Influence of the Oxidation-Reduction Process on the Surface and Sub-surface Microstructure of Intercritically Annealed CMnSi TRIP Steel
The surface and sub-surface of an intercritically annealed Si-bearing CMnSi TRIP steel were investigated by high resolution transmission electron microscopy after pre-oxidation in a N2+0.3 vol-% O2 atmosphere and subsequent reduction in a N2+10 vol-% H2 atmosphere. After the initial pre-oxidation the surface consisted of areas covered with a thick Fe3O4 oxide with an open porous structure and areas covered with a thin compact Fe3O4 film connected to the steel by xMnO.SiO2 oxide bridges connecting the oxide film to the matrix. Both porous and compact oxide contained embedded grains of MnO. A grain boundary network of amorphous SiO2 was formed in the steel subsurface under the porous Fe3O4 after the pre-oxidation treatment. No internal oxidation was observed below the compact Fe3O4 layer. Whereas, the Fe3O4 was fully reduced to metallic Fe after reduction annealing, the selective oxides MnO, c-xMnO.SiO2 (x>1) and a-xMnO.SiO2 (x<0.9) were unaffected by the reduction annealing. As no amorphous a-SiO2 or a-xMnO.SiO2 (x<0.9) film forming oxides were present at the surface of the Si-bearing TRIP steel after the pre-oxidation and reduction annealing, the treatment is believed to lead to an improved wettability of the surface by liquid Zn during hot dip galvanizing and Zn-coatings which are free of bare spot defects. Zn-coating adhesion problems may however arise from the formation of large pores at the Fe3O4/steel interface, which are still present after the oxide reduction.