Analysis of columnar-to-equiaxed transition experiment in lab scale steel casting by a multiphase model
Sachi Gowda SMiha ZaložnikHervé CombeauCharles‐André GandinMarvin GennessonJoëlle DemurgerM. StoltzIsabelle Poitrault
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Abstract Correct prediction of composition heterogeneities and grain structure across a steel ingot is critical in optimizing the industrial processing parameters for enhanced performance. The columnar to equiaxed transtion (CET) is a microstructural transition which is strictly controlled as it affects the mechanical properties of the final product along with the macrosegregation patterns. Larger equiaxed regions are preferred for most industrial applications. CET is significantly affected by the number density of equiaxed grains and by the nucleation undercooling. 8 kg 42CrMo4 alloy steel ingots (240 mm x 60 mm x 60 mm) were cast. The cast structure was characterized by ASCOMETAL. The experiments were simulated with a process-scale model of solidification that incorporates a multiscale description of the microstructure formation. The goal of the present study is to show the capabilities of such a process-scale solidification model to explain the observed structure distributions (extent of the columnar and equiaxed zones, equiaxed-to-columnar transition).Keywords:
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2024 Al alloy ingot with diameter of 100 mm is produced by conventional and combined electromagnetic fields (CEMF) horizontal direction chill (HDC) casting processes. The effect of CEMF on the ingot structure is investigated. The results show that the CEMF is effective in changing the inner structure of the 2024 HDC cast ingot. In the conventional HDC ingot, feathery grains mainly distribute over the majority of the area of the ingot, and coarse equiaxed grains mixed with coarse floating grains distribute in a crescent-shaped area near the bottom surface. With the application of the CEMF, the majority of the feathery grains transform to fine equiaxed grains, and the mixture of feathery and fine grains only exist in the area near the upper surface. In addition, the coarse floating grains completely disappeared in the ingot with the application of CEMF.
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A series of experiments were conducted by Al-2·0Si%Si alloy to investigate the effect of mould D/H ratio and shape on solidification structure under ultrasonic vibration (UV). Finally, the mechanism of grain refining was discussed. The results show that equiaxed grain occupancy is maximized when D/H is equal to 0·7 for ingots with mould volume of 80 and 120 cm3, however, the equiaxed grain occupancy has the maximum when D/H is equal to 2·0 for the ingot with mould volume of 200 cm3. Otherwise, the equiaxed grain occupancy has downtrend with increasing of mould volume. The columnar structure is easily formed in ultrasonic ingots with complex mould shape and more corners.
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A mathematical model of equiaxed grains movement and macro-segregation has been built to analyze the fluid flow and species distribution during the metal solidification. The mushy zone was divided into dendrites region and free equiaxed grains region by the dendritic coherency point. Macro-segregation of a steel ingot in a rectangular mold with a riser was simulated and the calculation result was compared with that of an experiment. It shows that the species distribution obtained by the grain movement model is more consistent with experiment comparing to that by the solid skeleton model in mush. The equiaxed grains move with the fluid and accumulate at the bottom center of the ingot during solidification. The cone-shape negative segregation forms after solidification. The positive segregation in the upper center and the negative segregation in the exterior region of the ingot are found at the same time.
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In the present work macrosegregation during solidification of a 2.45 ton steel ingot is simulated with a pure equiaxed model, which was tested previously via modeling of a benchmark experiment. While the columnar structure is not taken into account, a packed layer formed over inner walls of the mold at an early stage of solidification reproduces to some extent phenomena generally related to zones of columnar dendrites. Furthermore, it is demonstrated that interaction of free-floating equiaxed grains with ascending convective flow in the bulk liquid results in flow instabilities. This defines the irregular form of the negative segregation zone, the formation of which at the ingot bottom corresponds to experimental observation. Vertical channels reported in experimental measurements are reproduced in simulations. It is confirmed that intensification of ingot cooling may decrease segregation in the ingot.
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Abstract The control of the carbon macrosegregation level in steel ingots is important for the structural integrity of the final component. During solidification, the fragmentation of the columnar dendrites is an important source of equiaxed grains, and has a large influence on the macrosegregation and the grain structure. The goal of this study is to show that a numerical model that takes into account fragmentation can describe the formation of the structures and the macrosegregation during solidification of a large steel ingot. The present article describes the multiphase numerical model used in the simulations. The simulation results are compared to experimental data from a 9.8 t ingot cast in A5/6 steel by ArcelorMittal Industeel. The model can then be used to explain the formation of the observed structures. For example, we show that the structural transitions, first from columnar to equiaxed globular and then to equiaxed dendritic at the bottom of the ingot are a consequence of the concurrent transport and growth of the dendrite fragments from the columnar zone. Furthermore, we show that most of the structures are formed very early on during solidification, whereas macrosegregation develops much more gradually.
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