3D Study of Microstructural Influences on Retained Austenite Transformation in Q&P 1180 Steel

Advanced TRIP steels offer an attractive combination of strength and ductility because of the transformation-induced plasticity (TRIP) phenomenon. The retained austenite (RA) embedded in quenched and partitioning (Q&P) 1180 steel provides vital ductility, relating to the propensity of these grains to transform under applied deformation. It is well known that the characteristics of the RA grains (size, shape, orientation, etc.) have a strong influence on their stability, but few studies consider the accurate 3-dimensional character of the grains, due to the cost of extracting 3D data. This study observes the characteristics of RA grains in Q&P 1180 steel before and after applying tensile deformation. EBSD maps of serial sectioned layers are reconstructed using DREAM3D. The influence of 3D morphology and other factors on transformation of RA is studied. Apart from relatively traditional metrics, a novel shear affinity factor is introduced as a metric to describe the ease of transformation for an RA grain. The 3D nature of the information collected allows accurate classification of grain shape into the traditional globular/spherical and lamellar/lath categories, along with disk and needle shapes, and enables quantification of the evolution of the shape distributions.