1. Field of the Invention
This invention relates broadly to the field of metal production and casting. More specifically, this invention relates to an improved mold for a continuous casting system that has a longer useful life, improves the uniformity of heat removal, and turns out a better product than conventional continuous casting molds do.
2. Description of the Prior Art
A conventional continuous casting mold includes a number of liner plates, usually made of copper, and outer walls surrounding the liner plates. The liner plates define a portion of the mold that contacts the molten metal during the casting process. Parallel vertically extending water circulation slots or passageways are provided between the outer walls and the liner plates to cool the liner plates. During operation, water is introduced to these slots, usually at the bottom end of the mold, from a water supply via an inlet plenum that is in communication with all of the slots in a liner plate. The cooling effect so achieved causes an outer skin of the molten metal to solidify as it passes through the mold. The solidification is then completed after the semi-solidified casting leaves the mold by spraying additional coolant, typically water, directly onto the casting. This method of metal production is highly efficient, and is in wide use in the United States and throughout the world.
Briefly referring to FIG. 4, both the top and bottom ends of each slot or passageway 22 are conventionally radiused into the plenum 14 at a transition portion 28 in order to minimize flow resistance. Considering that plenum 22 extends along an entire side of a liner plate 20 while the slots 22 are spaced periodically, the plenum 14 is relatively large in a cross-section taken along a normal to the flow direction of the water when compared to the combined cross-sections of the slots 22. As a result, water flow velocity in the plenum area 14 and in the adjacent transition portion 28 tends to be materially less than the water flow velocity in the main portion of the slots 22. In one calculation that was done by the inventor, flow velocity in the plenum area was found to be 2-3 feet per second, while the flow rate in the main portion of the slots was estimated at 20-30 feet per second, a ten-fold difference. This flow velocity differential causes the liner plate to be cooled more effectively at its center than at its top and the bottom. In fact, the low velocity area at the top of a liner plate has, even when positioned adjacent to the meniscus of the molten metal in the mold, been measured to have as high a temperature than areas that are about two inches below the meniscus, when, if the cooling effect was even, it would be expected to have a lower temperature. This uneven cooling effect causes expansion stresses that substantially limit the life of the liner plates.
This invention solves the velocity problem by interposing a velocity plate between the plenum and the transition portion of the slot. The velocity plate increases the velocity of the coolant water at the top and the bottom of the liner plate.
Ideally, only the very top and bottom ends of the slot 22 in a conventional mold should be radiused, and it would be a fairly small radius. The mold that is disclosed in U.S. Pat. No. 3,763,920 to Auman et al. ("Auman") shows a relatively small radius. However, in practice the slot ends tend to end up with a much longer radius, as is shown in FIG. 4, because the slots 22 are cut into the liner plate 20 by a side mill cutter that has a relatively large diameter. This is almost certainly the case in real-world embodiments of the Auman mold. Auman also discloses a dispersion plate that is positioned between its plenum and cooling slots to break the momentum of the inflowing water and equalize flow rate among the different slots. In practice, the water flow in an Auman mold would be impeded because of the narrow gap that is defined between the dispersion plate and the larger-radius transition portion of the cooling slots that the mold would be likely to have. A need exists, then, to ensure that the velocity plate in this invention does not have similar problems.