Abstract
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.
Highlights
Reduction in vehicle weight becomes more emphasized for reducing exhaust emission and improving fuel efficiency[1,2,3]
The austenite is transformed to α′-martensite during deformation, which leads to excellent tensile strength (1200 MPa) and elongation (30%), depending on composition, size, orientation, distribution, and stacking fault energy of austenite
Ferrite grains are coarsened to 0.48 μm with increasing annealing temperature, while their volume fractions decrease (Fig. 1(b,c))
Summary
Reduction in vehicle weight becomes more emphasized for reducing exhaust emission and improving fuel efficiency[1,2,3]. In applications for structural reinforcement components composed of pillars, side sills, and front cross members, ultra-high yield strength over 1 GPa and sufficient stiffness are required These Giga-grade automotive sheet markets are largely divided by cold-rolled sheets and hot press forming (HPF) sheets, and HPF sheets are used alone in 1.5-GPa-grade applications. As a promising method to achieve ultra-high strengths of 1.0~1.5 GPa together with excellent ductility above 50%, utilization of non-recrystallization region in a cold-rolled and annealed Fe-Mn-Al-C-based steel (composition; Fe-0.5C-11.8Mn-2.9Al-1Si-0.32Mo-0.5 V (wt.%)) has been suggested in the present study. We show a plausible method of improving ductility as well as ultra-high strength by actively utilizing the non-recrystallization region in combination with TRIP mechanism. Our results demonstrate how the non-recrystallized austenite can be a metamorphosis in 1.5-GPa-grade steel sheet design
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