Method for manufacturing double pipe

Abstract

This method for manufacturing a double pipe comprises an outer pipe preparation step (S10), an inner pipe preparation step (S20), a composite billet preparation step (S30), a heating step (S40), and a rolling step (S50). In the outer pipe preparation step (S10), an outer pipe (6) made of carbon steel is prepared. In the inner pipe preparation step (S20), an inner pipe made of a metal material having a deformation resistance greater than the deformation resistance of carbon steel is prepared. In the composite billet preparation step (S30), a composite billet (10) including the outer pipe (6) and the inner pipe (8) disposed inside the outer pipe (6) is prepared. In the heating step (S40), the composite billet (10) is heated. In the rolling step (S50), the composite billet (10) is rolled to obtain a shell (12) in which the outer pipe (6) and the inner pipe (8) are in close contact with each other.

Description

Manufacturing method for double-walled tubes

[0001] This disclosure relates to a method for manufacturing a double-walled tube comprising an inner tube and an outer tube, and more particularly to a method for manufacturing a double-walled tube in which the outer circumferential surface of the inner tube is in close contact with the inner circumferential surface of the outer tube.

[0002] Pipes used to carry corrosive fluids require corrosion resistance. One such pipe is the CRA (Corrosion Resistance Alloy) line pipe. The entire CRA line pipe is made of CRA, a corrosion-resistant alloy. However, because CRA is expensive, manufacturing the entire pipe from CRA increases the cost of the pipe. Therefore, attempts are being made to construct the pipe with an outer pipe and an inner pipe placed inside the outer pipe. By making only the inner pipe from a corrosion-resistant material, the cost of the double-walled pipe can be reduced.

[0003] Japanese Patent Publication No. 62-148018 (Patent Document 1) and Japanese Patent Publication No. 59-163088 (Patent Document 2) disclose methods for manufacturing double-walled tubes. In the manufacturing method of Patent Document 1, first, a solid round billet is fitted into a tube body that forms the outer layer to form a laminated billet. At this point, there is a gap between the tube body and the round billet. Next, the laminated billet is heated and then kneaded and rolled in an inclined rolling mill. This brings the tube body and the round billet into close contact. After that, the laminated billet is perforated, stretched, and rolled to obtain a double-walled tube (clad tube).

[0004] In the manufacturing method described in Patent Document 2, first, an inner tube is inserted into an outer tube, and the inner tube is tightly fitted to the outer tube by hydraulic expansion to obtain a double-walled tube. Subsequently, the outer tube and inner tube are sealed welded at both ends, and then the double-walled tube is hot-rolled. When the double-walled tube is heated during hot-rolling, a gap may form between the outer tube and the inner tube due to the difference in thermal expansion between them. However, Patent Document 2 states that because the outer tube and inner tube are sealed welded, even if a gap forms, it will not be open to the outside, and therefore the inner surface of the outer tube and the outer surface of the inner tube will not oxidize.

[0005] JP-A-62-148018 JP-A-59-163088

[0006] However, as in the method described in Patent Document 1, when there is a gap between the tubular body (outer tube), which is the material for manufacturing the double tube, and the component placed inside it, it is not easy to bring these materials into close contact. Furthermore, both the methods in Patent Documents 1 and 2 involve many steps and have low production efficiency.

[0007] Therefore, the purpose of this disclosure is to provide a manufacturing method that can produce double-walled pipes, in which the outer and inner pipes are closely fitted together, with high production efficiency.

[0008] The method for manufacturing a double-walled tube according to this disclosure comprises an outer tube preparation step, an inner tube preparation step, a composite billet preparation step, a heating step, and a rolling step. In the outer tube preparation step, an outer tube made of carbon steel is prepared. In the inner tube preparation step, an inner tube is prepared, which is made of a metallic material having a deformation resistance greater than that of carbon steel, and has an outer diameter of 0.90 times or more and 0.99 times or less the inner diameter of the outer tube. In the composite billet preparation step, a composite billet comprising an outer tube and an inner tube disposed inside the outer tube is prepared.

[0009] In the heating process, the composite billet is heated. In the rolling process, an inclined rolling mill having inclined rolls and plugs is used to roll the composite billet by bringing the inclined rolls into contact with the outer surface of the heated composite billet and bringing the plugs into contact with the inner surface of the inner tube in the heated composite billet, thereby obtaining a shell in which the outer tube and inner tube are closely joined. In the heating process, the space between the outer tube and inner tube at the end of the composite billet is opened to the outside of the composite billet. In the rolling process, the composite billet is rolled using an inclined rolling mill under conditions that satisfy the following equation (1): ER ２ ×EL×(Ti / Ta) １／２ ×RI / SR ２ > 1.65 (1)

[0010] In equation (1), the meaning of each symbol is as follows: ER = Doa / Dob Doa: Set value for the outer diameter of the shell in the rolling process Dob: Outer diameter of the outer tube prepared in the outer tube preparation process EL = (S1 + S2) / S3 S1: Cross-sectional area of ​​the outer tube prepared in the outer tube preparation process S2: Cross-sectional area of ​​the inner tube prepared in the inner tube preparation process S3: Set value for the cross-sectional area of ​​the shell in the rolling process Ti = T1 + T2 T1: Wall thickness of the outer tube prepared in the outer tube preparation process T2: Wall thickness of the inner tube prepared in the inner tube preparation process Ta: Set value for the wall thickness of the shell in the rolling process RI = Izi / Iziv Izi: Second moment of area of ​​the inner tube prepared in the inner tube preparation process Izv: The second moment of area of ​​the inner tube, assuming that the inner tube is deformed so that its outer surface is in close contact with the inner surface of the outer tube prepared in the outer tube preparation process, while maintaining the cross-sectional area of ​​the inner tube. SR: The ratio of the deformation resistance of the inner tube to that of the outer tube at the heating temperature in the heating process.

[0011] The manufacturing method disclosed herein allows for the production of double-walled tubes in which the outer and inner tubes are closely joined, with high production efficiency.

[0012] Figure 1 is a flowchart of the manufacturing method according to this embodiment. Figure 2 is a perspective view of the composite billet. Figure 3 is a front view of the composite billet. Figure 4 is a top view of the inclined rolling mill. Figure 5 is a side view of the inclined rolling mill. Figure 6 is a front view of the shell. Figure 7 is a front view of the outer tube and the virtual inner tube. Figure 8 is a cross-sectional view of the double tube obtained by the manufacturing method according to this embodiment. Figure 9 is an optical microscope image of the inner tube after an oxalic acid etching test was performed after heat treatment. Figure 10 is an optical microscope image of the inner tube after an oxalic acid etching test was performed before heat treatment.

[0013] The manufacturing method for a double-walled tube according to this embodiment comprises an outer tube preparation step, an inner tube preparation step, a composite billet preparation step, a heating step, and a rolling step. In the outer tube preparation step, an outer tube made of carbon steel is prepared. In the inner tube preparation step, an inner tube is prepared, which is made of a metal material having a deformation resistance greater than that of carbon steel, and has an outer diameter of 0.90 times or more and 0.99 times or less the inner diameter of the outer tube. In the composite billet preparation step, a composite billet comprising an outer tube and an inner tube disposed inside the outer tube is prepared.

[0014] In the heating process, the composite billet is heated. In the rolling process, an inclined rolling mill having inclined rolls and plugs is used to roll the composite billet by bringing the inclined rolls into contact with the outer surface of the heated composite billet and bringing the plugs into contact with the inner surface of the inner tube in the heated composite billet, thereby obtaining a shell in which the outer tube and inner tube are closely joined. In the heating process, the space between the outer tube and inner tube at the end of the composite billet is opened to the outside of the composite billet. In the rolling process, the composite billet is rolled by an inclined rolling mill under conditions that satisfy the following equation (1) (first configuration). ER ２ ×EL×(Ti / Ta) １／２ ×RI / SR ２ > 1.65 (1)

[0015] In equation (1), the meaning of each symbol is as follows: ER = Doa / Dob Doa: Set value for the outer diameter of the shell in the rolling process Dob: Outer diameter of the outer tube prepared in the outer tube preparation process EL = (S1 + S2) / S3 S1: Cross-sectional area of ​​the outer tube prepared in the outer tube preparation process S2: Cross-sectional area of ​​the inner tube prepared in the inner tube preparation process S3: Set value for the cross-sectional area of ​​the shell in the rolling process Ti = T1 + T2 T1: Wall thickness of the outer tube prepared in the outer tube preparation process T2: Wall thickness of the inner tube prepared in the inner tube preparation process Ta: Set value for the wall thickness of the shell in the rolling process RI = Izi / Iziv Izi: Second moment of area of ​​the inner tube prepared in the inner tube preparation process Izv: The second moment of area of ​​the inner tube, assuming that the inner tube is deformed so that its outer surface is in close contact with the inner surface of the outer tube prepared in the outer tube preparation process, while maintaining the cross-sectional area of ​​the inner tube. SR: The ratio of the deformation resistance of the inner tube to that of the outer tube at the heating temperature in the heating process.

[0016] In the manufacturing method of the double-walled tube according to the first configuration, the composite billet is rolled in the rolling process under conditions that satisfy formula (1). As a result, a shell in which the outer tube and inner tube are closely joined can be obtained, and the tightness between the outer tube and inner tube is high even in the double-walled tube.

[0017] Generally, when heating a composite billet, to prevent the formation of oxide scale on the inner surface of the outer tube and the outer surface of the inner tube, for example, the outer and inner tubes are sealed with welding at both ends of the composite billet before heating. In contrast, in the first configuration, the space between the outer and inner tubes at the ends of the composite billet is open to the outside of the composite billet. Therefore, during the heating process, oxide scale is formed on the inner surface of the outer tube and the outer surface of the inner tube.

[0018] However, in the first configuration, during the rolling process, the oxide scale formed on the outer surface of the inner tube and the inner surface of the outer tube rubs against each other and peels off from their respective surfaces. Furthermore, the peeled oxide scale moves towards the rear end of the composite billet in the space between the outer and inner tubes and is discharged to the outside of the composite billet. Therefore, even if oxide scale is formed between the outer and inner tubes during the heating process, there is almost no oxide near the interface between the outer and inner tubes in the shell after the rolling process. Consequently, in the first configuration, processes performed to prevent the formation of oxide scale before heating, such as seal welding, can be omitted. Furthermore, if seal welding is omitted, machining of the ends of the outer and inner tubes and ensuring that the lengths of the outer and inner tubes are aligned, which are performed before seal welding, also become unnecessary.

[0019] Based on the above, according to the first configuration, a double-walled tube in which the outer tube and inner tube are closely joined can be manufactured with high production efficiency.

[0020] In the manufacturing method of the double-walled pipe according to the first configuration, the inner pipe prepared in the inner pipe preparation step may be made of austenitic stainless steel (second configuration). In this case, the inner surface of the manufactured double-walled pipe is corrosion-resistant. Therefore, according to the second configuration, a double-walled pipe can be manufactured for use in carrying corrosive fluids.

[0021] The method for manufacturing a double tube according to the second configuration may further include a quenching step and a tempering step. In the quenching step, the shell after the rolling step may be heated to a temperature of 925°C to 975°C, held at that temperature for 5 to 15 minutes, and then rapidly cooled. In the tempering step, the shell after the quenching step may be heated to a temperature of 625°C to 675°C, held at that temperature for 20 to 40 minutes, and then cooled (third configuration).

[0022] The method for manufacturing a double-walled tube according to this embodiment will be described in detail with reference to the drawings. Figure 1 is a flowchart of the manufacturing method according to this embodiment. This manufacturing method includes an outer tube preparation step S10, an inner tube preparation step S20, a composite billet preparation step S30, a heating step S40, and a rolling step S50.

[0023] <Outer pipe preparation process> In the outer pipe preparation process S10, an outer pipe 6 made of carbon steel is prepared. The outer pipe 6 has a cylindrical shape. The outer pipe 6 may be a seamless steel pipe or a welded steel pipe.

[0024] <Inner Tube Preparation Process> In the inner tube preparation process S20, an inner tube 8 made of a metal material different from carbon steel is prepared. That is, the inner tube 8 is made of a metal material different from the outer tube 6. The metal material that makes up the inner tube 8 has a deformation resistance greater than that of the carbon steel that makes up the outer tube 6. In other words, the deformation resistance ratio of the inner tube 8 to the outer tube 6 is greater than 1.0. The deformation resistance ratio of the inner tube 8 to the outer tube 6 is preferably 2.0 or less, and more preferably 1.5 or less. The inner tube 8 is less prone to deformation than the outer tube 6.

[0025] The inner pipe 8, like the outer pipe 6, has a cylindrical shape. The inner pipe 8, like the outer pipe 6, may be a seamless steel pipe or a welded steel pipe. The inner pipe 8 has an outer diameter of 0.90 times or more and 0.99 times or less the inner diameter of the outer pipe 6. That is, the outer diameter of the inner pipe 8 is 0.90 times or more and 0.99 times or less the inner diameter of the outer pipe 6. In other words, the ratio of the outer diameter of the inner pipe 8 to the inner diameter of the outer pipe 6 is 0.90 or more and 0.99 or less. The length of the inner pipe 8 may be substantially the same as the length of the outer pipe 6, or it may be different.

[0026] Here, the deformation resistance of a metallic material is determined by a hot tensile test using a Cermec Master testing machine as follows: First, a cylindrical test specimen with an outer diameter of 8 mm and a length of 130 mm is prepared. Next, under vacuum conditions, this test specimen is heated to a predetermined temperature at a heating rate of 10°C / second. This predetermined temperature is the heating temperature in heating step S40 described later. Furthermore, after holding the test specimen at the predetermined temperature for 10 seconds, a tensile test is performed on the test specimen at a strain rate of 30 / second. The maximum stress value obtained in this test is defined as the deformation resistance.

[0027] When the double-walled pipe obtained by this manufacturing method is used to carry a corrosive fluid through it, the inner pipe 8 is preferably made of a corrosion-resistant metal material. In this case, stainless steel is an example of the metal material that constitutes the inner pipe 8. The stainless steel is, for example, austenitic stainless steel. The austenitic stainless steel is preferably equivalent to SUS316L according to JIS standards. The stainless steel may also be martensitic stainless steel or ferritic stainless steel, etc. The metal material that constitutes the inner pipe 8 may also be a nickel alloy. The nickel alloy is preferably an alloy equivalent to NCF825 according to JIS standards. In this embodiment, in the inner pipe preparation step S20, the inner pipe 8 made of austenitic stainless steel is prepared.

[0028] The NCF825 equivalent alloy contains Ni: 38.0% to 46.0%, Cr: 19.5% to 23.5%, C: 0.025% or less, Mn: 1.0% or less, Si: 0.5% or less, Cu: 1.5% to 3.0%, Mo: 2.5% to 3.5%, Al: 0.2% or less, Ti: 0.6% to 1.2%, P: 0.020% or less, and S: 0.010% or less, with the remainder consisting of Fe and impurities.

[0029] SUS316L equivalent steel contains C: 0.030% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0.030% or less, Ni: 12.00% to 16.00%, Cr: 16.00% to 18.00%, and Mo: 2.00% to 3.00%, with the remainder consisting of Fe and impurities.

[0030] <Composite Billet Preparation Process> In the composite billet preparation process S30, the composite billet 10 is prepared. Figure 2 is a perspective view of the composite billet 10. The composite billet 10 comprises an outer tube 6 and an inner tube 8. The inner tube 8 is located inside the outer tube 6.

[0031] Figure 3 is a front view of the composite billet 10. Referring to Figure 3, let the inner diameter of the outer tube 6 be Dib and the outer diameter of the inner tube 8 be dob. In the composite billet 10, it is preferable that the difference between the inner diameter of the outer tube 6 and the outer diameter of the inner tube 8 (Dib - dob) is 10% or less of the outer diameter dob of the inner tube 8. That is, it is preferable that the inner diameter Dib of the outer tube 6 and the outer diameter dob of the inner tube 8 satisfy the condition "(Dib - dob) / dob ≤ 0.1". The difference between the inner diameter of the outer tube 6 and the outer diameter of the inner tube 8 (Dib - dob) corresponds to the average size of the gap between the outer tube 6 and the inner tube 8.

[0032] In the outer tube preparation step S10 and the inner tube preparation step S20, the outer tube 6 and the inner tube 8 are prepared individually, and then in the composite billet preparation step S30, the inner tube 8 is inserted into the outer tube 6. This allows a composite billet 10 to be obtained. At the ends of the outer tube 6 and the inner tube 8, they are not sealed by welding. Therefore, in the composite billet 10, the space between the outer tube 6 and the inner tube 8 is open to the outside of the composite billet 10. The inner tube 8 may be fixed to the outer tube 6 to prevent it from falling out of the outer tube 6 during transport of the composite billet 10. For example, the inner tube 8 can be fixed to the outer tube 6 by pressing both ends of the inner tube 8 inserted into the outer tube 6 in the axial direction, thereby expanding the inner tube 8 radially.

[0033] <Heating Step> In the heating step S40, the composite billet 10 is heated. The heating temperature is set to a temperature at which the composite billet 10 can be sufficiently deformed and desired rolling can be performed in the subsequent rolling step S50. For example, the heating temperature is preferably 900°C to 1300°C. More preferably, the heating temperature is 1010°C to 1300°C. The heating step S40 can be performed in an air atmosphere (oxidizing atmosphere). In this case, oxide scale is formed on the surfaces of the outer tube 6 and the inner tube 8. Oxide scale is also formed on the inner circumferential surface of the outer tube 6 and the outer circumferential surface of the inner tube 8. This is because the space between the outer tube 6 and the inner tube 8 is open to the outside of the composite billet 10.

[0034] <Rolling Step> FIG. 4 is a top view of the helical rolling mill 5, showing the composite billet 10 and the like with a part broken away. FIG. 5 is a side view of the helical rolling mill 5. In the rolling step S50, a helical rolling mill 5 having a pair of helical rolls 1 and a plug 2 is used. The pair of helical rolls 1 is brought into contact with the outer circumferential surface of the heated composite billet 10, and the plug 2 is brought into contact with the inner circumferential surface of the inner tube 8 in the heated composite billet 10. By rolling the composite billet 10 in this state, a shell 12 in which the outer tube 6 and the inner tube 8 are brought into close contact is obtained.

[0035] In the present embodiment, the pair of helical rolls 1 are arranged opposite to each other in the horizontal direction across the pass line PL. However, instead of the pair of helical rolls 1, three helical rolls arranged at an angular interval of 120° from each other around the pass line PL may be used. The helical rolling mill 5 has the same configuration as a Mannesmann type piercing and rolling mill used for piercing rolling. However, the composite billet 10 to be rolled already has a hole in the inner tube 8 before being processed by the helical rolling mill 5. For this reason, no piercing is performed in the rolling step S50.

[0036] Hereinafter, the inclined rolling mill 5 and the rolling step S50 will be described in detail. A pair of inclined rolls 1 are arranged on opposite sides to each other with respect to the pass line PL. The central axis (rotation axis) C of each inclined roll forms a predetermined crossing angle CA and a predetermined inclination angle FA with respect to the pass line PL. The crossing angle CA is an angle formed between the pass line PL and the central axis C projected onto a horizontal plane including the pass line PL (see Fig. 4). The inclination angle FA is an angle formed between the pass line PL and the central axis C projected onto a vertical plane including the pass line PL (see Fig. 5). The crossing angle CA can be, for example, 0 to 30°. The inclination angle FA can be, for example, 5 to 20°.

[0037] The plug 2 is arranged on the pass line PL approximately midway between the pair of inclined rolls 1. The plug 2 has a shell-shaped profile. The plug 2 is attached to the tip end of a mandrel 3. The plug 2 and the mandrel 3 are arranged such that their central axes coincide with the pass line PL.

[0038] When performing rolling, the composite billet 10 is moved along the pass line PL in a state where the central axis of the outer tube 6 of the composite billet 10 is aligned with the pass line PL. In Figs. 4 and 5, the moving direction of the composite billet 10 is indicated by hollow arrows. The tip end of the plug 2 faces in a direction opposite to the moving direction of the composite billet 10. The composite billet 10 is fed between the pair of inclined rolls 1. The rear end of the composite billet 10 may be pushed by a pusher until the composite billet 10 is sufficiently bitten by the pair of inclined rolls 1. The rear end of the composite billet 10 refers to the end portion on the opposite side of the pair of inclined rolls 1 among the two end portions of the composite billet 10.

[0039] As the composite billet 10 moves, on the downstream side in the moving direction, the inner peripheral surface of the inner tube 8 of the composite billet 10 comes into contact with the plug 2. Then, the composite billet 10 is rolled by the pair of inclined rolls 1 and the plug 2. As a result, the outer tube 6 and the inner tube 8 are deformed so as to be relatively close to each other, and finally the outer tube 6 and the inner tube 8 come into close contact with each other.

[0040] It is preferable that the maximum outer diameter of the plug 2 is larger than the inner diameter of the inner tube 8. In this case, the inner tube 8 is expanded by the plug 2, and the outer tube 6 and the inner tube 8 are rolled so as to be stretched in the axial direction by the pair of inclined rolls 1 and the plug 2. Expanding the inner tube 8 with the plug 2 allows the outer tube 6 and the inner tube 8 to be brought into close contact more easily than reducing the diameter of the outer tube 6 with the inclined rolls 1.

[0041] For the composite billet 10 shown in Figure 3, the outer diameter dob of the inner tube 8 prepared in the inner tube preparation step S20 is not less than 0.90 times and not more than 0.99 times the inner diameter Dib of the outer tube 6 prepared in the outer tube preparation step S10, so the inner tube 8 can be easily inserted into the outer tube 6 in the composite billet preparation step S30. In addition, in order to obtain the shell 12 in which the outer tube 6 and the inner tube 8 are in close contact through the rolling step S50, it is necessary to satisfy the following conditions (i) and (ii). (i) The outer diameter dob of the inner tube 8 prepared in the inner tube preparation step S20 is not less than 0.90 times and not more than 0.99 times the inner diameter Dib of the outer tube 6 prepared in the outer tube preparation step S10. (ii) Rolling is performed by a piercing mill 5 under conditions satisfying the following formula (1). ER ２ ×EL×(Ti / Ta) １／２ ×RI / SR ２ >1.65 (1)

[0042] Figure 6 is a front view of the shell 12. The shell 12 is constituted by the outer tube 6 after rolling and the inner tube 8 after rolling being in close contact with each other. Figure 7 is a front view of the outer tube 6 and a virtual inner tube 8v in the outer tube preparation step S10. The virtual inner tube 8v is an inner tube obtained when it is assumed that the inner tube 8 is deformed such that the outer circumferential surface of the inner tube 8 is in close contact with the inner circumferential surface of the outer tube 6 while maintaining the cross-sectional area of the inner tube 8 as the cross-sectional area thereof in the inner tube preparation step S20. In Figure 7, the inner circumferential surface of the virtual inner tube 8v is indicated by a two-dot chain line. Hereinafter, the parameters ER, EL, Ti, Ta, RI, and SR on the left side of formula (1) will be described with reference to Figure 3, Figure 6, and Figure 7.

[0043] ER is the tube expansion ratio. When the set value of the outer diameter of the shell 12 in the rolling step S50 is Doa, and the outer diameter of the outer tube 6 prepared in the outer tube preparation step S10 is Dob, ER is represented by the following formula. ER=Doa / Dob

[0044] The setting value Doa for the outer diameter of the shell 12 in the rolling process S50 is a setting value for the outer diameter of the shell 12 set for the inclined rolling mill 5. This setting value Doa corresponds to the hot dimensions of the shell 12 immediately after the composite billet 10 has been inclined-rolled in the inclined rolling mill 5, that is, the hot outer diameter.

[0045] EL is the elongation ratio. If S1 is the cross-sectional area of ​​the outer tube 6 prepared in the outer tube preparation process S10, S2 is the cross-sectional area of ​​the inner tube 8 prepared in the inner tube preparation process S20, and S3 is the set value of the cross-sectional area of ​​the shell 12 in the rolling process S50, then EL is expressed by the following formula: EL = (S1 + S2) / S3

[0046] The set value S3 for the cross-sectional area of ​​the shell 12 in the rolling process S50 is a set value for the cross-sectional area of ​​the shell 12 set for the inclined rolling mill 5. This set value S3 corresponds to the hot dimensions of the shell 12 immediately after the composite billet 10 has been inclined-rolled in the inclined rolling mill 5, i.e., the hot cross-sectional area. The set values ​​S1 and S2 for the cross-sectional areas of the outer tube 6 and inner tube 8, and the set value S3 for the cross-sectional area of ​​the shell 12, mean the area of ​​the cross-section perpendicular to the respective central axis.

[0047] Ti is the sum of the wall thickness T1 of the outer tube 6 prepared in the outer tube preparation process S10 and the wall thickness T2 of the inner tube 8 prepared in the inner tube preparation process S20. Specifically, Ti is expressed by the following formula: Ti = T1 + T2

[0048] Ta is the set value for the wall thickness of the shell 12 in the rolling process S50. The set value Ta for the wall thickness of the shell 12 in the rolling process S50 is a set value for the wall thickness of the shell 12 set for the inclined rolling mill 5. This set value Ta corresponds to the hot dimensions of the shell 12 immediately after the composite billet 10 has been inclined rolled in the inclined rolling mill 5, that is, the wall thickness when hot. In other words, the set value Ta corresponds to the sum of the hot wall thickness of the outer tube 6 and the hot wall thickness of the inner tube 8 in the shell 12 immediately after the composite billet 10 has been inclined rolled in the inclined rolling mill 5.

[0049] RI is the stiffness ratio. If Izi is the second moment of area of ​​the inner tube 8 prepared in the inner tube preparation process S20, and Izv is the second moment of area of ​​the virtual inner tube 8v, then RI is expressed by the following formula: RI = Izi / Izv

[0050] If the outer diameter of the inner tube 8 prepared in the inner tube preparation process S20 is dob, and the inner diameter of the inner tube 8 prepared in the inner tube preparation process S20 is dib, then Izi is expressed by the following formula: Izi = π(dob) ４ -dib ４ ) / 64

[0051] If the outer diameter of the virtual inner tube 8v is dov and the inner diameter of the virtual inner tube 8v is div, then Izv is expressed by the following formula: Izv = π(dov) ４ -div ４ ) / 64

[0052] Since the outer surface of the virtual inner tube 8v is in close contact with the inner surface of the outer tube 6 prepared in the outer tube preparation step S10, the outer diameter dov of the virtual inner tube 8v can be equal to the inner diameter Dib of the outer tube 6 prepared in the outer tube preparation step S10. Because the cross-sectional area S2 of the inner tube 8 prepared in the inner tube preparation step S20 is equal to the cross-sectional area of ​​the virtual inner tube 8v, the following equation holds: π(dob / 2) ２ -π(dib / 2) ２ = π(dov / 2) ２ -π(div / 2) ２

[0053] In other words, the following equation holds true: dob ２ -dib ２ =dov ２ -div ２ Therefore, the inner diameter div of the virtual inner tube 8v can be determined from dob, dib, and dov(Dib).

[0054] SR is the ratio of the deformation resistance of the inner tube 8 to that of the outer tube 6 at the heating temperature in heating step S40. SR can be determined as follows: For each of the materials of the outer tube 6 and the inner tube 8, the deformation resistance at the heating temperature in heating step S40 is measured by the hot tensile test described above. Then, SR can be determined by dividing the deformation resistance of the inner tube 8 material by the deformation resistance of the outer tube 6 material.

[0055] In each of the above formulas, the outer diameter Dob, inner diameter Dib, and wall thickness T1 of the outer tube 6 are the dimensions of the outer tube 6 in the outer tube preparation step S10. The outer diameter dob, inner diameter dib, and wall thickness T2 of the inner tube 8 are the dimensions of the inner tube 8 in the inner tube preparation step S20.

[0056] The manufacturing method according to this embodiment further includes a quenching step S60 and a tempering step S70. The quenching step S60 and the tempering step S70 are performed to increase the strength of the outer tube 6, which is made of carbon steel.

[0057] <Quenching Process> In the quenching process S60, the shell 12 after the rolling process S50 is heated to a temperature of 925°C to 975°C, held at this temperature for 5 to 15 minutes, and then rapidly cooled. For rapid cooling, for example, water cooling or oil cooling is performed on the shell 12.

[0058] <Tempering Process> In the tempering process S70, the shell 12 after the quenching process S60 is heated to a temperature of 625°C to 675°C, held at this temperature for 20 to 40 minutes, and then cooled.

[0059] The shell 12 obtained in the tempering process S70 may be used as the double-walled pipe product. Alternatively, the shell 12 obtained in the rolling process S50 may be used as the double-walled pipe product without performing heat treatment after the rolling process S50. In this case, the shell 12 obtained in the rolling process S50 may be subjected to further processing, such as stretching and rolling using a mandrel mill, to obtain the double-walled pipe product.

[0060] Figure 8 is a cross-sectional view of a double-walled tube 20 obtained by the manufacturing method according to this embodiment. The cross-section is a section perpendicular to the central axis of the double-walled tube 20. The double-walled tube 20 includes an outer tube 6 and an inner tube 8. The outer circumferential surface of the inner tube 8 is in close contact with the inner circumferential surface of the outer tube 6. The wall thickness of the outer tube 6 in the double-walled tube 20 is smaller than the wall thickness of the outer tube 6 before the rolling process S50 is carried out. Similarly, the wall thickness of the inner tube 8 in the double-walled tube 20 is smaller than the wall thickness of the inner tube 8 before the rolling process S50 is carried out.

[0061] <Effects> As in this embodiment, when the inner tube 8 is made of a corrosion-resistant metal material, a double-walled tube 20 with a corrosion-resistant inner surface can be manufactured by the method of this embodiment. Such a double-walled tube 20 can be used to carry corrosive fluids. Since the corrosive fluid flowing inside the double-walled tube 20 does not usually come into contact with the outer tube 6, the outer tube 6 can be made of carbon steel that does not have corrosion resistance. Generally, carbon steel is cheaper than corrosion-resistant metal materials. Therefore, by using an inexpensive metal material for the outer tube 6, the overall cost of the double-walled tube 20 can be reduced.

[0062] In this embodiment, in the rolling process S50, the composite billet 10 is rolled under conditions that satisfy formula (1), and a shell 12 is obtained. The inventors have found that in the double-walled tube 20 obtained from this shell 12, the carbon steel constituting the outer tube 6 and the metallic material constituting the inner tube 8 are in direct contact with each other in most of the vicinity of the interface between the outer tube 6 and the inner tube 8. As a result, the close contact between the outer tube 6 and the inner tube 8 is high. Therefore, according to this embodiment, a double-walled tube 20 in which the outer tube 6 and the inner tube 8 are closely connected can be obtained. Hereinafter, the space between the outer tube 6 and the inner tube 8 will be referred to as the "annular space". Furthermore, the inventors have found that even if the heating process S40 is performed with the annular space open to the atmospheric atmosphere outside the composite billet 10 at the end of the composite billet 10, there are almost no oxides near the interface between the outer tube 6 and the inner tube 8 in the double-walled tube 20 (composite billet 10 after rolling).

[0063] The reason for this is thought to be as follows: In the heating process S40, the oxide scale formed on the inner circumferential surface of the outer tube 6 and the oxide scale formed on the outer circumferential surface of the inner tube 8 rub against each other when the inner circumferential surface of the outer tube 6 and the outer circumferential surface of the inner tube 8 come into close contact in the rolling process S50, causing them to peel off from the inner circumferential surface of the outer tube 6 and the outer circumferential surface of the inner tube 8. The peeled-off oxide scale can move within the annular space and move toward the rear end of the composite billet 10, where it is discharged to the outside of the annular space.

[0064] Therefore, according to the method of this embodiment, in the heating step S40, the step of sealing the ends of the outer tube 6 and the inner tube 8 together, which is performed to prevent the formation of oxide scale on the inner surface of the outer tube 6 and the outer surface of the inner tube 8, is unnecessary. If sealing welding is performed as in the conventional technology, it is necessary to machine the ends of the tubes before sealing welding. In this embodiment, such machining is not necessary.

[0065] Furthermore, when performing seal welding as in conventional technology, the length of the outer tube and the length of the inner tube must be approximately the same before stretch rolling or the like. In this case, after stretch rolling, the inner tube becomes longer than the outer tube, and the portion of the inner tube that does not form a double tube is discarded as rolling slag. In contrast, in this embodiment, it is not necessary for the length of the outer tube 6 and the length of the inner tube 8 to be the same, so the amount of rolling slag generated can be reduced.

[0066] Furthermore, according to this embodiment, rolling using the inclined rolling mill 5 only needs to be performed once. In other words, in this embodiment, there is no need to perform pre-rolling on the composite billet 10 without using the plug 2.

[0067] As described above, according to the manufacturing method of this embodiment, a double-walled pipe 20 in which the outer pipe 6 and the inner pipe 8 are closely joined can be manufactured with high production efficiency.

[0068] 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.

[0069] The present disclosure will be further described below with reference to examples. However, the present disclosure is not limited to the following examples.

[0070] <First Embodiment> In the first embodiment, double-walled tubes were manufactured under multiple manufacturing conditions to demonstrate the effects of the manufacturing method according to the above embodiment. There were a total of 10 manufacturing conditions, corresponding to Comparative Examples 1 to 4 and Invention Examples 1 to 6. Table 1 shows the conditions of the composite billet used under each manufacturing condition. The material of the inner tube was JIS standard SUS316L equivalent steel (stainless steel) or NCF825 equivalent alloy (nickel alloy). The material of the outer tube was carbon steel. The dimensions of the inner and outer tubes were the dimensions when each tube was prepared.

[0071]

[0072] Table 2 shows the manufacturing conditions, the number of double-walled tubes manufactured under each condition (N), and the results of the seal quality evaluation. The seal quality was evaluated by penetrant testing as specified in JIS Z 2343-1:2017.

[0073]

[0074] After heating the prepared composite billets at the heating temperatures shown in Table 2, double-walled tubes were manufactured by inclined rolling under each manufacturing condition. Under all manufacturing conditions, the intersection angle was in the range of 0 to 30°, and the inclination angle was in the range of 9 to 16°. Under the manufacturing conditions of Comparative Examples 1 to 4, the left side of equation (1) was 1.65 or less, and equation (1) was not satisfied. In the cases of Comparative Examples 1 to 4, a gap existed between the inner and outer tubes in all of the manufactured double-walled tubes, indicating insufficient sealing.

[0075] In the manufacturing conditions of Examples 1 to 6 of the present invention, the left-hand side of equation (1) was greater than 1.65, thus satisfying equation (1). Furthermore, the outer diameter of the inner tube was between 0.90 and 0.99 times the inner diameter of the outer tube. In all of the double-walled tubes manufactured, there was no gap between the inner and outer tubes, indicating good airtightness. From these results, it became clear that by manufacturing a double-walled tube using a composite billet that satisfies the condition that the outer diameter of the inner tube is between 0.90 and 0.99 times the inner diameter of the outer tube, and manufacturing conditions that satisfy equation (1), a double-walled tube with high airtightness can be obtained.

[0076] <Second Example> Normally, heat treatment is applied to the shell after the rolling process to ensure the performance required for the final product. However, since the heat treatment conditions differ depending on the type of steel, attention must be paid to the heat treatment conditions in the case of a double-walled tube composed of two types of steel.

[0077] For example, in the case of austenitic stainless steel such as SUS316L, sensitization can occur depending on the heat treatment conditions. Sensitization is a phenomenon in which carbon and chromium in the grain boundaries combine to form carbon compounds, resulting in a decrease in chromium near the grain boundaries. When sensitization occurs, the corrosion resistance near the grain boundaries decreases due to the reduction in chromium, which can lead to intergranular corrosion.

[0078] Therefore, in the second embodiment, a double-walled tube composed of a carbon steel outer tube and a SUS316L equivalent steel inner tube was subjected to heat treatment, and the effect of the heat treatment on the double-walled tube was investigated. Specifically, the heat treatment of the above embodiment was performed on the double-walled tube after the rolling process, and it was investigated whether sensitization occurred in the inner tube. The heat treatment consisted of a quenching process and a tempering process. In the quenching process, the tube was heated to a temperature of 950°C, held at this temperature for 10 minutes, and then rapidly cooled. In the tempering process, the tube was heated to a temperature of 675°C, held at this temperature for 30 minutes, and then cooled.

[0079] An oxalic acid etching test was conducted on the inner tube after heat treatment to investigate whether sensitization occurred. In the oxalic acid etching test, a first test material cut from the inner tube was placed in a 10 wt% oxalic acid solution, and a current of 0.1 A was applied for approximately 2 minutes. The first test material was cut from the center of the inner tube's wall thickness to a size of 10 mm wide, 10 mm long, and 1 mm thick. To compare with the inner tube after heat treatment, a second test material was cut from the inner tube before heat treatment, and the oxalic acid etching test was performed on the second test material in the same manner as the first test material.

[0080] Figure 9 is an optical microscope image of the first test material after the oxalic acid etching test. Figure 10 is an optical microscope image of the second test material after the oxalic acid etching test. When sensitization occurs, the grain boundaries appear as thicker lines than usual. Looking at the state of the grain boundaries after heat treatment shown in Figure 9, it can be seen that it is almost the same as the state of the grain boundaries before heat treatment shown in Figure 10. In other words, under the heat treatment conditions of the second example, no Cr deposition at the grain boundaries due to heat treatment was observed, and it was found that sensitization did not occur. Therefore, it was confirmed that the corrosion resistance of the inner tube of the double tube does not decrease under these heat treatment conditions.

[0081] The temperature range at which sensitization can occur is generally considered to be around 600°C to 800°C. However, based on the results of this second embodiment, it is expected that sensitization will be minimal even if the quenching temperature in the quenching process S60 is 925°C to 975°C and the tempering temperature in the tempering process S70 is 625°C to 675°C.

[0082] 1: Inclined roll 2: Plug 3: Mandrel 5: Inclined rolling mill 6: Outer tube 8: Inner tube 8v: Virtual inner tube 10: Composite billet 12: Shell 20: Double tube C: Center axis of the inlined roll PL: Pass line

Claims

1. A method for manufacturing a double-walled tube, comprising: an outer tube preparation step of preparing an outer tube made of carbon steel; an inner tube preparation step of preparing an inner tube made of a metallic material having a deformation resistance greater than that of carbon steel, and having an outer diameter of 0.90 times or more and 0.99 times or less that of the inner diameter of the outer tube; a composite billet preparation step of preparing a composite billet comprising the outer tube and the inner tube disposed inside the outer tube; a heating step of heating the composite billet; and a rolling step of using an inclined rolling mill having inclined rolls and plugs, bringing the inclined rolls into contact with the outer circumferential surface of the heated composite billet, and bringing the plugs into contact with the inner circumferential surface of the inner tube in the heated composite billet, thereby rolling the composite billet to obtain a shell in which the outer tube and the inner tube are closely joined, wherein in the heating step, at the end of the composite billet, the space between the outer tube and the inner tube is opened to the outside of the composite billet. A method for manufacturing a double-walled pipe, wherein the rolling process is performed by the inclined rolling mill under conditions that satisfy the following formula (1). ２ ×EL×(Ti / Ta) １／２ ×RI / SR ２ >1.65 (1) In equation (1), the meaning of each symbol is as follows: ER = Doa / Dob Doa: Setting value for the outer diameter of the shell in the rolling process Dob: Outer diameter of the outer tube prepared in the outer tube preparation process EL = (S1 + S2) / S3 S1: Cross-sectional area of ​​the outer tube prepared in the outer tube preparation process S2: Cross-sectional area of ​​the inner tube prepared in the inner tube preparation process S3: Setting value for the cross-sectional area of ​​the shell in the rolling process Ti = T1 + T2 T1: Wall thickness of the outer tube prepared in the outer tube preparation process T2: Wall thickness of the inner tube prepared in the inner tube preparation process Ta: Setting value for the wall thickness of the shell in the rolling process RI = Izi / Iziv Izi: Second moment of area of ​​the inner tube prepared in the inner tube preparation process Izv: The second moment of area of ​​the inner tube, assuming that the inner tube is deformed so that its outer surface comes into close contact with the inner surface of the outer tube prepared in the outer tube preparation step, while maintaining the cross-sectional area of ​​the inner tube. SR: The ratio of the deformation resistance of the inner tube to that of the outer tube at the heating temperature in the heating step.

2. A method for manufacturing a double-walled pipe according to claim 1, wherein the inner pipe prepared in the inner pipe preparation step is made of austenitic stainless steel.

3. A method for manufacturing a double-walled tube according to claim 2, further comprising: a quenching step of heating the shell after the rolling step to a temperature of 925°C to 975°C, holding it at that temperature for 5 to 15 minutes, and then rapidly cooling it; and a tempering step of heating the shell after the quenching step to a temperature of 625°C to 675°C, holding it at that temperature for 20 to 40 minutes, and then cooling it.

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