Tailoring mechanical properties of Fe-32Mn-0.6C steel via deformation, dynamic recovery and recrystallization

The microstructures and mechanical properties of Fe-32Mn-0.6C steel after deformation, dynamic recovery and recrystallization were investigated. Dynamic recovery occurred during rolling at 800°C, and fined subgrains formed at deformation introduced lamellar boundaries. During tensile deformation, a large amount of deformation twins is activated, leading to a high work-hardening. Compared to the cold-rolled and coarse-grained recrystallized samples, the 800°C rolled sample shows an improved combination of strength and ductility at both room temperature (RT) and liquid nitrogen temperature (LNT). Specifically, the yield strength and uniform elongation are 702 MPa and 61%, respectively, at RT and are 1013 MPa and 48%, respectively, at LNT.


Introduction
High manganese austenitic steels have the advantages of high tensile strength, excellent ductility, high work-hardening ability, and low cost.Synergy of different deformation mechanisms, such as dislocation slip, twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effects, can be activated in high manganese steels during plastic deformation, resulting in excellent mechanical properties [1].In addition, by the dedicated chemical composition and microstructure design, the deformation behavior of high manganese steel at cryogenic temperatures can be controlled, resulting in remarkable mechanical performance in the cryogenic environment [2,3].However, the yield strength of high manganese steel is usually between 200 and 500 MPa [4], which limits the wide application of high manganese steels in structural components.
In recent years, numerous efforts have been made to increase the yield strength of high manganese steels through solid solution strengthening, precipitation strengthening, dislocation strengthening, and grain boundary refinement [5].Meanwhile, the yield strength of high manganese steels can also be improved through the addition of alloying elements such as Cr, Al, and Si [6][7][8].However, these approaches raise the production cost and is not environmentally friendly.Although the introduction of nano-precipitates is a promising approach to improve the yield strength via dislocation pinning [9], it may also lead to decreased cryogenic plasticity.The addition of carbon (C) has been shown to improve strength through the increase of lattice friction stress.It shall be noted that, a higher content of C can enhance dislocation multiplication and twin formation during plastic deformation, thereby enhancing the work-hardening ability of high manganese steels [10,11].Thus, proper C alloying is expected to result in a harmonious balance of strength and plasticity in high manganese steels.
The integration of high-density dislocations, twinning, and martensite through extensive plastic deformation is an effective strategy for enhancing the yield strength of high manganese steels.Nevertheless, following severe plastic deformation, the reduction in twinning and martensite transformation abilities results in a decrease in the work-hardening capacity and ductility during subsequent plastic deformation.Hence, an annealing treatment (at temperature between 800 and 1100C) is required to reduce the dislocation density and rejuvenate the work-hardening ability of high manganese steels [5].It is well known that the deformation mechanism of high manganese steels is affected by the stacking fault energy (SFE).For high manganese steels with a specific composition, an increase in SFE can impede the formation of twinning or martensitic transformation during deformation.To manipulate the SFE of high manganese steel, elevating the plastic deformation temperature is an effective way [12].By undergoing plastic deformation at a carefully selected temperature, dislocations can be introduced to enhance strength without prematurely depleting the twinning or phase transformation ability.Consequently, it is possible to attain high-manganese austenitic steels with a good combination of strength and ductility by regulating the temperature during plastic deformation.
This study aims to enhance the yield strength while preserving the work-hardening capacity of high manganese steels.The study on the microstructures and mechanical properties of Fe-32Mn-0.6Cfocuses on the effect of hot rolling at 800C, in comparison with cold rolling and recrystallization annealing.

Experimental
The Fe-32wt% Mn-0.6wt%C steel was prepared by melting 50 kg ingots in a vacuum induction furnace.The ingots were subjected to solid solution treatment at 1100°C for 4 h, and then forged to a thickness of 110 mm and a width of 40 mm.Subsequently, the forged samples were held at 800C for 2 h and further forged to a thickness of 50 mm while maintaining the same width.Finally, the samples were rolled to 12 mm thick plates with a reduction of 76%, after holding at 800C for 0.5 h.The cold-rolled sample was produced by further rolling the 12 mm thick plate to a final thickness of 5 mm at 200C.The fully recrystallized sample was produced by annealing the cold-rolled sample at 800C for 1 h.The cold-rolled, hot-rolled, and annealed samples were designated as CR, WR800, and A800, respectively.
The microstructures were analyzed using X-ray diffraction (XRD) and field emission gun scanning electron microscopy (FEGSEM) with electron backscattered diffraction (EBSD) and electron channeling contrast (ECC) imaging.The fracture surfaces of the tensile specimens were examined by FEGSEM.The specimens for XRD and EBSD analysis were cut from the longitudinal plane (the section containing the rolling direction and normal direction) of the plate.After standard mechanical grinding and polishing, the specimens were electrolytically polished for 60 s at 30 V and 25C to remove surface residual stress with a solution consisted of 10% perchloric acid and 90% alcohol.The XRD was operated with a Cu target, and the diffraction range was 20−100° with a scanning speed of 2°/min.EBSD analysis was performed with a voltage of 20 kV and a step size of 0.25 m and 0.05 m for initial microstructure and deformed microstructure, respectively.The tensile tests were performed using a Zwick tester with a homemade liquid nitrogen device.The tensile specimens were dog-bone shaped, with a gauge length of 15 mm, width of 3 mm, thickness of 2 mm, and length of 45 mm.At least three samples were tested in each condition to avoid artificial errors.

Results and discussion
Figure 1 shows the initial microstructure of WR800 and A800 samples.The WR800 sample exhibits the recovered subgrains during the rolling process at 800C.According to figure 1b, the WR800 sample displays only austenite diffraction peaks, suggesting the absence of martensitic transformation during rolling, which is supported by the SFE calculation (247.8 mJ/m 2 at 800C).The A800 sample exhibits a recrystallized structure of equiaxed grains, with an average size of 9.8 m (figure 1c).The mechanical properties of Fe-32Mn-0.6Cwere investigated at both RT and LNT, and the results are presented in figure and table 1. Figure 2a shows the engineering stress-strain curves where the CR sample exhibits the ultra-high strength and the limited elongation.At RT, the tensile strength of the CR sample reaches 2 GPa and increased to 2.5 GPa at LNT, whereas the total elongation is 5−6 %.The introduction of high-density defects during cold rolling significantly improves the strength but also deteriorates the ductility and work-hardening ability.In comparison with the A800 sample, the WR800 sample exhibits a significant increase in RT yield strength, rising from 408 MPa to 702 MPa, and a decrease in the uniform elongation, from 81% to 61%.Furthermore, the LNT yield strength of WR800 sample increases by 301 MPa when compared to the A800 sample, while the uniform elongation decreased modestly from 58% to 48%.Furthermore, serrations appear on the RT stress-strain curves due to the dynamic strain aging effect [13].However, the serration behavior disappears on the cryogenic stress-strain curve, because the diffusivity of carbon atoms decreases at cryogenic temperatures [14].
Figures 2c and d show the work-hardening rate and true stress-strain curves of Fe-32Mn-0.6Cat RT and LNT.The CR sample displays almost no work hardening ability.Meanwhile, the WR800 and A800 samples exhibit similar work-hardening rate at RT, but the WR800 sample exhibits a higher workhardening rate at LNT.It is noteworthy that, for the A800 and WR800 samples tested at RT, the fracture occurs only when the plastic instability criteria is satisfied (the work-hardening rate becomes smaller than the true stress).In contrast, during testing at LNT, the WR800 and A800 samples fracture before the plastic instability condition occurs.The fracture modes of the A800 sample tested at RT and LNT are shown in figure 3. The A800 specimen fracture surface at RT exhibits a ductile fracture mode The yield strength and uniform elongation of Fe-32Mn-0.6C,Fe-30Mn-0.11C[2] (with the mean grain sizes of 5.6 m and 47 m, respectively) and the 9%Ni steel (GB3513-2014) are put together in figure 2b.Compared with the Fe-30Mn-0.11Csteel, an increase in carbon content for the A800 sample shows a higher RT yield strength and elongation, but a reduced cryogenic ductility.The strength of the Fe-32Mn-0.6Csteel is further improved through rolling at 800°C .The strength and ductility of the WR800 sample exceed the requirements for the 9%Ni steel following the GB3513-2014, which is commonly used for cryogenic applications.In addition, the Fe-30Mn-0.11Csteel exhibits the inverse temperature sensitivity, while the Fe-32Mn-0.6Csteel does not, indicating that an increase in carbon content may be an important reason for the decrease in cryogenic ductility.It was reported that increasing the carbon content can promote the deformation twinning generation and increase the density of dislocations [10,11,15].Meanwhile, the temperature reduction can also contribute to the increase of twins and dislocations due to the reduction of SFE, which may result in the stress concentration at deformation twin boundaries and the eventual premature fracture [16].However, the effects of carbon content on the mechanical properties, deformation behavior and failure mechanisms of high manganese steels needs further detailed and systematic investigations, which may include the quantification of microstructural parameters [17][18][19] and the heterogeneous deformation [20] using advanced postmortem investigations [21][22][23] as well as in-situ electron microscopy mechanical testing [24].
High manganese austenitic steel possesses the excellent mechanical properties, which strongly depend on the stacking fault energy (SFE) [25].In this study, the SFE value was determined using the Allain's thermodynamic model [26].It should be noted that the grain size effect on the SFE is not considered in the present study.The calculated SFE of the Fe-32Mn-0.6Csteel is 41.3 mJ/m 2 at RT and 36.8 mJ/m 2 at LNT.With further cold rolling process, TWIP effect can be activated in the Fe-32Mn-0.6Csteel, leading to a depletion of work-hardening capacity and plasticity.Nevertheless, the deformation twinning has been suppressed during the rolling process at 800°C , as the elevated temperature has caused an increase in SFE.  Figure 4 shows the microstructure of the WR800 sample deformed at RT. Upon stretching to 20% engineering strain, deformation twinning bundles form in the recovered coarse layers (see figure 4a and b).After fracture, both the lamellar structure and subgrains extend along the tensile direction, as illustrated in figure 4c and d.Parallel twin bundles were observed within the layer, as indicated by the white arrows.Moreover, hot rolling is beneficial for the dynamic recovery, which promotes the storage of dislocations during subsequent plastic deformation, ultimately leading to higher work-hardening capacity.The moderate work-hardening capacity in the WR800 sample achieves high yield strength and tensile strength, while the uniform plasticity does not weaken significantly compared with that of the A800 sample.

Summary
After hot rolling at 800°C , the Fe-32Mn-0.6Csteel is composed of a recovered structure and subgrains.The subgrains are randomly distributed at the boundary of the recovered lamellar structure.The WR800 steel exhibits significantly improved ductility and work-hardening ability compared to the CR sample.Moreover, in comparison with the recrystallized sample (A800), the WR800 sample shows a 72% and 42% increase in the RT and LNT yield strength, respectively, with a slightly reduced elongation.

Table 1
Mechanical properties of Fe-32Mn-0.6Csteel at RT and LNT.