Thick steel plates having good combination of strength, toughness and weldability have been required for building large container ships. EH36 steel plates with heavy gauge were developed using newly installed accelerated cooling equipment. In order to develop EH40 grade steel, the hardenability of boron were carefully utilized, along with optimizing TMCP processing variables to obtain good mechanical properties and to lessen inhomogeneity in the thickness direction. These thick steel plates showed high strength and good toughness, even at the mid-thickness region, and excellent properties of welded joints at high heat input.
The foreign trade between countries has been growing and has been globalized at the very fast pace. This trend has accelerated the need for container ships and in turn the number of container ships has been increasing very fast. Also, the container ships are getting larger in order to enhance the transportation efficiency. In other words, larger container ships could reduce delivery cost per a container, compared to smaller ones. Nowadays, the container ships around 9000~10000 TEU (Twenty feet Equivalent Unit) are being manufactured in the shipyards. It is well known that the container ship has an open upper deck structure to load as many containers as possible. Its structural characteristics require heavy thickness steel plates having high strength and toughness, such as very thick EH36 or EH40 grade steels. These steel plates are used to construct an upper deck and a hatch coaming. These steel plates also need excellent weldability and welded joint properties which could lead to improvement of productivity in the shipyards by alleviating the restriction on preheating and short bead, and enabling high heat input welding which is around 300 kJ/cm. Thermo-mechanical control process (TMCP) is effective to achieve high strength and good toughness of thick steel plates, even with low carbon equivalent (Ceq) (Imai, 2002; Toyosada, 2002).
