Billet Production Line Introduction
A billet production line is a critical part of the steelmaking process. Billets are semi-finished, solid, square or rectangular sections of steel, and they serve as the raw material for a wide range of finished steel products like rebar, wire rod, structural shapes, and seamless pipes.
The primary method for producing billets in modern steel mills is through continuous casting, though older methods like ingot casting and rolling also exist. We'll focus on the continuous casting method as it's the most prevalent and efficient.
The continuous casting machine (CCM) production line is the key equipment that continuously casts molten steel from steelmaking furnaces into steel billets (such as slabs, square billets, round billets, shaped billets, etc.). It serves as the core link connecting steelmaking and rolling in iron and steel production. Employing “continuous solidification forming” technology, it replaces the traditional “ingot-primary rolling” process. Offering advantages such as high production efficiency, low energy consumption, high metal yield, and stable product quality, it stands as one of the hallmark pieces of equipment in modern steel manufacturing.
I. Basic Components of Continuous Casting Machines
Continuous casting machines consist of multiple subsystems working in coordination. Core components include:
1. Molten Steel Receiving and Conveying System
Ladle: Holds molten steel transported from steelmaking furnaces (converters, electric furnaces, etc.). Features a steel structure shell lined with refractory materials, with a bottom sliding nozzle controlling molten steel flow. Capacity is matched to steelmaking production capacity (e.g., 100t, 200t, 300t).
Ladle Turntable: Supports the ladle and enables 180° rotation, transferring molten steel from the “holding position” to the “pouring position” (above the tundish) to ensure seamless continuity during continuous casting from multiple furnaces.
2. Tundish System
Ladle: A refractory vessel positioned between the steel ladle and the mold, functioning as a “buffer” and “purifier.” Its roles include:
Stabilizing molten steel flow to mitigate fluctuations from direct ladle pouring;
Distributing molten steel for multi-stream continuous casting machines (e.g., multi-stream square billet casting);
Promoting the flotation of inclusions in molten steel (by expanding flow channels and extending residence time) to purify the steel.
Ladle Tundish Spout and Flow Control Device: Equipped with a bottom spout (paired with a tundish nozzle), it regulates molten steel flow into the mold via a stopper rod or sliding spout. Its precision directly impacts billet quality.
3. Crystallizer System (the “Heart” of the Continuous Casting Machine)
Crystallizer: The key equipment for the initial solidification and shaping of molten steel. Its internal cavity shape matches the billet cross-section (e.g., rectangular for square billets, flat for slab billets, circular for round billets).
Core component: Copper mold tube (made of high-purity purple copper or chromium-zirconium copper). Its inner wall is smooth, while the outer wall features cooling water channels. Forced cooling via circulating water rapidly solidifies the molten steel surface to form a shell.
Crystallizer Vibration System: Drives the crystallizer to vibrate vertically along a specific trajectory (e.g., sinusoidal curve), preventing adhesion between the initial solidified shell and the crystallizer's inner wall while reducing surface cracks in the cast billet.
Protective slag system: Introduces protective slag (powdered or granular) into the mold to cover the molten steel surface. This provides insulation, isolates the steel from air (preventing secondary oxidation), lubricates the shell, and adsorbs inclusions.
4. Secondary Cooling System
Located beneath the mold, this system comprises multiple sets of water nozzles or air-water spray devices. It further cools the billet (still liquid at its core) as it is drawn from the mold.
By controlling spray intensity (water volume and pressure), the system continuously thickens the billet shell until the core fully solidifies. This prevents excessive cooling that could induce internal stresses or cracks in the billet.
5. Drawing and Conveying System
Drawing Machine: Composed of multiple pairs of driven rolls, it provides traction to continuously draw the slab from the mold while simultaneously straightening it to eliminate bending deformation during solidification, ensuring slab straightness.
Conveyor Roller Table: Transfers the ingot from the straightening machine to subsequent stages like the cutting zone and cooling bed. Roller table speed is synchronized with ingot pulling speed to ensure smooth ingot conveyance.
6. Cutting System
Cuts the continuously moving ingot into predetermined lengths (e.g., 6m, 9m, 12m, adjusted per rolling mill requirements).
Flame Cutting Machine: Employs an oxygen-fuel (e.g., propane) flame to melt and sever the billet. Suitable for various cross-sections and steel grades, it is the mainstream equipment.
Mechanical Flying Shear: For small cross-section billets (e.g., square or round billets), high-speed flying shears can be used, offering higher cutting efficiency and superior dimensional accuracy.
7. Cooling Bed and Finishing System
Cooling Bed: Used to cool ingots, reducing temperatures from 800-1000°C to ambient via natural cooling or forced air cooling. This prevents internal stresses or deformation caused by rapid cooling (e.g., step-type cooling beds, chain-type cooling beds).
Finishing Equipment: Includes straighteners (to further correct billet curvature), surface cleaning equipment (to remove scale or defects), and marking equipment (to print furnace numbers, specifications, steel grades, etc.).
II. Working Principle of Continuous Casting Machines
Molten Steel Pouring: Refined molten steel (temperature approximately 1500-1580°C) is poured from the ladle into the tundish, then flows through the tundish nozzle into the mold.
Initial Solidification: Within the mold, molten steel contacts the cooled copper walls. The surface rapidly cools and solidifies, forming a shell of approximately 10-30 mm thickness, while the interior remains liquid.
Continuous Drawing: A drawing and straightening machine continuously draws the slab with its shell from the mold into the secondary cooling zone.
Complete solidification: In the secondary cooling zone, water mist continuously cools the slab, causing the shell to gradually grow toward the center until complete solidification.
Cut-to-length: The solidified slab is cut to specified lengths, cooled on a cooling bed, and finished to become qualified billets for use in the rolling mill.
III. Classification of Continuous Casters
Based on billet cross-section and application, continuous casters are categorized as:
Square Billet Continuous Caster: Produces square or rectangular cross-section billets (e.g., 150mm×150mm, 200mm×200mm) for rolling bars, wire rods, and small structural steel.
Slab continuous casting machines: Produce slab-shaped cast billets with high width-to-thickness ratios (e.g., thickness 200-300mm, width 1000-3000mm), used for rolling medium-thick plates, hot-rolled coils, etc.
Round billet continuous casting machine: Produces circular cross-section cast billets (diameter 100-400mm), primarily for rolling seamless steel pipes.
Special-Shaped Billet Continuous Casting Machine: Produces cast billets with special cross-sections (e.g., I-beam billets, H-beam billets), directly supplied to special-shaped rolling mills for processing.
Additionally, continuous casting machines can be classified by stream count into single-stream, dual-stream, and multi-stream (e.g., 6-stream, 8-stream) types. Multi-stream continuous casting machines can simultaneously produce multiple cast billets, enhancing efficiency.
