No abstract

The development of martensitic grades which can be processed in continuous galvanizing lines requires the reduction of the oxides formed on the steel during the hot dip process. This reduction mechanism was investigated in detail by means of High Resolution Transmission Electron Microscopy (HR-TEM) of cross-sectional samples. Annealing of a martensitic steel in a 10% H2 + N2 atmosphere with the dew point of-35 ℃ resulted in the formation of a thin c-xMnO.SiO2 (x>1) oxide film and amorphous a-xMnO.SiO2 oxide particles on the surface. During the hot dip galvanizing in Zn-0.13%Al, the thin c-xMnO.SiO2 (x>1) oxide film was reduced by the Al. The a-xMnO.SiO2 (x<0.9) and a-SiO2 oxides however remained embedded in the Zn coating close to the steel/coating interface. No Fe2Al5-xZnx inhibition layer formation was observed. During hot dip galvanizing in Zn-0.20%Al, the c-xMnO.SiO2 (x>1) oxide film was also reduced and the amorphous a-xMnO.SiO2 and a-SiO2 particles were embedded in the Fe2Al5-xZnx inhibition layer formed at the steel/coating interface during hot dipping. The results clearly show that Al in the liquid Zn bath can reduce the crystalline c-xMn.SiO2 (x>1) oxides but not the amorphous a-xMnO.SiO2 (x<0.9) and a-SiO2 oxides. These oxides remain embedded in the Zn layer or in the inhibition layer, making it possible to apply a Zn or Zn-alloy coating on martensitic steel by hot dipping. The hot dipping process was also found to deteriorate the mechanical properties, independently of the Zn bath composition.