Abstract
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.
Highlights
Medium-Mn bainitic steels with retained austenite belong to a group of steels dedicated to the automotive industry due to a great combination of mechanical and technological properties
The effects of temperature and time of isothermal holding in the bainitic range on the retained austenite stability were investigated in four medium-Mn steels with and without Nb microaddition
16–18% of γ phase was obtained for the 3Mn type steels, whereas 10–13% of retained austenite was detected in 5Mn type steels
Summary
Medium-Mn bainitic steels with retained austenite belong to a group of steels dedicated to the automotive industry due to a great combination of mechanical and technological properties. These steels can be produced as cold-rolled—when the required microstructure is obtained during heat treatment after cold rolling—or they can be obtained as thermomechanically rolled [1]. Medium-Mn sheet steels offer the best combination of strength and ductility at reasonable cost. They are much cheaper compared to fully austenitic high-Mn steels. The intermediate Mn content allows to stabilize a fraction of retained austenite between the high-Mn steels
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