Abstract
The effects of Nb addition on the recrystallization kinetics and the recrystallized grain size distribution after cold deformation were investigated by using Fe-30Ni and Fe-30Ni-0.044 wt pct Nb steel with comparable starting grain size distributions. The samples were deformed to 0.3 strain at room temperature followed by annealing at 950 °C to 850 °C for various times; the microstructural evolution and the grain size distribution of non- and fully recrystallized samples were characterized, along with the strain-induced precipitates (SIPs) and their size and volume fraction evolution. It was found that Nb addition has little effect on recrystallized grain size distribution, whereas Nb precipitation kinetics (SIP size and number density) affects the recrystallization Avrami exponent depending on the annealing temperature. Faster precipitation coarsening rates at high temperature (950 °C to 900 °C) led to slower recrystallization kinetics but no change on Avrami exponent, despite precipitation occurring before recrystallization. Whereas a slower precipitation coarsening rate at 850 °C gave fine-sized strain-induced precipitates that were effective in reducing the recrystallization Avrami exponent after 50 pct of recrystallization. Both solute drag and precipitation pinning effects have been added onto the JMAK model to account the effect of Nb content on recrystallization Avrami exponent for samples with large grain size distributions.
Highlights
NB is often added to steels to retard recrystallization during thermomechanical processing, through effects such as solute drag and precipitate pinning.[1,2] Solute drag effect refers to the retarding force caused by the alloying elements segregating to the migrating boundaries
Precipitate pinning effect is caused by strain-induced precipitates (SIPs) forming during annealing, which pin the migrating boundaries
These data indicate that in most of the cases represented there is a combination of Nb in solution and NbC SIP distributions. Both solute drag and Zener pinning can act to modify the recrystallization Avrami exponent. To account for both the effect of precipitate pinning and solute drag on recrystallization kinetics, the pinning force and reduced boundary mobility have been calculated using the data from Table VI
Summary
NB is often added to steels to retard recrystallization during thermomechanical processing, through effects such as solute drag and precipitate pinning.[1,2] Solute drag effect refers to the retarding force caused by the alloying elements segregating to the migrating boundaries. Precipitate pinning effect is caused by strain-induced precipitates (SIPs) forming during annealing, which pin the migrating boundaries. The spatial distribution of SIPs and their sizes are influenced by the dislocation structures.[3] Though the effect of Nb on retarding recrystallization in steels after hot deformation has been well studied in the past three decades, the available data of Nb precipitation kinetics and austenite recrystallization kinetics are rather limited. Model austenitic alloys allow cold deformation data to be garnered to establish a predictive model for recrystallization and precipitation in Nb-containing alloys
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