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Materials with superior cryogenic strength and good ductility are increasingly in demand for resident friendly liquid-hydrogen (20K) storage tanks. Additionally, the space industry also requires materials that retain excellent mechanical properties at extremely low temperatures. However, mechanical testing at such low temperatures is highly limited due to the difficulties in achieving and maintaining such conditions, while also providing adequate thermal insulation to prevent heat transfer from the surrounding environment. In this study, we present a novel tensile testing technique for a Fe-15Mn-13Cr-3Si-3Ni-0.1C (wt.%) steel at the temperature of liquid helium. To minimize the use of expensive liquid helium, we adopted a method of injecting liquid helium vapor and set the temperature for tensile testing at 6 K. The present alloy has a single face-centered cubic (FCC) structure with a large amount of stacking faults after annealing treatment. The Fe-Mn-Cr steel exhibited a superior ultimate tensile strength of 1200 MPa and good ductility of 35% at 6K. Moreover, compared with room temperature tensile tests, discontinuous plastic flow, i.e. serrated flow, occurred at extremely low temperature.