Materials Science Forum Vols. 467-470

Abstract: The distribution of the characteristic texture components between the ferrite grains of different size classes has been studied in a steel with 0.082%C, 1.54% Mn, 0.35% Si, 0.055%Nb and 0.078%V after different rolling schedules with a final rolling temperature above or below Ar3. Microstructures and textures were characterized by means of optical microscopy and orientation microscopy. A strong grain refining effect together with a bimodal grain size distribution was observed in the steel both after final rolling in the intercritical region or in the austenite region, close to the Ar3 d temperature. The differences in grain size were interpreted on the basis of three potentially acting mechanisms: (i) transformation- induced recrystallization, (ii) increased mobility of specific grain boundaries and (iii) fast nucleation of ferrite grains on specific sites of the parent austenite microstructure. The experimental data clearly favoured the third of these assumptions as the responsible mechanism for the observed bimodal grain size distributions.

Abstract: IF steel was homogeneously cold rolled between 30-95% reduction in thickness. The global cold rolling textures showed a gradual strengthening of both stable α and γ components with increasing reduction until ~80% after which γ remained effectively unchanged but α components intensified until 95% reduction of thickness. Deformation Banded (DB) and also fragmented microstructures were found exclusively in γ grains up until about 85% reduction after which DB was unexpectedly detected in α grains, becoming significant after 95% reduction. This is in sharp contrast with the rather undifferentiated microstructures found in α grains at low to medium levels of deformation. At lower reductions the annealing texture was a weak α, but the γ component increased with rolling strain and became dominant at ~80% rolling deformation. A peak type γ recrystallisation texture with orientations ranging from {554}<225> to {111}<123> was found in the 95% rolled sample. In addition to this a {411}<148> component began to intensify, reading 5R at 95% reduction. Microstructural analysis showed that DB provided the lattice curvature for nucleation in the α fibre.

Abstract: Obtaining optimal mechanical properties for highly formable ferritic stainless steel sheet requires careful control over recrystallization and texture. This is, in some cases, hampered by the slow approach to final recrystallization associated with the disappearance of deformed grains with particular orientations. The important mechanical properties for formability (e.g. the yield strength and r-value) are thus strongly dependent on the final few percent recrystallization. In this study, it has been attempted to correlate the microstructure and texture of ferritic stainless steel sheet to its mechanical properties as measured in uniaxial tension. It is shown that careful consideration of the evolution of texture and microstructure with recrystallization may explain the observed trends.

Abstract: This paper overviews the current state of development of the structure and properties formation in hot rolled low alloy strip steel after its coiling. To clarify the effect of coil cooling rate on the mechanical behavior, hydrogen content, corrosion , texture and coincidence site lattice industrial experiments were made at a hot strip mill. It is revealed that the texture and CSL has a notable influence on the complex of properties: strength, ductility, toughness and corrosion resistance.

Abstract: The softening kinetics of a 0.19 wt% C 1.5 wt% Mn steel deformed at two intercritical temperatures have been characterised using the stress relaxation technique. Recrystallisation of intercritical austenite has been modelled using a single grain model (Chen et al., 2002 [1]), whilst recovery of both intercritical austenite and ferrite has been modelled using a model in the literature [Verdier et al., 1999 [2]). The models are combined to predict the overall softening kinetics with a rule of mixtures formulation. Comparison of the model with experiment shows significant deviations. The reasons are discussed with reference to the mixture rule and to the local stress-strain distribution which exists in the deformed samples. A simple modification to the model is proposed which takes into account the effect of a local stress distribution in deformed austenite.

Abstract: A linear regression model consisting of the weighted sums of certain alloying elements has recently been developed to predict the activation energy (Qrex) and kinetics of static recrystallisation (SRX) for hot-deformed austenite based on stress relaxation test results for over 40 different carbon steels. The validity of the model has been further assessed here by determining the Qrex and the kinetics of SRX of certain high-Nb bearing steels, extra-low and low carbon Nb-Mo bainitic and high-Si dual phase and TRIP steels, and Nb-Ti grades with the varying N content. The validity of the model is shown to be fairly good for the Nb-Ti, Nb-Mo and Cr-Mo grades. The approach of maximum effective concentration of Nb and Si and the weight factor for Cr enable reasonable fit for DP, TRIP and Nb-Cr steels, as well. Possible influences of C and N on Qrex and the kinetics of SRX were checked, but none was observed in microalloyed steels.

Abstract: Ultra low-carbon interstitial free (IF) steel having ferrite (b.c.c.) single phase was deformed to various equivalent strains ranging from 0.8 to 5.6 by the accumulative roll bonding (ARB) process at 500°C. The microstructure and crystallographic feature of the deformed specimens were characterized mainly by FE-SEM/EBSD analysis. Grain subdivision during the plastic deformation up to very high strain was clarified quantitatively. After heavy deformation above 4.0 of strain, the specimens showed the lamellar boundary structure uniformly, in which the mean spacing of the lamellar boundaries was about 200nm and more than 80% of the boundaries were high-angle ones. Annealing behavior of the ARB processed IF steel strongly depended on the strain. The specimens deformed to medium strains exhibited discontinuous recrystallization characterized by nucleation and growth, while the specimens deformed above strain of 4.0 showed continuous recrystallization. The recrystallization behaviors are discussed on the basis of the microstructural and crystallographic parameters quantitatively measured in the as-deformed samples.

Abstract: Engineering properties of aluminum alloys can be optimized by a combination of thermomechanical processes. For example, a series of deformation and recrystallization processes is used to improve the deep drawing characteristics of aluminum sheet for beverage can fabrication or the forming characteristics in automotive applications. In other cases, suppression of recrystallization is desired: fracture toughness of high strength aluminum alloys is better for an unrecrystallized material than for a recrystallized material of comparable yield strength. Industrial control of recrystallization mainly focuses on control of texture for formability, control of grain size and degree of recrystallization for surface appearance and damage related properties. Such control is achieved by manipulating nucleation and growth of new grains. Nucleation density and crystallographic orientation of new grains is determined by inhomogeneities of the deformed structure. Grain boundary mobility is controlled by second phases, which at the correct size will exert a pinning force on the boundary. Additionally, solutes impose a drag on moving grain boundaries. The following will review the various applications of recrystallization in manufacturing of wrought aluminum products, summarize the current understanding of recrystallization processes in aluminum alloys, and point out needs for further understanding and necessary requirements for simulation of recrystallization processes.

Abstract: The evolution of texture as a function of recrystallization has been characterized for hotrolled AA1050. Samples prepared from hot rolled sheet were annealed isothermally for sufficient time to allow complete recrystallization. The microstructural variation and texture evolution in the samples was observed by automatic indexing of Electron Back Scatter Diffraction (EBSD) patterns in a Scanning Electron Microscope (SEM). The spatial orientation variation within the deformed microstructure of nucleation, growth and orientations of recrystallized grains was examined. The orientation spread within grains was found to be a useful quantity for partitioning recrystallized and unrecrystallized regions. Also the effect of deformation texture on the evolution and growth of various recrystallization texture components was analyzed. The analysis is aimed at obtaining a correlation between the deformation microstructure, texture development and recrystallization kinetics in the hot-rolled condition. Preliminary results suggest only a weak correlation between the rate of recrystallization and the deformation texture component.

Abstract: The X-ray rocking curve method (RCM) was used to characterize microstructure and texture of Al and alloys: Al-Mg (AMg6) and Al-Mg-Li (1420), Al- Mg-Sc (1570) after cold rolling to a 8-70 % reduction in thickness, and then annealing at 300-5500C. The fractions of recrystallized grains with {111}, {100}, {113} and {110} orientation in the sheet plane and its distribution on the sizes was measured by a RCM. For these alloys a grains with orientation of {111} and {100} grow much faster than grains {113} and particularly {110}. In an alloy 1420 at 300-4000C continuous recrystallization takes place with fine grains and weakening rolling texture