The Use of Water Cooling during the Continuous Casting of Steel and Aluminum Alloys J. SENGUPTA, B.G. THOMAS, and M.A. WELLS In both continuous casting of steel slabs and direct chill (DC) casting of aluminum alloy ingots, water is used to cool the mold in the initial stages of solidification, and then below the mold, where it is in direct contact with the newly solidified surface of the metal. Water cooling affects the product quality by (1) controlling the heat removal rate that creates and cools the solid shell and (2) gener- ating thermal stresses and strains inside the solidified metal. This work reviews the current state- of-the-art in water cooling for both processes, and draws insights by comparing and contrasting the different practices used in each process. The heat extraction coefficient during secondary cooling depends greatly on the surface temperature of the ingot, as represented by boiling water-cooling curves. Thus, the heat extraction rate varies dramatically with time, as the slab/ingot surface temperature changes. Sudden fluctuations in the temperature gradients within the solidifying metal cause thermal stresses, which often lead to cracks, especially near the solidification front, where even small tensile stresses can form hot tears. Hence, a tight control of spray cooling for steel, and practices such as CO 2 injection/pulse water cooling for aluminum, are now used to avoid sudden changes in the strand surface temperature. The goal in each process is to match the rate of heat removal at the surface with the internal supply of latent and sensible heat, in order to lower the metal surface temperature monotonically, until cooling is complete. I. INTRODUCTION CONTINUOUS casting processes for both steel and alu- minum alloys were developed several decades ago to pro- duce shapes for subsequent semifabrication processes such as extrusion or rolling. As-cast product shapes include bil- lets (square cross section with thickness less than 150 to 175 mm for steel), thick slabs/ingots (wide rectangular cross section with thickness between 50 and 300 mm for steel, and up to 500 to 750 mm for aluminum alloys), thin slabs (thickness between 50 and 75 mm for steel), strips (thick- ness between 1 and 12 mm for both steel and aluminum alloys), and rounds/extrusion billets (100- to 500-mm diameter for both steel and aluminum alloys). In recent decades, a dramatic growth of this primary metal process- ing technology has been realized in both steel and aluminum industries, owing to a substantial increase in yield, energy savings, and productivity over static casting. However, the technological advancement has taken distinctly different routes for these two metal industries. Over the years, the casting procedures for steel and aluminum alloy products have devel- oped distinctive features in terms of casting practices, machin- ery, and process and quality control methodologies. The productivity of both processes is controlled by the cast- ing speed, so higher speeds are always sought. However, the casting speed cannot be increased arbitrarily for several rea- sons. [1] First, the resulting increase in depth of the liquid pool and surface temperature of the strand prolongs the solidifica- tion process and increases the cooling requirements. In extreme cases, the structurally weak solid shell may rupture, leading to a “breakout” of liquid metal below the mold, or to excessive bulging if containment is exceeded for larger sections. Sec- ond, higher casting speeds often lead to cracks, caused by the higher thermal stresses. The practical range of operating speeds depends on alloy composition and product geometry. For steel slabs, the casting speed increases with decreasing thickness from 0.01 ms -1 (for 300-mm blooms) to over 0.08 ms -1 (for 50-mm thin slabs). Owing to cracking difficulties during start- up, aluminum alloy ingots and billets are cast at much lower speeds, increasing from 0.00075 to 0.001 ms -1[2] to steady speeds ranging from 0.001 to 0.003 ms -1[3] . The continuous casting machinery is comprised of the mold and secondary water-cooling systems. These are designed to extract superheat from the incoming liquid metal (5 pct of the total heat content in the metal), latent heat of fusion at the solidification front (20 pct of total heat content), and heat of phase transformation and sensible heat (75 pct of the total heat content) from the solidified metal. However, the cooling system features for casting steel and aluminum alloys are very different, as schematically illustrated by Figures 1(a) [4] (for steel) and (b) [5,6] (for aluminum alloys). In the conventional continuous (or strand) casting of steel, shown in Figure 1(a), liquid steel flows from the bottom of a ladle into a small intermediate vessel known as the tundish. It leaves the tundish bottom through a submerged nozzle, according to the position of a stopper-rod or slide-gate flow control system. The liquid flow is directed into the mold (usually 700 to 1200 mm in length), and freezes a thin shell against the water-cooled copper walls. At steady state, the solid shell exiting the mold forms a stable strand, which has adequate mechanical strength to support the liquid metal core (typically 5 to 30 m in depth, depending on the casting speed and thickness). Motor-driven drive rolls located far below the mold continuously withdraw the strand downward. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 36A, JANUARY 2005—187 J. SENGUPTA, NSERC (Canada) Postdoctoral Fellow, and B.G. THOMAS, W. Grafton & L.B. Wilkins Professor, are with the Department of Mechan- ical and Industrial Engineering, University of Illinois, Urbana, IL 61801. Contact e-mail: bgthomas@uiuc.edu M.A. WELLS, Assistant Professor, is with the Department of Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Manuscript submitted May 10, 2004.