Look at the trend of body materials through Mercedes-Benz “steel door”

Recently, the new domestically-made Mercedes-Benz E-Class long-wheelbase sedan was launched. Some netizens revealed that the domestically-made extended-wheelbase models have increased the weight by nearly 300 kilograms compared with the standard wheelbase models in overseas markets. It was replaced with steel materials when localized. Although Mercedes-Benz officially issued a statement afterwards, the domestic E-class cars and E-class cars sold in other markets are made of steel-aluminum hybrid materials, and they all comply with the safety standards stipulated by local laws. The incident still caused widespread concern in the industry. For a while, Mercedes-Benz was caught in the embarrassment of the “steel door” and was pushed to the forefront of public opinion.

Regardless of the incident itself, people are more concerned about the general trend of lightweight, which material should be used for the body structure? Steel body, aluminum body or composite material, which is good or bad, or each has its own advantages and disadvantages, how to choose? How will future body materials develop? Behind this battle of public opinion, the technology and development of body materials are moving from behind the scenes to the front.

■ The right material should be placed in the right place

At present, the materials used in mainstream passenger cars mainly include steel plate materials, aluminum alloys, high-performance plastics and composite materials. Carbon fiber composite materials can also be seen in some high-end passenger cars or electric vehicles. Pure steel plate materials are mostly found in low-end passenger car bodies. Steel-aluminum hybrid materials are mainly used in mid-to-high-end passenger car bodies, such as Mercedes-Benz E-Class and new Audi Q7. All-aluminum bodies are mostly used in high-end passenger cars, such as Land Rover. Range Rover, Audi A8 and other models.

It can be seen that steel and aluminum are currently the mainstream materials for passenger car bodies.

So, what factors will OEMs consider when choosing body materials? Zhang Zhan, head of lightweight and performance integration at Great Wall Motors Co., Ltd., told reporters that safe collision performance is usually the primary consideration for body material selection. The reasonable layout of material strength on the safe transmission path can effectively absorb the collision energy and ensure the safety of the passenger compartment. Different parts of the car body have different requirements for rigidity and impact energy absorption. Companies generally distribute materials of different strength levels based on the real force conditions and performance requirements of each part to meet collision safety, fatigue durability, and NVH static and dynamic stiffness. Such performance indicators should also be considered at the same time as the system cost.

Lu Hongzhou, head of the automotive field and senior manager of the CITIC Microalloying Technology Center, said that “how to put the right material in the right place” has been the main idea for selecting body materials in recent years, but in actual operation it is mainly based on experience and At the benchmarking level, there is no positive selection logic, so it is difficult to ensure the rationality of the selection. Therefore, Lu Hongzhou proposed the “Automotive Forward Material Selection System (FSM)” to cooperate with the forward development and performance development of automobiles. He said that the system comprehensively considers the functions of the components, the manufacturing process characteristics of the components, and the cost.

■ Steel body or aluminum body, which is better?

Industry experts said that steel body and aluminum body have their own advantages and disadvantages and cannot be generalized. According to Li Jun, the chief engineer of the Materials Engineering Department of Chery Automobile Engineering Research Institute, the advantages of steel body are mainly due to its relatively high technology maturity and relatively low cost, but there is no significant advantage in weight. The main advantage of aluminum alloy body is light weight, which can leave more weight space for high-end cars to increase configuration, such as improving power.

Higher cost is one of the reasons why aluminum bodywork has been slow to spread to low-end cars. “For traditional car factories that mainly use steel body technology, the aluminum body involves certain technological transformations, such as uncoiling and blanking, welding, painting, etc.” Zhang Zhan said that the current body aluminum is mostly 5 series and 6 series aluminum alloys. Mainly, after drawing and bake hardening, the yield strength can reach 220~260MPa, and the strength can only reach the strength level of general high-strength steel. Therefore, it needs to be compensated by cross-sectional structure design and connection technology, which poses a challenge to traditional steel body design engineers And the cost of aluminum is relatively expensive.

It is understood that the strength of the current industrialized high-strength steel can reach 1000MPa, while the aluminum alloy profile generally does not exceed 500MPa, “but this does not mean that aluminum alloy is unsafe. In fact, the strength of mechanical parts is not completely used as safety. Indexes need to be considered comprehensively with the rigidity and cross-section of the parts,” said Li Jun.

In addition, Lu Hongzhou said that the maintenance of aluminum car bodies is complicated and the price is higher. For example, when the collision deformation is not serious, steel parts can be repaired through sheet metal, but aluminum parts can only be replaced basically, which also brings cost pressure to consumers.

Although the price of aluminum is relatively high, for electric vehicles, the good lightweight effect of aluminum offsets its higher material cost, and it will become the preferred body material for electric vehicles. Recently, some new energy electric vehicles have also adopted all-aluminum bodies, such as Dongfeng E30 and Chery “Little Ant”.

■ The car body develops towards multi-material

The application of multi-material mixing is the inevitable choice of materials for the body and even the entire vehicle. During the interview, industry experts expressed the above views to reporters. Zhang Zhan explained that the multi-material application of the car body can give full play to the reasonable layout of the material strength on the car body, while ensuring that the cost is moderate, and it can also significantly reduce the weight.

Multi-material mixing will be the development trend of lightweight car bodies, and all-aluminum bodies will be the development trend of luxury cars. “Li Jun said that the current status of materials for mainstream European models is that passenger car body materials of less than 100,000 yuan are mainly high-strength steel; the trend of passenger car body materials for 100,000 to 300,000 yuan is high-strength steel and galvanized sheet. A small amount of application of aluminum alloy, etc.; the trend of passenger car body materials of 300,000 to 800,000 yuan is a mixture of multiple materials, including high-strength steel, aluminum alloy, plastic composite materials, etc.; the development trend of luxury cars of more than 800,000 yuan is all aluminum body , Or mainly carbon fiber composite body.

Lu Hongzhou believes that the prices of self-owned brand models are gradually increasing, so the amount of aluminum alloy on the body will also increase, especially for aluminum alloy profiles with lower technical requirements.

In addition, Zhang Zhan also said that as far as the current situation is concerned, the steel body has a wide distribution range, with low, medium and high prices, and it is still the most mainstream body material at present. “Because steel has comprehensive advantages in strength, plasticity, impact resistance, recycling and low cost, its dominant position in automotive materials is still unshakable, especially the application of high-strength steel and ultra-high-strength steel. There will be more room for growth.” Li Jun expressed similar views to reporters. At the same time, he also said that from the perspective of the development of independent brands, the research on high-strength steel application technology should be strengthened in the near future, and the light weight of high-strength steel should be deeply explored. Quantify the potential and improve the market competitiveness of self-owned brand passenger vehicles.

At the beginning, some people suspected that Mercedes-Benz “secretly changed body materials” in China. It is reported that the Mercedes-Benz E listed overseas has an aluminum body, but the domestically-made Mercedes-Benz body material is still mainly made of steel. Then I found the “fuel consumption mark” published by the Ministry of Industry and Information Technology and found that the curb weight of the new E-class long-wheelbase version is 1870 kg, which is 300 kg more weight than the overseas version.

With the further fermentation of this incident, the impact has become wider and wider. Mercedes-Benz officially issued a statement stating that the domestic E-Class uses a steel-aluminum hybrid material and complies with local regulations. In addition, some people who support Mercedes-Benz also said that the all-aluminum body is a concept, and it does not mean that the body is entirely aluminum.

Up to now, the discussion of the event is still in fermentation.