On September 18th, the team of the State Key Laboratory of Rolling Technology and Continuous Rolling Automation of DB University released a new high-toughness aluminum-silicon coating technology at the 2019 China Automotive Lightweight Conference. The team proposed a new theory of high carbon brittleness between the interface of the aluminum-silicon coating and the steel substrate, and achieved a breakthrough from "0 to 1" in the field of strengthening and toughening of automotive steel-silicon coatings.
Using the interface carbon reduction and toughening technology, and through batch industrial trial production of MGJT Steel Group and AGJT Automobile Steel Co., Ltd., high-toughness aluminum-silicon-plated hot stamped steel products were produced. This new material can reach the same strength as existing materials and achieve a 20-30% increase in toughness, helping to improve the safety performance of automotive parts.
Compared with the existing technology, the new technology is novel and creative in multiple dimensions, and has been authorized by a Chinese invention patent. This patent is expected to reduce the sales price and component costs of this type of steel. The technology has attracted great attention from the GM (North America) materials team from the beginning. GM officially requested MGJT in May this year to start preparing the complete technical data required for the certification of the new steel grade.
In GM ’s new global material standard GMW14400, announced in June of this year, for the first time, a high-toughness aluminum-silicon-plated hot stamped steel was added, and a technical requirement of 20% improvement in toughness was proposed. With the goal of promoting product application, MGJT and General Motors formed a joint technical team to conduct multi-round hot stamping verification of parts on the Dongfeng (Wuhan) Industrial mass production hot stamping line. Experiments have shown that the collision performance of parts made of new steel has been improved by 28%, the risk of delayed cracking has been greatly reduced, and various indicators such as coating performance meet the GMW14400 standard. At present, many domestic and foreign automobile manufacturers are continuously tracking and evaluating the interface carbon reduction and toughening technology.
Weight reduction, energy saving and safety improvement are two important directions for the development of the automotive industry. The application of ultra-high-strength steel is a cost-effective solution that can balance both. Compared with the traditional cold stamping forming technology, the hot stamping forming technology can overcome the technical pain points such as severe rebound of the steel plate, low strength, difficult forming, and low production efficiency. Therefore, it has become the mainstream solution for ultra-high-strength steel for body-in-white.
The traditional non-plated hot stamped steel will cause decarburization on the surface of the steel plate and the generation of scale on heating. The iron oxide scale is easy to fall off in the mold, which increases the friction coefficient between the steel plate and the abrasive tool, and reduces the service life of the mold. The iron oxide scale in the mold is regularly cleaned, which seriously reduces the production efficiency. In order to meet the requirements for subsequent processing of stamped parts, it is necessary to remove the oxide scale formed on the surface by shot peening. Shot peening will not only increase the cost, but also affect the dimensional accuracy of the part.
In order to avoid the oxidation and decarburization of the surface of hot stamped steel sheet and make it have high temperature resistance and corrosion resistance, Arcelor-Mittal Steel successfully developed aluminum-silicon coating technology in 1999. This technology effectively avoids the problem of surface oxidation of unplated hot stamped steel plates. At the same time, it also shows excellent performance in terms of abrasive tool protection, part dimensional accuracy, and corrosion resistance. Since then, the application of hot stamping steel on automobiles has been increasing year by year. With rapid growth, the global application of aluminum-silicon coated steel sheets has exceeded 3 million tons per year.
"Although there are many advantages to aluminum-silicon-coated automotive steel sheets, automotive companies have found an important defect in this type of steel sheet during use: insufficient toughness. Insufficient toughness can directly lead to the failure and fracture of automobile crash safety parts, and insufficient toughness can also cause component delay Cracking, that is, the parts are not cracked after stamping, but cracked after welding and assembly. If this bottleneck is cracked, the safety of car door crash beams, bumpers, B pillars, etc. can be further enhanced. " Yi Hongliang said.
"The defects and difficulties of the products must have their reasons, and we are determined to find out the reasons and then achieve targeted attack." Yi Hongliang said. Wang Guodong, an academician of the Chinese Academy of Engineering, said that the team started with a theoretical breakthrough and discovered the physical mechanism of carbon-rich brittleness between the interface between the ultra-high-strength hot-formed steel aluminum-silicon coating and the steel substrate, which is the basis of this "0" to "1" Theoretical exploration has enabled the development of an interface carbon reduction and toughening technology that breaks through the toughness bottleneck of hot stamped steel plates with silicon-aluminum coating. According to the design requirements of automotive companies for high-toughness hot stamping coating products, the requirements of hot stamping companies for products that reduce the risk of delayed cracking, and the demand for independent intellectual property products of steel companies, the team transformed the theory into breakthrough production technologies.
It was found that during the austenite heating process of the aluminum-silicon coated steel sheet, the alloying of the coating moved the interface toward the 22MnB5 substrate, forming an alloyed layer and a δ-ferrite diffusion layer with a high aluminum content. It contains carbon, which results in a large amount of carbon enrichment near the 22MnB5 substrate and near the interface of the diffusion layer, and the formation of high-carbon martensite during subsequent cooling. It is the extremely low toughness of this layer of high-carbon martensite that significantly reduces the toughness of aluminum-silicon coating products.
Based on this theory, Yi Hongliang's team worked out a combination of "reducing the thickness of the coating" and "optimizing the heating process" without changing the composition of the coating and the alloy of the substrate to reduce the alloying between the 22MnB5 substrate and the coating. Therefore, the thickness of the high-carbon martensite layer is reduced, the toughness of the aluminum-silicon coating product is greatly improved, and the bending fracture strain is greatly improved, and the delay cracking risk is greatly reduced.
"This technology started with theoretical innovation and formed independent intellectual property rights. Not only have Chinese invention patents been authorized, international patents are also being applied for, and the industrialization of this technology will exceed 3 million tons worldwide (China's nearly 1 million tons). It plays an important role on the stage. This is a contribution to the lightweight development of China's automobiles and also China's contribution to the world's automobile industry. "Said Mao Xinping, an academician of the Chinese Academy of Engineering.
This original innovation, which relies on scientific discoveries, has led to a new type of ultra-high-strength steel-aluminum-silicon coating with high toughness. This set of technology can be directly produced on the existing aluminum-silicon coated plate production line of the steel plant. The thinning of the coating can reduce the cost of the company by about 60 yuan per ton of ultra-high-strength automotive steel. Professor Yi Hongliang's team also proposed a hot stamping rapid heating process, which improves the heating efficiency by 10-20%. At the same time, the new high-toughness aluminum-silicon coating technology fully meets automotive companies' requirements for coating adhesion, stone strike resistance, and corrosion resistance.