Influence of Hot Band Annealing on Cold-Rolled Microstructure and Recrystallization in AA 6016

ORIGINAL RESEARCH ARTICLE Influence of Hot Band Annealing on Cold-Rolled Microstructure and Recrystallization in AA 6016 ELISA CANTERGIANI, IRMGARD WEIßENSTEINER, JAKOB GRASSERBAUER, GEORG FALKINGER, STEFAN POGATSCHER, and FRANZ ROTERS The influence of an intermediate heat treatment at the end of hot rolling and before cold rolling on Cube texture formation during the final solution annealing of AA 6016 is investigated. Three hot bands with different initial grain sizes and textures are considered: the first one without annealing before cold rolling, while the other two hot bands are heat treated at 540 C for 1 hour in air before being cold rolled. One of the heat-treated hot bands was left to cool down in air and the other inside the furnace. Electron-backscatter diffraction (EBSD) maps of the cold-rolled specimens and crystal plasticity simulations show no difference in the amount of Cube remaining in the microstructure at the end of cold rolling for all three specimens. The initial grain size of the hot band has no influence on the Cube texture fraction left in the microstructure at the end of cold rolling for thickness reductions higher than 65 pct. Nevertheless, the grain size of the hot band affects the shape and distribution of the Cube grains left in the microstructure and the kernel average misorientation in the cold-rolled specimens. Moreover, the heat treatment decreases the intensity of the beta fiber components (Brass, Copper, and S) in the hot band and promotes the formation of a cold-rolled microstructure with a low kernel average misorientation. Both these factors lower the probability of preferential Cube nucleation during solution annealing and keep the Cube volume fraction after recrystallization below 10 pct, while it reaches 25 pct without intermediate annealing. https://doi.org/10.1007/s11661-022-06846-4  The Author(s) 2022 I. INTRODUCTION OPTIMIZATION and tailoring of texture is fundamental to reduce anisotropy of aluminum sheets during forming, especially in 6xxx alloys because of their use in automotive components, where surface finish and shape must satisfy strict requirements. Alloys of 6xxx series are of interest in automotive applications because of their good combination of formability, corrosion resistance, and increase of strength during paint baking after forming.[1] Texture evolution depends on casting,[2] hot and cold rolling parameters (strain rate[3] and temperature), deformation history,[4–6] alloy composition,[7] ELISA CANTERGIANI and FRANZ ROTERS are with the Max-Planck-Institut für Eisenforschung, Max-Planck-Straße 1, 40237 Düsseldorf, Germany. Contact e-mail: IRMGARD WEIßENSTEINER and STEFAN POGATSCHER are with the Christian Doppler Laboratory for Advanced Aluminum Alloys, Montanuniversitaet Leoben, 8700 Leoben, Austria. Contact email: JAKOB GRASSERBAUER is with the HTL Leoben, Max-Tendler-Strasse 3, 8700 Leoben, Austria. GEORG FALKINGER is with the AMAG Rolling GmbH, 5282 Ranshofen, Austria. Manuscript submitted July 25, 2022; accepted September 29, 2022. METALLURGICAL AND MATERIALS TRANSACTIONS A and the presence of precipitates.[8,9] After cold rolling, the 6xxx sheets are solution annealed (SA) at a temperature between 500 C and 570 C followed by quenching to dissolve the hardening phases and retain the corresponding alloying elements in solid solution.[1,4] During this solution treatment, recrystallization of the cold-rolled sheet occurs resulting in a modified texture. Cube is considered a critical texture component during the recrystallization treatment because it can show preferential nucleation and growth.[10] Cube preferential growth during recrystallization is found mostly in high-purity aluminum alloys[11]; however, also 6xxx alloys stabilize a high fraction of Cube during hot rolling which can be difficult to remove during successive cold rolling.[11,12] This fraction of Cube left in 6xxx alloys can show preferential nucleation during the final solution annealing. Recrystallized Cube grains can originate from transition bands,[13] which are thin Cube seeds surrounded by high orientation gradients and that are caused by diverging rotations of unstable orientations according to the Dillamore and Katoh model.[14,15] However, also Cube fragments left in the microstructure at the end of cold rolling and not exactly corresponding to transition bands can act as nucleation sites.[16,17] During in situ annealing experiments and modeling of commercial AlFeSi alloy, Sukhopar and Gottstein[18] have shown that the intensity of Cube texture after recrystallization is due to nucleation at the Cube bands left in the deformed microstructure, while contribution to Cube nucleation from locations outside the Cube bands is negligible. Recrystallization in high purity and highly deformed (98 pct cold rolled) aluminum foil has shown that occurrence of extensive recovery and continuous recrystallization can retard the formation of Cube texture.[19] The recrystallization advantage of Cube grains decreases rapidly when the Cube present in the microstructure is misoriented at least 10 to 15 deg from its exact orientation.[20] Thus, several works were dedicated to increasing the Cube fragmentation during rolling through asymmetric cold rolling[4] or with the adjustment of shear loading.[21,22] Some experimental investigations have suggested that a fraction of Cube texture could be created by S-oriented grains during compression,[23] while elasto-viscoplastic fast Fourier transform (EVP-FFT) simulations have shown that depending on the amount of compression strain, some Cube can originate from random orientations or non-Cube grains misoriented within 10 to 20 deg from ideal Cube,[24] but the fraction of this newly formed Cube during rolling is extremely low and it does not play a key role during recrystallization. Solutions to reduce the amount of Cube already during casting were suggested through the use of continuous casting (CC) instead of direct casting (DC). For the same aluminum alloy, CC produces ND misoriented Cube microstructures, while DC forms microstructures containing a higher fraction of perfectly oriented Cube.[2] During cold rolling, Brass, Copper, and S develop faster in hot bands from CC than DC; moreover, microstructures from DC maintain a higher fraction of exact Cube orientation at the end of cold rolling.[25] In several aluminum alloys, the amount of Brass texture was increased and the Cube fraction was lowered by tailoring the alloy composition.[7,26,27] Cube recrystallization has been the object of several investigations at microtexture level using bicrystal plane strain compression tests.[28,29] Initially, the attention was focused on the role of grain boundaries and Cube transition bands in the nucleation of new grains. In particular, subgrain coalescence in transition bands helps Cube nuclei in reaching early the critical size to grow.[30] More recently, the influence of different amounts of iron content, deformatio (...truncated)