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%).