Rolling method for improving structure uniformity and mechanical property of BT14 titanium alloy large-specification seamless pipe

Abstract

The invention belongs to the technical field of titanium alloy material processing, and discloses a rolling method for improving the structure uniformity and the mechanical property of a BT14 titanium alloy large-specification seamless pipe. In order to solve the problems that in the periodic rolling process of an existing pipe, due to the temperature and strain difference of all parts of a blank, the structure is uneven, and the mechanical property is not matched with the toughness, optimization design is conducted on a traditional titanium alloy pipe rolling technology; according to the method, the uniformity of microscopic structures of the head and the tail of the pipe is improved by optimizing parameters such as the rolling temperature, the deformation rate and the deformation amount, meanwhile, refining of the microscopic structures is achieved through the recrystallization process, the BT14 titanium alloy large-specification seamless pipe with the improved structure uniformity and mechanical property is prepared, and the application requirement of the ocean engineering field is better met.

Description

Technical Field

[0001] The 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 a large-size seamless pipe of a BT14 titanium alloy. Background Art

[0002] Titanium alloy pipes have the characteristics of high strength and low density, which can effectively resist seawater erosion and corrosion and maintain structural stability. At the same time, they are resistant to low temperatures, have low expansion coefficients and are non-magnetic, making them suitable for manufacturing marine engineering equipment under low temperatures and complex sea conditions. In addition, the environmental friendliness and long-term economic benefits of titanium alloy pipes have made them widely used in ship pipelines, gas cylinders, seawater desalination and offshore platforms, improving the performance and life 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 the alloy in the annealed state can reach 850MPa, the impact toughness (KV2) at -60℃ is higher than 45J, and the strength can reach 1100MPa after strengthening heat treatment. Therefore, BT14 titanium alloy is suitable for manufacturing structural parts, welded parts and pressure tanks working in the range of -60 to 500℃.

[0004] The titanium alloy seamless tube prepared by the periodic 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, excessive temperature will lead to the deterioration of the mechanical properties of titanium alloy. Therefore, in order to avoid the "rolling card" phenomenon caused by insufficient rolling power during the rolling process and ensure the performance of the finished pipe, the current pipe rolling process usually sets the rolling temperature 20 to 40 ° C above the phase change. However, considering that the microstructure of titanium alloy is very sensitive to the hot working process parameters, the duration from the tube billet to the end of the periodic rolling process is long, and it is difficult to avoid a large temperature difference (up to 50 to 100 ° C) at the head and tail of the tube billet, resulting in the finished titanium alloy tube head being a Widmanstatten structure, and the tail being a dual or tri-state structure. The difference in mechanical properties between these two organizations causes the uniformity of the mechanical properties of the finished tube to be poor.

[0005] Studies have shown that the strength-plasticity matching of dual-state structure and equiaxed structure in titanium alloy is better than that of basket structure, and they also have better high-cycle fatigue performance than basket structure. In the rolling process of titanium alloy, basket structure is generally formed under the condition of heating above the phase transition point or starting deformation; dual-state structure is formed when the deformation temperature is less than the phase transition point, while equiaxed structure is produced when the deformation temperature is more than the phase transition point. Therefore, compared with the basket structure, the hot working process of obtaining dual-state structure and equiaxed structure has a larger "temperature window". At the same time, due to the large deformation amount of the tube in the cyclic rolling process, the temperature rise caused by the thermal effect of deformation (usually reaching about 40-60°C) can effectively compensate for the temperature loss during the rolling process, which can also improve the uniformity of the head and tail structure and performance of the tube to a certain extent. In addition, when the deformation temperature of BT14 titanium alloy is higher than 600°C, dynamic recrystallization will occur, and the degree of recrystallization of α and β phases will gradually increase with the decrease of strain rate. Utilizing this characteristic, the structure and performance of the tube can be further improved by regulating parameters such as deformation amount and deformation rate during tube forming. Summary of the invention

[0006] In view of this, in order to solve the above-mentioned problems in the prior art, it is necessary to optimize the rolling process of traditional titanium alloy pipes, improve the uniformity of the microstructure at 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 microstructure refinement, 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 tubes of BT14 titanium alloy, 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 blank during the periodic rolling process of existing tubes.

[0008] In order to achieve the above object, the present invention adopts the following technical solution.

[0009] On the one hand, the present invention provides a rolling method for improving the uniformity of structure and mechanical properties of large-size seamless pipes of BT14 titanium alloy, comprising the following steps:

[0010] S1. Grind the surface of BT14 titanium alloy forging bar to remove surface defects, and center machine a through hole of Φ40-80 mm on the ground bar;

[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 in the low temperature section and quickly heating in the 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 Grade 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 bar blank preheated in step S2 into a rough 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 rough tube obtained in step S3 with an anti-oxidation coating and preheating the rough tube, first 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 and keeping the temperature, so that the rough 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 pipe obtained in step S5, when the furnace temperature is ≤500°C, loading the rough pipe into the furnace; heating to 775°C-825°C at a heating rate of 100°C / h, keeping warm; then heating to 885°C-935°C at a heating rate of 120°C-160°C / h, keeping warm, 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-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-935°C at a heating rate of 120°C-160°C / h and then kept warm for 2-4 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-825°C at a heating rate of 100°C / h and then kept warm for 5 hours; the temperature is heated to 885°C-935°C at a heating rate of 120°C-160°C / h and then kept warm for 2-4 hours.

[0021] Further, in step S5, during the periodic 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, the temperature is heated to 775°C-825°C at a heating rate of 100°C / h and then kept warm for 5 hours; the temperature is heated to 885°C-935°C at a heating rate of 120°C-160°C / h and then kept 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 for the purpose of describing specific embodiments and are not intended to limit the present invention.

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

[0034] The temperature parameters in the present invention, if not specifically limited, are allowed to be either constant temperature treatment or to vary within a certain temperature range. It should be understood that the constant temperature treatment allows the temperature to fluctuate within the precision range controlled by the instrument. Fluctuations within the range of ±5°C, ±4°C, ±3°C, ±2°C, and ±1°C are allowed.

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

[0036] S1. Grind the surface of BT14 titanium alloy forging bar to remove surface defects, and center machine a through hole of Φ40-80 mm on the ground bar;

[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 in the low temperature section and quickly heating in the 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 Grade 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 bar blank preheated in step S2 into a rough 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 rough tube obtained in step S3 with an anti-oxidation coating and preheating the rough tube, first 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 and keeping the temperature, so that the rough 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 pipe obtained in step S5, when the furnace temperature is ≤500°C, loading the rough pipe into the furnace; heating to 775°C-825°C at a heating rate of 100°C / h, keeping the temperature; then heating 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 uniformity of structure and mechanical properties of large-size seamless tubes of BT14 titanium alloy, comprising the following steps:

[0044] S1, before loading into the furnace, coating the inner and outer surfaces of the rod blank machined in step S1 with an anti-oxidation coating, grinding the surface of the BT14 titanium alloy forging rod blank, and machining a through hole of Φ40-80 mm in the center of the rod blank;

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

[0046] S2, before loading into the furnace, coating the inner and outer surfaces of the rod blank machined in step S1 with an anti-oxidation coating, and preheating the rod blank machined by the grinding machine by slow heating in the low temperature section and rapid heating in the high temperature section;

[0047] Since titanium is very active, it is easy to react chemically with gases such as oxygen, hydrogen, and nitrogen, forming a hard and brittle oxygen-rich α layer on the surface of the billet. When deformed, surface microcracks will occur, affecting the quality of the pipe. Therefore, an anti-oxidation coating should be applied before heating the billet. In addition, the thermal conductivity of BT14 titanium alloy is low at room temperature, only 8.79W / (m·K); as the temperature increases, the thermal conductivity gradually increases; but when the temperature exceeds the phase transition point, the β grains grow rapidly, resulting in performance deterioration. Therefore, in order to ensure the quality of preheating of the billet, the 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℃, the billet is loaded into a resistance heating furnace that meets the requirements of GJB509A-1995 Grade 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 bar blank preheated in step S2 into a rough 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] Anti-oxidation coating is applied to the inner and outer surfaces of the rough tube prepared in step 3. In order to ensure that the rough tube obtained in step S3 is fully heated before forming and rolling, the rough tube is first heated to 775°C-825°C at a heating rate of 100°C / h after being loaded into the furnace, and kept warm for 5 hours; then heated to 885°C-935°C at a heating rate of 120-160°C / h, preferably 145°C-155°C / h, and kept warm for 2-4 hours. During the heating process, the heating temperature of the rough tube must be controlled within the range of ±10°C of the specified temperature.

[0052] S5, performing periodic rolling on 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;

[0053] The rough pipe preheated in S4 is rolled into shape using a periodic rolling device to prepare a rough pipe 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] In order to fully remove the internal stress generated during the tube forming process and improve the stability of the finished tube structure and performance, the titanium alloy rough tube obtained in step S5 is heat treated. The specific operation is: when the furnace temperature is ≤500°C, the rough tube is loaded into the furnace; heated to 775°C-825°C at a heating rate of 100°C / h, and kept warm for 5 hours; then heated to 885°C-935°C at a heating rate of 120-160°C / h, preferably 145°C-155°C / h, kept warm for 1-2 hours, and air-cooled.

[0056] S7, the rough pipe machine 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 specification of the rolling method of the present invention is 750kg-1500kg grade BT14 titanium alloy forging bar billet with a diameter of Φ400-500mm. The processing process mainly includes: surface grinding, machining → bar billet preheating → rough pipe oblique rolling and perforation → 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 in conjunction with specific embodiments and drawings of the specification. The specific embodiments described here are only used to explain the present invention, and the present invention is not limited thereto.

[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 uniformity of structure and mechanical properties of large-size seamless tubes of BT14 titanium alloy, the method comprising the following steps:

[0064] S1. Grind the surface of BT14 titanium alloy forging bar to remove surface defects, and center machine a through hole of Φ40-80mm on the ground bar:

[0065] The raw material used is a forged BT14 titanium alloy bar with a specification of Φ460mm, a weight of 1000kg, and a phase transition point of 950℃. The surface of the bar is ground to remove defects such as cracks, peeling, pits and folds; a Φ60±1mm through hole is machined in the center of the ground bar.

[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 in the low temperature section and quickly heating in the high temperature section:

[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 5h; then heated to 930℃ at a heating rate of 150℃ / h, and kept warm for 2.5h.

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

[0069] The BT14 titanium alloy bar blank after preheating treatment was made into a rough tube with a diameter of Φ470mm and a wall thickness of 50mm by using the perforation equipment for preparing large-diameter seamless pipes and the method of expanding, deforming and oblique hole perforation. The plug diameter used in the oblique hole perforation process is 350mm, and the perforation current is 3.5-4.5kA.

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

[0071] The inner and outer surfaces of the rough tube prepared in step S3 are coated with glass powder specially used for titanium alloy rolling process, and the rough tube is placed in a resistance heating furnace meeting 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 pipe after preheating treatment in step S4 is rolled into shape using a periodic rolling device to prepare a rough pipe 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. The specific operation is: 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 kept at this temperature for 5 hours; then heated to 930°C at a heating rate of 200°C / h, kept at this 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 been rolled and formed as described above are machined using sawing machines, lathes and deep hole boring machines, and finally BT14 titanium alloy finished seamless tubes with a diameter of Φ360mm, a wall thickness of 10mm and a length of 3500mm are obtained.

[0078] Comparative Example 1

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

[0080] S1. Grind the surface of BT14 titanium alloy forging bar to remove surface defects, and center machine a through hole of Φ40-80mm on the ground bar:

[0081] The raw material used is a forged BT14 titanium alloy bar with a specification of Φ460mm, a weight of 1000kg, and a phase transition point of 950℃. The surface of the bar is ground to remove defects such as cracks, peeling, pits and folds; a Φ60±1mm through hole is machined in the center of the ground bar.

[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 5h; then heated to 960℃ at a heating rate of 200℃ / h, and kept warm for 1.5h.

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

[0085] The BT14 titanium alloy rod blank after preheating treatment was made into a rough tube with a diameter of Φ480mm and a wall thickness of 30mm using a perforating device for preparing large-diameter seamless pipes. The diameter of the plug used in the oblique hole perforation process is 400mm, and the perforation current is 2-3kA.

[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 tube;

[0087] The inner and outer surfaces of the rough tube made of S3 are coated with special glass powder for titanium alloy rolling process, and the rough tube is placed in a resistance heating furnace that meets 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 rough pipe preheated in S4 was rolled into shape by using a periodic rolling device to prepare a rough pipe 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: 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 that have completed the above rolling forming are machined to finally obtain BT14 titanium alloy finished seamless tubes with a diameter of Φ360mm, a wall thickness of 10mm, and a length of 3500mm.

[0094] Figure 1 The microstructure diagram of the head and tail of the BT14 titanium alloy seamless tube prepared in Example 1; Figure 2 The microstructure diagram of the head and tail of the BT14 titanium alloy seamless pipe prepared in Comparative Example 1. 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 in Example 1 are both two-phase zone structures with good uniformity; while the microstructures of the head and tail parts of the BT14 titanium alloy seamless tube prepared in 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 of the BT14 titanium alloy seamless pipe. It can be seen from Table 1 that the performance of the BT14 titanium alloy seamless pipes of the two schemes meets the technical requirements, but the performance of the head and tail of the BT14 titanium alloy seamless pipe prepared by Example 1 is more uniform, and the strength-toughness matching is better.

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

[0097]

[0098] The working principle of the method of the present invention is: the optimal forging process is determined according to the mechanical properties of various structures of titanium alloy, the heating process of the blank at different stages is formulated according to the characteristics of the thermal conductivity of BT14 titanium alloy changing with temperature, and the pipe forming process is formulated according to the deformation behavior equipment capacity of titanium alloy at high temperature. The process of combining machined through-hole, one-fire oblique rolling perforation, and one-fire forming rolling improves the uniformity of the structure and mechanical properties of the BT14 titanium alloy finished pipe, and improves the quality of large-size pipes.

[0099] The preparation method of the present invention comprises the following steps: surface grinding and machining → preheating of bar billets → oblique rolling and piercing of rough tubes → preheating of rough tubes → rolling and forming of rough tubes → heat treatment of rough tubes → machining of finished products: in the billet heating stage, a method of slow heating in the low temperature section and rapid heating in the high temperature section is 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 tube; in the forming rolling stage, the feed amount and the rolling deformation amount are controlled, and the temperature rise generated in 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-size BT14 titanium alloy pipes prepared by the method of the present invention effectively improve the uniformity of the axial microstructure of the finished pipes, and at the same time improve the matching degree of strength-impact toughness, so that the mechanical properties of the pipes fully meet the application requirements of marine engineering. In addition, the preparation process has fewer firings, less process energy consumption, and a high yield rate, and is suitable for industrial production of BT14 titanium alloy pipes with a diameter of Φ350mm to Φ450mm, a wall thickness of 10 to 30mm, and a length of not less than 3500mm.

[0101] It should be noted that the above-described embodiments are only preferred embodiments of the present invention. For those skilled in the art, without departing from the principles of the present invention, the present invention may be modified, improved and replaced with equivalents, and these modifications, improvements and equivalent replacements are also considered to fall within the scope of protection of the claims of the present invention.

Claims

1. A rolling method for improving the uniformity of structure and mechanical properties of large-size seamless pipes of BT14 titanium alloy, characterized in that: The steps include: S1. Grind the surface of BT14 titanium alloy forging bar to remove surface defects, and center machine a through hole of Φ40-80 mm on the ground bar; 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 in the low temperature section and quickly heating in the 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 Grade 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; S3, using the method of diameter expansion deformation and oblique hole perforation to make the BT14 titanium alloy bar blank preheated in step S2 into a rough 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 rough tube obtained in step S3 with an anti-oxidation coating and preheating the rough tube, first 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 and keeping the temperature, so that the rough 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 pipe obtained in step S5, when the furnace temperature is ≤500°C, loading the rough pipe into the furnace; heating to 775°C-825°C at a heating rate of 100°C / h, keeping warm; then heating to 885°C-935°C at a heating rate of 120°C-160°C / h, keeping warm, 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 removing of surface defects 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 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.

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-825°C at a heating rate of 100°C / h and then kept warm for 5 hours; the temperature is heated to 885°C-935°C at a heating rate of 120°C-160°C / h and then kept warm for 2-4 hours.

6. The rolling method according to claim 1, characterized in that: In step S5, during the periodic 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-825°C at a heating rate of 100°C / h and then kept warm for 5 hours; the temperature is heated to 885°C-935°C at a heating rate of 120°C-160°C / h and then kept warm for 1-2 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 of BT14 titanium alloy, characterized in that: The present invention is obtained by rolling 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.

Images