Dissolving behaviour of second phase particles in Nb–Ti microalloyed steel

The dissolving behaviours of second phase particles in low carbon Nb–Ti microalloyed steel during isothermal holding at 1300 °C were investigated by carbon extraction replicas, TEM and EDX analysis. The experimental results indicate that there are two types of precipitates in as-forged steel: the coarsened Ti-rich precipitates originate from solidification, and the other finer Nb-rich particles attribute to strain-induced process. The strain-induced precipitates disappear after holding for 2 h at 1300 °C, while the coarsened particles from the solidification containing Nb-bearing (Nb,Ti)(C,N) still remain even holding for 48 h. With addition of Ti to the Nb-bearing microalloyed steel, the thermal stability of the carbonitrides would enhance remarkably. These results rationalize that formation and thermostability of precipitates are considerably influenced by interaction between Nb and Ti atoms.