Effects of austenite grain size and cooling rate on Widmanstätten ferrite formation in low-alloy steels

Deformation dilatometry is used to simulate the hot rolling of 0.20 pct C-1.10 pct Mn steels over a product thickness range of 6 to 170 mm. In addition to a base steel, steels with additions of 0.02 pct Ti, 0.06 pct V, or 0.02 pct Nb are included in the study. The transformation behavior of each steel is explored for three different austenite grain sizes, nominally 30, 55, and 100 µm. In general, the volume fraction of WidmanstÄtten ferrite increases in all four steels with increasing austenite grain size and cooling rate, with austenite grain size having the more significant effect. The Nb steel has the lowest transformation temperature range and the greatest propensity for WidmanstÄtten ferrite formation, while the amount of WidmanstÄtten ferrite is minimized in the Ti steel (as a result of intragranular nucleation of polygonal ferrite on coarse TiN particles). The data emphasize the importance of a refined austenite grain size in minimizing the formation of a coarse WidmanstÄtten structure. With a sufficiently fine prior austenite grain size (e.g., ≤30 µm), significant amounts of WidmanstÄtten structure can be avoided, even in a Nb-alloyed steel.