## 3 Results and Discussion
### Microstructure Changes  
Microstructures of samples for different temperatures of isothermal holding during bainitic transformation are shown in Figures 7-9. The obtained microstructures are fine-grained due to finishing the plastic deformation below the recrystallization temperature of austenite (850 \({}^{\circ}\)C) [5]. The microstructures were also affected by a high deformation level (\(\varepsilon\) = 1.7). The microstructures of 3 Mn type steels were composed of fine-grained bainitic-martensitic plates. The highest fraction of retained austenite was observed in 3 Mn and 3 MnNb steels after isothermal holding at 400 \({}^{\circ}\)C and 450 \({}^{\circ}\)C (Figure 7c-f). This was earlier confirmed by the X-ray diffraction method. The most optimal duration of isothermal holding in the bainitic region seems to be 300 s. The clear influence of Nb microaddition on the microstructure and the amount of retained austenite was not detected. It seems that the Nb microaddition remained dissolved in solid solution due to the relatively high manganese content [5]. Thus, its effect can be smaller than that expected for NbC precipitates [17]. Based on the  
Figure 6: (**a**) Influence of isothermal holding time at 450 \({}^{\circ}\)C on the hardness; (**b**) Influence of isothermal holding temperature for 300 s on the hardness. obtained diffraction patterns, it was found that the amount of retained austenite in steels with the Nb microaddition was lower by c.a. 2% (Table 3). Figure 7: Multiphase microstructures of 3 Mn and 3 MnNb steels isothermally held at: (**a,b**) 350 \({}^{\circ}\)C, (**c,d**) 400 \({}^{\circ}\)C, (**e,f**) 450 \({}^{\circ}\)C, (**g,h**) 500 \({}^{\circ}\)C, for 300 s (LM). Figure 8: Multiphase microstructure of 3 Mn and 3 MnNb steels isothermally held at: (**a,b**) 350 \({}^{\circ}\)C, (**c,d**) 400 \({}^{\circ}\)C, (**e,f**) 450 \({}^{\circ}\)C, (**g,h**) 500 \({}^{\circ}\)C, for 300 s (SEM). The more detailed information on the microstructure was revealed using the scanning electron microscope (SEM). Figure 8 shows the microstructures obtained for the specimens isothermally held  
Figure 9: Bainitic-martensitic microstructure of 5 Mn steel isothermally held at: (**a**,**b**) 350 \({}^{\circ}\)C, (**c**,**d**) 400 \({}^{\circ}\)C, (**e**,**f**) 450 \({}^{\circ}\)C, (**g**,**h**) 500 \({}^{\circ}\)C.  
between 350 and 500 \({}^{\circ}\)C. At 350 \({}^{\circ}\)C, the bainitic-martensitic laths were clearly observed and the retained austenite formed thin layers between the bainitic ferrite laths (Figure 8a,b). Increasing the bainitic transformation temperature up to 400 \({}^{\circ}\)C caused changes in the morphology of retained austenite. The fine-grained blocky retained austenite was observed at the boundary of bainitic-martensitic blocks (Figure 8c,d). Larger grains were subjected to partial martensitic transformation during cooling. As a consequence, they formed martensitic-austenitic regions (M-A). The confirmation of the phases using the EBSD (Electron Back-Scattered Diffraction) method can be found in our previous paper [5]. This indicates that the carbon content in the \(\gamma\) phase varies and depends on its size and morphology. In addition to the blocky-type grains of retained austenite, that phase also occurs in the form of continuous or intermittent layers characterized by various thicknesses, located inside the bainitic areas. This kind of microstructure is composed of degenerate upper bainite, where austenite occurs instead cementie along bainite laths [35]. While the bainitic transformation temperature was increased to 450 \({}^{\circ}\)C, the decomposition of the austenite into martensite began (Figure 8e,f). Under these conditions, only thin layers of retained austenite were thermodynamically stable. Bainitic-martensitic-austenitic (B-M-A) and martensitic-austenitic (M-A) areas occurred. Increasing the temperature to 500 \({}^{\circ}\)C resulted in the almost complete disappearance of the \(\gamma\) phase (Figure 8g,h).