Effect of N on microstructure evolution and strain hardening of high-Mn austenitic steel during tensile deformation

The present study investigated the microstructure evolution and strain hardening of two types of high-Mn austenitic steels, i.e., Fe- 18.5Mn-7Cr- 0.6C (00 N) and Fe- 18.5Mn-7Cr- 0.6C- 0.21 N (21 N), during tensile deformation. The results revealed that the addition of N increased the yield and tensile strength of high-Mn austenitic steel with almost no loss of elongation. The strain hardening of the 00 N and 21 N steels was closely related to the microstructure evolution. At the initial tensile deformation stage, dislocation slip was the dominant deformation mechanism of the two test steels. Due to the addition of N, the dislocation cross-slip in the 21 N steel was suppressed, leading to a higher strain hardening rate. With the increase of true strain, the mechanical twins became active. At a true strain of 0.18, a unique network structure of mechanical twins and dislocation cells was formed in the 00 N steel, and the thickness and spacing of mechanical twins were lower, which increased the resistance of dislocation motion and made its strain hardening rate slightly higher than that of the 21 N steel. However, at a true strain of 0.37, the thickness and spacing of mechanical twins in the 21 N steel decreased rapidly, which increased the difficulty of dislocation movement, resulting in a higher strain hardening rate than that of the 00 N steel. In short, the increased resistance of dislocation motion was the fundamental reason for the enhanced strain hardening rate, rather than mechanical twins.