Effect of niobium addition on hot deformation behaviors of medium carbon ultra-high strength steels

Abstract

The hot deformation behaviors of two medium carbon ultra-high strength steels with different niobium contents were investigated by using Zener-Hollmom parameter and processing map, and the effect of niobium addition on the hot deformation behavior of medium carbon steel was determined. The hot compression tests were conducted on a Gleeble-3500 thermo-mechanical simulator deformed at temperatures from 850 to 1 200 °C and strain rates from 0.001 to 1 s−1. The processing maps of two test steels were built at a true strain of 0.7 based on dynamic materials model (DMM). There are two peak efficiency domains and two flow instability regions in both test steels. However, the peak efficiency domains of Nb-bearing steel move to higher temperature due to the inhibition of dynamic recrystallization (DRX), and the instability domains of Nb-bearing steel are enlarged due to the precipitation of Nb-containing particles during hot deformation. The optimum process parameters of Nb-bearing and Nb-free steels for industrial production were determined according to the processing map and the microstructural observation.