Numerical modelling of grain refinement around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials by duplex technique

Microstructure evolution around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials have been investigated and discussed in the present work. Conditions leading to grain refinement during co-rolling stage of the duplex processing technique are analysed using the multi-level finite element based numerical model combined with three-dimensional frontal cellular automata. The model was capable to simulate development of grain boundaries and changes of the boundary disorientation angle within the metal structure taking into account crystal plasticity formulation. Appearance of a large number of structural elements, identified as dislocation cells, sub-grains and new grains, has been identified within the metal structure as a result of metal flow disturbance and consequently inhomogeneous deformation around oxide islets at the interfaces during the co-rolling stage. These areas corresponded to the locations of shear bands observed experimentally using SEM-EBSD analysis. The obtained results illustrate a significant potential of the proposed modelling approach for quantitative analysis and optimisation of the highly refined non-homogeneous microstructures formed around the oxidised interfaces during processing of such laminated materials

A microstructural study of the origins of γ recrystallization textures in 75% warm rolled IF steel

IF steel was warm rolled at 700 °C in a single pass. The resulting texture and microstructure were remarkably similar to those of the same steel after cold rolling. A detailed investigation of the microstructure by orientation imaging microscopy and scanning transmission electron microscopy showed microbands to have a mutual misorientation of less than 4° and shear bands to contain material misoriented from the parent matrix by less than 10°. Recrystallization did not occur preferentially at high-angle grain boundaries nor in shear bands. Instead the recrystallization nuclei were confined in the original hot band grain envelopes in crystals belonging to the γ fiber. These γ deformed grains had systematically developed deformation bands which consisted of elements that had rotated by up to ∼30° about the 〈1 1 1〉 parallel to the normal direction. This is essentially the same nucleation process as observed in cold rolled and annealed IF steel. © 2006 Acta Materialia Inc.link_to_subscribed_fulltex

Recrystallization texture and microstructure formation in heavily rolled if steel

This paper explains how the desirable ∼{111} texture of high intensity forms and an undesirable orientation component near {411} arises in heavily rolled IF steel. A commercial grade material was warm rolled 75% cold rolled 80% and annealed in a pre-heated air furnace at 710°C. The X-ray measured global texture showed the intensity of a fibre was stronger than γ fibre. During annealing this was replaced by a discontinuous and peak type y fibre. The maximum intensity spread from {554} to {111} which was accompanied by a weaker a component at {411}. The longitudinal and rolling plane microstructure of the deformed material showed that α and γ fibre grains maintain their typical microstructural appearance i.e. the a grains with coarse substructure and the y grains with fine fragmented substructure. The a grains, which are relatively uniform in terms of orientation at lower reductions, became unstable after cold rolling and produced deformation boundaries of the same nature as in y grains. These behaviors can be explained by the well-known Deformation Banding theory for BCC metals. The deformation banding in a grains produce recrystallized nuclei of {411} orientation and can be explained by either oriented nucleation or micro-growth selection for the particular orientation belonging to the α fibre. The high intensity spread from {554} to {111} component in the annealing texture is explained by nucleation at the intersection of two sets of in-grain shear bands, but such configuration are rare in conventionally cold rolled material up to 85% reduction.link_to_subscribed_fulltex

A comparative investigation of warm rolled IF steel and low carbon steel

X-ray and Electron metallography (SEM, OIM) has been used to examine the texture and substructure of warm rolled IF (Ti microalloyed Interstitial Free) steel and Low Carbon steel (LCS) after rolling reduction of 75% in a single pass at 700C. Although the rolling texture is similar for both the IF steel and LCS, the microstructure is very different. Very narrow in-grain shear bands only fractions of micrometers thick are quite common in IF steel and these make angles of 20-35 degrees with the rolling direction in the longitudinal section. On the other hand LCS produces no shear bands and the subgrain size is bigger than in the IF steel after the same rolling. Carbon in solution is obviously an important determining factor for shear band formation in high temperature rolling. Deformation banding is well developed in IF steel but not in LCS. On annealing, nucleation in the IF steel is associated with shear bands, deformation bands and grain boundaries, which almost always belong to the gamma fibre grains. The LCS, since it does not show such evidence of strain localization in the form of shear bands or deformation bands, reveals nucleation to be associated with grain boundaries. The macroscopic texture formed during static annealing at 700C is (111) for the IF steel, and is more random with small intensity along α fibre for the LCS. Partially annealed samples for both IF steel and LCS allow a detailed description of nucleation.link_to_subscribed_fulltex

Deformation banding and recrystallization of α fibre components in heavily rolled IF steel

Detailed SEM work has shown that about a quarter of crystals belonging to the α fibre formed by cold rolling up to reductions of 85%, are of uniform microstructure with relatively small misorientation. The remainder are indistinguishable from the polycrystalline mass, which is mostly of γ orientations. At rolling reductions above 85%, α grains begin to split their orientations by deformation banding, thus producing lattice curvature which allows recrystallization of α at the expense of other α components in the same original grain. This α recrystallization deteriorates the deep drawability of IF steel and since the process requires deformation banding a natural explanation of the optimum rolling reduction for good drawability is provided. The best cold rolling reduction is that which optimizes γ recrystallization by producing deformation banding in γ components without producing deformation banding in α components. Uniform microstructures and small lattice curvature allows α to be consumed by other orientations, and this condition is obtained in the 80-85% cold rolling range. © 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.link_to_subscribed_fulltex

Shear band formation in warm rolled IF steel

When IF steel is rolled to 75% reduction in a single pass at elevated temperatures within the ferrite phase, the rolling texture consists of the α and γ fibres. The microstructure revealed by Scanning Electron Microscopy (SEM) shows the γ fibre grains to be composed of microbands (MBs), while the α fibre grains have a coarse smooth structure. In addition to this, extremely fine grain scale shear bands (SBs) appear which cut through the microbands in the γ fibre grains but are not present in α fibre grains. The material inside the shear bands has ∼8-10° misorientation with the neighboring matrix and the rotation axis between shear band and the matrix material is not fixed to any of the samples reference directions. A model for shear band formation is proposed based on crystallographic slip system. Both {110} and {112} slip planes were considered to find the operative slip systems. The modeled shear band material rotates around the normal direction to the shear direction on the operative slip planes. Some experimental evidence, derived from SEM and OIM, supports this prediction.link_to_subscribed_fulltex

A rolling-annealing cycle for enhanced deep drawing properties in interstitial free steels

To give good drawability, steel needs high volume fractions of the annealing texture component {111} and a low fraction of ∼{100}. This is achieved in conventional Nb + Ti stabilized interstitial free (IF) steels by a cold rolling (CR) reduction of 85 per cent followed by annealing at 750-850°C for a few minutes. In this research, a detailed investigation of two-stage deformation processes was undertaken in which the total reduction in thickness was kept constant at 80 per cent, with a first and second rolling interrupted by recrystallization (RX) before the final recrystallization anneal was made. The texture produced is a rather flat γ recrystallization fibre of relatively high intensity at a reasonable final grain size. A second experiment, involving rolling ferrite at 700°C, produced a strong rolling texture and a well-developed {111} texture after annealing at 710°C, and so this material was also subjected to further rolling and annealing. The intermediate annealing between warm rolling (WR) and subsequent cold rolling significantly improved the intensity and uniformity of the final {111} texture compared with metal that was cold rolled without intermediate annealing. An investigation into the mechanisms involved in recrystallization revealed that the {111} oriented grains were subject to orientation splitting involving rotations around ND, and this process of deformation banding produced the necessary lattice curvature for nucleation of the {111} recrystallization texture components essential for good deep drawability. © IMechE 2004.link_to_subscribed_fulltex

Formation of the goss texture in a thin foil experiment on Fe-3.2%Si

Fe-3.2%Si steel from a commercial source supplied in the form of fully recrystallized sheet was selectively thinned to produce foils of different thicknesses which separated the subsurface layers in which the η texture dominated, from the central layer of γ texture. Goss oriented grains were more frequent in the subsurface layers, but were also present in the central portion of the material. Annealing these foils at ∼1000°C produced Goss texture in the surface foil, and {111 }(hkl) in the central foil. From these experiments it is clear that Goss secondaries grow easily in the η layer, but not in the γ layer and this was proved in a sequential heating experiment. A search of the misorientations between Goss oriented primary recrystallised grains in the η oriented volumes, for the most likely CSLs, i.e., Σ5 and Σ13a, proved to be unsuccessful. Some were found, but not in sufficient numbers to provide a satisfactory explanation for the formation of Goss secondaries. It is suggested however, that if CSLs are important in the selection of Goss secondaries, that Si segregation has also to be considered, for this will be less in special boundaries and thus provide less solute drag. © 2010 ISIJ.link_to_subscribed_fulltex

Increasing desirable recrystallization texture in IF steel by controlled rolling

A flat {111} texture of high intensity in IF steels following the final anneal produces good drawability and this is usually derived from a fairly random hot band material. In the first part of this work, conventional hot band was subjected to a cold roll-anneal-cold roll-final anneal procedure in which the total reduction was 80% (i.e. gage control) but in combinations of for example 70%+10% or 10%+70%, The purpose was to find whether final rolling of essentially a {111} (y) texture enhanced the intensity of {111} in the final annealed conditions. Also the effects of whether the material was fully recrystallized, partially recrystallized or merely recovered after the first annealing was investigated. The results are encouraging in that {111} could be increased when the texture before final cold rolling and annealing had a high y and low {hkl} (α) fibre. An explanation is provided for this based on Deformation Band (DB) theory. The second part of the work concerned warm rolling of the hot band in the a phase region, using a single pass reduction of 75%. This was either annealed and cold rolled or just cold rolled to 80% reduction followed by a standard recrystallization treatment. The results show the greatest intensity of {111} to be formed when the metal was cold rolled without intermediate annealing. Global textures were measured using X-rays, and the SEM techniques of EBSP and OIM coupled with conventional TEM and STEM were used for local texture and microstructures investigations.link_to_subscribed_fulltex