Understanding the Role Water-cooling Plays during Continuous Casting of Steel and Aluminum Alloys

Water-cooling plays a major role in extracting heat from both the mold and solidifying metal during the continuous casting of steel and aluminum alloys and is characterized by complex boiling phenomena. Heat extraction rates during water-cooling, which have strong dependence on the metal surface temperature, can rapidly change with time as the strand cools down. Consequently, uncontrolled cooling may cause fluctuations in the temperature gradients inside the solidifying shell and generate tensile thermal stresses at the solidification front that can ultimately lead to the appearance of hot tears/cracks in the final product. This paper compares and contrasts the water-cooling techniques used for casting steel and aluminum and discusses their implications in terms of final product quality based on fundamental studies and predictive mathematical models. Finally, optimal practices for the control of cooling in casting processes for both steel and aluminum alloys are evaluated. Introduction The technology used for continuous casting of steel and aluminum has progressed significantly over the past several decades, although the two processes have developed distinct differences. The productivity of the continuous casting process is generally controlled by the casting speed, which varies with alloy composition and product geometry. For steel billets, blooms, and slabs, the casting speed increases with decreasing thickness from 10 mm/s (for 300 mm blooms) to over 80 mm/s (for 50 mm thin slabs). Owing to cracking difficulties during startup, aluminum alloy ingots are cast at much lower speeds, increasing from ~0.75-1.0 mm/s during startup to steady state speeds ranging from 1.0-3.0 mm/s. 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 ~700-1200 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 (5~30 m in depth, depending on the casting speed and thickness). Motor-driven drive rolls located far below the mold continuously withdraw the strand downward. Many closely spaced support rolls prevent the outward bulging of the shell due to the ferrostatic pressure arising from the liquid steel core. Water sprays emerge from high-pressure nozzles, which are interspaced between the support rolls and cool the strand during the solidification process. Other strategically placed rolls bend the shell to follow a curved path and then straighten it flat prior to torch cut-off into individual slabs. This allows fully continuous operation. Start-up of this process is a relatively rare occurrence, and is achieved by inserting a “dummy” bar to plug the mold bottom. Thus, the first steel cast in a sequence can be routinely downgraded or scrapped for defects without incurring a significant yield loss. The D.C. casting process for aluminum alloys is shown schematically in Figure 1(b). In contrast to the continuous casting process for steel, D.C. casting is only semi-continuous, as the strand is withdrawn vertically for a short length (~10 m) until the process must be stopped and restarted when the cast ingot reaches the bottom of the casting pit. Thus, considerable attention must focus on the initial start-up stage, when defects are most likely to be initiated. To start the process, a bottom block is partially inserted into an open rectangular mold (usually ~100-150 mm in length). Superheated liquid aluminum flows through a launder, down the nozzle spout, through a distribution bag, and into the mold, at a predetermined, time-varying filling rate. Once the molten metal fills the bottom block to a prescribed height, the bottom block and cast ingot are lowered into a casting pit. The aluminum ingot is subjected to cooling by the transfer of heat to the water-cooled aluminum mold over a very short length (~70-90 mm), and to cooling through the contact of chill water with the solid shell after it emerges from the mold cavity. This water emerges from a series of holes, which surround the mold at MS&T 2004 Conference Proceedings, (New Orleans, LA), AIST, Warrendale, PA 179 its base. The defining character of the D.C. casting process is the extraction of heat due to this direct impingement of water on the ingot surface – typically more than 80% of the total heat is removed by this method under steady state conditions. The thermal field in this semi-continuous process can be considered to develop in two distinct stages. During the start-up, or Stage I, the liquid pool profile and thermal field continuously evolve with time. Finally, during steady state, or Stage II, the liquid pool profile remains essentially constant or “fully developed”, relative to the mold (typically ~200-500 mm in depth depending on the ingot size and alloy composition ). Steady state operation is usually achieved within a cast length of ~0.5-1 m.