3.2. The effect of niobium on the precipitates of low carbon, low alloyed steels
Table 3 presents chemical compositions of a few Nb-microalloyed linepipe steels fabricated in industries, adopted to study the effect of Nb concentration on the formation temperature and composition of nitride and carbonitride precipitates. There is a substantial deviation in the Manganese (Mn), Silicon (Si), Mo and V content of the selected steels. In order to fully realise the effect of these variations on the nature of precipitates, the phase diagrams of an abbreviated composition of a steel named High-Nb in Table 4, was calculated by varying Mn, Si and Mo composition within the range shown in the second row of Table 4. The steel with chemical composition of ”High-Nb” in table 3 represents steels 2, 3 and 4 in table 3 and ”Low-Nb” is representative of steel 1 in Table 3.
Interestingly, the deviations of Mn, Si and Mo contents within the designated composition range in Table 4, show no variation in the formation temperatures of carbides and nitrides (Figure 4a ). The precipitation temperatures of nitrides and carbonitrides as a function of Mo content for the High-Nb steel are shown in Figure 4(a) , as a model of obtained phase diagrams for these elements. It is worth noting that nitride and carbonitride precipitates contain both elements of Ti and Nb in these steels, specified by the calculated phase diagrams. Experimental microscopy analysis of rolled Ti-Nb microalloyed steels with similar compositions of the current study, presented ferrite-bainite microstructure which included precipitates of (Ti, Nb)N, 120-400nm in size, spherical or plate-like precipitates of (Nb, Ti)C within 10-120nm and needle-like 3-5nm (Nb, Ti)C precipitates [21]. Carbide precipitates are believed to be carbonitrides because the method was unable to analyse C and N in small precipitates [20]. Nitride precipitates which form at higher temperatures form larger precipitates than carbonitride precipitates.
Figure 4(b) shows that although the variation of V content has no effect on the formation temperature of nitride precipitates, it introduces vanadium carbonitride precipitates to the alloy at temperatures below ~1050K. It indicates that the composition, size and distribution of the carbonitride precipitates in steel 2 and 3 (table 3), is expected to be distinguishable from the rest of the alloys in table 3. The solubility limit of V in austenite is much greater than Ti and Nb in microalloyed steels, and therefore remains in solution during heat-treated processing in the austenite range to a much greater extent. V carbonitride precipitates have a thin, fibrous morphology and contribute to strengthening [20].
Table 3 Example Chemical composition of Nb -contained line pipesteels (wt%).