Atom probe analysis on interaction between Cr and N in bake-hardening steels with anti-aging properties at RT

One-dimensional atom probe (1DAP) analysis was performed on chromium and nitrogen added bake hardening steel sheets that have both high bake-hardenability and anti-aging properties at room temperature (RT). The atomic data of more than 2 million atoms were collected for sample steels with and without low-temperature aging after recrystallization annealing and quenching. The correlation in atomic position between chromium and nitrogen atoms in a solid solution was investigated by a statistical analysis using the binomial distribution function. In the samples with low-temperature aging, the probability that a chromium atom was observed near a nitrogen atom was significantly higher than that estimated from the null hypothesis that there was no attractive atomic interaction. This suggests that there is an attractive interaction between a nitrogen atom and a chromium atom in bcc iron, which led to the anti-aging properties at RT. In contrast, such correlation was not observed definitely in the samples without low-temperature aging, which implied that the atomic pair formation is a thermal activation process.