Development of Texture, Microstructure, and Grain Boundary Character Distribution in a High-Strength Boron-Added Interst
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INTRODUCTION
ALTHOUGH one of the major applications of interstitial-free (IF) steels is in the automobile sector,[1] use of IF steels in the packaging industry is also expected to be quite substantial.[2] For this purpose, it is necessary to produce steel sheets in much thinner gage sections than those used in the automobile industry. One way of achieving this thickness could be to go for cold deformation levels much higher than what is practiced (85 to 90 pct) currently.[3] A substantial amount of research so far has indicated that, in practical terms, the amount of cold rolling plays an important role in the development of the final annealing texture suitable for deep drawing. The amount of cold rolling to be applied to the material depends on various factors, and these have been discussed in depth in the literature.[1,4–7] However, the effect of severe cold rolling on the development of microstructure and texture during deformation and subsequent recrystallization is still not clear. Again, in order to produce high-strength steels with improved toughness and ductility, refining the final grain size of the steel has been thought of as one of the possibilities. Severe plastic deformation (SPD) has been considered by many for this purpose. Severe plastic deformation includes processes such as accumulative roll bonding (ARB),[8–12] equal channel angular pressing (ECAP),[13–15] high pressure torsion,[16,17] ball RAJIB SAHA, Researcher, and R.K. RAY, Visiting Scientist, are with the R&D Division, Tata Steel, Jamshedpur 831001, India. Contact e-mail: [email protected] Manuscript submitted June 13, 2008. Article published online July 14, 2009 2160—VOLUME 40A, SEPTEMBER 2009
milling,[18,19] folding and rolling,[20,21] etc. Application of these methods has shown that intense straining can produce grain sizes well below 100 nm, thereby improving the mechanical properties. A heavy amount of cold rolling followed by annealing has been explored to generate ultrafine grain structure in a Ti-stabilized IF steel.[22] More recently, severe cold rolling (93.6 pct) was applied to plain carbon steel whose initial microstructure was martensite.[23] Formation of the nanolayered structure of lath martensite with a mean thickness of 18.9 nm took place after heavy cold rolling. Annealing of the cold-rolled structure led to the formation of recrystallized grains of sizes around 52 nm. There has been quite a bit of work done to understand the effects of SPD produced by SPD techniques, such as ECAP and ARB on the grain boundary character distribution (GBCD), especially in fcc metals and to some extent in steels. Recently, Hughes et al.,[24,25] who worked on aluminum and nickel, observed that an increase in the cold rolling level significantly increased the fraction of high-angle grain boundaries (HAGBs). Hughes et al.[24] suggested that this could be due to the accumulation of dislocations across the boundaries, causing an increase in the individual misorientations. Similar observations were also made by Juul Jensen.[26] More recently, rese
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