Effect of dew point during annealing on phosphatability of ultra-high-strength steel

This study aims to investigate the impact of controlled surface oxides on the phosphatability of ultra-highstrength steel, particularly for automotive applications. Surface oxides were controlled by adjusting the dew point to -30 and + 10 degrees C during the annealing process, and the consequent alterations in phosphatability were examined. XPS and TEM were utilized to analyze the surface characteristics of the samples. At dew points of -30 degrees C, minimal phosphating reaction was observed, whereas a significant enhancement in phosphating performance was noted with an increase in dew point to 10 degrees C. This improvement was attributed to structural modification influenced by the chemical composition of surface oxides, particularly the reduction in Si content and relatively higher Mn content and the enhanced internal oxidation at a dew point of 10 degrees C. The present results provide a heat treatment approach as a viable method to enhance phosphatability in ultra-high-strength steel.