Generalized stability criterion for controlling solidification segregation upon twin-roll casting

Abstract

• Systematical framework integrating dendritic growth model, overall solidification kinetic model and generalized stability was constructed. • Processing parameters of the casting speed and the flow speed of cooling water in twin-roll casting are coupled. • Sub-rapid solidification kinetics, concentration and segregation by different processing parameters couple were established. • New strategy was proposed: high initial driving force suppresses centerline segregation, and high final generalized stability suppresses edge segregation. Macro- and micro-segregation formed upon twin-roll casting (TRC) can be inherited from sub-rapid solidification to solid-state transformation, even to plastic deformation, thus deteriorating drastically mechanical properties of as-produced thin sheets. Although many works focusing mainly on controlling fields of thermal, concentration and convection have been reported, how to control artificially and quantitatively the segregation using a theoretical connection between processing parameters and solidification models, has not been realized, yet. Regarding it, a systematical framework integrating non-equilibrium dendritic growth and overall solidification kinetics with the TRC parameters, was constructed applying a generalized stability (GS) conception deduced from transient thermodynamic driving force Δ G t and transient kinetic energy barrier Q eff t evolving upon solidification. Departing from this framework considering synergy of thermodynamics and kinetics (i.e., thermo-kinetic synergy), a criterion of high Δ G t -high GS guaranteed that the macro (i.e., the centerline) and the micro (i.e., the edge) segregation can be suppressed by increasing Δ G t and GS at the beginning and the ending stage of sub-rapid solidification, respectively. This typical thermo-kinetic combination producing the microstructure can be inherited into the plastic deformation, as reflected by corresponding strength-ductility combinations. This work realized quantitative controlling of TRC by a theoretical connection between processing parameters and solidification models, where, an optimization for sub-rapid solidification segregation using the GS conception including Δ G t and Q eff t has been performed.