Effect of prestrain and deformation temperature on the recrystallization behavior of steels microalloyed with niobium
- PDF / 1,170,017 Bytes
- 11 Pages / 605.52 x 785.28 pts Page_size
- 55 Downloads / 262 Views
Effect of Prestrain and Deformation Temperature on the Recrystallization Behavior of Steels Microalloyed with Niobium B. DUTTA and E.J. PALMIERE The evolution of microstructure during the hot working of steels microalloyed with Nb is governed by the recrystallization kinetics of austenite and the recrystallization-precipitation interaction. The present study focuses on the effects of prestrain and deformation temperature on the rectrystallization behavior in these steels. The extent of recrystallization is characterized by a softening parameter calculated from a series of interrupted plane strain compression tests carried out at different deformation temperatures and strain levels. The results indicate that at low temperatures, softening is caused by static recovery, while at higher temperatures, static recrystallization is the predominant mechanism. The recrystallization-stop temperature (T5pct ) and the recrystallization-limit temperature (T95pct ), marking the beginning and end of recrystallization, respectively, are determined as a function of strain. In order to achieve a homogeneous microstructure, finish rolling should be carried out outside the window of partial recrystallization (T5pct < T < T95pct ), as determined in this study. The Nb(CN) precipitation kinetics have been calculated using a model proposed in an earlier work, and these results are used to estimate the precipitate pinning force under the given processing conditions. Based on these estimations, a criterion has been proposed to predict the onset of recrystallization. The predicted results are found to be in reasonably good agreement with the experimental measurements.
I. INTRODUCTION
INDUSTRIAL production of flat-rolled products such as plate, sheet, and strip involves complex thermomechanical processing. In general, these products are aimed at achieving high strength as well as good impact toughness. The key to obtaining tailored microstructures and, hence, optimum properties is through obtaining a proper understanding of the microstructure evolution phenomenon during the processing, and evaluating the role of the different process parameters. Two rolling practices that are commonly in use are conventional controlled rolling (CCR) and recrystallization controlled rolling (RCR). The RCR is carried out at temperatures where the austenite microstructure is completely recrystallized (e.g., > T95pct ), where a sufficient amount of precipitate-induced grain-boundary pinning prevents a further grain growth and retains the fine recrystallized grain size. In contrast, CCR is carried out at temperatures below the recrystallisation-stop temperature (e.g., < T5pct ) whereby a completely deformed, but unrecrystallized, austenite microstructure is obtained. Subsequent cooling leads to nucleation of the low-temperature transformation product on grain boundaries and sub-boundaries of deformed austenite, and results in a refined and uniform microstructure.[1,2,3] However, if the finish rolling is carried out at a temperature between the two previously ment
Data Loading...
