Strain hardening and tensile deformation behaviour of two advanced high-strength steels with ferrite-bainite microstructure

Two advanced high-strength steels (AHSSs) with varying percentages of bainite present in the microstructure were examined for their deformation behaviour under tensile loading. Despite having comparable yield strength (680–700 MPa) and tensile strength (775–780 MPa), the two steels behaved differently in terms of deformation and damage during tensile loading. The strengthening mechanism was governed by the combined effect of precipitates and the harder bainite phase. Also, the two steels’ strain-hardening behaviour differed. At the early stages of the deformation, the amount of harder bainite phase determined the strain hardening rate, however, in the later stages, the strain hardening rate became sluggish and gradual due to restricted mobility of the dislocations caused by the nano-precipitates. This phenomenon was further confirmed by the EBSD and KAM maps, revealing differences in grain misorientation resulting from the deformation. Overall, steel with a reduced volume fraction of bainite phase demonstrated a ductile fracture behaviour in contrast to the steel with a predominantly bainitic microstructure. In this case, the quasi-cleavage kind of fracture was discovered.