## 3 Results and Discussion
### Effect of L Treatment Temperature on Mechanical Properties
 } & \multirow{2}{*}{**YR (\(\star\))**} & \multirow{2}{*}{**FATT (K)**} \\ \hline \multirow{2}{*}{Q-T} & \multirow{2}{*}{-} & \multirow{2}{*}{642} & \multirow{2}{*}{640} & \multirow{2}{*}{729} & \multirow{2}{*}{28} & \multirow{2}{*}{28} & \multirow{2}{*}{76} & \multirow{2}{*}{0.88} & \multirow{2}{*}{233} \\  & & & & & & & & & & \\  & & & & & & & & & & & \\  & & & & & & & & & & & & \\ \multirow{2}{*}{Q-L-T} & \multirow{2}{*}{1053} & \multirow{2}{*}{569} & \multirow{2}{*}{725} & \multirow{2}{*}{72} & \multirow{2}{*}{27} & \multirow{2}{*}{27} & \multirow{2}{*}{76} & \multirow{2}{*}{75} & \multirow{2}{*}{0.78} & \multirow{2}{*}{193} \\  & & & & & & & & & & & \\  & & & & & & & & & & & & \\  & & & & & & & & & & & & \\ \multirow{2}{*}{953} & \multirow{2}{*}{564} & \multirow{2}{*}{570} & \multirow{2}{*}{675} & \multirow{2}{*}{684} & \multirow{2}{*}{680} & \multirow{2}{*}{29} & \multirow{2}{*}{29} & \multirow{2}{*}{80} & \multirow{2}{*}{80} & \multirow{2}{*}{0.84} & \multirow{2}{*}{198} \\  & & & & & & & & & & & & \\  & & & & & & & & & & & & \\  & & & & & & & & & & & & \\ \multirow{2}{*}{Q-L-T} & \multirow{2}{*}{1003 phases were generated from the grain boundaries of the \(\alpha\) phase at 903 K, and the reverse transformed \(\gamma\) phase region spread with increasing temperature. Furthermore, the insides of grains were observed at 1003 K. When the temperature was raised to 1053 K or 1083 K, there was a complicated structure consisting of not-transformed \(\alpha\) phases and transformed \(\gamma\) phases. Over 1103 K, the microstructure consisted entirely of transformed \(\gamma\) phases. The in situ EBSD measurement results indicate that a fine and complicated microstructure is formed by L treatment at higher temperatures between A\({}_{\text{Cl}}\) and A\({}_{\text{C3}}\). The complicated structure seems to indicate that the crystal grain after the L treatment become extremely fine. The area ratio of the transformed \(\gamma\) phase depends not only on the temperature of the L treatment but also on the holding time [8]. Hence, it is necessary to grasp the relationship between the \(\gamma\) phase and holding time to develop the mechanical property-improving effect due to the complication of the microstructure caused by the L treatment. Figure 8 shows changes with time in the area ratio of the not-transformed \(\alpha\) phase and the transformed \(\gamma\) phase measured by in situ EBSD. The area ratios of the transformed \(\gamma\) phase when the temperature reached 1053 K and 1068 K were approximately 40% and 60%, respectively. At both temperatures, the proportion of the reverse transformation \(\gamma\) phase increased with the increase of the holding time, and the area ratio of the transformed \(\gamma\) phase reached approximately 60% at 1053 K and 90% at 1068 K after holding for 120 min. The area ratio of the \(\gamma\) phase at 1068 K also became almost constant beyond 100 min. Therefore, the necessary L treatment time is thought to be at least 100 min in order to stably obtain the mechanical property-improving effect. Figure 7: Phase maps of not-transformed \(\alpha\) phase and transformed \(\gamma\) phase at 823 K (**a**), 903 K (**b**), 1003 K (**c**), 1053 K (**d**), 1083 K (**e**), and 1103 K (**f**). As mentioned above, the material is Cu precipitation-strengthened steel. Hence, it is important to elucidate the Cu precipitation behavior in the not-transformed \(\alpha\) phase and in the transformed \(\gamma\) phase during L treatment. Figures 9 and 10 show TEM images and element (Ni, Cu, and Mn) maps using TEM-Energy Dispersive X-ray Spectroscopy (EDS), respectively. No Cu precipitates were observed in the as-Q sample, while Cu precipitated significantly in the as-L sample. It should be noted that there are Cu precipitated region and non-precipitated regions. The region containing coarse Cu precipitates seems to be in the not-transformed \(\alpha\) phase, because the \(\alpha\) phase tempered at high temperatures during L treatment.