# Grain Refinement and Improved Mechanical Properties of EUROFER97 by Thermo-Mechanical Treatments
## 3 Results and Discussion
### Recrystallization Behavior of EUROFER97 Steel  
Recrystallization takes place in all the heat-treated samples with kinetics depending on treatment temperature and CR ratio. Of course, greater CR ratios promote the start of recrystallization at lower temperatures; however, the samples deformed with CR ratio of 20% recrystallize at the highest temperature examined here (750 \({}^{\circ}\)C). This is shown by Figure 7 where a structure of new grains of very small size (\(\sim\)300 \(\upmu\)m) can be observed. In this work, attention was focused on the steel cold-rolled with a CR ratio of 80%, and SEM micrographs in Figure 8 illustrate its microstructure evolution following heat treatments at increasing temperature, i.e., 400, 450, 500, 550, 600 and 650 \({}^{\circ}\)C. The typical structure with elongated grains originating from cold rolling progressively weakens as treatment temperature increases, and at 650 \({}^{\circ}\)C, a population of equiaxed grains of sub-micrometric size can be observed indicating that the primary recrystallization has been completed. Figure 6: Effect of the CR ratio on hardness \(HV_{5}\) and yield strength YS measured by means of FIMEC test. Figure 7: EUROFER97 cold-rolled with a CR ratio of 20% and heat-treated at 750 \({}^{\circ}\)C for 1 h.  
For higher heat treatments temperatures (700 and 750 \({}^{\circ}\)C) some grains undergo an abnormal growth at the expense of neighboring ones (see Figure 9a,b). Some grains, usually aligned along the rolling direction, may reach a size of about 60-70 \(\upmu\)m (Figure 9a) while neighboring ones are about 350 nm (Figure 9b). The result is in agreement with those reported by [PERSON] et al. [49] who explained the abnormal grain growth as the synergic  
Figure 8: EUROFER97 deformed with a CR ratio of 80% and treated for 1 h at the temperature of: 400 \({}^{\circ}\)C (**a**), 450 \({}^{\circ}\)C (**b**), 500 \({}^{\circ}\)C (**c**), 550 \({}^{\circ}\)C (**d**), 600 \({}^{\circ}\)C (**e**), 650 \({}^{\circ}\)C (**f**). effect of high intrinsic grain boundary mobility, size advantage acquired in the early stages of annealing and the presence of local microstructural instabilities such as the dissolution and coarsening of M\({}_{23}\)C\({}_{6}\) carbides. The XRD patterns of EUROFER97 steel cold-rolled with CR ratio of 80% and heat-treated at temperatures in the range from 400 to 750 \({}^{\circ}\)C are shown in Figure 10. The relative intensities of XRD peaks determined from the patterns in Figure 10 are reported in Table 3.  
Figure 10: XRD patterns of EUROFER97 steel cold-rolled with CR ratio of 80% and heat-treated at increasing temperatures from 400 to 750 \({}^{\circ}\)C with steps of 50 \({}^{\circ}\)C.  
Figure 9: EUROFER97 deformed with a CR ratio of 80% and treated for 1 h at 750 \({}^{\circ}\)C. LM micrograph in (**a**) shows grains of abnormal size which are surrounded by quite smaller grains displayed by the SEM image in (**b**). The samples before heat treatments exhibit a cubic [100] texture with a secondary [211] component which progressively weaken as treatment temperature increases. Such trend observed up to 550 \({}^{\circ}\)C indicates that in a first stage of recrystallization the new grains form with the [110] orientation. In a second stage corresponding to the completion of recrystallization the new grains assume the same orientation of old deformed grains thus [100] and [211] texture components are still present. From the analysis of XRD peak profiles the mean grain size, D, and dislocation density, \(\rho\), have been determined (Figure 11). In the same figure the values of the not-treated steel (N.T.) are displayed for comparison. \begin{table}
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As treatment temperature increases, the dislocation density exhibits a continuously decreasing trend that is the consequence of the formation of new grains free from defects during recrystallization.