Effect of Sn Addition, Process Atmosphere pO2, and Annealing Time on the Selective Oxidation of a C-2Mn-1.7Si (Wt Pct) Advanced High-Strength Steel During Continuous Galvanizing

The effects of process atmosphere pO2, annealing time, and a minor Sn addition on the selective oxidation of a Fe-0.1C-2Mn-1.7Si (wt pct) advanced high-strength steel during continuous galvanizing heat treatments were determined. The reference and 0.05 wt pct Sn-added steels were intercritically annealed at 1113 K (840 °C) for annealing times between 60 and 600 seconds in a N2-5 vol pct H2 gas atmosphere, where the process atmosphere pO2 was controlled by varying the dew points at 223 K, 243 K, and 278 K (−50 °C, −30 °C and +5 °C). It was found that both the internal and external oxidation kinetics followed a parabolic rate law and were significantly reduced by the Sn addition to the alloy. For the lowest pO2 223 K (−50 °C) dew point atmosphere, the external oxides comprised compact, film-like MnSiO3, SiO2, and granular MnO. The addition of Sn to the alloy reduced the compactness of the granular oxides. Increasing the atmosphere dew point to 243 K and 278 K (−30 °C and +5 °C) reduced the thickness of the external oxides and increased the depth of internal oxidation for both alloys. Under the highest pO2 278 K (+5 °C) dew point atmosphere, although the reference steel surface was covered with small, closely packed MnSiO3 nodule-like particles, the addition of Sn altered their morphology to larger, more widely spaced nodules and decreased the depth of internal oxidation. The effect of Sn was found to be the result of its segregation at the surface oxide/substrate interface, as shown by 3D atom probe tomography. The alterations in oxide morphology resulting from the Sn addition are expected to enhance the reactive wetting of the substrate surfaces by the continuous galvanizing bath.