Grain Size Distributions after Single Hit Deformation of a Segregated, Commercial Nb-Containing Steel: Prediction and Ex
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INTRODUCTION
MECHANICAL properties of microalloyed steel plates are strongly influenced by the ferrite grain structure. In particular, a fine and uniform ferrite grain size is desired in the final microstructure as it provides high strength and toughness. The grain structure and any precipitate population are influenced by the thermomechanical processing (TMP) rolling schedule, through the interaction between recrystallization and precipitation effects.[1,2] However, it was found that in some Nb-microalloyed steel plates, bimodal grain structures consisting of abnormally large grains surrounded by small grains can develop. It was reported that certain processing conditions and compositions are more prone to the formation of bimodal grain structures.[3] Development of a bimodal grain structure is associated with microsegregation of microalloying elements during solidification.[4] However, the extent of segregation is not the same for all microalloying elements present in microalloyed steels. Niobium (Nb) was shown to be the most effective element for grain refinement during rolling; but it segregates strongly to the interdendritic regions along with C and N during solidification.[5–7] The segregation of Nb and its potent grain boundary pinning action (as Nb(C,N)) mean that there can be a substantial difference AMRITA KUNDU, Postdoctoral Research Student, CLAIRE DAVIS, Professor, and MARTIN STRANGWOOD, Senior Lecturer, are with the School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom. Contact e-mail: [email protected] Manuscript submitted December 6, 2010. Article published online May 21, 2011 2794—VOLUME 42A, SEPTEMBER 2011
in pinning force between the dendritic and interdendritic regions.[6] This difference in precipitate population develops on solidification. Reheating to a high (~1498 K to 1523 K (1225 C to 1250 C)) temperature causes complete dissolution of the precipitates regardless of the regions (dendritic or interdendritic) in which they are located removing all pinning particles, and a coarse austenite grain size without bimodality is observed.[6] Commercial reheating can be carried out at lower temperatures, between 1423 K and 1523 K (1150 C and 1250 C) for HSLA steels.[8–13] This temperature range may not give complete dissolution of the Nb-rich precipitates in the interdendritic regions, so that some pinning may remain and a bimodal austenite grain structure prior to deformation may develop. Even for complete dissolution of Nb(C,N), the segregated Nb distribution that remains leads to spatially inhomogeneous solute drag forces and tendency to precipitate. These can influence the recrystallization kinetics. It is therefore important to understand the effect of rolling deformation on grain size development in a segregated structure compared with a homogenized one.[7] The model most frequently referred to for straininduced precipitation, austenite recrystallization, and recrystallization-precipitation interaction in Nb-microalloyed steels is that proposed by
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