What is a Ladle Furnace?

Secondary Refining Ladle Furnace Overview

Secondary refining, also known as secondary metallurgy, refers to any post - steelmaking process carried out at a distinct station before casting and hot metal desulfurization. It excludes the normal alloying practices that occur in the furnace or at the ladle during tapping. However, this definition is not uniformly adopted, as many also incorporate operations at the tundish and electromagnetic stirring in the mold, among others.

The purposes of secondary refining are diverse, encompassing temperature homogenization or adjustment, chemical adjustments for elements such as carbon, sulfur, phosphorus, and oxygen, precise alloying, inclusion control, degassing, and more. Correspondingly, the equipment and processes involved are equally varied. Secondary refining processes can be carried out at atmospheric pressure or under a vacuum, with or without heating, and may involve the injection of solids and/or gases as well as stirring. For carbon steels alone, there are as many as nine different vacuum processes available on the market.

Description and Operation of a Ladle Furnace

A ladle furnace is employed to relieve the primary melter of the majority of secondary refining operations. Its primary functions are as follows:

Primary Functions

1. Reheating of Liquid Steel: It reheats liquid steel through electric power conducted by graphite electrodes. This ensures that the steel maintains the appropriate temperature for subsequent processing steps.
2. Homogenization of Steel Temperature and Chemistry: By using inert gas stirring, it achieves a uniform distribution of temperature and chemical composition throughout the steel. This helps in improving the quality and consistency of the final product.
3. Slag Layer Formation: It forms a slag layer on the surface of the liquid steel. This slag layer serves multiple purposes: it protects the refractory lining from arc damage, concentrates and transfers heat to the liquid steel, traps inclusions and metal oxides, and provides a means for desulfurization.

Secondary Functions

In addition to the primary functions, a ladle furnace can also perform the following secondary functions:

1. Alloy Additions: It allows for the addition of alloys to provide bulk or trim chemical control. This enables precise adjustment of the steel's chemical composition according to specific requirements.
2. Cored Wire Addition: Cored wire can be added for trimming or morphology control. This helps in achieving the desired microstructure and properties of the steel.
3. Deep Desulfurization: It provides a means for deep desulfurization, reducing the sulfur content in the steel to very low levels, which is crucial for certain applications where low - sulfur steel is required.
4. Dephosphorization: It can also be used for dephosphorization, removing phosphorus from the steel to improve its quality and performance.
5. Buffer for Down - stream Steelmaking Equipment: It acts as a buffer, regulating the flow of steel to downstream steelmaking equipment and ensuring a smooth and continuous production process.

Porous Plug and Gas Stirring

The porous plug in a ladle furnace is designed to provide gas stirring of the molten metal, promoting homogenization. Normal stirring operations are carried out by percolating argon gas through a purge plug arrangement located at the bottom of the ladle. In case the plug circuit in the ladle is temporarily inoperable, a top lance mechanism serves as a backup means for bath stirring. The gas supply connection to the ladle is automatically established when the ladle is placed on the transfer car.

Fume and Particulate Control

During heating and alloying operations at the ladle furnace, fumes and particulates are generated. These emissions exit the water - cooled ladle roof through various openings. They are then captured (entrained) by ambient air drawn into a lateral draft type fume collection hood mounted on supporting structures above the ladle roof. The ladle roof typically features a water - cooled design with a refractory center or delta section. It is configured to coordinate with existing ladles so that when in the operating (fully lowered) position, it completely covers the top portion of the ladle, effectively containing the emissions.
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