TWIP steel mold for cold extrusion forming of aluminum alloy and preparation method of TWIP steel mold

Abstract

The invention belongs to the field of die machining, and relates to a heat-treatment-free TWIP steel die for aluminum alloy cold extrusion precision forming and a preparation method of the heat-treatment-free TWIP steel die. TWIP steel is adopted as a mold material, firstly, a blank material with high yield strength (larger than or equal to 800 MPa) is obtained through medium-temperature plastic processing, then, the blank material is used for processing a semi-finished product of the cold extrusion mold, then, the semi-finished product mold is subjected to multiple times of extrusion treatment through an aluminum alloy bar under a pressing machine, so that a material on the surface of an inner cavity is subjected to small-amount plastic deformation to be hardened, and the cold extrusion mold is obtained. And the inner cavity of the die does not generate plastic deformation under the action of extrusion pressure. And finally, the inner cavity and the outer surface of the die are subjected to finish machining to the final size and precision through linear cutting, grinding and other machining. The high-pressure resistance and brittle failure resistance of the TWIP steel die are obviously enhanced, the service life is obviously prolonged, the production cost of an aluminum alloy extrusion product is obviously reduced, and a new material and a new process are provided for solving the problems that an aluminum alloy high-pressure cold extrusion forming die is easy to crack, short in service life and high in cost.

Description

Technical Field

[0001] The invention belongs to the field of aluminum alloy plastic forming die processing, and particularly relates to a heat treatment-free TWIP steel die for aluminum alloy cold extrusion precision forming and a preparation method thereof. Background Art

[0002] Cold extrusion is an important part of precision plastic bulk forming technology. During cold extrusion, the metal blank to be processed is first placed in the cold extrusion die, and then pressure is applied to the blank through the punch on the press at room temperature, so that the metal blank undergoes plastic deformation to fill the mold cavity, thereby obtaining an extruded part with the desired shape, size and mechanical properties. Because there is no need for heating, the extrusion pressure is high, and the filling speed is fast, cold extrusion has the advantages of high precision, high efficiency, high quality, and low consumption. It is one of the most widely used technologies in the plastic forming of metal materials, especially non-ferrous metal materials. For cold extrusion of aluminum alloys, due to the high pressure in the cavity, the strong adhesion and low fluidity of aluminum alloys, in order to ensure that all details on the parts can be precisely formed, the mold must be made of materials with outstanding strength and toughness to withstand high pressure and impact without breaking. Among them, toughness is one of the performance indicators that need to be given priority.

[0003] At present, the mold materials used for cold extrusion precision forming of aluminum alloys are mainly H13 and DC53. The former is the H13 air quenching hardening hot working die steel introduced from the United States, with the grade of 4Cr5MoSiV1 and a hardness of HRC56-58; the latter is a cold working die steel improved on the basis of Japanese SKD11 (Cr12MoV), with a hardness of HRC62-63 and a toughness twice that of Cr12MoV. Although these two materials have good effects on general and precision molds, they are prone to cracking due to the high forming pressure when extruding some aluminum alloy parts with fine structures, and the service life of the mold is very short. In order to solve this problem, many manufacturers have adjusted the heat treatment process to appropriately reduce the hardness and improve the toughness, but the effect is very limited, and the problem of mold cracking still occurs frequently. Therefore, finding suitable mold materials and improving the mold's resistance to high-pressure fracture is one of the problems that need to be solved in this field. Summary of the invention

[0004] One of the purposes of the present invention is to provide a method for preparing a heat-treatment-free TWIP steel die for cold extrusion precision forming of aluminum alloys, using TWIP steel with outstanding toughness as the die material, forging at a fixed ratio at medium temperature, rough machining the semi-finished cold extrusion die, and extruding the same aluminum alloy bar in the semi-finished cold extrusion die for 4 to 6 times, so that a small amount of plastic deformation occurs on the surface of the inner cavity of the die and hardens, and finally the inner cavity and outer surface of the die are finely machined to the final size and accuracy through wire cutting and grinding and other processing. The die processing technology is simple, the results are easy to control, the cost is greatly reduced, and it has the advantages of high toughness and long life.

[0005] To achieve the above object, the present invention adopts the following technical scheme: a method for preparing a TWIP steel die for cold extrusion of aluminum alloy, comprising the following steps:

[0006] S1. Melting: According to the formula of TWIP steel, weigh the single raw materials and the intermediate alloys V-Fe and N-Cr and add them into a melting furnace, melt them under the protection of argon gas and then cast them into steel ingots; the chemical composition of the TWIP steel is as follows in terms of mass percentage: C: 0.06-1.0; Mn: 17.0-30.0; Al: 0.1-3.0; Si: 0.4-0.8; P≤0.05; S≤0.01; V: 0.15-0.30; N: 0.10-0.5; the rest is Fe;

[0007] S2. High temperature forging: first heat the steel ingot to 1000-1100℃, keep it warm for 3-4h, and forge it into a square billet with a cross section of 200-300mm square and a length of ≥400mm in one or more fires;

[0008] S3, medium temperature forging; reheat the square billet to 650-750°C, keep warm for 1.5-2.0 hours, forge it into a round billet of φ120×600mm, cut off the riser and set aside;

[0009] S4, rough processing die: the forged round billet is processed into a semi-finished cold extrusion die with internal splines, and the inner cavity size is 0.10 to 0.15 mm smaller than the actual size.

[0010] S5, cold extrusion hardening treatment: put the aluminum alloy bar of suitable specifications into the cavity of the above rough processing mold, adopt the cold extrusion process, and fill the cavity with the aluminum alloy bar under 50-60 tons of pressure. After holding the pressure for 5-8 seconds, eject the aluminum alloy extrusion part, measure the inner cavity size of the mold, and repeat the above extrusion process 4-6 times until the inner cavity size of the semi-finished product of the cold extrusion mold no longer changes;

[0011] S6. Finishing: Finish the semi-finished cold extrusion die after hardening treatment, process the inner cavity of the die to the required dimensional accuracy and roughness, and obtain the final TWIP steel die for cold extrusion of aluminum alloy.

[0012] As a further improvement of the preparation method of TWIP steel die for cold extrusion of aluminum alloys:

[0013] Preferably, the TWIP steel belongs to the Fe-Mn-Al-Si system.

[0014] Preferably, the forged mechanical properties of the TWIP steel are as follows: yield strength R P0.2 :≥800MPa; tensile strength R m: ≥1150MPa; Elongation after fracture A: ≥30%.

[0015] Preferably, the preferred chemical composition of TWIP steel is divided into two types according to mass percentage: (1) high toughness type: C: 0.06-0.08; Mn: 28.0-30.0; Al: 3.0-3.2; Si: 3.0-3.2; P: ≤0.01; S: ≤0.01; V: 0.15-0.20; N: 0.10-0.20; the rest is Fe; (2) high strength type: C: 0.6-0.9; Mn: 18.0-20.0; Al: 0.1-0.2; Si: 0.4-0.6; P: ≤0.05; S: ≤0.005; V: 0.15-0.30; N: 0.10-0.5; the rest is Fe.

[0016] Preferably, in step S1, the single element raw materials are Fe, Mn, Al, Si and C.

[0017] Preferably, in step S1, the single elements Mn, Al and Si must be added after all other raw materials are melted, and at the same time, the power of the electric furnace is increased to the maximum to enable rapid melting.

[0018] Preferably, when multi-fire forging is performed in step S2, an annealing treatment is performed at 850-1100° C. and kept for 1-2 hours between each two fires.

[0019] Preferably, in step S4, turning or wire cutting is used to process the forged round billet into a cold extrusion die semi-finished product with internal splines.

[0020] Preferably, in step S6, the semi-finished cold extrusion die after hardening treatment is finely processed by wire cutting or precision grinding.

[0021] A second object of the present invention is to provide a TWIP steel mold for cold extrusion forming of an aluminum alloy obtained by the preparation method of the TWIP steel mold for cold extrusion forming of an aluminum alloy described in any one of the above.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1) The purpose of the present invention is to provide a heat-free, high-strength and tough TWIP steel die for cold extrusion precision forming of aluminum alloys and a preparation method thereof. The die has high-pressure fracture resistance and belongs to Fe-Mn-Al-Si series TWIP steel. The present invention uses TWIP steel with outstanding toughness as the die material. After medium-temperature constant ratio forging, the mechanical properties of the forged TWIP steel are: R P0.2 :800~900MPa;R m: 1150 - 1250 MPa; A: 25 - 35%, with a high yield strength (≥800 MPa). Then, a semi-finished cold extrusion die is machined from this blank material, and the inner cavity size is 0.10 - 0.15 mm smaller than the actual size. Before the formal extrusion production of aluminum alloy products, the same aluminum alloy bar is used for 4 - 6 extrusion treatments in the semi-finished cold extrusion die, causing a small amount of plastic deformation and hardening on the surface of the die inner cavity, and the size of the die inner cavity no longer changes. Finally, through machining such as wire cutting and grinding, the inner cavity and outer surface of the die are precision machined to the final size and accuracy. After nearly ten thousand extrusion tests, it is proved that under the same die, extrusion product, and extrusion conditions, the TWIP steel die is intact without any cracks.

[0024] Compared with traditional die steels, the TWIP steel die strengthened by cold extrusion deformation has significantly enhanced high-pressure resistance and brittle fracture resistance. The forming efficiency and dimensional accuracy of aluminum alloy cold extrusion products are significantly improved, and the production cost is significantly reduced, providing a new material and new process for solving the problems of easy cracking, low life, and high cost of aluminum alloy high-pressure cold extrusion forming dies. The present invention improves the surface hardness of the die inner cavity by work hardening, forming a gradient distribution of hardness in the thickness direction of the die, thereby ensuring that the die has high high-pressure resistance and fracture resistance. After work hardening, the surface hardness of the TWIP steel die inner cavity ≥ HRC35, and the thickness of the hardened layer ≥ 5 mm.

[0025] 2) The preferred TWIP steel composition of the present invention aims to reduce the stacking fault energy of TWIP steel from generally 35 mJ / m 2 to 30 mJ / m 2Next, during the plastic deformation of the alloy, the density of twins and stacking faults increases, and the capacity of the microstructure to accommodate dislocations, especially fixed dislocations, increases, thereby enhancing the alloy's work hardening ability, and a small amount of plastic deformation can produce significant strengthening and hardening effects. Since deformation and hardening only occur near the surface of the mold cavity during extrusion, the materials in most other areas still maintain their original hardness and toughness. After the extrusion process is completed, the hardness near the inner surface of the mold rises to HRC34.5, and the hardness of other parts is HRC17.0. Although the hardness of the inner surface is lower than that of traditional mold steel, it has extremely high deformation resistance due to hardening. Therefore, when the mold cavity is subjected to high pressure, the high-hardness material near the inner surface layer can resist the pressure without deformation, and at the same time transfer the high stress to the subsurface layer. The subsurface layer and the materials outside it have high toughness, so they can reduce stress concentration, thereby inhibiting the initiation of cracks. This kind of toughness distributed in a gradient along the thickness direction of the mold makes the mold show excellent characteristics of resisting high pressure without breaking, which is very suitable for aluminum alloy extrusion forming production. In addition, the current aluminum alloy cold extrusion die materials must be subjected to complex heat treatment before use, while the TWIP steel die used in the present invention only needs a small number of extrusion treatments to meet the use requirements without heat treatment. Therefore, the die processing technology is simple, the results are easy to control, and the cost is greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0027] Figure 1 This is the microhardness distribution of the mold from the inner cavity surface to the thickness direction of the mold after the cold extrusion hardening treatment in step S5 of Example 1.

[0028] Figure 2 This is a diagram of the parts of the TWIP steel die for cold extrusion of aluminum alloy prepared in Example 1.

[0029] Figure 3 This is the forging tensile stress-strain curve of the TWIP steel die blank for cold extrusion of aluminum alloy prepared in Example 1.

[0030] Figure 4 This is the forging tensile stress-strain curve of the blank of the TWIP steel die for cold extrusion of aluminum alloy prepared in Example 2.

[0031] Figure 5 This is the forging tensile stress-strain curve of the billet of the TWIP steel die for cold extrusion of aluminum alloy prepared in Example 3. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention.

[0033] Example 1

[0034] This embodiment provides a method for preparing a heat-treatment-free TWIP steel die for cold extrusion precision forming of aluminum alloy, and the specific implementation steps are as follows:

[0035] S1. Melting: According to the composition of TWIP steel (wt.%): C: 0.9; Mn: 19; Al: 2.8; Si: 0.7; P: ≤0.01; S: ≤0.01; V: 0.15; N: 0.20; the rest is Fe. Weigh the raw materials such as carbon, manganese, aluminum, silicon, iron, vanadium iron and nitrogen-chromium alloy, add them into the vacuum induction furnace, and melt them under the protection of argon. The single elements Mn, Al, Si and C must be added after all other raw materials are melted. The power of the electric furnace is increased to the maximum to melt the raw materials quickly. After the melting is completed, cast into a round steel ingot of about 160kg.

[0036] S2. High temperature forging: The steel ingot is heated to 1050℃, homogenized and kept warm for 2h, and then forged into a square billet with a cross section of 300mm square and a length of about 500mm in two fires. The first fire is to stretch the steel ingot axially with a forging ratio of 4; the second fire is to upset the billet with a forging ratio of 9;

[0037] S3, medium temperature forging: reheat the square billet to 700℃, keep it at this temperature for 3 hours, and draw it into 2 round billets of φ120×600mm, cut off the riser and set aside;

[0038] S4, rough machining of the die: using wire cutting to rough-machine the outer diameter of the TWIP steel round billet to the value marked on the drawing, and wire cutting the inner cavity to a size 0.15 mm smaller than the value marked on the drawing, to obtain a semi-finished cold extrusion die;

[0039] S5, cold extrusion hardening treatment: put the aluminum alloy bar of suitable specifications into the cavity of the rough processing mold, adopt the cold extrusion process, fill the cavity with the aluminum alloy bar under 60 tons of pressure, hold the pressure for 7 seconds, eject the aluminum alloy extrusion part, measure the inner cavity size of the mold, and repeat the above extrusion process 5 times until the inner cavity size of the semi-finished product of the cold extrusion mold no longer changes;

[0040] S6. Finishing: The semi-finished cold extrusion die after hardening treatment is finished by wire cutting and slow wire walking, and the inner cavity of the die is processed to the required dimensional accuracy and roughness, and finally the TWIP steel die for cold extrusion of aluminum alloy is obtained.

[0041] Observe the appearance of cracks in H13 and DC53 dies when extruding splined aluminum alloy products. At this time, the forming pressure is 60 tons and less than 1,000 aluminum alloy products are extruded. It can be seen that both H13 and DC53 dies have broken and the cracks have penetrated the entire die, showing typical brittle fracture characteristics. This shows that under high-pressure extrusion conditions, these two die materials can no longer meet production requirements.

[0042] The state of the TWIP steel die for cold extrusion of aluminum alloy prepared in Example 1 after nearly 10,000 extrusion tests was observed, which proved that under the same die, extrusion product and extrusion conditions, the TWIP steel die was intact without any cracks.

[0043] The change of the center hole diameter of the semi-finished cold extrusion die obtained in step S4 of Example 1 with the number of extrusions is shown in Appendix 1; after the cold extrusion hardening treatment in step S5 of Example 1, the microhardness distribution of the die from the inner cavity surface to the die thickness direction is shown in Appendix Figure 1 As shown in Table 1 and Figure 1 It can be seen that, since deformation and hardening only occur near the inner cavity surface of the mold during extrusion, the material in most other areas still maintains the original hardness and toughness. The inner surface of the mold has been hardened, and the thickness of the hardened layer is about 5 mm. The microhardness of the hardened layer reaches Hv350, and the macrohardness reaches HRC34.5. It is confirmed that the hardness of the mold obtained in Example 1 is distributed in a gradient from the inside to the outside, which can withstand high-pressure extrusion and suppress local stress concentration, so it is helpful to avoid the generation of cracks.

[0044] Table 1 Changes in inner cavity diameter during extrusion hardening of TWIP steel die (pressure: 60 tons)

[0045]

[0046]

[0047] Figure 2 This is a diagram of the parts of the TWIP steel die for cold extrusion of aluminum alloy prepared in Example 1.

[0048] The stress-strain curve of the round billet after forging in step S3 of Example 1 is shown in the attached figure. Figure 3 As shown. Figure 3It can be seen that after medium-temperature forging, the average yield strength and tensile strength of TWIP steel have exceeded 850MPa and 1250MPa respectively, while maintaining high ductility, and the average elongation after fracture is still as high as 30%.

[0049] Example 2

[0050] This embodiment provides a method for preparing a heat-treatment-free TWIP steel mold for cold extrusion precision forming of aluminum alloy, which specifically includes the following steps:

[0051] S1. Melting: According to the composition of TWIP steel (wt.%): C: 0.06; Mn: 30; Al: 3.0; Si: 3.0; P: ≤0.01; S: ≤0.01; V: 0.15; N: 0.20; the rest is Fe. Weigh the raw materials such as carbon, manganese, aluminum, silicon, iron, vanadium iron and nitrogen-chromium alloy, add them into the vacuum induction furnace, and melt them under the protection of argon. The single elements Mn, Al, Si and C must be added after all other raw materials are melted. The power of the electric furnace is increased to the maximum to melt the raw materials quickly. After the melting is completed, cast into a round steel ingot of about 160kg.

[0052] S2. High temperature forging: The steel ingot is heated to 1050℃, homogenized and kept warm for 2h, and then forged into a square billet with a cross section of 300mm square and a length of about 500mm in two fires. The first fire is to stretch the steel ingot axially with a forging ratio of 6.3; the second fire is to upset the billet with a forging ratio of 9;

[0053] S3, medium temperature forging: reheat the square billet to 750℃, keep it at this temperature for 4 hours, and draw it into 2 round billets of φ120×600mm, cut off the riser and set aside;

[0054] S4, rough machining of the die: using wire cutting to rough-machine the outer diameter of the TWIP steel round billet to the value marked on the drawing, and wire cutting the inner cavity to a size 0.15 mm smaller than the value marked on the drawing, to obtain a semi-finished cold extrusion die;

[0055] S5, cold extrusion hardening treatment: put the aluminum alloy bar of suitable specifications into the cavity of the rough processing mold, adopt the cold extrusion process, fill the cavity with the aluminum alloy bar under 60 tons of pressure, hold the pressure for 7 seconds, eject the aluminum alloy extrusion part, measure the inner cavity size of the mold, repeat the above extrusion process 5 times, until the inner cavity size of the semi-finished product of the cold extrusion mold no longer changes;

[0056] S6. Finishing: The semi-finished cold extrusion die after hardening treatment is finished by wire cutting and slow wire walking, and the inner cavity of the die is processed to the required dimensional accuracy and roughness, and finally the TWIP steel die for cold extrusion of aluminum alloy is obtained.

[0057] The stress-strain curve of the round billet after forging in step S3 of Example 2 is as follows: Figure 4 Compared with Example 1, the yield strength of the TWIP steel in this example is slightly decreased, but the tensile strength and elongation after fracture are improved, the average yield strength is 820MPa, the average tensile strength is 1265MPa, and the average elongation after fracture is as high as 43%.

[0058] Example 3

[0059] This embodiment provides a method for preparing a heat-treatment-free TWIP steel mold for cold extrusion precision forming of aluminum alloy, which specifically includes the following steps:

[0060] S1. Melting: According to the composition of TWIP steel (wt.%): C: 0.8; Mn: 20; Al: 0.2; Si: 0.4; P: ≤0.01; S: ≤0.01; V: 0.20; N: 0.30; the rest is Fe. Weigh the raw materials such as carbon, manganese, aluminum, silicon, iron, vanadium iron and nitrogen-chromium alloy, add them into the vacuum induction furnace, and melt them under the protection of argon. The single elements Mn, Al, Si and C must be added after all other raw materials are melted. The power of the electric furnace is increased to the maximum to melt the raw materials quickly. After the melting is completed, cast into a round steel ingot of about 160kg.

[0061] S2. High temperature forging: The steel ingot is heated to 1050℃, homogenized and kept warm for 2h, and then forged into a square billet with a cross section of 300mm square and a length of about 500mm in two fires. The first fire is to stretch the steel ingot axially with a forging ratio of 6.3; the second fire is to upset the billet with a forging ratio of 9;

[0062] S3, medium temperature forging: reheat the square billet to 650℃, keep it at this temperature for 5 hours, and draw it into 2 round billets of φ120×600mm, cut off the riser and set aside;

[0063] S4, rough machining of the die: using wire cutting to rough-machine the outer diameter of the TWIP steel round billet to the value marked on the drawing, and wire cutting the inner cavity to a size 0.15 mm smaller than the value marked on the drawing, to obtain a semi-finished cold extrusion die;

[0064] S5, cold extrusion hardening treatment: put the aluminum alloy bar of suitable specifications into the cavity of the rough processing mold, adopt the cold extrusion process, fill the cavity with the aluminum alloy bar under 60 tons of pressure, hold the pressure for 7 seconds, eject the aluminum alloy extrusion part, measure the inner cavity size of the mold, repeat the above extrusion process 5 times, until the inner cavity size of the semi-finished product of the cold extrusion mold no longer changes;

[0065] S6. Finishing: The semi-finished cold extrusion die after hardening treatment is finished by wire cutting and slow wire walking, and the inner cavity of the die is processed to the required dimensional accuracy and roughness, and finally the TWIP steel die for cold extrusion of aluminum alloy is obtained.

[0066] The stress-strain curve of the round billet after forging in step S3 of Example 3 is as follows: Figure 5 As shown. Figure 5 It can be seen that compared with Example 1, the strength of the TWIP steel in this example is significantly improved, the ductility is reduced, the average yield strength reaches 1283 MPa, the average tensile strength is as high as 1515 MPa, and the average elongation after fracture still reaches nearly 30%.

[0067] By observing the appearance of the TWIP steel mold for cold extrusion of aluminum alloy, it can be seen that the pre-pressed and hardened TWIP steel mold does not produce any cracks after use and can meet the production of aluminum alloy extrusion parts.

[0068] Those skilled in the art should understand that the above are only some specific embodiments of the present invention, rather than all embodiments. It should be noted that for those of ordinary skill in the art, many modifications and improvements can be made, and all modifications or improvements that do not exceed the scope of protection of the present invention should be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a TWIP steel die for cold extrusion of aluminum alloy, characterized in that: The steps include: S1. Melting: According to the formula of TWIP steel, weigh the single raw materials and the master alloy V-Fe and N-Cr and add them into a melting furnace, melt them under the protection of argon gas and then cast them into steel ingots; the chemical composition of the TWIP steel is as follows in terms of mass percentage: C: 0.06-1.0; Mn: 17.0-30.0; Al: 0.1-3.5; Si: 0.4-3.5; P≤0.05; S≤0.01; V：0.15～0.30； N: 0.10-0.5; the rest is Fe; S2. High temperature forging: first heat the steel ingot to 1000-1100℃, keep it warm for 3-4h, and forge it into a square billet with a cross section of 200-300mm square and a length of ≥400mm in one or more fires; S3, medium temperature forging; reheat the square billet to 650-750°C, keep warm for 1.5-2.0 hours, forge it into a round billet of φ120×600mm, cut off the riser and set aside; S4, rough processing die: the forged round billet is processed into a semi-finished cold extrusion die with internal splines, and the inner cavity size is 0.10 to 0.15 mm smaller than the actual size. S5, cold extrusion hardening treatment: put the aluminum alloy bar of suitable specifications into the cavity of the above rough processing mold, adopt the cold extrusion process, and fill the cavity with the aluminum alloy bar under 50-60 tons of pressure. After holding the pressure for 5-8 seconds, eject the aluminum alloy extrusion part, measure the inner cavity size of the mold, and repeat the above extrusion process 4-6 times until the inner cavity size of the semi-finished product of the cold extrusion mold no longer changes; S6. Finishing: Finish the semi-finished cold extrusion die after hardening treatment, process the inner cavity of the die to the required dimensional accuracy and roughness, and obtain the final TWIP steel die for cold extrusion of aluminum alloy.

2. The method for preparing a TWIP steel die for cold extrusion of aluminum alloy according to claim 1, characterized in that: The TWIP steel belongs to the Fe-Mn-Al-Si system.

3. The method for preparing a TWIP steel die for cold extrusion of aluminum alloy according to claim 1, characterized in that: The forged mechanical properties of the TWIP steel are as follows: Yield strength R P0.2 :≥800MPa; tensile strength R m : ≥1150MPa; Elongation after fracture A: ≥30%.

4. The method for preparing a TWIP steel die for cold extrusion of aluminum alloy according to claim 1 or 2, characterized in that: The preferred chemical composition of TWIP steel is divided into two types according to mass percentage: (1) High toughness type: C: 0.06-0.08; Mn: 28.0-30.0; Al: 3.0-3.2; Si: 3.0-3.2; P: ≤0.01; S：≤0.01； V：0.15～0.20； N: 0.10-0.20; the rest is Fe; (2) High strength type: C: 0.6-0.9; Mn: 18.0-20.0; Al：0.1～0.2； Si: 0.4~0.6; P：≤0.05； S：≤0.005； V：0.15～0.30； N: 0.10~0.5; the rest is Fe.

5. The method for preparing a TWIP steel die for cold extrusion of aluminum alloy according to claim 1, characterized in that: In step S1, the single-element raw materials are Fe, Mn, Al, Si and C.

6. The method for preparing a TWIP steel die for cold extrusion of aluminum alloy according to claim 5, characterized in that: In step S1, the single elements Mn, Al and Si must be added after all other raw materials are melted, and at the same time, the power of the electric furnace is increased to the maximum to make them melt quickly.

7. The method for preparing a TWIP steel die for cold extrusion of aluminum alloy according to claim 1, characterized in that: When multi-fire forging is performed in step S2, annealing treatment is performed at 850-1100° C. and kept for 1-2 hours between each two fires.

8. The method for preparing a TWIP steel die for cold extrusion of aluminum alloy according to claim 1, characterized in that: In step S4, the forged round blank is processed into a cold extrusion die semi-finished product with internal splines by turning or wire cutting.

9. The method for preparing a TWIP steel die for cold extrusion of aluminum alloy according to claim 1, characterized in that: In step S6, the semi-finished cold extrusion die is finely processed by wire cutting or precision grinding.

10. A TWIP steel die for cold extrusion of an aluminum alloy made by the method for preparing a TWIP steel die for cold extrusion of an aluminum alloy as claimed in any one of claims 1 to 9.

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