The Morphologies of Different Types of Fe2SiO4–FeO in Si-Containing Steel

Red scale defect is known to be mainly caused by net-like Fe2SiO4–FeO. In the present study, the morphology of Fe2SiO4–FeO in a Si-containing steel was investigated by simultaneous thermal analysis, high-temperature laser scanning confocal microscopy, scanning electron microscopy, and energy dispersive spectroscopy. Only liquid Fe2SiO4–FeO can form a net-like morphology. Liquid Fe2SiO4–FeO is classified into two types in this work. Type-1 liquid Fe2SiO4–FeO is formed by melting pre-existing solid Fe2SiO4–FeO that already exists before the melting point of Fe2SiO4–FeO. Type-2 liquid Fe2SiO4–FeO is formed at a temperature higher than the melting point of Fe2SiO4–FeO. The results show that type-1 liquid Fe2SiO4–FeO is more likely to form a net-like morphology than is type-2 liquid Fe2SiO4–FeO. The penetration depth of type-1 liquid Fe2SiO4–FeO is also larger at the same oxidation degree. Therefore, type-1 liquid Fe2SiO4–FeO should be avoided in order to eliminate red scale defect. Net-like Fe2SiO4–FeO may be alleviated by two methods: decreasing the oxygen concentration in the heating furnace before the melting point of Fe2SiO4–FeO is reached and increasing the reheating rate before the melting point. In addition, FeO is distributed with a punctiform or lamellar morphology on Fe2SiO4.