Regular Article

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

¹ School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
² School of Advanced Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
³ Xining Special Steel Co. Ltd, Qinghai Special Steel Engineering Technology Research Center, Xining 810005, PR China

^* lyzh@ustb.edu.cn

Received: 2 August 2017
Accepted: 4 January 2018

Abstract

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 A_C1, A_C3 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 °C/s to 5 °C/s and from 5 °C/s to 10 °C/s, respectively. (Ti, V)(C, N) particles on MnS is the nuclei of intragranular ferrite, and (1 0 0)_{(Ti, V)(C, N)} and (1̅ 0 1̅_α) are just misoriented by 6.7°. With the increase of nitrogen content, the number of intragranular ferrite is increased from 73 to 170 per · mm². The volume fraction of intragranular ferrite is increased from 0.23%∼0.79% to 0.79%∼4.6% at cooling rate of 1 °C/s∼0.1 °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.