Abstract
Effects of Nb addition on as-cast γ-austenite grain structure in 0.2 mass% carbon steel are investigated by means of furnace cooling and permanent mold casting experiments. In the furnace-cooled samples with Nb addition, Nb(C,N) particles crystallize from the last-solidifying liquid in non-equilibrium solidification condition and they act as pinning particles for γ grain growth just after the solidification completion. The Nb addition produces a strong pinning effect on the as-cast γ grain structure. In the permanent mold casting experiment, Coarse Columnar Grains (CCG) structure develops from the mold wall in the sample without Nb. The increase in Nb concentration gradually decreases the fraction of CCG region and increases the fraction of Fine Columnar Grains (FCG), thus leading to the grain refinement. This refinement could be ascribed to the pinning effect of Nb(C,N) particles.
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