表層超細粒化による高アレスト鋼板の諸特性

Abstract

A new type of steel plate with high crack arrestability has been developed by controlling its microstructure especially in surface layers of the plate. The microstructure of the surface layers of the plate is ultra-fine grained (so-called SUF), which provides excellent fracture toughness even at a cryogenic temperature. Crack arrestability of the plate has been further improved by the enhanced formation of side-ligaments (shear-lips) due to SUF, since unstable running crack behaviour is strongly affected by these shear-lips, which are formed in the surface layers of the plate after the unstable brittle crack has propagated in the mid-section of the plate.In the present report, the practical properties of the developed plate (so-called SUF plate) are investigated. Fracture tests confirm that the SUF plate possesses ultra high crack arrestability. The improved value of arrestability due to the SUF effect can be predicted by fracture mechanics.For practical use of the SUF plate, the damage to the surface layers in actual service should be considered, since the SUF region (surface layers) is essential for obtaining ultra high arrestability in the SUF plate. Considering corrosion loss on an un-painted surface of the plate, the SUF plate, which is prepared by cutting down one side surface to remove 'SUF', is evaluated for crack arrestability by ESSO tests. To simulate the welding damage to the SUF plate, bead-on-SUF plate specimens are tested for crack arrestability by ESSO tests. Both test results suggest that the SUF plate possesses sufficient crack arrestability even in actual service.Furthermore, it is also confirmed that the SUF plate has other characteristics, such as weldability and workability, which make it suitable for ship-building use, such characteristics being similar to those of a conventional-steel plate, including TMCP steel plate.