In hot strip mills, steel undergoes a complex transformation from a thick slab to a thin strip through a series of rolling stages. The first step in this process is the roughing mill, which reduces the slab’s thickness and prepares it for the final reductions in the finishing mill. However, during this critical transfer process, the steel strip—known as the transfer bar—experiences significant heat loss, presenting several challenges for hot strip manufacturers. The Encopanel® system from Primetals Technologies offers an effective solution to heat loss by providing superior insulation for transfer bars as they move from the roughing mill to the finishing mill, thereby reducing heat loss during this transfer process.
The Heat Loss Problem
As the transfer bar moves between the roughing and finishing mills, it loses valuable heat. This heat loss occurs through three main mechanisms: conduction, convection, and radiation. After the transfer bar leaves the reheating furnace, it is brought to a high temperature of 1100
1250°C, it cools down quickly.
- Conduction occurs when the transfer bar comes into contact with surfaces that absorb heat, gradually reducing its temperature.
- Convection happens when the bar passes through open areas between the mills, losing heat to the surrounding air. The longer the bar is exposed, the more heat it loses, especially in areas with high airflow.
- Radiation is the most significant form of heat loss. At high temperatures, the transfer bar emits radiation, which is lost to the environment, especially if the bar is exposed to open space without sufficient shielding. As much as 90% of the heat is lost through radiation.
This rapid heat loss becomes a significant concern if the transfer bar is not adequately insulated during transport. As a result, the reheating furnace is forced to work harder to maintain the bar’s required temperature, leading to several key challenges for manufacturers.
Consequences of Heat Loss
The heat loss during the transfer process between the roughing and finishing mills leads to a variety of operational issues:
- Increased Energy Demand: To compensate for the heat lost during transfer, the reheating furnace must consume more energy to discharge the product at a higher temperature, driving up operational costs and increasing environmental emissions.
- Energy Inefficiency: As the transfer bar cools during transportation, more energy is required to reheat it to the necessary rolling temperature. Increased passes are needed for the finishing mill, creating a cycle of inefficiency.
- Inconsistent Product Quality: Uneven temperatures during processing can result in defects such as surface imperfections, thickness variations, and inconsistent mechanical properties.
- Yield Loss: The cooling of the transfer bar can also cause issues in rolling, such as incomplete processing or difficulty in achieving the desired thickness and shape. This increases the amount of wasted material, lowering the mill’s efficiency and profitability.
Introducing the Encopanel® System
One solution to these challenges is the Encopanel® system, developed by Primetals Technologies in the UK. This innovative heat retention system conserves heat through thermal insulation, a reflective surface, and hot air containment, which reduces heat loss and minimizes the energy needed to reheat the transfer bars.
The Encopanel® system is strategically placed between the roughing and finishing mills and typically consists of modules 4 meters in length. These panels are critical in optimizing heat retention and ensuring smooth transfer bar movement. The panels are made with ceramic fiber insulation to minimize heat loss and maintain the transfer bar’s temperature. Additionally, they feature a unique alloy membrane face material resistant to high thermal cycling, ensuring durability in the demanding environment of hot strip mills. Each module contains up to 12 to 13 replaceable Encopanel® cells (top, bottom, and sides). The modular design allows easy maintenance, reduces downtime, and ensures long-term operational efficiency.
Beyond its optimized design, the Encopanel® system has several unique features to improve operations, including:
- Opening and closing top lids provide easy maintenance access, ensuring operational convenience and preventing damage to the Encopanel®.
- Scale Clearing Bottom Panels: Effectively removes oxide build-up from the transfer bar, ensuring smooth transport.
- Hydraulic Pushers: Facilitate the quick and efficient removal of the transfer bar from the system.
- Knockdown Unit: Positioned ahead of module 1, this unit pushes down on any upturned head ends, ensuring a smooth entry into the Encopanel® system.
- Bumper Bars: Guides the transfer bar and protects the panels from potential damage.
The Impact of the Encopanel® System
The effectiveness of the Encopanel® system is illustrated by the following example, which compares the transfer bar temperatures across scenarios:
- “Active” Scenario (Encopanel® system in the lowered position): In this scenario, the Encopanel® system creates a fully enclosed thermal tunnel, resulting in a more consistent temperature profile across the length of the transfer bar, from the head to the tail, as it moves down the roller table.
- Other/”Bottom Panel” Scenario (Encopanel® system in the raised position): In these scenarios, where the Encopanel® system is either absent or not fully lowered, the transfer bar experiences significant temperature inconsistencies, highlighting the importance of proper heat retention.
With the Encopanel® system producers take advantage of several benefits for their hot strip mill operations, including:
- Lower Reheat Furnace Temperatures: By minimizing heat loss during transfer, the system reduces the furnace’s energy consumption while still ensuring the steel meets the required material properties.
- Higher Mill Throughput: With the transfer bar maintained at the optimal temperature, processing in the roughing and finishing mills becomes smoother and faster, increasing overall throughput.
- Greater Product Envelope: Encopanel improve temperature control, enabling the production of a broader range of strip thicknesses and properties. This allows the mill to produce more varied steel products.
- Increased Yield: By maintaining the transfer bar at the optimal temperature, Encopanel reduce defects, scrap production, and yield losses, enhancing overall efficiency and cost-effectiveness.
- Reduced Centre-to-Surface Temperature Gradients: Minimizing the temperature difference between the center and surface of the steel strip ensures uniform deformation during rolling, resulting in better material properties and consistency.
The Success of the Encopanel® System
In a recent project, thyssenkrupp Steel has chosen Primetals Technologies to install the Encopanel® System at their hot strip mill in Bruckhausen, situated just outside Duisburg. The heat retention system is being implemented to specifically address the issue of edge cracks, reduce material waste, and extend thyssenkrupp Steel’s product portfolio.
Overall, the Encopanel® System intervenes at a key point between the roughing and finishing mills and is specifically designed to address heat loss in the production process. Integrating this heat retention system can improve product consistency, reduce waste, and lower energy consumption and costs.
