Thermodynamic analysis and isothermal bainitic transformation kinetics in lean medium-Mn steels

Abstract

The work presents the results of thermodynamic analysis of two medium manganese steels with different Mn contents. The steels containing 3.1 and 3.6% of manganese were subjected to theoretical thermodynamic calculations using MUCG83 software and dilatometric experiments. The steels were heat-treated in two different isothermal holding temperatures of 400 and 350 °C for 15 min. The bainite transformation kinetics at different temperatures for different manganese contents was investigated. In the steel including 3.1% Mn, a complete transformation was obtained. The results indicated a strong influence of the holding temperature on the kinetics of bainitic transformation. It was related to the driving force of this process. When the manganese content was increased by 0.5%, an incomplete bainite transformation occurred. The microstructure investigations after heat treatment were performed using light and scanning electron microscopy. The XRD analysis to determine retained austenite amount and its carbon enrichment was performed. The microstructure of 3MnNb steel consisted of bainite and retained austenite with filmlike and blocky morphologies. The steel with the higher Mn content contained also fresh martensite for both isothermal holding temperatures.

Effect of Hot Deformation on Phase Transformation Kinetics in Isothermally Annealed 3Mn-1.6Al Steel

“…The lowest hardness is exhibited by the 700ND sample, which contains the largest fraction of ferrite. The ferrite grains are arranged in bands, which is related to the segregation of manganese in medium-Mn steels, which was confirmed in earlier studies [31].…”

Bainite plate thickness reduction and microstructure tailoring by double austempering of Al-rich 3Mn steel

“…The obvious advantages of medium-Mn steels in terms of RA stabilization, however, may be associated with some problems -microsegregation, to which these steels are susceptible [41,42]: (i) grain boundary segregation and (ii) segregation banding [41]. The areas containing a lower content of Mn undergo the transformation faster due to the increased kinetics compared to the alloy with a bulk chemical composition [43]).…”