Mechanical behaviour and microstructure of Fe-20/27Mn-4Al-0.3C low magnetic steel at room and cryogenic temperatures

The stress-strain curves and strain hardening rate curves of Fe-20Mn-4Al-0.3C (20Mn) and Fe-27Mn-4Al-0.3C (27Mn) steels were obtained by tensile tests at room and cryogenic temperatures. The yield strength, tensile strength, and elongation of 20Mn and 27Mn steels at -196 degrees C were significantly improved, and the product of strength and ductility reached 92.6 GPa.% and 80.1 GPa.%, respectively. There were a large number of deformation twins in 20Mn and 27Mn steels, and the deformation-induced martensite effect also occurred in 20Mn steel. The twinning-induced plasticity (TWIP) and transformation -induced plasticity (TRIP) at -196 degrees C significantly improved the strength and plasticity of the steels, but TRIP has a stronger effect. The work hardening rate of 20Mn steel was higher than that of 27Mn steel. The fracture mechanism of 20Mn and 27Mn steels at -196 degrees C was ductile fracture. Plastic deformation of 20Mn steel at low temperature can be divided into the dislocation slipping, twinning, and deformation-induced martensitic transformation stages, whereby a high elongation (77.4%) was obtained, which was also ascribed to the formation of microbands and dislocation cross slipping during the deformation process. The deformation mechanism of 27Mn steel was mainly the TWIP effect, in which the deformation-induced martensite did not form. Therefore, the work hardening rate of 27Mn steel was lower than that of 20Mn steel, and the elongation was relatively low (68.3%). Although 27Mn steel exhibited large plastic deformation during low temperature deformation, there was almost no martensite, which has a more stable austenitic structure and better low magnetic property.

Automated summary

At low temperatures, the strength and plasticity of Fe-20Mn and Fe-27Mn steels are significantly improved, with TWIP and TRIP effects playing important roles. The work hardening rate of 20Mn steel is higher, but 27Mn steel has slightly lower ductility. Both steels exhibit ductile fracture at low temperatures.