Currently, the single-use converter process can achieve a phosphorus content in molten steel ranging from 40 to 100 ppm, contingent on the silicon and phosphorus levels in the molten iron. The silicon content of the molten iron is determined based on the slag volume, with P2O5 formation being essential during dephosphorization. A prevalent approach, particularly in Japan, involves less slag blowing in the converter after dephosphorizing the molten iron. Dephosphorization via the double converter method can result in molten steel with phosphorus levels as low as 40 ppm. However, it is crucial to note that hot metal dephosphorization necessitates prior desiliconization, leading to ultra-low silicon content in the converter and enabling less slag operation. Conversely, this process restricts the scrap loading ratio.
The two-stage converter process, or duplex method, involves stripping the slag from the first converter and returning it to the converter after tapping from the second converter for subsequent conversion. This approach can achieve a final phosphorus content of 30 ppm in the converter, referring to the content at the end of converter blowing. Any residual slag during tapping can cause P2O5 reduction in the slag, reintroducing phosphorus into the molten steel. Additionally, adding small amounts of phosphorus-containing alloying elements and ferromanganese can slightly elevate phosphorus levels, resulting in the final product having approximately 10 ppm more phosphorus than at the end of converter blowing.
Incorporating a ladle furnace and employing various secondary refining methods can further reduce phosphorus content. This process decreases the converter tapping temperature by about 50°C, necessitating heating in subsequent secondary refining to balance the reduced molten steel temperature. Comparisons of the phosphorus balance in the second converter of the double converter process, with converter slag containing about 18% iron, 0.4% P2O5, and a tapping temperature of 1700°C, demonstrate the potential to produce steel with 20 ppm phosphorus.
Desulfurization during BOF (Basic Oxygen Furnace) steelmaking occurs in three stages: molten iron desulfurization, in-converter desulfurization, and external molten steel desulfurization. Spraying calcium carbide, magnesium, or a blend of calcium oxide and magnesium can reduce sulfur content in molten iron to as low as 20 ppm, primarily depending on the desulfurization agent quantity. Desulfurization within the converter or electric furnace is minimal, leaving external molten steel desulfurization as the final step.
To achieve efficient desulfurization, sufficient aluminum must be added, and ladle slag must be saturated with calcium oxide. Ensuring vigorous boiling of molten steel in the ladle is crucial for optimal kinetic conditions. "Calcium oxide saturation" characterizes ladle top slag composition, with a saturation level of 1 indicating saturated calcium oxide, less than 1 indicating unsaturated, and greater than 1 indicating supersaturated. Tests reveal that under calcium oxide-saturated ladle top slag conditions, desulfurization rates can reach 95%. Unsaturated conditions decrease desulfurization efficiency due to reduced CaO activity, while supersaturated conditions also hinder efficiency.
Intense boiling in the ladle furnace induces other reactions alongside desulfurization. SiO2 in the ladle top slag reacts with [Al] to form Al2O3, increasing silicon content. Simultaneously, aluminum reacts with oxygen in the air, causing secondary oxidation. Considering these reactions, sulfur and aluminum contents vary over time, with minor aluminum loss during simple desulfurization. Reduced slag volume during boiling increases aluminum consumption, and reoxidation further burns out aluminum. Decreased Mn in ladle top slag slightly elevates furnace Mn content, while reduced SiO2 significantly increases silicon levels, adversely affecting desulfurization, especially for low-silicon steel production like thin plates and strips. Reduced boiling intensity weakens desulfurization efficiency, dropping from 92% under optimal conditions to 75% with diminished stirring, and only 35% with insufficient stirring and low airflow.
Under optimal desulfurization conditions, hot metal desulfurization can achieve sulfur contents as low as 10 or 20 ppm. For higher initial sulfur levels, molten steel desulfurization alone may suffice. Generally, molten iron desulfurization efficiency should reach 75%, with post-desulfurization sulfur content below 30 ppm. In cases of lower secondary refining efficiency (e.g., 35%), enhanced molten iron desulfurization is necessary, reducing sulfur content to about 30 ppm post-treatment to achieve a final minimum of 50 ppm. Typically, sulfur content must be reduced to 150 ppm after hot metal desulfurization to attain a final minimum of 100 ppm.
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