With the advancement of the automobile and machinery industries, there is a growing demand for high-performance materials used in fasteners. One of the most effective solutions is the use of high-strength bolt steel. However, as energy crises become more pressing, non-tempered steel has gained attention due to its energy efficiency, lower consumption, and simplified manufacturing process. Currently, high-strength U-bolts used in truck leaf spring applications are typically made from conventional quenched and tempered steels like 40Cr, 35CrMo, and 45Mn2. These materials require multiple steps such as spheroidizing annealing, pickling, phosphating, cold drawing, rolling, forming, quenching, tempering, and dehydrogenation annealing, which result in high energy usage and long production cycles.
Researchers at Yunnan University investigated the effects of cold-drawn surface reduction rate and aging treatment on the microstructure and mechanical properties of bainitic cold-worked non-tempered steel. They developed 10.9-grade U-bolts using this material. The study found that as the cold-drawing reduction rate increased, the material’s strength also rose significantly. At a 26% reduction rate, the tensile strength reached a maximum of 1134 MPa, attributed to the peak strain hardening index (n value) at this level. After aging treatments at temperatures ranging from 100°C to 500°C, the steel’s strength initially increased and then decreased. The highest tensile and yield strengths were achieved after aging at 300°C, due to changes in carbide precipitation behavior at different temperatures. At 300°C, the precipitated carbides were smaller in size and more densely distributed, leading to improved strength.
Based on these findings, an optimal production process was established. The trial-made bolts met the performance requirements of 10.9-grade U-bolts used in trucks, proving the feasibility of industrial-scale production. This research highlights the potential of non-tempered steel in reducing energy consumption while maintaining high mechanical performance, making it a promising alternative for future applications in the automotive industry.
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