Method for simultaneously improving strength and ductility of industrial pure titanium

Abstract

The invention discloses a method for simultaneously improving the strength and ductility of industrial pure titanium, and belongs to the technical field of material preparation. The method specifically comprises the steps that a pure titanium rod with the diameter being 20 mm is completely annealed for 2 h at the temperature of 700 DEG C, then rotary swaging treatment is carried out under the room temperature condition, a titanium rod with the diameter being 5.9-7.9 mm is obtained, finally, vacuum annealing treatment is carried out on the titanium rod obtained after rotary swaging for 15-60 min at the temperature of 450-525 DEG C, and pure titanium with the strength and ductility improved at the same time is obtained. The pure titanium is of a double-peak grain structure, mechanical experiment results show that the pure titanium of the structure has the yield strength of 530-660 MPa and the tensile strength of 620-785 MPa, and the uniform elongation is larger than or equal to 8%. According to the pure titanium with the double-peak grain structure, the problem that coarse-grain pure titanium is low in strength is solved, meanwhile, the pure titanium can have good ductility, the strength-ductility matching performance of the pure titanium is improved, and industrial application and production are facilitated.

Description

Technical Field

[0001] The invention relates to a method for simultaneously improving the strength and ductility of industrial pure titanium, and belongs to the technical field of material preparation. Background Art

[0002] The content of metallic titanium in the earth's crust is about 0.6%, ranking fourth, second only to aluminum, iron and magnesium. Titanium is known as a "strategic metal" and has important applications in steel, chemical industry, biomedicine, marine engineering, aerospace and other fields. Titanium and titanium alloys have excellent properties in many aspects, such as high specific strength, good plasticity, high corrosion resistance, good human adaptability and high chemical activity. Pure titanium has good ductility, but its lower strength greatly limits its further application in different fields. Therefore, improving the strength of pure titanium while giving it a certain ductility is the key to whether pure titanium can be widely used.

[0003] Severe plastic deformation (SPD) technology is a deformation method that uses equipment to plastically deform the material below the recrystallization temperature of the material, introduces high stress and strain into the material, and forms defects such as dislocations, stacking faults and twins in the material. With the increase of internal defects, a fine grain strengthening effect is achieved. Common severe plastic deformation processes include high pressure torsion (HPT), cumulative roll bonding (ARB), equal channel angular pressing / extrusion (ECAP / ECAE), surface mechanical attrition treatment (SMAT) and rotary swaging (RS). The SPD process can greatly improve the strength of industrial pure titanium, but its ductility will also decrease. Therefore, how to make pure titanium obtain high strength and high ductility at the same time has become a difficult problem that needs to be solved urgently.

[0004] In order to better alleviate or solve the inverted strength-ductility relationship of metal materials, researchers have proposed heterostructured metallic materials. Heterostructured metallic materials are materials with uneven internal microstructure distribution. They are composed of two soft and hard regions with different rheological stresses, and the performance of different regions differs by more than one times. The interaction between different regions produces a synergistic effect, making the overall performance of the material better than that of metal materials with a single structure. Heterostructured metallic materials include gradient structures, heterogeneous layered structures, double-peak structures of grain size, harmonic structures, and multi-scale grain distribution heterogeneous materials.

[0005] Wu et al. prepared heterogeneous layered titanium by asynchronous rolling combined with annealing process. The heterogeneous layered titanium exhibited strength comparable to that of ultrafine grains during tensile process, while maintaining the ductility of coarse grains, achieving very high yield strength and good ductility match.

[0006] Li et al. used hot pressing to prepare a multilayer heterostructure titanium with alternating coarse grains and fine grains, whose yield strength is higher than that of the fine grain layer alone, while its ductility is comparable to that of the coarse grain layer. Although these methods can effectively improve the strength and ductility matching of pure titanium, the processing equipment is large, the investment cost is high, and the equipment maintenance is difficult in the later stage, which is not conducive to industrial large-scale production.

[0007] Therefore, developing a method to obtain high strength and high ductility of pure titanium through rotary forging and annealing treatment has important significance and broad application prospects. Summary of the invention

[0008] The purpose of the present invention is to address the contradiction between the low strength of coarse-grained pure titanium and the low ductility of ultrafine-grained / nanocrystalline pure titanium, and to propose a method for simultaneously improving the strength and ductility of pure titanium. The specific steps are:

[0009] (1) The titanium rod was fully annealed at 700°C for 2 h.

[0010] (2) The surface of the titanium rod treated in step (1) is polished and then subjected to rotary forging.

[0011] (3) The titanium rod treated in step (2) is subjected to vacuum incomplete annealing at 450-525° C. for 15-60 min, and then air-cooled to room temperature.

[0012] Preferably, the components and mass percentages of the pure titanium in step (1) are: iron: 0.30%, carbon: 0.1%, nitrogen: 0.05%, hydrogen: 0.015%, oxygen: 0.2%, the content of other impurities is less than 0.2%, and the rest is titanium.

[0013] Preferably, the rotary forging treatment in step (2) is carried out at room temperature, the number of rotary forging times is 18 to 23 times, the total reduction is 60.5% to 70.5%, and the corresponding true strain is 1.86 to 2.45.

[0014] Preferably, the annealing in step (1) and step (3) is carried out in a tubular vacuum annealing furnace.

[0015] Beneficial effects of the present invention

[0016] (1) The pure titanium prepared by the method of the present invention has a bimodal grain structure, a yield strength between 530 and 660 MPa, a tensile strength between 620 and 785 MPa, and a uniform elongation of ≥8%.

[0017] (2) The present invention requires less equipment, has low investment and simple process, and can be applied to industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the EBSD microstructure image of industrial pure titanium after complete annealing in Comparative Example 1.

[0019] Figure 2 EBSD microstructure image of the bimodal pure titanium prepared in Example 4.

[0020] Figure 3 This is a statistical distribution diagram of the grain size of the bimodal structured pure titanium obtained in Example 4.

[0021] Figure 4 This is the engineering stress-engineering strain curve of coarse-grained pure titanium and bimodal structure pure titanium obtained after rotary forging and incomplete recrystallization annealing. DETAILED DESCRIPTION

[0022] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and through specific implementation methods. However, the following embodiments are only simple examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention shall be subject to the claims.

[0023] Example 1

[0024] In this embodiment, the initial diameter of the titanium rod is 20 mm, and the components and mass percentages are: iron: 0.30%, carbon: 0.1%, nitrogen: 0.05%, hydrogen: 0.015%, oxygen: 0.2%, other impurities content is less than 0.2%, and the rest is titanium.

[0025] A method for simultaneously improving the strength and ductility of pure titanium comprises the following steps:

[0026] (1) The titanium rod was completely annealed in a tubular vacuum annealing furnace at 700°C for 2 h to obtain a homogenized structure with an average grain size of 21 μm.

[0027] (2) The titanium rod treated in step (1) is polished with sandpaper, and then rotary forging is performed on the titanium rod at room temperature. The diameter of the original titanium rod is 20 mm. The rotary forging machine has a fixed diameter mold. Since the required diameter cannot be obtained directly, a gasket needs to be added. The reduction amount for each pass is 0.3 to 1 mm. After rotary forging, the diameter is 7.9 mm and the true strain is 1.86.

[0028] (3) The titanium rod treated in step (2) is subjected to an incomplete recrystallization annealing treatment at 450° C. for 60 minutes in a vacuum annealing furnace to obtain a titanium rod with improved strength and ductility.

[0029] The titanium rod prepared by the above method in this embodiment has a bimodal grain structure, a yield strength of 600MPa, a tensile strength of 750MPa, and a uniform elongation of 9.1%. The engineering stress-engineering strain curves of the coarse-grained pure titanium after complete annealing and the bimodal structure pure titanium obtained by incomplete recrystallization annealing after rotary forging are shown in FIG. Figure 4 As shown, it can be seen that the industrial pure titanium treated by the method of the present invention has a significantly improved yield strength and tensile strength compared with coarse-grained industrial pure titanium, while still retaining good ductility.

[0030] Example 2

[0031] In this embodiment, the initial diameter of the titanium rod is 20 mm, and the components and mass percentages are: iron: 0.30%, carbon: 0.1%, nitrogen: 0.05%, hydrogen: 0.015%, oxygen: 0.2%, other impurities content is less than 0.2%, and the rest is titanium.

[0032] A method for simultaneously improving the strength and ductility of pure titanium comprises the following steps:

[0033] (1) The titanium rod was completely annealed in a tubular vacuum annealing furnace at 700°C for 2 h to obtain a homogenized structure with an average grain size of 21 μm.

[0034] (2) The titanium rod treated in step (1) was polished with sandpaper, and then the titanium rod was rotary forged at room temperature. The diameter of the original titanium rod was 20 mm. The rotary forging machine had a fixed diameter die. Since the required diameter could not be obtained directly, a gasket was required. The reduction amount for each pass was 0.3 to 1 mm. After rotary forging, the diameter was 7.9 mm, and the true strain was 1.86.

[0035] (3) The titanium rod treated in step (2) is subjected to an incomplete recrystallization annealing treatment in a vacuum annealing furnace at 525° C. for 15 minutes to obtain a titanium rod with improved strength and ductility.

[0036] The titanium rod prepared by the above method in this embodiment has a bimodal grain structure, a yield strength of 540 MPa, a tensile strength of 650 MPa, and a uniform elongation of 13.7%. The engineering stress-engineering strain curves of the coarse-grained pure titanium after complete annealing and the bimodal structure pure titanium obtained by incomplete recrystallization annealing after rotary forging are as follows: Figure 4 As shown, it can be seen that the industrial pure titanium treated by the method of the present invention has a significantly improved yield strength and tensile strength compared with coarse-grained industrial pure titanium, while the uniform elongation exceeds that of coarse-grained pure titanium, and has a good strength-ductility match.

[0037] Example 3

[0038] In this embodiment, the initial diameter of the titanium rod is 20 mm, and the components and mass percentages are: iron: 0.30%, carbon: 0.1%, nitrogen: 0.05%, hydrogen: 0.015%, oxygen: 0.2%, other impurities content is less than 0.2%, and the rest is titanium.

[0039] A method for simultaneously improving the strength and ductility of pure titanium comprises the following steps:

[0040] (1) The titanium rod was completely annealed in a tubular vacuum annealing furnace at 700°C for 2 h to obtain a homogenized structure with an average grain size of 21 μm.

[0041] (2) The titanium rod treated in step (1) was polished with sandpaper, and then the titanium rod was rotary forged at room temperature. The diameter of the original titanium rod was 20 mm. The rotary forging machine had a fixed diameter die. Since the required diameter could not be obtained directly, a gasket was required. The reduction amount for each pass was 0.3 to 1 mm. After rotary forging, the diameter was 5.9 mm, and the true strain was 2.45.

[0042] (3) The titanium rod treated in step (2) is subjected to an incomplete recrystallization annealing treatment at 450° C. for 60 minutes in a vacuum annealing furnace to obtain a titanium rod with improved strength and ductility.

[0043] The titanium rod prepared by the above method in this embodiment has a bimodal grain structure, a yield strength of 660MPa, a tensile strength of 785MPa, and a uniform elongation of 8.0%. The engineering stress-engineering strain curves of the coarse-grained pure titanium after complete annealing and the bimodal structure pure titanium obtained by incomplete recrystallization annealing after rotary forging are as follows: Figure 4 As shown, it can be seen that the yield strength and tensile strength of pure titanium treated by the method of the present invention are greatly improved compared with coarse-grained industrial pure titanium, while still retaining good ductility.

[0044] Example 4

[0045] In this embodiment, the initial diameter of the titanium rod is 20 mm, and the components and mass percentages are: iron: 0.30%, carbon: 0.1%, nitrogen: 0.05%, hydrogen: 0.015%, oxygen: 0.2%, other impurities content is less than 0.2%, and the rest is titanium.

[0046] A method for simultaneously improving the strength and ductility of pure titanium comprises the following steps:

[0047] (1) The titanium rod was completely annealed in a tubular vacuum annealing furnace at 700°C for 2 h to obtain a homogenized structure with an average grain size of 21 μm.

[0048] (2) The titanium rod treated in step (1) was polished with sandpaper, and then the titanium rod was rotary forged at room temperature. The diameter of the original titanium rod was 20 mm. The rotary forging machine had a fixed diameter die. Since the required diameter could not be obtained directly, a gasket was required. The reduction amount for each pass was 0.3 to 1 mm. After rotary forging, the diameter was 5.9 mm, and the true strain was 2.45.

[0049] (3) The titanium rod treated in step (2) is subjected to an incomplete recrystallization annealing treatment in a vacuum annealing furnace at 525° C. for 15 minutes to obtain a titanium rod with improved strength and ductility.

[0050] The titanium rod prepared by the above method in this example has a bimodal grain structure, and its yield strength is 530MPa, tensile strength is 620MPa, and uniform elongation is 15.9%. The EBSD morphology and grain size statistics of bimodal pure titanium are as follows: Figure 2 and Figure 3 As shown, it can be seen that the bimodal structure pure titanium contains ~41% UFG and ~59% FG, with an average grain size of 1.2μm. The engineering stress-engineering strain curves of the coarse-grained pure titanium after complete annealing and the bimodal structure pure titanium obtained by incomplete recrystallization annealing after rotary forging are shown in Figure 4 As shown, it can be seen that the industrial pure titanium treated by the method of the present invention has a significantly improved yield strength and tensile strength compared with coarse-grained industrial pure titanium, while the uniform elongation exceeds that of coarse-grained pure titanium, and has a good strength-ductility match.

[0051] Comparative Example 1

[0052] For comparison, this comparative example only performs the complete annealing treatment of step (1) to obtain a coarse-crystalline titanium rod with a yield strength of 310 MPa, a tensile strength of 445 MPa, and a uniform elongation of 12.6%. The EBSD microstructure of the coarse-crystalline state is shown in FIG. Figure 1 As shown, all the grains are equiaxed, without twins, and the average grain size is 21 μm

[0053] The engineering stress-engineering strain curves of the titanium rods prepared in Examples 1 to 4 and Comparative Example 1 are as follows: Figure 4 As shown, it can be seen that the bimodal structure pure titanium obtained by the method of the present invention has greatly improved strength compared with coarse-grained pure titanium. The ductility of Example 2 and Example 4 even exceeds that of coarse-grained pure titanium, and has a good strength-ductility match, which can greatly promote industrial applications.

Claims

1. A method for simultaneously improving the strength and ductility of industrial pure titanium, characterized in that: The specific steps include: (1) The titanium rod was fully annealed at 700°C for 2 h; (2) grinding the surface of the titanium rod treated in step (1) and then performing rotary forging; (3) The titanium rod treated in step (2) is subjected to vacuum incomplete annealing at 450-525° C. for 15-60 min, and then air-cooled to room temperature.

2. The method for simultaneously improving the strength and ductility of industrial pure titanium according to claim 1, characterized in that: The components and mass percentages of the pure titanium in step (1) are: iron: 0.30%, carbon: 0.1%, nitrogen: 0.05%, hydrogen: 0.015%, oxygen: 0.2%, other impurities content is less than 0.2%, and the rest is titanium.

3. The method for simultaneously improving the strength and ductility of industrial pure titanium according to claim 1, characterized in that: The rotary forging treatment in step (2) is carried out at room temperature, the number of rotary forging times is 18 to 23 times, the total reduction is 60.5% to 70.5%, and the corresponding true strain is 1.86 to 2.

45.

4. The method for simultaneously improving the strength and ductility of industrial pure titanium according to claim 1, characterized in that: The annealing in step (1) and step (3) adopts a tubular vacuum annealing furnace.

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