A method for producing a 1500 MPa grade Ti-5553 titanium alloy bar for fasteners

Abstract

A preparation method of 1500MPa grade Ti-5553 titanium alloy rod for fasteners, the Ti-5553 titanium alloy rod is subjected to two-phase zone temperature control free forging, two-phase zone temperature control drawing forging and temperature control hot rolling technology, realizing the engineering preparation of fine-grained dual-phase Ti-5553 titanium alloy rod for fasteners. The 1500MPa grade fine-grained dual-phase Ti-5Al-5V-5Mo-3Cr titanium alloy rod for fasteners with different specifications, diameter of Φ6mm, Φ7mm, Φ9mm, Φ11mm, Φ13mm, Φ15mm, Φ17mm, Φ19mm and Φ21mm, yield strength higher than 1400MPa, tensile strength higher than 1500MPa, and elongation after fracture higher than 8% is obtained.

Description

Technical Field

[0001] This invention relates to the field of titanium alloy processing technology, and proposes an engineering preparation method for 1500MPa grade fine-grained dual-state Ti-5553 titanium alloy bars for fasteners, with a diameter range of 6mm to 21mm. Background Technology

[0002] 1500MPa-grade fasteners form the foundation of the main connection structure of an aircraft, and their typical material is GH4169 nickel-based alloy. This is because GH4169 alloy has a high density of 8.2 × 10⁻⁶. 3 kg / m 3 Furthermore, the large quantity of GH4169 fasteners used per aircraft makes their use detrimental to aircraft weight control. Replacing GH4169 alloy fasteners with 1500MPa-grade lightweight fasteners is of significant value in achieving aircraft weight reduction.

[0003] Ti-5553 titanium alloy is a commercially available high-strength titanium alloy, nominally known as Ti-5Al-5V-5Mo-3Cr. With a maximum hardenability diameter of up to 100 mm, Ti-5553 titanium alloy is currently used in thick-walled structures such as the landing gear and flaps of the Boeing 787 large passenger aircraft. A fine-grained bimodal microstructure is a typical microstructure of Ti-5553 titanium alloy, characterized by fine Beta grains with a grain diameter of less than 20 μm. This fine-grained bimodal Ti-5553 titanium alloy exhibits the following characteristics: First, its tensile strength is comparable to, and exceeds, that of GH4169 alloy (above 1500 MPa), while its density is only 57.3% that of GH4169 alloy, resulting in a significantly superior specific strength. Second, the room temperature elongation after fracture of the 1500 MPa grade fine-grained bimodal Ti-5553 titanium alloy exceeds 8%, which is beneficial for the cold rolling of MJ threads in fasteners. Therefore, fine-grained dual-phase Ti-5553 titanium alloy bars are ideal materials for preparing ultra-high strength lightweight fasteners with a strength of 1500MPa.

[0004] The key processing steps for Ti-5553 titanium alloy fasteners are: (1) solution heat treatment, (2) heading, (3) aging heat treatment, and (4) MJ thread rolling. The fasteners transform into a fine-grained structure after solution treatment and into a fine-grained bimodal structure after aging treatment. The diameters of currently used 1500MPa grade fasteners are 5mm, 6mm, 8mm, 10mm, 12mm, 14mm, 16mm, 18mm, and 20mm. To prepare 1500MPa grade titanium alloy fasteners using Ti-5553 titanium alloy, it is first necessary to prepare Ti-5553 titanium alloy bars with a diameter range of 6mm to 21mm and a tensile strength higher than 1500MPa, exhibiting a fine-grained bimodal structure.

[0005] Patent CN118751717B discloses a method for preparing fine-grained Ti-5553 titanium alloy wire for fasteners. This method utilizes a technical route of "melting → free forging in the single-phase region → free forging in the two-phase region → elongation forging in the two-phase region → radial forging in the two-phase region → hot rolling in the two-phase region → cold rolling" to prepare fine-grained bimodal Ti-5553 titanium alloy bars with a diameter range of 5.5 mm to 8.5 mm. During cold rolling, to fully fragment the Beta grains, the minimum cold rolling deformation of the bars is 67.9%. However, the maximum feed diameter of the cold rolling mill is 15 mm. Due to the limitations of the cold working conditions of the mill, this method can only produce bars with a diameter range of 5.5 mm to 8.5 mm, and cannot produce fine-grained Ti-5553 titanium alloy bars with a diameter range of 11 mm to 21 mm, thus failing to meet the diverse specifications of Ti-5553 titanium alloy bars required for 1500 MPa grade titanium alloy fasteners.

[0006] Patent CN106363021B discloses a rolling method for 1500MPa grade Ti-6Al-4Mo-4Zr-2Sn-2Cr-1Fe-1Nb titanium alloy bars. This method uses conventional continuous hot rolling. However, due to the significant temperature rise during continuous hot working of Ti-5553 titanium alloy, this continuous hot rolling method will cause significant coarsening of the Beta grains in the Ti-5553 alloy. Furthermore, the phase transformation point of Ti-5553 titanium alloy is 855℃, while the hot rolling temperature range of this method is 780℃~900℃. The 855℃~900℃ hot rolling temperature range is higher than the phase transformation point of Ti-5553 titanium alloy, and hot rolling in this temperature range cannot refine the Beta grains of the Ti-5553 titanium alloy bars to below 100μm. Therefore, this method is not suitable for preparing fine-grained bimorphic Ti-5553 titanium alloy rods with diameters ranging from 6 mm to 21 mm and tensile strengths exceeding 1500 MPa.

[0007] Patent CN120001822A discloses a method for preparing 1500MPa grade TB19 titanium alloy coiled wire. This method utilizes a "precision forging → hot rolling → roll drawing" technical route to prepare 1500MPa grade TB19 titanium alloy coiled wire with a diameter range of 6.3mm to 10.5mm. However, this method is also unsuitable for preparing fine-grained bimodal Ti-5553 titanium alloy bars with a diameter range of 6mm to 21mm and a tensile strength higher than 1500MPa. Firstly, the precision forging temperature range of this method is 910℃ to 970℃, which is 55℃ to 115℃ higher than the phase transformation point of Ti-5553 titanium alloy. After precision forging in this temperature range, the Beta grain size of Ti-5553 titanium alloy will surge to over 200μm. Secondly, the hot rolling temperature range of this method is 800℃~880℃ (some temperature ranges are higher than the phase transformation of Ti-5553 alloy), and the rolling method is traditional continuous hot rolling. Due to the significant thermal effect of Ti-5553 titanium alloy during continuous hot deformation, the continuous hot rolling cannot refine the Beta grains in the Ti-5553 alloy radial forging billet. After hot rolling in this temperature range, the Beta grains of Ti-5553 titanium alloy will still be between 100μm and 200μm. Thirdly, the deformation rate of the roll die hot drawing is too low, only 49.54%~71.34%. After hot drawing with this deformation rate, the Beta grains with a size between 100μm and 200μm in the hot-rolled Ti-5553 alloy cannot be refined to below 20μm. Fourth, the maximum feed diameter of the roller die is only 15mm. This method can only produce titanium alloy wires with a diameter range of 6.3mm to 10.5mm, and cannot produce Ti-5553 titanium alloy rods with a diameter range of 11mm to 21mm. It cannot meet the multi-specification requirements of 1500MPa grade titanium alloy fasteners for Ti-5553 titanium alloy rods.

[0008] In summary, the currently published methods for preparing fine-grained Ti-5553 titanium alloy bars, as well as methods for preparing 1500MPa grade titanium alloy bars of other compositions, are not suitable for preparing fine-grained bimodal Ti-5553 titanium alloy bars with diameters ranging from 6mm to 21mm and tensile strengths exceeding 1500MPa. To process 1500MPa grade Ti-5553 titanium alloy fasteners, there is an urgent need to develop an engineering-grade fabrication technology for fine-grained bimodal Ti-5553 titanium alloy bars with diameters ranging from Φ6mm to Φ21mm and tensile strengths exceeding 1500MPa. Summary of the Invention

[0009] To fill the gap in the engineering preparation technology of 1500MPa grade Ti-5553 titanium alloy bars for fasteners, this invention proposes a method for preparing 1500MPa grade Ti-5553 titanium alloy bars for fasteners.

[0010] The specific process for preparing 1500MPa grade Ti-5553 titanium alloy bars for fasteners proposed in this invention is as follows:

[0011] Step 1, Two-phase region temperature-controlled free forging:

[0012] The two-phase region temperature-controlled free forging is a two-phase region free forging process involving six passes of controlled upsetting temperature rise on the forging billet. The forging billet is a Ti-5553 titanium alloy forged in the single-phase region.

[0013] The micrograin size of the forging billet is better than grade 1, and its size is 400mm×400mm×800mm.

[0014] The specific process of the two-phase region temperature-controlled free forging is as follows:

[0015] The first step is single-fire forging. This single-fire forging includes heating and holding the billet, upsetting and drawing the billet, cooling, and grinding. The billet obtained after the first forging process is then obtained.

[0016] In the process of roughening, the displacement of the anvil head is 40-200mm, the number of displacements of the anvil head is 2-10, and the cooling time between displacements of the anvil head is 5-60s.

[0017] The process of one-time forging is as follows:

[0018] During the heating and holding of the forging billet, a box-type resistance furnace is used for heating. When the temperature of the box-type resistance furnace reaches 835℃, the forging billet is placed in a uniform temperature zone and held for 280 minutes. After the holding period, the forging billet is obtained.

[0019] The forging billet is transferred from the box-type resistance furnace to the lower anvil of the high-speed forging hydraulic press in a transfer time of less than 1 minute.

[0020] During the forging and drawing of the billet, each firing cycle includes two consecutive forging and drawing cycles.

[0021] The height of the forging billet is uplifted to 400mm. The uplifted forging billet is then drawn to 400mm×400mm×800mm. This completes the first uplift-drawing process.

[0022] The forging billet, after the first upsetting and drawing, undergoes a second upsetting and drawing process. The height of the forging billet is upset to 400mm. The billet after the second upsetting is then drawn to 400mm × 400mm × 800mm. This completes the second upsetting and drawing process.

[0023] The second step is the forging of the remaining fire stages.

[0024] Repeat the process of heating and holding the billet in the first forging cycle, upsetting and drawing the billet, cooling and grinding, and then complete the remaining forging cycles in sequence until the billet has undergone six cycles of two-phase temperature-controlled free forging, to obtain a Ti-5553 titanium alloy billet that has undergone two-phase temperature-controlled free forging.

[0025] The dimensions of the forging billet, which is subjected to temperature-controlled free forging in the two-phase region, are 400mm × 400mm × 800mm.

[0026] Step 2, temperature-controlled elongation forging in the two-phase region:

[0027] A round bar forging blank, obtained through multi-stage temperature-controlled drawing and forging in the two-phase region, is formed. The specifications of the round bar forging blank are Φ180mm×L; L is the length of the round bar forging blank.

[0028] The number of heat passes in the two-phase temperature-controlled forging process is n; n = 20. The deformation rate of each heat pass during elongation forging is < 9.5%.

[0029] During the two-phase region temperature-controlled elongation forging, the process of single-pass temperature-controlled elongation forging is as follows:

[0030] Ⅰ Heating and holding the forging billet

[0031] The box-type resistance furnace is heated to 835℃. The forging billet, which has undergone two-phase temperature-controlled free forging, is placed in a homogenization zone and held for 280 minutes. A forging billet forged in one pass is obtained.

[0032] After the heat preservation is completed, the forging billet is transferred from the box-type resistance furnace to the lower anvil of the high-speed forging hydraulic press; the transfer time is <1 minute.

[0033] After the forging billet is removed, the box-type resistance furnace is shut off without power and kept at 835℃.

[0034] II. Temperature-controlled drawing and forging.

[0035] A 400mm×400mm×800mm forging billet is drawn and forged into a 382mm×382mm×L1 forging billet; L1 is the length of the forging billet after one controlled temperature drawing.

[0036] Ⅲ Cooling the forged billet.

[0037] After the drawing and forging process is completed, the forging billet is placed on the lower anvil and air-cooled for 120 seconds.

[0038] Complete the single-fire controlled temperature drawing and forging of the forging billet.

[0039] During the two-phase region temperature-controlled elongation forging, the process of returning the forged billet to the furnace, temperature-controlled elongation forging, and cooling the forged billet is repeated to complete 2 to 19 heats of temperature-controlled elongation forging. Specifically:

[0040] Ⅰ. Forged billets are returned to the furnace.

[0041] The forged billet, after being forged in a single-pass temperature-controlled drawing process, is placed in the uniform temperature zone of a resistance furnace under heat preservation conditions and held for 15 minutes. After heat preservation, the forged billet is transferred from the box-type resistance furnace to the lower anvil of the forging hydraulic press; the transfer time is less than 1 minute.

[0042] II. Temperature-controlled drawing and forging.

[0043] The Ti-5553 titanium alloy forging billet that has been remelted is lengthened by drawing, and the 382mm×382mm×L1 Ti-5553 titanium alloy forging billet is lengthened by drawing to a 364mm×364mm×L2 billet. L2 is the length of the forging billet after the second temperature-controlled drawing.

[0044] The resulting billet undergoes a second temperature-controlled elongation forging process.

[0045] Ⅲ Cooling the forged billet.

[0046] The forged billet, which has undergone two temperature-controlled drawing and forging processes, is placed on the lower anvil and air-cooled for 115 seconds.

[0047] The temperature-controlled drawing and forging process is completed in 3 to 19 heats to obtain a forging billet that has undergone 19 heats of temperature-controlled drawing and forging.

[0048] During the two-phase region temperature-controlled elongation forging, the 20th temperature-controlled elongation forging process is as follows:

[0049] Ⅰ. Hot materials are returned to the furnace.

[0050] The forged billet, after undergoing the 19th temperature-controlled drawing and forging process, is returned to the furnace. The billet is then placed in a homogenization zone and held at that temperature for 5 minutes. After the holding period, the billet is transferred from the box-type resistance furnace to the lower anvil of the forging hydraulic press; the transfer time is less than 1 minute.

[0051] II. Temperature-controlled drawing and forging.

[0052] The dimensions obtained after the 19th forging are 166mm × 166mm × L. 19 The forging billet is drawn and forged into a round bar shape with a diameter of 180mm×L.

[0053] III. Cooling of the forged billet.

[0054] After the 20th temperature-controlled drawing forging of the forging billet is completed, the forging billet is air-cooled to room temperature.

[0055] IV. Sawing.

[0056] The obtained Φ180mm×L round bar forging billet is sawn into Φ180mm×1500mm round bar forging billets.

[0057] Step 3, temperature-controlled hot rolling in the two-phase region:

[0058] The two-phase region temperature-controlled hot rolling includes two-phase region temperature-controlled hot rolling of temperature-controlled hot-rolled bar preforms and two-phase region temperature-controlled hot rolling of temperature-controlled hot-rolled bar finished products:

[0059] The pre-formed temperature-controlled hot-rolled bar has a specification of Φ45mm×950mm. The finished temperature-controlled hot-rolled bar has a specification of Φ25mm×3000mm.

[0060] When processing the temperature-controlled hot-rolled bar preforms:

[0061] All the obtained Φ180mm×1500mm round bar forgings are processed into Φ45mm×950mm bars by two-phase zone temperature controlled hot rolling. The two-phase zone temperature controlled hot rolling is performed in 1 to 3 passes.

[0062] The specific process is as follows:

[0063] Ⅰ When the hot rolling process is 1 heat:

[0064] The first step is the heating and heat preservation of the round bar forging billet.

[0065] The box-type resistance furnace is heated to 835℃; the Φ180mm×1500mm round bar forging billet prepared in step 2 is placed in the uniform temperature zone, and the heat preservation coefficient of the round bar forging billet is 0.5min / mm.

[0066] The second step is temperature-controlled hot rolling. After the holding period, the round bar forging is rapidly transferred from the box-type resistance furnace to the rolling mill, with a transfer time of less than 1 minute. The round bar forging is then hot rolled. There are 15 rolling passes, with a deformation rate of 16.9% per pass. The rolling speed is 0.5 m / s. To control the temperature rise during hot rolling, the cooling time between each pass is 5 to 120 seconds. A round bar forging with a diameter of Φ45×L is obtained.

[0067] The third step is straightening and sawing. After hot rolling, the Φ45×L round bar forging billet is transferred to a straightening machine for hot straightening. The straightened round bar forging billet is then obtained. After air cooling to room temperature, the round bar forging billet is sawn to obtain a Φ45mm×950mm temperature-controlled hot-rolled bar preform.

[0068] II. When the hot rolling process involves 2 heat passes:

[0069] i. First Fire

[0070] The first step is the heating and heat preservation of the round bar forging billet.

[0071] The box-type resistance furnace is heated to 835℃; the Φ180mm×1500mm round bar forging billet prepared in step 2 is placed in the uniform temperature zone; the holding coefficient is 0.5min / mm. After the holding period, the raw material is quickly transferred from the box-type resistance furnace to the rolling mill, with a transfer time of <1min, and the round bar forging billet is hot rolled.

[0072] The second step is temperature-controlled hot rolling.

[0073] The temperature-controlled hot rolling process consists of 10 passes, with a deformation rate of 12.9% per pass. The rolling speed is 0.5 m / s. To control the temperature rise during hot rolling, the cooling time between each pass is 5 s to 120 s. This yields a Φ90×L round bar forging billet after hot rolling.

[0074] The third step is straightening and sawing.

[0075] The round bar forging billet is transferred to a straightening machine for hot straightening; after straightening, the round bar forging billet is air-cooled to room temperature; then sawn; the Φ90mm×L round bar forging billet is sawn into Φ90mm×2000mm round bar forging billets.

[0076] ii. Second fire

[0077] The first step is heating and heat preservation. The box-type resistance furnace is heated to 835°C. The Φ90mm×2000mm round bar forging billet is placed in the uniform temperature zone, and the heat preservation coefficient is 0.5min / mm.

[0078] The second step is temperature-controlled hot rolling. After the holding period, the hot-rolled bar is transferred from the box-type resistance furnace to the rolling mill for hot rolling, with a transfer time of less than 1 minute. There are 10 rolling passes, with a deformation rate of 12.9% per pass. The rolling speed is 0.5 m / s. To control the temperature rise during hot rolling, the cooling time between each pass is 5 to 120 seconds. This yields a temperature-controlled hot-rolled round bar forging.

[0079] The third step is straightening and sawing. After hot rolling, the round bar forgings that have undergone temperature-controlled hot rolling are hot straightened. The resulting hot-rolled bars are then air-cooled to room temperature and sawn to obtain Φ45mm×950mm temperature-controlled hot-rolled bar pre-products.

[0080] Ⅲ When the hot rolling process involves 3 heat passes:

[0081] i. First Fire

[0082] The first step is the heating and heat preservation of the round bar forging billet.

[0083] The box-type resistance furnace is heated to 835℃; the raw material is placed in the uniform temperature zone with a holding coefficient of 0.5 min / mm. After the holding period, the round bar forging billet is transferred from the box-type resistance furnace to the rolling mill in a time of less than 1 min.

[0084] The second step is temperature-controlled hot rolling.

[0085] The round bar forging billet is subjected to temperature-controlled hot rolling; the temperature-controlled hot rolling consists of 8 passes, with a deformation rate of 11.6% per pass. The rolling speed is 0.5 m / s. To control the temperature rise during hot rolling, the cooling time between each pass is 5 s to 120 s. After this round of hot rolling, a Φ180 mm × L round bar forging billet is obtained after the first round of temperature-controlled hot rolling.

[0086] The third step is to straighten it.

[0087] The Φ180mm×L round bar forging blank was transferred to a straightening machine for hot straightening. After straightening, it was air-cooled to room temperature. A straightened Φ110mm×L round bar forging blank was obtained.

[0088] ii. Second fire

[0089] The first step is heating and heat preservation. The box-type resistance furnace is heated to 835℃; the straightened Φ110mm×L round bar forging billet is placed in the uniform temperature zone with a heat preservation coefficient of 0.5min / mm.

[0090] The second step is temperature-controlled hot rolling. After the heat treatment, the Φ110mm×L round bar forging billet is rapidly transferred from the box-type resistance furnace to the rolling mill in a transfer time of less than 1 minute, and then hot rolled. The rolling process consists of 8 passes, with a deformation rate of 10.7% per pass. The rolling speed is 0.5 m / s. To control the temperature rise during hot rolling, the cooling time between each pass is 5 to 120 seconds.

[0091] The third step is straightening and sawing. After hot rolling, the hot-rolled bar is transferred to a straightening machine for hot straightening. After straightening, it is air-cooled to room temperature to obtain a Φ70mm×2000mm round bar forging billet that has undergone a second temperature-controlled hot rolling process.

[0092] ⅲ Third Fire

[0093] The first step is heating and heat preservation.

[0094] The box-type resistance furnace is heated to 835℃; the Φ70mm×2000mm round bar forging billet, which has undergone the second temperature-controlled hot rolling, is placed in the uniform temperature zone with a heat preservation coefficient of 0.5min / mm.

[0095] The second step is temperature-controlled hot rolling. After the holding period, the round bar forging is transferred from the box-type resistance furnace to the rolling mill for hot rolling; the transfer time is less than 1 minute. Eight rolling passes are used to obtain the round bar forging after the third temperature-controlled hot rolling. During the temperature-controlled hot rolling, the average deformation rate of each pass is 10.5%. The rolling speed is 0.5 m / s. To control the temperature rise during hot rolling, the cooling time between each pass is 5 s to 120 s.

[0096] The third step is straightening and sawing. After hot rolling, the round bar forging billet that has undergone the third temperature-controlled hot rolling is transferred to a straightening machine for hot straightening. This yields hot-rolled bars. After air cooling to room temperature, the hot-rolled bars are sawn into Φ45mm×950mm temperature-controlled hot-rolled bar pre-products.

[0097] When processing the temperature-controlled hot-rolled bar stock:

[0098] The first step is heating and heat preservation. The box-type resistance furnace is heated to 835℃, and the obtained Φ45mm×950mm hot-rolled bar preforms are placed in the uniform temperature zone with a heat preservation coefficient of 0.8min / mm.

[0099] The second step is temperature-controlled hot rolling. The hot-rolled bar preform is subjected to 6-10 passes of temperature-controlled hot rolling using a 250 two-roll mill to obtain a Φ25mm×950mm hot-rolled bar preform; the rolling speed is 0.5m / s. To control the temperature rise of the bar during the hot rolling process, the cooling time between each pass is 5s-120s.

[0100] The third step is straightening. After hot rolling, the Φ25mm×950mm hot-rolled bar pre-product is transferred to a straightening machine for hot straightening. After straightening, it is air-cooled to room temperature. This yields a Φ25mm×L hot-rolled bar semi-finished product.

[0101] Step four, flatten the top.

[0102] The two ends of the hot-rolled bar semi-finished product are sawn off to obtain a temperature-controlled hot-rolled bar finished product with a diameter of Φ25mm×3000mm.

[0103] Step 4, two-phase region temperature drawing

[0104] Take 9 hot-rolled bars with a diameter of 25mm × 3000mm prepared in step 3, and use a multi-pass two-phase zone warm drawing method to draw the 9 hot-rolled bars with a diameter of 25mm × 3000mm into warm-drawn bars of different specifications of Φ6mm, Φ7mm, Φ9mm, Φ11mm, Φ13mm, Φ15mm, Φ17mm, Φ19mm and Φ21mm respectively.

[0105] The specific process of the two-phase region temperature drawing is as follows:

[0106] The first step involves tipping each of the nine Φ25mm×3000mm hot-rolled bars, forming a small cylindrical segment at one end of each bar. This small cylindrical segment is 300mm long and has a diameter of D. 尖 .

[0107] D 尖 =D 模 -0.5mm, where D 尖为The diameter D of the small cylindrical segment of hot-rolled bar stock 模 To fix the diameter of the mold hole.

[0108] The second step is to coat the finished hot-rolled bar with graphite emulsion and then place it in the homogenization zone of a tube furnace for heat preservation.

[0109] The third step, after the heat treatment is completed, is to insert the hot-rolled bar into the drawing die, using the chuck of the drawing machine to hold the rolled tip area of ​​the bar. The drawing machine is then started to perform warm drawing on the hot-rolled bar at a speed of 30 mm / s. This yields a warm-drawn bar.

[0110] Based on the determined target diameter of each hot-rolled bar, the pass deformation rate in the two-phase region hot drawing is determined:

[0111] The number of drawing passes for Φ6mm hot-rolled bars is 11 to 21. The deformation rate for each drawing pass is determined to be 12.7% to 22.9%.

[0112] The number of drawing passes for Φ7mm hot-rolled bars is 9 to 19. The deformation rate per pass is determined to be 12.5% ​​to 24.6% for different drawing passes.

[0113] The number of drawing passes for Φ9mm hot-rolled bars is 8 to 18. The deformation rate per pass is determined to be 10.7% to 22.5% for different drawing passes.

[0114] The number of drawing passes for Φ11mm hot-rolled bars is 6 to 15. The deformation rate per pass is determined to be 13.9% to 23.9% for different drawing passes.

[0115] The number of drawing passes for Φ13mm hot-rolled bars is 5 to 12. The deformation rate per pass is determined to be 10.3% to 23.0% for different drawing passes.

[0116] The number of drawing passes for Φ15mm hot-rolled bars is 4 to 9. The deformation rate per pass is determined to be 10.7% to 22.5% for different drawing passes.

[0117] The number of drawing passes for Φ17mm hot-rolled bars is 3 to 7. The deformation rate per pass is determined to be 10.4% to 22.7% for different drawing passes.

[0118] The number of drawing passes for Φ19mm hot-rolled bars is 3 to 5. The deformation rate per pass is determined to be 10.4% to 22.7% for different drawing passes.

[0119] The drawing passes for Φ21mm hot-rolled bars are 2 or 3. The deformation rate for each pass is determined to be 10.9% to 16.0% for different drawing passes.

[0120] During the warm drawing process of warm-drawn bars with different target diameters, the deformation rate is the same for each pass.

[0121] The drawing temperature for all bars with different target diameters is 500–600°C.

[0122] Step 5, Solution treatment and aging heat treatment of warm drawn bars:

[0123] Solution treatment and aging heat treatment were performed on warm-drawn bars of different specifications to obtain Ti-5Al-5V-5Mo-3Cr titanium alloy bars with a fine grain dual-state structure of 1500MPa for fasteners.

[0124] I. Solution heat treatment

[0125] The specific process of the solution heat treatment is as follows:

[0126] The first step is to heat the tubular resistance furnace to 835℃. The temperature uniformity of the tubular resistance furnace is ±5℃, and the length of the uniform temperature zone is 5000mm.

[0127] The second step is to place the warm-drawn bars of different specifications prepared in step 4 into a uniform temperature zone and keep them at that temperature for 15 minutes.

[0128] The third step is straightening. After the heat preservation is completed, the drawn bars are transferred to the straightening machine for hot straightening.

[0129] The fourth step, after straightening, is to air-cool each warm-drawn bar to room temperature. This yields a solution-treated warm-drawn bar.

[0130] II. Aging Heat Treatment

[0131] The specific process for aging the obtained solution-treated warm-drawn bars is as follows:

[0132] The first step is to heat the tubular resistance furnace to 520°C.

[0133] The second step is to place the solution-treated warm-drawn bars of different specifications in a uniform temperature zone.

[0134] The third step is to keep it warm for 1 hour; after the warming period, air cool it to room temperature.

[0135] The core technologies of this invention are: 1. Two-phase region temperature-controlled free forging technology for Ti-5553 titanium alloy; 2. Two-phase region temperature-controlled drawing forging technology for Ti-5553 titanium alloy; 3. Temperature-controlled hot rolling technology for Ti-5553 titanium alloy bars; 4. Two-phase region temperature drawing technology for Ti-5553 titanium alloy bars. Through the adopted technical solutions, the engineering preparation of fine-grained biphase Ti-5553 titanium alloy bars for fasteners has been achieved. The bar diameters are Φ6mm, Φ7mm, Φ9mm, Φ11mm, Φ13mm, Φ15mm, Φ17mm, Φ19mm, and Φ21mm. The yield strength of these nine bar specifications is higher than 1400MPa, the tensile strength is higher than 1500MPa, and the elongation after fracture is higher than 8%.

[0136] Compared with the prior art, the present invention has the following beneficial effects:

[0137] I. Two-phase region temperature-controlled free forging

[0138] The existing two-phase free forging technology for Ti-5553 alloy is continuous free forging in the two-phase region. Due to the thermal effect of continuous free forging in the two-phase region, the temperature of the Ti-5553 forging billet during forging easily exceeds the dynamic recrystallization temperature range of the alloy, and even exceeds the phase transformation point of the alloy. The thermal effect of continuous free forging in the two-phase region will cause the Beta grains in the forging billet to grow dynamically to more than 200 μm. These coarse Beta grains are not easily eliminated in subsequent cold / hot working processes such as elongation forging, hot rolling, and drawing, and will remain in the finished Ti-5553 alloy bar. If coarse Beta grains are present in the finished Ti-5553 titanium alloy bar, the tensile strength after solution aging heat treatment will be <1500MPa and the elongation after fracture will be <8%. In summary, continuous free forging in the two-phase region is not conducive to the preparation of Ti-5553 titanium alloy bars with a fine-grained bimodal structure of 1500MPa.

[0139] To address the shortcomings of continuous free forging in the two-phase region of Ti-5553 alloy, this invention proposes a temperature-controlled free forging technology for the two-phase region of Ti-5553 alloy. Compared with continuous free forging in the two-phase region, the temperature-controlled free forging technology has the following characteristics: First, the temperature-controlled free forging technology decomposes the upsetting process into multiple equal-displacement upsetting sub-steps. Second, the temperature-controlled free forging technology introduces cooling steps between the equal-displacement upsetting sub-steps. The temperature-controlled free forging technology eliminates the thermal effects of traditional two-phase region free forging, ensuring that the temperature of each region of the forged billet remains within the dynamic recrystallization temperature range of the Ti-5553 titanium alloy during each forging cycle, and suppressing the alpha reaction that easily occurs during traditional two-phase region free forging. 初生 →The dynamic phase transformation of Beta and the dynamic growth of Beta grains ensure that the Beta grains are continuously refined as forging progresses.

[0140] II. Two-phase temperature-controlled elongation forging

[0141] Traditional two-phase zone drawing forging results in a high deformation rate per pass, typically between 30% and 50%, with a significant thermal effect in the forged billet core. During this process, the temperature in the core of the Ti-5553 titanium alloy billet easily exceeds the alloy's dynamic recrystallization temperature, and even surpasses its phase transformation point, leading to intense dynamic growth of the Beta grains in the core. After traditional two-phase zone drawing forging, a coarse-grained region of Beta grains exists in the core of the Ti-5553 titanium alloy billet, with Beta grains larger than 200 μm in size. It should be noted that this coarse-grained region in the core of the Ti-5553 titanium alloy billet is difficult to eliminate during subsequent hot rolling and hot drawing, resulting in large Beta grains in the finished Ti-5553 titanium alloy bars. If large Beta grains are present in the finished Ti-5553 titanium alloy bars, the tensile strength after solution aging heat treatment will be <1500 MPa, and the elongation after fracture will be <8%. In summary, traditional two-phase elongation forging is not conducive to the preparation of 1500MPa grade fine-grained biphase Ti-5553 titanium alloy bars.

[0142] To address the shortcomings of traditional two-phase region drawing and forging processes for Ti-5553 alloys, this invention proposes a two-phase region temperature-controlled drawing and forging technique. This method eliminates the thermal effects of traditional two-phase region drawing and forging by controlling the deformation rate of each drawing and forging pass, ensuring that the temperature of the billet core remains within the dynamic recrystallization temperature range of the Ti-5553 titanium alloy during each drawing and forging pass, and suppressing Alpha during the two-phase region forging process. 初生 →The dynamic phase transformation of Beta and the dynamic growth of Beta grains ensure that the Beta grains in the core of the forging blank are refined as the forging blank undergoes elongation thermoplastic deformation. After temperature-controlled elongation forging in the two-phase region as described in this application, the micrograin size of the Ti-5553 alloy can reach level 4.

[0143] III. Two-phase zone temperature-controlled hot rolling

[0144] Two-phase hot rolling is one of the important processing methods for titanium alloy bars. The characteristic of traditional two-phase hot rolling of titanium alloys is that the billet passes through rolls at high speed sequentially to achieve diameter reduction. However, due to the thermal effect of traditional two-phase hot rolling, Ti-5553 titanium alloy bars are prone to a sharp temperature rise, resulting in a temperature higher than the alloy's phase transformation point. This will induce local (or overall) Beta grain growth in the Ti-5553 titanium alloy bar, leading to the presence of coarse Beta grains in the hot-rolled bar. These coarse Beta grains are difficult to eliminate in subsequent cold / hot drawing processes, resulting in the continued presence of coarse Beta grains in the finished Ti-5553 titanium alloy bar. If coarse Beta grains are present in the finished bar, its tensile strength after solution aging heat treatment is <1500MPa, and its elongation after fracture is <8%. In summary, traditional two-phase hot rolling is not conducive to preparing Ti-5553 titanium alloy bars with a fine-grained two-phase microstructure of 1500MPa.

[0145] To address the shortcomings of traditional two-phase hot rolling processes for Ti-5553 alloys, this invention proposes a two-phase temperature-controlled hot rolling technology. This method eliminates the thermal effects of traditional two-phase hot rolling by introducing a cooling process between hot rolling passes and reducing the rolling speed. It ensures that the billet temperature remains within the alloy's dynamic recrystallization temperature range during each hot rolling pass, thus suppressing Alpha during traditional two-phase hot rolling. 初生 → Beta dynamic phase transformation and Beta grain dynamic growth ensure that the Beta grains in the billet become finer as the billet diameter decreases. After hot rolling in the two-phase region as described in this application, the micrograin size of Ti-5553 alloy bars can reach level 6.

[0146] IV. Two-phase zone temperature pull-out

[0147] The applicant studied the tensile mechanical properties of Ti-5553 titanium alloy in the temperature range of 200℃ to 600℃, and found that the tensile reduction of area of ​​the alloy was 87.0% at 500℃ and 96.3% at 600℃. These results indicate that Ti-5553 titanium alloy exhibits excellent diameter reduction plastic deformation capability in the 500℃–600℃ temperature range, and can achieve large plastic deformation warm drawing within this range. Two-phase region warm drawing of Ti-5553 alloy offers the following benefits: First, the Beta grains become increasingly refined with increasing cumulative drawing deformation rate. Second, due to the low strain rate and small deformation per pass during warm drawing, the thermal effect of Ti-5553 titanium alloy during warm drawing is extremely low, preventing dynamic growth of Beta grains in the bar. After two-phase temperature drawing, the micrograin size of Ti-5553 alloy bars can reach grade 7-8.

[0148] The fine-grained bimodal Ti-5553 titanium alloy bars prepared by this invention exhibit excellent strength and plasticity balance. To verify the effectiveness of this invention, Xi'an Hantang Analysis and Testing Co., Ltd. was commissioned to measure the room temperature tensile mechanical properties of fine-grained bimodal Ti-5553 titanium alloy bars with diameters of Φ6mm, Φ7mm, Φ9mm, Φ11mm, Φ13mm, Φ15mm, Φ17mm, Φ19mm, and Φ21mm according to the testing methods specified in GB / T228.1-2021. The results are shown in Table 1. The tensile strength of all bar specifications is >1500MPa, the yield strength is >1400MPa, and the elongation after fracture is >8%.

[0149] Table 1. Mechanical properties of fine-grained dual-state Ti-5553 titanium alloy bars

[0150] Bar diameter (mm) 6 7 9 11 13 15 17 19 21 Tensile strength (MPa) 1660 1642 1625 1597 1582 1564 1533 1525 1506 Yield strength (MPa) 1505 1490 1482 1469 1453 1442 1424 1410 1402 Elongation after fracture 8％ 8.5％ 8.5％ 9％ 9％ 9％ 10％ 10％ 10.5％ Reduction of area 21％ 24％ 26％ 30％ 31％ 34％ 37％ 41％ 43％ Detailed Implementation

[0151] This invention relates to a method for preparing 1500MPa grade fine-grained bimorphic Ti-5Al-5V-5Mo-3Cr titanium alloy bars for fasteners, and its technical solution is described in detail through the following nine embodiments.

[0152] The specific process of this invention is as follows:

[0153] Step 1, Two-phase region temperature-controlled free forging:

[0154] The two-phase region temperature-controlled free forging is performed using a 4500-ton high-speed forging hydraulic press, which conducts six passes of controlled upsetting temperature rise in the two-phase region forging of the billet. The billet is a Ti-5553 titanium alloy forged in the single-phase region.

[0155] The forged billet has a micrograin size better than grade 1, a phase transformation point of 855℃, external dimensions of 400mm×400mm×800mm, a surface roughness of Ra6.3 and no microcracks.

[0156] The specific process of the two-phase region temperature-controlled free forging is as follows:

[0157] The first step, one-time forging

[0158] Ⅰ Heating and holding of forging billets

[0159] The forging billet is heated in a box-type resistance furnace. When the furnace reaches a temperature of 835°C, the billet is placed in a uniform temperature zone and held for 280 minutes. After the holding period, the forging billet is obtained.

[0160] The forging billet is transferred from the box-type resistance furnace to the lower anvil of the high-speed forging hydraulic press in a transfer time of less than 1 minute.

[0161] II. Upsetting and Drawing of Forging Billets

[0162] Each firing cycle includes two consecutive rounds of thickening and stretching.

[0163] The forging billet is uplifted to a height of 400mm. The uplifted billet is then drawn to a length of 400mm × 400mm × 800mm using conventional methods. This completes the first uplift-drawing process.

[0164] The forging billet, after the first upsetting and drawing, undergoes a second upsetting and drawing process. The height of the forging billet is upset to 400mm. The billet after the second upsetting is then drawn to 400mm × 400mm × 800mm. This completes the second upsetting and drawing process.

[0165] In the roughing process, the anvil head displacement is 40-200mm, the number of anvil head displacements is 2-10, and the cooling time between anvil head displacements is 5-60s.

[0166] The process parameters for upsetting in each embodiment are shown in Table 2.

[0167] Table 2. Upsetting technical parameters for temperature-controlled free forging in the two-phase region.

[0168]

[0169] III. Cooling and Grinding. The forged billet, after the second upsetting and drawing process, is air-cooled to room temperature. The micro-cracks on the surface of the forged billet are ground to obtain the forged billet after the first forging.

[0170] The second step is the forging of the remaining fire stages.

[0171] The process of heating and holding the billet, upsetting and drawing the billet, and cooling and grinding in the first forging cycle is repeated sequentially to complete the remaining forging cycles until the billet undergoes six cycles of two-phase temperature-controlled free forging, resulting in a Ti-5553 titanium alloy billet that has undergone two-phase temperature-controlled free forging. The dimensions of the billet after two-phase temperature-controlled free forging are 400mm × 400mm × 800mm.

[0172] Step 2, temperature-controlled elongation forging in the two-phase region:

[0173] The number of firing cycles in the two-phase temperature-controlled forging process is n; n = 20. The forging temperature is 835℃.

[0174] Step 1: One-time temperature-controlled drawing and forging:

[0175] Ⅰ Heating and holding the forging billet

[0176] The box-type resistance furnace is heated to 835℃. The forging billet, which has undergone two-phase temperature-controlled free forging, is placed in a homogenization zone and held for 280 minutes. A forging billet forged in one pass is obtained.

[0177] After the heat preservation is completed, the forging billet is transferred from the box-type resistance furnace to the lower anvil of the high-speed forging hydraulic press; the transfer time is <1 minute.

[0178] After the forging billet is removed, the box-type resistance furnace is shut off without power and kept at 835℃.

[0179] II. Temperature-controlled drawing and forging

[0180] Using conventional methods, a 400mm×400mm×800mm forging billet is drawn and forged into a 382mm×382mm×L1 forging billet; L1 is the length of the forging billet after one temperature-controlled drawing.

[0181] Ⅲ Cooling forging billet

[0182] After the drawing and forging process is completed, the forging billet is placed on the lower anvil and air-cooled for 120 seconds.

[0183] Complete the single-fire controlled temperature drawing and forging of the forging billet.

[0184] The second step is a second-stage, temperature-controlled drawing and forging process:

[0185] Ⅰ. Forged billets are returned to the furnace.

[0186] The forged billet, after being forged in a single-pass temperature-controlled drawing process, is placed in the uniform temperature zone of a resistance furnace under heat preservation conditions and held for 15 minutes. After heat preservation, the forged billet is transferred from the box-type resistance furnace to the lower anvil of the forging hydraulic press; the transfer time is less than 1 minute.

[0187] II. Temperature-controlled drawing and forging.

[0188] A 4500-ton high-speed forging hydraulic press was used to forge the Ti-5553 titanium alloy forging billet that had been remelted, elongating it from 382mm×382mm×L1 to 364mm×364mm×L2. L2 is the length of the forging billet after the second temperature-controlled elongation.

[0189] The resulting billet undergoes a second temperature-controlled elongation forging process.

[0190] Ⅲ Cooling the forged billet.

[0191] The forged billet, which has undergone two temperature-controlled drawing and forging processes, is placed on the lower anvil and air-cooled for 115 seconds.

[0192] Complete the two-stage temperature-controlled elongation forging process.

[0193] The third step is temperature-controlled drawing and forging, which involves 3 to 19 heat treatments.

[0194] The process of forging billet return to the furnace, temperature-controlled elongation forging, and cooling of the billet in the two-stage forging process is repeated to complete forging processes n=3 to 19 in sequence, resulting in a billet that has undergone temperature-controlled elongation forging in the two-phase region after the 19th forging process. The technical parameters of temperature-controlled elongation forging in the two-phase region for the 3rd to 19th forging processes are shown in Table 3.

[0195] Table 3 Technical parameters for temperature-controlled drawing and forging in the two-phase region during the 3rd to 19th firing cycles.

[0196]

[0197] Step 4, the 20th temperature-controlled drawing and forging process:

[0198] Ⅰ Hot material remelting

[0199] The forged billet, after being forged in the 19th heat with controlled temperature drawing, is returned to the furnace. The billet is then placed in a homogenization zone and held for 5 minutes. After holding, the billet is transferred from the box-type resistance furnace to the lower anvil of the forging hydraulic press; the transfer time is <1 minute.

[0200] II. Temperature-controlled drawing and forging

[0201] The billet was elongated using a 4500-ton high-speed forging hydraulic press. The resulting dimensions after the 19th forging were 166mm × 166mm × L. 19 The forging billet is drawn and forged into a round bar shape with a diameter of 180mm×L.

[0202] III. Forging Billet Cooling

[0203] After the 20th temperature-controlled drawing forging of the forging billet is completed, the forging billet is air-cooled to room temperature.

[0204] IV. Sawing

[0205] The obtained Φ180mm×L round bar forging billet is sawn into Φ180mm×1500mm round bar forging billets.

[0206] A round bar forging billet was obtained by temperature-controlled elongation forging in the two-phase region.

[0207] Step 3: Process temperature-controlled hot-rolled bar stock into finished products

[0208] The process of processing temperature-controlled hot-rolled bar products includes processing temperature-controlled hot-rolled bar preforms through two-phase temperature-controlled hot rolling and processing temperature-controlled hot-rolled bar products through two-phase temperature-controlled hot rolling:

[0209] The pre-formed hot-rolled bar has a specification of Φ45mm×950mm. The finished hot-rolled bar has a specification of Φ25mm×3000mm.

[0210] When processing the temperature-controlled hot-rolled bar preforms:

[0211] Using a 650-type transverse rolling mill, all the Φ180mm×1500mm round bar forgings obtained in step 2 are processed into Φ45mm×950mm bars through two-phase zone temperature-controlled hot rolling. The two-phase zone temperature-controlled hot rolling is performed in 1 to 3 passes. The technical characteristics of the two-phase zone temperature-controlled hot rolling are shown in Table 4.

[0212] Table 4. Temperature-controlled hot rolling technical parameters for Φ180mm→Φ45mm Ti-5553 titanium alloy bars.

[0213]

[0214] The specific process of temperature-controlled hot rolling in the two-phase region is as follows:

[0215] Ⅰ When the hot rolling process is 1 heat:

[0216] The first step is the heating and heat preservation of the round bar forging billet.

[0217] When the box-type resistance furnace is heated to 835℃, the Φ180mm×1500mm round bar forging billet prepared in step 2 is placed in the uniform temperature zone, and the heat preservation coefficient of the round bar forging billet is 0.5min / mm.

[0218] The second step is temperature-controlled hot rolling. After the holding period, the round bar forging is rapidly transferred from the box-type resistance furnace to the rolling mill, with a transfer time of less than 1 minute. The round bar forging is then hot rolled. There are 15 rolling passes, with a deformation rate of 16.9% per pass. The rolling speed is 0.5 m / s. To control the temperature rise during hot rolling, the cooling time between each pass is 5 s to 120 s. After this hot rolling process, a round bar forging with a diameter of Φ45×L is obtained.

[0219] The third step is straightening and sawing. After hot rolling, the Φ45×L round bar forging billet is transferred to a straightening machine and hot straightened using conventional methods. This yields a straightened round bar forging billet. The straightened round bar forging billet is then air-cooled to room temperature and sawn to obtain a Φ45mm×950mm temperature-controlled hot-rolled bar preform.

[0220] II. When the hot rolling process involves 2 heat passes:

[0221] i. First Fire

[0222] The first step is the heating and heat preservation of the round bar forging billet.

[0223] The box-type resistance furnace is heated to 835℃. The Φ180mm×1500mm round bar forging billet prepared in step 2 is then placed in the homogenization zone; the holding coefficient is 0.5min / mm. After holding, the raw material is rapidly transferred from the box-type resistance furnace to the rolling mill in a transfer time of <1min, and the round bar forging billet is then hot-rolled.

[0224] The second step is temperature-controlled hot rolling.

[0225] The temperature-controlled hot rolling process consists of 10 passes, with a deformation rate of 12.9% per pass. The rolling speed is 0.5 m / s. To control the temperature rise during hot rolling, the cooling time between each pass is 5 s to 120 s. This yields a Φ90×L round bar forging billet after hot rolling.

[0226] The third step is straightening and sawing.

[0227] The round bar forging billet is transferred to a straightening machine for hot straightening. After straightening, the round bar forging billet is air-cooled to room temperature to obtain hot-rolled bar stock. The Φ90mm×L round bar forging billet is then sawn into Φ90mm×2000mm round bar forging billets.

[0228] After this hot rolling process, the diameter of the round bar forging billet is rolled from 180mm to 90mm.

[0229] ii. Second fire

[0230] The first step is heating and heat preservation. The box-type resistance furnace is heated to 835℃. The round bar forging billet with dimensions of Φ90mm×2000mm prepared in the first heat is placed in the uniform temperature zone, and the heat preservation coefficient is 0.5min / mm.

[0231] The second step is temperature-controlled hot rolling. After the holding period, the round bar forging is rapidly transferred from the box-type resistance furnace to the rolling mill for hot rolling, with a transfer time of less than 1 minute. There are 10 rolling passes, with a deformation rate of 12.9% per pass. The rolling speed is 0.5 m / s. To control the temperature rise during hot rolling, the cooling time between each pass is 5 to 120 seconds. This yields a round bar forging that has undergone temperature-controlled hot rolling.

[0232] The third step is straightening and sawing. After hot rolling, the obtained temperature-controlled hot-rolled round bar forging billet is transferred to a straightening machine and hot straightened using conventional methods.

[0233] After the obtained hot-rolled bars are air-cooled to room temperature, they are sawn into temperature-controlled hot-rolled bar pre-products with a diameter of Φ45mm×950mm.

[0234] Ⅲ When the hot rolling process involves 3 heat passes:

[0235] i. First Fire

[0236] The first step is the heating and heat preservation of the round bar forging billet.

[0237] In a heated box-type resistance furnace, when the furnace reaches 835℃, the raw material is placed in a uniform temperature zone with a holding coefficient of 0.5 min / mm. After the holding period, the round bar forging is transferred from the box-type resistance furnace to the rolling mill in a transfer time of <1 min, and then hot-rolled.

[0238] The second step is temperature-controlled hot rolling.

[0239] The round bar forging billet is subjected to temperature-controlled hot rolling; the temperature-controlled hot rolling has 8 rolling passes, and the deformation rate of each pass is 11.6%; the rolling speed is 0.5m / s; in order to control the temperature rise during hot rolling, the cooling time between each pass is 5s to 120s; after this hot rolling, a Φ180mm×L round bar forging billet after the first temperature-controlled hot rolling is obtained.

[0240] The third step is to straighten it.

[0241] The straightened Φ180mm×L round bar forging billet was transferred to a straightening machine and hot-straightened using conventional methods. After straightening, the bar was air-cooled to room temperature, resulting in a straightened Φ110mm×L hot-rolled bar.

[0242] ii. Second fire

[0243] The first step is heating and heat preservation. The box-type resistance furnace is heated to 835°C. The straightened Φ110mm×L round bar forging billet is placed in the uniform temperature zone, and the heat preservation coefficient is 0.5min / mm.

[0244] The second step is temperature-controlled hot rolling. After the holding period, the Φ110mm×L round bar forging is rapidly transferred from the box-type resistance furnace to the rolling mill in less than 1 minute for hot rolling. The rolling process consists of 8 passes, with a deformation rate of 10.7% per pass. The rolling speed is 0.5 m / s. To control the temperature rise during hot rolling, the cooling time between passes is 5 to 120 seconds. This yields a round bar forging that has undergone temperature-controlled hot rolling.

[0245] The third step is straightening and sawing. After hot rolling, the round bar forging billet that has undergone temperature-controlled hot rolling is transferred to a straightening machine for hot straightening. After straightening, the bar is air-cooled to room temperature to obtain a Φ70mm×2000mm hot-rolled bar that has undergone a second temperature-controlled hot rolling process.

[0246] ⅲ Third Fire

[0247] The first step is heating and heat preservation.

[0248] In a heated box-type resistance furnace, when the furnace reaches a temperature of 835℃, a Φ70mm×2000mm round bar forging billet that has undergone second-stage temperature-controlled hot rolling is placed in a uniform temperature zone with a holding coefficient of 0.5min / mm.

[0249] The second step is temperature-controlled hot rolling. After the heat treatment is completed, the round bar forging billet is transferred from the box-type resistance furnace to the rolling mill for hot rolling. The transfer time is less than 1 minute. The rolling passes are 8, resulting in a round bar forging billet that has undergone temperature-controlled hot rolling for the third time. During temperature-controlled hot rolling, the average deformation rate of each pass is 10.5%. The rolling speed is 0.5 m / s. To control the temperature rise during hot rolling, the cooling time between each pass is 5 s to 120 s.

[0250] The third step is straightening and sawing. After hot rolling, the round bar forging billet that has undergone the third temperature-controlled hot rolling is transferred to a straightening machine for hot straightening. After straightening, the bar is air-cooled to room temperature and then sawn. This yields a Φ45mm×950mm temperature-controlled hot-rolled bar pre-product.

[0251] When processing the temperature-controlled hot-rolled bar stock:

[0252] Using a 250-type tandem rolling mill, all the obtained Φ45mm×950mm hot-rolled bars were processed into Φ25mm×3000mm hot-rolled bars through two-phase zone temperature-controlled hot rolling. The technical characteristics of the two-phase zone temperature-controlled hot rolling are shown in Table 5.

[0253] Table 5. Temperature-controlled hot rolling technical parameters for Φ45mm→Φ25mm Ti-5553 titanium alloy bars.

[0254]

[0255] The specific implementation process for processing the Φ45mm×950mm hot-rolled bar into a Φ25mm×3000mm hot-rolled bar is as follows:

[0256] The first step is heating and heat preservation. The box-type resistance furnace is heated to 835℃. The obtained Φ45mm×950mm hot-rolled bar preforms are placed in the uniform temperature zone with a heat preservation coefficient of 0.8min / mm.

[0257] The second step is temperature-controlled hot rolling; the hot-rolled bar preform is subjected to 6 to 10 passes of temperature-controlled hot rolling using a 250 two-roll mill to obtain a Φ25mm×950mm hot-rolled bar preform; the rolling speed is 0.5m / s; in order to control the temperature rise of the bar during the hot rolling process, the cooling time between each pass is 5s to 120s.

[0258] The technical parameters for temperature-controlled hot rolling are shown in Table 5.

[0259] The third step is straightening. After hot rolling, the Φ25mm×L hot-rolled bar is transferred to a straightening machine for hot straightening. After straightening, the bar is air-cooled to room temperature. This yields a semi-finished hot-rolled bar with a specification of Φ25mm×L.

[0260] Step four, flatten the top.

[0261] The two ends of the hot-rolled bar semi-finished product are sawn, with a sawing length > 25mm.

[0262] The finished product is a temperature-controlled hot-rolled bar with a diameter of 25mm and a diameter of 3000mm.

[0263] Step 4, two-phase region temperature drawing:

[0264] Nine hot-rolled bars with a diameter of 25mm × 3000mm prepared in step 3 were taken and warm-drawn into bars with diameters of 6mm, 7mm, 9mm, 11mm, 13mm, 15mm, 17mm, 19mm and 21mm respectively by using a multi-pass two-phase zone warm drawing method.

[0265] During the warm drawing process of warm-drawn bars with different target diameters, the deformation rate is the same for each pass.

[0266] The drawing temperature for all bars with different target diameters is 500–600°C.

[0267] The drawing machine used is a 50-ton drawing machine. The die uses existing technology and is a fixed die. The die core material is polycrystalline diamond. The drawing lubricant is graphite emulsion. The heating equipment used for warm drawing is a tubular resistance furnace with a furnace temperature uniformity of ±10℃ and a uniform temperature zone length of 5000mm. Before drawing, the tubular resistance furnace is heated to the required temperature and held for <30min. During multiple drawing passes, the tubular resistance furnace remains open and at a constant temperature. The drawing speed is ≤50mm / s. The technical parameters for the warm drawing temperature and number of warm drawing passes for bars with different target diameters are shown in Table 6. The process follows a cycle of tipping → furnace loading → holding → drawing → air cooling until the required multiple warm drawing passes are completed and the bar is processed to the target diameter.

[0268] Table 6 Two-phase temperature drawing parameters for bars of different diameters

[0269]

[0270] The specific process of the two-phase region temperature drawing is as follows:

[0271] The first step involves tipping each of the nine Φ25mm×3000mm hot-rolled bars, forming a small cylindrical segment at one end of each bar. This small cylindrical segment is 300mm long and has a diameter of D. 尖 ;

[0272] D 尖 =D 模 -0.5mm, where D 尖 D is the diameter of the small cylindrical segment of the hot-rolled bar.模 To fix the diameter of the mold opening;

[0273] The second step is to coat the finished hot-rolled bar with graphite emulsion and then place it in the uniform temperature zone of a tube furnace for heat preservation.

[0274] The third step is to insert the hot-rolled bar into the drawing die after the heat preservation is completed, and use the chuck of the drawing machine to hold the rolled tip area of ​​the hot-rolled bar; start the drawing machine to perform warm drawing on the hot-rolled bar at a drawing speed of 30 mm / s; and obtain the warm-drawn bar.

[0275] Based on the determined target diameter of each hot-rolled bar, its pass deformation rate is determined using conventional methods:

[0276] The drawing passes for Φ6mm hot-rolled bars are 11 to 21, and the deformation rate for each pass is determined to be 12.7% to 22.9% for different drawing passes; the drawing passes for Φ7mm bars are 9 to 19, and the deformation rate for each pass is determined to be 12.5% ​​to 24.6% for different drawing passes.

[0277] The number of drawing passes for Φ9mm hot-rolled bars is 8 to 18. The deformation rate per pass is determined to be 10.7% to 22.5% for different drawing passes.

[0278] The number of drawing passes for Φ11mm hot-rolled bars is 6 to 15. The deformation rate per pass is determined to be 13.9% to 23.9% for different drawing passes.

[0279] The number of drawing passes for Φ13mm hot-rolled bars is 5 to 12. The deformation rate per pass is determined to be 10.3% to 23.0% for different drawing passes.

[0280] The number of drawing passes for Φ15mm hot-rolled bars is 4 to 9. The deformation rate per pass is determined to be 10.7% to 22.5% for different drawing passes.

[0281] The number of drawing passes for Φ17mm hot-rolled bars is 3 to 7. The deformation rate per pass is determined to be 10.4% to 22.7% for different drawing passes.

[0282] The number of drawing passes for Φ19mm hot-rolled bars is 3 to 5. The deformation rate per pass is determined to be 10.4% to 22.7% for different drawing passes.

[0283] The drawing passes for Φ21mm hot-rolled bars are 2 or 3. The deformation rate for each pass is determined to be 10.9% to 16.0% for different drawing passes.

[0284] During the warm drawing process of warm-drawn bars with different target diameters, the deformation rate is the same for each pass.

[0285] The drawing temperature for all bars with different target diameters is 500–600°C.

[0286] The drawing die uses existing technology.

[0287] Nine hot-rolled bars with a diameter of 25mm × 3000mm were warm-drawn into warm-drawn bars with diameters of 6mm, 7mm, 9mm, 11mm, 13mm, 15mm, 17mm, 19mm and 21mm respectively.

[0288] Step 5: Solution treatment and aging heat treatment of warm drawn bars

[0289] I. Solution heat treatment

[0290] The nine types of warm-drawn bars obtained in step 4 are simultaneously subjected to solution treatment. The specific process is as follows:

[0291] The first step is to heat the tubular resistance furnace to 835℃. The temperature uniformity of the tubular resistance furnace is ±5℃, and the length of the uniform temperature zone is 5000mm.

[0292] The second step involves placing the warm-drawn bars of different specifications prepared in step 4 into a uniform temperature zone and holding them there for 15 minutes.

[0293] The third step is straightening. After the heat preservation is completed, the drawn bars are transferred to the straightening machine for hot straightening.

[0294] The fourth step is to air-cool the warm-drawn bar to room temperature after straightening to obtain a solution-treated warm-drawn bar.

[0295] II. Aging Heat Treatment

[0296] The specific process for aging the obtained solution-treated rods is as follows:

[0297] The first step is to heat the tubular resistance furnace to 520°C.

[0298] The second step is to place the solution-treated warm-drawn bars of different specifications in a uniform temperature zone.

[0299] The third step is to keep it warm for 1 hour. After the warming period, air cool it to room temperature.

[0300] Thus, the preparation of 1500MPa grade fine-grained bimorphic Ti-5Al-5V-5Mo-3Cr titanium alloy rods for fasteners was completed. The diameter specifications of the rods are Φ6mm, Φ7mm, Φ9mm, Φ11mm, Φ13mm, Φ15mm, Φ17mm, Φ19mm and Φ21mm respectively.

Claims

1. A method for preparing 1500MPa grade Ti-5553 titanium alloy rods for fasteners, characterized in that, The specific process is as follows: Step 1, Two-phase region temperature-controlled free forging: The two-phase region temperature-controlled free forging is a two-phase region free forging process that involves six passes of controlled upsetting temperature rise on the forging billet; the forging billet is a Ti-5553 titanium alloy forged in the single-phase region. The microcrystalline grain size of the forging billet is better than grade 1, and the size of the forging billet is 400mm×400mm×800mm; The specific process of the two-phase region temperature-controlled free forging is as follows: The first step is single-fire forging; the single-fire forging includes heating and holding the billet, upsetting and drawing the billet, cooling and grinding; to obtain the billet after the first fire forging. In the process of roughening, the displacement of the anvil head is 40~200mm, the number of displacements of the anvil head is 2~10, and the cooling time between displacements of the anvil head is 5~60s. The second step is the forging of the remaining stages of fire; Repeat the process of heating and holding the billet in the first forging step, upsetting and drawing the billet, cooling and grinding, and then complete the remaining forging steps in sequence until the billet is completed by six two-phase temperature-controlled free forging steps, to obtain the Ti-5553 titanium alloy billet after two-phase temperature-controlled free forging. The dimensions of the forging billet, which has undergone temperature-controlled free forging in the two-phase region, are 400mm × 400mm × 800mm. Step 2, temperature-controlled elongation forging in the two-phase region: The number of heats in the two-phase region temperature-controlled drawing and forging process is n; n=20; the deformation rate of each heat-controlled drawing and forging process is <9.5%; the forging temperature is 835℃. Step 1: One-time temperature-controlled drawing and forging: Ⅰ Heating and holding the forging billet The box-type resistance furnace is heated to 835℃; the forging billet that has undergone two-phase temperature control free forging is placed in the uniform temperature zone and held for 280 minutes; a forging billet forged in one firing is obtained. After the heat preservation is completed, the forging billet is transferred from the box-type resistance furnace to the lower anvil of the high-speed forging hydraulic press; the transfer time is <1 minute; After the forging billet is removed, the box-type resistance furnace is shut off without powering off and is kept at 835℃. II. Temperature-controlled drawing and forging A forging billet with dimensions of 400mm×400mm×800mm is drawn and forged into a forging billet of 382mm×382mm×L1; L1 is the length of the forging billet after one controlled temperature drawing. III. Cooling the forging billet After the drawing and forging process is completed, the forging billet is placed on the lower anvil and air-cooled for 120 seconds. Complete the single-fire controlled-temperature elongation forging of the billet; The second step is a second-stage, temperature-controlled drawing and forging process: Ⅰ. Forged billets are returned to the furnace; The forging billet, which has undergone single-pass temperature-controlled drawing and forging, is placed in the uniform temperature zone of a box-type resistance furnace that is in a heat-preserving state and held for 15 minutes. After the heat preservation is completed, the forging billet is transferred from the box-type resistance furnace to the lower anvil of the forging hydraulic press. The transfer time is less than 1 minute. II. Temperature-controlled drawing and forging; A 4500-ton high-speed forging hydraulic press was used to forge the Ti-5553 titanium alloy forging billet that had been remelted, drawing the 382mm×382mm×L1 Ti-5553 titanium alloy forging billet to a 364mm×364mm×L2 forging billet; L2 is the length of the forging billet after secondary temperature-controlled drawing. A forging billet that has undergone secondary temperature-controlled elongation forging is obtained; III. Cooling the forging billet; The forged billet obtained after two-stage temperature-controlled drawing and forging was placed on the lower anvil and air-cooled for 115 seconds. Complete the second-stage temperature-controlled elongation forging process; The third step is temperature-controlled drawing and forging, which involves 3 to 19 heat treatments. Repeat the process of forging billet return to furnace, temperature-controlled elongation forging, and cooling billet in the two-stage forging process, and complete the forging process n=3~19 times in sequence to obtain the billet after the 19th two-phase region temperature-controlled elongation forging. The process of the 20th temperature-controlled drawing and forging is as follows: Ⅰ. Hot materials are returned to the furnace; The forging billet that has undergone the 19th temperature-controlled drawing and forging process is returned to the furnace; the forging billet is placed in a uniform temperature zone and held for 5 minutes; after the holding time is completed, the forging billet is transferred from the box-type resistance furnace to the lower anvil of the forging hydraulic press; the transfer time is <1 minute. II. Temperature-controlled drawing and forging; The billet is elongated by forging; the size of the billet obtained after the 19th forging is 166 mm x 166 mm x L 19 The billet is elongated by forging to a round bar-shaped billet of Φ180 mm x L III. Forging billet cooling; After the 20th temperature-controlled drawing forging of the forging billet is completed, the forging billet is air-cooled to room temperature; IV. Sawing; The round bar forging blank with a diameter of 180mm and a diameter of 1500mm was obtained by sawing. Step 3, temperature-controlled hot rolling in the two-phase region: The two-phase region temperature-controlled hot rolling includes two-phase region temperature-controlled hot rolling of temperature-controlled hot-rolled bar preforms and two-phase region temperature-controlled hot rolling of temperature-controlled hot-rolled bar finished products: The pre-form of the temperature-controlled hot-rolled bar has a specification of Φ45mm×950mm; the finished product of the temperature-controlled hot-rolled bar has a specification of Φ25mm×3000mm. When processing the temperature-controlled hot-rolled bar preforms: All the obtained Φ180mm×1500mm round bar forgings are processed into Φ45mm×950mm bars by two-phase zone temperature controlled hot rolling; the two-phase zone temperature controlled hot rolling is performed in 1 to 3 heat passes. The specific process is as follows: Ⅰ When the hot rolling process is 1 heat: The first step is the heating and holding of the round bar forging billet; The box-type resistance furnace is heated to 835℃; the Φ180mm×1500mm round bar forging billet prepared in step 2 is placed in the uniform temperature zone, and the heat preservation coefficient of the round bar forging billet is 0.5min / mm; The second step is temperature-controlled hot rolling. After the heat preservation is completed, the round bar forging billet is quickly transferred from the box-type resistance furnace to the rolling mill, with a transfer time of less than 1 minute. The round bar forging billet is then hot rolled. There are 15 rolling passes, and the deformation rate of each pass is 16.9%. The rolling speed is 0.5 m / s. In order to control the temperature rise during hot rolling, the cooling time between each pass is 5 s to 120 s. A round bar forging blank with a diameter of 45×L was obtained; The third step is straightening and sawing; after hot rolling, the Φ45×L round bar forging billet is transferred to a straightening machine for hot straightening. A straightened round bar forging blank is obtained; After the round bar forging billet is air-cooled to room temperature, it is sawn to obtain a temperature-controlled hot-rolled bar preform with a diameter of Φ45mm×950mm. II. When the hot rolling process involves 2 heat passes: i. First Fire The first step is the heating and holding of the round bar forging billet; The box-type resistance furnace is heated to 835℃; the Φ180mm×1500mm round bar forging billet prepared in step 2 is placed in the uniform temperature zone; The heat preservation coefficient is 0.5 min / mm; after the heat preservation is completed, the forging billet is quickly transferred from the box-type resistance furnace to the rolling mill, and the transfer time is <1 min, and the round bar forging billet is hot rolled. The second step is temperature-controlled hot rolling; The temperature-controlled hot rolling process consists of 10 passes, with a deformation rate of 12.9% for each pass; the rolling speed is 0.5 m / s; and the cooling time between each pass is 5 s to 120 s to control the temperature rise during hot rolling; resulting in a Φ90×L round bar forging billet after hot rolling. The third step is straightening and sawing; The round bar forging billet is transferred to a straightening machine for hot straightening; After straightening, the round bar forging billet is air-cooled to room temperature; then sawn; the Φ90mm×L round bar forging billet is sawn into Φ90mm×2000mm round bar forging billets; ⅱ Second Fire The first step is heating and heat preservation; the box-type resistance furnace is heated to 835°C, and the Φ90mm×2000mm round bar forging billet is placed in the uniform temperature zone with a heat preservation coefficient of 0.5min / mm. The second step is temperature-controlled hot rolling. After the heat preservation is completed, the round bar forging billet is transferred from the box-type resistance furnace to the rolling mill for hot rolling. The transfer time is <1min. The rolling passes are 10, and the deformation rate of each pass is 12.9%. The rolling speed is 0.5m / s. In order to control the temperature rise during hot rolling, the cooling time between each pass is 5s~120s. The temperature-controlled hot rolled round bar forging billet is obtained. The third step is straightening and sawing. After hot rolling, the round bar forging billet that has been hot rolled by temperature control is hot straightened. After straightening, it is air-cooled to room temperature. The round bar forging billet is air-cooled to room temperature and sawed to obtain a Φ45mm×950mm temperature-controlled hot-rolled bar preform. Ⅲ When the hot rolling process involves 3 heat passes: i. First Fire The first step is the heating and holding of the round bar forging billet; The box-type resistance furnace is heated to 835℃; the raw material is placed in the uniform temperature zone with a holding coefficient of 0.5 min / mm; after the holding period, the round bar forging billet is transferred from the box-type resistance furnace to the rolling mill, with a transfer time of <1 min; The second step is temperature-controlled hot rolling; The round bar forging billet is subjected to temperature-controlled hot rolling; the temperature-controlled hot rolling has 8 rolling passes, and the deformation rate of each pass is 11.6%; the rolling speed is 0.5m / s; in order to control the temperature rise during hot rolling, the cooling time between each pass is 5s~120s; after this hot rolling, a Φ180mm×L round bar forging billet after the first temperature-controlled hot rolling is obtained. The third step is to straighten it; The Φ180mm×L round bar forging billet is transferred to a straightening machine for hot straightening; after straightening, it is air-cooled to room temperature; thus, a straightened Φ110mm×L round bar forging billet is obtained. ⅱ Second Fire The first step is heating and heat preservation; the box-type resistance furnace is heated to 835℃; the straightened Φ110mm×L round bar forging billet is placed in the uniform temperature zone with a heat preservation coefficient of 0.5min / mm; The second step is temperature-controlled hot rolling. After the heat preservation is completed, the Φ110mm×L round bar forging billet is transferred from the box-type resistance furnace to the rolling mill. The transfer time is <1min. The round bar forging billet is then hot rolled. There are 8 rolling passes, and the deformation rate of each pass is 10.7%. The rolling speed is 0.5m / s. In order to control the temperature rise during hot rolling, the cooling time between each pass is 5s~120s. The round bar forging billet after temperature-controlled hot rolling is obtained. The third step is straightening and sawing. After hot rolling, the round bar forging billet that has undergone temperature-controlled hot rolling is transferred to a straightening machine for hot straightening. After straightening, it is air-cooled to room temperature to obtain a Φ70mm×2000mm round bar forging billet that has undergone temperature-controlled hot rolling in the second fire. iii. Third Fire The first step is heating and heat preservation; The box-type resistance furnace is heated to 835℃; the Φ70mm×2000mm round bar forging billet obtained after the second temperature-controlled hot rolling is placed in the uniform temperature zone with a heat preservation coefficient of 0.5min / mm; The second step is temperature-controlled hot rolling. After the heat preservation is completed, the round bar forging billet is transferred from the box-type resistance furnace to the rolling mill for hot rolling. The transfer time is less than 1 minute. There are 8 rolling passes to obtain the round bar forging billet after the third temperature-controlled hot rolling. In the temperature-controlled hot rolling, the average deformation rate of each pass is 10.5%. The rolling speed is 0.5 m / s. In order to control the temperature rise of hot rolling, the cooling time between each pass is 5 s to 120 s. The third step is straightening and sawing. After hot rolling, the round bar forging billet that has undergone the third temperature-controlled hot rolling is transferred to a straightening machine for hot straightening. After straightening, the bar is air-cooled to room temperature and sawn to obtain a hot-rolled bar pre-product with a diameter of 45mm and a diameter of 950mm. When processing the temperature-controlled hot-rolled bar stock: The first step is heating and heat preservation. The box-type resistance furnace is heated to 835℃. The obtained Φ45mm×950mm hot-rolled bar preforms are placed in the uniform temperature zone with a heat preservation coefficient of 0.8min / mm. The second step is temperature-controlled hot rolling; the hot-rolled bar preform is subjected to 6 to 10 passes of temperature-controlled hot rolling to obtain the hot-rolled bar preform. The rolling speed is 0.5 m / s; in order to control the temperature rise of the bar during the hot rolling process, the cooling time between each pass is 5 s to 120 s; The third step is to straighten it; After hot rolling, the hot-rolled bars are transferred to a straightening machine for hot straightening; after straightening, the bars are air-cooled to room temperature. A hot-rolled bar semi-finished product with a specification of Φ25mm×L was obtained; Step four, flat top; The two ends of the hot-rolled bar semi-finished product are sawn off to obtain a temperature-controlled hot-rolled bar finished product with a diameter of Φ25mm×3000mm; Step 4, two-phase region temperature drawing: Take 9 hot-rolled bars with a diameter of 25mm × 3000mm prepared in step 3, and use a multi-pass two-phase zone warm drawing method to draw the 9 hot-rolled bars with a diameter of 25mm × 3000mm into warm-drawn bars of different specifications of Φ6mm, Φ7mm, Φ9mm, Φ11mm, Φ13mm, Φ15mm, Φ17mm, Φ19mm and Φ21mm respectively. During the warm drawing process of warm-drawn bars with different target diameters, the deformation rate is the same for each pass; The drawing temperature for all bars with different target diameters is 500~600℃; The specific process of the two-phase region temperature drawing is as follows: First, the 9 Φ25mm*3000mm hot-rolled rod products are respectively sharpened to form a small cylindrical section at one end of each hot-rolled rod product; the length of the small cylindrical section is 300mm and the diameter is D 尖 ; D 尖 =D 模 -0.5 mm, wherein D 尖 is the diameter of the hot-rolled bar small cylindrical segment, D 模 is the hole diameter of the fixed die; The second step is to coat the finished hot-rolled bar with graphite emulsion and then place it in the uniform temperature zone of a tube furnace for heat preservation. The third step is to insert the hot-rolled bar into the drawing die after the heat preservation is completed, and use the chuck of the drawing machine to hold the rolled tip area of ​​the hot-rolled bar; start the drawing machine to perform warm drawing on the hot-rolled bar at a drawing speed of 30 mm / s; and obtain the warm-drawn bar. Step 5, Solution treatment and aging heat treatment of warm drawn bars: Solution treatment and aging heat treatment were performed on warm-drawn bars of different specifications to obtain Ti-5Al-5V-5Mo-3Cr titanium alloy bars with a fine grain dual-state structure of 1500MPa for fasteners.

2. The method for preparing 1500MPa grade Ti-5553 titanium alloy rods for fasteners as described in claim 1, characterized in that, The process of one-time forging is as follows: During the heating and holding of the forging billet, a box-type resistance furnace is used for heating. When the box-type resistance furnace reaches a temperature of 835°C, the forging billet is placed in a uniform temperature zone and held for 280 minutes. After the holding period, the forging billet is obtained. The forging billet is transferred from the box-type resistance furnace to the lower anvil of the high-speed forging hydraulic press in a transfer time of <1 min; During the forging and drawing of the billet, each firing cycle includes two consecutive forging and drawing cycles. The height of the forging billet is uplifted to 400mm; the uplifted forging billet is drawn to 400mm×400mm×800mm; the first uplift-drawing process is completed. The forging billet that has undergone the first upsetting and drawing process is then subjected to a second upsetting and drawing process; the height of the forging billet is upsetting to 400mm; the forging billet that has undergone the second upsetting process is then drawn to 400mm×400mm×800mm; the second upsetting and drawing process is then completed.

3. The method for preparing 1500MPa grade Ti-5553 titanium alloy rods for fasteners as described in claim 1, characterized in that, Based on the determined target diameter of each hot-rolled bar, the pass deformation rate in the two-phase region hot drawing is determined: The number of drawing passes for Φ6mm hot-rolled bars is 11 to 21, and the deformation rate per pass is determined to be 12.7% to 22.9% for different drawing passes. The number of drawing passes for Φ7mm hot-rolled bars is 9 to 19. The deformation rate per pass is determined to be 12.5% ​​to 24.6% for different drawing passes. The number of drawing passes for Φ9mm hot-rolled bars is 8 to 18, and the deformation rate per pass is determined to be 10.7% to 22.5% for different drawing passes. The number of drawing passes for Φ11mm hot-rolled bars is 6 to 15, and the deformation rate per pass is determined to be 13.9% to 23.9% for different drawing passes. The number of drawing passes for Φ13mm hot-rolled bars is 5 to 12, and the deformation rate per pass is determined to be 10.3% to 23.0% for different drawing passes. The drawing passes for Φ15mm hot-rolled bars are 4 to 9, and the deformation rate per pass is determined to be 10.7% to 22.5% for different drawing passes. The drawing passes for Φ17mm hot-rolled bars are 3 to 7, and the deformation rate per pass is determined to be 10.4% to 22.7% for different drawing passes. The drawing passes for Φ19mm hot-rolled bars are 3 to 5, and the deformation rate per pass is determined to be 10.4% to 22.7% for different drawing passes. The drawing passes for Φ21mm hot-rolled bars are 2 or 3. The deformation rate for each pass is determined to be 10.9%~16.0% for different drawing passes.

4. The method for preparing 1500MPa grade Ti-5553 titanium alloy rods for fasteners as described in claim 1, characterized in that, The solution treatment and aging heat treatment process for warm-drawn bars is as follows: Ⅰ Solution heat treatment The specific process of the solution heat treatment is as follows: The first step is to heat the tubular resistance furnace to 835℃; the temperature uniformity of the tubular resistance furnace is ±5℃ and the length of the uniform temperature zone is 5000mm. The second step is to place the warm-drawn bars of different specifications prepared in step 4 in a uniform temperature zone and keep them at that temperature for 15 minutes. The third step is to straighten it; After the heat preservation is completed, the drawn bars are transferred to the straightening machine for hot straightening. Fourth step: After straightening, the warm-drawn bar is air-cooled to room temperature; thus obtaining a solution-treated warm-drawn bar. II. Aging Heat Treatment The specific process for aging the obtained solution-treated warm-drawn bars is as follows: The first step is to heat the tubular resistance furnace to 520℃; The second step is to place the solution-treated rods of different specifications in a uniform temperature zone; The third step is to keep it warm for 1 hour; after the warming period, air cool it to room temperature.

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