In electric arc furnace steelmaking, slag is not just a byproduct. It plays a vital role in metallurgical reactions, especially through its oxidizing ability.
The oxidizing ability of slag refers to its capacity to supply oxygen to molten steel (the metal phase in the bath). In other words, it describes how slag removes impurities such as phosphorus, carbon, and inclusions from the steel through oxidation. At a given temperature, this ability can be measured by the amount of oxygen transferred from the slag to the steel per unit of time. It directly determines the effectiveness of dephosphorization, decarburization, and steel refining, making it a crucial factor in the steelmaking process.
How Does Slag Oxidize Molten Steel?
Slag can oxidize molten steel mainly through two mechanisms:
- FeO Partitioning
Iron oxide (FeO) in slag distributes between the slag and the steel bath. This transfer allows FeO to participate in oxidation reactions within the steel. - Oxygen Ion Transfer
Slag contains various ions such as Ca²⁺, Mn²⁺, Fe²⁺, O²⁻, and SiO₄⁴⁻. Among them, O²⁻ has the strongest affinity for iron and the smallest ionic radius, making it the easiest to migrate from slag to steel. This transfer is accompanied by the migration of Fe²⁺ to maintain electrical neutrality. In this way, FeO acts as the primary carrier of oxygen transfer in steelmaking reactions.
How to Express and Evaluate the Oxidizing Ability of Slag
In metallurgical practice, the oxidizing ability of slag is usually expressed by its iron oxide (FeO + Fe₂O₃) content. The key influencing factors are:
- Slag composition: A higher FeO content means stronger oxidizing power.
- Temperature: At a constant FeO content, higher steelmaking temperature increases the oxygen content in molten steel, which enhances the oxidizing ability of slag.
This explains why decarburization proceeds faster at higher temperatures compared to lower temperatures.
