## 1 Introduction  
Oil and natural gas are major energy sources, accounting for about 60% of the primary energy in the world. Ultra-deepwater development is expected to increase in the future, as demands for oil and natural gas increase [1]. Copper-containing low alloy steel based on the American Society for Testing and Materials (ASTM) A707 5L grade is widely used for structural parts of offshore wells because of its low carbon equivalent (C\({}_{\text{eq}}\)) and weld crack sensitivity composition (P\({}_{\text{cm}}\)) [2; 3; 4]. From the viewpoint of preventing failure and accidents, steel with high strength and excellent low-temperature toughness is required. Generally, the forging steel is subjected to quenching (Q) and tempering (T). The steel is reverse-transformed from \(\alpha\) to \(\gamma\) by holding the quenching temperature; after that, \(\gamma\) is transformed to bainite or martensite by water cooling. However, manufacturing large forged partsthat have homogeneous and excellent properties over the whole length and thickness is difficult, since there are various reasons such as the segregation of elements, mass effects, and so on.  
Therefore, in our previous research, we investigated the effect of intercritical quenching--which is called lamellarizing (L) treatment--from the dual-phase region of ferrite (\(\alpha\)) and austenite (\(\gamma\)) on the mechanical properties and microstructures of A707 modified steel using 50-kg test ingots [5]. L-treated steels such as structural steel and steel for very low temperatures have been studied [6; 7; 8; 9; 10; 11; 12; 13]. On the other hand, the effects of L treatment on Cu-containing low alloy steel have not been reported. The strength of Cu-containing low alloy steel is improved by Cu deposited during heat treatment [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. Therefore, for the A707 modified steel, the Cu precipitation state is very important to obtain the strength. On the other hand, it has been reported that the precipitation behavior of Cu-bearing steel differs depending on each stage of the refining heat treatment, especially intercritical quenching or tempering [15]. So, it is extremely important to evaluate the mechanical properties of the material whose L treatment conditions are changed and also show the optimum treatment conditions in A707 modified steel as well in order to apply for offshore structures. In our previous research, L treatment brought clear improvement in the balance between the strength and toughness of A707 modified steel [5]. Therefore, the purpose of this research is to clarify the mechanism of improvement of mechanical properties by L treatment in terms of reverse transformation behavior, because the behavior during L treatment is considered to affect the mechanical properties of heat-treated steel due to grain refinement and the hardness of each phase. Based on our previous results, we applied L treatment to a long forged product about 20,000 mm long, and studied the mechanical properties, including crack tip opening displacement (CTOD) and nil ductility transition temperature (NDTT) over the trial production to check whether or not L treatment has a good effect on the mechanical properties.  
In the manufacturing of offshore structures, the weldability of steel is very important because the structures have a lot of weld joints. In general, the equivalent amount of C is kept at a low level, and the component design is in consideration of the weldability of the A707 steel. However, since the element, such as C, Ni, and Mn, diffuses to the reverse-transformed \(\gamma\) phase during the L treatment and becomes concentrated locally [16], there is also a concern that the local non-uniformity of the element might reduce the weldability. Therefore, the weldability of the forging that applied L treatment was evaluated from the CTOD property of the welded part in this study.