Gradient cooling speed domain control heat treatment process suitable for TC18 piercing and rolling pipe

Abstract

The invention discloses a gradient cooling speed domain control heat treatment process suitable for a TC18 piercing and rolling pipe. The process comprises the following steps: 1, beta single-phase region solid solution treatment: placing the TC18 piercing and rolling pipe at a temperature higher than T beta for heat preservation and carrying out solid solution treatment; and secondly, gradient cooling speed domain control is conducted, specifically, high-temperature-stage rapid cooling is conducted, specifically, rapid cooling is conducted from a beta-phase region to be below T beta at the cooling speed not lower than 0.5 DEG C / s, and then low-temperature-stage narrow-domain controlled cooling is conducted, specifically, cooling is conducted to the room temperature at the cooling speed of 0.03-0.1 DEG C / s. According to the method, the beta single-phase region solid solution is combined with the gradient cooling speed region control heat treatment process, the cooling speed is accurately controlled, so that the homogenization reconstruction of beta grains of the pipe in the piercing and rolling state and the regulation and control of a multi-scale alpha-phase heterogeneous structure are efficiently realized, and the technical problem of strength-plasticity inversion of the TC18 pipe in the piercing and rolling state is solved; and the process is high in engineering applicability, high in tissue regulation and control precision and suitable for the field of key force bearing pipe fittings in the aerospace field.

Description

Technical Field

[0001] The present invention belongs to the technical field of titanium alloy heat treatment, and particularly relates to a gradient cooling rate domain control heat treatment process applicable to TC18 piercing and rolling tubes. Background Technique

[0002] As a core component in the field of advanced structural materials, TC18 tubes have become a key material carrier for high-end manufacturing such as aerospace, deep-sea equipment, and hydrogen energy storage and transportation due to their high specific strength, excellent plasticity, and outstanding corrosion resistance. In industrial manufacturing, the piercing and rolling technology has become the core process for realizing the high-precision, low-cost, and batch production of TC18 titanium tubes due to its near-net shaping ability and high material utilization rate. However, the inherent multi-pass local severe deformation characteristics above the β phase transformation point of this process result in obvious non-uniform grain size distribution in the tubes: the average size of large-scale grains is about 200 μm, while the average size of small-scale grains is about 20 μm. In addition, at the microscopic level, due to the imbalance of dynamic phase transformation kinetics, secondary α phase is abnormally enriched (volume fraction greater than 75%) and forms a submicron-scale needle-like morphology (average length less than 1 μm). This "non-uniform grain scale + α phase ultra-refinement" microstructure feature of the pierced and rolled state causes the yield strength of the material to exceed 1300 MPa while the elongation drops sharply to less than 3%, forming a typical strength-plasticity inversion effect. Therefore, while ensuring the strength of TC18 pierced and rolled tubes, improving the plasticity of the alloy is an urgent problem to be solved.

[0003] Existing studies have shown that α / β phase parameters (including α phase size, volume fraction, distribution morphology, and β grain size) are the main factors affecting the mechanical properties of titanium alloys. Specifically: fine needle-like α phase significantly improves the alloy strength through the strong pinning effect on dislocations; coarse lamellar or spherical α phase is more conducive to plasticity improvement through the crack propagation inhibition mechanism. Based on this, relevant studies have optimized the performance of TC18 alloy through different heat treatment paths: Patent CN115725916B uses a single-stage solution aging process to control the α precipitation phase size (5 μm - 0.2 μm), enabling the forged TC18 alloy to obtain a strong plasticity match with a tensile strength of 1108 MPa and an elongation of 9%; Patent CN115976441B constructs a duplex microstructure containing spherical primary α phase and needle-like secondary α phase through solution + multi-step aging, enabling the forged TC18 alloy to have a tensile strength exceeding 1400 MPa, but the elongation drops below 5%; Patent CN118186325B uses a four-stage multiple annealing process to synergistically control the grain boundary α phase and β grain size, and finally enables the forged TC18 alloy to achieve a comprehensive improvement in tensile strength of 1110 MPa - 1140 MPa and elongation of 11% - 14%. However, the above several methods have significant limitations in the engineering application of pierced and rolled tubes:

[0004] ① Organizational adaptability limitation: Existing heat treatment processes are all developed based on the forged microstructure and cannot effectively regulate the through-rolling microstructure formed by processing above the β phase transformation point.

[0005] ② Production efficiency bottleneck: Stepwise process parameters (such as using 2 aging temperatures in CN115976441B) significantly reduce the production line rhythm and are difficult to adapt to the large-batch production requirements of through-rolled tubes.

[0006] ③ Sharp increase in energy consumption cost: Complex multi-stage heat treatment (such as 4 temperature control conversions required for multiple annealing in CN118186325B) results in an increase in energy consumption cost by more than 40%, and the economic performance deteriorates significantly.

[0007] ④ Lack of precise cooling rate control: The existing process system overly focuses on optimizing time-temperature parameters (such as aging temperature gradient design and holding time control), but simply classifies the cooling rate as a process connection step and fails to achieve microstructure regulation by precisely changing the cooling rate. In addition, the random fluctuation of the wide-range cooling rate (such as the cooling rate range in the first stage of CN118186325B is 1 min / s to 20 min / s) easily leads to a significant increase in the dispersion degree of the α phase size, thus inducing abnormal fluctuations in the strength-plasticity matching. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a gradient cooling rate domain control heat treatment process suitable for TC18 through-rolled tubes in view of the above deficiencies of the existing technology. This process adopts an organizational optimization method combining β single-phase region solution treatment with gradient cooling rate domain control, realizes the β grain homogenization and α phase multi-scale microstructure reconstruction, improves the strength-plasticity matching relationship of TC18 tubes, and breakthroughly improves the comprehensive mechanical properties and engineering applicability of TC18 through-rolled tubes, and solves the bottleneck problems such as organizational adaptability limitation, lack of precise cooling rate control, low efficiency and high energy consumption, and insufficient mass production adaptability existing in the existing heat treatment processes.

[0009] To solve the above technical problems, the technical solution adopted by the present invention is: A gradient cooling rate domain control heat treatment process suitable for TC18 through-rolled tubes, characterized in that the process comprises the following steps:

[0010] Step 1, β single-phase region solution treatment: Place the TC18 through-rolled tube at a temperature of T β or above and hold for 0.5 h to 1 h for solution treatment;

[0011] Step 2, gradient cooling rate domain control: First, quickly cool the TC18 through-rolled tube after solution treatment in Step 1 at a cooling rate not lower than 0.5 °C / s from the β phase region to a temperature below T β and then perform narrow-domain controlled cooling in the low-temperature stage: Cool to room temperature at a cooling rate of 0.03 °C / s to 0.1 °C / s to obtain the heat-treated TC18 through-rolled tube.

[0012] The present invention innovatively proposes a gradient cooling rate domain control heat treatment process. On the basis of retaining the high-precision forming advantages of TC18 piercing and rolling tubes, the synergistic effect is exerted through "solution treatment in the β single-phase region + rapid high-temperature controlled cooling (not less than 0.5 °C / s) + narrow low-temperature domain controlled cooling (0.03 °C / s to 0.1 °C / s)". Specifically, first, a short-time solution treatment strategy in the β single-phase region is adopted. While suppressing the abnormal growth of β grains, the grain inhomogeneity of the original rolling structure in TC18 tubes is eliminated by the complete recrystallization of the β phase. Generally, the grain size dispersion can be reduced from the original ±180 μm to ±30 μm, significantly improving the anisotropy of the tubes. Then, a gradient cooling rate domain control process is adopted to break through the phase transformation regulation limitations of the traditional constant cooling rate process. First, rapid cooling in the high-temperature stage inhibits the overgrowth and coarsening of β grains and quickly provides the nucleation driving force for the α phase. Then, in the α+β two-phase region, the kinetics of α-phase precipitation is precisely controlled through the coupling effect of cooling rate - temperature - phase transformation in the low-temperature stage narrow domain controlled cooling, obtaining a multi-scale heterogeneous structure with coexisting micron-sized coarse α phase (10 μm to 22 μm) and fine α phase (less than 1 μm) inside the uniform β matrix, taking into account the dislocation pinning strengthening and crack propagation hindrance effects, thereby simultaneously improving the strength and plasticity of TC18 piercing and rolling tubes.

[0013] The above-mentioned gradient cooling rate domain control heat treatment process applicable to TC18 piercing and rolling tubes is characterized in that the chemical composition of the TC18 piercing and rolling tubes meets the requirements of TC18 in GB / T 3620.1-2020 "Titanium and Titanium Alloys - Designations and Chemical Compositions".

[0014] The above-mentioned gradient cooling rate domain control heat treatment process applicable to TC18 piercing and rolling tubes is characterized in that the initial tissue characteristics of the TC18 piercing and rolling tubes in step one are: the grain size dispersion is relatively large, where the average size of large-scale grains is close to 200 μm, while the average size of small-scale grains is close to 20 μm, the volume fraction of abnormally rich secondary α phase is greater than 75%, and it shows a submicron-scale needle-like morphology with an average length of less than 1 μm.

[0015] The above-mentioned gradient cooling rate domain control heat treatment process applicable to TC18 piercing and rolling tubes is characterized in that the temperature of the solution treatment in step one is T β 10 °C to 50 °C higher than the above.

[0016] The above-mentioned gradient cooling rate domain control heat treatment process applicable to TC18 piercing and rolling tubes is characterized in that the grain size dispersion of the TC18 piercing and rolling tubes after solution treatment in step one is lower than ±20 μm.

[0017] The above-mentioned gradient cooling rate domain control heat treatment process applicable to TC18 piercing and rolling tubes is characterized in that the cooling rate in the high-temperature stage in step two is 0.5 °C / s.

[0018] The above-mentioned gradient cooling rate domain control heat treatment process applicable to TC18 piercing and rolling tubes is characterized in that the rapid cooling end temperature of the rapid cooling in the high temperature stage in step two is T β below 30°C to 70°C. More preferably, the rapid cooling end temperature of the rapid cooling in the high temperature stage is T β below 50°C to 70°C.

[0019] Generally, there is a corresponding relationship between the solution treatment temperature in step one of the present invention and the rapid cooling end temperature of the rapid cooling in the high temperature stage in step two. Generally, the temperature difference is 80°C. For example, when the solution treatment temperature is T β +10°C, it corresponds to T β -70°C, T β +50°C corresponds to T β -30°C to accurately control the starting temperature and ending temperature of the first stage of the gradient cooling rate domain control, namely the rapid cooling in the high temperature stage, and ensure the unity of the temperature reduction temperature range.

[0020] The above-mentioned gradient cooling rate domain control heat treatment process applicable to TC18 piercing and rolling tubes is characterized in that the cooling rate of the narrow domain control cooling in the low temperature stage in step two is 0.06°C / s to 0.1°C / s.

[0021] The above-mentioned gradient cooling rate domain control heat treatment process applicable to TC18 piercing and rolling tubes is characterized in that the tissue characteristics of the TC18 piercing and rolling tubes after the heat treatment in step two are: β grain homogenization, the grain size dispersion is less than ±30μm, and the multi-scale heterogeneous microstructure is reconstructed. Among them, the size of the coarse α phase is 10μm to 22μm, and the size of the fine α phase is less than 1μm. More preferably, the size of the coarse α phase is 10μm to 15μm.

[0022] The above-mentioned gradient cooling rate domain control heat treatment process applicable to TC18 piercing and rolling tubes is characterized in that the tensile strength of the TC18 piercing and rolling tubes after the heat treatment in step two is 1052MPa to 1126MPa, the yield strength is 934MPa to 1012MPa, and the elongation is 13.0% to 15.6%. More preferably, the tensile strength of the TC18 piercing and rolling tubes after the heat treatment is 1110MPa to 1120MPa, the yield strength is 991MPa to 1004MPa, and the elongation is 13.0% to 14.1%.

[0023] The present invention has the following advantages compared with the prior art:

[0024] 1. The heat treatment process of the present invention adopts a heat treatment process of β single-phase region solution + high-temperature rapid controlled cooling + low-temperature narrow-range controlled cooling. By precisely controlling the temperature and cooling rate of the gradient cooling rate range control, the uniform reconstruction of β grains of the pierced and rolled tube is efficiently realized, and the multi-scale α-phase heterogeneous tissue is regulated. A multi-scale heterogeneous tissue with uniform β grains and containing micron-sized coarse α phases and nano-sized fine α phases is obtained, which improves the plasticity of the TC18 tube while ensuring the strength of the TC18 tube, enhances the strength-plasticity matching relationship of the TC18 tube, and provides key technical support for improving the service life of aerospace thin-walled components and deep-sea equipment.

[0025] 2. The heat treatment process of the present invention realizes the uniform control of β grains, eliminates the local stress concentration caused by the highly uneven grain size of the original pierced and rolled structure in the TC18 tube, improves the quality of the TC18 tube, reconstructs the multi-scale heterogeneous microstructure, realizes the comprehensive matching improvement of the strength and plasticity of the TC18 tube, and breakthroughly improves the tensile strength of the TC18 pierced and rolled tube to be 1052 MPa - 1126 MPa, the yield strength to be 934 MPa - 1012 MPa, and the elongation to be 13.0% - 15.6%.

[0026] 3. The heat treatment process of the present invention is simple. The whole process only requires single heating and continuous cooling. Compared with the traditional multi-stage heat treatment, the heat treatment process is shortened by more than 50%, the number of equipment starts and stops is reduced by 70%, and the production line processing efficiency is increased by more than 2 times. It has the characteristics of strong engineering applicability and high tissue regulation accuracy, meets the industrial needs of large quantities of pierced and rolled tubes, realizes the engineering application scenario of strength-plasticity matching, and can provide a reliable solution for the performance optimization of key load-bearing pipe fittings in the aerospace field.

[0027] The technical solution of the present invention will be further described in detail below through the accompanying drawings and embodiments. Description of the Drawings

[0028] Figure 1 It is the EBSD diagram of the original grain distribution of the TC18 pierced and rolled tube used in the present invention.

[0029] Figure 2 It is the SEM diagram of the original microstructure of the TC18 pierced and rolled tube used in the present invention.

[0030] Figure 3 It is the flow chart of the gradient cooling rate range control heat treatment process of the present invention.

[0031] Figure 4 It is the SEM diagram of the grain distribution of the TC18 pierced and rolled tube after solution treatment in Example 1 of the present invention.

[0032] Figure 5 It is the SEM diagram of the microstructure of the TC18 pierced and rolled tube after heat treatment in Example 1 of the present invention.

[0033] Figure 6 This is the SEM micrograph of the microstructure of the TC18 pierced and rolled tube after heat treatment in Example 2 of the present invention.

[0034] Figure 7 This is the SEM micrograph of the microstructure of the TC18 pierced and rolled tube after heat treatment in Example 3 of the present invention.

[0035] Figure 8 This is the SEM micrograph of the microstructure of the TC18 pierced and rolled tube after heat treatment in Example 4 of the present invention.

[0036] Figure 9 This is the SEM micrograph of the microstructure of the TC18 pierced and rolled tube after heat treatment in Example 5 of the present invention.

[0037] Figure 10 This is the SEM micrograph of the microstructure of the TC18 pierced and rolled tube after single-stage aging in Comparative Example 1 of the present invention.

[0038] Figure 11 This is the SEM micrograph of the microstructure of the TC18 pierced and rolled tube after single-stage aging in Comparative Example 2 of the present invention.

[0039] Figure 12 This is the SEM micrograph of the microstructure of the TC18 pierced and rolled tube after single-stage aging in Comparative Example 3 of the present invention. Detailed implementation manners

[0040] In Examples 1-5 and Comparative Examples 1-3 of the present invention, TC18 titanium alloy pierced and rolled tubes with a specification of φ102mm / 78mm were used as raw materials, and their chemical composition was: Ti-5Al-5Mo-5V-1Cr-1Fe, meeting the requirements of TC18 in GB / T3620.1-2020 "Titanium and Titanium Alloy Grades and Chemical Compositions", and the phase transformation point T β was 830 °C, and its original grain distribution and original microstructure were as Figures 1 to 2 shown. It can be seen from Figures 1 to 2 that the initial tissue characteristics of this TC18 pierced and rolled tube are: (1) The phenomenon of uneven grain size distribution: The grain size dispersion is relatively large, where the average size of large-scale grains is close to 200 μm, while the average size of small-scale grains is close to 20 μm; (2) The volume fraction of the secondary α-phase abnormal aggregation is greater than 75%, and it shows a sub-micron needle-like morphology with an average length of less than 1 μm. The yield strength of this TC18 pierced and rolled tube is 1300 MPa, and the elongation is 3%.

[0041] As Figure 3 shown, the process of the gradient cooling rate domain control heat treatment process of the present invention is: (1) Solution treatment in the β single-phase region: At T βSolution treatment is carried out at 10°C to 50°C (i.e., 840°C to 880°C) for 0.5 h to 1 h to eliminate the characteristics of the original microstructure; (2) Gradient cooling rate domain control: In the high-temperature stage, the solution-treated TC18 cross-rolled tube is rapidly cooled from the β phase region to T β below 30°C to 70°C (i.e., 760°C to 800°C) at a cooling rate of 0.03°C / s to 0.1°C / s in the low-temperature stage until room temperature is reached.

[0042] Example 1

[0043] This example includes the following steps:

[0044] Step 1: Solution in the β single-phase region: Place the TC18 cross-rolled tube at 840°C for 1 h for solution treatment;

[0045] Step 2: Gradient cooling rate domain control: First, rapidly cool the TC18 cross-rolled tube after solution treatment in Step 1 in the high-temperature stage: rapidly cool from the β phase region to 760°C at a cooling rate of 0.5°C / s, and then perform narrow-domain controlled cooling in the low-temperature stage: cool to room temperature at a cooling rate of 0.03°C / s to obtain the heat-treated TC18 cross-rolled tube.

[0046] Figure 4 is the SEM image of the grain distribution of the TC18 cross-rolled tube after solution treatment in this example. From Figure 4 it can be seen that the average grain size of the TC18 cross-rolled tube after solution treatment is 342 μm, and the grain size dispersion is ±19 μm.

[0047] Figure 5 is the SEM image of the microstructure of the heat-treated TC18 cross-rolled tube in this example. Region 1 is coarse α phase, and region 2 is fine α phase. From Figure 5 it can be seen that the microstructure of the heat-treated TC18 cross-rolled tube exhibits multi-scale heterogeneous characteristics. Among them, the average size of the coarse α phase is about 22 μm, and the volume fraction is about 30%. The average size of the fine α phase is less than 1 μm, and the volume fraction is about 10%.

[0048] Example 2

[0049] This example includes the following steps:

[0050] Step 1: Solution in the β single-phase region: Place the TC18 cross-rolled tube at 840 for 1 h for solution treatment;

[0051] Step 2. Gradient cooling rate domain control: First, quickly cool the solution-treated TC18 cross-rolled tube in Step 1 at a high temperature stage: rapidly cool from the β phase region to 760°C at a cooling rate of 0.5°C / s, and then perform narrow domain controlled cooling at a low temperature stage: cool to room temperature at a cooling rate of 0.06°C / s to obtain the heat-treated TC18 cross-rolled tube.

[0052] Figure 6 This is the SEM micrograph of the microstructure of the heat-treated TC18 cross-rolled tube in this example. Region 1 is the coarse α phase, and region 2 is the fine α phase. From Figure 6 It can be seen that the microstructure of the heat-treated TC18 cross-rolled tube shows multi-scale heterogeneous characteristics. Among them, the average size of the coarse α phase is about 15 μm, and the volume fraction is about 20%. The average size of the fine α phase is less than 1 μm, and the volume fraction is about 30%.

[0053] Example 3

[0054] This example includes the following steps:

[0055] Step 1. Solution treatment in the β single-phase region: Place the TC18 cross-rolled tube at 840°C and hold for 1 h for solution treatment;

[0056] Step 2. Gradient cooling rate domain control: First, quickly cool the solution-treated TC18 cross-rolled tube in Step 1 at a high temperature stage: rapidly cool from the β phase region to 760°C at a cooling rate of 0.5°C / s, and then perform narrow domain controlled cooling at a low temperature stage: cool to room temperature at a cooling rate of 0.1°C / s to obtain the heat-treated TC18 cross-rolled tube.

[0057] Figure 7 This is the SEM micrograph of the microstructure of the heat-treated TC18 cross-rolled tube in this example. Region 1 is the coarse α phase, and region 2 is the fine α phase. From Figure 7 It can be seen that the microstructure of the heat-treated TC18 cross-rolled tube shows multi-scale heterogeneous characteristics. Among them, the average size of the coarse α phase is about 10 μm, and the volume fraction is about 10%. The average size of the fine α phase is less than 1 μm, and the volume fraction is about 50%.

[0058] Example 4

[0059] This example includes the following steps:

[0060] Step 1. Solution treatment in the β single-phase region: Place the TC18 cross-rolled tube at 880°C and hold for 1 h for solution treatment;

[0061] Step 2. Gradient cooling rate domain control: The solution-treated TC18 pierced and rolled tubes in Step 1 are first rapidly cooled in the high-temperature stage: rapidly cooled from the β phase region to 800 °C at a cooling rate of 0.5 °C / s, and then cooled in a narrow domain in the low-temperature stage: cooled to room temperature at a cooling rate of 0.1 °C / s to obtain the heat-treated TC18 pierced and rolled tubes.

[0062] Figure 8 This is the SEM micrograph of the microstructure of the heat-treated TC18 pierced and rolled tubes in this example. Region 1 is coarse α phase, and region 2 is fine α phase. From Figure 8 It can be seen that the microstructure of the heat-treated TC18 pierced and rolled tubes exhibits a multi-scale heterogeneous structure. Among them, the average size of the coarse α phase is about 10 μm, and the volume fraction is about 10%. The average size of the fine α phase is less than 1 μm, and the volume fraction is about 50%.

[0063] Example 5

[0064] This example includes the following steps:

[0065] Step 1. Solution treatment in the β single-phase region: Place the TC18 pierced and rolled tubes at 880 °C and hold for 0.5 h for solution treatment;

[0066] Step 2. Gradient cooling rate domain control: The solution-treated TC18 pierced and rolled tubes in Step 1 are first rapidly cooled in the high-temperature stage: rapidly cooled from the β phase region to 800 °C at a cooling rate of 0.5 °C / s, and then cooled in a narrow domain in the low-temperature stage: cooled to room temperature at a cooling rate of 0.03 °C / s to obtain the heat-treated TC18 pierced and rolled tubes.

[0067] Figure 9 This is the SEM micrograph of the microstructure of the heat-treated TC18 pierced and rolled tubes in this example. Region 1 is coarse α phase, and region 2 is fine α phase. From Figure 9 It can be seen that the microstructure of the heat-treated TC18 pierced and rolled tubes exhibits a multi-scale heterogeneous structure. Among them, the average size of the coarse α phase is about 22 μm, and the volume fraction is about 30%. The average size of the fine α phase is less than 1 μm, and the volume fraction is about 10%.

[0068] Comparative Example 1

[0069] This comparative example includes the following steps:

[0070] Step 1. Solution treatment in the β single-phase region: Place the TC18 pierced and rolled tubes at 840 °C and hold for 1 h for solution treatment;

[0071] Step 2. Single-stage aging: The solution-treated TC18 pierced and rolled tubes in Step 1 are rapidly cooled from the β phase region to 750 °C at a cooling rate of 0.5 °C / s and held for 3 h for single-stage aging, and then water-cooled to room temperature to obtain the heat-treated TC18 pierced and rolled tubes.

[0072] Figure 10 This is the SEM micrograph of the TC18 pierced and rolled tube after single-stage aging in this comparative example. From Figure 10 it can be seen that the microstructure of the TC18 pierced and rolled tube after this heat treatment shows a homogeneous feature of micron-sized α precipitation phase. The average size of the α phase is about 20 μm, and the volume fraction is about 20%.

[0073] Comparative Example 2

[0074] This comparative example includes the following steps:

[0075] Step 1: Solution treatment in the β single-phase region: Place the TC18 pierced and rolled tube at 840 °C and hold for 1 h for solution treatment;

[0076] Step 2: Single-stage aging: First, quickly cool the TC18 pierced and rolled tube after solution treatment in Step 1 at a cooling rate of 0.5 °C / s from the β phase region to 650 °C, hold for 3 h for single-stage aging, and then water-cool to room temperature to obtain the heat-treated TC18 pierced and rolled tube.

[0077] Figure 11 This is the SEM micrograph of the TC18 pierced and rolled tube after single-stage aging in this comparative example. From Figure 11 it can be seen that the microstructure of the TC18 pierced and rolled tube after this heat treatment shows a homogeneous feature of micron-sized α precipitation phase. The average size of the α phase is about 5 μm, and the volume fraction is about 35%.

[0078] Comparative Example 3

[0079] This comparative example includes the following steps:

[0080] Step 1: Solution treatment in the β single-phase region: Place the TC18 pierced and rolled tube at 840 °C and hold for 1 h for solution treatment;

[0081] Step 2: Single-stage aging: First, quickly cool the TC18 pierced and rolled tube after solution treatment in Step 1 at a cooling rate of 0.5 °C / s from the β phase region to 550 °C, hold for 3 h for single-stage aging, and then water-cool to room temperature to obtain the heat-treated TC18 pierced and rolled tube.

[0082] Figure 12 This is the SEM micrograph of the TC18 pierced and rolled tube after single-stage aging in this comparative example. From Figure 12 it can be seen that the microstructure of the TC18 pierced and rolled tube after this heat treatment shows a homogeneous feature of micron-sized α precipitation phase. The average size of the α phase is less than 1 μm, and the volume fraction is about 55%.

[0083] The room temperature mechanical properties, average grain size, and grain size dispersion of the TC18 pierced and rolled tubes after heat treatment in Examples 1-5 and Comparative Examples 1-3 of the present invention are shown in Table 1 below.

[0084] Table 1

[0085]

[0086] Comparing Examples 1-5 of the present invention with Comparative Examples 1-3, it can be seen that each example of the present invention shows significant advantages in terms of mechanical properties and microstructural control. Specifically, Examples 1-5 exhibit a more superior strength and plasticity match, where the tensile strength (1052 MPa - 1126 MPa) and yield strength (934 MPa - 1012 MPa) are significantly better than those of Comparative Example 1 (907 MPa / 854 MPa) and Comparative Example 2 (1010 MPa / 942 MPa). At the same time, the elongation rate is generally high (13.0% - 15.6%), which is significantly better than that of Comparative Example 3 (only 1.5%). This indicates that each example, through optimizing the microstructure, avoids extreme embrittlement phenomena similar to Comparative Example 3 while increasing the strength. Therefore, in the multi-scale heterogeneous microstructure constructed by the heat treatment process of the present invention, the fine acicular α phase significantly improves the strength of the TC18 pierced and rolled tubes through a strong pinning effect on dislocations, while the coarse lamellar α phase ensures the plasticity of the TC18 pierced and rolled tubes through a crack propagation inhibition mechanism. On the contrary, only using a single-stage aging heat treatment process, in Comparative Example 1, due to the relatively high volume fraction (20%) of coarse α phase and the lack of fine α phase, the strength of the TC18 pierced and rolled tube is low; in Comparative Example 3, due to complete dependence on fine α phase (volume fraction 55%) and the absence of coarse α phase, although the tensile strength is as high as 1334 MPa, the elongation rate drops sharply to 1.5%, reflecting the limitations of a single strengthening mechanism; although Comparative Example 2 shows a relatively good strength and plasticity match, Examples 1-5 of the present invention have achieved a significant simultaneous increase in both strength and plasticity on this basis.

[0087] The above are only the preferred embodiments of the present invention and do not impose any limitations on the present invention. Any simple modifications, changes, and equivalent variations made to the above embodiments based on the technical essence of the invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A gradient cooling rate domain control heat treatment process applicable to TC18 pierced and rolled tubes, characterized in that, The process includes the following steps: Step 1. Solution treatment in the β single-phase region: Place the TC18 pierced and rolled tube at a temperature above T β and hold for 0.5 h to 1 h for solution treatment; Step 2. Gradient cooling rate domain control: First, quickly cool the TC18 pierced and rolled tubes after solution treatment in step 1 at a high temperature stage: rapidly cool from the β phase region to below T β at a cooling rate not lower than 0.5 °C / s, and then perform narrow domain controlled cooling at a low temperature stage: cool to room temperature at a cooling rate of 0.03 °C / s to 0.1 °C / s to obtain the heat-treated TC18 pierced and rolled tubes.

2. A gradient cooling rate domain control heat treatment process applicable to TC18 pierced and rolled tubes according to claim 1, characterized in that, The chemical composition of the TC18 pierced and rolled tube meets the requirements of TC18 in GB / T 3620.1-2020 "Titanium and Titanium Alloy Grades and Chemical Compositions".

3. A gradient cooling rate domain control heat treatment process applicable to TC18 pierced and rolled tubes according to claim 1, characterized in that, The initial microstructure characteristics of the TC18 pierced and rolled tube in Step 1 are as follows: the grain size has a large dispersion, where the average size of large-scale grains is close to 200 μm, while the average size of small-scale grains is close to 20 μm, the volume fraction of the abnormally enriched secondary α-phase is greater than 75%, and it shows a submicron-scale needle-like morphology with an average length less than 1 μm.

4. A gradient cooling rate domain control heat treatment process applicable to TC18 pierced and rolled tubes according to claim 1, characterized in that, The solution treatment temperature described in Step 1 is T β 10°C to 50°C higher than the above temperature.

5. A gradient cooling rate domain control heat treatment process applicable to TC18 pierced and rolled tubes according to claim 1, characterized in that, After solution treatment, the grain size dispersion of the TC18 pierced and rolled tube in Step 1 is lower than ±20 μm.

6. The gradient cooling rate domain control heat treatment process applicable to TC18 pierced and rolled tubes according to claim 1, characterized in that, The fast cooling rate in the high-temperature stage in Step 2 is 0.5 °C / s.

7. A gradient cooling rate domain control heat treatment process applicable to TC18 pierced and rolled tubes according to claim 1, characterized in that, The rapid cooling end temperature of the rapid cooling in the high temperature stage described in step two is T β to 30°C to 70°C below.

8. A gradient cooling rate domain control heat treatment process applicable to TC18 pierced and rolled tubes according to claim 1, characterized in that, The cooling rate of narrow-range controlled cooling in the low-temperature stage in Step 2 is 0.06 °C / s to 0.1 °C / s.

9. A gradient cooling rate domain control heat treatment process applicable to TC18 pierced and rolled tubes according to claim 1, characterized in that, The microstructure characteristics of the TC18 pierced and rolled tube after heat treatment in Step 2 are as follows: the β grains are homogenized, the grain size dispersion is lower than ±30 μm, and the multi-scale heterogeneous microstructure is reconstructed, where the size of the coarse α-phase is 10 μm to 22 μm, and the size of the fine α-phase is less than 1 μm.

10. A gradient cooling rate domain control heat treatment process applicable to TC18 pierced and rolled tubes according to claim 1, characterized in that, The tensile strength of the TC18 pierced and rolled tube after heat treatment in Step 2 is 1052 MPa to 1126 MPa, the yield strength is 934 MPa to 1012 MPa, and the elongation is 13.0% to 15.6%.

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