Tailoring the Mechanical Properties of a Twinning-Induced Plasticity Steel by Retention of Deformation Twins During Heat
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High manganese twinning-induced plasticity (TWIP) steels have gained extensive industrial and scientific interest due to their outstanding combination of ultrahigh strength along with high work hardening capacity and elongation, which is commonly attributed to a dynamic Hall–Petch effect.[1] However, a substantial shortcoming of this class of materials is their low yield strength of 200 to 400 MPa[2,3] that strongly restricts industrial applications. Apart from microalloying[4] and grain refinement after recrystallization,[5,6] a processing route combining cold rolling and recovery annealing was shown to result in increased yield strength and appreciable ductility.[7] This behavior was interpreted in terms of thermal stability of the mechanically induced, nanoscale twin boundaries up to the recrystallization temperature. As a consequence, the fine-grained microstructure is maintained during recovery. The current study aimed at proving the efficiency of this approach for aluminum-added TWIP steel. In addition to the previous studies by Bouaziz et al.,[7,8] not only the improvement of mechanical properties but CHRISTIAN HAASE, Doctoral Candidate, LUIS ANTONIO BARRALES-MORA, Research Fellow, Group Leader of Parallel Material Models, DMITRI A. MOLODOV, Professor, Group Leader of Interface Dynamics, and GU¨NTER GOTTSTEIN, Professor, are with the Institute of Physical Metallurgy and Metal Physics, RWTH Aachen University, Aachen 52074, Germany. Contact e-mail: haase@ imm.rwth-aachen.de Manuscript submitted June 26, 2013 Article published online August 13, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
also the establishment of a relation between the mechanical behavior of TWIP steels, the processes occurring during annealing, and specific features of its microstructure after cold rolling and subsequent recovery and/or recrystallization annealing were set into the focus of this work. A laboratory-produced steel Fe-23Mn-1.5Al-0.3C with estimated stacking fault energy (SFE) of about 25 mJ/m2[9] was used for experiments. In our previous study,[10] where the texture and microstructure evolution during cold rolling in a Fe-28Mn-0.3C alloy with similar SFE of about 27 mJ/m2 were investigated, it was found that severe deformation twinning set in at a rolling reduction of 30 pct. Therefore, in the current work, prior to annealing at 903 K (630 C), the investigated material was cold rolled to the same rolling degree. In order to identify the potential of the aforementioned production route for the material investigated, cold-rolled (CR), recovery-annealed, and recrystallization-annealed samples were mechanically characterized using uniaxial tensile testing. The microstructure of the material in different conditions was analyzed using electron backscatter diffraction (EBSD). The corresponding sample preparation comprised mechanical grinding up to 4000 SiC grit paper, mechanical polishing using 3 and 1 lm diamond suspension, and electropolishing for 20 seconds at 22 V and 293 K (20 C). The electrolyte consisted of 700 mL ethanol (C2H5OH),
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