Preparation method of BT14 titanium alloy large-size bar

Abstract

The invention discloses a preparation method of a BT14 titanium alloy large-specification bar, and aims to solve the problems of non-uniform structure and mismatching of mechanical property strength and toughness caused by temperature and strain difference of each part of a blank in the existing bar forging process. According to the titanium alloy structure evolution rule and the deformation distribution characteristic in the blank forging process, the optimal forging technology is determined, and the deformation uniformity of the titanium alloy is improved by adopting a deformation mode combining upsetting, conventional drawing, reversing drawing, diagonal drawing and inverted octagonal drawing. And the alloy structure and mechanical properties meeting the requirements are obtained through optimized forging process parameters, and the quality of large-specification bars is guaranteed. According to the BT14 titanium alloy large-size bar prepared through the method, the grain size in a microscopic structure is effectively reduced, meanwhile, a macrostructure is uniform, and the mechanical property meets the technical requirements of ocean engineering materials; in addition, the preparation heating number is small, energy consumption in the process is low, and the method is suitable for industrial production of BT14 titanium alloy bars with the diameter phi ranging from 400 mm to 500 mm.

Description

Technical Field

[0001] The invention belongs to the technical field of titanium alloy material processing, and in particular relates to a method for preparing a large-size BT14 titanium alloy bar. Background Art

[0002] Titanium alloys have the characteristics of high strength, low density, excellent seawater corrosion resistance, and good low-temperature performance and fatigue resistance. They can meet the stringent service requirements of ships and deep-sea equipment, and show broad application prospects in the field of marine engineering. With the development of marine resources and the advancement of marine engineering technology, ship structures, deep-sea equipment, seawater desalination equipment, etc. are gradually developing towards large-scale. Therefore, large-size titanium alloy bars have become a key force in promoting the upgrading of marine engineering equipment.

[0003] The nominal composition of BT14 titanium alloy is Ti-4.5Al-3Mo-1V. It is a heat-treatable and strengthened α+β titanium alloy developed in the late 1950s. The titanium alloy with similar composition in the United States is Ti-431 (nominal composition Ti-4Al-3Mo-1V), and the corresponding domestic grade is TC30 (nominal composition Ti-5Al-3Mo-1V). The Al equivalent of the alloy under the nominal composition is 5.36, the Mo equivalent is 3.92, the β stability coefficient (Kβ) is 0.36, and the β transformation temperature (Tβ) is 940-970℃. In this alloy, a large amount of hexagonal close-packed (hcp) α phase is the guarantee of good low temperature performance, high temperature performance and weldability; while a certain amount of body-centered cubic (bcc) β phase is the guarantee of good process plasticity and heat treatability. The strength of the alloy in the annealed state can reach 850MPa, the -60℃ impact toughness (KV2) is higher than 45J, and the strength can reach 1100MPa after strengthening heat treatment. Therefore, BT14 titanium alloy can be used for bar, plate and pipe processing, and for manufacturing structural parts, welded parts and pressure tanks working in the range of -60 to 500 ° C. In addition, the alloy also has a high resistance to seawater stress corrosion crack growth and heat salt stress corrosion, and is suitable for manufacturing components serving in marine environments. However, since the flow stress of BT14 titanium alloy is very sensitive to deformation temperature and deformation rate, and there are differences in the dynamic recrystallization ability of α phase and β phase in the alloy in the two-phase region, this poses a great challenge to the uniformity of the organization and performance control of large-size BT14 titanium alloy bars.

[0004] In view of this, it is necessary to optimize the forging process of traditional titanium alloy bars, improve the uniformity of the core and surface structure of the billet by controlling the heating temperature, holding time, deformation rate, deformation amount and deformation method, and use the dynamic and static recrystallization processes to achieve the refinement of the billet structure, thereby achieving large-scale and stable production of large-size BT14 titanium alloy bars to meet the application needs of marine engineering and other fields. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing large-size BT14 titanium alloy bars, which can solve the problems of uneven structure and mismatch between strength and toughness of mechanical properties caused by temperature and strain differences in various parts of the billet in the existing bar forging process.

[0006] In order to achieve the above object, the present invention adopts the following technical solution.

[0007] On the one hand, the present invention provides a method for preparing a large-size BT14 titanium alloy bar, comprising the following steps:

[0008] S1. Billet forging:

[0009] The BT14 titanium alloy ingot is heated and kept warm, and then cooled after 1 to 3 forgings. During the forging process, conventional drawing, reversing drawing and diagonal drawing processes are used to obtain a forged billet with a β structure having a grain size of 5 mm to 10 mm.

[0010] S2. Homogenization forging:

[0011] The forging blank obtained in step S1 is heated and kept warm, and is forged for 1 to 3 times at temperatures below and above the phase transition point, respectively, and then cooled to obtain a forging blank with uniform structure;

[0012] S3, intermediate forging:

[0013] The forging blank obtained in step S2 is heated and kept warm, and then cooled after 5 to 8 forgings. During the forging process, conventional drawing, reversing drawing and eight-way inversion processes are used to obtain a forging blank having an α+β two-phase structure with a grain size of 1 mm to 2 mm;

[0014] S4, Forming forging:

[0015] The forging blank obtained in step S3 is heated and kept warm, subjected to 1-2 rounding forgings, and then cooled to obtain a final formed bar blank.

[0016] S5, bar machining:

[0017] The formed bar blank obtained in step S4 is subjected to sawing, surface turning and polishing to obtain a final finished bar.

[0018] Furthermore, the step S1 specifically includes:

[0019] For BT14 titanium alloy ingots, 1 to 3 fire forgings are carried out at 100℃~200℃ above the phase transformation point, and 1 to 2 upsetting and drawing are completed in each fire forging to obtain a β structure with a grain size of 5mm~10mm; the cumulative total forging ratio is 6.2~17.4, and the forging methods are upsetting, conventional drawing, reversing drawing, diagonal drawing and inverted eight-sided drawing. After forging air cooling, the surface cracks of the material are polished; the starting forging temperature of each fire is 50℃~150℃ above the phase transformation point, and the final forging temperature is 50℃~100℃ below the phase transformation point.

[0020] Furthermore, the step S2 specifically includes:

[0021] The forging blank obtained in step S1 is forged 1 to 2 times at 20°C to 70°C below the phase transformation point, the forging methods are upsetting, conventional drawing and octagonal forging, the cumulative forging ratio of each time is 2.8 to 5.6, the initial forging temperature of each time is 20°C to 70°C below the phase transformation point, the final forging temperature is 100°C to 200°C below the phase transformation point, the cooling method after forging is air cooling, and the surface cracks of the blank are polished after forging air cooling; then, one forging is performed at 20°C to 50°C above the phase transformation point, the forging methods are upsetting, conventional drawing and octagonal forging, the cumulative forging ratio is 2.8 to 5.6, the initial forging temperature is 20°C to 50°C above the phase transformation point, the final forging temperature is 50°C to 100°C below the phase transformation point, the cooling method after forging is air cooling, and the surface cracks of the blank are polished after forging air cooling.

[0022] Furthermore, the step S3 specifically includes:

[0023] The forging blank obtained in step S2 is subjected to 2-3 fire forgings at 20°C-70°C below the phase transformation point, the forging methods are upsetting, conventional drawing and inverting, the cumulative forging ratio of each fire is 2.8-5.6, the initial forging temperature of each fire is 20°C-70°C below the phase transformation point, the final forging temperature is 100°C-200°C below the phase transformation point, the cooling method after forging is air cooling, and the surface cracks of the blank are polished after forging air cooling;

[0024] Then the forged billet is reheated at 20℃~70℃ below the phase transformation point, and the holding time is 6~8h, and then 2~3 times of forging are performed, and the forging method is upsetting, conventional drawing and inverting, and the cumulative forging ratio of each time is 2.8~5.6, and the initial forging temperature of each time is 20℃~70℃ below the phase transformation point, and the final forging temperature is 100℃~200℃ below the phase transformation point. The cooling method after forging is air cooling, and the surface cracks of the billet are polished after forging air cooling;

[0025] The billet that has completed the above forging is then reheated at 20℃~70℃ below the phase transformation point, the holding time is 6~8h, and then 1~2 rounds of forging are carried out. The forging methods are upsetting, conventional drawing and inverting. The cumulative forging ratio of each round is 2.8~5.6. The starting forging temperature of each round is 20℃~70℃ below the phase transformation point, and the final forging temperature is 100℃~200℃ below the phase transformation point. The cooling method after forging is air cooling. After forging air cooling, the surface cracks of the billet are polished.

[0026] Furthermore, the step S4 specifically includes:

[0027] The forging blank obtained in step S3 is heated to 20°C to 70°C below the phase transformation point, the holding time is 6 to 8 hours, and 1 to 2 fire forgings are performed, the forging ratio is 1.1 to 2.3, and the forging method is drawing and rounding; the starting forging temperature of each fire is 20°C to 70°C below the phase transformation point, the final forging temperature is 100°C to 200°C below the phase transformation point, and the cooling method after forging is air cooling.

[0028] On the other hand, the present invention also provides a BT14 titanium alloy large-size bar, characterized in that it is prepared by the above-mentioned BT14 titanium alloy large-size bar preparation method.

[0029] Furthermore, the diameter of the large-size BT14 titanium alloy bar is Φ400mm to Φ500mm.

[0030] Furthermore, the length of the large-size BT14 titanium alloy bar is not less than 1500 mm.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] In the ingot forging stage, large deformation upsetting deformation is used to refine the coarse cast grains in the original ingot, and conventional drawing, reversing drawing and diagonal drawing processes are used to obtain a β structure with a grain size of 5mm to 10mm; after the forging, homogenization forging is carried out, and the billet structure is homogenized by using two-stage upsetting deformation above and below the phase transformation point. Subsequently, two-phase zone multi-fire upsetting deformation is used, and conventional drawing, reversing drawing and inverted octagonal processes are used to further improve the uniformity of the billet structure, and an α+β two-phase structure with a grain size of 1mm to 2mm is obtained; in the forming forging stage, the rounding forging method and a smaller forging ratio are used, which can not only ensure uniform deformation in all directions during the forging process, but also realize precise control of the forging size.

[0033] The large-size BT14 titanium alloy rods prepared by the method of the present invention effectively reduce the grain size in the microstructure, the low-magnification structure of the finished rods is uniform, and the mechanical properties meet the technical requirements of marine engineering materials; at the same time, the preparation has fewer firings and less energy consumption in the process, which can meet the requirements of large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a low-magnification microstructure diagram of the end of a Φ460mm BT14 titanium alloy rod prepared in an embodiment of the present invention in R state;

[0035] Figure 2 This is the R-state microstructure at the end edge of the Φ460mm BT14 titanium alloy bar prepared in the embodiment of the present invention;

[0036] Figure 3 The R-state microstructure at the R / 2 position of the end of the Φ460mm BT14 titanium alloy bar prepared in the embodiment of the present invention;

[0037] Figure 4 This is the R-state microstructure at the center of the end of the Φ460mm BT14 titanium alloy rod prepared in an embodiment of the present invention. DETAILED DESCRIPTION

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used in the specification of the present invention are only used for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0039] In the present invention, when it comes to a numerical interval (i.e., a numerical range), unless otherwise specified, the optional numerical distribution is considered continuous within the above numerical interval, and includes the two numerical endpoints (i.e., the minimum value and the maximum value) of the numerical range, and each numerical value between the two numerical endpoints. Unless otherwise specified, when the numerical interval only refers to an integer within the numerical interval, it includes the two endpoint integers of the numerical range, and each integer between the two endpoints. In this article, it is equivalent to directly listing each integer, such as t is an integer selected from 1-10, indicating that t is any integer selected from the integer group consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10. In addition, when multiple ranges are provided to describe features or characteristics, these ranges can be combined. In other words, unless otherwise specified, the ranges disclosed herein should be understood to include any and all sub-ranges included therein.

[0040] The temperature parameters in the present invention, if not specifically limited, are allowed to be either constant temperature treatment or to vary within a certain temperature range. It should be understood that the constant temperature treatment allows the temperature to fluctuate within the precision range controlled by the instrument. Fluctuations within the range of ±5°C, ±4°C, ±3°C, ±2°C, and ±1°C are allowed.

[0041] The present invention provides a method for preparing a large-size BT14 titanium alloy bar, comprising the following steps:

[0042] S1. Billet forging:

[0043] The BT14 titanium alloy ingot is heated and kept warm, and then cooled after 1 to 3 forgings. During the forging process, conventional drawing, reversing drawing and diagonal drawing processes are used to obtain a forged billet with a β structure having a grain size of 5 mm to 10 mm.

[0044] S2. Homogenization forging:

[0045] The forging blank obtained in step S1 is heated and kept warm, and forged for 1 to 3 times at temperatures below and above the phase transition point, respectively, and then cooled to obtain a forging blank with uniform structure;

[0046] S3, intermediate forging:

[0047] The forging blank obtained in step S2 is heated and kept warm, and then cooled after 5 to 8 forgings. During the forging process, conventional drawing, reversing drawing and eight-way inversion processes are used to obtain a forging blank having an α+β two-phase structure with a grain size of 1 mm to 2 mm;

[0048] S4, Forming forging:

[0049] The forging blank obtained in step S3 is heated and kept warm, subjected to 1-2 rounding forgings, and then cooled to obtain a final formed bar blank;

[0050] S5. Bar machining:

[0051] The formed rod blank obtained in step S4 is subjected to sawing, surface turning and polishing treatment to obtain a large-size BT14 titanium alloy rod.

[0052] In some embodiments, the present invention provides a method for preparing a large-size BT14 titanium alloy bar, comprising the following steps:

[0053] S1. Open forging: heating and heat preservation of the BT14 titanium alloy ingot, cooling after 1 to 3 rounds of forging, and using conventional drawing, reversing drawing and diagonal drawing processes during the forging process to obtain a forging blank with a β structure having a grain size of 5 mm to 10 mm;

[0054] The specific process is: for BT14 titanium alloy ingots, 1 to 3 fire forgings are carried out at 100℃~200℃ above the phase transformation point, and 1 to 2 upsetting and drawing are completed in each fire forging to obtain a β structure with a grain size of 5mm~10mm; the cumulative total forging ratio is 6.2~17.4, and the forging methods are upsetting, conventional drawing, reversing drawing, diagonal drawing and inverted eight-sided drawing. After forging air cooling, the surface cracks of the material are polished; the starting forging temperature of each fire is 50℃~150℃ above the phase transformation point, and the final forging temperature is 50℃~100℃ below the phase transformation point.

[0055] In step S1, using large deformation upsetting can refine the coarse grains in the original ingot; using a variety of stretching and inverting operations can improve the deformation uniformity at each position.

[0056] S2, homogenization forging: heating and keeping the forging blank obtained in step S1, forging for 1 to 3 times at temperatures below and above the phase transition point, respectively, and then cooling to obtain a forging blank with uniform structure;

[0057] The specific process is: in order to obtain a billet with uniform structure, the forging billet that has completed the open forging is forged 1 to 2 times at 20℃~70℃ below the phase transformation point, the forging methods are upsetting, conventional drawing and inverting, the cumulative forging ratio of each fire is 2.8~5.6, the initial forging temperature of each fire is 20℃~70℃ below the phase transformation point, the final forging temperature is 100℃~200℃ below the phase transformation point, the cooling method after forging is air cooling, and the surface cracks of the billet are polished after air cooling; then, it is forged 1 time at 20℃~50℃ above the phase transformation point, the forging methods are upsetting, conventional drawing and inverting, the cumulative forging ratio is 2.8~5.6, the initial forging temperature is 20℃~50℃ above the phase transformation point, the final forging temperature is 50℃~100℃ below the phase transformation point, the cooling method after forging is air cooling, and the surface cracks of the billet are polished after air cooling.

[0058] S3, intermediate forging: heating and heat preservation of the forging blank obtained in step S2, cooling after 5 to 8 forgings, and using conventional drawing, reversing drawing and eight-way inversion processes in the forging process to obtain a forging blank with uniform and fine α+β two-phase structure;

[0059] The specific process is: in order to obtain a forging billet with fine and uniform α+β two-phase structure, the forging billet that has completed homogenization forging is forged 2 to 3 times at 20℃ to 70℃ below the phase transformation point, the forging method is upsetting, conventional drawing and inverting, the cumulative forging ratio of each fire is 2.8 to 5.6, the initial forging temperature of each fire is 20℃ to 70℃ below the phase transformation point, the final forging temperature is 100℃ to 200℃ below the phase transformation point, the cooling method after forging is air cooling, and the surface cracks of the billet are polished after forging air cooling;

[0060] Then the forged billet is reheated at 20℃~70℃ below the phase transformation point, and the holding time is 6~8h, and then 2~3 times of forging are performed, and the forging method is upsetting, conventional drawing and inverting, and the cumulative forging ratio of each time is 2.8~5.6, and the initial forging temperature of each time is 20℃~70℃ below the phase transformation point, and the final forging temperature is 100℃~200℃ below the phase transformation point. The cooling method after forging is air cooling, and the surface cracks of the billet are polished after forging air cooling;

[0061] The blank that has completed the above forging is then reheated at 20℃~70℃ below the phase transformation point, the holding time is 6~8h, and then 1~2 rounds of forging are performed, the forging methods are upsetting, conventional drawing and inverting, the cumulative forging ratio of each round is 2.8~5.6, the initial forging temperature of each round is 20℃~70℃ below the phase transformation point, the final forging temperature is 100℃~200℃ below the phase transformation point, the cooling method after forging is air cooling, and the surface cracks of the blank are polished after forging air cooling;

[0062] S4, forming forging: the forging blank after the above forging is heated and kept warm, and after 1 to 2 rounds of forging, it is cooled to obtain a formed bar blank.

[0063] The specific process is: the forging blank obtained in step S3 is heated to 20°C to 70°C below the phase transformation point, the heat preservation time is 6 to 8 hours, and 1 to 2 fire forgings are performed, the forging ratio is 1.1 to 2.3, and the forging method is drawing and rolling. The initial forging temperature of each fire is 20°C to 70°C below the phase transformation point, the final forging temperature is 100°C to 200°C below the phase transformation point, and the cooling method after forging is air cooling.

[0064] S5, bar machining:

[0065] The formed rod blank obtained in step S4 is subjected to sawing, surface turning and polishing to obtain a BT14 titanium alloy rod.

[0066] In some embodiments, the raw material specification of the processing object of the preparation method of the present invention is a 2500kg-5000kg grade BT14 titanium alloy ingot with a size of Φ720mm. The unit weight of the ingot after sawing is 1200kg-3000kg. The processing process mainly includes blanking forging → homogenization forging → intermediate forging → forming forging → bar machining. The final finished bar has a diameter of Φ400mm-Φ500mm and a length of not less than 1500mm, which fully meets the technical requirements of BT14 titanium alloy bars for marine engineering.

[0067] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below in conjunction with specific embodiments and drawings of the specification. The specific embodiments described here are only used to explain the present invention, and the present invention is not limited thereto.

[0068] Example 1: Preparation of Φ460mm BT14 titanium alloy large-size bar

[0069] This embodiment provides a method for preparing a large-size BT14 titanium alloy bar of Φ460 mm, the method comprising the following steps:

[0070] S1. Billet forging:

[0071] The BT14 titanium alloy ingot used has a specification of Φ720mm, a weight of 2700kg, and a phase transition point of 950℃.

[0072] The BT14 titanium alloy ingot with a unit weight of 1400kg after sawing is forged in three fires, with two upsetting and two drawing in each fire, and the cumulative forging ratio of each fire is controlled at 4.6-6.5. The forging methods are upsetting, conventional drawing, reversing drawing, and diagonal drawing. The cooling method after forging is air cooling, and the surface cracks of the material are polished after air cooling. The initial forging temperature of each fire is 1000-1200℃, and the final forging temperature is not less than 850℃ to obtain a forged billet.

[0073] S2. Homogenization forging:

[0074] The forging billet that has completed the open forging is forged twice at 950°C, the forging methods are upsetting, conventional drawing and inverting, the cumulative forging ratio is 6.5, the initial forging temperature is 900-920°C, the final forging temperature is not less than 800°C, the cooling method after forging is air cooling, and the surface cracks of the billet are polished after forging air cooling; then it is forged once at 990°C, the forging methods are upsetting, conventional drawing and inverting, the cumulative forging ratio is 3.2, the initial forging temperature is 970°C-1000°C, the final forging temperature is not less than 850°C, the cooling method after forging is air cooling, and the surface cracks of the billet are polished after forging air cooling to obtain the forging billet.

[0075] S3, intermediate forging:

[0076] The forging billet that has completed homogenization forging is forged at 950℃ for 3 times. The forging methods are upsetting, conventional drawing and inverting. The cumulative forging ratio of each time is 6.5. The initial forging temperature is 900-920℃, the final forging temperature is not less than 800℃, and the cooling method after forging is air cooling. After forging air cooling, the surface cracks of the billet are polished.

[0077] The bar billet is reheated to 950℃, kept at temperature for 6.5h, and forged for 3 times. The forging methods are upsetting, conventional drawing and inverting. The cumulative forging ratio of each time is 6.5. The initial forging temperature is 900-920℃, the final forging temperature is not less than 800℃, and the cooling method after forging is air cooling. After forging air cooling, the surface cracks of the billet are polished.

[0078] The bar billet is reheated to 950℃, the holding time is 6.5h, and then two rounds of forging are carried out. The forging methods are upsetting, reversing drawing and inverting. The cumulative forging ratio of each round is 6.5, the initial forging temperature is 900-920℃, the final forging temperature is not less than 800℃, and the cooling method after forging is air cooling. After forging air cooling, the surface cracks of the billet are polished to obtain the forged billet.

[0079] S4, Forming forging:

[0080] The forging billet after intermediate forging is heated to 950℃, the holding time is 6.5h, and two fire forgings are performed, the cumulative forging ratio is 3.3, and the forging method is drawing and rolling. The initial forging temperature of forming forging is 900-920℃, and the final forging temperature is not less than 800℃.

[0081] S5. Bar machining:

[0082] The forged rod blank is sawn, surface turned and polished to finally obtain a Φ460mm BT14 titanium alloy rod.

[0083] Figure 1 This is a low-magnification microstructure diagram of the R state at the end of the Φ460mm BT14 titanium alloy rod prepared in this embodiment; Figure 2 This is the R-state microstructure diagram of the end edge of the Φ460mm BT14 titanium alloy rod prepared in this embodiment; Figure 3 This is the R-state microstructure diagram of the R / 2 position at the end of the Φ460mm BT14 titanium alloy rod prepared in this embodiment; Figure 4 The R-state microstructure diagram of the center position of the end of the Φ460mm BT14 titanium alloy rod prepared in this embodiment; wherein R / 2 refers to the middle position of the rod in the radial direction. Figure 1 to Figure 4 It can be seen that the Φ460mm BT14 titanium alloy large-size bar prepared by this embodiment has good uniformity of structure. Table 1 shows the room temperature mechanical properties of the Φ460mm BT14 titanium alloy large-size bar prepared by this embodiment. It can be seen from Table 1 that the mechanical properties of the prepared Φ460mm BT14 titanium alloy bar meet the technical requirements and have a large margin, and the strength and toughness are well matched.

[0084] Table 1. Mechanical properties of Φ460mm BT14 titanium alloy bar at room temperature in R state

[0085]

[0086] The working principle of the method of the present invention is as follows: the optimal forging process is determined according to the evolution law of titanium alloy structure and the deformation distribution characteristics of the billet forging process, and the uniformity of titanium alloy deformation is improved by combining upsetting, conventional drawing, reversing drawing, diagonal drawing and inverted octagonal drawing. The alloy structure and mechanical properties that meet the requirements are obtained by optimizing the forging process parameters, thereby ensuring the quality of large-size bars. The preparation method of the invention comprises blank forging→homogenizing forging→intermediate forging→forming forging→rod machining: in the blank forging stage of the ingot, a large deformation is adopted to upset and refine the original coarse cast grains, and conventional drawing, reversing drawing and diagonal drawing processes are used to obtain a β structure with a grain size of 5 mm to 10 mm; after the blank forging, a homogenizing forging stage is added, and the blank structure is homogenized by using two-stage upsetting and drawing deformation above and below the phase transformation point, and then two-phase region multi-fire upsetting and drawing deformation is adopted, and conventional drawing, reversing drawing and inverting processes are used to further improve the uniformity of the rod blank structure, and an α+β two-phase structure with a grain size of 1 mm to 2 mm is obtained; in the forming forging stage, a rolling forging method and a relatively small forging ratio are used to ensure uniform deformation of the blank in all directions during the forging process, and precise control of the forging size is achieved.

[0087] The large-size BT14 titanium alloy rods prepared by the method of the present invention effectively reduce the grain size in the microstructure, and at the same time the macrostructure is uniform, and the mechanical properties meet the technical requirements of marine engineering materials; in addition, the preparation process requires fewer firings and consumes less energy, and is suitable for the industrial production of BT14 titanium alloy rods with a diameter of Φ400mm to Φ500mm and a length of not less than 1500mm.

[0088] It should be noted that the above-described embodiments are only preferred embodiments of the present invention. For those skilled in the art, without departing from the principles of the present invention, the present invention may be modified, improved and replaced with equivalents, and these modifications, improvements and equivalent replacements are also considered to fall within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a large-size BT14 titanium alloy bar, comprising the following steps: S1. Billet forging: The BT14 titanium alloy ingot is heated and kept warm, and then cooled after 1 to 3 forgings. During the forging process, conventional drawing, reversing drawing and diagonal drawing processes are used to obtain a forged billet with a β structure having a grain size of 5 mm to 10 mm. S2. Homogenization forging: The forging blank obtained in step S1 is heated and kept warm, and forged for 1 to 3 times at temperatures below and above the phase transition point, respectively, and then cooled to obtain a forging blank with uniform structure; S3, intermediate forging: The forging blank obtained in step S2 is heated and kept warm, and then cooled after 5 to 8 forgings. During the forging process, conventional drawing, reversing drawing and eight-way inversion processes are used to obtain a forging blank having an α+β two-phase structure with a grain size of 1 mm to 2 mm; S4, Forming forging: The forging blank obtained in step S3 is heated and kept warm, subjected to 1-2 rounding forgings, and then cooled to obtain a formed bar blank; S5, bar machining: The formed rod blank obtained in step S4 is subjected to sawing, surface turning and polishing treatment to obtain a large-size BT14 titanium alloy rod.

2. The method for preparing the BT14 titanium alloy large-size bar according to claim 1, characterized in that: The step S1 specifically includes: For BT14 titanium alloy ingots, 1 to 3 fire forgings are carried out at 100℃~200℃ above the phase transformation point, and 1 to 2 upsetting and drawing are completed in each fire forging to obtain a β structure with a grain size of 5mm~10mm; the cumulative total forging ratio is 6.2~17.4, and the forging methods are upsetting, conventional drawing, reversing drawing, diagonal drawing and inverted eight-sided drawing. After forging air cooling, the surface cracks of the material are polished; the starting forging temperature of each fire is 50℃~150℃ above the phase transformation point, and the final forging temperature is 50℃~100℃ below the phase transformation point.

3. The method for preparing the BT14 titanium alloy large-size bar according to claim 1, characterized in that: The step S2 specifically includes: The forging blank obtained in step S1 is forged 1 to 2 times at 20°C to 70°C below the phase transformation point, the forging methods are upsetting, conventional drawing and octagonal forging, the cumulative forging ratio of each time is 2.8 to 5.6, the initial forging temperature of each time is 20°C to 70°C below the phase transformation point, the final forging temperature is 100°C to 200°C below the phase transformation point, the cooling method after forging is air cooling, and the surface cracks of the blank are polished after forging air cooling; then, one forging is performed at 20°C to 50°C above the phase transformation point, the forging methods are upsetting, conventional drawing and octagonal forging, the cumulative forging ratio is 2.8 to 5.6, the initial forging temperature is 20°C to 50°C above the phase transformation point, the final forging temperature is 50°C to 100°C below the phase transformation point, the cooling method after forging is air cooling, and the surface cracks of the blank are polished after forging air cooling.

4. The method for preparing the BT14 titanium alloy large-size bar according to claim 1, characterized in that: The step S3 specifically includes: The forging blank obtained in step S2 is subjected to 2-3 fire forgings at 20°C-70°C below the phase transformation point, the forging methods are upsetting, conventional drawing and inverting, the cumulative forging ratio of each fire is 2.8-5.6, the initial forging temperature of each fire is 20°C-70°C below the phase transformation point, the final forging temperature is 100°C-200°C below the phase transformation point, the cooling method after forging is air cooling, and the surface cracks of the blank are polished after forging air cooling; Then the forged billet is reheated at 20℃~70℃ below the phase transformation point, and the holding time is 6~8h, and then 2~3 times of forging are performed, and the forging method is upsetting, conventional drawing and inverting, and the cumulative forging ratio of each time is 2.8~5.6, and the initial forging temperature of each time is 20℃~70℃ below the phase transformation point, and the final forging temperature is 100℃~200℃ below the phase transformation point. The cooling method after forging is air cooling, and the surface cracks of the billet are polished after forging air cooling; The billet that has completed the above forging is then reheated at 20℃~70℃ below the phase transformation point, the holding time is 6~8h, and then 1~2 rounds of forging are carried out. The forging methods are upsetting, conventional drawing and inverting. The cumulative forging ratio of each round is 2.8~5.

6. The starting forging temperature of each round is 20℃~70℃ below the phase transformation point, and the final forging temperature is 100℃~200℃ below the phase transformation point. The cooling method after forging is air cooling. After forging air cooling, the surface cracks of the billet are polished.

5. The method for preparing the BT14 titanium alloy large-size bar according to claim 1, characterized in that: The step S4 specifically includes: The forging blank obtained in step S3 is heated to 20°C to 70°C below the phase transformation point, the holding time is 6 to 8 hours, and 1 to 2 fire forgings are performed, the forging ratio is 1.1 to 2.3, and the forging method is drawing and rounding; the starting forging temperature of each fire is 20°C to 70°C below the phase transformation point, the final forging temperature is 100°C to 200°C below the phase transformation point, and the cooling method after forging is air cooling.

6. A BT14 titanium alloy large-size bar, characterized in that: The method is prepared according to claim 1.

7. The BT14 titanium alloy large-size bar according to claim 6, characterized in that: The diameter of the large-size BT14 titanium alloy bar is Φ400mm-Φ500mm.

8. The BT14 titanium alloy large-size bar according to claim 6 or 7, characterized in that: The length of the BT14 titanium alloy large-size bar is not less than 1500 mm.

Images