High-strength fastener and process for making same

Abstract

The application relates to the technical field of fasteners, and discloses a high-strength fastener and a preparation process thereof. The high-strength fastener is composed of the following components in percentage by weight: C 0.38%-0.42%, Mn 0.8%-0.9%, Cr 0.9%-1.1%, Mo 0.14%-0.17%, Ni 1.3%-1.5%, V 0.1%-0.2%, B 0.0005%-0.003%, Si 0.15%-0.35%, P<0.02%, S<0.001%, and the balance of Fe and inevitable impurities, wherein the components satisfy (Cr+Mo+V) / C=3-3.5. Through the technical scheme, the problem of insufficient fastener strength in the related art is solved.

Description

Technical Field

[0001] This invention relates to the field of fastener technology, specifically to a high-strength fastener and its manufacturing process. Background Technology

[0002] Fasteners are a class of mechanical parts used to mechanically connect two or more components into a whole. There are many types of fasteners, mainly including bolts, screws, nuts, washers, rivets, pins, and various anchors. With the rapid development of industrial manufacturing, fasteners have widely penetrated various key areas, covering multiple industries such as construction engineering and infrastructure, automotive and transportation, aerospace, electronics, energy and power, medical devices, and shipbuilding.

[0003] However, existing fasteners still suffer from insufficient strength. Insufficiently strong fasteners are prone to fatigue fracture, loosening, and other failures, which can lead to equipment downtime, production interruptions, and even equipment damage and personal injury.

[0004] Therefore, it is very necessary to develop a high-strength fastener. Summary of the Invention

[0005] This invention proposes a high-strength fastener and its manufacturing process, which solves the problem of insufficient fastener strength in related technologies.

[0006] To achieve this objective, the present invention adopts the following technical solution: In a first aspect, the present invention provides a high-strength fastener composed of the following components by weight percentage: C 0.38%~0.42%, Mn 0.8%~0.9%, Cr 0.9%~1.1%, Mo 0.14%~0.17%, Ni 1.3%~1.5%, V 0.1%~0.2%, B 0.0005%~0.003%, Si 0.15%~0.35%, P < 0.02%, S < 0.001%, with the balance being Fe and unavoidable impurities, wherein the components satisfy (Cr+Mo+V) / C = 3~3.5.

[0007] In one embodiment, the component satisfies Cr / C = 2.4~2.5.

[0008] Secondly, the present invention provides a manufacturing process for the above-mentioned high-strength fastener, comprising the following steps: S1. Mix the raw materials according to the target composition, melt them, and then cast them to obtain a billet; S2. The billet is heated to 1100~1200℃ and forged to obtain a semi-finished product; S3. The semi-finished product is heat-treated to obtain the high-strength fastener.

[0009] In one embodiment, the heat treatment sequentially includes normalizing, quenching, tempering, and annealing.

[0010] In one embodiment, the normalizing process includes the following steps: heating the semi-finished product to 880~920℃, holding it at that temperature for 30~60 minutes, and then air-cooling it to room temperature.

[0011] In one embodiment, the quenching includes the following steps: the normalized semi-finished product is heated to 860~870℃, held for 20~40 minutes, then air-cooled to 750~780℃, and then oil-quenched to 100~150℃.

[0012] In one embodiment, the oil temperature during oil quenching is 40~60°C.

[0013] In one embodiment, the tempering includes the following steps: the quenched semi-finished product is heated to 380~420℃, held for the first time, then heated to 520~550℃, held for the second time, and then water-cooled to 80~100℃.

[0014] In one implementation, the second heat preservation time is 2 to 3 times the first heat preservation time, and the first heat preservation time is 60 to 70 minutes.

[0015] In one embodiment, the annealing includes the following steps: heating the tempered semi-finished product to 200~250°C, holding it at that temperature for 180~240 minutes, cooling it in the furnace to 120~150°C, and then air-cooling it to room temperature.

[0016] To improve the strength of fasteners, this invention adjusts the composition content of the fasteners to satisfy (Cr+Mo+V) / C=3~3.5, which has the following beneficial effects: In this invention, a reasonable range of carbon elements ensures the basic hardness and strength of the matrix; manganese and silicon play a role in solid solution strengthening, deoxidation and desulfurization, refining the matrix structure and improving the overall strength, toughness and hardenability of the fastener; chromium, molybdenum and vanadium are synergistically combined with the toughening effect of nickel, and further supplemented by trace amounts of boron for grain boundary strengthening, which significantly refines the grains, suppresses temper brittleness, and greatly improves the strength of the fastener; the content of phosphorus and sulfur impurities is strictly controlled to reduce inclusions and reduce the risk of fracture from the source; the (Cr+Mo+V) / C ratio is limited to 3~3.5 to achieve a match between carbide precipitation and alloy solid solution strengthening, which avoids excessive carbon content leading to excessive brittleness, and ensures that the alloy elements fully exert their strengthening effect to improve the strength of the fastener. Detailed Implementation

[0017] 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.

[0018] Existing fasteners suffer from insufficient strength. To address this issue, this invention improves the strength of fasteners by adjusting the elemental composition and content of each component to promote synergy among the elements and ensuring that the (Cr+Mo+V) / C ratio is 3~3.5.

[0019] Specifically, in order to better understand the technical solution of the present invention, it is described in the following parts.

[0020] Part One This invention provides a high-strength fastener composed of the following components by weight percentage: C 0.38%~0.42%, Mn 0.8%~0.9%, Cr 0.9%~1.1%, Mo 0.14%~0.17%, Ni 1.3%~1.5%, V 0.1%~0.2%, B 0.0005%~0.003%, Si 0.15%~0.35%, P < 0.02%, S < 0.001%, with the balance being Fe and unavoidable impurities, wherein the composition satisfies (Cr+Mo+V) / C = 3~3.5.

[0021] The high-strength fastener of this invention contains 0.38% to 0.42% C, for example, it can be 0.38%, 0.39%, 0.40%, 0.41%, 0.42%, etc. Carbon is the most basic strengthening element in steel. Carbon can combine with alloying elements such as Cr, Mo, and V to form dispersed alloy carbides, which play a role in precipitation strengthening and grain refinement.

[0022] The high-strength fastener of this invention comprises 0.8%~0.9% Mn, for example, it can be 0.8%, 0.81%, 0.82%, 0.83%, 0.84%, 0.85%, 0.86%, 0.87%, 0.88%, 0.89%, 0.90%, etc.; manganese can preferentially combine with oxygen and sulfur in steel to form stable compounds, effectively removing free oxygen and fixing harmful sulfur elements, reducing sulfide and oxide inclusions; Mn can produce lattice distortion, achieving solid solution strengthening and improving the strength of the fastener.

[0023] The high-strength fastener of the present invention comprises 0.9% to 1.1% Cr, for example, it can be 0.9%, 0.92%, 0.94%, 0.95%, 0.96%, 0.98%, 1.0%, 1.02%, 1.04%, 1.05%, 1.06%, 1.08%, 1.1%, etc.; chromium can combine with carbon in the system to form dispersed and fine chromium alloy carbides, which are uniformly distributed in the matrix structure, thereby improving the structural stability of the fastener.

[0024] The high-strength fastener of the present invention comprises 0.14% to 0.17% Mo, for example, 0.14%, 0.15%, 0.16%, 0.17%, etc.; Mo can significantly inhibit the growth of austenite grains during heat treatment, refine the microstructure of the matrix after tempering, improve the overall strength and microstructure uniformity of the fastener from the microstructure level, and reduce stress concentration.

[0025] The high-strength fastener of this invention comprises 1.3% to 1.5% Ni, for example, it can be 1.3%, 1.32%, 1.34%, 1.35%, 1.36%, 1.38%, 1.4%, 1.42%, 1.44%, 1.45%, 1.46%, 1.48%, 1.5%, etc.; the nickel is dissolved in the steel matrix, which can improve the toughness and plasticity of the matrix and reduce the risk of brittle fracture of the fastener.

[0026] The high-strength fastener of the present invention contains 0.1% to 0.2% V, for example, it can be 0.1%, 0.12%, 0.14%, 0.15%, 0.16%, 0.18%, 0.2%, etc. Vanadium easily forms fine carbides and nitrides in steel, which can pin grain boundaries, inhibit the growth of austenite grains during heat treatment, significantly refine the matrix microstructure, and improve the strength of the fastener.

[0027] The high-strength fastener of this invention comprises 0.0005% to 0.003% B, for example, 0.0005%, 0.001%, 0.0015%, 0.002%, 0.0025%, 0.003%, etc.; it improves the hardenability of steel, and B can enable the fastener to be fully hardened as a whole, avoiding the problems of insufficient hardening of the core and large differences in mechanical properties between the inside and outside, and ensuring the overall high strength uniformity.

[0028] The high-strength fastener of this invention comprises 0.15% to 0.35% Si, for example, 0.15%, 0.20%, 0.25%, 0.3%, 0.35%, etc.; silicon is an excellent deoxidizing element in the steelmaking process, which can preferentially combine with oxygen in steel to form stable oxides, effectively reducing the oxygen content of the matrix, reducing oxide inclusion defects, improving the purity of steel, and avoiding the decline in the mechanical properties of fasteners caused by internal inclusions.

[0029] In some embodiments of the present invention, the composition satisfies Cr / C = 2.4~2.5.

[0030] In this invention, the composition of the fastener is limited to a Cr / C ratio of 2.4 to 2.5. Within this range, Cr and C stably form fine, dispersed chromium alloy carbides, avoiding excess carbon freeing or excess chromium solid solution wasting, maximizing the precipitation strengthening effect and improving the strength of the fastener. If the Cr / C ratio is too low, carbon is relatively abundant, easily forming coarse network carbides that break the matrix structure, resulting in decreased strength and increased brittleness; if the ratio is too high, excess chromium cannot fully form carbides, wasting alloying elements and resulting in insufficient strengthening effect; within this range, Cr and C can achieve a better synergistic level, improving the strength of the fastener.

[0031] Part Two This invention also provides a process for manufacturing high-strength fasteners, comprising the following steps: S1. Mix the raw materials according to the target composition, melt them, and then cast them to obtain a billet; S2. The billet is heated to 1100~1200℃ and forged to obtain a semi-finished product; S3. The semi-finished product is heat-treated to obtain high-strength fasteners.

[0032] In some embodiments of the present invention, the heat treatment includes normalizing, quenching, tempering and annealing in sequence.

[0033] In some embodiments of the present invention, normalizing includes the following steps: heating the semi-finished product to 880~920°C, holding it at that temperature for 30~60 minutes, and then air-cooling it to room temperature.

[0034] In this invention, the heat treatment of fasteners includes normalizing, which raises the temperature of the fastener semi-finished product to 880~920℃, which can fully recrystallize and refine the coarse grains, eliminate the defects of uneven grain size in the structure, and obtain a uniform and fine normalized structure, thereby improving the overall strength and structural consistency of the matrix from the basis of the microstructure.

[0035] In some embodiments of the present invention, quenching includes the following steps: the semi-finished product after normalizing is heated to 860~870°C, held for 20~40 minutes, then air-cooled to 750~780°C, and then oil-quenched to 100~150°C.

[0036] In the preparation process of the fasteners of this invention, during quenching, the semi-finished product after normalizing is heated to 860~870℃, held at that temperature, and then pre-cooled to 750~780℃ before oil quenching. By adding a pre-cooling and air-cooling process at 750~780℃, the problem of sudden changes in structure and surge in internal stress caused by excessive temperature difference during direct high-temperature oil quenching can be avoided. The pre-cooling stage moderately reduces the overall temperature of the workpiece and reduces the temperature difference between the inside and outside of the cross section, making the subsequent oil quenching cooling rate more gradual and controllable. On the one hand, it can suppress the formation of coarse martensite and promote the formation of fine and uniform martensite tempering precursor structure, thereby improving the strength of the fastener. On the other hand, it can avoid quenching cracks and structural distortion caused by direct high-temperature oil quenching, reduce grain boundary defects and stress concentration, and ensure that the cross-sectional structure and mechanical properties of the workpiece are uniform and consistent.

[0037] In this invention, the fastener is first pre-cooled to 750-780℃ by air cooling after removal from the furnace before oil quenching. Maintaining the temperature within this range prevents premature precipitation of coarse carbides from Cr, Mo, V, and C. Subsequent oil quenching and tempering allows for the precipitation of dispersed, fine strengthening phases, thus improving the fastener's strength. At higher temperatures, insufficient pre-cooling results in an overall high workpiece temperature, leading to an excessively large temperature difference during direct oil quenching. Conversely, at lower temperatures, premature coarsening and precipitation of alloy carbides hinders strength improvement.

[0038] In some embodiments of the present invention, the oil temperature is 40~60°C during oil quenching.

[0039] In some embodiments of the present invention, tempering includes the following steps: the quenched semi-finished product is heated to 380~420°C, held for the first time, then heated to 520~550°C, held for the second time, and then water-cooled to 80~100°C.

[0040] In the heat treatment process of the fasteners of this invention, a two-stage heating and holding method is adopted during tempering. The first stage involves heating to 380~420℃ and holding for a period of time. This allows for the slow release of the huge residual internal stress generated by quenching, avoiding microcracks and structural distortion caused by stress concentration during direct high-temperature tempering. The second stage involves heating to 520~550℃ and holding for a period of time. With the stress fully released and the structure initially stabilized, this further promotes the precipitation of fine alloy carbides of Cr, Mo, and V alloying elements, achieving precipitation strengthening and improving the strength of the fasteners.

[0041] In some embodiments of the present invention, the second heat preservation time is 2 to 3 times the first heat preservation time, and the first heat preservation time is 60 to 70 minutes.

[0042] In the tempering process of the fasteners in this invention, the first holding time is 60-70 minutes. This moderate holding time avoids both incomplete stress release and uneven microstructure transformation caused by too short a holding time, and premature grain coarsening and decreased matrix strength caused by too long a holding time, thus laying a uniform and stable microstructure foundation for the second tempering. The second holding time is 2-3 times that of the first holding time. The longer holding time allows alloying elements such as C, Cr, Mo, and V to diffuse fully and continuously precipitate fine alloy carbides, maximizing the precipitation strengthening effect and steadily improving the strength of the fasteners.

[0043] In some embodiments of the present invention, annealing includes the following steps: the tempered semi-finished product is heated to 200~250°C, held at that temperature for 180~240 minutes, cooled in the furnace to 120~150°C, and then air-cooled to room temperature.

[0044] In this invention, the heat treatment process of the fastener includes annealing, which can further release residual internal stress at grain boundaries and within the lattice, preventing deformation and dimensional deviation during subsequent processing and long-term service, and ensuring the structural stability and dimensional accuracy of the fastener.

[0045] method Tensile strength shall be tested in accordance with the test method specified in GB / T 228.1-2021 "Metallic materials, tensile testing - Part 1: Test method at room temperature".

[0046] The composition of the fasteners in Examples 1-8 and Comparative Examples 1-2 is shown in Table 1: Table 1. Composition of Examples 1-8 and Comparative Examples 1-2

[0047] The methods for preparing the fasteners in Examples 1-8 and Examples 1-2 all include the following steps: S1. Mix the raw materials according to the target composition, melt them, and then cast them to obtain a billet; S2. The billet is heated to 1150℃ and forged to obtain a semi-finished product; S3. The semi-finished product is heat-treated to obtain high-strength fasteners.

[0048] The detailed parameters and processes of heat treatment in the fastener preparation methods of Examples 1-3 are shown in Table 2: Table 2. Heat treatment process of fasteners in Examples 1-8 and Comparative Examples 1-2

[0049] The heat treatment processes of Examples 4-8 and Comparative Examples 1-2 are the same as those of Example 2.

[0050] The tensile strength test results of the fasteners prepared in Examples 1-8 and Comparative Examples 1-2 are shown in Table 3: Table 3 Test results of Examples 1-8 and Comparative Examples 1-2

[0051] As shown in Table 3, when the composition of the fastener is within the specified range and satisfies (Cr+Mo+V) / C=3~3.5, the strength of the fastener can be improved. When the composition also satisfies Cr / C=2.4~2.5, the strength of the fastener is even higher.

[0052] The heat treatment process during the preparation of the fasteners in Examples 9-13 is shown in Table 4: Table 4 Heat treatment processes of Examples 9-13

[0053] The composition of Examples 9-14 is the same as that of Example 2.

[0054] The tensile strength test results of the fasteners prepared in Examples 9-14 are shown in Table 5: Table 5 Test results of Examples 9-14

[0055] As shown in Table 5, during the preparation of fasteners, adding a pre-cooling process of air cooling to 750~780℃ after quenching can improve the strength of the fasteners. When tempering, adopting a two-stage heating and holding method, and the second holding time being 2~3 times longer than the first holding time, can further improve the strength of the fasteners.

[0056] 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 fastener, characterized in that, It is composed of the following components by weight percentage: C 0.38%~0.42%, Mn 0.8%~0.9%, Cr 0.9%~1.1%, Mo 0.14%~0.17%, Ni 1.3%~1.5%, V 0.1%~0.2%, B 0.0005%~0.003%, Si 0.15%~0.35%, P < 0.02%, S < 0.001%, with the balance being Fe and unavoidable impurities, wherein the components satisfy (Cr+Mo+V) / C = 3~3.

5.

2. The high-strength fastener according to claim 1, characterized in that, The composition satisfies Cr / C = 2.4~2.

5.

3. A manufacturing process for a high-strength fastener, used to manufacture the high-strength fastener according to any one of claims 1 to 2, characterized in that, Includes the following steps: S1. Mix the raw materials according to the target composition, melt them, and then cast them to obtain a billet; S2. The billet is heated to 1100~1200℃ and forged to obtain a semi-finished product; S3. The semi-finished product is heat-treated to obtain the high-strength fastener.

4. The manufacturing process of a high-strength fastener according to claim 3, characterized in that, The heat treatment includes normalizing, quenching, tempering and annealing in sequence.

5. The manufacturing process of a high-strength fastener according to claim 4, characterized in that, The normalizing process includes the following steps: heating the semi-finished product to 880~920℃, holding it at that temperature for 30~60 minutes, and then air-cooling it to room temperature.

6. The manufacturing process of a high-strength fastener according to claim 4, characterized in that, The quenching process includes the following steps: the semi-finished product after normalizing is heated to 860~870℃, held for 20~40 minutes, then air-cooled to 750~780℃, and then oil-quenched to 100~150℃.

7. The manufacturing process of a high-strength fastener according to claim 6, characterized in that, During oil quenching, the oil temperature is 40~60℃.

8. The manufacturing process of a high-strength fastener according to claim 4, characterized in that, The tempering process includes the following steps: the quenched semi-finished product is heated to 380~420℃, held for the first time, then heated to 520~550℃, held for the second time, and then water-cooled to 80~100℃.

9. The manufacturing process of a high-strength fastener according to claim 8, characterized in that, The second heat preservation time is 2 to 3 times the first heat preservation time, and the first heat preservation time is 60 to 70 minutes.

10. The manufacturing process of a high-strength fastener according to claim 4, characterized in that, The annealing process includes the following steps: the tempered semi-finished product is heated to 200~250℃, held for 180~240 minutes, cooled in the furnace to 120~150℃, and then air-cooled to room temperature.

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