Hyejin Song

Dramatic improvement of strain hardening and ductility to 95% in highly-deformable high-strength duplex lightweight steels

Ferrite + austenite duplex lightweight steels have been actively developed by adding low-density Al for overcoming a limitation of stiffness deterioration by a traditional approach to obtain a weight reduction. Multiple-stage deformation mechanism in lightweight steels, i.e., simultaneous formation of deformation-induced martensite and deformation twin and additional plasticity by twinning, has been nominated as an attractive strategy, but shows a steady flow behavior with early plastic instability. Here, we present a newly designed Fe-0.3C-9Mn-5Al steel in order to obtain an optimal level of stability of austenite and a resultant outstanding combination of tensile strength and ductility, e.g., 874 MPa and 72%, together with sufficiently high strain hardening. These enhanced properties are attributed to the decreased austenite stability by controlling the austenite size and alloying partitioning due to variation in austenite fraction inside duplex microstructures. The present work gives a promise for structural applications requiring both reduced specific weight and remarkable deformability.

Novel 1.5 GPa-strength with 50%-ductility by transformation-induced plasticity of non-recrystallized austenite in duplex steels

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.

Tensile property improvement in Ti/Al clad sheets fabricated by twin-roll casting and annealing

A new fabrication method of Ti/Al clad sheet by bonding a thin Ti sheet on to Al alloy melt during twin-roll casting was presented to broaden industrial applications of Ti alloys. In the twin-roll-cast Ti/Al clad sheet, a homogeneously solidified microstructure existed in the cast Al side, while a Ti/Al interface did not contain pores, cracks, or lateral delamination, which indicated a successful fabrication. When this cast sheet was annealed at 400 °C, metallurgical bonding was expanded by interfacial diffusion, thereby leading to improvement in tensile properties. After the 600 °C-annealing, a TiAl3 intermetallic compound was formed at the Ti/Al interface, which deteriorated tensile properties because of its brittle characteristics. The yield and tensile strengths of the 400 °C-30-min-annealed sheet (132 MPa and 218 MPa, respectively) were higher than those (103 MPa and 203 MPa, respectively) calculated by a rule of mixtures using tensile properties of a 5052-O Al alloy and a pure Ti. Its ductility was also higher than that of 5052-O Al alloy (25%) or pure Ti (25%) because the strain at the interfacial delamination point reached 41%. The 400 °C-30-min-annealed sheet showed the overall homogenous deformation behavior by complimenting drawbacks of mono-layer of Al or Ti.

Three-Ply Al/Mg/Al Clad Sheets Fabricated by Twin-Roll Casting and Post-treatments (Homogenization, Warm Rolling, and Annealing)

Abstract When thin Al alloy sheets are clad on to twin-roll-cast Mg alloy melt, inherent drawbacks of Mg alloys such as poor formability, corrosion resistance, and surface quality can be effectively complemented. In this study, three-ply Al/Mg/Al clad sheets were fabricated by twin-roll casting and post-treatments. Brittle interfacial layers composed of γ (Mg17Al12) and β (Mg2Al3) phases were inevitably formed, but their proper thickening during the post-treatments led to improvement of interfacial bonding and resultant tensile properties. In particular, warm rolling was an effective way to modify interfacial microstructures and tensile properties by minimizing deformation inhomogeneity and stress concentration.