Jean-Hubert Schmitt

Sequences of rolling-tension experiments were performed on polycrystalline copper sheets with two different grain sizes (20 and 250 μm). After prestrain in rolling (between 10 and 50%), the dislocation microstructure observed by transmission electron microscopy is qualitatively similar for both grain sizes. The main differences occur during the second path in tension for which the mechanical and microstructural behaviours have been investigated. From the macroscopic point of view, the effect of the prestrain on the strain hardening is higher the lower the initial grain size. This leads to an early strain localization in tension for the smaller grain size. At the microscopic level, different features are also observed. In 250 μm grain size specimens, microbands appear inside many grains just after yielding. The microbands are aligned with the trace of {111} planes, corresponding to the principal active slip plane in tension. By contrast, no trace of localized deformation is noted for the smaller grain size; the dislocation structure evolves in a more or less continuous manner by a partial dissolution of the prestrain substructure. These different microstructural behaviours are discussed on the basis of strain accommodation principles.

The mechanical behaviour and fracture were studied in a Fe–5Mn–2.5Al–0.2C (in wt-%) Medium Mn steel. Metallographic and magnetic measurements confirm the significant influence of the transformation kinetics of the strain-induced martensite on the mechanical properties and strain heterogeneities (Lüders and Portevin-Le Chatelier-like phenomena). An accurate study of isothermal evolutions of the microstructure, associated with atomistic calculations, complements current thermodynamic databases to quantify the nature and volume fraction of phases at different temperatures. A kinetic approach then predicts the influence of annealing conditions on the composition of retained austenite, key parameter for the martensitic transformation kinetics. This supports quantitative modelling of the influence of the intercritical annealing temperature on the ultimate tensile strength for industrial developments of these new grades.

Automotive industry asks for higher resistant steels to lighten parts and improve crash resistance. Keeping a good ductility while increasing tensile strength requires the development of new grades in which hardening mechanisms counteract the drop in elongation when enhancing mechanical resistance. This is mainly achieved with multiphase steels and completing dislocation hardening by twinning and martensite transformation during straining. This has led to high-strength steel families, some of them being already used in body in white (Dual Phase (DP) and TRIP steels). Others, still in development, will soon emerge on the market (Quenched and Partitioned (Q&P), medium-Mn steels or TWIP).

It is well established that cold‐drawing of pearlitic wire develops a strong anisotropy due to a preferential orientation of the ferrite phase parallel to the wire axis. The subsequent wire behavior during torsion, which mainly occurs during assembling of drawn wires, is correlated with the texture and its gradient along the wire radius. Although different attempts were made in the past to measure the texture after drawing, it is still challenging since the final wire diameter is on the order of 0.3 mm. The paper presents a simple and robust method to measure the texture gradient by X‐ray diffraction. First, to overcome the problems of a composite sample made of parallel wires, an isotropic reference sample is prepared by cycles of austenite transformation and quenching. The texture gradient is measured at different thicknesses by dissolving the external surface of the wires. This allows the investigation of the first 40 μm of depth. Finally, the results are compared with EBSD measurements on a longitudinal cross‐section to quantify the influence of the drawing conditions: A flat schedule with constant die angles favors a fiber texture with a weak gradient, while increasing the drawing angle for three dies increases the gradient.