Fig. 1. (a-c) EBSD inverse pole figure (IPF) maps and (d) XRD profiles of the 0Cr, 3Cr, and 6Cr steels. The average austenite grain sizes (Dγ) are ranged from 20 to 24 μm in the three steels, indicating the almost same austenitic microstructures.

Fig. 2. EPMA (a-c) Mn- and (d, e) Cr-distribution maps of the 0Cr, 3Cr, and 6Cr steels. Mn-rich and Mn-depleted areas exist in a banded shape in the three steels, and the Cr distribution of the 3Cr and 6Cr steels shows a similar trend of the Mn distribution.

Fig. 3. (a, b) Engineering tensile stress-strain curves and (c, d) true stress-strain curves obtained from the tensile test at 25 and -196 °C. The yield strength increases as Cr is added into the 0Cr steel, while the tensile strength is hardly varied within error ranges. The yield and tensile strengths at -196 °C are much higher than those at 25 °C, but the elongation is similar.

Fig. 4. SEM fractographs of the tensile specimens of the (a, d) 0Cr, (b, e) 3Cr, and (c, f) 6Cr steels fractured at 25 and -196 °C. The surfaces fractured at 25 and -196 °C consist mostly of ductile dimples in all the steels.

Fig. 5. Charpy impact energy as a function of test temperature for the three high-Mn steels. The absorbed energies at 25 °C are very high over 230 J in the three steels, and decreases as the Cr content increases. They gradually decrease with decreasing temperature to -196 °C. At -196 °C, the absorbed energy increases as the Cr content increases, which is opposite to the energy data at 25 °C.

Fig. 6. SEM fractographs of the near-notch-tip area of the Charpy impact specimen of the (a, d) 0Cr, (b, e) 3Cr, and (c, f) 6Cr steels fractured at 25 and -196 °C. The surfaces fractured at 25 and -196 °C consist mostly of ductile dimples in all the steels. It is noted that a few quasi-cleavage facets having river patterns are found in the 0Cr steel (arrow marks in (d)).

Fig. 7. EBSD IPF and (IQ + Phase) maps of the half-sectioned notch-tip area of the Charpy impact specimen fractured at -196 °C for the (a, d) 0Cr, (b, e) 3Cr, and (c, f) 6Cr steels. Many twins are populated in the three steels (arrow marks in Fig. 7(a-f)). When a red-dashed box area in (d) is magnified (g, h), fine particles are found in the 0Cr steel (red-colors in (h). These particles are identified to be ε-martensite because they well satisfy the Shoji-Nishiyama (S-N) orientation relationship, {111}γ//{0002}ε and < 101 >γ//< $11\bar{2}0$ >ε as shown in (i, j), which implies the existence of ε-martensite in the plastic zone formed near the notch-tip area.

Fig. 8. Dynamic compressive stress-strain curves at (a) 25 and (b) -196 °C. The yield and maximum compressive strengths at -196 °C are higher than those at 25 °C, while the plastic strain is similar. As Cr is added, the yield and maximum strengths increase at -196 °C, while the plastic strain hardly vary.

Fig. 9. IPF and IQ maps of the half-sectioned area of the dynamically compressed specimen at 25 °C for the (a, b) 0Cr, (c, d) 3Cr, and (e, f) 6Cr steels. In all the steels, twins are populated (arrow marks) without any martensite. Misorientation-angle distributions of austenite grains are shown in (g-i). Number fractions at the misorientation angle indicating twin boundaries (60°) are 6.3%, 4.2%, and 2.7% in the 0Cr, 3Cr, and 6Cr steels, respectively, which indicates the active twinning with decreasing Cr addition.

Fig. 10. EBSD (IQ+Phase) maps and EPMA Mn distribution maps of the half-sectioned area of the specimen dynamically-compressed at strains of 10% and 20% at -196 °C for the (a) 0Cr, (b) 3Cr, and (c) 6Cr steels. When the 10%-strain was applied to the 0Cr steel, the ε-martensite is observed, and its amount increases with increasing strain to 20%. Its volume fractions (Vε) were measured, and are indicated inside the maps. According to the Mn distribution map, the ε-martensite forms mostly along low-Mn bands.

Fig. 11. Fractions of equilibrium phases (liquid, austenite, ferrite, and M23C6 carbide) of the (a) 0Cr, (b) 3Cr, and (c) 6Cr steels. SEM images of the (d) 0Cr, (e) 3Cr, and (f) 6Cr steels, supporting that the M23C6 carbide is hardly observed for the Cr-added steels.