Q&P steels are alloyed with a medium amount of carbon, and small amounts of silicon and manganese. Their microstructure is composed of tempered martensite, fresh martensite and retained austenite which gives them a good balance of strength and ductility, fulfilling the increasing interest of automotive industry.
Uniaxial tensile tests are commonly employed to reveal the material tensile properties, e.g. strength, ductility and strain hardening. However, they do not give a good indication of the formability of a steel sheet under complex forming operations. In this work, stress triaxiality in steel sheets is engaged as a key indicator to evaluate the resistivity against crack propagation. Two different microstructures, one with a banded distribution of elements and one with a homogeneous distribution were subjected to quasi in-situ 3 point bending measurements of notched samples.
Subsequently they were investigated using various orientation microscopy techniques, mainly electron back-scattered diffraction (EBSD) and electron channelling contrast imaging (ECCI) to reveal the crystal orientations and dislocation structures close to the crack tip. The crack propagates more easily at regions where a large strength difference between the hard martensite and soft austenite/ferrite is observed, especially in areas with localised soft austenite/ferrite banding. Hence, the strain partitioning at austenite/martensite interfaces and the strength contrast between austenite and martensite play important roles on the crack initiation.