Secondary Oxidation of Molten Steel during Continuous Casting
Continuous casting is the process whereby molten steel is poured into a mould and solidified continuously in a single pass. This process is used in steel production and is often used in combination with an oxidation/stabilization treatment that involves subjecting the steel to controlled exposure to oxygen-containing atmosphere, allowing oxide walls to form between the liquid and solid surfaces. This oxide wall controls the localised stresses on the molten steel, thereby preventing surface cracking and wastage of the molten steel. However, during casting, molten steel can undergo secondary oxidation due to periodic exposure to external air, leading to losses of alloying elements, changes in surface properties and increased element segregation.
Secondary oxidation of molten steel during continuous casting is difficult to control and the formation of oxide scales can seriously affect the steel casting process. The oxide scales formed can reduce the strength and stability of the interface between the liquid and solid faces of the steel, increasing the probability of thermal shock when subject to cooling. The oxide scales can also decrease the quality of the steel product by reducing the metallic and fluid flow properties. To prevent these issues, careful oxidation/stabilization treatment is required.
The oxidation/stabilization treatment involves subjecting the molten steel to a controlled atmosphere containing oxygen-containing gases, typically nitrogen (N2) and oxygen (O2). The oxidation rate of the molten steel is typically controlled by the temperature and oxygen partial pressure of the atmosphere, or by a combination of these parameters. During the oxidation/stabilization treatment, the nitrogen and oxygen form an oxidizing agent, which slowly oxidizes the surface of the steel and forms a protective oxide film. The oxygen partial pressure should be sufficiently low to ensure that oxidation of the molten steel occurs within an acceptable rate and does not become excessive which can lead to substantial losses of alloying elements from the molten steel. This partial pressure is typically kept within a range of 0.0 – 0.05 atm O2.
The oxidation/stabilization treatment can also be used to improve the casting surface finish and to reduce the amount of scale deposit on the casting surface. This is achieved through the formation of a thin oxide film which acts as a lubricant, decreasing the friction between the casting and the surface. This reduced friction helps in preventing the build-up of oxide scales on the casting surface. The oxide film also acts as an insulative barrier protecting the casting surface from further oxidation.
To optimise the oxidation/stabilization process and to minimise secondary oxidation of molten steel during continuous casting, careful management of the oxygen partial pressure should be followed. The oxygen partial pressure should be kept as low as possible and should be monitored regularly using methods such as the oxygen-containometer and the oxygenation index. The rate of oxidation should also be monitored to ensure that it is within acceptable levels. Furthermore, surface inspection of the steel should be conducted periodically to ensure optimal steel quality and to detect any problems with the process.
In conclusion, secondary oxidation of molten steel during continuous casting is a complex process and can affect the quality of the steel product if not kept under control. Careful oxidation/stabilization treatments need to be applied and the oxygen partial pressure should be kept as low as possible and regulated to ensure that the oxidation rate is within acceptable limits. Additionally, regular monitoring of the steel should be conducted to ensure optimal quality.
