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.