Transformation Induced Plasticity in low alloyed TRIP-steels and microstructure response to a complex stress history

Presently, high strength thin sheet grades are gaining considerable market shares due to the intensified application in the automotive industry. Although several new high-strength steels for cold forming have been developed and continuously improved for some decades, there is still a necessity to increase the formability at a given strength level. TRIP steels are a consistent further step to meet this requirement.

By properly alloying with e.g. Si, Mn, Cu, Al,.. and applying special heat treatments it is possible to stabilize some austenite (retained austenite, RA) at room temperature even in low alloyed ferritic steels. Mechanical loading then initiates the strain induced austenite to martensite phase transformation, which enhances the ductility of the material and increases its strength via the TRIP-effect (transformation induced plasticity).

The stability of retained austenite against strain induced martensitic transformation depends on the chemical composition, the thermomechanical history, morphological and topological properties of the RA and the intensity and type of mechanical loading. Moreover, the kinetics (i.e. change of RA content as function of applied strain) strongly depends on the mutual crystallographic orientation of the phases (see images).

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Hence, an evaluation of the TRIP-steelís properties demands a thorough knowledge of the microstructural changes upon mechanical loading. In this project, several loading conditions are applied to industrially produced TRIP-steels of varying chemical composition and thermomechanical processing. The microstructural changes are monitored using light and electron microscopy. Numerical simulations support the interpretation of the microstructural investigations.

For further information on this project in cooperation with
Voest Alpine Stahl Linz at the Christian-Doppler-Laboratory for Modern Multiphase Steels, please contact Prof. E. Werner.