On the study of tensile and strain hardening behavior of a thermomechanically treated ferritic stainless steel

Ferritic stainless steels, having a lower price than their austenitic counterparts, can compete and be replaced with some austenitic grades in some particular applications if their properties could be enhanced both chemically and mechanically. The main purpose of this work is to investigate the mechanical properties of fine-grained AISI 430 ferritic stainless steel treated by an advanced thermomechanical process. Intense cold-rolling of the specimens up to 95% reduction in thickness followed by annealing at temperatures of 700°C and 800°C for 20s and 120s at each temperature, led to significant changes in the properties compared to the unprocessed specimen. The microstructural observations showed that by this thermomechanical treatment, the grain size reduced to nano-size scale (by a factor of 660). The important improvements in yield and tensile strengths up to about 1000 and 1200MPa, respectively, showed the capability of this alloy to attain nomination as an ultrahigh strength steel. Strain hardening behavior of the samples based on Hollomon analysis indicated that the logarithmic variation of stress versus strain is not linear and obeys a two-stage strain hardening mechanism. Although all the specimens had the similar trend of strain hardening rate, more intense variations in the strain hardening rates were seen as the annealing temperature decreased. Ductile fracture mode was also characterized from the tensile fracture surfaces in all the specimens.