Elsevier

Materials Science and Engineering: A

Volume 586, 1 December 2013, Pages 276-283
Materials Science and Engineering: A

Tensile deformation of a low density Fe–27Mn–12Al–0.8C duplex steel in association with ordered phases at ambient temperature

https://doi.org/10.1016/j.msea.2013.07.094Get rights and content

Abstract

Room temperature tensile behavior of a low density Fe–27Mn–12Al–0.8C duplex steel was correlated with both undeformed and deformed microstructures, focusing on the effects of the ordered phases on plastic deformation. Various ordered phases were formed by quenching of the steel after annealing at the (austenite+ferrite) two phase region. The B2 domains bounded by swirled thermal antiphase boundaries were formed in disordered ferrite matrix. In addition to the B2 domains, fine D03 phases were evenly distributed through both B2 domains and disordered ferrite matrix. The nano-sized κ carbides were precipitated in austenite. The steel exhibited the relatively high yield strength and the low strain hardening rate initially, leading to the moderate elongation. The specific strength of the steel reached ~146 MPa cm3/g. Deformed structure of ferrite is manifested by short, straight segments of paired dislocations (superdislocations) with narrow mechanical antiphase boundaries. In austenite, a single planar dislocation glide was dominant at low strains and multiple planar slip occurred at high strains. Based on these microstructural observations, it is suggested that strain hardening of the steel is dominated mainly by shearing of the ordered phases by superdislocations (in ferrite) and planar gliding dislocations (in austenite). In addition, the tensile deformation behavior of the present duplex steel was compared with that of other low density Fe3Mn3Al3C duplex steels.

Introduction

The steel heavy-alloyed with Mn and Al (hereafter, Fe3Mn3Al3C) has long been studied for the cryogenic structural applications [1], [2] and soft magnetic applications [3]. The Fe3Mn3Al3C system was recently reappraised as the promising automotive steel because of its low density, high strength and extended ductility [4], [5]. The low density, of course, result from Al addition. 1 wt% Al addition approximately reduces the density of ordinary steels by ~1.3% by the combined effects of the atomic mass and the lattice dilatation [5]. Depending on the relative amount of the alloying elements, the Fe3Mn3Al3C system is either austenitic or duplex (austenite+ferrite); Mn and C are the austenite former while Al is the strong ferrite former. Therefore, the mechanical properties of the Fe3Mn3Al3C system are dependent on deformation characteristics of the constituent phase(s). The austenitic Fe3Mn3Al3C system exhibits the remarkable combination of strength—ductility primarily due to extensive strain hardenability associated with dislocation planar glide [6], [7]. Some austenitic Fe–(28–30)Mn–(8–10)Al–(0.8–1.2)C steels have been reported to show the strength close to 1 GPa and the ductility over 80% which are not only superior to those of the conventional automotive steels but comparable to those of Fe3Mn3C twinning induced plasticity (TWIP) steels. By contrast, the mechanical properties of the duplex Fe3Mn3Al3C system are much inferior to those of the austenitic one, surely because of the presence of ferrite; for example, a duplex Fe–20Mn–9Al–0.6C steel exhibited~800 MPa strength and ~50% ductility [8].

Meanwhile, precipitation of various ordered phases (i.e. disorder–order transformation) inevitably occurs in the Fe3Mn3Al3C system due to heavy alloying of Mn and Al even in the quenched state after solution treatment. The representatives are the L′12 (perovskite) type κ [(Fe,Mn)3AlC] in austenite and the CsCl type B2 [(Fe,Mn)Al] and Fm3m type D03 [(Fe,Mn)3Al] phases in ferrite; their superlattices are shown schematically in Fig. 1. Therefore, various interactions between dislocations and these ordered phases are anticipated to greatly affect the deformation characteristic of the Fe3Mn3Al3C system. In the case of the as-quenched austenitic Fe3Mn3Al3C system, it was reported that dislocations gliding in the planar mode either destroy the ordered zone (the precursor of κ phase) or shear the nano-sized coherent κ phase, enhancing strain hardenability [9], [10]. The B2 and D03 phases appear in the austenitic Fe3Mn3Al3C system only after prolonged overaging by a transition of the κ phase to a (B2+D03) composite structure [11]. However, in the case of the as-quenched duplex Fe3Mn3Al3C system, the contradictory observations were reported. Wu et al. [12] observed the B2 domains with the finely distributed D03 particles in ferrite of the as-quenched Fe–28.6Mn–10.1Al–0.46C duplex steel by using electron diffraction, but they did not detect any precipitation in austenite. By contrast, Frommeyer and Brüx [13] detected the characteristic X-ray diffraction (XRD) peak of the κ phase in the as-quenched Fe–26Mn–11Al–1.15C duplex steel, and observed the weak D03 peak after tensile deformation at warm temperatures.

In order to develop the Fe3Mn3Al3C system with the lower density, more Al addition is essential and then it makes the Fe3Mn3Al3C system to be duplex. For the duplex Fe3Mn3Al3C system, the primary limiting factor of ductility is the deformation characteristics of ferrite with the B2 and D03 phases. While most current researches on the Fe3Mn3Al3C system for the structural use mainly focus on deformation of austenite whether it is austenitic or duplex, the characteristics of the order phases in ferrite and their effects on the overall deformation of the duplex system were not clearly demonstrated. This issue is addressed in the present study to provide guidance information on developing the advanced low density steel grades. For this purpose, a Fe–27Mn–12Al–0.8C duplex steel was tensile tested and the deformed microstructures were examined by transmission electron microscopy.

Section snippets

Experimental

A 50 kg ingot of a Fe–27Mn–12Al–0.8C steel was prepared by induction melting in an argon atmosphere. After homogenization at 1200 °C for 4 h, the ingot was hot-rolled to the plate of 10 mm thickness. The hot-rolled plate was annealed at 1000 °C for 1 h and then cold-rolled to the final thickness of 2.5 mm. The cold-rolled plate was solution treated at 1100 °C for 30 min and water-quenched. The phase diagram of Fe–27Mn–xAl–0.8C constructed by the Thermo-Calc software revealed that a duplex microstructure

As-quenched microstructures

Fig. 2a shows a representative optical micrograph of the as-quenched steel. Ferrite was embedded into austenite in the form of either the isolated island or the discontinuous band along the rolling direction. The austenite fraction was ~0.73 and the grain size of austenite and ferrite was ~13 μm and ~9 μm, respectively; the annealing twin boundary of austenite was considered as the grain boundary for the grain size measurement. The composition of austenite and ferrite in the as-quenched state

Conclusions

  • 1.

    Various ordered phases were formed in a low density Fe–27Mn–12Al–0.8C duplex steel even by water quenching after annealing at the two phase region. As a result, ferrite consisted of disordered ferrite and B2 domains with even distribution of fine D03 phases. In austenite, nano-sized κ phases were precipitated uniformly.

  • 2.

    The deformed substructure of ferrite was characterized by short, straight segments of paired dislocations (i.e. superdislocations) related to deformation of the B2 domains at low

Acknowledgments

This work was supported in part by Hanbat National University (2011 Research Fund) and in part by POSCO, Korea (contract # 2011XOO8).

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