Characterization of surface layers of oxidation-reduction treated Si and Mn added advanced high strength steel

Oxidation-reduction process is an effective way to improve the coating quality of hot dip galvanized advance high strength steel (AHSS). In order to figure out the interface characteristic of the oxidized and reduced steel, laboratory oxidation-reduction experiments were conducted on 1.8 mass-%Si and 2.7 mass-%Mn added AHSS and both the interfaces of ferric oxide/substrate and reduced iron/substrate were characterized by means of GD-OES and HR-TEM. It is found that both Fe and Mn are oxidized to form a ferric oxide on the surface and a thin layer of Fe, Si and Mn oxides already occupies the interface of the ferric oxide/substrate in the oxidation step. In the reduction step, the iron oxides are fully reduced into metallic iron in which the unreduced Mn oxides are embedded. Though the external oxidation of Si can be dramatically suppressed by the reduced iron layer, the external oxidation of Mn still occurs. Moreover, the enrichment of Mn and Si at the interface of reduced iron/substrate is enhanced and the thickness of the interfacial oxides is increased by the further outward diffusion of Mn and Si in the reduction step. In addition, it is found that the interfacial oxides close to the substrate have higher Si and lower Mn, while those close to the reduced iron have higher Mn and lower Si. A formation mechanism is discussed in the end. •A thin layer of Fe, Si and Mn oxides already occupies the interface of the scale/substrate in the oxidation step.•The enrichment of Si and Mn at the interface is enhanced in the following reduction step.•The interface oxides have a two-layer structure.