High-strength aviation fastener wire rod with excellent low-temperature performance and manufacturing method thereof

Abstract

The application discloses a high-strength aviation fastener wire rod with excellent low-temperature performance and a manufacturing method thereof. The alloy structural steel has the following chemical components (wt%): C: 0.37-0.43%, Si: 0.30-0.40%, Mn: 0.60-0.90%, P≤0.010%, S≤0.005%, Cr: 0.70-0.90%, Mo: 0.30-0.40%, Ni: 0.70-0.90%, Nb: 0.02-0.04%, and the rest is iron and trace impurities. The manufacturing process of the hot-rolled wire rod comprises the following steps: configuring raw materials according to the chemical components, and sequentially performing converter / electric furnace-LF refining-vacuum degassing-casting-breaking-intermediate billet surface treatment-wire rod controlled rolling-wire rod controlled cooling. The wire rod produced by the application has a grain size of 12 levels, and the single-side surface defect and decarburization are less than or equal to 0.05 mm. The fastener after the wire rod reforming, cold heading and quenching and tempering treatment has the characteristics of high tensile strength, high yield ratio and excellent low-temperature performance.

Description

Technical Field

[0001] This invention relates to the field of wire rod metallurgical production, specifically to a high-strength wire rod for aerospace fasteners with excellent low-temperature performance and its preparation method. Background Technology

[0002] Aerospace fasteners are fasteners used in aerospace applications, manufactured from specific materials, possessing special properties or uses to meet the unique technical requirements of aerospace. They are generally used on aircraft, satellites, rockets, and other aerospace vehicles, representing a typical high-end fastener and an essential basic component in spacecraft. Their performance requirements are much higher than those of fasteners in other fields. Aerospace fasteners need to be used in low-temperature, high-pressure, and high-strength environments, therefore their quality requirements are exceptionally high.

[0003] High-strength fasteners are primarily made of CrMo steel, suitable for general environments. However, in the cryogenic environments of aerospace, traditional CrMo steel fasteners often exhibit poor low-temperature impact performance. While there are existing patent documents concerning high-strength wire rods, such as Chinese patent CN103952633A which discloses a high-strength steel wire rod with good low-temperature impact toughness, its chemical composition does not include Ni and Nb elements, leaving room for improvement in low-temperature impact performance. Chinese patent CN105506500B discloses a high-elasticity steel wire rod with a strength exceeding 1200 MPa, but it is mainly used for cold-drawn high-strength steel wire and has a high carbon content (0.82-0.90%), making it unsuitable for cold-forged aerospace fasteners. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a wire rod for aerospace fasteners that has both high strength and high yield strength ratio and excellent impact toughness in low temperature environment, and a method for manufacturing the same.

[0005] The technical solution adopted by the present invention to solve the above problems is as follows: a high-strength aerospace fastener wire rod with excellent low-temperature performance, wherein the chemical composition of the wire rod by mass percentage is: C: 0.37~0.43%, Si: 0.30~0.40%, Mn: 0.60~0.90%, P ≤ 0.010%, S ≤ 0.005%, Cr: 0.70~0.90%, Mo: 0.30~0.40%, Ni: 0.70~0.90%, Nb: 0.02-0.04%, with the remainder being Fe and unavoidable impurities.

[0006] The chemical composition of this invention is designed based on the following: 1) Determination of C content Carbon (C) is the most basic strengthening element in steel, but an increase in carbon content in tempered martensite negatively impacts the toughness and plasticity of steel. In this invention, the C content is determined to be in the range of 0.37% to 0.43%.

[0007] 2) Determination of Si content Si is the main deoxidizing element in steel and has a strong solid solution strengthening effect. However, silicon can significantly increase the deformation resistance of steel, which is detrimental to cold heading and cold extrusion of steel. In addition, Si can promote the segregation of impurity elements at grain boundaries, increase the delayed fracture sensitivity of steel, and have an adverse effect on the toughness and plasticity of steel. The range of Si content in this invention is determined to be 0.30~0.40%.

[0008] 3) Determination of Mn content Manganese (Mn), as a deoxidizing element in the steelmaking process, can improve the hardenability of steel. Mn can also fix the form of sulfur in steel and form MnS and (Fe,Mn)S, which are less detrimental to steel properties, reducing or inhibiting FeS production and preventing hot embrittlement. Furthermore, manganese strengthens steel through solid solution, making it one of the most effective low-cost alloying elements for increasing steel strength. However, Mn promotes grain growth, increasing the steel's overheat sensitivity and cracking tendency. To obtain excellent microstructure and mechanical properties, the Mn content in this invention is determined to be in the range of 0.60% to 0.90%.

[0009] 4) Determination of P and S content Phosphorus (P) severely causes segregation during solidification in steel. P dissolves in ferrite, causing grain distortion and coarsening, and increasing cold brittleness, which is detrimental to the low-temperature performance of steel. In this invention, the P content is determined to be ≤0.010%. Sulfur (S) causes hot brittleness in steel, reduces the ductility and toughness of steel, and is detrimental to the low-temperature performance of steel. In this invention, the S content is determined to be ≤0.005%.

[0010] 5) Determination of Cr content Cr is the most commonly used alloying element in high-strength fasteners. It can effectively improve the hardenability and resistance to temper softening of steel to obtain the required high strength. However, excessive content will deteriorate the toughness and cold workability of the steel. In this invention, the Cr content is determined to be in the range of 0.70~0.90%.

[0011] 6) Determination of Mo content Mo is a commonly used alloying element in high-strength fasteners. It can significantly improve the hardenability and resistance to temper softening of steel. In this invention, the Mo content is determined to be in the range of 0.30~0.40%.

[0012] 7) Determination of Ni content Ni can stabilize austenite, enhance the hardenability of steel, and improve low-temperature toughness. The Ni content in this invention is determined to be in the range of 0.70~0.90%.

[0013] 8) Determination of Nb content Nitrogen (Nb) is a typical microalloying element with a strong affinity for both carbon and nitrogen (C and N), forming stable and fine carbonitrides. The precipitation strengthening and grain refinement of these carbonitrides significantly improve the strength, ductility, and toughness of steel. In this invention, the Nb content is determined to be in the range of 0.02% to 0.04%.

[0014] The present invention also provides a method for manufacturing the above-mentioned high-strength aerospace fastener wire rod with excellent low-temperature performance, the specific production process of which is as follows: Step 1: Smelting and Casting The raw materials are smelted in a converter / electric furnace, refined, vacuum degassed and cast in mold or continuously cast in sequence to produce steel billets with accurate chemical composition, good uniformity and high purity.

[0015] The smelting process utilizes high-quality blast furnace iron, scrap steel, and raw and auxiliary materials to reduce the content of harmful elements in the molten steel. The steelmaking process involves converter / electric furnace primary smelting, LF refining, vacuum degassing, KR iron pretreatment, and converter / electric furnace slag formation to remove carbon and phosphorus. Slag discharge must be avoided during tapping. One purpose of vacuum treatment is to control the hydrogen content below 0.5 ppm, effectively reducing the hazards of hydrogen embrittlement in aerospace fasteners. Continuous casting or ingot casting processes are used to cast the molten steel into billets. If continuous casting is used, the superheat during casting is 15-30°C. Light reduction and electromagnetic stirring processes are employed to improve billet segregation. Protective casting is used throughout the continuous casting process to isolate the billet from air and prevent secondary oxidation.

[0016] Step 2: Opening the blank The continuously cast billet is opened by a reversible rolling mill, and then the intermediate billet undergoes surface treatment to ensure that the surface is qualified.

[0017] The billet undergoes high-temperature diffusion to further remove hydrogen from the steel. The billet is then heated in a furnace at high temperatures (1130-1250℃) to fully dissolve the alloy components and homogenize the composition. Large square billets are then cut into 150mm diameter sections. 2 ~£200mm 2 The intermediate billet is slowly cooled after being cut and then its surface is treated to ensure the surface quality of the intermediate billet.

[0018] Step 3: Controlled rolling and controlled cooling of wire rod The billet is heated in stages in the furnace. The temperature of the preheating section is controlled at ≤720℃, and the temperature of the high-temperature section is controlled at 950~1050℃. The holding time in the high-temperature section is 40-60min, and the residual oxygen content in the furnace is ≤1.0%. After the holding time is completed, the billet is descaled and the surface oxide layer is removed before rolling. In the roughing stage, the initial rolling temperature is 900~950℃, and the total compression ratio is ≥60%. Secondary descaling is performed before intermediate rolling. In the intermediate and finishing rolling stage, the intermediate and finishing rolling temperature is 800~850℃, and the total compression ratio is ≥50%. The temperature of the sizing mill is 780~850℃, and the compression ratio of the last two passes is ≥50%. The wire drawing temperature is 780~860℃. After wire drawing, a slow cooling process is adopted, the roller speed is 0.10m / s~0.35m / s, and the insulation cover is completely closed.

[0019] This invention produces high-strength aerospace fastener wire rods with excellent low-temperature performance, with specifications ranging from Φ5.5 to Φ25mm. The resulting wire rods have a ferrite + pearlite structure and a grain size of grade 12. After subsequent wire rod modification, cold heading, and heat treatment (heating to 860-900℃ and holding for 40-90 minutes, followed by water or oil cooling, and then air cooling after holding at 550-600℃ for 40-90 minutes), the produced fasteners exhibit high tensile strength, high yield strength ratio, and excellent low-temperature performance. The tensile strength is ≥1050MPa, the yield strength is ≥970MPa, the yield strength ratio is ≥0.92, and the KU2 at -45℃ is ≥85J.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1) Composition design: (1) Strictly control the content of impurity elements such as P and S in steel, reduce grain boundary segregation, prevent grain boundary embrittlement, and improve the toughness of aerospace fasteners; (2) By adding Nb microalloying elements, refine the grains and generate dispersed carbonitrides to refine the austenite grains. While improving strength, it can also improve toughness and enhance the low-temperature impact performance of aerospace fasteners; (3) Add appropriate Cr and Mo elements to increase the tempering temperature while ensuring strength, so that the carbonitrides become fine and uniform spheroids, improve toughness, and enhance the low-temperature impact performance of aerospace fasteners; (4) Appropriately add Ni elements to increase the hardenability of steel and improve low-temperature toughness.

[0021] 2) The continuous casting integrated technology is used to control and roll the wire rod from the large billet, resulting in good wire rod uniformity. The billet surface is treated, and low-temperature heating and low-temperature rolling are carried out to improve the surface quality and decarburization of the wire rod, preventing failure in subsequent processing.

[0022] 3) Controlled rolling: Compared to conventional rolling, low-temperature heating and rolling are employed. The temperature of the intermediate and finishing rolling mills and the temperature entering the reducing and sizing mill are controlled below 850℃ in the austenitic non-recrystallization zone. Combined with a high compression ratio in multiple passes and the pinning effect of Nb, the final wire rod grain size is controlled at grade 12. Grain refinement can simultaneously improve strength and toughness, enhancing the low-temperature impact performance of aerospace fasteners.

[0023] 4) Fasteners produced from the wire rods after wire rod modification, cold heading, and heat treatment have the characteristics of high tensile strength, high yield strength ratio, and excellent low-temperature performance. The tensile strength is ≥1050Mpa, the yield strength is ≥970Mpa, the yield strength ratio is ≥0.92, and the KU2 of the low-temperature impact at -45℃ is ≥85J. Attached Figure Description

[0024] Figure 1 This invention ultimately yielded a 100X metallographic photograph of the wire rod. Figure 2 This invention ultimately yielded a 500X metallographic photograph of the wire rod. Detailed Implementation

[0025] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention. Example 1

[0026] This embodiment relates to a high-strength aerospace fastener wire rod with excellent low-temperature performance, with a specification of Φ10mm. The composition and its mass percentage are as follows: C: 0.40%, Si: 0.36%, Mn: 0.78%, P: 0.009%, S: 0.002%, Cr: 0.77%, Mo: 0.32%, Ni: 0.82%, Nb: 0.025%, with the remainder being Fe and unavoidable impurities.

[0027] Raw materials are prepared according to chemical composition, and the process proceeds in sequence as follows: converter / electric furnace—LF refining—vacuum degassing—casting (390mm*510mm)—billing (150-200mm2)—intermediate billet surface treatment (flaw detection + grinding)—wire rod controlled rolling—wire rod controlled cooling.

[0028] During continuous casting, the superheat temperature is 18°C. The continuous casting process uses light reduction and electromagnetic stirring to improve billet segregation. The entire continuous casting process uses protective pouring to isolate the billet from air and prevent secondary oxidation.

[0029] Controlled rolling and cooling of wire rod: The billet is heated in sections in the furnace, with the preheating section temperature controlled at 705℃, the high-temperature section temperature controlled at 994℃, the high-temperature section holding time at 54 minutes, and the residual oxygen content in the furnace at 0.6%. After holding, the billet is descaled and the surface oxide layer is removed before rolling. In the roughing stage: the initial rolling temperature is 945℃, and the total compression ratio is 68%; a second descaling is performed before intermediate rolling; in the intermediate and finishing rolling stage: the intermediate and finishing rolling temperature is 848℃, and the total compression ratio is 78%; the temperature entering the sizing mill is 823℃, and the compression ratio of the last two passes is 56%; the wire exit temperature is 828℃, a slow cooling process is adopted, the roller speed is 0.30m / s, and the rest is shut down.

[0030] The wire rod produced by the above manufacturing method has a grain size of grade 12. The fasteners produced after wire rod modification, cold heading, and heat treatment (heating to 860℃ for 50 minutes and water cooling, then holding at 580℃ for 45 minutes and air cooling) have a tensile strength of 1060 MPa, a yield strength of 990 MPa, a yield ratio of 0.93, and a low-temperature impact strength of -45℃ (KU2: 88J). Example 2

[0031] This embodiment relates to a high-strength aerospace fastener wire rod with excellent low-temperature performance, with a specification of Φ13mm. The composition and its mass percentage are as follows: C: 0.41%, Si: 0.33%, Mn: 0.81%, P: 0.008%, S: 0.001%, Cr: 0.81%, Mo: 0.31%, Ni: 0.80%, Nb: 0.028%, with the remainder being Fe and unavoidable impurities.

[0032] Raw materials are prepared according to chemical composition, and the process proceeds in sequence as follows: converter / electric furnace—LF refining—vacuum degassing—casting (390mm*510mm)—billing (150-200mm2)—intermediate billet surface treatment (flaw detection + grinding)—wire rod controlled rolling—wire rod controlled cooling.

[0033] During continuous casting, the superheat temperature is 24℃. The continuous casting process uses light reduction and electromagnetic stirring to improve billet segregation. The entire continuous casting process uses protective pouring to isolate the billet from air and prevent secondary oxidation.

[0034] Controlled rolling and cooling of wire rod: The billet is heated in sections in the furnace, with the preheating section temperature controlled at 665℃, the high-temperature section temperature controlled at 1014℃, the high-temperature section holding time at 56 minutes, and the residual oxygen content in the furnace at 0.5%. After holding, the billet is descaled and the surface oxide layer is removed before rolling. In the roughing stage: the initial rolling temperature is 935℃, and the total compression ratio is 63%; a second descaling is performed before intermediate rolling; in the intermediate and finishing rolling stage: the intermediate and finishing rolling temperature is 841℃, and the total compression ratio is 65%; the temperature entering the reducing and sizing mill is 813℃, and the compression ratio of the last two passes is 52%; the wire drawing temperature is 818℃, a slow cooling process is adopted, the roller speed is 0.20m / s, and the rest is shut down.

[0035] The wire rod produced by the above manufacturing method has a grain size of grade 12. The fasteners produced after wire rod modification, cold heading, and heat treatment (heating to 880℃ for 60 minutes and water cooling, then holding at 570℃ for 50 minutes and air cooling) have a tensile strength of 1080 MPa, a yield strength of 998 MPa, a yield ratio of 0.92, and a low-temperature impact strength of -45℃ (KU2: 91J). Comparative Example

[0036] To highlight the features and technical effects of this invention, comparisons were made with Example 1 using methods without Nb, without Ni, and with different rolling and cooling processes. See Table 1 below: Table 1

[0037] Note: 1. The components of Comparative Example 1 are the same as those of Example 1, except that the Nb content is 0.001% (residual).

[0038] 2. The components of Comparative Example 2 are the same as those of Example 1, except that the Ni content is 0.01% (residual).

[0039] 3. The components of Comparative Examples 3 and 4 are the same as those of Example 1.

[0040] Analysis of the effect of proportion: 1. Adding Nb (Example 1 vs Comparative Example 1) refines the grain size from level 8 to level 12, significantly improving low-temperature impact toughness (88J vs 65J).

[0041] 2. Adding Ni (Example 1 vs Comparative Example 2) significantly improves low-temperature impact toughness (88J vs 58J).

[0042] 3. Low-temperature non-recrystallization zone rolling (Example 1 vs. Comparative Example 3) refines grains, improves strength and low-temperature impact energy, and significantly improves decarburization.

[0043] 4. The slow cooling process (Example 1 vs. Comparative Example 4) significantly improves decarburization by improving the uniformity of the microstructure and avoiding abnormal microstructure.

[0044] Example 3 (Complete Application of Aerospace Fastener Manufacturing)

[0045] In this embodiment, the Φ10mm wire rod obtained in Example 1 is processed into aviation-grade hexagonal bolts (specification M8×40mm). The specific process flow is as follows: 1. Wire rod reform Pickling with lime → Drawing to Φ8.2mm → Spheroidizing annealing → Pickling and phosphating saponification → Drawing to Φ7.8mm 2. Cold heading The multi-station cold heading machine (5 stations) forms hexagonal head bolt blanks.

[0046] 3. Heat treatment (quenching and tempering) Quenching: Hold at 880℃ for 60 minutes, then oil cool. Tempering: Hold at 580℃ for 60 minutes, then air cool.

[0047] 4. Surface treatment Sandblasting → Zinc-nickel alloy plating → Passivation → Baking to remove hydrogen.

[0048] The performance of the obtained fasteners is shown in Table 2 below: Table 2

[0049] Example 3 shows that the wire rod of the present invention can still stably achieve the design goals of high strength, high yield strength ratio and excellent low temperature impact performance after actual fastener processing, meeting the requirements of aerospace fastener engineering applications.

[0050] In addition to the above embodiments, the present invention can also adjust parameters such as billet thickness and continuous casting process according to the production requirements of converters and electric furnaces of different tonnages. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A high-strength aerospace fastener wire rod with excellent low-temperature performance, wherein the chemical composition of the wire rod, by mass percentage, is: C: 0.37~0.43%, Si: 0.30~0.40%, Mn: 0.60~0.90%, P ≤ 0.010%, S ≤ 0.005%, Cr: 0.70~0.90%, Mo: 0.30~0.40%, Ni: 0.70~0.90%, Nb: 0.02-0.04%, with the remainder being Fe and unavoidable impurities.

2. The high-strength aerospace fastener wire rod with excellent low-temperature performance according to claim 1, characterized in that, The specifications of the wire rod are Φ5.5~Φ25mm.

3. The high-strength aerospace fastener wire rod with excellent low-temperature performance according to claim 1, characterized in that, The metallographic structure of the wire rod is ferrite + pearlite, with a grain size of 12 and a single-sided surface defect and decarburization of ≤0.05mm.

4. The high-strength aerospace fastener wire rod with excellent low-temperature performance according to claim 1, characterized in that, The fasteners produced after the wire rod is subsequently modified, cold-headed, and heat-treated have a tensile strength ≥1050 MPa, a yield strength ≥970 MPa, a yield ratio ≥0.92, and a low-temperature impact KU2 ≥85 J at -45℃. The heat treatment process is as follows: the processed fastener blank is heated to 860-900℃ and held for 40-90 minutes, then water-cooled or oil-cooled, and then held at 550-600℃ for 40-90 minutes before air-cooling.

5. A method for manufacturing high-strength aerospace fastener wire rod with excellent low-temperature performance as described in claim 1, characterized in that: Includes the following steps: (1) Smelting and casting: The raw materials are smelted in a converter / electric furnace, refined, vacuum degassed and cast in mold or continuously to produce steel billets with accurate chemical composition, good uniformity and high purity. (2) Billet opening: The continuous casting billet is opened by a reversible rolling mill. After the billet is opened, the intermediate billet surface is treated to ensure that the surface is qualified. (3) Controlled rolling and controlled cooling of wire rod: The billet is heated in sections in the heating furnace. The temperature of the preheating section is controlled at ≤720℃, the temperature of the high temperature section is controlled at 950~1050℃, the holding time of the high temperature section is 40-60min, and the residual oxygen content in the furnace is ≤1.0%. After the holding time is completed, the intermediate billet is descaled and the surface oxide layer is removed before rolling. In the rough rolling stage: the initial rolling temperature is 900~950℃, and the total compression ratio is ≥60%; a second descaling is performed before intermediate rolling; in the intermediate and finishing rolling stage: the intermediate and finishing rolling temperature is 800~850℃, and the total compression ratio is ≥50%; the temperature of the sizing mill is 780~850℃, and the compression ratio of the last two passes is ≥50%; the wire drawing temperature is 780~860℃, and a slow cooling process is adopted after wire drawing. The roller speed is 0.10m / s~0.35m / s, and the heat preservation cover is completely closed.

6. The method for manufacturing a high-strength aerospace fastener wire rod with excellent low-temperature performance according to claim 5, characterized in that, In step (1), high-quality molten iron, scrap steel and raw and auxiliary materials are selected for smelting to reduce the content of harmful elements in the molten steel. The molten steel smelting involves converter / electric furnace primary smelting, LF refining and vacuum degassing. The vacuum treatment controls the hydrogen content to be lower than 0.5ppm. The superheat of the casting during continuous casting is 15~30℃. The continuous casting process adopts light reduction and electromagnetic stirring process to improve the segregation of the billet. The entire continuous casting process adopts protective casting to isolate the billet from the air and prevent secondary oxidation.

7. The method for manufacturing a high-strength aerospace fastener wire rod with excellent low-temperature performance according to claim 4, characterized in that, In step (2), the steel billet is heated in a furnace to a high temperature of 1130-1250℃ to fully dissolve the alloy components in the steel and homogenize the composition. The large square billet is then cut into 150mm diameter billets. 2 ~£200mm 2 The intermediate billet is slowly cooled after being cut and then its surface is treated to ensure the surface quality of the intermediate billet.

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