One-piece forming preparation method for a hollow thin-walled hexagonal cover part

Abstract

This application relates to the field of stamping and stretching technology, and in particular to an integrated forming preparation method for a hollow thin-walled hexagonal cover. The preparation process includes multiple processes such as stamping, stretching, phosphating, cold heading, recrystallization annealing, machining, carburizing, electroplating, and electrophoresis. During stretching, multiple consecutive stretches are used, combined with multiple surface phosphating treatment processes, and the processes such as phosphating, surface carburizing, galvanizing, and electrophoresis are skillfully utilized to meet the performance requirements of high strength and corrosion resistance. The comprehensive requirements of high strength, corrosion resistance, hexagonal forming, and integrated forming of the parts are achieved. The traditional casting and welding processes are abandoned, avoiding defects such as shrinkage porosity, shrinkage cavity, welding deformation, and false welding, and meeting the integrated forming requirements of hollow, thin-walled, internal and external hexagons, thickening and narrowing at the mouth, high strength, and corrosion resistance.

Description

Technical Field

[0001] This application relates to the technical field of stamping and stretching, and in particular to an integrated forming preparation method for a hollow thin-walled hexagonal cover part. Background Art

[0002] Pickup trucks require a body bracket with internal and external hexagons, hollow, thin walls, variable wall thickness, high strength, corrosion resistance, and strong buffering to connect the vehicle chassis and the cockpit. The structure of the body bracket is to assemble a silica gel product between the hexagonal upper cover and the hexagonal lower cover with a wall thickness of only 2.5 - 3.0 mm, and use the elasticity of the silica gel pad to relieve the impact force during the movement of the pickup truck. It is connected and fixed to the internal thread of the hexagonal lower cover and the chassis with special bolts, and can withstand an impact tensile force and pressure of about 5T instantaneously. The structures of the hexagonal upper cover and the hexagonal lower cover are as Figure 1 and 2 shown.

[0003] For products similar to the hexagonal upper cover and the hexagonal lower cover, due to their requirements of being hollow, thin-walled, and having special-shaped (hexagonal or elliptical) shapes, the general manufacturing process usually adopts casting + machining process or machining + stamping + welding forming process. For such hollow thin-walled products, if the casting process is used, it can meet the integrated forming requirements. However, because the wall thickness is only 2.5 - 3.0 mm, it is very thin and not easy to be cast; and there are inevitably shrinkage cavities, shrinkage porosity and other defects inside the casting, which will affect its overall strength; moreover, the dimensional accuracy of the casting surface is difficult to guarantee compared with stamping or machining. If additional machining processes are added, the cost will increase relatively more, which is not suitable for mass production. If the bottom circular part is stamped and then welded to the machined middle shaft hole (cylindrical part), this process may cause potential problems such as welding deformation, false welding, and welding pores, that is, the welding process will have an adverse impact on the overall strength; and it will cause changes in the perpendicularity between the cylinder and the bottom cover, and it is not easy to meet the perpendicularity requirements, and the cost is also high. Summary of the Invention

[0004] The production of the hexagonal cover part of this application meets the integrated forming requirements of being hollow, thin-walled, with internal and external hexagons, thickening at the mouth part, high strength, and corrosion resistance, and further provides an integrated forming preparation method for a hollow thin-walled hexagonal cover part.

[0005] This application adopts the following technical solutions:

[0006] S1: Stamp and blank the steel plate raw material into a round sheet;

[0007] S2: Under normal temperature conditions, use a cylindrical core mold to gradually stretch the middle part of the round sheet into a convex cylindrical hollow workpiece with a decreasing diameter;

[0008] S3: Place the workpiece stretched in step S2 in a phosphate solution for phosphating to form a water-insoluble phosphate film on the surface of the workpiece;

[0009] S4: The first hexagonal forming of the columnar hollow core of the workpiece is carried out by using a hexagonal core mold to form a workpiece with a hexagonal columnar hollow core;

[0010] S5: The top of the hexagonal columnar hollow core is truncated so that the hexagonal columnar hollow core forms an axial through hole;

[0011] S6: Stamping is carried out at the connection between the hexagonal columnar hollow core of the workpiece and the bottom disc to complete the R corner chamfering;

[0012] S7: Stamping and flanging forming are carried out on the bottom disc of the workpiece;

[0013] S8: Machining is carried out on the workpiece;

[0014] S9: Surface treatment is carried out on the workpiece.

[0015] By adopting the above technical solutions, the stretching integral forming method is adopted, which solves the problems that when the hollow thin-walled parts are integrally formed by casting, the thin-walled parts are not easy to form, and there are defects such as shrinkage cavities and porosity in the casting; it also avoids potential problems such as welding deformation, false welding, and welding pores in the welding process, and improves the forming quality of the workpiece.

[0016] Further, the cylindrical hollow core in step S2 is formed by multi-stage stretching, and phosphating as described in step S3 is carried out after each stretching. Each stretching is multiple continuous stretchings, and the cumulative stretching coefficient of each continuous stretching is greater than the ultimate stretching coefficient of the workpiece steel raw material. The above phosphate solution is a zinc-based phosphating solution, the phosphating temperature is 60 - 70 °C, and the soaking time is not less than 10 min.

[0017] By adopting the above technical solutions, the application rationally utilizes the stretching limit coefficient of the material to design the number of stretchings, and adds a phosphating process in the middle to form a phosphating film on the surface, which can have a friction-reducing and lubricating effect in the subsequent stretching process, avoiding stretching defects such as wrinkles, breaks, and cracks, and smoothly completing the stretching process.

[0018] Further, before step S7, cold heading treatment is carried out on the top of the hexagonal columnar hollow core of the workpiece to increase the wall thickness of the hexagonal columnar hollow core, and at the same time, the second forming of the columnar hollow core of the workpiece is carried out by using a hexagonal core mold.

[0019] By adopting the above technical solutions, the purpose of increasing the wall thickness and narrowing the upper part of the hexagonal columnar hollow core of the hexagonal lower cover workpiece can be achieved, so as to realize the processing of internal threads and other dimensions.

[0020] Further, before cold heading, the whole workpiece is first subjected to recrystallization annealing, and the annealing temperature is 100 °C - 200 °C higher than the theoretical recrystallization temperature of the workpiece steel raw material.

[0021] By adopting the above technical solution, this application uses the recrystallization annealing process to solve the problem that multiple stretchings may cause the original grains of the material to break due to continuous extension. The addition of recrystallization annealing allows the grains to return from a slender shape to their original equiaxed shape, providing the original excellent ductility of low-carbon steel for subsequent stretching.

[0022] Furthermore, the surface treatment in step S9 includes carburizing, electroplating, and electrophoresis performed sequentially. The carburizing process is to heat the workpiece to 880°C-930°C for 2-4 hours, then perform quenching at 850°C-870°C and low-temperature tempering at 160°C-200°C for 60-90 minutes.

[0023] This application utilizes a carburizing heat treatment process to enhance component strength. Carburizing the surface after forming transforms the carbon content of the raw material (low-carbon steel) into high-carbon steel with a carbon content greater than 0.8-1%. After quenching and low-temperature tempering, the surface microstructure changes from the original ferrite with a small amount of pearlite to tempered martensite with a very high carbon content and a small amount of retained austenite. The hardness also increases from HB130-150 to HRC50-55 (equivalent to HB550), doubling the component's strength and meeting the requirements of tensile and torque testing. Meanwhile, the core maintains the original low-carbon steel structure of ferrite with a small amount of pearlite, maintaining a certain degree of toughness.

[0024] At the same time, the workpiece surface is coated with an anti-corrosion protective film through phosphating, galvanizing, and electrophoresis. The workpiece meets the requirements of a 500H neutral salt spray corrosion test, with no white spots or red spots. This ensures excellent corrosion resistance, allowing the workpiece to withstand erosion in both normal and harsh environments, preventing rust and other issues.

[0025] In summary, this application includes at least one of the following beneficial technical effects:

[0026] 1. This application adopts a preparation method that cleverly integrates multiple processes including stamping, drawing, cold heading, heat treatment (recrystallization annealing, carburizing), and surface treatment (phosphating, electroplating, and electrophoresis), achieving the comprehensive requirements of high strength, corrosion resistance, hexagonal forming, and one-piece molding of the parts. It abandons traditional casting and welding processes, avoids defects such as shrinkage cavities, shrinkage, welding deformation, and cold welds, ensures the overall strength of the parts, and guarantees the technical requirements of a perpendicularity of 0.2 mm between the hexagonal prism and the bottom.

[0027] 2. In the method of this application, the integral molding is achieved in steps, and the strength is improved by carburizing. After the tensile test, it can meet the minimum tensile force of 50KN, which is equivalent to being able to withstand a weight of 50x102=5100Kg=5.1 tons (Note: 1KN≈102Kg), meeting the requirement of bearing a weight of 5 tons;

[0028] 3. In the method of this application, the result of integral forming is achieved step by step; carburization is used to increase strength. By controlling the carburization process parameters reasonably, the carburized layer thickness and surface hardness are within a reasonable range, meeting the requirement of the minimum torque test of 122 N·M, and providing a sufficient safety factor for the part's usage scenario.

[0029] 4. In the method of this application, through two phosphating pretreatments during the process, final galvanizing, and electrophoretic treatment, a very firm anti-corrosion film is formed, achieving the result of passing the salt spray corrosion test for 500H (24H of neutral salt spray corrosion is equivalent to 1 year of natural environment corrosion, and 500H is approximately equal to 20.8 years without being corroded in the natural environment), without white spots and red rust. This greatly enhances the corrosion resistance of the part.

[0030] 5. In the method of this application, by exploring the unilateral clearance between the external hexagon of the hexagonal lower cover and the internal hexagon of the hexagonal upper cover to be between 0.15 and 0.20, the requirements that the hexagonal upper cover and the hexagonal lower cover can slide smoothly up and down, without shaking and rotating after assembly are met. Description of the Drawings

[0031] Figure 1 is a schematic structural diagram of the hexagonal upper cover;

[0032] Figure 2 is a schematic structural diagram of the hexagonal lower cover. Detailed Description of the Embodiment

[0033] The following is a further detailed description of this application in conjunction with the Figure 1 and the Figure 2 drawings.

[0034] As shown in the Figure 1 hexagonal upper cover and the Figure 2 hexagonal lower cover shown, by adding silicone pads (upper silicone pad, lower silicone pad) and frame lock parts between the hexagonal upper cover and the hexagonal lower cover, and assembling them with bolts, the body bracket structure of the pickup truck is formed. It can be seen from the figure that the hexagonal upper cover and the hexagonal lower cover need to meet the requirements of integral forming with a hollow, thin-walled, internal and external hexagon, thickened at the mouth, high strength, and corrosion resistance.

[0035] Both the hexagonal upper cover and the hexagonal lower cover of this application are made of DC01 material. The chemical composition of this steel is mainly iron Fe, carbon C, and a small amount of alloying elements. The carbon content is between 0.05% and 0.12%, belonging to low-carbon steel, and the total ultimate tensile coefficient is 0.24. DC01 low-carbon cold-rolled steel sheets are mainly used for manufacturing parts in industries such as automobiles, electrical appliances, and electronics. Compared with other steels, it has higher fatigue resistance, good plasticity and toughness.

[0036] The external hexagon size of the hexagonal lower cover is 19 0 -0.1 and the internal hexagon of the hexagonal upper cover is 19.350 -0.1 Nested fit, fit tolerance unilateral clearance: 19.35 - 19 = 0.35, unilateral clearance 0.35 / 2 = 0.175mm, that is, within 0.2mm, in order to meet the requirements that the upper hexagonal cover and the lower hexagonal cover can slide smoothly up and down, without shaking and without rotating after assembly. Embodiment 1

[0037] The preparation steps of the lower hexagonal cover are as follows:

[0038] S1. Blanking: Stamping into a round sheet with Ф135x3.

[0039] S2. First stretching: Stretching into a cylindrical hollow shape, with the inner hole of the middle column being Ф63 and the height being 38; the stretching coefficient is 0.466.

[0040] S3. Second stretching: Stretching the cylindrical hollow shape again, with the inner hole of the middle column being Ф47 and the height being 44; the stretching coefficient is 0.746.

[0041] S4. Third stretching: Continuing to stretch the cylindrical hollow shape, with the inner hole of the middle column being Ф37 and the height being 54; the stretching coefficient is 0.787.

[0042] S5. First phosphating: Using a zinc-based phosphating solution and medium-temperature phosphating process. Heating to about 65°C, working temperature 60 - 70°C, soaking for 10 minutes, the film layer is dark gray, and the film layer crystals are dense. A dense phosphate film with a thickness of 5 - 15um is formed on the workpiece surface, and subsequent stretching processes can be continued.

[0043] S6. Fourth stretching: Stretching the cylindrical hollow shape, with the inner hole of the middle column being Ф26.5 and the height being 62; the stretching coefficient is 0.716.

[0044] S7. Fifth stretching: Stretching the cylindrical hollow shape again, with the inner hole of the middle column being Ф19.5 and the height being 75; the stretching coefficient is 0.735.

[0045] S8. Sixth stretching: Continuing to stretch the cylindrical hollow shape, with the inner hole of the middle column being Ф14.5 and the height being 82; the stretching coefficient is 0.743.

[0046] S9. Second phosphating: Phosphating again, phosphating for 10 minutes at 60°C - 70°C, the composition of the phosphating solution is the same as that in step S5. After phosphating, the workpiece surface retains anti-rust ability and plays a role of reducing friction and lubrication for subsequent stretching deformation.

[0047] S10. Seventh stretching: Using a hexagonal cavity and core to perform the first forming of the inner and outer hexagons on the cylindrical hollow shape, that is, achieving a 14 inner hexagon and a 20 outer hexagon.

[0048] S11. Machining: Cutting off the upper opening of the column of the cylindrical hollow shape to ensure that the overall height is greater than 66.

[0049] S12. Stamping: Clear the R angle at the connection between the cylindrical hollow column and the bottom of the cover. The maximum R angle here is 1.2.

[0050] S13. Stamping: Punch out the bottom large circle, trim the edges, and cut into a large circle of Ø98.5.

[0051] S14, recrystallization annealing: temperature 600 ℃, keep warm for 1.0H, cool in the furnace.

[0052] S15, cold heading: The upper part of the cylindrical hollow column is cold headed, with an inner hole of 10.2 mm as the bottom hole of the M12x1.75 internal thread; the outer circle is 19 mm in diameter, and the single-side thickness is 4.4 mm, which is 1.4 mm thicker than the original thickness of 3 mm.

[0053] S16, stretching 8: Use the hexagonal cavity and core to perform the second forming of the inner and outer hexagons of the cylindrical hollow core, that is, the inner hexagon size reaches 13±0.2 and the outer hexagon size reaches 19 0 -0.1 Here, cold heading and hexagonal secondary forming are completed in one process.

[0054] S17, stamping: forming the bottom of the lid.

[0055] S18, machining: Check the height of the bottom flange of the hood and the upper part of the column to ensure the total height.

[0056] S19, make M12x1.75 internal thread.

[0057] S20, surface carburizing: heat the workpiece to 920℃, keep warm for 3.0 hours, cool to 860℃, take out and quench in water, then use low temperature 180℃, keep warm for 1.5 hours for tempering.

[0058] S21. Electroplating: Blue-white zinc plating, film thickness required ≥8µm. Process: Degreasing and rust removal cleaning (three passes with cold water), zinc plating time adjusted according to current and voltage, cold water rinse for polishing, blue-white passivation, cleaning (cold water rinse + hot water rinse), and drying.

[0059] S22, Black Electrophoresis: Film thickness required ≥ 25µm. Process: Workpiece pre-treatment, including degreasing, hot water washing, rust removal, cold water washing, phosphating, hot water washing, and passivation; anodic electrophoresis; workpiece post-treatment, including water washing and drying.

[0060] S23. Inspection: corrosion resistance test, tensile test, torque test.

[0061] The blanking and stretching processes of steps S1-S4 can be completed in one go through a progressive die or separately. Since the total limiting drawing coefficient of the material in this application is 0.24, the total drawing coefficient of S1-S4 has reached 0.27, which is very close to the limiting drawing coefficient of the material, 0.24. Therefore, stretching cannot continue. Similarly, if other materials are used, ensure that the total drawing coefficient during the stretching process is not lower than the total limiting drawing coefficient of the material to avoid drawing cracks and surface scratches due to excessive stretching.

[0062] After the blanking and stretching of S1-S4, the diameter and height of the cylindrical hollow core still do not reach the preset dimensions. However, since the total drawing coefficient of S2-S4 has reached 0.27, which is very close to the limiting drawing coefficient of the material, 0.24, stretching cannot continue. At this time, recrystallization annealing can be used to refine the grains of the overall microstructure of the workpiece, almost reaching the original state of the steel plate. However, considering the cost, the cost of recrystallization annealing is three times that of phosphating; and the time of the phosphating process is approximately 1 / 5 - 1 / 10 of the time of recrystallization annealing. Therefore, the phosphating process is adopted, which can not only meet the subsequent stretching requirements but also shorten the construction period and reduce costs. The phosphating process is to place the workpiece in a phosphate solution containing manganese, iron, and zinc, so that a water-insoluble phosphate film is formed on the surface of the workpiece. Its function is to seal the pores on the surface of the workpiece to achieve the purpose of rust prevention; most importantly, it plays a role in reducing friction and lubrication in the cold working deformation process.

[0063] Similarly, the stretching of steps S6-S8 can be completed in one go through a progressive die or separately.

[0064] After the three stretching processes of S6-S8, the phosphating film of S5 has been damaged due to stretching deformation. If further cold deformation processing is required, another phosphating process needs to be carried out to form a dense phosphate film on the surface of the workpiece, and then the stretching process can continue.

[0065] After the above six stretches of S2-S4 and S6-S8, cold deformation causes work hardening of the workpiece, generates residual internal stress, and the grains are distorted and elongated, showing great changes compared with the original equiaxed grains. Although phosphating treatment has been carried out twice, only a protective film is formed on the surface to enable the stretching process to continue. It is the most economical temporary process.

[0066] The recrystallization annealing in step S14 is annealing carried out to restore the plasticity of the whole workpiece, not just to change the grain shape on the surface, but also in the core, of the whole workpiece. When cold-deformed metal is heated at a relatively low temperature, the migration of some point defects and dislocations in the metal reduces the lattice distortion and gradually lowers the internal stress. This stage is called "recovery". When the temperature continues to rise, due to the increased activity of atoms, the microstructure of the metal changes from crushed, elongated or flattened grains into uniform, fine equiaxed grains. This is a process of grain re-nucleation and growth. At this time, the strength of the metal decreases, the plasticity increases, and the plastic deformation ability is restored, that is, the ductility is improved. It prepares the microstructure in advance for the subsequent cold heading and hexagonal forming processes.

[0067] Recrystallization temperature: The lowest temperature at which cold-deformed metal begins to recrystallize is called the recrystallization temperature.

[0068] For pure metal: T 再 = 0.4T 熔 (K)

[0069] Such as Fe: T 再 = 0.4(1538 + 273) - 273 = 451.4 °C

[0070] The DC01 material used in this application has a carbon content between 0.05% and 0.12%. The above temperature can be referred to for calculation. The actual recrystallization annealing temperature is 100 - 200 °C higher than the recrystallization temperature (the lowest) to improve production efficiency. Therefore, a temperature of 450 + 150 = 600 °C and a holding time of 1.0H are adopted. The effect of crystallization annealing is sufficient to meet the requirements of subsequent cold heading and stretching.

[0071] The recrystallization annealing process is designed between multiple stretching and cold heading forming processes. Its special significance lies in eliminating the work hardening phenomenon of parts caused by multiple stretching, eliminating residual internal stress, and the grains becoming plastic and excellent equiaxed grains through recrystallization, making full microstructure preparations for the subsequent cold heading process, the final stable forming size of the hexagon, and the carburizing process.

[0072] Generally, sheet metal stamping and stretching parts do not undergo recrystallization annealing because the cold working process causes less deformation to the internal grains, which is not enough to affect the dimensional forming and service performance.

[0073] For stretch parts similar to the cylinder in this application, it is indeed necessary to compare the total stretch deformation coefficient of the part with the ultimate stretch deformation coefficient of each material. The principle is that the total stretch deformation coefficient is greater than the ultimate stretch coefficient of the material. Otherwise, it is necessary to achieve the required shape through a temporary phosphating process or a recrystallization annealing process to achieve multiple stretch deformations.

[0074] The cold heading process in step S15 is one of the non-cutting metal forming processes. It is a processing method that utilizes the plastic deformation of metal under the action of external force and, with the aid of a die, redistributes and transfers the metal volume to form the required parts or blanks. The cold heading process is most suitable for producing standard fasteners such as bolts, screws, nuts, rivets, and pins. Due to the structural requirements of the hexagonal lower cover in this application, this process is applied.

[0075] For the cold heading process, a hot heading process was also used for trial production and comparison. However, the hot heading process has defects such as uneven local heating and material folding and delamination; and it requires heat input, increasing costs. For the workpiece in this application, the cold heading process is more optimal.

[0076] During the internal thread machining in step S19, since there will be subsequent surface treatment processes of blue-white zinc plating and black electrophoretic coating, there will be attachments on the thread surface, namely the electroplating layer and the electrophoretic layer. Here, a pre-plating tap is required to machine the thread. A pre-plating tap is a special tap for machining internal threads of products that need to be electroplated, and the major diameter and pitch diameter of the tap are intentionally enlarged. This provides space for the increased thickness during subsequent surface treatment to meet the requirements for the smooth passing of the thread plug gauge after electroplating and the assembly of the parts. The amount by which the pre-plating tap is enlarged needs to be calculated based on the thickness of the coating. Theoretical basis: According to trigonometric calculations, the size that needs to be enlarged for tapping is 4 times the thickness of the coating. After measurement, the thickness of zinc plating + electrophoretic paint is basically between 0.033 - 0.040 mm. Therefore, (0.033 - 0.040) x 4 = 0.13 - 0.16 mm, and a pre-plating tap with an enlargement of 0.15 mm is used for machining.

[0077] The surface carburizing in step S20 is a chemical heat treatment method. The workpiece made of low-carbon steel or low-carbon alloy steel is placed in a carbon-rich active medium (gas carburizing is used for this product), heated to 880 - 930 °C and held for 2.5 - 4 h, so that the carburizing medium produces active carbon atoms on the surface of the workpiece. Through surface absorption and diffusion, it penetrates into the surface layer of the workpiece, so that the carbon content of the surface layer reaches 0.8 - 1.0% of the heat treatment process. After carburizing, it is then processed by quenching (temperature 850 - 870 °C, water quenching) and low-temperature tempering (temperature 160 - 200 °C, holding for 60 - 90 min) to improve the surface hardness, wear resistance, and fatigue strength of the workpiece, while maintaining a certain strength and good toughness at the core. It is particularly suitable for the working scenarios of the hexagonal upper cover and hexagonal lower cover in this application, that is, it requires high strength and good toughness to cope with impact forces.

[0078] Therefore, the purpose of surface carburization is to improve the surface hardness of the hexagonal cover part, thereby enhancing the overall strength of the part. Eventually, the surface hardness of the workpiece reaches HRC50 - 55. Compared with the original steel plate with HB130 - 150 (corresponding to about HRC14, which is basically not suitable for evaluating this hardness because it is too low), the surface hardness of the part is greatly improved at this time, and the strength of the part is correspondingly increased.

[0079] Electroplating utilizes the principle of electrolytic deposition to reduce metal ions on the electrode and deposit the metal on the surface of the workpiece to enhance the hardness, corrosion resistance and other properties of the workpiece. Electrophoresis deposits a liquid containing substances such as pigments and resins on the surface of the workpiece through electrophoresis to form a film layer for the purpose of protection, decoration and improvement of surface properties. The difference between electroplating and electrophoresis lies in the intermediate object. Electroplating is metal ions, and electrophoresis is colloid. The results after treatment are also different. Electroplating deposits a layer of metal to perform functions such as protection, rust prevention and beauty, but the coating thickness is generally relatively thin, about 8 - 12um; electrophoresis is generally used for painting, that is, a layer of paint is applied on the surface, which is commonly used in the automotive industry, and the paint film thickness can reach 25 - 50um. And it has a shiny appearance, is beautiful and rust-proof. The surface anti-corrosion superposition treatment of phosphating + electroplating + electrophoresis is to obtain better anti-corrosion effects.

[0080] The hexagonal lower cover of this application is formed into a hexagonal column by the steps of S1 blanking, continuous stretching in S2 - S4, and continuous stretching in S6 - S8 to make the workpiece taller and thinner. During the stretching process, considering that the stretching deformation degree is almost close to the ultimate stretching coefficient of this material, phosphating in steps S5 and S9 is adopted, so that the stretching action can continue. Then through step S14 recrystallization annealing, the work hardening, residual stress and grain distortion caused by cold deformation in the previous multi-step processes are heated to make them recover and recrystallize into equiaxed grains again, providing a suitable equiaxed crystal microstructure basis for the subsequent S15 cold heading process and S20 carburization. Then through step S20 carburization, the surface hardness, wear resistance and fatigue strength of the workpiece are improved, and at the same time, a certain strength and good toughness can be maintained in the core. The carburization process cannot be adjusted. Moving the steps forward makes it extremely difficult to realize the processes of S15 cold heading, S16 stretching and S17 machining threads because the surface of the part has high hardness and high strength; if the steps are moved backward to after S21 electroplating and S22 electrophoresis, due to the existence of the protective film, the carburization effect is greatly reduced, and the protective film of galvanizing plus electrophoresis is damaged, which will cause S20, S21 and S22 to fail to achieve the expected effects.

[0081] Therefore, only through the sequence of steps S1 - S22 in this application can the dimensional changes of the workpiece, the changes in the internal grain shape of the material, work hardening, the changes in the surface structure of the material, and the continuous changes in hardness and strength achieve the expected effects.

[0082] The specific inspection requirements for S23 are as follows:

[0083] Verification method for corrosion resistance: According to the standard of GB / T 10125-2021, a salt spray corrosion test is conducted. The salt spray corrosion test simulates the marine climate. A 5% NaCl solution with a pH value between 6.5 and 7.2 is atomized and placed in a sealed environment at a temperature of 35°C and a humidity of 98%. The parts are placed continuously for 500H. After taking out, it is checked that there shall be no white spots, blisters, peeling, or red rust on the surface of the parts.

[0084] Tensile test: Align and weld the bottoms of two hexagonal lower covers in the reverse direction (full weld). Assemble corresponding bolts to the two M12x1.75 internal threads respectively. Use a universal material testing machine, and the chucks clamp the two bolts (M12x1.75) at both ends respectively to conduct a tensile test. The maximum force value measured is the tensile test value, and it is required to be greater than 50KN. Note: If the welding points at the bottoms of the two hexagonal lower covers break during the test, the test data is invalid; also note that during the test, it can be observed that the connection part between the column body and the bottom cover of the hexagonal lower cover will deform first, which is normal because this is the weak part of the strength of the hexagonal lower cover.

[0085] For the hexagonal lower cover without the carburizing process, the tensile test values are mostly in the range of 30 - 35KN; in comparison, after adding the carburizing process, the tensile test values are mostly in the range of 55 - 60KN, and the tensile test values increase by 70 - 80%, which indeed plays a role in increasing the strength.

[0086] Torque test: Use a special fixture to fix the hexagonal part and the bottom of the hexagonal lower cover on the vise, and use a torque wrench to clamp the periphery of the threaded head to conduct a torque test.

[0087] The torque required for the clamping part of the torque wrench to separate from the hexagonal part of the cover is the torque test value, and this value is required to be ≥122N.M. That is, set the torque value to 122N.M and conduct a torque test on the parts. At this torque value, if it is observed that the parts do not deform or break, it is considered qualified; if deformation, breakage, etc. occur between the head and the column body when the torque is less than 122N.M, it is determined as not meeting the requirements. Under the action of the torque wrench, the broken part generally appears at the joint of the hexagonal part of the column body and the circular part of the head, which is normal because both the shape and thickness change at this place, becoming the weak part of the strength.

[0088] For the hexagonal lower cover without the carburizing process, the torque test values are mostly in the range of 80 - 95N.M, which does not meet the requirement of torque value ≥122N.M; in comparison, after adding the carburizing process, the torque test values are mostly in the range of 140 - 155N.M, and the torque value increases by 63 - 75%, which indeed plays a role in increasing the strength.

[0089] The following is a comparison of the implementation examples of different carburizing process parameters and test results for the hexagonal lower cover:

[0090]

[0091] Examples 1 - 8. The carburizing temperature was continuously increased within a suitable range, and the carburizing holding time was controlled between 2.5 - 4.0 h. Combined with the quenching + low-temperature tempering process, a carburized layer with a thickness of 0.40 - 0.55 mm on one side was obtained. The surface hardness of the parts was between HRC50 - 55, the tensile strength was greater than 50 KN in the range of 55 - 60 KN, the torque value was between 140 - 155 N·m and greater than 122 N·m, and the deformation of the parts was small and acceptable. In particular, the carburizing process parameters of Example 1 obtained a moderate carburized layer thickness, the surface hardness of the carburized layer was moderate, the test values of tensile strength and torque also had sufficient safety factors, the deformation of the parts was small and acceptable, and the process cost was optimal.

[0092] Comparing Examples 1 - 8 with Comparative Example 1, it is obvious that there are significant differences in the surface hardness of the parts, the tensile test and torque test values of the parts between the presence and absence of the carburizing process. Comparative Example 1 has no carburizing process, and its tensile strength and torque values are both lower than the requirements. Examples 1 - 8 all have the carburizing process, which indeed plays a role in increasing the strength of the parts and can bear greater force requirements during use.

[0093] Comparing Examples 1 - 8 with Comparative Example 2, the difference lies in the carburizing temperature. Obviously, as the carburizing temperature increases in Examples 1 - 8, the thickness of the carburized layer increases, and the surface hardness of the carburized layer increases, resulting in different increases in both the tensile test and torque test. The carburizing temperature of Comparative Example 2 is 850 °C, which is lower than the minimum carburizing temperature of carbon steel parts. The carburized layer is relatively shallow, and the tensile and torque test values of the parts are both lower than the requirements. Of course, the deformation of the parts is also very small.

[0094] Comparing Examples 1 - 8 with Comparative Example 3, the difference also lies in the carburizing temperature. However, the carburizing temperature of Comparative Example 3 is 980 °C, which is higher than the upper limit of the carburizing temperature of carbon steel parts. As a result, the thickness of the carburized layer increases. Even with a tempering temperature of 200 °C, the surface hardness of the parts reaches HRC62 (the hardness value of cutting tools), the tensile strength increases significantly, but due to the increase in surface brittleness and the decrease in toughness of the parts, the torque test value decreases significantly. This fully shows that we cannot blindly pursue high hardness and high strength, and we need to consider the toughness requirements of the parts in the actual use situation. Moreover, due to the high carburizing temperature, the parts deform at high temperature, and the surface hardness is also high, resulting in difficult shaping, which is not advisable.

[0095] Comparing Examples 1 - 8 with Comparative Example 4, the main difference lies in the carburizing time. Excessive carburizing time not only affects the production turnover efficiency, but also the thickness of the carburized layer increases with time, the surface hardness increases, which is beneficial to the tensile test, but the torque test value will instead decrease. At the same time, the parts are heated at high temperature for a long time, which will cause the grains to grow coarsely and also cause deformation of the parts, which is also not advisable.

[0096] Summary: It can be seen from the above embodiments that for thin-walled parts such as the hexagonal lower cover (3mm thick) in the embodiment of the present application, in a certain carbon potential atmosphere of a carburizing furnace, the suitable carburizing heating temperature is 880-930°C and the carburizing holding time is 2.5-4.0H, combined with the appropriate tempering temperature of 160-200°C and the tempering time of 60-90Min, a single-sided carburized layer of 0.40mm-0.55mm is obtained, that is, a thin layer carburizing (the carburizing layer is less than 1mm), and the carburizing layers on both sides together account for 1 / 3 of the total thickness. It is possible to obtain parts with a hardness of HRC50-55, a tensile force ≥50KN, a torque ≥122N.M, and acceptable deformation, ensuring the various dimensions of the parts (the axial part of the hexagonal prism is not twisted or deformed, and the perpendicularity to the bottom is less than 0.2mm). If the carburizing heating temperature is too low, the carburized layer will be too shallow, failing to achieve the desired surface hardness, and both tension and torque will fall below the required values. Conversely, if the carburizing temperature is too high or the holding time is too long, the carburized layer will be too thick and the surface hardness will be too high, which will in turn cause a decrease in torque values. This is because excessive hardness increases the surface brittleness of the part and reduces its toughness, which is detrimental to torque testing. In actual use, excessive hardness can reduce the impact resistance of the part and even lead to brittle fracture.

[0097] The following is an example comparison of different surface treatment processes and corrosion resistance results for hexagonal bottom covers:

[0098]

[0099] Comparing Example 1 with Comparative Examples 1-3, Example 1 utilizes a combination of phosphating, electroplating, and electrophoresis, achieving superior corrosion resistance compared to a single surface treatment process, achieving 500 hours of corrosion resistance (24 hours of neutral salt spray corrosion is equivalent to 1 year of corrosion in a natural environment, and 500 hours is approximately equivalent to 20.8 years of no corrosion in a natural environment), with no white spots or red rust. Comparative Example 1, which utilizes only phosphating, exhibits corrosion resistance for only 12 hours, with sporadic red rust spots appearing on the surface. Comparative Example 2, which utilizes only electroplating (blue-white zinc plating), exhibits corrosion resistance for only 24 hours, with sporadic red rust spots appearing on the surface. Comparative Example 3, which utilizes only electrophoresis, exhibits corrosion resistance for 200 hours, with sporadic red rust spots appearing on the surface, as well as blistering and peeling.

[0100] The preparation process of the hexagonal upper cover is simpler than that of the hexagonal lower cover. The preparation steps do not require cold heading in S15 and threading in S19. At the same time, the S23 inspection only requires salt spray testing, and no tension and torque testing is required. The remaining steps and processing requirements are the same as those of the hexagonal upper cover.

[0101] The method of this application adopts integral forming, abandons casting and welding processes, avoids defects such as shrinkage cavities, shrinkage porosity, welding deformation, and false soldering, ensures the overall strength of the parts, and ensures the perpendicularity between the column and the bottom is 0.2 mm; it adopts 8 stretching processes, rationally utilizes the stretching limit coefficient of the material to design the number of stretching times, and cooperates with two phosphating processes and one recrystallization annealing process to form the parts and achieve the optimal cost; the application of the recrystallization annealing process is to prevent the continuous extension of the original grains of the material due to multiple stretching and cause fractures. A recrystallization annealing process is added in the middle to make the grains return from the slender shape to the equiaxed shape, providing the excellent ductility of low-carbon steel for subsequent stretching again, that is, making microstructural preparations for the cold heading process and carburizing process; the cold heading process is used to achieve necking down of the part mouth and increase the wall thickness, meeting the internal thread and other outer diameter size allowances required for machining; pre-plating taps are used to process the internal thread, leaving space for subsequent electroplating and electrophoretic coating. The pre-plating tap slightly increases the pitch diameter and major diameter dimensions of the thread (the specific dimensions can be calculated through the coating thickness) to ensure that the thread after electroplating and electrophoretic coating can pass the inspection of the thread plug gauge; carburizing treatment is adopted. Reasonable carburizing process parameters not only improve the surface strength and hardness of the parts, but also maintain the original toughness of the core, that is, meet the strength requirements of tensile force and torque, and meet the working requirements of impact toughness in the working occasion, making the parts have sufficient sense of security during work; the superposition of a series of surface anti-rust treatment processes of phosphating + galvanizing + electrophoretic coating meets the excellent corrosion resistance requirements of no white spots and red rust after the final neutral salt spray corrosion test for 500H (neutral salt spray corrosion for 24H is equivalent to 1 year of natural environment corrosion, and 500H is approximately equal to 20.8 years of natural environment corrosion) to cope with the erosion of the parts working in general environments or even harsh environments; for the nested clearance interval, by exploring, the unilateral clearance between the outer hexagon of the hexagonal lower cover and the inner hexagon of the hexagonal upper cover is between 0.15 - 0.20 mm, which can optimally meet the requirements that the hexagonal upper cover and the hexagonal lower cover can slide smoothly up and down, without shaking and without rotating after assembly.

[0102] The embodiments of this specific implementation manner are all preferred embodiments of this application, and do not limit the protection scope of this application accordingly. The same components are denoted by the same reference numerals. Therefore: All equivalent changes made according to the structure, shape, and principle of this application shall be covered within the protection scope of this application.

Claims

1. An integrated molding preparation method for a hollow thin-walled hexagonal cover part, characterized in that, The specific steps are as follows: S1: Stamp and blank the steel plate raw material into circular wafers. Among them, the circular wafers are made of DC01 material, and the carbon content is between 0.05% and 0.12%. S2: At room temperature, use a cylindrical core mold to gradually stretch the middle part of the circular wafer into a convex cylindrical hollow workpiece with a decreasing diameter. S3: Place the workpiece after stretching in step S2 into a phosphate solution for phosphating, so that a water-insoluble phosphate film is formed on the surface of the workpiece. S4: Use a hexagonal cavity and core to perform the first hexagonal forming on the cylindrical hollow of the workpiece to form a workpiece with a hexagonal cylindrical hollow. Among them, the first hexagonal forming uses a stretching process. S5: Cut off the top of the hexagonal cylindrical hollow to make the hexagonal cylindrical hollow form an axial through hole. S6: Punch the connection between the hexagonal cylindrical hollow of the workpiece and the bottom circular wafer to complete the R corner chamfering. S7: Perform stamping and flanging forming on the bottom circular wafer of the workpiece. S8: Perform machining on the workpiece. S9: Perform surface treatment on the workpiece. In step S2, the cylindrical hollow is formed by multi-stage stretching, and phosphating as described in step S3 is performed after each stretching. Each stretching is a multi-stage continuous stretching, and the cumulative stretching coefficient of each continuous stretching is greater than the ultimate stretching coefficient of the steel plate raw material of the workpiece. After step S6 and before step S7, it also includes recrystallization annealing. After annealing, cold heading treatment is performed on the upper part of the main body of the cylindrical hollow. After cold heading treatment, a hexagonal cavity and core are used to perform the second hexagonal forming on the cylindrical hollow. The second hexagonal forming uses a stretching process.

2. The integrated forming preparation method of a hollow thin-walled hexagonal cover part according to claim 1, characterized in that, The phosphate solution in step S3 is a zinc-based phosphating solution, the phosphating temperature is 60 - 70 °C, and the soaking time is not less than 10 min.

3. The integral molding preparation method of a hollow thin-walled hexagonal cover part according to claim 1, characterized in that, Before step S7, cold treatment is performed on the top of the hexagonal cylindrical hollow of the workpiece to increase the wall thickness of the mouth of the hexagonal cylindrical hollow, and at the same time, a hexagonal core mold is used to perform the second forming on the cylindrical hollow of the workpiece.

4. The integrally formed preparation method of a hollow thin-walled hexagonal cover member according to claim 3, characterized in that, Before cold treatment, recrystallization annealing is first performed on the whole workpiece, and the annealing temperature is 100 °C - 200 °C higher than the theoretical recrystallization temperature of the steel plate raw material of the workpiece.

5. The integrated molding preparation method of a hollow thin-walled hexagonal cover part according to claim 1, characterized in that, The surface treatment in step S9 includes carburizing, electroplating and electrophoresis performed in sequence.

6. The integrally formed preparation method of a hollow thin-walled hexagonal cover part according to claim 5, wherein, The carburizing process is to heat the workpiece to 880 °C - 930 °C and hold it for 2 - 4 hours, then perform quenching at 850 °C - 870 °C and low-temperature tempering at160 °C - 200 °C. The low-temperature tempering time is 60 - 90 min.

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