Recently, Shougang successfully developed a new type of 500–600 MPa hot-stamped steel for axle housings which solves the serious decrease in the strength of axle housings after hot stamping, reduces the strength loss ratio from 30% to 5%, greatly improves the fatigue strength and bearing capacity of axle housings, and enhances overall vehicle safety. Since the project’s implementation, the hot-stamped steel products for axle housings have been successfully applied by well-known commercial vehicle enterprises in China, and the product quality has won unanimous recognition from users, greatly enhancing the market influence of Shougang’s hot-stamped steel for axle housings.

The drive axle is an important structure for automobile bearing which is widely used in such fields as heavy trucks, semi-trailer tractors, buses and construction machinery. It is connected to the frame through suspension assembly, and wheels are installed at both ends to transmit various forces between the frame and wheels, supporting the load of the automobile. As such, its strength, stiffness, fatigue life and other performance indicators are crucial. The axle housing is a vital part of the drive axle and one of the main components of a drive system. The drive axle housing should be sufficiently strong and rigid, low in mass, and facilitate the disassembly and adjustment of the main retarder. Hot-stamped steel for axle housings firmly occupies the leading position in the production of medium and heavy truck axle housings due to its remarkable advantages of low deformation resistance at high temperature, low residual stress and high fatigue life. However, its strength is significantly reduced after hot stamping. This has long been a difficult problem in the industry, limiting the lightweight properties of axle housings and the improvement of vehicle carrying capacity, and seriously restricting the technical progress of the medium and heavy truck industry and green development. In order to solve this problem, the Shougang Research Institute of Technology and Jingtang Company set up a tackling team geared towards providing “differentiated and customized” services, and started to carry out research on new hot-stamped steel products and application technology for axle housings.

The tackling team first looked for a breakthrough in the design of its own product components. Under the careful guidance of Wang Quanli, Deputy Chief Engineer of the Group, the two project leaders Hui Yajun (Member of the Shougang Research Institute of Technology) and Wu Kemin (Director of the Shougang Hot-rolled Product Department) led the team members to go deep into the axle housing factory. By tracking the whole process of axle housing production, it was found that the biggest drawback of hot stamping technology lies in the uncontrollable cooling speed in air cooling mode, which leads to uncontrollable metal phase transformation and the precipitation of microalloy elements during cooling. It is difficult to achieve precise control due to the numerous influencing factors and complex mechanism. By analyzing the evolution laws of microstructures and the microalloyed second phase in the hot stamping process, the team conducted a series of basic studies on the hot rolling, continuous cooling transformation behavior, high temperature mechanical properties and hot stamping process simulation of seven composition systems at pilot scale, then determined the design idea for composition optimization. After hot stamping, the yield strength was increased by 50–60 MPa compared with conventional products, and the strength loss was decreased from about 30% to less than 15%. As for how to further improve the strength after hot stamping, the tackling team took a different approach and started their process research from the control of the cooling path after hot stamping. The team members systematically simulated the influence of process parameters such as heating temperature, stamping speed and pre-stamping holding time on the strength and plasticity of axle housing steel, and mastered basic theoretical data such as the high-temperature mechanical behavior of axle housing steel in the austenite region. On this basis, a new hot stamping process was put forward. After hot stamping, the yield strength was increased by 100–120 MPa compared with conventional products, and the strength loss ratio was further reduced to 5%. Meanwhile, the continuous casting process team led by Liu Guoliang and Yu Xuecheng and the welding process team led by Dong Xianchun also solved the problems of the surface cracking and fatigue cracking of axle housings, greatly improving their fatigue performance. These innovative achievements vividly demonstrate the alloy reduction design concept, leading to the successful development of a series of hot-stamped steel products for axle housings and becoming an excellent case of the Group using technology to reduce costs. At the same time, these achievements also contribute to the realization of vehicle lightweight properties, promote the technological progress of upstream and downstream industrial chains, and advance the green development of energy conservation and emission reduction in the national automobile industry.