JPS61137613A - Rolling method by plug mill and plug mill - Google Patents

Abstract

PURPOSE:To improve the quality of an expanded pipe by a simple mechanism and to prevent the spattering and overconsumption of a lubricant, by spraying the lubricant on a rolling part from a plug during plug-mill rolling, and blowing a high-pressure gas from the front end of a plug bar to the return direction during return stroke. CONSTITUTION:In plug mill rolling, a lubricant 14 is supplied 20 to the flow passage 19 of high-pressure gas 15 to supply the lubricant 14 from nozzles 39 to a rolling part through supplying pipe lines 17, 38 in order to lubricate the rolling part. In return stroke, a plug 3, 4, 35 is separated from a taper contact-surface 34, and the spraying is stopped by a spool 45 advanced by a spring 50, and only the high-pressure gas 15 is sent to a front-tip fittings 6 of plug by changing valves 20, 30 by the signal control 32 of a load cell 31 of roll 5b. Accordingly, the high pressure gas 15 is blown to the return direction from the flow passage and a nozzle 43, to descale and clean the inner surface of an expanded pipe 4, thereby improving the quality of the inner surface of pipe 4 and preventing the spattering and overconsumption of lubricant 14.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a plug mill rolling method among seamless steel pipe manufacturing methods and a plug mill for carrying out the method, and in particular to a method for supplying and descaling a rolling lubricant. The present invention relates to a method and apparatus for carrying out both of the above.

As is well known in the art, the plug mill rolling method is a tube manufacturing method using a plug mill as the main rolling machine.The raw tube made through a heating process and a perforation process is rolled thinly and long in a plug mill, and then rolled into a reeler. In this method, the inner and outer surfaces are finished cleanly using sipes, and finished to the specified dimensions using sipes.

In a plug mill, which is the main rolling mill that implements this method, a plug attached to the tip of a plug bar (mandrel bar) is inserted into the raw tube, and a pair of upper and lower circular hole rolls (caliber) are inserted into the raw tube. Normally, once rolling is completed, the tube is returned to the inlet side, rotated 90 degrees, rolled again, and then returned to the inlet side after rolling is completed. Rolling is carried out using so-called two buses.

Therefore, in a plug mill, since a frictional force is generated between the plug and the pipe, W8t is applied to the thread between the plug and the pipe in order to improve the life of the plug and reduce the rolling power. Conventionally, methods and devices for this purpose are disclosed, for example, in Japanese Patent Application Laid-open Nos. 49-112851 and 58-116.
910 and Japanese Patent Application Laid-open No. 58-167005. To briefly explain each of them, the device according to Japanese Patent Application Laid-open No. 49-113851 is a mandrel bar.
A plug is attached to the tip of a mandrel corresponding to , and an injection port is made to protrude forward at the center of the plug, and compressed air is supplied through an injection pipe provided inside the mandrel.
The structure is such that the S agent is injected from the injection port along the central axis of the raw pipe. Further, the method according to 5R Patent Publication No. 58-116910 is a method of continuously supplying lubricant by applying back pressure to the lubricant, and the method involves continuously supplying lubricant to the rolling part of the plug that is actually involved in rolling. A small diameter part is provided between the (bit part) and the guide part on the tip side, and lubricant under back pressure is supplied to the space formed by the small diameter part and the inner surface of the tube during rolling. As the process progresses, the am agent is introduced between the rolled part and the inner surface of the tube, thereby increasing mw over the entire length of the tube.
This is a method of supplying i-agent. Zarani Japanese Patent Publication No. 58-167
In the method according to No. 005, a guide plug is provided at a distance in front of the work plug, and high-pressure air for descaling is injected from a part between these plugs closer to the guide plug, and at the same time, a guide plug is provided in front of the work plug. This is a method in which lubricant is injected toward the inner surface of the tube from the location.

Problems to be Solved by the Invention However, among the above-mentioned conventional methods and devices, the first device disclosed in Japanese Patent Application Laid-open No. 112851/1980 is capable of discharging lubricant through an injection port protruding forward from the center of the plug. Since the lubricant is injected toward the inner surface of the tube, it is not necessarily possible to distribute the m'/R agent uniformly on the inner surface of the tube, and the lubricant is not efficiently applied to the rolling part of the plug that is directly used for rolling. Because the lubricant cannot be injected well, some of the lubricant may oxidize before it gets between the rolled part and the inner surface of the tube, and as a result, it is not possible to sufficiently extend the life of the plug or improve the quality of the inner surface of the tube. There's a problem.

Furthermore, the method according to JP-A No. 58-116910 is ffl
Although the lubricant is continuously supplied, since it is not directly supplied to the rolling section, a portion of the lubricant is oxidized or scattered, resulting in poor lubrication efficiency. In addition, in the plug mill rolling method, two-bath rolling is usually performed as mentioned above, but if back pressure is applied to the lubricant, the lubricant can be retained in the supply pipe line during the return process of the pipe. Therefore, there is a problem in that the piping is easily clogged, and the amount of lubricant supplied accordingly fluctuates. Generally speaking, this method has a problem in that the secondary scale generated in the first bath has an adverse effect, making it impossible to obtain a good lubrication effect.

Furthermore, the method according to Japanese Patent Application Laid-open No. 58-167005 mentioned last uses high pressure gas for descaling and fllW! In this method, the lubricant is injected at two locations close to each other in front of the plug, but if descaling is performed first and lubrication is performed subsequently, it is necessary to inject the descaling pressurized gas and the lubricant. It is difficult to control the timing of both, and if both are injected at the same time, part of the mPR agent will be blown away by the high-pressure descaling gas, and part of the scale will mix with the rA lubricant and cause damage to the plug and pipe inner surface. In any case, the lubrication efficiency was poor, and quality improvements such as prevention of bit flaws on the inner surface of the tube could not be sufficiently improved.

In view of the above circumstances, the present invention provides a plug mill rolling method that can effectively perform descaling and lubrication, thereby extending plug life, improving product quality, and reducing lubricant costs. It is an object of the present invention to provide a plug mill for carrying out the method.

Means for Solving the Problems In order to achieve the above object, the method of the present invention involves inserting a plug attached to the tip of a plug bar via a support fitting into a pre-drilled pipe to perform rolling. , during the rolling process, toward the rolling part that is in contact with the inner surface of the plug tube,
Inject rolling lubricant from the support fitting on the front side of the plug,
In addition, during the return process in which the tube is run in the opposite direction to the rolling direction, high-pressure gas for descaling is applied from the outer periphery of the tip of the plug bar diagonally rearward to the inner surface of the tube with respect to the running direction of the tube. This method is characterized by injection.

Furthermore, the plug mill of the present invention has a supply pipe for selectively flowing rolling lubricant and descaling gas inside the support fitting for attaching the plug to the tip of the plug bar and inside the plug bar along the axial direction. a lubricant injection port that communicates with the supply conduit and opens toward the rolled part that contacts the inner surface of the pipe of the plug, is formed on the outer peripheral part of the support fitting that protrudes toward the distal end side of the plug; In addition, a descaling gas injection port communicating with the supply pipe and opening diagonally rearward of the plug bar is formed on the outer periphery of the tip of the plug bar. Connect the pipe line to the lubricant injection port,
and a supply device that is provided with a switching mechanism that connects the supply pipe to the descaling gas injection port when the plug is removed from the plug bar, and further selectively sends rolling lubricant and descaling gas to the supply pipe. It is characterized by having the following.

In other words, in this invention, the plug attached to the tip of the plug bar is inserted into the inside of the pipe together with the plug bar to roll the pipe, and in that case, from the lubricant injection port that opens in front of the plug, Rolling lubricant is injected toward the rolled portion of the plug that is in contact with the inner surface of the tube. By attaching the plug to the tip of the plug bar, the supply pipe and the lubricant injection port are communicated with each other by a switching mechanism.
Further, the rolling lubricant is fed into the supply pipe by a supply device. When the first pass of rolling is completed,
A return process begins in which the pipe runs in the opposite direction. In that case, the plug is removed from the tip of the plug bar and falls to the outside of the tube, which causes the switching mechanism to switch and bring the supply line into communication with the descaling gas injection port, and the supply line A descaling gas is supplied by a supply device. Therefore, in the return process, descaling gas such as high-pressure air is injected from the descaling gas injection port opening diagonally rearward of the plug bar diagonally rearward with respect to the running direction of the tube and toward the inner surface of the tube.
The scale that causes bit defects is removed.

Therefore, according to the method and plug mill of this invention,
Descaling and lubrication are performed effectively.

Example Next, the present invention will be explained based on examples.

First, an embodiment of the plug mill according to the present invention will be described. FIG. 1 is a schematic diagram showing the overall configuration of the plug mill.
These plugs 2.3 are inserted into the tube 4, and the tube 4 is rolled by the work plug 2 and a pair of upper and lower rolls 5a and 5b. The plug bar 1 has a sealed rear end and a tip fitting 6 having the configuration shown in FIG. 2 at the tip.
The double tube 8 is coaxially inserted into the inside of the vibrator 7, which is screwed into the vibrator 7. Therefore, it has a substantially triple tube structure. Hollow part 9 at the outermost periphery inside the plugper 1
a and the intermediate hollow part 9b serve as cooling channels, and a water supply pipe 1o communicating with the intermediate hollow part 9b and a drain pipe 11 communicating with the outermost circumferential hollow part 9a are connected to the rear end of the plug par 1 with its radius. installed facing the direction. Furthermore, as shown in FIG. 2, inside the tip fitting 6, a folded hollow part 12 with a blind bow is formed which communicates with the outermost circumferential hollow part 9a and the middle hollow part 9b, so that water can be supplied from the water supply pipe 10. The cooling water is configured to cool the plug par 1 by flowing in the order of the intermediate hollow part 9b → the folded hollow part 112 → the outermost circumferential hollow part 9a and flowing out from the drain pipe 11. On the other hand, the pipe 13 forming a hollow portion along the axis passes through the tip fitting 6 and has an opening on the tip side of the plugper 1, and has a rear end protruding from the plugper 1, as shown in FIG. A supply device 16 for selectively supplying the ffl reagent 14 and high pressure gas 15 for descaling is connected to the protruding rear end thereof. That is, the inside of the pipe 13 serves as a supply conduit 17.

Here, the configuration of the supply device 16 will be explained.

ml of lubricant 14 is placed at the lower end of a hopper 18 that temporarily stores rolling lubricant 14 such as powdered or granular graphite.
A rotary pulp 20 is provided in the II agent pipe 19 to feed the pulp at a constant rate. On the other hand, the lubricant pipe 19
If! to a high pressure gas source (not shown) via the ta regulating valve 21 and the pressure reducing valve 22! On the other hand, it is connected to the pipe 13 via a cutoff valve 23 and a flexible pipe 24, and the lubricant 14 is transferred to the pipe 13 by using a gas obtained by reducing the pressure of the high-pressure gas 15 to, for example, 7 kg/car or less by a pressure reducing valve 22 as a carrier gas. It is designed to be fed into a supply pipe line 17. Also hopper 1
A lubricant supply pipe 25 is connected to 8, and
A pressure balance pipe 26 communicating with the lubricant pipe 19 is connected. That is, by communicating the hopper 18 and the 151WI agent pipe 19 through the pressure balance pipe 26, the pressures of both are balanced, and the feeding pressure is 1.
Rotary pulp 2 due to exceeding kg/clr
This is to prevent a reduction in the sealing performance of the 0 and the accompanying backflow of the ms agent 14 into the hopper 15.

A descaling gas pipe 27 is provided bypassing one of the lubricant pipes, which pipe 27 is connected on the one hand to a high pressure gas source via a flow regulating valve 28 and a pressure reducing valve 29, and on the other hand to the descaling gas pipe 27. It is connected to a branch between the valve 23 and the flexible tube 24 via a cutoff valve 30.

Note that the pressure reducing valve 29 on the descaling gas pipe 27 side is
The gas pressure is set to be, for example, 7 to 30 kMd. Each of the shutoff valves 23 and 30 is electrically controlled, so that during rolling, the shutoff valve 23 on the lubricant pipe 19 side is opened and the shutoff valve 30 on the descaling gas pipe 27 is closed, and the opposite is true during the return process. Shutoff valve 23 on the lubricant pipe 19 side
is closed, and the cutoff valve 30 on the descaling gas pipe 27 side is opened. Such control is achieved by, for example, attaching a load cell 31 to the drive mechanism of the rolls 5a and 5b to detect the magnitude of the load, as shown in FIG. This can be done by determining whether it is a process or a return process, and operating each shutoff valve 23, 30 based on the output signal of the v1m device 32.

By the way, the plug 2.3 is installed at the tip of the plugper 1 by the axial force during rolling, and in the return process, the rear end of the tube 4 during rolling slips out and is no longer supported by the tube. This structure allows the plug to naturally fall from the plugper 1 at the completion of rolling. That is, the guide plug 3 has a shape in which the maximum outer diameter is smaller than the maximum outer diameter of the workpiece plug 2, and each plug 2.3 is attached to a support metal fitting with the guide plug 3 disposed on the tip side as shown in FIG. It is integrally attached to 33. The support fitting 33 includes a shaft 35 having a truncated conical rear end, a tapered surface 34, and a threaded tip, and a spacer 36 that is fitted into the shaft 35 to keep the distance between the plugs 2.3. , shaft 3
After the work plug 2 is fitted to the rear end flange of the shaft 35, a spacer 36 is interposed between the work plug 2 and the guide plug. Each plug 2.3 is attached to the support fitting 33 by fitting the plug 2.3 onto the shaft 35 and tightening the nut 37 in this state.

Further, the shaft 35 has a supply conduit 38 extending along the axis at a predetermined length from the rear end thereof, and the supply conduit 38 is formed at a predetermined position on the front side of the workpiece 2 in the support fitting 33. A plurality of lubricant injection ports 39 are formed which communicate with each other. Specifically, the lubricant injection port 39 is formed on the outer periphery of the spacer 36, and as shown in FIG. It is opened so that the lubricant 14 is injected toward the rolling section 40 directly provided for use, more precisely toward the rolling start point 41.

Therefore, when the size of the work plug 2 changes, it is necessary to change the angle of the lubricant m118 port 39, and this can be done by replacing the spacer 36. Note that the inner diameter of the lubricant injection port 39 is the same as that of the lubricant 1.
It is necessary to configure settings to prevent blockage due to 4.
- For example, it is preferably 51 or more.

On the other hand, the tapered surface 34 formed at the rear end of the shaft 35 is for aligning the axial center of the support fitting 33, that is, the axial center of the plug 2.3 with the axial center of the plugper 1. At the center of the end fitting 6 attached to the
A tapered recess 42 that matches the tapered surface 34 is formed.

A plurality of descaling gas injection ports 4 are provided on the outer periphery of the tip of the plugper 1 and communicate with the supply pipe 17 inside the plugper 1.
3 is formed, and a switching mechanism 44 that switches the supply pipe line 17 to communicate with the descaling gas injection port 43 and the supply pipe line 38 of the support fitting 33 is built in the tip of the plugper 1. has been done. The descaling gas injection port 43 is for injecting a gas for removing scale, such as pressurized air, during the return process.
As shown in FIGS. 4 and 5, it opens obliquely in the opposite direction to the running direction of the pipe 4 during the return process, that is, toward the rear end of the plugper 1, and its angle θ2 applies a shearing force to the scale. A range of 15 to 45 degrees is suitable for giving the same effect, and it is effective if the inner diameter is 101111 or less and the number is 8 or more. In addition, the switching mechanism 44 is connected to the spool 4.
5, and the spool 45 is connected to the supply pipe 1 as shown in FIGS. 2, 4, and 5.
7, and is arranged so that it can be moved back and forth in the axial direction by a predetermined stroke S when attached to the inner circumferential surface of the tip fitting 6. Two communication holes 46 extending to the outer circumferential surface are formed, and a hollow portion 47 that accommodates the tip of the vibrator 13 and opens toward the tip is formed on the front end side thereof. On the other hand, a small hole 48 is formed in the circumferential portion of the distal end of the vibrator 13, which opens into the hollow portion 47 when the spool 45 is at the backward end, and which opens into the communication hole 46 when the spool 45 is at the forward end. has been done. Further, the inner circumferential surface of the tip metal fitting 6 is provided in communication with the descaling gas injection port 43 and with the vibrator 1.
An annular groove 49 facing the small hole 48 of No. 3 is formed.

Then, the spool 45 connects the support fitting 33 to the tip fitting 6.
When the shaft 3 is fitted into the tapered recess 42 of
The spring 50 is set to a length such that it is pushed by the rear end surface of the spool 45 and positioned at the retreating end, and a spring 50 is arranged on the rear end side of the spool 45 to push the spool 45 forward.

Next, the operation of the plug mill configured as described above, that is, the method of the present invention will be explained.

Each plug 2.3 is placed on the distal end side of the plugper 1 while attached to the support fitting 33, and by pushing the preheated tube 4 toward the plug 2.3, each plug 2.3 is attached to the support fitting 33.
3 is attached to the tip of the plugper 1 together with the support fitting 33, and the pipe 4 is guided by the guide plug 3 and fitted into these plugs 2.3, and the workpiece plug 2 and the roll 5a.

It is caught between 5b and 5b. In that case, the rear end tapered surface 34 of the shaft 35 in the support fitting 33 fits into the tapered recess 42 of the tip fitting 6 in the plugper 1, so that the axes of the shaft 35 and the plugper 1 coincide, and each plug 2.3 centering is performed. Further, by fitting the shaft 35 in this manner, the spool 45 is moved back against the spring 50 at its distal end, so that the supply conduit 17 in the plugper 1 is connected to the vibrator 13 as shown in FIG. It communicates with the supply pipe line 38 in the support fitting 33 via a small hole 48 formed at the tip and a hollow part 47 of the spool 45 . On the other hand, by catching the tube 4 between the work plug 2 and the rolls 5a and 5b,
Because the load increases, the drive 111 of the 0-rules 5a and 5b
The load cell 31 attached to the m structure detects an increase in load,
Accordingly, the vibrator device 32 outputs a signal to open the cutoff valve 23 on the lubricant pipe 19 side and close the cutoff valve 30 on the descaling gas pipe 27 side. In addition, when the time delay of injection is large, the injection timing may be determined by setting a timer from the start of pushing the tube.

Therefore, the pressure was reduced to, for example, 7 kaldLX by the pressure reducing valve 22, and the gas was discharged in a fixed amount from the rotary valve 20, the flow rate of which was controlled by the II adjustment valve 21.
lubricant 14 is fed into supply line 17. The lubricant 14 is injected from the lubricant injection port 39 in the support fitting 33 because the supply pipes 17 and 38 communicate with each other as described above. In that case, since the lubricant injection port 39 opens toward the rolling portion 40 in the workpiece plug 2, particularly toward the rolling start point 41, l? The ll agent 14 is applied to the work plug 2 without causing any waste such as scattering or oxidation.
and the inner surface of the tube 4. In addition, ff1ffl
In order to more reliably prevent scattering of the agent 14, the pressure of the carrier gas is set to 0 to o, zka/ad at the rolling start point 41.
(gauge pressure). Further, the flow rate of the lubricant 14 needs to be set according to conditions such as the type of lubricant and the rolling speed, but for example, in the case of granular graphite, the flow rate is 2000 Nm'/wa for a gas flow rate of 0.2 Nm'/wa.
~3500cc/me is suitable.

When the first bus rolling is completed as described above, each plug 2.3 protrudes from the pipe 4, and the pipe 4 is caused to run in the opposite direction by a return roll (not shown).
Move on to the return process. In the return process, the rear end of the tube 4 during rolling falls out and is no longer supported by the tube, so that each plug 2.3 is attached to the support fitting 33 at the end of rolling.
and fall to the tip side of plugper 1 as one. As a result, the spool 45 is pushed by the spring 50 and moves forward as shown in FIG. It coincides with the annular groove 49 and communicates between the two. That is, the supply pipe line 17 in the plugper 1 communicates with the descaling gas injection port 43. On the other hand, during the return process, the rolls 5a and 5b are separated from the pipe 4 and are not driven, so the load detected by the load cell 31 becomes zero, and therefore the output signal of the control device 32 switches the cutoff valves 23, 30. , fii
I? The cutoff valves 23 of the 19 IT agent pipes are closed, and the cutoff valves 30 of the descaling gas pipes 27111 are opened. As a result, the high pressure gas 15 is discharged by the pressure reducing valve 29, for example, from 7 to 3
The pressure is reduced to about 0 kMcd, and the flow is controlled by the R1 regulating valve 28 and sent to the supply pipe line 17 of the plugper 1.

The 7 to 30 kg fed into the supply pipe 17 in this way
A gas at a pressure of about Md, that is, a descaling gas, is
The descaling gas is injected from the descaling gas injection port 43 at the tip of the plugper 1 toward the inner surface of the plugper 14 through the communication hole 46 of the spool 45 .

In the process of returning the tube 4, there is a time delay of about 1 second until the rear end of the tube 4 returns to the position of the rolls 5a, 5b and fits into the tip of the plugper 1. The opening operation of the cutoff valve 30 on the descaling gas pipe 27 side is preferably delayed by a timer or the like. Since the injection port 43 of the descaling gas opens diagonally toward the rear of the plugper 1, the descaling gas is
As a result, scale, iron pieces, etc. on the inner surface of the tube 4 are blown away and removed.

When the return step of the first bus is completed in this manner, the plug 2.3 is again attached to the tip of the plugper 1, and the rolling and return steps are performed in the same manner as in the first bus. This descaling in the second bath removes scale, iron pieces, etc. from the inner surface of the tube, so that it is possible to effectively prevent the occurrence of inner surface defects in the next process of reel milling.

Here, an experimental example and a comparative example conducted to confirm the effects of the present invention will be shown.

Experimental Example A carbon steel pipe was used, and its dimensions were as follows.

Inlet side: outer diameter 249 x wall thickness, 15.85 x length 9910
(av) Output side: outer diameter 242 x wall thickness 12,35 x length 1
2807 (ms) The outer diameter of the plug was 216a+m.

Rough lubrication conditions were as follows: lubricant side flow: 13000 cc/m, carrier gas flow: 10.2 Nm'/mm.

The pressure of the descaling gas was 25 ka/d.

The materials and plugs used in the comparative example were the same as those in the above experimental example. Furthermore, lubrication is carried out by the method shown in JP-A No. 58-116910 mentioned above, and the lubricant flow rate in that case is 3.
000 cc/mesh, and the carrier gas flow rate was set to 0.2 Nm'/m.

When we investigated the occurrence of tube inner surface defects and plug life in the above experimental examples and comparative examples, we found that in the experimental examples according to the present invention, the quality deteriorated due to tube inner surface defects compared to the comparative example using the conventional method. (Degradation of product) and the maintenance rate for repairing defects decreased by about 5%. Additionally, the plug life has been improved by approximately 1.5 times.

Effects of the Invention As is clear from the above explanation, according to the method and plug mill of the present invention, a mm agent is injected toward the rolling part that contacts the inner surface of the plug tube during the rolling process, and a descaling gas is injected during the returning process. m
The WI agent is not unnecessarily scattered or oxidized without being used for lubrication, and scale is not mixed into the rIi lubricant, so lubrication can be performed efficiently. Therefore, it is possible to improve the life of the plug, prevent the occurrence of scratches on the inner surface of the tube, reduce the drop rate, and furthermore, reduce the cost required for lubricant. The plug mill of the present invention has a structure in which the B lubricant injection and the descaling gas 1IIa4 are selectively performed by the switching bridge that switches depending on the presence or absence of the plug, so the plug bar is smaller than the original one. FR'+ inside
'a can be simplified.

[Brief explanation of drawings]

Fig. 1 is a schematic overall information diagram showing one embodiment of the plug mill of the present invention, Fig. 20 is an enlarged sectional view of the tip of the plug bar, Fig. 3 is the shut plug bar of the support fitting,
2... Work plug, 4... Pipe, 5a, 5b.
... Roll, 14... Lubricant, 16... Supply device, 17.38... Supply pipe line, 33... Support fitting, 39... Lubricant injection port, 40... Rolling section , 4
DESCRIPTION OF SYMBOLS 1... Rolling start point, 43... Descaling gas injection port, 44... Switching mechanism, 45... Spool.

Claims (2)

[Claims] (1) When inserting a plug attached to the tip of a plug bar via a support fitting into a pre-drilled pipe and rolling it, a rolling lubricant is applied to the rolled part of the plug in contact with the inner surface of the pipe during the rolling process. In the return process in which the tube is run in the opposite direction from the rolling direction, the jet is injected from the support fitting on the front side of the plug, and in the return process in which the tube is run in the opposite direction from the rolling direction, the jet is injected diagonally backward to the running direction of the pipe and toward the inner surface of the tube. A plug mill rolling method characterized by injecting high pressure gas for scaling from the outer periphery of the tip of a plug bar. (2) In a rolling mill in which a plug is removably attached to the tip of a plug bar via a support fitting and the plug is inserted into a pre-drilled tube to perform rolling, rolling lubricant and descaling gas are selectively applied. A lubricant supply pipe is provided inside the plug bar and the support fitting along the axial direction thereof, and the supply pipe is connected to the supply pipe and opens toward the rolled part that contacts the inner surface of the pipe of the plug. A agent injection port is formed on the outer periphery of the support fitting that protrudes toward the tip end side of the plug, and a descaling gas injection port that communicates with the supply pipe and opens diagonally rearward of the plug bar;
The supply pipe is formed on the outer periphery of the tip of the plug bar, and when the plug is attached to the tip of the plug bar, the supply pipe is connected to the lubricant injection port, and when the plug is removed from the plug bar, the supply pipe is connected to the lubricant injection port. 1. A plug mill comprising: a switching mechanism for communicating a descaling gas with a descaling gas injection port; and a supply device for selectively feeding a rolling lubricant and a descaling gas into the supply pipe.