Process for improving the ability of a steel sheet to be punched

Abstract

The application discloses a process for improving the punching capacity of a steel plate, which comprises the following steps: punching and polishing a smooth round hole on the steel plate, smearing graphite cream on the upper and lower surfaces of the steel plate around the round hole and on the edge of the round hole, covering the graphite cream with a circular tantalum sheet, pressing the tantalum sheet and the graphite cream with a copper electrode, passing current to heat the steel plate to a temperature 50-80 DEG C higher than the complete austenitizing temperature and keeping the temperature for 10-30 s, removing the tantalum sheet in the high-temperature state, punching, then water quenching to generate martensite. The process can enhance the punching forming capacity of all steel types, and does not need to use high-expansion hole steel to replace high-strength steel, thus solving the contradiction between the high-strength steel punching difficulty, easy cracking and punching forming performance. The polished punching edge is smooth and flat, and the punching process deterioration caused by stress concentration is eliminated. The graphite cream lubricant solves the high-resistance problem between the steel plate and the electrode pressure head, the punching edge oxidation problem under high temperature, and the steel plate punching surface lubrication and protection problem.

Description

Technical Field

[0001] This invention belongs to the field of metal heat treatment technology, and specifically relates to a process for improving the ability of steel plates to be perforated. Background Technology

[0002] Flanging is a common process in steel plate forming. The conventional flanging method involves first punching a small hole in the steel plate, then using a clamping ring to hold the part in place, and finally using a punch of appropriate diameter to pass through the center of the hole to form the final shape of the part. Figure 1 As shown (D is the diameter of the enlarged hole, d is the diameter of the punched hole, and h is the height of the turned hole). Figure 2 The large hole in the center of the triangular arm component on the car body is manufactured using a hole-expanding process.

[0003] As market demands for automotive safety performance continue to rise, high-strength steel is increasingly being chosen for the manufacture of automotive parts. However, with the increase in strength, the forming performance of the steel sheet tends to decrease. Therefore, resolving the contradiction between steel sheet strength and forming performance has become one of the industry's challenges.

[0004] Replacing high-strength steel with high-expansion steel is a common method to solve this problem. For example, patents such as "A 780MPa grade hot-rolled high-strength high-expansion steel and its manufacturing method" (application number CN2015105956270), "A 980MPa grade hot-rolled high-strength high-expansion steel and its manufacturing method" (application number CN2016104512915), "A boron-containing high-expansion steel and its production method" (application number CN2019108894081), and "A Nb microalloyed high-strength high-expansion steel and its production method" (application number CN2019108887567) all involve different high-expansion steels.

[0005] High-strength steel contains a large amount of hard martensite. When the steel sheet is subjected to deformation, the martensite is difficult to deform, while other structures, such as ferrite, deform easily. This creates a deformation coordination problem between the undeformed martensite and the deformed structure, making it highly susceptible to microvoids and cracks. To avoid this drawback, high-hole-expansion steel does not contain martensite in its microstructure; instead, it uses bainite, which is less hard than martensite, to coordinate deformation. However, this method significantly reduces the strength of the steel sheet, thus affecting the safety performance of the vehicle body. Summary of the Invention

[0006] To overcome the shortcomings of the existing technology, the present invention provides a process for improving the turning capability of steel plates, which can enhance the turning capability of all steel grades without replacing high-strength steel with high-expansion steel; it can not only achieve the turning process with large deformation, but also does not affect the strength of the steel plate, thus solving the contradiction between high strength and turning performance.

[0007] A process for improving the perforation capability of steel plates, the process comprising the following steps:

[0008] ① Punch small round holes on a steel plate and grind the edges of the holes to eliminate burrs and micro-cracks caused by punching, resulting in smooth round holes.

[0009] Grinding and cutting round holes to make the edges of the holes smooth can prevent edge cracks from causing stress concentration during the hole turning process, which would cause the round holes to crack prematurely.

[0010] ② Apply graphite emulsion evenly to the upper and lower surfaces of the steel plate around the circular hole, as well as to the edge of the circular hole; the graphite emulsion covers the finished circular hole and exceeds the diameter of the finished hole by 10 to 12 mm.

[0011] ③ On the graphite emulsion on the upper and lower surfaces of the steel plate, a circular tantalum sheet with the same area as the graphite emulsion is covered; the tantalum sheet can be fixed to the surface of the steel plate by the adhesiveness of the graphite emulsion.

[0012] Graphite emulsion not only serves to fix tantalum sheets, but also has excellent electrical conductivity and can provide good lubrication between the steel plate and the mold during subsequent hole-turning deformation.

[0013] ④ Two copper electrodes are used to press the tantalum sheets and graphite emulsion on the upper and lower surfaces of the steel plate together; the overlapping positions of the copper electrode pressure head, tantalum sheets, graphite emulsion, and steel plate are as follows: Figure 4 As shown.

[0014] The purpose of tantalum sheets is to prevent the steel plate from coming into direct contact with the copper electrode, which would damage the steel's formability.

[0015] ⑤ When current is passed through the copper electrode, the steel plate will heat up rapidly due to the much higher resistance of the copper electrode, tantalum sheet, and graphite emulsion. The steel plate temperature should be raised to 50–80°C above the full austenitization temperature of the steel and held for 10–30 seconds.

[0016] After steel is fully austenitized, its plastic deformation capacity will increase significantly. It can not only achieve larger deformation of the hole, but also has lower requirements for equipment capacity and less wear on the mold.

[0017] ⑥ Under high temperature conditions, remove the steel plate, remove the tantalum sheet, and perform drilling.

[0018] ⑦ After drilling, water quenching or air cooling is performed until the hole is cooled to room temperature.

[0019] After quenching, local martensite forms around the hole, resulting in greater strength, higher hardness, and excellent performance.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] This invention provides a process to improve the flanging capability of steel plates, enhancing the flanging forming ability of all steel grades without requiring the replacement of high-strength steel with high-expansion steel. It not only enables flanging processes with large deformation but also does not affect the steel plate strength, thus resolving the contradiction between high strength and flanging forming performance. This process requires immediate grinding of the punched edges after punching to ensure a smooth and even finish, eliminating stress concentration-induced deterioration of the flanging process. The use of graphite emulsion grease simultaneously solves the problems of high resistance between the steel plate and the electrode indenter, oxidation of the punched edges at high temperatures, and lubrication and protection of the flanged surface. This process is applicable to all steel plates, especially addressing the difficulties and cracking issues associated with flanging high-strength steel. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a steel plate before and after hole turning in the prior art, where D is the enlarged hole diameter, d is the punching hole diameter, and h is the turning hole height;

[0023] Figure 2 Images of automotive parts with perforated features;

[0024] Figure 3 This is a schematic diagram showing the location of graphite emulsion application (the shaded area indicates the location of graphite emulsion application);

[0025] Figure 4 This is a schematic diagram showing the positions of the copper electrode indenter, tantalum sheet, graphite emulsion, and steel plate. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments, but this does not limit the invention in any way. To avoid redundancy, unless otherwise specified, the raw materials used in the following embodiments are all commercially available products, and the processes used are all conventional processes unless otherwise specified.

[0027] Five steel grades were used, and three flanging test schemes were conducted for each steel grade. The technical effects of the traditional flanging process and the process of this invention were compared. The geometric characteristics before and after flanging are as follows: Figure 1 As shown in Table 1, the effects of the two hole-flipping processes are compared. SP540F, 780HE, and 980HE in the experimental scheme are all high-strength steels, while DP5900 and QStE600TM are ordinary high-strength steels.

[0028] Example

[0029] A process for improving the perforation capability of steel plates includes the following steps:

[0030] ① Punch small round holes on a steel plate and grind the edges of the round holes to eliminate edge burrs and micro-cracks caused by punching, so as to obtain round holes with smooth edges;

[0031] ② Apply graphite emulsion evenly to the upper and lower surfaces of the steel plate surrounding the circular hole, as well as to the edge of the circular hole; for example Figure 3 As shown, the graphite emulsion covers the finished circular hole and exceeds the diameter of the finished hole by 12mm.

[0032] ③ On the graphite emulsion on both the upper and lower surfaces of the steel plate, a circular tantalum sheet with the same area as the graphite emulsion is covered.

[0033] ④ Two copper electrodes are used to press the tantalum sheets and graphite emulsion on the upper and lower surfaces of the steel plate together; the overlapping positions of the copper electrode pressure head, tantalum sheets, graphite emulsion, and steel plate are as follows: Figure 4 As shown.

[0034] ⑤ Pass an electric current through the copper electrode. The steel plate heats up under the action of the current. When the temperature of the steel plate rises to 50-80°C above the complete austenitization temperature of the steel, hold it at that temperature for 10-30 seconds.

[0035] ⑥ Under high temperature conditions, remove the steel plate, remove the tantalum sheet, and perform drilling;

[0036] ⑦ After drilling, water quenching or air cooling is used to cool the steel plate around the hole to room temperature.

[0037] Comparative Example

[0038] The traditional flanging process is used: first, a small hole is punched in the steel plate, then the part is fixed in place with a pressure ring, and then a punch of the appropriate diameter is used to pass through the middle of the hole to form the final shape of the part.

[0039] Table 1 Comparison of the effects of the two flanging techniques

[0040]

[0041] The porosity in Table 1 is calculated using the following equation.

[0042]

[0043] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A process for improving the perforation capability of steel plates, characterized in that, The process includes the following steps: ① Punch round holes on a steel plate and grind the edges of the round holes to remove burrs and micro-cracks, resulting in smooth round holes; ② Apply graphite emulsion to the upper and lower surfaces of the steel plate around the circular hole, as well as to the edge of the circular hole; the graphite emulsion covers the finished circular hole and exceeds the diameter of the finished hole by 10 to 12 mm; ③ On the graphite emulsion on the upper and lower surfaces of the steel plate, a circular tantalum sheet with the same area as the graphite emulsion is covered. ④ Two copper electrodes are used to press the tantalum sheets and graphite emulsion on the upper and lower surfaces of the steel plate together; ⑤ Pass an electric current through the copper electrode, and the steel plate heats up under the action of the current. When the temperature of the steel plate rises to 50°C to 80°C above the complete austenitization temperature of the steel, hold it at that temperature for 10 to 30 seconds. ⑥ Under high temperature conditions, remove the steel plate, remove the tantalum sheet, and perform drilling; ⑦ After drilling, water quenching or air cooling is performed until the hole is cooled to room temperature.

Images