Effect of nitrogen on transformation behavior and intragranular ferrite formation in a V-Ti microalloyed steel for forging

The transformation behavior and intragranular ferrite formation in V Ti microalloyed steel with a nitrogen content of 0.005 wt.% and 0.015 wt.% are studied by the DIL 805A dilatometer. The results show that increasing the nitrogen content has no significant effect on AC(1), AC(3) and M-S temperature. However, the continuous cooling transformation (CCT) diagrams are shifted to left side. The minimum cooling rates of bainitc and martensitic transformation are increased from 3 degrees C/s to 5 degrees C/s and from 5 degrees C/s to 10 degrees C/s, respectively. (Ti, V) (C, N) particles on MnS is the nuclei of intragranular ferrite, and (10 0) (Ti, V)(C, N) and ((1) over bar 0 (1) over bar)(alpha) are just misoriented by 6.7 degrees. With the increase of nitrogen content, the number of intragranular ferrite is increased from 73 to 170 per . mm(2). The volume fraction of intragranular ferrite is increased from 0.23%similar to 0.79% to 0.79% similar to 4.6% at cooling rate of 1 degrees C/s similar to 0.1 degrees C/s. According to the industrial production experiment, the toughness of forging crankshaft is improved by increasing the nitrogen content. It is benefit for achieving fair matching of high strength and high toughness of crankshaft.