Precise plastic forming die and blank for special-shaped steel pipe

Abstract

The utility model belongs to the technical field of metal material section processing, and particularly discloses a precision plastic forming die and a blank for a special-shaped steel pipe. The special-shaped steel pipe precision plastic forming die comprises an extrusion die and a cold drawing die, the cold drawing die comprises a cold drawing outer die and a cold drawing inner die, a second sizing hole with a cylindrical inner wall is formed in the cold drawing outer die in the axis direction in a penetrating mode, and a gap is formed between the outer wall of the cold drawing inner die and the inner wall of the second sizing hole. And the cold-drawing inner die is coaxially arranged in the second sizing hole in a penetrating manner. The die can produce the special-shaped steel pipe which is high in size precision, complete in forging streamline and compact in organization structure and has the outer circle and the inner polygon, meanwhile, the size precision and the surface smoothness of the outer wall and the inner hole of the special-shaped steel pipe are high, and the die is suitable for producing the special-shaped steel pipe.

Description

Technical Field

[0001] The utility model relates to the technical field of metal material section processing, in particular to a precision plastic forming die and blank for special-shaped steel pipes. Background Art

[0002] Compared to round steel pipes, special-shaped steel pipes offer greater resistance to bending and twisting, high structural strength, low weight, and reduced steel consumption. Currently, improvements to special-shaped steel pipes are primarily focused on cross-sectional shape, material quality, and performance. These pipes are widely used in aviation, automotive, shipbuilding, mining machinery, agricultural machinery, construction, textiles, and boiler manufacturing. Special-shaped steel pipes are produced by extrusion and cold drawing.

[0003] The extrusion process involves forming a single piece of steel under three-dimensional compressive stress, and is suitable for the production of stainless steel workpieces. Workpieces produced by the extrusion process boast high dimensional accuracy, streamlined forging, and a compact structure. Chinese utility model patent application number 202322586999.X discloses a production line and apparatus for internally variable-diameter special-shaped steel pipes. This method utilizes a special-shaped outer die and a variable-diameter mandrel to produce the special-shaped steel pipe body through extrusion. However, this method of producing special-shaped steel pipes solely through extrusion cannot guarantee the dimensional accuracy and surface finish of the polygonal inner bore sidewalls of the steel pipe.

[0004] The cold drawing process is widely used in the production of special-shaped steel pipes, offering advantages such as precise dimensions and excellent surface quality. By inserting a die with a cross-sectional shape identical to the inner bore of the special-shaped steel pipe into the cold drawing die, special-shaped steel pipes with polygonal internal bores can also be produced. However, the cold drawing process places high demands on raw materials, requiring the use of densely structured, high-quality steel. Using untreated, ordinary steel for cold drawing can compromise product quality.

[0005] Therefore, using the extrusion method alone or the cold drawing method alone to produce special-shaped steel pipes with an outer circle and an inner polygonal structure cannot simultaneously guarantee the dimensional accuracy, surface finish, and product quality of the polygonal inner hole. Utility Model Content

[0006] In order to solve the above-mentioned deficiencies in the prior art, the utility model aims to provide a precision plastic forming mold for special-shaped steel pipes, which includes an extrusion mold and a cold drawing mold. By using the cold drawing mold, the inner hole side wall of the special-shaped steel pipe after extrusion is polished for the second time to achieve the purpose of improving the dimensional accuracy of the inner hole side wall.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is as follows: a precision plastic forming mold for special-shaped steel tubes, including an extrusion mold, the extrusion mold including an extrusion outer mold and a core rod, a first sizing hole with a cylindrical inner wall is opened through the inner axial direction of the extrusion outer mold, the cross-section of the core rod is polygonal, there is a distance between the outer wall of the core rod and the inner wall of the first sizing hole, and the core rod is coaxially inserted into the first sizing hole, the precision plastic forming mold for special-shaped steel tubes also includes a cold drawing mold independent of the extrusion mold, the cold drawing mold includes a cold drawing outer mold and a cold drawing inner mold; a second sizing hole with a cylindrical inner wall is opened through the inner axial direction of the cold drawing outer mold, the diameter of the first sizing hole is larger than the diameter of the second sizing hole; the cross-sectional shape of the cold drawing inner mold is the same as the cross-sectional shape of the core rod, and the outer contour size of the core rod cross-sectional size is larger than the outer contour size of the cold drawing inner mold cross-sectional size; there is a distance between the outer wall of the cold drawing inner mold and the inner wall of the second sizing hole, and the cold drawing inner mold is coaxially inserted into the second sizing hole.

[0008] As a limitation of the present invention: the diameter of the first sizing hole is 1.02 to 1.03 times the diameter of the second sizing hole, and the outer contour size of the core rod cross section is 1.02 to 1.03 times the outer contour size of the cold drawing inner die cross section.

[0009] As a definition of the present utility model: the outer wall of the extrusion outer die is a conical surface with the bottom facing the extrusion side; a first inlet hole is coaxially penetrated through one end of the first sizing hole close to the extrusion side, and a first outlet hole is coaxially penetrated through one end of the first sizing hole close to the extrusion side; the inner wall of the first inlet hole is a conical surface with the bottom facing the extrusion side, and the inner wall of the first outlet hole is a conical surface with the bottom facing the extrusion side, and the diameters of the first inlet hole close to the extrusion side and the first outlet hole close to the extrusion side are the same as the diameter of the first sizing hole.

[0010] As a limitation of the present invention: the core rod is a long rod-shaped hexagonal prism structure, and the cross-section thereof is a hexagonal shape.

[0011] As a definition of the present utility model: the outer wall of the cold-drawn outer die is a conical surface with the bottom facing the extrusion side; a second inlet hole is coaxially penetrated through one end of the second sizing hole close to the extrusion side, and a second outlet hole is coaxially penetrated through one end of the second sizing hole close to the extrusion side; the inner wall of the second inlet hole is a conical surface with the bottom facing the extrusion side, and the inner wall of the second outlet hole is a conical surface with the bottom facing the extrusion side, and the diameters of the second inlet hole close to the extrusion side and the second outlet hole close to the extrusion side are the same as the diameter of the second sizing hole.

[0012] As a limitation of the present invention: the cold drawing outer die includes an outer shell and an inner insert, the outer wall of the outer shell is a conical surface with the bottom facing the extrusion side, the inner insert is detachably coaxially installed in the outer shell, and the second sizing hole is coaxially opened in the inner insert.

[0013] As a limitation of the present invention: the cold drawing inner die is a hexagonal prism structure, and the cross-section thereof is in the shape of a hexagon.

[0014] As a limitation of the present invention: a blank used in the said special-shaped steel pipe precision plastic forming mold, the blank is cylindrical, and a through hole with the same cross-sectional shape as the core rod is opened through the inner axial direction, and the outer contour size of the through hole cross section is larger than the outer contour size of the core rod cross section.

[0015] As a limitation of the present invention: the outer contour dimension of the through hole cross section is 7 mm to 15 mm larger than the outer contour dimension of the core rod cross section.

[0016] As a limitation of the present invention: the blank includes a blank body and a tail pad, both of which are cylindrical in shape, have the same diameter, and are coaxially welded together end to end. The end of the blank body away from the tail pad is rounded.

[0017] Due to the adoption of the above-mentioned technical solution, the present invention achieves the following beneficial effects compared to the prior art: in the extrusion die, the blank is initially shaped through the cavity formed by the mandrel and the first sizing hole, resulting in a special-shaped steel tube with a high dimensional accuracy, a well-formed forging streamline, and a dense structure. In the cold drawing die, the outer diameter and inner hole dimensions and sidewalls of the special-shaped steel tube are further adjusted through the cavity formed by the cold drawing inner die and the second sizing hole, thereby improving the dimensional accuracy and surface finish of the outer wall and inner hole of the special-shaped steel tube. By combining the extrusion die and the cold drawing die, it is possible to produce special-shaped steel tubes with high dimensional accuracy and surface quality, facilitating subsequent direct use.

[0018] Since a rounded corner is set at the head of the blank and the inner hole shape of the blank corresponds to the inner hole shape of the produced special-shaped steel pipe, the extrusion force of the mold on the blank is reduced, which is conducive to the smooth movement of the blank in the hollow structure of the mold. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments.

[0020] Figure 1 This is a cross-sectional view of the extrusion die of Example 2 of the present utility model;

[0021] Figure 2 This is a cross-sectional view of the cold drawing die of Example 2 of the present utility model;

[0022] Figure 3 This is a structural diagram of the extrusion outer die of Example 2 of the present utility model;

[0023] Figure 4 This is a cross-sectional view of the extrusion outer die of Example 2 of the present utility model;

[0024] Figure 5 It is the structure schematic view of the core rod of the utility model embodiment 2;

[0025] Figure 6 It is the sectional view of the core rod of the utility model embodiment 2;

[0026] Figure 7 It is the structure schematic view of the end face of the cold-drawing outer die of the utility model embodiment 2;

[0027] Figure 8 It is the A-A sectional view of the utility model embodiment 2; Figure 7

[0028] Figure 9 It is the structure schematic view of the end face of the cold-drawing inner die of the utility model embodiment 2;

[0029] Figure 10 It is the B-B sectional view of the utility model embodiment 2; Figure 9

[0030] Figure 11 It is the structure schematic view of the special-shaped steel pipe produced by the utility model embodiment 2;

[0031] Figure 12 It is the sectional view of the special-shaped steel pipe produced by the utility model embodiment 2;

[0032] Figure 13 It is the structure schematic view of the blank of the utility model embodiment 1;

[0033] Figure 14 It is the sectional view of the blank of the utility model embodiment 1;

[0034] Figure 15 It is the sectional view of the extrusion process of the utility model embodiment 2;

[0035] Figure 16 It is the sectional view of the cold-drawing process of the utility model embodiment 2;

[0036] In the drawing: 1-die holder, 2-extrusion outer die, 21-first inlet hole, 22-first sizing hole, 23-first outlet hole,

[0037] 3-die support, 4-lubricating pad, 5-extrusion cylinder, 6-hollow gasket, 7-core rod, 8-extrusion rod,

[0038] 9-blank, 901-blank body, 902-tail pad, 10-special-shaped steel pipe,

[0039] 11-cold-drawing outer die, 111-inner insert block, 112-outer shell, 113-second inlet hole, 114-second sizing hole, 115-second outlet hole,

[0040] ​​12-cold drawing inner die, 13-core rod, 14-clamp, 15-hydraulic device, 16-die frame, 17-pushing device. DETAILED DESCRIPTION

[0041] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the embodiments described herein are preferred embodiments and are only used to illustrate and explain the present invention, and do not constitute a limitation of the present invention.

[0042] The directional terms or positional relationships such as "up", "down", "left" and "right" described in the present invention are based on the directional relationships in the drawings of the present invention specification and are only for the convenience of describing the present invention and simplifying the description. They do not indicate or imply that the device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the content protected by the present invention.

[0043] Example 1

[0044] The structural diagram of the blank 9 required for the special-shaped steel pipe 10 produced in this embodiment is as follows Figure 13 and Figure 14 As shown, it can be used for precision plastic forming molds of special-shaped steel pipes. The blank 9 has a through hole inside, the shape of which corresponds to the cross-sectional profile of the core rod 7, and the outer contour size of the cross-sectional profile of the through hole is larger than the outer contour size of the cross-sectional profile of the core rod 7. Preferably, the outer contour size of the cross-sectional profile of the through hole is 7mm to 15mm larger than the outer contour size of the cross-sectional profile of the core rod 7, so as to facilitate the insertion of the core rod 7 into the through hole. Specifically, the blank 9 includes a blank body 901 and a tail pad 902, both of which are cylindrical in shape, have the same diameter, and are coaxially welded together from head to tail. They are made of stainless steel and carbon steel, respectively. The end of the blank body 901 away from the tail pad 902 is filleted, and the radius of the fillet is R, preferably 30 mm, and the length of the tail pad 902 is preferably 70 to 85 mm. The structure of the produced special-shaped steel pipe 10 is as follows. Figure 11 and Figure 12 As shown, it is a tubular structure with an outer circle and an inner hexagon.

[0045] Since a rounded corner is provided at the head of the blank 9, and the cross-sectional shape of the through hole of the blank 9 corresponds to the cross-sectional shape of the inner hole of the special-shaped steel pipe 10 to be produced, the extrusion force of the extrusion die on the outer wall of the blank 9 is reduced, which is conducive to the smooth movement of the blank 9 in the cavity of the extrusion die; at the same time, a tail pad 902 made of carbon steel is provided at the tail of the blank 9, which is conducive to the complete extrusion of the stainless steel blank 9, thereby improving the utilization rate of the stainless steel material.

[0046] Example 2

[0047] like Figure 1 and Figure 2As shown, a special-shaped steel pipe precision plastic forming mold includes an extrusion mold, and the extrusion mold includes an extrusion outer mold 2 and a core rod 7, both of which are made of H13 steel with a Rockwell hardness of 48 to 52. The outer wall of the extrusion outer mold 2 is a conical surface with the bottom facing the extrusion side, and the top angle is β, preferably 40±0.5°; a first sizing hole 22 with a cylindrical inner wall is opened in the inner axial direction of the extrusion outer mold 2, and the axial length of the first sizing hole 22 is L1, preferably 10.5 to 16 mm, and the diameter of the first sizing hole 22 is used to determine the outer diameter of the special-shaped steel pipe 10. The cross-section of the core rod 7 is polygonal, and the outer contour size of the cross-section of the core rod 7 is smaller than the cross-section size of the first sizing hole 22, and the core rod 7 is coaxially inserted into the first sizing hole 22. Preferably, the core rod 7 is a long rod-shaped hexagonal prism structure, and its cross-section is hexagonal. There is a distance between the outer wall of the core rod 7 and the inner wall of the extrusion outer mold 2 to form a cavity, and then the blank 9 is processed into Figure 11 and Figure 12 The cross section of the special-shaped steel pipe 10 shown in FIG. Figures 1 to 4 、 Figure 8 In the figure, the right side is the intrusion side and the left side is the extrusion side.

[0048] like Figures 3 to 6 As shown, the outer wall of the extrusion outer die 2 is a conical surface with its bottom facing the extrusion side. A first inlet hole 21 is coaxially formed through the end of the first sizing hole 22 near the inlet side, and a first outlet hole 23 is coaxially formed through the end of the first sizing hole 22 near the extrusion side. The inner wall of the first inlet hole 21 is a curved conical surface with its bottom facing the inlet side. The radius of the arcuate sidewall is R1, preferably 10-12 mm. The diameter of the end surface of the first inlet hole 21 near the inlet side is larger than the diameter of the end surface near the extrusion side, allowing the billet 9 to smoothly enter the interior of the extrusion outer die 2 from the inlet side. The inner wall of the first outlet hole 23 is a conical surface with its bottom facing the extrusion side, and its top angle is α, preferably 20±0.5°. The diameters of the first inlet hole 21 near the extrusion side and the first outlet hole 23 near the inlet side are the same as the diameter of the first sizing hole 22, which helps to improve the surface flatness of the billet 9.

[0049] In other possible cases, only a first calibrating hole 22 with a cylindrical inner wall is opened through the interior of the extrusion outer die 2 along the axial direction, and the blank 9 can enter the extrusion outer die 2 from the extrusion side for preliminary processing.

[0050] like Figure 15As shown, the extruder includes a cylindrical hollow die base 1 and a die support 3 with the same external diameter, which are respectively used to place the extrusion outer die 2 and to support the extrusion outer die 2 during the extrusion process; the extruder also includes a lubricating pad 4 for allowing the billet 9 to move smoothly toward the extrusion side, an extrusion cylinder 5 and an extrusion rod 8 for applying pressure to the billet, a cylindrical hollow gasket 6 coaxially fixedly arranged on the extrusion rod 8 close to the extrusion side, and a core rod 7 coaxially passed through the hollow gasket 6 and the extrusion rod 8.

[0051] The precision plastic forming die for special-shaped steel pipes also includes a cold drawing die that is independent of the extrusion die. The cold drawing die includes a cold drawing outer die 11 and a cold drawing inner die 12. The cold drawing outer die 11 and the cold drawing inner die 12 are used in conjunction to further cold draw the blank 9 to a suitable size and precision. A second sizing hole 114 with a cylindrical inner wall is provided through the inner axial direction of the cold drawing outer die 11. Its axial length is L2, preferably 12 to 16 mm. The diameter of the first sizing hole 22 is larger than that of the second sizing hole 114. Preferably, the diameter of the first sizing hole is 1.02 to 1.03 times the diameter of the second sizing hole. The shape of the cross section of the cold drawing inner die 12 is the same as the shape of the cross section of the core rod 7. The outer contour size of the cross section of the core rod 7 is larger than the outer contour size of the cross section of the cold drawing inner die 12. Preferably, the outer contour size of the cross section of the core rod 7 is 1.02 to 1.03 times the outer contour size of the cross section of the cold drawing inner die 12. At the same time, the cross-sectional outer contour of the cold-drawn inner die 12 is smaller than the cross-sectional dimension of the second calibrating hole 114, and the cold-drawn inner die 12 is coaxially inserted into the second calibrating hole 114. A gap is provided between the inner wall of the cold-drawn outer die 11 and the outer wall of the cold-drawn inner die 12, forming a cavity. When the extruded blank 9 passes through the cavity, the precision and quality of the inner wall of the special-shaped steel tube 10 can be further improved.

[0052] like Figures 7 to 10 As shown, the outer wall of the cold-drawn outer die 11 is a conical surface with the bottom facing the extrusion side; a second inlet hole 113 is coaxially penetrated at one end of the second sizing hole 114 near the extrusion side, and a second outlet hole 115 is coaxially penetrated at one end of the second sizing hole 114 near the extrusion side. The inner wall of the second inlet hole 113 is a conical surface with the bottom facing the extrusion side, and the vertex angle is γ1, preferably 24±0.5°. The inner wall of the second outlet hole 115 is a conical surface with the bottom facing the extrusion side, and the vertex angle is γ2, preferably 50±0.5°. The diameters of the second inlet hole 113 near the extrusion side and the second outlet hole 115 near the extrusion side are the same as the diameter of the second sizing hole 114, which is beneficial to improving the flatness of the surface of the blank 9.

[0053] Furthermore, the cold drawing outer die 11 includes an outer shell 112 and an inner insert 111. The outer wall of the outer shell 112 is a conical surface with the bottom facing the extrusion side, and the top angle is γ3, preferably 12±0.5°. The interior of the outer shell 112 is stepped, and the inner insert 111, with a cylindrical outer wall, is detachably mounted coaxially within the outer shell 112. A second calibrating hole 114 is coaxially defined through the inner insert 111.

[0054] The cold-drawing inner die 12 is a hexagonal prism with a cylindrical hollow inner bore. Its cross-section is hexagonal, and its axial length is greater than that of the cold-drawing outer die 11. The cross-sectional profile of the cold-drawing inner die 12 is adapted to the internal shape of the special-shaped steel tube 10 to improve the precision and smoothness of the inner wall of the special-shaped steel tube 10. Specifically, when the desired special-shaped steel tube 10 has a regular hexagonal inner bore, the cold-drawing inner die 12 is a regular hexagonal prism with a regular hexagonal cross-section to meet practical needs.

[0055] In other possible cases, only a second calibrating hole 114 with a cylindrical inner wall is opened inside the cold drawing outer die 11 along the axial direction, and the blank 10 can enter the cold drawing outer die 11 from the extrusion side for secondary processing.

[0056] like Figure 16 As shown, the cold drawing machine includes a core rod 13 for feeding the cold drawing inner die 12 into a specified position in the inner cavity of the cold drawing outer die 11. The diameter of the core rod 13 is smaller than the diameter of the second calibrating hole 114. The cold drawing machine also includes a clamp 14 for clamping the end of the blank 9 near the extrusion side, a hydraulic device 15 for providing clamping force to the clamp 14, a cylindrical hollow die frame 16 for accommodating the cold drawing outer die 11, and a pusher 17 for moving the blank 9 toward the extrusion side. The clamp 14 is fixedly connected to the hydraulic device 15, and they and the pusher 17 are respectively fixedly mounted on both sides of the die frame 16.

[0057] In the extrusion die, the cavity formed by the core rod 7 and the first sizing hole 22 initially shapes the blank 9, resulting in a special-shaped steel tube 10 with a high dimensional accuracy, streamlined forging, and a dense structure. In the cold drawing die, the cavity formed by the cold drawing inner die 12 and the second sizing hole 114 further adjusts the dimensions of the outer circle and inner hole, as well as the sidewalls, of the special-shaped steel tube 10, improving the dimensional accuracy and surface finish of the outer wall and inner hole of the special-shaped steel tube 10. By combining the extrusion die and the cold drawing die, it is possible to produce special-shaped steel tubes 10 with high dimensional accuracy and surface quality, facilitating their subsequent direct use.

[0058] The method for producing the special-shaped steel pipe 10 using the above-mentioned special-shaped steel pipe precision plastic forming mold and the blank 9 is divided into an extrusion forming process and a cold drawing process. Figure 15As shown, the extrusion process is: after the blank 9 is heated, it is placed horizontally in the extrusion cylinder 5, the hollow gasket 6 is attached to the right side of the blank 9, the extrusion rod 8 applies a pushing force to the left side, and the blank body 901 passes through the cavity composed of the extrusion outer die 2 and the core rod 7 to become the profiled steel pipe 10 with an outer circle and an inner hexagonal structure. Figure 16 As shown, the cold drawing process is: after the profiled steel pipe 10 obtained by the foregoing extrusion is heated, the surface of the profiled steel pipe 10 is cleaned and lubricated, and then the profiled steel pipe 10 is placed on the feeding rack, the profiled steel pipe 10 is pushed into the inner cavity of the cold drawing outer die 11 by a certain distance by the pushing device 17, the protruding part of the profiled steel pipe 10 is clamped by the clamp 14 for air drawing, when the profiled steel pipe 10 is displaced to the left by 100 mm, the cold drawing inner die 12 is sent into the inner cavity of the cold drawing outer die 11 by the core rod 13, and the clamp 14 is moved to the left, and the drawing is completed, so that the profiled steel pipe 10 is uniformly drawn in the outer diameter and the inner hole, and finally the profiled steel pipe 10 with high dimensional accuracy and good surface quality is obtained.

Claims

1. A precision plastic forming die for a special-shaped steel pipe, comprising an extrusion die, the extrusion die comprising an extrusion outer die and a core rod, a first sizing hole with a cylindrical inner wall being formed through the inner axial direction of the extrusion outer die, a polygonal cross-section of the core rod, a spacing being provided between the outer wall of the core rod and the inner wall of the first sizing hole, the core rod being coaxially inserted into the first sizing hole, and characterized in that: The special-shaped steel pipe precision plastic forming mold also includes a cold drawing mold that is independent of the extrusion mold, and the cold drawing mold includes a cold drawing outer mold and a cold drawing inner mold; a second sizing hole with a cylindrical inner wall is opened through the inner axial direction of the cold drawing outer mold, and the diameter of the first sizing hole is larger than the diameter of the second sizing hole; the cross-sectional shape of the cold drawing inner mold is the same as the cross-sectional shape of the core rod, and the outer contour size of the core rod cross-sectional area is larger than the outer contour size of the cold drawing inner mold cross-sectional area; there is a distance between the outer wall of the cold drawing inner mold and the inner wall of the second sizing hole, and the cold drawing inner mold is coaxially arranged in the second sizing hole.

2. The precision plastic forming die for special-shaped steel pipes according to claim 1, characterized in that: The diameter of the first sizing hole is 1.02 to 1.03 times the diameter of the second sizing hole, and the outer contour size of the core rod cross section is 1.02 to 1.03 times the outer contour size of the cold drawing inner die cross section.

3. The precision plastic forming die for special-shaped steel pipes according to claim 2, characterized in that: The outer wall of the extrusion outer die is a conical surface with the bottom facing the extrusion side; a first inlet hole is coaxially formed through one end of the first sizing hole close to the extrusion side, and a first outlet hole is coaxially formed through one end of the first sizing hole close to the extrusion side; The inner wall of the first inlet hole is a conical surface with the bottom facing the extrusion side, and the inner wall of the first outlet hole is a conical surface with the bottom facing the extrusion side. The diameters of the first inlet hole close to the extrusion side and the first outlet hole close to the extrusion side are the same as the diameter of the first sizing hole.

4. The precision plastic forming die for special-shaped steel pipes according to claim 3, characterized in that: The core rod is a long rod-shaped hexagonal prism structure, and its cross section is in the shape of a hexagon.

5. The special-shaped steel pipe precision plastic forming die according to claim 4, characterized in that: The outer wall of the cold-drawn outer die is a conical surface with the bottom facing the extrusion side; a second inlet hole is coaxially formed through one end of the second sizing hole close to the extrusion side, and a second outlet hole is coaxially formed through one end of the second sizing hole close to the extrusion side; The inner wall of the second inlet hole is a conical surface with the bottom facing the extrusion side, and the inner wall of the second outlet hole is a conical surface with the bottom facing the extrusion side. The diameters of the second inlet hole close to the extrusion side and the second outlet hole close to the extrusion side are the same as the diameter of the second sizing hole.

6. The precision plastic forming die for special-shaped steel pipes according to claim 5, characterized in that: The cold drawing outer die includes an outer shell and an inner insert. The outer wall of the outer shell is a conical surface with the bottom facing the extrusion side. The inner insert is detachably coaxially installed in the outer shell. The second calibrating hole is coaxially opened in the inner insert.

7. The special-shaped steel pipe precision plastic forming die according to claim 6, characterized in that: The cold drawing inner die is a hexagonal prism structure, and its cross-section is hexagonal.

8. A blank for use in a precision plastic forming die for a special-shaped steel pipe according to any one of claims 1 to 7, characterized in that: The blank is cylindrical, and a through hole with the same cross-sectional shape as that of the core rod is penetrated in the inner axial direction of the blank, and the outer contour size of the cross-sectional shape of the through hole is larger than the outer contour size of the cross-sectional shape of the core rod.

9. The blank according to claim 8, characterized in that: The outer contour dimension of the through hole cross section is 7 mm to 15 mm larger than the outer contour dimension of the core rod cross section.

10. The blank according to claim 9, characterized in that: The utility model comprises a blank body and a tail pad, both of which are cylindrical in shape, have the same diameter and are coaxially welded together. The end of the blank body away from the tail pad is rounded.

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