as in the case of DP450, a decrease of martensite hardness can be observed for 1.0 mm cycles as compared to water quenches. This may be due to an auto-tempering phenomenon, which can be activated by the decreasing cooling rates at the end of the cooling cycles (electrodes removal). Once again, the hardness levels are comparable with the ones obtained after welding, 1 mm He hardness being the closer to the value expected in the weld.As a conclusion for this section, subcritical and coarse grained HAZ weld microstructures could be successfully reproduced experimentally. These simulated specimens are used in the following in order to study their detailed mechanical properties. Local Constitutive BehaviourThe constitutive behaviour in tension of the different SCHAZ and CGHAZ is illustrated in Fig. 14. The true stress vs. true plastic strain is plotted until the onset of plastic localization. As expected from the microstructural observations and micro hardness of DP450, the different subcritical thermal cycles do not alter the base metal constitutive behaviour significantly. Both ultimate tensile strengths and uniform elongations remain in the same order of magnitude ( Fig. 14(a)). On the contrary, the different cooling cycles from 1 200°C result in a strong evolution of strain hardening and ultimate tensile strength. Martensitic microstructures (water quench and 1.0 mm cycle) are characterised by a high strength and low ductility, while the bainitic 3.0 mm microstructure exhibits a lower strength and increased ductility.In the case of DP980, martensite tempering in the SCHAZ highlighted previously allows an increase in ductility and a loss of yield and ultimate tensile strengths ( Fig. 14(b)). This softening is more pronounced for the 3.0 mm thermal cycle than for the 1.0 mm one, in accordance with a longer time in the subcritical temperature range. CGHAZ microstructures present high strengths and low ductilities. However, it can be noticed that the 1.0 mm and 3.0 mm microstructures exhibit lower strength in comparison with the water quench. This may be attributed to the auto-tempering of the martensite in formation at the end of the CGHAZ thermal cycles, where the cooling rates are reduced.Moreover, it is worth to mention that martensitic microstructures present low ductility, but not zero. A few percents uniform elongation can be obtained even with water quenched DP980. This can be related to the relatively low level of carbon content for such automotive steels. ConclusionA Finite element analysis of the resistance spot welding process was conducted in this paper in order to assess the order of magnitude of heating and cooling rates experienced in dual phase steel spot welds. The evolution of thermal cycles when welding sheets with thicknesses in the range [1.0-3.0] mm was highlighted. Spot welding thermal cycles could then be simulated experimentally with a Gleeble and their influences on microstructure and mechanical properties investigated. Gleeble simulations were targeted for the reproduction of the thermal ...

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