A Comprehensive Analysis of Macrosegregation Formation During Twin-Roll Casting
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DUCTION
MACROSEGREGATION is a term that denotes a concentration inhomogeneity at the scale of a casting. It poses a severe quality issue for big ingots or continuously cast slabs. It is generally believed that the smaller the dimension of a casting the less distinctive are macrosegregations. This opinion is motivated by the fact that in most cases. the relative motion between solute-enriched liquid and solute-depleted crystals necessary for the formation of macrosegregation is caused by (i) natural or forced convection, (ii) sedimentation of crystals, and/or (iii) solidification-induced feeding flow.[1–4] However, if deformation during solidification is the main reason for the relative motion between solid CHRISTIAN M. G. RODRIGUES, ANDREAS LUDWIG, and MENGHUAI WU are with the Metallurgy Department, Montanuniversitaet Leoben, Franz-Josef Street 18, 8700 Leoben, Austria, Contact e-mail: [email protected] ABDELLAH KHARICHA is with the Metallurgy Department, Montanuniversitaet Leoben and also with the Christian-Doppler Laboratory for Metallurgical Application of Magnetohydrodynamics, Montanuniversitaet of Leoben, Franz-Josef Street 18, 8700 Leoben, Austria. ALEXANDER VAKHRUSHEV is with the ChristianDoppler Laboratory for Metallurgical Application of Magnetohydrodynamics, Montanuniversitaet of Leoben. Manuscript submitted December 03 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
and liquid, then one can conclude that macrosegregation can also form in thin products such as in sheets produced by twin-roll casting. Twin-roll casting is a technology that has been used over the recent decades for commercial production of thin metal strips. During this process, the molten metal is injected into the gap between two water-cooled counter-rotating rolls. As a result, it solidifies and the corresponding solid shell that forms on each of the moving roll surfaces is subjected to a considerable amount of hot rolling before reaching the roll nip. This technology has many advantages compared with conventional casting techniques. The major advantage has to do with the reduced number of operational steps in the production line. This leads to a reduction in investment and processing costs, as well as less-rigorous logistics and labor requirements.[5,6] According to Barekar and Dhindaw,[7] it also yields fewer greenhouse gas emissions. Furthermore, due to the very high cooling rates, the final strand is expected to have a refined microstructure with improved mechanical properties.[8,9] On the other hand, combining metal solidification with hot rolling into one single step makes it more sensitive to process conditions,[10] and eventually susceptible to a number of casting defects.[11–14] In fact, over the years, as the emphasis has been growing for
improving productivity, increasing attention has been placed on the understanding of the corresponding complex melt flow and solidification patterns. The behavior of metallic alloys in the semisolid state is complex as it has been found that its behavior changes dramatically
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