Understanding Of Deformation And Fracture Behavior In Next Generation High Strength-High Ductility Steels

The industrial demand for low density high strength steels without compromising on ductility in is endless. In this regards, grain refinement and austenitic stability considered to the practical approach in order to meet the needs. The concept of phase reversion involving sever cold deformation of metastable austenite to generate strain-induced martensite, followed by time-temperature annealing sequence, was used to obtain varying grain size from nanograined/ultrafine-grained (NG/UFG) to coarse-grained (CG) regime. This concept was used to obtain high strength-high ductility combination in nano/ultrafine-grained (NG/UFG) austenite stainless steel. Using this concept, the objective of the study here is to elucidate the dependence on deformation mechanism, deformation-induced microstructural changes and fracture behavior w.r.t grain size. The objective was accomplished by combining depth-sensing nanoindentation experiments conducted at various strain rates and interrupted tensile testing at various strains and post-mortem analysis of deformed Fe-17Cr-7Ni (AISI 301LN). In the high strength NG/UFG steel, deformation twinning contributes to excellent ductility, while in the low strength coarse-grained (CG) steel, ductility was due to strain-induced martensite, implying clear distinction and fundamental transition in the deformation behavior of NG/UFG and CG austenitic stainless steels. TWIP effect was observed in NG/UFG steel, which effected the fracture behavior where it shows striations on the fracture surface and as the grain size, the fracture becomes microvoid-coalesce type due to the presence of strain-induced martensite (TRIP effect) in CG steels. Success of this concept motivated to develop varied grain sizes (NG/UFG & CG) of Fe-1.55Mn-0.1V-0.018N microalloyed superplastic steel and its deformation and fracture behavior was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The superplastic steel exhibits grain boundary migration during plastic deformation, which is an attribute to grain boundary sliding. The fracture surface was characterized by elongated cavities that nucleated and grew parallel to the applied tensile stress along with the presence of plastic deformation around the cavities. In order to explore the austenitic stability and their ability to improve mechanical properties, other set of advance high strength steels, medium-Mn TRIP steels is chosen for its TRIP behavior. The effect of Al content on deformation mechanism and fracture behavior have been explored using different characterization tenchniques. Steels with 2-4 wt% Al were characterized by only TRIP effect, while TWIP was also observed in conjunction with strain-induced martensite in 6Al-steel, a behavior attributed to increase in stacking fault energy with increase in Al-content such that the austenite is stable. The difference in mechanical properties was reflected in the fracture behavior. In the practical point of view, these approaches provide novel methods to develop metals and alloys with exceptional combination of strength and ductility