Influence of Carbide Precipitation and Dissolution on the Microstructure of Ultra-Fine-Grained Intercritically Annealed
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recently developed ultra-fine-grained (UFG), medium Mn (6 to 12 pct), ferrite + austenite steels the control of the room-temperature austenite stacking fault energy (SFE) and stability is achieved by the controlled partitioning of C and Mn to the intercritical austenite during an intercritical annealing treatment.[1–6] If the composition-dependent SFE is in the range of 15 to 45 mJ/m2, strain-induced twinning takes place in the austenite grains during room-temperature deformation. This strain-induced twinning takes place at low strains. In addition, the strain-induced transformation to martensite nucleates at twin–twin intersections after the process of strain-induced twinning saturates. Both deformation-induced twinning and transformation are known to result in effective plasticity-enhancement mechanisms known as the TWIP and the TRIP effect, respectively. Although UFG microstructures are usually characterized by a poor formability, the TWIP and
SANGWON LEE, Senior Researcher, is with the Metallic Materials Division, Titanium Department, Korea Institute of Materials Science, Changwon, Republic of Korea. BRUNO C. DE COOMAN, Professor and Director, is with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang, 51508, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted July 8, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
TRIP effects occurring in succession result in the combination of an ultra-high strength (>1 GPa) and high ductility (>40 pct) of UFG medium Mn ferrite + austenite steels. UFG medium Mn (6 to 12 pct) ferrite + austenite steels exhibit a high yield strength (>800 MPa) due to the presence of hard ferrite[5] and an UFG structure (
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