Jaap Moerman

Polycrystalline Model Predictions of Flow Stress and Textural Hardening during Monotonic Deformation

A series of mechanical tests in different specimen orientations was performed to study the anisotropic behavior of an IF steel (DC06). State-of-the-art polycrystalline models Alamel [1], VPSC [2], as well as the classical FC Taylor model were employed to predict flow stress curves. A two-stage Voce law was used to describe the single crystal shear stress-accumulated shear strain relationship. In this approach, the textural hardening and the dislocation hardening are effectively modeled separately. Results demonstrate that both the Alamel and VPSC models could reproduce the flow stress curves adequately. Also, the quantitative agreement of texture prediction is used to validate the model predictions. It is concluded that the better performance of grain interaction models compared to the FC Taylor model is mainly due to an improved prediction of the slip inside the constituting grains, and not in particular due to an improved prediction of texture evolution.

Validation of the Texture-Based ALAMEL and VPSC Models by Measured Anisotropy of Plastic Yielding

Several multilevel plasticity models that make use of the crystallographic texture have been developed in the past for the prediction of deformation textures. State-of-the-art models that consider grain interaction, such as Alamel and VPSC, are known to give superior deformation texture predictions compared to the well-known (full constraint) Taylor model. In this paper, these models are assessed on a different basis, namely their ability to predict plastic anisotropy in single-phase steel sheet. A wide range of mechanical tests is considered: uniaxial tension, plane strain tension, simple shear and sheet normal compression. Furthermore, the sensitivity of the anisotropy predictions is analyzed, considering the variability in textures measured by routine XRD. The considered grain interaction models clearly produce improved predictions of plastic anisotropy over the Taylor model.

Origin of the {h 1 1 } Fiber in Single Phase Ferrite Steels

In the present work, the oriented nucleation origin of the recrystallized {h11}<1/h,1,2> fibre is characterized. Aiming to investigate the substructural evolution of <110>//RD fibre grains and {001}<110> grains in particular, a detailed microstructure and texture analysis is performed by high resolution orientation scanning microscopy on a cross-rolled sample. The reason to work with cross-rolled material is the increased incidence of rotated cube orientations after cross rolling. The present data have revealed the presence in the deformed substructure of a crystallite volume that has rotated towards the {311}<136> component in the interior of <110>//RD fibre grains as a result of a grain fragmentation process. Preliminary simulations of the deformation texture suggested that the observed orientation fragmentation might be produced by strain localizations of a shear band nature.

Determination of the Non-Recrystallisation Temperature (Tnr) of Austenite in High Strength C-Mn Steels

In the present study non-recrystallisation (Tnr) and Ar3 temperatures have been determined for the C-Mn steels from multi-pass hot torsion experiments with continuous cooling in the temperature range of 1260°C to 600°C. Results show that Tnr decreases with increasing strain/pass, strain rate or interpass time. An alternative approach based on the work-hardening rate is proposed for the determination of Tnr and is shown to be more suitable in case the usual mean flow stress method does not provide a clear Tnr value.

Texture Evolution during Cold Rolling and Annealing in Dual Phase Steels

This paper investigates the bulk texture evolution during cold rolling and annealing of Dual Phase steels for different processing conditions, i.e. cold reduction within the reduction range of 45 to 73% and annealing at temperatures between 650 and 850°C, which includes the recovery, recrystallisation and partial phase transformation domains. Textures have been measured by X-ray diffraction. The results reveal that the rolling texture is strengthened during the recovery process or initial stage of recrystallisation while during recrystallisation a weak RD-ND type of texture appears. During subsequent phase transformation the RD-ND type of texture further weakens and later randomises as the second phase fraction increases beyond 75%.

Evaluation of geometrically necessary dislocations density (GNDD) near phase boundaries in dual phase steels by means of EBSD

The accumulation of dislocations around hard particles such as martensite in Dual Phase steel has a prominent influence on the mechanical properties of multiphase steels. The origin of these so-called Geometrically Necessary Dislocations (GNDs) is either due to the transformation strain, or to strain gradients that arise during deformation. The generation of deformation-GNDs is explained by Ashby’s theory [1] regarding deformation of a plastic mass that contains dispersed undeformable particles. It is argued that the GNDs pile up locally against the ferrite-martensite interface. This work reports the calculated density of GNDs from high resolution Electron BackScatter Diffraction (EBSD) measurements. By measuring the lattice orientation within the grains, the lattice curvature can be quan-tified, which can be directly related to the presence of GNDs. The density of the GNDs can then be estimated either directly through kernel average misorientations, or through the calculation of the dislo-cation tensor. From this first approximation of the GND density a GNDD map has been obtained by two recently developed approaches. This map shows an enhanced dislocation density around the mart-ensite particles due to volume change during transformation. The kernel choice and step size depend-ency of the results are also investigated.

Anisotropic sheet forming simulations based on the ALAMEL model: Application on cup deep drawing and ironing

The grain interaction ALAMEL model [1] allows predicting the evolution of the crystallographic texture and the accompanying evolution in plastic anisotropy. A FE constitutive law, based on this multilevel model, is presented and assessed for a cup deep drawing process followed by an ironing process. A Numisheet2011 benchmark (BM‐1) is used for the application. The FE material model makes use of the Facet plastic potential [2] for a relatively fast evaluation of the yield locus. A multi‐scale approach [3] has been recently developed in order to adaptively update the constitutive law by accommodating it to the evolution of the crystallographic texture. The identification procedure of the Facet coefficients, which describe instantaneous plastic anisotropy, is accomplished through virtual testing by means of the ALAMEL model, as described in more detail in the accompanying conference paper [4]. Texture evolution during deformation is included explicitly by re‐identification of Facet coefficients in the course...

Recrystallization Texture of Ferrite Steels: Beyond the γ-Fibre

The recrystallization texture of highly cold deformed IF steels is addressed. The latter is characterized by the //ND fibre and a certain spread towards the {311} orientation. The //ND fibre is the optimum texture for enhanced deep-drawing properties whereas the presence of any other component, such as {311}, will deteriorate the plastic anisotropy of the material. Previous works concluded that the recrystallized {311} orientation results from an oriented nucleation process related to the plastic instability of {001} deformed grains. In the present work, the microstructural nature of such plastic instability is investigated by high resolution orientation scanning microscopy on an annealed IF sample after cross-rolling. Present data indicate that localized deformation in near {001} grains plays an essential role in the nucleation of {311} orientations.

Orientation Gradients in α-Fibre Grains of Cold Rolled IF Steels

The plastic behavior of the <110>//RD orientations, and specially that of the {001}<110> orientation, under severe cold reductions is addressed. Based on the orientation dependence of the stored energy, the {001}<110> orientation is known to lack from structured misorientation gradients and significant dislocation storage after plastic deformation which makes the former orientation not particularly prone to enhancing the recrystallization process. Recent evidences, however, indicate that {001}<110> orientation plays a relevant role in the origin of {h11}<1/h,1,2> orientations (predominantly {311}<136> and {411}<148> orientations) observed in the recrystallization texture of severely deformed IF steels. The complete understanding of the development of the recrystallized {h11}<1/h,1,2> orientations in IF steels is, therefore, of relevance as it deteriorates the optimum γ-fibre texture required for deep-drawability applications. The plastic instability of {001}<110> grains in a cross-rolled IF steel is evaluated in the present work. The extensive characterization of the deformed substructure along with partially recrystallized data confirmed the oriented nucleation origin of {h11}<1/h,1,2> orientations from deformed {001}<110> grains. Innovative crystal plasticity calculations accounting for the position of the grain boundary plane suggested that the recrystallized {h11}<1/h,1,2> orientations could result from a low Taylor value nucleation criterion.