Determination of stacking fault energy of cold rolled medium manganese steel by X-ray diffraction

Objectives

To determine the stacking fault energy (SFE) of the austenite phase in cold-rolled medium Mn steel by x-ray diffraction technique, previously intercritically heat-treated for austenite stabilization. Further to investigate the deformation behaviour of the austenite phase and to find out the correlation between the SFE and deformation behaviour of austenite.

Technical Details

Over the last decade, research in the field of Advanced High Strength Steel (AHSS) for automotive applications has undergone a paradigm shift from low Mn1st-Gen AHSS grades such as TRIP steels and high Mn 2nd-Gen AHSS such as TWIP steels to medium Mn (3-10 wt.% Mn) TRIP and/or TWIP aided steels. This is primarily due to the unique combination of strength and ductility obtained in medium Mn steels. Furthermore, there are few inherent problems associated with these steels which challenge their suitability in modern automotive. In one case 1st-GenAHSS are becoming inferior w.r.t. modern car safety standards while in other case, 2nd-GenAHSS has problems associated with processing, weldability and high cost (Mn>20%). To overcome these, the concept of medium Mn steels has evolved in which enhancement of properties is achieved through post-processing treatments to engineer the composition and volume fraction of the austenite phase. It is well known that the deformation behaviour of medium Mn steels is directly associated with composition and volume fraction of the austenite phase. These medium Mn steels are new whose deformation behaviouris not yet fully understood. Since, the deformation behaviour of austenite is highly dependent on the SFE, it requires precise measurement of SFE by some suitable technique. The measured SFE value will decide the deformation mode of austenite phase (i.e. whether it will proceed by martensitic transformation or via formation of twins in the microstructure) and subsequently the deformation mode controls the final mechanical properties of the material.

Achievement (Output/outcome):

1.670 oC is the intercritical temperature for complete dissolution of carbides in the microstructure.

2. 640 oC is the intercritical temperature for obtaining austenite phase with optimum stability such that it yields highest UTS*TE value (~ 26.8 GPa%).

Fig.1: Schematic of heat treatment schedule showing the evolution of microstructure before and after the intercritical annealing.

Fig.2: SEM micrograph of the specimens annealed at different intercritical  temperatures for 1 hour.

Fig.3: (a) Bright field TEM micrograph showing retained austenite films indicated by arrows (b) Dark field TEM micrograph corresponding to the austenite diffraction spot.