Continuous casting
Continuous casting is a metallurgical process used to produce semi-finished metal products, such as slabs, billets, and blooms, by continuously solidifying molten metal—primarily steel—into a desired cross-sectional shape as it is withdrawn from a water-cooled mold.[1][2] In this method, molten metal flows from a ladle into a tundish for distribution, then enters an oscillating copper mold where initial solidification forms a solid shell around the liquid core; the partially solidified strand is then pulled through secondary cooling zones with water sprays to complete solidification before being cut to length.[1][3] This technique enables high-volume production at speeds of 1–8 meters per minute, transforming the metal in a single, efficient operation rather than discrete batches.[1] Developed as an alternative to traditional ingot casting, continuous casting originated from concepts proposed by Henry Bessemer in the 1850s and early experiments in the 1930s, but it achieved commercial viability in the 1950s with the first machines installed in Europe and North America.[4] By the 1960s, adoption accelerated, particularly in Japan, Korea, and Nordic countries, and as of 2024, it accounts for 97.5% of global steel production, solidifying approximately 1.84 billion tonnes annually.[4][1][5] The process offers significant advantages over ingot methods, including higher material yield (up to 95–100% due to minimized waste like contraction pipes), improved surface quality with fewer inclusions, and lower energy consumption through integrated casting and rolling.[3][4] However, it requires substantial capital investment and is best suited for simple, uniform cross-sections, limiting its use for complex shapes.[3] Key innovations have expanded its scope, including thin-slab casting introduced in the 1980s for mini-mills and near-net-shape technologies like twin-roll casting commercialized in the early 2000s, which produce thinner strips directly and reduce further processing needs.[4] While predominantly applied to steel, continuous casting is also used for non-ferrous metals like aluminum and copper, enabling the production of high-quality intermediates for industries such as automotive, construction, and aerospace.[1] Ongoing advancements focus on defect mitigation—such as segregation and cracking—through mathematical modeling and process controls to enhance efficiency and sustainability. Recent developments as of 2025 include increased integration of AI for real-time monitoring and efforts toward sustainable practices like electromagnetic stirring for better quality control.[2][4]Overview
Definition and principles
Continuous casting is a metallurgical process in which molten metal is introduced into an open-ended, water-cooled mold, where it begins to solidify from the outer surface inward, forming a solid shell that is continuously withdrawn as a semi-finished product, such as billets, blooms, or slabs, for further processing.[2] This method contrasts with traditional batch processes by enabling the production of long, uniform strands without interruption, accounting for over 97% of global steel output, approximately 1.84 billion tons annually as of 2024.[6] The core principles of continuous casting revolve around controlled heat extraction and mechanical support to ensure progressive solidification. Primary cooling occurs in the mold through direct contact with water-cooled copper walls, which rapidly extract heat to form a thin solid shell around the molten core; this shell thickens as the strand progresses.[2] To prevent the shell from adhering to the mold, the mold oscillates vertically at a frequency typically between 100 and 300 cycles per minute, creating lubrication films and oscillation marks on the surface that influence heat transfer uniformity.[2] Beyond the mold, secondary cooling zones apply water sprays or air-mist combinations to further solidify the interior, promoting a columnar grain structure and avoiding defects like cracks or center segregation.[2] Compared to traditional ingot casting, continuous casting offers significant advantages in efficiency, quality, and resource utilization. It achieves higher material yields, up to 95-98% of the molten metal converted to usable product, versus approximately 80% for ingot methods, due to minimized cropping losses and reduced oxidation.[7][8] Additionally, the process enhances quality control by producing more uniform microstructures with fewer inclusions and surface defects, while lowering energy consumption by eliminating intermediate reheating steps.[9][10] The basic process flow begins with molten metal transferred from a ladle to a tundish, which regulates the flow and distributes it evenly; from there, it passes through a submerged entry nozzle into the oscillating mold for initial solidification.[2] The partially solidified strand then enters secondary cooling zones, where it is bent and straightened to maintain shape before being cut to length by torches or shears into final semi-finished forms.[2] Key operational parameters include casting speed, which for steel typically ranges from 1 to 5 meters per minute depending on section size, influencing shell thickness and productivity; and superheat control, where the molten metal's temperature above its liquidus (often 15-30°C) is managed to optimize flow and prevent excessive turbulence or incomplete solidification.[2][11]Historical development
The concept of continuous casting emerged in the 19th century, with early patents laying the groundwork for more efficient metal solidification. In 1843, J. Laing patented a method in the United States for feeding liquid metal from a vertical reservoir through a trough into a preheated horizontal mold, aiming to enhance productivity by enabling continuous operation.[12] This idea was further advanced in 1856 by Henry Bessemer, who developed a water-cooled rotating twin-roll machine to cast metal strips continuously, influencing later processes despite initial limitations in scale.[13] Practical implementation, however, remained elusive for steel until the 20th century, as ingot casting dominated due to technological constraints. Significant progress occurred in the 1930s with non-ferrous metals, where Siegfried Junghans pioneered industrial applications. In 1927, Junghans perfected continuous casting for copper alloys like brass, producing rods and pipes to support his metal trading business, marking the first breakthrough in commercial non-ferrous production.[14] He filed a key patent in 1931 for steel continuous casting, followed by experimental work in the early 1930s that demonstrated potential yield improvements of 10-20% over ingot methods, though challenges like mold sticking persisted.[4] The first non-ferrous continuous casting machine was installed in 1937 at Scovill Manufacturing Company in Waterbury, Connecticut, using a design by Italian engineer Rossi. For steel, Junghans built the inaugural experimental machine in 1943 in Germany, but full commercialization awaited post-war advancements.[12] The 1950s marked the transition to industrial steel casting, led by German engineering firm Mannesmann. In 1952, Mannesmann commissioned the world's first commercial continuous caster at its Hüls works, producing small billets vertically and proving the process's viability for steel with oscillating molds to prevent sticking.[15] Adoption accelerated in the 1960s, particularly in the United States, where Bethlehem Steel established a pilot plant around 1960 in collaboration with partners to develop slab casting for automotive applications, validating the technology despite eventual abandonment of the facility in 1968 due to economic factors.[16] By the 1970s, the process expanded to non-ferrous metals like aluminum, with Pechiney conducting twin-roll casting trials starting in 1956 and achieving commercial scales in the 1960s for strip production. Copper continuous casting also matured, building on Junghans's work with horizontal and vertical systems for rod production.[17] Technological refinements in the 1970s and beyond drove widespread adoption. The shift from tall vertical machines to curved molds, introduced in the early 1970s, reduced building heights and improved efficiency by allowing horizontal withdrawal after bending, enabling higher casting speeds.[18] In the 1990s, integration with secondary metallurgy—such as ladle refining for composition control and inclusion removal—enhanced steel quality, making continuous casting suitable for high-grade alloys.[19] Globally, continuous casting overtook ingot methods by the mid-1980s, with adoption reaching over 50% of steel production by 1990 and exceeding 90% in many regions by the late 1980s. By 2024, it accounted for 97.5% of worldwide steel output, solidifying approximately 1.84 billion tons annually.[6][20]Equipment and Process
Core components
The continuous casting process relies on several primary components to handle and solidify molten metal into a continuous strand. The ladle serves as the initial vessel for holding and transporting molten metal from the furnace to the casting area, typically maintaining a slag cover to minimize oxidation and temperature loss.[2] The tundish acts as an intermediate reservoir that regulates the flow of molten metal into the mold, distributes it evenly through submerged entry nozzles, and facilitates the removal of inclusions via settling or flotation mechanisms.[2] The water-cooled copper mold is the critical zone for initial solidification, where the molten metal forms a solid shell as it contacts the mold walls, with the open-ended design allowing continuous withdrawal of the partially solidified strand.[2] Support systems enhance the stability and precision of the casting operation. Oscillators impart sinusoidal or other waveform vibrations to the mold at frequencies typically ranging from 1 to 3 Hz, preventing the strand from adhering to the mold walls and promoting smooth extraction while minimizing surface defects like cracks. Withdrawal rolls, arranged in a series along the casting path, provide mechanical support to the developing strand, counteracting ferrostatic pressure to prevent bulging and ensure uniform shape as the strand progresses through the cooling zones.[2] For product severance, cutting torches or shears are employed at the end of the solidification zone to slice the fully formed strand into desired lengths, such as slabs or billets, without interrupting the continuous flow.[2] Secondary cooling systems follow the mold to further control the solidification front and achieve uniform shell thickness. These consist of spray zones where water mist or air-water mixtures are directed onto the strand surface, extracting latent and sensible heat to refine the microstructure and prevent internal defects. The heat removal in these zones can be approximated by the equation for convective cooling via the spray medium: Q = \dot{m} C_p \Delta T where Q is the heat transfer rate, \dot{m} is the mass flow rate of the coolant, C_p is the specific heat capacity of the coolant, and \Delta T is the temperature difference between the coolant inlet and outlet. Material specifications are tailored for thermal efficiency and durability. Molds are constructed from copper alloys, such as Cu-Cr-Zr, offering high thermal conductivity around 350 W/m·K to rapidly extract heat from the molten metal. Roller designs vary by section size, with driven and idler rolls featuring adjustable spacing and materials like chilled cast iron to accommodate different strand dimensions, from small billets to wide slabs, while minimizing deformation.[2] Caster layouts influence operational efficiency and product types. Vertical casters, which maintain a straight downward path, are commonly used for billets and smaller sections due to their simplicity and lower height requirements. Horizontal casters, where the strand moves laterally after an initial vertical section, are less common and mainly used for billet production or non-ferrous metals in space-constrained facilities, requiring additional support to manage gravitational effects on the unsolidified core. Curved or bow-type casters predominate for slab production, balancing height requirements with effective support.[21] These components integrate seamlessly into the overall process flow to enable continuous production.[2]Steel casting process
The continuous casting process for steel begins with molten steel, typically at temperatures between 1550°C and 1600°C, being transferred from a ladle to a tundish, where it is distributed evenly to maintain a steady flow into the casting machine.[22] The tundish serves as a reservoir to regulate the metal flow and facilitate inclusion control through argon bubbling, which promotes the flotation of non-metallic inclusions to the surface for removal, enhancing steel cleanliness across various grades from ultra-low carbon (0.03% C) to high-carbon (up to 2% C) and stainless steels.[22][23] From the tundish, the molten steel is poured through a submerged entry nozzle into a water-cooled copper mold, initiating primary cooling where a solid shell forms rapidly on the outer surface due to the high thermal gradient.[2] The initial shell thickness develops within 0.5 to 2 seconds at the meniscus, following an empirical solidification model where shell thickness d approximates d = k \sqrt{t}, with k an empirical constant of approximately 20 to 30 mm/\sqrt{\text{min}} depending on steel composition and cooling intensity.[24] To ensure homogeneity in the high-temperature steel melt, electromagnetic stirring (EMS) is often applied in the mold region, generating Lorentz forces that refine the flow pattern, reduce macrosegregation, and minimize inclusion entrapment.[25] The partially solidified strand is then withdrawn from the mold by drive rolls at controlled speeds, entering the secondary cooling zone with water sprays or air-mist for further heat extraction, while the caster bends the strand at approximately 45° in slab configurations to transition from vertical to horizontal orientation.[22] Straightening occurs via support rolls to restore the strand's shape, and cooling continues until the core solidifies, with the strand exiting at 800°C to 1000°C for subsequent cutting into slabs, billets, or blooms.[23] Typical casting speeds range from 1 to 6 m/min, enabling production rates of 100 to 300 tons per hour per strand, with multi-strand casters accommodating up to 8 parallel strands in high-volume facilities to meet steel alloy demands efficiently.[23][2]Non-ferrous metal adaptations
Continuous casting processes for non-ferrous metals, such as aluminum and copper, require significant modifications from those used for steel due to differences in melting points, thermal conductivities, and susceptibility to oxidation. These adaptations focus on managing lower processing temperatures, preventing surface defects like oxides, and optimizing solidification to produce high-quality billets, rods, and slabs suitable for downstream rolling or extrusion.[26] For aluminum, which has a melting point of approximately 660°C, the direct chill (DC) casting method is predominantly employed in a horizontal or semi-horizontal configuration to produce billets and rods. This process involves pouring molten aluminum into a water-cooled mold, where direct chilling solidifies the metal while a starter block withdraws the forming ingot at controlled speeds typically ranging from 5 to 20 cm/min. To minimize oxide formation on the surface—a common issue due to aluminum's high affinity for oxygen—graphite-lined or hot-top molds are used; the hot-top design maintains an insulated layer of molten metal at the meniscus, reducing exposure to air and preventing skull formation.[27][28][29] Copper casting adaptations emphasize vertical upcasting techniques for producing oxygen-free rods, particularly through systems like the Southwire Continuous Rod (SCR) process, which integrates continuous casting with rolling for efficient production. In vertical upcasting, molten copper is drawn upward through a graphite die submerged in the melt, solidifying into rods as it emerges, with the SCR variant utilizing a horizontal casting wheel to form and roll the rod inline, achieving high conductivity for electrical applications. Wheel molds are specifically adapted for wire production, where the rotating wheel acts as a traveling mold to rapidly cool and shape thin sections. These processes handle both pure copper and alloys such as brass, with integrated annealing steps during or post-casting to relieve stresses and enhance ductility without separate heat treatment.[30][31][32] Key differences in non-ferrous casting include substantially reduced cooling rates compared to steel, with aluminum typically experiencing 10-50°C/s to prevent thermal cracking in its more brittle as-cast structure, versus steel's higher rates around 100°C/s enabled by its greater heat capacity. Lubrication poses unique challenges for these softer metals, as traditional oil-based systems can lead to inclusions or uneven withdrawal; instead, graphite-based dry lubrication or minimal wetting agents are preferred to avoid contamination while ensuring smooth mold release.[33] Typical outputs from these adapted processes include aluminum slabs up to 600 mm thick, ideal for plate production, and copper rods with diameters ranging from 8 to 40 mm, suited for wire drawing. Compared to traditional ingot casting followed by rolling, continuous casting for non-ferrous metals yields energy savings of 20-30% through reduced reheating cycles and higher material yield.[34][35][36]Product Variations
Standard section profiles
In continuous casting of steel, standard section profiles primarily consist of billets, blooms, and slabs, each tailored to specific downstream rolling and forming processes. Billets are typically square or round cross-sections with side diameters ranging from 100 to 200 mm, produced for further processing into long products such as bars, rods, and wires.[37][38] Blooms feature larger square or rectangular cross-sections, usually 200 to 400 mm per side, serving as intermediates for structural shapes like beams and heavy sections.[37][39] Slabs are rectangular thick plates with thicknesses of 150 to 250 mm and widths from 800 to 2000 mm, optimized for hot and cold rolling into flat products including sheets and plates.[40][41] For non-ferrous metals, continuous casting yields distinct profiles adapted to their lower melting points and applications. Aluminum ingots are commonly rectangular, with widths spanning 400 to 2000 mm and thicknesses around 400 to 600 mm, enabling efficient rolling into sheets, foils, and extrusions.[42][26] Copper billets are cylindrical, featuring diameters of 200 to 500 mm, primarily destined for drawing into wires, tubes, and busbars.[43][44] Standard dimensions and tolerances in continuous casting ensure compatibility with rolling mills, with variations based on plant scale. For instance, slab thickness tolerances are typically maintained at ±2 mm to minimize reheating distortions, while length tolerances can reach ±150 mm.[45][46] Mini-mills, often focused on billets and smaller blooms, offer greater scalability for capacities under 2 million tons annually, allowing flexible production of specialty steels, whereas integrated plants handle larger slab outputs exceeding 5 million tons per year for high-volume flat products.[47] These profiles underpin key applications in steel production, where billets feed bar and wire mills, and slabs supply over 70% of the feedstock for flat-rolled products like automotive sheets and construction plates.[48][49] In non-ferrous sectors, aluminum ingots support packaging and aerospace components, while copper billets dominate electrical conductor manufacturing.| Profile Type | Material | Shape and Standard Dimensions | Typical Applications |
|---|---|---|---|
| Billet | Steel | Square/round, 100-200 mm side/diameter | Bars, rods, wires |
| Bloom | Steel | Square/rectangular, 200-400 mm side | Beams, structural sections |
| Slab | Steel | Rectangular, 150-250 mm thick × 800-2000 mm wide | Sheets, plates |
| Ingot | Aluminum | Rectangular, 400-2000 mm wide × 400-600 mm thick | Sheets, extrusions, foils |
| Billet | Copper | Cylindrical, 200-500 mm diameter | Wires, tubes, busbars |