Monitoring the phase evolution and fracture behavior of advanced multiphase QP steel using EBSD technique and digital image correlation

A novel multiphase 0.17C-5Mn-0.88Si-0.76Al-0.22Mo-0.06Nb steel was subjected to complex thermomechanical processing schedules to produce quenching and partitioning (Q&P) microstructures. The tests were performed in the Gleeble 3800 simulator applying multiple deformations in a plane state of strain and final deformation was applied at 850 degrees C. The samples were controlled cooled to 240 degrees C and 260 degrees C to produce some martensite fraction and subsequently partitioned at a temperature of 450 degrees C to enrich austenite in carbon from the supersaturated martensite. Different primary/secondary martensite microstructures containing retained austenite were produced, which were subjected to detailed microstructural and fracture studies using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The mechanical behaviour of the samples was investigated in static tensile tests using sub-sized flat samples. Digital image correlation (DIC) measurements were performed to monitor the strain distribution in a micro scale. Relationships between processing conditions and fracture behaviour were assessed in fractographic tests.