Shape memory wide-width nickel-titanium alloy sheet and method of making

Abstract

The application discloses a shape memory wide-width nickel-titanium alloy plate and a preparation method thereof, and the preparation method comprises the following steps: mixing and smelting alloy raw materials to prepare a nickel-titanium ingot, and homogenizing the nickel-titanium ingot; cutting off the nickel-titanium ingot after the riser and then cutting the ingot into a to-be-forged ingot blank; adopting a combined forging process of high-frequency forging and low-frequency forging on the ingot blank, and the cumulative deformation is more than 90%; obtaining a hot-rolled plate blank through two-stage hot rolling, including one-stage initial rolling at a temperature of 750-900 DEG C and two-stage finish rolling at a temperature of 700-850 DEG C; cold rolling the hot-rolled plate blank to obtain a cold-rolled plate blank; and performing heat treatment strengthening on the cold-rolled plate blank to obtain a shape memory wide-width nickel-titanium alloy plate with a width of more than 600 mm. The wide-width nickel-titanium alloy plate prepared by the application has a thickness of 0.3-3.0 mm, a yield strength of more than 450 Mpa, a tensile strength of more than 850 Mpa, an elongation of more than 20%, and a shape recovery rate of more than 90%.

Description

A shape memory wide nickel-titanium alloy sheet and its preparation method Technical Field

[0001] This invention belongs to the field of nickel-titanium shape memory alloy technology, and particularly relates to a shape memory wide nickel-titanium alloy sheet and its preparation method. Background Technology

[0002] Shape memory alloys (SMA) are a new type of functional material with a unique shape memory effect (SME). This means that when an alloy is deformed by external force at a certain temperature, it retains its deformed shape after the force is removed. However, when the temperature rises to a certain level, the material automatically returns to its original shape. The unique microstructure of nickel-titanium shape memory alloys gives them excellent ductility, wear resistance, corrosion resistance, and superelasticity, among other comprehensive properties. These excellent characteristics make them applicable to almost all sectors of industry, including electronics, machinery, aerospace, transportation, construction, chemicals, medical, home appliances, and everyday consumer goods. Since their discovery in the 1960s, nickel-titanium shape memory alloys have aroused great interest among researchers. With in-depth research, the processing methods for nickel-titanium alloys have been continuously expanded, resulting in a wide variety of product types.

[0003] Currently, domestic processing of nickel-titanium alloys mainly focuses on wires, bars, forgings, and plates. Invention patent application CN107805741A discloses a method for preparing titanium-nickel shape memory alloy thin plates, mainly including the steps of smelting, annealing, forging, hot rolling, cold rolling and annealing, pickling, and inspection. The chemical composition of the titanium-nickel ingot is guaranteed to be Ni 54.5%–55.5%, C ≤ 0.04%, H ≤ 0.005%, O ≤ 0.050%, N ≤ 0.04%, with the balance being Ti. The titanium-nickel alloy thin plate is 700–900 mm long and 180–185 mm wide. Patent CN111112334B discloses a process for preparing cold-rolled ultrathin sheets of nickel-titanium shape memory alloy. The process involves cold rolling and finishing rolling of nickel-titanium shape memory alloy with a thickness of 1.0 mm to 1.5 mm using a finishing mill. The cold rolling process includes multiple treatments such as solution treatment under an inert atmosphere, cold rolling, finishing rolling, and post-treatment (including annealing and low-temperature aging). The final dimensions of the cold-rolled material are a thickness of 0.10 mm ± 0.03 mm to 0.30 mm ± 0.03 mm and a width of ≤ 300 mm. The nickel content in the aforementioned ultrathin nickel-titanium shape memory alloy sheet is between 48.0 at.% and 52.0 at.%, with the balance being titanium. However, wide sheets with a width exceeding 300 mm are rarely reported.

[0004] Different nickel-titanium alloy compositions, heat treatment processes, and processing conditions have a significant impact on the properties of nickel-titanium alloys, making sheet metal processing difficult. In actual production, the processing of wide-width nickel-titanium alloy sheets is also prone to defects such as cracks, peeling, folding, oxidation, and impurity indentation, resulting in a very low yield of high-quality wide-width nickel-titanium alloy sheets and making mass production difficult.

[0005] Therefore, how to comprehensively control the alloy composition and process conditions to prepare high-quality shape memory wide nickel-titanium alloy plates with a width of more than 600 mm has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the shortcomings of the existing technology, this invention provides a shape memory wide nickel-titanium alloy sheet and its preparation method. By controlling the conditions of multiple process steps such as melting, forging, rolling, and heat treatment, a shape memory wide nickel-titanium alloy sheet with excellent comprehensive performance and a width of more than 600 mm is obtained.

[0007] In a first aspect, the present invention provides a method for preparing shape memory wide nickel-titanium alloy plates, comprising the following steps:

[0008] Step 1: Mix the alloy raw materials, prepare nickel-titanium ingots through multiple VAR melting processes, and then homogenize the nickel-titanium ingots.

[0009] Step 2: After homogenization, the riser is removed from the nickel-titanium ingot and it is then cut into ingot blanks to be forged.

[0010] Step 3: The billet is subjected to a composite forging process combining high-frequency forging and low-frequency forging, with a cumulative deformation of over 90%.

[0011] Step 4: Two-stage hot rolling to obtain hot-rolled slab, including a first-stage rough rolling at 750-900℃ and a second-stage finish rolling at 700-850℃;

[0012] Step 5: Cold roll the hot-rolled slab to obtain a cold-rolled slab;

[0013] Step 6: Heat-treat the cold-rolled slab to strengthen it, and obtain shape memory wide nickel-titanium alloy plates with a width of more than 600mm.

[0014] First, the above method ensures the intrinsic quality and plasticity of the billet through full-process control from smelting, forging to rolling and heat treatment, especially through composite forging and high cumulative deformation. The effective combination of two-stage hot rolling and cold rolling processes ultimately produces wide nickel-titanium alloy plates with a width of over 600 mm, effectively solving the technical bottleneck of wide plate production in the background technology. Second, the homogenization treatment, composite forging (combination of high and low frequencies, with a cumulative deformation of over 90%), and multi-stage rolling process in the above method can effectively eliminate ingot composition segregation, break up coarse grains, and reduce internal defects (such as cracks and shrinkage cavities), thereby greatly reducing the risk of peeling, folding, and breakage during subsequent wide rolling and significantly improving the yield of high-quality wide plates. Finally, the cold rolling and final heat treatment strengthening steps can optimize the microstructure of the plate, giving the resulting wide plate high strength, high ductility, and excellent shape memory properties.

[0015] Furthermore, in step one, the homogenization treatment of the nickel-titanium ingot includes: holding at an inert gas temperature of 700-1000℃ for 1-5 hours.

[0016] First, prolonged high-temperature holding promotes the full diffusion of alloying elements (especially nickel and titanium), effectively eliminating dendritic segregation and microstructural inhomogeneity that may exist in the ingot after vacuum arc remelting (VAR), providing a billet with uniform composition and consistent microstructure for subsequent forging and rolling. Second, the homogenization treatment is carried out under inert gas protection, which effectively prevents the nickel-titanium ingot from reacting with oxygen in the air at high temperatures, thereby avoiding surface oxidation and the introduction of internal impurity gases, ensuring the purity of the material. Finally, this homogenization process can dissolve non-equilibrium phases in the ingot, initially refine coarse grains, and alleviate casting internal stress, thus significantly improving the plasticity and toughness of the ingot, reducing the risk of cracking during subsequent high-strength forging and rolling, and improving the stability and yield of the processing.

[0017] Furthermore, on an atomic percentage basis, the alloy raw materials contain the following elements:

[0018] Nickel content: 49.0-51.5 at.%

[0019] Titanium content: 47.5-50 at.%

[0020] The third element is 0.1-1.0 at.%;

[0021] The third element is selected from at least one of Cu, Fe, Pd, Hf, Zr, and Nb.

[0022] First, it is necessary to add a third element to nickel-titanium alloys to form ternary or multi-element nickel-titanium alloys. Depending on the application, adding a small amount of the third element can achieve a targeted synergistic effect. For example, adding Cu (copper) helps reduce hysteresis, improves phase transformation temperature stability, and also improves fatigue performance. Adding Fe (iron) is particularly suitable for the preparation of low-temperature hyperelastic materials, significantly reducing the phase transformation temperature. Conversely, adding Hf (hafnium) and Zr (zirconium) helps increase the high-temperature phase transformation temperature, expanding high-temperature application scenarios. Adding Pd (palladium) improves the alloy's corrosion resistance and biocompatibility, giving it good medical value. Adding Nb (niobium) refines the grains and improves the microstructure, which helps improve the alloy's corrosion resistance, strength, and elastic modulus. Secondly, strictly controlling the content of the third element within a low range of 0.1-1.0 at.% ensures that the excellent shape memory effect and hyperelasticity of nickel-titanium alloys are not compromised while achieving the above-mentioned synergistic effects, avoiding the problem of altered matrix phase transformation behavior or performance degradation due to excessive element addition.

[0023] Furthermore, in step one, the melting process includes at least three VAR melting operations:

[0024] One-time melting: The temperature of one-time melting T1 is 1400-1600℃, and the melting rate of one-time melting V1 is 10-20kg / min;

[0025] Secondary melting: The secondary melting temperature T2 is (85%-95%) × T1, and the secondary melting rate V2 is (1.1-1.5) × V1;

[0026] Three-stage smelting: The temperature for the three-stage smelting is: The value of V3 after three smeltings is: (V1+V2)×0.6≤V3≤(V1+V2)×1.2.

[0027] First, by performing at least three VAR melting processes sequentially, the high temperature of the electric arc can effectively remove volatile impurities and dissolved gases (such as H, O, and N) from the alloy melt, significantly reducing casting defects such as porosity and shrinkage cavities, and greatly improving the purity and density of the ingot. Second, by setting different temperatures and melting rates for the three melting processes (the first high temperature and moderate melting rate lays the foundation, the second slightly lower temperature and higher melting rate promotes homogenization, and the third melting temperature and melting rate are adjusted within the above range for final optimization), this multi-stage remelting process greatly promotes the full diffusion and uniform distribution of alloying elements, effectively eliminating compositional segregation and significantly refining the as-cast grains. Finally, by precisely defining the temperatures (T1, T2) and melting rates (V1, V2, V3) for each melting process, rather than simply repeating the operation, precise control of the melting process is achieved. This ensures that the effects of each remelting are superimposed to achieve the best purification and homogenization effect, while avoiding the introduction of new metallurgical defects due to uncontrolled melting parameters.

[0028] Furthermore, VAR melting is performed 3-6 times.

[0029] Existing technologies often employ vacuum induction melting (VIM) combined with vacuum arc remelting (VAR), which helps eliminate component segregation in the alloy and achieve a more uniform element distribution. However, VIM melting is inefficient, typically producing alloy ingots weighing no more than 500 kg per batch, and the melting process is time-consuming. After numerous comparative experiments, this invention preferentially uses multiple VAR melting processes to replace traditional methods. This allows for the production of large alloy ingots in a single batch. Multiple VAR melting processes help refine the alloy grains and improve compositional homogenization. Furthermore, the high vacuum environment and multiple melting operations effectively reduce the introduction of impurities and harmful gases, further eliminating defects such as shrinkage cavities and porosity, significantly improving the purity, mechanical properties, and reliability of the titanium alloy. The specific number of melting cycles can be selected based on the alloy's intended use. Generally, using three or more VAR melting processes can minimize segregation and achieve better homogenization. At the same time, setting 3-6 melting times avoids the possibility of component segregation or insufficient impurity removal caused by insufficient melting times (such as less than 3 times), and also prevents the decrease in production efficiency and unnecessary increase in cost caused by excessive melting times (such as more than 6 times). This ensures that high-quality ingots with low defects and uniform composition can be obtained stably and efficiently, regardless of how many melting times are selected within this range.

[0030] Furthermore, in step three, the composite forging process includes sequential high-frequency forging and low-frequency forging:

[0031] High-frequency forging involves forging along the height, length, or width of the billet, with a striking rate of 2-5 m / min, a forging frequency of 120-150 times / min, and a total deformation of 75-85%.

[0032] Low-frequency forging is carried out in a direction perpendicular to the high-frequency forging direction, with a striking rate of 2-5 m / min, a forging frequency of 80-100 times / min, and a total deformation of 70-80%.

[0033] By sequentially implementing high-frequency and low-frequency forging, the strip thickness is reduced to 80-150 mm, preferably 100-140 mm, after which the strip is cut into square billets for rolling. This combination of high- and low-frequency forging with large deformation (cumulative deformation of over 90%, preferably over 93%, more preferably over 95%) can efficiently break up coarse cast grains and dendrites from different directions, fully weld defects such as porosity and shrinkage cavities inside the ingot, significantly improve the density of the material, and provide uniform and dense billets for subsequent wide-width rolling. Secondly, the impact rate, forging frequency, and deformation amount of high- and low-frequency forging are precisely limited. The higher impact frequency combined with the large deformation amount achieves efficient processing, while the impact rate controlled within a reasonable range avoids adiabatic heating or internal stress concentration caused by excessively rapid deformation, effectively preventing the generation and propagation of cracks during forging, ensuring the stability of the forging process and the quality of the billet.

[0034] Furthermore, high-frequency forging is performed along the length of the billet, while low-frequency forging is performed along the width direction perpendicular to the length direction.

[0035] The direction of low-frequency forging is perpendicular to that of high-frequency forging. This multi-directional forging process can effectively disrupt the directional arrangement of grains, eliminate the anisotropy of the microstructure caused by single-direction forging, and make the mechanical properties of the billet more uniform in multiple directions. This greatly reduces the risk of edge cracks or fractures caused by anisotropy during subsequent wide-width rolling.

[0036] Furthermore, in step four, both the first-stage rough rolling and the second-stage finish rolling are performed asynchronously.

[0037] The speed ratio of the first upper and lower rolls in the first-stage primary rolling is (1.15-1.3):1, and the first-stage deformation is above 80%.

[0038] The second upper and lower roll speed ratio of the second-stage finishing mill is (1.1-1.2):1, and the second-stage deformation is above 70%.

[0039] The speed ratio of the first upper and lower rollers is greater than the speed ratio of the second upper and lower rollers.

[0040] A reversal operation is performed between the first-stage primary rolling and the second-stage finishing rolling.

[0041] Furthermore, two-stage hot rolling yields hot-rolled slabs with a thickness of 1-8 mm.

[0042] First, the initial rolling stage employs a larger upper and lower roll speed ratio. Through a strong shear deformation effect, this effectively enhances the penetration of deformation, ensuring that even with large deformation amounts, the core of the sheet can be fully deformed. This effectively breaks down the forging structure, refines the core grains, and avoids the common problem of uneven performance between the core and edges in wide-width sheets. Second, by setting the second-stage finishing rolling stage with a relatively smaller speed ratio and deformation amount, it focuses on further homogenizing the structure and improving the surface quality of the sheet, building upon the sufficient deformation achieved in the initial rolling. The setting of the first upper and lower roll speed ratio being greater than the second upper and lower roll speed ratio allows the process to transition from "strong shear crushing" to "warm smoothing optimization," reducing surface damage that may be caused by excessive shear stress while ensuring the total deformation amount.

[0043] Furthermore, the first-stage rough rolling is carried out along the length of the slab, and the second-stage finish rolling is carried out along the width of the slab. After the first-stage rough rolling, the slab can be cut according to the width of the hot rolling mill, and then the direction is reversed for the second-stage finish rolling.

[0044] With the above technical solution, firstly, the reversal operation between the two hot rolling stages—that is, the rolling direction of the second stage is perpendicular to the first stage—effectively disrupts the texture, eliminates directionality, significantly improves the uniformity of the longitudinal and transverse properties of wide plates, and enhances their overall mechanical properties. Secondly, the option to "selectively cut the first-stage slab according to the width of the hot rolling mill" provides process flexibility. For mills with sufficient width capacity, continuous production without cutting can be selected; for mills with limited width, ultra-wide slabs can be divided into suitable widths through intermediate cutting before being finished rolled separately with reversal. This flexibility avoids problems such as equipment inability to roll or poor edge quality due to excessively wide slabs, optimizes the production process, and improves equipment utilization and the quality and yield of the final product.

[0045] Furthermore, step five includes at least two cold rolling processes, with a total cold rolling deformation of more than 35%, wherein each cold rolling process includes multiple rolling passes, such as 5-35 passes, preferably 10-30 passes, and the deformation per pass is less than 5%, preferably 3-5%.

[0046] Intermediate annealing is performed between cold rolling processes. The intermediate annealing conditions include heating to 650-750℃ at a rate of 100-180℃ / h, holding at that temperature for 10-15 minutes, and then slowly cooling to room temperature.

[0047] Intermediate annealing between at least two cold rolling processes effectively eliminates high-density dislocations and work hardening generated in the previous cold rolling, promotes recrystallization, and restores the material's plasticity and toughness. This ensures smooth subsequent cold rolling and avoids rolling cracks or strip breakage caused by excessive work hardening. The deformation per cold rolling pass is less than 5%, preferably 3-5%. This strategy of small-pass deformation and multi-pass accumulation significantly reduces single-pass rolling force, minimizes mill bounce and sheet dimensional fluctuations, and facilitates precise control of sheet thickness tolerances. It also reduces surface scratches and other defects that may be caused by large deformation rolling, ensuring excellent dimensional accuracy and surface quality in the final wide sheet. Intermediate annealing effectively controls the nucleation and growth of recrystallized grains, obtaining a fine and uniform equiaxed grain structure, preparing the microstructure for subsequent final heat treatment. Slow cooling to room temperature helps reduce internal stress.

[0048] Furthermore, a reversal operation is performed between cold rolling processes and after intermediate annealing.

[0049] After a certain deformation is cold-worked, the martensite of the nickel-titanium alloy undergoes significant work hardening. In order to avoid excessive work hardening and reduced machinability, the alloy must be fully recrystallized to maintain good machinability. Therefore, it is essential to perform intermediate annealing between two cold rolling processes.

[0050] Furthermore, step six, the heat treatment strengthening, includes sequential solution treatment and low-temperature aging treatment:

[0051] Solution treatment: Heat to 750-850℃ at a rate of 50-100℃ / h, hold for 0.5-12h, and cool rapidly at a rate of 100-1000℃ / min, for example by water quenching or oil quenching, to suppress the precipitation of harmful phases and retain the shape memory properties of the alloy.

[0052] Low-temperature aging treatment: Keep at 350-550℃ for 0.5-4 hours, then slowly cool to room temperature.

[0053] Solution treatment facilitates the complete dissolution of elements through thermal diffusion at high temperatures, improving elemental and microstructural homogenization and forming a homogeneous single-phase matrix, thus preparing for subsequent aging treatment. However, the elements in the solution-treated nickel-titanium alloy may still not be completely homogenized, or trace precipitates (such as Ni4Ti3) may exist. Immediately following solution treatment, low-temperature aging is performed, followed by holding at a lower temperature for a certain period. This effectively controls the microstructure and phase structure of the alloy, thereby optimizing its shape memory, enhancing mechanical properties, and improving stability. After aging, slow cooling (e.g., air cooling) avoids the additional thermal stress introduced by rapid cooling or the triggering of unintended phase transformations.

[0054] Furthermore, the low-temperature aging treatment employs a staged aging process, including:

[0055] Heat to 300-450℃ at a rate of 100-180℃ / h, and hold for 0.5-1.5h;

[0056] Heat to 400-550℃ at a rate of 50-100℃ / h and hold for 0.5-1.5h.

[0057] Low-temperature aging treatment requires strict control of the temperature range and holding time. If the temperature is too low or the time is too short, atomic diffusion will be too slow and precipitates will be difficult to form. If the temperature is too high or the time is too long, it may lead to grain coarsening, which will reduce the alloy properties and weaken the strengthening effect.

[0058] The present invention provides a shape memory wide nickel-titanium alloy sheet prepared by the aforementioned method, wherein the sheet has a width of more than 600 mm and a thickness of 0.3-3.0 mm.

[0059] Preferably, the nickel-titanium alloy sheet satisfies at least one of the following:

[0060] (1) Yield strength above 450 MPa, tensile strength above 850 MPa, and elongation above 20%;

[0061] (2) The shape recovery rate is above 90%.

[0062] The beneficial effects of the technical solution of this invention include:

[0063] (1) This invention obtains a complete preparation process for shape memory wide nickel-titanium alloy plates by combining process steps such as component design, multiple VAR melting, composite forging, asynchronous hot rolling, multi-pass cold rolling and intermediate annealing, and graded aging, rather than simply superimposing them. It can prepare shape memory wide nickel-titanium alloy plates with a width of more than 600 mm and various thicknesses.

[0064] (2) Through the design and control of each process step, the present invention can precisely control the evolution of the microstructure of the board. The final board not only meets the size requirements, but also has high yield strength (≥450MPa), high tensile strength (≥850MPa), high elongation (≥20%) and excellent shape memory function (shape recovery rate ≥90%). It achieves a balance between large size, high strength and good memory function, and solves the technical problem that it is difficult to mass produce high-quality wide-width boards and the quality is low.

[0065] (3) This invention utilizes a multi-stage VAR melting process to replace the traditional vacuum induction melting (VIM) combined with vacuum arc remelting (VAR) method, which melts large-size alloy ingots in a single process. Through multiple VAR melting processes, the alloy grains are refined and the composition is made more uniform, further eliminating defects such as shrinkage cavities and porosity. This significantly improves the purity, mechanical properties and reliability of titanium alloys, effectively overcoming the problem that the mass of alloy ingots produced in a single VIM melting process is less than 500 kg, which reduces the overall production efficiency. Attached Figure Description

[0066] Figure 1 is a flowchart of the preparation process of the shape memory wide nickel-titanium alloy plate of the present invention;

[0067] Figure 2 is a schematic diagram of some process steps in the preparation method of the shape memory wide nickel-titanium alloy plate of the present invention. In the figure, Figure A is a display of the round ingot to be forged after the riser is cut off, Figure B is a display of the forged strip being cut into square billets, and Figure C is a thin plate after hot rolling and cold rolling.

[0068] Figure 3 is an optical microscope image of the nickel-titanium alloy plate sample of Example 20;

[0069] Figure 4 is a schematic diagram of the shape recovery rate test of the present invention. Detailed Implementation

[0070] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in further detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0071] The terminology used in the embodiments of this invention is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms “a,” “the,” and “the” as used in the embodiments of this invention and the appended claims are also intended to include the plural forms, and “multiple” generally includes at least two unless the context clearly indicates otherwise.

[0072] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that an article or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or device. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the article or device that includes said element.

[0073] As described in the background section, in actual production, the processing of wide nickel-titanium alloy sheets (width ≥ 600 mm) is prone to introducing various defects, resulting in a very low yield of high-quality wide nickel-titanium alloy sheets and making mass production difficult. Referring to Figures 1-2, this invention provides a method for preparing shape-memory wide nickel-titanium alloy sheets. This method can stably and efficiently prepare nickel-titanium alloy sheets with a width of ≥ 600 mm, a yield strength of ≥ 450 MPa, a tensile strength of ≥ 850 MPa, an elongation of ≥ 20%, and a shape recovery rate of ≥ 90%. Optionally, the sheet thickness is 0.3-3.0 mm. The preparation method includes the following steps:

[0074] Step 1: Mix and smelt the alloy raw materials to prepare nickel-titanium ingots, and then homogenize the nickel-titanium ingots, including the following steps:

[0075] S1. Select and mix 49.0-51.5 at.% nickel source, 47.5-50 at.% titanium source and 0.1-1.0 at.% third element source, based on atomic percentage; wherein the third element is selected from at least one of Cu, Fe, Pd, Hf, Zr and Nb.

[0076] S2. The mixed raw materials are subjected to 3-6 VAR smelting processes to prepare nickel-titanium ingots; the VAR smelting process includes the following steps:

[0077] S2.1, Primary melting: The primary melting temperature T1 is 1400-1600℃, and the primary melting rate V1 is 10-20kg / min;

[0078] S2.2 Secondary melting: The secondary melting temperature T2 is (85%-95%) × T1, and the secondary melting rate V2 is (1.1-1.5) × V1;

[0079] S2.3, Third Melting: The temperature for the third melting is:

[0080] The value of V3 after three smeltings is: (V1+V2)×0.6≤V3≤(V1+V2)×1.2;

[0081] Additional VAR melting sessions can be selected by adjusting the settings based on the three melting sessions.

[0082] S3. Homogenize the nickel-titanium ingot: Hold the nickel-titanium ingot at 700-1000℃ for 1-5 hours under inert gas protection.

[0083] Step 2: After the homogenization treatment, the riser is cut off and the nickel-titanium ingot is divided into ingot blanks to be forged, as shown in Figure 2A.

[0084] Step 3: The billet to be forged is subjected to high-frequency forging and low-frequency forging in sequence to obtain forged strip billet, which is then cut into square billets to be rolled, as shown in Figure 2B. The cumulative deformation is above 90%, preferably above 92%. Among them, high-frequency forging refers to forging along the height direction, length direction or width direction (preferably the length direction) of the billet, with a striking rate of 2-5 m / min, a forging frequency of 120-150 times / min, and a deformation of 75-85%.

[0085] Low-frequency forging refers to forging along a direction perpendicular to the high-frequency forging direction (preferably the width direction), with a striking rate of 2-5 m / min, a forging frequency of 80-100 times / min, a deformation of 70-80%, and a strip thickness of approximately 80-150 mm, preferably 100-140 mm.

[0086] Step 4: The billet to be rolled is subjected to two stages of hot rolling to obtain a hot-rolled slab, including a first-stage rough rolling at 750-900℃ and a second-stage finish rolling at 700-850℃; both the first-stage rough rolling and the second-stage finish rolling are asynchronous rolling processes, specifically including the following steps:

[0087] S4.1 The first-stage primary rolling is carried out along the length of the slab. The speed ratio of the first upper and lower rolls in the first-stage primary rolling is (1.15-1.3):1, and the first-stage deformation is above 80%. After the first-stage primary rolling, the first-stage hot-rolled slab can be cut according to the width of the hot rolling mill, and then the direction is reversed for the second-stage finishing rolling.

[0088] S4.2 The second-stage finishing rolling is carried out along the width direction of the slab. The speed ratio of the second upper and lower rolls in the second-stage finishing rolling is (1.1-1.2):1, and the second-stage deformation is above 70%. Ensure that the speed ratio of the first upper and lower rolls is greater than the speed ratio of the second upper and lower rolls. The thickness of the hot-rolled slab is about 1-8mm.

[0089] Step 5: Cold-roll the hot-rolled slab to obtain a cold-rolled slab; preferably, the hot-rolled slab is subjected to at least two cold-rolling processes, each cold-rolling process including multiple rolling passes, for example, each cold-rolling process includes 5-35 passes, preferably 10-30 passes, the deformation per pass is less than 5%, preferably 3-5%, and the total deformation of cold rolling is more than 35%, to obtain a cold-rolled slab, the thickness of the cold-rolled slab is reduced to 0.3-3mm; specifically including the following steps:

[0090] S5.1, One-time cold rolling, including 5-35 rolling passes;

[0091] S5.2. The cold-rolled slab obtained from the first cold rolling is subjected to intermediate annealing: the temperature is raised to 650-750℃ at a rate of 100-180℃ / h, held for 10-15min and then slowly cooled to room temperature; optionally, the cold-rolled slab is cut.

[0092] S5.3. Reverse the direction and perform secondary cold rolling. Secondary cold rolling includes 5-35 rolling passes to obtain cold-rolled slab. Depending on the situation, a third cold rolling and intermediate annealing step can also be set up. By using multiple passes with small deformation, the slab thickness can be reduced while ensuring stable and uniform plate quality.

[0093] Step Six: Strengthen the cold-rolled slab through heat treatment, including sequential solution treatment and low-temperature aging treatment, to obtain shape memory wide nickel-titanium alloy sheets with a width of 600mm or more; specifically including the following steps:

[0094] S6.1 Solution treatment: Heat to 750-850℃ at a rate of 50-100℃ / h, hold for 0.5-12h, and cool rapidly at a rate of 100-1000℃ / min, for example by water quenching or oil quenching, to suppress the precipitation of harmful phases and retain the shape memory properties of the alloy.

[0095] S6.2 Low-temperature aging treatment: Hold at 350-550℃ for 0.5-4 hours, then slowly cool to room temperature;

[0096] Preferably, the low-temperature aging treatment employs a staged aging process, specifically including:

[0097] S6.2.1. Heat to 350-450℃ at a rate of 100-180℃ / h, and hold for 0.5-1.5h;

[0098] S6.2.2, Heat to 450-550℃ at a rate of 50-100℃ / h, and hold for 0.5-1.5h.

[0099] Example 1

[0100] The shape memory wide nickel-titanium alloy plate sample of Example 1 has a thickness of approximately 0.3 mm and a width of 620 mm, and was prepared through the following steps:

[0101] Step 1: Mix and smelt the alloy raw materials to prepare nickel-titanium ingots, and then homogenize the nickel-titanium ingots, including:

[0102] S1. Raw material mixing: On an atomic percentage basis, the alloy raw materials contain 50.0 at.% nickel, 49.5 at.% titanium, and 0.5 at.% copper.

[0103] S2. Perform three VAR melting processes on the mixed raw materials:

[0104] S2.1, Primary melting: The primary melting temperature T1 is 1500℃, and the primary melting rate V1 is 15 kg / min;

[0105] S2.2 Secondary melting: The secondary melting temperature T2 is 1350℃ (90%×T1), and the secondary melting rate V2 is 18 kg / min (1.2×V1).

[0106] S2.3, Three-stage melting: The temperature of the three-stage melting is 1300℃, and the melting rate V3 is 22kg / min (approximately (V1+V2)×0.67).

[0107] S3. Homogenize the nickel-titanium ingot: Hold at 850±10℃ for 3 hours under Ar gas protection;

[0108] Step 2: After homogenization, the riser is removed from the nickel-titanium round ingots and they are cut into ingot blanks to be forged;

[0109] Step 3: The ingot to be forged is subjected to high-frequency forging and low-frequency forging in sequence to obtain a forged strip, which is then cut into square billets to be rolled. The cumulative deformation is 95%, including:

[0110] High-frequency forging: Forging is carried out along the length of the billet, with a striking rate of 4.0 m / min, a forging frequency of 135 times / min, and a deformation of 80%.

[0111] Low-frequency forging: Forging is carried out along the width direction with a striking rate of 3.5 m / min, a forging frequency of 95 times / min, and a deformation of 75%. The resulting strip thickness is about 100 mm, which is then cut into square billets to be rolled.

[0112] Step 4: Perform two-stage hot rolling on the square billet with a thickness of approximately 100 mm to obtain a hot-rolled slab with a thickness of approximately 2.5 mm, including:

[0113] S4.1 First-stage primary rolling: At 800℃, it is carried out along the length of the billet to be rolled. The speed ratio of the first upper and lower rolls is 1.2:1. The first-stage deformation is about 90%. The first-stage primary rolled plate is cut perpendicular to the length direction. The cut primary rolled plate is then reversed 90° and sent into the hot rolling mill (the subsequent rolling direction is actually along the width direction of the previously rolled plate).

[0114] S4.2 Second-stage finishing rolling: carried out at 750℃ along the width of the slab, with the second upper and lower roll speed ratio being 1.15:1, and the second-stage deformation being approximately 75%.

[0115] Step 5: Perform 30 cold rolling passes on the 2.5mm thick hot-rolled slab, with a total deformation of about 84% (about 6% per pass), to obtain a cold-rolled slab with a thickness of about 0.40mm and a width of about 620mm.

[0116] Step Six: The cold-rolled slab is subjected to heat treatment strengthening, including solution treatment and low-temperature aging treatment, in sequence:

[0117] S6.1 Solution treatment: Heat to 800℃ at a rate of 80℃ / h, hold for 2h, and then quench in water.

[0118] S6.2 Low-temperature aging treatment: Heat to 400℃ at a rate of 150℃ / h, hold for 2 hours, and then slowly cool to room temperature.

[0119] Example 2

[0120] The difference between Example 2 and Example 1 is that in step S2, the mixed raw materials are subjected to four VAR melting processes, including:

[0121] S2.1, Primary melting: The primary melting temperature T1 is 1500℃, and the primary melting rate V1 is 15 kg / min;

[0122] S2.2 Secondary melting: The secondary melting temperature T2 is 1350℃ (90%×T1), and the secondary melting rate V2 is 18 kg / min (1.2×V1).

[0123] S2.3, Three-stage melting: The temperature of the three-stage melting is 1300℃, and the melting rate V3 is 22kg / min (approximately (V1+V2)×0.67).

[0124] S2.4, Four-stage melting: The temperature for the four-stage melting is 1300℃, and the melting rate V4 for the four-stage melting is 25 kg / min.

[0125] Example 3

[0126] The difference between Example 3 and Example 2 is that in step five, the 2.5mm thick hot-rolled slab is subjected to two cold rolling processes, with a total cold rolling deformation of approximately 84%, resulting in a cold-rolled slab with a thickness of approximately 0.40mm, including:

[0127] S5.1, One-pass cold rolling: 20 passes of cold rolling (the deformation per pass is about 5%), the total deformation of one-pass cold rolling is about 64%, and the thickness is reduced to about 0.9 mm;

[0128] S5.2 Intermediate annealing: Heat to 700℃ at a rate of 150℃ / h, hold for 12 minutes, then slowly cool to room temperature and feed into the cold rolling mill at a 90° angle.

[0129] S5.3 Secondary cold rolling: 16 passes of cold rolling are performed (the deformation per pass is about 5%), and the total deformation of the secondary cold rolling is about 55%, resulting in a cold-rolled slab with a thickness of about 0.40 mm.

[0130] Example 4

[0131] The difference between Example 4 and Example 3 is that in step S6.2, a two-stage low-temperature aging treatment is performed, including:

[0132] S6.2.1, First stage of low temperature aging: Heat to 400℃ at a rate of 150℃ / h and hold for 1 hour;

[0133] S6.2.2 Second-stage low-temperature aging: Continue heating to 500℃ at a rate of 80℃ / h, hold for 1 hour, and then slowly cool to room temperature.

[0134] Examples 5-15

[0135] The difference between Examples 5-15 and Example 4 is that the alloy composition is different, as shown in Table 1.

[0136] Table 1

[0137]

[0138] Example 16

[0139] The difference between Example 16 and Example 4 is that the high-frequency forging and low-frequency forging parameters are different, and the cumulative deformation is 95%, including:

[0140] High-frequency forging: Forging is carried out along the width of the billet, with a striking rate of 4.5 m / min, a forging frequency of 140 times / min, and a deformation of 75%.

[0141] Low-frequency forging: Forging is carried out along the length direction, with a striking rate of 3.0 m / min, a forging frequency of 90 times / min, and a deformation of 80%.

[0142] Example 17

[0143] The difference between Example 17 and Example 4 lies in the different upper and lower roll speed ratios in the two-stage hot rolling, including:

[0144] S4.1 First-stage rolling: at 800℃, it is carried out along the length of the billet to be rolled, with the first upper and lower roll speed ratio being 1.25:1, the first-stage deformation being about 88%, and then cutting and reversing.

[0145] S4.2 Second-stage finishing rolling: At 750℃, the rolling is carried out along the width of the slab, with the second upper and lower roll speed ratio being 1.1:1. The second-stage deformation is about 79%, resulting in a hot-rolled slab with a thickness of about 2.5mm.

[0146] Example 18

[0147] The difference between Example 18 and Example 4 is that a 2.5mm thick hot-rolled slab is subjected to three cold-rolling processes, with a total deformation of 84%, resulting in a cold-rolled sheet with a thickness of approximately 0.40mm, comprising:

[0148] S5.1, One-pass cold rolling: 10 passes of cold rolling (each pass deformation is about 5%), reducing the thickness to about 1.5 mm;

[0149] S5.2 Intermediate annealing: Heat to 700℃ at a rate of 150℃ / h, hold for 12 minutes, then slowly cool to room temperature and feed into the cold rolling mill at a 90° angle.

[0150] S5.3 Secondary cold rolling: 15 passes of cold rolling (deformation per pass is about 4%), reducing the thickness to about 0.8 mm;

[0151] S5.4 Intermediate annealing: Heat to 680℃ at a rate of 120℃ / h, hold for 14 minutes, then slowly cool to room temperature, and feed into the cold rolling mill at a 90° reversal.

[0152] S5.5, Three-stage cold rolling: 20 passes of cold rolling (the deformation per pass is about 3%), reducing the thickness to about 0.4 mm.

[0153] Example 19

[0154] The difference between Example 19 and Example 4 lies in the different process parameters for the low-temperature aging treatment, including:

[0155] S6.2.1, First stage of low temperature aging: Heat to 350℃ at a rate of 120℃ / h and hold for 1 hour;

[0156] S6.2.2 Second-stage low-temperature aging: Heat to 450℃ at a rate of 60℃ / h, hold for 1 hour, and then slowly cool to room temperature.

[0157] Example 20

[0158] The shape memory wide nickel-titanium alloy plate sample of Example 20 is approximately 2.5 mm thick and 610 mm wide, and was prepared by the following steps:

[0159] Step 1: Mix and smelt the alloy raw materials to prepare nickel-titanium ingots, and then homogenize the nickel-titanium ingots, including:

[0160] S1. Raw material mixing: On an atomic percentage basis, the alloy raw materials contain 50.0 at.% nickel, 49.5 at.% titanium, and 0.5 at.% copper.

[0161] S2. Perform four VAR melting processes on the mixed raw materials:

[0162] S2.1, Primary melting: The primary melting temperature T1 is 1500℃, and the primary melting rate V1 is 15 kg / min;

[0163] S2.2 Secondary melting: The secondary melting temperature T2 is 1350℃ (90%×T1), and the secondary melting rate V2 is 18 kg / min (1.2×V1).

[0164] S2.3, Three-stage melting: The temperature of the three-stage melting is 1300℃, and the melting rate V3 is 22kg / min (approximately (V1+V2)×0.67).

[0165] S2.4, Four-stage melting: The temperature of the four-stage melting is 1300℃, and the melting rate V4 of the four-stage melting is 25 kg / min;

[0166] S3. Homogenize the nickel-titanium ingot: Hold at 850±10℃ for 3 hours under Ar gas protection;

[0167] Step 2: After homogenization, the riser is removed from the nickel-titanium round ingots and they are cut into ingot blanks to be forged;

[0168] Step 3: The ingot blank is subjected to high-frequency forging and low-frequency forging in sequence, with a cumulative deformation of approximately 93%, including:

[0169] High-frequency forging: Forging is carried out along the length of the billet, with a striking rate of 4.0 m / min, a forging frequency of 135 times / min, and a deformation of 75%.

[0170] Low-frequency forging: Forging is carried out along the width direction with a striking rate of 3.5 m / min, a forging frequency of 95 times / min, and a deformation of about 72%. The resulting strip thickness is about 140 mm, which is then cut into square billets to be rolled.

[0171] Step 4: Perform two-stage hot rolling on the square billet with a thickness of approximately 140 mm to obtain a hot-rolled slab with a thickness of approximately 7.0 mm, including:

[0172] S4.1 First-stage initial rolling: At 800℃, it is carried out along the length of the slab, with the first upper and lower roll speed ratio being 1.2:1 and the first-stage deformation being 80%. The first-stage initial rolled plate is cut perpendicular to the length direction, and the cut initial rolled plate is reversed at 90° and sent into the hot rolling mill (the subsequent rolling direction is actually along the width direction of the previously rolled plate).

[0173] S4.2 Second-stage finishing rolling: carried out at 750℃ along the width of the slab, with the second upper and lower roll speed ratio being 1.15:1, and the second-stage deformation being 75%.

[0174] Step 5: The hot-rolled slab with a thickness of approximately 7.0 mm is subjected to two cold rolling processes, with a total cold rolling deformation of approximately 64%, resulting in a cold-rolled slab with a thickness of approximately 2.5 mm, including:

[0175] S5.1, One-pass cold rolling: 10 passes of cold rolling (each pass deformation is about 5%), reducing the thickness to about 4.2 mm;

[0176] S5.2 Intermediate annealing: Heat to 700℃ at a rate of 150℃ / h, hold for 12 minutes, then slowly cool to room temperature and feed into the cold rolling mill at a 90° angle.

[0177] S5.3 Secondary cold rolling: 17 passes of cold rolling (the deformation per pass is about 3%), reducing the thickness to 2.5 mm.

[0178] Step Six: The cold-rolled slab is subjected to heat treatment strengthening, including solution treatment and low-temperature aging treatment, in sequence:

[0179] S6.1 Solution treatment: Heat to 850℃ at a rate of 80℃ / h, hold for 2h, and then quench in water.

[0180] S6.2, Perform two-stage low-temperature aging treatment, including:

[0181] S6.2.1, First stage of low temperature aging: Heat to 420℃ at a rate of 150℃ / h and hold for 1 hour;

[0182] S6.2.2 Second-stage low-temperature aging: Continue heating to 520℃ at a rate of 80℃ / h, hold for 1 hour, and then slowly cool to room temperature.

[0183] Comparative Example 1

[0184] The main difference between Comparative Example 1 and Example 1 is the smelting process. The mixed raw materials are smelted using both traditional VIM smelting and VAR smelting, including:

[0185] Vacuum Induction Melting (VIM): The electrodes are placed in the crucible of a vacuum induction furnace, and the vacuum is evacuated to below 10°C. -3 Pa, about 150kW medium frequency current is passed through, and the electrodes are heated by electromagnetic induction until they are completely melted, and the melting temperature is controlled at 1500±10℃.

[0186] Vacuum self-consumable melting (VAR): melting temperature 1300±10℃, melting rate 20kg / min.

[0187] Comparative Example 2

[0188] The main difference between Comparative Example 2 and Example 1 is the forging process. Comparative Example 2 uses a traditional forging process, including:

[0189] The ingot is forged along its length at a striking rate of 3.5 m / min and a forging frequency of 100 times / min, with a cumulative deformation of approximately 95%.

[0190] Comparative Example 3

[0191] The main difference between Comparative Example 3 and Example 1 is the hot rolling process. Comparative Example 3 uses a two-stage hot rolling process with simultaneous twin rolls, including:

[0192] First-stage hot rolling: rolling multiple times along the length of the slab at 800℃, with a first-stage deformation of 90%;

[0193] Second-stage hot rolling: at 750℃, multiple passes are rolled along the width of the slab, with a second-stage deformation of 75%.

[0194] Tests and Results

[0195] The mechanical properties, shape recovery rate, and in-plate differences of the shape memory wide nickel-titanium alloy plates obtained in Examples 1-20 and Comparative Examples 1-3 were tested, and the results are shown in Table 2.

[0196] (1) Mechanical properties: Yield strength, tensile strength and elongation were determined in accordance with the national standard GB / T 228.1-2021 "Metallic materials, tensile testing - Part 1: Test method at room temperature". Using a universal testing machine, samples were taken from the final obtained plate along the length direction, processed into standard tensile specimens, and tensile tests were carried out at room temperature until the specimens broke. The software automatically recorded and calculated the various mechanical property parameters.

[0197] (2) Shape recovery rate: determined by the following bending deformation method, see Figure 4: Cut plate samples were placed at room temperature. The thickness of the samples in Examples 1-16 and Comparative Examples 1-3 was 0.4 mm and the width was 10 mm. The thickness of the sample in Example 20 was 2.5 mm and the width was 10 mm. The samples were held in a 180° bend along a stainless steel round bar 2 with a diameter of 7 mm for 30 seconds, as shown in (a). Then, the plate was removed from the stainless steel round bar, and the radius of curvature R1 of the plate was obtained, as shown in (b). After that, the plate was placed in an electric furnace and heated to 400°C at a heating rate of 10°C / min or higher. Then, it was air-cooled to room temperature, and the radius of curvature R2 was measured, as shown in (c). The curvatures ρ1(1 / R1) and ρ2(1 / R2) were calculated based on the radii of curvature R1 and R2:

[0198] Shape recovery rate = [(ρ1-ρ2) / ρ1]×100(%).

[0199] (3) Size of microprecipitates: The morphology of the plate sample of Example 20 was observed using an optical microscope, as shown in Figure 3. As can be seen from the figure, the metallographic matrix of the 2.5 mm thick plate sample exhibits equiaxed, recrystallized austenite grains with a size between 10-20 μm, and there are basically no visible Ni4Ti3 precipitates. The austenite grains of the 0.4 mm thick plate samples of Examples 1-19 are even smaller, basically all below 8 μm, and no Ni4Ti3 precipitates were observed.

[0200] (4) Plate difference: Six measurement points were taken for each sample to measure the thickness, and the plate difference of each sample was calculated based on the difference between the maximum and minimum thickness (unit: mm).

[0201] Table 2 Test results of Examples 1-20 and Comparative Examples 1-3

[0202]

[0203] As can be seen from the test results in Table 2, compared with the optimized operations of Examples 2-4, Example 1 had fewer VAR melting cycles and relatively conventional cold rolling and low-temperature aging treatment. Although it improved the alloy melting efficiency and weight, compared with other examples that underwent four VAR melting cycles, its ingot composition segregation and impurity content were slightly higher. This defect spread during subsequent forging and rolling. Furthermore, the single cold rolling and relatively simple aging treatment resulted in slightly larger residual stress and dislocation defects inside the plate, which could not be completely eliminated by heat treatment. This led to a certain impact on the mechanical properties, elongation, shape recovery rate, and flatness of the finished plate.

[0204] Examples 5-15 involve adjusting the constituent elements according to different applications of the alloy. Within the range of 0.1-1.0 at% addition of the third element, the sheet preparation method of this invention, with strict control of the process parameters at each stage, can yield wide nickel-titanium alloy sheets with good comprehensive performance and a width of over 600 mm, providing excellent raw material guarantee for the application of large-size shape memory nickel-titanium alloys. Examples 16-19, based on Example 4, conducted comparative experiments on the process parameters of key stages, further demonstrating the overall advantages of the preparation method of this invention. For example, Example 18 utilizes the large deformation amount of multi-stage cold rolling to reduce the work hardening tendency, minimize the decrease in material plasticity, and intermediate annealing promotes recrystallization and softening, reducing residual stress and dislocation defects inside the cold-rolled slab. Combined with subsequent heat treatment, this further eliminates internal residual stress, resulting in finished sheet sheets with high mechanical properties, shape recovery rate, and dimensional stability. Compared to Example 4, Example 19's slower heating rate and lower annealing temperature resulted in slightly insufficient dissolution and diffusion of alloying elements, leading to a less uniform austenitic phase compared to Example 4. Consequently, the strength of the sheet metal, particularly the yield strength, was slightly lower, the shape memory effect was somewhat suppressed, and the shape recovery rate decreased.

[0205] Example 20 provides an example of a 2.5mm thick shape memory wide nickel-titanium alloy plate, which also exhibits excellent mechanical properties, elongation, shape recovery rate, and flatness. Furthermore, since a wide plate needs to be fabricated, the thickness of the plate prepared in this invention should not be too low. When the thickness is less than 0.3mm, even with heat treatment and other methods, excessive internal stress can easily lead to difficulties in material formation during large deformation processes involving thickening and widening. Therefore, the thickness of the shape memory wide nickel-titanium alloy plate of this invention is above 0.3mm, preferably above 0.4mm.

[0206] The test results of Comparative Example 1 show that the traditional combination of VIM melting and VAR melting can also produce good plates. However, since the mass of alloy ingots produced by VIM melting in a single run is usually no more than 500 kg, the production efficiency is far lower than that of the multiple VAR melting processes of this invention. Furthermore, multiple VAR melting processes effectively solve the defects such as ingot segregation and high impurities, forming a high-quality and high-efficiency melting process. Comparative Examples 2-3 conducted comparative experiments on key process steps. In Comparative Example 2, unidirectional forging is difficult to fully break the as-cast structure, and defects and coarse grains may still exist in the core of the slab. The microstructure refinement effect is poor, and the anisotropy is severe. During subsequent wide-width rolling, cracks are easily generated at the edges or in the core, resulting in insufficient and uneven distribution of mechanical properties and shape recovery rate of the final plate. In Comparative Example 3, synchronous rolling is mainly compression deformation with little shear deformation. The deformation is difficult to penetrate into the core of the wide plate, resulting in a coarse microstructure in the core and significant differences in properties compared to the edges. This increases the risk of edge cracks and poor plate shape, failing to meet the requirements for high-quality wide-width plates. It is evident that although the process of each comparative example appears to be simplified, the resulting board material is not as effective as that of the example, and the reliability and application scope of the product in later applications are significantly limited.

[0207] The preferred embodiments of the present invention have been described above, which are intended to make the spirit of the present invention clearer and easier to understand, and are not intended to limit the present invention. All modifications, substitutions and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a shape memory wide nickel-titanium alloy sheet, characterized in that, The process includes the following steps: Step 1: Mixing the alloy raw materials and preparing nickel-titanium ingots through multiple VAR melting processes, followed by homogenization treatment of the nickel-titanium ingots; at least three VAR melting processes are performed: First melting: First melting temperature T1 is 1400-1600℃, first melting rate V1 is 10-20 kg / min; Second melting: Second melting temperature T2 is (85%-95%) × T1, second melting rate V2 is (1.1-1.5) × V1; Third melting: Third melting temperature T3 is: The melting rate V3 for the three steps is: (V1+V2)×0.6≤V3≤(V1+V2)×1.2; Step 2: After homogenization, the nickel-titanium ingot is cut off from the riser and then divided into ingot blanks to be forged; Step 3: The ingot blanks are subjected to a composite forging process combining high-frequency forging and low-frequency forging, with a cumulative deformation of over 90%; Among them, the composite forging process includes high-frequency forging and low-frequency forging performed sequentially: high-frequency forging is performed along the height, length, or width direction of the ingot blank, with a striking rate of 2-5m / min and a forging frequency of 120-150 times / min, and the total deformation of high-frequency forging is... The deformation is 75-85%; low-frequency forging is carried out in a direction perpendicular to the high-frequency forging direction, with an impact rate of 2-5 m / min and a forging frequency of 80-100 times / min, and the total deformation of low-frequency forging is 70-80%; Step 4: two-stage hot rolling to obtain hot-rolled slabs, including a first-stage initial rolling at 750-900℃ and a second-stage finish rolling at 700-850℃; Step 5: cold rolling the hot-rolled slabs to obtain cold-rolled slabs; Step 6: heat-treating the cold-rolled slabs to strengthen them, to obtain shape memory wide nickel-titanium alloy plates with a width of more than 600mm.

2. The preparation method according to claim 1, characterized in that, On an atomic percentage basis, the alloy raw material contains the following elements: nickel 49.0-51.5 at.%; titanium 47.5-50 at.%; a third element 0.1-1.0 at.%; wherein the third element is selected from at least one of Cu, Fe, Pd, Hf, Zr, and Nb.

3. The preparation method according to claim 1 or 2, characterized in that, In step four, both the first-stage rough rolling and the second-stage finish rolling are asynchronous rolling processes; the first upper and lower roll speed ratio of the first-stage rough rolling is (1.15-1.3):1, and the first-stage deformation is above 80%; the second upper and lower roll speed ratio of the second-stage finish rolling is (1.1-1.2):1, and the second-stage deformation is above 70%; the first upper and lower roll speed ratio is greater than the second upper and lower roll speed ratio; a reversal operation is performed between the first-stage rough rolling and the second-stage finish rolling.

4. The preparation method according to claim 3, characterized in that, Step 5 includes at least two cold rolling processes, with a total cold rolling deformation of more than 35%. Each cold rolling process includes multiple rolling passes, with a deformation of less than 5% per pass. Intermediate annealing is performed between the cold rolling processes. The intermediate annealing conditions include heating to 650-750℃ at a rate of 100-180℃ / h, holding at that temperature for 10-15 minutes, and then slowly cooling to room temperature.

5. The preparation method according to claim 1 or 2, characterized in that, Step six, heat treatment strengthening, includes solution treatment and low-temperature aging treatment performed sequentially: Solution treatment: Heat to 750-850℃ at a rate of 50-100℃ / h, hold for 0.5-12h, and cool rapidly at a rate of 100-1000℃ / min; Low-temperature aging treatment: Hold at 350-550℃ for 0.5-4h, and cool slowly to room temperature.

6. The preparation method according to claim 5, characterized in that, The low-temperature aging treatment adopts a graded aging step, including: heating to 300-450℃ at a rate of 100-180℃ / h and holding for 0.5-1h; heating to 400-550℃ at a rate of 50-100℃ / h and holding for 0.5-1h.

7. A shape memory wide nickel-titanium alloy sheet, characterized in that, The plate is prepared by the preparation method described in any one of claims 1-6, and the width of the plate is more than 600 mm and the thickness is 0.3-3.0 mm.

8. The shape memory wide nickel-titanium alloy sheet as described in claim 7, characterized in that, The nickel-titanium alloy plate meets the following requirements: yield strength above 450 MPa, tensile strength above 850 MPa, and elongation above 20%.

9. The shape memory wide nickel-titanium alloy sheet as described in claim 7 or 8, characterized in that, The nickel-titanium alloy sheet meets the following requirement: shape recovery rate of over 90%.

Images