## 3 Results and Discussion
### Mechanism of Improvement of Mechanical Properties by L Treatment  
As mentioned above, the influences of the L treatment temperature on mechanical properties are as follows:  
1. The strength of the Q-L-T material becomes lower than that of the Q-T material in the cases where the L treatment was carried out in the temperature range from 1003 K to 1053 K (between A\({}_{\text{C1}}\) and A\({}_{\text{C3}}\)). 2. The toughness gradually improves as the L treatment temperature increases. Firstly, we discuss the main reason for phenomenon (1). The strength of Cu-added steel generally depends on the dispersed state of Cu precipitation resulting from heat treatment [2; 14], and the precipitated state is dependent on the L treatment temperature [15]. When the steel is heated to about 723 K to 823 K, the strength increases by age hardening, but the strength decreases in an overaging treatment at temperatures over 823 K. Additionally, L treatment is a heat treatment in the two phases (\(\alpha\) and \(\gamma\)). The investigation implied that the not-transformed \(\alpha\) phase is softened during L treatment, because it is heated up to 1068 K. Hence, the strength could be decreased in the softened not-transformed \(\alpha\) phase. The results shown in Figures 9 and 10 also indicate that the decreased strength is consistent with coarse Cu precipitates. On the other hand, Cu precipitates dissolve into transformed \(\gamma\) phases again, and then the \(\gamma\) phases are quenched. Hence, the \(\gamma\) phases harden due to Cu precipitation during tempering. L treatment at 953 K (right above A\({}_{\text{C1}}\)) has a trivial effect on the strength, because the not-transformed \(\alpha\) phase was tempered at low temperatures. However, as the L temperature for treating the steel ranged between 1003-1053 K, the strength was notably decreased due to the not-transformed \(\alpha\) phase treated with high-temperature tempering. For the L treatment at 1068 K (right below A\({}_{\text{C3}}\)), the strength of the sample was higher than that treated at approximately 1003 K to 1053 K. According to Figure 8, the transformed \(\gamma\) phase became the main phase after holding at 1068 K for 2 h; therefore, the harder phase inherited the \(\gamma\) phase, leading to improve strength. Secondly, we discuss the main reason for phenomenon (2). Previous research has suggested that the retained \(\gamma\) phase is stabilized by L treatment and is helpful for enhancing the low-temperature toughness of the 9% Ni steel [6]. However, the Cu-containing low alloy steel did not have a retained  
Figure 10: Element mapping images of TEM-EDS of Q–L samples heated to 950 K (**a**) and 1053 K (**b**). \(\gamma\) phase after L treatment, so the improvement of toughness owing to the L treatment is not related to the retained \(\gamma\) phase for this steel. Therefore, it is necessary to verify the mechanism of toughness improvement other than the retained \(\gamma\) phase. Thus, we focused on the EBSD grain size that indicates the distribution of high angle grain boundary (HAGB), because the HAGB generally has high crack propagation resistance [18]. In other words, the refinement of EBSD particle size indicates an increase in the frequency of HAGB. The EBSD grain size of each sample underwent L treatment at a temperature range from 953 K to 1068 K in order to investigate phenomenon (2). The results are shown in Table 5 and Figure 11. The EBSD grain size became finer as the L treatment temperature increased, and also, the grain size of the sample treated at an optimized temperature, which was 1063 K, was the finest among the four samples. These results suggest that the toughness improvement with the increasing L temperature is due to the grain refining. Furthermore, in past literature [19], it is reported that the effective grain size corresponding to the destructive unit of the brittle fracture is refined, and the toughness is improved along with the refinement of the EBSD grain size. This report supports our results.