Effects of Nitrogen Content and Austenitization Temperature on Precipitation in Niobium Micro-alloyed Steels

The influences of nitrogen content and austenitization temperature on Nb(C, N) precipitation in niobium micro-alloyed steels were studied by different methods: optical microscopy, tensile tests, scanning electron microscopy, transmission electron microscopy, physicochemical phase analysis, and small-angle X-ray scattering. The results show that the strength of the steel with high nitrogen content is slightly higher than that of the steel with low nitrogen content. The increase in the nitrogen content does not result in the increase in the amount of Nb(C, N) precipitates, which mainly depends on the niobium content in the steel. The mass fraction of small-sized Nb(C, N) precipitates (1–10 nm) in the steel with high nitrogen content is less than that in the steel with low nitrogen content. After austenitized at 1150 °C, a number of large cuboidal and needle-shaped particles are detected in the steel with high nitrogen content, whereas they dissolve after austenitized at 1200 °C and the Nb(C, N) precipitates become finer in both steels. Furthermore, the results also show that part of the nitrogen in steel involves the formation of alloyed cementite.