High-strength nut and method for manufacturing the same

Abstract

This invention relates to the field of nut technology, specifically to a high-strength nut and its preparation method. The high-strength nut is composed of the following components by mass percentage: C 0.28%-0.32%, Si 0.5%-0.8%, Mn 1.2%-1.6%, Cr 1.8%-2.2%, Ni 0.8%-1.2%, Mo 0.3%-0.5%, Ti 0.07%-0.15%, Nb 0.06%-0.15%, B 0.002%-0.005%, Ca 0.001%-0.003%, Zr 0.002%-0.006%, REM 0.005%-0.015%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities. This invention achieves a high-strength nut that possesses high strength, salt spray corrosion resistance, and excellent resistance to hydrogen-induced delayed fracture, even without a surface anti-corrosion coating.

Description

Technical Field

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

[0002] Nuts are indispensable connecting components in high-end equipment such as marine engineering, wind power, nuclear power, and bridge construction. Their service environment often involves high stress loads and corrosive media with high salt spray and high humidity. Therefore, modern industry has placed extremely stringent requirements on the strength grade and corrosion resistance of nuts.

[0003] Currently, the traditional method in the industry to improve the corrosion resistance of high-strength nuts is to use carbon steel or low alloy steel (such as 35CrMo, 42CrMo, etc.) as the base material to ensure strength, and then apply anti-corrosion coatings such as hot-dip galvanizing, Dacromet coating, electroplating cadmium, etc. on the surface. However, existing technologies have the following significant drawbacks: First, for ultra-high strength nuts with tensile strength ≥1200MPa, their hydrogen embrittlement sensitivity is extremely high. Traditional pickling, electroplating, or coating sintering processes easily introduce free hydrogen into the substrate, leading to unpredictable and catastrophic delayed fracture (hydrogen embrittlement) during service. Second, coatings with high corrosion resistance requirements are usually thick, increasing the thread pitch diameter and making high-precision thread mating difficult. Furthermore, after repeated disassembly and high-torque tightening, the coating is prone to peeling off at the root and flank of the thread, thus losing its corrosion resistance. Third, during the smelting and heat treatment of conventional high-strength steel, impurity atoms such as P and S tend to accumulate at grain boundaries. Conventional single quenching processes also result in significant residual thermal stress in the microstructure, which not only reduces the material's strength and toughness but also exacerbates intergranular corrosion. Therefore, developing an alloy material and nut preparation process that achieves both ultra-high strength and excellent corrosion resistance without relying on external coatings has become a major technical problem urgently needing to be solved in the fastener field. Based on this, this invention proposes a high-strength nut and its preparation method. Summary of the Invention

[0004] This invention proposes a high-strength nut and its preparation method, aiming to solve the technical problems of existing high-strength nuts, such as high risk of hydrogen embrittlement fracture, poor thread fit accuracy, easy peeling of coating, and large residual stress caused by surface coating corrosion protection, which rely on surface coating.

[0005] The technical solution of the present invention is as follows: In a first aspect, the present invention provides a high-strength nut, which is composed of the following components by mass percentage: C 0.28%-0.32%, Si 0.5%-0.8%, Mn 1.2%-1.6%, Cr 1.8%-2.2%, Ni 0.8%-1.2%, Mo 0.3%-0.5%, Ti 0.07%-0.15%, Nb 0.06%-0.15%, B 0.002%-0.005%, Ca 0.001%-0.003%, Zr 0.002%-0.006%, REM 0.005%-0.015%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities.

[0006] As a further technical solution, the nut is composed of the following components by mass percentage: C 0.28%-0.32%, Si 0.5%-0.8%, Mn 1.2%-1.6%, Cr 1.8%-2.2%, Ni 0.8%-1.2%, Mo 0.3%-0.5%, Ti 0.07%-0.15%, Nb 0.06%-0.15%, B 0.002%-0.005%, Ca 0.001%-0.003%, Zr 0.002%-0.006%, REM 0.005%-0.015%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities, and 0.14%≤(Ti+Nb)≤0.25%.

[0007] As a further technical solution, the nut is composed of the following components by mass percentage: C 0.28%-0.32%, Si 0.5%-0.8%, Mn 1.2%-1.6%, Cr 1.8%-2.2%, Ni 0.8%-1.2%, Mo 0.3%-0.5%, Ti 0.07%-0.15%, Nb 0.06%-0.15%, B 0.002%-0.005%, Ca 0.001%-0.003%, Zr 0.002%-0.006%, REM 0.005%-0.015%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities, and 0.8≤Ti / Nb≤1.5.

[0008] As a further technical solution, the nut is composed of the following components by mass percentage: C 0.28%-0.32%, Si 0.5%-0.8%, Mn 1.2%-1.6%, Cr 1.8%-2.2%, Ni 0.8%-1.2%, Mo 0.3%-0.5%, Ti 0.07%-0.15%, Nb 0.06%-0.15%, B 0.002%-0.005%, Ca 0.001%-0.003%, Zr 0.002%-0.006%, REM 0.005%-0.015%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities, and 0.010%≤(B+Zr+REM)≤0.025%.

[0009] As a further technical solution, the REM is a rare earth mixture containing at least two of La, Ce, and Pr.

[0010] This invention, based on low-alloy medium-carbon steel, strictly controls the total amount and ratio of Ti and Nb, particularly limiting it to 0.14%≤(Ti+Nb)≤0.25% and 0.8≤Ti / Nb≤1.5. By forming fine and dispersed Ti / Nb carbonitrides, it hinders austenite grain growth, achieving strong grain refinement and precipitation strengthening, resulting in a matrix tensile strength of over 1320 MPa. Simultaneously, the composite addition of trace amounts of B, Ca, Zr, and rare earth elements (REM), particularly limiting it to 0.010%≤(B+Zr+REM)≤0.025%, significantly purifies grain boundaries and inhibits the grain boundary segregation of harmful elements such as P and S. Without relying on any surface anti-corrosion coating, it greatly improves the matrix's resistance to intergranular corrosion and salt spray resistance (neutral salt spray initial rust time ≥240h).

[0011] On the other hand, the present invention provides a method for preparing a high-strength nut, comprising the following steps: S1. After batching, vacuum induction melting or non-vacuum induction melting is performed at a melting temperature of 1580-1620℃, the vacuum degree is controlled to be ≤10Pa and the degassing time is ≥15min during the refining period to obtain molten steel; S2. The molten steel obtained in step S1 is cast into a steel ingot at a casting temperature of 1520-1550℃, and then the steel ingot is heated to 1100-1200℃ for hot forging or hot rolling to obtain a nut blank; S3. The nut blank obtained in step S2 is subjected to gradient heat treatment, wherein the gradient heat treatment includes austenitizing treatment, pre-quenching treatment, air cooling to room temperature, secondary austenitizing treatment, final quenching treatment, cryogenic treatment and tempering treatment in sequence; S4. The nut blank after heat treatment in step S3 is precision machined to obtain a high-strength nut.

[0012] This invention employs a gradient heat treatment process consisting of "pre-quenching + secondary austenitization + final quenching + cryogenic treatment". After pre-quenching, a second austenitization at a lower temperature achieves secondary microstructural reconstruction, significantly refining the original austenite grains; subsequent cryogenic treatment completely eliminates residual austenite. This process fundamentally releases the phase transformation thermal stress within the material, and combined with the dispersed precipitates within the matrix acting as hydrogen traps, gives the high-strength nut extremely excellent resistance to hydrogen-induced delayed fracture.

[0013] The nut of this invention does not require surface coating treatment, thus avoiding the influence of coating thickness on the thread pitch diameter, ensuring the assembly accuracy of the thread, completely solving the problem of coating peeling failure caused by repeated disassembly and assembly, and greatly extending the service life of high-strength fasteners in harsh marine or industrial atmospheric environments.

[0014] As a further technical solution, in step S3, the austenitizing treatment includes heating to 880-920℃ and holding for 40-60 minutes; the pre-quenching treatment involves quenching in a nitrate bath at a temperature of 220-250℃ and holding for 5-10 minutes.

[0015] As a further technical solution, in step S3, the secondary austenitizing treatment step includes heating to 840-870℃ and holding for 20-30 minutes; the final quenching treatment is quenching in quenching oil at a temperature of 180-210℃ and holding for 15-20 minutes.

[0016] As a further technical solution, in step S3, the cryogenic treatment involves placing the sample in a cryogenic medium at -80℃ to -120℃ and holding it at that temperature for 1-2 hours.

[0017] As a further technical solution, the tempering process involves heating to 200-240℃, holding at that temperature for 3-4 hours, and then air-cooling to room temperature.

[0018] The working principle and beneficial effects of this invention are as follows: This invention relates to a high-strength nut that achieves synergistic optimization of multiple material properties through precise control of alloy composition, without the need for any surface anti-corrosion coating. The composite addition of Ti and Nb forms dispersed carbonitrides, achieving grain refinement and precipitation strengthening; the trace composite addition of B, Zr, Ca, and rare earth elements (REM) effectively purifies grain boundaries and inhibits impurity segregation, thereby significantly improving the matrix's resistance to intergranular corrosion. Simultaneously, it avoids traditional surface treatment processes such as electroplating and pickling, fundamentally eliminating the risk of hydrogen-induced delayed fracture. Therefore, this nut effectively solves the technical problems of existing high-strength nuts that rely on surface coatings for corrosion protection, such as high hydrogen embrittlement risk, poor thread fit accuracy, and easy coating peeling. 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 A high-strength nut, comprising the following components by mass percentage: C 0.30%, Si 0.65%, Mn 1.40%, Cr 2.00%, Ni 1.00%, Mo 0.40%, Ti 0.12%, Nb 0.09%, B 0.003%, Ca 0.002%, Zr 0.004%, REM 0.010%, P 0.008%, S 0.005%, with the balance being Fe and other unavoidable impurities; REM is a rare earth mixture containing La and Ce in a weight ratio of 1:1. The preparation method of high-strength nuts includes the following steps: S1. Smelting: Weigh the raw materials according to the proportion, use vacuum induction melting, the smelting temperature is 1600℃, the vacuum degree is controlled at 8Pa during the refining period, and the degassing time is 20min to obtain molten steel; S2. Casting: The obtained molten steel is cast into steel ingots at a casting temperature of 1530℃; after the steel ingots have cooled to room temperature, they are reheated to 1150℃ for hot forging to obtain nut blanks; S3. Gradient heat treatment: The nut blank is subjected to the following heat treatment: Austenitizing treatment: Heat to 900℃ and hold for 50 minutes; Pre-quenching treatment: Quickly quench in a nitrate bath at 240℃ and hold for 8 minutes; Intermediate cooling: Remove the nut blank from the nitrate bath and allow it to cool naturally to room temperature in still air; Secondary austenitizing treatment: Heat to 860℃ and hold for 25 minutes; Final quenching treatment: Quench in quenching oil at 200℃ and hold for 15 minutes; Cryogenic treatment: Place in a liquid nitrogen cryogenic medium at -100℃ and keep warm for 1.5 hours; Tempering treatment: Heat to 220℃, hold for 3.5 hours, and air cool to room temperature.

[0021] S4. Finishing: The heat-treated nut blank is subjected to thread machining and surface finishing to obtain a high-strength nut.

[0022] Example 2 A high-strength nut, comprising the following components by mass percentage: C 0.28%, Si 0.50%, Mn 1.20%, Cr 1.80%, Ni 0.80%, Mo 0.30%, Ti 0.08%, Nb 0.08%, B 0.002%, Ca 0.001%, Zr 0.003%, REM 0.005%, P 0.005%, S 0.005%, with the balance being Fe and other unavoidable impurities; REM is a rare earth mixture containing La and Ce in a weight ratio of 1:1. The preparation method and steps are the same as in Example 1, except that the parameters for step S3 are as follows: austenitization at 880℃ for 60 min; pre-quenching in a 250℃ nitrate bath for 5 min; after removal, air cooling to room temperature; secondary austenitization at 840℃ for 30 min; final quenching in oil at 210℃ for 15 min; cryogenic treatment at -80℃ for 2 h; and tempering at 240℃ for 3 h.

[0023] Example 3 A high-strength nut, comprising the following components by mass percentage: C 0.32%, Si 0.80%, Mn 1.60%, Cr 2.20%, Ni 1.20%, Mo 0.50%, Ti 0.14%, Nb 0.10%, B 0.004%, Ca 0.003%, Zr 0.006%, REM 0.012%, P 0.010%, S 0.008%, with the balance being Fe and other unavoidable impurities; REM is a rare earth mixture containing La and Ce in a weight ratio of 1:1. The preparation method and steps are the same as in Example 1, except that the parameters for step S3 are as follows: austenitization at 920℃ for 40 min; pre-quenching in a 220℃ nitrate bath for 10 min; after removal, air cooling to room temperature; secondary austenitization at 870℃ for 20 min; final quenching in oil at 180℃ for 20 min; cryogenic treatment at -120℃ for 1 h; and tempering at 200℃ for 4 h.

[0024] Example 4 The chemical composition of the nut and steps S1, S2, and S4 in this embodiment are exactly the same as in Example 1.

[0025] The difference is that in step S3, the "deep cryogenic treatment" step is not performed; the tempering process is performed directly after the final quenching.

[0026] Example 5 The chemical composition of the nut and steps S1, S2, and S4 in this embodiment are exactly the same as in Example 1.

[0027] The difference lies in the fact that step S3 adopts the traditional single quenching heat treatment process: the nut blank is heated to 900℃ and held for 50 minutes, then directly quenched in room temperature oil for cooling, without pre-quenching in a nitrate bath, intermediate air cooling and secondary austenitization, followed by deep cryogenic treatment and tempering.

[0028] Comparative Example 1 A high-strength nut is composed of the following components by mass percentage: Ti 0.06%, Nb 0.05%, with the remaining components being exactly the same as in Example 1; the preparation method is exactly the same as in Example 1.

[0029] Comparative Example 2 A high-strength nut is composed of the following components by mass percentage: Ti 0.18%, Nb 0.07%, with the remaining components being exactly the same as in Example 1; the preparation method is exactly the same as in Example 1.

[0030] Comparative Example 3 A high-strength nut is composed of the following components by mass percentage: B 0.001%, Zr 0.001%, REM 0.002%, with the remaining components being exactly the same as in Example 1; the preparation method is exactly the same as in Example 1.

[0031] Test Example 1: The nuts 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 nut specimen at room temperature were tested in accordance with the latest current standard GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test method at room temperature"; its Rockwell hardness (HRC) was tested in accordance with GB / T 230.1-2018 "Metallic materials - Rockwell hardness testing - Part 1: Test method". Neutral salt spray resistance: Refer to GB / T 10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test", use a 5% NaCl aqueous solution to conduct a continuous neutral salt spray test on the nut sample at 35℃, and record the time when obvious red rust appears on the surface. Delayed fracture test (hydrogen embrittlement resistance): Refer to GB / T 3098.17-2000 "Preload test for hydrogen embrittlement of fasteners - parallel support surface method", assemble the nut on the high-strength bolt, apply 0.9 times the guarantee load, and immerse the whole in a 5% NaCl aqueous solution. Test the time for delayed fracture of the nut under constant load (maximum observation time 500h, no fracture is recorded as >500h).

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

[0033] As can be seen from the foregoing, the high-strength nuts provided in Embodiments 1-3 of the present invention, through an optimized microalloying system and gradient heat treatment process, have successfully achieved ultra-high strength (tensile strength ≥1320MPa), excellent salt spray resistance, and outstanding resistance to delayed fracture without any anti-corrosion coating on the surface.

[0034] In Comparative Example 1, the total amount of (Ti+Nb) was insufficient, resulting in a significant decrease in its room temperature tensile strength and yield strength. This directly demonstrates that the adequate addition of Ti and Nb plays an irreplaceable and crucial role in generating fine-grained strengthening and carbonitride precipitation strengthening, ensuring that the material reaches ultra-high strength levels.

[0035] In Comparative Example 2, the Ti / Nb ratio was imbalanced (too high). Although its strength was still acceptable, the delayed fracture time dropped sharply to 180 hours. This indicates that when the Ti content is too high relative to Nb, the Ti(C,N) inclusions formed at high temperatures are excessive and coarse. The coarse precipitates become microcrack initiation points and stress concentration points within the matrix, severely deteriorating the material's toughness and resistance to hydrogen-induced fracture. The data validates that controlling the Ti / Nb ratio within the range of 0.8-1.5 is crucial for ensuring the stability of the internal microstructure of high-strength steel.

[0036] The total amount of (B+Zr+REM) in Comparative Example 3 was too low. The initial rusting time of this comparative example in neutral salt spray was significantly lower than that of the Example Group, and it fractured faster under constant load. This proves that the composite addition of trace amounts of B, Zr, and rare earth REM can effectively purify grain boundaries, reduce the segregation of impurities such as P and S at grain boundaries, and greatly improve the matrix's resistance to intergranular corrosion and cracking.

[0037] Furthermore, the cryogenic treatment step was omitted in Example 4. In this example, both hardness and strength decreased significantly, and delayed fracture occurred. This strongly demonstrates the core function of cryogenic treatment in completely eliminating retained austenite after quenching. Retained austenite easily induces phase transformation stress under stress, leading to a decrease in the material's resistance to hydrogen embrittlement.

[0038] In Example 5, the traditional single quenching heat treatment not only significantly reduced the salt spray tolerance time but also made the material highly susceptible to delayed fracture. This indicates that traditional quenching easily generates enormous phase transformation thermal stress within the material and induces microcracks. The "pre-quenching + secondary austenitization" gradient heat treatment of this invention can significantly release structural stress, thereby significantly improving the material's corrosion resistance and crack resistance while ensuring ultra-high strength, achieving a better balance between strength and toughness.

[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 nut, characterized by, The nut is composed of the following components by mass percentage: C 0.28%-0.32%, Si 0.5%-0.8%, Mn 1.2%-1.6%, Cr 1.8%-2.2%, Ni 0.8%-1.2%, Mo 0.3%-0.5%, Ti 0.07%-0.15%, Nb 0.06%-0.15%, B 0.002%-0.005%, Ca 0.001%-0.003%, Zr 0.002%-0.006%, REM 0.005%-0.015%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities.

2. A high strength nut according to claim 1, wherein The nut is composed of the following components by mass percentage: C 0.28%-0.32%, Si 0.5%-0.8%, Mn 1.2%-1.6%, Cr 1.8%-2.2%, Ni 0.8%-1.2%, Mo 0.3%-0.5%, Ti 0.07%-0.15%, Nb 0.06%-0.15%, B 0.002%-0.005%, Ca 0.001%-0.003%, Zr 0.002%-0.006%, REM 0.005%-0.015%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities, and 0.14%≤(Ti+Nb)≤0.25%.

3. A high strength nut according to claim 1, wherein The nut is composed of the following components by mass percentage: C 0.28%-0.32%, Si 0.5%-0.8%, Mn 1.2%-1.6%, Cr 1.8%-2.2%, Ni 0.8%-1.2%, Mo 0.3%-0.5%, Ti 0.07%-0.15%, Nb 0.06%-0.15%, B 0.002%-0.005%, Ca 0.001%-0.003%, Zr 0.002%-0.006%, REM 0.005%-0.015%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities, and 0.8≤Ti / Nb≤1.

5.

4. A high strength nut according to claim 1, wherein The nut is composed of the following components by mass percentage: C 0.28%-0.32%, Si 0.5%-0.8%, Mn 1.2%-1.6%, Cr 1.8%-2.2%, Ni 0.8%-1.2%, Mo 0.3%-0.5%, Ti 0.07%-0.15%, Nb 0.06%-0.15%, B 0.002%-0.005%, Ca 0.001%-0.003%, Zr 0.002%-0.006%, REM 0.005%-0.015%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities, and 0.010%≤(B+Zr+REM)≤0.025%.

5. A high strength nut according to claim 1 wherein, The REM is a rare earth mixture containing La and Ce in a weight ratio of 1:

1.

6. A method of producing a high-strength nut as claimed in any one of claims 1 to 5, characterized in that, The steps include: S1. After batching, the materials are smelted at a temperature of 1580-1620℃. During the refining process, the vacuum degree is controlled to be ≤10Pa and the degassing time is ≥15min to obtain molten steel. S2. The molten steel obtained in step S1 is cast into steel ingots at a casting temperature of 1520-1550℃. The steel ingots are then heated to 1100-1200℃ for hot forging or hot rolling to obtain nut blanks. S3. The nut blanks obtained in S2 are subjected to gradient heat treatment, which includes austenitizing treatment, pre-quenching treatment, air cooling to room temperature, secondary austenitizing treatment, final quenching treatment, cryogenic treatment, and tempering treatment in sequence. S4. The nut blank after heat treatment in step S3 is precision machined to obtain a high-strength nut.

7. The method for preparing a high-strength nut according to claim 6, characterized in that, In step S3, the austenitizing treatment includes heating to 880-920℃ and holding for 40-60 minutes; the pre-quenching treatment involves quenching in a nitrate bath at a temperature of 220-250℃ and holding for 5-10 minutes.

8. The method for preparing a high-strength nut according to claim 6, characterized in that, In step S3, the secondary austenitizing treatment includes heating to 840-870℃ and holding for 20-30 minutes; the final quenching treatment is quenching in quenching oil at a temperature of 180-210℃ and holding for 15-20 minutes.

9. The method for preparing a high-strength nut according to claim 6, characterized in that, In step S3, the cryogenic treatment involves placing the sample in a cryogenic medium at -80°C to -120°C for 1-2 hours.

10. The method for preparing a high-strength nut according to claim 6, characterized in that, The tempering process involves heating to 200-240℃, holding at that temperature for 3-4 hours, and then air-cooling to room temperature.

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