Effects of Nb and B on the microstructure and mechanical properties of nitrogen-strengthened austenitic stainless steel

The effects of Nb and B on the microstructure and mechanical properties of hot-rolled nitrogen-strengthened austenitic stainless steel were investigated using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and X-ray diffraction (XRD), in conjunction with room-temperature tensile and impact tests. The results indicate that the matrix structures of the base nitrogen-strengthened austenitic stainless steel (NSASS) and the experimental steels with additions of Nb (Nb-NSASS) and B (B-NSASS) all consist of coarse grains in the deformation areas and fine grains in the recrystallized areas. Furthermore, NSASS, Nb-NSASS, and B-NSASS are primarily composed of austenite at room temperature. NSASS contains a small amount of large-sized δ-ferrite phase. Nb-NSASS contains minor amounts of small-sized Laves phase, Z-phase, and Nb(C,N) particles, while B-NSASS exhibits no obvious secondary phases. The additions of Nb and B decreased both the degree of recrystallization and the proportion of coincident site lattice (CSL) boundaries Σ3 n , while the average dislocation density increases. The average grain size in the recrystallized area decreased in Nb-NSASS but increased in B-NSASS. In terms of mechanical properties, the additions of Nb and B improved the properties to varying degrees. Notably, the yield strength increased by 61.6 % with Nb addition, and the impact energy increased by 138 % with B addition. This study provides a theoretical basis for the microalloying design of hot-rolled nitrogen-strengthened austenitic stainless steels.