The work presented here demonstrated that changes to the near-surface microstructure could promote internal oxidation in a CMnSi advanced high strength steel (AHSS) grade. The starting material was industrially-supplied cold-rolled sheet. Samples with this microstructure were compared against two other conditions: annealed at 1200°C and 850°C prior to oxidation. The surface regions of both annealed samples were decarburized. All samples were then oxidized at 850°C in an argon atmosphere with approximately 10−20 atm oxygen for 5–90 minutes. The oxidation treatments were performed in a high-temperature confocal scanning laser microscope (CSLM) setup that enabled in-situ visualization of surface oxide formation. Analysis of CSLM and scanning electron microscopy (SEM) images of sample surfaces indicated there were differences in oxide morphology and coverage between samples with different starting microstructures. Cold-rolled samples showed little internal oxidation but extensive external oxidation. Samples annealed prior to oxidation exhibited both internal and external oxidation. The differences in behavior were attributed to the formation of a decarburized surface layer during the pre-anneal, which led to ferritic iron near the surface. Another result of the pre-anneal was a reduction in dislocation density, which potentially led to fewer preferential nucleation sites for oxides very close to the sample surface.
2017 by The Iron and Steel Institute of Japan