Effect of Cr on the Oxidation of Advanced High Strength Steels during Annealing Prior to Galvanizing

摘要

The effect of Cr in Fe-Mn based steels on the oxidation during isothermal annealing at different dew points was investigated. Specimens with different compositions (i.e. Fe-1.7%Mn, Fe-1.8%Mn-0.6%Cr-0.5%Si, Fe-1.8%Mn-l.l%Cr-0.5%Si and Fe-2.8%Mn-0.6%Cr-0.5%Si, all in mol%) were annealed at 950 °C for 1 hour in a gas mixture of N_2 + 5 vol.% H_2 with dew points of -45, -30, -10 and 10 °C, respectively. For comparison with the observed oxide phases, the phase diagrams of Fe-Mn-Cr and Fe-Mn-Cr-Si alloys in O_2 environment were computed. The identified oxide species after annealing are in good agreement with these phase diagrams. Adding Cr to the Fe-Mn steels leads to the formation of (Mn,Cr,Fe)_3O_4 spinel oxide during annealing. The addition of Cr promotes the formation of external oxides (Mn,Cr,Fe)_3O_4 spinel and (Mn,Fe)0. However, the amount of (Mn,Fe)0 increases while the amount of (Mn,Cr,Fe)_3O_4 spinel decreases with increasing dew point. The Si in the steels results in the formation (Mn,Fe)_2SiO_4 which is more stable than (Mn,Cr,Fe)_3O_4 spinel and (Mn,Fe)0. Adding Si to the alloy exerts an adverse effect as adding Mn on the formation of (Mn,Cr,Fe)_3O_4 spinel and (Mn,Fe)0.