Formation and decomposition of Widmanstätten austenite in GOES belt-casted strips

Abstract

This contribution deals with the formation and decomposition of Widmanstätten austenite during solidification of the thin belt-casted strips made of a grain oriented electrical steel (GOES). Solidification of liquid steel started with the formation of δ-ferrite. Cooling in the δ + γ phase field resulted in the formation of a small fraction of Widmanstätten austenite by displacive mechanism accompanied by carbon partition. Widmanstätten austenite laths formed flat low energy interface facets along ferrite grain boundaries. In order to minimize the interfacial energy, ferrite grain boundaries in the vicinity of flat austenite/ferrite facets migrated. It resulted in the formation of either straight or zig-zag ferrite boundaries. Intensive precipitation of sulphides along ferrite/austenite interfaces made it possible to study the early stages of austenite decomposition at the end of the δ + γ phase field. Complex sulphides containing chromium, manganese, iron and copper were identified as the Cr2CuS4 phase. Decomposition of austenite started with the formation of epitaxial ferrite. This was accompanied by further partitioning of carbon into remaining austenite. The growth of epitaxial ferrite into the flat ferrite/austenite interface facets along ferrite grain boundaries resulted in a wavy shape of these boundaries. Finally, remaining carbon rich austenite transformed either to pearlite or plate martensite.