

[0076] 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 fine pearlite regions are dispersed in the electric resistance welded joint 12. As a result, a hardened layer region 21 can be formed on the inner surface of the electric resistance welded joint 12. Consequently, excellent wear resistance can be obtained on the inner surface of the electric resistance welded joint 12.

[0077] Average circular equivalent diameter D of the perlite regionＷ The preferable upper limit is 9.5 μm, more preferably 9.0 μm, still more preferably 8.5 μm, still more preferably 8.0 μm, still more preferably 7.5 μm, still more preferably 7.0 μm, still more preferably 6.5 μm, still more preferably 6.0 μm, and still more preferably 5.5 μm. The average equivalent circle diameter D of the pearlite regions Ｗ is preferably as small as possible. However, an excessive reduction in the average equivalent circle diameter D of the pearlite regions Ｗ will increase manufacturing costs. Therefore, the preferable lower limit of the average equivalent circle diameter D of the pearlite regions Ｗ is 1.0 μm, more preferably 1.5 μm, and still more preferably 2.0 μm.

[0078] [Method for measuring average equivalent circle diameter D of pearlite regions in electric resistance welded portion 12 Ｗ The average equivalent circle diameter D of pearlite regions in electric resistance welded portion 12 Ｗ is measured by the following method. In the above-mentioned [Method for measuring microstructure of electric resistance welded portion 12], pearlite regions are identified by the above-mentioned method in SEM images of each of the five observation fields of view. Among the identified plurality of pearlite regions, pearlite regions having an area of 0.75 μm ２ or less are excluded from measurement objects. Among the identified plurality of pearlite regions, pearlite regions that partially protrude outside from one side of a 200 μm × 200 μm rectangular observation field of view, or are in contact with one side thereof are excluded from measurement objects. That is, pearlite regions that are entirely contained inside the sides of the rectangular observation field of view are taken as measurement objects.

[0079] The equivalent circle diameter (μm) of each pearlite region identified as a measurement object is obtained. The equivalent circle diameter is a value up to the first decimal place obtained by rounding off the second decimal place of the obtained value. The arithmetic mean value of the equivalent circle diameters of all pearlite regions to be measured is obtained, and this is taken as the average equivalent circle diameter D Ｗ (μm) of the pearlite regions. The average equivalent circle diameter D Ｗ is a value up to the first decimal place obtained by rounding off the second decimal place of the obtained value. Note that the equivalent circle diameter can be obtained, for example, with well-known image analysis software.

[0080] [Regarding the hardness of the hardened layer region 21 and the hardness of the base material region 22 in the base material portion 11, and the hardness of the hardened layer region 21 and the hardness of the base material region 22 in the electric resistance welded portion 12 (Features 5 and 6)] In the cylinder member of this embodiment, in the base material portion 11 of the pipe body 1, the nanoindentation hardness of the hardened layer region 21 on the inner surface is 5.0 GPa or more, and the nanoindentation hardness of the base material region 22 at a depth of 10.0 μm from the inner surface is 3.0 to 4.0 GPa. Furthermore, in the cylinder member of this embodiment, in the electric resistance welded portion 12 of the pipe body 1, the nanoindentation hardness of the hardened layer region 21 on the inner surface is 5.0 GPa or more, and the nanoindentation hardness of the base material region 22 at a depth of 10.0 μm from the inner surface is 3.0 to 4.0 GPa.

[0081] In the cylinder member of this embodiment, a hardened layer region 21 with extremely high hardness is formed in an extremely thin surface region less than 10 μm deep from the inner surface of the base material portion 11. Similarly, a hardened layer region 21 with extremely high hardness is formed in an extremely thin surface region less than 10 μm deep from the inner surface of the electric resistance welded portion 12. In other words, the thickness of the hardened layer region 21 is less than 10.0 μm.

[0082] The hardened layer region 21 formed in the ultrathin surface region of the base material portion 11 and the ultrathin surface region of the electric resistance welded portion 12 is formed by applying shear strain to a base material region 22 in which the ferrite area ratio is 50% or more, the pearlite area ratio is 10% or more, the sum of the ferrite area ratio and pearlite area ratio is 95% or more, and furthermore, the nanoindentation hardness is 3.0 to 4.0 GPa.

[0083] The nanoindentation hardness of the hardened layer region 21 of the base material portion 11 and the nanoindentation hardness of the hardened layer region 21 of the electric resistance welded portion 12 are both 5.0 GPa or higher, which is significantly higher than the nanoindentation hardness of the base material region 22 of the base material portion 11 (3.0 to 4.0 GPa) and the nanoindentation hardness of the base material region 22 of the electric resistance welded portion 12 (3.0 to 4.0 GPa). Therefore, cylinder members containing the hardened layer region 21 in the inner surface layer can be obtained with excellent wear resistance.

[0084] As described above, the hardened layer region 21 is formed by applying shear strain to the base material region 22 of the inner surface layer of the base material portion 11 and the electric resistance welded portion 12 of the pipe body 1. Therefore, numerous strains are introduced into the hardened layer region 21. As a result, it is extremely difficult to identify the microstructure of the hardened layer region 21. Furthermore, as described above, the hardened layer region 21 is formed in a range of less than 10.0 μm in depth from the inner surface and is extremely thin. Therefore, in this embodiment, the hardened layer region 21 is defined by nanoindentation hardness.

[0085] For example, when shot blasting is performed on the inner surface of a steel pipe, a hardened layer is formed on the inner surface. However, the thickness of the hardened layer formed on the inner surface by shot blasting is significantly thicker than 10.0 μm, and the thickness of the hardened layer is at least 50.0 μm. Alternatively, a hardened layer can be formed on the inner surface of a steel pipe by high-frequency induction hardening or carburizing. However, when a hardened layer is formed by high-frequency induction hardening or carburizing, the thickness of the hardened layer is at least 100.0 μm. Therefore, the hardened layer region 21 of this embodiment differs significantly in thickness, at least, from that formed by conventional methods.

[0086] In the hardened layer region 21 of this embodiment, the nanoindentation hardness is extremely high at 5.0 GPa. Therefore, the wear resistance of the inner surface of the cylinder member's tube body 1 is significantly improved.

[0087] In this embodiment, the cylinder member slides in the axial direction of the pipe on the inner surface of the pipe body 1. This sliding motion gradually wears away the hardened layer region 21, but at the same time, it applies shear strain to the inner surface of the pipe body 1. Therefore, even if the hardened layer region 21 is worn away, the shear strain applied to the inner surface due to the sliding motion generates a new hardened layer region 21 from the base material region 22 near the inner surface. In other words, in this embodiment, the hardened layer region 21 self-repairs as the cylinder device operates. As a result, the cylinder member can maintain excellent wear resistance even after long-term use.

[0088] The preferred lower limit for the nanoindentation hardness of the hardened layer region 21 of the base material portion 11 is 5.1 GPa, more preferably 5.2 GPa, more preferably 5.3 GPa, more preferably 5.5 GPa, and still more preferably 6.0 GPa. The upper limit for the nanoindentation hardness of the hardened layer region 21 of the base material portion 11 is not particularly limited. The preferred upper limit for this nanoindentation hardness is 10.0 GPa, more preferably 9.0 GPa, more preferably 8.0 GPa, and still more preferably 7.5 GPa. The preferred lower limit for the nanoindentation hardness of the base material region 22 of the base material portion 11 is 3.1 GPa, more preferably 3.2 GPa. The preferred upper limit for the nanoindentation hardness of the base material region 22 of the base material portion 11 is 3.9 GPa, more preferably 3.8 GPa, and still more preferably 3.7 GPa.

[0089] The preferred lower limit for the nanoindentation hardness of the hardened layer region 21 of the electric resistance welded joint 12 is 5.1 GPa, more preferably 5.2 GPa, more preferably 5.3 GPa, more preferably 5.5 GPa, and still more preferably 6.0 GPa. The upper limit for the nanoindentation hardness of the hardened layer region 21 of the electric resistance welded joint 12 is not particularly limited. The preferred upper limit for this nanoindentation hardness is 10.0 GPa, more preferably 9.0 GPa, more preferably 8.0 GPa, and still more preferably 7.5 GPa. The preferred lower limit for the nanoindentation hardness of the base material region 22 of the electric resistance welded joint 12 is 3.1 GPa, more preferably 3.2 GPa. The preferred upper limit for the nanoindentation hardness of the base material region 22 of the base material 11 is 3.9 GPa, more preferably 3.8 GPa, and still more preferably 3.7 GPa.

[0090] [Method for measuring the hardness of the hardened layer region 21 and the base material region 22 of the base material 11] The hardness of the hardened layer region 21 and the base material region 22 of the base material 11 is determined by the following method. A test piece is taken from a position in the base material 11 that is shifted 180° in the circumferential direction around the central axis of the electric resistance welded steel pipe from the center of the width of the electric resistance welded welded joint 12 of the electric resistance welded steel pipe constituting the pipe body 1, with the inner surface of the base material 11 as the surface. This inner surface is used as the measurement surface. On the measurement surface, 10 measurement points are selected at 1 μm intervals in the circumferential direction of the pipe body 1 and 10 points at 1 μm intervals in the axial direction of the pipe, for a total of 100 measurement points. At each measurement point, the nanoindentation hardness is determined in accordance with JIS Z 2255:2021.

[0091] A diamond Berkovich indenter is used to measure nanoindentation hardness. The test force is set to 5000 μN. The nanoindentation hardness measurement test is performed at room temperature in air. At each measurement point, the indentation hardness (GPa) at an indentation depth of 150 nm is determined. The arithmetic mean of the indentation hardness obtained at 100 measurement points is taken as the nanoindentation hardness (GPa) of the hardened layer region 21 of the base material 11. The nanoindentation hardness (GPa) is rounded to one decimal place by rounding the second decimal place of the obtained arithmetic mean.

[0092] Furthermore, a test specimen is taken from a position on the base material 11 that is offset 180° circumferentially around the central axis of the electric resistance welded steel pipe from the center of the width of the electric resistance welded joint 12 of the electric resistance welded steel pipe constituting the pipe body 1. The measurement surface of the test specimen is the cross section (L section) that includes the pipe axis direction and wall thickness direction of the pipe body 1. Note that the test specimen is taken so that one side of the measurement surface of the test specimen is on the inner surface of the base material 11.

[0093] On the measurement surface, 100 measurement points are selected at a depth of 10.0 μm from the inner surface. The pitch of each measurement point is 1 μm in the direction of the tube axis. At each measurement point, the nanoindentation hardness is determined in accordance with JIS Z 2255:2021. A diamond Birkovich indenter is used to measure the nanoindentation hardness. The test force is 5000 μN. The nanoindentation hardness measurement test is performed at room temperature and in air. At each measurement point, the indentation hardness (GPA) at an indentation depth of 150 nm is determined. The arithmetic mean of the indentation hardness obtained at the 100 measurement points is taken as the nanoindentation hardness (GPA) of the base material region 22 of the base material part 11. The nanoindentation hardness (GPA) is the value obtained by rounding the second decimal place of the obtained arithmetic mean to the first decimal place.

[0094] [Method for measuring the hardness of the hardened layer region 21 and the base material region 22 of the electric resistance welded joint 12] The hardness of the hardened layer region 21 and the base material region 22 of the electric resistance welded joint 12 is determined by the following method. A test piece is taken from the electric resistance welded steel pipe constituting the pipe body 1, with the inner surface of the electric resistance welded joint 12 as its surface. This inner surface is used as the measurement surface. On the measurement surface, 10 measurement points are selected at 1 μm intervals in the circumferential direction of the pipe body 1 and at 1 μm intervals in the axial direction of the pipe, for a total of 100 points. At each measurement point, the nanoindentation hardness is determined in accordance with JIS Z 2255:2021.

[0095] A diamond Berkovich indenter is used to measure nanoindentation hardness. The test force is set to 5000 μN. The nanoindentation hardness measurement test is performed at room temperature in air. At each measurement point, the indentation hardness (GPa) at an indentation depth of 150 nm is determined. The arithmetic mean of the indentation hardness obtained at 100 measurement points is taken as the nanoindentation hardness (GPa) of the hardened layer region 21 of the base material 11. The nanoindentation hardness (GPa) is rounded to one decimal place by rounding the second decimal place of the obtained arithmetic mean.

[0096] Furthermore, a test specimen is taken at the center of the width of the electric resistance welded joint 12 of the electric resistance welded steel pipe constituting the pipe body 1, and the specimen has a cross-section (L-section) that includes the pipe axis direction and the wall thickness direction. This cross-section will be used as the measurement surface. The test specimen is taken so that one side of the measurement surface of the test specimen is on the inner surface of the electric resistance welded joint 12.

[0097] On the measurement surface, 100 measurement points are selected at a depth of 10.0 μm from the inner surface. The pitch of each measurement point is 1 μm. At each measurement point, the nanoindentation hardness is determined in accordance with JIS Z 2255:2021. A diamond Birkovich indenter is used to measure the nanoindentation hardness. The test force is 5000 μN. The nanoindentation hardness measurement test is performed at room temperature and in air. At each measurement point, the indentation hardness (GPA) at an indentation depth of 150 nm is determined. The arithmetic mean of the indentation hardness obtained at the 100 measurement points is taken as the nanoindentation hardness (GPA) of the base material region 22 of the electric resistance welded joint 12. The nanoindentation hardness (GPA) is the value obtained by rounding the second decimal place of the arithmetic mean to the first decimal place.

[0098] [Effects of the cylinder member of this embodiment] The cylinder member of this embodiment satisfies the above-described features 1 to 6. Therefore, the cylinder member of this embodiment provides excellent wear resistance on the inner surface of the pipe body 1.

[0099] [Applications of the Cylinder Member of This Embodiment] The cylinder member of this embodiment provides excellent wear resistance on the inner surface of the pipe body 1. Therefore, it can be widely applied to applications where the above characteristics are required. The cylinder member of this embodiment is particularly suitable for hydraulic cylinder applications, such as the arm cylinders of construction machinery.

[0100] [Dimensions and Shape of the Cylinder Member in This Embodiment] The dimensions and shape of the cylinder member in this embodiment are not particularly limited. Preferably, the tube body 1 of the cylinder member 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.

[0101] [Method for Manufacturing the Cylinder Member of This Embodiment] The method for manufacturing the cylinder member of this embodiment will now be described. Note that the manufacturing method described below is illustrative and not limited to this. In other words, as long as a cylinder member satisfying features 1 to 6 can be manufactured, the method is not limited to the method described below. However, the manufacturing method described below is a preferred method for manufacturing the cylinder member.

[0102] An example of a method for manufacturing the cylinder member of this embodiment includes the following steps: (Step 1) Molding step (Step 2) Welding step (Step 3) Seam heat treatment step (Step 4) Sizing step (Step 5) Inner surface processing step Each step will be described below.

[0103] [(Process 1) Forming Process] In the forming process, the steel plate, which is the material for the electric resistance welded steel pipe that constitutes the pipe body 1, is bent to form a cylindrical open pipe. Specifically, a steel plate having a chemical composition that satisfies Feature 1 is prepared. The steel plate is formed into an open pipe shape (tubular shape) so that both ends face each other. Specifically, after the steel plate is unwound from the coil, both ends of the steel plate are bent upward by a group of breakdown rolls in the forming apparatus, causing the entire steel plate to curve into an arc shape. Furthermore, a group of fin pass rolls located downstream of the breakdown rolls performs finish forming on the steel plate so that the steel plate has a substantially circular cross-section. Through the above process, an open pipe-shaped steel plate is formed in which both ends of the steel plate are separated in the circumferential direction and face each other.

[0104] 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.

[0105] 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.

[0106] [Regarding the cooling stop temperature T0 (Condition 1)] If the cooling stop temperature T0 is too high, the hardness of the steel plate corresponding to the base material portion 11 of the pipe body 1 will be excessively low. In this case, the hardness of the base material region 22 of the base material portion 11 of the pipe body 1 of the manufactured cylinder member will be excessively low. On the other hand, if the cooling stop temperature T0 is too low, the pearlite area ratio will decrease and the hard structure (bainite and / or martensite) area ratio will increase in the microstructure of the manufactured steel plate. In this case, the microstructure of the base material portion 11 of the pipe body 1 will not satisfy characteristic 2. Therefore, the cooling stop temperature T0 should be set to 650 to 500°C.

[0107] [(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.

[0108] [(Step 3) Seam Heat Treatment Process] In the seam heat treatment process, the electric resistance welded (ERW) welds of the manufactured ERW steel pipe intermediates are subjected to pre-heating. Specifically, a predetermined area (specifically, 60 to 80 mm in the circumferential direction of the ERW steel pipe intermediate, centered on the width center of the ERW weld) is heated. A heating device with an induction element (heating coil) is used for heating. The heating device is placed above the ERW weld to heat the ERW weld. This pre-heats the ERW weld.

[0109] 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.

[0110] 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(Condition 3) The temperature should be 10 to 100°C. (Condition 4) The rapid cooling stop temperature T2 should be 500 to 300°C. (Condition 5) The second cooling rate CR2 should be 20°C / second or less. The following explains each condition.

[0111] [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 Transformation point + T Ｎ When expressed in (°C), the relative temperature T Ｎ The temperature range is 10 to 100°C.

[0112] 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 12 of the pipe body 1 made of electric resistance welded steel pipe is maintained. 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 joint become coarser. As a result, the average equivalent circle diameter D of the pearlite region in the electric resistance welded joint 12 of the pipe body 1 decreases. Ｗ It becomes excessively large. Therefore, the relative temperature T Ｎ The temperature should be between 10 and 100°C.

[0113] [Condition 3: 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.

[0114] 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 circular equivalent diameter D of the pearlite region in the base material region 22 of the electric resistance welded joint 12 of the pipe body 1 will be high. Ｗ The hardness 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 base material region 22 of the electric resistance welded joint 12 of the pipe body 1 becomes less than 10%, and the remainder becomes a microstructure mainly composed of martensite and / or bainite. As a result, the hardness of the base material region 22 becomes excessively high. Therefore, the first cooling rate CR1 should be set to 100 to 200°C / second.

[0115] [Regarding (Condition 4) 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. 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 in the base material region 22 of the electric resistance welded joint 12 of the pipe body 1 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, in the microstructure of the base material region 22 of the electric resistance welded joint 12 of the manufactured pipe body 1, the pearlite area ratio becomes less than 10%, and the remainder is mainly hard structure (bainite and / or martensite). As a result, the hardness of the base material region 22 becomes excessively high. Therefore, the rapid cooling stop temperature T2 should be set to 500 to 300°C.

[0116] 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.

[0117] [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).

[0118] If the second cooling rate CR2 exceeds 20°C / second, the cooling in the second cooling process is too fast. In this case, the pearlite area ratio in the base material region 22 of the electric resistance welded joint 12 of the pipe body 1 will be less than 10%, and the remainder will be a microstructure mainly composed of martensite and / or bainite. As a result, the hardness of the base material region 22 will become excessively high. Therefore, the second cooling rate CR2 should be set to 20°C / second or less.

[0119] [(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.

[0120] [(Step 5) Inner surface processing step] In the inner surface processing step, the following shear strain application process is carried out to form a hardened layer region 21 on the inner surface of the electric resistance welded steel pipe that constitutes the pipe body 1.

[0121] Figure 4 is a schematic diagram of a device for performing shear strain application (shear strain application device). Referring to Figure 4, the shear strain application device comprises a fixing device 31 and a surface pressure application tool 32. The surface pressure application tool 32 has, for example, an annular shape. The fixing device 31 is cylindrical and fixes and supports the surface pressure application tool 32. In Figure 4, the cylindrical fixing device 31 is fitted onto the inner surface of the annular surface pressure application tool 32 to fix the surface pressure application tool 32.

[0122] In the shear strain application process, a surface pressure application tool 32 fixed to a fixture 31 is inserted into a through-hole in the electric resistance welded steel pipe 200. At this time, the shape of the surface pressure application tool 32 is designed such that its outer surface contacts the inner surface IN of the electric resistance welded steel pipe 200, and the surface pressure application tool 32 applies a surface pressure P (MPa) in the wall thickness direction of the inner surface IN of the electric resistance welded steel pipe 200. The material of the surface pressure application tool 32 is made of a material harder than the electric resistance welded steel pipe 200. For example, the material of the surface pressure application tool 32 is a cemented carbide for wear-resistant tools as specified in JIS B 4054:2020.

[0123] With the pressure-applying tool 32 applying a pressure P (MPa) to the inner surface IN, the pressure-applying tool 32 is slid in the axial direction of the electric resistance welded steel pipe 200. Specifically, the electric resistance welded steel pipe 200 is fixed, and the pressure-applying tool 32 is moved back and forth in the axial direction of the pipe. While the pressure-applying tool 32 is moving back and forth in the axial direction of the pipe, lubricating oil is continuously supplied to the inner surface IN. The lubricating oil is hydraulic fluid. For example, the hydraulic fluid is a product of JX Nippon Oil & Energy Corporation, product name: Super Highland WP46.

[0124] In the internal surface processing process, shear strain application processing is carried out to satisfy the following conditions: (Condition 6) The surface pressure P is 100 MPa or more. (Condition 7) The number of reciprocating cycles of the surface pressure application tool 32 with surface pressure P applied to the internal surface is 5000 or more. Each condition is explained below.

[0125] [Condition 6: Regarding surface pressure P] Surface pressure P affects the amount of shear strain applied to the inner surface. If the surface pressure P is less than 100 MPa, the amount of shear strain applied to the inner surface layer of the electric resistance welded steel pipe 200 during the shear strain application process is too small. In this case, the nanoindentation hardness of the hardened layer region 21 of the base material portion 11 of the manufactured pipe body 1 and / or the nanoindentation hardness of the hardened layer region 21 of the electric resistance welded portion 12 will be less than 5.0 GPa. Therefore, the surface pressure P should be 100 MPa or more.

[0126] [Condition 7: Number of reciprocating motions] In the shear strain application process, the number of reciprocating motions of the surface pressure application tool 32 shall be 5,000 or more. If the number of reciprocating motions is less than 5,000, the amount of shear strain applied to the inner surface layer of the electric resistance welded pipe 200 will be too small. In this case, the nanoindentation hardness of the hardened layer region 21 of the base material portion 11 of the manufactured pipe body 1 and / or the nanoindentation hardness of the hardened layer region 21 of the electric resistance welded portion 12 shall be less than 5.0 GPa. Therefore, the number of reciprocating motions shall be 5,000 or more.

[0127] Through the above manufacturing process, a cylinder member comprising the pipe body 1 is manufactured.

[0128] The effects of the cylinder member of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the cylinder member of this embodiment. Therefore, the cylinder member of this embodiment is not limited to this one example of conditions.

[0129] Electric resistance welded steel pipes having the chemical compositions shown in Table 1 (Table 1A and Table 1B) were manufactured.

[0130]

[0131]

[0132] Electric resistance welded steel pipes for each test number were manufactured using the following method. First, steel plates were manufactured using the following process. After heating the slab to 1000-1300°C, rough rolling was performed to produce rough bars. Using a tandem rolling mill, finish rolling was performed on the rough bars to produce hot-rolled steel plates. Accelerated cooling was performed on the hot-rolled steel plates after finish rolling. The cooling rate during accelerated cooling was 30°C / second. Accelerated cooling was stopped at the cooling stop temperature T0 listed in Table 2, and then the plates were allowed to cool to room temperature. Through the above manufacturing process, steel plates having the chemical compositions shown in Table 1 (Table 1A and Table 1B) were manufactured.

[0133]

[0134] The manufactured steel plates were bent to form cylindrical open pipes (forming process). Electric resistance welding (ERW) was performed on the open pipes to produce ERW steel pipe intermediates (welding process). Seam heat treatment was performed on the ERW steel pipe intermediates (seam heat treatment process). Relative temperature T during the seam heat treatment process. Ｎ The temperature (°C), first cooling rate CR1 (°C / sec), rapid cooling stop temperature T2 (°C), and second cooling rate CR2 (°C / sec) were as shown in Table 2. A sizing process was performed on the electric resistance welded (ERW) steel pipe intermediate after the seam heat treatment process. The ERW steel pipes for each test number were manufactured using the above manufacturing process. The ERW steel pipes for each test number had an outer diameter of 100 mm and a wall thickness of 10.0 mm.

[0135] An internal surface processing step was performed on the manufactured electric resistance welded (ERW) steel pipes. Specifically, a hardened layer region 21 was formed on the entire internal surface of the ERW steel pipe using the shear strain application apparatus shown in Figure 4. The surface pressure P (MPa) and the number of reciprocating movements during the shear strain application process are shown in Table 2.

[0136] Through the manufacturing process described above, cylinder components, including the tube body for each test number, were produced. The tube body of each test number's cylinder component had an outer diameter of 100 mm and a wall thickness of 10.0 mm.

[0137] [Evaluation Tests] The following evaluation tests were conducted on the electric resistance welded steel pipes of each test number: (Test 1) Measurement test of ferrite area ratio and pearlite area ratio at the center of the wall thickness of the base material (Test 2) Measurement test of ferrite area ratio and pearlite area ratio at the center of the wall thickness of the electric resistance welded section (Test 3) Average equivalent circle diameter D of the pearlite area in the base material region of the electric resistance welded section Ｗ Measurement tests (Test 4) Nanoindentation hardness measurement test of the hardened layer region of the base material and the nanoindentation hardness measurement test of the base material region of the base material (Test 5) Nanoindentation hardness measurement test of the hardened layer region of the electric resistance welded part and the nanoindentation hardness measurement test of the base material region of the electric resistance welded part (Test 6) Abrasion resistance evaluation test Each test will be explained below.

[0138] [(Test 1) Measurement Test of Ferrite Area Ratio and Pearlite Area Ratio at the Center of the Wall Thickness of the Base Material] Based on the method described in [Method for Measuring the Microstructure of the Base Material 11] above, the ferrite area ratio (%) and pearlite area ratio (%) were determined at the center of the wall thickness of the base material of the tube body of the cylinder member for each test number. The obtained ferrite area ratio (%) is shown in the "Ferrite Area Ratio (%)" column of the "Base Material Area" column in the "Base Material" column of Table 3. The obtained pearlite area ratio (%) is shown in the "Pearlite Area Ratio (%)" column of the "Base Material Area" column in the "Base Material" column of Table 3. The sum of the obtained ferrite area ratio and pearlite area ratio is shown in the "Total (%)" column of the "Base Material Area" column in the "Base Material" column of Table 3. If the sum of the ferrite area ratio and pearlite area ratio is less than 100%, the remaining structure was a hard structure.

[0139]

[0140] [(Test 2) Measurement Test of Ferrite Area Ratio and Pearlite Area Ratio at the Center of the Wall Thickness of the Electric Welded Joint] Based on the method described in [Method for Measuring the Microstructure of the Electric Welded Joint 12] above, the ferrite area ratio (%) and pearlite area ratio (%) at the center of the wall thickness of the electric welded joint of the pipe body of the cylinder member for each test number were determined. The obtained ferrite area ratio (%) is shown in the "Ferrite Area Ratio (%)" column of the "Base Material Area" column in the "Electric Welded Joint" column of Table 3. The obtained pearlite area ratio (%) is shown in the "Pearlite Area Ratio (%)" column of the "Base Material Area" column in the "Electric Welded Joint" column of Table 3. The sum of the obtained ferrite area ratio and pearlite area ratio is shown in the "Total (%)" column of the "Base Material Area" column in the "Electric Welded Joint" column of Table 3. If the sum of the ferrite area ratio and pearlite area ratio is less than 100%, the remaining structure was a hard structure.

[0141] [(Test 3) Average circular equivalent diameter D of the perlite region in the base material region of the electric resistance welded joint Ｗ [Measurement Test] The average circular equivalent diameter D of the pearlite region of the electric resistance welded joint 12 as described above. Ｗ Based on the method described in [Measurement Method], the average equivalent circle diameter D of the perlite region in the base material region of the electric resistance welded portion of the cylinder member for each test number ＷThe (μm) value was calculated. The obtained average equivalent circle diameter D Ｗ (μm) is in the "Pearlite Region D" column of the "Base Material Region" column in the "Electric Welded Area" column of Table 3. Ｗ (μm)" is shown in the column.

[0142] [(Test 4) Measurement Test of Nanoindentation Hardness of the Hardened Layer Region of the Base Material and the Nanoindentation Hardness of the Substrate Region of the Base Material] Based on the method described in [Method for Measuring the Hardness of the Hardened Layer Region 21 of the Base Material 11 and the Hardness of the Substrate Region 22 of the Base Material 11] above, the nanoindentation hardness of the inner surface of the base material of the tube body of the cylinder member for each test number (hardness of the hardened layer region), and the nanoindentation hardness at a depth of 10.0 μm from the inner surface of the base material (hardness of the substrate region) were determined. The obtained inner surface nanoindentation hardness (GPa) is shown in the "Inner Surface Hardness (GPa)" column of the "Hardened Layer Region" column of the "Base Material" column in Table 3. The obtained nanoindentation hardness (GPa) at a depth of 10.0 μm is shown in the "10.0 μm Depth Hardness (GPa)" column of the "Substrate Region" column of the "Base Material" column in Table 3.

[0143] [(Test 5) Measurement Test of Nanoindentation Hardness of Hardened Layer Region and Base Material Region of Electric Resistance Welded Weld] Based on the method described in [Method for Measuring Hardness of Hardened Layer Region 21 and Base Material Region 22 of Electric Resistance Welded Weld 12] above, the nanoindentation hardness of the inner surface of the electric resistance welded weld of the pipe body of the cylinder member for each test number (hardness of the hardened layer region), and the nanoindentation hardness at a depth of 10.0 μm from the inner surface of the electric resistance welded weld (hardness of the base material region) were determined. The obtained inner surface nanoindentation hardness (GPa) is shown in the "Inner Surface Hardness (GPa)" column of the "Hardened Layer Region" column in the "Electric Resistance Weld" column in Table 3. The obtained nanoindentation hardness (GPa) at a depth of 10.0 μm is shown in the "10.0 μm Depth Hardness (GPa)" column of the "Base Material Region" column in the "Electric Resistance Weld" column in Table 3.

[0144] [(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 test. A cylindrical intermediate test piece with a length of 200 mm in the pipe axis direction was taken from the cylinder member of each test number. The intermediate test piece was cut in the pipe axis direction at a cross section including the central axis of the cylinder member to create a semi-cylindrical test piece (hereinafter referred to as the arc-shaped test piece). When the arc-shaped test piece is viewed in the pipe axis direction, the test piece has a convex arc shape, and the electric resistance weld was 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 polished using 1000-grit sandpaper. Note that even after finishing polishing with 1000-grit sandpaper, the hardness of the inner surface did not change before and after polishing. Furthermore, after alkaline degreasing the arc-shaped test specimens, their mass (g) was measured.

[0145] 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.

[0146] 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.

[0147] [Evaluation Results] Referring to Tables 1 to 3, test numbers 1 to 19 satisfied features 1 to 6. Therefore, the amount of wear in the abrasion resistance evaluation test was 10 g or less, and excellent abrasion resistance was obtained on the inner surface.

[0148] On the other hand, in test number 20, the carbon content was too low. As a result, the pearlite area ratio in the base material region of the base material was low, and the pearlite area ratio in the base material region of the electric resistance welded section was also low. Furthermore, the nanoindentation hardness in the hardened layer region of the base material was low, and the nanoindentation hardness in the hardened layer region of the electric resistance welded section was also low. As a result, the amount of wear in the abrasion resistance evaluation test exceeded 10 g, and sufficient abrasion resistance could not be obtained.

[0149] In test number 21, the carbon content was too high. As a result, the ferrite and pearlite area ratios in the base material region of the base material were low, and excessive hard structure formation occurred. Furthermore, the ferrite and pearlite area ratios in the base material region of the electric resistance welded section were low, and excessive hard structure formation occurred. Furthermore, the nanoindentation hardness in the hardened layer region of the base material was low, while the nanoindentation hardness in the base material region of the base material was high. Furthermore, the nanoindentation hardness in the hardened layer region of the electric resistance welded section was low, while the nanoindentation hardness in the base material region of the electric resistance welded section was high. As a result, the amount of wear in the abrasion resistance evaluation test exceeded 10 g, and sufficient abrasion resistance could not be obtained.

[0150] In tests 22 and 23, the cooling stop temperature T0 was low. As a result, the pearlite area ratio in the base material region was low, the sum of the ferrite area ratio and pearlite area ratio was low, and the hard structure was excessively formed. Consequently, the nanoindentation hardness in the hardened layer region of the base material was low. Therefore, the amount of wear in the wear resistance evaluation test exceeded 10 g, and sufficient wear resistance could not be obtained.

[0151] In tests 24 and 25, the cooling stop temperature T0 was high. As a result, the nanoindentation hardness in the hardened layer region of the base material was low, and the nanoindentation hardness in the substrate region of the base material was also low. Consequently, the amount of wear in the wear resistance evaluation test exceeded 10 g, and sufficient wear resistance could not be obtained.

[0152] In tests 26 and 27, the relative temperature T during the seam heat treatment process was Ｎ The value was low. Therefore, the average equivalent circle diameter D of the pearlite region in the electric resistance welded area was low. Ｗ The material was large. Furthermore, the nanoindentation hardness in the hardened layer region of the electric resistance weld was low. As a result, the amount of wear in the wear resistance evaluation test exceeded 10g, and sufficient wear resistance could not be obtained.

[0153] In tests 28 and 29, the relative temperature T during the seam heat treatment process was Ｎ The average circle equivalent diameter D of the pearlite region in the electric resistance welded area was high. Ｗ The material was large. Furthermore, the nanoindentation hardness in the hardened layer region of the electric resistance weld was low. As a result, the amount of wear in the wear resistance evaluation test exceeded 10g, and sufficient wear resistance could not be obtained.

[0154] In tests 30 and 31, 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. Ｗ The material was large. Furthermore, the nanoindentation hardness in the hardened layer region of the electric resistance weld was low. As a result, the amount of wear in the wear resistance evaluation test exceeded 10g, and sufficient wear resistance could not be obtained.

[0155] In tests 32 and 33, the rapid cooling stop temperature T2 was too low. As a result, the pearlite area ratio in the base material region of the electric resistance weld was low, the sum of the ferrite area ratio and pearlite area ratio was low, and excessive hard structure was generated. Consequently, the nanoindentation hardness in the hardened layer region of the electric resistance weld was low, while the nanoindentation hardness in the base material region of the electric resistance weld was high. As a result, the amount of wear in the abrasion resistance evaluation test exceeded 10 g, and sufficient abrasion resistance could not be obtained.

[0156] In tests 34 and 35, 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 material was large. Furthermore, the nanoindentation hardness in the hardened layer region of the electric resistance weld was low. As a result, the amount of wear in the wear resistance evaluation test exceeded 10g, and sufficient wear resistance could not be obtained.

[0157] In tests 36 and 37, the second cooling rate CR2 was fast. As a result, the pearlite area ratio in the base material region of the electric resistance weld was low, the sum of the ferrite area ratio and pearlite area ratio was low, and excessive hard structure was generated. Consequently, the nanoindentation hardness in the hardened layer region of the electric resistance weld was low. As a result, the amount of wear in the abrasion resistance evaluation test exceeded 10 g, and sufficient abrasion resistance could not be obtained.

[0158] In tests 38 and 39, the surface pressure P during the internal surface processing step was low. As a result, the nanoindentation hardness in the hardened layer region of the base material was low, and the nanoindentation hardness in the hardened layer region of the electric resistance welded area was also low. Consequently, the amount of wear in the abrasion resistance evaluation test exceeded 10g, and sufficient abrasion resistance could not be obtained.

[0159] In tests 40 and 41, the number of reciprocating strokes during the internal surface processing step was insufficient. As a result, the nanoindentation hardness in the hardened layer region of the base material was low, and the nanoindentation hardness in the hardened layer region of the electric resistance welded area was also low. Consequently, the amount of wear in the abrasion resistance evaluation test exceeded 10 g, and sufficient abrasion resistance could not be obtained.

[0160] 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.

[0161] 1. Pipe body 11. Base material 12. Electric resistance welded section 21. Hardened layer area 22. Base material area

Claims

1. A cylinder member comprising a tube body including a base material portion and an electric resistance welded portion, wherein the chemical composition of the tube body 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%, B: 0 to 0.0100%. It contains W: 0-0.10%, Ca: 0-0.0200%, Mg: 0-0.0200%, Zr: 0-0.0200%, and rare earth elements (REM): 0-0.0200%, with the remainder being Fe and impurities. At the center of the thickness of the base material, 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. At the center of the thickness of the electric resistance welded section, 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 average equivalent circle diameter D of the pearlite region, which consists of one or a plurality of continuously connected pearlite blocks, is 50% or more. Ｗ A cylinder member wherein the nanoindentation hardness of the hardened layer region on the inner surface of the tube body is 10.0 μm or less, and the tube body further includes a hardened layer region formed on the surface layer of the inner surface of the tube body and a base material region other than the hardened layer region having a lower hardness than the hardened layer region, and in the base material portion, the nanoindentation hardness of the hardened layer region on the inner surface is 5.0 GPa or more, and the nanoindentation hardness of the base material region at a depth of 10.0 μm from the inner surface is 3.0 to 4.0 GPa, and in the electric resistance welded portion, the nanoindentation hardness of the hardened layer region on the inner surface is 5.0 GPa or more, and the nanoindentation hardness of the base material region at a depth of 10.0 μm from the inner surface is 3.0 to 4.0 GPa.

2. A cylinder member according to claim 1, wherein the chemical composition of the tube body contains, by 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 (REM): 0.0001 to 0.0200%.

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