Dispersion Strengthening in Vanadium Microalloyed Steels Processed by Simulated Thin Slab Casting and Direct Rolling

摘要

The dispersion strengthening of five low carbon (～0.06wt.%), vanadium (～0.1wt.%), containing steels, V, V-N, V-Ti-N, V-Nb and V-Nb-Ti, where nitrogen was varied in the range 0.007-0.02 wt.%, produced by simulated thin slab casting and direct rolling, has been studied. The thin slab casting and direct rolling process route has modified the precipitation process, chemical composition, size distribution, and volume fraction of the dispersion strengthening fine particles. The fine particles were only observed in the final product of the steels. No interphase precipitation was found. A detailed TEM/PEELS analysis of the fine particles showed that they are essentially nitrides (N/Metal ≈ 0.63-0.82) containing increasingly less carbon as the nitrogen level increased. The atomic ratio of N/Metal in the fine particles is related to the ratio of the nitrogen and microalloying content in the steels. It was slightly lower with decreased equalisation temperature and increased end water cool temperature or when a titanium addition was made. For the multiple microalloyed steels, these fine particles were rich in vanadium (V/Metal = 0.69-0.88). The fine particles did not coarsen during the final stages of processing and about 75% of the fine particles, measured in the final product, remained a stable size of 4-8 nm. Depending on the equalisation and end water cool temperatures, the contribution of dispersion to yield strength can reach to 250 MPa. Increasing the nitrogen content in the steels led to higher dispersion strengthening contributions. However, the addition of titanium to the vanadium or vanadium-niobium steels resulted in a lower contribution to the dispersion strengthening.