Plug for manufacturing seamless steel pipe, piercing mill for manufacturing seamless steel pipe, and manufacturing method of seamless steel pipe

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 producing a seamless steel pipe by the Mannesmann piercing-mandrel mill system and a piercing mill for producing a seamless steel pipe equipped with the plug, and more specifically, to a plug whose life is improved by replacing only the tip portion of the plug, which is a rate-limiting factor of the plug life, a piercing mill equipped with the plug, and a method for producing a seamless steel pipe. [Background technology]

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

[0003] However, as the hollow becomes longer, the time that the plug stays in the billet becomes longer (for example, 15 to 30 seconds), and the time that the plug is exposed to high temperatures becomes longer. Furthermore, during piercing and rolling to turn the billet from a solid to a blank pipe, the plug is always in contact with the newly formed surface of the billet. In this case, when a conventional plug is used in a high-temperature, high-pressure environment, its tip may melt or deform, and it may become unusable for piercing-rolling thereafter. For example, in piercing-rolling of high-alloy steel having a Cr content of 5 mass % or more and high hot deformation resistance, one plug can only be used for piercing-rolling a few to 10-odd billets.

[0004] Such a shortened life of the plug increases the unit cost of the plug, which ultimately leads to an increase in the pipe manufacturing cost. Therefore, there is a demand for improving the life of the plug.

[0005] To date, various proposals have been made to specialize the shape and material of the plug with the aim of improving the life of the plug. For example, Patent Document 1 discloses a plug made of ceramic material. In addition, taking into consideration the melting damage of the tip of the plug, a proposal has been made to make the melted tip non-ferrous (for example, see Patent Document 2). On the other hand, in consideration of keeping the cost of plugs down and other factors, there has been a desire in recent years to improve the life of plugs by improving the shape of the plug without improving the material, and Patent Documents 3 to 5 cite techniques for making the tip of the plug detachable. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2001-87804 A [Patent Document 2] JP 2000-167606 A [Patent Document 3] International Publication No. 2005 / 087401 [Patent Document 4] International Publication No. 2014 / 030593 [Patent Document 5] JP 2021-164958 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, even if the front end of the plug is made detachable as in the technologies described in Patent Documents 3 and 4, the state of connection between the front end portion of the plug and the reeling portion cannot be said to be sufficient for improving the life of the plug. Specifically, with the conventional technologies, if the state of connection is made strong, there is a concern that the connected portion may be damaged during piercing-rolling, while if the state of connection is weakened, there is a concern that the front end portion of the plug may detach from the reeling portion during piercing-rolling, and it has not been possible to find an optimal state of connection, and therefore it has not been possible to obtain a sufficient life.

[0008] Furthermore, in the technique described in Patent Document 5, it takes time to manufacture the threaded and non-threaded portions, and there was a demand for the establishment of other techniques. As a result, there was a demand for the establishment of new plug technology that would make the plug tip detachable.

[0009] The present invention has been made to solve the problems of the prior art, and has an object to provide a plug for producing a seamless steel pipe having an improved life, a piercing mill for producing a seamless steel pipe equipped with the plug for producing a seamless steel pipe, and a method for producing a seamless steel pipe. [Means for solving the problem]

[0010] As a result of intensive investigations into solving the above-mentioned problems, the inventors focused on coupling the front edge portion of the plug and the rolled portion by a specific key shape as a mechanism which suppresses detachment of the front edge portion of the plug from the rolled portion and damage to the fitting protrusion portion and the fitted portion and impossibility of detachment due to wear during piercing-rolling, suppresses deterioration of the front edge portion of the plug, and enables the front edge portion of the plug to be easily detached from the rolled portion after piercing-rolling.

[0011] Specifically, the inventors discovered that the plug front end portion is formed with a fitting protrusion at its rear end in the piercing direction, and that the fitting protrusion portion has one or more convex portions (key portions) formed on its surface. They also discovered that the rolled portion is formed with a fitted portion, and that the fitted portion has a convex portion accommodating portion that allows the convex portion to rotate freely in the circumferential direction (the steel pipe circumferential direction) and enables the convex portion to be locked. Furthermore, it was found that it is preferable for the rolled portion to have a groove-shaped portion in the first hole portion formed on the drilling direction side of the mating portion, through which the convex portion of the mating protrusion portion (second axial portion) can be inserted, in order to easily attach and detach the mating protrusion portion. In this case, it has been found that the number of grooves conforming to the shape of the protrusions (keys) need only be equal to or greater than the number of protrusions (keys), and does not necessarily have to be the same. In addition, the inventors also found that, in order to allow the plug front end portion having the convex portion to be freely rotatable in the circumferential direction when the plug front end portion and the rolled portion are joined (connected), it is preferable that the convex portion accommodating portion has a columnar shape with dimensions capable of accommodating a rotating body that can be formed by 360° rotation of the convex portion (key portion). As described above, by providing a specific protrusion-accommodating structure in the front edge portion and the rolled portion of plug, it is possible to improve the life of plug, which has led to the completion of the present invention.

[0012] Based on the above findings and further investigation, the present invention has been completed as follows. [1] A plug for piercing and rolling a steel pipe material rotating in a circumferential direction, a plug tip portion for drilling a steel pipe material; a rolling section that is detachably connected to the plug front end section and expands the diameter of the pierced steel pipe material; Equipped with the plug front end portion has a fitting protrusion portion at a rear end portion in a drilling direction, The rolling portion has a fitted portion fitted to the fitting protrusion portion at a tip end in a drilling direction, The fitting protrusion has a protrusion formed on a side wall surface, a convex portion accommodating portion is formed in the fitted portion, which allows the convex portion to rotate freely in the circumferential direction and enables the convex portion to be engaged. [2] A plug for piercing and rolling a steel pipe material rotating in a circumferential direction, a plug tip portion for drilling a steel pipe material; a rolling section that is detachably connected to the plug front end section and expands the diameter of the pierced steel pipe material; Equipped with The rolling portion has a fitting protrusion at a tip end in a drilling direction, the plug front end portion has a fitted portion fitted to the fitting protrusion portion at a rear end portion in a drilling direction, The fitting protrusion has a protrusion formed on a side wall surface, a convex portion accommodating portion is formed in the fitted portion, which allows the convex portion to rotate freely in the circumferential direction and enables the convex portion to be engaged. [3] The convex portion is formed at the rear end portion in the drilling direction of the fitting projection portion, The fitting projection portion has, in the direction opposite to the drilling direction, a first axial center portion and a second axial center portion where the convex portion is formed, in this order, The convex portion accommodating portion is formed at the rear end portion in the drilling direction of the fitted portion, The fitted portion has, in the direction opposite to 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 this order, The drilling direction length LC1 of the first axial center portion and the drilling direction length lc1 of the first hole portion satisfy the following formula (1-1), The drilling direction length LK1 of the second axial center portion and the drilling direction length lk1 of the second hole portion satisfy the following formula (2-1), the plug for seamless steel pipe manufacturing according to the above [1]. LC1>lc1 ··· Formula (1-1) LK1<lk1 ··· Formula (2-1) [4] The convex portion is formed at the tip portion 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 this order, The convex portion accommodating portion is formed at the tip portion in the drilling direction of the fitted portion, The fitted portion has, in 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 this order, The drilling direction length LC2 of the first axial center portion and the drilling direction length lc2 of the first hole portion satisfy the following formula (1-2), The drilling direction length LK2 of the second axial center portion and the drilling direction length lk2 of the second hole portion satisfy the following formula (2-2), the plug for seamless steel pipe manufacturing according to the above [2]. LC2>lc2 ··· Formula (1-2) LK2<lk2 ··· Formula (2-2) [5] An anti-seizure layer is formed on the plug surface that contacts the steel pipe material during piercing rolling, the plug for seamless steel pipe manufacturing according to any one of the above [1] to [4]. [6] A piercing mill for producing a seamless steel pipe, comprising the plug for producing a seamless steel pipe according to any one of [1] to [4] above. [7] A method for producing a seamless steel pipe, comprising producing a seamless steel pipe by using a plug for producing a seamless steel pipe according to any one of [1] to [4] above. Effect of the Invention

[0013] According to the present invention, there are provided a plug for producing a seamless steel pipe having an improved life, a piercing mill for producing a seamless steel pipe provided with the plug for producing a seamless steel pipe, and a method for producing a seamless steel pipe. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram for explaining a method for producing a seamless steel pipe. [Diagram 2] FIG. 2 is a diagram showing a schematic configuration of a piercing-rolling mill. [Diagram 3] FIG. 3 is a diagram for explaining the configuration of the plug according to the first embodiment of the present invention. [Figure 4] FIG. 4 is an exploded perspective view of the plug. [Diagram 5] FIG. 5 is a diagram for explaining the process in which the front end portion of the plug and the rolled portion are joined together. [Figure 6] FIG. 6 is a diagram for explaining the configuration of a plug according to a modified example of the present invention. [Figure 7] FIG. 7 is a diagram for explaining the configuration of a plug according to a modified example of the present invention. [Figure 8] FIG. 8 is a diagram for explaining the configuration of a plug according to a modified example of the present invention. [Figure 9] FIG. 9 is a diagram for explaining the configuration of a plug according to a related art (comparative example). [Figure 10] FIG. 10 is a diagram for explaining the configuration of a plug according to a related art (comparative example). [Figure 11] FIG. 11 is a diagram for explaining the configuration of a plug according to a related art (comparative example). [Figure 12]FIG. 12 is a diagram for explaining the configuration of a plug according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] <Manufacturing method for seamless steel pipes> The present invention will be described with reference to the drawings. Note that the present invention is not limited to the embodiments.

[0016] Before describing the configuration and functions of the plug for producing a seamless steel pipe of the present invention in detail, a method for producing a seamless steel pipe by a Mannesmann piercing-mandrel mill system using the plug for producing a seamless steel pipe will be described with reference to Fig. 1. Fig. 1 is a schematic diagram for explaining the method for producing a seamless steel pipe.

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

[0018] Next, as shown in Fig. 1(d), a mandrel bar 102 is inserted in a skewer-like shape into the inner surface of the hollow S, and elongation rolling is performed while restraining the outer surface of the hollow S with grooved rolling rolls of a mandrel mill 103 consisting of 5 to 8 stands, thereby reducing the wall thickness to a predetermined thickness to obtain a mother tube. Thereafter, the mandrel bar 102 is removed from the mother tube, and as shown in Fig. 1(e), the reduced wall thickness of the mother tube is sizing rolled to a predetermined outer diameter by a reducer 104 to obtain a product (seamless steel tube X).

[0019] Here, the above-mentioned piercing and rolling will be described in more detail. Fig. 2 is a diagram showing a schematic configuration of a piercing and rolling mill 101, in which Fig. 2(a) shows a side view and Fig. 2(b) shows a plan view. Note that Fig. 2(b) omits the illustration of a plug 10. As shown in Fig. 2, the piercing mill 101 includes a pair of rolls 2a, 2b arranged with their rotation axes inclined to each other, disk shoes 4a, 4b arranged at a position rotated 90° relative to the rolls 2a, 2b on a plane perpendicular to the rolling direction, and a plug 10 whose rear end is supported by a mandrel 3. The pair of rolls 2a, 2b can be arranged with their rotation axes inclined by an inclination angle FA in opposite directions in a plan view about the rolling direction, and can rotate in the same direction. The plug 10 is arranged between the pair of rolls 2a, 2b and the disk shoes 4a, 4b, and pierces the billet B.

[0020] In order to pierce and roll a solid billet B (round billet) using the piercing mill 101, the billet B is first fed between a pair of rolls 2a and 2b. After the billet B is bitten between the pair of rolls 2a and 2b, a rotating force and a forward force in the axial direction (rolling direction) act simultaneously on the billet B due to the frictional force of the rolls 2a and 2b. Then, by the time the billet B reaches the tip of the plug 10, compressive stress and tensile stress are alternately and continuously applied to the center of the billet B by the rolls 2a and 2b (rotary forging effect), making it easy to form a hole. After that, when the billet B collides with the plug 10, a hole is opened in the center of the billet B, and thereafter, the billet B is subjected to a thinning process every half rotation between the rolls 2a and 2b and the plug 10, and a hollow S with a predetermined thickness and outer shape is obtained.

[0021] <Plugs 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 Fig. 3. Fig. 3 is a diagram for explaining the configuration of the plug of the first embodiment of the present invention. In Fig. 3, (a) is an overall view of the plug 10 (10A) of this embodiment, in Fig. 3, (b) is an enlarged view of the plug front end portion 11, and in Fig. 3, (c) is an enlarged view of the vicinity of the joint portion U1 between the plug front end portion 11 and the rolled portion 12. In addition, Fig. 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 convex portion accommodating portion 12P in a cross section perpendicular to the piercing direction.

[0022] The plug 10A of the present embodiment is a plug for producing a seamless steel pipe used in the piercing-rolling mill 101 as described above, and has a plug front end portion 11 and a rolling portion 12 that are detachably connected to each other. Specifically, as shown in Fig. 3(a), the plug 10A of the present embodiment is a plug for piercing-rolling a steel pipe material such as a billet B that rotates in the circumferential direction, and has a plug front end portion 11 that pierces the steel pipe material, and a rolling portion 12 that is detachably connected to the plug front end portion 11 and expands the diameter of the pierced steel pipe material. In addition, in FIG. 3(b), the plug front end portion 11 has a fitting protrusion portion 11X at its rear end in the drilling direction, and as shown in (c), the rolled portion 12 has a fitted portion 12X fitted into the fitting protrusion portion 11X at its front end in the drilling direction. Moreover, the fitting protrusion 11X has a convex portion 11P formed to protrude from a side wall surface, and the fitted portion 12X is formed with a convex portion accommodating portion 12P that allows the convex portion 11P to be engaged while allowing the convex portion 11P to rotate freely in the circumferential direction. The plug 10A of the present embodiment is thus characterized in that it has a convex portion accommodating structure in which the plug front portion 11 and the rolled portion 12 are coupled together with the convex portion 11P accommodated in the convex portion accommodating portion 12P.

[0023] The shape of the convex portion 11P is not particularly limited as long as the convex portion 11P is formed to protrude from the side wall surface of the mating protrusion portion 11X, and can freely rotate circumferentially within the convex portion accommodating portion 12P when the plug tip portion 11 and the rolled portion 12 are coupled (connected), and can be engaged with the convex portion accommodating portion 12P. In addition, the shape of the protrusion accommodating portion 12P is not particularly limited as long as the protrusion 11P can be locked while being rotatable in the circumferential direction in a state in which the plug front end portion 11 and the rolled portion 12 are joined (connected) to each other. The protrusion accommodating portion 12P may be a recessed portion formed in the circumferential direction on the side wall of the fitted portion 12X.

[0024] The plug front end portion 11 is not particularly limited as long as it can pierce a steel pipe material and has the fitting protrusion portion 11X as described above, but in order to perform piercing with higher accuracy, it is preferable that the piercing portion LF that pierces holes other than the fitting protrusion portion 11X has a tapered shape whose cross section gradually becomes smaller toward the front end in the piercing direction. The piercing portion LF comes into contact with the billet B.

[0025] The rolling section 12 is not particularly limited as long as it can expand the diameter of the steel pipe material pierced by the plug tip section 11 and has the fitted section 12X as described above, but the portion where the diameter of the steel pipe material is expanded other than the fitted section 12X may have, for example, a reeling section partially formed in a columnar (cylindrical) shape so as to smooth the inner surface of the steel pipe material. Also, the rolling section 12 may be partially formed in a shape whose cross section gradually becomes smaller toward the plug tip section 11 side (the piercing direction side) so as to be able to expand the diameter of the steel pipe material.

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

[0027] Before describing the plug of the modified example of this embodiment shown in Figures 6 to 8, the configuration of the plug of the related art will be compared with that of this embodiment with reference to Figures 9 to 11. Figures 9 to 11 are diagrams for explaining the configuration of the plug of the related art. 9, the front edge portion of the plug and the rolled portion are integrally formed, and the front edge portion of the plug and the rolled portion cannot be detached from each other. Since the front edge portion of the plug and the rolled portion cannot be detached from each other in this plug 110E, if the front edge of the plug is melted or deformed by piercing-rolling, it will become impossible to use the plug in the subsequent piercing-rolling, and the entire plug will have to be replaced.

[0028] A plug 110F shown in Fig. 10 does not have a convex portion accommodating structure at the joint between the front edge portion 11' of the plug and the rolled portion 12', and the fitting protrusion of the front edge portion 11' of the plug is fitted into the fitted portion of the rolled portion 12'. In this plug 110F, the front edge portion 11' of the plug is likely to detach from the rolled portion 12' during piercing-rolling, making it difficult to continue piercing-rolling. In addition, when the front edge portion 11' of the plug and the rolled portion 12' are joined, no gap is formed between the front edge portion 11' of the plug and the rolled portion 12'. Therefore, when the joint portion becomes hot during piercing-rolling, diffusion bonding occurs, and there is a possibility that the front edge portion 11' of the plug and the rolled portion 12' cannot be detached.

[0029] Like the plug 110F, a plug 110G shown in FIG. 11 does not have a convex portion accommodating structure at the joint between the plug front end portion 11'' and the rolled portion 12'', and the fitting protrusion of the plug front end portion 11'' is fitted into the fitted portion of the rolled portion 12''. In the plug 110G, the fitting protrusion is formed in a tapered shape, and specifically, the cross section perpendicular to the piercing direction of the fitting protrusion becomes gradually larger toward the rolled portion 12'' side. In this plug 110G, since the fitting protrusion has a tapered shape, the plug front end portion does not detach from the rolled portion during piercing-rolling, but even if the plug front end portion needs to be replaced after piercing-rolling, the plug front end portion cannot be easily detached. In addition, in order to firmly fasten the plug front end portion 11'' to the rolled portion 12'', the plug front end portion 11'' is easily deteriorated by the load during piercing-rolling. Moreover, the fitting projection of the plug front end portion 11'' has a so-called inverse tapered shape in which the diameter becomes smaller toward the front end of the plug (the piercing direction side). In other words, the fitting projection has a cross-section becoming smaller toward the root side of the plug front end portion 11''. Therefore, there is a possibility that the fitting projection will break near the root portion during piercing-rolling.

[0030] In this regard, in the plug 10A of the present embodiment, as described above, at the joint U1 between the plug front end portion 11 and the rolled portion 12, the fitting protrusion portion 11X of the plug front end portion 11 has a convex portion 11P formed to protrude from the side wall surface (see FIG. 3 again). Also, the fitted portion 12X of the rolled portion 12 has a convex portion accommodating portion 12P that allows the convex portion 11P to be locked while allowing the convex portion 11P to rotate freely in the circumferential direction. Note that, in the fitting protrusion portion 11X, the position in the drilling direction at which the convex portion 11P is formed is not particularly limited, but it is preferable that it is formed at the rear end in the drilling direction, as described later.

[0031] Here, specific shapes of the fitting protrusion 11X and the fitted portion 12X will be described with reference to Fig. 4 in addition to Fig. 3. Fig. 4 is an exploded perspective view of the plug 10A. As shown in FIG. 4, the convex portion 11P is formed at the rear end of the fitting protrusion 11X in the drilling direction, and the fitting protrusion 11X may have a first axial center portion 11A and a second axial center portion 11B in which the convex portion 11P is formed, in the opposite direction to the drilling direction. The first axial center portion 11A is connected to the rear end of the drilling portion LF in the drilling direction, and may have a shape extending toward the rear end side in the drilling direction. The second axial center portion 11B is formed at the rear end of the first axial center portion 11A in the drilling direction, and may have a shape extending toward the rear end side in the drilling direction. In the example shown in FIG. 4, the outer diameter of the second axial center portion 11B (the outer diameter of the cylindrical portion excluding the convex portion 11P) is larger than the outer diameter of the first axial center portion 11A in a cross section perpendicular to the tube axis direction, but this is not limited to this example, and the two outer diameters may be the same.

[0032] The fitted portion 12X may have a convex accommodating portion 12P formed at the rear end in the drilling direction of the fitted portion 12X, and may have, in the opposite direction to the drilling direction, a first hole portion 12A covering the outer periphery of the first axial portion 11A, and a second hole portion 12B in which the convex accommodating portion 12P is formed, in that order. Also, as shown in Fig. 4, a pair of the convex portions 11P may be formed facing each other in the circumferential direction of the second axial center portion 11B. In this case, as shown in Fig. 3, the first hole portion 12A of the fitted portion 12X is preferably formed in a shape that follows the first axial center portion 11A of the plug tip portion 11 and the second axial center portion 11B on which the convex portions 11P are formed. That is, as the shape of the coupling portion U1, the first hole portion 12A is preferably shaped so that the second axial center portion 11B on which the convex portions 11P are formed and the first axial center portion 11A can be inserted until the pair of convex portions 11P are accommodated in the convex portion accommodating portion 12P. Specifically, as shown in FIG. 3(d), it is preferable that the first hole portion 12A has a columnar shape with a circular protruding portion formed therein when viewed in a cross section perpendicular to the drilling direction.

[0033] In addition, the second hole portion 12B is formed with a convex portion accommodating portion 12P so that the convex portion 11P can be rotatably accommodated in the circumferential direction and can be locked when the plug front end portion 11 and the rolled portion 12 are joined. Since the convex portion accommodating portion 12P is formed in the circumferential direction, the second hole portion 12B is formed with a larger cross section perpendicular to the drilling direction than the cross section of the first hole portion 12A. Specifically, as shown in FIG. 3(d), the second hole portion 12B in which the convex portion accommodating portion 12P is formed is preferably circular in cross section perpendicular to the drilling direction so that the convex portion 11P can be rotatably accommodated in the circumferential direction of the pipe and can be locked. That is, the second hole portion 12B in which the convex portion accommodating portion 12P is preferably a cylindrical cavity.

[0034] With regard to the second axial portion 11B in which the convex portion 11P is formed and the second hole portion 12B in which the convex portion accommodating portion 12P is formed, when viewed in a cross section perpendicular to the drilling direction, it is preferable that 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 obtained by adding the outer diameter φC1 of the first axial portion 11A to the length B1 of the convex portion.

[0035] Here, a process in which the front edge portion 11 of the plug and the rolled portion 12 of the plug 10A of the present embodiment are bonded to each other will be described with reference to Fig. 5. Fig. 5 is a view for explaining the process in which the front edge portion 11 of the plug and the rolled portion 12 are bonded to each other. First, FIG. 5(a) shows a state where the plug tip 11 and the rolling part 12 are separated. As shown in FIG. 5(a), the plug tip 11 advances in the direction of the rolling part 12 (the direction opposite to the piercing direction, see the arrow X in FIG. 5(a)), and the fitting protrusion 11X of the plug tip 11 is fitted into the fitted part 12X of the rolling part 12 (see FIG. 5(b)). Then, as shown in FIG. 5(c), the convex part 11P provided on the fitting protrusion 11X of the plug tip 11 can rotate while being accommodated in the convex part accommodation part 12P provided in the fitted part 12X of the rolling part 12 (see the arrow α in FIG. 5(c)). In this way, with the convex part 11P being rotatable in the circumferential direction and lockable in the convex part accommodation part 12P, the connection between the plug tip 11 and the rolling part 12 is completed. The plug tip 11 is in a state of being loosely fitted without being fixed to the rolling part 12. During piercing and rolling, the plug tip 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 11 is relieved. Also, due to the above configuration, during piercing and rolling, since the plug tip 11 is in a state of being loosely fitted without being fixed to the rolling part 12, it is possible to prevent the detachment of the plug tip 11 from becoming impossible due to damage or seizure at the joint U.

[0036] LC1>lc1 ··· Equation (1-1) LK1<lk1 ··· Equation (2-1) As shown in FIG. 3, the fitting protrusion 11X may sequentially have a first axial center part 11A and a second axial center part 11B on the surface of which the convex part 11P is formed in the direction opposite to the piercing direction. Also, the fitted part 12X may sequentially have a first hole part 12A that covers the outer periphery of the first axial center part 11A and a second hole part 12B in which the convex part accommodation part 12P is formed in the direction opposite to 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 housing 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 housing 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 it is even more preferable to satisfy lk1 - LK1 ≥ 1 mm. On the other hand, if lk1 is made too large, since the volume of the rolling portion 12 becomes small (since the cavity portion of the rolling portion 12 becomes large), 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 it is even more preferable 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 by being unable to 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, there is a possibility that the convex portion 11P is 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 of the second axial center portion 11B in the drilling direction is as close as possible to the central position of the concave portion 12A in the drilling direction. 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. Moreover, it is more preferable to satisfy LC1 + LK1 ≤ lc1 + lk1. 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 direction opposite 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 made small, the volume of the convex portion accommodating portion 12P increases, the processing amount increases, and the cost of the tool increases. 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, regarding 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. In addition, in a cross section perpendicular to the drilling direction of the fitted portion 12X, the shape of the first hole portion 12A is preferably a shape in which a part protruding into a circle is formed. The first hole portion 12A is preferably a columnar shape with the part of this shape as the bottom surface. In addition, in a cross section perpendicular to the drilling direction of the fitted portion 12X, the shape of the second hole portion 12B in which the convex portion accommodating portion 12P is formed is preferably a circular shape. In other words, the second hole portion 12B in which the convex portion accommodating portion 12P is formed is preferably a cylindrical shape. At this time, it is preferable that the maximum diameter φK1 (in the example shown in Figs. 3 and 4, the length φK1 between the outer peripheries of a pair of protrusions 11P) in a cross section perpendicular to the drilling direction of the second axial portion 11B in which the protrusions 11P are formed and the diameter φk1 of a cross-sectional circle perpendicular to the drilling direction of the second hole portion 12B in which the protrusion accommodating portion 12P is formed satisfy the following formula (3-1). Also, at this time, it is preferable that the diameter φC1 of the cross-sectional circle 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 formula (4-1). φK1<φk1...Formula (3-1) φC1<φc1...Formula (4-1)

[0042] When the fitting protrusion 11X of the plug front end portion 11 is fitted into the fitted portion 12X of the rolling portion 12 and the plug front end portion 11 is rotated, it is desirable that the plug front end portion 11 can rotate without applying a load in the rotational direction to the second axial center portion 11B. Therefore, it is preferable that φK1<φk1 is satisfied, that is, φk1-φK1>0 mm is satisfied. Regarding the lower limit, if the difference between φk1 and φK1 is too small, the deflection of the plug front end portion 11 during drilling may cause the outer surface of the convex portion 11P to come into contact with the inner surface of the convex portion accommodating portion 12P, and a load in the rotational direction may be applied to the convex portion 11P, which may damage any portion of the fitting protrusion portion 11X. In addition, the above-mentioned contact may cause frictional heat to be generated, which may increase the plug temperature and reduce the deformation resistance of the plug 10A, which may damage the plug from within. For these reasons, it is more preferable to satisfy φk1-φK1 ≧ 1 mm. On the other hand, if φk1 is made too large, the distance between the outer surface of the rolled portion 12 and the circumferential inner surface of the convex portion accommodating portion 12P becomes small, and there is a possibility that the plug 10A will be damaged by heat input from the billet B during piercing. Therefore, it is more preferable to satisfy 10 mm ≧ φk1 - φK1.

[0043] In order to prevent a decrease in strength of the second axial portion 11B due to 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, φk1 / φK1 is preferably 1.01 or more, and more preferably 1.05 or more. On the other hand, if φk1 is too large, the wall thickness in the rolling direction (direction perpendicular to the piercing direction) of the rolled portion 12 at the position in the piercing direction where the second hole portion 12B is present will be thin, that is, the distance from the outer surface of the second hole portion 12B will be small, and the strength will be insufficient against the rolling stress, 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 in a cross section perpendicular to the drilling direction) and the outer diameter of the second axial portion 11B (outer diameter in a cross section perpendicular to the drilling direction) becomes small, and the engagement of the protrusion 11P with the protrusion accommodating portion 12P may become insufficient. From these points of view, φk1 / φK1 is preferably 2.00 or less, and more preferably 1.50 or less.

[0044] Similarly, after completion of joining of the plug front portion 11 and the rolled portion 12, the first axial portion 11A and the second axial portion 11B rotate in the circumferential direction during piercing-rolling. 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 portion 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 the load in the rotational direction and heat generated by friction during drilling occur, but handling and mating during mating will also be impaired. The mating work is not limited to humans or robots, but requires high precision and can exceed 10 kg depending on the size of the plug, making mating even more difficult. Therefore, 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 front end portion 11 sags when it is fitted. If piercing-rolling is performed in a sagging state, the front end portion 11 becomes eccentric, and a load such as bending stress is applied, which may cause the first axial portion 11A to break. Therefore, it is preferable to satisfy 3 mm ≧ φc1-φC1, and it is more preferable to satisfy 2 mm ≧ φc1-φC1.

[0045] In order to prevent a decrease in 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 adhesion between the first axial portion 11A and the first hole portion 12A, it is preferable that φc1 / φC1 is 1.01 or more. In addition, in consideration of operability when attaching and detaching the plug front end portion 11, it is more preferable that φc1 / φC1 is 1.05 or more. On the other hand, if the gap between the first axial portion 11A and the first hole portion 12A is wide, the plug front end portion 11 will sag during operations after attachment and detachment, and the direction in which the plug front end portion 11 is about to drill a hole will not coincide with (become parallel to) the direction in which the plug 10 should drill a hole. If rolling is performed in this state, the plug front end portion 11 will become eccentric, and the first axial portion 11A will be easily damaged by 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 drilling direction of the convex portion 11P and the length b1 (= (φk1 - φc1) / 2) of the cross-section perpendicular to the drilling direction of the concave convex portion accommodating portion 12P formed in the second hole portion 12B preferably satisfy B1 < b1 so that non-contact is achieved 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 that b1 is large. 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 inevitable impurities. On the other hand, like the plug front end portion 11, the material of the rolled portion 12 is not limited to steel. Furthermore, the plug front end portion 11 and the rolled portion 12 may have the same or different chemical compositions. For example, the chemical composition of the plug front end portion 11 may contain a Co alloy having high strength at high temperatures, and the chemical composition of the rolled portion 12 may contain a dissimilar alloy such as ceramic having high wear resistance.

[0048] (Anti-seizure layer) In the plug 10A of the present embodiment, an anti-seizure layer for preventing seizure of the plug is preferably formed on the plug surface that comes into contact with the steel pipe material such as the billet B during piercing and rolling. By forming the anti-seizure layer, contact between the base steel (base material) other than the anti-seizure layer and the steel pipe material can be avoided, and the life of the plug 10A can be further improved. In particular, the surface hardness of the anti-seizure layer (hardness based on JIS Z2244 (2009)) is preferably equal to or greater than the hardness of the plug base steel, and more preferably greater than the hardness of the plug base steel. Moreover, it is preferable that the anti-seizure layer containing a large amount of S, P, etc. is not attached to the inner surface of the steel pipe material such as the billet B that has been pierced, that is, it is preferable that the anti-seizure layer does not contain a large amount of S or P. Moreover, it is preferable that the anti-seizure layer does not undergo a chemical reaction with the steel pipe material during piercing and rolling so as not to change the properties of the steel pipe material. Moreover, the surface roughness of the anti-seizure layer (roughness based on JIS B0601 (2001)) is preferably lower than the surface roughness of the body of the plug 10A (the base material of the plug 10A before the anti-seizure layer is formed).

[0049] As specific examples of the anti-seizure layer, a solid lubricant layer and an alloy layer will be described below.

[0050] Solid Lubricant Layer As described above, in the present invention, the front end portion 11 of plug can be easily detached from the rolled portion 12. As a result, the seized front end portion 11 of plug can be replaced, thereby improving the life of the plug 10A. At this time, it is preferable that an anti-seizure 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 piercing and rolling. Among the members constituting the plug 10A, the front end portion 11 of the plug can be replaced as needed, and therefore it is particularly preferable that an anti-seizure layer such as a solid lubricant layer is formed on the surface of the rolling portion 12. In addition, since the oxide scale formed at the joint between the front end portion 11 of the plug and the rolling portion 12 (the contact portion between the front end portion 11 of the plug and the rolling portion 12 at the portion that comes into contact with the steel pipe material during piercing and rolling) is easily peeled off, and the steel pipe material is easily seized at the peeled portion, it is preferable that an anti-seizure layer such as a solid lubricant layer is formed at this joint. In particular, it is preferable that an anti-seizure layer such as a solid lubricant layer is formed at the joint on the side of the rolling portion 12. In this manner, by forming an anti-seizure layer on the surface of the plug 10A, the number of times the plug can be used before the front end portion 11 of the plug needs to be replaced can be increased, and the life of the plug 10A can be improved.

[0051] The solid lubricant layer is preferably formed on top of an oxide scale layer formed on the plug base steel (base material), so that the solid lubricant layer prevents seizure, and even if the solid lubricant layer peels off, the oxide scale can prevent seizure.

[0052] The solid lubricant layer can be formed by applying a solid lubricant to the surface of the plug 10A. Examples of the solid lubricant include a lubricant containing graphite, a lubricant containing water glass, a lubricant containing iron oxide, or a lubricant containing two or more of these. The solid lubricant layer preferably has a dynamic friction coefficient of 0.2 or less under simulated rolling conditions. Specifically, the dynamic friction coefficient under simulated rolling conditions is obtained by measuring a steel plate having the composition of the steel pipe material used and a test material having the composition of the solid lubricant layer based on JIS P8147 (2010). The method of applying the solid lubricant is not particularly limited, but examples include application by compressed spray, application with a brush, and application by the PVD method. In order to fully secure the effect of improving the life due to the formation of the solid lubricant layer, it is necessary to make the roughness of the surface of the solid lubricant layer lower than the roughness of the plug base steel. Moreover, if the thickness (film thickness) of the solid lubricant layer is made too large, it becomes more likely to peel off, and there is also a possibility that the coefficient of friction between the steel pipe material such as billet B and the plug 10 will become high. Therefore, when the surface roughness of the plug base steel is R and the film thickness of the solid lubricant layer is h (mm), it is preferable that 0.8≦h / R≦5.0, and it is more preferable that 0.9≦h / R≦2.0. It is preferable to apply the lubricant to the plug 10 by compressed spray, brush, PVD method, or the like so that the ratio of the surface roughness R of the plug base steel to the film thickness h of the solid lubricant layer satisfies the above-mentioned range.

[0053] alloy layer The anti-seizure layer may be an alloy layer, such as a Mo-based alloy layer, a Co-based alloy layer, a Ni-based alloy layer, or a W-based alloy layer. As with the solid lubricant layer, the alloy layer can also prevent seizure and thereby improve the lifespan. Although not particularly limited, the Mo-based alloy layer preferably contains, by mass%, 60 to 99.9% Mo. The Co-based alloy layer preferably contains, by mass%, 43 to 50% Co. The Ni-based alloy layer preferably contains, by mass%, 45 to 65% Ni. The W-based alloy layer preferably contains, by mass%, 90 to 97% W.

[0054] The alloy layer has higher adhesion to the plug base steel than the solid lubricant layer. Therefore, as described above in the explanation of the solid lubricant layer, it is not necessary to form an oxide scale on the surface of the plug base steel, and a plug having a Mo-based alloy layer formed thereon can have a longer life than a plug having a solid lubricant layer formed thereon. In addition, the Mo-based alloy, Co-based alloy, Ni-based alloy, and W-based alloy constituting the alloy layer are different alloys from the alloy of the steel pipe material containing Fe, etc., and therefore, the plug 10A is suppressed from seizing.

[0055] The mechanism by which an alloy layer prevents seizure will be described in detail using a Mo-based alloy layer as an example of an alloy layer. The Mo-based alloy layer formed by surface coating through thermal spraying of a Mo-based alloy has an intermediate layer that suppresses metal-to-metal contact between the billet B and the plug base steel. The Mo-based alloy layer has a film that is formed between the intermediate layer and a steel pipe material such as the billet B as a result of Mo sublimating at about 800°C. Since this film is a gas, it is presumed that the heat insulating effect suppresses a decrease in the hot strength of the plug 10. It is believed that such a mechanism can also be used to further improve the life of a plug in which an alloy layer such as a Mo-based alloy layer is formed.

[0056] The Mo-based alloy layer can be formed by surface coating of a Mo-based alloy by thermal spraying onto the surface of the plug 10A. The Co-based alloy layer, Ni-based alloy layer and W-based alloy layer can be formed by surface coating of a Co-based alloy, a Ni-based alloy and a W-based alloy, respectively, by thermal spraying onto the surface of the plug 10A in the same manner. The conditions for the thermal spraying are not particularly limited, but in order to prevent early peeling during rolling, exposing the plug base steel and causing seizure due to metal contact, the thermal spraying may be performed by a plasma build-up welding method, a plasma thermal spraying method or a PVD method. It is preferable to carry out the 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-seizure layer is an alloy layer, it is preferable that the plug base steel and the alloy layer have the same or similar linear expansion coefficients measured in accordance with JIS Z2285 (2003) (the linear expansion coefficient of the alloy layer is 50 to 150% of that of the plug base steel.) This can prevent the occurrence of cracks, etc. in the plug 10A during thermal spraying for forming the alloy layer, and can also prevent the occurrence of cracks, etc. when the temperature of the plug 10A increases during piercing-rolling.

[0058] (Modification) Next, the configuration of a plug that is a modification of this embodiment will be described with reference to Figures 6 to 8. Figures 6 to 8 are diagrams for explaining the configuration of a plug that is a modification of this embodiment.

[0059] 6 has a plug front edge portion and a rolled portion, like plug 10A, but the configuration of the rolled portion 12 is the same as that of plug 10A and is therefore not shown here. Whereas plug 10A has a pair (two) of convex portions 11P formed opposite to each other in the circumferential direction of the second axial portion 11B, plug 10B has only one convex portion 11P2 formed in the circumferential direction on the surface of the second axial portion 11B2 of plug front edge portion 11-2. As shown in Fig. 3(d), in a cross section perpendicular to the drilling direction, the first hole portion 12A of the rolled portion 12 has a pair of groove-like portions formed to protrude from a circular center portion in the circumferential direction so as to enable the fitting protrusion portion 11X having a pair of convex portions 11P formed therein to fit therein. In this case, since the plug 10B of this modified example has only one convex portion 11P2 formed in the second axial portion 11B2, the plug 10B may have a shape in which the above-mentioned groove-like portion is formed in only one place, but it is preferable to use the rolled portion 12 in which a pair of groove-like portions are formed as shown in Fig. 3(d). If there was only one groove portion, the plug front end portion 11-2 could be rotated 360 degrees relative to the rolled portion and could only be attached and detached at one specific location. In contrast, by forming a pair of groove portions (two locations), one protrusion 11P2 can be inserted into each groove portion, and the plug front end portion 11-2 can be detached at two locations. In this manner, in plug 10B of this modified example, the frequency of contact between convex portion 11P2 and convex portion accommodating portion 12P during attachment and detachment can be further reduced, and therefore damage to convex portion 11P2 can be further reduced. Therefore, in the plug 10B of this modified example, it is preferable to use a rolled portion 12 in which a pair of groove-like portions are formed, as shown in FIG. 3(d). As long as the plug tip portion 11 can be attached and detached, the number of grooves and protrusions 11P2 is not particularly limited, but as in the above example, the number of grooves may be an integer multiple of the number of protrusions 11P2.

[0060] 7, compared to the plug 10A, the boundary surface between the plug front end portion 11 (piercing portion LF) and the rolled portion 12 (front end in the piercing direction) is tapered. More specifically, this boundary surface has a tapered shape that is inclined toward the piercing direction from the fitting protrusion portion 11X (fitted portion 12X) side toward the outer circumferential direction. The plug 10C having such a configuration can reduce the load applied to the above-mentioned boundary surface. Furthermore, when the plug front end portion 11 in contact with the steel pipe material is pressed into the rolled portion 12 during piercing-rolling, it is possible to prevent the axis of the plug front end portion 11 in the piercing direction from being misaligned with the central axis of the plug 10C in the piercing direction.

[0061] 8, the mating protrusion 11X and the mated portion 12X have opposing surfaces formed in a tapered shape, compared to the plug 10A. More specifically, the mating protrusion 11X and the mated portion 12X are formed in a tapered shape so that the cross-sectional area (area of ​​a cross section perpendicular to the drilling direction) of the outer circumferential surface of the mating protrusion 11X gradually decreases in the direction opposite to the drilling direction. Because the plug 10D has such a configuration, the plug tip portion 11 is less likely to break even if a load is applied to the plug tip portion 11 due to eccentricity or rotation. Furthermore, the volume of the protrusion 11P can be made smaller, thereby reducing the weight of the entire plug tip portion 11.

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

[0063] The plug of the first embodiment of the present invention has been described above. According to this embodiment, a specific key shape is adopted and a mechanism is provided in which the front edge portion of the plug and the rolling portion are detachable, so that the front edge portion of the plug, which is likely to be subjected to load during piercing-rolling, is less likely to deteriorate (melt and / or deform). Even if the front edge portion of the plug deteriorates, it is possible to easily replace only the front edge portion of the plug. In this way, the life of the plug can be improved.

[0064] In addition, in this embodiment, a piercing mill for producing a seamless steel pipe having the plug is also provided. In addition, in this embodiment, a method for producing a seamless steel pipe, in which the plug is used to produce a seamless steel pipe, is also provided.

[0065] [Second embodiment]

[0066] Furthermore, the plug of the present invention is not limited to the example described in the first embodiment, so long as the front end portion of the plug and the rolled portion have a convex portion accommodating structure and the front end portion of the plug can rotate. For example, the plug is not limited to the case where a convex portion 11P is formed in the front end portion 11 of the plug and a convex portion accommodating portion 12P is formed in the rolled portion 12 as shown in Fig. 3 etc., but may have a convex portion formed in the rolled portion and a convex portion accommodating portion formed in the front end portion of the plug.

[0067] FIG. 12 is a diagram for explaining the configuration of a plug 20 according to a second embodiment of the present invention. As shown in FIG. 12, a plug (a plug for manufacturing a seamless steel pipe) 20 of a second embodiment of the present invention is a plug for piercing and rolling a steel pipe material while rotating it in the circumferential direction, and comprises a plug front end portion 21 that pierces the steel pipe material, and a rolling portion 22 that is detachably connected to the plug front end portion 21 and expands the diameter of the pierced steel pipe material, the rolling portion 22 has a fitting protrusion 22X at its front end in the piercing direction, the plug front end portion 21 has a fitted portion 21X that fits into the fitting protrusion 22X at its rear end in the piercing direction, the fitting protrusion 22X has a convex portion 22P formed to protrude from a side wall surface, and the fitted portion 21X is formed with a convex portion accommodating portion 21P that allows the convex portion 22P to be rotatable in the circumferential direction and enables the convex portion 22P to be engaged.

[0068] That is, the plug 10 (10A, B, C, D) of the first embodiment has a fitting protrusion 11X formed in the plug front end portion 11 and a fitted portion 12X formed in the rolled portion 12, whereas the plug 20 of the present embodiment has a fitted portion 21X in the plug front end portion 21 and a fitting protrusion 22X in the rolled portion 22. Although the plug 20 differs from the plug 10 in these configurations, the other configurations and 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 (circular columnar) shape. Also, regarding the position in the perforation 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 perforation 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 perforation direction of the fitting protrusion portion 22X. The fitting protrusion portion 22X has, in the perforation 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 housing portion 21P is formed at the tip portion in the perforation direction of the fitted portion 21X. The fitted portion 21X has, in the perforation 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 housing portion 21P is formed, in this order. It is preferable that the perforation direction length LC2 of the first axial center portion 22A and the perforation direction length lc2 of the first hole portion satisfy the following formula (1-2), and the perforation direction length LK2 of the second axial center portion 22B and the perforation 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, lk2 / LK2 is preferably 1.01 or more, and more preferably 1.50 or more, in order to make the tip portion 21 of the plug rotatable while further suppressing the load on the fitting protrusion portion 22X during rolling. On the other hand, if lk2 is made too large, the volume of the tip portion 21 of the plug becomes small, that is, the hollow portion of the plug 20 becomes large, and there is a possibility of deformation because it cannot withstand the load (force in the perforation direction) during rolling and the force in the rolling direction (direction perpendicular to the perforation direction). Also, if LK2 is made too small, there is a possibility that the convex portion 22P will be damaged when the tip portion 11 of the plug is detached. From these points, lk1 / LK1 is preferably 5.00 or less, and more preferably 2.00 or less.

[0071] With regard to lc2 and LC2 shown in FIG. 12, LC2 / lc2 is preferably 1.01 or more in order to prevent a decrease in strength of the first axial portion 22A due to 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, and is more preferably 1.05 or more in consideration of operability when attaching and detaching the plug tip portion 21. On the other hand, if lc2 is too small, the convex portion 22P comes into contact with the bottom of the convex portion accommodating portion 21P (the tip of the convex portion accommodating portion 12P in the drilling direction), which makes it easier for heat generation and adhesion to occur during rolling. Furthermore, making lc2 small results in increasing the volume of the convex portion accommodating portion 21P, which increases the amount of processing and the cost of tools. From these points of view, 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 be satisfied. Also, in order to make the longitudinal center position of the protrusion 22P and the longitudinal center position of the protrusion accommodating portion 21P the same, it is more preferable to satisfy (lc2+lk2 / 2)=(LC2+LK2 / 2).

[0073] In addition, it is preferable that the maximum diameter φK2 (in the example shown in FIG. 12, the length φK2 between the outer peripheries of the pair of protrusions 22P) in a cross section perpendicular to the drilling direction of the second axial portion 22B in which the protrusions 22P are formed and the diameter φk2 of a cross-sectional circle perpendicular to the drilling direction of the second hole portion 21B in which the protrusion accommodating portion 21P is formed satisfy the following formula (3-2). In this case, it is preferable that the diameter φC2 of the cross-sectional circle 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 formula (4-2). φK2<φk2...Formula (3-2) φC2<φc2...Formula (4-2)

[0074] Regarding φk2 and φK2 shown in FIG. 12, φk2 / φK2 is preferably 1.01 or more, and more preferably 1.05 or more, in order to prevent a decrease in strength of the second axial portion 22B due to 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 front edge portion 21 of the plug in the rolling direction (direction perpendicular to the drilling direction) at the position where the second hole portion 21B is present in the drilling direction will be thin, that is, the distance from the outer surface of the second hole portion 21B will be small, and the strength will be insufficient against the rolling stress, which may cause damage to the plug 20. Furthermore, if φK2 is too small, the difference between the outer diameter of the first axial portion 22A (outer diameter in a cross section perpendicular to the drilling direction) and the outer diameter of the second axial portion 22B (outer diameter in a cross section perpendicular to the drilling direction) becomes small, and the engagement of the protrusion 22P with the protrusion accommodating portion 21P may become insufficient. From these points of view, φk2 / φK2 is preferably 2.00 or less, and more preferably 1.50 or less.

[0075] In order to prevent a decrease in 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 adhesion between the first axial portion 22A and the first hole portion 21A, it is preferable that φc2 / φC2 is 1.01 or more. In addition, in consideration of operability when attaching and detaching the plug front end portion 21, it is more preferable that φc2 / φC2 is 1.05 or more. 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 will sag during operations such as attachment and detachment, and the direction in which the plug tip portion 21 is attempting to drill will no longer match (become non-parallel) with the direction in which the plug 20 should originally drill. If rolling is performed in this state, plug front end portion 21 becomes eccentric, and first axial portion 22A becomes susceptible to damage due to bending stress. Therefore, 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 so that, except for the locking portions, the convex portion 22P and the convex portion accommodating portion 21P are non-contact. 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 it difficult for the plug tip portion 21 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 it difficult for the plug tip portion 21 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 as well as 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.

[0078] Regarding the plug 20, the size is not particularly limited, but similar to the plug 10, the plug diameter may be 30 to 500 mm, and the total length of the plug may be 50 to 1000 mm.

[0079] Also provided is a piercing mill for manufacturing seamless steel pipes having the plug 20 of the present embodiment. Also, even when using this plug 20, seamless steel pipes can be manufactured by the same manufacturing method as that of the plug 10.

Example

[0080] The present invention will now be described in more detail with reference to examples. It should be noted that the present invention is not limited to the following Examples. In order to confirm the effects of the present invention, plugs Nos. 1 to 4 and 8 to 14 as invention examples and plugs Nos. 5 to 7 as comparative examples shown in Table 1 were prepared, and the number of times that they could be pierced was investigated.

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

[0082] The plug 10A shown in FIG. 3 is a plug for piercing and rolling a steel pipe material rotating in the circumferential direction, and has a plug front portion 11 that pierces the steel pipe material, and a rolling portion 12 that is detachably connected to the plug front portion 11 and expands the diameter of the pierced steel pipe material, the plug front portion 11 has an engaging protrusion 11X at a rear end in the piercing direction, the rolling portion 12 has a fitted portion 12X that fits into the fitting protrusion 11X at its front end in the piercing direction, the fitting protrusion 11X has a convex portion 11P formed to protrude from a side wall surface, and the fitted portion 12X is formed with a convex portion accommodating portion 12P that allows the convex portion 11P to rotate freely in the circumferential direction and enables the convex portion 11P to be engaged. In contrast to the plug 10A in which a pair (two) convex portions 11P2 are formed facing each other in the circumferential direction of the second axial portion 11B, the plug 10B shown in FIG. 6 has only one convex portion 11P2 formed in the circumferential direction on the surface of the second axial portion 11B2 of the plug front end portion 11-2. In a plug 10C shown in FIG. 7, the boundary surface between the plug front end portion 11 and the rolled portion 12 is tapered, as compared with the plug 10A. In comparison with plug 10A, plug 10D shown in FIG. 8 has opposing surfaces of fitting protrusion 11X and fitted portion 12X formed in a tapered shape so that the cross-sectional area (area of ​​a cross section perpendicular to the drilling direction) of the outer peripheral surface of fitting protrusion 11X gradually decreases in the opposite direction to the drilling direction. A plug 10E shown in FIG. 9 is a plug in which the front edge portion and the rolled portion are integrally formed, and the front edge portion and the rolled portion cannot be detached from each other. In the plug 110F shown in FIG. 10, no convex portion or convex portion accommodating portion is formed at the joint between the plug front end portion 11' and the rolled portion 12', and the fitting protrusion portion of the plug front end portion 11' is fitted into and connected to the fitted portion of the rolled portion 12'. In plug 110G shown in FIG. 11, like plug 110F, no convex portion or convex portion accommodating portion is formed at the joint between plug front end portion 11″ and rolled portion 12″, and the fitting protrusion portion of plug front end portion 11″ is fitted into the fitted portion of reeling portion 12″, and the fitting protrusion portion is formed in a tapered shape. The plug 20 shown in FIG. 12 has a fitted portion 21X in a front end portion 21 of the plug and a fitted protrusion 22X in a rolled portion 22, as compared to the plug 10 in which a fitting protrusion 11X is formed in the front end portion 11 of the plug and a fitted portion 12X is formed in the rolled portion 12.

[0083] Each plug had a plug diameter of Φ49 mm and a total length of 130 mm.

[0084] As the plug components of each plug, hot working tool steel SKD61 was used for both the plug tip portion and the rolled portion. The plug tip portion and the rolled portion that were produced in a separate state were integrated, and for each of plugs No. 1 to 14 equipped with a plug tip portion 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 then heated to form an oxide scale having a thickness of 0.4 to 0.7 mm on the surface.

[0085] In addition, in plug No. 9, a solid lubricant layer was formed on the plug surface as an anti-seizure layer. To form the solid lubricant layer, first, a solid lubricant containing 30 mass % graphite was heated to 60°C to reduce the 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 a solid lubricant layer. The coefficient of dynamic friction under the rolling simulation conditions was 0.02, and the ratio h / R of the film thickness h (mm) of the solid lubricant layer to the surface roughness R of the plug steel was 2.0.

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

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

[0088] The material of the billet used in the evaluation experiments of this embodiment was SUS420J2 (steel having a composition containing, by mass%, C: 0.3%, Si: 1.0%, Mn: 1.0%, Cr: 13%, and the remainder Fe), and the billet dimensions were φ58 mm × 250L mm.

[0089] The billet was heated at an atmospheric temperature of 1280°C for 1 h, and then piercing was performed using each plug. The drilling conditions were as follows: the roll gap (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] Since the plugs become hot due to friction after piercing and heat conduction from the billet, they were used again only when the surface temperature of the plug reached 30 degrees or less as measured by a contact thermometer. The plug lifespan was compared when the plugs were used repeatedly under the above conditions.

[0091] In Table 1, the plug life was determined by 3D shape measurement. Specifically, the surface temperature of the plug after rolling was reduced to 30°C or less by water cooling or air cooling, and the measurement was performed using a plug which had been left to stand for one day or more in an environment at an atmospheric temperature of 25°C. The measurement device used was an ATOS Compact Scan manufactured by GOM Corporation, and the shape of the plug was measured. In this case, the life of a plug refers to the number of times that the plug is used (the total number of times of use in Table 1) until a deformation occurs that exceeds a volume rate of 10% relative to the volume of each plug before the first piercing-rolling, or until the billet no longer pierces midway and even a part of it does not become a mother pipe. In the present invention, a plug life of 7 or more cycles was deemed to be acceptable. However, for detachable plugs, even if the front end of the plug was deformed by more than 10%, if it was still detachable, it was not considered to have reached the end of its life, and the front end of the plug was replaced and piercing continued. The life of a plug was determined to be when the front end of the plug (excluding the fitting protrusion or mated portion) was deformed to the point where it could not be detached (tip deformation in Table 1), or when the rolled portion 12 was deformed and more than 10% of the plug was deformed (rolled portion deformation in Table 1), or when there was no damage to the front end or rolled portion of the plug but the fitting protrusion or mated portion was damaged, making it impossible to detach the front end of the plug again (damage to the fitting protrusion or mated portion in Table 1), and the number of repeated uses was also counted to determine the total number of uses of the plug.

[0092] The various results are shown in Table 1. In Table 1, the total number of times the plugs were used (total number of times used in Table 1) and the number of times the plug tip was replaced (number of times the tip was replaced in Table 1) are listed, and a check mark is placed next to those that were judged to have been factors in shortening the final plug life.

[0093] [Table 1]

[0094] As shown in Table 1, it became clear that a long life could be obtained by using the plug of the present invention. In the plug of No. 1, which is an example of the present invention, the front edge portion 11 of plug could be replaced five times before the rolled portion 12 was deformed. Moreover, in all of the other plugs of the present invention examples, the front edge portion of plug could be replaced two or more times, and the plug life was seven or more times, which was an improvement in life compared to the conventional plug of No. 5. In the case of No. 9, an example of the present invention in which an anti-seizure layer was formed, the plug front end portion 11 could be replaced six times before the rolled portion 12 was deformed. In the case of Inventive Example No. 10, the front edge portion 11 of plug could be replaced eight times before deformation of the rolled portion 12 occurred. In the case of Inventive Example No. 10, the wear resistance of the rolled portion 12 was improved by the Mo-based alloy layer having high hardness, which extended the life of the plug, and it was revealed that the number of times the front edge portion 11 of plug had to be replaced was increased, thereby achieving a longer life.

[0095] On the other hand, in the comparative example of a conventional plug, No. 5, the front edge portion and the rolled portion were integrally formed, and the total number of uses of the plug was 5 times. In addition, in the case of Comparative Example No. 6, the fitting protrusion portion was heated to a high temperature during the piercing-rolling process and was joined to the fitted portion, making it impossible to detach the front edge portion of the plug, and the total number of uses of the plug was 5. In addition, in the case of Comparative Example No. 7, since the fitting protrusion and the mated portion did not have a convex portion accommodating structure formed thereon, damage occurred to the fitting protrusion and the mated portion, and the plug tip could not be attached or detached. The total number of times the plug was used was 5. [Explanation of symbols]

[0096] 10, 10A, 10B, 10C, 10D plug 11, 11-2 Plug tip 11X mating protrusion 11A First axis center 11B Second axis center 11P convex part LF perforation part 12 Rolling section 12X Mated part 12A First hole 12B Second hole 12P Convex part housing 13 Parallel section 20 Plug 21 Plug tip 21X Mated part 21A First hole 21B Second hole 21P Convex part accommodation part 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 Mandrel 4a, 4b Disc shoe 100 Rotary hearth heating furnace 101 Piercing and rolling mill 102 Mandrel bar 103 Mandrel Mill 104 Reducer B Billet S Hollow

Claims

1. A plug for piercing and rolling a steel pipe material rotating in a circumferential direction, a plug tip portion for drilling a steel pipe material; a rolling section that is detachably connected to the plug front end section and expands the diameter of the pierced steel pipe material; Equipped with the plug front end portion has a fitting protrusion portion at a rear end portion in a drilling direction, The rolling portion has a fitted portion fitted to the fitting protrusion portion at a tip end in a drilling direction, The fitting protrusion has a protrusion formed on a side wall surface, a convex portion accommodating portion is formed in the fitted portion, which allows the convex portion to rotate freely in the circumferential direction and enables the convex portion to be engaged.

2. A plug for piercing and rolling a steel pipe material rotating in a circumferential direction, a plug tip portion for drilling a steel pipe material; a rolling section that is detachably connected to the plug front end section and expands the diameter of the pierced steel pipe material; Equipped with The rolling portion has a fitting protrusion at a tip end in a drilling direction, the plug front end portion has a fitted portion fitted to the fitting protrusion portion at a rear end portion in a drilling direction, The fitting protrusion has a protrusion formed on a side wall surface, a convex portion accommodating portion is formed in the fitted portion, which allows the convex portion to rotate freely in the circumferential direction and enables the convex portion to be engaged.

3. The protrusion is formed at a rear end of the fitting protrusion in the drilling direction, The fitting protrusion has, in order in a direction opposite to the drilling direction, a first axis portion and a second axis portion on which the protrusion is formed, The protrusion accommodating portion is formed at a rear end portion of the fitted portion in a drilling direction, The fitted portion has, in a direction opposite to a drilling direction, a first hole portion covering an outer periphery of the first axial portion and a second hole portion in which the protrusion accommodating portion is formed, in that order; The length LC of the first axial portion in the drilling direction １ and the length lc of the first hole portion in the drilling direction. １ and satisfy the following formula (1-1), The length LK of the second axial portion in the drilling direction １ and the drilling direction length lk of the second hole portion １ and satisfy the following formula (2-1): LC １ > lc １ ... Equation (1-1) LK １ <lk １ ・・・ Formula (2-1)

4. The protrusion is formed at a tip end of the fitting protrusion in a drilling direction, The fitting protrusion has, in order in a drilling direction, a first axis portion and a second axis portion on which the protrusion is formed, The protrusion accommodating portion is formed at a tip end of the fitted portion in a drilling direction, The fitted portion has, in order in a drilling direction, a first hole portion covering an outer periphery of the first axial portion and a second hole portion in which the protrusion accommodating portion is formed, The length LC of the first axial portion in the drilling direction ２ and the length lc of the first hole portion in the drilling direction. ２ and satisfy the following formula (1-2), The length LK of the second axial portion in the drilling direction ２ and the drilling direction length lk of the second hole portion ２ and satisfies the following formula (2-2): The plug for producing a seamless steel pipe according to claim 2. LC ２ > lc ２ ... Equation (1-2) LK ２ <lk ２ ・・・ Formula (2-2)

5. 5. The plug for producing a seamless steel pipe according to claim 1, wherein an anti-seizure layer is formed on a plug surface which comes into contact with the steel pipe material during piercing-rolling.

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

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

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