Stirring is an essential operation in secondary steelmaking. By creating controlled circulation within molten steel, it improves heat and mass transfer, resulting in more uniform temperature, consistent chemical composition, and better steel cleanliness. Depending on the refining process, stirring can be achieved mechanically, by gravity, through gas injection, or by electromagnetic force.
Mechanical Stirring
Mechanical stirring uses rotating impellers or similar devices to agitate molten metal. Although widely used in industries operating at lower temperatures, it has only limited application in steelmaking because of the harsh conditions of molten steel. Today, it is mainly found in hot metal pretreatment rather than secondary steel refining.
Gravity-Assisted Stirring
Some refining processes generate circulation naturally through molten steel flow. For example, RH and DH vacuum refining continuously circulate steel between the ladle and the vacuum vessel, creating mixing without dedicated stirring equipment. While effective for these specific processes, the stirring intensity is largely determined by the process itself and cannot be adjusted independently.
Gas Stirring
Gas stirring is the most widely used stirring method in modern secondary refining. It is commonly applied in processes such as VD, VOD, AOD, LOD, and LRF.
In a typical ladle refining system, argon is injected through porous plugs or bottom nozzles. As the gas bubbles rise, they carry molten steel upward, while surrounding steel flows downward along the ladle wall to replace it. This continuous circulation—often referred to as the bubble pump effect—creates efficient mixing throughout the molten bath.
The controlled circulation provides several important metallurgical benefits:
- Promotes uniform temperature and chemical composition throughout the ladle
- Accelerates refining reactions and improves deoxidation efficiency
- Increases the flotation and removal of non-metallic inclusions
- Improves alloy recovery through better mixing
- Reduces segregation and casting defects while enhancing steel cleanliness
- Helps minimize secondary oxidation under controlled refining conditions
Because of these advantages, bottom gas stirring has become a standard feature in modern secondary steelmaking and is suitable for producing structural steel, bearing steel, stainless steel, electrical steel, and many other steel grades.
SME Group’s IF + LOD + LRF Secondary Refining Solution
SME Group has developed an integrated IF + LOD + LRF secondary refining process for induction furnace steelmaking.
A key feature of the LOD (Ladle Oxygen Decarburization) process is its flexible bottom-blowing system. In addition to conventional argon stirring, the process can utilize recovered industrial gases with CO₂ bottom blowing under suitable operating conditions. This approach supports efficient refining while improving industrial gas utilization.
Combined with LRF refining, the integrated process helps achieve stable temperature control, effective mixing, and improved steel cleanliness for a wide range of steel products.
Electromagnetic Stirring
Electromagnetic stirring generates molten steel flow through electromagnetic induction instead of gas injection. It provides accurate control of flow patterns and can also reduce heat loss during stirring. Electromagnetic stirring is widely used in large-capacity furnaces and continuous casting, where precise control of molten steel flow is particularly important.
Choosing the Right Stirring Method
Each stirring technology serves a different purpose. Mechanical and gravity-assisted stirring are limited to specific applications, while electromagnetic stirring is typically selected for large-scale installations. For most ladle refining operations, however, bottom gas stirring remains the preferred solution because it combines excellent mixing performance, flexible process control, and proven metallurgical benefits.
For steel plants using induction furnaces, SME Group has developed an integrated IF+LOD+LRF refining solution. Because induction furnaces offer limited refining capability, scrap quality is normally a critical constraint. The addition of LOD with bottom gas stirring enables decarburization and dephosphorization, significantly broadening scrap selection. Together with LRF, the process provides a practical solution for producing higher-quality steel from induction furnace-based operations.
