Compositional and microstructural changes which attend reheating and grain coarsening in steels containing niobium
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I.
INTRODUCTION
THE influence of microalloying elements such as Nb on the response of austenite to thermomechanical processing can be understood by the way in which Nb affects the three critical temperatures of austenite. These temperatures are the grain-coarsening temperature (T~c), the recrystallization-stop temperature (TRxH), and the transformation temperature (Ar3 o r Bs). [I-4] While all three of these temperatures are important to the effectiveness of thermomechanical processing, this present study focuses only on the T~o The TGc is defined as that temperature above which abnormal grain coarsening or secondary recrystallization commences. This temperature can be considered where an equilibrium exists between the driving force for grain coarsening and the pinning force opposing boundary motion. The driving force is taken to be inversely related to the initial grain diameter, while the pinning force is related to the ratio of the particle volume fraction to the particle radius. The pinning force is, therefore, governed by the thermodynamic stability of second-phase particles in austenite. 15,6,7j Since the reheating of slabs or billets represents the initial stage for a given deformation process, knowledge of where the TGc exists relative to the reheating temperature (TRHr) is important. Hence, vastly different microstructures and microalloy solute levels can result depending upon whether the steel has been reheated above or below the TGc. The reheating practice is usually dictated by the requirements of the final product. For example, in products where toughness is very important (e.g., highstrength plate), reheating can take place below the TGc such that grain coarsening is kept to a minimum. In this E.J. PALMIERE, Assistant Professor, C.I. GARCIA, Associate Research Professor, and A.J. DeARDO, Professor, are with the Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261. Manuscript submitted February 17, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
case, second-phase particles remain relatively undissolved in austenite and will not be available for strengthening by reprecipitation during subsequent processing. Therefore, products such as high-strength plate are able to achieve the combined benefits of high-strength and low-impact transition temperatures through grain and subgrain refinement, tl~ Conversely, for products where the toughness is not as important as strength (e.g., strip), reheating can take place above the TGc. Hence, although some coarsening may occur during strip processing when TRHr > T~c, most of the microalloying elements are put into solution in austenite. These elements in solution are thus available for precipitation strengthening during deformation and/or after the ferrite or bainite transformation, tS-~u However, the ability to choose a particular reheat practice can only be accomplished through an understanding of the Tcc and those elements which affect it. The present investigation has considered the solution behavior of Nb, C, and N
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