A method for preparing high-strength and high-toughness equiaxed ultrafine-grained pure titanium

Abstract

The present invention belongs to the field of material preparation, and specifically relates to a method for preparing high-strength and high-toughness equiaxed ultrafine-grained pure titanium. It comprises the following steps: (1) multi-directional hot forging: selecting original coarse-grained block pure titanium, and hot forging the block pure titanium below the recrystallization temperature; (2) rolling: rolling the block pure titanium after multi-directional hot forging in step (1) at room temperature, with the total rolling reduction being greater than 70%; (3) transient heat treatment: subjecting the titanium block after rolling in step (2) to transient heat treatment for 10 to 180 seconds above the recrystallization temperature, and then air cooling to prepare equiaxed ultrafine-grained pure titanium. The product prepared by the method of the present invention has an ultrafine-grained structure metal material with extremely low dislocation density, and achieves good strength-ductility matching performance. The present invention can be implemented with simple equipment, is easy to control economic costs and production costs, and is easy to mass-produce industrially.

Description

Technical Field

[0001] The invention belongs to the field of material preparation, and in particular relates to a method for preparing high-strength and high-toughness equiaxed ultrafine-grained pure titanium. Background Art

[0002] Plastic deformation techniques are widely used to produce superior ultrafine-grained (UFG) metal materials, enhancing their strength. For example, UFG metal materials produced through intense plastic deformation methods such as equal channel angular pressing, high-pressure torsion, and cumulative rolling have significantly improved their strength, hardness, and wear resistance, but their toughness and ductility have been significantly reduced. This is primarily due to the fact that intense plastic deformation creates a large number of deformed structures, defects, and substructures within the material, significantly increasing the overall deformation energy storage and leading to structural instability.

[0003] Annealing heat treatment is a traditional treatment method for improving microstructural defects and releasing residual stress of materials. However, conventional annealing usually leads to problems such as grain coarsening inside the material, causing the performance of the metal material to deteriorate. For example, Yanxia Gu et al. published a paper titled "Simultaneously improving mechanical properties and corrosion resistance of pure Ti by continuous ECAPplus short-duration annealing" (Materials Characterization, 2018, 138: 38-47) in Materials Characterization. Ultrafine-grained pure titanium materials were prepared by continuous equal channel angular extrusion technology and low-temperature short-time annealing. The material structure prepared by this processing method contains a large number of defects such as low-angle grain boundaries and high-density dislocations, which hinder dislocation movement, resulting in a significant reduction in the plasticity of the material, causing the risk of early fracture, and limiting its practical application as a structural component. For example, Chinese patent CN105821360A proposes a preparation method that combines asynchronous rolling and short-time annealing to improve the strength and plasticity of metallic titanium. This processing method requires asynchronous rolling and strict control of the subsequent recrystallization annealing holding time. This process is complex and unstable, with poor controllability, which is not conducive to the promotion of the technology. Therefore, it is urgent to invent a method for efficiently preparing high-strength and high-toughness ultrafine-grained pure titanium that can achieve both high strength and high toughness to a certain extent, significantly improving the overall performance of industrial pure titanium to meet the needs of industrial production. Summary of the Invention

[0004] The purpose of the present invention is to provide a low-cost and repeatable method for preparing high-strength and high-toughness equiaxed ultrafine-grained pure titanium, and the high-strength and high-toughness equiaxed ultrafine-grained pure titanium material is prepared by combining three steps of multi-directional hot forging, rolling and transient heat treatment.

[0005] The technical solution for achieving the purpose of the present invention is: a method for preparing high-strength and high-toughness equiaxed ultrafine-grained pure titanium, comprising the following steps:

[0006] Step (1): Multi-directional hot forging: Select original coarse-grained bulk pure titanium and perform hot forging on the bulk pure titanium below the recrystallization temperature;

[0007] Step (2): rolling: rolling the block of pure titanium after multi-directional hot forging in step (1) at room temperature, with the total rolling reduction being greater than 70%;

[0008] Step (3): Transient heat treatment: The titanium block after rolling in step (2) is subjected to transient heat treatment above the recrystallization temperature for 10 to 180 seconds, and then air-cooled to prepare equiaxed ultrafine-grained pure titanium.

[0009] Furthermore, the multi-directional hot forging in step (1) is specifically as follows:

[0010] After the pure titanium sample is heated at a constant temperature, it is forged once in three directions, namely, the X-axis, Y-axis, and Z-axis directions, below the recrystallization temperature. This is considered as a forging cycle, and the reduction in each forging is 5%-30%. The multi-directional hot forging treatment includes at least three forging cycles. After each forging cycle is completed, it is reheated to the set temperature.

[0011] Furthermore, the rolling in step (2) is specifically as follows: rolling is performed at a rolling speed of not less than 10 r / min, with a reduction of 5±1% per pass.

[0012] Furthermore, the transient heat treatment in step (3) is specifically as follows: placing the rolled pure titanium into a high-frequency induction furnace for heating, with a heating rate of ≥100°C / s.

[0013] A high-strength and high-toughness equiaxed ultrafine-grained pure titanium is prepared by the above method, and the grain size of the pure titanium is less than 1 μm.

[0014] The present invention is based on the concept of grain boundary relaxation stabilization of ultrafine-grained metal materials. Specifically, the present invention proposes a technical concept for producing high-strength, high-toughness, equiaxed ultrafine-grained pure titanium by controlling the annealing conditions during the preparation process to induce strong grain boundary relaxation. Specifically, the present invention demonstrates that, compared with grain coarsening, the recovery of deformation defects at or near grain boundaries after severe plastic deformation exhibits a more pronounced dependence on heating rate. Defects such as vacancies and dislocations within the grains are significantly reduced through transient heat treatment. Simultaneously, due to the short heat treatment time, the grain size remains essentially unchanged, thereby producing a high-strength, high-toughness, equiaxed ultrafine-grained pure titanium material.

[0015] Compared with the prior art, the present invention has the following significant advantages:

[0016] (1) The present invention adopts a method combining three simple processes of multi-directional hot forging, rolling and transient heat treatment, which has a simple operation process, high production efficiency, low cost, and no material size restriction, and is suitable for industrial large-scale production.

[0017] (2) Compared with the traditional process, the ultrafine-grained pure titanium prepared by the present invention has a uniform microstructure and low dislocation density inside the organization, which can overcome the defects caused by structural heterogeneity and has better strength-plasticity matching.

[0018] (3) Compared with conventional metal strengthening and toughening treatment methods, the method for preparing high-strength and high-toughness equiaxed ultrafine-grained pure titanium provided by the present invention has short heat treatment time, high temperature and strong controllability. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the processing flow of the present invention.

[0020] Figure 2 This is an EBSD morphology image of the equiaxed ultrafine grained industrial pure titanium with low dislocation density obtained after being processed by the preparation method of the present invention in Example 1.

[0021] Figure 3 The engineering stress-engineering strain curves of coarse-grained industrial pure titanium and ultrafine-grained industrial pure titanium obtained after being processed by the preparation method of the present invention in Example 1 are shown.

[0022] Description of Reference Numerals

[0023] 1-Box annealing equipment; 2-Forging equipment; 3-Industrial pure titanium; 4-Rolling process; 5-Pure titanium sample after hot forging; 6-High-frequency induction heating equipment, 7-Crucible, 8-Heating coil. DETAILED DESCRIPTION

[0024] The present invention is further described in detail below with reference to the accompanying drawings.

[0025] The present invention discloses a method for preparing high-strength and high-toughness equiaxed ultrafine-grained pure titanium. The method first performs a multi-directional hot forging treatment on a block of pure titanium material below the recrystallization temperature, significantly refining the original billet structure and making it more uniform. The multi-directional hot forged sample is then subjected to room temperature rolling with a deformation of more than 70% to prepare an ultrafine structure, which contains a large number of deformation defects and dynamic recrystallization structure during the deformation process. Finally, the material is subjected to a transient heat treatment for 10 to 180 seconds above its recrystallization temperature to prepare equiaxed ultrafine-grained (less than 1 μm) pure titanium. The material has a low internal dislocation density, small grain size, high strength, and good toughness and ductility. The ultrafine-grained pure titanium prepared by this preparation method has a room temperature tensile test showing that its yield strength is increased by about 60% and its tensile strength is increased by about 40%.

[0026] Example

[0027] First, this example uses the following equipment: a forging machine, a hot and cold two-roll mill, and a high-frequency induction heating device. The initial block of industrial pure titanium sample used in this example has a size of 30mm × 30mm × 40mm (length × width × thickness). The specific operation is as follows:

[0028] (1) Isothermal forging of rectangular block-shaped pure titanium specimens was performed. The forging temperature was set at 450°C and kept in a box-type annealing furnace for 30 minutes. The pure titanium specimen was forged once in each of the three directions (X-axis, Y-axis, and Z-axis) as a forging cycle. The reduction for each forging pass was 25%. The multi-directional hot forging treatment included at least three forging cycles to significantly refine the grains. After each forging cycle, the specimen was returned to the furnace and heated to the forging temperature.

[0029] (2) Rolling the hot-forged sample at room temperature with a roller speed of 35 Hz, with a reduction of 0.2 mm per pass, so that the total deformation of the original sample reaches more than 70%;

[0030] (3) Transient heat treatment is performed on the above-mentioned pure titanium plate sample with severe plastic deformation. The above-mentioned pure titanium plate after cold rolling is placed in a high-frequency induction heating device, the heat treatment temperature is set to above 550°C, and the heat treatment time is controlled within 180s.

[0031] The high-strength and high-toughness equiaxed ultrafine-grained pure titanium obtained by the above method in this embodiment has a microstructure as shown in FIG. Figure 2 The figure shows a uniform, dislocation-free ultrafine grain structure. Room temperature tensile tests show a tensile strength of ~502 MPa and a uniform elongation of ~18%.

[0032] The above embodiments clearly show that the present invention provides a method for preparing high-strength and high-toughness ultrafine-grained pure titanium with simple operation and repeatability. The ultrafine-grained pure titanium obtained by the preparation method of the present invention has low internal dislocation density, small grain size, high strength, good toughness and plasticity, and eliminates size restrictions. It can be seen that the industrial pure titanium treated by the method of the present invention has both high strength and high toughness to a certain extent compared with coarse-grained pure titanium, greatly improving the comprehensive performance of ultrafine-grained metal materials, having good strength-plasticity matching, and is universal for various conventional engineering metal materials, and has very broad application prospects.

Claims

1. A method for preparing high-strength, high-toughness, equiaxed ultrafine-grained pure titanium, characterized in that: The steps include: Step (1): Multi-directional hot forging: Select original coarse-grained bulk pure titanium and perform hot forging on the bulk pure titanium below the recrystallization temperature; Step (2): rolling: rolling the block of pure titanium after multi-directional hot forging in step (1) at room temperature, with the total reduction of rolling being greater than 70%; Step (3): transient heat treatment: the titanium block after rolling in step (2) is subjected to transient heat treatment above the recrystallization temperature within 10 to 180 seconds, and then air-cooled to prepare equiaxed ultrafine-grained pure titanium; The multi-directional hot forging in step (1) is specifically as follows: After the pure titanium sample is heated at a constant temperature, it is forged once in each of the three directions of the pure titanium sample, the X-axis, Y-axis, and Z-axis, below the recrystallization temperature. This is considered a forging cycle, with a reduction of 5%-30% each time. The multi-directional hot forging process includes at least three forging cycles. After each forging cycle, the sample is reheated to the set temperature. The transient heat treatment in step (3) is specifically as follows: the rolled pure titanium is placed in a high-frequency induction furnace for heating, with a heating rate of ≥100°C / s.

2. The method according to claim 1, characterized in that The rolling in step (2) is specifically as follows: rolling is performed at a rolling speed of not less than 10 r / min, with a reduction of 5±1% per pass.

3. A high-strength and high-toughness equiaxed ultrafine-grained pure titanium, characterized in that: The method according to any one of claims 1 to 2 is used for preparation, and the grain size of the pure titanium is less than 1 μm.

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