Effects of N addition and annealing treatment on microstructures and mechanical properties of ODS steels

In this paper, the effects of nitrogen (N) addition and annealing treatment on the microstructures and mechanical properties of ODS steels were investigated. The ODS steels with 0 wt%, 0.1 wt%, 0.3 wt%, 0.5 wt% nitrogen were fabricated by mechanical alloying (MA) and spark plasma sintering(SPS) method. The microstructure evolution duo to N addition and annealing treatment were characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Results showed that 0.5N-ODS steel had smaller grain size and more precipitates than 0N-ODS steel. Tensile tests showed that both ultimate tensile strength and total elongation of the annealed 0.5N-ODS steel were higher with 1387 MPa and 7.6% respectively than 0N-ODS steel. It was considered that the concurrently improvement of strength and toughness were related with the microstructure feature that there are the bimodal grain size distribution, with micrometer-sized grains and nano-sized grains uniformly and a large number of precipitates in the N-ODS steel duo to N addition. Among the precipitates, there were some “core/shell” structural particles, in which the “core” is rich in Y, Ti and O and the “shell” is rich in Cr, V, N. These feature lead to a better toughness. Using a linear model to evaluate the yield strength and obtained a good agreement with the experimental results. •N element not only contributes to the strength but also the ducitility.•Bimodal grain size distribution characterized by EBSD.•“Core/shell” structural precipitates was observed by TEM.