Effect of C on the superplasticity of medium Mn steel

Abstract

We investigated the effect of C content on the superplasticity of medium Mn steel through high-temperature tensile testing of Fe–7Mn-(0–0.35)C (wt%) steels at temperatures ranging from 490 °C to 720 °C with an initial strain rate of 1.0 × 10−3 s−1. Superplasticity became evident at 550 °C for C-added steels and at 634 °C for the C-free steel. As all the C-added steels exhibited the highest elongation at 650 °C, we conducted in-depth analyses using the specimens tested at 650 °C. The elongation significantly increased with increasing C content up to 0.20 wt%. The results of strain rate sensitivity, apparent activation energy, and microstructural observations indicated that the low elongation of the 0C steel was attributed to the occurrence of dislocation creep, whereas the high elongations of both 0.10C and 0.20C steels were due to the occurrence of grain boundary sliding. The 0.20C steel exhibited higher elongation than the 0.10C steel due to its smaller average grain size and a larger area of α/γ interphase boundaries compared to the 0.10C steel. Meanwhile, the elongation of the 0.35C steel was not significantly improved compared to the 0.20C steel, despite having a smaller average grain size. This was because the 0.35C steel had a lower area of α/γ interphase boundaries, which resulted from the higher γ fraction. We believe the optimal C content for enhancing the superplasticity of Fe–7Mn steel is approximately 0.20 wt%, considering the 0.35C_T steel requires an additional tempering process, and its elongation was comparable to the 0.20C steel.

Keywords

• Superplasticity
• Medium Mn steel
• Grain refinement
• Grain boundary sliding
• Interphase boundary