Materials Science & Engineering A 768 (2019) 138458 Available online 27 September 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved. Effect of prior austenite grain size on the evolution of microstructure and mechanical properties of an intercritically annealed medium manganese steel A.K. Chandan a , G.K. Bansal a , J. Kundu b , J. Chakraborty a , S. Ghosh Chowdhury a, * a Materials Engineering Division, CSIR – National Metallurgical Laboratory, Jamshedpur, 831007, India b Metallurgy and Materials Engineering Department, IIEST, Shibpur, Howrah, 711103, India A R T I C L E INFO Keywords: Medium manganese steel Prior austenite grain Intercritical annealing Retained austenite ABSTRACT This paper presents the infuence of prior austenite grain (PAG) size on the evolution of microstructure and mechanical properties in an intercritically annealed medium manganese steel (Fe- 8.1 Mn - 0.3 C- 1.1 Si- 1.5 Al). The hot forged alloy was austenitized at two different temperature-time combination to achieve the PAG size of ~20 μm (sample 1) and ~40 μm (sample 2). Both the samples retained a small amount of austenite after the austenitization treatment. The sample 1 with smaller PAG size stabilized a signifcantly higher amount of austenite (~54.4 � 0.6 vol%) after the intercritical annealing treatment than the sample 2 (~39.3 � 0.7 vol%). Composition as well as dimension of the austenite flms were found to be different in the two samples. Sample 1 displayed a better combination of strength and ductility (1096 � 30 MPa and 32 � 4.2%, respectively) as compared to the sample 2 (1026 � 30 MPa and 22 � 1.9%, respectively). The superior mechanical properties of sample 1 were ascribed to the cumulative effect of the higher volume fraction and lower stability of retained austenite in sample 1 as compared to the sample 2. Fracture surface investigation revealed that the sample 1 experienced microvoid mediated ductile fracture, whereas sample 2 displayed brittle fracture facilitated by crack propagation along certain cleavage facets. 1. Introduction Intercritically heat treated medium manganese (Mn) steels have attracted considerable research attention owing to its ability to over- come the so-called strength-ductility trade-off which is usually observed in advanced high strength steels (AHSS). Medium Mn steel generally contains 3–10 wt % Mn along with other alloying elements such as carbon, aluminum, silicon etc [1–3]. Such steels are subjected to the intercritical annealing (IA) process in order to stabilize the austenite phase in the fnal microstructure [1,2,4–7]. Introduction of the austenite phase in the microstructure improves the toughness and impact prop- erties of such steels [8–10]. The improvement in the mechanical prop- erties is attributed to the active transformation induced plasticity (TRIP) phenomena in the austenite phase [11–15]. The TRIP mechanism work hardens the steel during the course of deformation and thus results in a superior combination of strength and ductility [11–15]. The effcacy of the austenite phase to furnish the desired work hardening is dependent on its volume (vol.) fraction in the microstructure and its mechanical stability [16–20]. The higher is the amount of retained austenite, greater will be the degree of work hardening. Furthermore, the mechanical stability of the austenite phase should be such that the metastable austenite transforms progressively to martensite over a broad range of strain [21,22]. An austenite phase with extremely high stability may result in slower kinetics of austenite to martensite transformation and hence may not furnish the desired work hardening [23]. In this respect, it is important to study the various factors affecting the austenite vol. fraction after the IA and its stability during the deformation. Quite a few literatures on medium Mn steels have identifed several factors infuencing the vol. fraction and stability of the austenite phase, namely chemical composition of austenite phase [23–25], IA parameters [2,4,24], grain size [25,26], morphology of the austenite phase [27,28], orientation [29] etc. Among these, the prior austenite grain (PAG) size is realized to be an important factor. A few studies have tried to reveal the signifcance of PAG size in medium Mn steels in the context of austenite vol. fraction and its stability [30,31]. Han et al. [30] have correlated the austenite stability with the tensile properties in specimens with varying * Corresponding author. Materials Engineering Division, CSIR-National Metallurgical Laboratory, P.O.: Burmamines, Jamshedpur, Jharkhand, 831007, India. E-mail address: sgc@nmlindia.org (S.G. Chowdhury). Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea https://doi.org/10.1016/j.msea.2019.138458 Received 17 March 2019; Received in revised form 16 August 2019; Accepted 26 September 2019