Communication On the Austenite Stability of a New Quality of Twinning Induced Plasticity Steel, Exploring New Ranges of Mn and C RAFAEL AGNELLI MESQUITA, REINHOLD SCHNEIDER, KATHARINA STEINEDER, LUDOVIC SAMEK, and ENNO ARENHOLZ A new high-manganese, low-silicon TWIP steel was studied to evaluate austenite stability after different heat treatment conditions. To determine the phase transfor- mations, dilatometric experiments were performed, and the microstructure was characterized by light optical microscopy, X-ray diffraction, and transmission elec- tron microscopy. Precipitation of lamellar cementite was observed in the microstructure for extended treatment times at 823 K (550 °C). Long isothermal holding at this temperature also caused epsilon martensite formation during cooling, resulting from a decrease in austenite stability due to carbon depletion in the matrix when a quantifiable amount of cementite is formed. DOI: 10.1007/s11661-013-1741-8 Ó The Minerals, Metals & Materials Society and ASM International 2013 Since the discovery of high-Mn effects in steels by Robert Hadfield at the end of the nineteenth century, [1] there has been strong interest in industry and academia in the properties and phase transformations of austenitic high- Mn steels. More recently, compositions in the range from 0.6 to 1.2 pct C and from 12 to 22 pct Mn have been the focus of several studies due to the twinning-induced- plasticity (TWIP) effect. [2] The microstructure of such steels consists of austenite stabilized by high amounts of C and Mn. The addition of Al, Si, or other elements controls the stacking fault energy (SFE) which determines the balance between martensite and twinning formation upon plastic deformation. [3] When properly designed, the microstruc- ture of TWIP steels enables intense mechanical twin formation, which increases the strength by decreasing the free path for dislocation movement (similar to a Hall–Petch effect, [4] ). At the same time, the FCC austenitic phase maintains a high ductility level in the material. As a consequence, a combination of high strength and high ductility has been obtained in TWIP steels, thus identifying them as interesting candidates for automotive applications. Although high-Mn steels are an interesting solution for low-cost austenite stabilization, inconveniences have been observed related to the elevated Mn content. First, the inherent cost of Mn alloying may be a disadvantage compared with other automotive grades, such as transformation-induced-plasticity (TRIP) or high- strength low-alloy (HSLA) steels. Second, the high tendency of positive segregation of Mn is of concern because it leads to heterogeneous microstructures after solidification and rolling. [5] Therefore, exploring the new fields of Mn ranges is important, combining the chem- ical composition with a corresponding C content. To illustrate this correlation, Figure 1 presents a traditional Schumann diagram for steels with different contents of C and Mn. This diagram summarizes the metastable phases vs C and Mn contents after rapid cooling and additional plastic deformation, as detailed in Reference 6. The straight lines in Figure 1(a), although giving a good indication for the different fields of metastable phases, lead to an oversimplification in some areas, especially in the ranges from 0.6 to 0.8 pct C and from 15 to 20 pct Mn, which are interesting ranges for steels with a TWIP effect. By carefully analyzing this area of the Schumann diagram, it turns out that the austenite field can be expanded, as shown in Figure 1(b). To confirm this hypothesis, new compositions were pro- duced (large dots in Figure 1(b)), which proved to remain austenitic at room temperature and after a plastic deformation of more than 50 pct in tensile testing. In addition, previous results have shown that different compositions in this new Mn-C range, specif- ically 16 pct Mn and 0.8 pct C, are interesting candi- dates in terms of their mechanical properties. [7,8] In fact, a proprietary alloy was developed in this range, as detailed in Reference 9. In the current article, the austenite stability of this new steel composition was studied to determine possible microstructural changes during thermal processing, an important factor for steel production. The final compo- sition of the evaluated steel is 0.79 pct C, 15.9 pct Mn, 0.001 pct Si, 0.005 pct Al, 0.031 pct Cr, 0.027 pct Ni, with the SFE primarily controlled by the N content and with a distinct balance of P. [9] Transmission electron microscopy (TEM) and Mo-targeted X-ray diffraction (XRD) were used for phase characterization. TEM preparation consisted of grinding and final polishing in a chromic acid electrolyte (150 g CrO 3 dissolved in 810 mL CH 3 COOH + 42 mL H 2 O) using a Tenupol facility operating at 288 K (15 °C) and 40 V. A double-tilt JEOL120 kV transmis- sion electron microscope was used for the analysis. All thermal treatments were performed in a Ba¨ hr 805 A/D-dilatometer to determine length vs time and tem- perature. The transformation areas of the determined time–temperature–transformation (TTT) diagram for this alloy were obtained by dilatometric measurements RAFAEL AGNELLI MESQUITA, Professor, is with the Department of Industrial Engineering, Universidade Nove de Julho, Av. Francisco Matarazzo 612, Sa˜ o Paulo, SP 05001-100, Brazil. Contact e-mail: rafael.mesquita@uninove.br REINHOLD SCHNEIDER, Professor, and KATHARINA STEINEDER, Student, are with the University of Applied Sciences Upper Austria, Wels, Austria. LUDOVIC SAMEK, Researcher, and ENNO ARENHOLZ, Head of Research, are with the voestalpine Stahl GmbH, Linz, Austria. Manuscript submitted November 17, 2012. Article published online June 13, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 44A, SEPTEMBER 2013—4015