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A dual phase (DP500/800) steel sheet (1.6 mm thickness) was evaluated by means of low cycle fatigue (LCF). The fatigue properties of the material were assessed by means of the cyclic elastic modulus determination at half-life as well as by the plastic strain amplitude measurements. Both methods show a good correlation and high reliability, however, the plastic strain amplitude approach tends to fit better to the experimental results. From a parametric analysis performed, it was observed that the plastic strain amplitude approximation could be more suitable to assess the fatigue properties for LCF of the material due to an improved correlation of the computed cyclic elastic modulus in comparison to the cyclic elastic modulus measurements at half-life. Both methods showed that, for strain amplitudes higher than εa = 0.004, the cyclic elastic modulus tends to be lower than 210 GPa, meanwhile for lower strain amplitudes, the cyclic elastic modulus tends to increase. An evaluation by X-ray Diffraction technique allowed to determine that the increment of residual equivalent strain contributes to the cyclic elastic modulus diminution.

Dual phase (DP) steels have high strength, while maintaining outstanding elongation capacities. This is possible using a well-controlled thermomechanical process that produces a perfect phase combination in the DP microstructures. However, automotive makers are required to weld the DP steels, which generates a soft zone in the microstructure. In this work, 1.6 mm-thick DP980 steel sheets were welded by gas metal arc welding process to analyze the response of the welded soft zone to cyclic loading conditions. Conducted macrographic and metallography analyses revealed good quality in the appearance of the welded joints, with a complete fusion of the DP980 joint and without the presence of discontinuities. Low cycle fatigue tests of the DP welded joints were conducted under a constant amplitude strain control mode. The welded joints experienced a fatigue life reduction with respect to the DP980 steel of ~16% at strain amplitudes of 0.2, 0.3, and 0.4%. For strain amplitudes larger than 0.6%, the fatigue life of the welded joint was reduced by 39%. Weld thermal cycles combined with metallography analysis indicated that a tempered process of the martensite during the welding was responsible for the soft-zone formation and the poor fatigue response.