Plug for manufacturing seamless steel pipes, piercing mill for manufacturing seamless steel pipes, and method for manufacturing seamless steel pipes

Abstract

To provide a plug for manufacturing a seamless steel pipe which enables improvement of its life, and to provide a piercing mill for manufacturing the seamless steel pipe including the same and a manufacturing method of the seamless steel pipe.SOLUTION: A plug 10 for manufacturing a seamless steel pipe is used to pierce and roll a steel pipe material which rotates in a circumferential direction and includes: a plug tip part 11 which pierces the steel pipe material; and a rolling part 12 which is detachably connected to the plug tip part 11 and expands a diameter of the pierced steel pipe material. The plug tip part 11 has a fitting protrusion 11X at a rear end in a piercing direction. The rolling part 12 has a fitted part 12X in which the fitting protrusion 11X is fitted at a tip in the piercing direction. The fitting protrusion 11X has protruding parts 11P which are formed protruding on side wall surfaces. The fitted part 12X is provided with a protruding part housing unit 12P which allows the protruding parts 11P to rotate in the circumferential direction and makes the protruding parts 11P engageable.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a plug for manufacturing a seamless steel pipe by the Mannesmann piercing-mandrel mill method and a piercing mill for manufacturing a seamless steel pipe provided with the plug. More specifically, the present invention relates to a plug having an improved lifespan by replacing only the tip of the plug, which is the rate-determining factor for the plug lifespan, a piercing mill provided with the plug, and a method for manufacturing a seamless steel pipe.

Background Art

[0002] In Mannesmann piercing for manufacturing seamless steel pipes, piercing rolling is performed on a steel pipe material using a PCM (Piercing Mill) plug. In order to increase the pipe manufacturing efficiency of seamless steel pipes and improve the yield, it is preferable to increase the length of the hollow (for example, about 9 to 12 m in length).

[0003] However, due to the increase in the length of the hollow, the time for the plug to stay in the billet becomes longer (for example, 15 to 30 seconds), and the time for the plug to be exposed to high temperatures becomes longer. Furthermore, during piercing rolling to convert the billet from solid to a hollow pipe, the plug is constantly in contact with the newly formed surface of the billet. At this time, in the case of a conventional plug, when used in a high-temperature and high-pressure environment, the tip may be melted or deformed, and thereafter it cannot be used during piercing rolling. For example, in the piercing rolling of high-alloy steel with a Cr content of 5% by mass or more and high hot deformation resistance, one plug can only be used for the piercing rolling of several to a dozen billets.

[0004] Such a decrease in the lifespan of the plug increases the unit cost of the plug, resulting in an increase in the pipe manufacturing cost. Therefore, it is required to improve the lifespan of the plug.

[0005] To date, various proposals have been made to improve the lifespan of these plugs by specializing their shape and material. For example, Patent Document 1 discloses a plug made of ceramic material. In addition, considering that the tip of the plug may melt, a proposal has been made to make the melted tip a non-ferrous material (see, for example, Patent Document 2). On the other hand, in recent years, there has been a desire to improve the lifespan of plugs by improving their shape without improving the material, taking into consideration factors such as reducing the cost of plugs. Patent documents 3 to 5 describe a technology that makes the plug tip detachable. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2001-87804 [Patent Document 2] Japanese Patent Publication No. 2000-167606 [Patent Document 3] International Publication No. 2005 / 087401 [Patent Document 4] International Publication No. 2014 / 030593 [Patent Document 5] Japanese Patent Publication No. 2021-164958 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, even when the plug tip is made detachable, as in the technologies described in Patent Documents 3 and 4, the connection between the plug tip and the reeling part was not sufficient to improve the plug's lifespan. Specifically, in the conventional technology, there was a concern that if the connection was too strong, the connection would break during drilling and rolling, while on the other hand, if the connection was too weak, there was a concern that the plug tip would detach from the reeling part during drilling and rolling. As a result, an optimal connection state could not be found, and a sufficient lifespan could not be achieved.

[0008] Furthermore, the technology described in Patent Document 5 required time for manufacturing the threaded and non-threaded parts, necessitating the establishment of other technologies. Thus, there was a need to establish a new plug technology that made the plug tip detachable.

[0009] The present invention aims to solve the problems of the prior art and to provide a plug for manufacturing seamless steel pipes with improved lifespan, a piercing mill for manufacturing seamless steel pipes equipped with the plug, and a method for manufacturing seamless steel pipes. [Means for solving the problem]

[0010] As a result of diligent research to solve the above problems, the inventors focused on a mechanism that connects the plug tip and the rolled part with a specific key shape, which suppresses detachment of the plug tip from the rolled part, damage to the fitting projection and the fitted part, and inability to detach due to wear during drilling and rolling, as well as suppressing deterioration of the plug tip, and allows for easy detachment of the plug tip and the rolled part after drilling and rolling.

[0011] Specifically, the inventors have found that the tip of the plug has a fitting projection at the rear end in the direction of drilling, and that the fitting projection has one or more protrusions (key portions) that protrude from its surface. They have also found that the rolled portion has a fitted portion, and that the fitted portion has a protrusion housing portion that allows the protrusion to rotate freely in the circumferential direction (circumferential direction of the steel pipe) and to lock the protrusion. Furthermore, it was found that the rolled portion preferably has a groove-like portion in the first hole formed on the drilling direction side of the fitted portion, which allows the protrusion of the fitted projection (second axial portion) to be inserted, in order to facilitate attachment and detachment with the fitted projection. At this time, we also found that the number of groove-like sections that follow the shape of the protrusion (key section) only needs to be greater than or equal to the number of protrusions (key sections), and they do not necessarily need to be identical. Furthermore, it was found that, in order to allow the plug tip having a protrusion to rotate freely in the circumferential direction when the plug tip and the rolled portion are joined (connected), it is preferable that the protrusion housing portion has a columnar shape with dimensions that can accommodate a rotating body that can be formed by the 360° rotation of the protrusion (key portion). As described above, by providing the plug tip and rolled portion with a specific protrusion-receiving structure, the plug life can be improved, leading to the completion of the present invention.

[0012] Based on the above findings, and after further consideration, the gist of the present invention is as follows. [1] A plug for drilling and rolling a steel pipe material that rotates in the circumferential direction, The tip of the plug that drills into the steel pipe material, The tip of the plug is detachably connected to a rolling section that expands the diameter of the perforated steel pipe material, Equipped with, The tip of the plug has a fitting projection at its rear end in the direction of drilling. The rolling portion has a fitted portion at its leading edge in the drilling direction that is fitted to the fitting projection, The fitting projection has a protrusion formed on the side wall surface, A plug for manufacturing seamless steel pipes, wherein the fitting portion has a projection housing portion formed therein that allows the projection to rotate circumferentially and to lock into place. [2] A plug for drilling and rolling a steel pipe material that rotates in the circumferential direction, The tip of the plug that drills into the steel pipe material, The tip of the plug is detachably connected to a rolling section that expands the diameter of the perforated steel pipe material, Equipped with, The rolling portion has a fitting projection at its leading edge in the direction of drilling. The tip of the plug has a fitted portion at its rear end in the direction of drilling that is fitted to the fitting projection, The fitting projection has a protrusion formed on the side wall surface, A plug for manufacturing seamless steel pipes, wherein the fitting portion has a projection housing portion formed therein that allows the projection to rotate circumferentially and to lock into place. [3] The convex portion is formed at the rear end in the drilling direction of the fitting projection portion, The fitting projection portion has, in the reverse direction of the drilling direction, a first axial center portion and a second axial center portion where the convex portion is formed, in that order, The convex portion accommodating portion is formed at the rear end in the drilling direction of the fitting portion, The fitting portion has, in the reverse direction of the drilling direction, a first hole portion covering the outer periphery of the first axial center portion and a second hole portion where the convex portion accommodating portion is formed, in that order, The length LC1 in the drilling direction of the first axial center portion and the length lc1 in the drilling direction of the first hole portion satisfy the following formula (1-1), The length LK1 in the drilling direction of the second axial center portion and the length lk1 in the drilling direction of the second hole portion satisfy the following formula (2-1), the plug for seamless steel pipe manufacturing according to [1] above. LC1>lc1 ··· formula (1-1) LK1<lk1 ··· formula (2-1) [4] The convex portion is formed at the tip in the drilling direction of the fitting projection portion, The fitting projection portion has, in the drilling direction, a first axial center portion and a second axial center portion where the convex portion is formed, in that order, <00​​​​​​​​​​​​​​[6] A piercing mill for manufacturing seamless steel pipes, comprising a plug for manufacturing seamless steel pipes as described in any of [1] to [4] above. [7] A method for manufacturing seamless steel pipes, comprising manufacturing a seamless steel pipe using a plug for manufacturing seamless steel pipes described in any of [1] to [4] above. [Effects of the Invention]

[0013] According to the present invention, a plug for manufacturing seamless steel pipes with improved lifespan, a piercing mill for manufacturing seamless steel pipes equipped with the plug for manufacturing seamless steel pipes, and a method for manufacturing seamless steel pipes are provided. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is a schematic diagram illustrating the manufacturing method of seamless steel pipes. [Figure 2] Figure 2 shows a schematic configuration of a drilling and rolling mill. [Figure 3] Figure 3 is a diagram illustrating the configuration of a plug according to the first embodiment of the present invention. [Figure 4] Figure 4 is an exploded perspective view of the plug. [Figure 5] Figure 5 is a diagram illustrating the process by which the plug tip and the rolled portion are joined together. [Figure 6] Figure 6 is a diagram illustrating the configuration of a modified plug according to the present invention. [Figure 7] Figure 7 is a diagram illustrating the configuration of a modified plug according to the present invention. [Figure 8] Figure 8 is a diagram illustrating the configuration of a modified plug according to the present invention. [Figure 9] Figure 9 is a diagram illustrating the configuration of a plug that is a related technology (comparative example). [Figure 10] Figure 10 is a diagram illustrating the configuration of a plug that is a related technology (comparative example). [Figure 11] Figure 11 is a diagram illustrating the configuration of a plug that is a related technology (comparative example). [Figure 12]Figure 12 is a diagram illustrating the configuration of a plug according to a second embodiment of the present invention. [Modes for carrying out the invention]

[0015] <Method for manufacturing seamless steel pipes> The present invention will be described with reference to the drawings. However, the present invention is not limited by this embodiment.

[0016] Before describing the details of the configuration and function of the plug for manufacturing seamless steel pipes of the present invention, a method for manufacturing seamless steel pipes using the plug for manufacturing seamless steel pipes by the Mannesmann drilling-mandrel mill method will be described with reference to Figure 1. Figure 1 is a schematic diagram illustrating the manufacturing method for seamless steel pipes.

[0017] In the Mannesmann piercing-mandrel mill method, first, the billet B (steel pipe material) shown in Figure 1(a) is heated to 1200-1300°C in a rotary hearth heating furnace 100 (see Figure 1(b)). Then, in the piercing and rolling process, the plug 10 (PCM (Piercing Mill plug)) and the rolling rolls 2a and 2b of the piercing and rolling mill 101 are used to insert the plug 10 into the billet B in the longitudinal direction (axial direction), and piercing and rolling is performed on the billet B. This produces the hollow S (see Figure 1(c)).

[0018] Next, as shown in Figure 1(d), a mandrel bar 102 is inserted in a skewer-like manner into the inner surface of the hollow S, and the outer surface of the hollow S is restrained by the perforated rolling rolls of a mandrel mill 103 consisting of 5 to 8 stands while stretch rolling is performed to reduce the wall thickness to a predetermined thickness and obtain a raw pipe. After that, the mandrel bar 102 is removed from the raw pipe, and as shown in Figure 1(e), the reduced-wall thickness raw pipe is rolled to a predetermined outer diameter using a reducer 104 to obtain a product (seamless steel pipe X).

[0019] Here, the perforation rolling process described above will be explained in more detail. Figure 2 shows the schematic configuration of the perforation rolling mill 101, with Figure 2(a) showing a side view and Figure 2(b) showing a top view. Note that the plug 10 is not shown in Figure 2(b). As shown in Figure 2, the perforating rolling mill 101 comprises a pair of rolling rolls 2a and 2b arranged with their rotation axes tilted relative to each other, disc shoes 4a and 4b positioned at a 90° rotation relative to the rolling rolls 2a and 2b in a plane perpendicular to the rolling direction, and a plug 10 whose rear end is supported by the mandrel 3. The pair of rolling rolls 2a and 2b can be arranged with their rotation axes tilted in opposite directions by an inclination angle FA in a plan view, around the rolling direction, and can rotate in the same direction relative to each other. The plug 10 is positioned between the pair of rolling rolls 2a and 2b and between the disc shoes 4a and 4b to perforate the billet B.

[0020] To drill and roll a solid billet B (round billet) using the drilling and rolling mill 101, the billet B is first fed between a pair of rolling rolls 2a and 2b. After the billet B is engaged with the pair of rolling rolls 2a and 2b, the frictional force of the rolling rolls 2a and 2b acts on the billet B simultaneously, causing rotational force and axial (rolling direction) forward force. As the billet B reaches the tip of the plug 10, the center of the billet B is subjected to alternating compressive and tensile stresses by the rolling rolls 2a and 2b (rotational forging effect), making it easier to form a hole. Subsequently, when the billet B collides with the plug 10, a hole is made in the center of the billet B, and thereafter, it undergoes thinning every half rotation between the rolling rolls 2a and 2b and the plug 10, resulting in a hollow S with a predetermined thickness and outer shape.

[0021] <Plug for manufacturing seamless steel pipes> [First Embodiment] The plug configuration of the plug 10 of the first embodiment of the present invention will be described below with reference to Figure 3. Figure 3 is a diagram illustrating the configuration of the plug of the first embodiment of the present invention. In Figure 3, (a) is an overall view of the plug 10 (10A) of this embodiment, (b) is an enlarged view of the plug tip portion 11, and (c) is an enlarged view of the vicinity of the joint U1 between the plug tip portion 11 and the rolled portion 12. In addition, Figure 3(d) shows an example of the shape of the first hole portion 12A of the rolled portion 12, and an example of the shape of the second hole portion 12B and the protrusion housing portion 12P in a cross-sectional view perpendicular to the drilling direction.

[0022] The plug 10A of this embodiment is a plug for manufacturing seamless steel pipes used in the perforating and rolling mill 101 as described above, and the plug tip 11 and the rolling section 12 are detachably connected. Specifically, as shown in Figure 3 (a), the plug 10A of this embodiment is a plug for perforating and rolling a steel pipe material such as a billet B that rotates in the circumferential direction, and has a plug tip 11 for perforating the steel pipe material and a rolling section 12 which is detachably connected to the plug tip 11 and expands the diameter of the perforated steel pipe material. Furthermore, as shown in (b) of Figure 3, the plug tip portion 11 has a fitting projection 11X at its rear end in the drilling direction, and as shown in (c), the rolled portion 12 has a fitted portion 12X that is fitted to the fitting projection 11X at its tip in the drilling direction. Furthermore, the fitting projection 11X has a protrusion 11P that is formed to protrude from the side wall surface, and the fitted portion 12X has a protrusion housing portion 12P that allows the protrusion 11P to rotate freely in the circumferential direction while also enabling the protrusion 11P to be locked in place. The plug 10A of this embodiment is characterized in that it has a protrusion housing structure in which the plug tip portion 11 and the rolled portion 12 are joined together with the protrusion 11P housed in the protrusion housing portion 12P.

[0023] The shape of the protrusion 11P is not particularly limited, as long as the protrusion 11P is formed to protrude from the side wall surface of the fitting projection 11X, and is rotatable in the circumferential direction within the protrusion housing 12P when the plug tip 11 and the rolled portion 12 are joined (connected), and can be locked into the protrusion housing 12P. Furthermore, the shape of the protrusion housing portion 12P is not particularly limited, as long as it allows the protrusion 11P to rotate freely in the circumferential direction while being able to be locked in place when the plug tip portion 11 and the rolled portion 12 are joined (connected). The protrusion housing portion 12P may be a concave portion formed in the circumferential direction on the side wall of the fitted portion 12X.

[0024] The plug tip 11 is not particularly limited as long as it can perforate steel pipe material and has a fitting projection 11X as described above, but to enable more precise perforation, it is preferable that the perforation portion LF, which performs perforation other than the fitting projection 11X, has a tapered shape in which the cross-section gradually decreases toward the tip in the direction of perforation. The perforation portion LF comes into contact with the billet B.

[0025] The rolling portion 12 can expand the diameter of the steel pipe material drilled by the plug tip portion 11, and as described above, it is not particularly limited as long as it has a fitting portion 12X, but the portion that expands the diameter of the steel pipe material other than the fitting portion 12X may have a reeling portion that is partially formed in a columnar (cylindrical) shape to smooth the inner surface of the steel pipe material. Alternatively, it may be formed in a shape in which the cross-section gradually decreases toward the plug tip portion 11 side (drilling direction side) in order to expand the diameter of the steel pipe material.

[0026] Furthermore, the plug 10A may have a parallel portion 13 formed in a columnar (cylindrical) shape.

[0027] Before describing the modified plugs of this embodiment shown in Figures 6-8, we will now compare the configuration of the plug of this embodiment with that of related technologies by referring to Figures 9-11. Figures 9-11 are diagrams illustrating the configuration of related technologies. The plug 110E shown in Figure 9 has a plug tip and a rolled portion that are integrally formed, and the plug tip and the rolled portion cannot be detached. In this plug 110E, because the plug tip and the rolled portion cannot be detached, the tip may melt or deform during drilling and rolling, making it impossible to use the plug in subsequent drilling and rolling operations, and requiring the entire plug to be replaced.

[0028] The plug 110F shown in Figure 10 does not have a protrusion-receiving structure at the joint between the plug tip 11' and the rolled portion 12', and the fitting projection of the plug tip 11' is fitted into the fitting portion of the rolled portion 12'. With this plug 110F, the plug tip 11' is prone to detaching from the rolled portion 12' during drilling and rolling, making it difficult to continue drilling and rolling. Also, when the plug tip 11' and the rolled portion 12' are joined, no gap is formed between the plug tip 11' and the rolled portion 12', so when the joint becomes hot during drilling and rolling, diffusion bonding may occur, potentially preventing the plug tip 11' and the rolled portion 12' from detaching.

[0029] The plug 110G shown in Figure 11, like the plug 110F, does not have a protrusion-receiving structure at the joint between the plug tip 11'' and the rolled portion 12'', and the fitting projection of the plug tip 11'' is fitted into the mating portion of the rolled portion 12''. In the plug 110G, the fitting projection is formed in a tapered shape, specifically, the cross-section of the fitting projection perpendicular to the drilling direction gradually increases toward the rolled portion 12'' side. In this plug 110G, because the fitting projection has a tapered shape, the plug tip does not detach from the rolled portion during drilling and rolling, but even if the plug tip needs to be replaced after drilling and rolling, it cannot be easily detached. Also, in order to firmly fasten the plug tip 11'' to the rolled portion 12'', the plug tip 11'' is prone to deterioration due to the load during drilling and rolling. Furthermore, the fitting projection of the plug tip 11'' has a so-called reverse taper shape, where the diameter decreases towards the plug tip (towards the drilling direction). In other words, the cross-section of the fitting projection decreases towards the base of the plug tip 11'', so there is a possibility that it may break near the base during drilling and rolling.

[0030] In this regard, in the plug 10A of this embodiment, as described above, at the joint U1 between the plug tip 11 and the rolled portion 12, a protrusion 11P is formed on the side wall surface of the fitting projection 11X of the plug tip 11 (see Figure 3 again). Furthermore, the fitted portion 12X of the rolled portion 12 has a protrusion housing portion 12P that allows the protrusion 11P to rotate freely in the circumferential direction while also enabling the protrusion 11P to be locked. The position of the protrusion 11P on the fitting projection 11X in the drilling direction is not particularly limited, but as described later, it is preferable that it be formed at the rear end in the drilling direction.

[0031] Here, with reference to Figure 3 and further to Figure 4, we will explain the specific shapes of the mating projection 11X and the mated portion 12X. Figure 4 is an exploded perspective view of the plug 10A. As shown in Figure 4, the protrusion 11P is formed at the rear end of the fitting projection 11X in the drilling direction, and the fitting projection 11X may have a first axial portion 11A and a second axial portion 11B on which the protrusion 11P is formed, in the opposite direction to the drilling direction. The first axial portion 11A is connected to the rear end of the drilling portion LF in the drilling direction and may have a shape that extends toward the rear end in the drilling direction. The second axial portion 11B is formed at the rear end of the first axial portion 11A in the drilling direction and may have a shape that extends toward the rear end in the drilling direction. In the example shown in Figure 4, the outer diameter of the second axial portion 11B (the outer diameter of the cylindrical portion excluding the protrusion 11P) is larger than the outer diameter of the first axial portion 11A in a cross-sectional view perpendicular to the pipe axis direction, but the example is not limited to this example, and the two outer diameters may be the same.

[0032] The fitted portion 12X has a protrusion-receiving portion 12P formed at the rear end of the fitted portion 12X in the direction of drilling, and may sequentially have a first hole portion 12A that covers the outer circumference of the first axial portion 11A and a second hole portion 12B in which the protrusion-receiving portion 12P is formed, in the opposite direction of drilling. Furthermore, as shown in Figure 4, the protrusions 11P may be formed in pairs facing each other in the circumferential direction of the second axial portion 11B. In this case, as shown in Figure 3, it is preferable that the first hole 12A of the fitted portion 12X is formed in a shape that aligns with the first axial portion 11A of the plug tip portion 11 and the second axial portion 11B on which the protrusions 11P are formed. That is, as the shape of the coupling portion U1, it is preferable that the first hole 12A has a shape that allows the second axial portion 11B on which the protrusions 11P are formed and the first axial portion 11A to be inserted until the pair of protrusions 11P are accommodated in the protrusion housing portion 12P. Specifically, as shown in Figure 3(d), it is preferable that the first hole portion 12A has a columnar shape in which a circularly protruding portion is formed when viewed in a cross-sectional view perpendicular to the drilling direction.

[0033] Furthermore, the second hole 12B has a protrusion housing portion 12P formed therein so as to accommodate the protrusion 11P so as to be rotatable in the circumferential direction and so as to be able to lock the protrusion 11P when the plug tip portion 11 and the rolled portion 12 are joined together. Because the protrusion housing portion 12P is formed in the circumferential direction of the second hole 12B, the cross section perpendicular to the drilling direction is formed to be larger than the cross section of the first hole 12A. Specifically, as shown in Figure 3(d), it is preferable that the cross section perpendicular to the drilling direction of the second hole 12B, in which the protrusion housing portion 12P is formed, is circular in shape so as to accommodate the protrusion 11P so as to be rotatable in the circumferential direction of the pipe and so as to be able to lock the protrusion 11P. That is, it is preferable that the second hole 12B in which the protrusion housing portion 12P is formed is a cylindrical cavity.

[0034] With respect to the second axial portion 11B on which the protrusion 11P is formed and the second hole portion 12B on which the protrusion housing portion 12P is formed, it is preferable that, in a cross-sectional view perpendicular to the drilling direction, the diameter φk1 of the second hole portion 12B is greater than or equal to the longest diameter φK1 of the second axial portion 11B, which is the sum of the outer diameter φC1 of the first axial portion 11A and the length B1 of the protrusion.

[0035] Now, referring to Figure 5, the process by which the plug tip 11 and the rolled portion 12 are joined in the plug 10A of this embodiment will be explained. Figure 5 is a diagram illustrating the process by which the plug tip 11 and the rolled portion 12 are joined. First, Fig. 5(a) shows a state where the plug tip portion 11 and the rolling portion 12 are separated. As shown in Fig. 5(a), the plug tip portion 11 advances in the direction of the rolling portion 12 (the opposite direction of the piercing direction, see the arrow X in Fig. 5(a)), and the fitting projection portion 11X of the plug tip portion 11 is fitted into the fitting portion 12X of the rolling portion 12 (see Fig. 5(b)). Then, as shown in Fig. 5(c), the convex portion 11P provided on the fitting projection portion 11X of the plug tip portion 11 can rotate while being accommodated in the convex portion accommodating portion 12P provided on the fitting portion 12X of the rolling portion 12 (see the arrow α in Fig. 5(c)). In this way, the convex portion 11P can rotate circumferentially in the convex portion accommodating portion 12P and can be locked, and the connection between the plug tip portion 11 and the rolling portion 12 is completed. The plug tip portion 11 is in a state of being loosely fitted without being fixed to the rolling portion 12. During piercing and rolling, the plug tip portion 11 can also rotate in accordance with the rotation of the steel pipe material such as the billet B, so the load on the plug tip portion 11 is alleviated. Also, due to the above configuration, during piercing and rolling, the plug tip portion 11 is in a state of being loosely fitted without being fixed to the rolling portion 12, so it is possible to prevent the detachment of the plug tip portion 11 from becoming impossible due to damage or seizure at the joint portion U.

[0036] LC1>lc1 ··· Equation (1-1) LK1<lk1 ··· Equation (2-1) As shown in Fig. 3, the fitting projection portion 11X may sequentially have a first axial center portion 11A and a second axial center portion 11B on which the convex portion 11P is formed on the surface in the opposite direction of the piercing direction. Further, the fitting portion 12X may sequentially have a first hole portion 12A that covers the outer periphery of the first axial center portion 11A and a second hole portion 12B in which the convex portion accommodating portion 12P is formed in the opposite direction of the piercing direction. At this time, in the joint portion U1, it is preferable that there is a gap between the outer surface of the convex portion 11P and the convex portion accommodating portion 12P even when the plug tip portion 11 and the rolling portion 12 are connected. For this purpose, it is preferable that the perforation direction length LK1 of the second axial center portion 11B (convex portion 11P) and the perforation direction length lk1 of the second hole portion 12B (convex portion accommodating portion 12P) satisfy LK1 < lk1, that is, lk1 - LK1 > 0 mm. Regarding the lower limit of lk1 - LK1, in order to prevent load and frictional heat generation due to contact with the steel pipe material, it is more preferable to satisfy lk1 - LK1 ≥ 0.5 mm, and further preferably to satisfy lk1 - LK1 ≥ 1 mm. On the other hand, if lk1 is made too large, the volume of the rolling portion 12 becomes small (because the cavity portion of the rolling portion 12 becomes large), so the plug 10A is likely to be damaged by external pressure during piercing. Therefore, it is more preferable to satisfy 10 mm ≥ lk1 - LK1, and further preferably to satisfy 3 mm ≥ lk1 - LK1.

[0037] lk1 / LK1 is preferably 1.01 or more, and more preferably 1.50 or more in order to make the plug tip portion 11 rotatable while further suppressing the load on the fitting protrusion portion 11X during rolling. On the other hand, if lk1 is made too large, the volume of the rolling portion 12 becomes small, that is, the hollow portion of the plug 10A becomes large, and there is a possibility of deformation because it cannot withstand the load (force in the piercing direction) during rolling and the force in the reduction direction (direction perpendicular to the piercing direction). Also, if LK1 is made too small, the convex portion 11P may be damaged when the plug tip portion 11 is detached or attached. From these points, lk1 / LK1 is preferably 5.00 or less, and more preferably 2.00 or less.

[0038] Also, LK1 is preferably 1 mm or more in consideration of the ease of processing of the convex portion 11P and ensuring strength.

[0039] Here, LC1 is the perforation direction length of the first axial center portion 11A, and lc1 is the perforation direction length of the first hole portion 12A. When the connection between the plug tip portion 11 and the rolling portion 12 is completed, it is preferable that the central position in the drilling direction of the second axial center portion 11B is as close as possible to the central position in the drilling direction of the concave portion 12A. Further, it is preferable that the convex portion 11P can maintain a non-contact state with the convex portion accommodating portion 12P. In order to realize this non-contact state, it is preferable to satisfy LK1 < lk1 and LC1 > lc1, that is, to satisfy LC1 - lc1 > 0 mm. More preferably, LC1 + LK1 ≤ lc1 + lk1 is satisfied. Also, in order to make the longitudinal center position of the convex portion 11P the same as the longitudinal center position of the convex portion accommodating portion 12P, it is more preferable to satisfy (lc1 + lk1 / 2) = (LC1 + LK1 / 2).

[0040] LC1 / lc1 is preferably 1.01 or more in order to prevent the strength reduction of the first axial center portion 11A due to the heat generated by the friction between the first axial center portion 11A and the first hole portion 12A during rolling and the adhesion between the first axial center portion 11A and the first hole portion 12A. Considering the operability during the detachment of the plug tip portion 11, it is more preferably 1.05 or more. On the other hand, when lc1 is too small, the convex portion 11P contacts the bottom of the convex portion accommodating portion 12P (the end portion (rear end portion) in the opposite direction to the drilling direction of the convex portion accommodating portion 12P), and heat generation and adhesion are likely to occur during rolling. Also, since lc1 is reduced, the volume of the convex portion accommodating portion 12P is increased, the processing amount is increased, and the cost of the tool is increased. From these points, LC1 / lc1 is preferably 1.50 or less, and more preferably 1.20 or less.

[0041] Also, as shown in FIGS. 3 and 4, with respect to the vertical cross-section perpendicular to the drilling direction of the fitting protrusion portion 11X, the shape of the first axial center portion 11A and the shape of the portion excluding the convex portion 11P in the second axial center portion 11B are preferably circular shapes respectively. That is, it is preferable that the first axial center portion 11A and the portion excluding the convex portion 11P in the second axial center portion 11B each have a cylindrical shape. Furthermore, regarding the cross-section of the fitted portion 12X perpendicular to the drilling direction, it is preferable that the shape of the first hole 12A has a circular projection. It is preferable that the first hole 12A has a columnar shape with this shaped portion as its base. Also, regarding the cross-section of the fitted portion 12X perpendicular to the drilling direction, it is preferable that the shape of the second hole 12B in which the convex portion housing portion 12P is formed has a circular shape. That is, it is preferable that the second hole 12B in which the convex portion housing portion 12P is formed has a cylindrical shape. In this case, it is preferable that the longest diameter φK1 in the cross section perpendicular to the drilling direction of the second axial portion 11B where the protrusion 11P is formed (in the example shown in Figures 3 and 4, the length between the outer circumferences of the pair of protrusions 11P, φK1) and the diameter φk1 of the circle in the cross section perpendicular to the drilling direction of the second hole portion 12B where the protrusion housing portion 12P is formed satisfy the following equation (3-1). Furthermore, in this case, it is preferable that the diameter φC1 of the circle in the cross section perpendicular to the drilling direction of the first axial portion 11A and the diameter φc1 of the circle in the cross section perpendicular to the drilling direction of the first hole portion 12A satisfy the following equation (4-1). φK1<φk1...Formula (3-1) φC1<φc1...Formula (4-1)

[0042] When the plug tip 11 is rotated by fitting the fitting projection 11X of the plug tip 11 into the fitted portion 12X of the rolled portion 12, it is desirable that the second axis portion 11B can rotate without being subjected to rotational load. For this reason, it is preferable that φK1 < φk1, that is, that φk1 - φK1 > 0 mm be satisfied. Regarding the lower limit, if the difference between φk1 and φK1 is too small, the vibration of the plug tip 11 during drilling may cause the outer surface of the protrusion 11P to come into contact with the inner surface of the protrusion housing 12P, placing a rotational load on the protrusion 11P and potentially damaging one of the parts of the fitting projection 11X. Furthermore, this contact may cause frictional heat generation, raising the plug temperature and reducing the deformation resistance of the plug 10A, potentially causing the plug to break from the inside. For these reasons, it is more preferable to satisfy φk1-φK1≧1mm. On the other hand, if φk1 is made too large, the distance between the outer surface of the rolled portion 12 and the inner surface of the convex portion housing portion 12P in the circumferential direction becomes small, and the plug 10A may be damaged by heat input from the billet B during drilling. For this reason, it is more preferable to satisfy 10 mm ≥ φk1 - φK1.

[0043] φk1 / φK1 is preferably 1.01 or greater, and more preferably 1.05 or greater, in order to prevent a decrease in the strength of the second axial portion 11B due to the heat generated by friction between the second axial portion 11B and the second hole portion 12B during rolling, and to prevent adhesion between the second axial portion 11B and the second hole portion 12B. On the other hand, if φk1 is too large, the wall thickness of the rolled portion 12 in the reduction direction (direction perpendicular to the drilling direction) at the location where the second hole portion 12B exists in the drilling direction will be thin, that is, the distance from the outer surface of the second hole portion 12B will be small, and the strength against the reduction stress will be insufficient, which may cause damage to the plug 10A. Furthermore, if φK1 is too small, the difference between the outer diameter of the first axial portion 11A (outer diameter of the cross section perpendicular to the drilling direction) and the outer diameter of the second axial portion 11B (outer diameter of the cross section perpendicular to the drilling direction) becomes small, which may result in insufficient locking of the protrusion 11P to the protrusion housing portion 12P. For these reasons, φk1 / φK1 is preferably 2.00 or less, and more preferably 1.50 or less.

[0044] Similarly to the above, after the plug tip 11 and the rolling portion 12 are joined, during drilling and rolling, the first axial portion 11A and the second axial portion 11B rotate in the circumferential direction. Therefore, in a cross section perpendicular to the drilling direction, it is preferable that the outer diameter φC1 of the first axial portion 11A and the diameter φc1 of the circular portion of the first hole 12A satisfy φC1 < φc1, that is, φc1 - φC1 > 0 mm. Regarding the lower limit, if φc1-φC1 is too small, there is a concern that not only will there be rotational load and frictional heat generation due to contact during drilling, but also poor handling and fit during mating. Mating work is not limited to humans or robots, but it requires a high degree of precision, and depending on the size of the plug, it can exceed 10 kg, making mating more difficult. For this reason, it is more preferable to satisfy φc1-φC1≧0.5 mm, and even more preferable to satisfy φc1-φC1≧1 mm. On the other hand, if φc1 is made too large, that is, if φc1-φC1 is made too large, the plug tip 11 will sag when fitted. If drilling and rolling is performed in a sagging state, the tip 11 will become eccentric, and loads such as bending stress will be applied, which may cause the first axial portion 11A to break. For this reason, it is preferable to satisfy 3 mm ≥ φc1-φC1, and more preferably 2 mm ≥ φc1-φC1.

[0045] The ratio of φc1 / φC1 is preferably 1.01 or greater to prevent a decrease in the strength of the first axial portion 11A due to heat generated by friction between the first axial portion 11A and the first hole portion 12A during rolling, and to prevent adhesion between the first axial portion 11A and the first hole portion 12A. Furthermore, considering the operability when attaching and detaching the plug tip portion 11, it is more preferable that the ratio of φc1 / φC1 is 1.05 or greater. On the other hand, if the gap between the first axial portion 11A and the first hole portion 12A is wide, the plug tip portion 11 will droop during operations after attachment or detachment, and the direction in which the plug tip portion 11 attempts to drill will no longer coincide with the drilling direction that the plug 10 should originally advance (they will no longer be parallel). If rolling is performed in this state, the plug tip portion 11 will become eccentric, and the first axial portion 11A will be more susceptible to damage due to bending stress. For this reason, φc1 / φC1 is preferably 1.50 or less, and more preferably 1.20 or less.

[0046] Also, in FIG. 3, the length B1 (= (φK1 - φC1) / 2) of the cross-section perpendicular to the perforation direction of the convex portion 11P and the length b1 (= (φk1 - φc1) / 2) of the cross-section perpendicular to the perforation direction of the concave convex portion accommodating portion 12P formed in the second hole portion 12B preferably satisfy B1 < b1 so that they are non-contact except for the locking portions in the convex portion 11P and the convex portion accommodating portion 12P. Also, in order to obtain the strength of the first axial center portion 11A, it is desirable to increase φC1, that is, to decrease B1. On the other hand, in order to make it difficult for the plug tip portion 11 to come off from the rolling portion 12 due to the locking of the convex portion 11P to the convex portion accommodating portion 12P, it is desirable to increase B1. From these points, B1 is preferably 1.0 to 50.0% of φK1. Also, in order to reduce the processing amount of the convex portion accommodating portion 12P, it is desirable to decrease b1. On the other hand, in order to make it difficult for the plug tip portion 11 to come off from the rolling portion 12 due to the locking of the convex portion 11P to the convex portion accommodating portion 12P, similar to B1, it is desirable to increase b1. From these points, b1 is preferably 1.0 to 50.0% of φk1.

[0047] The material of the plug tip portion 11 is not limited to steel, and the plug tip portion 11 may have, for example, a Co alloy or Mo alloy having high strength at high temperatures, or a ceramic such as silicon nitride or silicon carbide having high wear resistance. The component composition of the rolling portion 12 is not particularly limited and can be appropriately adjusted according to the material of the billet. However, in order to suppress plug deformation even in a high-temperature and high-pressure environment in the piercing mill, for example, in mass%, C: 0.25 to 0.35%, Si: 0.15 to 0.55%, Mn: 0.30 to 0.70%, P: 0.035% or less, S: 0.030% or less, Al: 0.025 to 0.045%, Ni: 1.25 to 1.75%, Cr: 0.25 to 1.00%, Mo: 0.25 to 1.00%, and a total of 1 or more selected from Nb, W, and Co: 3.00 to 5.00% are contained, and the balance is composed of Fe and unavoidable impurities. On the other hand, the material of the rolled portion 12 is not limited to steel, just like the plug tip portion 11. Furthermore, the plug tip portion 11 and the rolled portion 12 may have the same or different component compositions. For example, the component composition of the plug tip portion 11 may include a Co alloy that has high strength at high temperatures, while the component composition of the rolled portion 12 may include a different alloy such as ceramic that has high wear resistance.

[0048] (Anti-seize layer) In the plug 10A of this embodiment, it is preferable that an anti-seize layer is formed on the plug surface that comes into contact with the steel pipe material such as billet B during drilling and rolling to prevent the plug from seizing. By forming an anti-seize layer, contact between the base metal (base material) other than the anti-seize layer and the steel pipe material can be avoided, thereby further improving the lifespan of the plug 10A. In particular, it is preferable that the surface hardness of the anti-seize layer (hardness based on JIS Z2244 (2009)) is equal to or greater than the hardness of the plug base metal, and more preferably greater than the hardness of the plug base metal. Furthermore, it is preferable that the anti-seize layer containing a large amount of S and P does not adhere to the inner surface of the drilled steel pipe material such as billet B; in other words, it is preferable that the anti-seize layer does not contain a large amount of S and P. In addition, it is preferable that the anti-seize layer does not undergo a chemical reaction with the steel pipe material during drilling and rolling so as not to change the properties of the steel pipe material. Furthermore, it is preferable that the surface roughness of the anti-seize layer (roughness based on JIS B0601 (2001)) is lower than the surface roughness of the plug 10A body (the base material of the plug 10A before the anti-seize layer is formed).

[0049] Below, we will describe specific examples of anti-seize layers, such as solid lubricant layers and alloy layers.

[0050] Solid lubricant layer As described above, the present invention allows for easy attachment and detachment of the plug tip 11 and the rolled portion 12. This improves the lifespan of the plug 10A by replacing the seized plug tip 11. In this case, it is preferable that an anti-seize layer, such as a solid lubricant layer, is formed on the surface of the plug 10A that comes into contact with the steel pipe material during drilling and rolling. Of the components constituting the plug 10A, the plug tip 11 can be replaced as needed, so it is particularly preferable that an anti-seize layer, such as a solid lubricant layer, is formed on the surface of the rolling section 12. Furthermore, oxide scale formed at the joint between the plug tip 11 and the rolling section 12 (the part that comes into contact with the steel pipe material during drilling and rolling, and the contact area between the plug tip 11 and the rolling section 12) is easily peeled off, and the steel pipe material is prone to seizing at the peeled-off areas, so it is preferable that an anti-seize layer, such as a solid lubricant layer, is formed at this joint. In particular, it is preferable that an anti-seize layer, such as a solid lubricant layer, is formed at the joint on the rolling section 12 side. In this way, by forming an anti-seize layer on the surface of the plug 10A, the number of times the plug can be used before the plug tip 11 needs to be replaced can be increased, thereby improving the lifespan of the plug 10A.

[0051] Preferably, the solid lubricant layer is formed on top of the oxide scale layer formed on the plug base metal (base material). This prevents seizing by the solid lubricant layer, and even if the solid lubricant layer peels off, seizing can be prevented by the oxide scale.

[0052] The solid lubricant layer can be formed by applying a solid lubricant to the surface of the plug 10A. Examples of solid lubricants include lubricants containing graphite, lubricants containing water glass, lubricants containing iron oxide, or lubricants containing two or more of these. The solid lubricant layer preferably has a dynamic friction coefficient of 0.2 or less under rolling simulation conditions. Specifically, the coefficient of dynamic friction under simulated rolling conditions is obtained by measuring it using a test material consisting of a steel plate with the component composition of the steel pipe material used and a test material consisting of the component composition of the solid lubricant layer, based on JIS P8147 (2010). Furthermore, the method of applying the solid lubricant is not particularly limited, but examples include application by compressed spray, brush, or PVD (Physical Vapor Deposition) method. To ensure sufficient life-extending effects through the formation of a solid lubricant layer, the surface roughness of the solid lubricant layer must be lower than that of the plug base metal. Furthermore, if the thickness (film thickness) of the solid lubricant layer is too large, it becomes prone to peeling, and the coefficient of friction between the steel pipe material (such as billet B) and the plug 10 may increase. Therefore, when the surface roughness of the plug base metal is R and the thickness of the solid lubricant layer is h (mm), it is preferable that 0.8 ≤ h / R ≤ 5.0, and more preferably that 0.9 ≤ h / R ≤ 2.0. It is preferable to apply the lubricant to the plug 10 by compressed spray, brush, or PVD method, etc., such that the ratio of the surface roughness R of the plug base metal to the film thickness h of the solid lubricant layer satisfies the above range.

[0053] alloy layer The anti-seize layer may be an alloy layer. Examples of alloy layers include Mo-based alloy layers, Co-based alloy layers, Ni-based alloy layers, and W-based alloy layers. Similar to the solid lubricant layer, the alloy layer can also be improved in terms of lifespan by preventing seizing. While not particularly limited, the Mo-based alloy layer preferably contains Mo: 60-99.9% by mass. The Co-based alloy layer preferably contains Co: 43-50% by mass. The Ni-based alloy layer preferably contains Ni: 45-65% by mass. The W-based alloy layer preferably contains W: 90-97% by mass.

[0054] The alloy layer has better adhesion to the plug base metal than the solid lubricant layer. Therefore, as mentioned above in the explanation regarding the solid lubricant layer, there is no need to form oxide scale on the plug base metal surface, and plugs with a Mo-based alloy layer can have a longer lifespan than plugs with a solid lubricant layer. In addition, since the Mo-based alloy, Co-based alloy, Ni-based alloy, and W-based alloy that make up the alloy layer are different alloys from the alloys of steel pipe materials containing Fe, etc., seizure is suppressed in plug 10A.

[0055] Furthermore, regarding the mechanism of preventing seizing by the alloy layer, the Mo-based alloy layer will be explained in detail using the Mo-based alloy layer as an example. The Mo-based alloy layer, formed by surface coating by thermal spraying of the Mo-based alloy, has an intermediate layer that suppresses metal-to-metal contact between the billet B and the plug base metal. In addition, the Mo-based alloy layer has a film formed between the intermediate layer and the steel pipe material such as the billet B as the Mo sublimates at around 800°C. Since this film is a gas, it is presumed that the decrease in the hot strength of the plug 10 can be suppressed by its heat insulating effect. It is thought that the lifespan of plugs with an alloy layer such as a Mo-based alloy layer can be further improved by such a mechanism.

[0056] The Mo-based alloy layer can be formed by surface coating of the Mo-based alloy onto the surface of the plug 10A by thermal spraying. Similarly, the Co-based alloy layer, Ni-based alloy layer, and W-based alloy layer can be formed by surface coating of the Co-based alloy, Ni-based alloy, and W-based alloy onto the surface of the plug 10, in that order. Furthermore, while the conditions for thermal spraying are not particularly limited, to avoid early peeling during rolling, exposing the plug base metal and preventing seizing due to metal-to-metal contact, thermal spraying may be performed using plasma overlay welding, plasma thermal spraying, or PVD methods. It is preferable to perform thermal spraying so that the thickness of the alloy layer is 0.1 to 1.5 mm.

[0057] In the present invention, when the anti-seize layer is an alloy layer, it is preferable that the linear expansion coefficients of the plug base metal and the alloy layer are the same or approximate, as measured according to JIS Z2285 (2003) (the linear expansion coefficient of the alloy layer is 50-150% of that of the plug base metal). This prevents cracks from occurring in the plug 10A during thermal spraying when forming the alloy layer, and also prevents cracks from occurring when the temperature of the plug 10A rises during drilling and rolling.

[0058] (modified version) Next, the configuration of a modified plug of this embodiment will be described with reference to Figures 6-8. Figures 6-8 are diagrams illustrating the configuration of a modified plug of this embodiment.

[0059] The plug 10B shown in Figure 6, like the plug 10A, has a plug tip and a rolled portion, but the structure of the rolled portion 12 is the same as that of the plug 10A, so it is not shown here. In the plug 10A, the protrusions 11P are formed in pairs (two) facing each other in the circumferential direction of the second axial portion 11B, whereas in the plug 10B, only one protrusion 11P2 of the plug tip 11-2 is formed in the circumferential direction on the surface of the second axial portion 11B2. As shown in Figure 3(d), the first hole 12A of the rolled portion 12 has a pair of groove-like portions that protrude from the circular center and are formed in the circumferential direction, so as to enable the fitting of a fitting projection 11X with a pair of protrusions 11P formed thereon in a cross-sectional view perpendicular to the drilling direction. In this modified plug 10B, only one protrusion 11P2 is formed on the second axial portion 11B2, so it may have a shape in which only one of the above-mentioned groove-like portions is formed. However, as shown in Figure 3(d), it is preferable to use a rolled portion 12 in which a pair of groove-like portions are formed. If there is only one groove-shaped section, the plug tip 11-2 can only be attached and detached at one specific location by rotating it 360 degrees relative to the rolling section. In contrast, by forming a pair (two) groove-shaped sections, each groove-shaped section can accommodate one protrusion 11P2, thus providing two positions for attaching and detaching the plug tip 11-2. Thus, in this modified plug 10B, the frequency of contact between the protrusion 11P2 and the protrusion housing 12P during attachment and detachment can be further reduced, thereby reducing damage to the protrusion 11P2. Therefore, in the modified plug 10B, it is preferable to use a rolled portion 12 in which a pair of groove-shaped portions are formed, as shown in Figure 3(d). The number of grooves and protrusions 11P2 is not particularly limited as long as the plug tip 11 can be attached and detached, but as in the example above, the number of grooves may be an integer multiple of the number of protrusions 11P2.

[0060] Furthermore, compared to plug 10A, plug 10C shown in Figure 7 has a tapered interface between the plug tip 11 (perforation portion LF) and the rolling portion 12 (perforation direction tip). More specifically, this interface has a tapered shape that slopes toward the perforation direction as it moves from the fitting projection 11X (fitted portion 12X) side toward the outer circumference side. Plug 10C with such a configuration can reduce the load on the interface. In addition, when the plug tip 11 that is in contact with the steel pipe material is pushed into the rolling portion 12 during perforation rolling, it is possible to prevent the axis of the plug tip 11 in the perforation direction from shifting away from the central axis of plug 10C in the perforation direction.

[0061] Furthermore, compared to plug 10A, plug 10D shown in Figure 8 has tapered surfaces on the mating projection 11X and the mated portion 12X. More specifically, the mating projection 11X and the mated portion 12X are tapered such that the cross-sectional area of ​​the outer surface of the mating projection 11X (area of ​​the cross-section perpendicular to the drilling direction) gradually decreases as you move in the opposite direction to the drilling direction. Because plug 10D has this configuration, plug tip 11 is less likely to break even if it is subjected to load due to eccentricity or rotation. Moreover, the volume of the protrusion 11P can be reduced, thereby reducing the overall weight of plug tip 11.

[0062] Regarding plug 10 (10A, 10B, 10C, 10D), the size is not particularly limited, but the plug diameter (the maximum diameter when viewed in a cross-sectional area perpendicular to the drilling direction) may be 30 to 500 mm, and the total length of the plug (length in the drilling direction) may be 50 to 1000 mm.

[0063] The plug according to the first embodiment of the present invention has been described above. According to this embodiment, a specific key shape is adopted, and the plug tip and the rolling part are made detachable, so that the plug tip, which is prone to being subjected to load during drilling and rolling, is less likely to deteriorate (melt and / or deform). Furthermore, even if the plug tip deteriorates, only the plug tip can be easily replaced. In this way, the lifespan of the plug can be improved.

[0064] Furthermore, this embodiment also provides a piercing mill for manufacturing seamless steel pipes having this plug. Also, this embodiment provides a method for manufacturing seamless steel pipes using this plug.

[0065] [Second Embodiment]

[0066] Furthermore, the plug of the present invention is not limited to the example described in the first embodiment, as long as the plug tip and the rolled portion have a convex portion housing structure and the plug tip can rotate. For example, it is not limited to the case where a convex portion 11P is formed on the plug tip 11 and a convex portion housing portion 12P is formed on the rolled portion 12 as shown in Figure 3, etc., but a convex portion may be formed on the rolled portion and a convex portion housing portion may be formed on the plug tip.

[0067] Figure 12 is a diagram illustrating the configuration of a plug 20 according to a second embodiment of the present invention. As shown in Figure 12, the plug (plug for manufacturing seamless steel pipes) 20 of the second embodiment of the present invention is a plug for drilling and rolling a steel pipe material while rotating it in the circumferential direction, and comprises a plug tip portion 21 for drilling the steel pipe material and a rolling portion 22 that is detachably connected to the plug tip portion 21 and expands the diameter of the drilled steel pipe material, the rolling portion 22 having a fitting projection 22X at its tip in the drilling direction, the plug tip portion 21 having a fitted portion 21X that is fitted to the fitting projection 22X at its rear end in the drilling direction, the fitting projection 22X having a protrusion 22P that is formed to protrude from the side wall surface, and the fitted portion 21X has a protrusion housing portion 21P that allows the protrusion 22P to rotate in the circumferential direction and to lock the protrusion 22P.

[0068] In other words, the plug 10 (10A, B, C, D) of the first embodiment has a fitting projection 11X formed on the plug tip 11 and a fitted portion 12X formed on the rolled portion 12, whereas the plug 20 of this embodiment has a fitted portion 21X on the plug tip 21 and a fitting projection 22X on the rolled portion 22. In these configurations, the plug 20 and the plug 10 differ, but the other configurations and the functions based thereon are substantially the same. For example, as shown in FIG. 12, the plug 20 may have a parallel portion 23 formed in a columnar (cylindrical) shape. Further, with respect to the position in the punching direction where the convex portion 22P is formed in the fitting protrusion portion 22X, there is no particular limitation, but as will be described later, it is preferably formed at the tip portion in the punching direction.

[0069] As an example of the plug 20 of the present embodiment, as shown in FIG. 12, as the structure of the coupling portion U2, the convex portion 22P is formed at the tip portion in the punching direction of the fitting protrusion portion 22X, and the fitting protrusion portion 22X has, in the punching direction, a first axial center portion 22A and a second axial center portion 22B where the convex portion is formed in this order. The convex portion accommodating portion 21P is formed at the tip portion in the punching direction of the fitting portion 21X, and the fitting portion 21X has, in the punching direction, a first hole portion 21A that covers the outer periphery of the first axial center portion 22A and a second hole portion 21B where the convex portion accommodating portion 21P is formed in this order. It is preferable that the punching direction length LC2 of the first axial center portion 22A and the punching direction length lc2 of the first hole portion satisfy the following formula (1-2), and the punching direction length LK2 of the second axial center portion 22B and the punching direction length lk2 of the second hole portion satisfy the following formula (2-2). LC2>lc2 ··· Formula (1-2) LK2<lk2 ··· Formula (2-2)

[0070] Regarding lK2 and LK2 shown in FIG. 12, for lk2 / LK2, in order to make the plug tip portion 21 rotatable while further suppressing the load of the fitting protrusion portion 22X during rolling, it is preferably 1.01 or more, and more preferably 1.50 or more. On the other hand, if lk2 is made too large, the volume of the plug tip portion 21 becomes small, that is, the hollow portion of the plug 20 becomes large, and there is a possibility of deformation by not being able to withstand the load (force in the punching direction) during rolling and the force in the rolling direction (direction perpendicular to the punching direction). Also, if LK2 is made too small, there is a possibility that the convex portion 22P will be damaged when the plug tip portion 11 is detached. From these points, it is preferable that lk1 / LK1 is 5.00 or less, and more preferably 2.00 or less.

[0071] Furthermore, regarding lc2 and LC2 shown in Figure 12, LC2 / lc2 is preferably 1.01 or greater in order to prevent a decrease in the strength of the first axial portion 22A due to the heat generated by friction between the first axial portion 22A and the first hole portion 21A during rolling, and to prevent adhesion between the first axial portion 22A and the first hole portion 22A. It is even more preferably 1.05 or greater in order to consider the operability when attaching and detaching the plug tip portion 21. On the other hand, if lc2 is too small, the protrusion 22P will come into contact with the bottom of the protrusion housing 21P (the tip of the protrusion housing 12P in the drilling direction), making it easier for heat generation and adhesion to occur during rolling. Also, reducing lc2 increases the volume of the protrusion housing 21P, which increases the amount of processing and increases the cost of the tool. For these reasons, LC2 / lc2 is preferably 1.50 or less, and more preferably 1.20 or less.

[0072] Furthermore, it is more preferable that LC2 + LK2 ≤ lc2 + lk2 is satisfied. Furthermore, in order to make the longitudinal center position of the protrusion 22P the same as the longitudinal center position of the protrusion housing 21P, it is more preferable to satisfy (lc2 + lk2 / 2) = (LC2 + LK2 / 2).

[0073] Furthermore, it is preferable that the longest diameter φK2 in the cross-section perpendicular to the drilling direction of the second axial portion 22B where the protrusion 22P is formed (in the example shown in Figure 12, the length between the outer circumferences of the pair of protrusions 22P, φK2) and the diameter φk2 of the circle in the cross-section perpendicular to the drilling direction of the second hole portion 21B where the protrusion housing portion 21P is formed satisfy the following equation (3-2). In addition, it is preferable that the diameter φC2 of the circle in the cross-section perpendicular to the drilling direction of the first axial portion 22A and the diameter φc2 of the circle in the cross-section perpendicular to the drilling direction of the first hole portion 21A satisfy the following equation (4-2). φK2<φk2...Formula (3-2) φC2<φc2...Formula (4-2)

[0074] Regarding φk2 and φK2 shown in Figure 12, φk2 / φK2 is preferably 1.01 or greater, and more preferably 1.05 or greater, in order to prevent a decrease in the strength of the second axial portion 22B due to the heat generated by friction between the second axial portion 22B and the second hole portion 21B during rolling, and to prevent adhesion between the second axial portion 22B and the second hole portion 21B. On the other hand, if φk2 is too large, the wall thickness of the plug tip 21 in the reduction direction (direction perpendicular to the drilling direction) at the location where the second hole 21B exists in the drilling direction becomes thin, that is, the distance from the outer surface of the second hole 21B becomes small, and the plug 20 may be damaged due to insufficient strength against the reduction stress. Furthermore, if φK2 is too small, the difference between the outer diameter of the first axial portion 22A (outer diameter of the cross section perpendicular to the drilling direction) and the outer diameter of the second axial portion 22B (outer diameter of the cross section perpendicular to the drilling direction) becomes small, which may result in insufficient locking of the protrusion 22P to the protrusion housing portion 21P. For these reasons, φk2 / φK2 is preferably 2.00 or less, and more preferably 1.50 or less.

[0075] The ratio of φc2 / φC2 is preferably 1.01 or greater to prevent a decrease in the strength of the first axial portion 22A due to heat generated by friction between the first axial portion 11A and the first hole portion 12A during rolling, and to prevent adhesion between the first axial portion 22A and the first hole portion 21A. Furthermore, considering the ease of operation when attaching and detaching the plug tip portion 21, it is more preferable that the ratio of φc2 / φC2 is 1.05 or greater. On the other hand, if the gap between the first axial portion 22A and the first hole portion 21A is wide, the plug tip portion 21 may droop during operation after attachment or detachment, causing the direction in which the plug tip portion 21 attempts to drill and the drilling direction that the plug 20 should ideally advance to no longer coincide (they will no longer be parallel). If rolling is performed in that state, the plug tip portion 21 will become eccentric, and the first axial portion 22A will be prone to damage due to bending stress. For this reason, φc2 / φC2 is preferably 1.50 or less, and more preferably 1.20 or less.

[0076] In Fig. 12, the length B2 (= (φK2 - φC2) / 2) of the cross-section perpendicular to the drilling direction of the convex portion 22P and the length b2 (= (φk2 - φc2) / 2) of the cross-section perpendicular to the drilling direction of the concave convex portion accommodating portion 21P formed in the second hole portion 21B preferably satisfy B2 < b2 in order to make the portions other than the locking portions non-contact between the convex portion 22P and the convex portion accommodating portion 21P. Also, in order to obtain the strength of the first axial center portion 22A, it is desirable to increase φC2, that is, to decrease B2. On the other hand, in order to make the plug tip portion 21 difficult to come off from the rolling portion 22 due to the locking of the convex portion 22P to the convex portion accommodating portion 21P, it is desirable to increase B2. From these points, B2 is preferably 1.0 to 50.0% of φK2. Also, in order to reduce the processing amount of the convex portion accommodating portion 21P, it is desirable to decrease b2. On the other hand, in order to make the plug tip portion 21 difficult to come off from the rolling portion 22 due to the locking of the convex portion 22P to the convex portion accommodating portion 21P, similar to B2, it is desirable to increase b2. From these points, b2 is preferably 1.0 to 50.0% of φk2.

[0077] Also, similar to the plug 10 of the first embodiment, an anti-seizure layer described above may be formed on the plug surface. Also, the material of the plug tip portion 21 may be the same as that of the plug tip portion 11, and the material of the rolling portion 22 may be the same as that of the rolling portion 12. <00​​​​​​​​​​​​​​The present invention will be described in more detail below based on examples. The present invention is not limited to the following embodiments. To confirm the effects of the present invention, plugs No. 1-4 and 8-14 of the present invention example shown in Table 1 and plugs No. 5-7 of the comparative example were prepared and the number of times they could be punctured was investigated.

[0081] In Table 1, the plug shapes are classified as A to H. Shape A is the shape of plug 10A shown in Figure 3. Shape B is the shape of plug 10B shown in Figure 6. Shape C is the shape of plug 10C shown in Figure 7. Shape D is the shape of plug 10D shown in Figure 8. Shape E is the shape of plug 110E shown in Figure 9. Shape F is the shape of plug 110F shown in Figure 10. Shape G is the shape of plug 110G shown in Figure 11. Shape H is the shape of plug 20 shown in Figure 12.

[0082] The plug 10A shown in Figure 3 is a plug for drilling and rolling a steel pipe material that rotates in the circumferential direction, and has a plug tip portion 11 for drilling the steel pipe material and a rolling portion 12 which is detachably connected to the plug tip portion 11 and expands the diameter of the drilled steel pipe material. The plug tip portion 11 has a fitting projection 11X at its rear end in the drilling direction, and the rolling portion 12 has a fitted portion 12X that is fitted to the fitting projection 11X at its front end in the drilling direction, and the fitting projection 11X has a protrusion 11P that is formed to protrude from the side wall surface, and the fitted portion 12X has a protrusion housing portion 12P that allows the protrusion 11P to rotate freely in the circumferential direction and to lock the protrusion 11P. In contrast to plug 10A, which has a pair (two) of plugs formed opposite each other in the circumferential direction on the second axial portion 11B, plug 10B, shown in Figure 6, has only one convex portion 11P2 of the plug tip 11-2 formed in the circumferential direction on the surface of the second axial portion 11B2. Compared to plug 10A, plug 10C, shown in Figure 7, has a tapered interface between the plug tip 11 and the rolled portion 12. In the plug 10D shown in Figure 8, compared to the plug 10A, the opposing surfaces of the fitting projection 11X and the fitted portion 12X are formed in a tapered shape such that the cross-sectional area of ​​the outer surface of the fitting projection 11X (area of ​​the cross-section perpendicular to the drilling direction) gradually decreases as you move in the opposite direction to the drilling direction. The plug 10E shown in Figure 9 is a plug in which the plug tip and the rolled portion are integrally formed, and the plug tip and the rolled portion cannot be detached. In the plug 110F shown in Figure 10, there is no protrusion or protrusion housing formed at the joint between the plug tip 11' and the rolled portion 12', and the fitting projection of the plug tip 11' is fitted into the fitted portion of the rolled portion 12' to connect them. The plug 110G shown in Figure 11, like the plug 110F, does not have a protrusion or protrusion housing formed at the joint between the plug tip 11'' and the rolled portion 12''. Instead, the fitting projection of the plug tip 11'' is fitted into the mating portion of the reeling portion 12'', and the fitting projection is formed in a tapered shape. Compared to plug 10, which has a fitting projection 11X formed on the plug tip 11 and a fitted portion 12X formed on the rolled portion 12, plug 20, shown in Figure 12, has a fitted portion 21X on the plug tip 21 and a fitting projection 22X on the rolled portion 22.

[0083] Each plug has a diameter of Φ49mm and a total length of 130mm.

[0084] For each plug, SKD61 hot work tool steel was used for both the plug tip and the rolled portion. Plugs that were manufactured with the plug tip and rolled portion separated were then integrated. For plugs No. 1 to 14, each equipped with both a plug tip and a rolled portion, the plug material was held in an atmospheric furnace at 950°C for 4 hours, then cooled to room temperature, and subsequently heated to create an oxide scale with a thickness of 0.4 to 0.7 mm on its surface.

[0085] Furthermore, plug No. 9 has a solid lubricant layer formed on its surface as an anti-seize layer. To form the solid lubricant layer, first, a solid lubricant containing 30% by mass of graphite was heated to 60°C to reduce its viscosity and make it easier to apply to the plug surface. Then, this solid lubricant was applied to the plug surface with a brush to form the solid lubricant layer. The coefficient of dynamic friction under simulated rolling conditions was 0.02, and the ratio h / R between the thickness h (mm) of the solid lubricant layer and the surface roughness R of the plug base metal was 2.0.

[0086] Furthermore, for plug No. 10, a Mo-based alloy layer was formed on the plug surface as an anti-seize layer. The Mo-based alloy layer was formed by spraying a Mo-based (TZM) alloy, consisting of Mo: 98.5%, Ti: 0.5%, and Zr: 1.0% by mass, onto the plug surface using a plasma spraying method. The thickness of the alloy layer was 0.8 mm. Regarding the coefficient of linear expansion measured in the plug base metal and the Mo-based alloy layer according to JIS Z2285 (2003), it was confirmed that the coefficient of linear expansion of the Mo-based alloy layer was 55% of that of the plug base metal. Furthermore, it was confirmed that the surface hardness of the Mo-based alloy layer (hardness according to JIS Z2244 (2009)) was greater than that of the plug base metal.

[0087] In Table 1, LC / lc, lk / LK, φc / φC, and φk / φK correspond to LC1 / lc1, lk1 / LK1, φc1 / φC1, and φk1 / φK1, respectively, as explained with reference to Figure 3, for plugs No. 1-4, 6, 7, and 9-14. Also, LC / lc, lk / LK, φc / φC, and φk / φK correspond to LC2 / lc2, lk2 / LK2, φc2 / φC2, and φk2 / φK2, respectively, as explained with reference to Figure 12, for plug No. 8.

[0088] The billet material used in the evaluation experiment of this embodiment was SUS420J2 (a steel with a composition consisting of C:0.3%, Si:1.0%, Mn:1.0%, Cr:13% by mass, with the remainder being Fe), and the billet dimensions were φ58mm × 250Lmm.

[0089] The billet was heated at an ambient temperature of 1280°C for 1 hour, after which holes were drilled using each plug. The drilling conditions are as follows: The gap between rolls (distance from the bottom dead center of the roll) was 52.5 mm, the roll rotation speed was 90 rpm, the shoe spacing (SH) was 65 mm, the shoe rotation speed was 1 rpm, and the lead was 15 mm (see Figure 2 for SH).

[0090] Because the spark plugs become hot after drilling due to friction and heat conduction from the billet, they were reused only after the surface temperature of the plug had dropped below 30 degrees Celsius as measured by a contact thermometer. The lifespan of the spark plugs was compared when they were repeatedly used under the above conditions.

[0091] In Table 1, plug life was determined by 3D shape measurement. Specifically, the surface temperature of the plug after rolling was reduced to 30°C or below by water cooling or air cooling, and measurements were taken using plugs that had been left in an environment with an ambient temperature of 25°C for at least one day. The shape of the plug was measured using the ATOS Compact Scan from gom. In this context, plug life refers to the number of times a plug is used (total number of uses in Table 1) until the volume of each plug deforms by more than 10% relative to its volume before the first drilling and rolling, or until the billet stops drilling midway and no part of it becomes a raw tube. In this invention, plugs with a lifespan of 7 cycles or more were deemed acceptable. However, for detachable plugs, even if the plug tip was deformed by more than 10%, if it was still detachable, it was not considered the end of the plug's lifespan, and the plug tip was replaced and drilling continued. The end of the plug's lifespan was defined as when the plug tip (excluding the mating projection or mated part) became deformed, making detachment impossible (tip deformation in Table 1), or when the rolled part 12 was deformed, resulting in more than 10% deformation of the plug (rolled part deformation in Table 1), or when the plug tip or rolled part was undamaged but the mating projection or mated part was damaged, making it impossible to detach the plug tip again (damage to mating projection or mated part in Table 1). The total number of plug uses was calculated by adding up the number of times each plug was used.

[0092] Table 1 shows the various results. In Table 1, the total number of times the plug was used (total number of uses in Table 1) and the number of times the plug tip was replaced (number of tip replacements in Table 1) are listed, and a checkmark (✓) is placed next to the item that was determined to be the factor in the final plug lifespan.

[0093] [Table 1]

[0094] As shown in Table 1, it has become clear that a long lifespan can be obtained by using the plug of the present invention. In example No. 1 of the present invention, the plug tip 11 could be replaced five times before the rolled portion 12 deformed. In addition, all other plugs in examples of the present invention were able to have their plug tips replaced two or more times, resulting in a plug life of seven or more replacements, which is an improvement over the life of conventional plug No. 5. In example No. 9 of the present invention, which has an anti-seize layer formed on it, the plug tip 11 could be replaced six times until the rolled portion 12 was deformed. In example No. 10 of the present invention, the plug tip 11 could be replaced eight times before the rolled portion 12 deformed. In example No. 10 of the present invention, the wear resistance of the rolled portion 12 was improved by the high-hardness Mo-based alloy layer, resulting in a longer lifespan, which increased the number of times the plug tip 11 could be replaced, and thus resulted in an even longer lifespan.

[0095] On the other hand, in the comparative example No. 5 of the conventional plug, the plug tip and the rolled portion were integrally formed, and the total number of times the plug was used was 5. Furthermore, in comparative example No. 6, the mating projection became hot during the drilling and rolling process and bonded to the mating portion, making it impossible to detach the plug tip, and the plug was used a total of 5 times. Furthermore, in comparative example No. 7, because the mating projection and mating portion lacked a structure to accommodate the protrusion, damage occurred to the mating projection and mating portion, making it impossible to attach or detach the plug tip, and the plug was used a total of 5 times. [Explanation of Symbols]

[0096] 10, 10A, 10B, 10C, 10D plugs 11, 11-2 Plug tip 11X mating protrusion 11A First axis center 11B Second axis center 11P protrusion LF perforation part 12 Rolling section 12X Mated part 12A First hole 12B Second hole 12P Convex part housing 13 Parallel section 20 plugs 21 Plug tip 21X Mated part 21A First hole 21B Second hole 21P Convex part housing 22 Rolling section 22X mating protrusion 22A First axis center 22B Second axis center 22P Convex part 23 Parallel section U1, U2 joint 2a, 2b Rolling rolls 3 Mandrels 4a, 4b Disc shoe 100 Rotary hearth type heating furnace 101 Perforated Rolling Mill 102 Mandrel Bar 103 Mandrel Mill 104 Reducer B Billet S Hollow

Claims

1. A plug for drilling and rolling a steel pipe material that rotates in the circumferential direction, The tip of the plug that drills into the steel pipe material, The tip of the plug is detachably connected to a rolling section that expands the diameter of the perforated steel pipe material, Equipped with, The tip of the plug has a fitting projection at its rear end in the direction of drilling. The rolling portion has a fitted portion at its leading edge in the drilling direction that is fitted to the fitting projection, The fitting projection has a protrusion formed on the side wall surface, A plug for manufacturing seamless steel pipes, wherein the fitting portion has a projection housing portion formed therein that allows the projection to rotate circumferentially and to lock into place.

2. A plug for drilling and rolling a steel pipe material that rotates in the circumferential direction, The tip of the plug that drills into the steel pipe material, The tip of the plug is detachably connected to a rolling section that expands the diameter of the perforated steel pipe material, Equipped with, The rolling portion has a fitting projection at its leading edge in the direction of drilling. The tip of the plug has a fitted portion at its rear end in the direction of drilling that is fitted to the fitting projection, The fitting projection has a protrusion formed on the side wall surface, A plug for manufacturing seamless steel pipes, wherein the fitting portion has a projection housing portion formed therein that allows the projection to rotate circumferentially and to lock into place.

3. The aforementioned protrusion is formed at the rear end of the fitting projection in the drilling direction, The fitting projection has, in the opposite direction to the drilling direction, a first axial portion and a second axial portion on which the protrusion is formed, in that order. The aforementioned protrusion housing portion is formed at the rear end of the fitted portion in the direction of drilling, The fitting portion has, in the opposite direction to the drilling direction, a first hole that covers the outer circumference of the first axial portion and a second hole in which the protrusion receiving portion is formed, in that order. The drilling direction length LC of the first axial portion １ and the drilling direction length lc of the first hole １ And satisfy the following equation (1-1), The drilling direction length LK of the second axial portion. １ and the drilling direction length lk of the second hole. １ The plug for manufacturing seamless steel pipes according to claim 1, wherein the plug satisfies the following formula (2-1). LC １ >lc １ ... Equation (1-1) LK １ <lk １ ・・・ Equation (2-1)

4. The aforementioned protrusion is formed at the tip of the fitting projection in the direction of drilling, The fitting projection has, in the direction of drilling, a first axial portion and a second axial portion on which the protrusion is formed, in that order. The aforementioned protrusion housing portion is formed at the tip of the fitted portion in the direction of drilling, The fitting portion has, in the direction of drilling, a first hole that covers the outer circumference of the first axial portion and a second hole in which the protrusion housing portion is formed, in that order. The drilling direction length LC of the first axial center part ２ and the drilling direction length lc of the first hole part ２ satisfy the following formula (1-2), The drilling direction length LK of the second axial portion. ２ and the drilling direction length lk of the second hole. ２ And satisfy the following equation (2-2): A plug for manufacturing seamless steel pipes according to claim 2. LC ２ > lc ２ ... Equation (1-2) LK ２ <lk ２ ... Formula (2-2)

5. A plug for manufacturing seamless steel pipes according to any one of claims 1 to 4, wherein an anti-seize layer is formed on the plug surface that comes into contact with the steel pipe material during drilling and rolling.

6. A piercing mill for manufacturing seamless steel pipes, comprising a plug for manufacturing seamless steel pipes according to any one of claims 1 to 4.

7. A method for manufacturing seamless steel pipes, comprising manufacturing a seamless steel pipe using a plug for manufacturing seamless steel pipes according to any one of claims 1 to 4.

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