High-strength anti-theft bolt and preparation method thereof

Abstract

The invention relates to the technical field of bolts, and provides a high-strength anti-theft bolt and a preparation method thereof.The bolt is composed of C, Si, Mn, Cr, W, Mo, V, Re, Nb, Al, N, Ca, B, P, S and the balance Fe and other inevitable impurities. According to the technical scheme, the ultrahigh strength, excellent toughness and delayed fracture resistance of the bolt are achieved, the corrosion resistance of the bolt is improved, and the service life of the bolt is prolonged.

Description

Technical Field

[0001] This invention relates to the field of bolt technology, specifically to a high-strength anti-theft bolt and its preparation method. Background Technology

[0002] Bolts, as a key basic component in the field of mechanical connections, are widely used in critical engineering fields such as bridges, buildings, rail transit, and aerospace. Their performance directly determines the safety and reliability of the overall structure. With the continuous improvement of material performance requirements in modern engineering, traditional bolts are no longer able to meet the needs of complex working conditions in terms of strength, toughness, corrosion resistance, and resistance to delayed fracture.

[0003] Patent CN120210668A discloses a high-strength bolt and its manufacturing method. The bolt is composed of the following components by mass percentage: C 0.28%~0.35%, Si 0.3%~0.35%, Mn 0.5%~0.6%, Cr 1.2%~1.5%, W 0.18%~0.22%, Co 0.1%~0.3%, Mo 0.2%~0.3%, Nb 0.03%~0.05%, Ti 0.02%~0.05%, Al 0.015%~0.03%, N 0.008%~0.015%, V 0.18%~0.22%, Ce 0.05%~0.12%, Y 0.03%~0.08%, Re 0.03%~0.06%, B 0.002%~0.005%, with the balance being Fe and other unavoidable impurities. This invention improves the strength of bolts and further enhances their high-temperature stability and corrosion resistance.

[0004] High-strength steel is prone to failure due to hydrogen-induced delayed fracture during long-term service. While existing technologies can improve resistance to delayed fracture to some extent by adding certain alloying elements (such as niobium (Nb) and titanium (Ti), the effect is limited. This is especially true for high-strength anti-theft bolts, whose service environments are often harsher, placing higher demands on the material's resistance to delayed fracture. To address the aforementioned problems in existing technologies, this invention proposes a high-strength anti-theft bolt and its preparation method. Summary of the Invention

[0005] This invention proposes a high-strength anti-theft bolt and its preparation method, which achieves ultra-high strength, excellent toughness and resistance to delayed fracture of the bolt, improves the corrosion resistance of the bolt, and extends the service life of the bolt.

[0006] The technical solution of the present invention is as follows: In a first aspect, the present invention provides a high-strength anti-theft bolt, the bolt being composed of the following components by mass percentage: C 0.30%~0.38%, Si 0.15%~0.30%, Mn 0.40%~0.70%, Cr 0.80%~1.20%, W 0.80%~1.20%, Mo 0.40%~0.60%, V 0.25%~0.35%, Re 0.08%~0.15%, Nb 0.03%~0.06%, Al 0.020%~0.040%, N 0.010%~0.020%, Ca 0.001%~0.005%, B 0.001%~0.003%, P≤0.010%, S≤0.010%, with the balance being Fe and other unavoidable impurities.

[0007] As a further technical solution, the bolt is composed of the following components by mass percentage: C 0.30%~0.38%, Si 0.15%~0.30%, Mn 0.40%~0.70%, Cr 0.80%~1.20%, W 0.80%~1.20%, Mo 0.40%~0.60%, V 0.25%~0.35%, Re 0.08%~0.15%, Nb 0.03%~0.06%, Al 0.020%~0.040%, N 0.010%~0.020%, Ca 0.001%~0.005%, B 0.001%~0.003%, P≤0.010%, S≤0.010%, with the balance being Fe and other unavoidable impurities, and the (W+Mo) / V weight ratio is 3.5~5.5.

[0008] As a further technical solution, the bolt is composed of the following components by mass percentage: C 0.30%~0.38%, Si 0.15%~0.30%, Mn 0.40%~0.70%, Cr 0.80%~1.20%, W 0.80%~1.20%, Mo 0.40%~0.60%, V 0.25%~0.35%, Re 0.08%~0.15%, Nb 0.03%~0.06%, Al 0.020%~0.040%, N 0.010%~0.020%, Ca 0.001%~0.005%, B 0.001%~0.003%, P≤0.010%, S≤0.010%, with the balance being Fe and other unavoidable impurities, and the Re / (P+S) weight ratio ≥8.

[0009] As a further technical solution, the bolt is composed of the following components by mass percentage: C 0.30%~0.38%, Si 0.15%~0.30%, Mn 0.40%~0.70%, Cr 0.80%~1.20%, W 0.80%~1.20%, Mo 0.40%~0.60%, V 0.25%~0.35%, Re 0.08%~0.15%, Nb 0.03%~0.06%, Al 0.020%~0.040%, N 0.010%~0.020%, Ca 0.001%~0.005%, B 0.001%~0.003%, P≤0.010%, S≤0.010%, with the balance being Fe and other unavoidable impurities, and the Ca / S weight ratio being 0.5~2.0.

[0010] As a further technical solution, the bolt surface is sequentially deposited with an Al-Zn-Mg alloy layer and a diamond-like carbon film.

[0011] As a further technical solution, the thickness of the Al-Zn-Mg alloy layer is 2~3μm; the thickness of the diamond-like carbon film is 1~2μm.

[0012] This invention constructs a composite coating system with active protection by sequentially depositing an Al-Zn-Mg alloy layer and a diamond-like carbon film on the surface of a bolt. The Al-Zn-Mg alloy layer, acting as a "sacrificial anode layer," not only provides a physical barrier to the bolt, preventing the intrusion of corrosive media, but also provides active and long-lasting protection to the steel substrate through preferential corrosion when the coating is damaged. The diamond-like carbon film, with its high hardness, low coefficient of friction, and good chemical stability, further enhances the wear resistance and corrosion resistance of the coating. By controlling the mass percentage content of each element in the Al-Zn-Mg alloy layer and the coating thickness (e.g., an Al-Zn-Mg alloy layer thickness of 2-3 μm and a diamond-like carbon film thickness of 1-2 μm), the synergistic complementarity of the two coatings in terms of protective performance is achieved, effectively improving the corrosion resistance of the bolt and extending its service life.

[0013] As a further technical solution, in the Al-Zn-Mg alloy layer, the mass percentage content of Al is 50%~70%, the mass percentage content of Zn is 25%~40%, and the mass percentage content of Mg is 5%~15%.

[0014] On the other hand, the present invention also provides a method for preparing a high-strength anti-theft bolt, the steps of which include: T1. Melting and forming: The smelting process adopts a combination of vacuum induction melting and electroslag remelting, and the oxygen content is controlled to be ≤15ppm; the ingot is then opened, forged, and processed into bolt blanks. T2. Gradient austenitizing treatment: Under a protective atmosphere, the bolt blank is first heated to 900~920℃ and held for 30~60min, then rapidly heated to 1050~1100℃ and held for 10~20min. T3. Two-phase quenching treatment: Quench the workpiece treated in step T2 to 200~250℃ and hold for 5~15s, then continue to cool to room temperature; then immediately transfer it to a salt bath at 300~350℃ for isothermal treatment for 2~5min. T4. Multi-stage tempering treatment: Perform tempering three times in sequence. The first tempering temperature is 500~520℃ and the holding temperature is 2h; the second tempering temperature is 540~560℃ and the holding temperature is 2h; the third tempering temperature is 300~350℃ and the holding temperature is 3h. T5. Surface coating deposition: Al-Zn-Mg alloy layer and diamond-like carbon film are sequentially deposited on the bolt surface using high-power pulsed magnetron sputtering technology.

[0015] The heat treatment process of this invention combines gradient austenitization, dual-phase quenching, and multi-stage tempering. Gradient austenitization first involves holding the workpiece at a lower temperature to homogenize the microstructure, followed by rapid heating to a higher temperature and holding to promote the full growth and homogenization of austenite grains, providing a good foundation for subsequent microstructure transformation. Dual-phase quenching involves quenching the workpiece to a specific temperature and holding it briefly, then cooling it to room temperature and immediately performing isothermal treatment. This process refines the grains and may introduce a small amount of stable, reversible austenite and other toughening phases. Multi-stage tempering, through tempering at different temperatures and times, eliminates quenching stress, adjusts the microstructure, and promotes uniform precipitation of carbides, further improving the material's strength and toughness. This innovative heat treatment process overcomes the limitation of achieving an optimal balance between strength and toughness in a single martensitic microstructure. While ensuring ultra-high strength, it significantly improves the material's plasticity and toughness, achieving a superior strengthening and toughening effect.

[0016] As a further technical solution, the current for electroslag remelting in step T1 is 4000~6000A, and the melting rate is 3~5kg / min.

[0017] As a further technical solution, in step T3, the cooling rate from 1050~1100℃ to 200~250℃ is ≥50℃ / s; the continued cooling to room temperature is achieved by high-speed argon spray quenching.

[0018] The working principle and beneficial effects of this invention are as follows: This invention designs a W-Mo-V-Re alloy system, where the synergistic effect of each element significantly improves bolt performance. Tungsten (W) and molybdenum (Mo) can form M2C-type secondary hardening carbides rich in tungsten and molybdenum, which precipitate during heat treatment, playing a secondary hardening role and effectively improving the high-temperature strength and microstructure stability of the material. Vanadium (V) can form stable carbides and nitrides, refining grains and improving the strength and toughness of the material. Rhenium (Re) plays a key role in purifying grain boundaries, inhibiting the segregation of harmful elements at grain boundaries and significantly improving the delayed fracture resistance of high-strength steel. By controlling the mass percentage range of each element and the weight ratio between specific elements, such as (W+Mo) / V weight ratio of 3.5-5.5 and Re / (P+S) weight ratio ≥8, the synergistic effect of each element in strengthening, toughening, and delayed fracture resistance is achieved, giving the bolts ultra-high strength, excellent toughness, and outstanding resistance to delayed fracture. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1 This embodiment provides a high-strength anti-theft bolt, which is composed of the following components by mass percentage: C 0.34%, Si 0.22%, Mn 0.55%, Cr 1.00%, W 1.05%, Mo 0.50%, V 0.30%, Re 0.12%, Nb 0.045%, Al 0.030%, N 0.015%, Ca 0.003%, B 0.002%, P 0.008%, S 0.006%, with the balance being Fe and other unavoidable impurities; An Al-Zn-Mg alloy layer and a diamond-like carbon film are sequentially deposited on the surface of the bolt. The Al-Zn-Mg alloy layer has a thickness of 2.5 μm, with an Al mass percentage content of 60%, a Zn mass percentage content of 30%, and a Mg mass percentage content of 10% (Zn / Mg=3.0). The diamond-like carbon film has a thickness of 1.5 μm. The preparation method of high-strength anti-theft bolts includes the following steps: T1. Melting and forming: Weigh the raw materials according to the proportion, and smelt them using a dual process of vacuum induction melting and electroslag remelting, controlling the oxygen content to 12ppm; the electroslag remelting current is 5000A and the melting rate is 4kg / min; the ingot is then shaped, forged, and processed into bolt blanks. T2. Gradient austenitizing treatment: Under argon protection, the bolt blank is first heated to 910℃ and held for 45 min, and then rapidly heated to 1080℃ at a rate of >100℃ / min and held for 15 min. T3. Two-phase quenching treatment: The workpiece treated in step T2 is quickly quenched in a salt bath to 220°C and held for 10 seconds. Then, it is cooled to room temperature by high-speed argon spray quenching (the cooling rate from 1080°C to 220°C is about 60°C / s). After that, it is immediately transferred to a 330°C salt bath for isothermal treatment for 3 minutes. T4. Multi-stage tempering treatment: Perform tempering three times in sequence. The first tempering temperature is 510℃ and the holding temperature is 2 hours. The second tempering temperature is 550℃ and the holding temperature is 2 hours. The third tempering temperature is 320℃ and the holding temperature is 3 hours. Air cool to room temperature. T5. Surface coating deposition: The heat-treated bolts are cleaned and dried. Using high-power pulsed magnetron sputtering technology, an Al-Zn-Mg alloy layer is first deposited. Then, without breaking the vacuum, the target material is switched to deposit a diamond-like carbon film.

[0021] Example 2 This embodiment provides a high-strength anti-theft bolt, which is composed of the following components by mass percentage: C 0.38%, Si 0.15%, Mn 0.70%, Cr 0.80%, W 0.90%, Mo 0.60%, V 0.30%, Re 0.15%, Nb 0.06%, Al 0.040%, N 0.020%, Ca 0.005%, B 0.003%, P 0.005%, S 0.005%, with the balance being Fe and other unavoidable impurities; An Al-Zn-Mg alloy layer and a diamond-like carbon film are sequentially deposited on the surface of the bolt. The Al-Zn-Mg alloy layer has a thickness of 3.0 μm, with an Al mass percentage content of 55%, a Zn mass percentage content of 35%, and a Mg mass percentage content of 10% (Zn / Mg=3.5). The diamond-like carbon film has a thickness of 2.0 μm. The preparation method of the high-strength anti-theft bolt is the same as that in Example 1, except that the parameters for step T2 are: first, heat to 900℃ and hold for 60 min, then rapidly heat to 1100℃ and hold for 10 min; the parameters for step T3 are: quench to 250℃ and hold for 5 s, then transfer to 350℃ for isothermal treatment for 2 min; the parameters for tempering T4 are: 500℃×2h, 560℃×2h, 300℃×3h.

[0022] Example 3 This embodiment provides a high-strength anti-theft bolt, which is composed of the following components by mass percentage: C 0.30%, Si 0.30%, Mn 0.40%, Cr 1.20%, W 1.20%, Mo 0.40%, V 0.35%, Re 0.08%, Nb 0.03%, Al 0.020%, N 0.010%, Ca 0.001%, B 0.001%, P 0.004%, S 0.002%, with the balance being Fe and other unavoidable impurities; An Al-Zn-Mg alloy layer and a diamond-like carbon film are sequentially deposited on the surface of the bolt. The Al-Zn-Mg alloy layer has a thickness of 2.0 μm, with an Al mass percentage content of 65%, a Zn mass percentage content of 25%, and a Mg mass percentage content of 10% (Zn / Mg=2.5). The diamond-like carbon film has a thickness of 1.0 μm. The preparation method of the high-strength anti-theft bolt is the same as that in Example 1, except that the parameters for step T2 are: first, heat to 920℃ and hold for 30 min, then rapidly heat to 1050℃ and hold for 20 min; the parameters for step T3 are: quench to 200℃ and hold for 15 s, then transfer to 300℃ for isothermal treatment for 5 min; the parameters for tempering T4 are: 520℃×2h, 540℃×2h, 350℃×3h.

[0023] Example 4 The chemical composition and heat treatment process (T1-T4) of the bolts in this embodiment are exactly the same as those in Example 1.

[0024] During the T5 surface coating deposition, only a diamond-like carbon film is deposited with a thickness adjusted to 2 μm, and no Al-Zn-Mg alloy layer is deposited.

[0025] Example 5 The chemical composition of the bolt in this embodiment is exactly the same as that in Example 1.

[0026] In its preparation method, steps T2 and T3 are eliminated and replaced with: heating directly to 1080℃ and holding for 1 hour under a protective atmosphere, and then oil quenching to room temperature.

[0027] The subsequent T4 multi-stage tempering and T5 surface coating deposition were the same as in Example 1.

[0028] Comparative Example 1 This comparative example provides a high-strength anti-theft bolt, which is composed of the following components by mass percentage: C 0.34%, Si 0.22%, Mn 0.55%, Cr 1.00%, W 1.05%, Mo 0.50%, V 0.30%, Re 0.01%, Nb 0.045%, Al 0.030%, N 0.015%, Ca 0.003%, B 0.002%, P 0.008%, S 0.006%, with the balance being Fe and other unavoidable impurities; The surface coating and preparation method are exactly the same as in Example 1.

[0029] Comparative Example 2 This comparative example provides a high-strength anti-theft bolt, which is composed of the following components by mass percentage: C 0.34%, Si 0.22%, Mn 0.55%, Cr 1.00%, W 0.40%, Mo 0.20%, V 0.30%, Re 0.12%, Nb 0.045%, Al 0.030%, N 0.015%, Ca 0.003%, B 0.002%, P 0.008%, S 0.006%, with the balance being Fe and other unavoidable impurities; The surface coating and preparation method are exactly the same as in Example 1.

[0030] Comparative Example 3 This comparative example provides a high-strength anti-theft bolt, which is composed of the following components by mass percentage: C 0.34%, Si 0.22%, Mn 0.55%, Cr 1.00%, W 1.05%, Mo 0.50%, V 0.30%, Re 0.12%, Nb 0.045%, Al 0.030%, N 0.015%, Ca 0%, B 0.002%, P 0.008%, S 0.006%, with the balance being Fe and other unavoidable impurities; The surface coating and preparation method are exactly the same as in Example 1.

[0031] Test Example 1: The bolts prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to the following tests: Mechanical properties: The tensile strength and yield strength of the bolts at room temperature were tested in accordance with GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test method at room temperature", and the tensile strength of the bolts at different temperatures were tested in accordance with GB / T 228.2-2015 "Metallic materials - Tensile testing - Part 2: Test method at high temperature". Neutral salt spray resistance: Refer to GB / T 10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test", and conduct continuous neutral salt spray tests on intact bolts and bolts with artificial scratches (single scratches penetrating all coatings to the substrate) at 35±2℃ using a 5% NaCl aqueous solution. Record the initial rust time of intact samples and the maximum single-sided rust spread width of scratched samples after 500 hours of testing.

[0032] The results are shown in Table 1 below: Table 1

[0033] As can be seen from the foregoing, the high-strength anti-theft bolts provided in Embodiments 1-3 of the present invention, through the optimized W-Mo-V-Re alloy system and the composite coating with active protection function, have successfully achieved comprehensive performance of ultra-high strength, excellent toughness, outstanding resistance to delayed fracture and long-term active corrosion protection.

[0034] Comparative Example 1, with its extremely low Re content, exhibited similar room-temperature and high-temperature mechanical properties to Example 1, but its resistance to hydrogen-induced delayed fracture decreased dramatically. The delayed fracture test resulted in fracture within 95 hours, significantly lower than the 720 hours observed in Example 1. This directly demonstrates the irreplaceable and crucial role of sufficient rhenium (Re) in purifying grain boundaries and significantly improving the delayed fracture resistance of high-strength steel. An excessively low Re / (P+S) ratio fails to effectively inhibit the segregation of harmful elements at grain boundaries, which is the root cause of premature failure.

[0035] Comparative Example 2 had an excessively low (W+Mo) / V ratio, resulting in significantly lower high-temperature strength (500°C, 1180 MPa) and high-temperature strength retention rate (76.1%) compared to all other examples. This indicates that when the (W+Mo) / V ratio is too low (2.0 in this comparative example), the precipitation of tungsten- and molybdenum-rich M2C-type secondary hardening carbides is insufficient, leading to severely inadequate microstructural stability and load-bearing capacity of the material at high temperatures. The data validates that controlling the (W+Mo) / V ratio between 3.5 and 5.5 is crucial for ensuring the strength stability of bolts under high temperatures or long-term service.

[0036] Comparative Example 3, without Ca addition, showed a significantly lower elongation after fracture (9.5%) than the Example Group, indicating deteriorated plasticity. Due to the lack of calcium (Ca) to control the morphology of sulfides, elongated MnS inclusions became stress concentration points and crack initiation sites, severely disrupting the matrix and leading to significant material anisotropy. This demonstrates that adding trace amounts of Ca and controlling the Ca / S ratio is crucial for improving the toughness and plasticity of ultra-high strength steel, especially enhancing its transverse properties.

[0037] Furthermore, in Example 4, the absence of the Al-Zn-Mg alloy intermediate layer resulted in a decrease in corrosion resistance. The initial salt spray rust time for the complete coating was only 400 hours, and when the coating was scratched, corrosion spread rapidly at the scratch (up to 3.2 mm wide), completely losing its protective capability. This contrasts sharply with Examples 1-3, where corrosion at the scratches was strictly inhibited (0.5-0.7 mm). These results strongly demonstrate the core function of the Al-Zn-Mg alloy layer as a "sacrificial anode layer": it not only provides a physical barrier but also provides active and long-lasting protection to the steel substrate through preferential corrosion when the coating is damaged.

[0038] In Example 5, the conventional single heat treatment process resulted in a comprehensive deterioration of the overall mechanical properties of the comparative example: the room temperature strength (1580 MPa) was the lowest, and the plasticity and toughness (elongation after fracture 10.0%, low impact toughness) were also poor. This indicates that a single martensitic structure is insufficient to achieve the optimal balance between strength and toughness. The process in this invention can refine the grains and may introduce a small amount of stable reversible austenite and other toughening phases, thereby significantly improving the plasticity and toughness of the material while ensuring ultra-high strength, achieving a superior strengthening and toughening effect.

[0039] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-strength anti-theft bolt, characterized in that, The bolt is composed of the following components by mass percentage: C 0.30%~0.38%, Si 0.15%~0.30%, Mn 0.40%~0.70%, Cr 0.80%~1.20%, W 0.80%~1.20%, Mo 0.40%~0.60%, V 0.25%~0.35%, Re 0.08%~0.15%, Nb 0.03%~0.06%, Al 0.020%~0.040%, N 0.010%~0.020%, Ca 0.001%~0.005%, B 0.001%~0.003%, P≤0.010%, S≤0.010%, with the balance being Fe and other unavoidable impurities.

2. A high-strength anti-theft bolt according to claim 1, characterized in that, The bolt is composed of the following components by weight percentage: C 0.30%~0.38%, Si 0.15%~0.30%, Mn 0.40%~0.70%, Cr 0.80%~1.20%, W 0.80%~1.20%, Mo 0.40%~0.60%, V 0.25%~0.35%, Re 0.08%~0.15%, Nb 0.03%~0.06%, Al 0.020%~0.040%, N 0.010%~0.020%, Ca 0.001%~0.005%, B 0.001%~0.003%, P≤0.010%, S≤0.010%, with the balance being Fe and other unavoidable impurities, and the (W+Mo) / V weight ratio is 3.5~5.

5.

3. A high-strength anti-theft bolt according to claim 1, characterized in that, The bolt is composed of the following components by weight percentage: C 0.30%~0.38%, Si 0.15%~0.30%, Mn 0.40%~0.70%, Cr 0.80%~1.20%, W 0.80%~1.20%, Mo 0.40%~0.60%, V 0.25%~0.35%, Re 0.08%~0.15%, Nb 0.03%~0.06%, Al 0.020%~0.040%, N 0.010%~0.020%, Ca 0.001%~0.005%, B 0.001%~0.003%, P≤0.010%, S≤0.010%, with the balance being Fe and other unavoidable impurities, and the Re / (P+S) weight ratio ≥8.

4. A high-strength anti-theft bolt according to claim 1, characterized in that, The bolt is composed of the following components by weight percentage: C 0.30%~0.38%, Si 0.15%~0.30%, Mn 0.40%~0.70%, Cr 0.80%~1.20%, W 0.80%~1.20%, Mo 0.40%~0.60%, V 0.25%~0.35%, Re 0.08%~0.15%, Nb 0.03%~0.06%, Al 0.02%~0.04%, N 0.01%~0.02%, Ca 0.001%~0.005%, B 0.001%~0.003%, P≤0.01%, S≤0.01%, with the balance being Fe and other unavoidable impurities, and the Ca / S weight ratio being 0.5~2.

0.

5. A high-strength anti-theft bolt according to claim 1, characterized in that, The bolt surface is sequentially deposited with an Al-Zn-Mg alloy layer and a diamond-like carbon film.

6. A high-strength anti-theft bolt according to claim 5, characterized in that, The thickness of the Al-Zn-Mg alloy layer is 2~3μm; the thickness of the diamond-like carbon film is 1~2μm.

7. A high-strength anti-theft bolt according to claim 5, characterized in that, In the Al-Zn-Mg alloy layer, the mass percentage content of Al is 50%~70%, the mass percentage content of Zn is 25%~40%, and the mass percentage content of Mg is 5%~15%.

8. A method for preparing a high-strength anti-theft bolt as described in any one of claims 1-7, characterized in that, step... include: T1. Melting and forming: The smelting process adopts a combination of vacuum induction melting and electroslag remelting, and the oxygen content is controlled to be ≤15ppm; the ingot is then opened, forged, and processed into bolt blanks. T2. Gradient austenitizing treatment: Under a protective atmosphere, the bolt blank is first heated to 900~920℃ and held for 30~60min, then heated to 1050~1100℃ and held for 10~20min. T3. Two-phase quenching treatment: Quench the workpiece treated in step T2 to 200~250℃ and hold for 5~15s, then continue to cool to room temperature; then transfer it to a salt bath at 300~350℃ for isothermal treatment for 2~5min. T4. Multi-stage tempering treatment: Perform tempering three times in sequence. The first tempering temperature is 500~520℃ and the holding temperature is 2h; the second tempering temperature is 540~560℃ and the holding temperature is 2h; the third tempering temperature is 300~350℃ and the holding temperature is 3h. T5. Surface coating deposition: Al-Zn-Mg alloy layer and diamond-like carbon film are sequentially deposited on the bolt surface using high-power pulsed magnetron sputtering technology.

9. The method for preparing the high-strength anti-theft bolt according to claim 8, characterized in that, In step T1, the current for electroslag remelting is 4000~6000A, and the melting rate is 3~5kg / min.

10. The method for preparing the high-strength anti-theft bolt according to claim 8, characterized in that, In step T3, the cooling rate from 1050~1100℃ to 200~250℃ is ≥50℃ / s; the continued cooling to room temperature is achieved by high-speed argon spray quenching.

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