Microstructural Evolution and Mechanical Properties of Nb-Containing Medium-Mn Low-Density Steels

Microalloying of Nb has been commonly utilized to enhance the structure and mechanical properties of advanced high-strength steel (AHSS), but its application in medium-Mn low-density steel has been relatively understudied. This study aims to investigate the evolution of nano-scale precipitates and mechanical properties of Fe-12Mn-9Al-3Cr-1.4C-0.02/0.04Nb low-density steels. The findings reveal that the austenite grain size of 0.04Nb steels is approximately half that of 0.02Nb steels due to the precipitation of NbC particles. Moreover, the addition of Nb is found to elevate the formation energy of kappa-carbide, thereby impeding its growth and coarsening. Consequently, the intragranular kappa-carbides in 0.04Nb steels are consistently smaller than those in 0.02Nb steels, with no coarsened intergranular kappa-carbides detected in either steel variant after aging treatment at 723-823K. Despite a slight reduction on precipitation strengthening of kappa-carbides with higher Nb content, the yield strength of 0.04Nb steels exceeds that of 0.02Nb steels by approximate to 71MPa, mainly due to additional fine grain strengthening and dislocation strengthening. The strengthening mechanisms in the Nb-containing low-density steels are analyzed quantitatively. The role of Nb in regulating the microstructure and mechanical properties of medium-Mn low-density steels is comprehensively discussed, offering valuable insights for the alloy design of low-density steel.