Effect of Austenite Grain Size on the Hot Ductility of Nb-Bearing Peritectic Steel

Austenitic grain size is an important factor affecting the hot ductility of steel slabs during continuous casting. In the present study, the effects of austenite grain sizes of approximately 250, 550, and 1000 mu m on the hot ductility of peritectic and Nb-bearing peritectic steels were measured with a Gleeble-3500 thermomechanical simulator. The morphology of the proeutectoid ferrite and the distribution of Nb(C, N) precipitates were detected by metallurgical microscopy and transmission electron microscopy, respectively. Based on these results, a mechanism was proposed for the effects of the austenite grain size on the hot ductility of peritectic and Nb-bearing peritectic steels. The results showed that the volume fraction of film-like proeutectoid ferrite decreased from 17.2 to 12.6 pct, and the minimum value of the reduction in area (RA) decreased from 42.32 to 35 pct, at 750 degrees C as the austenite grain size increased from 250 to 1000 mu m. The decrease in the proeutectoid ferrite fraction facilitated strain concentration for a long time, which caused hot ductility to decrease. The average diameter of Nb(C, N) decreased from 17.3 to 7.2 nm, and the minimum RA value decreased from 41.5 to 25.4 pct, at 800 degrees C, as the austenite grain size increased from 250 to 1000 mu m in peritectic steel with 0.048 wt pct Nb addition. The decreases in the average diameter of Nb(C, N) precipitates and the volume fraction of film-like proeutectoid ferrite caused strain concentration at the austenite grain boundaries, which decreased hot ductility. Moreover, the decrease in the RA of Nb-bearing peritectic steel increased significantly with increasing austenite grain size.

Automated summary

Austenitic grain size is an important factor affecting the hot ductility of steel slabs during continuous casting. The increase in austenite grain size decreases the volume fraction of film-like proeutectoid ferrite and the average diameter of Nb(C, N) precipitates, leading to strain concentration and a decrease in hot ductility.