MS-4-P-6009 TEM Investigation of the Effect of NbC Precipitates on Microband Alignment during Multipass Hot Deformation of Model Fe-30Ni-Nb Microalloyed Steel

Strain-induced NbC precipitation in multipass hot rolling of microalloyed steels is of a high practical importance, however, its direct study is precluded by the phase transformation on cooling from the hot rolling temperatures in these steels. The current investigation was performed using Fe-30Ni-Nb model austenitic steel, that preserves the hot deformed microstructure on cooling to ambient temperature. The hot deformation was carried out in two passes, separated by isothermal holding (10, 100 and 300 s), in uniaxial compression at 925 °C at a strain rate of 1 s-1 using the first pass strain of 0.2 and the second pass strain of 0.2 to 0.6. High resolution EBSD and a wide range of TEM imaging and diffraction techniques were employed to characterise the dislocation substructure and its interaction with strain-induced precipitates. EBSD investigation after the first pass revealed that the microstructure in the <110> fibre grains consists of the crystallographic microbands (MBs) aligned parallel to highly stressed {111} slip planes. This is consistent with reported findings that, during straining to medium levels, the MBs represent “transient” microstructure features that maintain their crystallographic character, through continuously rearranging themselves, and do not follow the rigid body rotation imposed by the plastic deformation. During inter-pass holding after the first pass, the strain-induced NbC particles were observed to nucleate preferentially on the nodes of periodic dislocation networks constituting MB walls. Over the increase of holding time from 10 to 300 s, the precipitates grew in size from 5 to 31 nm and the MB walls became increasingly disintegrated. Interestingly, after second pass deformation the MBs maintained their crystallographic character irrespective of holding time (Fig, 1). For shorter holding time, NbC particles still occupied the MB nodes (Fig, 1a), which indicates that during reloading these particles remained strongly pinned and became dragged by the rearranging MB walls. During reloading after increased holding time the particles tended to become increasingly detached from the MB walls and to follow the rigid body rotation (Fig. 1b). Ultimately, precipitate-free MBs were created for the prolonged inter-pass holding (Fig. 1c). This might be attributed to both the progressively reduced pinning force exerted by coarsening precipitates and the reduced dislocation density in MB walls obtained with an increase in the inter-pass holding time, which would allow the precipitates to move out of these walls during reloading. The present observations made on partially pinned MB walls (see Fig, 1b) revealed that their rearrangement on reloading occurred through cooperative migration of the corresponding dislocation networks.