Effect of low-temperature ausforming on bainitic transformation and mechanical properties in ultrahigh-strength bainitic steel

Low-temperature ausforming (LT-AF) prior to bainitic transformation leads to a noticeable acceleration of bainitic transformation kinetics; however, the effect of LT-AF on the retained austenite (RA) features and the resulting mechanical properties is still unclear. LT-AF was applied to ultrahigh-strength bainitic steel before austempering. The deformation behavior and the resulting dislocation substructures were investigated by thermomechanical simulator and transmission electron microscopy (TEM). The planar dislocation structures produced during deformation at 350 °C accelerate the bainitic transformation kinetics during isothermal holding. The effect of LT-AF on the bainitic transformation kinetics and the features of RA was elucidated via dilatometer measurement, TEM, scanning electron microscopy, and X-ray diffraction. It is observed that LT-AF not only retains more RA content but also facilitates improved RA stability. This trend is mainly due to the large amounts of planar dislocations in RA and bainitic laths inherited from undercooled austenite caused by LT-AF, the decrease in bainitic sheaves size, and the increase in filmy RA content compared to the sample without ausforming. A large fraction of filmy RA with high stability and the refinement of bainitic sheaves obtained by LT-AF remarkably enhance the strain hardening capacity and achieve significantly better ductility compared to the directly austempered sample.