1. What is blowing loss? What components make it up?
During the converter blowing process, the amount of steel tapped is always less than the amount charged. The amount of metal lost during this process is referred to as blowing loss.
Blowing loss consists of several components, including chemical loss, dust loss, loss due to iron beads and iron oxide in the slag, and spatter loss.
(1) Chemical Losses: To convert molten iron and scrap steel into molten steel, carbon, phosphorus, and sulfur must be removed, while silicon and manganese also undergo oxidation.
(2) Fume Losses: During the blowing process, iron in the central zone of the oxygen lance is oxidized, producing reddish-brown fumes that are lost as they are expelled with the furnace gas. Fume losses generally account for 0.8% to 1.3% of the metal charge.
(3) Iron Oxide Loss in Slag: The slag contains iron oxide, and iron is lost when the slag is tapped during slag removal.
(4) Iron Bead Loss in Slag: In converter slag, some molten metal droplets suspend in the slag to form iron beads, which are lost along with the slag during tapping.
(5) Splatter Losses During converter blowing, improper operational control can cause splatter, potentially ejecting some metal from the furnace along with the slag, resulting in metal loss. The amount of splatter varies depending on raw material conditions and operational proficiency, generally accounting for 0.5% to 2.5% of the charge.
The primary component of blowing losses is chemical loss, followed by slag iron loss and spatter loss. While chemical loss is unavoidable, measures can be taken to reduce slag iron loss and spatter loss.
Iron and Steel uses top-and-bottom blowing in their 50-metric-ton converter. This process produces Q195 steel with minimal spatter.
Lower spatter levels reduce slag FeO content. This results in decreased iron and spatter losses.
Measured Blowing Loss Data:
Total Loss: 7.43%.
Chemical Loss: 4.38%.
Slag Iron Loss: 1.38%.
Spatter Loss: 0.96%.
Dust Loss: 0.71%.
2. Why is preventing spatter so important during converter operation?
Spatter is a common phenomenon during the operation of top-blown converters. Types of spatter include explosive spatter, foamy spatter, and metal spatter. Statistics show that during major spatter events, metal loss is approximately 3.6%, while even minor spatter results in metal loss of around 1.2%. Some researchers estimate that spatter causes metal losses ranging from 0.5% to 5%. Preventing spatter is equivalent to increasing steel output. Spatter caused by improper operation can severely erode the furnace lining and lead to accidents such as steel adhesion to the oxygen lance. The ejection of large amounts of slag also impairs the removal of phosphorus (P) and sulfur (S), results in heat loss, affects operational stability, and limits further increases in oxygen supply intensity. Therefore, preventing spatter is of utmost importance during converter operation.
3. What is the root cause of explosive spatter?
The root cause of explosive spatter is the uneven progression of the carbon-oxygen reaction within the molten pool, which leads to the instantaneous generation of large amounts of CO gas.
Top-blown converters decarburize rapidly, resulting in a large instantaneous discharge of CO gas. This gas possesses significant energy, which drives the movement of molten steel in the bath. High Decarburization Rate: When the rate is 0.4% to 0.6%, the molten pool releases massive amounts of CO gas.
Large Gas Volume: The CO discharge per second is 3 to 4.5 times the volume of the molten steel.
High Agitation Energy: This gas generates 7 to 9 times the agitation energy of the oxygen flow.
Forced Eruption: Discharging this high-energy gas instantaneously forces molten steel and slag out of the furnace nozzle.
Under normal conditions, carbon oxidizes uniformly, and the resulting CO gas is discharged evenly, preventing violent spattering. During intense carbon oxidation, carbon is highly sensitive to temperature changes. Operational factors can cause the molten pool to cool abruptly. This inhibits the carbon-oxygen reaction and causes FeO to accumulate rapidly.
Once the bath temperature exceeds 1470°C and $w(TFe)$ exceeds 20%, the reaction resumes intensely. This causes a sudden, high-energy surge of CO gas to escape through the furnace mouth. Consequently, this eruption carries significant amounts of molten steel and slag out of the furnace.
Example: Improper Charging: Adding the second batch of slag charge incorrectly can trigger this sudden, major spatter shortly after the addition.
It can be concluded that explosive spatter is likely to occur when the slag is excessively oxidizing and the molten pool temperature is suddenly cooled.
4. What are the principles and specific measures for preventing explosive spatter?
The principles for preventing explosive spatter are as follows.
① Control the molten pool temperature effectively. Here are the operational guidelines for temperature control:
Balance Temperature: Avoid excessively low early-stage temperatures and excessively high middle-to-late stage temperatures.
Ensure Uniformity: Maintain a steady and uniform temperature rise throughout the process.
Prevent Sudden Cooling: Strictly prohibit sudden cooling of the molten pool.
Eliminate Explosions: These steps ensure the carbon-oxygen reaction proceeds evenly and prevents explosive reactions.
② Control the TFe content in the slag to ensure that TFe does not accumulate, thereby preventing excessive foaming of the slag or triggering explosive carbon-oxygen reactions.
Specifically, attention should be paid to the following situations.
① Overemphasizing rapid slag formation in the early stage. Operating with an excessively high lance position causes the temperature to rise slowly in the early stage, leading to excessive TFe accumulation. Once carbon begins to oxidize violently, this often results in major spattering.
② Using a method of adding small batches multiple times helps prevent major sputtering caused by the second batch of slag material cooling the molten pool.
③ When addressing “slag drying out” or accelerating the formation of the final slag, adding excessive fluorite or operating at an excessively high lance position can cause the final slag to form too early or lead to TFe accumulation. Since carbon oxidation is still intense at this stage, this can also result in major sputtering.
④ When the final slag is essentially fully basic and the lance is lowered too early or too low, the carbon content in the melt pool remains relatively high, causing the rate of carbon oxidation to surge sharply and resulting in severe spattering. Therefore, the timing of lowering the lance at the end of the process must be carefully controlled.
⑤ In the early stages of a furnace run, when the furnace chamber is small, temperatures are low, and the TFe content in the slag is relatively high, care must be taken to lower the lance in a timely manner to prevent the TFe content from becoming too high and to avoid spattering.
⑥ After furnace repair, the furnace lining temperature is relatively low, causing the early-stage blowing temperature to drop accordingly and resulting in strong oxidizing conditions. Care must be taken to lower the lance in a timely manner to control the TFe content in the slag and prevent spatter.
⑦ If a slag retention operation is used, the retained slag has a high TFe content. If measures are not taken to cool the slag before mixing with molten iron, explosive spatter may also occur.
The above phenomena can be summarized as follows: slag spatter in the early stage indicates that the furnace temperature is too low; slag spatter in the middle stage indicates that the furnace temperature is too high.
5. How should the lance position be adjusted after explosive spatter occurs?
Once spatter occurs during the blowing process, do not lower the lance lightly, because doing so will intensify the carbon oxidation reaction and actually exacerbate the spatter. At this point, the lance should be raised slightly. Aise the Lance: This slows carbon oxidation and melt pool heating, while using the oxygen jet to clear slag and assist gas escape.
Cool and Thicken: If the temperature is too high, raise the lance and add lime. This cools the melt pool and thickens the slag to suppress spattering.
Avoid Over-cooling: Do not cool the melt pool excessively.
Use Anti-spatter Agents: Add low-density materials like scrap insulation or wooden blocks during spattering. These reduce slag TFe content and minimize spatter intensity. Additionally, appropriately reducing the oxygen flow rate can also lessen the intensity of spatter.
6. What are the causes of foamy spatter and the preventive measures?
(1) Causes: When molten iron is high in Si and P, the slag becomes rich in SiO₂ and P₂O₅. Combined with a large slag volume and high TFe content, this reduces the slag’s surface tension.
Obstruction: Excessive slag foam blocks the smooth escape of CO gas.
Layer Buildup: The slag layer thickens significantly and can rise to the furnace mouth.
Spatter: While the carbon-oxygen reaction force is low, it can still propel this foam out of the furnace.At the same time, foamy slag effectively covers the surface of the molten pool, hindering gas escape. Therefore, severe foaming of the slag is the primary cause of foamy spattering.
Clearly, a large amount of slag makes spattering more likely to occur; in converters with a higher furnace-to-capacity ratio, gas can escape more freely, reducing the likelihood of significant spattering. Foamy spatter, due to the high TFe content in the slag, is often accompanied by explosive spatter.
(2) Preventive Measures
① Control Impurities: Regulate Si and P content in the molten iron.
Pretreat Iron: Use molten iron pretreatment for “triple removal” when possible.
Tap Acidic Slag: If pretreatment is unavailable, tap out part of the acidic foamy slag during blowing.
Use Secondary Slagging: Apply secondary slagging technology to prevent mid-process foamy spatter.
② Control the total iron (TFe) content in the slag to prevent TFe accumulation, thereby avoiding excessive foaming of the slag.
7. What are the causes of metal spatter and the corresponding preventive measures?
(1) Causes: When the TFe content in the slag is too low, the slag’s fluidity becomes poor. The oxygen stream then comes into direct contact with the molten metal surface. As the carbon-oxygen reaction expels CO gas, it carries molten metal droplets out of the furnace, causing metal spatter. These droplets adhere to the oxygen lance nozzle. This severely deteriorates the nozzle’s cooling conditions, leading to nozzle damage. Metal spatter is also known as “dry spatter.” As can be seen, the causes of metal spatter are exactly the opposite of those for explosive spatter. Metal spatter may occur when operating with the oxygen lance in a low position for an extended period, when the second batch of charge is added too early, or when the lance is lowered for decarburization before the slag has fully melted.
(2) Optimize Charging: Increase the charge volume reasonably through a phased, quantitative system.
Prevent Overfilling: Avoid overfilling to prevent the melt pool from becoming too deep.
Manage Furnace Bottom: Promptly address any rise in the furnace bottom caused by slag splashing.
Monitor Levels: Frequently measure the molten metal level.
Control Lance Position: Use these measurements to prevent improper lance position control.
Proper control of the lance position, thorough slag melting, and avoidance of an excessively low lance position and excessively low TFeO content all contribute to the prevention of metal spatter.
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