Lamellar structure formation of hot-stamped galvannealed coating during tempering

A significant amount of research and development on partial softening techniques, including tempering, is being conducted. Coated steel sheets, including galvannealed (GA) steel sheets, have been applied to automobile components. It is thought that tempering has a great influence on the coating structure in heating treatment after the hot-stamping process. In this study, a change of hot-stamped GA coating during tempering was investigated. By analyzing hot-stamped GA coating tempered at different temperatures, it was elucidated that the coating structure of hot-stamped GA may change during tempering. Tempering at more than 500°C separates the Fe-Zn solid solution, which is the hot-stamped GA coating, into two phases, a r phase and an Fe-Zn solid solution, which contain less Zn than the original. This coating structure change results in lamellar structure formation. Lamellar structure formation is thought to be caused by discontinuous precipitation from the Fe-Zn supersaturated solid solution. The main factor of generating lamellar structure formation is the diffusion velocity of Fe and Zn, determined by the tempering temperature. In addition, the driving force of the phase separation has an effect on this reaction.