In steelmaking, sulfur is considered one of the most harmful impurities. While sulfur can dissolve infinitely in liquid steel, its solubility in solid iron is extremely low, only about 0.015%–0.020%. During the solidification of molten steel, sulfur precipitates as FeS (iron sulfide), which further combines with iron to form a low-melting Fe-FeS eutectic compound. These compounds distribute along the grain boundaries in a continuous or discontinuous network, making the steel vulnerable to cracking during hot working.
If the molten steel contains high levels of FeO, FeS may combine with FeO to form another eutectic compound with an even lower melting point of 940°C, intensifying the risk of hot shortness. Since sulfur tends to segregate at the final solidification stage, the formation of such compounds becomes more pronounced, significantly reducing steel quality.
The Harmful Effects of Sulfur in Steel
Sulfur can cause multiple problems in steel products, including:
- Reduced mechanical properties– Sulfur and sulfide inclusions weaken the transverse mechanical performance of steel, particularly lowering impact toughness.
- Accelerated corrosion– In weathering steel and corrosion-resistant steel, sulfides are the root cause of localized pitting corrosion.
- Impaired magnetic properties– In ferromagnetic materials, sulfur increases iron loss and decreases magnetic permeability.
- Poor weldability– Manganese sulfide inclusions can cause hot tearing in the heat-affected zone of welds, reducing the weldability of steel.
In continuous casting, high sulfur content may also lead directly to transverse cracks in billets, posing serious challenges to production stability.
Sources of Sulfur in Steel
Sulfur enters steel mainly through raw materials, including:
- Hot metal (pig iron)
- Coke
- Ferroalloys
- Slag materials
- Scrap steel
Effective control of sulfur in raw materials is therefore essential for producing high-quality steel.
Why Is Desulfurization Impossible Without Slag?
In electric furnace steelmaking and secondary refining, most desulfurization reactions occur through the interaction between molten steel and slag. Steel slag absorbs sulfur and reacts with injected desulfurizing agents, forming sulfide-rich droplets that separate from steel. A smaller portion of sulfur is removed in the form of gaseous sulfides.
This means that slag plays an irreplaceable role in the desulfurization process. Without slag, it would be extremely difficult to achieve effective sulfur removal during steelmaking.
