Hot stamping technology and application progress o

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MFC: high strength steel hot stamping technology and application progress

in order to protect our environment, the fuel consumption and carbon dioxide emissions of vehicles must be reduced in the future. With the increasing number of cars around the world, it is necessary to further reduce the fuel consumption of single cars. The automotive industry is facing new challenges. One way to achieve this goal is to reduce the weight of the car. With the increase of road traffic flow, the risk of accidents is also increasing, which means that cars must meet higher safety requirements. For body design, the current challenge is to improve the stiffness of important parts and reduce the weight of the body. One of the perfect solutions to meet these needs is the automobile body parts manufactured by hot forming technology. In the past, thermoforming was known as an inefficient and expensive manufacturing process. Nowadays, the newly developed manufacturing process not only improves the manufacturing speed, but also provides better part quality. This makes hot forming develop from a temporary solution to a long-term production mode of automobile structural parts manufacturing in the next five years. The output of the new generation hot forming line is very close to that of cold stamping. These newly developed high-yield and high-quality production lines involve a lot of basic research, including heating, sheet metal mold contact, forming, cooling and trimming processes. The end result of these developments is a hot forming line with high production efficiency for manufacturing high-quality and high-quality stable parts

demand of automobile lightweight for hot formed parts

automobile lightweight technology was first developed and patented in Sweden, but was not applied in the automobile industry at first. Plannja, a Swedish company, is the patent owner and uses this technology to produce saw blades and lawn mower blades. In the early 1980s, the first automobile parts were produced. The side anti-collision beams of passenger cars were made of advanced steel materials such as steel for basic parts and high-performance marine engineering steel. Other components, such as bumper, A-pillar and B-pillar, and chassis components are also produced successively. The parts produced by hot stamping for each vehicle are very limited, and the price of parts used by OEMs is high due to the inefficient production process. After the patent protection expired in the mid-1980s, three companies will reach 1.5 million tons by 2020 and can provide all the required parts. The number of parts produced will increase from 30000 pieces per year in 1987 to 8million pieces per year in 1997. Since 2000, more kinds of hot stamping parts have been used in automobiles, and the production volume has increased to more than 1million pieces in 2000

after 2008, the hot forming technology of high-strength steel plate has developed rapidly in the automotive manufacturing industry at home and abroad, and has broad application prospects in the automotive field. According to the data published by Chs2 in October 2012 (Figure 1), the global consumption of high-strength steel will increase from 1.8 million tons in 2012 to 2.5 million tons in 2014, and the number of high-strength steel hot formed parts will reach more than 500million. The number of parts used in each vehicle has also greatly increased, and some companies are studying the possibility of using hot formed parts to construct the overall body structure

Figure 1 prediction of hot stamping parts and material demand of high strength steel

limit of thinning structure

an important direction for the development of hot forming technology is the improvement of material strength. At present, 1500-1600mpa can be obtained under general hot stamping conditions, that is, people usually start to talk about 2gpa. The way to realize the lightweight of car body parts is to improve the material strength (at present, 1800-2000mpa ultra-high strength steel has appeared) and optimize the part structure to make the parts thinner and thinner, so as to reduce the weight. However, the thinning amount is limited. In the relevant impact energy calculation system, the impact load is a function of material thickness. When the strength increases and the parts become thinner, they will break due to the instability of structural materials. In this case, the bending strength of the material is more important. On the one hand, light weight can be achieved by strengthening and thinning materials, and on the other hand, instability and fracture protection measures should be taken. Under the condition of instability, how much weight can be reduced depends on the amount of structural thinning. For the existing conventional car body structure, the weight can be reduced by about 20% - 30% by using hot formed car body parts

variable strength forming technology

from the current research situation, a lot of work needs to be done on the variable strength forming technology (tailored properties), because this is another major topic, which requires that the design of materials should be more closely combined with the forming technology. This technology can obtain different strength and elongation in different areas of the part without homogenizing the strength and elongation by only one hot stamping. Automobile designers hope to protect the passengers from collision and intrusion through the ultra-high strength of the car body. For example, for typical lateral collision, they rely on the B-pillar to provide protection for the passengers. However, if the same body structural parts can absorb part of the impact energy while resisting the impact intrusion, the impact safety index can be greatly improved. Variable strength forming is to soften the strength of some areas of the part, so it is necessary to improve the elongation of the part material and increase the absorption capacity of impact energy. Therefore, it is of great significance to tailor the hot formed components for a certain vehicle model to meet its strong plastic distribution

Figure 2 variable strength formed part of automobile B-pillar

as shown in Figure 2, it is a variable strength formed part of automobile B-pillar. The strength at position a is 1500MPa and the elongation is 6%; The strength at position B is 700MPa and the elongation is 15%. There are many methods to achieve this requirement. Similar to tailor welded blanks, 1500MPa strength may be required in an impact intrusion resistant area, but the elongation is only 6% - 8%. In another area where hot stamping strength is not required, it may be 600MPa or 700MPa, but the elongation can reach 15%. In contrast, the improvement of toughness helps to absorb collision energy

as shown in Fig. 3a, the forming test die is similar to the B-pillar variable strength joint. The wht1500 sheet metal is heated to 930 ℃ and then formed in a specially designed die at 850 ℃. The temperature of T-shaped head mold is kept at 400 ℃, while the temperature of rod mold is kept at 20 ℃. The temperature and hardness distribution of parts simulated by variable strength forming are shown in Fig. 3b and C. The test results show that it is a simple and feasible variable strength forming method by setting die gap and die temperature difference. The key to engineering application is to control the distribution of the mechanical properties of the part transition zone formed by the die gap

Figure 3 temperature and mechanical properties of parts during variable strength forming and heat preservation for 20s

this goal can be achieved in the process flow, such as using customized heating. The heating of some billets is protected and controlled. They will never reach the austenitic temperature and will not be quenched into martensite during forming. In addition, die steels with different heat transfer coefficients can be used. When forming at the same billet temperature, some areas are cooled faster to obtain more martensite, and some areas are cooled slower to obtain bainite. Martensite is obtained by stable quenching, so it is very hard, and the local part of the blank will be quenched to bainite. 30 ℃ per second is the critical cooling rate. Martensite is formed above this rate, while bainite (or ferrite) will be formed below this rate, so it is relatively soft. Bainite is a stage in the formation of austenite. Bainite is the medium strength structure of steel, so a part can be formed. It has 1500MPa strength in one impact zone and 600-700mpa strength in the other zone, and has higher elongation

another method is tempering. In this case, the part is completely quenched during hot forming, so its strength is consistent. Then different tempering heat treatment is applied to some areas to obtain lower strength and higher elongation

based on the principle of variable strength forming and process test results, people pay attention to the comprehensive application of variable strength from different angles (Fig. 4): it is required that energy can be effectively absorbed in some parts, and the soft flange edge can be cut by die punching instead of laser cutting. Or in the customized area, such as at the fastener connection, it has better flexibility and reduces brittleness

variable strength forming and hot cutting edge

hot cutting edge die

forming without or with less cutting edge

Figure 4 comprehensive application of variable strength technology

development of zinc based coated sheet

traditional uncoated hot stamping parts will cause decarburization and oxide scaling on the surface of stamping steel plate during heating. Decarburization will reduce the surface strength of the steel, while the oxide skin increases the friction coefficient between the steel plate and the mold, reducing the service life of the mold. At the same time, the oxide skin needs to be cleaned regularly, which seriously reduces the production efficiency

at present, hot stamping steel coatings mainly include aluminum silicon coating (al-10si) coating, hot-dip pure zinc (GI) coating, alloyed zinc iron (GA) coating and electro galvanized nickel (zn-10ni) coating. The hot stamping steel plate with coating has a good working environment during stamping, high surface quality of parts, and no subsequent shot peening process. It can not only prevent surface oxidation and decarburization in the forming process, but also improve the anti-corrosion performance after coating. In the process of hot stamping, it also has excellent characteristics in die protection and matrix corrosion resistance, and has been widely used in hot stamping steel

in order to meet the requirements of hot stamping steel for coating notch protection performance and heating window, Germany Thyssen company has developed gamma protect coating technology. The coating is a zn-10ni coating, and the part near the substrate is composed of single-phase γ- Nizn21 is composed of metal compound, which makes the melting point of the coating as high as 880 ℃, which is conducive to avoiding the formation of liquid zinc phase during heating and the cracking of matrix grain boundary during forming. The main purpose of adding nickel is to stabilize the zinc rich phase. The coating has a wide hot working window (880-920 ℃), and the oxide layer on the Zn Ni surface is more stable than that on the zinc coating during heating, which can effectively prevent the volatilization of zinc on the surface. stable γ The coating has excellent friction properties by combining the phase with the oxide layer on the surface. With the large-scale production of zinc based plated sheet, it will cause the technical change of hot stamping, and has a wide application prospect

early intervention service and high-performance simulation platform

automobile high-strength steel plate and forming technology are also issues of concern to automobile manufacturers. How to intervene in the development of new cars of automobile enterprises in the early stage and evaluate the formability and part performance in the part design stage is an effective way to promote the application of high-strength steel hot forming technology and materials. In the car body design stage of automobile manufacturers, manufacturers of hot formed parts focus on the selection and evaluation of hot formed materials, formability analysis and manufacturing process analysis of car body parts, which can greatly reduce the defects of design and manufacturing, optimize the design and manufacturing process, and reduce the manufacturing cost. Evi service is also a process for hot forming part manufacturers and automobile designers to jointly implement hot forming design criteria. Modern evi services mainly use hot stamping simulation to evaluate the formability and impact mechanical properties of car body parts. Evi technology center with hot forming test line can also develop low-cost tests to reduce the use of materials and assembly time, which is expected to continue to generate 100 billion level market space molds for hot forming tests in the future, so as to ensure that the key parts of the new design scheme meet the design indicators, as shown in Figure 5

Fig. 5 synchronization of hot forming materials, forming process and car body design

the outstanding workload of early intervention service is that the calculation workload of forming simulation is large, and the forming simulation and repair

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