Selective Oxidation of a C-2Mn-1.3Si (Wt Pct) Advanced High-Strength Steel During Continuous Galvanizing Heat Treatments

The influence of oxygen partial pressure and annealing time on the selective oxidation of a Fe-0.1C-2Mn-1.3Si (wt pct) advanced high-strength steel during intercritical annealing was investigated. The steel was intercritically annealed at 1093 K (820 °C) for annealing times in the range of 60 to 600 seconds in a N₂-5 vol pct H₂ gas atmosphere with dew points ranging from 223 K to 278 K (− 50 °C to + 5 °C). The morphology, thickness, and chemistry of the oxides were determined as a function of process atmosphere oxygen partial pressure and annealing time by means of SEM, XPS, and TEM. It was found that for the 223 K (− 50 °C) dew point atmosphere, a compact, film-like external oxide comprising MnSiO₃, Mn₂SiO₄, and SiO₂ and nodule-like MnO oxides covered the surface. On the other hand, for the 278 K (+ 5 °C) dew point atmosphere, MnSiO₃, Mn₂SiO₄, and MnO nodule/plate-like external oxides were formed, while Si was oxidized internally. Longer annealing times resulted in increasing either the thickness of the external oxide layer or the depth of internal oxidation. A comparison between the predictions of the Wagner internal-to-external transition model found good agreement between the experimental results for Mn and Si in ferrite.