At present, there are many technical defects in the cold stamping process, such as difficulty in forming, poor product quality, and easy deformation after forming. The research and application of hot stamping technology for ultra-high strength steel have been put on the development agenda. Although hot stamping technology has emerged in the field of automobile manufacturing in China, there are still some technical issues that urgently need to be addressed. This article briefly elaborates and analyzes the technical characteristics and related research progress of existing steel hot stamping forming.

1、 Hot stamping forming technology for steel

1.1  Principle of hot stamping forming

Hot stamping forming technology refers to the process of heating boron alloy steel plates above the recrystallization temperature (about 900 ℃), utilizing the significant increase in plasticity and ductility of the metal plate at high temperatures, and the rapid decrease in yield strength. The steel is stamped and formed in a mold at a cooling rate greater than 27 ℃/s, and then rapidly cooled and quenched to transform from austenite to martensite.

1.2  Characteristics of Hot Stamping Forming Process

In recent years, with the continuous research on steel stamping technology in China, a new stamping technology - hot stamping has emerged. Stamping technology has received widespread attention in the stamping industry. The use of hot stamping technology can obtain advanced ultra-high strength stamped parts, significantly improving the tensile strength, plasticity, and ductility of steel. Using conventional cold stamping technology may result in problems such as high stamping force required for the workpiece, easy deformation, and poor product quality. However, hot stamping technology has solved the problems of poor formability and shape freezing caused by traditional cold stamping technology, and has developed rapidly in recent years, widely used in modern automotive manufacturing.

1.3  Hot stamping forming materials

In the traditional cold stamping process, advanced high-strength automotive steel plates such as TRIP steel and martensitic steel have been processed and formed, and the performance indicators of the parts remain basically unchanged. In the hot pressing process, boron steel is widely used, with its main component being 22MnB5. The tensile strength at room temperature is only 500-700MPa, but after hot stamping and quenching, it transforms into a martensitic structure, significantly improving the mechanical properties of the material. Its tensile strength can reach 1500 megapascals, yield strength can reach over 1000 megapascals, and hardness can reach 50HRC.

1.4 Hot stamping forming mold

In cold stamping technology, molds are only used to form parts, and the mechanical strength of the parts will not change significantly before and after stamping. In hot stamping process, the mold needs to be formed through forming, quenching and other processes, with a complex structure. The formed parts have higher strength and better comprehensive performance.

Foreign literature has reported the use of Al2O3/Cu composite materials in hot stamping molds, and the reasonable selection of mold materials is crucial for hot stamping technology. During the forming, cooling, and quenching processes, hot stamping die materials should be able to achieve rapid and uniform cooling, while also possessing the functions of rapid heat transfer and cooling.

Therefore, molds not only need to have good thermal mechanical properties, wear resistance, corrosion resistance, and thermal fatigue resistance, but also need to be designed with reasonable dimensions for convex and concave molds to ensure that the quality of the produced parts is qualified and reliable. To avoid uneven hardness of hot stamping parts caused by inconsistent local cooling rates of parts, a cooling water circuit system is set up to cool the mold and provide continuous cooling capability. Moreover, repeated stamping can cause the mold temperature to continuously rise and fall, which can reduce the lifespan of the mold. Therefore, we urgently need to find a mold that can improve cooling capacity and is durable.

2、 The advantages of hot stamping forming technology

The traditional cold stamping process includes heating, forming, cooling, and subsequent processing, and the process parameters vary in different processing steps. The hot stamping forming process mainly consists of several processes, including cutting, pre forming, heating, transfer, stamping forming, quenching, and subsequent treatment.

The main advantages of hot stamping technology are: easy control of rebound, low resistance to deformation, surface hardness of parts, and good hardness characteristics. High dimensional accuracy and good dent resistance of parts; Steel has good stamping performance and low forming resistance; The production cycle of car body parts is short and the strength is high; Reducing the thickness of parts and the number of reinforcing plates is beneficial for achieving effective weight reduction of the vehicle body.

In the hot stamping process, in order to shorten the production cycle and improve production efficiency, the following precautions should be taken: the heating temperature should not be too high and should be kept above the recrystallization temperature; The furnace needs to be kept warm for a certain period of time, and the insulation time should not be too long; Has a high stamping speed and a certain holding time; In the phase transition of cooling and quenching, the cooling rate cannot be too low; The thickness of stamped sheet metal should not be too thick; Mold materials should be selected and designed reasonably.

3、 Research progress of hot stamping technology

Hot stamping technology was widely used in Europe and America, which can effectively reduce vehicle weight, lower costs, and improve vehicle collision stability. At present, this technology has attracted widespread attention in countries around the world. We have successively developed boron steel hot stamping technology and put it into production on components such as car door bumpers, A, B, C pillars, and bumper reinforcement beams. After putting this technology into production practice, it can effectively reduce the weight of automobiles, improve stability, and reduce production costs. Therefore, it is highly favored in the market and has achieved rapid development and application.

3.1  Abroad Research Status

The global automotive industry has invested a lot of energy in the research of hot stamping forming technology, mainly distributed in relatively developed European countries, almost monopolizing the global hot stamping forming market. As a new forming technology, hot stamping can achieve high strength of stamped parts, which has aroused the interest of many researchers in the research and development of hot stamping technology.

3.2  Domestic research status

Due to the fact that hot stamping forming technology is an advanced automotive manufacturing technology, its application can better meet the requirements of weight reduction and environmental protection, improve the impact resistance and collision stability of automobiles, and therefore has received attention and importance from China and even the world. Although China's research on this technology is relatively lagging behind, it can be said that in recent years, China has developed rapidly, keeping up with the trend of the times, and the pace of research and development is also very fast. In China, various enterprises and research institutions have conducted research on hot stamping forming technology.

4、 Conclusion

With the application of hot stamping technology in automobile production, the preparation of advanced ultra-high strength stamping parts in China has become more widespread and the technology has become more mature. The structure and performance of the mold are gradually becoming idealized, and the comprehensive performance of the molded parts is also better, with higher dimensional accuracy. Compared with cold stamping, the mechanical properties of the workpiece have been significantly improved after hot stamping. In the future, China's hot forming technology will continue to improve, and its application in automobile production will continue to expand. The competition in the hot forming market will become more intense.