## 2 Materials and Methods  
The investigations were carried out on four medium-Mn steels characterized by manganese content c.a. 3% and 5%. The increased Mn content was added to stabilize the retained austenite. A silicon addition was partially replaced by aluminum to improve the susceptibility of sheet steel to hot dip galvanizing and to prevent carbide precipitation. Al also accelerates bainite nucleation, which is advantageous due to the possibility of the short duration of isothermal holding at the bainitic area [17]. Table 1 shows the chemical composition of the investigated steels. They are characterized by high metallurgical purity and low contents of S and P. Nb microaddition was intended to increase the strength through grain refinement and precipitation strengthening [18]. Mo was added for solid solution strengthening. A relatively low carbon content (max. 0.17%) should not deteriorate the weldability of the steels [19; 20] and ensures the optimal enrichment of the austenite by this element during isothermal bainite transformation. [PERSON] et al. [21] showed that the 0.15C-10 Mn-1.5 Al steel is weldable despite the medium Mn content. The investigated steel was melted in a Balzers VSG-50 vacuum induction furnace under Ar atmosphere. After melting and casting, the ingots were hot forged between 1200 to 900 \({}^{\circ}\)C. The specimens 15 \(\times\) 20 \(\times\) 35 mm for the thermomechanical processing using the Gleeble 3800 simulator were prepared. Detailed information on deformation conditions performed in 7 steps (T\({}_{1}\)-T\({}_{7}\)) in a temperature range 1200-850 \({}^{\circ}\)C (Figure 1) was listed in work [8]. The 5 Mn and 5 MnNb steels were continuously cooled to the bainitic transformation temperature T\({}_{\rm B}\) and isothermally treated in temperatures: 350, 400, 450, 475, and 500 \({}^{\circ}\)C. The duration of isothermal holding was: 300 s, and for 450 \({}^{\circ}\)C it was also 60, 600, and 1800 s, respectively. The aim was to determine the effect of isothermal holding time on the thermodynamic stability of retained austenite. 3 Mn and 3 MnNb steels after deformation were cooled according to the parameters given in Table 2. The duration and temperatures of isothermal holding at the bainitic region were the same as for 5 Mn and 5 MnNb steels (Figure 1). The X-ray investigations were done using the X' Pert PRO diffractometer with cobalt radiation and a graphite monochromator on a diffracted beam. The phase identification was carried out \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Steel Type** & **C\%** & **Mn\%** & **Al\%** & **Si\%** & **Mo\%** & **Nb\%** & **S\%** & **P\%** & **N\%** \\ \hline
3 Mn & 0.17 & 3.3 & 1.7 & 0.22 & 0.23 & - & 0.014 & 0.010 & 0.0043 \\
3 MnNb & 0.17 & 3.1 & 1.6 & 0.22 & 0.22 & 0.04 & 0.005 & 0.008 & 0.0046 \\
5 Mn & 0.16 & 4.7 & 1.6 & 0.20 & 0.20 & - & 0.004 & 0.008 & 0.0039 \\
5 MnNb & 0.17 & 5.0 & 1.5 & 0.21 & 0.20 & 0.03 & 0.005 & 0.008 & 0.0054 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of investigated steels in wt. %. \begin{table}
\begin{tabular}{c c c} \hline
**Operation Number** & **Temperature Range, \({}^{\circ}\)C** & **Cooling Rate, \({}^{\circ}\)C/s** & **Duration of Isothermal Holding, s** \\ \hline
1 & 850\(\rightarrow\)700 & 30 & - \\
2 & 700\(\rightarrow\)650 & 5 & - \\
3 & 650\(\rightarrow\)T\({}_{\rm B}\) & 40 & - \\  & 350 & & 300 \\  & 400 & & 300 \\
4 & 450 & - & 60, 300, 600, 1800 \\  & 475 & & 300 \\  & 500 & & 300 \\
5 & T\({}_{\rm B}\)\(\rightarrow\)RT & 0.5 & - \\ \hline \end{tabular}
\end{table} 
Table 2: Parameters of cooling after deformation for 3 Mn and 3 MnNb steels. Figure 1: Thermomechanical processing conditions of investigated steels (for specimens isothermally held at 400 \({}^{\circ}\)C). according to the data from International Centre for Diffraction Data ICDD. X-ray data obtained from the measurements served in the quantitative determination of the volume fraction of retained austenite.