Effect of As-Cast Structure and Macrosegregation on Mechanical Properties in Direct-Quenched Low-Alloy Ultrahigh-Strengt
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HOWE (1916) describes the basic principles of the formation of segregation in steel castings. Macroscopic variations in the composition of a casting by the gradual separation of the different chemical constituents during solidification are called macrosegregation and correspondingly the chemical heterogeneity at the scale of the microstructure is known as microsegregation. Both macro- and microsegregation originates from the partitioning of alloying elements between the solid and liquid phases coupled with the movement of the coexisting liquid and solid phases in the mushy zone.[1] Flemings[2] summarized the state of knowledge about macrosegregation in the twentieth century in his review article. Generally, macrosegregation is the result of S. KOSKENNISKA, A. KAIJALAINEN, D. PORTER, and J. KO¨MI are with the Materials and Mechanical Engineering, Centre for Advanced Steel Research, University of Oulu, P.O. Box 8000, 90014, Oulu, Finland. Contact e-mail: sami.koskenniska@oulu.fi T. PIKKARAINEN and S. MEHTONEN are with the SSAB Europe Oy, Rautaruukintie 155, P.O. Box 93, 92101 Raahe, Finland. Manuscript submitted December 13, 2019; accepted October 2, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
interdendritic flow, driven by i.e., geometry, gravity, shrinkage, bulging, and solid deformation and in specific circumstances the settling of solid in the early steps of solidification.[2] Ghosh[3] explained the fundamentals of microsegregation, which is confined to the micron-scale interdendritic spaces of the solidifying melt and can be alleviated by subsequent homogenization heat treatments unlike macrosegregation, which is therefore more harmful to the finished steel properties. Later refinements include treatment of dendrite tip undercooling and further research of microsegregation to more complex alloys. In the case of low-alloy steel solidification, accumulation of solute-enriched liquid leads to positive macrosegregation, for example on the centerline of the casting, and the settling of solute-lean solid regions leads to negative segregation.[3] A columnarto-equiaxed transition (CET) zone is formed once the equiaxed grains in the central part of the solidifying casting are present to prevent the columnar front movement. For example, Choudhary and Ghosh[4] explained that the formation of an equiaxed zone demands the existence of solidification nuclei in the liquid and circumstances under which the nuclei growth and number density is sufficient to arrest the propagation of the columnar dendrites.[4] High superheat
increases the amount of columnar dendritic structure and when the superheat is high enough, the columnar dendrites can reach to the centerline of a continuously steel cast slab, while a low superheat can lead to a fully equiaxed dendritic structure as showed by Choudhary and Ganguly.[5] With intermediary superheat in casting, the positive macrosegregation may appear at the CET boundary as Pikkarainen et al. proposed in their article.[6] Zhang et al. have investigated the effect of filling velocity on positive macr
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