## 3 Results and Discussion
### Hardness Changes  
The mechanical properties of the steels were assessed by hardness tests, which are sensitive to any possible phase transformation of the austenite upon cooling. On the one hand, this allows to indicate when the austenite is thermodynamically stable and remains at room temperature. On the other hand, one can easily assess when partial martensitic transformation upon cooling takes place and martensitic-austenitic (M-A) constituents are formed. Hence, the microstructure changes are reflected in sensitive hardness measurements.  
The hardness of the steel samples was measured using the Vickers method. The obtained results are presented in Figure 6 as a function of time and temperature. The data presented in Figure 6a show that the lowest hardness values were obtained for specimens isothermally held within 300 s, both for 3 Mn and 5 Mn type steels. However, it is worth to note that higher hardness values were detected for the steels containing 5% Mn, especially for the steels containing the Nb microaddition. This can suggest the higher degree of the transformation into martensite. Figure 6b shows the effect of the temperature of isothermal holding during bainitic transformation on the hardness changes. In the case of 3 Mn, 3 MnNb, and 5 Mn type steels, the lowest hardness was obtained at 450 \({}^{\circ}\)C, whereas for the 5 MnNb steel at 400 \({}^{\circ}\)C. Higher hardness values were detected for the steels with 5% Mn.  
Generally, the steels containing the Nb microaddition were characterized by higher hardness than the specimens without this element. However, the effect of Nb is not clear, probably due to its presence in the solid solution [11], as the manganese content increases the solubility of Nb in the austenite raises. It seems that the prior austenite grain size is similar for all the steels investigated. Hence, niobium is dissolved and its effect is at least partially masked (not so clear).  
The increase in hardness is directly related to the higher amount of martensite or M-A constituents. Generally, the steels containing 3% Mn showed lower hardness than the 5% Mn steels. This is due to the higher fraction of retained austenite observed in the 3 Mn and 3 MnNb steels. Increasing the temperature to 500 \({}^{\circ}\)C caused a significant increase in martensite fraction, which was reflected in the higher hardness values in Figure 6b. The same effect is observed when the time of the isothermal holding is too short, that is, 60 s (Figure 6a).  
In our previous study, [PERSON] et al. [5] analyzed the mechanical stability of the retained austenite in the investigated steels using larger samples based on the work hardening behavior and elongation changes. The yield stress of 3 Mn steels was near 700 MPa and above 200 MPa higher for the steels containing 5% Mn. These properties are slightly higher for Nb-containing steels. The total elongation was about 14-15% for the steels containing 3% Mn, whereas the 5 Mn type steels showed lower elongation values. The tensile test data are in good agreement with the hardness changes presented in the current paper. For further mechanical stability data, the reader is referred to the text in Reference [5].