In steelmaking, ferroalloys are added following the principle of deoxidizing ability from weak to strong. All ferroalloys shall be charged after slag skimming and carburization are completed, and reducing slag is formed by adding slag materials. Based on the physical and chemical properties and application scenarios of different ferroalloys, the feeding time, operational requirements, precautions and recovery rates are detailed as follows:

1. Ferrosilicon

Ferrosilicon is mainly used for alloying in the production of silicon steel, spring steel, heat-resistant steel and other steel grades. When added in large quantities, it must be thoroughly preheated to red heat. This removes hydrogen contained in ferrosilicon to avoid adverse impacts on molten steel quality and accelerates its melting.

Ferrosilicon reacts to produce silicon dioxide after being charged, which tends to reduce the basicity of local slag and impair steel quality. For this reason, an appropriate amount of lime shall be added before and after feeding ferrosilicon to stabilize slag basicity. Increase the voltage and hold power supply for several minutes to ensure full reaction of slag and form uniform white slag.

In the refining process, add ferrosilicon only when the chemical composition, temperature and slag condition of molten steel meet requirements. Tapping shall be finished within 10~25 minutes after feeding. Too short an interval will lead to incomplete melting and uneven element distribution, while an excessively long interval will cause molten steel to absorb gas and degrade steel performance.

Recovery rate: 90%~98%.

2. Ferromanganese

Ferromanganese can be added simultaneously with the formation of reducing slag. In actual production, the manganese content after initial feeding shall be controlled at the lower limit of the standard range to facilitate subsequent fine adjustment according to assay results. Manganese features stable chemical properties with low oxidation loss and simple application.

Recovery rate: Over 95%.

3. Copper

Copper is primarily applied in weathering steel production to improve steel hardenability and corrosion resistance. With excellent oxidation resistance, copper can be charged during furnace charging or oxidation period.

Considering the high cost of pure copper, manufacturers prefer to add most copper via copper-bearing pig iron, scrap steel and iron ore during the melting stage to cut costs. Only a small amount of pure copper is used for composition trimming in the reduction period to minimize the consumption of pure copper.

Recovery rate: Over 95%.

4. Ferrochrome

Chromium has a much stronger affinity with oxygen than iron and is prone to oxidation. Ferrochrome is strictly prohibited from being added during melting and oxidation periods, and shall be charged at the early stage of reduction period.

If the slag turns green after ferrochrome addition, it indicates poor deoxidation. Oxidized chromium will not only cause alloy loss, but also thicken the slag and hinder dephosphorization and normal smelting operations. Intensify reduction treatment to reduce chromium oxide in slag until the slag turns white. Chromium loss is minimized under qualified white slag conditions.

Recovery rate: Over 95% under white slag conditions.

5. Ferrovandium

Vanadium has an extremely strong affinity with oxygen and is easily oxidized. In addition, the addition of ferrovandium will cause molten steel to absorb nitrogen from the air and seriously deteriorate steel quality. Therefore, ferrovandium shall not be added too early and shall be charged in the late reduction period shortly before tapping.

Standard operation requires feeding ferrovandium 10~35 minutes before tapping. Select the lower limit for small feeding quantity and the middle to upper limit for large feeding quantity to ensure sufficient melting time.

Recovery rate: Similar to that of ferrosilicon.

6. Ferromolybdenum

Ferromolybdenum is a refractory alloy with a slow melting rate. To ensure complete melting of ferromolybdenum and uniform composition of molten steel and prevent element segregation, it shall be added at the early stage of reduction period.

Adding ferromolybdenum in the late refining stage shortly before tapping will result in incomplete melting and uneven composition, and inevitably prolong the overall smelting cycle and reduce production efficiency.

Recovery rate: Over 98%.

7. Ferroniobium

Niobium has a weak affinity with oxygen and stable chemical properties, making its composition easy to control during smelting. In conventional processes, ferroniobium is added at the early stage of reduction period. Tapping shall be conducted after more than 20 minutes to guarantee thorough melting and uniform distribution.

For non-oxidation smelting processes, ferroniobium can be charged together with furnace materials during furnace charging to simplify operation.

Recovery rate: Over 95%.

8. Ferrotungsten

Ferrotungsten is characterized by high density and melting point. It tends to sink to the furnace bottom after feeding and is difficult to melt. Meanwhile, tungsten has a strong affinity with oxygen. Tungsten will be oxidized during melting period and exist in slag in the form of calcium tungstate, causing alloy loss and increasing difficulty in composition control.

The bulk of ferrotungsten shall be added at the early stage of reduction period, with only a small portion reserved for composition trimming in the late reduction period. Mass addition during melting and late refining stages is forbidden. Ferrotungsten shall be crushed into small pieces and preheated to red heat to improve melting efficiency.

Recovery rate: Over 95%.

9. Aluminum (Alloying Element)

Aluminum has ultra-strong deoxidizing capacity and is extremely susceptible to oxidation, so it is added at the latest stage right before tapping. Two operating methods are adopted according to aluminum content in steel grades:

For steel with aluminum content below 0.2%: No slag skimming is required. Insert aluminum blocks into the furnace 2~5 minutes before tapping. The recovery rate is about 50% for common steel, and can reach 55% for titanium-containing steel.

For high-aluminum steel: To prevent silicon reversion in slag after aluminum feeding which leads to excessive silicon content in finished products, remove all reducing slag completely before adding aluminum blocks. Then add lime accounting for 2%~3% of molten steel weight and low-silicon fluorite. Turn on power to homogenize slag and then tilt the furnace for tapping. The recovery rate ranges from 65% to 88%.

10. Ferroboron

Boron readily combines with oxygen and nitrogen in molten steel to form harmful inclusions. Appropriate aluminum and titanium must be added in advance for pre-deoxidation and nitrogen stabilization before feeding ferroboron. Ferroboron is added right before or during tapping via two mainstream processes:

Ladle feeding method: Enlarge the tapping hole in advance and increase furnace tilting speed with strict slag retaining measures. Spread lime at the tapping hole and block it with wood rake for slag retention. Let molten steel flow into the ladle first, and add ferroboron by throwing or inserting when the ladle is filled with about one-third molten steel, then allow slag to flow out.

Furnace insertion method: After adding aluminum and titanium, fix ferroboron on an iron rod and wrap it with aluminum sheet or straw paper. Insert it into molten steel quickly, stir thoroughly and tap immediately. This method delivers more uniform boron distribution and better internal quality of finished steel.

The recovery rates of the two methods are basically the same.

Recovery rate: 45%~85%, which can be further increased under special working conditions.

11. Ferrotitanium

Titanium has an extremely strong affinity with oxygen and nitrogen and is easily oxidized and nitrided to form inclusions in steel. Ferrotitanium shall be added after stable white slag is formed, and tapping must be completed within 5~15 minutes after feeding.

Place ferrotitanium near the furnace door and away from the electric arc to reduce burning loss. Ferrotitanium has low density and floats on the interface between slag and molten steel before gradually melting into the steel. Its recovery rate fluctuates greatly affected by furnace temperature, slag condition and operating parameters. Long retention of ferrotitanium in the furnace will not only reduce the recovery rate significantly, but also deteriorate the quality of molten steel.

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