JP7276370B2 - Seamless steel pipe manufacturing plug, seamless steel pipe manufacturing piercing mill, and seamless steel pipe manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a plug for manufacturing seamless steel pipes by the Mannesmann piercing-mandrel mill method and a piercing mill for manufacturing seamless steel pipes equipped with the plug, and more specifically, to a plug that is a rate-limiting factor in the service life of the plug. The present invention relates to a plug whose life is improved by replacing only the tip, a piercing mill equipped with the plug, and a method for manufacturing a seamless steel pipe.

In Mannesmann piercing in the production of seamless steel pipes, a steel pipe material is pierced and rolled by a PCM (Piercing Mill) plug.
In order to increase the pipe-making efficiency of the seamless steel pipe and improve the yield, it is preferable to lengthen the hollow (for example, about 9 to 12 m in length).

However, lengthening the hollow lengthens the time that the plug stays in the billet (for example, 15 to 30 seconds) and increases the time that the plug is exposed to high temperatures. Furthermore, the plug is always in contact with the nascent surface of the billet during piercing-rolling for turning the billet from solid to blank.
At this time, when the conventional plug is used in a high-temperature, high-pressure environment, the tip may be eroded or deformed, after which it cannot be used for piercing-rolling. For example, in the piercing-rolling of high-alloy steel with a Cr content of 5% by mass or more and high resistance to hot deformation, one plug can only be used for piercing-rolling several to ten-odd billets.

Such a shortened life of the plug increases the unit consumption of the plug, resulting in an increase in pipemaking costs. Therefore, it is required to improve the life of the plug.

Various proposals have been made so far to specialize the shape and material of the plug for the purpose of extending the service life of the plug. For example, Patent Literature 1 discloses a plug made of ceramic. In addition, considering that the tip of the plug is eroded, it has been proposed to make the eroded tip non-ferrous (see, for example, Patent Document 2).

On the other hand, in consideration of cost reduction of the plug, there has recently been a demand to improve the plug life by improving the shape of the plug without improving the material. 4 describes the technology to make the tip of the plug detachable.

JP-A-2001-87804 JP-A-2000-167606 WO 2005/087401 WO2014/030593

However, even if the tip of the plug is detachable as in the techniques described in Patent Documents 3 and 4, the state of connection between the tip of the plug and the reeling portion is not sufficient for extending the service life of the plug. Specifically, in the prior art, if the bonded state is strengthened, there is a concern that the bonded portion may be damaged during piercing-rolling. There was a concern that it would end up being unusable, and the optimal bonding state could not be found, and sufficient life could not be obtained. Thus, there has been a demand for the establishment of a new plug technology in which the tip of the plug is detachable.

DISCLOSURE OF THE INVENTION The present invention has been made to solve the problems of the prior art, and provides a seamless steel pipe manufacturing plug having an improved service life, a seamless steel pipe manufacturing piercing mill equipped with the seamless steel pipe manufacturing plug, and a seamless steel pipe manufacturing plug. An object of the present invention is to provide a method for manufacturing a steelless pipe.

As a result of intensive studies to solve the above problems, the present inventors have found that during piercing-rolling, the tip of the plug is prevented from detaching from the reeling part, and deterioration of the tip of the plug is suppressed. Later, as a mechanism for easily attaching and detaching the plug tip portion and the reeling portion, attention was paid to connecting the plug tip portion and the reeling portion by a specific screwing method.
Specifically, a partially threaded structure in which threads are partially formed is adopted in each of the connecting portions of the plug tip and the reeling portion, and the plug tip portion and the reeling portion having this structure are connected. It has been found that the life of the plug can be improved by

Based on the above findings, the gist and configuration of the present invention, which was completed by adding further studies, is as follows.
[1] A plug for piercing and rolling a steel pipe material,
a plug tip for drilling a steel pipe material;
a reeling part that is detachably connected to the tip part of the plug and expands the diameter of the drilled steel pipe material;
with
The plug front end portion has a fitting projection portion at the rear end portion in the drilling direction,
The reeling part has a fitted part fitted to the fitting projection part at the leading end in the drilling direction,
A first threaded portion and a first non-threaded portion are formed in order in the drilling direction of the fitting protrusion,
A second non-threaded portion and a second threaded portion that can be screwed into the first threaded portion are sequentially formed in the fitted portion in the drilling direction,
A plug for manufacturing seamless steel pipes, wherein the first threaded portion and the second threaded portion are not screwed together, and the tip end portion of the plug and the reeling portion are connected.
[2] A plug for piercing and rolling a steel pipe material,
a plug tip for drilling a steel pipe material;
a reeling part that is detachably connected to the tip part of the plug and expands the diameter of the drilled steel pipe material;
with
The reeling part has a fitting protrusion at the leading end in the drilling direction,
The plug front end portion has a fitted portion fitted to the fitting projection portion at the rear end portion in the drilling direction,
A first threaded portion and a first non-threaded portion are formed in order in the drilling direction of the fitted portion,
A second non-threaded portion and a second threaded portion that can be screwed into the first threaded portion are sequentially formed in the fitting protrusion in the drilling direction,
A plug for manufacturing seamless steel pipes, wherein the first threaded portion and the second threaded portion are not screwed together, and the tip end portion of the plug and the reeling portion are connected.
[3] The drilling direction length α1 of the first threaded portion and the drilling direction length β2 of the second non-threaded portion satisfy the following formula (1),
The production of seamless steel pipe according to [1] or [2] above, wherein the drilling direction length α2 of the second threaded portion and the drilling direction length β1 of the first non-threaded portion satisfy the following formula (2): for plug.
1.00<β2/α1≦5.00 Expression (1)
1.00<β1/α2≦5.00 Expression (2)
[4] The end portion of the first threaded portion on the drilling direction side and the end portion of the second threaded portion on the side opposite to the drilling direction are spaced apart by 5 to 20 mm, and the tip end portion of the plug and the reeling portion are separated from each other. The plug for manufacturing seamless steel pipes according to any one of [1] to [3], wherein the plug is connected to the
[5] The plug for producing seamless steel pipes according to any one of [1] to [4], wherein an anti-seizure layer is formed on the surface of the plug that comes into contact with the steel pipe material during piercing-rolling.
[6] A piercing mill for manufacturing seamless steel pipes, comprising the plug for manufacturing seamless steel pipes according to any one of [1] to [5] above.
[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 [1] to [5].

ADVANTAGE OF THE INVENTION According to the present invention, there are provided a seamless steel pipe manufacturing plug with improved service life, a seamless steel pipe manufacturing piercing mill equipped with the seamless steel pipe manufacturing plug, and a seamless steel pipe manufacturing method.

FIG. 1 is a schematic diagram for explaining a method of manufacturing a seamless steel pipe. FIG. 2 is a diagram showing a schematic configuration of a piercing rolling mill. FIG. 3 is a diagram for explaining the configuration of the plug according to the first embodiment of the invention. 4A and 4B are diagrams for explaining the process of coupling the plug tip portion 11 and the reeling portion 12. FIG. FIG. 5 is a diagram for explaining the configuration of a plug that is a modification of the present invention. FIG. 6 is a diagram for explaining the configuration of a plug that is a modification of the present invention. FIG. 7 is a diagram for explaining the configuration of a plug that is related technology (comparative example). FIG. 8 is a diagram for explaining the configuration of a plug that is related technology (comparative example). FIG. 9 is a diagram for explaining the configuration of a plug that is related technology (comparative example). FIG. 10 is a diagram for explaining the configuration of a plug that is related technology (comparative example). FIG. 11 is a diagram for explaining the configuration of a plug that is related technology (comparative example). FIG. 12 is a diagram for explaining the shape of the thread. FIG. 13 is a diagram for explaining the configuration of the plug according to the second embodiment of the invention.

<Method for manufacturing seamless steel pipe>
The present invention will be described with reference to the drawings. It should be noted that the present invention is not limited by this embodiment.

Before describing the details of the configuration and functions of the seamless steel pipe manufacturing plug of the present invention, referring to FIG. explain how to FIG. 1 is a schematic diagram for explaining a method of manufacturing a seamless steel pipe.

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)). Thereafter, as a piercing-rolling step, a plug 10 (PCM (Piercing Mill) plug) and rolling rolls 2a and 2b of a piercing-rolling mill 101 are used to insert the plug 10 into the billet B in the longitudinal direction (axial direction). Then, the billet B is pierced and rolled to manufacture a hollow S (see FIG. 1(c)).

Next, as shown in FIG. 1(d), a mandrel bar 102 is inserted into the inner surface of the hollow S in a skewed manner, and the outer surface of the hollow S is constrained by grooved rolling rolls of a mandrel mill 103 consisting of 5 to 8 stands. By stretching and rolling, the wall thickness is reduced to a predetermined thickness to obtain a mother pipe. After that, the mandrel bar 102 is pulled out from the raw pipe, and as shown in FIG. A steel pipe X) is obtained.

Here, the piercing-rolling mentioned above will be described in more detail. 2A and 2B show a schematic configuration of the piercing and rolling mill 101. FIG. 2A shows a side view, and FIG. 2B shows a plan view. In addition, illustration of the plug 10 is abbreviate|omitted in FIG.2(b).
As shown in FIG. 2, the piercing-rolling mill 101 includes a pair of rolling rolls 2a and 2b arranged with their rotation axes tilted with respect to each other, and the rolling rolls 2a and 2b rotated by 90° in a plane perpendicular to the rolling direction. Disc shoes 4a and 4b arranged at positions close to each other and a plug 10 whose rear end is supported by the mandrel 3 are provided. The pair of rolling rolls 2a and 2b can be arranged with their rotation axes tilted in opposite directions in a plan view by an inclination angle FA about the rolling direction, and can rotate in the same direction. The plugs 10 are arranged between the pair of rolling rolls 2a, 2b and between the disk shoes 4a, 4b to pierce the billet B. As shown in FIG.

In order to pierce-roll a solid billet B (round billet) using the piercing-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 billet B is simultaneously subjected to a rotating force and an axial (rolling direction) advancing force due to the frictional force of the rolling rolls 2a and 2b. By the time the billet B reaches the tip of the plug 10, the rolling rolls 2a and 2b continuously and alternately apply compressive stress and tensile stress (rotary forging effect) to the central portion of the billet B, causing a hole. is likely to form. After that, when the billet B collides with the plug 10, a hole is formed in the center of the billet B, and thereafter, it is subjected to a thinning process every half rotation between the rolling rolls 2a, 2b and the plug 10, and a predetermined wall thickness is obtained. A hollow S with an outer shape is obtained.

<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. FIG. 3 is a diagram for explaining the configuration of the plug according to the first embodiment of the invention. In FIG. 3, (a) is an overall view of the plug 10A of this embodiment, and (b-1) and (b-2) in FIG. An enlarged view of the vicinity of the joint U is shown.

The plug 10A of the present embodiment is a seamless steel pipe manufacturing plug used in the piercing and rolling mill 101 as described above, and the plug tip 11 and the reeling 12, each having a partially threaded structure, are detachably connected. ing. Specifically, as shown in FIG. 3(a), the plug 10 of the present embodiment is a plug for piercing and rolling a steel pipe material such as a billet B, and is a plug tip portion for piercing the steel pipe material. 11, and a reeling portion 12 which is detachably connected to the plug tip portion 11 and expands the diameter of the drilled steel pipe material. In addition, as shown in (b-1) in FIG. 3, the plug tip portion 11 has a fitting projection portion 11X at the rear end portion in the drilling direction, and as shown in (b-2), a reeling portion. 12 has a to-be-fitted portion 12X fitted to the fitting protrusion 11X at the leading end in the drilling direction. In addition, a first threaded portion 11A and a first non-threaded portion 11B are formed in order in the drilling direction of the fitting protrusion 11X, and the fitted portion 12X is formed in the drilling direction of A second non-threaded portion 12B and a second threaded portion 12A that can be screwed into the first threaded portion 11A are formed in this order. The plug tip portion 11 and the reeling portion 12 are connected in a non-threaded state.

The plug tip portion 11 is not particularly limited as long as it can pierce a steel pipe material and has the fitting protrusion 11X as described above. It is preferable that the portion where the drilling is performed has a tapered shape in which the cross section gradually decreases toward the tip in the drilling direction.

The reeling portion 12 can expand the diameter of the steel pipe material pierced by the plug tip portion 11, and is not particularly limited as long as it has the fitted portion 12X as described above. The portion of the steel pipe material other than the diameter-expanding portion may be partially formed in a columnar shape (cylindrical shape) so as to smooth the inner surface of the steel pipe material, for example. Moreover, it may be partially formed in a shape such that the cross section gradually decreases toward the plug tip portion 11 side so that the diameter of the steel pipe material can be expanded.

Further, the plug 10A may have a parallel portion 13 formed in a columnar (cylindrical) shape. The plug 10A may have a configuration in which the plug tip portion 11, the reeling portion 12, and the parallel portion 13 are formed in order in the opposite direction (rolling direction) to the drilling direction.

Here, before describing the plug of the modified example of the present embodiment shown in FIGS. 5 and 6, referring to FIGS. . 7 to 11 are diagrams for explaining the configuration of a plug that is related technology.

In the plug 110D shown in FIG. 7, the plug tip portion and the reeling portion are integrally formed, and the plug tip portion and the reeling portion cannot be detached. In this plug 110D, since the tip portion of the plug and the reeling portion cannot be detached from each other, the tip end may be eroded or deformed by piercing-rolling, making it unusable for subsequent piercing-rolling. you'll have to replace the whole thing.

In the plug 110E shown in FIG. 8, the connection portion between the plug tip portion 11' and the reeling portion 12' is not threaded, and the fitting protrusion of the plug tip portion 11' is covered by the reeling portion 12'. It is fitted in the fitting portion. With this plug 110E, the tip end portion 11' of the plug tends to come off from the reeling portion 12' during piercing-rolling, and it is difficult to continue piercing-rolling. Further, when the plug tip portion 11′ and the reeling 12′ are joined, no gap is formed between the plug tip portion 11′ and the reeling 12′. As a result, there is a possibility that the plug tip portion 11' and the reeling portion 12' cannot be detached.

Similarly to the plug 110E, the plug 110F shown in FIG. portion is fitted to the fitted portion of the reeling portion 12''. In the plug 110F, the fitting protrusion is formed in a tapered shape. Specifically, the vertical cross section of the fitting protrusion in the drilling direction gradually increases toward the reeling part 12''. In this plug 110F, since the fitting protrusion has a tapered shape, the tip of the plug does not separate from the reeling part during piercing and rolling, but even if the tip of the plug needs to be replaced after piercing and rolling, the tip of the plug does not come off. cannot be easily removed. Further, since the plug front end portion 11'' is firmly fastened to the reeling portion 12'', the plug front end portion 11'' is likely to deteriorate due to the load during piercing and rolling. Further, the fitting protrusion of the plug tip 11'' has a so-called reverse tapered shape in which the diameter decreases toward the plug tip (piercing direction side). Since the cross section becomes smaller toward the root side of 11'', there is a possibility that it will break near the root portion during piercing and rolling.

A plug 110G shown in FIG. 10 and a plug 110H shown in FIG. is provided.
First, in the plug 110G, the threaded portions of the plug distal end portion 11''' and the reeling portion 12''' are screwed together, and the plug distal end portion 11'''' and the reeling portion 12''' are detachable. concatenate with Piercing rolling by a piercing mill having a plug is tilt rolling in which a steel pipe material is pierced spirally. Therefore, when the direction of rotation of the threaded portions of the plug tip portion 11''' and the reeling portion 12''' is the same as the direction of rotation of the steel pipe material, the fastening of the threaded portions is made more firm during piercing and rolling. As a result, the threaded structure may be damaged or seized, after which detachment becomes impossible. On the other hand, if the threaded portion and the steel pipe material rotate in opposite directions, the thread loosens and the plug tip portion 11''' separates from the reeling portion 12''' during piercing-rolling. As a result, the total length of the plug becomes longer, which makes it impossible to carry out piercing-rolling as planned, and there is a possibility that the thickness, length, etc. of the resulting steel pipe will not be as desired. In addition, a recess is formed at the boundary between the plug tip portion 11''' and the reeling portion 12''', and the recess may be transferred to the inner surface of the steel pipe by piercing-rolling, degrading the quality of the inner surface of the steel pipe. Moreover, there is a possibility that the entire plug tip portion 11''' will be detached from the reeling portion 12''', making it impossible to continue piercing-rolling.

Further, in the plug 110H, each of the plug tip portion 11'''' and the reeling portion 12'''' is provided with a non-threaded portion in addition to the threaded portion. ' is not screwed by the threaded part when attaching and detaching '. In this plug 110H, the plug front end portion 11'''' lacks stability such that the plug front end portion 11'''' tends to detach from the reeling portion 12'''' during piercing and rolling.

In this regard, in the plug 10A of the present embodiment, not all of the fitting protrusions 11X of the plug tip 11 are threaded at each of the connecting portions U between the plug tip 11 and the reeling portion 12. In addition, not all of the fitted portion 12X of the reeling portion 12 is threaded, but a partially threaded structure is adopted, and a plug tip having this structure is adopted. The part 11 and the reeling part 12 are connected (see FIG. 3 again).
The plug 110H of the related art described above also employs a partially threaded structure, but unlike this plug 110H, in the plug 10A of the present embodiment, a portion of the tip side of the fitting protrusion 11X of the plug tip portion 11 is threaded. Only a portion of the reeling portion 12 on the opening side of the fitted portion 12X is threaded.

More specifically, as shown in FIG. 3(a), in the plug 10A of the present embodiment, the fitting protrusion 11X of the plug distal end portion 11 has a first threaded portion 11A and a first non-threaded portion in the drilling direction. A second non-threaded portion 12B and a second threaded portion 12A are formed in order in the drilling direction in the fitted portion 12X of the reeling portion 12. .

The first threaded portion 11A functions as a male thread having a threaded outer surface, and can be screwed into a second threaded portion 12A which similarly has a threaded outer surface and functions as a female thread. can. The first non-threaded portion 11B is not particularly limited as long as it has a shape that does not screw together with the second threaded portion 12A. For example, the outer surface can be a smooth surface as shown in FIG.
The second non-threaded portion 12B is not particularly limited as long as it has a shape that does not screw together with the first threaded portion 11A. For example, as shown in FIG. 3, the outer surface can be made smooth.

Therefore, even though the first threaded portion 11A and the second threaded portion 12A are not screwed together, the threads of the first threaded portion 11A and the second threaded portion 12A act as barriers to each other, thereby While preventing the fitting protrusion 11X of the reeling part 12 from being detached from the fitted part 12X of the reeling part 12, the plug tip part 11 and the reeling part 12 can be maintained in a firmly connected state.

Further, since the first threaded portion 11A and the second threaded portion 12A are not screwed together during piercing and rolling, the plug tip portion 11 is rotatable. Therefore, deterioration (erosion or deformation) of the plug tip portion 11 due to the load during piercing and rolling can be suppressed.

Further, in the plug 10A of the present embodiment, even if the plug tip portion 11 becomes eroded or deformed by repeated piercing and rolling and cannot be used, only the plug tip portion 11 can be replaced. Plug 10A can be reused. Further, when it becomes necessary to replace the plug tip portion 11 after piercing and rolling, the plug tip portion 11 can be easily removed from the reeling portion 12 by using the threaded structure (screwed structure).

Here, with reference to FIG. 4, the process of coupling the plug distal end portion 11 and the reeling portion 12 in the plug 10A of the present embodiment will be described. 4A and 4B are diagrams for explaining the process of coupling the plug tip portion 11 and the reeling portion 12. FIG.

First, FIG. 4A shows a state in which the plug tip portion 11 and the reeling portion 12 are separated. As shown in FIG. 4(A), the plug tip portion 11 advances spirally in the direction of the reeling portion 12 (the direction opposite to the drilling direction, see the arrow in FIG. 4(A)). The first threaded portion 11A is screwed into the second threaded portion 12A of the reeling portion 12 (see FIG. 4B).

From the threaded state shown in FIG. 4(B), the plug distal end portion 11 can further advance spirally in the direction opposite to the drilling direction, gradually connecting the first threaded portion 11A and the second thread. The threaded state with the processed portion 12A is gradually released.

Thereafter, as shown in FIG. 4(C), the threaded state between the first threaded portion 11A and the second threaded portion 12A of the plug tip portion 11 is completely disengaged, and the leading end of the reeling portion 12 in the drilling direction is removed. , and the root portion of the fitting protrusion 11X of the plug tip portion 11 (the rear end portion of the main body portion of the plug tip portion 11 in the drilling direction), the connection is completed.
At this time, the first threaded portion 11A and the second non-threaded portion 12B may face each other, and the second threaded portion 12A and the first non-threaded portion 11B may face each other.
Also, the first threaded portion 11A and the second non-threaded portion 12B may be in contact with each other.
Also, the second threaded portion 12A and the first non-threaded portion 11B may be in contact with each other.

Further, when the plug tip portion 11 is detached from the reeling portion 12, by moving the plug tip portion 11 in the drilling direction, the drilling direction side tip portion of the first threaded portion 11A and the second threaded portion are separated. The rear end of 12A on the drilling direction side is brought into contact, and then the screws are screwed together. By releasing the screwed state, the plug tip portion 11 is easily separated from the reeling portion 12 .

(Threaded lengths α1 and α2 and non-threaded lengths β1 and β2)
Next, the drilling direction length α1 of the first threaded portion 11A, the drilling direction length β1 of the first non-threaded portion 11B, the drilling direction length α2 of the second threaded portion 12A, and the drilling direction of the second non-threaded portion 12B Expressions (1) and (2) that the length β2 preferably satisfies will be described.
1.00<β2/α1≦5.00 Expression (1)
1.00<β1/α2≦5.00 Expression (2)
In the relationship between the drilling direction length α1 of the first threaded portion 11A and the drilling direction length β2 of the second non-threaded portion 12B, α1<β2. That is, 1.00<β2/α1.
Further, in the relationship between the drilling direction length α2 of the second threaded portion 12A and the drilling direction length β1 of the first non-threaded portion 11B, α2<β1. That is, 1.00<β1/α2.

By satisfying α1<β2 and α2<β1, the first threaded portion 11A and the second threaded portion 12A are in a non-threaded state when the plug tip portion 11 and the reeling 12 are connected. can be done. Also, at this time, a gap is generated between the fitting projection portion 11X and the fitted portion 12X. More specifically, a gap is formed between the first threaded portion 11A of the plug tip portion 11 and the second threaded portion 12A of the reeling portion 12 .

Due to this gap, the plug tip portion 11 is loosely fitted to the reeling portion 12 without being fixed, so that the plug tip portion 11 can be freely rotated along with the rotation of the steel pipe material such as the billet B during piercing and rolling. Therefore, the load on the plug tip portion 11 is alleviated.
In addition, with the above configuration, the first threaded portion 11A of the plug tip portion 11 and the second threaded portion 12A of the reeling portion 12 are not meshed with each other during piercing and rolling, so damage or seizure at the connecting portion U may occur. Therefore, it is possible to prevent the plug front end portion 11 from becoming impossible to attach and detach.

On the other hand, when at least one of α1>β2 and α2>β1 holds, when the plug tip portion 11 and reeling portion 12 are connected, the first threaded portion 11A and the second threaded portion 12A are at least partially displaced. The piercing-rolling is performed in a state where the plug is screwed into the piercing-rolling.

As the connected state is strengthened, the first threaded portion 11A and/or the second threaded portion 12A in the connecting portion U may be damaged or seized, making it impossible to separate the plug tip portion 11 from the reeling portion 12. . In addition, due to the weakening of the connected state, the plug tip portion 11 is detached from the reeling portion 12 during piercing-rolling, making it impossible to continue piercing-rolling.

Further, even when α=β, the threaded state of the first threaded portion 11A and the second threaded portion 12A is formed during piercing-rolling as described above, and the plug tip portion 11 is separated from the reeling portion 12. may leave. In addition, the gap between the fitting protrusion 11X and the fitted portion 12X (between the end of the first threaded portion 11A of the plug tip portion 11 in the drilling direction and the second threaded portion 12A of the reeling portion 12) Since there is no gap between the end portion on the opposite side of the piercing direction (hereinafter simply referred to as a gap), a load is applied to at least one of the first threaded portion 11A and the second threaded portion 12A during piercing and rolling. It can get stuck and get damaged.

As described above, in the present embodiment, it is preferable to satisfy α1<β2 (1.00<β2/α1) and α2<β1 (1.00<β1/α2).

Also, if the above clearance is set too large, that is, if at least one of β2/α1 and β1/α2 shows a large value, the plug tip 11 is likely to detach from the reeling portion 12, resulting in poor handling. can be difficult. Further, when the plug tip portion 11 is about to come off from the reeling portion 12, the screw portions of the plug tip portion 11 and the reeling portion 12 (the first threaded portion 11A and the second threaded portion 12A) Contact can easily cause damage. Here, the preferred sizes of α1 and α2 are each 5 mm or more. If at least one of α1 and α2 is less than 5 mm, the pitch at the time of threading becomes small, which may make threading difficult. In addition, repeated attachment and detachment of the plug tip 11 and the reeling part 12 may cause the tip of the screw to wear out. In a screw with at least one of α1 and α2 less than 5 mm, the threaded part may wear out. There is a possibility of unintentional detachment.
For example, when α1 and α2 are 5 mm, in consideration of the above points, the preferable maximum value of the gap is 20 mm, and β1 and β2 are 25 mm. At this time, the ratios of α1 and β2 and between α2 and β1 are 5.00. Thus, β2/α1 is preferably 5.00 or less and β1/α2 is preferably 5.00 or less.
Also, β2/α1 is more preferably 1.50 or more and 4.00 or less. Also, β1/α2 is more preferably 1.50 or more and 4.00 or less.

Also, α2+β2>α1+β1 may be satisfied, or α1+β1=α2+β2.
When α1+β1=α2+β2, that is, when the drilling direction length of the fitting protrusion 11X and the drilling direction length of the fitted protrusion 12X are equal, α1=α2 and β1=β2 may be satisfied.

Further, it is preferable that the relationship between α and β satisfy β2=α1+1 to 20 mm and β1=α2+1 to 20 mm. That is, the end of the first threaded portion 11A in the drilling direction and the end of the second threaded portion 12A in the opposite direction to the drilling direction are spaced apart by 1 to 20 mm so that the plug tip portion 11 and the reeling portion 12 are separated. It is preferred that they are connected. More preferably, β2=α1+5 to 15 mm and β1=α2+5 to 15 mm are satisfied.

The gap (the gap between the fitting protruding portion 11X and the fitted portion 12X (the gap between the first threaded portion 11A of the plug front end portion 11 in the drilling direction and the second threaded portion 12A of the reeling portion 12) If the gap between the end on the opposite side of the drilling direction) is less than 1 mm, the gap is too small, so contact between the first threaded portion 11A and the second threaded portion 12A occurs during handling of the plug 10A. There is a possibility that the plug tip portion 11 may be unintentionally detached from the reeling portion 12. Therefore, it is preferable to secure a clearance of 1 mm or more.
Further, the temperature of the steel pipe material during piercing-rolling is as high as 1200 to 1300° C., and the plug 10A is also heated during piercing-rolling.
Oxidation scale is generated on the plug 10A due to the temperature rise of the plug 10A. Since the plug tip portion 11 and the reeling portion 12 are joined together, there is a possibility that oxidized scale will be generated at the joint portion U, even if the intrusion of oxygen is small. For example, hot work tool steel SKD61 (0.35% C-1.0% Si-0.35% Mn-5.0% Cr-1.25% Mo-1.0% V-balance Fe) When piercing-rolling is repeated, an oxide scale of 1 mm or more on one side may be generated. Therefore, an oxide scale of 2 mm or more in total is generated at the joint U.
Therefore, β2≧α1+2 mm and β1≧α2+2 mm are preferable, and the optimum gap is β2≧α1+3 mm and β1≧α2+3 mm because a gap of 1 mm is necessary for the above reason.

On the other hand, when β2 exceeds α1+20 mm, when β1 exceeds α2+20 mm, a large bending stress is applied to the connecting portion U of the plug when the plug tip portion 11 is eccentrically aligned with the steel pipe material during piercing-rolling. Furthermore, when β2 exceeds α1+20 mm, and when β1 exceeds α2+20 mm, since the connecting portion U is long, a large moment is applied compared to 20 mm or less, and the connecting portion U may break. In addition, the plug tip portion 11 and the reeling portion 12 must be machined for 2α+20 mm, which takes time. Furthermore, since the weight of the plug tip portion 11 increases, it becomes difficult to handle, and it becomes difficult to attach and detach the plug tip portion 11 and the reeling portion 12 . Therefore, β2≦α1+20 mm and β1≦α2+20 mm are preferable, and β2≦α1+10 mm and β2≦α1+10 mm are more preferable.

From the above, the ranges of β2≦α1+20 mm and β1≦α2+20 mm are preferable, and the more preferable ranges are β2=α1+3 to 10 mm and β1=α2+3 to 10 mm.

A preferred size for α(α1, α2) is 5 to 200 mm. If α is less than 5 mm, the pitch in threading is small, making threading difficult. Further, repeated attachment and detachment of the plug tip portion 11 may wear the tips of the threaded portions 11A and 12A. As a result, the plug distal end portion 11 may be unintentionally detached from the reeling portion 12 .
On the other hand, the larger α is, the longer the first threaded portion 11A of the plug tip portion 11 becomes. In view of these, α is preferably 5 to 200 mm, more preferably 5 to 100 mm.

Although the overall length of the plug in the drilling direction is not particularly limited, it is preferably 100 to 500 mm. If the overall length is less than 100 mm, the inner surface of the steel pipe may be damaged because the reeling portion, which serves to scratch the inner surface of the steel pipe, cannot be sufficiently secured. As for the inner surface flaws of the steel pipe, the flaws are sufficiently smoothed if the overall length of the plug is 500 mm, and lengths exceeding 500 mm are unnecessary because the weight of the plug increases. In addition, the reeling part plays a role in obtaining a hollow with a predetermined thickness by maintaining a constant gap between the roll and the plug. unnecessary. Therefore, the total length of the plug in the drilling direction is preferably 100 to 500 mm.

Next, the preferred number of threads in the first threaded portion 11A and the second threaded portion 12A will be described. The number of threads is preferably 1 to 5 so that the plug tip 11 can be attached and detached easily at any timing. Considering that it is not necessary to rotate 11 more than necessary when detaching 11 from reeling portion 12, it is more preferable to have 3 to 4 ridges.

Furthermore, the thread shape will be described with reference to FIG. FIG. 12 is a diagram for explaining the shape of the thread. The thread is not particularly limited as long as the first threaded portion 11A of the plug tip portion 11 and the second threaded portion 12A of the reeling portion 12 can be screwed together. Eccentricity can occur due to various factors such as segregation and piercing mill stiffness. Since the plug tip portion 11 is not fixed to the reeling portion 12, it is eccentric together with the steel pipe material. At that time, if the thread shape is a triangular shape such as a metric thread (see FIG. 12(b)) or a Whitworth thread (see FIG. 12(c)), a screw thread is required to support the plug tip 11 with a line. There is a possibility that the mountain will be deformed due to the load and it will not be possible to attach and detach. Therefore, it is preferable that the thread shape be a trapezoidal thread (see FIG. 12(d)) or a square thread (see FIG. 12(e)) that can support the plug tip 11 on its surface. Further, a round screw (see FIG. 12(a)) is more preferable because the stress applied to the threaded portion is relaxed and the temperature gradient of the threaded portion is uniform.

The component composition of the plug tip portion 11 is not particularly limited, and for example, it may be a component composition having Co/Mo or Co-based alloy/Mo-based alloy, which has a higher hot deformation resistance than iron. It may also have a ceramic with high wear resistance. Also, a steel having a composition different from that of the reeling portion 12 may be used.

The component composition of the reeling part 12 is not particularly limited, and can be appropriately adjusted according to the material of the billet. At, 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%, Nb, W and Co 3.00 to 5.00% in total of one or more selected from among them, and the balance being Fe and unavoidable impurities. The component composition of the plug tip portion 11 may be the same as that of the reeling portion 12, or a different steel may be used.

(Seizure prevention layer)
In the plug 10A of the present embodiment, it is preferable that an anti-seizure layer for preventing the plug from seizing is formed on the surface of the plug 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 iron (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 (hardness based on JIS Z2244 (2009)) of the anti-seizure layer is preferably equal to or higher than the hardness of the plug base iron, and more preferably higher than the plug base iron. In addition, it is preferable that an anti-seizure layer containing a large amount of S, P, etc. is not adhered to the inner surface of the steel pipe material such as the pierced billet B, that is, the anti-seizure layer does not contain much S or P. is preferred. Also, the anti-seizure layer preferably does not chemically react with the steel pipe material during piercing-rolling so as not to change the properties of the steel pipe material. The surface roughness of the anti-seizure layer (roughness based on JIS B0601 (2001)) should be lower than the surface roughness of the main body of the plug 10A (the base material of the plug 10A before forming the anti-seizure layer). is preferred.

A solid lubricant layer and an alloy layer will be described below as specific examples of the anti-seizure layer.

Solid Lubricant Layer As described above, according to the present invention, the plug tip portion 11 and the reeling portion 12 can be easily attached and detached. By replacing the seized plug tip 11, the service life of the plug 10A can be extended.
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. Of the members constituting the plug 10A, the plug tip portion 11 may be replaced as appropriate, so it is particularly preferable that an anti-seizure layer such as a solid lubricant layer is formed on the reeling portion 12 surface. In addition, the oxide scale formed at the joint between the plug tip 11 and the reeling portion 12 (the contact portion between the plug tip 11 and the reeling portion 12 that contacts the steel pipe material during piercing-rolling) is easily exfoliated. A seizure prevention layer such as a solid lubricant layer is preferably formed on the seam because the steel pipe material is likely to seize at the peeled portion. In particular, it is preferable that an anti-seizure layer such as a solid lubricant layer is formed on the joint on the reeling portion 12 side.
By forming the anti-seizure layer on the surface of the plug 10A in this way, the number of times the plug can be used until the plug tip 11 is replaced can be increased, and the life of the plug 10A can be extended.

The solid lubricant layer is preferably formed on an oxide scale layer formed on the plug base iron (base material). As a result, seizure can be prevented by the solid lubricant layer, and seizure can be prevented by the oxide scale even if the solid lubricant layer is peeled off.

A 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, and lubricants 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 measured using a steel plate having the chemical composition of the steel pipe material used and a test material having the chemical composition of the solid lubricant layer based on JIS P8147 (2010). obtained by
The method of applying the solid lubricant is not particularly limited, but examples include application by compression spray, brush, and PVD method.
In order to sufficiently secure the effect of improving the service life by forming the solid lubricant layer, it is necessary to make the surface roughness of the solid lubricant layer lower than the roughness of the plug base iron. Also, if the thickness (film thickness) of the solid lubricant layer is excessively increased, it may become easy to separate, 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 iron 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 0.9. More preferably, ≤h/R≤2.0.
Lubricant can be applied to the plug 10 by a compression spray, brush, PVD method, or the like so that the ratio of the surface roughness R of the plug base iron to the film thickness h of the solid lubricant layer satisfies the above range. preferable.

Alloy Layer The anti-seizure 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.
As with the solid lubricant layer, the alloy layer can also improve the service life by preventing seizure.
Although not particularly limited, the Mo-based alloy layer preferably contains 60 to 99.9% by mass of Mo. The Co-based alloy layer preferably contains Co: 43 to 50% by mass. The Ni-based alloy layer preferably contains Ni: 45 to 65% by mass. The W-based alloy layer preferably contains 90 to 97% by mass of W.

The alloy layer has higher adhesion to the plug base iron 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 iron, and the plug having the Mo-based alloy layer formed thereon is more stable than the plug having the solid lubricant layer formed thereon. can also improve life. In addition, since the Mo-based alloy, Co-based alloy, Ni-based alloy, and W-based alloy forming the alloy layer are different alloys from the steel pipe material alloy containing Fe and the like, seizure of the plug 10A is suppressed. .

Further, the mechanism of seizure prevention by the alloy layer will be described in detail by taking the Mo-based alloy layer among the alloy layers as an example. It has an intermediate layer that suppresses metal contact with the plug base iron. Further, the Mo-based alloy layer has a film formed between the intermediate layer and the steel pipe material such as the billet B by sublimation of Mo at about 800°C. Since this film is a gas, it is presumed that the reduction in the hot strength of the plug 10 can be suppressed by the heat insulating effect. It is considered that such a mechanism can further improve the life of a plug having an alloy layer such as a Mo-based alloy layer formed thereon.

The Mo-based alloy layer can be formed by surface coating by spraying the Mo-based alloy onto the surface of the plug 10A. The Co-based alloy layer, the Ni-based alloy layer, and the W-based alloy layer are formed by coating the surface of the plug 10 with a Co-based alloy, a Ni-based alloy, and a W-based alloy, respectively, by thermal spraying. can be done.
Although the conditions for thermal spraying are not particularly limited, in order to avoid seizure due to metal contact resulting from early peeling during rolling, the plug base iron is exposed, and thermal spraying is performed by plasma overlay welding, plasma spraying, or PVD. you can
Thermal spraying is preferably carried out so that the alloy layer has a thickness of 0.1 to 1.5 mm.

In the present invention, when the anti-seizure layer is an alloy layer, the plug base iron and the alloy layer have the same or similar coefficients of linear expansion measured based on JIS Z2285 (2003) (alloy layer is preferably 50 to 150% of the plug base iron). As a result, it is possible to prevent cracks from occurring in the plug 10A during thermal spraying for forming the alloy layer, and to prevent cracks from occurring when the temperature of the plug 10A rises during piercing-rolling. .

(Modification)
Next, referring to FIGS. 5 and 6, the configuration of a plug that is a modification of this embodiment will be described. 5 and 6 are diagrams for explaining the configuration of a plug that is a modification of this embodiment.

A plug 10B shown in FIG. 5 has a tapered boundary surface between a plug tip portion 11 and a reeling portion 12 as compared with the plug 10A. More specifically, this boundary surface has a tapered shape that is inclined toward the drilling direction as it goes from the fitting protrusion 11X (fitted portion 12X) side toward the outer peripheral direction side. The plug 10B having such a configuration can reduce the load applied to the interface. Also, when the plug tip 11 in contact with the steel pipe material is pushed into the reeling portion 12 during piercing and rolling, it is possible to prevent the axis of the plug tip 11 in the piercing direction from deviating from the center axis of the plug 10B in the piercing direction. .

Further, in the plug 10C shown in FIG. 6, the facing surfaces of the fitting protrusion 11X and the fitted portion 12X are tapered compared to the plug 10A. More specifically, the fitting protrusion 11X and the fitted portion are arranged so that the cross-sectional area of the outer peripheral surface of the fitting protrusion 11X (the area of the vertical cross section in the drilling direction) gradually decreases in the direction opposite to the drilling direction. 12X is tapered. Since the plug 10C has such a configuration, the fitting protrusion 11X can be fitted into the non-fitting portion 12X without bringing the threaded portion of the plug distal end portion 11 into contact with all of the threaded portions on the reeling portion 12 side. can be combined. Therefore, compared to the plugs 10A and 10B shown in FIGS. 3 and 5, the plug tip portion 11 can be attached to the reeling portion 12 with fewer rotations. Also, when the plug tip portion 11 is detached from the reeling portion 12, it can be detached with a small number of revolutions.

The plug according to the first embodiment of the present invention has been described above. According to this embodiment, a partially threaded structure is adopted and the plug tip and reeling part are detachable. or deformation). Moreover, even if the plug tip portion deteriorates, only the plug tip portion can be easily replaced. In this way, the life of the plug can be improved.

The present embodiment also provides a piercing mill for manufacturing seamless steel pipes having this plug.

[Second embodiment]
Further, in the plug of the present invention, threading of the plug distal end portion 11 and the reeling portion 12 can be performed by connecting the plug distal end portion 11 and the reeling portion 12 and then allowing the plug distal end portion 11 to rotate. It is not limited to the examples described in the embodiments. For example, as shown in FIG. 3 and the like, the threading on the plug front end portion 11 side is not limited to the crest and the threading on the reeling portion 12 side is the trough.

FIG. 13 is a diagram for explaining the configuration of the plug 20 according to the second embodiment of the invention.

In the plug 20 of the present embodiment, the reeling portion 22 has the fitting protrusion 22X at the leading end in the drilling direction, and the plug leading end 21 is fitted to the fitting protrusion 22X at the trailing end in the drilling direction. It has a fitted portion 21X. At this time, the first threaded portion 21A and the first non-threaded portion 21B are formed in order in the drilling direction on the fitted portion 21X, and the second threaded portion 22X in the drilling direction is formed on the fitting protrusion 22X. A non-threaded portion 22B and a second threaded portion 22A that can be screwed into the first threaded portion 21A are formed in this order. , the plug tip portion 21 and the reeling portion 22 are connected.

That is, in the plug 10 (10A) of the first embodiment, the plug front end portion 11 is formed with the fitting projection portion 11X and the reeling portion 12 is formed with the fitted portion 12X. The plug 20 of FIG. In these configurations, the plug 20 and the plug 10 (10A) are different, but other configurations and functions based thereon are substantially the same.

For example, in FIG. 13, as shown in (b-1), the drilling direction length α1 of the first threaded portion 21A, the drilling direction length β2 of the second non-threaded portion 22B, and the drilling direction of the second threaded portion 22A The relationship between the length α2 and the drilling direction length β1 of the first non-threaded portion 21B preferably satisfies the following equations (1) and (2), as in the plug 10 of the first embodiment.
1.00<β2/α1≦5.00 Expression (1)
1.00<β1/α2≦5.00 Expression (2)
Further, similarly to the plug 10 of the first embodiment, the end portion of the first threaded portion 21A on the drilling direction side and the end portion of the second threaded portion 22A on the side opposite to the drilling direction are spaced apart by 1 to 20 mm. It is preferable that the plug tip portion 21 and the reeling portion 22 are connected.
Further, the above-described anti-seizure layer may be formed on the surface of the plug in the same manner as the plug 10 of the first embodiment.
A piercing mill for manufacturing seamless steel pipes having the plug 20 of this embodiment is also provided.
Moreover, even if this plug 20 is used, a seamless steel pipe can be manufactured by the same manufacturing method as that of the plug 10 .

The present invention will be further described in detail below based on examples. In addition, the present invention is not limited to the following examples. In order to confirm the effects of the present invention, plug Nos. of the present invention shown in Table 1 were tested. 1 to 9, 15, 16 to 17 and comparison plug Nos. 10 to 14 were prepared, and the number of possible perforations was investigated.

In Table 1, the plug shapes are classified as A to I. Shape A is the shape of the plug 10A shown in FIG. Shape B is the shape of the plug 10B shown in FIG. Shape C is the shape of the plug 10C shown in FIG. Shape D is the shape of plug 110D shown in FIG. Shape E is the shape of plug 110E shown in FIG. Shape F is the shape of plug 110F shown in FIG. Shape G is the shape of plug 110G shown in FIG. Shape H is the shape of plug 110H shown in FIG. Shape I is the shape of the plug 20 shown in FIG.

A plug 10A shown in FIG. 3 is a plug for piercing and rolling a steel pipe material, and is detachably connected to a plug tip 11 for piercing the steel pipe material. , the plug tip 11 has a fitting protrusion 11X at the rear end in the drilling direction, and the reeling part 12 has a fitting protrusion at the front end in the drilling direction A first threaded portion 11A and a first non-threaded portion 11B are formed in this order in the drilling direction of the fitting protrusion 11X. In the fitted portion 12X, a second non-threaded portion 12B and a second threaded portion 12A that can be screwed into the first threaded portion 11A are formed in order in the drilling direction. The plug distal end portion 11 and the reeling portion 12 are connected while the threaded portion 11A and the second threaded portion 12A are not screwed together.
In the plug 10B shown in FIG. 5, the interface between the plug tip portion 11 and the reeling portion 12 has a tapered shape as compared with the plug 10A.
In the plug 10C shown in FIG. 6, compared with the plug 10A, the cross-sectional area of the outer peripheral surface of the fitting protrusion 11X (the area of the vertical cross section in the drilling direction) is gradually reduced in the direction opposite to the drilling direction. The facing surfaces of the mating protrusion 11X and the fitted portion 12X are tapered.
A plug 110D shown in FIG. 7 is a plug in which a plug tip portion and a reeling portion are integrally formed, and the plug tip portion and the reeling portion cannot be detached.
In the plug 110E shown in FIG. 8, threading is not applied to the connecting portion between the plug tip portion 11′ and the reeling portion 12′, and the fitting protrusion of the plug tip portion 11′ is covered by the reeling portion 12′. It is fitted and connected to the fitting portion.
In the plug 110F shown in FIG. 9, similarly to the plug 110E, threading is not applied to the connecting portion between the plug tip portion 11'' and the reeling portion 12'', and the fitting projection of the plug tip portion 11'' is not threaded. is fitted to the fitted portion of the reeling portion 12'', and the fitting protrusion is formed in a tapered shape.
In the plug 110G shown in FIG. 10, unlike the plug 10A, only the threaded portion is formed in the fitting protrusion, and the non-threaded portion is not formed. Also, only the threaded portion is formed in the fitted portion, and the non-threaded portion is not formed. In the plug 110G, these threaded portions are screwed together to connect the plug tip portion 11''' and the reeling portion 12'''.
In the plug 110H shown in FIG. 11, a non-threaded portion and a threaded portion are formed in order in the drilling direction in the fitting protrusion, and a threaded portion and a non-threaded portion are formed in the drilling direction in the fitted portion. Processed parts are formed in order.
In the plug 20 shown in FIG. 13, a fitting protrusion is formed at the tip of the plug and a fitted part 21X is formed at the tip of the plug 10A, unlike the plug 10A in which the fitted part is formed at the reeling part. , and the reeling portion 22 is formed with a fitting projection portion 22X.

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

The definition of the thread ridge shape of the threaded portion of the present invention in Table 1 will be described with reference to FIG. As shown in FIG. 12( a ), circle means that the outer surface has a fan-shaped thread shape. As shown in FIG. 12(b), mm indicates a triangular shape with a thread angle of 60 degrees. As shown in FIG. 12(c), the wit indicates a triangular shape with a thread angle of 55 degrees. As shown in FIG. 12(d), a trapezoid indicates that the angle of the thread is 30 degrees and the crest and valley tops are flat. As shown in FIG. 12(e), corner refers to a square cross-section of the thread. The height of the thread is 1 mm. Plug No. Since 10 to 12 do not have a threaded portion, the thread shape is indicated as "-".

Also, plug No. 1 having a plug shape of A, B, C or I is used. For 1 to 9 and 15, the drilling direction length α1 of the first threaded portion 11A (21A), the drilling direction length β1 of the first non-threaded portion 11B (21B), and the drilling of the second threaded portion 12A (22A) Regarding the direction length α2 and the drilling direction length β2 of the second non-threaded portion 12B (22B), α1=α2 and β1=β2. Hereafter, α1 and α2 are represented as α, and β1 and β2 are represented as β.

The drilling direction length (screw length) α of the threaded portions 11A (21A) and 12A (22A) was all set to 7 mm. The gap (mm) between the plug tip portion 11 (21) and the reeling portion 12 (22) is defined as β-α, and the gap and β/α of each plug are as shown in Table 1. Plug No. Since plugs No. 10 to 12 do not have a threaded portion, plug Nos. Since 13 does not have a non-threaded part, the gap is marked as "-".

Hot work tool steel SKD61 was used as the plug component of each plug for both the plug tip portion and the reeling portion. Then, each plug No. 1 having a plug tip portion and a reeling portion, which was manufactured in a state in which the plug tip portion and the reeling portion were separated, was integrated. For Nos. 1 to 15, the plug material was held in an air furnace at 950° C. for 4 hours, 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.

Moreover, the plug No. No. 16 formed a solid lubricant layer on the surface of the plug as an anti-seizure layer.
Regarding the formation of the solid lubricant layer, first, a solid lubricant containing 20 to 30% by mass of graphite was heated to 40 to 80° C. in order to lower its viscosity and make it easier to apply to the plug surface. A solid lubricant layer was formed by applying this solid lubricant to the surface of the plug with a brush.
The coefficient of dynamic friction under simulated rolling conditions was 0.08, and the ratio h/R between the film thickness h (mm) of the solid lubricant layer and the surface roughness R of the plug base iron was 2.0.

Moreover, the plug No. No. 17 formed a Mo-based alloy layer as an anti-seizure layer on the surface of the plug. The Mo-based alloy layer is formed by thermally spraying a Mo-based (TZM) alloy consisting of 98.5% Mo, 0.5% Ti, and 1.0% Zr on the plug surface using a plasma thermal spray method. formed. The thickness of the alloy layer was 0.8 mm.
Regarding the coefficient of linear expansion of the plug base iron and the Mo-based alloy layer measured based on JISZ2285 (2003), it was confirmed that the linear expansion coefficient of the Mo-based alloy layer was 55% of that of the plug base iron. It was also confirmed that the surface hardness of the Mo-based alloy layer (hardness based on JISZ2244 (2009)) is higher than the hardness of the plug base iron.

The billet material used in the evaluation experiment of this example was SUS420J2 (mass%, C: 0.3%, Si: 1.0%, Mn: 1.0%, Cr: 13%, and the balance was Fe (steel having a chemical composition consisting of), and the billet size was φ58 mm × 250 L mm.

After heating the billet at an ambient temperature of 1280° C. for 1 hour, each plug was used to perforate the billet.
The perforation conditions are shown below. 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 interval SH was 65 mm, the shoe rotation speed was 1 rpm, and the lead L was 15 mm (see FIG. 2 for SH and L). .

Since the temperature of the plug increased due to friction after piercing and heat conduction from the billet, the plug was used again after the surface temperature of the plug reached 30°C or less according to the contact thermometer. The plug life was compared when the plugs were repeatedly used under the above conditions.

In Table 1, the plug life was determined by 3D shape measurement. Specifically, the surface temperature of the plug after rolling was set to 30°C or less by water cooling or air cooling, and the measurement was performed using the plug that was left in an environment with an atmospheric temperature of 25°C for one day or longer. The shape of the plug was measured using an ATOS Compact Scan manufactured by gom as a measuring device.
At this time, the life of the plug is limited to the case where the volume ratio of each plug before the first piercing-rolling is deformed by more than 10%, or the billet stops being pierced in the middle and even a part of it is intact. It is the number of times the plug has been used (total number of uses in Table 1) until it does not become a tube.
In the present invention, plugs with a plug life of 7 times or more were considered acceptable.
However, for the detachable plug, even if the plug tip 11 was deformed by more than 10%, if the plug was detachable, the plug tip 11 was replaced and the plug was continuously drilled if the plug was detachable. When the deformation of the plug tip 11 (excluding the threaded portion 11A) makes it impossible to attach or detach (tip deformation in Table 1), or when the reeling portion 12 is deformed, the plug is deformed by more than 10%. (deformation of the reeling portion in Table 1), or although the tip end portion 11 and the reeling portion 12 were not damaged, the threaded portions 11A and 12A were damaged, making it impossible to attach and detach the plug tip portion 11 again. The life of the plug was defined as the time when the plug was damaged (thread damage in Table 1).

Table 1 shows various results. In Table 1, the total number of times the plug has been used (total number of uses in Table 1) and the number of times the plug tip T has been replaced (the number of times the tip has been replaced in Table 1) are listed, and the final plug life factor is determined. I put "re" in the top.

Example No. of the present invention. In 1 and 2, the plug tip portion 11 could be replaced four times until the reeling portion 12 was deformed. In addition, all of the plugs of the other examples of the present invention also allow the plug tip 11 to be replaced at least once, and have a lifespan of 7 times or more. It was possible to improve the life compared to 10.

From the above, it has been clarified that the use of the plug of the present invention provides a long service life.

Plug No. with an anti-seizure layer formed. 16 was able to replace the plug tip portion 11 four times until the reeling portion 12 was deformed. In addition, since the solid lubricant layer serves as a protective layer, the number of times of use until the plug tip portion 11 is replaced once has increased.

Plug No. In No. 17, the plug tip portion 11 could be replaced five times until the reeling portion 12 was deformed. Plug No. 17, No. As with 16, not only did the number of times of use before replacing the tip portion 11 of the plug increase once, but also the wear resistance of the reeling portion 12 was improved by the Mo-based alloy layer with high hardness, which extended the service life of the plug. It was found that the number of exchanges of the tip portion 11 increased and a longer life was obtained.

On the other hand, the No. 1 of the comparative example of the conventional plug. In No. 10, the tip portion of the plug and the reeling portion were integrally formed, and the number of times the plug was used was 5 times.
Moreover, No. of the comparative example. In No. 11, during the piercing and rolling process, the fitting protrusion was heated to a high temperature and joined to the fitted part, so that the tip of the plug could not be removed. 5 times as well as 10.

Moreover, No. of the comparative example. 12 is No. As in 11, the fitting protrusion is heated to a high temperature during the piercing and rolling process and is joined to the fitted part. Since the cross-section of the opening of the reeling portion is smaller than that of the reeling portion, the tip portion of the plug cannot be attached and detached. 5 times as well as 10.

Moreover, No. of the comparative example. In 13, the non-threaded portion is not formed on the fitting protrusion in the drilling direction, only the threaded portion is formed, and the non-threaded portion is not formed on the fitted portion. Since only the threaded portion was formed, the threaded portion was damaged, and the tip of the plug could not be removed.

Moreover, No. of the comparative example. In 14, unlike the plug of the present invention, a non-threaded portion and a threaded portion are formed in order on the fitting protrusion in the piercing direction, and a threaded portion and a non-threaded portion are formed on the fitted portion. Furthermore, since the drilling direction length α of the threaded portion and the drilling direction length β of the non-threaded portion were the same and no gap was formed, the threaded portion was damaged and the plug The tip part could not be attached and detached, and the total number of times the plug was used was 5 times.

10, 10A, 10B, 10C Plug 11 Plug tip 11X Fitting projection 11A First threaded portion 11B First non-threaded portion 12 Reeling portion 12X Fitted portion 12A Second threaded portion 12B Second non-threaded Processed portion 13 Parallel portion 20 Plug 21 Plug tip portion 21X Fitted portion 21A First threaded portion 21B First non-threaded portion 22 Reeling portion 22X Fitting protrusion 22A Second threaded portion 22B Second non-threaded portion Part U, U2 Joining part 2a, 2b Rolling roll 3 Mandrel 4a, 4b Disk shoe 100 Rotary hearth type heating furnace 101 Piercing mill 102 Mandrel bar 103 Mandrel mill 104 Reducer B Billet S Hollow

Claims (8)

A plug for piercing and rolling a steel pipe material,
a plug tip for drilling a steel pipe material;
a reeling part that is detachably connected to the tip part of the plug and expands the diameter of the drilled steel pipe material;
with
The plug front end portion has a fitting projection portion at the rear end portion in the drilling direction,
The reeling portion has a fitted portion that is fitted to the fitting projection portion at the leading end portion in the drilling direction,
A first threaded portion and a first non-threaded portion are formed in order in the drilling direction of the fitting protrusion,
A second non-threaded portion and a second threaded portion that can be screwed into the first threaded portion are sequentially formed in the fitted portion in the drilling direction,
the first threaded portion and the second threaded portion are in a non-threaded state, and the plug distal end portion and the reeling portion are connected;
The drilling direction length α1 of the first threaded portion and the drilling direction length β2 of the second non-threaded portion satisfy α1<β2,
A plug for manufacturing seamless steel pipes, wherein the drilling direction length α2 of the second threaded portion and the drilling direction length β1 of the first non-threaded portion satisfy α2<β1. A plug for piercing and rolling a steel pipe material,
a plug tip for drilling a steel pipe material;
a reeling part that is detachably connected to the tip part of the plug and expands the diameter of the drilled steel pipe material;
with
The reeling part has a fitting protrusion at the leading end in the drilling direction,
The plug front end portion has a fitted portion fitted to the fitting projection portion at the rear end portion in the drilling direction,
A first threaded portion and a first non-threaded portion are formed in order in the drilling direction of the fitted portion,
A second non-threaded portion and a second threaded portion that can be screwed into the first threaded portion are sequentially formed in the fitting protrusion in the drilling direction,
the first threaded portion and the second threaded portion are in a non-threaded state, and the plug distal end portion and the reeling portion are connected;
The drilling direction length α1 of the first threaded portion and the drilling direction length β2 of the second non-threaded portion satisfy α1<β2,
A plug for manufacturing seamless steel pipes, wherein the drilling direction length α2 of the second threaded portion and the drilling direction length β1 of the first non-threaded portion satisfy α2<β1. The drilling direction length α1 of the first threaded portion and the drilling direction length β2 of the second non-threaded portion satisfy the following formula (1),
3. The plug for manufacturing a seamless steel pipe according to claim 1, wherein the drilling direction length α2 of the second threaded portion and the drilling direction length β1 of the first non-threaded portion satisfy the following formula (2).
1.00<β2/α1≦5.00 Expression (1)
1.00<β1/α2≦5.00 Expression (2) The end portion of the first threaded portion on the drilling direction side and the end portion of the second threaded portion on the side opposite to the drilling direction are separated by 1 to 20 mm, and the tip end portion of the plug and the reeling portion are connected. The plug for manufacturing seamless steel pipes according to any one of claims 1 to 3, wherein The plug for producing seamless steel pipes according to any one of claims 1 to 4, wherein an anti-seizure layer is formed on a surface of the plug that comes into contact with the steel pipe material during piercing-rolling. 6. The plug for manufacturing seamless steel pipes according to claim 1, wherein the threads of said first threaded portion and said second threaded portion are round threads, trapezoidal threads or square threads. A piercing mill for manufacturing seamless steel pipes, comprising the plug for manufacturing seamless steel pipes according to any one of claims 1 to 6 . 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 6 .

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