Steelmaking process – secondary refining
For purposes of this article, Secondary Refining or Secondary Metallurgy is defined as any the post steelmaking process performed at a separate station prior to casting and hot metal desulfurization. It does not include normal alloying practices in the furnace or at the ladle during tapping. This definition is not universally applied and many include operations at the tundish, electromagnetic stirring in the mold, etc.
The purposes of secondary refining are many: temperature homogenization or adjustment; chemical adjustments for carbon, sulfur, phosphorus, oxygen and precise alloying; inclusion control; degassing, and others.
The equipment and processes are equally varied. Secondary Refining processes are performed at atmospheric pressure or under a vacuum, with or without heating, solids and/or gas injection, and stirring. Fully nine different vacuum processes are sold for carbon steels alone.
A Ladle Furnace is used to relieve the primary melter of most secondary refining operations and its primary functions are:
- Reheating of liquid steel through electric power conducted by graphite electrodes
- Homogenization of steel temperature and chemistry through inert gas stirring
- Formation of a slag layer that protects refractory from arc damage, concentrates and transfers heat to the liquid steel, trap inclusions, and metal oxides, and provide the means for desulphurization.
Secondary functions that can be included with a ladle furnace are:
- Alloy additions to provide bulk or trim chemical control
- Cored wire addition for trimming or morphology control
- Provide a means for deep desulphurization
- Provide a means for dephosphorization
- Act as a buffer for downstream steelmaking equipment
The function of the porous plug is to provide gas stirring of the molten metal to promote homogenization. Normal stirring operations are performed by percolating argon gas through a purge plug arrangement in the bottom of the ladle. A top lance mechanism serves as a backup means for bath stirring in the event the plug circuit in the ladle is temporarily inoperable. The gas supply connection to the ladle is automatically made when the ladle is placed on the transfer car.
The ladle roof is typically a water-cooled design with a refractory center or delta section and is configured to coordinate with existing ladles such that the roof will completely cover the top portion of the ladle when in the operating (i.e. fully lowered) position.