Development of YP460 Class Steel Plate with Excellent Brittle Crack Arrestability for Large Container Ships

摘要

Since brittle fracture occurred at a hull structure of large size container ships has a possibility to cause catastrophic damage, structural integrity should be ensured by prevention to initiate brittle crack and / or prevention of brittle crack propagation in case it may occur (arrestability). Especially, research for crack arrestability of steel plates has been widely conducted in order to compile guidelines for arrestability requirements. In addition, a recent trend in container ships is a huge increase in ship size over 15,000 TEU class appropriate for transport efficiency and low cost operation. Consequently high strength (YP460) steel plates with not only heavy thickness but high crack arrestability performance are in high demand with the trend of enlargement of ships. In general, crack arrestability of steel plate is related to the impact property of the plate through microstructure refining by TMCP process, particularly by means of lowering heating temperature and rolling temperature. JFE Steel Corporation has developed an innovative solution for compatibility of high arrestability and high strength for heavy thickness steel plate over 70mm, by means of texture controlled TMCP process with chemical compositions optimization. The developed YP460 (EH47 grade) heavy thickness steel plate has an excellent arrestability over 15,000 N/mm~(3/2), adequately more than double for guidelines on brittle crack arrest design issued by some classification societies. Characteristics of the plate, manufactured by TMCP process, are highly oriented transformation texture for prevention of brittle crack propagation in addition to excellent impact toughness. Alloy design of the developed steel is relatively high carbon equivalent (Ceq.) but low carbon content in order to satisfy required tensile properties for every thickness portion. Moreover, the steel also exhibits low cold cracking susceptibility equal to conventional YP390 steel plates due to its low PCM design, consequently good weldability.