Outstanding cracking resistance of fibrous dual phase steels

Dual phase (DP) steels are widely used for their excellent combination of strength-ductility-cost perfor- mance. However, several forming and/or structural applications are affected by cracking resistance issues. We show here that DP steels with a ‘Thomas-fibers’ type platelet martensite morphology exhibit sig- nificantly larger cracking resistance compared to corresponding equiaxed microstructures. The superior cracking resistance is demonstrated both for thin and thick specimens processed with different marten- site volume fractions and thicknesses. The cracking of thin sheets involves complex intermingling of work spent for crack tip necking and for material fracture as quantified using the essential work of fracture method, complemented with fractography and microstructure characterization. The better cracking resis- tance is shown to originate from a remarkable resistance to damage nucleation related to the alignment of the platelets upon deformation and from their small size. This finding offers a new path to optimize the fracture toughness of DP steels without compromising the strength and/or changing the chemical composition.