Abstract
A new type of steel plate with high fracture toughness has been developed by further improving TMCP (Thermo Mechanical Control Process) technology. The steel plate possesses s__-urface layers of u__-ltra f__-ine grain microstructure (SUF) of 1 to 3 micrometers in diameter, of which thickness is one sixth of the plate thickness. The SUF provides various advantages: for example, the SUF makes it difficult for unstable brittle crack to penetrate into the plate from its surface, dramatically improves crack arrest toughness against unstable brittle crack propagation, and so on. In the present report, the fracture toughness (fracture properties against unstable brittle crack initiation from through-thickness notch) of the developed plate with SUF has been investigated. The results suggest the following: (1) Fracture toughness, Kc value in the surface layer region of the developed plate is hardly deteriorated by tensile pre-strain of about 10 to 18%. (2) A new equation has been developed in order to predict Kc value from the result of Charpy V-notch impact test using sub-size specimens. (3) It is little expected that the SUF region significantly improves the fracture toughness against the unstable crack initiation from the through-thickness notch or crack.
