抄録
The microstructure and toughness of steels corresponding to the chemical composition of the segregated zone in the plate by continuous casting process or in the ingot by non-continuous process were investigated by means of microstructural observations and notch tensile tests to clarify the degree of embrittlement of the segregated zone.
The nil-ductility transition temperature (NDTT) in notch tensile tests of the martensite-bainite duplex structure due to the high manganese and high phosphorus contents is above +200°C, while that of the ferrite-pearlite structure containing low manganese contents with even high phosphorus contents is below -100°C. Therefore, the embrittlement caused by the independent increase of phosphorus content is comparatively small, whereas the one caused by the formation of low-temperature transformed structure due to the increase of both manganese and phosphorus contents is large. Here the embrittlement becomes small by tempering at 650°C even when the low-temperature transformed structure is formed. The fracture mode is transferred from cleavage to intergranular with the increase of phophorus content, which bring about lower fracture strength.
