## 3 Results and Discussion  
### X-ray Results  
X-ray diffraction analysis was carried out to identify the phases and to determine a retained austenite amount and its carbon content. It was necessary to assess the stability of retained austenite. Figure 2 shows selected X-ray diffraction patterns of the examined steels. The phase identification revealed the presence of diffraction lines from \(\alpha\) phase and retained austenite. The detailed information on the fraction of retained austenite and carbon content in this phase are listed in Table 3. The obtained results are also presented in Figures 3-5 as a function of temperature and time. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Steel Type**} & **Theromechanical** & **Fraction of** & **Carbon Content** & \multirow{2}{*}{**Steel Type**} & **Theromechanical** & **Fraction of** & **Carbon Content** \\  & **Treatment Conditions** & **Retalened** & **In Retained** & **In Retained** & **Treatment Conditions** & **Retained** & **in Retained** \\  & **Austenite, \%** & **Austenite, wt. \%** & & & & **Austenite, \%** & **Austenite, wt. \%** \\ \hline  & 880-700  
In the case of 3 Mn-type steel, the highest fraction of retained austenite was obtained during isothermal holding at 400 \({}^{\circ}\)C (16.8%). 3 MnNb steel possessed a slightly higher fraction of \(\gamma\) phase--17.7%. Similar results were reported by [PERSON] et al. [32] in 0.2C-1.5 Mn-1.5 Si type steels with and without Nb microaddition. Lowering the bainitic transformation temperature to 350 \({}^{\circ}\)C caused a decrease in the amount of retained austenite to c.a. 10% for both steels containing 3% Mn (Table 3). This was due to the decrease in the diffusion rate while the temperature dropped. The amount of retained austenite slightly decreased when the temperature of bainitic transformation increased to 450 \({}^{\circ}\)C. Further increasing the temperature to 500 \({}^{\circ}\)C caused a significant decrease in the amount of retained austenite (Figure 3a). A similar tendency was observed in 0.1C-1.5 Mn-1.5 Si and 0.2C-1.5 Mn-1.5 Si steels with lower Mn content [6]. The maximum amount of retained austenite was obtained at 425 \({}^{\circ}\)C. However, it was lower (-7%) than the values obtained in the present study. A small number of points did not allow to interpolate the data. However, from the physical point of view, it seems that the points should form a parabolic tendency with a peak value at a particular temperature. Such a behavior was observed by [PERSON] et al. [12] in 0.2C-1.5 Mn-0.7 Si-1 Al-0.5 Cu steel. In their steel, peak temperature was 440 \({}^{\circ}\)C. This was related to the initiation of the carbide precipitation and the resulting transformation to martensite of some austenite fraction with the smallest carbon content. In our case, the drop in the amount of retained austenite is earlier, that is, above 400 \({}^{\circ}\)C. This is due to the chemistry of the steels. The increased manganese content seems to play a major role in this behavior. Figure 2: (**a**) X-ray diffraction pattern of 3 Mn type steel isothermally held at 350 \({}^{\circ}\)C for 300 s; (**b**) X-ray diffraction pattern of 3 Mn type steel isothermally held at 400 \({}^{\circ}\)C for 300 s; (**c**) X-ray diffraction pattern of 3 MnNb type steel isothermally held at 400 \({}^{\circ}\)C for 300 s; (**d**) X-ray diffraction pattern of 5 MnNb type steel isothermally held at 400 \({}^{\circ}\)C for 300 s.  
Steels containing 5% Mn are less vulnerable to temperature changes. The amount of retained austenite in these steels is almost constant at the bainitic transformation temperature range from 350 to 450 \({}^{\circ}\)C (Figure 3a). The amount of retained austenite (RA) in this case was from 9 to 12% (Table 3) and it was higher for the steel with the Nb microaddition. The effect of isothermal holding time at 450 \({}^{\circ}\)C on the amount of retained austenite is shown in Figure 3b.