STUDY ON SEGREGATION OF DIRECT CHILL CAST PURE ALUMINIUM SLABS IN EACH STAGE OF PRODUCTION OF THIN SHEETS

Abstract

Direct Chill Casting of pure aluminium has special solidification process different from the Book Mold Casting.To acpuire the essential characteristics of Direct Chill Casting, the samples were taken from the standard slabs produced in commercial plant, and, in order of producing operation, the properties of molten metal, slab, hot rolled plate and cold rolled coil were studied systematically.The results of the studies on the relations of producing processes and on the residual effects of the preceding operations are as follows:1) Molten metal: Chemical composition is uniform at the holding stage.Mechanical properties are improved by holding and degassing, but at the first stage of pouring, are dateriorated temporarily and according to stabilization of operation come back to the improved state again.2) Slab: Macro-structure is fine and uniform except the part close to the sprue and other thin skin of the slab.Impurities of outer layer are very high, those of the layer from 2-10mm deep are lowest and those of the part deeper than 10mm are uniform which shows the mean value of the original molten metal.Hardness distribution is similar to that of composition.Tensile properties and impact value are uniform.Microstructure consists of three parts, namely, finely chilled part, slowly cooled part of columner structure, and granular, dendritic part in which “Schweb-kristall” is observed.We can't observe the periodic sweating and dark zone in which may not be able to exist in such slab made under the special casting control as this.3) Hot rolled plate:Segregation of surface in longitudinal or transverse direction is not recognized, but in thickness direction it exists due to the residual effect of the slab. Impurities are high at skin and low at center.Tensile strength in longitudinal direction is lower at central part due to the effect of different distribution of hot rolling temperature and cooling rate.Micro-structure in thickness direction differs remarkably at skin and center.4) Cold rolled coil: Local difference of chemical composition through the coil is neglisible. Tensile strength as rolled state is uniform in the coil, but after annealing it is lower at the center part of the coil. Grain size at center part is finer than outer part, which seems to be due to the residual effect of hot rolling condition.