Figure 5 Fe-C phase diagram for High-Nb steel, showing the formation temperature of titanium nitrides and carbonitrides varies significantly by carbon concentration.
The variation of niobium and nitrogen concentrations on the transformation and precipitation temperatures, have been studied for the High-Nb steel. The niobium concentration up to 0.12 wt% has no influence on the precipitation temperature of nitrides in this steel (Figure 6(a) ), whereas the formation temperature of niobium carbides is increased considerably up to 0.03 wt% Nb. Further addition of niobium gradually increases the precipitation temperature of carbides. The calculated chemical composition of precipitates at 1400K and 1300K are compared for steels with niobium concentrations of 0.03 and 0.11 wt% respectively in table 5. The nitrides contain mostly titanium, and niobium forms carbides.
A study of non-isothermal austenite grain growth, in the Heat Affected Zone (HAZ) of an X80 microalloyed steel, has shown that the increasing peak temperature from 1223 K to 1523 K results in a significant growth of the grain size [8]. The chemical composition of the studied alloy is similar to the High-Nb steel of the current report with 0.034 wt% Nb. The calculated phase diagram in Figure 6(a) indicates that the niobium carbonitride precipitates are only partially dissolved at a peak temperature of 1223K, and therefore remaining precipitates suppress the austenite grain growth via pinning of the grain boundaries. Whereas austenite grains can grow rapidly at a peak temperature of 1523K, where niobium carbonitrides are dissolved completely in the matrix. Titanium nitride precipitates are formed at temperatures above 1700K, and remain undissolved in the austenite matrix at both selected peak temperatures. The nitride precipitates which form at high temperatures are generally large in size and therefore, are not expected to contribute considerably in suppressing the austenite grain coarsening [5, 6].
Table 5 The chemical composition of precipitates in the High-Nb steel for nominated niobium concentrations.