Needs for steel designs of ultra-high strength and excellent ductility have been an important issue in worldwide automotive industries to achieve energy conservation, improvement of safety, and crashworthiness qualities. Because of various drawbacks in existing 1.5-GPa-grade steels, new development of formable cold-rolled ultra-high-strength steels is essentially needed. Here we show a plausible method to achieve ultra-high strengths of 1.0~1.5 GPa together with excellent ductility above 50% by actively utilizing non-recrystallization region and TRansformation-Induced Plasticity (TRIP) mechanism in a cold-rolled and annealed Fe-Mn-Al-C-based steel. We adopt a duplex microstructure composed of austenite and ultra-fine ferrite in order to overcome low-yield-strength characteristics of austenite. Persistent elongation up to 50% as well as ultra-high yield strength over 1.4 GPa are attributed to well-balanced mechanical stability of non-crystallized austenite with critical strain for TRIP. Our results demonstrate how the non-recrystallized austenite can be a metamorphosis in 1.5-GPa-grade steel sheet design.

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This study showed the complex interactions of alloying elements and their prevailing microstructure on the pitting corrosion. This interaction was illustrated by electrochemical tests of three grades of wrought duplex stainless steels. It was shown that SAF2507 had the highest pitting potential, followed by SAF2205 and SAF2304. However, SAF2205 had higher corrosion potential than SAF2507. SAF2205 and SAF2507 were immune to pitting, while SAF2304 was susceptible to pitting. It was also found in the experiment that the austenite‐ferrite interface was the most susceptible to corrosion, followed by the austenite and finally the ferrite phase.

It is shown that grain boundaries containing intrinsic grain boundary dislocations act as preferential nucleation site for the martensitic transformation. The microstructure and crystallography of BCC α'-martensite formed in a sensitized AISI 304 stainless steel was studied in detail by means of convergent beam Kikuchi pattern analysis. The orientation relationship between martensite and austenite was determined to be Kurdjumov-Sachs type. The presence of intrinsic grain boundary dislocations was observed at a grain boundary 10.83° from the exact Σ11 CSL boundary orientation, where a martensite nucleus formed by the faulting and extension of intrinsic grain boundary dislocations. The selection of the crystallographic variant for the martensite nucleus was not related to a reduction in interfacial energy at nucleation, nor was it related to the accommodation of transformation strain by easy slip in the austenite.