## 3 Results and Discussion
### X-ray Results
5 Mn and 5 MnNb steels showed fewer changes in the amount of \(\gamma\) phase due to variable duration of bainitic transformation. The optimal time of isothermal holding for all the steels seems to be 300 s. The highest fraction of retained austenite was detected for 3 MnNb steel (Table 3)--17.7%. It was found that the \(\gamma\) phase was not enriched in enough carbon during isothermal holding at 450 \({}^{\circ}\)C for 60 s (Table 3). Therefore, the thermodynamic stability of the austenite in the steel containing Nb addition seems to be slightly lower. This is especially true for the steels with the higher Mn content. The stability of retained austenite is higher in the case of 3 MnNb steel in comparison to 5 MnNb steel, due to the higher carbon content. In general, the Nb-containing steels show slightly smaller carbon contents (compare the values in Table 3). However, there are  
Figure 3: (a) Influence of temperature on the amount of retained austenite for steels isothermally held for 300 s; (b) Influence of isothermal holding time on the amount of retained austenite for steels treated at 450 \({}^{\circ}\)C.  
also some disturbances in this tendency. For example, the 3 MnNb steel sample treated at 350 \({}^{\circ}\)C and 5 MnNb steel sample treated at 400 \({}^{\circ}\)C show a little bit higher carbon content in the RA compared to the reference steels without Nb. This indicates the complex interactions of the chemistry in TRIP steels containing microadditions [12; 17] and it will be analyzed in future investigations. Increasing the time of bainitic transformation up to 1800 s caused a drop in the amount of RA (Figure 3b). In some cases, X-ray diffraction cannot provide the correct retained austenite content values because strain-induced martensitic transformation takes place easily near the specimen surface. In such cases, the metastable retained austenite fraction should be determined using neutron diffraction. This can be found in works [28; 29; 30]. The amount of retained austenite is closely related to the carbon content in this phase (Figure 4a). The obtained results showed that increasing the temperature of isothermal bainitic transformation leads to lowering the carbon content in retained austenite (Figure 4a). This tendency is similar for all  
Figure 4: (**a**) Influence of isothermal holding temperature on the carbon content in retained austenite for the steels treated for 300 s; (**b**) Influence of the isothermal holding time at 450 \({}^{\circ}\)C on the carbon content in retained austenite. the tested steels. However, the highest carbon content in retained austenite was detected for the 3 Mn and 3 MnNb steels. The lowest C content was obtained for the 5 MnNb steel. For the steel containing 5% Mn, isothermal holding for 60 s is too short to enrich the austenitic phase in carbon (Figure 4b). The optimal duration of isothermal bainitic transformation for these steels is 300 s, similar to the steels, which contain 3% Mn. In fact, there is a lack of literature data concerning the effects of isothermal holding time on the fraction of retained austenite in medium-Mn steels and its carbon content. The data available in the literature concern mostly the 1 st generation AHSS, with a Mn content below \(\sim\)2%. [PERSON] et al. [7] reported that for 0.1C-1.5 Mn-1.5 Al steel isothermally held at 375 \({}^{\circ}\)C, the maximum carbon content in retained austenite was obtained after 1000 s. According to [PERSON] et al. [12], the maximum carbon content in the 0.2C-1.5 Mn-0.7 Si-1 Al-0.5 Cu steel was measured at a level of 0.91% at 440 \({}^{\circ}\)C.  
Figure 5: (**a**) Influence of isothermal holding temperature on the product of the retained austenite amount and its carbon content for the steels treated for 300 s; (**b**) Influence of isothermal holding time at 450 \({}^{\circ}\)C on the product of the retained austenite amount and its carbon content. [PERSON] et al.