Method for manufacturing double pipe

Abstract

This manufacturing method comprises: a preparation step (S10) for preparing a composite billet comprising a cylindrical outer pipe and a cylindrical inner pipe disposed inside the outer pipe; a heating step (S20) for heating the composite billet; and a rolling step (S30) for obtaining a double pipe in which the outer-circumferential surface of the inner pipe and the inner-circumferential surface of the outer pipe closely contact one another by rolling the composite billet using an inclined rolling mill having an inclined roller and a plug. The preparation step (S10) includes: an arrangement step (S11) for arranging the inner pipe inside the outer pipe in a manner such that at least one of the two end sections of the inner pipe in the axial direction protrudes from the outer pipe; and a compression step (S12) for causing a pair of dies to respectively contact each of the two axial-direction end surfaces of the inner pipe positioned inside the outer pipe, compressing the inner pipe in the axial direction by using the pair of dies, and bringing at least a part of the outer-circumferential surface of the inner pipe into contact with the inner-circumferential surface of the outer pipe.

Description

Method for manufacturing double pipe

[0001] The present disclosure relates to a method for manufacturing a double pipe including an inner pipe and an outer pipe, and more particularly to a method for manufacturing a double pipe in which the outer circumferential surface of the inner pipe is in close contact with the inner circumferential surface of the outer pipe.

[0002] Corrosion resistance is required for pipes through which corrosive fluids flow. As such a pipe, there is a CRA (Corrosion Resistance Alloy) line pipe. A CRA line pipe is entirely made of CRA, that is, a corrosion-resistant alloy. However, since CRA is expensive, manufacturing the entire pipe from CRA increases the cost of the pipe. Accordingly, attempts have been made to configure a pipe with an outer pipe and an inner pipe disposed inside the outer pipe. By forming only the inner pipe from a corrosion-resistant material, the cost of the double pipe can be reduced.

[0003] Japanese Patent Application Laid-Open No. 56-148486 (Patent Document 1) discloses a method for manufacturing a double pipe (composite billet). In this manufacturing method, first, a hollow inner cylinder billet is inserted into a hollow outer cylinder billet. Next, an elastic body is inserted into the inner cylinder billet. Thereafter, pressure is applied to the elastic body to bring the outer surface of the inner cylinder billet into close contact with the inner surface of the outer cylinder billet.

[0004] Japanese Patent Application Laid-Open No. 2001-1014 (Patent Document 2) discloses a method for manufacturing a clad steel pipe. In this manufacturing method, first, a billet for manufacturing a clad steel pipe is manufactured. The billet for manufacturing a clad steel pipe includes a square cross-section steel piece having a hole formed therein, and a round cross-section steel piece inserted into the hole. The square cross-section steel piece and the round cross-section steel piece are welded around the entire circumference at both ends of these pieces. The gap between the square cross-section steel piece and the round cross-section steel piece is evacuated. Thereafter, the billet for manufacturing a clad steel pipe is rolled by a press roll piercing machine to obtain a clad steel pipe.

[0005] Japanese Patent Application Laid-Open No. 56-148486 Japanese Patent Application Laid-Open No. 2001-1014

[0006] However, in the manufacturing method described in Patent Document 1, the outer and inner billets are not fixed to each other until the outer surface of the inner billet is in close contact with the inner surface of the outer billet. Therefore, if the outer and inner billets are transported with the inner billet inserted inside the outer billet, the inner billet may fall out of the outer billet.

[0007] Furthermore, in the manufacturing method described in Patent Document 1, it is necessary to seal both ends of the inner cylinder billet with seal rings and apply pressure to the elastic body using a hydraulic cylinder. In the manufacturing method described in Patent Document 2, it is necessary to machine the ends of the square-section steel billet and the round-section steel billet before performing full-circumferential welding. Also, the time required for full-circumferential welding is long. For these reasons, the manufacturing methods described in Patent Documents 1 and 2 have long manufacturing times.

[0008] Therefore, the purpose of this disclosure is to provide a method for manufacturing a double-walled tube that can suppress the inner tube from falling out of the outer tube and shorten the manufacturing time.

[0009] The method for manufacturing a double-walled tube according to this disclosure comprises a preparation step, a heating step, and a rolling step. In the preparation step, a composite billet comprising a cylindrical outer tube and a cylindrical inner tube disposed inside the outer tube is prepared. In the heating step, the composite billet is heated. In the rolling step, an inclined rolling mill having inclined rolls and plugs is used to press the inclined rolls against the outer circumferential surface of the heated composite billet, and the plugs against the inner circumferential surface of the inner tube in the heated composite billet, thereby rolling the composite billet to obtain a double-walled tube in which the outer circumferential surface of the inner tube and the inner circumferential surface of the outer tube are in close contact. Here, the preparation step includes a placement step and a compression step. In the placement step, the inner tube is placed inside the outer tube such that at least one of the axial ends of the inner tube protrudes from the outer tube. In the compression process, a pair of dies are brought into contact with both axial ends of the inner tube, which is placed inside the outer tube. The dies compress the inner tube in the axial direction, causing at least a portion of the outer surface of the inner tube to come into contact with the inner surface of the outer tube.

[0010] The method for manufacturing a double-walled tube according to this disclosure can suppress the inner tube from falling out of the outer tube and shorten the manufacturing time.

[0011] Figure 1 is a flowchart illustrating the manufacturing method according to the embodiment. Figure 2 is a cross-sectional view illustrating the preparation process in the manufacturing method according to the first embodiment. Figure 3 is a top view of the inclined rolling mill. Figure 4 is a side view of the inclined rolling mill. Figure 5 is a cross-sectional view of the double-walled pipe obtained by the manufacturing method according to the first embodiment. Figure 6 is a cross-sectional view illustrating the preparation process in the manufacturing method according to the second embodiment. Figure 7 is a cross-sectional view illustrating the preparation process in the manufacturing method according to the third embodiment.

[0012] The manufacturing method for a double-walled tube according to this embodiment comprises a preparation step, a heating step, and a rolling step. In the preparation step, a composite billet comprising a cylindrical outer tube and a cylindrical inner tube disposed inside the outer tube is prepared. In the heating step, the composite billet is heated. In the rolling step, an inclined rolling mill having inclined rolls and plugs is used to press the inclined rolls against the outer circumferential surface of the heated composite billet, and the plugs against the inner circumferential surface of the inner tube in the heated composite billet, thereby rolling the composite billet to obtain a double-walled tube in which the outer circumferential surface of the inner tube and the inner circumferential surface of the outer tube are in close contact. Here, the preparation step includes an arrangement step and a compression step. In the arrangement step, the inner tube is arranged inside the outer tube such that at least one of the axial ends of the inner tube protrudes from the outer tube. In the compression process, a pair of dies are brought into contact with both axial ends of the inner tube, which is placed inside the outer tube, and the inner tube is compressed in the axial direction by the pair of dies, causing at least a portion of the outer surface of the inner tube to come into contact with the inner surface of the outer tube (first configuration).

[0013] In the first configuration, the outer and inner tubes can be fixed together by performing a compression process during the preparation process. This prevents the inner tube from falling out of the outer tube. Furthermore, in the compression process, it is sufficient to compress the inner tube axially using a pair of dies. Therefore, the compression process can be performed in a short time (for example, several tens of seconds). In addition, it is not necessary to perform any preliminary processing on the outer and inner tubes before performing the compression process. As described above, the first configuration makes it possible to manufacture double-walled tubes while preventing the inner tube from falling out of the outer tube and shortening the manufacturing time.

[0014] In the manufacturing method of the double tube according to the first configuration, each of the pair of dies may be a flat die in which the surface facing the inner tube is flat (second configuration).

[0015] In the manufacturing method of the double-walled pipe according to the second configuration, the inner pipe may be positioned inside the outer pipe in the arrangement step such that only one of the axial ends of the inner pipe protrudes from the outer pipe. In this case, it is preferable that the length Lo of the outer pipe, the length Li1 of the inner pipe before the compression step, and the outer diameter Di1 of the inner pipe before the compression step satisfy the following formula (1) (third configuration): 1.00 ≤ (Li1 - Lo) / Di1 ≤ 3.00 (1)

[0016] In the third configuration, since the value of ((Li1 - Lo) / Di1) in equation (1) is 3.00 or less, the amount of protrusion of the inner tube relative to the outer tube is not excessive before the compression process is carried out. In this case, the inner tube is less likely to buckle during the compression process. Therefore, in the composite billet after the compression process, the gap between the outer surface of the inner tube and the inner surface of the outer tube can be reduced across the entire surface. Furthermore, in the third configuration, since the value of ((Li1 - Lo) / Di1) in equation (1) is 1.00 or more, sufficient fixing force of the inner tube relative to the outer tube can be ensured.

[0017] In the manufacturing method of the double-walled pipe according to the second configuration, the inner pipe may be placed inside the outer pipe in the arrangement step such that both axial ends of the inner pipe protrude from the outer pipe. In this case, it is preferable that the length Lo of the outer pipe, the length Li1 of the inner pipe before the compression step, and the outer diameter Di1 of the inner pipe before the compression step satisfy the following formula (2) (fourth configuration): 1.00 ≤ (Li1 - Lo) / 2 / Di1 ≤ 3.00 (2)

[0018] In the fourth configuration, since the value of ((Li1 - Lo) / 2 / Di1) in equation (2) is 3.00 or less, the axial ends of the inner tube do not protrude excessively from the outer tube before the compression process is carried out. In this case, the inner tube is less likely to buckle during the compression process. Therefore, in the composite billet after the compression process, the gap between the outer surface of the inner tube and the inner surface of the outer tube can be reduced across the entire surface. Furthermore, in the fourth configuration, since the value of ((Li1 - Lo) / 2 / Di1) in equation (2) is 1.00 or more, sufficient fixing force of the inner tube to the outer tube can be ensured.

[0019] In the manufacturing method of the double-walled tube according to the first configuration, one of the pair of dies may be a protruding die with a projection on the surface facing the inner tube, and the other of the pair of dies may be a flat die with a flat surface facing the inner tube. In this case, the projection may have a cylindrical outer surface. The outer diameter of the projection is smaller than the inner diameter of the outer tube and larger than the inner diameter of the inner tube (fifth configuration).

[0020] When a composite billet is rolled in an inclined rolling mill, the outer tube is rolled so that it extends forward in the direction of movement of the composite billet relative to the inner tube. In other words, the outer tube is moved forward relative to the inner tube in the direction of movement. Therefore, if the end faces of the outer tube and the inner tube are flush at the rear of the composite billet, a portion not covered by the outer tube may be created at the rear of the inner tube of the rolled double-walled pipe. In other words, a single-tube section may be created in the rolled double-walled pipe. This single-tube section is discarded as rolling slag.

[0021] In contrast, in the fifth configuration, one of the pair of dies is a protruding die. Therefore, at the rear of the composite billet, the inner tube can be compressed without the inner tube being inside the outer tube. In other words, the inner tube can be fixed in a state where the outer tube protrudes to the rear in the direction of movement relative to the inner tube. When such a composite billet is rolled, a single-tube section is less likely to occur at the rear of the double tube after rolling. Therefore, the amount of slag generated can be suppressed according to the fifth configuration.

[0022] In the manufacturing method of the double-walled pipe according to the fifth configuration, it is preferable in the compression step to compress the inner pipe in the axial direction such that the length of the inner pipe becomes shorter than the length of the outer pipe (sixth configuration).

[0023] In the manufacturing method of the double-walled pipe according to the fifth or sixth configuration, the inner pipe may be placed inside the outer pipe in the arrangement step such that only one of the axial ends of the inner pipe protrudes from the outer pipe. In this case, it is preferable that the axial length S of the protruding portion, the length Lo of the outer pipe, the length Li1 of the inner pipe before the compression step, and the outer diameter Di1 of the inner pipe before the compression step satisfy the following formula (3) (seventh configuration): 1.00 ≤ (Li1 - (Lo - S)) / Di1 ≤ 3.00 (3)

[0024] In the seventh configuration, since the value of ((Li1 - (Lo - S)) / Di1) in equation (3) is 3.00 or less, the axial ends of the inner tube do not protrude excessively from the outer tube before the compression process is carried out. In this case, the inner tube is less likely to buckle during the compression process. Therefore, in the composite billet after the compression process, the gap between the outer surface of the inner tube and the inner surface of the outer tube can be reduced across the entire surface. Furthermore, in the seventh configuration, since the value of ((Li1 - (Lo - S)) / Di1) in equation (3) is 1.00 or more, sufficient fixing force of the inner tube to the outer tube can be ensured.

[0025] In the manufacturing method of a double-walled pipe according to any of the first to seventh configurations, it is preferable that the average outer diameter Di2 of the inner pipe after the compression process, the outer diameter Do of the outer pipe, and the wall thickness to of the outer pipe satisfy the following formula (4) (eighth configuration): 0.900 ≤ Di2 / (Do - 2 × to) ≤ 1.000 (4)

[0026] In the eighth configuration, by satisfying equation (4), the contact area between the outer surface of the inner tube and the inner surface of the outer tube is increased, and in the composite billet after the compression process, the outer tube and the inner tube are firmly fixed to each other. As a result, by rolling the composite billet, a double tube in which the outer tube and the inner tube are closely joined is easily obtained.

[0027] The method for manufacturing a double-walled tube according to this embodiment will be described in detail with reference to the drawings. In the following description, unless otherwise specified, "%" in relation to chemical composition means "mass%".

[0028] Figure 1 is a flowchart of the manufacturing method according to this embodiment. This manufacturing method comprises a preparation step S10, a heating step S20, and a rolling step S30.

[0029] [First Embodiment] <Preparation Step> In preparation step S10, a composite billet 10 is prepared. Figure 2 is a cross-sectional view illustrating preparation step S10 in the manufacturing method according to the first embodiment. The composite billet 10 comprises an outer tube 6 and an inner tube 8 disposed inside the outer tube 6. The inner tube 8 is longer than the outer tube 6. Both the outer tube 6 and the inner tube 8 are cylindrical steel pipes. Each of the outer tube 6 and the inner tube 8 may be a welded steel pipe or a seamless steel pipe. The inner diameter of the outer tube 6 is larger than the outer diameter of the inner tube 8. Therefore, there is a gap between the outer tube 6 and the inner tube 8.

[0030] When a double-walled pipe obtained by this manufacturing method is used to carry a corrosive fluid through it, the inner pipe 8 may be made of the following materials. Preferably, the inner pipe 8 is made of a corrosion-resistant material. Examples of corrosion-resistant materials include JIS standard NCF625 equivalent alloy, NCF825 equivalent alloy, SUS316L equivalent steel, and SUS329J3L equivalent steel, as well as ASTM standard S39274 equivalent steel and S41426 equivalent steel.

[0031] The NCF625 equivalent alloy contains Ni: 58.0% or more, Cr: 20.0% to 23.5%, Fe: 5.0% or less, C: 0.10% or less, Mn: 0.50% or less, Si: 0.50% or less, Mo: 8.0% to 10.0%, Co: 1.0% or less, Al: 0.40% or less, Ti: 0.40% or less, one or more of Nb and Ta: 3.15% to 4.15% in total, P: 0.015% or less, and S: 0.015% or less, with the remainder being impurities.

[0032] 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.

[0033] 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.

[0034] Steel equivalent to S39274 contains C: 0.030% or less, Si: 0.80% or less, Mn: 1.0% or less, P: 0.030% or less, S: 0.020% or less, Cu: 0.20% to 0.80%, Cr: 24.0% to 26.0%, Ni: 6.0% to 8.0%, Mo: 2.50% to 3.50%, N: 0.24% to 0.32%, and W: 1.50% to 2.50%, with the remainder consisting of Fe and impurities.

[0035] SUS329J3L equivalent steel contains C: 0.030% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.030% or less, S: 0.020% or less, Cr: 21.0% to 23.0%, Ni: 4.50% to 6.50%, Mo: 2.50% to 3.50%, and N: 0.08% to 0.20%, with the remainder consisting of Fe and impurities.

[0036] Steel equivalent to S41426 contains C: 0.030% or less, Si: 0.50% or less, Mn: 0.50% or less, P: 0.020% or less, S: 0.005% or less, Cr: 11.5% to 13.5%, Ni: 5.50% to 7.50%, Mo: 1.50% to 3.00%, Ti: 0.01% to 0.50%, and V: 0.50% or less, with the remainder consisting of Fe and impurities.

[0037] On the other hand, the outer tube 6 does not need to be made of a corrosion-resistant material. Therefore, carbon steel can be used for the outer tube 6.

[0038] Preparation step S10 includes arrangement step S11 and compression step S12.

[0039] <Placement Process> In placement process S11, the inner tube 8 is placed inside the outer tube 6 such that at least one of the axial ends of the inner tube 8 protrudes from the outer tube 6. In this embodiment, only one end of the inner tube 8 protrudes from the outer tube 6. In the example in Figure 2, only the right end of the inner tube 8 protrudes from the outer tube 6. At the other end of the inner tube 8, the end face of the inner tube 8 and the end face of the outer tube 6 are flush. In the example in Figure 2, at the left end of the inner tube 8, the end face of the inner tube 8 and the end face of the outer tube 6 are flush.

[0040] <Compression Process> In the compression process S12, a pair of dies 11 are brought into contact with both axial ends of the inner tube 8, which is placed inside the outer tube 6. The inner tube 8 is then compressed in its axial direction by the pair of dies 11. In Figure 2, the direction of compression is indicated by a white arrow. At this time, the outer tube 6 can be fixed, for example, by a clamp.

[0041] Each die 11 is a flat die having no protrusion on the surface facing the inner tube 8. The surface facing the inner tube 8 in each die 11 is flat. In the present specification, this surface is referred to as a flat surface 11f. However, the flat surface 11f may be formed with a groove or the like used for alignment with the inner tube 8. In the example shown in FIG. 2, the flat surface 11f of one die 11 shown on the right side is in contact with the end face of the inner tube 8, and the flat surface 11f of the other die 11 shown on the left side is in contact with the end face of the inner tube 8 and the end face of the outer tube 6. It is preferable that the flat surface 11f of each die 11 has a size capable of coming into contact with the entire end face of the inner tube 8.

[0042] FIG. 2 shows the composite billet 10 immediately before the compression step S12 is performed after the arrangement step S11 is performed. At this point, in the composite billet 10, a gap is formed between the outer tube 6 and the inner tube 8 along the axial direction thereof. The outer tube 6 and the inner tube 8 are not fixed to each other. Therefore, in the composite billet 10 in this state, when a strong force is applied to the inner tube 8 in one direction along the axial direction of the inner tube 8, the inner tube 8 falls out of the outer tube 6.

[0043] In the compression step S12, by compressing the inner tube 8 in the axial direction, at least a part of the outer peripheral surface of the inner tube 8 is brought into contact with the inner peripheral surface of the outer tube 6. Specifically, by performing the compression step S12, the inner tube 8 is deformed. The length of the inner tube 8 along the axial direction is shortened, and the average outer diameter and wall thickness of the inner tube 8 are increased. Thereby, the contact area between the outer peripheral surface of the inner tube 8 and the inner peripheral surface of the outer tube 6 is increased. As a result, the outer tube 6 and the inner tube 8 are fixed to each other. In the composite billet 10 in this state, even if a strong force is applied to the inner tube 8 in one direction along the axial direction of the inner tube 8, the inner tube 8 does not easily fall out of the outer tube 6. Due to the compression, the outer diameter of the inner tube 8 does not have to increase uniformly in the radial direction.

[0044] After the completion of the compression step S12, the length of the inner tube 8 may be the same as the length of the outer tube 6. In this case, on the one die 11 side, the end face of the inner tube 8 and the end face of the outer tube 6 are flush with each other. Further, after the completion of the compression step S12, the length of the inner tube 8 may be longer than the length of the outer tube 6. In this case, on the one die 11 side, the inner tube 8 protrudes from the outer tube 6.

[0045] If the protruding amount of the inner tube 8 from the outer tube 6 is too large before compression, the inner tube 8 is prone to buckling in the compression step S12. It is difficult to obtain a double pipe in which the outer peripheral surface of the inner tube 8 is in close contact with the inner peripheral surface of the outer tube 6 over the entire surface from the composite billet 10 where the inner tube 8 is buckled. Therefore, in the present embodiment, it is preferable that the length Lo of the outer tube 6, the length Li1 of the inner tube 8 before performing the compression step S12, and the outer diameter Di1 of the inner tube 8 before performing the compression step S12 satisfy the following formula (1). Here, it is assumed that the length Lo of the outer tube 6 does not change before and after performing the compression step S12. 1.00≦(Li1−Lo) / Di1≦3.00 (1)

[0046] When the value of ((Li1−Lo) / Di1) in formula (1) is 3.00 or less, buckling of the inner tube 8 in the compression step S12 can be suppressed. This means that buckling is suppressed by limiting the protruding amount of the inner tube 8 from the outer tube 6 to 3 times or less the outer diameter Di1 of the inner tube 8 before performing the compression step S12.

[0047] When the value of ((Li1−Lo) / Di1) in formula (1) is 1.00 or more, sufficient fixing force of the inner tube 8 relative to the outer tube 6 can be ensured. When the value of ((Li1−Lo) / Di1) in formula (1) is less than 1.00, for example, the inner tube 8 may retract relative to the outer tube 6 during rolling of the composite billet 10. However, even if the value of ((Li1−Lo) / Di1) in formula (1) is less than 1.00, a fixing force sufficient to prevent the inner tube 8 from falling out of the outer tube 6 during transportation is ensured.

[0048] It is preferable that the average outer diameter Di2 of the inner tube 8 after performing the compression step S12, the outer diameter Do of the outer tube 6, and the wall thickness to of the outer tube 6 satisfy the following formula (4). Here, it is assumed that the outer diameter Do and the wall thickness to of the outer tube 6 do not change before and after performing the compression step S12. The average outer diameter Di2 of the inner tube 8 herein is a cold dimension after the compression step S12 and before the heating step S20. 0.900≦Di2 / (Do−2×to)≦1.000 (4)

[0049] If the value of (Di2 / (Do-2×to)) in equation (4) is 0.900 or greater, the contact area between the outer surface of the inner tube 8 and the inner surface of the outer tube 6 increases, and the outer tube 6 and the inner tube 8 are firmly fixed to each other. In other words, if the average outer diameter Di2 of the inner tube 8 after compression is 0.900 times or more the inner diameter of the outer tube 6, the outer tube 6 and the inner tube 8 are firmly fixed to each other. As a result, rolling the composite billet makes it easier to obtain a double tube in which the outer tube and inner tube are closely fitted. Also, since the inner tube 8 is still located inside the outer tube 6 after the compression process S12, the average outer diameter Di2 of the inner tube 8 is less than or equal to the inner diameter of the outer tube 6 (Do-2×to). That is, the value of (Di2 / (Do-2×to)) in equation (4) is 1.000 or less.

[0050] The inner tube 8 does not necessarily deform uniformly in the circumferential direction due to the compression process S12. Therefore, in this embodiment, the average outer diameter Di2 of the inner tube 8 after the compression process S12 is calculated by the following method. Assuming that the axial compressive strain of the inner tube 8 due to the compression process S12 is x, the average increase in strain of the outer diameter of the inner tube 8 is 0.5x, and the average increase in strain of the wall thickness of the inner tube 8 is 0.5x, the average outer diameter Di2 is calculated by the following formula (a). The axial compressive strain x is the value obtained by subtracting the length of the inner tube 8 after the compression process S12 from the length of the inner tube 8 before the compression process S12, and dividing the result by the length of the inner tube 8 before the compression process S12, Li1. This assumption is based on the fact that the volume of the inner tube 8 remains unchanged before and after the compression process S12. Di2 = Di1 + 0.5x × Di1 (a)

[0051] <Heating Process> In the heating process S20, the composite billet 10 is heated. The heating temperature is set to a temperature at which the composite billet 10 is sufficiently deformed in the next rolling process S30 and the desired rolling can be performed. For example, if both the outer tube 6 and the inner tube 8 are steel pipes, the heating temperature is preferably 900°C to 1300°C. The furnace for heating the composite billet 10 may be, for example, a walking beam furnace. When heating in a walking beam furnace, vibrations are applied to the composite billet 10 as it moves. However, because the outer tube 6 and the inner tube 8 are fixed to each other in the composite billet 10, the inner tube 8 will not fall out of the outer tube 6 or become misaligned.

[0052] <Rolling Process> Figure 3 is a top view of the inclined rolling mill 5, showing a partially cut composite billet. Figure 4 is a side view of the inclined rolling mill 5. In the rolling process S30, an inclined rolling mill 5 having a pair of inclined rolls 1 and a plug 2 is used. Rolling is performed by pressing the pair of inclined rolls 1 against the outer circumferential surface of the heated composite billet 10, and simultaneously pressing the plug 2 against the inner circumferential surface of the inner tube 8 in the heated composite billet 10. By rolling the composite billet 10 in this way, a double tube 20 is obtained in which the inner circumferential surface of the outer tube 6 and the outer circumferential surface of the inner tube 8 are in close contact.

[0053] In this embodiment, a pair of inclined rolls 1 are arranged facing each other horizontally, straddling the pass line PL. However, instead of a pair of inclined rolls 1, three inclined rolls arranged around the pass line PL at 120° intervals from each other may be used. The inclined rolling mill 5 has a configuration similar to that of a Mannesmann type perforated rolling mill used for perforated rolling. However, the composite billet 10 to be rolled already has the holes of the inner tube 8 before being processed by the inclined rolling mill 5. Therefore, perforation is not performed in the rolling process S30.

[0054] The inclined rolling mill 5 and the rolling process S30 will be described in detail below. The central axis (rotation axis) C of each inclined roll 1 forms a predetermined intersection angle CA and inclination angle FA with respect to the pass line PL. The intersection angle CA is the angle between the central axis C and the pass line PL when projected onto a horizontal plane containing the pass line PL (see Figure 3). The inclination angle FA is the angle between the central axis C and the pass line PL when projected onto a vertical plane containing the pass line PL (see Figure 4). The intersection angle CA can be, for example, 0 to 30°. The inclination angle FA can be, for example, 5 to 20°.

[0055] Plug 2 is positioned approximately midway between the pair of inclined rolls 1 on the pass line PL. Plug 2 has a bullet-shaped form. Plug 2 is attached to the tip of the core metal 3. Plug 2 and core metal 3 are positioned so that their central axes coincide with the pass line PL.

[0056] During rolling, the composite billet 10 is moved along the pass line PL with the central axis of the outer tube 6 aligned with the pass line PL. Figures 3 and 4 show the direction of movement of the composite billet 10 with white arrows. The tip of the plug 2 is pointed in the opposite direction to the direction of movement of the composite billet 10. The composite billet 10 is fed between a pair of inclined rolls 1. The rear end of the composite billet 10 may be pushed with a pusher until the composite billet 10 is sufficiently engaged with the pair of inclined rolls 1.

[0057] As the composite billet 10 moves, the inner surface of the inner tube 8 and the plug 2 come into contact on the downstream side in the direction of movement. The composite billet 10 is then rolled by the pair of inclined rolls 1 and the plug 2. As a result, the outer tube 6 and the inner tube 8 deform to be relatively close together, and finally, the inner surface of the outer tube 6 and the outer surface of the inner tube 8 come into close contact.

[0058] The maximum outer diameter of the plug 2 is preferably larger than the average inner diameter of the inner tube 8 after the compression process S12. In this case, the inner tube 8 is expanded by the plug 2, and the outer tube 6 and inner tube 8 are rolled so as to be stretched axially by the pair of inclined rolls 1 and the plug 2. Expanding the inner tube 8 with the plug 2 makes it easier to bring the outer tube 6 and inner tube 8 into close contact than reducing the diameter of the outer tube 6 with the inclined rolls 1.

[0059] Figure 5 is a cross-sectional view of a double-walled pipe 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 pipe 20. The double-walled pipe 20 includes an outer pipe 6 and an inner pipe 8. The outer circumferential surface of the inner pipe 8 is in close contact with the inner circumferential surface of the outer pipe 6.

[0060] <Effects> When the inner pipe 8 is made of a corrosion-resistant material, the manufacturing method of this embodiment can be used to produce a double-walled pipe 20 with a corrosion-resistant inner surface. Such a double-walled pipe 20 can be used to carry corrosive fluids. Since the corrosive fluid flowing inside the double-walled pipe 20 does not usually come into contact with the outer pipe 6, the outer pipe 6 can be made of a material that does not have corrosion resistance. Generally, materials that do not have corrosion resistance are cheaper than materials that do have corrosion resistance. Therefore, by using a material that is cheaper for the outer pipe 6, the overall cost of the double-walled pipe 20 can be reduced.

[0061] As described above, the outer tube 6 and the inner tube 8 can be fixed to each other by performing the compression step S12. This prevents the inner tube 8 from falling out of the outer tube 6. In the compression step S12, it is sufficient to compress the inner tube 8 in its axial direction using a press device equipped with a pair of dies 11. For this reason, the compression step S12 can be performed in a short time (for example, several tens of seconds). Furthermore, it is not necessary to perform any preliminary processing on the outer tube 6 and the inner tube 8 before performing the compression step S12. Thus, with the manufacturing method of this embodiment, it is possible to manufacture a double-walled tube 20 while preventing the inner tube 8 from falling out of the outer tube 6 and shortening the manufacturing time.

[0062] [Second Embodiment] Figure 6 is a cross-sectional view illustrating the preparation step S10 in the manufacturing method according to the second embodiment. The inner tube 8 is longer than the outer tube 6. In the second embodiment, in the placement step S11, the inner tube 8 is placed inside the outer tube 6 such that both axial ends of the inner tube 8 protrude from the outer tube 6. In the compression step S12, similar to the first embodiment, the inner tube 8 is compressed in its axial direction by a pair of dies (flat dies) 11. As a result, the average outer diameter and wall thickness of the inner tube 8 increase. Consequently, the contact area between the outer surface of the inner tube 8 and the inner surface of the outer tube 6 increases. As a result, the outer tube 6 and the inner tube 8 are fixed to each other.

[0063] In the second embodiment, it is preferable that the length Lo of the outer tube 6, the length Li1 of the inner tube 8 before the compression process S12, and the outer diameter Di1 of the inner tube 8 before the compression process S12 satisfy the following formula (2). Here, it is assumed that the length Lo of the outer tube 6 does not change before and after the compression process S12. 1.00 ≤ (Li1 - Lo) / 2 / Di1 ≤ 3.00 (2)

[0064] By keeping the value of ((Li1 - Lo) / 2 / Di1) in equation (2) at 3.00 or less, buckling of the inner tube 8 during the compression process S12 can be suppressed. To fully obtain this effect, it is preferable that the amount of protrusion of the inner tube 8 from one end face of the outer tube 6 is substantially equal to the amount of protrusion of the inner tube 8 from the other end face of the outer tube 6. In this case, a value of ((Li1 - Lo) / 2 / Di1) in equation (2) at 3.00 or less means that buckling of the inner tube 8 is suppressed by keeping the amount of protrusion of the inner tube 8 from each end face of the outer tube 6 to three times or less the outer diameter Di1 of the inner tube 8 before the compression process S12 is performed.

[0065] If the value of ((Li1 - Lo) / 2 / Di1) in equation (2) is 1.00 or greater, sufficient fixing force of the inner tube 8 to the outer tube 6 can be ensured. If the value of ((Li1 - Lo) / 2 / Di1) in equation (2) is less than 1.00, for example, the inner tube 8 may retract relative to the outer tube 6 during the rolling of the composite billet 10. However, even if the value of ((Li1 - Lo) / 2 / Di1) in equation (2) is less than 1.00, sufficient fixing force is ensured so that the inner tube 8 does not fall out of the outer tube 6 during transport.

[0066] In this embodiment as well, it is preferable that the average outer diameter Di2 of the inner tube 8 after the compression process S12, the outer diameter Do of the outer tube 6, and the wall thickness to of the outer tube 6 satisfy the above formula (4). This makes it possible to achieve the same effects as in the first embodiment.

[0067] [Third Embodiment] Figure 7 is a cross-sectional view illustrating the preparation step S10 in the manufacturing method according to the third embodiment. In the third embodiment, the inner tube 8 may be longer than the outer tube 6, shorter than the outer tube 6, or substantially the same length as the outer tube 6. In the arrangement step S11, the inner tube 8 is placed inside the outer tube 6 such that only one end of the inner tube 8 (hereinafter referred to as the "first end") protrudes from the outer tube 6. The other end of the inner tube 8 (hereinafter referred to as the "second end") is located inside the outer tube 6. In the inner tube 8 shown in Figure 7, the end shown on the right is the first end, and the end shown on the left is the second end. The end face of the second end of the inner tube 8 is not flush with the end face of the outer tube 6.

[0068] The flat surface 11f of the die 11 is opposed to the first end of the inner tube 8. On the other hand, the die (projecting die) 11A is opposed to the second end of the inner tube 8. That is, in the third embodiment, unlike the first and second embodiments, one of the pair of dies is a projecting die 11A, and the other of the pair of dies is a flat die 11. A projection 11p is provided on the surface of the die 11A that is opposed to the second end of the inner tube 8. The projection 11p has a generally cylindrical outer surface. In this embodiment, the shape of the projection 11p is cylindrical. However, the projection 11p may be cylindrical with sufficient wall thickness. The tip surface of the projection 11p is flat. The tip surface of the projection 11p is opposed to the second end of the inner tube 8 at the projection 11p.

[0069] The outer diameter of the projection 11p is smaller than the inner diameter of the outer tube 6. This allows the projection 11p to be positioned coaxially with the outer tube 6 and inserted into the outer tube 6. Also, the outer diameter of the projection 11p is larger than the inner diameter of the inner tube 8. In the die 11A, the axial length (projection amount) of the projection 11p is greater than the distance between the end face of the second end of the inner tube 8 and the end face of the outer tube 6 on the same side. As a result, the tip surface of the projection 11p can be brought into contact with the end face of the second end of the inner tube 8.

[0070] The outer diameter of the protrusion 11p may vary in the axial direction of the protrusion 11p. In this case, the maximum outer diameter of the protrusion 11p shall be smaller than the inner diameter of the outer tube 6. Furthermore, if the outer diameter of the protrusion 11p varies in its axial direction, the outer diameter of the protrusion 11p at its tip surface shall be larger than the inner diameter of the inner tube 8.

[0071] In the compression step S12, the flat surface 11f of the die 11 is brought into contact with the end face of the first end of the inner tube 8, and the tip surface of the projection 11p of the die 11A is brought into contact with the end face of the second end of the inner tube 8. Then, the inner tube 8 is compressed in its axial direction by the dies 11 and 11A. If the length of the inner tube 8 before compression is longer than the length of the outer tube 6, it is preferable to compress the inner tube 8 in the axial direction so that the length of the inner tube 8 becomes shorter than the length of the outer tube 6.

[0072] The compression step S12 fixes the outer tube 6 and the inner tube 8 together. This prevents the inner tube 8 from falling out of the outer tube 6. Furthermore, the manufacturing method of this embodiment yields a composite billet 10 in which the second end of the inner tube 8 is located inside the outer tube 6.

[0073] In the third embodiment, it is preferable that the axial length S of the protrusion 11p, the length Lo of the outer tube 6, the length Li1 of the inner tube 8 before the compression process S12, and the outer diameter Di1 of the inner tube 8 before the compression process S12 satisfy the following formula (3). Here, it is assumed that the length Lo of the outer tube 6 does not change before and after the compression process S12. 1.00 ≤ (Li1 - (Lo - S)) / Di1 ≤ 3.00 (3)

[0074] By keeping the value of ((Li1 - (Lo - S)) / Di1) in equation (3) to 3.00 or less, buckling of the inner tube 8 during the compression process S12 can be suppressed. This means that buckling is suppressed by keeping the amount of protrusion of the inner tube 8 from the outer tube 6 to three times or less the outer diameter Di1 of the inner tube 8 before the compression process S12 is performed.

[0075] If the value of ((Li1 - (Lo - S)) / Di1) in equation (3) is 1.00 or greater, sufficient fixing force of the inner tube 8 to the outer tube 6 can be ensured. If the value of ((Li1 - (Lo - S)) / Di1) in equation (3) is less than 1.00, for example, the inner tube 8 may retract relative to the outer tube 6 during the rolling of the composite billet 10. However, even if the value of ((Li1 - (Lo - S)) / Di1) in equation (3) is less than 1.00, sufficient fixing force is ensured so that the inner tube 8 does not fall out of the outer tube 6 during transport.

[0076] In this embodiment as well, it is preferable that the average outer diameter Di2 of the inner tube 8 after the compression process S12, the outer diameter Do of the outer tube 6, and the wall thickness to of the outer tube 6 satisfy the above formula (4). This makes it possible to achieve the same effects as in the first and second embodiments.

[0077] In the rolling process S30, the first end of the inner tube 8 is positioned towards the front in the direction of movement of the composite billet 10 (hereinafter simply referred to as the "direction of movement"), and it is rolled in the inclined rolling mill 5. At this time, the outer tube 6 is rolled so as to extend forward in the direction of movement relative to the inner tube 8. That is, the outer tube 6 is sent forward in the direction of movement relative to the inner tube 8. For this reason, for example, if the end face of the outer tube 6 and the end face of the inner tube 8 are flush on the rear side in the direction of movement, a portion of the inner tube 8 of the double tube 20 after rolling that is not covered by the outer tube 6 (hereinafter referred to as the "single tube portion") may be created. The single tube portion will be discarded as rolling slag.

[0078] In this embodiment, at the rear in the direction of movement, there is a portion inside the outer tube 6 where the inner tube 8 does not exist. In other words, at the rear in the direction of movement, there is an excess of the outer tube 6 relative to the inner tube 8. Therefore, in the rolling process S30, the formation of single-tube sections at the rear in the direction of movement can be suppressed. That is, the amount of slag generated can be suppressed.

[0079] 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.

[0080] The present disclosure will be further described below with reference to examples. However, the present disclosure is not limited to the following examples.

[0081] <First Embodiment> In the first embodiment, the effects of the manufacturing method according to the first embodiment were verified. In the preparation step, eight types of inner tubes with different dimensions were prepared for one type of outer tube. The material of the outer tube was carbon steel. The material of the inner tubes was SUS316L equivalent steel according to JIS standards. The inner tubes were placed inside the outer tube so that one end of the inner tube protruded from the outer tube. The inner tubes were compressed in the axial direction to prepare eight types of composite billets. The prepared composite billets were transported and heated at 1200°C. The heated composite billets were rolled in an inclined rolling mill to manufacture double tubes. The rolling conditions were a stretching ratio of 2.60 [dimensionless], an expansion ratio of 1.07 [dimensionless], an inclination angle of the rolling rolls of 10°, and an intersection angle of the rolling rolls of 0° to 30°. The stretching ratio is the ratio of the sum of the cross-sectional areas of the inner tube and the outer tube before compression to the cross-sectional area of ​​the manufactured double tube. The expansion ratio is the ratio of the outer diameter of the manufactured double-walled tube to the outer diameter of the outer tube before the rolling process. The dimensions (cross-sectional area and outer diameter) of the inner tube, outer tube, and double-walled tube here are cold dimensions measured at room temperature.

[0082] The fixation state of the inner tube to the outer tube was evaluated for each composite billet before rolling. The fixation state was evaluated by whether the inner tube did not fall out of the outer tube during transport. Table 1 shows the dimensions of the outer tube, the dimensions of the inner tube before and after compression, the value of (Di2 / (Do-2×to)) in equation (4) [dimensionless], and the value of ((Li1-Lo) / Di1) in equation (1) [dimensionless]. The outer diameter Di2 of the inner tube after compression corresponds to the average outer diameter Di2 of the inner tube after compression. Both the inner and outer tube dimensions were cold dimensions.

[0083]

[0084] In all of the tests from 1 to 8, the inner tube did not detach from the outer tube during transport, indicating that the inner tube was securely fixed to the outer tube.

[0085] Upon observation of the composite billet after compression, buckling occurred in the inner tube at the end of the tube in test number 5. In test number 5, the value of ((Li1-Lo) / Di1) in equation (1) was greater than 3.00. In contrast, in test numbers 1-4 and 6-8, where the value of ((Li1-Lo) / Di1) in equation (1) was 3.00 or less, the inner tube was sound at the end of the tube.

[0086] During the rolling of the composite billet, in test number 3, the inner tube was observed to retract relative to the outer tube. In contrast, in test numbers 1, 2, and 4-8, no retraction of the inner tube relative to the outer tube occurred during rolling. Therefore, from the viewpoint of the fixing force of the inner tube relative to the outer tube, it was found that the value of ((Li1-Lo) / Di1) in equation (1) is preferably 1.00 or higher. However, as mentioned above, even in composite billets where the value of ((Li1-Lo) / Di1) in equation (1) is less than 1.00, sufficient fixing force was ensured to prevent the inner tube from falling out of the outer tube during transport. From these results, it was found that it is preferable to satisfy equation (1) from the viewpoint of inner tube buckling and fixing force.

[0087] The tight seal between the outer and inner tubes was evaluated in the double-walled tubes (test numbers 1-8) after the rolling process. The tight seal was evaluated by penetrant testing as specified in JIS Z 2343-1:2017. In test numbers 2-4, 7, and 8, the tight seal between the outer and inner tubes was sufficient. In test numbers 2-4, 7, and 8, the value of (Di² / (Do-²×to)) in equation (4) was 0.900 or higher, thus satisfying equation (4).

[0088] In tests 1, 5, and 6, the seal between the outer and inner tubes was partially insufficient. In tests 1 and 6, the value of (Di² / (Do-²×to)) in equation (4) was less than 0.900, and equation (4) was not satisfied. In test 5, equation (4) was satisfied, but equation (1) was not. As a result, in test 5, slight buckling occurred in the inner tube during compression, which resulted in partially insufficient seal. From these results, it was found that satisfying equations (1) and (4) is preferable from the viewpoint of seal in double-walled tubes.

[0089] <Second Embodiment> In the second embodiment, the effects of the manufacturing method according to the second embodiment were verified. In the preparation step, six types of inner tubes with different dimensions were prepared for one type of outer tube. The material of the outer tube was carbon steel. The material of the inner tubes was SUS316L equivalent steel according to JIS standards. The inner tubes were placed inside the outer tubes so that both ends of the inner tubes protruded from the outer tubes. The inner tubes were compressed in the axial direction to prepare six types of composite billets. The prepared composite billets were transported and heated at 1200°C. The heated composite billets were rolled in an inclined rolling mill to produce double tubes. The rolling conditions were a stretching ratio of 2.60 [dimensionless], an expansion ratio of 1.07 [dimensionless], an inclination angle of the rolling rolls of 10°, and an intersection angle of the rolling rolls of 0° to 30°.

[0090] The state of fixation of the inner tube to the outer tube was evaluated for each composite billet before rolling. The fixation state was evaluated in the same manner as in the first embodiment. Table 2 shows the dimensions of the outer tube, the dimensions of the inner tube before and after compression, the value of (Di2 / (Do-2×to)) in equation (4) [dimensionless], and the value of ((Li1-Lo) / 2 / Di1) in equation (2) [dimensionless].

[0091]

[0092] In all of the tests from 9 to 14, the inner tube did not detach from the outer tube during transport, indicating that the inner tube was securely fixed to the outer tube.

[0093] Upon observation of the composite billet after compression, buckling occurred in the inner tube at the end of the tube in test number 11. In test number 11, the value of ((Li1-Lo) / 2 / Di1) in equation (2) was greater than 3.00. In contrast, in test numbers 9, 10, and 12-14, where the value of ((Li1-Lo) / 2 / Di1) in equation (2) was 3.00 or less, the inner tube was sound at the end of the tube.

[0094] During the rolling of the composite billet, in test number 12, the inner tube was observed to retract relative to the outer tube. In contrast, in test numbers 9-11, 13, and 14, no retraction of the inner tube relative to the outer tube occurred during rolling. Therefore, from the viewpoint of the fixing force of the inner tube relative to the outer tube, it was found that the value of ((Li1-Lo) / 2 / Di1) in equation (2) is preferably 1.00 or higher. However, as mentioned above, even in composite billets where the value of ((Li1-Lo) / 2 / Di1) in equation (2) is less than 1.00, sufficient fixing force was ensured to prevent the inner tube from falling out of the outer tube during transport. From these results, it was found that it is preferable to satisfy equation (2) from the viewpoint of inner tube buckling and fixing force.

[0095] In the double-walled tubes of test numbers 9 to 14 after the rolling process, the tightness between the outer and inner tubes was evaluated. The tightness evaluation was performed in the same manner as in the first embodiment. In test numbers 10 and 12 to 14, the tightness between the outer and inner tubes was sufficient. In test numbers 10 and 12 to 14, the value of (Di² / (Do-²×to)) in equation (4) was 0.900 or higher, thus satisfying equation (4).

[0096] In tests 9 and 11, the seal between the outer and inner tubes was partially insufficient. In test 9, the value of (Di² / (Do-2 × to)) in equation (4) was less than 0.900, and equation (4) was not satisfied. In test 11, equation (4) was satisfied, but equation (2) was not. As a result, in test 11, slight buckling occurred in the inner tube during compression, which resulted in partially insufficient seal. From these results, it was found that satisfying equations (4) and (2) is preferable from the viewpoint of seal in a double-walled tube.

[0097] <Third Embodiment> In the third embodiment, the effects of the manufacturing method according to the third embodiment were verified. In the preparation step, eight types of inner tubes with different dimensions were prepared for one type of outer tube. The material of the outer tube was carbon steel. The material of the inner tubes was SUS316L equivalent steel according to JIS standards. The inner tubes were placed inside the outer tube so that one end of the inner tube protruded from the outer tube. A flat die was placed on one end of the inner tube, and a protruding die was placed on the other end of the inner tube. The inner tubes were compressed in the axial direction to prepare eight types of composite billets. The prepared composite billets were transported and heated at 1200°C. The heated composite billets were rolled in an inclined rolling mill to produce double tubes. The rolling conditions were a stretching ratio of 2.60 [dimensionless], an expansion ratio of 1.07 [dimensionless], an inclination angle of the rolling rolls of 10°, and an intersection angle of the rolling rolls of 0° to 30°.

[0098] The state of fixation of the inner tube to the outer tube was evaluated for each composite billet before rolling. The fixation state was evaluated in the same manner as in the first and second embodiments. Table 3 shows the axial length of the projection of the projection die, the dimensions of the outer tube, the dimensions of the inner tube before and after compression, the value of (Di2 / (Do-2×to)) in equation (4) [dimensionless], and the value of ((Li1-(Lo-S)) / Di1) in equation (3) [dimensionless].

[0099]

[0100] In all of the tests from 15 to 22, the inner tube did not detach from the outer tube during transport, indicating that the inner tube was securely fixed to the outer tube.

[0101] Upon observation of the composite billet after compression, buckling occurred in the inner tube at the end of the tube in test number 19. In test number 19, the value of ((Li1 - (Lo - S)) / Di1) in equation (3) was greater than 3.00. In contrast, in test numbers 15-18 and 20-22, where the value of ((Li1 - (Lo - S)) / Di1) in equation (3) was 3.00 or less, the inner tube was sound at the end of the tube.

[0102] During the rolling of the composite billet, in test number 17, the inner tube was observed to retract relative to the outer tube. In contrast, in test numbers 15, 16, and 18-22, no retraction of the inner tube relative to the outer tube occurred during rolling. Therefore, from the viewpoint of the fixing force of the inner tube relative to the outer tube, it was found that the value of ((Li1 - (Lo - S)) / Di1) in equation (3) is preferably 1.00 or higher. However, as mentioned above, even in composite billets where the value of ((Li1 - (Lo - S)) / Di1) in equation (3) is less than 1.00, sufficient fixing force was ensured to prevent the inner tube from falling out of the outer tube during transport. From these results, it was found that it is preferable to satisfy equation (3) from the viewpoint of inner tube buckling and fixing force.

[0103] In the double-walled tubes (test numbers 15-22) after the rolling process, the tightness between the outer and inner tubes was evaluated. The tightness evaluation was performed using the same method as in the first and second embodiments. In test numbers 16-18, 21, and 22, the tightness between the outer and inner tubes was sufficient. In test numbers 16-18, 21, and 22, the value of (Di² / (Do-²×to)) in equation (4) was 0.900 or higher, thus satisfying equation (4).

[0104] In tests 15, 19, and 20, the seal between the outer and inner tubes was partially insufficient. In tests 15 and 20, the value of (Di² / (Do-²×to)) in equation (4) was less than 0.900, and equation (4) was not satisfied. In test 19, equation (4) was satisfied, but equation (3) was not. As a result, in test 19, slight buckling occurred in the inner tube during compression, which resulted in partially insufficient seal. From these results, it was found that satisfying equations (3) and (4) is preferable from the viewpoint of seal in double-walled tubes.

[0105] 1: Inclined roll 2: Plug 3: Core 5: Inclined rolling mill 6: Outer tube 8: Inner tube 10: Composite billet 11: Die (flat die) 11A: Die (projecting die) 11p: Projection 20: Double tube C: Center axis of the inclined roll PL: Pass line

Claims

1. A method for manufacturing a double-walled tube, comprising: a preparation step of preparing a composite billet comprising a cylindrical outer tube and a cylindrical inner tube disposed within 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, pressing the inclined rolls against the outer circumferential surface of the heated composite billet and pressing the plugs against the inner circumferential surface of the inner tube in the heated composite billet, thereby rolling the composite billet to obtain a double-walled tube in which the outer circumferential surface of the inner tube and the inner circumferential surface of the outer tube are in close contact, wherein the preparation step includes: an arrangement step of arranging the inner tube within the outer tube such that at least one of the axial ends of the inner tube protrudes from the outer tube; and a compression step of bringing a pair of dies into contact with both axial end faces of the inner tube disposed within the outer tube, and using the pair of dies to compress the inner tube in the axial direction, thereby bringing at least a portion of the outer circumferential surface of the inner tube into contact with the inner circumferential surface of the outer tube.

2. A method for manufacturing a double-walled tube according to claim 1, wherein each of the pair of dies is a flat die with a flat surface facing the inner tube.

3. A method for manufacturing a double-walled tube according to claim 2, wherein in the arrangement step, the inner tube is arranged inside the outer tube such that only one of the axial ends of the inner tube protrudes from the outer tube, and the length of the outer tube Lo, the length of the inner tube Li1 before the compression step, and the outer diameter Di1 of the inner tube before the compression step satisfy the following formula (1): 1.00 ≤ (Li1 - Lo) / Di1 ≤ 3.00 (1) 4. A method for manufacturing a double-walled pipe according to claim 2, wherein in the arrangement step, the inner pipe is arranged inside the outer pipe such that both axial ends of the inner pipe protrude from the outer pipe, and the length of the outer pipe Lo, the length of the inner pipe Li1 before the compression step, and the outer diameter Di1 of the inner pipe before the compression step satisfy the following formula (2): 1.00 ≤ (Li1 - Lo) / 2 / Di1 ≤ 3.00 (2) 5. A method for manufacturing a double-walled tube according to claim 1, wherein one of the pair of dies is a protruding die with a projection on the surface facing the inner tube, the other of the pair of dies is a flat die with a flat surface facing the inner tube, the projection has a cylindrical outer surface, and the outer diameter of the projection is smaller than the inner diameter of the outer tube and larger than the inner diameter of the inner tube.

6. A method for manufacturing a double-walled pipe according to claim 5, wherein in the compression step, the inner pipe is compressed in the axial direction such that the length of the inner pipe becomes shorter than the length of the outer pipe.

7. A method for manufacturing a double-walled tube according to claim 5, wherein in the arrangement step, the inner tube is arranged inside the outer tube such that only one of the axial ends of the inner tube protrudes from the outer tube, and the axial length S of the protruding portion, the length Lo of the outer tube, the length Li1 of the inner tube before the compression step, and the outer diameter Di1 of the inner tube before the compression step satisfy the following formula (3): 1.00 ≤ (Li1 - (Lo - S)) / Di1 ≤ 3.00 (3) 8. A method for manufacturing a double-walled pipe according to any one of claims 1 to 7, wherein the average outer diameter Di2 of the inner pipe after the compression step, the outer diameter Do of the outer pipe, and the wall thickness to of the outer pipe satisfy the following formula (4): 0.900 ≤ Di2 / (Do - 2 × to) ≤ 1.000 (4)

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