The Question
In ferrosilicon smelting, coke is used as a reducing agent to make the self-baking electrode easier to carburize. Coke bricks are used to build the tap hole and the iron flow trough, graphite powder may be used to coat the ingot mold, and a carbon sample spoon is used to take liquid samples. In short, during the smelting of ferrosilicon from the reaction in the furnace until the iron is tapped, there are many opportunities for contact with carbon during the pouring process. Yet the carbon content of ferrosilicon is less than 0.1 percent, generally 0.05-0.07 percent. This raises an obvious question: why is the carbon content so low?
The Explanation
The solubility curve of carbon in ferrosilicon at about 1187 deg C shows that the higher the silicon content in ferrosilicon, the lower the carbon content. According to published data, when the silicon content in ferrosilicon is greater than about 30 percent, most of the carbon in ferrosilicon exists in the state of silicon carbide, SiC.
Silicon carbide is easily oxidized and reduced by silicon dioxide or silicon monoxide in the crucible. Silicon carbide has very little solubility in ferrosilicon, especially when the temperature is low, and it is easy to precipitate and float. Therefore, the silicon carbide remaining in ferrosilicon is very low, so the carbon content of ferrosilicon is very low.
Why Carbon Control Matters
The carbon content in ferrosilicon needs to be controlled, because carbon easily reacts with oxygen to produce carbon monoxide, which can create bubbles in the molten steel and affect the quality of the steel. During the production of ferrosilicon, the carbon content is inherently low for the metallurgical reasons described above, which is exactly what downstream steelmakers require from the alloy.
Frequently Asked Questions
Why does high-silicon ferrosilicon contain less carbon? The higher the silicon content, the lower the carbon solubility; most carbon exists as SiC, which has very low solubility and is easily removed by oxidation and precipitation.
What happens to SiC in ferrosilicon? It is oxidized and reduced by SiO2 or SiO in the crucible, and because its solubility is low it precipitates and floats out of the alloy.
Why does carbon content matter to steelmakers? Carbon reacting with oxygen forms CO bubbles in molten steel, which can affect steel quality.
What is the typical carbon content of ferrosilicon? Less than 0.1 percent, generally 0.05-0.07 percent.
At what temperature is the carbon solubility curve relevant? The solubility relationship is often referenced at about 1187 deg C.
Where does carbon come into contact with ferrosilicon during smelting? Coke as reductant and electrode material, coke bricks in the tap hole, graphite powder on the mold, and carbon sample spoons all create contact opportunities.
