Effect of Dew Point on the Selective Oxidation of Advanced High Strength Steels

The effect of the dew point (and therefore oxygen partial pressure) on the selective oxidation of Advanced High Strength Steels was investigated. Steels with different Si contents, 0.2 wt% Si, 0.8 wt% Si and 1.5 wt% Si were used. The steel samples were annealed at 840 °C for 60 s and various gas atmospheres prior to hot-dip galvanized at 460 °C. The dew point of the 5 % H2-N2 annealing atmosphere was lowered from-30 °C (equivalent to 380 ppm H2O) to-58 °C (equivalent to 14 ppm H2O) in order to investigate surface segregation of alloying elements Si, Mn and Cr. These conditions are reducing for Fe, but oxidizing the oxygen-affine elements. Oxide morphology changed from a complete covering surface at high dew point to separated oxide spots at grain boundaries at low dew point. At the low dew point Cr was not oxidized. Oxides with a low Mn/Si-ratio seems to be amorphous. The Si-oxides are especially located at grain boundaries, Mn-oxides tend to cover the surface. Oxides covering the steel surfaces are detrimental for subsequent procedures as hot dip galvanizing, painting and welding.