Relationship between Microstructure, Mechanical Properties and Damage Mechanisms in High Martensite Fraction Dual Phase Steels

The relationships between microstructure, mechanical properties and damage mechanisms in as-quenched and in quenched-and-tempered high martensite fraction (>60%) dual phase (DP) steels were investigated. The mechanical behaviour was determined by tensile and hole expansion (HE) tests. In the as-quenched condition, the HE ratio decreased with increasing ferrite content and showed a non-linear inverse relation to the uniform elongation. Tempering significantly improved the HE ratio for all studied martensite fractions; the increase in HE was found to be monotonic for tempering temperatures between 230°C and 460°C, even though the yield stress dependence was complex in this range. Tempering studies showed that, at constant martensite fractions, there was a linear dependence between the HE ratio and the ductile fracture strain, εf. However, the parameters of the linear relation changed significantly when the martensite fraction was varied. The dominant damage mechanism in simple tensile tests evolved from ferrite/martensite or martensite/martensite interface decohesion in the as-quenched state to martensite/carbide interface decohesion after tempering. The damage mechanisms were qualitatively described using the Beremin local criteria.