Programme nominated for the Innovation of the Year Steelie Award 2023
With the aim of reducing CO2 emissions, gasoline engine vehicles are improving fuel efficiency by reducing body weight, and sales of electric vehicles are increasing.
Additionally, the protection of occupants and the battery during a collision is also important, and therefore, in recent years, the number of parts with strength exceeding 1.5 GPa has been increasing. Hot stamping is widely used to produce these parts. It has excellent formability compared with the conventional cold press but has problems with material properties and CO2 emissions in the heating process.
In order to overcome these problems, JFE Steel has developed a full-martensite type 1.5 GPa steel (1.5G-MS) and cold press forming technologies that can produce parts with complex shapes. 1.5G-MS is produced by a unique water-quench with a cooling rate exceeding 1,000°C/s based on a continuous annealing process and has superior delayed fracture properties due to high uniformity of microstructure and low alloy design, and superior crash-worthiness due to higher yield strength compared to hot stamped parts.
On the other hand, in the cold press using 1.5G-MS, fractures during pressing and poor dimensional accuracy of parts due to springback are major challenges. In response to these problems, JFE Steel proposed a ‘pre-forming concept’, and then developed a ‘stress reverse formingTM’ for long curved parts for which it is difficult to ensure dimensional accuracy.
This technology utilises the Bauschinger effect (the phenomenon whereby yield stress is reduced when stress is reversed), which occurs remarkably in high-strength steel sheets.
In this technology, the stress at the press bottom dead centre, which is a factor of spring back, is greatly reduced, and therefore, the dimensional accuracy can be greatly improved.
Substitution of cold press-formed 1.5G-MS for parts that had been produced by the hot stamping process until now is progressing. In addition to the world’s first use in roof centre reinforcements, 1.5G-MS has been adopted in many structural parts of the Toyota Prius.
JFE Steel believe these technologies will contribute significantly to the spread of electric vehicles in the future.