Formation Mechanism of Band Delta-Ferrite in 416 Stainless Steel and Its Relationship with MnS and M23C6

AISI 416 steel with a banded structure (BS) that is formed by element segregation, and its MnS significantly affects its microstructure and properties during the hot working process. In this study, we investigated the formation mechanism of the BS and its relationship with MnS in 416 steel by carrying out thermodynamic calculations and microscopic characterization measurements. The BS was identified to be a delta-ferrite phase rich in Cr. In addition, when the volume fraction of delta-ferrite was the largest during the 1665 K, the BS was most likely to precipitate. The fine M23C6 particles around the BS enhanced the segregation of Cr and worked with the MnS inclusions to pin the surrounding grains. The metastable-banded structure formed near the BS transformed into a martensite phase. The MnS inclusions pinned and refined the grains and showed four types of distributions.