Phenomenological Model for Deformation-Induced Ferrite Transformation
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INTRODUCTION
CURRENTLY, a significant research effort is devoted to novel fine-grained materials. In particular, an emphasis of this research has been on ultrafinegrained steels, the goal being to develop a new class of high-strength low-carbon steels in which the use of costly alloying elements is dramatically reduced. A number of processing routes have been evaluated for this purpose, primarily with laboratory simulations.[1–3] An advantage of steel compared to many other alloys is that the solid-solid transformation from austenite to ferrite can be exploited to obtain grain refinement. Using conventional industrial processing routes, including accelerated cooling, a ferrite grain size limit of approximately 5 lm has in general been observed for CMn steels. However, the novel technique of deformation-induced ferrite transformation (DIFT), also known as strain-induced transformation (SIT), dynamic straininduced transformation (DSIT), strain-induced dynamic transformation (SIDT), and strain-assisted transformation (SAT), provides the potential to produce an ultrafine ferrite (UFF) structure with grain sizes that typically fall into the 1-lm range.[3–7] The DIFT processing is based on quickly cooling austenite just above the transformation start temperature that would be associated with this cooling condition. Heavy deformation of the strongly undercooled, metastable austenite then triggers the formation of UFF. An important M. MILITZER, Professor, is with The Centre for Metallurgical Process Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Contact e-mail: [email protected] Y. BRECHET, Professor, is with the SIMAP Laboratory, Institut National Polytechnique de Grenoble, F-38402 St Martin d’He`res Cedex, France. Manuscript submitted December 17, 2008. Article published online August 7, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
advantage of this technique is that it can, at least in principle, be applied in sheet production. Priestner and de los Rios[4] were among the first who had observed this phenomenon, and Yada et al.[8] had already claimed in the 1980s to have used this technique to produce hotrolled coils with a ferrite grain size of approximately 1 lm. The main observation made was that, during hot rolling, an ultrafine-grained surface zone can be obtained that can be attributed to DIFT.[5,6] Initially, it was thought that a coarse austenite microstructure (austenite grain size of the order 200 lm) in combination with surface chilling by contact with the rolls and the significantly higher deformation degree in the surface region leads to the required conditions for DIFT. Recently, Morimoto et al.[9] reported industrial production of a fine-grained plain-carbon steel that is attributed to DIFT, which was initiated in the final stands of a seven-stand finish mill by a combination of asymmetric rolling and proper interstand cooling. In the laboratory, DIFT was observed for a wide range of chemistries with little effect of the steel chemistry on the resulting ferrite grain sizes.[6]
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