JPH0747173B2 - Hot extrusion tube making method and mandrel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hot-extrusion pipe manufacturing method for producing a seamless pipe by hot extrusion and a mandrel used therefor.

[Conventional technology]

According to the Eugene extrusion method, which is a typical hot extrusion tube manufacturing method, as shown in FIG. 9, first, a heated hollow hollow billet 3 is charged into the container 1. The container 1 has a die holder 6 with a die 2 attached to its front end.
Abutting against.

Next, the mandrel 4 is inserted through the hollow billet 3, and the hollow billet 3 is pushed forward by a stem (not shown) via a dummy block (not shown). Mandrel 4
Since the tip of the hollow die is inserted into the die 2 with a gap, by pushing the hollow billet 3 forward, the hollow billet 3 is pushed forward from the annular gap between the die 2 and the mandrel 4, and the extrusion pipe Become P.

Since the mandrel 4 moves forward as the stem advances, the mandrel 4 remains in the bottom portion of the extruded tube P at the end of extrusion. Therefore, after the extrusion, the mandrel 4 is pulled out rearward from the inside of the bottom portion of the extrusion pipe P.

In the production of a seamless pipe by such a Eugene method, the outer diameter of the extruded pipe P extruded in front of the die gradually increases in the so-called middle part from the part excluding the top end part to the bottom end part. It is known that the outer diameter of the bottom end portion increases sharply toward the end surface. The extreme large-diameter phenomenon at the bottom end is attributed to the fact that the mandrel 4 hinders thermal contraction of the extruded tube P that has been extruded.

That is, the extruded pipe P extruded in front of the die 2 is gradually cooled, and the middle part of the extruded pipe P contracts with the cooling, but the bottom end portion in which the mandrel 4 is inserted is Since the inner surface side is constrained by the mandrel 4, shrinkage due to cooling does not occur. Therefore, the outer diameter of the bottom end portion becomes extremely larger than the outer diameter of the middle portion, and as a result, the dimensional tolerance is likely to deviate in this portion.

In order to solve this problem, for example, JP-A-63-235020
Japanese Patent Publication discloses a hot extrusion tube manufacturing method using a mandrel having a tip end enlarged in diameter. According to this pipe manufacturing method, the bottom end portion into which the mandrel is inserted at the end of pipe manufacturing has a portion facing the expanded diameter portion of the mandrel tip,
The inner peripheral surface is cooled in a restrained state, but in other parts, the inner peripheral surface is not restricted by the outer peripheral surface of the mandrel and cooling proceeds,
As a result, the extreme expansion phenomenon at the bottom end is alleviated.

[Problems to be Solved by the Invention]

However, in this hot extrusion tube manufacturing method, although the extreme large-diameter phenomenon at the bottom end of the extruded tube is mitigated, the moderate large-diameter phenomenon in the middle part from the vicinity of the top to the bottom end is almost eliminated. Even if the bottom end is not eliminated, the outer diameter tends to gradually increase toward the end face. That is, the phenomenon that the outer diameter gradually increases in almost the entire region of the extruded pipe from the middle portion to the bottom end excluding the top end still remains. The phenomenon of large diameter after the middle part becomes more remarkable as the amount of increase in pipe diameter due to the enlarged portion of the mandrel tip increases, especially when extruding a long pipe, the fluctuation of the pipe outer diameter in the axial direction becomes large, and There are major problems such as a decrease in yield due to the removal of the outlying portion and a decrease in efficiency due to the forced extrusion of the short tube.

The outer diameter of the extruded pipe top portion gradually increases toward the top side, but since the amount is small and the range is narrow, the outer diameter fluctuation does not pose a problem as much as the portion after the middle portion.

The present invention has been made in view of the above circumstances, and is not limited to an extreme large-diameter phenomenon at the bottom end portion, but can also significantly reduce a moderate large-diameter phenomenon after the middle portion. And to provide mandrels.

[Means for Solving the Problems]

Regarding the phenomenon of the large diameter of the extruded pipe in the hot extruded pipe, the extreme phenomenon of the bottom end is caused by the constraint of the heat shrinkage of the extruded pipe by the mandrel, but the outer diameter gradually increases after the middle part. The exact cause of the phenomenon has not been clarified. However, recent research by the present inventors has revealed the following facts, and has also clarified the cause of the gradual increase in diameter after the middle part.

i) The gradual increase in the outer diameter of the extruded pipe after the middle portion becomes more noticeable as the extrusion speed decreases, and when using a mandrel with an enlarged tip, as described above, the pipe diameter increases due to the enlarged portion. The larger the amount, the more prominent.

ii) The mandrel part (hereinafter referred to as the upset part) located in the die at the time of upset that presses the billet inserted into the container to fill the inside of the container. The temperature rises significantly compared to the site, and sometimes even red heat.

iii) The change in wall thickness is small compared to the increase in the outer diameter of the extruded tube.

From the above facts, it is found that the increase in the outer diameter after the middle part of the extruded pipe is caused by the gradually increasing diameter of the extruded pipe by the upset part of the mandrel that is most significantly thermally expanded on the exit side of the die. It was That is, as the extrusion progresses, especially the upset portion of the mandrel continues to expand due to heat absorption and frictional heat generation from the extruded tube, and the outer diameter of the extruded tube gradually increases accordingly.

The present invention has been made based on the above findings, and the outer diameter thermal expansion of the mandrel body made of a metal material is smaller than that of the mandrel body, and the outer diameter thereof is 0.5 to 3% larger than that of the mandrel body. The gist is a hot extrusion tube-making method, which is characterized by using a mandrel having a large portion.

The mandrel of the present invention used in the hot extrusion tube manufacturing method is a mandrel body made of a metal material, and is held concentrically at the tip of the mandrel body, and is 0.5 to 3% of the outer diameter of the mandrel body. A mandrel having an annular body or a cylindrical body made of a material having a larger outer diameter and a smaller coefficient of thermal expansion than the metal material, a body made of a metal material, and concentrically held at the tip of the mandrel body. A heat insulating coating layer having an outer diameter larger than that of the mandrel body by 0.5 to 3% and made of the metal material or a metal material equivalent to the metal material and having heat resistance and wear resistance at least on the outer peripheral surface thereof. Mandrel having an annular body or a cylindrical body formed by forming a core, and further, concentrically and integrally formed at the tip of the mandrel body made of a metal material, and having heat resistance and abrasion resistance on its outer peripheral surface. It forms a thermal barrier coating layer having a sex, a mandrel having a large diameter portion having a 0.5% to 3% larger outer diameter than the outer diameter of the mandrel body.

[Action]

In the present invention, the extruded tube whose diameter is expanded by the upset portion of the mandrel main body is further expanded by the large-diameter portion formed directly on the tip of the mandrel main body or by holding the annular body or the cylindrical body. The large-diameter portion has a smaller thermal expansion than the mandrel body even when it slides on the extruding tube during the extruding work, and the outer diameter is kept substantially constant throughout the extruding period.

That is, as the material of the mandrel used for hot extrusion, a metal material conventionally represented by JIS-SKD61, a so-called hot tool steel or for the purpose of improving seizure resistance, the outer surface thereof is plated with Cr or A scaled product is used, and the mandrel body used in the present invention is also made of the same or similar material, which is very simple and inexpensive. However, the large-diameter portion provided at the tip of the mandrel has a smaller thermal expansion than the above metal material with respect to sliding with the extruded tube involved in the extrusion work, and therefore has a larger dimension than the upset portion of the mandrel body. The fluctuation is small. Therefore, the outer diameter of the middle portion of the extruded pipe extruded forward from the large-diameter portion with small dimensional variation is kept substantially constant. Also, the bottom end of the extruded tube, into which the mandrel is inserted at the end of extrusion, is finished to have the same outer diameter as the other parts by passing through the large-diameter portion as the mandrel is pulled out. With a conventional mandrel with an expanded tip,
The reason why the variation of the outer diameter of the middle part cannot be suppressed is that the expanded diameter part at the tip of the middle part undergoes thermal expansion to the same extent as the mandrel body part.

Incidentally, in the large-diameter portion of the mandrel tip, as a method of making the thermal expansion smaller than a metal material such as hot tool steel used for the mandrel body, the large-diameter portion is an annular body independent of the mandrel body, A method for producing an annular body from a ceramic such as silicon nitride, sialon, cermet, alumina, zirconia, or a material having a small coefficient of thermal expansion such as cemented carbide, molybdenum-based alloy, niobium-based alloy, and the annular body as a mandrel body. Alumina, zirconia, silicon nitride, titanium nitride, titanium carbide, vanadium carbide, chromium carbide, which has the same or equivalent metal material as that of at least the annular body and has heat resistance and wear resistance on the outer peripheral surface on which the annular body slides with the extruded pipe, By forming a heat-insulating coating layer made of chromium oxide or the like, heat absorption of the annular body due to the diameter expansion work is suppressed, As a result, there may be mentioned a method of reducing thermal expansion, and a method of forming the large-diameter portion of the tip of the mandrel integrally with the main body and forming the heat insulating coating layer on the surface of the large-diameter portion.

In the above two methods of forming the large-diameter portion by the annular body independent of the mandrel, the annular body is formed into a cylindrical body in view of the workability and toughness of the material, and the mandrel body is held by a screwing method or the like. Good. In addition,
The heat-insulating coating layer is preferably formed in a thickness of 0.1 to 1.5 mm. That is, if the thickness is less than 0.1 mm, there is no effect of suppressing the thermal expansion of the large diameter portion, and if it exceeds 1.5 mm, peeling easily occurs.

In the present invention, the diameter enlargement ratio with respect to the mandrel body of the large diameter portion is less than 0.5%, in most cases the extruded pipe expanded by the upset portion passes through the large diameter portion,
The diameter of the extruded pipe is not made constant in the large diameter part. Therefore, it is difficult to make the outer diameter uniform over the entire length of the middle portion. Even if the upset part of the mandrel has a small thermal expansion and the large diameter part can come into contact with the extruded pipe, fluctuations in the outer diameter of less than 0.5% do not pose a practical problem and the large diameter part is purposely set. There is no need to adopt a mandrel that you have.

The reason why the upper limit of the diameter enlargement ratio is set to 3% is that the conventional outer diameter variation in the axial direction of the middle portion is about 3% at the maximum. That is, even if the large diameter ratio exceeds 3%, the fluctuation of the outer diameter is suppressed, but the diameter of the extruded pipe is unnecessarily enlarged, and the load on the large diameter portion becomes larger than necessary.

FIG. 1 is a partial cross-sectional view schematically showing an embodiment of the present invention, and FIG. 2 is an enlarged view of a main part thereof. In the figure, 1 is a container, 2 is a die, 3 is a hollow billet, 4 is a mandrel, and 5 is a stem.

The container 1 is provided with an accommodating portion 1c which is surrounded by a liner 1a fixed by a liner holder 1b at the center thereof, and a die 2 uses a die holder 6 and a die backer 6a so as to face an opening on one end side thereof. And a mandrel 4 and a stem 5 which are held by a mandrel holder 7 are concentrically arranged on the other end opening side of the accommodating portion 1c and are individually supported by a hydraulic or hydraulic cylinder so as to be movable back and forth. Has been done.

The mandrel 4 includes a mandrel main body 41 in the shape of a round bar made of a conventional metal material, and an annular body 42 held at the tip thereof.
And a holding body 43 for holding the annular body 42 at the tip of the mandrel body 41.

The mandrel body 41 is a screw hole that opens at the end face at the tip axial center part.
41a. The holding body 43 is composed of a trapezoidal head 43a and a bolt portion 43b extending from the rear axial center portion thereof, and the end portion of the bolt portion 43b is a screw portion 43c screwed into the screw hole 41a of the mandrel body 41. Has become. The annular body 42 is externally fitted to the bolt portion 43b of the holding body 43, and the screw portion 43c of the bolt portion 43b is screwed into the screw hole 41a of the mandrel body 41, so that the holding body 43 between the head 43a and the mandrel body 41. Are held concentrically.

The annular body 42 is a material having a coefficient of thermal expansion smaller than that of the metal material forming the mandrel body 41, and examples of the material include silicon nitride, silicon carbide, sialon, cermet, alumina, ceramics such as zirconia described above, or Cemented carbide (WC), molybdenum-based alloy, niobium-based alloy, etc. can be mentioned.

The outer diameter of the annular body 42 is 0.5 than the outer diameter of the mandrel body 41.
~ 3% larger, considering the amount of increase in outer diameter in the case of not relying on the present invention that is expected from past results, etc., appropriately selected within the above range so that the annular body 42 expands the diameter beyond this. To be done.

Other points to be noted regarding the annular body 42 are as follows.

Materials such as ceramics having a smaller coefficient of thermal expansion than the metal material used for the mandrel body are generally more brittle than metal materials, so the annular body 42 is shown in the illustrated example in order to avoid unnecessary stress concentration. As described above, it is preferable that the corners before and after the outer and inner circumferences have smooth curved surfaces, and the front and rear ends of the outer circumference have a smooth curved surface from the viewpoint that the extruded pipe P should be smoothly expanded in diameter. It should have been

Considering that the annular body 42 receives a small load in the extrusion direction from the extruding pipe P and is brought into contact with the holding body 43 during the extruding operation and during the mandrel extracting operation after the extruding is completed, an illustrated example is provided between the two. It is desirable to interpose an annular shim 44 made of mild steel as described above. Of course, when the mandrel 4 is inserted into the inner hole 3a of the hollow billet 3, in consideration of the possibility that the annular body 42 is strongly abutted against the mandrel body 41 in the opposite direction to the above, the mandrel body 41 and the annular body are considered. A similar annular shim can also be interposed between 42 and 42.

In the illustrated example, the annular body 42 is fixed to the distal end portion of the mandrel 4 by using the rear end portion of the holding body 43 as a bolt and the distal end portion of the mandrel body 41 as a screw hole. It may be fixed in any way, as long as 42 does not fall off during the extrusion operation.

Since the outer peripheral surface of the holding body 43 must not touch the inner peripheral surface of the extruded pipe P, at least the outer diameter dimension thereof must be smaller than the outer diameter dimension of the annular body 42.

When the extruding work is performed between the annular body 42 and the die 2, the extruding pipe P is extruded from the die 2 at this portion because the wall thickness of the extruding pipe P becomes thinner than the other portions. Previously, the installation position of the annular body 42 on the mandrel 4 and the forward movement of the mandrel 4 with respect to the hollow billet 3 must be determined so that the annular body 42 is located on the exit side of the die 2.

When the mandrel 4 is pulled out after the extrusion is completed, the outer peripheral portion of the annular body 42 is caught on the inner peripheral portion of the rear end portion of the hollow billet 3 which is left on the entry side of the die 2, and the mandrel 4 cannot be removed. When there is a possibility of damaging the annular body 42, by making the outer diameter dimension of the rear end portion 41b of the mandrel body 41 approximately equal to or slightly larger than the outer diameter dimension of the annular body 42, the hollow The inner diameter of the unpressed portion of the billet 3 may be made larger than before when the extrusion is completed. If the large-diameter portion of the rear end portion 41b becomes long, the wall thickness of the rear end portion of the extruded pipe P becomes unnecessarily thin, which may impair the dimensional accuracy of the extruded pipe P as a product. It is preferable to determine the length of the part so that the tip of the part is located immediately below the die 2 at the end of extrusion.

-Of course, the outer diameter of the annular body 42 must be set smaller than the inner diameter of the die 2.

In the extrusion by the mandrel 4 provided with such an annular body 42, pipe production is performed as follows.

The hollow billet 3 is preliminarily formed with a core hole 3a by mechanical processing or press processing, and after being heated to a predetermined temperature, the glass disk 8, the hollow billet 3 and the dummy block 9 are individually or in order. The container 1 is inserted into the container 1c of the container 1 at the same time. Then, the hollow billet 3 is first upset-pressurized by the stem 5, and then the hollow billet 3 is extruded through the annular space between the die 2 and the mandrel 4 to form the extruded pipe P.

The extruded tube P whose wall thickness is determined between the die 2 and the mandrel body 41 during the extrusion process is continuously located in the die 2 at the start of extrusion and is thermally expanded together with the extruded mandrel 4.
The diameter is enlarged by the upset portion and finally by the annular body 42 located further on the exit side than the upset portion. Since the annular body 42 is made of a material having a coefficient of thermal expansion smaller than that of a metal material, for example, ceramics such as silicon nitride, the dimensional variation due to heat absorption and heat generation by abrasion from the extruded tube is extremely small, and the extruded product passing through the annular body 42 is extruded. The outer diameter variation of the pipe P is extremely small. After the extrusion, the mandrel 4 is pulled out by the mandrel holder 7 in the opposite direction to the extrusion direction.
The bottom end of the extruded pipe P that has not passed through the annular body 42 during the extruding operation is also expanded to have a constant inner diameter like the other portions. Moreover, the inner diameter of the portion of the hollow billet 3 that is left unpressed in the container 1 is enlarged by the large diameter portion of the rearmost end portion 41b of the mandrel as necessary, so that the mandrel 4 including the annular body 42 can be smoothly extracted. Complete.

The annular body 42 may be a cylindrical body depending on its material.

FIG. 6 to FIG. 8 are enlarged views of essential parts when another mandrel is used.

In FIG. 6, an annular body 42 in which a heat insulating coating layer 42a made of alumina, zirconia or the like is formed on the outer peripheral surface of an annular body 42b made of the same or equivalent metal material as the mandrel body 41.
A mandrel 4 with is used. The annular body 42 has an outer diameter larger than that of the mandrel body 41 by 0.5 to 3%, and is fixed concentrically to the tip of the mandrel body 41 by a holding body 43 like the mandrel shown in FIGS. 1 and 2. There is.

Since the ring-shaped body 42b is made of the same or equivalent metal material as the mandrel body 41, the ring-shaped body 42b can be formed into a columnar shape and screwed directly to the mandrel body 41 by screws or the like. When this is embodied, it becomes as shown in FIG. 7, for example.

In FIG. 7, a columnar body 45 is a columnar body 45b made of the same or equivalent metal material as the mandrel body 41, and an insulating coating layer 45a similar to the above is formed on the outer peripheral surface of the body 45. The outer diameter of the mandrel body 41 is 0.5 to 3% larger, and the mandrel body 41 is concentrically screwed to the tip of the mandrel body 41 by a bolt portion 45c extending from the rear axial center of the cylindrical body 45.

Further, in FIG. 8, the mandrel 4 having the large diameter portion 46 integrally with the tip of the mandrel body 41 is used. A heat insulating coating layer 46a similar to the above is formed on the outer peripheral surface of the large-diameter portion 46, and the large-diameter portion 46 is concentrically with the mandrel body 41 and has an outer diameter larger by 0.5 to 3%.

The mandrel used in the embodiment of FIGS. 6 to 8 has a tip having a diameter 0.5 to 3% larger than that of the mandrel body, and has a heat insulating coating layer on at least the outer peripheral surface thereof. During the extrusion of the large-diameter portion, the heat-insulating coating layer suppresses the temperature rise of the large-diameter portion body due to heat absorption and frictional heat generation from the extruded tube when the diameter of the extruded pipe is expanded during the extrusion operation. Is small, and the outer diameter fluctuation in the middle portion of the extruded pipe is correspondingly small. The extrusion work can be carried out in exactly the same manner as the embodiment shown in FIGS. 1 and 2.

As a result, the increase in the outer diameter of the extruded pipe P after the middle portion is extremely small.

Example 1 An example of the present invention will be described below in comparison with a conventional example.

SUS304 hollow billet with outer diameter 173mm, inner diameter 44mm, length 700mm is extruded after heating to 1200 ℃, outer diameter 46.7mm, wall thickness 3.5m
A pipe was manufactured aiming at m and a length of about 30 m, and after cooling and pickling, the outer diameter was measured over the entire length of the extruded pipe.

The dies and mandrels used in the pipe making test were the following three combinations. However, the total length of the mandrels was 900 mm.

Mandrel A: Straight mandrel with a constant outer diameter over the entire length, the outer diameter is 40.3 mm, and the material is SK plated with Cr.
It is D61. The inner diameter of the die combined with this is 47.7 mm.

Mandrel B: An enlarged tip mandrel with the outer diameter of the tip of the mandrel set to 40.3 mm and the outer diameter of the other parts set to 39.9 mm.
The material is the same as the mandrel A. The inner diameter of the die combined with this is 47.3 mm.

Mandrel C: It has the structure shown in Fig. 1 and Fig. 2, and the mandrel body and holder have an outer diameter of 39.9 mm and are made of Cr.
It consists of plated SKD61. The annular body is made of silicon nitride and has an outer diameter of 40.3 mm, a wall thickness of 10.0 mm and a length of 20.0 mm. The inner diameter of the die combined with this is 47.3 mm.

The results of the tests using the mandrels A to C are shown in FIGS.
Shown in the figure. The outer diameter is investigated at 100 mm pitch in the longitudinal direction,
At each point, the outer diameter in four directions was measured at a pitch of 45 ° in the circumferential direction, and the average was taken as the average outer diameter.

When using the mandrel A (straight mandrel), about 1 m, which corresponds to about 2% of the target outer diameter in the middle part of the extruded tube
Outer diameter variation of up to m occurs, and the outer diameter variation of about 1 mm is added at the bottom end.

When the mandrel B (diameter expanding mandrel at the tip end) is used, the extreme outer diameter variation at the bottom end is considerably suppressed, but the outer diameter variation at the middle portion remains close to 1 mm.

On the other hand, when the mandrel C (mandrel with ceramic annular body) is used, the outer diameter variation in the middle part is significantly suppressed to about 0.3 mm (about 0.6% of the target outer diameter), The outer diameter variation at the bottom end is also small.

Example 2 Next, Table 1 shows the effect of suppressing the outer diameter variation of the extruded pipe middle portion when the hot extrusion pipe production is performed according to another example of the present invention.

In Table 1, CASEI is a JI used for the mandrel body by forming the large diameter part of the mandrel tip with an annular body.
When integrally formed with various materials that have a smaller thermal expansion than S-SKD61, CASEII is an annular body and various heat insulation coating layers (1 mm) are formed on the outer peripheral surface of JIS-SKD61, and CASEIII is integrally formed on the tip of the mandrel. This is a case where a large diameter portion is provided and various heat insulating coating layers (1 mm) are formed on the outer peripheral surface of the large diameter portion.

The outer diameters of the mandrel body and large diameter part are 39.9 mm and 40.3 m, respectively.
m, the inner diameter of the die is 47.3 mm, and other conditions are the same as in Example 1. The average outer diameter variation amount of the extruded pipe middle part is shown as an index showing the outer diameter variation suppression effect.

According to Table 1, in any of the embodiments of the present invention,
Compared to the conventional technique using the mandrel A of Example 1, the average outer diameter fluctuation amount of the extruded tube middle part is suppressed low.

The configuration of the large diameter portion provided at the tip of the mandrel for carrying out the present invention is not limited to Table 1.
It does not matter if the large-diameter portion has a smaller thermal expansion than the mandrel body during extrusion. In other words, the material of the large diameter portion and its heat insulation coating layer is not limited to the combination shown in Table 1, and the method of forming the coating layer is not limited to thermal spraying and build-up welding.
Examples include CVD and PVD.

As described above, according to the present invention, it is possible to significantly improve the outer diameter dimensional accuracy in the middle portion of the extruded pipe. Further, conventionally, the extruded pipe is practically used due to the deviation of the outer diameter tolerance due to the increase of the outer diameter in the longitudinal direction. Extremely long extruded pipes (for example, extruded pipe length 100m), which was impossible to achieve, can be realized.

In consideration of the purpose of the present invention, if a mandrel (a tip-diameter expanding mandrel) is extruded as a silicon nitride monolith, an extruded tube having excellent outer diameter accuracy can be obtained, but the tool manufacturing cost is enormous. However, it is not practical in view of the above (material cost) and the processing cost being extremely high).

〔The invention's effect〕

As is apparent from the above description, according to the hot extrusion pipe manufacturing method and the mandrel of the present invention, the outer diameter increase phenomenon at the extruded pipe middle portion is significantly suppressed, and the outer diameter increase at the bottom end is also small. Therefore, excellent outer diameter accuracy is assured over substantially the entire length of the extruded tube. Therefore, a decrease in yield due to deviation of the outer diameter tolerance is prevented, and it becomes possible to extrude a long tube having excellent outer diameter dimension accuracy.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view schematically showing an embodiment of the present invention, FIG. 2 is an enlarged view of a main part thereof, and FIGS. 3 and 4 show a longitudinal outside diameter distribution of an extruded pipe according to a conventional example. FIG. 5 is a graph showing the outer diameter distribution in the longitudinal direction of the extruded pipe according to the example of the present invention, FIGS. 6 to 8 are enlarged views of the essential parts showing another embodiment of the present invention, and FIG. It is sectional drawing which shows the outline of an extrusion pipe manufacturing method. In the figure, 1: container, 2: die, 3: hollow billet, 4: mandrel, 41: mandrel body, 42: annular body, 43: holding body, 45: cylindrical body: 42a, 45a: heat insulating coating layer, 46: Large diameter part.

Claims (4)

[Claims] 1. An annular shape formed by loading a heated cylindrical billet into a container, and between a die located in front of the container and a mandrel inserted through the billet into the die. In the hot extrusion tube manufacturing method in which the billet is extruded forward from the gap to form a metal tube, the mandrel as the mandrel has a tip end portion of a mandrel body having a smaller outer diameter thermal expansion than the mandrel body, and A method for hot-extrusion pipe making, which comprises using a mandrel having a large diameter portion whose outer diameter is 0.5 to 3% larger than that of the mandrel body. 2. A mandrel for use in the hot extrusion tube manufacturing method according to claim 1, wherein the mandrel body is made of a metal material, and the mandrel body is concentrically held at the tip of the mandrel body. And an annular body or a cylindrical body made of a material having a thermal expansion coefficient smaller than that of the metal material, the outer diameter being 0.5 to 3% larger than the outer diameter of the mandrel. 3. A mandrel used in the hot extrusion tube manufacturing method according to claim 1, wherein the mandrel body is made of a metallic material, and is held concentrically at the tip of the mandrel body. A heat-insulating coating layer having heat resistance and wear resistance formed on at least the outer peripheral surface of the metal material or a metal material equivalent to the metal material having an outer diameter larger than the outer diameter of 0.5 to 3%. A mandrel, comprising: 4. A mandrel used in the hot extrusion tube manufacturing method according to claim 1, wherein the mandrel body made of a metal material is concentrically formed integrally with the tip portion of the mandrel body, and the outer peripheral surface of the mandrel has heat resistance. Forming a heat-insulating coating layer having wear resistance,
A mandrel comprising a large-diameter portion having an outer diameter that is 0.5 to 3% larger than the outer diameter of the mandrel body.