In recent years, problems have arisen of reducing CO2 gas, which is said to be a cause of global warming, or the future exhaustion of oil or other fossil fuels. To address these problems, recyclable natural energy has been actively used. Wind power is one form of promising recyclable natural energies, and large-scale wind power generators have been increasingly constructed.
The most suitable area for wind power generators to be constructed is an area where strong wind is expected to blow constantly. Thus, off-shore wind power generators are under planning or actually in operation all over the world (see Patent Documents 1 to 4).
In order to build a tower for wind power generation at sea, it is necessary to drive a foundation portion of the tower into the seabed. Further, in order to obtain sufficient height of turbine blades of the wind power generator from the sea level, a foundation portion of the tower is required to have sufficient length.
Thus, the foundation portion of the tower of the wind power generator employs a steel pipe structure having a wall thickness exceeding 50 mm, for example, of approximately 100 mm, and a large diameter in cross-section of approximately 4 m. Further, the total height of the tower is as high as 80 m or more. In recent years, a large steel-structure such as a tower for wind power generation has been required to be welded and built through electron-beam welding on the coast near the construction site in an easy and efficient manner.
In other words, under the circumstances described above, there arises a new technical demand for welding an ultra-thick steel plate having a thickness of 100 mm on-site in a highly efficient manner.
In general, a high-energy-density beam welding such as electron-beam welding and laser beam welding exhibits high efficiency. However, the thickness of the steel plate to be welded with laser beam has been limited. Further, the conventional electron-beam welding is required to be performed in a vacuum chamber under a high vacuum state. Thus, the thickness or size of the steel plate that can be welded through the high-energy-density beam welding largely depends on the capacity of welding equipment or the capacity of the vacuum chamber.
In recent years, to address the circumstances described above, an electron-beam welding method has been proposed that employs reduced pressure in the vicinity of a weld-target portion, thereby efficiently welding an ultra-thick steel plate with a thickness of approximately 100 mm on-site. For example, Welding Institute of the United Kingdom has developed a welding method (reduced pressured electron-beam welding: RPEBW) enabling working under a low vacuum state (see Patent Document 5).
With the method of RPEBW, it is possible to efficiently perform welding, by locally reducing the pressure of the portion to be welded to be a vacuum state in a case where a large-scale steel structure such as the tower of the wind power generator is constructed. The RPEBW method is a welding method that is performed in a state in which the degree of vacuum is low as compared with the method of performing welding in the vacuum chamber.
In general, a fracture toughness value δc based on fracture mechanics is known as an index for quantitatively evaluating the safety of a welded structure. The fracture toughness value δc can be obtained through a CTOD (crack tip opening displacement) test. The fracture toughness is affected by a size of a test piece. Thus, although favorable results can be obtained through a small-sized test such as the conventional V-notch Charpy impact test, it is not always true that the favorable fracture toughness value δc can be obtained through the CTOD test with a welded joint in the large-scale steel structure.
The electron-beam welding method is a method employing energy of the electron-beam to once melt and solidify the base metal of a welded portion to weld. Normally, the compositions of the molten metal portion in the electron-beam welding method are almost the same as those of the base metal (steel). On the other hand, in large-heat input arc welding such as electro gas welding, mechanical properties such as hardness and the fracture toughness value δc of the welded metal is adjusted by using, for example, welding wire. It is difficult to use such a method in the electron-beam welding method.
In view of the above-described circumstances, a method of optimizing the hardness or cleanliness of the welded metal (WM) has been proposed to improve the fracture toughness value δc of the electron-beam welded joint (see, for example, Patent Documents 6 and 7). Patent Document 6 proposes setting the hardness of the welded metal to be more than 110% and not more than 220% of that of the base metal, and setting the width of the welded metal to be 20% or less of the thickness of the steel. Further, Patent Document 7 proposes setting the amount of O in the welded metal to 20 ppm or more, and the number of oxides having a diameter of 2.0 μm or more to 10 pieces/mm2 or less.
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