Solidification modelling of microstructures in near-net-shape casting of steels

Abstract

The relationship between heat transfer conditions and microstructure in thin slab casting and twin-roller thin strip casting is analysed theoretically utilizing a one-dimensional solidification model. Experimentally determined secondary dendrite arm spacing characteristics λ 2( x) in thin slabs and thin strips as functions of the distance x from the chill surface could well be approached by the model applying the well-known empirical relations between local solidification time and λ 2 for different steel grades. Stab-mould heat transfer coefficients of the order of 1.5–4.5 kW m −2 K −1 were obtained from the best fits of the experimentally determined λ 2( x) characteristics of thin slabs. The magnitude of heat transfer coefficients depends on the cast steel grade and well exceeds the heat transfer in conventional continuous casting. Microsegregation between dendrite arms in thin slabs was shown to be slightly enhanced compared with conventional slabs. Overall λ 2( x) characteristics of thin strips could be fitted with strip-roller heat transfer coefficients h ≈ 5–8 kW m −2 K −1. Detailed investigation of lateral inhomogeneities of the dendrite structure, however, revealed localized areas of excellent heat contact exceeding h ≈ 10 kW m −2 K −1.