Relations between microstructure and bendability on TRIP-aided steels for automotive products

Abstract : In order to limit fuel consumption, automotive industries push the steelmakers to develop thinner sheets with higher strength. Ultra High Strength Steels (UHSS) such as low alloy TRIP (TRansformation Induced Plasticity)-aided steels enable to get complex shapes for automotive parts. However, understanding the formability of these steels appears to be complex and involves a detailed study of failure mechanisms during forming tests. Therefore, links between microstructural features, in particularly banded structures, and formability were investigated in order to better predict the bending ability of steels as a function of their microstructure. In this study, four low alloy TRIP-aided steels exhibiting different bending performances were characterized using both air and stretch bending tests. From scanning electron and light microscopy observations, cracks initiate from the outer surface or from just below in air-bending, but from the central segregation bands in stretch bending. Fracture surfaces after bending tests are ductile and damage mainly appears by ferrite/martensite interface decohesion and occasionally as microcracks within martensite islands. An original procedure based on thickness-reduced specimens, with various locations of the main hard band within the specimen thickness, was set up. It enabled to propose a relationship between cracking, local thickness of hard band and local strain during air-bending. In order to develop a local fracture criterion that could be used for any bending test, stress and strain fields were computed by numerical simulation of the two bending tests. For that purpose, an experimental database including tensile tests on smooth and notched samples as well as shear tests was established. Material parameters of constitutive equations, accounting for anisotropic elasto-plastic behaviour with mixed hardening, were determined from this database. Finite element simulation of both bending tests associated with a ductile damage criterion made it possible to satisfactorily predict strain fields, bending load-displacement curves and fracture angle. The possibility of using simplifying assumptions in the model (such as an isotropic flow criterion, pure isotropic hardening, two-dimensional assumptions and simplified boundary conditions) is also discussed.
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Delphine Reche. Relations between microstructure and bendability on TRIP-aided steels for automotive products. Engineering Sciences [physics]. École Nationale Supérieure des Mines de Paris, 2011. English. ⟨NNT : 2011ENMP0016⟩. ⟨pastel-00973376⟩

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