A reheating furnace is a key part of a steel rolling mill. Its main function is to heat billets, blooms, or slabs to the required rolling temperature and ensure a sufficiently uniform temperature across the billet section before entering the rolling mill.
Proper furnace operation directly affects steel quality, rolling mill productivity, fuel consumption, and operating costs. Understanding the basic principles of combustion, heat transfer, furnace pressure, and temperature control is therefore essential for stable rolling mill operation.
Basic Knowledge of Reheating Furnaces
Energy, Temperature and Pressure
Several basic units are commonly used in reheating furnace operation.
- 1 kcal ≈ 4,185 J
- 1 J ≈ 0.000239 kcal
- Absolute temperature: T = 273 + t, where Tis the absolute temperature in Kelvin and t is the temperature in °C.
- 1 mmH₂O ≈ 9.81 Pa
- 1 kg/cm² ≈ 98,100 Pa
- 1 mmHg ≈ 133.32 Pa
- 1 standard atmosphere = 101,325 Pa
For fuel consumption calculations, standard fuel or standard coal can also be used as a reference. One kilogram of standard coal is conventionally assigned a calorific value of approximately 7,000 kcal.
Common Fuels for Steel Reheating Furnaces
The choice of fuel depends on plant conditions, availability, furnace design, and operating requirements.
Natural gas is a high-calorific-value fuel, with methane (CH₄) as its primary component. It can provide a high combustion temperature and is widely used in modern industrial furnaces.
Coke oven gas (COG) is a by-product of coke production. Its main combustible components include hydrogen and methane, giving it a considerably higher calorific value than blast furnace gas.
Blast furnace gas (BFG) is a by-product of blast furnace ironmaking. It contains a relatively high proportion of nitrogen and carbon monoxide and has a comparatively low calorific value. Typical BFG composition includes approximately 25–31% CO, 2–3% H₂, 0.3–0.5% CH₄, and 55–58% N₂.
Actual fuel composition and calorific value can vary depending on plant conditions and gas treatment systems, so furnace operation should be based on the actual fuel properties available at the site.
Three Basic Heating Stages
The heating process in a steel reheating furnace can generally be understood through three stages: preheating, heating, and soaking.
1. Preheating
At the beginning of heating, the heating rate needs to be controlled to avoid excessive thermal stress, particularly when processing billets with residual stress.
High-carbon steels and high-alloy steels may require greater attention to heating rates. The objective of the preheating stage is to allow the billet temperature to increase progressively while limiting excessive temperature differences between the surface and the core.
2. Heating
During the heating stage, the furnace supplies most of the required thermal energy.
Within the limits specified by the furnace and steel grade, increasing the temperature difference between the furnace atmosphere and billet surface can enhance heat transfer and accelerate heating. Radiant heat transfer is particularly important at high furnace temperatures.
However, excessive heating intensity can produce a large temperature difference between the billet surface and its center. Furnace temperature and heat input therefore need to be matched with billet size, steel grade, and production rate.
3. Soaking
The main objective of the soaking stage is to reduce the temperature difference across the billet section.
Instead of continuing to increase the surface temperature, heat is allowed to conduct from the surface toward the billet center. The furnace temperature and heat input should therefore be reduced appropriately while maintaining the required billet temperature.
Proper soaking helps achieve a more uniform temperature distribution and reduces the risk of overheating and burning. Furnace atmosphere control is also important during this stage.
Common Reheating Furnace Operating Problems
Excessive Furnace Exit Temperature
A high furnace exit or flue-gas temperature generally means more heat is carried away with the exhaust gas, increasing heat loss and reducing thermal efficiency.
Common causes include:
- Incomplete combustion and excessively long flames
- Excessive fuel input
- Billets remaining in the furnace longer than required
- Heat input not being adjusted to the actual production rate
Corrective actions include optimizing the fuel-air ratio and combustion condition and reducing heat input to match the rolling mill production rate.
Low Furnace Exit Temperature
Furnace exit temperature should be consistent with the production rate. If it is too low, billet heating may become insufficient and furnace productivity can be affected.
Possible causes include excessive production speed relative to the available furnace heat input.
If the furnace has not reached its maximum heat load, fuel input can be increased appropriately. If the furnace is already operating at its maximum capacity, the production rate may need to be reduced.
Low Billet Head Temperature
Low billet-end temperature can result from insufficient furnace pressure, air infiltration through poorly sealed side doors, or uneven temperature distribution across the furnace width.
Possible corrective measures include improving furnace pressure control, sealing furnace doors, and adjusting heat input across the furnace width.
Overheated Billet Ends
Billet ends can become overheated when the furnace temperature near the side walls is excessive or when short billets expose a larger surface area to the furnace atmosphere.
Excessive bottom heating can also contribute to overheating at the billet ends.
Appropriate adjustment of bottom heating and the heat input around short billets can help control this problem.
Low Billet Bottom Temperature
Low temperature on the billet bottom surface may be associated with:
- Insufficient bottom heating
- An inadequate soaking zone
- Damage or loss of insulation around skid pipes
- Poor sealing of bottom furnace doors
These conditions should be checked during furnace inspection and maintenance.
Furnace Pressure Control
Furnace pressure is an important operating parameter. Poor pressure control can increase air infiltration and fuel consumption or interfere with stable combustion.
Difficulty in controlling furnace pressure may be caused by excessive deposits of iron oxide scale in the flue system, water accumulation, leakage points in the stack or flue, or excessive stack draft.
The appropriate solution depends on the actual cause. Maintenance may include removing accumulated scale, eliminating water accumulation, repairing leakage points, and adjusting the exhaust system.
In some cases, a modernized furnace with significantly lower fuel consumption may have an exhaust system with excessive capacity relative to the new operating conditions. Appropriate modification of the flue system can then be considered to improve pressure control.
How to Improve Reheating Furnace Energy Efficiency
A key principle of efficient furnace operation is to avoid supplying more heat than the production process requires.
When production decreases, the fuel distribution between furnace zones should be adjusted accordingly rather than maintaining the heat load used at normal production rates.
One practical approach is the fixed-point heating method, in which a defined location in the furnace is selected as a reference point for monitoring billet temperature. Under stable production conditions, the billet should reach a specified temperature at this point.
If the billet temperature is already higher than the target at the reference point, the heat input may be excessive. If it is lower, the heat input may be insufficient.
The purpose is not simply to maximize furnace temperature, but to provide the required amount of heat at the right location and at the right production rate.
Another useful operating principle is what can be described as controlled heating according to steel grade, billet size, production rate, and furnace temperature/heat-load schedules. Stable and repeatable furnace operation requires these parameters to be coordinated rather than adjusted independently.
Furnace Commissioning and Operator Training
The performance of a reheating furnace depends not only on its design but also on commissioning, operation, maintenance, and operator training.
For a newly constructed furnace, the refractory-lined stack and related structures require proper drying and heating before normal operation. This removes moisture from the refractory materials and establishes the thermal conditions required for stable draft and operation.
During commissioning and operation, furnace pressure, combustion, temperature distribution, fuel consumption, billet temperature, and exhaust temperature should be monitored systematically.
SME Group Reheating Furnace and Rolling Mill EPC Services
As an EPC contractor for steel rolling mills, SME Group provides services covering the complete project cycle, from rolling mill design and equipment procurement to construction, commissioning, trial operation, and production management.
Our service can also include detailed training for local operators and maintenance personnel. The training focuses on practical furnace operation, temperature and pressure control, combustion management, production coordination, equipment maintenance, and troubleshooting.
The objective is not simply to complete the installation and hand over the equipment, but to help the local operating team develop the knowledge and skills required for stable reheating furnace operation and proper maintenance after project delivery.
A well-designed reheating furnace, combined with appropriate commissioning, operator training, and production management, provides a solid foundation for efficient and reliable steel rolling mill operation.
Editorial Note
This article is adapted from training materials originally prepared by SME Group for employees of a previous steel rolling mill project. The original materials were developed for the specific operating conditions and requirements of that project and have been edited and reorganized here for general technical reference.
If you identify any inaccuracies, omissions, or points that require further clarification, we welcome you to contact SME Group for discussion and technical exchange.
