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Highlights • The austenite-based, duplex and triplex structures were developed. • Different types of intra-granular and grain boundaries κ -carbides were observed. • The microstructure-dependent mechanical responses were analyzed. • This work can be used to tailor phase constituents of medium-Mn lightweight steels. Abstract We elucidate three distinguished microstructural features and their contribution to the overall properties and strengthening in a Fe–11Mn–10Al–1.25C alloy treated at various temperatures. At 1000 °C, the nano-scaled ordered κ -carbides in the austenitic matrix contributed to YS (1041 MPa) without scarifying ductility (∼30%). At 900 °C, the significantly refined triplex microstructure with GB κ -carbides caused similar YS but limited ductility (∼8%). At 800 °C, a jump of YS but almost no ductility resulted from the synergic effects of the hard ferrite and lamellar microstructure. The microstructure-dependent mechanical responses were further clarified by analyses of flow curves and fractographs. [ABSTRACT FROM AUTHOR]

The present work aims to reveal the influence of κ-carbides precipitated in grain interior (GI) and along grain boundaries (GBs) with various characteristics on the strain hardening behavior of Fe–11Mn-(7, 10)Al-(0.6, 0.9, 1.2)C steels. Because of the higher probability for moving dislocations meeting with κ-carbides and shearing them, the increase in the volume fraction of shearable nano κ-carbides is more favorable for the improvement of the yield strength (YS) but thus induces the lower strain hardening rate (SHR), especially at low strains. The SHR maintains almost constant values even gradually increases during straining as the enhanced κ-carbide precipitation is beneficial to the planar glide softening and thus promotes the slip bands refinement. EBSD and TEM analysis reveal that both the unshearable coarse and sub-micron κ-carbides formed along GBs and in GI hinder the dislocation movement, and the former results in the significant strain localization in α/κ interfaces, consequently bringing about high SHR but premature failure. It is hoped that these findings will help achieving better synergy of strength and ductility of lightweight steels by utilization of the unshearable sub-micron κ-carbide in addition to the shearable one and partial-recrystallization. [ABSTRACT FROM AUTHOR]

This paper aimed to study the effects of annealing temperature on the microstructures and yield strength (YS) in the Fe-11Mn-xAl-yC medium-Mn lightweight steels, and reveal the structure-strength relationships and strengthening mechanisms. As the annealing temperature decreased from 1100 °C to 850 °C, the YS of the IG κ-carbide-free Fe-11Mn-7Al-0.6C steel (0.6C) could be improved from 482 MPa to 630 MPa by grain size refinement strengthening. When the 0.6C steel was annealed below 800 °C, partial recrystallization occurred and brought about a great increase in the YS up to 1.1 GPa by the high dislocation density (8.2–10.9 × 1014 m−2). The increases in the contents of C and Al promoted the precipitation of both grain boundary (GB) and intra-granular (IG) κ-carbides. Thus, the 1000 °C-annealed Fe-11Mn-10Al-1.2C steel possessed an austenite matrix containing nano κ-carbide and a small amount of α-ferrite, and achieved ultrahigh YS of 1242 MPa by relying on the remarkable precipitation strengthening effect (679 MPa). This study provided a new insight into comprehensively understanding the relationships among alloy composition, thermomechanical process (TMP), microstructures and various strengthening mechanisms in the lightweight steels so as to improve the YS. [ABSTRACT FROM AUTHOR]

In the present study, the variations of microstructure and tensile properties were investigated as a function of aging time using a Fe–11Mn–10Al-1.2C medium-Mn lightweight steel. The 1000 °C-annealed specimen revealed an austenite matrix containing nano-sized particles of intra-granular (IG) κ-carbide and a small amount of α-ferrite. After aging at 550 °C for 3 min, pre-existing IG κ-carbide grew up rapidly and relatively coarse κ-carbide formed at the γ-austenite grain boundary (GB). With aging for 10 min, the C-depleted zones, close to the coarsened GB κ-carbide particles, were transformed to filmy grain boundary ferrite (GBF). When aging time increased to 30 min, the eutectoid reaction (γ → α + κ) occurred preferentially at the coarse particles of GB κ-carbide. The eutectoid reaction continued with increasing aging time up to 720 min, and finally the specimen had a eutectoid matrix (α+κ) with a few of α-ferrite. The annealed specimen possessed high strengths (yield strength (YS) of 1242 MPa and ultimate tensile strength (UTS) of 1314 GPa) with moderate elongation (El) of 22.2%. The YS value of aged specimens was improved to ~1.35 GPa by the growth of IG κ-carbide particles after aging for 3 min, and increased further up to ~1.54 GPa by the formation of eutectoid structure after aging for 720 min. [ABSTRACT FROM AUTHOR]

The dynamic recrystallization (DRX) and dynamic precipitation (DP) of grain boundary/intra-granular (GB/IG) κ-carbides in a novel medium-Mn lightweight steel are investigated by conducting compression tests over a wide range of temperatures (800–1100 °C) and strain rates (0.001–10 s−1). Two types of distinct flow behaviors have been observed: the flow curves show typical DRX features at high temperatures (≥ 950 °C), whereas the flow stress shows the significant softening after peak stress at low temperatures (≤ 900 °C), which is closely associated with the DP of GB/IG κ-carbides. According to the distinguished flow behaviors, two constitutive equations have been developed for the low and high temperature regimes, showing disparate hot deformation energy (Q) values (i.e. 600.58 kJ/mol and 355.51 kJ/mol). At low temperatures, the κ-carbides inhibit dislocation motion, and the pinning effect of fine GB κ-carbide particles retards the austenite grain boundary migration, which greatly improves the peak stress level and thus increases the energy barrier (i.e. Q). Furthermore, the volume fraction and grain size of GB κ-carbides increase with the decrease in strain rate, resulting in the consumption of stored deformation energy, which coupled with the continuous DRX (CDRX) in δ-ferrite are responsible for the significant softening. The DRX takes place more sufficiently with increasing temperature, and the deformation induced α-ferrite transformation (DIFT) and morphology evolution of δ-ferrite are observed at 950 °C and 1100 °C, respectively. [ABSTRACT FROM AUTHOR]

In the present work, the Fe-13Mn-8Al-(0.7, 1.2)C steels were subjected to different post-cold rolling annealing treatments in a temperature range of 800–1000 °C for 15 min to investigate the microstructural evolution and the mechanical properties. Both steels annealed at 800 °C consisted of fine austenite and ferrite grains along with intergranular κ-carbides formed by eutectoid reaction, which contributed to the high strength but deteriorated ductility. With annealing temperature increasing, the volume fraction of ferrite and intergranular κ-carbides progressively decreased. Meanwhile, the precipitation of intragranular κ′-carbides enhanced the yield strength, whereas coarse austenite was harmful to both strength and elongation. The increase in C content increased the volume fraction of intergranular κ-carbides and austenite as well as retarded the austenite recrystallization, which further increased strength and strain hardening rate but deteriorate ductility. The deformation mechanism in austenite was massive planar slip. The microbands were widely observed in both steels at an annealing temperature above 900 °C. [ABSTRACT FROM AUTHOR]