Microstructure Evolution in Belt-Casted Strip of Grain Oriented Electrical Steel

Abstract

This paper deals with the formation and decomposition of Widmanstätten austenite during solidification of the thin belt-casted strip made of a grain oriented electrical steel (GOES). Solidification of liquid steel starts with the formation of d-ferrite. Cooling in the delta + gama phase field results in the formation of a small fraction of Widmanstätten austenite by displacive transformation accompanied by carbon partition. Widmanstätten austenite laths have an orientation relationship with the ferrite grain into which they grow. Furthermore, they form a flat low energy interface along the ferrite grain boundary. In order to minimize the interfacial energy, ferrite grain boundaries in the vicinity of flat austenite/ferrite interface facets are forced to migrate which results in straightening of these grain boundaries. If parallel Widmanstätten austenite laths form in two adjacent ferrite grains, zig–zag ferrite grain boundaries arise. Precipitation of sulphides along ferrite/austenite interfaces make it possible to study the early stages of austenite decomposition under the delta + gama phase field. It starts with the formation of epitaxial ferrite 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 results in a wavy shape of these ferrite grain boundaries. Finally austenite transforms either to pearlite or to plate martensite.