جستجوی مقالات مرتبط با کلیدواژه « dual phase steels » در نشریات گروه « مواد و متالورژی »

The effect of microstructural refinement and intercritical annealing on the mechanical properties and work-hardening response of a low carbon St12 steel was studied. It was revealed that intercritical annealing of the ferritic-pearlitic sheet results in the formation of a coarse-grained DP microstructure with discrete martensite islands normally formed in place of pearlitic colonies, which results in the minor enhancement of mechanical properties with disappearance of the yield-point elongation. On the other hand, a fine-grained DP steel with chain-network martensite morphology can be obtained by intercritical annealing of the cold rolled martensitic microstructure, which shows superior work hardening rate, low yield ratio, and high tensile strength. In this way, it is possible to enhance the mechanical properties of St12 steel toward those of DP300/600 steel. Compared with the conventional DP350/600 grade, a significant enhancement in the work-hardening behavior can be achieved with acceptable strength-ductility balance compared with the usual trend seen in steels. As a result, it was concluded that cold rolling of the initial martensitic microstructure before intercritical annealing is a viable approach for processing DP steels with enhanced mechanical properties for industrial applications.

The effect of the initial microstructure and intercritical annealing on mechanical properties and work-hardening response of a high-formability low carbon steel were studied. The work-hardening analysis was based on the modified Crussard–Jaoul method. The ferritic-pearlitic sheets showed low strength and high total elongation with the appearance of the yield point phenomenon. The occurrence of yield-point phenomenon resulted in the very low work-hardening rates at the initial stages of deformation. However, after intercritical annealing, a good combination of tensile strength and ductility along with much better work hardening response was observed. Intercritical annealing of martensite initial microstructure was found to be a viable technique for graining refinement of DP steels with much better tensile strength compared with that obtained by intercritical annealing of the normalized microstructure. These observations were related to the much finer microstructure and enhancement of work-hardening behavior in the former, where its work-hardening rate at each given true stress was considerably higher.

The microstructure of dual phase steels can be considered as a matrix of ferrite phase reinforced by martensite particles. Recent measurements show that the mechanical properties of the ferrite phase are changed with the distance from the martensite grains. In this paper, a new method has been proposed to consider this phenomenon in finite element modeling of dual phase steels microstructure. In this method, ferrite mechanical properties were imported to the model as a continuous function of the distance from martensite boundary. A unit cell model of dual phase steel was constructed based on the experimental measurements. The tensile test was simulated in both cases of considering the ferrite phase as the homogeneous and inhomogeneous matrix. It was observed that by considering the ferrite phase inhomogeneity, the model could predict macro stress precisely. Considering the ferrite phase inhomogeneity also led to the better prediction of shear band formation in the unit cell, as compared to the other model. A different stress distribution prediction was also observed in these two models and ferrite phase maximum stress was higher when inhomogeneity was included. These observations could be crucial in the investigation of dual phase steels damage. It was observed that martensite volume fraction and the grain size had a stronger effect on the model with the inhomogeneous ferrite phase.

Two different dual phase steels were prepared from low carbon manganese steel after the elimination of the banding using the direct quenching (DQ) and the step quenching (SQ) procedures. Different heat treatments resulted in different martensite morphologies, microstructures and mechanical properties. The heat treatments were designed in such a way to obtain a 0.25 volume fraction of martensite (Vm). For this reason, an intercritical temperature of 725 oC was applied. Furthermore, the tensile and impact properties were discussed. The results from the impact and tensile tests at different temperatures showed that the ductile-brittle transition temperatures for the DQ and SQ treatments were -49 and -6 oC respectively, while the DQ had better toughness.

The effect of ferrite grain size on the fatigue and tensile properties of dual phase steels with a 0.25 volume fraction of martensite (Vm) under different heat treatments was investigated. The heat treatments were homogenized at 1200 oC along with several subsequent normalizations at 910 oC, resulting in different microstructures and mechanical properties. After heat treatment, the obtained steels, with different ferrite grain sizes, were heat treated to obtain a dual phase ferritic-martensitic microstructure. In order to process the dual phase steels, low carbon manganese steel was used. Fatigue tests were carried out at room temperature and the fracture surfaces of the fatigue specimens were studied by SEM. The data obtained by the fatigue tests indicated that the fatigue strength at 107 cycles had a linear increase with decreasing the grain size of ferrite, while higher applied stress had a little effect on the grain size and the fatigue strength. The fracture surface of the fatigue specimens showed two distinct regions, namely, the fatigue fracture and the final fracture. Striation lines were clearly seen in the region of the fatigue failure. Furthermore, for all microstructures, the final fracture was mainly brittle.