Usage of Cold Stamping Process

Due to the rapid development of automobile industry, the application of lightweight automobile and high-strength steel has become an important development direction. However, due to the improvement of the strength of high-strength steel plates, the traditional cold stamping process is prone to crack during the forming process, which cannot meet the processing requirements of high-strength steel plates. In the case that the forming conditions cannot be met, the hot stamping technology of ultra-high strength steel plate is gradually studied internationally. This technology is a new process that integrates forming, heat transfer and phase transformation of structure. It is mainly a process of forming through die by taking advantage of the characteristics of increased plasticity and reduced yield strength of sheet metal in high temperature austenite state. However, hot forming requires in-depth research on process conditions, metal phase transformation and CAE analysis technology. At present, this technology is monopolized by foreign manufacturers, and its development is slow in China.
Analysis on the Characteristics and Forming Problems of High Strength Steel
According to the research of the Energy Department of the American Iron and Steel Institute, even if the value of high-strength steel is reduced, its stretching is still much more difficult than that of traditional cold plate. The ductility of high-strength steel is only half of that of ordinary steel. At the same time, when the material is pressed, it will harden. Different steels will harden to different degrees. Generally, the high strength low alloy steel increases only slightly by 20MPa, less than 10%. Note: The yield strength of dual phase steel has increased by 140 MPa, more than 40%! Metal in the forming process, will become completely different, unlike before the beginning of stamping. The yield strength of these steels increases a lot after being stressed. The higher yield stress of the material plus work hardening is equal to a great increase in the flow stress. Therefore, cracking, springback, wrinkling, workpiece size, die wear, micro welding wear and so on have become the focus of problems in the forming process of high-strength steel.
Based on the characteristics and characteristics of high-strength steel, if the metal flow cannot be changed and the friction cannot be reduced, the cracking and texture inhomogeneity of high-strength steel (HSS) may cause the increase of component scrap rate. The high kilopound force per square inch (KSI) of this material (the unit for measuring the yield force), the enhanced springback, the tendency to work hardening, and the ability to operate at elevated molding temperatures are challenges for the mold.
Effect of High Performance Lubrication Technology on Cold Stamping of High Strength Steel
It is an indisputable fact that lubrication affects formability. Therefore, high performance lubricants can provide a higher performance film to protect the metal from fracture, fracturing or being welded to the mold. A good lubricant can also reduce the friction heat, make the metal flow uninterrupted and control the wrinkling or fracture. To solve this problem, IRMCO, a world famous professional company engaged in lubrication research and manufacturing, has successfully developed a new water-based lubrication technology 30 years ago to solve the forming problem of high-strength steel. Now, based on years of research and use, I would like to talk about my views and opinions from the perspective of lubrication and the characteristics of this lubrication technology. As follows:
First, lubrication guarantee under severe conditions.
1. Because the yield force of high-strength steel is large, it requires more energy to yield it than ordinary plates. In addition to deformation, this energy is largely due to the collision (friction) between plates and molds. Therefore, as the deformation temperature of metal in the stamping die continues to rise, the oil-based lubricating oil will become thinner. In some cases, it will reach the flash point or burn (smoke), causing lubrication failure. IRMCO polymer extreme temperature lubricant contains extreme pressure resistant materials and is “heat seeking” and will stick to metal. With the increase of temperature, a tough protective barrier will be formed on the surface of the mold and plate, thus reducing friction and temperature, helping the workpiece to better extend, so as to control friction and metal flow; At the same time, it can protect the metal from overheating, blackening, cracking and bonding.
At the same time, according to the problems in the actual test, we replicate the results of the field test in the laboratory. Oil based and conventional (soap) based synthetic lubricants with different formulations do not work properly on high-strength steel. The scrap rate and die wear (scratch) are out of control.
Test Report of Dual Phase Steel DP600-UHSS Ultra High Strength Steel
Green Leaf Technology Laboratory of the United States carried out molding contrast experiments on DP780 ultra-high strength steel of the United States for IRMCO products and certified drawing oils and synthetic lubricants in the automotive industry. Interlaken tensile forming test method of Ohio State University was adopted. This method has been proved to be very accurate for testing actual results. IRMCO reached a value of 145, which is more than 25% than 115 of the automobile industry certified stretching oil and 110 of the synthetic lubricant. The larger the value, the better the forming ability.
2. Due to the temperature generated during the stamping process, and due to the large heat capacity of the oil, the thermal diffusion is relatively poor. The IRMCO water-based lubricant can not only effectively reduce the temperature and protect the mold, but also effectively control the micro welding wear caused by the temperature due to the formation of water vapor to take away the heat due to the temperature. At the same time, it can also improve the stroke times.
Secondly, the performance of tool coating is closely related to the quality of lubricant.
Traditional physical vapor deposition (PVD) and chemical vapor deposition (CVD) coatings require lubrication externally to maximize performance. IRMCO pointed out that: “The thermal diffusion process can reduce the problem of friction and wear. The vanadium carbide produced by thermal diffusion can form a durable and hard surface, which will greatly improve the service life of tools when forming and stamping die surfaces are harsh. If the lubricant used cannot withstand high temperature, wear and the work hardening of advanced high-strength steel, the value will be greatly reduced. Therefore, the use of lubricants that can meet the needs of high-strength steel is to achieve the goal of coating The key to maximize the service life. ”
According to the first implementation of IRMCO technology and years of application experience, high-strength steel stamping dies can meet the production needs and extend the service life of the dies without expensive coating costs. It can be seen from (Figure 7 on the right) that the IRMCO lubrication technology has a protective effect on the mold. After punching 3000 workpieces, the surface quality of the workpieces and the die surface are significantly improved (the surface scratches are significantly reduced).
Third, high-performance lubrication can make the workpiece quality more stable.
Die clearance and geometric design of the die are important contents to achieve dimensional accuracy of the workpiece. Therefore, this is directly related to the technical requirements for metal materials and lubricants. If the clearance is smaller and the friction force is larger, the requirements for lubricants are higher, but the forming accuracy of the workpiece is also relatively higher. Since IRMCO can better control friction and metal flow, it can ensure more stable workpiece quality.
The Application of IRMCO in Some High Strength Steel Cold Stamping Parts
For the control arm (NMS rocker arm) produced by a large chassis parts enterprise, the plate material is FB780, the plate thickness is 4mm, and the seven station progressive stamping die is used. At the same time, the IRMCO test is used to ensure the quality of the workpiece. Compared with the oil-based lubricant under the same environmental conditions, the working temperature of the die decreases by 10-20 ℃. Here, it is important to note that the reduction of temperature means the reduction of friction, which can not only protect the mold, but also improve the stroke times. The picture of workpiece is as follows: