The tensile failure of mild steel oxides under hot rolling conditions

The failure of oxide scales formed on mild steel (mass contents of 0.17% C, 0.13% Si, 0.72% Mn) was investigated using a high-temperature tensile test technique over the temperature range 830 - 1150 degrees C. Strain and strain rates used were 1.5 - 20% and 0.02 - 4.0 s(-1), respectively. The scales were 10 - 300 mu m thick. The variations of these test parameters were chosen to approximate the tensile loading of the oxidised surface layer at the slab faces just before roll contact at the upper or lower surfaces. Oxide scales formed cannot be assumed either to be perfectly adhering during tensile loading, in the sense of slipping, or to be fully brittle. Two limit modes leading to oxide spallation have been observed which are strongly influenced by the temperature, strain rate and strain. For the first mode initial through-scale cracking occurs followed by initiation and propagation of a crack along the oxide-metal interface between adjacent cracks. The second mode corresponds to the slipping of the oxide scale relative to the metal surface before spallation. The delamination and following slipping of the oxide raft can take place along the interface within the non-homogeneous oxide scale.