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

Highlights: •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. - Abstract: 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.