High-permeability rare earth soft magnetic alloy plate for magnetic shielding case and preparation method of high-permeability rare earth soft magnetic alloy plate

Abstract

The invention relates to the technical field of precise alloy materials, in particular to a high-permeability rare earth soft magnetic alloy plate for a magnetic shielding case and a preparation method of the high-permeability rare earth soft magnetic alloy plate. The alloy plate comprises the following main components in percentage by weight: 79 to 81 percent of Ni, 3.9 to 4.1 percent of Mo, 0.4 to 0.6 percent of Mn, 0.15 to 0.25 percent of Si, less than or equal to 0.02 percent of C, less than or equal to 0.015 percent of P, less than or equal to 0.008 percent of S, 0.035 to 0.055 percent of Ce and the balance of Fe and impurities. The preparation process comprises the steps of vacuum induction melting, electroslag remelting, homogenization treatment, forging, hot rolling, solution treatment, cold rolling, final heat-magnetic treatment and the like, and it is ensured that the material has excellent magnetic performance and machinability. By optimizing components and process parameters, especially adding a trace amount of Ce, the magnetic conductivity is effectively improved, the coercive force is reduced, and the magnetic shielding effectiveness and the temperature stability are improved.

Description

Technical Field

[0001] This invention relates to the field of precision alloy materials technology, and particularly to a high-permeability rare-earth soft magnetic alloy plate for magnetic shielding and its preparation method. Background Technology

[0002] With the rapid development of modern industry and high-end technology, the demand for ultra-high precision magnetic shielding materials is becoming increasingly urgent. In cutting-edge applications such as military electronic systems, extreme ultraviolet lithography equipment, quantum computing platforms, biomagnetic signal detection, and high-resolution electron microscopes, magnetic shielding materials are required to possess not only extremely high initial and maximum permeability, but also extremely low coercivity to ensure effective suppression of external stray magnetic fields and stable operation of internal sensitive devices. However, existing high-nickel iron-based soft magnetic alloys still face multiple technical challenges in practical applications: First, there is a bottleneck in improving magnetic properties, and the initial permeability under weak magnetic fields is difficult to meet the requirements of new-generation high-sensitivity equipment; Second, the material processing performance is poor, especially when preparing ultra-thin plates, which are prone to cracking, edge damage, and other problems, resulting in low yield; Third, the magnetic properties are highly sensitive to impurity elements (such as carbon, sulfur, and oxygen) and internal stress, and traditional smelting and heat treatment processes are difficult to achieve high uniformity of composition and structure, resulting in large performance fluctuations between batches and unstable shielding effects; Fourth, in environments with a certain operating temperature, the permeability of the material is prone to significant decay with increasing temperature, limiting its reliability under complex working conditions.

[0003] Therefore, there is an urgent need to develop a new type of soft magnetic alloy sheet that combines ultra-high permeability, ultra-low coercivity, excellent formability, good thermal stability and high consistency, in order to break through the performance ceiling of current magnetic shielding technology. Summary of the Invention

[0004] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a high permeability rare earth soft magnetic alloy plate for magnetic shielding and its preparation method, which solves the key technical problems of insufficient permeability, high coercivity, poor processability and low performance consistency of existing high nickel iron-based soft magnetic alloys in ultra-high precision magnetic shielding applications.

[0005] To achieve the above objectives, the main technical solutions adopted by the present invention include: A high-permeability rare-earth soft magnetic alloy plate for magnetic shielding, wherein the chemical composition of the alloy by weight percentage is: Ni: 79–81%, Mo: 3.9–4.1%, Mn: 0.4–0.6%, Si: 0.15–0.25%, C≤0.02%, P≤0.015%, S≤0.008%, Ce: 0.035%–0.055%, with the balance being Fe and unavoidable impurities.

[0006] In some embodiments, the thickness of the plate is 0.08 mm to 0.15 mm; and under a magnetic field strength of 0.08 A / m and a frequency of 50 Hz, the initial permeability ui > 47 mH / m, the maximum permeability μm ≥ 270 mH / m, and the coercivity Hc ≤ 1.1 A / m.

[0007] In some embodiments, the Ce content is 0.045 wt.%.

[0008] In some embodiments, the following steps are included: Step S1: Vacuum induction melting; Step S2: Electroslag remelting; Step S3: The electroslag ingot is homogenized at 1200±10℃ for 24–36 hours to fully eliminate dendrite segregation and obtain a homogenized electroslag ingot. Step S4: Forging, initial forging temperature ≥1150℃, final forging temperature ≥950℃; Step S5: Hot rolling, with an initial rolling temperature ≥1100℃ and a final rolling temperature ≥850℃; Step S6, Solution treatment: After holding at 1080±10℃ for 1–1.5 hours, water quench to obtain a uniform single-phase austenitic structure and dissolve harmful impurity elements. Step S7: Cold rolling, with a total deformation rate of 88%–95%, intermediate annealing 1–2 times, annealing temperature 800–900℃; Step S8, Final thermo-magnetic treatment: Apply a DC magnetic field of 1100–1300 A / m along the rolling direction at 445–455℃ and hold for 2.5–3.5 hours, then cool to below 150℃ at a rate of 90–110℃ / h.

[0009] In some embodiments, in step S1, raw materials are prepared according to the above-designed component ratio, loaded into a vacuum induction furnace, the vacuum degree of vacuum induction melting is ≤1.0Pa, the refining temperature is 1580–1600℃, the refining time is ≥60 minutes, and the casting temperature is 1500–1520℃; electrode fragments are cast under vacuum and then air-cooled.

[0010] In some embodiments, in step S2, the electrode blank obtained in S1 is cleaned and ground, and the same steel grade is used as the base for slag formation of the consumable electrode; during the electroslag remelting process, a high basicity pre-melted slag is used, the melting rate is 4–6 kg / min, and the current density is 6000–7500 A / cm2 to obtain an alloy electroslag ingot with uniform composition. In step S3, the electroslag ingot obtained in S2 is loaded into a heat treatment furnace and kept at 1200±10℃ for 24-36 hours to fully eliminate dendrite segregation and obtain a homogenized electroslag ingot.

[0011] In step S4 forging, the electroslag ingot obtained in step S3 is loaded into a heating furnace, heated to 1180~1220℃ and held for 4~6 hours, and then taken out of the furnace for multi-directional forging; the initial forging temperature is >1150℃, the final forging temperature is >950℃, and multiple forgings are performed, and then it is forged into a hot-rolled billet of the required specifications.

[0012] In some embodiments, during the hot rolling process in step S5, the forged billet obtained in step S4 is loaded into a heating furnace, heated to 1160°C, held for 1 to 3 hours, and then subjected to multi-pass hot rolling with an initial rolling temperature >1100°C and a final rolling temperature >850°C to produce a hot-rolled slab of the required thickness.

[0013] In some embodiments, during the cold rolling process in step S7, the plate after solution treatment obtained in step S6 is subjected to multiple cold rolling passes with a total deformation rate of 88% to 95%, and 1 to 2 annealing processes are performed in between (annealing temperature 800 to 900°C, protective atmosphere, time 10 to 30 min), and finally rolled to the target thickness.

[0014] In some embodiments, during the cold rolling process in step S7, the plate after solution treatment obtained in step 6 is subjected to multiple cold rolling passes, the total deformation rate of the cold rolling is 88% to 95%, and 1 to 2 intermediate annealings are performed during the cold rolling process, the intermediate annealings are held at 800 to 900°C under a protective atmosphere.

[0015] In some embodiments, the specific parameters for the final thermo-magnetic treatment in step S8 are as follows: the plate obtained in step S7 is processed under vacuum or high-purity hydrogen protection using the following procedure: Step 1: Heat to 445-455℃ at a rate of 180-220℃ / h; Step 2: Hold at 445-455℃ for 2.5-3.5 hours, while simultaneously applying a DC magnetic field along the rolling direction with a field strength of 1100-1300 A / m; Step 3: After the heat preservation is completed, slowly cool to below 150℃ at a controlled rate of 90-110℃ / h, and then cool with the furnace or remove from the furnace.

[0016] In some embodiments, the finished sheet material obtained in step S8 is packaged and stored.

[0017] Beneficial effects: This application provides a high-permeability rare-earth soft magnetic alloy sheet for magnetic shielding. The chemical composition of the alloy, by weight percentage, is: Ni: 79–81%, Mo: 3.9–4.1%, Mn: 0.4–0.6%, Si: 0.15–0.25%, C≤0.02%, P≤0.015%, S≤0.008%, Ce: 0.035%–0.055%, with the balance being Fe and unavoidable impurities. The addition of trace amounts of Ce effectively purifies grain boundaries, refines inclusions, and reduces domain wall pinning forces, thereby increasing μi and μm, and significantly reducing Hc. Examples demonstrate that the μi of the alloy of this invention is increased by an average of more than 15% compared to the control group without Ce, and the Hc is reduced by more than 20%.

[0018] Through systematic optimization of the three-factor matrix of composition, process, and specifications, the robustness of the process window was ensured. The process capability index (Cpk) of the magnetic property parameters (μi, Hc) in batch production is ≥ 1.67, indicating that the process capability is highly stable.

[0019] In the 1-200Hz ultra-low frequency band, the shielding cover made of the plate of this invention has a shielding effectiveness (dB value) that is 5-10dB higher than that made of traditional plate without rare earth elements, and has lower stress sensitivity and temperature drift.

[0020] Through the effects of high cold rolling deformation rate and strip field annealing, a stronger recrystallization texture is formed, which is the surface of nickel-iron alloy that is most easily magnetized.

[0021] Ce forms high-melting-point stable compounds with impurities such as S and O, reducing harmful inclusions at grain boundaries and improving the smoothness of magnetic domain movement.

[0022] The Kernel Average Misorientation (KAM) and Grain Orientation Spread (GOS) values ​​of the alloy of this invention are significantly lower than those of the control group, indicating that its lattice distortion and residual stress levels are extremely low.

[0023] Samples processed by the process of this invention have wider and straighter magnetic domain structures and less resistance to domain wall movement. Detailed Implementation

[0024] These embodiments are provided to make the application thorough and complete, and to fully express the scope of the application to those skilled in the art. It should be noted that, unless otherwise specifically stated, the relative arrangement of components and steps, material composition, numerical expressions, and values ​​illustrated in these embodiments should be interpreted as merely exemplary and not as limiting.

[0025] Unless otherwise specified, percentages in this document are weight percentages.

[0026] Example 1 Ingredients: The ingredients are prepared according to the basic composition, with the following weight percentages: Ni: 80.2%, Mo: 4.05%, Mn: 0.52%, Si: 0.22%, C: 0.018%, P: 0.012%, S: 0.005%, Ce: 0.035 wt.%, with the balance being Fe and unavoidable impurities.

[0027] Step 1, Vacuum Melting: The raw materials are prepared according to the above-designed composition ratio and placed in a vacuum induction furnace for melting. The melting vacuum degree is 1.0 Pa. Refining temperature: 1590℃; Refining time: 60 minutes; Casting temperature: 1510℃; The electrode blank is cast under vacuum and then air-cooled.

[0028] The second step, electroslag remelting (ESR), involves cleaning the electrode billet obtained in the first step, using the same steel grade as the base, and creating slag with consumable electrodes. The ESR process uses high-basicity pre-melted slag, controlling the melting rate at 5 kg / min and the current density at 6750 A / cm². This yields a homogeneous alloy electroslag ingot.

[0029] The third step is homogenization: The electroslag ingot obtained in the second step is loaded into a heat treatment furnace and held at 1200℃ for 30 hours to fully eliminate dendrite segregation and obtain a homogenized electroslag ingot.

[0030] Step 4: Forging. The electroslag ingot obtained in step 3 is loaded into a heating furnace, heated to 1200℃ and held for 5 hours, then removed from the furnace for multi-directional forging. The initial forging temperature is 1150℃, and the final forging temperature is ≥950℃. Multiple forging processes are performed to produce hot-rolled billets of the required specifications.

[0031] Step 5: Hot rolling: The forged billet obtained in step 4 is loaded into a heating furnace, heated to 1160°C, held for 2 hours, and then hot rolled in multiple passes at an initial rolling temperature of 1100°C and a final rolling temperature of 850°C to produce a hot-rolled slab of the required thickness.

[0032] The sixth step is solution treatment: the hot-rolled plate obtained in the fifth step is subjected to water quenching at 1080℃ for 1.25h to obtain a uniform single-phase austenitic structure and to dissolve harmful impurity elements.

[0033] Step 7: Cold rolling: The plate obtained after solution treatment in step 6 is subjected to multiple cold rolling passes with a total deformation rate of 91%. An intermediate annealing is performed once (annealing temperature 850℃, protected nitrogen atmosphere, time 20min), and finally rolled to the target thickness of 0.10mm.

[0034] Step 8: Final thermo-magnetic treatment: Using the material obtained in Step 7, perform the following procedure under vacuum protection: (1) Heat to 450°C at a rate of 200°C / h.

[0035] (2) Keep at this temperature for 3 hours, and at the same time apply a DC magnetic field along the rolling direction with a field strength of 1200A / m.

[0036] (3) After the heat preservation is completed, the temperature is slowly cooled to 150°C at a controlled rate of 100°C / h, and then cooled with the furnace or removed from the furnace.

[0037] Step 9: Finished Product: The finished boards obtained in Step 8 are packaged and stored.

[0038] The test conditions for this embodiment and subsequent embodiments and comparative examples are based on GB / T 13012-2008.

[0039] Performance results: μi = 47200μ, μm = 271000μ, Hc = 1.09 A / m.

[0040] Example 2 This embodiment is the same as Embodiment 1 in all other respects, except that the amount of Ce added is 0.045 wt.%.

[0041] Performance results: μi = 48500, μm = 282000, Hc = 1.02 A / m.

[0042] Example 3 This embodiment is the same as Embodiment 1 in all other respects, except that the amount of Ce added is 0.055 wt.%.

[0043] Performance results: μi = 47800, μm = 275000, Hc = 1.07 A / m.

[0044] Comparative Example 1 This comparative example is the same as Example 1 in all other respects, except that the amount of Ce added is 0%.

[0045] Performance results: μi = 41200, μm = 240000, Hc = 1.42 A / m.

[0046] Comparative Example 2 This comparative example is the same as Example 1 in all other respects, except that the amount of Ce added is 0.045%, and there is no magnetic field during the final heat treatment process.

[0047] Performance results: μi = 44100, μm = 252000, Hc = 1.28 A / m.

[0048] Comparative Example 3 This comparative example is the same as Example 1 in all other respects, except that the amount of Ce added is 0.045%.

[0049] Final heat treatment: electric field strength 600 A / m, cooling rate 150 ℃ / h.

[0050] Performance results: μi = 42800, μm = 245000, Hc = 1.35 A / m.

[0051] Comparative Example 4 This comparative example is the same as Example 1 in all other respects, except that the amount of Ce added is 0.045%.

[0052] Final heat treatment: holding temperature 400℃, electric field strength 600 A / m.

[0053] Performance results: μi = 47500, μm = 272000, Hc = 1.05 A / m.

[0054] Comparative Example 5 This comparative example is the same as Example 1 in all other respects, except that the amount of Ce added is 0.045%.

[0055] Final heat treatment: holding temperature 400℃, cooling rate 150℃ / h.

[0056] Performance results: μi = 47800, μm = 276000, Hc = 1.04 A / m.

[0057] Comparative Example 6 This embodiment is the same as Embodiment 1 in all other respects, except that the amount of Ce added is 0.08 wt.%.

[0058] Performance results: μi = 42000, μm = 250000, Hc = 1.36 A / m.

[0059] Comparative Example 7 This embodiment is the same as Embodiment 1 in all other respects, except that the amount of Ce added is 0.02 wt.%.

[0060] Performance results: μi = 41400, μm = 241000, Hc = 1.40 A / m.

[0061] Comparative Example 8 This embodiment is the same as Embodiment 1 in all other respects, and is finally rolled to the target thickness of 0.06 mm.

[0062] Performance results: μi = 43200, μm = 248000, Hc = 1.38 A / m.

[0063] Comparative Example 9 This embodiment is the same as Embodiment 1 in all other respects, and is finally rolled to the target thickness of 0.18 mm.

[0064] Performance results: μi = 44500, μm = 255000, Hc = 1.32 A / m.

[0065] The experimental data above show that, using electric field strength (A), holding temperature (B), and cooling rate (C) as factors, and μi and Hc as response values, the established response surface model reveals a significant performance plateau region in the central area of ​​the parameter range. Comparative Examples 3-5 are placed outside the performance plateau region determined by the response surface model (electric field strength 1100–1300 A / m, cooling rate 90–110℃ / h, holding temperature 445–455℃). The results show that even deviations in only two key factors lead to a significant decrease in μi and a significant increase in Hc, further verifying the synergy and sensitivity of the process window of this invention. Comparative Examples 3-5 are located outside this plateau region, exhibiting significant performance degradation.

[0066] Comparative Examples 6 and 7 show that when the Ce content exceeds the 0.035%–0.055% range specified in this invention, the magnetic properties deteriorate significantly: excessive Ce leads to the formation of coarse and brittle rare earth phases, disrupting matrix continuity, hindering magnetic domain movement, and causing μi to decrease and Hc to increase; while insufficient Ce has a limited effect on purifying grain boundaries and cannot effectively suppress the segregation of impurities such as S and O at grain boundaries, also resulting in unsatisfactory μi and Hc data. This, together with Examples 1–3 and Comparative Example 1, constitutes a "volcano-like" relationship, confirming that 0.035%–0.055% is the optimal performance window for Ce addition.

[0067] Although comparative examples 8 and 9 do not directly provide magnetic property data, a thickness of 0.06 mm or less exacerbates rolling stress concentration and edge defects, making it difficult to maintain high texture integrity and low residual stress; while a thickness of 0.18 mm weakens the cold-rolled induced {111} <112> Recrystallization texture intensity reduces magnetic permeability and may increase coercivity.

[0068] In Example 2, Ce formed fine, spherical Ce₂O₂S inclusions with residual S and O, which were uniformly distributed within the crystal, thus avoiding the formation of continuous FeMnS and other harmful films at the grain boundaries. In Comparative Example 1, obvious grain boundary sulfide films were observed.

[0069] {111} of Example 2 <112> The texture volume fraction is as high as 75%, far exceeding the 58% of Comparative Example 1. Its average KAM value is as low as 0.35°, and the GOS value is also significantly lower, indicating more complete recrystallization and extremely low lattice distortion and microscopic residual stress.

[0070] Using a magnetic force microscope, it was observed that the sample of Example 2 had a wide and flat 180° magnetic domain structure with clear domain walls in the demagnetized state; while the magnetic domains of Comparative Examples 1 and 2 were fragmented and the domain walls had a large number of pinning points.

[0071] Systematic experiments conducted at 10 points (n=6 per point) within the range of 0-0.08% showed that Ce content exhibited a volcano-like relationship with μi and Hc. When Ce < 0.03%, the purification effect was insufficient; when Ce > 0.06%, excessive Ce formed a coarse and brittle phase, disrupting the matrix continuity and leading to performance degradation. The optimal window was 0.035%-0.055%.

[0072] Example 4: Stability test.

[0073] Three independent production batches (≥100 pieces per batch) of the finished product of the formulation in Example 2 were tested using the final thermo-magnetic treatment method. The coercivity Hc P10 / P50 / P90 values ​​were 1.00 / 1.02 / 1.05 A / m, and the calculated Cpk value was as high as 1.85, demonstrating exceptional batch stability.

[0074] Example 5: Application performance tests were performed on the board material of Example 2 and Comparative Example 1.

[0075] The shielding cylinder made from the 0.10mm plate of this invention has a shielding effectiveness of 28dB at 1Hz and 56dB at 50Hz. Throughout the 1-200Hz frequency band, its effectiveness curve is always 5-12dB higher than that of the plate in Comparative Example 1.

[0076] After applying a tensile stress of 150 MPa, the μi attenuation rate of the plate of the present invention is <3%, while the attenuation rate of Comparative Example 1 is >12%.

[0077] Within the temperature range of -55℃ to +85℃, the temperature coefficient of magnetic permeability α(μi) of the material of this invention is ≤ 1.0×10⁻⁶. -6 / ℃, exhibiting extremely high temperature stability.

[0078] The embodiments of this application have now been described in detail. To avoid obscuring the concept of this application, some details known in the art have not been described. Those skilled in the art can fully understand how to implement the technical solutions disclosed herein based on the above description.

[0079] While specific embodiments of this application have been described in detail by way of examples, those skilled in the art should understand that the above examples are for illustrative purposes only and are not intended to limit the scope of this application. Those skilled in the art should understand that modifications can be made to the above embodiments or equivalent substitutions can be made to some technical features without departing from the scope and spirit of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any manner.

Claims

1. A high-permeability rare-earth soft magnetic alloy plate for magnetic shielding, characterized in that, The chemical composition of the alloy, by weight percentage, is as follows: Ni: 79–81%, Mo: 3.9–4.1%, Mn: 0.4–0.6%, Si: 0.15–0.25%, C≤0.02%, P≤0.015%, S≤0.008%, Ce: 0.035%–0.055%, with the balance being Fe and unavoidable impurities.

2. The high permeability rare-earth soft magnetic alloy plate as described in claim 1, characterized in that, The thickness of the plate is 0.08 mm to 0.15 mm; and under the conditions of magnetic field strength of 0.08 A / m and 50 Hz, the initial permeability ui>47 mH / m, the maximum permeability μm≥270 mH / m, and the coercivity Hc≤1.1 A / m.

3. The high permeability rare-earth soft magnetic alloy plate as described in claim 1, characterized in that, The Ce content is 0.045 wt.%.

4. A method for preparing a high-permeability rare-earth soft magnetic alloy plate as described in claim 1, characterized in that, Includes the following steps: Step S1: Vacuum induction melting; Step S2: Electroslag remelting; Step S3: Homogenize the electroslag ingot at 1200±10℃ for 24–36 hours; Step S4: Forging, initial forging temperature ≥1150℃, final forging temperature ≥950℃; Step S5: Hot rolling, with an initial rolling temperature ≥1100℃ and a final rolling temperature ≥850℃; Step S6, Solution treatment: After holding at 1080±10℃ for 1–1.5 hours, water quench to obtain a uniform single-phase austenitic structure and dissolve harmful impurity elements. Step S7: Cold rolling, with a total deformation rate of 88%–95%, intermediate annealing 1–2 times, annealing temperature 800–900℃; Step S8, Final thermo-magnetic treatment: Apply a DC magnetic field of 1100–1300 A / m along the rolling direction at 445–455℃ and hold for 2.5–3.5 hours, then cool to below 150℃ at a rate of 90–110℃ / h.

5. The preparation method according to claim 4, characterized in that, In step S1, the vacuum degree of vacuum induction melting is ≤1.0Pa, the refining temperature is 1580–1600℃, the refining time is ≥60 minutes, and the casting temperature is 1500–1520℃; the electrode is cast under vacuum and then air-cooled.

6. The preparation method according to claim 4, characterized in that, In the electroslag remelting process of step S2, a high-basicity pre-melted slag is used, the melting rate is 4–6 kg / min, and the current density is 6000–7500 A / cm2, to obtain an alloy electroslag ingot with uniform composition. In step S4 forging, the electroslag ingot obtained in step S3 is loaded into a heating furnace, heated to 1180~1220℃ and held for 4~6 hours, and then taken out of the furnace for multi-directional forging; the initial forging temperature is >1150℃, the final forging temperature is >950℃, and multiple forgings are performed, and then it is forged into a hot-rolled billet of the required specifications.

7. The preparation method according to claim 4, characterized in that, In step S5, during the hot rolling process, the forged billet obtained in step S4 is loaded into a heating furnace, heated to 1160°C, held for 1 to 3 hours, and then subjected to multiple hot rolling passes with an initial rolling temperature >1100°C and a final rolling temperature >850°C to produce a hot-rolled slab of the required thickness.

8. The preparation method according to claim 4, characterized in that, In the cold rolling process of step S7, the plate after solution treatment obtained in step S6 is subjected to multiple cold rolling passes with a total deformation rate of 88% to 95%. One to two annealing processes are performed in between, with an annealing temperature of 800 to 900°C and a holding time of 10 to 30 minutes under a protective atmosphere. Finally, the plate is rolled to the target thickness.

9. The preparation method according to claim 4, characterized in that, In step S7, during the cold rolling process, the plate obtained in step 6 after solution treatment is subjected to multiple cold rolling passes. The total deformation rate of the cold rolling is 88% to 95%, and 1 to 2 intermediate annealings are performed during the cold rolling process. The intermediate annealings are held at 800 to 900°C under a protective atmosphere.

10. The preparation method according to claim 4, characterized in that, The specific parameters for the final thermo-magnetic treatment in step S8 are as follows: The plate obtained in step S7 is processed under vacuum or high-purity hydrogen protection using the following procedure: Step 1: Heat to 445-455℃ at a rate of 180-220℃ / h; Step 2: Hold at 445-455℃ for 2.5-3.5 hours, while simultaneously applying a DC magnetic field along the rolling direction with a field strength of 1100-1300 A / m; Step 3: After the heat preservation is completed, slowly cool to below 150℃ at a controlled rate of 90-110℃ / h, and then cool with the furnace or remove from the furnace.