| Strip casting process has been developed to manufacture thin steel strip, 2 to 4 ㎜ thick, directly from molten metal fed in between two water-cooled casting rolls, and is being applied to the industrial steel making plants. The cast strip contains typically 2 - 6% of delta ferrite in the microstructure at room temperature. This work studied the dissolution kinetics of delta ferrite in strip cast specimen for 1050℃ - 1200℃ temperature range by varying the annealing time. The morphology of delta ferrite in the cast strip was in vermicular type with a three-dimensional honeycomb network structure. At early stage of dissolution of delta ferrite, more than 50% of delta ferrite dissolved very rapidly regardless of annealing temperature. The reason for the rapid dissolution was suggested that Cr depletion in the delta ferrite excessively occurred due to (Fe, Cr) carbide precipitation at interface boundary between delta ferrite and austenite phases. Subsequently, the dissolution was controlled by volume diffusion of Cr and Ni in austenite phase. The analytical solution assuming the austenite phase as semi-infinite can be applied to the analysis of experimental data at 1150℃. The activation energy for the dissolution was measured as 251.4 kJ/㏖e, which is coincident with activation energy of Cr volume diffusion constant. |
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