High temperature deformation behavior of two as-cast high-manganese TWIP steels A. Khosravifard a , A.S. Hamada b,c , M.M. Moshksar a , R. Ebrahimi a , D.A. Porter b , L.P. Karjalainen b,n a Department of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz 71348, Iran b Centre for Advanced Steels Research, University of Oulu, PO Box 4200, FI-90014 Oulu, Finland c Metallurgical and Materials Engineering Department, Faculty of Petroleum and Mining Engineering, Suez Canal University, Box 43721, Suez, Egypt article info Article history: Received 24 May 2013 Received in revised form 5 June 2013 Accepted 8 June 2013 Available online 15 June 2013 Keywords: High-manganese steel As-cast structure Micro-segregation Flow stress Softening Dynamic recrystallization abstract The high-temperature behavior of two as-cast high-manganese steels with different levels of carbon (0.49 and 0.07 wt%) has been studied by employing hot compression tests at different temperatures (900, 1000 and 1100 1C) and strain rates (0.01, 0.1,1 and 10/s). Microstructures of the deformed specimens have been examined using SEM–EBSD. The steels are compared in terms of their flow stress level, activation energy of deformation, critical stress and strain for the initiation of softening, and extent of grain refinement. The two steels behave quite differently: flow stress levels at small strains are higher for the high-carbon steel than for the low-carbon one but softening, starting at very small strains, is very pronounced in the former, whereas only slight softening is observed for the low-carbon steel. This peculiar behavior of the high-carbon steel is due to the localization of strain along segregation bands and possibly the presence of ferrite at high temperatures in the highly segregated regions of the cast structure. Effective grain refinement occurs by dynamic recrystallization in both the steels. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Among the various austenitic steels, those showing twinning induced plasticity (TWIP) are well known for their excellent ductility at room temperature due to intense strain hardening up to large values of strain [1]. A high-manganese content of about 15–30 wt% is necessary to maintain austenite as the stable phase down to room temperature. Additionally, certain amounts of Al and Si may also be added to adjust the stacking fault energy of the steel to make twinning the dominant deformation mechanism [2]. The generated twins are responsible for the very high hardening rate of TWIP steels during their plastic deformation. Physical and mechanical metallurgy of these steels are comprehensively reviewed in a recent work [3]. As with other metals and alloys, hot working is an integral part of the production of TWIP steels and the investigation of its hot deformation behavior is of industrial importance. Li et al. [4] and Dobrzanski et al. [5–7] have studied the high-temperature beha- vior of high-manganese steels extensively. Li et al. considered the flow stress and dynamic recrystallization (DRX) characteristics of a low-carbon 20Mn–3Si–3Al steel (all concentrations are hereafter in wt%). Dobrzanski et al. investigated the flow stress levels and the conditions for the occurrence of static recrystallization (SRX), DRX and metadynamic recrystallization (MDRX), and microstruc- ture evolution during multi-pass deformation in a low-carbon 26Mn TWIP steel with Ti–Nb microalloying and various Al and Si additions. The flow stress was observed to be quite high but it was effectively reduced by DRX. Hamada et al. [8] confirmed that Al alloying increases the high-temperature flow resistance of the steel and retards the onset of DRX. However, in another work, only a minor influence of Al on the SRX kinetics was found [9]. By conducting double-hit compression tests, Hajkazemi et al. [10] showed that in a low-carbon 29Mn-Si-Al TWIP steel, SRX results in softening of the steel during the inter-pass time, while simulta- neously, static strain aging causes a considerable hardening effect. In some very recent work [11,12], the flow resistance of low- carbon Si–Al-alloyed TWIP steels at a variety of temperatures and deformation rates was modeled by the Arrhenius-type hyperbolic sine equation. All the above-mentioned studies concerned wrought high- manganese steels with low carbon contents (less than 0.15%). Hamada et al. [13] investigated the high-temperature flow stress and recrystallization behavior of several wrought TWIP steels with somewhat higher carbon contents up to 0.27%. In another research Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.06.014 n Corresponding author. Tel.: +358 294 482140; fax: +358 8 553 2165. E-mail addresses: akhosravi@shirazu.ac.ir (A. Khosravifard), atef.hamada@oulu.fi (A.S. Hamada), moshksar@shirazu.ac.ir (M.M. Moshksar), ebrahimy@shirazu.ac.ir (R. Ebrahimi), david.porter@oulu.fi (D.A. Porter), pentti.karjalainen@oulu.fi (L.P. Karjalainen). Materials Science & Engineering A 582 (2013) 15–21