Localized Deformation in Multiphase, Ultra-Fine-Grained 6 Pct Mn Transformation-Induced Plasticity Steel SEAWOONG LEE, SEOK-JAE LEE, S. SANTHOSH KUMAR, KYOOYOUNG LEE, and B.C. DE COOMAN Multiphase, ultra-fine-grained transformation-induced plasticity (MP UFG TRIP) steel con- taining 6 mass pct Mn was obtained by cold rolling and intercritical annealing of an initially fully martensitic microstructure. UFG microstructures with an average grain size less than 300 nm were obtained. The amount of austenite in the microstructures, speculated to be formed by diffusionless transformation, was controlled by changing the intercritical temperature. The tensile properties were strongly influenced by the volume amount and the stability of the reversely transformed austenite. The MP UFG TRIP steel was characterized by pronounced localization of the deformation. The deformation band properties were analyzed in detail. DOI: 10.1007/s11661-011-0636-9 Ó The Minerals, Metals & Materials Society and ASM International 2011 I. INTRODUCTION THE interest in ultra-fine-grained (UFG) and nano- grain-sized materials is directly related to their ultra- high strength. Bulk metallic materials are usually subjected to severe plastic deformation processes such as equal channel angular pressing, [1–3] accumulative roll bonding, [4,5] or hot pressure torsion [6] in order to achieve an UFG microstructure. Whereas these processes were successful in terms of realizing ultra-high strengths, two essential features, i.e., retaining ductility and fracture toughness for shape forming, have proven difficult. UFG materials fracture almost immediately after the onset of necking with little or no plastic strain due to a lack of work hardening. It is essential to attain a higher rate of strain hardening, in order to benefit from ultra-high-strength materials with UFG microstructures. Several methods were suggested to limit the drawbacks resulting from grain refinement. The introduction of a second phase consisting of fine carbides formed by warm annealing of the cold-rolled and as-quenched carbon-supersaturated martensite was reported to enhance elongation in UFG steel. [5] Dual-phase microstructures were also reported to be promising in terms of high strength and ductil- ity. [1,5] Incorporation of a hetero-nanostructure in the form of a ductile second phase such as dendrites, particles, or simply grains of the same composition of larger size was also suggested as a possible route to prevent severe strain localization and to avoid plastic instability. It was also suggested that a bimodal grain size distribution could be beneficial. [7] Another method proposed to improve the ductility involves the formation of nanostructures with an enhanced strain hardening rate, resulting from the room-temperature grain bound- ary sliding of high-angle and nonequilibrium grain boundaries. [8] A combination of adequate tensile elongation and strength can be achieved by thermomechanical treatment of strain-induced martensite and its reverse transforma- tion into metastable austenitic steel with a stable ultra- fine grain structure. Steels with Mn contents less than 10 mass pct were shown to combine high strength and ductility in the UFG grain size regime. Mn contents in the range of 5 to 6 mass pct are adequate to generate UFG microstructures, which possess good combinations of strength and elongation. A Fe-5.7Mn-0.11C (mass pct) alloy with an austenite volume amount between 20 and 30 pct was reported to exhibit an UFG microstruc- ture and good mechanical properties. [9] Recent work on Fe-0.1C (mass pct) steels with Mn content in the range of 5.2 to 7.1 mass pct showed values for the product of ultimate tensile strength (UTS) and uniform elongation in the range of 20,000 to 30,000 MPa pct, as a result of the high volume amount of austenite obtained after a prolonged annealing in the 863 K to 953 K (590 °C to 680 °C) range. [10] Fe-7.1 pct Mn-0.1 pct C (mass pct) steel was reported to combine a tensile strength of 1074 MPa with a total elongation of 33.6 pct. [11] The present work focuses on the mechanical behavior of a multiphase (MP) UFG transformation-induced plasticity (TRIP) steel containing 6 mass pct Mn, in which metastable austenite was formed by reverse transformation of deformed martensite. The volume amount of metastable austenite was varied by changing the intercritical annealing temperature without changing the average grain size of the microstructure, which was less than 300 nm. The mechanical properties of the MP UFG 6 mass pct Mn TRIP steel were found to depend strongly on the amount of austenite in the microstruc- ture; some of the samples in the present work showed SEAWOONG LEE, Master Student, SEOK-JAE LEE, Research Professor, S. SANTHOSH KUMAR, Postdoctoral Researcher, and B.C. De COOMAN, Professor, are with the Graduate Institute of Ferrous Technology, POSTECH, Pohang 790-784, South Korea. Contact e-mail: decooman@postech.ac.kr KYOOYOUNG LEE, Researcher, is with POSCO Technical Research Laboratories, Gwangyang 545-711, South Korea. Manuscript submitted June 10, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS A