Effect of increased nitrogen content on continuous cooling transition of γ→α in hot-deformed low-C Mo–V–Ti steels

Abstract

The two-stage controlled rolling and cooling at 0.5–50 °C/s of low-carbon Mo–V–Ti steels with the increasing nitrogen content from 0.0032 to 0.0081 and 0.0123 wt.% were simulated through a Gleeble 3500 system. The continuous cooling transition (CCT) of γ→α in each steel was estimated via microstructure characterization and CCT diagram. The results indicated that CCT diagram for each steel was divided into three regions of γ→ferrite, γ→pearlite and γ→bainite, and the increasing N content elevated all the starting temperatures for γ→α. Consequently, the polygonal ferrite (PF) and pearlite formed in each steel cooled at 1 °C/s and, however, the increasing N content led to slightly coarser ferrite grain and pearlite colony. With the increased cooling rate to 10 and 30 °C/s, a mixed microstructure of acicular ferrite (AF), granular bainite (GB) and lath bainite (LB) formed in 32N steel and in contrast, the mixture of PF+AF+GB in 81N and 123N steels. The increasing N content promoted (Ti,V)(C,N) precipitation, enhanced the intragranular PF/AF nucleation, increased martensite/austenite constituent and depressed LB. In addition, the mechanisms dominating the effect of increasing N on this CCT of γ→α were discussed.