Today's global automotive industry is promoting the rapid development of automotive products in the direction of green, low carbon, energy saving, environmental protection and safety. Lightweight vehicles are one of the key links to achieve this direction. According to the statistics of international authoritative departments, about 60% of automobile fuel is consumed by the weight of the automobile, and the vehicle quality is reduced by 10%, which can reduce fuel consumption by 6% to 8%. For no-load, about 70% of fuel consumption is used in the body quality, while the quality of automobile body panels and structural parts ranks first, accounting for more than 30% of the total mass of the car. Therefore, the lightweight of the car body is of vital importance for reducing the quality of the car's curb and improving fuel economy to achieve energy conservation and environmental protection.
The automobile structural parts produced by the hot stamping forming integrated control complete equipment production line have many advantages such as ultra high strength, high hardness, light weight (thickness can be reduced by 35% compared with ordinary steel plates), almost no rebound (high manufacturing precision), and the like. The technical advantage is very obvious. In addition, stampings that require multiple sets of molds to be formed multiple times in cold stamping can be formed at one time by a hot forming process, thereby reducing the number of molds and the forming process. Since this technology requires sufficient cooling rate for quenching while thermoforming, there are specific requirements for equipment and molds.
In 2011, with the support of the national science and technology major project “Ultra-high-strength automotive structural parts hot stamping technology and equipment production lineâ€, our company cooperated with Changchun Weifute Auto Parts Co., Ltd. to carry out hot stamping of ultra-high-strength automotive structural parts. Technical breakthroughs in technology and equipment production lines, and the use of domestic key equipment to independently develop equipment production lines, breaking the monopoly of foreign technology, filling the gap in China's hot stamping forming production line equipment production line, and now the production line has been successfully applied.
This paper introduces the hot stamping production line of domestic high-strength structural parts, analyzes the composition of the production line, expounds the key equipment and performance indicators of the production line, compares the performance of the production line with the level of similar foreign products, and the main performance indicators of the production line have reached the developed level. The technical level of national advanced products has exceeded foreign products even in certain indicators.
Production line composition
The project team carried out innovative research on the technology of hot stamping technology and equipment production line for ultra-high-strength automotive structural parts, solved the key technical problems of commonality, and developed an energy-saving ring-type bottom-end anti-oxidation heating furnace with independent intellectual property rights. A high-speed, high-efficiency, stable and reliable central control automation production line for dedicated water-cooled molds and variable-speed adjustable high-speed hydraulic presses is shown in Figure 1. The development of the entire production line integrates experts and technologies in hydraulic machines, industrial furnaces, heat treatment, automatic transfer equipment, stamping dies, automobile stamping processes, system automation, etc., and uses fieldbus industrial network control technology to adjust various key equipments. According to the production cycle and process parameters, stable and reliable hot-formed parts are produced in large quantities, with a production capacity of 600,000 pieces per year. The production line has completely independent intellectual property rights and has the advantage of low cost, which can replace the imported hot stamping production line. Compared with similar foreign technology and production line, energy saving is nearly 30%, and the preform can be in-mold quenched, which solves the problem of forming many complicated structural parts that cannot be formed under high temperature conditions, thereby expanding the application of hot stamping parts. The range can meet the market demand of the automotive and aerospace industries.
Figure 1 Hot stamping technology and equipment production line for ultra-high-strength automotive structural parts
Key equipment and performance indicators
The production line consists of key equipment such as anti-oxidation continuous heating furnace, high-speed transmission device, high-speed hot stamping hydraulic press, water-cooled hot stamping die, etc. The system structure is shown in Figure 2.
Figure 2 Centralized automated production line
1. Anti-oxidation heating furnace 2. High-speed transmission device 3. High-speed hydraulic machine 4. Water-cooled hot stamping die
1. Energy-saving ring-type bottom heating furnace
The heating furnace is heated by an electric hybrid ring type bottoming method, and a high temperature resistant special material having a very low expansion ratio is used as a core spindle of a high temperature heating furnace filled with an oxidation preventing gas. The furnace design process uses infrared temperature test, thermocouple temperature test and other techniques to analyze the no-load temperature distribution inside the furnace to ensure that the full load temperature distribution is optimal. In order to ensure the reliability of the heating furnace, it also tests and verifies the high temperature resistance and thermal fatigue performance of the key heat-resistant parts in the furnace, and simultaneously carries out the reliability test under the overall operating state of the heating furnace to ensure the heating furnace is in the mass production line. Reliability. The internal structure of the heating furnace is shown in Figure 3. The main performance indicators are shown in Table 1.
Figure 3 Internal structure of the heating furnace
Table 1 Main performance indicators of heating furnace 
2. High speed conveyor
The high-speed conveyor (see Figure 4) uses a linear reciprocating composite motion mechanism that is simple, flexible, and reliable, making it easier to replace different products. The control system of the gripper adopts high-reliability and stable PLC equipment, and the external related equipments are controlled and interlocked by means of Modbus protocol through communication. High temperature end pickers are designed and manufactured for automotive structural parts of different sizes, shapes and weights. The end picker cooperates with the high-speed intelligent conveying device to complete the process of grasping and conveying the high-temperature steel plate under the premise of meeting the process requirements. In order to achieve reliable mass production, under the central control system, a control system for the high-speed intelligent transmission device and a temperature monitoring device attached to the transmission device are established. The temperature monitoring device and the mold temperature monitoring device are integrated to form a temperature collecting system on the production line for recording the temperature process parameters at the same time of production, thereby facilitating process optimization, steel sheet performance prediction, and mold fatigue performance according to actual production data. Forecasting, etc.
Figure 4 high speed conveyor
3. High speed hot stamping hydraulic press
The mainframe structure of the high-speed hot stamping hydraulic press uses computer simulation analysis to analyze the coupling stiffness between the whole machine, the mold and the stamping parts, so that the main machine can fully meet the requirements in terms of strength, rigidity and life. The hydraulic cylinder seal design adopts the "Stern seal-Y-ring-guide ring" seal combination method to ensure the sealing effect of the hydraulic press under high-speed operation, and the pressure remains stable. The hydraulic system adopts the control technology of precise closed-loop control of block position and proportional control of pressure closed loop, and realizes functions such as proportional pressure regulation, four-corner proportional pressure regulation, four-point pressure regulation of the flanger proportional slider, and variable pressure control. The speed proportional control technology is used to effectively reduce the impact of speed conversion. The high-speed hot stamping hydraulic press can realize the movement pressure, displacement and speed of the slider can be set arbitrarily. It has digital display and numerical control function, and the repeating control precision of the slider running reaches ±0.1mm. The high-speed hot stamping hydraulic press is shown in Figure 5. The main performance indicators are shown in Table 2. The special requirement is that the feeding mechanism is a high-speed motion intelligent control feeding mechanism that can match the firing beat of the heating furnace.
Figure 5 high-speed hot stamping hydraulic press
Table 2 Main performance indicators of high-speed hot stamping hydraulic press 
4. Water-cooled mold for hot stamping
The relationship between the cooling rate and the die surface temperature, the fatigue failure mechanism of the mold surface under cold and hot alternating conditions in mass production, the manufacturing process technology to improve the service life of the mold, the development of water-cooled mold materials and the new technology of water-cooled pipe system The study formed a thermoformed mold structure with internal cooling channels. The piping of the inner wall of the cavity is optimized, such as a cooling water pipe system uniformly distributed along the shape of the hot stamping. A new technology for cooling tube forming has been developed, which can reduce the manufacturing cost of the piping system, improve the life of the mold, smooth circulation of the cooling water, uniform temperature field distribution, and uniform martensite in the mold. The hot stamping forming die material adopts a material with high thermal conductivity and good thermal stability, and the cooling speed of the mold surface is ≥100 ° C / s, and the service life is more than 200,000 times.
Production line level comparison
The equipment developed has been tested in Changchun Weifute Auto Parts Co., Ltd., and the effect is good. Compared with similar foreign products, the main performance indicators of the development production line have reached the technical level of advanced products in developed countries, and even exceeded the foreign products in some indicators. Table 3 compares the performance of this line with foreign production lines.
Table 3 Comparison of the level of this production line with similar foreign products 
Conclusion
The production line can be used for hot stamping of high-strength automotive structural parts, and has the advantages of high efficiency, safety, reliability, energy saving and environmental protection. The development of the production line has broken the monopoly of foreign technology, and it is of great significance to revitalize China's automobile manufacturing industry and parts processing industry, replace imports, and promote the production skills of the machine tool and mold industry.
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