When it comes to laminage d'acier, one of the most critical steps that directly affects product quality, efficacité énergétique, and production cost is the heating of the steel billet. Getting the heating temperature right isn’t just about turning up the furnace—it’s a precise science that balances metallurgy, equipment capability, and process economics.
Why Heating Temperature Matters in Steel Rolling
Before a steel billet enters the rolling mill, it must be heated to a temperature where it becomes sufficiently ductile for deformation without cracking. Si la température est trop basse, the steel is brittle and prone to surface or internal defects. If it’s too high, excessive oxidation (scale formation), grain coarsening, and even melting of low-melting-point phases can occur—especially in alloy steels.
The ideal heating range for most carbon and low-alloy steels falls between 1050°C and 1250°C. This window is not arbitrary—it’s rooted in the iron-carbon phase diagram, where austenite (γ-Fe) is the dominant phase. Austenite has a face-centered cubic (FCC) structure that allows for easy slip and plastic deformation, making it perfect for hot rolling.
How Steel Grade Influences Heating Temperature
Not all steel billets are created equal. The chemical composition—especially carbon content and alloying elements like chromium, nickel, or vanadium—shifts the optimal heating temperature. Par exemple:
| Steel Type | Typical Carbon Content (%) | Recommended Heating Temp (°C) | Remarques |
|---|---|---|---|
| Low-carbon steel | ≤ 0.25 | 1150–1250 | High ductility; lower risk of overheating |
| Medium-carbon steel | 0.25–0.60 | 1100–1200 | Watch for decarburization at high temps |
| High-carbon steel | > 0.60 | 1050–1150 | Risk of grain boundary melting above 1200°C |
| Low-alloy steel (par ex., Cr-Mo) | Varie | 1100–1180 | Alloying elements raise recrystallization temp |
| Stainless steel (austenitic) | ~0.08 (304) | 1150–1200 | Avoid sensitization zone (450–850°C) during cooling |
As you can see, pushing every billet to 1250°C is a recipe for trouble. High-carbon grades, for instance, begin to form liquid phases at grain boundaries above 1200°C—a phenomenon known as “hot shortness.” This drastically reduces hot workability and can cause catastrophic cracking during rolling.
The Role of Billet Size and Section
Billet dimensions also play a major role in determining both heating temperature and time. Thicker sections take longer to heat uniformly. If the surface reaches 1200°C while the core is still at 900°C, thermal stresses can cause internal cracks—even before rolling begins.
Industry practice often uses these rules of thumb for heating time:
- Time per cm of thickness: 8–12 minutes/cm for carbon steels in a well-tuned reheating furnace.
- Maximum heating rate: Up to 300°C/hour in the initial stage (below 700°C), but slowed to 100–150°C/hour above 900°C to avoid thermal shock.
Par exemple, un 150 mm × 150 mm square billet (15 cm thick) typically requires 2 à 3 hours of total heating time in a walking-beam furnace to achieve uniform temperature throughout.
Furnace Types and Their Impact on Heating Control
Modern steel mills use different furnace designs, each with distinct heating characteristics:
| Type de four | Max Temp (°C) | Heating Uniformity | Best For |
|---|---|---|---|
| Walking-beam furnace | 1300 | Excellent (±15°C) | High-volume, consistent billet sizes |
| Pusher-type furnace | 1250 | Modéré (±30°C) | Legacy mills; smaller billets |
| Induction heater (pre-heater) | 1200 | Very fast, surface-focused | Supplemental heating; energy savings |
Walking-beam furnaces are now the industry standard for new installations because they minimize billet skid marks and offer superior temperature control. Advanced systems use infrared pyrometers and mathematical models to adjust burner zones in real time, ensuring the billet exits at the exact target temperature—say, 1180°C ±10°C—for the next rolling stand.
Real-World Consequences of Poor Temperature Control
Let’s look at what happens when things go wrong:
- Underheating (<1050°C): Leads to high rolling loads, motor overloads, and surface cracking. In extreme cases, the mill may stall.
- Surchauffe (>1250°C): Causes excessive scale (up to 3–5% metal loss), coarse grains, and reduced mechanical properties in the final product.
- Non-uniform heating: Results in “banana bends” or twisting during rolling due to uneven deformation resistance across the billet cross-section.
A study from a European long-product mill showed that reducing temperature variation from ±50°C to ±15°C cut roll wear by 22% and improved yield strength consistency by 8% in rebar products.
Practical Tips for Operators and Process Engineers
If you’re involved in billet heating or rolling operations, here are actionable guidelines:
- Always match heating temp to steel grade. Don’t use a “one-size-fits-all” setting. Keep a lookup table near the furnace HMI.
- Monitor core temperature, not just surface. Use thermocouples in test billets or infer core temp via thermal models if direct measurement isn’t possible.
- Optimize soaking time. Once the billet reaches target temp, hold it just long enough for homogenization—typically 15–30 minutes. Longer soaking increases scale and fuel use.
- Control atmosphere. A slightly oxidizing atmosphere minimizes decarburization in medium/high-carbon steels, but too much oxygen accelerates scale formation.
- Track fuel-to-billet ratio. A typical efficient furnace consumes 1.8–2.2 GJ/ton of billet. Spikes in consumption often signal burner misalignment or excess air.
Remember: the goal isn’t just to get the billet hot—it’s to deliver it to the first rolling stand with the right microstructure, minimal defects, and consistent temperature from head to tail.
Looking Ahead: Smart Heating and Digital Twins
Leading steel producers are now deploying digital twin technology—virtual replicas of their reheating furnaces fed by real-time sensor data. These systems predict billet temperature profiles and automatically adjust burner settings, damper positions, and conveyor speeds. Early adopters report 5–8% reductions in natural gas consumption and tighter temperature tolerances without operator intervention.
While such systems require investment, even small mills can benefit from basic data logging: recording billet entry/exit temps, fuel flow, and rolling outcomes helps build a knowledge base for continuous improvement.
In the end, mastering the heating temperature of steel billets isn’t just about following a number on a chart. It’s about understanding the interplay between chemistry, physics, and machinery—and using that knowledge to produce better steel, more efficiently, every single shift.
