Iron oxide scale burn-off is a common technical problem in the operation of regenerative pusher reheaters, with significant consequences. On the one hand, excessive iron oxide scale increases metal consumption, reduces steel yield, and directly impacts the company’s economic benefits. On the other hand, detached iron oxide scale can clog the furnace bottom slide and regenerator channels, affecting the normal operation of the reheater and increasing equipment maintenance costs. Simultaneously, the oxidation burn-off process involves significant heat consumption, reducing the reheater’s thermal efficiency and increasing energy consumption. Furthermore, iron oxide scale adhering to the billet surface can affect the product quality of subsequent rolling processes, leading to surface defects in the rolled products. Therefore, a thorough analysis of the causes of severe iron oxide scale burn-off and the development of scientific and effective solutions are of great significance for improving the operating efficiency of the reheater and ensuring steel quality.
Analysis of the reasons for severe iron oxide scale burn-off:
(i) Unreasonable heating process parameters
Heating temperature and heating time are the core process parameters affecting oxidation loss. If the heating temperature is too high, exceeding the critical oxidation temperature of the steel grade (generally, the oxidation reaction rate of carbon steel will significantly accelerate after the heating temperature exceeds 1000℃), it will cause the iron elements on the surface of the steel billet to undergo a violent oxidation reaction with the oxygen in the furnace, generating a large amount of iron oxide scale. At the same time, if the heating time is too long, the steel billet will remain in the high-temperature zone for too long, causing the oxidation reaction to continue and increasing the cumulative oxidation loss. In addition, improper control of the heating rate, such as excessively rapid temperature rise in the low-temperature stage, will lead to an excessive temperature difference between the inside and outside of the steel billet, causing thermal stress, and may also accelerate oxidation due to a sudden increase in surface temperature.
(ii) Improper control of the atmosphere inside the furnace
The furnace atmosphere is a key factor determining the degree of oxidation loss. When the excess air coefficient is too high, excessive oxygen is introduced during combustion, creating a highly oxidizing atmosphere that significantly accelerates the oxidation reaction on the steel billet surface. In regenerative furnaces, the preheating and mixing of air and gas directly affect combustion completeness and atmosphere properties. An imbalance in the air-fuel ratio, such as excessive air supply or insufficient gas supply, will result in unburned oxygen remaining in the furnace, enhancing oxidation. Furthermore, an unreasonable regenerator commutation cycle can lead to unstable combustion, large fluctuations in the furnace atmosphere, and potential oxygen enrichment in localized areas, exacerbating localized oxidation loss.
(iii) Poor equipment operating condition
1. Poor Furnace Sealing: Inadequate sealing of the furnace door, slide rails, observation holes, etc., allows a large amount of cold air to enter the furnace. This cold air not only lowers the furnace temperature but also introduces extra oxygen, altering the furnace atmosphere and increasing oxidizing properties. Furthermore, areas where the cold air contacts the high-temperature steel billet are prone to localized accelerated oxidation.
2. Burner Arrangement and Performance Issues: Improper burner installation positions or angle deviations can lead to uneven temperature distribution in the heating zone, resulting in localized overheating. The oxidation reaction rate on the steel billet surface in overheated areas is much higher than in normal areas, causing severe localized iron oxide scale burn-off. In addition, poor burner atomization or spraying effect leads to incomplete combustion and unstable flame patterns, further exacerbating temperature unevenness and atmospheric disturbance.
3. Regenerator Failure: If the regenerator becomes clogged, coke-prone, or its heat exchange efficiency decreases after long-term use, it will result in insufficient preheating temperatures for air and gas. When low-temperature air and gas enter the furnace for combustion, more air is required to ensure complete combustion, which indirectly increases the excess air coefficient. At the same time, the decrease in combustion temperature will also affect the stability of the temperature field inside the furnace, which is not conducive to the control of oxidation loss.
(iv) The influence of the billet itself and the furnace charging conditions
The original surface condition of the steel billet has a certain impact on oxidation loss. If the surface of the steel billet is contaminated with oil, rust, or moisture before entering the furnace, the burning of the oil will consume reducing gases in the furnace during heating, while producing oxidation products; the evaporation of moisture will cause an oxidation reaction when it comes into contact with the high-temperature steel billet, thus exacerbating surface oxidation. In addition, different steel grades have different oxidation resistance properties. Low-carbon steel is more prone to surface oxidation reaction than high-carbon steel or alloy steel. If the heating process is not adjusted according to the characteristics of the steel grade, it will also lead to an increase in the amount of iron oxide scale loss.
(v) Unreasonable steel pushing rhythm and furnace charging method
A mismatch between the billet pushing speed and the heating capacity will result in the billet remaining in the furnace for too long. If the pushing speed is too slow, the billet’s residence time in each heating section of the furnace will exceed process requirements, leading to a significant cumulative effect of oxidation. Overly dense billet loading will result in insufficient spacing between billets, hindering flue gas circulation, causing uneven oxygen supply in localized areas, and obstructing heat transfer, easily leading to uneven heating and accelerated localized oxidation. Furthermore, friction and collision between billets during pushing may cause the original oxide scale to peel off, exposing the fresh metal surface to a high-temperature oxidizing atmosphere, which will accelerate new oxidation reactions.
III. Measures to address severe iron oxide scale burn-off:
(a) Optimize heating process parameters
1. Develop a precise temperature profile: Based on the characteristics of the steel (such as carbon steel, alloy steel, etc.) and specifications, develop a reasonable heating temperature range and strictly control the maximum heating temperature to avoid exceeding the critical oxidation temperature of the steel. For example, the heating temperature of ordinary carbon steel is generally controlled between 1150 and 1250℃, and the specific temperature is adjusted according to the rolling requirements.
2. Reasonable control of heating time: Calculate the optimal heating time based on the billet thickness and heating temperature to ensure the billet reaches the required rolling temperature while minimizing the dwell time in the high-temperature zone. Optimize the temperature distribution of each heating section (preheating section, heating section, and soaking section) to achieve a balance between rapid and uniform heating, reducing the cumulative oxidation reaction time.
3. Optimize heating rate: In the low-temperature preheating stage, adopt a slow and uniform heating to avoid a sudden rise in surface temperature; after entering the high-temperature heating section, under the premise of ensuring the heat penetration of the billet, reasonably control the heating rate to reduce the surface oxidation time.
(ii) Precise control of the furnace atmosphere
1. Optimize air-fuel ratio control: Based on the type of gas (such as blast furnace gas, converter gas, mixed gas, etc.), precisely adjust the air and gas supply according to the theoretical air-fuel ratio, and control the excess air coefficient within a reasonable range of 1.05-1.15 (specifically adjusted according to the calorific value and combustion characteristics of the gas) to reduce the excess oxygen content in the furnace.
2. Stabilize the regenerator reversing cycle: Based on the furnace heat load and regenerator performance, set a reasonable reversing cycle (generally 30-120 seconds) to ensure stable combustion and reduce furnace atmosphere fluctuations. An advanced automatic control system ensures a smooth transition of air and gas flow during reversing, avoiding oxygen peaks at the moment of reversal.
3. Use atmosphere control technology: When conditions permit, inert or reducing gases (such as nitrogen or coal gas) can be introduced to fine-tune the atmosphere inside the furnace, reduce the oxygen concentration in local areas, and inhibit oxidation reactions.
(III) Strengthen equipment maintenance and optimization
1. Strengthen furnace sealing: Inspect and modify easily leaking parts such as furnace doors, slides, and seals; replace aged seals with high-temperature resistant sealing materials to ensure furnace sealing. Regularly inspect the furnace structure and promptly repair any deformed or damaged parts to reduce cold air intrusion.
2. Optimize burner configuration and performance: Based on the furnace structure and heating requirements, re-optimize the burner placement and angle to ensure uniform flame coverage and avoid localized high temperatures. Regularly clean and maintain the burners, inspecting components such as atomizing plates and injection nozzles to ensure complete combustion and stable flame pattern.
3. Maintain the normal function of the heat storage medium: Develop a plan for regular cleaning and replacement of the heat storage medium, removing accumulated ash, coke, and other debris to ensure its heat exchange efficiency. By monitoring the temperature difference between the inlet and outlet of the heat storage medium, promptly assess its performance status and ensure that the preheating temperatures of air and gas meet design requirements (generally, air preheating temperature can reach 800-1000℃, and gas preheating temperature can reach 600-800℃).
(iv) Optimize billet charging conditions and production organization
1. Pre-treatment of steel billets before furnace loading: Clean the surface of the steel billets to remove impurities such as oil and rust, reducing the impact of combustion or reaction of impurities on the furnace atmosphere after loading. For easily oxidized steel grades, surface coating technology (such as high-temperature resistant and oxidation-resistant coatings) can be used to form a protective layer on the surface of the steel billets, preventing oxygen from contacting the surface of the steel billets.
2. Optimize the pushing rhythm and charging method: Adjust the pushing speed reasonably according to the heating furnace capacity and rolling rhythm to ensure that the billet residence time in the furnace matches the heating process. When charging the furnace, ensure that there is a reasonable gap between the billets (generally 50-100mm, adjusted according to the billet specifications) to ensure smooth flue gas flow and uniform temperature and atmosphere distribution in the furnace.
(v) Establish an online monitoring and intelligent control system
Install furnace temperature monitoring devices (such as thermocouples and infrared thermometers) and atmosphere analysis equipment (such as zirconia oxygen analyzers) to monitor the temperature and oxygen content in each area of the furnace in real time. Link the monitoring data with heating process parameters and equipment operating parameters through an industrial control system to automatically adjust parameters such as the air-fuel ratio, steel pushing speed, and reversing cycle, ensuring that the furnace temperature and atmosphere are always under optimal control. Simultaneously, establish a regular oxidation loss rate monitoring mechanism, using weighing methods or sampling analysis to evaluate the effectiveness of loss control and continuously optimize the control strategy.
Severe iron oxide scale burn-off in regenerative pusher reheating furnaces is the result of multiple factors, including process, equipment, and operation. Solving this problem requires a systemic approach, addressing the root causes through comprehensive management. Optimizing heating process parameters, precisely controlling the furnace atmosphere, strengthening equipment maintenance, improving billet feeding conditions, and establishing an intelligent monitoring system can effectively reduce the iron oxide scale burn-off rate. This not only reduces metal consumption and improves steel quality but also increases furnace thermal efficiency and reduces energy consumption, creating significant economic and social benefits for enterprises. In practical applications, it is necessary to continuously optimize the solution based on the specific furnace model, steel characteristics, and production needs to ensure the scientific validity and effectiveness of the measures.
