Integrated forming preparation method of hollow-core thin-wall hexagonal cover piece

Abstract

The invention relates to the technical field of stamping and stretching, in particular to an integral forming preparation method of a hollow-core thin-wall hexagonal cover part, the preparation process comprises multiple procedures of stamping, stretching, phosphating, cold heading, recrystallization annealing, machining, carburizing, electroplating and electrophoresis, during stretching, multiple continuous stretching is adopted, multiple surface phosphating treatment procedures are matched, and the hollow-core thin-wall hexagonal cover part is obtained. And the performance requirements of high strength and corrosion resistance are met by skillfully utilizing the procedures of phosphorization, surface carburization, galvanization, electrophoresis and the like. And the comprehensive requirements of high strength, corrosion resistance, hexagonal forming and integrated forming of parts are met, traditional casting and welding processes are abandoned, the defects of shrinkage cavities, shrinkage porosity, welding deformation, pseudo soldering and the like are overcome, and the integrated forming requirements of hollow cores, thin walls, internal and external hexagons, narrowed and thickened openings, high strength and corrosion resistance are met.

Description

Technical Field

[0001] The present application relates to the field of stamping and stretching technology, and in particular to an integral molding preparation method for a hollow thin-walled hexagonal cover. Background Art

[0002] Pickup trucks need a body bracket with hexagonal inner and outer, hollow, thin wall, variable wall thickness, high strength, corrosion resistance, and strong buffering to connect the chassis and cockpit. The structure of the body bracket is to assemble silicone products between the hexagonal upper cover and the hexagonal lower cover with a wall thickness of only 2.5-3.0mm. The elasticity of the silicone pad is used to alleviate the impact force of the pickup truck during movement. The special bolts are connected and fixed to the internal thread of the hexagonal lower cover and the chassis to withstand the instantaneous impact tension and pressure of about 5T. The structure of the hexagonal upper cover and the hexagonal lower cover is as follows Figure 1 and 2 shown.

[0003] For products like hexagonal upper cover and hexagonal lower cover, since they require hollow core, thin wall, and special shape (hexagonal or elliptical), the general manufacturing process will adopt casting + machining process or machining + stamping + welding process. If such hollow and thin-walled products are cast, they can meet the requirements of one-piece molding, but because the wall thickness is only 2.5-3.0mm, it is not easy to cast; and the shrinkage holes and shrinkage defects inside the casting will inevitably affect its overall strength; in addition, the dimensional accuracy of the casting surface is difficult to guarantee compared with stamping or machining. If the machining process is added, the cost will increase relatively, which is not suitable for mass production. If the bottom circular part is stamped and then welded with the machined intermediate shaft hole (column part), this process may cause hidden dangers such as welding deformation, cold welding, welding porosity, etc., that is, the welding process will have an adverse effect on the overall strength; and it will cause the verticality of the column and the bottom cover to change, which is not easy to meet the verticality requirements and the cost is also high. Summary of the invention

[0004] The production of the hexagonal cover of the present application meets the requirements of hollow, thin-walled, hexagonal inside and outside, narrowed and thickened mouth, high strength, and corrosion resistance, thereby providing an integrated molding preparation method for a hollow thin-walled hexagonal cover.

[0005] This application adopts the following technical solutions: S1: punching and blanking the steel plate raw material into round pieces; S2: Under normal temperature, using a cylindrical core mold, gradually stretching the middle portion of the disc into a convex cylindrical hollow workpiece with decreasing diameter; S3: placing the workpiece stretched in step S2 in a phosphate solution for phosphating, so that a phosphate film that is insoluble in water is formed on the surface of the workpiece; S4: using a hexagonal core mold to perform a first hexagonal forming on the cylindrical hollow core of the workpiece to form a hexagonal cylindrical hollow core workpiece; S5: cutting off the top of the hexagonal columnar hollow core to form an axial through hole in the hexagonal columnar hollow core; S6: Punch the connection between the hexagonal columnar hollow core and the bottom disc of the workpiece to complete the R corner cleaning; S7: stamping and flanging the bottom disc of the workpiece; S8: machining the workpiece; S9: Perform surface treatment on the workpiece.

[0006] By adopting the above technical solution and the stretching one-piece molding method, the problem that hollow thin-walled parts are difficult to mold and have shrinkage holes and shrinkage defects inside the casting when the hollow thin-walled parts are molded by casting is solved; it also avoids the hidden dangers of welding deformation, cold welding, welding porosity and other hidden dangers that may exist in the welding process, thereby improving the molding quality of the workpiece.

[0007] Furthermore, the cylindrical hollow core in step S2 is formed by multiple stretching, and the phosphating in step S3 is performed after each stretching. Each stretching is multiple continuous stretching, and the cumulative stretching coefficient of each continuous stretching is greater than the ultimate stretching coefficient of the workpiece steel raw material. The above-mentioned phosphate solution is a zinc-based phosphating solution, the phosphating temperature is 60-70°C, and the immersion time is not less than 10 minutes.

[0008] By adopting the above technical scheme, the present application reasonably utilizes the tensile limit coefficient of the material to design the number of stretching times, 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 the occurrence of wrinkles, breaks, cracks and other stretching defects, and the stretching process is successfully completed.

[0009] Furthermore, before step S7, the top of the hexagonal columnar hollow core of the workpiece is cold-forging treated to increase the wall thickness of the hexagonal columnar hollow core, and the columnar hollow core of the workpiece is molded for the second time using a hexagonal core mold.

[0010] By adopting the above technical solution, the purpose of thickening and narrowing the wall thickness of the hexagonal columnar hollow upper part of the hexagonal lower cover workpiece can be achieved, thereby realizing the processing of internal threads and other dimensions.

[0011] Furthermore, before cold heading, the workpiece is subjected to recrystallization annealing as a whole, and the annealing temperature is 100° C.-200° C. higher than the theoretical recrystallization temperature of the workpiece steel raw material.

[0012] By adopting the above technical solution, the present application utilizes the recrystallization annealing process to solve the problem that the original grains of the material may be fractured due to continuous extension due to multiple stretching. The recrystallization annealing is added to restore the grains from a slender shape to their original equiaxial shape, providing the original excellent ductility of low-carbon steel for subsequent stretching.

[0013] Furthermore, the surface treatment in step S9 includes carburizing, electroplating and electrophoresis performed in sequence. The carburizing process is to heat the workpiece to 880℃-930℃ and keep it for 2-4 hours, then perform quenching at 850℃-870℃ and low-temperature tempering at 160℃-200℃, and the low-temperature tempering time is 60-90min.

[0014] This application adopts a carburizing heat treatment process that can improve the strength of parts. By carburizing the surface after forming, the carbon content of the raw material (low carbon steel) is infiltrated into high carbon steel with a carbon content greater than 0.8-1%. After quenching + low temperature tempering, the surface microstructure changes from the original ferrite + a small amount of pearlite to tempered martensite with extremely high carbon content + a small amount of residual austenite, and the hardness changes from the original HB130-150 to HRC50-55 (equivalent to HB550), which means that the strength of the parts is doubled, meeting the tensile test and torque test of the parts; at the same time, the core maintains the original ferrite + a small amount of pearlite structure of low carbon steel, which has a certain toughness.

[0015] At the same time, the workpiece surface is given an anti-corrosion protective film through phosphating + galvanizing + electrophoresis, which meets the excellent corrosion resistance of 500H neutral salt spray corrosion test without white spots or red rust, to cope with the erosion of general environment or even harsh environment, and avoid rust and other problems.

[0016] In summary, the present application includes at least one of the following beneficial technical effects: 1. This application adopts a preparation method that cleverly integrates multiple processes such as stamping, stretching, 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 forming of parts, abandoning traditional casting and welding processes, avoiding defects such as shrinkage cavities, shrinkage, welding deformation, and cold welding, ensuring the overall strength of the parts, and ensuring the verticality of the hexagonal prism to the bottom of 0.2mm and other technical requirements; 2. In the present application method, the one-piece 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), which meets the requirement of bearing a weight of 5 tons; 3. In the method of this application, the result of one-piece molding is achieved in steps; carburizing improves strength, and the thickness of the carburized layer and the surface hardness are controlled within a reasonable range through reasonable carburizing process parameters, meeting the minimum torque test requirement of 122N.M, giving the parts a sufficient safety factor for use scenarios; 4. In the present application method, through two phosphating pretreatments, the final galvanizing and electrophoresis treatments, a very strong anti-corrosion film is formed, achieving a 500-hour salt spray corrosion test (neutral salt spray corrosion for 24 hours is equivalent to 1 year of corrosion in the natural environment, and 500 hours is approximately equal to 20.8 years of no corrosion in the natural environment), with no white spots or red rust. The corrosion resistance of the parts is greatly enhanced; 5. In the present application method, the outer hexagon of the hexagonal lower cover and the inner hexagon of the hexagonal upper cover are nested to have a single-side clearance between 0.15 and 0.20, which satisfies the requirement that the hexagonal upper cover and the hexagonal lower cover can slide smoothly up and down without shaking or rotating after being assembled. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the structure of the hexagonal upper cover; Figure 2 It is a structural schematic diagram of the hexagonal lower cover. DETAILED DESCRIPTION

[0018] The following is combined with Figure 1 and attached Figure 2 , further details of this application are given.

[0019] like Figure 1 The hexagonal cover shown and Figure 2 The hexagonal lower cover shown in the figure, with silicone pads (upper silicone pad, lower silicone pad) and frame lock parts added between the hexagonal upper cover and the hexagonal lower cover, assembled by bolts, forms the body support structure of the pickup truck. As can be seen from the figure, the hexagonal upper cover and the hexagonal lower cover need to meet the requirements of hollow core, thin wall, hexagonal inside and outside, narrow and thick mouth, high strength, and corrosion resistance.

[0020] The hexagonal upper cover and the hexagonal lower cover of this application are both made of DC01 material. The chemical composition of this steel is mainly iron Fe, carbon C and a small amount of alloy elements. The carbon content is between 0.05% and 0.12%. It is low-carbon steel and the total ultimate tensile coefficient is 0.24. DC01 low-carbon cold-rolled steel plate is mainly used for the manufacture of parts in the automotive, electrical and electronic industries. Compared with other steels, it has higher fatigue resistance, good plasticity and toughness.

[0021] Hexagonal bottom cover outer hexagonal size 19 0 -0.1 Hexagonal cover with inner hexagonal 19.35 0 -0.1 Nested fit, fit tolerance single-side clearance: 19.35-19=0.35, single-side clearance 0.35 / 2=0.175mm, that is, within 0.2mm, to meet the requirements that the hexagonal upper cover and the hexagonal lower cover can slide up and down smoothly without shaking or rotating after assembly. Example 1

[0022] The preparation steps of the hexagonal lower cover are as follows: S1. Blanking: Punching into Ф135x3 discs.

[0023] S2, stretching 1: stretching into a cylindrical hollow core, the inner hole of the middle column is 63 Ф, the height is 38; the stretching coefficient is 0.466.

[0024] S3, stretching 2: stretch the cylindrical hollow core again, the inner hole of the middle column is Φ47, the height is 44; the stretching coefficient is 0.746.

[0025] S4, stretching 3: Continue to stretch the cylindrical hollow core, the inner hole of the middle column is Ф37, the height is 54; the stretching coefficient is 0.787.

[0026] S5, Phosphating 1: Use zinc phosphating solution and medium temperature phosphating process. Heat to about 65℃, working temperature 60-70℃, soak for 10min, the film is dark gray, and the film is densely crystalline. A dense phosphate film is formed on the surface of the workpiece, with a thickness of 5-15um, and subsequent stretching process can be continued.

[0027] S6, stretching 4: stretching cylindrical hollow core, the inner hole of the middle column is Ф26.5, the height is 62; the stretching coefficient is 0.716.

[0028] S7, stretching 5: stretch the cylindrical hollow core again, the inner hole of the middle column is Ф19.5, the height is 75; the stretching coefficient is 0.735.

[0029] S8, stretching 6: Continue to stretch the cylindrical hollow core, the inner hole of the middle column is Ф14.5, the height is 82; the stretching coefficient is 0.743.

[0030] S9, phosphating 2: phosphating again, phosphating for 10 minutes at 60℃-70℃, the composition of the phosphating solution is the same as step S5. After phosphating, the surface of the workpiece retains the ability to resist rust, which plays a role in reducing friction and lubrication for subsequent tensile deformation.

[0031] S10, stretching 7: using a hexagonal cavity and core to perform the first internal and external hexagonal molding of the cylindrical hollow core, that is, achieving an internal hexagon of 14 and an external hexagon of 20.

[0032] S11, machining: cut off the upper opening of the cylindrical hollow column to ensure that the overall height is greater than 66.

[0033] S12. Stamping: Clear the R angle at the connection between the cylindrical hollow column and the bottom of the cover, where the maximum R is 1.2.

[0034] S13, stamping: punch out the bottom big circle, trim the edges, and cut into a big circle of Φ98.5.

[0035] S14, recrystallization annealing: temperature 600 ℃, keep warm for 1.0H, cool in the furnace.

[0036] S15, cold heading: cold heading treatment is performed on the upper part of the cylindrical hollow cylinder, the inner hole is Ф10.2, which serves as the bottom hole of the M12x1.75 internal thread; the outer circle is Ф19mm, and the single-side thickness is 4.4mm, which is 1.4mm more than the original thickness of 3mm.

[0037] 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.

[0038] S17, stamping: forming the bottom of the cover.

[0039] S18, Machining: The height of the bottom flange of the hood and the upper part of the column are required to ensure the total height.

[0040] S19, make M12x1.75 internal thread.

[0041] S20, surface carburizing: heat the workpiece to 920℃, keep it warm for 3.0 hours, cool it to 860℃, take it out and quench it in water, then use the tempering process at low temperature of 180℃ and keep it warm for 1.5 hours.

[0042] S21, electroplating: blue-white zinc plating, the film thickness is required to be ≥8um. Process: degreasing and rust removal cleaning (three times of cold water), adjusting the time of zinc plating according to the current and voltage, cold water cleaning for brightening, blue-white passivation, cleaning (cold water cleaning + hot water cleaning), drying.

[0043] S22, black electrophoresis: film thickness ≥ 25um. Process: workpiece pre-treatment, including degreasing, hot water washing, rust removal, cold water washing, phosphating, hot water washing, passivation; anodic electrophoresis; workpiece post-treatment, including water washing and drying.

[0044] S23, Inspection: corrosion resistance test, tension test, torque test.

[0045] The blanking and stretching process of steps S1-S4 can be completed at one time through a continuous die, or they can be completed separately. Since the total ultimate tensile coefficient of the material of this application is 0.24, the total tensile coefficient of S1-S4 has reached 0.27, which is very close to the ultimate tensile coefficient of the material of 0.24, so stretching cannot be continued. Similarly, if other materials are used, the total tensile coefficient must not be lower than the total ultimate tensile coefficient of the material during the stretching process to avoid tensile cracks and surface scratches due to excessive stretching.

[0046] After blanking and stretching of S1-S4, the diameter and height of the cylindrical hollow core still do not reach the preset size, but because the total stretch coefficient of S2-S4 has reached 0.27, which is very close to the material's ultimate stretch coefficient of 0.24, it cannot be stretched further. 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 3 times that of phosphating; and the phosphating process time is about 1 / 5-1 / 10 of the recrystallization annealing time. Therefore, the phosphating process can not only meet the subsequent stretching requirements, but also shorten the construction period and reduce costs. The phosphating process is a process in which the workpiece is placed in a phosphate solution containing manganese, iron, and zinc to form a layer of phosphate film that is insoluble in water on the surface of the workpiece. The function is to close the pores on the surface of the workpiece to prevent rust; the most important thing is to reduce friction and lubricate in the cold processing deformation process.

[0047] Similarly, the stretching of steps S6-S8 can be completed at one time through a continuous die, or can be completed separately.

[0048] After three stretching processes from S6 to S8, the S5 phosphate film has been damaged due to stretching deformation. If cold deformation processing is required, another phosphating process is required to form a dense phosphate film on the surface of the workpiece before the stretching process can continue.

[0049] After the above-mentioned S2-S4, S6-S8 stretching, the cold deformation causes work hardening of the workpiece, generates residual internal stress, and the grains are distorted and elongated, which has changed greatly compared with the original equiaxed grains. Although it has been phosphated twice, it only forms a protective film on the surface, allowing the stretching process to continue, which is the most economical temporary process.

[0050] The recrystallization annealing in step S14 is to restore the plasticity of the workpiece as a whole. It not only changes the grain shape on the surface, but also the core and the workpiece as a whole. When the cold-deformed metal is heated to a low temperature, the migration of some point defects and dislocations in the metal reduces the lattice distortion and gradually reduces the internal stress. This stage is called "recovery". When the temperature continues to rise, due to the increase in atomic activity, the metal's microstructure changes from broken, elongated or flattened grains to uniform and fine equiaxed grains. This is a process of re-nucleation and growth of grains. 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. Prepare the early microstructure for the subsequent cold heading and hexagonal forming processes.

[0051] Recrystallization temperature: The lowest temperature at which cold deformed metal begins to recrystallize is called recrystallization temperature.

[0052] For pure metals: T 再 =0.4T 熔 (K) Such as Fe:T 再 =0.4(1538+273)-273=451.4℃ The DC01 material used in this application has a carbon content between 0.05% and 0.12%. The above temperature calculation can be referred to. The actual recrystallization annealing temperature is 100 to 200°C higher than the recrystallization temperature (minimum) in order to improve production efficiency. Therefore, a temperature of 450+150=600°C and a holding time of 1.0H are used. The effect of the crystallization annealing is sufficient to meet the requirements of subsequent cold heading and stretching.

[0053] The recrystallization annealing process is designed between multiple stretching and cold heading processes. Its special significance lies in eliminating the work hardening phenomenon of parts caused by multiple stretching, eliminating residual internal stress, and transforming grains into equiaxed grains with excellent plasticity through recrystallization, thus making full microstructural preparation for the subsequent cold heading process, the final hexagonal forming dimensional stability, and the carburizing process.

[0054] Generally, sheet metal stamping and drawing parts are not subjected to recrystallization annealing because the cold working process causes little deformation of the internal grains, which is not enough to affect the size forming and performance.

[0055] For the cylindrical tensile parts similar to the present application, it is indeed necessary to compare the ultimate tensile deformation coefficient of each material with the total tensile deformation coefficient of the parts. The principle is that the total tensile deformation coefficient is greater than the ultimate tensile coefficient of the material. Otherwise, it is necessary to achieve multiple tensile deformations through temporary phosphating process or recrystallization annealing process to achieve the required shape.

[0056] The cold heading process in step S15 is one of the non-cutting metal pressure processing processes. It is a processing method that utilizes the plastic deformation of metal under the action of external force, and with the help of a mold, 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. This process is applied to the hexagonal lower cover of this application due to structural requirements.

[0057] In addition to the cold heading process, the hot heading process was also used for trial production and comparison. However, the hot heading process will result in uneven local heating, resulting in defects such as material folding and stratification; and it requires heat input, which increases costs. For the workpiece of this application, the cold heading process is more optimal.

[0058] When processing the internal thread in step S19, there will be blue-white zinc plating and black electrophoresis surface treatment processes in the later stage, which will leave attachments on the thread surface, namely the electroplating layer and the electrophoresis layer. Here, a pre-plating tap is needed to process the thread. The pre-plating tap is a tap specially used for processing internal threads of products that need to be electroplated. The large diameter and medium diameter of the tap are intentionally enlarged. Provide space for the thickness increased by subsequent surface treatment to meet the smooth passage of the thread plug gauge after electroplating and the assembly requirements of parts. How much 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 measuring the thickness of galvanizing + electrophoretic paint is basically 0.033-0.040mm, so (0.033-0.040)x4=0.13-0.16mm, use a pre-plating tap that is enlarged by 0.15mm for processing.

[0059] The surface carburizing of 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 (this product uses gas carburizing), heated to 880-930℃ and kept warm for 2.5-4 hours, so that the carburizing medium produces active carbon atoms on the surface of the workpiece, which penetrate into the surface layer of the workpiece through surface absorption and diffusion, so that the carbon content of the surface layer reaches 0.8-1.0%. After carburizing, it is treated by quenching (temperature 850-870℃, water quenching) and low-temperature tempering (temperature 160-200℃, insulation 60-90min) to improve the hardness, wear resistance and fatigue strength of the workpiece surface, while maintaining a certain strength and good toughness in the core. It is particularly suitable for the working scenarios of the hexagonal upper cover and hexagonal lower cover of this application, that is, it requires very high strength and has good toughness to cope with the impact force.

[0060] Therefore, the purpose of surface carburizing is to increase the surface hardness of the hexagonal cover, thereby improving the overall strength of the part. The workpiece eventually reaches a surface hardness of HRC50-55, compared with the original steel plate HB130-150 (corresponding to HRC about 14, which is basically not suitable to use HRC to evaluate this hardness because it is too low), the surface hardness of the part is greatly improved, and the strength of the part is correspondingly improved.

[0061] Electroplating is the process of reducing metal ions to metal on the electrode and depositing them on the surface of the workpiece by the principle of electrolytic deposition, so as to enhance the hardness and corrosion resistance of the workpiece. Electrophoresis is the process of depositing a liquid containing pigments, resins and other substances on the surface of the workpiece to form a film layer through electrophoresis, so as to achieve the purpose of protection, decoration and improvement of surface performance. The difference between electroplating and electrophoresis is that the intermediate objects are different. Electroplating is metal ions, and electrophoresis is colloids. The results after treatment are also different. Electroplating is to plate a layer of metal, which has the functions of protection, rust prevention and beauty, but the coating thickness is generally thin, about 8-12um; electrophoresis is generally used for spray painting, that is, a layer of paint is applied on the surface, which is often used in the automotive industry, and the thickness of the paint film will reach 25-50um. And the appearance is glossy, beautiful and rust-proof. The surface anti-corrosion superposition treatment of phosphating + electroplating + electrophoresis is to obtain a better anti-corrosion effect.

[0062] The hexagonal lower cover of the present application is formed into a hexagonal column by blanking in step S1, continuous stretching in S2-S4, and continuous stretching in S6-S8 to make the workpiece taller and thinner. During the stretching process, the degree of tensile deformation is considered to be almost close to the ultimate tensile coefficient of this material, and then the phosphating in steps S5 and S9 is adopted to allow the stretching action to continue. Then, through step S14 recrystallization annealing, the work hardening, residual stress, and grain distortion and elongation caused by cold deformation in multiple previous processes are heated to recover and crystallize again into equiaxed grains, providing a suitable equiaxed crystal microstructure foundation for the subsequent S15 cold heading process and S20 carburizing. Then, through step S20 carburizing, the hardness, wear resistance and fatigue strength of the workpiece surface are improved, while a certain strength and good toughness can be maintained in the core. The carburizing process cannot be adjusted. Moving the steps forward will make the surface hardness and strength of the parts high, making the S15 cold heading, S16 stretching, and S17 machine threading processes extremely difficult to implement. If the steps are moved back to after S21 electroplating and S22 electrophoresis, the carburizing effect will be greatly reduced due to the presence of the protective film, and the protective film of galvanizing and electrophoresis will be destroyed, which will make S20, S21, and S22 fail to achieve the expected results.

[0063] Therefore, only through the sequence of steps S1-S22 of the present application can the change in workpiece size, the change in the internal grain shape of the material, work hardening, the change in the surface structure of the material, and the continuous change in hardness and strength be achieved to achieve the expected effect.

[0064] S23 specific inspection requirements are: Verification method of corrosion resistance: salt spray corrosion test is carried out in accordance with GB / T 10125-2021 standard. Salt spray corrosion test simulates marine climate. A 5% NaCl solution with a pH between 6.5-7.2 is atomized and placed in a closed environment at 35°C and 98% humidity for 500 hours. Take out and check that there are no white spots, bubbles, shedding, or red rust on the surface of the parts.

[0065] Tensile test: Align the bottoms of the two hexagonal lower covers in the opposite direction and weld them (full welding), respectively assemble the corresponding bolts at two M12x1.75 internal threads, use a universal material testing machine, clamp the two bolts (M12x1.75) with the chuck, and perform a tensile test. The maximum force value of the test is the tensile test value, which is required to be greater than 50KN. Note: If the two welding points at the bottom of the hexagonal lower cover are broken during the test, this test data is invalid; in addition, pay attention to the observation that during the test, the connection between the hexagonal lower cover column and the bottom cover will be deformed first, which is normal, because this is the weak link in the strength of the hexagonal lower cover.

[0066] When the hexagonal lower cover that has not undergone the carburizing process was subjected to a tensile test, the tensile values ​​were mostly in the range of 30-35KN. In comparison, after the carburizing process was added, the tensile values ​​were mostly in the range of 55-60KN, and the tensile value increased by 70-80%, which did play a role in increasing strength.

[0067] Torque test: Use a special fixture to fix the hexagonal part of the hexagonal lower cover and the bottom of the cover in a vise, use a torque wrench to clamp the outer periphery of the threaded head, and perform a torque test.

[0068] The torque required to separate the clamping part of the torque wrench from the hexagonal part of the cover is the torque test value, which is required to be ≥122N.M. That is, set the torque value to 122N.M, and perform a torque test on the parts. At this torque value, if the parts are not deformed or broken, they are qualified; if the head and the column are deformed or broken when the torque is less than 122N.M, it is judged as not meeting the requirements. Under the action of the torque wrench, the broken part generally appears at the junction of the hexagonal column and the round head, which is normal, because the shape and thickness of this part change, becoming a weak link in strength.

[0069] When the hexagonal lower cover without carburizing process was tested for torque, most of the torque values ​​were in the range of 80-95N.M, which did not meet the torque value ≥122N.M. In comparison, after the carburizing process was added, most of the torque values ​​were in the range of 140-155N.M, and the torque value increased by 63-75%, which did play a role in increasing strength.

[0070] The following is an example comparison of different carburizing process parameters and test results for hexagonal lower covers:

[0071] In Example 1 to Example 8, the appropriate carburizing temperature range is continuously increased, the carburizing holding time is controlled between 2.5-4.0H, and the quenching + low-temperature tempering process is combined to obtain a carburized layer of 0.40-0.55mm on one side, the surface hardness of the part is between HRC50-55, the tensile force value is greater than 50KN in the range of 55-60KN, the torque value is greater than 122N.M between 140-155N.M, and the deformation of the part is small and acceptable. In particular, the carburizing process parameters of Example 1 obtain a moderate carburizing layer thickness, a moderate surface hardness of the carburizing layer, and the test values ​​of the tensile force and torque also have sufficient safety factors, the deformation of the part is small and acceptable, and the process cost is optimal.

[0072] Comparing Example 1 to Example 8 with Comparative Example 1, it is obvious that there is a great difference in the hardness of the part surface, the tensile test value and the torque test value of the part between the carburizing process and the non-carburizing process. Comparative Example 1 has no carburizing process, and its tensile force and torque values ​​are both lower than the requirements. Examples 1 to Examples 8 all have carburizing processes, which do increase the strength of the parts and can bear greater force requirements during use.

[0073] The difference between Example 1 to Example 8 and Comparative Example 2 is the carburizing temperature. Obviously, as the carburizing temperature increases, the carburized layer thickness increases, and the surface hardness of the carburized layer increases, which improves the tensile test and torque test to varying degrees. 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 shallow, and the tensile and torque test values ​​of the parts are lower than the requirements. Of course, the deformation of the parts is also very small.

[0074] Compared with Example 3, Example 1 to Example 8 also differ in the carburizing temperature. However, the carburizing temperature of 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 if the tempering temperature is 200°C, the surface hardness of the part reaches HRC62 (the hardness value of the tool), and the tensile value increases a lot. However, due to the increased brittleness of the part surface and the reduced toughness, the torque test value decreases a lot. It is enough to show that we cannot blindly pursue high hardness and high strength, and we need to take into account the demand for toughness in the occasions where the parts are used. In addition, due to the high carburizing temperature, the parts are deformed at high temperatures, and the surface hardness is high, which makes plastic shaping difficult, which is not desirable.

[0075] Compared with Example 4, the main difference between Example 1 to Example 8 is the carburizing time. Too long carburizing time not only affects the production turnover efficiency, but also the carburized layer increases with time, the surface hardness increases, which is beneficial to the tensile test, but the torque test value will decrease. At the same time, heating the parts at high temperature for a long time will cause coarse grains and deformation of the parts, which is also undesirable.

[0076] 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 carburizing furnace carbon potential atmosphere, 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, that is, a thin layer carburizing (the carburizing layer is less than 1mm), and the carburized 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 dimensions of the parts (the axial part of the hexagonal prism is not twisted or deformed, and the verticality with the bottom is less than 0.2mm). If the carburizing heating temperature is too low, the carburized layer is too shallow, the appropriate surface hardness cannot be obtained, and the tension and torque are lower than the required values; on the contrary, if the carburizing temperature is too high or the holding time is too long, the carburized layer is too thick and the surface hardness of the carburized layer is too high, which will cause the torque value to decrease. This is because if the hardness value is too high, the surface brittleness of the part increases and the toughness decreases, which is not conducive to the torque test. In actual application, the impact resistance of the parts will deteriorate due to the high hardness, and even brittle fracture may occur.

[0077] The following is a comparison of different surface treatment processes and corrosion resistance results of the hexagonal lower cover:

[0078] Compared with Comparative Examples 1-3, Example 1 uses the superposition of multiple surface treatment processes of phosphating + electroplating + electrophoresis to obtain a better anti-corrosion effect than a single surface treatment process, with a corrosion resistance of 500H (neutral salt spray corrosion for 24H is equivalent to corrosion in the natural environment for 1 year, and 500H is approximately equal to 20.8 years of no corrosion in the natural environment), and no white spots or red rust. Comparative Example 1 only uses phosphating treatment, and the corrosion resistance is only 12H, and sporadic red rust corrosion points will appear on the surface; Comparative Example 2 only uses electroplating (blue and white zinc plating) process, and the corrosion resistance is only 24H, and sporadic red rust corrosion points will appear on the surface; Comparative Example 3 only uses electrophoresis process, and the corrosion resistance is 200H, and sporadic red rust corrosion points will appear on the surface, blistering, shedding, etc.

[0079] 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, 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.

[0080] The method of the present application adopts one-piece forming, abandons casting and welding processes, avoids defects such as shrinkage cavities, shrinkage, welding deformation, and cold welding, ensures the overall strength of the parts, and ensures that the verticality of the column and the bottom is 0.2mm; adopts 8 stretching processes, reasonably utilizes the tensile limit coefficient of the material to design the number of stretching times, cooperates with two phosphating processes and one recrystallization annealing process, and the parts are formed and the cost is optimized; the application of the recrystallization annealing process, in order to prevent the continuous extension of the original grains of the material due to multiple stretching and cause fracture, adds a recrystallization annealing process in the middle, so that the grains are restored from a slender shape to an equiaxed shape, and provide the original excellent ductility of low-carbon steel for subsequent re-stretching, that is, prepares the microstructure for the cold heading process and the carburizing process; adopts the cold heading process to achieve the shrinkage and wall thickness increase of the part mouth, and meets the internal thread and other external circle size allowances required for machining; adopts a pre-plating tap to process the internal thread, leaving space for attachments for subsequent electroplating and electrophoresis. Before plating, the tap slightly enlarges the middle diameter and major diameter of the thread (the specific size can be calculated by the thickness of the plating layer) to ensure that the thread after electroplating and electrophoresis can pass the inspection of the thread plug gauge; carburizing treatment is adopted, and reasonable carburizing process parameters can 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 tension and torque, and meet the requirements of the workplace with impact toughness, so that the parts have sufficient sense of security during work; the superposition of a series of anti-rust treatment processes such as phosphating + galvanizing + electrophoresis is adopted to meet the final neutral salt spray resistance After 500H of corrosion test, excellent corrosion resistance with no white spots or red rust is required (24H of neutral salt spray corrosion is equivalent to 1 year of corrosion in natural environment, and 500H is approximately equal to 20.8 years of corrosion in natural environment) to cope with the erosion of parts working in general environment or even harsh environment; in the nesting gap range, by exploring the single-side gap between the outer hexagon of the hexagonal lower cover and the inner hexagon of the hexagonal upper cover, it is found that the nesting gap between the outer hexagon of the hexagonal lower cover and the inner hexagon of the hexagonal upper cover is between 0.15-0.20mm, which can optimally meet the requirements of smooth sliding up and down, no shaking, and no rotation of the hexagonal upper cover and the hexagonal lower cover after assembly.

[0081] The embodiments of this specific implementation are all preferred embodiments of the present application, and are not intended to limit the protection scope of the present application. The same components are represented by the same figure marks. Therefore, any equivalent changes made based on the structure, shape, and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method for integrally forming a hollow thin-walled hexagonal cover, characterized in that: The specific steps are as follows: S1: punching and blanking the steel plate raw material into round pieces; S2: Under normal temperature, using a cylindrical core mold, gradually stretching the middle portion of the disc into a convex cylindrical hollow workpiece with decreasing diameter; S3: placing the workpiece stretched in step S2 in a phosphate solution for phosphating, so that a phosphate film that is insoluble in water is formed on the surface of the workpiece; S4: using a hexagonal core mold to perform a first hexagonal forming on the cylindrical hollow core of the workpiece to form a hexagonal cylindrical hollow core workpiece; S5: cutting off the top of the hexagonal columnar hollow core to form an axial through hole in the hexagonal columnar hollow core; S6: Punch the connection between the hexagonal columnar hollow core and the bottom disc of the workpiece to complete the R corner cleaning; S7: stamping and flanging the bottom disc of the workpiece; S8: machining the workpiece; S9: Perform surface treatment on the workpiece.

2. The one-piece forming preparation method of a hollow thin-walled hexagonal cover according to claim 1, characterized in that: The cylindrical hollow core in step S2 is formed by stretching in batches, and the phosphating in step S3 is performed after each stretching.

3. The one-piece forming preparation method of a hollow thin-walled hexagonal cover according to claim 2, characterized in that: Each stretching is a plurality of continuous stretchings, and the cumulative stretching coefficient of each continuous stretching is greater than the limit stretching coefficient of the workpiece steel plate raw material.

4. The one-piece forming preparation method of a hollow thin-walled hexagonal cover 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 immersion time is not less than 10 minutes.

5. The one-piece forming preparation method of a hollow thin-walled hexagonal cover according to claim 1, characterized in that: Before step S7, the top of the hexagonal columnar hollow core of the workpiece is cold-forging treated to increase the wall thickness of the mouth of the hexagonal columnar hollow core, and at the same time, the columnar hollow core of the workpiece is molded for the second time using a hexagonal core mold.

6. The one-piece forming preparation method of a hollow thin-walled hexagonal cover according to claim 5, characterized in that: Before cold heading, the 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 plate raw material.

7. The one-piece forming preparation method of a hollow thin-walled hexagonal cover according to claim 1, characterized in that: The surface treatment in step S9 includes carburizing, electroplating and electrophoresis performed in sequence.

8. The one-piece forming preparation method of a hollow thin-walled hexagonal cover according to claim 7, characterized in that: The carburizing process is to heat the workpiece to 880℃-930℃ and keep it warm for 2-4 hours, then quench it at 850℃-870℃ and low-temperature temper it at 160℃-200℃. The low-temperature tempering time is 60-90min.

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