Papers by Keyword: ULC Steels

Abstract: Ultra low carbon (ULC) steel samples were deformed in near plane-strain compression mode with different strains, strain rates and temperatures. Different aspects of microstructural developments, for deformed γ (ND//) and θ (ND//) fibre grains, were investigated using X-ray line profiles and high resolution electron diffraction. The study clearly showed increase in grain interior strain localizations and in-grain misorientation at the intermediate deformation temperature. This effect was more apparent in γ-fibre and can best be explained through orientation sensitive recovery. γ-fibre also demonstrated higher potential for increase in dislocation density. This was observed experimentally and simulated through discrete dislocation dynamic simulations. Higher textural softening with stronger increase in dislocation density and possible effects of orientation sensitive recovery appears to define the orientation dependent recovery in low carbon steels.

Abstract: In this paper, effects of ferritic rolling process on microstructures and mechanical properties especially drawability of ultra-low carbon (ULC) and Ti-stabilized interstitial-free (Ti-IF) steels were investigated and the precipitates of ferritic-rolled Ti-IF steels were also analyzed. The results show that good deep drawability are achieved as IF steels are ferritic rolled in good lubricant condition; on the contrary, lubricant condition has less influence on the deep drawability of ULC steels. And with the decrease of rolling temperature in ferrite region, deep drawability of Ti-IF steels are improved, but for ULC steels, when ferritic rolled at low temperature, the r-value is still less than one, which means insignificant deep drawability.

Abstract: Arcelor produces « Bake-Hardening » steels for automotive outer panels, which present the advantages of a remarkable drawability combined with a significant hardening after stamping and paint baking by the car maker. This hardening enables to increase the dent resistance of those automotive parts. In order to give easy design criterion and support the development for new “bake-hardening” steels, a physically-based model for Bake-Hardening steels has been developed. It is suitable to predict: -the physical phenomenon of strain ageing based on Cottrell atmospheres formation. A detailed description of the strain ageing kinetics is given based on a generalized form of the Harper model taking into account the diffusion of carbon atoms in the stress field of a dislocation, progressive carbon depletion in the matrix and saturation of the available dislocation sites. -the plastic instabilities propagation during tensile testing according to Piobert–Lüders phenomenon using the finite element method. A local mechanical behaviour is introduced whose shape schematically describes the local dislocation behaviour. The effect of the grain size on the velocity of the Lüders’ band front is especially enlightened. -the effect of ageing process on dent resistance. To do so, a physical extension of the former approach to more complex loading paths is proposed.

Abstract: During annealing of cold rolled steel sheets, the evolution of the texture, to produce the characteristic texture of a recrystallised material, depends on a series of factors, including the starting deformation texture, composition and processing conditions. In the present work, electron backscattered diffraction (EBSD) techniques have been used to investigate the deformation substructures being developed by cold rolling into grains belonging to different texture components. The strain localisation, the deformation banding, the spread in orientation into grains with different orientations are some of the aspects that have been considered. Annealing cycles at different temperatures have also been carried out in order to promote recovery and initial stages of recrystallisation . The obtained substructures have been compared to the as-cold rolled ones. It has been observed that recovery induces the dislocation structures to arrange into subgrains. Associated to the ND fibre, a network of relatively high angle boundaries develop within the subgrain structure. The recrystallisation nuclei have been observed to evolve from such a network.