## 3 Results  
### Microstructural Characterization and Stability of RA  
The performed thermomechanical processing realized using the Gleeble simulator provided the microstructures composed of bainite, acicular ferrite and RA (Figure 1). The semi-industrial process characterized by less precise control of temperature-time parameters (greater temperature inertia) resulted in the formation of an additional small fraction of pearlite in the microstructure (Figures 1b and 2b).  
The RA in both cases is fine and evenly distributed. The majority of the RA is in a form of fine grains or elongated layers at the interfacial boundaries of acicular ferrite and bainitic ferrite, or it is located inside bainitic areas (Figure 2). The austenite deformation conditions below the recrystallization finish temperature of this phase favor the refinement of the ferritic matrix grains. Therefore, a significant part of the RA occurs in this phase. The retained austenite grains located in ferrite tend to elongate in the direction of plastic flow, as evidenced by an aspect ratio \(\sim\)1.4.  
Figure 1: Light microscopy images showing the microstructures of samples produced in (**a**) laboratory scale—Gleeble; (**b**) semi-industrial scale—thermomechanical processing; RA—retained austenite, B-RA—bainitic-ausenitic regions, F—ferrite, P—pearlite.  
Figure 2: SEM images showing the microstructures of samples produced in (**a**) laboratory scale—Gleeble; (**b**) semi-industrial scale—thermomechanical processing; RA—retained austenite, B-RA—bainitic-ausenitic regions, F—ferrite, P—pearlite.  
Comparing the steel microstructures obtained after the semi-industrial rolling and in the Gleeble simulator, it can be concluded that they generally show good agreement. The differences may be caused by the different cooling rate in individual stages of sample cooling, which is constant (in individual temperature ranges) in laboratory conditions, and changes during the cooling of sheets in the air and immersion cooling. The laboratory-scale production process resulted in obtaining \(\sim\)10% of the RA in the microstructure with a carbon content above 1.5 wt.% (Table 1). The pearlite formation in case of semi-industrial production process didn't affect the RA fraction. However, there is a significant reduction in the C content (\(\sim\)1 wt.%) in this phase.  
During tensile deformation, the largest retained austenite grains distributed in the ferrite are transformed into strain-induced martensite first (Figure 3a). The transformation of grains present in ferrite progresses with the increase of strain up to 10%. The fine-plate martensite formed under these conditions covers almost the entire surface of the larger grains, while the finer, blocky grains and thin layers of RA do not transform.  
After increasing the strain value to 15% (Figure 3b,c), the fraction of retained austenite decreases to about 4% (Table 1). Figure 3 shows that the martensitic transformation is subject to layered areas with a width of about 1 \(\upmu\)m, located within the bainitic ferrite laths, whereas the RA films retain mechanically stability. The \(\gamma\) phase is most often in the form of a shell around the formed martensite or small areas in the bainitic ferrite. The amount of untransformed \(\gamma\) phase after sample breaking is about 2.5%. The relatively rapid transformation of austenite into martensite in the initial stage of deformation results from the carbon content (1.08 wt.%) in the retained austenite (Table 1).