Titanium alloy oil pipe for engine and preparation method of titanium alloy oil pipe

Abstract

The invention discloses an engine titanium alloy oil pipe and a preparation method thereof. The preparation method comprises the following steps that a titanium alloy cast ingot is obtained through three times of smelting in a vacuum consumable electric arc furnace; the titanium alloy cast ingot is subjected to free forging treatment, and a titanium alloy forged piece is obtained; carrying out hot rolling treatment on the titanium alloy forge piece to obtain a titanium alloy round bar; the titanium alloy round rod is machined into a blank pipe; and the blank pipe is subjected to cold rolling deformation, and the finished engine titanium alloy oil pipe is obtained through aftertreatment. Wherein vacuum annealing is conducted on the titanium alloy pipe obtained after each pass of cold rolling, and the vacuum annealing temperature of the titanium alloy pipe obtained after each pass of cold rolling is determined according to the phase transformation point temperature T and the deformation rate epsilon of each pass of cold rolling; and the vacuum annealing heat preservation time of the titanium alloy pipe obtained through cold rolling of each pass is determined according to the wall thickness of the titanium alloy pipe obtained through cold rolling of each pass. The prepared titanium alloy is uniform and stable in component, structure and performance, reasonable in grain orientation distribution, good in strength and plasticity matching performance and excellent in fatigue performance.

Description

Technical Field

[0001] The present invention relates to the technical field of materials and metallurgy, and more particularly to an engine titanium alloy oil pipe and a preparation method thereof. Background Art

[0002] Titanium alloy oil pipes for aircraft engines are typically manufactured from small-diameter, thick-walled seamless pipes. Due to their small size, cold rolling of titanium pipes can easily lead to cracks on the inner surface of the pipe. Furthermore, due to the small inner diameter of the pipe, these inner surface cracks are difficult to remove using conventional methods. The production process from the billet to the finished pipe requires multiple cold rolling passes, each of which results in the continuous generation and expansion of cracks on the inner surface of the titanium alloy pipe. Ultimately, these cracks can affect the quality of the finished titanium alloy pipe, causing it to fail flaw detection testing, resulting in a low pipe yield. Existing preparation methods do not offer a comprehensive solution. Summary of the Invention

[0003] The purpose of the present invention is to overcome the above-mentioned defects in the prior art and provide an engine titanium alloy oil pipe and a preparation method thereof.

[0004] To achieve the above object, the technical solution of the present invention is as follows:

[0005] A method for preparing an engine titanium alloy oil pipe comprises the following steps:

[0006] S1. According to the element ratio of the engine titanium alloy oil pipe, weigh the master alloy and sponge titanium raw materials, and smelt them in a vacuum consumable arc furnace three times to obtain a titanium alloy ingot. The content of the titanium alloy ingot is controlled to be 3.6% to 3.7% aluminum, 3.1% to 3.2% vanadium, 1.1% to 1.3% chromium, 0.1% to 0.2% iron, ≤0.06% oxygen, and the balance titanium, by mass percentage;

[0007] S2. performing free forging on the titanium alloy ingot to obtain a titanium alloy forging;

[0008] S3, hot rolling the titanium alloy forging to obtain a titanium alloy round rod;

[0009] S4, machining the titanium alloy round rod into a billet tube;

[0010] S5. Cold-rolling the billet tube and performing post-processing to obtain a finished engine titanium alloy oil pipe; wherein, the billet tube is cold-rolled, the rolling speed of each cold rolling pass is ≥200 times / minute, the cold rolling deformation rate ε of each pass is ≥65%, the temperature of the titanium alloy tube during each cold rolling pass is ≥160°C, and the titanium alloy tube after each cold rolling pass is vacuum annealed, wherein the vacuum annealing temperature of the titanium alloy tube obtained by each cold rolling pass is determined according to the phase transition point temperature T and the cold rolling deformation rate ε of each pass, and the vacuum annealing holding time of the titanium alloy tube obtained by each cold rolling pass is determined according to the wall thickness of the titanium alloy tube obtained by each cold rolling pass; the outer diameter D of the titanium alloy tube obtained by cold rolling with a cold rolling deformation rate of ε is n+1 , wall thickness is S n+1 The calculation formula for the vacuum annealing temperature of the titanium alloy tube is (0.5T+330×ε)℃, and the calculation formula for the annealing holding time is (45×S n+1 )min; where ε is the cold rolling deformation rate per pass, in %; T is the phase transition temperature, in °C; S n+1 It is the wall thickness of the titanium alloy tube after each cold rolling, in mm.

[0011] Optionally, in step S1, the titanium sponge is high-purity titanium sponge with a hardness of ≤ HB85.

[0012] Optionally, in step S2, the free forging process includes the following steps:

[0013] S21, performing a three-forging and three-draw free forging process on the titanium alloy ingot, wherein the heating temperatures of the four fires are 1140° C., 1100° C., 1060° C., and 1020° C., respectively, and the heating and holding time of each fire is the minimum size of the titanium alloy forging × 0.43 min;

[0014] S22. The titanium alloy forging obtained in step S21 is subjected to three-fire, two-blow, two-draw free forging treatment, wherein the heating temperature of the three fires is T-50° C., and the heating and holding time of each fire is the minimum size of the titanium alloy forging × 0.63 min.

[0015] Optionally, in step S3, the temperature of the hot rolling treatment is T-60°C; and the metallographic structure of the titanium alloy round rod is an equiaxed structure.

[0016] Optionally, the billet tube obtained in step S4 is inspected and reaches AA grade of GB / T5193 standard; the wall thickness of the billet tube is 4 times the wall thickness of the finished engine titanium alloy oil pipe, and the diameter of the billet tube is 15 times the outer diameter of the finished engine titanium alloy oil pipe.

[0017] Optionally, in step S5, the deformation rate ε is calculated as:

[0018] ε=((D n -Sn )×Sn-(D n+1 -S n+1 )×S n+1 ) / ((D n -S n )×Sn);

[0019] Among them, D n is the outer diameter of the titanium alloy tube before each cold rolling, in mm; S n is the wall thickness of the titanium alloy tube before each cold rolling, in mm; D n+1 is the outer diameter of the titanium alloy tube after each cold rolling, in mm; S n+1 It is the wall thickness of the titanium alloy tube after each cold rolling, in mm.

[0020] Optionally, in step S5, the post-processing includes vibrating finishing treatment of the annealed titanium alloy tube, and then performing deep freezing treatment at -155°C for 5h, and cold rolling, vacuum annealing, straightening, performance testing and flaw detection; wherein, the titanium alloy tube after deep freezing treatment is cold rolled, the rolling speed of each cold rolling pass is ≥200 times / min, the cold rolling deformation rate of each pass is ≥65%, the temperature of each cold rolling pass is ≥160°C, and the titanium alloy tube after each cold rolling is vacuum annealed, wherein the vacuum annealing temperature of the titanium alloy tube obtained by each cold rolling pass is determined according to the phase transition point temperature T and the cold rolling deformation rate ε of each pass as (0.5T+330×ε)°C, and the annealing holding time is determined according to the wall thickness of the titanium alloy tube obtained by each cold rolling pass as (45×S n+1 )min; where ε is the cold rolling deformation rate per pass, in %; T is the phase transition temperature, in °C; S n+1 It is the wall thickness of the titanium alloy tube after each cold rolling, in mm.

[0021] The present invention also discloses an engine titanium alloy oil pipe prepared by the above preparation method, comprising the following components in mass percentage: aluminum: 3.6% to 3.7%, vanadium: 3.1% to 3.2%, chromium: 1.1% to 1.3%, iron: 0.1% to 0.2%, oxygen ≤0.06%, and the balance being titanium.

[0022] The implementation of the present invention will have the following beneficial effects:

[0023] The present invention ensures uniform composition of titanium alloy ingots through three vacuum consumable arc furnace smelting processes, selects high-purity sponge titanium with a hardness of ≤HB85, reduces the impurity content in the titanium alloy, and improves the plasticity and fatigue properties of the titanium alloy. The optimized titanium alloy composition design obtained based on a large number of experimental results ensures the mechanical properties of the finished titanium alloy oil pipe and reduces the tendency of cracks to form on the inner surface of the pipe during cold rolling.

[0024] The present invention ensures that the metallographic structure of the titanium alloy round bar is an equiaxed structure through the design of titanium alloy composition and the combination of forging and hot rolling processes, ensures that the flaw detection of the titanium alloy round bar can reach the AA level of the GB / T5193 standard, and makes good organizational preparation for subsequent cold rolling. The blank tube is prepared by punching using a machining process, and the specifications of the blank tube can be flexibly determined according to the specifications of the finished tube, so as to ensure a reasonable cold rolling deformation process and reduce the tendency of cracks to be generated on the inner surface of the tube during cold rolling.

[0025] The present invention enables the temperature of the titanium alloy tube to be ≥160°C during cold rolling by cold rolling at a high speed and a large deformation rate, thereby reducing the tendency of cracks to form on the inner surface of the tube during cold rolling. At the same time, the annealing heating temperature of the titanium alloy tube is set according to the cold rolling deformation rate, and the annealing holding time is set according to the wall thickness, thereby ensuring sufficient annealing of the titanium alloy tube and reducing the tendency of cracks to form on the inner surface of the tube during cold rolling.

[0026] The present invention uses vacuum annealing and vibration aging to reduce the residual tensile stress generated on the inner surface of the titanium alloy tube due to the previous cold rolling, and then generates residual compressive stress on the inner surface of the titanium alloy tube through deep cryogenic treatment. The residual compressive stress generated on the inner surface should be ≥ 0.6 times the yield strength of the titanium alloy tube, thereby inhibiting the initiation and expansion of cracks on the inner surface of the titanium alloy tube during the next cold rolling process, and solving the problem that small-diameter thick-walled titanium alloy seamless tubes cannot form compressive stress by shot peening on the inner surface of the tube due to their small inner diameter.

[0027] The preparation method of the present invention provides a new option for the production of titanium alloy oil pipes and has broad application prospects. After application, the monthly output can reach 1 ton / month, which has great economic benefits. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to specific examples, but the present invention is not limited thereto in any way.

[0029] The present invention discloses a method for preparing an engine titanium alloy oil pipe, comprising the following steps:

[0030] S1. According to the element ratio of the engine titanium alloy oil pipe, the intermediate alloy and sponge titanium raw materials are weighed and smelted three times in a vacuum consumable arc furnace to obtain a titanium alloy ingot. By mass percentage, the content of the titanium alloy ingot is controlled to be 3.6% to 3.7% aluminum, 3.1% to 3.2% vanadium, 1.1% to 1.3% chromium, 0.1% to 0.2% iron, ≤0.06% oxygen, and the balance is titanium.

[0031] In a specific embodiment, in step S1 , the titanium sponge is high-purity titanium sponge with a hardness of ≤ HB85.

[0032] S2. Free forging the titanium alloy ingot to obtain a titanium alloy forging.

[0033] In a specific embodiment, in step S2, the free forging treatment comprises the following steps:

[0034] S21, the titanium alloy ingot is subjected to 4-fire three-mound three-pull free forging treatment, the heating temperature of each fire is 1140℃, 1100℃, 1060℃, 1020℃, preferably 1140℃, 1100℃, 1060℃, 1020℃, and the heating holding time of each fire is the minimum size of the titanium alloy forging piece x 0.43 min;

[0035] S22, the titanium alloy forging piece obtained in step S21 is subjected to 3-fire two-mound two-pull free forging treatment, the heating temperature of each fire is T-50℃, and the heating holding time of each fire is the minimum size of the titanium alloy forging piece x 0.63 min.

[0036] S3, the titanium alloy forging piece is subjected to hot rolling treatment to obtain a titanium alloy round bar.

[0037] In a specific embodiment, in step S3, the temperature of the hot rolling treatment is T-60℃, and the metallographic structure of the titanium alloy round bar is equiaxed structure.

[0038] S4, the titanium alloy round bar is machined into a blank pipe.

[0039] In a specific embodiment, the blank pipe obtained in step S4 is subjected to flaw detection, and reaches AA level of GB / T5193 standard; the wall thickness of the blank pipe is 4 times, preferably 4 times, of the wall thickness of the finished engine titanium alloy oil pipe, and the diameter of the blank pipe is 15 times, preferably 15 times, of the outer diameter of the finished engine titanium alloy oil pipe.

[0040] S5, the blank pipe is subjected to cold rolling deformation, and is subjected to post-treatment to obtain a finished engine titanium alloy oil pipe; wherein the blank pipe is subjected to cold rolling deformation, the rolling speed of each pass of cold rolling is ≥200 times / min, the deformation rate ε of each pass of cold rolling is ≥65%, the temperature of the titanium alloy pipe during each pass of cold rolling is ≥160℃, the titanium alloy pipe after each pass of cold rolling is subjected to vacuum annealing, wherein the vacuum annealing temperature of the titanium alloy pipe obtained after each pass of cold rolling is determined according to the phase transition point temperature T and the deformation rate ε of each pass of cold rolling, and the vacuum annealing holding time of the titanium alloy pipe obtained after each pass of cold rolling is determined according to the wall thickness of the titanium alloy pipe obtained after each pass of cold rolling; the vacuum annealing temperature of the titanium alloy pipe obtained after cold rolling deformation with a deformation rate of ε is D n+1 , and the wall thickness is S n+1 , the calculation formula of the vacuum annealing temperature is (0.5T+330×ε)℃, and the calculation formula of the annealing holding time is (45×S n+1 )min; wherein ε is the deformation rate of each pass of cold rolling, in %; T is the phase transition point temperature, in ℃; S n+1 is the wall thickness of the titanium alloy pipe after each pass of cold rolling, in mm.

[0041] In a specific embodiment, in step S5, the deformation rate ε is calculated according to the following formula:

[0042] ε = ((D n -S n ) × Sn- (D n+1 -S n+1 ) × S n+1 ) / ((D n -S n ) × Sn);

[0043] wherein D n is the outer diameter of the titanium alloy pipe before each pass of cold rolling, in mm; S n is the wall thickness of the titanium alloy pipe before each pass of cold rolling, in mm; D n+1 is the outer diameter of the titanium alloy pipe after each pass of cold rolling, in mm; and S n+1 is the wall thickness of the titanium alloy pipe after each pass of cold rolling, in mm.

[0044] In a specific embodiment, in step S5, the post-processing includes performing vibration polishing treatment on the titanium alloy pipe after annealing, then performing deep cooling treatment at -155℃ for 5h, and performing cold rolling, vacuum annealing, straightening, performance detection and flaw detection; wherein the titanium alloy pipe after deep cooling treatment is subjected to cold rolling, the rolling speed of each pass of cold rolling is ≥200 times / min, the deformation rate of each pass of cold rolling is ≥65%, the temperature of each pass of cold rolling is ≥160℃, the titanium alloy pipe after each pass of cold rolling is subjected to vacuum annealing, wherein the vacuum annealing temperature of the titanium alloy pipe after each pass of cold rolling is calculated according to the formula (0.5T+330×ε)℃ according to the phase transition point temperature T and the deformation rate ε of each pass of cold rolling, and the annealing holding time is calculated according to the formula (45×S n+1 )min according to the wall thickness of the titanium alloy pipe after each pass of cold rolling; wherein ε is the deformation rate of each pass of cold rolling, in %; T is the phase transition point temperature, in ℃; and S n+1 is the wall thickness of the titanium alloy pipe after each pass of cold rolling, in mm.

[0045] The application further discloses an engine titanium alloy oil pipe prepared by the preparation method.

[0046] The following is a specific embodiment

[0047] Embodiment 1

[0048] The production specification of the embodiment is a preparation method of an engine titanium alloy oil pipe with a specification of Φ8×2mm, and the method comprises the following steps:

[0049] S1, according to the element ratio of the engine titanium alloy oil pipe, the intermediate alloy and high-purity titanium raw material with hardness ≤ HB85 are weighed, and a circular titanium alloy ingot with a diameter of Φ690 and a length of 3200mm is obtained by three times of vacuum self-consumption arc furnace smelting, according to mass percentage, the aluminum in the titanium alloy ingot is 3.65%, the vanadium is 3.12%, the chromium is 1.2%, the iron is 0.17%, the oxygen is 0.05%, and the balance is titanium. The phase transition point T of the titanium alloy is 916℃.

[0050] S2, after the titanium alloy ingot is peeled and the head and tail are removed, the size is Φ680*3000mm, and the free forging treatment is carried out to obtain the titanium alloy forge piece; the free forging treatment includes the following steps:

[0051] S21, the titanium alloy ingot is subjected to 4 fire times of three-dump three-pull free forging treatment, and the heating temperature of 4 fire times is 1140℃, 1100℃, 1060℃ and 1020℃ respectively, and the heating holding time of each fire is the minimum size of the titanium alloy forge piece × 0.43min;

[0052] S22, the titanium alloy forge piece obtained in step S21 is subjected to 3 fire times of two-dump two-pull free forging treatment, and the heating temperature of 3 fire times is 866℃, and the heating holding time of each fire is the minimum size of the titanium alloy forge piece × 0.63min.

[0053] S3, the titanium alloy forge piece is subjected to hot rolling treatment, and the temperature of the hot rolling treatment is 856℃, and a titanium alloy round bar with a diameter of Φ125mm is obtained, and the metallographic structure of the titanium alloy round bar is equiaxed structure.

[0054] S4, the titanium alloy round bar is machined into a Φ120×8mm blank pipe (8×15, 2×4), and the blank pipe is detected, which reaches the AA level of GB / T5193 standard.

[0055] S5, the blank pipe is subjected to cold rolling deformation, and a finished product engine titanium alloy oil pipe with a diameter of Φ56×6mm is obtained after post-treatment; wherein the blank pipe is subjected to cold rolling deformation, the rolling speed of each pass of cold rolling is 205 times / min, the cold rolling deformation rate of each pass is calculated according to the following formula, which is 66.5%, the temperature of the titanium alloy pipe during each pass of cold rolling is ≥160℃, the titanium alloy pipe after each pass of cold rolling is subjected to vacuum annealing, the vacuum annealing temperature is 677℃ (916 / 2+330×0.665), and the annealing holding time is 270min (45×6).

[0056] S6, the titanium alloy pipe annealed out of the furnace is subjected to vibration polishing treatment, then deep cryogenic treatment at -155℃ for 5h, and cold rolling into Φ30x4 titanium pipe by LG40 two-roller cold rolling pipe machine, the deformation rate ε is calculated to be 65.3% according to the following calculation formula, the rolling speed is 205 times / min, the vacuum annealing temperature is 673℃ (916 / 2+330x0.653), the vacuum annealing holding time is 180min (45x4). After vibration aging, deep cryogenic treatment at -155℃ for 5h, the titanium pipe is cold rolled into Φ15x3 titanium pipe by LG20 two-roller cold rolling pipe machine, ε is 65.4%, the rolling speed is 205 times / min, the temperature is 674℃ (916 / 2+330x0.654), the vacuum annealing holding time is 135min (45x3), then vibration aging, deep cryogenic treatment at -155℃ for 5h, the titanium pipe is cold rolled into Φ8x2 titanium pipe by LG10 two-roller cold rolling pipe machine, ε is 66.7%, the rolling speed is 205 times / min, the temperature is 678℃, the vacuum annealing holding time is 90min, the finished pipe is straightened, the performance is detected, and the flaw detection is performed.

[0057] The deformation rate ε calculation formula is:

[0058] ε = ((D n -S n ) x Sn- (D n+1 -S n+1 ) x S n+1 ) / ((D n -S n ) x Sn)

[0059] Wherein, D n is the outer diameter of the titanium alloy pipe before each pass of cold rolling, in mm; S n is the wall thickness of the titanium alloy pipe before each pass of cold rolling, in mm; D n+1 is the outer diameter of the titanium alloy pipe after each pass of cold rolling, in mm; S n+1 is the wall thickness of the titanium alloy pipe after each pass of cold rolling, in mm.

[0060] The engine titanium alloy oil pipe with the specification of Φ8x2mm prepared in the embodiment has a yield strength of 616Mpa, a tensile strength of 721Mpa, an elongation of 19%, and a first-time flaw detection qualified rate of 96%

[0061] Example 2

[0062] The production specification of the engine titanium alloy oil pipe with the specification of Φ10x2mm prepared in the embodiment includes the following steps:

[0063] S1. Based on the elemental composition for engine titanium alloy oil pipes, a master alloy and high-purity sponge titanium with a hardness of ≤HB85 were weighed and smelted three times in a vacuum consumable arc furnace to produce a circular titanium alloy ingot with a diameter of Φ690 and a length of 3200 mm. The ingot's mass percentages were controlled to be 3.65% aluminum, 3.12% vanadium, 1.2% chromium, 0.17% iron, 0.05% oxygen, and the balance titanium. The phase transition point of the titanium alloy was measured to be 916°C.

[0064] S2. The titanium alloy ingot is peeled and the head and tail are removed to a size of Φ680*3000mm, and then subjected to free forging to obtain a titanium alloy forging. The free forging process includes the following steps:

[0065] S21, performing a three-forging and three-draw free forging process on the titanium alloy ingot, wherein the heating temperatures of the four fires are 1140° C., 1100° C., 1060° C., and 1020° C., respectively, and the heating and holding time of each fire is the minimum size of the titanium alloy forging × 0.43 min;

[0066] S22. The titanium alloy forging obtained in step S21 is subjected to three-fire, two-blow, two-draw free forging treatment, wherein the heating temperature of the three fires is 866° C., and the heating and holding time of each fire is the minimum size of the titanium alloy forging×0.63 min.

[0067] S3. Hot rolling the titanium alloy forging at a temperature of 856° C. to obtain a titanium alloy round rod with a diameter of Φ155 mm. The metallographic structure of the titanium alloy round rod is an equiaxed structure.

[0068] S4. The titanium alloy round rod is machined into a Φ150×8 mm billet tube (10×15, 2×4), and flaw detection is performed on it, which meets the AA grade of GB / T5193 standard.

[0069] S5. Cold-rolling the billet tube and post-processing to obtain a finished engine titanium alloy oil pipe of Φ70×6 mm; wherein the billet tube is cold-rolled, the rolling speed of each cold rolling pass is 205 times / minute, the cold rolling deformation rate of each pass is calculated according to the following calculation formula to be 66.2%, the temperature of the titanium alloy tube during each cold rolling pass is ≥160°C, and the titanium alloy tube after each cold rolling pass is vacuum annealed, the vacuum annealing temperature is 676°C (916 / 2+330×0.662), and the annealing holding time is 270 min (45×6).

[0070] S6. The annealed titanium alloy tube is subjected to vibration finishing, vibration aging, and cryogenic treatment at -155°C for 5 hours. It is cold-rolled on an LG40 two-roll cold rolling mill into a Φ37.5×4 titanium tube with an ε of 65.1%, a rolling speed of 205 times / min, a temperature of 673°C (916 / 2+330×0.651), and a holding time of 180 minutes (45×4). It is then subjected to vibration aging, cryogenic treatment at -155°C for 5 hours, and cold-rolled on an LG20 two-roll cold rolling mill into a Φ 18.5×3 titanium tube, ε is 65.3%, rolling speed is 205 times / min, temperature is 673℃ (916 / 2+330×0.653), holding time is 135min (45×3), vacuum annealing, then vibration aging, deep cold treatment at -155℃ holding time for 5h, LG10 two-roll cold rolling mill is cold-rolled into Φ10×2 titanium tube, ε is 66.7%, rolling speed is 205 times / min, holding time is 678° for 90min. The finished tube is vacuum annealed, straightened, performance tested, and flaw detected.

[0071] The calculation formula of deformation rate ε is:

[0072] ε=((D n -S n )×Sn-(D n+1 -S n+1 )×S n+1 ) / ((D n -S n )×Sn);

[0073] Among them, D n is the outer diameter of the titanium alloy tube before each cold rolling, in mm; S n is the wall thickness of the titanium alloy tube before each cold rolling, in mm; D n+1 is the outer diameter of the titanium alloy tube after each cold rolling, in mm; S n+1 It is the wall thickness of the titanium alloy tube after each cold rolling, in mm.

[0074] The engine titanium alloy oil pipe with a specification of Φ10×2mm prepared in this embodiment has a yield strength of 623 MPa, a tensile strength of 731 MPa, an elongation of 20%, and an ultra-scanning first-time qualified rate of 97.1%.

[0075] The above-described embodiments merely represent several implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for preparing an engine titanium alloy oil pipe, characterized in that: The following steps are involved: S1. According to the element ratio of the engine titanium alloy oil pipe, weigh the master alloy and sponge titanium raw materials, and smelt them three times in a vacuum consumable arc furnace to obtain a titanium alloy ingot. Control the aluminum content of the titanium alloy ingot by mass percentage: 3.6% to 3.7%, vanadium: 3.1% to 3.2%, chromium: 1.1% to 1.3%, iron: 0.1% to 0.2%, oxygen ≤ 0.06%, and the balance is titanium; S2. performing free forging on the titanium alloy ingot to obtain a titanium alloy forging; S3, hot rolling the titanium alloy forging to obtain a titanium alloy round rod; S4, machining the titanium alloy round rod into a billet tube; S5. Cold-rolling the billet tube and performing post-processing to obtain a finished engine titanium alloy oil pipe; wherein, during the cold-rolling of the billet tube, the rolling speed of each cold-rolling pass is ≥200 times / minute, the cold-rolling deformation rate ε of each pass is ≥65%, the temperature of the titanium alloy tube during each cold-rolling pass is ≥160°C, and the titanium alloy tube after each cold-rolling pass is vacuum annealed, wherein the vacuum annealing temperature of the titanium alloy tube obtained by each cold-rolling pass is determined according to the phase transition point temperature T and the cold-rolling deformation rate ε of each pass, and the vacuum annealing holding time of the titanium alloy tube obtained by each cold-rolling pass is determined according to the wall thickness of the titanium alloy tube obtained by each cold-rolling pass; The outer diameter obtained by cold rolling with a cold rolling deformation rate of ε is D n+1 , wall thickness is S n+1 The calculation formula for the vacuum annealing temperature of the titanium alloy tube is (0.5T+330×ε)℃, and the calculation formula for the annealing holding time is (45×S n+1 )min; Wherein, ε is the cold rolling deformation rate per pass, unit is %; T is the phase transition point temperature, unit is ℃; S n+1 It is the wall thickness of the titanium alloy tube after each cold rolling, in mm.

2. The method for preparing an engine titanium alloy oil pipe according to claim 1, characterized in that: In step S1, the titanium sponge is high-purity titanium sponge with a hardness of ≤ HB85.

3. The method for preparing an engine titanium alloy oil pipe according to claim 1, characterized in that: In step S2, the free forging process includes the following steps: S21, performing a three-forging and three-draw free forging process on the titanium alloy ingot, wherein the heating temperatures of the four fires are 1140° C., 1100° C., 1060° C., and 1020° C., respectively, and the heating and holding time of each fire is the minimum size of the titanium alloy forging × 0.43 min; S22. The titanium alloy forging obtained in step S21 is subjected to three-fire, two-blow, two-draw free forging treatment, wherein the heating temperature of the three fires is T-50° C., and the heating and holding time of each fire is the minimum size of the titanium alloy forging × 0.63 min.

4. The method for preparing an engine titanium alloy oil pipe according to claim 1, characterized in that: In step S3, the temperature of the hot rolling treatment is T-60°C; the metallographic structure of the titanium alloy round rod is an equiaxed structure.

5. The method for preparing an engine titanium alloy oil pipe according to claim 1, characterized in that: The blank tube obtained in step S4 is inspected and found to meet the AA grade of GB / T5193 standard; the wall thickness of the blank tube is 4 times the wall thickness of the finished engine titanium alloy oil pipe, and the diameter of the blank tube is 15 times the outer diameter of the finished engine titanium alloy oil pipe.

6. The method for preparing an engine titanium alloy oil pipe according to claim 1, characterized in that: In step S5, the deformation rate ε is calculated as follows: ε=((D n -S n )×Sn-(D n+1 -S n+1 )×S n+1 ) / ((D n -S n )×Sn); Among them, D n is the outer diameter of the titanium alloy tube before each cold rolling, in mm; S n is the wall thickness of the titanium alloy tube before each cold rolling, in mm; D n+1 is the outer diameter of the titanium alloy tube after each cold rolling, in mm; S n+1 It is the wall thickness of the titanium alloy tube after each cold rolling, in mm.

7. The method for preparing an engine titanium alloy oil pipe according to claim 6, characterized in that: In step S5, the post-processing includes vibrating finishing treatment of the annealed titanium alloy tube, and then performing deep cryogenic treatment at -155°C for 5h, and cold rolling, vacuum annealing, straightening, performance testing and flaw detection; wherein, the titanium alloy tube after deep cryogenic treatment is cold rolled, the rolling speed of each cold rolling pass is ≥200 times / minute, the cold rolling deformation rate of each pass is ≥65%, the temperature of each cold rolling pass is ≥160°C, and the titanium alloy tube after each cold rolling is vacuum annealed, wherein the vacuum annealing temperature of the titanium alloy tube obtained by each cold rolling pass is determined according to the phase transition point temperature T and the cold rolling deformation rate ε of each pass as (0.5T+330×ε)°C, and the annealing holding time is determined according to the wall thickness of the titanium alloy tube obtained by each cold rolling pass as (45×S n+1 )min; Wherein, ε is the cold rolling deformation rate per pass, unit is %; T is the phase transition point temperature, unit is ℃; S n+1 It is the wall thickness of the titanium alloy tube after each cold rolling, in mm.

8. An engine titanium alloy oil pipe prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The invention comprises the following components in mass percentage: aluminum: 3.6% to 3.7%, vanadium: 3.1% to 3.2%, chromium: 1.1% to 1.3%, iron: 0.1% to 0.2%, oxygen ≤ 0.06%, and the balance being titanium.