In the field of offshore structures, there is an ongoing trend of advancing development of energy resources in very deep waters, expanding resource exploration/mining zone to an extreme sea area such as Arctic sea, and growing in the equipment size. Against this background of technical trend, a high-strength and high-toughness design of the offshore structure is proceeding, and more stringent requirements are imposed on the performance of the weld joint.
On the other hand, as for the welding material, in view of higher efficiency, a flux-cored wire for all-position welding is demanded. However, a conventional flux-cored wire for all-position welding forms a weld metal having a high oxygen amount and therefore, when gas-shielded arc welding is performed using the wire, the weld joint part can be hardly assured of having low-temperature toughness.
Conventionally, there has also been proposed a flux-cored wire for all-position welding, in which for the purpose of enhancing the low-temperature toughness of a weld metal, the ratio of the content of MgO to the content of TiO2 is set to a specific range and the relationship with a slag forming flux is specified (see, Patent Document 1). Patent Document 1 describes that when welding is performed using the flux-cored wire of Examples, a weld metal having a 0.2% yield strength of 620 MPa or more and a Charpy impact value at −60° C. of 27 J or more is obtained and the welding usability is good.