Influence of Gas Atmosphere Oxygen Partial Pressure on the Selective Oxidation during Intercritical Annealing of a 6Mn-2Si Advanced High Strength Steel

摘要

Selective oxidation of a 6Mn-2Si advanced high strength steel at an intercritical annealing temperature (IAT) of 665°C as a function of IAT holding time and process atmosphere oxygen partial pressure was investigated. Based on the modified Wagner model for the prediction of the transition between external/internal oxidation, the effect of process atmosphere oxygen partial pressure on the alloy selective surface oxidation was assessed by employing dew points (dp) of -50°C, -30°C and +5°C in a N_2-5%H_2 process atmosphere such that different modes of oxidation could be studied. SEM surface observations of the samples annealed under the -50°C dp process atmosphere clearly showed external enrichment of the alloying elements in the form of a coarse-grained oxide film covering the surface, particularly at extended holding times. At the intermediate oxygen partial pressure range (-30°C dp), a 200 - 500 nm oxide particles were detected on the surface along with smaller nodule-like particles formed between them. This changed to a rather oxide-free surface after annealing at the +5°C dp, indicative of internal oxidation. However, sparse surface enrichment was revealed at longer holding times at this oxygen partial pressure. Auger elemental maps acquired from the -30°C dp + 600s sample provided a much better understanding of the surface oxides, showing enrichment of Si in some areas of the aforementioned samples.