The Role of New Ferrite on Retained Austenite Stabilization in Al-TRIP Steels

Microstructure and mechanical properties of two high Al, low-Si TRIP steels with different Cr and Mo contents were studied using continuous galvanizing line (CGL) laboratory simulation. Combined use of specific etching methods, optical and electron microscopy observations and EBSD characterization led to verify the epitaxial growth of ferrite during cooling at a moderate rate from the intercritical annealing to the isothermal holding temperature. The amounts of “new” ferrite formed during cooling and retained austenite obtained after processing are much higher in the steel with lower content of hardenability-promoting elements. Measured tensile properties and mechanical behavior of the steel strongly depend on the amount of new ferrite and retained austenite. It is found that the formation of new epitaxial ferrite from intercritical austenite can effectively contribute to the chemical and particle size stabilization of untransformed austenite as well as to obtain the desired TRIP effect under processing conditions highly compatible with industrial practice, i.e. cooling rates near 15°C/s and isothermal holding times at 460°C shorter than 60 s.