Precipitate behaviour of large-sized Nb(C, N) in micro-alloyed steel

The large-sized Nb-rich particles become a crack source easily because it is very brittle and will resist deformation under stress. HT-CLSM experiments and thermodynamic calculations were performed to investigate the precipitate behaviour of large-sized Nb(C, N) in micro-alloyed steel. The precipitate composition in the casting specimen was detected to be MnS and Nb-rich particles distributed along the crack. The results suggest the large-sized Nb-rich particles precipitate at the solidification process, instead of liquid. The precipitates for the HT-CLSM specimen is Nb(C, N), and Nb(C, N) is mainly distributed in a chain-like, clustered way in which the growth of Nb(C, N) is limited by the last to solidify region. The size of most Nb(C, N) is between 10 and 30 μm in the length direction, and the size of Nb(C, N) in the width direction tends to decrease with the increase of cooling rate attributed to larger super-cooling degree and weaker diffusion ability of solute atom. Because of solute segregation, a hypo-eutectic reaction occurs, L→Nb(C, N)+ γ-Fe. Nb(C, N) and γ-Fe are interactively precipitated to form a clustered Nb(C, N). When the holding temperature is high, and the cooling rate is rapid, a little liquid exists in the dendrite that is close to the eutectic composition. Because γ-Fe preferentially attaches to the previous dendrite, a divorce eutectic reaction occurs, forming a single Nb(C, N). MnS can act as the nucleation site of Nb(C, N) with the orientation relationship of MnS(100)//NbC(100).