High-oxygen pure-titanium large-specification fine-grain pipe and preparation method thereof

Abstract

The invention discloses a high-oxygen pure-titanium large-size fine-grain pipe and a preparation method thereof, and the preparation method comprises the following steps: cast ingot smelting: selecting sponge titanium, pressing the sponge titanium into an electrode, smelting the electrode, and then carrying out post-treatment to obtain a high-oxygen pure-titanium cast ingot; the high-oxygen pure titanium cast ingot is heated to the set temperature above the beta phase transformation point, heat preservation is conducted, and then high-temperature reversing cogging forging is conducted; low-temperature forging: heating the cogged blank to be below a beta phase transformation point, carrying out low-temperature reversing upsetting-drawing and drawing-out procedures after heat preservation, and machining to obtain a smooth-surface bar blank; a through hole is machined in the center of the smooth-surface bar blank, then the smooth-surface bar blank is heated to be below the beta phase transformation point, cross piercing is conducted after heat preservation, and a pierced billet is obtained; low-temperature rolling is conducted, specifically, the pierced billet is heated to be below the beta phase transformation point and subjected to periodic rolling after heat preservation, and a tube blank is obtained; and a finished pipe is prepared, specifically, the pipe blank is sequentially subjected to stress relief annealing and machining, and the high-oxygen pure-titanium large-specification fine-grain pipe is prepared. According to the method, the grain size can reach 7-8 grades.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy pipe technology, specifically relating to a high-oxygen pure titanium large-size fine-grained pipe and its preparation method, which is particularly suitable for applications in the fields of petrochemicals, marine engineering equipment and shipbuilding. Background Technology

[0002] Titanium, with its low density, high specific strength, excellent corrosion resistance, and fatigue resistance, has become an ideal material for advanced lightweight structures and is widely used in petrochemical, marine engineering equipment, and shipbuilding industries. Among these, pure titanium tubing, especially large-diameter seamless tubing, is a key development area due to its excellent structural performance and good adaptability to various working conditions, making it highly compatible with the development of petrochemical, marine engineering, and shipbuilding industries and possessing broad application prospects. Currently, the production of most pure titanium seamless tubing is still mainly focused on medium and small-sized tubing with an outer diameter of less than Φ400mm. For example, patent CN101708511B discloses a production method for manufacturing pure titanium seamless tubing with an outer diameter of Φ10~89mm, and patents CN104889163B and CN107234145B produce pure titanium seamless tubing with an outer diameter not exceeding 114mm.

[0003] With the development of related fields, the demand for large-size seamless pure titanium tubes with an outer diameter of Φ400mm or more is increasing. However, current research on large-size seamless pure titanium tubes with an outer diameter of Φ400mm or more mainly focuses on tube forming technology, with less attention paid to the microstructure control technology. The complexity of the application environment also places higher demands on the comprehensive mechanical properties of seamless pure titanium tubes. Large-size seamless pure titanium tubes prepared by existing methods have coarse grains and low strength, resulting in low comprehensive mechanical properties. For example, large-size seamless pure titanium tubes are often obtained by skew rolling piercing billet followed by rolling. However, in current processing methods using skew rolling piercing billet followed by rolling, the heating regime is often selected to be higher or slightly lower than the β phase transformation temperature. For example, patent CN111167862A uses heating temperatures of 900℃ (higher than the β phase transformation temperature) and 850℃ (about 30~50℃ lower than the β phase transformation temperature) to pierce and thermally expand pure titanium billets. High heating temperatures and deformation temperature rises easily lead to grain growth, severely reducing the overall mechanical properties of seamless pure titanium tubes. For example, patent CN110170543A uses a piercing temperature of 890~990℃ to prepare seamless pure titanium tubes. Although these tubes have an elongation of ≥45%, the grains are coarse, with an average size ≥80μm, and the strength is low, with a tensile strength ≥320MPa and a yield strength ≥235MPa. Furthermore, existing manufacturing methods also have high production costs.

[0004] Therefore, there is an urgent need to develop a method for preparing large-size seamless pure titanium tubes with uniform and fine microstructure, combining strength and plasticity, to ensure their reliability and stability in service environments. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and propose a high-oxygen pure titanium large-size fine-grained tube and its preparation method. Improving the comprehensive mechanical properties of pure titanium tubes can be achieved through two methods: interstitial solid solution strengthening and fine-grain strengthening. Therefore, in the composition design process, this invention increases the oxygen content in the ingot by expanding the range of sponge titanium selection. O dissolved in the HCP unit cell of pure titanium influences the nucleation of dislocations within the pure titanium. The addition of oxygen forms O-Ti bonds within the pure titanium. The bond energy of the formed O-Ti bonds is higher than that of the Ti-Ti bonds in pure titanium, increasing the lattice resistance during dislocation slip. Secondly, the increased bond energy of the pure titanium bond (O-Ti bond) hinders the shear of tensile twins. Therefore, through the combined effects of influencing dislocation nucleation, increasing the bond energy within the titanium alloy, and increasing the resistance during dislocation slip, the strength of large-size seamless pure titanium tubes is effectively improved. In the process of cost control, oxygen is the most effective and economical interstitial solid solution strengthening element. The high oxygen content composition design can simplify the smelting and hot working process, improve the yield, and reduce the process cost.

[0006] The manufacturing process employs a combined forging and rolling production mode, which offers advantages such as a relatively simple production process, low material loss, and low production cost. During the forging process, a low-temperature, high-deformation hot working process, incorporating a reversing upsetting technique, effectively breaks down the as-cast microstructure of the pure titanium ingot, forming a fine and uniform microstructure. Utilizing the fine-grain effect, this significantly improves the strength, hardness, and fatigue performance of large-diameter seamless pure titanium tubes while retaining a certain degree of plasticity. Furthermore, the fine-grained microstructure provides better cold and hot formability, facilitating further precision machining of the tubes. During the tube rolling process, rolling is performed within a temperature range of 40℃ to 150℃ below the phase transformation point. This allows for the production of large-diameter fine-grained high-oxygen pure titanium tubes with a grain size of 7-8 grades and a tensile strength exceeding 580 MPa, and also facilitates mass production.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a method for preparing large-diameter fine-grained high-oxygen pure titanium tubing, comprising the following steps: Step 1, Ingot Smelting: According to the set chemical composition weight percentage range: O 0.26~0.30%, Fe≤0.4%, C≤0.1%, N≤0.05%, H≤0.015%, balance Ti, sponge titanium is selected, and then it is pressed into electrodes. After the electrodes are smelted, post-processing is performed to obtain high oxygen pure titanium ingots. Step 2, High-temperature billet forging: The high-oxygen pure titanium ingot obtained in Step 1 is heated to a set temperature above the β phase transformation point and held at that temperature before high-temperature reversal billet forging.

[0008] Step 3, Low-temperature forging: After the billet in Step 2 is opened, it is heated to a set temperature below the β phase transformation point and held at that temperature. Then, a combination of low-temperature reversing upsetting and drawing processes is performed, followed by machining to obtain a smooth bar billet. Step 4, medium-temperature skew rolling piercing: A through hole is machined at the center of the smooth bar billet obtained in Step 3, and then heated to a set temperature below the β phase transformation point and held at that temperature before skew rolling piercing to obtain a rough tube; Step 5, Low-temperature rolling: The rough tube obtained in Step 4 is heated to a set temperature below the β phase transformation point and held at that temperature before being rolled in cycles to obtain the tube blank; Step 6: Prepare finished pipe: The pipe blank obtained in step 5 is subjected to stress-relief annealing and machining treatment in sequence to obtain high oxygen pure titanium large-size fine-grained pipe.

[0009] Further, in step 1, the particle size of the sponge titanium is 0.83~25.4mm. The electrode is subjected to two vacuum consumable arc melting processes. After the riser and ingot bottom are removed, the oxide scale on the surface of the ingot is removed by machining to prepare a high oxygen pure titanium ingot with a specification of Ф620~720×L mm, where L is the length.

[0010] Optionally, the particle size of the sponge titanium is 0.83~5mm, 5.01~10mm, 10.01~15mm, 15.01~20mm, or 20.01~25mm.

[0011] Furthermore, in step 2, the high-oxygen pure titanium ingot is heated to 50-100°C above the β phase transformation point, held at that temperature for 150-180 minutes, and then subjected to high-temperature reversal forging, with a total deformation of 80%-90%. The high-temperature reversing forging process includes shaft upsetting and side upsetting.

[0012] Further, in step 3, the billet after the blanking in step 2 is polished and heated to 100~150℃ below the β phase transformation point, held for 270~300min, and then subjected to 2~3 low-temperature reversing upsetting and 1 drawing process. The total deformation of each reversing upsetting is 70%~80%, and the total deformation of the drawing process is 30%~45%. The oxide scale and defects on the surface of the billet are removed by machining to prepare a smooth billet with a diameter of Ф500~Ф600×L mm. The specific process of the 2-3 low-temperature reversing upsetting and drawing is as follows: the reversing upsetting and drawing process is the same for each batch, and the shaft upsetting and side drawing are performed first, followed by side upsetting and shaft drawing to complete one reversing process, and then air cooling and grinding are performed.

[0013] Further, in step 4, a through hole of Ф80~100mm is machined at the center of the smooth bar billet, and then heated to 50~100℃ below the β phase transformation point and held for 360~400min. A rough tube with an outer diameter of Ф520~600 / inner diameter of Ф390~400×L mm is prepared by skew rolling and piercing using a Ф380mm high alloy mandrel. The total deformation is 40%~60%.

[0014] Further, in step 5, the rough tube is heated to 100~150℃ below the β phase transformation point and held for 240~300min, and then periodically rolled to prepare a tube blank with an outer diameter of Ф400~500 / inner diameter of Ф360~380×L mm, with a total deformation of 40%~60%.

[0015] Furthermore, in step 6, the stress-relief annealing involves heating the tube blank to 600~640℃ and holding it at that temperature for 90~120 minutes, followed by residual heat straightening.

[0016] Furthermore, in step 6, the machining process involves turning, boring, sawing, flattening, and polishing the stress-relieved tube blank to obtain large-size fine-grained high-oxygen pure titanium tubes.

[0017] On the other hand, the present invention provides a large-size fine-grained tube of high-oxygen pure titanium, which is prepared based on the preparation method described above, and its grain size reaches level 7 to 8.

[0018] Furthermore, the large-diameter fine-grained tube has a room temperature tensile strength > 580 MPa, a yield strength > 440 MPa, an elongation after fracture > 20%, and a reduction of area > 40%.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. Compared with existing technologies, this invention allows the use of sponge titanium with higher oxygen content, which can significantly reduce raw material costs (and the limited O content of 0.26~0.30% will not cause oxidation). The high oxygen content composition design not only simplifies the melting and hot working process, improves the yield, and further reduces process costs, but also improves the material strength through solid solution strengthening of oxygen.

[0020] 2. This invention employs a medium-temperature, large-deformation skew rolling piercing billet process, which does not lead to grain growth and has a high yield. It solves the problem that high heating temperatures and deformation temperature rises easily lead to grain growth, severely reducing the comprehensive mechanical properties of pure titanium seamless tubes. The tube preparation process using low-temperature hot rolling sizing allows for larger deformation per pass compared to cold rolling, resulting in a shorter process flow, higher production efficiency, and precise control of lower processing temperatures and large deformation amounts to obtain a uniform and refined microstructure. Fine grain strengthening further improves the strength and plasticity of the tube.

[0021] 3. The high-oxygen pure titanium large-size fine-grained tubes with outer diameters of Ф400~Ф500mm and inner diameters of Ф360~Ф380mm prepared by this invention can achieve a low-magnification microstructure without cracks or obvious metallurgical defects; the microstructure is uniform and fine, and the grain size meets the 7~8 grade specified in GB / T 6394-2017; its room temperature tensile strength is >580MPa, yield strength is >440MPa, elongation after fracture is >20%, and reduction of area is >40%.

[0022] 4. The high-oxygen pure titanium large-size pipes prepared by this invention have good microstructure uniformity, good batch stability, and excellent comprehensive performance, meeting the technical requirements and can further expand the application fields of pure titanium large-size pipes. Attached Figure Description

[0023] The accompanying drawings are incorporated in and form part of this specification, and together with the description serve to explain the principles of the invention.

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a flowchart of the preparation method of the high-oxygen pure titanium large-size fine-grained tube of the present invention; Figure 2 Transverse microstructure of the TA3 pure titanium Ф444 / Ф380×6000mm large-diameter pipe prepared in Example 1; Figure 3 The longitudinal microstructure of the TA3 pure titanium Ф444 / Ф380×6000mm large-diameter pipe prepared in Example 1; Figure 4 The transverse microstructure of the TA3 pure titanium Ф420 / Ф380×5000mm large-diameter pipe prepared in Example 2 is shown in Figure 2. Figure 5 The image shows the longitudinal microstructure of the TA3 pure titanium Ф420 / Ф380×5000mm large-diameter pipe prepared in Example 2.

[0026] Figure 6 Transverse microstructure of the TA3 pure titanium Ф460 / Ф380×6500mm large-diameter pipe prepared in Example 3; Figure 7 The image shows the longitudinal microstructure of the TA3 pure titanium Ф420 / Ф380×6500mm large-diameter pipe prepared in Example 3. Detailed Implementation

[0027] Exemplary embodiments will now be described in detail. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples consistent with some aspects of the invention as detailed in the appended claims.

[0028] This invention provides a method for preparing large-diameter fine-grained high-oxygen pure titanium tubing, comprising the following steps: Step 1, Ingot Smelting: According to the set chemical composition weight percentage range: O 0.26~0.30%, Fe≤0.4%, C≤0.1%, N≤0.05%, H≤0.015%, balance Ti, sponge titanium is selected, and then it is pressed into electrodes. After the electrodes are smelted, post-processing is performed to obtain high oxygen pure titanium ingots. Step 2, High-temperature blanking forging: The high-oxygen pure titanium ingot obtained in Step 1 is heated to a set temperature above the β phase transformation point and held at that temperature before high-temperature reversing blanking forging. The purpose is to eliminate the original as-cast structure and obtain a blanking forging with uniform structure through large deformation and reversing upsetting.

[0029] Step 3, Low-temperature forging: After the billet in Step 2 is opened, it is heated to a set temperature below the β phase transformation point and held at that temperature. Then, a combination of low-temperature reversing upsetting and drawing processes is performed, followed by machining to obtain a smooth bar billet. Step 4, medium-temperature skew rolling piercing: A through hole is machined at the center of the smooth bar billet obtained in step 3, and then heated to a set temperature below the β phase transformation point and held at that temperature before skew rolling piercing to obtain a rough tube; Step 5, Low-temperature rolling: The rough tube obtained in Step 4 is heated to a set temperature below the β phase transformation point and held at that temperature before being rolled in cycles to obtain the tube blank; Step 6: Prepare finished pipe: The pipe blank obtained in step 5 is subjected to stress-relief annealing and machining treatment in sequence to obtain high oxygen pure titanium large-size fine-grained pipe.

[0030] Further, in step 1, the particle size of the sponge titanium is 0.83~25.4mm. The electrode is subjected to two vacuum consumable arc melting processes. After the riser and ingot bottom are removed, the oxide scale on the surface of the ingot is removed by machining to prepare a high oxygen pure titanium ingot with a specification of Ф620~720×L mm, where L is the length.

[0031] Furthermore, in step 2, the high-oxygen pure titanium ingot is heated to 50-100°C above the β phase transformation point, held at that temperature for 150-180 minutes, and then subjected to high-temperature reversal forging, with a total deformation of 80%-90%. The high-temperature reversing forging process includes shaft upsetting and side upsetting.

[0032] Further, in step 3, the billet after the blanking in step S2 is heated to 100~150℃ below the β phase transformation point, held for 270~300min, and then subjected to 2~3 low-temperature reversing upsetting and drawing and 1 drawing process. The total deformation of each reversing upsetting is 70%~80%, and the total deformation of the drawing process is 30%~45%. The oxide scale and defects on the surface of the billet are removed by machining to prepare a smooth billet with a diameter of Ф500~Ф600×L mm. The specific process of the 2-3 low-temperature reversing upsetting and drawing is as follows: the reversing upsetting and drawing process is the same for each batch, and the shaft upsetting and side drawing are performed first, followed by side upsetting and shaft drawing to complete one reversing process, and then air cooling and grinding are performed.

[0033] Further, in step 4, a through hole of Ф80~100mm is machined at the center position of the smooth bar billet, and then heated to 50~100℃ below the β phase transformation point and held for 360~400min. A rough tube with an outer diameter of Ф520~600 / inner diameter of Ф390~400×L mm is prepared by skew rolling and piercing using a Ф380mm high alloy mandrel. The total deformation is 40%~60%.

[0034] Further, in step 5, the rough tube is heated to 100~150℃ below the β phase transformation point and held for 240~300min, and then periodically rolled to prepare a tube blank with an outer diameter of Ф400~500 / inner diameter of Ф360~380×L mm, with a total deformation of 40%~60%.

[0035] Furthermore, in step 6, the stress-relief annealing involves heating the tube blank to 600~640℃ and holding it at that temperature for 90~120 minutes, followed by residual heat straightening.

[0036] Furthermore, in step 6, the machining process involves turning, boring, sawing, flattening, and polishing the stress-relieved tube blank to produce large-size fine-grained tubes of high-oxygen pure titanium.

[0037] On the other hand, the present invention provides a large-size fine-grained tube of high-oxygen pure titanium, which is prepared based on the preparation method described above, and its grain size reaches level 7 to 8.

[0038] Furthermore, the room temperature tensile strength (Rm) of the large-diameter fine-grained tubing is >580 MPa, and the yield strength (Rp) is >580 MPa. 0.2 >440MPa, elongation after fracture (A) >20%, reduction of area (Z) >40%.

[0039] Example 1: Preparation of large-diameter TA3 pure titanium tubing with dimensions Ф444 / Ф380×6000mm Includes the following steps: Step 1, Ingot Melting: Weigh standard-sized sponge titanium (0.83~25.4mm) and perform two vacuum arc melting processes. After removing the riser and ingot bottom, machine the ingot surface to remove the oxide scale. The ingot is then eccentrically cut to prepare a Ф700×1100mm high-oxygen pure titanium ingot. The measured chemical composition (weight percentage) of the ingot is shown in Table 1.

[0040] Step 2, High-temperature forging: The Ф700×1100mm pure titanium ingot from Step 1 is heated to 1000℃ and held for 150 minutes before reversing forging. It is then shaped into Ф650×1270mm by shaft upsetting and side upsetting. The intermediate dimensions of shaft upsetting and side upsetting are Ф930×625mm and Ф855×740mm, respectively, and the total deformation is 85%.

[0041] Step 3: Low temperature forging. After grinding, the Ф650×1270mm billet from Step 2 is heated to 800℃, held for 270 minutes, and then subjected to two low temperature reversing upsetting and one drawing process. The two low-temperature reversing upsetting and drawing processes were identical. In each process, the forging billet was shaft-upset to Ф825×790mm, then side-drawn to Ф650×1270mm, then reversing side-upset to Ф825×790mm, and finally shaft-drawn to Ф650×1270mm. After that, the forging billet was air-cooled and ground. The deformation amount of the two reversing upsetting and drawing processes was 75.5%. In the first drawing process, the forging billet is drawn from Ф650×1270mm to Ф540×1840mm; the deformation is 30.98%. Then, the surface oxide scale is removed to prepare a bright bar billet of Ф530×1800mm.

[0042] Step 4, medium-temperature skew rolling piercing: Machining a Ф80mm through hole in the center of the Ф530×1800mm bar billet from Step 3, heating to 860℃ and holding for 380min, and using a Ф380mm mandrel to perform skew rolling piercing to prepare a rough tube with an outer diameter of Ф540 / inner diameter of Ф400×3750mm, with a deformation of 52%.

[0043] Step 5, Low-temperature rolling: The rough tube from Step 4 with an outer diameter of Ф540 / inner diameter of Ф400×3750mm is heated to 780℃ and held for 270min. A tube blank with an outer diameter of Ф460 / inner diameter of Ф370×6600mm is prepared by low-temperature cyclic rolling using a Ф370mm mandrel. The rolling deformation is 43.2%.

[0044] Step 6, Stress-relieving annealing: Heat the tube blank with an outer diameter of Ф460 / inner diameter of Ф370×6600mm from step 5 to 620℃ and hold for 100 minutes, then straighten it under residual heat.

[0045] Step 7, Machining: The tube blank after stress relief annealing in step 6 is turned, bored, sawed, flattened, and polished to produce a large-size fine-grained tube of high-oxygen pure titanium with an outer diameter of Ф444 and an inner diameter of Ф380×6000mm.

[0046] Table 1 Chemical composition of ingots in Example 1 Example 2: Preparation of large-diameter TA3 pure titanium tubing with dimensions Ф420 / Ф380×5000mm Includes the following steps: Step 1, Ingot Smelting: Weigh standard-sized sponge titanium (0.83~25.4mm) and perform two vacuum arc melting processes. After removing the riser and ingot bottom, machine the ingot surface to remove the oxide scale. The ingot is then eccentrically cut to prepare a Ф700×1300mm high-oxygen pure titanium ingot. The measured chemical composition (weight percentage) of the ingot is shown in Table 2.

[0047] Step 2, High-temperature forging: The Ф700×1300mm pure titanium ingot from Step 1 is heated to 1000℃ and held for 160 minutes before reversing forging. After shaft upsetting and side upsetting, it is drawn to Ф685×1350mm. The intermediate dimensions of shaft upsetting and side upsetting are Ф910×765mm and Ф900×790mm, respectively, and the total deformation is 82.4%.

[0048] Step 3: Low temperature forging. After grinding, the Ф685×1350mm billet from Step 2 is heated to 800℃, held for 300 minutes, and then subjected to two-stage reversing upsetting and one-stage drawing process. The two low-temperature reversing upsetting and drawing processes were identical. In each process, the forging billet was shaft-upset to Ф855×870mm, then side-drawn to Ф685×1350mm, then reversing side-upset to Ф855×870mm, and finally shaft-drawn to Ф685×1350mm. After that, the forging billet was air-cooled and ground. The deformation amount of the two reversing upsetting and drawing processes was 71.4%. In the first drawing process, the forging billet is drawn from Ф685×1350mm to Ф520×2345mm; the deformation is 42.4%; then the surface oxide scale is removed, and according to the final finished product size, billet size and equipment processing size limitations, it is determined that a Ф510×1100mm smooth bar billet needs to be prepared by splitting.

[0049] Step 4, medium-temperature skew rolling piercing: Machining a Ф80mm through hole in the center of the Ф510×1100mm bar billet from Step 3, heating to 860℃ and holding for 360min, and using a Ф380mm mandrel to perform skew rolling piercing to prepare a rough tube with an outer diameter of Ф520 / inner diameter of Ф396×2450mm, with a deformation of 55.23%.

[0050] Step 5, Low-temperature rolling: The rough tube from Step 4 with an outer diameter of Ф520 / inner diameter of Ф396×2450mm is heated to 780℃ and held for 240min. A tube blank with an outer diameter of Ф432 / inner diameter of Ф370×5550mm is prepared by low-temperature cyclic rolling using a Ф370mm mandrel. The rolling deformation is 56.22%.

[0051] Step 6, Stress-relieving annealing: Heat the tube blank with an outer diameter of Ф432 / inner diameter of Ф370×5550mm from step 5 to 600℃ and hold for 90 minutes, then straighten it under residual heat.

[0052] Step 7, Machining: The tube blank after stress relief annealing in Step 6 is machined by turning, boring, sawing, flattening, and polishing to produce large-size fine-grained tubes of high-oxygen pure titanium with an outer diameter of Ф420 and an inner diameter of Ф380×5000mm.

[0053] Table 2 Chemical composition of ingots in Example 2 Example 3: Preparation of large-diameter TA3 pure titanium tubing with dimensions Ф460 / Ф380×6500mm Includes the following steps: Step 1, Ingot Melting: Weigh standard-sized sponge titanium (0.83~25.4mm) and perform two vacuum arc melting processes. After removing the riser and ingot bottom, machine the surface oxide scale to remove the ingot. The ingot is then eccentrically cut to prepare a Ф720×1200mm high-oxygen pure titanium ingot. The measured chemical composition (weight percentage) of the ingot is shown in Table 3.

[0054] Step 2, High-temperature forging: The Ф720×1200mm pure titanium ingot from Step 1 is heated to 1000℃ and held for 180 minutes before reversing forging. After shaft upsetting and side upsetting, it is drawn to Ф670×1300mm. The intermediate dimensions of shaft upsetting and side upsetting are Ф950×650mm and Ф900×730mm, respectively, and the total deformation is 89.6%.

[0055] Step 3: Low temperature forging. After grinding, the Ф670×1300mm billet from Step 2 is heated to 800℃, held for 290 minutes, and then subjected to three-stage reversing upsetting and one-stage drawing process. The three low-temperature reversing upsetting and drawing processes were identical. In each process, the forging billet was shaft-upset to Ф845×820mm, then side-drawn to Ф670×1300mm, then reversing side-upset to Ф845×820mm, and finally shaft-drawn to Ф670×1300mm. After that, the forging billet was air-cooled and ground. The deformation amount of the three reversing upsetting and drawing processes was 74.1% in all three processes. In the first drawing process, the forging billet is drawn from Ф670×1300mm to Ф560×1965mm, with a deformation of 30.14%; then the surface oxide scale is removed to prepare a bright bar billet of Ф550×1900mm.

[0056] Step 4, medium-temperature skew rolling piercing: Machining a Ф80mm through hole in the center of the Ф550×1900mm bar billet from Step 3, heating to 860℃ and holding for 400min, and using a Ф380mm mandrel to perform skew rolling piercing to prepare a rough tube with an outer diameter of Ф560 / inner diameter of Ф400×3650mm, with a deformation of 48.1%.

[0057] Step 5, Low-temperature rolling: The rough tube from Step 4 with an outer diameter of Ф560 / inner diameter of Ф400×3650mm is heated to 780℃ and held for 300min. A tube blank with an outer diameter of Ф470 / inner diameter of Ф370×6650mm is prepared by low-temperature cyclic rolling using a Ф370mm mandrel. The rolling deformation is 45.3%.

[0058] Step 6, Stress-relieving annealing: Heat the tube blank with an outer diameter of Ф470 / inner diameter of Ф370×6650mm from step 5 to 640℃ and hold for 120 minutes, then straighten it under residual heat.

[0059] Step 7, Machining: The tube blank after stress relief annealing in Step 6 is turned, bored, sawed, flattened, and polished to produce a large-size fine-grained tube with an outer diameter of Ф460 / inner diameter of Ф380×6500mm.

[0060] Table 3 Chemical composition of ingots in Example 3 The large-diameter fine-grained pure titanium tubes obtained in Examples 1-3 were subjected to microstructure rating according to GB / T 6394-2017 standard and room temperature mechanical property tests according to GB / T 228.1-2021. Specific results are shown in Table 4, where L represents the longitudinal sampling direction, T represents the transverse sampling direction, and M represents the annealed state of the tube. The microstructure is equiaxed α-structure with an average grain size of 7.5-8.0, indicating that the prepared large-diameter fine-grained pure titanium tubes have a uniform and fine microstructure. The mechanical property test results show that the prepared large-diameter fine-grained pure titanium tubes have uniform mechanical property distribution and significant excess, and all mechanical properties meet the technical standard requirements.

[0061] Table 4 Performance of large-diameter fine-grained pure titanium tubing The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention.

[0062] It should be understood that the present invention is not limited to the content already described above, and various modifications and changes can be made without departing from its scope. The scope of the present invention is limited only by the appended claims.

Claims

1. A method for preparing large-diameter fine-grained high-oxygen pure titanium tubing, characterized in that, Includes the following steps: Step 1, Ingot Smelting: According to the set chemical composition weight percentage range: O 0.26~0.30%, Fe≤0.4%, C≤0.1%, N≤0.05%, H≤0.015%, balance Ti, sponge titanium is selected, and then it is pressed into electrodes. After the electrodes are smelted, post-processing is performed to obtain high oxygen pure titanium ingots. Step 2, High-temperature billet forging: The high-oxygen pure titanium ingot obtained in Step 1 is heated to a set temperature above the β phase transformation point and held at that temperature before high-temperature reversal billet forging. Step 3, Low-temperature forging: After the billet in Step 2 is opened, it is heated to a set temperature below the β phase transformation point and held at that temperature. Then, a combination of low-temperature reversing upsetting and drawing processes is performed, followed by machining to obtain a smooth bar billet. Step 4, medium-temperature skew rolling piercing: A through hole is machined at the center of the smooth bar billet obtained in Step 3, and then heated to a set temperature below the β phase transformation point and held at that temperature before skew rolling piercing to obtain a rough tube; Step 5, Low-temperature rolling: The rough tube obtained in Step 4 is heated to a set temperature below the β phase transformation point and held at that temperature before being rolled in cycles to obtain the tube blank; Step 6: Prepare finished pipe: The pipe blank obtained in step 5 is subjected to stress-relief annealing and machining treatment in sequence to obtain high oxygen pure titanium large-size fine-grained pipe.

2. The preparation method according to claim 1, characterized in that, In step 1, the particle size of the sponge titanium is 0.83~25.4mm. The electrode is subjected to two vacuum consumable arc melting processes. After the riser and ingot bottom are removed, the oxide scale on the surface of the ingot is removed by machining to prepare a high oxygen pure titanium ingot with a specification of Ф620~720×L mm, where L is the length.

3. The preparation method according to claim 1, characterized in that, In step 2, the high-oxygen pure titanium ingot is heated to 50-100°C above the β phase transformation point, held at that temperature for 150-180 minutes, and then subjected to high-temperature reversal forging, with a total deformation of 80%-90%. The high-temperature reversing forging process includes shaft upsetting and side upsetting.

4. The preparation method according to claim 1, characterized in that, In step 3, the billet after the blanking in step 2 is polished and heated to 100~150℃ below the β phase transformation point. After holding at this temperature for 270~300min, it undergoes 2~3 low-temperature reversing upsetting and 1 drawing process. The total deformation of each reversing upsetting is 70%~80%, and the total deformation of the drawing process is 30%~45%. The oxide scale and defects on the surface of the billet are removed by machining to prepare a smooth billet with a diameter of Ф500~Ф600×L mm. The specific process of the 2-3 low-temperature reversing upsetting and drawing is as follows: the reversing upsetting and drawing process is the same for each batch, and the shaft upsetting and side drawing are performed first, followed by side upsetting and shaft drawing to complete one reversing process, and then air cooling and grinding are performed.

5. The preparation method according to claim 1, characterized in that, In step 4, a through hole is machined at the center of the smooth bar billet, and then heated to 50~100℃ below the β phase transformation point and held for 360~400 min. A mandrel is used to perform skew rolling and piercing to prepare a rough tube, with a total deformation of 40%~60%.

6. The preparation method according to claim 1, characterized in that, In step 5, the raw tube is heated to 100-150°C below the β phase transformation point and held for 240-300 minutes, and then rolled periodically to prepare the tube blank, with a total deformation of 40%-60%.

7. The preparation method according to claim 1, characterized in that, In step 6, the stress-relief annealing involves heating the tube blank to 600~640℃ and holding it at that temperature for 90~120 minutes, followed by residual heat straightening.

8. The preparation method according to claim 1, characterized in that, In step 6, the machining process involves turning, boring, sawing, flattening, and polishing the stress-relieved tube blank to obtain large-size fine-grained tubes of high-oxygen pure titanium.

9. A large-diameter fine-grained tube of high-oxygen pure titanium, characterized in that, The crystallite is prepared by the preparation method according to any one of claims 1 to 8, and its grain size reaches level 7 to 8.

10. The high-oxygen pure titanium large-size fine-grained tube according to claim 9, characterized in that, The large-diameter fine-grained tube has a room temperature tensile strength > 580 MPa, a yield strength > 440 MPa, an elongation after fracture > 20%, and a reduction of area > 40%.

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