CN109070166B

CN109070166B - Drawing core rod

Info

Publication number: CN109070166B
Application number: CN201780018876.0A
Authority: CN
Inventors: 崔秉福; 尹元锡; 金武成; 金畅贤
Original assignee: Iljin Steel Corp Co ltd
Current assignee: Iljin Steel Corp Co ltd
Priority date: 2016-02-01
Filing date: 2017-01-19
Publication date: 2020-06-26
Anticipated expiration: 2037-01-19
Also published as: WO2017135598A2; CN109070166A; KR101859936B1; WO2017135598A3; KR20170091394A

Abstract

The tube is disposed between the drawing plug and the drawing die according to the embodiment of the present invention, the drawing plug is inserted inside the inner surface of the tube, and the tube is processed. The drawing plug according to the invention comprises: a large diameter portion formed to have a first outer diameter; a contact portion contacting one side surface of the large diameter portion and including an inclined portion having one side contacting the large diameter portion and formed to have a first outer diameter and having an outer diameter decreasing toward the other side of the inclined portion; an insertion portion inserted into both the large diameter portion and the contact portion to connect the large diameter portion and the contact portion, wherein the inclined portion has a stepped portion stepped in an insertion direction, and at least a portion of the inclined portion is made of a diamond sintered body.

Description

Drawing core rod

Technical Field

The present disclosure relates to a drawing plug (drawing plug). More particularly, the present disclosure relates to a drawing mandrel that can reduce defects and improve the roughness of the tube being machined.

Background

Generally, the process for cold-producing stainless steel pipe may include cold pilger rolling, cold drawing, or a combination of cold pilger rolling and subsequent cold drawing.

In the manufacture of stainless steel pipes, the dimensional accuracy and roughness are improved as the number of steps of cold pilger rolling or cold drawing or a combination of cold pilger rolling and subsequent cold drawing is increased.

However, there are limits to the surface roughness that can be achieved in cold forming within the maximum RA (area reduction)% of the material. In order to improve the surface roughness, the number of cold forming processes may need to be increased. However, this leads to an increase in manufacturing cost.

Therefore, there is a need for a forming technique that substantially improves the roughness in cold forming processes using existing methods.

Disclosure of Invention

Technical purpose

The present disclosure is designed to solve the above-mentioned problems.

The present disclosure aims to improve the inner surface roughness by using a diamond core rod when drawing a stainless steel pipe, compared to a drawing method using an existing die. The present disclosure aims to improve the inner surface roughness in cold-draw forming by coating diamond material on conventionally used mandrel structures.

Technical scheme

In one aspect of the present disclosure, there is provided a drawing plug for processing a tube, wherein the drawing plug is introduced into the tube to process the tube when the tube is disposed between a drawing die and the drawing plug, wherein the drawing plug includes: a body portion having a first outer diameter; a contact portion extending from the body portion, wherein the contact portion includes an inclined portion extending from the body portion, wherein an outer diameter of the inclined portion gradually decreases from the first outer diameter to a second outer diameter as the inclined portion extends away from the body portion; and an inner connection part simultaneously inserted into the body part and the contact part to connect the body part and the contact part to each other,

wherein the inclined portion has a step portion formed thereon and extending in an introduction direction of the mandrel into the tube, wherein at least a portion of the inclined portion is made of sintered diamond.

In one embodiment of the drawing plug, a portion of the contact portion including the step portion is made of sintered diamond.

In one embodiment of the drawing plug, the step portion extends in a closed curve having a symmetrical inclination with respect to a vertical plane coinciding with a central axis of the contact portion.

In one embodiment of the drawing plug, the step portion extends in a spiral shape symmetrical with respect to a vertical plane coinciding with a central axis of the contact portion.

In one embodiment of the drawing plug, the start and end points of the spiral shape are on a line.

In one embodiment of the drawing plug, the contact portion further includes: a first non-inclined portion having a first outer diameter and having a constant outer diameter; and a second non-inclined portion having a second outer diameter smaller than the first outer diameter and having a constant outer diameter.

In one embodiment of the drawing plug, the inclined portion is located between the first non-inclined portion and the second non-inclined portion.

In one embodiment of the drawing mandrel, at least one of the main body portion and the contact portion is made of polycrystalline sintered diamond.

In one embodiment of the drawing mandrel, the body portion has a first internal female thread on a groove defined in the body portion, wherein the contact portion includes a second internal female thread on a hole defined through the contact portion and a head receiving space defined in the contact portion and having an inner diameter greater than an inner diameter of the hole,

wherein the inner connection part includes: a head accommodated in the head accommodating space; and an external male thread on an elongated hole defined in the inner connection part along a length direction of the inner connection part, wherein the external male thread is simultaneously engaged with the first and second internal female threads to connect the main body part and the contact part to each other.

In one embodiment of the drawing plug, an exposed surface of the head accommodated in the head accommodating space is coplanar with a free end surface of the contact portion.

In one embodiment of the drawing mandrel, a first inner fluid passage is defined in the body portion and a second inner fluid passage is defined in the inner connecting portion, wherein the first inner fluid passage and the second inner fluid passage are in fluid communication with each other when the inner connecting portion is inserted into the body portion.

In one embodiment of the drawing plug, the arithmetic mean roughness of the inner surface of the processed tube in contact with the contact portion is Ra 0.1 μm or less.

Effects of the disclosure

According to the present disclosure, the surface roughness of the inner surface of the tube due to pressure and thermal shock generated when the tube is processed may be improved.

Further, the present disclosure can secure durability of a contact portion contacting the tube, thereby securing processing efficiency and durability of the tube.

In addition, the present disclosure may increase the smoothness of the inner surface of the tube, which may greatly reduce defects.

Further, the present disclosure may suppress vacuum pressure that may be additionally generated in the tube during the tube processing, thereby preventing personal injury and ensuring durability of the machine to reduce replacement/repair costs.

Drawings

FIG. 1 is a perspective view, partially in section, illustrating a process of drawing a tube using a drawing die and a drawing mandrel in accordance with one embodiment of the present disclosure.

FIG. 2 is a schematic perspective view of a drawing mandrel according to one embodiment of the present disclosure.

FIG. 3 is a side view of a drawing mandrel according to one embodiment of the present disclosure.

FIG. 4 is a partial cross-sectional view illustrating a drawing mandrel according to one embodiment of the present disclosure.

FIG. 5 is a cross-sectional view of a drawing mandrel according to one embodiment of the present disclosure.

FIG. 6 is a schematic perspective view of a drawing mandrel according to another embodiment of the present disclosure.

FIG. 7 is a side view of a drawing mandrel according to another embodiment of the present disclosure.

Fig. 8 and 9 are sectional views illustrating the processing of a tube using a drawing die and a drawing plug according to another embodiment of the present disclosure.

Best mode for carrying out the invention

When the drawing mandrel 10 is connected to the draw bar 4, the movement of the drawing mandrel 10 is prevented by the draw bar 4. Therefore, when the pipe P located between the drawing plug 10 and the drawing die D moves, the drawing plug 10 does not move together with the pipe P.

The drawing die D includes an inclined portion 2 for receiving the outer surface of the pipe. The inclined portion 2 includes a receiving section (R1, see fig. 9) having an inner diameter larger than the outer diameter of the pipe before machining, a first contact section (R2, see fig. 9) having an inner diameter identical to the outer diameter of the pipe before machining, and a second contact section (R3, see fig. 9) having an inner diameter smaller than the outer diameter of the pipe before machining.

Detailed Description

Examples of various embodiments are further shown and described below. It is to be understood that the description herein is not intended to limit the claims to the particular embodiments described. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the disclosure as defined by the appended claims.

For simplicity and clarity of illustration, elements in the figures have not necessarily been drawn to scale. The same reference numbers in different drawings identify the same or similar elements and, thus, perform similar functions.

Referring to fig. 1, a pipe P is placed between a drawing mandrel 10 and a drawing die D according to one embodiment of the present disclosure. The mandrel 10 may be implemented as a floating draw mandrel that is inserted into the pipe and used to machine the pipe P.

The pipe P may be a workpiece made of stainless steel. The tube moves while contacting the drawing die D and the drawing plug 10 (particularly, the inclined contact portion 14 of the drawing plug 10). Thus, the cross-section of the tube is reduced. That is, as the pipe P passes through the space between the drawing die D and the drawing plug 10, the outer diameter and thickness of the pipe decrease. Thus, the tube can be drawn to have the desired final dimensions.

In view of this, when the drawing plug 10 is attached to the draw bar 4, the movement of the plug 10 is restricted by the draw bar 4. Therefore, when the pipe P between the drawing mandrel 10 and the drawing die D is moved, the mandrel 10 is not moved together with the pipe P.

Further, the drawing die D includes a non-inclined portion 3 extending from the inclined portion 2 and having a constant inner diameter. The size of the inner diameter of the non-inclined portion 3 is equal to the size of the outer diameter of the tube having the final dimensions as desired by the user.

In one embodiment, the roughness of the tube is determined by the strength and shape of the member in contact with the drawing plug 10. According to the present disclosure, a drawing plug 10 is provided that can significantly improve the roughness of the pipe P and reduce defects thereof.

In one embodiment, when the same area Reduction (RA) percentage was applied to the present disclosure and the case using the cermet drawing mandrel or the hard metal drawing mandrel, the roughness after drawing of the tube having an outer diameter of 12.7mm and a thickness of 1.0mm in the cold drawing using the drawing mandrel including the sintered diamond according to the present disclosure had improved roughness compared to the comparative case.

When the cermet drawing plug is used, the arithmetic mean roughness Ra of the tube after working is 0.2 μm to 0.35. mu.m. When the same area Reduction (RA)% is applied, the arithmetic mean roughness RA of the tube machined using the drawing plug containing sintered diamond is 0.06 to 0.08 μm.

Accordingly, the present disclosure may be applied to various fields requiring tubes having high inner surface roughness quality, such as cleaning tubes, heat transfer tubes of steam generators, clad tubes for nuclear fuel, and the like.

Referring to fig. 2-6, a drawing mandrel 10 is shown according to one embodiment of the present disclosure. FIG. 2 is a schematic perspective view of a drawing mandrel according to one embodiment of the present disclosure. FIG. 3 is a side view of a drawing mandrel according to one embodiment of the present disclosure. FIG. 4 is a partial cross-sectional view illustrating a drawing mandrel according to one embodiment of the present disclosure. FIG. 5 is a cross-sectional view of a drawing mandrel according to one embodiment of the present disclosure. FIG. 6 is a schematic perspective view of a drawing mandrel according to another embodiment of the present disclosure.

The drawing mandrel 10 according to one embodiment of the present disclosure includes a main body portion 12, a contact portion 14, and an inner connection portion 16.

The body portion 12 has a cylindrical shape and has a first outer diameter. The body portion 12 may be made of a material selected from the group consisting of polycrystalline sintered diamond, hard metal, steel, carbon steel, specialty steel, cast iron, non-ferrous metals, and combinations thereof.

In view of this, the free end of the body portion 12 may have a tie rod receiving hole 125 defined therein for receiving at least a portion of the tie rod 4.

The contact portion 14 extends from the main body portion 12. The contact portion 14 has a first outer diameter portion 144 contacting the body portion 12 and an inclined portion 140 extending from the first outer diameter portion and having a gradually decreasing outer diameter.

Further, the contact portion 14 may include a first non-inclined portion 144 extending from the body portion 12 and having a first outer diameter and extending a predetermined length, an inclined portion 140 extending from the first non-inclined portion 144 and having a gradually decreasing outer diameter, and a second non-inclined portion 146 extending from the inclined portion 140 and having a second outer diameter smaller than the first outer diameter and extending a predetermined length.

That is, the first and second

non-inclined portions

144 and 146 have constant first and second outer diameters, respectively. The angled portion 140 is located between the first non-angled portion 144 and the second non-angled portion 146 and has an outer diameter that decreases from a first outer diameter to a second outer diameter. In FIG. 3, a first outer diameter S1 and a second outer diameter S2 are shown to compare the size of the first outer diameter to the size of the second outer diameter.

In view of this, the length of the inclined portion 140 may be determined by the difference between the first outer diameter and the second outer diameter. In particular, the inclination and length of the inclined portion 140 may vary according to the difference between the first outer diameter and the second outer diameter such that the outer diameter of the inclined portion 140 gradually varies from the first outer diameter to the second outer diameter.

The inclined part 140 may have a truncated cone shape having an outer diameter gradually decreasing along a length direction. The slope part 140 may contact the inner surface of the pipe P and define the size of the inner diameter of the pipe P when the pipe P is processed through the drawing die.

In other words, the outer surface of the pipe P contacts the drawing die D while the inner surface of the pipe P contacts the drawing plug 10. In this state, as the tube moves along the space defined between the drawing die D and the drawing plug 10, the tube P may be processed into a shape corresponding to the space defined between the drawing die D and the drawing plug 10. In view of this, the dimensions of the drawing plug 10 may be adjusted to obtain a pipe P having the final dimensions as desired by the user.

Further, the contact portion 14 according to the present embodiment has the step portion 145, and the step portion 145 is located at the minimum outer diameter position and between the second non-inclined portion 146 and the inclined portion 140. The stepped portion 145 generates a significant difference between the outer diameter of the second non-inclined portion 146 and the outer diameter of the inclined portion 140.

The contact portion 14 in this embodiment may be made of a material selected from the group consisting of polycrystalline sintered diamond, hard metal, steel, carbon steel, specialty steel, cast iron, non-ferrous metals, and combinations thereof. In particular, the inclined portion 140 may be made of polycrystalline sintered diamond. Alternatively, the portion of the inclined portion including the stepped portion 145 is preferably made of sintered diamond.

This extends the life of the edge surface of the inclined portion 140 receiving a large contact load from the pipe P. This also prevents the inside of the pipe P from being damaged due to abrasion of the inclined portion 140 caused by repeated contact of the inclined portion 140 with the pipe P.

Further, this configuration improves the workability of the inclined portion 140, thus extending the life of the die and slowing down the replacement time of the drawing plug 10. Further, when replacing the drawing plug 10, this may allow only one of the contact portion 14, the main body portion 12, and the inner connecting portion 16 to be replaced, if necessary.

Therefore, the material cost can be reduced.

Further, due to high workability of the inclined part 140, when the inclined part 140 having high strength is in contact with the pipe P, the pipe P having a uniform quality may be continuously supplied while improving the inner surface roughness of the pipe P.

In another embodiment, the main body portion 12, the contact portion 14, and the inner connection portion 16, as well as the inclined portion 140, which are the respective components of the drawing plug 10, may be made of polycrystalline sintered diamond. Alternatively, the components of the drawing plug 10 other than the inclined portion 140 may be made of a hard metal or a cermet material.

Referring to fig. 4, the drawing plug 10 includes an inner connecting portion 16. The inner connection part 16 is simultaneously inserted into the body part 12 and the contact part 14 and connects the body part 12 and the contact part 14.

The body portion 12 has a partially formed first female internal thread 121. The contact portion 14 includes a second internal female thread 141 passing through substantially most of the entire length thereof and a head receiving space 143 defined therein, the head receiving space 143 having an inner diameter greater than that of the second internal female thread 141.

The first internal female thread 121 may be defined in a groove extending from an end of the body portion 12 in contact with the contact portion 14 toward a free end of the body portion 12. At least a portion of the external male threads 163 of the inner connection portion 16 may be inserted into the slot defining the first internal female threads 121.

Further, the second internal female thread 141 may be defined in a long opening extending from an end of the contact portion 14 in contact with the body portion 12. The opening may extend less than the entire length of the contact portion 14.

The head receiving space 143 communicates with an opening defining the second internal female screw 141. The head receiving space 143 has an inner diameter larger than that of the second internal female screw 141. The head 161 of the inner connecting portion 16 is seated in the head receiving space 143. That is, the combination of the head receiving space 143 and the opening defining the second internal female screw 141 defines an elongated hole penetrating the contact portion 14 in the length direction.

The inner connecting portion 16 includes a head 161 and an outer male thread 163 accommodated in the head accommodating space 143. The external male thread 163 extends from the head portion and is simultaneously fastened to the first internal female thread 121 and the second internal female thread 141 to connect the body portion 12 and the contact portion 14.

In other words, the external male screw 163 of the internal connection portion 16 is fastened to the first internal female screw 121 of the body portion 12 and the second internal female screw 141 of the contact portion 14. The fastening may be achieved by means of bolts/nuts.

Further, the head 161 of the inner connecting portion 16 is accommodated in the head accommodating space 143 of the contact portion 14. The top surface of the head 161 may be exposed to the outside. In view of this, the exposed top surface of the head 161 may be coplanar with the free end surface of the contact 14. That is, the head 161 may be hidden within the contact portion 14.

In addition, the inner connection portion 16 has a second inner fluid passage 165 defined therein. The second inner fluid passage 165 may be implemented as a hole penetrating the inner connection part 16 in a length direction.

The body portion 12 has a first internal fluid passage 123 defined therein. The first internal fluid passage 123 may be implemented as a hole penetrating the body portion 12 from the first internal female thread 121 of the body portion 12 toward the free end portion of the body portion 12.

In view of this, when the inner connection part 16 connects the main body part 12 and the contact part 14 to each other, the first internal fluid passage 123 of the main body part 12 and the second internal fluid passage 165 of the inner connection part 16 may define a through hole (H, see fig. 9). A through hole (H, see fig. 9) extends from one end to the other end of the drawing plug 10.

The through hole H defined by the first inner fluid passage 123 and the second inner fluid passage 165 prevents breakage of the drawing plug 10 which might otherwise be caused by vacuum pressure generated inside the tube P during the drawing process of the tube P.

In particular, when the pipe P is pulled to pull the pipe P, a vacuum pressure is generated inside the pipe P. In view of this, when the end of the pipe P drawn under vacuum pressure is separated from the drawing plug 10 and the drawing die D, the vacuum pressure is instantaneously eliminated. Therefore, the drawing die D or the drawing plug 10 may be damaged/broken due to the impact caused by such instantaneous cancellation.

However, the fluid can freely move through the through hole H defined by the first inner fluid passage 123 and the second inner fluid passage 165. Therefore, no difference is generated between the internal pressure of the pipe P and the external pressure of the pipe P. Therefore, damage or breakage of the drawing die D or the drawing plug 10 can be prevented.

As previously shown, in the drawing plug according to the present embodiment, the portion of the contact portion 14 including the step portion 145 may be made of sintered diamond. Referring to fig. 5, a portion of the inclined portion 140a including the stepped portion 145 may be made of sintered diamond and have an annular shape.

The inner surface of the tube in contact with the contact portion 14 is polished by the edge generated by the presence of the step 145 and thus may have improved roughness. That is, when the inner surface of the tube contacts the contact portion 14, metal chips may be generated due to high pressure and heat, or an uneven section due to a contact failure or a folded section due to pushing may occur. However, in the present disclosure, such metal debris may be removed by the step 145, or an uneven section or a folded section may be polished by the step 145, thereby improving roughness.

When the inclined portion 140a is made of sintered diamond, the strength of the stepped portion 145, on which the load is concentrated during the drawing process, is reinforced. This can increase the life of the drawing plug itself and improve the roughness of the tube as a workpiece.

Referring to FIG. 6, a drawing mandrel is shown according to another embodiment. FIG. 6 is a schematic perspective view of a drawing mandrel according to another embodiment of the present disclosure. FIG. 7 is a side view of a drawing mandrel according to another embodiment of the present disclosure.

Referring to fig. 6 and 7, a drawing plug 20 according to another embodiment of the present disclosure has the same configuration as the drawing plug 10 according to the embodiment illustrated with reference to fig. 1 to 5 except for the step portion 245. Hereinafter, description of components other than the step 245 will be omitted.

The step 245 extends in a spiral shape along the direction X in which the drawing plug 20 is introduced into the pipe P. That is, the stepped portion 245 extends in a spiral shape on and along the outer surface of the contact portion 24.

In particular, the step 245 extends from a starting point located on the top edge of the contact portion 24 of truncated cone shape with a predetermined inclination. Then, the step portion 245 extends downward in a curved manner along the outer surface of the contact portion 24. Then, the step portion 245 extends curvedly upward before reaching the bottom edge of the contact portion 24. Then, the step portion 245 returns to the starting point on the top edge of the contact portion 24. Therefore, the stepped portion 245 has a spiral shape.

That is, the contact portion 24 has a gradually decreasing outer diameter while being inclined from one edge having the first outer diameter to the other edge having the second outer diameter. The presence of step 245 extending along the outer surface of contact 24 may create a region of abrupt decrease in the outer diameter of contact 24 and a region of abrupt increase in the outer diameter of contact 24.

Accordingly, the inner surface of the tube, which is in contact with the contact portion 24, is polished by the edge due to the presence of the stepped portion 245 and thus may have improved roughness. That is, when the inner surface of the tube contacts the contact portion 24, metal chips may be generated due to high temperature and heat, or an uneven section due to a contact failure or a folded section due to pushing may occur. However, in the present disclosure, such metal debris may be removed by step 245, or uneven sections or folded sections may be polished by step 245, thereby improving roughness.

In the case of the drawing plug 20 according to the present embodiment, the step portion 245 extends in a spiral shape as compared with the drawing plug described above. Therefore, while the polishing effect of the stepped portion 245 can be maintained, the pipe can be prevented from being rapidly processed. Therefore, the surface roughness of the final product can be further improved.

Referring to fig. 8 and 9, a process for machining a pipe P using the drawing mandrel 20 of the present disclosure is shown. Fig. 8 and 9 are sectional views illustrating the processing of a tube using a drawing die and a drawing plug according to another embodiment of the present disclosure.

For ease of illustration, while the drawing process using the drawing plug according to the embodiment shown in fig. 6 and 7 is shown, the description is equally applicable to drawing processes using drawing plugs according to other embodiments.

First, the pipe P before processing may be placed on the inclined part 2 of the drawing die. Specifically, the outer surface of the pipe P is contacted to the contact section R2 of the inclined part 2, and the contact section R2 has the same inner diameter as the outer diameter of the pipe P before machining.

Then, after the drawing plug 20 is inserted into the pipe P, the pipe P is placed between the drawing plug 20 and the drawing die D.

Subsequently, with the drawing plug 20 fixed to the drawing die D, the pipe P is inserted into a space defined between the drawing plug 20 and the drawing die D. In view of this, one end of the drawing plug 20 is connected to the draw bar 4 and does not move with the tube P being drawn.

In view of this, the outer surface of the pipe P moves along the contact section R3 of the drawing die D, while the inner surface of the pipe P contacts the contact portion 24 of the drawing plug 20.

The inclination of the contact section R3 of the drawing die D is equal to the inclination of the inclined portion 240 of the drawing plug 20. The outer surface of the pipe P is in contact with the contact section R3 of the drawing die D, while the inner surface thereof is in contact with the inclined portion 240 of the drawing plug 20. Therefore, the thickness of the pipe P is kept constant.

That is, the interval between the contact section R3 of the drawing die D and the inclined portion 240 of the drawing plug 20 corresponds to the thickness of the pipe P being processed.

Then, as the tube is drawn along the contact section R3 of the drawing die D and the inclined portion 240 of the drawing plug 20, the inner diameter of the tube P is gradually reduced.

In view of this, the presence of the stepped portion 245 formed on the inclined portion 240 may significantly improve the roughness of the inner surface of the pipe P.

After the tube is drawn along the contact section R3 of the drawing die D and the inclined portion 240 of the drawing mandrel 20, the tube P passes along the non-inclined portion 3 of the drawing die D and the second non-inclined portion 246 of the drawing mandrel 20. At this time, the inner diameter of the pipe P becomes constant.

That is, the size of the inner diameter of the pipe P is determined by the non-inclined portion 3 of the drawing die D and the second non-inclined portion 246 of the drawing plug 20.

Further, as the pipe P moves along the drawing die D and the drawing plug 20, the pressure inside the pipe P becomes vacuum. However, a through-hole defined along the length of the drawing mandrel 10 (i.e., the combination of the first inner fluid passage 123 and the second inner fluid passage 165) (see fig. 6) may communicate the inner and outer spaces of the mandrel 10 with each other. Accordingly, the fluid moves through the through-hole H so that the inner pressure of the pipe P and the outer pressure of the pipe P are maintained equal to each other.

As described above, embodiments according to the present disclosure are respectively shown. However, the embodiments may be combined, modified or changed in various ways within the spirit and scope of the present disclosure.

It will be understood by those skilled in the art that the present disclosure may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. It is therefore to be understood that the above described embodiments are illustrative and not restrictive in all respects. The scope of the invention is defined by the claims rather than the specific embodiments above. All changes or modifications within the spirit and scope of the claims and equivalents thereof should be construed as being included in the scope of the present invention.

Claims

1. A drawing mandrel for processing a tube, wherein the drawing mandrel is introduced into the tube to process the tube when the tube is disposed between a drawing die and the drawing mandrel, wherein the drawing mandrel comprises:

a body portion having a first outer diameter;

a contact portion extending from the body portion, wherein the contact portion includes an inclined portion extending from the body portion, wherein an outer diameter of the inclined portion gradually decreases from the first outer diameter to a second outer diameter as the inclined portion extends away from the body portion; and

an inner connection part simultaneously inserted into the body part and the contact part to connect the body part and the contact part to each other,

wherein the inclined portion is made of sintered diamond and has a ring shape,

wherein the inclined portion has a step portion formed thereon and extending in an introduction direction of the drawing plug into the tube,

wherein the step portion extends in a closed curve having a symmetrical spiral shape with respect to a vertical plane coinciding with a central axis of the contact portion.

2. The drawing mandrel of claim 1, wherein the contact portion further comprises:

a first non-inclined portion having a first outer diameter and having a constant outer diameter; and

a second non-inclined portion having a second outer diameter smaller than the first outer diameter and having a constant outer diameter.

3. The drawing mandrel of claim 2, wherein the inclined portion is located between the first non-inclined portion and the second non-inclined portion.

4. The drawing mandrel of claim 1, wherein at least one of the body portion and the contact portion is made of polycrystalline sintered diamond.

5. The drawing mandrel of claim 1, wherein the body portion has a first internal box thread located on a groove defined in the body portion,

wherein the contact portion includes:

a second internal female thread on a bore defined through the contact portion; and

a head receiving space defined in the contact portion and having an inner diameter greater than an inner diameter of the hole,

wherein the inner connection part includes:

a head accommodated in the head accommodating space; and

an external male thread on an elongated hole defined in the inner connection part along a length direction of the inner connection part, wherein the external male thread is simultaneously engaged with the first and second internal female threads to connect the main body part and the contact part to each other.

6. The drawing mandrel of claim 5, wherein an exposed surface of the head received in the head receiving space is coplanar with a free end surface of the contact portion.

7. The drawing mandrel of claim 6, wherein a first internal fluid passage is defined in the body portion and a second internal fluid passage is defined in the internal connection portion,

wherein the first inner fluid passage and the second inner fluid passage are in fluid communication with each other when the inner connecting portion is inserted into the body portion.

8. The drawing mandrel according to claim 1, wherein the arithmetic mean roughness of the inner surface of the processed tube in contact with the contact portion is Ra 0.1 μm or less.