Combined screw and manufacturing method and application thereof

Abstract

The invention relates to a combined screw and a manufacturing method and application thereof, and the manufacturing method of the combined screw comprises the following steps: sequentially carrying out heat treatment and composite coating electroplating on an assembly obtained by combining a tile pad and a screw rod to obtain the combined screw, carburizing is carried out in the heat treatment, and the carburizing concentration C is controlled to meet the condition that the carburizing concentration C is larger than or equal to the screw carbon content C1 and smaller than the tile pad carbon content C2. Carburization is carried out in heat treatment, and the carburization concentration is controlled to be not lower than the carbon content of the screw and lower than the carbon content of the tile pad, so that the hardness of the screw can be improved, meanwhile, the hardness of the tile pad does not exceed the standard to cause brittle rupture, and the purpose of improving the torque performance is achieved; in addition, compared with a traditional single electroplated layer, the electroplated composite coating can avoid electrochemical corrosion, and then the salt mist resistance of the whole machine is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of fastener manufacturing, and in particular to a combination screw and a manufacturing method and application thereof. Background Art

[0002] Combination screws are a type of fastener pre-assembled from a screw and a washer. They are widely used in low-voltage electrical appliances such as switches, circuit breakers, contactors, and distribution boxes to provide electrical connection, mechanical fixation, and anti-loosening functions.

[0003] Currently, the low-voltage electrical appliance industry places high demands on the salt spray resistance of its entire product. The electroplating layer of the combination screws used in low-voltage electrical appliances is typically zinc. For example, CN222577099U discloses a salt spray corrosion-resistant metal fastener that reduces salt spray corrosion by adding a zinc coating to the hexagonal bolt head and nut mechanism. However, typically, the product's contact plate is typically made of copper, such as brass. The potential difference between zinc and copper is significant. When the combination screw and contact plate are combined into a terminal, the salt solution promotes a galvanic chemical reaction between the two, leading to electrochemical corrosion. This leaves much room for improvement in the overall salt spray resistance of the device.

[0004] Nickel has a higher potential than zinc and a smaller potential gap with copper, making it more suitable as an electroplating material for salt spray protection. For example, CN101713072A discloses a method for surface treatment of fasteners using a gas multi-element co-penetration and electrodeposition composite technology, comprising the following steps: preliminary processing of the fasteners, gas multi-element co-penetration treatment, electroplating copper plating, and electroplating nickel plating. After the above steps, fasteners that meet high strength requirements and can withstand a 2600-hour salt spray test without red rust can be obtained. Even with the technical means of nickel plating, the potential difference between nickel and copper still exists, and nickel plating has more pores. In a salt spray environment, without considering the potential difference, its own protection effect is not as good as that of zinc plating. Therefore, there are still technical bottlenecks in achieving salt spray resistance for the entire machine.

[0005] In addition, the torque performance of the combination screw is closely related to the hardness of the screw and its thread. The screw in the combination screw is usually made of ML20Mn material, and the tile washer is made of 45 steel material. Since the carbon content and hardness of 45 steel material are higher than those of ML20Mn material, if the hardness of the screw is increased, the hardness of the tile washer will also increase simultaneously. If the hardness of the tile washer is too high, the combination screw may cause the tile washer to break during use.

[0006] Currently, the industry typically manufactures fasteners using boron-containing materials such as German 19MnB4 and 22B2, and American 10B21. Heat treatment is achieved through an austempering salt bath process, with the quenching temperature controlled at 860-880°C and then cooled in a salt bath at 320-340°C. This approach has the drawback of high raw material and heat treatment costs.

[0007] In summary, there is an urgent need to develop a combination screw that can achieve salt spray resistance, high torque and low cost for the entire product. Summary of the Invention

[0008] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a combination screw and its manufacturing method and application. The combination screw can be used in low-voltage electrical products to achieve salt spray resistance of the entire machine, and can improve its torque and reduce the occurrence of breakage while maintaining cost-effectiveness.

[0009] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present invention provides a method for manufacturing a combination screw, the manufacturing method comprising the following steps:

[0011] The assembly obtained by combining the tile pad and the screw rod is subjected to heat treatment and electroplating composite coating in sequence to obtain the combined screw;

[0012] Carburizing is performed during the heat treatment, and the carburizing concentration C is controlled to satisfy the following conditions: screw carbon content C1≤carburizing concentration C<pad carbon content C2.

[0013] The manufacturing method for the combination screw provided by the present invention significantly improves the screw hardness by carburizing during heat treatment, controlling the carburization concentration to be no less than the screw's carbon content and lower than the pad's carbon content. This method also significantly improves the screw's hardness while still meeting the pad's hardness requirements. After carburizing, the screw's surface hardness reaches HV ≥ 330, while the pad's hardness does not exceed the specified hardness, leading to brittle cracking. This achieves the goal of improving torque performance. Furthermore, compared to traditional single electroplating layers, the electroplated composite coating can prevent electrochemical corrosion, thereby improving the overall salt spray resistance of the device.

[0014] The unit of carbon content C1 of the screw of the present invention is %, and the unit of carbon content C2 of the pad is %.

[0015] Preferably, the carburizing concentration C is closer to the screw carbon content C1 than the shoe carbon content C2.

[0016] In the present invention, the carburizing concentration is controlled to be slightly higher than or equal to the carbon content of the screw and lower than the carbon content of the pad, thereby increasing the hardness of the screw to the upper limit of HRC20-45, while the hardness of the pad still meets the requirement of HRC35-45.

[0017] Preferably, the screw carbon content C1 is 0.18-0.23%, for example, 0.18%, 0.19%, 0.2%, 0.21% or 0.23%, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0018] Preferably, the carbon content C2 of the tile pad is 0.42-0.5%, for example, it can be 0.42%, 0.44%, 0.46%, 0.48% or 0.5%, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

[0019] Preferably, the carburizing concentration C is 0.2-0.27%, for example, 0.2%, 0.22%, 0.24%, 0.26% or 0.27%, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0020] Preferably, the tile pad is made of carbon structural steel.

[0021] Preferably, the carbon structural steel includes No. 45 steel.

[0022] Preferably, the tile pad is obtained by stamping a steel billet.

[0023] Preferably, the material of the screw includes ML20Mn.

[0024] The ML20Mn described in the present invention corresponds to the SWRCH22A material of the Japanese standard.

[0025] Preferably, the screw is obtained by sequentially cold heading, screw head forming and thread cold forming of a wire rod.

[0026] Preferably, the combination is to squeeze and fix the tile pad on the polished rod of the screw.

[0027] Preferably, the heat treatment includes quenching and tempering, and the carburizing is performed during the quenching process.

[0028] Preferably, the quenching temperature is 860-880°C, for example, 860°C, 865°C, 870°C, 875°C or 880°C, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0029] Preferably, the quenching temperature is divided into zones, with the temperature of the first zone being 870°C, the temperature of the middle zone being 880°C, and the temperature of the tail zone being 860°C.

[0030] Preferably, the quenching time is 50-80 min, for example, 50 min, 55 min, 60 min, 70 min or 80 min, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0031] Preferably, cooling is performed after quenching and before tempering.

[0032] Preferably, the quenching medium used for quenching includes quenching oil.

[0033] Preferably, the temperature of the quenching oil is 60-80° C., and the oil cooling quenching time is 6-10 minutes.

[0034] The temperature of the quenching oil is 60-80°C, for example, 60°C, 65°C, 70°C, 75°C or 80°C, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0035] The oil cooling quenching time of the quenching oil is 6-10 minutes, for example, it can be 6 minutes, 7 minutes, 8 minutes, 9 minutes or 10 minutes, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0036] Preferably, the tempering temperature is 400-440°C, for example, 400°C, 410°C, 420°C, 430°C or 440°C, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0037] Preferably, the tempering temperature is divided into zones, with the temperature of the first zone being 440°C, the temperature of the middle zone being 440°C, and the temperature of the tail zone being 430°C.

[0038] Preferably, the tempering time is 70-100 min, for example, 70 min, 75 min, 90 min, 90 min or 100 min, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0039] Preferably, water cooling or air cooling is used for cooling after the tempering.

[0040] Preferably, the loading thickness of the assembly during carburizing is ≤8 cm, for example, it can be 8 cm, 6 cm, 5 cm, 3 cm or 1 cm, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0041] In the present invention, the loading thickness of the assembly during carburizing is controlled so that the assembly is fully in contact with the carburizing medium, thereby ensuring the fluidity of the carburizing medium and the uniformity of carburizing.

[0042] Preferably, the composite plating layer includes a copper-nickel plating layer or a nickel-graphene plating layer.

[0043] Preferably, the thickness of the nickel graphene coating is 1-8 μm, for example, 1 μm, 3 μm, 5 μm, 6 μm or 8 μm, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

[0044] Preferably, the copper-nickel plating layer comprises a copper plating layer and a nickel plating layer electroplated sequentially on the surface of the assembly.

[0045] Preferably, the thickness of the copper plating layer is 3-7 μm, for example, 3 μm, 4 μm, 5 μm, 6 μm or 7 μm, but is not limited to the listed values. Other values not listed within the numerical range are also applicable, preferably 5 μm.

[0046] Preferably, the thickness of the nickel plating layer is 5-10 μm, for example, 5 μm, 6 μm, 8 μm, 9 μm or 10 μm, but is not limited to the listed values. Other values not listed within the numerical range are also applicable, preferably 5 μm.

[0047] When electroplating copper-nickel coatings in the present invention, the copper coating is first electroplated and then the nickel coating is electroplated, and the thickness of the two is controlled. On the one hand, the setting of the copper coating can make the potential difference between the contact plate and the assembly essentially non-existent, thereby avoiding electrochemical corrosion. On the other hand, the crystal type of electroplated copper is a compact crystal type, the crystals are relatively dense, the particles are relatively fine, and the coating surface is relatively smooth, while the crystal type of electroplated nickel is a dendritic crystal type, its crystals grow in different directions, the crystal particles are large, and the surface is rough and uneven. The copper coating can play a role of dense protection, avoiding salt spray intrusion into the substrate. In addition, the corrosion resistance of the copper coating is poor, while the corrosion resistance of the nickel coating is strong, so the nickel coating needs to be externally protected from corrosion. At the same time, the copper coating is relatively soft, while the nickel coating is relatively hard. The nickel coating is external, which can significantly improve the hardness. When the combination screw is subsequently tightened for wiring, the coating structure is not easily destroyed. Among them, the thickness of the copper coating and the nickel coating is preferably 5 μm, that is, the coating structure of Cu5Ni5 can reduce the coating thickness as the electroplating process develops, the density and performance are improved.

[0048] The composite coating can also be a nickel graphene coating, or the coating type can be increased by using a copper-nickel-tin coating electroplating method and adding a tin coating to increase the solderability alternative.

[0049] In a second aspect, the present invention provides a combination screw, which is manufactured by the manufacturing method of the combination screw described in the first aspect.

[0050] The combination screw provided by the present invention, when used in an AC contactor, has good salt spray resistance of the entire machine. The hardness of the screw in the combination screw can reach the upper limit of HRC20-45, and the hardness of the pad still meets the requirement of HRC35-45, indicating that the combination screw has good salt spray resistance and high torque performance of the entire machine.

[0051] In a third aspect, the present invention provides an application of a combination screw, which is manufactured by the manufacturing method of the combination screw described in the first aspect. The combination screw is used for low-voltage electrical appliances, and the low-voltage electrical appliances include contactors, thermal relays, starters, buttons and signal lights.

[0052] Compared with the prior art, the present invention has the following beneficial effects:

[0053] The manufacturing method of the combination screw provided by the present invention performs carburization during heat treatment, and controls the carburization concentration to be no less than the carbon content of the screw and less than the carbon content of the pad. This improves the hardness of the screw while preventing the pad from exceeding the hardness standard and causing brittle cracking, thereby achieving the purpose of improving torque performance. In addition, compared with traditional single electroplating layers, the electroplated composite coating can avoid electrochemical corrosion, thereby improving the salt spray resistance of the entire device. The combination screw obtained by the present invention is used in low-voltage electrical products and has good salt spray resistance, high torque, and low cost, and has high application value. DETAILED DESCRIPTION

[0054] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0055] Example 1

[0056] This embodiment provides a combination screw, and a manufacturing method of the combination screw includes the following steps:

[0057] The 45 steel billet is stamped to obtain a tile with a carbon content of 0.44%. The ML20Mn wire is sequentially cold-headed, screw head formed and thread cold-formed to obtain a screw with a carbon content of 0.2%. The tile is extruded and fixed on the polished rod of the screw to obtain an assembly.

[0058] The obtained assembly is quenched and tempered, wherein the temperature of the first zone of quenching is 870°C, the temperature of the middle zone is 880°C, the temperature of the tail zone is 860°C, the quenching time is 70 minutes, the quenching medium used for quenching is 70°C quenching oil, and the oil-cooled quenching time is 8 minutes; then tempering is performed, the temperature of the first zone of tempering is 440°C, the temperature of the middle zone is 440°C, the temperature of the tail zone is 430°C, the tempering time is 90 minutes, and then water cooling is performed; carburizing is performed during the quenching process, the carburizing concentration C is 0.22%, and the loading thickness of the assembly during carburizing is 8 cm.

[0059] The surface of the assembly is electroplated with a copper plating layer with a thickness of 5 μm and a nickel plating layer with a thickness of 5 μm in sequence to obtain the assembly screw.

[0060] Example 2

[0061] This embodiment provides a combination screw, and a manufacturing method of the combination screw includes the following steps:

[0062] The 45 steel billet is stamped to obtain a tile with a carbon content of 0.42%. The ML20Mn wire is sequentially cold-headed, screw head formed and thread cold-formed to obtain a screw with a carbon content of 0.18%. The tile is extruded and fixed on the polished rod of the screw to obtain an assembly.

[0063] The obtained assembly is quenched and tempered, wherein the temperature of the first zone of quenching is 870°C, the temperature of the middle zone is 880°C, the temperature of the tail zone is 860°C, the quenching time is 50 minutes, the quenching medium used for quenching is 60°C quenching oil, and the oil-cooled quenching time is 10 minutes; then tempering is performed, the temperature of the first zone of tempering is 440°C, the temperature of the middle zone is 440°C, the temperature of the tail zone is 430°C, the tempering time is 70 minutes, and then water cooling is performed; carburizing is performed during the quenching process, the carburizing concentration C is 0.2%, and the loading thickness of the assembly during carburizing is 5 cm.

[0064] The surface of the assembly is electroplated in sequence with a copper plating layer having a thickness of 3 μm and a nickel plating layer having a thickness of 10 μm to obtain the assembly screw.

[0065] Example 3

[0066] This embodiment provides a combination screw, and a manufacturing method of the combination screw includes the following steps:

[0067] The 45 steel billet is stamped to obtain a tile with a carbon content of 0.5%. The ML20Mn wire is sequentially cold-headed, screw head formed and thread cold-formed to obtain a screw with a carbon content of 0.23%. The tile is extruded and fixed on the polished rod of the screw to obtain an assembly.

[0068] The obtained assembly is quenched and tempered, wherein the temperature of the first zone of quenching is 870°C, the temperature of the middle zone is 880°C, the temperature of the tail zone is 860°C, the quenching time is 80 minutes, the quenching medium used for quenching is 80°C quenching oil, and the oil-cooled quenching time is 6 minutes; then tempering is performed, the temperature of the first zone of tempering is 440°C, the temperature of the middle zone is 440°C, the temperature of the tail zone is 430°C, the tempering time is 100 minutes, and then air cooling is performed; carburizing is performed during the quenching process, the carburizing concentration C is 0.27%, and the loading thickness of the assembly during carburizing is 3 cm.

[0069] The surface of the assembly is electroplated in sequence with a copper plating layer having a thickness of 7 μm and a nickel plating layer having a thickness of 5 μm to obtain the assembly screw.

[0070] Example 4

[0071] This embodiment provides a combination screw. The difference between the manufacturing method of the combination screw and that of embodiment 1 is that, except for adjusting the carburizing concentration C to 0.35%, the rest is the same as that of embodiment 1.

[0072] Example 5

[0073] This embodiment provides a combination screw. The difference between the manufacturing method of the combination screw and that of embodiment 1 is that, except for adjusting the loading thickness of the assembly during carburizing to 9 cm, the rest is the same as that of embodiment 1.

[0074] Example 6

[0075] This embodiment provides a combination screw. The manufacturing method of the combination screw is different from that of Example 1 in that the tempering is not divided into multiple zones, the tempering temperature is adjusted to 390° C., and the rest is the same as that of Example 1.

[0076] Example 7

[0077] This embodiment provides a combination screw. The manufacturing method of the combination screw is different from that of Example 1 in that the tempering is not divided into multiple zones, the tempering temperature is adjusted to 450° C., and the rest is the same as that of Example 1.

[0078] Example 8

[0079] This embodiment provides a combination screw. The manufacturing method of the combination screw is different from that of Example 1 in that a nickel graphene layer with a thickness of 8 μm is electroplated on the surface of the assembly. The rest is the same as that of Example 1.

[0080] Comparative Example 1

[0081] This comparative example provides a combination screw. The manufacturing method of the combination screw is different from that of Example 1 in that, except for adjusting the carburizing concentration C to 0.15%, the rest is the same as that of Example 1.

[0082] Comparative Example 2

[0083] This comparative example provides a combination screw. The difference between the manufacturing method of the combination screw and that of Example 1 is that, except for adjusting the carburizing concentration C to 0.45%, the rest is the same as that of Example 1.

[0084] Comparative Example 3

[0085] This comparative example provides a combination screw. The manufacturing method of the combination screw is different from that of Example 1 in that only a copper layer with a thickness of 10 μm is electroplated on the surface of the assembly, and the rest is the same as that of Example 1.

[0086] Comparative Example 4

[0087] This comparative example provides a combination screw. The manufacturing method of the combination screw is different from that of Example 1 in that only a nickel plating layer with a thickness of 10 μm is electroplated on the surface of the assembly, and the rest is the same as that of Example 1.

[0088] Performance testing:

[0089] The combined screws provided in Examples 1-8 and Comparative Examples 1-4 were installed on an AC contactor assembly, and a neutral salt spray test was performed in a salt spray corrosion test chamber using the GB / T10125 / ISO9227 method. The results are shown in Table 1.

[0090] The combined screws provided in Examples 1-8 and Comparative Examples 1-4 were tested for screw and tile hardness using a Vickers hardness tester according to GB / T 4340.1 method and a Rockwell hardness tester according to GB / T 230.1 method. The results are shown in Table 1.

[0091] The combination screws provided in Examples 1-8 and Comparative Examples 1-4 were subjected to fracture tests, and 10,000 pieces were processed to test the fracture ratio. The results are shown in Table 1. Note: In Example 5, the surface hardness became more discrete, the surface hardness of the screw did not meet the requirements, and the torque of the combination screw did not meet the standards. In addition, the surface hardness of the combination screw in Comparative Example 1 was too low, and the torque of the combination screw did not meet the standards. In Example 7, the core hardness of the screw and the tile washer were both low, and the low core hardness of the tile washer resulted in insufficient rigidity, making it difficult to hold the wires down during wiring, resulting in connection failure.

[0092] Table 1

[0093]

[0094] It can be seen from Table 1 that the combination screw provided by the present invention is used on low-voltage electrical appliances and has good salt spray resistance and high torque.

[0095] Comparing Example 1 with Examples 2 and 3, it can be seen that the electroplating scheme of 5μm copper plating and 5μm nickel plating has a better salt spray resistance effect for the whole machine than other plating thickness schemes; comparing Example 1 with Example 4, it can be seen that a high carburizing concentration, that is, the carbon content of the screw is closer to the carbon content of the tile pad, will lead to an increase in the proportion of tile pad fractures and an increase in the risk of fracture; comparing Example 1 with Example 5, it can be seen that the loading thickness during carburizing is too large, so that some assemblies cannot fully contact the carburizing medium, reducing the carburizing effect, increasing the surface hardness dispersion, and thus reducing the torque. There is a situation where the dispersion is too large and the standard is not met; comparing Example 1 with Example 6, it can be seen that when tempering is not divided into multiple zones, the tempering temperature is too low, which will lead to high tile pad hardness and brittle fracture. Comparing Example 1 with Example 8, it can be seen that the electroplated nickel graphene coating can also achieve good salt spray resistance and high torque for the whole machine, but the cost of Example 8 is relatively high.

[0096] From the comparison between Example 1 and Comparative Examples 1 and 2, it can be seen that if the carburizing concentration is lower than the carbon content of the screw and the tile pad, surface decarburization will occur and the surface hardness will be too low; if the carburizing concentration is higher than the carbon content of the tile pad, the surface hardness of the tile pad will be too high and will cause breakage during use; from the comparison between Example 1 and Comparative Examples 3 and 4, it can be seen that using a single copper plating layer or nickel plating layer for electroplating will lead to a significant decrease in salt spray performance.

[0097] In summary, the manufacturing method of the combination screw provided by the present invention performs carburizing during heat treatment and controls the carburizing concentration to be slightly higher or equivalent to the carbon content of the screw, while being lower than the carbon content of the pad. This can increase the hardness of the screw to the upper limit of HRC20-45, while the pad hardness still meets the HRC35-45 requirement. After carburizing the surface of the screw, the surface hardness HV ≥ 330, and the pad hardness will not exceed the standard to cause brittle cracking, thereby achieving the purpose of improving torque performance. In addition, compared with the traditional single electroplating layer, the electroplated composite coating can avoid electrochemical corrosion, thereby improving the salt spray resistance of the entire machine. The combination screw obtained by the present invention is used in low-voltage electrical products, has good salt spray resistance of the entire machine, high torque and low cost, and has high application value.

[0098] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for manufacturing a combination screw, characterized in that: The manufacturing method comprises the following steps: The assembly obtained by combining the tile pad and the screw rod is subjected to heat treatment and electroplating composite coating in sequence to obtain the combined screw; Carburizing is performed during the heat treatment, and the carburizing concentration C is controlled to satisfy the following conditions: screw carbon content C1≤carburizing concentration C<pad carbon content C2.

2. The method for manufacturing a combination screw according to claim 1, characterized in that: The carburizing concentration C is closer to the screw carbon content C1 than the shoe carbon content C2.

3. The method for manufacturing a combination screw according to claim 2, characterized in that: The screw carbon content C1 is 0.18-0.23%; Preferably, the carbon content C2 of the tile pad is 0.42-0.5%; Preferably, the carburizing concentration C is 0.2-0.27%.

4. A method for manufacturing a combination screw according to any one of claims 1 to 3, characterized in that: The material of the tile pad includes carbon structural steel; Preferably, the carbon structural steel comprises No. 45 steel; Preferably, the tile pad is obtained by stamping a steel billet; Preferably, the material of the screw comprises ML20Mn; Preferably, the screw is obtained by sequentially cold heading, screw head forming and thread cold forming of a wire rod; Preferably, the combination is to squeeze and fix the tile pad on the polished rod of the screw.

5. A method for manufacturing a combination screw according to any one of claims 1 to 4, characterized in that: The heat treatment includes quenching and tempering, and the carburizing is performed during the quenching process; Preferably, the quenching temperature is 860-880°C; Preferably, the quenching temperature is divided into zones, with the temperature of the first zone being 870°C, the temperature of the middle zone being 880°C, and the temperature of the tail zone being 860°C; Preferably, the quenching time is 50-80 min; Preferably, cooling is performed after quenching and before tempering; Preferably, the quenching medium used for quenching includes quenching oil; Preferably, the temperature of the quenching oil is 60-80° C., and the oil cooling quenching time is 6-10 minutes.

6. The method for manufacturing a combination screw according to claim 5, characterized in that: The tempering temperature is 400-440°C; Preferably, the tempering temperature is divided into zones, with the temperature of the first zone being 440°C, the temperature of the middle zone being 440°C, and the temperature of the tail zone being 430°C; Preferably, the tempering time is 70-100 min; Preferably, water cooling or air cooling is used for cooling after the tempering.

7. A method for manufacturing a combination screw according to any one of claims 1 to 6, characterized in that: During the carburizing process, the loading thickness of the assembly is ≤8 cm.

8. A method for manufacturing a combination screw according to any one of claims 1 to 7, characterized in that: The composite plating layer includes a copper-nickel plating layer or a nickel-graphene plating layer; Preferably, the thickness of the nickel graphene coating is 1-8 μm; Preferably, the copper-nickel plating layer comprises a copper plating layer and a nickel plating layer electroplated sequentially on the surface of the assembly; Preferably, the thickness of the copper plating layer is 3-7 μm, preferably 5 μm; Preferably, the thickness of the nickel plating layer is 5-10 μm, preferably 5 μm.

9. A combination screw, characterized in that: The combination screw is manufactured by the manufacturing method of a combination screw according to any one of claims 1 to 8.

10. An application of a combination screw, characterized in that: The combination screw is manufactured by the manufacturing method of a combination screw according to any one of claims 1 to 8. The combination screw is used for low-voltage electrical appliances, and the low-voltage electrical appliances include contactors, thermal relays, starters, buttons and signal lights.

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