The secondary steelmaking process is critical to production of high quality steels and providing a buffer between the primary steelmaking furnaces and casters.
Slag chemistry using high calcium lime, dolomitic lime and lime blended products with argon stirring are vital in this process which may include use of a ladle metallurgical furnace (LMF) or stir station for desulfurization, chemistry trim additions, temperature adjustments and removal of inclusions from the liquid steel in the ladle. Vacuum degassers such as the tank degasser (VD) and vacuum argon degasser (VAD) provide a final step in this process to reduce unwanted dissolved hydrogen and reduce dissolved oxygen contents by lowering the atmospheric pressure of the liquid steel in the ladle in a contained sealed environment.
In the stainless steelmaking process, the argon oxygen decarburization furnace (AOD) receives liquid stainless steel from the EAF and refines the steel for carbon removal and chemistry control. The use of lime products is key to neutralizing “acid oxides” from the oxidation of silicon and aluminum during the oxidation process of stainless steelmaking in the (AOD). It is the CaO/SiO2 +Al2O3 relationship that influences the MgO demand of the slag. The slag condition during the reduction stage must provide a medium where the oxidized chrome can be returned to the liquid steel, a cost effective practice.
- High Calcium pebble lime is key to producing a fluid reducing slag that provides for the removal of sulfur, insulation from atmospheric re-oxidation, slag line protection from arc flare in the LMF and chemical attack on the refractory from additional acidic oxides during deoxidation of the liquid steel.
- Dolomitic pebble lime additions provide the MgO saturation for basicity conditions to provide the refractory protection. The slag contains dissolved calcium compounds and acidic compounds from alloy additions during transfer from the primary furnaces that cause MgO to dissolve from refractory brick into the slag. Dolomitic lime provides the MgO to reduce refractory consumption.
- FerroDolime is a specialized product that provides the same benefits as dolomitic lime in the AOD stainless steelmaking, but due to the di-calcium ferrite coating on the pebble it provides additional environmental benefits due to its mechanical strength from the coating and provides protection from hydration. Additionally it helps reduce the consumption of oxygen as a result of the iron oxide content of the product during the oxidation process.