Papers by Author: Roumen H. Petrov

Abstract: Qualitative and quantitative texture changes are investigated in a severely deformed 5xxx series aluminum alloy. Unusual recrystallization textures are observed after a rolling strain of more than 97% in thickness reduction. The influence of both strain amplitude and strain mode on the development of the deformation and recrystallization textures is discussed based on experimental data and results of crystal plasticity calculations.

Abstract: Recrystallization was investigated in the context of its effect on the roping phenomenon in a 6000 series Al alloy. The findings suggest that, in general, the recrystallized grain size affects the material's susceptibility to roping more than the actual area fraction of specific texture components. For example, bands of Cube grains were typically observed in the final processed samples in cases which had coarse, recrystallized grain sizes from the earlier stages of processing. In contrast, bands were not observed for cases having fine, intermediate recrystallized grain sizes. Although microstructure and texture are highly intertwined, these findings suggest that the microstructural characteristics, such as the recrystallized grain size, are more important than those related to the texture.

Abstract: Recrystallization and austenite formation in a TRIP-assisted steel during conventional and ultra fast reheating for intercritical annealing are studied with the purpose to clarify the possibility for grain refinement. Partially recrystallized (or transformed) samples were prepared by reheating and water quenching to temperatures between 650 and 1050°C at reheating rates of 10, 50, and 3000 °C/s, respectively, without isothermal soaking from 95% cold rolled steel sheet with ferrite-pearlite microstructure. By monitoring the hardness and microstructure, it was shown that the increase of the reheating rate from 10 to 3000°C/s causes grain refinement from 5µm to 1µm in diameter and the final ferrite grain size depends significantly on both reheating temperature and reheating rate. It was observed that after an extreme reheating rate of 3000°C/s the α-γ phase transformation starts before the completion of the recrystallization. This opens up possibilities for further structural refinement and alternative texture control.

Abstract: The sheet manufacturing process, which involves various solid-state transformations such as phase transformations, plastic deformation and thermally activated recovery processes, determines the texture of steel and aluminium sheet. The conventional process of flat rolling and annealing only offers limited degrees of freedom to modify the texture of the final product. After annealing a {111} recrystallization fibre in BCC alloys and a cube dominated recrystallization texture in FCC metals is commonly obtained. Many applications, however, require other texture components than the ones achievable by conventional processing. In the present paper it is shown that by asymmetric rolling of a Si-alloyed ultra-low carbon steel a texture can be obtained with increased intensity on the {001} fibre, which is of interest for magnetic applications. Also in aluminium alloys the strong cube annealing texture can be drastically modified by the process of asymmetric rolling. It is argued that by observing the proper rolling and annealing conditions a recrystallization texture with improved normal and planar anisotropy of the mechanical properties may be produced.

Abstract: 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.

Abstract: Texture formation during annealing of a 95% cold rolled HSLA steel with 10°C/s and ~3000°C/s was studied with the purpose to investigate the interaction between the recrystallization and the austenite formation as well as the possibility of grain refinement. The recrystallization and austenite formation were monitored by means of optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). It was found that at extreme reheating rates of 1000°C/s and higher, the α-γ phase transformation starts before the completion of recrystallization and the deformation texture is partially retained in the samples after annealing. The crystallographic texture observed in the martensitic phase which is the product of intercritical austenite transformation in as-quenched samples, i.e. after the double α-γ-α’ transformation, is the same as the initial cold rolling texture, which is an indication for the texture memory effect. After ultra fast reheating with average reheating rates of 1000°C/s and higher a significant grain refinement was observed with an average ferrite grain size of ~1µm. The obtained final ferrite grain size depends significantly on both the reheating temperature and the reheating rate.

Abstract: Texture formation during an austempering treatment of a TRIP-assisted steel was studied by in-situ texture measurements with a high energy source (synchrotron). Samples from a cold rolled sheet were subjected to a complete heat treatment cycle for TRIP steels including reheating to the intercritical (α+γ) temperature region, isothermal soaking and bainitic holding (austempering) at 400°C for 600s. At specific points of the thermal cycle {200}γ, {220}γ {222}γ, {331}γ and {200}α, {211}α and {220}α Debye rings were recorded and the corresponding incomplete pole figures were calculated. The latter were used to derive the orientation distribution functions (ODFs) of BCC and FCC phases at specific steps of the annealing process after assuming the orthotropic sample symmetry. The acquired data for the texture evolution during the α–γ–α phase transformation showed that during the reheating for intercritical annealing the gamma phase with {011} orientation is among the first to nucleate from the recrystallized α phase during heating and the Goss and Cube orientations are among the principal gamma phase components which transform to BCC phase after cooling.

Abstract: Low ductile crack arrestability in a full-scale burst test of 1420 mm-diameter X80 steel line pipes was accompanied by a high intensity of fracture surface separation. The texture of the steel plates was studied using different techniques in order to evaluate the influence of {001} planes oriented parallel to the rolling plane on the separation intensity during fracture. Though no clear correlation between the content of {001} planes parallel to the rolling plane and intensity of separation was found, the local distribution of the {001}<110> texture component among the microstructure components was different in different steels providing long areas suitable for cleavage fracture parallel to the rolling plane in steel with low arrestability.

Abstract: In crystal plasticity models the crystal anisotropy of the yield strength is accounted for by the yield locus. In the present paper the full constraint Taylor model is used to calculate the yield strength anisotropy of a heavily cold rolled and annealed IF steel. In addition to the crystallographically induced anisotropy also the grain shape anisotropy was taken into account. To this purpose a model is presented in which the grain size that appears in the Hall-Petch relation is substituted by an effective grain size that is dependent of the grain-shape morphology and the crystal orientation. The grain shape of a specific crystal orientation is approximated by an ellipsoid volume of which the major axes are obtained from experimental data. The effective grain size of a specific crystal orientation is determined by the intersection of the most active crystal slip plane of this orientation and the ellipsoid volume.

Abstract: Aluminium alloys for car body manufacturing often show a specific type of band-shaped surface roughening upon stretching, called “ridging” or “roping”. Experimental research as well as modelling attempts have indicated that the evolving surface roughness profiles cannot be understood based on banding of individual surface texture components, like cube or Goss, only. Therefore, it is proposed to study banding on the “mesoscopic” level of texture banding rather than on the “microscopic” one of orientation banding. In mechanical terms, such patterning in the texture would lead to patterning in, for instance, the Lankford coefficient (r-value), so that the mechanical response can be calculated on an intermediate length scale. The present contribution presents a method for calculating r-value patterning from Electron Backscatter diffraction (EBSD) orientation maps. In a first test case of a strongly ridging AA6xxx sheet, indeed patterning in the r-value is found which corresponds to reported patterning of the surface roughness.