Effect of intragranular κ carbides and intergranular precipitates on the hot deformation mechanism and dynamic recrystallization mechanism of Fe–28Mn–11Al–1.5C–5Cr lightweight steel

The hot compression tests of Fe–28Mn–11Al–1.5C–5Cr (wt.%) lightweight steel were carried out by Gleeble-3500 at temperature of 900, 1050 and 1150 °C with strain rates of 0.01 and 1 s⁻¹. The effect of various precipitates on the hot deformation mechanism and dynamic recrystallization (DRX) mechanism was studied by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). When deformed at 900 °C, the dislocations tend to cut through intragranular κ carbides to form unidirectional microbands and deformation twins. The intergranular precipitates (intergranular κ carbides, B2 (FeAl type) precipitates and (Cr,Mn)₂₃C₆ precipitates) hinder the migration of the original grain boundaries, so that the discontinuous DRX (DDRX) is suppressed at the initial stage, but with the increase of strain, the dislocations accumulate at the grain boundaries to promote the nucleation of DRX grains. Meanwhile, the nucleation of DRX grains by continuous DRX (CDRX) mechanism within grains is also activated. Therefore, bimodal distribution of DRX grains is formed at low strain rate of 0.01 s⁻¹, while the degree of DRX is low at high strain rate of 1 s⁻¹. When the deformation temperature is 1050 °C, the content of microbands and deformation twins decreases. There are only a few B2 precipitates at the grain boundaries, so the original grain boundaries tend to bulge to promote DDRX nucleation. When deformed at 1150 °C, large-sized κ carbides are easily formed near the dislocations due to the fast diffusion rate of atoms, which inhibits the formation of microbands and deformation twins, and only slip bands are observed. There is no precipitate on the grain boundary, and the DDRX is the dominant mechanism of DRX. Moreover, the special yield-point-like elongation phenomenon has been proved to be caused by intragranular κ carbides.