Intergranular and Internal Oxidation during Hot-rolling Process in Ultra-low Carbon Steel

Intergranular and internal oxidation in slab reheating process for hot-rolling was investigated for ultra-low carbon steel with 0.01 mass% Si. Intergranular oxidation depth significantly increased at 1 473 K because of melting of Fe–Si–O complex oxides and decreased with increasing oxidation temperature at above 1473 K. The depth increased with increasing oxidation time in the range of 1800–5 400 sec and decreasing partial pressure of oxygen in the range of 1–10%. The maximum interval between intergranular oxidation was 210μm, while austenite grain size observed at high temperature by a confocal laser microscope was over 1 mm. The size of "quasi-austenite" grains considered sites of ferrite grains was consistent with this interval. Internal oxides were formed underneath the scale and around the intergranular oxides. The thickness of internal oxidation layer extending beyond the furthest point of permeation by intergranular oxide increased with increasing oxidation temperature, time and with decreasing partial pressure of oxygen. The oxidation loss of substrate increased with increasing oxidation temperature, time and partial pressure of oxygen. The intergranular oxidation depth and the thickness of internal oxidation layer can be theoretically calculated by using oxidation loss rate of substrate and oxygen diffusion rate in austenite.