Metallurgist, Vol. 53, Nos. 5–6, 2009
IMPROVED FEED OF CASTING POWDERS INTO CONTINUOUS-CASTING-MACHINE MOLDS
A. V. Kuklev, Yu. M. Aizin, I. F. Goncharevich, and A. V. Manuilov
UDC 669.18:621.746.047
The surface condition of cast metals depends to a great extent on the conditions under which the ingot skin forms in the mold, as well as the volume and uniformity of feed for the steel mixture poured into the mold. A computer model was developed for the mold–lubricant–ingot system. The effect of the volume of lubricant fed into the mold on the magnitude and type of loads on the skin of the ingot as it passes through the mold. An experimental stand was developed as a model for a simple device for feeding casting powders into continuous-casting machine molds. Key words: continuous-casting machine, mold, casting powder, thermal conditions during crystallization.
As the ingot passes through the mold, friction against the walls of the mold causes undesirable adverse effects due to tensile stress that may lead to tears in the skin. Rocking the mold in a special pattern enables the interaction between the mold walls and the ingot skin to be controlled by reducing the frictional force between them, thereby also reducing the tensile stress. A similar effect is achieved by adding casting powders to the mold; at high temperature, these casting powders act as a lubricant on the ingot. The surface quality of the slabs (number of longitudinal cracks, transverse cracks, and spider cracks, cracks resulting from traces of the mold rocking, and surface slag inclusions) and the macrostructure of the metal are highly dependent on the composition and physical properties of the casting powders. These powders should provide the following: minimization of the force required to draw the slab from the mold for specified intervals of pour speeds, mold rocking frequency and amplitude, optimization of the distribution of thermal flux from the ingot skin to the mold walls, and reduction of the likelihood of direct contact between the slab skin and the mold. The required properties of the casting powder as a lubricant also depend on the mold rocking amplitude, mold rocking frequency, and shape of the mold rocking curve, i.e., on the amount by which the mold leads and the mold taper. Thus, as the lead time increases, the specific consumption of casting powder increases, and the size of the rocking motions requires a reduction in the viscosity and melting point of the mixture. The role of the lubricant is as follows: in the absence of lubricant, dry friction forces and plastic deformation forces predominate; when sufficient lubricant is present, viscous drag forces (which are much smaller) are the primary forces acting on the contact surfaces. In this connection, one important means of ensuring stability of the continuous-casting process is to ensure that lubricant is added in sufficient quantities to fill the gap between the ingot skin and the mold. Volume, uniformity of feed, and uniformity of distribution over the surface of the melt in the mold are primary considerations in this process. Available information [1] indicates that proper arrangements for lubrication of contact surfaces reduces the drag as the ingot moves through the mold by a factor of 1.5–2.5, and virtually eliminates cases in which the ingot skin becomes “welded” to the walls of the mold. The surface condition of continuous-cast ingots depends to a great extent on the conditions under which the skin of the ingot forms in the mold, as well as the volume and uniformity of feed for the steel mixture poured into the mold. This Korad ZAO, 21 Zaraiskaya St., 109428 Moscow, Russia; e-mail:
[email protected]. Translated from Metallurg, No. 5, pp. 40–41, May, 2009. Original article submitted April 1, 2009. 274
0026-0894/09/0506-0274 ©2009 Springer Science+Business Media, Inc.
Fig. 1. Load on ingot skin with insufficient (a), average (b), and sufficient (c) lubricant.
attempt to improve the quality of the surface and subsurface layers in the continuous-cast ingot (reduce the cracks from mold rocking, reduce the height of “folds,” eliminate microeddies, cracks, etc.) leads to the need to optimize the rocking modes based on a variety of criteria, and, in particular, the conditions for addition of lubricant to the mold. Korad specialists have developed a software-based model for the crystallizer–lubricant–ingot system. Mathematical interpretation of this system includes an inertial (two-mass) model of the continuous-cast-ingot lubricant with three contact zones (between the mold wall and first layer of lubricant, between the first and second layers of lubricant, and between the second layer of lubricant and the ingot), whose elements fall into the aforementioned zones. Use of lubricant during the model study enabled the actual deformation processes occurring in the lubricant layer to be analyzed, and for their effect both on the nature of the loads generated on the ingot skin in the mold, and on the defects occurring in the ingot skin. Moreover, use of the techniques developed by Korad will support: analysis of the effects of mold rocking modes on formation of the ingot surface and drag during passage of ingot through mold; optimization of the process for addition of casting powder; selection of the most efficient rocking configurations and parameters; and many other items. The methods developed were used to analyze the impact of the lubricant feed volume on the magnitude and type of loads on the ingot skin as the ingot passes through the mold (see Fig. 1). The graphs clearly indicate that when insufficient lubricant is used, dry friction forces (plastic deformations) predominate, and tensile stresses (the most hazardous) are especially large (see Fig. 1a). The transitions from compression to tension and from tension to compression in the ingot skin are very abrupt. These transitions are when tears in the ingot skin can occur. As the lubricant feed increases (see Fig. 1b), the loads decrease, and viscous drag forces start to become dominant. The transitions from tension to compression and from compression to tension involve smaller changes in load, and are therefore less hazardous. Further increases in lubricant feed rate (see Fig. 1c) reduce the loads even further, and, especially importantly, reduce the tensile loads, which are most hazardous. The research performed also enabled analysis of the mechanisms behind sticking friction between the ingot skin and the mold walls when lubricant feed is too low or the lubricant is not uniformly distributed around the perimeter of the ingot. 275
There are two approaches for efficient feed of lubricant into continuous-casting molds: the first involves ensuring a stable feed as required and simultaneously ensuring a uniform distribution of casting powder (which is converted into lubricant upon melting) over the entire mold cross section, while the second involves selecting the mold rocking modes in such a way that a sufficient quantity of molten casting powder/lubricant is fed into the gap between the mold walls and the ingot skin. Modern practice (primarily foreign) calls for the use of granulated casting powders, and they are generally fed into the mold using pneumatic feed systems. However, pneumatic systems are rarely used to feed the powder-type casting powders generally used at domestic enterprises, due to the risk of the mixture becoming segregated during transportation because of the large variation in the aerodynamic properties of the powder components. Casting powder is generally fed into molds using complex, expensive, equipment with programmable controls. Most of these are fully-enclosed screw conveyors on mobile carriages attached to each side of the casting ladles. The casting powder is fed into the mold through a cut in the screw conveyor pipe. The casting powder is distributed over the surface of the melt via periodic rotation of the pipe in the space between the mold walls. Based on the research performed, Korad developed an experimental test stand (based on vibration technology) as a model for a simple device for feeding casting powders into continuous-casting machine molds. Tests using this test stand indicate that the vibrational feed (which has a simple design) provides uniform simultaneous distribution of powder over the entire surface of the melt, such that: the same amount of powder is fed to each point on the open surface of the melt at any given time, so that the temperature remains constant over the entire free surface of the melt This mold operating mode will presumably eliminate a variety of technological problems in continuous casting of steel. Another simple mechanism consisting of a vibrating nozzle on a mass-produced flexible screw conveyor is currently being developed. Not all of the casting powder feed devices currently in use enable the casting powder to be fed simultaneously to all points on the open surface of the melt. Thus, these newly developed casting-powder feed systems, which provide uniform delivery of the mixture to the entire cross section, will enable more reliable control over heat and mass exchange processes in the melt. Use of more efficient devices for casting-powder feed mechanization will have the following effects: optimization of thermal conditions in the metal through more uniform feed of casting powders into the gap between the mold walls and the ingot, reduction in the drag force opposing passage of the ingot through the mold (due to guaranteed uniformity of powder feed), and improved surface quality of the continuous-cast ingots.
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