Thermal stability of retained austenite and mechanical properties of medium-Mn steel during tempering treatment

The thermal stability of retained austenite (RA) and the mechanical properties of the quenched and intercritical annealed 0. 1C-5Mn steel with the starting ultrafine lamellar duplex structure of ferrite and retained austenite during tempering within the range from 200 to 500 degrees C were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and tensile testing. The results showed that there was a slight decrease in the RA volume fraction with increasing tempering temperature up to 400 degrees C. This caused a slight increase in the ultimate tensile strength (UTS) and a slight decrease in the total elongation (TE); thus, the product of UTS to TE (UTS X TE) as high as 31 GPa center dot % was obtained and remained nearly unchanged. However, a portion of the RA began to decompose when tempered at 500 degrees C and thus caused a similar to 35% decrease of the RA fraction and a similar to 16% decrease of the value of UTS X TE. It is concluded that the ultrafine lamellar duplex structure is rather stable and the excellent combination of strength and ductility could he retained with tempering temperature up to 400 degrees C. Thus, thermal processes such as galvanization are feasible for the tested steel provided that their temperatures are not higher than 400 degrees C.