Effects of Nb on Bainite Transformation Behavior and Mechanical Properties of Low‐Carbon Bainitic Steels

Abstract

The bainite transformation behavior and the mechanical properties of lowcarbon bainitic steels with and without niobium (Nb) are investigated by two different heat-treatment methods À isothermal transformation process (ITP) and continuous cooling process (CCP). The results indicate that for ITP, the kinetics of bainitic transformation for the Nb-added sample is accelerated at the beginning of the isothermal holding process due to more nucleation sites followed by its further deceleration as a consequence of the formation of ferrite prior to austempering. For all ITPs, Nb addition resulted in a decrease in the final amount of transformed bainites. In addition, different from the generally accepted result, the tensile strength of the steel is degraded by Nb addition during both ITP and CCPs, and the addition of Nb did not improve the final comprehensive properties of the steels. Moreover, Nb addition caused an increase in the critical temperature of high-temperature transformation (Ar 3 ); however, it yielded no significant effects on critical temperatures of Ac 1 and Ac 3 . Materials and Methods MaterialsNb-free (Fe-0.22C-1.55Si-2.01Mn (wt%); used as a reference sample) and 0.025Nb-bearing (wt%) steels were first refined in a vacuum induction furnace and then cast into 50 kg ingots. The