Manufacturing method for double-walled tubes

Abstract

The manufacturing method comprises an outer tube preparation step (S10) for preparing an outer tube, an internal member preparation step (S20) for preparing an internal member having a cylindrical outer surface, a composite billet forming step (S30) for inserting the internal member into the outer tube to form a composite billet comprising the outer tube and the internal member, a heating step (S40) for heating the composite billet, and a rolling step (S60) for rolling the composite billet using an inclined rolling mill having inclined rolls and a plug to obtain a double tube in which the inner surface of the outer tube and the outer surface of the inner tube formed from the internal member are in close contact. The manufacturing method further comprises a positioning step (S50) after the heating step (S40) and before the internal member comes into contact with the plug of the rolling mill in the rolling step (S60), in which the rear end surface of the internal member opposite to the inclined roll side is positioned closer to the inclined roll than the rear end surface of the outer tube.

Description

[Technical Field]

[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. [Background Art]

[0002] Pipes for passing corrosive fluids are required to have corrosion resistance. As such a pipe, there is a CRA (Corrosion Resistance Alloy) line pipe. The entire CRA line pipe is composed 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. Therefore, attempts have been made to employ a double pipe composed of an outer pipe and an inner 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. 2001-1014 (Patent Document 1) discloses a method for manufacturing a clad steel pipe as a double 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 hollow square-section steel billet having a hole formed therein, and a round-section steel billet inserted into the hollow square-section steel billet. The square-section steel billet and the round-section steel billet are welded around the entire circumference at both ends of these billets. The gap between the square-section steel billet and the round-section steel billet 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, that is, a double pipe. [Prior Art Literature] [Patent Literature]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-1014 [Summary of the Invention] [Problem to be Solved by the Invention]

[0005] In the manufacturing method described in Patent Document 1, in order to perform full-circumference welding, the lengths of the square-section steel billet and the round-section steel billet had to be made the same, and the positions of one and the other end of the square-section steel billet were to be aligned with the positions of one and the other end of the round-section steel billet, respectively, in the axial direction of these steel billets. Rolling by a press roll drilling machine was performed on the square-section steel billet and the round-section steel billet in this state.

[0006] However, during rolling with a press roll drilling machine, the outer rectangular steel billet is rolled so that it extends forward in the direction of movement relative to the inner round steel billet. In other words, the rectangular steel billet is moved forward in the direction of movement relative to the round steel billet. As a result, in a double-walled pipe, at the rear of the direction of movement during rolling, a portion of the inner pipe formed from the round steel billet that is not covered by the outer pipe (hereinafter referred to as "rolling slag") may be generated. The rolling slag will be discarded.

[0007] Rolling slag can detach from the double-walled tube and accumulate inside the rolling equipment. In this case, the equipment needs to be regularly maintained and the rolling slag removed. Rolling slag can also adhere to the piercer plugs of the drilling machine. Such rolling slag can fall into the double-walled tube when the piercer plugs are withdrawn from the tube. In this case, if further rolling is performed on the double-walled tube using a mandrel mill, the rolling slag may cause damage to the inner surface of the double-walled tube and the mandrel bar.

[0008] Therefore, the object of this disclosure is to provide a method for manufacturing a double-walled pipe that can suppress the generation of rolled slag. [Means for solving the problem]

[0009] The method for manufacturing a double-walled tube according to this disclosure comprises an outer tube preparation step, an internal member preparation step, a composite billet formation step, a heating step, and a rolling step. In the outer tube preparation step, an outer tube having a cylindrical shape is prepared. In the internal member preparation step, an internal member having a cylindrical outer surface is prepared. In the composite billet formation step, the internal member is inserted into the outer tube to form a composite billet comprising the outer tube and the internal member. In the heating step, the composite billet is heated. In the rolling step, an inclined rolling mill equipped with an inclined roll and a plug is used to press the inclined roll against the outer surface of the heated composite billet, and the plug is pressed against the internal member in the heated composite billet, thereby rolling the composite billet to obtain a double-walled tube in which the inner surface of the outer tube and the outer surface of the inner tube formed from the internal member are in close contact.

[0010] The inclined rolling mill further includes a pusher positioned on the pass line, closer to the rolling direction entry side than the rolling mill body. The pusher has a pusher body and a pusher head located closer to the rolling mill body than the pusher body. The maximum outer diameter of the pusher head is smaller than the inner diameter of the outer tube.

[0011] The manufacturing method for the double-walled tube further comprises a positioning step. In the positioning step, after the heating step is performed and before the internal member comes into contact with the plug in the rolling step, the rear end surface of the internal member opposite to the inclined roll side is pushed with a pusher head so that the rear end surface of the internal member is positioned closer to the inclined roll side than the rear end surface of the outer tube opposite to the inclined roll side. [Effects of the Invention]

[0012] The method for manufacturing a double-walled tube according to this disclosure can suppress the generation of rolled slag. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is a flowchart showing the manufacturing method of a double-walled tube according to the first and second embodiments. [Figure 2] Figure 2 is a perspective view of a composite billet equipped with an internal tube as an internal component. [Figure 3] Figure 3 is a perspective view of a composite billet with a solid billet as an internal component. [Figure 4] Figure 4 is a top view of an inclined rolling mill. [Figure 5] Figure 5 is a longitudinal cross-sectional view of the composite billet after the arrangement process has been carried out. [Figure 6] Figure 6 is a top view of an inclined rolling mill during the rolling process. [Figure 7] Figure 7 is a side view of an inclined rolling mill during the rolling process. [Figure 8] Figure 8 is a cross-sectional view of the double-walled pipe. [Figure 9] Figure 9 is a top view of the inclined rolling mill and composite billet before the arrangement step is performed in the manufacturing method of the second embodiment. [Modes for carrying out the invention]

[0014] The manufacturing method for a double-walled tube according to this embodiment comprises an outer tube preparation step, an internal member preparation step, a composite billet formation step, a heating step, and a rolling step. In the outer tube preparation step, an outer tube having a cylindrical shape is prepared. In the internal member preparation step, an internal member having a cylindrical outer surface is prepared. In the composite billet formation step, the internal member is inserted into the outer tube to form a composite billet comprising the outer tube and the internal member. In the heating step, the composite billet is heated. In the rolling step, an inclined rolling mill equipped with an inclined roll and a plug is used to press the inclined roll against the outer surface of the heated composite billet, and the plug is pressed against the internal member in the heated composite billet, thereby rolling the composite billet to obtain a double-walled tube in which the inner surface of the outer tube and the outer surface of the inner tube formed from the internal member are in close contact.

[0015] The inclined rolling mill further includes a pusher positioned on the pass line, closer to the rolling direction entry side than the rolling mill body. The pusher has a pusher body and a pusher head located closer to the rolling mill body than the pusher body. The maximum outer diameter of the pusher head is smaller than the inner diameter of the outer tube.

[0016] The method for manufacturing a double tube further comprises a positioning step. In the positioning step, after the heating step is performed and before the inner member contacts the plug in the rolling step, a rear end surface of the inner member opposite to the inclined roll side is pressed by a pusher head, such that the rear end surface of the inner member is positioned closer to the inclined roll side than a rear end surface of the outer tube opposite to the inclined roll side (first configuration).

[0017] In the rolling step, rolling is performed such that the outer tube extends toward the front side in the movement direction relative to the inner member. That is, the outer tube is fed to the front side in the movement direction relatively to the inner member. Therefore, in the first configuration, in the positioning step, the rear end surface of the inner member is positioned more toward the front side in the movement direction than the rear end surface of the outer tube. This reduces the portion of the inner tube formed from the inner member that is not covered by the outer tube on the rear side in the movement direction of the double tube after the rolling step is performed. Accordingly, according to the first configuration, generation of rolling dross is suppressed.

[0018] In the method for manufacturing a double tube according to the first configuration, it is preferable that a distance d between the rear end surface of the outer tube and the rear end surface of the inner member after performing the positioning step satisfies the following formula (1) (second configuration). 0.3×Di<d<3.0×Di (1) where Di is the outer diameter of the inner member in the composite billet formed in the composite billet forming step.

[0019] In the second configuration, since formula (1) is satisfied, the generation amount of rolling dross can be reduced, and the length of the portion of the outer tube that does not constitute the double tube can also be shortened.

[0020] In the method for manufacturing a double tube according to the first or second configuration, the piercing mill may further comprise a clamp device disposed between the mill main body and the pusher. In this case, in the positioning step, with the outer tube fixed by the clamp device, the rear end surface of the inner member can be pressed by the pusher head (third configuration).

[0021] In the manufacturing method of the double-walled pipe according to the first or second configuration, the pusher may include a first part including the portion of the pusher body furthest from the rolling mill body, and a second part including the portion of the pusher body adjacent to the first part and the pusher head. The second part is rotatable around the axis of the pusher body relative to the first part. In this case, during the placement process, with the composite billet engaged in the inclined roll, the rear end face of the internal member can be pushed by the pusher head of the second part (fourth configuration).

[0022] When the composite billet engages with the inclined roll, the composite billet rotates around the pass line. In the fourth configuration, the second part of the pusher is rotatable relative to the first part of the pusher. Therefore, even when the rear end face of the internal member is pressed by the pusher head while the composite billet is rotating, the second part of the pusher can rotate in accordance with the rotation of the composite billet. Thus, the pusher does not hinder the rotation of the composite billet.

[0023] In the manufacturing method of a double tube according to any of the first to fourth configurations, the pusher may include a first part including the portion of the pusher body furthest from the rolling mill body, and a second part including the portion of the pusher body adjacent to the first part and the pusher head. The second part is rotatable around the axis of the pusher body relative to the first part. In this case, when rolling the composite billet with the inclined rolls and plug during the rolling process, the second part of the pusher can push the rear end of the composite billet opposite to the inclined roll side (fifth configuration).

[0024] In the rolling process, the plug is pressed against the internal member, creating resistance to the internal member's forward movement. Therefore, depending on the rolling conditions, the internal member may recede relative to the outer tube. In contrast, in the fifth configuration, the rear end of the composite billet is pushed by the second part of the pusher during rolling. In this case, back pressure is applied to the internal member by the pusher head, and back pressure is applied to the outer tube by the pusher body. Therefore, the internal member does not recede relative to the outer tube, and the axial distance between the rear end face of the internal member and the rear end face of the outer tube is maintained. This more reliably suppresses the generation of rolling slag and promotes close contact between the inner tube formed from the internal member and the outer tube.

[0025] The method for manufacturing the 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%".

[0026] Figure 1 is a flowchart common to the manufacturing method of this embodiment. Referring to Figure 1, the manufacturing method includes an outer tube preparation step S10, an internal component preparation step S20, a composite billet formation step S30, a heating step S40, an arrangement step S50, and a rolling step S60. Each step S10 to S60 will be described in detail below.

[0027] [First Embodiment] <Outer tube preparation process> In the outer pipe preparation step S10, the outer pipe is prepared. The outer pipe is made of, for example, carbon steel. The outer pipe has a cylindrical shape. The outer pipe may be a welded steel pipe or a seamless steel pipe.

[0028] <Internal component preparation process> In the internal component preparation step S20, the internal component is prepared. The internal component has a cylindrical outer surface. In this case, the internal component has a hollow cylindrical shape. The internal component may also have a solid cylindrical shape.

[0029] When a double-walled pipe obtained by the manufacturing method according to this embodiment is used to carry a corrosive fluid through it, the internal components may be made of the following materials. Preferably, the internal components are made of corrosion-resistant materials. 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.

[0030] NCF625 equivalent alloy is Ni: 58.0% or more, Cr: 20.0%~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%~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, It contains less than 0.015% sulfur, with the remainder consisting of impurities.

[0031] NCF825 equivalent alloy is Ni: 38.0%~46.0% Cr: 19.5%~23.5% C: 0.025% or less, Mn: 1.0% or less, Si: 0.5% or less, Cu: 1.5%~3.0% Mo: 2.5%~3.5%, Al: 0.2% or less, Ti: 0.6%~1.2% P: 0.020% or less, It contains less than 0.010% S, with the remainder consisting of Fe and impurities.

[0032] SUS316L equivalent steel is, 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%~16.00% Cr: 16.00%~18.00%, and It contains 2.00% to 3.00% Mo, with the remainder consisting of Fe and impurities.

[0033] Steel equivalent to S39274 is 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%~0.80% Cr: 24.0%~26.0% Ni: 6.0%~8.0% Mo: 2.50%~3.50% N: 0.24%~0.32%, and It contains 1.50% to 2.50% W, with the remainder consisting of Fe and impurities.

[0034] SUS329J3L equivalent steel is, 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%~23.0% Ni: 4.50%~6.50% Mo: 2.50%~3.50%, and It contains N: 0.08% to 0.20%, with the remainder consisting of Fe and impurities.

[0035] Steel equivalent to S41426 is, 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%~13.5% Ni: 5.50%~7.50% Mo: 1.50%~3.00%, Ti: 0.01%~0.50%, and V: Contains 0.50% or less, with the remainder consisting of Fe and impurities.

[0036] <Composite billet formation process> In the composite billet forming process S30, an internal member is inserted into the outer tube to form a composite billet. An example of the composite billet's configuration will be described below with reference to Figures 2 and 3. Figure 2 is a perspective view of the composite billet 10. The composite billet 10 comprises an outer tube 16 and an internal member 18. In the composite billet forming process S30, the internal member 18 is inserted into the outer tube 16, and the internal member 18 is positioned within the outer tube 16. This forms a composite billet 10 comprising the outer tube 16 and the internal member 18. In the composite billet 10, the internal member 18 is a hollow inner tube with a cylindrical shape. In this case, the internal member 18 may be a welded steel pipe or a seamless steel pipe. Instead of an inner tube, a cylindrical metal plate formed by winding a metal sheet (for example, a steel plate) may be used as the internal member 18.

[0037] Furthermore, as shown in Figure 3, a cylindrical solid billet may be used as the internal member 18A. Figure 3 is a perspective view of a composite billet 10A having a solid billet as the internal member 18A. The composite billet 10A comprises an outer tube 16 and an internal member 18A of a solid billet. In the composite billet forming process S30, the internal member 18A is inserted into the outer tube 16, and the internal member 18A is positioned inside the outer tube 16. This forms a composite billet 10A comprising the outer tube 16 and the internal member 18A. Hereafter, unless otherwise specified, the internal member will be described as a plain inner tube.

[0038] The internal member 18 and the outer tube 16 may have the same length or different lengths. The inner diameter of the outer tube 16 is larger than the outer diameter Di of the internal member 18. Therefore, there is a gap between the outer tube 16 and the internal member 18. However, if the internal member 18 is a cylindrical metal plate, the outer circumferential surface of the internal member 18 and the inner circumferential surface of the outer tube 16 can be brought into contact over almost the entire surface in the composite billet 10 formed in the composite billet forming process S30. In this case, the gap between the outer tube 16 and the internal member 18 becomes very small, and the outer diameter Di of the internal member 18 can be considered to correspond to the inner diameter of the outer tube 16.

[0039] <Heating process> In heating step S40, the composite billet 10 is heated. The heating temperature is set to a temperature at which the composite billet 10 will be sufficiently deformed in the subsequent rolling step S60 and the desired rolling can be performed. For example, if both the outer tube 16 and the internal member 18 are steel tubes, the heating temperature is preferably 900 to 1300°C.

[0040] Furthermore, after the composite billet forming process S30 and before the heating process S40, the internal member 18 may be fixed to the outer tube 16 to the extent that it does not fall out of the outer tube 16 when the composite billet 10 is transported. Specifically, the internal member 18 may be fixed to the outer tube 16 by friction by bringing the inner circumferential surface of the outer tube 16 and the outer circumferential surface of the internal member 18 into contact over most of the surface.

[0041] <Placement process> In the arrangement process S50, the inclined rolling mill 5 is used. The inclined rolling mill 5 is also used in the next rolling process S60. Figure 4 is a top view of the inclined rolling mill 5. Figure 4 shows a cross-section of the composite billet, etc. The inclined rolling mill 5 comprises a rolling mill body 4, a pusher 6, and a clamping device 7. The rolling mill body 4 comprises a pair of inclined rolls 1 and a plug 2.

[0042] The pusher 6 is positioned on the pass line PL, on the side of the rolling direction inlet that is closer to the rolling mill body 4. The rolling direction inlet side is the side that supplies the composite billet 10 to the rolling mill body 4. The pusher 6 has a pusher body 6A and a pusher head 6B that is located closer to the rolling mill body 4 than the pusher body 6A. The pusher body 6A has a cylindrical shape. The outer diameter of the pusher body 6A is larger than the inner diameter of the outer tube 16. The pusher head 6B has a cylindrical shape. The pusher body 6A and the pusher head 6B are positioned coaxially with the pass line PL. The outer diameter of the pusher head 6B is smaller than the inner diameter of the outer tube 16. Also, the outer diameter of the pusher head 6B is larger than the inner diameter of the raw inner tube, which is an internal component 18.

[0043] The pusher head 6B may have a shape that is slightly deformed from a cylinder. In that case, the maximum outer diameter of the pusher head 6B shall be smaller than the inner diameter of the outer tube 16 and larger than the inner diameter of the inner tube which is the internal component 18.

[0044] The pusher 6 includes a first part 6s and a second part 6r that is rotatable relative to the first part 6s. The first part 6s includes the portion of the pusher body 6A furthest from the rolling mill body 4. The second part 6r includes the portion of the pusher body 6A adjacent to the first part 6s and the pusher head 6B. That is, the second part 6r includes the pusher head 6B and the portion of the pusher body 6A adjacent to the pusher head 6B. The second part 6r is rotatable relative to the first part 6s about the axis of the pusher body 6A.

[0045] The clamping device 7 is positioned between the rolling mill body 4 and the pusher 6. Figure 4 shows the state immediately before the positioning process S50 is carried out. The outer surface of the composite billet 10 is held by the clamping device 7. This fixes the outer tube 16 in place.

[0046] The positioning step S50 is performed after the heating step S40 and before the internal member 18 comes into contact with the plug 2 in the next rolling step S60. In the positioning step S50, the rear end surface 18b of the internal member 18, opposite to the inclined roll 1 side, is pushed by the pusher head 6B. As a result, the internal member 18 moves forward (towards the inclined roll 1) relative to the outer tube 16. In other words, the rear end surface 18b of the internal member 18 is positioned on the inclined roll 1 side of the outer tube 16 than the rear end surface 16b opposite to the inclined roll 1 side. Because the outer diameter of the pusher head 6B is smaller than the inner diameter of the outer tube 16 and larger than the inner diameter of the internal member 18, the pusher head 6B can push the rear end surface 18b of the internal member 18 into the outer tube 16 without interfering with the outer tube 16.

[0047] Because the outer diameter of the pusher body 6A is larger than the inner diameter of the outer tube 16, when the pusher head 6B further pushes the rear end surface 18b of the internal member 18, the pusher body 6A comes into contact with the rear end surface 16b of the outer tube 16. The arrangement process S50 ends at this point.

[0048] Figure 5 is a longitudinal cross-sectional view of the composite billet 10 after the placement process S50 has been carried out. A longitudinal cross-section is a cross-section obtained when the composite billet 10 is cut by a plane containing the central axis of the composite billet 10. The pusher 6 is also shown in Figure 5. The pusher head 6B is in contact with the rear end surface 18b of the internal member 18. The pusher body 6A is in contact with the rear end surface 16b of the outer tube 16. In the axial direction of the outer tube 16, the rear end surface 18b of the internal member 18 is separated from the rear end surface 16b of the outer tube 16. In the axial direction of the outer tube 16, the distance between the rear end surface 18b of the internal member 18 and the rear end surface 16b of the outer tube 16 (hereinafter referred to as "end face spacing") is equal to the amount of protrusion of the pusher head 6B from the pusher body 6A (hereinafter referred to as "head protrusion amount").

[0049] In the examples shown in Figures 4 and 5, the length of the internal member 18 and the length of the outer tube 16 are the same. Therefore, at the front end (inclined roll 1 side) of the composite billet 10, the internal member 18 protrudes from the outer tube 16 by the amount of head protrusion. If it is necessary for the internal member 18 to protrude from the outer tube 16 by a smaller amount than the amount of head protrusion at the front end of the composite billet 10, or if the internal member 18 does not need to protrude from the outer tube 16, an internal member 18 shorter than the outer tube 16 can be used.

[0050] Let d be the end face spacing (see Figure 5). Let Di be the outer diameter of the internal member 18 in the composite billet 10 formed in the composite billet forming process S30 (hereinafter referred to as the "initial outer diameter") (see Figure 2). Specifically, the outer diameter Di of the internal member 18 is the outer diameter of the internal member 18 after it has been inserted into the outer tube 16. In this embodiment, when the internal member 18 is a plain inner tube, the outer diameter Di of the internal member 18 is the same as the outer diameter of the internal member 18 prepared in the internal member preparation process S20. It is preferable that d and Di satisfy the following formula (1). 0.3 × Di <d<3.0×Di (1)

[0051] As a result, in the composite billet 10 (double tube) after the rolling process S60 described later, the axial position of the rear end surface 16b of the outer tube 16 and the rear end surface of the inner tube formed from the internal member 18 can be almost aligned.

[0052] <Rolling process> Figure 6 is a top view of the inclined rolling mill 5 during the rolling process S60. Figure 7 is a side view of the inclined rolling mill 5 during the rolling process S60. In the rolling process S60, rolling is performed by pressing a pair of inclined rolls 1 against the outer circumferential surface of the heated composite billet 10, while simultaneously pressing a plug 2 against the inner circumferential surface of the internal member 18 in the heated composite billet 10. By rolling the composite billet 10 in this manner, a double-walled pipe is obtained in which the inner circumferential surface of the outer tube 16 and the outer circumferential surface of the inner tube formed from the internal member 18 are in close contact.

[0053] In this embodiment, a pair of inclined rolls 1 are arranged facing each other horizontally across 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, in the composite billet 10 to be rolled, the internal members 18 have holes before being processed by the inclined rolling mill 5. Therefore, perforation is not performed in the rolling process S60. Note that if a composite billet 10A equipped with a solid billet internal member 18A is used instead of the internal member 18 of the raw inner tube (see Figure 3), then perforation is also performed in the rolling process S60.

[0054] The central axis (rotation axis) C of each inclined roll 1 makes 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 6). 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 7). 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 16 aligned with the pass line PL. Figures 6 and 7 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 composite billet 10 is released from the clamping device 7 before the front end of the composite billet 10 contacts the pair of inclined rolls 1.

[0057] It is preferable to press the rear end of the composite billet 10 (the end opposite to the pair of inclined rolls 1) with the second portion 6r of the pusher 6 until the composite billet 10 is sufficiently engaged with the pair of inclined rolls 1.

[0058] As the composite billet 10 moves, the inner circumferential surface of the internal member 18 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 16 and the internal member 18 deform to move closer together, and finally, the inner circumferential surface of the outer tube 16 and the outer circumferential surface of the inner tube formed from the internal member 18 come into close contact.

[0059] When the composite billet 10 is loaded into a pair of inclined rolls 1 and then rolled by the pair of inclined rolls 1 and the plug 2, it is preferable to push the rear end of the composite billet 10 with the second part 6r of the pusher 6. In this case, back pressure is applied to the internal member 18 by the pusher head 6B and to the outer tube 16 by the pusher body 6A. Therefore, even if there is forward resistance to the outer tube 16 and the internal member 18 during rolling, the end face gap can be maintained. Specifically, during rolling, the internal member 18 moves forward together with the outer tube 16, but the plug 2 is pressed against the internal member 18. Therefore, the plug 2 acts as forward resistance to the internal member 18, and there is a possibility that the internal member 18 will recede relative to the outer tube 16. However, since back pressure is applied to the composite billet 10, the internal member 18 will not recede relative to the outer tube 16, and the end face gap is maintained.

[0060] In this specification, back pressure refers to the pressure applied to the composite billet 10 that is engaged with the inclined roll 1, directed toward the inclined roll 1 and the plug 2. When rolling begins, the outer tube 16 and the internal members 18 in the composite billet 10 rotate together around the pass line axis. The second part 6r rotates in accordance with the rotation of the internal members 18 and the outer tube 16. The integral rotatability of the second part 6r allows back pressure to be applied to the composite billet 10 without hindering the rotation of the outer tube 16 and the internal members 18.

[0061] The maximum outer diameter of the plug 2 is preferably larger than the inner diameter of the raw inner tube, which is the internal member 18, before the rolling process S60 is carried out. In this case, the plug 2 expands the internal member 18 (raw inner tube), and the outer tube 16 and the internal member 18 are rolled so as to be stretched axially by a pair of inclined rolls 1 and the plug 2. Expanding the internal member 18 with the plug 2 is more effective in bringing the inner tube formed from the outer tube 16 and the internal member 18 into close contact than reducing the diameter of the outer tube 16 with the inclined rolls 1.

[0062] Figure 8 is a cross-sectional view of a double-walled pipe obtained by the manufacturing method according to this embodiment. The double-walled pipe 20 includes an outer pipe 16 and an inner pipe 19 formed from an internal member 18. The outer circumferential surface of the inner pipe 19 formed from the internal member 18 is in close contact with the inner circumferential surface of the outer pipe 16. Therefore, according to the manufacturing method according to this embodiment, a double-walled pipe 20 can be obtained in which the inner circumferential surface of the outer pipe 16 and the outer circumferential surface of the inner pipe 19 formed from the internal member 18 are in close contact.

[0063] <effect> If the internal component 18 is made of a corrosion-resistant material, a double-walled pipe 20 with a corrosion-resistant inner surface can be manufactured by the manufacturing method of this embodiment. Such a double-walled pipe 20 can be used to carry a corrosive fluid. The corrosive fluid flowing inside the double-walled pipe 20 does not normally come into contact with the outer pipe 16. For this reason, the outer pipe 16 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 made of an inexpensive material for the outer pipe 16, the overall cost of the double-walled pipe 20 can be reduced.

[0064] In the rolling process S60, the outer tube 16 is rolled so that it extends forward in the direction of movement relative to the inner member 18. That is, the outer tube 16 is moved forward in the direction of movement relative to the inner member 18. Here, in the positioning process S50, the rear end surface 18b of the inner member 18 is positioned forward in the direction of movement of the outer tube 16b. As a result, in the double tube 20 after the rolling process S60, the portion of the inner tube 19 formed from the inner member 18 that is not covered by the outer tube 16 is reduced on the rear side in the direction of movement during rolling. That is, the generation of rolling slag is suppressed.

[0065] In particular, when d and Di satisfy the above formula (1), the amount of slag generated can be reduced, and the length of the portion of the outer tube 16 that does not constitute the double tube 20 can be shortened. That is, if d is (0.3 × Di) or less, in the double tube 20 after the rolling process S60, at the rear side in the direction of movement during rolling, the portion of the inner tube 19 formed from the internal member 18 that is not covered by the outer tube 16, i.e., slag, is likely to be generated. Also, if d is (3 × Di) or more, although the generation of slag can be suppressed, in the double tube 20, at the rear side in the direction of movement during rolling, the portion of the outer tube 16 that does not have the inner tube 19, i.e., a portion that does not constitute the double tube 20, is likely to be generated.

[0066] Furthermore, in this embodiment, the rear end of the composite billet 10 is pressed by the second portion 6r of the pusher 6 during rolling. Therefore, even in the initial stages of rolling, before the inner tube 19 formed from the internal member 18 and the outer tube 16 begin to come into close contact, the internal member 18 does not recede relative to the outer tube 16, and the end face spacing is maintained. This makes it possible to more reliably suppress the generation of rolling slag and promotes close contact between the inner tube 19 and the outer tube 16.

[0067] For example, when using a composite billet 10A equipped with an internal member 18A of a solid billet (see Figure 3), in the arrangement step S50, the rear end surface of the internal member 18A is positioned closer to the inclined roll 1 than the rear end surface 16b of the outer tube 16. Then, in the rolling step S60, the composite billet 10A in this state is perforated and rolled to obtain a double tube. In this case, in the rolling step S60, rolling is performed by pressing a pair of inclined rolls 1 against the outer circumferential surface of the heated composite billet 10A, and pressing the plug 2 against the internal member 18A of the heated composite billet 10A. In this case as well, the above-mentioned effects can be obtained, similar to when using a composite billet 10 equipped with an internal member 18 of a plain inner tube.

[0068] [Second Embodiment] In the manufacturing method of the second embodiment, the outer tube preparation step S10 to the heating step S40 are carried out in the same manner as in the first embodiment, and then the arrangement step S50 is carried out as follows. Figure 9 is a top view of the inclined rolling mill 5A and the composite billet 10 before the arrangement step S50 is carried out. In the manufacturing method of the second embodiment, the clamping device 7 (see Figure 4) is not used. Therefore, the inclined rolling mill 5A does not need to be equipped with the clamping device 7.

[0069] In the positioning process S50, the front end of the composite billet 10 is engaged with a pair of inclined rolls 1. This restrains the outer tube 16. More specifically, the outer tube 16 moves at a constant speed in its axial direction and rotates around the pass line PL. The internal member 18 moves at a constant speed in its axial direction and rotates around the pass line PL in accordance with the movement of the outer tube 16.

[0070] With the composite billet 10 engaged in the inclined roll 1 in this manner, the pusher head 6B of the second section 6r pushes the rear end surface 18b of the internal member 18. As a result, the rear end surface 18b of the internal member 18 is pushed into the outer tube 16. Finally, as shown in Figure 5, in the axial direction of the outer tube 16, the rear end surface 18b of the internal member 18 moves away from the rear end surface 16b of the outer tube 16 by the amount of the head protrusion.

[0071] Subsequently, the rolling process S60 is carried out in the same manner as in the first embodiment. This causes the outer circumferential surface of the inner tube 19 formed from the internal member 18 to come into close contact with the inner circumferential surface of the outer tube 16, resulting in a double tube in which the outer tube 16 and the inner tube 19 are in close contact. In the arrangement process S50, by shifting the rear end surface 18b of the internal member 18 from the rear end surface 16b of the outer tube 16 as described above, the axial positions of the rear end surface 16b of the outer tube 16 and the rear end surface of the inner tube 19 formed from the internal member 18 are almost aligned in the double tube obtained by the rolling process S60. In other words, the generation of rolling slag can be suppressed.

[0072] [Differentiation] Before the placement process S50 is carried out, the composite billets 10, 10A may be partially and firmly fixed to the outer tube 16 and the internal members 18, 18A (raw inner tube or solid billet). For example, the internal members 18, 18A may be firmly fixed to the outer tube 16 at the front end of the composite billets 10, 10A before the placement process S50. In this case, during the placement process S50, with the outer tube 16 restrained by the clamping device 7 or a pair of inclined rolls 1, the internal members 18, 18A are compressed by the pusher head 6B pressing against their rear end faces. The length of the internal members 18, 18A may be shortened by compression as long as the distance between the rear end face 16b of the outer tube 16 and the rear end face of the internal members 18, 18A in the axial direction of the outer tube 16 is a predetermined size. Since the internal members 18 and 18A are compressed, a predetermined distance can be secured between the rear end surface 16b of the outer tube 16 and the rear end surfaces of the internal members 18 and 18A, thus suppressing the generation of rolling slag in the double tube after rolling.

[0073] 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. [Examples]

[0074] The present disclosure will be further described below with reference to examples. However, the present disclosure is not limited to the following examples.

[0075] In this embodiment, the effects of the manufacturing method of the above embodiment were verified. Multiple composite billets were prepared and heated at 1200°C. A cylindrical inner tube was used as the internal component of the composite billet. The material of the inner tube was SUS316L equivalent steel according to JIS standards. The material of the outer tube was carbon steel. The heated composite billet was rolled in an inclined rolling mill to produce a double tube. The rolling conditions were a draw 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 draw ratio is the ratio of the sum of the cross-sectional areas of the inner tube in the internal component preparation process and the outer tube in the outer tube preparation process to the cross-sectional area of ​​the manufactured double tube. The expansion ratio is the ratio of the outer diameter of the manufactured double tube to the outer diameter of the outer tube in the outer tube preparation process. The dimensions (cross-sectional area and outer diameter) of the inner tube, outer tube, and double tube described here are cold dimensions measured at room temperature.

[0076] In multiple composite billets, the end face spacing after the arrangement process was changed to produce double-walled tubes of comparative examples and inventive examples 1 to 5. The presence or absence of rolling slag in the produced double-walled tubes was evaluated. Table 1 shows the dimensions of the raw inner and outer tubes constituting the composite billet, the end face spacing d, and the evaluation results of rolling slag. The dimensions of the raw inner and outer tubes were those of each tube as it was when it was prepared.

[0077] [Table 1]

[0078] The double-walled pipe in the comparative example was manufactured under the condition that the end face spacing d was 0 mm. In other words, the placement process was not performed before the rolling process. As a result, slag was generated in the double-walled pipe of the comparative example.

[0079] In the double-walled pipes of Examples 1 to 5 of the present invention, the arrangement process was carried out before the rolling process. Specifically, the composite billet was rolled with the end face spacing d shown in Table 1. As a result, no rolling slag was generated in the double-walled pipes of Examples 1 to 5 of the present invention.

[0080] Unlike the double-walled tubes of Examples 2 to 5 of the present invention, in the double-walled tube of Example 1 of the present invention, burrs were generated at the rear end of the inner tube formed from the internal material (raw inner tube). This is thought to be because the end face spacing d is short, causing the inner tube to slightly protrude from the rear end of the outer tube after rolling. Therefore, it is preferable that the end face spacing d be greater than 8.7 mm. In this embodiment, 8.7 mm corresponds to 0.3 times the outer diameter Di (=29.0 mm) of the raw inner tube. Therefore, it is preferable that the end face spacing d be greater than 0.3 times the outer diameter Di of the raw inner tube.

[0081] In the double-walled tube of Example 5 of the present invention, the axial length of the portion where the inner tube is absent at the rear end of the outer tube was longer compared to the double-walled tubes of Examples 1 to 4 of the present invention. This is because the end face spacing d was longer. If the axial length of the portion where the inner tube is absent at the rear end of the outer tube becomes too long, the yield of the double-walled tube will decrease. Therefore, it is preferable that the end face spacing d be smaller than 87.0 mm. In this embodiment, 87.0 mm corresponds to 3.0 times the outer diameter Di (=29.0 mm) of the raw inner tube. Therefore, it is preferable that the end face spacing d be smaller than 3.0 times the outer diameter Di of the raw inner tube.

[0082] From the above results, it became clear that the generation of rolling slag is suppressed when a configuration process is performed before the rolling process, and the composite billet is rolled with the rear end face of the inner tube, which is an internal component, positioned forward in the direction of movement of the rear end face of the outer tube. Furthermore, it was found that it is preferable for the end face spacing d to satisfy equation (1) in the configuration process. [Explanation of symbols]

[0083] 1: Inclined Roll 2: Plug 3: Mandrel 4: Rolling mill body 5.5A: Inclined rolling mill 6: Pusher 6A: Pusher body 6B: Pusher head 6s: 1st part 6r: 2nd part 7: Clamping device 10,10A: Composite billet 16:Outer tube 16b: Rear end face of the outer tube 18,18A: Internal components 18b: Rear end face of internal member 19: Inner tube 20:Double tube C: Center axis of the inclined roll PL: Passline

Claims

1. A method for manufacturing a double tube, An outer tube preparation step in which an outer tube having a cylindrical shape is prepared, An internal member preparation step involves preparing an internal member having a cylindrical outer surface, A composite billet forming step involves inserting the internal member into the outer tube to form a composite billet comprising the outer tube and the internal member, A heating step for heating the composite billet, The rolling process involves using an inclined rolling mill equipped with an inclined roll and a plug, pressing the inclined roll against the outer circumferential surface of the heated composite billet, and pressing the plug against the internal member in the heated composite billet, thereby rolling the composite billet to obtain a double tube in which the inner circumferential surface of the outer tube and the outer circumferential surface of the inner tube formed from the internal member are in close contact, The inclined rolling mill further comprises a pusher positioned on the pass line on the rolling direction entry side of the rolling mill body, The pusher comprises a pusher body and a pusher head located closer to the rolling mill body than the pusher body. The maximum outer diameter of the pusher head is smaller than the inner diameter of the outer tube. The manufacturing method for the double tube further includes, after the heating step and before the internal member comes into contact with the plug in the rolling step, a positioning step in which the rear end surface of the internal member opposite to the inclined roll side is pressed with the pusher head so that the rear end surface of the internal member is positioned on the inclined roll side of the outer tube compared to the rear end surface opposite to the inclined roll side. A method for manufacturing a double-walled pipe, wherein, after the arrangement step is performed, the distance d between the rear end face of the outer pipe and the rear end face of the inner member is 0.3 times or more and 3.0 times or less the outer diameter Di of the inner member in the composite billet formed in the composite billet forming step.

2. A method for manufacturing a double tube according to claim 1, A method for manufacturing a double-walled pipe, wherein the distance d between the rear end face of the outer pipe and the rear end face of the inner member after the arrangement step is performed satisfies the following formula (1). 0.3×Di<d<3.0×Di (1) However, Di is the outer diameter of the internal member in the composite billet formed in the composite billet forming process.

3. A method for manufacturing a double tube according to claim 1, The inclined rolling mill further includes a clamping device positioned between the rolling mill body and the pusher. A method for manufacturing a double-walled pipe, wherein, in the arrangement step, the outer pipe is fixed by the clamping device, and the rear end surface of the inner member is pressed with the pusher head.

4. A method for manufacturing a double tube according to claim 1, The pusher includes a first part which includes the portion of the pusher body furthest from the rolling mill body, and a second part which is rotatable around the axis of the pusher body relative to the first part and includes the portion of the pusher body adjacent to the first part and the pusher head. A method for manufacturing a double-walled pipe, wherein, in the arrangement step, the composite billet is engaged with the inclined roll, and the pusher head of the second portion presses the rear end face of the internal member.

5. A method for manufacturing a double tube according to any one of claims 1 to 4, The pusher includes a first part which includes the portion of the pusher body furthest from the rolling mill body, and a second part which is rotatable around the axis of the pusher body relative to the first part and includes the portion of the pusher body adjacent to the first part and the pusher head. A method for manufacturing a double-walled pipe, wherein, in the rolling process, when rolling the composite billet with the inclined roll and the plug, the rear end of the composite billet opposite to the inclined roll side is pressed with the second portion of the pusher.

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