SummaryBainite is thought to play an important role for the chemical and mechanical stabilization of metastable austenite in lowalloyed TRIP steels. Therefore, in order to understand and improve the material properties, it is important to locate and quantify the bainitic phase. To this aim, electron backscatter diffraction-based orientation microscopy has been employed. The main difficulty herewith is to distinguish bainitic ferrite from ferrite because both have bcc crystal structure. The most important difference between them is the occurrence of transformation induced geometrically necessary dislocations in the bainitic phase. To determine the areas with larger geometrically necessary dislocation density, the following orientation microscopy maps were explored: pattern quality maps, grain reference orientation deviation maps and kernel average misorientation maps. We show that only the latter allow a reliable separation of the bainitic and ferritic phase. The kernel average misorientation threshold value that separates both constituents is determined by an algorithm that searches for the smoothness of the boundaries between them.
Modern metal forming and crash simulations are usually based on the finite element method. Aims of such simulations are typically the prediction of the material shape, failure, and mechanical properties during deformation. Further goals lie in the computer assisted lay‐out of manufacturing tools used for intricate processing steps. Any such simulation requires that the material under investigation is specified in terms of its respective constitutive behavior. Modern finite element simulations typically use three sets of material input data, covering hardening, forming limits, and anisotropy. The current article is about the latter aspect. It reviews different empirical and physically based concepts for the integration of the elastic‐plastic anisotropy into metal forming finite element simulations. Particular pronunciation is placed on the discussion of the crystallographic anisotropy of polycrystalline material rather than on aspects associated with topological or morphological microstructure anisotropy. The reviewed anisotropy concepts are empirical yield surface approximations, yield surface formulations based on crystallographic homogenization theory, combinations of finite element and homogenization approaches, the crystal plasticity finite element method, and the recently introduced texture component crystal plasticity finite element method. The paper presents the basic physical approaches behind the different methods and discusses engineering aspects such as scalability, flexibility, and texture update in the course of a forming simulation.
The recrystallization of titanium-alloyed interstitial-free steel (IF steel) has been investigated by high-resolution electron backscattered diffraction (EBSD) measurements and transmission electron microscopy (TEM) observations. The deformed microstructure of the cold rolled material can be subdivided into three different groups. These three types of microstructure are characterized by their orientations and internal local misorientations. The development of these three regions during recrystallization annealing has been observed. Nucleation from c-fiber orientations but also from other orientations was found. Comparison of EBSD and TEM results indicates some limitations of high-resolution EBSD measurements concerning the observation of subgrain structures.Motivation: IF steels are mainly used as sheets for deepdrawing applications (car bodies, cans etc.) for which a high r value (ratio of strain in width direction to that in thickness direction) is required, in order to reduce thinning. It is well known that a high r value is related to a high fraction of crystals with a <111>//ND crystal orientation fiber (called c-fibre). [1] Homogeneous occupation of this orientation fiber also leads to a low anisotropy within the sheet plane. As the c-fibre is mainly created during recrystallization after heavy cold rolling, this process is of high significance for applications of IF steel. Another important crystal orientation fiber is <110>//RD. The latter is called a-fibre and is formed typically when rolling bcc metals.The problem with recrystallization of IF steels is related to the heterogeneity of the process. Figure 1 shows an example of a recrystallization process taken from a work by Raabe. [2] In this case, recrystallization evolved in a strongly heterogeneous way throughout the material. In the lower part of the picture, recrystallization has not taken place at all, whereas, in the upper part, two different recrystallization regimes are visible, showing different grain sizes. All three different kinds of microstructure, as expected, display different mechanical behaviors and lead to unwanted surface effects (such as orange peel structure) during deep-drawing operations. In order to understand and predict phenomena such as the one shown in Figure 1, it is necessary to improve our knowledge of the recrystallization process. The basic problem in this context is understanding which features of the microstructure enable nucleation for the recrystallization process.It has been well established that the ability to form a nucleus is strongly dependent on the orientation of the material [3] and it is therefore important to study the local orientation distribution and microstructure with high spatial resolution. EBSD applied in a high-resolution scanning electron microscope (SEM) is the most appropriate tool for this task as it gives both high resolution and large observable areas. In this communication we describe a study of the recrystallization progress in IF steel. The deformed and several partially recrystallized...
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