Novel Concept of TWIP-Assisted Steel

Abstract

TWIP steels are known for their high strength with elongation but have manufacturing and cost disadvantages due to their high manganese content. The novel concept of TWIP-assisted steels has been introduced in analogywith TRIP-assisted steels. The aim was to produce local regions of austenite which contains manganese high enough to exhibit localized TWIP during thedeformation. To achieve the aim, the cementite particles in ferrite matrix were formed by tempering in the ferrite+cementite field. The cementite would enrich withMn after appropriate time. Then, this enriched cementite was supposed to transform into austenite by isothermal heat treatment. The austenite would inherit the high manganese content and be retained on cooling. The finalmicrostructure would contain Mn-rich austenite islands in ferrite matrix.Three alloys were designed using thermodynamic calculation considering the volume fraction and Mn content of cementite. The appropriate time to enrich cementite was decided using thermodynamic calculation with a finitedifference kinetic model. A series of heat treatment and microstructural and microanalytical characterization were done. The most important result was that austenite which formed at ferrite/cementite interface grew by consuming both ferrite and cementite simultaneously. Even though cementite bears high manganese, growth into ferrite resulted in dilution of Mn. The tensile testing was attempted but the samples were so large that they showed low heating rate. They resulted in different microstructure from the ones produced by dilatometry, thus, exhibiting disappointing tensile properties.