A rolling and drawing combined forming method for titanium alloy small-diameter thick-wall pipes

Abstract

The application discloses a rolling-drawing combined forming method for a titanium alloy small-diameter thick-wall pipe in the technical field of metal pipe processing, and aims at the technical problems of poor size uniformity of the titanium alloy small-diameter thick-wall pipe, defects such as easy folding and scratching of an inner wall, and low flaw detection qualified rate in the prior art, and provides a rolling-drawing-rolling combined forming system integration method, realizes synchronous optimization of pipe size precision, surface quality and internal organization through a complete process chain of vertical extrusion blanking, surface treatment and finishing forming, annealing straightening and machining finishing, composite coating protection and multi-pass drawing, precision finishing and multi-pass rolling annealing and finished product flaw detection treatment. The method adopts a composite lubrication system combining copper plating and MoS2 solid coating, cooperates with coupling regulation and control of deformation and Q value and a differential interpass annealing system, effectively inhibits the generation and inheritance of inner wall defects in the forming process, and significantly improves production efficiency and product quality stability.

Description

Technical Field

[0001] This invention relates to the field of metal pipe processing technology, and in particular to a rolling and drawing combined forming method for small-diameter thick-walled titanium alloy pipes. Background Technology

[0002] Seamless titanium alloy tubes, with their high specific strength, corrosion resistance, and fatigue resistance, have become key materials for core pressure-bearing pipelines in high-end equipment, finding widespread application in nuclear power, aerospace, and marine equipment. Increasing the tube wall thickness is a common design solution to further enhance the pressure-bearing capacity and corrosion resistance of the pipelines. However, for small-diameter, thick-walled titanium alloy tubes, the small radius of curvature and low hardness of the inner wall make them prone to defects such as folds and scratches. As the number of deformation passes increases, these inner wall defects are inherited and worsen, resulting in a low ultrasonic testing pass rate for such tubes.

[0003] To address the aforementioned shortcomings, traditional processes employ multiple small-deformation passes combined with finishing treatments. For example, patent document CN115647106B discloses a method for producing small-diameter, thick-walled seamless titanium alloy tubes. This method uses inter-pass metallographic inspection combined with internal honing and pickling to remove defects, thus producing small-diameter, thick-walled tubes. However, this process has low production efficiency, and end-metallographic sampling cannot accurately assess the defect situation of a batch of products, easily leading to the scrapping of the entire batch of tubes. Patent document CN121156686B discloses a method for producing small-diameter, thick-walled near-α-type titanium alloy tubes and its application, proposing the use of coated strip warm rolling to avoid tube rolling cracks. However, high-temperature deformation leads to poorer dimensional stability of the tubes, increasing the difficulty of production quality control. Summary of the Invention

[0004] To overcome the above-mentioned shortcomings of existing small-diameter thick-walled titanium alloy tube production methods, the technical problem to be solved by the present invention is to provide a rolling and drawing combined forming method for small-diameter thick-walled titanium alloy tubes that can simultaneously optimize tube dimensional accuracy, surface quality and internal structure, and improve the tube flaw detection pass rate.

[0005] The technical solution adopted by this invention to solve its technical problem is: A method for forming a small-diameter thick-walled titanium alloy tube by a combination of rolling and drawing includes the following steps: S1. Vertical extrusion billet forming: The titanium alloy billet is hot extruded using a vertical extruder. The preheating temperature of the billet is controlled to be 50°C below the phase transformation point. The extrusion cylinder and die are preheated to 300°C. The extrusion speed is 50-70 mm / s. S2. Surface treatment and finishing: The extruded rough tube is degreased and pickled in sequence, and then finished in one pass with small deformation using a two-roll Pilger mill. The deformation amount is 20%-30% and the Q value is ≤0.9. S3. Annealing, straightening and machining finishing: Vacuum annealing, straightening and machining are performed on the finished pipes; S4. Composite coating protection and multi-pass drawing: After degreasing and pickling, the tube blank is plated with copper with a thickness of 15-20μm. Then, it is treated with MoS2 to form a solid coating. It is then drawn in 3-4 passes using a roll die, with a deformation of 20%-30% per pass and a Q value ≤0.9. Online short-time annealing is performed between passes. S5. Precision finishing and multi-pass rolling annealing: After finishing the drawn tube, it undergoes multi-pass cold rolling and intermediate annealing. S6. Finished Product Inspection and Treatment: After annealing, the finished pipes are straightened, followed by inspection and water pressure testing. Qualified pipes are then pickled and cut to length.

[0006] Furthermore, in step S1, the titanium alloy ingot is first rolled into a bar that meets the specifications of the vertical extrusion press. Then, the bar is drilled and used as an extrusion billet. During extrusion, the outer diameter tolerance of the extrusion tube is controlled to be ≤ ±0.3 mm and the wall thickness tolerance is controlled to be ≤ ±0.5 mm.

[0007] Furthermore, in step S2, after removing the extruded copper scale and oxide scale by degreasing and pickling, the tube is washed with water and dried. During the finishing rolling process, the outer diameter tolerance of the rolled tube is controlled to be ≤ ±0.1 mm and the wall thickness tolerance is controlled to be ≤ ±0.3 mm.

[0008] Furthermore, in step S3, the heating rate of vacuum annealing is 10℃ / min, the annealing temperature is 700-750℃, the holding time is 120min, and after furnace cooling to 300℃, argon gas is introduced to cool to room temperature; a vertical straightener is used for straightening, and the straightness is controlled to be ≤1mm / m. Subsequently, external turning and internal boring are performed, and the outer diameter tolerance is controlled to be ≤±0.05mm, the wall thickness tolerance is controlled to be ≤±0.2mm, and the surface roughness Ra of the inner and outer surfaces of the pipe is guaranteed to be ≤1.6μm.

[0009] Furthermore, in step S4, copper is plated on the surface of the tube blank using an electroplating process, followed by immersion in an aqueous MoS2 solvent to form a MoS2 solid coating with a thickness of 5-8 μm. Before drawing with a tube drawing machine, the inner wall of the tube is coated with a mixture of rolling oil and MoS2 lubricant. The annealing atmosphere between drawing passes is argon, the annealing temperature is 400-500℃, and the holding time is 10 min. The outer diameter tolerance of the drawn tube is controlled to be ≤ ±0.04 mm, and the wall thickness tolerance is controlled to be ≤ ±0.15 mm.

[0010] Furthermore, in step S5, the drawn tube is first subjected to external polishing with a sanding belt and internal grinding with a diamond grinding head, with external polishing of 0.05mm and internal grinding of 0.05mm, so that the surface roughness Ra of the inner and outer surfaces is ≤0.4μm, the coating on the tube surface is removed, and then degreasing and pickling are performed to obtain a tube with a metallic luster.

[0011] Furthermore, in step S5, the pipe is pickled twice. The first pickling involves soaking the pipe in a solution with a mass percentage of sulfuric acid:nitric acid:deionized water = 2:1:7 for 5 minutes. The second pickling involves soaking the pipe in a solution with a mass percentage of nitric acid:hydrofluoric acid:deionized water = 30:3:67 for 3 minutes, followed by an internal flow pickling for 2 minutes.

[0012] Furthermore, in step S5, before the pipe is subjected to multiple cold rolling passes, the pickled pipe is first subjected to recrystallization annealing. The annealing environment is a vacuum, the temperature is 700-750℃, and the holding time is 60 minutes.

[0013] Furthermore, in step S5, four passes of cold rolling are performed. In the first pass, the cold rolling deformation is 55%-65%, and the Q value is controlled at 1.0-1.2. After cold rolling, degreasing, and pickling, stress-relief annealing is performed in a vacuum environment at a temperature of 500-600℃ for 30 minutes. The second and third passes have the same cold rolling parameters, with a deformation of 40%-45% and a Q value controlled at 1.0-1.2. After cold rolling, degreasing, and pickling, incomplete recrystallization annealing is performed in an argon atmosphere at a temperature of 500-600℃ for 30 minutes. In the fourth pass, the cold rolling deformation is 40%-45%, and the Q value is controlled at 1.2-1.4. After cold rolling, degreasing, and pickling, stress-relief annealing is performed in an argon atmosphere at a temperature of 500-600℃ for 30 minutes.

[0014] The beneficial effects of this invention are: through a complete process chain of vertical extrusion billet preparation, surface treatment and finishing, annealing and straightening and machining finishing, composite coating protection and multi-pass drawing, precision finishing and multi-pass rolling annealing, and finished product flaw detection and treatment, this invention achieves simultaneous optimization of the dimensional accuracy, surface quality and internal structure of titanium alloy small-diameter thick-walled tubes, and significantly improves the tube flaw detection pass rate. Among these advancements, the deep synergy between vertical extrusion billet preparation and rolling-drawing-rolling composite forming processes significantly improved the uniformity of metal flow and the dimensional accuracy of the tubes, suppressing the generation of internal wall defects from the source. Through the synergy of composite coating protection and multi-pass lubrication, a multi-layer composite lubrication system covering the inner and outer surfaces of the tubes was constructed, significantly reducing the friction coefficient during the drawing process and preventing the generation and deterioration of defects such as internal wall folds and scratches. In the rolling process, the coupling control of deformation amount and Q value, combined with differentiated inter-pass annealing regimes, achieved orderly stress release and fine control of microstructure, effectively eliminating processing stress and causing recrystallization of the tubes without significant grain growth, thereby reducing cold rolling defects and improving the internal microstructure uniformity and flaw detection pass rate of the finished tubes. Attached Figure Description

[0015] Figure 1 This is a flaw detection curve diagram of the pipe material according to an embodiment of the present invention; Figure 2 These are illustrations of inner surface defects of tubes from different cold-rolling passes in the comparative examples of this invention; Figure 3 This is a comparative example of the flaw detection curves for pipes according to the present invention. Detailed Implementation

[0016] The present invention will be further described below with reference to the accompanying drawings, embodiments, and comparative examples.

[0017] The present invention provides a rolling and drawing combined forming method for small-diameter thick-walled titanium alloy tubes, comprising the following steps: S1. Vertical extrusion billet forming: The titanium alloy billet is hot extruded using a vertical extruder. The preheating temperature of the billet is controlled to be 50°C below the phase transformation point. The extrusion cylinder and die are preheated to 300°C. The extrusion speed is 50-70 mm / s. S2. Surface treatment and finishing: The extruded rough tube is degreased and pickled in sequence, and then finished in one pass with small deformation using a two-roll Pilger mill. The deformation amount is 20%-30% and the Q value is ≤0.9. S3. Annealing, straightening and machining finishing: Vacuum annealing, straightening and machining are performed on the finished pipes; S4. Composite coating protection and multi-pass drawing: After degreasing and pickling, the tube blank is plated with copper with a thickness of 15-20μm. Then, it is treated with MoS2 to form a solid coating. It is then drawn in 3-4 passes using a roll die, with a deformation of 20%-30% per pass and a Q value ≤0.9. Online short-time annealing is performed between passes. S5. Precision finishing and multi-pass rolling annealing: After finishing the drawn tube, it undergoes multi-pass cold rolling and intermediate annealing. S6. Finished Product Inspection and Treatment: After annealing, the finished pipes are straightened, followed by inspection and water pressure testing. Qualified pipes are then pickled and cut to length.

[0018] In step S1, the titanium alloy ingot is first rolled into a bar that meets the specifications of the vertical extrusion press. Then, the bar is drilled and used as the extrusion billet. During extrusion, the outer diameter tolerance of the extrusion tube is controlled to be ≤±0.3mm, and the wall thickness tolerance is controlled to be ≤±0.5mm. Compared with horizontal extrusion, vertical extrusion results in more uniform metal flow and smaller wall thickness differences in the extrusion tube, providing a high-quality billet base for subsequent cold working.

[0019] In step S2, after degreasing and pickling to remove the extruded copper scale and oxide scale, the tube is washed with water and dried. During finish rolling, the outer diameter tolerance of the rolled tube is controlled to be ≤ ±0.1 mm, and the wall thickness tolerance is controlled to be ≤ ±0.3 mm. This step, through small deformation finishing, effectively improves the dimensional deviation of the extruded rough tube, providing a tube blank with uniform dimensions for subsequent machining and drawing.

[0020] In step S3, the vacuum annealing heating rate is 10℃ / min, the annealing temperature is 700-750℃, the holding time is 120min, and after furnace cooling to 300℃, argon gas is introduced for cooling to room temperature to ensure complete recrystallization of the tube and eliminate work hardening and internal stress generated during extrusion and finishing. Subsequently, a 10-roll vertical straightener is used for straightening, controlling the straightness to ≤1mm / m. Then, external turning and internal boring are performed, controlling the outer diameter tolerance to ≤±0.05mm and the wall thickness tolerance to ≤±0.2mm, ensuring the inner and outer surface roughness Ra of the tube is ≤1.6μm. This step, through the synergy of annealing softening and machining finishing, removes residual oxides and surface defects from extrusion, further improving the dimensional accuracy and surface quality of the tube blank.

[0021] In step S4, after degreasing and pickling, copper is electroplated onto the surface of the tube blank. It is then immersed in an aqueous MoS2 solvent for 10 minutes and air-dried at room temperature, forming a 5-8 μm thick MoS2 solid coating on both the inner and outer surfaces of the tube blank. Before drawing with a tube drawing machine, a mixture of rolling oil and MoS2 lubricant is applied to the inner wall of the tube. The annealing atmosphere between drawing passes is argon, the annealing temperature is 400-500℃, and the holding time is 10 minutes. This annealing process effectively removes cold drawing stress while avoiding oxidation of the composite coating and Ti-Cu reaction, and induces a small amount of recrystallization in the tube. This controls the outer diameter tolerance of the drawn tube to be ≤±0.04 mm and the wall thickness tolerance to be ≤±0.15 mm. The synergistic effect of the composite coating protection and multi-pass lubrication significantly reduces frictional resistance during the drawing process and suppresses defects such as inner wall folds and scratches.

[0022] In step S5, the drawn tube is first subjected to external polishing with a sanding belt and internal grinding with a diamond grinding head, with an external polishing thickness of 0.05 mm and an internal grinding thickness of 0.05 mm, so that the surface roughness Ra is ≤0.4 μm, thoroughly removing the coating on the tube surface. Subsequently, degreasing and two pickling processes are performed to obtain a tube with a metallic luster. The first pickling uses a solution with a mass percentage of sulfuric acid:nitric acid:deionized water = 2:1:7, immersing the tube for 5 minutes. The second pickling uses a solution with a mass percentage of nitric acid:hydrofluoric acid:deionized water = 30:3:67, immersing the tube for 3 minutes, followed by internal flow pickling for 2 minutes. The two pickling processes, through the synergistic effect of different acid ratios, effectively remove the surface oxide layer and residual coating, ensuring the surface quality for subsequent cold rolling. Before multi-pass cold rolling, the pickled tube undergoes recrystallization annealing in a vacuum environment at a temperature of 700-750℃ for 60 minutes, allowing for sufficient recrystallization and providing the microstructure basis for subsequent multi-pass cold rolling.

[0023] Four passes of cold rolling were then performed, with the following parameters and annealing regime for each pass: The first pass involved a cold rolling deformation of 55%-65%, with a Q value controlled at 1.0-1.2. After cold rolling, degreasing, and pickling, stress-relief annealing was performed in a vacuum environment at 500-600℃ for 30 minutes. The second and third passes had the same cold rolling parameters, with a deformation of 40%-45% and a Q value controlled at 1.0-1.2. After cold rolling, degreasing, and pickling, incomplete recrystallization annealing was performed in an argon atmosphere at 500-600℃ for 30 minutes. The fourth pass involved a cold rolling deformation of 40%-45%, with a Q value controlled at 1.2-1.4. After cold rolling, degreasing, and pickling, stress-relief annealing was performed in an argon atmosphere at 500-600℃ for 30 minutes. The intermediate cold rolling pass uses short-time low-temperature annealing, which effectively removes the cold rolling stress of the tube and causes recrystallization of the tube without significant grain growth, thereby reducing cold rolling defects and improving the uniformity of the internal structure of the tube.

[0024] The present invention will be further described below with reference to embodiments and comparative examples.

[0025] Example: A TA16 titanium alloy bar with a length of 500 mm and a diameter of φ220 mm was selected. After hollowing out the center, it was held at 900℃ for 2 hours and then extruded using a vertical extrusion press to obtain a φ81×17 mm black-skinned tube. The extruded tube was then degreased and pickled to remove the surface copper and oxide scale, resulting in a cold-rolled tube with dimensions close to φ80×16 mm. Following the conditions shown in Table 1, a combined rolling-drawing-rolling process was performed to finally produce a φ10×2 mm finished tube. The flaw detection results are as follows: Figure 1 As shown, the flaw detection curve of the finished pipe prepared in this embodiment is stable and no obvious defect echo appears, indicating that the internal structure of the pipe is uniform and there are no obvious defects, and the flaw detection pass rate reaches 92%.

[0026] Table 1. Transformation Pass Allocation in Embodiments

[0027] Comparative example: To verify the technical effectiveness of the process of this invention, a comparative experiment was conducted using a traditional process. A TA16 titanium alloy bar with a length of 500mm and a diameter of φ220mm was selected. After hollowing out the center, it was held at 900℃ for 2 hours and then extruded using a horizontal extrusion press to obtain a φ82×17mm black tube. After external turning and internal boring, an 80×14mm cold-rolled rough tube was obtained. This rough tube was then subjected to multi-pass rolling according to the conditions shown in Table 2 to prepare a φ10×2mm finished tube. Figure 2 and Figure 3As shown in the comparative example, two passes of cold rolling resulted in obvious scratches on the inner surface of the tube, with multiple abnormal echoes appearing in the flaw detection curve, and a flaw detection pass rate of only 15%. The comparative results demonstrate that this invention, through the synergistic effect of vertical extrusion billet preparation, rolling-drawing-rolling composite forming process, composite coating protection, and the coupled control of deformation amount and Q value, effectively suppresses the generation and inheritance of inner wall defects, significantly improving the flaw detection pass rate of small-diameter thick-walled titanium alloy tubes.

[0028] Table 2. Proportional Deformation Pass Allocation

Claims

1. A method of forming a titanium alloy small-diameter thick-wall pipe by a combination of rolling and drawing, characterized by, Includes the following steps: S1. Vertical extrusion billet forming: The titanium alloy billet is hot extruded using a vertical extruder. The preheating temperature of the billet is controlled to be 50°C below the phase transformation point. The extrusion cylinder and die are preheated to 300°C. The extrusion speed is 50-70 mm / s. S2. Surface treatment and finishing: The extruded rough tube is degreased and pickled in sequence, and then finished in one pass with small deformation using a two-roll Pilger mill. The deformation amount is 20%-30% and the Q value is ≤0.

9. S3. Annealing, straightening and machining finishing: Vacuum annealing, straightening and machining are performed on the finished pipes; S4. Composite coating protection and multi-pass drawing: After degreasing and pickling, the tube blank is plated with copper with a thickness of 15-20μm. Then, it is treated with MoS2 to form a solid coating. It is then drawn in 3-4 passes using a roll die, with a deformation of 20%-30% per pass and a Q value ≤0.

9. Online short-time annealing is performed between passes. S5. Precision finishing and multi-pass rolling annealing: After finishing the drawn tube, it undergoes multi-pass cold rolling and intermediate annealing. S6. Finished Product Inspection and Treatment: After annealing, the finished pipes are straightened, followed by inspection and water pressure testing. Qualified pipes are then pickled and cut to length.

2. The rolling and drawing combined forming method for small-diameter thick-walled titanium alloy tubes as described in claim 1, characterized in that, In step S1, the titanium alloy ingot is first rolled into a bar that meets the specifications of the vertical extrusion press. Then, the bar is drilled and used as the extrusion billet. During extrusion, the outer diameter tolerance of the extrusion tube is controlled to be ≤ ±0.3 mm and the wall thickness tolerance is controlled to be ≤ ±0.5 mm.

3. The rolling and drawing combined forming method for small-diameter thick-walled titanium alloy tubes as described in claim 1, characterized in that, In step S2, after degreasing and pickling to remove the extruded copper scale and oxide scale, the tube is washed with water and dried. During the finishing rolling, the outer diameter tolerance of the rolled tube is controlled to be ≤ ±0.1 mm and the wall thickness tolerance is controlled to be ≤ ±0.3 mm.

4. The rolling and drawing combined forming method for small-diameter thick-walled titanium alloy tubes as described in claim 1, characterized in that, In step S3, the heating rate of vacuum annealing is 10℃ / min, the annealing temperature is 700-750℃, the holding time is 120min, and after furnace cooling to 300℃, argon gas is introduced to cool to room temperature; a vertical straightener is used for straightening, and the straightness is controlled to be ≤1mm / m. Then, external turning and internal boring are performed, and the outer diameter tolerance is controlled to be ≤±0.05mm, the wall thickness tolerance is controlled to be ≤±0.2mm, and the surface roughness Ra of the inner and outer surfaces of the pipe is guaranteed to be ≤1.6μm.

5. The rolling and drawing combined forming method for small-diameter thick-walled titanium alloy tubes as described in claim 1, characterized in that, In step S4, copper is plated on the surface of the tube blank using an electroplating process. Then, a MoS2 solid coating with a thickness of 5-8 μm is formed by immersion in an aqueous MoS2 solvent. Before drawing with a tube drawing machine, the inner wall of the tube is coated with a mixture of rolling oil and MoS2 lubricant. The annealing atmosphere between drawing passes is argon, the annealing temperature is 400-500℃, and the holding time is 10 min. The outer diameter tolerance of the drawn tube is controlled to be ≤ ±0.04 mm, and the wall thickness tolerance is controlled to be ≤ ±0.15 mm.

6. The rolling and drawing combined forming method for a small-diameter thick-walled titanium alloy tube as described in claim 1, characterized in that, In step S5, the drawn tube is first polished with a sanding belt and ground with a diamond grinding head. The external polishing is 0.05 mm and the internal grinding is 0.05 mm, so that the surface roughness Ra of the inner and outer surfaces is ≤0.4 μm. The coating on the tube surface is removed, followed by degreasing and pickling to obtain a tube with a metallic luster.

7. The rolling and drawing combined forming method for a small-diameter thick-walled titanium alloy tube as described in claim 6, characterized in that, The pipes are pickled twice. The first pickling is done by soaking in a solution with a mass percentage of sulfuric acid:nitric acid:deionized water = 2:1:7 for 5 minutes. The second pickling is done by soaking in a solution with a mass percentage of nitric acid:hydrofluoric acid:deionized water = 30:3:67 for 3 minutes, followed by an internal flow pickling for 2 minutes.

8. The rolling and drawing combined forming method for a small-diameter thick-walled titanium alloy tube as described in claim 1, characterized in that, In step S5, before the pipe is subjected to multiple cold rolling passes, the pickled pipe is first subjected to recrystallization annealing. The annealing environment is a vacuum, the temperature is 700-750℃, and the holding time is 60 minutes.

9. A method for forming a small-diameter thick-walled titanium alloy tube by a combination of rolling and drawing as described in claim 1, characterized in that, In step S5, four passes of cold rolling are performed. In the first pass, the deformation is 55%-65%, and the Q value is controlled at 1.0-1.

2. After cold rolling, degreasing, and pickling, stress-relief annealing is performed in a vacuum environment at a temperature of 500-600℃ for 30 minutes. The second and third passes have the same cold rolling parameters, with a deformation of 40%-45% and a Q value controlled at 1.0-1.

2. After cold rolling, degreasing, and pickling, incomplete recrystallization annealing is performed in an argon atmosphere at a temperature of 500-600℃ for 30 minutes. In the fourth pass, the deformation is 40%-45%, and the Q value is controlled at 1.2-1.

4. After cold rolling, degreasing, and pickling, stress-relief annealing is performed in an argon atmosphere at a temperature of 500-600℃ for 30 minutes.

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