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Description
The selective oxidation of Boron in bake hardening steel sheet is investigated in a hot dip process simulator for a typical DFF RTF cycle with two levels of pre-oxidation. The time in the RTF (85, 125 and 220 seconds), the RTF dew point (-40, -25 and -10 oC) and the hydrogen content (1, 3 and 5 %) of the RTF atmosphere are varied. The samples were analysed with FEG-SEM, EDX and GDOES. A few cases were analysed with Auger to get more information about the condition of the outer surface.
Both the annealed substrate as the inhibition layer formed in a MagiZinc bath containing 1.6 % Aluminium and 1.6 % Magnesium was analysed.
The amount of Boron enrichment at the surface increases when the dewpoint is decreased from -10 to -40 oC which is an indication of more external oxidation of Boron. Pre oxidation leads to a lower Boron enrichment at the surface. The effect of pre oxidation is biggest for the cases with lower dewpoints (-40 and -25 oC) and it is limited for a dewpoint of -10 oC which is in accordance with results from literature. The longer times in the RTF and soak showed saturation of Boron at the surface for a dewpoint of -40 oC, this did not happen for the higher dewpoints.
The inhibition layer showed small holes where we could still detect oxides on the substrate. In some cases evidence of an inhibition layer was still found that can indicate reduction of oxides by the MagiZinc bath.
Keywords
Galvanising, selective oxidation, boron, dew-point, pre-oxidation