DISTRIBUTION OF DELTA-FERRITE CONTENT IN CONTINUOUSLY CAST TYPE-304-STAINLESS-STEEL SLABS

Abstract

The delta ferrite distribution and morphologies in continuously cast type 304 stainless steel slabs were studied. The M type distribution of delta ferrite content through the thickness of a slab could be explained by the different solidification modes and cooling rates at various depths of a slab. The delta ferrite morphology, solidification structure, and secondary dendrite arm spacing were examined. The finite difference method was used for the analysis of heat transfer in a slab during continuous casting. Unidirectional solidification experiments were also carried out to investigate how the delta ferrite content and morphology changed during solidification and subsequent cooling. In the 60 mm region below the surface of a slab, the delta ferrite content increased with depth. in this region, the secondary dendrite arm spacing was the main factor determining the final delta ferrite content. In the 60-95 mm deep region, the delta ferrite content increased as a result of two factors. One was the decreased diffusion time and the other was the different delta ferrite morphology originating from equiaxed dendrites. In the central region, the delta ferrite content decreased due to the different solidification mode caused by centre segregation. (C) 1995 The Institute of Materials. Manuscript received 11 August 1994.