Dissolution kinetics of delta ferrite in AISI 304 stainless steel produced by strip casting process

Strip casting process has been developed to manufacture thin steel strip, 2–4 mm thick, directly from molten metal fed in between two water-cooled casting rolls, and is being applied to the industrial steelmaking plants. The cast strip contains typically 2–6% delta ferrite in the microstructure at room temperature. This work studied the dissolution kinetics of delta ferrite in strip cast specimen in the temperature range of 1050–1200 °C by varying the annealing time. The morphology of delta ferrite in the cast strip was a vermicular type with a three-dimensional honeycomb network structure. At the early stage of delta ferrite dissolution, more than 50% of delta ferrite dissolved very rapidly regardless of annealing temperature. The rapid dissolution was mainly attributed to the excess Cr depletion in the delta ferrite resulting from the (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 the austenite phase. The analytical solution assuming the austenite phase as semi-infinite was successfully applied for the analysis of experimental data at 1150 °C. The activation energy for the dissolution was measured as 251.4 kJ mol−1, which is consistent with activation energy for volume diffusion of Cr.