## 3 Results and Discussion
### Mechanism of Improvement of Mechanical Properties by L Treatment
Reference [8] reported that the optimum L treatment of SA508 Gr.3 steel is at a relatively low temperature between the A\({}_{\text{C1}}\) and A\({}_{\text{C3}}\) of the steel, and the trend is different from the research results using A707 modified steel. In A707 modified steel, when the L treatment temperature is relatively high--that is, near the A\({}_{\text{C3}}\) point--a good balance of strength and toughness could be obtained. This is considered to be the difference in whether the mechanical properties after the L treatment are due to the precipitation or dispersion of carbides, or the area ratio of the reverse transformed \(\gamma\) phase and the Cu precipitates. In the SA508 Gr.3 steel, the C content is about 0.20 mass%, and the carbides precipitated during the L treatment have an effect on the mechanical properties. On the other hand, the carbides might be slightly precipitated during the treatment in A707 modified steel, because the steel has 0.03 mass% C. Accordingly, the refinement of the grain size by reverse transformation during the L treatment and precipitation state of the Cu become dominant with respect to the mechanical properties. Figure 11: Relationship between L treatment temperature and maximum EBSD grain size. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**L Treatment Temperature (K)**} & \multicolumn{2}{c}{**EBSD Grain Size (\(\mu\)m)**} \\ \cline{2-3}  & **Average** & **Maximum** \\ \hline
953 & 12.5 & 129 \\
1003 & 10.8 & 96 \\
1053 & 10.5 & 84 \\
1068 & 7.2 & 49 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The electron back scatter diffraction (EBSD) grain size of Q–L–T samples treated at various temperature. From these results, it is considered that the optimum L treatment condition differs depending on the steel type, because the influence factor affecting the mechanical properties is different. Based on these results, schematic illustrations regarding changes in the microstructure and precipitates during the L treatment of A707 modified steel are shown in Figure 12. Reverse transformation does not proceed in the state at 953 K that is shown in Figure 12a, so the grain refining and the toughness improvement are not achieved. At 1053 K, as shown in Figure 12b, the toughness improves due to the refined grain size with the reverse transformation, but the strength is decreased by the not-transformed \(\alpha\) phase containing coarse Cu precipitates, because the area ratio of the not-transformed \(\alpha\) phase still remain approximately 40% after L treatment. At 1068 K, as shown in Figure 12c, the \(\alpha\) phase almost transforms to the \(\gamma\) phase after 2 h. The transformed \(\gamma\) phase bears the strength of the steel, because it becomes fresh bainitic ferrite during the cooling process of L treatment, and furthermore, Cu precipitates in a matrix during tempering. It is important to retain a small amount of not-transformed \(\alpha\) phase during L treatment for grain refining. From the above hypotheses, it is believed to be possible to enhance the mechanical properties of Cu-containing low alloy steel by controlling the area ratio of the reverse transformed \(\gamma\) phase and by selecting the appropriate L treatment temperature. Figure 12: Schematic illustrations of changes in microstructure and precipitates during L treatment at 953 K (**a**), 1053 K (**b**), and 1068 K (**c**).