Influence of Annealing Atmosphere on the Phosphatability of Ultra-High-Strength Automotive Steels

This study investigates the effect of surface oxide control on the phosphatability of ultra-high-strength steel (UHSS) for automotive applications. Surface oxides were manipulated by adjusting the dew point to -50 °C and 0 °C during the annealing process, and the corresponding changes in phosphating behavior were examined. The surface characteristics of the samples were analyzed using X-ray photoelectron spectroscopy (XPS) and field-emission transmission electron microscopy (FE-TEM), while the phosphatability of the samples was evaluated through electrochemical measurements. The sample annealed at a dew point of -50 °C formed continuous Si and Mn oxide films (~10 nm), which significantly suppressed the phosphatability. In contrast, when annealed at 0 °C, internal oxidation occurred along the grain boundaries to a depth of about 3 μm, resulting in the formation of discontinuous Si and Mn oxides on the surface, which greatly enhanced phosphatability. This difference was also supported by OCP measurements: the -50 °C specimen showed a gradual increase in potential, whereas the 0 °C specimen rapidly reached -0.59 V and then stabilized. The findings of this study demonstrate that optimizing the annealing atmosphere provides an effective approach to enhance the phosphating performance of UHSS without the need for additional surface treatments.