An increase in the size of container ships has led to an increase in the thickness of steel materials used, thus causing problems such as making welding work difficult as well as increasing hull weight. In order to alleviate these problems, Nippon Steel Corporation has developed YP47 class steel plates instead of conventional YP40 class steel plates in cooperation with Mitsubishi Heavy Industries, Ltd. and ClassNK, which were then applied to actual ships. Double integrity for safety was the concept established as a development goal. That is, the goal is to put steel plates into practical application which are hard to initiate brittle cracks, and are capable of arresting brittle cracks even in the event of brittle crack occurrence. YP47steel plates have been manufactured on the basis of TMCP utilizing a variety of technologies; for example the technology for dispersing fine oxidation products into steel for enhancing weld toughness, and the technology for rolling in short time period at low temperature for enhancing arrestability. Large-size container ships, which use or are scheduled to use YP47steel plate, amount to approximately 50 vessels since the delivery in 2006.
Marine container transport has begun in the 1960s aiming for efficient harbor loading and unloading, realization of land and sea intermodal transport, reduction in transport time and cost, and so on, and has expanded rapidly since then. Now almost all major regular lines in the world employ containerization, thus making it main means for transport supporting the world's trade and economy. The most effective means in terms of reduction in environmental burdens and reduction in transport cost is to make the size of ships larger, and the trend toward increasing the size of ships is shown in Fig. 1.
