A method for improving the shrinkage strain ratio of TA18 titanium alloy pipe

Abstract

The application discloses a method for improving the shrinkage strain ratio of TA18 titanium alloy pipes, and aims at solving the problem that the shrinkage strain ratio of the existing TA18 titanium alloy pipes fluctuates greatly after multiple-pass periodic reciprocating cold rolling, the batch reproducibility is poor, and the consistency requirement of the pipe bending plastic deformation capacity in the field of aviation and aerospace cannot be met. The pipe is treated by adopting a segmented temperature and speed control vacuum incomplete annealing process, the existing preparation process does not need to be changed, the pipe stress elimination and organization recovery process can be accurately controlled, the shrinkage strain ratio can be stably controlled in the ideal range of 1.4-2.0, the process compatibility is good, and the method is suitable for industrialized batch production.

Description

Technical Field

[0001] This application relates to the field of titanium alloy processing technology, specifically to a method for improving the shrinkage strain ratio of TA18 titanium alloy tubing. Background Technology

[0002] Among various titanium alloy tubing, TA18 (Ti-3Al-2.5V) titanium alloy seamless tubing stands out for its comprehensive performance. It can be heat-treated to strengthen and improve its service performance, and also possesses excellent thermal strength and stability, as well as good weldability. It is widely used in aircraft hydraulic lines and engine fuel lines. Currently, the demand for high-performance, high-precision, and highly formable TA18 titanium alloy tubing is increasing in aerospace, military, and other industrial sectors. However, research on titanium alloy tubing in my country started relatively late, and its production and manufacturing technologies are not yet mature. Particularly in the later stages of development and application of TA18 titanium alloy tubing, the control over the bending plastic deformation capacity of the tubing is not precise enough, requiring further breakthroughs. Existing technology uses the contraction strain ratio (CSR) to judge the performance of the tubing. If the CSR value is too low, too high, or has poor reproducibility, it indicates a poor internal microstructure and insufficient plastic deformation capacity. Therefore, optimizing the manufacturing process to improve the contraction strain ratio is of great significance to the microstructure and performance of TA18 tubing. Summary of the Invention

[0003] To address the issue of unstable CSR (Contraction Strain Ratio) in TA18 pipes prepared by existing technologies, which tends to be too high or too low, this application provides a method to improve the shrinkage strain ratio of TA18 titanium alloy pipes. By using a temperature and speed control method for incomplete annealing, the internal stress generated in the finished pipes during cold deformation processing and other forming processes is eliminated, significantly reducing the complex effects of microscopic changes and restoring the internal structure, grain shape, and size to their original state. This improves the pipe structure, enhances its plastic deformation capacity, and solves the problem of low or high CSR.

[0004] To achieve the above objectives, this application provides a method for improving the shrinkage strain ratio of TA18 titanium alloy tubing, the specific steps of which are as follows:

[0005] Step 1: Prepare TA18 titanium alloy tubing;

[0006] Step 2: Perform vacuum heat treatment on the tubes prepared in Step 1. The steps of the vacuum heat treatment are as follows:

[0007] The temperature was increased to 250℃~550℃ using a heating rate of 1℃ / min~5℃ / min, held for 130min~150min, and then cooled to room temperature to complete the vacuum heat treatment.

[0008] Preferably, the preparation process in step 1 is as follows:

[0009] Step a. Melting and casting ingots → Step b. Forging bars → Step c. Extruding tubes → Step d. Vacuum annealing → Step e. Rolling to target dimensions → Step f. Surface treatment → Step g. Vacuum annealing.

[0010] Preferably, steps d → e → f are repeated 6 to 10 times to ensure that the extruded tube blank has a relatively uniform internal structure after cold rolling.

[0011] Preferably, in step d, the annealing temperature is 550℃~850℃, the holding time is 1h~3h, and the furnace is cooled.

[0012] Preferably, in step e, the pipe feed amount is not less than 60% of the mill feed amount design value, and the mill adopts a periodic reciprocating rolling process with a working speed of 65 times / min to 85 times / min.

[0013] Preferably, in step f, the pipe is subjected to surface treatments such as sandblasting, external polishing, degreasing, and pickling. The treatment accuracy is 0.01mm~0.02mm, which effectively removes macroscopic defects such as micro-scratches, pores, and acid spots without affecting the pipe size, and ensures that the annealed pipe has a smooth inner and outer surface quality after cold rolling.

[0014] Preferably, in step 2, the temperature is heated to 250~350℃ at 5℃ / min and held at this temperature for 20min~30min; then heated to 400℃~530℃ at 5℃ / min and held at this temperature for 10min; then heated to 420℃~550℃ at 1℃ / min and held at this temperature for 100min~120min; finally, the temperature is cooled to 250℃~350℃ at a cooling rate of 1℃ / min, and then further cooled to below 50℃ at a cooling rate of 1℃ / min~3℃ / min before being removed from the furnace.

[0015] Compared with the prior art, this application has the following beneficial effects:

[0016] 1. This application addresses the technical problem in the prior art where TA18 titanium alloy tubes, after undergoing multiple cyclic reciprocating cold rolling processes, accumulate uneven residual internal stress and form uneven cold deformation structures. This results in large fluctuations in the tube's shrinkage strain ratio, which is prone to abnormally high or low values ​​and exhibits poor batch-to-batch reproducibility, failing to meet the aerospace field's requirements for consistent tube bending plastic deformation capabilities. The application employs a segmented temperature and speed-controlled vacuum incomplete annealing process. By setting gradient heating rates, multi-segment holding temperatures and corresponding holding times, and segmented cooling rates, the final heat treatment of the finished tubes is performed without altering existing pre-processing techniques such as vacuum consumable melting, multi-fire forging, hot extrusion, and multi-pass cold rolling-annealing cycles. This heat treatment process can precisely control the elimination of residual internal stress inside the pipe, achieving uniform release of uneven residual internal stress. At the same time, it can control the uniform recovery process of cold-deformed structure, avoiding the adverse effects of excessive grain growth during full annealing on the strength and hardness of the pipe. The uniform elimination of residual internal stress provides a uniform thermodynamic driving force for structure recovery, while the uniform structure recovery further stabilizes the microstructure state after the internal stress is eliminated. The two work together to improve the uniformity of stress distribution in the pipe during plastic deformation.

[0017] 2. The heat treatment process used in this application is carried out in a high vacuum environment, which can prevent surface oxidation, hydrogen absorption and contamination of the pipe during the heat treatment process, and ensure the surface smoothness and internal metallurgical quality of the pipe. This effect interacts with the aforementioned uniform elimination of internal stress and uniform restoration of structure, avoiding surface defects and internal metallurgical defects from becoming stress concentration points and crack initiation sources during plastic deformation, and further improving the consistency of the plastic deformation behavior of the pipe.

[0018] 3. The method described in this application only optimizes and adjusts the final annealing process of the finished pipe, without requiring additional equipment investment or process flow. Therefore, it has excellent process compatibility and industrial applicability, and ultimately effectively solves the technical problems of unstable shrinkage strain ratio and poor batch-to-batch reproducibility of existing TA18 titanium alloy pipes. It enables the shrinkage strain ratio of the pipe to be stably controlled within the ideal forming performance range of 1.4 to 2.0, providing reliable technical support for the stable mass production of high-performance TA18 titanium alloy pipes in the aerospace field. Attached Figure Description

[0019] Figure 1 This is a schematic diagram showing the position of the laser infrared marking when laser marking is performed on the outer surface of the pipe samples after laser marking in the examples and comparative examples. Detailed Implementation

[0020] This application will clearly and completely describe the technical solutions in the embodiments of this application in conjunction with the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0021] Unless otherwise specified in the specific circumstances, the numerical ranges listed herein include upper and lower limits, as well as all integers and fractions within that range, but are not limited to the specific values ​​listed when the range is defined.

[0022] I. A method for improving the shrinkage strain ratio of TA18 titanium alloy pipes

[0023] To address the technical problems of large fluctuations in the shrinkage strain ratio, abnormally high or low values, and poor batch-to-batch reproducibility in TA18 titanium alloy tubes produced by multiple cold rolling processes, this application considers that the instability of the shrinkage strain ratio mainly stems from the uneven distribution of residual internal stress and the inhomogeneity of the cold-deformed microstructure generated during cold deformation. Traditional finished product annealing processes, which use a single-temperature direct heating and holding method, cannot achieve precise control over the degree of internal stress relief and the microstructure recovery process. Based on this understanding, this application approaches the issue from the perspective of the final heat treatment process of the finished tubes, employing a segmented temperature and rate controlled incomplete annealing method. By setting gradient heating rates, multi-segment holding temperatures and corresponding holding times, and segmented cooling rates, the internal stress relief process and microstructure evolution process of the tubes can be precisely controlled. The specific steps are as follows:

[0024] Step 1: Prepare TA18 titanium alloy tubing;

[0025] Step 2: Perform vacuum heat treatment on the tubes prepared in Step 1. The steps of the vacuum heat treatment are as follows:

[0026] The temperature was increased to 250℃~550℃ using a heating rate of 1℃ / min~5℃ / min, held for 130min~150min, and then cooled to room temperature to complete the vacuum heat treatment.

[0027] This application has achieved significant technical effects in practical application: Under the same maximum annealing temperature, the shrinkage strain ratio of TA18 titanium alloy pipes treated by the method described in this application is reduced by about 0.5 compared to pipes treated by the traditional single-temperature holding annealing process. Even though the total holding time of this application is only extended by 25% to 40% compared to the traditional process, the reduction in shrinkage strain ratio still far exceeds the expected effect of simply extending the holding time. As the maximum annealing temperature gradually increases from 420℃ to 550℃, the shrinkage strain ratio of the pipes treated by this application shows a stable linear decreasing trend, which can be stably controlled within the ideal forming performance range of 1.4 to 2.0. In contrast, the shrinkage strain ratio of pipes treated by the traditional single-temperature holding annealing process decreases by a similar rate as that of this application, but the overall value is significantly higher than that of the pipes treated by this application, and cannot reach the lower limit of the ideal range of 1.4 to 2.0. The shrinkage strain ratio of all tested samples is in the high range. Furthermore, this application can effectively control the shrinkage strain ratio by simply adjusting the temperature-time curve of the finished product annealing, without changing the pre-cold rolling process parameters of the tube (including mill operating speed, feed rate and deformation per pass). Moreover, the control effect is not significantly affected by the early cold deformation history and specifications of the tube, and can be applied to TA18 titanium alloy finished tubes of different production batches and specifications.

[0028] In some embodiments of this application, the process for the pipe prepared in step 1 is not limited; in fact, TA18 titanium alloy pipes prepared by existing technologies can all undergo the heat treatment method described in this application. The preparation process of step 1 is as follows:

[0029] Step a. Melting and casting ingots → Step b. Forging bars → Step c. Extruding tubes → Step d. Vacuum annealing → Step e. Rolling to target dimensions → Step f. Surface treatment → Step g. Vacuum annealing.

[0030] In some embodiments of this application, steps d→e→f are repeated 6 to 10 times to ensure that the extruded tube blank has a relatively uniform internal structure after cold rolling.

[0031] In some embodiments of this application, in step d, the annealing temperature is 550℃~850℃, the holding time is 1h~3h, and the material is cooled in the furnace. In practical applications, different pre-rolling holding temperatures and holding times can be selected according to the pipe specifications.

[0032] In some embodiments of this application, in step e, the pipe feed amount is not less than 60% of the mill feed amount design value, and the mill adopts a periodic reciprocating rolling process with a working speed of 65 times / min to 85 times / min.

[0033] In some embodiments of this application, in step f, the pipe is subjected to surface treatments such as sandblasting, external polishing, degreasing, and pickling. The treatment accuracy is 0.01mm~0.02mm, which effectively removes macroscopic defects such as micro-scratches, pores, and acid spots without affecting the pipe size, and ensures that the annealed pipe has a smooth inner and outer surface quality after cold rolling.

[0034] In some embodiments of this application, in step 2, the temperature is heated to 250~350℃ at a rate of 5℃ / min and held at this temperature for 20min~30min; then heated to 400℃~530℃ at a rate of 5℃ / min and held at this temperature for 10min; then heated to 420℃~550℃ at a rate of 1℃ / min and held at this temperature for 100min~120min; finally, the temperature is cooled to 250℃~350℃ at a rate of 1℃ / min, and then further cooled to below 50℃ at a rate of 1℃ / min~3℃ / min before being removed from the furnace. The segmented vacuum incomplete annealing process of this application achieves precise control over the elimination of internal stress and the recovery of microstructure in TA18 titanium alloy tubing through four continuous and interrelated temperature-time stages. Any deviation of the process parameters from the set range in any stage will affect the control effect of the preceding process and be transmitted to subsequent stages, ultimately causing the tubing shrinkage strain ratio (CSR) to deviate from the ideal range or reduce reproducibility. For the first stage, which involves heating at 5℃ / min to 250℃~350℃ and holding for 20min~30min for low-temperature preheating and preliminary internal stress relief, if the holding temperature is below 250℃, the low-energy residual internal stress generated by multiple cold rolling passes inside the pipe cannot be effectively released. This stress will remain in the subsequent high-temperature stage and superimpose with the high-temperature internal stress, resulting in uneven overall internal stress relief. If the holding temperature is above 350℃, uneven dislocation movement and microstructure recovery will occur prematurely in localized areas of the pipe, disrupting the synchronicity of microstructure evolution during subsequent gradient heating. If the heating rate is faster than 5℃ / min, a significant internal and external temperature difference will occur due to the lag in heat conduction along the pipe wall thickness, thereby inducing new thermal stress and offsetting part of the internal stress relief effect. If the heating rate is slower than 5℃ / min, the production cycle will be unnecessarily extended without any additional benefits from internal stress relief. If the holding time is less than 20 minutes, the overall temperature of the pipe will not reach a uniform state, and the initial internal stress will not be sufficiently eliminated. If the holding time is longer than 30 minutes, the low-energy internal stress has been basically released, and continuing to hold the temperature will not produce any beneficial effect, but will instead reduce production efficiency. For the second stage, the medium-temperature transition and temperature homogenization process, which involves heating at 5℃ / min to 400℃~530℃ and holding for 10 minutes, its core function is to achieve a uniform temperature distribution across the pipe before the main recovery stage, laying the foundation for the subsequent slow heating process. If the holding temperature is below 400℃, the temperature difference between the inside and outside of the pipe will not be completely eliminated, and the non-uniformity of the structural recovery will be further amplified after entering the third stage of slow heating. If the holding temperature is above 530℃, it will trigger the medium-temperature recovery process in some areas, leading to non-uniform changes in dislocation configuration and affecting the control accuracy of the main recovery stage. If the heating rate is faster than 5℃ / min, thermal stress will be generated again and superimposed on the internal stress remaining from the first stage; if the heating rate is slower than 5℃ / min, the transition time will be prolonged and production efficiency will be reduced.If the holding time is less than 10 minutes, the temperature uniformity along the pipe wall thickness will not meet the requirements, leading to asynchronous evolution of the inner and outer layers during the third-stage main recovery process. If the holding time is longer than 10 minutes, the internal stress at medium temperature has been largely eliminated, and continuing to hold the temperature will not improve the uniformity; instead, it may cause premature over-recovery of local structures. The third-stage main recovery process, which involves heating at 1℃ / min to 420℃~550℃ and holding for 100min~120min, is the most critical core stage for controlling CSR in the entire process, directly determining the sufficiency of internal stress elimination and the uniformity of structure recovery. If the maximum holding temperature is below 420℃, the recovery driving force of the cold-deformed structure is insufficient, the dislocation annihilation and rearrangement processes are incomplete, and a large amount of residual internal stress remains inside the pipe, resulting in a high and fluctuating CSR value. If the maximum holding temperature is above 550℃, it will exceed the temperature limit of incomplete annealing, triggering the recrystallization process and causing excessive grain growth. This will not only significantly reduce the strength and hardness of the pipe but also cause the CSR value to fall below the ideal lower limit of 1.4, affecting the bending and forming performance of the pipe. If the heating rate is faster than 1℃ / min, the difference in heating rate between the inner and outer layers of the pipe will lead to inconsistent recovery of the structure between the inner and outer layers, forming a structure gradient and internal stress gradient along the wall thickness direction, ultimately resulting in poor batch-to-batch reproducibility of the CSR value. If the heating rate is slower than 1℃ / min, it will significantly prolong the production cycle and will not significantly improve the uniformity of structure recovery. If the holding time is less than 100 minutes, the main recovery process is not complete, internal stress is not sufficiently eliminated, and the CSR value is high and unstable. If the holding time is longer than 120 minutes, the recovery process is basically complete, and continuing to hold will lead to misalignment rearrangement or even the formation of subgrains, which will also cause the CSR value to deviate from the ideal range and reduce production efficiency. For the fourth stage, which involves cooling at 1℃ / min to 250℃~350℃ and then at 1℃ / min~3℃ / min to below 50℃ before unloading, its core function is to avoid the generation of new residual internal stress due to excessively rapid temperature changes during the cooling process. If the cooling rate in the first stage is faster than 1℃ / min, significant thermal stress will be generated due to the difference in cooling rates between the inner and outer layers of the pipe. This thermal stress will be superimposed on the already stress-relieved matrix, resulting in the reappearance of uneven residual internal stress inside the final pipe, causing increased fluctuations in the CSR value. If the cooling rate in the first stage is slower than 1℃ / min, it will prolong the cooling time and reduce production efficiency. If the cooling rate is switched to a faster speed when the temperature drops below 250°C, the pipe temperature will still be high and the atomic diffusion ability will be strong, resulting in greater thermal stress during rapid cooling. If the cooling rate is switched to a speed above 350°C, the thermal stress will increase further during subsequent cooling due to the excessively high temperature.If the cooling rate in the second stage is faster than 3℃ / min, new residual internal stress will be generated, which will offset the control effect of the previous process; if the cooling rate in the second stage is slower than 1℃ / min, the production cycle will be extended meaninglessly, and no additional quality benefits will be brought.

[0035] II. Comparative Experiment

[0036] 1. The specifications of the TA18 titanium alloy finished tube samples involved in the examples and comparative examples are all 20×1.6mm. The hot working process and cold rolling process are the same, and will be described uniformly here:

[0037] (1) Melting and casting: The raw materials used for melting and casting TA18 (Ti-3Al-2.5V) alloy ingots are sponge titanium, vanadium-aluminum alloy, titanium-iron alloy, titanium dioxide and aluminum granules. The initial TA18 titanium alloy ingot is obtained by three vacuum self-consumption melting processes. The ingot is then polished and machined to remove pores (cracks) and pits from the head, tail and annular outer surface of the titanium ingot. Finally, a TA18 titanium alloy test ingot with a diameter of Φ740×L mm is obtained.

[0038] (2) Forging bar stock: Heat the TA18 titanium alloy ingot to 1200℃, hold for 1 hour, and then perform four upsetting and four drawing in one heat. After cooling the obtained titanium alloy forging bar, grind off the burrs, heat to 1050℃, hold for 1 hour, and then perform three upsetting and three drawing in the second heat. After cooling and grinding, heat to 900℃, hold for 1.5 hours, and then perform two upsetting and two drawing in the third heat. After cooling, grinding and finishing, heat to 880℃, hold for 1.5 hours, and then perform two upsetting and two drawing in the fifth heat. The heating temperature for the sixth heat is 860℃, hold for 1.5 hours, perform two upsetting and two drawing, and then cool and grind. The heating temperature for the seventh heat is 840℃, hold for 1 hour, and then perform two upsetting and two drawing. After the eighth heat, the heating temperature is 820℃, hold for 1 hour, and then draw and roll to Φ135mm. The surface is finished by lathe to remove defects such as non-metallic inclusions, cracks, peeling, holes, and oxide scale, and finally obtains 120mm TA18 titanium alloy bars.

[0039] (3) Extrusion of billet: Remove cracks, burrs, holes and oxide scale from the surface of titanium alloy forging rods. Hot extrusion is used. The specific process is as follows: punching and cladding → heating → hot extrusion → annealing → straightening → finishing. The surface of the titanium rod is clad in a double layer of copper and iron, with the inner layer being iron and the outer layer being copper. Extrusion speed: 60~100mm / s; lubricant is a mixture of graphite and MoS2 with cylinder oil (binder) at a mass ratio of 2:1; preheating temperature of the die cylinder and extrusion die: 150~400℃; after the billet exits the furnace, it is transferred to the extrusion cylinder for extrusion at a temperature ≥800℃; the extruded tube blank is heated to 770℃ in the furnace, held for 1~2.5h, and then air-cooled after exiting the furnace; straightening is performed using the residual heat after annealing, with an interval of ≤3 hours between annealing and straightening, and the straightness after straightening is ≤4mm / m; remove visible macroscopic defects such as oil stains, cracks, and peeling from the inner and outer surfaces of the tube blank, but the outer diameter and wall thickness must not exceed their allowable deviations, finally obtaining a Φ42×10mm cold-rolled TA18 titanium alloy tube blank.

[0040] (4) Cold-rolled finished pipe: Grind and trim 42mm×10mm TA18 titanium alloy tube blanks, and use vacuum annealing, two-roll cold rolling and surface treatment in alternating cycles until the target size is achieved. a. The rolling process is Φ42×10mm→Φ34×7mm→Φ29×4.8mm→Φ25×3.5mm→Φ22×2.2mm→Φ20×1.6mm, with a feed rate not exceeding 3mm, and the two-roll mill speed is set to 65~75 times / min. b. Vacuum annealing process for each heat cycle: The first heat cycle is 830℃ / 2h, cooled in the furnace to below 90℃ before exiting the furnace; the second heat cycle is 810℃ / 1h, cooled in the furnace to below 90℃ before exiting the furnace; the third heat cycle is 760℃ / 1h, cooled in the furnace to below 80℃ before exiting the furnace; the fourth heat cycle is 710℃ / 1h, cooled in the furnace to below 80℃ before exiting the furnace; the fifth heat cycle is 660℃ / 1h, cooled in the furnace to below 70℃ before exiting the furnace; the sixth heat cycle is 610℃ / 1h, cooled in the furnace to below 70℃ before exiting the furnace. The finished pipe is rolled using a two-roll mill to obtain Φ20×1.6mm finished pipe. c. Surface treatment: Based on the internal and external surface condition of the pipe before cold rolling and annealing in each heat cycle, sandblasting, external polishing, degreasing, and pickling are performed to remove visible macroscopic defects such as oil stains, scratches, pores, and acid spots. The precision of sandblasting, external polishing, and pickling is 0.01~0.02mm.

[0041] 2. Method for testing shrinkage strain ratio

[0042] (1) Principle: The CSR value, which characterizes the anisotropy of the pipe, is the ratio of the true circumferential plastic strain Ec to the true plastic strain Er of the wall thickness, i.e., CSR = Ec / Er. According to the principle of constant volume, Ea + Ec + Er = 0. By measuring Ea (axial true plastic strain) and Ec (circumferential plastic strain), Er (true plastic strain of wall thickness) can be obtained, and the shrinkage strain ratio (CSR) of the pipe can be calculated, i.e.:

[0043] (a)

[0044] in,

[0045] (b)

[0046] (c)

[0047] (2) Sample preparation: The gauge length of the pipe sample is 50 mm, and the length of the plug at the clamping end is 65 mm. Laser marking is performed on the outer surface of the pipe. First, three crosshairs, A1, A2, and A3, are laser-marked at the 0° longitudinal axis to meet the L... A1A2 =25mm, L A2A3 =25mm, such as Figure 1 As shown. Then, the pipe is rotated counterclockwise axially at 90°, 180°, and 270°, and the positions B1, B2, and B3 are marked in the same way. Repeat this step to determine the remaining 6 positions and mark them C1, C2, C3, D1, D2, and D3.

[0048] (3) Measurement of Ea and Ec: The initial outer diameter d of the pipe sample before deformation was measured using an image measuring instrument. 0A1C1 d 0A1C2 d 0A3C3 d 0B1D1 d 0B2D2 d 0B3D3 With gauge length L 0A1A3 L 0B1B3 L 0C1C3 L 0D1D3 The average initial outer diameter d0 and the average axial gauge length L0 of the sample tube were calculated. Then, the sample tube was subjected to tensile treatment at a tensile rate of 0.04 mm / mm / min. The tensile test was stopped when the total elongation was 4.20%, and the average outer diameter d and average gauge length L of the sample tube after tensile deformation were remeasured using the same measurement method as the initial measurement.

[0049] (4) Substitute d0, L0, d, and L from step (3) into formulas (a), (b), and (c) to obtain the CSR value.

[0050] 3. Examples and Comparative Examples

[0051] Example 1

[0052] One 230mm sample tube was cut from the head, middle, and tail sections of the finished Φ20×1.6mm pipe. The annealing method was temperature and rate controlled, using a vacuum tube annealing furnace. The pipe samples were heated to 250℃ at 5℃ / min and held for 20min; then heated to 400℃ at 5℃ / min and held for 10min; then heated to 420℃ at 1℃ / min and held for 120min; finally, cooled to 250℃ at 1℃ / min and then cooled in the furnace to below 50℃ before being removed from the furnace. The three sample tubes obtained under these conditions were tested, and the CSR values ​​at the head, middle, and tail sections were found to be 1.77, 1.75, and 1.80, respectively.

[0053] Example 2

[0054] The annealing process was adjusted based on Example 1, with the following differences: The annealing parameters were as follows: heating to 250°C at 5°C / min and holding for 20 min; then heating to 450°C at 5°C / min and holding for 10 min; then heating to 470°C at 1°C / min and holding for 120 min; finally cooling to 250°C at 1°C / min and then furnace-cooled to below 50°C before being removed from the furnace. All other steps were identical to those in Example 1. Three sample tubes obtained after treatment under these conditions were tested, and the CSR values ​​at the head, middle, and tail were found to be 1.62, 1.58, and 1.64, respectively.

[0055] Example 3

[0056] The annealing process was adjusted based on Example 1, with the following differences: The annealing parameters were as follows: heating to 350°C at 5°C / min and holding for 30 min; then heating to 490°C at 5°C / min and holding for 10 min; then heating to 510°C at 1°C / min and holding for 100 min; finally cooling to 350°C at 1°C / min and then furnace-cooled to below 50°C before being removed from the furnace. All other steps were identical to those in Example 1. Three sample tubes obtained after treatment under these conditions were tested, and the CSR values ​​at the head, middle, and tail were found to be 1.57, 1.54, and 1.55, respectively.

[0057] Example 4

[0058] The annealing process was adjusted based on Example 1, with the following differences: The annealing parameters were as follows: heating to 350°C at 5°C / min and holding for 30 min; then heating to 530°C at 5°C / min and holding for 10 min; then heating to 550°C at 1°C / min and holding for 100 min; finally cooling to 350°C at 1°C / min and then furnace-cooled to below 50°C before being removed from the furnace. All other steps were identical to those in Example 1. Three sample tubes obtained after treatment under these conditions were tested, and the CSR values ​​at the head, middle, and tail were found to be 1.44, 1.41, and 1.45, respectively.

[0059] Comparative Example 1

[0060] The experiment was adjusted based on Example 1, with the difference being the different annealing process parameters. Specifically, the parameters were as follows: heating to 420°C at a rate of 5°C / min, holding at that temperature for 120 minutes, and then cooling in the furnace to below 50°C before removal from the furnace. Three sample tubes obtained under these conditions were tested, and the CSR values ​​at the head, middle, and tail were found to be 2.50, 2.47, and 2.54, respectively.

[0061] Comparative Example 2

[0062] The experiment was adjusted based on Example 1, with the difference being the different annealing process parameters. Specifically, the parameters were as follows: heating to 470°C at a rate of 5°C / min, holding at that temperature for 120 minutes, and then cooling in the furnace to below 50°C before removal from the furnace. Three sample tubes obtained under these conditions were tested, and the CSR values ​​at the head, middle, and tail were found to be 2.25, 2.29, and 2.27, respectively.

[0063] Comparative Example 3

[0064] The method was adjusted based on Example 1, with the difference being the different annealing process parameters. Specifically, the heating was carried out at 5℃ / min to 510℃, held for 100min, and then the heating process was terminated. The tubes were then cooled in the furnace to below 50℃ before being removed from the furnace. Three sample tubes obtained under these conditions were tested, and the CSR values ​​at the head, middle, and tail were found to be 2.16, 2.12, and 2.18, respectively.

[0065] Comparative Example 4

[0066] The experiment was adjusted based on Example 1, with the difference being the different annealing process parameters. Specifically, the parameters were as follows: heating to 550°C at a rate of 5°C / min, holding at that temperature for 100min, and then cooling in the furnace to below 50°C before removal from the furnace. Three sample tubes obtained under these conditions were tested, and the CSR values ​​at the head, middle, and tail were found to be 2.09, 2.07, and 2.10, respectively.

[0067] The results from the examples and comparative examples show that:

[0068] (1) The embodiments adopted the segmented temperature and speed controlled incomplete annealing process described in this application, while the comparative examples adopted the traditional single-temperature direct heating and holding annealing process. By detecting the shrinkage strain ratio (CSR) values ​​of three different parts of the pipe (head, middle, and tail), the technical effect of the technical solution of this application was systematically verified. Under the condition that the maximum annealing temperature is completely consistent, the pipe treated by the segmented annealing process of this application showed a significant reduction in CSR value compared with the pipe treated by the traditional single-temperature annealing process, and the reduction was far greater than the expected effect that could be achieved by simply extending the holding time. Specifically, when the maximum annealing temperature was 420°C, the CSR value of Example 1 was stable between 1.75 and 1.80, while the CSR value of Comparative Example 1 was as high as 2.47 to 2.54; when the maximum annealing temperature was increased to 550°C, the CSR value of Example 4 was controlled between 1.41 and 1.45, while the CSR value of Comparative Example 4 remained in the high range of 2.07 to 2.10.

[0069] (2) As the maximum annealing temperature gradually increases from 420°C to 550°C, the CSR value of the embodiments of this application shows a steady linear decreasing trend. The CSR values ​​of all embodiments are stable within the ideal forming performance range of 1.4 to 2.0. The CSR value can be continuously and accurately controlled by adjusting the maximum annealing temperature. Although the CSR values ​​of the traditional process comparative examples show a similar decreasing trend as the annealing temperature increases, their overall values ​​are significantly higher than those of the embodiments of this application. Moreover, the CSR values ​​of all comparative examples are higher than 2.0, which cannot reach the lower limit of the ideal forming performance range and cannot meet the process requirements of subsequent precision bending forming of the pipe. Meanwhile, the CSR values ​​of the pipes processed by the segmented annealing process of this application fluctuate very little at different parts of the head, middle and tail, indicating that the internal stress distribution and microstructure of the pipes along the axial direction are highly uniform. Since the process of this application is controlled only by adjusting the temperature-time curve of the finished product annealing, it does not need to change the key parameters such as the mill working speed, feed rate and deformation per pass of the preceding cold rolling process. Moreover, the control effect is not significantly affected by the early cold deformation history and specifications of the pipes. Therefore, it can effectively solve the problem of poor batch-to-batch reproducibility of CSR values ​​in traditional processes and is suitable for TA18 titanium alloy finished pipes of different production batches and specifications.

[0070] (3) The above-mentioned technical effects of this application are achieved by the coordinated and precise control of the internal stress elimination process and the microstructure evolution process of the segmented annealing process. The low-temperature preheating and preliminary internal stress elimination stage releases the low-energy residual internal stress generated by multiple cold rolling and avoids the premature uneven recovery of local microstructure, which creates a good foundation for the temperature homogenization of the intermediate temperature transition stage. The uniform temperature distribution of the pipe as a whole achieved in the intermediate temperature transition stage ensures the synchronicity of the microstructure evolution of the inner and outer layers in the main recovery stage, so that the internal stress elimination and microstructure recovery can be carried out uniformly in the entire pipe wall thickness direction and axial direction. The main recovery stage, through slow heating and precise heat preservation at 1℃ / min, fully completes the dislocation annihilation and rearrangement process without exceeding the incomplete annealing temperature limit or triggering recrystallization to cause excessive grain growth, thus achieving effective elimination of residual internal stress and uniform recovery of microstructure. The subsequent segmented speed-controlled cooling stage, through the control of the cooling rate from slow to fast, avoids the generation of new residual internal stress due to the lag in heat conduction between the inner and outer layers of the pipe during the cooling process, further consolidating the control effect of the previous stage. The aforementioned process stages are interconnected, sequentially transmitted, and synergistically work together to achieve a significant reduction in CSR values, stable and controllable performance, and stable batch-to-batch reproducibility.

[0071] In summary, this application effectively solves the technical problems of traditional single-temperature annealing processes in the prior art, which cannot finely control the degree of stress relief and microstructure recovery process of TA18 titanium alloy tubes, resulting in high shrinkage strain ratios, large fluctuations, and abnormally high or low values, as well as poor batch-to-batch reproducibility. It can stably control the shrinkage strain ratio of the tubes within the ideal forming performance range of 1.4 to 2.0, significantly improving the forming quality and production consistency of TA18 titanium alloy tubes.

[0072] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of this application without departing from the spirit and scope of this application should be covered within the scope of the claims of this application.

Claims

1. A method for improving the shrinkage strain ratio of TA18 titanium alloy tubing, characterized in that, The specific steps are as follows: Step 1: Prepare TA18 titanium alloy tubing; Step 2: Perform vacuum heat treatment on the tubes prepared in Step 1. The steps of the vacuum heat treatment are as follows: The temperature was increased to 250℃~550℃ using a heating rate of 1℃ / min~5℃ / min, held for 130min~150min, and then cooled to room temperature to complete the vacuum heat treatment.

2. The method according to claim 1, characterized in that, The preparation process for step 1 is as follows: Step a. Melting and casting ingots → Step b. Forging bars → Step c. Extruding tubes → Step d. Vacuum annealing → Step e. Rolling to target dimensions → Step f. Surface treatment → Step g. Vacuum annealing.

3. The method according to claim 2, characterized in that, Repeat steps d, e, and f 6 to 10 times.

4. The method according to claim 3, characterized in that, In step d, the annealing temperature is 550℃~850℃, and the holding time is 1h~3h.

5. The method according to claim 3, characterized in that, In step e, the pipe feed rate is not less than 60% of the mill feed rate design value. The mill adopts a periodic reciprocating rolling process with a working speed of 65 times / min to 85 times / min.

6. The method according to claim 3, characterized in that, In step f, the pipe is subjected to surface treatments such as sandblasting, external polishing, degreasing, and pickling, with a treatment accuracy of 0.01mm~0.02mm, to remove surface defects of the pipe.

7. The method according to claim 1, characterized in that, In step 2, the temperature is heated to 250~350℃ at 5℃ / min and held at this temperature for 20min~30min; then heated to 400℃~530℃ at 5℃ / min and held at this temperature for 10min; then heated to 420℃~550℃ at 1℃ / min and held at this temperature for 100min~120min; finally, the temperature is cooled to 250℃~350℃ at a cooling rate of 1℃ / min, and then further cooled to below 50℃ at a cooling rate of 1℃ / min~3℃ / min before being removed from the furnace.

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