Effect of Nb contents on microstructure characteristics and yielding behavior of Fe–4Mn–2Al-0.2C steel

Abstract

The microstructure characteristics and yielding behavior of two Fe–4Mn–2Al-0.2C steels with different Nb contents, i.e. 0.02 wt% and 0.1 wt%, were investigated in the present study. The results show that the increase of Nb content will bring about 50–100 MPa more fine-grain strengthening effect. But on the other hand, Nb promotes the precipitation of cementite and delays recrystallization, so that the stability of austenite decreases and presents a single lathy morphology. The partial martensitic transformation of austenite in the cooling process promotes the diffusion of carbon from martensite to retained austenite, and causes the segregation of carbon at the austenite phase boundaries, leading to the Lüders effect. The steel with 0.02 wt% Nb undergoes a more persistent TRIP effect during the deformation process due to two austenite morphologies with higher volume fractions and exhibits excellent comprehensive mechanical properties with the yield strength of 570 MPa, ultimate tensile strength of 1011 MPa, total elongation of 35.6% and product of strength and elongation of 36 GPa·% after intercritical annealing at 725 °C for 30 min without the appearance of Lüders effect. Thus, this paper is of great importance to the effect of Nb contents on microstructure and yielding behavior of medium Mn steels.