Effects of C Content on Microstructure and Properties of Fe-Mn-Al-C Low-Density Steels

The lightweight Fe-Mn-Al-C steels (so-called low-density steels) have received great attentions as promising candidate for automobile structure applications due to their excellent combination of density reduction, mechanical properties and corrosion resistance. In previous studies, most examinations of the Fe-Mn-Al-C alloys focused on the deformation mechanisms and the relationship between the microstructure and mechanical properties. It is well known that chemical composition, especially C content, which enhances strength as the interstitial element and reduces the density of steels, plays an important role in the control of microstructure and performance. However, the influence of C element in the alloy with high Mn content is barely studied. In this work, the effects of C content on microstructure and mechanical properties of four Fe-30Mn-10Al-xC (x=0.53, 0.72, 1.21, 1.68, mass fraction, %) alloys were studied by EBSD, TEM, XRD and universal testing machine. The results show that with the increase of C content, the amount of austenite gradually increases and the ferrite/austenite dual-phase microstructure transforms into single phase austenite. In addition, the strength increases monotonously, while the elongation increases and then decreases ultimately with increasing C content. Statistical analysis reveals that the strain coordination capacity of austenite is higher than that of ferrite. Therefore, with the increase of austenite content, the ductility of the dual-phase steel remarkably increases, while the strength increases slightly. For single austenite steels, the yield strength increases but the elongation and work hardening ability decrease with increasing C content, which is related to the precipitation of kappa' carbides.