Effects of Temperature and Time of Isothermal Holding on Retained Austenite Stability in Medium-Mn Steels

Effects of isothermal holding time and temperature on the stability of retained austenite in medium manganese bainitic steels with and without Nb microaddition were investigated. The amount of retained austenite for various variants of thermomechanical processing was determined by X-ray diffraction. Relationships between processing conditions and microstructure were revealed using light microscopy and scanning electron microscopy techniques. The isothermal holding temperatures changed from 500 to 300 °C and the time was from 60 to 1800 s. The optimal time and temperature of isothermal holding for all the investigated steels were 400 °C and 300 s, respectively. The relationships between the Mn content, amount of retained austenite, and carbon enrichment of the retained austenite (RA) were observed. The noticeable effect of Nb microaddition on the amount of retained austenite was not observed. In general, the carbon content in RA was slightly lower for the steels containing Nb. The optimum gamma phase amount was up to 18% for the 3% Mn steels, whereas it was c.a. 13% for the steels with 5% Mn. It was found that the morphology of blocky/interlath retained austenite depends substantially on the isothermal holding temperature.