Surface oxidation of low carbon steel strip during batch annealing

Selective oxidation of low carbon strip steels industrially batch annealed in 4% hydrogen 96% nitrogen atmosphere was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the selective oxidation was not uniform across the width of the steel strip. The non-uniformity of the selective oxidation across the strip width produced the localised formation of (MnFe)O and MnCr2O4 particles along the ferrite grain boundaries, mainly in the vicinity of the strip edge region. The distribution of the MnCr2O, particle sizes within this region can explain the generation of a defect on the final tinplate product, which manifests as a localised band of low reflectivity after tinplating of batch annealed strip, 'edge defect' that can be produced on batch annealed tinplate. The propensity for nitrogen absorption was also found to vary significantly across the strip width and was limited in the region where selective oxidation occurred. This non-uniformity in selective oxidation and nitrogen absorption is probably due to the changes in the dewpoint or oxygen potential of the annealing gas across the width of the strip.