The Effect of Multiple Deformations on the Formation of Ultrafine Grained Steels
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DYNAMIC strain-induced transformation (DSIT) is one of the routes that has recently been used to produce ultrafine ferrite (UFF) grains in steels. The DSIT mechanism is clearly different from conventional grain refinement practices, which can be viewed as strainassisted transformation. In the latter case, the austenite is deformed above the transformation temperature and deformation is stored in the microstructure until the start of transformation (i.e., there is no recrystallization between the deformation and transformation events). Here, the ferrite grain size transformed from the deformed austenite is significantly refined (5 lm) compared with the ferrite grains transformed from the undeformed austenite, with the role of the deformation being the introduction of new nucleation sites.[1] The important feature here is that this is a static process and the deformation sets up the necessary conditions, but does not directly interact with the transformation. In the case of DSIT, most of the work to date suggests that the ferrite refinement can be further enhanced through the combined effect of deformation and transformation (1 to 3 lm), i.e., a dynamic process.[2–6] Here, the steel is deformed at temperatures significantly below the Ae3 (i.e., equilibrium temperature of austenite to HOSSEIN BELADI, Postdoctoral Research Fellow, GEORGINA L. KELLY, Research Fellow and PETER D. HODGSON, Professor, and ARC Federation Fellow, are with the Centre for Material and Fibre Innovation, Deakin University, Geelong, Victoria, Australia 3217. Contact e-mail: [email protected] Manuscript submitted September 4, 2006. Article published online April 12, 2007. 450—VOLUME 38A, MARCH 2007
ferrite transformation) and above the Ar3 (i.e., the empirical temperature of the austenite to ferrite transformation during continuous cooling), and in most cases, it is clear that the transformation occurs during deformation. There are some serious limitations that prevent mass production of UFF steels, although some reports are available that indicate UFF strip steels have been produced with pilot scale rolling.[6] One of the most important issues is the strain required for UFF formation through DSIT, because this is quite high compared with other thermomechanical processes (i.e., conventional controlled rolling). According to recent studies,[2,4,7] even with the addition of shear, this requires at least 40 pct reduction for a single rolling pass, although this could be affected by steel composition.[8] Furthermore, the critical strain for UFF formation (i.e., eC,UFF) varies between 1.5 and 3 in hot torsion testing and depends on the thermomechanical parameters.[9] One proposal to overcome this barrier is to accumulate strain through multiple deformations in the nonrecrystallization region and then to use a lower reduction in the intercritical region to activate the dynamic process. This requires an appropriate thermomechanical schedule designed to avoid any recrystallization between passes (i.e., the Ae3-Tnr region, where Tnr is the nonrecry
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