# Grain Refinement and Improved Mechanical Properties of EUROFER97 by Thermo-Mechanical Treatments
## 3 Results and Discussion
### Recrystallization Behavior of EUROFER97 Steel
The mean grain size has an analogous behavior up to 650 \({}^{\circ}\)C because the heavily deformed matrix of the steel has a large number of preferred nucleation sites for new grains. Many nucleation sites and slow growth due to the relatively low temperature produce a population of grains of small size. Above 650 \({}^{\circ}\)C grain growth occurs thus an increase in mean grain size can be observed. XRD results are in agreement with SEM observations displayed in Figures 8 and 9. The effect on hardness of heat treatments for 1 h at eight temperatures on samples submitted to cold rolling with five CR ratios is displayed in Figure 12; the value of standard EUROFER97 is shown for comparison. As already shown by the plot in Figure 6, hardness of cold-rolled samples is always greater than that of the material prepared by the standard treatment. It decreases as heat treatment temperature increases and the change is more pronounced in the samples cold-rolled with greater CR ratios. This result is evident in Table 4 which reports the percentage softening values at the maximum temperature (750 \({}^{\circ}\)C) for different CR ratios. The softening percentage was calculated as the ratio between the hardness after 1 h at 750 \({}^{\circ}\)C and that of the as-deformed material. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**CR Ratio \%** & 20 & 40 & 50 & 60 & 80 \\ \hline
**Softening \%** & 10.8 & 48.7 & 50.7 & 53 & 54.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Softening percentage of EUROFER97 steel, in the temperature range of the annealing heat treatment from 400 \({}^{\circ}\)C to 750 \({}^{\circ}\)C.  
Figure 12: Effect of the CR ratio and the heat treatment temperature on hardness of EUROFER97 steel. Heat treatment was performed for 1 h in argon atmosphere. In the case of CR ratios of 60% and 80%, and for heat treatment temperatures above 650 \({}^{\circ}\)C a rapid degradation of hardness is observed with values lower than those of the steel in standard condition. The same is true for CR ratios of 40% and 50%, but for higher temperature (above 700 \({}^{\circ}\)C). On the contrary, for the CR ratio of 20%, the hardness stays above the standard value, independent of the heat treatment temperature. After the heat treatment at 650 \({}^{\circ}\)C, the hardness is \(\sim\)230 HV, i.e., significantly lower than that of the cold-rolled steel (\(\sim\)300 HV) but still 15% greater than steel after the standard preparation route (\(\sim\)200 HV). For higher heat treatment temperatures (700 and 750 \({}^{\circ}\)C), abnormal grain growth takes place leading to hardness dropping below that of EUROFER97 prepared in standard conditions. On the basis of the results displayed in Figure 12 some samples (CR: 80%, heat-treated at 400 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, 600 \({}^{\circ}\)C and 650 \({}^{\circ}\)C) have been selected for tensile tests. The curves are reported in Figure 13 while Figure 14 displays the corresponding values of YS, UTS and A%. As expected, ultimate tensile stress (UTS) and yield stress (YS) decrease as temperature increases, while total elongation (A%) exhibits the opposite behavior. Figure 14: UTS, YS and A% of EUROFER97 steel cold rolled with CR of 80% and heat-treated at 400 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, 600 \({}^{\circ}\)C and 650 \({}^{\circ}\)C for 1 h. The values of the steel prepared by standard treatment [23] are reported for comparison. Figure 13: Tensile test curves of EUROFER97 steel cold rolled with CR of 80% and heat-treated at 400 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, 600 \({}^{\circ}\)C and 650 \({}^{\circ}\)C for 1 h.  
After the heat treatments at 400, 500 and 600 \({}^{\circ}\)C the values of YS and UTS are significantly larger than those of standard EUROFER97 [23] but such strengthening leads to lower ductility. On the contrary, the treatment at 650 \({}^{\circ}\)C increases YS of about 12% with a comparable total elongation. Therefore, this work demonstrated that it is possible to strengthen EUROFER97 steel without compromising its ductility.