A rolling method for improving the microstructure uniformity and mechanical properties of large-diameter seamless pipes of BT14 titanium alloy

Abstract

The application belongs to the technical field of titanium alloy material processing, and discloses a rolling method for improving the microstructure uniformity and mechanical properties of a BT14 titanium alloy large-size seamless pipe. In view of the problems of non-uniform microstructure and mismatch of strength and toughness of the existing pipe in the periodic rolling process caused by the temperature and strain differences of each part of the blank, the rolling process of the traditional titanium alloy pipe is optimized and designed, the rolling temperature, deformation rate, deformation amount and other parameters are optimized to improve the uniformity of the microstructure of the head and tail of the pipe, and the recrystallization process is used to refine the microstructure, so that the BT14 titanium alloy large-size seamless pipe with improved microstructure uniformity and mechanical properties is prepared, so as to better meet the application requirements in the field of ocean engineering.

Description

Technical Field

[0001] The present invention belongs to the technical field of titanium alloy material processing, and in particular relates to a rolling method for improving the structural uniformity and mechanical properties of large-scale seamless BT14 titanium alloy pipes. Background Art

[0002] Titanium alloy pipes possess high strength and low density, effectively resisting seawater erosion and corrosion while maintaining structural stability. They are also cryogenically resistant, have a low coefficient of expansion, and are non-magnetic, making them suitable for the manufacture of marine engineering equipment operating in low-temperature and complex sea conditions. Furthermore, their environmental friendliness and long-term economic benefits have led to their widespread application in ship piping, gas cylinders, seawater desalination, and offshore platforms, enhancing the performance and lifespan of marine equipment.

[0003] The nominal composition of BT14 titanium alloy is Ti-4.5Al-3Mo-1V. It is a heat-treatable α+β titanium alloy developed by the Soviet Institute of Aviation Materials in the late 1950s. The titanium alloy with similar composition in the United States is Ti-431 (nominal composition Ti-4Al-3Mo-1V), and the corresponding domestic grade is TC30 (nominal composition Ti-5Al-3Mo-1V). The strength of this alloy in the annealed state can reach 850MPa, and the impact toughness (KV2) at -60℃ is higher than 45J. After strengthening heat treatment, the strength can reach 1100MPa. Therefore, BT14 titanium alloy is suitable for manufacturing structural parts, welded parts and pressure tanks that work in the range of -60 to 500℃.

[0004] The titanium alloy seamless tube prepared by the cyclic rolling process not only has high dimensional accuracy, but also can obtain good surface quality, which is conducive to reducing subsequent processing losses. Since the rolling resistance of titanium alloy increases sharply with the decrease of temperature, and the high temperature will cause the mechanical properties of titanium alloy to deteriorate. Therefore, in order to avoid the "rolling jam" phenomenon caused by insufficient rolling power during the rolling process and to ensure the performance of the finished pipe, the current pipe rolling process usually sets the rolling temperature 20 to 40 ° C above the phase transformation. However, considering that the microstructure of titanium alloy is very sensitive to the hot working process parameters, the duration from the tube billet being taken out of the furnace to the end of the cyclic rolling process is long, and it is inevitable to produce a large temperature difference (up to 50 to 100 ° C) at the head and tail of the tube billet. As a result, the head of the finished titanium alloy tube is a Widmanstätten structure, while the tail is a dual or triple structure. The difference in mechanical properties of these two organizations causes the uniformity of the mechanical properties of the finished tube to be poor.

[0005] Research has shown that the strength-ductility match between duplex and equiaxed structures in titanium alloys is superior to that of basketweave structures, and they also exhibit better high-cycle fatigue performance than basketweave structures. During the rolling process of titanium alloys, the basketweave structure generally forms when heating or deformation begins above the phase transition point; the duplex structure forms when the deformation temperature is slightly below the phase transition point, while the equiaxed structure forms when the deformation temperature is significantly below the phase transition point. Therefore, compared to the basketweave structure, the hot working processes that produce the duplex and equiaxed structures have a wider "temperature window." Furthermore, due to the large deformation during the cyclic rolling process of tubes, the thermal effect temperature rise caused by the deformation (typically reaching approximately 40-60°C) can effectively compensate for the temperature loss during rolling, which can, to a certain extent, improve the uniformity of the microstructure and properties at the head and tail of the tube. Furthermore, BT14 titanium alloy undergoes dynamic recrystallization at deformation temperatures above 600°C, and the degree of recrystallization of the α and β phases gradually increases with decreasing strain rate. This characteristic can be exploited to further improve the microstructure and properties of the tube by manipulating parameters such as the deformation amount and deformation rate during the tube forming process. Summary of the Invention

[0006] In view of this, in order to solve the above-mentioned problems in the existing technology, it is necessary to optimize the rolling process of traditional titanium alloy pipes, improve the uniformity of the microstructure of the head and tail of the pipe by optimizing parameters such as rolling temperature, deformation rate, and deformation amount, and use the recrystallization process to achieve the refinement of the microstructure, thereby achieving the improvement of the mechanical properties of large-size BT14 titanium alloy pipes to better meet the application needs in the field of marine engineering.

[0007] The purpose of the present invention is to provide a rolling method for improving the uniformity of the structure and mechanical properties of large-size seamless BT14 titanium alloy pipes, which can solve the problems of uneven structure and mismatch of mechanical properties between strength and toughness caused by temperature and strain differences in various parts of the billet during the cyclic rolling process of existing pipes.

[0008] In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions.

[0009] In one aspect, the present invention provides a rolling method for improving the microstructure uniformity and mechanical properties of large-scale seamless BT14 titanium alloy pipes, comprising the following steps:

[0010] S1. Surface grinding of BT14 titanium alloy forging bar blanks is performed to remove surface defects, and a through hole of Φ40 to 80 mm is center machined on the ground bar blanks;

[0011] S2. Before loading into the furnace, the inner and outer surfaces of the rod blank machined in step S1 are coated with an anti-oxidation coating, and the rod blank machined by the grinding machine is preheated by slowly heating at a low temperature section and rapidly heating at a high temperature section. When the furnace temperature is ≤500°C, the rod blank is loaded into a resistance heating furnace meeting the requirements of GJB509A-1995 Level III, and heated to 775°C to 825°C at a heating rate of 100°C / h, and kept warm; then heated to 885°C to 935°C at a heating rate of 120°C to 160°C / h, and kept warm;

[0012] S3, using the method of diameter expansion deformation and oblique hole perforation to make the BT14 titanium alloy billet preheated in step S2 into a capillary tube with a diameter of 450-550 mm and a wall thickness of 30-60 mm;

[0013] S4, coating the inner and outer surfaces of the capillary tube obtained in step S3 with an anti-oxidation coating and performing a preheating treatment, first heating to 775°C to 825°C at a heating rate of 100°C / h and keeping warm; then heating to 885°C to 935°C at a heating rate of 120°C to 160°C / h and keeping warm, so that the capillary tube is fully heated before forming and rolling;

[0014] S5, using a periodic rolling device to periodically roll the rough pipe obtained in step S4 to obtain a rough pipe with a diameter of 350-450 mm and a wall thickness of 10-30 mm;

[0015] S6. Heat-treating the rough tube obtained in step S5. When the furnace temperature is ≤500°C, loading the rough tube into the furnace; heating the rough tube to 775°C-825°C at a heating rate of 100°C / h, and keeping the temperature; then heating the rough tube to 885°C-935°C at a heating rate of 120°C-160°C / h, keeping the temperature, and air-cooling;

[0016] S7. Use a sawing machine, a lathe and a deep hole boring machine to machine the rough pipe that has been heat treated in step S6 to prepare a large-size seamless pipe of BT14 titanium alloy.

[0017] Furthermore, in step S1, the removing of surface defects includes removing cracks, peeling, pits, and folds.

[0018] Further, in step S2, the temperature is heated to 775°C to 825°C at a heating rate of 100°C / h and then kept warm for 5 hours, and the temperature is heated to 885°C to 935°C at a heating rate of 120°C to 160°C / h and then kept warm for 2 to 4 hours.

[0019] Furthermore, in step S3, the diameter of the plug used in the oblique hole punching process is 300-400 mm, and the punching current is 3-6 kA.

[0020] Further, in step S4, the temperature is heated to 775°C to 825°C at a heating rate of 100°C / h and then kept warm for 5 hours; the temperature is heated to 885°C to 935°C at a heating rate of 120°C to 160°C / h and then kept warm for 2 to 4 hours.

[0021] Furthermore, in step S5, during the cyclic rolling process, the deformation amount is 60% to 75%, the feeding amount is 20 to 40 mm / time, and the rolling current is 5 to 9 kA.

[0022] Further, in step S6, heating to 775°C-825°C at a heating rate of 100°C / h and then keeping warm for 5 hours; heating to 885°C-935°C at a heating rate of 120°C-160°C / h and then keeping warm for 1-2 hours.

[0023] Furthermore, the anti-oxidation coating is glass powder specially used in titanium alloy rolling process.

[0024] On the other hand, the present invention also provides a large-size seamless pipe of BT14 titanium alloy, which is rolled by the above-mentioned rolling method.

[0025] Furthermore, the diameter of the BT14 titanium alloy large-size seamless pipe is 350-450 mm and the wall thickness is 10-30 mm.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The method of the present invention can obtain a finished titanium alloy seamless pipe with good strength-toughness matching and uniform microstructure in the axial direction of the pipe; improve the yield rate of titanium alloy seamless pipes, reduce energy consumption and production costs, and meet the requirements of large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a microstructure diagram of the head of the BT14 titanium alloy seamless pipe prepared in Example 1 of the present invention;

[0029] Figure 2 This is a microstructure diagram of the tail portion of the BT14 titanium alloy seamless tube prepared in Example 1 of the present invention;

[0030] Figure 3 This is a microstructure diagram of the head of the BT14 titanium alloy seamless pipe prepared in Comparative Example 1 of the present invention;

[0031] Figure 4 This is a microstructure diagram of the tail portion of the BT14 titanium alloy seamless pipe prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used in the specification of the present invention are only used to describe the purpose of specific embodiments and are not intended to limit the present invention.

[0033] In the present invention, when referring to a numerical interval (i.e., a numerical range), unless otherwise specified, the optional numerical distribution is considered continuous within the above numerical interval and includes the two numerical endpoints (i.e., the minimum and maximum values) of the numerical range, as well as each numerical value between the two numerical endpoints. Unless otherwise specified, when a numerical interval refers only to integers within the numerical interval, it includes the two endpoint integers of the numerical range, as well as each integer between the two endpoints. In this article, it is equivalent to directly listing each integer, such as t is an integer selected from 1-10, indicating that t is any integer selected from the group consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10. In addition, when multiple ranges are provided to describe features or characteristics, these ranges can be merged. In other words, unless otherwise specified, the ranges disclosed herein should be understood to include any and all subranges therein.

[0034] The temperature parameters in the present invention, unless otherwise specified, may be either constant temperature or fluctuating within a certain temperature range. It should be understood that the constant temperature treatment allows for temperature fluctuations within the precision range of the instrument control. Fluctuations within ranges such as ±5°C, ±4°C, ±3°C, ±2°C, and ±1°C are permitted.

[0035] The present invention provides a rolling method for improving the microstructure uniformity and mechanical properties of large-scale seamless BT14 titanium alloy pipes, comprising the following steps:

[0036] S1. Surface grinding of BT14 titanium alloy forging bar blanks is performed to remove surface defects, and a through hole of Φ40 to 80 mm is center machined on the ground bar blanks;

[0037] S2. Before loading into the furnace, the inner and outer surfaces of the rod blank machined in step S1 are coated with an anti-oxidation coating, and the rod blank machined by the grinding machine is preheated by slowly heating at a low temperature section and rapidly heating at a high temperature section. When the furnace temperature is ≤500°C, the rod blank is loaded into a resistance heating furnace meeting the requirements of GJB509A-1995 Level III, and heated to 775°C to 825°C at a heating rate of 100°C / h, and kept warm; then heated to 885°C to 935°C at a heating rate of 120°C to 160°C / h, and kept warm;

[0038] S3, using the method of diameter expansion deformation and oblique hole perforation to make the BT14 titanium alloy billet preheated in step S2 into a capillary tube with a diameter of 450-550 mm and a wall thickness of 30-60 mm;

[0039] S4, coating the inner and outer surfaces of the capillary tube obtained in step S3 with an anti-oxidation coating and performing a preheating treatment, first heating to 775°C to 825°C at a heating rate of 100°C / h and keeping warm; then heating to 885°C to 935°C at a heating rate of 120°C to 160°C / h and keeping warm, so that the capillary tube is fully heated before forming and rolling;

[0040] S5, using a periodic rolling device to periodically roll the rough pipe obtained in step S4 to obtain a rough pipe with a diameter of 350-450 mm and a wall thickness of 10-30 mm;

[0041] S6. Heat-treating the rough tube obtained in step S5. When the furnace temperature is ≤500°C, loading the rough tube into the furnace; heating the rough tube to 775°C-825°C at a heating rate of 100°C / h, and keeping the temperature; then heating the rough tube to 885°C-935°C at a heating rate of 120-160°C / h, keeping the temperature, and air-cooling;

[0042] S7. Use a sawing machine, a lathe and a deep hole boring machine to machine the rough pipe that has been heat treated in step S6 to prepare a large-size seamless pipe of BT14 titanium alloy.

[0043] In some embodiments, the present invention provides a rolling method for improving the microstructure uniformity and mechanical properties of large-scale seamless BT14 titanium alloy pipes, comprising the following steps:

[0044] S1. Before charging into the furnace, the inner and outer surfaces of the rod blank machined in step S1 are coated with an anti-oxidation coating, the surface of the BT14 titanium alloy forging rod blank is ground, and a through hole of 40 to 80 mm in diameter is machined in the center of the rod blank;

[0045] The surface of the forged BT14 titanium alloy forging bar is ground, including removing defects such as cracks, peeling, pits and folds; at the same time, in order to ensure the smooth progress of the piercing process, a through hole of Φ40 to 80 mm is machined in the center of the ground bar.

[0046] S2. Before loading into the furnace, the inner and outer surfaces of the rod blank machined in step S1 are coated with an anti-oxidation coating, and the rod blank machined by the grinding machine is preheated by slowly heating at a low temperature and rapidly heating at a high temperature;

[0047] Because titanium is very active, it easily reacts chemically with gases such as oxygen, hydrogen, and nitrogen, forming a hard and brittle oxygen-rich α layer on the surface of the billet. This can cause surface microcracks during deformation, affecting the quality of the pipe. Therefore, an anti-oxidation coating should be applied to the billet before heating. In addition, the thermal conductivity of BT14 titanium alloy is low at room temperature, only 8.79 W / (m·K); as the temperature rises, the thermal conductivity gradually increases; but when the temperature exceeds the phase transition point, the β grains grow rapidly, resulting in deterioration in performance. Therefore, to ensure the quality of the billet preheating, a method of slow heating in the low-temperature section and rapid heating in the high-temperature section is adopted: when the furnace temperature is ≤500°C, the billet is placed in a resistance heating furnace that meets the requirements of GJB509A-1995 Level III. Heat to 775°C~825°C at a heating rate of 100°C / h and keep warm for 5 hours; then heat to 885°C~935°C at a heating rate of 120°C~160°C, preferably 145°C~155°C / h, and keep warm for 2~4 hours.

[0048] S3, using the method of diameter expansion deformation and oblique hole perforation to make the BT14 titanium alloy billet preheated in step S2 into a capillary tube with a diameter of 450-550 mm and a wall thickness of 30-60 mm;

[0049] The diameter of the plug used in the oblique hole punching process is 300-400 mm, preferably 330-400 mm, and the punching current is 3-6 kA.

[0050] S4, coating the inner and outer surfaces of the capillary tube obtained in step S3 with an anti-oxidation coating and performing a preheating treatment;

[0051] An anti-oxidation coating is applied to the inner and outer surfaces of the shell produced in step 3. To ensure sufficient heating of the shell produced in step S3 before forming and rolling, the shell is first heated to 775°C to 825°C at a heating rate of 100°C / h after loading into the furnace and held at this temperature for 5 hours. It is then heated to 885°C to 935°C at a heating rate of 120°C to 160°C / h, preferably 145°C to 155°C / h, and held at this temperature for 2 to 4 hours. During the heating process, the shell temperature must be controlled within the specified range of ±10°C.

[0052] S5, performing periodic rolling on the rough tube obtained in step S4 to obtain a rough tube with a diameter of 350-450 mm and a wall thickness of 10-30 mm;

[0053] The rough tube preheated in S4 is rolled into shape using a periodic rolling device to prepare a rough tube with a diameter of 350-450 mm and a wall thickness of 10-30 mm; the deformation amount during the rolling process is 65-75%, the feed amount is 20-40 mm / time, and the rolling current is 5-9 kA.

[0054] S6. Heat treating the rough pipe obtained in step S5;

[0055] To fully remove the internal stress generated during the tube forming process and improve the stability of the finished tube's microstructure and performance, the titanium alloy rough tube obtained in step S5 is heat treated. Specifically, the rough tube is loaded into the furnace when the furnace temperature is ≤500°C; heated to 775°C-825°C at a heating rate of 100°C / h and held at that temperature for 5 hours; then heated to 885°C-935°C at a heating rate of 120°C-160°C / h, preferably 145°C-155°C / h, held at that temperature for 1-2 hours, and then air-cooled.

[0056] S7, the rough pipe is processed into titanium alloy seamless pipe;

[0057] The rough pipe processed in step S6 is processed into a large-size seamless pipe of BT14 titanium alloy with a diameter of 350 to 450 mm and a wall thickness of 10 to 30 mm by using a sawing machine, a lathe and a deep hole boring machine.

[0058] Preferably, the anti-oxidation coating is glass powder specially used in titanium alloy rolling process.

[0059] The raw material specifications of the rolling method of the present invention are 750kg-1500kg grade BT14 titanium alloy forging rods with a diameter of Φ400-500mm. The processing process mainly includes: surface grinding, machining → rod billet preheating → rough pipe oblique rolling and piercing → rough pipe preheating → rough pipe rolling and forming → rough pipe heat treatment → finished product machining. The final finished pipe has a diameter of Φ350mm-Φ400mm, a wall thickness of 10-30mm, and a length of not less than 3500mm, which fully meets the technical requirements of BT14 titanium alloy pipes for marine engineering.

[0060] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. The specific embodiments described here are only used to explain the present invention and the present invention is not limited thereto.

[0061] Example 1

[0062] The rolling method of the present invention is used to prepare a BT14 titanium alloy seamless pipe with a diameter of 360 mm, a wall thickness of 10 mm, and a length of 3500 mm.

[0063] This embodiment provides a rolling method for improving the microstructure uniformity and mechanical properties of large-scale seamless BT14 titanium alloy pipes, the method comprising the following steps:

[0064] S1. Surface grinding of BT14 titanium alloy forging bar blanks is performed to remove surface defects, and a through hole of Φ40-80 mm is center machined on the ground bar blanks:

[0065] The raw material used is a forged BT14 titanium alloy billet with a size of 460 mm, a weight of 1,000 kg, and a phase transition point of 950°C. The billet is surface-ground to remove defects such as cracks, peeling, pits, and folds. A through-hole of 60 ± 1 mm is then machined in the center of the ground billet.

[0066] S2. Before loading into the furnace, the inner and outer surfaces of the rod blank machined in step S1 are coated with glass powder specially used for titanium alloy rolling process, and the rod blank machined by the grinding machine is preheated by slowly heating at low temperature and rapidly heating at high temperature.

[0067] The inner and outer surfaces of the rod blank are coated with special glass powder for titanium alloy rolling process; then, when the furnace temperature is ≤500℃, the rod blank is loaded into a resistance heating furnace that meets the requirements of GJB509A-1995 Level III, and heated to 800℃ at a heating rate of 100℃ / h and kept warm for 5 hours; then heated to 930℃ at a heating rate of 150℃ / h and kept warm for 2.5 hours.

[0068] S3, using the method of diameter expansion, deformation and oblique hole perforation to make the BT14 titanium alloy bar blank that has been preheated in step S2 into a rough tube:

[0069] Using piercing equipment designed for large-diameter seamless pipe production, the preheated BT14 titanium alloy billet was transformed into a 470mm diameter, 50mm wall thickness rough pipe using a method of expansion, deformation, and oblique hole perforation. The oblique hole perforation process used a 350mm diameter plug and a perforation current of 3.5 to 4.5 kA.

[0070] S4, coating the inner and outer surfaces of the capillary tube obtained in step S3 with glass powder specially used for titanium alloy rolling process and preheating the capillary tube so that the capillary tube is fully heated before forming and rolling;

[0071] The inner and outer surfaces of the capillary tube prepared in step S3 are coated with glass powder specially used for titanium alloy rolling process. The capillary tube is placed in a resistance heating furnace that meets the requirements of GJB509A-1995 Level III, and heated to 800°C at a heating rate of 100°C / h and kept warm for 5 hours; then heated to 930°C at a heating rate of 150°C / h and kept warm for 2 hours.

[0072] S5, performing periodic rolling on the rough pipe to prepare a rough pipe;

[0073] The rough tube preheated in step S4 is rolled into shape using a periodic rolling device to prepare a rough tube with a diameter of 380±2mm and a wall thickness of 18±2mm; the deformation amount is 60%, the feed amount is 30mm / time, and the rolling current is 6-8kA.

[0074] S6. Heat treating the rough pipe obtained in step S5;

[0075] The titanium alloy rough tube obtained in step S5 is heat treated as follows: when the furnace temperature is ≤500°C, the rough tube is loaded into the furnace; heated to 800°C at a heating rate of 100°C / h and held at that temperature for 5 hours; then heated to 930°C at a heating rate of 200°C / h, held at that temperature for 1.5 hours, and air-cooled.

[0076] S7, machining the rough pipe obtained in step S6 into a titanium alloy seamless pipe;

[0077] The tube blanks that have completed the above rolling forming are machined using a sawing machine, a lathe and a deep hole boring machine, and finally a BT14 titanium alloy finished seamless tube with a diameter of Φ360mm, a wall thickness of 10mm and a length of 3500mm is obtained.

[0078] Comparative Example 1

[0079] This comparative example provides a method for preparing a large-scale seamless pipe of BT14 titanium alloy, which comprises the following steps:

[0080] S1. Surface grinding of BT14 titanium alloy forging bar blanks is performed to remove surface defects, and a through hole of Φ40-80 mm is center machined on the ground bar blanks:

[0081] The raw material used is a forged BT14 titanium alloy billet with a size of 460 mm, a weight of 1,000 kg, and a phase transition point of 950°C. The billet is surface-ground to remove defects such as cracks, peeling, pits, and folds. A through-hole of 60 ± 1 mm is then machined in the center of the ground billet.

[0082] S2. Before loading into the furnace, the inner and outer surfaces of the rod blank machined in step S1 are coated with glass powder specially used for titanium alloy rolling process, and the rod blank is preheated:

[0083] The surface of the rod blank is coated with special glass powder for titanium alloy rolling process; then, when the furnace temperature is ≤500℃, the rod blank is loaded into a resistance heating furnace that meets the requirements of GJB509A-1995 Level III, and heated to 850℃ at a heating rate of 100℃ / h and kept warm for 5 hours; then heated to 960℃ at a heating rate of 200℃ / h and kept warm for 1.5 hours.

[0084] S3, using the method of diameter expansion, deformation and oblique hole perforation to make the BT14 titanium alloy bar blank that has been preheated in step S2 into a rough tube:

[0085] Using piercing equipment designed for large-diameter seamless pipe production, preheated BT14 titanium alloy billets were formed into rough pipes with a diameter of 480 mm and a wall thickness of 30 mm. The oblique hole piercing process used a plug diameter of 400 mm and a piercing current of 2 to 3 kA.

[0086] S4, coating the inner and outer surfaces of the capillary tube obtained in step S3 with glass powder specially used for titanium alloy rolling process and preheating the capillary tube;

[0087] Glass powder specially used for titanium alloy rolling process was coated on the inner and outer surfaces of the capillary made of S3, and the capillary was placed in a resistance heating furnace meeting the requirements of GJB509A-1995 Level III, and heated to 850°C at a heating rate of 100°C / h and kept warm for 5 hours; then heated to 960°C at a heating rate of 200°C / h and kept warm for 1 hour.

[0088] S5, performing periodic rolling on the rough pipe to prepare a rough pipe;

[0089] The preheated blank in S4 was rolled into shape using a periodic rolling device to prepare a blank with a diameter of 380±2mm and a wall thickness of 18±2mm; the deformation amount was 50%, the feed amount was 60mm / time, and the rolling current was 4-5kA.

[0090] S6. Heat treating the rough pipe obtained in step S5;

[0091] The titanium alloy rough tube obtained in step S5 is heat treated. The specific operation is as follows: when the furnace temperature is ≤500°C, the rough tube is loaded into the furnace; heated to 850°C at a heating rate of 100°C / h and kept at this temperature for 5 hours; then heated to 960°C at a heating rate of 200°C / h, kept at this temperature for 1 hour, and air-cooled.

[0092] S7, machining the rough pipe obtained in step S6 into a titanium alloy seamless pipe;

[0093] The tube blanks after the above rolling forming are machined to finally obtain BT14 titanium alloy finished seamless tubes with a diameter of Φ360 mm, a wall thickness of 10 mm, and a length of 3500 mm.

[0094] Figure 1 The microstructure diagrams of the head and tail parts of the BT14 titanium alloy seamless tube prepared in Example 1 are as follows; Figure 2 The microstructure of the head and tail of the BT14 titanium alloy seamless pipe prepared in Comparative Example 1 is shown in FIG. Figure 1 and Figure 2 It can be seen that the microstructures of the head and tail parts of the BT14 titanium alloy seamless tube prepared by Example 1 are both two-phase region structures with good uniformity; while the microstructures of the head and tail parts of the BT14 titanium alloy seamless tube prepared by Comparative Example 1 are composed of a mixture of basketweave structure and dual-state structure, and there are obvious differences in the morphology and content of the two structures, and the uniformity is poor.

[0095] Table 1 shows the room temperature tensile and -50°C impact properties of the head and tail sections of the BT14 titanium alloy seamless tube. As can be seen from Table 1, the performance of the BT14 titanium alloy seamless tubes in both solutions meets the technical requirements, but the BT14 titanium alloy seamless tube prepared in Example 1 has more uniform performance at the head and tail sections, and a better strength-toughness match.

[0096] Table 1. Room temperature tensile and -50°C impact properties of BT14 titanium alloy seamless pipes in Example 1

[0097]

[0098] The method of this invention works by determining the optimal forging process based on the mechanical properties of various titanium alloy structures, formulating heating processes for the billets at different stages based on the temperature-dependent thermal conductivity of BT14 titanium alloy, and developing a tube forming process based on the equipment's deformation behavior at high temperatures. The method combines machining through-holes, single-pass oblique rolling, and single-pass forming rolling to improve the microstructure uniformity and mechanical properties of the finished BT14 titanium alloy tubes, enhancing the quality of large-scale tubes.

[0099] The preparation method of the present invention includes surface grinding and machining → preheating of the rod billet → oblique rolling and piercing of the rough pipe → preheating of the rough pipe → rolling and forming of the rough pipe → heat treatment of the rough pipe → machining of the finished product: in the billet heating stage, a slow heating method in the low temperature section and a rapid heating method in the high temperature section are formulated by utilizing the characteristic that the thermal conductivity of BT14 titanium alloy gradually increases with increasing temperature; in the oblique rolling and piercing stage, the deformation process is ensured to be smooth and uniform, thereby improving the surface quality of the rough pipe; in the forming rolling stage, the feed amount and the rolling deformation amount are controlled, and the temperature rise generated by the deformation process is utilized to compensate for the heat loss in the rolling process, thereby achieving forging structure homogenization and dimensional precision control.

[0100] The large-scale BT14 titanium alloy pipes produced by the method of the present invention effectively improve the uniformity of the axial microstructure of the finished pipes, while also improving the matching degree of strength and impact toughness, so that the mechanical properties of the pipes fully meet the requirements of marine engineering applications. In addition, this preparation process has a low number of firings, low process energy consumption, and a high yield rate. It is suitable for the industrial production of BT14 titanium alloy pipes with diameters ranging from 350mm to 450mm, wall thicknesses of 10 to 30mm, and lengths of at least 3500mm.

[0101] It should be noted that the above-described embodiments are merely preferred embodiments of the present invention. Persons skilled in the art will appreciate that various modifications, improvements, and equivalent substitutions may be made to the present invention without departing from the principles of the present invention, and such modifications, improvements, and equivalent substitutions are deemed to fall within the scope of protection of the claims of the present invention.

Claims

1. A rolling method for improving the uniformity of structure and mechanical properties of large-scale seamless BT14 titanium alloy pipes, characterized in that: The steps include: S1. Surface grinding of BT14 titanium alloy forging bar blanks is performed to remove surface defects, and a through hole of Φ40 to 80 mm is center machined on the ground bar blanks; S2. Before loading into the furnace, the inner and outer surfaces of the rod blank machined in step S1 are coated with an anti-oxidation coating. The rod blank machined by the grinding machine is preheated by slowly heating at a low temperature and rapidly heating at a high temperature. When the furnace temperature is ≤500°C, the rod blank is loaded into a resistance heating furnace meeting the requirements of GJB509A-1995 Level III, heated to 775°C to 825°C at a heating rate of 100°C / h, and kept warm; then heated to 885°C to 935°C at a heating rate of 120°C to 160°C / h, and kept warm; S3, using the method of diameter expansion deformation and oblique hole perforation to make the BT14 titanium alloy billet preheated in step S2 into a capillary tube with a diameter of 450-550 mm and a wall thickness of 30-60 mm; S4, coating the inner and outer surfaces of the capillary tube obtained in step S3 with an anti-oxidation coating and performing a preheating treatment, first heating to 775°C to 825°C at a heating rate of 100°C / h and keeping warm; then heating to 885°C to 935°C at a heating rate of 120°C to 160°C / h and keeping warm, so that the capillary tube is fully heated before forming and rolling; S5, using a periodic rolling device to periodically roll the rough pipe obtained in step S4 to obtain a rough pipe with a diameter of 350-450 mm and a wall thickness of 10-30 mm; S6. Heat-treating the rough tube obtained in step S5. When the furnace temperature is ≤500°C, loading the rough tube into the furnace; heating the rough tube to 775°C-825°C at a heating rate of 100°C / h, and keeping the temperature; then heating the rough tube to 885°C-935°C at a heating rate of 120°C-160°C / h, keeping the temperature, and air-cooling; S7. Use a sawing machine, a lathe and a deep hole boring machine to machine the rough pipe that has been heat treated in step S6 to prepare a large-size seamless pipe of BT14 titanium alloy.

2. The rolling method according to claim 1, characterized in that In step S1, the surface defect removal includes removing cracks, peeling, pits, and folds.

3. The rolling method according to claim 1, characterized in that In step S2, the temperature is heated to 775°C to 825°C at a heating rate of 100°C / h and then kept warm for 5 hours, and then heated to 885°C to 935°C at a heating rate of 120°C to 160°C / h and then kept warm for 2 to 4 hours.

4. The rolling method according to claim 1, characterized in that In step S3, the diameter of the plug used in the oblique hole punching process is 300-400 mm, and the punching current is 3-6 kA.

5. The rolling method according to claim 1, characterized in that In step S4, the temperature is heated to 775°C to 825°C at a heating rate of 100°C / h and then kept warm for 5 hours; the temperature is heated to 885°C to 935°C at a heating rate of 120°C to 160°C / h and then kept warm for 2 to 4 hours.

6. The rolling method according to claim 1, characterized in that In step S5, during the cyclic rolling process, the deformation amount is 60% to 75%, the feed amount is 20 to 40 mm / time, and the rolling current is 5 to 9 kA.

7. The rolling method according to claim 1, characterized in that In step S6, the temperature is heated to 775°C to 825°C at a heating rate of 100°C / h and then kept warm for 5 hours; the temperature is heated to 885°C to 935°C at a heating rate of 120°C to 160°C / h and then kept warm for 1 to 2 hours.

8. The rolling method according to claim 1, characterized in that The anti-oxidation coating is glass powder specially used in titanium alloy rolling process.

9. A large-size seamless pipe made of BT14 titanium alloy, characterized in that: The present invention is obtained by rolling the product by the rolling method according to any one of claims 1 to 8.

10. The BT14 titanium alloy large-size seamless pipe according to claim 9, characterized in that: The BT14 titanium alloy large-size seamless pipe has a diameter of 350 to 450 mm and a wall thickness of 10 to 30 mm.

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